A Cationic (N‐Heterocyclic carbene)silver Complex as Catalyst for Bulk Ring‐Opening Polymerization ofL‐Lactides

Abstract

AbstractSynthetic, theoretical, and catalysis studies of a cationic functionalized N‐heterocyclic carbene complex of silver, namely[{1‐isopropyl‐3‐(N‐phenylacetamido)imidazol‐2‐ylidene}2Ag]+Cl–(1b), is reported. Specifically,1bwas synthesized by the reaction of 1‐isopropyl‐3‐(N‐phenylacetamido)imidazolium chloride (1a) with Ag2O in 64 % yield;1awas synthesized by the alkylation reaction of 1‐isopropylimidazole withN‐phenyl chloroacetamide in 90 % yield. The molecular structure of1bwas determined by X‐ray diffraction studies and was found to be active for polymerization ofL‐lactide at elevated temperatures under solvent‐free melt conditions to give polylactide of moderate molecular weight with narrow molecular weight distribution. Density functional theory studies of the cationic species2b, derived from NHC silver complex1b, were employed to obtain an understanding of the structure, bonding, and electronic features of the molecule. Bonding in complex2bhas been probed with the help of charge decomposition analysis (CDA), the atoms in molecules (AIM) approach as well as natural bond orbital (NBO) methods. The Ag–NHC bond has been found to be more covalent with NHC acting as an effective σ‐donor. The π‐back‐bonding from the metal atom to the ligand was found to be negligible. It has been noticed that the imidazole rings remain nearly orthogonal with respect to each other, in contrast to the experimental geometry. Intramolecular hydrogen bonds as well as Ag···O interaction involving the carbonyl oxygen atoms have been identified as additional stabilizing factors contributing towards the lower energy conformer of2bcompared to the corresponding planar geometry.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)