Monomeric and Dimeric Iron(III) Complexes of 5‐Hydroxy‐10,15,20‐triphenylporphyrin: Formation of Cyano and Pyridine Complexes of (5‐Oxo‐10,15,20‐triphenylphlorin)iron

Abstract

The reaction of [(TrPP)FeII(py)2] (TrPP = dianion of 5,10,15-triphenylporphyrin) with hydrogen peroxide results in oxygenation of the (porphyrin)iron and the formation of (10,15,20-triphenyl-5-oxophlorin)iron [(5-O-TrPP)Fe(py)2]. Coupled oxidation of [(TrPP)FeIIICl] in the presence of ascorbic acid and potassium cyanide in methanol leads to oxidative hydroxylation of the porphyrin ring to form a cyclic dimer [(5-O-TrPP)FeIII]2. Three out of four pyrrole 1H NMR resonances of the dimeric complex present the characteristic displacement with respect to the typical position for the high-spin (tetraphenylporphyrin)iron(III) complexes. The linewidths of the β-H pyrrole resonances correlate with specific locations of the respective protons within the dimeric structure. Addition of HCl to [(5-O-TrPP)FeIII]2 results in cleavage of the dimer producing [(5-OH-TrPP)FeIIICl]. [(5-O-TrPP)FeIII]2 is also split with acetic anhydride to give (5-acetoxy-10,15,20-triphenylporphyrin)iron(III). In the presence of [D5]pyridine or a large excess of cyanide ion, [(5-O-TrPP)FeIII]2 undergoes cleavage to form air-sensitive [(5-O-TrPP)Fe(py)2] or [(5-O-TrPP)Fe(CN)2]2−. The pyrrole pattern observed for [(5-O-TrPP)Fe(py)2] is similar to that determined on the basis of an analysis carried out for four (oxophlorin)iron regioisomers obtained from hemes. For comparison, the characteristic 1H NMR features of the typical (5-substituted-10,15,20-triphenylporphyrin)iron(III) complexes have been investigated for the series of high-spin and low-spin complexes. The spin-density distribution in the (oxophlorin)iron skeleton is dominated by the radical-like electronic structure. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)