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

Association energies of 3 porphyrins bearing meso-4-pyridinium, 4-anil inium, or 4-benzoate substituents (TPPyP, TPA, TCP) are evaluated in w ater with 23 different ligands. UV titrations show excellent fit and c onstant values (usually +/-10%) for the equilibrium constants K evalua ted 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 lig ands by up to -8.0 ppm, and of up to -3.4 ppm on the porphyrins with a romatic ligands; they agree with tight face-to-face conformations. Ion ic 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 brid ge 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. Dev iations are observed with bulky substituents like (+)NMe(3) or SO3- gr oups which allow no close contact between the aromatic planes, as evid ent from CHARMm simulations. Heterocyclic electroneutral ligands show the same constant Delta G(vdw) values, independent of number and posit ion of the heteroatoms. Comparison of 33 different Delta G(vdw) contri butions with the number of double bonds occurring in the ligands yield s, for the first time, a comprehensive description of stacking interac tions with an increment of 1.4 +/- 0.15 kJ per mol and pi-electron. Me asurements with saturated ligands of comparable surface show no Delta G(vdw) contributions, indicating that the stacking is not driven by so lvophobic but by van der Waals forces. Copper(II) in the porphyrins (C uTPPyP) leads within +/-0.4 kJ mol(-1) to the same association energie s as without metal, whereas introduction of zinc leads to a decrease b y usually 3.4 kJ mol(-1). Axial orientations of the ligands are observ ed with complexes of alpha,omega-diaminoalkanes with a gable or sandwi ch 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 h ydrophobicity parameters. Linear correlations are also found for the f irst time between the complexation-induced Soret band wavelength chang es and the corresponding Delta G(cplx) values.