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(C₆F₅)Cl(bpzm*)] (5) [bpzm* = bis(3,5-dimethylpyrazol-1-yl)methane] was characterized by ¹H,¹H-TOCSY, ¹H-NOE difference spectra, ¹H,¹⁹F-HOESY and ¹³C,¹H-HMBC 2D-NMR techniques. Chemical exchange of the methylene protons from ¹H,¹H-NOESY cross peaks and exchange of the ortho- and meta-fluorine atoms, respectively, from ¹⁹F,¹⁹F-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 ¹⁹F-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 10⁶ 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 σ-donor and more weakly bound aliphatic amine ligands showed higher polymerization activities than compounds 3–5 with modest π-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, ¹H-, ¹³C- and ¹⁹F-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.