Synthesis and structural, spectroscopic, and magnetic characterization of (NH4)[Fe-3(mu(3)-OH)(H2L)(3)(HL)(3)] (H3L = orotic acid) presenting two novel metal-binding modes of the orotate ligand: The case of a spin-frustratedsystem

The use of orotic acid (H3L) in iron(III) chemistry has yielded a structura lly, magnetically, and spectrochemically very interesting complex. The 1:3: 3 FeCl3. 6H(2)O/H3L/aqueous NH3 reaction system in MeOH gives red (NH4)-[Fe -3(mu(3)-OH)(H2L)(3)(HL)(3)]. 7.75H(2)O (1) in high yield. 1. crystallizes in the hexagonal space group P (3) over bar with (at 25 degrees C) a = 14.8 13(7) Angstrom, c = 15.084(7) Angstrom, and Z = 2. There is a C-3 axis pass ing through the central hydroxo oxygen; thus, the three Fe-III atoms form a n equilateral triangle with an edge of 3.312 Angstrom and the Fe3Ohydroxy c ore is planar. The orotates present two novel coordination modes; each H2L- ligand bridges two Fe-III atoms through its syn,syn eta(1):eta(1):mu(2) ca rboxylate group, while HL2- simultaneously chelates one Fe-III atom through one carboxylate oxygen and the deprotonated amide nitrogen, and is bonded to a second metal ion through the adjacent carbonyl oxygen. The three H2L- and the three HL2- ligands lie above and below the Fe3Ohydroxy plane, respe ctively. Hydrogen bonds between the orotates result in a 3D network. Magnet ic measurements of I in the 1.8-300 K temperature range were fitted using a 2J model with mean-field corrections and show antiferromagnetic interactio ns between the metal ions in the trinuclear moiety as well as between the t rimers due to the 3D H-bonded network. The case of spin frustration is disc ussed extensively as well as the possible antisymmetric exchange interactio ns. The solid state X-band EPR spectrum of 1 at 4 K is consistent with the magnetic measurements showing that the S = (1)/(2) ground state is very clo se to the first excited S = (3)/(2), which is also populated at this temper ature. Furthermore, the simulation of the EPR spectrum reveals the anisotro pic character of the system.