Group 10 Metal Complexes Featuring Rigid Phenylene-Bridged NHC/Pyridyl Ligands: Controlling Coordination Modes, Electrochemical Properties, and CO2 Reduction Catalysis

An NHC/pyridyl hybrid ligand L, bearing a rigid phenylene linker between two NHC donors and methylene-tethered pyridyl arms, allows controlled access to two coordination modes with Group 10 metals (M = Ni, Pd, Pt). A silver(I) carbene complex of L was isolated as the tetranuclear chain [Ag4L2]4+, which serves as a practical carbene-transfer reagent for the formation of both ML2 (κ2-NHC) and ML (κ4-NHC/pyridyl) complexes. Single-crystal X-ray diffraction reveals that [Ag4L2]4+ forms a zigzag chain of four Ag(I) centers bridged by two L ligands, with argentophilic Ag···Ag separations of 3.165–3.263 Å. Across Group 10 metals, ML complexes exhibit a four-coordinate distortion from ideal square-planar geometry (higher τ4 values) but shorter M–C(carbene) bonds than their ML2 counterparts, consistent with reduced trans influence in the mixed NHC/pyridyl environment. Electrochemically, ML2 complexes undergo reductive decomposition, whereas ML complexes show enhanced stability under reducing conditions. [NiL]2+ catalyzes CO2 reduction under electrochemical conditions (formate 39.5% FE; H2 37.9% FE), whereas [PdL]2+ affords only H2. Under visible-light irradiation (450 nm) with Ir(ppy)3, the product selectivity depends on the sacrificial electron donor (BIH or TEA), providing 7.2–43.3% selectivity for formate. These product profiles are consistent with a catalytic pathway involving a nickel-hydride intermediate.