Molecular Catalysts with Intramolecular Re-O Bond for Electrochemical Reduction of Carbon Dioxide.

A new Re bipyridine-type complex, namely, -Re(pmbpy)(CO)Cl (pmbpy = 4-phenyl-6-(2-hydroxy-phenyl)-2,2'-bipyridine), , carrying a single OH moiety as local proton source, has been synthesized, and its electrochemical behavior under Ar and under CO has been characterized. Two isomers of , namely, characterized by the proximity of Cl to OH and , are identified. The interconversion between and is clarified by DFT calculations, which reveal two transition states. The energetically lower pathway displays a non-negligible barrier of 75.5 kJ mol. The 1e electrochemical reduction of affords the neutral intermediate , formally derived by reductive deprotonation and loss of Cl from . , which exhibits an entropically favored intramolecular Re-O bond, has been isolated and characterized. The detailed electrochemical mechanism is demonstrated by combined chemical reactivity, spectroelectrochemistry, spectroscopic (IR and NMR), and computational (DFT) approaches. Comparison with previous Re and Mn derivatives carrying local proton sources highlights that the catalytic activity of Re complexes is more sensitive to the presence of local OH groups. Similar to (2OH = 4-phenyl-6-(phenyl-2,6-diol)-2,2'-bipyridine), and display a selective reduction of CO to CO. In the case of the Re bipyridine-type complex, the formation of a relatively stable Re-O bond and a preference for phenolate-based reactivity with CO slightly inhibit the electrocatalytic reduction of CO to CO, resulting in a low TON value of 9, even in the presence of phenol as a proton source.