Disorder and intermolecular interactions in a family of tetranuclear Ni(II) complexes probed by high-frequency electron paramagnetic resonance

High-frequency electron paramagnetic resonance (HFEPR) data are presented for four closely related tetranuclear Ni(II) complexes, [Ni(hmp)(MeOH)Cl-4 center dot H2O (1a), [Ni(hmp)(MeOH)Br](4) center dot H2O (1b), [Ni(hmp)(EtOH)Cl](4) center dot H2O (2), and [Ni(hmp)(dmb)Cl](4) (3) (where hmp(-) is the anion of 2-hydroxymethylpyridine and. dmb is 3,3'-dimethyl-1-butanol), which exhibit magnetic bistability (hysteresis) and fast magnetization tunneling at low temperatures, properties which suggest they are single-molecule magnets (SMMs). The HFEPR spectra confirm spin S = 4 ground states and dominant uniaxial anisotropy (D (S) over cap (2)(z), D < 0) for all four complexes, which are the essential ingredients for a SMM. The individual fine structure peaks (due to zero-field splitting) for complexes la, 1b, and 2 are rather broad. They also exhibit further (significant) splitting, which can be explained by the fact that there exists two crystallographically distinct Ni-4 sites in the lattices for these complexes, with associated differences in metal-ligand bond lengths and different zero-field splitting (ZFS) parameters. The broad EPR lines, meanwhile, may be attributed to ligand and solvent disorder, which results in additional distributions of microenvironments. In the case of complex 3, there are no solvate molecules in the structure, and only one distinct Ni4 molecule in the lattice. Consequently, the HFEPR data for complex 3 are extremely sharp. As the temperature of a crystal of complex 3 is decreased, the HFEPR spectrum splits abruptly at similar to 46 K into two patterns with very slightly different ZFS parameters. Heat capacity data suggest that this is caused by a structural transition at 46.6 K. A single-crystal X-ray structure at 12(2) K indicates large thermal parameters on the terminal methyl groups of the dmb (3,3-dimethyl-1-butanol) ligand. Most likely there exists dynamic disorder of parts of the dmb ligand above 46.6 K; an order-disorder structural phase transition at 46.6 K then removes some of the motion. A further decrease in temperature (<6 K) leads to further fine structure splittings for complex 3. This behavior is thought to be due to the onset of short-range magnetic correlations/coherences between molecules caused by weak intermolecular magnetic exchange interactions.