Mild thermolysis of tantalaborane [(Cp*Ta) 2B 5H 11], 1 (Cp* = η 5-C 5Me 5) in presence of halogen sources affords the open cage clusters [(Cp*TaX) 2B 5H 11], 2-4 (2: X = Cl; 3: X = Br; and 4: X = I) in good yields. In contrast, the tetraborohydride cluster, [(Cp*Ta) 2B 4H 9(μ-BH 4)], 5, under the same reaction conditions forms the B-H substituted cluster [(Cp*Ta) 2B 4H 8I(μ-BH 4)], 6. All the new metallaboranes have been characterized by mass spectrometry, 1H, 11B, 13C NMR spectroscopy, and elemental analysis, and the structural types were established by crystallographic analysis of clusters 3, 4, and 6. Density functional theory (DFT) calculations at the BP86/TZ2P ZORA level reveal geometries in agreement with the structure determinations, large gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in accord with their stabilities. B3LYP-computed 11B chemical shifts accurately reflect the experimentally measured shifts. Clusters 2-4 can be viewed as 7-sep 7-vertex oblatoarachno M 2B 5 clusters which can be generated from a 7-sep 9-vertex oblatocloso M 2B 7 cluster by removal of two equatorial boron atoms. Cluster 6 can be considered as an electron-deficient 6-sep 6-vertex oblatoarachno M 2B 4 cluster derived from an 8-vertex oblatocloso hexagonal bipyramidal cluster, in which BH 4 - anion is weakly bonded in a bidentate mode. © 2012 American Chemical Society.

Sundargopal Ghosh

Department Of Chemistry

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IIT Madras

Inorganic Chemistry

The reaction of nido-[1,2-(Cp*RuH)2B3H 7] (1 a, Cp*=Î·5-C5Me 5) with [Mo(CO)3(CH3CN)3] under mild conditions yields the new metallaborane arachno-[(Cp*RuCO) 2B2H6] (2). Compound 2 catalyzes the cyclotrimerization of a variety of internal- and terminal alkynes to yield mixtures of 1,3,5- and 1,2,4-substituted benzenes. The reactivities of nido-1 a and arachno-2 with alkynes demonstrates that a change in geometry from nido to arachno drives a change in the reaction from alkyne-insertion to catalytic cyclotrimerization, respectively. Density functional calculations have been used to evaluate the reaction pathways of the cyclotrimerization of alkynes catalyzed by compound 2. The reaction involves the formation of a ruthenacyclic intermediate and the subsequent alkyne-insertion step is initiated by a [2+2] cycloaddition between this intermediate and an alkyne. The experimental and quantum-chemical results also show that the stability of the metallacyclic intermediate is strongly dependent on the nature of the substituents that are present on the alkyne. Everything changes but Ru: The reactivities of nido-[1,2-(Cp*RuH)2B3H7] and arachno-[(Cp*RuCO)2B2H6] with alkynes show that a change in geometry (nido to arachno) drives a change from alkyne-insertion to catalytic cyclotrimerization, respectively. Copyright © 2012 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.

Chemistry - A European Journal

A mechanistic study of the utilization of arachno-diruthenaborane [(Cp*RuCO)2B2H6] as an active alkyne-cyclotrimerization catalyst

Molecular metallaboranes, which feature complexes with M-B rather than M-C bonds, constitute a class of compounds which has revealed new possibilities for the way metal and borane fragments interact to generate novel structures, demonstrating an important role for the transition metal in the structural arrangement. Early transition-metal dimetallaboranes in particular adopt deltahedra with the same total connectivities as the borane anions or late transition-metal metallaboranes but with flattened geometries rather than spherical shapes. They are characterised by high metal coordination numbers, M-M cross-cluster distances within the single bond range, and formal (apparent) cluster electron counts three skeletal electron pairs short of that required for a canonical structure of the same nuclearity. A selection of these boron-rich dimetallaboranes is theoretically investigated and compared to illustrate the role of the transition metal in their cluster bonding. © 2012 Elsevier Masson SAS. All rights reserved.

Solid State Sciences

Molecular transition-metal boron compounds. Any interest?

The reaction of [Cp*TaCl4], 1 (Cp* = η5-C5Me5), with [LiBH4· THF] at -78 °C, followed by thermolysis in the presence of excess [BH 3·THF], results in the formation of the oxatantalaborane cluster [(Cp*Ta)2B4H10O], 2 in moderate yield. Compound 2 is a notable example of an oxatantalaborane cluster where oxygen is contiguously bound to both the metal and boron. Upon availability of 2, a room temperature reaction was performed with [Fe2(CO) 9], which led to the isolation of [(Cp*Ta)2B 2H4O{H2Fe2(CO)6BH}], 3. Compound 3 is an unusual heterometallic boride cluster in which the [Ta 2Fe2] atoms define a butterfly framework with one boron atom lying in a semi-interstitial position. Likewise, the diselenamolybdaborane, [(Cp*Mo)2B4H4Se2], 4 was treated with an excess of [Fe2(CO)9] to afford the heterometallic boride cluster [(Cp*MoSe)2Fe6(CO) 13B2(BH)2], 5. The cluster core of 5 consists of a cubane [Mo2Se2Fe2B2] and a tricapped trigonal prism [Fe6B3] fused together with four atoms held in common between the two subclusters. In the tricapped trigonal prism subunit, one of the boron atoms is completely encapsulated and bonded to six iron and two boron atoms. Compounds 2, 3, and 5 have been characterized by mass spectrometry, IR, 1H, 11B, 13C NMR spectroscopy, and the geometric structures were unequivocally established by crystallographic analysis. The density functional theory calculations yielded geometries that are in close agreement with the observed structures. Furthermore, the calculated 11B NMR chemical shifts also support the structural characterization of the compounds. Natural bond order analysis and Wiberg bond indices are used to gain insight into the bonding patterns of the observed geometries of 2, 3, and 5. © 2011 American Chemical Society.

Synthesis, characterization, and electronic structure of new type of heterometallic boride clusters

Vanadaborane, [(CpV)2(B2H6)2] (Cp = η5-C5H5), 1 reacts with elemental sulfur to afford the hexasulflde cluster [(CpV)2S4(μ- η1-S2)], 2 In high yield. Compound 2 is a notable example of an organovanadium sulfide cluster In which the [V2S 4] atoms define a bicapped tetrahedron framework, with μ-η1-S2 ligand bridged the two (CpV) moieties. The sulfur atom In [V2S4] core in 2 Is a four-skeletal- electron donor isoelectronlc with the BH3 unit; therefore, the replacement of boron hydride in 1 by four sulfur atoms necessitates the formation of a bicapped tetrahedron [V2S4] framework. Furthermore, this Is the only reported example of a bimetallic hexasulflde cluster containing vanadium. Pyrolysls of 1 with bis-chalcogenide ligands such as Ph2S2 and Bz2Se2 (Bz = PhCH 2), results In the formation of substituted vanadahexaboranes [(CpV)2B4H12-xLx], 3-5 (3: L = SPh: x= 3; 4: L = SPh, x= 2; 5: L = SeBz: x= 1) in modest yield. All these new compounds have been characterized by mass, 1H, 11B, 13C NMR spectroscopy, and elemental analysis, and the structural types were unequivocally established by crystallographic analysis of compounds 2-5. © 2010 American Chemical Society.

Chemistry of vanadaboranes: Synthesis, structures, and characterization of organovanadium sulfide clusters with disulfido linkage

In addition to (η 5-C 5Me 5W) 25H 9, (η 5-C 5Me 5W) 3(μ-H)B 8H 8, and (η 5-C 5Me 5W) 2B 7H 9 reported earlier, thermolysis of {η 5-C 5Me 5 W}H 3B 4H 8 (1) with BH 3·THF leads to the formation of bicapped closo-{η 5C 5Me 5W} 2B 6H 10 (2) in parallel with a B-B linked djmer of {η 5-C 5Me 5W} 2B 5H 9 (4) and bis-{η 5-C 5Me 5W) 2B 5H 8} 2 (3). Reaction of 4 with a 3-fold excess of Fe 2(CO) 9 in hexane generates an iron analogue of 2, (η 5-C 5Me 5W) 2B 5H 9Fe(CO) 3 (5) in-46% yield. AU the metallaboranes, 2, 3, and 5, have been characterized by 1H, 11B NMR spectra and crystal structure determinations. The structures of 2 and 5 appear to follow the regular electron counting rules as they possess seven skeletal electron pairs (sep) appropriate for bicapped octahedral structures. However, their electronic structures revealed by a molecular orbital analysis is more closely related to that of [{η 5-C 5Me 5Re} 2B 7H 7], a cluster that does not obey the classical electron counting rules. Clusters 2 and 5 are notable examples of closo clusters containing four bridging hydrogens. 3 is a coupled-cage structure in which two bicapped trigonal bipyramidal frameworks, W 2B 5, are joined by an exopolyhedral boron-boron bond. © 2007 American Chemical Society.

Organometallics

Linked and fused tungstaborane clusters: Synthesis, characterization, and electronic structures of bis(η 5C 5Me 55W) 2B 5H 8} 2 and (η 5-C 5Me 55W) 2{Fe(CO) 3} n,B 6-nH 10-n, n = 0, 1

Synthesis and characterization of hypoelectronic tantalaboranes: Comparison of the geometric and electronic structures of [(Cp*TaX) 2B 5H 11] (X = Cl, Br, and I)

Journal	Inorganic Chemistry
ISSN	00201669
Open Access	No