DFT study of substitution effect on the geometry, IR spectra, spin state and energetic stability of the ferrocenes and their pentaphospholyl analogues

Optimal structures, vibrational spectra and thermochemical characteristics of ferrocenes and pentaphosphaferrocenes containing up to five methyl substituents in the cyclopentadienyl ring are found with the use of DFT computations. It is shown that the non-substituted ferrocenes and pentaphosphaferrocenes as well as the species with small number of methyl substituents adopt eclipsed conformations. Increasing the number of the methyl substituents results either in the conformational equilibria between the eclipsed and staggered forms or in complete shift to the staggered conformations. These effects, as well as elongation of CC bonds of the Cp ring and growth of Fe–Cp and Cp–P5 distances with growing number of methyl groups can be explained by repulsive interactions of the methyl groups with each other or with phosphorus atoms of the pentaphospholyl ligands. The pentaphosphaferrocenes are predicted to be stable towards disproportionation to homoleptic sandwich complexes. This stability grows as the number of methyl substituents in cyclopentadienyl ring increases, which is a result of stronger Fe–P5 bonding in the methyl substituted pentaphosphaferrocenes relative to the non-substituted species, and destabilization of the homoleptic ferrocenes because of Me⋯Me repulsion. A huge energy gap between the low- and high-spin states of the ferrocenes and pentaphosphoferrocenes is predicted, though introduction of methyl substituents causes a moderate decrease of the gap. Transition from low- to high-spin state should result in pronounced changes of geometry and IR spectra of the ferrocenes, which suggests that IR spectroscopy can be used for diagnostics of spin state of iron complexes.