Reactivity of trialkylphosphine oxide complexes of divalent cobalt, nickel, copper and zinc with sulfur dioxide. Reversible co-ordination of SO2 in ligand-bound adducts

Abstract

Bis(trialkylphosphine oxide) complexes, [M(OPPrⁿ₃)₂Br₂](M = Co, Ni, Cu or Zn) and [M(OPEt₃)₂X₂](M = Co, X = I; M = Zn, X = Br), have been synthesised. Magnetic susceptibility measurements have been made (M = Co, Cu or Ni), and electronic (M = Cu or Co), ³¹P-{¹H} NMR (M = Zn), and infrared spectra (all complexes) have been recorded. Characterisation strongly supports the hypothesis that the complexes are pseudo-tetrahedral, with significant distortion in the case of copper. On exposure to SO₂ all the complexes display mass increases commensurate with the absorption of 2 equivalents of SO₂. During this process solid complexes become liquid, whilst oils appear visibly less viscous. The co-ordination of SO₂ is easily reversed in a flow of dinitrogen or at reduced pressures and the original complexes are recovered without apparent decomposition. These properties, along with the infrared absorptions observed for co-ordinated SO₂, are indicative of the formation of ligand-bound adducts, [M{O(SO₂)PR₃}₂X₂]. The equilibrium between free and co-ordinated SO₂ has been examined for the complex [Zn(OPEt₃)₂Br₂] using thermogravimetric analysis; K_eqm= 3.96 atm^–2(295 K), ΔH=–26.1 kJ mol^–1 per binding site, whilst a Hill plot indicates no significant co-operativity between the co-ordination sites. Several complexes and their sulfur dioxide adducts were investigated by visible (M = Cu or Co), ESR (M = Cu) and ³¹P-{¹H} NMR (M = Zn) spectroscopy. Visible spectroscopy indicates that little change occurs in the cobalt co-ordination sphere as a result of binding of SO₂, but [Cu(OPPrⁿ₃)₃Br₂] appears to change ground state and possibly geometry on co-ordination. The ESR spectra indicate that changes do occur in the metal environment for [Cu(OPPrⁿ₃)₂Br₂], but fail to provide detailed information. The ³¹P-{¹H} NMR spectra for [Zn(OPR₃)₂Br₂](R = Et, Prⁿ, Buⁿ or C₈H₁₇) and their adducts, surprisingly, show small negative (upfield) shifts on co-ordination of SO₂. Since the effect is opposite to that observed when SO₂ binds to free OPR₃ ligands, it is suggested that the net deshielding caused by zinc(II) and SO₂ in the [Zn{O(SO₂)PR₃}₂Br₂] complexes is comparable to that of zinc(II) alone in the [Zn(OPR₃)₂Br₂] precursors, and thus that significant competition occurs between the two Lewis–acid species in the [Zn{O(SO₂)PR₃}₂Br₂] complexes.