Enhancing the Acceptor Character of Conjugated Organoborane Macrocycles: A Highly Electron‐Deficient Hexaboracyclophane

We introduce a new boron‐doped cyclophane, the hexabora[1₆]cyclophane B6‐^FMes , in which six tricoordinate borane moieties alternate with short conjugated p‐phenylene linkers. Exocyclic 2,4,6‐tris(trifluoromethyl)phenyl (^FMes) groups serve not only to further withdraw electron density but at the same time sterically shield the boron atoms, resulting in a macrocycle that is both highly electron‐deficient and stable. The optical and electronic properties are compared with those of related linear oligomers and the electronic structure is further evaluated by computational methods. The studies uncover unique properties of B6‐^FMes , including a low‐lying and extensively delocalized LUMO and a wide HOMO–LUMO gap, which arise from the combination of a cyclic π‐system, strong electronic communication between the closely spaced borons, and the attachment of electron‐deficient pendent groups. The binding of small anions to the electron‐deficient macrocycle and molecular model compounds is investigated and emissive exciplexes are detected in aromatic solvents.     

Versatile Design Principles for Facile Access to Unstrained Conjugated Organoborane Macrocycles

A facile and versatile approach was developed to access ambipolar boron‐containing macrocycles. Two examples of new conjugated cyclic motifs are presented with carbazole moieties as donors and borane moieties as acceptors embedded into the ring system. They were first predicted using computational methods. Possible targets with appropriately shaped π‐conjugated bridges that minimize the overall ring strain were identified and their geometry was optimized by DFT methods. The synthetic demonstration was then accomplished using organometallic condensation reactions under high dilution conditions. The resulting monodisperse macrocycles provide important insights into the design principles necessary for the preparation of new unstrained macrocycles with interesting optical and electronic characteristics. The current research also offers a more general approach to conjugated ambipolar B/N macrocycles as a promising new family of (opto)electronic materials.     

Planar Chiral Organoborane Lewis Acids Derived from Naphthylferrocene

Enantiomerically pure metalated 2‐(1‐naphthyl)ferrocene (NpFc) derivatives NpFcM (M=SnMe₃, HgCl) were prepared and characterized by multinuclear NMR and UV/Vis spectroscopy, cyclic voltammetry, and elemental analysis. Optical rotation measurements were performed and the absolute configuration of the new planar chiral ferrocene species was confirmed by single‐crystal X‐ray diffraction analysis. The mercuriated species NpFcHgCl proved suitable as a reagent for the preparation of the chiral organoborane Lewis acid NpFcBCl₂, which can in turn be converted to other ferrocenylboranes by replacement of Cl with nucleophiles. The highly Lewis acidic perfluoroarylborane derivatives NpFcB(C₆F₅)Cl and NpFcB(C₆F₅)₂ were successfully prepared by treatment with CuC₆F₅. The structures were studied by single‐crystal X‐ray diffraction and variable‐temperature ¹⁹F NMR spectroscopy, which suggested that π stacking of a C₆F₅ group on boron with the adjacent naphthyl group is energetically favorable. UV/Vis absorption spectroscopy and cyclic voltammetry measurements were performed to examine the electronic properties of these novel redox‐active chiral Lewis acids.     

Highly Electron‐Deficient and Air‐Stable Conjugated Thienylboranes

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.     

5,5,15,15-Tetraoxo-5,15-Dithiaporphyrin as a Highly Electron-Deficient Porphyrinic Ligand

Oxidation of 5,15-dithiaporphyrin with meta-chloroperbenzoic acid afforded the corresponding S,S-tetraoxide in good yield. The resultant 5,5,15,15-tetraoxo-5,15-dithiaporphyrin exhibited the highly electron-deficient nature as elucidated by the electrochemical analysis and theoretical calculations. Treatment of tetraoxodithiaporphyrin with zinc(II) acetate and nickel(II) acetate provided the corresponding metal complexes efficiently. Owing to its enhanced Lewis acidity of the metal center by the electron-deficient ligand, the nickel complex underwent facile axial ligation to form pentacoordinate and hexacoordinate high-spin (S=1) complexes in solution and solid, respectively. The binding constant of pyridine to the Ni^II center was significantly higher than those of conventional porphyrin Ni^II complexes. Temperature-dependent magnetic susceptibility measurements of the high-spin Ni^II complex revealed the presence of weak ferromagnetic interactions.     

Electron-Deficient Conjugated Materials via p–π* Conjugation with Boron: Extending Monomers to Oligomers,Macrocycles, and Polymers

The extension of conjugated organoboranes from monomeric species to oligomers, macrocycles, and polymers offers access to a plethora of fascinating new materials. The p–π* conjugation between empty orbitals on boron and the conjugated linkers not only affects the electronic structure and optical properties, but also enables mutual interactions between electron-deficient boron centers. The unique properties of these electron-deficient π-conjugated systems are exploited in highly luminescent materials, organic optoelectronic devices, and sensing applications.     

Boron(II) Cations: Interplay between Lewis‐Pair‐Acceptor and Electron‐Donor Properties

Boron(III) cations are widely used as highly Lewis acidic reagents in synthetic chemistry. In contrast, boron(II) cations are extremely rare and their chemistry almost completely unknown. They are both Lewis acids and electron donors, properties that are commonly associated with catalytically active late‐transition‐metal complexes. This double reactivity pattern ensures a rich and diverse chemistry. Herein we report the facile synthesis of several new boron(II) cations starting with a special diborane with two easily exchangeable triflate substituents. By increasing the π‐acceptor character of the neutral σ‐donor reaction partners, first reactions were developed in which the combined Lewis acidity and electron‐donor properties of boron(II) cations are applied for the reduction of organic molecules. The results of our study pave the way for applications of these unusual compounds in synthetic chemistry.     

Controlling the Optoelectronic Properties of Pyrene by Regioselective Lewis Base-Directed Electrophilic Aromatic Borylation

Given that pyrene represents one of the most versatile chromophores, the development of new selective routes for its functionalization and tuning of its emission properties is highly desirable. Pyrene-based BN Lewis pair (LP)-functionalized polycyclic aromatic hydrocarbons (PAHs) were prepared by regioselective Lewis base-directed electrophilic aromatic substitution. The requisite 1,6-dipyridylpyrene ligands were accessed by Suzuki–Miyaura cross-coupling of 1,6-bis(pinacolatoboryl)pyrenes with 2-bromopyridine derivatives. Subsequent electrophilic borylation with BCl₃ in the presence of AlCl₃ and 2,6-di-tert-butylpyridine as a hindered base produced the dichloroborane complexes, which were then in situ reacted with diphenyl or diethyl zinc. The presence or absence of alkyl chains in the 3,8-positions of the pyrene moiety determined the position of the B−C bond formation (2,7 in the non-K region versus 5,10 in the K region) and thereby also the size of the BN heterocycle (five- versus six-membered). The impact of the regioisomeric borylation on the electrochemical, photophysical and structural properties was investigated and the conclusions supported by theoretical calculations. The rapid synthesis of derivatives that are borylated in the K region also suggests strong potential for the development of pyrene derivatives that are otherwise difficult to access.     

Selective N,O‐Addition of the TEMPO Radical to Conjugated Boryldienes

B(C₆F₅)₂‐containing boryldienes 4 underwent the addition of two molar equivalents of TEMPO to give N,O‐bonded four‐membered heterocyclic products 7 . The reaction is a metal‐free example of the generation of reactive nitrogen‐centered TEMPO radical derivatives, in this case by the addition of TEMPO to the borane, followed by carbon–nitrogen bond formation and subsequent trapping of the resulting allyl radical by the second equivalent of TEMPO.     

One-step Oxidative Monofluorination of Electron-Deficient Sulfoxides to Access Highly Lewis Acidic Sulfur(VI) Cations

The strongly oxidizing, powerful electrophilic fluorination reagent [FXe][OTf] is shown to effect direct oxidative monofluorination of sulfoxides. This one-step, chloride promoter-free methodology provides access to so far inaccessible, yet highly desirable strongly Lewis acidic fluorosulfoxonium cations from electron-deficient and/or sterically demanding sulfoxides that are shown to be practically unreactive towards the previously reported XeF₂/NEt₄Cl system. Experimental and density functional theory studies have been conducted to assess the Lewis acidities of the prepared sulfur(VI) cations. Preliminary results obtained with chiral sulfoxides provide early insights into the mechanism of these fluorination reactions.     

Boron‐Containing Polycyclic Aromatic Hydrocarbons: Facile Synthesis of Stable,Redox‐Active Luminophores

Herein we show that replacing the two meso carbon atoms of the polycyclic aromatic hydrocarbon (PAH) bisanthene by boron atoms transforms a near‐infrared dye into an efficient blue luminophore. This observation impressively illustrates the impact of boron doping on the frontier orbitals of PAHs. To take full advantage of this tool for the targeted design of organic electronic materials, the underlying structure–property relationships need to be further elucidated. We therefore developed a modular synthesis sequence based on a Peterson olefination, a stilbene‐type photocyclization, and an Si–B exchange reaction to substantially broaden the palette of accessible polycyclic aromatic organoboranes and to permit a direct comparison with their PAH congeners.     

9,10‐Dihydro‐9,10‐diboraanthracene: Supramolecular Structure and Use as a Building Block for Luminescent Conjugated Polymers

Building bridges : The title compound forms an unprecedented polymeric structure with bridging B–H–B three‐center two‐electron bonds in the solid state. This organoborane serves as an efficient precursor for the preparation of boron‐doped π‐conjugated polymers by hydroboration polymerization with a functionalized 1,4‐diethynylbenzene (see picture). These polymers form thin films that show intense green luminescence.

     

Diazadibora[1.1.1.1]m,p,m,p‐cyclophanes: Ambipolar Conjugated Macrocycles with Different B–π–N Embedded Patterns

Are different B(boron)–π–N(nitrogen) embedded patterns to bring about significant different (opto)electronic properties for the same macrocyclic molecular backbone? A series of B–π–N‐embedded alternate‐meta‐para‐linked cyclophanes 1 – 3 have been prepared and characterized as a new class of ambipolar π‐conjugated B–π–N macrocycles. The answer to the opening question is yes. These macrocycles revealed the intramolecular charge transfer in the oxidized states and the intriguing photophysical proprerties in accordance with the embedded patterns, suggesting the electronic structures are tunable by introducing multiple B–π–N moieties.     

Highly Luminescent Microporous Organic Polymer with Lewis Acidic Boron Sites on the Pore Surface: Ratiometric Sensing and Capture of F− Ions

Reversible and selective capture/detection of F^? ions in water is of the utmost importance, as excess intake leads to adverse effects on human health. Highly robust Lewis acidic luminescent porous organic materials have potential for efficient sequestration and detection of F^? ions. Herein, the rational design and synthesis of a boron‐based, Lewis acidic microporous organic polymer (BMOP) derived from tris(4‐bromo‐2,3,5,6‐tetramethylphenyl)boron nodes and diethynylbiphenyl linkers with a pore size of 1.08 nm for selective turn‐on sensing and capture of F^? ion are reported. The presence of a vacant p_π orbital on the boron center of BMOP results in intramolecular charge transfer (ICT) from the linker to boron. BMOP shows selective turn‐on blue emission for F^? ions in aqueous mixtures with a detection limit of 2.6 μM . Strong B–F interactions facilitate rapid sequestration of F^? by BMOP. The ICT emission of BMOP can be reversibly regenerated by addition of an excess of water, and the polymer can be reused several times.     

Optical Characteristics of Hybrid Macrocycles with Dithienogermole and Tricoordinate Boron Units

The introduction of unconventional elements into π-conjugated systems has been studied to manipulate the electronic states and properties of compounds. Herein, boron- and germanium-containing hybrid macrocycles, as a new class of element-hybrid conjugated systems, have been synthesized. The palladium-catalyzed Stille cross coupling of bis(bromothienyl)borane and bis(trimethylstannylthienyl)- or bis(trimethylstannylphenyl)-substituted dithienogermoles as the boron- and germanium-containing building blocks, respectively, produced a mixture of several macrocyclic compounds. Single-crystal X-ray analysis of the 2:2 coupling product revealed a planar structure with a cavity inside the macrocycle. The optical properties of the macrocyclic products indicated rather small electronic interactions between the building units. However, intramolecular photoenergy transfer from the dithienogermole unit to the boron unit was clearly observed with respect to the fluorescence spectra.     

Controlled Generation of 9-Boratriptycene by Lewis Adduct Dissociation: Accessing a Non-Planar Triarylborane

A highly bent triarylborane, 9-boratriptycene, was generated in solution by selective protodeboronation of the corresponding tetra-aryl boron ate complex with the strong Brønsted acid HNTf₂. The iptycene core confers enhanced Lewis acidity to 9-boratriptycene, making it unique in terms of structure and reactivity. We studied the stereoelectronic properties of 9-boratriptycene by quantifying its association with small N- and O-centered Lewis bases, as well as with sterically hindered phosphines. The resultant Lewis adducts exhibited unique structural, spectroscopic, and photophysical properties. Beyond the high pyramidalization of the 9-boratriptycene scaffold and its low reorganization energy upon Lewis base coordination, quantum chemical calculations revealed that the absence of π donation from the triptycene aryl rings to the boron vacant p_z orbital is one of the main reasons for its high Lewis acidity.     

Controlled Generation of 9‐Boratriptycene by Lewis Adduct Dissociation: Accessing a Non‐Planar Triarylborane

A highly bent triarylborane, 9‐boratriptycene, was generated in solution by selective protodeboronation of the corresponding tetra‐aryl boron ate complex with the strong Brønsted acid HNTf₂. The iptycene core confers enhanced Lewis acidity to 9‐boratriptycene, making it unique in terms of structure and reactivity. We studied the stereoelectronic properties of 9‐boratriptycene by quantifying its association with small N‐ and O‐centered Lewis bases, as well as with sterically hindered phosphines. The resultant Lewis adducts exhibited unique structural, spectroscopic, and photophysical properties. Beyond the high pyramidalization of the 9‐boratriptycene scaffold and its low reorganization energy upon Lewis base coordination, quantum chemical calculations revealed that the absence of π donation from the triptycene aryl rings to the boron vacant p_z orbital is one of the main reasons for its high Lewis acidity.