Manganese(II) complexes of trialkylphosphine oxide ligands and their reaction with sulfur dioxide to form ligand-bound adducts

Abstract

Manganese(II) complexes of the rarely studied ligands OPR₃(R = Et, Prⁿ or Buⁿ), [Mn(OPR₃)₂X₂](R = Et, Prⁿ or Buⁿ) and [Mn(OPR₃)_n(NCS)₂](R = Et or Buⁿ; n= 1, 2 or 4) have been prepared and characterised. The free ligands react rapidly with sulfur dioxide to form liquid 1 : 1 adducts R₃PO·SO₂, and over a longer period to form (R₃P·SO₂)(SO₂)_0.5. The adduct process is reversed by applying a vacuum; the stability of the R₃PO·SO₂ adducts is in the order R = Buⁿ > Prⁿ, Et. The [Mn(OPR₃)₂X₂] complexes do not react with SO₂ when slurried in toluene but take up 2 mol of SO₂ in the solid state. Moreover, [Mn(OPR₃)_n(NCS)₂](n= 2 or 4) take up 2 and 4 mol of SO₂, respectively, in the solid state, i.e. the number of SO₂ molecules bound is proportional to the number of ligands in the complex. Other evidence is presented which confirms ligand-bound SO₂, including the observation that Buⁿ₃PO·SO₂ reacts with MnCl₂ to form [Mn{O(SO₂)PBuⁿ₃}₂Cl₂]. Consistent with previous studies of ligand-bound SO₂, the binding is weak and the SO₂ is not activated to reaction with dioxygen. Aluminium trichloride replaces Mn from [Mn(OPBuⁿ₃)₂Cl₂] to form Buⁿ₃PO·AlCl₃.