Composés d'addition des halogénures de niobium(V) et de tantale(V). V. Stabilité relative des composés des chlorures avec quelques oxydes et sulfures organiques

The adducts of niobium(V) and tantalum(V) chlorides with some aliphatic and cyclic oxides and sulfides, studied by NMR. spectroscopy in CHCl₃, are found to have 1:1 stoechiometry, at room temperature and lower. In the thioxane complex TaCl₅ · C₄H₈OS two species are present with the ligand coordinated by the sulfur atom or by the oxygen atom, respectively, in a proportion which has been determined. The thioxane adduct of niobium(V) chloride, however, is preferentially coordinated by the sulfur atom. There is also evidence for the species 2MCl₅ · C₄H₈OS. The relative basicity of each donor atom in dioxane, thioxane and dithiane is calculated and discussed. In contrast to the nitrile adducts, whose stability was found earlier to be controlled by inductive factors, the steric factors are more important for the ether and sulfide adducts: MCl₅ · Me₂X is more stable than the corresponding MCl₅ · Et₂X (M = Nb, Ta; X = O, S). Both niobium(V) and tantalum(V) chlorides have a soft behaviour, but NbCl₅ is a weaker Lewis acid than TaCl₅ and shows also a softer behaviour.     

Photoluminescence of Uranium(VI): Quenching Mechanism and Role of Uranium(V)

The photoluminescence of uranium(VI) is observed typically in the wavelength range 400–650 nm with the lifetime of several hundreds μs and is known to be quenched in the presence of various halide ions (case A) or alcohols (case B). Here, we show by density functional theory (DFT) calculations that the quenching involves an intermediate triplet excited state that exhibits uranium(V) character. The DFT results are consistent with previous experimental findings suggesting the presence of photoexcited uranium(V) radical pair during the quenching process. In the ground state of uranyl(VI) halides, the ligand contributions to the highest occupied molecular orbitals increase with the atomic number (Z) of halide ion allowing larger ligand‐to‐metal charge transfer (LMCT) between uranium and the halide ion. Consequently, a larger quenching effect is expected as Z increases. The quenching mechanism is essentially the same in cases A and B, and is driven by an electron transfer from the quencher to the UO₂²⁺ entity. The relative energetic stabilities of the triplet excited state define the “fate” of uranium, so that in case A uranium(V) is oxidized back to uranium(VI), while in case B uranium remains as pentavalent.     

Composés d'addition des halogénures de niobium(V) et de tantale(V) IV. Stabilité relative des composés de chlorures avec quelques nitriles

The relative stability constants of the adducts of MCl₅ (M ? Nb, Ta) with acetonitrile, halogenoacetonitriles, pivalonitrile, acrylonitrile and benzonitriles are determined in dilute solutions by NMR. methods. The stability of the adducts is controlled by inductive factors. Chemical shifts and analysis of the new compounds are reported.     

Composés d'addition des halogénures de niobium(V) et de tantale(V). VII. Composés du chlorure de niobium(V) avec l'oxytrichlorure et l'oxytribromure de phosphore

The adducts NbCl₅ · OPCl₃ and NbCl₅ · OPBr₃ are observed in chloroform solution by ³¹P-NMR spectroscopy. The enthalpy and entropy of activation for the exchange reaction between bulk and coordinated OPCl₃ are found equal to 17 ± 3 kcal/mole and 18 ± 10 cal/°mole. The stability of NbCl₅ · OPCl₃ is compared on a semi-quantitative basis to the stability of other adducts NbCl₅ · OPR₃ (R = Br, OMe, NMe₂).     

Composés d'addition des halogénures de niobium(V) et de tantale(V): VI. Réactions d'échange des composés du chlorure de niobium(V) avec quelques nitriles

The ligand exchange reactions NbCl₅·?N + RCN* ? NbCl₅ · RCN* + RCN are studied by NMR. spectroscopy for R = Me₃C, Me, FCH₂, CICH₂, BrCH₂, ICH₂. The reaction is of zero order in RCN and of first order in NbCl₅ · RCN and thus a dissociative mechanism is suggested for all the ligands studied. The enthalpies and entropies of activation are determined over 50° to 90° temperature ranges. There is a linear correlation between ΔG≠ and the free enthalpy of formation of NbCl₅ RCN. However this correlation is shown to hold only for series of adducts having the same donor group.     

Adducts of Niobium(V) and Tantalum(V) Halogenides. IX. Relative stability of the adducts of the chlorides and bromides with some dialkylchalcogenides

The relative stability of adducts formed by Nb(V) and Ta(V) pentachlorides and bromides with some dimethylchalcogenides and nitriles has been determined by ¹H-NMR. in dichloromethane at ? 60°. The stabilities are explained in terms of the HSAB principle and Jørgensen's symbiotic effect. A good correlation exists between the ionisation potential of the valence p orbital of the chalcogen atom in the ligand and the logarithm of the relative stability of the adduct formed with a given acid.     

Structural Characterization of (Arylimido)vanadium(V) Compounds with 2,6‐Difluorophenoxide Ligand

The (arylimido)vanadium(V) compound, [(p‐MeOC₆H₄N)V(OiPr)₃] was demonstrated to undergo ligand exchange reaction with one or two equivalents of 2,6‐difluorophenol, affording the (arylimido)vanadium(V) compounds, [(p‐MeOC₆H₄N)V(OiPr)₂(O‐2,6‐F₂Ph)] and [(p‐MeOC₆H₄N)V(OiPr)(O‐2,6‐F₂Ph)₂]. Their X‐ray crystallographic analyses elucidated the μ‐isopropoxido‐bridged dimeric structures, wherein each vanadium atom has a trigonal‐bipyramidal arrangement with the imido and bridging isopropoxide ligands in the apical positions. The isopropoxide ligand was selectively employed as a bridging ligand between two central vanadium atoms. On the other hand, the reaction of the (arylimido)vanadium(V) compound, [(p‐MeOC₆H₄N)VCl₃] and three equivalents of lithium 2,6‐difluorophenoxide gave the (arylimido)vanadium(V) compound, [(p‐MeOC₆H₄N)V(O‐2,6‐F₂Ph)₃]. In the crystal packing, the thus‐obtained compound showed a distorted trigonal‐bipyramidal environment at the vanadium atoms with the μ‐phenoxido‐bridged dimeric structure, wherein the 2,6‐difluorophenoxide ligand was found to serve as a bridging ligand.     

Applications of catalytic reactions. Kinetic microdetermination of W(VI) and V(V)

Summary Applications of Catalytic Reactions. Kinetic Microdetermination of W(VI) and V(V) A kinetic method is described for the microdetermination of tungsten and vanadium. The method is based on the catalytic action of tungstate and vanadate ions on the oxidation of 2,4-diaminophenol with hydrogen peroxide. The effect of reagent concentration is studied and the maximum tolerable amounts of interfering ions are determined. Procedures for the determination of 0.46 to 73.5g/ml tungsten are given with a relative error of about 1.5%. Quantities of 2.02·10^–2 to 2.5g/ml vanadium could be determined with a relative error of about 2%.Furthermore a kinetic determination of tungstate and vanadate ions in their mixtures is proposed.Part of PhD Thesis-University of Thessaloniki, 1975.     

Chlorobromoantimonates(V): New synthesis and 121Sb-NMR study

The new tetraethylammonium tetrachlorodibromoantimonate(V) and dichlorotetrabromoantimonate(V) have been prepared and have been given a cis-structure from their vibrational spectra. The ways of synthesis of all the terms of the series NEt₄⁺SbCl_6?nBr_n^? are discussed; the ¹²¹Sb spectra of their solutions in CH₃CN are given and show halide exchange around Sb(V).     

The preparation of new oxoniobium(V) complexes from hydrated Niobium(V) Oxide: the crystal and molecular structure of Oxotris(2-pyridinolato-<Emphasis Type="Italic">N</Emphasis>-oxide)niobium(V)

Three oxoniobium(V) complexes ONbL₃ with HL = tropolone (1), 2-pyridinol-N-oxide (2) or, 3-hydroxy-1,2-dimethyl-4(1H)-pyridone (3) are obtained in good yield by one step reactions from commercially available hydrated niobium(V) oxide in aqueous media. The products are characterized by elemental analysis, FT-IR spectroscopy, NMR spectroscopy (¹H- and ¹³C-) and thermogravimetry. The crystal structure of oxotris(2-pyridinolato-N-oxide)niobium(V) is determined by single crystal X-ray diffraction, showing discrete molecules with pentagonal bipyramidal coordinated niobium. The ¹H-NMR spectra of CDCl₃ solutions indicate a fluxional behaviour for the three complexes.     

Etudes photochimiques en série hétérocyclique. V. Mise en évidence d'oxazoles par photooxydation de thiazoles

Formation of oxazoles as by-products of photooxydation of thiazoles In the presence of air, light (high pressure mercury lamp) and iodine as sensitizer, benzene solution of arylthiazoles 2 , 3 and 13 (see scheme 1) were converted into a mixture of photoisomers and aryloxazoles (see scheme 2). Upon shorter times of irradiation under oxygen in chloroform containing methylene blue as a sensitizer 2,4,5-triphenylthiazole (7) is converted in 2,4,5-triphenyloxazole. Under usual photooxidation conditions, in methanol as solvent, most of arylthiazoles studied gave a mixture of nitriles, amides, acids esters and diketones. In the case of thiazole 7 , the corresponding oxazole was also found. A possible mechanism for the photochemical oxidation of arylthiazoles to aryloxazoles, via 2,5- and 4,5-endo-peroxides, may be proposed (see schemes 5 and 6).     

Two square‐pyramidal chromium(V)–nitride complexes: bis­(2‐methyl­quinolin‐8‐olato)nitridochromium(V) and nitridobis(2‐sulfidopyridine N‐oxide)chromium(V)

Two new chromium(V)–nitride complexes with a coordination sphere completed by bidentate ligands have been synthesized and structurally characterized. Bis(2‐methyl­quinolin‐8‐olato)nitridochromium(V), [Cr(C₁₀H₈NO)₂(N)], has the coordination sphere completed by an equatorial N₂O₂ set of ligators. The compound crystallizes with the five‐coordinate complexes at sites with twofold rotational symmetry and all Cr—N bond directions aligned with the crystallographic b axis. Nitridobis(2‐sulfidopyridine N‐oxide)chromium(V), [Cr(C₅H₄NOS)₂(N)], crystallizes with the mol­ecules on general positions and has an equatorial S₂O₂ coordination environment, which is unprecedented among nitride complexes of the first‐row transition metals. In both systems, Cr[triple‐bond]N bonds are short at ca 1.56 Å.     

Exchange interactions and electron delocalization in the mixed-valence cluster V(4)(IV)V(2)(V)O(7)(OC(2)H(5))(12)

The mixed-valence cluster compound V4IVV2VO7(OC2H5)12 was studied by X-band electron paramagnetic resonance (EPR) in the temperature range of 4.2-293 K. According to X-ray diffraction study, the crystal structure of the compound was described by a R3m space group at 295 K (four d1 electrons are equally delocalized on all vanadium ions) and changed to a P21/n space group on cooling the crystals to 173 K (the electrons are preferably localized on the four equatorial vanadium ions). The EPR spectra originate from the S = 1 total spin states with the fine structure averaged to a single Lorentzian line and from the S = 2 total spin states with fine structure partly averaged in the temperature range of 295-200 K and well averaged below 45-50 K. The states of S = 1 and S = 2 of comparable energy (DeltaE approximately 2 cm(-1); ES=1 < ES=2) were shown to be the lowest ones. The VIV <--> VV unpaired electron transfers together with isotropic Heisenberg exchange were shown to determine the total spin states composition and the intracluster dynamics of the compound. Two types of electron transfers were assumed: the single-jump transfer leading to the averaged configurations of the V4IVV2V <--> V3IVVV VIVVV type and to the splitting of the total spin states by intervals comparable in magnitude with the isotropic exchange parameter J approximately 100 cm-1 and the double-jump transfer resulting in dynamics. Temperature dependence of the transition rates nutr was observed. In the range of 295-210 K, the value of nutr = (0.5-0.6) x 10(10) s(-1) is sufficient for averaging the fine structure of the S = 1 states, and below 45 K the value of nutr approximately 1.5 x 10(10) s(-1) also averages the fine structure of the S = 2 state. A change in the localization plane of the VIV ions in the temperature range of 40-50 K was discovered.     

Synthèse par cyclisation nucléophile de dérivés de sucres portant un hétérocycle inséré en spiro. Note de Laboratoire

Synthesis of Sugar Derivatives Bearing a Spiro Heterocycle via Nucleophilic Cyclization Treated with the 1,4-binucleophiles 1,2-diaminoethane, 2-aminoethanol, 2-aminoethanethiol, L -cysteine, o-phenylenediamine, o-aminophenol or o-aminothiophenol the ketosugar derivative 1 gave in good yields the corresponding spiro derivatives 2–8 . In each case, the reaction was stereospecific leading to the isomer bearing the N-atom on the endo face of the bicyclic starting material. Starting from the sugar enone 9 , the aromatic 1,4-binucleophiles led stereospecifically to the spirobenzo [b]-diazepine 10 , -oxazepine 11 or -thiazepine 12 . In one case, an imine (13) was isolated. As 13 cyclized to 6 , the intermediate formation of these kind of derivatives could be considered as a common step for all these reactions.     

Cation exchange extraction of neptunium(V) and protactinium(V) from molten nitrate salt

Cu-di-(2-ethylhexyl)phosphate was used as the cation exchange extractant from molten nitrate salt. IR absorption spectra of di-(2-ethylhexyl)phosphoric acid and Cu-di-(2-ethylhexyl)phosphate were compared and it was proved that the acidic form of the extractant is not Cu-di-(2-ethylhexyl)-phosphate. Using LiNO₃−NH₄NO₃ eutectic melt, it was shown that the back-extraction of Cu²⁺ is a cation exchange reaction. Np(V) and Pa(V) were extracted by Cu-di-(2-ethylhexyl)phosphate from LiNO₃−NaNO₃−KNO₃ eutectic melt. The distribution ratio of Np(V) was greater than that of Pa(V) on the contrary of their distribution ratios in the aqueous extraction system. A possible cation exchange extraction reaction was proposed for the extraction of Np(V).     

Interaction entre chromophores non-conjugués: Diméthylidène-2,3-époxy-5,6(exo)-norbornane et diméthylidène-2,3-époxy-5,6(exo)-oxa-7-norbornane

Preparation of the title compounds 7 and 17 is described. A hypsochromic shift of the V ← N transition of the s-cis-butadiene chromophore in the UV. spectra of the epoxydienes 7 and 17 is observed relative to 2,3-dimethylidene-norbornanes ( 20, 29 ), and to 2,3-dimethylidene-7-oxa-norbornanes ( 22, 23 ). This effect is discussed in terms of geometry changes of the s-cis-butadiene and in terms of variation of the hyperconjugation of this chromophore with the σ-skeleton of the bicyclic system.     

Vielseitige Koordinationschemie von Vanadium(V): Substitutionsreaktionen,Umlagerungen und Kondensationsreaktionen von Oxovanadium(V)‐Komplexen des tripodalen Sauerstoffliganden LOMe− = [η5‐(C5H5)Co{P(OMe)2(O)}3]−

Multifaceted Coordination Chemistry of Vanadium(V): Substitution, Rearrangement Reactions, and Condensation Reactions of Oxovanadium(V) Complexes of the Tripodal Oxygen Ligand L_OMe^? = [η⁵‐(C₅H₅)Co{P(OMe)₂(O)}₃]^? The octahedral oxovanadium(V) complex [V(O)F₂L_OMe] of the tripodal oxygen ligand L_OMe^? = [η⁵‐(C₅H₅)Co{P(OMe)₂(O)}₃]^? reacts with alcohols and phenol with substitution of one fluoride ligand to form alkoxo complexes [V(O)F(OR)L_OMe], R = Me, Et, i‐Prop, Ph. In the presence of water, however, both fluoride ions are substituted and a complex with the composition VO₂L_OMe can be isolated. The crystal structure shows that the oxo‐bridged trimer [{V(O)(L_OMe)O}₃] was synthesized. In the presence of BF₃ the fluoride ligand in the alkoxo‐complex [V(O)F(OEt)L_OMe] can be exchanged for pyridine to yield [V(O)(OEt)pyL_OMe]BF₄. Analogous attempts to exchange the fluoride ligand for tetrahydrofuran and acetonitrile induces a rearrangement reaction that leads to the vanadium complex [V(O)(L_OMe)₂]BF₄. The crystal structure of this compound has been determined. Its ¹H and ³¹P‐NMR spectra show that it is a highly fluxional vanadium complex at ambient temperature in solution. The two tripodal ligands L_OMe^? coordinate the vanadium centre as bidentate or tridentate ligands. The exchange bidentate/tridentate becomes slow on the NMR time scale below about 200 K.     

Spin dimer analysis of the magnetic structures of A2V3O9 (A = Ba,Sr) and η-Na9V14O35: Importance of the V4+–O⋯O–V4+ super-superexchange interactions mediated by the O–V5+–O bridges

Spin dimer analysis was carried out for the magnetic oxides of V⁴⁺ (d¹) ions, A₂V₃O₉ (A = Ba, Sr) and η-Na₉V₁₄O₃₅, to account for their magnetic structures. After identifying the V(O_eq)₄ square planes containing the magnetic orbitals of the V⁴⁺ (d¹) ions, we analyzed the relative strengths of the various V–O_eq–V and V–O_eq⋯O_eq–V spin exchange interactions. The V–O_eq⋯O_eq–V spin exchange interactions mediated by O_eq–V⁵⁺–O_eq bridges are found to be crucial in determining the magnetic structures of A₂V₃O₉ (A = Ba, Sr) and η-Na₉V₁₄O₃₅. Our analysis suggests that η-Na₉V₁₄O₃₅ should have two different spin gaps.     

Réactivité de l'hypochlorite de tertiobutyle dans l'oxydation du benzothiophene

In reaction with benzothiophene, t-butyl hypochlorite acts as an oxidizing reagent and a chlorination reagent. A mixture of 3-ehlorobenzothiophene, three 2,3-dichloro-2,3-dihydrobenzothio-phene 1-oxide isomers (trans-anti: trans-syn; cis-anti) and 2-chlorobenzothiophene 1-oxide was obtained. With a large exces of t-butyl hypochlorite, the reaction leads to 2,3-dichlorobenzothio-phene, 2,3-dichlorobenzothiophene 1-oxide. 2,2,3-trichloro-2,3-(lihydrobenzothiophene 1-oxide and 2,3-dichloro-3-oxobenzothio-phene 1-oxide. In any case, oxidation stops at the level of the sulfoxide.     

Décomposition du percarbonate de O,O-t-butyle et O-isopropényle en solution: Acétonylation des esters,acides et nitriles

The Free-Radical Decomposition of O,O-t-Butyl and O-Isopropenyl Peroxycarbonate in Solution: the Acetonylation of Esters, Acides and Nitriles The free-radical decomposition of O, O-t-butyl and O-isopropenyl peroxycarbonate in substratres possessing mobile H-atoms (S? H) consists mainly in an induced chain process leading to acetonylated derivatives of the solvent. Fairly good yields are obtained but teh acetonylation of functional substrates often gives mixtures of isomers. In the case of methyl acetate, the acetonylation occurs on the C-atoms adjacent to the carbonyl (acylox moiety) and to the O-atom (alkox moiety). However, the relative amounts of the isomeric products depend on the concentration of the peroxycarbonate solutions; at lowest concentration, methyl 4-oxopentanoate (acyloxy moiety) is obtained selectively. It is assumed that the free radicals issued from the solvent are able to abstract H-atoms of other molecules of solvent before adding to the double bond of the peroxycarbonate; the more the peroxycarbonate solution is diluted the more the transfers from the C-atom adjacent to the carbonyl to the radicals adjacent to the O-atom are favoured. In the case of methyl alkanoates, H-transfers from the α-C-atoms to β-radicals of teh acyloxy moiety may account for the orientation of the process. Owing to similar H-transfer processes, the acetonylation of functional esters, of acids and nitriles is selective in most cases.