Linear tricobalt compounds with Di(2-pyridyl)amide (dpa) ligands:: Temperature dependence of the structural and magnetic properties of symmetrical and unsymmetrical forms of Co3(dpa)4Cl2 in the solid state

The linear trinuclear compound Co-3(dpa)(4)Cl-2 (1; dpa = di(2-pyridyl)amide anion) crystallizes from CH2Cl2 solution in two forms simultaneously, namely, an orthorhombic form 1 . CH2Cl2 and a tetragonal form 1 . 2CH(2)Cl(2). The three linearly arranged cobalt atoms in 1 are supported by four dpa ligands in a spiral configuration. The chain of cobalt atoms is symmetrical in 1 . CH2Cl2, but unsymmetrical in 1 . 2CH(2)Cl(2). Both crystal structures have been studied at various temperatures. A reversible second-order phase transition (165 K) from orhorhombic (Pnn2) to monoclinic (Pn) symmetry for the crystal of 1 . CH2Cl2 has been documented by X-ray studies at 296, 168, and 109 K as well as a neutron diffraction study at 20 K. The linear tricobalt unit in 1 . CH2Cl2 becomes slightly unsymmetrical at low temperature although the two Co-Co bonds remain statistically equivalent (Co-Co approximate to 2.32 Angstrom) throughout the experimental temperature range. No phase transition was observed for the tetragonal form 1 . 2CH(2)Cl(2) at low temperature, but the Co-Co distances in 1 changed from 2.299(1) and 2.471(1) Angstrom at 298 K to 2.3035(7) and 2.3847(8) Angstrom at 20 K. Magnetic susceptibility measurements indicate that the two compounds are in an S = 1/2 ground state at low temperature and exhibit gradual spin-crossover at higher temperature.