Electrochemical and DFT studies of the oxidative decomposition of the trihydride complexes Cp*M(dppe)H3 (M = Mo, W) in acetonitrile

• Autores: R. Poli, R. Meunier-Prest, M. Baya
• Localización: New journal of chemistry, ISSN 1144-0546, Vol. 30, Nº 5, 2006, págs. 759-773
• Idioma: inglés
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• Resumen

  • A detailed electrochemical study of the oxidative decomposition of the trihydride complexes Cp*M(dppe)H3 (M = Mo, W) in acetonitrile is presented. For the Mo complex, the decomposition occurs by four different pathways involving classical and non-classical tautomers, whereas only the classical form is accessible for the W derivative. Each of the decomposition pathways has been quantitatively assessed by analyses of the linear sweep voltammograms. In addition to the previously established (B. Pleune, D. Morales, R. Meunier-Prest, P. Richard, E. Collange, J. C. Fettinger and R. Poli, J. Am. Chem. Soc., 1999, 121, 2209–2225) deprotonation, disproportionation, and H2 reductive elimination occurring via the non-classical tautomer of the 17-electron complex [Cp*Mo(dppe)H3]+ (obtained by oxidation at E1/2 = −0.33 V vs. Ag/AgCl), a new decomposition pathway from the more stable classical tautomer has been identified following a second oxidation process. In addition, the oxidatively induced H2 reductive elimination, previously evidenced only in THF or CH2Cl2, has been quantitatively assessed in MeCN. This process occurs preferentially by an associative mechanism (k = 0.020(4) M−1s−1) via the 19-electron [Cp*Mo(dppe)H(H2)(MeCN)]+ intermediate and is therefore in direct competition with the disproportionation mechanism. The resulting 17-electron [Cp*Mo(dppe)H(MeCN)]+ product is further oxidized at ca. 0.2 V. The oxidation of [Cp*Mo(dppe)H3]+ occurs at ca. 1.0 V and the resulting 16-electron [Cp*Mo(dppe)H3]2+ complex immediately delivers a proton to the starting material, giving [Cp*Mo(dppe)H4]+ and [Cp*Mo(dppe)H2]+. The latter coordinates MeCN in a rate determining step to afford [Cp*Mo(dppe)H2(MeCN)]+. The mechanistic details are consistent with studies at different scan rates and different MeCN concentrations, and are backed up by DFT calculations.