Synthesis, Structure, and Spectroscopic and Magnetic Characterization of [Mn₁₂O₁₂(O₂CCH₂Bu^t)₁₆(MeOH)₄]·MeOH, a Mn₁₂ Single-Molecule Magnet with True Axial Symmetry

The synthesis and properties are reported of a rare example of a Mn₁₂ single-molecule magnet (SMM) in truly axial symmetry (tetragonal, <i>I</i>4̅). [Mn₁₂O₁₂(O₂CCH₂Bu^t)₁₆(MeOH)₄]·MeOH (<b>3</b>·MeOH) was synthesized by carboxylate substitution on [Mn₁₂O₁₂(O₂CMe)₁₆(H₂O)₄]·2MeCO₂H·4H₂O (<b>1</b>). The complex was found to possess an <i>S</i> = 10 ground state, as is typical for the Mn₁₂ family, and displayed both frequency-dependent out-of-phase AC susceptibility signals and hysteresis loops in single-crystal magnetization vs DC field sweeps. The loops also exhibited quantum tunneling of magnetization steps at periodic field values. Single-crystal, high-frequency electron paramagnetic resonance spectra on <b>3·</b>MeOH using frequencies up to 360 GHz revealed perceptibly sharper signals than for <b>1</b>. Moreover, careful studies as a function of the magnetic field orientation did not reveal any satellite peaks, as observed for <b>1</b>, suggesting that the crystals of <b>3</b> are homogeneous and do not contain multiple Mn₁₂ environments. In the single-crystal ⁵⁵Mn NMR spectrum in zero applied field, three well-resolved peaks were observed, which yielded hyperfine and quadrupole splitting at three distinct sites. However, observation of a slight asymmetry in the Mn⁴⁺ peak was detectable, suggesting a possible decrease in the local symmetry of the Mn⁴⁺ site. Spin–lattice (<i>T</i>₁) relaxation studies were performed on single crystals of <b>3</b>·MeOH down to 400 mK in an effort to approach the quantum tunneling regime, and fitting of the data using multiple functions was employed. The present work and other recent studies continue to emphasize that the new generation of truly high-symmetry Mn₁₂ complexes are better models for thorough investigation of the physical properties of SMMs than their predecessors such as <b>1