Enhancing Magnetic Hysteresis in Single-Molecule Magnets by Ligand Functionalisation

The design criteria for single-molecule magnets (SMMs) of dysprosium(III) to show large thermal energy barriers to magnetic reversal have been established and proven, and the challenge to enhance their performance is understanding and control of vibrational modes that govern slow relaxation. We have prepared a new SMM, [Dy(L)2(py)5][BPh4] 1 (where HL = (S)-(-)-1-phenylethanol), which is based on the archetype [Dy(OtBu)2(py)5][BPh4] 2. Compounds 1 and 2 have similarly large energy barriers of Ueff = 1130(20) cm-1 and Ueff = 1250(10) cm-1, and yet 1 shows magnetic hysteresis at a far higher temperature of 22 K (cf. TH = 4 K for 2). Ab initio calculation of the spin dynamics show that substitution of the alkoxide ligand in fact enhances relaxation over the energy barrier for 1 over 2, in agreement with experiment, and that the higher temperature of magnetic hysteresis owes to reduced quantum tunnelling at low temperatures.