[Mn2(Fpymo)4(H2O)4]: synthesis, structure, magnetism and thermally induced solid-to-solid polymerisation reactions

The reaction of an aqueous solution of Mn(ClO4)2 · 6H2O with 5-fluoro-2-hydroxypyrimidine (HFpymo) and NaOH in 1:2:1 ratio affords a species analysing as Mn(Fpymo)2(H2O)2 (1) in 70% yield. Single crystal X-ray analysis reveals that 1 consists of [Mn2(μ-Fpymo-N1,O2)2(Fpymo-O2)2(H2O)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 μeff 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-N1,N3 kind, within a polymeric network. The N2 and CO2 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 m2 g-1) and its BJH pore size distribution.