Kinetic Isotope Effects and the Mechanism of CO 2 Insertion into the Metal-Hydride Bond of fac -(bpy)Re(CO) 3 H.

The 1,2-insertion reaction of CO 2 into metal-hydride bonds of d 6 -octahedral complexes to give κ 1 - O -metal-formate products is the key step in various CO 2 reduction schemes and as a result has attracted extensive mechanistic investigations. For many octahedral catalysts, CO 2 insertion follows an associative mechanism in which CO 2 interacts directly with the coordinated hydride ligand instead of the more classical dissociative mechanism that opens an empty coordination site to bind the substrate to the metal prior to a hydride migration step. To better understand the associative mechanism, we conducted a systematic quantum chemical investigation on the reaction between CO 2 and fac -(bpy)Re(CO) 3 H ( 1 -Re-H; bpy = 2,2'-bipyridine) starting with the gas phase and then moving to THF and other solvents with increased dielectric constants. Detailed analyses of the potential energy surfaces (PESs) and intrinsic reaction coordinates (IRCs) reveal that the reaction is enabled in all media by an initial stage of making a 3c-2e bond between the carbon of CO 2 and the metal-hydride bond that is most consistent with an organometallic bridging hydride Re-H-CO 2 species. Once CO 2 is bent and anchored to the metal-hydride bond, the reaction proceeds by a rotation motion via a cyclic transition state TS2 that interchanges Re-H-CO 2 and Re-O-CHO coordination. The combined stages provide an asynchronous-concerted pathway for CO 2 insertion on the Gibbs free energy surface with TS2 as the highest energy point. Consideration of TS2 as a rate-determining TS gives activation barriers, inverse KIE s, substituent effects, and solvent effects that agree with the experimental data available in this system. An important new insight revealed by the analyses of the results is that the initial stage of the reaction is not a hydride transfer step as has been assumed in some studies. In fact, the loose vibration of the TS that can be identified for the first stage of the reaction in solution ( TS1 ) does not involve the Re-H stretching vibrational mode. Accordingly, the imaginary frequency of TS1 is insensitive to deuteration, and therefore, TS1 leads to no significant KIE .