Pradyot Banerjee

Synthesis, crystal structures and magnetic properties of 1D polymeric [MnIII(salen)N3] and [MnIII(salen)Ag(CN)2] complexes

The syntheses, crystal structures and variable temperature (5–300 K) magnetic susceptibility measurements of two salen complexes of manganese(III) [where H2salen = N,N′-bis(salicylidene)-1,2-diaminoethane] having azido and dicyano argentate(I) as bridging ligands are reported. Both complexes belong to one-dimensional systems in which the azido functions as μ-(1,3) and Ag(CN)2− as ⋯NC– Ag–CN⋯ bridging ligands. The azido-bridged compound is antiferromagnetic with an intrachain interaction constant J = −4.52(4) cm−1 (Weng model) or J = −5.19(8) cm−1 (Fisher model), and it is a candidate to observe the Haldane gap in a S = 2 system. The silver cyanide bridged complex shows single ion behaviour of the Mn(III) ion, perhaps in combination with a very weak antiferromagnetic interaction.

Four μ4-oxo-bridged copper(II) complexes: magnetic properties and catalytic applications in liquid phase partial oxidation reactions

Four copper(II) complexes, [Cu(4)(O)(L(n))(2)(CH(3)COO)(4)] with N(2)O-donor Schiff-base ligands, where HL(1) = 4-methyl-2,6-bis(cyclohexylmethyliminomethyl)phenol for complex 1, HL(2) = 4-methyl-2,6-bis(phenylmethyliminomethyl)phenol for complex 2 x CH(3)CN, HL(3) = 4-methyl-2,6-bis(((3-tri-fluoromethyl)phenyl)methyliminomethyl)phenol for complex 3, HL(4) = 4-methyl-2,6-bis(((4-tri-fluoromethyl)phenyl)methyliminomethyl)phenol for complex 4, were synthesized and characterized by elemental analysis, FT-IR, UV-vis spectroscopy and finally by single crystal X-ray diffraction study. X-Ray analysis reveals that all of these are mu(4)-oxo-bridged tetrameric copper(II) complexes. Four copper atoms arrange themselves around an oxygen atom tetrahedrally. Magnetic susceptibility measurements show the existence of very strong antiferromagnetic coupling among these ions (J = -210.1 to -271.3 cm(-1)), mediated by the oxygen atoms. Catalysis of the epoxidation of cyclohexene, styrene, alpha-methylstyrene and trans-stilbene by these complexes has been carried out homogeneously as well as heterogeneously by immobilizing the metal complexes over 2D-hexagonal mesoporous silica. The results obtained in both the catalytic conditions show that the olefins are converted to the respective epoxides in good yield together with high selectivity.

Electron transfer reactions of nickel(III) and nickel(IV) complexes

Abstract This review narrates the electron transfer reactions of various nickel(III) and nickel(IV) complexes reported during the period 1981 until today. The reactions have been categorized mainly with respect to the type of nickel complexes. The reactivity of nickel(III) complexes of macrocycles, 2,2′-bipyridyl and 1,10-phenanthroline, peptides and oxime–imine, and of nickel(IV) complexes derived from oxime–imine, oxime and miscellaneous ligands with various organic and inorganic electron donors have been included. Kinetic and mechanistic features associated with such interactions have been duly analyzed. The relevance of Marcus cross-relation equations in the delineation of the electron transfer paths has also been critically discussed.

Electron exchange and transfer reactions of heteropoly oxometalates

A. Introduction B. Electron exchange in dodecatungstocobaltate(II/III) and dodecatungstocuprate(I/II) C. Electron transfer reactions of dodecatungstocobaltate(II1) (i) Reactions with inorganic reagents (ii) Reactions with thiols and related species (iii) Reactions with carboxylates, cc-hydroxycarboxylates and amine-N-polycarboxylates (iv) Reactions with carbonyl compounds and esters (v) Reactions with carbohydrates (vi) Reactions with substituted alkyl aromatic compounds (vii) Miscellaneous reactions D. Electron transfer reactions of dodecatungstocobaltate(I1) (i) Reactions of [CO”W,,O,,]~with peroxodisulphate and periodate (ii) Reactions of [Co”W,,O,,]‘~ and [CO”W,,O,,]~with polyhalogenated alkanes E. Electron transfer reactions of dodecamolybdocerate(IV) F. Electron transfer reactions of nonamolybdonickelate(IV) and nonamolybdomanganate(IV) G. Conclusions Note added in proof References . . . .._..._ _........__._.......__.....

Kinetics and mechanism of the reduction of 12-tungstocobaltate(III) by formate. Catalysis by alkali metal ions through outersphere bridging

Abstract The kinetics of the reduction of 12-tungstocobaltate(III) by the molecular and anionic forms of formic acid follow the rate laws -d[Co III M]/d t = 2 k 1 [Co III M] 2 [HCOOH] and -d[Co III M]/d t = 2 k 2 - [HCOO − ] [Co III M] 2 respectively at a constant alkali metal ion concentration. Although the low pH rate ( k 1 ) remains uninfluenced by the alkali cations, the magnitude of the apparent third-order rate constant ( k 2 ) for the oxidation of formate ion largely depends on both the nature and the concentration of the cation present. The cation catalytic order, K + > Na + >Li + , has been successfully explained with the help of the polarizability concept. A first-order dependence of k 2 on [K + ] but second-order dependence on [Na + ] and [Li + ] is observed. These cation assisted paths have been studied thoroughly and the rate constants for these cation accelerated paths along with that for the spontaneous process have been evaluated. Activation parameters corresponding to the rate constants of the spontaneous and sodium ion assisted paths have been determined. A cation bridged outersphere electron transfer mechanism with the generation of free radicals is suggested.

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′].

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.

Oxygenation of 4-tert-butylcatechol catalysed by a manganese(II) complex: implications for extradiol catechol dioxygenases

An N donor tetradentate manganese complex, [MnII(bispicen)Cl2] (A) [bispicen = N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine)] catalyses the oxidative cleavage of 4-tert-butylcatechol (1) in the presence of O2. The oxygenated products were isolated by t.l.c. and column chromatography and characterised by 1H-, 13C-n.m.r., DEPT, i.r. and u.v.–vis. spectroscopy. The oxygenated products as well as other spectral evidence suggest that the oxygenation occurs via a 4-tert-butylsemiquinone bound complex, [MnII(bispicen)(4-sq)]+ (4-sq = 4-tert-butylsemiquinone). 1H-n.m.r. spectroscopy suggests that the oxygenation follows multiple pathways. Isolation of the products suggests that the oxygenations proceed in an extradiol fashion and a probable mechanism is suggested. Some intradiol cleaved products have also been detected. E.s.r. spectroscopy suggests that manganese(II) is ultimately converted into the manganese(IV) species.

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.