Microwave-assisted synthesis of (N-heterocyclic carbene)MCl complexes of group 11 metals

Article abstract of DOI:10.1002/ejic.201200248

The use of microwave heating for the synthesis of (N-heterocyclic carbene)-bearing complexes of Cu, Ag and Au allows for a drastic reduction of the reaction times required by conventional heating, while affording comparable or better yields of the desired complexes. Copyright

Full text of DOI:10.1002/ejic.201200248

SHORT COMMUNICATION
DOI: 10.1002/ejic.201200248
Microwave-Assisted Synthesis of (N-Heterocyclic carbene)MCl Complexes of
Group 11 Metals
Brant Landers[a] and Oscar Navarro*[a]
Keywords: Microwave heating / N-Heterocyclic carbenes / Copper / Silver / Gold
The use of microwave heating for the synthesis of (N-hetero-
cyclic carbene)-bearing complexes of Cu, Ag and Au allows
for a drastic reduction of the reaction times required by con-
ventional heating, while affording comparable or better
yields of the desired complexes.
Introduction
Results and Discussion
The vast number of research articles and reviews on N-
heterocyclic carbenes (NHCs) in the literature is a clear sign
of the impact and ubiquity of these σ-donor ligands in now-
adays organometallic chemistry.[1–3] Since their first use as
ligands in a metal-catalysed process by Herrmann and co-
workers in 1995,[4] the number of metals utilized and the
applications developed has constantly increased.
Cazin and co-workers recently reported on the use of tol-
uene or water as solvents the for the direct synthesis of
(NHC)CuCl and (NHC)AgCl complexes.[12] In their com-
munication the authors synthesized, among others, (IPr)-,
(SIPr)-, (IMes)- and (SIMes)MCl (Figure 1) in very good
yields at refluxing temperatures after a reaction time of
24 h. In our laboratories, attempts to speed up the synthesis
in water by applying microwave heating did only reduce the
reaction times to about 4–5 h, in the best cases.
In the last years, the interest in NHC-bearing complexes
of Group 11 metal complexes has grown considerably. Most
applications of silver(I) complexes focused in their ability
to transmetalate the NHC to a less “friendly” metal that
would not accept the NHC directly (or would do it in very
low yields),[5] although some other uses of (NHC)AgI com-
plexes have also been reported.[1,6] On the other hand and
since their appearance, (NHC)CuI and lately (NHC)AuI
complexes have increasingly found interesting applications
in catalytic processes.[1,3,7] While straightforward, the syn-
thesis of these families of complexes requires in most cases
long reaction times at high reaction temperatures and there-
fore a considerable energy cost. Among the 12 principles of
green chemistry,[8] “energy efficiency by design” is consid-
ered a top priority.[9] In addition, quick access to libraries
of catalysts is crucial when optimizing reactions, both in
academia and industry, and fast, robust synthetic protocols
to obtain them are always desirable. In that regard, our
group has reported that the use of microwave heating for
the direct synthesis (from the imidazolium or imidazolidin-
ium salt and a metal salt) of (NHC)PdII[10] complexes al-
lows for a drastic reduction of the reaction times and there-
fore a considerable reduction of energy employed.[11] Herein,
we wish to report the application of this technology to the
synthesis of (NHC)MCl (M = Cu, Ag, Au) complexes.
Figure 1. (NHC)MCl complexes (M = Cu, Ag, Au).
Switching the solvent to THF, on the other hand, al-
lowed for the synthesis of the complexes in comparable
yields after 0.5 h (48 times faster than the preparations in
water or toluene) at 110 °C (Table 1).[13] For Cu, this proto-
col afforded higher yields for (IPr)- and (SIPr)-bearing
complexes (Table 1, Entries 1, 2), and lower ones for
(IMes)- and (SIMes)-bearing complexes (Table 1, Entries 3,
4). The trend was different for Ag: saturated ligands af-
forded higher yields (Table 1, Entries 6, 8), while lower
[a] Department of Chemistry, University of Hawaii at Manoa,
2545 McCarthy Mall, Honolulu, HI 96825, USA
Fax: +1-808-956-5908
E-mail: oscarnf@hawaii.edu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200248.
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Eur. J. Inorg. Chem. 2012, 2980–2982

(N-Heterocyclic carbene)MCl Complexes of Group 11 Metals
yields were obtained with the unsaturated counterparts ing able to synthesize (IMes)AuCl and (SIMes)AuCl in very
(Table 1, Entries 5, 7). For convenience, these reactions were good yields under microwave heating after 5 min
set up in a glovebox and using dry THF as solvent, but we (Scheme 2).
also carried out same-scale reactions without an inert gas
and using technical-grade THF as solvent to prepare com-
plexes 1 and 2, affording 88% and 82%, respectively, of the
desired products. In addition, a larger-scale reaction for the
synthesis of 1 was carried out without an inert gas, afford-
Scheme 2. Microwave-assisted synthesis of (IMes)- and (SIMes)-
ing 93% yield after 1 h.[14]
AuCl complexes. Reaction conditions: (Me2S)AuCl (0.25 mmol),
(NHC)CuCl (0.25 mmol), DCM (1 mL). [a] Transmetallation from
(NHC)CuCl, 40 °C, DCM, 1 h.[17] [b] Transmetallation from
Table 1. Microwave-assisted synthesis of (NHC)CuCl and (NHC)-
(NHC)AgCl, r.t., DCM, 7 h.[16]
AgCl complexes.
Conclusions
We have developed a microwave-assisted protocol for the
synthesis of a series of (NHC)CuCl, (NHC)AgCl and
(NHC)AuCl complexes. Most of these complexes could be
prepared directly from the corresponding imidazolium or
imidazolidinium salts and a metal salt in very short reaction
times (12–48 times faster than protocols involving conven-
tional heat)[13] and obtaining comparable or higher yields
than those reported in the literature.
[a] M2O (0.65 mmol), NHC·HCl (1 mmol), THF (2 mL). [b] Yields
in parentheses for conventional heating and water as solvent,
24 h.[12] [c] Reaction carried out in toluene.[12]
After these experiments, we directed our attention to the
synthesis of (NHC)AuCl complexes. To the best of our
knowledge, the direct synthesis of these complexes from AuI
salts has not been reported.[15] Reported protocols involve
the combination of free carbene and an Au source, or the
transmetallation from an (NHC)AgI complex.[16] (NHC)-
CuI complexes can also be used as transmetallating agents
for the synthesis of some (NHC)Au complexes under very
mild conditions and in very good yields.[17] Our interest fo-
Experimental Section
General Procedure for the Synthesis of (NHC)CuCl and (NHC)AgCl
Complexes: A 10 mL microwave vial equipped with magnetic stir
bar was loaded with NHC·HCl (1.0 mmol), metal oxide
(0.65 mmol) and of solvent (2 mL). The reaction mixture was
heated and stirred in the microwave reactor at 110 °C for 30 min.
After the reaction was complete, the mixture was cooled to room
temperature. The crude mixture was then filtered through Celite
cused in circumventing the need to synthesize and purify an (DCM). The filtrate was concentrated and the complex precipitated
with hexanes. Filtering left the desired product as a white solid.
The solid was vacuum-dried affording the desired (NHC)MCl com-
plexes as a white powder.
intermediate organometallic reagent. After an optimization
process, we found that the direct synthesis of (IPr)- and
(SIPr)AuCl (3 and 6) could be achieved by using microwave
heating in technical-grade xylenes in the presence of K2CO3
(Scheme 1). High yields were obtained, particularly keeping
in mind that the synthesis of the (NHC)Cu or -Ag interme-
diates, which are not quantitative, was not necessary. Unfor-
tunately, this methodology was only successful with those
complexes, and mixtures of products were obtained when
we attempted the synthesis of (IMes)- and (SIMes)AuCl (9
and 12). For these complexes, we applied a transmetallation
route starting from the corresponding Cu counterparts, be-
Supporting Information (see footnote on the first page of this arti-
cle): Experimental protocols and complex characterization.
Acknowledgments
The authors acknowledge the National Science Foundation
(CHE 0924324) for support.
[1] a) C. S. J. Cazin (Ed.), Heterocyclic Carbenes in Transition
Metal Catalysis and Organocatalysis, Springer, London, 2010;
b) F. Glorius (Ed.), N-Heterocyclic Carbenes in Transition
Metal Catalysis, Springer, Berlin, 2007.
[2] Some recent reviews: a) M. J. Ingleson, R. A. Layfield, Chem.
Commun. 2012, 48, 3579–3589; b) M. S. Viciu, O. Navarro,
Annu. Rep. Prog. Chem., Sect. B: Org. Chem. 2011, 107, 226–
243; c) T. Dröge, F. Glorius, Angew. Chem. Int. Ed. 2010, 49,
6940–6952.
Scheme 1. Microwave-assisted synthesis of (IPr)- and (SIPr)AuCl
complexes. Reaction conditions: (Me2S)AuCl (0.5 mmol),
[3] S. P. Nolan, Acc. Chem. Res. 2011, 44, 91–100; A. Correa, S. P.
Nolan, L. Cavallo, Top. Curr. Chem. 2011, 302, 131–155.
NHC·HCl (0.5 mmol), xylenes (1 mL). [a] Transmetallation from [4] W. A. Herrmann, M. Elison, J. Fischer, C. Köcher, G. R. J.
(NHC)CuCl, 40 °C, DCM, 2 h.[17] [b] Reaction with free carbene,
r.t., THF, 12 h.[16]
Artus, Angew. Chem. 1995, 107, 2602; Angew. Chem. Int. Ed.
Engl. 1995, 34, 2371–2374.
Eur. J. Inorg. Chem. 2012, 2980–2982
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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B. Landers, O. Navarro
SHORT COMMUNICATION
[5] a) T. Ramnial, C. D. Abernethy, M. D. Spicer, I. D. McKenzie,
I. D. Gay, J. A. C. Clyburne, Inorg. Chem. 2003, 42, 1391–1393;
b) P. de Frémont, N. M. Scott, E. D. Stevens, T. Ramnial, O. C.
Lightbody, C. L. B. Macdonald, J. A. C. Clyburne, C. D. Aber-
nethy, S. P. Nolan, Organometallics 2005, 24, 6301–6309.
[6] a) M.-T. Chen, B. Landers, O. Navarro, Org. Biomol. Chem.
2012, 10, 2206–2208; b) Y. Li, X. Chen, Y. Song, L. Fang, G.
Zou, Dalton Trans. 2011, 40, 2046–2052.
[7] Silvia Díez-González (Ed.), N-Heterocyclic Carbenes: From
Laboratory Curiosities to Efficient Synthetic Tools, RSC Catal-
ysis Series, RSC Publishing, Cambridge, 2011.
[8] R. A. Sheldon, I. Arends, U. Hanefield, Green Chemistry and
Catalysis, Wiley-VCH, Weinheim, 2008.
[9] D. Dallinger, C. O. Kappe, Chem. Rev. 2007, 107, 2563–2591.
[10] O. H. Winkelmann, O. Navarro, Adv. Synth. Catal. 2010, 352,
212–214.
[11] On a laboratory scale and for most chemical transformations,
energy savings up to 85-fold can be expected by using micro-
waves as an energy source: M. J. Gronnow, R. J. White, J. H.
Clark, D. J. Macquarrie, Org. Process Res. Dev. 2005, 9, 516–
518.
[12]
[13]
C. A. Citadelle, E. L. Nouy, F. Bisaro, A. M. Z. Slawing,
C. S. J. Cazin, Dalton Trans. 2010, 39, 4489–4491.
The choice of solvent also had an effect on the reaction: with
conventional heating, complex 1 could be prepared in 99%
yield in refluxing THF after 6 h (51% yield after 3 h).
[14]
See Supporting Information for details.
[15] Just recently, a direct synthesis from NaAuCl4·H2O in 3-Cl-
pyridine, at 80 °C for 24 h was reported: S. Zhu, R. Liang, L.
Chen, C. Wang, Y. Ren, H. Jiang, Tetrahedron Lett. 2012, 53,
815–818.
[16] P. de Frémont, N. M. Scott, E. D. Stevens, S. P. Nolan, Organo-
metallics 2005, 24, 2411–2418.
[17] M. R. L. Furst, C. S. J. Cazin, Chem. Commun. 2010, 46, 6924–
6925.
Received: March 10, 2012
Published Online: May 16, 2012
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 2980–2982