Electronic Control of the Asymmetry in Heteroatom Monosubstituted Olefin Bonding to the Cyclopentadienyl Iron(II) Dicarbonyl Cation. A Hammett Correlation Study

Abstract

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.

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

“…This p overlap has been experimentally demonstrated [4,5] to displace the Fp moiety along the olefin face toward the remaining olefinic carbon (as in II, Eq. (2)), thereby activating [4] displacement is not a prerequisite for nucleophilic substitution chemistry on the coordinated olefin [6], limited displacement has been observed (both experimentally and theoretically) to facilitate substitution at the site of heteroatom substitution [4,[7][8][9] ð2Þ When the entering nucleophile is an alcohol (or alkoxide), the unsymmetric dialkoxy intermediate product results in a competition between similar p donors.…”

“…As such, they reveal the competitive p donation between the oxygen at C 1 and the nitrogen at C 2 . We know from Taskinen's work with mixed cis-dialkoxyethylenes [11], as well as our previous work with various Fp olefin complexes [5], that when competition for p-p overlap exists, the 13 C shift of the carbon bonded to the dominant p donor will move downfield while the remaining olefin carbon moves upfield. The distance between these peaks (in an analogous series) is therefore a reasonable measure of each heteroatom's ability to compete for p overlap with the olefin.…”

“…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.…”

“…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)).…”

“…The increased separation of the 13 C peaks was structurally correlated to the increased displacement of the Fp fragment toward carbon 1 by comparing the X-ray structures of Fpðg 2 -CH 2 CHðOMeÞÞ þ BF À 4 and Fpðg 2 -CH 2 CHðNMe 2 ÞÞ þ BF À 4 [4], with those of derivatives 2 and 6 [5].…”