FULL PAPER DOI: 10.1002/ejic.200600407 First Example of a Gold(I) N-Heterocyclic-Carbene-Based Initiator for the Bulk Ring-Opening Polymerization of L-Lactide Lipika Ray, [a] Vimal Katiyar, [a] Mustafa J. Raihan, [a] Hemant Nanavati, [a] Mobin M. Shaikh, [a] and Prasenjit Ghosh* [a] Keywords: Carbenes / Functionalized NHC / Ring-opening polymerization (ROP) / Organometallic catalysis / Silver–NHC complex / Gold–NHC complex Synthesis, structure, and catalysis studies of two Au- and Ag- based initiators, namely, [3-(N-tert-butylacetamido)-1-(2-hy- droxycyclohexyl)imidazol-2-ylidene]AuCl (1c) and [3-(N- tert-butylacetamido)-1-(2-hydroxycyclohexyl)imidazol-2- ylidene]AgCl (1b), for the bulk ring-opening polymerization of L-lactide are reported. Specifically, gold complex 1c was obtained from silver complex 1b by the transmetalation reac- tion with (SMe 2 )AuCl. Silver complex 1b was synthesized by the treatment of 3-(N-tert-butylacetamido)-1-(2-hydroxycy- clohexyl)imidazolium chloride (1a) with Ag 2 O. Compound 1a was synthesized directly from the reaction of N-tert-butyl- 2-chloroacetamide, cyclohexene oxide, and imidazole. The Introduction The long-standing notion of gold being an expensive and unreactive coinage metal with limited utility is gradually changing. Various new applications of Au I complexes in pharmaceuticals, [1] chemical vapor depositions, [2] and in homogeneous catalysis [3] have been recently reported. Com- pared to the other coinage metals (i.e. Cu and Ag), Au has seen relatively few applications and poses a formidable chal- lenge in utility-oriented research. Quite significantly, a re- cent report detailed the use of a Au I complex supported over a sterically demanding N-heterocyclic carbene (NHC) ligand as a catalyst for the ethyl diazoacetate-assisted car- bene-transfer reaction; [3] this represents an important breakthrough in this field of chemistry and adds further promise to the use of gold in chemical catalysis. The grow- ing utility of NHCs in homogeneous catalysis is now well- recognized, and a plethora of NHC-based catalysts have been developed in recent years for a wide variety of trans- formations, such as C–C coupling reactions, [4,5] olefin me- tatheses, [6] hydrogenations, [7,8] hydroformylations, [9] hydro- silylations, [10] CO-ethylene copolymerizations, [11] hydrobor- ation [12] reactions, and so forth. Interestingly enough, de- spite the numerous NHC-transition metal complexes that [a] Department of Chemistry and Department of Chemical Engi- neering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India Fax: +91-22-2572-3480 E-mail: pghosh@chem.iitb.ac.in © 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2006, 3724–3730 3724 molecular structures of 1a, 1b, and 1c have been determined by X-ray diffraction studies. The formation of neutral mono- meric complexes with linear geometries at the metal centers was observed for both 1b and 1c. The Au and Ag complexes 1c and 1b successfully catalyzed the bulk ring-opening poly- merization of L-lactide at elevated temperatures under sol- vent-free melt conditions to produce moderate to low molec- ular weight polylactide polymers with narrow molecular weight distributions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) exist, the Au–NHC complexes are surprisingly few in number and have seldomly been employed in catalysis. [13] A growing number of new applications have appeared lately for the Ag I –NHC counterparts however, and their catalytic utility has just begun to unfold. [14,15] Specifically, the Ag I –NHC complexes have been found to be active cata- lysts for several chemical transformations, such as, ethyl di- azoacetate (EDA)-assisted carbene-transfer reactions, [16] catalytic preparation of 1,2-bis(boronate) esters, [17] transes- terification reactions, and ring-opening lactide polymeriza- tion reactions. [18] It is noteworthy that even though the ap- plications of Ag I –NHC complexes in chemical catalysis is a recent phenomenon, they are long-known for their use as transmetalation agents for the synthesis of other transition metal–NHC complexes. [14,15] Like its heavier congener, the Au analogue, biomedical applications, particularly with re- gard to antimicrobial activities, have recently been reported for Ag I –NHC complexes. [19,20] Thus far, the popularity of Ag–NHC complexes can be ascribed to their broadly based applications, to their synthetic accessibility, and to their air and moisture stabilities, which make them user-friendly and hence convenient to handle. [14,15] As the catalytic utility of Au–NHC complexes remains largely unexplored and that of the Ag–NHC complexes is just beginning to emerge, we became interested in the design of Au and Ag complexes for their potential application in homogeneous catalysis. In particular, we were interested in