Previous work in our lab has highlighted the advantage of tailoring ligand field symmetry to metal ground J charge distribution in the design of mononuclear f-element single-molecule magnets. As further demonstration of this approach, we present magnetic characterization of a series of homoleptic sandwich complexes [Ln(COT)2]? (COT2? = cyclooctatetraene dianion). The predominantly equatorial ligand field is highly selective for engineering slow relaxation in the prolate ErIII ion. Notably, at low temperatures, magnetic dilution of the ErIII? complex substantially quenches fast zero-field relaxation, leading to magnetic hysteresis as high as 10 K.Upon moving to an axial ligand field afforded by the scorpionate ligands dihydrobis(methylimidazolyl)borate ([BcMe]�) and dihydrobis(methypyrazolyl)borate ([BpMe]�), slow relaxation is now selective for the oblate ions TbIII, DyIII, and UIII over ErIII. Importantly, while these ligands generate isostructural series of complexes, relaxation of the magnetization is orders of magnitude slower for complexes of the N-heterocyclic carbene donor, [BcMe]�. Further characterization through EPR and 1H NMR spectroscopies suggests greater magnetic anisotropy is at the root of this slower relaxation in both the lanthanide and uranium complexes. These results collectively highlight that, in addition to ligand field symmetry, donor strength can be tuned to optimize magnetic anisotropy and design better single-molecule magnets.

Impact of ligand field symmetry and donor strength on slow magnetic relaxation in f-element complexes

248th National Meeting (2014)

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