ca alys s
Re iew
O ganome allic Nanopa icles Liga ed by NHCs:
Syn hesis, Su ace Chemis y and Ligand E ec s
Ch is ian Ce ezo-Na a e e 1, Pa icia La a 2,* and Luis M. Ma ínez-P ie o 1,*
1ITQ, Ins i u o de Tecnología Química, Uni e si a Poli ècnica de València (UPV), A . de los Na anjos S/N,
46022 Valencia, Spain; [email p o ec ed].es
2
Ins i u o de In es igaciones Qu
í
micas (IIQ), Depa amen o de Qu
í
mica Ino g
á
nica and Cen o de Inno aci
ó
n
en Química A anzada (ORFEO-CINQA), CSIC—Uni e sidad de Se illa, 41092 Se illa, Spain
*Co espondence: [email p o ec ed] (P.L.); [email p o ec ed] (L.M.M.-P.)
Recei ed: 23 Sep embe 2020; Accep ed: 30 Sep embe 2020; Published: 3 Oc obe 2020
Abs ac :
O e he las 20 yea s, he use o me allic nanopa icles (MNPs) in ca alysis has awakened
a g ea in e es in he scien i ic communi y, mainly due o he many ad an ages o his kind o
nanos uc u es in ca aly ic applica ions. MNPs exhibi he cha ac e is ic s abili y o he e ogeneous
ca alys s, bu wi h a highe ac i e su ace a ea han con en ional me allic ma e ials. Howe e , despi e
hei highe ac i i y, MNPs p esen a wide a ie y o ac i e si es, which makes i di icul o con ol
hei selec i i y in ca aly ic p ocesses. An e icien way o modula e he ac i i y/selec i i y o MNPs
is he use o coo dina ing ligands, which ans o ms he MNP su ace, subsequen ly modi ying he
nanopa icle ca aly ic p ope ies. In ela ion o his, he use o N-he e ocyclic ca benes (NHC) as
s abilizing ligands has demons a ed o be an e ec i e ool o modi y he size, s abili y, solubili y
and ca aly ic eac i i y o MNPs. Al hough NHC-s abilized MNPs can be p epa ed by di e en
syn he ic me hods, his e iew is cen e ed on hose p epa ed by an o ganome allic app oach. He e,
an o ganome allic p ecu so is decomposed unde H
2
in he p esence o non-s oichiome ic amoun s
o he co esponding NHC-ligand. The esul ing o ganome allic nanopa icles p esen a clean su ace,
which makes hem pe ec candida es o ca aly ic applica ions and su ace s udies. In sho , his
e ision s udy emphasizes he g ea e sa ili y o NHC ligands as MNP s abilize s, as well as hei
in luence on ca alysis.
Keywo ds:
o ganome allic nanopa icles; N-he e ocyclic ca benes; o ganome allic app oach; su ace
chemis y; ligand e ec s; ca alysis; hyd ogena ion eac ions; H/D exchanges
1. In oduc ion
Since he beginning o his cen u y, me allic nanopa icles (MNPs) ha e eme ged in he ield
o chemis y as new nano-objec s wi h a g ea po en ial in ca alysis. The eason o his in e es
esides in he ac ha hey combine he ad an ages o homogeneous (molecula complexes) and
he e ogeneous (me als dispe sed on suppo s) ca alys s [
1
,
2
]. MNPs p esen he dis inc i e s abili y
o he e ogeneous ca alys s, bu wi h a highe ac i e su ace a ea. Due o hei cha ac e is ic small
size (be ween 1 and 100 nm), MNPs p esen a high p opo ion o su ace a oms, which esul s in a
la ge numbe o ac i e cen e s ha acili a e he ca alys –subs a e in e ac ions. Howe e , his g ea
numbe o ac i e su ace si es no mally con e he subs a es in di e en ways; he e o e, o achie e
a p ecise con ol o MNP selec i i y is c ucial in his eme ging ield. An e ec i e way o con ol
he eac i i y and selec i i y o MNPs is by using ancilla y ligands as s abilize s. As well as in
o ganome allic chemis y, coo dina ing ligands a e able o al e he s e ic and elec onic p ope ies o
he me allic su ace, and hus o change he MNP ca aly ic p ope ies. Indeed, he e a e many pa allels
be ween ancilla y ligands liga ed o MNPs and molecula complexes [
3
]. Fo example, in bo h cases,
Ca alys s 2020,10, 1144; doi:10.3390/ca al10101144 www.mdpi.com/jou nal/ca alys s
Ca alys s 2020,10, 1144 2 o 30
ligands a e usually chemically bound o he me al a oms h ough he dona ion o a ee elec on pai
om he ligand (which beha es as a Lewis base) o he me al. In his con ex , N-he e ocyclic ca benes
(NHC) ha e appea ed as e icien ligands o MNP s abiliza ion, which no only can modi y he size,
s abili y, and solubili y o MNPs, bu also a e able o modula e hei ca aly ic eac i i y.
NHCs ha e been ex ensi ely s udied o e he pas 30 yea s [
4
,
5
]. Since Be and and A duengo
isola ed and cha ac e ized he i s example o his kind o neu al compounds (Figu e 1a), i s in e es
as a ligand has signi ican ly inc eased [
6
,
7
]. The g ea e s abili y o NHC ligands in espec o o he
ype o ca benes is mainly due o hei speci ic elec onic s uc u e. The wo adjacen N a oms o he
ca benic ca bon a om s abilize he emp y p-o bi al o he las one by elec onic esonance and pa ially
emo e he elec onic densi y om i s occupied
σ
-o bi al h ough he C–N bonds (Figu e 1b) [
8
].
Thanks o hei speci ic elec onic p ope ies, NHCs p esen nume ous ad an ages as ancilla y ligands.
Fo example, due o hei elec on-dona ing abili y, hese ligands s ongly bind o ansi ion me als.
In addi ion, in con as o phosphines, hey do no easily oxidize as hei chemical s uc u e only
con ains C, H and N a oms. As a esul , NHCs ha e been p omo ed om a scien i ic cu iosi y o one o
he mos e sa ile ligands in coo dina ion chemis y [
9
–
11
]. As excellen ligands o ansi ion me al
complexes, hey ha e been applied in some o he mos impo an ca aly ic p ocesses in chemis y. The
abo e-men ioned cha ac e is ic o NHCs no only make hem pe ec candida es o he o ma ion o
homogeneous ca alys s, bu also allow hei use in MNP syn hesis, showing a g ea abili y o s abilize
MNPs [
12
–
16
]. Indeed, du ing he las 10 yea s, NHC ligands ha e been employed o s abilize a
conside able numbe o MNPs o di e en me als, such as Au [
17
], Pd [
18
,
19
], Ru [
20
] and I [
21
]. The
g ea e sa ili y o hese s abilizing ligands has allowed he explo a ion o he in luence o he s uc u e
o he NHCs (i.e., N-subs i uen s and he g oups bound on he backbone o he imidazolium ing) in
he MNP p ope ies, such as size, solubili y, s abili y, su ace s a e and eac i i y. The e o e, i has been
obse ed ha he na u e and he amoun o NHC employed in he p epa a ion o he MNPs ha e a
clea in luence and modi y he physicochemical p ope ies o NHC-s abilized MNPs. In o he wo ds,
depending on he N-subs i uen (elec on dono /accep o o bulky g oups), o he numbe o NHC
equi alen s used as s abilize , he esul ing MNPs will ha e di e en a ailable ac i e su ace si es o
ca alysis, enabling o each a p ecise con ol o he eac i i y and selec i i y o hese colloids. Fo all his,
MNPs s abilized wi h NHC ligands ha e been success ully used in many ca aly ic p ocesses. Indeed,
hey ha e shown a high ac i i y and selec i i y in nume ous hyd ogena ion eac ions, speci ically in
he hyd ogena ion o a oma ic and ke ones g oups [
12
–
14
,
22
]. Fu he mo e, NHC-based MNPs ha e
also been applied in oxida ion [
13
], hyd obo a ion [
23
,
24
] and deu e a ion [
25
,
26
] eac ions among
o he s, making possible he s udy o he in luence o he ligands in mos o he cases.
Ca alys s2020,10,11442o 30
bo hcases,ligandsa eusuallychemicallybound o heme ala oms h ough hedona iono a ee
elec onpai om heligand(whichbeha esasaLewisbase) o heme al.In hiscon ex ,N‐
he e ocyclicca benes(NHC)ha eappea edase icien ligands o MNPs abiliza ion,whichno only
canmodi y hesize,s abili y,andsolubili yo MNPs,bu alsoa eable omodula e hei ca aly ic
eac i i y.
NHCsha ebeenex ensi elys udiedo e hepas 30yea s[4,5].SinceBe andandA duengo
isola edandcha ac e ized he i s exampleo hiskindo neu alcompounds(Figu e1a),i sin e es
asaligandhassigni ican lyinc eased[6,7].Theg ea e s abili yo NHCligandsin espec oo he
ypeo ca benesismainlydue o hei speci icelec onics uc u e.The woadjacen Na oms o he
ca benicca bona oms abilize heemp yp‐o bi alo helas onebyelec onic esonanceandpa ially
emo e heelec onicdensi y omi soccupiedσ‐o bi al h ough heC–Nbonds(Figu e1b)[8].
Thanks o hei speci icelec onicp ope ies,NHCsp esen nume ousad an agesasancilla y
ligands.Fo example,due o hei elec on‐dona ingabili y, heseligandss onglybind o ansi ion
me als.Inaddi ion,incon as ophosphines, heydono easilyoxidizeas hei chemicals uc u e
onlycon ainsC,HandNa oms.Asa esul ,NHCsha ebeenp omo ed omascien i iccu iosi y o
oneo hemos e sa ileligandsincoo dina ionchemis y[9–11].Asexcellen ligands o ansi ion
me alcomplexes, heyha ebeenappliedinsomeo hemos impo an ca aly icp ocessesin
chemis y.Theabo e‐men ionedcha ac e is ico NHCsno onlymake hempe ec candida es o
he o ma iono homogeneousca alys s,bu alsoallow hei useinMNPsyn hesis,showingag ea
abili y os abilizeMNPs[12–16].Indeed,du ing helas 10yea s,NHCligandsha ebeenemployed
os abilizeaconside ablenumbe o MNPso di e en me als,suchasAu[17],Pd[18,19],Ru[20]
andI [21].Theg ea e sa ili yo heses abilizingligandshasallowed heexplo a iono he
in luenceo hes uc u eo heNHCs(i.e.,N‐subs i uen sand heg oupsboundon hebackboneo
heimidazolium ing)in heMNPp ope ies,suchassize,solubili y,s abili y,su aces a eand
eac i i y.The e o e,i hasbeenobse ed ha hena u eand heamoun o NHCemployedin he
p epa a iono heMNPsha eaclea in luenceandmodi y hephysicochemicalp ope ieso NHC‐
s abilizedMNPs.Ino he wo ds,dependingon heN‐subs i uen (elec ondono /accep o o bulky
g oups),o henumbe o NHCequi alen susedass abilize , he esul ingMNPswillha edi e en
a ailableac i esu acesi es o ca alysis,enablingap ecisecon olo he eac i i yandselec i i y
o hesecolloids obe eached.Fo all his,MNPss abilizedwi hNHCligandsha ebeensuccess ully
usedinmanyca aly icp ocesses.Indeed, heyha eshownahighac i i yandselec i i yinnume ous
hyd ogena ion eac ions,speci icallyin hehyd ogena iono a oma icandke onesg oups[12–
14,22].Fu he mo e,NHC‐basedMNPsha ealsobeenappliedinoxida ion[13],
hyd obo a ion
[23,24]anddeu e a ion[25,26] eac ionsamongo he s,makingpossible hes udyo hein luenceo
heligandsinmos o hecases.
Figu e1.(a)Fi s exampleo anisola edN‐he e ocyclicca benes(NHC)ligand.(b)Elec onic
s uc u eo anN‐he e ocyclicca beneligand.
NHC‐s abilizedMNPsha ebeenmainlysyn hesizedbydi ec decomposi iono NHC‐based
me alcomplexes,byaligand‐exchangep ocess,o by he educ iono ame alp ecu so in he
p esenceo aselec edNHCligand.Wi hin hisla e ,wecan ind heo ganome allicapp oach,which
consis so hecon olleddecomposi iono ahigh‐ene gyo ganome allicp ecu so unde a educ i e
Figu e 1.
(
a
) Fi s example o an isola ed N-he e ocyclic ca benes (NHC) ligand. (
b
) Elec onic s uc u e
o an N-he e ocyclic ca bene ligand.
NHC-s abilized MNPs ha e been mainly syn hesized by di ec decomposi ion o NHC-based
me al complexes, by a ligand-exchange p ocess, o by he educ ion o a me al p ecu so in he p esence
o a selec ed NHC ligand. Wi hin his la e , we can ind he o ganome allic app oach, consis s in he
con olled decomposi ion o a high-ene gy o ganome allic p ecu so unde a educ i e gas (H
2
o
CO) in he p esence o a non-s oichiome ic amoun o an NHC ligand ha con ols he coalescence
Ca alys s 2020,10, 1144 3 o 30
p ocess (Figu e 2). The o ganome allic app oach has nume ous ad an ages compa ed o he es o he
syn he ic me hods. Fi s , he use o a p e- educed o ganome allic complex pe mi s he decomposi ion
unde mild condi ions (H
2
p essu e and oom empe a u e), con olling he MNP g ow h, size and
dispe sion. Second, unlike he con en ional MNP syn hesis ia chemical educ ion, he non-u iliza ion
o seconda y educing agen s allows ob aining MNPs wi h a clean su ace, ee o con aminan s ha can
come om he educ an o he me al sal [
27
]. Howe e , despi e hei nume ous ad an ages, i is no a
commonly used syn he ic me hodology due o he ac ha i is necessa y o employ an o ganome allic
p ecu so , which is no always comme cially a ailable o equi es a complex syn hesis. Mo eo e ,
hese o ganome allic p ecu so s a e usually e y sensi i e compounds ha need o be s o ed and
manipula ed unde ine a mosphe e. Despi e his ac , since he i s desc ibing o NHC-s abilized Ru
nanopa icles [
12
], he e has been a solid scien i ic line consis ing in he de elopmen o he syn hesis,
su ace s udies and ca aly ic applica ions o MNPs liga ed by NHCs ob ained om an o ganome allic
app oach [28].
Ca alys s2020,10,11443o 30
gas(H
2
o CO)in hep esenceo anon‐s oichiome icamoun o anNHCligand ha con ols he
coalescencep ocess(Figu e2).Theo ganome allicapp oachhasnume ousad an agescompa ed o
he es o hesyn he icme hods.Fi s , heuseo ap e educedo ganome alliccomplexpe mi s
decomposi ionunde mildcondi ions(H
2
p essu eand oom empe a u e),con olling heMNP
g ow h,sizeanddispe sion.Second,unlike hecon en ionalMNPsyn hesis iachemical educ ion,
henon‐u iliza iono seconda y educingagen sallowsMNPswi hacleansu ace, eeo
con aminan s ha cancome om he educ an o heme alsal obeob ained[27].Howe e ,despi e
hei nume ousad an ages,i isno acommonlyusedsyn he icme hodologydue o he ac ha i
isnecessa y oemployano ganome allicp ecu so ,whichisno alwayscomme ciallya ailableo
equi esacomplexsyn hesis.Mo eo e , heseo ganome allicp ecu so sa eusually e ysensi i e
compounds ha need obes o edandmanipula edunde anine a mosphe e.Despi e his ac ,
since he i s desc ibingo NHC‐s abilizedRunanopa icles[12], he ehasbeenasolidscien i icline
consis ingin hede elopmen o hesyn hesis,su aces udiesandca aly icapplica ionso MNPs
liga edbyNHCsob ained omano ganome allicapp oach[28].
Figu e2.Fo ma iono NHC‐s abilizedme allicnanopa icles(MNPs)s a ing omahigh‐ene gy
o ganome allicp ecu so ( ed,o ganome allicapp oach)o ame allicsal (blue,con en ionalMNP
syn hesis).
Al houghdu ing ecen yea s henumbe o su aces udieshasp oli e a ed, he eiss illaclea
need o hede elopmen o undamen als udieso hecoo dina ion,loca ionanddynamicso
su aceligandsonMNPs,as heyin luence hei ca aly icp ope ies.In his espec ,o ganome allic
NPsliga edbyNHCsha edemons a ed obesui ablenano‐objec s o su aces udies.Thei clean
su ace,s ongligand–me alin e ac ionand hepossibili y ocon ol hei ca aly icp ope ies
h ough hes abilizingligandsmake hempe ec candida es o su acein es iga ions.
T adi ionally,COhasbeenusedasap obemolecule o hede e mina iono loca ionanddynamics
o coo dina edligandsonme alsu acesbymagic‐anglespinningsolid‐s a eNMR(MAS‐NMR)and
Fou ie T ans o mIn a ed(FT‐IR).Apionee in a edspec oscopys udyo B adleye al.abou CO
coo dina iononPdNPsde ined hecoo dina ionmodeso COon heMNPsu ace.WhileCO
coo dina eswi h heMNP acesinab idgingmode(CO
b
),i adop sa e minalmode(CO
)when
adso bedon hemos exposeda omso hesu ace(apexesandedges)[29].Asho imela e ,in
collabo a ionwi hChaud e , heydemons a ed ha heCO
/CO
b
a iodependson hepa iclesize
andinc easeswhen heMNPsizedec eases[30].The e o e, h ough hecoo dina iono CO, he ypes
o a ailableac i esi es ha exis a heMNPsu acecanbeeasilyiden i ied.Following heses udies,
in2010,No ioe al.de e mined heloca ionanddynamicso su aceligandsbysolid‐s a eNMR
Figu e 2.
Fo ma ion o NHC-s abilized me allic nanopa icles (MNPs) s a ing om a high-ene gy
o ganome allic p ecu so (
ed
, o ganome allic app oach) o a me allic sal (
blue
, con en ional
MNP syn hesis).
Al hough du ing ecen yea s he numbe o su ace s udies has p oli e a ed, he e is s ill a clea
need o he de elopmen o undamen al s udies o he coo dina ion, loca ion and dynamics o su ace
ligands on MNPs, as hey in luence hei ca aly ic p ope ies. In his espec , o ganome allic NPs liga ed
by NHCs ha e demons a ed o be sui able nano-objec s o su ace s udies. Thei clean su ace, s ong
ligand–me al in e ac ion and he possibili y o con ol hei ca aly ic p ope ies h ough he s abilizing
ligands make hem pe ec candida es o su ace in es iga ions. T adi ionally, CO has been used as
a p obe molecule o he de e mina ion o loca ion and dynamics o coo dina ed ligands on me al
su aces by magic-angle spinning solid-s a e NMR (MAS-NMR) and Fou ie T ans o m In a ed (FT-IR).
A pionee in a ed spec oscopy s udy o B adley e al. abou CO coo dina ion on Pd NPs de ined he
coo dina ion modes o CO on he MNP su ace. While CO coo dina es o he MNP aces in a b idging
mode (CO
b
), i adop s a e minal mode (CO
) when adso bs on he mos exposed a oms o he su ace
(apexes and edges) [
29
]. A sho ime la e , in collabo a ion wi h Chaud e , hey demons a ed ha he
CO
/CO
b
a io depends on he pa icle size and inc eases when he MNP size dec eases [
30
]. The e o e,
h ough he coo dina ion o CO, he ypes o a ailable ac i e si es ha exis a he MNP su ace can be
easily iden i ied. Following hese s udies, in 2010,
No io e al.
de e mined he loca ion and dynamics
Ca alys s 2020,10, 1144 4 o 30
o su ace ligands by solid-s a e NMR [
31
], which also gi es in o ma ion abou he coo dina ion o he
capping ligands on he me allic su ace. Fu he mo e, X- ay pho oelec on spec oscopy (XPS) has been
ecen ly p esen ed as a p omising ool o s udy he coo dina ion modes o ancilla y ligands on he MNP
su ace, apa om p o iding chemical in o ma ion abou he me allic su ace a oms [
32
]. The e o e,
FT-IR, XPS and solid-s a e MAS-NMR spec oscopy a e es ablished echniques o su ace s udies,
which allow he iden i ica ion o he su ace ac i e si es as well as he in es iga ion o he coo dina ion,
loca ion and dynamics o su ace ligands on NHC-s abilized MNPs.
The aim o his e iew is o p o ide a gene al o e iew o o ganome allic NPs liga ed by
NHC ligands, ocusing he s udy on he ad an ages o he use o molecula ools o hei syn hesis
h ough an o ganome allic app oach. The i s pa includes a e ision o he h ee di e en
me hodologies, based on he o ganome allic app oach, ha a e commonly employed o he syn hesis o
NHC-based NPs. The second pa will deal wi h he su ace s udies o hese nano-objec s, enume a ing
he di e en spec oscopic echniques used and highligh ing he ele ance o CO adso p ion o hese
s udies. The las pa o his e iew will ocus on he e ec o s abilizing NHC ligands ela ed o he
s abili y, solubili y and ca aly ic ac i i y o MNPs.
2. Syn hesis o NHC-S abilized MNPs ollowing he O ganome allic App oach
Since he ea ly nine ies, whe e in a collabo a i e e o be ween B adley and Chaud e ,
Ru(COD)(COT) (COD =cyclooc adiene; COT =cyclooc a iene) was decomposed o he i s ime
unde a hyd ogen s eam o o m Ru NPs [
33
], he educ ion o high ene gy o ganome allic p ecu so s
as a clean and easy ou e o he o ma ion o MNPs has been widely used as a syn he ic me hod.
This o ganome allic app oach is based on he adequa e selec ion o he p ecu so and he s abilize
ha a e going o be used. The o ganome allic p ecu so s a e usually ze o- o low- alen me al
complexes, which unde mild condi ions (e.g., 3 ba H
2
and oom empe a u e) easily decompose
o o m he co esponding nanopa icles (Figu e 3). No mally, his decomposi ion occu s h ough
he clea age o he M–C bond, which can be a sigma bond (alkyl, a yl o allyl), pi bond (ole ins)
o a combina ion o bo h [
34
]. In addi ion o hese p ecu so s, amides and ni ogen con aining
complexes ha e been also used o MNP syn hesis h ough he o ganome allic app oach (Figu e 3).
Each ype o p ecu so p esen s a se ies o bene i s and disad an ages. Fo example, ole inic complexes
a e especially app op ia e since he unsa u a ed ligands a e easily educed unde H
2
owa d he
co esponding ine alkane, which does no in e ac wi h he MNP su ace. On he o he hand, amides
o ni ogen-con aining complexes can p oduce amines a e hei decomposi ion ha coo dina e on
he pa icle su ace and may modi y he shape o he esul ing MNPs. The decomposi ion a e o
he o ganome allic p ecu so s also plays an impo an ole in he MNP syn hesis, as i di ec s he
nuclea ion and g ow h s eps. When he decomposi ion a e is slow, he nuclea ion s ep is limi ed,
and NPs a e bigge han when he decomposi ion a e is as . The e o e, o ganome allic NPs wi h
di e en sizes and mo phologies ha e been epo ed by using di e se o ganome allic p ecu so s on
his syn he ic ou e, including colloidal sys ems o noble me al NPs (Ru, P , Pd) [
27
,
28
] o magne ic
NPs o he i s ansi ion se ies (Fe, Co) [35,36].
Ano he key pa ame e o ake in o accoun in MNP syn hesis is he s abilize , which can be
a polyme [
37
,
38
], ligand [
39
], ionic liquid (IL) [
40
], sol en [
41
] o e en a solid suppo [
42
–
44
].
Apa om con olling he p ocess o coalescence, he s abilize s a e a co ne s one in MNP syn hesis,
since hey go e n hei inal size and mo phology. Addi ionally, s abilize s can con ol he physical
and chemical p ope ies o MNPs and hus modula e hei su ace eac i i y. Among all o hem,
ancilla y ligands a e excellen s abilize s o MNPs p epa ed by he o ganome allic app oach, since,
as al eady men ioned, hey a e able o ans o m he MNP su ace p ope ies. The o ma ion o chemical
bonds be ween he o ganic ligands and su ace me al a oms esul s in a s ong elec onic s abiliza ion,
which leads o impo an modi ica ions o he chemical eac i i y o MNPs. In his con ex , NHCs ha e
demons a ed o be a amily o e sa ile s abilizing ligands ha a e able o modi y he elec onic o
s e ic p ope ies o MNPs by modi ying hei N-subs i uen s o imidazolium s uc u e [12–16,45].
Ca alys s 2020,10, 1144 5 o 30
Ca alys s2020,10,11445o 30
Figu e3.Exampleo o ganome allicp ecu so susedin heo ganome allicapp oach o he
p epa a iono MNPs.CuMes(Mes=mesi ylene);Ru(COD)(COT)(COD=cyclooc adiene;COT=
cyclooc a iene);P (DME)(COD)(DME=dime hyl);Rh(allyl)
3
(allyl=η
3
‐C
3
H
5
);{Fe[N‐(SiMe
3
)
2
]
2
}
2
.
Ano he keypa ame e o akein oaccoun inMNPsyn hesisis hes abilize ,whichcanbea
polyme [37,38],ligand[39],ionicliquid(IL)[40],sol en [41]o e enasolidsuppo [42–44].Apa
omcon olling hep ocesso coalescence, hes abilize sa eaco ne s oneinMNPsyn hesis,since
heygo e n hei inalsizeandmo phology.Addi ionally,s abilize scancon ol hephysicaland
chemicalp ope ieso MNPsand husmodula e hei su ace eac i i y.Amongallo hem,ancilla y
ligandsa eexcellen s abilize s o MNPsp epa edby heo ganome allicapp oach,since,asal eady
men ioned, heya eable o ans o m heMNPsu acep ope ies.The o ma iono chemicalbonds
be ween heo ganicligandsandsu aceme ala oms esul sinas ongelec onics abiliza ion,which
leads oimpo an modi ica ionso hechemical eac i i yo MNPs.In hiscon ex ,NHCsha e
demons a ed obea amilyo e sa iles abilizingligands ha a eable omodi y heelec onico
s e icp ope ieso MNPsbymodi ying hei N‐subs i uen so imidazoliums uc u e[12–16,45].
Thesyn hesiso NHC‐s abilizedo ganome allicNPswas i s epo edin2011byLa ae al.,
whoused1,3‐bis(2,6‐diisop opylphenyl)imidazol‐2‐ylidene(IP )andN,N‐di( e ‐bu yl)imidazol‐2‐
ylidene(I Bu) o hes abiliza iono RuNPs(Figu e4a)[12].Mo especi ically, heseNHC‐s abilized
MNPswe eob ainedbya educ ioninRu(COD)(COT)a oom empe a u eunde 3ba H
2
andin
hep esenceo I BuandIP (0.2and0.5equi alen s).In e es ingdi e encesinsize,s abili yand
ca aly icac i i ywe eobse edasa unc iono heamoun o s abilize usedand hena u eo he
N‐subs i uen s.Fo example,in hecaseo hebulkyI Bu,i wasnecessa y oadd0.5equi . o
s abilizeRuNPs,sincewhenemploying0.2equi .,ablackp ecipi a ewasobse ed.Su aces udies
allowed heau ho s oobse e ha I Bumos lycoo dina eswi h hemo eexposedsu acea oms
(apexesandedges),as hebulky Bug oupsp e en hei coo dina iono henanopa icle aces.On
heo he hand,s ableRuNPswe eob ainedbyusingei he 0.2o 0.5equi .o IP .In hiscase, he
highe heamoun o s abilize , hesmalle heMNPsize ound.Thisbeha io wasp e iously
obse edonRuNPss abilizedwi ho he cappingligandssuchasalkylamineso alkyl hiols[46].
Thesyn hesiso Rhnanopa icless abilizedbyNHCligandswasdesc ibeda ewyea sla e by
Goda de al.[47].Thesenanopa icleswe ealsop epa edby hedecomposi iono he
o ganome allicp ecu so Rh(η
3
‐C
3
H
5
)
3
(Figu e3)in hep esenceo non‐s oichiome icamoun so
IP ligand( om0.2 o0.6equi .).Smallandmonodispe senanopa iclesdisplayingmeansizes
be weenc.a.1.3 o1.7nmwe eob ained.Again,smalle meandiame e swe ede ec edas he
ligand/me al a ioinc eased.Thes ongin e ac ionbe ween heNHCand heRhsu acewas
con i medbyliganddisplacemen expe imen swi hhighCOp essu es,PPh
3
andP(OPh)
3
.
Addi ionally, heycon i medbysolu ionandsolid‐s a eNMR heco‐exis enceo IP and he
p o ona edca bene(IP
.
H
+
)a heMNPsu ace.Theca aly icac i i yo hesecolloidswass udied
Figu e 3.
Example o o ganome allic p ecu so s used in he o ganome allic app oach o he p epa a ion
o MNPs. CuMes (Mes =mesi ylene); Ru(COD)(COT) (COD =cyclooc adiene; COT =cyclooc a iene);
P (DME)(COD) (DME =dime hyl); Rh(allyl)3(allyl =η3-C3H5); {Fe[N-(SiMe3)2]2}2.
The syn hesis o NHC-s abilized o ganome allic NPs was i s epo ed in 2011 by La a e al., who
used 1,3-bis(2,6-diisop opylphenyl)imidazol-2-ylidene (IP ) and N,N-di( e -bu yl)imidazol-2-ylidene
(I Bu) o he s abiliza ion o Ru NPs (Figu e 4a) [
12
]. Mo e speci ically, hese NHC-s abilized MNPs
we e ob ained by educ ion o Ru(COD)(COT) a oom empe a u e unde 3 ba H
2
and in he p esence
o I Bu and IP (0.2 and 0.5 equi alen s). In e es ing di e ences in size, s abili y and ca aly ic ac i i y
we e obse ed as a unc ion o he amoun o s abilize used and he na u e o he N-subs i uen s.
Fo example, in he case o he bulky I Bu, i was necessa y o add 0.5 equi . o s abilize Ru NPs,
since when employing 0.2 equi ., a black p ecipi a e was obse ed. Su ace s udies allowed he au ho s
o obse e ha I Bu mos ly coo dina es o he mo e exposed su ace a oms (apexes and edges), as he
bulky Bu g oups p e en hei coo dina ion o he nanopa icle aces. On he o he hand, s able Ru NPs
we e ob ained by using ei he 0.2 o 0.5 equi . o IP . In his case, he highe he amoun o s abilize , he
smalle he MNP size ound. This beha io was p e iously obse ed on Ru NPs s abilized wi h o he
capping ligands such as alkylamines o alkyl hiols [
46
]. The syn hesis o Rh nanopa icles s abilized
by NHC ligands was desc ibed a ew yea s la e by Goda d e al. [
47
]. These nanopa icles we e
also p epa ed by he decomposi ion o he o ganome allic p ecu so Rh(
η3
-C
3
H
5
)
3
(Figu e 3) in he
p esence o non-s oichiome ic amoun s o IP ligand ( om 0.2 o 0.6 equi .). Small and monodispe se
nanopa icles displaying mean sizes be ween c.a. 1.3 o 1.7 nm we e ob ained. Again, smalle mean
diame e s we e de ec ed as he ligand/me al a io inc eased. The s ong in e ac ion be ween he
NHC and he Rh su ace was con i med by ligand displacemen expe imen s wi h high CO p essu es,
PPh
3
and P(OPh)
3
. Addi ionally, hey con i med by solu ion and solid-s a e NMR he co-exis ence o
IP and he p o ona ed ca bene (IP
.
H
+
) a he MNP su ace. The ca aly ic ac i i y o hese colloids
was s udied in di e en educ ion eac ions (phenol de i a i es and N-he e oa oma ics), being ac i e
bu highly sensi i e o he size o he subs a es.
The syn he ic me hodology employed in he p e iously men ioned wo ks implies he isola ion
o he ee NHC ligand in a p elimina y s ep. Howe e , mos o NHC ligands a e non-isolable due
o he high eac i i y o he ca benic ca bon, he eby limi ing he numbe o NHCs ha could be
used o MNP s abiliza ion. Wi h he in en ion o ci cum en his limi a ion, Ma
í
nez-P ie o e al.
de eloped a new app oach o s abilize Ru NPs wi h non-isolable NHCs [
20
], which is cen e ed on he
in-si u o ma ion o he ee ca bene. A e he dep o ona ion o he imidazolium sal wi h KO Bu
in solu ion, he co esponding non-isolable NHC ligand is ans e ed o he eac o con aining he
o ganome allic p ecu so by il a ion h ough Celi e. Then, he eac ion mix u e is p essu ized wi h H
2
,
Ca alys s 2020,10, 1144 6 o 30
o ming he NHC-s abilized MNPs. The il a ion s ep ensu es he elimina ion o he ino ganic sal s
o med in he dep o ona ion s ep ha could coo dina e o he su ace nanopa icle, poisoning i ,
and blocking he ac i e si es. Ma
í
nez-P ie o e al. alida ed he new me hodology by he o ma ion
o Ru NPs liga ed by 1,3-dicyclohexylimidazol-2-ylidene (Ru/ICy) by he compa ison o he wo
syn he ic ou es: om (1) he ee ca bene ICy (o iginal me hodology, Scheme 1), and (2) by he
in-si u dep o ona ion o he co esponding imidazolium sal , ICy
·
HCl (new me hodology,Scheme 1).
When compa ing he wo syn he ic me hods by T ansmission Elec on Mic oscopy (TEM), bo h showed
a simila size and dis ibu ion (1.23 and 1.25 nm o he o iginal and new me hodology, espec i ely;
see Scheme 1), hus es ablishing he new p ocedu e o he o ma ion o NHC-liga ed MNPs. In sum,
his new ou e ci cum en ed he main limi a ion o he o iginal me hod, whe e i was i s necessa y o
isola e he ee NHC, a oiding any possible con amina ion on he me allic su ace de i ed om he
imidazolium sal s and inc easing he numbe o NHC ligands ha could be used as s abilize s. In ac ,
ollowing his new me hodology, a g ea numbe o new NHC-s abilized o ganome allic NPs ha e
been p epa ed, including a ious me als such as, Ru, P , Ni and I , and nume ous non-isolable NHCs
wi h di e en s uc u es and mo phologies, such as chi al [
20
], long-chain [
13
,
23
] o hyd osoluble [
48
]
NHCs (Figu e 4).
Ca alys s2020,10,11446o 30
indi e en educ ion eac ions(phenolde i a i esandN‐he e oa oma ics),beingac i ebu highly
sensi i e o hesizeo hesubs a es.
Thesyn he icme hodologyemployedin hep e iouslymen ionedwo ksimplies heisola ion
o he eeNHCligandinap elimina ys ep.Howe e ,mos o NHCligandsa enon‐isolabledue o
hehigh eac i i yo heca benicca bon, he ebylimi ing henumbe o NHCs ha couldbeused
o MNPs abiliza ion.Wi h hein en ion oci cum en hislimi a ion,Ma ínez‐P ie oe al.
de elopedanewapp oach os abilizeRuNPswi hnon‐isolableNHCs[20],whichiscen e edon
hein‐si u o ma iono he eeca bene.A e hedep o ona iono heimidazoliumsal wi hKO Bu
insolu ion, heco espondingnon‐isolableNHCligandis ans e ed o he eac o con aining he
o ganome allicp ecu so by il a ion h oughCeli e.Then, he eac ionmix u eisp essu izedwi h
H
2
, o ming heNHC‐s abilizedMNPs.The il a ions epensu es heelimina iono heino ganic
sal s o medin hedep o ona ions ep ha couldcoo dina e o hesu acenanopa icle,poisoningi ,
andblocking heac i esi es.Ma ínez‐P ie oe al. alida ed henewme hodologyby he o ma ion
o RuNPsliga edby1,3‐dicyclohexylimidazol‐2‐ylidene(Ru/ICy)by hecompa isono he wo
syn he ic ou es: om(1) he eeca beneICy(o iginalme hodology,Scheme1),and(2)by hein‐
si udep o ona iono heco espondingimidazoliumsal ,ICy∙HCl(newme hodology,Scheme1).
Whencompa ing he wosyn he icme hodsbyT ansmissionElec onMic oscopy(TEM),bo h
showedasimila sizeanddis ibu ion(1.23and1.25nm o heo iginalandnewme hodology,
espec i ely;seeScheme1), huses ablishing henewp ocedu e o he o ma iono NHC‐liga ed
MNPs.Insum, hisnew ou eci cum en ed hemainlimi a iono heo iginalme hod,whe ei was
i s necessa y oisola e he eeNHC,a oidinganypossiblecon amina ionon heme allicsu ace
de i ed om heimidazoliumsal sandinc easing henumbe o NHCligands ha couldbeusedas
s abilize s.In ac , ollowing hisnewme hodology,ag ea numbe o newNHC‐s abilized
o ganome allicNPsha ebeenp epa ed,including a iousme alssuchas,Ru,P ,NiandI ,and
nume ousnon‐isolableNHCswi hdi e en s uc u esandmo phologies,suchaschi al[20],long‐
chain[13,23]
o hyd osoluble[48]NHCs(Figu e4).
Scheme1.Le :Syn hesiso Ru/ICy ollowing he(1)o iginaland he(2)newme hodology.Righ :
TEMmic og aphso Ru/ICy.Adap ed om e e ence20.Copy igh 2015Wiley‐VCH.
Scheme 1.
Le : Syn hesis o Ru/ICy ollowing he (
1
) o iginal and he (
2
) new me hodology. Righ :
TEM mic og aphs o Ru/ICy. Adap ed om e e ence 20. Copy igh 2015 Wiley-VCH.
Ca alys s 2020,10, 1144 7 o 30
Ca alys s2020,10,11447o 30
Figu e4.(a–b)IsolableNHCsusedbyLa ae al.in e e ences12and69.Non‐isolable(c)long‐chain,
(d)chi aland(e)hyd osolubleNHCsusedbyMa ínez‐P ie oe al.in e e ences13,23,20and48.
A hi dme hodologybasedin hedeca boxyla iono azwi e ionicCO2adduc hasbeen
ecen ly epo edbyCla e e al.[49].He e,1,3‐dialkylimidazolium‐2‐ca boxyla e,o gene al
o mulaR2Im‐CO2,wasusedasaca benep ecu so o he o ma iono NHC‐s abilized
o ganome allicNPs.Mo especi ically,Ni(COD)2was educedunde 3ba H2a 60°Cin hep esence
o di e en amoun so Me2Im‐CO2(0.1,0.2,0.5and1equi .) oa o dsmallandwell‐de inedNHC‐
s abilizedNiNPs(Figu e5).Asp e iouslyobse edbyLa ae al.wi hRu/I Bu[12],i wasnecessa y
oin oducea leas 0.5equi .o Me2Im‐CO2 oob ainacolloidalsuspensionwi hou agglome a ions
(Figu e5b).Thedi ec deca boxyla iono hisimidazoliumca boxyla ea oids hep e iousisola ion
o he eeca beneo anybasicp e ea men s.Howe e , henumbe o CO2adduc s ha canbeused
asaca benep ecu so islimi ed,andi canno beconside edagene alsyn he icme hod.
Figu e 4.
(
a
,
b
) Isolable NHCs used by La a e al. in e e ences 12 and 69. Non-isolable (
c
) long-chain,
(d) chi al and (e) hyd osoluble NHCs used by Ma ínez-P ie o e al. in e e ences 13, 23, 20 and 48.
A hi d me hodology based in he deca boxyla ion o a zwi e ionic CO
2
adduc has been
ecen ly epo ed by Cla e e al. [
49
]. He e, 1,3-dialkylimidazolium-2-ca boxyla e, o gene al o mula
R
2
Im-CO
2
, was used as ca bene p ecu so o he o ma ion o NHC-s abilized o ganome allic NPs.
Mo e speci ically, Ni(COD)
2
was educed unde 3 ba H
2
a 60
◦
C in he p esence o di e en amoun s
o Me
2
Im-CO
2
(0.1, 0.2, 0.5 and 1 equi .) o a o d small and well-de ined NHC-s abilized Ni NPs
(Figu e 5). As p e iously obse ed by La a e al. wi h Ru/I Bu [
12
], i was necessa y o in oduce a
leas 0.5 equi . o Me
2
Im-CO
2
o ob ain a colloidal suspension wi hou agglome a ions (Figu e 5b).
The di ec deca boxyla ion o his imidazolium ca boxyla e a oids he p e ious isola ion o he ee
ca bene o any basic p e ea men s. Howe e , he numbe o CO
2
adduc s ha can be used as ca bene
p ecu so , and i canno be conside ed a gene al syn he ic me hod.
Ca alys s 2020,10, 1144 8 o 30
Ca alys s2020,10,11448o 30
Figu e5.(a)Syn hesiso Ni/Me
2
Imasanexampleo NHC‐basedo ganome allicMNP om
imidazoliumca boxyla e.(b)TEMimagesandsizedis ibu iono Ni
0.2
,Ni
0.5
yNi
1
/Me
2
ImNPs.
Adap ed om e e ence49.Copy igh 2017TheRoyalSocie yo Chemis y.
Al hough heo ganome allicapp oachhasbeenp o en obea e ycon enien syn he ic
me hod o hesyn hesiso MNPss abilizedwi hNHCligands, heycanalsobep epa edbyo he
syn he icme hods(Scheme2).B ie ly,NHC‐s abilizedMNPsa emainlyp epa edby h eedi e en
app oaches:i)byme allicp ecu so educ ionin hep esenceo anNHCligand,ii)byligand
exchangeo p esyn hesizedMNPsandiii)bydi ec decomposi iono unc ionalizedme alNHC
complexes[50].Thesyn hesiso NHC‐s abilizedMNPs ialigandexchangeconsis so he
displacemen o su aceligands omnanopa iclesu acebyabe e s abilizingligand,which o ms
as onge bondwi h heme alsu ace(Scheme2,ii).In hisway,asu acemodi ica ion ha leads
oachangein hephysicalo chemicalp ope ieso heNPin e mso solubili y,s abili yo ca aly ic
p ope iesisachie ed.Thep epa a iono NHC‐s abilizedMNPsbyligandexchangehasbeenwidely
employedsincei was i s epo edbyFai lambandChechik[17].
Al hough heysuccess ully
displaceda hioe he (dodecylsul ide)weaklybound om heMNPsu aceby headdi iono I Bu,
he esul an NHC‐s abilizedAuNPsp esen edalimi eds abili yinsolu ion,sincegoldleaching o
o mcomplexeso agg ega eswasobse ed.Following hesames a egy(ligandexchangeo
hioe he ‐coa edMNPs)bu usingNHCssubs i u edwi hlongalkylchains(LC‐NHCs),Ra ooand
Glo iussyn hesizedNHC‐s abilizedPdNPs[51].Bycompa ing woligandswi hdi e en s e ic
hind ance,namelyLC‐IMeandLC‐IP (Figu e4c), heyobse ed ha N‐me hylsubs i uen s
acili a e heligandexchange,ob ainingsolubleands ableMNPs.Howe e ,wi h hes e ically
demandingLC‐IP , hesame endencyo leachingal eady epo edbyFai lambandChechikwas
obse ed,and heNHC‐s abilizedPdNPsquicklyagg ega edinsolu ion.
Figu e 5.
(
a
) Syn hesis o Ni/Me
2
Im as an example o NHC-based o ganome allic MNP om
imidazolium ca boxyla e. (
b
) TEM images and size dis ibu ion o Ni
0.2
, Ni
0.5
y Ni
1
/Me
2
Im NPs.
Adap ed om e e ence 49. Copy igh 2017 The Royal Socie y o Chemis y.
Al hough he o ganome allic app oach has been p o en o be a e y con enien syn he ic me hod
o he syn hesis o MNPs s abilized wi h NHC ligands, hey can also be p epa ed by o he syn he ic
me hods (Scheme 2). B ie ly, NHC-s abilized MNPs a e mainly p epa ed by h ee di e en app oaches:
(i) by me allic p ecu so educ ion in he p esence o an NHC ligand, (ii) by ligand exchange o
p esyn hesized MNPs and (iii) by di ec decomposi ion o unc ionalized me al NHC complexes [
50
].
The syn hesis o NHC-s abilized MNPs ia ligand exchange consis s o he displacemen o su ace
ligands om nanopa icle su ace by a be e s abilizing ligand, which o ms a s onge bond wi h
he me al su ace (Scheme 2ii). In his way, a su ace modi ica ion ha leads o a change in he
physical o chemical p ope ies o he NP in e ms o solubili y, s abili y o ca aly ic p ope ies
is achie ed. The p epa a ion o NHC-s abilized MNPs by ligand exchange has been widely employed
since i was i s epo ed by Fai lamb and Chechik [
17
]. Al hough hey success ully displaced a
hioe he (dodecylsul ide) weakly bound om he MNP su ace by he addi ion o I Bu, he esul an
NHC-s abilized Au NPs p esen ed a limi ed s abili y in solu ion, since gold leaching o o m complexes
o agg ega es was obse ed. Following he same s a egy (ligand exchange o hioe he -coa ed MNPs)
bu using NHCs subs i u ed wi h long alkyl chains (LC-NHCs), Ra oo and Glo ius syn hesized
NHC-s abilized Pd NPs [
51
]. By compa ing wo ligands wi h di e en s e ic hind ance, namely LC-IMe
and LC-IP (Figu e 4c), hey obse ed ha N-me hyl subs i uen s acili a e he ligand exchange,
ob aining soluble and s able MNPs. Howe e , wi h he s e ically demanding LC-IP , he same endency
o leaching al eady epo ed by Fai lamb and Chechik was obse ed, and he NHC-s abilized Pd NPs
quickly agg ega ed in solu ion.
The di ec decomposi ion o an NHC o ganome allic complex is a popula bo om-up
syn he ic me hod (Scheme 2iii), whe e he NHC–me al complex is educed by a educing agen
(
e.g., NaBH4) [52,53]
, o he mally decomposed o o m NHC-s abilized MNPs. In he i s case,
he MNP size can be easily con olled by modi ying he concen a ion o he me al p ecu so and
he educing agen [
54
]. In 2014, Baque o e al. epo ed he he mal decomposi ion o a sul ona ed
NHC-P complex in wa e , which leads o he o ma ion o ul a-s able wa e -soluble P NPs [
55
,
56
].
Simila esul s we e ob ained by Asensio e al. wi h he analogous Pd hyd osoluble complexes [
57
,
58
].
Ca alys s 2020,10, 1144 9 o 30
Ca alys s2020,10,11449o 30
Scheme2.Di e en syn hesisme hodologies o NHC‐s abilizedMNPs.
Thedi ec decomposi iono anNHCo ganome alliccomplexisapopula bo om‐upsyn he ic
me hod(Scheme2,iii),whe e heNHC–me alcomplexis educedbya educingagen (e.g.NaBH
4
)
[52,53],o he mallydecomposed o o mNHC‐s abilizedMNPs.In he i s case, heMNPsizecan
beeasilycon olledbymodi ying heconcen a iono heme alp ecu so and he educingagen
[54].In2014,Baque oe al. epo ed he he maldecomposi iono asul ona edNHC‐P complexin
wa e ,whichleads o he o ma iono ul a‐s ablewa e ‐solubleP NPs[55,56].
Simila esul swe e
ob ainedbyAsensioe al.wi h heanalogousPdhyd osolublecomplexes[57,58].
3.Su aces udies
Asp e iouslymen ionedin hein oduc ion, he eisas ongneed o adeepe unde s anding
o hecoo dina ionmode,loca ionanddynamicso su aceligandsand hein luenceo such
s abilize sin hechemicalandphysicalp ope ieso MNPs[59–63].B adley’sg oupwasoneo he
i s ousespec oscopic echniques o hesupe icialcha ac e iza iono highlydispe sedcolloids
insolu ion[29].Mo especi ically, heype o medin a edandNMRs udieso ca bonmonoxide
adso p ionon ansi ionme alcolloids[64–67].In hisway,B adleye al.es ablishedCOadso p ion
asasui able echnique os udyMNPsu aces h ough heloca iono heac i esi eso MNPs.
Mo eo e , h ough hesesu aces udiesi ispossible oin es iga e heposi ionanddynamicso
su aceligands,aswasla e desc ibedbyNo ioe al.[31].Fo hispu pose, heau ho sexamined
hecoo dina iono COa heme allicsu aceo wose so o ganome allicRuNPs,s abilizedei he
byapolyme (poly inylpy olidone;PVP),Ru/PVP,o byanancilla yligand
(bisdiphenylphosphinobu ane;dppb),Ru/dppb,[37,46]unde mildcondi ions oa oidany
supe icialmodi ica iono heMNP.Fi s ,bycombina iono FT‐IRandNMRspec oscopies,No io
e al.e idenced hecoo dina ionmodesanddynamicso COa heme alsu ace(Figu e6)—i.e.,in
ab idgemode(CO
b
)on he aceso heNPso ina e minalmode(CO
)on hemos exposeda oms
(edgesandapexes).MAS‐NMRo Ru/PVPshowedag ea mobili yo heCOadso bed,sincea sho
pe iodso COexposu e, he eismainlyab oadsignalcen e eda ca.250ppmwhichco esponds o
CO
b
,whilea longe exposu e imes,asha psignala ca.200ppm(CO
)appea sa hecos o he
b oadone(Figu e7,le ).On heo he hand,MAS‐NMRo Ru/dppb e ealedsignalsa ca.240(CO
b
)
and200ppm(CO
) ha didno showanydisplacemen e ena longe COexposu e imes(Figu e7,
igh ),whichindica ed helacko mobili yo COon hisMNPsu ace.Mo eo e , hec oss‐
pola iza ion(CP)NMRspec umo Ru/dppbshowedadec easein heCO
b
signalcompa ed o he
e minalone,indica ing ha CO
wasclose o hedppbligands,p obablyloca edon heapexesand
edges.Thiswo khighligh s hein luenceo su aceligandson hedynamicso adso bedCO.While
bulkyligandsled oaslow‐down luxionali yo adso bedmolecules, heabsenceo hesepe mi ed
hei mobili y.
Scheme 2. Di e en syn hesis me hodologies o NHC-s abilized MNPs.
3. Su ace S udies
As p e iously men ioned in he in oduc ion, he e is a s ong need o a deepe unde s anding o
he coo dina ion mode, loca ion and dynamics o su ace ligands and he in luence o such s abilize s
in he chemical and physical p ope ies o MNPs [
59
–
63
]. B adley’s g oup was one o he i s
o use spec oscopic echniques o he supe icial cha ac e iza ion o highly dispe sed colloids in
solu ion [
29
]. Mo e speci ically, hey pe o med in a ed and NMR s udies o ca bon monoxide
adso p ion on ansi ion me al colloids [
64
–
67
]. In his way, B adley e al. es ablished CO adso p ion as
a sui able echnique o s udy MNP su aces h ough he loca ion o he ac i e si es o MNPs. Mo eo e ,
h ough hese su ace s udies i is possible o in es iga e he posi ion and dynamics o su ace ligands,
as was la e desc ibed by No io e al. [
31
]. Fo his pu pose, he au ho s examined he coo dina ion
o CO a he me allic su ace o wo se s o o ganome allic Ru NPs, s abilized ei he by a polyme
(poly inylpy olidone; PVP), Ru/PVP, o by an ancilla y ligand (bisdiphenylphosphinobu ane; dppb),
Ru/dppb, [
37
,
46
] unde mild condi ions o a oid any supe icial modi ica ion o he MNP. Fi s , by
combina ion o FT-IR and NMR spec oscopies, No io e al. e idenced he coo dina ion modes and
dynamics o CO a he me al su ace (Figu e 6)—i. e., in a b idge mode (CO
b
) on he aces o he NPs
o in a e minal mode (CO
) on he mos exposed a oms (edges and apexes). MAS-NMR o Ru/PVP
showed a g ea mobili y o he CO adso bed, since a sho pe iods o CO exposu e, he e is mainly a
b oad signal cen e ed a ca. 250 ppm which co esponds o CO
b
, while a longe exposu e imes, a
sha p signal a ca. 200 ppm (CO
) appea s a he cos o he b oad one (Figu e 7, le ). On he o he
hand, MAS-NMR o Ru/dppb e ealed signals a ca. 240 (CO
b
) and 200 ppm (CO
) ha did no show
any displacemen e en a longe CO exposu e imes (Figu e 7, igh ), which indica ed he lack o
mobili y o CO on his MNP su ace. Mo eo e , he c oss-pola iza ion (CP) NMR spec um o Ru/dppb
showed a dec ease in he CO
b
signal compa ed o he e minal one, indica ing ha CO
was close o
he dppb ligands, p obably loca ed on he apexes and edges. This wo k highligh s he in luence o
su ace ligands on he dynamics o adso bed CO. While bulky ligands led o a slow-down luxionali y
o adso bed molecules, he absence o hese pe mi ed hei mobili y.
Ca alys s 2020,10, 1144 16 o 30
4.1. Con olling he Solubili y and S abili y
One o he majo challenges in nanopa icle chemis y is he syn hesis o esis an NPs o hei
la e use in ca aly ic applica ions, since mos o hem end o agglome a e a e he ca aly ic p ocess as
a esul o hei limi ed s abili y unde eac ion condi ions, wi h he consequen loss o hei ca aly ic
p ope ies [
85
]. Usually non-pola NHC-s abilized MNPs a e soluble in o ganic sol en s bu p esen
s abili y p oblems—e.g., Au/I Bu NPs apidly agg ega e in DMSO, CH
3
CN o CH
2
Cl
2
[
17
]. Howe e ,
LC-NHC ligands con e a g ea s abili y and solubili y o MNPs in o ganic sol en s because o he long
alipha ic chains loca ed in he imidazole backbone (Figu e 4c) [
86
]. In 2014, Ra oo in collabo a ion
wi h Glo ius used hese long chain ligands o he i s ime o s abilize Pd NPs, c ea ing a p o ec i e
monolaye ha p e en s MNP agg ega ion [
51
]. In hei wo k, hey p o ed he impo ance o he
N-subs i uen o he imidazole g oup in he s abiliza ion o Pd NPs, whe e, in o de o minimize
he s e ic epulsion be ween he NHC and he MNP su ace, he N-subs i uen s should be as small
and lexible as possible (Figu e 14a). Ano he clea example o he impo ance o he na u e o he
N-subs i uen s was epo ed by Bakke e al., whose wo k in es iga ed he in luence o he alkyl side
g oups on he coo dina ion mode o NHCs in Au me al su aces [
87
]. In sho , hey dis inguished wo
di e en coo dina ion modes depending on he leng h o he alkyl chain by combined DFT calcula ions,
STM (Scanning Tunneling Mic oscopy) and XPS analyses. IBu wi h long alkyl chain coo dina es in a
Down
su
con igu a ion and hen emains in a la -lying IBu-Au-IBu complex (Down
ad
). In con as ,
he ca bene wi h a sho alkyl chain (IMe) p e e s o coo dina e in an up-s anding con igu a ion
wi h an Au ada om (Up
ad
; Figu e 14b). Howe e , o he s udies in his espec poin o di e en
coo dina ion modes depending on he bulkiness o he N-bound o ganic subs i uen s. NHCs bea ing
bulky N-subs i uen s bind me allic su aces (Au, Ag o Cu) in a monocoo dina ed Up
ad
, while NHCs
wi h small N-subs i uen s adop a bis-coo dina ed Downad binding mode [88].
Ca alys s2020,10,114416o 30
4.In luenceo NHCsinMNPP ope ies
Asp e iouslypoin edou in hein oduc ion, hechemicalandphysicalp ope ieso MNPs
canbemodi ieddependingon hes abilizingliganduseddu ing hesyn hesisp ocesso ina
subsequen unc ionaliza ions ep, ialigand‐exchange.In hiscon ex ,NHCsa ep esen edas
e sa ileligandswhichcaneasilymodula e heMNPp ope iessuchassize,solubili y,s abili y,
su aces a eo ca aly icac i i y.The e o e, heamoun o na u eo heNHCemployedasas abilize
willgo e n heca aly icp ope ieso MNPs.
4.1.Con olling heSolubili yandS abili y
Oneo hemajo challengesinnanopa iclechemis yis hesyn hesiso esis an NPs o hei
la e useinca aly icapplica ions,sincemos o hem end oagglome a ea e heca aly icp ocess
asa esul o hei limi eds abili yunde eac ioncondi ions,wi h heconsequen losso hei
ca aly icp ope ies[85].Usuallynon‐pola NHC‐s abilizedMNPsa esolubleino ganicsol en sbu
p esen s abili yp oblems—e.g.,Au/I BuNPs apidlyagg ega einDMSO,CH
3
CNo CH
2
Cl
2
[17].
Howe e ,LC‐NHCligandscon e ag ea s abili yandsolubili y oMNPsino ganicsol en s
becauseo helongalipha icchainsloca edin heimidazolebackbone(Figu e4c)[86].In2014,Ra oo
incollabo a ionwi hGlo iusused heselongchainligands o he i s ime os abilizePdNPs,
c ea ingap o ec i emonolaye ha p e en sMNPagg ega ion[51].In hei wo k, heyp o ed he
impo anceo heN‐subs i uen o heimidazoleg oupin hes abiliza iono PdNPs,whe e,ino de
ominimize hes e ic epulsionbe ween heNHCand heMNPsu ace, heN‐subs i uen sshould
beassmalland lexibleaspossible(Figu e14a).Ano he clea exampleo heimpo anceo he
na u eo heN‐subs i uen swas epo edbyBakke e al.,whosewo kin es iga ed hein luenceo
healkylsideg oupson hecoo dina ionmodeo NHCsinAume alsu aces[87].Insho , hey
dis inguished wodi e en coo dina ionmodesdependingon heleng ho healkylchainby
combinedDFTcalcula ions,STM(ScanningTunnelingMic oscopy)andXPSanalyses.IBuwi hlong
alkylchaincoo dina esinaDown
su
con igu a ionand hen emainsina la ‐lyingIBu‐Au‐IBu
complex(Down
ad
).Incon as , heca benewi hasho alkylchain(IMe)p e e s ocoo dina einan
up‐s andingcon igu a ionwi hanAuada om(Up
ad
;Figu e14b).Howe e ,o he s udiesin his
espec poin odi e en coo dina ionmodesdependingon hebulkinesso heN‐boundo ganic
subs i uen s.NHCsbea ingbulkyN‐subs i uen sbindme allicsu aces(Au,Ago Cu)ina
monocoo dina edUp
ad
,whileNHCswi hsmallN‐subs i uen sadop abis‐coo dina edDown
ad
bindingmode[88].
Figu e14.(a)Rep esen a iono NHCligandswi h wodi e en N‐subs i uen s.Ex ac edwi h
pe mission om e e ence51.Copy igh 2014TheRoyalSocie yo Chemis y.(b)Di e en binding
modeso NHCligands(IBuandIMe) o heme alsu aces.Adap ed om e e ence87wi h
pe missiono Ame icanChemicalSocie y.
Figu e 14.
(
a
) Rep esen a ion o NHC ligands wi h wo di e en N-subs i uen s. Ex ac ed wi h
pe mission om e e ence 51. Copy igh 2014 The Royal Socie y o Chemis y. (
b
) Di e en binding
modes o NHC ligands (IBu and IMe) o he me al su aces. Adap ed om e e ence 87 wi h pe mission
o Ame ican Chemical Socie y.
Taking ad an age o he g ea solubili y and s abili y ha hese long alkyl chain NHC ligands
con e o MNPs, Ma
í
nez-P ie o e al. p epa ed wo ypes o P NPs s abilized wi h he same
LC-NHC ligands (LC-IMe and LC-IP ; Figu e 4c) [
23
]. Due o he high solubili y in o ganic sol en s o
hese P NPs, hey we e cha ac e ized by solu ion NMR, Di usion-O de ed Spec oscopy (DOSY) and
ESI-MS, among o he echniques, ob aining aluable in o ma ion abou he NHC-me al in e ac ion
and he nanopa icle size dis ibu ion. Fo example, he di usion coe icien s o he ee ca benes
(LC-IMe and LC-IP ) obse ed by DOSY spec oscopy we e highe han hose co esponding o he
P NPs, con i ming he expec ed slowe di usion o he ligands bound o he nanopa icle su ace in
Ca alys s 2020,10, 1144 17 o 30
compa ison wi h he ee ones (Figu e 15). In addi ion, only one di usion coe icien was obse ed o
P /LC-IP , which indica es a na ow size dis ibu ion o hese NPs, in con as o he obse a ion o
di e en di usion coe icien s o P /LC-IMe, which poin s o a la ge size dis ibu ion.
Ca alys s2020,10,114417o 30
Takingad an ageo heg ea solubili yands abili y ha heselongalkylchainNHCligands
con e oMNPs,Ma ínez‐P ie oe al.p epa ed wo ypeso P NPss abilizedwi h hesameLC‐
NHCligands(LC‐IMeandLC‐IP ;Figu e4c)[23].
Due o hehighsolubili yino ganicsol en so
heseP NPs, heywe echa ac e izedbysolu ionNMR,Di usion‐O de edSpec oscopy(DOSY)
andESI‐MS,amongo he echniques,ob aining aluablein o ma ionabou heNHC‐me al
in e ac ionand henanopa iclesizedis ibu ion.Fo example, hedi usioncoe icien so he ee
ca benes(LC‐IMeandLC‐IP )obse edbyDOSYspec oscopywe ehighe han hose
co esponding o heP NPs,con i ming heexpec edslowe di usiono heligandsbound o he
nanopa iclesu aceincompa isonwi h he eeones(Figu e15).Inaddi ion,onlyonedi usion
coe icien wasobse ed o P /LC‐IP ,whichindica esana owsizedis ibu iono heseNPs,in
con as o heobse a iono di e en di usioncoe icien s o P /LC‐IMe,whichpoin s oala ge
sizedis ibu ion.
Figu e15.
1
HDOSYspec ao P /LC‐IP (black,a)andP /LC‐IMe(black,(b), oge he wi h hei
co espondingca benep ecu so sLC‐IP ∙HB ( ed,a)andLC‐IMe∙HI( ed,b)inTHF‐d
8
.Ex ac ed
wi hpe mission om e e ence23.Copy igh 2017Willey‐VCH.
Mos o NHCligandslackhyd ophilicg oups,whichp o ide o heNHC‐s abilizedMNPs he
pola i yneeded obewa e ‐ esis an .Howe e ,sul ona edNHCligands(Figu e4e)ha ebeen
success ullyusedasancilla yligands o hes abiliza iono wa e ‐solubleMNPs[55–58,89,90].The
inco po a iono hesul ona edg oup o heimidazoleco econ e s o heNHCligandsagood
solubili yinpola sol en sandmakesMNPss ableinaqueousmedia.Ma inez‐P ie oe al.p epa ed
MNPss abilizedwi hsul ona edNHCligands o he i s ime ollowing heo ganome allic
app oach[48],sincep e iousexampleso sul ona edNHC‐s abilizedMNPswe eob ainedbydi ec
decomposi iono me alNHCcomplexes[55–58]o ligandexchange[89].
In hiswo k, aking
ad an ageo hehyd o‐solubili yo heRuNPs,i waspossible omoni o hein e ac ionbe ween
he
L
‐lysineand heMNPsu acebychemicalshi pe u ba ion(CSP)inH/Dexchange eac ions.
CSPexpe imen s,no mallyused oin es iga e hep o ein/subs a ein e ac ions[91–93],we e
success ullyemployed omeasu e hesubs a e/MNPin e ac ionbysolu ionNMR.Thein o ma ion
abou hebindingsi eswasob ainedby
1
H,
13
C‐HSQCNMRa e moni o ingdi e en quan i ieso
L
‐lysinein oa1mg∙mL
‐1
solu iono RuNPsinD
2
Oa di e en pH alues.A pH10.4 heαandε
posi ionso
L
‐lysinewe ealmos comple elydeu e a eda e 42ha 55Cunde 1ba D
2
.He e, he
L
‐lysinecoo dina es h oughαandεinasimul aneouso al e na eway h ough heaminog oups,
explaining hehighdeu e a ionon heseposi ions.A pH13.2,αandεposi ionswe enea ly ully
deu e a ed,andγposi ionwasalsodeu e a eddue o hesimul aneousin e ac iono heamino
g oupswi h henanopa icle,poin ing hep o onsinγ o heme allicsu ace.A lowpH,inwhich
heaminog oupsa ep o ona ed, hesubs a e‐nanopa iclein e ac ionisnon‐exis en ,and he
ac i i yisp ac icallysupp essed(seeFigu e16).Thesamewa e ‐solubleRuNPswe e ecen lyused
byPie e se al. o hehyd ogeniso opeexchange(HIE)o nucleobasepha maceu icalsand
Figu e 15. 1
H DOSY spec a o P /LC-IP (black, (
a
)) and P /LC-IMe (black, (
b
)), oge he wi h hei
co esponding ca bene p ecu so s LC-IP
·
HB ( ed, (
a
)) and LC-IMe
·
HI ( ed, (
b
)) in THF-d
8
. Ex ac ed
wi h pe mission om e e ence 23. Copy igh 2017 Willey-VCH.
Mos o NHC ligands lack hyd ophilic g oups, which p o ide o he NHC-s abilized MNPs
he pola i y needed o be wa e - esis an . Howe e , sul ona ed NHC ligands (Figu e 4e) ha e
been success ully used as ancilla y ligands o he s abiliza ion o wa e -soluble MNPs [
55
–
58
,
89
,
90
].
The inco po a ion o he sul ona ed g oup o he imidazole co e con e s o he NHC ligands a
good solubili y in pola sol en s and makes MNPs s able in aqueous media. Ma inez-P ie o e al.
p epa ed MNPs s abilized wi h sul ona ed NHC ligands o he i s ime ollowing he o ganome allic
app oach [48], since p e ious examples o sul ona ed NHC-s abilized MNPs we e ob ained by di ec
decomposi ion o me al NHC complexes [
55
–
58
] o ligand exchange [
89
]. In his wo k, aking ad an age
o he hyd o-solubili y o he Ru NPs, i was possible o moni o he in e ac ion be ween he
L
-lysine
and he MNP su ace by chemical shi pe u ba ion (CSP) in H/D exchange eac ions. CSP expe imen s,
no mally used o in es iga e he p o ein/subs a e in e ac ions [
91
–
93
], we e success ully employed
o measu e he subs a e/MNP in e ac ion by solu ion NMR. The in o ma ion abou he binding
si es was ob ained by
1
H,
13
C-HSQC NMR a e moni o ing di e en quan i ies o
L
-lysine in o a
1 mg
·
mL
−1
solu ion o Ru NPs in D
2
O a di e en pH alues. A pH 10.4 he
α
and
ε
posi ions o
L
-lysine we e almos comple ely deu e a ed a e 42 h a 55
◦
C unde 1 ba D
2
. He e, he
L
-lysine
coo dina es h ough
α
and
ε
in a simul aneous o al e na e way h ough he amino g oups, explaining
he high deu e a ion on hese posi ions. A pH 13.2,
α
and
ε
posi ions we e nea ly ully deu e a ed,
and
γ
posi ion was also deu e a ed due o he simul aneous in e ac ion o he amino g oups wi h he
nanopa icle, poin ing he p o ons in
γ
o he me allic su ace. A low pH, in which he amino g oups
a e p o ona ed, he subs a e-nanopa icle in e ac ion is non-exis en , and he ac i i y is p ac ically
supp essed (see Figu e 16). The same wa e -soluble Ru NPs we e ecen ly used by Pie e s e al. o he
hyd ogen iso ope exchange (HIE) o nucleobase pha maceu icals and oligonucleo ides, showing a good
deu e ium inco po a ion due o he high solubili y in wa e o hese hyd osoluble nanopa icles [26].
Ca alys s 2020,10, 1144 18 o 30
Ca alys s2020,10,114418o 30
oligonucleo ides,showingagooddeu e iuminco po a iondue o hehighsolubili yinwa e o hese
hyd osolublenanopa icles[26].
Figu e16.Coo dina iono
L
‐lysineonRuNPsu aceasa unc iono pHinD
2
O.Redpo sco espond
o hesi eso heinco po a iono deu e iumin hehyd ogeniso opeexchange eac ions.Adap ed
om e e ence48wi hpe missiono TheRoyalSocie yo Chemis y.
Using hesameCSPme hod,Bouzoui ae al.pe o medasimila in e ac ions udyby
employing his imewa e ‐solublebime allicRuP NPs[25].Th eedi e en ypeso IP SO
3
‐s abilized
RuP NPswi hdis inc su acecomposi ionswe eob ainedbyplayingwi h hedecomposi ion a e
o heo ganome allicp ecu so suseddu ing hesyn hesis.Ingene al e ms, heslowe he
decomposi ion a eo hepla inump ecu so , he iche inpla inum hesu acewas.Su aces udies
pe o medbysolid‐s a eNMRandFT‐IRspec oscopiesallowed heanalysiso he ela i esu ace
composi iono hesebime allicsys ems.Bime allicNPsp epa edbyco‐decomposi iono
Ru(COD)(COT)andP (CH
3
)
2
(COD)we e hosewi h hehighe P /Ru a ioon hei su ace.He e,in
con as omonome allicRuNPs, he
L
‐lysinecoo dina edwi h heRuP su ace h ough heamino
andca boxyla eg oups,inachela ingway(Figu e17).Thiss onge in e ac ion,asSaba ie ’s
p inciplep edic s[94],was e lec edinaslowe dissocia iono hedeu e a ionp oduc ha caused
aslowe eac ion a einαposi ion.
Figu e17.
L
‐lysinein e ac ionwi hRuandRuP NPs.Ex ac edwi hpe mission om e e ence25.
Copy igh 2019TheRoyalSocie yo Chemis y.
4.2.Modi ying heCa aly icP ope ies
Asdiscussedin hein oduc ion,su aceligandsingene alandNHCsinpa icula a eable o
modi y heca aly icpe o mancesamongo he p ope ieso MNPs.Byplayingwi h heamoun o
s abilizingNHCligand,o hena u eo heca bene(i.e.,bulkyN‐subs i uen s,longalkylchains
g oupsin heimidazoliumbackbone,e c.),i ispossible ocon ol heac i i y/selec i i yo he
nanoca alys s— o example,NHC‐s abilizedo ganome allicRuNPs epo edbyLa ae al.[12]
exhibi eda ema kableligande ec in hehyd ogena iono a oma iccompounds[95].Gene ally,
Ru/IP
0.2
showed hehighes ac i i yincompa ison oRu/IP
0.5
andRu/I Bu
0.5
.Fi s , hep esenceo
anexcesso ligandcoo dina ed o he u heniumsu acesloweddown heac i i yo heca alys s.
Inaddi ion, heno ableligande ec obse ed(Ru/IP
0.5
beingmuch
mo eac i e han
Ru/I Bu
0.5
)was
explainedin e mso di e encesbe weenligand–su acein e ac ions.Thein e ac iono hebulky
a oma icsubs i uen so Ru/IP wi h heme alsu acewasweake han heonewi h he Bug oups,
allowinganeasie accesso hea oma icsubs a esemployedin heca aly icp ocesses.Taking
Figu e 16.
Coo dina ion o
L
-lysine on Ru NP su ace as a unc ion o pH in D
2
O. Red po s co espond
o he si es o he inco po a ion o deu e ium in he hyd ogen iso ope exchange eac ions. Adap ed
om e e ence 48 wi h pe mission o The Royal Socie y o Chemis y.
Using he same CSP me hod, Bouzoui a e al. pe o med a simila in e ac ion s udy by employing
his ime wa e -soluble bime allic RuP NPs [
25
]. Th ee di e en ypes o IP SO
3
-s abilized RuP
NPs wi h dis inc su ace composi ions we e ob ained by playing wi h he decomposi ion a e o he
o ganome allic p ecu so s used du ing he syn hesis. In gene al e ms, he slowe he decomposi ion
a e o he pla inum p ecu so , he iche in pla inum he su ace was. Su ace s udies pe o med by
solid-s a e NMR and FT-IR spec oscopies allowed he analysis o he ela i e su ace composi ion
o hese bime allic sys ems. Bime allic NPs p epa ed by co-decomposi ion o Ru(COD)(COT) and
P (CH
3
)
2
(COD)we e hosewi h hehighe P /Ru a ioon hei su ace. He e, incon as omonome allic
Ru NPs, he
L
-lysine coo dina ed o he RuP su ace h ough he amino and ca boxyla e g oups, in a
chela ing way (Figu e 17). This s onge in e ac ion, as Saba ie ’s p inciple p edic s [
94
], was e lec ed
in a slowe dissocia ion o he deu e a ion p oduc ha caused a slowe eac ion a e in αposi ion.
Ca alys s2020,10,114418o 30
oligonucleo ides,showingagooddeu e iuminco po a iondue o hehighsolubili yinwa e o hese
hyd osolublenanopa icles[26].
Figu e16.Coo dina iono
L
‐lysineonRuNPsu aceasa unc iono pHinD
2
O.Redpo sco espond
o hesi eso heinco po a iono deu e iumin hehyd ogeniso opeexchange eac ions.Adap ed
om e e ence48wi hpe missiono TheRoyalSocie yo Chemis y.
Using hesameCSPme hod,Bouzoui ae al.pe o medasimila in e ac ions udyby
employing his imewa e ‐solublebime allicRuP NPs[25].Th eedi e en ypeso IP SO
3
‐s abilized
RuP NPswi hdis inc su acecomposi ionswe eob ainedbyplayingwi h hedecomposi ion a e
o heo ganome allicp ecu so suseddu ing hesyn hesis.Ingene al e ms, heslowe he
decomposi ion a eo hepla inump ecu so , he iche inpla inum hesu acewas.Su aces udies
pe o medbysolid‐s a eNMRandFT‐IRspec oscopiesallowed heanalysiso he ela i esu ace
composi iono hesebime allicsys ems.Bime allicNPsp epa edbyco‐decomposi iono
Ru(COD)(COT)andP (CH
3
)
2
(COD)we e hosewi h hehighe P /Ru a ioon hei su ace.He e,in
con as omonome allicRuNPs, he
L
‐lysinecoo dina edwi h heRuP su ace h ough heamino
andca boxyla eg oups,inachela ingway(Figu e17).Thiss onge in e ac ion,asSaba ie ’s
p inciplep edic s[94],was e lec edinaslowe dissocia iono hedeu e a ionp oduc ha caused
aslowe eac ion a einαposi ion.
Figu e17.
L
‐lysinein e ac ionwi hRuandRuP NPs.Ex ac edwi hpe mission om e e ence25.
Copy igh 2019TheRoyalSocie yo Chemis y.
4.2.Modi ying heCa aly icP ope ies
Asdiscussedin hein oduc ion,su aceligandsingene alandNHCsinpa icula a eable o
modi y heca aly icpe o mancesamongo he p ope ieso MNPs.Byplayingwi h heamoun o
s abilizingNHCligand,o hena u eo heca bene(i.e.,bulkyN‐subs i uen s,longalkylchains
g oupsin heimidazoliumbackbone,e c.),i ispossible ocon ol heac i i y/selec i i yo he
nanoca alys s— o example,NHC‐s abilizedo ganome allicRuNPs epo edbyLa ae al.[12]
exhibi eda ema kableligande ec in hehyd ogena iono a oma iccompounds[95].Gene ally,
Ru/IP
0.2
showed hehighes ac i i yincompa ison oRu/IP
0.5
andRu/I Bu
0.5
.Fi s , hep esenceo
anexcesso ligandcoo dina ed o he u heniumsu acesloweddown heac i i yo heca alys s.
Inaddi ion, heno ableligande ec obse ed(Ru/IP
0.5
beingmuch
mo eac i e han
Ru/I Bu
0.5
)was
explainedin e mso di e encesbe weenligand–su acein e ac ions.Thein e ac iono hebulky
a oma icsubs i uen so Ru/IP wi h heme alsu acewasweake han heonewi h he Bug oups,
allowinganeasie accesso hea oma icsubs a esemployedin heca aly icp ocesses.Taking
Figu e 17. L
-lysine in e ac ion wi h Ru and RuP NPs. Ex ac ed wi h pe mission om e e ence 25.
Copy igh 2019 The Royal Socie y o Chemis y.
4.2. Modi ying he Ca aly ic P ope ies
As discussed in he in oduc ion, su ace ligands in gene al and NHCs in pa icula a e able o
modi y he ca aly ic pe o mances among o he p ope ies o MNPs. By playing wi h he amoun
o s abilizing NHC ligand, o he na u e o he ca bene (i.e., bulky N-subs i uen s, long alkyl chains
g oups in he imidazolium backbone, e c.), i is possible o con ol he ac i i y/selec i i y o he
nanoca alys s— o example, NHC-s abilized o ganome allic Ru NPs epo ed by La a e al. [
12
]
exhibi ed a ema kable ligand e ec in he hyd ogena ion o a oma ic compounds [
95
]. Gene ally,
Ru/IP
0.2
showed he highes ac i i y in compa ison o Ru/IP
0.5
and Ru/I Bu
0.5
. Fi s , he p esence o
an excess o ligand coo dina ed o he u henium su ace slowed down he ac i i y o he ca alys s.
In addi ion, he no able ligand e ec obse ed (Ru/IP
0.5
being much mo e ac i e han Ru/I Bu
0.5
)
was explained in e ms o di e ences be ween ligand–su ace in e ac ions. The in e ac ion o he
bulky a oma ic subs i uen s o Ru/IP wi h he me al su ace was weake han he one wi h he
Bu g oups, allowing an easie access o he a oma ic subs a es employed in he ca aly ic p ocesses.
Taking ad an age o he new ou e o s abilize MNPs wi h non-isolable NHCs (see Sec ion 2, Scheme 1),
Ma
í
nez-P ie o e al. syn hesized Ru NPs s abilized wi h chi al ca benes (Figu e 4d; Ru/SIDPhNp and
Ru/SIPhOH) wi h he aim o inducing enan ioselec i i y in he hyd ogena ion o di e en eac an s [
20
].
Ca alys s 2020,10, 1144 19 o 30
The ac i i y o hese MNPs was es ed in he hyd ogena ion o nume ous p ochi al subs a es,
wi hou obse ing any enan iome ic excess. Howe e , hey showed good le els o ca aly ic ac i i y
and in e es ing di e ences in he hyd ogena ion o C=C and C=O bonds, Ru/SIDPhNp being he
mos ac i e. The gene al lowe con e sion obse ed o Ru-SIPhOH was explained in e ms o he
s ong in e ac ion o hese ligands wi h he MNP su ace, which educes he access o he subs a es o
he ac i e si es. This s ong me al–ligand in e ac ion was mainly due o he
π
-in e ac ions be ween
he me al su ace and he a oma ic ings and he p obable coo dina ion o he –OH g oups o me al
ac i e si es. In a collabo a i e wo k La a, Ma
í
nez-P ie o and co-wo ke s syn hesized and s udied he
su ace o a g ea numbe o NHC-s abilized Ru NPs (Figu e 4b), con aining an asymme ic NHC [
69
].
He e, again, no enan iome ic excess was obse ed, in spi e o he high ac i i y and chemoselec i i y
o hese chi al MNPs. The absence o chi al induc ion in hese asymme ic hyd ogena ion eac ions
demons a es ha al hough su ace ligands a e able o con ol he ca aly ic beha io o MNPs, as well
as in o ganome allic chemis y, i is o en no easible o ex apola e he ca aly ic pe o mance o
molecula complexes o ligand-s abilized MNPs.
In 2014, La a e al. epo ed he i s example o P NPs s abilized wi h NHCs and hei use as
ca alys s in he selec i e hyd ogena ion o ni oa oma ics [
14
]. In hei s udy, he na u e o he NHC
ligand employed as a s abilize and he MNP su ace co e age (0.2 and 0.5 equi . o wo di e en
NHCs (IP and IiP
2
Me
2
; Figu e 4a,b)) showed an impo an in luence on he ca aly ic ac i i y in he
hyd ogena ion o ni oa oma ics. P /IP
0.2
exhibi ed he bes ca aly ic esul s, hyd ogena ing a g ea
numbe o unc ionalize ni o compounds unde mild condi ions (1 ba H
2
, 30
º
C), wi h high le els o
chemoselec i i y (>99% a ull con e sion). The highe con e sion o P /IP
0.2
s. P /(IiP
2
Me
2
)
0.2
and
P /IP
0.5
indica es ha bo h he ype o he ligand and su ace co e age con ol he ca alys s ac i i y.
In gene al, a highe su ace co e age p o ides slowe ca alys s, since subs a e access is mo e hinde ed,
demons a ing once again ha he p ope choice o he numbe o su ace ligands is essen ial o a o d
ac i e ca alys s.
Ano he illus a i e example o he modula ion o he ca aly ic ac i i y o ligand-s abilized MNPs
by adjus ing he amoun o su ace ligands was epo ed by Ma
í
nez-P ie o e al. [
71
]. Mo e speci ically,
he au ho s p epa ed Ru NPs wi h di e en quan i ies (0.1, 0.2 and 0.5 equi .) o a de i ed-NHC
ligand (ICy
·(p- ol)
NCN), which s ongly coo dina es o he me al su ace due o i s zwi e ionic cha ac e
(Figu e 18a). The e, a clea dependence on NP size acco ding o he amoun o he amidina e ligand
employed o he s abiliza ion was obse ed: he MNP size dec eases as he numbe o equi alen s
o s abilizing ligand inc eases. This size co ela ion was also ound o NHC-s abilized MNPs ( ide
sup a) [
12
] and o he simila sys ems [
46
,
96
]. The smalles NPs (Ru/ICy
·(p- ol)
NCN
0.2
; ca. 1.0 nm),
wi h a mino numbe o ac i e si es loca ed on he aces han he la ge ones (Ru/ICy
·(p- ol)
NCN
0.1
;
ca. 1.3 nm), showed a g ea e selec i i y in he hyd ogena ion o s y ene o e hylbenzene. Since a ailable
aces a e essen ial o he hyd ogena ion o a oma ic g oups, he smalle size o Ru/ICy
·(p- ol)
NCN
0.2
,
along wi h he highe numbe o su ace ligand, es ablished he ca aly ic ac i i y o hese ul a-small
NPs in he on ie be ween molecula complexes and ace ed NPs. Sho ly a e , Ma
í
nez-P ie o e al.
we e also able o con ol he ca aly ic ac i i y o P NPs by modi ying he elec onic cha ac e o he
N-a yl subs i uen s o hese no el amidina e ligands [
70
]. The ca aly ic ac i i y o hese P NPs in he
hyd ogena ion o ac i a ed ke ones was highly dependen on he elec on dono /accep o capaci y
(–Me, –OMe, –Cl) o he N-subs i uen s (Figu e 18). The sys ems con aining he elec on-dona ing
g oup (–OMe) showed he highes ca aly ic ac i i y o hese hyd ogena ion eac ions, while hose
wi h elec on-wi hd awing g oups (–Cl) we e he leas ac i e. He e, i was clea ly e idenced ha small
modi ica ions on he s abilizing ligands modula e he ca aly ic ac i i y o MNPs. A simila ligand
e ec was ecen ly epo ed by L
ó
pez Vinasco e al. in magne ic Ni NPs liga ed by he same amidina e
ligands in he pa ial hyd ogena ion o alkynes o alkenes [
97
]. The la e examples e idenced ha bo h
he numbe and na u e o he s abilizing ligand play an impo an ole in he ac i i y and selec i i y o
MNPs in ca alysis.
Ca alys s 2020,10, 1144 20 o 30
Ca alys s2020,10,114420o 30
Figu e18.(a)Zwi e ionicadduc o N,N‐dicyclohexylimidazolideneanddia ylca bodiimide
(ICy∙
(A )
NCN;A =p‐ ol,p‐anisyl,p‐ClC
6
H
4
)coo dina ed o heMNPsu ace.(b)Ligande ec in he
hyd ogena iono ac i a edke oneswi hP NPss abilizedbyICy∙
(A )
NCN.Rep in edwi hpe mission
om e e ence70.Copy igh 2020Sp inge .
Thein luenceo su aceligandsinMNPca alysiswasalsoobse edonRuNPss abilizedwi h
LC‐NHC(Figu e4c) epo edbyMa ínez‐P ie oe al.[13].Thehighs abili yo heseRuNPsdue o
helongalkylchainlinked o hebackboneallowed hei useasca alys sinhyd ogena ionand
oxida ion eac ions.Theywe ee enac i einanunp eceden ed eac ionbasedinone‐po
oxida ion/hyd ogena ionp ocess,whe ea e heoxida iono hesubs a es, hea mosphe eo O
2
was eplacedbyH
2
and heoxygena ionp oduc swe eselec i elyhyd ogena ed(Figu e19a).An
in e es ingligande ec wasobse eddependingon heN‐subs i uen o heNHCligand(–Meo –
(iP )
2
Ph).Thelessbulky heN‐subs i uen , helowe henumbe o eeac i esi esand hepoo e
heac i i y.In ac ,whileRu/LC‐IP o allyhyd ogena edace ophenone oe hylcyclohexane,Ru/LC‐
IMewasno able ohyd ogena ei ,showinganex emelydi e en ca aly icbeha io .The e o e, he
ca aly ic eac i i yo heseLC‐NHC‐s abilizedRuNPswaseasilymodi iedbychanging hei N‐
subs i uen s.P NPss abilizedwi h hesameLC‐NHCswe ealso epo edbyMa inez‐P ie oe al.
in2017as he i s exampleo hyd obo a iono phenylace ylenebynon‐suppo edpla inum
nanopa icles[23].In hesamewayasin hep e iouswo k,ano ableligande ec on heca aly ic
ac i i yo heseP NPswasobse ed.Again, hebulkinesso heN‐subs i uen scon olled heMNP
ca alysis.P /LC‐IP wi h hebulkie subs i uen showedahigh eac i i yandselec i i yin
hyd obo a ion eac ion,aswellasmolecula P complexes,bu P /LC‐IMeNPswi hla ge diame e
we eno ac i ein his ypeo eac ions,ashe e ogeneousP ca alys s(Figu e19b).Recen ly,i has
been epo edbyMo aese al. ha P NPss abilizedbyN‐he e ocyclic hiones(Figu e20)a ealso
ac i einhyd obo a iono alkynes.Themos ac i enanopa icleswe e hoseliga ed oNHT
Mes
,wi h
lowsu aceco e age,whichp esen edexcellen selec i i ies o hean i‐Ma koniko monobo yla ed
p oduc [24].
Figu e 18.
(
a
) Zwi e ionic adduc o N,N-dicyclohexylimidazolidene and dia ylca bodiimide
(ICy
·(A )
NCN; A =p- ol, p-anisyl, p-ClC
6
H
4
) coo dina ed o he MNP su ace. (
b
) Ligand e ec in he
hyd ogena ion o ac i a ed ke ones wi h P NPs s abilized by ICy
·(A )
NCN. Rep in ed wi h pe mission
om e e ence 70. Copy igh 2020 Sp inge .
The in luence o su ace ligands in MNP ca alysis was also obse ed on Ru NPs s abilized wi h
LC-NHC (Figu e 4c) epo ed by Ma
í
nez-P ie o e al. [
13
]. The high s abili y o hese Ru NPs
due o he long alkyl chain linked o he backbone allowed hei use as ca alys s in hyd ogena ion
and oxida ion eac ions. They we e e en ac i e in an unp eceden ed eac ion based in one-po
oxida ion/hyd ogena ion p ocess, whe e a e he oxida ion o he subs a es, he a mosphe e o
O
2
was eplaced by H
2
and he oxygena ion p oduc s we e selec i ely hyd ogena ed (Figu e 19a).
An in e es ing ligand e ec was obse ed depending on he N-subs i uen o he NHC ligand (–Me o
–(iP )
2
Ph). The less bulky he N-subs i uen , he lowe he numbe o ee ac i e si es and he poo e he
ac i i y. In ac , while Ru/LC-IP o ally hyd ogena ed ace ophenone o e hylcyclohexane, Ru/LC-IMe
wasno able o hyd ogena e i , showingan ex emely di e en ca aly icbeha io . The e o e, he ca aly ic
eac i i y o hese LC-NHC-s abilized Ru NPs was easily modi ied by changing hei N-subs i uen s.
P NPs s abilized wi h he same LC-NHCs we e also epo ed by
Ma inez-P ie o e al.
in 2017 as he
i s example o hyd obo a ion o phenylace ylene by non-suppo ed pla inum nanopa icles [
23
].
In he same way as in he p e ious wo k, a no able ligand e ec on he ca aly ic ac i i y o hese P
NPs was obse ed. Again, he bulkiness o he N-subs i uen s con olled he MNP ca alysis. P /LC-IP
wi h he bulkie subs i uen showed a high eac i i y and selec i i y in hyd obo a ion eac ion, as well
as molecula P complexes, bu P /LC-IMe NPs wi h la ge diame e we e no ac i e in his ype o
eac ions, as he e ogeneous P ca alys s (Figu e 19b). Recen ly, i has been epo ed by
Mo aes e al.
ha P NPs s abilized by N-he e ocyclic hiones (Figu e 20) a e also ac i e in hyd obo a ion o
alkynes. The mos ac i e nanopa icles we e hose liga ed o NHT
Mes
, wi h low su ace co e age,
which p esen ed excellen selec i i ies o he an i-Ma koniko monobo yla ed p oduc [24].
Ano he in e es ing ligand e ec , his ime in ol ing NHCs de i ed om choles e ol [
98
],
was epo ed by Rake s e al. in 2018 (Figu e 21) [
99
]. These Ru NPs showed ema kable ac i i y in
he hyd ogena ion o a ene de i a i es, and a no iceable in luence o he ligand due o hei s e ic
di e ences. Gene ally, Ru NPs liga ed by NHCs wi h he choles e ol pa on he N-subs i uen
(Ru/IMe-chol) displayed highe eac i i y han hose wi h he NHCs bea ing he lipophilic agmen
in he backbone o he imidazolium ing (Ru/chol-IMe). The highe ac i i y o Ru/IMe-chol is again
explained by he bulkiness o his ca bene, which gene a es Ru NPs wi h a lowe ligand co e age
han Ru/chol-IMe, and hus, mo e a ailable aces ha a e essen ial o he hyd ogena ion o a oma ic
subs a es. Apa om hei ac i i y in he hyd ogena ion o a oma ics, as hey a e de i ed om
choles e ol, hese new nanosys ems could ha e in e es ing possibili ies as biological ecogni ion sys ems.
Ca alys s 2020,10, 1144 21 o 30
Ca alys s2020,10,114421o 30
Figu e19.(a)One‐po oxida ion‐hyd ogena iono ge aniolca alyzedbyRuNPss abilizedwi hLC‐
NHCs.(b)Hyd obo a iono phenylace yleneca alyzedbyP s abilizedwi hLC‐NHC.Adap edwi h
pe mission om e e ence23.Copy igh 2017Wiley‐VCH.
Figu e20.N‐He e ocyclic hionesusedin e e ence24byMo aese al.Rep oducedwi hpe mission
om e e ence24.Copy igh 2020TheRoyalSocie yo Chemis y.
Ano he in e es ingligande ec , his imein ol ingNHCsde i ed omcholes e ol[98],was
epo edbyRake se al.in2018(Figu e21)[99].TheseRuNPsshowed ema kableac i i yin he
hyd ogena iono a enede i a i es,andano iceablein luenceo heliganddue o hei s e ic
di e ences.Gene ally,RuNPsliga edbyNHCswi h hecholes e olpa on heN‐subs i uen
(Ru/IMe‐chol)displayedhighe eac i i y han hosewi h heNHCsbea ing helipophilic agmen
in hebackboneo heimidazolium ing(Ru/chol‐IMe).Thehighe ac i i yo Ru/IMe‐cholisagain
explainedby hebulkinesso hisca bene,whichgene a esRuNPswi halowe ligandco e age
hanRu/chol‐IMe,and hus,mo ea ailable aces ha a eessen ial o hehyd ogena iono a oma ic
subs a es.Apa om hei ac i i yin hehyd ogena iono a oma ics,as heya ede i ed om
choles e ol, hesenewnanosys emscouldha ein e es ingpossibili iesasbiological ecogni ion
sys ems.
Figu e 19.
(
a
) One-po oxida ion-hyd ogena ion o ge aniol ca alyzed by Ru NPs s abilized
wi h LC-NHCs. (
b
) Hyd obo a ion o phenylace ylene ca alyzed by P s abilized wi h LC-NHC.
Adap ed wi h pe mission om e e ence 23. Copy igh 2017 Wiley-VCH.
Ca alys s2020,10,114421o 30
Figu e19.(a)One‐po oxida ion‐hyd ogena iono ge aniolca alyzedbyRuNPss abilizedwi hLC‐
NHCs.(b)Hyd obo a iono phenylace yleneca alyzedbyP s abilizedwi hLC‐NHC.Adap edwi h
pe mission om e e ence23.Copy igh 2017Wiley‐VCH.
Figu e20.N‐He e ocyclic hionesusedin e e ence24byMo aese al.Rep oducedwi hpe mission
om e e ence24.Copy igh 2020TheRoyalSocie yo Chemis y.
Ano he in e es ingligande ec , his imein ol ingNHCsde i ed omcholes e ol[98],was
epo edbyRake se al.in2018(Figu e21)[99].TheseRuNPsshowed ema kableac i i yin he
hyd ogena iono a enede i a i es,andano iceablein luenceo heliganddue o hei s e ic
di e ences.Gene ally,RuNPsliga edbyNHCswi h hecholes e olpa on heN‐subs i uen
(Ru/IMe‐chol)displayedhighe eac i i y han hosewi h heNHCsbea ing helipophilic agmen
in hebackboneo heimidazolium ing(Ru/chol‐IMe).Thehighe ac i i yo Ru/IMe‐cholisagain
explainedby hebulkinesso hisca bene,whichgene a esRuNPswi halowe ligandco e age
hanRu/chol‐IMe,and hus,mo ea ailable aces ha a eessen ial o hehyd ogena iono a oma ic
subs a es.Apa om hei ac i i yin hehyd ogena iono a oma ics,as heya ede i ed om
choles e ol, hesenewnanosys emscouldha ein e es ingpossibili iesasbiological ecogni ion
sys ems.
Figu e 20.
N-He e ocyclic hiones used in e e ence 24 by Mo aes e al. Rep oduced wi h pe mission
om e e ence 24. Copy igh 2020 The Royal Socie y o Chemis y.
All he examplesdiscussed un ilnow we e ocused on hepossibili y ocon ol he physicochemical
p ope ies o ligand-s abilized colloidal MNPs by modula ing he NHC ligands p esen a hei su ace.
Ne e heless, i is also possible o con ol he su ace p ope ies o suppo ed MNPs h ough hei
unc ionaliza ion wi h NHCs [
18
,
52
,
100
], hus modi ying hei selec i i y o ac i i y. As is known,
since imp o ing he selec i i y o he e ogeneous ca alys s is always a challenge, he modi ica ion o
suppo ed MNPs wi h su ace NHCs ligands can ha e a g ea po en ial o indus ial applica ions [
101
].
In his line, he e a e ew ecen examples o con olling he ac i i y and selec i i y o suppo ed Ru
NPs by he coo dina ion o NHC ligands on he me allic su ace. Fo example, in 2016, Glo ius in
collabo a ion wi h Mu a sugu epo ed he unc ionaliza ion o Ru NPs suppo ed on po assium-doped
alumina (Ru/K-Al
2
O
3
) wi h wo di e en NHC ligands (ICy and IMes) [
102
]. These NHC-modi ied
suppo ed he e ogeneous ca alys s we e es ed in he hyd ogena ion o nume ous eac an s wi h he
Ca alys s 2020,10, 1144 22 o 30
in en ion o s udying he e ec o he su ace unc ionaliza ion on hei ca aly ic p ope ies. Fi s ,
a clea dependence o he ca aly ic ac i i y wi h he amoun o su ace ligands was obse ed in
he hyd ogena ion o ans-s ilbene and ace ophenone. The ac i i y dec eased as he numbe o
coo dina ed NHC ligands inc eased, his end being mo e ma ked o K-Al
2
O
3
-suppo ed Ru NPs
unc ionalized wi h he bulkies NHC ligand (IMes), which in p inciple blocks mo e su ace ac i e si es.
Mo eo e , a ema kable in luence o he su ace co e age was obse ed in he selec i e hyd ogena ion
o phenylace ylene o e hylbenzene; he la ge he amoun o su ace NHC ligands, he highe he
selec i i y o e hylbenzene (Figu e 22a). Again, his end was mo e p onounced o Ru/K-Al
2
O
3
@IMes,
which p esen ed he bulkie N-subs i uen . Some yea s be o e, he same g oup epo ed he p epa a ion
o Pd nanopa icles suppo ed on Fe
3
O
4
and hei la e ea men wi h a chi al NHC in o de o modi y
he su ace o he ca alys and induce enan ioselec i i y in asymme ic eac ions [
18
]. Mo e p ecisely, he
suppo ed Pd/Fe
3
O
4
NPs we e modi ied by a ea men wi h he imidazolium sal o an enan iome ically
pu e NHC in he p esence o a base (Figu e 22b). The esul ing ca alys was ully cha ac e ized by a
combina ion o di e en echniques (XPS, FT-IR, TEM, SEM), and i s ca aly ic ac i i y was e alua ed in
he asymme ic
α
-a yla ion o ke ones, showing in a couple o cases enan iome ic excess highe han
80%. The magne ic ecycling o he ca alys was also s udied, showing he p ese a ion o hei ca aly ic
ac i i y and selec i i y a e i e ca aly ic cycles. Ano he in e es ing example o unc ionaliza ion
o he e ogeneous Ru ca alys s wi h NHC ligands was ecen ly epo ed by Pie e s e al. [
103
]. The
au ho s ound ha i is possible o con ol he ac i i y and selec i i y o a comme cially a ailable
he e ogeneous Ru ca alys (Ru on ca bon; Ru/C) in hyd ogen iso ope exchange (HIE) eac ions by
adding NHC ligands, being he i s example o ligand induced modi ica ion o he e ogeneous ca alys s
in he con ex o C–H ac i a ion. Mo e speci ically, hey epo ed a gene al me hod o he selec i e
C–H deu e a ion o
α
-posi ions on alcohols, pha maceu ical ele an he e ocycles and aldehydes,
p omo ing he H/D exchange p ocesses o e educ ion side eac ions (Figu e 23). In gene al, he use o
Ru/C p oduces a mix u e o labeled (HIE) and educed p oduc s ( educ i e deu e a ion). Howe e , he
ca alys modi ica ion by adding NHCs p omo es he H/D exchange o e he educ i e deu e a ion.
The e o e, he unc ionaliza ion o Ru/C wi h NHCsallowed he selec i e deu e a ion o a oma ic
compounds wi h pha maceu ical in e es ha o he wise canno be ob ained due o educ i e side
eac ions. In addi ion o hese examples, he e a e a ew mo e s udies whe e i is possible o con ol he
s e eoselec i i y o chemoselec i i y o suppo ed MNPs h ough hei unc ionaliza ion wi h NHC
ligands [
52
,
100
]. The e o e, pos -ligand unc ionaliza ion o suppo ed MNPs is an eme ging ield
wi h g ea indus ial in e es , since i combines he ad an ages o homogeneous and he e ogeneous
ca alysis (i.e., ac i i y, selec i i y and s abili y).
Ca alys s2020,10,114422o 30
Figu e21.Choles e ol(le )andNHCligandsde i ed omcholes e ol( igh )used o RuNP
s abiliza ion.Adap ed om e e ence99.Copy igh 2018TheRoyalSocie yo Chemis y.
All heexamplesdiscussedun ilnowwe e ocusedon hepossibili y ocon ol he
physicochemicalp ope ieso ligand‐s abilizedcolloidalMNPsbymodula ing heNHCligands
p esen a hei su ace.Ne e heless,i isalsopossible ocon ol hesu acep ope ieso suppo ed
MNPs h ough hei unc ionaliza ionwi hNHCs[18,52,100], husmodi ying hei selec i i yo
ac i i y.Asisknown,sinceimp o ing heselec i i yo he e ogeneousca alys sisalwaysachallenge,
hemodi ica iono suppo edMNPswi hsu aceNHCsligandscanha eag ea po en ial o
indus ialapplica ions[101].In hisline, he ea e ew ecen exampleso con olling heac i i yand
selec i i yo suppo edRuNPsby hecoo dina iono NHCligandson heme allicsu ace.Fo
example,in2016,Glo iusincollabo a ionwi hMu a sugu epo ed he unc ionaliza iono RuNPs
suppo edonpo assium‐dopedalumina(Ru/K‐Al
2
O
3
)wi h wodi e en NHCligands(ICyand
IMes)[102].TheseNHC‐modi iedsuppo edhe e ogeneousca alys swe e es edin he
hyd ogena iono nume ous eac an swi h hein en iono s udying hee ec o hesu ace
unc ionaliza ionon hei ca aly icp ope ies.Fi s ,aclea dependenceo heca aly icac i i ywi h
heamoun o su aceligandswasobse edin hehyd ogena iono ans‐s ilbeneandace ophenone.
Theac i i ydec easedas henumbe o coo dina edNHCligandsinc eased, his endbeingmo e
ma ked o K‐Al
2
O
3
‐suppo edRuNPs unc ionalizedwi h hebulkies NHCligand(IMes),which
inp incipleblocksmo esu aceac i esi es.Mo eo e ,a ema kablein luenceo hesu aceco e age
wasobse edin heselec i ehyd ogena iono phenylace ylene oe hylbenzene; hela ge he
amoun o su aceNHCligands, hehighe heselec i i y oe hylbenzene(Figu e22a).Again, his
endwasmo ep onounced o Ru/K‐Al
2
O
3
@IMes,whichp esen ed hebulkie N‐subs i uen .Some
yea sbe o e, hesameg oup epo ed hep epa a iono Pdnanopa iclessuppo edonFe
3
O
4
and
hei la e ea men wi hachi alNHCino de omodi y hesu aceo heca alys andinduce
enan ioselec i i yinasymme ic eac ions[18].Mo ep ecisely, hesuppo edPd/Fe
3
O
4
NPswe e
modi iedbya ea men wi h heimidazoliumsal o anenan iome icallypu eNHCin hep esence
o abase(Figu e22b).The esul ingca alys was ullycha ac e izedbyacombina iono di e en
echniques(XPS,FT‐IR,TEM,SEM),andi sca aly icac i i ywase alua edin heasymme ic
α‐a yla iono ke ones,showinginacoupleo casesenan iome icexcesshighe han80%.The
magne ic ecyclingo heca alys wasalsos udied,showing hep ese a iono hei ca aly icac i i y
andselec i i ya e i eca aly iccycles.Ano he in e es ingexampleo unc ionaliza iono
he e ogeneousRuca alys swi hNHCligandswas ecen ly epo edbyPie e se al.[103].The
au ho s ound ha i ispossible ocon ol heac i i yandselec i i yo acomme ciallya ailable
he e ogeneousRuca alys (Ruonca bon;Ru/C)inhyd ogeniso opeexchange(HIE) eac ionsby
addingNHCligands,being he i s exampleo ligandinducedmodi ica iono he e ogeneous
ca alys sin hecon ex o C–Hac i a ion.Mo especi ically, hey epo edagene alme hod o he
selec i eC–Hdeu e a iono α‐posi ionsonalcohols,pha maceu ical ele an he e ocyclesand
aldehydes,p omo ing heH/Dexchangep ocesseso e educ ionside eac ions(Figu e23).In
gene al, heuseo Ru/Cp oducesamix u eo labeled(HIE)and educedp oduc s( educ i e
deu e a ion).Howe e , heca alys modi ica ionbyaddingNHCsp omo es heH/Dexchangeo e
he educ i edeu e a ion.The e o e, he unc ionaliza iono Ru/Cwi hNHCsallowed heselec ion
o deu e a eda oma iccompoundswi hpha maceu icalin e es ha o he wisecanno beob ained
Figu e 21.
Choles e ol (
le
) and NHC ligands de i ed om choles e ol (
igh
) used o Ru NP
s abiliza ion. Adap ed om e e ence 99. Copy igh 2018 The Royal Socie y o Chemis y.
Ca alys s 2020,10, 1144 23 o 30
Ca alys s2020,10,114423o 30
due o he educ ioninside eac ions.Inaddi ion o heseexamples, he ea ea ewmo es udies
whe ei ispossible ocon ol hes e eoselec i i yo chemoselec i i yo suppo edMNPs h ough
hei unc ionaliza ionwi hNHCligands[52,100].The e o e,pos ‐ligand unc ionaliza iono
suppo edMNPsisaneme ging ieldwi hg ea indus ialin e es ,sincei combines head an ages
o homogeneousandhe e ogeneousca alysis(i.e.,ac i i y,selec i i yands abili y).
Figu e22.(a)In luenceo NHC‐loadingon hechemoselec i i yin hehyd ogena iono
phenylace ylene.Ex ac ed om e e ence102.Copy igh 2016Ame icanChemicalSocie y.(b)
Syn hesiso Fe
3
O
4
/PdNPsmodi iedbychi alNHCs.Rep oduced om e e ence18.Copy igh 2010
Willey‐VCH.
Figu e 22.
(
a
) In luence o NHC-loading on he chemoselec i i y in he hyd ogena ion o
phenylace ylene. Ex ac ed om e e ence 102. Copy igh 2016 Ame ican Chemical Socie y.
(
b
) Syn hesis o Fe
3
O
4
/Pd NPs modi ied by chi al NHCs. Rep oduced om e e ence 18. Copy igh
2010 Willey-VCH.
Ca alys s2020,10,114424o 30
Figu e23.Top:Syn hesiso Ruonca bonmodi iedwi hNHCs(NHC@Ru/C).Bo om:Ca aly ic
ac i i yswi chdue o he unc ionaliza iono Ru/Cwi hNHCs.Rep oduced om e e ence103.
Copy igh 2020Willey‐VCH.
5.Conclusions
In his e iew,weha eexplo ed heabili yo NHCligands os abilizeandmodula e he
ca aly icp ope ieso MNPs, ocusing hesu eyono ganome allicnanopa iclesp epa edbyan
o ganome allicapp oach,whichp oducescleansu aceMNPsideal o su aces udiesandca aly ic
applica ions.Since he i s exampleo RuNPss abilizedwi hisolableNHCswas epo ed[12], hei
syn hesishase ol ed,as hes abiliza iono o ganome allicNPswi hnon‐isolableNHCsisnow
possible.Thiscanbepe o medby hein‐si u o ma iono he eeca bene[20]o byusinganNHC‐
CO2adduc asca benep ecu so [49].Thesenewapp oachesha ep o en obequi e e sa ile o he
s abiliza iono mul i udeo NHC‐s abilizedo ganome allicNPs(P ,Ru,Ni,e c.).
Thes ongligand–me alin e ac ionandcleansu aceo heseNHC‐s abilizedo ganome allic
NPsmake hempe ec nano‐objec s o su acein es iga ions.MASNMR,FT‐IRandXPSha ebeen
es ablishedase ec i e ools ode e minebo h hecoo dina iono NHCligandson heMNPsu ace
and heloca iono hei a ailableac i esi es.Fo example,NHCcoo dina ionhasbeende ini i ely
p o enby13CMAS‐NMR h ough hede e mina iono he13C–195P couplingcons an be ween he
labeledligandand heMNP[55]and heobse a iono Knigh shi in hecoo dina edca benic
ca bon[57].Inaddi ion,byusingCOasap obemolecule, hedi e en su aces a eso NHC‐
s abilizedMNPscanbeeasilyde e minedbyFT‐IRandMAS‐NMR.Asa ule, heloca ionand
numbe o ac i esi esmainlydependon hena u eand heamoun o su aceNHCligands.
Mo eo e ,ashasbeenobse edalong his e ision, henumbe o equi alen sand hes uc u eo
hes abilizingNHCligandalsoin luence heMNPp ope iessuchass abili y,solubili yand
ca aly icac i i y/selec i i y.Changeson heN‐subs i uen s(e.g.,inco po a iono abulkyg oup)o
adjus men sin heimidazolium ing(e.g.,p esenceo alongalkylchain)a e e lec edin he
Figu e 23.
(
Top
): Syn hesis o Ru on ca bon modi ied wi h NHCs (NHC@Ru/C). (
Bo om
): Ca aly ic
ac i i y swi ch due o he unc ionaliza ion o Ru/C wi h NHCs. Rep oduced om e e ence 103.
Copy igh 2020 Willey-VCH.
Ca alys s 2020,10, 1144 24 o 30
5. Conclusions
In his e iew, we ha e explo ed he abili y o NHC ligands o s abilize and modula e he
ca aly ic p ope ies o MNPs, ocusing he su ey on o ganome allic nanopa icles p epa ed by an
o ganome allic app oach, which p oduces clean su ace MNPs ideal o su ace s udies and ca aly ic
applica ions. Since he i s example o Ru NPs s abilized wi h isolable NHCs was epo ed [
12
],
hei syn hesis has e ol ed, as he s abiliza ion o o ganome allic NPs wi h non-isolable NHCs is
now possible. This can be pe o med by he in-si u o ma ion o he ee ca bene [
20
] o by using an
NHC-CO
2
adduc as ca bene p ecu so [
49
]. These new app oaches ha e p o en o be qui e e sa ile
o he s abiliza ion o mul i ude o NHC-s abilized o ganome allic NPs (P , Ru, Ni, e c.).
The s ong ligand–me al in e ac ion and clean su ace o hese NHC-s abilized o ganome allic
NPs make hem pe ec nano-objec s o su ace in es iga ions. MAS NMR, FT-IR and XPS ha e been
es ablished as e ec i e ools o de e mine bo h he coo dina ion o NHC ligands on he MNP su ace
and he loca ion o hei a ailable ac i e si es. Fo example, NHC coo dina ion has been de ini i ely
p o en by
13
C MAS-NMR h ough he de e mina ion o he
13
C–
195
P coupling cons an be ween
he labeled ligand and he MNP [
55
] and he obse a ion o Knigh shi in he coo dina ed ca benic
ca bon [
57
]. In addi ion, by using CO as a p obe molecule, he di e en su ace s a es o NHC-s abilized
MNPs can be easily de e mined by FT-IR and MAS-NMR. As a ule, he loca ion and numbe o ac i e
si es mainly depend on he na u e and he amoun o su ace NHC ligands. Mo eo e , as has been
obse ed along his e ision, he numbe o equi alen s and he s uc u e o he s abilizing NHC ligand
also in luence he MNP p ope ies such as s abili y, solubili y and ca aly ic ac i i y/selec i i y. Changes
on he N-subs i uen s (e.g., inco po a ion o a bulky g oup) o adjus men s in he imidazolium ing
(e.g., p esence o a long alkyl chain) a e e lec ed in he modi ica ion o he physicochemical p ope ies
o MNPs. Fo example, he bulkie he N-subs i uen , he la ge he amoun o ee ac i e si es and
highe he ca aly ic ac i i y esul s.
Finally, NHCs ha e also demons a ed hei capaci y o une he ca aly ic p ope ies o suppo ed
MNPs, being able o modi y hei ac i i y o selec i i y, which esul s in g ea in e es due o he ac
ha eaching a good con ol o he selec i i y o he e ogeneous ca alys s is one o he majo challenges
in indus ial ca alysis. These ca aly ic sys ems uni y he bene i s o he e ogeneous ca alys s (s abili y)
and NHC-s abilized MNPs (selec i i y).
The e o e, o summa ize, in his e iew, we ha e shown he capabili y o NHC ligands o modi y
he ca aly ic p ope ies o MNPs, p o iding impo an insigh s in an eme ging ield: in luence o
s abilizing ligands in me al nanopa icle ca alysis.
Au ho Con ibu ions:
Indi idual con ibu ions o all au ho s a e speci ied as ollows: w i ing—o iginal d a
p epa a ion, C.C.-N.; w i ing— e iew and edi ing, P.L., L.M.M.-P.; supe ision, L.M.M.-P. All au ho s ha e ead
and ag eed o he published e sion o he manusc ip .
Funding:
This esea ch was unded by: P oyec os In amu ales Especiales (201880E079), P ime o P oyec os
de In es igaci
ó
n PAID-06-18 (SP20180088), Agencia Es a al de In es igaci
ó
n (PID2019-104159GB-I00/AEI/
10.13039/501100011033) and Jun a de Andalucía (PY18-3208).
Acknowledgmen s:
The au ho s hank Ins i u o de Tecnolog
í
a Qu
í
mica (ITQ), Consejo Supe io de
In es igaciones Cien
í
icas (CSIC), Uni e si a Poli
è
cnica de Val
è
ncia (UPV), Depa amen o de Qu
í
mica Ino g
á
nica
(Uni e si y o Se ille), Ins i u e o Chemical Resea ch (IIQ) o he acili ies and Se e o Ochoa excellence
p og amme, C.C.-N. hanks Gene ali a Valenciana o he p edoc o al GVA ellowship (ACIF/2019/076). We
g a e ully acknowledge B. Chaud e o his in aluable con ibu ion o his esea ch a ea and his since e iendship.
Con lic s o In e es : The e a e no con lic s o decla e.
Re e ences
1. As uc, D. Nanopa icles and Ca alysis; Wiley-VCH: Weinheim, Ge many, 2008.
2. Se p, P.; Philippo , K. Nanoma e ials in Ca alysis; Wiley-VCH: Weinheim, Ge many, 2013.
Ca alys s 2020,10, 1144 25 o 30
3.
Ma
í
nez-P ie o, L.M.; Van Leeuwen, P.W.N.M. Ligand e ec s in u henium nanopa icle ca alysis. In Recen
Ad ances in Nanopa icle Ca alysis; Sp inge Science and Business Media: New Yo k, NY, USA, 2020;
pp. 407–448.
4.
Hopkinson, M.N.; Rich e , C.; Schedle , M.; Glo ius, F. An o e iew o N-he e ocyclic ca benes. Na u e
2014
,
510, 485–496. [C ossRe ]
5.
Smi h, C.A.; Na ouz, M.R.; Lummis, P.A.; Singh, I.; Nazemi, A.; Huang, C.-J.; C udden, C.M. N-he e ocyclic
ca benes in ma e ials chemis y. Chem. Re . 2019,119, 4986–5056. [C ossRe ]
6.
A duengo, A.J.; Ha low, R.L.; Kline, M. A s able c ys alline ca bene E a um o documen ci ed in
CA114(7):62009 . J. Am. Chem. Soc. 1991,113, 2801. [C ossRe ]
7.
Bou issou, D.; Gue e , O.; Gabbaï, F.P.; Be and, G. S able ca benes. Chem. Re .
2000
,100, 39–92. [C ossRe ]
8.
De F
é
mon , P.; Ma ion, N.; Nolan, S.P. Ca benes: Syn hesis, p ope ies, and o ganome allic chemis y.
Co-O d. Chem. Re . 2009,253, 862–892. [C ossRe ]
9.
He mann, W.A.; Goo
β
enL, J.; Spiegle , M. Func ionalized imidazoline -2- ylidene complexes o hodium
and palladium. J. O ganome . Chem. 1997,547, 357–366. [C ossRe ]
10.
Poya os, M.; Ma a, J.A.; Pe is, E. Complexes wi h poly (N-he e ocyclic ca bene) ligands: S uc u al ea u es
and ca aly ic applica ions. Chem. Re . 2009,109, 3677–3707. [C ossRe ]
11.
Schape , L.-A.; Hock, S.J.; He mann, W.A.; Kühn, F.E. Syn hesis and applica ion o wa e -soluble NHC
ansi ion-me al complexes. Angew. Chem. In . Ed. 2012,52, 270–289. [C ossRe ]
12.
La a, P.; Coneje o, S.; Ri ada-Wheelaghan, O.; Po eau, R.; Philippo , K.; Chaud e , B. Ru henium nanopa icles
s abilized by N-he e ocyclic ca benes: Ligand loca ion and in luence on eac i i y. Angew. Chem. In . Ed.
2011,50, 12080–12084. [C ossRe ]
13.
Ma
í
nez-P ie o, L.M.; Fe y, A.; Rake s, L.; Rich e , C.; Lecan e, P.; Philippo , K.; Chaud e , B.; Glo ius, F.
Long-chain NHC-s abilized RuNPs as e sa ile ca alys s o one-po oxida ion/hyd ogena ion eac ions.
Chem. Commun. 2016,52, 4768–4771. [C ossRe ]
14.
La a, P.; Su
á
ez, A.; Colli
è
e, V.; Philippo , K.; Chaud e , B. Pla inum N-he e ocyclic ca bene nanopa icles as
new and e ec i e ca alys s o he selec i e hyd ogena ion o ni oa oma ics. ChemCa Chem
2013
,6, 87–90.
[C ossRe ]
15.
Rangana h, K.V.S.; Schä e , A.; Glo ius, F. Compa ison o supe pa amagne ic Fe
3
O
4
-suppo ed N-he e ocyclic
ca bene-based ca alys s o enan ioselec i e allyla ion. ChemCa Chem 2011,3, 1889–1891. [C ossRe ]
16.
Molla -Cuni, A.; Ven u a-Espinosa, D.; Ma
í
n, S.; Mayo al, A.; Bo ja, P.; Ma a, J.A. S abiliza ion o
nanopa icles p oduced by hyd ogena ion o palladium–N-he e ocyclic ca bene complexes on he su ace o
g aphene and implica ions in ca alysis. ACS Omega 2018,3, 15217–15228. [C ossRe ]
17.
Hu s , E.C.; Wilson, K.; Fai lamb, I.J.S.; Chechik, V. N-He e ocyclic ca bene coa ed me al nanopa icles.
New J. Chem. 2009,33, 1837–1840. [C ossRe ]
18.
Rangana h, K.V.S.; Kloesges, J.; Schä e , A.H.; Glo ius, F. Asymme ic nanoca alysis: N-he e ocyclic ca benes
as chi al modi ie s o Fe
3
O
4
/Pd nanopa icles. Angew. Chem. In . Ed.
2010
,49, 7786–7789. [C ossRe ]
[PubMed]
19.
Planellas, M.; Pleixa s, R.; Sha i , A. Palladium nanopa icles in Suzuki c oss-couplings: Tapping in o he
po en ial o is-imidazolium sal s o nanopa icle s abiliza ion. Ad . Syn h. Ca al.
2012
,354, 651–662.
[C ossRe ]
20.
Ma
í
nez-P ie o, L.M.; Fe y, A.; La a, P.; Rich e , C.; Philippo , K.; Glo ius, F.; Chaud e , B. New ou e o
s abilize u henium nanopa icles wi h non-isolable chi al N-he e ocyclic ca benes. Chem. A Eu . J.
2015
,21,
17495–17502. [C ossRe ] [PubMed]
21.
Hin e mai , U.; Hashmi, S.M.; Elimelech, M.; C ab ee, R.H. Pa icle o ma ion du ing oxida ion ca alysis
wi h Cp i idium complexes. J. Am. Chem. Soc. 2012,134, 9785–9795. [C ossRe ]
22.
Axe , M.; Coneje o, S.; Ge be , I.C. Ligand e ec s on he selec i e hyd ogena ion o ni obenzene o
cyclohexylamine using u henium nanopa icles as ca alys s. ACS Appl. Nano Ma e .
2018
,1, 5885–5894.
[C ossRe ]
23.
Ma
í
nez-P ie o, L.M.; Rake s, L.; L
ó
pez-Vinasco, A.M.; Cano, I.; Coppel, Y.; Philippo , K.; Glo ius, F.;
Chaud e , B.; Van Leeuwen, P.W.N.M. Soluble pla inum nanopa icles liga ed by long-chain N-he e ocyclic
ca benes as ca alys s. Chem. A Eu . J. 2017,23, 12779–12786. [C ossRe ]
