Nitrosyliron(III) porphyrinates: Porphyrin core conformation and FeNO geometry. Any correlation?

The synthesis, structural, and spectroscopic characterization of (nitrosyl)iron(ill) 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-Melm)(NO)]ClO4.C6H5-Cl (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-N-p = 1.990(9) Angstrom, Fe-N(NO) = 1.650(2) Angstrom, Fe-N(L) = 1.983(2) Angstrom, and Fe-N-O = 177.0(3)degrees. For the complex [Fe(OEP)(2-MeHlm)(NO)]ClO4.0.5CH(2)Cl(2) (H2OEP = octaethylporphyrin), there are two independent molecules in the asymmetric unit. The cation denoted [Fe(OEP)(2-MeHlm)(NO)](+)(pla) has a close-to-planar porphyrin core. For this cation, Fe-N-p = 2.014(8) Angstrom, Fe-N(NO) = 1.649(2) Angstrom, Fe-N(L) = 2.053(2) Angstrom, and Fe-N-O = 175.6(2)degrees. The second cation, [Fe(OEP)(2-MeHlm)(NO)](+)(ruf), has a ruffled core: Fe-N-p = 2.003(7) Angstrom, Fe-N(NO) = 1.648(2) Angstrom, Fe-N(L) = 2.032(2) Angstrom, and Fe-N-O = 177.4(2)degrees. 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-MeHlm)(NO)]ClO4 and 1871 cm(-1) for [Fe(OETPP)-(1-Melm)(NO)]ClO4 are well within the range seen for linear Fe-N-O groups. Mossbauer data for [Fe(OEP)(2-MeHlm)(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-Melm)(NO)]ClO4.C6H5Cl, monoclinic, space group P2(1)/c, Z = 4, with a = 12.9829(6) Angstrom, b = 36.305(2) Angstrom, c = 14.0126(6) Angstrom, beta = 108.087(1)degrees; [Fe(OEP)(2-MeHim)(NO)]ClO4.0.5CH(2)Cl(2), triclinic, space group P (1) over bar, Z = 4, with a = 14.062(2) Angstrom, b = 16.175(3) Angstrom, c = 19.948(3) Angstrom, alpha = 69.427(3)degrees, beta = 71.504(3)degrees, gamma = 89.054(3)degrees.