Spin-state dependence of the structural and vibrational properties of solvated iron(ii) polypyridyl complexes from AIMD simulations: aqueous [Fe(bpy)3]Cl2, a case study

The accurate description of transition metal complexes in liquid solutions is a challenging fundamental research problem, which must be tackled when it comes to understanding the role of the solvent in the photoinduced low-spin (LS) → high-spin (HS) transition in solvated Fe(II) complexes. We report an in-depth ab initio molecular dynamics (AIMD) study of the spin-state dependence of the structural and vibrational properties of the prototypical [Fe(bpy)3]2+ (bpy = 2,2′-bipyridine) LS complex in water. The description achieved for the LS and HS solution structures of aqueous [Fe(bpy)3]2+ significantly improves on and actually supersedes the one from our previous AIMD study [Lawson Daku and Hauser, J. Phys. Chem. Lett., 2010, 1, 1830], thanks to substantially longer simulation times and the use of the dispersion-corrected BLYP-D3 functional in place of the standard BLYP functional. The present results confirm the ≈0.19 Å lengthening of the Fe–N bonds and the increased thermal fluctuation of the molecular edifice stemming from the weakening of the Fe–N bonds upon the LS → HS change of states. Revisiting our previous finding on the solvation of [Fe(bpy)3]2+, they indicate that the number of water molecules in its first hydration shell actually increases from ∼15 in the LS state to ∼17 in the HS state. The vibration modes and associated vibrational density of states (VDOS) of [Fe(bpy)3]2+ have been determined from a generalized normal coordinate analysis. The VDOS of the Fe–N stretching and bending modes are located in the far-IR region. For LS [Fe(bpy)3]2+, the peak positions of the VDOS of the Fe–N stretching modes agree very well with the experimental Fe–N stretching frequencies. For HS [Fe(bpy)3]2+, the spanned frequency range encompasses the Fe–N stretching frequency range reported for HS polypyridine Fe(II) complexes. The LS and HS IR spectra of the complex have also been calculated in the 0 ≤ ≤ 2500 cm−1 range from the dynamics of the Wannier function centers. The calculated LS IR spectrum matches available experimental data. The predicted HS–LS IR difference spectrum of aqueous [Fe(bpy)3]2+ shows mostly an increase in intensity upon the LS → HS change of states.