Sundargopal Ghosh

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.

Borylene-Based Direct Functionalization of Organic Substrates: Synthesis, Characterization, and Photophysical Properties of Novel π-Conjugated Borirenes

Room temperature photolysis of aminoborylene complexes, [(CO)(5)M=B=N(SiMe(3))(2)] (1: M = Cr, 2: Mo) in the presence of a series of alkynes and diynes, 1,2-bis(4-methoxyphenyl)ethyne, 1,2-bis(4-(trifluoromethyl)phenyl)ethyne, 1,4-diphenylbuta-1,3-diyne, 1,4-bis(4-methoxyphenyl)buta-1,3-diyne, 1,4-bis(trimethylsilylethynyl)benzene and 2,5-bis(4-N,N-dimethylaminophenylethynyl)thiophene led to the isolation of novel mono and bis-bis-(trimethylsilyl)aminoborirenes in high yields, that is [(RC=CR)(mu-BN(SiMe(3))(2)], (3: R = C(6)H(4)-4-OMe and 4: R = C(6)H(4)-4-CF(3)); [{(mu-BN(SiMe(3))(2)(RC=C-)}(2)], (5: R = C(6)H(5) and 6: R = C(6)H(4)-4-OMe); [1,4-bis-{(mu-BN(SiMe(3))(2)(SiMe(3)C=C)}benzene], 7 and [2,5-bis-{(mu-BN(SiMe(3))(2) ((C(6)H(4)NMe(2))C=C)}-thiophene], 8. All borirenes were isolated as light yellow, air and moisture sensitive solids. The new borirenes have been characterized in solution by (1)H, (11)B, (13)C NMR spectroscopy and elemental analysis and the structural types were unequivocally established by crystallographic analysis of compounds 6 and 7. DFT calculations were performed to evaluate the extent of pi-conjugation between the electrons of the carbon backbone and the empty p(z) orbital of the boron atom, and TD-DFT calculations were carried out to examine the nature of the electronic transitions.

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.

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.

Chemistry of Molybdaboranes: Synthesis, Structures, and Characterization of a New Class of Open-Cage Dimolybdaheteroborane Clusters

Reaction of [Cp*MoCl(4)], 1 (Cp* = eta(5)-C(5)Me(5)), with [LiBH(4).thf] in toluene at -70 degrees C, followed by pyrolysis with excess dichalcogenides RE-ER (R = Ph, CH(2)Ph, 2,6-((t)Bu)(2)-C(6)H(2)OH, (CH(3))(3)C = (t)Bu); E = S, Se) yielded a new class of hybrid clusters, 3-8: (3, [(Cp*Mo)(2)(mu-eta(1)-SPh)(2)(mu(3)-S)(H(2)BSPh)]; 4, [(Cp*Mo)(2)B(5)H(8)(SPh)]; 5, [(Cp*Mo)(2)B(5)H(8)(SePh)]; 6, [(Cp*Mo)(2)B(2)S(2)H(2)(mu-eta(1)-S)]; 7, [(Cp*Mo)(2)B(2)H(5)(BSR)(2)(mu-eta(1)-SR)], (R = 2,6-((t)Bu)(2)-C(6)H(2)OH); and 8, [(Cp*Mo)(2)B(2)H(5)(BSePh)(2)(mu-eta(1)-SePh)]. Compounds 3-8 have been isolated in modest yields as green or brown crystalline solids. In parallel with 3-8, [(Cp*Mo)(2)B(5)H(9)] was isolated as a major product in all cases. The isolation and structural characterization of compounds 3 and 6-8 provided the first direct evidence of the existence of [(Cp*Mo)(2)B(4)H(8)], 2, an intermediate in the formation of [(Cp*Mo)(2)B(5)H(9)]. These new compounds have been characterized in solution by mass spectrometry, (1)H, (11)B, (13)C NMR, and IR spectroscopy, and elemental analysis. The structural types were unequivocally established by X-ray crystallographic analysis of compounds 3-8.

A Mechanistic Study of the Utilization of arachno‐Diruthenaborane [(Cp*RuCO)2B2H6] as an Active Alkyne‐Cyclotrimerization Catalyst

The reaction of nido-[1,2-(Cp*RuH)(2)B(3)H(7)] (1a, Cp*=η(5)-C(5)Me(5)) with [Mo(CO)(3)(CH(3)CN)(3)] under mild conditions yields the new metallaborane arachno-[(Cp*RuCO)(2)B(2)H(6)] (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-1a 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.