Molybdenum complexes protonation studies

In mechanistic studies, the molybdacyclobutane of a MAP catalyst was found to break up to ethylene/methylidene intermediates 4500 times faster than the corresponding tungstacycle (at 40°C) [19]. Syn and anti proton exchange were also found to be significantly faster (up to lOOx s) for molybdacycles than for tungsta-cycles. Methylidene rotation about the M=C bond was determined to be comparatively slower for molybdenum complexes (<0.2 s ) than for tungsten complexes (3.6-230 s ). Schrock and coworkers proposed that many of these properties contribute to the superior efficiency of the tungsten MAP catalysts relative to their molybdenum counterparts. 

Further support for these arguments comes from the X-ray structure of the molybdenum complex (jr-CsH5)2MoH2 (Figure 35) [213], and from proton resonance studies which show that the hydrogens of the trihydride complexes form an A2B group and hence are not equivalent [212]. 

Hydride transfer reactions from [Cp2MoH2] were discussed above in studies by Ito et al. [38], where this molybdenum dihydride was used in conjunction with acids for stoichiometric ionic hydrogenations of ketones. Tyler and coworkers have extensively developed the chemistry of related molybdenocene complexes in aqueous solution [52-54]. The dimeric bis-hydroxide bridged dication dissolves in water to produce the monomeric complex shown in Eq. (32) [53]. In D20 solution at 80 °C, this bimetallic complex catalyzes the H/D exchange of the a-protons of alcohols such as benzyl alcohol and ethanol [52, 54]. 

Hunt s group (50, 51) have pioneered the application of the Cl source to organometallics such as the iron tricarbonyl complex of heptafulvene, whose electron impact spectrum shows (M—CO)+ as the heaviest ion, in contrast to the methane Cl spectrum with the ion as base peak. Boron hydrides (52) and borazine (53) have also been studied. The methane Cl spectrum of arenechromium and -molybdenum (54) show protonation at the metal giving a protonated parent or molecular ion. Risby et al. have studied the isobutane Cl mass spectra of lanthanide 2,2,6,6-tetramethylheptane-3,5-dionates[Ln(thd)3] (55) and 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-oetanedione [H(fod)] lanthanide complexes (56). These latter complexes have been suggested as a means of analysis for the lanthanide elements. 

An in situ 15N-NMR study of the protonation reactions of molybdenum and tungsten dinitrogen complexes has shown that signals arise in the early stages of the reaction which can be assigned to relatively stable hydrazido(2-) species (4). With careful choice of acid and conditions, thermally stable hydrazido(2-) complexes can be isolated. They often yield ammonia and/or hydrazine upon further protonation (4,105), for example, reaction (43). 

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