η5-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

Dinuclear complexes of palladium(II), containing two bridging halogen (Cl or Br) ligands, [NnBu4]2[(X5C6)2Pd(μ-Cl)2Pd(C6X5)2] and [(X5C6)(L)Pd(μ-Y)2Pd(C6X5)(L)] (X = F, Cl; Y = Cl, Br), readily react with cyclopentadienylthallium, C5H5Tl, to give the corresponding air stable half-sandwich, pseudo-trigonal η5-cyclopentadienylpalladium complexes, [NnBu4][(η5-C5H5)Pd(C6X5)2] (X = F 1, Cl 2) and (η5-C5H5)Pd(C6X5)(L) (X = F, L = CNBut3, PPh34, PMe2Ph 5, PEt36, AsPh37, SbPh38; X = Cl, L = PMe2Ph 9, PEt310), respectively. With tetraphenylcyclopentadienylthallium, C5Ph4HTl or pentabenzylcyclopentadienylthallium, C5Bn5Tl (Bn = CH2Ph) the air stable half-sandwich complexes (η5-C5Ph4H)Pd(C6F5)(AsPh3), 12 and (η5-C5Bn5)Pd(C6F5)(AsPh3), 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 106 gPNB/(molPd·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 PPh3 ligand of 4 (versus AsPh3 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.