Correlated ligand dynamics in oxyiron picket fence porphyrins: structural and Mössbauer investigations

Disorder in the position of the dioxygen ligand is a well-known problem in dioxygen complexes and, in particular, those of picket fence porphyrin species. The dynamics of Fe-O2 rotation and tert-butyl motion in three different picket fence porphyrin derivatives has been studied by a combination of multitemperature X-ray structural studies and Mössbauer spectroscopy. Structural studies show that the motions of the dioxygen ligand also require motions of the protecting pickets of the ligand binding pocket. The two motions appear to be correlated, and the temperature-dependent change in the O2 occupancies cannot be governed by a simple Boltzmann distribution. The three [Fe(TpivPP)(RIm)(O2)] derivatives studied have RIm = 1-methyl-, 1-ethyl-, or 2-methylimidazole. In all three species there is a preferred orientation of the Fe-O2 moiety with respect to the trans imidazole ligand and the population of this orientation increases with decreasing temperature. In the 1-MeIm and 1-EtIm species the Fe-O2 unit is approximately perpendicular to the imidazole plane, whereas in the 2-MeHIm species the Fe-O2 unit is approximately parallel. This reflects the low energy required for rotation of the Fe-O2 unit and the small energy differences in populating the possible pocket quadrants. All dioxygen complexes have a crystallographically required 2-fold axis of symmetry that limits the accuracy of the determined Fe-O2 geometry. However, the 80 K structure of the 2-MeHIm derivative allowed for resolution of the two bonded oxygen atom positions and provided the best geometric description for the Fe-O2 unit. The values determined are Fe-O = 1.811(5) Å, Fe-O-O = 118.2(9)°, O-O = 1.281(12) Å, and an off-axis tilt of 6.2°. Demonstration of the off-axis tilt is a first. We present detailed temperature-dependent simulations of the Mössbauer spectra that model the changing value of the quadrupole splitting and line widths. Residuals to fits are poorer at higher temperature. We believe that this is consistent with the idea that population of the two conformers is related to the concomitant motions of both Fe-O2 rotations and motions of the protecting tert-butyl pickets.