Muktimoy Chaudhury

Mono- and dinuclear oxovanadium (IV) compounds containing VO(ONS) basic core: synthesis, structure and spectroscopic properties

Abstract Reaction of [VO(acac)2] with S-methyl 3-(2-hydroxyphenyl) methylenedithiocarbazate (H2L1) and its nitro- (H2L2) and bromo derivatives (H2L3) as tridentate heterodonor (ONS)2− ligands offers a basic VO(ONS) core. In the presence of bidentate (N,N′) donor ancillar ligands, various mono- and dinuclear oxovanadium(IV) complexes [VOL(2,2′-bipy)] (1–3), [VOL(Phen)] (4, 5) and [LOV(μ-4,4′-bipy)VOL] (4, 5) and [LOV(μ-4,4′-bipy)VOL] (6–8) were obtained. The compounds have been characterized by EPR, electronic and FAB mass spectroscopic studies. The X-ray crystal structure of a representative compound [VOL2(2,2′-bipy)] (2) is reported and shows the presence of the tridentate ONS ligand occupying the meridional sites leading to distorted octahedral geometry. The nitrogen donor, occupying an apical position has a trans-labilizing influence, resulting in elongation of the VO terminal bond. Magnetic susceptibility and EPR spectral studies indicate non-interacting nature (vbJ vb ≈ 0) of the vanadium(IV) unpaired electrons in the dinuclear complexes (6–8).

Single Crystal-to-Single Crystal Irreversible Transformation from a Discrete Vanadium(V)−Alcoholate to an Aldehydic-Vanadium(IV) Oligomer

An unprecedented single crystal-to-single crystal transformation occurs when a binuclear oxovanadium(V) compound [V(V)(2)O(2)(L)(2)] 1 involving 2,6-bis(hydroxymethyl)-p-cresol (H(3)L) as a bridging ligand is exposed simultaneously to white light and aerial oxygen to generate an oligomeric compound [V(IV)(2)O(2)(L*)(2)] 2 (H(2)L* is 3-hydroxymethyl-5-methylsalicylaldehyde). Each vanadium(V) center in 1 is reduced to vanadium(IV) in 2 at the expense of a two-electron alcohol-to-aldehyde oxidation in the coordinated ligand. The additional electron being released is possibly consumed by molecular oxygen to generate hydrogen peroxide.

Reporting a unique example of electronic bistability observed in the form of valence tautomerism with a copper(II) helicate of a redox-active nitrogenous heterocyclic ligand.

Valence tautomeric compounds involving nondixolene-type ligands are rare. The triple-helicate copper(II) complex [Cu(II)(2)(L)(3)](ClO(4))(4)·3CH(3)CN (1) containing a redox-active N-heterocyclic ligand (L) has been prepared and displays VT equilibrium in solution, as established by electronic spectroscopy, electron paramagnetic resonance spectroscopy, and cyclic and differential pulse voltammetry carried out at variable temperatures. The process involves intramolecular transfer of an electron from one of the L ligands to a copper(II) center, leading to the oxidation of L to an L(•+) radical with concomitant reduction of the Cu(II) center to Cu(I), as shown by the equilibrium [Cu(II)Cu(I)L(•+)L(2)](4+) ⇄ [Cu(II)(2)L(3)](4+).

Vanadium-Induced Nucleophilic IPSO Substitutions in a Coordinated Tetrachlorosemiquinone Ring: Formation of the Chloranilate Anion as a Bridging Ligand

In basic media, the coordinated semiquinone radical in the spin-coupled [(bipy)ClV(IV)O(TCSQ)] 1 (HTCSQ = tetrachlorosemiquinone) undergoes nucleophilic ipso substitution (OH- for Cl-) to generate the chloranilate anion (CA(2-)) that bridges the vanadium(IV) centers, forming a binuclear compound [(bipy)ClV(IV)O(CA)OV(IV)Cl(bipy)] 2.

Coordination asymmetry in divanadium(V) compounds containing a V2O3 core: synthesis, characterization, and redox properties.

A general protocol for the synthesis of micro-oxo divanadium(V) compounds [LOV(micro-O)VO(Salen)] (1-5) incorporating coordination asymmetry has been developed for the first time. One of the vanadium centers in these compounds has an octahedral environment, completed by tetradentate Salen ligand, while the remaining center has square pyramidal geometry, made up of tridentate biprotic Schiff-base ligands (L2-) with ONO (1-3) and ONS (4, 5) type donor combinations. Single crystal X-ray diffraction analysis, ESI-MS, and NMR (both 1H and 51V) spectroscopy have been used extensively to establish their identities. The V(1)-O(6)-V(2) bridge angle in these compounds, save 3, lie in a narrow range (166.20(9)-157.79(16) degrees) with the V2O3 core having a rare type of twist-angular structure, somewhat intermediate between the regular anti-linear and the syn-angular modes. For 3, however, the bridge angle is sufficiently smaller 117.92(8) degrees that it forces the V2O3 core to adopt an anti-angular geometry. The V(1)...V(2) separations in these molecules (3.7921(7)-3.3084(6) A) are by far the largest compared to their peers containing a V2O3 core. The molecules retain the binuclear structures also in solution as confirmed by NMR spectroscopy. Their redox behaviors appear quite interesting, each undergoing a one-electron reduction in the positive potential range (E1/2, 0.42-0.45 V vs Ag/AgCl) to generate a trapped-valence mixed-oxidation products [LVVO-(micro-O)-OVIV(salen)]1-, confirmed by combined coulometry-EPR experiments. The bent V-O-V bridge in these molecules probably prevents the symmetry-constrained vanadium d xy orbitals, containing the unpaired electron, to overlap effectively via the ppi orbitals of the bridging oxygen atom, thus accounting for the trapped-valence situation in this case.

Metal complexes of sulphur–nitrogen chelating agents. Part 11. Synthesis, characterisation, and thermodynamics of nucleophilic substitution reactions of monohalogeno nickel(II) complexes of tridentate ligands of the type SNN in solution and the solid state

Square-planar nickel(II) complexes [NiLX](X = Cl, Br, I, or SCN) have been obtained through deprotonation of the ligands methyl 2-(β-aminoethylamino)cyclopent-1-enedithiocarboxylate (HL1), methyl 2-(β-aminoisopropylamino)cyclopent-1-enedithiocarboxylate (HL2), and methyl 2-(β-diethylaminoethylamino)cyclopent-1-enedithiocarboxylate (HL3). Diamagnetic complexes of composition [NiL(MeCN)][ClO4](L = L1–L3) and [Ni2L2(HL)][ClO4]2(L = L1 or L2) have also been synthesised. The equilibrium constants for the nucleophilic substitution reaction [NiL2X]+ B [NiL2(B)]++ X–(X = Cl, Br, or I; B = pyridine and its methyl-substituted derivatives) have been determined spectrophotometrically in the temperature range 25–50 °C. A linear correlation has been obtained between the ΔH⊖ and ΔS⊖ values. The enthalpy changes (ΔH2) and activation energies (Ea*) for the thermal dissociation [NiL2(B)]X(s)→[NiL2X](s)+ B(g) have been determined. A linear correlation has been found between the (ΔS2) and Ea* values.

Tri- and tetranuclear nickel(II) inverse metallacrown complexes involving oximato oxygen linkers: role of the guest anion (oxo versus alkoxo) in controlling the size of the ring topology.

A trinuclear oximato complex, [(NiHL(1))(3)(μ(3)-O)]ClO(4) (1), with inverse metallacrown 9-MC-3 topology has been synthesized using a Schiff-base ligand (H(2)L(1)) formed by condensation of ethanolamine (Hea) and diacetylmonoxime (Hdamo). The diamagnetic compound has been characterized by electrospray ionization mass spectrometry as well as by single-crystal X-ray diffraction analysis. In the solid state, the alcoholic OH group in this molecule stays away from coordination. Surprisingly in a similar chemical reaction, when intact Hea and Hdamo have been used as ligands instead of their Schiff-base forms, the product obtained is a 12-MC-4-type metallacrown, (Et(3)NH)[Ni(4)(damo)(4)(Hea)(2)(ea)(2)](ClO(4))(3) (2), with a larger cavity size needed to accommodate a pair of hydrogen-bonded (O-H···O)(-) anions. Unlike in 1, the alcoholic OH groups in 2 take part in metal coordination. Compound 2 on being refluxed with lithium hydroxide in methanol is converted to 1 in almost quantitative yield. This appears to be a novel reaction type, leading to contraction of a metallacrown ring size. A family of 12-MC-4 Ni(4) metallacrowns in inverse topology, viz., [Ni(4)(damo)(4)(H(2)dea)(2)(Hdea)(2)](ClO(4))(2)·2H(2)O (3), [Ni(4)(dpko)(4)(Hea)(2)(ea)(2)](ClO(4))(2)·4H(2)O (4), and [Ni(4)(mpko)(4)(Hmea)(2)(mea)(2)](ClO(4))(2) (5), have been synthesized following a methodology similar to that adopted for 2, using different combinations of free oximes [viz., dipyridylketonoxime (Hdpko) and methylpyridylketonoxime (Hmpko)] and amino alcohols [viz., diethanolamine (H(2)dea), and N-methylethanolamine (Hmea)]. Crystal and molecular structures of 3-5 have been reported, each involving either a quasi (in 3) or a perfect (in 4 and 5) square plane (S(4) symmetry) with four octahedral Ni centers occupying the corners, and serve as a backbone of puckered metallacrown rings that accommodate a pair of hydrogen-bonded (O-H···O)(-) anions. Antiferromagnetic interactions within the [Ni(4)] core [J/k(B) ≈ -20 to -27 K based on the following spin Hamiltonian: H = -2J(S(1)·S(2) + S(2)·S(3) + S(3)·S(4) + S(4)·S(1))] lead to an S(T) = 0 ground state for these complexes.

Building Metallacrown Topology around a Discrete [M3(μ3-O)] (M = Ni(II) and Pd(II)) Core Using Oximato Oxygen Linkers: Synthesis, Structures, and Spectroscopic Characterization of a New Family of Compounds with an Inverse-9-MC-3 Motif

A family of trinuclear oximato complexes [(M(II)L)(3)(mu(3)-O)]ClO(4) (M = Ni, 1-3; Pd, 4 and 5) (HL = 2-alkylamino-3-oximobutane) involving a discrete [M(3)(mu(3)-O)](4+) core have been synthesized in moderate to high yields by a simple one-pot reaction. The products were characterized by ESI-mass and (1)H NMR spectroscopy as well as by single-crystal X-ray diffraction analysis of representative compounds viz., 1, 2, and 4. The oximato oxygen atoms from the ligands and the central mu(3)-O atom connect the metal centers, forming an inverse metallacrown topology in these complexes. In the isostructural nickel compounds (1, 2), the metal centers are situated at the vertices of an equilateral triangle with its centroid position being occupied by the mu(3)-O atom; the Ni-O-Ni angles vary in the range 119.0(2)-120.2(2) degrees . In the palladium complex 4, the geometry of the Pd(3)O core is better described as a regular trigonal pyramid with the metal centers and the mu(3)-O atom occupying the apexes; the Pd-O-Pd angles are close to 109 degrees . The coordination square planes around the individual palladium centers bend appreciably from each other (dihedral angles vary in the range 28.62-34.53 degrees ), providing more of a bowl shape compared to the overall metallacrown topology that remains virtually planar in the nickel complexes. The mu(3)-oxygen atom in 4 is displaced by 0.687 A from the center of the triangular plane with corners occupied by the Pd(II) ions. The protons of the metallacrown peripheral rings in 4 and 5 are more deshielded compared to their nickel(II) counterparts, as revealed from their (1)H NMR spectra in dichloromethane-d(2) solution.

Targeted synthesis of heterobimetallic compounds containing a discrete vanadium(V)-μ-oxygen-iron(III) core.

Heterobimetallic compounds [L(1)OV(V)═O→Fe(metsalophen)(H(2)O)] (1) and [L(2)OV(V)═O→Fe(metsalophen)(H(2)O)]CH(3)CN (2), where H(2)L(1) and H(2)L(2) are tridentate dithiocarbazate-based Schiff base ligands, containing a discrete V(V)-μ-O-Fe(III) angular core have been synthesized for the first time through a targeted synthesis route: confirmation in favor of such a heterobimetallic core structure has come from single-crystal X-ray diffraction analysis and electrospray ionization mass spectrometry.

Electron transport and magnetic properties of some mixed-valent alkalithiocuprates

Abstract Electrical transport properties of some two-dimensional isostructural compounds KCu 4 S 3 , RbCu 4 S 3 , and CsCu 4 S 3 and a one dimensional conductor Na 3 Cu 4 S 4 have been investigated. Conductivities have been measured on pelletized materials in the temperature range 300 to 150 K. All of them behave as metallic conductors. The room temperature conductivities of these compounds are KCu 4 S 3 , 1600 Ω −1 cm −1 ; RbCu 4 S 3 , 1400 Ω −1 cm −1 ; CsCu 4 S 3 , 1250 Ω −1 cm −1 ; Na 3 Cu 4 S 4 , 700 Ω −1 cm −1 . The observed trend in conductivities among the isostructural compounds may be rationalized either on the basis of shortest CuCu distances or on the carrier charge concentrations per unit volume. All of these compounds behave as Pauli paramagnetic materials.