Synthesis and Structural, Spectroscopic, and Magnetic Characterization of (NH₄)[Fe₃(μ₃-OH)(H₂L)₃(HL)₃] (H₃L = Orotic Acid) Presenting Two Novel Metal-Binding Modes of the Orotate Ligand: The Case of a Spin-Frustrated System

The use of orotic acid (H3L) in iron(III) chemistry has yielded a structurally, magnetically, and spectrochemically very interesting complex. The 1:3:3 FeCl3·6H2O/H3L/aqueous NH3 reaction system in MeOH gives red (NH4)[Fe3(μ3-OH)(H2L)3(HL)3]·7.75H2O (1) in high yield. 1 crystallizes in the hexagonal space group P3̄ with (at 25 °C) a = 14.813(7) Å, c = 15.084(7) Å, and Z = 2. There is a C3 axis passing through the central hydroxo oxygen; thus, the three FeIII atoms form an equilateral triangle with an edge of 3.312 Å and the Fe3Ohydroxy core is planar. The orotates present two novel coordination modes; each H2L- ligand bridges two FeIII atoms through its syn,syn η1:η1:μ2 carboxylate group, while HL2- simultaneously chelates one FeIII 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, respectively. Hydrogen bonds between the orotates result in a 3D network. Magnetic measurements of 1 in the 1.8−300 K temperature range were fitted using a 2J model with mean-field corrections and show antiferromagnetic interactions between the metal ions in the trinuclear moiety as well as between the trimers due to the 3D H-bonded network. The case of spin frustration is discussed extensively as well as the possible antisymmetric exchange interactions. 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 close to the first excited S = 3/2, which is also populated at this temperature. Furthermore, the simulation of the EPR spectrum reveals the anisotropic character of the system.