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, K_O¹, for the dioxygenation reaction have been deduced. The general observation that K_O2 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.