Comparison of the Molecular and Electronic Structures of (2,3,7,8, 12,13,17,18-Octaethylporphyrinato)iron(II) and (trans-7,8-Dihydro-2,3,7,8,12,13,17,18-octaethyl-porphyrinato)iron(II)

The molecular and electronic structures of (2,3,7,8,12,13,17,18-octaethylporphyrinato)iron(II), Fe(OEP), and (trans-7,8-dihydro-2,3,7,8,12,13,17,18-octaethylporphyrinato)iron(II), Fe(OEC), are compared and show important differences and similarities. The compound Fe(OEP) crystallizes in space group C_i¹-P1* (Z = 1) with unit cell dimensions a = 4.812 (1) Å, b = 13.252 (1) Å, c = 13.369 (1) Å, α = 113.13 (1)°, β = 92.20 (1)°, γ = 93.20 (1)°, and V= 781.2 ų. The structure has been refined to an R index on F_o²of 0.126 on the basis of 2022 reflections (293 K) and 187 variables. The molecule has a crystallographically imposed inversion center. The compound Fe(OEC) crystallizes in space group D¹⁴_2h-Pbcn (Z = 4) with unit cell dimensions a = 21.880 (9) Å, b = 15.795 (6) Å, c = 8.554 (4) Å and V = 2956.2 ų. The structure has been refined to an R index on F_o^{2 of} 0.109 on the basis of 3873 reflections (123 K) and 187 variables. The molecule has a crystallographically imposed 2-fold axis passing through the unique pyrroline nitrogen atom, the iron atom, and a pyrrole nitrogen atom. The iron and four nitrogen atoms are rigorously planar in both Fe(OEP) and Fe(OEC), with iron-nitrogen bond lengths of 1.984 (5) and 2.007 (5) Å for Fe(OEP) and 1.969 (3), 1.987 (4), and 2.002 (4) Å for Fe(OEC). The last value given for Fe(OEC) is the distance from the iron atom to the pyrroline nitrogen atom. Despite the similarity in these bond distances, the porphyrin macrocycle of Fe(OEP) is essentially planar while the chlorin macrocycle of Fe(OEC) is significantly S₄ruffled. Spectroscopic and magnetic susceptibility data demonstrate that the compounds do not have congruent electronic structures despite the fact that they both possess an intermediate-spin (S = 1) ground state. Solid-state effective magnetic moments (296 K) are 4.6 (1) μ_B for Fe(OEP) and 3.5 (1)μB for Fe(OEC). Mössbauer spectra at 4.2 K show simple quadrupole doublets for both compounds. Isomer shifts are equal to within experimental error, 0.62 (1) mm/s for Fe(OEP) and 0.63 (1) mm/s for Fe(OEC), but the quadrupole splittings are significantly different, 1.71 (1) mm/s for Fe(OEP) and 2.55 (1) mm/s for Fe(OEC). An analysis of variable-temperature ¹H NMR spectra shows that Fe(OEC) possesses rhombic magnetic anisotropy. This is the first example of resolvable ring-induced rhombicity in a metallohydroporphyrin.