Solvent-Dependent Formation of Os(0) Complexes by Electrochemical Reduction of [Os(CO)(2,2′-bipyridine)(L)Cl2]; L = Cl–, PrCN

Cyclic voltammetry and ultraviolet–visible/infrared (UV–vis/IR) spectroelectrochemistry were used to study the cathodic electrochemical behavior of the osmium complexes mer-[OsIII(CO) (bpy)­Cl3] (bpy = 2,2′-bipyridine) and trans(Cl)-[OsII(CO) (PrCN)­(bpy)­Cl2] at variable temperature in different solvents (tetrahydrofuran (THF), butyronitrile (PrCN), acetonitrile (MeCN)) and electrolytes (Bu4NPF6, Bu4NCl). The precursors can be reduced to mer-[OsII(CO) (bpy•–)­Cl3]2– and trans(Cl)-[OsII(CO)­(PrCN) (bpy•–)­Cl2]−, respectively, which react rapidly at room temperature, losing the chloride ligands and forming Os(0) species. mer-[OsIII(CO) (bpy)­Cl3] is reduced in THF to give ultimately an Os–Os-bonded polymer, probably [Os0(CO) (THF)­(bpy)]n, whereas in PrCN the well-soluble, probably mononuclear [Os0(CO) (PrCN)­(bpy)], species is formed. The same products were observed for the 2 electron reduction of trans(Cl)-[OsII(CO)­(PrCN) (bpy)­Cl2] in both solvents. In MeCN, similar to THF, the [Os0(CO) (MeCN)­(bpy)]n polymer is produced. It is noteworthy that the bpy ligand in mononuclear [Os0(CO) (PrCN)­(bpy)] is reduced to the corresponding radical anion at a significantly less negative potential than it is in polymeric [Os0(CO) (THF)­(bpy)]n: ΔE1/2 = 0.67 V. Major differences also exist in the IR spectra of the Os(0) species: the polymer shows a broad ν­(CO) band at much smaller wavenumbers compared to the soluble Os(0) monomer that exhibits a characteristic ν­(Pr-CN) band below 2200 cm–1 in addition to the intense and narrow ν­(CO) absorption band. For the first time, in this work the M0-bpy (M = Ru, Os) mono- and dicarbonyl species soluble in PrCN have been formulated as a mononuclear complex. Density functional theory (DFT) and time-dependent-DFT calculations confirm the Os(0) oxidation state and suggest that [Os0(CO) (PrCN)­(bpy)] is a square planar moiety. The reversible bpy-based reduction of [Os0(CO) (PrCN)­(bpy)] triggers catalytic reduction of CO2 to CO and HCOO–.