Silver(I) and Palladium(II) Complexes of an Ether-Functionalised Quasi-pincer Bis-carbene Ligand and Its Alkyl Analogue

Silver and palladium complexes of bis-NHC ligands bearing a weakly coordinating ether functionality have been prepared as counterparts to the well-studied pyridine-functionalized bis-NHC pincer ligands. The Ag(I) complex of bis[2-(3-methylimidazolin-2-yliden-1-yl)ethyl] ether (MeCOC) was prepared through the reaction of the corresponding bis-imidazolium salt with Ag2O and crystallographically characterized as the triflate salt. The structure reveals a dinuclear [Ag-2(MeCOC)(2)](OTf)(2) formulation and the presence of two separate forms in the crystal lattice arising from varying conformations of the ether bridging groups. The related pentamethylene-bridged bis-NHC ligand 1,5-bis(3-methylimidazolin-2-yliden-1-yl)pentane was analogously complexed to Ag(I) and crystallized as the [AgBr2](-) salt. A single-crystal X-ray study of the pentamethylene-bridged complex revealed continuous ladders of quasi-planar dinuclear cations linked by [AgBr2](-) anions; the divergent metal-ligand associations in the two Ag(I) compounds are attributed to the influence of the coordinating [AgBr2](-) anions rather than any effect of the ether functionality of MeCOC. Transmetalation of the bis-NHC ligand from [Ag-2(MeCOC)(2)](BF4)(2) to Pd(II) gave the structurally characterized complex [PdCl(MeCN)(MeCOC)](BF4), in which the NHC groups adopt a cis configuration with the oxygen atom remaining uncoordinated. This Pd-II(NHC) complex exhibited good activity in a model Heck coupling reaction.