Fast magnetization tunneling in tetranickel(II) single-molecule magnets

A series of Ni-4 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 approximate to -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 (S-4 site symmetry) Ni-4 complexes in the crystal of complex 3, and the tunneling rate can even be enhanced by destroying the S-4 site symmetry, as is the case for complex 4, where there are two crystallographically different Ni4 molecules, one with C-2 and the other with C-1 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 similar to 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.