Crystal lattice desolvation effects on the magnetic quantum tunneling of single-molecule magnets

High-frequency electron paramagnetic resonance (HFEPR) and alternating current (ac) susceptibility measurements are reported for a new high-symmetry Mn₁₂ complex, [Mn₁₂O₁₂(O₂CCH₃)₁₆(CH₃OH)₄]⋅CH₃OH. The results are compared to those of other high-symmetry spin S=10 Mn₁₂ single-molecule magnets (SMMs), including the original acetate, [Mn₁₂(O₂CCH₃)₁₆(H₂O)₄]⋅2CH₃CO₂H⋅4H₂O, and the [Mn₁₂O₁₂(O₂CCH₂Br)₁₆(H₂O)₄]⋅4CH₂Cl₂ and [Mn₁₂O₁₂(O₂CCH₂Bu^t)₁₆(CH₃OH)₄]⋅CH₃OH complexes. These comparisons reveal important insights into the factors that influence the values of the effective barrier to magnetization reversal, U_eff, deduced on the basis of ac susceptibility measurements. In particular, we find that variations in U_eff can be correlated with the degree of disorder in a crystal which can be controlled by desolvating (drying) samples. This highlights the importance of careful sample handling when making measurements on SMM crystals containing volatile lattice solvents. The HFEPR data additionally provide spectroscopic evidence suggesting that the relatively weak disorder induced by desolvation influences the quantum tunneling interactions and that it is under-barrier tunneling that is responsible for a consistent reduction in U_eff that is found upon drying samples. Meanwhile, the axial anisotropy deduced from HFEPR is found to be virtually identical for all four Mn₁₂ complexes, with no measurable reduction upon desolvation.