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V(IV)-catalyzed cyclohexane oxygenation promoted by oxalic acid: Mechanistic study
Abstract
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.
... 23 (Oxalic acid primarily grants the hidden-radical mechanism of the process, catalyst reduction, and renders proper electrochemical characteristics to the reaction medium.) 24,25 Then, the use of NHPI allows us to reduce the toxicity of the operation and expensive organic initiators and promoters. 26 Elucidating the mechanism of oxidation in the presence of NHPI, OxalH, its mixture and different oxidants may be beneficial for the process efficiency as well as the impact of natures of oxidants on process productivity looks having a sense. ...
... In the case of C 6 H 12 , the free-radical-piloted transformations dominate, 50 albeit the nonradical ways of substrate oxidation may not be fully excluded from consideration. 25 By contrast, due to the lack of peroxides formation during the PhCH 3 oxidation (Table 2), one may conclude that free radicals are not involved in the process mechanism on a large scale (Scheme 2). Nevertheless, at least, partial involvement of hidden-radical steps in the overall mechanism cannot be ignored. ...
... 58 For that reason, the CV curves of solutions consisting of VO(acac) 2 , NHPI, and VO(acac) 2 + NHPI have been drawn before and after supplementing it with H 2 O 2 . (The influence of H 2 O 2 on the redox properties of the systems composed of VO(acac) 2 and VO(acac) 2 + OxalH has been reported previously 25 ). Based on the acquired results, one can rationale that a combination of VO(acac) 2 with NHPI notably modifies the voltammogram of initial VO(acac) 2 (Figure 7). ...
The oxygenation of cyclohexane and toluene by O2 and H2O2 catalyzed by VO(acac)2 and Co(acac)2 was studied at 40–100 °C and 1–10 atm. Upon such conditions, the process can be remarkably (30× times) enhanced by the minute (6–15 mM) additives of oxalic acid (OxalH) or N-hydroxyphthalimide (NHPI). The revealed effect of OxalH on H2O2-piloted oxidation is closely associated with the nature of the catalyst cation and boosted by VO(acac)2. Whereas the effectiveness of Co(acac)2-based systems was curbed by the addition of OxalH and remained much below the one displayed with the previous system. The observed conspicuous difference in activity was attributed to the substantially higher solubility of in situ formed VO(IV)oxalate compared to that of Co(II)oxalate. The exploration of H2O2 for the NHPI-promoted process leads to the decisively lower (5–7 times) yield in comparison to the O2-driven reaction. Similarly, for the O2-operated protocol, the yield cannot be improved by addition of OxalH either to VO(acac)2 + NHPI or to Co(acac)2 + NHPI mixture. By contrast, the combination of NHPI with VO(acac)2 or Co(acac)2 and particularly with the above two mixtures in O2-piloted oxidation enhances the yield of the aimed products 3–6 times regardless of the substrate used. The revealed significant synergetic effect of the cobalt + vanadyl bicomponent catalyst was due to the participation of each of its moiety in the different stages of the process mechanism. Only benzyl alcohol and benzaldehyde were identified in VO(acac)2- or Co(acac)2-catalyzed toluene oxidation, while cyclohexane oxidation yields cyclohexylhydroperoxide in line with cyclohexanol and cyclohexanone. The putative mechanism of investigated processes is highlighted and discussed.
... Inspired by the cited studies, we scrutinized the activation abilities of glyoxal as a simpler analog of butanedione where CH 3 -groups are substituted by H-atoms. 12 On the other hand, since oxalic acid (OxalH) was the product of glyoxal oxidation, 14 its potential impact on the studied process was also investigated. As revealed, the yield of the products can be sufficiently improved by application of the VO(acac) 2 catalyst together with OxalH. ...
... Among others, it leads to the species composed of the VO(h 2 -O) 2 core, which manifests itself as a selective (contrary to the free radicals) and active oxygenation agent. 14 Most previously undertaken studies involve traditional chemical approaches to assess the enhancing effect of catalysts and additives on the process efficiency. On the other hand, the literature that elucidates the impact of electrochemical parameters on the process, apart from cyclic voltammetry (CV) studies, remains rather short and covers pure solvents only. ...
... More rationales concerning the mechanism of the revealed features were reported earlier. 14 In order to evaluate the impact of cation nature, C 6 H 12 oxidation was also carried out in the presence of Co(acac) 2 (Table 1, entries 9-11). For this catalyst, whether oxalic acid was present in the initial reaction solution or not, the set of products (COL, CON, and CHHP) remained the same as in case of VO(acac) 2 . ...
Cyclohexane oxidation by H2O2 to cyclohexanol, cyclohexanone, and cyclohexylhydroperoxide under mild (40 °C, 1 atm) conditions is significantly enhanced in the system composed of VO(acac)2 (starting catalyst) and small additives of oxalic acid (process promoter). In corroboration of this, several times higher yield of the desired products was obtained compared to that obtained in the acid-free process. The revealed advantage was addressed to elevate the electrical conductance G (or vice versa, decreasing the resistance, 1/G) of the reaction medium. On the other hand, the content of oxalic acid (20–30 mM) was compulsory to optimize the process parameters. The last value of concentration affords, besides the lowest 1/G, the utmost impact on pH, redox potential, and current–voltage relationships. Exceeding this level leads to an increase in 1/G of the reaction solution, ceases the impact on pH, ORP, and CV profiles, and is detrimental for the product yield. The putative mechanism of the revealed effects has been envisaged.
... Recently, we have reported the mechanistic study on VO(acac) 2 (1)-catalyzed C 6 H 12 oxidation enhanced by small amounts of oxalic acid (2) [12][13][14]. The yield of cyclohexanone and cyclohexanol, increased in the presence of 2, was assigned to the relatively (compare to 2-free process) high level of metal-peroxo intermediates supposedly responsible for selective non-radical oxidation. ...
... This speculation is justified with the net rate of CHHP formation (W I = k I 9 1. Giving the inset b (Fig. 2), the rate of H 2 O 2 decay decreased when InH concentration grows both in the absence or presence of 2. This confirms the important role of free-radical mechanism in H 2 O 2 decomposition (Scheme 1 below) and correlates with our previous results [12][13][14]. The profile of H 2 O 2 decay rate (W H 2 O 2 ) in the absence of oxalic acid indicates that a very small amount (B 0.0025 M) of InH inhibits the oxidation almost completely (circles, Fig. 2, inset b), whereas in the presence of 2, the influence of InH on the process characteristics was notably lower. ...
... H 2 O 2 mixture (Fig. 5, insets). The described differences can be a consequence of: (i) the protection of the central cation of originated VO(oxalate) 2 [12] from a rapid irreversible oxidation due to the known reduction properties of 2; ...
The efficiency of the vanadyl(IV)acetylacetonate (1)–catalyzed oxidation of cyclohexane by hydrogen peroxide in acetonitrile increased in the presence of oxalic acid (2). The addition of 2 leads to the substitution of acac ligands by oxalate resulting in formation of vanadyl(IV)oxalate (3). That promotes the formation of cyclohexanol, cyclohexanone and cyclohexylhydroperoxide principal products. The observed alteration in the product ratio was subjected to the mixed mechanism of substrate transformation: mainly the free radical in 2–free process and radical/non-radical one in the presence of oxalic acid. In the last case, the putative VO(η²-O2)(oxalate) intermediates can directly transfer the bounded oxygen onto the substrate without the involvement of free radicals. The obviousness of two mechanisms has been shown by comparison of 2 and the influence of free radical scavenger N-benzylidene-tert-butylamine (InH) on the process kinetics.
... Vanadium metal has a characteristic feature of having versatile oxidation states (from − III to +V), which makes its chemistry exceptionally intriguing. Furthermore, the vanadium compounds possessing oxidation states, +IV and +V are the most common and shows significant roles in various fields like bio-chemical, pharmacological, and catalytic activities [1][2][3][4][5][6][7][8][9][10][11]. Vanadium also plays a major role in some enzymatic action and present in enzyme like nitrogenases, nitrate hydrolytic stability and defined hydrolysis products [21]. ...
... Based on these facts, we observed the absorption spectra of both BSA and HSA in the absence and presence of complexes 1-3, which are depicted in Fig. 8 and S12. From Fig. 8, it is clear that upon continuous addition of complex (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) μM), absorption intensity of both BSA and HSA increases drastically at 228 nm along with a slight red shift of 6-10 nm inducing static quenching in all the three complexes. The slight red shift in wavelength can be due to the perturbation in the secondary structure of protein or due to loosening and unfolding of the protein skeleton [106,119]. ...
Three ONNO donor tetradentate diamino bis(phenolato) “salan” ligands, N, N′-dimethyl-N, N′-bis-(5-chloro-2-hydroxy-3-methyl-benzyl)-1,2-diaminoethane (H2L¹), N, N′-dimethyl-N, N′-bis-(5-chloro-2-hydroxy-3-isopropyl-6-methyl-benzyl)-1,2-diamino-ethane (H2L²) and N, N′-bis-(5-chloro-2-hydroxy-3-isopropyl-6-methyl-benzyl) -1,2-diaminocyclohexane (H2L³) have been synthesized by following Mannich condensation reaction. Reaction of these ligands with their corresponding vanadium metal precursors gave one oxidomethoxidovanadium(V) [VVOL¹(OCH3)] (1) and two monooxido-bridged divanadium (V, V) complexes [VVOL2–3]2(μ-O) (2–3). The complexes were characterized by IR, UV–vis, NMR and ESI mass spectrometry. Also, the structure of all the complexes (1–3) was confirmed by the Single-Crystal X-ray diffraction analysis, which revealed a distorted octahedral geometry around the metal centres. The solution behavior of the [VVOL¹(OCH3)] (1) reveals the formation of two different types of V(V) species in solution, the structurally characterized compound 1 and its corresponding monooxido-bridged divanadium (V, V) complex [VVOL¹]2(μ-O), which was further studied by IR, and NMR spectroscopy. The electrochemical behavior of all the complexes was evaluated through cyclic voltammetry. Interaction of the salan-V(V) complexes with human serum albumin (HSA) and bovine serum albumin (BSA) were analysed through fluorescence quenching, UV–vis absorption titration, synchronous fluorescence, circular dichroism studies, and förster resonance energy transfer (FRET). Finally, the in vitro cytotoxicity of the complexes was investigated against MCF-7 and HT-29 and NIH-3 T3 cell lines. Cytotoxicity value of complexes in both MCF-7 and HT-29 follows the same trend that is 3 > 1 > 2 which is in line with protein binding affinity of the complexes.
... The mechanisms of some vanadium-mediated oxidation reactions of alkanes have been studied, most of them providing evidence for the involvement of radical species and a few suggesting non-radical pathways in the presence of a Lewis acid or oligovanadate complexes in solution [77,[82][83][84][85][86][87][88][89][90]. Because most of the reactions are not likely to occur through either a direct metal-mediated C-H activation involving carbon-metal bond or a mechanism involving the usual metal-mediated coupling pathways comprising oxidative addition, transmetalation and reductive elimination steps, they are beyond the scope of this review and will not be extensively covered herein. ...
Several valuable biologically active molecules can be obtained through C–H activation processes. However, the use of expensive and not readily accessible catalysts complicates the process of pharmacological application of these compounds. A plausible way to overcome this issue is developing and using cheaper, more accessible, and equally effective catalysts. First-row transition (3d) metals have shown to be important catalysts in this matter. This review summarizes the use of 3d metal catalysts in C–H activation processes to obtain potentially (or proved) biologically active compounds.
... The high activity of V-based catalysts when used in conjunction with pre-formed H 2 O 2 to catalyse the oxidation of alkanes has been widely linked to the V 4+ /V 5+ redox cycle [56][57][58][59]. With the enhanced activity of the VPd supported catalyst in this work attributed to the dual functionality of the catalyst, with Pd catalysing the synthesis of H 2 O 2 , which is then subsequently activated by V (Scheme 1). ...
The oxidation of cyclohexane via the in-situ production of H 2 O 2 from molecular H 2 and O 2 offers an attractive route to the current industrial means of producing cyclohexanone and cyclohexanol (KA oil), key materials in the production of Nylon. The in-situ route has the potential to overcome the significant economic and environmental concerns associated with the use of commercial H 2 O 2 , while also allowing for the use of far lower reaction temperatures than those typical of the purely aerobic route to KA oil. Herein we demonstrate the efficacy of a series of bi-functional Pd-based catalysts, which offer appreciable concentrations of KA oil, under conditions where limited activity is observed using O 2 alone. In particular the introduction of V into a supported Pd catalyst is seen to improve KA oil concentration by an order of magnitude, compared to the Pd-only analogue. In particular we ascribe this improvement in catalytic performance to the development of Pd domains of mixed oxidation state upon V incorporation as evidenced through X-ray photoelectron spectroscopy.
Graphic Abstract
Hypercrosslinked polymers (HCPs) are a type of porous organic materials with high surface area and maneuverable preparation process. Metal-functionalization is a popular and effective post-synthesis modification strategy to construct catalytically active HCPs. Tannic acid (TA) is a biomolecule containing polyphenol moieties that possess the ability to conduct Friedel–Crafts alkylation reaction and metal-bonding active sites. We utilized TA to prepare the hypercrosslinked polymer (TA-HCP) with 1,2-dichloroethane as the crosslinker. After post-synthesis reaction, the vanadium-functionalized TA-HCP-VO was obtained with well-maintained chemical structure and moderate surface area. TA-HCP-VO exhibits an excellent catalytic performance in thioether oxidation reaction to produce sulfoxide compounds. Various thioether substrates with different substituents are transformed to sulfoxide products with high conversion rates (99%) and selectivities (86–95%) under the catalysis of TA-HCP-VO. Furthermore, heterogeneous TA-HCP-VO retains fine catalytic activity and chemical structure after four cyclic reactions, which demonstrates the superiority of heterogeneous porous catalysts.
Every year, thousands of tons of polystyrene are produced and discarded, filling landfills and polluting the marine environment. Although several degradation alternatives have been proposed, the need for an effective procedure for the chemical recycling of polystyrene still remains. Here, a vanadium‐catalyzed reaction, assisted by visible light, promoted the direct, selective conversion of tertiary alkylbenzenes into acetophenone and other ketone derivatives. Likewise, standard polystyrene samples as well as polystyrenes from insulation and packaging waste could be chemically recycled into acetophenone in a scalable way regardless of their molecular weight, polydispersity, or form. Preliminary mechanistic investigations revealed the participation of singlet oxygen, superoxide, and hydroxyl radical species in this homogenously catalyzed process. Acetophenone could be used as an additive to accelerate the reaction and to increase the yields in some cases.
The centrosymmetric oxidovanadium(IV) complex (Et3NH)2[{VO(OH2)(ox)}2(μ–ox)] (I), where ox²⁻ = oxalate, was synthesized and characterized by X-ray diffraction (single-crystal and powder, PXRD), thermogravimetric (TGA), magnetic susceptibility (at room temperature) and spectroscopic analyses (infrared, Raman and electron paramagnetic resonance, EPR, spectroscopies). In the solid state, each vanadium center is coordinated by the oxygen atoms of a bis-bidentate oxalate bridging ligand, a terminal oxalate, an oxo group and one water molecule. The electronic structure of the binuclear complex was investigated by density functional theory (DFT) calculations, both in vacuum and in a simulated aqueous environment, employing the ωB97XD functional and the def2TZVP basis set. The cytotoxicity of I was evaluated in vitro in the human hepatocellular carcinoma cell line HepG2, giving an IC50 value of 15.67 µmol L⁻¹ after incubation for 24 h. The EPR analysis of I in aqueous solution suggested the maintenance of the binuclear structure, while in the hyperglycemic medium DMEM the complex suffers dissociation to give a mononuclear oxidovanadium(IV) species. HepG2 cell treatment with 0.10 and 0.50 µmol L⁻¹ of I in DMEM increased 2-NBDG (2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxy-d-glucose) uptake significantly (up to 91% as compared to HepG2 in hyperglycemic condition, 59 %). These results indicate a promising activity of I to be investigated further in additional antidiabetic studies.
The cyclohexane oxidation by H2O2 using VO(acac)2 as starting catalyst in the presence of oxalic acid (OA) was studied. The dissociation of OA and VO(oxalate) formed in situ by interaction of VO(acac)2 with OA is the essence of the electrical conductance G elevation (or vice versa 1/G dropping). As follows from the electronic and cyclic voltammetry spectra taken alongside 1/G, the substitution of weak field ligands (acac) of VO(acac)2 by the middle-field (oxalate) ones strengthens the cation-ligand bonds and postpone the irreversible catalyst oxidation. In the absence of OA, 1/G was several times larger than the value intrinsic to VO(acac)2 + OA mixture. The last feature corresponds with the considerable process productivity enhancement in presence of OA. The experimental part of this work was complemented with DFT calculation of the key quantum chemical characteristics as catalyst d-d-splitting, HOMO–LUMO gap and Gibbs energy. Bringing together the experimental and theoretical data led to deduce that the oxidation process efficiency relates, among others, with the modification the outer-sphere electronic configuration of metalocomplexes possibly leading to metal-peroxo species e.g. VO(η2-O2) generation. On the other hand, oxalate anions, besides decreasing 1/G, may facilitate the cations and H2O2 interaction. Mentioned peculiarities may be responsible for the noteworthy yield enhancement in the presence of OA.
Fascinating oscillations of color, kinetics of pH, cyclic voltammetry (CV) and UV‐vis spectra, ensued from supplementing of VO(acac)2‐acetonitrile solutions with micro‐amounts of H2O2, were greatly affected by the small additives of glyoxal and oxalic acid. Such effect was consisted in sufficient decreasing the sensitivity of samples by means of it pH, CV and UV‐vis response onto H2O2 additives. The revealed peculiarities substantiate the responsibility of free radicals (particularly .OH generated in the course of H2O2 homolysis) for the discussed parameters alteration. In view of .OH are identified as most active and destructive (especially, in respect to the cellular constituents) agents can be originated in vivo, it overproduction is responsible for developing of many related severe diseases. In practical aspect the undertaken research may be recommended for the oxidative stress evaluation and tracking.
Three new anionic dioxidovanadium(V) complexes (HNEt3)[VO2(L)1-3] (1–3) of tridentate binegative aroylhydrazone ligands containing azobenzene moiety have been synthesized and structurally characterized. The aroylhydrazone ligands (H2L1-3) were derived from the condensation of 5-(arylazo) salicylaldehyde derivatives with corresponding aroyl hydrazides. All the synthesized ligands and metal complexes were successfully characterized by several physicochemical techniques namely, elemental analysis, electrospray ionization mass spectrometry, spectroscopic methods (IR, UV–Vis and NMR), and cyclic voltammetry. Single crystal X-ray diffraction crystallography of 1–3 revealed five-coordinate geometry, where the ligand coordinates to the metal centre in a binegative tridentate O,N,O coordinating anion and two oxido–O atoms, resulting in a distorted towards square pyramidal structure. The complexes were further evaluated for their in vitro cytotoxicity against HeLa and HT-29 cancer cell lines. All the complexes manifested cytotoxic potential that was found to be comparable with clinical referred drugs. While, complex 3 proved to be the most cytotoxic among the three complexes for both the cell lines, which may be due to the synergistic effect of the napthyl substituent in the azohydrazone ligand environment coordinated to the vanadium metal. The synthesized complexes 1–3 were probed as catalysts for the oxidative bromination of thymol and styrene as a functional mimic of vanadium haloperoxidases (VHPOs). All the reactions gave a high percentage of conversion (>90%) with a high turnover frequency (TOF), in the presence of the catalysts 1–3. In particular, for the oxidative bromination of thymol the percentage of conversion and TOF lies in the range of 98–99% and 5380–7173 (h−1) respectively. Besides, 3 bearing the napthyl substituent showed the highest TOF among all the complexes for both oxidative bromination of thymol and styrene.
Homobimetallic vanadium(V) complex of the composition [(CH3)2NH2⁺]2[(VO2)2(sloxCl)].4H2O was synthesized from the reaction of V2O5 with bis(5‐chlorosalicylaldehyde)oxaloyldihydrazone ligand in a 1:1 molar ratio in methanol. The structure of the complex was established by X‐ray crystallography. Reactivity of the complex with H2O2 leads to bis (monooxidoperoxidovanadate(V)) [{VO(O2)}2(sloxCl)]²⁻ formation and with HCl, oxidohydroxido complex of composition [(VO (OH)(sloxCl)]²⁻ was formed. Binding interaction of the complex was also investigated toward protein (BSA) and it was found to be 2.21 x 10⁸ M⁻¹. The catalytic activity of the complex in the oxidation of alcohols and oxidative bromination of some organic substrates was also studied, and it showed a great potent as a catalyst.
C 1 -symmetric vanadyl Schiff-base complexes were synthesized by reacting vanadium(IV) acetylacetonate with (S)-3-[(1-(2-hydroxynaphthalen-1-yl)naphthalen-2-ylimino]methyl]-phenanthrene-4-ol and (S)-2-{[1-(2-hydroxynaphthalen-1-yl)naphthalen-2-ylimino]methyl}tetraphene-1-ol. The complexes were characterized by MALDI-TOF-MS, UV–vis, and circular dichroism (CD) spectroscopy. The catalysts showed moderate activity for the oxidation of thioanisole to methyphenylsulfoxide with hydrogen peroxide, tert-butyl hydroperoxide, and cumene hydroperoxide as the oxidants.
The efficient selective oxidation of cyclohexane to cyclohexanone and cyclohexanol (KA-oil) under mild conditions remains one of the most challenging and interesting projects in catalytic chemistry. Here, photocatalytic oxidation of cyclohexane was carried out in acetonitrile (MeCN) solution of VOCl 2 under Xe lamp irradiation and pure O 2 atmosphere. The results show that VOCl 2 has high photocatalytic activity in pure MeCN solution, and a cyclohexane conversion of 17.8% with ca. 99% selectivity for KA-oil can be obtained after light irradiation of 4 h. It is worth noting that the use of hydrochloric acid as an additive can effectively enhance the photo-catalytic oxidation of cyclohexane, providing a cyclohexane conversion of 23.3%, which may be due to the important role of HCl in improving the light absorption, decreasing the oxidation-reduction potential of V ⁵⁺ /V ⁴⁺ and promoting the redox process of photocatalyst. The effects of photocatalyst concentration, hydrochloric acid addition amount, O 2 pressure and reaction time on photocatalytic performance were also studied. It was found that the optimization of these reaction conditions can further improve the conversion of cyclohexane and selectivity of KA-oil. A possible reaction mechanism for the photocatalytic oxidation of cyclohexane was proposed.
The chemistry of vanadium has seen remarkable activity in the past 50 years. In the present review, reactions catalyzed by homogeneous and supported vanadium complexes from 2008 to 2018 are summarized and discussed. Particular attention is given to mechanistic and kinetics studies of vanadium-catalyzed reactions including oxidations of alkanes, alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species, as well as oxidative C–C and C–O bond cleavage, carbon–carbon bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation, dehydrogenation, ring-opening metathesis polymerization, and oxo/imido heterometathesis. Additionally, insights into heterogeneous vanadium catalysis are provided when parallels can be drawn from the homogeneous literature.
Osmium derivatives are much less popular among catalytic chemists in comparison with, for example, compounds of iron (an analog of osmium), copper, manganese… Indeed, this metal is expensive, poisoneous and volatile. Nevertheless, in the recent decades, osmium complexes have found applications not only in cis-hydroxylation of olefins but also in oxygenation of C‒H compounds (saturated and aromatic hydrocarbons and alcohols) by hydrogen peroxide as well as organic peroxides. Thus, the authors of the present review, using certain additives, developed extremely active systems that transform alkanes into corresponding alkyl hydroperoxides which can be easily reduced to alcohols. These new catalytic systems are record holders in the field of metal-complex catalysts.
Five monomeric oxovanadium(V) complexes [VO(OMe)(N∩O)2] with the nitro or halogen substituted quinolin-8-olate ligands were synthesized and characterized using Fourier transform infrared, 1H and 13C NMR, high-resolution mass spectrometry-electrospray ionization as well as X-ray diffraction and UV-vis spectroscopy. These complexes exhibit high catalytic activity toward oxidation of inert alkanes to alkyl hydroperoxides by H2O2 in aqueous acetonitrile with the yield of oxygenate products up to 39% and turnover number 1780 for 1 h. The experimental kinetic study, the C6D12 and 18O2 labeled experiments, and density functional theory (DFT) calculations allowed to propose the reaction mechanism, which includes the formation of HO· radicals as active oxidizing species. The mechanism of the HO· formation appears to be different from those usually accepted for the Fenton or Fenton-like systems. The activation of H2O2 toward homolysis occurs upon simple coordination of hydrogen peroxide to the metal center of the catalyst molecule and does not require the change of the metal oxidation state and formation of the HOO· radical. Such an activation is associated with the redox-active nature of the quinolin-8-olate ligands. The experimentally determined activation energy for the oxidation of cyclohexane with complex [VO(OCH3)(5-Cl-quin)2] (quin = quinolin-8-olate) is 23 ± 3 kcal/mol correlating well with the estimate obtained from the DFT calculations.
There is something intriguing and at the same time fascinating that a simple reaction (of Fe 2+ ions with H 2O 2), which was observed by H.J.H. Fenton over 110 years ago, proves to be very difficult to describe and understand. As yet the nature of the oxidizing species obtained in Fenton reaction is still a subject of deliberation, which may be explained by the fact that it is very common in both chemical and biological systems and in natural environment. It is a paradox that the Fenton reaction is successfully used in environmental protection (for example in wastewater treatment and remediation of groundwater) and it is thought to be a factor, which causes damage to biomolecules and plays a major role in the aging process and a variety of diseases. This article presents a short review on radical and non-radical mechanisms of the Fenton reaction postulated in literature, possible reaction pathways as well as various points of view in this field.
The solvation structure of the vanadyl ion (VO2+) in methanol and in water-methanol mixtures has been investigated by application of I H and I3C electron nuclear double resonance (ENDOR) spectroscopy. The ligand origins of the proton ENDOR resonances have been assigned with use of materials selectively enriched with 2H. The principal hyperfine coupling (hfc) components of both 1H and 13C in solvent molecules coordinated to the VO2+ ion have been determined by analysis of the H0 dependence of the ENDOR spectra. The hfc components of 1H and I3C of both metal-bound water and methanol exhibit axial symmetry. Under the point-dipole approximation the anisotropic hfc components yield estimates of the separation between the paramagnetic center and the 'H and I3C nuclei of axially and equatorially bound solvent molecules. Axially coordinated H2O molecules exhiibit metal-proton distances of 2.92 and 3.42 A, respectively, corresponding to an inner-sphere coordination site on one side of the equatorial plane and to a site with hydrogen bonding to the V=O group on the other side. Equatorially coordinated H 2 0 molecules exhibit metal-proton distances of 2.60 and 4.80 A, corresponding to an inner-sphere coordination site and a hydrogen-bonded outer-sphere coordination site. In pure methanol there are similarly inner-sphere-and outer-sphere-bound methanol molecules in equatorial and axial positions, and the coordination structure determined on the basis of the metal-proton distances is confirmed by ENDOR spectroscopy with 13C-enriched methanol. The ENDOR results provide unambiguous evidence that in water-methanol mixtures only [VO(H2O)5]2+ and [VO(H2O)4(CH3OH)]2+ species are formed. In pure methanol the [VO(CH3OH)5]2+ species is observed. The coordination geometries of the VO2+ complexes are deduced from ENDOR estimates of metal-nucleus distances by using computer-based molecular graphics. It is shown on the basis of molecular modeling that the ENDOR-determined metal-nucleus distances are best accounted for by complexes of tetragonal-pyramidal geometry.
Hydroxylation of alkanes by a mononuclear nonheme iron(v)-oxo complex, [Fe(v)(O)(TAML)](-), is initiated by a rate-determining hydrogen atom (H-atom) abstraction, followed by an oxygen non-rebound process. Evidence for the H-atom abstraction-oxygen non-rebound mechanism is obtained experimentally and supported by DFT calculations.
A sustainable V(v) and Mo(vi) catalysed two-phase procedure for bromination of toluene under quite mild conditions is proposed; H2O2 is the primary oxidant and KBr is the bromine source; metal precursors are commercially available salts. The reaction is efficient without any additional solvent. By using PhCH3 as a solvent/substrate good yields, together with interesting selectivity toward the formation of PhCH2Br, are obtained with both metal ions. Recycling of the catalytic phase is also possible. Useful information on the V-peroxido chemistry was obtained.
The hydration of the hydroxyl OH radical has been investigated by microsolvation modeling and statistical mechanics Monte Carlo simulations. The microsolvation approach was based on density functional theory DFT calculations for OH– (H 2 O) 1–6 and (H 2 O) 1–7 clusters. The results from microsolvation indicate that the binding enthalpies of the OH radical and water molecule to small water clusters are similar. Monte Carlo simulations predict that the hydration enthalpy of the OH radical, hyd H(OH,g), is 39.1 kJ mol 1 . From this value we have estimated that the band gap of liquid water is 6.88 eV, which is in excellent agreement with the result of Coe et al. J. Chem. Phys. 107, 6023 1997. We have compared the structure of the hydrated OH solution with the structure of pure liquid water. The structural differences between the two systems reflect the strong role played by the OH radical as a proton donor in water. From sequential Monte Carlo/DFT calculations the dipole moment of the OH radical in liquid water is 2.20.1 D, which is 33% above the experimental gas phase value 1.66 D.© 2003 American Institute of Physics.
Many efforts have been made to develop new catalysts to oxidize cyclohexane under mild conditions. Herein, we review the most interesting systems for this process with different oxidants such as hydrogen peroxide, tert-butyl hydroperoxide and molecular oxygen. Using H2O2, Na-GeX has been shown to be a most stable and active catalyst. Mesoporous TS-1 and Ti-MCM-41 are also stable, but the use of other metals such as Cr, V, Fe and Mo leads to leaching of the metal. Homogeneous systems based on binuclear manganese(IV) complexes have also been shown to be interesting. When t-BuOOH is used, the active systems are those phthalocyanines based on Ru, Co and Cu and polyoxometalates of dinuclear ruthenium and palladium. Microporous metallosilicates containing different transition metals showed leaching of the metal during the reactions. Molecular oxygen can be used directly as an oxidant and decreases the leaching of active species in comparison to hydrogen peroxide and tert-butyl hydroperoxide. Metal aluminophosphates (metal: Mn, Fe, Co, Cu, Cr V) are active and relatively stable under such conditions. Mn-AlPO-36 yields directly adipic acid, but large amounts of carboxylic acids should be avoided, as they cause metal leaching from the catalysts. Rare earth exchanged zeolite Y also shows good selectivity and activity. In the last part of the review, novel alternative strategies for the production of cyclohexanol and cyclohexanone and the direct synthesis of adipic acid are discussed.
Understanding the energy it takes to build or break chemical bonds is essential for scientists and engineers in a wide range of innovative fields, including catalysis, nanomaterials, bioengineering, environmental chemistry, and space science. Reflecting the frequent additions and updates of bond dissociation energy (BDE) data throughout the literature, the Comprehensive Handbook of Chemical Bond Energies compiles the most recent experimental BDE data for more than 19,600 bonds of 102 elements. The author organizes the data by bond type, functional group, bond order, bond degree, molecular size, and structure for ease of use. Data can also be located using the Periodic table. The book presents data for organic molecules, biochemicals, and radicals as well as clusters, ions, hydrogen- and surface-bonded species, van der Waals complexes, isotopic species, and halogen-clusters/complexes. It also introduces entirely new data for inorganics and organometallics. The final chapter summarizes the heats of formation for atoms, inorganic/organic radicals, and monoatomic ions in the gas phase. The Comprehensive Handbook of Chemical Bond Energies offers quick access to experimental BDE data in the most inclusive, well-organized, and up-to-date collection available today.
So many compounds, so many experiments reported by so many researchers using so many methods…Finding reliable data on bond dissociation energies (BDEs) can be like looking for a needle in a haystack. But these data are crucial to work in chemical kinetics, free radical chemistry, organic thermochemistry, and physical organic chemistry-so where does one turn? With data on almost 2900 bonds in 2500 organic compounds, the Handbook of Bond Dissociation Energies in Organic Compounds is the first comprehensive collection of experimental BDE data. Author Dr. Yu-Ran Luo has spent more than 10 years collecting, assessing, and tabulating BDE experiment results and presents them here in a well-organized, easy-to-use series of tables. For each compound, the tables list up to five recent experimental values, indicate which is the recommended value, give the method used to determine each value, and provide references for each value. An outstanding format and complete indexing both by compound name and by compound class make searching for data quick and easy. While experimental BDE values carry inherent uncertainties, the value of this compilation does not. There is simply no other resource that provides as much data. With this book, when you need data on a particular bond dissociation energy, you'll find it within just a few minutes.
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.
H[sub 2]O[sub 2] oxidizes cyclohexane and other alkanes in CH[sub 3]CN at 20-70[degrees]C in the presence of the catalyst Bu[sub 4]NVO[sub 3]-pyrazine-2-carboxylic acid (ratio 1:4) to afford, after reduction with PPh[sub 3], the corresponding alcohol and carbonyl derivatives (the ratio decreases from 12 to 4 on raising the temperature from 20 to 70[degrees]C). The product yield reaches 46%, based on H[sub 2]O[sub 2], and the turnover number is ca. 1000. The oxidation appears to give predominantly alkyl peroxides, which are partially decomposed in the course of the reaction. Peroxides can be formed when alkyl radicals, generated from alkanes, and radical-like species react with O[sub 2] or abstract OOH groups from the vanadium complex. Benzene and its alkyl derivatives are oxidized to produce phenols as well as side-chain oxidation derivatives. 11 refs., 2 figs., 1 tab.
Vanadyl(IV)-acetylacetonate-catalyzed oxidation of cyclohexane with H(2)O(2), at 40 degrees C under air atmosphere, has been studied in the presence of small quantities of oxalic acid. The process efficiency is increased by this additive and depends on the nature of the solvent (MeCN >= MeOH > Me(2)CO >= 2-PrOH > EtOH). The relationships between the results (conversion, yield) and the electrochemical characteristics of the reaction solution (relative permittivity, redox-potential) are highlighted and discussed.
A new protocol for the effective oxidation of cyclohexane in acetonitrile at 40°C and atmospheric pressure into cyclohexanol, cyclohexanone and cyclohexyl hydroperoxide using hydrogen peroxide as the oxidant, vanadyl(IV)-acetylacetonate as the catalyst, oxalic acid and glyoxal as additives is presented with some reaction mechanism proposals.
This brief essay consists of a few "exciting stories" devoted to relations within a metal-complex catalyst between a metal ion and a coordinated ligand. When, as in the case of a human couple, the rapport of the partners is cordial and a love cements these relations, a chemist finds an ideal married couple, in other words he obtains a catalyst of choice which allows him to functionalize C-H bonds very efficiently and selectively. Examples of such lucky marriages in the catalytic world of ions and ligands are discussed here. Activity of the catalyst is characterized by turnover number (TON) or turnover frequency (TOF) as well as by yield of a target product. Introducing a chelating N,N- or N,O-ligand to the catalyst molecule (this can be an iron or manganese derivative) sharply enhances its activity. However, the activity of vanadium derivatives (with additionally added to the solution pyrazinecarboxylic acid, PCA) as well as of various osmium complexes does not dramatically depend on the nature of ligands surrounding metal ions. Complexes of these metals are very efficient catalysts in oxidations with H2O2. Osmium derivatives are record-holders exhibiting extremely high TONs whereas vanadium complexes are on the second position. Finally, elegant examples of alkane functionalization on the ions of non-transition metals (aluminium, gallium etc.) are described when one ligand within the metal complex (namely, hydroperoxyl ligand HOO(-)) helps other ligand of this complex (H2O2 molecule coordinated to the metal) to disintegrate into two species, generating very reactive hydroxyl radical. Hydrogen peroxide molecule, even ligated to the metal ion, is perfectly stable without the assistance of the neighboring HOO(-) ligand. This ligand can be easily oxidized donating an electron to its partner ligand (H2O2). In an analogous case, when the central ion in the catalyst is a transition metal, this ion changing its oxidation state can donate an electron to the coordinated H2O2 fragment. This provokes the O-O bond rupture in the hydrogen peroxide molecule as is assumed for the role of Fe(2+) ions in the Fenton system.
An electron paramagnetic resonance (EPR) study was carried out to examine structural aspects of vanadium-containing bromoperoxidase from the brown seaweed Ascophyllum nodosum. At high pH, the reduced form of bromoperoxidase showed an apparently axially symmetric EPR signal with 16 hyperfine lines. When the pH was lowered, a new EPR spectrum was formed. When EPR spectra of the reduced enzyme were recorded in the pH range from 4.2 to 8.4, it appeared that these changes were linked to a functional group with an apparent pK/sub a/ of about 5.4. In DâO this value for the pK/sub a/ was 5.3. It is suggested that these effects arise from protonation of histidine or aspartate/glutamate residues near the metal ion. The values for the isotropic hyperfine coupling constant of the reduced enzyme at both high and low pH are also consistent with a ligand field containing nitrogen and/or oxygen donor atoms. When reduced bromoperoxidase was dissolved in DâO or Hâ¹â·O instead of Hâ¹â¶O, vanadium (IV) hyperfine line widths were markedly affected, demonstrating that water is a ligand of the metal ion. Together with previous work these findings suggest that vanadium (IV) is not involved in catalytic turnover and confirm the model in which the vanadium (V) ion of the native enzyme only serves to bind both hydrogen peroxide and bromide. After excess vanadate was added to a homogeneous preparation of purified bromoperoxidase, the extent of vanadium bound to the protein increased from 0.5 to 1.1, with a concomitant enhancement of enzymic activity. Finally, it is demonstrated that both vanadate (VOâ/sup 3 -/) and molybdate (MoOâ/sup 2 -/) compete for the same site on apobromoperoxidase.
The kinetics of H2O2 cleavage catalyzed by cobalt(II)-acetylacetonate has been studied at 40 °C in water, acetic acid, acetonitrile, tetrahydrofuran,
2-propanol, t-butanol, morpholine, and 1,4-dioxane by kinetic and spectroscopic techniques. As revealed, the nature of the solvent has
a decisive influence on process. Medium polarizability and polarity have been found to be the rate determining factors.
Calcined vanadium phosphorus oxide (VPO) prepared by an organic route is found to be an active and effective catalyst for the oxidation of various alkanes such as cyclopentane, cyclohexane, n-hexane, cycloheptane, cyclooctane, cyclodecane and adamantane in acetonitrile solvent using the environmentally benign oxidant, hydrogen peroxide, where the oxidation mechanism is believed to involve a reversible V4+/V5+ redox cycle.
The kinetic and mechanistic features of alkane oxidations to the corresponding alkyl hydroperoxides (main primary products), alcohols and ketones (secondary products) in the systems composed of tetracopper(II) triethanolaminate catalyst [O⊂Cu4{N(CH2CH2O)3}4(BOH)4][BF4]2 (1), aqueous hydrogen peroxide, acetonitrile solvent and an acid promoter (co-catalyst), have been investigated based on the combination of experimental kinetic, selectivity, ESR and UV–vis methods. The nature of acid promoter (hydrochloric, sulfuric, nitric and trifluoroacetic acid) is shown to be a key factor affecting significantly the rate of alkane oxidation. Although all these acids exhibit noticeable promoting effect, it has been observed that in the presence of HCl the reaction proceeds extremely rapidly, being one order faster than those promoted by the other acids, and allowing to achieve the remarkably high turnover frequencies (TOFs) of ca. 600h−1. The unusual rate-accelerating role of water has also been disclosed in the oxidation of cyclohexane catalyzed by 1+HCl or 1+CF3COOH systems. Furthermore, uncommon second-order reaction kinetics with respect to the catalyst have been found. A mechanism of the alkane oxygenation has been proposed, which includes the formation of hydroxyl radicals attacking the alkane molecule. Hydroxyl radicals are formed via the interaction between H2O2 and catalytically active Cu(I) species, the latter being reversibly generated from 1 under the action of an acid, H2O2 and water.
Reduction of an electron acceptor (oxidant), A, or oxidation of an electron donor (reductant), A2−, is often achieved stepwise via one-electron processes involving the couples A/A⋅− or A⋅−/A2− (or corresponding prototropic conjugates such as A/AH⋅ or AH⋅/AH2). The intermediate A⋅−(AH⋅) is a free radical. The reduction potentials of such one-electron couples are of value in predicting the direction or feasibility, and in some instances the rate constants, of many free-radical reactions. Electrochemical methods have limited applicability in measuring these properties of frequently unstable species, but fast, kinetic spectrophotometry (especially pulse radiolysis) has widespread application in this area. Tables of ca. 1200 values of reduction potentials of ca. 700 one-electron couples in aqueous solution are presented. The majority of organic oxidants listed are quinones, nitroaryl and bipyridinium compounds. Reductants include phenols, aromatic amines, indoles and pyrimidines, thiols and phenothiazines. Inorganic couples largely involve compounds of oxygen, sulfur, nitrogen and the halogens. Proteins, enzymes and metals and their complexes are excluded.
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ESI mass spectrometry and 51V-NMR spectroscopy have been used to study the reactions occurring between bisperoxo vanadates and a number of histidine and histidine-like ligands, in aqueous alcoholic solutions. Coordination of one and two molecules of ligand is observed with all the compounds investigated affording [VO5L]- and [VO52L]-, respectively. Characterization of these species has been achieved by MS(n) experiments, which have allowed specific fragmentations of the peroxidic moiety to be distinguished. In particular, with [VO52L]-, two distinct modes of decomposition were observed, depending on the presence in the ligand of a free carboxylic function. - Possible biochemical implications related to vanadium haloperoxidase enzymes are discussed.
Vanadium peroxides are known as very effective oxidants of different organic and inorganic substrates. In this short account reactivity, structural and mechanistic studies concerning the behaviour of peroxovanadates toward a number of different substrates are collected. Homogeneous and two-phase systems are presented, in addition, interesting synthetic results obtained with the use of ionic liquids as reaction media are also presented.
A new binuclear oxovanadium(v) complex [{VO(OEt)(EtOH)}2L] () where H4L is bis(2-hydroxybenzylidene)terephthalohydrazide has been synthesized and fully characterized. The combination of with pyrazine-2-carboxylic acid (PCA; a cocatalyst) affords a catalytic system for the efficient oxidation of saturated hydrocarbons, RH, with hydrogen peroxide and air in acetonitrile solution at 50 °C to produce alkyl hydroperoxides, ROOH, as the main primary products. Very high turnover numbers (TONs) have been attained in this reaction: for example, after 2220 min, TON = 44 000 and initial TOF (turnover frequency) = 3300 h(-1) per molecule of complex . The estimated activation energy of the cyclohexane oxygenation in the presence of /PCA is Ea = 16 ± 2 kcal mol(-1). This value is identical to that obtained for the cyclohexane oxidation with H2O2 catalyzed by the (n-Bu4N)[VO3]/PCA combination (17 ± 2 kcal mol(-1)). The dependences of initial oxidation rates W0 on the initial concentrations of all components of the reaction mixture have been determined. Based on these kinetic data and on the regio- and bond-selectivity parameters measured in the oxidation of linear and branched alkanes a mechanism of the oxidation has been proposed which includes the generation of hydroxyl radicals in the crucial stage.
Pulse radiolyses of oxalic acid and oxalates in aqueous solution were carried out. The following rate constants were determined: k(eaq- + HC2O4-) = (3.2 ± 0.6) × 109 M-1 sec-1 and k(eaq- + C2O42-) = (1.7 ± 0.5) × 107 M-1 sec-1. With CNS- ions as competitive solutes k(OH + H2C2O4) = (1.0 ± 0.1) × 106 M-1 sec-1; k(OH + HC2O4-) = (3.2 ± 0.1) × 107 M-1 sec-1, and k(OH + C2O42-) = (5.3 ± 0.3) × 106 M-1 sec-1 were also obtained. With N2O as a scavenger for eaq- and H2 (45 atm) for OH the absorption spectra of several radicals were obtained. Their absorption maxima are between 230 and 250 nm. Reaction mechanisms and kinetic data of the radicals are discussed.
The controlled cleavage of strong C–H bonds such as those of methane poses a significant and industrially important challenge for chemists. In nature, methane is oxidized to methanol by soluble methane monooxygenase via a diiron(iv) intermediate called Q. However, the only two reported diiron(iv) complexes have activities towards C–H bonds that fall far short of the activity of this biological catalyst. In this paper, we model the chemistry of MMO-Q by generating an oxo-bridged diiron(iv) complex by electrochemical oxidation. This species is a more effective oxidant. It can attack C–H bonds as strong as 100 kcal mol-1 and reacts with cyclohexane 100- to 1,000-fold faster than mononuclear FeIV=O complexes of closely related ligands. Strikingly, this species can also cleave the strong O–H bonds of methanol and t-butanol instead of their weaker C–H bonds, representing the first example of O–H bond activation for iron complexes.
Two mononuclear mixed-ligand ruthenium(III) complexes with oxalate dianion (ox2−) and acetylacetonate ion (2,4-pentanedionate, acac−), K2[Ru(ox)2(acac)] (1) and K[Ru(ox)(acac)2] (2), were prepared as a candidate for a building block. In fact, reaction of complex 2 with manganese(II) sulfate gave a heterometallic tetranuclear complex, TBA[MnII{(μ-ox)RuIII(acac)2}3] (5) in the presence of tetrabutylammonium (TBA) bromide. The 1H NMR, UV–Vis, selected IR and FAB mass spectral data of these complexes are presented. Both mixed-ligand ruthenium(III) complexes gave a Nernstian one-electron reduction step in 0.1 mol dm−3 Na2SO4 aqueous solution on a mercury electrode at 25 °C. Comparison of observed reversible half-wave potentials with calculated values for a series of [Ru(ox)n(acac)3−n]n− (n=0–3) complexes by using Lever’s ligand electrochemical parameters is presented.
The formation and disappearance of a relatively stable chromium(V) intermediate during the reaction of chromic acid with oxalic acid can be conveniently observed by electron spin resonance as well as by spectrophotometric studies. The esr signals indicate the existence of two chromium(V) species which are in rapid equilibrium and whose relative concentrations depend on the composition of the solvent. Absorption spectra of chromium(V) in 50 % acetic acid and water have been constructed. It is shown that chromium(V) resembles chromium-(VI) in most of its kinetic properties, and under most conditions is only two to three times more reactive toward oxalic acid.
Recent attempts to quantify the rate constant for radical capture in cytochrome P-450 hydroxylations employing substrates that are precursors to radicals that rearrange very rapidly have given widely differing apparent rate constants, suggesting that the consensus hydroxylation mechanism is incomplete or incorrect or that the probe substrates behave in an unexpected manner. We report cytochrome P-450 hydroxylations of a new, calibrated hypersensitive radical probe substrate, (trans,trans-2-tert-butoxy-3-phenylcyclopropyl)methane (1a), that permits discrimination between radical and cationic intermediates. Cytochrome P-450 oxidation of the methyl group in 1a gave unrearranged and rearranged hydroxylation products. Most of the rearranged products derived from a cationic intermediate apparently produced during the course of the hydroxylation reaction; this unanticipated process is the origin of the confusing results obtained with other probes. The radical species in the hydroxylation reaction has a lifetime of only 70 fs; it is not a true intermediate but part of a reacting ensemble. The small amount of radical rearrangement occurs because the insertion reaction is nonsynchronous with C-H bond cleavage leading C-O bond formation. The short radical lifetime also requires that the oxygen atom is within bonding distance of carbon at the instant of hydrogen abstraction; that is, a ''side-on'' approach of oxygen to the C-H bond is suggested as opposed to a linear C-H-O array of a conventional abstraction.
The dioxovanadium(V) complexes VO2(bpg) (1), [VO2(pmida)]- (2), and [VO2(ada)]- (3) have been synthesized and characterized as models for the vanadium haloperoxidases. These compounds react with hydrogen peroxide in acetonitrile to form the corresponding peroxovanadium(V) complexes that have been previously studied by stopped-flow spectrophotometry. 1H and 51V NMR spectra of the VO2+ complexes in aqueous solution provide a clear picture of the solution structure of each complex. The results of these kinetic studies suggest an associative mechanism in which peroxide binds to a protonated form of the vanadium complex, followed by loss of a bound hydroxide or water molecule in the rate-determining step of the reaction and rapid rearrangement to the final product. The addition of acid to the reaction mixture results in rapid increases in the rate of peroxide binding by vanadium as a result of increased protonation of the complex. As in previous studies of similar reactions in aqueous solution, the reaction is first order in [H+] for substoichiometric amounts of acid, but when acid is present in excess, the dependence on [H+] becomes more complex, implicating the presence of hydroxide- and water-ligated intermediates. Under conditions in which no acid is added to the reaction mixture, the rate constants for formation of the peroxovanadium complex from the vanadium−peroxide adduct are 0.12 ± 0.04 s-1 for 1, 0.33 ± 0.03 s-1 for 2, and 0.29 ± 0.06 s-1 for 3. The implications of this study with respect to catalysis by the vanadium-dependent haloperoxidase enzymes are discussed.
The electrochemistry of V(acac) 3 and VO(acac) 2 (acac - = acetylacetonate anion) has been studied by cyclic voltammetry and controlled-potential coulometry in dimethyl sulfoxide at a platinum electrode. VO(acac) 2 is irreversibly reduced by one electron at -1.9 V vs. SCE to a stable V III product. In the presence of excess ligand, VO(acac) 2 is reduced by two electrons to V(acac) 3-, with the V III species mentioned above and V(acac) 3 as intermediates. V(acac) 3 is reversibly reduced to V(acac) 3- at -1.42 V. The one-electron oxidation of VO(acac) 2 and the two-electron oxidation of V(acac) 3 (at +0.81 and +0.76 V, respectively) give the same vanadium(V) product. The reduction products of this species depend on whether or not free ligand is present.
There is increased recognition by the world’s scientific, industrial, and political communities that the concentrations of greenhouse gases in the earth’s atmosphere, particularly CO_2, are increasing. For example, recent studies of Antarctic ice cores to depths of over 3600 m, spanning over 420 000 years, indicate an 80 ppm increase in atmospheric CO_2 in the past 200 years (with most of this increase occurring in the past 50 years) compared to the previous 80 ppm increase that required 10 000 years.2 The 160 nation Framework Convention for Climate Change (FCCC) in Kyoto focused world attention on possible links between CO2 and future climate change and active discussion of these issues continues.3 In the United States, the PCAST report4 “Federal Energy Research and Development for the Challenges of the Twenty First Century” focused attention on the growing worldwide demand for energy and the need to move away from current fossil fuel utilization. According to the U.S. DOE Energy Information Administration,5 carbon emission from the transportation (air, ground, sea), industrial (heavy manufacturing, agriculture, construction, mining, chemicals, petroleum), buildings (internal heating, cooling, lighting), and electrical (power generation) sectors of the World economy amounted to ca. 1823 million metric tons (MMT) in 1990, with an estimated increase to 2466 MMT in 2008-2012 (Table 1).
Representative atomic and molecular systems, including various inorganic and organic molecules with covalent and ionic bonds, have been studied by using density functional theory. The calculations were done with the commonly used exchange-correlation functional B3LYP followed by a comprehensive analysis of the calculated highest-occupied and lowest-unoccupied Kohn-Sham orbital (HOMO and LUMO) energies. The basis set dependence of the DFT results shows that the economical 6-31+G* basis set is generally sufficient for calculating the HOMO and LUMO energies (if the calculated LUMO energies are negative) for use in correlating with molecular properties. The directly calculated ionization potential (IP), electron affinity (EA), electronegativity (c), hardness (h), and first electron excitation energy (t) are all in good agreement with the available experimental data. A generally applicable linear correlation relationship exists between the calculated HOMO energies and the experimental/calculated IP's. We have also found satisfactory linear correlation relationships between the calculated LUMO energies and experimental/calculated EA's (for the bound anionic states), between the calculated average HOMO/LUMO energies and c values, between the calculated HOMO-LUMO energy gaps and h values, and between the calculated HOMO-LUMO energy gaps and experimental/calculated first excitation energies. By using these linear correlation relationships, the calculated HOMO and LUMO energies can be employed to semi-quantitatively estimate ionization potential, electron affinity, electronegativity, hardness, and first excitation energy.
The activation hardness concept is defined and used to predict the orientation of electrophilic aromatic substitution. With the transition state defined much in the manner of Wheland, it is shown that the activation energy is the negative of twice the change in hardness in going from reactant to transition state (called the activation hardness). The smaller the activation hardness is the faster the reaction is; the harder the transition state is the better. Calculations presented show that the activation hardness is an excellent index for predicting orientation effects. Several new principles of maximum hardness or softness are stated and discussed.
Trinuclear carbonyl hydride cluster, Os3(CO)10(μ- H)2, catalyzes oxidation of cyclooctane to cyclooctyl hydroperoxide by hydrogen peroxide in acetonitrile solution. The hydroperoxide partly decomposes in the course of the reaction to afford cyclooctanone and cyclooctanol. Selectivity parameters obtained in oxidations of various linear and branched alkanes as well as kinetic features of the reaction indicated that the alkane oxidation occurs with the participation of hydroxyl radicals. A similarmechanism operates in transformation of benzene into phenol and styrene into benzaldehyde. The system also oxidizes 1-phenylethanol to acetophenone. The kinetic study led to a conclusion that oxidation of alcohols does not involve hydroxyl radicals as main oxidizing species and apparently proceeds with the participation of osmyl species, 'Os = O'.
ESI mass spectrometry and 51V-NMR spectroscopy have been used to study the reactions occurring between bisperoxo vanadates and a number of histidine and histidine-like ligands, in aqueous alcoholic solutions. Coordination of one and two molecules of ligand is observed with all the compounds investigated affording [VO5L]− and [VO52L]−, respectively. Characterization of these species has been achieved by MSn experiments, which have allowed specific fragmentations of the peroxidic moiety to be distinguished. In particular, with [VO52L]−, two distinct modes of decomposition were observed, depending on the presence in the ligand of a free carboxylic function. – Possible biochemical implications related to vanadium haloperoxidase enzymes are discussed.
Ab initio restricted Hartree Fock and density functional theory (DFT), as well as semiempirical Austin model 1 and parametrization method 3 molecular orbital calculations were carried out for a range of chlorinated dioxin molecules to obtain molecular descriptors such as HOMO–LUMO gaps (HOMO = highest occupied molecular orbital, LUMO = lowest unoccupied molecular orbital) and partial atomic charges. The HOMO–LUMO gap is an indicator of stability in a molecule: the larger the gap the greater the stability of the molecule toward further reaction. These calculations indicate that with increasing extent of chlorination, the gap decreases. The observed charge pattern shows that carbon atoms in the peri (1,4,6,9) ring positions have a partial negative charge while those in the lateral (2,3,7,8) position have a partial positive or small partial negative charge. The descriptors, from the more precise DFT method, were used to rationalize experimental observations of dechlorination of dioxins. Reductive dechlorination pathways from two different experimental studies were examined using partial charges and estimated Gibbs free energy of dechlorination. In both experimental studies, highly thermodynamically favorable and less thermodynamically favorable pathways were observed. For a given chlorinated dioxin, when more than one degradation pathway was possible, dechlorination in the most thermodynamically favored pathway occurred at the most positively charged carbon atom in the ring, which was usually a lateral carbon atom. These results are discussed in light of a possible mechanism for reductive dechlorination.
The enhancement of the reactivity of peroxides, particularly hydrogen peroxide and alkylhydroperoxides, in the presence of vanadium catalysis is a very well known process. The catalytic effect is determined by the formation of an intermediate whose nature depends on the peroxides used and on its interaction with the metal precursor, high-valent peroxo vanadium species being usually the reactive oxidants. During the last decades the mechanistic details for several types of oxidation reactions have been elucidated. Interestingly, in a number of cases theoretical calculations offered support to the proposed reaction pathways.In general, V(V) peroxo species behave as electrophilic oxygen transfer reagents thus reacting preferentially with the more nucleophilic functional group present in the molecule. In several instances the chemoselectivity observed in such processes is very high when not absolute. As far as vanadium peroxides are concerned, a radical oxidative reactivity toward alkanes and aromatics has been also observed; also for this latter chemistry, diverse research groups studied in detail the mechanism. On the other hand, no clear-cut evidence of nucleophilic reactivity of vanadium peroxo complexes has been obtained.Here we collect a selection of recent achievements concerning the reaction mechanisms in the vanadium catalysed oxidation and bromination reactions with peroxides.
The principles of the Rio Conference (1992) and Agenda 21 address the pressing problems of today and also aim at preparing the world for the challenges of this century. The conservation and management of resources for development are the main foci of interest, to which chemistry will have to make a considerable contribution. Since base chemicals are produced in large quantities and important product lines are synthesized from them, their resource-saving production is especially important for a sustainable development. New processes based on renewable feedstocks are significant here. Most products that are obtained from renewable raw materials may, at present, not be able to compete with the products of the petrochemical industry, but this will change as oil becomes scarcer and oil prices rise. The design of chemical products should make sustainable processing and recycling possible, and should prevent their bioaccumulation. Methods and criteria to assess their contribution to a sustainable development are necessary. The time necessary to introduce the new more sustainable processes and products has to be diminished by linking their development with operational innovation management and with efficient environmental-political control procedures. To cite this article: J.O. Metzger, C. R. Chimie 7 (2004).
Hydrogen peroxide is a strong oxidizing agent commercially available in aqueous solution over a wide concentration range. Its many uses involve its varied chemistry and are based on oxidations, reductions, substitutions, molecular additions, and decompositions. Product from hydrogen peroxide decomposition are the environmentally benign oxygen and water. Manufacture and use, especially in industrial bleaching applications, are on the rise, fueled by environmental concerns and ecological pressure to replace chlorine‐based bleaching with the cleaner peroxide chemistry.
Manufactured primarily in large, strategically located anthrahydroquinone autoxidation processes, the primary use for hydrogen peroxide is in bleaching wood pulp. Hydrogen peroxide is also produced electrolytically. The various manufacturing processes are discussed. Safe use and handling require adherence to approved procedures and use of proper protective equipment.
An innovation of the aerobic oxidation of hydrocarbons through catalytic carbon radical generation under mild conditions was achieved by using N-hydroxyphthalimide (NHPI) as a key compound. Alkanes were successfully oxidized with O2 or air to valuable oxygen-containing compounds such as alcohols, ketones, and dicarboxylic acids by the combined catalytic system of NHPI and a transition metal such as Co or Mn. The NHPI-catalyzed oxidation of alkylbenzenes with dioxygen could be performed even under normal temperature and pressure of dioxygen. Xylenes and methylpyridines were also converted into phthalic acids and pyridinecarboxylic acids, respectively, in good yields. The present oxidation method was extended to the selective transformations of alcohols to carbonyl compounds and of alkynes to ynones. The epoxidation of alkenes using hydroperoxides or H2O2 generated in situ from hydrocarbons or alcohols and O2 under the influence of the NHPI was demonstrated and seems to be a useful strategy for industrial applications. The NHPI method is applicable to a wide variety of organic syntheses via carbon radical intermediates. The catalytic carboxylation of alkanes was accomplished by the use of CO and O2 in the presence of NHPI. In addition, the reactions of alkanes with NO2 and SO2 catalyzed by NHPI provided efficient methods for the synthesis of nitroalkanes and sulfonic acids, respectively. A catalytic carbon-carbon bond forming reaction was achieved by allowing carbon radicals generated in situ from alkanes or alcohols to react with alkenes under mild conditions.
The principles of the United Nations Conference on Environment and Development (UNCED), held in June 1992 in Rio de Janeiro, and Agenda 21, the comprehensive plan of action for the 21st century, adopted 10 years ago by more than 170 governments, address the pressing problems of today and also aim at preparing the world for the challenges of this century. The conservation and management of resources for development are the main focus of interest, to which the sciences will have to make a considerable contribution. Natural, economic, and social sciences will have to be integrated in order to achieve this aim. In their future programs, the associations of the chemical industries in Europe, Japan, and the USA have explicitly accepted their obligation to foster a sustainable development.In this review we investigate innovations in chemistry exemplarily for such a development with regard to their ecological, economical, and social dimensions from an integrated and interdisciplinary perspective. Since base chemicals are produced in large quantities and important product lines are synthesized from them, their resource-saving production is especially important for a sustainable development. This concept has been shown, amongst others, by the example of the syntheses of propylene oxide and adipic acid. In the long run, renewable resources that are catalytically processed could replace fossil raw materials. Separation methods existing today must be improved considerably to lower material and energy consumption. Chemistry might become the pioneer of an innovative energy technique.The design of chemical products should make possible a sustainable processing and recycling and should prevent their bio-accumulation. Methods and criteria to assess their contribution to a sustainable development are necessary. The time taken to introduce the new more sustainable processes and products has to be diminished by linking their development with operational innovation management and with efficient environmental-political control procedures.
Bis(acetylacetonato)oxovanadium C10H14O5V (I) and (S)-[2-(N-salicylidene)aminopropionate]oxovanadium monohydrate C10H9NO5V (II) are synthesized. The crystal structures of compounds I and II are determined using single-crystal X-ray diffraction. Crystals of compound I are triclinic, a = 7.4997(19) Å, b = 8.2015(15) Å, c = 11.339(3) Å, α = 91.37(2)°, β = 110.36(2)°, γ = 113.33(2)°, Z = 2, and space group
P[`1]P\bar 1
. Crystals of compound II are monoclinic, a = 8.5106(16) Å, b = 7.373(2) Å, c = 9.1941(16) Å, β = 101.88(1)°, Z = 2, and space group P21. The structures of compounds I and II are solved by direct methods and refined to R
1 = 0.0382 and 0.0386, respectively. The oxovanadium complexes synthesized are investigated by vibrational spectroscopy.
The mechanism of reaction of hexaquo iron(II) with hydrogen peroxide has been unresolved for 70 years. Most scientists, perhaps by default, have accepted the free radical chain mechanism of Barb et al. (Trans. Faraday Soc. 47 (1957) 462). However an earlier proposal involved formation of the ferryl ion, FeO2+ (J. Am. Chem. Soc. 54 (1932) 2124). Recent work has favored a mechanism involving FeO2+ and FeOFe5+ species. Similarly there are differences of opinion on the mechanism of reaction of iron(III), both hexaquo and chelated, with hydrogen peroxide. These differences have fostered a recent burst of activity, with claims on one hand that hydroxyl radicals play a key role, and on the other, that there is a non-free radical mechanism. In contrast, the mechanism of reaction of the heme-containing peroxidase and catalase enzymes with hydrogen peroxide, orders of magnitude faster than reactions of iron(II)/(III), now appears to be well established. In this paper I attempt, as objectively as possible, to delineate the proposed mechanisms, discuss their possible physiological relevance, and summarize the current state of knowledge.
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.