Ethylene dimerization by cationic palladium(II) alkyl complexes that contain bis(heterocycle)methane ligands

The catalytic ethylene dimerization reactions of (N boolean AND N)PdMe(L)(+) species that contain bidentate nitrogen donor ligands were studied (N boolean AND N = (1-Me-imidazol-2-yl)(2)CH2 (a); (1-Me-imidazol-2-yl)(2)CH(C6H13) (b), 1,1'-di(triphenylmethyl)-4,4'-biimidazole (c), (5-Me-pyridin-2-yl)(2)CH2 (d), (pyrazol-1-yl)(2)CH2 (e), (3,5-Me-2-pyrazol-1-yl)(2)CH2 (f), (4-Me-C6H4)N=CMeCMe=N(4-Me-C6H4) (g), and (2,6-Pr-i(2)-C6H3)N=CMeCMe=N(2,6-(Pr2C6H3)-Pr-i) (h)). (N boolean AND N)PdMe2 (2a-e,g) and (N boolean AND N)Pd(Me)Cl (3f-h) complexes were converted to [(N boolean AND N)Pd{C(=O)Me}CO][B(C6F5)(4))] (7a,c-h), [(N boolean AND N)Pd(Me)(H2C=CH2)][B(C6F5)4)] (8a-g), and [(N boolean AND N)Pd(Me)(H2C=CH2)][SbF6] (8f'). The v(CO) values for 7a,c-f show that there is weak back-bonding in these species, the donor ability of the N boolean AND N ligand varies in the order imidazole > pyridine > pyrazole, and variation of the chelate ring size does not strongly affect the electron density at Pd. 8a,c-g and 8f' dimerize ethylene by an insertion/beta-H elimination mechanism. The catalyst resting state is (N boolean AND N)Pd(Et)(H2C=CH2)(+) (9a,c-g). First-order rate constants for ethylene insertion of 8a-g and 8f' (k(insert,Me)) and 9a,c-g (kin,,,,Et) were determined by NMR. The k(insert,Me) and k(insert,Et) values for analogous (N boolean AND N)Pd(R)(H2C=CH2)(+) species are similar. Increasing the electrophilic character and the steric bulk of the (N boolean AND N)Pd unit leads to moderate increases in ethylene insertion rates.