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 [(C(5)Me(5))(2)(MesO)U-E(C(5)Me(5))], E = Al, Ga, Mes = 2,4,6-Me(3)C(6)H(2), and their reactivity with dihydrogen. The reaction of H(2) with the U(iii)–Al(i) complex affords a trihydroaluminate complex, [(C(5)Me(5))(2)(MesO)U(μ(2)-(H)(3))–Al(C(5)Me(5))] through a formal three-electron metal-based reduction, with concomitant formation of a terminal U(iv) hydride, [(C(5)Me(5))(2)(MesO)U(H)]. Noteworthy is that neither U(iii) complexes nor [(C(5)Me(5))Al](4) are capable of reducing dihydrogen on their own. To make the terminal hydride in higher yields, the reaction of [(C(5)Me(5))(2)(MesO)U(THF)] with half an equivalent of diethylzinc generates [(C(5)Me(5))(2)(MesO)U(CH(2)CH(3))] or treatment of [(C(5)Me(5))(2)U(i)(Me)] with KOMes forms [(C(5)Me(5))(2)(MesO)U(CH(3))], which followed by hydrogenation with either complex cleanly affords [(C(5)Me(5))(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.