Mössbauer spectroscopy of ferrocene complexes. XIII. Conformational analysis of arylferrocenes using Mössbauer, 1H NMR, and 13C NMR spectroscopy

Abstract

A series of arylferrocenes (FcC6H4X) have been studied using 1H and 13C NMR spectroscopy as well as 27Fe Mössbauer spectroscopy. The Hβ resonances δ(Hβ) gave a biphasic plot against both σ and σ+, electron-withdrawing groups having about three times the effect of electron releasing substituents. For ortho-phenyl derivatives δ(Hβ) was relatively insensitive to the nature of the substituent, but δ(Hα) varied significantly. The effect is discussed in terms of twist angle θ between the phenyl and cyclopentadienyl rings. From 13C data, average values of the ferrocenyl substituent effect on the chemical shifts (CCS) of the phenyl carbons was evaluated. From a plot of the values for the para carbon vs. σ and σ+ for monosubstituted benzenes, σ and σ+ values for the ferrocenyl substituent of -0.01 and -0.19, respectively, were found. This indicates that the ferrocenyl substituent is not a particularly strong electron releaser in this system. Further comparison of CCS data with those for the related biphenyls enabled an assessment to be made of θ for meta and para substituents. θ values for the ortho derivatives were discussed in a qualitative manner. Mössbauer data reveal a small mesomeric effect on the quadrupole splitting (QS) of the para substituted derivatives. These splittings were significantly lower than that of ferrocene. The introduction of an ortho substituent of any nature resulted in QS values very close to ferrocene. These results were also discussed in terms of changes in θ.