Synthesis and Crystal Structure of a Self-Assembled, Octanuclear Oxo-Tantalum(V) Derivative Containing the First Example of a Transition Metal M₈(μ-O)₁₂ Cage

By reduction with CoCp*2 in THF or toluene, the tantalum(V) chlorocarbamato complex TaCl2(O2CNEt2)3, 1, gives high yields of the tantalum(III) N,N-diethylcarbamate, Ta(O2CNEt2)3, 2. On the other hand, good yields of the octanuclear μ-oxo-N,N-diethylcarbamato cage compound Ta8(μ-O)12(O2CNEt2)16, 3, were secured by reaction of 1 with sodium in THF. Compound 3 has been characterized by analytical and spectroscopic methods and by X-ray diffractometry. Crystal data for C80H160N16O44Ta8: M = 3497.84, triclinic, space group = P1̄ (no. 2), 1̄a = 15.440(3) Å, b = 15.710(1) Å, c = 16.090(2) Å, α = 101.53(4)°, β = 107.51(5)°, γ = 118.79(3)°, V = 2971.4(3) Å3, Z = 1, Dc = 1.955 Mg/m3, μ = 7.416 mm-1, F(000) = 1688, R1 = 0.0350, wR2 = 0.0798, T = 213 K. The eight tantalum atoms are located at the vertexes of a cube whose edges are occupied by 12 bridging oxo groups. Hexacoordination at tantalum is completed by bridging and terminal N,N-diethylcarbamato ligands. Both THF and the carbon dioxide fragments of the carbamato groups are involved in the formation of the μ-oxo cage oxides.