Sumana Gangopadhyay

Oxidation reactions of mononuclear manganese (III) complexes

Abstract This review encompasses the oxidation reactions of various manganese(III) coordinated molecules. The reactions have been categorized primarily with respect to the type of manganese(III) complexes. Emphasis has been given to the reactivity of the manganese(III) complexes derived from aminopolycarboxylic acids, acetylacetone, porphyrins, bipyridine and pyrophosphoric acid with various organic, inorganic and biochemical electron donors. Kinetic and mechanistic features associated with the interactions have been highlighted and analysed critically. The utility and scope of the catalytic oxidation of hydrocarbons and secondary amines by manganese(III) porphyrins are discussed at length.

Mixed ligand palladium(II) complexes of oxalate and malonate with vitamin-B6 molecules: synthesis, crystal structure and kinetics

The synthesis of six mononuclear palladium complexes of general formula [Pd(ox)/(mal)L2] and [Pd(ox)/(mal)L′] (ox = oxalate, mal = malonate, both L and L′ are vitamin-B6 molecules (I), L = pyridoxine, pyridoxal and L′ = pyridoxamine) has been achieved. The structures of these compounds were established by elemental analysis, i.r. and 13C-n.m.r. [Pd(oxalate)(pyridoxine)2] was analyzed by single crystal X-ray diffraction. It exhibits square planar coordination with bond lengths 2.015 (2) Å for Pd—N and 2.010 (2) Å for Pd—O. The interaction of [Pd(ox)2]2− and [Pd(mal)2]2− with L has been followed kinetically in order to look into the nature of products and the mechanism of formation under the conditions [PdII-chelate] ≫ [L] and [L′].

A novel rhodamine-3,4-dihydro-2H-1,3-benzoxazine conjugate as a highly sensitive and selective chemosensor for Fe3+ ions with cytoplasmic cell imaging possibilities

A novel, highly sensitive and selective fluorescent chemosensor ‘rhodamine-3,4-dihydro-2H-1,3-benzoxazine’ [RH-BZN (1)] has been synthesized and characterized by single crystal X-ray diffraction and other physicochemical techniques. In 3:7 water:MeCN (v/v) at pH 7.2 (10 mM HEPES buffer, μ = 0.05 M LiCl) it selectively recognizes Fe3+ through 1:1 complex formation resulting in a 240-fold fluorescence enhancement and a binding constant (Kf) of 1.50 × 104 M−1. The otherwise non-fluorescent spirolactam form of the probe results in dual changes in absorbance and fluorescence arising out of opening of the spirolactam ring through coordination to Fe3+ ions. This probe could suitably be employed for cytoplasmic intracellular imaging of Fe3+ without notable cytotoxicity. The reversible binding of RH-BZN to Fe3+ was confirmed by reacting with tetra-n-butylammonium fluoride both in extra- and intra-cellular conditions.

Kinetics and mechanism of the oxidation of histidine by dodecatungstocobaltate(III) and trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetatomanganate (III) in aqueous medium

The oxidation of histidine with dodecatungstocobaltate(III), CoW12O405– and trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetatomanganate(III), [MnIII(cdta)]– have been investigated in the range pH 3.0–9.5 at variable reductant concentrations, at a constant ionic strength and temperature. Both reactions are found to be dependent on the first powers of the concentration of the substrate and oxidants and follow the general rate law, -d[Ox]/dt= 2k[Ox][Hist], where 2 is the stoichiometric factor. In the reduction of [MnIII(cdta)]–, a bell-shaped curve is obtained for the variation of second-order rate constant (K) as a function of pH, and this has been explained by considering the hydroxo form of the complex as being unreactive towards the reductant. An attempt has been made to verify the effect of alkali cation catalysis on the reaction rate. The observed alkali cation catalysis for the reduction of CoW12O405– is consistent with a rate law, k=k0+kc[M+] where k0 and kc are the rate constants for the spontaneous and catalysed paths respectively. For the reduction of [MnIII(cdta)]–, a negligible dependence of rate on [M+] was noted. All the observations are successfully explained by considering outer-sphere mechanistic pathways for both reactions.

Kinetic studies on the reduction of nickel(IV) and nickel(III) oxime–imine complexes by ascorbic acid

The oxidation of ascorbic acid by [NiIIIL1]2+ and [NiIVL32]2+ complexes (where HL1= 15-amino-3-methyl-4,7,10,13-tetraazapentadec-3-en-2-one oxime and HL3= 6-amino-3-methyl-4-azahex-3-en-2-one oxime) has been investigated by stopped-flow spectrophotometry in the range pH 2.50–8.20, with /= 0.20 mol dm–3 NaClO4 and T= 30 °C, using variable concentrations of ascorbic acid. At a particular pH both the reactions are second order, first order with respect to ascorbic acid and the complexes, and follow the general rate law –1/m d[NiLxn+]/dt=k[NiLxn+][H2A]T where H2A = ascorbic acid and m represents the stoichiometric factor (m= 2 for [NiIIIL1]2+ and 1 for [NiIVL32]2+). In the reduction of [NiIVL32]2+, the monophasic reaction traces throughout the experimental pH range (2.50–8.20) imply the involvement of nickel(III) complexes in the rate-determining step. A detailed evaluation of the reduction was achieved by considering suitable pH regions and employing appropriate computer programs to fit the experimental data. Application of the Marcus theory in calculating the theoretical rate constants and a comparison of these constants with the respective experimental values reveals the occurrence of an outer-sphere mechanism for the oxidation of A2– by both [NiIIIL1]2+ and [NiIVL32]2+ as well as by [NiIIIL2]+ where H2L2= 3,14-dimethyl-4,7,10,13-tetra-azahexadec-3,13-diene-2,15-dione dioxime. The oxidation of H2A and HA– seems to follow a concerted electron–proton transfer with initial association of the reactants.

Oxidation of thioglycolic acid and glutathione by (trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetato)manganate(III) in aqueous media

The kinetics of the electron-transfer reactions of the manganese(III) complex of trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetate (cdta4–) with two thiols thioglycolic acid and glutathione has been investigated at 30 °C in aqueous media in the range pH 2.0–10.33 with varying reductant concentrations at constant ionic strength, I= 0.20 mol dm–3(NaClO4). The reactions are first order both in complex and reductants and follow the general rate law, –d[MnIII(cdta)–]/dt=Kobs[MnIII(cdta)–]=K[MnIII(cdta)–][reductant]. Both the reactions have been assumed to proceed via an inner-sphere mechanism with support for this coming from the observation of a rapid initial increase in absorption followed by a slower decay. This indicates the formation of an inner-sphere associated species which decomposes unimolecularly leading to the transfer of the electron from the thiol to the oxidant. Additional support for this mechanism comes from a comparison of the water-exchange rate of [Mn(cdta)(H2O)]– with the higher limit of the electron-transfer rates. The pH–rate profiles are bell-shaped curves and were successfully modelled by fitting the experimental data to a computer-fitted program thereby evaluating the reactivity of all the reacting species of the reductants.

Kinetics and mechanism of the oxidation of iminodiacetate, nitrilotriacetate and ethylenediaminetetraacetate by trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetatomanganate(III) in aqueous media

Kinetic studies of the oxidation of iminodiacetate (ida), nitrilotriacetate (nta) and ethylene-diaminetetraacetate (edta) by trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetatomanganate(III), [MnIII(cdta)]–, have been made in aqueous solution in the range pH 3.0–10.0 with varying reductant concentrations at constant ionic strength, I= 0.50 mol dm–3(NaClO4), and temperature, 30°C. All the reactions are first order both in complex and reductant concentration, and follow the general rate law –d[MnIII]/dt=kobs[MnIII]=(kd+ks[R])[MnIII], where kd denotes the autodecomposition rate of the complex, ks the electron-transfer rate and R is the reductant irrespective of the nature and type of the reacting species. The complex [MnIII(cdta)]– showed an interesting behaviour in acidic and alkaline media. For ida oxidation both the aqua- and hydroxo-forms of the complex are reactive and an inner-sphere mechanism has been proposed. However, for nta and edta oxidations, the aqua form is the sole reacting species and an outer-sphere mechanism has been proposed. The rate parameters and proton equilibrium constants of the complex and reductants were obtained by fitting of the experimental data by appropriate rate equations using computer-fit programs. Thus the reactivities of all the species of the polycarboxylates available in the reaction solutions have been evaluated individually. The reactivity orders are Hida– < ida2–, H2nta– < Hnta2– < nta3– and H3edta– < H2edta2– < Hedta3– < edta4–.

Kinetic studies on the oxidation of sulfur(IV) by nickel(IV) oxime–lmine complexes

The oxidation of sulfite by the two nickel(IV) oxime–imine complexes [NiIVL12+]2 and [NiIVL2]2+(HL1= 6-amino-3-methyl-4-azahex-3-en-2-one oxime and H2L2= 3,14-dimethyl-4,7,10,13-tetra-azahexadeca-3,13-diene-2,1 5-dione dioxime) has been investigated in aqueous medium at 30 °C over the range pH 2.0–8.0. Single-step two-electron transfer reactions were encountered in the regions 2.0 ⩽ pH ⩽ 5.50 for [NiIVL12]2+ and 2.0 ⩽ pH ⩽ 4.25 for [NiIVL2]2+ reductions. A distinct biphasic process with faster initial one-electron reduction of NiIV to NiIII followed by a slower conversion of NiIII into NiII was observed above pH 5.75 and 4.50 for the respective complexes. Attempts were made to evaluate the reactivity of all the reacting species of the complexes as well as of sulfur(IV) by considering suitable pH regions. The stoichiometries of the reactions ([SIV]:[NiIV]) were found to vary within the region ≈2.0–1.0 depending upon the mole ratios of the reactants. At pH 4.50 with an excess of complex over the reductant the ratio [SIV]:[NiIV] is less than 2:1 for both complexes giving a mixture of dithionate and sulfate as the oxidation products of sulfite. However, with an excess of sulfite over the complex this ratio reaches a limiting value of ≈2:1 at [SIV]:[NiIV] > 8 giving dithionate as the major oxidation product. The observed stoichiometric ratios for both complexes are in accord with the distribution of the sulfate and dithionate products. Attempts were made to correlate the experimentally observed rate constants with those obtained from Marcus cross-relation calculations. Calculated rate constants for the oxidation of SO32– by various nickel-(IV) and -(III) species were found to be one to two orders of magnitude lower than the corresponding experimental values. The oxidation of SO2·H2O and HSO3– is proposed to proceed through the formation of a hydrogen-bonded adduct.

Kinetics and mechanism of the oxidation of some carboxylates by a nickel (III) oxime-imine complex

The kinetics of the oxidation of formate, oxalate, and malonate by |NiIII(L1)|2+ (where HL1 = 15-amino-3-methyl-4,7,10,13-tetraazapentadec-3-en-2-one oxime) were carried out over the regions pH 3.0–5.75, 2.80–5.50, and 2.50–7.58, respectively, at constant ionic strength and temperature 40°C. All the reactions are overall second-order with first-order on both the oxidant and reductant. A general rate law is given as - d/dt|NiIII(L1)2+| = kobs|NiIII(L1)2+| = (kd + nks |R|)|NiIII(L1)2+|, where kd is the auto-decomposition rate constant of the complex, ks is the electron transfer rate constant, n is the stoichiometric factor, and R is either formate, oxalate, or malonate. The reactivity of all the reacting species of the reductants in solution were evaluated choosing suitable pH regions. The reactivity orders are: kHCOOH > k; k > k > k, and k > k < k for the oxidation of formate, oxalate, and malonate, respectively, and these trends were explained considering the effect of hydrogen bonded adduct formation and thermodynamic potential.

Kinetics of the interaction of imidazole, benzimidazole and pyrazole with trans-[Pd(PN)2Cl2] (PN = pyridoxine) in dimethyl sulfoxide

The kinetics of the interaction of trans-[Pd(PN)2Cl2] (PN = pyridoxine) with nitrogen heterocycles e.g., imidazole, benzimidazole and pyrazole, in dimethyl sulfoxide (DMSO) have been carried out at 30 °C using the stopped-flow technique and u.v.–vis. spectrophotometry. Trans-[Pd(PN)2(DMSO)2] was assumed to be the actual reactive species in solution. Four reaction steps can be proposed from an analysis of absorbance-time data and where [Pd(HB)4]Cl2 (HB – heterocyclic bases) is the final reaction product. Rate constants for each step have been evaluated and interpreted.