Pinkowicz, D.; Southerland, H.I.; Avendano, C.; Prosvirin, A.; Sanders, C.; Wernsdorfer, W.; Pedersen, K.S.; Dreiser, J.; Clerac, R.; Nehrkorn, J.; Simeoni, G.; Schnegg, A.; Holldack, K.; Dunbar, K.R.: Cyanide Single-Molecule Magnets Exhibiting Solvent Dependent Reversible "On" and "Off" Exchange Bias Behavior. Journal of the American Chemical Society 137 (2015), p. 14406-14422
10.1021/jacs.5b09378
Abstract:
The syntheses, structures and magnetic properties of four new complex salts, (PPN){[MnIII(salphen)]2[MIII(CN)6]}•7MeOH (Mn2M•7MeOH) (M = Fe, Ru, Os and Co; PPN+ = bis(triphenylphosphoranylidene)ammonium cation; H2salphen = N,N'-bis(salicylidene)-1,2-diaminobenzene) and a mixed metal Co/Os analogue (PPN){[MnIII(salphen)]2[CoIII0.9OsIII¬0.1(CN)6]} •7MeOH were undertaken. It was found that the cyanide-bridged Mn2Fe, Mn2Ru, Mn2Os and the Mn2Co/Os exhibit switchable Single-Molecule Magnet (SMM) and exchange-bias behavior depending on interstitial methanol content. The Mn2Co analogue exhibits the onset of a frequency dependence in the AC magnetic susceptibility data only at very low temperatures. The pristine (solvated) form of the OsIII trinuclear species (PPN){[Mn(salphen)(MeOH)]2[Os(CN)6]}•7MeOH (Mn2Os•7MeOH) behaves as an SMM with an effective barrier for the magnetization reversal, (Ueff/kB), of 17.1 K. Upon desolvation (PPN){[MnIII(salphen)(MeOH)]2[OsIII(CN)6]} (Mn2Os) exhibits an increase of (Ueff/kB) to 45.2 K and an opening of the hysteresis loop observable at 1.8 K at a 0.0044 T/s average magnetic field sweep rate. Mn2Os•7MeOH shows also exchange-bias behavior with magnetic hysteresis loops exhibiting a shift in the quantum tunneling to 0.25 T from zero-field. The FeIII and RuIII analogues (PPN){[MnIII(salphen)]2[FeIII(CN)6]}•7MeOH (Mn2Fe•7MeOH) and (PPN){[MnIII(salphen)]2[RuIII(CN)6]}•7MeOH (Mn2Ru•7MeOH) were prepared as reference compounds for assessing the effect of the 5d versus 4d and 3d metal ions on the SMM properties. These compounds are also SMMs and exhibit similar effects but with lower energy barriers (Ueff/kB) of 14.7 K and 22.2 K for pristine and desolvated Mn2Fe and 15.9 K and 24.4 K for pristine and desolvated Mn2Ru, respectively. These findings underscore the importance of introducing heavy transition elements into SMMs to improve their properties. The (PPN){[MnIII(salphen)]2[CoIII(CN)6]}•7MeOH (Mn2Co•7MeOH) analogue with a diamagnetic CoIII central atom and the mixed Co/Os (PPN){[MnIII(salphen)]2[CoIII0.9OsIII¬0.1(CN)6]} •7MeOH (Mn2Co/Os•7MeOH) “magnetically diluted” system with a 9:1 Co:Os metal ratio were prepared in order to further probe the nature of the energy barrier increase upon desolvation of Mn2Os. While weak intermolecular interactions are responsible for the exchange bias behavior of the pristine compounds, it was found that the “on” and “off” modulation of the SMM behavior for the diluted Mn2Co/Os and undiluted Mn2Os compounds is very similar suggesting the key role of the intramolecular interactions rather than intermolecular ones. In addition, inelastic neutron scattering and frequency-domain Fourier-transform THz electron paramagnetic resonance spectra obtained on Mn2Os•7MeOH and Mn2Os in combination with the magnetic data, reveal the presence of anisotropic exchange interactions between MnIII and OsIII ions.