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Metal catalysis in oxidation by peroxides. Part 10. On the nature of the peroxovanadium(V) species in non-aqueous solvents
Abstract
The equilibrium formation of mono- and diperoxovanadium(V) compounds from vanadate esters and hydrogen peroxide in ethanol and dioxane—ethanol mixtures has been studied by electron spectroscopy. The acid—base behaviour of the peroxo species identified by the spectroscopic investigation was studied potentiometrically. The results show that in ethanol a monoperoxo species, which behaves as a weak acid, is formed at low hydrogen peroxide concentrations and that such a species is transformed into a diperoxo compound, which behaves as a strong acid, at higher [H2O2]0. In dioxane—ethanol, on the other hand, only the monoperoxo species can be detected, even at high hydrogen peroxide concentrations. The consistency of these findings with the outcome of previous kinetic investigations is discussed.
... Simple vanadate ion is thought to inhibit phosphatase enzymes by coordinating to the active site to form a stable trigonal bipyramidal transition state analogue of the binding of a phosphate ester during hydrolysis [26]. The combination of distorted trigonal bipyramidal geometry and strong oxidizing properties [27][28][29] observed for peroxovanadium compounds may enable them to inhibit PTP not only through a competitive mechanism but also by modifying the active site of the enzyme via oxidation. The oxidative chemistry of pV compounds has recently been reviewed by Butler et al. [30]. ...
A series of bisperoxovanadium complexes (bpV) of the general formula K3[VO(O2)2(L-L′)] has been prepared and characerized. where L-L′ is a pyridinedicarboxylate (2,3-pdc, 2,4-pdc, 2,5-pdc) or 3-acetatoxypicolinate (3-acetpic). Two structures have been determined: bpV(2,4-pdc), P21, , , , Z = 8, R/Rw = 0.049/0.055, and bpV(3-acetpic), P1, , , , α = 110.742(16)°, β = 95.600(19)°, γ = 100.195(19)°, , Z = 2, R/Rw=0.036/0.026. The oxygen atom of the 2-carboxylate group and the oxo ligand are situated in apical positions of a pseudotrigonal plane formed by the nitrogen atom and the centres of the two peroxo groups. The compounds rapidly oxidize sodium tris(3-sulfonatophenyl)phosphine, in water, to the corresponding oxide. The results are discussed with reference to the reported insulin mimetic activity of this class of compounds.
Study the mild cyclohexane oxygenation using vanadyl(IV)acetylacetonate as the starting catalyst and H2O2 as the oxidant has shown that oxalic acid as activator alters the products ratio, increases yield and catalyst turnover number. According to the instrumental (ESI-MS, NMR, EPR, UV–vis, GC, pH, titrimetric) investigations both the parental VO(acac)2 and originated in situ vanadyl(IV)oxalate can consequently interact with H2O2 led, among others, to intermediates comprise VO(η²-O2) metal core. Such vanadium-peroxo species manifest itself as the additional oxygenation agent which generation is inspired by oxalic acid additives. The enhanced aimed products yield and improved process selectivity revealed in the presence of oxalic acid may be stipulated with these non-radical intermediates.
According to IUPAC recommendations the abbreviation Va(V) is used to indicate pentavalent vanadium.
Novel vanadium peroxo complexes with pyridine ligands were prepared and characterized by NMR and IR spectroscopy and were shown to have activity in the selective oxidation of dialkyl and alkylaryl disulfides to corresponding sulfoxides.
The kinetic features of the redox reaction between sodium tetrahydroborate and vanadium(V) ions in aqueous acidic medium have been studied. The rate of the reaction was observed to be dependent on the first powers of the concentrations of the reactants. Evidence has been presented for the formation of newer vanadium(IV) compounds. A plausible mechanism for the reaction has been suggested.
As outlined in chapter 1 the activation of H2O2 in the presence of transition metal complexes to give metal peroxo species is one of the major pathways through which hydrogen peroxide can be employed as primary oxidant for the synthesis of oxygenated compounds. In this chapter it will be discussed how this activation will lead to peroxo complexes having either electrophilic or nucleophilic characteristics and how this guideline may be helpful in suggesting the appropriate catalyst for a given reaction. However, before doing so it seems appropriate to summarize some general physico-chemical features of hydrogen peroxide.
Substituted aromatic molecules can be oxidized under mild conditions using Ti or V substituted molecular sieves. The nature and the selectivities of the products formed strongly depend on the oxidant used in the catalytic reaction. H2O2 favours the hydroxylation of the aromatic ring whereas tert-butyl hydroperoxide is very selective in the side-chain oxidation. For V-substituted jnolecular sieves, we have proposed a mechanism which involves the redox system V5+ /V4+.
V-doped iron oxides are used as heterogeneous catalysts to oxidize the dye methylene blue in an aqueous medium containing hydrogen peroxide. XRD and Mössbauer spectroscopy reveal that vanadium is incorporated into the iron oxide structure. The H2O2 pretreatment of the solid catalyst promotes important surface and structural changes in the iron oxides primarily due to the formation of peroxo-vanadium complexes, which specifically enhance the catalytic properties of the material. Transmission electron microscope images show that the H2O2 treatment also tends to decrease the mean particle size of the material grains. V-doped iron oxides were found to play an important role as solid catalysts in H2O2 reactions. The prepared vanadium containing iron oxide was confirmed to exhibit remarkable catalytic activity for the oxidation of methylene blue, an important contaminant from textile industry. In fact, the ESI-MS spectrum obtained for methylene blue after reaction with the V-doped oxide shows hydroxylation by hydroxyl radicals in solution forming species with m/z = 130 and m/z = 110.
The oxidation to the corresponding sulphoxides of two dialkyl sulphides, Bun—S—Bun and Bun—S—But, and three aryl alkyl sulphides, p-methyl-phenyl sulphide, p-chlorophenyl methyl sulphide and phenyl t-butyl sulphide, with hydrogen peroxide in the presence of catalytic amounts of TiO-(acac)2 has been studied in aqueous ethanol, at 25 °C. Kinetic experiments show that the oxidation rate is first-order in the catalyst and almost zero-order in hydrogen peroxide. This latter feature is interpreted as arising from a large value of the association constant H2O2—Ti(IV) to form a side-bonded peroxotitanium(IV) species. A non-linear dependence of rates on substrate concentration is observed for sulphides which are not sterically hindered, whereas for the two t-butyl derivatives, a kinetic first order is found.This behaviour suggests that non-bulky sulphides coordinate to Ti(IV) in solution.The consenquences of such a process on the efficiency of the oxidizing system are discussed and, in particular, the suggestion is made that an ‘inhibitive complexation’ by the substrate operates, as the sulphide-complexed peroxotitanium compounds is a less effective electrophilic oxidant than the uncomplexed one.
The rates of oxidation to the corresponding sulphoxides of two dialkyl sulphides, Bun—S—Bun and Bun—S—But, and two arylalkyl sulphides, Ph—S—CH3 and Ph—S—But, with t-butylhydroperoxide and hydrogen peroxide in the presence of catalytic amounts of vanadium (VO(acac)2) and molybdenum (MoO2(acac)2), have been measured in ethanol. The steric retardations observed are compared with those previously found in the acid-catalyzed oxidation of the same substrates, with hydrogen peroxide in ethanol, a typical ‘electrophilic’ oxidation. This comparison indicates very similar behaviour of the various oxidizing systems, thus confirming the generality of the external ‘electrophilic’ oxidation by metalperoxo species.
The oxidation of p-chlorophenylmethyl sulphide and of a series of phenylsubstituted arylmethyl sulphides, of p-chlorophenylmethylsulphoxide and 1-methylcyclohexene with hydrogen peroxide in the presence of catalytic amounts of three molybdenum derivatives, MoO2(acac)2, Mo(CO)6 and MoO5·HMPT·H2O has been studied in ethanol. The reactions afford the corresponding sulphoxides, the sulphone and the epoxide, respectively, in quantitative yields.Kinetic studies indicate that the oxidation rate is first-order in the substrate and catalyst but zero-order in hydrogen peroxide. The reactivity order observed for the three different substrate parallels their nucleophilicity, i.e. sulphide > sulphoside > alkene. The rates of sulphide oxidation, on the other hand, are very little affected by substitution in the phenyl ring.On the basis of the identity of the catalytic efficiency of the three Mo species employed, the oxidizing agent formed in solution is suggested to be the peroxocomplex MoO5(EtOH)2.The results will be discussed in the light of the two alternative mechanisms proposed for the oxygen transfer process, i.e. the normal ‘electrophilic’ mechanism and the two-step one involving prior coordination of the substrate.
The oxidation of benzene to phenol by the vanadium(V) peroxo complex VO(Oâ)(PIC)(HâO)â (1) has been investigated in various solvents and in the presence of both one-electron donors and radical traps. The hypothesis is advanced that the active oxidizing species is a vanadium complex resulting from the monoreduction of 1, i.e., formally a radical-anion derivative. Such a process readily occurs in noncoordinating solvents (CHâNOâ, CâHâ, and also CHâCN) whereas it is almost suppressed in coordinating ones (CHâOH and DMF). The PIC ligand plays a role, likely in stabilizing the active species, which, however, may be formed also from simple peroxo complexes such as VO(Oâ)(OR). As far as the synthetic applications of 1 are concerned, preliminary data indicate that by running the oxidation in a two-phase system the main drawback of the process, i.e., the fast decomposition of the oxidant, may be, at least in part, overcome.
The oxidation of di-n-butyl sulfide and phenylmethyl sulfide by two vanadium (V) peroxocomplexes, e.g. VO(O2)OCH3, 1, and VO(O2)(PIC)(H2O)2, 2, in methanol, was investigated. The kinetic analysis of the reactions revealed significant mechanistic differences between
the two peroxometal oxidants. In particular, 1 behaves as an electrophilic oxidant so that the transition state of sulfide
oxidations by 1 may be described as a S
N
2 displacement on the peroxide oxygen whereas 2 acts as a radical oxidant in a process which has a substantial SET character. The role of the picolinaoo ligand in determining the reactivtty of 2 is discussed in terms of a decrease of the electrophilicity of the peroxide oxygen compared with 1 and of a stabilization of the radical metal species.
The effect of the addition of HMPT on the oxidation rates of p-chlorophenylmethyl sulphide and of a series of phenyl-substituted arylmethyl sulphides with hydrogen peroxide in the presence of catalytic amounts of MoO2(acac)2 in ethanol at 25 °C was studied. HMPT added at relatively low concentration inhibits the oxidation of p-ClC6H4SCH3, whereas at higher HMPT the oxidation rates become independent of its concentration. Under these conditions the substituent effect on the oxidation rates of the set of arylmethyl sulphides follows the Hammett free-energy relationship (p = −0.7) contrary to the complex behaviour observed previously. These findings, while ruling out the possibility that the oxidation occurs at the complexed sulphide, strongly support the electrophilic mechanism for these oxidation reactions.
Evidence concerning the nature of peroxometal intermediates in the oxidation of organic substrates by hydrogen peroxide or t-butyl hydroperoxide by vanadium(V) compounds is critically reviewed; it suggests formation of either metal μ-peroxoester (or peroxoacid) 2 or side-bonded peroxometal intermediates 3. Kinetic and spectroscopic data, as well as comparison with analogous systems, indicate that an entirely different situation arises in the interaction of alkyl hydroperoxides or hydrogen peroxide with the metal center, in that while alkyl hydroperoxides generate metal μ-peroxoester intermediates, H2O2 produces vanadium(V)—side-bonded peroxocomplexes. It is suggested that formation constant (Kt) values of the peroxometal species and kinetic behaviour in catalytic oxidation permit distinction between these two significantly different modes of binding of the peroxide to the metal.
Vanadium(V) complexes formed in the reaction between vanadyl acetylacetonate and alkylhydroperoxides are characterized according to their1H and51V NMR spectra and the reactivity of these complexes towards cyclohexene is studied.
Aerobic oxidation of PriOH to acetone with parallel reduction of dioxygen to hydrogen peroxide is observed for acid PriOH–H2O solutions of peroxovanadates. Spectroscopic evidence, obtained with a combined use of 51V-NMR, UV–VIS and ESI-MS techniques, is presented for a monoperoxovanadium-catalysed inner-sphere oxidation of the alcohol coordinated to the metal center.
The reaction of vanadium (V) derivatives with hydrogen peroxide or with alkyl hydroperoxides leads to peroxovanadium complexes which are effective and selective oxidants of organic and inorganic substrates either under stoichiometric or catalytic conditions. Examples of synthetically significant oxidations of various classes of organic compounds are reported together with details on the commonly accepted mechanisms operating. When possible a comparison is made between peroxovanadium complexes of other do metal derivative, e.g. Ti(IV), Mo(VI) and W(VI).
The catalytic behaviour of the two peroxo complexes MO(O2)2HMPT· H2O (M = W(VI), Mo(VI); HMPT = hexamethylphosphoric acid triamide) in the oxidations by hydrogen peroxide in ethanol has been examined. Rate data, obtained in a model reaction, i.e. the oxidation of organic sulphides to the corresponding sulphoxides, indicate that in ‘neutral’ ethanol the Mo(VI)-catalyzed system is more efficient than the W(VI) one. Spectroscopic (1H NMR), potentiometric and kinetic experiments suggest that this is mainly due to the much stronger acidity of the W(VI)-peroxo complex which is almost completely dissociated into its anionic form, and which is a poor electrophilic oxidant. In fact, when the anions are neutralized upon addition of methanesulphonic acid, the neutral peroxotungsten complex appears to be a much more effective oxidizing agent than the structurally similar peroxomolybdenum compound.An additional advantage of the W(VI)-catalyzed system may be found in the reduced affinity of W(VI)-peroxo species, as compared with Mo(VI), for neutral ligands. Therefore HMPT, even at the highest catalytic concentrations employed, is completely removed from the coordination sphere of tungsten.
The oxidation of diphenyl sulfide (Ph2S) by hydrogen peroxide in the presence of a catalytic amount of sodium metavanadate (NaVO3) has been studied kinetically by means of iodometry of hydrogen peroxide. The reaction rate is expressed as: v = k[NaVO3]st[Ph2S]2, when the concentration of catalyst is very low and [Ph2S]0/[H2O2]0 > 2, where []st and []0 mean stoichiometric and initial concentration, respectively. The effective oxidant may consist of polymeric as well as monomeric peroxyvanadate in view of the effect of concentration of catalyst on the rate. The main oxidizing species at low concentration of catalyst seems to be diperoxyvanadate VO5−. The rate constant k2 in v = k2[Ph2S]2 tends to decrease with initial concentration of H2O2, which is present in excess of the catalyst. A probable mechanism for the oxidation is discussed.
Vanadium complexes formed from VO (acac)2 as the starting material for the catalysts have been characterized in situ with 51V and 1H NMR and EPR in the course of three types of catalytic reactions: (1) decomposition of cumene and tert-butyl hydroperoxides in C6D6, (2) epoxidation of cyclohexene in C6D6:cyclohexene=1:1 mixture and (3) oxidation of cyclohexane in C6D6: cyclohexane = 1:1 mixture. In the course of all three reactions VO (acac)2 transforms initially into the alkylperoxo complex VO(acac)2OOR and the alkoxo complex VO(acac)2OR, which further decompose to produce VO (OR)3 and two other vanadium (V) species: an alkylperoxo complex I and an alkoxo complex II. VO (acac)2OOR reacts with cyclohexene to form VO (acac)2OR and cyclohexene oxide. I reacts with cyclohexene to form II and cyclohexene oxide and with ROH to form II and VO(OR)3. II does not react with cyclohexene but reacts with ROOH to form I. No acetylacetonate ligands have been detected in I or II with 1H NMR. I is suggested to be the key species that permits cyclohexene epoxidation with ROOH under steady-state conditions. No complexes of I with cyclohexene are observed with 51V NMR. I does not react directly with cyclohexane, but through the equilibrium VvO(OOR) ⇌ VIVO+RO2· produces free radical RO2· that initiates oxidation of cyclohexane.
The rates of oxidation of 4-chlorophenylmethyl suphide, 1, 2-[(4-chlorophenyl) thio]ethanol, 2, 1-methylcyclohexene, 3, and geraniol, 4, by t-butylhydroperoxide and hydrogen peroxide in the presence of V(V) and Mo(IV) catalysts have been measured at various temperatures. The kinetic data and the Arrhenius parameters obtained allow a comparison among the substrates and the oxidizing systems. On the basis of such a comparison, the effect of the hydroxo group on the behaviour of 2 and 4 may be estimated.The suggestion is made that the enhanced reactivity of such substrates, in some cases, is related to the specific interaction of the OH group in the transition state with the peroxide oxygen rather than to the formation of a ground-state alkoxo-peroxo complex.
Kinetic and 18O-labeling evidence is presented of a radical pathway in the oxidation of 2-propanol to acetone by V(V)- and Mo(VI)-peroxo complexes. When the apparent pH of the reaction mixture is varied, thus affecting the relative amounts of neutral and anionic peroxo complexes, a different oxidative behavior of such species is observed. A mechanistic scheme is presented where the oxidation of the alcohol coordinated to the metal is proposed.
Der durch VO(acac)2 und M002 (acac)2 katalysierte Sauerstoffaus- tausch zwischen H2O2 und H2O in ethanolischer Lösung wird mit Hilfe von H2′8O2 untersucht.
Efficient, mild syntheses of the three major metabolites 2–4 of the important antipsychotic drug thioridazine (1) have been developed. The cardiotoxic metabolite 2 with a ring sulfoxide moiety was prepared in 96% yield by oxidation of 1 with NaIO4 under acidic conditions. Four different procedures were elaborated for the selective side-chain sulfide oxidation of 1 to mesoridazine (3), giving rise to yields of up to 91%. Finally, sulforidazine (4) was synthesised via oxidation of the sulfoxide 3 in the presence of either KMnO4 or t-BuOOH under basic conditions. Except for the oxidation with t-BuOOH, all reactions took place under mild conditions within a few minutes, were nicely reproducible, and afforded medium-to-high yields of the desired products, which could be readily purified by column chromatography.
ESI-MS and 51V NMR techniques have been used to detect the formation of vanadium triperoxo complexes in protic solvents. RHF/6-311++G(d,p) ab initio calculations were also carried out to address the structure and the energy requirements of vanadium peroxo species formed from vanadates and hydrogen peroxide. Such studies point to a step-by-step decomposition process involving the triperoxo vanadium complex, as highlighted by ESI-MSn experiments. The reactivity of the triperoxo compound has been examined in an alcohol/water mixture with the aim of elucidating its oxidative behavior; evidence for the formation of the HOO− anion as an active oxidant in solution has been obtained.
Introduction
Limitations and Scope
Synthesis and Structural Characterization
Reactivity
Conclusions
Acknowledgments
In acid isopropanol/water solution and aerobic conditions, (Bu)4N+VO3− in the presence of an initial amount of H2O2, catalyzes the autoxidation of isopropanol to acetone and the contextual dioxygen reduction to hydrogen peroxide, which accumulates in solution. We have observed that, in the system under examination, the build-up of H2O2 concentration shows an oscillatory behavior. Speciation of the peroxovanadium complexes in iPrOH/H2O has been explored with the combined use of 51V NMR, UV–Vis and ESI-MS techniques.
The oxidation of aniline has been performed in the liquid phase at moderate temperatures over a series of transition-metal-substituted molecular sieves. Experimental data have shown that TS-1 was not the best catalyst for this reaction but that large pore zeolites or mesoporous silicas were preferred. Indeed, these solids could be used with either hydrogen peroxide or tert-butyl hydroperoxide (TBHP) as oxidant. For low oxidant/aniline ratios, azoxybenzene (AZY) was the major product formed over Ti-containing catalysts, but azobenzene (AZO) and nitrosobenzene (NSB) were also detected. The selectivity in AZO was always higher with TBHP than with H2O2. In contrast, V-substituted zeolites were only active with TBHP and led to the very selective formation of nitrobenzene.
This chapter describes different types of peroxide. The solid hydrated peroxides of elements such as copper, praseodymium, or cerium may well contain O2–ions: the only roughly stoichiametric peroxides of thorium and plutonium represent a slightly different class of compound, but may still be essentially ionic. The peroxides of transition metals either in complexes or in high oxidation states (or in both), however, contain peroxide linked to the metal by directed bonds, and are closely related to the oxygenated O2-carrying complexes. The characteristic and interesting properties of transition metal complexes are largely associated with compounds of the last two classes. Most of the known transition metal peroxides have been obtained by the action of H2O2 on some derivative of the metal; in aqueous solution, the proportion of peroxide in the product increases with increasing pH, though this may not be true in solvents other than water. Peroxides of rhenium and cobalt, and of course the oxygenated O2-carrying species containing iron, iridium, or copper, have been made from O2 instead of H2O2; there is evidence that peroxy derivatives of uranium have been obtained by heating compounds of uranium in air, while peroxides are often postulated as unstable intermediates in the oxidation by air of such species as chromium.
The reduction of quinquevalent vanadium by hydrogen peroxide in acid solution depends upon the VO++-VO2+ and the H2O2-O2 redox system, which, in turn, are affected by the nature and concentration of acid, the time, and the temperature. Simultaneous peroxidation of quinquevalent vanadium renders the mechanism of reduction rather complex. A scheme is suggested in which pale yellow vanadyl(V), red-brown monoperoxyvanadyl(V), yellow diperoxyorthovanadate, and blue vanadyl(IV) ions are related to hydrogen-ion and hydrogen peroxide concentrations.
Alcohol exchange with trialkyl arsenates and vanadates is rapid and appears to proceed via a 5-coordinate transition state wherein the degree of bond-breaking is governed by the steric size of the alkyl group.
Kinetic and spectroscopic evidence suggests that, in alcoholic solvents (ROH) and in the presence of t-butyl hydroperoxide, bis(acetylacetonato)oxovanadium(IV)[VO(acac)2] is rapidly converted to vanadate esters VO(OR)3. These vanadium(V) triesters undergo acid–base equilibria leading to anionic or cationic species depending on the acidity of the medium. Kinetic studies on the vanadium(V) catalysed oxidation of di-n-butyl sulphide by t-butyl hydroperoxide in methanol, ethanol, and propan-2-ol, respectively, at 25° indicate that the most active catalyst is the neutral species VO(OR)3. The significant depression of oxidation rates observed when vanadium anion species are present, even at low concentration, suggests that vanadium(V) species might be involved in aggregation phenomena which become more significant as the acidity decreases. Thus, the seemingly large differences observed in the rates of catalysed oxidation in the three alcohols mentioned stem from changes in the position of acid–base equilibria involving the catalyst species in the three solvents. Indeed, the dependence of oxidation rates upon the medium acidity shows that rather small differences in rates are found in the range where the neutral catalyst ester VO(OR)3 is the dominant species.
The oxidation of p-chlorophenyl methyl sulphide, geraniol and cyclohexene with hydrogen peroxide in the presence of catalytic amounts of bis acetylacetonato oxovanadium(IV) [VaO(acac)2] in dioxane and dioxaneethanol has been studied, p-Chlorophenyl methyl sulphoxide and 2,3-epoxygeraniol are produced in quantitative yield; oxidation of cyclohexene is, on the other hand, much less selective, affording 2-cyclohexen-1-ol, 2-cyclohexen-1-one and cyclohexene oxide as major products. Kinetic studies indicate that the rate of sulphide oxidation, at relatively low hydrogen peroxide concentration, is first-order in sulphide, oxidant and catalyst whereas, at higher [H2O2]o, zero-order dependence on hydrogen peroxide is observed.Autodecomposition of H2O2, which takes place at [VaO(acac)2] much higher than that used in oxidation experiments shows zero-order dependence on [H2O2]o and second-order dependence on [VaO(acac)2].The equilibrium formation of monoperoxovanadium(V) species as the oxidizing agent is suggested. The relevance of acid—base equilibria, involving vanadium peroxoacid, on its oxidizing efficiency is discussed, also on the ground of potentiometric experiments. The mechanism of geraniol epoxidation, which accounts for the high reactivity and regioselectivity observed, is also discussed.
The kinetics of formation and decomposition in perchloric and other acids of blue chromium(VI) diperoxy and the formation of red vanadium(V) monoperoxy, yellow vanadium(V) diperoxy, and orange titanium-(IV) monoperoxy species have been examined by flow methods. Rate laws have been established, and mechanisms are suggested for these reactions.
The electric moments of a series of alkyl orthovanadates have been determined in benzene solution at 25°. The following moments were found: Et3VO4, 1.18 D.; Pr3nVO4, 1.15 D.; Pr3iVO4, 1.23 D.; Bu3nVO4, 1.12 D.; Bu3iVO4, 1.10 D.; Bu3nVO4, 1.01 D.; Bu3tVO4, 1.16 D.; Am3tVO4, 1.11 D. The densities, dielectric constants and refractive indices of the pure esters were also measured. The small electric moments observed show that rotation of the alkoxy groups is considerably restricted.
The preparation and characterization of a dipicolinatooxovanadium(V) complex and a protonated form containing μ-hydroxo bridges are reported. Formation of [VO(O2)(dipic)H2O]- was monitored for the reaction of this complex which appears to be trimeric in solution with H2O2. With the trimer in excess first-order rate constants kobsd, [H+] = 0.1-1.0 M, fit a rate dependence of kobsd = kaK[H+][trimer]/(1 + K[H+]), where K, the trimer protonation constant, is 1.37 M-1 and ka for reaction of the protonated complex is 5.25 × 103 M-1 s-1; ΔHa‡ = 5.6 kcal mol-1 and ΔSa‡ = -23 cal K-1 mol-1 at 25 °C, μ = 1.0 M (LiClO4). At low [H+], 10-3-6.2 × 10-5 M, the unprotonated form reacts with H2O2 (kb) and the much stronger nucleophile HO2- (kc): rate = (kb + kcKH2O/[H+])[H2O2] [trimer], where KH2O2 is the dissociation constant of H2O2 (∼10-12 M), kb = 1.1 M-1 s-1, and kc ≈ 108 M-1 s-1 at 25°C, μ = 1.0 M. Two different transient diperoxovanadium(V) complexes (HO2- being coordinated as a monodentate ligand) are formed with excess H2O2 from dipicolinatovanadium(V) fragments which have been generated after reaction of the trimer with one H2O2 ligand and subsequent bridge-cleavage processes (rate-determining steps). Each of these diperoxo species is converted to the final [VO(O2)(dipic)H2O]- product via two kinetically distinguishable steps: rate = {1/(a + b[H2O2])} [H+] [diperoxovanadium(V)]. This is consistent with the proton-assisted dissociation of one H2O2 ligand and formation of an intermediate monoperoxovanadium(V) complex which is then transformed to the resulting complex V independently of [H2O2] and inversely dependent on [H+] via an intramolecular process. This process is tentatively assigned to the intramolecular ring closure of the peroxo ligand.
The kinetics of the reactions of hydrogen peroxide with nitrilotriacetatodioxovanadate(V) (VVNTA, VO2(nta)2-) and dioxo(2,6-pyridinedicarboxylato)vanadate(V) (VVPDA, VO2(pda)(H2O)-) have been studied spectrophotometrically using a stopped-flow technique at an ionic strength of 1.0 mol dm-3 (NaClO4) in the pH range 1.5-5 between 15 and 35°C. The rate of the peroxo complex formation of VVNTA is expressed as d[VO(O2)(nta)2-]/dt = (k1 + k2[H+])[VO2(nta)2-][H 2O2] where k1 = 7.41 mol-1 dm3 s-1 (ΔH‡ = 39 ± 1 kJ mol-1, ΔS‡ = -96 ± 9 J K-1 mol-1) and k2 = 1.86 × 104 mol-2 dm6 s-1 (ΔH‡ = 39 ± 1 kJ mol-1, ΔS‡ = -33 ± 9 J K-1 mol-1). The rate of the peroxo complex formation of VVPDA is as follows: d[VO(O2)(pda)(H2O)-]/dt = {k1[H+][VO2(pda)(H2O)-] + k2[VO2(pda)(H2O)-] + k3[VO2(pda)(OH)2-]}[H2O2] where k1 = 4.1 × 102 mol-2 dm6 s-1 (ΔH‡ = 24 ± 1.5 kJ mol-1, ΔS‡ = -114 ± 9 J K-1 mol-1), k2 = 0.39 mol-1 dm3 s-1 (ΔH‡ = 46 ± 2 kJ mol-1, ΔS‡ = -97 ± 9 J K-1 mol-1), and k3 = 28 mol-1 dm3 s-1 (ΔH‡ = 35 ± 2 kJ mol-1, ΔS‡ = -100 ± 9 J K-1 mol-1). Mechanisms are proposed for the H2O2 insertion reaction and in terms of activation parameters some discussions are made on the difference in the reactivities of these complexes. We have studied the protonation equilibria of PDA where log K1 = 4.62 ± 0.02 (ΔH1 = -27 ± 4 kJ mol-1, ΔS1 = -2 ± 10 J K-1 mol-1), log K2 = 2.18 ± 0.02, and log K3 = 0.49 ± 0.02, the complexation equilibrium of the VV-PDA complex where [VO2(pda)(H2O)-]/[VO2+][pda2-] = 108.65±0.10, and the hydrolysis of the VV-PDA complex where [VO2(pda)(OH)2-][H+]/[VO 2(pda)(H2O)-] = 10-5.87±0.02 (ΔH = 7 ± 7 kJ mol-1, ΔS = -78 ± 20 J K-1 mol-1).
The crystal structure at -100° of the compound NH4[VO(O2)(H2O)(C5H 3N(COO)2)]·xH2O (x ≈ 1.3) has been determined from three-dimensional X-ray intensity data collected by counter methods on a computer-controlled diffractometer using a Joule-Thomson low-temperature device. The compound crystallizes in the monoclinic space group C2/c with eight formula units in a cell having lattice constants a = 11.307 (2) Å, b = 25.490 (5) Å, c = 8.316 (2) Å, and β = 96.90 (1)° (at temperature -100 (2)°). The structure was solved by direct methods and refinement by full-matrix least-squares methods has given a conventional R value of 3.1% for the 1331 observed reflections. The structure is comprised of two crystallographically different ammonium ions (one lying on a twofold axis and the other on a center of symmetry) and a vanadium-based anion. These ions are held together by both electrostatic forces and extensive hydrogen bonding. The vanadium atom environment is a seven-coordinate distorted pentagonal bipyramid, with a vanadyl oxygen and a water molecule at the apices and a peroxy group, the nitrogen from the pyridine ring, and one oxygen atom from each carboxylate group forming an approximate pentagonal plane. The vanadium atom is displaced 0.25 Å from the "plane" toward the vanadyl oxygen atom. Interatomic distances within the anion are 1.870 (2) and 1.872 (2) Å for the V-Qperoxo bonds, 1.579 (2) A for the V=O bond, 2.053 (2) and 2.064 (2) Å for the V-Ocarboxylate distances, 2.211 (2) Å for the V-Owater distance, 2.088 (2) Å for the V-N distance, and 1.441 (2) Å for the O-Operoxo bond.