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.