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

The syntheses of the new ligand 3-bis(1,4,7-triaza-1-cyclononyl)-2-hyd roxypropane, 2, and the tetranuclear iron(III) complex which it stabil izes are presented. The ligand contains two distinct, macrocyclic bind ing sites covalently linked via a 2-hydroxypropyl group. The reaction of 2Fe2 With triethylamine in either the presence or absence of a carb oxylate source results in the formation of a green tetranuclear iron(I II) complex, 4. In addition, the reaction of ligand 2, Fe(BF4)2, and s odium benzoate, followed by exposure to oxygen, produces a green solid which is spectroscopically identical to 4. Complex 4, C36H86N12O10P4F 24Fe4, 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) ang strom, b = 16.150(3) angstrom, c = 25.225(3) angstrom, V = 6257(2) ang strom3, and Z = 4. The structure of 4 contains a quadruply-charged, te tranuclear iron(III) cation capped by two molecules of 2 which act as ditopic, hepatadentate ligands. The Fe4O6 core is composed of a tetrah edron of iron atoms bridged by six oxygen atoms (two oxo, two hydroxo, and two alkoxo groups from ligand 2). This results in an adamantane-l ike geometry with the iron atoms occupying the bridgehead positions. T he 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 th ree unique coupling pathways in 4, we have modeled the magnetic behavi or by emphasizing only the dominant oxo-mediated pathway. In this case , there is excellent agreement between theory and experiment with a va lue for the magnetic exchange coupling constant, J, of -93(2) cm-1.