Proximal effects

Few molecules have had their conformational properties characterized as exhaustively as have metalloporphyrins.

a. Porphyrin Conformation and M-Np Separations The cyclic aromatic 24atom porphyrinato skeleton offers a tightly constrained metal-binding site. The conformation of least strain is planar, and the radius of the hole of the dianion is close to 2.00 A,11O leading to metal-porphyrinato nitrogen-atom separations, M-Np , of 2.00 A if the metal is centered in the square plane defined by the four porphyrinato nitrogen atoms. Small deviations from planarity are generally observed and attributed to crystal packing effects; large deviations may be induced by bulky substituents on the porphyrin skeleton, especially at the meso positions, by the crystal matrix,6s or by the highly anisotropic protein matrix. The 2.00 A radius hole neatly accommodates low-spin (S = 0) and intermediatespin (S = 1) iron(II), low-spin (S = !) iron(III), and cobalt(II) and cobalt(III) ions.9]* With few exceptions the metal is centered in or above the central hole for mononuclear porphyrin species; only rarely do M-Np bonds show a significant (though still small) scatter about their mean value.
 

b. M ... Porph Displacement For five-coordinate complexes the magnitude of the displacement of the metal from the plane of the four nitrogen atoms, M ... porph, is a consequence of the electronic configuration of MLs complexes. Of course, the effect is augmented if the 3dx2- y 2 orbital (directed along M-Np bonds, Figure 4.16) is occupied. Compare a displacement of 0.14 A for Co(TPP)(1,2-Me2Im) (no 3dx2-v2 occupancy)]]] with 0.43 A for Fe(PF)(2-MeIm) (3dx2- v2 occupied). 169 For six-c~ordinate complexes where the two axial ligands, L1 and L2, are different, the M . . . porph displacement usually reflects relative trans influences.
 

Generally, displacement of the metal from the plane of the porphyrinatonitrogen atoms is within 0.04 A of the displacement from the 24-atom mean plane of the entire porphyrin skeleton. On occasions this second displacement may be much larger, for example in Fe(TPP)(2-MeIm), where it is 0.15 A largerllO than it is for Fe(PF)(2-MeIm). This effect is called doming, and it is usually attributed to crystal packing forces. Interaction of the porphyrin with protein side chains leads to considerable doming or folding of the heme in vertebrate hemoglobins.
 

c. M-L Separations The metal-axial ligand separations, M-L (when more than one, L] denotes the heterocyclic axial base), are dependent on the nature of the ligand, L. When L1and L2 are different, the M-L separations are sensitive to the relative trans influences of L1 and L2 as well as to steric factors. For example, for Fe(TPP)(l-Melmh, the Fe-N1m bond length is 2.016(5) A,IIO whereas for Fe(TPP)(l-Melm)(NO) it is 2.180(4) A.ll1 For sterically active ligands, such as 2-methylimidazole compared to I-methylimidazole (4.34), the longer Co-N1m bond occurs for the 2-Melm ligand because of steric clash
between the 2-methyl group and the porphyrin. III 

It is possible that combinations of intrinsic bonding and steric factors may give rise to a double minimum and two accessible axial ligand conformations (see Figure 4.26). This situation seems to occur in the solid state for Fe(PF)(2-

Melm)(02)' EtOH, where a short Fe-N1m and a long Fe-O bond are observed both from the structure revealed by single-crystal x-ray diffraction methods and by EXAFS data. On the other hand, for solvate-free Fe(PF)(2-MeIm)(02) and for Fe(PF)(l,2-Me2Im)(02)' the EXAFS patterns are interpreted in terms of a short Fe-O and long Fe-1m bond.l?l
 

d. The Angle <P This parameter is the minimum angle that the plane of the axial base (e.g., pyridine, substituted imidazole, etc.) makes with a plane defined by the Np , M, and L l atoms (Figure 4.25).65 If there are two axial ligands, e.g., l-methylimidazole and O2, then, as before, the angle the axial base makes is denoted <PI and the other angle <P2. For a linear CO ligand bound perpendicularly to the porphyrin plane, <P2 is undefined. Note that the orientation of the second ligand is influenced by distal effects.
 

When <P = 0, the axial base eclipses a pair of M-Np bonds; contacts with the porphyrin are maximized. When <p = 45°, contacts are minimized. Unless the axial base has a 2-substituent, however, the contacts are not excessively close for any value of <p. With a 2-methyl substituent, the contacts are sufficiently severe that the M-NL1 vector is no longer perpendicular to the porphyrin plane, and the imidazole group is rotated so that the M-NL1 vector no longer approximately bisects the imidazole C-N-C bond angle, as illustrated III
Figure 4.25.