Distinctive coordination behavior of a pyrazole imine-oxime compound towards Co(II) and Ni(II).

The polytopic Schiff base 5-methyl-1H-pyrazole-3-carboxylic acid 2-(hydroxyimino-1-methyl-propylidene)-hydrazide (H2L)was synthesized by the condensation of 5-methyl pyrazole-3-carbohydrazide and 3-(hydroxyimino)butan-2-one and its coordination ability was tested against cobalt (II) and nickel (II) nitrates. The ligand exhibited two different binding modes to form a unique binuclear triply bridged Co(III) cationic complex [Co2(1κN 2:2κN 2-L) (1κN 3:2κO 1-HL)2](NO3)2 (1). With the Ni(II) precursor, H2L was hydrolyzed to N',N˝-butane-2,3-diylidenebis (5-methyl-1H-pyrazole-3-carbohydrazide) (H2L1) which bound the metal cation in a tetradentate N3O1 fashion leading to the neutral square planar complex [Ni(κN 3 O 1-L1)]·MeOH (2·MeOH). Complexes 1 and 2 were characterized by IR, NMR, UV-Vis and single crystal X-ray crystallography. The probable mechanism for the Ni(II) mediated transformation of H2L into H2L1 has been investigated by ESI-MS.

Alkali metal directed assembly of heterometallic V(v)/M (M = Na, K, Cs) coordination polymers: structures, topological analysis, and oxidation catalytic properties.

The reactions of [VO(acac)2] with bis(salicylaldehyde)-oxaloyldihydrazone (H4L) and an alkali metal carbonate M2CO3 (M = K, Na, Cs), in EtOH/H2O medium upon reflux, resulted in the generation of three new heterometallic V(V)/M materials, namely the 1D [(VO2)2(µ4-L){Na2(µ-H2O)2(H2O)2}]n (1), 2D [{V(µ-O)2}2(µ4-L){K2(µ-H2O)2(H2O)2}]n (2), and 3D [{V(µ-O)(µ3-O)}2(µ8-L){Cs2(µ-H2O)2(H2O)2}]n (3) coordination polymers. They were isolated as air-stable solids and fully characterized by IR, UV-vis, (1)H, and (51)V NMR spectroscopy, ESI-MS(±), elemental, thermal, and single-crystal X-ray diffraction analyses, the latter showing that 1-3 are constructed from the resembling [(VO2)2(µ(4/8)-L)](2-) blocks assembled by the differently bound aqua-metal [M2(µ-H2O)2(H2O)2](2+) moieties (M = Na, K, Cs). The main distinctive features of 1-3 arise from the different coordination numbers of Na (5), K (7), and Cs (9) atoms, thus increasing the complexity of the resulting networks from the ladder-like 1D chains in 1 to double 2D layers in 2, and layer-pillared 3D framework in 3. The topological analysis of 2 disclosed a uninodal 4-connected underlying net with a rare kgm [Shubnikov plane net (3.6.3.6)/kagome pattern] topology, while 3 features a trinodal 4,7,8-connected underlying net with an unprecedented topology. Compounds 1-3 also show solubility in water (S(25 °C) ≈ 4-7 mg mL(-1)) and were applied as efficient precatalysts for the homogeneous oxidation of cyclohexane by aqueous H2O2, under mild conditions (50 °C) in MeCN/H2O medium and in the presence of an acid promoter. Total yields (based on substrate) of cyclohexanol and cyclohexanone up to 36% and turnover numbers (TONs) up to 5700 were achieved.

Ferro- and anti-ferromagnetically coupled tetracopper(II) 2 x 2 homoleptic rectangular grids supported by both mu-O and mu-(N-N) bridges derived from a new pyrazole based polydentate Schiff base ligand-magneto-structural correlations and DFT calculation.

The pyrazole derived Schiff base polytopic ligand 5-methyl-N'-[1-(pyridin-2-yl)ethylidene]-1H-pyrazole-3-carbohydrazide (PzCAP), prepared by the reaction between 5-methylpyrazole-3-carbohydrazide and 2-acetyl pyridine, has two potentially bridging functional groups [mu-O and mu-(N-N)] and consequently can exhibit different coordination conformations. Two tetranuclear homoleptic copper(II) 2 x 2 rectangular grid-complexes [Cu(4)(PzCAP)(4)(NO(3))(2)] (NO(3))(2).8H(2)O (1) and [Cu(4)(PzCAP)(4)(ClO(4))(2)] (ClO(4))(2) (2) were formed by a strict self-assembly process employing metal and ligand under 1:1 mol proportion. Each pair of the ligand molecules in the two complexes are arranged in roughly parallel fashion but under different conformations. The ligand PzCAP contains terminal pyridine and pyrazole residues bound to a central flexible diazine subunit (N-N). The rectangular Cu(II) 2 x 2 grid complexes having [Cu(4)(mu-N-N)(2)(mu-O)(2)] core involve a mixture of two diazine (Cu-N-N-Cu approximately 160 degrees ) and two alkoxo (Cu-O-Cu approximately 138 degrees ) bridges along the length and breadth respectively. In the [Cu(4)(mu-N-N)(2)(mu-O)(2)] core in , out of the four Cu(II) centers, all are hexa-coordinated but there are two penta-coordinated and two hexa-coordinated Cu(II) centers in the same core of . Each complex having the central [Cu(4)(mu-N-N)(2)(mu-O)(2)] core, exhibits quite different magnetic interactions among the metal centers. The paramagnetic Cu(II) centers bridged through the diazine fragment are involved in anti-ferromagnetic interaction while a dominant ferromagnetic interaction prevails between the alkoxo-bridged Cu(II) centers. The [Cu(4)(mu-N-N)(2)(mu-O)(2)] cluster in shows both ferromagnetic and anti-ferromagnetic interaction (J(1) = -0.80 cm(-1) and J(2) = +3.49 cm(-1)), a very unusual characteristic in this system while the same cluster in exhibits dominant anti-ferromagnetic coupling (J(1) = -89.1 cm(-1) and J(2) = +5.5 cm(-1)) through the trans Cu-(N-N)-Cu bridging arrangement, typical for systems of this sort. Both the complexes and have been characterized structurally, magnetically and spectroscopically. The exchange pathways parameters (J(1) and J(2)) have also been evaluated from density functional theoretical calculations to corroborate the bridging signatures with experimental findings.

Effect of silica nanoparticles on reinforcement of poly(phenylene ether) based thermoplastic elastomer.

Reinforcement of a novel poly(phenylene ether) (PPE) based thermoplastic elastomer (TPE), i.e., styrene-ethylene-butylene-styrene (SEBS)/ethylene vinyl acetate (EVA) and PPE-polystyrene (PS), was studied to develop a reinforced thermoplastic elastomer or thermoplastic vulcanizate (TPV). An effort was made to reinforce selectively the elastomeric dispersed phase of EVA by silica nanoparticles and silica sol-gel precursors, like alkoxy orthosilanes, using twin-screw extrusion and injection molding processes. Improvement of tensile strength and percent elongation at break was observed both with silica nanoparticles and tetraethoxy orthosilane (TEOS). Addition of TEOS transformed the dispersed EVA lamellar morphology into semispherical domains as a consequence of possible crosslinking. Soxhlet extraction was done on the silica and TEOS reinforced materials. The insoluble residues collected from both the silica and TEOS reinforced samples were analyzed in detail using both morphological and spectroscopic studies. This extensive study also provided an in-depth conceptual understanding of the PPE based TPE behavior upon reinforcement with silica nanoparticles and silica sol-gel precursors and the effect of reinforcement on recycling behavior.

Copper(II) mediated anion dependent formation of Schiff base complexes.

A tetranuclear mixed ligand copper(II) complex of a pyrazole containing Schiff base and a hydroxyhexahydropyrimidylpyrazole and copper(II) and nickel(II) complexes of the Schiff base having N-donor atoms have been investigated. A 2 equiv amount of 5-methyl-3-formylpyrazole (MPA) and 2 equiv of 1,3-diamino-2-propanol (1,3-DAP) on reaction with 1 equiv of copper(II) nitrate produce an unusual tetranuclear mixed ligand complex [Cu4(L1)2(L2)2(NO3)2] (1), where H2L1 = 1,3-bis(5-methyl-3-formylpyrazolylmethinimino)propane-2-ol and HL2 = 5-methyl-3-(5-hydroxyhexahydro-2-pyrimidyl)pyrazole. In contrast, a similar reaction with nickel(II) nitrate leads to the formation of a hygroscopic intractable material. On the other hand, the reaction involving 2 equiv of MPA and 1 equiv each of 1,3-DAP and various copper(II) salts gives rise to two types of products, viz. [Cu(T3-porphyrinogen)(H2O)]X2 (X = ClO4, NO3, BF4 (2)) (T3-porphyrinogen = 1,6,11,16-tetraza-5,10,15,20-tetrahydroxy-2,7,12,17-tetramethylporphyrinogen) and [Cu(H2L1)X]X x H2O (X = Cl (3), Br (4)). The same reaction carried out with nickel(II) salts also produces two types of compounds [Ni(H2L1)(H2O)2]X2 [X = ClO4 (5), NO3 (6), BF4 (7)] and [Ni(H2L1)X2] x H2O [X = Cl (8), Br (9)]. Among the above species 1, 3, and 5 are crystallographically characterized. In 1, all four copper atoms are in distorted square pyramidal geometry with N4O chromophore around two terminal copper atoms and N5 chromophore around two inner copper atoms. In 3, the copper atom is also in distorted square pyramidal geometry with N4Cl chromophore. The nickel atom in 5 is in a distorted octahedral geometry with N4O2 chromophore, where the metal atom is slightly pulled toward one of the axial coordinated water molecules. Variable-temperature (300 to 2 K) magnetic susceptibility measurements have been carried out for complex 1. The separations between the metal centers, viz., Cu(1)...Cu(2), Cu(2)...Cu(2)A, and Cu(2)A...Cu(1)A are 3.858, 3.89, and 3.858 A, respectively. The overall magnetic behavior is consistent with strong antiferromagnetic interactions between the spin centers. The exchange coupling constants between Cu(1)...Cu(2) and Cu(2)...Cu(2A) centers have turned out to be -305.3 and -400.7 cm(-1), respectively, resulting in a S = 1/2 ground state. The complexes are further characterized by UV-vis, IR, electron paramagnetic resonance, and electrochemical studies.