Dialkylgallium complexes with alkoxide and aryloxide ligands possessing N-heterocyclic carbene functionalities: Synthesis and structure

Article abstract of DOI:10.1021/om400863y

Methods for the synthesis of dialkylgalium compounds with alkoxide or aryloxide ligands possessing N-heterocyclic carbene functionalities have been established. As a result, the synthesis of a series of dialkylgallium complexes Me₂Ga(O,C) (1, 3-5), and Me₂Ga(O,C)·Me ₃Ga (2, 6) is described, where (O,C) represents an alkoxide or aryloxide monoanionic chelate ligand with an N-heterocyclic carbene functionality. All complexes have been fully characterized using spectroscopic and X-ray techniques. The presence of a strongly basic NHC functionality in alkoxide or aryloxide ligands resulted in the formation of monomeric Me ₂Ga(O,C) species. The reaction of those complexes with the Lewis acid Me₃Ga leads to Me₂Ga(O,C)·Me₃Ga adducts (2 and 6) with a strong Me₃Ga-O dative bond. The effect of (O,C) ligands with various steric and electronic properties on the structure of obtained Me₂Ga(O,C) and Me₂Ga(O,C)·Me₃Ga has been discussed on the basis of spectroscopic data. Finally, the bond valence vector model has been used to estimate the effect of a chelating (O,C) ligand on strains in complexes 1-6 on the basis of X-ray data.

Full text of DOI:10.1021/om400863y

Article
pubs.acs.org/Organometallics
Dialkylgallium Complexes with Alkoxide and Aryloxide Ligands
Possessing N‑Heterocyclic Carbene Functionalities: Synthesis and
Structure
Paweł Horeglad,*,† Osman Ablialimov,† Grzegorz Szczepaniak,† Anna Maria Dąbrowska,† Maciej Dranka,‡
and Janusz Zachara‡
†
̇
Faculty of Chemistry, University of Warsaw, Zwirki i Wigury 101, 02-089 Warsaw, Poland
‡Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
S
* Supporting Information
ABSTRACT: Methods for the synthesis of dialkylgalium com-
pounds with alkoxide or aryloxide ligands possessing N-heterocyclic
carbene functionalities have been established. As a result, the
synthesis of a series of dialkylgallium complexes Me2Ga(O,C) (1,
3−5), and Me2Ga(O,C)·Me3Ga (2, 6) is described, where (O,C)
represents an alkoxide or aryloxide monoanionic chelate ligand with
an N-heterocyclic carbene functionality. All complexes have been
fully characterized using spectroscopic and X-ray techniques. The
presence of a strongly basic NHC functionality in alkoxide or
aryloxide ligands resulted in the formation of monomeric Me2Ga-
(O,C) species. The reaction of those complexes with the Lewis acid
Me3Ga leads to Me2Ga(O,C)·Me3Ga adducts (2 and 6) with a strong Me3Ga−O dative bond. The effect of (O,C) ligands with
various steric and electronic properties on the structure of obtained Me2Ga(O,C) and Me2Ga(O,C)·Me3Ga has been discussed
on the basis of spectroscopic data. Finally, the bond valence vector model has been used to estimate the effect of a chelating
(O,C) ligand on strains in complexes 1−6 on the basis of X-ray data.
LA.8 Although they showed no stereoselectivity, Zn and Mg9 as
INTRODUCTION
■
well as Y and Ti10 complexes with alkoxide ligands possessing
Transition-metal complexes with N-heterocyclic carbenes
NHC functionalities were also active in the polymerization of
(NHCs) have been widely applied as catalysts in various
lactide. Despite the fact that Li,11 Na,12 K,13 Mg,10,12 and Zn10
areas of chemistry.1 Although the strongly basic properties of
complexes with alkoxide- or aryloxide-linked NHC ligands
NHCs allow their use as ligands for the synthesis of main-
(NHC-alkoxide or NHC-aryloxide) have been synthesized and
group-metal complexes, the number of well-characterized main-
characterized, there is still a paucity of data concerning the
group-metal complexes with N-heterocyclic carbenes is
structure and methods of synthesis of main-group-metal
limited.2 For instance, among main-group-metal alkoxides and
complexes with such ligands. To our knowledge, no gallium
aryloxides widely investigated in both catalysis and material
or other group 13 element complexes with NHC-alkoxide or
chemistry, there are only a few reports concerning structurally
-aryloxide ligands have been reported so far. Here we report the
characterized Zn,3 Ga,4 and Ge5 alkoxides and aryloxides and
synthesis and structure of a series of Me2Ga(O,C) complexes,
boron alkoxides6 with NHCs. Of the former, zinc complexes
where (O,C) is a monoanionic alkoxide or aryloxide ligand
were found to be highly active catalysts for the controlled and
possessing an N-heterocyclic carbene functionality.
stereoselective polymerization of rac-lactide3a,c as well as ε-
caprolactone and cyclohexyl oxide.3b Recently we have shown
RESULTS AND DISCUSSION
■
that dialkylgallium alkoxides with NHCs are highly active and
We used two strategies for the synthesis of Me2Ga(O,C)
complexes. The first one, presented in Scheme 1, included the
reaction of an NHC salt possessing an alkoxide or aryloxide
group ([H2L]X) with Me3Ga in order to obtain a
dimethylgallium alkoxide or aryloxide with an imidazolinium
salt functionality: [(Me2Ga(HL))X]. Subsequent reaction with
potassium hydride was performed in order to form the N-
isoselective catalysts in the polymerization of rac-LA under mild
conditions and allow for a facile stereoselectivity switch.4 The
latter, supported by the promising catalytic properties of
gallium complexes in the ring-opening polymerization (ROP)
of cyclic esters,7 indicates the need for construction of new
gallium catalysts and encouraged us to investigate whether it is
possible to synthesize gallium complexes with alkoxide or
aryloxide ligands possessing NHC functionalities. It is note-
worthy that Ti and Zr bisaryloxides with NHC functionalities
were active and stereoselective in the polymerization of rac-
Received: August 27, 2013
Published: December 10, 2013
© 2013 American Chemical Society
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Scheme 1. First Approach to the Synthesis of Me2Ga(O,C) Complexes
heterocyclic carbene functionality and the desired Me2Ga(O,C)
complexes.
In the second approach (Scheme 2) the alkoxide or aryloxide
salt [H2L]X was first treated with KH, resulting in the
formation of the cyclic compound [HL], which was further
reacted with Me3Ga.
insoluble in common organic solvents: e.g., CH2Cl2, toluene,
and n-hexane (Scheme 4). Further reaction with 1 equiv of KH
in toluene was carried out at room temperature in the presence
of a catalytic amount of KOtBu acting as a proton transfer agent
and required due to essential insolubility of both [(Me2Ga-
(HL1))X] and KH in toluene. The final product Me2GaL1 (1)
was crystallized by slow addition of n-hexane to the toluene
solution to give colorless crystals in moderate yield. The X-ray
diffraction analysis revealed monomeric species with the
coordination sphere of gallium atoms adopting a distorted-
tetrahedral geometry (Figure 1). The presence of the strongly
basic NHC functionality coordinated to gallium, resulting in the
six-membered chelate GaO···Ccarbene ring, caused a decrease of
gallium acidity and therefore prevented association to e.g.
Scheme 2. Second Approach to the Synthesis of
Me2Ga(O,C) Complexes
1
dimeric species via Ga−O−Ga bridges. H and 13C NMR
spectroscopy in CD2Cl2 revealed higher field shifts of signals
corresponding to Me groups, at −0.88 and −2.5 ppm,
respectively, in comparison with dimethylgallium alkoxides,14
which indicates the strong coordination of carbene carbon to
gallium in solution. Similarly to 1, such a higher field shift was
observed for Me2GaOR(SIMes) (SIMes = 1,3-bis(2,4,6-
trimethylphenyl)imidazolin-2-ylidene; OR = methoxide, (S)-
methyl lactate) monomeric complexes recently reported by us.4
Finally, a single set of signals in 1H NMR, similarly to the case
for Me2GaOR(SIMes), was in line with the monomeric
structure of 1 in solution. The presence of monomeric species
due to a strong Ga−Ccarbene interaction is not surprising in light
of the structure of Me2GaOR(SIMes).4 However, it must be
noted that examples of monomeric dialkylgallium alkoxides are
rare, due to the strong tendency of dialkylgallium alkoxides to
form Ga−O−Ga bridges, which results most often in the
formation of dimeric species.14 Except for monomeric
Me2GaOR(SIMes),4 only recently we reported the dialkylgal-
While the second method has been already described in the
literature and used for the synthesis of Mg and Zn complexes,10
the first approach, as far as we are concerned, has not been
described for the synthesis of metal alkoxides or aryloxides. The
low tendency of dimethylgallium alkoxides for (i) exchange
reactions with the halogen anions and (ii) reactions with the
acidic proton of e.g. an imidazolinium functionality14 should
allow for the isolation of [(Me2Ga(HL))X] and further
formation of Me2Ga(O,C) due to the reaction of [(Me2Ga-
(HL))X] with KH (Scheme 1). Therefore, we developed
methods for the synthesis of a series of imidazolinium salt
derivatives of alcohols and phenols ([H2L]X) (Scheme 3),
which were further used as substrates for the synthesis of the
desired gallium complexes.
Synthesis and Structure of Alkoxide Complexes. The
reaction of [H2L1]I with Me3Ga in a 1:1 molar ratio resulted in
the evolution of methane and formation of a white solid that is
a
Scheme 3. Imidazolinium Salts [H2L]X Used in the Study
a
[H2L5]Cl has been reported in the literature.15 [H2L1]I and [H2L2]I were synthesized according to the method proposed in the literature for the
synthesis of asymmetric NHC salts.16 [H2L3]Cl and [H2L4]Cl were synthesized according to the method described in the literature for the synthesis
of similar compounds.17
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Scheme 4. Synthesis of Me2GaL1 (1)
1
90 °C for 24 h further evolution of gas was not observed. H
NMR analysis revealed a complex mixture of products, from
which we were unable to crystallize [Me2GaL2] or any other
pure product. Therefore, in this case we applied the second
strategy for the synthesis of Me2Ga(O,C) complexes, which is
presented in Scheme 2. [H2L2]I was initially reacted with
potassium hydride, which resulted in the formation of cyclic
[HL2] in the form of a pale yellow oil, in essentially quantitative
yield (Scheme 5). The structures of such products have already
been confirmed by X-ray diffraction studies and reported in the
literature.21 The reaction of [HL2] with Me3Ga (1:1) in toluene
proceeded, as evidenced by evolution of methane gas, after
heating of the reaction mixture to about 50 °C. Surprisingly,
after crystallization from a toluene/n-hexane solution colorless
crystals of the adduct Me2GaL2·Me3Ga (2), rather than the
expected Me2GaL2, were isolated in high yield. X-ray diffraction
analysis revealed monomeric species with the Me3Ga
coordinated to the alkoxide oxygen of Me2GaL2 (Figure 2).
The coordination sphere of gallium in the Me2GaL2 moiety
adopts a distorted-tetrahedral geometry. The coordination of
the NHC functionality to gallium resulted in the formation of a
seven-membered GaO···Ccarbene ring. Notably, the coordination
of Me3Ga to the alkoxide oxygen resulted in significant
lengthening of the Me2Ga(1)−O bond (1.9414(8) Å) in
comparison with that in 1 (1.8946(8) Å). The strong
coordination of Me3Ga to alkoxide oxygen in solution, and
the lack of exchange of Me groups between gallium centers, was
Figure 1. Molecular structure of 1 with thermal ellipsoids at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Ga(1)−C(1) 2.0789(10), Ga(1)−
C(4) 1.9792(11), Ga(1)−C(5) 1.9790(11), Ga(1)−O(1) 1.8946(8);
C(5)−Ga(1)−C(4) 120.50(5), O(1)−Ga(1)−C(1) 92.60(4), O(1)−
Ga(1)−C(4) 109.84(4), O(1)−Ga(1)−C(5) 108.26(4), C(4)−
Ga(1)−C(1) 111.00(4), C(5)−Ga(1)−C(1) 111.03(5), N(2)−
C(1)−N(1) 108.90(9).
lium alkoxide, which adopts a monomeric structure exclusively
due to the increased basicity of the alkoxide ligand
functionality.7b In the latter case the presence of the alkoxide
ligand with an organosuperbase functionality resulted in the
formation of a strong Ga−N bond. The other monomeric
dialkylgallium species were isolated only in the cases where the
basicity of alkoxide oxygen is decreased by (i) conjugation with
the ligand,18 (ii) electron-withdrawing substituents,19 and (iii)
the engagement of oxygen in hydrogen bonding.20
1
further confirmed by H NMR spectroscopy in toluene-d8,
which revealed two sharp singlets at −0.40 and 0.02 ppm
corresponding to Me2Ga and Me3Ga protons, respectively. This
lack of exchange of Me groups between the gallium centers of
Me2GaL2·Me3Ga is in sharp contrast to analogous dialkylalu-
minum alkoxides with Me3Al coordinated to alkoxide oxygen,
for which exchange of methyl groups between aluminum
centers was observed, resulting in one broad signal for the Al-
Me protons.22 The formation of 2 can be explained by the
coordination of Me3Ga present in solution to in situ formed
Me2GaL2 and therefore a decrease of reactivity of Me3Ga in
adduct 2 toward HL2. The reaction of 2 with HL2 was not
observed up to 100 °C. This can be explained by the impeded
coordination of Me3Ga to HL2 due to strong coordination of
Me3Ga to the alkoxide oxygen of Me2GaL2. It is worth noting
that among main-group metals there is only one example of a
similar compound, namely a monomeric Li complex bearing an
As a result of the reaction between [H2L2]I and Me3Ga in a
1:1 molar ratio, the ionic compound [(Me2Ga(HL2))I] was
1
obtained in essentially quantitative yield. H NMR confirmed
the formation of this salt due to the reaction of the OH group
of [H2L2]I with Me3Ga, while the proton of the imidazolinium
ring remained unreacted. We were, however, unable to
crystallize the obtained compound for X-ray analysis, as any
attempts at crystallization led to the formation of a heterophase
system, most probably due to its ionic nature. Although in the
next step the obtained [(Me2Ga(HL2))I] initially reacted with
KH in the presence of a catalytic amount of KOtBu, the
reaction ceased after several minutes and even after heating to
Scheme 5. Synthesis of Me2GaL2·Me3Ga (2)
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Figure 3. Molecular structure of Me3Ga(SIMes) with thermal
ellipsoids at the 50% probability level. Hydrogen atoms are omitted
for clarity. Selected bond lengths (Å) and angles (deg): Ga(1)−C(1)
2.121(3), Ga(1)−C(4) 2.001(3), Ga(1)−C(5) 1.994(3), Ga(1)−C(6)
2.002(3); C(4)−Ga(1)−C(1) 106.02(12), C(5)−Ga(1)−C(1)
108.70(11), C(6)−Ga(1)−C(1) 107.41(12).
Figure 2. Molecular structure of 2 with thermal ellipsoids at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Ga(1)−C(1) 2.0865(10), Ga(1)−
C(4) 1.9697(12), Ga(1)−C(5) 1.9745(12), Ga(1)−O(1) 1.9414(8),
Ga(2)−O(1) 2.0003(8), Ga(2)−C(22) 1.9945(12), Ga(2)−C(23)
1.9799(13), Ga(2)−C(24) 1.9904(12); C(5)−Ga(1)−C(4)
122.01(6), O(1)−Ga(1)−C(1) 95.66(4), O(1)−Ga(1)−C(4)
106.69(5), O(1)−Ga(1)−C(5) 107.93(4), C(4)−Ga(1)−C(1)
113.34(5), C(5)−Ga(1)−C(1) 107.83(5), N(2)−C(1)−N(1)
109.03(9), Ga(1)−O(1)−Ga(2) 123.63(4).
(O,O′)(SIMes) is more likely due to the formation of dimeric
dialkylgallium alkoxide. Therefore, the presence of a chelating
(O,C) ligand in Me2GaL2 is one of the main reasons for the
different reactivity of Me2Ga(O,C) with Me3Ga, in comparison
with Me2GaOR(NHC).
Synthesis and Structure of Aryloxide Complexes. The
first approach (Scheme 1) was applied for the synthesis of
dimethylgallium aryloxides with N-heterocyclic carbene func-
tionalities. In the first step, reactions of [H2L3]Cl, [H2L4]Cl,
and [H2L5]Cl with Me3Ga in 1:1 molar ratios led to the
formation of the dimethylaryloxide salts [(Me2Ga(HL3−5))Cl]
with imidazolinium functionalities (Scheme 7). 1H NMR
spectra of obtained complexes were indicative of the presence
of an imidazolinium salt functionality, which showed that
methane formation was exclusively due to the reaction of
Me3Ga with phenol groups of [H2L3−5]Cl. Similarly to
[(Me2Ga(HL2))I], attempts to crystallize [(Me2Ga(HL3−5))-
Cl] led to the formation of heterophase liquid systems, which
did not give monocrystals suitable for X-ray studies. The
reactions of [(Me2Ga(HL3−5))Cl] with 1 equiv of KH in
toluene at room temperature resulted in the formation of
Me2GaL3−5 (3−5), which were crystallized from toluene/n-
hexane solution to give colorless crystals in moderate to high
yields. X-ray diffraction analysis revealed monomeric species
with the coordination sphere of gallium adopting a distorted-
tetrahedral geometry (Figures 4−6). The Ga−Ccarbene inter-
action led to the formation of a six-membered (5) or seven-
membered (3, 4) GaO···Ccarbene ring. The formation of
monomeric dialkylgallium aryloxides with NHC functionalities
(3−5) is in accordance with the weaker basicity of phenolate
oxygen in comparison with alkoxides and therefore the lower
tendency for the formation of e.g. dimeric dimethylgallium
aryloxides. Particularly for dimethylgallium aryloxides with
alkoxide ligand with an NHC functionality with a coordinated
Lewis acid (Sc(CH2SiMe3)3) to alkoxide oxygen.23 Interest-
ingly, a search of the CSD24 revealed that 2 is the only example
of a structurally characterized gallium alkoxide with Me3Ga
coordinated to alkoxide oxygen.
The reactivity of Me3Ga toward Me2GaL2, resulting in the
formation of 2, is noteworthy in light of the reaction of
Me2Ga(O,O′)(SIMes) ((O,O′) = (S)-methyl lactate) with
Me3Ga in an equimolar ratio (Scheme 6). The latter resulted in
the formation of a [Me2Ga(O,O′)]2 and Me3Ga(SIMes)
adduct, due to the reaction of Me3Ga with the SIMes ligand
bonded to gallium, rather than the alkoxide oxygen of
Me2Ga(O,O′)(SIMes). Me3Ga(SIMes) is the second example
of a crystallographically characterized Me3Ga adduct with
NHC25 and the first with a saturated N-heterocyclic carbene.
Colorless crystals of Me3Ga(SIMes) were obtained from a
toluene/hexane solution, and X-ray analysis revealed a
monomeric complex with tetrahedral gallium (Figure 3).
Interestingly, the 13C NMR shifts of the carbene carbon signals
of Me2Ga(O,O′)(SIMes) (199.3 ppm)4 and Me3Ga(SIMes)
(206.1 ppm) revealed a weaker Ga···Ccarbene bond in the case of
Me3Ga(SIMes). In this case the reactivity of Me3Ga toward the
carbene carbon of SIMes bonded to the gallium of Me2Ga-
Scheme 6. Reaction of Me2Ga(O,O′)(SIMes) with Me3Ga
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Scheme 7. Synthesis of Me2GaL3‑5 (3−5)
Figure 4. Molecular structure of 3 with thermal ellipsoids at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Ga(1)−C(1) 2.0802(11), Ga(1)−
C(4) 1.9762(12), Ga(1)−C(5) 1.9725(12), Ga(1)−O(1) 1.9137(8);
C(5)−Ga(1)−C(4) 124.06(5), O(1)−Ga(1)−C(1) 100.45(4),
O(1)−Ga(1)−C(4) 103.28(4), O(1)−Ga(1)−C(5) 105.28(4),
C(4)−Ga(1)−C(1) 108.53(5), C(5)−Ga(1)−C(1) 112.11(5),
N(2)−C(1)−N(1) 108.83(9).
Figure 6. Molecular structure of 5 with thermal ellipsoids at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Ga(1)−C(1) 2.0559(12), Ga(1)−
C(4) 1.9596(13), Ga(1)−C(5) 1.9857(13), Ga(1)−O(1) 1.9209(9);
C(5)−Ga(1)−C(4) 121.50(6), O(1)−Ga(1)−C(1) 88.30(4), O(1)−
Ga(1)−C(4) 110.05(5), O(1)−Ga(1)−C(5) 106.49(5), C(4)−
Ga(1)−C(1) 114.33(5), C(5)−Ga(1)−C(1) 110.79(5), N(2)−
C(1)−N(1) 109.43(11).
respectively, which were similar to those of 1, 2, and
Me2GaOR(SIMes).4 The 13C NMR shift of 193.9 ppm in the
case of 5 was indicative of even stronger coordination of NHC
to gallium in comparison with the complexes mentioned above.
Despite the weaker basicity of aryloxide oxygen, we examined
whether Me3Ga may coordinate to the aryloxide oxygen of
Me2Ga(O,C) strongly enough to form complexes analogous to
2. The reaction between 4 and Me3Ga (1:1) at ambient
temperature led to the formation of Me2GaL4·Me3Ga (6) as a
white crystalline solid in essentially quantitative yield.
Monocrystals of the toluene solvate of 6 were obtained by
crystallization from a toluene/n-hexane solution. X-ray
diffraction analysis revealed a monomeric species essentially
isostructural with 2, with the Me3Ga coordinated to the
aryloxide oxygen (Figure 7). The weaker basicity of aryloxide
oxygen of 6 was reflected by much longer Me3Ga−O bonds of
2.0678(14) and 2.0622(14) Å for the two independent
molecules of the asymmetric unit in comparison with the
Figure 5. Molecular structure of 4 with thermal ellipsoids at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Ga(1)−C(1) 2.0664(13), Ga(1)−
C(4) 1.9787(15), Ga(1)−C(5) 1.9700(14), Ga(1)−O(1) 1.9045(10);
C(5)−Ga(1)−C(4) 119.45(6), O(1)−Ga(1)−C(1) 99.23(5), O(1)−
Ga(1)−C(4) 107.13(6), O(1)−Ga(1)−C(5) 107.42(5), C(4)−
Ga(1)−C(1) 106.96(6), C(5)−Ga(1)−C(1) 114.52(5), N(2)−
C(1)−N(1) 109.14(11).
analogous bond in the alkoxide derivative 2 (2.0003(8) Å). 13
C
Lewis base functionalities, such as amines and Schiff bases,
dimers with weakened Ga−O−Ga bridges26 or monomeric
species27 are reported. For 3−5 1H NMR revealed in each case
one set of signals, in agreement with the presence of
monomeric species. The shifts of Ga−Me protons in toluene-
d8 of −0.34 ppm (3), −0.35 ppm (4), and −0.21 ppm (5) were
similar to that of the alkoxide derivative 2 (−0.40 ppm),
indicating the strong coordination of the NHC functionality to
gallium. The latter was also confirmed by 13C NMR shifts of
198.4 and 198.2 ppm for the carbene carbons of 3 and 4,
NMR of coordinated Me3Ga was indicative of the same trend
in solution and revealed signals shifted to higher field at −3.2
and −1.9 ppm for 2 and 6, respectively. The strong
coordination of Me3Ga to aryloxide oxygen in solution and
the lack of exchange of Me groups between gallium atoms was
1
shown by H NMR, which revealed a sharp singlet for Me3Ga.
For 6 the presence of a broad signal of Me2Ga, along with
benzylic CH2, should be interpreted in terms of slow exchange,
on the NMR time scale, between two asymmetric isomers of
the Me2GaL4 unit, which is associated with the conformation of
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reaction mixture revealed the presence of two signals of Ga-Me
at −0.02 and −0.36 (corresponding to 3), in a 1:1.7 ratio
(Figure S32, Supporting Information). Subsequent recrystalli-
zations led to the isolation of colorless crystals of 3 in about
15% yield. Although the first approach (Scheme 1) was more
efficient for the synthesis of 3, the experiment described above
shows that the second approach may be used for the synthesis
of dialkylgallium aryloxide derivatives with NHC functionalities
(Scheme 8). However, some limitations of this approach were
shown by the lack of reactivity of [HL4] toward Me3Ga even at
elevated temperatures up to 100 °C.
Discussion and Bond Valence Vector Analysis. We
have analyzed the structure of compounds 1−6 in order to
demonstrate how the steric and electronic parameters of (O,C)
ligands influence the structure of discussed complexes. In order
to analyze the role of the NHC functionality in an alkoxide
chelate ligand, the simple dimethylgallium alkoxide with the
NHC Me2GaOMe(SIMes)4 served as a reference.
As indicated above, a strong Ga−Ccarbene bond has
considerable influence on the structure of the investigated
complexes and is responsible for their monomeric structure
both in solution and the solid state. An analysis of bond lengths
for 1−6 on the basis of X-ray crystallography revealed
important information on the coordination of the (O,C) ligand
containing an NHC functionality to gallium. For 1−6 the Ga−
Ccarbene bond length decreased in comparison with the
analogous bond in Me2GaOMe(SIMes) (2.1007(13) Å) by
0.014−0.045 Å and was greater than for GaCl3(NHC)30
(2.023−2.027 Å) with a saturated NHC. In solution the
strength of the carbene carbon interaction with the metal center
can be estimated on the basis of the 13C NMR carbene carbon
signal.31 Although similar chemical shifts of carbene carbons for
1−4, 6, and the reference complex Me2GaOMe(SIMes)
(198.2−200.5 ppm) indicate strong coordination of gallium
with the carbene carbon, they do not in all cases correlate with
the length of the Ga−Ccarbene bond. However, for 5 13C NMR
analysis revealed the carbene carbon signal at 193.9 ppm, which
correlates well with the shortest Ga−Ccarbene bond (2.0559(12)
Å). The stronger coordination of the NHC functionality for 1−
6 in comparison with that for Me2GaOMe(SIMes) is expected
to be the result of a chelate effect. It should be noted that
stronger coordination of the NHC functionality due to a
chelate effect could be responsible for the reactivity of Lewis
acids with Me2Ga(O,C) at oxygen rather than the carbene
carbon. In comparison with the Ga−O bond of Me2GaOMe-
(SIMes) (1.8778(11) Å) the length of the Ga−O bond was
slightly increased for alkoxide derivative 1 (1.8946(8) Å) and
Figure 7. Molecular structure of one of two unique molecules of 6
with thermal ellipsoids at the 50% probability level. Hydrogen atoms
are omitted for clarity. Selected bond lengths (Å) and angles (deg):
Ga(1)−C(1) 2.0701(19), Ga(1)−C(4) 1.967(2), Ga(1)−C(5)
1.964(2), Ga(1)−O(1) 1.9637(14), Ga(2)−O(1) 2.0678(14),
Ga(2)−C(25) 1.984(2), Ga(2)−C(26) 1.982(2), Ga(2)−C(27)
1.983(2), C(5)−Ga(1)−C(4) 123.38(10); O(1)−Ga(1)−C(1)
98.27(7), O(1)−Ga(1)−C(4) 107.26(8), O(1)−Ga(1)−C(5)
105.68(8), C(4)−Ga(1)−C(1) 104.59(9), C(5)−Ga(1)−C(1)
114.62(9), N(2)−C(1)−N(1) 109.10(17), Ga(1)−O(1)−Ga(2)
121.13(7).
the chelate ring (vide supra). Interestingly, a search of the
CSD22 revealed only one example of a structurally characterized
complex in which Me3Ga is coordinated to a gallium aryloxide
moiety.28
The weaker bonding of Me3Ga to aryloxide oxygen in
comparison with alkoxide oxygen, as demonstrated for
compounds 6 and 2, respectively, encouraged us to verify the
possibility of using the second strategy presented in Scheme 2
for the synthesis of 3 and 4. In both cases the reactions of
[H2L3]Cl and [H2L4]Cl with 1 equiv of KH resulted in the
formation of [HL3] and [HL4], respectively, and in both cases
colorless oils were obtained in essentially quantitative yield, as
evidenced by 1H NMR spectroscopy. The X-ray structure of an
analogous compound has been reported in the literature.29
A
subsequent reaction of [HL3] with Me3Ga proceeded after
heating of the reaction mixture to 80 °C with evolution of
methane. After the reaction ceased, the 1H NMR analysis of the
Scheme 8. Synthesis of Me2GaL3 (3) and Me2GaL4 (4) via the Second Approach
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Table 1. Values of Selected Bond Distances and Angles for Compounds 1−6 and Me2GaOMe(SIMes)4
compound
Ga−Ccarbene (Å)
Ga−O (Å)
Ga−C (Å)
Ga−C (Å)
yaw angle (deg)
pitch angle (deg)
O−Ga−Ccarbene (deg)
1
2
3
4
5
6
2.0789(10)
2.0865(10)
2.0802(11)
2.0664(13)
2.0559(12)
2.0701(19)
2.0782(19)
2.1007(13)
1.8946(8)
1.9414(8)
1.9137(8)
1.9045(10)
1.9209(9)
1.9637(14)
1.9667(14)
1.8778(11)
1.9792(11)
1.9697(12)
1.9762(12)
1.9787(15)
1.9596(13)
1.967(2)
1.9790(11)
1.9745(12)
1.9725(12)
1.9700(14)
1.9857(13)
1.964(2)
5.8
3.6
6.2
5.1
6.6
4.1
5.9
4.5
4.9
4.0
6.7
2.8
1.3
2.9
5.4
4.3
92.60(4)
95.66(4)
100.45(4)
99.23(5)
88.30(4)
98.27(7)
97.65(7)
101.99(5)
a
1.967(2)
1.955(2)
Me2GaOMe(SIMes)
1.9798(17)
1.9838(16)
a
For two unique molecules of 6.
aryloxide derivatives 3 (1.9137(8) Å), 4 (1.9045(10) Å), and 5
(1.9209(9) Å). The coordination of Me3Ga to the alkoxide
oxygen of 2 and 6 resulted in a more significant elongation of
the Me2Ga−O bond, which was found to be 1.9414(8) Å for 2
and 1.9637(14) and 1.9667(14) Å for 6. It is worth noting that
in the latter case the elongation of the Me2Ga−O bond did not
result in shortening of the Ga−Ccarbene bond, and a comparison
of 4 and 6 serves as a good example. No major modification of
the Ga−Ccarbene bond was compensated by the shorter Ga−
C(Me) bonds (the difference is greater than 3σ) with
subsequent widening of the C(Me)−Ga−C(Me) angle. As
the Ga−Ccarbene bond was shorter and the Ga−O bond longer
for 1−6 in comparison with Me2GaOMe(SIMes), we used the
bond valence model32 to calculate Ga−Ccarbene and Ga−O bond
valencies in order to evaluate the relationship between these
bonds. For 1 (1.27), 3 (1.24), 4 (1.28), 5 (1.28), and
Me2GaOMe(SIMes) (1.26) the sum of Ga−Ccarbene and Ga−O
bond valencies (in valence units) were essentially the same
within method accuracy. For 2 (1.19) and 6 (1.19, 1.17) the
coordination of Me3Ga to alkoxide or aryloxide oxygen resulted
in a decrease of 0.1 (see the Supporting Information).
While the latter analysis concerns the strength of the (C,O)
ligand coordination to gallium, the strains resulting from the
chelate GaO−Ccarbene ring can be reflected by selected angles.
The strains should be mainly seen by the deformations around
carbene and gallium centers. Yaw (in plane tilting) and pitch
(out of plane tilting) angles are usually used for the description
of the former.33 For compounds 1−6 and Me2GaOMe-
(SIMes)4 pitch angles are less than 7° (Table 1). Similarly
low values for yaw angles (Scheme 9, Table 1) for
Me2GaOMe(SIMes) and investigated Me2Ga(O,C) complexes
indicate that the binding mode for 1−6 should not be affected
considerably by chelate effects.29,34 On the other hand,
OGaCcarbene bite angles were analyzed as a possible indication
of the chelating ring strain. For 3 (100.45(4)°) the value of the
OGaCcarbene angle is closest to the analogous angle in
Me2GaOMe(SIMes) (101.99(5)°). Smaller angles are observed
for compounds 4 (99.23(5)°), 6 (98.27(7)° and 97.65(7)°), 2
(95.66(4)°), 1 (92.60(4)°), and 5 (88.30(4)°). The smallest
OGaC(NHC) angles for 1 and 5 may indicate higher ring
strains for complexes with six-membered OGa···C(NHC) rings.
An alternative method we used to estimate the strains at the
gallium center was the bond valence vector model (BVVM)
proposed by Zachara.35 The bond valence vector model was
shown to be a convenient tool for the estimation of constraints
of the geometry of the coordination sphere, which was
demonstrated for five-coordinate aluminum organometallic
complexes, four-coordinate phosphate derivatives, and three-
coordinate carbonates.31 Such constraints were shown to
correlate with the resultant bond valence vector, which is the
sum of individual valence vectors of all Ga−X bonds, where X is
the ligating atom in the coordination sphere. For compounds
1−6 and Me2GaOMe(SIMes) the resultant bond valence
vectors were calculated according to the method proposed in
the literature31 (for details see the Supporting Information). In
the case of 1 and 5 with six-membered GaO···Ccarbene rings the
resultant bond valence vectors are the longest and are equal to
0.077 and 0.108 vu (valence units), respectively. They are
directed toward the (O,C) ligand, as depicted in Figure 8,
therefore indicating the largest strains caused by the chelate
ring. The strain magnitude was smaller for 2 (0.036 vu), 3
(0.066 vu), 4 (0.039 vu), 6 (0.059 and 0.040 vu), and
Me2GaOMe(SIMes) (0.043 vu), as evidenced by the resultant
bond valence vectors. It must be noted that the large strains
revealed by the BVV model for 1 and 5 are in line with the
smallest O−Ga−Ccarbene bite angles observed for these
complexes, which indicates that the latter is a good measure
for the estimation of ring strain in metal alkoxides with NHC
functionalities.
Another cause of conformational differences for 1−6 is the
presence of a large substituent on the N(2) atom. The most
pronounced changes concern the arrangement of a mesityl or
Scheme 9. Coordination Sphere of Me2Ga(O,C) Complexes
i
C6H4 Pr2 substituent vs Me groups bonded to gallium, which
results in a twist of the Me2GaO entity around the Ga−Ccarbene
bond. The orientation of Me2GaO can be described by the
O(1)−Ga−C(1)−N(1) torsion angle. In the case of 1
(12.41(9)°) and 3 (−18.72(10)°) with mesityl substituents
the low values indicate an essentially symmetric arrangement, as
demonstrated in Figure 9a for 3. The switch from mesityl to
i
C6H4 Pr2 caused the asymmetrization of 4 (47.46(12)°), 5
(−35.45(10)°), and 6 (41.17(19)°, 43.82(19)°), which is
demonstrated in Figure 9b for 4. Interestingly, the coordination
of GaMe3 to the alkoxide oxygen in 2 resulted in the increase of
the analyzed torsion angle to −37.42(9)° despite the presence
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CONCLUSIONS
■
In summary, we have focused on the synthesis of dialkylgallium
alkoxides and aryloxides with alkoxide or aryloxide ligands
possessing N-heterocyclic carbene functionalities and devel-
oped two approaches for their synthesis. The most important
features of obtained complexes include a monomeric structure
due to the presence of a N-heterocyclic carbene with strongly
Lewis basic properties, despite the high tendency of
dialkylgallium alkoxides and aryloxides to form aggregates
with an increase of coordination number. The role of the
chelating ligand possessing an NHC functionality was revealed
in the reaction of Me2Ga(O,C) with Me3Ga, a Lewis acid,
which occurred at the alkoxide or aryloxide oxygen rather than
at the carbene carbon of the N-heterocyclic carbene
functionality, leading to Me2Ga(C,O)·GaMe3 adducts. Finally,
the obtained complexes constitute a platform of compounds
which can serve as substrates for the further synthesis of gallium
catalysts for the polymerization of heterocyclic monomers,
which is the topic of our current research.
EXPERIMENTAL SECTION
■
General Procedures. The synthesis of gallium complexes was
carried out under dry argon using standard Schlenk techniques.
Solvents and reagents were purified and dried prior to use. Solvents
were dried over potassium (toluene, n-hexane) or calcium hydride
(methylene chloride). Me3Ga was purchased from STREM Chemicals,
Inc., and used as received. N1-Mesitylethane-1,2-diaminium chloride
was prepared according to the literature.16 Potassium hydride (50% in
paraffin) was used, and the paraffin was washed out with n-hexane
before the reaction. 1H and 13C NMR spectra were recorded on
Agilent 400-MR DD2 400 MHz and Varian UnityPlus 200 MHz
spectrometers with shifts given in ppm according to the deuterated
solvent shift. Visualization of TLC plates was performed by UV light
with either KMnO4 or I2 stain. Flash chromatography was performed
using silica gel 60 (230−400 mesh). Melting points were recorded on
an OptiMelt SRS instrument with a heating rate of 10 °C/min. High-
resolution electrospray mass spectra (HRMS) were recorded on a
Quattro LC (triple-quadrupole mass spectrometer). The mass
spectrometer was calibrated with an internal standard solution of
sodium formate in MeOH. Elemental analysis was performed on a
Vario EL III instrument (Heraeus).
Figure 8. Resultant bond valence vector at the gallium center (in
purple) for (a) 1 and (b) 5. For the graphical presentation it was
assumed that 1 vu is equal to 10 Å.
Synthesis of NHC Salts. Synthesis of [H2L1]I and [H2L2]I. N1-
Mesitylethane-1,2-diaminium chloride (5.0 g, 20.0 mmol) was refluxed
in 15 equiv of trimethyl orthoformate for 6 h. Then, the volatiles were
removed under reduced pressure. The residue was dissolved in DCM
(100 mL) and precipitated by the addition of diethyl ether (250 mL).
The precipitate was dissolved in EtOH (50 mL) and stirred with 1.05
equiv of KOH for 30 min. Then, the solvent was removed and toluene
(100 mL) was added, and the resulting solution was filtered through a
Figure 9. Molecular structures (views along the Ga−Ccarbene bond)
and space-filling diagrams of (a) 3 transformed by a glide plane and
(b) 4.
Buchner funnel. Finally toluene was removed under vacuum to give 1-
̈
mesityl-4,5-dihydro-1H-imidazole as a white solid in 93% yield (3.50
g). Spectral data are in agreement with those reported in the
literature.36 A mixture of 1-mesityl-4,5-dihydro-1H-imidazole (1.88 g,
10.0 mmol) and 1.1 equiv of 2-iodoethanol (0.8 mL, 10.0 mmol) or 3-
iodo-1-propanol (1.0 mL, 10.0 mmol) in toluene (20 mL) was stirred
at 80 °C for 16 h in a pressure flask. After the solution was cooled to
room temperature, the solvent and volatiles were removed under
vacuum. The residue was dissolved in DCM (20 mL) and precipitated
by the gradual addition of diethyl ether (100 mL) to give a white
crystalline solid. This operation was repeated three times. The final
precipitate of 3-(2-hydroxyethyl)-1-mesityl-4,5-dihydro-1H-imidazoli-
nium iodide (2.34 g, 65%) or 3-(3-hydroxypropyl)-1-mesityl-4,5-
dihydro-1H-imidazolinium iodide (3.07 g, 8.2 mmol, 82%) was dried
under vacuum.
of the sterically less demanding mesityl substituent. The latter
shows that electronic changes at the alkoxide substituent may
lead to asymmetrization. In solution the fast exchange between
asymmetric conformers led to one sharp signal corresponding
to GaMe2 protons, as shown by 1H NMR. Only in the case of 6
is the exchange slow on NMR time scale, which results in the
broadening of the signal for GaMe2 protons. In this case such
behavior in solution is the result of a more difficult inversion
due to (i) coordination of alkoxide oxygen to Me3Ga and (ii)
3-(2-Hydroxyethyl)-1-mesityl-4,5-dihydro-1H-imidazolinium io-
i
1
dide ([H2L1]I): MS (ESI) m/z 233.5 [M − I]+. H NMR (400 MHz,
the presence of a more sterically demanding C6H4 Pr2
substituent.
CDCl3) 2.25 (s, 3H, CH3), 2.30 (s, 6H, CH3), 3.83−3.96 (m, 4H,
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3
CH2), 4.05 (br t, 1H, OH), 4.22 (dd, J(H,H) = 12.5, 9.2 Hz, 2H,
CH2), 4.44 (dd, 3J(H,H) = 12.5, 9.2 Hz, 2H, CH2), 6.89 (d, 3J(H,H) =
0.5 Hz, 2H, CHAr), 8.56 (s, 1H, CH); 13C NMR (100 MHz, CDCl3)
18.7, 21.1, 48.8, 50.6, 51.2, 56.8, 130.0, 130.5, 135.5, 140.3, 158.7.
3-(3-Hydroxypropyl)-1-mesityl-4,5-dihydro-1H-imidazolinium io-
silica gel chromatography (c-Hex/EtOAc, 3/7) yielded the product
[H2L5]Cl as a white powder which was dried in vacuo (1.2 g, 64%,
over two steps). Spectral data for [H2L5]Cl are in agreement with data
reported in the literature.15
Synthesis of Gallium Complexes. Synthesis of 1. A stirred
solution of [H2L1]I (774.0 mg, 2.15 mmol) in CH2Cl2 (10 mL) was
cooled to −70 °C, and 1 mL of a CH2Cl2 solution of Me3Ga (247.0
mg, 2.15 mmol) was added dropwise. After addition, the cooling bath
was removed and the reaction mixture was warmed. Before the
reaction mixture reached room temperature, the evolution of gas and
formation of a white precipitate was observed. Then, the reaction
mixture was stirred for 1 h and the solvent and volatile residues were
removed under vacuum to give a white crystalline solid, essentially
insoluble in common organic solvents, which was assumed on the basis
of other results (see below) to be [(Me2Ga(HL1))I]. Next, [Me2Ga(S-
1)] (692 mg, 1.51 mmol) and KH (60.5 mg, 1.151 mmol) were placed
in a Schlenk vessel and toluene (35 mL) was added at room
temperature. After subsequent addition of around 5 mol % of KOtBu
the evolution of gas was observed and the reaction mixture was stirred
for 2 h until gas evolution essentially ceased. The solution was filtered
through a 0.45 μm hydrophobic syringe filter, and the solvent and
volatile residues were then removed under vacuum to give a white
crystalline solid. Crystallization from CH2Cl2/toluene solution resulted
in the formation of colorless crystals of 1 (190 mg, 38%). Anal. Calcd
for 1, C16H25GaN2O: C, 58.04; H, 7.61; N, 8.46. Found: C, 58.08; H,
dide ([H2L2]I): MS (ESI) m/z 247.6 [M − I]+. H NMR (400 MHz,
1
CDCl3) 1.95−2.05 (m, 2H, CH2), 2.26 (s, 3H, Mes-CH3), 2.30 (s, 6H,
3
CH3), 2.98 (br, 1H, OH), 3.76 (t, J(H,H) = 5.4 Hz, 2H, CH2), 3.97
(t, 3J(H,H) = 6.2 Hz, 2H, CH2), 4.18 (dd, 3J(H,H) = 12.6, 9.2 Hz, 2H,
CH2), 4.38 (dd, 3J(H,H) = 12.6, 9.2 Hz, 2H, CH2), 6.87−6.92 (m, 2H,
CHAr), 8.85 (s, 1H, CH); 13C NMR (100 MHz, CDCl3) 18.6, 21.1,
29.0, 46.8, 49.6, 51.0, 59.1, 130.0, 130.6, 135.5, 140.3, 158.5.
Synthesis of [H2L3]Cl. In a 100 mL round-bottomed flask N1-(2,4,6-
trimethylphenyl)-N2-(2-hydroxybenzyl)-1,2-diaminoethane (2.0 g,
7.03 mmol) was dissolved in 50 mL of Et2O. Then HCl (10 mL,
2.5 equiv) 1 M in dioxane was added (dropwise), and the mixture was
stirred for 2 h. Next, the white precipitate was filtered and washed with
cold Et2O, triethyl orthoformate (5.23 mL, 5 equiv) was added, and
the resulting mixture was stirred for 12 h at 100 °C. After that, the
reaction mixture was cooled to room temperature and the resulting
beige precipitate was filtered through the Schott filter and washed with
cold n-hexane, giving the desired product [H2L3]Cl as white crystals
which were dried in vacuo (1.8 g, 77%); mp 249−251 °C; MS (ESI)
m/z 295.2 [M − Cl]+; HRMS calcd 295.1810, found 295.1801; H
1
NMR (400 MHz, C2D6SO, 100 °C) 2.24 (s, 6H, CH3), 2.27 (s, 3H,
CH3), 3.87−4.20 (m, 4H, CH2), 4.72 (s, 2H, CH2), 6.80−6.89 (m,
1H, CHAr), 7.03−7.07 (m, 3H, CHAr), 7.19−7.34 (m, 2H, CHAr), 9.02
(s, 1H, CH), 10.37 (br, 1H, OH); 13C NMR (100 MHz, C2D6SO, 100
°C) 17.2, 20.5, 47.7, 50.4, 115.7, 119.1, 129.3, 130.1, 130.7, 131.2,
135.5, 139.2, 156.4, 159.3.
1
7.80; N, 8.46. H NMR (CD2Cl2, 400 MHz): −0.88 (s, 6H, GaCH3),
2.18 (s, 6H, CH3), 2.29 (s, 3H, CH3), 3.35 (m, 2H, CH2), 3.70 (m,
2H, CH2), 3.85 (m, 2H, CH2), 4.08 (m, 2H, CH2), 6.93 (s, 2H, CHAr).
13C{1H} NMR (CD2Cl2, 50 MHz): −2.5 (GaMe3), 18.2, 21.1, 49.4,
49.7, 51.2, 130.2, 130.3, 130.8, 135.7, 140.9, 158.6, 177.3. Because of
its intensity, the signal at 177.3 ppm should not be interpreted as a
carbene carbon signal. The carbene carbon signal, expected at around
200 ppm, was not observed despite long measurement times.
Synthesis of [H2L4]Cl. In a 100 mL round-bottomed flask N1-(2,6-
diisopropylphenyl)-N2-(2-hydroxybenzyl)-1,2-diaminoethane (1.8 g,
5.51 mmol) was dissolved in 50 mL of Et2O. Then HCl (8.27 mL,
3 equiv) 1 M in dioxane was added (dropwise), and the mixture was
stirred for 2 h. Next the white precipitate was filtered and washed with
cold Et2O, triethyl orthoformate (4.7 mL, 5 equiv) was added, and the
resulting mixture was stirred for 12 h at 100 °C. After that the reaction
mixture was cooled to room temperature and the resulting beige
precipitate was filtered through the Schott filter and washed with cold
n-hexane, giving the desired product [H2L4]Cl as white crystals which
were dried in vacuo (1.75 g, 85%): mp 282−284 °C; MS (ESI) m/z
Synthesis of [(Me2Ga(HL2))I]. A stirred solution of [H2L2]I (810.1
mg, 2.16 mmol) in CH2Cl2 (10 mL) was cooled to −70 °C, and 1 mL
of a CH2Cl2 solution of Me3Ga (248.0 mg, 2.16 mmol) was added
dropwise. After addition, the cooling bath was removed and the
reaction mixture was warmed. Before the reaction mixture reached
room temperature, the evolution of gas and formation of a white
precipitate was observed. Then the reaction mixture was stirred for 1 h
and the solvent and volatile residues were removed under vacuum to
give [(Me2Ga(HL2))I)] as a white crystalline solid in essentially
quantitative yield. 1H NMR (CD2Cl2, 400 MHz): −0.17 (s, 6H,
GaCH3), 1.97 (m, 2H, CH2), 2.30 (s, 3H, CH3), 2.32 (s, 6H, CH3),
3.79 (br, 2H, CH2), 3.95 (br t, 2H, CH2), 4.16−4.30 (m, 4H, CH2),
6.97 (s, 2H, CHAr), 9.23, 9.43 (br, 1H, CH).
337.2 [M − Cl]+; HRMS calcd 337.2280, found 337.2264; H NMR
1
(400 MHz, C2D6SO) 1.12−1.27 (m, 12H, CH3); 2.87−2.97 (m, 1H,
CH), 3.95−4.16 (m, 4H, CH2),), 4.75 (s, 2H, CH2), 6.85 (td, 1H,
3J(H,H) = 8.4, 1.2 Hz, CHAr), 7.08−7.14 (m, 1H, CHAr), 7.25 (td, 1H,
3J(H,H) = 9.2, 1.6, CHAr), 7.28−7.38 (m, 3H, CHAr), 7.46−7.52 (m,
1H, CHAr), 9.18 (s, 1H, CH), 10.34 (br, 1H, OH); 13C NMR (100
MHz, C2D6SO) 23.9, 25.1, 28.2, 47.7, 48.2, 53.4, 116.1, 119.3, 119.4,
124.9, 130.5, 130.8, 130.9, 131.0, 146.8, 156.9, 159.2.
Synthesis of 2. [H2L2]I (200.0 mg, 0.53 mmol) and KH (21.4 mg,
0.53 mmol) were placed in a Schlenk vessel, and toluene (20 mL) was
added at room temperature. After subsequent addition of around 5
mol % of KOtBu the evolution of gas was observed and the reaction
mixture was stirred for 3 h until gas evolution essentially ceased. The
solution was filtered through a 0.45 μm hydrophobic syringe filter, and
the solvent and volatile residues were then removed under vacuum to
Synthesis of [H2L5]Cl. N1-(2,6-Diisopropylphenyl)-N2-(2-hydroxy-
benzyl)-1,2-diaminoethane (1.7 g, 5.21 mmol) was dissolved in 4.3 mL
(5 equiv) of triethyl orthoformate in a 25 mL round-bottomed flask,
and then 2.86 mL (2.2 equiv) of 4 M HCl in dioxane was added and
the resulting mixture was stirred for 12 h at 100 °C. After that, the
reaction mixture was cooled to room temperature and the resulting
beige precipitate was filtered through the Schott filter and washed with
cold n-hexane, giving white crystals which were dried in vacuo. The
resulting NHC salt was used in the next step without purification. A
100 mL Schlenk was equipped with a stirring bar, and then the
imidazolinium salt (1.43 g, 3.85 mmol) was dissolved in 20 mL of
CH2Cl2 (DCM, dry) under an Ar atmosphere and the resulting
solution was cooled to 0 °C. BBr3 (7.71 mL, 1 mol/L in n-hexane, 2
equiv) was added dropwise and the mixture was stirred for 30 min.
After that, the ice bath was removed and the resulting mixture was
stirred at room temperature. Control of the reaction was carried out by
TLC (DCM/MeOH 9/1). After 6 h a saturated solution of aqueous
sodium bicarbonate was added (100 mL). The CH2Cl2 layer was
separated, and the aqueous layer was washed with EtOAc (2 × 20
mL). The combined organic phases were dried over anhydrous
MgSO4, filtered, and concentrated. Purification of the crude mixture by
1
give a pale yellow oil of [HL2] in 92% yield (120 mg). H NMR
(toluene-d8, 200 MHz): 0.58 (m, 1H, CH2), 1.70 (m, 1H, CH2), 2.15
(s, 3H, CH3), 2.41 (s, 6H, CH3), 2.70−2.85 (m, 2H, CH2), 2.98−3.45
(m, 5H, CH2), 3.76 (m, 2H, CH2), 3.85 (m, 2H, CH2), 5.05 (s, 1H,
CH), 6.78 (s, 2H, CHAr). To a stirred solution of [HL2] (110.0 mg,
0.45 mmol) in toluene (5 mL) was added Me3Ga (50 mg, 0.44 mmol)
at −70 °C. The reaction mixture was warmed, but no gas evolution
was observed until room temperature. Heating to around 50 °C
resulted in gas evolution, which ceased after 30 min. Next, the solvent
and volatile residues were removed under vacuum to give a crystalline
solid, which was subsequently crystallized from toluene/n-hexane to
give a white crystalline solid (70 mg) in 88% yield, based on Ga
content. Anal. Calcd for 2, C20H36Ga2N2O: C, 52.23; H, 7.89; N, 6.09.
1
Found: C, 52.09; H, 7.88; N, 6.05. H NMR (toluene-d8, 400 MHz):
−0.40 (s, 6H, GaCH3), 0.02 (s, 9H, GaCH3), 1.49 (m, 2H, CH2), 1.88
(s, 6H, CH3), 2.02 (s, 3H, CH3), 2.63 (m, 2H, NCH2), 2.79 (m, 2H,
NCH2), 3.28 (br t, 3J(H,H) = 5.9 Hz, 2H, NCH2), 3.76 (t, 3J(H,H) =
108
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5.9 Hz, 2H, OCH2), 6.58 (s, 2H, CHAr), 13C{1H} NMR (toluene-d8,
100 MHz): −6.7, −3.2, (GaMe3), 17.4, 20.8, 29.1, 43.4, 49.5, 50.6,
60.2, 129.7, 133.9, 135.7, 139.1, 199.3 (carbene).
Synthesis of 4. A stirred solution of [H2L4]Cl (1.04 g, 2.79 mmol)
in CH2Cl2 (15 mL) was cooled to −70 °C, and 1 mL of a CH2Cl2
solution of Me3Ga (320.0 mg, 2.79 mmol) was added. After addition,
the cooling bath was removed and the reaction mixture was warmed.
Before the reaction mixture reached room temperature, the evolution
of gas and formation of a white precipitate was observed. Then the
reaction mixture was stirred for 1 h and the solvent and volatile
residues were removed under vacuum to give [(Me2Ga(HL4))Cl)] as
a white crystalline solid in essentially quantitative yield. 1H NMR
(CD2Cl2, 400 MHz): −0.42 (s, 6H, GaCH3), 1.25 1.26 (d, 3J(H,H) =
Reaction of Me2Ga(O,O′)(SIMes) with Me3Ga. To a stirred
solution of Me2Ga(O,O′)(SIMes) ((HO,O′) = (S)-methyl lactate)
(302.1 mg, 0.59 mmol) in toluene (10 mL) was added 1 mL of a
toluene solution of Me3Ga (68.2 mg, 0.59 mmol) dropwise at room
temperature. Then the reaction mixture was stirred for 10 min and the
solvent and volatile residues were removed under vacuum to give a
white crystalline solid. The product was dissolved and crystallized from
toluene/hexane to give colorless crystals of Me3Ga(SIMes) (205 mg,
3
6.7 Hz, 12H, CH3), 2.85 (sept, J(H,H) = 6.7 Hz, 2H, CH), 4.05 (m,
3
1
4H, CH2), 4.68 (s, 2H, CH2), 6.63 (td, J(H,H) = 7.4, 1.2 Hz, 1H,
82%). H NMR (toluene-d8, 400 MHz): −0.74 (s, 9H, GaCH3), 2.09
CHAr), 6.71 (dd, 3J(H,H) = 8.2, 1.2 Hz, 1H, CHAr), 7.12 (dd, 3J(H,H)
(s, 6H, CH3), 2.20 (s, 12H, CH3), 3.09 (s, 4H, CH2), 6.74 (s, 4H,
CHAr) 13C{1H} NMR (toluene-d8, 100 MHz): −5.8 (GaMe3), 17.9,
21.0, 50.9, 128.5, 129.6, 135.6, 136.0, 138.3, 206.1 (carbene).
= 7.4, 2.0 Hz, 1H, CHAr), 7.18 (m, 1H, CHAr), 7.27 (d, 3J(H,H) = 7.8
3
Hz, 2H, CHAr), 7.46 (t, J(H,H) = 7.8 Hz, 1H, CHAr), 8.85 (s, 1H,
CH). Next, [(Me2Ga(HL4))Cl)] (1.037 g, 2.20 mmol) and KH (88.1
mg, 2.20 mmol) were placed in a Schlenk vessel, and toluene (30 mL)
was added at room temperature. After subsequent addition of around 5
mol % of KOtBu the evolution of gas was observed and the reaction
mixture was stirred for 2 h until gas evolution essentially ceased. The
solution was filtered through a 0.45 μm hydrophobic syringe filter, and
the solvent and volatile residues were then removed under vacuum to
give a pale yellow crystalline solid. The obtained solid was
subsequently dissolved in 3 mL of toluene, and the slow addition of
15 mL of n-hexane resulted in the formation of a white crystalline
solid, which was washed twice with 10 mL of n-hexane and dried under
vacuum to give 4 in 47% yield (450 mg). Anal. Calcd for 4,
C24H33GaN2O: C, 66.23; H, 7.64; N, 6.44. Found: C, 66.36; H, 7.64;
Synthesis of 3. A stirred solution of [HL3]Cl (546.4 mg, 1.65
mmol) in CH2Cl2 (10 mL) was cooled to −50 °C, and 1 mL of a
CH2Cl2 solution of Me3Ga (190.0 mg, 1.65 mmol) was added. After
addition, the cooling bath was removed and the reaction mixture was
warmed. Before the reaction mixture reached room temperature, the
evolution of gas and formation of a white precipitate was observed.
Then the reaction mixture was stirred for 1 h and the solvent and
volatile residues were removed under vacuum to give a white
crystalline solid of [(Me2Ga(HL3))Cl] in essentially quantitative yield.
1H NMR (CD2Cl2, 200 MHz): −0.40 (s, 6H, GaCH3), 2.27 (s, 6H,
CH3), 2.31 (s, 3H, CH3), 4.00 (s, 4H, CH2), 4.65 (s, 2H, CH2), 6.58−
6.72 (m, 2H, CHAr), 6.99 (s, 2H, CHAr), 7.08−7.21 (m, 2H, CHAr),
8.73 (s, 1H, CH). Next, [(Me2Ga(HL3))Cl)] (500.0 mg, 1.16 mmol)
and KH (46.5 mg, 1.16 mmol) were placed in a Schlenk vessel and
toluene (20 mL) was added at room temperature. After subsequent
addition of around 5 mol % of KOtBu the evolution of gas was
observed and the reaction mixture was stirred for 2 h until gas
evolution essentially ceased. The solution was filtered through a 0.45
μm hydrophobic syringe filter, and the solvent and volatile residues
were then removed under vacuum to give a yellow oil. The obtained
oil was subsequently dissolved in 3 mL of toluene, and the slow
addition of 20 mL of n-hexane resulted in the formation of a white
crystalline solid, which was washed twice with 10 mL of n-hexane and
dried under vacuum to give 3 in 46% yield (210 mg). Anal. Calcd for
3, C21H27GaN2O: C, 64.15; H, 6.92; N, 7.12. Found: C, 64.15; H,
7.05; N, 7.14. 1H NMR (toluene-d8, 400 MHz): −0.34 (s, 6H,
GaCH3), 1.80 (s, 6H, CH3), 1.98 (s, 3H, CH3), 2.78 (m, 2H, CH2),
3.00 (m, 2H, CH2), 4.22 (br, 2H, CH2), 6.56 (s, 2H, CHAr), 6.67 (td,
1
N, 6.26. H NMR (toluene-d8, 400 MHz): −0.35 (s, 6H, GaCH3),
0.92, 1.10 (d, 3J(H,H) = 6.7 Hz, 12H, CH3), 2.56 (sept, 3J(H,H) = 6.7
Hz, 2H, CH), 3.00 (m, 4H, CH2), 4.30 (br, 2H, CH2), 6.62 (td,
3
3J(H,H) = 7.4, 1.2 Hz, 1H, CHAr), 6.82 (dd, J(H,H) = 7.4, 2.0 Hz,
1H, CHAr), 6.84 (d, 3J(H,H) = 7.8 Hz, 2H, CHAr), 7.06 (t, 3J(H,H) =
7.8 Hz, 1H, CHAr), 7.12 (m, 1H, CHAr), 7.20 (m, 1H, CHAr) 13C{1H}
NMR (toluene-d8, 100 MHz): −5.8 (GaMe3), 23.7, 24.9, 28.5, 49.0,
50.1, 53.3, 114.6, 122.9, 124.1, 124.4, 129.6, 130.0, 130.7, 134.3, 146.5,
165.9, 198.2 (carbene).
Second Approach. [H2L4]Cl (464.0 mg, 1.24 mmol) and KH (50.0
mg, 1.25 mmol) were placed in a Schlenk vessel, and toluene (20 mL)
was added at room temperature. After subsequent addition of a
catalytic amount of KOtBu (<5%) the evolution of gas was observed
and the reaction mixture was stirred for 1.5 h until gas evolution
essentially ceased. The solution was filtered through a 0.45 μm
hydrophobic syringe filter, and the solvent and volatile residues were
then removed under vacuum to give a yellow oily solid of [HL4] in
3
3J(H,H) = 7.3, 1.5 Hz, 1H, CHAr), 6.91 (dd, J(H,H) = 7.3, 2.0 Hz,
1H, CHAr), 7.14 (dd, 3J(H,H) = 8.3, 1.5 Hz, 1H, CHAr), 7.21 (m, 1H,
CHAr), 13C{1H} NMR (toluene-d8, 100 MHz): −7.2 (GaMe3), 17.4,
20.8, 49.8, 50.0, 50.1, 115.3, 122.8, 126.0, 129.5, 130.7, 134.4, 135.9,
138.7, 166.0, 198.4 (carbene).
1
65% yield (272 mg). H NMR (toluene-d8, 400 MHz): 1.21, 1.31 (d,
3
3J(H,H) = 6.7 Hz, 12H, CHCH3), 2.92 (t, J(H,H) = 6.7 Hz, 2H,
CH2), 3.20 (t, 3J(H,H) = 6.7 Hz, 2H, CH2), 3.66 (sept, 3J(H,H) = 6.7
Hz, 2H, CHCH3), 3.79 (s, 2H, CH2), 5.75 (s, 1H, CH), 6.77 (m, 2H,
Second Approach. [H2L3]Cl (724.0 mg, 2.19 mmol) and KH (88.0
mg, 2.19 mmol) were placed in a Schlenk vessel, toluene (20 mL) was
added at room temperature, and evolution of gas was observed. The
reaction mixture was stirred for 2 h until gas evolution essentially
ceased and the yellow solution was filtered through a 0.45 μm
hydrophobic syringe filter, and the solvent and volatile residues were
then removed under vacuum to give the yellow crystalline solid [HL3]
in 60% yield (391 mg). 1H NMR (toluene-d8, 400 MHz): 2.15 (s, 3H,
CH3), 2.32 (s, 6H, CH3), 2.91 (m, 2H, CH2), 3.07 (m, 2H, CH2), 3.83
(s, 2H, CH2), 5.90 (br, 1H, CH), 6.73−6.79 (m, 4H, CHAr), 6.87 (br
d, 1H, CHAr), 6.98−7.10 (m, 2H, CHAr). To a stirred solution of
[HL3] (612.0 mg, 2.08 mmol) in toluene (20 mL) was added Me3Ga
(239.0 mg, 2.08 mmol) at −50 °C to give a white precipitate. The
reaction mixture was warmed and slowly heated, and gas evolution was
observed at around 50 °C. Then, the reaction mixture was stirred at 80
°C for 3.5 h, at which point evolution of gas essentially ceased. Next
the solvent and volatile residues were removed under vacuum to give a
yellow oil. Crystallization from a toluene/n-hexane solution followed
by washing of the yellow oily precipitate with cold toluene (cooled to
around −70 °C) and n-hexane gave colorless crystals of 3 in 15% yield
3
CHAr), 6.77 (m, 2H, CHAr), 6.88 (d, J(H,H) = 6.7 Hz, 1H, CHAr),
6.97−7.12 (m, CHAr) 7.20 (m, 1H, CHAr). To the stirred solution of
[HL4] (250.0 mg, 0.75 mmol) in toluene (10 mL) was added Me3Ga
(86 mg, 0.75 mmol) at −70 °C. The reaction mixture was warmed, but
no gas evolution was observed until room temperature. Heating to
around 90 °C resulted in only a slight evolution of gas, which ceased
after 0.5 h. Next the solvent and volatile residues were removed under
1
vacuum to give a yellow oily solid. H NMR of the reaction mixture
was complex (see the Supporting Information), indicating the
presence of unreacted CH of [HL4].
Synthesis of 5. A stirred solution of [H2L5]Cl (640 mg, 1.78 mmol)
in CH2Cl2 (15 mL) was cooled to −70 °C, and 1 mL of a CH2Cl2
solution of Me3Ga (205.0 mg, 1.79 mmol) was added. After addition,
the cooling bath was removed and the reaction mixture was warmed.
Before the reaction mixture reached room temperature, the evolution
of gas and formation of a white precipitate was observed. Then the
reaction mixture was stirred for 1 h and the solvent and volatile
residues were removed under vacuum to give [(Me2Ga(HL5))Cl] as a
white crystalline solid in essentially quantitative yield. 1H NMR
3
(CD2Cl2, 400 MHz): −0.21 (s, 6H, GaCH3), 1.31 (d, J(H,H) = 6.7
1
3
(125 mg). H 13C NMR was the same as that reported above for 3.
Hz, 12H, CH3), 3.01 (br sept, J(H,H) = 6.7 Hz, 2H, CH), 4.20 (m,
109
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Organometallics
Article
3
2H, CH2), 4.65 (m, 2H, CH2), 6.78 (br t, J(H,H) = 7.4, 1H, CHAr),
Å, β = 91.9516(7)°, V = 2285.98(3) Å3, Z = 4, μ(Mo Kα) = 2.366
mm−1, Dcalc = 1.336 g cm−3, 141485 reflections measured, 6358 unique
(Rint = 0.0285), F(000) = 960.0, R1 = 0.0176 (I > 2σ(I)), wR2 =
0.0483 (all data).
3
3
6.94 (br d, J(H,H) = 8.2, 1H, CHAr), 7.04 (br d, J(H,H) = 8.2 Hz,
1H, CHAr), 7.13 (br d, 3J(H,H) = 7.4 Hz, 1H, CHAr), 7.32 (d, 3J(H,H)
= 7.8 Hz, 2H, CHAr), 7.51 (t, 3J(H,H) = 7.8 Hz, 1H, CHAr), 10.15 (br,
1H, CH). Next, [(Me2Ga(HL5))Cl] (502.9 mg, 1.10 mmol) and KH
(44.1 mg, 1.10 mmol) were placed in a Schlenk vessel and toluene (20
mL) was added at room temperature. After subsequent addition of
around 5 mol % of KOtBu the evolution of gas was observed and the
reaction mixture was stirred for 6 h until gas evolution essentially
ceased. The solution was filtered through a 0.45 μm hydrophobic
syringe filter, and the solvent and volatile residues were then removed
under vacuum to give a pale yellow oily solid. The obtained product
was purified by washing with a toluene/n-hexane (1/1) mixture cooled
to around 0 °C, and subsequent crystallization from toluene/n-hexane
solution gave 5 in 39% yield (180 mg). Anal. Calcd for 5,
C23H31GaN2O: C, 65.58; H, 7.42; N, 6.65. Found: C, 65.64; H,
7.44; N, 6.54. 1H NMR (toluene-d8, 400 MHz): −0.39 (s, 6H,
Crystal data for 3: C21H27GaN2O, Mr = 393.16, triclinic, space
group P1, a = 7.9057(4) Å, b = 8.0823(3) Å, c = 17.5181(5) Å, α =
̅
89.256(3)°, β = 82.908(3)°, γ = 61.207(4)°, V = 972.05(8) Å3, Z = 2,
μ(Mo Kα) = 1.426 mm−1, Dcalc = 1.343 g cm−3, 39584 reflections
measured, 5189 unique reflections (Rint = 0.0352), F(000) = 412.0, R1
= 0.0210 (I > 2σ(I)), wR2 = 0.0559 (all data).
Crystal data for 4: C24H33GaN2O, Mr = 435.24, monoclinic, space
group P21/c, a = 13.3098(3) Å, b = 12.43120(19) Å, c = 14.6027(3) Å,
β = 111.684(2)°, V = 2245.13(8) Å3, Z = 4, μ(Mo Kα) = 1.241 mm−1,
Dcalc = 1.288 g cm−3, 39832 reflections measured, 6233 unique
reflections (Rint = 0.0445), F(000) = 920.0, R1 = 0.0272 (I > 2σ(I)),
wR2 = 0.0673 (all data).
Crystal data for 5: C23H31GaN2O, Mr = 421.22, monoclinic, space
group P21/c, a = 12.2017(8) Å, b = 13.8740(9) Å, c = 13.9308(9) Å, β
= 113.816(2)°, V = 2157.5(2) Å3, Z = 4, μ(Mo Kα) = 1.290 mm−1,
Dcalc = 1.297 g cm−3, 37459 reflections measured, 5662 unique
reflections (Rint = 0.0249), F(000) = 888.0, R1 = 0.0251 (I > 2σ(I)),
wR2 = 0.0646 (all data).
3
GaCH3), 1.00, 1.20 (d, J(H,H) = 6.7 Hz, 12H, CH3), 2.85 (sept,
3J(H,H) = 6.7 Hz, 2H, CH), 3.16 (m, 4H, CH2), 6.53 (td, J(H,H) =
3
3
8.2, 1.6 Hz, 1H, CHAr), 6.66 (dd, J(H,H) = 7.4, 1.6 Hz, 1H, CHAr),
6.95 (d, 3J(H,H) = 7.8 Hz, 2H, CHAr), 7.05 (td, 3J(H,H) = 7.4, 1.6 Hz,
1H, CHAr), 7.10 (t, 3J(H,H) = 7.8 Hz, 1H, CHAr), 7.29 (dd, 3J(H,H) =
8.2, 1.6 Hz, 1H, CHAr) 13C{1H} NMR (toluene-d8, 100 MHz): −8.1
(GaMe2), 23.4, 25.9, 28.4, 49.0, 51.4, 114.9, 117.7, 123.1, 124.5, 126.6,
130.1, 134.1, 147.0, 158.0, 193.9 (carbene).
Crystal data for 6: 4(C27H42Ga2N2O)·C7H8, Mr = 2292.39,
triclinic, space group P1, a = 10.64328(19) Å, b = 17.3159(3) Å, c
̅
= 17.6567(3) Å, α = 98.6975(16)°, β = 103.4380(16)°, γ =
106.7528(17)°, V = 2946.31(10) Å3, Z = 1, μ(Mo Kα) = 1.850
mm−1, Dcalc = 1.292 g cm−3, 178277 reflections measured, 12415
unique reflections (Rint = 0.0681), F(000) = 1202.0, R1 = 0.0304 (I >
2σ(I)), wR2 = 0.0841 (all data).
Synthesis of 6. To a stirred solution of 4 (213.0 mg, 0.49 mmol) in
toluene (6 mL) was added 1 mL of a toluene solution of Me3Ga (56.0
mg, 0.49 mmol) dropwise at room temperature. Then the reaction
mixture was stirred for 10 min and the solvent and volatile residues
were removed under vacuum to give a white crystalline solid in
essentially quantitative yield. Anal. Calcd for 6, C27H42Ga2N2O: C,
Crystal data for Me3Ga(SIMes): C24H35GaN2, Mr = 421.26,
orthorhombic, space group Pca21, a = 17.6087(4) Å, b = 16.5494(4)
Å, c = 15.9289(3) Å, V = 4641.88(17) Å3, Z = 8, μ(Mo Kα) = 0.7107
mm−1, Dcalc = 1.206 g cm−3, 62740 reflections measured, 12156 unique
reflections (Rint = 0.0397), F(000) = 1792.0, R1 = 0.0269 (I > 2σ(I)),
wR2 = 0.0647 (all data).
1
58.95; H, 7.70; N, 5.09. Found: C, 59.97; H, 7.60; N, 5.06. H NMR
(toluene-d8, 400 MHz): −0.33 (br, 6H, GaCH3), 0.01 (s, 9H,
GaCH3), 0.84 (d, 3J(H,H) = 6.7 Hz, 6H, CH3), 0.96 (br d, 6H, CH3),
2.32 (br, 2H, CH), 2.80−2.97 (m, 4H, CH2), 3.5−4.9 (br, 2H, CH2),
6.69−6.76 (m, 2H, CHAr), 6.82 (d, 3J(H,H) = 7.8 Hz, 2H, CHAr), 7.01
(t, J(H,H) = 7.8 Hz, 1H, CHAr), 7.07 (br d, 2H, CHAr) 13C{1H}
3
ASSOCIATED CONTENT
* Supporting Information
■
NMR (toluene-d8, 100 MHz): −5.5 (br), −1.9 (GaMe3), 23.2, 25.5,
28.4, 49.0, 50.4, 53.3, 120.0, 124.4, 125.6, 128.3, 128.5, 129.8, 129.9,
130.3, 133.7, 146.4, 159.3, 200.5 (carbene).
S
CIF files of 1−6 and Me3Ga(SIMes), synthetic details of NHC
salts, NMR FTIR data, and calculations of bond valence vectors
for gallium centers. This material is available free of charge via
the Internet at http://pubs.acs.org.
Crystallographic Studies. Single crystals of 1−6 and Me3Ga-
(SIMes) 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 at
100(2) K with graphite-monochromated Mo Kα radiation on an
Oxford 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.37 Absorption effects
were corrected analytically from the crystal shape. The structures were
solved by direct methods using the SHELXS-97 structure solution
program and refined by full-matrix least squares against F2 with
SHELXL-201338 and OLEX2.39 All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were added to the structure model at
geometrically idealized coordinates and refined as riding atoms. The
unit cell of 6 contains a toluene molecule which is disordered about a
center of inversion, with the center of gravity of the six-membered ring
displaced slightly from the inversion center. The atoms of one entire
toluene molecule were defined with the site occupation factors equal
to 0.5. The atoms of the six-membered ring of the toluene molecule
were constrained to an ideal hexagon, while neighboring atoms within
each orientation of the disordered toluene molecule were restrained to
have similar atomic displacement parameters.
AUTHOR INFORMATION
Corresponding Author
*E-mail for P.H.: horeglad@chem.uw.edu.pl.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors wish to thank the Foundation for Polish Science
“Team Programme” cofinanced by the EU European Regional
Development Fund, Operational Program Innovative Economy
2007/2013, for financial support.
REFERENCES
■
(1) See for instance: (a) Nolan, S. P., Ed. N-Heterocyclic Carbenes in
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