Vanadium complexes equilibria

Scheme 43. The Fischer-type see Fischer-type Carbene Complexes) vanadium complexes are prepared by treatment of (butadiene)hafhocene (81) or (butadiene)zirconocene with CpV(CO)4 (Scheme 44). The equilibrium mixture reacts with cyclopentanone to give the adduct complex (82). - ...

The equilibrium constant for the formation of [V02(H2edta)] has been estimated as 1.66 x 10. Vanadium)v)-hydroxycarboxylato- and -polyalcohol complexes have been observed in acidic media, and were shown to be primarily tetravanadato-complexes which are broken down by an excess of... 

However, oxidation with H2O2 in acetone resulted in a high diol selectivity with an equilibrium mixture of the cis- and trans-diols, illustrating the role of the residual acidity of the support The reaction is suggested to occur via heterolyhc cleavage of the vanadium peroxo species. Less than 0.5% leaching of the bipy complex was observed over 50 h of operation. 

A similar preparative procedure was used to isolate tetrahedral complexes of bidentate donors (L = bipy or phen) [VL2](SCN)3. However, complexes of vanadium(lll) chloride or bromide with these same bidentate donors have stoichiometries depending on conditions.199 With excess of bipy or phen, complexes [VX JX were obtained. In EtOH or in MeCN two series of complexes were isolated. VC13LL (L = bipy or phen L = EtOH or MeCN) and [VC12L2]+ + [VCUL]-. In the reaction of vanadium(III) with phenanthroline202 203 or bipyridine204 there is evidence for extensive hydrolysis. Spectrophotometry was used to estimate the equilibrium constant (Kcq = 0.43) for the reaction in equation (7). 03... 

Aromatic polyalcohols act as strong coordinating agents and Table 17 summarizes reported formation constants. The complexes are quite stable this behaviour has been used for the qualitative and quantitative determination of vanadium (e.g. refs. 494 and 495). At pH 3-4, an initial vanadyl catechol complex slowly converts to a tris complex.496 In fact complexes with 1 3 metal-ligand stoichiometry have been isolated (see below), but since in the equilibrium (30) no protons are consumed or liberated, [VO(cat)2]2- and [V(cat)3]2 are not distinguishable by potentiometric studies. 

Early publications on [VO(/3-diketonato)2] have been reviewed.355 More recently, complexes with benzoyl m-nitroacetanilide, benzoyl acetanilide545 and l,l -(l,3-phenylene)-bis(butane-1,3-dione546 have been synthesized. Other [VO(/S-dik)2] adducts have been isolated, for example [VO(acac)2] adducts with a series of pyridine N-oxides547 and several pyridine carboxamides, 48 and [VO(bzac)2] adducts with pyridine, methylamine, isoquinoline and 4-picoline.549 Equilibrium constants of 1 1 and 2 1 adducts of pyrazine with [VO(tfacac)2] have been determined (equation 38).550 In the 2 1 complex, the pyrazine bridge between two equatorial sites of adjacent vanadium atoms promotes a weak exchange interaction. The nitroxide radical 2,2,6,6-tetramethylpiperidinyl N-oxide also forms an adduct with [VO(hfacac)2] in which there is a strong interaction between the electrons on the metal and nitroxide.551... 

The reaction of vanadium salts (VC14, V202(S04)2, V(OCOMe)4, VO(acac)2) and porphyrin gives a complex which, after work-up, is isolated as a very stable oxovanadium(IV) species, VO(Por) (for VO(OEP), V—O = 1.620(2)A, V—= 2.102A, A(N4) = 0.543 A).17 -21 In the presence of a large excess of nitrogenous ligand such as pyridine and piperidine, it forms a six-coordinate complex with a small equilibrium constant (K = 10 I-10 21 M-1).18 The effect of / substituents on the association constant (Kx) is expressed by the Hammett equation (equation 4). 

Crans, D.C., F. Jiang, I. Boukhobza, I. Bodi, and T. Kiss. 1999. Solution characterization of vanadium(V) and -(IV) V-(phosphonomethyl)iminodiacetate complexes Direct observation of one enantiomer converting to the other in an equilibrium mixture. Inorg. Chem. 38 3275-3282. 

The retarding effect of alcohols on the rate of epoxidation manifests itself in the observed autoretardation by the alcohol coproduct.428,434 446,447 The extent of autoretardation is related to the ratio of the equilibrium constants for the formation of catalyst-hydroperoxide and catalyst-alcohol complexes. This ratio will vary with the metal. In metal-catalyzed epoxidations with fe/T-butyl hydroperoxide, autoretardation by tert-butyl alcohol increased in the order W < Mo < Ti < V the rates of Mo- and W-catalyzed epoxidations were only slightly affected. Severe autoretardation by the alcohol coproduct was also observed in vanadium-catalyzed epoxidations.428 434 446 447 The formation of strong catalyst-alcohol complexes explains the better catalytic properties of vanadium compared to molybdenum for the epoxidation of allylic alcohols.429 430 452 On the other hand, molybdenum-catalyzed epoxidations of simple olefins proceed approximately 102 times faster than those catalyzed by vanadium.434 447 Thus, the facile vanadium-catalyzed epoxidation of allyl alcohol with tert-butyl hydroperoxide may involve transfer of an oxygen from coordinated hydroperoxide to the double bond of allyl alcohol which is coordinated to the same metal atom,430 namely,... 

Applications of NMR spectroscopy to structural, thermodynamic, and dynamic processes have been described. A brief discussion of the types of problems appropriate for study by this technique has been included. H and 13C NMR spectroscopy has been applied to define the ligand coordination in complexes. These experiments, combined with 170-labeling experiments, allowed deduction of the coordination number of the vanadium atom. Integration of NMR spectra allowed measurement of the formation constants and equilibrium constants. 2D 13C and 51V EXSY experiments were used in a qualitative and quantitative manner to examine intra- and intermolecular dynamic processes, of which several examples are discussed. The interpretation of the rate matrix and its relationship to the chemical processes under examination were also described. 2D EXSY spectroscopy has great potential as a tool with which to probe mechanisms in complex reactions however, such uses often requires estimation of errors. The major source of error in 2D 51V EXSY NMR studies on a two- and four-site vanadate system were found to be baseline distortion and the errors were estimated. Our results suggest... 

The bimetallic pentalene-bridged complex [Cp2V]2Pn (21, Pn = CgHe) containing a V-V triple bond, in which the vanadium centers adopt a iyn-coordination, is synthesized from Li2Pn xDME (DME = 1,2-dimethoxyethane) with 2 equiv of [CpVCl THF] . The complex shows a high-spin/low-spin (see High-spin Low-spin Compounds) equilibrium in both solution and solid states. 

---