The elusive abnormal CO2 insertion enabled by metal-ligand cooperative photochemical selectivity inversion

Direct hydrogenation of CO2 to CO, the reverse water–gas shift reaction, is an attractive route to CO2 utilization. However, the use of molecular catalysts is impeded by the general reactivity of metal hydrides with CO2. Insertion into M–H bonds results in formates (MO(O)CH), whereas the abnormal insertion to the hydroxycarbonyl isomer (MC(O)OH), which is the key intermediate for CO-selective catalysis, has never been directly observed. We here report that the selectivity of CO2 insertion into a Ni–H bond can be inverted from normal to abnormal insertion upon switching from thermal to photochemical conditions. Mechanistic examination for abnormal insertion indicates photochemical N–H reductive elimination as the pivotal step that leads to an umpolung of the hydride ligand. This study conceptually introduces metal-ligand cooperation for selectivity control in photochemical transformations.