Isolation of a triplet benzene dianion

Similarities and Differences in Benzene Reduction with Ca, Sr, Yb and Sm: Strong Evidence for Tetra-Anionic Benzene

Inverse sandwich complexes of Yb and Sm stabilized by a bulky β-diketiminate (BDI) ligand have been prepared: (BDI)Ln(η6,η6-C6H6)Ln(BDI); Ln=lanthanide. Coordinated benzene ligands can be neutral, di-anionic or, often controversially discussed, even tetra-anionic. The formal charge on benzene is correlated to assignment of the metal oxidation state which generally poses a problem. Herein, we take advantage of the structural similarities found when comparing CaII with YbII, and SrII with SmII complexes. In this work, we found an excellent overlap of the Ca/Yb inverse sandwich structures but a striking difference for the Sr/Sm pair. The much shorter Sm-N and Sm-C6H6 distances are strong evidence for a SmIII-benzene-4-SmIII assignment. This was further supported by NMR spectroscopy, magnetic susceptibility, reactivity and comprehensive computational investigation.

Ab initio-based determination of lanthanoid-radical exchange as visualised by inelastic neutron scattering

Magnetic exchange coupling can modulate the slow magnetic relaxation in single-molecule magnets. Despite this, elucidation of exchange coupling remains a significant challenge for the lanthanoid(iii) ions, both experimentally and computationally. In this work, the crystal field splitting and 4f-π exchange coupling in the erbium-semiquinonate complex [ErTp2dbsq] (Er-dbsq; Tp- = hydro-tris(1-pyrazolyl)borate, dbsqH2 = 3,5-di-tert-butyl-1,2-semiquinone) have been determined by inelastic neutron scattering (INS), magnetometry, and CASSCF-SO ab initio calculations. A related complex with a diamagnetic ligand, [ErTp2trop] (Er-trop; tropH = tropolone), has been used as a model for the crystal field splitting in the absence of coupling. Magnetic and INS data indicate antiferromagnetic exchange for Er-dbsq with a coupling constant of Jex = -0.23 meV (-1.8 cm-1) (-2Jex formalism) and good agreement is found between theory and experiment, with the low energy magnetic and spectroscopic properties well modelled. Most notable is the ability of the ab initio modelling to reproduce the signature of interference between localised 4f states and delocalised π-radical states that is evident in the Q-dependence of the exchange excitation. This work highlights the power of combining INS with EPR and magnetometry for determination of ground state properties, as well as the enhanced capability of CASSCF-SO ab initio calculations and purposely developed ab initio-based theoretical models. We deliver an unprecedentedly detailed representation of the entangled character of 4f-π exchange states, which is obtained via an accurate image of the spin-orbital transition density between the 4f-π exchange coupled wavefunctions.

From Stable Radicals to Thermally Robust High-Spin Diradicals and Triradicals

Stable radicals and thermally robust high-spin di- and triradicals have emerged as important organic materials due to their promising applications in diverse fields. New fundamental properties, such as SOMO/HOMO inversion of orbital energies, are explored for the design of new stable radicals, including highly luminescent ones with good photostability. A relation with the singlet-triplet energy gap in the corresponding diradicals is proposed. Thermally robust high-spin di- and triradicals, with energy gaps that are comparable to or greater than a thermal energy at room temperature, are more challenging to synthesize but more rewarding. We summarize a number of high-spin di- and triradicals, based on nitronyl nitroxides that provide a relation between the experimental pairwise exchange coupling constant J/k in the high-spin species vs experimental hyperfine coupling constants in the corresponding monoradicals. This relation allows us to identify outliers, which may correspond to radicals where J/k is not measured with sufficient accuracy. Double helical high-spin diradicals, in which spin density is delocalized over the chiral π-system, have been barely explored, with the sole example of such high-spin diradical possessing alternant π-system with Kekulé resonance form. Finally, we discuss a high-spin diradical with electrical conductivity and derivatives of triangulene diradicals.

Reversible Single Electron Redox Steps Convert Polycycles with a C3 P3 Core to a Planar Triphosphinine

Reaction of the imidazolium-stabilized diphosphete-diide IDP with trityl phosphaalkyne affords a mixture which contains the molecules 1 a and 1 b with a central C₃ P₃ core, which formally carries a two-fold negative charge. In order to avoid the formation of an antiaromatic 8π electron system within a conjugated dianionic six-membered [C₃ P₃ ]^2- ring, 1 a adopts a bicyclic [3.1.0] and 1 b a tricyclic [2.2.0.0] structure which are in a dynamic equilibrium. 1 a, b can be reversibly oxidized to a triphosphinine dication [5]²⁺ with a central flat aromatic six-membered C₃ P₃ ring. This two-electron redox reaction occurs in two single-electron transfer steps via the 7π-radical cation [4]⋅⁺ , which could also be isolated and fully characterized. The profound reversible structural change observed for the two-electron redox couple [5]²⁺ /1 a, b is in sharp contrast to the C₆ H₆ /[C₆ H₆ ]^2- couple, which undergoes only a modest structural deformation.

Modulation of slow magnetic relaxation in Gd(III)‐tetrahalosemiquinonate complexes

Incorporating lanthanoid(III)‐radical magnetic exchange coupling is a possible route to improving the performance of lanthanoid (Ln) single‐molecule magnets (SMMs), molecular materials that exhibit slow relaxation and low temperature quantum tunnelling of the magnetization. Complexes of Gd(III) can conveniently be used as model systems to study the Ln‐radical exchange coupling, thanks to the absence of the orbital angular momentum that is present for many Ln(III) ions. Two new Gd(III)‐radical compounds of formula [Gd(18‐c‐6)X₄SQ(NO₃)].I₃ (18‐c‐6=18‐crown‐6, X₄SQ⋅⁻=tetrahalo‐1,2‐semiquinonate, 1: X=Cl, 2: X=Br) have been synthesized, and the presence of the dioxolene ligand in its semiquinonate form confirmed by X‐ray crystallography, UV‐Visible‐NIR spectroscopy and voltammetry. Static magnetometry and EPR spectroscopy indicate differences in the low temperature magnetic properties of the two compounds, with antiferromagnetic exchange coupling of J_Gd‐SQ∼−2.0 cm⁻¹ (H_ex=−2J_Gd‐SQ(S_GdS_SQ)) determined by data fitting. Interestingly, compound 1 exhibits slow magnetic relaxation in applied magnetic fields while 2 relaxes much faster, pointing to the major role of packing effects in modulating slow relaxation of the magnetization.

Three-Dimensional Fully π-Conjugated Macrocycles: When 3D-Aromatic and When 2D-Aromatic-in-3D?

Several fully π-conjugated macrocycles with puckered or cage-type structures were recently found to exhibit aromatic character according to both experiments and computations. We examine their electronic structures and put them in relation to 3D-aromatic molecules (e.g., closo-boranes) and to 2D-aromatic polycyclic aromatic hydrocarbons. Using qualitative theory combined with quantum chemical calculations, we find that the macrocycles explored hitherto should be described as 2D-aromatic with three-dimensional molecular structures (abbr. 2D-aromatic-in-3D) and not as truly 3D-aromatic. 3D-aromatic molecules have highly symmetric structures (or nearly so), leading to (at least) triply degenerate molecular orbitals, and for tetrahedral or octahedral molecules, an aromatic closed-shell electronic structure with 6n + 2 electrons. Conversely, 2D-aromatic-in-3D structures exhibit aromaticity that results from the fulfillment of Hückel’s 4n + 2 rule for each macrocyclic path, yet their π-electron counts are coincidentally 6n + 2 numbers for macrocycles with three tethers of equal lengths. It is notable that 2D-aromatic-in-3D macrocyclic cages can be aromatic with tethers of different lengths, i.e., with π-electron counts different from 6n + 2, and they are related to naphthalene. Finally, we identify tetrahedral and cubic π-conjugated molecules that fulfill the 6n + 2 rule and exhibit significant electron delocalization. Yet, their properties resemble those of analogous compounds with electron counts that differ from 6n + 2. Thus, despite the fact that these molecules show substantial π-electron delocalization, they cannot be classified as true 3D-aromatics.

Electronic structure, stability, and aromaticity of M2B6 (M = Mg, Ca, Sr, and Ba): an interplay between spin pairing and electron delocalization

It has been shown in previous studies that the Be₂B₆ complex exhibits a triplet ground state with double aromaticity. In this work, the stability, electronic structure, and aromaticity of the homologous series M₂B₆ (M = Mg, Ca, Sr and Ba) were examined and compared to those of Be₂B₆. At the CCSD(T)/def2-TZVP//B3LYP/def2-TZVP level of theory, the target molecules were found to be more stable in the singlet than in the triplet spin state. Magnetically induced current densities and multicentre delocalization index (MCI) were employed to assess the aromatic character of the studied complexes. Both employed methods agree that M₂B₆ (M = Mg, Ca, Sr and Ba) are π aromatic and σ nonaromatic in the singlet ground state, and double aromatic in the triplet state. It was demonstrated that the electron counting rules of aromaticity cannot be used to correctly predict the aromaticity and relative stability of the examined molecules in different spin states.

A Uranium(II) Arene Complex That Acts as a Uranium(I) Synthon

Two-electron reduction of the amidate-supported U(III) mono(arene) complex U(TDA)₃ (2) with KC₈ yields the anionic bis(arene) complex [K[2.2.2]cryptand][U(TDA)₂] (3) (TDA = N-(2,6-di-isopropylphenyl)pivalamido). EPR spectroscopy, magnetic susceptibility measurements, and calculations using DFT as well as multireference CASSCF methods all provide strong evidence that the electronic structure of 3 is best represented as a 5f⁴ U(II) metal center bound to a monoreduced arene ligand. Reactivity studies show 3 reacts as a U(I) synthon by behaving as a two-electron reductant toward I₂ to form the dinuclear U(III)-U(III) triiodide species [K[2.2.2]cryptand][(UI(TDA)₂)₂(μ-I)] (6) and as a three-electron reductant toward cycloheptatriene (CHT) to form the U(IV) complex [K[2.2.2]cryptand][U(η⁷-C₇H₇)(TDA)₂(THF)] (7). The reaction of 3 with cyclooctatetraene (COT) generates a mixture of the U(III) anion [K[2.2.2]cryptand][U(TDA)₄] (1-crypt) and U(COT)₂, while the addition of COT to complex 2 instead yields the dinuclear U(IV)-U(IV) inverse sandwich complex [U(TDA)₃]₂(μ-η⁸:η³-C₈H₈) (8). Two-electron reduction of the homoleptic Th(IV) amidate complex Th(TDA)₄ (4) with KC₈ gives the mono(arene) complex [K[2.2.2]cryptand][Th(TDA)₃(THF)] (5). The C-C bond lengths and torsion angles in the bound arene of 5 suggest a direduced arene bound to a Th(IV) metal center; this conclusion is supported by DFT calculations.