A series of iron(III) complexes of the imines of L-amino acids derived from salicylaldehyde, Fe(sal-L-aa)Cl (aa = ala, val, phe, his), and the corresponding complex obtained from histamine, Fe(sal-him)Cl2, have been prepared and characterized by various spectroscopic techniques. The complexes are high spin and have five- or six-coordinate structures. They generally exist in solution as equilibrium mixtures of monomeric and dimeric forms, the ratio of which depends upon the solvent and the presence of externally added ligands. Information on the two species has been obtained by combined ESR and NMR measurements. The NMR spectra, in particular, show two sets of paramagnetically shifted proton resonances that can be attributed to the salicylaldimine protons of the two species, the dimeric species exhibiting smaller NMR shifts than the monomeric species. The optical spectra of the complexes are largely determined by transitions originating in the iron salicylaldiminate chromophore. An assignment of the low-energy phenolate to iron(III) LMCT bands has been made possible by the analysis of the electronic and CD spectra of the complexes and their adducts with donor bases and by MO calculations performed on the ligand analogues A-methylsalicylaldiminate and p-methylphenolate anions. The spectral features of the iron complexes and their adducts have been discussed in relation to those of the iron tyrosinate proteins. of particular relevance are the adducts of the complexes with catecholate anions since their spectral properties often mimic those of the enzyme-substrate complexes of the catechol dioxygenases.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry