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

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.