Access To Metal Centers And Fluxional Hydride Coordination Integral For Co2 Insertion Into [Fe-3(Mu-H)(3)](3+) Clusters

CO2 insertion into tri(mu-hydrido) triiron(II) clusters ligated by a tris(beta-diketiminate) cyclophane is demonstrated to be balanced by sterics for CO2 approach and hydride accessibility. Time-resolved NMR and UV-vis spectra for this reaction for a complex in which methoxy groups border the pocket of the hydride donor (Fe3H3L2, 4) result in a decreased activation barrier and increased kinetic isotope effect consistent with the reduced sterics. For the ethyl congener Fe3H3L1 (2), no correlation is found between rate and reaction solvent or added Lewis acids, implying CO2 coordination to an Fe center in the mechanism. The estimated hydricity (50 kcal/mol) based on observed H/D exchange with BD3 requires Fe-O bond formation in the product to offset an endergonic CO2 insertion. mu(3)-hydride coordination is noted to lower the activation barrier for the first CO2 insertion event in DFT calculations.