An alternative synthesis method for [Os(NN)(CO)2X2] complexes (NN = 2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine; X = Cl, Br, I). Electrochemical and photochemical properties and behavior

Abstract

A novel synthesis method is introduced for the preparation of [Os(NN)(CO)₂X₂] complexes (X = Cl, Br, I, and NN = 2,2′-bipyridine (bpy) or 4,4′-dimethyl-2,2′-bipyridine (dmbpy)). In the first step of this two-step synthesis, OsCl₃ is reduced in the presence of a sacrificial metal surface in an alcohol solution. The reduction reaction produces a mixture of trinuclear mixed metal complexes, which after the addition of bpy or dmbpy produce a trans(Cl)-[Os(NN)(CO)₂Cl₂] complex with a good 60–70% yield. The halide exchange of [Os(bpy)(CO)₂Cl₂] has been performed in a concentrated halidic acid (HI or HBr) solution in an autoclave, producing 30–50% of the corresponding complex. All of the synthesized trans(X)-[Os(bpy)(CO)₂X₂] (X = Cl, Br, I) complexes displayed a similar basic electrochemical behavior to that found in the ruthenium analog trans(Cl)-[Ru(bpy)(CO)₂Cl₂] studied previously, including the formation of an electroactive polymer [Os(bpy)(CO)₂]_n during the two-electron electrochemical reduction. The absorption and emission properties of the osmium complexes were also studied. Compared to the ruthenium analogues, these osmium complexes display pronounced photoluminescence properties. The DFT calculations were made in order to determine the HOMO–LUMO gaps and to analyze the contribution of the individual osmium d-orbitals and halogen p-orbitals to the frontier orbitals of the molecules. The electrochemical and photochemical induced substitution reactions of carbonyl with the solvent molecule are also discussed.