Strong van der Waals Interactions in Water. Molecular Complexes of Porphyrins and Quinones in Organic and Aqueous Media

Abstract

The stability of the molecular complexes between 5,10,15,20-tetraarylporphyrins and quinones in organic solvents has been compared with that in water. 5,10,15,20-Tetrakis(mono-substituted phenyl)porphines (RTPPs) hardly form molecular complexes with p-benzoquinone (BQ), 2,3,5,6-tetramethyl-p-benzoquinone (DQ), and 2,3,5,6-tetrachloro-p-benzoquinone (CQ) in dichloromethane (K>60 mol−1 dm3). In contrast, the stability constants (K<60 mol−1 dm3) for the molecular complexes of cationic porphyrins (TMPyP and TAPP) with BQ in water are much larger than those in methanol. The thermodynamic parameters clearly indicate that complexation in water is an enthalpically favorable process, suggesting that the van der Waals interactions are the main binding forces and that hydrophobic interaction does not play an important role in complex formation in water. The larger stability of the molecular complexes in water can be interpreted in terms of the nature of water, as demonstrated by Smithrud and Diederich (D. B. Smithrud and F. Diederich, J. Am. Chem. Soc., 112, 339 (1990)). The microscopically hydrophobic porphyrin ring many hardly be hydrated and interact with BQ through the van der Waals interactions without an extensive dehydration process. The water molecules bound to BQ should be released upon complexation, and the released water molecules may be incorporated tightly into the hydrogen-bonding networks of water. Such complexation in water seems to be an enthalpically favorable process.