Influence of alkali metal cations on the formation of the heterobimetallic actinide tert-butoxides [AnM3(OtBu)7] and [AnM2(OtBu)6] (AnIV = Th, U; MI = Li, Na, K, Rb, Cs)

Abstract

Heterobimetallic tert-butoxides of alkali metal cations with tetravalent actinide centers exhibit two distinctive structural motifs, [AnM₂(O^tBu)₆] and [AnM₃(O^tBu)₇] (An^IV = Th, U and M^I = Li, Na, K, Rb, Cs), evidently governed by the size of the alkali metal ions. Both [AnM₃(O^tBu)₇] AnM3 (An^IV = U, M^I = Li; An^IV = Th, M^I = Li, Na) and [AnM₂(O^tBu)₆] AnM2 (An^IV = U, M^I = Na–Cs; An^IV = Th, M^I = K–Cs) compounds are obtained in nearly quantitative yields by reacting actinide and alkali metal silyl amides with an excess of tert-butyl alcohol. The AnM3 complexes form a cubane-type coordination motif, whereas the AnM2 complexes display a geometry resembling two face-shared bipyramids. The sodium derivatives of thorium and uranium (ThNa3 and UNa2) allow the determination of the structural transition threshold as a function of the ratio of the ionic radii r_i(An^IV)/r_i(M^I). The AnM3 complexes are formed for ratios above 0.92 and the AnM2 type is formed for ratios below 0.87. All compounds are unambiguously characterized in both solution and solid states by NMR and IR spectroscopic studies and single crystal X-ray diffraction analyses, respectively.