eterotopic Bidentate Ligands and Their Roles in Catalysis – Mechanistic Approach to Reactions of Ethylene-Norbornene Copolymerization and Hydrogenation
Date Issued
2012
Date
2012
Author(s)
Yu, Kuo-Hsuan
Abstract
Part I
Cationic methylpalladium complexes with hemilabile heterotopic bidentate ligands of α-amino-pyridines, in the formula of {[R1HNCR2H(o-C6H5N)]Pd(Me)(NCMe)}(BF4) (R1 = iPr, tBu, Ph, 2,6-Me2C6H3, 2,6-iPr2C6H3 R2= H, Me), have been found to be effective precursors for catalytic copolymerization of ethylene (E) and norbornene (N) under mild conditions. Both of the square planar methyl and norbornyl palladium(II) complexes exhibit facile reversible geometrical isomerization. The geometric isomers show distinct reactivity toward olefin-insertion reactions. Detailed kinetic studies by means of variable-temperature NMR technique on ethylene-insertion, norbornene-insertion, as well as the geometric isomerization have been examined. Corresponding DFT calculations for such fundamental paths have also been investigated. Both approaches reveal consistent results, indicating that the cis-isomers undergo olefin insertion more facile than the trans-isomers. An unprecedented mechanism in which the reversible geometric isomerization and the geometric isomer-distinguished kinetics lead to the alternating E-N copolymerization will be presented.
Part II
The sulfoxide(SO)-imidazolium salts have been successfully synthesized to react with palladium precursors via transmetallation of resulting silver complexes. The SO-NHC ligand behaving like monodentate results in the formation of dinuclear species in the formula of [(SO-NHC)PdCl2]2. The zwitterionic (SO-imidazolium)PdCl3 complex with the configuration of coordinated sulfoxide was also synthesized. Subsequent deprotonation afforded the trans-(SO-NHC)2PdCl2 complex, which was resulted from the transformation to unusual anionic PdCl4 complex bearing two SO-imidazolium zwitterions in solvents.
A series of (SO-NHC)Pd(R-allyl)X (R = H, Me, Ph and X = Cl, PF6) complexes have been found to be effective catalysts for alkene hydrogenation. These complexes revealing cis configuration of NHC and unsubstituted allylic group were characterized by the NMR techniques and X-ray crystallography. A rapid exo-endo exchange of allyl group via η3-η1-η3 isomerization was observed. In variable-temperature 1H NMR spectrometry, the envelope movement of five-membered metallacyclic structure of [(SO-NHC)Pd(allyl)]PF6 complex affords four exo- and endo- diastereomers. The energy of such fluxion was also examined to give the ΔG‡ in the range of 12.4 - 13.5 kcal/mol.
Subjects
amino-pyridine
copolymerization
ethylene
norbornene
kinetic study
mechanism
sulfoxide-NHC
carbene
hydrogenation
Type
thesis
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