Properties of a binaphthyl-bridged porphyrin-iron complex bearing hydroxy groups inside its cavity

Hydrogen-bond formation with the terminal oxygen atom is considered to be the basis of dioxygen molecule activation by cytochrome P450. In order to verify the effect of this hydrogen bond, we have undertaken the synthesis of a model complex: a binaphthyl-bridged porphyrin bearing hydroxy groups at suitable positions ("single coronet" or "SC"). The reactivity of the iron complex of the synthesized compound towards basic ligands, dioxygen and carbon monoxide has been studied. When a bulky axial ligand such as 1-methyladamantyl-2-methylimidazole is used, only the pentacoordinated complex is obtained, and, as expected, dioxygen binds as the sixth ligand only in the cavity of the compound. Under unusually low dioxygen partial pressures and in rigorously anhydrous toluene, the pentacoordinated iron complex is completely transformed into a new species which absorbs in the visible region at 420 and 559 nm, and which we have identified as the oxygenated complex. Surprisingly, this reaction seems to occur irreversibly, based on the fact that the initial complex is not recovered after bubbling nitrogen through the solution for several hours. On the other hand, saturation of the solution with carbon monoxide transforms the complex slowly but completely into the Fe(II)-CO complex which is stable in a dioxygen-saturated toluene solution at 0 degrees C. However, by raising the temperature, it is spontaneously transformed back into the dioxygen complex; this verifies the reversibility of the dioxygen binding process. Compared with its affinity towards carbon monoxide, the SC iron complex has a much stronger affinity towards dioxygen. This remarkable property may be partly explained by hydrogen bonding between the terminal atom of the dioxygen molecule and the hydroxy groups attached to the binaphthyl bridges of the porphyrin, and also by polar neighbouring-group effects. Ligand binding and debinding constants have been determined by laser flash photolysis. Ligand-iron bond strength and hydrogen bonding have been investigated by IR and Raman spectroscopy. The role of the hydroxy groups has also been emphasized by comparing the properties of SC with those of a binaphthyl-bridged porphyrin bearing methoxy groups instead of hydroxy groups.