Reactions of [Rh(κ²-O,O-acac)(PMe₃)₂] (acac=acetylacetonato) and α,ω-bis(arylbutadiynyl)alkanes afford two isomeric types of MC₄ metallacycles with very different photophysical properties. As a result of a [2+2] reductive coupling at Rh, 2,5-bis(arylethynyl)rhodacyclopentadienes (A) are formed, which display intense fluorescence (Φ=0.07–0.54, τ=0.2–2.5 ns) despite the presence of the heavy metal atom. Rhodium biphenyl complexes (B), which show exceptionally long-lived (hundreds of μs) phosphorescence (Φ=0.01–0.33) at room temperature in solution, have been isolated as a second isomer originating from an unusual [4+2] cycloaddition reaction and a subsequent β-H-shift. We attribute the different photophysical properties of isomers A and B to a higher excited state density and a less stabilized T₁ state in the biphenyl complexes B, allowing for more efficient intersystem crossing S₁T_n and T₁S₀. Control of the isomer distribution is achieved by modification of the bis- (diyne) linker length, providing a fundamentally new route to access photoactive metal biphenyl compounds.

The mono- and bis-reduction of 6,13-bis((triisopropylsilyl)ethynyl)quinoxalino[2,3-b]phenazine (1) with potassium anthracenide in THF is reported. Both the radical anion 1^.− and the dianion 1²⁻ were isolated and characterized by optical and structural (single-crystal X-ray diffraction) methods. Solutions of the radical anion 1^.− were stable in air for several hours and characterized by EPR spectroscopy. Dianion 1²⁻ is highly fluorescent and photostable.

Das Mono- und das Dianion von 6,13-Bis((triisopropylsilyl)ethinyl)chinoxalino[2,3-b]phenazin wurden durch Reduktion mit Kaliumanthracenid in THF synthetisiert und anschließend charakterisiert. Sowohl das Radikalanion, 1^.−, als auch das Dianion, 1²⁻, konnten isoliert sowie durch optische Methoden und Einkristallstrukturanalyse charakterisiert werden. Lösungen des Radikalanions 1^.− sind mehrere Stunden lang luftstabil und wurden durch EPR-Spektroskopie charakterisiert. Das Dianion 1²⁻ ist hochfluoreszierend und photostabil.

The nickel-catalyzed alkyl–alkyl cross-coupling (CC bond formation) and borylation (CB bond formation) of unactivated alkyl halides reported in the literature show completely opposite reactivity orders in the reactions of primary, secondary, and tertiary alkyl bromides. The proposed Ni^I/Ni^III catalytic cycles for these two types of bond-formation reactions were studied computationally by means of DFT calculations at the B3LYP level. These calculations indicate that the rate-determining step for alkyl–alkyl cross-coupling is the reductive elimination step, whereas for borylation the rate is determined mainly by the atom-transfer step. In borylation reactions, the boryl ligand involved has an empty p orbital, which strongly facilitates the reductive elimination step. The inability of unactivated tertiary alkyl halides to undergo alkyl–alkyl cross-coupling is mainly due to the moderately high reductive elimination barrier.

A zinc-catalyzed combined CX and CH borylation of aryl halides using B₂pin₂ (pin=OCMe₂CMe₂O) to produce the corresponding 1,2-diborylarenes under mild conditions was developed. Catalytic CH bond activation occurs ortho to the halide groups if such a site is available or meta to the halide if the ortho position is already substituted. This method thus represents a novel use of a group XII catalyst for CH borylation. This transformation does not proceed via a free aryne intermediate, but a radical process seems to be involved.

1,2-Diborylarene konnten durch Zink-katalysierte C-X- und C-H-Borylierung von Arylhalogeniden bei milden Reaktionsbedingungen unter Verwendung von B₂pin₂ (pin=OCMe₂CMe₂O) synthetisiert werden. Die katalytische C-H-Bindungsaktivierung erfolgt in ortho-Position zum Halogensubstituenten, sofern diese Stelle zugänglich ist, oder in meta-Position, falls die ortho-Position substituiert ist. Der Prozess beinhaltet somit eine neuartige Verwendung von Gruppe-XII-Katalysatoren zur C-H-Borylierung. Die Reaktion verläuft nicht über ein freies Arin als Zwischenstufe, vielmehr scheint ein radikalischer Prozess vorzuherrschen.

The push–pull character of a series of donor–bithienyl–acceptor compounds has been tuned by adopting triphenylamine or 1,1,7,7-tetramethyljulolidine as a donor and B(2,6-Me₂-4-RC₆H₂)₂ (R=Me, C₆F₅ or 3,5-(CF₃)₂C₆H₃) or B[2,4,6-(CF₃)₃C₆H₂]₂ as an acceptor. Ir-catalyzed CH borylation was utilized in the derivatization of the boryl acceptors and the tetramethyljulolidine donor. The donor and acceptor strengths were evaluated by electrochemical and photophysical measurements. In solution, the compound with the strongest acceptor, B[2,4,6-(CF₃)₃C₆H₂]₂ ((FMes)₂B), has strongly quenched emission, while all other compounds show efficient green to red (Φ_F=0.80–1.00) or near-IR (NIR; Φ_F=0.27–0.48) emission, depending on solvent. Notably, this study presents the first examples of efficient NIR emission from three-coordinate boron compounds. Efficient solid-state red emission was observed for some derivatives, and interesting aggregation-induced emission of the (FMes)₂B-containing compound was studied. Moreover, each compound showed a strong and clearly visible response to fluoride addition, with either a large emission-color change or turn-on fluorescence.

A new catalytic system based on a Zn^II NHC precursor has been developed for the cross-coupling reaction of alkyl halides with diboron reagents, which represents a novel use of a Group XII catalyst for CX borylation. This approach gives borylations of unactivated primary, secondary, and tertiary alkyl halides at room temperature to furnish alkyl boronates, with good functional-group compatibility, under mild conditions. Preliminary mechanistic investigations demonstrated that this borylation reaction seems to involve one-electron processes.

Introduced herein is a series of conjugated thienylboranes, which are inert to air and moisture, and even resist acids and strong bases. X-ray analyses reveal a coplanar arrangement of the thiophene rings, an arrangement which facilitates p–π conjugation through the boron atoms despite the presence of highly bulky 2,4,6-tri-tert-butylphenyl (Mes*) or 2,4,6-tris(trifluoromethyl)phenyl (^FMes) groups. Short B⋅⋅⋅F contacts, which lead to a pseudotrigonal bipyramidal geometry in the ^FMes species, have been further studied by DFT and AIM analysis. In contrast to the Mes* groups, the highly electron-withdrawing ^FMes groups do not diminish the Lewis acidity of boron toward F⁻ anions. These compounds can be lithiated or iodinated under electrophilic conditions without decomposition, thus offering a promising route to larger conjugated structures with electron-acceptor character.

Als neuartiges Beispiel eines Gruppe-XII-Katalysators zur C-X-Borylierung wurde ein katalytisches System basierend auf einer Zn^II-NHC-Vorstufe für die Kreuzkupplung von Alkylhalogeniden mit Diborreagentien entwickelt. Durch diesen Ansatz wurde die Borylierung von nicht-aktivierten primären, sekundären und tertiären Alkylhalogeniden zu Alkylboronaten bei Raumtemperatur erreicht. Die Umsetzungen erfolgen unter milden Bedingungen bei guter Toleranz anderer funktioneller Gruppen. Erste Ergebnisse bezüglich des Reaktionsmechanismus der Borylierungsreaktion deuten auf die Beteiligung von Ein-Elektronen-Prozessen hin.

A series of 2,5-bis(arylethynyl)rhodacyclopentadienes has been prepared by a rare example of regiospecific reductive coupling of 1,4-(p-R-phenyl)-1,3-butadiynes (RH, Me, OMe, SMe, NMe₂, CF₃, CO₂Me, CN, NO₂, −CC-(p-C₆H₄NHex₂), −CC(p-C₆H₄CO₂Oct)) at [RhX(PMe₃)₄] (1) (X=−CCSiMe₃ (a), −CC-(p-C₆H₄NMe₂) (b), −CCCC(p-C₆H₄NPh₂) (c) or CC{p-C₆H₄-CC(p-C₆H₄-N(C₆H₁₃)₂)} (d) or Me (e)), giving the 2,5-bis(arylethynyl) isomer exclusively. The rhodacyclopentadienes bearing a methyl ligand in the equatorial plane (compound 1 e) have been converted into their chloro analogues by reaction with HCl etherate. The rhodacycles thus obtained are stable to air and moisture in the solid state and the acceptor-substituted compounds are even stable to air and moisture in solution. The photophysical properties of the rhodacyclopentadienes are highly unusual in that they exhibit, exclusively, fluorescence between 500–800 nm from the S₁ state, with quantum yields of Φ=0.01–0.18 and short lifetimes (τ=0.45–8.20 ns). The triplet state formation (Φ_ISC=0.57 for 2 a) is exceptionally slow, occurring on the nanosecond timescale. This is unexpected, because the Rh atom should normally facilitate intersystem crossing within femto- to picoseconds, leading to phosphorescence from the T₁ state. This work therefore highlights that in some transition-metal complexes, the heavy atom can play a more subtle role in controlling the photophysical behavior than is commonly appreciated.

Introduced herein is a series of conjugated thienylboranes, which are inert to air and moisture, and even resist acids and strong bases. X-ray analyses reveal a coplanar arrangement of the thiophene rings, an arrangement which facilitates p–π conjugation through the boron atoms despite the presence of highly bulky 2,4,6-tri-tert-butylphenyl (Mes*) or 2,4,6-tris(trifluoromethyl)phenyl (^FMes) groups. Short B⋅⋅⋅F contacts, which lead to a pseudotrigonal bipyramidal geometry in the ^FMes species, have been further studied by DFT and AIM analysis. In contrast to the Mes* groups, the highly electron-withdrawing ^FMes groups do not diminish the Lewis acidity of boron toward F⁻ anions. These compounds can be lithiated or iodinated under electrophilic conditions without decomposition, thus offering a promising route to larger conjugated structures with electron-acceptor character.

Quadrupolar oligothiophene chromophores composed of four to five thiophene rings with two terminal (E)-dimesitylborylvinyl groups (4 V–5 V), and five thiophene rings with two terminal aryldimesitylboryl groups (5 B), as well as an analogue of 5 V with a central EDOT ring (5 VE), have been synthesized via Pd-catalyzed cross-coupling reactions in high yields (66–89 %). Crystal structures of 4 V, 5 B, bithiophene 2 V, and five thiophene-derived intermediates are reported. Chromophores 4 V, 5 V, 5 B and 5 VE have photoluminescence quantum yields of 0.26–0.29, which are higher than those of the shorter analogues 1 V–3 V (0.01–0.20), and short fluorescence lifetimes (0.50–1.05 ns). Two-photon absorption (TPA) spectra have been measured for 2 V–5 V, 5 B and 5 VE in the range 750–920 nm. The measured TPA cross-sections for the series 2 V–5 V increase steadily with length up to a maximum of 1930 GM. We compare the TPA properties of 2 V–5 V with the related compounds 5 B and 5 VE, giving insight into the structure–property relationship for this class of chromophore. DFT and TD-DFT results, including calculated TPA spectra, complement the experimental findings and contribute to their interpretation. A comparison to other related thiophene and dimesitylboryl compounds indicates that our design strategy is promising for the synthesis of efficient dyes for two-photon-excited fluorescence applications.

A new synthetic approach to hydroboranes catechol-, pinacol-, and neopentylglycolborane has been developed. Starting from diboranes(4) B₂cat₂, B₂pin₂, or B₂neop₂, the respective boranes were obtained by heterogeneously catalyzed cleavage of the BB bond in the respective diboranes with hydrogen. Group 10 metals were found to be effective catalysts for this reaction.

Four linear π-conjugated systems with 1,3-diethyl-1,3,2-benzodiazaborolyl [C₆H₄(NEt)₂B] as a π-donor at one end and dimesitylboryl (BMes₂) as a π-acceptor at the other end were synthesized. These unusual push–pull systems contain phenylene (1,4-C₆H₄; 1), biphenylene (4,4′-(1,1′-C₆H₄)₂; 2), thiophene (2,5-C₄H₂S; 3), and dithiophene (5,5′-(2,2′-C₄H₂S)₂; 4) as π-conjugated bridges and different types of three-coordinate boron moieties serving as both π-donor and π-acceptor. Molecular structures of 2, 3, and 4 were determined by single-crystal X-ray diffraction. Photophysical studies on these systems reveal blue-green fluorescence in all compounds. The Stokes shifts for 1, 2, and 3 are notably large at 7820–9760 cm⁻¹ in THF and 5430–6210 cm⁻¹ in cyclohexane, whereas the Stokes shift for 4 is significantly smaller at 5510 cm⁻¹ in THF and 2450 cm⁻¹ in cyclohexane. Calculations on model systems 1′–4′ show the HOMO to be mainly diazaborolyl in character and the LUMO to be dominated by the empty p orbital at the boron atom of the BMes₂ group. However, there are considerable dithiophene bridge contributions to both orbitals in 4′. From the experimental data and MO calculations, the π-electron-donating strength of the 1,3-diethyl-1,3,2-benzodiazaborolyl group was found to lie between that of methoxy and dimethylamino groups. TD-DFT calculations on 1′–4′, using B3LYP and CAM-B3LYP functionals, provide insight into the absorption and emission processes. B3LYP predicts that both the absorption and emission processes have strong charge-transfer character. CAM-B3LYP which, unlike B3LYP, contains the physics necessary to describe charge-transfer excitations, predicts only a limited amount of charge transfer upon absorption, but somewhat more upon emission. The excited-state (S₁) geometries show the borolyl group to be significantly altered compared to the ground-state (S₀) geometries. This borolyl group reorganization in the excited state is believed to be responsible for the large Stokes shifts in organic systems containing benzodiazaborolyl groups in these and related compounds.

An efficient synthetic route to 2- and 2,7-substituted pyrenes is described. The regiospecific direct CH borylation of pyrene with an iridium-based catalyst, prepared in situ by the reaction of [{Ir(μ-OMe)cod}₂] (cod=1,5-cyclooctadiene) with 4,4′-di-tert-butyl-2,2′-bipyridine, gives 2,7-bis(Bpin)pyrene (1) and 2-(Bpin)pyrene (2, pin=OCMe₂CMe₂O). From 1, by simple derivatization strategies, we synthesized 2,7-bis(R)-pyrenes with R=BF₃K (3), Br (4), OH (5), B(OH)₂ (6), and OTf (7). Using these nominally nucleophilic and electrophilic derivatives as coupling partners in Suzuki–Miyaura, Sonogashira, and Buchwald–Hartwig cross-coupling reactions, we obtained 2,7-bis(R)-pyrenes with R=(4-CO₂C₈H₁₇)C₆H₄ (8), Ph (9), C≡CPh (10), C≡C[{4-B(Mes)₂}C₆H₄] (11), C≡CTMS (12), C≡C[(4-NMe₂)C₆H₄] (14), C≡CH (15), N(Ph)[(4-OMe)C₆H₄] (16), and R=OTf, R′=C≡CTMS (13). Lithiation of 4, followed by reaction with CO₂, yielded pyrene-2,7-dicarboxylic acid (17), whilst borylation of 2-tBu-pyrene gave 2-tBu-7-Bpin-pyrene (18) selectively. By similar routes (including Negishi cross-coupling reactions), monosubstituted 2-R-pyrenes with R=BF₃K (19), Br (20), OH (21), B(OH)₂ (22), [4-B(Mes)₂]C₆H₄ (23), B(Mes)₂ (24), OTf (25), C≡CPh (26), C≡CTMS (27), (4-CO₂Me)C₆H₄ (28), C≡CH (29), C₃H₆CO₂Me (30), OC₃H₆CO₂Me (31), C₃H₆CO₂H (32), OC₃H₆CO₂H (33), and O(CH₂)₁₂Br (34) were obtained from 2. These derivatives are of synthetic and photophysical interest because they contain donor, acceptor, and conjugated substituents. The crystal structures of compounds 4, 5, 7, 12, 18, 19, 21, 23, 26, and 28–31 have also been obtained from single-crystal X-ray diffraction data, revealing a diversity of packing modes, which are described in the Supporting Information. A detailed discussion of the structures of 1 and 2, their polymorphs, solvates, and co-crystals is reported separately.