Cyclic porphyrin dimers as hosts for coordinating ligands

Bicovalently linked tetraphenylporphyrins bearing dioxypentane groups at the opposite (transoid, H4A) and adjacent (cisoid, H4B) aryl groups have been synthesised. Protonation of the free-base porphyrins leads to fully protonated species H8A4+/H8A4+ accompanied by expansion of cavity size of the bisporphyrins. The electrochemical redox studies of these porphyrins and their Zinc(II) derivatives revealed that the first ring oxidation proceeds through a two-electron process while the second ring oxidation occurs at two distinct one-electron steps indicating unsymmetrical charge distribution in the oxidized intermediate. The axial ligation properties of the Zinc(Il) derivatives of H4A/H4B with DABCO and PMDA investigated by spectroscopic and single crystal X-ray diffraction studies showed predominant existence of 1: I complex. The Zn2A.DABCO complex assumes an interesting eclipsed structure wherein DABCO is located inside the cavity between the two porphyrin planes with Zn-N distances at 2.08 and 2.22 Å. The Zn atoms are pulled into the cavity due to coordination towards nitrogen atoms of DABCO and deviate from the mean porphyrin plane by 0.35 Å. The electrochemical redox potentials of the axially ligated metal derivatives are found to be sensitive function of the relative coordinating ability of the ligands and the conformation of the hosts

Synthesis and structure of ferrocene-linked Schiff base porphyrins.

A series of mono ferrocene Schiff-base porphyrins in which the ferrocene unit is disposed in different orientations to the porphyrin core have been synthesized from meso-5(o- or p)aminophenyl-10,15,20-triphenyl-21,23-H-porphyrin and ferrocenecarboxaldehyde. The optical and magnetic resonance spectral features of these derivs. reveal weak interactions between ferrocene and porphyrin units. The kinetics of metal incorporation into the free-base ferrocenyl porphyrins indicate the steric features induced by the ferrocene unit. Electrochem. redox behavior of the compds. shows that the ferrocene and porphyrin entities behave as independent units. Steady-state photolysis expts. reveal no internal electron-transfer, although the energies of the CT state of porphyrin-ferricenium ion indicate the feasibility of electron transfer in these systems