SUPRAMOLECULAR CHEMISTRY .49. LIGAND-PORPHYRIN COMPLEXES - QUANTITATIVE-EVALUATION OF STACKING AND IONIC CONTRIBUTIONS

Association energies of 3 porphyrins bearing meso-4-pyridinium, 4-anilinium, or 4-benzoate substituents (TPPyP, TPA, TCP) are evaluated in water with 23 different ligands. UV titrations show excellent fit and constant values (usually +/-10%) for the equilibrium constants K evaluated at different wavelengths as well as isosbestic points, securing 1:1 complexes as well as negligible self-association of the porphyrins. Complexation-induced proton NMR shifts (CIS) show shielding of the ligands by up to -8.0 ppm, and of up to -3.4 ppm on the porphyrins with aromatic ligands; they agree with tight face-to-face conformations. Ionic binding contributions can be factorized with 5.2 +/- 1.1 kJ per mol and per salt bridge (or ion pair). After subtraction of the salt bridge increments, constant binding free energies are observed with Delta G(vdw) = 7.2 +/- 1.5 kJ mol(-1) for all benzene derivatives and Delta G(vdw) = 15.8 +/- 1.8 kJ mol(-1) for all naphthalene-like, and Delta G(vdw) = 18.5 +/- 0.5 kJ mol(-1) for phenanthrene-like derivatives. Deviations are observed with bulky substituents like (+)NMe(3) or SO3- groups which allow no close contact between the aromatic planes, as evident from CHARMm simulations. Heterocyclic electroneutral ligands show the same constant Delta G(vdw) values, independent of number and position of the heteroatoms. Comparison of 33 different Delta G(vdw) contributions with the number of double bonds occurring in the ligands yields, for the first time, a comprehensive description of stacking interactions with an increment of 1.4 +/- 0.15 kJ per mol and pi-electron. Measurements with saturated ligands of comparable surface show no Delta G(vdw) contributions, indicating that the stacking is not driven by solvophobic but by van der Waals forces. Copper(II) in the porphyrins (CuTPPyP) leads within +/-0.4 kJ mol(-1) to the same association energies as without metal, whereas introduction of zinc leads to a decrease by usually 3.4 kJ mol(-1). Axial orientations of the ligands are observed with complexes of alpha,omega-diaminoalkanes with a gable or sandwich Zn-porphyrin dimer. The Delta(cplx) values, measured in chloroform, increase with the match between the Zn and the N atoms. Measurements of K in binary methanol-water mixtures with four complexes (e.g. TPPyP + phenanthroline) show linear correlations with solvent polarity or hydrophobicity parameters. Linear correlations are also found for the first time between the complexation-induced Soret band wavelength changes and the corresponding Delta G(cplx) values.