Cooperative dihydrogen activation with unsupported uranium-metal bonds and characterization of a terminal U(IV) hydride

Cooperative chemistry between two or more metal centres can show enhanced reactivity compared to the monometallic fragments. Given the paucity of actinide-metal bonds, especially those with group 13, we targeted uranium(III)-aluminum(I) and -gallium(I) complexes as we envisioned the low-valent oxidation state of both metals would lead to novel, cooperative reactivity. Herein, we report the molecular structure of [(C5Me5)(2)(MesO)U-E(C5Me5)], E = Al, Ga, Mes = 2,4,6-Me3C6H2, and their reactivity with dihydrogen. The reaction of H-2 with the U(III)-Al(I) complex affords a trihydroaluminate complex, [(C5Me5)(2)(MesO)U(mu(2)-(H)(3))-Al(C5Me5)] through a formal three-electron metal-based reduction, with concomitant formation of a terminal U(IV) hydride, [(C5Me5)(2)(MesO)U(H)]. Noteworthy is that neither U(III) complexes nor [(C5Me5)Al](4) are capable of reducing dihydrogen on their own. To make the terminal hydride in higher yields, the reaction of [(C5Me5)(2)(MesO)U(THF)] with half an equivalent of diethylzinc generates [(C5Me5)(2)(MesO)U(CH2CH3)] or treatment of [(C5Me5)(2)U(I)(Me)] with KOMes forms [(C5Me5)(2)(MesO)U(CH3)], which followed by hydrogenation with either complex cleanly affords [(C5Me5)(2)(MesO)U(H)]. All complexes have been characterized by spectroscopic and structural methods and are rare examples of cooperative chemistry in f element chemistry, dihydrogen activation, and stable, terminal ethyl and hydride compounds with an f element.