Synthesis of Cobaltaborane Clusters from [Cp*CoCl]₂ and Monoboranes. New Structures and Mechanistic Implications

The reaction of [Cp*CoCl]₂, Cp* = η⁵-C₅Me₅, with BH₃·THF leads to the formation of nido-1-(η⁵-C₅Me₅)Co-2-(η⁴-C₅Me₅H)CoB₃H₈, 1 (60%, monoclinic P2₁/n, a = 8.076(1), b = 20.359(2), c = 13.880(2) Å, β = 91.23(1)°, V = 2281.7(6) ų, d(calcd) = 1.251g/cm³, Z = 4), and arachno-(η⁵-C₅Me₅)CoB₄H₁₀, 2 (2–10%, monoclinic P2₁/c, a = 13.954(3), b = 14.185(4), c = 29.383(7) Å, β = 100.61(2)°, V= 5717(2) ų, d(calcd) = 1.150 g/cm³, Z = 16) via the metastable paramagnetic intermediate [{(η⁵-C₅Me₅)Co}(BH₃Cl)]₂, 9, with the coproduct BH₂Cl. Heating 1 leads to nido-2,4-{(η⁵-C₅Me₅)Co}₂B₃H₇, 3 (80%, tetragonal P4₂/n, a = b = 23.440(4), c = 8.317(2) Å, V = 4570(1) ų, d(calcd) = 1.244 g cm^-3, Z = 8). The reaction of [Cp*CoCl]₂ with LiBH₄ leads to the formation of closo-2,3,4-{(η⁵-C₅Me₅)Co}₃B₂H₄, 4 (20%, rhombohedral, R3, a = 17.994 (1) Å, c = 15.986 (1) Å, V = 4477.3 ų, d(calcd) = 1.347 g cm⁻³, Z = 6) and lesser amounts of closo-1,2,3-{(η⁵-C₅Me₅)Co}₃B₃H₅, 5 (triclinic, P1, a = 8.461 (2) Å, b = 10.718 (2) Å, c = 17.591 (4) Å, V = 1511.0 (8) ų, d(calcd) = 1.363 g cm⁻³, Z = 2), and closo-1,2,3,6-{(η⁵-C₅Me₅)Co}₄B₂H₄, 6, via the sequential formation of the metastable intermediates [{(η⁵-C₅Me₅)Co}(BH₄)]_n, 7 (paramagnetic), and {(η⁵-C₅Me₅)Co}₂B₂H₆, 8 (diamagnetic). These results demonstrate that a labile metal precursor permits control of the number of metal atoms in the metallaborane and the characterization of unusual and metastable cobaltaboranes. A kinetic study of the conversion of 1 to 3, which involves the intramolecular dehydrogenation of a η⁴-C₅Me₅H ligand and skeletal rearrangement, is reported and the mechanistic implications for the origin of 1 are discussed.