Comprehensive study of a novel PFe-OAc: Insights from X-ray crystallography, spectroscopic characterizations, DFT-modeling, and docking assessments on bonding mechanism and electronic properties

This study describes the preparation of the acetato iron(II) porphyrinate complex formulated as K(2,2,2-crypt)Fe II (TpivPP)(OAc), ( PFe-OAc ), where OAc − denotes the acetate ligand, TpivPP corresponds to the α,α,α,α-tetrakis( o -pivalamidophenyl)porphyrinate anion, and 2,2,2-crypt refers to cryptand−222. The compound was fully characterized by UV–visible and infrared spectroscopy, as well as single-crystal X-ray diffraction analysis. The crystallographic study indicates that the complex crystallizes in the monoclinic P2₁/n space group and is composed of one anionic Fe II (TpivPP)(OAc) − unit associated with one K(2,2,2-crypt) + counterion. The porphyrin macrocycle adopts a markedly non-planar geometry, dominated by dome- and saddle-type distortions. Mean equatorial Fe Np bond distance (2.084(3) Å) and displacement of iron atom from 24-atom porphyrin core (Fe-PC = 0.582(6) Å) are both larger than those reported for related five-coordinate high-spin iron(II) porphyrin complexes, attributed to increased electronic repulsion between d x 2 -y 2 and d xy orbitals and negatively charged pyrrolic nitrogen donors. Single-crystal X-ray diffraction further reveals a high-spin Fe(II) center with a ground-state electronic configuration described as (d xy ) 2 (d xz ) 1 (d yz ) 1 (d z 2 ) 1 (d x 2 –y 2 ) 1 . Structural features confirm the high-spin nature of the acetato complex and demonstrate that the coordination environment of the metal center is strongly influenced by the monodentate axial acetate ligand coordinated from the pocket side of the TpivPP framework. DFT and structural and electronic properties of the complex showed that the optimized geometry of Fe(II) porphyrin core, as well as the coordination environment of the cryptand-222 unit, is in very good agreement with crystallographic data. Molecular electrostatic potential analysis highlights axial acetate ligand and amide functionalities as the most nucleophilic regions of the molecule. Frontier molecular orbital analysis indicates a relatively narrow HOMO-LUMO energy gap of approximately 2.23 eV, with main electronic transitions predominantly arising from metal-to-ligand charge-transfer processes. In the solid state, the crystal packing is stabilized by a dense network of H ··· H van der Waals interactions, complemented by directional O ··· H hydrogen-bonding contacts.