Gerald S. Laurence

Equilibrium and kinetic studies of complexes of the pendant donor macrocycles N,N′,N″,N‴-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane (THEC-12) and N, N′, N″, N‴-tetrakis(2-hydroxyethyl)-1,4,8,12-tetraazacyclopentadecane (THEC-15)

Abstract Investigations of the thermodynamic and kinetic properties of the Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Pb2+ complexes of N, N′, N″, N‴-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane (THEC-12) and the newly synthesised N, N′, N″, N‴-tetrakis(2-hydroxyethyl)-1,4,8,12-tetraazacyclopentadecane (THEC-15) have been undertaken. Formation constants for the ML2+ complexes indicate that the hydroxyethyl groups cause significant destabilisation of the complexes with respect to those of the non-hydroxyethylated macrocycles. For decomplexation of [M(THEC-12)]2+ in aqueous HNO3 the observed rate constants =kobs = kO + kH[H+], where at 298.2 K, kO = (3.06 ± 1.86) × 10−5 s−1 when M  Co, kO ≈ s−1 when M  Ni and Cu, and 105 kH = 61.4 ± 2.9, 5.78 ± 1.42 and 2.81 ± 0.09 dm3 mol−1 s−1 when M  Co, Ni and Cu, respectively. For [M(THEC-15)]2+, decomplexation is very slow with t 1 2 >24 h (in 1.00 mol dm−3 HNO3) when M  Co and Ni, but considerably faster with t 1 2 ≈ s (1.00 mol dm−3 HNO 3) when M  Cu.

Kinetics of oxidation of transition-metal ions by halogen radical anions. Part I. The oxidation of iron(II) by dibromide and dichloride ions generated by flash photolysis

The radical anions Br2– and Cl2–, generated by flash photolysis of the corresponding halide solutions or of the iron(III) complexes FeBr2+ and FeCl2+, oxidize iron(II) to iron(III). The product of the reaction between the ions Br2– and Fe2+ is the FeBr2+ complex. The reaction proceeds by an inner-sphere substitution-controlled path with a rate constant of (3·6 ± 0·4)×106 l mol–1 s–1 at 25 °C (ΔH‡= 25·2 ± 2 kJ mol–1; ΔS‡=–42 ± 12 J K–1 mol–1). The reaction between the ions Cl2– and Fe2+ proceeds by two paths at 25 °C, an inner-sphere substitution-controlled path with a rate constant of (4·0 ± 0·6)× 106 l mol–1 s–1(ΔH‡= 31·5 ± 4 kJ mol–1; ΔS‡=–21 ± 15 J K–1 mol–1), and an outer-sphere path with a rate constant of (1·0 ± 0·2)× 107 l mol–1s–1(ΔH‡= 22·7 ± 4 kJ mol–1; ΔS‡=–42 ± 15 J K–1 mol–1).

Aqueous chemistry of thallium(II). Part I. Kinetics of reaction of thallium(II) with cobalt(II) and iron(III) ions and oxidation–reduction potentials of thallium(II)

Reactions of Tl2+ ions [generated by flash photolysis of thallium(III) solutions] with cobalt(II) and iron(III) have been studied. For the reaction Tl2++ Co2+⇌ Tl++ Co3+ the forward rate constant is (6·2 ± 0·5)× 103 l mol–1 s–1(25 °C, [H+]= 0·25, I= 0·75M) and the equilibrium constant calculated from this rate constant and that of the reverse reaction is 2·0 × 106. The forward rate constant for the reaction Tl2++ Fe3+⇌ Tl3++ Fe2+ is (1·1 ± 0·15)× 106 l mol–1 s–1(25 °C, [H+]= 0·25, I= 0·30M) and the equilibrium constant calculated from this value and that of the reverse reaction is 2·4 × 107. Standard reduction potentials for the reactions Tl3++e -⇌ Tl2+(E10) and Tl2++ e–⇌ Tl+(E20) are +0·33 ± 0·05 and +2·22 ± 0·05 V respectively. The thermal electron-exchange reaction between thallium(I) and thallium(III) is discussed and a mechanism involving intermediate formation of Tl2+ ions is shown to be inconsistent with equilibrium and kinetic data. The disproportionation reaction Tl2++ Tl2+→ Tl++ Tl3+ has a rate constant of (5·5 ±0·5)× 108 l mol–1 s–1(25 °C, [H+]= 0·25, I= 0·25M) and is diffusion controlled with an activation energy of 7·9 ± 1·5 kJ mol–1.

Intramolecular exchange between square antiprismatic enantiomers of the pendant arm macrocyclic ligand complex N,N′,n″,n‴-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecanelead(II)

Variable temperature 13C NMR studies of the pendant arm macrocyclic complex N,N′,N″,N‴-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecanelead(II) and its cadmium(II) and mercury(II) analogues in [2H4]-methanol indicate that these complexes undergo rapid intramolecular exchange between two square antiprismatic enantiomers.

Computer-based systems for the acquisition and treatment of kinetic data from flash photolysis, laser photolysis and pulse radiolysis experiments

Abstract Computer-based systems for the acquistion and treatment of data from flash photolysis, laser photolysis and pulse radiolysis experiments are described. For the flash photolysis and laser photolysis systems, data acquisition of transient optical absorbance signals takes place via a Biomation 610B transient recorder or a Digital Equipment Corp. AR-11 real-time subsystem. The digitised data are stored in a PDP-11/10 computer and treated to obtain kinetic and spectral parameters such as rate constants. For the pulse radiolysis system, data acquisition also takes place via a Biomation 610B transient recorder, and data storage and reduction is carried out with a DEC PDP-11/03 LSI computer. Auxiliary equipment for control of the data acceptance is described, together with the computer programs used for data reduction. Examples of the use of the systems are given, and the advantages of a computer-based system over a manual system discussed.

Oxidation of (5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra-azacyclotetradeca-4,11-diene)copper(II) by radicals produced by flash photolysis and reactions of the oxidized copper complex

The title complex, [CuL1]2+, is oxidized in aqueous solution by OH and [Cl2]– radicals generated by flash photolysis. The oxidation by OH radicals is initiated by H-atom abstraction from the ligand (k 1.5 × l010 dm3 mol–1 s–1). The spectrum of the transient oxidized copper complex is more characteristic of a copper(II)–ligand-radical species than of a copper(III) macrocyclic complex. The oxidized complex is itself a powerful oxidant, oxidizing Cl– to Cl2(k 1.5 × 103 dm3 mol–1 s–1), [N3]– to N2(k 2.5 × 104 dm3 mol–1 s–1), and H2O2 to HO2(k 7.0 × 102 dm3 mol–1 s–1) as well as HO2 and [O2]– radicals. The oxidized [CuL1]2+ species is probably an intermediate in the decay of [CuL1]3+ in acetonitrile.

Kinetics of oxidation of transition-metal ions by halogen radical anions. Part IV. The oxidation of vanadium(II) and chromium(II) by di-iodide, dibromide, and dichloride ions generated by pulse radiolysis

The radical ions I2–, Br2–, and Cl2–, generated by nanosecond pulse radiolysis of solutions containing the halide ions, oxidise vanadium(II) to vanadium(III) and chromium(II) to chromium(III). At 22 ± 3 °C and I= 0·5 mol l–1 rate constants for the reactions X2–+ V2+aq→ V3+aq+ 2X– are (1·43 ± 0·2)× 108, (1·48 ± 0·2)× 109, and (1·95 ± 0·2)× 109 l mol–1 s–1(X = I, Br, and Cl) and the reaction mechanism is outer sphere in all cases. Corresponding rate constants for the reactions X2–+ Cr2+aq→[Cr(OH2)5X]2++ X– are (1·5 ± 0·2)× 109, (1·9 ± 0·2)× 109, and (2·4 ± 0·3)× 109 l mol–1 s–1. The reaction mechanisms were established by gamma-radiolysis and flash-photolysis experiments. The reaction between the ions Cl2– and Cr2+aq proceeds by parallel inner- and outer-sphere paths with approximately equal probability. The radical ions I2– and Br2– react with Cr2+aq ions entirely by inner-sphere paths.

Radical oxidation of nickel(II) complexes of tetra-azamacrocyclic ligands and the reactions of the resulting nickel(III) complexes: a pulse-radiolysis and flash-photolysis study

The nickel(II) complexes [NiL]2+[L = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra-azacyclotetradecane (L1), 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra-azacyclotetradeca-4,11-diene (L2), or 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra-azacyclotetradeca-1,4,8,11-tetraene (L3)] are oxidised to the nickel(III) complexes in aqueous solutions by the radical ions [Cl2]–, [Br2]–, and [(SCN)2]–, generated by pulse radiolysis and flash photolysis. The outer-sphere electron-transfer reactions are diffusion controlled. Nickel(III) complexes of the ligands which contain NH groups (L1 and L2) are converted into nickel(II) complexes of radical forms of the ligands at pH >3. Hydroxyl radicals oxidise the nickel(II) complexes by initial H-atom abstraction from the ligand, and the nickel(III) complexes are then formed by proton-assisted intramolecular electron transfer. The nickel(II) ligand-radical complexes are more stable than the nickel(II) complexes and are intermediates in the decay of the nickel(III) complexes in water. The nickel(III) complexes are powerful oxidants and the rates of oxidation of Cl–, Br–, [SCN]–, and [N3]– are dependent on the reduction potentials of the complexes. Hydrogen peroxide is oxidised by the nickel(III) and nickel(II) ligand-radical complexes at similar rates. Oxidation of [O2]– radical ion by the nickel(III) complexes is diffusion controlled; the rates of oxidation of HO2 radicals are much smaller and are the same for all three complexes. Perhydroxyl radicals oxidise the nickel(II) complex of the saturated ligand, L1, to the nickel(III) complex.

Luminescence from [Ru(bipy)3]2+ ions adsorbed on ion exchange surfaces. Dynamic quenching reactions between adsorbed cations

Luminescence lifetime measurements show that when both luminescent and quencher ions are adsorbed on cation exchange resin luminescence quenching takes place by dynamic bimolecular processes involving energy transfer or electron transfer at rates which are of the same order as those in homogeneous solution.

Aqueous chemistry of thallium(II). Part II. Kinetics of reaction of thallium(II) with manganese(II), iron(II), and cobalt(III) ions

Thallium(II) ions generated by flash photolysis of thallium(III) solutions oxidise manganese(II) and iron(II) and reduce cobalt(III). Rate constants for the reactions are: (Tl2++ Mn2+→ Tl++ Mn3+)(1·9 ± 0·2)× 104(0·5M-acid, I= 0·75M, and 22 ± 3 °C); (Tl2++ Fe2+→ Tl++ Fe3+)(2·6 ± 0·1)× 106(0·25M-acid. I= 0·30M, and 25 °C); ΔH‡= 4·8 ± 2 KJ mol–1; and (Tl2++ Co3+→ Tl3++ Co2+)(9·5 ± 0·5)× 106 l mol–1 s–1(0·5M-acid. I= 0·55M, and 22 ± 3 °C). The reactions are apparently outer sphere in type, and comparison of the standard free energies of reaction and the activation free energies for these and other one-electron-transfer reactions involving thallium(II) with the same parameters for two-electron transfers of thallium(III) shows that the latter have activation free energies which are some 25 KJ mol–1 larger than those for one-electron-transfer reactions with the same standard free-energy change, reflecting the increased organisation needed in the transition state for the transfer of two electrons.