Calculations of 57Fe Mössbauer parameters of mononuclear iron(II) N4 Schiff-base complexes by HF and DFT quantum-chemical approaches

A series of iron(II)-containing octahedral complexes of the general composition [Fe(L)(NCS)2] (1–9) (Published in N. Bréfuel, I. Vang, S. Shova, F. Dahan, J.P. Costes, J.P. Tuchagues, Polyhedron 26 (2007) 1745; and N. Bréfuel, S. Shova, J.P. Tuchagues, Eur. J. Inorg. Chem. (2007) 4326) involving tetradentate N4 Schiff-base type ligands (L), where L represents N,N′-bis-(1H-imidazol-2-ylmethylene)-propane-1,3-diamine a, N,N′-bis[(1H-imidazol-4-yl)methylene]-propane-1,3-diamine b, N-[(1-H-imidazol-2-yl)methylene]-N′-(1-pyridin-2-yl-ethylidene)-propane-1,3-diamine c, N-[(1-H-imidazol-4-yl)methylene]-N′-(1-pyridin-2-yl-ethylidene)-propane-1,3-diamine d, and N,N′-bis(1-pyridin-2-ylethylidene)-propane-1,3-diamine e, has been investigated by quantum-chemical calculations based on the Hartree–Fock (HF) and density functional (DFT) theories. Based on the optimized geometries of the complexes 1–9 as well as their geometries following from single crystal X-ray determinations, the Mössbauer parameters {isomer shift (δ) and quadrupole splitting ΔEQ} have been calculated and evaluated. The theoretically obtained results have been compared with those experimentally determined. The evaluation showed that geometry optimization at the HF level provided, as expected, geometries with generally elongated bonds and deformed bond angles, while the B3LYP optimization led to molecular geometries generally comparable with those observed experimentally. Higher values of the absolute mean errors in the HF calculated bond lengths (0.1264 Å contrary to 0.0674 Å determined by B3LYP) and bond angles (4.14° contrary to 2.18° of B3LYP) led to higher inaccuracy in calculations of both Mössbauer parameters. The average deviations of isomer shift are equal to 0.286 mm s−1 in the case of HF geometries, and 0.062 mm s−1 in the case of those obtained by the B3LYP approach. Similarly, the average deviations of quadrupole splitting equal 0.430 and 0.300 mm s−1 for the HF, and B3LYP optimized geometries, respectively.