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

A program has been initiated to develop Fe/oxo cluster chemistry with the "pseudocarboxylate" ligand dimethylarsinate (MeAsO) for comparison with the well investigated Fe/oxo/carboxylate cluster area. The synthesis and characterization of three polynuclear Fe complexes are reported, [FeO(OCBu)(OAsMe)(HO)]Cl (), NaFeNaO(OAsMe)(NO)(MeAsOH)(HO) (), and Fe(OAsMe)(MeAsOH)(hqn) (), where hqnH is 8-hydroxyquinoline. The Fe core of is a type never previously encountered in Fe carboxylate chemistry, consisting of two Fe units each of which comprises two {Fe(μ-O)} units bridged by three MeAsO groups and linked into an Fe loop structure by two η:η:μ MeAsO groups, a bridging mode extremely rare with carboxylates. also consists of two Fe units, differing in their ligation from those in , and this time linked together into a linear structure by a central {Na(NO)} bridging unit. is a linear Fe complex with no monatomic bridges between Fe ions, a very rare situation in Fe 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 Fe pairwise interactions to be antiferromagnetic. For and , the different couplings were estimated by use of a magnetostructural correlation for high nuclearity Fe-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 = 0 ground states. The values were then used as input values to give excellent fits of the experimental χ vs data. For , the fits of the experimental χ vs data to the Van Vleck equation or with PHI gave a very weak = -0.8(1) cm ( = -2· convention) between adjacent Fe ions and an = / ground state. These initial Fe 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.