BASE BINDING TO ZINC PICKET FENCE PORPHYRINS - ATTRACTIVE INTRAMOLECULAR INTERACTIONS IN ORGANIC-SOLVENTS

Thermodynamic values for the binding of pyridine and isoquinoline to α4 and trans-α2 atropisomers of four kinds of zinc(II) picket fence porphyrins, which contain pivalamido, (isopropylcarbonyl)amino, (neopentylcarbonyl)amino, or (butylcarbonyl)amino pickets, were determined in toluene. The binding constants K of the trans-α2 complexes were remarkably larger than those of the corresponding α4 complexes, depending on both shape and size of pickets; the stability enhancements by pivalamido pickets were a factor of 20 and 34 in K for pyridine and isoquinoline, respectively, while those by (butylcarbonyl)amino pickets were only a factor of 2.6 and 3.7. These stabilizations of the trans-α2 complexes were due to increased binding energy (−ΔΔH = 0.8–2.3 kcal/mol), although accompanied by a slight decrease of entropy (−ΔΔS = 0.1–2.2 eu). It was concluded that the stability enhancements of the trans-α2 complexes can be ascribed as the attractive intramolecular CH–π interactions induced by the London dispersion force in base adducts rather than solvation effects in the weakly polar solvent. Comparisons of the shape of picket fences with both thermodynamic values and 1H NMR chemical shifts suggested that preorganization of the cavity is essential to the stability enhancements. The variation of binding constants in several noncoordinating solvents also suggested that desolvation processes upon the ligation of free base considerably affect the stability factor K(trans-α2)/K(α4) in this case. © 1990, American Chemical Society. All rights reserved.