Synthesis and crystal structure of two synthetic oxofluoride framework compounds--Co2TeO3F2 and Co2SeO3F2.

Two new isostructural Co(2+) containing tellurium and selenium oxofluoride compounds Co(2)TeO(3)F(2) and Co(2)SeO(3)F(2) are synthesized and their structures determined by single crystal X-ray diffraction. They crystallize in the orthorhombic space group Pnma with the unit cell parameters a = 7.3810(5) Å, b = 10.1936(7) Å, c = 5.3013(3) Å and a = 7.2655(8) Å, b = 10.0013(13) Å, c = 5.3564(6) Å, respectively. The Co(II) ion has octahedral coordination [CoO(3)F(3)] and builds up a 3D framework by corner- and edge sharing. The Se(IV) and the Te(IV) ions have the coordinations [SeO(3)E] and [TeO(3)E] respectively where E is the lone-pair electrons. The Se(IV) and Te(IV) ions are isolated from each other and bond only to the [CoO(3)F(3)] polyhedra. The electronegative element fluorine takes the role of a network builder like oxygen and helps to form the 3D framework structure. This is a difference compared to many oxohalide compounds containing Cl and Br where the halide ions are terminating ions preventing a 3D network from being formed. Long range antiferromagnetic interactions dominate at temperatures < 20 K. The magnetic susceptibility follows the Curie-Weiss law above 25 K with the Curie constant C = 5.62 emu K mol(-1), the Weiss temperature θ = -56 K and the effective magnetic moment μ(eff) = 4.74 μ(B) per cobalt atom.