Ethylene Polymerization by Sterically Crowded Palladium(II) Complexes that Contain Bis(heterocycle)methane Ligands

Abstract

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)2CHCH(p-tolyl)2 (1i), (1-Me-imidazol-2-yl)2CHC(p-tolyl)3 (1j), (5-Me-pyridin-2-yl)2CHC(p-tolyl)2)3 (1k), (5-Me-pyridin-2-yl)2CHCH(p-tolyl)2 (1l), (3-Me-pyridin-2-yl)2CHCH(p-tolyl)2 (1m), (5-Me-pyridin-2-yl)2CHCH(m-xylyl)2 (1n), (3-Me-pyridin-2-yl)2CHCH(m-xylyl)2 (1o), (quinol-2-yl)2CHCH(m-xylyl)2 (1p), and (3-Me-quinol-2-yl)2CHCH(m-xylyl)2 (1q). (N∧N)PdMe2 (2i,j,q) and (N∧N)Pd(Me)Cl (3m−o) complexes were converted to (N∧N)Pd{C(═O)Me}CO+ complexes (7i,j,m−o,q). The νCO 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 3l−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 (Mw = 34 000). Polyethylene produced by 8j,l,m contains 100 branches/103 C, while that from 8n,o,q contains 60 branches/103C.