SYNTHESIS, CHARACTERIZATION, AND SPECTRAL PROPERTIES OF NEW PERPENDICULARLY LINKED OSMIUM(II) PORPHYRIN OLIGOMERS

A series of new osmium(II) porphyrin oligomers having tetraarylporphyr ins with mixed 4-pyridyl (Py) and phenyl (P) groups, H(2)Py(n)P(4-n)P, as an axial or a bridging ligand, Os(OEP)(CO)(H(2)PyP(3)P), 1, cis-[O s(OEP)(CO)](2)(H(2)Py(2)P(2)P), 2, trans-[Os(OEP)(CO)](2)(H(2)Py(2)P(2 )P), 3, [Os(OEP)(CO)](3)(H(2)Py(3)PP), 4, and [Os(OEP)(CO)](4)(H(2)TPy P), 5, were synthesized and characterized by spectroscopic methods. Th e oligomerizations were confirmed by H-1 NMR. The construction of the oligomers enabled to observe directly the tautomerization of internal N-H protons in the axial porphyrins at low temperatures. The activatio n energy of the tautomerism: E(a) = 40.5 kJ mol(-1), and the exchangin g rate constant, k = 7200 +/- 100 s(-1) at 298 K, were comparable to t he values previously reported for various porphyrin monomers. The abso rption spectra of these oligomers are essentially superpositions of th e spectra of the constituent monomeric porphyrins. The cyclic voltammo grams of all the oligomers showed the redox waves of parent porphyrins and the osmium ions, each current reflecting the number of constituen ts. The peak separations of these Os(II/III) redox waves are nearly th e same among the five oligomers. The redox and electronic spectral mea surements of the oligomers indicated that there are no distinct intera ctions between the macrocycles in the ground state electrochemically a nd visible spectrally.