Shubhankar Kumar Bose

Zinc‐Catalyzed Borylation of Primary, Secondary and Tertiary Alkyl Halides with Alkoxy Diboron Reagents at Room Temperature

A new catalytic system based on a Zn(II) NHC precursor has been developed for the cross-coupling reaction of alkyl halides with diboron reagents, which represents a novel use of a Group XII catalyst for CX borylation. This approach gives borylations of unactivated primary, secondary, and tertiary alkyl halides at room temperature to furnish alkyl boronates, with good functional-group compatibility, under mild conditions. Preliminary mechanistic investigations demonstrated that this borylation reaction seems to involve one-electron processes.

Metallaboranes of the Early Transition Metals: Direct Synthesis and Characterization of [{(η5-C5Me5)Ta}2BnHm] (n=4, m=10; n=5, m=11), [{(η5-C5Me5)Ta}2B5H10(C6H4CH3)], and [{(η5-C5Me5)TaCl}2B5H11]

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.

Boron Beyond the Icosahedral Barrier: A 16‐Vertex Metallaborane

A neutral metallaborane comprising a Rh4B12 polyhedron with icosioctahedron geometry with 16 vertices and 28 triangular faces was prepared (see structure; Rh: red, B: green). The cage has the shape of a 12-membered truncated tetrahedron with four capped hexagonal faces.

Fine Tuning of Metallaborane Geometries: Chemistry of Metallaboranes of Early Transition Metals Derived from Metal Halides and Monoborane Reagents

Reaction of [Cp(n)MCl(4-x)] (M=V: n=x=2; M=Nb: n=1, x=0) or [Cp*TaCl(4)] (Cp=eta(5)-C(5)H(5), Cp*=eta(5)-C(5)Me(5)), with [LiBH(4).thf] at -70 degrees C followed by thermolysis at 85 degrees C in the presence of [BH(3).thf] yielded the hydrogen-rich metallaboranes [(CpM)(2)(B(2)H(6))(2)] (1: M=V; 2: M=Nb) and [(Cp*Ta)(2)(B(2)H(6))(2)] (3) in modest to high yields. Complexes 1 and 3 are the first structurally characterized compounds with a metal-metal bond bridged by two hexahydroborate (B(2)H(6)) groups forming a symmetrical complex. Addition of [BH(3).thf] to 3 results in formation of a metallaborane [(Cp*Ta)(2)B(4)H(8)(mu-BH(4))] (4) containing a tetrahydroborate ligand, [BH(4)](-), bound exo to the bicapped tetrahedral cage [(Cp*Ta)(2)B(4)H(8)] by two Ta-H-B bridge bonds. The interesting structural feature of 4 is the coordination of the bridging tetrahydroborate group, which has two B-H bonds coordinated to the tantalum atoms. All these new metallaboranes have been characterized by mass, (1)H, (11)B, and (13)C NMR spectroscopy and elemental analysis and the structural types were established unequivocally by crystallographic analysis of 1-4.

From Metallaborane to Borylene Complexes: Syntheses and Structures of Triply Bridged Ruthenium and Tantalum Borylene Complexes

Reaction of [1,2-(Cp*RuH)(2)B(3)H(7)] (1; Cp*=η(5)-C(5)Me(5)) with [Mo(CO)(3)(CH(3)CN)(3)] yielded arachno-[(Cp*RuCO)(2)B(2)H(6)] (2), which exhibits a butterfly structure, reminiscent of 7 sep B(4)H(10). Compound 2 was found to be a very good precursor for the generation of bridged borylene species. Mild pyrolysis of 2 with [Fe(2)(CO)(9)] yielded a triply bridged heterotrinuclear borylene complex [(μ(3)-BH)(Cp*RuCO)(2)(μ-CO){Fe(CO)(3)}] (3) and bis-borylene complexes [{(μ(3)-BH)(Cp*Ru)(μ-CO)}(2)Fe(2)(CO)(5)] (4) and [{(μ(3)-BH)(Cp*Ru)Fe(CO)(3)}(2)(μ-CO)] (5). In a similar fashion, pyrolysis of 2 with [Mn(2)(CO)(10)] permits the isolation of μ(3)-borylene complex [(μ(3)-BH)(Cp*RuCO)(2)(μ-H)(μ-CO){Mn(CO)(3)}] (6). Both compounds 3 and 6 have a trigonal-pyramidal geometry with the μ(3)-BH ligand occupying the apical vertex, whereas 4 and 5 can be viewed as bicapped tetrahedra, with two μ(3)-borylene ligands occupying the capping position. The synthesis of tantalum borylene complex [(μ(3)-BH)(Cp*TaCO)(2)(μ-CO){Fe(CO)(3)}] (7) was achieved by the reaction of [(Cp*Ta)(2)B(4)H(9)(μ-BH(4))] [corrected] at ambient temperature with [Fe(2)(CO)(9)]. Compounds 2-7 have been isolated in modest yield as yellow to red crystalline solids. All the new compounds have been characterized in solution by mass spectrometry; IR spectroscopy; and (1)H, (11)B, and (13)C NMR spectroscopy and the structural types were unequivocally established by crystallographic analysis of 2-6.

Synthesis and structural characterization of new divanada- and diniobaboranes containing chalcogen atoms.

The reaction of [Cp(n)MCl(4-x)] (M=V: n=2, x=2; M=Nb: n=1, x=0; Cp=η(5)-C(5) H(5)) with LiBH(4)⋅THF followed by thermolysis in the presence of dichalcogenide ligands E(2)R(2) (E=S, Te; R=2,6-(tBu)(2)-C(6)H(2)OH, Ph) and 2-mercaptobenzothiazole (C(7)H(5)NS(2)) yielded dimetallaheteroboranes [{CpV(μ-TePh)}(2)(μ(3) -Te)BH⋅thf] (1), [(CpV)(2)(BH(3)S)(2)] (2), [(CpNb)(2)B(4)H(10)S] (3), [(CpNb)(2)B(4)H(11)S(tBu)(2)C(6)H(2)OH] (4), and [(CpNb)(2)B(4)H(11)TePh] (5). In cluster 1, the V(2)BTe atoms define a tetrahedral framework in which the boron atom is linked to a THF molecule. Compound 2 can be described as a dimetallathiaborane that is built from two edge-fused V(2)BS tetrahedron clusters. Cluster 3 can be considered as an edge-fused cluster in which a trigonal-bipyramidal unit (Nb(2)B(2)S) has been fused with a tetrahedral core (Nb(2)B(2)) by means of a common Nb(2) edge. In addition, thermolysis of an in-situ-generated intermediate that was produced from the reaction of [Cp(2)VCl(2)] and LiBH(4)⋅THF with excess BH(3)⋅THF yielded oxavanadaborane [(CpV)(2)B(3)H(8)(μ(3)-OEt)] (6) and divanadaborane cluster [(CpV)(2)B(5)H(11)] (7). Cluster 7 exhibits a nido geometry with C(2v) symmetry and it is isostructural with [(Cp*M)(2)B(5)H(9+n)] (M=Cr, Mo, and W, n=0; M=Ta, n=2; Cp*=η(5)-C(5)Me(5)). All of these new compounds have been characterized by (1)H NMR, (11)B NMR, and (13)C NMR spectroscopy and elemental analysis and the structural types were established unequivocally by crystallographic analysis of compounds 1-4, 6, and 7.

Efficient Synthesis of Aryl Boronates via Zinc-Catalyzed Cross-Coupling of Alkoxy Diboron Reagents with Aryl Halides at Room Temperature

A zinc(II)/NHC system catalyzes the borylation of aryl halides with diboron (4) reagents in the presence of KOMe at rt. This transformation can be applied to a broad range of substrates with high functional group compatibility. Radical scavenger experiments do not support a radical-mediated process.

A Family of Heterometallic Cubane-Type Clusters with an exo-Fe(CO)3 Fragment Anchored to the Cubane†

Metal-rich boron clusters are members of a rapidly growing family of metallaborane “hybrid” systems which bridges the gap between metal clusters and polyhedral boranes. Clusters such as [Cp4Ni4B4H4] [2] and [Cp4Co4B4H4] [3] (Cp = h-C5H5) can be linked with the well-studied class of tetranuclear metal complexes known as cubanes. Recently, a metal-rich metallaborane with the same geometry, namely, [(Cp*Ru)3(m3CO)Co(CO)2B3H3] (Cp* = h -C5Me5), was reported by Fehlner et al. Cubane is quite a popular shape, especially for combinations of heterometal and main group elements. These compounds are of interest not only because of their contribution to the development of organometallic chemistry, but also for their potential use as models for various industrial and biological catalytic processes. As a part of our ongoing studies on metallaboranes and their derivatives, we recently reported the synthesis of [(Cp*Mo)2B4H4E2] [11] (1: E = S; 2 : E = Se) and arachno[(Cp*RuCO)2B2H6] [12] (3) in good yields. Until now we have focused on the chemistry of metallaboranes with boranes, main group elements, and small organic molecules. We have now extended our studies to transition metal carbonyl compounds such as [Fe2(CO)9], [Mn2(CO)10], and [Co2(CO)8], since earlier work suggested their potential as versatile reagents in cluster-building reactions. Reaction with [Co2(CO)8] led to decomposition, whereas mild pyrolysis of [Fe2(CO)9] with 1–3 in hexane led to hybrid clusters [(Cp*M)2(m3-E)2B2H(m-H){Fe(CO)2}2Fe(CO)3] (4 : M = Mo, E = S; 5 : M = Mo, E = Se; 6 : M = Ru, E = CO). The identities of 5 and 6 were established by a single-crystal X-ray diffraction study, which together with spectroscopic studies demonstrated the existence of novel capped-cubane cluster cores. Although X-ray quality crystals of 4 have not been obtained yet, its identity is inferred by comparison to selenium analogue 5. The overall structure of 5 is intriguing and its geometry can be viewed in a few different ways. The more obvious approach is to recognize the cubane shape made of two Mo, two Fe, two Se, and two B atoms, capped by a third Fe atom attached to one of the B-Fe-Fe faces of the cube (Figure 1). An alternative description of 5 is as a Mo2Fe2 tetrahedron face-capped by two selenium and two boron atoms. Capping one of the resulting Fe2B faces with another Fe(CO)3 group generates capped cubane 5.

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

Vanadaborane, [(CpV)(2)(B(2)H(6))(2)] (Cp = eta(5)-C(5)H(5)), 1 reacts with elemental sulfur to afford the hexasulfide cluster [(CpV)(2)S(4)(mu-eta(1)-S(2))], 2 in high yield. Compound 2 is a notable example of an organovanadium sulfide cluster in which the [V(2)S(4)] atoms define a bicapped tetrahedron framework, with one mu-eta(1)-S(2) ligand bridged the two (CpV) moieties. The sulfur atom in [V(2)S(4)] core in 2 is a four-skeletal-electron donor isoelectronic with the BH(3) unit; therefore, the replacement of boron hydride in 1 by four sulfur atoms necessitates the formation of a bicapped tetrahedron [V(2)S(4)] framework. Furthermore, this is the only reported example of a bimetallic hexasulfide cluster containing vanadium. Pyrolysis of 1 with bis-chalcogenide ligands such as Ph(2)S(2) and Bz(2)Se(2) (Bz = PhCH(2)), results in the formation of substituted vanadahexaboranes [(CpV)(2)B(4)H(12-x)L(x)], 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, (1)H, (11)B, (13)C NMR spectroscopy, and elemental analysis, and the structural types were unequivocally established by crystallographic analysis of compounds 2-5.

Synthesis and Structure of Dirhodium Analogue of Octaborane-12 and Decaborane-14

We present the results of our investigation of a thermally driven cluster expansion of rhodaborane systems with BH(3)·THF. Four novel rhodaborane clusters, for example, nido-[(Cp*Rh)(2)B(6)H(10)], 1; nido-[(Cp*Rh)B(9)H(13)], 2; nido-[(Cp*Rh)(2)B(8)H(12)], 3; and nido-[(Cp*Rh)(3)B(8)H(9)(OH)(3)], 4 (Cp* = η(5)-C(5)Me(5)), have been isolated from the thermolysis of [Cp*RhCl(2)](2) and borane reagents in modest yields. Rhodaborane 1 has a nido geometry and is isostructural with [B(8)H(12)]. The low temperature (11)B and (1)H NMR data demonstrate that compound 1 exists in two isomeric forms. The framework geometry of 2 and 3 is similar to that of [B(10)H(14)] with one BH group in 2 (3-position), and two BH groups in 3 (3, 4-positions) are replaced by an isolobal {Cp*Rh} fragment. The 11 vertex cluster 4 has a nido structure based on the 12 vertex icosahedron, having three rhodium and eight boron atoms. In addition, the reaction of rhodaborane 1 with [Fe(2)(CO)(9)] yielded a condensed cluster [(Cp*Rh)(2){Fe(CO)(3)}(2)B(6)H(10)], 5. The geometry of 5 consists of [Fe(2)B(2)] tetrahedron and an open structure of [(Cp*Rh)(2)B(6)], fused through two boron atoms. The accuracy of these results in each case is established experimentally by spectroscopic characterization in solution and structure determinations in the solid state.