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)­{[MnIII(salphen)(MeOH)]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)­(MeOH)]2[CoIII0.92OsIII0.08(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, (Ueff/kB), of 17.1 K. Upon desolvation, Mn2Os exhibits an increase of Ueff/kB 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 FeIII and
RuIII 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)­{[MnIII(salphen)(MeOH)]2[CoIII(CN)6]}·7MeOH (Mn2Co·7MeOH) analogue with a diamagnetic
CoIII central atom and the mixed Co/Os (PPN)­{[MnIII(salphen)(MeOH)]2[CoIII0.92OsIII0.08(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 MnIII and OsIII ions.