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

Abstract

The palladium(II) complex [Pd(C(6)F(5))Cl(bpzm*)] (5) [bpzm* = bis(3,5-dimethylpyrazol-1-yl)methane] was characterized by (1)H,(1)H-TOCSY, (1)H-NOE difference spectra, (1)H,(19)F-HOESY and (13)C,(1)H-HMBC 2D-NMR techniques. Chemical exchange of the methylene protons from (1)H,(1)H-NOESY cross peaks and exchange of the ortho- and meta-fluorine atoms, respectively, from (19)F,(19)F-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 C(6)F(5) atom positions. The presence of three (19)F-NMR signals for 2F(o) : 1F(p) : 2F(m) of the C(6)F(5) ligand for complexes [Pd(C(6)F(5))Cl(tmeda)] (1) and [Pd(C(6)F(5))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 C(6)F(5) rotation is not limited to 5 but is also present in 1 and 3 due to ligand repulsion. Complexes [Pd(C(6)F(5))Cl(tmeda)] (1), [Pd(C(6)F(5))OH(tmeda)] (2), [Pd(C(6)F(5))Cl(bipy)] (3), [Pd(C(6)F(5))OH(bipy)] (4) and [Pd(C(6)F(5))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" Pd(2+) cations. For the activation mechanism, (1)H-, (13)C- and (19)F-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 C(6)F(5) takes place with a redox reaction giving Pd metal and C(6)F(5)-CH(3) in the absence of norbornene monomer.