Experimental Spin State Determination of Iron(II) Complexes by Hirshfeld Atom Refinement

Abstract

In this study, we present the first experimental determination of the spin state of transition metal complexes by using Hirshfeld Atom Refinement. For the demonstration, the two iron(II) complexes, (NH₄)₂Fe(SO₄)₂ ⋅ 6 H₂O and lFe(pic)₃jCl₂ ⋅ EtOH were investigated. The method involves the refinement using wavefunctions of different spin multiplicity and comparison against experimental diffraction data by means of refinement indicators and residual electron density. Our results show a clear distinction between high-spin and low-spin configurations, even for compounds with spin-crossover behavior. The presented work demonstrates the potential of Hirshfeld Atom Refinement for the experimental determination of spin states in transition metal complexes.

Keywords: Hirshfeld Atom Refinement; Iron; Solid-state structures; Spin state determination; X-ray diffraction.

Figure 1

Structural arrangement of the formula units, ellipsoids are drawn at 50 % probability. Classical hydrogen bonds are shown as dashed lines; (a) visualization of 1 (${\left({\mathrm{NH}}_4\right)}_2\mathrm{Fe}{\left({\mathrm{SO}}_4\right)}_2·{6\mathrm{H}}_2\mathrm{O}$ ) within the crystal structure, the iron(II) center is coordinated by six water molecules in an octahedral fashion and located in a special position (Wykoff letter d); (b) visualization of 2 ($\left[\mathrm{Fe}{\left(\mathrm{pic}\right)}_3\right]{\mathrm{Cl}}_2·\mathrm{EtOH}$ ) within the crystal structure. The iron(II) center is coordinated by three pic ligands in an octahedral fashion, the disorder present in one pic moiety and the ethanol is depicted as transparent drawing of the minor component.

Figure 2

Difference Fourier maps (F_obs ‐ F_calc) of the HS (a) and LS (c) conigured structure models and respective Hirshfeld deformation maps (F_IAM ‐ F_HAR) of HS (b) and LS (d) models. View towards Fe‐O2‐O3 plane. The scale is given in eA^‐3, where (a) and (c) use the left, (b) and (d) uses the right scale.

Figure 3

Left: difference of the HS and LS molecular density (ρ _HS‐ρ _LS) resulting from an ab initio single point calculation (using ORCA[ ⁵⁶ , ⁵⁷ , ⁵⁸ , ⁵⁹ , ⁶⁰ ]) applying the same starting geometry as the crystal structure, isosurface drawn at ±0.04 atomic units. Right: experimental residual density (F_obs ‐ F_calc) of the LS model, isosurface at ±0.4 eÅ^–3. Green represents positive and red negative electron density difference.

Figure 4

Difference Fourier isosurfaces (F_obs ‐ F_calc) of the different structure models (IAM, HS, LS from top to bottom) of 2. The isosurfaces are drawn at ±0.3 eÅ^–3. Only the iron(II) cation and the directly coordinating nitrogen atoms are pictured as ellipsoids (50 % probability), the disorder and EtOH is omitted for clarity and a numbering is exemplary depicted at the IAM structure. Green represents positive and red negative residual density.

Figure 5

Residual density iso‐surface of the iron coordination sphere of the LS model at different resolution cutoffs given as d‐spacing. a: d_min=0.50 Å, b: d_min=0.60 Å, c: d_min=0.70 Å, d: d_min=0.80 Å. The iso‐surface is drawn at ±0.3 eÅ^–3. e: d_min=0.50 Å, f: d_min=0.80 Å. The iso‐surface is drawn at ±0.15 eÅ^–3. Ellipsoids are drawn at 50 % probability.