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 -SC 6 H 4 X-4)] 2 (Cp # = η 5 -C 5 H 5 ,(Cp), η 5 -C 5 H 5 Me, (Cp′); X=Cl, Br) with phosphines or Cp # Ni(PR 3 )Br with HSC 6 H 4 X-4 and Et 3 N or NaSeC 6 H 4 Cl-4. The complexes that were isolated have the general formula CpNi(PR 3 )(EC 6 H 4 X-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(PBu 3 )(SC 6 H 4 Cl-4) ( 2a ) and Cp'Ni(PPh 3 )(SeC 6 H 4 Cl-4) ( 9a ) were subjected to single-crystal X-ray diffraction studies. While the solution and solid state structures of CpNi(PBu 3 )(SC 6 H 4 Cl-4) were found to be the same, that of Cp'Ni(PPh 3 )(SeC 6 H 4 Cl-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 SO 2 to form SO 2 adducts and the reversibility could be monitored by visible and 1 H-NMR spectroscopy. Electrochemical studies show that 2a and 6a have reversible couples whereas 1a and 7a are only quasi-reversible. The PBu 3 complexes are easier to oxidise as compared with the PPh 3 analogues.