UV photolysis of M(C p)(CO) 3 (M ) Mn or Re) in liquid or supercritical CO 2 (scCO 2) solution led to the formation of the CO 2 complexes M(C p)(CO) 2(CO 2), which were detected by nanosecond time-resolved infrared spectroscopy (TRIR). The coordination of CO 2 to the metal centers was confirmed by carrying out experiments in supercritical Kr (scKr) and scKr doped with CO 2. Differences between the positions of the v(C-O) IR bands of the CO ligands in Mn(C p)(CO) 2(CO 2) and Re(C p)(CO) 2(CO 2) suggest that the CO 2 ligand has different coordination modes to the metal centers in these complexes. The rate constants and activation enthalpies for the reactions of M(C p)(CO) 2(CO 2) with CO, H 2, and N 2 in scCO 2 have been measured and compared with those previously reported for the analogous Xe and heptane complexes in scXe and heptane, respectively. Striking differences and similarities between these kinetic data and also the IR spectra of the CO 2, Xe, and heptane complexes provided evidence that the CO 2 coordination mode is η 1-O end-on bound in Mn(C p)(CO) 2(CO 2), and η 2-C,O side-on bound in Re(C p)(CO) 2(CO 2). These different coordination modes lead to dramatic differences in reactivity with CO, H 2, and N 2, with the Re complexes being significantly less reactive. To provide more evidence for the nature of the CO 2 binding modes, a series of DFT calculations were performed at the B3LYP/SDD-6-311G** level. The calculations supported the experimentally proposed CO 2 coordination modes. For Mn(C p)(CO) 2(CO 2), a two-electron stabilizing interaction leads to the η1-O coordination mode, with a major component of the bonding being an electrostatic attraction, with little charge transfer between CO 2and Mn. For Re(C p)(CO) 2(CO 2), the ? 2-C,O structure was more stable than η 1-O by 1.7 kcal mol -1. A significant charge transfer from Re to CO 2 occurs, resulting in partial oxidation of Re. Frequency calculations corroborate these conclusions and also reveal that the IR band observed at ca. 1860 cm -1 in the TRIR spectrum of Re(C p)(CO) 3 obtained after irradiation in scCO 2 is due to the asymmetric CO 2 stretch of bound CO 2. Reaction enthalpies and activation barriers were also calculated for the reactions of the CO 2 complexes with CO, H 2, and N 2.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry