The co-ordination of small molecules by manganese(II) phosphine complexes. Part 11. The co-ordination of dioxygen by manganese(II) complexes containing long-chain phosphine ligands

Abstract

Manganese(II) complexes of long-chain phosphines, [MnX2(phosphine)][X = Cl, Br, or I; phosphine = P(C12H25)3, P(C14H29)3, P(C16H33)3, PPh(C12H25)2, PPh(C14H29)2, or PPh(C16H33)2], have been prepared. They form highly coloured 1 : 1 adducts with dioxygen, [MnX2(phosphine)(O2)], in both toluene and tetrahydrofuran (thf) solution. Infrared and e.s.r. spectra strongly suggest that these [MnX2(phosphine)] complexes are pseudotetrahedral dimers in the solid state and in toluene, contrasting vividly with the previously reported structure of [Mnl2(PPhMe2)]. Some molecular weight data indicate the product of dioxygenation in toluene to be the monomeric [MnX2(phosphine)(O2)] species. Dioxygen-binding curves for toluene and thf solutions have been obtained and equilibrium constants, KO1, for the dioxygenation reaction have been deduced. The general observation that KO2 increases and P50, the partial pressure when 50% of sites are deoxygenated, decreases for changes in halide ligand from Cl to Br to I in both solvents mirrors earlier observations with complexes of short-chain phosphines. The affinity of the complexes for O2 in thf is in the order phenyldialkylphosphines < trialkylphosphines. Hill coefficients, n, are approximately 1 for thf solutions of the chloride and bromide complexes, but for the iodides in thf and all of the complexes in toluene n lies in the range 1.10–1.64 indicating co-operativity for O2 binding, which may be related to the dimeric structure proposed for the complexes in toluene.