WATER-SOLUBLE FERRIC PORPHYRINATES - SOLUTION AND SOLID-STATE SPECIES

The nature of the aqueous solution axial ligands of the water-soluble iron(III) porphyrinates [Fe(TMPyP)]5+ and [Fe(TPPS)]3- has been examined as a function of pH by H-1 NMR spectroscopy. The NMR shifts provide indirect evidence for the existence of [Fe(TPPS)(H2O)2]3-, [Fe(TPPS)(H2O)(OH)]4-, and [(Fe(TPPS))2O]8- in the [Fe(TPPS)]3- system and [Fe(TMPyP)(H2O)2]5+, [Fe(TMPyP)(H2O)(OH)]4+, [(Fe(TMPyP))2O]8+, and [Fe(TMPyP)(OH)2]3+ in the [Fe(TMPyP)]5+ system. Estimates of the distribution of the species from the NMR spectra allows approximation of the various equilibrium constants in the [Fe(TPPS)]3- system. The pH-dependent equilibria for [Fe(TPPS)]3- and [Fe(TMPyP)]5+ correspond qualitatively with previously published results. Attempts to prepare crystalline species at various pH's were successful only for the high pH form of [Fe(TMPyP)]5+. A single-crystal structure determination shows that this species has the idealized composition [(Fe(TMPyP))2O](ClO4)8.4H2O. The five-coordinate iron atom has an axial Fe-O bond distance of 1.750 (2) angstrom, while the average Fe-N(p) bond distance is 2.080 (8) angstrom; the iron(III) atom is 0.47 angstrom out of the porphyrin plane. Crystal data: a = 32.06 (2) angstrom, b = 18.492 (9) angstrom, c = 17.17 (2) angstrom, beta = 108.6 (1)o, monoclinic, space group C2/c, V = 9650 (2) angstrom-3, Z = 4, Fe2Cl8O37N16C88H80, 5188 observed data, R1(F(o)) = 0.125, R2(F(o)) = 0.140, all observations at 118 K.