Synthesis, characterization and catalytic activity of stable [(NHC)H][ZnXY2] (NHC?=?N-Heterocyclic carbene, X, Y?=?Cl, Br) species

Article abstract of DOI:10.1016/j.molcata.2016.05.031

The synthesis and characterization of imidazol(in)ium-based zinc(II) halide salts are reported. These compounds present interesting structural features and exhibit high stability. Their catalytic activity was explored in the methylation of amines with CO₂ and PhSiH₃.

Full text of DOI:10.1016/j.molcata.2016.05.031

Accepted Manuscript
Title: Synthesis, characterization and catalytic activity of
stable [(NHC)H][ZnXY2] (NHC = N-Heterocyclic carbene, X,
Y = Cl, Br) species
Author: Orlando Santoro Fady Nahra David B. Cordes
Alexandra M.Z. Slawin Steven P. Nolan Catherine S.J. Cazin
PII:
S1381-1169(16)30213-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.05.031
MOLCAA 9906
To appear in:
Journal of Molecular Catalysis A: Chemical
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Revised date:
Accepted date:
20-4-2016
30-5-2016
31-5-2016
Please cite this article as: Orlando Santoro, Fady Nahra, David B.Cordes, Alexandra
M.Z.Slawin, Steven P.Nolan, Catherine S.J.Cazin, Synthesis, characterization
and catalytic activity of stable [(NHC)H][ZnXY2] (NHC=N-Heterocyclic
carbene, X, Y=Cl, Br) species, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.05.031
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Synthesis, characterization and catalytic activity of stable
[(NHC)H][ZnXY2] (NHC = N-Heterocyclic carbene, X, Y =
Cl, Br) species
Orlando Santoro,a Fady Nahra,b David B. Cordes,a Alexandra M. Z. Slawin,a Steven P.
Nolanb,c and Catherine S. J. Cazina*ccazin@gmail.com
aEaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, UK.
bUniversiteit Gent, Department of Inorganic and Physical Chemistry, Krijgslaan 281, S-3, B-
9000 Ghent, Belgium.
cKing Saud University, Department of Chemistry, College of Sciences, P.O. Box 2455,
Riyadh, 11451, Saudi Arabia.
Corresponding Author
Dr. Catherine Cazin
Tel: +44 (0)1334 464 808; Fax: +44 (0)1334 463 808

Graphical Abstract

Highlights
• A library of stable [(NHC)H][ZnXY2] species was synthesized
• The structure of the salts was unambiguously assessed by X-ray diffraction analyses
• The catalytic activity of the novel species was tested in the methylation of amines with
CO2 as carbon feedstock

Abstract
The synthesis and characterization of imidazol(in)ium-based zinc(II) halide salts are reported.
These compounds present interesting structural features and exhibit high stability. Their
catalytic activity was explored in the methylation of amines with CO2 and PhSiH3.
Keywords
N-Heterocyclic carbenes, Zinc, methylation, CO2

1. Introduction
During the past decades, transition metal complexes bearing NHC ligands (NHC = N-
Heterocyclic Carbene) have gained increasing attention. Indeed, these have shown to be
efficient catalysts in several reactions as well as enabling interesting synthetic applications
[1]. In order to develop more cost-effective processes, the use of complexes featuring
inexpensive and Earth-abundant metals has been widely investigated [2]. In this context,
Zn(II)-NHC complexes represent a remarkable example. They have shown to be particularly
active in reactions involving the fixation of CO2, such as the methylation of amines [3], or the
synthesis of cyclic carbonates from epoxides [4]. In addition, they are employed in the
synthesis of polyurethanes [5] and in the polymerization of D,L-lactide [6]. To date, the
synthesis of Zn(II)-NHC complexes involves the addition of a free carbene to a zinc salt. The
carbene can either be a priori isolated or generated in situ from an appropriate precursor
(NHC salt or NHC-CO2 adduct) [3-7]. Alternatively, the NHC salt can be directly reacted
with diethylzinc, affording a mixed halide-ethyl complex [7b]. In spite of their general
applicability, these procedures involve the use of sensitive and/or pyrophoric precursors and,
consequently, require strictly anaerobic conditions. The development of a straightforward
synthetic procedure leading to Zn(II)-NHC complexes would be most useful. Herein we
report synthetic attempts towards Zn(II)-NHC complexes that led to the isolation of highly
air- and moisture-stable [(NHC)H][ZnXY2] (X = Cl, Br) species. Such compounds were fully
characterized and their catalytic activity in the methylation of amines with CO2 was
investigated.
2. Experimental
2.1 General Considerations
All reactions were carried out in air unless otherwise stated. Chemicals were used as received
1
unless otherwise noted. Dry solvents were obtained from a solvent purification system. H
and 13C-{1H} Nuclear Magnetic Resonance (NMR) spectra were recorded on a Bruker-400
MHz or 300 MHz spectrometers using the residual solvent peak as reference (CDCl3: δH =
7.26 ppm, δC = 77.16 ppm) at 298K. Elemental analyses were performed by the London
Metropolitan University.
2.2 General procedure for the synthesis of the zincates 4-7

A 20 mL vial was charged with the NHC·HCl (300 mg, 1 equiv.) and the zinc salt (1 equiv.).
Tetrahydrofuran (5 mL) was added, the vial was sealed with a screw-cap and the reaction was
stirred at 60 ⁰C for two hours. The mixture was allowed to reach room temperature and the
solvent was removed under reduced pressure to afford the desired product.
2.2.1 Synthesis of [IPrH][ZnCl3] (4a)
Colorless solid (368 mg, 93%). 1H NMR (400 MHz, CDCl3, 298 K): δ = 1.14 (d, 3JH-H = 6.9
3
3
Hz, 12H, CH-CH3), 1.31 (d, JH-H = 6.9 Hz, 12H, CH-CH3), 2.53 (sept, JH-H = 6.9 Hz, 4H,
3
3
CH-CH3), 7.32 (d, JH-H = 7.7 Hz, 4H, CH phenyl), 7.55 (t, JH-H = 7.8 Hz, 2H, CH phenyl),
8.00 (s, 1H, H2), 8.46 (s, 2H, H4 and H5). 13C-{1H} NMR (75 MHz, CDCl3, 298 K): δ = 24.0
(s, CH-CH3), 25.0 (s, CH-CH3), 29.0 (s, CH-CH3), 124.8 (s, CH Ar), 128.6 (s, CH Ar), 129.9
(s, CIV), 132.2 (s, CIV), 134.3 (s, C2), 145.6 (s, C4 and C5). Anal. Calcd for C27H37Cl3N2Zn: C,
57.77; H, 6.64; N, 4.99. Found: C, 57.84; H, 6.61; N, 5.05.
2.2.2 Synthesis of [IPrH][ZnClBr2] (4b)
Colorless solid (414 mg, 90%). 1H NMR (400 MHz, CDCl3, 298 K): δ = 1.16 (d, 3JH-H = 6.9
3
3
Hz, 12H, CH-CH3), 1.30 (d, JH-H = 6.9 Hz, 12H, CH-CH3), 2.51 (sept, JH-H = 6.9 Hz, 4H,
3
3
CH-CH3), 7.33 (d, JH-H = 7.7 Hz, 4H, CH phenyl), 7.56 (t, JH-H = 7.8 Hz, 2H, CH phenyl),
8.19 (s, 1H, H2), 8.37 (s, 2H, H4 and H5). 13C-{1H} NMR (75 MHz, CDCl3, 298 K): δ = 24.1
(s, CH-CH3), 24.4 (s, CH-CH3), 29.1 (s, CH-CH3), 124.9 (s, CH Ar), 128.3 (s, CH Ar), 129.8
(s, CIV), 132.3 (s, CIV), 134.9 (s, C2), 145.4 (s, C4 and C5). Anal. Calcd for C27H37Br2ClN2Zn:
C, 49.87; H, 5.74; N, 4.31. Found: C, 50.04; H, 5.81; N, 4.45.
2.2.3 Synthesis of [SIPrH][ZnCl3] (5a)
Colorless solid (363 mg, 92%). 1H NMR (400 MHz, CDCl3, 298 K): δ = 1.18 (d, 3JH-H = 6.8
3
3
Hz, 12H, CH-CH3), 1.40 (d, JH-H = 6.8 Hz, 12H, CH-CH3), 3.14 (sept, JH-H = 7.3 Hz, 4H,
CH-CH3), 4.97 (s, 4H, H4 and H5), 7.25 (d, 3JH-H = 7.7 Hz, 4H, CH phenyl), 7.36 (s, 1H, H2),
3
7.44 (t, JH-H = 7.8 Hz, 2H, CH phenyl). 13C-{1H} NMR (75 MHz, CDCl3, 298 K): δ = 24.0
(s, CH-CH3), 25.6 (s, CH-CH3), 29.1 (s, CH-CH3), 55.7 (s, C4 and C5), 125.0 (s, CH Ar),
129.5 (s, CH Ar), 131.5 (s, CIV), 146.8 (s, CIV), 156.6 (s, C2). Anal. Calcd for C27H39Cl3N2Zn:
C, 57.57; H, 6.98; N, 4.97. Found: C, 57.38; H, 6.83; N, 5.05.

2.2.4 Synthesis of [SIPrH][ZnClBr2] (5b)
Colorless solid (408 mg, 89%). 1H NMR (400 MHz, CDCl3, 298 K): δ = 1.18 (d, 3JH-H = 6.8
3
3
Hz, 12H, CH-CH3), 1.40 (d, JH-H = 6.8 Hz, 12H, CH-CH3), 3.11 (sept, JH-H = 7.3 Hz, 4H,
CH-CH3), 4.94 (s, 4H, H4 and H5), 7.25 (d, 3JH-H = 7.7 Hz, 4H, CH phenyl), 7.38 (s, 1H, H2),
3
7.44 (t, JH-H = 7.8 Hz, 2H, CH phenyl). 13C-{1H} NMR (75 MHz, CDCl3, 298 K): δ = 24.1
(s, CH-CH3), 25.6 (s, CH-CH3), 29.1 (s, CH-CH3), 55.6 (s, C4 and C5), 125.1 (s, CH Ar),
129.4 (s, CH Ar), 131.6 (s, CIV), 146.7 (s, CIV), 156.8 (s, C2). Anal. Calcd for
C27H37BrCl2N2Zn: C, 49.72; H, 6.03; N, 4.29. Found: C, 49.60; H, 5.76; N, 4.77.
2.2.5 Synthesis of [IMesH][ZnCl3] (6a)
Colorless solid (370 mg, 88%). 1H NMR (400 MHz, CDCl3, 298 K): δ = 2.14 (s, 12H, CH3),
2.34 (s, 6H, CH3), 7.01 (s, 4H, CH phenyl), 7.89 (s, 2H, H4 and H5), 8.71 (s, 1H, H2). 13C-
{1H} NMR (75 MHz, CDCl3, 298 K): δ = 17.7 (s, CH3), 21.3 (s, CH3), 126.0 (s, CIV), 129.9
(s, CH Ar), 130.6 (s, CIV), 134.4 (s, CIV), 136.4 (s, C2), 141.3 (s, C4 and C5). Anal. Calcd for
C21H25Cl3N2Zn: C, 52.86; H, 5.28; N, 5.87. Found: C, 52.93; H, 5.11; N, 5.93.
2.2.6 Synthesis of [IMesH][ZnClBr2] (6b)
Colorless solid (428 mg, 86%). 1H NMR (400 MHz, CDCl3, 298 K): δ = 2.10 (s, 12H, CH3),
2.30 (s, 6H, CH3), 6.96 (s, 4H, CH phenyl), 7.75 (s, 2H, H4 and H5), 9.01 (s, 1H, H2). 13C-
{1H} NMR (75 MHz, CDCl3, 298 K): δ = 17.8 (s, CH3), 21.2 (s, CH3), 125.7 (s, CIV), 129.8
(s, CH Ar), 130.5 (s, CIV), 134.3 (s, CIV), 136.6 (s, C2), 141.2 (s, C4 and C5). Anal. Calcd for
C21H25Br2ClN2Zn: C, 44.56; H, 4.45; N, 4.95. Found: C, 44.61; H, 4.44; N, 5.13.
2.2.7 Synthesis of [IPr*H][ZnCl3] (7a)
1
Colorless solid (315 mg, 92%). H NMR (400 MHz, CDCl3, 298 K): δ = 2.15 (s, 6H, CH3),
5.29 (s, 4H, CH-Ph), 5.59 (s, 2H, H4 and H5), 6.80 (m, 12H, CH Ar), 7.12-7.25 (m, 32H, CH
Ar), 10.76 (s, 1H, H2). 13C-{1H} NMR (75 MHz, CDCl3, 298 K): δ = 21.9 (s, CH3), 51.4 (s,
CH3-Ph), 123.8 (s, C4 and C5), 125.7 (s, CIV), 127.0 (s, CH Ar), 127.1 (s, CH Ar),128.7 (s,
CH Ar), 128.8 (s, CH Ar), 129.3 (s, CH Ar), 130.1 (s, CH Ar), 130.3 (s, CH Ar), 131.1 (s,
CH Ar), 140.5 (s, CIV), 141.5 (s, C2), 141.9 (s, CIV), 142.6 (s, CIV). Anal. Calcd for
C69H57Cl3N2Zn: C, 76.32; H, 5.29; N, 2.58. Found: C, 76.23; H, 5.17; N, 2.67.
2.2.8 Synthesis of [IPr*H][ZnClBr2] (7b)
1
Colorless solid (331 mg, 89%). H NMR (400 MHz, CDCl3, 298 K): δ = 2.19 (s, 6H, CH3),
5.13 (s, 4H, CH-Ph), 5.71 (s, 2H, H4 and H5), 6.80 (m, 12H, CH Ar), 7.10 (m, 8H, CH Ar),
7.15-7.20 (m, 16H, CH Ar), 7.26-7.30 (m, 8H, CH Ar), 10.11 (s, 1H, H2). 13C-{1H} NMR (75

MHz, CDCl3, 298 K): δ = 22.0 (s, CH3), 51.5 (s, CH3-Ph), 124.4 (s, C4 and C5), 127.2 (s,
CIV), 127.4 (s, CH Ar), 128.9 (s, CH Ar), 128.9 (s, CH Ar), 129.2 (s, CH Ar), 129.7 (s, CH
Ar), 129.9 (s, CH Ar), 131.1 (s, CH Ar), 138.9 (s, CH Ar), 140.5 (s, CIV), 141.5 (s, C2), 141.9
(s, CIV), 142.3 (s, CIV). Anal. Calcd for C69H57Br2ClN2Zn: C, 70.54; H, 4.89; N, 2.38. Found:
C, 70.45; H, 4.75; N, 2.51.
2.3 General procedure for the Zn-catalyzed N-methylation of amines with CO2
Under an argon atmosphere, a 3 mL vial was charged with 6b (5 mol%), KOtBu (5 mol%)
and CPME (2 mL). Substrate 8 (0.28 mmol, 1 equiv.) and PhSiH3 (3 equiv.) were added and
the vial was sealed with a septum cap. The septum cap was pierced with a syringe needle and
placed into a six-slot steal autoclave. The autoclave was sealed, purged twice with CO2 and
heated at 100 ⁰C (oil bath) under CO2 atmosphere (1 bar) for 20 hours. After this time the
reaction mixture was allowed to cool and the gas was carefully released. The reaction mixture
was analyzed by gas chromatography (GC).
3. Results and Discussion
3.1 Synthetic attempts towards Zn(II)-NHC complexes
Recently, NHC complexes of gold(I) and copper(I) have been synthesized by reacting the
corresponding imidazol(in)ium salts with the metal precursors in the presence of K2CO3 in air
under mild conditions [8,9]. Based on these reports, attempts towards the synthesis of Zn(II)-
NHC complexes were carried out using a similar strategy (Scheme 1) [10]. NHC salts were
reacted with ZnBr2 or ZnCl2 in the presence of K2CO3 at 60 0C, under inert atmosphere [11].
Although full transformation of the starting NHC salt was achieved in all cases, the desired
1
Zn(II)-NHC complexes were not obtained. The H NMR spectra of the products showed the
presence of the acidic H signal (N-CH-N) significantly shifted upfield, suggesting the
formation of new imidazolium-based species [10].
Crystals suitable for X-ray analyses were obtained for compounds 1, 2 and 3, isolated from
the reaction of ZnBr2 with K2CO3 and IPr*·HCl, IPrCl·HCl and IPr·HBr, respectively (IPr* =

N,N’-bis-[2,6-bis-(diphenylmethyl)-4-methylphenyl]imidazol-2-ylidene, IPrCl = N,N’-bis-
[2,6-(di-iso-propyl)phenyl]-4,5-dichloroimidazol-2-ylidene and IPr = N,N’-bis-[2,6-(di-iso-
propyl)phenyl]imidazol-2-ylidene) [12]. X-ray analyses showed that the obtained compounds
were the zincate salts and not the expected Zn(II)-NHC complexes. The molecular structure
of 1 is reported in figure 1. In this species, two NHC moieties surround a tetrahedral [ZnX4]2-
unit, which acts as counterion [13]. Interesting short-contacts between imidazolium protons
and halides of the zinc moieties were observed (e.g. H(1)-Br(1) 2.787 Å), which would
probably justify the absence of deprotonation when K2CO3 is used. Compound 2 displayed
structural features similar to 1 [14]. In this case, the anionic dimer [(ZnBr2Cl)-]2 is in contact
with two IPrCl moieties. Short-contacts were found between the Cl atoms on the backbone of
the NHC and the halides of the zinc anion.
Compound 3 exhibited a peculiar molecular structure (Figure 2). In this species, a tetrahedral
[ZnBr4]2- anion is surrounded by four IPr moieties. Short-contacts between the Br atoms and
the hydrogen atoms on the backbone of the NHC were also observed (e.g. H(33)-Br(2) 2.726
Å) [15], further highlighting the stability of such species. Bond lengths and angles around the
imidazole unit of compounds 1-3 are similar to those reported for the corresponding NHC
salts [16].
3.2 Synthesis of a library of [(NHC)H][ZnXY2] salts
Compounds 1-3 are considered analogues to the metallate intermediates obtained during the
synthesis of Au(I)- and Cu(I)-NHC complexes via the reaction of NHC salts with gold and
copper precursors in the absence of a base [8,9]. In order to generate a library of zincate salts
that we could later use to generate the Zn(II)-complexes, we reacted various imidazol(in)ium
salts with ZnCl2 and ZnBr2 in the absence of base (Scheme 2). In all cases full conversion to
the corresponding zincates was observed.
All species were isolated in micro-analytically pure form and in good to excellent yields.
1
Similarly to zincates 1-3, the H NMR spectra of these species displayed an upfield shift of
the signal assigned to the acidic proton (N-CH-N) with respect to their corresponding
NHC·HX salts [17]. Synthetic attempts towards Zn(II)-NHC complexes by treatment of these
compounds with bases such as K2CO3 or KOtBu were unsuccessful, further underlining the
high stability of these salts [18]. Crystals suitable for X-ray analyses of 4a and 5b were
obtained by slow diffusion of pentane into a saturated solution of the salts in chloroform [12].
Their molecular structures are reported in Figure 3 and Figure 4, respectively.

In both species, two [ZnX3]- moieties are bound via bridging halides and interacting with two
[(NHC)H] cations. Similarly to compound 2, the sites of the anion in 5b are partially
occupied by both Cl and Br atoms. Short-contacts between the hydrogen atoms of the NHC
backbone and the halides of the counterion have been observed in both structures. Bond
lengths and angles of the NHC moieties are similar to those found in salts 1-3 [16]. It is clear
that the structure of the zincates and the nature of the short-contacts are highly dependent on
the NHC used (steric hindrance) and on the type of halides present on the zinc (mixed
bromide-chloride, fully brominated or chlorinated). In addition, in the absence of K2CO3,
only [(ZnX3)2]- dimers were observed as counter-ions, which indicated that the base was
probably responsible for the disproportionation of the [(ZnX3)2]- into the [ZnX4]2- observed in
compounds 1 and 3.
3.3 Zn-catalysed methylation of amines with CO2 as carbon feedstock
The catalytic activity of these interesting species was tested in the N-methylation of N-
methylaniline (8a) using CO2 (Table 1) [3,19]. Under the optimized conditions reported for
[Zn(Cl)2(IPr)] [3], none of the zincates 4a-7a were catalytically active (Table 1, entries
1-4) [20]. To our delight, in the presence of a catalytic amount of KOtBu (5 mol%), the
desired product 9a was obtained (Table 1, entry 5). It should also be noted that mixed
chloride-bromide salts performed better than their fully chlorinated or fully
brominated analogues (Table 1, entries 5-13). In particular, in the presence of
[IMesH][ZnClBr2] (6b) 80% conversion towards the desired methylated compound
was obtained (Table 1, entry 12). Finally, by adding a further equivalent of PhSiH3, full
conversion of the starting material was observed (Table 1, entry 14). With these optimal
conditions in hand, an exploration of the substrate scope was undertaken (Scheme 3). While
full conversion of N-methylaniline 8a into 9a was achieved, substrates 8b and 8c only
afforded 66% and 64% conversion, respectively, into the corresponding methylated products.
These results are in line with those reported for the same reaction catalyzed by [Zn(Cl)2(IPr)]
[3].

Thus, these observations clearly indicate that the air- and moisture-stable zincates could be
considered as viable alternatives to the highly sensitive well-defined Zn(II)-NHC systems.
Primary amines were also investigated. Under the optimal conditions, 39% conversion
towards the dimethylated compound 9d was observed when aniline 8d was used, whereas the
more hindered 2,6-diisopropylaniline 8e only afforded 9% conversion into the dimethylated
product 9e. Nevertheless, by doubling the amount of catalyst, KOtBu and phenylsilane, in
both cases, the formation of the dimethylated product drastically increased (48% for 9d and
70% for 9e).
Insights into the mechanism of this transformation can be obtained by analyzing the
composition of the reaction mixtures for substrates 8d and 8e (Table 2). Under the standard
conditions, the reaction mixture was composed of mono- and dimethylated amines (I and III)
and by the carbamate II, with the latter being the major product in both cases (Table 2,
entries 1 and 2).
When the amounts of catalyst, silane and base were doubled, the conversion of the substrate
into III improved and the formation of carbamate IV was also observed (Table 2, entries 3
and 4).
A possible mechanism for this transformation, based on our observations and those of
previous reports, is depicted in Scheme 4 [3,19]. The first step involves the incorporation of
CO2 into the primary aniline 8 affording carbamate II. The reduction of the latter under
hydrosilylation conditions leads to the formation of the corresponding methylated aniline I.
Similarly, the insertion of CO2 into the methylated aniline I generates amide IV, whose
reduction liberates the corresponding dimethylated compound III. Previous reports have also
identified species II and IV as possible intermediates of this transformation. Moreover, the
reduction of such carbonyl compounds under hydrosilylation conditions have also been
shown to yield the desired methylated products I and III [3,19]. Considering this pathway,
KOtBu could have two roles: 1) assist the in situ formation of Zn(II)-NHC and 2) accelerate
the hydrosilylation step. In fact, it has been shown that the use of a catalytic amount of an
alkoxide salt has a positive effect on the rate of reduction of carbonyl compounds via
hydrosilylation [21]. Both synthetic and computational works are ongoing in our laboratories
in order to identify the actual catalytically active species and to obtain a better understanding
of the mechanism of this transformation.

4. Conclusions
In conclusion, we have described the synthesis of [(NHC)H][ZnXY2] species. These
compounds present interesting structural and spectroscopic features. Their stability could be
explained by taking into account the short-distance interactions between the imidazol(in)ium
moieties and the central anion. In addition, these species were shown to be catalytically active
in the methylation of amines with CO2 and phenylsilane, proving to be stable alternatives to
the highly air- and moisture-sensitive well-defined Zn(II)-NHC complexes. The mechanism
of the methylation and other possible catalytic applications of these novel compounds are
being investigated in our laboratories.
Acknowledgements
The authors gratefully acknowledge the Royal Society (University Research Fellowship to
C.S.J.C) and the EPSRC (EP/J500549/1, EP/K00445X/1) for funding. S.P. N. acknowledges
support from KSU.
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[11] This methodology was first attempted under inert atmosphere because of the air-
sensitivity of the expected Zn-NHC complexes. Although acetone was first used, it was later
replaced by THF because the latter allowed an easier recovery of the product.
[12] CCDC-1447299 (1), CCDC-1447302 (2), CCDC-1447301 (3), CCDC-1447298 (4a) and
CCDC-1447300 (5b) contain the supplementary crystallographic data for this contribution.
[13] Ideally, both Cl and Br partially occupy the sites of the anion; it was not possible to
discriminate between the two halides in the refinement process.
[14] For the molecular structure of 2, see Supporting Information, section 6.
[15] All H-Br contacts drawn are shorter than the sum of the van der Waals radii of H and Br.
[16] A. J. Arduengo III, R. Krafczyk, R. Schmutzler, Tetrahedron, 55 (1999) 14523.
[17] For further details, see Supporting Information, Section 2.
[18] See Supporting Information, Table S1.
[19] For more examples of N-methylation of amines with CO2 see: (a) Y. Fang, X. Li, K.
Junge, M. Beller, Angew. Chem. Int. Ed., 52 (2013) 9568. (b) T. Beydoun, K. vom Stein, J.
Klankermayer, W. Leitner, Angew. Chem. Int. Ed., 52, (2013) 9554. (c) Y. Li, I. Sorribes, T.
Yan, K. Junge, M. Beller, Angew. Chem. Int. Ed., 52 (2013) 12156. (d) L. González-
Sebastián, M. Flores-Alamo, J. J. García, Organometallics, 34 (2015) 763. (e) E. Blondiaux,
J. Pouessel, T. Cantat, Angew. Chem. Int. Ed., 53 (2014) 12186. (f) S. Das, F. D. Bobbink, G.
Laurenczy, P. J. Dyson, Angew. Chem. Int. Ed., 53 (2014) 12876. (g) O. Santoro, F. Lazreg,
Y. Minenkov, L. Cavallo, C. S. J. Cazin, Dalton Trans., 44 (2015) 18138.
[20] See Supporting Information, Table S2.
[21] T. Vergote, F. Nahra, A. Merschaert, O. Riant, D. Peeters, T. Leyssens, Organometallics,
33 (2014) 1953.

Figure Captions
Scheme 1. Initial attempts towards Zn(II)-NHC complexes via the K2CO3 route.
Scheme 2 Synthesis of [(NHC)H][ZnClX2] species.

Scheme 3. Zn-catalyzed N-methylation of amines with CO2 and PhSiH3.a
Scheme 4. Proposed mechanism for the Zn-catalyzed N-methylation of amines with CO2 and PhSiH3 [3,19].

Fig. 1. Molecular structure of [IPr*H]2[ZnX4] (1) (X = Br, Cl). Ellipsoids are represented at the 50% probability
level. Hydrogen atoms (except for N-CH-N) are omitted for clarity. Selected bond lengths (Å) and angles (°)
(esd): C(1)-N(2) 1.331(5), N(2)-C(3) 1.393(6), C(3)-C(4) 1.334(6), Zn(1)-Cl(1) 2.30(2), Zn(1)-Br(1) 2.397(3),
N(2)-C(1)-N(5) 108.0(5), C(1)-N(2)-C(3) 108.9(4), N(2)-C(3)-C(4)-107.1(4), Cl(1)-Zn(1)-Cl(1) 111.8(7), Br(1)-
Zn(1)-Br(1) 111.76(12).
Fig. 2. Molecular structure of [IPrH]4[ZnBr4][ZnBr3]2 (3). Ellipsoids are represented at the 50% probability
level. Hydrogen atoms (except for N-CH-CH-N), N-aryl substituents and two [ZnBr3]- counter-ions are omitted
for clarity. Selected bond lengths (Å) and angles (°) (esd): C(31)-N(32) 1.312(10), N(32)-C(33) 1.391(11),
C(33)-C(34) 1.352(13), Zn(1)-Br(1) 2.4297(13), N(32)-C(31)-N(35) 107.7(7), C(31)-N(32)-C(33) 110.4(7),
N(32)-C(33)-C(34) 105.5(8), Br(1)-Zn(1)-Br(2) 107.94(5), Br(1)-Zn(1)-Br(4) 108.78(5).

Fig. 3 Molecular structure of [IPrH][ZnCl3] (4a). Ellipsoids are represented at the 50% probability level.
Hydrogen atoms (except for N-CH-N) are omitted for clarity. Selected bond lengths (Å) and angles (°) (esd):
C(31)-N(32) 1.334(3), N(32)-C(33) 1.381(2), C(33)-C(34) 1.340(4), Zn(31)-Cl(31) 2.2299(6), Zn(31)-Cl(32)
2.3316(8), N(32)-C(31)-N(35) 107.95(15), C(31)-N(32)-C(33) 108.87(18), N(32)-C(33)-C(34)-107.1(2),
Cl(31)-Zn(31)-Cl(32) 114.45(3), Zn(31)-Cl(32)-Zn(31) 88.20(2).
Fig. 4 Molecular structure of [SIPrH][ZnClBr2] (5b). Ellipsoids are represented at the 50% probability level.
Hydrogen atoms (except for N-CH2-CH2-N) are omitted for clarity. Selected bond lengths (Å) and angles (°)
(esd): C(1)-N(2) 1.304(3), N(2)-C(3) 1.483(3), C(3)-C(4) 1.533(3), Zn(1)-Br(1) 2.442(7), Zn(1)-Cl(1)
2.381(10), N(2)-C(1)-N(5) 115.13(19), C(1)-N(2)-C(3) 109.56(17), N(2)-C(3)-C(4)-102.89(17), Br(1)-Zn(1)-
Br(2) 109.9(2), Cl(1)-Zn(1)-Cl(2) 103.3(4), Zn(1)-Br(1)-Zn(1) 85.0(2), Zn(1)-Cl(1)-Zn(1) 86.7(3).
Table
Table 1 Optimization of the reaction conditions for the Zn-catalyzed N-methylation of 8a with CO2 and
PhSiH3.a
Entry
1
[Zn]
Conversion (%)a
[IPrH][ZnCl3] (4a)
[SIPrH][ZnCl3] (5a)
[IMesH][ZnCl3] (6a)
[IPr*H][ZnCl3] (7a)
[IPrH][ZnCl3] (4a)
[SIPrH][ZnCl3] (5a)
[IMesH][ZnCl3] (6a)
[IPr*H][ZnCl3] (7a)
[IPrH][ZnClBr2] (4b)
[IPrH][ZnBr3] (3)
[SIPrH][ZnClBr2] (5b)
[IMesH][ZnClBr2] (6b)
-
2
-
3
-
4
5b
6b
7b
8b
9b
10b
11b
12b
-
51
-
65
44
58
50
6
80

13b
14b,c
[IPr*H][ZnClBr2] (7b)
[IMesH][ZnClBr2] (6b)
50
>99 (78)
Reaction conditions: 8a (1 equiv.), PhSiH3 (2 equiv.), [Zn] (5 mol%),
CO2 (1 bar), CPME (2 mL), 100 oC, 20 h. aConversions determined by
GC based on 8a, average of two runs. Isolated yield in parentheses.
bKOtBu (5 mol%). cPhSiH3 (3 equiv.).
Table 2. Composition of the reaction mixtures for the methylation of primary amines with CO2 catalyzed by
6b.a
Conversiona
Entry
Substrate
I
II
III
IV
-
1
8d
8e
8d
8e
10%
2%
6%
18%
58%
62%
14%
4%
32%
9%
2
-
3b
4b
48%
70%
14%
6%
aDetermined by GC based on the substrate, average of two runs. b6b (10 mol%), KOtBu (10
mol%), PhSiH3 (6 equiv.).