Studies of the Temperature-Dependence of the Structure and Magnetism of a Hexagonal Bipyramidal Dysprosium(III) Single-Molecule Magnet

The hexagonal-bipyramidal lanthanide(III) complex [Dy(OtBu)Cl(18-C-6)][BPh4] (18-C-6 = 1,4,7,10,13,16-hexaoxacycloocta-decane ether 1) displays an energy barrier for magnetization reversal (Ueff) of ca. 1000 K in zero DC field. Temperature dependent X-ray diffraction studies of 1 down to 30 K of compound reveal bending of the Cl-Ln-OtBu angle at low temperature. Using ab initio calculations, we show that the significant bending of the O-Dy-Cl angle upon cooling from 273 K to 100 K leads to a ca. 10% decrease in the energy of the excited electronic states. A thorough exploration of the temperature- and field-dependencies of the magnetic relaxation rate reveals that magnetic relaxation is dictated by five mechanisms in different regimes: Orbach, Raman-I, QTM and Raman-II, in addition to the observation of a phonon bottleneck effect.