Measurement of Olefin Bond Rotation Barriers in a Series of Vinylaniline Complexes of the Cyclopentadienyliron(II) Dicarbonyl Cation and Their Correlation to Metal−Olefin Bond Asymmetry

Abstract

1H NMR techniques (selective inversion, total line shape analysis, and T1ρ measurement) have been used to measure the olefin bond rotation rates in a series of compounds of the general formula CpFe(CO)2[η2-CH2C(H)NH(p−C6H4X)], where X = OMe (4), H (6), CN (9), NO2 (10), Cl (11), COOMe (12). Complexes 4, 6, 11, and 12 enabled the use of all three methods to yield data sets that spanned a wide temperature range and allowed for reliable determination of the thermodynamic activation parameters for olefin bond rotation. When these three methods were combined with a bootstrap statistical analysis of the Eyring plots, it was possible to resolve the small differences in the ΔH⧧ values between three of the four complexes. A clear trend is established between increasing electron donation from the aniline ligand and the facilitation of olefin bond rotation. These results are placed in the context of previous work, providing additional experimental evidence that electronic control from the para position of the anilin...