Metal Effect on Cationic [Cp2MH]+ (M = Group 4 and 5)-Mediated CO2 Hydrogenation in the Gas Phase.

Abstract

Effective CO2 hydrogenation has recently attracted quite some attention for producing more valuable chemical oxygenates (such as methanol, formate) in mild conditions. However, the influence of the metal center on the CO2 activation remains unclear. First, electrospray ionization mass spectrometry (ESI-MS) was employed to explore the direct CO2 hydrogenation to formic acid mediated by [Cp2MH]+ (M = Zr, Hf) in the gas phase at room temperature. The key formate intermediate [Cp2M(O2CH)]+ (M = Zr, Hf) was confirmed by traveling wave ion mobility spectrometry (TWIMS). Second, to gain insights into the metal effect, the CO2 hydrogenation process involving Group 4 (i.e., Ti, Zr, Hf) transition metals was calculated along with Group 5 (i.e., V, Nb, Ta) by density functional theory (DFT) methods. The CO2 insertion process was found to be the rate-limiting step. For [Cp2TiH]+, [Cp2ZrH]+, [Cp2HfH]+, [Cp2VH]+, [Cp2NbH]+, and [Cp2TaH]+, the barriers are +7.7, +6.5, +5.9, +9.2, +8.0, and +6.3 kcal/mol, respectively. [Cp2HfH]+-mediated CO2 hydrogenation occurs the most rapidly, as revealed by MS. According to the orbital analysis on the CO2 insertion transition state, the electron-deficient metal center resulting in a low-lying lowest unoccupied molecular orbital (LUMO) could interact more favorably with the π bond of deformed CO2, which was also consistent with the natural bond orbital (NBO) results. Last but not the least, NBO charges on the metal centers were found to correlate linearly well with the CO2 insertion barriers rather than hydride affinity. Thus, the reactivity of different metal hydride complexes with CO2 to produce a formate could be estimated by the NBO charge on metals. Our findings might provide a series of candidates for the catalyst as well as guidance for catalyst design in mild CO2 hydrogenation.