Application of the Hirshfeld Atom Refinement to Determine the Spin State of the Novel Fe(III) Schiff Base Complex

The novel complex [FeIII(HL)(L)]·2dmf (1) was successfully synthesized by reacting iron chloride with dimethylformamide solution of the Schiff base ligand (H2L). The ligand was prepared in situ through the condensation of 5-bromo-salicylaldehyde and benzhydrazide. The compound 1 was investigated by single crystal X-ray diffraction, IR spectroscopy, CASSCF/NEVPT2 and DFT theoretical calculations, revealing the high-spin (S = 5/2) state of Fe(III). The Hirshfeld Atom Refinement (HAR) of the crystal structure confirmed the sextet ground spin state of 1 through the comparison of R-factors and Hirshfeld deformation maps of the structures refined assuming high, indeterminate (S = 3/2) and low (S = 1/2) ground spin states. A series of basis sets and DFT functionals was screened, indicating that in most cases the combinations allow the correct determination of the ground state of 1. The def2-SVP double-zeta basis set combined with the GGA (Generalized Gradient Approximation) functional such as PBE or BP86 was found to be a sufficiently precise low-cost computational level. While further investigation is necessary, the results of the work tentatively suggest that the HAR could be successfully used for determination of the ground spin state of Fe(III) complexes even in the case of regular quality X-ray structures.