η 5-Cyclopentadienylpalladium(II) complexes: Synthesis, characterization and use for the vinyl addition polymerization of norbornene and the copolymerization with 5-vinyl-2-norbornene or 5-ethylidene-2-norbornene

Abstract

Dinuclear complexes of palladium(II), containing two bridging halogen (Cl or Br) ligands, [N n Bu 4] 2[(X 5C 6) 2Pd(μ-Cl) 2Pd(C 6X 5) 2] and [(X 5C 6)(L)Pd(μ-Y) 2Pd(C 6X 5)(L)] (X = F, Cl; Y = Cl, Br), readily react with cyclopentadienylthallium, C 5H 5Tl, to give the corresponding air stable half-sandwich, pseudo-trigonal η 5-cyclopentadienylpalladium complexes, [N n Bu 4][(η 5-C 5H 5)Pd(C 6X 5) 2] (X = F 1, Cl 2) and (η 5-C 5H 5)Pd(C 6X 5)(L) (X = F, L = CNBu t 3, PPh 3 4, PMe 2Ph 5, PEt 3 6, AsPh 3 7, SbPh 3 8; X = Cl, L = PMe 2Ph 9, PEt 3 10), respectively. With tetraphenylcyclopentadienylthallium, C 5Ph 4HTl or pentabenzylcyclopentadienylthallium, C 5Bn 5Tl (Bn = CH 2Ph) the air stable half-sandwich complexes (η 5-C 5Ph 4H)Pd(C 6F 5)(AsPh 3), 12 and (η 5-C 5Bn 5)Pd(C 6F 5)(AsPh 3), 13 are synthesized accordingly. The molecular structures were verified by NMR-spectroscopy, X-ray crystallography ( 7, 12, 13) and electron impact-mass spectrometry (EI-MS). The precatalysts 4 and 7 can be activated with methylalumoxane (MAO) for the homopolymerization of norbornene (NB) and 5-ethylidene-2-norbornene (ENB) and for the copolymerization of NB with 5-vinyl-2-norbornene (VNB) or ENB with activities of more than 10 6 g PNB/(mol Pd·h). The higher activity of 7/MAO over 4/MAO towards NB homopolymerization was reversed when the olefin-substituted VNB or ENB were added. Then, the more strongly bound PPh 3 ligand of 4 (versus AsPh 3 of 7) can compete with the olefin functionality of VNB or ENB and assume a directing role for the insertion of the ring double bond. As a consequence 4/MAO shows almost the same activity in NB and ENB homopolymerization.