Molecular Recognition of Ubiquinone Analogues. Specific Interaction between Quinone and Functional Porphyrin via Multiple Hydrogen Bonds

Abstract

meso-α,α,α,α-Tetrakis(2-hydroxy-1-naphthyl)porphyrin (1) and 5,15-cis-bis(2-hydroxy-1-naphthyl)-10,20-diphenylporphyrin (2) were prepared as receptors for ubiquinone analogues. UV−vis, IR, NMR, and X-ray crystal data reveal that these porphyrins bind a series of quinones with a cofacial structure linked by hydrogen bonds. The binding constants of 1 for quinones increase with the number of MeO substituents bound to a quinone ring. Compared to 2,5-dimethoxy-p-benzoquinone, 2,3-dimethoxy-p-benzoquinone shows more favorable negative changes in ΔG° and ΔH° upon binding with 1, whereas the interaction between 2 and quinone is not affected by the position and number of MeO substituents. Thus the two adjacent MeO substituents cooperatively act as the third recognition site for OH groups in 1 to form the bifurcated hydrogen bonding. Particularly, tetramethoxy-p-benzoquinone (4f) shows an extremely large affinity and favorable ΔH° with 1 due to the formation of simultaneous multiple hydrogen bonds; ΔG° = −7.9 kcal/mol at 298 K in toluene. The van't Hoff plots obtained from titrimetric measurement at 0.9−5.2 mol % ethanol in toluene reveal a nonlinear relationship between R ln K and 1/T. The curvature of these plots is explained in terms of the temperature dependence of the degree of functional groups solvation in 1 and 4f with a negative heat capacity change; ΔCp° = −5.6 × 102 cal/mol·K at 0.9 mol % ethanol, indicating that the solvent−solute interaction has a crucial effect on the stability of the host−guest complexes.