Single-Molecule Magnets: High-Field Electron Paramagnetic Resonance Evaluation of the Single-Ion Zero-Field Interaction in a Zn3Ni Complex

High-field electron paramagnetic resonance (HFEPR) spectra were collected at several frequencies for single crystal [Zn3.91Ni0.09(hmp)4(dmb)4Cl4] (1), where dmb is 3,3-dimethyl-1-butanol and hmp is the monoanion of 2-hydroxymethylpyridine. This crystal is isostructural to [Ni4(hmp)4(dmb)4Cl4] (2), which has been characterized to be a single-molecule magnet (SMM) with fast quantum tunneling of its magnetization (QTM). The single NiII ion zero-field-splitting (zfs) parameters D [= -5.30(5) cm-1] and E [= +/-1.20(2) cm-1] in the doped complex 1 were evaluated by rotation of a crystal in three planes. The easy-axes of magnetization associated with the single-ion zfs interactions were also found to be tilted 15 degrees away from the crystallographic c-direction. This inclination provides a possible explanation for the fast QTM observed for complex 2. The single-ion zfs parameters are then related to the zfs parameters for the Ni4 molecule by irreducible tensor methods to give D = -0.69 cm-1 for the S = 4 ground state of the SMM, where the axial zfs interaction is given by DSz2.