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-dimelhyl-3,11,19,27,33,35-hexaazapentacyclo [27.3.1.1.(5,9)1.(13,17)1.(21,25)]-hexatriaconta-5,7,9(33),- 13,15,1 7(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 KCI supporting electrolyte (0.100 M) at 25.0 degrees 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. (C) 2000 Elsevier Science S.A. All rights reserved.