A monometallic lanthanide bis(methanediide) single molecule magnet with a large energy barrier and complex spin relaxation behaviour

We report a dysprosium(iii) bis(methanediide) single molecule magnet (SMM) where stabilisation of the highly magnetic states and suppression of mixing of opposite magnetic projections is imposed by a linear arrangement of negatively-charged donor atoms supported by weak neutral donors. Treatment of [Ln(BIPM^TMS)(BIPM^TMSH)] [Ln = Dy, <b>1Dy</b>; Y, <b>1Y</b>; BIPM^TMS = {C(PPh₂NSiMe₃)₂}^2-; BIPM^TMSH = {HC(PPh₂NSiMe₃)₂}^-] with benzyl potassium/18-crown-6 ether (18C6) in THF afforded [Ln(BIPM^TMS)₂][K(18C6)(THF)₂] [Ln = Dy, <b>2Dy</b>; Y, <b>2Y</b>]. AC magnetic measurements of <b>2Dy</b> in zero DC field show temperature- and frequency-dependent SMM behaviour. Orbach relaxation dominates at high temperature, but at lower temperatures a second-order Raman process dominates. Complex <b>2Dy</b> exhibits two thermally activated energy barriers (<i>U</i> _eff) of 721 and 813 K, the largest <i>U</i> _eff values for any monometallic dysprosium(iii) complex. Dilution experiments confirm the molecular origin of this phenomenon. Complex <b>2Dy</b> has rich magnetic dynamics; field-cooled (FC)/zero-field cooled (ZFC) susceptibility measurements show a clear divergence at 16 K, meaning the magnetic observables are out-of-equilibrium below this temperature, however the maximum in ZFC, which conventionally defines the blocking temperature, <i>T</i> _B, is found at 10 K. Magnetic hysteresis is also observed in 10% <b>2Dy</b>@<b>2Y</b> at these temperatures. <i>Ab initio</i> calculations suggest the lowest three Kramers doublets of the ground ⁶H_15/2 multiplet of <b>2Dy</b> are essentially pure, well-isolated |±15/2, |±13/2 and |±11/2 states quantised along the C[double bond, length as m-dash]Dy[double bond, length as m-dash]C axis. Thermal relaxation occurs <i>via</i> the 4^th and 5^th doublets, verified experimentally for the first time, and calculated <i>U</i> _eff values of 742 and 810 K compare very well to experimental magnetism and luminescence data. This work validates a design strategy towards realising high-temperature SMMs and produces unusual spin relaxation behaviour where the magnetic observables are out-of-equilibrium some 6 K above the formal blocking temperature.