Structure and Spectroscopic Properties of Five-Coordinate (2-Methylimidazolato)-and Six-Coordinate (Imidazole)(imidazolato)iron(II) “Picket-Fence” Porphyrins

The synthesis and characterization of the sodium 222 cryptated salts of the five-coordinate high-spin (2-methylimidazolato)iron(II) derivative 1 and the six-coordinate (imidazole)(imidazolato)iron(II) complex 2 of “picket-fence” porphyrin are reported. 1 crystallizes as the 2-methylimidazole bis(chlorobenzene) solvate, C₁₀₂H₁₂₁N₁₄O₁₀NaCl₂Fe, in the monoclinic space group P2₁ with two formula units in the unit cell. At −100 °C, the unit cell parameters are a = 13.430 (4) Å, b = 26.163 (6) Å, c = 14.398 (4) Å, and β = 103.12 (2)°. The structure was solved with 3393 observed reflections, and refinements led to conventional R and weighted R_w factors of 0.068 and 0.086, respectively. The mean Fe–N_p bond length is 2.106 (20) Å. The displacements of the iron atom relative to the 4N_p and 24-atom-core mean planes are respectively 0.524 (10) and 0.647 (10) Å. The 2-methylimidazolate axial ligand is located inside the molecular cavity of the picket-fence porphyrin. The Fe–N(2-MeIm) (2-MeIm = 2-methylimidazolate) bond distance of 2.002 (15) Å is not very accurate but, nevertheless, significantly shorter than the Fe–N(2-MeHIm) (2-MeHIm = 2-methylimidazole) bond length of 2.095(6) Å in [FeTP_pivP(2-MeHIm)] (TP_pivP = picket-fence porphyrin). 2 crystallizes as the THF solvate (THF = tetrahydrofuran), C₉₂H₁₁₅N₁₄O₁₁NaFe, in the monoclinic space group P2₁/n with four formula units in the unit cell. At −100 °C, the unit cell parameters are a = 30.974 (9) Å, b = 22.753 (5) Å, c = 12.596 (4) Å, and β= 111.71 (2)°. The structure of 2 was solved by using 3757 observed reflections and refined to R and R_w values, respectively, of 0.052 and 0.069. The average Fe–N_p bond distance of 1.995 (9) Å is consistent with those of other six-coordinate low-spin ferrous porphyrins. The Fe–N(ligand) bond lengths are 1.930 (5) and 1.953 (5) Å. The neutral HIm ligand (HIm = imidazole) cannot be distinguished from the anionic deprotonated Im group (Im = imidazolate) by comparison of the Fe–N bond distances, especially not when one takes into account the orientation of the ligand mean planes relative to the porphyrin ring. However, consideration of the structures of several iron picket-fence porphyrin derivatives with anionic ligands indicates that the group located inside the molecular cavity is, most probably, the deprotonated imidazole. For compound 1, UV–visible and Mössbauer spectral results are consistent with the presence of a high-spin five-coordinate ferrous porphyrin in chlorobenzene solution and in the solid state. For compound 2, the spectral properties are consistent with the presence of a low-spin six-coordinate ferrous derivative in THF solution and in the solid state. The high-spin five-coordinate species 1 with a nitrogen donor axial ligand shows also an unusually large quadrupole splitting of ΔE_q = 3.51 mm/s at 77 K.