Zero-Field Quantum Tunneling of the Magnetization in a Series of High Energy-Barrier Dysprosium (III) Single-Molecule Magnets

Energy barriers to magnetisation reversal (U_eff) in single-molecule magnets (SMMs) have vastly increased recently, but only for the dysprosocenium SMM [Dy(Cp^ttt)₂][B(C₆F₅)₄] (Cp^ttt = C₅H₂^tBu₃-1,2,4) has this translated into a considerable increase in magnetic hysteresis temperatures. The lack of concomitant increases in hysteresis temperatures with U_eff values is due to efficient magnetic relaxation at zero-field, referred to as quantum tunnelling of the magnetisation (QTM); however, the exact nature of this phenomenon is unknown. Recent hypotheses suggest that both transverse dipolar magnetic fields and hyperfine coupling play a significant role in this process for Dy(III) SMMs. Here, by studying the compounds [Dy(^tBuO)Cl(THF)₅][BPh₄] (1), [K(18-crown-6-ether)(THF)₂][Dy(BIPM)₂] (2, BIPM = C{PPh₂NSiMe₃}₂), and [Dy(Cp^ttt)₂][B(C₆F₅)₄] (3), we show conclusively that neither of these processes are the main contributor to zero-field QTM for Dy(III) SMMs, and suggest that its origin instead owes to molecular flexibility. By analysing the vibrational modes of the three molecules, we show that the modes that most impact the magnetic ion occur at the lowest energies for 1, at intermediate energies for 2 and at higher energies for 3, in correlation with their ability to retain magnetisation. Therefore, we conclude that SMM performance could be improved by employing more rigid ligands with higher-energy metal-ligand vibrational modes.