Five- to Six-Coordination in (Nitrosyl)iron(II) Porphyrinates: Effects of Binding the Sixth Ligand

We report structural and spectroscopic data for a series of six-coordinate (nitrosyl)iron(II) porphyrinates. The structures
of three tetraphenylporphyrin complexes [Fe(TPP)(NO)(L)], where L = 4-(dimethylamino)pyridine, 1-methylimidazole,
4-methylpiperidine, are reported here to a high degree of precision and allow observation of several previously
unobserved structural features. The tight range of bonding parameters for the {FeNO} moiety for these three
complexes suggests a canonical representation for six-coordinate systems (Fe−Np = 2.007 Å, Fe−N(NO) = 1.753
Å, ∠FeNO = 138.5°). Comparison of these data with those obtained previously for five-coordinate systems allows
the precise determination of the structural effects of binding a sixth ligand. These include lengthening of the Fe−N(NO) bond and a decrease in the Fe−N−O angle. Several other aspects of the geometry of these systems are
also discussed, including the first examples of off-axis tilting of a nitrosyl ligand in a six-coordinate {FeNO}7 heme
system. We also report the first examples of Mössbauer studies for these complexes. Measurements have been
made in several applied magnetic fields as well as in zero field. The spectra differ from those of their five-coordinate
analogues. To obtain reasonable fits to applied magnetic field data, rotation of the electrical field gradient is required,
consistent with differing g-tensor orientations in the five- vs six-coordinate species.