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

摘要

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, [LnFc _(3) (THF) _(2) Li _(2) ] ~(?) , 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 (Fc ~(2?) ) average twist angle with decreasing ionic radius ( r _(ion) ) of the central Ln ion, resulting in the largest average Fc ~(2?) twist angles for the Lu ~(3+) compound 6 . Such high sensitivity of the twist angle to changes in r _(ion) is unique to the here presented ferrocenophane complexes and likely due to the large trigonal plane separation enforced by the ligand (>3.2 ?). This geometry also allows the non-Kramers ion Ho ~(3+) 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 [HoFc _(3) (THF*) _(2) Li _(2) ] ~(?) ( 2-THF* ) and [HoFc _(3) (py) _(2) Li _(2) ] ~(?) ( 2-py ) motifs. We find that larger average Fc ~(2?) twist angles (in 2-THF* and 2-py as compared to in 2 ) result in faster magnetic relaxation times at a given temperature.