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The reaction of dibasic tetradentate ONNO donor Mannich bases, derived from ethylenediamine and 2,4–di–tert–butylphenol (H2L1) (I), 2,4–di–methylphenol (H2L2) (II), 2–tert–butyl–4–methylphenol (H2L3) (III) and 2,4–di–chlorophenol (H2L4) (IV), with UVIO2(MeCOO)2·2H2O in a 1:1 M ratio in refluxing MeOH gave the corresponding mononuclear trans-dioxidouranium(VI) complexes of the type trans-[UVIO2L(MeOH)] (H2L = H2L1 to H2L4) (1–4). The synthesized complexes are stable in air, reddish-brown in color and soluble in most solvents. These complexes are characterized by elemental analysis, various spectroscopic (FT-IR, UV/Vis, 1H and 13C NMR) techniques and single-crystal X-ray analysis of 3 and 4. The complexes adopt distorted pentagonal bipyramidal geometry around the metal centre. The ligand acts as tetradentate, coordinating through two phenolato oxygen and two imino nitrogen atoms; two oxido groups are trans to each other. These complexes are used as catalysts to study the oxidative bromination of thymol and styrene. The catalytic oxidative bromination of thymol resulted in the formation of three products namely, 2-bromothymol, 4-bromothymol and 2,4-dibromothymol while oxidative bromination of styrene gave two products, 2-bromo-1-phenylethane-1-ol and 1-phenylethane-1,2-diol. In order to find out the optimized reaction conditions for the fixed concentration (10 mmol) of substrate, effects of different amounts of catalyst, KBr, HClO4, and oxidant (H2O2) have been investigated. Under the optimized reaction conditions, all the complexes have shown good catalytic potentials for the oxidative bromination of substrates, establishing the functional similarity to vanadium dependent haloperoxidases. Changes in the UV–visible absorption spectra of dioxidouranium(VI) complexes upon addition of H2O2 suggest the formation of the corresponding oxidoperoxidouranium(VI) complexes.

The stable dibasic tetradentate ligand 1,4-bis-(2-hydroxy-3,5-dimethylbenzyl)piperazine (H2pip-2,4-dmp, I) prepared by reacting 2,4-dimethylphenol with piperazine in the presence of formaldehyde reacts with [MoVIO2(acac)2] and [UVIO2(CH3COO)2] in equimolar ratio to give neutral hexa-coordinated [MoVIO2(pip-2,4-dmp)] (1) and hepta-coordinated [UVIO2(pip-2,4-dmp)(MeOH)] (2), respectively. After characterizing these complexes by spectroscopic (IR, UV/Vis, 1H and 13C NMR) data, elemental and thermal analyses (and single crystal X-ray diffraction study of uranium complex), they are used as catalysts to study the oxidative bromination of thymol. Such catalytic reactions are observed by many model vanadium complexes and are considered as a functional mimic of haloperoxidases. The catalytic oxidation resulted in the formation of three products namely, 2-bromothymol, 4-bromothymol and 2,4-dibromothymol. The optimized reaction conditions are obtained considering concentration of KBr, HClO4, and oxidant for the maximum yield of brominated products. Under the optimized reaction conditions, the product selectivity for both the prepared complexes is investigated. They are found to be competent homogeneous catalysts to afford the products in good yield.

The tetradentate diaminobis(phenolato) “salan” ligand precursors, N,N′-bis-(2-hydroxy-3-iPr-5-Cl-6-Me-benzyl)-4-methylimidazolidine (H2L1) is synthesized. Reaction of H2L1 with MoO2(acac)2, FeCl3, VOSO4 and Ni(OAc)2, resulted in the formation of [MoO2(L1A)] (1), [(Fe(L1A))2(µ-Cl)(µ-OCH3)], (2), [(VO(L1A))2(µ-O)]·THF (3) and [Ni(L1A)] (4), wherein (L1A)2− resulted from the loss of the capping –CH2- moiety of H2L1 upon complex formation. However the capping –CH2- moiety between the two N-amine atoms is retained in the reaction of H2L1 with Ti(OiPr)4, resulting in formation of the dinuclear complex [(Ti(L1)(OiPr))2(µ-O)] (5). All compounds obtained are characterized by various physicochemical techniques (elemental analysis, IR, UV–Vis and NMR spectroscopy, and ESI-MS), and by single crystal X-ray diffraction analysis in the case of complexes 1–3. The several factors relevant for the formation of the compounds, as well as cleavage or non-cleavage of the –CH2- bridge in the salan ligands, are elucidated with the help of DFT calculations.

Two oxidoperoxidotungsten(VI) complexes and the corresponding dioxidotungsten(VI) complexes of tridentate ONO donor Schiff base ligands, H2hap-nah (I) and H2hap-bhz (II) (Hhap = 2-hydroxyacetophenone, nah = nicotinoylhydrazide and bhz = benzoylhydrazide) have been reported. Complexes [WVIO(O2)(hap-nah)(MeOH)] (1), [WVIO(O2)(hap-bhz)(MeOH)] (2), [WVIO2(hap-nah)(MeOH)] (3) and [WVIO2(hap-bhz)(MeOH)] (4) have been characterized by elemental analysis, spectroscopic (IR, UV–Vis, 1H and 13C NMR) and thermogravimetric studies. Single crystal X-ray analysis of 2 and [WO(O2)(hap-inh)(MeOH)] (inh = isonicotinoylhydrazide) (whose synthesis has already been reported in the literature) confirms dibasic tridentate behavior of the ligands and their coordination to the metal ion through two oxygen and a nitrogen atoms. These complexes catalyze oxidative bromination of thymol, a model oxidative halogenation reaction normally shown by vanadium complexes, under acidic (HClO4) condition using 30% H2O2 in the presence of KBr. Under optimized reaction conditions, the selectivity of products follows the order: 4-bromothymol > 2,4-dibromothymol > 2-bromothymol.

Four dioxidomolybdenum(VI) complexes, [MoVIO2(bp-bhz)(MeOH)] (1), [MoVIO2(bp-inh)(MeOH)] (2), [MoVIO2(bp-nah)(MeOH)] (3) and [MoVIO2(bp-fah)(MeOH)] (4) have been isolated with ONO tridentate Schiff base ligands, H2bp-bhz (I), H2bp-inh (II), H2bp-nah (III) and H2bp-fah (IV) [where ligands I–IV are derived from 4-benzoyl-3-methyl-1-phenyl-2-pyrazoline-5-one (Hbp) and isonicotinoyl hydrazide (Hinh), benzoyl hydrazide (Hbhz), nicotinoyl hydrazide (Hnah) or furoyl hydrazide (Hfah), respectively] and characterized. Structures of complexes [MoVIO2(bp-bhz)(MeOH)] (1) and [MoVIO2(bp-nah)(DMSO)] · 0.5H2O (3a) have been confirmed by single crystal X-ray study. Using conventional and microwave-assisted methods, these complexes have been tested as catalysts for the oxidation of secondary alcohols (1-phenylethanol, 2-propanol and 2-butanol) in the presence of 30% H2O2 as an oxidant. Various parameters such as amounts of catalyst, oxidant, solvent and temperature of the reaction mixture have been taken into consideration for the maximum conversion of substrates. Under the atmospheric optimized reaction conditions, secondary alcohols give high yield of the corresponding ketones selectively. Addition of an N-based additive reduces the reaction time considerably. Microwave irradiation effect on the reaction time and yield has been taken into consideration to conclude that microwave method is more efficient compared to conventional liquid phase method.

The tridentate ONN donor ligand, Hbzpy-fah (I) (Hbzpy-fah = Schiff base derived from 2-benzoylpyridine and 2-furoylhydrazide) upon reaction with [VIVO(acac)2] in methanol followed by aerial oxidation give a dinuclear complex, [{VVO(bzpy-fah)}2(μ-O)2] (1) having [{VVO}2(μ-O)2]2+ core. Treatment of 1 with H2O2 in methanol results in the formation of dinuclear peroxido complex, [{VVO(bzpy-fah)}2(μ-O2)2] (2). The molecular structure of 2 was determined by single-crystal X-ray diffraction, confirming the ONN binding mode of I and μ-bis(peroxido) with side-on coordination. Reaction of 1 with imidazolomethylpolystyrene cross-linked with 5% divinylbenzene (PS-im) in DMF gives the polymer-supported complex 3 (abbreviated as PS-im[VVO2(bzpy-fah)]) which was characterized by field-emission scanning electron micrographs (FE-SEM) as well as energy dispersive X-ray (EDAX), atomic force microscopy (AFM) and thermal analysis along with usual spectroscopic techniques. Complexes 1 and 3 have been used as catalyst precursors for the oxidation of isoeugenol in the presence of aqueous H2O2 as oxidant and gave vanillin, vanillic acid and dehydrodiisoeugenol. The polymer-supported complex showed higher conversion than its neat counterpart. This has also allowed for recyclable catalytic system with increased catalyst lifetime and thus proved to be better over the homogeneous counterpart.

Three neat complexes [VVO2(acpy-bhz)] (1) [VVO2(acpy-inh)] (2) and [VVO2(acpy-nah] (3) and the corresponding polymer-supported (PS) dioxidovanadium(V) complexes having monobasic tridentate ONN donor ligands, abbreviated herein as PS-im[VVO2(acpy-bhz)] (4) PS-im[VVO2(acpy-inh)] (5) and PS-im[VVO2(acpy-nah)] (6) are isolated through covalent bonding of imidazolomethylpolystyrene, obtained by reacting chloromethylated polystyrene cross-linked with 5% divinylbenzene with imidazole, with the corresponding neat complexes 1–3. All compounds are characterized in solid state and in solution, namely by spectroscopic techniques (IR, UV–Vis, 51V NMR, thermal and scanning electron micrograph studies). The monomeric form {[VO2(acpy-nah)]·DMSO (3·DMSO)} (3a) of complex 3 is also isolated from its solution in DMSO and its molecular structure is confirmed by single crystal X-ray diffraction. Polymer-supported as well as neat complexes are used as catalyst precursors for the oxidative bromination of styrene and trans-stilbene using 30% aqueous H2O2 as an oxidant, the compounds acting as functional models of vanadium dependent haloperoxidases. 1-Phenylethane-1,2-diol, 2-bromo-1-phenylethane-1-ol (bromohydrin) and 1,2-dibromo-1-phenylethane are the reaction products of styrene after 1 h of reaction, while those of trans-stilbene are: 2,3-diphenyloxirane (trans-stilbene oxide), 2-bromo-1,2-diphenylethanol and 1,2-dibromo-1,2-diphenylethane. It is also shown that all these compounds are catalyst precursors for the catalytic oxidation of benzoin by peroxide, the products being benzil, methylbenzoate, benzoic acid and benzaldehyde-dimethylacetal. An outline of the mechanism is proposed and plausible intermediates involved in the catalytic processes are proposed/established by UV–Vis and 51V NMR studies.

The formation of decavanadates containing amine counter-ions is explored to get a better insight into the resulting crystal packing. From mixtures containing the Schiff base H2(SO3-sal)2en and vanadate, two different compounds containing decavanadate units have been characterised by X-ray diffraction. The structure of 1 consists of [V10O28]6− monomers united by K+, ethylenediamine cations and H2O molecules, which act as bridging atoms between the layers, forming a three-dimensional array. The structure of 2 contains vanadium triprotonated decavanadate anions, [H3V10O28]3−, in one-dimensional vertical chains, bonded by interdecavanadate hydrogen bonds. The interchain region is populated by (H2en)2+ cations and H2O molecules, which form a network of hydrogen bonds. From the filtrate solution of the reaction mixture containing VIVOSO4, d , l -diaminopropionic acid and salicylaldehyde, which also contained triethylamine, a diprotonated decavanadate 3 was obtained. Compound 3 consists of decavanadate vertical dimers, surrounded by triethylamonium cations that preclude the interaction between the dimers. Bond valence calculations were used to confirm the degree of protonation of the decavanadate anions.

The redox behavior of the potential oral insulin mimetic compound bis[3-hydroxy-1,2-dimethyl-4-pyridinonate]oxovanadium(IV), VO(dmpp) 2 , was studied in aqueous solution, under aerobic conditions and at different pH values and metal-to-ligand ratios, as well as the speciation of its V(V) oxidation products. Dissolution of this compound in water originates a violet solution with pH 4.7, which is partially oxidized to two V(V) species, VO 2 L and [VO 2 L 2 ] − , detected through their 51 V NMR signals at −485 and −504 ppm. However, the VO(dmpp) 2 compound still remains to some extent in solution, as confirmed by EPR Spectroscopy. High pH values favor the oxidation of the V(IV) complex in solution, whereas at low pH the V(IV) species are predominant. Aqueous solutions containing vanadate–Hdmpp mixtures spontaneously yield V(IV) species, reflecting the reducing capacity of the ligand. This reduction process is favored by ligand excess and low pH. For a ligand excess of fivefold or higher, the [VO 2 L 2 ] − complex, the only species present in solution, produces, at pH 2 (H 2 O)] and [VO(H 2 O) 5 ] 2+ , as detected by EPR. At higher vanadate–hdmpp ratios [VO(dmpp)(H 2 O) 2 ] + is also produced. The V(V) species formed in solution were studied through a parallel 51 V and 1 H NMR study of the vanadate–Hdmpp system, as a function of metal/ligand ratio and pH. The p K a values of the two main mononuclear V(V) octahedral complexes, VO 2 L and [VO 2 L 2 ] − (L=dmpp), were determined from the pH dependence of their 51 V and 1 H NMR chemical shifts. For VO 2 L, p K a values of 5.88±0.03 and 3.30±0.07 were assigned, respectively, to protonation of [VO 2 (dmpp)(OH)(H 2 O)] − at the coordinated hydroxo ligand and at the ligand nitrogen atom. For [VO 2 L 2 ] − , the three p K a values obtained, 3.96±0.06, 5.62±0.03, and 2.9±0.4, were attributed, respectively, to protonation of [VO 2 (dmpp) 2 ] − at the nitrogen atom of one of the ligands, at one oxygen of the VO 2 + core, and at its resulting VOH group, with formation of the VO 3+ core. The stability constants of these two complexes (log K 1 =10.48±0.05; log K 2 =5.25±0.04; log β 2 =15.73±0.05) were obtained from 51 V NMR data, at pH 8.2, ionic strength 0.1 M, by changing the metal/ligand ratio at constant metal concentration.

Copper(II) complexes with a series of bibracchial lariat ethers are described. Independently of the nature of the counterion present (nitrate or perchlorate), the lariat ether N,N'-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 (L1) always forms mononuclear complexes, whereas the lariat ethers N,N'-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L2) and N,N'-bis(2-salicylaldiminobenzyl)-4,13-diaza-18-crown-6 (L3) only give binuclear compounds. The X-ray crystal structure of [CuL1](ClO4)2 shows a seven-coordinated copper(II) ion in a distorted (axially compressed) pentagonal-bipyramidal geometry. The X-ray crystal structure of [Cu2(L3-2H)](ClO4)2 confirms the binuclear nature of the compound with both metal ions having identical coordination environments and each one placed out of the crown hole but efficiently encapsulated by the corresponding pendant arm; each copper(II) ion is five-coordinated with an intermediate geometry between trigonal-bipyramidal and square-pyramidal ([tau]=0.40). The EPR spectra in frozen solution samples are in accordance with a stable coordinate pattern for the metal centre of ligand L1, yielding a rhombic distorted complex with axial compression in solution, in agreement with the X-ray crystal structure of [CuL1](ClO4)2. For the binuclear complexes of L2 and L3, the Cu(II) centres in solution can be distorted from their tetragonally elongated structures via interaction with ethanol and/or the nitrate counterion, leading to more than one species. http://www.sciencedirect.com/science/article/B6TG5-433PCM9-T/1/7815e8fe94b2b6b2129e76252c3517d0