Synthesis and Characterization of Chelating Alkyl-borohydride Complexes of Thorium and Uranium: Th(CH₂NMe₂BH₃)₄ and U(CH₂NMe₂BH₃)₄

This paper describes the synthesis and characterization of two actinide alkyls: thorium and uranium compounds of stoichiometry M(CH2NMe2BH3)4 (M = Th, U) where CH2NMe2BH3– is the boronato­dimethyl­aminomethyl group (abbreviated BDAM). The crystal structures of these compounds show that the coordination geometry of the metal center can be described in terms of a D2d dodecahedron, whose eight vertices are defined by the four metal-bound carbon atoms and the four boron atoms of the BH3 groups. Two of the BH3 groups in each compound bind to the metal centers in a κ2 fashion (i.e., by means of two B–H–M bridges) and two in a κ1 fashion. Thus, both compounds can also be described as ten-coordinate in which four carbon atoms and six hydrogen atoms form the inner coordination sphere. The structures of both complexes and the nature of the BH3–M interactions were investigated computationally. The NMR spectra of the diamagnetic thorium(IV) compound and paramagnetic uranium(IV) compound are discussed; both compounds are fluxional in solution even at −124 °C. Variable temperature magnetization studies of the paramagnetic uranium(IV) compound in the solid state are consistent with its tetravalent oxidation state.