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 <i>mer</i>-[Os^III(CO) (bpy)­Cl₃] (bpy = 2,2′-bipyridine) and <i>trans</i>(Cl)<i>-</i>[Os^II(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 <i>mer</i>-[Os^II(CO) (bpy^•–)­Cl₃]^2– and <i>trans</i>(Cl)-[Os^II(CO)­(PrCN) (bpy^•–)­Cl₂]⁻, respectively, which react rapidly at room temperature, losing the chloride ligands and forming Os(0) species. <i>mer</i>-[Os^III(CO) (bpy)­Cl₃] is reduced in THF to give ultimately an Os–Os-bonded polymer, probably [Os⁰(CO) (THF)­(bpy)]_<i>n</i>, 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 <i>trans</i>(Cl)-[Os^II(CO)­(PrCN) (bpy)­Cl₂] in both solvents. In MeCN, similar to THF, the [Os⁰(CO) (MeCN)­(bpy)]_<i>n</i> 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)]_<i>n</i>: Δ<i>E</i>_1/2 = 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(<i>Pr</i>-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<sup>–