Structural Insights into Ligand Dynamics: Correlated Oxygen and Picket Motion in Oxycobalt Picket Fence Porphyrins

Two different oxygen-ligated cobalt porphyrins have been
synthesized
and the solid-state structures have been determined at several temperatures.
The solid-state structures provide insight into the dynamics of Co–O2 rotation and correlation with protecting group disorder.
[Co­(TpivPP)­(1-EtIm)­(O2)] (TpivPP = picket fence porphyrin)
is prepared by oxygenation of [Co­(TpivPP)­(1-EtIm)2] in
benzene solution. The structure at room temperature has the oxygen
ligand within the ligand binding pocket and disordered over four sites
and the trans imidazole is disordered over two sites. The structure
at 100 K, after the crystal has been carefully annealed to yield a
reversible phase change, is almost completely ordered. The phase change
is reversed upon warming the crystal to 200 K, whereupon the oxygen
ligand is again disordered but with quite unequal populations. Further
warming to 300 K leads to greater disorder of the oxygen ligands with
nearly equal O2 occupancies at all four positions. The
disorder of the tert-butyl groups of the protecting
pickets is correlated with rotation of the O2 around the
Co–O­(O2) bond. [Co­(TpivPP)­(2-MeHIm)­(O2)] is synthesized by a solid-state oxygenation reaction from the
five-coordinate precursor [Co­(TpivPP)­(2-MeHIm)]. Exposure to 1 atm
of O2 leads to incomplete oxygenation, however, exposure
at 5 atm yields complete oxygenation. Complete oxygenation leads to
picket disorder whereas partial (40%) oxygenation does not. Crystallinity
is retained on complete degassing of oxygen in the solid, and complete
ordering of the pickets is restored. The results should provide basic
information needed to better model M–O2 dynamics
in protein environments.