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

• Published in: Organometallics 2019-10, Vol.38 (19), p.3688-3695
• Main Authors: Pérez-Iglesias, María, Infante, Rebeca, Casares, Juan A, Espinet, Pablo
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/acs.organomet.9b00460

Ligand PPh2(p-C6H4–C6F5), L HF , is an example of monodentate biphenyl phosphine that allows for cis coordination of two phosphines to Pd, as in complex cis-[PdPf2Pd­(L HF )2] (A) (Pf = C6F5). At 25 °C, complex A undergoes easy reductive elimination to decafluorobiphenyl and, competitively, isomerizes to trans-[Pd­(C6F5)2Pd­(L HF )2] via functionally three-coordinate intermediates cis-[PdPf2(L HF )­(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 L HF . The reason for this is that two cis L HF ligands play the role of a chelate with a large bite angle and some tetrahedral distortion. As a matter of fact, using L HF 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 same as tBuBrettPhos. On the other hand, complex (μ-Cl)2[Pd2Rf2(L HF )2] (B) (Rf = C6F3Cl2 = 3,5-dichloro-2,4,6-trifluorophenyl), obtained by reaction of (μ-Cl)2[Pd2Rf2(tht)2] (tht = tetrahydrothiophene) with L HF , presents in the 19F NMR COSY spectrum a very intriguing through-space coupling pattern of the Fortho atoms of the C6F5 group in L HF and the 3-5-C6F3Cl2 group on Pd. The intermittent coupling mechanism proposed is based on the switching of π–π-stacking of C6F5 from one Ph group to another Ph group of L HF , which gives rise to enantiomers at the chiral P atom. Rotation around the P–biphenyl bond under hindered rotation around the C–C6F5 bond produces the intriguing selective coupling observed.