Reversible carbon monoxide binding by Rh2(O2CCH3)n(HNOCCH3)4-n. A spectroscopic and electrochemical investigation

Abstract

The binding constants for carbon monoxide addition to a series of dirhodium complexes of the form Rh/sub 2/(O/sub 2/CCH/sub 3/)/sub n/(HNOCCH/sub 3/)/sub 4-n/, where n = 0, 2, 3, and 4, were determined in 1,2-dichloroethane and in acetonitrile. At room temperature, either 1:1 or 1:2 CO adducts are observed depending upon the particular combination of bridging ligands and the nature of the solvent. In addition, the stability constants of the CO adducts were found to increase with increasing number of acetamidate bridging ions in the dirhodium complex. Cyclic voltammetry was carried out for the oxidation of Rh/sub 2/(ac)/sub n/(acam)/sub 4-n/ in acetonitrile and dichloroethane under various CO partial pressures. The value of E/sub 1/2/ for the formation of RH/sup II/Rh/sup III/ shifted positively with increasing CO pressure. This positive shift can be associated with log K for the 1:1 CO adduct formation. A linear relationship between log K and E/sub 1/2/ for the first oxidation of Rh/sub 2/(O/sub 2/CCH/sub 3/)/sub n/(HNOCCH/sub 3/)/sub 4-n/ under nitrogen was observed for complexes having n = 0, 1, and 2. Finally, solution infrared spectroscopy of Rh/sub 2/(O/sub 2/CCH/sub 3/)/sub n/(HNOCCH/sub 3/)/sub 4-n/CO species showed that the CO stretching frequencies are lowered as the numbermore » of HNOCCH/sub 3/ bridging ligands is increased. The lowest CO stretching frequency in this series occurs for Rh/sub 2/(HNOCCH/sub 3/)/sub 4/CO, which has upsilon/sub CO/ = 2028 cm/sup -1/.« less