Iron Complexes of N-Confused Pyriporphyrin: NMR Studies

Abstract

Insertion of iron(II) into 6,11,16,21-tetraaryl-3-aza-m-benziporphyrin (N-confused pyriporphyrin, (PyPH)H) yielded the high-spin iron(II) complex, (PyPH)Fe(II)Br. The coordination of iron(II) to the perimeter nitrogen atom of (PyPH)Fe(II)Br resulted in the formation of the diiron species. Oxidation and oxygenation of (PyPH)Fe(II)Br were followed by 1H NMR spectroscopy. The addition of Br2 to the solution of (PyPH)Fe(II)Br in the absence of dioxygen results in a one-electron oxidation yielding the high-spin iron(III) N-confused pyriporphyrin [(PyPH)Fe(III)Br]+ which preserves the side-on interaction between the inverted pyridine ring and metal ion. The reaction of (PyPH)Fe(II)Br with dioxygen ends up with the formation of a five-coordinate species (PyPO)Fe(III)Br] ((PyPOH)H = 3-aza-22-hydroxy-m-benziporphyrin, PyPO = the corresponding dianion) which is formed by oxygenation at the C(22) position. Coordination of a metal ion by 3-aza-22-hydroxy-benziporphyrin imposes a steric constraint on the geometry of the ligand. The halide ligand of (PyPO)Fe(III)Br coordinates on one of the two inequivalent faces of the macrocycle, leading to two distinct species: syn and anti. The (1)H NMR spectra of paramagnetic iron(II) and iron(III) N-confused pyriporphyrin complexes have been examined. The characteristic patterns of pyrrole and pyridine resonances have been found to be diagnostic of the ground electronic state of iron and the donor nature of the C(22)H and N(3) centers. The enormous downfield H(22) paramagnetic shift, determined for the iron(II) N-confused pyriporphyrin, provides a distinct resonance in a peculiar spectroscopic window (350-800 ppm) for a series of axial ligands which can be considered as a diagnostic sign of an agostic Fe(II)...{C(22)-H} interaction. Coordination of the pyridine moiety via the perimeter N(3) atom is reflected unambiguously by the H(2/4) resonance at 201 ppm.