Boryl‐Functionalized σ‐Alkynyl and Vinylidene Rhodium Complexes: Synthesis and Electronic Properties

The synthesis, reactivity, and properties of boryl‐functionalized σ‐alkynyl and vinylidene rhodium complexes such as trans‐[RhCl(?C?CHBMes₂)(PiPr₃)₂] and trans‐[Rh(C?CBMes₂)(IMe)(PiPr₃)₂] are reported. An equilibrium was found to exist between rhodium vinylidene complexes and the corresponding hydrido σ‐alkynyl complexes in solution. The complex trans‐[Rh(C?CBMes₂)(IMe)(PiPr₃)₂] (IMe=1,3‐dimethylimidazol‐2‐ylidene) was found to exhibit solvatochromism and can be quasireversibly oxidized and reduced electrochemically. Density functional calculations were performed to determine the reaction mechanism and to help rationalize the photophysical properties of trans‐[Rh(C?CBMes₂)(IMe)(PiPr₃)₂].     

Photoionization and Pyrolysis of a 1,4‐Azaborinine: Retro‐Hydroboration in the Cation and Identification of Novel Organoboron Ring Systems

The photoionization and dissociative photoionization of 1,4‐di‐tert‐butyl‐1,4‐azaborinine by means of synchrotron radiation and threshold photoelectron photoion coincidence spectroscopy is reported. The ionization energy of the compound was determined to be 7.89 eV. Several low‐lying electronically excited states in the cation were identified. The various pathways for dissociative photoionization were modeled by statistical theory, and appearance energies AE_0K were obtained. The loss of isobutene in a retro‐hydroboration reaction is the dominant pathway, which proceeds with a reverse barrier. Pyrolysis of the parent compound in a chemical reactor leads to the generation of several yet unobserved boron compounds. The ionization energies of the C₄H₆BN isomers 1,2‐ and 1,4‐dihydro‐1,4‐azaborinine and the C₃H₆BN isomer 1,2‐dihydro‐1,3‐azaborole were determined from threshold photoelectron spectra.     

Antiaromaticity to Aromaticity: From Boroles to 1,2‐Azaborinines by Ring Expansion with Azides

We have exploited the reactivity of antiaromatic boroles, gaining access to aryl‐substituted monocyclic 1,2‐azaborinines. The observed ring‐expansion reaction of inherently electron‐deficient boroles with organometallic and organic azides is demonstrated for representative examples. This substance class is expected to provide a new avenue into 1,2‐azaborinine chemistry, especially in the area of functional organoboron materials. Our results are based on NMR and UV/Vis spectroscopy as well as single‐crystal X‐ray crystallography and provide a virtually quantitative approach that also offers numerous points of variation.     

Isolation of a Neutral Boron‐Containing Radical Stabilized by a Cyclic (Alkyl)(Amino)Carbene

Utilizing a cyclic (alkyl)(amino)carbene (CAAC) as a ligand, neutral CAAC‐stabilized radicals containing a boryl functionality could be prepared by reduction of the corresponding haloborane adducts. The radical species with a duryl substituent was fully characterized by single‐crystal X‐ray structural analysis, EPR spectroscopy, and DFT calculations. Compared to known neutral boryl radicals, the isolated radical species showed larger spin density on the boron atom. Furthermore, the compound that was isolated is extraordinarily stable to high temperatures under inert conditions, both in solution and in the solid state. Electrochemical investigations of the radical suggest the possibility to generate a stable formal boryl anion species.     

Reversible Photochemical and Thermal Isomerization of Azaboratabisnorcaradiene to Azaborabenzotropilidene

Two new tricyclic 1,2‐azaboratabisnorcaradiene molecules ( 1 b and 2 b ) generated through the photoisomerization of N‐methyl‐2‐phenylimidazolyl‐chelated dimesitylboranes ( 1 a and 2 a ) have been found to undergo unusual photoisomerization, producing the first examples of 1,2‐azaborabenzotropilidenes ( 1 c and 2 c ), accompanied by a distinct color change, upon irradiation at 350 nm. Compounds 1 c and 2 c contain a conjugated alkylideneborane unit and can be fully reverted back to 1 b and 2 b , and subsequently to 1 a and 2 a upon heating. The mechanistic pathway of the new isomerism has been established to involve “walk” rearrangements by DFT computational studies.     

Tapered copolymers of styrene and 4-vinylbenzocyclobutene via carbanionic polymerization for crosslinkable polymer films

Well-defined polystyrene homopolymers with surface-adhesive triethoxysilyl end group were synthesized via living carbanionic polymerization, epoxide end-functionalization and subsequent hydrosilylation with triethoxysilane. Grafting-to performance of polymers with various molecular weight (M_n = 3000–14,000 g mol⁻¹) to a silicon surface was examined in dependence of reaction time, polymer concentration, solvent and number of alkoxysilyl end groups. Crosslinkable polymers for surface modification were synthesized by statistical carbanionic copolymerization of 4-vinylbenzocyclobutene (4-VBCB) and styrene, followed by epoxide end-functionalization and triethoxysilane modification (M_n = 4000–14,000 g mol⁻¹). The copolymers were characterized by ¹H-NMR, THF-SEC, and matrix-assisted laser desorption and ionization time-of-flight mass spectrometry. In situ ¹H-NMR kinetic studies in cyclohexane-d₁₂ provided information regarding the monomer gradient in the polymer chains, with styrene being the more reactive monomer (r_s = 2.75, r_4-VBCB = 0.23). Thin polymer films on silicon wafers were prepared by grafting-to surface modification under conditions derived for the polystyrene homopolymer. The traceless, thermally induced crosslinking reaction of the benzocyclobutene units was studied by DSC in bulk as well as in 3–6 nm thick polymer films. Crosslinked films were analyzed by atomic force microscopy, ellipsometry, and nanoindentation, showing smooth polymer films with an increased modulus. © 2019 The Authors. Journal of Polymer Science published by Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 181–192     

An Iterative Divergent Approach to Conjugated Starburst Borane Dendrimers

Using a new divergent approach, conjugated triarylborane dendrimers were synthesized up to the 2^nd generation. The synthetic strategy consists of three steps: 1) functionalization, via iridium catalyzed C−H borylation; 2) activation, via fluorination of the generated boronate ester with K[HF₂] or [N(nBu₄)][HF₂]; and 3) expansion, via reaction of the trifluoroborate salts with aryl Grignard reagents. The concept was also shown to be viable for a convergent approach. All but one of the conjugated borane dendrimers exhibit multiple, distinct and reversible reduction potentials, making them potentially interesting materials for applications in molecular accumulators. Based on their photophysical properties, the 1^st generation dendrimers exhibit good conjugation over the whole system. However, the conjugation does not increase further upon expansion to the 2^nd generation, but the molar extinction coefficients increase linearly with the number of triarylborane subunits, suggesting a potential application as photonic antennas.     

Light-Activated Carbon Monoxide Prodrugs Based on Bipyridyl Dicarbonyl Ruthenium(II) Complexes

Two photoactivatable dicarbonyl ruthenium(II) complexes based on an amide-functionalised bipyridine scaffold (4-position) equipped with an alkyne functionality or a green-fluorescent BODIPY (boron-dipyrromethene) dye have been prepared and used to investigate their light-induced decarbonylation. UV/Vis, FTIR and ¹³C NMR spectroscopies as well as gas chromatography and multivariate curve resolution alternating least-squares analysis (MCR-ALS) were used to elucidate the mechanism of the decarbonylation process. Release of the first CO molecule occurs very quickly, while release of the second CO molecule proceeds more slowly. In vitro studies using two cell lines A431 (human squamous carcinoma) and HEK293 (human embryonic kidney cells) have been carried out in order to characterise the anti-proliferative and anti-apoptotic activities. The BODIPY-labelled compound allows for monitoring the cellular uptake, showing fast internalisation kinetics and accumulation at the endoplasmic reticulum and mitochondria.     

Boron: Its Role in Energy‐Related Processes and Applications

Boron's unique position in the Periodic Table, that is, at the apex of the line separating metals and nonmetals, makes it highly versatile in chemical reactions and applications. Contemporary demand for renewable and clean energy as well as energy‐efficient products has seen boron playing key roles in energy‐related research, such as 1) activating and synthesizing energy‐rich small molecules, 2) storing chemical and electrical energy, and 3) converting electrical energy into light. These applications are fundamentally associated with boron's unique characteristics, such as its electron‐deficiency and the availability of an unoccupied p orbital, which allow the formation of a myriad of compounds with a wide range of chemical and physical properties. For example, boron's ability to achieve a full octet of electrons with four covalent bonds and a negative charge has led to the synthesis of a wide variety of borate anions of high chemical and electrochemical stability—in particular, weakly coordinating anions. This Review summarizes recent advances in the study of boron compounds for energy‐related processes and applications.     

trans‐Selective Insertional Dihydroboration of a cis‐Diborene: Synthesis of Linear sp3‐sp2‐sp3‐Triboranes and Subsequent Cationization

The reaction of aryl‐ and amino(dihydro)boranes with dibora[2]ferrocenophane 1 leads to the formation 1,3‐trans‐dihydrotriboranes by formal hydrogenation and insertion of a borylene unit into the B=B bond. The aryltriborane derivatives undergo reversible photoisomerization to the cis‐1,2‐μ‐H‐3‐hydrotriboranes, while hydride abstraction affords cationic triboranes, which represent the first doubly base‐stabilized B₃H₄⁺ analogues.