Models for Verdoheme Hydrolysis. Paramagnetic Products from the Ring Opening of Verdohemes, 5-Oxaporphyrin Complexes of Iron(II), with Methoxide Ion

Studies of the reaction of 5-oxaporphyrin iron complexes (verdohemes) with methoxide ion have
been undertaken to provide models for the initial step in hydrolysis of verdohemes. Treatment of both low-spin [(py)2FeII(OEOP)]Cl and high-spin {ClFeII(OEOP)} (OEOP is the monoanion of octaethyl-5-oxaporphyrin)
with methoxide ion in either pyridine or methanol solution is shown to cause ring opening and addition of
methoxide to one end of the tetrapyrrole. Black {FeII(OEBOMe)}2 has been isolated in crystalline form from
the reaction of {ClFeII(OEOP)} with methoxide in methanol and studied by single-crystal X-ray diffraction.
The molecule has a centrosymmetric dimeric structure composed of two helical Fe(OEBOMe) units with FeN4
coordination which are connected by a pair of Fe−O bonds. Each iron ion has approximately trigonal
bipyramidal geometry. Evidence is presented which suggests that {FeII(OEBOMe)}2 dissolves in dichloromethane in the presence of methanol or pyridine to form high-spin {(MeOH)nFeII(OEBOMe)} or {(py)nFeII(OEBOMe)} (n = 1 or 2) through rupture of the Fe−O bonds in the dimer. The electronic absorption
spectra of these open chain complexes show characteristic low-energy features at ca. 720 and 810 nm. The
1H NMR spectrum of {FeII(OEBOMe)}2 at 23 °C shows a characteristic upfield methoxy resonance, 16
methylene resonances in the 47 to 5 ppm region, and meso resonances in the 51 to 29 ppm region. The
temperature dependence of these spectra shows marked deviations from the Curie law, which are consistent
with a dimeric structure.