Synthesis of [Os(bpy)2(py)(OH2)](PF6)x substituted pyridine complexes; characterization of a singly bridged H3O2− ligand†

Abstract

Proton-coupled electron transfer (PCET) underpins energy conversion processes in biological systems and fuel-forming reactions. Interrogation of the dynamics of electron and proton transfer in PCET processes requires tunable models, with synthetic transition metal aquo complexes being particularly well-explored examples. A previous study on a PCET model, [Os^II(bpy)₂(py)(OH₂)]²⁺ (bpy = 2,2′-bipyridine; py = pyridine), reported synthetic intractability which limits access to this class of models. Herein, we report an improved protocol to synthesize a family of [Os^II(bpy)₂(py)(OH₂)]²⁺ complexes enabling the modular tuning of the pyridine ligand (py^L) with electron-donating or -withdrawing groups at the para-position. The modification of the electron density about the osmium center is reflected in Hammett plots of half-wave peak potential for the Os^II/Os^III couples and pK_a values of the coordinated water. Moreover, a hydrogen-bonded osmium dinuclear structure featuring a short, strong hydrogen bonding network in the solid state was observed; we find the dinuclear Os structure is likely not maintained in solution. Our work expands access to osmium aquo complexes and provides an avenue to understand how modification of supporting ligands can influence PCET processes.