ca alys s
A icle
Modeling he OEC wi h Two New Biomime ic
Models: P epa a ions, S uc u al Cha ac e iza ion,
and Wa e Pho olysis S udies o a Ba–Mn Box Type
Complex and a Mn4N6Plana -Diamond Clus e
La a Rouco 1, M. Isabel Fe nández-Ga cía1, Rosa Ped ido 2, Luis M. Bo ana 3,
Da id Es eban-Gómez 4, Ca los Pla as-Iglesias 4and Ma celino Manei o 1,*
1Depa amen o de Química Ino gánica, Facul ade de Ciencias, Uni e sidade de San iago de Compos ela,
27002 Lugo, Spain; [email p o ec ed] (L.R.); misabel. e nandez.ga [email p o ec ed] (M.I.F.-G.)
2Depa amen o de Química Ino gánica, Facul ade de Química, Uni e sidade de San iago de Compos ela,
15782 San iago de Compos ela, Spain; [email p o ec ed]
3Depa amen o Pha macology, Facul y o Ve e ina y, Uni e si y o San iago de Compos ela, 27002 Lugo,
Spain; [email p o ec ed]
4
Cen o de In es igacións Cien í icas A anzadas (CICA) and Depa amen o de Química, Campus Zapa ei a,
Uni e sidade da Co uña, 15008 A Co uña, Spain; [email p o ec ed] (D.E.-G.);
[email p o ec ed] (C.P.-I.)
*Co espondence: ma celino.manei [email p o ec ed]; Tel.: +34-982-824-106
Recei ed: 28 July 2018; Accep ed: 3 Sep embe 2018; Published: 7 Sep embe 2018
Abs ac :
The oxygen-e ol ing complex (OEC) is he na i e enzyme ha ca alyzes he oxida ion
o wa e in na u al pho osyn hesis. Two new classes o manganese clus e complexes o o mula
Ba2Mn2L12(H3L1)2(CH3OH)41and Mn4L26Cl22we e p epa ed (H4L1=N,N0-(e hane-1,2-diyl)bis
(2-hyd oxybenzamide); L
2
= me hyl picolinimida e) and cha ac e ized by s anda d echniques
including mic oanalysis, IR spec oscopy, ESI spec ome y, and magne ic suscep ibili y
measu emen s. X- ay di ac ion s udies o hese complexes e ealed (i) a box- ype s uc u e o
1
o med by wo edox-ac i e manganese(III) ions and wo ba ium(II) ions connec ed by wo b idging
bisamido-bisphenoxy ligand molecules; and (ii) a plana -diamond a ay o Mn
4
N
6
clus e
2
whe e
he picolinimida es ac as chela ing ligands h ough he wo ni ogen a oms. The abili y o
1
and
2
o spli wa e has been s udied by means o wa e pho olysis expe imen s. In hese expe imen s,
he oxygen e olu ion was measu ed in aqueous media in he p esence o p-benzoquinone (ac ing
as he hyd ogen accep o ), he educ ion o which was ollowed by UV-spec oscopy. The ele an
pho oly ic ac i i y ound o
1
is in con as o he inac i i y o
2
in he pho oly ic expe imen s.
This di e en beha io is discussed on he basis o he s uc u e o he biomime ic models and he
p oposed eac ion mechanism o his p ocess suppo ed by DFT calcula ions.
Keywo ds: a i icial pho osyn hesis; pho oca alys ; wa e spli ing; manganese
1. In oduc ion
The oxygen-e ol ing complex (OEC) is he ca aly ic si e ha ca alyzes, in na u al pho osyn hesis,
he oxida ion o wa e in o dioxygen, p o ons, and elec ons [
1
–
3
]. The na i e Mn
4
CaO
5
complex,
loca ed a he dono si e o pho osys em II (PSII), is esponsible o bo h he a mosphe ic oxygen ha
we b ea he, and also he con e sion o sola ene gy in o chemical ene gy. In his p ocess, wa e spli ing
is he key s ep ha d i es he chain o elec on- ansduc ion eac ions o o m he ene gy anspo ing
molecule NADPH, which holds he elec ons in his chain [
4
,
5
]. The na u al ca alys con ains ou
Ca alys s 2018,8, 382; doi:10.3390/ca al8090382 www.mdpi.com/jou nal/ca alys s
Ca alys s 2018,8, 382 2 o 17
manganese a oms and one calcium a om, coo dina ed by ou wa e molecules, one imidazole, and six
ca boxyla e g oups. In addi ion, wo chlo ide ions a e also in he icini y o he Mn
4
Ca clus e [
1
,
6
].
Much o wha we know abou his ca aly ic complex was lea ned h ough s uc u al and unc ional
a i icial models o he OEC [
7
–
10
]. S uc u al unde s anding o his ca aly ic si e has been achie ed
hanks o mul iple echniques, such as EPR [
11
–
13
], XAS [
14
–
16
] o XRD [
17
,
18
], o en based on
he in o ma ion ob ained h ough mime ic models. The unde s anding o he mechanism o wa e
oxida ion is also be e unde s ood on he basis o da a ob ained om bioino ganic modeling o he
OEC [19–21].
We ha e p e iously epo ed ha a numbe o manganese model compounds a e ca alys s o
wa e pho olysis [
22
,
23
]. In his sense, manganese-Schi base
µ
-aqua dime s we e ound o be ac i e
sys ems unde ligh i adia ion and in he p esence o p-benzoquinone, which ac s as a hyd ogen
accep o . The ONNO se o he e aden a e-Schi base (using he iminic ni ogen and phenoxy
oxygen a oms) p o ides a s ong chela ing e ec which con e s an inc eased obus ness o his ype o
complex in compa ison o o he sys ems [
24
,
25
]. We p opose a mechanism o his ca alysis in ol ing
successi e hyd ogen abs ac ions om wa e molecules bound o he me al ions by op ically exci ed
p-benzoquinone. Pho ogene a ed dioxygen is e ol ed in he o e all p ocess, while p-benzoquinone is
educed o hyd oquinone (Scheme 1).
Ca alys s 2018, 8, x FOR PEER REVIEW 2 o 18
anspo ing molecule NADPH, which holds he elec ons in his chain [4,5]. The na u al ca alys
con ains ou manganese a oms and one calcium a om, coo dina ed by ou wa e molecules, one
imidazole, and six ca boxyla e g oups. In addi ion, wo chlo ide ions a e also in he icini y o he
Mn4Ca clus e [1,6]. Much o wha we know abou his ca aly ic complex was lea ned h ough
s uc u al and unc ional a i icial models o he OEC [7–10]. S uc u al unde s anding o his
ca aly ic si e has been achie ed hanks o mul iple echniques, such as EPR [11–13], XAS [14–16] o
XRD [17,18], o en based on he in o ma ion ob ained h ough mime ic models. The unde s anding
o he mechanism o wa e oxida ion is also be e unde s ood on he basis o da a ob ained om
bioino ganic modeling o he OEC [19–21].
We ha e p e iously epo ed ha a numbe o manganese model compounds a e ca alys s o
wa e pho olysis [22,23]. In his sense, manganese-Schi base µ-aqua dime s we e ound o be ac i e
sys ems unde ligh i adia ion and in he p esence o p-benzoquinone, which ac s as a hyd ogen
accep o . The ONNO se o he e aden a e-Schi base (using he iminic ni ogen and phenoxy
oxygen a oms) p o ides a s ong chela ing e ec which con e s an inc eased obus ness o his ype
o complex in compa ison o o he sys ems [24,25]. We p opose a mechanism o his ca alysis
in ol ing successi e hyd ogen abs ac ions om wa e molecules bound o he me al ions by
op ically exci ed p-benzoquinone. Pho ogene a ed dioxygen is e ol ed in he o e all p ocess, while
p-benzoquinone is educed o hyd oquinone (Scheme 1).
Scheme 1. Pho ogene a ion o dioxygen om wa e molecules bound o he me al ion in
manganese-Schi base µ-aqua dime s.
In ou sea ch o mo e e icien ca alys s, we a e looking o sys ems wi h highe nuclea i y,
di e en opologies, and o he dono a oms beyond he iminic ni ogen o he phenoxy oxygen
a oms. We a e also in e es ed in checking he hypo hesis o he ease o coo dina ion o he wa e
molecules o achie e ac i e ca alys s. In o de o add ess some o hese issues, in he wo k desc ibed
he e, we epo on wo new po en ial biomime ic models o he OEC using o he ype o o ganic
ligands, such as he bisamido-bisphenoxy H4L1 o he me hyl-picolinimida e L2 (see Scheme 2). H4L1
con ains six po en ial dono a oms: wo amide ni ogen, wo phenoxy, and wo amide oxygen a oms.
The la e dono a oms poin ou wa d wi h espec o he ligand ca i y which may acili a e he
ex ension o he s uc u e dimensionali y. In addi ion, he syn he ic p ocedu e used o ob ain he
manganese complex inco po a es an alkaline ea h me al (i.e., ba ium) which may be impo an in
s abilizing he s uc u e aking in o accoun he p esence o o he alkaline ea h me al ion, calcium,
in he na u al OEC. In he case o he L2 ligand, i s s uc u e may a o he o ma ion o
high-nuclea i y clus e s.
(a) (b)
Scheme 2. (a) S uc u e o H4L1. (b) Me hanolysis p ocess o explain he o ma ion o he L2 ligand
om he ini ial 2-cyanopy idine.
Scheme 1.
Pho ogene a ion o dioxygen om wa e molecules bound o he me al ion in manganese-Schi
base µ-aqua dime s.
In ou sea ch o mo e e icien ca alys s, we a e looking o sys ems wi h highe nuclea i y,
di e en opologies, and o he dono a oms beyond he iminic ni ogen o he phenoxy oxygen a oms.
We a e also in e es ed in checking he hypo hesis o he ease o coo dina ion o he wa e molecules o
achie e ac i e ca alys s. In o de o add ess some o hese issues, in he wo k desc ibed he e, we epo
on wo new po en ial biomime ic models o he OEC using o he ype o o ganic ligands, such as
he bisamido-bisphenoxy H
4
L
1
o he me hyl-picolinimida e L
2
(see Scheme 2). H
4
L
1
con ains six
po en ial dono a oms: wo amide ni ogen, wo phenoxy, and wo amide oxygen a oms. The la e
dono a oms poin ou wa d wi h espec o he ligand ca i y which may acili a e he ex ension o he
s uc u e dimensionali y. In addi ion, he syn he ic p ocedu e used o ob ain he manganese complex
inco po a es an alkaline ea h me al (i.e., ba ium) which may be impo an in s abilizing he s uc u e
aking in o accoun he p esence o o he alkaline ea h me al ion, calcium, in he na u al OEC. In he
case o he L2ligand, i s s uc u e may a o he o ma ion o high-nuclea i y clus e s.
Ca alys s 2018, 8, x FOR PEER REVIEW 2 o 18
anspo ing molecule NADPH, which holds he elec ons in his chain [4,5]. The na u al ca alys
con ains ou manganese a oms and one calcium a om, coo dina ed by ou wa e molecules, one
imidazole, and six ca boxyla e g oups. In addi ion, wo chlo ide ions a e also in he icini y o he
Mn4Ca clus e [1,6]. Much o wha we know abou his ca aly ic complex was lea ned h ough
s uc u al and unc ional a i icial models o he OEC [7–10]. S uc u al unde s anding o his
ca aly ic si e has been achie ed hanks o mul iple echniques, such as EPR [11–13], XAS [14–16] o
XRD [17,18], o en based on he in o ma ion ob ained h ough mime ic models. The unde s anding
o he mechanism o wa e oxida ion is also be e unde s ood on he basis o da a ob ained om
bioino ganic modeling o he OEC [19–21].
We ha e p e iously epo ed ha a numbe o manganese model compounds a e ca alys s o
wa e pho olysis [22,23]. In his sense, manganese-Schi base µ-aqua dime s we e ound o be ac i e
sys ems unde ligh i adia ion and in he p esence o p-benzoquinone, which ac s as a hyd ogen
accep o . The ONNO se o he e aden a e-Schi base (using he iminic ni ogen and phenoxy
oxygen a oms) p o ides a s ong chela ing e ec which con e s an inc eased obus ness o his ype
o complex in compa ison o o he sys ems [24,25]. We p opose a mechanism o his ca alysis
in ol ing successi e hyd ogen abs ac ions om wa e molecules bound o he me al ions by
op ically exci ed p-benzoquinone. Pho ogene a ed dioxygen is e ol ed in he o e all p ocess, while
p-benzoquinone is educed o hyd oquinone (Scheme 1).
Scheme 1. Pho ogene a ion o dioxygen om wa e molecules bound o he me al ion in
manganese-Schi base µ-aqua dime s.
In ou sea ch o mo e e icien ca alys s, we a e looking o sys ems wi h highe nuclea i y,
di e en opologies, and o he dono a oms beyond he iminic ni ogen o he phenoxy oxygen
a oms. We a e also in e es ed in checking he hypo hesis o he ease o coo dina ion o he wa e
molecules o achie e ac i e ca alys s. In o de o add ess some o hese issues, in he wo k desc ibed
he e, we epo on wo new po en ial biomime ic models o he OEC using o he ype o o ganic
ligands, such as he bisamido-bisphenoxy H4L1 o he me hyl-picolinimida e L2 (see Scheme 2). H4L1
con ains six po en ial dono a oms: wo amide ni ogen, wo phenoxy, and wo amide oxygen a oms.
The la e dono a oms poin ou wa d wi h espec o he ligand ca i y which may acili a e he
ex ension o he s uc u e dimensionali y. In addi ion, he syn he ic p ocedu e used o ob ain he
manganese complex inco po a es an alkaline ea h me al (i.e., ba ium) which may be impo an in
s abilizing he s uc u e aking in o accoun he p esence o o he alkaline ea h me al ion, calcium,
in he na u al OEC. In he case o he L2 ligand, i s s uc u e may a o he o ma ion o
high-nuclea i y clus e s.
(a) (b)
Scheme 2. (a) S uc u e o H4L1. (b) Me hanolysis p ocess o explain he o ma ion o he L2 ligand
om he ini ial 2-cyanopy idine.
Scheme 2.
(
a
) S uc u e o H
4
L
1
. (
b
) Me hanolysis p ocess o explain he o ma ion o he L
2
ligand
om he ini ial 2-cyanopy idine.
Ca alys s 2018,8, 382 3 o 17
2. Resul s
2.1. P epa a ion and Cha ac e iza ion o Biomime ic Model 1
The mul iden a e bisamido-bisphenoxy H
4
L
1
eadily eac s wi h manganese(II) ace a e in he
p esence o ba ium hyd oxide and ai , as de ailed in he expe imen al sec ion, o gi e biomime ic
model
1
(Ba
2
Mn
2
L
12
(H
3
L
1
)
2
(CH
3
OH)
4
). The alkaline ea h-me al hyd oxide p o ides he basic
condi ions equi ed o achie e e adep o ona ion o he ligand and oxida ion o manganese(II) o
manganese(III) in he p esence o oxygen. Elemen al analysis o he complex indica ed a s oichiome y
Ba
2
Mn
2
L
12
(H
3
L
1
)
2
(CH
3
OH)
4
, which is consis en wi h he o ma ion o neu al species, whe e L
iden i ies he ligand in i s e a-anionic o m, and H
3
L iden i ies he ligand in i s monoanionic o m.
The analy ical, magne ic, spec oscopic, and mass spec ome y da a o model
1
a e gi en in he
Ma e ials and Me hods sec ion. Complex
1
seems o be s able in ai as well as he mally s able, mel ing
abo e 300
◦
C wi hou decomposi ion. I is spa ingly soluble in wa e , pa ially soluble in common
o ganic sol en s such as me hanol, and e y soluble in pola coo dina ing sol en s such as DMF and
DMSO. The o mula ion o model
1
is in ag eemen wi h he mola conduc i i y measu ed in 10
−3
M
DMF solu ion, which is 23 µS cm−1, ypical o non-elec oly e complexes [26].
The alue o he magne ic momen a oom empe a u e, 4.8 B.M., is e y close o he spin-only
alue o 4.89 B.M. expec ed o a high-spin magne ically dilu ed d
4
manganese(III) ion. The ESI
(elec osp ay ioniza ion) mass spec um (Figu e 1) egis e ed in me hanol shows a peak co esponding
o he agmen [MnL + H
+
]
+
, indica ing coo dina ion o he manganese ion wi h he dep o ona ed
ligand. O he mino signals, assigned o [MnL(H
3
L) + H
+
]
+
, [Mn
2
L(H
3
L) + H
+
]
+
, [MnL(H
3
L)Ba +
H
+
]
+
and [MnL(H
3
L)Ba
2
+ H
+
]
+
also con i m he o ma ion o he polynuclea complex including
manganese, ba ium, and he ligand bo h in i s e a-anionic (L
−4
) and monoanionic (H
3
L
−
) o ms.
IR spec oscopy also sugges s he o ma ion o model
1
, exhibi ing he se o bands a ibu able o
he e a-anionic coo dina ed ligand, bu also he co esponding bands ha can be assigned o he
monoanionic o m o he ligand. Thus, he s ong band a 1605 cm
−1
, cha ac e is ic o he
ν
(amide)I
(C=O) s e ching mode, is shi ed 38 cm
−1
o lowe wa enumbe s wi h espec o he ee ligand
(1643 cm
−1
), and he band 1540 o he
ν
(amide)II s e ching mode, is shi ed 12 cm
−1
wi h espec o
he ee ligand (1552 cm
−1
). These da a sugges coo dina ion o he L
−4
ligand h ough he amide
ni ogen a oms, bu addi ional bands a 1641 cm
−1
and 1551 cm
−1
can be assigned o he H
3
L
−
ligand which does no coo dina e h ough hese amide ni ogen a oms. The band a ibu ed o he
ν
(C–O) mode is also shi ed 11 cm
−1
o lowe equencies wi h espec o he ee ligand, indica ing
coo dina ion h ough he hyd oxyl g oups. A s ong band cen e ed a ca 3400 cm
−1
can be assigned o
he ν(O–H) o me hanol molecules.
Ca alys s 2018, 8, x FOR PEER REVIEW 3 o 18
2. Resul s
2.1. P epa a ion and Cha ac e iza ion o Biomime ic Model 1
The mul iden a e bisamido-bisphenoxy H4L1 eadily eac s wi h manganese(II) ace a e in he
p esence o ba ium hyd oxide and ai , as de ailed in he expe imen al sec ion, o gi e biomime ic
model 1 (Ba2Mn2L12(H3L1)2(CH3OH)4). The alkaline ea h-me al hyd oxide p o ides he basic
condi ions equi ed o achie e e adep o ona ion o he ligand and oxida ion o manganese(II) o
manganese(III) in he p esence o oxygen. Elemen al analysis o he complex indica ed a
s oichiome y Ba2Mn2L12(H3L1)2(CH3OH)4, which is consis en wi h he o ma ion o neu al species,
whe e L iden i ies he ligand in i s e a-anionic o m, and H3L iden i ies he ligand in i s
monoanionic o m. The analy ical, magne ic, spec oscopic, and mass spec ome y da a o model 1
a e gi en in he Ma e ials and Me hods sec ion. Complex 1 seems o be s able in ai as well as
he mally s able, mel ing abo e 300 °C wi hou decomposi ion. I is spa ingly soluble in wa e ,
pa ially soluble in common o ganic sol en s such as me hanol, and e y soluble in pola
coo dina ing sol en s such as DMF and DMSO. The o mula ion o model 1 is in ag eemen wi h he
mola conduc i i y measu ed in 10−3 M DMF solu ion, which is 23 µS cm−1, ypical o non-elec oly e
complexes [26].
The alue o he magne ic momen a oom empe a u e, 4.8 B.M., is e y close o he spin-only
alue o 4.89 B.M. expec ed o a high-spin magne ically dilu ed d4 manganese(III) ion. The ESI
(elec osp ay ioniza ion) mass spec um (Figu e 1) egis e ed in me hanol shows a peak
co esponding o he agmen [MnL + H+]+, indica ing coo dina ion o he manganese ion wi h he
dep o ona ed ligand. O he mino signals, assigned o [MnL(H3L) + H+]+, [Mn2L(H3L) + H+]+,
[MnL(H3L)Ba + H+]+ and [MnL(H3L)Ba2 + H+]+ also con i m he o ma ion o he polynuclea complex
including manganese, ba ium, and he ligand bo h in i s e a-anionic (L−4) and monoanionic (H3L−)
o ms. IR spec oscopy also sugges s he o ma ion o model 1, exhibi ing he se o bands
a ibu able o he e a-anionic coo dina ed ligand, bu also he co esponding bands ha can be
assigned o he monoanionic o m o he ligand. Thus, he s ong band a 1605 cm−1, cha ac e is ic o
he ν(amide)I (C=O) s e ching mode, is shi ed 38 cm−1 o lowe wa enumbe s wi h espec o he
ee ligand (1643 cm−1), and he band 1540 o he ν(amide)II s e ching mode, is shi ed 12 cm−1 wi h
espec o he ee ligand (1552 cm−1). These da a sugges coo dina ion o he L−4 ligand h ough he
amide ni ogen a oms, bu addi ional bands a 1641 cm−1 and 1551 cm−1 can be assigned o he H3L−
ligand which does no coo dina e h ough hese amide ni ogen a oms. The band a ibu ed o he
ν(C–O) mode is also shi ed 11 cm−1 o lowe equencies wi h espec o he ee ligand, indica ing
coo dina ion h ough he hyd oxyl g oups. A s ong band cen e ed a ca 3400 cm−1 can be assigned
o he ν(O–H) o me hanol molecules.
Figu e 1. Elec osp ay ioniza ion (ESI) mass spec um o complex 1.
The elec onic abso p ion spec um o 1 shows wo b oad bands: a b oad shoulde ob ained a
510 nm (ε = 700 M−1 cm−1), a ibu able o a d–d ansi ion, and a b oad band a 484 nm (ε = 3100 M−1
Figu e 1. Elec osp ay ioniza ion (ESI) mass spec um o complex 1.
Ca alys s 2018,8, 382 4 o 17
The elec onic abso p ion spec um o
1
shows wo b oad bands: a b oad shoulde ob ained
a 510 nm (
ε
= 700 M
−1
cm
−1
), a ibu able o a d–d ansi ion, and a b oad band a 484 nm
(ε= 3100 M−1cm−1)
, assigned o he phenola e
→
manganese(III) cha ge ans e . The ene gy
and in ensi y o hese wo bands a e in ag eemen wi h hose epo ed o ela ed manganese(III)
complexes [
27
,
28
]. The pa amagne ic
1
H NMR spec um (Figu e S1 con ains an up- ield p o on
esonance ou side he diamagne ic egion a –24.17 ppm (Figu e S1) due o he iso opic shi ing o he
ligand p o ons o high-spin manganese(III) complexes in an oc ahed al ield. This signal co esponds
o he p o ons in o ho posi ions ela i e o he hyd oxyl g oups [
22
,
23
,
29
] and se es o subs an ia e
he o ma ion o a manganese(III) complex.
Single c ys als o complex
1
sui able o X- ay di ac ion s udies we e ob ained as desc ibed in he
Ma e ials and Me hods sec ion. The main c ys al da a and s uc u e e inemen de ails a e shown in
Tables 1and 2; de ailed c ys allog aphic da a is shown in Tables S1–S5 o he Supplemen a y Ma e ials.
Di e en d awings showing he c ys al s uc u e a e shown in Figu es 2and 3.
Table 1. C ys al da a and s uc u e e inemen pa ame e s o compounds 1and 2.
Compound 1 2
Empi ical o mula C34H35BaMnN4O10 C21H24ClMn2N6O3
Fo mula weigh 851.93 553.79
Tempe a u e (K) 100(2) 293(2)
Wa eleng h (Å) 0.71073 0.71069
C ys al sys em Monoclinic Monoclinic
Space g oup P21/c P21/n
a(Å) 12.245(2) 11.953(5)
b(Å) 17.345(3) 11.256(5)
c(Å) 18.041(4) 17.889(5)
α(◦) 90 90
β(◦) 106.38(3) 99.051(5)
γ(◦) 90 90
Volume (Å3)3676.2(13) 2376.9(16)
Z 4 4
Dcalcd. (g cm−3)1.525 1.548
µ(mm−1)1.467 1.21
F (000) 1712 1132
θmin/max (◦) 2.62/21.14 1.92/24.73
Goodness-o - i on F21.005 1.067
To al da a 27,410 4038
Unique da a 6303 4038
Da a/ es ains/pa ame e s 6303/3/440 4038/0/299
Final Rindices (I> 2σ(I)) R1= 0.0521; wR2= 0.1220 R1= 0.0894; wR2= 0.2658
R indices (all da a) R1= 0.0941; wR2=0.1356 R1= 0.1223; wR2=0.2812
Ca alys s 2018, 8, x FOR PEER REVIEW 5 o 18
Figu e 2. ORTEP iew o he BaMnL1(H3L1)(CH3OH)2 asymme ic uni o compound 1, wi h a oms
showing he a omic numbe ing scheme.
The asymme ic uni comp ises a ba ium ion, a manganese ion, a e aanionic (L1)4− ligand, a
monoanionic (H3L1)− en i y, and a me hanol sol en molecule. The wo manganese ions o 1 ha e
simila coo dina ion en i onmen s.
(a)
(b)
(c)
Figu e 3. (a) S ick diag am o sup amolecula box 1 (ba ium ca ion in g een, manganese ion in
pu ple, oxygen in ed, ni ogen in blue, and ca bon in g ey); (b) coo dina ion en i onmen a ound
he manganese cen e in 1, showing he squa e-py amidal geome y o his ion; (c) coo dina ion
en i onmen a ound he ba ium cen e in 1.
The geome y a ound he manganese(III) ion is a i e-coo dina ed dis o ed squa e-py amidal
geome y (Figu e 3b). The coo dina ion sphe e a ound each manganese cen e comp ises he plana
e aanionic bisamido-bisphenoxy L4− ligand, which is igh ly bound o he me al ion h ough he
inne N2O2 compa men by he Namide and Ophenol a oms. The Mn-Namide (1.938 Å and 1.944 Å) and
Mn-Ophenol (1.877 Å and 1.883 Å) bond leng hs a e in he ange expec ed o he e adep o ona ion o
he ligand [23]. The i h coo dina ion posi ion is comple ed wi h a phenolic oxygen a om (O571)
Figu e 2.
ORTEP iew o he BaMnL
1
(H
3
L
1
)(CH
3
OH)
2
asymme ic uni o compound
1
, wi h a oms
showing he a omic numbe ing scheme.
Ca alys s 2018,8, 382 5 o 17
Table 2. Selec ed bond leng hs (Å) and angles (°) o 1.
Mn(2)–N(1) 1.938(5) Ba(1)–O(411) 2.636(5)
Mn(2)–N(4) 1.944(6) Ba(1)–O(511) 2.648(5)
Mn(2)–O(471) 1.877(5) Ba(1)–O(811) 2.658(5)
Mn(2)–O(171) 1.883(5) Ba(1)–O(171) 2.754(5)
Mn(2)–O(571) 2.125(5) Ba(1)–O(471) 2.816(5)
Ba(1)–Mn(2) 3.5510(12) Ba(1)–O(571) 3.074(5)
Ba(1)–O(200)
Ba(1)–O(100)
O(471)–Mn(2)–O(171) 88.6(2) O(411)–Ba(1)–O(511) 120.81(16)
O(471)–Mn(2)–N(1) 173.4(2) O(411)–Ba(1)–O(811) 75.91(14)
O(171)—Mn(2)–N(1) 92.4(2) O(511)–Ba(1)–O(811) 91.95(14)
O(471)—Mn(2)–N(4) 92.7(2) O(411)–Ba(1)–O(171) 97.69(15)
O(171)—Mn(2)–N(4) 164.2(2) O(511)–Ba(1)–O(171) 140.07(14)
N(1)–Mn(2)–N(4) 84.6(2) O(811)–Ba(1)–O(171) 87.29(14)
O(471)–Mn(2)–O(571) 84.4(2) O(411)–Ba(1)–O(471) 139.51(16)
O(171)–Mn(2)–O(571) 92.40(19) O(511)–Ba(1)–O(471) 94.13(15)
N(1)–Mn(2)–O(571) 102.1(2) O(811)–Ba(1)–O(471) 126.37(14)
N(4)–Mn(2)–O(571) 103.4(2) O(171)–Ba(1)–O(471) 56.24(14)
O(411)–Ba(1)–O(571) 142.68(14)
O(511)–Ba(1)–O(571) 82.08(13)
O(811)–Ba(1)–O(571) 74.13(13)
O(171)–Ba(1)–O(571) 59.34(13)
O(471)–Ba(1)–O(571) 54.23(13)
Ca alys s 2018, 8, x FOR PEER REVIEW 5 o 18
Figu e 2. ORTEP iew o he BaMnL1(H3L1)(CH3OH)2 asymme ic uni o compound 1, wi h a oms
showing he a omic numbe ing scheme.
The asymme ic uni comp ises a ba ium ion, a manganese ion, a e aanionic (L1)4− ligand, a
monoanionic (H3L1)− en i y, and a me hanol sol en molecule. The wo manganese ions o 1 ha e
simila coo dina ion en i onmen s.
(a)
(b)
(c)
Figu e 3. (a) S ick diag am o sup amolecula box 1 (ba ium ca ion in g een, manganese ion in
pu ple, oxygen in ed, ni ogen in blue, and ca bon in g ey); (b) coo dina ion en i onmen a ound
he manganese cen e in 1, showing he squa e-py amidal geome y o his ion; (c) coo dina ion
en i onmen a ound he ba ium cen e in 1.
The geome y a ound he manganese(III) ion is a i e-coo dina ed dis o ed squa e-py amidal
geome y (Figu e 3b). The coo dina ion sphe e a ound each manganese cen e comp ises he plana
e aanionic bisamido-bisphenoxy L4− ligand, which is igh ly bound o he me al ion h ough he
inne N2O2 compa men by he Namide and Ophenol a oms. The Mn-Namide (1.938 Å and 1.944 Å) and
Mn-Ophenol (1.877 Å and 1.883 Å) bond leng hs a e in he ange expec ed o he e adep o ona ion o
he ligand [23]. The i h coo dina ion posi ion is comple ed wi h a phenolic oxygen a om (O571)
Figu e 3.
(
a
) S ick diag am o sup amolecula box 1 (ba ium ca ion in g een, manganese ion in
pu ple, oxygen in ed, ni ogen in blue, and ca bon in g ey); (
b
) coo dina ion en i onmen a ound
he manganese cen e in
1
, showing he squa e-py amidal geome y o his ion; (
c
) coo dina ion
en i onmen a ound he ba ium cen e in 1.
The asymme ic uni comp ises a ba ium ion, a manganese ion, a e aanionic (L
1
)
4−
ligand,
a monoanionic (H
3
L
1
)
−
en i y, and a me hanol sol en molecule. The wo manganese ions o
1
ha e
simila coo dina ion en i onmen s.
Ca alys s 2018,8, 382 6 o 17
The geome y a ound he manganese(III) ion is a i e-coo dina ed dis o ed squa e-py amidal
geome y (Figu e 3b). The coo dina ion sphe e a ound each manganese cen e comp ises he plana
e aanionic bisamido-bisphenoxy L
4−
ligand, which is igh ly bound o he me al ion h ough he
inne N
2
O
2
compa men by he N
amide
and O
phenol
a oms. The Mn-N
amide
(1.938 Å and 1.944 Å) and
Mn-O
phenol
(1.877 Å and 1.883 Å) bond leng hs a e in he ange expec ed o he e adep o ona ion
o he ligand [
23
]. The i h coo dina ion posi ion is comple ed wi h a phenolic oxygen a om (O571)
om a monoanionic (H
3
L
1
) ligand. Thus, wo di e en coo dina ion beha io s a e ound in
1
o he
pa en bisamido-bisphenoxy ligand: he inne compa men o he e a-anionic (L
1
)
4−
o ms h ee
chela e ings ( wo six-membe ed and one i e-membe ed) once he manganese ion is coo dina ed,
while he monoanionic (H
3
L
1
)
−
ligand coo dina es o one posi ion o he manganese ion coo dina ion
en i onmen , ac ing as a b idging ligand be ween he manganese ion and he a hes ba ium ion.
The axial Mn–O571 leng h o 2.125 Å is longe han he o he Mn–O
phenol
leng hs due o he Jahn–Telle
e ec de i ed om he Mn(III) d
4
high-spin con igu a ion. The angles be ween he O–(o N)–Mn–O571,
ep esen ing O o N dono a oms a he equa o ial posi ions occupied by he inne N
2
O
2
compa men
o (L
1
)
4−
, ange om 84.4
°
o 103.4
°
, e ealing ce ain dis o ions o he squa e-py amidal geome y
a ound each Mn(III) ion. Mo eo e , he amide a oms o (H
3
L
1
)
−
and (L
1
)
4−
, which poin owa ds
he ou side o he inne ca i y, play c ucial oles in o ex ending he dimensionali y o he s uc u e,
since hey a e bound o he ba ium ions.
The wo ba ium a oms o
1
ha e simila coo dina ion en i onmen s. The ba ium a om is bound
o eigh oxygen a oms (Figu e 3c): wo phenoxy oxygen a oms (O171 and O471) om he e a-anionic
L
4−
ligand, also bound o he manganese ion; wo me hanolic oxygen a oms (O100 and O200); and
h ee amidic oxygen a oms (O411, O511, and O811) om wo di e en neighbo ing ligands, (L
1
)
4−
and
(H
3
L
1
)
−
. The e o e, each ba ium a om is coo dina ed wi h ou di e en esidues, which esul s in
Ba–O dis ances anging om 2.636 o 3.073 Å. The ela i ely b oad ange o Ba–O dis ances is also
associa ed wi h inc eased s uc u al lexibili y and an inc eased capaci y o close con ac s be ween he
ba ium ion and a oma ic ings [
30
]. Ne e heless, hese dis ances a e also in he expec ed ange o
Ba–O bonds [31].
The ba ium a oms and he b idging (H
3
L
1
)
−
ligands play c ucial oles in he assembly o he
inal sup amolecula s uc u e o
1
, which can be also isualized as a Ba
2
Mn
2
L
12
(H
3
L
1
)
2
(CH
3
OH)
4
sup amolecula box (Figu e 3a). The size o he ca i y inside he box is la ge han hose displayed
by sup amolecula boxes induced by alkali me al ions [
23
]. Thus, he Ba–Ba dis ance in
1
is 8.339 Å,
while he M–M dis ances o alkali me al ions ange om 4.47 o 4.87 Å o po assium, ubidium,
and cesium sup amolecula boxes. The dis ance be ween he manganese ions is 7.886 Å, which is
long enough o p e en in e me allic in e ac ions o be es ablished, in ag eemen wi h he obse ed
spin-only magne ic momen o 4.8 B.M.
2.2. P epa a ion and Cha ac e iza ion o Biomime ic Model 2
The eac ion o 2-cyanopy idine and Mn(II) in me hanol solu ion leads o he o ma ion o
biomime ic model
2
(Mn
4
L
26
Cl
2
) con aining O-me hyl picolinimida e L
2
as he chela e ligand.
The me hanolysis o he ini ial 2-cyanopy idine akes place upon coo dina ion wi h he Mn(II) ion as a
chela ing biden a e ligand h ough he wo ni ogen a oms o he py idine ing and he ca boni ile
g oup. As obse ed p e iously, he coo dina ion o 2-cyanopy idine o some di alen me al ions
ac i a es he CN iple bond and makes i much mo e amenable owa d nucleophilic a ack by
CH
3
OH molecules [
32
–
34
]. The p oposed s oichiome y o complex
2
, Mn
4
L
26
Cl
2
, in which six
O-me hyl picolinimida e ligands a e in a monoanionic mode (L
2
)
−
, was con i med by analy ical and
spec oscopic da a. Mo eo e , ec ys alliza ion om he mo he liquo s a o ded X- ay quali y c ys als
o
2
. Complex
2
mel s abo e 300
◦
C wi hou decomposi ion. I is insoluble in wa e and spa ingly
soluble in common o ganic sol en s such as me hanol, bu soluble in pola coo dina ing sol en s such
as DMF and DMSO. The mola conduc i i y measu emen in 10
−3
M DMF solu ion o 18
µ
S cm
−1
is also consis en wi h he o ma ion o he neu al Mn
4
L
26
Cl
2
species [
26
]. The alue o he oom
Ca alys s 2018,8, 382 7 o 17
empe a u e magne ic momen wi h diamagne ism co ec ions is 5.6 B.M., which is compa ible wi h
high-spin magne ically dilu ed d
5
manganese(II) ions. This alue does no allow o di e en ia ion
be ween oc ahed al and bipy amid igonal coo dina ion modes, since he ligand ield heo y aises
he same numbe o unpai ed elec ons in bo h ypes o geome ies.
The ESI-MS o he CH
2
Cl
2
solu ion o
2
gi es peaks a m/z1108.4 and 1129.5, which co esponds
o [
2
+ H]
+
and [
2
+ Na]
+
(posi i e mode), sugges ing he s abili y o his biomime ic model in solu ion
(Figu e S2). The IR spec um o
2
also con i ms he me hanolysis eac ion o he 2-cyanopy idine
o gi e he O-me hyl picolinimida e ligand. Thus, he spec um (Figu e S3) has a sha p band wi h a
medium in ensi y a 3237 cm
−1
, cha ac e is ic o he N–H ib a ion o O-me hyl picolinimida e [
35
].
The C–H s e ching ib a ions o he me hyl g oups o he ca boxamide appea a 2981 and 2940 cm
−1
,
while he absence o he
ν
(C
≡
N) band (which should ha e appea ed a abou 2240 cm
−1
) is indica i e
ha he ni ile g oup has been con e ed o a ca boxamide one. An addi ional s ong band a
1659 cm
−1
is also assigned o
ν
(C=NH) o he ca boxamide g oup. The C–H s e ching ib a ions
o he py idine ings appea a 3072 cm
−1
. Di e en medium and s ong bands obse ed in he
ange 1631–1591 cm
−1
a e assigned o C=N, C=C, and C–C s e ching ib a ions [
36
]. The abso p ion
band a 1379 cm
−1
is assigned o he
ν
(=C–O–) s e ching ib a ion which mixes wi h
δ
(NH) o he
imino e he g oup. The
νas
(C–O–C) and
νs
(C–O–C) abso p ion bands appea a 1138 and 965 cm
−1
,
espec i ely. The abso p ion band obse ed a 1206 cm
−1
is assigned o
δ
(O–CH
3
). The medium
abso p ion band obse ed a he a -in a ed spec um egion a 303 cm
−1
is assigned o Mn–Cl
s e ching ib a ions [37], indica ing he coo dina ion o he chlo ide o he manganese ions.
Single c ys als o complex
2
, sui able o X- ay di ac ion s udies, we e ob ained by slow
e apo a ion o he mo he liquo s a oom empe a u e. The main c ys al da a and s uc u e
e inemen de ails a e collec ed in Tables 1and 3; de ailed c ys allog aphic da a is collec ed
in Tables S6–S10. Figu es 4and 5show di e en iews o he s uc u e o
2
, which displays
a plana -diamond co e o he e ame ic clus e . The c ea ion o bioinspi ed ca alys s o ep oduce
he basic chemis y o he na u al OEC has a oused g ea in e es in he p epa a ion o e anuclea
manganese clus e s [3,7,9,10,13,15,38–43].
Table 3. Selec ed bond leng hs (Å) and angles (°) o 2.
Mn(1)–N(28) 2.048(8) Mn(1)–Mn(2)#1 3.206(2)
Mn(1)–N(8) 2.051(8) Mn(2)–N(28)#1 1.939(8)
Mn(1)–N(18)#1 2.120(7) Mn(2)–N(8) 1.971(8)
Mn(1)–N(1) 2.123(9) Mn(2)–N(11) 2.081(11)
Mn(1)–N(18) 2.141(8) Mn(2)–Cl(31) 2.305(4)
Mn(1)–N(21) 2.143(10) Mn(2)–N(18) 2.353(8)
Mn(1)–Mn(2) 3.203(2) Mn(2)–Mn(1)#1 3.206(2)
N(28)–Mn(1)–N(8) 175.2(3) N(28)#1–Mn(2)–N(8) 124.9(4)
N(28)–Mn(1)–N(18)#1
80.3(3)
N(28)#1–Mn(2)–N(11)
118.5(4)
N(8)–Mn(1)–N(18)#1 102.2(3) N(8)–Mn(2)–N(11) 101.3(4)
N(28)–Mn(1)–N(1) 98.5(3)
N(28)#1–Mn(2)–Cl(31)
101.6(2)
N(8)–Mn(1)–N(1) 77.1(3) N(8)–Mn(2)–Cl(31) 102.9(2)
N(18)#1–Mn(1)–N(1) 98.9(3) N(11)–Mn(2)–Cl(31) 105.2(3)
N(28)–Mn(1)–N(18) 102.7(3)
N(28)#1–Mn(2)–N(18)
76.8(3)
N(8)–Mn(1)–N(18) 81.9(3) N(8)–Mn(2)–N(18) 78.4(3)
N(18)#1–Mn(1)–N(18)
78.2(3) N(11)–Mn(2)–N(18) 75.4(4)
N(1)–Mn(1)–N(18) 157.7(3) Cl(31)–Mn(2)–N(18) 178.4(2)
N(28)–Mn(1)–N(21) 77.9(4)
N(8)–Mn(1)–N(21) 100.4(4)
N(18)#1–Mn(1)–N(21)
155.7(4)
N(1)–Mn(1)–N(21) 94.8(4)
N(18)–Mn(1)–N(21) 96.2(4)
Ca alys s 2018,8, 382 8 o 17
Ca alys s 2018, 8, x FOR PEER REVIEW 8 o 18
Figu e 4. ORTEP iew o he asymme ic uni o 2 wi h a oms showing he a omic numbe ing
scheme.
The biomime ic model Mn4L26Cl2 (2) con ains six O-me hyl picolinimida e and wo chlo ide
ligands. The s uc u e o 2 also e eals he decomposi ion o he 2-cyanopy idine ia he addi ion o
me hanol ac oss he C≡N iple bond o o m a chela ing ligand O-me hyl picolinimida e (L2)−.
The coo dina ion numbe s a e six and i e o Mn1 and Mn2, espec i ely. The me al
coo dina ion geome y is desc ibed as dis o ed oc ahed al o Mn1 and dis o ed igonal
bipy amidal o Mn2 [44]. Analysis o he shape de e mining angles o Mn2, using he app oach o
Reedijk and cowo ke s [45], yielded τ [(α-β)/60, being wi h α and β being he wo g ea es alence
angles o he coo dina ion cen e ] ha ing a alue o 0.9 o Mn2 (τ = 0.0 and 1.0 o squa e-py amidal
and igonal bipy amidal geome ies espec i ely). Thus, i we apply he same app oach o he
i e-coo dina ed manganese ion in 1, we ob ain a alue o 0.15 o τ, co esponding o he desc ibed
squa e-py amidal geome y. Al hough M(II) me al complexes end o s abilize in oc ahed al
geome ies, which is he case o Mn1 in 2, he symme ical high-spin con igu a ion o he Mn(II) ion
p o ides no c ys al ield s abiliza ion ene gy (CFSE), and he s abili y cons an s o i s high-spin
complexes a e consequen ly lowe han hose o co esponding complexes o neighbo ing M(II) ions.
This may be one o he easons o he occu ence o di e en geome ies such as he igonal
bipy amidal displayed by Mn2 in 2.
Each manganese a om in 2 is coo dina ed o h ee o ou di e en O-me hyl picolinimida e
ligands, depending on whe he he ion is igonal bipy amidal o oc ahed al. In he case o Mn1,
which has a oc ahed al geome y, wo chela ing (L2)− a e bound ia he py idyl ni ogen dono
(Mn1–N1 = 2.123(9) Å and Mn1–N21 = 2.143(10)) and he imine ni ogen a oms (Mn1–N8 = 2.051(8) Å
and Mn1–N28 = 2.048 Å), wo addi ional monoden a e (L2)− ligands a e also bound h ough hei
imine ni ogen a oms (Mn1–N18 = 2.141(8) and 2.120(7) Å). Fo Mn2, h ee (L2)− a e bound; one o
hem beha es as he chela ing ligand h ough he py idyl and he imine ni ogen a oms (Mn2–N11 =
2.081(11) Å and Mn2–N18 = 2.353(8) Å), while wo (L2)− ac as monoden a es ia he imine ni ogen
a oms (Mn2–N28 = 1.939(8) and Mn2–N8 = 1.971(8)). The i h coo dina ion posi ion o Mn2 is
comple ed wi h a chlo ide ion. Acco dingly, each one o he six O-me hyl picolinimida e ligands
chela es a manganese ion bu also b idges wo manganese cen e s ia he imine ni ogen a om.
Table 3. Selec ed bond leng hs (Å) and angles (˚) o 2.
Mn(1)–N(28) 2.048(8) Mn(1)–Mn(2)#1 3.206(2)
Mn(1)–N(8) 2.051(8) Mn(2)–N(28)#1 1.939(8)
Mn(1)–N(18)#1 2.120(7) Mn(2)–N(8) 1.971(8)
Mn(1)–N(1) 2.123(9) Mn(2)–N(11) 2.081(11)
Mn(1)–N(18) 2.141(8) Mn(2)–Cl(31) 2.305(4)
Mn(1)–N(21) 2.143(10) Mn(2)–N(18) 2.353(8)
Figu e 4.
ORTEP iew o he asymme ic uni o
2
wi h a oms showing he a omic numbe ing scheme.
Ca alys s 2018, 8, x FOR PEER REVIEW 9 o 18
Mn(1)–Mn(2) 3.203(2) Mn(2)–Mn(1)#1 3.206(2)
N(28)–Mn(1)–N(8) 175.2(3) N(28)#1–Mn(2)–N(8) 124.9(4)
N(28)–Mn(1)–N(18)#1 80.3(3) N(28)#1–Mn(2)–N(11) 118.5(4)
N(8)–Mn(1)–N(18)#1 102.2(3) N(8)–Mn(2)–N(11) 101.3(4)
N(28)–Mn(1)–N(1) 98.5(3) N(28)#1–Mn(2)–Cl(31) 101.6(2)
N(8)–Mn(1)–N(1) 77.1(3) N(8)–Mn(2)–Cl(31) 102.9(2)
N(18)#1–Mn(1)–N(1) 98.9(3) N(11)–Mn(2)–Cl(31) 105.2(3)
N(28)–Mn(1)–N(18) 102.7(3) N(28)#1–Mn(2)–N(18) 76.8(3)
N(8)–Mn(1)–N(18) 81.9(3) N(8)–Mn(2)–N(18) 78.4(3)
N(18)#1–Mn(1)–N(18) 78.2(3) N(11)–Mn(2)–N(18) 75.4(4)
N(1)–Mn(1)–N(18) 157.7(3) Cl(31)–Mn(2)–N(18) 178.4(2)
N(28)–Mn(1)–N(21) 77.9(4)
N(8)–Mn(1)–N(21) 100.4(4)
N(18)#1–Mn(1)–N(21) 155.7(4)
N(1)–Mn(1)–N(21) 94.8(4)
N(18)–Mn(1)–N(21) 96.2(4)
(a) (b)
Figu e 5. (a) S ick diag am o plana -diamond clus e 2. (b) Plana -diamond clus e co e showing he
a omic numbe ing scheme.
2.3. Pho oly ic S udies
The pho oly ic expe imen s we e ca ied ou in he p esence o p-benzoquinone, a
wa e -soluble hyd ogen a om accep o [46]. The expe imen al de ails a e gi en in Sec ion 4.5, and
he expe imen al se up was imp o ed by us [22,23] wi h espec o p e ious expe imen s [47,48] in
o de o ob ain a be e sealing o he sys em. The magne ic s i e used in he o iginal expe imen al
se up had a de imen al e ec on he ep oducibili y o he dissol ed oxygen measu emen s, while
now he use o a me hac yla e ba h allows magne ic s i ing, so he se up is mo e ai igh .
The pho oly ic ac i i y o he biomime ic models was ollowed in wo ways: quan i a i e
oxygen e olu ion and a ia ion o he elec onic spec um o he BQ du ing pho olysis. I was no ed
ea lie ha exci ed-s a e benzoquinone abs ac s a hyd ogen a om di ec ly om wa e [49] a pH 7
and is g ea e in aqueous solu ions. The inal p oduc s we e hyd oquinone and
2-hyd oxybenzoquinone.
Figu e 5.
(
a
) S ick diag am o plana -diamond clus e 2. (
b
) Plana -diamond clus e co e showing he
a omic numbe ing scheme.
The biomime ic model Mn
4
L
26
Cl
2
(
2
) con ains six O-me hyl picolinimida e and wo chlo ide
ligands. The s uc u e o
2
also e eals he decomposi ion o he 2-cyanopy idine ia he addi ion o
me hanol ac oss he C≡N iple bond o o m a chela ing ligand O-me hyl picolinimida e (L2)−.
The coo dina ion numbe s a e six and i e o Mn1 and Mn2, espec i ely. The me al coo dina ion
geome y is desc ibed as dis o ed oc ahed al o Mn1 and dis o ed igonal bipy amidal o Mn2 [
44
].
Analysis o he shape de e mining angles o Mn2, using he app oach o Reedijk and cowo ke s [
45
],
yielded
τ
[(
α
-
β
)/60, being wi h
α
and
β
being he wo g ea es alence angles o he coo dina ion
cen e ] ha ing a alue o 0.9 o Mn2 (
τ
= 0.0 and 1.0 o squa e-py amidal and igonal bipy amidal
geome ies espec i ely). Thus, i we apply he same app oach o he i e-coo dina ed manganese
ion in
1
, we ob ain a alue o 0.15 o
τ
, co esponding o he desc ibed squa e-py amidal geome y.
Al hough M(II) me al complexes end o s abilize in oc ahed al geome ies, which is he case o Mn1
in
2
, he symme ical high-spin con igu a ion o he Mn(II) ion p o ides no c ys al ield s abiliza ion
ene gy (CFSE), and he s abili y cons an s o i s high-spin complexes a e consequen ly lowe han
hose o co esponding complexes o neighbo ing M(II) ions. This may be one o he easons o he
occu ence o di e en geome ies such as he igonal bipy amidal displayed by Mn2 in 2.
Each manganese a om in
2
is coo dina ed o h ee o ou di e en O-me hyl picolinimida e
ligands, depending on whe he he ion is igonal bipy amidal o oc ahed al. In he case o Mn1,
which has a oc ahed al geome y, wo chela ing (L
2
)
−
a e bound ia he py idyl ni ogen dono
(Mn1–N1 = 2.123(9) Å and Mn1–N21 = 2.143(10)) and he imine ni ogen a oms (Mn1–N8 = 2.051(8) Å
Ca alys s 2018,8, 382 9 o 17
and Mn1–N28 = 2.048 Å), wo addi ional monoden a e (L
2
)
−
ligands a e also bound h ough
hei imine ni ogen a oms (
Mn1–N18 = 2.141(8)
and 2.120(7) Å). Fo Mn2, h ee (L
2
)
−
a e bound;
one o hem beha es as he chela ing ligand h ough he py idyl and he imine ni ogen a oms
(
Mn2–N11 = 2.081(11) Å
and Mn2–N18 = 2.353(8) Å), while wo (L
2
)
−
ac as monoden a es ia he
imine ni ogen a oms (Mn2–N28 = 1.939(8) and Mn2–N8 = 1.971(8)). The i h coo dina ion posi ion
o Mn2 is comple ed wi h a chlo ide ion. Acco dingly, each one o he six O-me hyl picolinimida e
ligands chela es a manganese ion bu also b idges wo manganese cen e s ia he imine ni ogen a om.
2.3. Pho oly ic S udies
The pho oly ic expe imen s we e ca ied ou in he p esence o p-benzoquinone, a wa e -soluble
hyd ogen a om accep o [
46
]. The expe imen al de ails a e gi en in Sec ion 4.5, and he expe imen al
se up was imp o ed by us [
22
,
23
] wi h espec o p e ious expe imen s [
47
,
48
] in o de o ob ain
a be e sealing o he sys em. The magne ic s i e used in he o iginal expe imen al se up had
a de imen al e ec on he ep oducibili y o he dissol ed oxygen measu emen s, while now he use
o a me hac yla e ba h allows magne ic s i ing, so he se up is mo e ai igh .
The pho oly ic ac i i y o he biomime ic models was ollowed in wo ways: quan i a i e oxygen
e olu ion and a ia ion o he elec onic spec um o he BQ du ing pho olysis. I was no ed ea lie
ha exci ed-s a e benzoquinone abs ac s a hyd ogen a om di ec ly om wa e [
49
] a pH 7 and is
g ea e in aqueous solu ions. The inal p oduc s we e hyd oquinone and 2-hyd oxybenzoquinone.
The concen a ions o O
2
in he solu ions du ing he expe imen s inc eased linea ly om
abou 2% dissol ed oxygen o 14.4% o pho olysis ca alyzed by biomime ic model
1
, whe eas
he concen a ions o O
2
emained almos cons an o hou s in he case o pho oly ic expe imen s
using biomime ic model
2
(Figu e 6). The abili y o
1
o spli wa e was also been es ed by he
educ ion o p-benzoquinone in o hyd oquinone, which was de e mined by spec opho ome ic
moni o ing o he eac ion. Benzoquinone in wa e has majo abso p ion a 246 nm (
ε
= 2.2
×
10
4
M
–1
cm
–1
), which dec eased du ing he expe imen s, whils a cha ac e is ic hyd oquinone peak a 290 nm
de eloped (Figu e 7).
Ca alys s 2018, 8, x FOR PEER REVIEW 10 o 18
The concen a ions o O2 in he solu ions du ing he expe imen s inc eased linea ly om abou
2% dissol ed oxygen o 14.4% o pho olysis ca alyzed by biomime ic model 1, whe eas he
concen a ions o O2 emained almos cons an o hou s in he case o pho oly ic expe imen s using
biomime ic model 2 (Figu e 6). The abili y o 1 o spli wa e was also been es ed by he educ ion o
p-benzoquinone in o hyd oquinone, which was de e mined by spec opho ome ic moni o ing o
he eac ion. Benzoquinone in wa e has majo abso p ion a 246 nm (ε = 2.2 × 104 M–1 cm–1), which
dec eased du ing he expe imen s, whils a cha ac e is ic hyd oquinone peak a 290 nm de eloped
(Figu e 7).
0 5 10 15 20 25
0
4
8
12
16
% sa O2
ime (h)
Figu e 6. Plo o he pe cen age o O2 dissol ed in solu ion s ime o complexes 1 () and 2 ().
Figu e 7. Ul a iole spec a a ia ion o p-benzoquinone du ing he wa e pho olysis expe imen s
using bioca alys 1, showing he dec ease in he 246 nm band (disappea ance o benzoquinone) and
inc ease in he 290 nm band ( o ma ion o hyd oquinone). G ey colo co esponds o spec um a
ime ze o, and pink colo co esponds o spec um a 24 h.
The pho oly ic expe imen o 2 showed simila beha io o an aqueous solu ion con aining
only benzoquinone (wi hou any complex)—a slow dec ease in he amoun o benzoquinone,
ollowed by he o ma ion o a mix u e o hyd oquinone and 2-hyd oxy-p-benzoquinone wi hou
he gene a ion o molecula oxygen [49]. The ac ha 2-hyd oxy-p-benzoquinone was no obse ed
in he UV-VIS spec um o he p esen s udies wi h 1 indica es ha i is p obably s abilized by he
manganese complex, p esumably due o he app oaching o he quinones o he complex. In his
sense, i is wo h no ing ha no dioxygen e ol es when he s e ically hinde ed
Figu e 6. Plo o he pe cen age o O2dissol ed in solu ion s ime o complexes 1 (N) and 2 ( ).
The pho oly ic expe imen o
2
showed simila beha io o an aqueous solu ion con aining only
benzoquinone (wi hou any complex)—a slow dec ease in he amoun o benzoquinone, ollowed by
he o ma ion o a mix u e o hyd oquinone and 2-hyd oxy-p-benzoquinone wi hou he gene a ion
o molecula oxygen [
49
]. The ac ha 2-hyd oxy-p-benzoquinone was no obse ed in he UV-VIS
spec um o he p esen s udies wi h
1
indica es ha i is p obably s abilized by he manganese
complex, p esumably due o he app oaching o he quinones o he complex. In his sense, i is wo h
no ing ha no dioxygen e ol es when he s e ically hinde ed 2,5- e -bu yl-p-benzoquinone is used
a he han p-benzoquinone, showing a s e ic equi emen in he hyd ogen abs ac ion p ocess.
Ca alys s 2018,8, 382 16 o 17
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