Exchange bias in Ni4 single-molecule magnets

The syntheses and physical properties are reported for three single-molecule magnets (SMMs) with the composition Ni(hmp)(ROH)Cl]₄, where R is CH₃ (complex 1), CH₂CH₃ (complex 2) or CH₂CH₂C(CH₃)₃ (complex 3) and hmp^? is the monoanion of 2-hydroxymethylpyridine. The core of each complex is a distorted cube formed by four Ni^II ions and four alkoxide hmp^? oxygen atoms at alternating corners. Ferromagnetic exchange interactions give a S=4 ground state. Single crystal high-frequency EPR spectra clearly indicate that each of the complexes has a S=4 ground state and that there is negative magnetoanisotropy, where D is negative for the axial zero-field splitting D?_z². Magnetization versus magnetic field measurements made on single crystals with a micro-SQUID magnetometer indicate these Ni₄ complexes are SMMs. Exchange bias is seen in the magnetization hysteresis loops for complexes 1 and 2.