Leonidas Phillips

The aromaticity of ferrocene and some derivatives, ruthenocene and dibenzenechromium as determined via ring current assessment and 13C anisotropic contributions to the 1H NMR shielding

The concept of aromaticity is examined in terms of a delocalised ring current contribution to the proton NMR shielding of ferrocene and related compounds. The source of the ring current is derived as a delocalised contribution to the experimentally determined chemical shift, where the local contribution is calculated from predictions based on the experimental 13C shielding tensors. Examined are dibenzenechromium, ruthenocene, benzene, hexamethylbenzene and several derivatives of ferrocene with attention to: (i) the effects of complexation of the benzene ring, (ii) the protons in an aliphatic bridge on a ferrocene cyclopentadienyl ring, and (iii) the electric field effects of the permanent electric dipole moment in pentamethylferrocene.

Probing the magnetizability distribution of ferrocene as determined via anisotropic contributions to the NMR shielding and its application to several substituted ferrocenes

Molecular magnetic anisotropies of ferrocene derivatives are derived from solution-state measurements of the Cotton–Mouton and Kerr constants. A calculation is presented of chemical shifts induced at the hydrogen and carbon nuclei, arising from the experimental magnetic anisotropy. Ferrocene, ruthenocene and analogously substituted benzenes were also examined. Comparisons of the chemical shifts were used to probe the effects of metal–carbon bonding in the metallocenes. The experimental chemical shifts for pentamethylferrocene have been interpreted, allowing for magnetic anisotropy contributions, to give carbon atom charges that are in close agreement with predictions from the measured electric dipole moment.

Variable-temperature one- and two-dimensional 13C CP/MAS NMR studies of the dynamics of monohaptocyclopentadienyl rings of hafnium and titanium tetracyclopentadienyl in the solid state

Abstract Organometallic compounds with cyclopentadienyl ligands (C5H5) can undergo fluxional motion. We have studied the Group IVB transition metal complexes of the formula M(η5-C5H5)2(η1-C5H5)2, where M=Ti and Hf, in the solid state using variable-temperature 13C CP/MAS NMR spectroscopy. We present data which indicate that intramolecular rearrangement proceeds via a sigmatropic shift. Using two-dimensional (2D) exchange NMR, we can follow the rearrangement of the single bonded cyclopentadienyl ligands.

The electronic polarizability distribution of several substituted ferrocenes and Di(η6-benzene)chromium

The polarizability tensor was investigated as a function of group additivity for a series of substituted ferrocenes and di(η6-benzene)chromium. The findings yielded the sign of the polarizability anisotropy for several molecules. An analysis of the stretching force constants gave the metal atom charges for ferrocene, decamethylferrocene and di(η6-benzene)chromium.

Analysis of substituted ferrocenes by infrared spectroscopy

The i.r. spectra of substituted ferrocenes were investigated to determine and analyse the appropriate ring–metal stretching force constants. Assignment of the ring-mode vibrations is attempted.

Dipole rotation in some substituted ferrocenes

Abstract The dielectric relaxation times (τ′) of dilute cyclohexane and benzene solutions of several mono-, di- and penta-substituted ferrocenes are determined. From these we derive the corresponding relaxation times (τ) for intra- and overall molecular dipole rotation, as well as the associated thermodynamic parameters. Our analysis of the modes of dipole reorientation in 1,1′-dichloroferrocene (DiClFc) takes into account the simultaneous operation of both intra- and overall molecular rotation, in accordance with standard theory. This was found necessary in order to account for the observation that the dielectric relaxation time of chloroferrocene (ClFc) in cyclohexane (14.4 ps) is greater than that for the larger molecule DiClFc (9.0 ps). We show that intra-molecular dipole rotation in DiClFc is less hindered than in ClFc.

Electrochemistry of chlorinated ferrocenes: stability of chlorinated ferrocenium ions

The cyclic voltammetries of 1,2,3,4,5-pentachloro- and decachloro-ferrocene have been studied in acetonitrile. The complexes undergo an irreversible two-electron oxidation consistent with an electrochemical–chemical–electrochemical mechanism at scan rates up to 10 V s–1. However, at the faster scan rates (up to 160 V s–1) available to electrodes of small radius and microelectrodes, chemically reversible one-electron oxidations to the chlorinated ferrocenium ions, [C10H10–xClxFe]+ are obtained under ambient conditions. The reversible [C10H10–xClxFe]+/0 couples when x= 10,5,2 and 1 are observed at + 1.246, +0.774, +0.315 and +0.168 V vs. ferrocenium–ferrocene, respectively. A plot of Efvs.Σσp(σp= Hammett para coefficient for the chloro substituent) shows that the neutral molecules are stabilised with respect to the corresponding ferrocenium cations by 0.16–0.12 V per Cl. The rate constants of decomposition of the [Fe(η-C5Cl5)2] and [Fe(η-C5H5)(η-C5Cl5)]+ cations were calculated by both digital simulation and the method of Nicholson and Shain to be 40 ± 20 and 200 ± 50 s–1, respectively, at room temperature (ca. 20 °C). The complexes [Fe(η-C5H4Cl)2] and [Fe(η-C5H5)(η-C5H4Cl)] exhibit reversible oxidations at all scan rates down to 0.100 V s–1 under the same conditions. Both [Fe(η-C5Cl5)2] and [Fe(η-C5H5)(η-C5Cl5)] undergo a series of irreversible two-electron reductions at potentials negative of –1.8 V, which lead to reductive dechlorination consistent with an electrochemical–chemical–electrochemical–chemical reaction.

The charge distribution in substituted benzenechromiumtricarbonyls: An interpretation of spectroscopic data in terms of bond dipoles and charges

Abstract The charge distribution in a series of methyl-substituted benzenechromiumtricarbonyls was investigated as a function of bond charge displacement. The modelling is based on vibrational spectroscopic data, group and bond dipole moment additivity and molecular orbital calculations. An effective radius for the chromium atom in these complexes is determined. The EXAFS spectra are related to the proposed model.

Mean magnetic susceptibility and its solvent dependence for several substituted ferrocenes, ruthenocene and di(η6-benzene)chromium

The mean magnetic susceptibility of several ferrocene derivatives, ruthenocene and di(η6-benzene)-chromium as solutes in a number of solvents was determined via an NMR technique. The solvent dependence of this property was investigated and the variation in the magnetic susceptibility of the ferrocenyl group, due to substituent effects was analysed.

A solution-state dipole moment and microwave dielectric loss study of Ti, Zr, Hf and Sn cyclopentadienyl compounds

Abstract Dielectric properties of cyclopentadienyl complexes Cp2MX2 and Cp4M (M = Ti, Zr, Hf or Sn; X = Cl or Br; Cp = cyclopentadienyl) have been investigated in dilute benzene or 1,4-dioxane solution using the standard refractivity and microwave dielectric absorption techniques. The highly dipolar nature of the molecules Cp2MX2 has been confirmed for the solution-state and it has been shown that atomic distortion polarisalions for these compounds are small. The dielectric relaxation times for the Cp2MX2 molecules are consistent with a process of overall molecular tumbling. The compounds Cp4Zr and Cp4Hf are also highly dipolar indicating non-equivalent Cp rings about the metal. The predominant component of dielectric relaxation for these is by molecular tumbling although weakly absorbing intramolecular modes must also exist. The complex Cp4Sn has a low dipole moment and is weakly absorbing, this being consistent with a (η1-Cp)4Sn structure.