Differentiation between Chelate Ring Inversion and Aryl Rotation in a CF3-Substituted Phosphine-Sulfonate Palladium Methyl Complex

Abstract

The solution conformations and dynamic properties of the CF3-sbustituted (ortho-phosphinoarenesulfonate)Pd complexes (PO-CF3)PdMe(L) ([PO-CF3]− = 2-{(o-CF3-Ph)2P}-4-Me-benzenesulfonate, L = 2,6-lutidine (3), pyridine (4)) were studied by NMR spectroscopy, taking particular advantage of 31P–19F through-space couplings and 1H–1H and 1H–19F nuclear Overhauser effects. In CD2Cl2 solution in the temperature range of −80 to 20 °C, 3 adopts an exo2 conformation. One o-CF3-Ph ring is positioned such that the CF3 group points toward Pd (exo) and exhibits through-space 4JPF coupling. The other o-CF3-Ph ring is positioned such that the CF3 group points away from Pd (endo) and does not exhibit through-space 4JPF coupling, and the o-H lies in the deshielding region near an axial site of the Pd square plane and exhibits a low-field chemical shift (δ > 9). Complex 4 exists as a 2:1 mixture of exo2 and exo3 isomers in CD2Cl2 solution at −90 °C. In exo2-4, one CF3 group is exo and exhibits through-space 4JPF coupling, while the other CF3 group is endo and does not exhibit through-space 4JPF coupling. In exo3-4, both CF3 groups are exo and exhibit through-space 4JPF couplings. Complex 4 undergoes two dynamic processes: rotation of the axial o-CF3-Ph ring (AaR), which interconverts exo2-4 and exo3-4 (ΔG⧧ = 9.9(5) kcal/mol), and chelate ring inversion (RI), which permutes the axial and equatorial o-CF3-Ph rings (ΔG⧧ = 21(1) kcal/mol).