Lanthanide(III) Sandwich and Half‐Sandwich Complexes with Bulky Cyclooctatetraenyl Ligands: Synthesis, Structures, and Magnetic Properties

Abstract

Five new lanthanide(III) sandwich and half‐sandwich complexes with bulky cyclooctatetraenyl ligands have been prepared and fully characterized, including single‐crystal X‐ray structure determination. The treatment of anhydrous lanthanide(III) chlorides, LnCl 3 (Ln = Pr, Tb, Yb), with 2 equivalents of Li 2 COT′′ [COT′′ = 1,4‐bis(trimethylsilyl)cyclooctatetraene dianion] followed by crystallization in the presence of a coordinating solvent afforded the anionic sandwich complexes [Li(THF) 4 ][Pr(COT′′) 2 ] ( 1 ), [Li(DME) 3 ][Tb(COT′′) 2 ] ( 2 ; DME = 1,2‐dimethoxyethane), and [Li(TMEDA) 2 ][Yb(COT′′) 2 ] ( 3 ; TMEDA = N , N , N′ , N′ ‐tetramethylethylenediamine). Attempted oxidation of 2 with silver iodide did not afford the neutral terbium(IV) sandwich complex [Tb(COT′′) 2 ]. Instead, the tri(µ‐iodido)‐bridged dinuclear half‐sandwich complex [Li(DME) 2 ][Tb 2 (µ‐I) 3 (COT′′) 2 ] ( 4 ) was isolated in 72 % yield. In this complex, the Li(DME) 2 fragment is attached to one of the µ‐iodido ligands. A closely related binuclear lutetium complex, [Li(DME) 3 ][Lu 2 (µ‐Cl) 3 (COT big ) 2 ] · DME ( 5 ), was obtained by using the “superbulky“ COT big ligand [COT big = 1,4‐bis(triphenylsilyl)cyclooctatetraenyl dianion]. In addition to the spectroscopic and structural characterization, the magnetic properties of the paramagnetic terbium(III) derivative 2 have been investigated and further complemented by ab initio computational methods. These results have been used to discuss the structural requirements for potential terbium(III) single‐ion magnets.