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.

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−˙.

The kinetics and mechanism for the bleaching of Calmagite (H3CAL, 3-hydroxy-4-(2-hydroxy-5-methylphenylazo)naphthalene-1-sulfonic acid) in aqueous solution at pH 8.00 and 23 ± 1 °C using in situ generated H2O2 is described. Complete mineralisation of H3CAL results with turnover frequencies (TOF = moles of H3CAL bleached per mole of manganese per hour) of 40 h−1. The monohydroxy azo dyes Me–H2CAL, Orange G and Orange II are not bleached which indicates that a requirement of dye bleaching is the coordination of the dye to the Mn centre. Spectroscopic studies show the formation of Mn(CAL)2 and Mn(CAL) species but in the presence of Tiron (1,2-dihydroxybenzene-3,5-disulfonate, disodium salt, monohydrate, Na2TH2·H2O), [Mn(CAL)(T)] is formed. It is proposed that a Mn(III)–hydroperoxide species is generated, [Mn(O2H)(CAL)(TQ)] from the in situ generated H2O2, where TQ represents the o-quinone form of Tiron, and this is the active species in the bleaching of coordinated CAL; the formation of this hydroperoxide species is supported by UV/VIS and ESI-MS data. The formation of a Mn(III) species is supported by EPR studies which also show some evidence for the presence of a labile d5 Mn(II) species in the presence of the reducing substrate hydroxylamine (NH2OH). This would enable rapid ligand exchange for both in situ H2O2 generation and dye bleaching to occur; there is no evidence for the presence of MnIVO species. The virtue of low local concentrations of in situ generated H2O2 is shown to be important in preventing over oxidation of the catalyst and thus contributing to a robust catalytic system.

Nucleophilic aromatic substitution of tetrachloro-o-benzoquinone by pyridine and reduction of the o-quinone to the catechol by hydroxylamine forms 1,2-dihydroxy-3,5,6-trichlorobenzene-4-pyridinium chloride. This compound reacts with manganese(II) acetate in air to form chlorobis(3,5,6-trichlorobenzene 4-pyridinium catecholate)manganese(III), which represents the first complex of this ligand class to be structurally characterized by X-ray diffraction; this complex is active in the catalytic reduction of dioxygen to hydrogen peroxide under ambient conditions and turnover frequencies (TOFs) >10000 h−1 can be obtained.

Hydrogen peroxide (H2O2) 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 °C with de-protonated Tiron (1,2-dihydroxybenzene-3,5-disulfonate, disodium salt) acting as an essential co-ligand.

The mononuclear complexes (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)] and (Bu4N)2[Mn(Cl4Cat)3] (H2Cat=1,2-dihydroxybenzene) have been synthesised and characterised by X-ray diffraction. This work provides a direct, independent, synthesis of these complexes and an interesting example of how solvent effects can promote the formation of either a manganese(III) or manganese(IV) complex of the same ligand. The characterisation of (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)] supports previous work that manganese(III) is extremely reluctant to form tris (catecholato) complexes due to the short `bite distance' of catecholate oxygen atoms (2.79 Å) which are unable to span the elongated coordination axes of the Jahn-Teller distorted Mn(III) ion and explains the 2:1 and 3:1 tetrachlorocatechol:manganese ratios in the Mn(III) and Mn(IV) complexes, respectively. Hydrogen peroxide production using dioxygen and hydroxylamine as substrates in acetonitrile/water mixtures, under ambient conditions, can be demonstrated with both complexes, suggesting that neither labile coordination sites nor the oxidation state of the manganese are important to the catalytic system. Turn over frequencies (TOF, moles of H2O2 per moles of manganese per hour) of ∼10 000 h−1 are obtained and this compares very favourably with the commercial production of hydrogen peroxide by the autoxidation of 2-ethylanthrahydroquinone (AO process).The mononuclear complexes (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)] and (Bu4N)2[Mn(Cl4Cat)3] (H2Cat=1,2-dihydroxybenzene) have been prepared by a direct method and structurally characterised. This work provides an example of how solvent effects can promote the formation of either a manganese(III) or manganese(IV) complex of the same ligand. In aqueous-acetonitrile mixtures, under physiological conditions, both complexes are efficient catalysts for the reduction of dioxygen to hydrogen peroxide using hydroxylamine as substrate and therefore neither labile co-ordination sites nor the oxidation state at manganese are important to the catalysis

The kinetics and mechanism for the bleaching of Calmagite (H3CAL, 3-hydroxy-4-(2-hydroxy-5-methylphenylazo)naphthalene-1-sulfonic acid) in aqueous solution at pH 8.00 and 23 ± 1 °C using in situ generated H2O2 is described. Complete mineralisation of H3CAL results with turnover frequencies (TOF = moles of H3CAL bleached per mole of manganese per hour) of 40 h−1. The monohydroxy azo dyes Me–H2CAL, Orange G and Orange II are not bleached which indicates that a requirement of dye bleaching is the coordination of the dye to the Mn centre. Spectroscopic studies show the formation of Mn(CAL)2 and Mn(CAL) species but in the presence of Tiron (1,2-dihydroxybenzene-3,5-disulfonate, disodium salt, monohydrate, Na2TH2·H2O), [Mn(CAL)(T)] is formed. It is proposed that a Mn(III)–hydroperoxide species is generated, [Mn(O2H)(CAL)(TQ)] from the in situ generated H2O2, where TQ represents the o-quinone form of Tiron, and this is the active species in the bleaching of coordinated CAL; the formation of this hydroperoxide species is supported by UV/VIS and ESI-MS data. The formation of a Mn(III) species is supported by EPR studies which also show some evidence for the presence of a labile d5 Mn(II) species in the presence of the reducing substrate hydroxylamine (NH2OH). This would enable rapid ligand exchange for both in situ H2O2 generation and dye bleaching to occur; there is no evidence for the presence of MnIVO species. The virtue of low local concentrations of in situ generated H2O2 is shown to be important in preventing over oxidation of the catalyst and thus contributing to a robust catalytic system.

Nucleophilic aromatic substitution of tetrachloro-o-benzoquinone by pyridine and reduction of the o-quinone to the catechol by hydroxylamine forms 1,2-dihydroxy-3,5,6-trichlorobenzene-4-pyridinium chloride. This compound reacts with manganese(II) acetate in air to form chlorobis(3,5,6-trichlorobenzene 4-pyridinium catecholate)manganese(III), which represents the first complex of this ligand class to be structurally characterized by X-ray diffraction; this complex is active in the catalytic reduction of dioxygen to hydrogen peroxide under ambient conditions and turnover frequencies (TOFs) >10000 h−1 can be obtained.

Hydrogen peroxide (H2O2) 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 °C with de-protonated Tiron (1,2-dihydroxybenzene-3,5-disulfonate, disodium salt) acting as an essential co-ligand.