J. David Miller

Coordinative cross-linking of pyridyl- and bipyridyl-based hydrogel polymer membranes

Abstract The preparation of a series of hydrogel copolymer membranes of 2-hydroxyethyl methacrylate with 4-vinyl pyridine (VPy) in the range of 0.5–17 wt%, or 4′-vinyl 4-methyl 2,2′-bipyridine (VBpy) in the range 0.5–8 wt%, are described. The membranes are shown to coordinate selected transition metal ions from aqueous solution. Interactions with Cu( ii ) and Fe( ii ) were isolated for individual attention on the basis that, under certain conditions, bands appear in the visible spectra characteristic of the Cu( ii ) tetra(VPy) and the Fe( ii ) tris(VBpy) complexes. The complexation of more than one ligand group per metal ion leads to the potential anchoring of two or more previously unconnected polymer chains at a metal-centred cross-link. The effects of ‘coordinative cross-linking’ on the tensile properties of the hydrogel polymers were studied in elongation at 25°C and 100% humidity, until failure of the membranes occurred. The tensile strengths of both the Cu( ii ) and Fe( ii )-coordinated polymers are found to increase in conjunction with copolymer ligand content, as a consequence of a greater degree of coordinative cross-linking. In more classical hydrogel systems, the equilibrium water content of the polymer is caused to fall with degree of cross-linking. Here, this is not the case, as the equilibrium water content remains relatively unchanged owing to the incorporation of further water-structuring groups in the guise of metal complexes. Together with the capability of a reversible cross-linking effect by metal leaching under acid conditions, these ligand-modified systems offer a novel and unique route to the enhancement of mechanical properties in hydrogel-type materials.

Synthesis, characterisation and complexation behaviour of a series of pyridyl- and bipyridyl-based hydrogel membranes

The syntheses of hydrophilic membranes composed of copolymers of 4-vinylpyridine or 4-vinyl-4′-methyl-2,2′-bipyridine with 2-hydroxyethyl methacrylate are described. These coordinating membranes readily take up and complex divalent metal ions (Cu, Co and Fe) from aqueous solution. The types of complexes formed are identified by comparison of analytical and spectroscopic data for the immobilised species with those for the complexes of the monomeric ligands in aqueous solution. Changes in the number or stereochemical arrangements of coordinated ligands occur when the hydrogel membranes are dehydrated. Additionally, it is demonstrated that the redox chemistry of the metal centre can be exploited as if in solution. In the case of 4-vinyl-4′-methyl-2,2′-bipyridine, pre-complexed copolymer membranes were prepared from the tris complexes of CoII, FeII and RuII Data from these specimens can be used in the calculation of the total ion-binding capacity of other membranes. For FeII ion-binding, up to 80% of the ligands present readily become involved in tris complex formation. This reflects the high degree of polymer-chain mobility possessed by the hydrogel. However, the extent of FeII coordination is shown to be anion dependent, with the concentration of FeII partitioned within the gel matrix influencing the position of an equilibrium that is established between the mono and tris complexes.

Kinetic and mechanistic aspects of iron(II) coordination to bipyridyl-based hydrogel pofymer membranes

A range of hydrophilic membranes composed of copolymers of 4-methyl-4′-vinyl-2, 2′-bipyridyl with 2-hydroxyethyl methacrylate have been synthesized. These membranes readily coordinate iron(II) from aqueous solution to form the tris(2,2′-bipyridyl)iron(II) species, but at a rate very much slower than that of the free ligand in solution. Kinetic studies on the rate of development of these colour centres have shown the process to be anion-dominated and pseudo-first order for iron(II) sulfate and second order for the chloride and perchlorate. The extent of coordination within the membrane is also dependent on the salt used, as the anion influences the concentration of FeII partitioned within the gel matrix, and ultimately the position of the equilibrium established between the mono and tris complexes. Mechanisms are proposed incorporating the known water-structuring effects that anions impose on this type of hydrogel environment and accounting for the presence of ion pairs and their effect on the molecular reorganisations that are necessary in order for tris complexation to occur.

Iron(II) coordination to bipyridyl-based hydrogel polymers

Abstract Hydrophilic membranes composed of copolymers of 0.3% by weight 4-methyl-4′-vinyl-2,2′-bipyridyl with 2-hydroxyethyl methacrylate were prepared, and the coordination of iron(II) from aqueous solutions of the sulfate, perchlorate and chloride salts to form the tris(2,2′-bipyridyl)iron(II) species was studied at a range of temperatures in order to extend earlier findings. In the case of the chloride the rate equation was found to differ from that previously reported. Values for the enthalpy and entropy changes for an equilibrium involving the obstruction of potential ligand sites by existing coordination centres, and the activation enthalpies and entropies for the slow steps during complex formation, are determined. They are interpreted as showing that the slow steps in all cases involve segmental rotation within the copolymer chains to permit further coordination to occur.

A Kinetic and mechanistic study of the reduction of an aryltellurium trichloride by chromium(II) and vanadium(II) ions

Abstract The reduction of an aryltellurium trichloride by Cr 2+ and V 2+ in aqueous dioxane solutions has been investigated. The reactions are quantitative forming the ditelluride and M III products. The reaction sequence begins with hydrolysis which leads to rapidly maintained equilibria between RTeO(OH), RTeOCl and RTeO 2 − . The first reductive step is rate determining, and is believed to be a one-electron, innersphere process. Reactions involving the use of the Teue5f8OH group to form the bridge are slower than those using Teue5f8Cl or Teue5f8O − . Rate data were measured over a range of experimental conditions; rate and activation parameters are quoted. A few experiments using the outer-sphere reductant [Ru(NH 3 ) 6 ] 2+ are also reported. Reduction still occurs, but at a slower rate than with the C r2+ and V 2+ ions

A kinetic and mechanistic study of the reaction between bis-p-ethoxyphenyl ditelluride and molecular iodine in solution

The title reaction in toluene has been examined spectrophotometrically. The data are consistent with equations (1)–(3)(R =p-EtOC6H4). The rate constants k1 and k3 are 56 ± 2 dm3 mol–1 s–1 at R2Te2+ [graphic omitted] 2RTel (1) RTel + I2 [graphic omitted] RTe(I3)(2) RTe(I3) [graphic omitted] RTel(I)3(3) 7 °C and 6.2 × 10–4 s–1 at 25 °C, respectively. The equilibrium constant K2 is 1 570 dm3 mol–1 at 25 °C, and ΔH2=–22 ± 4 kJ mol–1. The activation parameters for the final step are ΔH‡= 44 ± 5 kJ mol–1 and ΔS‡=–160 ± 15 J K–1 mol–1.

Complex formation between polymers containing the 2, 2′-bipyridyl group and iron(II)

Condensation polymers derived from the reaction of butane-1,4-diol and either the 4,4′- or 5,5′-di-acid derivatives of 2,2′-bipyridyl have been prepared. The formation of strongly coloured complexes between iron(II) and soluble portions of the products has been investigated. In the case of the polymer derived from the 5,5′-disubstituted bipyridyl the polymeric ligand appears to behave as a collection of independent ligand groups, but approximately one half of these groups are prevented from taking part in the formation of a ‘tris’ complex. The polymer derived from the 4,4′-disubstituted ligand behaves differently. The polymer chain develops meta to the ring–ring bond of the bipyridyl group. The difficulty of rotation of this bond produces two different types of bipyridyl repeat unit. One behaves as a ligand in the same way as the monomeric ligand, the other does not. Presumably these are fixed cis and trans configurations. The inflexibility of the polymer chain prevents the majority of the potential ligand groups from participating in the formation of a ‘tris’ complex, which formation involves only two polymer chains. Data are quoted for the stability constants relating to both polymers.

Kinetic and mechanistic aspects of copper(II)coordination to bis-N,N′-(salicylidene)-1,2-diaminoethane-based hydrogel polymer membranes,and the permeation of cations through them

A range of hydrophilic membranes composed of copolymers of nbis-N,N′-(5-vinylsalicylidene)-1,2-diaminoethane nwith 2-hydroxyethylmethacrylate have been synthesised. Over a period of napproximately 2 h these membranes coordinate copper(ii) ions from aqueous nsolution to yield tetradentate species in a first-order process. However, nonly a small fraction of the potentially ligating sites are fully used in nthis way. Kinetic studies of the interactions with the nitrate, chloride nand sulfate salts of copper(ii) are described and a detailed mechanism is nproposed. Molecular rotations at the ligand site are suggested to be the nrate determining steps of the overall process. Values of individual rate nand equilibrium constants have been determined, and shown to be consistent nwith the equivalent data found for simpler ligands involved in reactions nin homogeneous solutions. The permeation of the nitrates of nCo n II n , Ni n II n and Cu n II n through membranes of nthese copolymers is also described. Due to the slow rates of complex nformation, the ligand sites have no significant effect on either the npermeability of the salts through the membrane, or the time lags before nsalt passage is detected. For comparison purposes, permeation data for the npassage of the nitrate of the substitutionally inert n[Cr(H n 2 nO) n 6 n ] n 3+ n ion through n4-methyl-4′-vinyl-2,2′-bipyridyl-containing membranes are also nreported.

A kinetic and mechanistic study of the oxidation of a diarylditelluride by hydrogen peroxide in aqueous tetrahydrofuran

Abstract When a diarylditelluride (RTeTeR), in solution in aqueous tetrahydrofuran, reacts with hydrogen peroxide it is first oxidised to RTeOH in a chain reaction involving RTe·(O2H2) and RTe radicals, which are in equilibrium with each other. If dioxygen is present a further equilibrium, involving RTe·O2, slows the overall oxidation. The chain reaction, rather than a concerted four-centre mechanism, occurs because of the difference in Teue5f8Te and Oue5f8O bond lengths. The subsequent two-electron oxidation of RTeOH to RTeO(OH) by another molecule of H2O2 occurs at a very similar rate. Finally there is a very slow oxidation to RTe(OH)3 The effect of various experimental parameters on the rate of the first oxidation stage has been studied in detail and kinetic data have been determined. A mechanistic scheme is proposed.

A kinetic and mechanistic study of the solvolysis of aryitellurium trihalides in organic solvents

When an aryltellurium trichloride reacts with methanol in a mixed solvent also containing benzene or dioxane the reaction can be followed by monitoring the generation of acid. The first Te–Cl bond cleavage is acid catalysed and is rate determining. It exhibits a small activation enthalpy, but a large and negative activation entropy. The other trihalides have similar activation parameters, The mechanism is believed to involve protonation of a co-ordinated Cl, with dissociative loss of HCl being rate determining. The compounds RTe(O)Cl and RTe(OMe)2Cl have been isolated as solid products but, since the reactions occurring in solution are reversible, the products in dilute solution may be different from these. Kinetic experiments to study the variation of rate with the solvent composition and the nature of the alcohol are also reported, as are results of reactions in aqueous dioxane.