Strong Impact of Intramolecular Hydrogen Bonding on the Cathodic Path of [Re(3,3'-dihydroxy-2,2'-bipyridine)(CO)3Cl] and Catalytic Reduction of Carbon Dioxide.

Abstract

Herein, we present the cathodic paths of the Group-7 metal complex [Re(3,3′-DHBPY)(CO)3Cl] (3,3′-DHBPY = 3,3′-dihydroxy-2,2′-bipyridine) producing a moderately active catalyst of electrochemical reduction of CO2 to CO. The combined techniques of cyclic voltammetry and IR/UV–vis spectroelectrochemistry have revealed significant differences in the chemistry of the electrochemically reduced parent complex compared to the previously published Re/4,4′-DHBPY congener. The initial irreversible cathodic step in weakly coordinating THF is shifted toward much less negative electrode potentials, reflecting facile reductive deprotonation of one hydroxyl group and strong intramolecular hydrogen bonding, O–H···O–. The latter process occurs spontaneously in basic dimethylformamide where Re/4,4′-DHBPY remains stable. The subsequent reduction of singly deprotonated [Re(3,3′-DHBPY-H+)(CO)3Cl]− under ambient conditions occurs at a cathodic potential close to that of the Re/4,4′-DHBPY-H+ derivative. However, for the stabilized 3,3′-DHBPY-H+ ligand, the latter process at the second cathodic wave is more complex and involves an overall transfer of three electrons. Rapid potential step electrolysis induces 1e–-reductive cleavage of the second O–H bond, triggering dissociation of the Cl– ligand from [Re(3,3′-DHBPY-2H+)(CO)3Cl]2–. The ultimate product of the second cathodic step in THF was identified as 5-coordinate [Re(3,3′-DHBPY-2H+)(CO)3]3–, the equivalent of classical 2e–-reduced [Re(BPY)(CO)3]−. Each reductive deprotonation of the DHBPY ligand results in a redshift of the IR ν(CO) absorption of the tricarbonyl complexes by ca. 10 cm–1, facilitating the product assignment based on comparison with the literature data for corresponding Re/BPY complexes. The Cl– dissociation from [Re(3,3′-DHBPY-2H+)(CO)3Cl]2– was proven in strongly coordinating butyronitrile. The latter dianion is stable at 223 K, converting at 258 K to 6-coordinate [Re(3,3′-DHBPY-2H+)(CO)3(PrCN)]3–. Useful reference data were obtained with substituted parent [Re(3,3′-DHBPY)(CO)3(PrCN)]+ that also smoothly deprotonates by the initial reduction to [Re(3,3′-DHBPY-H+)(CO)3(PrCN)]. The latter complex ultimately converts at the second cathodic wave to [Re(3,3′-DHBPY-2H+)(CO)3(PrCN)]3– via a counterintuitive ETC step generating the 1e– radical of the parent complex, viz., [Re(3,3′-DHBPY)(CO)3(PrCN)]. The same alternative reduction path is also followed by [Re(3,3′-DHBPY-H+)(CO)3Cl]− at the onset of the second cathodic wave, where the ETC step results in the intermediate [Re(3,3′-DHBPY)(CO)3Cl]•– further reducible to [Re(3,3′-DHBPY-2H+)(CO)3]3– as the CO2 catalyst.