Studies on 24-membered macrocyclic mononuclear and dinuclear iron complexes: Stability and catalytic hydroxylation of adamantane by divalent iron complexes

The dinucleating 24-membered hexaazadiphenol macrocyclic ligand 3,6,9,17,20,23-hexaaza-29,30-dihydroxy-13,27-dimethyltricyclo[23.3.1.1(11,15)]triaconta-1(28),11,13,15(30),25,26-hexaene (L or [24]BDBPH) is prepared by the NaBH4 reduction of the Schiff base obtained from [2 + 2] template condensation of 2,6-diformyl-p-cresol with diethyltriamine. The ligand maintains dinuclear integrity for both iron(II,II) and iron(III,III) complexes, while facilitating the formation of bridging phenolate diiron cores. Potentiometric equilibrium studies indicate that a variety of protonated, mononuclear and dinuclear iron(II) and iron(III) complexes form from p[H] 2-11 in an aqueous solution. The protonation constants and stability constants of the 1:1, 1:2 [ligand/iron(II) or ligand/iron(III)], and 1:1:1 [ligand/iron(III)/iron(II)] complexes were determined in a KCl supporting electrolyte (mu = 0.100 M) at 25 degrees C. The mechanisms for the formation of dinuclear iron(II), iron(III), sind mixed-valence iron(II,III) complexes are described. Preliminary results showed that the dinuclear iron(II) complexes catalyze hydroxylation of adamantane in the presence of H2S as a two-electron reductant.