Exploring N2 activation using novel Lewis acid/base pairs: computational insight into frustrated Lewis pairs reactivity

Dal on
T ansac ions
PAPER
Ci e his: Dal on T ans., 2025, 54,
4338
Recei ed 10 h Decembe 2024,
Accep ed 6 h Feb ua y 2025
DOI: 10.1039/d4d 03425b
sc.li/dal on
Explo ing N
2
ac i a ion using no el Lewis acid/
base pai s: compu a ional insigh in o us a ed
Lewis pai eac i i y†
Xuban Gas ea ena,
a
Jon M. Ma xain *
a
and Fe nando Ruipé ez *
b
The ac i a ion o dini ogen (N
2
) is a c ucial s ep in syn hesizing ni ogen-based compounds and emains
a significan challenge due o i s s ong iple bond. Cu en ly, indus ial N
2
con e sion elies on he
Habe –Bosch p ocess, a highly ene gy-in ensi e me hod ha u ilizes ansi ion me al-based ca alys s.
F us a ed Lewis pai s (FLPs) ha e eme ged as a p omising al e na i e o N
2
ac i a ion wi hou he need
o ansi ion me als. In his wo k, we employ densi y unc ional heo y (DFT) o in es iga e he ac i a ion
o N
2
by ansi ion me al- ee Lewis acids (LAs) and bases (LBs). Ou s udy demons a es ha LAs play a
c ucial ole in cap u ing N
2
and de e mining he he modynamics o ac i a ion, while LBs play a comp-
lemen a y ole by educing he bond o de o he N
2
molecule, he eby p omo ing ac i a ion. The
efficiency o N
2
cap u e is di ec ly linked o he elec oaccep ing cha ac e is ics o he LAs. A p incipal
componen analysis (PCA) e eals ha he key ac o s influencing he elec oaccep ing powe o LAs a e
he deg ee o py amidaliza ion and o bi al occupa ion a he acidic si e, as well as he local elec ophilici y
index. The LA-N
2
in e ac ion is ound o be elec os a ic wi h pa ially co alen cha ac e . Among he 21
LAs analyzed, ip ycene-based sys ems exhibi he highes s abili y in o ming LA-N
2
complexes, high-
ligh ing hei po en ial as effec i e N
2
-cap u ing agen s. Howe e , he N
2
iple bond emains la gely
in ac , necessi a ing he in ol emen o LBs in LA-N
2
-LB complexes o ull ac i a ion, in a “push–pull”
mechanism. Six LBs a e analyzed in complexes wi h he mos p omising LAs. Bonding analysis indica es
ha he LB-N
2
in e ac ion can be ega ded as a co alen bond, which may explain he main ole o he LB
in he educ ion o he N
2
bond o de . Fu he mo e, he bond ac i a ion is significan ly enhanced by
inc easing he nucleophilici y o he LB. Among all he LA–LB pai combina ions, only h ee exhibi he
defining cha ac e is ics o us a ed Lewis pai s (FLPs), wi h mode a e in e ac ion ene gies and subs an ial
LA–LB dis ances. Ou findings sugges ha FLPs composed o ip ycene-based LAs and is- e -bu yl-
phosphine ep esen he mos p omising candida es o N
2
ac i a ion.
1 In oduc ion
Ni ogen is a i al elemen o all li e o ms and se es as a p e-
cu so o essen ial ni ogen-con aining compounds such as
amino acids, DNA, and e ilize s.
1
I is he mos abundan
elemen in Ea h’s a mosphe e, comp ising 78% o he ai in
i s dia omic o m, N
2
. Howe e , mos o ganisms a e unable o
di ec ly u ilize ni ogen in i s gaseous o m; i mus i s be
con e ed o “ ixed”in o a mo e accessible o m. This p ocess
occu s na u ally h ough wo p ima y mechanisms. Ligh ning
can con e a mosphe ic ni ogen in o ni ogen oxides (NO
x
),
2
while ni ogenase enzymes, ound in ce ain bac e ia, play a
mo e c i ical ole. These enzymes acili a e he mul i-elec on
educ ion o N
2
o NH
3
, a eac ion ca alyzed by he i on-molyb-
denum co ac o (FeMoco).
3,4
The ni ogen ixa ion p ocess
equi es he hyd olysis o a leas 16 equi alen s o adenosine
iphospha e (ATP) and achie es up o 65% selec i i y.
Howe e , biological N
2
ixa ion emains kine ically slow due o
i s dependence on elec on unneling, making i inadequa e o
suppo he demands o mode n in ensi e ag icul u e.
5
O e
he pas cen u y, ni ogen ac i a ion has p edominan ly been
achie ed h ough he Habe –Bosch p ocess, in which a mos-
phe ic N
2
is educed by hyd ogen gas o p oduce ammonia,
which is hen con e ed in o a ious e ilize s. Despi e i s
indus ial success, he ini ial s ep o ni ogen educ ion is
†Elec onic supplemen a y in o ma ion (ESI) a ailable. See DOI: h ps://doi.o g/
10.1039/d4d 03425b
a
Kimika Fakul a ea, Euskal He iko Unibe si a ea UPV/EHU and Donos ia
In e na ional Physics Cen e (DIPC), Paseo Manuel La dizábal 4, 20018 Donos ia,
Euskadi, Spain
b
POLYMAT and Physical Chemis y Depa men , Facul y o Pha macy, Uni e si y o
he Basque Coun y UPV/EHU, 01006 Vi o ia –Gas eiz, Euskadi, Spain.
E-mail: [email p o ec ed]
4338 |Dal on T ans.,2025,54,4338–4352 This jou nal is © The Royal Socie y o Chemis y 2025
Open Access A icle. Published on 07 Feb ua y 2025. Downloaded on 3/4/2025 11:13:53 AM.
This a icle is licensed unde a
C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence.
View A icle Online
View Jou nal
| View Issue
inhe en ly challenging due o he ex eme s abili y o he N
2
iple bond. This necessi a es ha sh eac ion condi ions (ele -
a ed empe a u e and p essu e) and he use o a me al ca alys ,
o en powe ed by ossil uels, leading o he emission o ossil-
de i ed CO
2
as a by-p oduc .
6–9
Consequen ly, he ac i a ion o
N
2
has become a ocal poin o scien i ic esea ch, wi h effo s
ocused on de eloping en i onmen ally sus ainable al e na-
i es ha ope a e unde mild condi ions and u ilize main-
g oup elemen s a he han ansi ion me als.
The widesp ead use o ansi ion me als in dini ogen ac i-
a ion is due o hei abili y o p o ide bo h unoccupied and
occupied d o bi als ha a e ene ge ically and symme ically
sui ed o accep ing elec on densi y om N
2
and back-dona -
ing i in o he molecule’s an ibonding o bi als, he eby weak-
ening i s iple bond.
10–13
Recen ly, he “push–pull”hypo hesis
has been in oduced, sugges ing ha elec on deple ion
caused by Lewis acid complexa ion ( he pull effec ) enhances
he delocaliza ion o d elec ons om he me al cen e in o N
2
an ibonding o bi als ( he push effec ), u he p omo ing bond
weakening. A ep esen a i e example in ol es bo on, which
ac s as a Lewis acid h ough sp
3
hyb idiza ion wi h i s 2s
2
2p
1
elec onic con igu a ion. When combined wi h low- alen i on
(Fe), molybdenum (Mo), and ungs en (W) cen e s, is(pen a-
luo ophenyl)bo ane (B(C
6
F
5
)
3
) signi ican ly enhances he ac i-
a ion o N
2
in M–N–N–B(C
6
F
5
)
3
complexes.
14
Concep ually, he push–pull eac i i y obse ed in us a ed
Lewis pai s (FLPs) can be compa ed o ha o ansi ion me al
sys ems, whe e a Lewis acid (LA) and Lewis base (LB) coope a e
o ac i a e small molecules. In bo h cases, he eac i i y is
d i en by he syne gis ic ac ion o a s ong LA pulling elec on
densi y and a bulky LB pushing i s elec on pai in o he an i-
bonding o bi als o a subs a e, acili a ing ac i a ion.
15
In his
con ex , he combina ion o ee LA and LB, which a e p e-
en ed om in e ac ing due o s e ic hind ance om bulky
subs i uen s, exhibi s in iguing ca aly ic p ope ies ha
mimic he ole o me allic ca alys s (Fig. 1). The LA mimics he
emp y d o bi als o a me al by in e ac ing wi h ni ogen lone
pai s ia σ-bonding, while he LB pe o ms he π-backdona ion
in o he π* an ibonding o bi als o he ni ogen molecule. In
ecen yea s, FLPs and o he main g oup sys ems ha e demon-
s a ed he abili y o eplica e he eac i i y o a ious an-
si ion me al complexes, enabling bo h s oichiome ic and ca a-
ly ic eac ions ha we e once hough o be exclusi e o an-
si ion me als.
16
This ealiza ion has led o he hypo hesis ha
main g oup compounds could se e as iable al e na i es o
ansi ion me al complexes o N
2
ixa ion.
17
Examples include
expe imen al s udies on bo ylene-based FLPs, such as cyclic
alkyl(amino)ca bene (CAAC)-suppo ed bo ylene complexes
[(Du )B(CAAC)].
18–20
Addi ionally, compu a ional in es iga ions
o N-he e ocyclic ca benes (NHC) emphasize he ole o a oma-
ici y in he ac i a ion p ocess.
21–23
The ac i a ion po en ial o
bo ylenes lies in he ambiphilic cha ac e o bo on, which pos-
sesses a sp hyb idiza ion wi h one emp y 2p o bi al and one
illed, allowing hem o unc ion as bo h LA and LB, simila o
ansi ion me als. Howe e , mos FLP sys ems s udied so a
a e based on in amolecula FLPs, whe e bo h acidic and basic
si es a e inco po a ed in o he same molecule. Mo eo e , hese
FLPs ypically in ol e ca benes as he basic si es and bo ylenes
as he acidic si es.
24,25
The highly eac i e na u e o bo ylenes
necessi a es s abiliza ion as Lewis base adduc s, usually wi h
NHC o CAAC. The elec onic cha ac e is ics o bo ylenes limi
he selec ion o LA and LB o N
2
ac i a ion, making i challen-
ging o de elop al e na i e sys ems wi h he same ac i a ion
capaci y bu wi h g ea e s abili y and lexibili y.
In he ealm o me al- ee N
2
ac i a ion, he use o common
icoo dina e bo ane species p esen s a p omising al e na i e,
po en ially b oadening he ange o LAs and LBs ha can be
u ilized. Despi e his po en ial, a emp s o de elop such
sys ems a e limi ed and s ill in he ea ly s ages. One o he pio-
nee ing effo s in ol ed explo ing he eac i i y o diphenyldia-
zome hane (Ph
2
CNN) wi h B(C
6
F
5
)
3
,
26
whe e he adduc
o med can be iewed as a sys em whe e N
2
is effec i ely
apped be ween a bo ane LA and a ca bene LB. Howe e , his
adduc was ound o be uns able, o en eleasing N
2
a he
han main aining i in a apped s a e. O he app oaches ha e
sough o cap u e and ac i a e N
2
h ough LA-N
2
adduc s. Fo
ins ance, he (N
2
)BF
3
species has been ansien ly gene a ed,
albei unde speci ic condi ions o 170 K and 600 To ,
27
sugges ing ha me al- ee N
2
ac i a ion wi h icoo dina e
bo anes may be easible. Consequen ly, he binding o N
2
o
Lewis acids has been a ocus o se e al compu a ional s udies.
Gi en he low Lewis basici y o N
2
, i is necessa y o use
s ong LAs o supe acids. Compu a ional s udies ha e shown
examples such as he use o ca bo anes,
28
which ha e demon-
s a ed success ul N
2
ac i a ion, al hough he ini ial LA-N
2
adduc s we e no ound o be he modynamically s able.
Addi ionally, compu a ional s udies on B(SiMe
3
)
3
and B(CF
3
)
3
ha e shown ha hese species can o m s able adduc s wi h N
2
,
wi h s abiliza ion ene gies a ound −15 kcal mol
−1
.
29
The py ami-
daliza ion o he acidic cen e has been shown o signi ican ly
enhance Lewis acidi y by lowe ing he eo ganiza ion ene gy and
educing he s uc u al changes equi ed du ing complex o -
ma ion.
30
This dec ease in eo ganiza ion ene gy inc eases he
effec i eness o he Lewis acid in accep ing elec on pai s om
Lewis bases.
31–35
Addi ionally, his cha ac e is ic con ibu es o
he s abili y o Lewis acid–Lewis base (LA–LB) complexes, o en
esul ing in highe dissocia ion ene gies o complexes in ol ing
py amidal Lewis acids.
36
Compu a ional s udies indica e ha
py amidal bo on-con aining Lewis acids, such as 9-bo a ip y-
Fig. 1 T ansi ion me al (M-N
2
) s. us a ed Lewis pai s (LA-N
2
-LB)
complexes (le ) and N
2
alence molecula o bi al diag am ( igh ).
Dal on T ansac ions Pape
This jou nal is © The Royal Socie y o Chemis y 2025 Dal on T ans.,2025,54,4338–4352 | 4339
Open Access A icle. Published on 07 Feb ua y 2025. Downloaded on 3/4/2025 11:13:53 AM.
This a icle is licensed unde a
C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence.
View A icle Online
cene, exhibi signi ican ly g ea e Lewis acidi y compa ed o
hei plana analogs.
37
Fo ins ance, py amidal compounds like
bo aadaman ane and alaadaman ane a e capable o o ming
dono –accep o complexes wi h noble gases.
38
Fu he mo e,
Lewis acidi y can be enhanced h ough luo ina ion, due o he
elec on-wi hd awing p ope ies o luo ine a oms,
37,39
o ia ca -
ionic effec s.
32–34
Despi e hese ad ances, inding common i-
coo dina e bo ane species ha can o m s able adduc s wi h N
2
emains a signi ican challenge.
Based on all his p e ious expe ience and challenges, he
main goal o his wo k is o de e mine which a e he key
ac o s ha de e mine and imp o e he dini ogen cap u e and
ac i a ion by LAs and LBs, espec i ely. In o de o do so,
diffe en complexes o dini ogen and a ious icoo dina e
bo on-based LAs and LBs om diffe en amilies wi h di e se
elec onic and s uc u al cha ac e is ics ha e been s udied.
Then, he na u e o he in e ac ions be ween LAs, LBs and dini-
ogen was ho oughly analyzed by means o compu a ional
ools o unde s and and p edic he binding pa e ns. These
esul s will allow in he design o imp o ed LA/LB and FLP
combina ions o u he dini ogen educ ion o ammonia.
2 Compu a ional de ails
All geome y op imiza ions and ib a ional equency calcu-
la ions we e ca ied ou wi hin densi y unc ional heo y
(DFT)
40,41
using he Gaussian 16 p og am package.
42
Speci ically, geome ies we e op imized using he ωB97XD unc-
ional,
43
combined wi h he 6-31+G(d) basis se .
44
Ha monic
ib a ional equencies we e ob ained by analy ical diffe en-
ia ion o g adien s, a he same le el o heo y, o iden i y i he
cha ac e ized s uc u es we e minima in he po en ial ene gy
su ace. Such equencies we e hen used o e alua e he ze o-
poin ib a ional ene gy (ZPVE) and he he mal (T= 298 K)
ib a ional co ec ions o he en halpy (H) and he Gibbs ee
ene gy (G). These co ec ions a e calcula ed in gas phase, while
expe imen s would be ca ied ou in solu ion. The simpli ica ion
o ou eac ion model in oduces en opy o e es ima ion e o s
associa ed o he o e es ima ion o he en ophy due o he
neglec o empe a u e-dependen sol a ion effec s. To pa ially
o e come his o e es ima ion, he Gibbs ee ene gy co ec ion
p oposed by Finkels ein and Janin is applied:
45
Gco ¼HT½S ib þ1=2ðS ans þS o Þ ð1Þ
This co ec ion mi iga es he o e es ima ion o en opy
changes, hus p o iding mo e accu a e esul s o compa ison
wi h expe imen al alues. The non-co ec ed Gibbs ee ene gy
alues a e a ailable in he ESI.†
To e ine he elec onic ene gies single-poin calcula ions
wi h he 6-311++G(2d ,2p) basis se
46
we e ca ied ou in he
op imized s uc u es. The na u al cha ges we e compu ed by
he na u al bonding o bi al (NBO) me hodology.
47–49
The com-
pu a ional app oach used in his wo k has been alida ed
agains highe -le el heo e ical da a om he li e a u e
29,32,50
(see Tables S1, S2, S3 and S4 in he ESI†).
2.1 Calcula ion o acidi y and basici y
The elec ophilici y o he Lewis acids has been es ima ed by
se e al pa am e s: hyd ide ion affini y (HIA),
51
global elec o-
philici y index (ω)
52
and local elec ophilici y index (ω
B
).
53
The
HIA is de ined as he en halpy change (ΔH) in he eac ion
be ween and acid (LA) and a hyd ide anion (H
−
) in gas phase:
LA þH!½LA Hð2Þ
The HIA alues ha e been calcula ed using he ollowing
isodesmic eac ion:
50
LA þMe3Si H!½LA HþMe3Siþð3Þ
a e subs ac ing he eac ion:
Me3Si H!Me3SiþþHð4Þ
The la ge he en halpy (mo e nega i e o highes absolu e
alue) he la ge he acidi y o he LA. The global elec ophili-
ci y index (ω) is a measu e o he o e all abili y o a molecule
o accep elec ons om any elec on- ich species (nucleo-
phile), and is de ined as:
ω¼χ2
2ηð5Þ
whe e χ≈−
1
2(ε
H
+ε
L
) is Mulliken’s elec onega i i y, ε
H
and ε
L
a e he ene gy o HOMO and LUMO o bi als, and η≈ε
H
−ε
L
is he chemical ha dness. The local elec ophilici y index (ω
B
)
desc ibes he elec ophilic na u e o speci ic a oms o egions
wi hin a molecule and can be de ined as he p oduc o he
global elec ophilici y wi h a local Fukui unc ion (
B+
) on he
bo on a om (o he acidic a om):
ωB¼ω Bþð6Þ
whe e he Fukui unc ion can be con enien ly exp essed om
he elec on popula ion o bo on (Q
B
) in he sys em o Nand N
+ 1 elec ons:
32
Bþ¼QBðNþ1ÞQBðNÞ¼ΔQBð7Þ
The ωand ω
B
indexes a e quan i a i e and base-indepen-
den me ics o Lewis acidi y and p o ide he elec oaccep ing
capaci y o he LA, globally and locally ( e e ed o he 2p
o bi al o B). La ge absolu e alues o hese pa ame e s
should indica e a highe affini y owa ds binding he N
2
. The
basici y is es ima ed wi h he empi ical global nucleophilici y
index (N), a ela i e scale based on he HOMO ene gy o he
base (LB) e e ed o e acyanoe hylene (TCE). This compound
shows he lowes HOMO in a la ge se o p e iously s udied
molecules
54
and allows a posi i e scale o basici ies:
N¼εHðLBÞεHðTCEÞð8Þ
2.2 Analysis o bonding in e ac ions
The na u e o he in e ac ion was analyzed using he Quan um
Theo y o A oms in Molecules (QTAIM)
55–58
and Elec on
Decomposi ion Analysis (EDA)
59,60
me hodologies. In QTAIM,
Pape Dal on T ansac ions
4340 |Dal on T ans.,2025,54,4338–4352 This jou nal is © The Royal Socie y o Chemis y 2025
Open Access A icle. Published on 07 Feb ua y 2025. Downloaded on 3/4/2025 11:13:53 AM.
This a icle is licensed unde a
C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence.
View A icle Online
he bond c i ical poin (BCP) o a pa icula bond is cha ac e -
ized by he elec on densi y (ρ), i s Laplacian (
∇
2
ρ) and he
kine ic (V), po en ial (G) and o al (H) elec on ene gy densi y.
A nega i e alue o
∇
2
ρdeno es elec on densi y concen a ion
in he in e a omic egion, co esponding o co alen bonds.
The condi ion |V|≥2Galso deno es a co alen in e ac ion. A
posi i e Laplacian indica es deple ion o elec on densi y,
which is usually a ibu ed o non-co alen in e ac ions. I he
Laplacian is posi i e bu His nega i e, he in e ac ion can be
classi ied as pa ially co alen . Las ly, i he densi y alue is
la ge enough (mo e han 0.03 a.u.) and His nega i e, he in e -
ac ion is pa ially co alen .
The EDA me hod is based on Mo okuma’s ene gy pa i ion-
ing scheme,
61
and examines he ins an aneous in e ac ion
ene gy (ΔE
in
) be ween wo agmen s (A and B) wi hin a bond
(A–B). This analysis is pe o med in he speci ic elec onic
e e ence s a e and wi h he ozen geome y o he AB
complex. The in e ac ion ene gy is ypically di ided in o wo
main componen s, ΔE
els a
and ΔE
o b
, along wi h an addi ional
dispe sion e m (ΔE
disp
). The e m ΔE
els a
ep esen s he
quasi-classical elec os a ic in e ac ion be ween he unpe -
u bed cha ge dis ibu ions o he p epa ed a oms, which is
gene ally a ac i e. Las ly, ΔE
o b
accoun s o cha ge ans e
and pola iza ion effec s wi hin he sys em.
3 Resul s and discussion
The ac i a ion o N
2
was s udied h ough a wo-s ep p ocess:
cap u ing ni ogen ollowed by i s ac i a ion. Ini ially, he
cap u e mechanism was in es iga ed by examining in e -
ac ions wi h indi idual Lewis acids (LAs) o Lewis bases (LBs).
This was ollowed by an analysis o he ac i a ion p ocess ia
he o ma ion o LX-N
2
-LX complexes, whe e LX deno es ei he
a LA o a LB. This s udy p o ides an in-dep h analysis o he
elec oaccep ing and elec odona ing capaci ies o LAs and
LBs, he molecula geome ies o he o med complexes, hei
in e ac ion ene gies, and he na u e o he in e ac ions
in ol ed.
3.1 N
2
cap u e by Lewis acids
The cap u e o a small molecule like N
2
can occu h ough
in e ac ions wi h ei he a Lewis acid o a Lewis base. When
in e ac ing wi h a Lewis acid, he p ocess ypically in ol es a σ-
ype in e ac ion be ween he ni ogen lone pai and he emp y
o bi al o he acid. In con as , in e ac ion wi h a Lewis base
o en in ol es πdona ion o elec on densi y in o he π* an i-
bonding o bi als o N
2
. In his wo k, we ha e employed a se o
21 Lewis acids (see Fig. 2) and 6 Lewis bases (see Fig. 3) o
explo e hei po en ial o cap u ing N
2
.
A emp s o cap u e ee N
2
using Lewis bases ha e been
unsuccess ul, as none o he es ed bases we e able o o m a
s able LB-N
2
complex. This ou come was an icipa ed because a
Lewis base is unlikely o in e ac a o ably wi h ano he Lewis
base, such as N
2
. As a esul , he p oposed mechanism in ol-
ing πdona ion om he Lewis base o he an ibonding o bi-
als o N
2
is inefficien o cap u ing N
2
. The e o e, he analysis
o he N
2
cap u e is pe o med only wi h Lewis acids.
N
2
is a e y weak Lewis base due o i s unique elec onic
s uc u e. The lone pai s in N
2
a e loca ed in sp hyb id o bi-
als, which a e mo e localized and less a ailable o in e ac ion
compa ed o sp
3
hyb ids o pu e a omic p o bi als.
Addi ionally, he high elec onega i i y o ni ogen u he
educes he a ailabili y o hese lone pai s o bonding wi h
o he molecules. The linea s uc u e o N
2
can also hinde
effec i e o e lap wi h he o bi als o o he molecules app oach-
ing om diffe en angles. The e o e, he acidi y and elec ophi-
lici y o Lewis acids a e c ucial ac o s in achie ing effec i e
cap u e o N
2
. The acidi y o he LAs ep esen ed in Fig. 2 is
s udied using he ollowing pa ame e s: hyd ide ion affini y
(HIA), global (ω) and local (ω
B
) elec ophilici y indexes and
occupa ion numbe o he 2p emp y o bi al o bo on a om
(η(2p)). No e ha , wi h he excep ion o wo, all he acids
unde conside a ion a e bo on-based. The in e ac ion o LAs
wi h N
2
is e alua ed using he in e ac ion (ΔH
in
,ΔG
in
) and
Fig. 2 Lewis acids s udied in his wo k: bo aadaman ane (1), 1-bo aba -
elene (2), B-cubane (3a, X = CH), B,N,N,N-cubane (3b, X = N), BN
3
(4),
bo a ip ycene (5a, Y = B; X = CH; R = H), Al- ip ycene (5b: Y = Al; X =
CH; R = H), Ga- ip ycene (5c: Y = Ga; X = CH; R = H), ca ionic
S-bo a ip ycene (5d: Y = B; X = S; R = H), F-bo a ip ycene (5e:Y=B;X
= CH; R = F), ca ionic F-S-bo a ip ycene (5 : Y = B; X = S; R = F), Cl-bo -
a ip ycene (5g: Y = B; X = CH; R = Cl), cage-shaped bo a e es e (6),
subpo phy in bo enium ca ion (7) and he BX
3
de i a i es (8a:X=H,8b:
X=F;8c: X = Cl; 8d: X = B ; 8e:X=CH
3
;8 :X=CF
3
and 8g:X=C
6
F
5
).
Fig. 3 Lewis bases s udied in his wo k: guanidine (1), iphenyl-
phosphine (2), is- -bu ylphosphine (3), phosphazene (4), Ve kade base
(5) and cyclic ben allene (CBA) (6).
Dal on T ansac ions Pape
This jou nal is © The Royal Socie y o Chemis y 2025 Dal on T ans.,2025,54, 4338–4352 | 4341
Open Access A icle. Published on 07 Feb ua y 2025. Downloaded on 3/4/2025 11:13:53 AM.
This a icle is licensed unde a
C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence.
View A icle Online
de o ma ion (E
de
) ene gies, as well as he py amidaliza ion
angle (α). The in e ac ion ene gy is es ima ed as bo h he
en halpy (ΔH) and he Gibbs ee ene gy (ΔG) change in he
eac ion be ween an acid and N
2
, while he de o ma ion
ene gy co esponds o he en halpy change associa ed o he
geome y ea angemen o he acid ollowing he o ma ion o
he LA-N
2
complex. The αangle e lec s he ini ial geome ical
con igu a ion o he LA be o e in e ac ing wi h N
2
(see Fig. 4).
The deg ee o py amidaliza ion can in luence E
de
, wi h highe
alues expec ed o plana LAs. Addi ionally, plana LAs may
acili a e π-backbonding om subs i uen s o he emp y o bi al
a he acidic si e (bo on), he eby inc easing o bi al occupa ion
(η(2p)) and educing he capaci y o accep elec on densi y
om N
2
.
3.1.1 Elec oaccep ing powe o he Lewis acids. In his
subsec ion, he acidi y and elec oaccep ing capaci y o he 21
LAs om Fig. 2 a e e alua ed ia he HIA, ω,ω
B
and η(2p), and
he esul s summa ized in Table 1. Gene al ends can be
obse ed o he HIA ac oss all s udied LAs. Typically, he
highes HIA absolu e alues a e associa ed wi h compounds
ha ea u e a py amidal geome y a ound he bo on a om and
elec on-wi hd awing subs i uen s (EWGs), pa icula ly in com-
pounds 5a–g, wi h HIA alues anging om −103.87 kcal
mol
−1
o −225.77 kcal mol
−1
, in ag eemen wi h p e ious
esul s.
32,33,62
In con as , lowe HIA alues a e obse ed o
plana LAs wi h elec on-dona ing subs i uen s (EDGs), such
as 6(−76.31 kcal mol
−1
) and 8e (−50.92 kcal mol
−1
).
EWG g oups enhance he acidi y o LAs by wi hd awing
elec on densi y om he bo on a om h ough induc i e
effec s.
37
This end is e iden when compa ing compounds
such as 8a (R = H, −70.62 kcal mol
−1
), 8e (R = CH
3
,
−50.92 kcal mol
−1
), and 8 (R = CF
3
,−138.38 kcal mol
−1
). The
elec onic na u e o he subs i uen s also impac s he occu-
pa ion o he bo on 2p o bi al, wi h EDGs leading o highe
alues. Fo example, η(2p) is la ge in 8e (0.15) compa ed o 8
(0.09). In gene al, η(2p) is in luenced by he e odona ion
abili y o he ligands, which depends on bo h hei elec onic
na u e (EWG o EDG) and he geome y o he acids. Fo 8a,
η(2p) is 0.00 due o hyd ogen’s inabili y o pa icipa e in e o-
dona ion. Subs i uen s wi h ni ogen, oxygen o halogens can
exhibi dual cha ac e is ics, ac ing as EWGs ia induc i e
effec s (due o high elec onega i i y) and as EDGs h ough
lone-pai conjuga ion. Ni ogen a oms di ec ly bonded o
bo on inc ease bo h HIA and η(2p), as seen in compounds 3a
and 3b. No ably, η(2p) alues o BCl
3
and BB
3
a e no
epo ed due o subs an ial e odona ion om halogen a oms,
which leads o he o ma ion o pa ial double bonds.
The impo ance o he local geome y a he acidic si e lies
in educing he ene gy penal y associa ed wi h he geome ic
ea angemen du ing hyd ide ion binding.
31,37,63
Speci ically,
non-plana geome ies minimize his penal y while also hin-
Fig. 4 Defini ion o he py amidaliza ion angle, α.
Table 1 In e ac ion (ΔH
in
,ΔG
in
) and de o ma ion (E
de
) ene gy, in kcal mol
−1
, hyd ide ion affini y (HIA, in absolu e alue), in kcal mol
−1
, global (ω)
and local (ω
B
) elec ophilici y indexes, in eV, occupancy o emp y 2p o bi al o bo on (η(2p)), py amidaliza ion angle (α), in deg ees, B–N and N–N
bond dis ances (R
BN
and R
NN
), in Å, and Wibe g bond index o he B–N (WBI
1
) and N–N (WBI
2
) bonds
LA ΔH
in
ΔG
in
E
de
HIA ωω
B
η(2p) αR
BN
R
NN
WBI
1
WBI
2
12.98 6.46 8.21 70.17 0.85 −0.48 0.15 11.0 1.616 1.103 0.656 2.874
2−7.00 −3.02 7.71 91.92 0.83 −1.02 0.09 15.3 1.569 1.102 0.723 2.866
3a −0.47 3.37 9.73 79.49 0.84 −0.90 0.25 30.0 1.492 1.110 0.806 2.738
3b ———88.25 0.87 −0.03 0.33 24.4 ————
4−51.90 −47.79 9.71 169.62 3.20 −3.91 0.28 49.4 1.426 1.111 0.860 2.687
5a −10.71 −6.99 7.33 110.40 0.93 −1.25 0.08 15.4 1.565 1.101 0.727 2.876
5b −8.55 −6.98 0.63 108.56 1.18 −0.89 0.04
a
21.8 2.190
b
1.099 0.312
c
2.991
5c −5.46 −3.64 −0.01 109.24 1.20 −0.88 0.04
d
21.9 2.276
e
1.099 0.292
2.999
5d
g
−15.36 −11.48 7.23 200.70 3.71 −5.22 0.07 13.8 1.589 1.099 0.706 2.908
5e −17.78 −13.52 7.93 146.12 1.76 −2.45 0.08 16.6 1.589 1.098 0.714 2.906
5
g
−22.60 −18.17 8.43 230.47 5.05 −7.18 0.07 14.8 1.596 1.097 0.704 2.912
5g −0.42 4.78 15.86 138.68 1.60 −1.32 0.12 15.8 1.590 1.097 0.707 2.904
6———81.00 0.78 0.00 0.42 0.0 ————
7———153.23 4.42 0.03 0.45 0.0 ————
8a −5.19 −0.89 9.33 75.32 1.51 −1.82 0.00 0.0 1.579 1.102 0.860 2.687
8b ———72.14 1.35 −1.19 0.32 0.0 ————
8c ———96.42 1.38 −1.22 —0.0 ————
8d ———104.59 1.40 −0.96 —0.0 ————
8e ———55.61 0.95 −0.57 0.15 0.0 ————
8 −12.39 −10.99 10.88 143.07 2.72 −3.22 0.09 0.0 1.619 1.098 0.669 2.915
8g ———114.62 2.07 −1.05 0.21 0.0 ————
a
Al 3p o bi al.
b
Al–N bond dis ance.
c
Al–N bond index.
d
Ga 4p o bi al.
e
Ga–N bond dis ance.
Ga–N bond index.
g
Ca ionic.
Pape Dal on T ansac ions
4342 |Dal on T ans.,2025,54,4338–4352 This jou nal is © The Royal Socie y o Chemis y 2025
Open Access A icle. Published on 07 Feb ua y 2025. Downloaded on 3/4/2025 11:13:53 AM.
This a icle is licensed unde a
C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence.
View A icle Online

de ing po en ial e odona ion om subs i uen s o he bo on
2p o bi al, which would o he wise educe acidi y. This effec is
e iden when compa ing he calcula ed HIA alues o 8g
(−109.93 kcal mol
−1
) and 5e (−141.42 kcal mol
−1
), whe e 5e
can be conside ed a py amidal analogue o 8g. The non-plana
geome y o 5e leads o a highe HIA, as well as a lowe η(2p)
alue (0.08 s. 0.21), due o he o hogonal a angemen
be ween he ip ycene a yl p-o bi als and he bo on 2p o bi al,
which p e en s π-conjuga ion ha would o he wise inc ease
η(2p) (see Fig. S1 in he ESI†). The highes HIA alues a e
obse ed o he ca ionic species 5 (−225.77 kcal mol
−1
) and
5d (−196.00 kcal mol
−1
), whe e he combined effec s o EWGs,
non-plana geome y and a ca ionic sul u a om, which
s ongly enhances acidi y h ough i s elec on-wi hd awing
cha ac e , a e e iden . No ably, his s ong acidi y occu s
wi hou affec ing η(2p). Finally, compounds 3b,6,7,8b–eand
8g we e ound o no bind o N
2
.
When examining he elec ophilici y pa ame e s, ω(global)
and ω
B
(local), a end simila o ha obse ed o HIA is
appa en . The highes ω alues a e ound in py amidal LAs
wi h EWGs, speci ically 5a–g, whe e ω anges om 0.93 eV o
5.05 eV. Con e sely, plana LAs wi h EDGs display he lowes ω
alues, such as 6(0.78 eV) and 8e (1.51 eV). The la ges ωis
obse ed in ca ionic 5 (5.05 eV), while he smalles co es-
ponds o 6(0.78 eV), mi o ing he HIA ends. Signi ican dis-
c epancies be ween global (ω) and local (ω
B
) elec ophilici y
indexes a e e iden in compounds like 1(ω= 0.85 eV, ω
B
=
−0.48 eV), 3b (ω= 0.87 eV, ω
B
=−0.03 eV), 6(ω= 0.78 eV, ω
B
=
0.00 eV) and 7(ω= 4.42 eV, ω
B
= 0.03 eV). This diffe ence
a ises because he global index e lec s he o e all elec on-
accep ing capabili y o he en i e molecule, while he local
index speci ically measu es elec ophilici y a he bo on si e.
Fo example, compound 7has a high global elec ophilici y (ω
= 4.42 eV), likely due o i s ca ionic na u e, bu shows low
bo on-si e elec ophilici y (ω
B
= 0.03 eV), illus a ing he non-
uni o m dis ibu ion o elec ophilic cha ac e ac oss he
molecule.
3.1.2 Geome ical and elec onic cha ac e is ics o he
LA-N
2
complexes. In his subsec ion, he in e ac ion be ween
he N
2
lone pai and he emp y o bi al o he LAs will be ana-
lyzed using in e ac ion (E
in
) and de o ma ion (E
de
) ene gies,
and geome ic pa ame e s o he LA-N
2
complex, such as bond
leng hs (R
BN
,R
NN
) and Wibe g bond indexes (WBI
1
, WBI
2
), see
Table 1.
Fi s ly, i is impo an o no e ha se e al acids (3b,6,7,
8b–eand 8g) o m weakly bonded an de Waals complexes
and will he e o e no be conside ed in he discussion. These
acids exhibi sha ed ea u es, such as plana i y, inc eased
occupa ion o he bo on 2p o bi al and diminished elec ophi-
lici y, which con ibu e o hei limi ed in e ac ion wi h N
2
.
The emaining LAs o m he modynamically a o able LA-N
2
complexes h ough B–N in e ac ions in almos all cases, wi h
in e ac ion ene gies (ΔG
in
) anging om −0.89 kcal mol
−1
o
8a o −47.79 kcal mol
−1
o 4, excep o compounds 1,3a and
5g which show posi i e alues. Compounds 5b and 5c exhibi
unusually low de o ma ion ene gies (0.63 kcal mol
−1
and
−0.01 kcal mol
−1
, espec i ely), which can be a ibu ed o he
highly elec os a ic na u e o he in e ac ion, as will be dis-
cussed la e . Compound 5g shows a ela i ely high posi i e
de o ma ion ene gy (15.86 kcal mol
−1
), likely due o s e ic hin-
d ance be ween he ni ogen and he lone pai s o chlo ine
a oms, which hinde s effec i e B–N coo dina ion.
Rega ding he geome ies o he LA-N
2
complexes, he B–N
bond leng hs ange om 1.426 Å o 1.619 Å, wi h Wibe g bond
indices (WBI
1
) be ween 0.706 and 0.860, indica ing a weak
single bond in mos cases. Howe e , 5b and 5c exhibi signi i-
can ly longe B–N bond dis ances (2.190–2.276 Å) and lowe
WBI
1
alues (0.292–0.312), consis en wi h he elec os a ic
na u e o he in e ac ion. The N–N bond leng hs ange om
1.097 Å o 1.111 Å, closely ma ching he expe imen al bond
dis ance o 1.0977 Å,
64
sugges ing ha in e ac ion wi h he LA
causes only a sligh weakening o he N
2
bond. This minimal
weakening is u he e lec ed in he Wibe g bond indices o
N
2
(WBI
2
), which ange om 2.687 (indica ing a sligh ly wea-
kened iple bond) o 2.999.
3.1.3 P incipal componen analysis o he LA-N
2
in e -
ac ions. Gi en he complexi y o he in e ac ions be ween he
LAs and N
2
, iden i ying he p ima y ac o s in luencing his
in e ac ion is challenging. To add ess his, P incipal
Componen Analysis (PCA) was employed o educe he dimen-
sionali y o he da ase and highligh he mos signi ican
pa ame e s go e ning he in e ac ion. PCA simpli ies he ana-
lysis by ans o ming he la ge se o a iables in o a smalle
subse , which e ains he essen ial pa e ns and ends o he
in e ac ion while educing he o e all numbe o a iables.
P incipal componen s a e newly cons uc ed, unco ela ed
a iables o med as linea combina ions o he o iginal ones,
cap u ing he di ec ions wi h he highes a iance in he
da ase . The g ea e he a iance wi hin a p incipal com-
ponen , he mo e in o ma ion i e ains, indica ing ha he
componen e lec s a subs an ial amoun o he unde lying
s uc u e o he in e ac ion.
Fig. 5 illus a es he esul s o he PCA analysis and he
co esponding co ela ion ma ix o he acidi y and elec o-
philici y pa ame e s. In he PCA analysis (le panel), he i e
o iginal pa ame e s ha e been educed o wo p incipal com-
ponen s (PC1 and PC2). The o ange do s ep esen LA-N
2
com-
plexes (labeled as 1), while he blue do s indica e LAs ha do
no bind o N
2
(labeled as 0). The PCA esul s e eal ha η(2p)
and αa e he mos in luen ial pa ame e s. Speci ically, a
highe η(2p) alue is associa ed wi h a educed endency o
bind o N
2
, whe eas a highe α alue co esponds o an
inc eased endency o bind. HIA, ωand ω
B
exhibi lowe ec o
magni udes in he PCA, indica ing hei lesse ele ance in he
in e ac ion wi h N
2
. Addi ionally i seems o be a connec ion
be ween he size o he lone pai and he in e ac ion wi h N
2
in he bo a ip ycenes (5a–g) and compound 2(see Fig. S2 and
S3 in he ESI†). These indings a e co obo a ed by he co e-
la ion ma ix ( igh panel), which shows ha he pa ame e s
mos s ongly co ela ed wi h in e ac ion ene gy a e η(2p)
(−0.68) and α(0.54). ω
B
also shows a mode a e co ela ion
(−0.46), while ωand HIA ha e he weakes co ela ions (−0.36
Dal on T ansac ions Pape
This jou nal is © The Royal Socie y o Chemis y 2025 Dal on T ans.,2025,54,4338–4352 | 4343
Open Access A icle. Published on 07 Feb ua y 2025. Downloaded on 3/4/2025 11:13:53 AM.
This a icle is licensed unde a
C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence.
View A icle Online
and 0.12, espec i ely). The e o e, HIA and global elec ophili-
ci y indices alone a e insufficien o ully explain he binding
pa e ns wi h N
2
. Ins ead, he ends in LA-N
2
in e ac ions can
be mo e accu a ely a ionalized by ocusing on α,ω
B
, and
η(2p). Gene ally, non-plana compounds end o ha e lowe
de o ma ion ene gy (E
de
) alues (7.23–9.73 kcal mol
−1
) com-
pa ed o plana compounds (9.33–10.88 kcal mol
−1
), and hese
non-plana compounds also p e en back-dona ion o he 2p
o bi al, esul ing in lowe η(2p) alues. Addi ionally, highe ω
B
alues a e indica i e o g ea e elec oaccep ing powe .
Acoo ding o his esul s, he LAs can be classi ied in o h ee
g oups based on hese h ee pa ame e s. In Fig. 6, each ci cle
ep esen s LAs ha mee he ollowing c i e ia: α>0°,ω
B
>0.9
eV and η(2p) < 0.09. This classi ica ion highligh s how diffe en
combina ions o hese pa ame e s in luence he exo he mic
na u e o he LA-N
2
binding. The g oup o LAs wi h he la ges
in e ac ion ene gies is ound a he in e sec ion o he h ee
ci cles. This in e sec ion ep esen s LAs ha mee all he speci-
ied pa ame e h esholds. No ably, his g oup includes
membe s o he 2and 5 amilies, wi h he excep ion o 5g.
The nex g oup o LAs includes hose wi h α= 0°, bu
mee ing he condi ions o he o he wo pa ame e s (in e -
sec ion o he ed and g een ci cles): 8a and 8 . These LAs a e
plana , bu hei subs i uen s a ached o bo on (H o 8a and
CF
3
o 8 ) do no pa icipa e in back-dona ion o elec on
densi y, esul ing in low η(2p) alues. The hi d g oup com-
p ises LAs ha sa is y he c i e ia o αand ω
B
bu no o
η(2p) (in e sec ion o he g een and blue ci cles): 3a,4, and 5g.
No ably, 4demons a es a e y high ω
B
alue (−3.91 eV), which
is likely due o i s unusual geome y and sp
2
hyb idiza ion o
he bo on a om, acili a ing a s ong in e ac ion wi h N
2
(ΔG
in
=−47.79 kcal mol
−1
). In con as , 3a and 5g exhibi signi i-
can ly lowe ω
B
alues (−0.90 eV and −1.32 eV, espec i ely).
Addi ionally, 5g shows an unusually high de o ma ion ene gy
(15.86 kcal mol
−1
), p obably due o s e ic hind ance be ween
he Cl a oms and N
2
, esul ing in a posi i e in e ac ion ene gy
(ΔG
in
= 4.78 kcal mol
−1
). The emaining LAs ei he mee only
one o he speci ied pa ame e s o none a all, leading hem o
ei he no bind wi h N
2
o o m complexes wi h posi i e ΔG
in
.
3.1.4 Na u e o he bonding in LA-N
2
complexes. The ana-
lysis o he in e ac ion wi h N
2
using he Quan um Theo y o
A oms in Molecules (QTAIM) me hodology e eals he ollow-
ing (see Table 2): o all complexes, he Laplacian o he elec-
on densi y is posi i e (∇
2
ρ> 0), indica ing ha he LA-N
2
in e ac ion is p edominan ly elec os a ic. Despi e his, he
la ge alues o elec on densi y (ρ> 0.03) and he nega i e
alues o he ene gy densi y (H) sugges ha he in e ac ion
also exhibi s a signi ican co alen cha ac e . This pa ial
co alency a ises om he dono –accep o in e ac ion be ween
he lone pai o he ni ogen a om and he emp y 2p o bi al o
he bo on a om. The esul s o he EDA analysis a e also sum-
ma ized in Table 2, p o iding addi ional complemen a y
insigh s. No ably, LAs such as 5b and 5c, which a e cha ac e -
ized by p edominan ly elec os a ic in e ac ions, exhibi lowe
ρ alues, longe LA-N
2
dis ances, and weake in e ac ion ene -
gies. In con as , LAs wi h g ea e o bi al con ibu ions show
highe ρ alues, leading o s onge in e ac ion ene gies and
sho e LA-N
2
dis ances, especially in he case o compound 4.
Fu he mo e, conside ing ha EDA and QTAIM analyses a e
Fig. 5 Le : esul s o he P incipal Componen Analysis (PCA). Righ : co ela ion ma ix o he acidi y/elec ophilici y pa ame e s.
Fig. 6 Classifica ion o he LAs in e ms o he mos impo an pa a-
me e s in he o ma ion o he LA-N
2
complexes: α,ω
B
and η(2p).
Pape Dal on T ansac ions
4344 |Dal on T ans.,2025,54,4338–4352 This jou nal is © The Royal Socie y o Chemis y 2025
Open Access A icle. Published on 07 Feb ua y 2025. Downloaded on 3/4/2025 11:13:53 AM.
This a icle is licensed unde a
C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence.
View A icle Online
complemen a y, as suppo ed by he li e a u e,
65
he ollowing
discussion o bonding na u e will be based solely on he
QTAIM esul s.
3.2 Ac i a ion o N
2
wi h Lewis acids and bases
In he p e ious sec ion, i was obse ed ha se e al LAs can
cap u e N
2
h ough a dono –accep o in e ac ion be ween he
ni ogen lone pai and he emp y o bi al o he acid. Howe e ,
his in e ac ion does no ac i a e he s ong N
2
iple bond. To
acili a e his ac i a ion, a second ac i e species is equi ed.
Fo example, a Lewis base can in e ac wi h he LA-N
2
complex
o o m a LA-N
2
-LB “push–pull”complex (see Fig. 7). In his
sys em, he acid wi hd aws elec on densi y om he N
2
bond
(“pull”), while he base dona es elec on densi y o he an i-
bonding π* o bi als o N
2
(“push”). This combined effec
weakens he N
2
iple bond in a manne simila o ha
obse ed wi h ansi ion me als.
15,66,67
To explo e he ole o he bases in ac i a ing he N
2
bond,
he in e ac ion o se e al LA-N
2
complexes wi h he LBs shown
in Fig. 3 was in es iga ed. Ini ially, he basici y o hese LBs
was assessed using he global nucleophilici y index (N), as his
p ope y is expec ed o be c i ical o he ac i a ion p ocess.
The mos basic LBs iden i ied a e LB-5 and LB-6, wi h nucleo-
philici y alues o 4.80 eV and 4.79 eV, espec i ely (see
Table 3). Compound LB-6 is a cyclic ben allene (CBA), a ype
o py azolin-4-ylidene wi h wo ni ogens in he ing. This
s uc u e ansmi s a s ong σ-dona ion capaci y o he cen al
ca bon a om.
68–70
LB-5 is Ve kade’s base, a iaminophosphine
known o i s excep ionally high basici y, which su passes
many adi ional bases.
71
LB-4 is a phosphazene (R
2
P N ype
compound), whose basici y is signi ican ly a ibu ed o he
esonance s abiliza ion and he elec onic cha ac e is ics o
he P N bond, which inc eases elec on densi y o e he ni o-
gen.
72
Addi ionally, he well-known guanidine (LB-1), i-
phenylphosphine (LB-2), and is- e -bu ylphosphine (LB-3)
we e also included in he s udy o compa ison.
3.2.1 Geome ical and elec onic cha ac e is ics o he
LA-N
2
-LB complexes. In o de o in es iga e he effec o he
Lewis bases, only he complexes ha showed he s onges
LA-N
2
in e ac ions ha e been selec ed, namely, LA-4,LA-5a,
LA-5b, ca ionic LA-(5d– ) and LA-8 . In Table 3 a e collec ed
he LA-N
2
-LB in e ac ion ene gies, de ined as he en halpy
(ΔH) and Gibbs ee ene gy (ΔG) change in he ollowing
eac ion:
LA þLB þN2!LA N2LB ð9Þ
LA-4, as well as he ca ionic acids LA-5d and LA-5 , o m
s able LA-N
2
-LB complexes wi h all bases, excep o LA-5d
wi h LB-1, which exhibi s a sligh ly posi i e in e ac ion ene gy
(ΔG
in
= 2.22 kcal mol
−1
). Simila ly, LA-5e and LA-8 o m
s able complexes wi h all LBs, excep LB-1 and LB-2.On he
o he hand, LA-5a and LA-5b only o m s able complexes wi h
he excep ionally nucleophilic LB-6. The esul s show a co e-
la ion be ween he capaci y o he LAs o cap u e N
2
(ΔG
in
,
Table 1) and he s abili y o he LA-N
2
-LB complexes (ΔG
in
,
Table 3), wi h he mos a o able LA-N
2
-LB in e ac ions ollow-
ing his o de (excep o he exchange be ween LA-5e and ca -
ionic LA-5d): LA-4 >LA-5 >LA-5d >LA-5e >LA-8 >LA-5a >
LA-5b. Fo he ip ycenes (LA-5 amily), hese ends can be
a ibu ed o he ω
B
pa ame e , due o hei iden ical geo-
me ic s uc u e. Besides, o he same acid, he in e ac ion
ene gy appea s o co ela e wi h he nucleophilici y index (N),
e lec ing he in luence o he base. S onge bases yield mo e
nega i e in e ac ion ene gies. LB-6 shows signi ican ly mo e
nega i e in e ac ion ene gies, no able g ea e han LB-5,
despi e ha ing simila s eng h, e en o LA-5a, which shows
Table 2 EDA and QTAIM analyses o selec ed LA-N
2
complexes. Elec os a ic in e ac ion (ΔE
els a
), o bi al a ac ion (ΔE
o b
) and dispe sion ene gy
(ΔE
disp
), in kcal mol
−1
. Values in b acke s a e he pe cen age con ibu ions o he o al a ac i e in e ac ions: ΔE
els a
+ΔE
o b
+ΔE
disp
. Elec on
densi y (ρ), i s Laplacian (
∇
2
ρ), o al elec on ene gy densi y (H), po en ial elec on ene gy densi y (G) and kine ic elec on ene gy densi y (V)
LA ΔE
els a
ΔE
o b
ΔE
disp
ρ∇
2
ρVGH
1−220.65 (43.6) −271.35 (53.7) −13.51 (2.7) 0.093 0.611 −0.243 0.198 −0.045
2−212.60 (40.3) −304.34 (57.7) −10.33 (2.0) 0.107 0.674 −0.283 0.226 −0.057
3a −343.31 (42.6) −451.46 (56.1) −10.49 (1.3) 0.135 0.775 −0.369 0.281 −0.088
4−214.42 (30.0) −495.90 (69.3) −5.45 (0.8) 0.169 0.867 −0.479 0.348 −0.131
5a −214.61 (39.7) −308.14 (57.0) −17.76 (3.3) 0.109 0.680 −0.288 0.229 −0.059
5b −71.78 (46.8) −72.47 (47.3) −8.99 (5.9) 0.030 0.152 −0.337 0.358 0.021
5c −69.21 (50.1) −59.95 (43.4) −9.09 (6.6) 0.041 0.144 −0.507 0.433 −0.073
5d
a
−700.39 (34.0) −1341.12 (65.1) −19.01 (0.9) 0.104 0.615 −0.267 0.210 −0.057
5e −197.69 (37.2) −311.95 (58.7) −22.03 (4.1) 0.106 0.592 −0.269 0.208 −0.060
5
a
−637.70 (32.3) −1315.59 (66.6) −22.55 (1.1) 0.107 0.553 −0.264 0.201 −0.063
5g −239.46 (40.8) −323.09 (55.1) −24.26 (4.1) 0.107 0.583 −0.269 0.207 −0.061
8a −191.55 (38.8) −297.98 (60.4) −3.95 (0.8) 0.103 0.688 −0.279 0.225 −0.053
8 −191.02 (37.6) −300.02 (59.1) −16.89 (3.3) 0.099 0.570 −0.249 0.196 −0.053
a
Ca ionic.
Fig. 7 Rep esen a i e molecula geome ies o he LA-N
2
(le ) and
LA-N
2
-LB ( igh ) complexes.
Dal on T ansac ions Pape
This jou nal is © The Royal Socie y o Chemis y 2025 Dal on T ans.,2025,54,4338–4352 | 4345
Open Access A icle. Published on 07 Feb ua y 2025. Downloaded on 3/4/2025 11:13:53 AM.
This a icle is licensed unde a
C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence.
View A icle Online
un a o able in e ac ions wi h he es o he LBs and a s able
complex is ound wi h LB-6. This sugges s ha he ema kable
s abili y p o ided by LB-6 is also linked o he na u e o he
in e ac ion, as will be explained la e . Thus, bo h he elec onic
cha ac e is ics o he LA and LB de e mine he easibili y and
s eng h o he in e ac ion.
The deg ee o ac i a ion o he N
2
molecule can also be
deduced om he geome y o he complexes, especially om
he N
2
bond leng h. Upon eac ion wi h a Lewis base, he
LA-N
2
complex unde goes a signi ican geome y ea ange-
men (see Fig. 7). Speci ically, while he LA-NuN bond angles
in LA-N
2
complexes a e nea ly linea (close o 180°), he
LA-N N and N N-LB bond angles in he esul ing LA-N
2
-LB
complexes shi o a ound 120°, and can also be ep esen ed
as he zwi e ionic o m LA
−
-N
2
-LB
+
. This s uc u al ans o m-
a ion e lec s he o ma ion o a co alen bond be ween he
Lewis base and N
2
.
In Table 4 a e collec ed selec ed bond dis ances and bond
o de s. The N–N bond leng h oscilla es be ween 1.208–1.281 Å,
no ably longe han he bond leng h in he isola ed molecule
(1.10 Å). This ea u e is e lec ed in he educ ion o he bond
o de om iple o double (WBI
3
, a ound 2.000), clea ly indi-
ca ing an effec i e bond ac i a ion. Fu he mo e, in some
cases a weakened double bond is obse ed, as in he com-
plexes o med by he acid LA-4 and bases LB-2,LB-3,LB-5 and
LB-6 (WBI
3
less han 1.800). The LA-N
2
bond dis ances ange
be ween 1.415–1.563 Å and WBI
1
alues o 0.756–1.002, indi-
ca ing a bonding in e ac ion. Fo LA-5b, no ably longe dis-
ances and lowe WBI
1
alues a e calcula ed, sugges ing a
weake in e ac ion wi h his acid. LB-N
2
bond dis ances
be ween 1.370–1.786 Å and WBI
2
alues be ween 0.790–1.111
a e calcula ed, he e o e, a bonding in e ac ion is also
expec ed.
3.2.2 Na u e o he bonding in LA-N
2
-LB complexes.
QTAIM me hodology has been employed o unde s and he
in e ac ions in LA-N
2
-LB complexes. The esul s o he LA-N
2
and LB-N
2
bond c i ical poin s a e collec ed in he ESI (see
Table S4†). Rega ding he LA-N
2
in e ac ion, he same pa e n
obse ed in he LA-N
2
complexes is ep oduced he e in all
cases (∇
2
ρ>0,|V|≤2G and ρ> 0.03) and, he e o e, he in e -
ac ion can be ega ged as elec os a ic wi h pa ially co alen
cha ac e . On he o he hand, all he LB-N
2
in e ac ions can be
conside ed as co alen (∇
2
ρ> 0 and |V|≥2G), which may
explain he main ole o he base in he educ ion o he bond
o de o N
2
. The ema kable σ-dona ion abili y o LB-6 is
e lec ed in la ge ρ alues and bond o de s, and sho e bond
leng hs.
In summa y, he esul s suppo he p oposed model o N
2
ac i a ion, whe e he Lewis acid plays a key ole in cap u ing
N
2
and is p ima ily esponsible o he he modynamics o he
p ocess, s abilizing he complex and in luencing he o e all
ene gy p o ile o he eac ion. On he o he hand, he Lewis
base is p ima ily esponsible o he kine ics o he ac i a ion,
since he weakening o he bond di ec ly impac s he a e a
which he N
2
molecule is ac i a ed. To in es iga e he kine ics
o he Lewis base a ack on he LA-N
2
complex, we compu ed
Table 3 Nucleophilici y index (N), in eV, o he Lewis bases (LB) and LA-N
2
-LB in e ac ion ene gy (ΔH
in
and ΔG
in
), in kcal mol
−1
LB
LA-4 LA-5a LA-5b LA-5d
a
LA-5e LA-5
a
LA-8
NΔH
in
ΔG
in
ΔH
in
ΔG
in
ΔH
in
ΔG
in
ΔH
in
ΔG
in
ΔH
in
ΔG
in
ΔH
in
ΔG
in
ΔH
in
ΔG
in
LB-1 2.64 −29.04 −21.97 26.32 32.33 —— −3.83 2.22 6.83 13.85 −21.10 −14.15 7.09 10.87
LB-2 3.48 −40.25 −34.05 16.81 22.50 ——−15.02 −9.35 −2.70 3.11 −32.57 −25.93 −2.29 1.15
LB-3 3.74 −50.06 −43.35 6.78 13.93 ——−22.40 −15.37 −11.73 −4.02 −38.17 −29.91 −13.51 −8.36
LB-4 4.00 −41.75 −33.29 ————−23.30 −14.76 −10.57 −13.9 −39.31 −32.99 −13.66 −6.24
LB-5 4.80 −57.65 −50.52 0.15 6.11 9.65 16.81 −32.68 −25.24 −20.05 −13.06 −44.45 −35.65 −13.66 −9.33
LB-6 4.79 −80.50 −72.71 −25.52 −17.71 −18.94 −12.39 −62.46 −54.64 −49.27 −40.97 −83.77 −75.04 −49.23 −42.80
a
Ca ionic.
Pape Dal on T ansac ions
4346 |Dal on T ans.,2025,54, 4338–4352 This jou nal is © The Royal Socie y o Chemis y 2025
Open Access A icle. Published on 07 Feb ua y 2025. Downloaded on 3/4/2025 11:13:53 AM.
This a icle is licensed unde a
C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence.
View A icle Online