Nuclearity control in molecular iron phosphates through choice of iron precursors and ancillary ligands

Abstract

One to five, one at a time: The choice of iron precursors and the amine co-ligand are employed as switches to control the nuclearity in iron phosphate molecular clusters. Using this control, it is possible to isolate selectively either a mono, di, tri, tetra, or pentanuclear cluster, for a given set of reactants. The potential of a mono-organophosphate ester in assembling low to medium nuclearity iron complexes with novel topological architectures has been investigated. Reaction of 2,6-diisopropylphenyl dihydrogen phosphate (dipp-H₂) with ferrous acetate under an inert atmosphere resulted in the formation of mononuclear, [Fe^||(dipp-H)₂(py)₄] (1) (py=pyridine), dinuclear [Fe^|||₂O(dipp-H)₄(3,5-dmpz)₃(thf)]·(3,5-dmpz)(thf)₃ (2) (3,5-dmpz=3,5-dimethylpyrazole, thf=tetrahydrofuran), and trinuclear [Fe^|||₂Fe^||O(dipp-H)₆(thf)₃]·(collidine) (3) complexes by changing the ancillary amine used in the reaction. Use of a preformed μ3-oxo bridged trinuclear complex, [Fe^|||₃O(O₂CR)₆(H₂O)₃]X (X=Cl and NO₃), as the precursor yielded two tetranuclear iron phosphates, [Fe^|||₄O(dipp)₃(py)₄(PhCOO)₄]·(toluene)₃ (4) and [Fe^III₄O(dipp)₃(OAc)₄(py)₄]·(py)₂ (5), having a core structure similar to those found in tetranuclear iron phosphonates. When FeCl₃·₆H₂O was used as the iron precursor, an unprecedented pentanuclear iron phosphate complex, [Fe^|||₅O(dipp)₆(py)₄Cl₂][pyH] (6) with a novel structure was isolated. Another pentanuclear complex having a similar core structure, [Fe₅O(dipp)₆(HO₂CPh)₃(CH₃CN)₃Cl] (7) was isolated on treatment of dipp-H₂ with the triangular complex, [Fe^|||₃O(O₂CPh)₆(H₂O)₃]Cl in the absence of any added amine co-ligand. Compounds 1-7 have been characterized by analytical techniques, spectroscopic studies, single crystal X-ray diffraction studies, and magnetic measurements. The Mössbauer studies carried out at room temperature support the formulation of all the compounds.