Solvent Effects upon the Conformational Equilibrium for the Cofacial Zinc(II) Tetraphenylporphyrin Cage Molecule

Abstract

Experimental values for conformational equilibrium constants of the zinc(II) tetraphenylporphyrin cage molecule in seventeen nonaqueous solvents have been published recently. That data collection has now been critically re-examined in order to identify the characteristics of the solvent associated with the observed changes in the distribution of the solute between the introverted and extroverted forms of the porphyrin-quinone. Based on a linear solvation energy analysis for a wide range of solvent types, it now appears that the primary influences which are attributable to the solvent environment include the dipolarity, inductive polarizability, and hydrogen bonding by the solvent species. The biochemical metalloporphyrins have been studied extensively as chemical agents in oxygen transfer and storage and in electron transfer processes. A number of synthetic metalloporphyrins have been examined as well in order to isolate and more fully identify particular structural features contributing to photochemical, mechanistic and catalytic properties of their more complex natural analogues. Among these synthetic compounds, zinc(II) tetraphenyl-porphyrin (ZnTPP) and its derivatives have served as model structures for the investigation of the influence of axial ligation on electronic spectra (Nappa and Valentine, 1978) and thermodynamic changes which accompany modifications in the side-chain substituents on the porphyrin (Walker and Benson, 1980). Of the many ZnTPP derivatives, one structurally unique and interesting porphyrin-quinone cage molecule has been recently synthesized and fully characterized by Lisicki et al. (1988). This molecule can most easily be pictured as having an overall hemispherical shape with the base of the hemisphere being the Zn-porphyrin planar structure and the quinone portion located at the polar position above the center of the porphyrin ring. Thus, the quinone portion is attached to the perimeter of the ZnTPP plane by four bridging arms or spacer groups.