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

Palladium­(II) complexes
featuring the hybrid anionic ligand 1′-(diphenylphosphino)­ferrocene-1-sulfonate
(L–), viz., trans-(Et3NH)2[Pd­(μ-Cl)­Me­(L-κP)]2 (1), [Pd­(Me)­(dmap-κN1)­(L-κ2O,P)]­(2; dmap = 4-(dimethylamino)­pyridine),
and [Pd­(η3-allyl)­(L-κ2O,P)] (6), were synthesized
and together with the previously reported compounds trans-(Et3NH)2[PdCl2(L-κP)2] and [Pd­(LCY)­(L-κ2O,P)] (LCY =
2-[(dimethylamino-κN)­methyl]­phenyl-κC1 and 2-[(methylthio-κS)­methyl]­phenyl-κC1) 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 ≈ 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.