Ethylene Polymerization by Sterically Crowded Palladium(II) Complexes that Contain Bis(heterocycle)methane Ligands
作者:Christopher T. Burns、Richard F. Jordan
DOI:10.1021/om700768j
日期:2007.12.1
The catalytic ethylene polymerization reactions of (N<^>N)PdMe(L)(+) species that contain bulky bis(heterocycle)methane ligands were studied (N<^>N = (1-Me-imidazol-2 -yl)(2)CHCH(P-tolyl)(2) (1i), (Me-imidazol-2-yl)(2)CHC(p-tolyl)(3) (1j), (5-Me-pyridin-2-yl)(2)CHC(P-tOlyl)(2))(3) (1k), (5-Me-pyridin-2yl)(2)CHCH(p-tolyl)(2) (1l), (3-Me-pyridin-2-yl)(2)CHCH(p-tolyl)(2) (1m), (5-Me-pyridin-2-yl)(2)CHCH(m-xylyl)(2) (1n), (3-Me-pyridin-2-yl)(2)CHCH(m-xylyl)(2) (1o), (quinol-2-yl)(2)CHCH(m-xylyl)(2) (1p), and (3-Me-quinol2-yl)(2)CHCH(m-xylyl)(2) (1q). (N<^>N)PdMe2 (2i,j,q) and (N<^>N)Pd(Me)Cl (3m-o) complexes were converted to (N<^>N)PdC(=O)Me}CO+ complexes (7i,j,m-o,q). The nu(CO) values for 7i,j,m-o,q show that there is weak back-bonding in these species. Complexes 2i,j and (N<^>N)Pd(Me)Cl complexes 31-o were converted to (N<^>N)Pd(Me)(H2C=CH2)(+) (8i,j,l-o and 8q (not directly observed)). First-order rate constants for ethylene insertion of 8i,j,l-o were determined by NMR. Increasing the electrophilic character and the steric bulk of the (N<^>N)Pd unit increases the rate of ethylene insertion. In-situ-generated 8j,l-n,q produce low molecular weight polyethylene (Mw <= 4200), while 8o produces higher molecular weight polyethylene (M, = 34 000). Polyethylene produced by 8j,l,m contains 100 branches/10(3) C, while that from 8n,o,q contains 60 branches/ 10(3)C.