Dinitrogen activation and reduction is one of the most challenging and important subjects in chemistry. Herein, we report the N 2 binding and reduction at the well-defined Ta 3 N 3 H − and Ta 3 N 3 − gas-phase clusters by using mass spectrometry (MS), anion photoelectron spectroscopy (PES), and quantum-chemical calculations. The PES and calculation results show clear evidence that N 2 can be adsorbed and completely activated by Ta 3 N 3 H − and Ta 3 N 3 − clusters, yielding to the products Ta 3 N 5 H − and Ta 3 N 5 − , but the reactivity of Ta 3 N 3 H − is five times higher than that of the dehydrogenated Ta 3 N 3 − clusters. The detailed mechanistic investigations further indicate that a dissociative mechanism dominates the N 2 activation reactions mediated by Ta 3 N 3 H − and Ta 3 N 3 − ; two and three Ta atoms are active sites and also electron donors for the N 2 reduction, respectively. Although the hydrogen atom in Ta 3 N 3 H − is not directly involved in the reaction, its very presence modifies the charge distribution and the geometry of Ta 3 N 3 H − , which is crucial to increase the reactivity. The mechanisms revealed in this gas-phase study stress the fundamental rules for N 2 activation and the important role of transition metals as active sites as well as the new significant role of metal hydride bonds in the process of N 2 reduction, which provides molecular-level insights into the rational design of tantalum nitride-based catalysts for N 2 fixation and activation or NH 3 synthesis.
The reaction of cobalt cluster anions Con– (3 ≤ n ≤ 17) with CO2 was studied experimentally and theoretically to explore the size-specific activation mode of CO2 by Con–. Mass spectrometric measurements revealed that the reactivity depends strongly on cluster size: the reactivity emerges abruptly at n = 7, peaks at n = 8–10, and then gradually decreases with increasing n. Infrared multiple photon dissociation spectra of ConCO2– exhibit a single peak at ∼1870 cm–1, similarly to the previously reported spectra of ConCO–. Density functional theory calculations for Co7CO2– as an example revealed that the dissociative adsorption of CO2 into CO and O is energetically more favorable than nondissociative adsorption. The infrared spectra calculated for dissociated isomers Co7(CO)O– reproduced the experimental results, whereas those for nondissociated isomers Co7CO2– did not. The photoelectron spectra of ConCO2– were shifted dramatically toward higher energies relative to those of Con–, suggesting electron transfer from Con– to the CO and O ligands. These results indicate that the CO2 molecule adsorbs dissociatively on Con–, in sharp contrast to its nondissociative adsorption onto the Co monomer anion
The reactivity of gas-phase cluster anions TaN with CH under thermal collision conditions was studied by mass spectrometry in conjunction with density functional theory calculations. The full dehydrogenation of the CH molecule was observed, with the formation of two dihydrogen molecules. Interestingly, the two carbon atoms originating from the first CH molecule are used to construct another cluster TaNC, which can activate one more CH releasing one H molecule. Therefore, three dihydrogen molecules are liberated from two ethene molecules in the overall reaction. The full dehydrogenation of CH by gas-phase anions as well as the structure and reactivity of M-N-C (M: transition metal) cluster is reported for the first time. The properties of TaN and TaNC elucidated herein are of use in providing fundamental information that is necessary to tailor the design of new and effective catalysts by applying the related materials.
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