Heteroleptic Rare-Earth Tris(metallocenes) Containing a Dibenzocyclooctatetraene Dianion

Isolable heteroleptic tris(metallocenes) containing five-membered and larger rings remain extremely scarce. The utilization of tripositive rare-earth-metal ions with ionic radii >1 Å allowed access to unprecedented and sterically congested dibenzocyclooctatetraenyl (dbCOT) metallocenes, [K(crypt-222)][Cp^tet₂RE(η²-dbCOT)] (RE = Y (1), Dy (2); Cp^tet = tetramethylcyclopentadienyl), through a salt metathesis reaction involving Cp^tet₂RE(BPh₄) and the potassium salt of the dbCOT dianion. The solid-state structures were investigated by single-crystal X-ray diffraction, magnetometry, and IR spectroscopy and provided evidence for the first crystallographically characterized (dbCOT)^2- anion in a complex containing d- or f-block metals. Remarkably, the (Cp^tet)^- ligands force a distortion from planarity within the (dbCOT)^2- moiety, engendering a rare η²-bonding motif, as opposed to the classical η⁸ conformation observed in complexes bearing a (COT)^2- ion. The η² coordination mode was proven crystallographically between 100 and 298 K and computationally (DFT and NBO). Furthermore, nucleus independent chemical shift (NICS) calculations uncovered significant ring current within the dbCOT ligand. The solution-state properties of 1 and 2 were analyzed via cyclic voltammetry, NMR, and UV-vis spectroscopy. Cyclic voltammograms of 1 and 2 exhibit a quasi-reversible feature indicating the accessibility of complexes with dbCOT in two oxidation states (dbCOT^2-/3-•). Importantly, the dysprosium congener, 2, is a zero-field single-molecule magnet (SMM).