Slow magnetic relaxation in a homoaxially phosphine oxide coordinated pentagonal bipyramidal Dy(III) complex.

We report the synthesis of [(L)Dy III (Cy 3 PO) 2 ]·[BPh 4 ] (1-Dy) (where H 2 L = 2,6-diacetylpyridine bis-benzoylhydrazone and Cy = cyclohexyl) which crystallized in the triclinic, P 1̄ space group. The local geometry around Dy(III) in 1-Dy was found to be pentagonal bipyramidal (pseudo- D 5h ). The AC magnetic susceptibility measurements performed on 1-Dy and on its diluted 1-Y(Dy) samples showed a typical single-molecule magnet signature revealed by the appearance of AC-frequency dependent out-of-phase susceptibility signals in the absence of a static magnetic field. The out-of-phase AC susceptibility signals were well resolved on the application of a small magnetic field ( H DC = 500 Oe) and yielded an energy barrier for magnetization flipping of U eff / k B = 50 K for the diluted derivative. The magnetic studies on 1-Dy and 1-Y(Dy) and data analysis further confirm that Raman and QTM under-barrier magnetic relaxations play a crucial role in lowering U eff despite the almost axial nature of the Dy(III) ion in 1-Dy. We have rationalized these observations through detailed ab initio calculations performed on the X-ray crystal structure of 1-Dy.