Enhancement and control of the selectivity in light-driven ketone versus water reduction using aminopyridine cobalt complexes

Abstract

Cobalt(II) complexes with the general structure [Co^II(OTf)(^Y,XPy₂^Tstacn)](OTf) (1^R, where ^Y,XPy₂^Tstacn is 1,4-di(p-Y,m-X-picolyl)-7-R-1,4,7-triazacyclononane; 1^H, 1^CO2Et, 1^DMM) and [Co^II(OTf)₂(^Y,XPy^Metacn)] (2^R, where ^Y,XPy^Metacn is 1-(p-Y,m-X-picolyl)-7,4-di-methyl-1,4,7-triazacyclononane; 2^CO2Et, 2^Cl, 2^H, 2^DMM, 2^NMe2) were active in both light-driven acetophenone (3a) and water reduction. Competition studies show that aromatic ketone/water reduction selectivity ranks from 0.2 to 8.0. Nevertheless, considering the concentrations of water and ketone in catalysis (ratio H₂O/3a ∼ 2000) the highest selectivity obtained is greater than 15 000. The selectivity correlates well with the Co^I/II redox potential within the same cobalt catalyst series (span 240 mV (1^R) and 290 mV (2^R)), with electron donating ligands favoring ketone reduction over H₂ evolution. Based on this finding, the operative mechanism for the reduction of aromatic ketones is consistent with a single electron transfer (SET) followed by a hydrogen atom transfer (HAT) mechanism. This new insight will be a guide to develop selective catalytic systems to produce fine solar chemicals in water.