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be transmitted through this system and would not affect frustrated chemists for decades. Many claims of the coupling of ‘“C-2 to H-5”. The large amount of ferrous complexes that reversibly bind molecular oxyelectron density in the C-2--0-2 bond of 2 is extended but none have been fully subgen have been in this direction as seen on the difference map. The stantiated, some have later proved to be invalid,I”,11 decrease in the residual electron density of the C-2-N-1 and most have rested on the observation of a single and N-1-C-1’ bonds in 2 as compared to that in 1 can physical parameter. often a change in the visible specaccount for the anomalously low value for 2 of 3J12C.2,H.1f trum, as a criterion of oxygen uptake and reversibility. Low temperature seems to sufficiently retard irreversible (6.6 Hz for a torsional angle of - 175”). The residual electron density is decreased in the C-2-N-1 bond but is oxidation in some systems8a9,12 to permit the observaincreased in the N-1-C-6 bond of 2 from that of 1 and tion of some reversible oxygenation. However, in view the 3J18C.2,H.6for 2 of 7.2 Hz for a torsion angle of of the now well-recognized facile ligand redox re-174’ falls closer to the Karplus curve. actions of iron c o m p l e ~ e s , ’claims ~ of reversible oxyThe comparison of the residual electron density begenation which rely solely on the criterion of solution tween compounds 2 and l has been made with only one spectral change must be treated with utmost caution. We communicate here the synthesis and full characterof the independent molecules in the asymmetric unit of 1. The reason for this particular comparison is that ization of a crystalline dioxygenI4 complex derived from a specifically designed ferrous porphyrin comthe second independent molecule of 1 forms a hydrogen bond with N-3 of the uracil ring. The formation of plex. this hydrogen bond distorts the electron density in the Reversible oxygenation of myoglobin and hemoring in the manner shown in Figure 3. In this map the globin appears to result from a five-coordinate highonly significant change in the electron density distribuspin iron(I1) porphyrin immobilized within a hydrotion involves atom N-3 of the molecule forming a hyphobic pocket. l5 Apparently irreversible oxidation of iron results from either a bimolecular interaction indrogen bond. Electron density is withdrawn from the bonds N-3-C-2 and N-342-4 and resides in the revolving two Fe(I1) complexes with one O2 molecule16 maining sp2 orbital on N-3 and is directed toward the or protonation affording H 2 0 ? which subsequently reacts with the Fe(I1). Exploiting the concept of biproton of the hydrogen bond donor. This work thus also gives evidence of a shift in electron density, from phenyl-type atropisomerism” in ortho-substituted mesotetraphenylporphyrins, we have constructed a “picket the bonding orbitals of a nitrogen atom to a nonbonding fence” porphyrin whose steric bulk on one side creates orbital of this atom, as a result of the formation of a a nonprotic cavity for the coordination of small ligands hydrogen bond. These combined results from X-ray diffraction and while also protecting such ligands from bimolecular nmr studies indicate that anomalous bonding electron reactions. meso-Tetra(o-aminopheny1)porphyrin( H2TamPP)lsasb density can account for non-Karplus-type behavior rewas prepared by SnCl? reduction of meso-tetra(olating torsional angles to observed coupling constants nitropheny1)porphyrin and separated (silica gel chromaand therefore care should be exercised in the interpretatography) into its four a t r o p i s ~ m e r s . ~The ~ slowest tion of nmr results in cases where anomalous bonding is concerned. (1) A. H. Corwin and 2.Reyrs, J . Atner. Chem. Soc., 78,2437 (1956). The National Research Council crystallographic pro(2) J. H. Wang, ibid., 80, 3168 (1958). grams of Ahmed, et al., l 1 were used in the computations. (3) J. F. Drake and R. J. P. Williams, Nature (London), 182, 1084 (1958). Support for this research came from the National Re(4) W. R. McClellan and R. E. Benson, J. Amer. Chem. Soc., 88, 5165 search Council of Canada (Grant A-I72 to R. U. L.) (1966). and the Medical Research Council of Canada (Grant (5) J. 0. Alben, W. H. Fuchsman, C. A. Beaudreau, and W. S. Caughey, Biochemistry, 7, 624 (1968). MA-3406 to M. N. G. J.). We wish to thank Dr. (6) D. Vonderschmitt, I,of Alberta Ednroiitoii, Alberta, Catrudu Receiaed April 17, 1973
Reversible Oxygen Adduct Formation in Ferrous Complexes Derived from a “Picket Fence” Porphyrin. A Model for Oxymyoglobin Sii.:
The synthetic challenge of preparing oxymyoglobin and oxyhemoglobin models has both fascinated and Journul oj. the American C h e m i c a l S o c i e t y
Proc. Nut. Acad. Sci. U.S., 69, 2396 (1972). (8) J. E. Baldwin and J. Huff, J. Amer. Chem. Soc., 95, 5757 (1973). (9) L. I
