The Chemistry of Transition Metal Complexes Containing Catechol and Semiquinone Ligands

Reaction of VO(acac)2 with 2-mercaptophenol (mpH2) in the presence of triethylamine gives the mononuclear tris complex (Et3NH)2[V(mp)3] (1), in which the vanadyl oxygen has been displaced. An analogous reaction using 2-mercapto-4-methylphenol (mmpH2) afforded (Et3NH)(PNP)[V(mmp)3] (2), which was structurally characterized. 2 crystallizes in the orthorhombic space group Pna21 with unit cell parameters (at −163 °C) a = 23.974(7) A, b = 9.569(4) A, c = 25.101(6) A, and Z = 4. The coordination geometry around the vanadium is between octahedral and trigonal prismatic. Reaction of VO(acac)2 with the sodium salt of 2-mercaptophenol produces the vanadyl(IV) complex Na(Ph4P)[VO(mp)2]·Et2O (3), which crystallizes in the triclinic space group P1 with unit cell parameters (at −135 °C) a = 12.185(4) A, b = 12.658(4) A, c = 14.244(4) A, α = 103.19(2)°, β = 100.84(2)°, and γ = 114.17(2)°. The unit cell of 3 contains a pair of symmetry-related [VO(mp)2]2- units bridged through vanadyl and ligand oxygen atoms by a pair o...

Vanadium(IV) Complexes with Mixed O,S Donor Ligands. Syntheses, Structures, and Properties of the Anions Tris(2-mercapto-4-methylphenolato)vanadate(IV) and Bis(2-mercaptophenolato)oxovanadate(IV).

The concerned diols (general abbreviation, H(2)L) are catechol (H(2)L(1)) and its 3,5-Bu(t)(2) derivative (H(2)L(2)). Esters of the type VO(xsal)(HL), 2, are obtained by reacting H(2)L with VO(xsal)(H(2)O) or VO(xsal)(OMe)(HOMe), where xsal(2-) is the diionized salicylaldimine of glycine (x = g), L-alanine (x = a), or L-valine (x = v). The reaction of VO(acac)(2) with H(2)L and the salicylaldimine (Hpsal) of 2-picolylamine has furnished VO(psal)(L), 3. In the structures of VO(gsal)(HL(1)), 2a, and VO(vsal)(HL(2)), 2f, the HL(-) ligand is O,O-chelated, the phenolic oxygen lying trans to the oxo oxygen atom. The xsal(2-) coligand has a folded structure and the conformation of 2f is exclusively endo. In both 2a and 2f the phenolic oxygen atom is strongly hydrogen bonded (O...O, 2.60 A) to a carboxylic oxygen atom of a neighboring molecule. In VO(psal)(L(2)).H(2)O, 3b, the diionized diol is O,O-chelated to the metal and the water molecule is hydrogen bonded to a phenoxidic oxygen atom (O.O, 2.84 A). The C-O and C-C distances in the V(diol) fragment reveal that 2 is a pure catecholate and 3 is a catecholate-semiquinonate hybrid. In solution each ester gives rise to a single (51)V NMR signal (no diastereoisomers), which generally shifts downfield with a decrease in the ester LMCT band energy. The V(V)/V(IV) and catecholate-semiquinonate reduction potentials lie near -0.75 and 0.35, and 1.10 and 0.70 V vs SCE for 2 and 3, respectively. Molecular oxygen reacts smoothly with 2 quantitatively furnishing the corresponding o-quinone, and in the presence of H(2)L the reaction becomes catalytic. In contrast, type 3 esters are inert to oxygen. The initial binding of O(2) to 2 is proposed to occur via hydrogen bonding with chelated HL(-).

A family of vanadate esters of monoionized and diionized aromatic 1,2-diols: synthesis, structure, and redox activity

The reaction of the tripodal tetradentate dibasic ligand 6,6′-(2-(pyridin-2-yl)ethylazanediyl)bis(methylene)bis(2,4-di-tert-butylphenol), H2L1I, with [VIVO(acac)2] in CH3CN gives the VVO-complex, [VVO(acac)(L1)] 1. Crystallisation of 1 in CH3CN at ∼0 °C gives dark blue crystals of 1, while at room temperature it affords dark green crystals of [{VVO(L1)}2μ-O] 3. Upon prolonged treatment of 1 in MeOH, [VVO(OMe)(MeOH)(L1)] 2 is obtained. All three complexes were analysed by single-crystal X-ray diffraction, depicting a distorted octahedral geometry around vanadium. In the reaction of H2L1 with VIVOSO4 partial hydrolysis of the tripodal ligand results in the elimination of the pyridyl fragment of L1 and the formation of H[VVO2(L2)] 4 containing the ONO tridentate ligand 6,6′-azanediylbis(methylene)bis(2,4-di-tert-butylphenol), H2L2II. Compound 4, which was not fully characterised, undergoes dimerization in acetone yielding the hydroxido-bridged [{VVO(L2)}2μ-(OH)2] 5 having a distorted octahedral geometry around each vanadium. In contrast, from a solution of 4 in acetonitrile, the dinuclear compound [{VVO(L2)}2μ-O] 6 is obtained, with a trigonal bipyramidal geometry around each vanadium. The methoxido complex 2 is successfully employed as a functional catechol-oxidase mimic in the oxidation of catechol to o-quinone under air. The process was confirmed to follow a Michaelis–Menten type kinetics with respect to catechol, the Vmax and KM values obtained being 7.66 × 10−6 M min−1 and 0.0557 M, respectively, and the turnover frequency is 0.0541 min−1. A similar reaction with the bulkier 3,5-di-tert-butylcatechol proceeded at a much slower rate. Complex 2 was also used as a catalyst precursor for the oxidative bromination of thymol in aqueous medium. The selectivity shows quite interesting trends, namely when not using excess of the primary oxidizing agent, H2O2, the para mono-brominated product corresponds to ∼93% of the products and no dibromo derivative is formed.

Vanadium(V) complexes of a tripodal ligand, their characterisation and biological implications

The 655 articles cited in this review cover developments in the fields of ultraviolet light absorption spectrometry and light absorption spectrometry during the period December 1977 through November 1979, primarily documented in Chemical Abstracts. An extensive section of the review is devoted to the chemical aspects involved in the development of suitable reagents, absorbing systems, and methods for spectrophotometric analysis. Principles associated with the measurement of ultraviolet and visible radiant energy as well as the instrumentation used for these measurements are reviewed in the section entitled, Physics. Spectrophotometric methods for the determination of metals and of certain organic compounds are tabulated with matrix material, method or reagent, and appropriate reference number from the bibliography being specified. (BLM)

Ultraviolet and Light Absorption Spectrometry

A study of vanadium(IV)-3-methylcatechol-quaternary ammonium systems

Spectrophotometric study and extraction of niobium(V) complexes with catechol and its derivatives as ion-pairs with quaternary ammonium ions: Application to the separation of niobium from other refractory metal ions

J.A. Catoggio

Papers

A study of vanadium(IV)-3-methylcatechol-quaternary ammonium systems

Spectrophotometric study and extraction of niobium(V) complexes with catechol and its derivatives as ion-pairs with quaternary ammonium ions: Application to the separation of niobium from other refractory metal ions

Spectrophotometric determination of vanadium with 3-methylcatechol in alloy steels