Turning it off! Disfavouring hydrogen evolution to enhance selectivity for CO production during homogeneous CO(2) reduction by cobalt–terpyridine complexes

Understanding the activity and selectivity of molecular catalysts for CO(2) reduction to fuels is an important scientific endeavour in addressing the growing global energy demand. Cobalt–terpyridine compounds have been shown to be catalysts for CO(2) reduction to CO while simultaneously producing H(2) from the requisite proton source. To investigate the parameters governing the competition for H(+) reduction versus CO(2) reduction, the cobalt bisterpyridine class of compounds is first evaluated as H(+) reduction catalysts. We report that electronic tuning of the ancillary ligand sphere can result in a wide range of second-order rate constants for H(+) reduction. When this class of compounds is next submitted to CO(2) reduction conditions, a trend is found in which the less active catalysts for H(+) reduction are the more selective towards CO(2) reduction to CO. This represents the first report of the selectivity of a molecular system for CO(2) reduction being controlled through turning off one of the competing reactions. The activities of the series of catalysts are evaluated through foot-of-the-wave analysis and a catalytic Tafel plot is provided.