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Issue 15, 2010
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Palladium(II) complexes with pentafluorophenyl ligands: structures, C6F5 fluxionality by 2D-NMR studies and pre-catalysts for the vinyl addition polymerization of norbornene

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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 2Fo : 1Fp : 2Fm 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 gpolymer/(molPd 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.

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

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Publication details

The article was received on 07 Dec 2009, accepted on 04 Feb 2010 and first published on 05 Mar 2010


Article type: Paper
DOI: 10.1039/B925674A
Citation: Dalton Trans., 2010,39, 3609-3619
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    Palladium(II) complexes with pentafluorophenyl ligands: structures, C6F5 fluxionality by 2D-NMR studies and pre-catalysts for the vinyl addition polymerization of norbornene

    F. Blank, H. Scherer, J. Ruiz, V. Rodríguez and C. Janiak, Dalton Trans., 2010, 39, 3609
    DOI: 10.1039/B925674A

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