Stabilities of Fe(III) mononuclear and Fe(III)–M(II) (M=Cu, Zn, Cd and Mn) heterodinuclear complexes with a dinucleating hexaazadiphenol macrocycle

The 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 (24RBPyBC) forms a number of protonated and hydroxo forms of Fe(III) mononuclear complexes as well as Fe(III)–M(II) (M(II)=Cu, Zn, Cd and Mn) heterodinuclear complexes in aqueous solution. Stability constants of these complexes are measured by pH potentiometric titration in KCl supporting electrolyte (0.100 M) at 25.0°C and distribution of the species present in the solution as a function of p[H] are determined. Combination of potentiometric and visible spectral studies suggest that hydrolysis of the Fe(III) mononuclear complexes occurs in lower p[H] than does the deprotonation reaction of the uncoordinated ammoniums of the macrocycle. In the Fe(III)–M(II) heterodinuclear systems, the hydroxo heterodinuclear complexes predominate in basic solutions, while the protonated Fe(III) mononuclear complex predominates in acidic solutions except the case of Fe(III)–Cu(II) where the heterodinuclear complex [FeCuL]3+ is formed.