Compound I and Compound II Analogues from Porpholactones

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Abstract
The tetraaza macrocycles 2-oxa-3-oxotetramesitylporphine (|H2 1|) and 2-oxa-3-oxotetrakis(2,6-dichlorophenyl)porphine (|H2 2|) and the corresponding iron complexes (|FeIII(X) 1| and |FeIII(X) 2|; X= Cl-, OH-, or SO3CF3-) have been synthesized. These macrocycles are derived from porphyrins by transformation of one pyrrole ring to an oxazolone ring. The resulting lactone functionality serves to restrict but not completely block π-conjugation around the periphery. These complexes thus share properties with both porphyrins and chlorins. The ferric and high-valent iron complexes have been characterized by a variety of spectroscopic techniques. The molecular structure of |FeIII(Cl) 2| has been obtained by X-ray crystallography and shows that the structural changes at the macrocycle periphery do not perturb the coordination sphere of iron relative to the corresponding porphyrin complexes. This is illustrated by the observation that Fe−O frequencies in the resonance Raman spectra of the porpholactone analogues of compounds I and II are not substantially different from those of porphyrins and by the axial appearance of the EPR signals of the high-spin ferric complexes. This is consistent with reports that the FeO unit of oxidized porphyrins and chlorins is relatively insensitive to alteration of macrocycle symmetry. Nevertheless, probes of properties of the porpholactone macrocycle (1H NMR, resonance Raman skeletal modes) show effects of the asymmetry induced by the oxazolone ring. On the basis of 1H NMR, EPR, Mössbauer, and resonance Raman data, the singly occupied molecular orbital of oxoferryl porpholactone π-cation radicals correlates with the a1u molecular orbital of porphyrins under D4h symmetry. Moreover, the paramagnetic properties and the intramolecular exchange interaction of ferryl iron and the porpholactone π-radical have been characterized by EPR and magnetic Mössbauer measurements and spin-Hamiltonian analyses. The values J0 = 17 cm-1 and J0 = 11 cm-1 obtained for the exchange coupling constants of the oxoferryl porpholactone π-cation radical complexes |FeIVO 1|+ and |FeIVO 2|+, respectively, are among the lowest found for synthetic compound I analogues.