Rich structural and magnetic chemistry of cobalt(II) pyrimidin-2-olate and pyrimidin-4-olate complexes. Synthesis, X-ray powder diffraction studies, and thermal behavior

Two new cobalt(II) species containing the pyrimidin-4-olate ligand (4-pymo) have been prepared and fully characterized by spectroscopic, thermal, and ab initio X-ray powder diffraction methods. The magnetic properties of both these species and Co(2-pymo)(2), an extended cobalt(II) compound containing the pyrimidine-2-olate ligand (2-pymo), are also reported. Co(4-pymo)(2)(H2O)(4) (1) [orthorhombic, Pcab, a = 13.5233(4) Angstrom, b = 12.9617(3) Angstrom, and c = 6.7925(2) Angstrom] consists of D-4h octahedral monomers, bearing axial 4-pymo ligands, interlinked by an extensive network of OH...X (X = O, N) hydrogen bonds. Upon heating, it loses water and transforms into an amorphous (above 150 degreesC) (2a) or a polycrystalline above 320 degreesC) Co(4-pymo)(2) phase (2b) [orthorhombic, Imma, a = 6.5720(8) Angstrom, b = 6.6209(8) Angstrom, and c = 20.688(2) Angstrom]. In the latter, C-2v pseudo-tetrahedral cobalt(II) ions are linked by 4-pymo ligands in the unusual N,O-exo-bidentate mode, generating 2D layers of nearly square meshes, thus significantly differing from the Co(2-pymo)(2) analogue (3), in which N,N'-exo-bidentate bridges generate an acentric, 3D diamondoid network. The thermal dependence of the magnetic susceptibility has been studied for all the above compounds (1, 2a, 2b, and 3) in the 2-300 K temperature range. The magnetic behavior of 1 is dominated by spin-orbit coupling of magnetically isolated octahedral Co(II) centers. The extended materials 2a and 2b show antiferromagnetic exchange between distorted tetrahedral metal centers, whereas 3 behaves as a spin-canted antiferromagnet, a ferromagnetic ordering taking place below a critical temperature, T-c = 23 K; 3 can thus be considered as a molecular magnet. Indeed, magnetic hysteresis studies on 3 at 4.8 K yield a coercitive field H-coer = 3900 G and a remnant magnetization M-rem = 279 cm(3) G mol(-1).