A cationic (N-heterocyclic carbene)silver complex as catalyst for bulk ring-opening polymerization of L-lactides

Synthetic, theoretical, and catalysis studies of a cationic functionalized N-heterocyclic carbene complex of silver, namely [{1-isopropyl-3-(N- phenylacetamido)imidazol-2-ylidene}₂-Ag]⁺Cl^- (1b), is reported. Specifically, 1b was synthesized by the reaction of 1-isopropyl-3-(N-phenylacetamido)imidazolium chloride (1a) with Ag₂O in 64 % yield; 1a was synthesized by the alkylation reaction of 1-isopropylimidazole with N-phenyl chloroacetamide in 90 % yield. The molecular structure of 1b was determined by X-ray diffraction studies and was found to be active for polymerization of L-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 species 2b, derived from NHC silver complex 1b, were employed to obtain an understanding of the structure, bonding, and electronic features of the molecule. Bonding in complex 2b has 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 of 2b compared to the corresponding planar geometry.