In view of the very unusual structural motif found in the intermediate Ag3 complex and its synthetic utility for other d8 carbene-hybrid complexes, the researchers of the Department of Chemistry, University of Singapore and State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, CAS, have investigated the coordination of other related ligands and found that the N-substituent on the imidazolin-2-ylidene ring has a profound effect on the structures on the transmetallation Ag(I) intermediate and the Ni(II) and Pd(II) products.
However, it does not appear to influence the course or efficiency of the transmetallation process. Its use in the benzimidazol-imidazolin-2-ylidene difunctional C–N ligand enables the isolation of the intermediate Ag(I) complexes of different nuclearities, and both geometric isomers of the homoleptic products. Crystallographic analysis of the latter reveals that the metal is almost completely engulfed by the ligands. This suggests that in the catalytic activation process, some form of ligand dissociation process would be inevitable for the metal to be exposed to substrate approach. This is in contrary to the classical chelating ligands and illustrates the value of using a more labile donor such as the nitrogen-donating benzimidazole. Current experiments are directed at the stoichiometric reactions of these active d8 dications with key catalytic substrates in an attempt to isolate and identify products that could shed clearer light on the mode of action of catalytically important species.
N-Heterocyclic carbenes (NHCs) have attracted considerable interest for their diverse applications in coordination chemistry and homogeneous catalysis. In recent times, much attention has been turned to the design of carbene hybrid ligands, namely, NHCs that are functionalized with other donor functions. It provides a ready method to tune the electronic and steric characters of the resulting complexes, as well as to raise the ligand hemilability, both of which are essential considerations in catalytic designs. Nitrogen donors of different dissociability are among the successful examples that can be hybridized with pure NHC. Previously, the researchers have reported the use of benzimidazole functionalized imidazolium NHC ligand to support the trinuclear Ag3Cl2(µ-Cl)(µ-LMe)2 and its use as a transmetallation agent to give PdCl2(ŋ-LMe), which is a Suzuki-active catalyst with high TON (~11 750).
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Introduction
