Dominance of eclipsed ferrocene conformer in solutions revealed by the IR spectra between 400 and 500 cm-1

https://doi.org/10.1016/j.radphyschem.2021.109590Get rights and content

Abstract

A combined Fourier transform infrared (FTIR) spectra of ferrocene (Fc) and density functional theory (DFT) based quantum mechanical calculations confirmed the dominance of the eclipsed Fc conformer in the fingerprint region of 400–500 cm−1 in solutions. Solution IR spectra of Fc measured in acetonitrile (ACN, ε = 35.69), dichloromethane (DCM, ε = 8.93), tetrahydrofuran (THF, ε = 7.43) and dioxane (DOX, ε = 2.21) show two well-defined bands in the 480-500 cm−1 region with the higher-wavenumber band higher in intensity. The band profile agrees well with the earlier IR spectra of Lippincott and Nelson (1958) in tetrachloromethane solution as well as more recent measurement in dichloromethane solution of Duhović and Diaconescu (2013). DFT based quantum mechanical calculations predict that the eclipsed (D5h) Fc conformer is lower in energy than the staggered (D5d) conformer and that the room-temperature solution spectrum of Fc is dominated by that of the D5h form (Best et al., 2016). The present study confirms that solvent effects enhanced the dominance of the D5h Fc conformer which resulted in switch of the IR profile patterns in 400–500 cm−1, from low-wavenumber band higher in intensity pattern in gas phase to higher-wavenumber band higher in intensity in solutions. It further suggests that the effects of solvents on the IR spectra of Fc in this region are small and the solvent model effects are also small for the IR spectrum in the region of 400–500 cm−1 of Fc.

Introduction

Since its discovery some fifty years ago (Kealy and Pauson, 1951), dicyclopentadienyl iron (FeCp2 or Fc) continues to be the focus of many studies in its own right and in its relation to the metal complexes of substituted cyclopentadienyl (Cp) and other cyclic derivatives. Exploitation of its well-defined redox behaviour, and ease of derivatization (Long, 1998) have resulted in a broad range of Fc derivatives which have found key roles in homogeneous catalysis, polymer chemistry, molecular sensing, and nonlinear optical materials (Yamaguchi et al., 1998), organic solar cells (OSC), as well as biological applications (Osella et al., 2000; Fouda et al., 2007).

The ground-state conformation of the two parallel cyclopentadienyl rings of Fc, eclipsed (D5h symmetry) or staggered (D5d symmetry) is surprisingly difficult to resolve unambiguously, this being due to the small difference in energy between the rotameric forms and the small energy barrier between these forms (Cotton and Wilkinson, 1988; Mohammadi et al., 2012). This small rotational energy barrier, and attendant dynamic behaviour, challenges in the experimental studies of Fc since its discovery in 1951 (Kealy and Pauson, 1951). The debate on the most stable conformer of Fc, whether it is the eclipsed (D5h) or the staggered (D5d), with both experimental and other information (Wilkinson et al., 1952; Bohn and Haaland, 1966; Fischer, 1952), has been continuing without conclusive evidences until our ground-breaking study of Fc using IR spectroscopy (Mohammadi et al., 2012). A few years after the discovery of Fc, Lippincott and Nelson, 1953, 1955, 1958 reported the first infrared (IR) measurements. Notwithstanding the quality of the early measurements, the IR spectra of Fc were assigned to the staggered (D5d) Fc conformer (Lippincott and Nelson, 1958), this being based on the interpretation of the crystal structure. Subsequent reinterpretation of the X-ray structure in terms of statically or dynamically disordered D5h Fc (Dunitz, 1995)] is also consistent with the vibrational spectra as reflected by normal coordinate analysis of Fc (D5h) (Brunvoll et al., 1971). It is important to note that the predicted number of IR and Raman-active modes is the same for both staggered (D5d) and eclipsed (D5h) conformers of Fc.

Detailed structural understanding of the Fc conformers is very important as Fc derivatives may inherit particular properties which only exist in a particular conformer (Duhović and Diaconescu, 2013; Yamaguchi et al., 2007). For example, additional ligands coordinating to the metal and the Cp rings while maintaining certain symmetry is preferred for a particular geometry of Fc conformer (Duhović and Diaconescu, 2013; Cooper et al., 2011; Gryaznova et al., 2010). Design of synthesis pathways and understanding of the mechanics and reaction dynamics of the Fc derivatives require detailed information of the structure, symmetry and properties of the Fc conformers. The stability of eclipsed and staggered conformers of Fc has been a challenge issue and both structures were discussed in textbooks (Mehrotra, 2007; Veera, 1998; Atkins et al., 2010). A number of articles such as Coriani et al. (2006), Roy et al. (2008), Gryaznova et al. (2010) and Bean et al. (2011) have well documented the history and current status of the Fc studies.

Properties of the staggered and eclipsed conformers of Fc are not markedly different due to the shape of the conformers (both contain D5 subsymmetry). The differences between most of the calculated properties of the Fc conformers are either zero or too small to differentiate (e.g., within the error of the methods), such as dipole moment and rotational constants (Mohammadi et al., 2012). This underlies why previous conclusions regarding the conformation of Fc are qualified or contradictory (Wilkinson et al., 1952; Bohn and Haaland, 1966; Fischer, 1952). Despite the one to one correspondence of the form of the IR-active modes for the D5h and D5d conformers, calculations of the IR spectra reveal a predicted signature of the different conformers (Mohammadi et al., 2012; Lippincott and Nelson, 1958) and it is timely to review the assignment of the observed IR spectra to the staggered (D5d) conformer of Fc (Lippincott and Nelson, 1958).

Literature reveals that infrared (IR) spectral simulations of Fc (Lippincott and Nelson, 1953, 1958; Duhović and Diaconescu, 2013; Mohammadi et al., 2012; Gryaznova et al., 2010; Best et al., 2016; Wang and Vasilyev, 2020; Trevorah et al., 2020) that the DFT methods can be employed to interpret the measured IR spectra of Fc. However, the basis sets employed in the DFT models (Duhović and Diaconescu, 2013; Gryaznova et al., 2010) were not particularly suitable to the iron atom of Fc, the simulated IR spectrum of Fc needed scaling factors to fit the measurements (Duhović and Diaconescu, 2013; Gryaznova et al., 2010; Trevorah et al., 2020). As a result, the simulated Fc IR spectra may be useful for interpretation but hardly suitable for prediction purposes for dynamic systems. In a combined experimental and theory study, we further revealed (Best et al., 2016) that, owing to population of higher vibrational levels of the reaction coordinate the IR spectra of both rotameric forms are needed to calculate to temperature-dependence of the IR spectrum. This approach was shown to allow the self-consistent interpretation of the IR spectra of Fc in wax (solid) solution, CCl4 solution and the gas phase, where in each case the eclipsed conformer of Fc is lowest in energy, but that the relative energies of the D5h and D5d forms is phase-dependent (Best et al., 2016) and temperature dependent (Wang and Vasilyev, 2020). Most important, if the energy difference between the D5h and D5d forms is larger than ca. 10 kJ mol−1 then the RT spectrum will be dominated by the more stable form.

The DFT based Fc study (Mohammadi et al., 2012) suggested while there are only small differences in the calculated wavenumbers of the remaining normal modes for the D5h and D5d conformers, the spectral splitting within the IR region of 480–500 cm−1 provides a means of distinguishing between the eclipsed and staggered Fc conformers. There are two vibrations of Fc in this region, one dominated by the translation of the Fe atom between the Cp rings (Fe translation), the other is best described as a tilting of the Cp rings towards each other and a translation of the Fe atom normal to the tilt axis of the Cp rings (Cp tilt). Only the latter of these modes is sensitive to the relative conformation of the Cp rings, consequently there is a difference in the relative energies of the two normal modes according to whether Fc has a D5h or a D5d geometry (Mohammadi et al., 2012). Importantly, there is a difference in the relative intensities of these modes which can be used to assist the analysis. For the D5h conformer the more intense Cp-tilt mode is higher in wavenumber and there is a significant energy difference between the two modes (8-10 cm−1) whereas for the D5d form the two modes are nearly coincident (within 2 cm−1). (Mohammadi et al., 2012). These signatures provide a means of using the IR measurements to identify whether there is a single conformer or a mixture of the two forms. The DFT based B3LYP/m6-31G(d) model were shown to simulate accurately the IR spectra of Fc without scaling, if used with an appropriate basis set (Mitin et al., 2003). It was shown that the central Fe atom of the sandwich compound plays a significant role (Mohammadi et al., 2012) and the modified 6-31G(d) basis set, i.e., m6-31G(d) (Mitin et al., 2003) is so far the most appropriate basis set for accurately modelling the complexes containing Fe.

In the present study, we provide a combined study of DFT calculations with FTIR experimental measurements in a range of solvents, in order to explore the impact of the environment on Fc conformation.

Section snippets

Experimental details and computational methods

Infrared (IR) spectra of Fc dissolved in non-polar solvents such as acetonitrile (ACN, ε = 35.69), dichloromethane (DCM, ε = 8.93), tetrahydrofuran (THF, ε = 7.43) and dioxane (DOX, ε = 2.21) were obtained using a Bruker Tensor 27 FTIR and a conventional solution cell fitted with KBr windows and a 100 μm spacer. A resolution of 1 cm-1 was used for all measurements. Concentrated solutions were prepared in each case and these spectra were compared with those obtained from the corresponding

Results and discussion

Fig. 1 shows the analysis of the Fc IR spectrum in the acetonitrile (ACN, ε = 35.69) solution in the region of 400–1200 cm−1. Five major spectral peaks (four bands) are measured in this region as given in Fig. 1(a). The four measured major peaks (five bands) of Fc are analysed (Voigt fits) at 480.28 ± 0.05 cm−1 (Int: 0.12 ± 0.001) and 496.25 ± 0.03 cm−1 (Int: 0.19 ± 0.001) in Fig. 1 (b1), 823.69 ± 0.03 cm−1 (Int: 0.20 ± 0.002) in Fig. 1 (b2), at 1006.60 ± 0.02 cm−1 (Int: 0.16 ± 0.001) in Fig. 1

Conclusions

The present study reveals the dominance of the D5h ferrocene conformer in solutions at room temperature. It combines Fourier transform infrared (FTIR) spectral measurements with theoretical simulation for the IR spectra of ferrocene in solvents, such as ACN, DCM, THF and DOX, respectively, in a region of 400–500 cm−1. The measurements consistently agree well with previously available Fc IR spectra in CCl4 solution of Lippincott and Nelson (1958) as well as the most recent IR spectral

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We acknowledge the National Computational Infrastructure (NCI) at the Australian National University for an award under the Merit Allocation Scheme, and the Swinburne University Supercomputing Facilities (OzSTAR). NM acknowledges Vice-Chancellors’ Postgraduate Research Award at Swinburne University of Technology.

References (42)

  • D. Osella et al.

    On the mechanism of the antitumor activity of ferrocenium derivatives

    Inorg. Chim. Acta.

    (2000)
  • L. Selvam et al.

    Solvent effects on blue shifted improper hydrogen bond of C–H⋯O in deoxycytidine isomers

    Chem. Phys. Lett.

    (2010)
  • Y. Yamaguchi et al.

    Electronic structure, spectroscopy, and photochemistry of group 8 metallocenes

    Coord. Chem. Rev.

    (2007)
  • P. Atkins et al.

    Shriver and Atkins' Inorganic Chemistry

    (2010)
  • J. Baker et al.

    Direct scaling of primitive valence force Constants: an alternative approach to scaled quantum mechanical force fields

    J. Phys. Chem.

    (1998)
  • V. Barone et al.

    Quantum calculation of molecular energies and energy gradients in solution by a conductor solvent model

    J. Phys. Chem.

    (1998)
  • S.P. Best et al.

    Reinterpretation of dynamic vibrational spectroscopy to determine the molecular structure and dynamics of ferrocene

    Chem. Eur J.

    (2016)
  • S. Coriani et al.

    The equilibrium structure of ferrocene

    ChemPhysChem

    (2006)
  • M. Cossi et al.

    Analytical second derivatives of the free energy in solution by polarizable continuum models

    J. Chem. Phys.

    (1998)
  • M. Cossi et al.

    Energies, structures, and electronic properties of molecules in solution with the C-PCM solvation model

    J. Comput. Chem.

    (2003)
  • F.A. Cotton et al.

    Advanced Inorganic Chemistry

    (1988)
  • Cited by (4)

    • Solvent contribution to ferrocene conformation: Theory and experiment

      2021, Radiation Physics and Chemistry
      Citation Excerpt :

      The IR spectroscopy and dynamics related to interconversion between the rotameric forms of ferrocene (di-cyclopentadienyl iron, FeCp2 or Fc) is an important example (Fischer and Pfab, 1952; Wang et al., 2021; Wilkinson et al., 1952). Fc is among the most recognisable molecules in science (Seeman and Cantrill, 2016), due to the unusual structure and bonding (Dunitz and Orgel, 1955; Dunitz, 1993; Coriani et al., 2006), the low-energy difference and subsequent interconversion dynamics (Fischer and Pfab, 1952; Wang et al., 2021; Wilkinson et al., 1952) between the high-symmetry conformers (Haaland and Nilsson, 1968; Kubo et al., 1981; Appel et al., 2015) and the extensive range of applications dependent on molecules having the Fc core structure (Pietschnig, 2016; Larik et al., 2017; Yu and Shi, 2017; Zhao et al., 2017; Hao et al., 2018). In particular, different independent methods have confirmed that the D5h conformation is lowest in energy (Bohn and Haaland, 1966; Seiler and Dunitz, 1979; Bourke et al., 2016).

    • Infrared and Raman spectroscopy: Principles and applications

      2023, Infrared and Raman Spectroscopy: Principles and Applications
    View full text