Reaction of [FeL4X2][FeX4](L = OPPh3 or OAsPh3, X = Cl or Br) with sulphur dioxide to yield some rare examples of iron(III) complexes of sulphur dioxide, [FeL4{OS(O)X}2][FeX4]. The oxidation of sulphur dioxide by the iron(III) complexes and the isolation of the sulphuric acid derivatives (PPh3O)(PPh3OH)(HSO4) and (AsPh3OH)(HSO4). Crystal structure of the latter

Abstract

The complexes [FeL₄X₂][FeX₄](L = OPPh₃ or OAsPh₃, X = Cl or Br), first prepared by Cotton and Gibson, have been further characterised and their reactions with sulphur dioxide investigated; these reactions have been found to be dependent on the nature of the ligand, the halide, and the reaction medium. In the solid state only the iron(III) complexes containing OAsPh₃ ligands react with sulphur dioxide to form [Fe(OAsPh₃)₄{OS(O)X}₂][FeX₄](X = Cl or Br). However, in toluene slurries [FeL₄{OS(O)X}₂][FeX₄](L = OPPh₃, X = Br; L = OAsPh₃, X = Cl or Br) can be isolated, but [Fe(OPPh₃)₄Cl₂][FeCl₄] is recovered unchanged. Nonetheless, filtrates from all four reactions, when exposed to moist air yield the sulphuric acid derivatives (PPh₃O)(PPh₃OH)(HSO₄) and (AsPh₃OH)(HSO₄); the formation of sulphuric acid can be indefinitely delayed by maintaining dry anaerobic conditions. The compound (AsPh₃OH)(HSO₄) has been characterised by single-crystal X-ray analysis which shows hydrogen-bonded chains of HSO₄^– anions running parallel to the c axis; each hydrogensulphate ion is then hydrogen bonded to a [AsPh₃OH]⁺ cation. The As–O bond distance observed, 1.727(5)Å, is substantially longer than that previously reported for Ph₃AsO, 1.645 Å.