Spin-state dependence of the structural and vibrational properties of solvated iron(ii) polypyridyl complexes from AIMD simulations: II. aqueous [Fe(tpy)2]Cl2

We report a detailed ab initio molecular dynamics (AIMD) study of the structural and vibrational properties of aqueous [Fe(tpy)2]2+ (tpy = 2,2′:6′,2′′-ter-pyridine) in the low-spin (LS) and high-spin (HS) states, which extends our previous work on aqueous [Fe(bpy)3]2+ (bpy = 2,2′-bipyridine) [L. M. Lawson Daku and A. Hauser, J. Phys. Chem. Lett., 2010, 1, 1830–1835; L. M. Lawson Daku, Phys. Chem. Chem. Phys., 2018, 20, 6236–6253]. Upon the LS → HS change of states, the axial and distal Fe–N bonds of aqueous [Fe(tpy)2]2+ are predicted to lengthen by 0.226 Å and 0.206 Å, respectively, in excellent agreement with experiments [X. Zhang et al., J. Phys. Chem. C, 2015, 119, 3312–3321; G. Vankó et al., J. Phys. Chem. C, 2015, 119, 5888–5902]. The lengthening of the Fe–N bonds results from their weakening, which also gives rise to an increased thermal fluctuation of the molecular structure. As with [Fe(bpy)3]2+, the first hydration shell of [Fe(tpy)2]2+ consists in a chain of hydrogen-bonded water molecules wrapped around the ligands. The predicted hydration number of [Fe(tpy)2]2+ in both spin states is ≈15 while it was found for [Fe(bpy)3]2+ to increase from ≈15 to ≈17 on passing from the LS to the HS state. Due to the ≈0.2 Å lengthening of the Fe–N bonds in both complexes upon the LS → HS transition, the water molecules can get closer to the Fe atom: by ≈0.4 Å for [Fe(tpy)2]2+ and by ≈0.2 Å for [Fe(bpy)3]2+. This difference is ascribed to the fact that the bpy ligands with their trigonal coordination motif provides a bulkier environment to the Fe atom than the tpy ligands with their tetragonal coordination motif. In other words, upon the tpy → bpy substitution, the bulkier environment provided by the bpy ligands repels the hydration shell, the displacement being greater in the HS state than in the LS state. As a consequence of their chemical similarity, the calculated LS and HS IR spectra of aqueous [Fe(tpy)2]2+ closely resemble those of aqueous [Fe(bpy)3]2+, the LS → HS transition translating into a global increase in intensity. The present work taken with the one on aqueous [Fe(bpy)3]2+ enhances our understanding of the spin-state dependence of the structural and vibrational properties of Fe(II) polypyridyl complexes in water, highlighting the strong interpenetration between their coordination sphere and their hydration shell.