Metal Complexes of Porphycene, Corrphycene, and Hemiporphycene: Stability and Coordination Chemistry

• Published in: Chemistry : a European journal Vol. 8; no. 15; pp. 3485 - 3496
• Main Authors: Fowler, Christopher J., Sessler, Jonathan L., Lynch, Vincent M., Waluk, Jacek, Gebauer, Andreas, Lex, Johann, Heger, Andreas, Zuniga-y-Rivero, Fernando, Vogel, Emanuel
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 02.08.2002; WILEY‐VCH Verlag; Wiley
• Subjects: Binding Sites; Chemistry; Chemistry, Multidisciplinary; coordination chemistry; Drug Stability; ligand binding; metalloporphyrins; Metals; Models, Molecular; Molecular Conformation; Molecular Structure; Physical Sciences; porphyrinoids; Porphyrins - chemistry; Science & Technology; Spectrophotometry; stability; MOLECULAR-STRUCTURE; PERIMETER MODEL; SIGN VARIATION; FREE-BASE PORPHYRINS; metalloporphyrins; RAY CRYSTAL-STRUCTURE; ELEMENT-CARBON BOND; porphyrinoids; coordination chemistry; PI-ELECTRON SYSTEMS; AXIAL LIGANDS; MAGNETIC CIRCULAR-DICHROISM; ligand binding; stability; ALGEBRAIC-SOLUTION
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/1521-3765(20020802)8:15<3485::AID-CHEM3485>3.0.CO;2-9

Porphyrin (P), porphycene (Pc), corrphycene (Cn), and hemiporphycene (Hpc) represent a series of well defined “4‐N in” constitutional porphyrin isomers. These isomers, in the form of their octaethyl derivatives, represent a congruent set of porphyrinoids whose properties can be compared. In this study we report how variations in electronic structure and nitrogen‐core size in the free‐base forms of these four systems are reflected in the properties of their corresponding metal complexes. Specifically, the effects that these differences have on the axial ligation properties of the ZnII, MgII, NiII, and CoII complexes of P, Pc, Cn, and Hpc in toluene using pyridine as the axial ligand are detailed. Also reported are the relative stabilities of these complexes under acidic conditions. It is shown that for the zinc, magnesium, and cobalt complexes, there are distinct differences in the ability to maintain four‐, five‐, or six‐coordinate geometries in the presence of similar concentrations of pyridine. By contrast, no apparent differences in axial ligand binding affinity are seen for the four nickel complexes. Little difference in stability was likewise seen when these same complexes were subject to acid‐mediated demetallation, with all four falling into stability class II, according to the accepted porphyrin stability ranking system. High stabilities were also seen in the case of the cobalt complexes, with the Pc and Cn complexes being of stability class III and the P and Hpc derivatives falling into stability class II. The ZnII and MgII complexes were all far less stable than the corresponding NiII and CoII complexes. In this case, semiquantitative analyses of the rate of acid‐induced decomposition revealed the following stability sequence P>Cn>Hpc>Pc for both the ZnII and MgII complexes. Single‐crystal X‐ray diffraction structures were solved for the ZnII, MgII, and NiII complexes of the octaethyl derivatives of Hpc, Cn, and Pc as well as a CoII octamethylcorrphycene and are reported as part of this study. These solid‐state structures confirm four‐coordinate species for the NiII complexes, four‐ and five‐coordinate species for the MgII and ZnII complexes, and a six‐coordinate species for the lone CoII complex. In contrast to what might be expected the “N‐4 in” isomers of porphyrin, even though they have very different core shapes (see picture), act as surprisingly good ligands for a variety of metal cations.