Ethylene polymerization using catalysts derived from activation of zirconocene aluminohydride complexes with either methyl aluminoxane or B(C6F5)3 is reported. Variable-temperature NMR spectra of mixtures of Cp*2ZrH3AlH2 or Cp‘2ZrH3AlH2 and excess B(C6F5)3 reveal the formation of di- or polynuclear metallocenium ion-pairs featuring terminal or both terminal and bridging borohydride counteranions HB(C6F5)3 arising from hydride abstraction. At higher T, ion-pairs featuring the terminal HB(C6F5)3 counterion decompose, and the AlH3 that is liberated degrades B(C6F5)3 to furnish mixtures of (C6F5) n AlH3 - n and, in the case of Cp*2ZrH3AlH2, a new ion-pair partnered with the diborohydride counteranion [Cp*2ZrH][(μ-H)2B(C6F5)2]. The latter compound was independently prepared from Cp*2ZrH2 and HB(C6F5)2 and is active in ethylene polymerization; however it is 1000 times less active than the catalyst formed from Cp*2ZrH3AlH2 and B(C6F5)3 and so cannot account for the multisite behavior of the latter combination. There is evidence of chemical exchange between “free” or terminal HB(C6F5)3 and excess B(C6F5)3 in these mixtures, and on the basis of model studies with [ n Bu4N][HB(C6F5)3] and B(C6F5)3, this involves reversible formation of [ n Bu4N][(C6F5)3B)(μ-H)B(C6F5)3], which can be detected by 19F NMR spectroscopy in solution at low T.

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Previously reported heterobimetallic rhodium−aluminum and iridium−aluminum alkyl complexes are shown to activate hydrogen, generating the corresponding alkane. Kinetic data indicate a mechanistic difference between the iridium-and rhodium-based systems. In both cases the transition metal is an active participant in the release of alkane from the aluminum center. For iridium− aluminum species, experimental mechanistic data suggest that multiple pathways occur concomitantly with each other: one being the oxidative addition of hydrogen followed by proton transfer resulting in alkane generation. Computational data indicate a reasonable barrier to formation of an iridium dihydride intermediate observed experimentally. In the case of the rhodium−aluminum species, hydrides are not observed spectroscopically, though a reasonable barrier to formation of this thermodynamically unstable species has been calculated. Alternative mechanistic possibilities are discussed and explored computationally. Cooperative hydrogenolysis mechanisms are computed to be energetically unfeasible for both metal centers.

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