Heterobimetallic aluminium/group 4 complexes derived from single-electron processes initiated by an aluminyl anion

The reactivity of sodium and potassium aluminyls with group 4 metallocene dichloride compounds is described. The potassium aluminyl K[Al(NON)] reacts with TiCp₂Cl₂ at low temperature to afford an unstable heterobimetallic complex that contains a double hydride bridge and a reduced Ti(III) centre. Upon warming to room temperature this compound undergoes an intramolecular C-H activation to form a tridentate κN,N',C-ligand at aluminium, retaining a single μ-H bridge between the Al and Ti. The corresponding reaction of the potassium aluminyl with ZrCp₂Cl₂ afforded a bimetallic Al/Zr(IV) complex containing a terminal Zr-H bond in addition to a Al(μ-H)₂Zr core. Replacing the potassium aluminyl with the sodium analogue formed a tetrametallic Al₂Zr₂ complex, which may be considered as an alane adduct of Schwartz's reagent. Attempts to extend this work to hafnium afforded a mixture of compounds identified crystallographically as a homobimetallic Hf compound and a potassium chloroaluminate that are the products of inter- and intra-molecular C-H activation, respectively. Although it has not been possible to analyse the mechanisms leading to the formation of these products, the results suggest that single electron pathways are prevalent, affording hydride species derived from C-H activation of solvent and/or ligand substituents.