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

Abstract

Reactions of tert-butylphosphonic acid (^tBuPO₃H₂) with two different Fe(III) precursors have been investigated. The reaction of precursor complex [Fe₃(μ₃-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]·3.5py (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]·4py·5H₂O (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 T product in 2 reveals dominant antiferromagnetic interactions between Fe(III) centers.