Marios Stylianou

Vanadium(V) Compounds with the Bis-(hydroxylamino)-1,3,5-triazine Ligand, H2bihyat : Synthetic, Structural, and Physical Studies of [V2VO3(bihyat)2] and of the Enhanced Hydrolytic Stability Species cis-[VVO2(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.

Oxovanadium(IV)-sulfite compounds : Synthesis and structural and physical studies

Reaction of VIVOCl2 in strongly acidic aqueous solution with either (NH4)2SO3 or Na2SO3 and Bu4NBr at ~70°C in the pH range 2.5-4.5 gives the clusters (NH4)2{[V4IV(μ4-O)2(μ3-OH)2](VIVO)2(μ3-SO3)4O4(H2O)2} and (n-Bu4N)2{[V4IV(μ4-O)2(μ3-OH)2](VIVO)2(μ3-SO3)4O4(H2O)2}, respectively. Reaction of NH4VVO3 with (NH4)2SO3 resulted in the isolation of the first compound. When the latter reaction is carried out in the presence of MgO, compound (NH4)[VIVO(SO3)1.5H2O]∞.2.5H2O was isolated instead. Compound (n-Bu4N)2{[V4IV(μ4-O)2(μ3-OH)2](VIVO)2(μ3-SO3)4O4(H2O)2} and (NH4)[VIVO(SO3)1.5H2O]∞.2.5H2O were characterized by X-ray structure analysis. The crystal structure of species (n-Bu4N)2{[V4IV(μ4-O)2(μ3-OH)2](VIVO)2(μ3-SO3)4O4(H2O)2} revealed a unprecedented hexanuclear cluster consisting of a cubane core [M4(μ4-O)2(μ3-OH)2] connected to two other metal atoms through the core oxo-groups and four μ3-SO3 bridges. Compound (NH4)[VIVO(SO3)1.5H2O]∞.2.5H2O represents a rare example of an open-framework species prepared under mild conditions. Cyclic voltammetric examination of compound (n-Bu4N)2{[V4IV(μ4-O)2(μ3-OH)2](VIVO)2(μ3-SO3)4O4(H2O)2} revealed a redox process which was assigned to the oxidation of one core of vanadium(IV) to vanadium(V).

Molybdenum(VI) Coordination Chemistry of the N,N-Disubstituted Bis(hydroxylamido)-1,3,5-triazine Ligand, H2bihyat. Water-Assisted Activation of the MoVI═O Bond and Reversible Dimerization of cis-[MoVIO2(bihyat)] to [MoVI2O4(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.

Synthesis, solution, and structural characterization of tetrahydrofuranyl-2,2-bisphosphonic acid disodium salt

Bisphosphonates are biologically relevant therapeutics for bone disorders and cancer. Reaction of γ-chlorobutyric acid, phosphorus acid, and phosphorus trichloride without the use of solvent gave the tetrahydrofuranyl-2,2-bisphosphonate sodium salt (Na2H2L). The Na2H2L was isolated, characterized in solution by 1H, 13C, and 31P NMR spectroscopy and in solid state by single X-Ray crystallography. The crystal structure showed that the Na2H2L forms in the crystal infinite two-dimensional sheets stacked one parallel to the other. A comparison of the chelating properties of H2L2− with similar hydroxyl bisphosphonate ligands shows that the strength of the Na–O(furanyl/hydroxyl) bond is directly related to the total charge of the ligand anion.

σ-Bonded p-Dioxolene Transition Metal Complexes

Metal ions are known to lie in close proximity with these species in biological systems, thus resulting in immediate interaction. The two coupled, metal and organic redox centers have been found to participate in several biological processes such as, the oxidative maintenance of biological amine levels, (Klinman, 1996) tissue (collagen and elastin) formation, (Klinman, 1996) photosynthesis (Calvo, et al., 2000) and respiration (Iwata, et al., 1998). Although the crystal structures of many of these enzymes have been solved, the role of the metal ions in these reactions is still controversial. From another point of view, quinonoid metal complexes exhibit rich redox, magnetic and photochemical properties and thus can underpin key technological advances in the areas of energy storage, sensors, catalysis and “smart materials” (Evangelio & Ruiz-Molina, 2005; Stylianou, et al., 2008). Metal ions interact with hydroquinone systems, through σ-bonding to the oxygen atoms and/or through π-bonding to the carbocyclic ring. The structurally characterized σ-bonded hydroquinone metal complexes are surprisingly limited. Structures of metal ions with psemiquinones and quinones are even rarer, mainly due to the absence of a chelate coordination site in simple p-(hydro/semi)quinone and the low pK values of the semiquinone and quinone oxygen atoms. A strategy to synthesize stable metal complexes with hydroquinone species is to use substituted hydroquinones in o-position with substituents containing one or more donor atoms, enabling in this way the metal atom to