American Chemical Society
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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

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journal contribution
posted on 2003-05-06, 00:00 authored by Norberto Masciocchi, Simona Galli, Angelo Sironi, Elisa Barea, Jorge A. R. Navarro, Juan M. Salas, Leandro C. Tabares
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) Å, b = 12.9617(3) Å, and c = 6.7925(2) Å] consists of D4h 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 °C) (2a) or a polycrystalline (above 320 °C) Co(4-pymo)2 phase (2b) [orthorhombic, Imma, a = 6.5720(8) Å, b = 6.6209(8) Å, and c = 20.688(2) Å]. In the latter, C2v 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, Tc = 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 Hcoer = 3900 G and a remnant magnetization Mrem = 279 cm3 G mol-1.