Dynamic properties and reactivity of electronically unsymmetrical palladium(II) alkyl complexes.

摘要

This thesis describes investigation of dynamic properties and reactivity of new palladium(II) based complexes. These complexes bear bidentate chelating electronically unsymmetrical ligands, which contain both strong and weak donor groups.;Chapter One introduces palladium(II) alkyl catalysts that contain ancillary phosphine-sulfonate (PO) ligands. This class of catalysts is one of the few that catalyze the direct copolymerization of ethylene and polar vinyl monomers to afford highly linear copolymer with functional groups incorporated into the polymer backbone. The "electronic asymmetry" of PO ligands is believed to be important for the unique reactivity of (PO)Pd alkyl complexes. Recent development of this system is also discussed.;Chapter Two describes study of the solution conformations and dynamic properties of the CF3-sbustituted (ortho-phosphino-arenesulfonate)Pd complexes (PO-CF3)PdMe(L) ([PO-CF3]- = 2-{(o-CF3-Ph)2P}-4-Me-benzenesulfonate, L = 2,6-lutidine (3), pyridine (4)) by NMR spectroscopy, taking particular advantage of 31P-19F through-space couplings and 1H-1H and 1H- 19F nuclear Overhauser effects (NOEs). In CD2Cl2 solution in the temperature range of -80 to 20 °C, 3 adopts an exo2 conformation. One o-CF3-Ph ring is positioned such that the CF3 group points toward Pd ( exo) and exhibits through-space 4J PF coupling. The other o-CF3-Ph ring is positioned such that the CF3 group points away from Pd ( endo) and does not exhibit through-space 4 JPF coupling, and the o-H lies in the deshielding region near an axial site of the Pd square plane and exhibits a lowfield chemical shift (delta > 9). Complex 4 exists as a 2/1 mixture of exo2 and exo 3 isomers in CD2Cl2 solution at -90 °C. In exo2-4, one CF3 group is exo and exhibits through-space 4J PF coupling, while the other CF3 group is endo and does not exhibit through-space 4J PF coupling. In exo3-4, both CF3 groups are exo and exhibit through-space 4JPF couplings. Complex 4 undergoes two dynamic processes: rotation of the axial o-CF3-Ph ring (AaR), which interconverts exo2-4 and exo3-4 (DeltaG‡ = 9.9(5) kcal/mol), and chelate ring inversion (RI), which permutes the axial and equatorial o-CF3-Ph rings (DeltaG ‡ = 21(1) kcal/mol).;Chapter Three describes the synthesis, characterization and ethylene polymerization behavior of a new phosphine-sulfonate palladium complex that contains a m-terphenyl substituent on phosphorus. In both solid state and solution, one of the two flanking phenyl rings of the m-terphenyl unit is pi-pi stacked with the chelate ArSO 3 ring, and this interaction positions the other flanking phenyl ring at the axial site of Pd, providing excellent blockage of potential incoming monomers during chain transfer. This new catalyst is highly active toward ethylene polymerization, but surprisingly, the produced polyethylene has low molecular weight (Mn = 2700). The acceleration of chain transfer is attributed to a charge transfer interaction between the axial flanking phenyl ring and the Pd center.;Chapter Four describes the synthesis, and characterization of six new cationic [(PN)PdMe(L)][X] complexes (L = pyridine, 2,6-lutidine, X = [PF 6], [B(C6F5)4]) bearing bidentate electronically unsymmetrical phosphine-amine (PN) ligands L1-L4. These complexes show very low or no activity toward ethylene polymerization and oligomerization. A comprehensive series of NMR experiments on one case, [(L3)PdMe(py)][B(C 6F5)4] (L3 = 2-(tBu 2P)-NMe2-benzene), enabled characterization of [(L3 )PdR]+ species with both primary (beta-agostic, 8+) and secondary (possibly beta-agostic, 9 +) alkyl ligands. 8+ binds ethylene at low temperature, but 9+ does not. It is the inherent low reactivity of these species, rather than their low stability, that engenders the poor performance of this catalyst. The low reactivity is ascribed to the steric hindrance of the "L site" when ethylene must coordinate prior to insertion, that results from the presence of the bulky --NMe2 donor group.