A Planar Carboxylate-Rich Tetrairon(II) Complex and Its Conversion to Linear Triiron(II) and Paddlewheel Diiron(II) Complexes

We report a series of oligonuclear carboxylate-rich high-spin iron(II) complexes with three different [FeIIn(μ-O2Cbiph)2n(L)m] (n = 2−4; m = 2 or 4) structural motifs, where -O2Cbiph is 2-biphenylcarboxylate and L is an
exogenous ligand bound to terminal iron atoms. Solid compounds were isolated and their structural, spectroscopic,
and magnetic properties thoroughly investigated. The discrete tetranuclear complexes [Fe4(μ-O2Cbiph)8(L)2] crystallize
in a planar tetrairon(II) motif in which two diiron paddlewheel units are linked in an unprecedented manner involving
a μ3-1,1,3-bridging mode. X-ray crystallography reveals average Fe−Oanti bond lengths of 2.081[2] Å at the dimer−dimer interface. Terminal axial positions are capped by ligands L, where L is tetrahydrofuran (THF) (1), indazole
(2), pyrazole (3), 3,5-dimethylpyrazole (4), or acetamide (5). Reaction of 1 with an excess of acetonitrile affords the
linear compound [Fe3(μ-O2Cbiph)6(MeCN)4] (6). The acetonitrile ligands in 6 can be replaced by THF or
dimethoxyethane at elevated temperatures with retention of the structure to afford 7 and 8, respectively. Reaction
of 1 or 6 with pyridine or 1-methylimidazole results in the isolation of paddlewheel dimers 9 and 10, respectively,
with [Fe2(μ-O2Cbiph)4(L)2] composition. Mössbauer spectroscopy confirms the presence of high-spin ferrous ions
and indicates that the two iron sites of the dimer are geometrically indistinguishable. For the tri- and tetrairon
compounds, two quadrupole doublets are observed, suggesting that the iron centers do not have identical geometries.
Plots of magnetic susceptibility versus temperature reveal intramolecular antiferromagnetic exchange coupling for
all complexes under study. The magnetic data were fit to a theoretical model incorporating exchange coupling,
single-ion zero-field splitting, and g-tensor anisotropy. The resulting magnetic parameters reveal in most cases
weak antiferromagnetic exchange coupling (J typically <3 cm-1) and dominant zero-field-splitting parameters.