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

Matthew Gregson; Nicholas F Chilton; Ana-Maria Ariciu; Floriana Tuna; Iain F Crowe; William Lewis; Alexander J Blake; David Collison; Eric J L McInnes; Richard E P Winpenny; Stephen T Liddle

doi:10.1039/c5sc03111g

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

Chem Sci. 2016 Jan 1;7(1):155-165. doi: 10.1039/c5sc03111g. Epub 2015 Nov 23.

Affiliations

¹ School of Chemistry , The University of Manchester , Oxford Road , Manchester , M13 9PL , UK . Email: steve.liddle@manchester.ac.uk ; Email: richard.winpenny@manchester.ac.uk.
² School of Chemistry and Photon Science Institute , The University of Manchester , Oxford Road , Manchester , M13 9PL , UK.
³ School of Electrical and Electronic Engineering and Photon Science Institute , The University of Manchester , Oxford Road , Manchester , M13 9PL , UK.
⁴ School of Chemistry , University of Nottingham , University Park , Nottingham , NG7 2RD , UK.

Abstract

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, 1Dy; Y, 1Y; 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, 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 10% 2Dy@2Y at these temperatures. Ab initio calculations suggest the lowest three Kramers doublets of the ground ⁶H_15/2 multiplet of 2Dy 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 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.