OXO-BRIDGED IRON CLUSTERS - SYNTHESIS OF 1,3-BIS(1,4,7-TRIAZA-1-CYCLONONYL)-2-HYDROXYPROPANE AND ITS STABILIZATION OF THE FE4O6 CORE

The syntheses of the new ligand 1,3-bis(1,4,7-triaza-1-cyclononyl)-2-hydroxypropane, 2, and the tetranuclear iron(III) complex which it stabilizes are presented. The ligand contains two distinct, macrocyclic binding sites covalently linked via a 2-hydroxypropyl group. The reaction of 2Fe2 With triethylamine in either the presence or absence of a carboxylate source results in the formation of a green tetranuclear iron(III) complex, 4. In addition, the reaction of ligand 2, Fe(BF4)2, and sodium benzoate, followed by exposure to oxygen, produces a green solid which is spectroscopically identical to 4. Complex 4, C36H86N12O10P4F24Fe4, forms green crystals in the orthorhombic space group P2(1)2(1)2, (No. 19), with the following unit cell parameters: a = 15.360(3) angstrom, b = 16.150(3) angstrom, c = 25.225(3) angstrom, V = 6257(2) angstrom3, and Z = 4. The structure of 4 contains a quadruply-charged, tetranuclear iron(III) cation capped by two molecules of 2 which act as ditopic, hepatadentate ligands. The Fe4O6 core is composed of a tetrahedron of iron atoms bridged by six oxygen atoms (two oxo, two hydroxo, and two alkoxo groups from ligand 2). This results in an adamantane-like geometry with the iron atoms occupying the bridgehead positions. The iron atoms are antiferromagnetically coupled (mu(eff) = 1.81 mu(B)/Fe at 300 K; 0.26 mu(B) at 40 K). Unable to determine precisely the three unique coupling pathways in 4, we have modeled the magnetic behavior by emphasizing only the dominant oxo-mediated pathway. In this case, there is excellent agreement between theory and experiment with a value for the magnetic exchange coupling constant, J, of -93(2) cm-1.