V. Ramkumar

Salen Complexes of Zirconium and Hafnium: Synthesis, Structural Characterization, Controlled Hydrolysis, and Solvent-Free Ring-Opening Polymerization of Cyclic Esters and Lactides

Dinuclear salen compounds of zirconium and hafnium are efficient initiators for the solvent-free ring-opening polymerization of cyclic ester monomers and lactides. There is a correlation between the theoretical and experimental number-average molecular weights (M(n)s) in these polymerizations. Polymerization of β-butyrolactone gives poly(3-hydroxybutyrate) with a good M(n) and molecular weight distribution.

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.

Synthesis, spectral, crystal and antimicrobial studies of biologically potent oxime ethers of nitrogen, oxygen and sulfur heterocycles.

Three series of oxime ethers viz, 2,6-diarylpiperidin-4-one O-benzyloximes 5a-o, 2,6-diaryltetrahydropyran-4-one O-benzyloximes 7a-e and 2,6-diaryltetrahydrothiopyran-4-one O-benzyloximes 11a-b and 12a-c were synthesized and stereochemistry is established by their spectral and single crystal analysis. A SAR study has been carried out for the above oxime ethers against a panel of antibacterial (Pseudomonas aeruginosa, Staphylococcus aureus, Salmonella typhi and Escherichia coli) and antifungal agents (Candida albicans, Candida-51, Rhizopus sp., Aspergillus niger, Aspergillus flavus and Cryptococcus neoformans), respectively, using Ciprofloxacin and Amphotericin B as standards. Most of the chloro/methyl/methoxy substituted compounds exerted moderate to good activity against all the tested organisms; moreover, some compounds (5i, 5l, 5n, 5o, 7c2, 7d1, 7d2, 7e, 11b and 12c) exhibited promising activity than standard drugs.

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.

Synthesis, stereochemistry and antimicrobial studies of novel oxime ethers of aza/diazabicycles

Two series of bicyclic oxime ethers viz, 2,4-diaryl-3-azabicyclo[3.3.1]nonan-9-one O-benzyloximes 13-24 and 2,4,6,8-tetraaryl-3,7-diazabicyclo[3.3.1]nonan-9-one O-benzyloximes 31-36 were synthesized and stereochemistry was established by their spectral (1D and 2D NMR) and crystal studies. Synthesized oxime ethers were screened for their in vitro antimicrobial activity against a set of pathogenic bacteria (Pseudomonas aeruginosa, Staphylococcus aureus, Salmonella typhi, Escherichia coli and Klebsiella pneumoniae) and fungi (Candida albicans, Candida-51, Rhizopus sp., Aspergillus niger and Aspergillus flavus) by twofold serial dilution method, respectively, using Ciprofloxacin and Amphotericin B as standards. Most of the molecules expressed promising antimicrobial profile against the tested pathogens and even a few compounds 16, 21, 22, 33 and 34 were better than standard drugs.

Group IV complexes containing the benzotriazole phenoxide ligand as catalysts for the ring-opening polymerization of lactides, epoxides and as precatalysts for the polymerization of ethylene

A series of Ti(IV), Zr(IV) and Hf(IV) benzotriazole phenoxide (BTP) complexes were synthesized and characterized by various spectroscopic techniques, elemental analysis and X-ray crystallography. The monosubstituted Zr(IV) BTP complexes [(μ-L)Zr(O(i)Pr)3]2 1-3 [L = (C1)BTP-H (1), (TCl)BTP-H (2), (pent)BTP-H (3)] and tetrasubstituted Zr(IV), Hf(IV) complexes ZrL4 4-6 [L = (C1)BTP-H (4), (TCl)BTP-H (5), (pent)BTP-H (6)] and HfL4 7-9 [L = (C1)BTP-H (7), (TCl)BTP-H (8), (pent)BTP-H (9)] were prepared by the reaction of Zr(O(i)Pr)4·((i)PrOH) and Hf(O(t)Bu)4 in toluene with the respective ligands in different stoichiometric proportions. The reaction between BTP and TiCl4 and ZrCl4 and HfCl4 in a 2 : 1 stoichiometric reaction resulted in the formation of disubstituted group IV chloride complexes L2MCl2 10-12 [L = (C1)BTP-H, M = Ti, Zr and Hf]. The molecular structures of complexes 1, 4, 7, 10, 11, and 12 were determined by single-crystal X-ray studies. The X-ray structure of 1 reveals a dimeric Zr(IV) complex containing a Zr2O2 core bridged through the oxygen atoms of the phenoxide groups. Each Zr atom is distorted from an octahedral symmetry. These complexes were found to be active towards the ring-opening polymerization (ROP) of L-lactide (L-LA) and rac-lactide (rac-LA). Complex 1 produced highly heterotactic poly(lactic acid) (PLA) from rac-LA under melt conditions with narrow molecular weight distributions (MWDs) and well controlled number average molecular weights (M(n)). Additionally, epoxide polymerizations using rac-cyclohexene oxide (CHO), rac-propylene oxide (PO), and rac-styrene oxide (SO) were also carried out with these complexes. The yield and molecular weight of the polymer was found to increase with the extension of reaction time. Compounds 1-12 were activated by methylaluminoxane (MAO) and show good activity for ethylene polymerization and produced high molecular weight polyethylene.

Imino phenoxide complexes of group 4 metals: synthesis, structural characterization and polymerization studies

A complete library of new alkoxides containing group 4 metals and the imino phenol ligand were synthesized in high yields and purity. These compounds showed catalytic activity towards the polymerization of L-lactide (L-LA), rac-lactide (rac-LA), e-caprolactone (CL), δ-valerolactone (VL), rac-β-butyrolactone (rac-BL), ethylene and propylene. The zirconium catalysts were found to yield better polymerization results in comparison to the titanium and hafnium analogues. The number average molecular weight (Mn) values of the polymers are very high for LA and CL with controlled molecular weight distributions (MWDs). Analyses of MALDI-TOF and 1H NMR spectra of low Mn oligomers reveal that the ligand initiates the ring-opening polymerization (ROP). For ethylene and propylene polymerization, we have achieved moderate to good activity using MAO as a co-catalyst.

Chlorinated Hypoelectronic Dimetallaborane Clusters: Synthesis, Characterization, and Electronic Structures of (η5-C5Me5W)2B5HnClm (n = 7, m = 2 andn=8,m=1)

Pyrolysis of (eta(5)-C(5)Me(5)WH(3))B(4)H(8), 1, in the presence of excess BHCl(2) x SMe(2) in toluene at 100 degrees C led to the isolation of (eta(5)-C(5)Me(5)W)(2)B(5)H(9), 2, and B-Cl inserted (eta(5)-C(5)Me(5)W)(2)B(5)H(8)Cl, 3, and (eta(5)-C(5)Me(5)W)(2)B(5)H(7)Cl(2), 4-7 (four isomers). All the chlorinated tungstaboranes were isolated as red and air and moisture sensitive solids. These new compounds have been characterized in solution by (1)H, (11)B, (13)C NMR, and the structural types were unequivocally established by crystallographic analysis of compounds 3, 4, and 7. Density functional theory (DFT) calculations were carried out on the model molecules of 3-7 to elucidate the actual electronic structures of these chlorinated species. On grounds of DFT calculations we demonstrated the role of transition metals, bridging hydrogens, and the effect of electrophilic substitution of hydrogens at B-H vertices of metallaborane structures.