METAL-COMPLEXES WITH TETRAPYRROLE LIGANDS .59. REDOX POTENTIALS, NEAR-INFRARED ABSORPTION, IONIC-RADII, AND RING-RING DISTANCE IN METAL(III) BIS(TETRAPHENYLPORPHYRINATE) SYSTEMS WITH PORPHYRIN RINGS IN DIFFERENT OXIDATION-STATES

An extended series of tetrabutylammonium salts of bisporphyrinate double decker anions NBu4[M(III)(TPP)2]2) (M = Y, La-Lu except Pm) is described. The complexes are characterized by UV/Vis/NIR and H-1-NMR spectroscopy, cyclic voltammetry, and spectroelectrochemistry. The redox potentials of the new sandwich compounds and the near-infrared absorption energies of their corresponding oxidation products M(TPP)2 and [M(TPP)2]+ are shown to depend linearly on the ionic radii of the central ions. The gradients of the first and second oxidation potentials are positive whereas those of the third and fourth oxidation potentials are negative. This result is interpreted by means of a qualitative molecular orbital diagram describing the pi-pi-interactions of the four frontier orbitals of the two macrocycles in the double decker molecules.