Alexander McAuley

The Redox Chemistry of Nickel

Publisher Summary The discovery of nickel(III) and nickel(I) in methanogenic bacteria and in other biological systems has focused attention on the redox chemistry of nickel. This chapter discusses the redox chemistry of nickel basing on the most recent advances. This chapter presents a discussion on the structural and electronic requirements of the four oxidation states—I, II, III, and IV— and of the primary physical techniques and probes used. A description of the most commonly used ligand types follows, together with an indication of recent trends in the area. Discussions conclude with a review of the recent mechanistic chemistry of the oxidation states in turn. The nickel(II) ion has a d 8 electronic configuration and, with weak-field ligands such as H 2 O, it forms a six-coordinate ion with approximately octahedral symmetry and a paramagnetic (two unpaired electrons) 3 A 2 ground state. The X-ray crystal structures of a variety of high- and low-oxidation-state nickel complexes are now known. The technique most widely applied as a structural probe in the chemistry of both nickel(III) and nickel(I) is electron paramagnetic resonance. The mechanistic chemistry of nickel(IV) is dominated by substitution inert complexes and outer-sphere electron transfer reactions.

Metal-ion oxidations in solution. Part XII. Oxidation of thiourea and NN′-ethylenethiourea by chromium(VI) in perchlorate media

The oxidation of thiourea and its NN′-ethylene derivative (L) proceeds via the formation of 1 : 1 complexes, [O3CrL], which are considered to be sulphur bonded. These intermediates, formed within the time of mixing in the stopped-flow apparatus, have been characterised spectroscopically and thermodynamic parameters for reactions (i) have been measured. At 25 °C for thiourea. K= 380 ± 25 l–2 mol–2, ΔH=–9.8 ± 0.6 kcal mol–1, H++[(HO)CrO3]–+ L [graphic omitted] [O3CrL]+ H2O (i), and ΔS=–21 ± 3 cal K–1 mol–1, whilst for NN′-ethylenethiourea K= 211 ± 20 I–2 mol–2, ΔH=–8.8 ± 0.9 kcal mol–1, and ΔS=–19 ± 4 cal K–1 mol–1. The kinetics of the electron-transfer reactions to yield CrIII have been investigated. The reaction products in both cases are [Cr(H2O)6]3+ and [Chromium(III)–L]3+ complexes. In both systems, the principal path involves a reaction of overall stoicheiometry (ii) with the formation of a CrIV, 2H++[O3CrL]+ L [graphic omitted] CrIV+ L – L (ii), intermediate which undergoes further reduction. For L = thiourea at 25 °C, k= 33 I3 mol–3 s–1, ΔH‡=⩽1 kcal mol–1, and ΔS‡=–45 ± 6 cal K–1 mol–1, whilst for NN′-ethylenethiourea k= 47.5 I3 mol–3 s–1, ΔH⩽1.5 kcal mol–1, and ΔS‡=–45 ± 5 cal K–1 mol–1. Details of other hydrogen ion-catalysed reactions are presented and the data are compared with those for other systems of this type.

The synthesis, X-ray structure and hydrolysis kinetics of chloro(2-aminomethylpyridine) (triamine)chromium(III) salts

The chloro(diamine)(triamine)chromium(III) complexes, [CrCl(ampy)(dpt)](ClO4)2 (1) and [CrCl(ampy)(2,3-tri)]ZnCl4·H2O (2) were prepared from [CrCl3(tri)] and 2-aminomethylpyridine (ampy) (tri = dpt = 1,5,9-triazanonane or 2,3-tri = 1,4,8-triazaoctane) and the isomeric configurations were established by single crystal X-ray structural analysis. 1: orthorhombic, P212121 a = 8.281(1), b = 13.390(1), c = 19.091(2) A, V = 2116.73(30) A3, Z = 4. 2: orthorhombic, P212121, a = 8.515(1), b = 9.636(1), c = 26.542(1) A, V = 2177.79(26) A3, Z = 4. In both complexes the tridentate polyamine adopts the mer configuration with the sec-N proton remote (exo) from the coordinated chloro ligand. For the dpt system the pyridine end of the ampy ligand is trans to the chloro ligand, but in the 2,3-tri system the pyridine end of the ampy ligand is trans to the sec-NH group of the tridentate. Thus the cations are formulated as exo-trans(py)-mer-[CrCl(ampy)(dpt)]2+ (1) and exo-cis(py)-mer-[CrCl(ampy)(2,3-tri)]2+ (2). The rates of thermal acid hydrolysis (kH), Hg2+-assisted acid hydrolysis (kHg) and base hydrolysis (kOH) have been measured for 1 and 2. Kinetic parameters (kx (25 °C), ΔH# (kJ mol−1), ΔS# (J K−1 mol−1)) for 1 are 107 kH = 3.16 s−1, 101, −32; 106 kHg = 57.2 M−1 s−1, 102, +16; kOH = 2.17 M−1 s−1, 113, +140 and for 2 are 107 kH = 0.21 s−1, 109, −25; 106 kHg = 4.4 M−1 s−1, 106, +9; kOH = 0.355 M−1 s−1, 108, + 110. The incorporation of a coordinated pyridine ligand cis to the leaving group in 2 does not appear to have a significant acceleratory role in the base hydrolysis, with respect to the analogous exo-mer- [CrCl(en)(2,3-tri)]2+ cation.

Novel geometries exhibited by three palladium(II) macrocyclic complexes: crystal and solution structures

Abstract The synthesis of three related macrocyclic complexes of palladium(II) is reported, together with their structural characterization by crystallography and NMR. The crystal structure of [PdL 1 ](Cl)(PF 6 ) (L 1 = 1,4,7-trithia-11-azacyclotetradecane) ( P 1 (No. 2), a = 10.127(2) A , b = 12.568(4) A , c = 7.141(2) A , α = 87.99(2)°, β = 95.55(2)°, γ = 91.02(2)°, Z = 2, R = 0.0422, R w = 0.0486 exhibits an ‘endo’ coordination of the metal ion. For the related pendant-arm macrocylic ligand, N -(2′-pyridylmethyl)-1,4,7-trithia-11-azacyclotetradecane (L 2 ), the solid state structures of the [PdL 2 Cl](PF 6 ) ( P 2 1 2 1 2 1 (No. 19), a = 13.305(3) A , b = 12.413(4) A , c = 14.060(3) A , Z = 4, R = 0.0538, R w = 0.0529) and [PdL 2 ](BF 4 ) 2 ·0.5H 2 O ( I 2/ a (No. I5) (non-standard setting of the space group C 2/ c ), a = 19.045(8) A , b = 16.952(4) A , c = 16.635(6) A , β = 113.47(3)°, Z = 8, R = 0.0532, R w = 0.0560) compounds exhibit a palladium ion that is ‘chelated’ by the macrocyclic ligand and is only partially coordinated by the macrocyclic donor set. Each of these latter complexes exhibits fluxional NMR spectra. The former complex ion undergoes a simple inner-sphere substitution while the latter exhibits more complex behaviour.

Initial state and transition state solvation effects in the cobaltitungstate oxidation of iodide in binary aqueous solvent mixtures

SummaryRate constants are reported for 12-tungstocobaltate(III) [CoW12O40]5− oxidation of iodide in water and in binary aqueous solvent mixtures containing up to 40% methanol, 40% acetonitrile, or 60% dimethyl sulphoxide. From these kinetic results, solubility measurements on potassium 12-tungstocobaltate(III), and published data on Gibbs free energies of transfer of appropriate ions, it has been deduced that the dominant factor in determining the marked decrease in rate observed on going from water into the binary aqueous solvent mixtures is destabilisation of the transition state for the electron-transfer reaction.

Metal-ion oxidations in solution. Part 16. The oxidation of α-hydroxycarboxylic acids by cerium(IV) in perchioric acid media

The redox reactions between cerium(IV) ions and α-hydroxycarboxylic acids (HA) proceed via an inner-sphere mechanism. Investigations over the range 6.4–30 °C using stopped-flow methods have demonstrated the existence of intermediate complexes. The complex [CeA]3+is thermodynamically more stable than the protonated [Ce(HA)]4+ [graphic omitted] H++[CeA]3+ [graphic omitted] CeIII+ A·(i) form [Ce(HA)]4+and is more reactive kinetically. Formation constants for the complexes have been obtained both from initial optical-density changes and kinetic data. For reactions (ii) the formation constants at 25 °C and [Ce(OH)]3++ HA =+ [graphic omitted] [CeA]3+(ii) thermodynamic parameters are: glycolic acid, K2= 119 ± 13 dm3mol–1, ΔH2=–14.8 ± 2.5 kcal mol–1, ΔS2, =–40 ± 5 cal K–1mol–1: lactic acid, K2= 191 ± 14, ΔH2, =–12.8 ± 0.75, ΔS2, = 33 ± 4; 2-hydroxy-2-methyl-propanoic acid, K2= 372 ± 46, ΔH2, =–18.5 ± 3.3, ΔS2, =–50 ± 6; phenylglycolic acid. K2= 645 ± 120 (17.5 °C), ΔH2=–14.7 ± 1.5. ΔS2=–37 ± 5. Rate constants, k2, for the intramolecular redox reactions at 25 °C for the four substrates are 0.37, 2.25, 2.57, and 92 s–1 respectively, the corresponding ΔH‡ values being 28.8 ± 1.2, 25.0 ± 0.8, 28.5 ± 0.4, and 23.4 ± 1.3 kcal mol–1. No medium effects have been observed on substitution of lithium for sodium as the counter ion of the background electrolyte. Attempts to characterise the radical A· using flow-e.s.r. techniques have been unsuccessful. The rate data are compared with those in sulphuric acid media and possible similarities in the reaction paths are discussed.

Reduction of the copper(II) cation in acetonitrile by trimethyl phosphite. Evidence for an intermediate copper(II) complex

Abstract Reduction of the solvated copper(II) cation by trimethyl phosphite in acetonitrile occurs via a short-lived purple intermediate believed to be a copper(II)-phosphite complex.

Reactions between copper(II) and 2-mercaptosuccinic acid in aqueous perchlorate solution

The anaerobic oxidation of 2-mercaptosuccinic acid (H3L) by copper(II) in aqueous acidic perchlorate media (pH 2–4) proceeds by the formation of a sulphur-bonded transient [CuL]– which absorbs maximally at 350 nm with an absorption coefficient of 1 600 ± 50 dm3 mol–1 cm–1 In conditions of a large excess of substrate the major redox path involves the acid-catalysed dimerization of [CuL]–, equation (i), with a rate constant k3= 3 dm3[Cu(HL)]+[CuL]–→[Cu2HL2]–(i) mol–1 s–l at /= 0.1 mol dm–3 and 25 °C. Subsequent attack by substrate according to the overall equation (ii)[Cu2HL2]–+ 2H3L →[Cu2(H2L)2]+[H2L′–L′H]–(ii) results in formation of the corresponding disulphide and a copper(I) dimer which is fully formed when the ratio [H3L]0 : [Cu2+]0 exceeds 2 : 1. The mechanism is discussed in terms of the formation of a two-electron redox template which allows direct production of disulphide without formation of high-energy radical species. Various side reactions are noted.

Synthesis and characterization of Ni(II) and Cu(II) complexes of 6-(β-(3,4-dimethoxyphenylethyl))cyclam (L1) and 6-(β-(3,4-dihydroxyphenylethyl))cyclam (H2L2) (cyclam=1,4,8,11-tetraazacyclotetradecane). X-ray crystal structures of [Cu(L1)Br2] and [Cu(H2(BrL2))Br]Br·H2O and metal ion templated formation of multinuclear macrocyclic complexes

The synthesis of 6-(β-(3,4-dimethoxyphenyl)ethyl)cyclam (L1) and the corresponding Ni(II) and Cu(II) complexes is described. Demethylation of the complexes of L1 was carried out with BBr3 and the corresponding Ni(II) and Cu(II) complexes of H2L2 (H2L2=6-(β-(3,4-dihydroxyphenyl)ethyl)cyclam) have been isolated and characterized. Reaction of [Cu(L1)Br2] with Br2 resulted in bromination of the phenyl group, to yield [Cu(BrL1)Br2]. Demethylation of [Cu(BrL1)Br2] yielded the corresponding bromo-catechol appended macrocyclic complex [Cu(H2(BrL2))]Br2, where H2(BrL2)=6-(β-(6-bromo-3,4-dihydroxyphenyl)ethyl)cyclam. The crystal structures of [Cu(L1)Br2] and [Cu(H2(BrL2))Br]Br·H2O have been determined. In both complexes, the Cu(II) ion is within the macrocyclic cavity with an average CuN distance of (2.01 A). In [Cu(L1)Br2], the Cu(II) is pseudo-octahedral, with CuBr(1)=2.9996(3) and Cu Br(2)=2.925(3) A whereas in [Cu(H2(BrL2))Br]Br·H2O, the Cu(II) is square pyramidal with a CuBr distance of 2.904(2) A. Reaction of [M(H2L2)]2+ (M=Ni2+ and Cu2+) ions with Fe(III) in basic aqueous media led to the formation of the tetranuclear species [Fe(M(L2)Br2)3]3−, which has been monitored by UV–Vis and EPR spectroscopy.

Metal-ion oxidations in solution. Part 17. The kinetics and mechanism of the oxidation of malonic acid by cerium(IV) in perchloric acid media

The redox reaction between CeIV and malonic acid (H2L) proceeds via an inner-spheremechanism. Using stopped-flow methods, the oxidation has been investigated over the range 2.4–35 °C. A notable feature of the reaction is that, whilst at low temperatures there is kinetic and spectroscopic evidence for intermediate complex formation. at higher temperatures (30 and 35 °C) the reaction order changes to unity with respect to the reductant concentration. The reaction is catalysed by hydrogen ions. The data are rationalized in termsof thefollowing reactionscheme [graphic omitted] in which the principal path involves reaction of [Ce(H2L)]4+(K5 < 1 mol dm–3). The significance of the relative magnitudes and temperature dependences of the equilibrium constants is discussed and the incorporation of the kinetic term k2 provides a general mechanism for cerium(IV) oxidations. Flow e.s.r. techniques have been used to characterize the radical R˙, and its stability compared with radical intermediates fromtheoxidationofother substrates is discussed.