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

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

Electronic control of the competitive π donation in heteroatom substituted CpFe(CO)2(olefin)BF4 complexes and the resulting influence on the symmetry of the metal to olefin bond

“…Second, any increase in the double bond character resulting from increased bonding symmetry in 10-14 should be reflected in an increased barrier to rotation about the C 1 -C 2 bond relative to the facile rotation seen with the strongly asymmetric complexes 1-9 [16,17]. While the 13 C NMR argues for some displacement of the Fp group toward carbon 1, the 1 H NMR shows no evidence for olefin rotation at room temperature.…”

“…The ability to regulate electron donation to C 2 from the p-position of the vinyl aniline, previously demonstrated with 1-9 [5,16], affords the opportunity to control the competition between these p donors and their influence on the metal olefin bond asymmetry. Control of this competition should determine a molecule's placement along the continuum between resonance structures III and IV (Eq.…”

“…While the 13 expected to differ based on O vs. N substitution, the movement of both peaks in response to changes in the p-position of the aniline demonstrates that these shifts reflect changes in the competition between p donors. As the electron withdrawing ability of X increases (r para increases), the overlap between the nitrogen lone pair and C 2 decreases slightly [16], increasing the p donation from the oxygen at C 1 . This causes the 13 C shift of C 1 to increase and that of C 2 to decrease as the oxygen p donor competes more effectively.…”

“…In previous work, we demonstrated [5,16,17] this control in singly substituted olefin complexes by synthesizing the series of para substituted, Fp vinyl aniline complexes, 1-9 (Eq. (4)).…”

Dynamic NMR Study of Ethene Exchange in Cationic CNN-Type Platinum(II) Complexes

“…Olefin complexes typically show dynamical structures even in the solid state, ,, due to rotation, rocking, and/or librational motions . Actually, to explain the dynamic behavior of the olefin ligand, the following mechanisms have been taken into account: (1) propeller-like rotation, , (2) metal−olefin bond cleavage, followed by recombination, , (3) metal−ligand bond cleavage (i.e., the ligand trans to the olefin moiety), followed by ligand rotation and recombination, ,,, and/or (4) intermolecular exchange with the free olefin ligand. , In most cases the fluxional behavior of the organometallic compounds has been reported only without a complete discussion of the molecular mechanism involved. However, definitive arguments for a correct mechanistic choice might be provided by NMR line-shape analysis and by the derived kinetic parameters.…”