Mechanisms of catalytic reduction of CO(2) with heme and nonheme metal complexes

The catalytic conversion of CO(2) into valuable chemicals and fuels has attracted increasing attention, providing a promising route for mitigating the greenhouse effect of CO(2) and also meeting the global energy demand. Among many homogeneous and heterogeneous catalysts for CO(2) reduction, this mini-review is focused on heme and nonheme metal complexes that act as effective catalysts for the electrocatalytic and photocatalytic reduction of CO(2). Because metalloporphyrinoids show strong absorption in the visible region, which is sensitive to the oxidation states of the metals and ligands, they are suited for the detection of reactive intermediates in the catalytic CO(2) reduction cycle by electronic absorption spectroscopy. The first part of this review deals with the catalytic mechanism for the one-electron reduction of CO(2) to oxalic acid with heme and nonheme metal complexes, with an emphasis on how the formation of highly energetic CO(2)˙ is avoided. Then, the catalytic mechanism of two-electron reduction of CO(2) to produce CO and H(2)O is compared with that to produce HCOOH. The effect of metals and ligands of the heme and nonheme complexes on the CO or HCOOH product selectivity is also discussed. The catalytic mechanisms of multi-electron reduction of CO(2) to methanol (six-electron reduced product) and methane (eight-electron reduced product) are also discussed for both electrocatalytic and photocatalytic systems.