Trends in trigonal prismatic Ln-[1]ferrocenophane complexes and discovery of a Ho3+ single-molecule magnet

Abstract

Lanthanide metallocenophanes are an intriguing class of organometallic complexes that feature rare six-coordinate trigonal prismatic coordination environments of 4f elements with close intramolecular proximity to transition metal ions. Herein, we present a systematic study of the structural and magnetic properties of the ferrocenophanes, [LnFc3(THF)2Li2]-, of the late trivalent lanthanide ions (Ln = Gd (1), Ho (2), Er (3), Tm (4), Yb (5), Lu (6)). One major structural trend within this class of complexes is the increasing diferrocenyl (Fc2-) average twist angle with decreasing ionic radius (rion) of the central Ln ion, resulting in the largest average Fc2- twist angles for the Lu3+ compound 6. Such high sensitivity of the twist angle to changes in rion is unique to the here presented ferrocenophane complexes and likely due to the large trigonal plane separation enforced by the ligand (>3.2 Å). This geometry also allows the non-Kramers ion Ho3+ to exhibit slow magnetic relaxation in the absence of applied dc fields, rendering compound 2 a rare example of a Ho-based single-molecule magnet (SMM) with barriers to magnetization reversal (U) of 110-131 cm-1. In contrast, compounds featuring Ln ions with prolate electron density (3-5) don't show slow magnetization dynamics under the same conditions. The observed trends in magnetic properties of 2-5 are supported by state-of-the-art ab initio calculations. Finally, the magneto-structural relationship of the trigonal prismatic Ho-[1]ferrocenophane motif was further investigated by axial ligand (THF in 2) exchange to yield [HoFc3(THF∗)2Li2]- (2-THF∗) and [HoFc3(py)2Li2]- (2-py) motifs. We find that larger average Fc2- twist angles (in 2-THF∗ and 2-py as compared to in 2) result in faster magnetic relaxation times at a given temperature.