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 [(?4-Cb'''')Dy(?5-C5Me4tBu)(BH4)]- (1), [(?4-Cb'''')Dy(?8-Pn†)K(THF)] (2) and [(?4-Cb'''')Dy(?8-Pn†)]- (3) leads to larger energy barriers to magnetization reversal (Cb''''=C4(SiMe3)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 Zrel are introduced to facilitate comparisons of the SMM performance of 1–3 with a benchmark SMM. Increases in Z and Zrel when Cb''' replaces Cp signposts a route to SMMs with properties that could surpass leading systems.