Diborabutatrien: ein elektronenarmes Cumulen

An Unsymmetrical, Cyclic Diborene Based on a Chelating CAAC Ligand and its Small-Molecule Activation and Rearrangement Chemistry

A one-pot synthesis of a CAAC-stabilized, unsymmetrical, cyclic diborene was achieved via consecutive two-electron reduction steps from an adduct of CAAC and B2 Br4 (SMe2 )2 . Theoretical studies revealed that this diborene has a considerably smaller HOMO-LUMO gap than those of reported NHC- and phosphine-supported diborenes. Complexation of the diborene with [AuCl(PCy3 )] afforded two diborene-AuI π complexes, while reaction with DurBH2 , P4 and a terminal acetylene led to the cleavage of B-H, P-P, and C-C π bonds, respectively. Thermal rearrangement of the diborene gave an electron-rich cyclic alkylideneborane, which readily coordinated to AgI via its B=C double bond.

A Neutral Beryllium(I) Radical

The reduction of a cyclic alkyl(amino)carbene (CAAC)-stabilized organoberyllium chloride yields the first neutral beryllium radical, which was characterized by EPR, IR, and UV/Vis spectroscopy, X-ray crystallography, and DFT calculations.

Taming the Antiferromagnetic Beast: Computational Design of Ultrashort Mn-Mn Bonds Stabilized by N-Heterocyclic Carbenes

The development of complexes featuring low-valent, multiply bonded metal centers is an exciting field with several potential applications. In this work, we describe the design principles and extensive computational investigation of new organometallic platforms featuring the elusive manganese-manganese bond stabilized by experimentally realized N-heterocyclic carbenes (NHCs). By using DFT computations benchmarked against multireference calculations, as well as MO- and VB-based bonding analyses, we could disentangle the various electronic and structural effects contributing to the thermodynamic and kinetic stability, as well as the experimental feasibility, of the systems. In particular, we explored the nature of the metal-carbene interaction and the role of the ancillary η6 coordination to the generation of Mn2 systems featuring ultrashort metal-metal bonds, closed-shell singlet multiplicities, and positive adiabatic singlet-triplet gaps. Our analysis identifies two distinct classes of viable synthetic targets, whose electrostructural properties are thoroughly investigated.

Linear Group 13 E≡E Triple Bonds in E2 Li6 2

The triply bonded heavier main-group compounds have a textbook trans-bent geometry, in contrast to a familiar linear form found for the lightest analogues. Strikingly, the unexpected linear group 13 E≡E triple bonds were herein found in the D_4h -symmetry E₂ Li₆ ²⁺ clusters, and they possess a large barrier (>18.0 kcal/mol) towards the dissociation of Li⁺ . The perfectly surrounded Li₄ motifs and two linear coordinated Li atoms strongly suppress the increasing nonbonded electron density of heavier E atoms, making two degenerate π bonds and one multi-center σ bond in linear heavier main-group triple bonds. The surrounding Li₆ motifs not only creates an effective electronic structure to form a linear E≡E triple bond, but the resulting electrostatic interactions account for the highly stable global E₂ Li₆ ²⁺ clusters.

Twisting versus Delocalization in CAAC- and NHC-Stabilized Boron-Based Biradicals: The Roles of Sterics and Electronics

Twisted boron-based biradicals featuring unsaturated C2 R2 (R=Et, Me) bridges and stabilization by cyclic (alkyl)(amino)carbenes (CAACs) were recently prepared. These species show remarkable geometrical and electronic differences with respect to their unbridged counterparts. Herein, a thorough computational investigation on the origin of their distinct electrostructural properties is performed. It is shown that steric effects are mostly responsible for the preference for twisted over planar structures. The ground-state multiplicity of the twisted structure is modulated by the σ framework of the bridge, and different R groups lead to distinct multiplicities. In line with the experimental data, a planar structure driven by delocalization effects is observed as global minimum for R=H. The synthetic elusiveness of C2 R2 -bridged systems featuring N-heterocyclic carbenes (NHCs) was also investigated. These results could contribute to the engineering of novel main group biradicals.

cAAC-Stabilized 9,10-diboraanthracenes-Acenes with Open-Shell Singlet Biradical Ground States

Narrow HOMO-LUMO gaps and high charge-carrier mobilities make larger acenes potentially high-efficient materials for organic electronic applications. The performance of such molecules was shown to significantly increase with increasing number of fused benzene rings. Bulk quantities, however, can only be obtained reliably for acenes up to heptacene. Theoretically, (oligo)acenes and (poly)acenes are predicted to have open-shell singlet biradical and polyradical ground states, respectively, for which experimental evidence is still scarce. We have now been able to dramatically lower the HOMO-LUMO gap of acenes without the necessity of unfavorable elongation of their conjugated π system, by incorporating two boron atoms into the anthracene skeleton. Stabilizing the boron centers with cyclic (alkyl)(amino)carbenes gives neutral 9,10-diboraanthracenes, which are shown to feature disjointed, open-shell singlet biradical ground states.

π-Complexes of Diborynes with Main Group Atoms

We present herein an in-depth study of complexes in which a molecule containing a boron-boron triple bond is bound to tellurate cations. The analysis allows the description of these salts as true π complexes between the B-B triple bond and the tellurium center. These complexes thus extend the well-known Dewar-Chatt-Duncanson model of bonding to compounds made up solely of p block elements. Structural, spectroscopic and computational evidence is offered to argue that a set of recently reported heterocycles consisting of phenyltellurium cations complexed to diborynes bear all the hallmarks of π-complexes in the π-complex/metallacycle continuum envisioned by Joseph Chatt. Described as such, these compounds are unique in representing the extreme of a metal-free continuum with conventional unsaturated three-membered rings (cyclopropenes, azirenes, borirenes) occupying the opposite end.

Reactivity of a Tetra(o-tolyl)diborane(4) Dianion as a Diarylboryl Anion Equivalent

Lithium and magnesium salts of tetra(o-tolyl)diborane(4) dianion, having B=B double bond character, were synthesized. It was clarified that the lithium salt of the dianion has a high-lying HOMO and a narrow HOMO-LUMO gap, which were perturbed by dissociation of Li⁺ cation, as judged by UV/Vis spectroscopy and DFT calculations. The lithium salt of the dianion reacted as two equivalents of a diarylboryl anion with CH₂ Cl₂ or S₈ to give boryl-substituted products.

Heterodiatomic Multiple Bonding in Group 13: A Complex with a Boron-Aluminum π Bond Reduces CO2

Heterodiatomic multiple bonds have never been observed within Group 13. Herein, we disclose a method that generates [(CAAC)PhB=AlCp^3t ] (1), a complex featuring π bonding between boron and aluminum through the association of singlet fragments. We present the properties of this multiple bond as well as the reactivity of the complex with carbon dioxide, which yields a boron CO complex via an unusual metathesis reaction.

Coordination of Asymmetric Diborenes towards Cationic Coinage Metals (Au, Ag, Cu)

Synthesis of a series of cationic coinage metal (Au: 2-4, Ag: 5, and Cu: 6) complexes of asymmetric diborenes (1 a, 1 b) is reported. X-ray diffraction analysis reveals that the metal center interacts unsymmetrically with the B₂ moiety and the terminal boron atom exhibits pronounced pyramidal geometry. A computational study shows that in complexes 2-6, the bonding between the B₂ units and the metal center is dominated by electrostatic interactions concomitant with non-negligible covalent contributions.

Synthesis, Spectroscopic Characterization, and Redox Reactivity of a Cyclic (Alkyl) Amino Carbene-Derived Arsamethine Cyanine Dye

In our efforts to prepare a diarsenic allotrope supported by two cyclic alkyl amino carbene (CAAC) ligands we stumbled upon the synthesis of the first carbene-supported chloroarsinidene 3, which has been fully characterized by a combination of NMR spectroscopic and XRD methods. Although further reduction of 3 was not possible, we found that addition of a second equivalent of CAAC in refluxing toluene afforded the first example of a crystallographically characterized arsamethine cyanine dye (4). The arsenic(I) dye is structurally similar to phosphorus analogues, and contains an arsenide anion with two stereochemically active lone pairs supported by two iminium ligands. The UV/Visible spectrum and redox chemistry of 4 were also explored. Upon reduction with one equivalent of KC₈ , 3 is reduced to the originally targeted CAAC₂ As₂ allotrope 6, whereas oxidation provides access to the first example of an arsenic(II) radical dication (5).

CO2 Binding and Splitting by Boron-Boron Multiple Bonds

The room-temperature, ambient-pressure reactions of CO₂ with two species containing boron-boron multiple bonds led to the incorporation of either one or two CO₂ molecules. The structural characterization of a thermally unstable intermediate in one case indicates that an initial [2+2] cycloaddition is the key step in the reaction mechanism.

Borylenes: An Emerging Class of Compounds

Free borylenes (R-B:) have only been spectroscopically characterized in the gas phase or in matrices at very low temperatures. However, in recent years, a few mono- and bis(Lewis base)-stabilized borylenes have been isolated. In both of these compounds the boron atom is in the formal oxidation state +I which contrasts with classical organoboron derivatives wherein the element is in the +III oxidation state. Mono(Lewis base)-stabilized borylenes are isoelectronic with singlet carbenes, and their reactivity mimics to some extent that of transition metals. They can activate small molecules, such as H₂ , and coordinate an additional ligand; in other words, they are boron metallomimics. Bis(Lewis base)borylene adducts are isoelectronic with amines and phosphines. In contrast to boranes, which act as electron acceptors and thus Lewis acids, they are electron-rich and act as ligands for transition metals.

Titanium Imido Complexes via Displacement of -SiMe3 and C-H Bond Activation in a Ti(III) Amido Complex, Promoted by a Cyclic (Alkyl)(Amino) Carbene (cAAC)

A strong σ-donating cyclic (alkyl)(amino) carbene (cAAC) triggers rearrangement of the silyl(aryl) amido ligand -N(SiMe3)Dipp (Dipp = 2,6-diisopropylphenyl) in the coordination sphere of titanium(III) to afford a novel zwitterionic titanium imido complex with a TiCH2SiMe2[cAAC] linkage. Reduction of this species produces a new DippN=Ti imido complex containing a cAAC-centered radical species, characterized by single crystal diffraction analysis and electron paramagnetic resonance spectroscopy.

Uncatalyzed Hydrogenation of First-Row Main Group Multiple Bonds

Room temperature hydrogenation of an SIDep-stabilized diboryne (SIDep=1,3-bis(diethylphenyl)-4,5-dihydroimidazol-2-ylidene) and a cAAC-supported diboracumulene (cAAC=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) provided the first selective route to the corresponding 1,2-dihydrodiborenes. DFT calculations showed an overall exothermic (ΔG=19.4 kcal mol^-1 ) two-step asynchronous H₂ addition mechanism proceeding via a bridging hydride.

Neutral Diboron Analogues of Archetypal Aromatic Species by Spontaneous Cycloaddition

Among the numerous routes organic chemists have developed to synthesize benzene derivatives and heteroaromatic compounds, transition-metal-catalyzed cycloaddition reactions are the most elegant. In contrast, cycloaddition reactions of heavier alkene and alkyne analogues, though limited in scope, proceed uncatalyzed. In this work we present the first spontaneous cycloaddition reactions of lighter alkene and alkyne analogues. Selective addition of unactivated alkynes to boron-boron multiple bonds under ambient conditions yielded diborocarbon equivalents of simple aromatic hydrocarbons, including the first neutral 6 π-aromatic diborabenzene compound, a 2 π-aromatic triplet biradical 1,3-diborete, and a phosphine-stabilized 2 π-homoaromatic 1,3-dihydro-1,3-diborete. DFT calculations suggest that all three compounds are aromatic and show frontier molecular orbitals matching those of the related aromatic hydrocarbons, C6 H6 and C4 H4 (2+) , and homoaromatic C4 H5 (+) .

Highly Strained Heterocycles Constructed from Boron-Boron Multiple Bonds and Heavy Chalcogens

The reactions of a diborene with elemental selenium or tellurium are shown to afford a diboraselenirane or diboratellurirane, respectively. These reactions are reminiscent of the sequestration of subvalent oxygen and nitrogen in the formation of oxiranes and aziridines; however, such reactivity is not known between alkenes and the heavy chalcogens. Although carbon is too electronegative to affect the reduction of elements with lower relative electronegativity, the highly reducing nature of the B-B double bond enables reactions with Se(0) and Te(0) . The capacity of multiple bonds between boron atoms to donate electron density is highlighted in reactions where diborynes behave as nucleophiles, attacking one of the two Te atoms of diaryltellurides, forming salts consisting of diboratellurenium cations and aryltelluride anions.