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

Abstract

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.

κ3‐(N^C^C)Gold(III) Carboxylates: Evidence for Decarbonylation Processes

“…However, alternatives to this general trend have also been reported, involving decarbonylation pathways for complexes bearing strong coordinating ligands as a result of a less facile β‐hydride elimination [Scheme A, Eq. (2)] . Examples of dinuclear (μ‐CO 2 ‐κ 2 C,O) complexes have also been found as a result of the acid‐base reaction between metallocarboxylic acids and their corresponding hydroxides [Scheme A, Eq.…”

Carbon Dioxide Insertion into Bridging Iron Hydrides: Kinetic and Mechanistic Studies

“…There are few reports detailing the kinetic parameters for hydride transfer from a transition metal center to CO 2 . Independent of this dearth of detailed kinetic studies, two mechanistic pathways are proposed: normal insertion wherein an H – attacks the π* orbital of CO 2 , and abnormal insertion in which a metallocarboxylate is generated , . The latter pathway is typically invoked for CO 2 reduction to CO and H 2 O, whereas the former leads to formate.…”

Photochemically Driven Reverse Water‐Gas Shift at Ambient Conditions mediated by a Nickel Pincer Complex

“…NMR spectroscopic monitoring of the reaction indicates initial formation of 5 BArF and build‐up of 4 at higher conversions. This observation suggests that the formation of hydrocarbonate 4 might be attributed to competing photochemical conversion of intermediate 3 at low acid concentrations …”

“…This selectivity is predetermined by the favored normal CO 2 insertion into catalyst M−H bonds giving formates (MO 2 CH) instead of the metallacarboxylate (MCO 2 H) isomers . We recently reported the first well‐defined abnormal CO 2 insertion reaction for the nickel(II) hydride [NiH(PNP)] ( 1 ; PNP=N(CHCHP t Bu 2 ) 2 ) . In contrast to the thermal insertion product [Ni(O 2 CH)(PNP)] ( 2 ), under photochemical conditions the hydroxycarbonyl isomer [Ni(CO 2 H)(PNP)] ( 3 ) is obtained selectively (Scheme ).…”

“…This interpretation is confirmed by chemical reduction. Nickel(I) complex [Ni I (CO)(PNP)] ( ν CO =1910 cm −1 ), which we previously reported from reaction of [Ni(PNP)] with CO, is obtained upon reduction of 5 + with Mg. Reduction of [Ni I (CO)(PNP)] with 1 equiv NaC 10 H 8 selectively gives a diamagnetic species, which exhibits C s symmetry below −30 °C on the 1 H NMR timescale.…”