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.

Abstract

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 ≤ [small nu, Greek, macron] ≤ 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.