Synthesis, structure and solution chemistry of mixed-ligand oxovanadium(IV) and oxovanadium(V) complexes incorporating tridentate ONO donor hydrazone ligands

In the title family, the ONO donor ligands are the acetylhydrazones of salicylaldehyde (H₂L¹) and 2-hydroxyacetophenone (H₂L²) (general abbreviation, H₂L). The reaction of bis(acetylacetonato)oxovanadium(IV) with a mixture of tridentate H₂L and a bidentate NN donor [e.g., 2,2′-bipyridine(bpy) or 1,10-phenanthroline(phen), hereafter B] ligands in equimolar ratio afforded the tetravalent complexes of the type [V^IVO(L)(B)]; complexes (1)-(4) whereas, if B is replaced by 8-hydroxyquinoline(Hhq) (which is a bidentate ON donor ligand), the above reaction mixture yielded the pentavalent complexes of the type [V^VO(L)(hq)]; complexes (5) and (6). Aerial oxygen is most likely the oxidant (for the oxidation of V^IV → V^V) in the synthesis of pentavalent complexes (5) and (6). [V^IVO(L)(B)] complexes are one electron paramagnetic and display axial EPR spectra, while the [V^VO(L)(hq)] complexes are diamagnetic. The X-ray structure of [V^VO(L²)(hq)] (6) indicates that H₂L² ligand is bonded with the vanadium meridionally in a tridentate dinegative fashion through its phenolic-O, enolic-O and imine-N atoms. The general bond length order is: oxo < phenolato < enolato. The V-O (enolato) bond is longer than V-O (phenolato) bond by ∼0.07 Å and is identical with V-O (carboxylate) bond. ¹H NMR spectrum of (6) in CDCl₃ solution indicates that the binding nature in the solid state is also retained in solution. Complexes (1)-(4) display two ligand-field transitions in the visible region near 820 and 480 nm in DMF solution and exhibit irreversible oxidation peak near +0.60 V versus SCE in DMSO solution, while complexes (5) and (6) exhibit only LMCT band near 535 nm and display quasi-reversible one electron reduction peak near −0.10 V versus SCE in CH₂Cl₂ solution. The VO³⁺-VO²⁺E_1/2 values shift considerably to more negative values when neutral NN donor is replaced by anionic ON donor species and it also provides better VO³⁺ binding via phenolato oxygen. For a given bidentate ligand, E_1/2 increases in the order: (L²)²⁻ < (L¹)²⁻.     

Synthesis, structure and solution chemistry of quaternary oxovanadium(V) complexes incorporating hydrazone ligands

[V^IVO(acac)₂] reacts with an equimolar amount of benzoyl hydrazone of 2-hydroxyacetophenone (H₂L¹) or 5-chloro-2-hydroxyacetophenone (H₂L²) in the presence of excess pyridine (py) in methanol to produce the quaternary [V^VO(L¹)(OCH₃)(py)] (1) and [V^VO(L²)(OCH₃)(py)] (2) complexes, respectively, while under similar condition, the benzoyl hydrazones of 2-hydroxy-5-methylacetophenone (H₂L³) and 2-hydroxy-5-methoxyacetophenone (H₂L⁴) afforded only the methoxy bridged dimeric [V^VO(L³/L⁴)(OCH₃)]₂ complexes. The X-ray structural analysis of 1 and 2 indicates that the geometry around the metal is distorted octahedral where the three equatorial positions are occupied by the phenolate-O, enolate-O and the imine-N of the fully deprotonated hydrazone ligand in its enolic form and the fourth one by a methoxide-O atom. An oxo-O and a pyridine-N atom occupy two axial positions. Quaternary complexes exhibit one quasi-reversible one-electron reduction peak near 0.25 V versus SCE in CH₂Cl₂ and they decompose appreciably to the corresponding methoxy bridged dimeric complex in CDCl₃ solution as indicated by their ¹H NMR spectra. These quaternary VO³⁺ complexes are converted to the corresponding -complexes simply on refluxing them in acetone and to the -complexes on reaction with KOH in methanol. An equimolar amount of 8-hydroxyquinoline (Hhq) converts these quaternary complexes to the ternary [V^VO(L)(hq)] complexes in CHCl₃.     

Antimicrobial activity of organotin(IV) complexes with the ligand benzil bis(benzoylhydrazone) and 4,4'-bipyridyl as coligand

Several methyltin(IV) and butyltin(IV) complexes with the ligand benzil bis(benzoylhydrazone) and 4,4′-bipyridyl as coligand were synthesised and characterized by elemental analysis and by IR, ¹H, ¹³C and ¹¹⁹Sn NMR spectroscopies. Some of them were also analyzed using single crystal X-ray diffraction. The title compounds were evaluated for their in vitro antimicrobial properties. All buthyltin complexes showed significant inhibition of Gram positive bacteria, resulting Bacillus subtilis, Sarcina lutea and both methicillin-susceptible and methicillin-resistant Staphylococcus epidermidis the most sensitive strains. Furthermore, they were able to inhibit the growth of Gram negative bacteria, especially Proteus vulgaris, whereas no activity was exhibited against fungi. All methyltin complexes were devoid of antimicrobial properties.     

Three Mn(II) supramolecular coordination complexes containing carboxylate-tetrazolate ligands

Three novel complexes [Mn(atza)₂(H₂O)₄] (1), [Mn(nptza)₂(CH₃OH)₄] (2), and [Mn(a4-ptz)₂(H₂O)₂]_n · 2nH₂O] (3) [atza = 5-aminotetrazole-1-acetato, nptza = 5-[(4-nitryl)phenyl] tetrazole-1-acetato, a4-ptz = 5-[N-acetato(4-pyridyl)] tetrazole] containing carboxylate-tetrazolate ligands have been synthesized and characterized by element analysis. X-ray crystallography shows that complexes 1 and 2 both contain mononuclear structure. The complex 3 is a 1D polymeric chain structure. Compounds 1-3 are self-assembled to form supramolecular structures through hydrogen bonds interactions.     

The monofunctionalized 1,4,7-triazacyclononane derivatives 1,4,7-triazacyclononane-N-acetate (L) and N-(2-hydroxybenzyl-1,4,7-triazacyclononane (HL) and their complexes with vanadium(IV)/(V). Localized and delocalized electronic structures in compounds containing the mixed valent [OV---O---VO] core

The synthesis of the tetradentate pendant arm macrocycles 1,4,7-triazacyclononane-N-acetate (L¹) and N-(2-hydroxybenzyl)-1,4,7-triazacyclononane (HL²) and their coordination chemistry with vanadium(IV) and (V) are reported. The following mononuclear species have been prepared and characterized by UV-Vis, IR spectroscopy: [L¹V^IVO(NCS)] (1), [L¹VO₂]·H₂O (2), [L²VO(NCS)] (3), [L²VO(NCS)]Cl (4), and [L²VO₂] (5). In addition, the dinuclear, mixed valent complexes [L₂¹V₂O₃]Br (6), [L₂²V₂O₃](ClO₄)·0.5acetone (7), and the homovalent complex [L₂²V₂O₃](ClO₄)₂ (8) have been synthesized. Complexes 2, 3, 6 and 7 have been characterized by single crystal X-ray crystallography. Crystal data: 2, space group P2₁c,a=9.944(4),b=6.701(3),c=18.207(8)Å, β=102.88(3)°, V=1182.7 ų, Z=4, D_calc=1.51 g cm⁻³, R=0.049 based on 4760 reflections; 3, space group Pbca, A=11.003(6), b=14.295(7), C=20.21(1) Å, V=3178.8 ų, Z=8, D_calc=1,50 g cm⁻³, R=0.057 based on 1049 reflections; 6, space Pbcn, a=12.922(3), B=13.852(3), C=12.739(3) Å, V=2280.3 ų, Z=4, D_calc=1,75 g cm⁻³, R=0.047 based on 1172 reflections; 7, space group C2/c, A=23.553(9), B=13.497(5), C=20.951(8) Å, β=90.03(3)°, V=6660.2 ų, Z=8, D_calc=1.49 g cm⁻³, R=0.053 based on 3698 reflections. Complexes 6 and 7 are mixed valent V(IV)/(V) complexes containing the [OV---O---VO]³⁺ core. In the solid state 6 belongs to class III (delocalized) and 7 to class I (localized) according to the Robin and Day classification of mixed valent compounds. A rationale for these differing electronic structures is given.     

Reduction of vanadium(V) with ascorbic acid and isolation of the generated oxovanadium(IV) species

The interaction of sodium metavanadate and VOCl₃ with ascorbic acid, one of the possible natural reducing agents of vanadium(V) to oxovanadium(IV), has been investigated. Three new VO²⁺ complexes could be isolated as microcrystalline powders. One of them, of composition K_1.5Na_0.5[VO(HAsc)(OH)₃], contains ascorbic acid as a monodentate ligand. In the other two, K[VO(Diketo)(OH)]·H₂O and Na₃[VO(Diketo)₂(OH)], the enolized form of 2,3-diketogulonic acid (one of the oxidation products of ascorbic acid), acts as a bidentate ligand. The complexes were characterized by means of electronic (absorption and reflectance) and infrared spectroscopy and magnetic susceptibility measurements. Their thermal behavior was investigated by thermogravimetric and differential thermal analyses. The interest of the investigated system in relation to vanadium detoxification is also discussed.     

Synthesis and characterisation of dinuclear oxomolybdenum(V) complexes with thienyl carboxylate ligands

The first structurally characterised oxomolybdenum(V) complexes with thienyl carboxylate ligands were prepared by the reaction of [Mo₂O₃(C₅H₇O₂)₄] or (NH₄)₂[MoOCl₅] with the corresponding acid (2-thiophenecarboxylic, 5-methyl-2-thiophenecarboxylic or 3-(3-thienyl)acrylic acid). Complexes [Mo₂O₃(μ-OC₂H₅)(μ-O₂CR)(C₅H₇O₂)₂](R = -C₄H₃S (1), -C₄H₂S(CH₃) (2) or -CHCHC₄H₃S (3)) were obtained upon substitution of two acetylacetonate ligands from [Mo₂O₃(C₅H₇O₂)₄] with RCOO⁻ in dry ethanol. Reactions of (NH₄)₂[MoOCl₅] with the corresponding thienyl carboxylic acid in the presence of γ-picoline (C₆H₇N) yielded complexes (C₆H₇NH)[Mo₂O₄(μ-O₂CR)Cl₂(C₆H₇N)₂] (R = -C₄H₃S (4), -C₄H₂S(CH₃) (5) or -CHCHC₄H₃S (6)). All of the six new complexes were characterised as dinuclear. The molecular structures of 1, 3, 4·0.5CH₃CN and 5 were determined by the single crystal X-ray diffraction method. In the complexes the two molybdenum atoms are doubly bridged either by one oxygen and one ethoxy-oxygen, or alternatively by two oxo-oxygens, and are additionally bridged by the thienyl carboxylate ion in a didentate bridging manner. All complexes were further characterised by means of chemical analysis, IR spectroscopy, TG and in some cases by the one and two-dimensional NMR method.     

Synthesis and characterization of 8-quinolinolato vanadium(IV) complexes

Reaction of vanadium(III) chloride with 8-quinolinol (Hqn) gave a mononuclear vanadium(IV) complex, [VOCl₂(H₂O)₂] 1) · 2H₂qn · 2Cl · CH₃CN, and three dinuclear vanadium(IV) complexes: [V₂O₂Cl₂(qn)₂(H₂O)₂] (2) · Hqn, [V₂O₂Cl₂(qn)₂(C₃H₇OH)₂] (3), and [V₂O₂Cl₂(qn)₂(C₄H₉OH)₂] (4). Reaction of vanadium(III) chloride with 5-chloro-8-quinolinol (HClqn) gave four dinuclear vanadium(IV) complexes: [V₂O₂Cl₂(Clqn)₂(H₂O)₂] (5) · 2HClqn, [V₂O₂Cl₂(Clqn)₂(C₃H₇OH)₂] (6), [V₂O₂Cl₂(Clqn)₂(C₆H₅CH₂OH)₂] (7), and [V₂O₂Cl₂(Clqn)₂(C₄H₉OH)₂] (8) · 2C₄H₉OH. Reaction of vanadium(III) chloride with 5-fluoro-8-quinolinol (HFqn) gave two dinuclear vanadium(IV) complexes: [V₂O₂Cl₂(Fqn)₂(H₂O)₂] (9) · HFqn · 2H₂O and V₂O₂Cl₂(Fqn)₂(C₃H₇OH)₂] (10). X-ray structures of 1 · 2H₂qn · 2Cl · CH₃CN, 3, 4, 6, 7, 8 · 2 t-BuOH, and 10 have been determined. As to the mononuclear species 1 · 2H₂qn · 2Cl · CH₃CN, coordination of Hqn to vanadium does not occur, but protonation to Hqn occurs to give H₂qn⁺, which links 1’s through hydrogen bonding, while each of the dinuclear species has a terminal and a bridging qn (or Clqn, Fqn) ligand, giving rise to a (V-O)₂ ring. Magnetic measurements of 3, 4, 6, 7, and 10 in solid form show very weak antiferromagnetic behavior, and the effective magnetic moments are close to spin only value (2.44) of d¹-d¹ system, while ESR of 3 in THF shows dissociation to monomeric species. Change from mononuclear, 1, to dinuclear, 2, species was followed by the change of electronic spectrum.     

Synthesis, structural characterization and electrochemical activity of oxidovanadium(IV/V) complexes of a diprotic ONS chelating ligand

The present work reports the chemistry of a few oxidovanadium(IV) and (V) complexes of the ONS chelating ligand S-benzyl-β-N-(2-hydroxyphenylethylidine) dithiocarbazate (H₂L). Major objective of this work is to arrive at some general conclusions about the influence of binding environment generated by the replacement of an O-donor center by a S-donor point in a ligand (of a similar arrangement of the other O- and N-donor points) on the redox behavior and on the structural features of comparable [VO(OEt)(ONS)] and [VO(OEt)(ONO)] complexes. Synthesis, characterization by various physicochemical techniques (UV-Vis, IR, EPR and elemental analysis), exploration of electrochemical activity of the oxidovanadium(V) complex [V^VO(OEt)L] (1), the mixed ligand complex [V^VO(N-O)L] (3) (where N-O is the mono anion of 8-hydroxyquinoline) and a binuclear complex [V^VO(OEt)L]₂(μ-4,4′-bipy) (2) are reported. Similar studies on of mixed ligand oxidovanadium(IV) complexes of the formula [V^VO(N-N)L] (4,5) (where N-N = 2,2′-bipy and o-phen) are also presented here. The [V^VO(OEt)L] complex is pentacoordinated and distorted square pyramidal, while the [V^IV(N-N)L] complexes are hexacoordinated and octahedral. Structural features of the complex 1 were compared with the corresponding aspects of the previously reported analogous complex [V^VO(OEt)(ONO)] (1′).     

Novel dioxomolybdenum(VI) and oxomolybdenum(V) complexes with pyridoxal thiosemicarbazone ligands: Synthesis and structural characterisation

New molybdenum complexes were prepared by the reaction of [Mo^VIO₂(acac)₂] or (NH₄)₂[Mo^VOCl₅] with different N-substituted pyridoxal thiosemicarbazone ligands (H₂L¹ = pyridoxal 4-phenylthiosemicarbazone; H₂L² = pyridoxal 4-methylthiosemicarbazone, H₂L³ = pyridoxal thiosemicarbazone). The investigation of monomeric [MoO₂L¹(CH₃OH)] or polymeric [MoO₂L^1-3] molybdenum(VI) complexes revealed that molybdenum is coordinated with a tridentate doubly-deprotonated ligand. In the oxomolybdenum(V) complexes [MoOCl₂(HL^1-3)] the pyridoxal thiosemicarbazonato ligands are tridentate mono-deprotonated. Crystal and molecular structures of molybdenum(VI) [MoO₂L¹(CH₃OH)]·CH₃OH, and molybdenum(V) complexes [MoOCl₂(HL¹)]·C₂H₅OH, as well as of the pyridoxal thiosemicarbazone ligand methanol solvate H₂L³·MeOH, were determined by the single crystal X-ray diffraction method.     

Synthesis and characterization of two stable paramagnetic octahedral chromium(IV) complexes with dianionic tridentate SNO donor ligands and of a chromium(III) complex with a ONO donor ligand

Two novel paramagnetic octahedral chromium(IV) complexes with dianionic tridentate SNO donor ligands containing extended π-system have been synthesized while only a paramagnetic octahedral chromium(III) complex is obtained when a related dianionic tridentate ONO donor ligand is used under similar conditions. These bischelate complexes [Cr(abtsal)₂] (1) (abtsalH₂ is the Schiff base of o-aminobenzenethiol and salicylaldehyde), [Cr(4-PhTSCsal)₂] · H₂O (2) (4-PhTSCsalH₂ is the Schiff base of 4-phenylthiosemicarbazide and salicylaldehyde) and K[Cr(sap)₂] · H₂O (3) (sapH₂ is the tridentate Schiff base of salicylaldehyde and o-aminophenol) are characterized by elemental analyses, magnetic moment measurements, IR, UV-Vis and EPR spectroscopic studies. Compound 3 has been structurally characterized by X-ray crystallography. Measured room temperature (RT) magnetic moment values are 2.98 BM for 1 and 2.83 BM for 2 indicating a d² system with a triplet ground state in both the cases. On the other hand, the magnetic moment value for 3 is found to be 3.74 BM at RT and is consistent with the presence of three unpaired electrons for a d³ Cr(III) ion. The magnetic moment values rule out the large spin-orbit coupling which is substantiated by the presence of RT EPR signals. Compounds 1 and 2 exhibit very similar powder EPR spectra at RT and LNT, which show the allowed transition ΔM_s = ±1 (g = 2.004 for both 1 and 2) as well as the “forbidden” half-field transition (ΔM_s = ±2) at g = 4.105 for 1 and g = 4.318 for 2, respectively. The X-band LNT frozen glass EPR spectrum of 1 in DMF shows the presence of zero-field split rhombic symmetry character, and results in the parameters g ≅ 2.0, D = 740 G, and E = 260 G. It suggests that the intensity of ΔM_s = ±2 forbidden transition is large due to the large D value. The X-band frozen glass EPR spectrum of compound 3 in DMF is found to be very similar to that reported for trans-[Cr(py)₄F₂]⁺ in DMF-H₂O-MeOH glass. The large difference (∼700 mV) in the reduction potential for the two octahedral complexes 1 (−1.40 V) and 3 (−0.70 V) is attributed to the difference in their metal ion oxidation states.     

Synthesis and structural studies of dimolybdenum(V), palladium(II) and dicopper(I) complexes that contain mixed pyridyl-iso-propylphosphine ligands

The reaction of Mo₂(μ-O₂CCH₃)₄ with 2-pyridyl(diisopropylphosphino)methane (NP) affords the dimolybdenum(V) complex Mo₂(μ-O)₂O₂Cl₂(η²-NP)₂ (1). Complexes of the related 2-pyridylbis(diisopropylphosphino)methane ligand (NP²) have been isolated, namely, a mixed bromo/chloro complex of composition PdBr_1.09Cl_0.91(η²-NP²) (2) and the dicopper(I) complex [Cu₂(μ-η³-NP²)₂](BF₄)₂ (3). The structures of 1, 2 and 3 have been established by X-ray crystallography.     

Copper(II) complexes with 2-N-(picolinylidene)-n-R-phenols: Solvatochromism, self-assembly and supramolecular structures

A series of copper(II) complexes having the formula [Cu(n-R-pyp)X] with the N,N,O-donor Schiff base system 2-N-(picolinylidene)-n-R-phenol (n-R-Hpyp) (where n = 3, 4, 5 and 6, when R = Me and n = 4 when R = Cl) and halide (X⁻ = Cl⁻ or Br⁻) as an ancillary ligand have been synthesized. The complexes are characterized by microanalytical, magnetic and various spectroscopic measurements. They display solvatochromic behavior. Single crystal X-ray structures of all the complexes are determined. In coordinatively unsaturated species such as a square-planar complex, the metal ion can interact with a fifth atom and if this atom is metal bound, dimeric or polymeric aggregate is formed. In the present series of complexes, the metal ions are square-planar and distorted square-pyramidal when there is an intermolecular Cu···X interaction. In addition to this Cu···X interaction, presence of intermolecular weak non-covalent interactions namely O-H···O, C-H···O, C-H···X and π···π are perceived. The supramolecular architectures formed by the molecules of these complexes via these interactions are scrutinized. The observed supramolecular structural motifs can be classified as staircase, ladder, brick-wall and square-grid. Except for R = Cl the analogous chloride and bromide coordinated complexes show similar structural features.     

Synthesis, structure and solution chemistry of dioxidovanadium(V) complexes with a family of hydrazone ligands. Evidence of formation of centrosymmetric dimers via H-bonds in the solid state

[V^IVO(acac)₂] reacts with the methanol solution of tridentate ONO donor hydrazone ligands (H₂L^1-4, general abbreviation H₂L; are derived from the condensation of benzoyl hydrazine with 2-hydroxyacetophenone and its 5-substituted derivatives) in presence of neutral monodentate alkyl amine bases having stronger basicity than pyridine e.g., ethylamine, diethylamine, triethylamine and piperidine (general abbreviation B) to produce BH⁺[VO₂L]⁻ (1-16) complexes. Five of these sixteen complexes are structurally characterized revealing that the vanadium is present in the anionic part of the molecule, [VO₂L]⁻ in a distorted square pyramidal environment. The complexes 5, 6, 15 and 16 containing two H-atoms associated with the amine-N atom in their cationic part (e.g., diethylammonium and piperidinium ion) are involved in H-bonding with a neighboring molecule resulting in the formation of centrosymmetric dimers while the complex 12 (containing only one hydrogen atom in the cationic part) exhibits normal H-bonding. The nature of the H-bonds in each of the four centrosymmetric dimeric complexes is different. These complexes have potential catalytic activity in the aerial oxidation of l-ascorbic acid and are converted into the [VO(L)(hq)] complexes containing VO³⁺ motif on reaction with equimolar amount of 8-hydroxyquinoline (Hhq) in methanol.     

Synthesis, structures and spectroscopic properties of palladium(II) complexes with tridentate piperidyl-containing pincer ligands

A new series of square planar palladium(II) complexes with pincer ligands, pip₂NCN⁻ (pip₂NCNH = 1,3-bis(piperidylmethyl)benzene) and pip₂NNN (2,6-bis(piperidylmethyl)pyridine), has been prepared: Pd(pip₂NCN)X (X = Cl, Br, I), [Pd(pip₂NCN)(L)](BF₄) (L = pyridine, 4-phenylpyridine), and [Pd(pip₂NNN)Cl]Cl. The X-ray crystal structures of Pd(pip₂NCN)Br, [Pd(pip₂NCN)(L)]BF₄, and [Pd(pip₂NNN)Cl]Cl confirm the tridentate coordination geometries of the pincer ligands. For the pip₂NCN⁻ complexes, each piperidyl ring adopts a chair conformation with the metal center at an equatorial position on the N(piperidyl) atom. However, one of the piperidyl groups of Pd(pip₂NNN)Cl⁺ adopts a previously unobserved coordination geometry, effectively placing the metal center at an axial position on the N(piperidyl) atom. ¹H NMR and UV-Vis absorption measurements provide additional insight into the electronic structures of these complexes. The ¹H NMR spectra of Pd(pip₂NCN)X (X = Cl, Br, I) are consistent with deshielding of the pip₂NCN⁻ ligand resonances along the Cl < Br < I series, in opposition to the relative halogen electronegativities. It is suggested that this trend is consistent with decreasing filled/filled repulsions between the dπ orbitals of the metal center and the lone pair orbitals of the halide ligands along this series. Electronic absorption spectra support the notion that ligand-to-metal charge-transfer states are stabilized in these palladium(II) complexes relative to their platinum(II) analogues.     

Syntheses, redox and UV-Vis spectroelectrochemical properties of mono- and dinuclear tris(pyrazolyl)borato-oxomolybdenum(IV) complexes with pyridine ligands

Reaction of [Mo^VI(Tp^Me,Me)(O)₂Cl] with a variety of pyridine-based ligands [pyridine (py), 4,4′-bipyridine (bpy), 4-phenylpyridine (phpy) and 1,2′-bis(4-pyridyl)ethene (bpe)] in toluene in the presence of Ph₃P affords the mononuclear oxo-Mo(IV) complexes [Mo(Tp^Me,Me)(O)Cl(L)] (L=py, phpy or monodentate bpy; abbreviated as Mo(py), Mo(phpy) and Mo(bpy), respectively) and the dinuclear complexes [{Mo(Tp^Me,Me)(O)Cl}₂(μ-L)] (L=bpy, bpe; abbreviated as Mo₂(bpy), Mo₂(bpe), respectively). The complex Mo₂(bpy), together with the by-product [{Mo(Tp^Me,Me)(O)Cl}₂(μ-O)], have been crystallographically characterised. Electrochemical studies on the oxo-Mo(IV) complexes reveal the presence of reversible Mo(IV)/Mo(V) couples at around −0.3 V versus ferrocene/ferrocenium in every case. For the dinuclear complexes Mo₂(bpy) and Mo₂(bpe) these redox processes are coincident, indicating that they are largely metal-centred and not significantly delocalised across the bridging ligand. In contrast, Mo₂(bpe) alone shows two reversible reductions, separated by 320 mV; these could be described as ligand-centred reductions of the bpe bridge, or as Mo(IV)/Mo(III) couples which—because of their separation—are substantially delocalised onto the bridging ligand. UV-Vis spectroelectrochemical studies using an OTTLE cell at 243 K revealed that oxidation of the complexes results in spectral changes (collapse of the Mo(IV) d-d transitions, loss in intensity of the Mo→pyridine MLCT transition) consistent with the formation of a Mo(V) state following metal-centred oxidation, but that one-electron reduction of Mo₂(bpe) results in appearance of numerous intense transitions more characteristic of a ligand radical following ligand-centred reduction.     

Homogeneous green catalysts for olefin oxidation by mono oxovanadium(V) complexes of hydrazone Schiff base ligands

Three mono oxovanadium(V) complexes of tridentate Schiff base ligands [VO(OMe)L¹] (1), [VO(OMe)L²] (2) and [VO(OMe)L³] (3) obtained by monocondensation of 3-hydroxy-2-naphthohydrazide and aromatic o-hydroxyaldehydes have been synthesized (H₂L¹ = (E)-3-hydroxy-N′-(2-hydroxy-3-methoxybenzylidene)-2-naphthohydrazide, H₂L² = (E)-3-hydroxy-N′-(2-hydroxybenzylidene)-2-naphthohydrazide and H₂L³ = (E)-N′-(5-bromo-2-hydroxybenzylidene)-3-hydroxy-2-naphthohydrazide). The complexes were characterized by spectroscopic methods in the solid state (IR) and in solution (UV-Vis, ¹H NMR). Single crystal X-ray analyses were performed with 1 and 2. The catalytic potential of these complexes has been tested for the oxidation of cyclooctene using H₂O₂ as the terminal oxidant. The effects of various parameters including the molar ratio of oxidant to substrate, the temperature, and the solvent have been studied. The catalyst 2 showed the most powerful catalytic activity in oxidation of various terminal, cyclic and phenyl substituted olefins. Excellent conversions have been obtained for the oxidation of cyclic and bicyclic olefins.     

Synthesis, characterization and crystal structures of the organotin(IV) compounds with the Schiff base ligands of pyruvic acid thiophene-2-carboxylic hydrazone and salicylaldehyde thiophene-2-carboxylic hydrazone

A series of organotin (IV) compounds of the type [R₃SnL]₂, R is Me (1), Bu (2), [R₂SnL]₂, R is Ph (3), Me (4), Bu (5), L is pyruvic acid thiophene-2-carboxylic hydrazone, and R₂SnL, R is Me (6), Bu (7), Ph (8), L is salicylaldehyde thiophene-2-carboxylic hydrazone have been synthesized in 1:1 molar ratio. All compounds were characterized by elemental analysis, IR, ¹H NMR, ¹³C NMR and ¹¹⁹Sn NMR spectra. The crystal structure of compounds 1, 3, 4, 8 have been determined by X-ray single crystal diffraction analyses, study found that the compounds 1 and 3 are rendered one-dimensional chain structure and the tin atoms are five-coordinated in a distorted trigonal-bipyramidal geometry. The compound 4 has a dimeric structure and the central tin atom is rendered seven-coordinate in a distorted pentagonal-bipyramid configuration. While the compound 8 is a monomer in which the tin atom adopts five-coordinated in distorted trigonal-bipyramidal geometry.     

Ligand substitution in hexahalorhenate(IV) complexes: Synthesis, crystal structures and magnetic properties of NBu4[ReX5(DMF)] (X = Cl and Br)

The preparation, crystal structures, and magnetic properties of two rhenium(IV) mononuclear compounds of formula NBu₄[ReX₅(DMF)] with X = Cl (1) and Br (2) are reported. 1 and 2 are isostructural complexes which crystallize in the monoclinic system with the space group P2₁/n. The rhenium atom is six-coordinated with five X atoms and a DMF molecule forming a somewhat distorted octahedral surrounding [values of Re-X varying in the ranges 2.317(1)-2.358(1) (1) and 2.495(1)-2.518(1) Å (2)]. Magnetic susceptibility measurements on samples of 1 and 2 in the temperature range 1.9-300 K are interpreted in terms of magnetically isolated spin quartets with large values of the zero-field-splitting (|2D| is ca. 20.2 and 39.2 cm⁻¹ for 1 and 2, respectively).     

The reaction of the cis-dioxorhenium(V) core with N,O-donor ligands: mono and bidentate coordination of 3-methyl-2-aminophenol

The reaction of cis-[ReO₂I(PPh₃)₂] with 3-methyl-2-aminophenol (H₂map) in ethanol led to the formation of trans-[Re(map)(Hmap)I(PPh₃)₂]I. The X-ray crystal structure shows that the ligand map is coordinated monodentately through the doubly deprotonated amino nitrogen and is therefore present as an imide. The chelate Hmap is coordinated via the neutral amino nitrogen and the singly deprotonated phenolate oxygen, which is coordinated trans to the imido nitrogen. In benzene equimolar quantities of cis-[ReO₂I(PPh₃)₂] and 4-methyl-2-aminophenol (H₂amp) produced the five-coordinate cis-[ReO₂(Hamp)(PPh₃)]. A twofold molar excess of 2-aminophenol (H₂ap) yielded the six-coordinate [ReO(Hap)(ap)(PPh₃)]. The chelate ap is coordinated through the deprotonated phenolate oxygen and the singly deprotonated amino nitrogen (amide).