A Unique Dinuclear CuII Compound with Two Phase Transitions Clearly Visible from Thermal and Magnetic Behaviour; Synthesis, Magnetism and X-ray Structures at Three Temperatures of Bis(di-2-pyridylamine)bis(μ-hydroxo)bis(μ-perchlorato-O,O)dicopper(II)

A new compound, [Cu(dpyam)(OH)]2(ClO4)2 (dpyam = di-2-pyridylamine) is studied in detail and characterised by magnetic susceptibility and heat capacity measurements. This unique compound exhibits two phase transitions at 192 and 116 K. To obtain more insight into the origin of this transition, X-ray crystal structures at 3 different temperatures, below (1A, 110 K), between (1B, 150 K) and above (1C, 200 K) the transitions are measured. The structure consists of a dinuclear CuII unit with two bridging hydroxo groups and with two ligands bound to each copper atom via N, while the axial sites are occupied by oxygen atoms of bridging perchlorate anions, providing a distorted tetragonal environment with a Cu−Cu distance which varies from 2.9328(6) to 2.9633(6) Å, and a Cu−O−Cu angle which varies from 99.00(9) to 100.53(9)°. In all three structures, the Cu2O2L2 unit does not change significantly. The only changes involve the position of the perchlorate anions and their H bonds with the bridging OH groups. These anions have slightly different positions at different temperatures and appear to be the major origin of the crystallographic phase-transitions. Fitting the magnetic susceptibility measurements revealed a singlet-triplet separation (J) of −37.2 cm−1 (antiferromagnetic), in the temperature ranges 275−190 K and 115−50 K. In the range 190−115 K a best fit, albeit inaccurate, of 29.3 cm−1 for J (ferromagnetic) is found. A small magnetic hysteresis of 0.5 K has been observed in the phase transition at 116 K. The heat capacity measurements have confirmed the transitions and the excess entropy (ΔS) and enthalpy (ΔH) have been calculated as 4.9 and 2.0 J K−1 mol−1 for ΔS (for 192.3 K and 116.4 K, respectively), and 0.96 and 0.25 kJ mol−1 for ΔH (for 192.3 K and 116.4 K, respectively). (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)