Matrix Isolation Infrared Spectroscopic And Theoretical Studies Of The Reactions Of Group V Metal Oxides Molecules With Dihydrogen And Dinitrogen

The reaction of group V metal oxides molecules with dihydrogen and dinitrogen have been investigated by matrix isolation infrared spectroscopic and quantum chemical calculations. The reaction intermediates and products are identified by isotopic substitution and density functional theoretical calculations. The reaction mechanism and periodic trends are discussed.The ground state VO₂ molecule is able to cleave dihydrogen heterolytically and spontaneously in forming the HVO(OH) molecule in solid argon.The observation of spontaneous reaction is consistent with theoretical predictions that only under UV-visible excitation via a weakly bound the dihydrogen cleavage process is both thermodynamically exothermic and kinetically facile.In contrast, the reaction of VO₂ with dideterium to form DVO(OD) proceeds only under UV-visible excitation via a weakly bound VO₂(η2-D2) complex.The niobium and tatalum dioxide molecules react with dihydrogen to give primarily the side-on bonded metal dioxide bis-dihydrogen complexes, NbO₂(η2-H₂)2 and TaO₂(η2-H₂)2,which are further transferred to the HNbO(OH) and HTaO(OH) molecules via photoisomerization in combination with H₂ elimination under UV-visible light excition. The reactions are summarized in Scheme1: (?)The Laser-evaporated tantalum atoms react with H₂/O₂ mixtures to give primarily the tantalum dioxide molecule as well as the side-on bonded superoxo-tantalum dioxide complex, TaO₂(η2-O₂), which interact with dihydrogen to form the HTaO(OH)(η2-O₂) complex spontaneously on annealing via a weakly bound TaO₂(η2-O₂)(η2-H₂) intermediate. The observation of the spontaneous reaction is consistent with theoretical predictions that the TaO₂(η2-O₂) + H₂â†'TaO₂(η2-O₂)(η2-H₂)â†'HTaO(OH)(η2-O₂) reactions are thermodynamically exothermic and both elementary reaction steps are kinetically facile, requiring little activation energy. The HTaO(OH)(η2-O₂) complex further rearranges to the Ta(OH)2 molecule under broad-band UV irradiation, which is characterized to have a quartet ground state with linear geometry. The reactions are summarized in Scheme2: (?)The reactions of transitional metal dioxides MO₂(M=V,Nb,Ta) with dinitrogen in solid argon. The metal dioxide molecules produced from laser evaporation of bulk M2O5 target reacted with dinitrogen to form the MO₂(η1-NN)x (x=1-3) complexes on annealing, in which the N₂ ligands are end-on bonded to the metal center. The MO₂(η1-NN)3 complex decomposed to MO₂(η1-NN)2 under infrared irradiation. The MO₂(η1-NN)2 and MO₂(η1-NN)3 complexes rearranged to the less stable MO₂(η1-NN)(η2-N₂) and MO₂((η1-NN)2(η2-N₂) isomers under visible light excitation. The reactions of MO₂(M=V,Nb,Ta) with dinitrogen are summarized in Scheme3: (?)Both the mono- and bis-dinitrogen complexes were predicted to have 2A′or 2A1 ground states arising from the 2A1 ground state of MO₂, whereas the two tri-dinitrogen complexes were predicted to have 2B2 ground states with C2v symmetry, which are derived from the 2B1 excited state of MO₂.