Silver(I) and Palladium(II) Complexes of an Ether-Functionalized 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 [Ag2(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 [Ag2(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 PdII(NHC) complex exhibited
good activity in a model Heck coupling reaction.