Concentration- and solvent-dependent electron spin resonance signals and dimer formation of high-spin iron(III) porphyrins

Abstract

High-spin iron(III) prophyrins in frozen solutions show extra e.s.r. signals, in addition to the well known parallel (g= 2) and perpendicular (g= 6) absorption, which are remarkably sensitive to the concentration, solvent composition, pH, or the temperature. For example, two signals with g values around 3 and 11, respectively, are observed in moderately concentrated solutions of chloro(meso-nitro-octaethylporphyrinato)iron(III), in neutral and alkaline solutions of hemin when methanol is added, or in acidic solutions of tetrasodium phenylporphyrin-tetrasulphonatoferrate(III). The signals to higher field with g ca. 3 are interpreted as belonging to a dimeric species, on the basis of a simulation made by assuming a structure having two porphyrin planes arranged parallel. Another characteristic signal, which was observed in alkaline solutions of hemin or the tetrasodium ferrate, appears as a pair of shoulders with the g= 6 monomer peak in the middle. This type of signal is assigned to a dimer having a large Fe–Fe distance as well as skew angle. The effect of adding methanol, ethanol, or sucrose to the alkaline solutions of hemin, inducing further additional signals, and that of NaCl addition, which suppresses the overall intensity, are described and discussed in relation to previous studies by spectrophotometric and magnetic susceptibility measurements.