Electron transfer. 144. Reductions with germanium(II)

Solutions 0.2-0.4 M in Ge(II) and 6 M in HCl, generated by reaction of Ge(IV) with H3PO2, are stable for more than 3 weeks and can be diluted 200-fold with dilute HCl to give GeCl3- preparations to be used in redox studies. Kinetic profiles for the reduction of Fe(III) by Ge(II), as catalyzed by Cu(II), implicate the odd-electron intermediate, Ge(III), which is formed from Cu(II) and Ge(II) (k = 30 M-1 s-1 in 0.5 M HCl at 24 degrees C) and which is consumed by reaction with Fe(III) (k = 6 x 10(2) M-1 s-1). A slower direct reaction between Ge(II) and Fe(III) (k = 0.66 M-1 s-1) can be detected in 1.0 M HCl. The reaction of Ge(II) with I3- in 0.01-0.50 M iodide is zero order in oxidant and appears to proceed via a rate-determining heterolysis of a Ge(II)-OH2 species (k = 0.045 s-1) which is subject to H(+)-catalysis. Reductions of IrCl6(2-) and PtCl6(2-) by Ge(II) are strongly Cl(-)-catalyzed. The Ir(IV) reaction proceeds through a pair of 1e- changes, of which the initial conversion to Ge(III) is rate-determining, whereas the Pt(IV) oxidant probably utilizes (at least in part) an inner-sphere PtIV-Cl-GeII bridge in which chlorine is transferred (as Cl+) from oxidant to reductant. The 2e- reagent, Ge(II), like its 5s2 counterpart, In(I), can partake in 1e- transactions, but requires more severe constraints: the coreagent must be more powerfully oxidizing and the reaction medium more halide-rich.     

Electron Transfer. 130. Reductions with Indium(I)(1)

Aqueous solutions of the hypovalent state indium(I) have been prepared by treatment of In(Hg) with silver triflate in acetonitrile, followed by dilution with oxygen-free water. These solutions are stable for over 5 h at 25 degrees C. In(I)(aq) reacts with oxidants of the type [(NH(3))(5)Co(III)(Lig)](2+) (In(I) + 2Co(III) --> In(III) + 2Co(II)), and kinetic profiles are consistent with a two-step sequence proceeding with formation of the metastable state In(II), which reacts rapidly with Co(III). Rate ratios for reductions of halogeno-substituted oxidants point to predominance of halide-bridged paths for the chloro, bromo and iodo complexes. Reductions of carboxylato-substituted derivatives are slow but appear to entail inner-sphere precursors if aided by an O-donor group in a position favorable for chelation. In no case is there evidence for reaction via initial reduction of the ligand (the radical-cation mechanism) although the potential of the In(I,II) couple (-0.40 V) allows this path for carbonyl-substituted oxidants. Reductions by In(I), like those by Eu(II), make no significant use of bridging by heterocyclic donor nitrogen centers in pyridine and pyrazine complexes.     

Electron transfer. 141. Reactions of indium(I) with transition metal center oxidants

Aqueous solutions of the hypovalent state In(I) reduce a series of complexes of Fe(III) and selected derivatives of Ir(IV), Cr(V), Pt(IV), Ag(III), and Ni(IV). All reactions yield In(III). Ions derived from Fe(III) and Ir(IV) undergo net le- conversions, whereas the other oxidants change by two units. Reductions of Fe(III) are strongly promoted by increasing pH or by adding Cl-, Br-, NCS-, or N3-. Reaction appears to proceed through monoligated complexes, FeIII(Lig-)2+, and the kinetic response to alteration of added ligand indicates initiation mainly via a bridged transition state FeIII-Lig-InI. Oxidations by Fe(CN)6(3-) are exceptionally rapid, those by FeIII(EDTA) are unusually slow, and redox is blocked by addition of excess F-. Reduction of IrCl6(2-) proceeds somewhat more slowly than predicted by the marcus model for outer-sphere reactions. Conversions of the 2e- oxidants are rapid. For these, multistep routes initiated by le- changes are reasonable, but direct 2e-paths involving oxygen transfer (from CrVO) or Cl+ transfer (from PtCl6(2-)) cannot be ruled out. Whether inner-sphere 2e- transactions without transfer of atomic species can be competitive remains an open question.     

Electron transfer reactions of V(IV) with Cl(I), Br(I) and I(I). A kinetic study

A kinetic study on the oxidation of V(IV) by chloramine-T (CAT) at pH 6.85 by N-bromo succinimide (NBS) in aqueous acetic acid–perchloric acid media and by N-iodo succinimide (NIS) in aqueous perchloric acid medium has been carried out. In all the systems studied the order with respect to the oxidant is unity. NBS and CAT oxidation reactions exhibited Michaelis–Menten type kinetics, and the NIS study indicated unit dependence on [substrate]. Independence on acidity has been observed in the case of CAT and NBS reactions, but NIS reactions exhibited inverse unit dependence on [acid]. Novel solvent influences have been noticed in the case of CAT reactions, but with NIS and NBS reactions retardation in the rate has been observed with an increase in the percentage of acetic acid. Plausible mechanisms consistent with the results have been postulated, and suitable rate laws in consonance with the postulated mechanisms have been derived.     

Electron transfer. 123. Competitive redox reactions involving a Cobalt(I) intermediate

he hypophosphito derivative of Co^III, (NH₃)₅CoO₂PH₂ ²⁺, decomposes in basic media, yielding Co(II) quantitatively, along with a 1:1 mixture of hypophosphite and phosphite. Earlier studies point to a cobalt (I) intermediate, which rapidly reduces a second molecule of Co(III) reactant to Co(II). In the presence of an external cobalt (III) oxidant (Co³X), the latter competes with the hypophosphito complex for Co(I), lowering the yield of free hypophosphite. From the ratio of phosphorus products (P³/P¹), evaluated by proton decoupled ³¹P NMR, the relative reactivities (k_t/k_c) of the external Co(III) “traps” and the hypophosphito complex may be determined. A log-log plot comparing values of k_t/k_c with rates for reductions of the same series of Co(III) oxidants with Ru(NH₃)₆ ²⁺ (k_Ru values) is badly scattered with a slope of only 0.23, well below the value of unity stipulated by the Marcus model, indicating that an outer-sphere mechanism cannot operate for all of the Co(III)-Co(I) reactions and may not operate for any, except for that with Co(NH₃)₅(py)₃₊. Among the carboxylato-substituted oxidants, there are no rate enhancements by neighbouring pyridine, -SR,-OH, -CHO, or -SO₃H functions, in contrast to the accelerations previously observed for reductions by Cr(II), Eu(II) and Ti(III), which are attributed to intermediacy of chelate-stabilized precursor complexes. It is suggested that the Co(III)-Co(I) reactions, which determine the ratio of phosphorus products, are much more rapid than the ligand substitution at the Co(I) center which must precede chelate formation, and that the latter therefore does not intervene significantly in the redox processes.     

Isolated cationic crown ether complexes of gallium(I) and indium(I)

The recently reported homologous low-valent indium and gallium salts M(+)[Al(OR(F))(4)](-) (M = Ga, In; R(F) = C(CF(3))(3)) were used to extend the coordination chemistry of Ga(I) and In(I) to the isolated [18]crown-6 complexes [M([18]crown-6)(PhF)(2)](+)[Al(OR(F))(4)](-) in fluorobenzene solution (PhF = C(6)H(5)F). In contrast to known ion-paired compounds for M = In, our complexes are undisturbed and in the solid state free of contacts to the anion. A peculiar combination of very weak η(1)- and η(6)-coordination to the PhF-solvent was observed that allows speculation about the presence of a stereochemically active lone pair at M(I). Structure and energetics of these novel salts were rationalized on the basis of DFT calculations.     

Electron transfer. 137. Formal potentials involving indium(II)

Aqueous solutions of indium(III) undergo 1e⁻ reductions by Sm(II) (E⁰-1.55 V) and by Yb(II) (−1.05 V), but not by U(III) (−0.52 V). The facile and irreversible reduction, by In⁺ _aq, of Ru(NH₃)₆ ³⁺ (E⁰+0.067 V) at reagent concentrations near 10⁻³ M implies a potential more negative than −0.23 V for In(II,I) and, in conjunction with the known value of −0.44 V for In(III,I), a complementary potential less negative than −0.65 V for In(III,II). These observations, taken together, support le⁻ indium potentials E_II,I ^o−0.2 V and E_III,II ⁰−0.6 V.     

Electron transfer. 138. Potentials involving chelated chromium(V)

The bis(chelated) complex of Cr^V(0) derived from the dianion (L^{2 −}) of 2-ethyl-2-hydroxybutanoic acid is readily reduced to a bis(chelate of Cr^III, featuring the monoanion (LH⁻) [Cr ^V(0)(L²⁻)₂]⁻+4H⁺+H₂O+2e⁻→[Cr^III(OH₂)₂(LH⁻ ₂]⁺ of this acid. Potentials estimated by Ghosh in 1993 for this 2e⁻ change, E⁰ (pH 0) 1.32 V, E^eff (pH 3.3) 0.93 V, are in accord with the nearly irreversible reductions of the Cr(V) species (in 1∶1 ligand buffer) by Fe²⁺, V0²⁺, IrCl₆ ^{3 −} and I⁻, whereas lower values reported by Bose in 1996, E⁰ (pH 0) 0.84 V, E^eff (pH 3.3) 0.45 V, are potentiometrically inconsistent with these conversions. A similar discrepancy is noted for potentials for Cr(V,IV) estimated in 1996, E⁰ (pH 0) 0.84 V, E^eff (pH 3.3) 0.46 V, which, wholly contrary to observation, predict that the reductions of excess Cr(V) to CR(IV) by Fe²⁺, V0²⁺, and I⁻ are thermodynamically disfavored.     

Electron transfer reaction of oxo(salen)chromium(V) ion with anilines

The kinetics of oxidation of 16 meta-, ortho-, and para-substituted anilines with nine oxo(salen)chromium(V) ions have been studied by spectrophotometric, ESIMS, and EPR techniques. During the course of the reaction, two new peaks with lambda(max) at 470 and 730 nm appear in the absorption spectrum, and these peaks are due to the formation of emeraldine forms of oligomers of aniline supported by the ESIMS peaks with m/z values 274 and 365 (for the trimer and tetramer of aniline). The rate of the reaction is highly sensitive to the change of substituents in the aryl moiety of aniline and in the salen ligand of chromium(V) complexes. Application of the Hammett equation to analyze kinetic data yields a rho value of -3.8 for the substituent variation in aniline and +2.2 for the substituent variation in the salen ligand of the metal complex. On the basis of the spectral, kinetic, and product analysis studies, a mechanism involving an electron transfer from the nitrogen of aniline to the metal complex in the rate controlling step has been proposed. The Marcus equation has been successfully applied to this system, and the calculated values are compliant with the measured values.     

Electron transfer. 157. Reactions of hypervalent manganese species with S(2) metal-ion reductants

Solutions of the complexes of hypervalent manganese, [Mn(III)(C(2)O(4))(3)](3)(-) (in oxalate buffers), [Mn(IV)(bigH)(3)](4+) (in biguanide buffers), and [(bipy)(2)Mn(III)(O)(2)Mn(IV)(bipy)(2)](3+) (in bipyridyl buffers) may be reduced by s(2) center reductants In(I), Sn(II), and Ge(II), yielding Mn(II) quantitatively. In all cases, rates are determined by the initial act of electron transfer, giving an s(1) transient (In(II), Sn(III), or Ge(III)); subsequent steps are rapid and kinetically silent. The In(I)-Mn(III) and Ge(II)-Mn(III) reactions are inhibited by added oxalate, whereas the Sn(II)-(Mn(III)Mn(IV)) reaction is strongly accelerated by Cl(-). The In(I)-Mn(IV) reaction is complicated by formation of a 1:1 addition compound In(I).Mn(IV). We find no evidence for two-unit steps in any of these systems.     

Electron transfer. 153. Internal electron transfer to bound cobalt(III) induced by hydroxyl radical

Hydroxyl radicals, generated in aqueous solution from Fe²⁺ and H₂O₂, react with the formato, glycolato, lactato and mandelato complexes of (NH₃)₅Co^III, extracting H·, releasing CO₂ and inducing the internal reduction of Co^III to Co²⁺; decomposition of peroxynitrous acid (O=N—OOH) in the presence of these complexes also yields Co²⁺, indicating partial utilization (15% at 22°C and pH 1) of a path involving OH·.     

Electron transfer in electroactive poly(glutamic acid)

Electroactive biopolymer was synthesized by incorporation of ferrocene moieties onto poly(glutamic acid) polymer chains. In the presence of the electron acceptor methyl viologen dichloride, the ferrocene-containing poly(glutamic acid) exhibits efficient photoinduced electron transfer. This redox-active polymer’s electrocatalytic activity for the decomposition of hydrogen peroxide and ascorbic acid was investigated by using cyclic voltammetry. As for hydrogen peroxide, the reduction peak current shows proportional response to peroxide concentration in the wide range of 10-100 mM; as for ascorbic acid, the oxidation peak current displays linear dependence on the ascorbic acid concentration under 80 mM, which could lead to the electroactive biopolymer’s applications in catalysis, photosensitizer, sensors, etc. The study could offer a strategy for developing environment-friendly electroactive and photoactive biopolymers.     

The distinction of gallium(I) tetrachloroaluminate,Ga[AlCl4], from gallium dichloride

Ga[AlCl₄] is obtained as single crystals via the reduction of gallium trichloride with aluminum and slow cooling of the melt. It crystallizes in the monoclinic system, space group p2₁/n, with a = 716.4(1), b = 1017.9(2), c = 926.7(1) pm, β = 93.21(1)°. Unlike in gallium dichloride, Ga[GaCl₄]], where Ga⁺ has a dodecahedral, eight-coordinate surrounding, a 6 + 2 + 1 coordination is adopted in Ga[AlCl₄] that is similarly observed not only in Ga₂[Ga₂Br₆] and in tin (II) and lead (II) chalcogenides but also in (the almost isotypic) K[AlCl₄].     

Electron transfer reactions of iron(III)-polypyridyl complexes with organic sulfoxides

The redox reactions of four iron(III)-polypyridyl complexes with six aryl methyl sulfoxides have been investigated by spectrophotometric technique. The reaction follows clean second order kinetics and proceeds through rate determining electron transfer (ET) from organic sulfoxides to iron(III). The Marcus cross-reaction relation has been applied to obtain the self exchange rate constant for the ArSOR/ArS⁺(O)R couple as 1.3×10⁷ M⁻¹ s⁻¹. The application of Marcus theory to this ET reaction shows that the contribution of inner sphere reorganization energy is 0.4 eV. The rate constant and reaction constant values observed with organic sulfoxides are small compared with organic sulfides towards the same oxidant Fe(NN)₃³⁺.     

Direct titrimetric microdetermination of thallium(I), indium,and gallium(III) separately. I. Microdetermination of indium-thallium(I) and indium-gallium(III) in one solution without separation

Thallium, indium, and gallium are determined in micro amounts quantitatively by direct titrations against hippuric acid separately and in the presence of each other in different sets without separation. Thallium, indium, and gallium form complexes with hippuric acid in the ratio of 1:1 and 1:3. Maximum error in determination of thallium, indium, and gallium is 1.0 and 1.2%, respectively. In separate titrations of the three metals ions interference by different metals ions has also been observed; and the order of interference by metals ions on Tl, In, and Ga is Tl > In > Ga.