Palladium(II) complexes of imidazolin-2-ylidene N-heterocyclic carbene ligands with redox-active dimethoxyphenyl or (hydro)quinonyl substituents
作者:Louise A. Berben、Donald C. Craig、Carolina Gimbert-Suriñach、Andrew Robinson、Kristian H. Sugiyarto、Stephen B. Colbran
DOI:10.1016/j.ica.2011.02.007
日期:2011.5
Four novel imidazolium salts, precursors to N-heterocyclic carbene (NHC) ligands, with 2,5-dimethoxybenzyl or 2,5-dihydroxybenzyl (i.e., p-hydroquinone) substituents have been prepared. The crystal structure of the hydroquinone-substituted imidazolium salt (H3LBr)-Br-3 reveals Br center dot center dot center dot H-O bridged chiral chains of alternating [H3L3](+) cations and Br counter-ions parallel to the x-axis. Palladium(II) complexes were accessible from reactions of the dimethoxyphenyl-substituted imidazolium precursors with palladium(II) acetate, but not from reactions of imidazolium cations with hydroquinonyl substituents. The crystal structure of the bis(dimethoxybenzyl)-substituted bis(NHC)Pd complex, cis-[PdBr2(L-2)] (2), is described. Puckering of the bis(NHC) ligand leads to a cleft in which an included molecule of dimethylformamide is situated. The cleft is closed by one of the dimethoxybenzyl groups which p-stacks with the dimethylformamide; the other dimethoxybenzyl group points away from the cleft and Pd(II) centre. Reaction of complex 2 with BBr3 afforded the targeted bis(hydroquinone)-substituted bis(NHC) Pd(II) complex 3 (97% yield) which, in turn, was oxidised by 2,3-dichloro-5,6-dicyano-benzoquinone to the corresponding p-benzoquinone-substituted bis(NHC) Pd(II) complex 4 (98% yield). The cyclic voltammograms of the Pd(II) complexes 2-4 reveal waves that are attributed to an admix of the anticipated ligand-centred and [Pd(C-NHC)(2)Br-2]-centred processes. (C) 2011 Elsevier B.V. All rights reserved.