Unusual co-ordination of water to iron(II) amino acid basket-handle porphyrins

Abstract

Iron(II) hanging-imidazole basket-handle porphyrins in which one or two amino-acids [(L)phenylalanine or (L)-valine] are inserted into one of the two handles, have been prepared in order to increase the steric hindrance and polarity of the distal side cavity. These complexes resulting from reduction of their hematine forms in a heterogeneous two-phase system, organic solvent–aqueous sodium dithionite solution, have been characterized by spectroscopic and magnetic methods. In wet toluene solution, they are found to be a mixture of high (S= 2) and low (S= 0) spin states at a temperature of 295 K. At a lower temperature of 243 K they give complexes in a pure magnetic low spin state. On the contrary, in solution of a dry solvent, they exhibit a high spin state. This uncommon magnetic property is due to a water molecule present at the sixth co-ordination site of the iron(II) atom stabilized in the distal cavity by hydrogen bond(s). Rate and affinity constants for the unusual water ligation have been measured. Water binding is shown to be controlled by diffusion when toluene is used as solvent. The dissociation rate constants decrease at least 100-fold in the presence of amino acid residues, relative to those of a single hanging-imidazole basket-handle porphyrin. Carbon monoxide and oxygen binding kinetics have been determined and it is concluded that the polarity increase of the distal cavity has no additional stabilizing effects, but the steric effect due to the presence of amino-acid(s) interferes with O₂ and CO ligation.