Tippu S. Sheriff

Manganese catalysed reduction of dioxygen to hydrogen peroxide: structural studies on a manganese(III)–catecholate complex

Abstract The complex [Na]5[Mn(3,5-(SO3)2Cat)2]·10H2O (H2Cat=1,2-dihydroxybenzene) has been synthesised and characterised by X-ray diffraction (triclinic system, space group P 1 ). The characterisation of this complex supports previous work that manganese(III) is extremely reluctant to form tris (catecholato) complexes due to the short ‘bite distance’ of catecholate oxygen atoms (2.60 A) which are unable to span the elongated coordination axes of the Jahn–Teller distorted Mn(III) ion. Hydrogen peroxide production is demonstrated with [Na]5[Mn(3,5-(SO3)2Cat)2]·10H2O using dioxygen and hydroxylamine as substrates at pH 8.0 in aqueous solution under ambient conditions. In the presence of added Tiron (1,2-dihydroxybenzene-3,5-disulfonate, disodium salt monohydrate) turn over frequency (TOF) in H2O2 (the number of moles of H2O2 per moles of manganese per hour) of ∼10 000 h−1 are obtained. The redox and structural motif characteristics imposed on this system by catecholate ligands together with the precise electronic requirements determined by the substituents on the catechol ring provide the enzyme-like characteristics of this systems unique ability to activate O2 for reduction to hydrogen peroxide.

Calmagite dye oxidation using in situ generated hydrogen peroxide catalysed by manganese(II) ions

Hydrogen peroxide (H(2)O(2)) generated from the manganese(II) catalysed reduction of dioxygen has been shown to efficiently oxidize Calmagite (3-hydroxy-4-(2-hydroxy-5-methylphenylazo)naphthalene-1-sulfonic acid) in aqueous solution at pH 8.0 and 20 +/- 1 degrees C with de-protonated Tiron (1,2-dihydroxybenzene-3,5-disulfonate, disodium salt) acting as an essential co-ligand.

The production of hydrogen peroxide from dioxygen using hydroxylamine as substrate catalysed by Mn2+-exchanged montmorillonite clay

Abstract Mn 2+ -exchanged montmorillonite clay, in milligramme quantities, efficiently catalyses the production of hydrogen peroxide from dioxygen (or air) and hydroxylamine at pH 8.0 in aqueous solution; concentrations of hydrogen peroxide >0.40 mol dm −3 (∼ 1.5% w/v H 2 O 2 , ∼ 86% based on hydroxylamine) in 50 min and turnover numbers [H 2 O 2 ]/[Mn 2+ ] >10 5 were obtained. The mechanism for hydrogen peroxide production in this system is discussed together with the possibilities of developing a dual system to use the in situ generated hydrogen peroxide to effect some organic oxidation transformations.

Selective oxidative degradation of azo dyes by hydrogen peroxide catalysed by manganese(II) ions

Manganese(II) ions catalyse the oxidative degradation of Calmagite (H3CAL) dye in aqueous solution at 20 ± 1 °C in the pH range 7.5–9.0 using hydrogen peroxide (H2O2) as oxidant by a mechanism that involves strong complexation to the MnII centre. It is proposed that [MnIII(CAL)(O2H)]− i.e. a dye coordinated hydroperoxyl (O2H−) MnIII complex is formed and bleaching of the dye is initiated by an electron-transfer to MnIII, with the binding of H2O2 being the rate determining step. At pH 9.0 in (bi)carbonate, HCO3−, H3CAL is rapidly bleached via the in situ formation of coordinated peroxycarbonate (HCO4−); a TOF (TOF = moles of dye bleached per mole of manganese per hour) of ∼5000 h−1 can be achieved. The bleaching of the related azo dyes Orange II and Orange G is different because, unlike Calmagite, they lack an o,o-dihydroxy motif so are unable to complex strongly to MnII and no oxidation to MnIII occurs. At pH 8.0 (phosphate buffer) Orange II and Orange G are not bleached but bleaching can be achieved at pH 9.0 (HCO3− buffer); the rate determining step is dye coordination and it is proposed bleaching is achieved via an outer-sphere oxygen atom transfer. Mechanisms for dye bleaching at pH 8.0 and pH 9.0 are proposed using data from EPR, UV/VIS and ESI-MS. MnII/H2O2/HCO3− form a potent oxidising mixture that is capable of removing stubborn stains such as curcumin.

Selective detection of hydrogen peroxide vapours using azo dyes

A rapid and selective visual colour method is described for detection of hydrogen peroxide (H2O2) and peroxide based explosive (PBE) vapours by the combination of three azo dyes – Calmagite, Orange G and Orange II. The bleaching of these dyes by H2O2 is catalysed by MnII. At pH 8.0 (EPPS, N-2-hydroxyethyl-piperazine-N′-3-propanesulfonic acid) Calmagite is quickly degraded but under these conditions Orange G and Orange II are not perceptibly bleached, especially in the presence of ethylenediaminetetraacetic acid (H4edta). However, fast bleaching of Orange G and Orange II was observed at pH 9.0 (Na2CO3, sodium carbonate) due to the in situ formation of the carbonate radical (CO3−˙). Hence by a combination of Calmagite at pH 8.0, and Orange G and Orange II at pH 9.0, selectivity to H2O2 vapours against Cl2, NO2 and O3 can be demonstrated. Initial studies were carried out on filter papers but reaction times were slow. With Calmagite rapid colour changes, within 15 minutes, were found when the dye was deposited on to polyvinyl alcohol (PVA) polymer which had been coated onto a borosilicate glass plate. However, with Orange G and Orange II the colour changes on the PVA/plates were slow and this may be due to the limited availability of CO2, as the activating species, in generating CO3−˙.

Nucleophile and base differentiation of pyridine with tetrahalocatechols and the formation of manganese(III)-catecholate and pyridinium-catecholate complexes for the in situ generation of H2O2 from O2

Crystal structures of two novel pyridinium catecholate compounds (1,2-dihydroxy-3,5,6-trichlorobenzene-4-pyridinium chloride and 1,2-dihydroxy-3,5,6-tribromobenzene-4-pyridinium bromide) were obtained by the reaction of pyridine with tetrachloro-o-benzoquinone (in the presence of hydroxylamine) and tetrabromocatechol respectively. A similar reaction with tetrachlorocatechol as a starting substrate showed pyridine to act as a base rather than a nucleophile, with a crystal structure of the pyridinium-catecholate salt obtained. The role of a number of manganese-catecholate complexes as catalysts in the reduction of dioxygen to hydrogen peroxide was also investigated. Diaqua-bis(3,5,6-tribromobenzene-4-pyridinium catecholate)manganese(III) bromide·MeOH, [pyH][MnIII(Br4Cat)2(H2O)(py)] and [4-MepyH][MnIII(Br4Cat)2(H2O)(4-Mepy)] (where CatH2 = catechol) were synthesised and characterised by melting point, FTIR, CHN (and Mn) analysis, mass spectrometry and UV-Vis spectroscopy. All showed catalytic behaviour in dioxygen reduction at 20 ± 1 °C and pH 8.00 in the presence of hydroxylamine as reducing substrate, with initial rates of hydrogen peroxide generation and turnover frequencies of up to 11.2 × 10−5 mol dm−3 s−1 and 8060 h−1 respectively in the presence of a 30-fold molar excess of ligand.

Crystal structure of bromido-nitro-syl-bis(tri-phenyl-phosphane-κP)nickel(II).

The asymmetric unit of the title complex, [NiBr(NO){P(C6H5)3}2], comprises two independent molecules each with a similar configuration. The NiII cation is coordinated by one bromide anion, one nitrosyl anion and two triphenylphosphane molecules in a distorted BrNP2 tetrahedral coordination geometry. The coordination of the nitrosyl group is non-linear, the Ni—N—O angles being 150.2 (5) and 151.2 (5)° in the two independent molecules. In the crystal, molecules are linked by weak C—H⋯Br hydrogen bonds and weak C—H⋯π interactions into a three-dimensional supramolecular architecture.