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 with electron-donating or -withdrawing groups on 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 pKa values of the coordinated water. Moreover, a hydrogen-bonded osmium dimeric structure featuring a short, strong hydrogen bonding network in the solid state was observed; we find the dimeric Os structure is likely not maintained in solution. Our work expands access to osmium aquo complexes and provides a venue to understand how modification of supporting ligands can influence PCET processes.