Spectroscopic and Computational Assessment of the Rotational Barrier of a Ferrocenyl-Stabilized Cyclopentadienyl Cation:  Evidence for the First Hydroxyfulvalene Ligand

Abstract

Protonation of 3-ferrocenyl-2,4,5-triphenylcyclopentadienone with trifluoroacetic acid results in the formation of the first hydroxyfulvalene complex. The dramatic deshielding and decoalescence of the 1H and 13C NMR signals in the ferrocenyl region offer evidence for the participation of the metal in the stabilization of the positive charge, which results in restricted rotation of the ferrocenyl group. As determined by NMR spectroscopy, the maximum barrier to rotation in the neutral species is ca. 9 kcal mol-1, whereas the minimum barrier in the cation considerably exceeds 13 kcal mol-1. The difference in rotational barriers for the neutral and protonated species is rationalized by using DFT calculations.

From allenes to tetracenes: Syntheses, structures, and reactivity of the intermediates

“…It is well established that organometallic moieties, such as ferrocenyl [26,27], or tetrahedral alkynyl-dimetallic clusters, (RC≡C)(ML n ) 2 where ML n = Co(CO) 3 or Mo(CO) 2 (C 5 H 5 ) [28], are well able to alleviate positive charge, while cyclopentadienide, ester, or nitrile substituents can play the opposite role. We therefore chose to develop synthetic routes to such potentially zwitterionic systems with the goal of probing the carbenic character of the central carbon.…”

An NMR study of the rotational barriers in cobalt-stabilized carbocations: X-ray crystal structures of (η4-C4Ph4)Co-(η5-C5H4R), where R is CH3CO, CHO, CH(tBu)OH

“…Another approach is to probe the degree of double bond character developed between the complexed ring and the cationic carbon since this will result in an enhanced barrier to rotation about the C@CR 1 R 2 linkage. This approach has recently been used to demonstrate the ability of a dicobalt-alkyne cluster, or of a ferrocenyl substituent, to stabilize antiaromatic fluorenyl, indenyl or cyclopentadienyl cations [15,16]. The particular focus of the current investigation is to explore the cation-stabilizing ability of systems of the type [(g 4 -C 4 Ph 4 )Co(g 5 -C 5 H 4 -CHR)] þ and to compare the results with those for other sandwich compounds.…”

Different Rearrangement Behaviour of the Cation or Anion Derived from the Diels–Alder Adduct of 9‐Ferrocenylanthracene and 1,4‐Benzoquinone: Ring‐Opening or Paddlewheel Formation

“…One can rationalize this result by invoking the formation of a particularly favourable cation adjacent to the ferrocenyl substituent. The ability of many organometallic systems, such as ferrocenyl, [13] (h 4 -cyclobutadiene)cobalt(h 5 -cyclopentadienyl), [14] or (alkynyl)dicobalthexacarbonyl, [15] to delocalize positive charge through overlap of a filled metal orbital with the vacant 2p orbital on the neighbouring carbocationic site is a well-documented phenomenon. In the present case, as shown in Scheme 2, protonation of the carbonyl group adjacent to the ferrocenyl substituent, as in 5, cleavage of the C9ÀC12 bond to form the ferrocenyl-stabilized cation 6, loss of a proton, leading to the keto-enol 7, and finally tautomerization of 7 leads directly to the observed product, 4.…”

“…NMR spectra were acquired on Varian VnmrS 400 and Inova 500 MHz spectrometers at 30 8C unless otherwise stated. Assignments were based on standard 1 H-1 H and 1 H- 13 C two-dimensional techniques, and nOe measurements. Melting points were determined on a Gallenkamp instrument in air, and are uncorrected.…”