Journal article

Group 11 Metal Complexes of N-Heterocyclic Carbene Ligands: Nature of the Metal-Carbene Bond

The silver, copper and gold complexes of tripodal N-heterocyclic carbene (NHC) ligand, 1,1,1-tris[(3-methylimidazol-1-yl-2-ylidene)methyl]ethane were prepd. and characterized. The silver complex of the title ligand, TIMEMe was prepd. by reaction of [H3TIMEMe][PF6]3 with Ag2O, which afforded trinuclear bis-carbene silver complex [(TIMEMe)2Ag3](PF6)3 (3). Reaction of 3 with CuBr or (Me2S)2AuCl yielded the corresponding D3-sym. copper(I) and gold(I) complexes [(TIMEMe)2Cu3](PF6)3 (4) and [(TIMEMe)2Au3](PF6)3 (5). Single-crystal x-ray diffraction, spectroscopic, and computational studies of this series of metal NHC complexes are described. The group 11 metal complexes of the TIMEMe ligand exhibit isostructural geometries, with three metal ions bridging two of the TIMEMe ligands. Each metal ion is linearly coordinated to two carbene centers, with each of the carbenoid carbons stemming from a different ligand. Overall, the mols. possess D3 symmetry. The electronic structure of these newly prepd. compds. was elucidated with the aid of DFT calcns. In contrast to the common assumption that NHCs are pure sigma-donor ligands, the orbital energy calcns. for complexes 3-5, bis(1,3-di-tert-butylimidazol-2-ylidene)palladium (6), model D2h complexes bis(1,3-dimethylimidazol-2-ylidene)silver, -copper, and -gold (8-10, resp.) reveal the existence of both sigma- and pi-type interactions between the metal ions and the carbenoid carbons. In these diaminocarbene models the contribution of pi-back-bonding interactions is approx. 15-30% of the overall orbital interaction energy of the complexes. [on SciFinder (R)]


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