Aromaticity and ring currents in ferrocene and two isomeric sandwich complexes

Differentiation of ferrocene D5d and D5h conformers using IR spectroscopy

“…As a result, the orbital irreducible representations which are correlated as 0 and 00 in D 5h become g and u in D 5d . The highest occupied molecular orbital (HOMO) of D 5h conformer is a doubly degenerate e 2 0 state and the next HOMO (HOMO-1) is a 1 0 but the lowest unoccupied molecular orbital (LUMO) is e 1 00 which agrees well with previous DFT based B3LYP/6-31G** calculations [2]. On the other hand, the HOMO for D 5d is a doubly degenerate e 2g state, the HOMO-1 is a 1g and LUMO is e 1g .…”

“…This is in good agreement with other theoretical studies [9e11]. Although the graphic user interface (GUI) tools for computational chemistry packages such as ADFView [28] and Molden [29] almost always indicate the ten localised FeÀC bonds existing in both D 5h and D 5d conformers of ferrocene in display, the ferrocene orientations are likely to adopt stacking structures due to the ligand p-orbitals and aromaticity as noted by Bean et al [2]. Infrared (IR) discussion herein will also provide evidences of the stacking structures rather than the conventional FeeC bonds for ferrocene.…”

“…Since its discovery some fifty years ago [1], a literature search in 2010 yields over 23,000 articles studying ferrocene [2] and new discoveries of ferrocene continue. For example, a most recent article in Nature Chemistry established ferrocene as a redox couple for organic dye sensitizer solar cell (DSSC) [3].…”

“…The development of ferrocene study since its discovery in 1951 [1] has been well documented in recent articles such as Coriani et al [9], Roy et al [10], Gryaznova et al [11] and Bean et al [2]. It has been stated [11] that a staggered ferrocene structure of D 5d point group symmetry dominates experiments in the condensed phase [12e15], whereas the eclipsed structure of D 5h point group symmetry is found in the gas phase [16e18].…”

“…It begins to recognize that the CpFeCp structure does not confine to the conventional FeeC "bonds" [2]. Infrared (IR) spectra of ferrocene [11,24] reveal torsional or swing vibrations of CpFeCp in the low energy region of <600 cm À1 , indicating that it is unlikely that such ten localised FeeC bonds exist in the ferrocene structures as proposed [9e11,24].…”