COORDINATION CHEMISTRY AND GEOMETRIES OF SOME 4,4'-BIPYRIDYL-CAPPED PORPHYRINS - PROTON-INDUCED AND LIGAND-INDUCED SWITCHING OF CONFORMATIONS

The NMR spectra of 3 4,4''-bipyridyl-capped porphyrins are analyzed in terms of a previously described ring current model to yield molecular geometries. The compound with the shortest chain connecting porphyrin and cap takes up a distorted conformation, either as a result of .pi.-.pi. interactions or to minimize strain. Protonation of porphyrin and cap yields a similar conformation for all 3 compounds, with a large separation between the 2 components. The NMR and UV/visible spectra of the Zn complexes of these porphyrins are strongly dependent on the presence or absence of pyridine. This is a result of competition between strained intramolecular binding and the entropically less favorable, but unstrained, intermolecular binding of the Zn. Detailed geometries are obtained for both states of each molecule, and the strain involved in intramolecular binding is estimated. The criteria to be satisfied for effective molecular switching of conformations are discussed.