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)_nCF₃}(PMe₃)₂]⁺X^- [n = 1, 2, 3, 5, 7, 9, 11; X = I, OT_f]

Abstract

A series of primary perfluoroalkyl iridium complexes [IrCp*{(CF 2 ) n CF 3 }(PMe 3 ) 2 ] + X -[n ) 1, 2, 3, 5, 7, 9, 11; X ) I, OT f ] has been prepared. X-ray crystallographic studies of a representative example, [IrCp*{(CF 2 ) 9 CF 3 }(PMe 3 ) 2 ] + I -, shows the ground state structure of the cation in the solid to be of C 1 symmetry, with the perfluoroalkyl ligand adopting a conformation in which the fluorines on each CF 2 group reside in different chemical environments. In solution the 19 F NMR spectra of all these compounds indicate that the R-CF 2 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 C s symmetry; Eyring plots of the rate constant/ temperature data provide values of ∆H q of ∼33 ( 2 kJ/mol for each system; the values are independent of the counterion. While there is clearly a difference between ∆S q 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 PMe 3 ligands to the O atoms of acetylacetonate in IrCp*(R F )(acac) results in no decoalescence of CF 2 resonances at low temperatures, even though the X-ray structure of IrCp*(CF 2 -CF 3 )(acac) shows an identical unsymmetrical ground state conformation for the perfluoroethyl ligand to that observed in the bis(trimethylphosphine) analogues.

Spatially Resolved Raman and UV‐visible‐NIR Spectroscopy on the Preparation of Supported Catalyst Bodies: Controlling the Formation of H2PMo11CoO405− Inside Al2O3 Pellets During Impregnation

“…Notably, while a single conformer is populated in solution, other conformations are not prohibitively high in energy, and conformational exchange is rapid on the NMR timescale. As illustrated in Figure 3, we have established that there is a low rotation barrier about the Ir–C bond (Δ H ‡ ≈ 33 ± 2 kJ mol –1 ), which interconverts the environments of F 1 and F 2 in [IrCp*(CF 2 R F )(PMe 3 ) 2 ] + cations and in IrCp*(CF 2 R F )(acac) 190. Likewise, HOESY studies on the H and CH 3 migration products allow the conformations and relative stereochemistries of the Ir and C stereocenters to be established for the major ( R Ir , R C )( S Ir , S C ) and minor ( R Ir , S C )( S Ir , R C ) diastereomers, as shown in Figure 2 181,191.…”

Conversion of Carbon–Fluorine Bonds α to Transition Metal Centers to Carbon–Hydrogen, Carbon–Carbon, and Carbon–Heteroatom Bonds

“…As illustrated in Figure 3, we have established that there is a low rotation barrier about the Ir-C bond (∆H ‡ ≈ 33 Ϯ 2 kJ mol -1 ), which interconverts the environments of F 1 and F 2 in [IrCp*(CF 2 R F )(PMe 3 ) 2 ] + cations and in IrCp*(CF 2 R F )(acac). [190] Likewise, HOESY studies on the H and CH 3 migration products allow the conformations and relative stereochemistries of the Ir and C stereocenters to be established for the major (R Ir ,R C )(S Ir ,S C ) and minor (R Ir ,S C )(S Ir ,R C ) diastereomers, as shown in Figure 2. [181,191] It is noteworthy that of all the solution HOESY measure- [191] Because only small concentrations of HCl are available at any given point, overall reactions are slower than those with one full equivalent of acetic acid; measured in CD 2 Cl 2 , the overall values of ∆G ‡ (298K) are 84 Ϯ 4 kJ mol -1 for the reaction of 14-H and LutHCl and 64 Ϯ 2 kJ mol -1 for reaction of 14-H with acetic acid; [172] in each case this is a significantly higher barrier than that for any conformational change in the starting materials.…”

Reactions of Half-Sandwich Ethene Complexes of Rhodium(I) toward Iodoperfluorocarbons: Perfluoro-alkylation or -arylation of Coordinated Ethene versus Oxidative Addition

“…Perfluoroiodocarbons usually react with complexes of the type [M­(η 5 -C 5 R 5 )­L 2 ] (M = Co, Rh or Ir; R = H or Me; L = CO or PF 3 ) by oxidative addition to give complexes [M­(η 5 -C 5 R 5 )­I­(R F )­L], where R F is a perfluorinated alkyl, aryl, or benzyl group. − However, reactions involving perfluoroalkylation of ligands have been observed in a few cases (Scheme ). For instance, clean perfluoroalkylation at the η 5 -Cp ring was observed in the reactions of [M­(η 5 -Cp)­(PMe 3 ) 2 ] (M = Co or Rh) with I n C 3 F 7 or ICF­(CF 3 ) 2 , ,, whereas mixtures of products resulting from perfluoroalkylation at the metal, CO, or η 5 -Cp ligands were formed in the reactions of [M­(η 5 -Cp)­(PMe 3 )­(CO)] (M = Rh or Ir) with the same perfluoroalkyl iodides .…”