Tetrahydroborate complexes of uranium: Preparation and crystal structure of mixed-valent [Na(THF)6] [(C5Me5)U(BH4)3

Abstract

Reaction of U(BH4)3·nTHF with Cp*2Th(PPh2)2 in a THF/toluene solution containing NaCl resulted in isolation of the novel tetrahydroborate complex [Na(THF)6][Cp*U(BH4)3]2, Cp*=C5Me5. The complex crystallizes in the space group P1 with cell parameters of a=11.110(3), b=15.140(3), c= 17.856(3) Å, α=88.27(1), β=74.49(2) and γ= 85.42(2)o, Z=2 and Dcalc=1.49 g/cm3. The unit cell contains two crystallographically independent uranium atoms in general positions, each coordinated by three tridentate, hydrogen-bridged BH4− groups and one η5-pentamethylcyclopentadienyl ligand. Two Na(THF)6+ cations, which occupy centers of crystallographic symmetry, complete the structure. Metrical parameters are not significantly different between the two independent uranium centers (av. UB distances 2.61 Å) or the two independent sodium containing cations. Crystallographically independent Cp*U(BH4)3 moieties pack to form two separate chains parallel to the crystallographic a axis. The stoichiometry and accompanying black color for the crystal suggest a mixed-valent charge transfer compound for which the average uranium oxidation state is 3.5. An unexpected Cp* transfer from thorium to uranium is also noted in the reaction chemistry.