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

Abstract

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).