Nucleophilic substitution of CpFe(CO) 2 (η 2 -H 2 CC(H)OEt) + PF 6with para-substituted anilines was used to prepare a series (4-10) of η 2 -vinyl aniline Fp + complexes of the general formula CpFe(CO) 2 [η 2 -CH 2 C(H)NH(p-C 6 H 4 X)] + PF 6 -, where X ) OMe (4), Me ( 5), H, (6), Br (7), COMe (8), CN (9), and NO 2 (10). Correlation of the Hammett σ para parameters with the 13 C NMR shifts of the metal-coordinated vinyl carbons demonstrated the ability to control the position of the Fp + moiety along the olefin face. As the electron-donating character of the para substituent was increased, the Fp + moiety was displaced away from the nitrogenbearing carbon, increasing the asymmetry of the metal-olefin bonding. These conclusions were further supported by determining the X-ray crystal structure of the p-Me (5) and the p-acetyl (8) derivatives.
H NMR techniques (selective inversion, total line shape analysis, and T 1 F measurement) have been used to measure the olefin bond rotation rates in a series of compounds of the general formula CpFe(CO) 2 [η 2 -CH 2 C(H)NH(p-C 6 H 4 X)], where X ) OMe ( 4), H (6), CN (9), NO 2 (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 ∆H q 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 aniline influences the overlap of the nitrogen lone pair with the adjacent olefinic carbon. This overlap controls the asymmetry in the metal-olefin bond by displacing the Fp + moiety away from the nitrogen-bearing carbon, which is reflected in a drop in the
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