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The 1,1-azoliumdithiocarboxylate cAACMe-CS2 (1a) was prepared and its redox chemistry was evaluated and compared to NHC based dithiocarboxylates IDipp-CS2 (1b) and IMes-CS2 (1c). Radical anions [carbene-CS2]˙− were prepared by metallic reduction as the potassium or magnesium ion complexes [K(18-crown-6)(cAACMe-CS2)] (2a), [K(18-crown-6)(NHC-CS2)] (NHC = IDipp: 2b, IMes: 2c), and [Mg(DippNacNac)(cAACMe-CS2)] (3) and extensively characterized (SC-XRD, EPR, UV/VIS/NIR, DFT). These complexes represent the first examples of isolated radical anions of 1,1-dithiolenes. [ABSTRACT FROM AUTHOR]

Herein, we simultaneously synthesized three different double B←N Lewis pair-functionalized naphthalenes including the asymmetric BNNY and BNNO and the mirror-symmetric BNNR, via a one-pot method. The annulation modes of the B←N Lewis pairs and the substituents on the boron atoms efficiently tune the molecular frontier orbitals and emission colors. α-Position fusion leads to a significant enhancement of the HOMO energy level and a decrease in the HOMO-LUMO gap, while the electron-withdrawing groups attached to the boron atom make the LUMO energy level more stable. In particular, the LUMO energy level of BNNO is as low as -3.12 eV. BNNY, BNNO, and BNNR emit cyan, green, and red fluorescence at 509, 534, and 620 nm, respectively, with fluorescence quantum yields of 24 %, 49 %, and 30 %, respectively., (© 2024 Wiley-VCH GmbH.)

Herein, we report nickel(0)-catalyzed cross-coupling reactions of NHC/CAAC-based carbodicarbene (NHC = N -heterocyclic carbene and CAAC = cyclic(alkyl)(amino)carbene) with different aryl chlorides, bromides, and iodides. The resulting aryl-substituted cationic carbodicarbene derivatives are prone to one-electron oxidation yielding radical-dications, which, depending on the aryl motif employed, follow different modes of radical-radical dimerization and constitute an entry point to carbon/nitrogen- and nitrogen/nitrogen-centered diradicaloids. Subsequently, this coupling strategy was strategically applied to the synthesis of p -phenylene- and p , p' -biphenylene-bridged carbon/carbon-centered electron-deficient diradicaloids. The employed π-conjugated spacer plays a crucial role in determining the triplet population at room temperature by modulation of the singlet-triplet gap: EPR inactive for p -phenylene vs EPR active for p , p' -biphenylene. Nearly two decades after the disclosure of carbodicarbenes as donor-stabilized atomic carbon equivalents by Tonner and Frenking in 2007, we demonstrate their cross-couplings with a series of aryl halides/dihalides and, based on this, developed a modular methodology for the systematic synthesis of various electron-deficient diradicaloids.

The influence of the electron density of a bridge connecting two redox centers on both the intervalence hole transfer and the magnetic superexchange was investigated in a series of bridged bis-triarylamine mono- and dications. In this series, the bridge was 2,7-fluorenyl, where the bridge electron density was modified by substituents at the 9-position. For the mixed-valence monocations, the observation of both an intervalence charge transfer (IVCT) band and an absorption band associated with an electron transfer from the bridging fluorene to the triarylamine radical cation centers allowed determination of the electron transfer couplings in the framework of the three-state generalized Mulliken-Hush theory. Comparison of the derived couplings with those obtained from a classical two-state approach demonstrates an enhancement of the electronic coupling which increases with decreasing bridge state energy. For the dicationic diradical counterparts, the singlet-triplet gap (exchange interaction) was determined both experimentally and by quantum chemical methods. Hereby, an increase of antiferromagnetic coupling with a lowering of the bridge state energy by electron donating substituents was observed. Analysis of the involved molecular orbitals suggests that the ferromagnetic coupling is inversely proportional to the square of the bridge energy, which is also supported by the experimental findings. This influence of the bridge state energy on both types of interactions, electron transfer and magnetic exchange, provides a design guideline for fine-tuning the properties of electronically coupled organic redox dyads by variation of the bridge electron density.

We synthesized 2-(1-1,2-dicarbadodecaboranyl(12))-6,6,12,12-tetramethyl-7,8,11,12-tetrahydro-6H,10H-phenaleno[1,9-fg]pyrido[3,2,1-ij]quinoline (4), a julolidine-like pyrenyl-o-carborane, with pyrene substituted at the 2,7-positions on the HOMO/LUMO nodal plane. Using solid state molecular structures, photophysical data, cyclic voltammetry, DFT and LR-TDDFT calculations, we compare o-carborane and B(Mes) 2 (Mes=2,4,6-Me 3 C 6 H 2 ) as acceptor groups. Whereas the π-acceptor strength of B(Mes) 2 is sufficient to drop the pyrene LUMO+1 below the LUMO, the carborane does not do this. We confirm the π-donor strength of the julolidine-like moiety, however, which raises the pyrene HOMO-1 above the HOMO. In contrast to the analogous pyrene-2-yl-o-carborane, 2-(1-1,2-dicarbadodecaboranyl(12))-pyrene VI, which exhibits dual fluorescence, because the rate of internal conversion between locally-excited (LE) and charge transfer (CT) (from the pyrene to the carborane) states is faster than the radiative decay rate, leading to a thermodynamic equilibrium between the 2 states, 4 shows only single fluorescence, as the CT state involving the carborane as the acceptor moiety in not kinetically accessible, so a more localized CT emission involving the julolidine-like pyrene moiety is observed., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The chemistry of unsaturated boron heterocycles, including five-membered boroles, continues to attract substantial interest. Herein, we report the synthesis of 1,2,3-triphenyl-1-boraindene, a benzene-fused borole, and examine its Lewis acidic, electrophilic, and antiaromatic properties relative to non-fused and bis-benzannulated boroles (9-borafluorenes). Reactivity studies with organic azides reveal that the boraindene behaves similarly to other boroles, undergoing ring expansion to a BN-naphthalene through insertion of a nitrogen atom., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Conjugation between three-dimensional (3D) carboranes and the attached substituents is commonly believed to be very weak. In this paper, we report that reducing 1,12-bis(BMes 2 )- p -carborane ( B 2 p Cab ) with one electron gives a radical anion with a centrosymmetric semiquinoidal structure. This radical anion shows extensive electron delocalization between the two boron centers over the p -carborane bridge due to the overlap of carborane lowest unoccupied molecular orbital (LUMO) and the BMes 2 LUMO. Unlike dianions of other C 2 B 10 H 12 carboranes, which rearrange to a nido -form, two-electron reduction of B 2 p Cab leads to a rearrangement into a basket-shaped intermediate.

Radicals of the lightest group 13 element, boron, are well established and observed in numerous forms. In contrast to boron, radical chemistry involving the heavier group 13 elements (aluminum, gallium, indium, and thallium) remains largely underexplored, primarily attributed to the formidable synthetic challenges associated with these elements. Herein, we report the synthesis and isolation of planar and twisted conformers of a doubly CAAC (cyclic alkyl(amino)carbene)-radical-substituted dialane. Extensive characterization through spectroscopic analyses and X-ray crystallography confirms their identity, while quantum chemical calculations support their open-shell nature and provide further insights into their electronic structures. The dialane-connected diradicals exhibit high susceptibility to oxidation, as evidenced by electrochemical measurements and reactions with o-chloranil and a variety of organic azides. This study opens a previously uncharted class of dialuminum systems to study, broadening the scope of diradical chemistry and its potential applications., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Herein, we report the syntheses and electronic structures of crystalline dianionic as well as neutral diboron-centered classical diradicaloids as boron analogues of classical Thiele, Chichibabin, and Müller (this only for dianionic diradicaloids!) hydrocarbons. These are based on borane radical anion and NHC-stabilized boryl radical spin carriers, respectively. All these dianionic diboron-centered diradicaloids exhibit triplet population at room temperature regardless of the π-conjugated spacer: p -phenylene, p , p' -biphenylene, or p , p″ -terphenylene. In the case of neutral diboron-centered diradicaloids, the employed π-conjugated spacer plays a crucial role for the triplet population at room temperature: EPR inactive for p -phenylene vs EPR active for p , p' -biphenylene. The findings emphasize the importance of the spin carriers for the resulting ground-state: borane radical anion vs NHC-stabilized boryl radical along with the pivotal role of the π-conjugated spacer as spin-coupler between two spins. Notably, 100 years (a century) after the first report by Krause of the triphenyl borane radical-anion, being isoelectronic to the triphenylmethyl radical, we convey borane radical anion-based diradicaloids. Furthermore, while donor-stabilized boryl radicals were introduced in the 1980s by Giles and Roberts, said concept is herewith being extended to NHC-stabilized boryl radical-based diradicaloids.

The study focuses on the structural and photophysical characteristics of neutral and oxidized forms of N-tolanyl-phenochalcogenazines PZX-tolan with X=O, S, Se, and Te. X-ray crystal structure analyses show a pseudo-equatorial (pe) structure of the tolan substituent in the O, S, and Se dyads, while the Te dyad possesses a pseudo-axial (pa) structure. DFT calculations suggest the pe structure for O and S, and the pa structure for Se and Te as stable forms. Steady-state and femtosecond-time resolved optical spectroscopy in toluene solution indicate that the O and S dyads emit from a CT state, whereas the Se and Te dyads emit from a tolan-localized state. The T 1 state is tolan-localized in all cases, showing phosphorescence at 77 K. The heavy atom effect of chalcogens induces intersystem crossing from S 1 to T x , resulting in a decreasing S 1 lifetime from 2.1 ns to 0.42 ps. The T 1 states possess potential for singlet oxygen sensitization with a high quantum yield (ca. 40 %) for the O, S, and Se dyads. Radical cations exhibit spin density primarily localized at the heterocycle. EPR measurements and quasirelativistic DFT calculations reveal a very strong g-tensor anisotropy, supporting the pe structure for the S and Se derivatives., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Crystalline polymeric structures of trans -1,4-cyclohexylene bridged N -tethered bis-CAACs in the form of their LiOTf adducts were synthesized and isolated. These were further used as building blocks for the synthesis of crystalline (amino)(carboxy)-based diradicals. The triplet diradical character of these compounds was unambiguously confirmed by the presence of a half-field signal in their EPR spectra. Theoretical calculations show that the singlet state is marginally more stable than the triplet state.

The one-electron reduction of [(CAAC)Be(Dur)Br] (CAAC = cyclic alkyl(amino)carbene, Dur = 2,3,5,6-tetramethylphenyl = duryl) with lithium sand in diethyl ether yields the first neutral, tricoordinate, and moderately stable beryllium radical, [(CAAC)(Et 2 O)BeDur] • ( 2-Et 2 O ), which undergoes a facile second one-electron reduction concomitant with the insertion of the beryllium center into the endocyclic C-N CAAC bond and a cyclopropane-forming C-H bond activation of an adjacent methyl group. In situ generation of 2-Et 2 O and addition of PMe 3 yield the stable analogue, [(CAAC)(Me 3 P)BeDur] • ( 2-PMe 3 ), which serves as a platform for PMe 3 -ligand exchange with stronger donors, generating the radicals [(CAAC)LBeDur] • ( 2-L , L = isocyanides, pyridines, and N -heterocyclic carbenes). X-ray structural analyses show trigonal-planar beryllium centers and strong π backbonding from the metal to the CAAC ligand. The EPR signals of all six isolated [(CAAC)LBeDur] • radicals display significant, albeit small, hyperfine coupling to the 9 Be nucleus. DFT calculations show that the spin density is mostly delocalized over the CAAC π framework and, where present, the isocyanide CN moiety, with only a small proportion (3-6%) on the beryllium center. 2-PMe 3 proved thermally unstable at 80 °C, first undergoing radical hydrogen abstraction with the solvent, followed by insertion of beryllium into the endocyclic C-N CAAC bond and PMe 3 transfer to the former carbene carbon atom. The reactions with diphenyl disulfide and phenyl azide occur at the beryllium center and yield the corresponding Be(II) phenyl sulfide and amino complexes, respectively, the latter concomitant with radical transfer and hydrogen abstraction by the beryllium-bound nitrogen center.

In this work, we report the reactivity of various annulated borole derivatives toward chalcogen (O, S, and Se) insertion. Among a series of 9-borafluorenes with different boron substituents (Ph, Br, or o -carboranyl) and a mixed thiophene-benzene-fused derivative, only the 9- o -carboranyl-substituted 9-borafluorene yielded the complete set of chalcogen-containing heteroarenes, including the first 1,2-selenaborinine derivative. To evaluate the aromaticity of this heterocyclic analogue of phenanthrene, nucleus-independent chemical shift (NICS) values were computed and compared to those of its lighter group 16 congeners.

The first poly(ferrocenylene iminoborane), that is, a polyferrocene-based metallopolymer featuring CC-isoelectronic/-isosteric BN linking units, and a series of monodisperse ferrocenylene iminoborane oligomers are presented. Our studies provide important insight into the structural and electronic nature of this novel class of hybrid materials.

A modular approach for the synthesis of isolable crystalline Schlenk hydrocarbon diradicals from m-phenylene bridged electron-rich bis-triazaalkenes as synthons is reported. EPR spectroscopy confirms their diradical nature and triplet electronic structure by revealing a half-field signal. A computational analysis confirms the triplet state to be the ground state. As a proof-of-principle for the modular methodology, the 4,6-dimethyl-m-phenylene was further utilized as a coupling unit between two alkene motifs. The steric conjunction of the 4,6-dimethyl groups substantially twists the substituents at the nonbonding electron bearing centers relative to the central coupling m-phenylene motif. As a result, the spin delocalization is decreased and the exchange coupling between the two unpaired spins, hence, significantly reduced. Notably, 108 years after Schlenk's m-phenylene-bis(diphenylmethyl) synthesis as a diradical, for the first time we were able to isolate its derivative with the same spacer, i.e. m-phenylene, between two radical centers in a crystalline form., (© 2023 Wiley-VCH GmbH.)

While the dimerization of heavier group 13 carbene analogues to the corresponding alkene analogues is known and relatively well understood, the dimerization of dicoordinate borylenes (LRB:, L = neutral donor; R = anionic substituent) to the corresponding diborenes (LRB═BRL) has never been directly observed. In this study we present the first example of a formal borylene-to-diborene dimerization through abstraction of a labile phosphine ligand from the tricoordinate hydroborylene precursor (CAAC)(Me 3 P)BH (CAAC = cyclic alkyl(amino)carbene) by bulky Lewis-acidic dihaloboranes (BX 2 Y, X = Cl, Br, Y = aryl, boryl), generating the corresponding dihydrodiborene (CAAC)HB═BH(CAAC) and (Me 3 P)BX 2 Y as the byproduct. An in-depth experimental and computational mechanistic analysis shows that this seemingly simple process (2 LL'BH + 2 BX 2 Y → LHB═BHL + 2 L'BX 2 Y) is in fact based on a complex sequence of finely tuned processes, involving the one-electron oxidation of and PMe 3 abstraction from the borylene precursor by BX 2 Y, multiple halide transfers between (di)boron intermediates and BX 2 Y/[BX 3 Y] - , and multiple one-electron redox processes between diboron intermediates and the borylene precursor, which make the reaction ultimately autocatalytic in [(CAAC)(Me 3 P)BH] •+ . The findings suggest that [LBXR] • boryl radicals are more likely coupling partners than dicoordinate LRB: borylenes in the reductive coupling of base-stabilized LBX 2 R boranes to LRB═BRL diborenes.

The synthesis of an alkene is reported which is concurrently twisted (twist angle = 86.6(8)°), push-pull (dipole moment = 7.48 D), and electron-rich ( E 1/2 = -1.45 V and -0.52 V vs. Fc/Fc + ) in nature, comprising a unique trinity combination for the alkene class of compounds. Subsequently, this newly synthesized alkene-motif was used as a donor for the synthesis of a zwitterionic boron-containing π-conjugated compound (dipole moment = 12.17 D) through an intramolecular charge transfer process exploiting the π-conjugated donor-acceptor system.

We report the synthesis of a series of group 11 metal complexes with sterically demanding anionic nitrogen ligands based on the 1,2-azaborinine motif. The ligands, which share structural similarities with m -terphenyls, have been used to stabilize two-coordinate phosphine complexes and dimeric complexes with close contacts between the metal centers. Spectroscopic, crystallographic, and theoretical investigations reveal close parallels to the related m -terphenyl complexes, including metallophilic interactions in the dimers.

Carbene-stabilized symmetrical and unsymmetrical dicationic tetraboranes, featuring an electron-precise tetraborane chain, were synthesized and fully characterized. Reactions of these tetraboranes with reductants/bases give rise to different outcomes according to the conditions employed, including: 1) reduction and rearrangement of the tetraborane chain to give a zwitterionic alkylidene borate-borenium species; 2) cleavage of the tetraborane chain to afford a 1,3-azaborinine; and 3) reduction of the supporting ligands to provide a diamino dipotassium salt. The zwitterionic alkylidene borate-borenium species can be viewed as an analogue of the base-stabilized diborenes. NMR spectroscopy and DFT calculations reveal a highly polarized B-B bond in the zwitterionic alkylidene borate-borenium, in which the formal oxidation states of the boron atoms can be considered as -1 and +2. These results suggest the considerable potential of tetraboranes as synthons for low-valent boron species., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

In this contribution, we reported the three-dimensional (3D) analogues of N-heterocyclic carbene (NHC)-stabilized 9-borafluorenium and 9-borafluorene radical. The radical was fully characterized by cyclic voltammetry (CV), UV-Vis absorption spectroscopy, electron paramagnetic resonance (EPR) and single-crystal X-ray diffraction analyses. The distinct boron-centered radical character of 9-borafluorene radical was corroborated by DFT calculations and EPR analysis., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The alumole Cp 3t AlC 4 Et 4 (Cp 3t = 1,2,4-tris( tert -butyl)cyclopentadienyl) is reported to be capable of transferring its butadiene moiety to aryl(dihalo)boranes to generate boroles through aluminum-boron exchange. The products feature a rare alkyl-substituted backbone, which, as shown in other examples, often leads to dimerization due to insufficient steric protection of the antiaromatic borole ring. Sterically crowded aryl groups bound to the boron atom are shown to prevent dimerization, allowing access to the first monomeric derivatives of this type. Results from UV-vis spectroscopy, electrochemistry, and DFT calculations reveal that the alkyl substituents cause remarkable modifications in the optical and electronic properties of the boroles compared to their perarylated counterparts., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

Low-valent main group chemistry involves a balancing act between steric and electronic stabilization of the electron-rich low oxidation state main group centers and their desired reactivity. Herein we show that the combination of sterically shielding mesityl and rotationally flexible 2-thienyl groups, the latter having the potential to be either electronically stabilizing or activating, at a diborane(4) provides a platform for both radical anion stabilization and unusual bond activation and rearrangement reactions. The addition of a Lewis base to a 1,2-dimesityl-1,2-dithienyldiborane(4) (1) results in direct and unprecedented C-H borylation of one thienyl substituent with cleavage of the B-B bond. The facile one-electron reduction of 1 yields a stable diboron radical anion through delocalization of its unpaired electron over the entire planar 1,2-dithienyldiboron framework, as evidenced by EPR spectroscopy and DFT calculations. The two-electron reduction of 1 with magnesium anthracene under more forcing conditions results in B-B bond cleavage and replacement of one thienyl sulfur atom by a mesitylboron moiety, leading to the formation of a magnesium complex of an η 5 -diborafulvene dianion. Salt metathesis of the latter with [(η 6 -p-cymene)RuCl 2 ] affords a mixed ruthenium sandwich complex of an η 5 -borylborole dianion. Calculations highlight both the structural and electronic changes in the boron-substituted heterocyclic C 4 B dianion upon switching coordination from magnesium (diborafulvene dianion) to ruthenium (borylborole dianion)., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

This combined experimental and theoretical study examines the influence of acene elongation, boron atom position, and acene substitution pattern on the structure and electronics of cyclic alkyl(amino)carbene (CAAC)-stabilized diboraacenes and presents the first syntheses of neutral diboranaphthalene (DBN) and diborapentacene (DBP). Whereas 2,3-diethyl-substituted 1,4-(CAAC) 2 - Et2 DBN is isolated as a mixture of a planar (structurally characterized) NMR-active conformer and a presumably bent EPR-active conformer, 6,13-(CAAC) 2 -DBP resembles 9,10-(CAAC) 2 -DBA (DBA = diboraanthracene), with a highly puckered 6,13-DBP core and a typical biradical EPR signal. Both species are easily reduced to their puckered dianions. DFT calculations confirm that 6,13-(CAAC) 2 -DBP is only stable in its bent conformation, whereas 1,4-(CAAC) 2 - Et2 DBN exists as both flat closed-shell and bent open-shell biradical conformers, which interchange by thermally activated ethyl and CAAC rotation/diboraacene bending processes. An in-depth computational study of the series of unsubstituted, CAAC-stabilized, symmetrically diboron-doped acenes from 1,4-(CAAC) 2 -DBN to 6,13-(CAAC) 2 -DBP was carried out. The results show interesting trends dependent on the position of the boron atoms within the acene framework as well as on the relative orientation of the CAAC ligands, which enable fine-tuning of the electronic and structural features.

A series of linear oligo( p -phenylene iminoboranes), which are BN-modified congeners of oligo( p -phenylene vinylenes), featuring pendent ferrocene groups have been prepared. Stoichiometric reaction of a bis-silylamine with a bisborane led to selective formation of an unprecedented macrocycle, without the use of a template.

Methyl and methylene compounds of arsenic and antimony have been studied by photoelectron photoion coincidence spectroscopy to investigate their relative stability. While for As both HAs=CH 2 , As-CH 3 and the methylene compound As=CH 2 are identified in the spectrum, the only Sb compound observed is Sb-CH 3 . Thus, there is a step in the main group 15 between As and Sb, regarding the relative stability of the methyl compounds. Ionisation energies, vibrational frequencies and spin-orbit splittings were determined for the methyl compound from photoion mass-selected photoelectron spectra. Although the spectroscopic results for organoantimony resemble those for the previously investigated bismuth compounds, EPR spectroscopic experiments indicate a far lower tendency for methyl transfer for Sb(CH 3 ) 3 compared to Bi(CH 3 ) 3 . This study concludes investigations on low-valent organopnictogen compounds., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Dimethylnonacethrene is the first derivative of the cethrene family that is energetically more stable than the product of its electrocyclic ring closure. Compared to the shorter homologue dimethylcethrene, the new system is EPR-active, because of a significantly lowered singlet-triplet gap, and displays remarkable stability. Our results suggest that adjustment of the steric bulk in the fjord region can enable realisation of diradicaloid-based magnetic photoswitches.

Direct insertion of unsaturated substrates into a five-membered borole ring is a useful method to obtain valuable heterocycles containing one or more three-coordinate boron atoms. A highly Lewis acidic 9-o-carboranyl-9-borafluorene, in which the o-carboranyl substituent is connected via one of the cluster carbon atoms to the boron atom of the 9-borafluorene unit, was found to react with a vast array of unsaturated molecules, such as alkynes, aldehydes and various organic azides, to form larger boraheterocyclic products. The ring expansion reactions of the central borole ring proceed rapidly at room temperature, cementing the role of the o-carboranyl substituent in enhancing the insertion reactivity of 9-borafluorenes., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

We report a detailed computational and experimental study of the fixation and reductive coupling of dinitrogen with low-valent boron compounds. Consistent with our mechanistic findings, the selectivity toward nitrogen fixation or coupling can be controlled through either steric bulk or the reaction conditions, allowing for the on-demand synthesis of nitrogen chains. The electronic structure and intriguing magnetic properties of intermediates and products of the reaction of dinitrogen with borylenes are also elucidated using high-level computational approaches.

Herein we report the development of a new methodology for the synthesis of various quinodimethane derivatives under two-electron oxidation of bis - N -heterocyclic olefins linked by different π-conjugated aromatic spacers. In case of para - and ortho -phenylene bridge, we obtained air and moisture stable diimidazolium para - and ortho -quinodimethane derivatives. Analogues of the para -phenylene spacer such as tetrafluoro- p -phenylene and p -anthracene also led to the corresponding air and moisture stable quinodimethane derivatives. This emphasizes the influence of imidazolium substituents which facilitate the air and moisture stability of the quinodimethane derivatives. Differences were observed for the electron transfer processes: two one-electron vs one two-electron redox transitions between bis - N -heterocyclic olefins and diimidazolium-quinodimethanes depending on the employed π-conjugated aromatic spacer. The formation of the π-conjugated radical-cations, transient redox intermediates between bis - N -heterocyclic olefins and diimidazolium-quinodimethanes, was addressed by an EPR investigation.

A cyclic alkyl(amino)carbene (CAAC)-stabilized dicoordinate aminoborylene is synthesized by the twofold reduction of a [(CAAC)BCl 2 (TMP)] (TMP=2,6-tetramethylpiperidyl) precursor. NMR-spectroscopic, X-ray crystallographic and computational analyses confirm the cumulenic nature of the central C=B=N moiety. Irradiation of [(CAAC)B(TMP)] (2) resulted in an intramolecular C-C bond activation, leading to a doubly-fused C 10 BN heterocycle, while the reaction with acetonitrile resulted in an aryl migration from the CAAC to the acetonitrile nitrogen atom, concomitant with tautomerization of the latter to a boron-bound allylamino ligand. One-electron oxidation of 2 with CuX (X=Cl, Br) afforded the corresponding amino(halo)boryl radicals, which were characterized by EPR spectroscopy and DFT calculations. Placing 2 under an atmosphere of CO afforded the tricoordinate (CAAC,CO)-stabilized aminoborylene. Finally, the twofold oxidation of 2 with chalcogens led, in the case of N 2 O and sulfur, to the splitting of the B-C CAAC bond and formation of the 2,4-diamino-1,3,2,4-dichalcogenadiboretanes and CAAC-chalcogen adducts, whereas with selenium a monomeric boraselenone was isolated, which showed some degree of B-Se multiple bonding., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

We present the reduction of two azaacenes (a benzo-[3,4]cyclobuta[1,2- b ]phenazine and a benzo[3,4]cyclobuta[1,2- b ]naphtho[2,3- i ]phenazine derivative), featuring a single cyclobutadiene unit, to their radical anions and dianions. The reduced species were produced using potassium naphthalenide in the presence of 18-crown-6 in THF. Crystal structures of the reduced representatives were obtained and their optoelectronic properties evaluated. Charging these 4 n Hückel systems gives dianionic 4 n + 2 π-electron systems with increased antiaromaticity, according to NICS(1.7) zz calculations, featuring unusually red-shifted absorption spectra.

Herein, a new type of carbodicarbene (CDC) comprising two different classes of carbenes is reported; NHC and CAAC as donor substituents and compare the molecular structure and coordination to Au(I)Cl to those of NHC-only and CAAC-only analogues. The conjugate acids of these three CDCs exhibit notable redox properties. Their reactions with [NO][SbF 6 ] were investigated. The reduction of the conjugate acid of CAAC-only based CDC with KC 8 results in the formation of hydrogen abstracted/eliminated products, which proceed through a neutral radical intermediate, detected by EPR spectroscopy. In contrast, the reduction of conjugate acids of NHC-only and NHC/CAAC based CDCs led to intermolecular reductive (reversible) carbon-carbon sigma bond formation. The resulting relatively elongated carbon-carbon sigma bonds were found to be readily oxidized. They were, thus, demonstrated to be potent reducing agents, underlining their potential utility as organic electron donors and n-dopants in organic semiconductor molecules., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Tetraiodotetraazapentacene I 4 TAP, the last missing derivative in the series of halogenated silylated tetraazapentacenes, was synthesized via condensation chemistry from a TIPS-ethynylated diaminobenzothiadiazol in three steps. Single and double reduction furnished its air-stable monoanion and relatively air-stable dianion, both of which were characterized by crystallography. All three species are structurally and spectroscopically compared to non-halogenated TAP and Br 4 TAP. I 4 TAP is an n-channel material in thin-film transistors with average electron mobilities exceeding 1 cm 2 (Vs) -1 ., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

A naphthyridine diimine (NDI) supported bis(germylene) NDI-Ge 2 containing two dicoordinate, coplanar Ge(II) atoms has been synthesised. Computational investigations on NDI-Ge 2 indicated the two Ge(II) atoms are nearly independent. The EDA-NOCV analysis of the [NDI-Ge 2 ][Fe 2 (CO) 6 ] complex revealed the six-electron donor behavior of NDI-Ge 2 , the first example for group-14-element-based bidentate ligands.

The choice of backbone linker for two ortho -bis-(9-borafluorene)s has a great influence on the LUMO located at the boron centers and, therefore, the reactivity of the respective compounds. Herein, we report the room temperature rearrangement of 1,2-bis-(9-borafluorenyl)- ortho -carborane, C 2 B 10 H 10 -1,2-[B(C 12 H 8 )] 2 ([2a]) featuring o -carborane as the inorganic three-dimensional backbone and the synthesis of 1,2-bis-(9-borafluorenyl)benzene, C 6 H 4 -1,2-[B(C 12 H 8 )] 2 (2b), its phenylene analog. DFT calculations on the transition state for the rearrangement support an intramolecular C-H bond activation process via an S E Ar-like mechanism in [2a], and predicted that the same rearrangement would take place in 2b, but at elevated temperatures, which indeed proved to be the case. The rearrangement gives access to 3a and 3b as dibora-benzo[ a ]fluoroanthene isomers, a form of diboron polycyclic aromatic hydrocarbon (PAH) that had yet to be explored. The isolated compounds 2b, 3a, and 3b were fully characterized by NMR, HRMS, cyclic voltammetry (CV), single-crystal X-ray diffraction analysis, and photophysical measurements, supported by DFT and TD-DFT calculations., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The stepwise reduction of a doubly cyclic alkyl(amino)carbene (CAAC)-stabilized 2,3-bis(dibromoboryl)naphthalene enables the isolation of the corresponding mono- and bis(boryl) radicals (one- and two-electron reduction), a 2π-aromatic 1,2-diborete (four-electron reduction), which shows biradical character in the solid-state EPR spectrum, and its cyclic bis(alkylidene)diboron dianion (six-electron reduction). The X-ray crystallographic analysis of the diborete shows a highly strained and twisted four-membered ring with a formal cis -diborene motif featuring a very elongated B-B double bond. Calculations based on DFT and multireference approaches reveal that the diborete possesses an open-shell singlet biradicaloid ground state, which is slightly energetically preferred to its EPR-active triplet-state congener. The addition of CO to the diborete resulted in B-B bond splitting and the formation of the corresponding closed-shell singlet, doubly Lewis base-stabilized bis(borylene), whereas a twofold γ insertion of phenyl azide generates a 1,3-bis(diazenyl)-1,3,2,4-diazadiboretidine.

A modular synthesis of both difurooxa- and difuroazadiborepins from a common precursor is demonstrated. Starting from 2,2'-bifuran, after protection of the positions 5 and 5' with bulky silyl groups, formation of the novel polycycles proceeds through opening of the furan rings to a dialkyne and subsequent re-cyclization in the borylation step. The resulting bifuran-fused diborepins show pronounced stability, highly planar tricyclic structures, and intense blue light emission. Deprotection and transformation into dibrominated building blocks that can be incorporated into π-extended materials can be performed in one step. Detailed DFT calculations provide information about the aromaticity of the constituent rings of this polycycle., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

While spin-orbit coupling does not play a decisive role in the photophysics of unsubstituted perylene diimides (PDI), this changes dramatically when two phenylselenyl or phenyltelluryl substituents were attached to the PDI bay positions. In the series of PhO-, PhS-, PhSe-, and PhTe-substituted PDIs we observed strongly decreasing fluorescence quantum yield as a consequence of strongly increasing intersystem crossing (ISC) rate, measured by transient absorption spectroscopy with fs- and ns-time resolution as well as by broadband fluorescence upconversion. Time-dependent density functional calculations suggest increasing spin-orbit coupling due to the internal heavy-atom effect as the reason for fast ISC. In case of the selenium PDI derivative we found significant singlet oxygen sensitization via the PDI triplet state. The corresponding radical anions of the chalcogen substituted PDIs were also prepared and investigated by optical and EPR spectroscopy. Here, the increasing SOC results in an increase of the g -tensor anisotropy, and of the isotropic g -value in solution, albeit quasirelativistic density functional calculations show only a relatively small fraction of the spin density to be located on the chalcogen atom.

Three [Cr 6 E 8 (PEt 3 ) 6 ] cluster molecules with E = S, Se, and Te have been synthesized by reaction of stoichiometric mixtures of Cr(II) and Cr(III) metal salts with silylated chalcogen reagents E(SiMe 3 ) 2 (E = S, Se, Te) in the presence of L = PEt 3 = triethylphosphine. For the sulfide- and selenide-bridged clusters two crystallographic forms (trigonal R 3̄ and triclinic P 1̄), which differ in the presence of lattice solvent molecules, have been isolated. Structural data, optical spectra and quantum chemical calculations reveal the presence of low-lying excited states in [Cr 6 E 8 (PEt 3 ) 6 ] (E = S, Se), which would help in rationalizing the non-vanishing magnetic moments at 2 K revealed by DC magnetic measurements and EPR spectroscopy. These findings are partially in contrast to a previous report by Saito and co-workers (S. Kamiguchi, H. Imoto, T. Saito, Inorg. Chem. , 1998, 37 , 6852-6857.), who postulated an incorporated hydrogen atom as the source of paramagnetism at low temperatures for the trigonal forms of [Cr 6 E 8 (PEt 3 ) 6 ] (E = S, Se).

The isolation of carbon-centered diradicals is always challenging due to synthetic difficulties and their limited stability. Herein we report the synthesis of a trans -1,4-cyclohexylene bridged bis-NHC-CAAC dimer derived thermally stable dicationic diradical. The diradical character of this compound was confirmed by EPR spectroscopy. The variable temperature EPR study suggests the singlet state to be marginally more stable than the triplet state (2 J = -5.5 cm -1 (Δ E ST = 0.065 kJ mol -1 )). The presence of the trans -1,4-cyclohexylene bridge is instrumental for the successful isolation of this dicationic diradical. Notably, in the case of ethylene or propylene bridged bis-NHC-CAAC dimers, the corresponding dicationic diradicals are transient and rearrange to hydrogen abstracted products., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Three novel tetracationic bis-triarylboranes with 3,4-ethylenedioxythiophene (EDOT) linkers, and their neutral precursors, showed significant red-shifted absorption and emission compared to their thiophene-containing analogues, with one of the EDOT-derivatives emitting in the NIR region. Only the EDOT-linked trixylylborane tetracation was stable in aqueous solution, indicating that direct attachment of a thiophene or even 3-methylthiophene to the boron atom is insufficient to provide hydrolytic stability in aqueous solution. Further comparative analysis of the EDOT-linked trixylylborane tetracation and its bis-thiophene analogue revealed efficient photo-induced singlet oxygen production, with the consequent biological implications. Thus, both analogues bind strongly to ds-DNA and BSA, very efficiently enter living human cells, accumulate in several different cytoplasmic organelles with no toxic effect but, under intense visible light irradiation, they exhibit almost instantaneous and very strong cytotoxic effects, presumably attributed to singlet oxygen production. Thus, both compounds are intriguing theranostic agents, whose intracellular and probably intra-tissue location can be monitored by strong fluorescence, allowing switching on of the strong bioactivity by well-focused visible light., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

We report the reduction of bulky ferrocenyl-based NHC-stabilised aluminium(iii) diiodide [Fc*(NHC)AlI 2 ] (Fc* = 2,5-bis(3,5-di- tert -butylphenyl)-1-ferrocenyl) in different hydrocarbon solvents (hexane, benzene, toluene, and p -xylene), which results in different outcomes. Reduction in hexane with an equivalent amount of KC 8 generates the diiododialane [(Fc*(NHC)AlI) 2 ], whereas complete reduction in hexane leads to an unusual C-H activation at an N-Me group of one NHC unit. In contrast, reaction in aromatic solvents result in hitherto unknown Birch-type reductions of the corresponding solvent molecules by transient aluminium radicals of the type [LAlR 2 ]˙, which is ultimately bound to two aluminium centers., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

[VCl 3 (THF) 3 ] offers a convenient entrance point into the chemistry of carbene stabilized V(III) complexes. Herein we report the paramagnetic mono- and biscarbene complexes [VCl 3 (cAAC Me )] 1, [VCl 3 (cAAC Me )(THF)] 1(thf), [VCl 3 (IMes)] 2, [{VCl 2 (IiPr Me )(μ-Cl)} 2 ] 3, [VCl 3 (IDipp)] 4, [VCl 3 (SIDipp)] 5, [VCl 3 (SIDipp)(THF)] 5(thf), [VCl 3 (I t Bu)] 6, [VCl 3 (cAAC Me ) 2 ] 7 and [VCl 3 (IiPr Me ) 2 ] 8. Reaction of 1 with MesMgCl, MesLi and LiNPh 2 afforded the complexes [VCl 2 (Mes)(cAAC Me )] 9, [cAAC Me H] + [VCl 2 Mes 2 ] - 10 and [VCl 2 (NPh 2 )(cAAC Me )] 11. The V(V) complexes [V(O)Cl 3 (IDipp)] 12 and [V(O)Cl 3 (SIDipp)] 13 were selectively prepared from oxygen oxidation of 4 and 5. [V(O)Cl 3 (IDipp)] 12 and [V(O)Cl 3 (IMes)] react with isocyanates to yield the NHC-ligated imido complexes [V(N- p -CH 3 C 6 H 4 )Cl 3 (IDipp)] 14, [V(N- p -FC 6 H 4 )Cl 3 (IDipp)] 15, [V(N- p -CH 3 C 6 H 4 )Cl 3 (SIDipp)] 16, [V(N- p -FC 6 H 4 )Cl 3 (SIDipp)] 17, [V(N- p -CH 3 C 6 H 4 )Cl 3 (IMes)] 18 and [V(N- p -FC 6 H 4 )Cl 3 (IMes)] 19.

A series of bis-(4'-pyridylethynyl)arenes (arene=benzene, tetrafluorobenzene, and anthracene) were synthesized and their bis-N-methylpyridinium compounds were investigated as a class of π-extended methyl viologens. Their structures were determined by single crystal X-ray diffraction, and their photophysical and electrochemical properties (cyclic voltammetry), as well as their interactions with DNA/RNA were investigated. The dications showed bathochromic shifts in emission compared to the neutral compounds. The neutral compounds showed very small Stokes shifts, which are a little larger for the dications. All of the compounds showed very short fluorescence lifetimes (<4 ns). The neutral compound with an anthracene core has a quantum yield of almost unity. With stronger acceptors, the analogous bis-N-methylpyridinium compound showed a larger two-photon absorption cross-section than its neutral precursor. All of the dicationic compounds interact with DNA/RNA; while the compounds with benzene and tetrafluorobenzene cores bind in the grooves, the one with an anthracene core intercalates as a consequence of its large, condensed aromatic linker moiety, and it aggregates within the polynucleotide when in excess over DNA/RNA. Moreover, all cationic compounds showed highly specific CD spectra upon binding to ds-DNA/RNA, attributed to the rare case of forcing the planar, achiral molecule into a chiral rotamer, and negligible toxicity toward human cell lines at ≤10 μM concentrations. The anthracene-analogue exhibited intracellular accumulation within lysosomes, preventing its interaction with cellular DNA/RNA. However, cytotoxicity was evident at 1 μM concentration upon exposure to light, due to singlet oxygen generation within cells. These multi-faceted features, in combination with its two-photon absorption properties, suggest it to be a promising lead compound for development of novel light-activated theranostic agents., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

New borylation methodologies have been reported recently, wherein diboron(4) compounds apparently participate in free radical couplings via the homolytic cleavage of the B-B bond. We report herein that bis-NHC adducts of the type (NHC) 2 ·B 2 (OR) 4 , which are thermally unstable and undergo intramolecular ring expansion reactions (RER), are sources of boryl radicals of the type NHC-BR 2 ˙, exemplified by Me 2 Im Me ·Bneop˙ 1a (Me 2 Im Me = 1,3,4,5-tetramethyl-imidazolin-2-ylidene, neop = neopentylglycolato), which are formed by homolytic B-B bond cleavage. Attempts to apply the boryl moiety 1a in a metal-free borylation reaction by suppressing the RER failed. However, based on these findings, a protocol was developed using Me 2 Im Me ·B 2 pin 2 3 for the transition metal- and additive-free boryl transfer to substituted aryl iodides and bromides giving aryl boronate esters in good yields. Analysis of the side products and further studies concerning the reaction mechanism revealed that radicals are likely involved. An aryl radical was trapped by TEMPO, an EPR resonance, which was suggestive of a boron-based radical, was detected in situ , and running the reaction in styrene led to the formation of polystyrene. The isolation of a boronium cation side product, [(Me 2 Im Me ) 2 ·Bpin] + I - 7, demonstrated the fate of the second boryl moiety of B 2 pin 2 . Interestingly, Me 2 Im Me NHC reacts with aryl iodides and bromides generating radicals. A mechanism for the boryl radical transfer from Me 2 Im Me ·B 2 pin 2 3 to aryl iodides and bromides is proposed based on these experimental observations., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

The synthesis of 9-borafluorene with an electron-withdrawing o -carboranyl substituent and its reactions with a series of alkenes are described. The o -carboranyl substituent is bonded via one of the cluster carbon atoms to the boron atom of the 9-borafluorene moiety. In all cases, the reactions afford partly saturated analogs of borepins ( i.e. 6,7-dihydroborepins) by unprecedented alkene insertion into the endocyclic B-C bond of the borole ring. Comparative studies with 9-bromo-9-borafluorene illustrate the superior insertion reactivity of the carboranyl-substituted derivative. A suite of experimental and computational techniques disclose the unique properties of the 9-borafluorene and provide insight into how the 9-carboranyl substituent affects its chemical reactivity., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Herein, we disclose cyclic(alkyl)(amino)carbenes (CAACs) to be one-electron reductants under the formation of a transient radical cation as indicated by EPR spectroscopy. The disclosed CAAC reducing reactivity was used to synthesize acyclic(amino)(aryl)carbene-based Thiele and Chichibabin hydrocarbons, a new class of Kekulé diradicaloids. The results demonstrate CAACs to be potent organic reductants. Notably, the acyclic(amino)(aryl)carbene-based Chichibabin's hydrocarbon shows an appreciable population of the triplet state at room temperature, as evidenced by both variable-temperature NMR and EPR spectroscopy., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The synthesis and characterisation of a homologous series of quinoid sulfur-containing imidazolyl-substituted heteroacenes is described. The optoelectronic and magnetic properties were investigated by UV/vis, fluorescence and EPR spectroscopy as well as quantum-chemical calculations, and were compared to those of the corresponding benzo congener. The room-temperature and atmospherically stable quinoids display strong absorption in the NIR region between 678 and 819 nm. The dithieno[3,2-b:2',3'-d]thiophene and the thieno[2',3':4,5]thieno[3,2-b]thieno[2,3-d]thiophene derivatives were EPR active at room temperature. For the latter, variable-temperature EPR spectroscopy revealed the presence of a thermally accessible triplet state, with a singlet-triplet separation of 14.1 kJ mol -1 ., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)