Synthesis,electrochemistry and in situ spectroelectrochemistry of a new Co(III) thio Schiff-base complex with N,N′-bis(2-aminothiophenol)-1,4-bis(carboxylidene phenoxy)butane

A new cobalt Schiff-base complex, [Co(L)(OH)(H₂O)] (where L = [N,N′-bis(2-aminothiophenol)-1,4-bis(carboxylidene phenoxy)butane), was synthesized and its electrochemical and spectroelectochemical properties were investigated using cyclic voltammetry (CV), differential pulse voltammetry (DPV) and thin-layer spectro-electrochemistry in solutions of dimethyl sulfoxide (DMSO) and dichloromethane (CH₂Cl₂). The [Co(L)(OH)(H₂O)] complex displays two well-defined reversible reduction processes with the corresponding anodic waves. The half-wave potentials of the first and second reduction processes were displayed at E_1/2 = 0.08 V and E_1/2 = −1.21 V (scan rate: 0.100 Vs⁻¹) in DMSO, and E_1/2 = −0.124 V and E_1/2 = −1.32 V (scan rate: 0.100 Vs⁻¹) in CH₂Cl₂. The potentials of the reduction processes in DMSO are shifted toward negative potentials (0.220–0.112 V) compared to those in CH₂Cl₂. The electrochemical results are assigned to two one-electron reduction processes; [Co(III)L] + ⁻e → [Co(II)L]⁻ and [Co(II)L]⁻ + ⁻e → [Co(I)L]²⁻. The six-coordination of the complex remains unchanged during the reduction processes and the electron transfer processes were not followed by a chemical reaction upon scan reversal. It was also seen that [Co(L)(OH)(H₂O)] was reduced at a more positive potential than the corresponding salen analogs. The shift and reversibility are apparently related to the high degree of electron delocalization of the [Co(L)(OH)(H₂O)] complex, having a N₂O₂S₂ donor set and two additional benzene units. Additionally, in situ spectroelectrochemical measurements support Co(III)/Co(II) and Co(II)/Co(I) reversible reduction processes with the observation of the corresponding spectral changes with the applied potentials E_app = −0.40 and −1.60 V. Application of the spectroelectrochemical results allowed the determination ofE_1/2 and n (the number of electrons) from the spectra of the fully oxidized and reduced species in one unified experiment as well. The results obtained by this method are in agreement with those by the CV and DPV methods.     

Strong antiferromagnetic coupling in doubly N,O oximato-bridged dinuclear copper(II) complexes

The use of di-2-pyridyl ketone oxime, (py)pkoH, and phenyl 2-pyridyl ketone oxime, ppkoH, in copper(II) hexafluoroacetylacetonate chemistry is reported. The reaction of CuCl₂·2H₂O with one and two equivalents of ppkoH and Na(hfac), respectively, in CH₂Cl₂ affords the dinuclear complex [Cu₂(hfac)₂(ppko)₂] (1) in excellent yield. The replacement of ppkoH by (py)pkoH gives the isostructural compound [Cu₂(hfac)₂{(py)pko}₂] (2) in good yield. The Cu^II atoms in both 1 and 2 are doubly bridged by the oximate groups of two η¹:η¹:η¹:μ₂ ppko⁻ and (py)pko⁻ ligands, respectively. The bridging Cu–(R–NO)–Cu′ units are not planar, with the torsion angles being 23.2° (1) and 20.3° (2). A bidentate chelating hfac⁻ ligand completes five-coordination at each square pyramidal metal ion. The hfac⁻-free reaction system CuCl₂·2H₂O/(py)pkoH/NEt₃ (1:2:1) gives instead the mononuclear complex [CuCl{(py)pko}{(py)pkoH}] (3) in very good yield. The Cu^II atom is coordinated by two N,N′-bidentate (py)pko⁻/(py)pkoH chelates and a monodentate chloride anion resulting in a distorted square pyramidal geometry around the metal center. Variable-temperature, solid-state dc magnetic studies were carried out on the representative dinuclear complex 1 in the 2.0–300 K range. The data indicate a very strong antiferromagnetic exchange interaction and a resulting S = 0 ground state, which is well isolated from the S = 1 excited state. The J value of −720 cm⁻¹ was derived from the fitting of the experimental data using the Hamiltonian H = −J(S₁ · S₂).     

Syntheses,structures and hydrolytic properties of copper(II) complexes of asymmetrically N-functionalised 1,4,7-triazacyclononane ligands

A series of new asymmetrically N-substituted derivatives of the 1,4,7-triazacyclononane (tacn) macrocycle have been prepared from the common precursor 1,4,7-triazatricyclo[5.2.1.0^4,10]decane: 1-ethyl-4-isopropyl-1,4,7-triazacyclononane (L1), 1-isopropyl-4-propyl-1,4,7-triazacyclononane (L2), 1-(3-aminopropyl)-4-benzyl-7-isopropyl-1,4,7-triazacyclononane (L3), 1-benzyl-4-isopropyl-1,4,7-triazacyclononane (L4) and 1,4-bis(3-aminopropyl)-7-isopropyl-1,4,7-triazacyclononane (L5). The corresponding monomeric copper(II) complexes were synthesised and were found to be of composition: [Cu(L1)Cl₂] · 1/2 H₂O (C1), [Cu(L4)Cl₂] · 4H₂O (C2), [Cu(L3)(MeCN)](ClO₄)₂ (C3), [Cu(L5)](ClO₄)₂ · MeCN · NaClO₄ (C4) and [Cu(L2)Cl₂] · 1/2 H₂O (C5). The X-ray crystal structures of each complex revealed a distorted square-pyramidal copper(II) geometry, with the nitrogen donors on the ligands occupying 3 (C1 and C2), 4 (C3) or 5 (C4) coordination sites on the Cu(II) centre. The metal complexes were tested for the ability to hydrolytically cleave phosphate esters at near physiological conditions, using the model phosphodiester, bis(p-nitrophenyl)phosphate (BNPP). The observed rate constants for BNPP cleavage followed the order k_C1 ≈ k_C2 > k_C5 ? k_C3 > k_C4, confirming that tacn-type Cu(II) complexes efficiently accelerate phosphate ester hydrolysis by being able to bind phosphate esters and also form the nucleophile necessary to carry out intramolecular cleavage. Complexes C1 and C2, featuring asymmetrically disubstituted ligands, exhibited rate constants of the same order of magnitude as those reported for the Cu(II) complexes of symmetrically tri-N-alkylated tacn ligands (k ∼ 1.5 × 10⁻⁵ s⁻¹).     

Kinetics of the CH3 + HCl/DCl → CH4/CH3D + Cl and CD3 + HCl/DCl → CD3H/CD4 + Cl reactions: An experimental H atom tunneling investigation

The kinetics of the radical reactions of CH₃ with HCl or DCl and CD₃ with HCl or DCl have been investigated in a temperature controlled tubular reactor coupled to a photoionization mass spectrometer. The CH₃ (or CD₃) radical, R, was produced homogeneously in the reactor by a pulsed 193 nm exciplex laser photolysis of CH₃COCH₃ (or CD₃COCD₃). The decay of CH₃/CD₃ was monitored as a function of HCl/DCl concentration under pseudo-first-order conditions to determine the rate constants as a function of temperature, typically from 188 to 500 K. The rate constants of the CH₃ and CD₃ reactions with HCl had strong non-Arrhenius behavior at low temperatures. The rate constants were fitted to a modified Arrhenius expression k = QA exp (−E_a/RT) (error limits stated are 1σ + Students t values, units in cm³ molecule⁻¹ s⁻¹): k(CH₃ + HCl) = [1.004 + 85.64 exp (−0.02438 × T/K)] × (3.3 ± 1.3) × 10⁻¹³ exp [−(4.8 ± 0.6) kJ mol⁻¹/RT] and k(CD₃ + HCl) = [1.002 + 73.31 exp (−0.02505 × T/K)] × (2.7 ± 1.2) × 10⁻¹³ exp [−(3.5 ± 0.5) kJ mol⁻¹/RT]. The radical reactions with DCl were studied separately over a wide ranges of temperatures and in these temperature ranges the rate constants determined were fitted to a conventional Arrhenius expression k = A exp (−E_a/RT) (error limits stated are 1σ + Students t values, units in cm³ molecule⁻¹ s⁻¹): k(CH₃ + DCl) = (2.4 ± 1.6) × 10⁻¹³ exp [−(7.8 ± 1.4) kJ mol⁻¹/RT] and k(CD₃ + DCl) = (1.2 ± 0.4) × 10⁻¹³ exp [−(5.2 ± 0.2) kJ mol⁻¹/RT] cm³ molecule⁻¹ s⁻¹.     

Ruthenium-catalysed asymmetric transfer hydrogenation of N-(tert-butanesulfinyl)imines

The ruthenium complex prepared from [RuCl₂(p-cymene)]₂ and (1S,2R)-1-amino-2-indanol is a very efficient catalyst for the asymmetric transfer hydrogenation of (R)-N-(tert-butanesulfinyl)ketimines in isopropanol. By carefully removing all possible moisture from the reaction medium, chiral primary amines with very high optical purities (up to >99% ee) can be easily prepared in excellent yields by the diastereoselective reduction of the imines followed by removal of the sulfinyl group under mild acidic conditions. Reaction times of 1-4 h were needed to complete the reduction reactions when they were performed at 40 °C.     

Conformation and coordination of β-diketonates in mixed-ligand copper(II) chelates of 1,2-diamines. Theoretical approach and experimental verification. Crystal structure of [bis(N,N′-dimethyl-1,2-diaminoethane) bis(3-cyano-2,4-pentanedionate)copper(II)]dihydrate

The reaction of the bis(2,4-pentanedionato)copper(II), Cu(acac)₂, and its substituted derivatives (Cu(NC-acac)₂, Cu(O₂N-acac)₂, and Cu(tfac)₂), with 1,2-diaminoethane (en), 1,2-diaminopropane (pn) and certain N-substituted derivatives of 1,2-diaminoethane, enR, is reported. The reaction products were found to depend on the reaction conditions, the extent of N-substitution of the diamine and the nature of the β-diketonate anion, β⁻. The [Cu(enR)β₂] addition compounds are not always sufficiently stable and in most instances the tetragonal species [Cu(enR)₂β₂] prevail as the final product. The 1,2-diamine molecules in [Cu(enR)₂β₂] form chelate rings attaining the gauche conformation while the β-diketonato anions essentially confer electrical neutrality. Density functional theory (DFT) calculations suggest that the active sites of the β-diketonato anion and its conformation depend on their ability to create hydrogen bonds and on the substituents in 3-position. The Fukui indices of chemical reactivity favor the carbonyl oxygens as binding sites of most anions while in the case of the 3-cyano-2,4-pentanedionato anion, NC-acac⁻, bonding through the cyano nitrogen is envisaged with the S(E, Z) conformer having the lowest energy. These findings are in accord with experimental data and further proof is given by single crystal X-ray diffraction analysis of the structure of [Cu(MeenMe)₂(NC-acac)₂] · 2H₂O (MeenMe denoting N,N′-dimethyl-1,2-diaminoethane). In this compound the NC-acac⁻ attains the S(E, Z) conformation and interacts with the square-planar entity [Cu(MeenMe)₂]²⁺ through the cyano nitrogen, while intermolecular hydrogen bonding involving the water molecules leads to supramolecular structure.     

Radical polymerization behavior of 3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate in water

The homogeneous polymerization of 3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate (MDAPS) with potassium peroxydisulfate (KPS) was kinetically in situ investigated in water by means of FT-near IR spectroscopy. The overall activation energy of the polymerization was calculated to be 16.0 kcal/mol. The initial polymerization rate (R_p) at 40 °C was expressed by R_p=k[KPS]^0.65[MDAPS]^1.0. The presence of alkaline metal salts was observed to accelerate the polymerization. The order of acceleration at 40 °C was CsCl > KCl > NaCl > LiCl when the chloride salts were used. NaCl showed higher acceleration effect than NaF. NaBr and NaI exhibited retardation and inhibition effect, respectively, because of reduction of KPS and its primary radical with bromide and iodide ions. The polymerization of MDAPS with KPS in water in the presence of NaCl at 2.0 mol/l gave R_p=k[KPS]^0.70[MDAPS]^1.4 at 40 °C. The overall activation energy of the polymerization in the presence of NaCl was estimated to be 11.6 kcal/mol being considerably lower value compared with that in its absence. The syndiotacticity of poly(MDAPS) tended to increase with rising temperature and decrease in the presence of NaCl.     

Calorimetric investigation of the effect of hydroxyanthraquinones in Rheum officinale Baill on Staphylococcus aureus growth

The inhibitory effects of five hydroxyanthraquinones (HAQs) from root and rhizoma of Rheum officinale Baill, a traditional Chinese medicinal (TCM) herb, on Staphylococcus aureus growth were investigated by calorimetry. The power-time curves of S.aureus with and without HAQ were acquired and the extent and duration of inhibitory effects on the metabolism evaluated by growth rate constants (k₁, k₂), half inhibitory ratio (IC₅₀), maximum heat output (P_max) and peak time (t_p). The value of k₁ and k₂ of S. aureus in the presence of the five HAQs decreased with the increasing concentrations of HAQs. Moreover, P_max was reduced and the value of t_p increased with increasing concentrations of the five drugs. The inhibitory activity varied for different drugs. IC₅₀ of the five HAQs was 4 μg ml⁻¹ for emodin, 3.5 μg ml⁻¹ for rhein, 10 μg ml⁻¹ for aloe-emodin, 1000 μg ml⁻¹ for chrysophanol, 1600 μg ml⁻¹ for physcion. The sequence of antimicrobial activity of the five HAQs: rhein > emodin > aloe-emodin > chrysophanol > physicion.     

Synthesis and characterization of fluorous (S)- and (R)-1-phenylethylamines that effect heat facilitated resolution of (±)-2-(8-carboxy-1-naphthylsulfinyl)benzoic acid via diastereomeric salt formation and study of their circular dichroism

Perfluoroalkyl- or nonafluoro-tert-butoxy-alkyl-substituted enantiopure amines having the structure PhCHCH₃(NR¹R²) [R¹ = H, CH₃; R² = (CH₂)₃C₈F₁₇, (CH₂)₂OC(CF₃)₃; R¹ = R² = (CH₂)₃C₈F₁₇, (CH₂)₂OC(CF₃)₃] are obtained in high yields, when (S)-(−)-1-phenylethylamine is reacted with readily accessible alkylating reagents or fluorous 2° amines (R¹ = H; R² = (CH₂)₃C₈F₁₇, (CH₂)₂OC(CF₃)₃) are methylated in a Leuckart-Wallach reaction. The solubility patterns of these novel chiral amines and their hydrochlorides are qualitatively described for a broad spectrum of solvents and the fluorous partition coefficients of the free bases are determined by GC. A novel method for the resolution of enantiomers is disclosed here, which involves the use a half-equivalent of the selected resolving agent in solvent water that displays low solubility for the crystalline diastereomeric salt(s) formed even at temperatures near to its boiling point. Compound (S)-(−)-PhCHCH₃[NH(CH₂)₃C₈F₁₇] is found to satisfy all the latter conditions and successfully used for the heat facilitated resolution of the title racemic acid. The circular dichroism (CD) spectra of six novel fluorous (S)-(−)-1-phenylethylamine derivatives are measured in ethanol, trifluoroethanol and hexafluoropropan-2-ol and discussed in detail.     

On the α-lithiation-rearrangement of N-toluensulfonyl aziridines: mechanistic and synthetic aspects

A detailed study of the rearrangement of five cycloalkene N-toluenesulfonyl (tosyl) aziridines using sec-butyllithium (with and without added ligands such as (−)-sparteine and TMEDA) has been carried out. Allylic sulfonamides were the main products from the cyclopentene and cyclohexene aziridines whereas bicyclic sulfonamides were obtained from the cycloheptene and cyclooctene aziridines. In most cases, p-toluenesulfonamide (TsNH₂) was produced as a by-product and a mechanistic explanation for its formation is forwarded. These reactions are believed to involve α-lithiation to a lithiated aziridine which can then partition through two pathways: (i) rearrangement to allylic or bicyclic sulfonamides via C-H insertion reactions or (ii) reductive alkylation to alkenes via attack by sec-butyllithium and subsequent elimination of TsNH₂. In the (−)-sparteine reactions, the products were generated with 38-66% ee and the sense of asymmetric induction involved lithiation of the S-aziridine stereocentre. This is opposite to that observed with epoxides.     

Complexes derived from the general copper(II)/maleamic acid/N,N′,N′′-chelate reaction systems: Synthetic,reactivity, structural and spectroscopic studies

The synthetic investigation of the Cu^II/maleamate(−1) ion (HL⁻)/N,N′,N′′-chelate general reaction system has allowed access to compounds [Cu₂(HL)₂(bppy)₂](ClO₄)₂·H₂O (1·H₂O), [Cu(HL)(bppy)(ClO₄)] (2) and [Cu(HL)(terpy)(H₂O)](ClO₄) (4) (bppy = 2,6-bis(pyrazol-1-yl)pyridine, terpy = 2,2′;6′,2′′-terpyridine). In the absence of externally added hydroxides, compound [Cu₂(L′)₂(bppy)₂](ClO₄)₂ (3) was obtained from MeOH solutions; L′⁻ is the monomethyl maleate(−1) ligand which is formed in situ via the Cu^II-assisted HL⁻ → L′⁻ transformation. In the case of tptz-containing (tptz = 2,4,6-tris(2-pyridyl)-1,3,5-triazine) reaction systems, the Cu^II-assisted hydrolysis of tptz to pyridine-2-carboxamide (L1) afforded complex [Cu(L1)₂(NO₃)₂] (5). The crystal structures of 1–5 are stabilized by intermolecular hydrogen bonding and π–π stacking interactions. Characteristic IR bands of the complexes are discussed in terms of the known structures and the coordination modes of the ligands.     

Stereospecific preparation of symmetrical (1Z, 3Z)- and (1E, 3E)-2,3-difluoro-1,4-disubstituted-buta-1,3-dienes from 1-bromo-1-fluoroalkenes

A straightforward method to prepare symmetrical (1Z, 3Z)- and (1E, 3E)-2,3-difluoro-1,4-disubstituted-buta-1,3-dienes is described. High E/Z ratio 1-bromo-1-fluoroalkenes, prepared by isomerization from the E/Z ≈ 1:1 isomeric mixtures, reacted with Bu₃SnSnBu₃ and Pd(PPh₃)₄ to afford (1Z, 3Z)-2,3-difluoro-1,4-disubstituted-buta-1,3-dienes in good yield. (Z)-1-Bromo-1-fluoroalkenes, which were prepared by kinetic reduction from 1-bromo-1-fluoroalkenes (E/Z ≈ 1:1), can undergo similar reaction with Bu₃SnSnBu₃ and Pd(PPh₃)₄/CuI to prepare (1E, 3E)-2,3-difluoro-1,4-disubstituted-buta-1,3-dienes.     

Hydridic reactivity of W(CO)(H)(NO)(PMe3)3 - Dihydrogen bonding and H2 formation with protic donors

The hydridic reactivity of the complex W(CO)(H)(NO)(PMe₃)₃ (1) was investigated applying a variety of protic donors. Formation of organyloxide complexes W(CO)(NO)(PMe₃)₃(OR) (R = C₆H₅ (2), 3,4,5-Me₃C₆H₂ (3), CF₃CH₂ (4), C₆H₅CH₂ (5), Me (6) and iPr (7)) and H₂ evolution was observed. The reactions of 1 accelerated with increasing acidity of the protic donor: Me₂CHOH (pK_a = 17) < MeOH (pK_a = 15.5) < C₆H₅CH₂OH (pK_a = 15) < CF₃CH₂OH (pK_a = 12.4) < C₆H₂Me₃OH (pK_a = 10.6) < C₆H₅OH (pK_a = 10).Regioselective hydrogen bonding of 1 was probed with two of the protic donors furnishing equilibrium formation of the dihydrogen bonded complexes ROH···HW(CO)(NO)(PMe₃)₃ (R = 3,4,5-Me₃C₆H_2,3a and iPr, 7a) and the O_NO hydrogen bonded species ROH···ONW(CO)(H)(PMe₃)₃ (R = C₆H₂Me_3,3b and iPr, 7b) which were studied in hexane and d₈-toluene solutions using variable temperature IR and NMR spectroscopy. Quantitative IR experiments at low temperatures using 3,4,5-trimethylphenol (TMP) confirmed the two types of competitive equilibria: dihydrogen bonding to give 3a (ΔH₁ = −5.8 ± 0.4 kcal/mol and ΔS₁ = −15.3 ± 1.4 e.u.) and hydrogen bonding to give 3b (ΔH₂ = −2.8 ± 0.1 kcal/mol and ΔS₂ = −5.8 ± 0.3 e.u.). Additional data for the hydrogen bonded complexes 3a,b and 7a,b were determined via NMR titrations in d₈-toluene from the equilibrium constants K_(Δδ) and K_(ΔR1) measuring either changes in the chemical shifts of H_W(_Δδ) or the excess relaxation rates of H_W (ΔR₁) (3a,b: ΔH_(Δδ) = −0.8 ± 0.1 kcal/mol; ΔS_(Δδ) = −1.4 ± 0.3 e.u. and ΔH_(ΔR1) = −5.8 ± 0.4 kcal/mol; ΔS_(ΔR1) = −22.9 ± 1.9 e.u) (7a,b: ΔH_(Δδ) = −2.3 ± 0.2 kcal/mol; ΔS_(Δδ) = −11.7 ± 0.9 e.u. and ΔH_(ΔR1) = −2.9 ± 0.2 kcal/mol; ΔS_(ΔR1) = −14.6 ± 1.0 e.u). Dihydrogen bonding distances of 1.9 Å and 2.1 Å were derived for 3a and 7a from the NMR excess relaxation rate measurements of H_W in d₈-toluene. An X-ray diffraction study was carried out on compound 2.     

Stereoselective synthesis of (E)-N,N-diethyl-3-(imidazo[1,2-a]pyrid-2-yl)-2-(phenylsulfonyl)propenamide

The title compound, gem-amidovinylsulfone 3, was synthesized stereoselectively by aldolic condensation of N,N-diethylphenylsulfonylacetamide 1 on imidazo[1,2-a]pyridine-2-carbaldehyde 2 adding Et₃N at the end. The X-ray crystal structure of 3 [C₂₀H₂₁N₃O₃S: M_r=383.5, monoclinic, P2₁, a=8.191(4) Å, b=21.132(2) Å, c=11.752(1) Å, β=96.40(2)°, V=2022(1) Å³, Z=4 (two molecules per asymmetric unit), D_calc=1.260 g cm⁻³, λ(Mo Kα)=0.71073 Å, μ=0.184 mm⁻¹, F(000)=808, T=293(2)K, R=0.059 for 5105 observed reflections with I≥2σ(I)] was determined, and confirmed the (E) configuration.     

New ternary rare-earth metal boride carbides R15B4C14 (R=Y, Gd-Lu) containing BC2 units: Crystal and electronic structures, magnetic properties

The ternary rare-earth boride carbides R₁₅B₄C₁₄ (R=Y, Gd-Lu) were prepared from the elements by arc-melting followed by annealing in silica tubes at 1270 K for 1 month. The crystal structures of Tb₁₅B₄C₁₄ and Er₁₅B₄C₁₄ were determined from single crystal X-ray diffraction data. They crystallize in a new structure type in space group P4/mnc (Tb₁₅B₄C₁₄: a=8.1251(5) Å, c=15.861(1) Å, Z=2, R₁=0.041 (wR₂=0.088) for 1023 reflections with I_o>2σ(I_o); Er₁₅B₄C₁₄: a=7.932(1) Å, c=15.685(2) Å, Z=2, R₁=0.037 (wR₂=0.094) for 1022 reflections with I_o>2σ(I_o)). The crystal structure contains discrete carbon atoms and bent CBC units in octahedra and distorted bicapped square antiprisms, respectively. In both structures the same type of disorder exists. One R atom position needs to be refined as split atom position with a ratio 9:1 indicative of a 10% substitution of the neighboring C⁴⁻ by C₂⁴⁻. The actual composition has then to be described as R₁₅B₄C_14.2. The isoelectronic substitution does not change the electron partition of R₁₅B₄C₁₄ which can be written as (R³⁺)₁₅(C⁴⁻)₆(CBC⁵⁻)₄•e⁻. The electronic structure was studied with the extended Hückel method. The investigated compounds Tb₁₅B₄C₁₄, Dy₁₅B₄C₁₄ and Er₁₅B₄C₁₄ are hard ferromagnets with Curie temperatures T_C=145, 120 and 50 K, respectively. The coercive field B_C=3.15 T for Dy₁₅B₄C₁₄ is quite remarkable.     

Halogenation of ketones with N-halosuccinimides under solvent-free reaction conditions

Several aryl substituted ketones, cyclic ketones, 1,3-diketones and a β-ketoamide were halogenated with N-halosuccinimides under solvent-free reaction conditions (SFRC) at various temperatures (20-80 °C), whereas less enolized ketones required the presence of an acid catalyst (p-toluenesulfonic acid, PTSA). Bromination of substituted acetophenones obeys first order kinetics v=k_Br[ketone] and the following correlation with the keto-enol equilibrium constant: log k_Br=0.3pK_E+C₁, less enolized substrates being more reactive; the moderate positive charge developed in the rate determining step was confirmed by the Hammett correlation (ρ=−0.5). On the other hand, in cyclic ketones an opposite relation was observed: log k_Br=−0.6pK_E+C₂, indicating higher reactivity of substrates with higher enolization constant (K_E). The important role of the nature of the solvent (MeCN, MeOH) in preorganization of the ketone-NBS-PTSA mixture prior to SFRC bromination was found.     

The iron potassium diarsenate KFeAs2O7 structural, electric and magnetic behaviors

Crystals of a new potassium iron (III) diarsenate (KFeAs₂O₇) have been grown and characterized by single crystal X-ray diffraction. It crystallizes in the triclinic space group , with a=7.662(1) Å, b=8.402(2) Å, c=10.100(3) Å, α=90.42(3)°, β=89.74(2)°, γ=106.39(2)°, V=623.8(3) Å³ and Z=4. The final agreement factors are R=0.0342, wR=0.0889, S(F²)=1.01; the structural model is validated by bond valence sum (BVS) and charge distribution (CD) methods. The structure consists of corner-sharing FeO₆ octahedra and As₂O₇ diarsenate groups, the three-dimensional framework delimits tunnels running along [0 1 0] direction where the potassium ions reside. The crystal structure of the title compound is different from that of the monoclinic KAlP₂O₇ type but structural relationships exist between the frameworks. Impedance measurements (frequency/temperature ranges: 5-13,000 Hz/526-668 K) show KFeAs₂O₇ an ionic conductor being the conductivity 2.76×10⁻⁷ S cm⁻¹ at 568 K and E_a is 0.47 eV. The BVS model suggests that the most probable potassium conduction pathway is along b-direction. Magnetic measurements reveal the Curie—Weiss type paramagnetic behavior over the range 30-300 K and ferromagnetic below 29.3 K.     

Spectroscopic and structural characterization of Na2[(VO)2(ttha)]·8H2O (ttha = triethylenetetraamine-N,N,N′,N″,N′″,N′″-hexaacetate): Interpreting the results with density functional theory

Na₂[(V^IVO)₂(ttha)]·8 H₂O (ttha = triethylenetetraamine–N,N,N′,N″,N′″,N′″–hexaacetate ion), prepared by treating [VO(H₂O)₅][(VO)₂(ttha)]·4 H₂O with Na₆(ttha), has been characterized by single crystal X-ray diffraction, infrared spectroscopy, UV–Vis absorption spectroscopy, electron spin resonance spectroscopy, and modeled by density functional theory (DFT). The X-ray structure revealed a distorted octahedral geometry around each vanadium center. The electronic absorption spectrum of [(VO)₂(ttha)]²⁻ (aq) features absorptions at ca. 200 nm (ε > 13900 L mol⁻¹ cm⁻¹), 255 nm (ε = 3480 L mol⁻¹ cm⁻¹), 586 nm (ε = 33 L mol⁻¹ cm⁻¹), and 770 nm (ε = 38 L mol⁻¹ cm⁻¹). The time-dependent density functional theory (TDDFT) calculated electronic absorption spectrum was remarkably similar to the actual spectrum, and TDDFT predicts absorption peaks at 297, 330, 458, 656, and 798 nm. TDDFT assigned the peak at 798 nm to be the α spin HOMO → LUMO transition. Hence, the peak at 770 nm in the actual spectrum is most likely the α spin HOMO → LUMO transition. Moreover, the TDDFT calculations revealed that the α spin HOMO and LUMO are partly comprised of d orbitals on both vanadium centers, and the first derivative electron spin resonance spectrum also suggests that the two unpaired electrons in [(VO)₂(ttha)]²⁻ are localized near the vanadium centers.     

New one-dimensional uranyl and neptunyl iodates: crystal structures of K3[(UO2)2(IO3)6](IO3)·H2O and K[NpO2(IO3)3]·1.5H2O

The uranyl and neptunyl(VI) iodates, K₃[(UO₂)₂(IO₃)₆](IO₃)·H₂O (1) and K[NpO₂(IO₃)₃]·1.5H₂O (2), have been prepared and crystallized under mild hydrothermal conditions. The structures of 1 and 2 both contain one-dimensional _∞¹[AnO₂(IO₃)₃]¹⁻(An=U,Np) ribbons that consist of approximately linear actinyl(VI) cations bound by iodate anions to yield AnO₇ pentagonal bipyramids. The AnO₇ units are linked by bridging iodate anions to yield chains that are in turn coupled by additional iodate anions to yield ribbons. The edges of the ribbons are terminated by monodentate iodate anions. For 1 and 2, K⁺ cations and water molecules separate the ribbons from one another. In addition, isolated iodate anions are also found between _∞¹[UO₂(IO₃)₃]¹⁻ ribbons in 1. In order to aid in the assignment of oxidation states in neptunyl containing compounds, a bond-valence sum parameter of 2.018 Å for Np(VI) bound exclusively to oxygen has been developed with b=0.37 Å. Crystallographic data (193 K, MoKα, λ=0.71073): 1, triclinic, , a=7.0609(4) Å, b=14.5686(8)  Å, c=14.7047(8)  Å, α=119.547(1)°, β=95.256(1)°, γ=93.206(1)°, Z=2, R(F)=2.49% for 353 parameters with 6414 reflections with I>2σ(I); (203 K, MoKα, λ=0.71073): 2, monoclinic, P2₁/c, a=7.796(4)  Å, b=7.151(3)  Å, c=21.79(1)  Å, β=97.399(7)°, Z=4, R(F)=6.33% for 183 parameters with 2451 reflections with I>2σ(I).     

Synthesis, characterization and Schlenk equilibrium studies of methylmagnesium compounds with O- and N-donor ligands - the unexpected behavior of [MgMeBr(pmdta)] (pmdta = N,N,N′,N″,N″-pentamethyldiethylenetriamine)

MgMe₂ (1) was found to react with 1,4-diazabicyclo[2.2.2]octane (dabco) in tetrahydrofuran (thf) yielding a binuclear complex [{MgMe₂(thf)}₂(μ-dabco)] (2). Furthermore, from reactions of MgMeBr with diglyme (diethylene glycol dimethyl ether), NEt₃, and tmeda (N,N,N′,N′-tetramethylethylenediamine) in etheral solvents compounds MgMeBr(L), (L = diglyme (5); NEt₃ (6); tmeda (7)) were obtained as highly air- and moisture-sensitive white powders. From a thf solution of 7 crystals of [MgMeBr(thf)(tmeda)] (8) were obtained. Reactions of MgMeBr with pmdta (N,N,N′,N″,N″-pentamethyldiethylenetriamine) in thf resulted in formation of [MgMeBr(pmdta)] (9) in nearly quantitative yield. On the other hand, the same reaction in diethyl ether gave MgMeBr(pmdta) · MgBr₂(pmdta) (10) and [{MgMe₂(pmdta)}₇{MgMeBr(pmdta)}] (11) in 24% and 2% yield, respectively, as well as [MgMe₂(pmdta)] (12) as colorless needle-like crystals in about 26% yield. The synthesized methylmagnesium compounds were characterized by microanalysis and ¹H and ¹³C NMR spectroscopy. The coordination-induced shifts of the ¹H and ¹³C nuclei of the ligands are small; the largest ones were found in the tmeda and pmdta complexes. Single-crystal X-ray diffraction analyses revealed in 2 a tetrahedral environment of the Mg atoms with a bridging dabco ligand and in 8 a trigonal-bipyramidal coordination of the Mg atom. The single-crystal X-ray diffraction analyses of [MgMe₂(pmdta)] (12) and [MgBr₂(pmdta)] (13) showed them to be monomeric with five-coordinate Mg atoms. The square-pyramidal coordination polyhedra are built up of three N and two C atoms in 12 and three N and two Br atoms in 13. The apical positions are occupied by methyl and bromo ligands, respectively. Temperature-dependent ¹H NMR spectroscopic measurements (from 27 to −80 °C) of methylmagnesium bromide complexes MgMeBr(L) (L = thf (4); diglyme (5); NEt₃ (6); tmeda (7)) in thf-d₈ solutions indicated that the deeper the temperature the more the Schlenk equilibria are shifted to the dimethylmagnesium/dibromomagnesium species. Furthermore, at −80 °C the dimethylmagnesium compounds are predominant in the solutions of Grignard compounds 4-6 whereas in the case of the tmeda complex7 the equilibrium constant was roughly estimated to be 0.25. In contrast, [MgMeBr(pmdta)] (9) in thf-d₈ revealed no dismutation into [MgMe₂(pmdta)] (12) and [MgBr₂(pmdta)] (13) even up to −100 °C. In accordance with this unexpected behavior, 1:1 mixtures of 12 and 13 were found to react in thf at room temperature yielding quantitatively the corresponding Grignard compound 9. Moreover, the structures of [MgMeBr(pmdta)] (9c), [MgMe₂(pmdta)] (12c), and [MgBr₂(pmdta)] (13c) were calculated on the DFT level of theory. The calculated structures 12c and 13c are in a good agreement with the experimentally observed structures 12 and 13. The equilibrium constant of the Schlenk equilibrium (2 9c ? 12c + 13c) was calculated to be K_gas = 2.0 × 10⁻³ (298 K) in the gas phase. Considering the solvent effects of both thf and diethyl ether using a polarized continuum model (PCM) the corresponding equilibrium constants were calculated to be K_thf = 1.2 × 10⁻³ and K_ether = 3.2 × 10⁻³ (298 K), respectively.