SYNTHESIS OF COBALTABORANE CLUSTERS FROM [CP-ASTERISK-COCL](2) AND MONOBORANES - NEW STRUCTURES AND MECHANISTIC IMPLICATIONS

The reaction of [Cp*CoCl](2), Cp* = eta(5)-C(5)Me(5), with BH3.THF leads to the formation of nido-1-(eta(5)-C(5)Me(5))Co-2-(eta(4)-C(5)Me(5)H)CoB3H8, 1 (60%, monoclinic P2(1)/n, a = 8.076(1), b = 20.359(2), c = 13.880(2) Angstrom, beta = 91.23(1)degrees, V = 2281.7(6) Angstrom(3), d(calcd) = 1.251g/cm(3), Z = 4), and arachno-(eta(5)-C(5)Me(5))CoB4H10, 2 (2-10%, monoclinic P2(1)/c, a = 13.954(3), b = 14.185(4), c = 29.383(7) Angstrom, beta = 100.61(2)degrees, V = 5717(2) Angstrom(3), d(calcd) = 1.150 g/cm(3), Z = 16) via the metastable paramagnetic intermediate [{(eta(5)-C(5)Me(5))Co}(BH3Cl)](2), 9, with the coproduct BH2Cl. Heating 1 leads to nido-2,4-{(eta(5)-C(5)Me(5))Co}2B3H7, 3 (80%, tetragonal P4(2)/n, a = b = 23.440(4), c = 8.317(2) Angstrom, V = 4570(1) Angstrom(3), 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-{(eta(5)-C(5)Me(5))Co}3B2H4, 4 (20%, rhombohedral, R $($) over bar$$ 3, a = 17.994 (1) Angstrom, c = 15.986 (1) Angstrom, V = 4477.3 Angstrom(3), d(calcd) = 1.347 g cm(-3), Z = 6) and lesser amounts of closo-1,2,3-{(eta(5)-C(5)Me(5))Co}3B3H5, 5 (triclinic, P $($) over bar$$ 1, a = 8.461 (2) Angstrom, b = 10.718 (2) Angstrom, c = 17.591 (4) Angstrom, V = 1511.0 (8) Angstrom(3), d(calcd) = 1.363 g cm(-3), Z = 2), and close-1,2,3,6-{(eta(5)-C(5)Me(5))Co}4B2H4, 6, via the sequential formation of the metastable intermediates [{(eta(5)-C(5)Me(5))Co}(BH4)](n), 7 (paramagnetic), and {(eta(5)-C(5)Me(5))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 eta(4)-C(5)Me(5)H ligand and skeletal rearrangement, is reported and the mechanistic implications for the origin of 1 are discussed.