Selective Ethylene Dimerization by Palladium(II) Complexes Bearing a Phosphinoferrocene Sulfonate Ligand

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Palladium(II) complexes featuring the hybrid anionic ligand 1′-(diphenylphosphino)ferrocene-1-sulfonate ( L – ), viz., trans -(Et 3 NH) 2 [Pd(μ-Cl)Me( L -κ P )] 2 ( 1 ), [Pd(Me)(dmap-κ N 1 )( L -κ 2 O, P )]( 2; dmap = 4-(dimethylamino)pyridine), and [Pd(η 3 -allyl)( L -κ 2 O, P )] ( 6 ), were synthesized and together with the previously reported compounds trans -(Et 3 NH) 2 [PdCl 2 ( L -κ P ) 2 ] and [Pd(L CY )( L -κ 2 O, P )] (L CY = 2-[(dimethylamino-κ N )methyl]phenyl-κ C 1 and 2-[(methylthio-κ S )methyl]phenyl-κ 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[PF 6 ] 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 ≈ 95 h –1 ) and high selectivity for 1-butene (95%) at 21 °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.