Palladium(II) complexes with pentafluorophenyl ligands: structures, C6F5 fluxionality by 2D-NMR studies and pre-catalysts for the vinyl addition polymerization of norbornene

The palladium(II) complex [Pd(C6F5)Cl(bpzm*)] (5) [bpzm* = bis(3,5-dimethylpyrazol-1-yl)methane] was characterized by H-1, H-1-TOCSY, H-1-NOE difference spectra, H-1, F-19-HOESY and C-13, H-1-HMBC 2D-NMR techniques. Chemical exchange of the methylene protons from H-1, H-1-NOESY cross peaks and exchange of the ortho-and meta-fluorine atoms, respectively, from F-19,F-19-EXSY cross peaks indicates that the Pd-bpzm* chelate ring boat-to-boat inversion occurs at a rate slower than the NMR time scale together with a concomitant change of the C6F5 atom positions. The presence of three F-19-NMR signals for 2F(o) : 1F(p) : 2F(m) of the C6F5 ligand for complexes [Pd(C6F5)Cl(tmeda)] (1) and [Pd(C6F5)Cl(bipy)] (3) (tmeda = N,N,N',N'-tetramethylethylenediamine; bipy = 2,2'-bipyridine) is interpreted as being due to identical hemi-spaces above and below an apparent symmetry plane coinciding with the Pd-coordination plane instead of free ring rotation. The molecular structures of 1, 3 and 5 from single-crystal studies suggest that the hindered C6F5 rotation is not limited to 5 but is also present in 1 and 3 due to ligand repulsion. Complexes [Pd(C6F5) Cl(tmeda)] (1), [Pd(C6F5) OH(tmeda)] (2), [Pd(C6F5) Cl(bipy)] (3), [Pd(C6F5) OH(bipy)] (4) and [Pd(C6F5) Cl(bpzm*)] (5) have been applied as pre-catalysts for the vinyl homopolymerization of norbornene in combination with the cocatalyst methylaluminoxane (MAO). Activities of more than 10(6) g(polymer)/(mol(Pd) h) could be reached with these catalytic systems. Based on the spectrochemical series, pre-catalysts 1 and 2 with the pure sigma-donor and more weakly bound aliphatic amine ligands showed higher polymerization activities than compounds 3-5 with modest pi-accepting and stronger bound aromatic substituents. This is reasoned with a kinetic activation effect through a faster removal of the more weakly bound ligands upon reaction with MAO together with the chloro or hydroxo ligands to give the active, almost "naked" Pd2+ cations. For the activation mechanism, H-1-, C-13- and F-19-NMR studies of the MAO activated complex 5 showed about 13% chlorine-to-methyl exchange for a molar Pd : Al ratio of 1 : 10. For 5: MAO at a Pd: Al ratio of 1 : 100 abstraction of C6F5 takes place with a redox reaction giving Pd metal and C6F5-CH3 in the absence of norbornene monomer.