J. L. Hoard

SOME ASPECTS OF METALLOPORPHYRIN STEREOCHEMISTRY

The authors critical appraisal of metalloporphyrin stereochemistry, published within the period of this writing, precludes any immediate need for another comprehensive review of the subject. In the published review, however, attention is focused on the iron porphyrins and their roles as hemes in the hemoproteins; several aspects of metalloporphyrin stereochemistry that are accorded only passing mention, yet are not without interest to porphyrin chemists, are appropriate topics for elaboration in this supplementary paper. It is readily demonstrated, for example, that a moderately nonplanar conformation of the porphinato core in an externally unconstrained molecule of a metalloporphyrin can be stabilized through interaction with the axial ligand(s). An especially interesting question is whether purely internal strain within either the most strongly contracted core of a nickel(I1) porphyrin or the most highly expanded core of a dichlorotin(IV) porphyrin may be so relieved in a nonplanar conformation as to outweigh the accompanying inimical effect on the delocalized 71 bonding. No unqualified answer to either part of this question can be given, but detailed stereochemical analysis of the most favorable nonplanar conformations does sharply distinguish between the relative probabilities of spontaneous deformation from planarity by the contracted and expanded cores. The quantitative data of metalloporphyrin stereochemistry come from X-ray determination of structure for single crystals of (at last count) 18 metalloporhyrins; in at least half of these analyses, however, the precision attained is unfortunately, if understandably, low. But in the most precisely determined structure, that of the tetragonal crystal of a,/3,y,6-tetraphenylporphinatodichlorotin(IV) * (CI, SnTPP), each structurally independent C-C bond length carries an estimated standard deviation (esd) of only 0.003 A. Although the symmetry required of the CI, SnTPP molecule in the crystal is just C4,, , the dimensional variations from Dqh geometry are objectively altogether trivial. The stereochemical parameters of the porphinato core in this molecule, averaged in agreement with D4,, symmetry, are entered on the diagram in FIGURE 1 ; the averaged bond distances also are included in the more general listings of TABLE 1. Carbon atoms of the three chemical types are designated C,, Cb, C, in both FIGURE 1 and TABLE 1. Still less symmetry is required of the metalloporphyrin molecules in the other crystalline arrangements that are determined with rather high precision. For these structures, nonetheless, it is generally observed that the mean deviation from the averaged length of each chemical class of bond in the porphinato core is as small as, or smaller than, the esd of any independently determined bond length of the class. In these circumstances, the set of averaged bond lengths of the several chemical types represents a simple idealization that retains the physically objective content