title: Metallaboranes of the Early Transition Metals: Direct Synthesis and Characterization of [{(eta5-C5Me5)Ta}2BnHm] (n=4, m=10; n=5, m=11), [{(eta5-C5Me5)Ta}2B5H10(C6H4CH3)], and [{(eta5-C5Me5)TaCl}2B5H11].
abstract: Reaction of [Cp*TaCl4] (Cp*=eta5-C5Me5) with a sixfold excess of LiBH(4)thf followed by BH3thf in toluene at 100 degrees C led to the isolation of hydrogen-rich metallaboranes [(Cp*Ta)2B4H10] (1), [(Cp*Ta)2B5H11] (2), [(Cp*Ta)2B5H10(C6H4CH3)] (3), and [(Cp*TaCl)2B5H11] (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 C2v symmetry and is isostructural with [(Cp*M)2B4H8] (M=Cr and Re); whereas 2 and 3 are C3v-symmetric and isostructural with [(Cp*M)2B5H9] (M=Cr, Mo, W) and [(Cp*ReH)2B5Cl5]. 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 A). Cluster 4 is a rare example of electronically unsaturated metallaborane containing four TaHB bonds. All these new metallaboranes have been characterized by mass spectrometry, 1H, 11B, and 13C NMR spectroscopy, and elemental analysis, and the structural types were unequivocally established by crystallographic analysis of 1-4.
authors.affiliation: Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036 (India), Fax: (+91) 44-2257-4202.
authors.forename: Shubhankar Kumar
authors.initials: SK
authors.lastname: Bose
authors.affiliation: Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036 (India), Fax: (+91) 44-2257-4202.
authors.forename: K
authors.initials: K
authors.lastname: Geetharani
authors.affiliation: Sophisticated Analytical Instruments Facility, Indian Institute of Technology Madras, Chennai 600 036 (India).
authors.forename: Babu
authors.initials: B
authors.lastname: Varghese
authors.affiliation: National Single Crystal X-ray Diffraction Facility, Indian Institute of Technology Bombay, Mumbai 400 076 (India).
authors.forename: Shaikh M
authors.initials: SM
authors.lastname: Mobin
authors.affiliation: Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036 (India), Fax: (+91) 44-2257-4202.
authors.forename: Sundargopal
authors.initials: S
authors.lastname: Ghosh