Variable-Temperature NMR Determination of the Barriers to Rotation about the Ir−C σ-Bond in a Series of Primary Perfluoroalkyl Iridium Complexes [IrCp*{(CF₂)<i>_n</i>CF₃}(PMe₃)₂]⁺X^- [<i>n</i> = 1, 2, 3, 5, 7, 9, 11; X = I, OT_f]

Abstract

A series of primary perfluoroalkyl iridium complexes [IrCp*{(CF2)nCF3}(PMe3)2]+X- [n = 1, 2, 3, 5, 7, 9, 11; X = I, OTf] has been prepared. X-ray crystallographic studies of a representative example, [IrCp*{(CF2)9CF3}(PMe3)2]+I-, shows the ground state structure of the cation in the solid to be of C1 symmetry, with the perfluoroalkyl ligand adopting a conformation in which the fluorines on each CF2 group reside in different chemical environments. In solution the 19F NMR spectra of all these compounds indicate that the α-CF2 fluorines are diastereotopic at low temperatures, consistent with the solid state structure. On warming, these resonances coalesce, indicating a rotation or windshield-wiper motion of the perfluoroalkyl ligand that samples a conformation of Cs symmetry; Eyring plots of the rate constant/temperature data provide values of ΔH⧧ of ∼33 ± 2 kJ/mol for each system; the values are independent of the counterion. While there is clearly a difference between ΔS⧧ values for the perfluoroethyl compounds compared to longer chain analogues, the precision of the data is insufficient to quantify this effect. The barrier to perfluoroalkyl ligand rotation is ascribed to steric effects; changing the Cp* ligand to Cp or changing from PMe3 ligands to the O atoms of acetylacetonate in IrCp*(RF)(acac) results in no decoalescence of CF2 resonances at low temperatures, even though the X-ray structure of IrCp*(CF2CF3)(acac) shows an identical unsymmetrical ground state conformation for the perfluoroethyl ligand to that observed in the bis(trimethylphosphine) analogues.