Beryllium Dimer Reactions with Acetonitrile: Formation of Strong Be-Be Bonds

Laser ablated Be atoms have been reacted with acetonitrile molecules in 4 K solid neon matrix. The diberyllium products BeBeNCCH3 and CNBeBeCH3 have been identified by D and 13C isotopic substitutions and quantum chemical calculations. The stabilization of the diberyllium species is rationalized from the formation of the real Be-Be single bonds with bond distances as 2.077 and 2.058 Å and binding energies as -27.1 and -77.2 kcal/mol calculated at CCSD (T)/aug-cc-pVTZ level of theory for BeBeNCCH3 and CNBeBeCH3, respectively. EDA-NOCV analysis described the interaction between Be2 and NC···CH3 fragments as Lewis "acid-base" interactions. In the complexes, the Be2 moiety carries positive charges which transfer from antibonding orbital of Be2 to the bonding fragments significantly strengthen the Be-Be bonds that are corroborated by AIM, LOL and NBO analyses. In addition, mono beryllium products BeNCCH3, CNBeCH3, HBeCH2CN and HBeNCCH2 have also been observed in our experiments.

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.

Activation of CO2 by Alkaline-Earth Metal Hydrides: Matrix Infrared Spectra and DFT Calculations of HM(O2CH) and (MH2)(HCOOH) Complexes (M = Sr, Ba)

Reactions of MH₂ (M = Sr, Ba) with CO₂ were explored in pure parahydrogen at 3.5 K using matrix isolation infrared spectroscopy and quantum chemical calculations. The formate complex HM(η²-O₂CH) and formic acid complex (MH₂)(HCOOH) were trapped and identified by isotopic substitutions and density functional theory (DFT) frequency calculations. Natural population analysis and the CO₂ reduction mechanism demonstrate that hydride ion transfer from a metal hydride to a CO₂ moiety facilitates the stabilization of such complexes.