Probing the M-CNHC Bond and Its Effect on the Synthesis, Structure, and Reactivity of R2MOR(NHC) (M = Al, Ga, In) Complexes

Article abstract of DOI:10.1021/acs.organomet.8b00570

The studies on the reactivity of dialkylaluminum alkoxides towards N-heterocyclic carbenes (NHCs) has allowed investigation of not only the factors controlling the synthesis and properties of Me₂AlOR(NHC) complexes. Additionally, we have focuse

Full text of DOI:10.1021/acs.organomet.8b00570

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Cite This: Organometallics XXXX, XXX, XXX−XXX
Probing the M−CNHC Bond and Its Effect on the Synthesis, Structure,
and Reactivity of R2MOR(NHC) (M = Al, Ga, In) Complexes
†
,§
Rafał Zaremba,†,‡ Maciej Dranka,‡ Bartosz Trzaskowski, Lilianna Chęcinska,*
́
and Paweł Horeglad*,†
†Centre of New Technologies, University of Warsaw, Banacha 2c, 02-097, Warsaw, Poland
‡Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664, Warsaw, Poland
§Faculty of Chemistry, University of Lodz, Pomorska 163/165, 90-236, Lodz, Poland
S
* Supporting Information
ABSTRACT: The studies on the reactivity of dialkylalumi-
num alkoxides towards N-heterocyclic carbenes (NHCs) has
allowed investigation of not only the factors controlling the
synthesis and properties of Me2AlOR(NHC) complexes.
Additionally, we have focused on the effect of group 13
metals on the synthesis, structure, and reactivity of Me2MOR-
(NHC) (M = Al, Ga, In) complexes, with regard to the
strength and character of M−CNHC bonds. The reactions of
simple dimethylaluminum alkoxides with NHCs lead to the
monomeric Me2AlOR(NHC) complexes, as shown by the
isolation of Me2AlOMe(NHC) (NHC = IMes (1a), SIMes
(1b)) (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-yli-
dene, SIMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-
ylidene). Despite their tendency to disproportionate, the interaction of the additional NHC molecule in Me2Al-
(OCH2CH2OMe)(IMes)/IMes (2a/IMes) with the fifth coordinate site of aluminum has resulted in stabilization and allowed
for the isolation of Me2Al(OCH2CH2OMe)(IMes) (2a). In contrast, the limited accessibility of the fifth coordinate site of
aluminum in the case of five-coordinate [Me2Al(μ-OCH(Me)CO2Me)]2 or four-coordinate [Me2Al(μ-OR)]2 alkoxides with
bulky alkoxide ligands, has affected the formation of Al−CNHC bonds and allowed only for the synthesis and isolation of stable
Me2Al(OCPh2Me)(NHC) (NHC = IMes (3a), SIMes) complexes. Additionally stable aryloxide derivatives Me2M-
(OC6H4OMe)(NHC) (NHC = IMes (4a) and SIMes (4b)) have been isolated and characterized. More ionic Al−CNHC bonds
of Me2AlOR(NHC), in comparison with analogous Ga−CNHC and In−CNHC bonds, have been decisive for the reactivity of
aluminum complexes, which includes their tendency for ligand disproportionation and activity of 1a, 4a, and 4b in the ring
opening polymerization (ROP) of lactide, initiated in each case by the insertion of lactide into mainly an ionic Al−CNHC bond.
The ionic character of M−CNHC bonds, decreasing in the series Al−CNHC > In−CNHC > Ga−CNHC, has been reflected by the
reactivity of investigated complexes and determined by density functional theory (DFT) calculations using real-space bonding
indicators (RSBIs). The structure of investigated aluminum complexes and the strength of Al−CNHC bonds have been
investigated using spectroscopic methods and X-ray diffraction studies. The strength of M−CNHC bonds of investigated
aluminum complexes, as well as their gallium and indium analogues, have been also determined by DFT calculations of their
bond dissociation energies.
selective fashion. Noteworthy, mentioned examples have
shown that the M−CNHC bond strongly influences the catalytic
activity of such complexes, as demonstrated, for instance, by
the high activity and stereoselectivity of heteroselective Zn
alkoxides with NHCs,8a,c as well as isoselective Me2GaOR-
(NHC) complexes,5a,b in the polymerization of rac-LA.
Importantly, the character of M−CNHC bonds was crucial for
the differences between the catalytic properties of Zn and Mg
complexes possessing alkoxide chelate ligands with NHC
INTRODUCTION
■
In recent years, main-group metal complexes with N-
heterocyclic carbenes (NHCs) have become a focus of
scientists, both from the point of view of fundamental studies
as well as their applications in catalysis.1,2 In this regard, NHC-
stabilized main group metal alkoxides,3 including aluminum,4
gallium,5 indium,5c,6 magnesium,7 and zinc,7,8 alkoxides, as well
as Me3Al(NHC)9 and [R2Al(NHC)]+ in the presence of
benzyl alcohol,10 have been shown to catalyze ring-opening
polymerization (ROP) of rac-lactide (rac-LA), as well as other
heterocyclic monomers, such as cyclohexyl oxide, trimethylene
carbonate, and ε-caprolactone, in controlled and stereo-
Received: August 9, 2018
© XXXX American Chemical Society
A
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termini (O,CNHC), as demonstrated by Arnold et al.,7 while we
showed its significance for Me2GaOR(NHC)5a,b and
Me2InOR(NHC) complexes.6 In the former cases, insertion
of lactide into the M−CNHC bond was indicated for Mg
alkoxides in contrast to Zn alkoxides, with the Zn−CHNC bond
remaining intact and resulting in the exclusive insertion of rac-
LA into Zn−Oalkoxide bond,7 similarly to Zn alkoxides reported
by Tolman and co-workers.8a,c In the case of our studies on
R2MOR(NHC) (M = Ga, In, NHC = IMes, SIMes)
complexes, which are highly active and isoselective catalysts
for the polymerization of rac-LA, we observed the insertion of
rac-LA exclusively into Ga−Oalkoxide bond of gallium alkoxides,
while strong Ga−CNHC bonds remained intact during
polymerization.5a,b Noteworthy, even for Me2GaOR(6-Mes)
(6-Mes = 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyr-
imidin-1-ylidene), with a much weaker Ga−C6‑Mes bond in
comparison with Ga−CNHC (NHC = IMes, SIMes), the
insertion of lactide to the latter could not be evidenced.5c On
the contrary, in the case of Me2InOR(NHC), rac-LA could
insert under the same conditions into either an In−O bond or
In−CNHC bond, the latter considerably weaker in comparison
with Ga−CNHC of Me2GaOR(NHC).5c,6 Notably, the insertion
of lactide into the M−CNHC bond was observed by Dagorne
and co-workers for the ROP of lactide catalyzed with
Me3Al(NHC) complexes.9 In addition to the effect of the
M−CNHC bond on the activity of the main group complexes in
ROP, Dagorne and co-workers showed recently that changing
of NHC coordination mode from normal to abnormal and vice
versa, which should be expected to affect considerably the
character of M−CNHC bond, had a profound effect on the
insertion of CO2 into either Zn−CNHC or Zn−Me of zinc
cationic complexes stabilized with NHCs.11 With regard to the
catalytic properties of main group metal alkoxides with NHCs,
we believe that there is a need for systematic research
concerning the effect of the character of M−CNHC bond on the
structure and reactivity of main group metal alkoxides.12 Such
studies should enable researchers to understand the factors
controlling reactivity of main group metal alkoxides with NHC
and, as a result, rationally design new catalysts for ROP and
beyond. It should also prove interesting for the synthesis of
main group metal complexes with NHCs, in light of the
stabilization and isolation of B,13 Al,14 and Ga and In
complexes15 at unusual oxidation states, which have been
facilitated by the formation of M−CNHC bonds.
and co-workers.12h In our case, the use of different alkoxide
groups allowed us to evaluate the strength of the Al−CNHC
bond, but also showed the considerable effect of OR and OAr
groups on their synthesis and stability. With the use of both
spectroscopic techniques and DFT calculations, including
calculations of bond dissociation energies (BDEs) and real
space bond indicators (RSBIs) of M−CNHC bonds, we show
how the character of M−CNHC bonds, rather than their
strength, influence the synthesis, structure, and reactivity of
Me2MOR(NHC) (M = Al, Ga, In) complexes (Scheme 1).
Scheme 1. Character vs Reactivity of M−CNHC Bond of
R2MOR(NHC) (M = Al, Ga, In) Complexes
RESULTS AND DISCUSSION
■
Reactivity of Simple Dialkylaluminum Alkoxides
toward NHC. Dialkylaluminum derivatives [Me2Al(μ-OR)]n
(OR = OMe, OCH2CH2OMe, n = 3 or 2) were initially
selected for the reaction with NHCs, IMes and SIMes.
Although NMR spectroscopy of the reaction mixtures
confirmed instant formation of Me2AlOMe(NHC) (NHC =
IMes (1a), SIMes (1b)) (Schemes 2 and 4), the observation of
Scheme 2. Synthesis and Disproportionation of
Me2AlOMe(NHC) (NHC = SIMes, IMes; Al/NHC = 1:1)
With regard to the above, we have decided to investigate the
effect of M−CNHC on the synthesis structure and reactivity,
including activity in the ROP of lactide, of group 13 metal
dialkylalkoxides and dialkylaryloxides with NHCs - Me2MOR-
(NHC) and Me2MOAr(NHC) (M = Al, Ga, In; OAr =
aryloxide group) respectively. Therefore, in the first step, we
have extended our studies on the synthesis, structure, and
catalytic activity of Me2MOR(NHC) (M = Ga,5 In5c,6
)
complexes in ROP, to their aluminum analogues in order to
determine the strength of the Al−CNHC bond and its effect on
the synthesis structure and reactivity, including both stability
and activity of aluminum complexes in the ROP of lactide.
Subsequently, in order to fully determine the strength and
character of M−CNHC (M = Al, Ga, In) bonds, we applied
density functional theory (DFT) calculations. We hereby
report the synthesis, structure, and reactivity of Me2AlOR-
(NHC) and Me2AlOAr(NHC) (NHC = IMes, SIMes)
complexes, which constitute a barely explored class of
complexes only recently reported in the literature by Camp
trace amounts of Me3Al(IMes) and Me3Al(SIMes),16
respectively, indicated that 1a and 1b undergo disproportio-
nation, similar to analogues dimethylindium alkoxides with
NHCs.6 Further decomposition of 1a and 1b in time led to the
formation of Me3Al(NHC) (NHC = IMes, SIMes) and a
yellow crystalline precipitate. The isolated precipitate was
essentially insoluble in common solvents including THF and
pyridine, which precluded its further analysis. However, its
general formula corresponding to [MeAl(OMe)2] could be
tentatively proposed.
B
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Despite the tendency of Me2AlOMe(NHC) to dispropor-
tionate, colorless crystals of 1a and 1b, suitable for X-ray
analysis, were isolated from a toluene/hexane solution and
toluene, respectively, within minutes after mixing the reagents.
Similarly to Me2GaOMe(NHC),5a,b Me2InOMe(SIMes),6 and
Me2InOMe(IMes) (see the Supporting Information), the X-
ray analysis of 1a and 1b revealed the presence of asymmetrical
aluminum complexes with the coordination sphere of
aluminum of a distorted tetrahedral geometry (Figure 1).
(107.9(3)°, Δ = 3.19°) in comparison with free IMes18 and
SIMes,19 respectively. The strength of the Al−CIMes bond in 1a
in the solid state was comparable to Ga−CNHC and In−CNHC
bonds of analogous Me2MOMe(IMes) (M = Ga, In), while the
smaller ΔNCN (Table 1) of IMes upon coordination could be
explained by the steric hindrances caused by a smaller radius of
aluminum in comparison with gallium or indium. Interestingly,
a similar valency of M−CIMes bonds for Me2MOMe(IMes)
(M= Al, Ga, In) complexes was surprising in the light of our
recent work, which revealed significantly weaker In−CNHC in
comparison with Ga−CNHC bonds in the case of Me2MOMe-
(NHC) (M = Ga, In; NHC = IMes, SIMes). However, in the
case of 1b, the strength of Al−CSIMes bond in the solid state
was comparable to the analogous In−CSIMes bond of unstable
Me2InOMe(SIMes) and significantly weaker in comparison
with Ga−CSIMes bond of stable Me2GaOMe(SIMes) (Table 1).
Noteworthy, the distance of Al−CSIMes was significantly longer
in comparison with recently reported (iBu)2Al(O,CNHC),12h
where O,CNHC represents monoanionic chelate alkoxide ligand
with saturated NHC termini.
The 1H NMR of 1a in toluene-d8 and 1b in THF-d8, as well
as the corresponding reaction mixtures of [Me2Al(μ-OMe)]3
and IMes/SIMes (Al/NHC = 1:1), revealed the instant ligand
disproportionation, which was evidenced by the presence of
signals corresponding to Me2AlOMe(NHC) and Me3Al-
(NHC) (NHC = IMes, SIMes) complexes (see the Supporting
Information). Notably, the 1H NMR of the reaction mixture of
[Me2Al(μ-OMe)]3 and IMes (Al/IMes = 1:1) in toluene-d8
showed essentially a single set of signals, and only traces of
decomposition products (Me2AlOMe(IMes)/Me3Al(NHC) =
96:4). However, in all cases discussed above, further
decomposition in time was observed (see the Supporting
Information). For both 1a and 1b, considerably higher-field
shifted signals corresponding to Al−Me protons (1a: − 1.09
ppm, toluene-d8, 1b: − 1.65 ppm, THF-d8) were in line with
the formation of Al−CNHC bonds and Me2AlOMe(NHC)
complexes. Although carbene carbon signals in 13C NMR can
be indicative for the strength of M−CNHC bonds,20 they could
not be observed for both 1a and 1b, as well as the
corresponding reaction mixtures, even over prolonged
acquisition times.
In order to further investigate the structure of Me2AlOR-
(NHC) complexes, including the strength of Al−CNHC bond,
we initially attempted to synthesize Me2Al(OCH2CH2OMe)-
(IMes) (2a), which formed instantly in the reaction between
[Me2Al(μ-OCH2CH2OMe)]2 and IMes (Scheme 3, path A,
Scheme 4). Contrary to analogous Me2In(OCH2CH2OMe)-
(NHC) indium complexes that could be barely observed due
to instant and complete disproportionation,6 2a was dominant
in the reaction mixture. Alas, in this case also, ligand
disproportionation leading to the complex mixture of products,
including Me3Al(NHC) (NHC = IMes, SIMes), precluded its
isolation. However, colorless crystals of 2a, suitable for X-ray
analysis, were isolated from the reaction mixture of [Me2Al(μ-
OCH2CH2OMe)]n and IMes (Al/IMes = 1:2) − 2a/IMes
(Scheme 3, path B). The coordination of the IMes ligand to
aluminum in 2a resulted in the formation of complexes with
essentially symmetric alignment of IMes ligand, similarly to the
analogous dimethylgallium derivative, which is evidenced by
C(Me)−Al−C(1)−N(1) torsion angles (Figure 2). Impor-
tantly, the distance and valency of Al−CIMes bond (2.074(2) Å,
0.65 vu - valence units21) indicated the similar strength of
Al−CIMes in 2a and Ga−CIMes in Me2Ga((S)-OCH(Me)-
Figure 1. Molecular structure of 1a (above) and 1b (below) with
thermal ellipsoids at the 50% probability level. Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and angles (deg) for 1a:
Al(1)−C(4) 1.983(2), Al(1)−C(5) 1.965(2), Al(1)−C(1) 2.082(2),
Al(1)−O(1) 1.752(1), C(1)−Al(1)−C(4) 107.10(7), C(1)−Al(1)−
C(5) 109.50(7), C(1)−Al(1)−O(1) 103.42(6), N(1)−C(1)−N(2)
103.55(12), C(5)−Al(1)−C(1)−N(1) 173.26(12), C(4)−Al(1)−
C(1)−N(1) 61.95(14), O(1)−Al(1)−C(1)−N(1) 53.63(13), NHC
tilt 2.0(1).17 for 1b: Al(1)−C(4) 1.971(4), Al(1)−C(5) 1.971(5),
Al(1)−C(1) 2.110(3), Al(1)−O(1) 1.746(3), C(1)−Al(1)−C(4)
108.88(16), C(1)−Al(1)−C(5) 108.63(16), C(1)−Al(1)−O(1)
104.25(14), N(1)−C(1)−N(2) 107.9(3), C(5)−Al(1)−C(1)−N(1)
− 117.39(1), C(4)−Al(1)−C(1)−N(1) 5.14(1), O(1)−Al(1)−
C(1)−N(1) 121.70(1), NHC tilt 4.5(1).17
The lack of symmetry was due to the orientation of the NHC
ligand respective to Al−CMe bonds. In the case of both 1a and
1b, CH···O interactions, between the NHC heterocyclic ring
and the OMe group, led to the formation of chains in the solid
state, akin to those observed in Me2MOMe(NHC) (M = Ga,
In; NHC = IMes, SIMes).5a,b,6 Most importantly, the
formation of strong Al−CNHC bonds in 1a and 1b, in
comparison with Me2MOR(NHC) (M = Ga, In; NHC =
IMes, SIMes), was evidenced by the distances, and bond
valencies (Table 1), of Al−CNHC bonds. The strength of Al−
CNHC bonds was further evidenced by the small NHC tilts,
which is split into pitch angles (out of NHC plane tilting) and
yaw angles (in plane tilting),17 as well as a significant increase
of NCN angles of 1a (103.55(12)°, Δ = 2.13°) and 1b
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Table 1. Selected Structural and 13C NMR Data for Me2MOR(NHC) (M = Al, Ga, In; NHC = IMes, SIMes)
a
e
N−C−N (deg)
M−CNHC valency (vu)
13C NMR Ccarbene (ppm)
b
b
b
b
Me2AlOMe(IMes) (1a)
103.55(12) (Δ = 2.1)
103.83(9) (Δ = 2.4)
0.64
0.65
0.65
0.59
0.63
0.60
0.65
0.68
b
c
Me2GaOMe(IMes)5b
176.1 (Δ = −43.6)
Me2InOMe(IMes) (SI2)
103.98(15) (Δ = 2.6)
b
Me2AlOMe(SIMes) (1b)
Me2GaOMe(SIMes)5a
Me2InOMe(SIMes)6
Me2AlOCH2CH2OMe(IMes) (2a)
Me2GaOCH2CH2OMe(IMes) (SI1)
Me2AlOCH2CH2OMe(SIMes)
Me2GaOCH2CH2OMe(SIMes)5b
Me2Al(OCPh2Me)(IMes) (3a)
Me2Ga(OCPh2Me)(IMes)6
107.9(3) (Δ = 3.19)
108.57(12) (Δ = 3.9)
b
108.0(5) (Δ = 3.3)
103.52(15) (Δ = 2.1)
104.06(16) (Δ = 2.6)
174.9 (Δ = −44.8)
176.3 (Δ = −43.4)
198.2 (Δ = −46.2)
200.2 (Δ = −44.2)
175.0 (Δ = −44.7)
176.6 (Δ = −43.1)
108.45(11) (Δ = 3.7)
102.52(17) (Δ = 1.1)
0.65
0.64
0.64
0.64
0.61
0.61
b
b
c
103.8(3) (Δ = 2.4)
d
b
b
103.9(3) (Δ = 2.5)
Me2In(OCPh2Me)(IMes)6
105.4(7) (Δ = 4.0)
181.5 (Δ = −38.2)
b
c
d
104.8(7) (Δ = 3.4)
Me2Al(OCPh2Me)(SIMes)
198.1 (Δ = −46.3)
199.9 (Δ = −44.5)
205.5 (Δ = −38.9)
175.5 (Δ = −44.2)
198.6 (Δ = −45.8)
Me2Ga(OCPh2Me)(SIMes)6
Me2In(OCPh2Me)(SIMes)6
Me2Al(OC6H4OMe)(IMes) (4a)
Me2Al(OC6H4OMe)(SIMes) (4b)
Me2Ga(OC6H4OMe)(SIMes)22
c
c
103.52(10) (Δ = 2.1)
0.65
199.2 (Δ = −45.2)
a
b
c
d
Refers to the N(1)−C(1)−N(2) angle. In comparison with SIMes or IMes. In comparison with SIMes or IMes in toluene-d8. For the second
independent molecule of the asymmetric unit. Bond valence was calculated according to the method described by Brown and Altermatt.21
e
ppm), could be associated with its interaction with aluminum,
leading to the stabilization of the Me2Al(OCH2CH2OMe)-
(IMes) species. Analogously, the Me2Al(OCH2CH2OMe)-
(SIMes) species were more stable in the case of Me2Al-
(OCH2CH2OMe)(SIMes)/SIMes mixture, in comparison
with Me2Al(OCH2CH2OMe)(SIMes), and characterized
using NMR spectroscopy (Table 1, see the Supporting
Information). Notably, the stabilization of both 2a and
Me2Al(OCH2CH2OMe)(SIMes), in the case of 2a/IMes and
Me2Al(OCH2CH2OMe)(SIMes)/SIMes, respectively, was in
contrast to the stability of indium Me2InOR(NHC) (NHC =
IMes, SIMes) species, which revealed the increased tendency
for the ligand disproportionation in the presence of excess of
NHC.6 Interestingly, despite the tendency of Me2Al-
(OCH2CH2OMe)(NHC) (NHC = IMes, SIMes) to dis-
proportionate, the shift of carbene carbon of IMes/SIMes
upon coordination to aluminum, indicated the presence of an
Al−CNHC bond of a similar strength to Ga−CNHC in respective
stable Me2Ga(OCH2CH2OMe)(NHC) complexes (Table 1).
With regard to the previously investigated Me2M(OCH-
(Me)CO2Me)(NHC) complexes, which mimic active species
with a growing PLA chain, in the polymerization of lactide with
Me2MOR(NHC) (M = Ga, In; NHC = IMes, SIMes),5,6 we
approached the synthesis of analogous Me2Al((S)-OCH(Me)-
CO2Me)(NHC) (Me2Al((S)-melac)(NHC)) complexes.
However, the reaction of [Me2Al(μ-(S)-melac)]2 with IMes
Scheme 3. Reaction of [Me2Al(μ-OCH2CH2OMe)]2 with
IMes
CH2OMe)(IMes) (vu = 0.64),5b however slightly weaker in
comparison with Ga−CIMes in Me2Ga(OCH2CH2OMe)-
(IMes) (SI1, vu = 0.68) (Table 1).
1
While 2a was unstable in solution, H NMR of 2a/IMes
revealed the presence of a single set of signals corresponding to
Me2Al(OCH2CH2OMe)(IMes) (2a) and additional signals
corresponding to IMes. In the latter case, the excess of IMes
resulted in the stabilization of Me2AlOR(NHC) species,
although slow disproportion was observed in time. Notably,
the presence of essentially uncoordinated IMes molecule of
2a/IMes was evidenced by the carbene carbon signal in 13C
NMR (217.3 ppm), considerably shifted to lower field in
comparison with the IMes coordinated to aluminum of 2a
(174.9 ppm, Δ = −44.8 ppm). The stability of 2a in 2a/IMes
could be explained by the interaction of IMes with aluminum
of 2a resulting in the exchange of coordinated/uncoordinated
IMes molecules, which was confirmed by ROESY (Rotating
frame Overhause Effect SpectroscopY) experiment (Figure
S15). Moreover, in light of the exchange of coordinated/
uncoordinated IMes molecule, the shift of uncoordinated IMes
by approximately 2 ppm, in comparison with free IMes (219.7
1
or SIMes resulted in oily products with complex H NMR
spectra (see the Supporting Information). Interestingly, the
presence of several singlets corresponding to Al−Me protons
suggested in this case the formation of asymmetric dimeric
species with coordinated NHC rather than monomeric
Me2Al((S)-melac)(NHC) complexes, although the formation
of the latter could not be excluded unequivocally. However,
the detailed investigation of the structure of resulting
complexes, as well as the possibility of the formation of
Me2Al((S)-melac)(NHC) in time, was severely limited by the
D
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Scheme 4. Reaction of IMes towards [Me2Al(μ-OR)]n (n =
2,3) Complexes with Variable Accessibility of the Fifth
Coordinate Site of Aluminum (Al/IMes = 1:1) at Room
Temperature
Figure 2. Molecular structure of 2a with thermal ellipsoids at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg) for 2a: Al(1)−C(4) 1.977(2),
Al(1)−C(5) 1.984(2), Al(1)−C(1) 2.074(2), Al(1)−O(1) 1.756(2),
C(1)−Al(1)−C(4) 108.38(8), C(1)−Al(1)−C(5) 109.48(8), C(1)−
Al(1)−O(1) 99.68(8), N(1)−C(1)−N(2) 103.52(15), C(5)−
Al(1)−C(1)−N(1) 26.44(13), C(4)−Al(1)−C(1)−N(1)
147.48(14), O(1)−Al(1)−C(1)−N(1) 92.02(12), NHC tilt 1.6(1).17
disproportion does not seem to be likely. On the other hand,
limited accessibility of aluminum in [Me2Al(μ-(S)-melac)]2 for
NHC could promote different reaction pathways, as we have
demonstrated for instance for the reactions of [Me2M(μ-(S)-
melac)]2 (M = Ga, In) complexes with bulky NHCs such as
SIPr (l,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene)5b or
6-Mes.5c As the smaller ionic radius of aluminum, in
comparison with gallium and indium, could be expected to
affect the formation of Al−CNHC bonds (see below), it should
not limit, to such an extent, the formation of Me2Al((S)-
melac)(NHC) given the instant reaction of [Me2Al(μ-
OCH2CH2OMe)]2 toward IMes/SIMes (Scheme 4). On the
contrary, the strongest chelate CO···Al bond, among C
O···M of [Me2M(μ-(S)-melac)]2(M = Al,23 Ga,24 In25)
complexes, could rather be responsible for limited reactivity
of [Me2Al(μ-(S)-melac)]2 toward SIMes/IMes, and the
impaired formation of the Al−CNHC bond of Me2Al((S)-
melac)(NHC) complexes. Such a chelate bond of high
directionality, typical of five-coordinate dialkylaluminum
derivatives, should be expected to severely limit the access of
NHC to the CCO face of the tetrahedral coordination sphere
of aluminum in [Me2Al(μ-OR)]2, trans to the Al−Obridging
bond (Scheme 4),26 which has already been shown to be
crucial for the structure of [R2Al(O,X)]n (n = 1,2; O,X =
monoanionic alkoxide/aryloxide ligand with Lewis Base
termini),27 reactivity of [Me2Al(μ-(S)-melac)]2 in the ROP
of cyclic esters,28 and activity of dialkylaluminum alkoxides
toward molecular oxygen.29 Similarly, our results on the
reactivity of dialkylaluminum alkoxides toward NHCs,
discussed below, revealed that the accessibility of the fifth
coordinate site of aluminum was the main factor affecting the
formation of Al−CNHC and Me2AlOR(NHC) complexes.
However, the character of the Al−CNHC bond could also be
responsible for the difficulties in breaking of Al2O2 bridges and
the subsequent formation of Me2Al((S)-melac)(NHC) com-
plexes.
ligand disproportionation, which was evidenced by the
formation of, among others, Me3Al(NHC) (see the Supporting
Information). While the ligand disproportionation of dialky-
laluminum (S)-melac derivatives upon interaction with NHCs
should not be surprising in light of the low stability of
Me2AlOR(NHC), the lack of instant formation of Me2Al((S)-
melac)(NHC) upon addition of IMes/SIMes to [Me2Al(μ-(S)-
melac)]2 should be surprising in light of facile synthesis of both
stable Me2Ga((S)-melac)(NHC)5a,b and unstable Me2In((S)-
melac)(NHC)6 (NHC = IMes, SIMes) complexes. With regard
to the higher stability of Me2AlOR(NHC) in comparison with
Me2InOR(NHC) complexes (see above), the instant for-
mation of Me2Al((S)-melac)(NHC) followed by its quick
The synthesis of stable Me2AlOR(NHC) and Me2AlOAr-
(NHC) (OAr = aryloxide group) (NHC = IMes, SIMes) was
essential in order to investigate the strength and character of
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Article
the Al−CNHC bond using spectroscopic techniques. On the
other hand, the careful choice of alkoxide and aryloxide ligands
of different steric and electronic properties (Scheme 4) was
crucial in order to explain how far the access to the fifth
coordinate site is important for the synthesis of dialkylalumi-
num alkoxides and aryloxides with NHCs. Therefore for the
synthesis of Me2AlOR(NHC) and Me2AlOAr(NHC) com-
plexes, we chose [Me2Al(μ-OCPh2Me)]2, [Me2Al(μ-OCPh-
MeH)]2, and [Me2Al(μ-OC6H4OMe)]2 in which bulky
alkoxide/aryloxide groups of different steric demands should
to a different extent limit the access to the fifth coordinate site
of Al in trans position to one of Al−Obridging bonds (Scheme 4).
Importantly, the reaction of selected dialkylaluminum alk-
oxides and aryloxides (Figure 3) should lead to the synthesis of
Me2M(OCPhMeH)(IMes) in the reaction of IMes with
[Me2Al(μ-OCPhMeH)]2 (Scheme 4). The decrease of steric
hindrances of the alkoxide ligand in the case of [Me2Al(μ-
OCPhMeH)]2 resulted in considerably shorter reaction times
of the latter with IMes, leading to the formation of stable
Me2Al(OCPhMeH)(IMes) with 90% conversion after 3 days
at room temperature (see the Supporting Information).
However, the monocrystals suitable for X-ray analysis were
obtained only for 3a (Figure 3). Similarly to previously
reported Me2M(OCPh2Me)(NHC) (M = Ga, In) complexes,
the coordination of IMes ligand to aluminum in 3a resulted in
the formation of asymmetrical complexes, due to the
orientation of the IMes ligand respectively to M−CMe bonds,
with the aluminum coordination sphere adopting a distorted
tetrahedral geometry. In contrast to 1a, no CH···O interactions
were observed in the solid state. Instead, CH···π interactions
and CH···HCH2Al dihydrogen bonds30 could be distinguished.
The presence of the strong Al−CNHC bond was mainly
supported by the length and valency of Al−CNHC bond
(2.080(2) Å, 0.64 vu) (Table 1) and further confirmed by a
small NHC tilt (Figure 3). Noteworthy the strength of the Al−
CIMes bond in 3a was comparable to Ga−CIMes (0.64 vu), and
considerably stronger than In−CIMes (0.61 vu), in analogous
Me2M(OCPh2Me)(IMes) (M = Ga, In) complexes,6 while the
opposite trend was observed for ΔNCN of 3a (1.1°),
Me2Ga(OCPh2Me)(IMes) (2.4° and 2.5°) and Me2In-
(OCPh2Me)(IMes) (4.0° and 3.4°) complexes. For 3a the
observed tendency resulted, most probably, from the smallest
ionic radii of Al, and therefore the proximity of IMes mesityl
rings and Al−Me groups, which precluded the increase in
ΔNCN beyond 1.1°, as it would require pushing mesityl rings
even further toward Al−Me groups (Figure 3). Interestingly,
the latter did not affect the strength of the Al−CIMes bond,
which was comparable to 1a and 2a. It also did not cause
noticeable constraints within the coordination sphere of
aluminum, which was evidenced by the short resultant bond
valence vector (BVV)31 of 0.049 vu (Figure S58).32 It must
therefore be noted that ΔNCN is sensitive to steric hindrances,
especially in the presence of sterically bulky ligands, and as
such should not be considered as a precise measure of M−
CNHC bond strength.20
Figure 3. Molecular structure of 3a with thermal ellipsoids at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Al(1)−C(4) 1.976(2), Al(1)−
C(5) 1.978(2), Al(1)−C(1) 2.080(2), Al(1)−O(1) 1.750(2), C(1)−
Al(1)−C(4) 109.70(9), C(1)−Al(1)−C(5) 107.37(9), C(1)−
Al(1)−O(1) 98.09(8), N(1)−C(1)−N(2) 102.52(17), C(5)−
Al(1)−C(1)−N(1) 23.13(13), C(4)−Al(1)−C(1)−N(1)
143.63(14), O(1)−Al(1)−C(1)−N(1) 98.08(12), NHC tilt 3.2(1).17
stable Me2AlOR(NHC) (OR = OCPh2Me, OCPhMeH) and
Me2AlOAr(NHC) (OAr = OC6H4OMe) complexes, similarly
to stable indium complexes - Me2In(OCPh2Me)(NHC)
(NHC = IMes, SIMes, 6-Mes) recently reported by us, despite
the observed tendency of Me2InOR(NHC) to disproportio-
nate.5c,6
The 1H NMR of both 3a and Me2Al(OCPh2Me)(SIMes) in
toluene-d8 revealed a single set of signals with Al-Me protons at
−1.23 ppm and −1.31 ppm, respectively, shifted to
considerably higher-field in comparison with [Me2Al(μ-
OCPh2Me)]2. Importantly, in contrast to 1 and 2, both 3a
and Me2Al(OCPh2Me)(SIMes) were stable in solution over
weeks. The 13C NMR carbene carbon shift of 3a, and the
difference between the latter and free IMes, are indicative of a
strong Al−CIMes bond (175.0 ppm, Δ = −44.7 ppm), slightly
stronger than Ga−CNHC (176.6 ppm, Δ = −43.1 ppm), and
significantly stronger in comparison with the In−CNHC bond
(181.5 ppm, Δ = −38.2 ppm) of Me2M(OCPh2Me)(NHC)
(M = Ga, In) (Table 1). Similarly to the latter, the largest shift
of the carbene carbon among Me2M(OCPh2Me)(SIMes) (M
= Al (198.1 ppm, Δ = −46.3 ppm), Ga (199.9 ppm, Δ = −44.5
ppm),6 and In (205.5 ppm, Δ = −38.9 ppm)6 complexes was
observed for the aluminum derivative, indicating the strongest
Al−CNHC bond among M−CNHC bonds of Me2M(OCPh2Me)-
(NHC) (M = Al, Ga, In; NHC = IMes, SIMes) complexes.
Although we have recently shown that the shift of the carbene
carbon should not be considered a precise measure of the
strength of M−CNHC bonds,5c it should be noted that the
Synthesis and Structure of Me2AlOR(NHC) (OR =
OCPh2Me, OCPhMeH) Alkoxide Complexes with Bulky
Alkoxide Groups. Although initially we did not observe any
reaction between NHC (NHC = IMes, SIMes) and [Me2Al(μ-
OCPh2Me)]2 (Al/NHC = 1:1), which was evidenced, among
others by the presence of essentially free NHC in solution, the
slow formation of stable Me2Al(OCPh2Me)(NHC) (NHC =
IMes (3a), SIMes) complexes was observed within days and
evidenced by 1H NMR spectroscopy (see the Supporting
Information). The reaction of [Me2Al(μ-OCPh2Me)]2 with
IMes led to 97% conversion after 11 days at r.t., which allowed
for the isolation of 3a as colorless crystals in high yield.
Although Me2Al(OCPh2Me)(SIMes) was also formed in the
analogous reaction, much longer reaction times at r.t. were
required (85% conversion after 77 days). In order to confirm
that limited accessibility of the fifth coordinate site of
aluminum in [Me2Al(μ-OCPh2Me)]2 was crucial for its
reactivity with NHC, we investigated the synthesis of
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strength of M−CNHC bond of Me2M(OCPh2Me)(NHC) (M =
Al, Ga, In; NHC = IMes, SIMes) complexes in solution,
estimated on the basis of 13C NMR spectroscopy, i.e., Al−
over one of the methyl groups, while the phenyl ring of 2-
methoxyphenol faced the other mesityl group of IMes, which
likely resulted from the π-stacking of the aromatic rings.
Alternatively, it could be also caused by the (mesityl)H2CH···
Oaryloxide and (IMes central ring)CH···OMe weak interactions,
leading to the chain structure of 4a in a solid state (see the
Supporting Information). Interestingly, the orientation of
OC6H4OMe precluded in this case the interaction of OMe
group to the fifth coordinate site of aluminum. The lack of Al···
OMe chelate interaction in the solid state could also be
rationalized by the presence of strong Al−CNHC. Importantly,
the strength of Al−CNHC of 4a in the solid state was evidenced
mainly by the distance and bond valence of Al−CIMes bond
(2.0733(13) Å, 0.65 vu). However, as a result of the smaller
steric hindrance of the aryloxide ligand, in comparison with the
OCPh2Me alkoxide ligand, we observed a more significant
increase of the NCN angles of 4a (103.52(10)°, Δ = 2.1°), in
comparison with free IMes18 and SIMes,19 much larger than in
the case 3a (102.52(17)°, Δ = 1.1°) and similar to methoxide
derivative 1a (103.55(12)°, Δ = 2.1°).
In solution, complexes 4a and 4b showed no tendency for
ligand disproportionation similarly to analogous Me2In-
(OC6H4OMe)(NHC) aryloxides.5c The 13C NMR carbene
carbon shifts, as well as the differences between free and
coordinated NHC, were indicative in the case of 4a (176.4
ppm, Δ = −43.3 ppm) and 4b (198.6 ppm, Δ = −45.8 ppm)
of the strength of Al−CNHC bonds (Table 1). They revealed
the similar strength of Al−CNHC and Ga−CNHC bonds of
Me2Ga(OC6H4OMe)(NHC) (NHC = IMes - 176.0 ppm, Δ =
−43.7 ppm; SIMes −199.2 ppm, Δ = −45.2 ppm) as well as
significantly stronger Al−CNHC bonds in comparison with the
In−CNHC bonds of Me2In(OC6H4OMe)(NHC) (NHC =
IMes -183.3 ppm, Δ = −34.0 ppm; SIMes - 206.7 ppm, Δ =
−37.3 ppm).20 The formation of stable Me2M(OC6H4OMe)-
(NHC) as well as Me2Al(OCPh2Me)(NHC) complexes is not
surprising in the light of high stability of analogous gallium and
indium complexes. However, given the relatively strong Al−
CNHC bond, evidenced for both Me2AlOAr(NHC) and
Me2AlOR(NHC), the tendency of the latter to dispropor-
tionate was in opposition to our earlier conclusion on the effect
of the strength of M−CNHC bond (M = Ga, In) on the stability
of Me2MOR(NHC) (M = Ga, In) complexes.6 Although in
our previous studies we anticipated that much weaker In−
CNHC in comparison with Ga−CNHC should be responsible for
the tendency of dialkylindium complexes with NHC to
disproportionate, the above observations concerning the
strength of Al−CNHC bonds indicated that increased ionic
character of M−CNHC bonds, rather than only their strength,
should be responsible for the reactivity of Me2M(OCPh2Me)-
(NHC) (M = Al, Ga, In) complexes (see below). Therefore,
the stability of these complexes should also be affected by the
ionic/covalent character of M−CNHC bonds.
CNHC> Ga−CNHC ≫ In−CNHC, is essentially in line with Al−
CNHC ≈ Ga−CNHC > In−CNHC determined on the basis of M−
CIMes bond lengths (and valency) in the solid state (Table 1).
The examples above not only showed that stable Me2AlOR-
(NHC) with bulky alkoxide ligands can be synthesized
similarly to their stable gallium and indium analogues.6 They
also confirmed that the directionality of the effective
interaction of the NHC to the fifth coordinate site of dimeric
[Me2Al(μ-OR)]2 is crucial for the coordination of NHC to
aluminum, the subsequent breaking of Al2O2 bridges, and
finally the formation of monomeric Me2AlOR(NHC) com-
plexes.
Synthesis and Structure of Me2AlOAr(NHC) Aryloxide
Complexes. The reaction between NHC (NHC = IMes,
SIMes) and [Me2Al(μ-OC6H4OMe)]2 (Al/NHC = 1:1) at
room temperature led to the instant formation of Me2Al-
(OC6H4OMe)(NHC) (NHC = IMes (4a) and SIMes (4b)).
Instant reaction of [Me2Al(μ-OC6H4OMe)]2 and NHC (NHC
= IMes or SIMes) should be interpreted in terms of much
higher availability of CCO face of tetrahedral coordination
sphere of aluminum in [Me2Al(μ-OC6H4OMe)]2 in compar-
ison with [Me2Al(μ-OCPh2Me)]2 and [Me2Al(μ-OCPh-
MeH)]2 (Scheme 4), which should be due to the presence
of weak chelate bond between aluminum and methoxide group
of 2-methoxyphenol, similarly to [Me2Al(μ-OCH2CH2OMe)]2
(see above). Moreover, significantly weaker Al2O2 bridges in
the case of dialkylaluminum aryloxides should facilitate the
formation of Me2Al(OC6H4OMe)(NHC). 4a and 4b were
isolated as colorless crystals in high yields. However, crystals
suitable for X-ray analysis could only be obtained in the case of
4a. The X-ray analysis of 4a revealed the presence of a
monomeric aluminum complex with the coordination sphere
of aluminum adopting a distorted tetrahedral geometry. The
orientation of the IMes ligand was asymmetric with respect to
Al−CMe bonds, which was reflected by C(Me)−Al−CNHC−N
torsion angles (Figure 4). As a result, the ipso carbon of the
mesityl group of IMes was located, similarly to 1a, essentially
Ring Opening Polymerization (ROP) of rac-LA with
Me2AlOR(NHC) and Me2AlOAr(NHC) Complexes. With
regard to the catalytic activity of Me2AlOR(NHC) complexes
in the ROP of lactide, it must be noted that Me2AlOR(NHC)
(OR = OMe, OCH2CH2OMe, (NHC = IMes, SIMes)), as
well as Me2Al(OCH(Me)CO2)(NHC) which mimic alumi-
num catalytic species formed by the insertion of rac-LA into
Al−Oalkoxide bond of Me2AlOR(NHC), disproportionate read-
ily. Moreover, according to the literature, the resulting free
NHC catalyzes a polymerization of lactide under investigated
conditions (see below).33 Therefore, out of unstable
Figure 4. Molecular structure of 4a with thermal ellipsoids at the 30%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Al(1)−C(4) 1.978(1), Al(1)−
C(5) 1.971(1), Al(1)−C(1) 2.073(1), Al(1)−O(1) 1.764(1), C(1)−
Al(1)−C(4) 107.23(5), C(1)−Al(1)−C(5) 109.87(5), C(1)−
Al(1)−O(1) 104.39(5), N(1)−C(1)−N(2) 103.52(10), C(5)−
Al(1)−C(1)−N(1) 12.71(10), C(4)−Al(1)−C(1)−N(1)
108.74(10), O(1)−Al(1)−C(1)−N(1) 128.16(9), NHC tilt 3.6(1).17
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dimethylaluminumalkoxides discussed above, we investigated
only the catalytic activity of Me2AlOMe(IMes) (1a) (as the
reaction mixture of [Me2Al(μ-OMe)]3 and IMes/SIMes; Al/
NHC = 1:1), for which only traces of decomposition products
were evidenced (Figure S1)). However, in the light of the
catalytic activity of Me3Al(NHC),9 as well as Me2InOR-
(NHC),6 which allowed for the initiation of rac-LA polymer-
ization by NHC, it was interesting to investigate the effect of
the character of the Al−CNHC bond on the catalytic properties
of stable Me2Al(OCPh2Me)(IMes) (3a), Me2Al-
(OC6H4OMe)(IMes) (4a), and Me2Al(OC6H4OMe)(SIMes)
(4b).
the analogous Me2Ga(OC6H4OMe)(SIMes) complex, initi-
ated the ROP of rac-LA. Therefore, the reactivity of the M−
C
NHC bond in the ROP of lactide should be expected to result
from the ionic character of M−CNHC bonds, rather than simply
their strength. With regard to the effect of M−CNHC bonds on
the structure and reactivity of Me2MOR(NHC) (M = Al, Ga,
In; NHC = IMes, SIMes) complexes, we decided to
investigated both the strength and character of M−CNHC
bonds using DFT methods.
Computational Studies of Bond Dissociation Ener-
gies (BDEs) of M−CNHC Bonds. Bond dissociation energies
(BDEs) of the M−CNHC (M = Al, Ga, In; NHC = IMes,
SIMes) bonds were estimated, using the same approach as in
our previous work,5b by calculating the energy of systems 1a,
1b, 2a, and 3a, as well as analogous gallium and indium
complexes, and subtracting the sum of the energies of NHC
and aluminum/gallium/indium parts (with the counterpoise
correction) (Table 2). In the case of Me2MOMe(NHC) (M =
The polymerization of rac-LA with Me2AlOMe(IMes) (1a),
3a, 4a, and 4b was performed at −20 °C similarly to previously
investigated Me2MOR(NHC) (M = Ga, In) complexes.5,6
Notably, under these conditions, the insertion of rac-LA into
In−CNHC of Me2InOR(NHC) was observed,5c,6 while the
significantly stronger Ga−CNHC bond in Me2GaOR(NHC)
remained intact.5 For the discussed dimethylaluminum
alkoxide complexes with IMes, 3a showed essentially no
activity in the polymerization of rac-LA at −20 °C, even after
24 h, in contrast to Me2In(OCPh2Me)(IMes), which
polymerized rac-LA due to insertion of the latter into the
In−CIMes bond. On the other hand analogous Me2Ga-
(OCPh2Me)(IMes) did not reveal any reactivity toward rac-
LA under the same conditions. However, the observed lack of
activity of 3a should be associated with the steric hindrances of
the bulky alkoxide group, limiting the effective interaction of
lactide with 3a, rather than low reactivity of the Al−CIMes
bond, in the light of the much more ionic character of the Al−
CNHC bond in comparison with Ga−CNHC and In−CNHC
bonds and the reactivity of Me2M(OC6H4OMe)(NHC) (M
= Al, Ga) (see below). Noteworthy, steric congestion around
aluminum center can be crucial for the reactivity of Al−NHC
motif, which has been also recently observed by Camp and co-
workers for aluminum alkoxides and aryloxides stabilized with
NHCs.12h Moreover, the activity of Me3Al(NHC) is strongly
dependent on the steric hindrances in the Al coordination
sphere, in this case on bulkiness of NHC ligand.9 Such
reasoning was supported by the catalytic activity of 4a and 4b,
as well as Me2AlOMe(IMes) (1a), which polymerized rac-LA
already at −20 °C. In all cases, the resulting PLA was
dominantly isotactic (Pm = 0.76−0.80). However, the average
molecular weight (Mn ≈ 100 000 Da), obtained by gel
permeation chromatography (see the Supporting Information),
was much higher than expected from a rac-LA/Al ratio of 50,
indicating that only small fraction of aluminum centers
initiated the polymerization of rac-LA. Additionally a broad
molecular weight distribution was indicative of the uncon-
trolled nature of the polymerization. MALDI-TOF analyses of
high molecular weight PLA, obtained with Me2AlOMe(IMes)
(1a), 4a, and 4b, did not allow for the determination of end
groups. Moreover, no insertion of rac-LA into Al-OC6H4OMe
or Al−OMe bonds, leading to PLA with OH and OR end
Table 2. (BDEs) of M−CNHC Bonds of Me2MOR(NHC) (M
= Al, Ga, In; NHC = IMes, SIMes) Complexes
BDE
(kcal/mol)
DFT
BDE (kcal/mol)
DLPNO−CCSD(T)
Me2AlOMe(IMes) (1a)
Me2GaOMe(IMes)
Me2InOMe(IMes) (SI2)
Me2AlOMe(SIMes) (1b)
Me2GaOMe(SIMes)
Me2InOMe(SIMes)
44.4
45.6
40.7
43.5
41.6
38.9
42.1
45.1
39.65b
42.2
42.5
38.75b
41.7
Me2AlOCH2CH2OMe(IMes)
(2a)
46.2
Me2GaOCH2CH2OMe(IMes)
(SI1)
42.5
42.3
Me2Al(OCPh2Me)(IMes) (3a)
Me2Ga(OCPh2Me)(IMes)
Me2In(OCPh2Me)(IMes)
50.0
46.1
45.8
48.7
45.8
44.2
Al, Ga, In; NHC = IMes, SIMes) complexes, BDEs of M−
CIMes bonds are higher in comparison with corresponding M−
C
SIMes bonds, which is in full agreement with the bond strength
obtained from X-ray data (Table 1). On the other hand,
calculated BDEs indicate that the strength of M−CNHC bonds
increases in series: Ga−CNHC < In−CNHC < Al−CNHC, which is
in sharp contrast to the strength of M−CIMes and M−CSIMes
bonds, determined by X-ray analysis (Table 1). Although, in
the case of Me2M(OCPh2Me)(IMes) (M = Al, Ga, In),
calculated BDIs revealed slightly stronger Ga−CIMes bonds in
comparison with In−CIMes, the strength of M−CIMes bonds
increasing in a row, In−CIMes < Ga−CIMes < Al−CIMes, is also
not in line with bond valencies calculated on the basis of M−
CIMes bond distances (Al−CIMes ≈ Ga−CIMes > In−CIMes).
Finally, the highest BDEs of Al−CNHC bonds, among all
investigated Me2MOR(NHC) complexes (Table 2) explain
neither the stability nor the reactivity of Me2AlOR(NHC)
complexes in comparison with their gallium and indium
analogues, which rather depends on the ionic/covalent
character of M−CNHC bonds (see below).
1
groups, could be evidenced by H NMR spectroscopy. All the
above strongly suggested the insertion of rac-LA into the Al−
CNHC bond, similarly to Me3Al(NHC).9 Although in the case
of Me2AlOMe(IMes) (1a) initiation by free NHC resulting
from its disproportionation should also be considered, the
insertion of LA exclusively into the Al−CNHC bond should be
expected for stable 4a and 4b complexes. Interestingly, both a
relatively strong Al−CNHC bond in 4a and 4b, as well as a weak
In−CNHC bond,6 in comparison with inert Ga−CNHC bonds in
Probing the Character of M−CNHC Bonds in Me2MOR-
(NHC) Using AIM and ELI-D Analysis. Although in our
previous studies we suggested that the strength of M−CNHC
bonds was decisive for the synthesis, structure, and reactivity of
Me2MOR(NHC) (M = Ga, In) complexes,5 the detailed
H
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a
b
Table 3. Calculated Bond Topological and Integrated Bond Descriptors for M−CNHC of Selected Me2MOR(NHC) and
c
Me2MOAr(NHC)
bond
d
ρbcp
∇2ρbcp
ε
G/ρbcp
H/ρbcp
Me2AlOMe(IMes) (1a)
Me2GaOMe(IMes)
Me2InOMe(IMes)
Me2AlOMe(SIMes) (1b)
Me2GaOMe(SIMes)
Me2InOMe(SIMes)
Me2Al(OCPh2Me)(IMes) (3a)
Me2Ga(OCPh2Me)(IMes)
Me2Ga(OCPh2Me)(6-Mes)
Me2In(OCPh2Me)(IMes)
Me2In(OCPh2Me)(6-Mes)
Me2Al(OC6H4OMe)(IMes) (4a)
Me2Ga(OC6H4OMe)(IMes)
Me2In(OC6H4OMe)(IMes)
Al−CNHC
Ga−CNHC
In−CNHC
Al−CNHC
Ga−CNHC
In−CNHC
Al−CNHC
Ga−CNHC
Ga−CNHC
In−CNHC
In−CNHC
Al−CNHC
Ga−CNHC
In−CNHC
2.082
2.089
2.278
2.110
2.101
2.301
2.080
2.096
2.139
2.301
2.331
2.073
2.072
2.292
0.41
0.58
0.50
0.39
0.58
0.48
0.41
0.58
0.55
0.48
0.47
0.42
0.60
0.49
ELIpop
5.3
4.1
4.5
4.8
3.9
4.2
5.2
4.0
3.2
4.2
3.8
5.3
4.3
4.3
0.05
0.04
0.03
0.03
0.04
0.03
0.01
0.03
0.01
0.03
0.06
0.04
0.09
0.01
1.13
0.90
0.90
1.08
0.88
0.87
1.12
0.89
0.83
0.87
0.83
1.12
0.91
0.88
−0.23
−0.41
−0.27
−0.23
−0.41
−0.26
−0.23
−0.41
−0.41
−0.26
−0.26
−0.24
−0.41
−0.26
RJI (%)
ELI
basin
δ
V001
ELImax
ΔELI
RJI (e)
d
d
Me2AlOMe(IMes) (1a)
Me2GaOMe(IMes)
Me2InOMe(IMes)
Me2AlOMe(SIMes) (1b)
Me2GaOMe(SIMes)
Me2InOMe(SIMes)
Me2Al(OCPh2Me)(IMes) (3a)
Me2Ga(OCPh2Me)(IMes)
Me2Ga(OCPh2Me)(6-Mes)
Me2In(OCPh2Me)(IMes)
Me2In(OCPh2Me)(6-Mes)
Me2Al(OC6H4OMe)(IMes) (4a)
Me2Ga(OC6H4OMe)(IMes)
Me2In(OC6H4OMe)(IMes)
Al−CNHC
Ga−CNHC
In−CNHC
Al−CNHC
Ga−CNHC
In−CNHC
Al−CNHC
Ga−CNHC
Ga−CNHC
In−CNHC
In−CNHC
Al−CNHC
Ga−CNHC
In−CNHC
0.20
0.46
0.48
0.20
0.46
0.47
0.20
0.46
0.45
0.47
0.47
0.21
0.49
0.46
11.11
10.64
11.21
9.75
2.54
2.57
2.48
2.34
2.38
2.28
2.54
2.55
2.35
2.44
2.29
2.55
2.60
2.48
2.32
2.08
2.06
2.43
2.16
2.14
2.34
2.10
2.19
2.09
2.14
2.32
2.06
2.08
0.028
0.057
0.053
0.046
0.070
0.064
0.008
0.132
0.057
0.149
0.026
0.042
0.087
0.146
2.41(0.09 )
95.0(3.7 )
2.25
2.24
87.5
90.3
94.8(4.0 )
d
d
2.21(0.09 )
9.24
9.81
2.05
2.05
86.4
89.8
95.0(3.7 )
d
d
11.22
10.28
8.99
10.85
9.94
11.27
10.91
11.11
2.42(0.09 )
2.24
2.02
2.20
2.04
87.5
85.7(13.0)
90.3
89.3(9.1)
d
d
2.42(0.97 )
94.9(3.8 )
2.25
2.25
86.5
90.7
a
Definitions and units: Bond lengths (d) in Å and bond topological properties: electron density ρbcp in e·Å−3 and its corresponding
b
Laplacian∇2ρbcp, in e·Å−5; ε, bond ellipticity; G/ρbcp and H/ρbcp, kinetic and total energy density over ρbcp ratios in he−1. Definitions and units: δ −
the delocalization index; V001ELI, volume of the ELI-D basin in Å3 cut at 0.001 au; ELIpop, electron population within the ELI-D basin in e, ELImax
ELI-D value at the attractor position; ΔELI, the distance in Å of the attractor position perpendicular to the atom−atom axis; RJI, Raub-Jansen index
,
c
d
in e and %. M = Al, Ga, In; NHC = IMes, SIMes, and 6-Mes. Raub−Jansen index value for contribution of aluminum atom.
analysis of Me2MOR(NHC) (M = Al, Ga, In) complexes (see
above) indicated that in addition to the strength of M−CNHC
bonds their character should also be considered. In order to
estimate the character of M−CNHC bonds in the investigated
complexes, we used real-space bonding indicators (RSBIs)
obtained from density functional theory (DFT) calculations
and topological analysis of the computed electron and pair
densities according to the atoms-in-molecules (AIM)34 and
electron localizability indicator (ELI-D)35 space partitioning
schemes, respectively, which provide a set of topological and
integrated bonding and atomic properties. RSBIs have been
recently discussed with regard to the determination of the
character of interactions between main group elements,36 as
well as used for the determination of the character of M−C
Starting from the experimental coordinates, gas-phase
structures of Me2MOR(NHC) (M = Al, Ga, In; NHC =
IMes, SIMes) complexes such as Me2MOMe(IMes),
Me2MOMe(SIMes), Me2M(OCPh2Me)(IMes), and Me2M-
(OC6H4OMe)(IMes),39 as well as Me2M(OCPh2Me)(6-Mes)
(M = Ga, In), were obtained by single-point calculations at the
B3PW91/6-311+G(2df,p) level of theory in order to reveal the
effect of metal and NHC on the character of M−CNHC bonds.
For the investigated complexes, a set of topological and
integrated real-space bonding indicators (RSBI) derived from
the electron and electron pair densities was calculated and
collected in Table 3 (see also the Supporting Information for
details). For all M−CNHC bonds, positive values of the
corresponding Laplacian ∇2ρbcp were indicative of strongly
polar bonds with dominant ionic contribution. However, the
electron densities ρbcp at the M−CNHC bond critical points
ranged from 0.39−0.42 e·Å−3 (Al−CNHC), 0.48−0.50 e·Å−3
(In−CNHC) and 0.58−0.60 e·Å−3 (Ga−CNHC), which indicated
the decrease of ionic contribution in a row Al−CNHC > In−
CNHC > Ga−CNHC bonds. The delocalization index values
confirmed the higher covalent contribution for Ga−CNHC and
In−CNHC bonds (for both from 0.46 to 0.49) compared to Al−
CNHC (δ = 0.2). Additionally, the kinetic energy density over
ρbcp ratios G/ρbcp, showing the degree of ionicity, and
38
bonds, including Zn−C37 and Te−CNHC bonds. In our
studies, we used RSBIs to explain the effect of the character of
M−CNHC bonds in Me2MOR(NHC) (M = Al, Ga, In, NHC =
IMes, SIMes) complexes on their synthesis, structure, and
reactivity. Additionally, for a more thorough understanding of
the character of M−CNHC bonds, we calculated RSBIs for
Me2MOR(6-Mes) (M = Ga, In) complexes recently reported
by us, in which weaker M−C6‑Mes bonds, in comparison with
M−CIMes and M−CSIMes bonds, are present.5c
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Article
especially the total energy density over ρbcp ratios H/ρbcp
,
showing the degree of covalency, indicated the significantly
higher (polar) covalent character of Ga−CNHC bonds (G/ρbcp
= 0.88−0.91 he−1, H/ρbcp = −0.41 he−1) in comparison with
In−CNHC(G/ρbcp = 0.87−0.9 he−1, H/ρbcp= −0.26 − (−0.27)
he−1) and even more ionic Al−CNHC(G/ρbcp = 1.08−1.13 he−1,
H/ρbcp = −0.23 − (−0.24) he−1) bond. Noteworthy, G/ρbcp of
M−CNHC (M = Al, Ga, In) bonds follow the trend of Pauling
electronegativity differences, which decrease in the order Al−
CNHC (0.94) > In−CNHC (0.77) > Ga−CNHC (0.74), similarly
to X3ADY3(A = B, Al; D = N, P) recently discussed by Mebs
and Beckmann.36b Interestingly, the differences of H/ρbcp for
M−CNHC (M = Al, Ga, In) followed electronegativity
differences, taking into account the Allred and Rochow
electronegativity scale (Al−CNHC (1.03) ≈ In−CNHC(1.01) >
Ga−CNHC(0.68)). As a result, ionicity and covalency of M−
CNHC bonds could be better described by Allen electro-
negativity differences, which decrease in the order Al−CNHC
(0.931) > In−CNHC(0.888) > Ga−CNHC(0.788).40 Consis-
tently, in the case of Me2MOR(NHC) (NHC = IMes, SIMes),
the Raub−Jansen indexes (RJIs)41 of Ga−CNHC (86−88%)
were significantly lower in comparison with In−CNHC(89−
90%) and mainly ionic Al−CNHC (95%) bonds. The difference
between the mainly ionic Al−CNHC bonds and predominantly
polar covalent Ga−CNHC bonds are visible in the ELI-D
distribution mapped onto the corresponding bonding basins
(Figure 5). The smallest electron-contribution of Al atom in
the M−CNHC bond is indicated by the green color area in the
direction to the Al atom compared to the blue-color area for In
and Ga; however, the In−CNHC basin is flattened in the metal
direction. The volumes of the ELI-D basin V001ELI (cut at
0.001 au), which range from around 9 to 11 A,3 are relatively
large in comparison with other bonds in the coordination
sphere of aluminum gallium and indium. For any series of
investigated Me2MOR(NHC) (M = Al, Ga, In) complexes,
smaller volumes were observed in the case of gallium
complexes, in comparison with aluminum or indium.
Interestingly, significantly smaller volumes were observed for
the Me2MOR(SIMes), as well as Me2MOR(6-Mes) com-
plexes, with saturated NHCs in comparison with Me2MOR-
(IMes) complexes with unsaturated IMes carbene. The latter,
which affect the electron density distribution in the vicinity of
carbene carbon, could be responsible for a more significant
shift of carbene carbon in 13C NMR in the case of
Me2MOR(NHC) complexes with saturated NHCs (Table
1). The most striking example includes a larger shift of carbene
carbon of Me2M(OCPh2Me)(6-Mes) (M = Ga, In) in
comparison with Me2M(OCPh2Me)(IMes), although a
stronger Al−CIMes bond was clearly evidenced by X-ray
analysis in the solid state.5c
Figure 5. ELI-D distributions onto V2(Al, CNHC), V2(Ga, CNHC), and
V2(In, CNHC) bonding (disynaptic valence) basins of 1a.
Although the strength of M−CNHC bonds, determined on
the basis of both X-ray analysis calculations of BDEs, could not
explain the reactivity of investigated Me2MOR(NHC) (M =
Al, Ga, In; NHC = IMes, SIMes) complexes, the ionic
character of M−CNHC bonds, which increased in series: Ga−
CNHC < In−CNHC < Al−CNHC could be much more easily
associated with the reactivity, including stability, of Me2MOR-
(NHC) complexes. With regard to the latter, noteworthy is the
reactivity of the Al−CNHC bond of both aluminum alkoxides/
aryloxides with NHCs,12h as well as trialkylaluminum NHC
adducts,42 the latter additionally supported by DFT studies. In
the case of the catalytic activity of Me2MOR(NHC) complexes
in ROP, the most polar covalent character of Ga−CNHC was
intact toward lactide in contrast to In−CNHC and Al−CNHC
bonds. The character of M−CNHC (M = Al, Ga, In) bonds was
also interesting in the light of the recent debate on the proper
Lewis formula representation of donor−acceptor complexes,43
which was recently discussed by Mebs and Beckmann with
regard to RSBIs of A−D bonds of X3ADY3(A = B, Al; D = N,
P) complexes.36b For discussed Me2MOR(NHC) complexes,
both structure/reactivity studies and calculations concerning
Me2MOR(NHC) complexes suggest the dominating canonical
form for Ga−CNHC and arrow notation for Al−CNHC bonds, In
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Article
from a toluene solution of the reaction mixture. IMes and SIMes were
synthesized as described by us previously.5a,b 1H and 13C NMR
spectra were recorded on an Agilent 400-MR DD2 400 MHz
spectrometer with shifts given in ppm according to the deuterated
solvent shift. Elemental analysis was performed on a Vario EL III
instrument (Heraeus). ROP of rac-LA with 1a, 3a, 4a, and 4b was
conducted according to the previously described procedure.5a,b,6
Compounds 1a, 1b, 3a, and 4a were isolated in high yields (see
below) and characterized in each case by elemental analysis (EA),
NMR spectroscopy, and X-ray analysis. Although obtained EA results
are low in carbon by 1.56% (1a), 1.57% (1b), and 3.71% (3a), they
are correct with regard to the content of hydrogen and nitrogen, and
represent the best values we could obtain experimentally. Compound
2a was not isolated pure in bulk, as it cocrystallized with free IMes
present in solution; however, it could be purified by crystallization,
which was evidenced by isolation of monocrystals of 2a suitable for X-
ray analysis. The structure of 2a was additionally confirmed by 1H and
13C NMR spectroscopy in the presence of 1 equiv of IMes.
Compounds Me2Al(OCPh2Me)(SIMes) and Me2Al(OCPhMeH)-
(IMes) were not isolated; however, their formation and structure were
the case of In−CNHC, a combination of these two notations
would be much more suitable.
CONCLUSIONS
■
We have focused on the effect of group 13 metals on the
synthesis, structure, and reactivity of Me2MOR(NHC) (M =
Al, Ga, In) complexes, with regard to the strength and
character of M−CNHC bonds. Therefore, we have investigated
the effect of alkoxide ligands and NHCs on the formation,
stability, and structure of Me2AlOR(NHC) alkoxides, as well
as and their reactivity, including catalytic activity in the ring-
opening polymerization (ROP) of lactide, in comparison with
their gallium and indium analogues that have been already
reported by us. The synthesis of Me2AlOR(NHC), and the
formation of Al−CNHC bond, with aluminum possessing a
smaller radius in comparison with gallium and indium, is
strongly dependent on the accessibility of the fifth coordinate
site of aluminum in [Me2Al(μ-OR)]n (n = 2, 3) aggregates.
However, the structure of the resulting Me2AlOR(NHC)
complexes, their stability or tendency for ligand disproportio-
nation, as well as reactivity toward lactide in the ROP, is
strongly dependent on both the strength and character of Al−
CNHC bonds, rather than only their strength, as we suggested
previously for Me2MOR(NHC) (M = Ga, In) complexes.
While the strength of M−CNHC bonds was determined using
spectroscopic techniques, as well as calculations of their bond
dissociation energies (BDEs), real-space bonding indicators
(RSBIs), obtained from density functional theory (DFT)
calculations and topological analysis of the computed electron
and pair densities according to the atoms-in-molecules (AIM)
and electron localizability indicator (ELI-D) space partitioning
schemes, were used for the determination of the character of
M−CNHC bonds of Me2MOR(NHC) (M = Al, Ga, In)
complexes. The character of M−CNHC bonds, changing from
polar covalent in the case Ga−CNHC bonds to essentially only
ionic for Al−CNHC bonds (polar covalent character: Ga−CNHC
> In−CNHC > Al−CNHC) much better reflects the properties of
Me2MOR(NHC) complexes than the strength of M−CNHC
bonds determined either by spectroscopic techniques or DFT
calculations of bond dissociation energies (BDEs). Our studies
not only show the important role of the character of the M−
CNHC bond on the properties, including catalytic properties of
metal NHC complexes, but also confirm that the character of
M−CNHC can be estimated using RSBIs.
1
evidenced by H and 13C NMR spectroscopy.
Synthesis of Aluminum, Gallium, and Indium Complexes.
Synthesis of 1a. To a stirred solution of [Me2Al(μ-OMe)]3 (37 mg,
0.14 mmol) in toluene (2 mL) 2 mL of a toluene solution of IMes
(128 mg, 0.42 mmol) was added at room temperature. The resulting
solution was stirred for 20 min. Then toluene was removed under a
vacuum to give a white crystalline solid, which was recrystallized from
toluene/hexane solution at −18 °C, and colorless crystals were dried
under a vacuum (115 mg, 70%). Anal. Calcd for C24H33AlN2O: C,
1
73.44, H, 8.47, N, 7.14. Found: C, 71.88, H, 8.45, N, 6.92. H NMR
(toluene-d8, 400 MHz): −1.09 (s, 6H, AlCH3), 2.03 (s, 12H, CH3),
2.10 (s, 6H, CH3), 3.38 (s, 3H, OCH3), 6.18 (br s, 2H, CH), 6.73 (s,
4H, CHAr). 13C {1H} NMR (toluene-d8, 100 MHz): −9.5 (AlMe2),
17.6, 21.0, 50.6, 122.6, 129.2, 135.4, 137.4, 139.3.
Synthesis of 1b. To a stirred solution of [Me2Al(μ-OMe)]3 (30
mg, 0.11 mmol) in toluene (2 mL) 2 mL of a toluene solution of
SIMes (101 mg, 0.33 mmol) was added at room temperature. The
resulting solution was stirred for 20 min. Then toluene was removed
under a vacuum to give a white crystalline solid, which was
recrystallized from toluene solution at −18 °C, and colorless crystals
were dried under a vacuum (98.5 mg, 76%). Anal. Calcd for
C24H35AlN2O: C, 73.06, H, 8.94, N, 7.10. Found: C, 71.49, H, 9.05,
1
N, 7.17. H NMR (THF-d8, 400 MHz), 13C NMR (THF-d8, 100
MHz): dissolving 1b immediately caused its partial decomposition; in
this case signals corresponding to 1b could not be clearly
distinguished (see the Supporting Information).
Synthesis of 2a/IMes. To a stirred solution of [Me2Al(μ-
OCH2CH2Me)]2 (32 mg, 0.12 mmol) in toluene (4 mL) 2 mL of
a toluene solution of IMes (146 mg, 0.48 mmol) was added at room
temperature. The resulting solution was stirred for 30 min. Then
toluene was removed under a vacuum to give a white crystalline solid
1
EXPERIMENTAL SECTION
of 2a and IMes. H NMR (toluene-d8, 400 MHz): 2a: −1.12 (s, 6H,
■
AlCH3), 2.02 (s, 12H, CH3), 2.09 (s, 6H, CH3), 3.17 (s, 3H, OCH3),
3.31 (t, J = 6.4 Hz, 2H, CH2) 3.63 (t, J = 6.4 Hz, 2H, CH2), 6.27 (s,
2H, CH), 6.74 (s, 4H, CHAr); IMes: 2.04 (s, 12H, CH3), 2.14 (s, 6H,
CH3), 6.51 (s, 2H, CH), 6.72 (s, 4H, CHAr). 13C {1H} NMR
(toluene-d8, 100 MHz): 2a: −9.0 (AlMe2), 17.6, 21.3, 58.4, 62.0, 77.2,
122.8, 129.2, 135.5, 137.7, 139.0, 174.9 (Ccarbene); IMes: 18.0, 21.0,
120.6, 129.0, 135.2, 137.1, 139.1, 217.3 (Ccarbene).
General Procedures. All operations were carried out under dry
argon using standard Schlenk techniques or in a glovebox (MBraun.
UniLab). Solvents and reagents were purified and dried prior to use.
Solvents were purified using MBRAUN Solvent Purification Systems
(MB-SPS-800) and stored over molecular sieves. Deuterated solvents
were either dried over potassium (toluene-d8, THF-d8) or calcium
hydride (CD2Cl2). rac-Lactide was purchased from Sigma-Aldrich and
further purified by crystallization from anhydrous toluene and
sublimation. (S)-Methyl lactate, 2-methoxyethanol, and methanol
were purchased from Sigma-Aldrich, dried over molecular sieves, and
distilled under argon. 1-Phenylethanol and o-metoxyphenol (guaiacol)
were purchased from Sigma-Aldrich and dried over molecular sieves.
1,1-Diphenylethanol was purchased from Sigma-Aldrich and used as
received. Me3Al and Me3Ga and Me3In were purchased from STREM
Chemicals, Inc. and used as received. Me2GaOCH2CH2OMe(IMes)
was synthesized analogously to other dialkylgallium alkoxides (see the
Supporting Information).5 Me2InOMe(IMes) was synthesized as
described previously,6 while crystals suitable for X-ray were obtained
Synthesis of 3a. The first step involved the synthesis of [Me2Al(μ-
OCPh2Me)]2. A stirred solution of Me3Al (265 mg, 3.68 mmol) in
methylene chloride (10 mL) was cooled to −78 °C, and a methylene
chloride solution (5 mL) of 1,1-diphenylethanol (728 mg, 3.68
mmol) was added dropwise. The cooling bath was removed, and the
reaction mixture was warmed slowly to room temperature, while the
evolution of gas was observed, and stirred for an additional 1 h. The
solvent and volatiles were removed under a vacuum to give a
yellowish crystalline solid, which was recrystallized from methylene
chloride/hexane at −18 °C, and colorless crystals of [Me2Al(μ-
1
OCPh2Me)]2 were dried under a vacuum (749 mg, 80%). H NMR
K
DOI: 10.1021/acs.organomet.8b00570
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(toluene-d8, 400 MHz): −0.82 (s, 6H, AlCH3), 2.01 (s, 3H, CH3),
7.02−7.06 (m, 2H, CHAr), 7.10−7.15 (m, 4H, CHAr), 7.35−7.38 (m,
2H, CHAr). 13C {1H} NMR (toluene-d8,100 MHz): −5.7 (AlMe2),
31.3, 81.8, 127.9, 128.1, 128.3, 145.5. To a stirred solution of
[Me2Al(μ-OCPh2Me)]2 (92 mg, 0.18 mmol) in toluene (4 mL) 2 mL
of a toluene solution of IMes (109 mg, 0.36 mmol) was added at
room temperature. The resulting solution was stirred for 11 days.
Then toluene was removed under a vacuum to give a yellowish
crystalline solid, which was recrystallized from toluene/hexane
solution at −18 °C, and colorless crystals were dried under a vacuum
(162 mg, 81%). Anal. Calcd for C37H43AlN2O: C, 79.54, H, 7.76, N,
Diffraction κ-CCD Gemini A Ultra diffractometer. Cell refinement
and data collection as well as data reduction and analysis were
performed with the CRYSALISPRO software.44 Using Olex2,45 the
structure was solved with the ShelXT46 structure solution program
using Intrinsic Phasing and refined with the SHELXL program
refinement package,47 using Least Squares minimization. The crystal
data and experimental parameters are summarized in Table S1.
Hydrogen atoms were added to the structure model at a geometrically
idealized coordinates and refined as riding atoms. CCDC 1852729−
1852732 and 1852735−1852737 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.ca-
m.ac.uk/data > request/cif.
Crystal Data for 1a. C24H33N2OAl (M = 392.50 g/mol):
monoclinic, P21/n, a = 7.8428(2) Å, b = 23.1863(5) Å, c =
12.5518(4) Å, β = 92.450(2)°, V = 2280.40(11) Å3, Z = 4, T =
120.0(1) K, μ(MoKα) = 0.105 mm−1, 36 935 reflections measured
(6.964° ≤ 2Θ ≤ 53.74°), 4886 unique (Rint = 0.0517, Rsigma = 0.0286)
which were used in all calculations. The final R1 was 0.0405 (I >
2σ(I)) and wR2 was 0.1088 (all data).
Crystal Data for 1b. C24H35AlN2O (M = 394.52 g/mol):
orthorhombic, Pna21, a = 22.7296(11) Å, b = 7.9504(4) Å, c =
13.1086(10) Å, V = 2368.8(2) Å3, Z = 4, T = 293(1) K, μ(MoKα) =
0.101 mm−1, 15287 reflections measured (6.984° ≤ 2Θ ≤ 52.246°),
4667 unique (Rint = 0.0406, Rsigma = 0.0462) which were used in all
calculations. The final R1 was 0.0447 (I > 2σ(I)) and wR2 was 0.1116
(all data).
Crystal Data for 2a. C26H37N2O2Al (M = 436.55 g/mol): triclinic,
P, a = 8.1815(4) Å, b = 8.5023(4) Å, c = 19.5640(7) Å, α =
94.985(4)°, β = 91.892(4)°, γ = 107.000(4)°, V = 1294.07(10) Å3, Z
= 2, T = 120.0(1) K, μ(MoKα) = 0.101 mm−1, 11 113 reflections
measured (6.87° ≤ 2Θ ≤ 51.75°), 4995 unique (Rint = 0.0304, Rsigma
= 0.0440) which were used in all calculations. The final R1 was 0.0501
(I > 2σ(I)) and wR2 was 0.1320 (all data).
1
5.01. Found: C, 75.83, H, 7.64, N, 4.98. H NMR (toluene-d8, 400
MHz): −1.23 (s, 6H, AlCH3), 1.86 (s, 3H, CH3), 1.99 (s, 12H, CH3),
2.14 (s, 6H, CH3), 5.98 (s, 2H, CH), 6.74 (s, 4H, CHAr), 7.03−7.07
(m, 2H, CHAr), 7.11−7.16 (m, 4H, CHAr), 7.36−7.39 (m, 4H, CHAr).
13C {1H} NMR (toluene-d8,100 MHz): −6.1 (AlMe2), 17.8, 21.0,
32.8, 76.4, 122.6, 125.3, 127.3, 127.4, 129.3, 135.5, 135.6, 139.2,
153.5, 175.0 (Ccarbene).
Synthesis of Me2Al(OCPh2Me)(SIMes). To a stirred solution of
[Me2Al(μ-OCPh2Me)]2 (66 mg, 0.13 mmol) in toluene (2 mL) 2 mL
of a toluene solution of SIMes (80 mg, 0.26 mmol) was added at
room temperature. The resulting solution was stirred for 3 months.
Then toluene was removed under a vacuum to give a white crystalline
solid, which was recrystallized from a toluene/hexane solution at −18
°C, and colorless crystals were dried under a vacuum. The isolation of
pure Me2Al(OCPh2Me)(SIMes) was not possible due to cocrystal-
lization of Me2Al(OCPh2Me)(SIMes) and SIMes; however, signals
corresponding to Me2Al(OCPh2Me)(SIMes) could be clearly
distinguished. 1H NMR (toluene-d8, 400 MHz): −1.31 (s, 6H,
AlCH3), 1.83 (s, 3H, CH3), 2.12 (s, 12H, CH3), 2.18 (s, 6H, CH3),
3.05 (s, 2H, CH2), 6.73 (s, 4H, CHAr), 7.12−7.20 (m, 6H, CHAr),
7.11−7.16 (m, 4H, CHAr), 7.35−7.37 (m, 4H, CHAr). 13C {1H} NMR
(toluene-d8,100 MHz): −6.0 (AlMe2), 18.1, 21.0, 32.7, 51.0 76.4,
125.4, 127.3, 127.4, 129.7, 135.5, 136.2, 138.4, 153.4, 198.1 (Ccarbene).
Synthesis of 4a. To a stirred suspension of [Me2Al(μ-
OC6H4OMe)]2 (68 mg, 0.19 mmol) in toluene (10 mL) 2 mL of a
toluene solution of IMes (116 mg, 0.38 mmol) were added at room
temperature, and the resulting solution was stirred for 0.5 h. Then
toluene was removed under a vacuum to give a white crystalline solid,
which was recrystallized from toluene/hexane at −18 °C to give white
crystals, which were dried under a vacuum (123 mg, 69%). Anal.
Calcd for C30H37AlN2O2: C, 74.35; H, 7.70; N, 5.78. Found: C,
Crystal Data for 3a. C37H43N2OAl (M = 558.71 g/mol): triclinic,
P, a = 9.9657(8) Å, b = 10.4394(7) Å, c = 17.1951(13) Å, α =
82.894(6)°, β = 81.462(7)°, γ = 72.973(7)°, V = 1685.4(2) Å3, Z = 2,
T = 293.1(2) K, μ(MoKα) = 0.089 mm−1, 18 014 reflections
measured (6.722° ≤ 2Θ ≤ 52.114°), 6638 unique (Rint = 0.0474,
R
sigma = 0.0583) which were used in all calculations. The final R1 was
0.0514 (I > 2σ(I)) and wR2 was 0.1450 (all data).
Crystal Data for 4a. C30H37AlN2O2 (M = 484.59 g/mol): triclinic,
P, a = 7.6209(3) Å, b = 8.4804(3) Å, c = 23.5527(7) Å, α =
82.549(2)°, β = 86.426(3)°, γ = 64.056(4)°, V = 1357.18(9) Å3, Z =
2, T = 120.0(1) K, μ(MoKα) = 0.103 mm−1, 22 514 reflections
measured (6.876° ≤ 2Θ ≤ 54.34°), 5997 unique (Rint = 0.0260, Rsigma
= 0.0192) which were used in all calculations. The final R1 was 0.0393
(I > 2σ(I)) and wR2 was 0.1099 (all data).
Computational Methodology (BDEs). The geometry optimiza-
tion of all studied systems were carried out at the ωB97X-D level of
theory using the 6-311++G** triple-ζ basis set for all atoms except In,
for which we used def2-ECP basis set.48 The input coordinates were
generated from the X-ray single-crystal data. BDE was defined as the
difference between the energy of the complex and the sum of the
energy of the carbene and the remaining part of the complex. For
DFT calculations, we used the Boys−Bernardi counterpoise scheme
to correct for the basis set superposition error.49 All DFT calculations
were performed in the Gaussian09 program.50 We also performed
single-point DLPNO−CCSD(T)51 energy calculations in the def2-
svp basis set using the Orca v4.0.0.1 program52 and used it
independently to estimate BDEs.
Computational Methodology (RSBIs). The single-point calcu-
lations of Me2MOMe(IMes) (M = Al (1a), Ga, In), Me2MOMe-
(SIMes) (M = Al (1b), Ga, In), Me2M(OCPh2Me)(IMes) (M = Al
(3a), Ga, In), Me2M(OCPh2Me)(6-Mes) (M = Ga, In), Me2M-
(OC6H4OMe)(IMes) (M = Al (4a), Ga, In) were carried out at the
B3PW91 level of theory,53 using effective core potentials for In54
along with the associated triple-ζ basis sets55 and the 6-311+G(2df,p)
basis set for all other atoms. The input coordinates were generated
from the X-ray single-crystal data. The distances to hydrogen atoms
1
74.21; H, 7.86; N, 5.54. H NMR (toluene-d8, 400 MHz): −1.03 (s,
6H, AlCH3), 2.08 (s, 6H, CH3), 2.10 (s, 12H, CH3), 3.40 (s, 3H,
OCH3), 6.06 (s, 2H, CH), 6.57 (dd, 1H, J = 7.9, 1.7 Hz, CHAr), 6.62
(dd, 1H, J = 7.3, 1.6 Hz, CHAr) 6.68 (s, 4H, CHAr), 6.72 (dd, 1H, J =
7.9, 1.6 Hz, CHAr), 6.84−6.90 (m, 1H, CHAr). 13C {1H} NMR
(toluene-d8, 100 MHz): −8.2 (AlMe2), 17.6, 21.0, 54.6, 111.7, 116.1,
119.9, 121.4, 122.6, 129.3, 135.4, 139.2, 151.0, 152.3, 175.5 (Ccarbene).
Synthesis of 4b. To a suspension of [Me2Al(μOC6H4OMe)]2 (54
mg, 0.15 mmol) in toluene (10 mL) 2 mL of a toluene solution of
IMes (91 mg, 0.30 mmol) was added at room temperature, and the
resulting solution was stirred for 0.5 h. Then toluene was removed
under a vacuum to give a white crystalline solid, which was
recrystallized from toluene at −18 °C to give white crystals, which
were dried under a vacuum (92 mg, 63%). Anal. Calcd for
C30H39AlN2O2: C, 74.05; H, 8.08; N, 5.76. Found: C, 74.13; H,
8.07; N, 5.72. 1H NMR (toluene-d8, 400 MHz): −1.14 (s, 6H,
AlCH3), 2.05 (s, 6H, CH3), 2.29 (s, 12H, CH3), 3.16 (s, 4H, CH2),
3.35 (s, 3H, OCH3), 6.51 (dd, 1H, J = 7.9, 1.6 Hz, CHAr), 6.60 (td,
1H, J = 7.6, 1.6 Hz, CHAr) 6.67 (s, 4H, CHAr), 6.71 (dd, 1H, J = 7.9,
1.6 Hz, CHAr), 6.85 (td, 1H, J = 7.6, 1.7, CHAr). 13C {1H} NMR
(toluene-d8, 100 MHz): −8.1 (AlMe2), 17.9, 21.0, 51.1, 54.5, 111.6,
116.1, 118.9, 121.3, 129.7, 135.4, 136.2, 138.3, 151.0, 152.1, 198.6
(Ccarbene).
X-ray Structure Determination. Single crystals suitable for X-
ray diffraction studies were selected under a polarizing microscope,
mounted in inert oil and transferred to the cold gas stream of the
diffractometer. Diffraction data were measured with graphite-
monochromated MoKα (λ = 0.71073) radiation on the Oxford
L
DOI: 10.1021/acs.organomet.8b00570
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Article
were elongated to mean values from neutron-diffraction experi-
ments.56 All computations were performed by using the Gaussian09
program.50 Subsequently, topological and integrated AIM and ELI-D
parameters were derived using AIMAll57 and DGID-4.658 on the basis
of wave function and checkpoint files, respectively. For the grid
calculations, a step size of 0.05 bohr was applied. All molecular graphs
are AIMAll representations. ELI-D graphs were shown with
MOLISO59 with all protonated basins (H atoms) in transparent
mode for clarity reasons.
Ugarte, R. A.; Perera, T. A. Main Group Complexes with N-
Heterocyclic Carbenes: Bonding, Stabilization and Applications in
Catalysis. In N-Heterocyclic Carbenes: From Laboratory Curiosities to
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Efficient Synthetic Tools; Diez-Gonzales, S., Ed.; RSC Catalysis Series
No. 27; The Royal Society of Chemistry, 2017; pp 178−237.
(c) Nesterov, V.; Reiter, D.; Bag, P.; Frisch, P.; Holzner, R.; Porzelt,
A.; Inoue, S. Chem. Rev. 2018, 118, 9678−9842.
(2) See the representative reviews on NHC complexes of: group 13
́
metals (a) Fliedel, C.; Schnee, G.; Aviles, T.; Dagorne, S. Coord.
Chem. Rev. 2014, 275, 63−86. group 1 and 2 metals - (b) Bellemin-
Laponnaz, S.; Dagorne, S. Chem. Rev. 2014, 114, 8747−8774.
(3) Byers, J. A.; Biernesser, A. B.; Delle Chiaie, K. R.; Kaur, A.; Kehl,
J. A. Adv. Polym. Sci. 2017, 279, 67−118.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00570.
■
S
(4) Romain, C.; Fliedel, C.; Bellemin-Laponnaz, S.; Dagorne, S.
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NMR spectra. Bond valence vector and bond valencies
for 3a. Calculated bond topological and integrated bond
descriptors, molecular graphs, isosurface representations
of the localization domains of the ELI-D, ELI-D
distributions onto V2(M, CNHC) bonding (disynaptic
valence) basins of Me2MOMe(IMes) (M = Al (1a), Ga,
In), Me2MOMe(SIMes) (M = Al (1b), Ga, In),
Me2M(OCPh2Me)(IMes) (M = Al (3a), Ga, In),
Me2M(OCPh2Me)(6-Mes) (M = Ga, In), Me2M-
(OC6H4OMe)(IMes) (M = Al (4a), Ga, In) (PDF)
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Accession Codes
CCDC 1852729−1852732 and 1852735−1852737 contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*(P.H.) E-mail: phoreglad@uw.edu.pl. Tel. +48 22 5543670.
*(L.C.) E-mail: lilianna.checinska@chemia.uni.lodz.pl. Tel.
+48 42 6354273.
́
(11) Specklin, D.; Fliedel, C.; Gourlaouen, C.; Bruyere, J−C.; Aviles,
■
T.; Boudon, C.; Ruhlmann, L.; Dagorne, S. Chem. - Eur. J. 2017, 23,
5509−5519.
(12) For the representative examples of main group metal alkoxides
and aryloxides see group 1: (a) Arnold, P. L.; Rodden, M.; Davis, K.
M.; Scarisbrick, A. C.; Blake, A. J.; Wilson, C. Chem. Commun. 2004,
1612−1613. (b) Arnold, P. L.; Rodden, M.; Wilson, C. Chem.
Commun. 2005, 1743−1745. (c) Zhang, D.; Kawaguchi, H.
Organometallics 2006, 25, 5506−5509. (d) Yao, H.; Zhang, J.;
Zhang, Y.; Sun, H.; Shen, Q. Organometallics 2010, 29, 5841−5846.
(e) Arnold, P. L.; Turner, Z. R.; Bellabarba, R.; Tooze, R. P. J. Am.
Chem. Soc. 2011, 133, 11744−11756. Group 2: see ref 7 and 12c.
Group 12: see ref 8 (f) Anantharaman, G.; Elango, K. Organometallics
2007, 26, 1089−1092. (g) Jochmann, P.; Stephan, D. W. Chem. - Eur.
J. 2014, 20, 8370−8378. group 13: see ref 4-6 (h) Dardun, V.;
Escomel, L.; Jeanneau, E.; Camp, C. Dalton Trans. 2018, 47, 10429−
10433. group 14: (i) Rupar, P. A.; Jennings, M. C.; Baines, K. M.
Organometallics 2008, 27, 5043−5051. (j) Ruddy, A. J.; Rupar, P. A.;
Bladek, K. J.; Allan, C. J.; Avery, J. C.; Baines, K. M. Organometallics
2010, 29, 1362−1367. (k) Rupar, P. A.; Staroverov, V. N.; Baines, K.
M. Organometallics 2010, 29, 4871−4881. (l) Paul, D.; Heins, F.;
Krupski, S.; Hepp, A.; Daniliuc, C. G.; Klahr, K.; Neugebauer, J.;
Glorius, F.; Hahn, E. Organometallics 2017, 36, 1001−1008.
(13) Silva Valverde, M. F.; Schweyen, P.; Gisinger, D.; Bannenberg,
T.; Freytag, M.; Kleeberg, C.; Tamm, M. Angew. Chem., Int. Ed. 2017,
56, 1135−1140.
ORCID
Maciej Dranka: 0000-0003-0987-5762
́
Lilianna Chęcinska: 0000-0002-3546-920X
Paweł Horeglad: 0000-0002-2813-1251
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Science
Centre of Poland (SONATA BIS2 Programme, Grant No.
DEC-2012/07/E/ST5/02860). The authors thank Dr. Łukasz
Dobrzycki from University of Warsaw for help with X-ray
measurements of Me2InOMe(IMes), Prof. Janusz Zachara
from Warsaw University of Technology for calculations of the
resultant bond valence vector of 3a and helpful discussions,
and Dr. Andrzej Plichta from Warsaw University of
Technology for GPC measurements. TOC/abstract artwork
was done by Anna Maria Dabrowska (full.nonmetal.chemist@
̨
gmail.com).
́
(14) (a) Tan, G.; Szilvasi, T.; Inoue, S.; Blom, B.; Driess, M. J. Am.
Chem. Soc. 2014, 136, 9732−9742. (b) Li, B.; Kundu, S.; Stuckl, A.
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