The balance between side-chain and backbone-driven association in folding of the α-helical influenza A transmembrane peptide.

The correct balance between attractive, repulsive and peptide hydrogen bonding interactions must be attained for proteins to fold correctly. To investigate these important contributors, we sought a comparison of the folding between two 25-residues peptides, the influenza A M2 protein transmembrane domain (M2TM) and the 25-Ala (Ala25 ). M2TM forms a stable α-helix as is shown by circular dichroism (CD) experiments. Molecular dynamics (MD) simulations with adaptive tempering show that M2TM monomer is more dynamic in nature and quickly interconverts between an ensemble of various α-helical structures, and less frequently turns and coils, compared to one α-helix for Ala25 . DFT calculations suggest that folding from the extended structure to the α-helical structure is favored for M2TM compared with Ala25 . This is due to CH⋯O attractive interactions which favor folding to the M2TM α-helix, and cannot be described accurately with a force field. Using natural bond orbital (NBO) analysis and quantum theory atoms in molecules (QTAIM) calculations, 26 CH⋯O interactions and 22 NH⋯O hydrogen bonds are calculated for M2TM. The calculations show that CH⋯O hydrogen bonds, although individually weaker, have a cumulative effect that cannot be ignored and may contribute as much as half of the total hydrogen bonding energy, when compared to NH⋯O, to the stabilization of the α-helix in M2TM. Further, a strengthening of NH⋯O hydrogen bonding interactions is calculated for M2TM compared to Ala25 . Additionally, these weak CH⋯O interactions can dissociate and associate easily leading to the ensemble of folded structures for M2TM observed in folding MD simulations.

The pseudo-π model of the induced magnetic field: fast and accurate visualization of shielding and deshielding cones in planar conjugated hydrocarbons and spherical fullerenes.

The induced magnetic fields originating from the π system of planar conjugated polycyclic hydrocarbons and spherical fullerenes are accurately reproduced by their corresponding hydrogen skeletal models (HSMs). Moreover, the individual contribution per molecular orbital is also reproduced unraveling simple symmetry rules related to canonical molecular orbitals. Hence, fast, handy and accurate 3D visualization of shielding and deshielding cones is realized, enabling the interpretation of global and local π aromaticity and antiaromaticity of PAHs and spherical species in a simple and concise manner to facilitate further interpretations of large sized hydrocarbon systems.

Canonical orbital contributions to the magnetic fields induced by global and local diatropic and paratropic ring currents.

The induced magnetic field (IMF) of naphthalene, biphenyl, biphenylene, benzocyclobutadiene, and pentalene is dissected to contributions from the total π system, canonical π-molecular orbitals (CMO), and HOMO→π* excitations, to evaluate and interpret relative global and local diatropicity and paratropicity. Maps of the IMF of the total π system reveal its relative strength and topology that corresponds to global and local diatropic and paratropic ring currents. The total π magnetic response is determined by this of canonical HOMOs and particularly by paratropic contributions of rotational excitations from HOMOs to unoccupied π* orbitals. Low energy excitations and similar nodal structure of HOMO and π* induce strong paratropic fields that dominate on antiaromatic rings. High energy excitations and different nodal structures lead to weak paratropic contributions of canonical HOMOs, which are overwhelmed by diatropic response of lower energy canonical orbitals in aromatic rings. CMO-IMF analysis is found in agreement with ring current analysis. © 2017 Wiley Periodicals, Inc.

Study of Electron Delocalization in 1,2-, 1,3-, and 1,4-Azaborines Based on the Canonical Molecular Orbital Contributions to the Induced Magnetic Field and Polyelectron Population Analysis.

The electron delocalization in 1,2-azaborine, 1,3-azaborine, and 1,4-azaborine is studied using canonical molecular orbital contributions to the induced magnetic field (CMO-IMF) method and polyelectron population analysis (PEPA). Contour maps of the out-of-plane component of the induced magnetic field (Bz(ind)) of the π system show that the three azaborines, in contrast with borazine, sustain much of benzene's π-aromatic character. Among them, 1,3-azaborine exhibits the strongest π delocalization, while 1,4-azaborine is the weakest. Contour maps of Bz(ind) for individual π orbitals reveal that the differentiation of the magnetic response among the three isomers originates from the π-HOMO orbitals, whose magnetic response is governed by rotational allowed transitions to unoccupied orbitals. The low symmetry of azaborines enables a paratropic response from HOMO to unoccupied orbitals excitations, with their magnitude depending on the shape of interacting orbitals. 1,3-Azaborine presents negligible paratropic contributions to Bz(ind) from HOMO to unoccupied orbitals transitions, where 1,2- and 1,4-azaborine present substantial paratropic contributions, which lead to reduced diatropic response. Natural bond orbital (NBO) analysis employing PEPA shows that only the 1,3-azaborine contains π-electron fully delocalized resonance structures.

Interaction of chromium(III) with a N,N'-disubstituted hydroxylamine-(diamido) ligand: a combined experimental and theoretical study.

Reaction of hydroxylamine hydrochloride with prop-2-enamide in dichloromethane in the presence of triethylamine resulted in the isolation of the N,N'-disubstituted hydroxylamine-(diamido) ligand, 3,3'-(hydroxyazanediyl)dipropanamide (Hhydia). The ligand Hhydia was characterized by multinuclear NMR, high-resolution electrospray ionization mass spectrometry (ESI-MS), and X-ray structure analysis. Interaction of Hhydia with trans-[Cr(III)Cl2(H2O)4]Cl·2H2O in ethanol yields the ionization isomers [Cr(III)(Hhydia)2]Cl3·2H2O(1·2H2O) and cis/trans-[Cr(III)Cl2(Hhydia)2]Cl·2H2O (2·2H2O). The X-ray structure analysis of 1 revealed that the chromium atom in [Cr(III)(Hhydia)2](3+) is bonded to two neutral tridentate O,N,O-Hhydia ligands. The twist angle, θ, in [Cr(III)(Hhydia)2](3+) is 54.5(6)(0), that is, very close to an ideal octahedron. The intramolecular hydrogen bonds developed between the N-OH group of the first ligand and the amidic oxygen atom of the second ligand and vice versa contribute to the overall stability of the cation [Cr(III)(Hhydia)2](3+). The reaction rate constant of the formation of Cr(III) complexes 1·2H2O and 2·2H2O was found to be 8.7(±0.8) × 10(-5) M(-1) s(-1) at 25 °C in methyl alcohol and follows a first-order law kinetics based on the biologically relevant ligand Hhydia. The reaction rate constant is considerably faster in comparison with the corresponding water exchange rate constant for the hydrated chromium(III). The modification of the kinetics is of fundamental importance for the chromium(III) chemistry in biological systems. Ultraviolet-visible and electron paramagnetic resonance studies, both in solution and in the solid state, ESI-MS, and conductivity measurements support the fact that, irrespective of the solvent used in the interaction of Hhydia with trans-[Cr(III)Cl2(H2O)4]Cl·2H2O, the ionization isomers[Cr(III)(Hhydia)2]Cl3·2H2O (1·2H2O) and cis/trans-[Cr(III)Cl2(Hhydia)2]Cl·2H2O (2·2H2O) are produced.The reaction medium affects only the relevant percentage of the isomers in the solid state. The thermodynamic stability of the ionization isomers 1·2H2O and cis/trans-2·2H2O, their molecular structures as well as the vibrational spectra and the energetics of the Cr(III)- Hhydia/hydia(-) were studied by means of density functional theory calculations and found to be in excellent agreement with our experimental observations.

Application of density functional theory in combination with FTIR and DSC to characterise polymer drug interactions for the preparation of sustained release formulations between fluvastatin and carrageenans.

In the present study, ι- and λ-carrageenans were used as appropriate carriers for sustained release formulations of fluvastatin drug. From viscosity measurements, it was found that both carrageenans can give miscible blends with fluvastatin due to the interactions between the sulfate groups of carrageenans and hydroxyl groups of fluvastatin. This was predicted by computational analysis using density functional theory and proved by FTIR spectroscopy. These interactions, which are in higher intensity using ι-carrageenan, lead to the formation of complexes between polymeric matrices and fluvastatin drug. DSC experiments also confirmed that miscible blends between carrageenans and fluvastatin can be formed since in all concentrations only one glass transition temperature was recorded. Fluvastatin release depends on the drug content and in all formulations of λ-carrageenans containing 10, 25 and 50 wt% drug, almost sustained release profiles were observed. Fluvastatin/carrageenan complexes have lower dissolution profiles compared with physical mixtures. Polymer swelling seems to be the dominant drug release mechanism. Besides to neat ι- and λ-carrageenans, their blends can be also used as effective matrices for sustained release.

Interpretation of electron delocalization in benzene, cyclobutadiene, and borazine based on visualization of individual molecular orbital contributions to the induced magnetic field.

The magnetic response of the valence molecular orbitals (MOs) of benzene, cyclobutadiene, and borazine to an external magnetic field has been visualized by calculating the chemical shielding in two-dimensional grids of points on the molecular plane and on a plane perpendicular to it, using gauge-including atomic orbitals (GIAOs). The visualizations of canonical MO contributions to the induced magnetic field (CMO-IMF) provide a clear view of the spatial extension, the shape, and the magnitude of shielding and deshielding areas within the vicinity of the molecule, originating from the induced currents of each valence orbital. The results are used to investigate the delocalization of each valence MO and to evaluate its contribution to the aromatic character of systems under study. The differentiation of the total magnetic response among the three molecules originates exclusively from π-HOMO orbitals because the magnetic response of the subsets of the remaining MOs is found to be almost identical. Borazine is classified as nonaromatic as the four electrons that occupy the π-HOMO are found to be strongly localized on nitrogen centers. CMO-IMF can clarify the interpretation of various NICS indexes and can be applied for the investigation of various types of electron delocalization.

Copper(I) complexes of 1,10-phenanthroline and heterocyclic thioamides: an experimental and theoretical (DFT) investigation of the photophysical characteristics.

A series of luminescent mixed ligand complexes of copper(I) halides with 1,10-phenanthroline and the heterocyclic thioamides pyridine-2(1H)-thione (py2SH), pyrimidine-2(1H)-thione (pymtH), 4,6-dimethylpyrimidine-2(1H)-thione (dmpymtH), 1,4,5,6-tetrahydropyrimidine-2-thione (tHpymtH), 1,3-imidazolidine-2-thione (imtH(2)) and 4,5-diphenyl-2-oxazolethiol (dpoxtH) have been synthesized and characterized. The molecular structures of two representative compounds have been established by single-crystal X-ray diffraction. The mononuclear complexes feature the metal in a distorted tetrahedral environment surrounded by the two N atoms of the chelating 1,10-phenanthroline, the thione-S atom of the thioamide, and the halogen atom. The molecular structure, the electronic and photophysical properties and the energetics of the metal-ligand interactions for [CuI(phen)(py2SH)] have been studied by means of density functional calculations.

Molybdenum(VI) coordination chemistry of the N,N-disubstituted bis(hydroxylamido)-1,3,5-triazine ligand, H2bihyat. Water-assisted activation of the Mo(VI)═O bond and reversible dimerization of cis-[Mo(VI)O2(bihyat)] to [Mo(VI)2O4(bihyat)2(H2O)2].

Reaction of the N,N-disubstituted bis(hydroxylamino) ligand 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine (H(2)bihyat) with cis-[Mo(VI)O(2)(acac)(2)] in tetrahydrofuran resulted in isolation of the mononuclear compound cis-[Mo(VI)O(2)(bihyat)] (1). The treatment of Na(2)Mo(VI)O(4)·2H(2)O with the ligand H(2)bihyat in aqueous solution gave the dinuclear compounds cis-[Mo(VI)(2)O(4)(bihyat)(2)(H(2)O)(2)] (2) and trans-[Mo(VI)(2)O(4)(bihyat)(2)(H(2)O)(2)] (3) at pH values of 3.5 and 5.5, respectively. The structures for the three molybdenum(VI) compounds were determined by X-ray crystallography. Compound 1 has a square-pyramidal arrangement around molybdenum, while in the two dinuclear compounds, each molybdenum atom is in a distorted pentagonal-bipyramidal environment of two bridging and one terminal oxido groups, a tridentate (O,N,O) bihyat(2-) ligand that forms two five-membered chelate rings, and a water molecule trans to the terminal oxido group. The dinuclear compounds constitute rare examples containing the {Mo(2)(VI)O(2)(µ(2)-O(2))}(4+) moiety. The potentiometry revealed that the Mo(VI)bihyat(2-) species exhibit high hydrolytic stability in aqueous solution at a narrow range of pH values, 3-5. A subtle change in the coordination environment of the five-coordinate compound 1 with ligation of a weakly bound water molecule trans to the oxido ligand (1w) renders the equatorial oxido group in 1w more nucleophilic than that in 1, and this oxido group attacks a molybdenum atom and thus the dinuclear compounds 2 and 3 are formed. This process might be considered as the first step of the oxido group nucleophilic attack on organic substrates, resulting in oxidation of the substrate, in the active site of molybdenum enzymes such as xanthine oxidase. Theoretical calculations in the gas phase were performed to examine the influence of water on the dimerization process (1 → 2/3). In addition, the molecular structures, cis/trans geometrical isomerism for the dinuclear molybdenum(VI) species, vibrational spectra, and energetics of the metal-ligand interaction for the three molybdenum(VI) compounds 1-3 have been studied by means of density functional theory calculations.

Theoretical study of hydrogen bond interactions of fluvastatin with ι-carrageenan and λ-carrageenan.

The binding of the reductase inhibitor drug fluvastatin, hydroxy-3-methylglutaryl coenzyme A, with the hydrophilic ι- or λ-carrageenan polymers, serving as potential controllers of the drug's release rate, have been studied at the density functional level of theory with the B3LYP exchange correlation functional. Three low energy conformers of fluvastatin have been calculated. The vibrational spectroscopic properties calculated for the most stable conformer were in satisfactory agreement with the experimental data. A series of hydrogen bonded complexes of the most stable conformer of fluvastatin anion with low molecular weight models of the polymers have been fully optimized. In almost all, intermolecular H-bonds are formed between the sulfate groups of ι- or λ-carrageenan and fluvastatin's hydroxyls, resulting in a red shift of the fluvastatin's O - H stretching vibrations. Cooperative intramolecular H-bonds within fluvastatin or ι-, λ-carrageenan are also present. The BSSE and ZPE corrected interaction energies were estimated in the range 281-318 kJ mol⁻¹ for ι-carrageenan - fluvastatin and 145-200 kJ mol⁻¹ for λ-carrageenan - fluvastatin complexes. The electron density (ρ (bcp)) and Laplacian (∇²ρ (bcp)) properties at critical points of the intermolecular hydrogen bonds, estimated by AIM (atoms in molecules) calculations, have a low and positive character (∇²ρ(bcp) > 0), consistent with the electrostatic character of the hydrogen bonds. The structural and energetic data observed, as well as the extent of the red shift of the fluvastatin's O - H stretching vibrations upon complex formation and the properties of electron density show a stronger binding of fluvastatin to ι- than to λ-carrageenan.

Density functional investigation and bonding analysis of pentacoordinated iron complexes with mixed cyano and carbonyl ligands.

The equilibrium structures and vibrational frequencies of the iron complexes [Fe(0)(CN)(n)(CO)(5-n)](n-) and [Fe(II)(CN)(n)(CO)(5-n)](2-n) (n = 0-5) have been calculated at the BP86 level of theory. The Fe(0) complexes adopt trigonal bipyramidal structures with the cyano ligands occupying the axial positions, whereas corresponding Fe(2+) complexes adopt square pyramidal structures with the cyano ligands in the equatorial positions. The calculated geometries and vibrational frequencies of the mixed iron Fe(0) carbonyl cyanide complexes are in a very good agreement with the available experimental data. The nature of the Fe-CN and Fe-CO bonds has been analyzed with both charge decomposition and energy partitioning analysis. The results of energy partitioning analysis of the Fe-CO bonds shows that the binding interactions in Fe(0) complexes have 50-55% electrostatic and 45-50% covalent character, whereas in Fe(2+) 45-50% electrostatic and 50-55% covalent character. There is a significant contribution of the pi- orbital interaction to the Fe-CO covalent bonding which increases as the number of the cyano groups increases, and the complexes become more negatively charged. This contribution decreases in going from Fe(0) to Fe(2+) complexes. Also, this contribution correlates very well with the C-O stretching frequencies. The Fe-CN bonds have much less pi-character (12-30%) than the Fe-CO bonds.

A DFT study on the radical scavenging activity of maritimetin and related aurones.

The radical scavenging activity of maritimetin and a series of synthetic aurones has been studied by using density functional theory with the B3LYP exchange correlation functional. The computation of various molecular descriptors that could assist the elucidation of hydrogen atom and electron donating ability of the selected compounds was carried out in the gas phase and in the liquid phase (benzene, methanol, water) with the aid of IEF-PCM. For reasons of comparison a series of simple phenols of known activity were also included in the study. The results are discussed with regards to the structure-activity relationship principles of flavonoids and in particular to the capacity of the selected aurones to scavenge 1,1-diphenyl-2-picrylhydrazyl (DPPH(*)) and superoxide anion (O(2)(-*)) radicals. The O-H bond dissociation enthalpy (BDE) seems to be the most proper parameter to characterize the antiradical properties of the studied compounds. The hydroxylation pattern in ring B defines the order of activity, while the extended conjugation and especially the presence of a catechol moiety in ring A are responsible for the high activity observed experimentally for the selected aurones.

Vanadium(V) compounds with the bis-(hydroxylamino)-1,3,5-triazine ligand, H2bihyat: synthetic, structural, and physical studies of [V2(V)O3(bihyat)2] and of the enhanced hydrolytic stability species cis-[V(V)O2(bihyat)]-.

Reaction of the ligand 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine (H(2)bihyat) with NaV(V)O(3) in aqueous solution followed by addition of either Ph(4)PCl or C(NH(2))(3)Cl, respectively, gave the mononuclear vanadium(V) compounds Ph(4)P[V(V)O(2)(bihyat)].1.5H(2)O (1) and C(NH(2))(3)[V(V)O(2)(bihyat)] (2). Treatment of V(IV)OSO(4).5H(2)O with the ligand H(2)bihyat in methyl alcohol under specific conditions gave the oxo-bridged dimer [V(V)(2)O(2)(mu(2)-O)(bihyat)(2)] (3). The structures for 1 and 3 were determined by X-ray crystallography and indicate that these compounds have distorted square-pyramidal arrangement around vanadium. The ligand bihyat(2-) is bonded to vanadium atom in a tridentate fashion at the pyridine-like nitrogen atom and the two deprotonated hydroxylamino oxygen atoms. The high electron density of the triazine ring nitrogen atoms, which results from the resonative contribution of electrons of exocyclic nitrogen atoms (Scheme 4 ), leads to very strong V-N bonds. The cis-[V(V)O(2)(bihyat)](-) species exhibits high hydrolytic stability in aqueous solution over a wide pH range, 3.3-11.0, as it was evidenced by (1)H and (51)V NMR spectroscopy and potentiometry. The high affinity of the H(2)bihyat ligand for the V(V)O(2)(+) unit, its tridentate character, as well as its small size, paves the way for potential applications in medicine, analysis, and catalysis for the C(NH(2))(3)[V(V)O(2)(bihyat)] compound. The molecular structures, vibrational and electronic spectra, and the energetics of the metal-ligand interaction for compounds 1 and 3 have been studied by means of density functional calculations.

Solution identification and solid state characterisation of a heterometallic polyoxometalate {Mo(11)V(7)}: [Mo(VI)(11)V(V)(5)V(IV)(2)O(52)(mu(9)-SO(3))](7-).

A polyoxomolybdenum/vanadium-sulfite {M(18)} cluster-based compound, [Mo(VI)(11)V(V)(5)V(IV)(2)O(52)(mu(9)-SO(3))](7-), is reported that exhibits a unique structural motif, arising from the incorporation of five V(V) and two V(IV) ions into a {M(18)} cluster framework templated by SO(3)(2-); this cluster compostion was first identified using cryospray mass spectrometry.

Polyoxomolybdenum(V/VI)-sulfite compounds: synthesis, structural, and physical studies.

Reaction of Na(2)Mo(VI)O(4) x 2H(2)O with (NH(4))(2)SO(3) in the mixed-solvent system H(2)O/CH(3)CN (pH = 5) resulted in the formation of the tetranuclear cluster (NH(4))(4)[Mo(4)(VI)SO(16)] x H(2)O (1), while the same reaction in acidic aqueous solution (pH = 5) yielded (NH(4))(4)[Mo(5)(VI)S(2)O(21)] x 3H(2)O (2). Compound {(H(2)bipy)(2)[Mo(5)(VI)S(2)O(21)] x H(2)O}(x) (3) was obtained from the reaction of aqueous acidic solution of Na(2)Mo(VI)O(4) x 2H(2)O with (NH(4))(2)SO(3) (pH = 2.5) and 4,4'-bipyridine (4,4'-bipy). The mixed metal/sulfite species (NH(4))(7)[Co(III)(Mo(2)(V)O(4))(NH(3))(SO(3))(6)] x 4H(2)O (4) was synthesized by reacting Na(2)Mo(VI)O(4) x 2H(2)O with CoCl(2) x 6H(2)O and (NH(4))(2)SO(3) with precise control of pH (5.3) through a redox reaction. The X-ray crystal structures of compounds 1, 2, and 4 were determined. The structure of compound 1 consists of a ring of four alternately face- and edge-sharing Mo(VI)O(6) octahedra capped by the trigonal pyramidal sulfite anion, while at the base of the Mo(4) ring is an oxo group which is asymmetrically shared by all four molybdenum atoms. Compound 3 is based on the Strandberg-type heteropolyion [Mo(5)(VI)S(2)O(21)](4-), and these coordinatively saturated clusters are joined by diprotonated 4,4'-H(2)bipy(2+) through strong hydrogen bonds. Compound 3 crystallizes in the chiral space group C2. The structure of compound 4 consists of a novel trinuclear [Co(III)Mo(2)(V)SO(3)(2-)] cluster. The chiral compound 3 exhibits nonlinear optical (NLO) and photoluminescence properties. The assignment of the sulfite bands in the IR spectrum of 4 has been carried out by density functional calculations. The cobalt in 4 is a d(6) octahedral low-spin metal atom as it was evidenced by magnetic susceptibility measurements, cw EPR, BVS, and DFT calculations. The IR and solid-state UV-vis spectra as well as the thermogravimetric analyses of compounds 1-4 are also reported.

Investigation of the release mechanism of a sparingly water-soluble drug from solid dispersions in hydrophilic carriers based on physical state of drug, particle size distribution and drug-polymer interactions.

In the present study the release mechanism of the sparingly water-soluble drug felodipine (FELO) from particulate solid dispersions in PVP or PEG was investigated. FT-IR data indicated that a N-H...O hydrogen bond is formed between FELO and polymers. The drug-polymer interaction was theoretically studied with the density functional theory with the B3LYP exchange correlation function. The interaction energies have been estimated at -31.8 kJ/mol for PVP and -18.8 kJ/mol for PEG. Also, detailed vibrational analysis of the complexes showed that the red shift of the N-H bond stretching in FELO molecule due to H-bonding was higher in the FELO-PVP complex than in the FELO-PEG complex. Both the experimental and theoretical data indicated that a stronger interaction of FELO with PVP than with PEG was developed. The interactions of FELO with the polymer appeared to control the physical state (amorphous or crystalline) and the particle size of FELO in the solid dispersions. In the FELO/PVP dispersions, the drug is found as amorphous nanoparticles whereas in FELO/PEG dispersions the drug is dispersed as crystalline microparticles. The size of drug particles in the dispersion was also influenced by drug proportion, with an increase in drug content of the dispersion resulting in increased drug particle size. The particle size of drug, the proportion of drug in the dispersion and the properties of the polymer (molecular weight) appeared to determine the mechanism of drug release from the solid dispersions, which was drug diffusion (through the polymer layer)-controlled at low drug contents and drug dissolution-controlled at high drug contents. In situ DLS measurements indicate that the large initial particles of FELO/PVP and FELO/PEG solid dispersions with low drug content (10-20 wt%) are very rapidly decreased to smaller particles (including nanoparticles) during dissolution, leading to the observed impressive enhancement of FELO release rate from these dispersions.

A novel series of vanadium-sulfite polyoxometalates: synthesis, structural, and physical studies.

Reaction of NH4VO3 with sulfur dioxide affords the hexanuclear cluster (NH4)2(Et4N)[(V(IV)O)6(mu4-O)2(mu3-OH)2(mu3-SO3)4(H2O)2]Cl x H2O (1), and the decapentanuclear host-guest compound (Et4N)5{Cl subset [(VO)15(mu3-O)18(mu-O)3]} x 3 H2O (2). Sequential addition of magnesium oxide to an acidic aqueous solution of NH4VO3 (pH approximately 0) followed by (NH4)2SO3 resulted in the formation of either the non-oxo polymeric vanadium(IV) compound trans-(NH4)2[V(IV)(OH)2(mu-SO3)2] (3) or the polymeric oxovanadium(IV) sulfite (NH4)[V(IV)O(SO3)1.5(H2O)] x 2.5 H2O (4) at pH values of 6 and 4, respectively. The decameric vanadium(V) compound {Na4(mu-H2O)8(H2O)6}[Mg(H2O)6][V(V)10(O)8(mu6-O)2(mu3-O)14] x 3 H2O (5) was synthesised by treating an acidic aqueous solution of NH4VO3 with MgO and addition of NaOH to pH approximately 6. All the compounds were characterised by single-crystal X-ray structure analysis. The crystal structure of compound 1 revealed an unprecedented structural motif of a cubane unit [M4(mu4-O)2(mu3-OH)2] connected to two other metal atoms. Compound 3 comprises a rare example of a non-oxo vanadium(IV) species isolated from aqueous solution and in the presence of the reducing agent SO3(2-), while compound 4 represents a rare example of an open-framework species isolated at room temperature (20 degrees C). In addition to the synthesis and crystallographic studies, we report the IR and magnetic properties (for 1, 2 and 3) of these vanadium clusters as well as theoretical studies on compound 3.

Synthesis, structure, and solution dynamics of UO22+-hydroxy ketone compounds [UO2(ma)2(H2O)] and [UO2(dpp)(Hdpp)2(H2O)]ClO4 [ma = 3-hydroxy-2-methyl-4-pyrone, Hdpp = 3-hydroxy-1,2-dimethyl-4(1H)-pyridone].

Reaction of [UO(2)(NO(3))(2)] with the hydroxy ketones 3-hydroxy-2-methyl-4-pyrone (Hma) and 3-hydroxy-1,2-dimethyl-4(1H)-pyridone (Hdpp) in aqueous acidic solutions (pH approximately 3) yields the compounds [UO(2)(ma)(2)(H(2)O)].H(2)O (1.H(2)O) and [UO(2)(dpp)(Hdpp)(2)(H(2)O)]ClO(4) (2), respectively. X-ray diffraction shows that the geometry around the metal ion in both complexes is pentagonal bipyramid. Uranium ion in the crystal structure of 1 were found to be ligated with two chelate ma(-) groups and one unidentate H(2)O molecule (C coordination mode) at the equatorial plane, while in 2 with two single-bonded Hdpp there were one chelate dpp(-) and one H(2)O molecule (P coordination mode). Crystal data (Mo Kalpha; 293(2) K) are as follows: (1) monoclinic space group C2/c, a = 14.561(7) A, b = 14.871(9) A, c = 7.250(4) A, beta = 95.40(4) degrees , Z = 4; (2) monoclinic space group P2(1)/c, a = 19.080(2) A, b = 9.834(1) A, c = 15.156(2) A, beta = 104.62(1) degrees , Z = 4. (1)H NMR measurements indicate that complex 2 retains its structure in CD(3)CN solution; however, in DMSO-d(6) both complexes adopt the C structure. Line-shape analysis for the (1)H NMR peaks of 2 at various temperatures shows a fast intramolecular exchange process between the chelate dpp(-) and one of the single bonded Hdpp ligands and one slower exchange between all three ligands. The activation parameters and the decrease of the exchange rate by replacing unidentate ligand with DMSO indicate the dissociation of the unidentate ligand as the rate-determining step for the former exchange. Density functional calculations (DFT) support this mechanism and give a quantitative interpretation of the electronic structure of the two ligands and the geometries adopted by the complexes.

Monomeric oxovanadium(IV) compounds of the general formula cis-[V(IV)(=O)(X)(L(NN))(2)](+/0) [X = OH(-), Cl(-), SO(4)(2)(-) and L(NN) = 2,2'-bipyridine (bipy) or 4,4'-disubstituted bipy].

Reaction of [V(IV)OCl(2)(THF)(2)] in aqueous solution with 2 equiv of AgBF(4) or AgSbF(6) and then with 2 equiv of 2,2'-bipyridine (bipy), 4,4'-di-tert-butyl-2,2'-bipyridine (4,4'-dtbipy), or 4,4'-di-methyl-2,2'-bipyridine (4,4'-dmbipy) affords compounds of the general formula cis-[V(IV)O(OH)(L(NN))(2)]Y [where L(NN) = bipy, Y = BF(4)(-) (1), L(NN) = 4,4'-dtbipy, Y = BF(4)(-) (2.1.2H(2)O), L(NN) = 4,4'-dmbipy, Y = BF(4)(-) (3.2H(2)O), and L(NN) = 4,4'-dtbipy, Y = SbF(6)(-) (4)]. Sequential addition of 1 equiv of Ba(ClO(4))(2) and then of 2 equiv of bipy to an aqueous solution containing 1 equiv of V(IV)OSO(4).5H(2)O yields cis-[V(IV)O(OH)(bipy)(2)]ClO(4) (5). The monomeric compounds 1-5 contain the cis-[V(IV)O(OH)](+) structural unit. Reaction of 1 equiv of V(IV)OSO(4).5H(2)O in water and of 1 equiv of [V(IV)OCl(2)(THF)(2)] in ethanol with 2 equiv of bipy gives the compounds cis-[V(IV)O(OSO(3))(bipy)(2)].CH(3)OH.1.5H(2)O (6.CH(3)OH.1.5H(2)O) and cis-[V(IV)OCl(bipy)(2)]Cl (7), respectively, while reaction of 1 equiv of [V(IV)OCl(2)(THF)(2)] in CH(2)Cl(2) with 2 equiv of 4,4'-dtbipy gives the compound cis-[V(IV)OCl(4,4'-dtbipy)(2)]Cl.0.5CH(2)Cl(2) (8.0.5CH(2)Cl(2)). Compounds cis-[V(IV)O(BF(4))(4,4'-dtbipy)(2)]BF(4) (9), cis-[V(IV)O(BF(4))(4,4'-dmbipy)(2)]BF(4) (10), and cis-[V(IV)O(SbF(6))(4,4'-dtbipy)(2)]SbF(6) (11) were synthesized by sequential addition of 2 equiv of 4,4'-dtbipy or 4,4'-dmbipy and 2 equiv of AgBF(4) or AgSbF(6) to a dichloromethane solution containing 1 equiv of [V(IV)OCl(2)(THF)(2)]. The crystal structures of 2.1.2H(2)O, 6.CH(3)OH.1.5H(2)O, and 8.0.5CH(2)Cl(2) were demonstrated by X-ray diffraction analysis. Crystal data are as follows: Compound 2.1.2H(2)O crystallizes in the orthorhombic space group Pbca with (at 298 K) a = 21.62(1) A, b = 13.33(1) A, c = 27.25(2) A, V = 7851(2) A(3), Z = 8. Compound 6.CH(3)OH.1.5H(2)O crystallizes in the monoclinic space group P2(1)/a with (at 298 K) a = 12.581(4) A, b = 14.204(5) A, c = 14.613(6) A, beta = 114.88(1) degrees, V = 2369(1), Z = 4. Compound 8.0.5CH(2)Cl(2) crystallizes in the orthorhombic space group Pca2(1) with (at 298 K) a = 23.072(2) A, b = 24.176(2) A, c = 13.676(1) A, V = 7628(2) A(3), Z = 8 with two crystallographically independent molecules per asymmetric unit. In addition to the synthesis and crystallographic studies, we report the optical, infrared, magnetic, conductivity, and CW EPR properties of these oxovanadium(IV) compounds as well as theoretical studies on [V(IV)O(bipy)(2)](2+) and [V(IV)OX(bipy)(2)](+/0) species (X = OH(-), SO(4)(2)(-), Cl(-)).

NMR and theoretical investigations on the structures and dynamics of octahedral bis(chelate)dichloro V(III) compounds isolated by an unusual reduction of non-oxo V(IV) species.

Reaction of the non-oxo V(IV) species [V(IV)Cl(2)(L(OO))(2)] [L(OO) = acetylacetonate (acac(-)) or benzoylacetonate (bzac(-))] with a chelate nitrogen-donor ligand L(NN) in acetonitrile leads to the reduction of V(IV) to V(III) and the formation of the mononuclear V(III) compounds of the general formula [V(III)Cl(2)(L(OO))(L(NN))] (L(OO) and L(NN) are acac(-) and bipy for 1; acac- and 5,5'-me(2)bipy for 2; acac(-) and 4,4'-tb(2)bipy for 3; acac(-) and phen for 4; bzac(-) and bipy for 5; bzac(-) and phen for 6). The reduction of the V(IV) complexes was monitored by GC-MS and (1)H NMR spectroscopy. Both one- and two-dimensional (2D COSY and 2D EXSY) (1)H NMR techniques were used to assign the observed (1)H NMR resonances of 1-6 in CD(2)Cl(2) or CDCl(3) solution. It appeared that in solution these V(III) complexes form two isomers which are in equilibrium: cis-[V(III)Cl(2)(L(OO))(L(NN))] <==> trans-[V(III)Cl(2)(L(OO))(L(NN))]. 2D EXSY cross-peaks were clearly observed between bipy- and acac-hydrogen atoms of the two geometrical isomers of 1-3 as well as between bipy and acac(-) protons of the cis isomer, indicating a dynamic process that corresponds to cis-trans isomerization and a cis-cis racemization. The thermodynamic and kinetic parameters of the equilibrium between these two isomers were calculated for compounds 1 and 2 by using variable temperature (VT) NMR data. Both cis-trans isomerization and cis-cis racemization processes probably proceed with an intramolecular twist mechanism involving a trigonal prismatic transition state. Density functional calculations (DFT) also indicated such a rearrangement mechanism.