Acceleration of CO₂ insertion into metal hydrides: ligand, Lewis acid, and solvent effects on reaction kinetics

Abstract

Kinetic studies of CO2 insertion into transition metal hydrides provide evidence about the nature of the rate-determining transition state.

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

“…The value corresponds to a minimal normal KIE of 1.08(9) for the first hydride insertion event. This result contrasts the inverse KIE reported for insertion of the terminal hydride into CO 2 by a rhenium hydride complex (0.52–0.58) or a nickel hydride species (0.61–0.79) , . Assuming that this differences arises from the greater bond strength of one or both Fe–(µ‐H) bonds for the hydride being transferred from 1 to CO 2 as compared to the reported monometallic complexes, we surmise that one or more Fe–H bonds is substantially weakened during hydride transfer to CO 2 (Scheme ).…”

“…Executing the reaction in toluene at room temperature leads to greater product speciation as compared to THF as evidenced by the number of C=O stretches in IR spectra of the reaction mixture. Previously, hydridicity of metal hydrides has been correlated to solvent acceptor number, consistent with the observed solvent effect for reaction of 1 with CO 2 , , , . Taken together with the reactivity of Fe 3 H 3 L Et/Me with CO which results in H 2 reductive elimination, we proposed that the hydrides in these clusters are coordinatively fluxional and capable of adopting semi‐bridging or terminal modes.…”

“…Contrastingly, the outer sphere mechanism results from hydride insertion followed by reorientation and coordination of the resultant formate, and is expected for coordinatively saturated complexes . Here, the highly polarized transition state can be stabilized by exogenous Lewis acids or secondary coordination sphere interactions , …”

“…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 , .…”

“…With respect to terminal metal hydrides, the two generally accepted normal insertion mechanisms and their respective rate controlling steps are depicted in Scheme , . Coordinatively unsaturated complexes typically proceed through an inner sphere mechanism: CO 2 binds to the metal centers at a vacant coordination site, followed by a concerted M–H bond breaking and C–H bond forming event to generate the (formato)metal complex . Contrastingly, the outer sphere mechanism results from hydride insertion followed by reorientation and coordination of the resultant formate, and is expected for coordinatively saturated complexes .…”