Co-ordination chemistry of higher oxidation states. Part 38. Synthesis, spectroscopic and electrochemical studies of some trans-dihalogenoosmium complexes. Crystal structure of trans-[Os(PMe3)4Cl2]BF4

Abstract

The complexes trans-[Os(PMe₃)₄X₂](X = Cl or Br) have been obtained from [Os(PPh₃)₃X₂] and PMe₃, and trans-[OsL₄X₂][L = PMe₂Ph, AsMe₃, SbPh₃ or pyridine (py)] by reduction of appropriate osmium(III) complexes in the presence of L. The complexes cis-[Os(PR₃)₄X₂] are formed by isomerisation of the trans analogues in chlorinated solvents, and in other ways. Air oxidation of the osmium(II) complexes gives trans-[OsL₄X₂]BF₄(L = PMe₃ or AsMe₃), but formation of trans-[OsL₄X₂]BF₄(L = PMe₂Ph, SbPh₃ or py) and cis-[Os(PMe₂Ph)₄Cl₂]BF₄ requires HNO₃ as oxidant. Use of concentrated HNO₃ gives trans-[OsL₄X₂]²⁺(L = PMe₃, PMe₂Ph or AsMe₃) in solution, but these have not been isolated. The complexes have been characterised by IR, UV–VIS, and NMR spectroscopies, and the effect of stereochemistry and L and X upon the Os^II–Os^III and Os^III–Os^IV redox potentials probed by cyclic voltammetry. The crystal structure of trans-[Os(PMe₃)₄Cl₂]BF₄ has been determined: orthorhombic, space group Fddd, a= 8.104(4), b= 32.195(11), c= 38.540(9)Å, and Z= 16. The cation has Os–P 2.419(5) and 2.398(5)Å and Os–Cl 2.352(4)Å, and shows deviations of the OsP₄ unit from planarity due to steric interactions. There is no evidence that mer-[IrL₃Cl₃](L = PEtPh₂, AsMe₂Ph, SbPh₃ or SMe₂) can be oxidised either chemically or electrochemically to stable iridium(IV) cations.