Synthesis, structure, thermal, magnetic properties and quantum mechanical calculations of bridged [bis(di(2-methylpyridyl)amine)-(μ2-1,2-bis(4-pyridyl)ethane)-tetraperchlorato-dicopper(II)] dihydrate complex

摘要

The title compound [Cu2(DPA)2(μ-bpe)(ClO4)4]·2H2O, I where DPA = di(2-methylpyridyl)amine and bpe = 1,2-bis(4-pyridyl)ethane was studied by single crystal X-ray crystallography at different temperatures, spectroscopic methods, differential scanning calorimetry (DSC) and by magnetic susceptibility methods. DSC studies were recorded between 125 K and 647 K and magnetic studies between 2 K and 300 K. The X-ray diffraction studies were carried out at 293, 200, 150 and 100 K. Cooling the crystal from 293 K to 100 K produces a significant change in cell constants but no change in space group. X-ray and DSC studies revealed that significant phase transitions occur at temperatures between 200 K and 100 K that are described herein. The X-ray studies revealed a thermally-induced case of polymorphism whereby the cell volume triples on going from 293 to 100 K. This transition was reversible even though there is a marked hysteresis in the process. The major structural change between 200 K and 100 K is due to the displacement of the central bpe pyridyl rings relative to the plane defining CuN1N2N3 and the bending angle Cu-N1-C3. This transitional phase change was indirectly supported by quantum mechanical calculations which indicated the absence of significant π-π stacking interaction between the central bpe pyridyl rings forming the two parallel dinuclear species. This was attributed to the long 8.28 A distances between the two indicated moieties. Lack of energetic and spatial constraints justified the pyridyl ring displacement along the N-C3 axis. The temperature dependence of the magnetic behavior of I was that of a simple paramagnet down to 2 K. The complex revealed a very weak to non-existent magnetic coupling between the Cu(II) centers (J = -0.34 cm~(-1)).