Mn 2(Fpymo) 4(H 2O) 4]: Synthesis, structure, magnetism and thermally induced solid-to-solid polymerisation reactions

Abstract

The reaction of an aqueous solution of Mn(ClO 4) 2 · 6H 2O with 5-fluoro-2-hydroxypyrimidine (HFpymo) and NaOH in 1:2:1 ratio affords a species analysing as Mn(Fpymo) 2(H 2O) 2 ( 1) in 70% yield. Single crystal X-ray analysis reveals that 1 consists of [Mn 2(μ-Fpymo- N 1, O 2) 2(Fpymo- O 2) 2(H 2O) 4] dinuclear units, in which each Mn(II) ion shows a slightly distorted trigonal bipyramidal stereochemistry. Thermal treatment of 1 above 150 °C gives an anhydrous, amorphous material analysing as Mn(Fpymo) 2 ( 2a). Further heating of this compound above 250 °C results in the formation of the microcrystalline Mn(Fpymo) 2 species ( 2b). The thermal dependence of the magnetic susceptibility χ has been studied for species 1 and 2b in the 2–300 K temperature range at 100, 300 and 5000 Oe field strengths. The fitting of the χ values of 1 to the Curie–Weiss equation gives values of C = 2.450(2) and θ = 1.0(2) K, which is indicative of an almost negligible magnetic interaction between the Mn(II) centres. At variance, 2b shows a significant antiferromagnetic behaviour, with a decrease of the μ eff values upon cooling. The fitting of the χ values of 2b to the Curie–Weiss equation gives the respective C and θ values of 4.26(1) and −14.8(3) K, which agrees with an efficient coupling of the magnetic Mn(II) centres, possibly through bridges of the Fpymo- N 1, N 3 kind, within a polymeric network. The N 2 and CO 2 gas adsorption measurements at 77 K and 293 K, respectively, show that the 2b phase is not microporous, which is reflected in its low BET surface (19 m 2 g −1) and its BJH pore size distribution.