Hydride oxidation from a titanium-aluminum bimetallic complex: insertion, thermal and electrochemical reactivity

We report the synthesis and reactivity of paramagnetic heterometallic complexes containing a Ti(III)-mu-H-Al(III) moiety. Combining different stoichiometries amounts of Cp2TiCl and KH3AlC(TMS)(3) (Cp = cyclopentadienyl, TMS = trimethylsilyl) resulted in the formation of either bimetallic Cp2Ti(mu-H)(2)(H) AlC(TMS)(3) (2) or trimetallic (Cp2Ti)(2)(mu-H)(3)(H)AlC(TMS)(3) (3) via salt metathesis pathways. While these complexes were indefinitely stable at room temperature, the bridging hydrides were readily activated upon exposure to heteroallenes, heating, or electrochemical oxidation. In each case, formal hydride oxidation occurred, but the isolated product maintained the +3 oxidation state at both metal centers. The nature of this reactivity was explored using deuterium labelling experiments and Density Functional Theory (DFT) calculations. It was found that while C-H activation from the Ti(III) bimetallic may occur through a sigma-bond metathesis pathway, chemical oxidation to Ti(IV) promotes bimolecular reductive elimination of dihydrogen to form a Ti(III) product.