Coordination frameworks containing the pyrimidine-4-olate ligand. Synthesis, thermal, magnetic, and ab-initio XRPD structural characterization of nickel and zinc derivatives

Extended coordination frameworks containing the pyrimidin-4-olate ligand (4-pymo) and ZN(II) and NI(II) metal ions have been obtained by solid state reactions and have been fully characterized by spectroscopic, thermal, and magnetic measurements and by ab initio XRPD. The reaction of ZnO and 4-Hpymo at 140 °C gives a solid microcrystalline phase, Zn(4-pymo)2 (1). Its 3D framework contains ZN(II) centers linked by 4-pymo ligands acting in two different coordination modes, namely, the N,N′- and the N,O-exo-bidentate ones, which result in a pseudotetrahedral ZnN3O chromophore. Thermal treatment of the "molecular" Ni(4-pymo)2(H2O)4 complex (2) above 140 °C gives an anhydrous amorphous material analyzing as Ni(4-pymo)2 (3a). Further heating of this material above 388 °C results in the formation of the microcrystalline layered Ni(4-pymo)2 species (3b), in which NI(II) centers are bridged by N,O-exo-bidentate 4-pymo ligands (assisted by longer Ni⋯N contacts). The thermal dependence of the magnetic susceptibility has been studied for the paramagnetic species 2 and 3a. 2 shows a weak antiferromagnetic interaction [J = -0.313(5) cm^-1] transmitted through the multiple H-bonding interactions between the exocyclic pyrimidine and water oxygen atoms coordinated to the metal centers. 3a behaves as a 2D Heisenberg antiferromagnet with J = -4.11(3) cm^-1.