Reversible sulfur dioxide reactions with cyclopentadienylnickel(II) organochalcogenide complexes

A series of cyclopentadienylnickel(II) organochalcogenides containing different phosphines have been prepared by either reacting [CpNi(μ-SC₆H₄X-4)]₂ (Cp^#=η⁵-C₅H₅,(Cp), η⁵-C₅H₅Me, (Cp′); X=Cl, Br) with phosphines or Cp^#Ni(PR₃)Br with HSC₆H₄X-4 and Et₃N or NaSeC₆H₄Cl-4. The complexes that were isolated have the general formula CpNi(PR₃)(EC₆H₄X-4) {E=S, X=Cl, R=Ph (1a), Bu (2a), OPh (3a), OEt (4a), OMe (5a); X=Br, R=Bu (6a), Ph (7a)}. In addition to spectroscopic characterisation, the complexes CpNi(PBu₃)(SC₆H₄Cl-4) (2a) and Cp'Ni(PPh₃)(SeC₆H₄Cl-4) (9a) were subjected to single-crystal X-ray diffraction studies. While the solution and solid state structures of CpNi(PBu₃)(SC₆H₄Cl-4) were found to be the same, that of Cp'Ni(PPh₃)(SeC₆H₄Cl-4) were different. The orientation of the methyl group of 9a in the solid state is almost trans to the phosphine, but solution NMR data indicate that the methyl substituent on the cyclopentadienyl ligand is cis to the phosphorus. All the complexes, 1a-9a, were found to react reversibly with SO₂ to form SO₂ adducts and the reversibility could be monitored by visible and ¹H-NMR spectroscopy. Electrochemical studies show that 2a and 6a have reversible couples whereas 1a and 7a are only quasi-reversible. The PBu₃ complexes are easier to oxidise as compared with the PPh₃ analogues.