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 CH 2 Cl 2 solution in two forms simultaneously, namely, an orthorhombic form 1 ·CH 2 Cl 2 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 ·CH 2 Cl 2, 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 orthorhombic ( Pnn 2) to monoclinic ( Pn ) symmetry for the crystal of 1 ·CH 2 Cl 2 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 ·CH 2 Cl 2 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 ·2CH 2 Cl 2 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.