Selective Ethylene Dimerization by Palladium(II) Complexes Bearing a Phosphinoferrocene Sulfonate Ligand
Abstract
Palladium(II) complexes featuring the hybrid anionic ligand 1'-(diphenylphosphino)ferrocene-1-sulfonate (L-), viz., trans-(Et3NH)(2)[Pd-(mu-Cl)Me(L-kappa P)](2) (1), [Pd(Me)(dmap-kappa N-1)(L-kappa O-2,P)](2; dmap = 4-(dimethylamino)pyridine), and [Pd(eta(3)-allyl)(L-kappa O-2,P)] (6), were synthesized and together with the previously reported compounds trans- (Et3NH)(2)[PdCl2(L-kappa P)(2)] and [Pd(L-CY)(L-kappa O-2,P)] (L-CY = 2-[(dimethylamino-kappa N)methyl]phenyl-kappa C-1 and 2-[(methylthio-kappa S)methyl]phenyl-kappa C-1) tested as precatalysts for Pd-catalyzed ethylene dimerization. Only compound 1 gave rise to an active catalyst after activation by sequential halogen removal with Tl[PF6] and Na[BAr'(4)] (Ar' = 3,5-bis(trifluoromethyl)phenyl) in chloroform.
Thus, the formed catalyst efficiently mediated the dimerization of ethylene showing both good activity (TOF approximate to 95 h(-1)) and high selectivity for 1-butene (95%) at 21 degrees C and 30 bar of ethylene pressure. DFT calculations have shown that the dimerization reaction is thermodynamically preferred over the formation of higher oligomers and that O,P-chelate coordination of the phosphinosulfonate ligand in all Pd(II) reaction intermediates is vital for the catalytic process.
In particular, the O,P-chelating phosphinoferrocene sulfonate ligand stabilizes and electronically differentiates the reaction intermediates and favors concerted ethyl migration to coordinated ethylene giving rise to 1-butene.