Tetra- and decanuclear iron(III) phosphonates: observance of a rare P-C bond cleavage in a homogeneous medium.

Reactions of tert-butylphosphonic acid (tBuPO3H2) with two different Fe(III) precursors have been investigated. The reaction of precursor complex [Fe3(mu3-O)(O2CPh)6(H2O)3]Cl with tBuPO3H2 in pyridine (py) leads to the formation of tetranuclear iron(III)phosphonate [Fe4O(tBuPO3)3(O2CPh)3(py)3Cl] x 3.5 py (1) as single crystals. The change of the Fe(III) source to FeCl3 under similar reaction conditions results in the isolation of decanuclear complex [Fe10(OH)8(HPO4)(tBuPO3)8(tBuPO3H)4(py)8] x 4 py x 5 H2O (2). Compounds 1 and 2 have been characterized by elemental analysis, spectroscopic studies, and single-crystal X-ray diffraction studies. While the structure of 1 could be described as a tetrahedral cluster supported by benzoate and phosphonate ligands, the molecular structure of 2 is unprecedented in metal phosphonate chemistry. In the course of formation of 2, tBuPO3H2 undergoes a rare P-C bond cleavage at room temperature and produces the phosphate anion, which then acts as a template for the construction of a novel decanuclear iron-phosphate-phosphonate with a hitherto unknown core architecture. The temperature dependence of the chiT product in 2 reveals dominant antiferromagnetic interactions between Fe(III) centers.