High turnover catalysis of water oxidation by Mn(ii) complexes of monoanionic pentadentate ligands

Abstract

Capillary electrophoresis (CE) and electrospray ionisation (ESI) mass spectra of aqueous solutions of manganese(II) complexes of the monoanions of the pentadentate ligands N-methyl-N′-carboxymethyl-N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine (mcbpen⁻) and N-benzyl-N′-carboxymethyl-N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine (bcbpen⁻), show the presence of a mixture of closely related Mn(II) species, assigned to the mono, di-, tri- and poly-cationic complexes [Mn^II(L)(H₂O)]_nⁿ⁺, L = mcbpen⁻ or bcbpen⁻ with n = 1, 2, 3, etc. In solution, these complexes are reversibly oxidized by tert-butyl hydrogen peroxide (TBHP), (NH₄)₂[Ce(NO₃)₆], Ce(ClO₄)₄, oxone and [Ru(bipy)₃]³⁺ to form metastable (t_½ = min to h) higher valent (hydr)oxide species, showing a collective maximum absorbance at 430 nm. The same species can be produced by [Ru(bipy)₃]²⁺-mediated photooxidization in the presence of an electron acceptor. TBHP oxidation of the complexes, in large excesses of the TBHP, is concurrent with an O₂ evolution with turnovers of up to 1.5 × 10⁴ mol of O₂ per mol of [Mn] and calculated rate constants from two series of experiments of 0.039 and 0.026 mol[O₂] s⁻¹M⁻². A 1 : 1 reaction of TBHP with [Mn] is rate determining and the resultant species is proposed to be the mononuclear, catalytically competent, [Mn^IV(O)(mcbpen)]⁺. At very close m/z values [Mn^III(OH)(mcbpen)]⁺, [Mn₂^III/IV(O)₂(mcbpen)₂]⁺ and [Mn^IV₂(O)₂(mcbpen)₂]²⁺are detected by ESI MS and CE when the concentration of TBHP is comparable to or lower than that of [Mn]. These are conditions that occur post catalysis and these species are derived from [Mn^IV(O)(mcbpen)]⁺ through condensation reactions.