Gas-phase reactions of Nb sup + and Ta sup + with alkanes and alkenes. C-H bond activation and ligand-coupling mechanisms

• Published in: Inorganic chemistry 1989-09, Vol.28:18
• Main Authors: Buckner, S.W., MacMahon, T.J., Byrd, G.D., Freiser, B.S.
• Format: Article
• Language: English
• Subjects: 400201 - Chemical & Physicochemical Properties; ALKANES; ALKENES; CATIONS; CHARGED PARTICLES; CHEMICAL REACTIONS; DATA; EXPERIMENTAL DATA; FLUIDS; GASES; HYDROCARBONS; INFORMATION; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; IONS; NIOBIUM COMPOUNDS; NUMERICAL DATA; ORGANIC COMPOUNDS; REFRACTORY METAL COMPOUNDS; TANTALUM COMPOUNDS; TRANSITION ELEMENT COMPOUNDS
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic00317a024

The gas-phase reactions of Nb{sup +} with small alkanes and alkenes are reported. Nb{sup +} sequentially activates C-H bonds in ethene six times to produce NbC{sub 2n}H{sub 2n}{sup +} (n = 1-6). The C{sub 2}H{sub 2} ligands in this reaction couple via migratory insertion of the acetylene units. Ethane sequentially reacts with Nb{sup +} five times. Ligand coupling is proposed for these reactions as well. Comparison of NbC{sub 4}H{sub 6}{sup +} produced from isobutane, n-butane, and ethane indicates the presence of at least two isomeric forms. NbC{sub 4}H{sub 4}{sup +} ions produced from ethene, cyclobutane, and 1-butene produced similar CID spectra, supporting the presence of one structure. Excited Nb{sup +} produced by direct laser desorption reacts with methane to produce NbCH{sub 2}{sup +} and NbCH{sub 3}{sup +}, but collisional cooling with argon quenches the reaction. Primary reactions indicate a strong preference for C-H insertion, and reaction products indicate 1,3-hydrogen migration when {beta}-hydrogens are not present. The reactions with alkanes are compared to those with Ta{sup +}, which reacts with CH{sub 4} four times to produce TaC{sub 4}H{sub 8}{sup +}. Ta{sup +} and Nb{sup +}, which react similarly, are by far the most reactive metal ions toward C-H bond activation reported to date. CID spectra of the products form the Nb{sup +} reactions indicate that the extensive dehydrogenation reactions occur via ligand coupling and, for larger alkanes (C{sub 5} and C{sub 6}), dehydrocyclization. For example, Nb{sup +} undergoes a novel dehydrocyclization reaction with pentane to generate Nb(Cp){sup +} (Cp = cyclopentadienyl). Nb{sup +} reacts with cyclopentane to generate Nb(Cp){sub 2}{sup +} as a second product. 34 refs., 3 figs., 2 tabs.