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.