Metal-Rich Oxametallaboranes of Group 5 Metals: Synthesis and Structure of a Face-Fused µ7-Boride Cluster.

Aerobic oxidation of metallaborane compounds is an unexplored field apart from the few reports on accidental oxidation leading to oxametallaboranes. An effective method for the synthesis of group 5 oxametallaboranes has been developed by the oxidation of [(Cp*M)2(B2H6)2] (M = Ta/Nb) (Cp* = η5-C5Me5). The reaction of [(Cp*M)2(B2H6)2] (M = Ta/Nb) with O2 gas at room temperature yielded oxametallaboranes [(Cp*M)2(B4H10O)] (for 1, M = Nb; for 2, M = Ta). Density functional theory calculations signify an increase in the HOMO-LUMO energy gap for 1 and 2 as compared to that for the parent metallaboranes, [(Cp*M)2(B2H6)2] (M = Ta/Nb). Reaction of 1 and 2 with [Ru3(CO)12] led to the isolation of fused metallaborane clusters [(Cp*Nb)2(B2H4O){Ru(CO)2}2(B2H4){Ru(CO)3}2{µ-H}4] (3) and [(Cp*Ta)2(B3H4O){Ru(CO)2}3{µ7-B}{µ-CO}2{µ-H}4] (4). The structure of 3 may be considered as a fusion of five subunits [two tetrahedra (Td), two square pyramids (sqp), and one trigonal bipyramid (tbp)]. One of the key features of cluster 4 is the presence of a µ7-boride atom that shares three cluster units (one monocapped trigonal prism and two Td). All the compounds have been characterized by mass spectrometry, infrared spectroscopy, and 1H, 13C, and 11B nuclear magnetic resonance spectroscopy, and the structural types were unequivocally established by crystallographic analysis of compounds 1, 3, and 4.

Trimetallic Cubane-Type Clusters: Transition-Metal Variation as a Probe of the Roots of Hypoelectronic Metallaheteroboranes.

In an effort to synthesize chalcogen-rich metallaheteroborane clusters of group 5 metals, thermolysis of [Cp*TaCl4] (Cp* = η5-C5Me5) with thioborate ligand Li[BH2S3] was carried out, affording trimetallic clusters [(Cp*Ta)3(µ-S)3(µ3-S)3B(R)], 1-3 (1, R = H; 2, R = SH; and 3, R = Cl). Clusters 1-3 are illustrative examples of cubane-type organotantalum sulfido clusters in which one of the vertices of the cubane is missing. In parallel to the formation of 1-3, the reaction also yielded tetrametallic sulfido cluster [(Cp*Ta)4(µ-S)6(µ3-S)(µ4-O)], 6, having an adamantane core structure. Compound 6 is one of the rarest examples containing the µ4-oxo unit with a heavier early transition metal, i.e., tantalum. In an effort to isolate selenium analogues of clusters 1-3, we have isolated the trimetallic cluster [(Cp*Ta)3(µ-Se)3(µ3-Se)3B(H)], 4, from the thermolytic reaction of [Cp*TaCl4] and Li[BH2Se3]. In contrast, the thermolysis of [Cp*TaCl4] with Li[BH2Te3] under the same reaction conditions yielded tantalum telluride complex [(Cp*Ta)2(µ-Te)2], 5. Compounds 1-4 are hypo-electronic clusters with an electron count of 50 cluster valence electrons. All these compounds have been characterized by 1H, 11B{1H}, and 13C{1H} NMR spectroscopy; infrared spectroscopy; mass spectrometry; and single-crystal X-ray crystallography. The density functional theory calculations were also carried out to provide insight into the bonding and electronic structures of these molecules.

Electron Precise Group 5 Dimetallaheteroboranes [{CpV(µ-EPh)}2{µ-η2:η2-BH3E}] and [{CpNb(µ-EPh)}2{µ-η2:η2-B2H4E}] (E = S or Se).

Synthesis and structural elucidation of various electron precise group 5 dimetallaheteroboranes have been described. Room temperature reaction of [Cp2VCl2] with Li[BH3(EPh)], generated from the treatment of LiBH4·THF and Ph2E2 (E = S or Se), for 1 h in toluene, followed by thermolysis, led to the formation of bimetallic complexes [{CpV(µ-EPh)}2{µ-η2:η2-BH3E}], 1 and 2 (1: E = S and 2: E = Se), and [{CpV(µ-SePh)}2{µ-η2:η2-BH(OC4H8)Se}], 3. One of the striking features of these compounds is that they represent a rare class of distorted tetrahedral clusters having bridging hydrogens. Evaluating the skeletal electron pairs and bonding types, compounds 1, 2, and 3 may be considered as isoelectronic with our earlier reported [(CpV)2(B2H6)2]. In an attempt to synthesize the Nb analogues of 1-3, room temperature reactions of [CpNbCl4] and Li[BH3(EPh)] (E = S or Se) were carried out that afforded compounds [{CpNb(µ-EPh)}2{µ-η2:η2-B2H4E}], 4 and 5 (4: E = S and 5: E = Se). The solid-state X-ray structures of both 4 and 5 exemplify electronically saturated [M2B3] systems, and their geometries are analogous to that of [(Cp*MoCl)2B3H7]. For the extension of this work, reaction of [Cp*TaCl4] (Cp* = η5-C5Me5) with Li[BH3(SePh)] was carried out that yielded a tantalaselenaborane cluster [(Cp*Ta)2(µ-Se)B3H6Se(C6H5)] (6). All the new compounds have been characterized using 1H, 11B{1H}, 13C{1H} NMR, UV-vis absorption, and IR spectroscopy, mass spectrometry, and X-ray diffraction studies.