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Synthesis, Structural Characterization, Computational Study, and Catalytic Activity of Metal Complexes Based on Tetradentate Pyridine/N-Heterocyclic Carbene Ligand

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posted on 05.12.2005, 00:00 by Pei Ling Chiu, Chun-Liang Lai, Chiung-Fang Chang, Ching-Han Hu, Hon Man Lee
The ligand precursor [LH2]2+·2Br- (1) for the bis(NHC) ligand with N-picolyl moieties is synthesized in 83% yield. Complexation of 1 with palladium and nickel acetates produced [PdL]2+·2Br- (2) and [NiL]2+·2Br- (3), respectively, in quantitative yields. Metathesis reactions of 2 and 3 with AgPF6 give the corresponding bromide-free complexes [PdL]2+·2PF6- (4) and [NiL]2+·2PF6- (5). Complexes 24 were characterized by X-ray structural determinations, which reveal a highly twisted helical coordination of L around the metal ions. In solution, however, the tetradentate chelate ring undergoes a rapid fluxional process of ring twisting. A theoretical study confirms that the tetradentate coordination of L in 2 and 3 is energetically more favorable than the bidentate chelation mode with dangling picolyl groups. A preliminary application of [NiL]2+·2Br- in Suzuki coupling of aryl halides with phenylboronic acid shows effective activity.