Ethylene Dimerization by Cationic Palladium(II) Alkyl Complexes that Contain Bis(heterocycle)methane Ligands

The catalytic ethylene dimerization reactions of (N ∧ N)PdMe(L) + species that contain bidentate nitrogen donor ligands were studied (N ∧ N = (1-Me-imidazol-2-yl) 2 CH 2 ( a ); (1-Me-imidazol-2-yl) 2 CH(C 6 H 13 ) ( b ), 1,1′-di(triphenylmethyl)-4,4′-biimidazole ( c ), (5-Me-pyridin-2-yl) 2 CH 2 ( d ), (pyrazol-1-yl) 2 CH 2 ( e ), (3,5-Me 2 -pyrazol-1-yl) 2 CH 2 ( f ), (4-Me-C 6 H 4 )N═CMeCMe═N(4-Me-C 6 H 4 ) ( g ), and (2,6- i Pr 2 -C 6 H 3 )N═CMeCMe═N(2,6- i Pr 2 -C 6 H 3 ) ( h )). (N ∧ N)PdMe 2 ( 2a – e, g ) and (N ∧ N)Pd(Me)Cl ( 3f – h ) complexes were converted to [(N ∧ N)Pd{C(═O)Me}CO][B(C 6 F 5 ) 4 )] ( 7a, c – h ), [(N ∧ N)Pd(Me)(H 2 C═CH 2 )][B(C 6 F 5 ) 4 )] ( 8a − g ), and [(N ∧ N)Pd(Me)(H 2 C═CH 2 )][SbF 6 ] ( 8f′ ). The ν CO values for 7a, c − f show that there is weak back-bonding in these species, the donor ability of the N ∧ 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/β-H elimination mechanism. The catalyst resting state is (N ∧ N)Pd(Et)(H 2 C═CH 2 ) + ( 9a, c − g ). First-order rate constants for ethylene insertion of 8a − g and 8f′ ( k insert,Me ) and 9a, c − g ( k insert,Et ) were determined by NMR. The k insert,Me and k insert,Et values for analogous (N ∧ N)Pd(R)(H 2 C═CH 2 ) + species are similar. Increasing the electrophilic character and the steric bulk of the (N ∧ N)Pd unit leads to moderate increases in ethylene insertion rates.