Glenn A. Taylor and Minoru Tsutsui Texas A&M University College Station. 77843
I I
Out-of-Plane Metalloporphyrin Complexes
There is a growing interest in metalloporphyrins among chemists because of the unique nature of the cwrdination chemistry, for both the porphyrin ligand as well as the metal ion, of these materials. Recent progress in the chemistry of synthetic metalloporphyrins has shown that the porphyrin moiety can act as a hi, tri, or hexadentate ligand, as well as the usual tetradentate ligand. In addition, the metal ion has been observed to possess 4-, 5-, 6-, or 8-coordination. Furthermore, they are of interest to chemists hecause of their obvious relevance as biological models. This high level of interest in porphyrins and metalloporphyrins is fully justified by their behavior as complex physicochemical systems and the hiological circumst+ce that iron porphyrins serve as the heme or prosthetic groups in several classes of the hemeproteins (1) (Fig. 1). For example, the varied chemical and physical properties of metalloporphyrins are reflected in the hiological significance of porphyrinic materials (such as chlorophyll, hemoglobin, cytochrome, and vitamin BIZ)in photosynthesis, gas transport, enzymatic catalysis, metabolic regulation and control, electron-transport, etc. (2). Also, changes or modification in the general porphyrin metabolism characteristics have been associated with cancer, drug metaholism, and specific disease syndromes. Clearly, the better understanding of the structure and chemical properties of metalloporphyrins thiough studies of model complexes are of great importance toward understanding their biological functions (3). Certainly most of the research in porphyrins stems from interest in the biological systems and their potential medicinal value, and would he enough justification for a great deal of research interest. However, metalloporphyrins are studied for other reasons as well, such as: the search for new semiconductors (4),superconductors (5), anti-cancer drugs (6), catalysts (7). and chemical shift reagents (8). Several porphyrin-related compounds, particularly the phthalocyanines (9), have proved useful as dyes, such as the fast drying blue inks, as well as the blue and green coloring in security notes and bonds. Even without their hiological and industrial implications, metalloporphyrins would be studied for their purely theoretical importance. Clearly, even to the non-specialists, a knowledge of the more recent or unusual synthetic metalloporphyrins should he useful. While in the last few years we have seen a substantial growth in metalloporphyrin research, a rapid development in a unique area of chemistry, out-of-plane metalloporphyrins, has been particularly noticeable. I t would be impossible to cover all the developments in the space available; thus, we will confine our discussion to the synthesis and structure of the out-of-plane complexes with unusual geometries. Before we begin, let us first explain the terminology to be used to describe the various types of compounds; mono and dinuclear refers to the number of porphyrin or porphyrin type rings present in the unit chain of the compound; mono, di, or trimetallic refers to the number of metal ions present in (associated with) the ring(s). Synthesis Porphyrins are compounds formed hy adding suhstituents to the pyrrole rings of porphine (Fig. 2). The naturally occurring porphyrins are generally formed by adding substituenb to positions 1-8 and are named according to the
number and type of suhstituent (10). Upon removal of the pyrrole protons, porphyrins behave, normally, as tetradentate ligands and readily complex with a variety of metals. In most cases, the metal ion sits in the center of the plane of the porphyrin ring (11). The major problem in the traditional method (prior to 1966) of metalloporphyrin syntheses is the difficulty in dissolving both the free porphyrin and the metallic salt simultaneously into the same solution under reactive conditions. This is due to the fact that good solvents for the porphyrins in their unionized forms are generally poor solvents for simple metallic ions and vice versa. Acidic mediums typically require a large excess of metallic salt, usually a thousand fold or greater, in order to force the reaction toward metal insertion. This is particularly true where the thermodynamic stability of the complex is low, such as in the formation of zinc complexes, or where the formation of the porphyrin acid cation, P o r - H P , is facile. In the basic medium, on the other hand, the porphyrin must compete, often disadvantageously, with the solvent as a complexing agent for the metal ion. Recently, new synthetic techniaues, which have been shown to be caoable of circumventing this prohlem, have been developed The first of the new svnthetic methods to be discovered was the insertion of a metal from a carbonyl complex (12,
