Generation of a Mn(IV)–Peroxo or Mn(III)–Oxo–Mn(III) Species upon Oxygenation of Mono- and Binuclear Thiolate-Ligated Mn(II) Complexes

A thiolate-bridged binuclear complex [PPN]₂[(Mn^II(^TMSPS3))₂] (1, PPN = bis­(triphenylphosphine)­iminium and ^TMSPS3H₃ = (2,2′,2″-trimercapto-3,3′,3″-tris­(trimethylsilyl)­triphenylphosphine)), prepared from the reaction of MnCl₂/[PPN]Cl and Li₃[^TMSPS3], converts into a mononuclear complex [PPN]­[Mn^II(^TMSPS3)­(DABCO)] (2) in the presence of excess amounts of DABCO (DABCO = 1,4-diazabicyclo[2.2.2]­octane). Variable temperature studies of solution containing 1 and DABCO by UV–vis spectroscopy indicate that 1 and 2 exist in significant amounts in equilibrium and mononuclear 2 is favored at low temperature. Treatment of 1 or 2 with the monomeric O₂-side-on-bound [PPN]­[Mn^IV(O₂)­(^TMSPS3)] (3) produces the mono-oxo-bridged dimer [PPN]₂[(Mn^III(^TMSPS3))₂(μ-O)] (4). The electrochemistry of 1 and 2 reveals anodic peak(s) for a Mn^III/Mn^II redox couple at shifted potentials against Fc/Fc⁺, indicating that both complexes can be oxidized by dioxygen. The O₂ activation mediated by 1 and 2 is investigated in both solution and the solid state. Microcrystals of 2 rapidly react with air or dry O₂ to generate the Mn­(IV)–peroxo 3 in high yield, revealing a solid-to-solid transformation and two-electron reduction of O₂. Oxygenation of 1 or 2 in solution, however, is affected by diffusion and transient concentration of dioxygen in the two different substrates, leading to generation of 3 and 4 in variable ratios.