Metallaboranes of the Early Transition Metals: Direct Synthesis and Characterization of [{eta(5)-C(5)Me(5))Ta}(2)B(n)H(m)] (n=4, m=10; n=5, m=11), [{(eta(5)-C(5)Me(5))Ta}(2)B(5)H(10)(C(6)H(4)CH(3))], and [{(eta(5)-C(5)Me(5))TaCl}(2)B(5)H(11)]

Abstract

Reaction of [Cp*TaCl(4)] (Cp* = eta(5)-C(5)Me(5)) with a sixfold excess of LiBH(4)center dot thf followed by BH(3)center dot thf in toluene at 100 degrees C led to the isolation of hydrogen-rich metallaboranes [(CP*Ta)(2)B(4)H(10)] (1), [(CP*Ta)(2)B(5)H(11)] (2), [(CP*Ta)(2)B(5)H(10)(C(6)H(4)CH(3))] (3), and [(Cp*TaCl)(2)B(5)H(11)] (4) in modest yield. Compounds 1-3 are air- and moisture-sensitive but 4 is reasonably stable in air. Their structures are predicted by the electron-counting rules to be a bicapped tetrahedron (1), bicapped trigonal bipyramids (2, 3), and a nido structure based on a closo dodecahedron (4). Yellow tantalaborane 1 has a nido geometry with C(2v) symmetry and is isostructural with [(Cp*M)(2)B(4)H(8)] (M = Cr and Re); whereas 2 and 3 are C(3v)-Symmetric and isostructural with [(Cp*M)(2)B(5)H(9)] (M = Cr, Mo, W) and [(Cp*ReH)(2)B(5)Cl(5)]. The most remarkable feature of 4 is the presence of a hydride ligand bridging the ditantalum center to form a symmetrical tantalaborane cluster with a long Ta-Ta bond (3.22 angstrom). Cluster 4 is a rare example of electronically unsaturated metallaborane containing four TaHB bonds. All these new metal-laboranes have been characterized by mass spectrometry, (1)H, (11)B, and (13)C NMR spectroscopy, and elemental analysis, and the structural types were unequivocally established by crystallographic analysis of 1-4.