Nitrosyliron(III) Porphyrinates: Porphyrin Core Conformation and FeNO Geometry. Any Correlation?

The synthesis, structural, and spectroscopic characterization of (nitrosyl)iron(III) porphyrinate
complexes designed to have strongly nonplanar porphyrin core conformations is reported. The species
have a nitrogen-donor axial ligand trans to the nitrosyl ligand and display planar as well as highly nonplanar
porphyrin core conformations. The systems were designed to test the idea, expressly discussed for the
heme protein nitrophorin (Roberts, et al. Biochemistry 2001, 40, 11327), that porphyrin core distortions
could lead to an unexpected, bent geometry for the FeNO group. For [Fe(OETPP)(1-MeIm)(NO)]ClO4·C6H5Cl (H2OETPP = octaethyltetraphenylporphyrin), the porphyrin core is found to be severely saddled. However,
this distortion has little or no effect on the geometric parameters of the coordination group:  Fe−Np =
1.990(9) Å, Fe−N(NO) = 1.650(2) Å, Fe−N(L) = 1.983(2) Å, and Fe−N−O = 177.0(3)°. For the complex
[Fe(OEP)(2-MeHIm)(NO)]ClO4·0.5CH2Cl2 (H2OEP = octaethylporphyrin), there are two independent
molecules in the asymmetric unit. The cation denoted [Fe(OEP)(2-MeHIm)(NO)]+(pla) has a close-to-planar
porphyrin core. For this cation, Fe−Np = 2.014(8) Å, Fe−N(NO) = 1.649(2) Å, Fe−N(L) = 2.053(2) Å, and
Fe−N−O = 175.6(2)°. The second cation, [Fe(OEP)(2-MeHIm)(NO)]+(ruf), has a ruffled core:  Fe−Np =
2.003(7) Å, Fe−N(NO) = 1.648(2) Å, Fe−N(L) = 2.032(2) Å, and Fe−N−O = 177.4(2)°. Thus, there is no
effect on the coordination group geometry caused by either type of nonplanar core deformation; it is unlikely
that a protein engendered core deformation would cause FeNO bending either. The solid-state nitrosyl
stretching frequencies of 1917 cm-1 for [Fe(OEP)(2-MeHIm)(NO)]ClO4 and 1871 cm-1 for [Fe(OETPP)(1-MeIm)(NO)]ClO4 are well within the range seen for linear Fe−N−O groups. Mössbauer data for [Fe(OEP)(2-MeHIm)(NO)]ClO4 confirm that the ground state is diamagnetic. In addition, the quadrupole splitting
value of 1.88 mm/s and isomer shift (0.05 mm/s) at 4.2 K are similar to other (nitrosyl)iron(III) porphyrin
complexes with linear Fe−N−O groups. Crystal data:  [Fe(OETPP)(1-MeIm)(NO)]ClO4·C6H5Cl, monoclinic,
space group P21/c, Z = 4, with a = 12.9829(6) Å, b = 36.305(2) Å, c = 14.0126(6) Å, β = 108.087(1)°;
[Fe(OEP)(2-MeHIm)(NO)]ClO4·0.5CH2Cl2, triclinic, space group P1̄, Z = 4, with a = 14.062(2) Å, b =
16.175(3) Å, c = 19.948(3) Å, α = 69.427(3)°, β = 71.504(3)°, γ = 89.054(3)°.