Dianionic and Neutral Diboron-Centered Classical Diradicaloids.

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.

Introducing an orthogonally polarized electron-rich alkene: synthesis of a zwitterionic boron-containing π-conjugated system.

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 (E1/2 = -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.

Bis-Olefin Based Crystalline Schlenk Hydrocarbon Diradicals with a Triplet Ground State.

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.

Metal-Free Organic Radical Spin Source.

Organic radicals have long been suggested as candidates for organic magnets and components in organic spintronic devices. Herein, we demonstrate spin current emission from an organic radical film via spin pumping at room temperature. We present the synthesis and the thin film preparation of a Blatter-type radical with outstanding stability and low roughness. These features enable the fabrication of a radical/ferromagnet bilayer, in which the spin current emission from the organic radical layer can be reversibly reduced when the ferromagnetic film is brought into simultaneous resonance with the radical. The results provide an experimental demonstration of a metal-free organic radical layer operating as a spin source, opening a new avenue for the development of purely organic spintronic devices and bridging the gap between potential and real applications.

Deciphering the ground state of a C3-symmetrical Blatter-type triradical by CW and pulse EPR spectroscopy.

3,3',3''-(Benzene-1,3,5-triyl)tris(1-phenyl-1H-benzo[e][1,2,4]triazin-4-yl) (1) is a C3-symmetrical triradical comprised of three Blatter radical units connected at the 1, 3, 5 positions of a central trimethylenebenzene core. This triradical has an excellent air, moisture, and thermal stability. Single-crystal XRD indicates that triradical 1 adopts a propeller-like geometry with the benzotriazinyl moieties twisted by 174.1(2)° and packs in 1D chains along the c axis to form an extensive network of weak intermolecular interactions. Frozen solution continuous wave (CW) EPR spectra and variable-temperature field-sweep echo-detected (FSED) spectra revealed an intramolecular ferromagnetic exchange within the spin system, supporting a quartet S = 3/2 ground state. DFT calculations further supported these experimental findings.

Air and Moisture Stable para- and ortho-Quinodimethane Derivatives Derived from bis-N-Heterocyclic Olefins.

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.

Carbodicarbenes and Striking Redox Transitions of their Conjugate Acids: Influence of NHC versus CAAC as Donor Substituents.

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][SbF6 ] were investigated. The reduction of the conjugate acid of CAAC-only based CDC with KC8 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.

A bis-NHC-CAAC dimer derived dicationic diradical.

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 (2J = -5.5 cm-1 (ΔE 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.

Realizing 1,1-Dehydration of Secondary Alcohols to Carbenes: Pyrrolidin-2-ols as a Source of Cyclic (Alkyl)(Amino)Carbenes.

Herein we report secondary pyrrolidin-2-ols as a source of cyclic (alkyl)(amino)carbenes (CAAC) for the synthesis of CAAC-CuI -complexes and cyclic thiones when reacted with CuI -salts and elemental sulfur, respectively, under reductive elimination of water from the carbon(IV)-center. This result demonstrates a convenient and facile access to CAAC-based CuI -salts, which are well known catalysts for different organic transformations. It further establishes secondary alcohols to be a viable source of carbenes-realizing after 185 years Dumas' dream who tried to prepare the parent carbene (CH2 ) by 1,1-dehydration of methanol. Addressed is also the reactivity of water towards CAACs, which proceeds through an oxidative addition of the O-H bond to the carbon(II)-center. This emphasizes the ability of carbon-compounds to mimic the reactivity of transition-metal complexes: reversible oxidative addition and reductive elimination of the O-H bond to/from the C(II)/C(IV)-centre.

Activation of O2 across a C(sp3)-C(sp3) bond.

The activation of atmospheric molecular dioxygen (O2) is reported, which occurred across a C(sp3)-C(sp3) bond of a piperazine derivative without any catalyst at ambient conditions under the formation of 1,2,4,7-dioxadiazoctane, an 8-membered (larger-ring) cyclic organic peroxide.

Synthesis and reactivity of NHC-coordinated phosphinidene oxide.

Here we report the synthesis of an N-heterocyclic carbene (NHC)-stabilised phosphinidene oxide by the controlled oxygenation of a phosphinidene under ambient conditions. This compound can be further oxygenated to a phosphinidene dioxide. The stoichiometric reduction of a phosphinidene oxide with KC8 resembles the pinacol coupling reaction-the reduction of a carbonyl compound. We also looked at the stoichiometric oxidation of NHC-coordinated phosphinidene, phosphinidene oxide and phosphinidene dioxide with [NO][SbF6].

α,α'-Diamino-p-tetrafluoroquinodimethane: Stability of One- and Two-Electron Oxidized Species and Fixation of Molecular Oxygen.

Herein, we report the synthesis, characterization, and reactivity of α,α'-diamino-p-tetrafluoroquinodimethane, a p-tetrafluorophenylene-bridged monosubstituted carbene-based Thiele's hydrocarbon A. The compound exhibits a reversible two-step one-electron oxidation with a marginally stable radical cation state B. The in situ formation of the radical cation could be confirmed by electron paramagnetic resonance spectroscopy. Interestingly, α,α'-diamino-p-tetrafluoroquinodimethane fixates atmospheric oxygen to form a 16-membered peroxide-bridged macrocyclic compound C.

Correction: Tethered CAAC-CAAC dimers: oxidation to persistent radical cations and bridging-unit dependent reactivity/stability of the dications.

Correction for 'Tethered CAAC-CAAC dimers: oxidation to persistent radical cations and bridging-unit dependent reactivity/stability of the dications' by Mithilesh Kumar Nayak et al., Chem. Commun., 2021, 57, 1210-1213, DOI: 10.1039/D0CC07385G.

Tethered CAAC-CAAC dimers: oxidation to persistent radical cations and bridging-unit dependent reactivity/stability of the dications.

Herein, we report tethered cyclic(alkyl)(amino)carbene (CAAC) dimers in which two CAAC-motifs are connected by an ethylene-, trans-1,2-cyclohexylene- and propylene-spacer through their N-centres. The 1-electron oxidized radical cations are isolable, whereas a significant influence of the bridging unit on the chemical reactivity becomes apparent in and with the 2-electron oxidized products.

Molecular enneanuclear CuII phosphates containing planar hexanuclear and trinuclear sub-units: syntheses, structures, and magnetism.

Highly symmetric enneanuclear copper(ii) phosphates [Cu9(Pz)6(µ-OH)3(µ3-OH)(ArOPO3)4(DMF)3] (PzH = pyrazole, Ar = 2,6-(CHPh2)2-4-R-C6H2; R = Me, 2MeAr; Et, 2EtAr; iPr, 2iPrAr; and Ar = 2,6-iPr2C6H3, 2Dip) comprising nine copper(ii) centers and pyrazole, hydroxide and DMF as ancillary ligands were synthesized by a reaction involving the arylphosphate monoester, 1, copper(i)chloride, pyrazole, and triethylamine in a 4 : 9 : 6 : 14 ratio. All four complexes were characterized by single crystal structural analysis. The complexes contain two distinct structural motifs within the multinuclear copper scaffold: a hexanuclear unit and a trinuclear unit. In the latter, the three Cu(ii) centres are bridged by a µ3-OH. Each pair of Cu(ii) centers in the trinuclear unit are bridged by a pyrazole ligand. The hexanuclear unit is made up of three dinuclear Cu(ii) motifs where the two Cu(ii) centres are bridged by an -OH and a pyrazole ligand. The three dinuclear units are connected to each other by phosphate ligands. The latter also aid the fusion of the trinuclear and the hexanuclear motifs. Magnetic studies reveal a strong antiferromagnetic exchange between the Cu(ii) centres of the dinuclear units in the hexanuclear part and a strong spin frustration in the trinuclear part leading to a degenerate ground state.

Crystal structure of benzo[h]quinoline-3-carbox-amide.

The title com-pound, C14H10N2O, crystallizes in the monoclinic space group P21/c with four mol-ecules in the unit cell. All 17 non-H atoms of one mol-ecule lie essentially in one plane. In the unit cell, two pairs of mol-ecules are exactly coplanar, while the angle between these two orientations is close to perfectly perpendicular at 87.64 (6)°. In the crystal, mol-ecules adopt a 50:50 crisscross arrangement, which is held together by two nonclassical and two classical inter-molecular hydrogen bonds. The hydrogen-bonding network together with off-centre π-π stacking inter-actions between the pyridine and outermost benzene rings, stack the mol-ecules along the b-axis direction.

Structural Diversity in Supramolecular Organization of Anionic Phosphate Monoesters: Role of Cations.

Syntheses and structures of anionic arylphosphate monoesters [ArOP(O)2(OH)]- (Ar = 2,6-CHPh2-4-R-C6H2; R = Me/Et/iPr/tBu) with different counter cations are reported. The counter cations were varied systematically: imidazolium cation, 2-methyl imidazolium cation, N-methyl imidazolium cation, N,N'-alkyl substituted imidazolium cation, 1,4-diazabicyclo[2.2.2]octan-1-ium cation, 4,4'-bipyridinium dication, and magnesium(II) dication. The objective was to examine if the supramolecular structure of anionic arylphosphate monoesters could be modulated by varying the cation. It was found that an eight-membered P2O4H2-hydrogen-bonded dimeric motif involving intermolecular H-bonding between the [P(O)(OH)] unit of the anionic phosphate monoester along with the counter cation is formed with 2-methyl imidazolium cation, N-methyl imidazolium cation, N,N'-alkyl substituted imidazolium cation, 1,4-diazabicyclo[2.2.2]octan-1-ium cation, and magnesium(II) dication; both discrete and polymeric H-bonded structures are observed. In the case of imidazolium cations and 1,4-diazabicyclo[2.2.2]octan-1-ium cation, the formation of one-dimensional polymers (single lane/double lane) was observed. On the other hand, two types of phosphate motifs, intermolecular H-bonded dimer and an open-form, were observed in the case of 4,4'-bipyridinium dication.

Pd/PTABS: Low-Temperature Thioetherification of Chloro(hetero)arenes.

The thioetherification of heteroaryl chlorides is an essential synthetic methodology that provides access to bioactive drugs and agrochemicals. Due to their (actual or potential) industrial importance, the development of efficient and low-temperature protocols for accessing these compounds is a requirement for economic and ecologic reasons. A particular highly effective catalytic protocol using the Pd/PTABS system at only 50 °C was developed accordingly. The coupling between chloroheteroarenes and a variety of less reactive arylthiols and alkylthiols was carried out with a high efficiency. Heteroarenes of commercial relevance such as purines and pyrimidines were also found to be useful substrates for the reported transformation. The commercial drug Imuran (azathioprine) was synthesized as an example, and its preparation could be optimized. DFT studies were performed to understand the electronic effects of the tested ligands on the catalytic reaction.

Influence of N-Substitution on the Formation and Oxidation of NHC-CAAC-Derived Triazaalkenes.

We have studied the effect of N-substitution on the course of the reaction of imidazolium triflate. The reaction of N-heterocyclic carbene with N-tBu-substituted pyrrolinium triflate afforded 2-(pyrrolidin-2-yl)-imidazolium triflate, 3R. Treatment of 3R with potassium bis(trimethylsilyl)amide (KHMDS) leads to either the dealkylation product 4 or the deprotonation product, triazaalkene 5, depending on the N-substitution at the imidazolium moiety. Density functional studies using the B3LYP/TZVP setup have been employed to explore various pathways for the dealkylation reaction and the calculated energies support the dealkylation by a large energy margin compared to the deprotonatation process. Theoretical calculations revealed that dealkylation reaction is thermodynamically more favorable than deprotonation. The triazaalkene 5 could be oxidized by AgOTf to the corresponding radical cation 6 and dication 7 in-situ. While 6 and 7 could not be isolated, the formation of the former is inferred by electron paramagnetic resonance spectroscopy and its abstraction of a H-atom to afford 3Me. Similarly, the formation of the dication 7 is inferred by its ready elimination of isobutylene affording 8.

Comparison of molybdenum and rhenium oxo bis-pyrazine-dithiolene complexes - in search of an alternative metal centre for molybdenum cofactor models.

A pair of structurally precise analogues of molybdenum and rhenium complexes, [Et4N]/K2[MoO(prdt)2] and K[ReO(prdt)2] (prdt = pyrazine-2,3-dithiolene), were synthesized. These complexes serve as structural models for the active sites of bacterial molybdenum cofactor containing enzymes. They were comprehensively characterized and investigated by NMR, computationally supported IR and resonance Raman spectroscopy, cyclic voltammetry, mass spectrometry, elemental analysis and single-crystal X-ray diffraction. All compiled data are discussed in the context of comparing chemical and electronic structures and consequences thereof. This study constitutes the first investigation of a potential alternative Moco model system bearing rhenium as the central metal in an identical coordination environment to its molybdenum analogue. Structural evaluation revealed a slightly stronger M[double bond, length as m-dash]O bond in the rhenium complex in accordance with spectroscopic results, i.e. observed bond strengths. Thermodynamic parameters for the redox processes MoIV ↔ MoV and ReIV ↔ ReV were obtained by temperature dependent cyclic voltammetry. In contrast to molybdenum, rhenium loses entropy upon reduction and its redox potential is more temperature sensitive, indicating more significant differences than the respective diagonal relationship between the two metals in the periodic table might suggest and questioning rhenium's suitability as a functional artificial active site metal.