Investigation of the Electronic Structures of Organolanthanide Sandwich Complex Anions by Photoelectron Spectroscopy: 4f Orbital Contribution in the Metal–Ligand Interaction

New kind of electride sandwich complexes based on the cyclooctatetraene ligand M12(η8-C8H8)2M22 (M1 = Na, K and M2 = Ca, Mg): a theoretical study

In the present study, the electronic structures of a series of binuclear sandwich complexes based on the cyclooctatetraene ligand M¹₂(η⁸-C₈H₈)₂M²₂ (M¹ = Na, K and M² = Ca, Mg) are studied theoretically. Each cyclooctatetraene ligand binds with the metal in the η⁸ binding mode. The M²-M² bond length agrees well with the reported bimetallic covalent Ca₂ and Mg₂ bond lengths. The Wiberg bond index (WBI) also indicates the presence of covalent M²-M² bonds, which gives additional stability to the complex. A non-nuclear attractor (NNA) is found in-between the M²-M² bond and the negative Laplacian of the electron density is found at the NNA. Noncovalent interaction (NCI) plot shows that electron density is localized at the M²-M² bond. Based on the performed analysis, we have concluded that the designed sandwich complexes are electrides. We herein report, for the first time, the electride sandwich complexes of the cyclooctatetraene ligand. Due to the presence of a diffuse electron system, the electride complexes exhibit higher values of the static second hyperpolarizability within the range of 2.6 × 10⁵ to 1.4 × 10⁶ a.u. Among the studied complexes, M¹₂(η⁸-C₈H₈)₂Ca₂ exhibit a higher value of static second hyperpolarizability.

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

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 YCl₃ with K₂dbCOT 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 π-electrons 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 ⁸⁹Y 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.

Anionic Stacks of Alkali-Interlinked Yttrium and Dysprosium Bicyclooctatetraenes in Isolation

Electrospray ionization of THF solutions of preformed [K(18-c-6)][M(COT)₂] (M = Dy(III), Y(III); COT = C₈H₈^2-,18-c-6 = C₁₂H₂₄O₆) yields the isolated species [(M(COT)₂)_n+1 + nK]^- with n = 0-3. High-resolution ion mobility spectrometry combined with density functional theory calculations performed for the n = 0-2 aggregates indicate that anionic multidecker stacks interlinked by potassium cations are formed. The alternating metal ions are aligned linearly: COT^2--M³⁺-COT^2--K⁺-COT^2--M³⁺-COT^2-. The different M³⁺ ionic radii lead to slight but resolvable changes in mobility and thus collision cross sections indicative of different overall heights of the multidecker stacks. CID measurements show that the aggregates fragment by cleavage at the K⁺ interconnections.

New Photoelectron-Valence Electron Interactions Evident in the Photoelectron Spectrum of Gd2O. J Phys Chem A 2021;125:9892-9903. [PMID: 34730978 DOI: 10.1021/acs.jpca.1c07818]

Evidence of strong photoelectron-valence electron (PEVE) interactions has been observed in the anion photoelectron (PE) spectra of several lanthanide suboxide clusters, which are exceptionally complex from an electronic structure standpoint and are strongly correlated systems. The PE spectrum of Gd₂O^-, which should have relatively simple electronic structure because of its half-filled 4f subshell, exhibits numerous electronic transitions. The electron affinity determined from the spectrum is 0.26 eV. The intensities of transitions to excited states increase relative to the lower-energy states with lower photon energy, which is consistent with shakeup transitions driven by time-dependent electron-neutral interactions. A group of intense spectral features that lie between electron binding energies of 0.7 and 2.3 eV are assigned to transitions involving detachment of an electron from outer-valence σ_u and σ_g orbitals that have large Gd 6s contributions. The spectra show parallel transition manifolds in general, which is consistent with detachment from these orbitals. However, several distinct perpendicular transitions are observed adjacent to several of the vertical transitions. A possible explanation invoking interaction between the ejected electron and the high-spin neutral is proposed. Specifically, the angular momentum of electrons ejected from σ_u or σ_g orbitals, which is l = 1, can switch to l = 0, 2 with an associated change in the M_s of the remnant neutral, which is spin-orbit coupling between a free electron and the spin of a neutral.

Intermediate Valence States in Lanthanide Compounds

Over more than 50 years, intermediate valence states in lanthanide compounds have often resulted in unexpected or puzzling spectroscopic and magnetic properties. Such experimental singularities could not be rationalised until new theoretical models involving multiconfigurational electronic ground states were established. In this minireview, the different singularities that have been observed among lanthanide complexes are highlighted, the models used to rationalise them are detailed and how such electronic effects may be adjusted depending on energy and symmetry considerations is considered. Understanding and tuning the ground-state multiconfigurational behaviour in lanthanide complexes may open new doors to modular and unusual reactivities.

Probing the structures, electronic and bonding properties of multidecker lanthanides: Neutral and anionic Lnn(COT)m (Ln = Ce, Nd, Eu, Ho and Yb; n, m = 1, 2) complexes

The ground state structures of neutral and anionic Ln_n(COT)_m (Ln = Ce, Nd, Eu, Ho and Yb; n, m = 1, 2) complexes have been identified by density functional theory. Ln(COT)_1,2^0/- and Ln₂(COT)₂^0/- complexes are found to possess sandwich ground state structures in which Ln atoms and COT molecules are alternately stacked except for Nd₂COT₂^0/-. Our calculated AEA and VDE values show good agreement with the available experimental values, which validates that our obtained ground state structures are credible. Based on the frontier molecular orbitals, we find that the bond formation between the 4f electrons of Ln atoms and the π clouds of COT molecules is weak. Then, the bond strength within these complexes is further analyzed based on the topological analysis of electron density at bond critical point. By analyzing Hirshfeld charge, we find Ln_n(COT)_m^0/- are charge-transfer complexes with weak bonding feature.

A base-free synthetic route to anti-bimetallic lanthanide pentalene complexes

Homobimetallic complexes of Yb, Eu and Sm bridged by a silylated pentalene ligand have been prepared and intermetallic communication studied by magnetic measurements and cyclic voltammetry.

Theoretical Study on the Photoelectron Spectra of Ln(COT)2-: Lanthanide Dependence of the Metal-Ligand Interaction

We have performed a theoretical analysis of the recently reported photoelectron (PE) spectra of the series of sandwich complex anions Ln(COT)₂^- (Ln = La-Lu, COT = 1,3,5,7-cyclooctatetraene), focusing on the Ln dependence of the vertical detachment energies. For most Ln, the π molecular orbitals, largely localized on the COT ligands, have the energy order of e_1g < e_1u < e_2g < e_2u as in the actinide analogues, reflecting the substantial orbital interaction with the Ln 5d and 5p orbitals. Thus, it would be expected that the lanthanide contraction would increase the orbital interaction so that the overlaps between the COT π and Ln atomic orbitals tend to increase across the series. However, the PE spectra and theoretical calculations were not consistent with this expectation, and the details have been clarified in this study. Furthermore, the energy level splitting patterns of the anion and neutral complexes have been studied by multireference ab initio methods, and the X peak splittings observed in the PE spectra only for the middle-range Ln complexes were found to be due to the specific interaction between the Ln 4f and ligand π orbitals of the neutral complexes in e_2u symmetry. Because the magnitude of this 4f-ligand interaction depends critically on the final state 4f electron configuration and the spin state, a significant Ln dependence in the PE spectra is explained.

Rare-earth metal π-complexes of reduced arenes, alkenes, and alkynes: bonding, electronic structure, and comparison with actinides and other electropositive metals

Rare-earth metal complexes of reduced π ligands are reviewed with an emphasis on their electronic structure and bonding interactions.