The first systematic stability study of mononuclear and dinuclear iron(II) and iron(III) complexes incorporating a dinucleating macrocyclic ligand in aqueous solution†

Abstract

The dinucleating 24-membered hexaazadiphenol macrocyclic ligand 15,31-dimethyl-3,11,19,27,33,35-hexaazapentacyclo[27.3.1.1.^5,91.^13,171.^21,25]-hexatriaconta-5,7,9(33),13,15,17(34),21,23,25(35),29,31,1(36)-dodecaene-34,36-diol ([24]RBPyBC), prepared by the NaBH₄ reduction of the Schiff base obtained from the [2 + 2] condensation between 2,6-diformylpyridine and 2,6-bis(aminomethyl)-p-cresol, forms a variety of anionic and cationic species in aqueous solution. The structure of ([24]RBPyBC)·4HCl·6CH₃OH was determined by X-ray crystallographic methods. The ligand maintains dinuclear integrity for both iron(II,II) and iron(III, III) states, while facilitating the formation of bridging µ-phenolate diiron cores. Potentiometric equilibrium studies indicate that a variety of protonated, mononuclear and dinuclear iron(II) and iron(III) complexes form through p[H] 2 to 11 in aqueous solution. The protonation constants of the ligand and all associated stability constants of the 1∶1, 1∶2 [ligand:iron(II) or iron(III)], and 1∶1∶1 [ligand∶iron(II)∶iron(III)] complexes were determined in KCl supporting electrolyte (0.100 M) at 25.0 °C. The mechanisms of the formation of dinuclear iron(II), iron(III) and the mixed-valence iron(II,III) complexes are described.