Fast Magnetization Tunneling in Tetranickel(II) Single-Molecule Magnets

A series of Ni4 cubane complexes with the composition [Ni(hmp)(ROH)Cl]4 complexes 1−4 where R= −CH3 (complex
1), −CH2CH3 (complex 2), −CH2CH2(C4H9) (complex 3), −CH2CH2CH2(C6H11) (complex 4), hmp- is the anion of
2-hydroxymethylpyridine, t-Buhmp- is the anion of 4-tert-butyl-2-hydroxymethylpyridine, and dmb is 3,3-dimethyl-1-butanol] and [Ni(hmp)(dmb)Br]4 (complex 5) and [Ni(t-Buhmp)(dmb)Cl]4 (complex 6) were prepared. All six complexes
were characterized by dc magnetic susceptibility data to be ferromagnetically coupled to give an S = 4 ground
state with significant magnetoanisotropy (D ≈ −0.6 cm-1). Magnetization hysteresis measurements carried out on
single crystals of complexes 1−6 establish the single-molecule magnet (SMM) behavior of these complexes. The
exchange bias observed in the magnetization hysteresis loops of complexes 1 and 2 is dramatically decreased to
zero in complex 3, where the bulky dmb ligand is employed. Fast tunneling of magnetization is observed for the
high-symmetry (S4 site symmetry) Ni4 complexes in the crystal of complex 3, and the tunneling rate can even be
enhanced by destroying the S4 site symmetry, as is the case for complex 4, where there are two crystallographically
different Ni4 molecules, one with C2 and the other with C1 site symmetry. Magnetic ordering temperatures due to
intermolecular dipolar and magnetic exchange interactions were determined by means of very low-temperature ac
susceptibility measurements; complex 1 orders at 1100 mK, complex 3 at 290 mK, complex 4 at ∼80 mK, and
complex 6 at <50 mK. This confirms that bulkier ligands correspond to more isolated molecules, and therefore,
magnetic ordering occurs at lower temperatures for those complexes with the bulkiest ligands.