Iron(III)-Oxo Cluster Chemistry with Dimethylarsinate Ligands: Structures, Magnetic Properties, and Computational Studies.

A program has been initiated to develop FeIII/oxo cluster chemistry with the "pseudocarboxylate" ligand dimethylarsinate (Me2AsO2-) for comparison with the well investigated FeIII/oxo/carboxylate cluster area. The synthesis and characterization of three polynuclear FeIII complexes are reported, [Fe12O4(O2CtBu)8(O2AsMe2)17(H2O)3]Cl3 (1), Na2[Fe12Na2O4(O2AsMe2)20(NO3)6(Me2AsO2H)2(H2O)4](NO3)6 (2), and [Fe3(O2AsMe2)6(Me2AsO2H)2(hqn)2](NO3) (3), where hqnH is 8-hydroxyquinoline. The Fe12 core of 1 is a type never previously encountered in FeIII carboxylate chemistry, consisting of two Fe6 units each of which comprises two {Fe3(μ3-O2-)} units bridged by three Me2AsO2- groups and linked into an Fe12 loop structure by two anti-anti η1:η1:μ Me2AsO2- groups, a bridging mode extremely rare with carboxylates. 2 also consists of two Fe6 units, differing in their ligation from those in 1, and this time linked together into a linear structure by a central {Na2(NO3)2} bridging unit. 3 is a linear Fe3 complex with no monatomic bridges between FeIII ions, a very rare situation in FeIII chemistry with any ligands and unprecedented in Fe carboxylate chemistry. The distinct differences observed in arsinate vs carboxylate ligation modes are rationalized largely based on the greater basicity of the former vs the latter. Variable-temperature dc and ac magnetic susceptibility data reveal all Fe2 pairwise interactions to be antiferromagnetic. For 1 and 2, the different Jij couplings were estimated by use of a magnetostructural correlation for high nuclearity FeIII-oxo clusters and by density functional theory calculations using broken symmetry methods, allowing identification of their relative spin vector alignments and thus rationalization of their S = 0 ground states. The Jij values were then used as input values to give excellent fits of the experimental χMT vs T data. For 3, the fits of the experimental χMT vs T data to the Van Vleck equation or with PHI gave a very weak J12 = -0.8(1) cm-1 (H = -2JŜi·Ŝj convention) between adjacent FeIII ions and an S = 5/2 ground state. These initial FeIII arsinate complexes also provide structural parameters that help validate literature assignments of arsinate binding modes to iron oxide/hydroxide minerals as part of environmental concerns of using arsenic-containing herbicides in agriculture.