Reactivity of cobalt(II) complexes [CO(OEPh3)2X2](E = P or as; X = Cl, Br, I or NCS) with SO2 under aerobic and anaerobic conditions. Crystal structure of cocrystallised [Co(OPMe3)3(OH2)2]I2 and [Co(OPMe3)3(OH2)3]I2

Abstract

Pseudo-tetrahedral complexes [Co(OEPh₃)₂X₂](E = P or As; X = Cl, Br, I or NCS) and [Co(OPMe₃)₂I₂] have been isolated by conventional synthesis from ethanolic solutions of the appropriate hydrated metal salt and Ph₃EO. Their reactivity with sulfur dioxide has been examined in the solid state (anaerobic conditions) and for toluene slurries or solutions (aerobic and anaerobic conditions). The [Co(OEPh₃)₂X₂](X = Cl or Br) complexes show no reactivity and are recovered unchanged from all forms of exposure to SO₂. However, [Co(OEPh₃)₂X₂](E = P or As, X = I or NCS) reacted slowly with SO₂ in the solid state to form 1:1 adducts, as evidenced by elemental analyses and mass changes. The visible spectra of the precursor complexes and their adducts are similar, suggesting that SO₂ is ligand-bound. On exposure to air, SO₂-saturated toluene slurries and solutions of [Co(OPPh₃)₂I₂] underwent a complex oxidation process resulting in the quantitative isolation of CoSO₄·H₂O, (Ph₃PO)(Ph₃POH)(HSO₄) and I₂. Visible spectroscopy indicates at least one iodine-containing intermediate, which may be analogous to the cocrystallised [Co(OPMe₃)₃(OH₂)₂]I₂ and [Co(OPMe₃)₃(OH₂)₃]I₂ recovered from the reaction of [Co(OPMe₃)₂I₂] with SO₂ and air. SO₂-Saturated solutions of [Co(OAsPh₃)₂I₂] afforded (Ph₃AsOH)(HSO₄), I₂, Ph₃AsI₄ and CoSO₄·H₂O in the presence of air, although I₂ and Ph₃AsI₄ appear to be produced even under anaerobic conditions. Aerobic reactivity was also observed for [Co(OEPh₃)₂(NCS)₂](E = P or As); CoSO₄·H₂O and ligand derivatives of sulfuric acid were again isolated, however, the fate of the thiocyanate anion remains unclear. The inactivity of [Co(OPPh₃)₂(NCS)₂] under anaerobic conditions contrasts with its subsequent aerobic reactivity and suggests that simple adduct formation and oxidation of SO₂ to sulfuric acid are not closely linked processes.