STRUCTURES AND MAGNETIC-PROPERTIES OF IRON(III) DINUCLEAR COMPLEXES WITH ALKOXO AND CARBOXYLATO BRIDGES

A series of diiron(III) complexes containing alkoxo and carboxylato bridges, [Fe-2(dhpta)(OOCR)(H2O)(2)] (H-5-dhpta = 1,3-diamino-2-hydroxypropane-N,N,N',N'-tetraacetic acid; R = C6H4OH (1), CH2CH=CH2 (2), C6H4-NH2 (3), CH2CH2Br (4); CH=CHCH3 (5)) have been synthesized. Their molecular structures and magnetic properties were investigated. For 1 and 2, the crystal structures have been determined by X-ray analysis. Crystal data: for 1, dihydrate, triclinic, P $($) over bar$$ 1, a = 9.929(3) Angstrom, b = 18.955(6) Angstrom, c = 7.862(2) Angstrom, alpha = 113.60(2)degrees, beta = 107.57(2)degrees, gamma = 61.79(3)degrees, V = 1182.2(7) Angstrom(3), Z = 2, R = 0.050; for 2, triclinic, P $($) over bar$$ 1, a = 10.020(1) Angstrom, b = 15.952(1) Angstrom, c = 6.7523(7) Angstrom, alpha = 95.536(7)degrees, beta = 97.968(8)degrees, gamma = 86.612(7)degrees, V = 1062.7(2) Angstrom(3), Z = 2, R = 0.050. For both complexes, two Fe(III) ions take rather distorted octahedral structure and are bridged by the alkoxyl group of dhpta and the carboxylate ion. Fe-O(R) distances are 2.006(2) and 1.985(2) Angstrom for 1 and 1.971(4) and 1.998(3) Angstrom for 2, and the Fe-O(R)-Fe angle is 130.0(1)degrees for 1 and 130.2(2)degrees for 2. EXAFS, XANES, and IR data support that all the complexes have similar (mu-alkoxo)(mu-carboxylato)diiron dimer structures. Independent of the bridging carboxylate ligand, all these complexes exhibit similar weak antiferromagnetic exchange interactions between the spins of the ferric ions through the O(-R) bridge (-J = 14-16 cm(-1)). Among these complexes, a small but significant increase in the -J value with shorter Fe-(mu-O) distance is noted. This indicates that the spin exchange interaction is very sensitive to the bond length between Fe and bridging oxygen atoms also for the (mu-alkoxo)diiron(III) complexes. Both 1 and 2 have the crystal structures with a separate arrangement of hydrophilic and hydrophobic parts and stacking of the pi system.