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

We report a monometallic dysprosium( iii ) single molecule magnet with record energy barriers and unusual 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 TMS H)] [Ln = Dy, 1Dy ; Y, 1Y ; BIPM TMS = C(PPh 2 NSiMe 3 ) 2 2− ; BIPM TMS H = HC(PPh 2 NSiMe 3 ) 2 − ] with benzyl potassium/18-crown-6 ether (18C6) in THF afforded [Ln(BIPM TMS ) 2 ][K(18C6)(THF) 2 ] [Ln = Dy, 2Dy ; Y, 2Y ]. AC magnetic measurements of 2Dy 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 2Dy exhibits two thermally activated energy barriers ( U eff ) of 721 and 813 K, the largest U eff values for any monometallic dysprosium( iii ) complex. Dilution experiments confirm the molecular origin of this phenomenon. Complex 2Dy 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, T B is found at 10 K. Magnetic hysteresis is also observed in 10Dy @ 2Y at these temperatures. Ab initio calculations suggest the lowest three Kramers doublets of the ground 6 H 15/2 multiplet of 2Dy are essentially pure, well-isolated textbar±15/2〉, textbar±13/2〉 and textbar±11/2〉 states quantised along the CDyC axis. Thermal relaxation occurs via the 4 th and 5 th doublets, verified experimentally for the first time, and calculated U 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.