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

Abstract

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 tet 2 RE(η 2 -dbCOT)] (RE = Y (1), Dy (2); Cp tet = tetramethylcyclopentadienyl), through a salt metathesis reaction involving Cp tet 2 RE(BPh 4 ) and the potassium salt of the dbCOT dianion. The solid-state structures were investigated by singlecrystal 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 η 2 -bonding motif, as opposed to the classical η 8 conformation observed in complexes bearing a (COT) 2− ion. The η 2 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).

Linear, Electron-Rich, Homoleptic Rare Earth Metallocene and Its Redox Activity

“…The reproducible quasi-reversible feature at −1.632 ± 0.108 V vs Fc 0 /Fc + is ascribed to the dbCOT 2–/3–• redox couple, indicative of a mononuclear complex with the open-shell dbCOT trianion, dbCOT 3–• , being accessible on the timescale of the electrochemical measurement. The dbCOT 3–• radical trianion was also observed electrochemically when exposing the dianionic potassium salt, K 2 dbCOT, or the heteroleptic tris(metallocene) yttrium complex [K(crypt-222)][Cp tet 2 Y(dbCOT)] to electrons . Relative to the free K 2 dbCOT ligand (−1.991 V vs Fc 0 /Fc + ), the redox potential of 1 is cathodically shifted.…”

“…The dbCOT 3−• radical trianion was also observed electrochemically when exposing the dianionic potassium salt, K 2 dbCOT, 25 or the heteroleptic tris-(metallocene) yttrium complex [K(crypt-222)][Cp tet 2 Y-(dbCOT)] to electrons. 25 Relative to the free K 2 dbCOT ligand (−1.991 V vs Fc 0 /Fc + ), the redox potential of 1 is cathodically shifted. This possibly arises from the highly Lewis acidic Y III ion, which decreases the energy of the lowest unoccupied molecular orbital (LUMO) of (dbCOT) 2− , facilitating the uptake of an extra electron.…”

“…The shift toward positive potentials upon complexation with Y III is in accordance with the redox potential of −1.527 V monitored for [K(crypt-222)][Cp tet 2 Y(dbCOT)]. 25 Furthermore, the two irreversible redox events observed toward more positive potentials, −0.381 ± 0.108 and −0.032 ± 0.108 V vs Fc 0 / Fc + , may be assigned to the dbCOT 2−/1−• and dbCOT 1−•/0 respectively. The 0.35 V difference in redox potentials between dbCOT 2−/1−• and dbCOT 1−•/0 is akin to the 0.43 V difference for the first two reduction waves of dbCOT, albeit was measured relative to the saturated calomel electrode (s.c.e.).…”

“…This possibly arises from the highly Lewis acidic Y III ion, which decreases the energy of the lowest unoccupied molecular orbital (LUMO) of (dbCOT) 2– , facilitating the uptake of an extra electron. The shift toward positive potentials upon complexation with Y III is in accordance with the redox potential of −1.527 V monitored for [K­(crypt-222)]­[Cp tet 2 Y­(dbCOT)] . Furthermore, the two irreversible redox events observed toward more positive potentials, −0.381 ± 0.108 and −0.032 ± 0.108 V vs Fc 0 /Fc + , may be assigned to the dbCOT 2–/1–• and dbCOT 1–•/0 redox couples, respectively.…”

“… − The highly modular COT scaffold has also allowed for the development of larger arene ligands, such as dibenzocyclooctatetraene (dbCOT). The 16 π-antiaromatic ligand adopts a tub conformation and provides an increased π-surface area, which fostered various binding modes to transition metal ions, − and stronger coordination to metal centers in comparison to the 1,5-cyclooctadiene (COD) ligand. , A two-electron reduction of dbCOT with alkali metals led to the first isolation of an aromatic and planar (dbCOT) 2– dianion in the form of M 2 (dbCOT) (where M = Li, Na, K, Rb, Cs). − The first coordination compound containing a (dbCOT) 2– dianion was achieved as a set of sterically congested heteroleptic tris­(metallocene) complexes, [K­(crypt-222)]­[Cp tet 2 RE­(dbCOT)] (where RE = Dy, Y) . The steric bulk engendered by the (dbCOT) 2– ligand gave rise to a rare η 2 -coordination mode, which was crystallographically confirmed from 100 to 298 K, and the dysprosium congener exhibited single-molecule magnet (SMM) behavior.…”