<p>Energy barriers to magnetisation reversal (U<sub>eff</sub>)
in single-molecule magnets (SMMs) have vastly increased recently, but only for
the dysprosocenium SMM [Dy(Cp<sup>ttt</sup>)<sub>2</sub>][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]
(Cp<sup>ttt</sup> = C<sub>5</sub>H<sub>2</sub><sup>t</sup>Bu<sub>3</sub>-1,2,4)
has this translated into a considerable increase in magnetic hysteresis
temperatures. The lack of concomitant increases in hysteresis temperatures with
U<sub>eff</sub> 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(<sup>t</sup>BuO)Cl(THF)<sub>5</sub>][BPh<sub>4</sub>]
(<b>1</b>), [K(18-crown-6-ether)(THF)<sub>2</sub>][Dy(BIPM)<sub>2</sub>]
(<b>2</b>, BIPM = C{PPh<sub>2</sub>NSiMe<sub>3</sub>}<sub>2</sub>),
and [Dy(Cp<sup>ttt</sup>)<sub>2</sub>][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]
(<b>3</b>), 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 <b>1</b>, at intermediate energies for <b>2</b> and at higher energies for <b>3</b>, 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.</p>
Studies of hysteresis and quantum tunnelling of the magnetisation in dysprosium(iii) single molecule magnets