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 t he basis of dioxygen molecule activation by cytochrome P450. In order to ve rify the effect of this hydrogen bond, we have undertaken the synthesis of a model complex: a binaphthyl-bridged porphyrin bearing hydroxy groups at s uitable positions ("single coronet" or "SC"). The reactivity of the iron co mplex of the synthesized compound towards basic ligands, dioxygen and carbo n monoxide has been studied. When a bulky axial ligand such as 1-methyladam antyl-2-methylimidatele is used, only the pentacoordinated complex is obtai ned, and, as expected, dioxygen binds as the sixth ligand only in the cavit y of the compound. Under unusually low dioxygen partial pressures and in ri gorously 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. Surpris ingly, this reaction seems to occur irreversibly, based on the fact that th e initial complex is not recovered after bubbling nitrogen through the solu tion for several hours. On the ether 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 d egreesC. However, by raising the temperature, it is spontaneously transform ed back into the dioxygen complex; this verifies the reversibility of the d ioxygen binding process. Compared with its affinity towards carbon monoxide , the SC iron complex has a much stronger affinity towards dioxygen. This r emarkable property may be partly explained by hydrogen bonding between the terminal atom of the dioxygen molecule and the hydroxy groups attached to t he binaphthyl bridges of the porphyrin, and also by polar neighbouring-grou p effects. Ligand binding and debinding constants have been determined by l aser flash photolysis. Ligand-iron bond strength and hydrogen bonding have been investigated by IR and Raman spectroscopy. The role of the hydroxy gro ups has also been emphasized by comparing the properties of SC with those o f a binaphthyl-bridged porphyrin bearing methoxy groups instead of hydroxy groups.