Metal catalysed activation of dioxygen (air) for selective substrate oxidation
Supervisor: Dr Tippu Sheriff
Stereo- and regio-controlled oxidation processes are central to contemporary synthetic chemistry and a plethora of processes exist . In recent years there has been a significant drive to reduce the environmental impact of such chemistries and particular effort has focussed on the nature of the oxidant used. Dioxygen (O2) represents the ultimate in “green” oxidants as it is atom economic and freely available in the environment. Whilst Nature has established many elegant methods for the utilisation of this oxidant , its successful implementation in catalytic systems is rather limited . After O2, hydrogen peroxide (H2O2) is generally considered to be the next most environmentally benign oxidant available because it shares high atom economy with dioxygen (47% versus 50%) and has the significant advantage over many other oxidants that the by-product formed is usually water . As a result of these favourable properties it has also found extensive use in a range of other applications including laundry detergents and the bleaching of wood pulp in the manufacture of paper . Whilst H2O2 may be viewed as an ideal oxidant in these terms, the current method of industrial production (AO Process), as well as the transport, storage and handling of bulk H2O2 (in both liquid and solid forms) impact negatively on its favourable environmental characteristics . The realisation of efficient but uncomplicated methods for the in situ generation of H2O2 from O2 (or air) to provide synthetically useful concentrations under ambient conditions would therefore be a significant development.
The manganese(II) catalysed reduction of O2 to H2O2 under ambient conditions using either hydroxylamine (equation 1) or hydrazine (N2H4) as substrates has been described previously .
O2(g) (air) + 2 NH2OH(aq) H2O2(aq) + N2(g) + 2 H2O(l) (1)
Electron deficient catechols (quinoids, o-dioxolenes) such as tetrachlorocatechol (1,2-dihydroxybenzene-3,4.5,6-tetrachlorocatechol) were found to be essential ligand requirements for this system. In this case turnover frequencies (TOFs = moles of H2O2 generated moles per moles of catalyst used per hour) of >10,000 hr–1 can be obtained.
There is great interest in the development of new redox biomemtic catalysts that mimic enzymes that mediate selective aerobic oxidation reactions. These may be oxygenases that incorporate one or both oxygen atoms from O2 into organic substrates, or oxidases, which couple substrate oxidation with the reduction of O2 to water or H2O2. Catalysts that combine selective bond activation and O2 reduction must incorporate a multi-electron capacity to avoid the loss of specificity that typically accompanies odd-electron autoxidation. Traditional inorganic and organometallic catalysts for such reactions often utilize 2nd and 3rd row transition metal ions with redox-inert ancillary ligands. In these systems, the multi-electron redox activity is entirely metal-derived. In contrast, redox-active “non innocent” ligands may impart a multi-electron redox capacity to mononuclear 1st row tranistion metal complexes which typically prefer only single-electron redox changes. Apart from metalloporphyrin complexes, such ligand derived multi-electron reaction chemistry has been largely unexplored for redox transformations of small-molecule substrates but is receiving increased interest . We have developed multi-electron redox cycles based on the ability of catechol ligands and their derivatives to store and deliver charge in reactions with small-molecule substrates.
The reactions of O2 with complexes containing the anionic bis(catecholato)manganese(III) core, [MnIII(XmCat)2]n– (where H2Cat = 1,2-dihydroxybenzene and X = electronegative atom or group e.g. SO3–, Br, Cl, NO2), particularly the catalytic production of H2O2 from O2 with hydroxylamine (NH2OH) as a sacrificial reductant have been established .
The aim of this project will be the synthesis of novel electron-deficient catecholate complexes of manganese and testing their ability to activate O2 for selective substrate oxidation and / or the formation of in situ generated H2O2 using established methods . We have already successfully shown that in situ generated H2O2 from these systems can effectively degrade dye molecules  and there is the need to tame this reaction to enable selective oxidation to occur. We are also interested to model the active site of O2 activation so would seek to prepare novel mixed complexes containing both catecholate and amine ligands e.g. NH2OH and MeNHOH.
Eligibility and applying
International students must provide evidence of proficient English language skills. See our entry requirements page for further information.
Potential candidates should contact Dr Sheriff by e-mail (firstname.lastname@example.org) and submit their CV and a cover letter explaining their eligibility and interest in this project.
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