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; Keqm= 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.