Size‐Controlled Hapticity Switching in [Ln(C₉H₉)(C₈H₈)] Sandwiches

Abstract

Sandwich complexes of lanthanides have recently attracted a considerable amount of interest due to their applications as Single Molecule Magnet (SMM). Herein, a comprehensive series of heteroleptic lanthanide sandwich complexes ligated by the cyclononatetraenyl (Cnt) and the cyclooctatetraenyl (Cot) ligand [Ln(Cot)(Cnt)] (Ln=Tb, Dy, Er, Ho, Yb, and Lu) is reported. The coordination behavior of the Cnt ligand has been investigated along the series and shows different coordination patterns in the solid‐state depending on the size of the corresponding lanthanide ion without altering its overall anisotropy. Besides the characterization in the solid state by single‐crystal X‐ray diffraction and in solution by 1H NMR, static magnetic studies and ab initio computational studies were performed.

Molecular Lanthanide Switches for Magnetism and Photoluminescence

“…Upon addition of THF to solid samples of these [(CNT)Ln(η 8 ‐COT)] (Ln=La, Ce, Nd, Tb, Er) complexes, an immediate colour change of the insoluble materials can be observed (La: orange to colourless; Ce: green to light‐yellow (at −10 °C) or green to red (at room temperature, see below); Nd: yellow to green; Tb: orange to colourless; Er: orange to light‐pink). A colour change is often indicative for the transformation of the chemical environment of lanthanide ions; in the present scenario, the colour change is very likely due to THF coordination [10a,c] …”

“…Lifetimes of the excited states of 4 were determined to be ≈95 μs at room temperature and ≈570 μs at 77 K (Table S15), being in the range of literature values [18] . Although PLE spectra differ in energetic position and band shape, which is assigned to the different ligands, emission spectra of 4 and [Tb(COT)I(thf) 2 ] [10c] look very alike, confirming the emission taking place from the lanthanide centre regardless of the differences in the coordination environment (see Figure S19).…”

“…This observation is a rather unexpected finding in view of the classic chemistry of lanthanide‐cyclopentadienyl derivatives, which generally coordinates to the lanthanide ion in almost all cases in an irreversible manner [12] . Inspired by the labile nature of the CNT ligand, we envisioned that this original property could also be observable in heteroleptic trivalent sandwich compounds of the type [(CNT)Ln III (η 8 ‐COT)], which were recently reported [10b, c] …”

“…In this work, a key feature is that the CNT ligand in homoleptic sandwich compounds can be readily replaced by tetrahydrofuran or acetonitrile [10a] . In the case of trivalent dysprosium, a partial displacement has also been observed in the heteroleptic compound [(CNT)Dy III (η 8 ‐COT)] [10c] . This observation is a rather unexpected finding in view of the classic chemistry of lanthanide‐cyclopentadienyl derivatives, which generally coordinates to the lanthanide ion in almost all cases in an irreversible manner [12] .…”

Back to the future of organolanthanide chemistry

“…183 The use of heteroleptic ligands for the formation of neutral -sandwich complexes was also investigated and led to good SMMs properties, 184,185 particularly with the large monoanionic Cnt ligand. 186,187 The Cot-Er pair is highly relevant as an excellent example of the perfect adequation between the metal and the ligand coordination for maximal anisotropy: this is referred to as ligand-metal pair anisotropy. 188,189 The concept has been extended to divalent thulium, an ion with prolateshaped density.…”