Dominance of Cyclobutadienyl Over Cyclopentadienyl in the Crystal Field Splitting in Dysprosium Single-Molecule Magnets.

Replacing a monoanionic cyclopentadienyl (Cp) ligand in dysprosium single-molecule magnets (SMMs) with a dianionic cyclobutadienyl (Cb) ligand in the sandwich complexes [(η ⁴ -Cb'''')Dy(η ⁵ -C ₅ Me ₄ ^t Bu)(BH ₄ )] ^- (1), [(η ⁴ -Cb'''')Dy(η ⁸ -Pn ^† )K(THF)] (2) and [(η ⁴ -Cb'''')Dy(η ⁸ -Pn ^† )] ^- (3) leads to larger energy barriers to magnetization reversal (Cb''''=C ₄ (SiMe ₃ ) ₄ , Pn ^† =1,4-di(tri-isopropylsilyl)pentalenyl). Short distances to the Cb'''' ligands and longer distances to the Cp ligands in 1-3 are consistent with the crystal field splitting being dominated by the former. Theoretical analysis shows that the magnetic axes in the ground Kramers doublets of 1-3 are oriented towards the Cb'''' ligands. The theoretical axiality parameter and the relative axiality parameter Z and Z _rel are introduced to facilitate comparisons of the SMM performance of 1-3 with a benchmark SMM. Increases in Z and Z _rel when Cb''' replaces Cp signposts a route to SMMs with properties that could surpass leading systems.
(© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)