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 Co₃(dpa)₄Cl₂ (1; dpa = di(2-pyridyl)amide anion) crystallizes from CH₂2Cl₂ solution in two forms simultaneously, namely, an orthorhombic form 1·CH₂Cl₂ and a tetragonal form 1·2CH₂Cl₂. 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 1·2CH₂Cl₂. Both crystal structures have been studied at various temperatures. A reversible second-order phase transition (165 K) from orthorhombic (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 ≃ 2.32 Å) throughout the experimental temperature range. No phase transition was observed for the tetragonal form 1·2CH₂Cl₂ at low temperature, but the Co-Co distances in 1 changed from 2.299(1) and 2.471(1) Å at 298 K to 2.3035(7) and 2.3847(8) Å 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.