om700768j_si_004.cif (27.28 kB)

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

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posted on 31.12.2007, 00:00 by Christopher T. Burns, Richard F. Jordan
The catalytic ethylene polymerization reactions of (NN)PdMe(L)+ species that contain bulky bis(heterocycle)methane ligands were studied (NN = (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). (NN)PdMe2 (2i,j,q) and (NN)Pd(Me)Cl (3mo) complexes were converted to (NN)Pd{C(O)Me}CO+ complexes (7i,j,mo,q). The νCO values for 7i,j,mo,q show that there is weak back-bonding in these species. Complexes 2i,j and (NN)Pd(Me)Cl complexes 3lo were converted to (NN)Pd(Me)(H2CCH2)+ (8i,j,lo and 8q (not directly observed)). First-order rate constants for ethylene insertion of 8i,j,lo were determined by NMR. Increasing the electrophilic character and the steric bulk of the (NN)Pd unit increases the rate of ethylene insertion. In-situ-generated 8j,ln,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.