Journal article

Cyanide Single-Molecule Magnets Exhibiting Solvent Dependent Reversible "On" and "Off" Exchange Bias Behavior

The syntheses, structures, and magnetic properties of four new complex salts, (PPN){[Mn-III(salphen)(MeOH)](2)[M-III(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){[Mn-III(salphen)(MeOH)](2)[(Co0.92Os0.08III)-Os-III(CN)(6)]} 7MeOH were undertaken. It was found that all compounds exhibit switchable single-molecule magnet (SMM) and exchange-bias behavior depending on the interstitial methanol content. The pristine (PPN){[Mn(salphen)(MeOH)](2)[Os(CN)(6)]} 7MeOH (Mn2Os 7MeOH) behaves as an SMM with an effective barrier for the magnetization reversal, (U-eff/k(B)), of 17.1 K. Upon desolvation, Mn2Os exhibits an increase of U-eff/k(B) to 42.0 K and an opening of the hysteresis loop observable at 1.8 K. 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 Fe-III and Ru-III analogues 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. These findings underscore the importance of introducing heavy transition elements into SMMs to improve their slow relaxation of the magnetization properties. The (PPN){[Mn-III(salphen)(MeOH)](2)[Co-III(CN)(6)]} 7MeOH (Mn2Co 7MeOH) analogue with a diamagnetic Co-III central atom and the mixed Co/Os (PPN){[Mn-III(salphen)(MeOH)](2)[(Co0.92Os0.08III)-Os-III(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. 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 revealed the presence of anisotropic exchange interactions between Mn-III and Os-III ions.


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