Palladium(II) complexes of imidazolin-2-ylidene N-heterocyclic carbene ligands with redox-active dimethoxyphenyl or (hydro)quinonyl substituents

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 H3L3Br reveals Br−⋯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(L2)] (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 π-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)2Br2]-centred processes.

Palladium(II) complexes of novel N-heterocyclic carbene ligands with redox-active p-(hydro)quinonyl substituents have been prepared and characterised.Figure optionsDownload as PowerPoint slideResearch highlights
► Precursors to N-heterocyclic carbene ligands with p-(hydro)quinonyl substituents made.
► Palladium(II) complexes of the carbene ligands characterised.
► An admix of ligand and palladium-centred redox processes is observed.
► The complexes are inactive in oxidations of styrene under Wacker conditions.