Donor atom dependent geometric isomers in mononuclear oxo-molybdenum(V) complexes: implications for coordinated endogenous ligation in molybdoenzymes

We have previously demonstrated that the complex [(L1O)MoOCl(2)], where L1OH = (2-hydroxy-3-tert-butyl-5-methylphenyl)bis(3,5-dimethylpyrazolyl)methane, exists as both cis and trans isomers (Kail, B.; Nemykin, V. N.; Davie, S. R.; Carrano, C. J.; Hammes, B. S.; Basu, P. Inorg. Chem. 2002, 41, 1281-1291). Here, the cis isomer is defined as the geometry with the heteroatom in the equatorial position, and the trans isomer is designated as the geometry with the heteroatom positioned trans to the terminal oxo group. The trans isomer represents the thermodynamically more stable geometry as indicated by its spontaneous formation from the cis isomer. In this report, we show that for complexes of [(LO)MoOCl(2)], where LOH is the sterically less restrictive (2-hydroxyphenyl)bis(3,5-dimethylpyrazolyl)methane, only the trans isomer could be isolated, while in the corresponding thiolate containing ligand (2-dimethylethanethiol)bis(3,5-dimethylpyrazolyl)methane (L3SH) only the cis isomer could be observed. In addition, we have isolated and structurally characterized the complex [(L1O)MoO(OPh)(Cl)], a rare example of a species possessing both cis and trans phenolates. Using DFT calculations, we have investigated the origins of the differences in stability between the cis and trans isomers in these complexes and suggest that they are related to the trans influence of the oxo-group. Crystal data for [(LO)MoOCl(2)] (1) include that it crystallizes in the triclinic space group P(-)1 with cell dimensions a = 8.9607 (12) A, b = 10.596 (4) A, c = 13.2998 (13) A, alpha = 98.03 (2) degrees, beta = 103.21 (2) degrees, gamma = 110.05(2) degrees, and Z = 2. [(L1O)MoO(OPh)Cl].2CH(2)Cl(2) (2.2CH(2)Cl(2)) crystallizes in the triclinic space group P(-)1 with cell dimensions a = 12.2740 (5) A, b = 13.0403 (5) A, c = 13.6141 (6) A, alpha = 65.799 (2) degrees, beta = 64.487 (2) degrees, gamma = 65.750 (2) degrees, and Z = 2. [(L3S)Mo(O)Cl(2)] (3) crystallizes in the orthorhombic space group Pna2(1), with cell dimensions a = 13.2213 (13) A, b = 8.817 (2) A, c = 15.649 (4) A, and Z = 4. The implications of these results on the function of mononuclear molybdoenzymes such as sulfite oxidase, and the DMSO reductase, are discussed.     

Hydrogen activation by high-valent oxo-molybdenum(VI) and -rhenium(VII) and -(V) compounds

The high-valent oxo-molybdenum(VI) and -rhenium(VII) and -(V) derivatives MoO2Cl2, ReCH3O3 (MTO) and ReIO2(PPh3)2 catalyze the selective hydrogenation of alkynes to alkenes at 80 degrees C under 40 atm of pressure. The reduction of sulfoxides to sulfides has also been performed by oxo-rhenium and -molybdenum complexes using hydrogen as a reducing agent. Activation of hydrogen by MoO2Cl2 and MoO2(S2CNEt2)2 was shown by means of DFT calculations to proceed by H-H addition to the Mo=O bond, followed by hydride migration to yield a water complex.     

Solvent effects on the electrode kinetics of simple redox couples: Investigations on the electrochemical behaviour of the V(III)/V(II) system in water + acetonitrile mixtures

Parallel optical and electrochemical studies on the V(III)/V(II) system in H₂O + acetonitrile (AN) + CF₃SO₃H mixtures have been performed. It was found, on the basis of the spectra of vanadium ions in the visible range, that V(III) was totally hydrated in mixtures up to x_AN ⋍ 0.6 while V(II) was specifically solvated by AN molecules, even at a molar fraction of acetonitrile in H₂O + AN mixtures as low as 0.02. In agreement with this, the formal potentials of the V(III)/V(II) system expressed versus the ferrocene electrode move to less negative potentials with an increase in AN concentration.Straightforward correlations of the electrode kinetics of the V(III)/V(II) system at a mercury electrode in H₂O + AN mixtures with both the electrode surface coverage by AN molecules and the resolvation of vanadium ions in the bulk solution were found.     

Solvent effects on oxygen atom transfer reaction between manganese(V)-oxo corrole and alkene

Pseudo-first order reaction rate constants of 5,10,15-tris（pentafluorophenyl）corrole Mn（V）-oxo (F₁₅CMn（V）-oxo),5,15-bis（pentafluorophenyl）-10-（phenyl）corrole Mn（V）-oxo(F₁₀CMn（V）-oxo),5,15- bis（phenyl）-10-（pentafluorophenyl）corrole Mn（V）-oxo（F₅CMn（V）-oxo） and 5,10,15-tris（phenyl）corrole Mn（V）-oxo（F₀CMn（V）-oxo） with a series of alkene substrates in different solvents were determined by UV-vis spectroscopy.The results indicated that the oxygen atom transfer pathway between Mn（V）-oxo corrole and alkene is solvent-dependent.     

Solvent reorganization of electron transitions in viscous solvents

We develop a model of electron transfer reactions at conditions of nonergodicity when the time of solvent relaxation crosses the observation time window set up by the reaction rate. Solvent reorganization energy of intramolecular electron transfer in a charge-transfer molecule dissolved in water and acetonitrile is studied by molecular dynamics simulations at varying temperatures. We observe a sharp decrease of the reorganization energy at a temperature identified as the temperature of structural arrest due to cage effect, as discussed by the mode-coupling theory. This temperature also marks the onset of the enhancement of translational diffusion relative to rotational relaxation signaling the breakdown of the Stokes-Einstein relation. The change in the reorganization energy at the transition temperature reflects the dynamical arrest of the slow, collective relaxation of the solvent related to the relaxation of the solvent dipolar polarization. An analytical theory proposed to describe this effect agrees well with both the simulations and experimental Stokes shift data. The theory is applied to the analysis of charge-transfer kinetics in a low-temperature glass former. We show that the reorganization energy is substantially lower than its equilibrium value for the low-temperature portion of the data. The theory predicts the possibility of discontinuous changes in the dependence of the electron transfer rate on the free energy gap when the reaction switches between ergodic and nonergodic regimes.     

Solvent reorganization energy of electron-transfer reactions in polar solvents

A microscopic theory of solvent reorganization energy in polar molecular solvents is developed. The theory represents the solvent response as a combination of the density and polarization fluctuations of the solvent given in terms of the density and polarization structure factors. A fully analytical formulation of the theory is provided for a solute of arbitrary shape with an arbitrary distribution of charge. A good agreement between the analytical procedure and the results of Monte Carlo simulations of model systems is achieved. The reorganization energy splits into the contributions from density fluctuations and polarization fluctuations. The polarization part is dominated by longitudinal polarization response. The density part is inversely proportional to temperature. The dependence of the solvent reorganization energy on the solvent dipole moment and refractive index is discussed.     

Solvent reorganization entropy of electron transfer in polar solvents

We report the results of molecular dynamics simulations of the solvent reorganization energy of intramolecular electron transfer in a charge-transfer molecule dissolved in water and acetonitrile at varying temperatures. The simulations confirm the prediction of microscopic solvation theories of a positive reorganization entropy in polar solvents. The results of simulations are analyzed in terms of the splitting of the reorganization entropy into the contributions from the solute-solvent interaction and from the alteration of the solvent structure induced by the solute. These two contributions mutually cancel each other, resulting in the reorganization entropy amounting to only a fraction of each component.     

Solvent effects on the optical rotation of (S)-(-)-alpha-methylbenzylamine

The optical rotation of (S)-(-)-alpha-methylbenzylamine at 589 nm has been measured in 39 different solvents at five different concentrations: 0.25, 0.50, 1.00, 2.00, and 3.00 M. A correlation of the intrinsic rotations (i.e., extrapolation of specific rotations to zero concentration) with Kamlet's and Taft's solvent parameters (alpha, beta, and pi) is established. The polarity/polarizability, pi, and solvent acidity, alpha, terms are found to have a greater effect upon the optical rotation than the basicity of the solvent, beta. The specific rotation for (S)-(-)-alpha-methylbenzylamine has been calculated with Gaussian03 using a PCM model (B3LYP aug-cc-pVDZ) for all 39 solvated systems. Comparisons between the experimental and calculated values show the importance of hydrogen bonding on specific rotation.     

Solvent effects on the aquation of chloropentammine-cobalt(III) perchlorate in ethanol-water

Summary The aquation kinetics of the chloropenta-aminecobalt(III) ion in H₂O–EtOH mixtures have been determined. A new correlation is described for calculating the chemical potential from kinetic data and molar thermodynamic excess functions for binary mixtures. The rate constants correlate well with Grunwald-Winstein solvating power Y parameter and with the dielectric constant of the medium. The data supports the D mechanism.     

Solvent effects on electron transfer reactions. The system iron(III)-phenothiazine in water-alcohol mixtures

Summary The effect of solvent composition (aqueous mixtures of methanol, ethanol, propan-1-ol and propan-2-ol) on the rate constants and activation parameters of the electron transfer between iron(III) and phenothiazine has been investigated. The dependence of the kinetic parameters on the solvent composition is discussed with reference to previously investigated systems.     

Solvent effects on the dimensionality of oxamato-bridged manganese(II) compounds

Abstract

Two new oxamate-containing manganese(II) complexes, [{Mn(H₂edpba)(H₂O)₂}₂]_n (1) and [Mn(H₂edpba)(dmso)₂]?dmso?CH₃COCH₃?H₂O (2) (H₄edpba = N,N′-ethylenediphenylenebis(oxamic acid) and dmso = dimethylsulfoxide), have been synthesized and the structures of 1 and 2 were characterized by single crystal X-ray diffraction. The structure of 1 consists of neutral honeycomb networks in which each manganese(II) is six-coordinate by one H₂edpba^2? ligand and two carboxylate–oxygens from two other H₂edpba^2? ligands building the equatorial plane. Each manganese is connected to its nearest neighbor through two carboxylate(monoprotonated oxamate) bridges in an anti-syn conformation. A dmso solution of single crystals of 1 was placed under acetone atmosphere affording 2, whereas putting 2 in equimolar water:ethanol mixture results in 1. The molecular structure of 2 is made up of mononuclear manganese(II) units which are interlinked by weak C–H?π and edge-to-face π-stacking interactions leading to supramolecular chains along the crystallographic b axis. Magnetic measurements reveal the occurrence of an antiferromagnetic coupling between two manganese(II) ions through anti-syn carboxylate bridges for 1 [J = ?1.18 cm^?1, the Hamiltonian being defined as H = ?J S₁^.S₂] and very weak intrachain ferromagnetic interactions in 2 [J = + 0.046 cm^?1, H = ?J ∑_iS_i^.S_i + 1].     

Solvent extraction behavior of hexachlorotechnetate(IV) ion in the HCl-TBP system

The solvent extraction behavior of [TcCl₆]²⁻ in the HCl-TBP system was studied together with that of its aqua complexes. It was deduced from the dependence of the distribution coefficient of [TcCl₆]²⁻ both on the HCl and TBP concentrations that a possible form extracted into TBP is H₂[TcCl₆](TBP)₄. The distribution coefficients of aqua complexes of [TcCl₆]²⁻ were found to be in the order of [TcCl₄(H₂O)₂]<[TcCl₆]²⁻<[TcCl₅(H₂O)]⁻.     

Solvent extraction of arsenic(V) with dispersed ultrafine magnetite particles.

The solvent extraction of arsenic(V) was investigated using heptane containing ultrafine magnetite particles and hydrophobic ammonium salt. Arsenic(V) was favorably extracted from aqueous solutions of pH ranging over 2-7, where the distribution ratio (10(3)) was independent of the pH. Although the addition of alkyl ammonium salt improved the phase separation, no notable influence was observed on the extraction of arsenic(V). Oleic acid suppressed the distribution ratio of arsenic(V) when the concentration exceeded 10(-2) M. Sulfate did not interfere with the extraction, while the presence of more than 10(-3) M phosphate decreased the distribution ratio. Metal cations including calcium(II), manganese(II), cobalt(II), nickel(II), copper(II), zinc(II) and lanthanum(III) did not give any serious interference up to the 10(-4) M level. According to equilibrium and kinetic studies, the extraction of arsenic(V) can be interpreted by the adsorption of H2AsO4- onto the surface of dispersed magnetite particles. The relationship between the amount of arsenic(V) extracted in the organic phase and that remaining in an aqueous phase followed a Langmuir-type equilibrium equation. The maximum uptake capacity was determined to be 4.8 x 10(-4) mol/g-magnetite (36 mg As/g). The arsenic(V) extracted in the organic phase was quantitatively recovered by back-extraction with an alkaline solution.     

Solvent effects on the anation ofcis-diaquo-bis-ethylenediamine cobalt(III) by L-proline

The anation kinetics of the title complex were investigated spectrophotometrically in aqueous methanol, ethanol, ipropanol, t-butanol and dioxane, and the following rate law was established:

Solvent and substituent effects on the photoreduction of cobalt(III)-aryl amine complexes in an aquo-organic solvent medium

The photoreduction of a series of Co^III complex ions, [Coen₂Cl(RC₆H₄NH₂)]²⁺ where R = p-OMe, p-OEt, p-Me, m-Me, H, and p-F, has been studied using a low pressure Hg vapour lamp as the light source (254 nm) in an aquo-organic solvent [15–40% (v/v) 2-methylpropan-2-ol] medium. Quantum yields for Co^II production by redox decomposition have been determined in all cases. The quantum yield increases considerably with the increase in concentration of the organic co-solvent in the binary solvent mixture under investigation. The Hammett correlation is linear, affording negative reaction constants, which indicate that the excited state is electron deficient. Correlation of the experimental results with Kamlet–Taft's solvatochromic parameters indicates that the solvent hydrogen bond donor acidity plays a dominant role in governing the reactivity.     

Solvent effects in NMR spectra induced by benzene in amidoximes—V

Proton chemical shifts as well as solvent shifts induced by benzene in several amidoximes are examined with respect to their configuration and are compared to the solvent shifts induced in benzalanilines. The geometry of the benzene-solute ‘collision-complex’ is also discussed.     

Solvent effects on the redox potentials of tetraethylammonium hexacyanomanganate(III) and hexacyanoferrate(III)

Tetraethylammonium hexacyanomanganate(III) was studied in formamide, N-methylformamide, methanol, propylenecarbonate, N,N-dimethylthioformamide, N-methylthiopyrrolidinone(2), butyrolactone, acetonitrile, dimethylsulfoxide, N,N-dimethylformamide, N-methylpyrrolidinone(2), nitromethane and tetramethylenesulfone employing polarographic and voltammetric techniques. Reversible or nearly reversible behaviour for the reaction Mn(CN)₆^3?/Mn(CN)₆^2? was observed in most solvents on the stationary platinum electrode. The reaction Mn(CN)₆^3?/Mn(CN)₆^4? was studied on both the dropping mercury electrode and the stationary platinum electrode. Besides the reaction Mn(CN)₆^3?/Mn(CN)₆^4? several anodic waves due to successive reactions of mercury with the cyano-ligand of the complex occurred at the dropping mercury electrode. No redox reaction for (et₄N)₃Mn(CN)₆ was found in nitromethane. The polarographic behaviour of tetraethylammonium hexacyanoferrate(III) was studied in formamide, N-methylformamide, N-methylpyrrolidinone(2) and butyrolactone. The variation of E_1/2 and 1/2 (E_pa+E_pc) values versus bisphenylchromium(I)/bisbiphenylchromium(0) as reference redox system of the processes Mn(CN)₆^3?/Mn(CN)₆^2?, Mn(CN)₆^3?/Mn(CN)₆^4? and Fe(CN)₆^3?/Fe(CN)₆^4? with the nature of the solvent is discussed within the donor-acceptor concept. Correlations between the E_1/2 and 1/2(E_pa+E_pc) values and the acceptor properties of the solvent have been observed. The preparation of tetraethyl- and tetrabutylammonium hexacyanomanganate(III) is described.     

Solvent extraction study of protactinium(V) with methyl-iso-butyl carbinol and methyl-iso-butyl ketone

In order to study the extraction pattern of protactinium in different types of extracting agents and compare the similarity of patterns of extraction with dubnium and thereby unraveling its chemistry, solvent extraction of protactinium(V) with methyl-iso-butyl carbinol (MIBC) and methyl-iso-butyl ketone (MIBK) was studied using ²³³Pa as a radiotracer. The extraction efficiencies of Pa were determined as a function of shaking time, concentrations of mineral acids, and extractant concentrations using the two extractants. The results show that MIBK is more suitable for the extraction of protactinium than MIBC in benzene. Furthermore, the effect of the F anion is also discussed. The text was submitted by the authors in English.