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

Abstract

The first homoleptic sandwich complex of dibenzocyclooctatetraene (dbCOT), representing a large cyclooctatetraene (COT) ligand with two fused benzene moieties, for any metal was accessed through salt metathesis of YCl 3 with K 2 dbCOT in the presence of 2.2.2-cryptand. Single-crystal X-ray diffraction analysis on red-brown [K(crypt-222)][Y(dbCOT) 2 ], 1, revealed a remarkably linear anionic yttrocene complex featuring a centroid−yttrium−centroid angle of 180.0°. The anionic moiety adopts a pseudo D 2d geometry, where the carbon atoms of the central COT ring exhibit a staggered geometry. In total, 36 πelectrons are stored on both dbCOT anions, rendering it the largest isolated sandwich complex containing only fused aromatic rings. The solution-state structure of 1 was probed through a series of techniques involving cyclic voltammetry, UV−vis, and 1D and 2D nuclear magnetic resonance (NMR) spectroscopy, including 89 Y NMR. The density functional theory (DFT) and natural bond orbital (NBO) analysis uncovered an ionic bonding interaction between the (dbCOT) 2− ligands and Y III ion. NICS calculations support the experimentally observed aromatic character of 1, despite the deviation from planarity found in the dbCOT moieties. The cyclic voltammograms allude to the accessibility of a radical oxidation state, dbCOT 3−• , based on a quasi-reversible feature. Excitingly, the chemical one-electron reduction of 1 through exposure to potassium graphite yielded a paramagnetic molecule, which was detected by electron paramagnetic resonance (EPR) techniques. Notably, this EPR spectrum is the first one for any sandwich complex containing a COT radical. Remarkably, 1 is thermally stable, and its isolation may provide access to mono-and multinuclear complexes comprising heavier metals with applications in small-molecule activation, single-molecule magnetism, and molecular nanowires.

Linear, Electron-Rich Erbium Single-Molecule Magnet with Dibenzocyclooctatetraene Ligands

“…By contrast, the 1D and 2D NMR spectra of 2 revealed aromatic signals, consistent with the expected aromatic character of 2 following a 2-fold reduction of dbCOT 0 (Figures S12−S15). The UV−vis spectrum of 62 and exhibits two broad absorption maxima at 2.40 × 10 4 and 3.41 × 10 4 cm −1 (Figure 3). The high-energy transition in the UV region arises from π−π* transitions of the dianionic dbCOT ligand, consistent with the ligand−ligand charge-transfer bands for dianionic COT 2− and dbCOT 2− ligands.…”

“…Each [K(DME)] 2 [dbCOT] unit of the chain is rigorously linear with a K−Cnt−K angle (where Cnt = COT ring centroid) of 180.0°and C−C distances consistent with the THF adducts of the dianionic M 2 dbCOT salts of the alkali metals (where M = Li, Na, K, Rb, Cs) (Figure S8). The homoleptic, anionic bis(dibenzocyclooctatetraenyl)erbate metallocene, [K(crypt-222)][Er(dbCOT) 2 ], 1, was isolated by extending the previously reported synthetic route used to access the analogous yttrocene complex, 62 The salt metathesis reaction of ErCl 3 with K 2 dbCOT, in the presence of 2.2.2-cryptand, in THF at 50 °C (eq 1), affords dark red-brown, block-shaped crystals of 1 suitable for single-crystal X-ray diffraction analysis from a concentrated THF solution at −35 °C over the course of 3 days in a 46% yield (Figure 2A). The erbocene sandwich complex, 1, crystallizes in the C 2 /c space group, isostructural with [K(crypt-222)][Y(dbCOT) 2 ], and features a trivalent erbium ion ligated by two dianionic (η 8 -dbCOT) ligands.…”

Strong magnetic exchange coupling in Ln2 metallocenes attained by the trans-coordination of a tetrazinyl radical ligand

“…Y( iii ) possesses a stable isotope ( 89 Y) with a natural abundance of 100%, a nuclear spin of ½ and its ionic radius is similar to that of Gd( iii ), Tb( iii ) and Dy( iii ). 47 These enable detailed information of the electronic structures when subjecting the corresponding Y( iii ) complexes to invaluable characterization methods such as EPR spectroscopy, as well as to computational studies. 48 The EPR spectrum of 4 , revealed the characteristic nine-line pattern attributed to the tetrazine ring of the bpytz˙ − ( i.e.…”

“…Y(III) possesses a stable isotope ( 89 Y) with a natural abundance of 100%, a nuclear spin of 1 2 and its ionic radius is similar to that of Gd(III), Tb(III) and Dy(III). 47 These enable detailed information of the electronic structures when subjecting the corres- ponding Y(III) complexes to invaluable characterization methods such as EPR spectroscopy, as well as to computational studies. 48 The EPR spectrum of 4, revealed the characteristic nine-line pattern attributed to the tetrazine ring of the bpytz •− (i.e., simulated using a model based on the hyperfine coupling of four 14 N nuclei with two slightly different isotropic hyperfine coupling constants (IHCCs) for the coordinated and non-coordinated 14 N atoms; a N1 = 0.596 mT and a N2 = 0.468 mT; Fig.…”

Homoleptic Rare-Earth-Metal Sandwiches with Dibenzo[a,e]cyclooctatetraene Dianions

“…, which is built by a clathrated K + cation and a sandwich-type anion with a Y:DBCOT ratio of 1:2 (Scheme 1e). 31 Inspired by these recent works, we hypothesized that a homoleptic Dy(III) complex prepared with the doubly reduced DBCOT 2− anion could also behave as a SMM. Moreover, it would be of interest to evaluate how the replacement of two (Cp tet ) − ligands with a single DBCOT 2− ligand affects the SMM behavior.…”