Syntheses, Characterization, and Structural Studies of Several (Nitro)(nitrosyl)iron(III) Porphyrinates: [Fe(Porph)(NO₂)(NO)]

The preparation and characterization of several (nitro)(nitrosyl)iron(III) complexes, [Fe(Porph)(NO2)(NO)], where
Porph = tetraphenylporphyrin, octaethylporphyrin, tetra-p-methoxyphenylporphyrin, or picket fence porphyrin,
are described. Crystal data:  [Fe(Tp-OCH3PP)(NO2)(NO)]·CH2Cl2, tetragonal, space group I41/a, Z = 4, with a
= 15.620(3) Å, c = 18.649(2) Å; [Fe(TpivPP)(NO2)(NO)]·C6H5Cl (1), tetragonal, P4/ncc, Z = 4, with a =
18.094(3) Å, c = 18.939(4) Å, and (2) with a = 18.117(2) Å, c = 19.0838(8) Å; [Fe(TpivPP)(NO2)(NO)] (3),
monoclinic, C2/c, Z = 2, with a = 16.1559(5) Å, b = 18.6920(8) Å, c = 19.7779(10) Å, and β = 90.971(6)°;
[Fe(TpivPP)(NO2)(NO)]·C7H8 (4), monoclinic, P21/n, Z = 4, with a = 12.7764(10) Å, b = 27.192(2) Å, c =
19.1716(8) Å, and β = 105.063(5)°. Of the several crystal structures described, two (out of four characterized)
picket fence porphyrin derivatives exhibited no crystallographic disorder that led to problems defining the axial
ligand geometry. In these two derivatives the Fe−axial ligand distances are Fe−N(NO) = 1.668(2) or 1.671(2)
Å and Fe−N(NO2) = 2.002(2) or 1.998(2) Å; NO does not cause a significant structural trans effect. The average
Fe−Np distance of 1.998(4) Å is consistent with a low-spin state for iron. The linear or near-linear (169.3(2)°)
Fe−N−O angle observed is that expected for {FeNO}6 (i.e., iron(III) nitrosyl) species. The solid-state NO stretching
frequencies (1871−1893 cm-1) are also consistent with a linear Fe−N−O group. Mössbauer spectra show that
the complexes have quadrupole splittings ΔEq = 1.35−1.50 mm/s and isomer shifts δ = 0.0−0.11 mm/s; the
isomer shifts display a strong temperature dependence with lower values at high temperature. Mössbauer spectra
in applied magnetic field at 4.2 K show that the molecules have diamagnetic ground states which is presumed to
result from antiferromagnetic coupling between the low-spin iron(III) and the S = 1/2 nitric oxide center.