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Synthesis of cis- and trans-Diisothiocyanato−Bis(NHC) Complexes of Nickel(II) and Applications in the Kumada−Corriu Reaction

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posted on 2010-09-13, 00:00 authored by Ramasamy Jothibasu, Kuo-Wei Huang, Han Vinh Huynh
Metathetical reaction of AgSCN with a series of trans-dihalido-bis(carbene) nickel(II) complexes in CH3CN readily afforded the novel diisothiocyanato-bis(carbene) complexes [Ni(NCS)2(NHC)2] (trans-2a, NHC = 1,3-diisopropylbenzimidazolin-2-ylidene; trans-2b, NHC = 1,3-diisobutylbenzimidazolin-2-ylidene; trans-2c, NHC = 1,3-dibenzylbenzimidazolin-2-ylidene; cis-2d, NHC = 1,3-di(2-propenyl)benzimidazolin-2-ylidene; cis-2e, NHC = 1-propyl-3-methylbenzimidazolin-2-ylidene) as greenish-yellow powders in moderate to good yields. While dihalido-bis(carbene) Ni(II) complexes exclusively form trans-complexes, a transcis isomerization occurs upon halido−isothiocyanato exchange with complexes bearing less bulky carbene ligands, i.e., cis-2d/e. DFT calculations indicated that this isomerization can be attributed to a reduced energy difference between trans- and cis-isomers of diisothiocyanato complexes. All complexes have been characterized by multinuclear NMR spectroscopy, ESI mass spectrometry, and X-ray diffraction analysis. A catalytic study revealed that cis-complexes generally exhibit greater activities in the Kumada−Corriu coupling reaction.

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