Experimental and Theoretical Investigation into the Formation and Reactivity of M(Cp)(CO)2(CO2) (M = Mn or Re) in Liquid and Supercritical CO2 and the Effect of Different CO2 Coordination Modes on Reaction Rates with CO, H2, and N2

Abstract

Nanosecond time-resolved infrared spectroscopy (TRIR) has been used to study the coordination of CO₂ to the metal centers by UV photolysis of M(Cp)(CO)₃ (M = Mn or Re) in liquid or supercritical CO₂ (scCO₂) solution, which led to the formation of the CO₂ complexes M(Cp)(CO)₂(CO₂). Differences between the positions of the ν(C−O) IR bands of the CO ligands in Mn(Cp)(CO)₂(CO₂) and Re(Cp)(CO)₂(CO₂) suggest that the CO₂ ligand has different coordination modes to the metal centers in these complexes. The kinetic data and the IR spectra of the CO₂, Xe, and heptane complexes provided evidence that the CO₂ coordination mode is η¹-O end-on bound in Mn(Cp)(CO)₂(CO₂), and η²-C,O side-on bound in Re(Cp)(CO)₂(CO₂). These different coordination modes lead to dramatic differences in reactivity with CO, H₂, and N₂, with the Re complexes being significantly less reactive. To provide more evidence for the nature of the CO₂ binding modes, a series of DFT calculations were performed at the B3LYP/SDD-6-311G** level. The calculations supported the experimentally proposed CO₂ coordination modes. A significant charge transfer from Re to CO₂ occurs, resulting in partial oxidation of Re.