The Reductive Elimination of Methane from ansa-Hydrido(methyl)metallocenes of Molybdenum and Tungsten: Application of Hammond’s Postulate to Two-State Reactions
Résumé
The energetic profile of the methane reductive elimination from a selected number of hydrido(methyl)molybdenocene and -tungstenocene derivatives has been calculated by DFT methods. The calculations were carried out for the CH2(C5H4)2M (a-M), SiH2(C5H4)2M (a-H2Si–M), and SiMe2(C5Me4)2M (a-Me2Si–M*) ansa-metallocene systems for M = Mo, W. They include the full optimization of minima [the hydrido(methyl) starting complexes, M(H)(CH3), the intermediate methane complexes, M(CH4), and the metallocene products in the singlet and triplet configurations, (3M and 1M)], transition states (for the methyl hydride reductive elimination, M–TSins, and for the hydrogen exchange, M–TSexch), and the minimum energy crossing point (M–MECP) leading from the singlet methane complexes to the corresponding triplet metallocenes. The results are compared with those previously obtained for the simpler (C5H5)2M (Cp2M) systems (J. C. Green, J. N. Harvey, and R. Poli, J. Chem. Soc., Dalton Trans. 2002, 1861). The calculated energy profiles, notably the relative energies of M–TSins and M–MECP, are in agreement with available experimental observations for the a-Me2Si–M* systems. The comparison of the energies and geometries of the rate-determining M–TSins and M–MECP structures with those of the thermodynamically relevant minima for the various systems show the applicability of Hammond’s postulate to two-state reactions. However, one notable exception serves to show that the principle is only quantitatively reliable when all the potential energy surfaces for the set of analogous reactions have similar shapes. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
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