Intriguing Behavior of an Apparently Simple Coupling Promoter Ligand, PPh2(p-C6H4–C6F5), in Their Pd Complexes

Abstract

Ligand PPh2(p-C6H4–C6F5), LHF, is an example of monodentate biphenyl phosphine that allows for cis coordination of two phosphines to Pd, as in complex cis-[PdPf2Pd(LHF)2] (A) (Pf = C6F5). At 25 °C, complex A undergoes easy reductive elimination to decafluorobiphenyl and, competitively, isomerizes to trans-[Pd(C6F5)2Pd(LHF)2] via functionally three-coordinate intermediates cis-[PdPf2(LHF)(S)] with the fourth position empty or weakly protected (S= THF, OH2, or π-aryl). Unexpectedly, the direct reductive C6F5–C6F5 elimination is faster from the four-coordinate complex A than from the intermediates with only one strong LHF. The reason for this is that two cis LHF ligands play the role of a chelate with a large bite angle and some tetrahedral distortion. As a matter of fact, using LHF in excess (Pd:L ≪ 1:2), a Pf–Pf coupling barrier ΔG‡Pf–Pf = 23.1 kcal·mol–1 is measured, which ranks its efficiency for coupling and formation of the corresponding Pd0 catalyst as better than XantPhos or PhPEWO-F and about the sa...