Boron cluster chemistry

Copper-catalyzed electrochemical direct chalcogenations of o-carboranes was established at room temperature. Thereby, a series of cage C-sulfenylated and C-selenylated o-carboranes anchored with valuable functional groups was accessed with high levels of position-and chemo-selectivity control. The cupraelectrocatalysis provided efficient means to activate otherwise inert cage C-H bonds for the latestage diversification of o-carboranes.

Reactions of the diborane(4) B 2 (o-tolyl) 4 and monohydridoboranes are shown to give B(o-tolyl) 3 and (otolyl)BR 2 (R 2 = (C 8 H 14) 3, cat 4, pin 5, (C 6 F 5) 2 6) as the major products. The corresponding reaction with BH 3-sources gives complex mixtures, resulting from hydride/aryl exchange, dimerization and borane elimination. This led to the isolation of the first tetra-substituted pentaborane(9) HB[B(o-tolyl)(m-H)] 4 8. The reaction pathways are probed experimentally and by computations.

Reactions of the diborane(4) B 2 (o-tolyl) 4 and monohydridoboranes are shown to give B(o-tolyl) 3 and (otolyl)BR 2 (R 2 = (C 8 H 14) 3, cat 4, pin 5, (C 6 F 5) 2 6) as the major products. The corresponding reaction with BH 3-sources gives complex mixtures, resulting from hydride/aryl exchange, dimerization and borane elimination. This led to the isolation of the first tetra-substituted pentaborane(9) HB[B(o-tolyl)(m-H)] 4 8. The reaction pathways are probed experimentally and by computations.

MPtBu2 (M = Li, Na, K), KH and KN(SiMe3)2 are shown to activate HD reversibly. In the case of MPtBu2 this leads to isotopic scrambling and the formation of H2, D2, H(D)PtBu2 and MH(D) in C6D6. In toluene, KPtBu2 reacts with H2 but also leads to isotopic scrambling into the methyl groups of the solvent toluene. DFT calculations reveal that these systems effect H2 activation via cooperative interactions with the Lewis acidic alkali metal and the basic phosphorus, carbanion or hydride centers, mimicking frustrated Lewis pair (FLP) behaviour.

Reactions of the diborane(4) B 2 (o-tolyl) 4 and monohydridoboranes are shown to give B(o-tolyl) 3 and (otolyl)BR 2 (R 2 = ( ) as the major products. The corresponding reaction with BH 3 -sources gives complex mixtures, resulting from hydride/aryl exchange, dimerization and borane elimination. This led to the isolation of the first tetra-substituted pentaborane( ) HB[B(o-tolyl)(m-H)] 4 8. The reaction pathways are probed experimentally and by computations. [*] K.

We report a discovery that perfunction-alized icosahedral dodecaborate clusters of the type B12 (OCH2Ar)12 (Ar = Ph or C6F5) can undergo photo-excitation with visible light, leading to a new class of metal-free photooxidants. Excitation in these species occurs as a result of the charge transfer between low-lying orbitals located on the benzyl substituents and an unoccupied orbital delocalized throughout the boron cluster core. Here we show how these species, photo-excited with a benchtop blue LED source, can exhibit excited-state reduction potentials as high as 3 V and can participate in electron-transfer processes with a broad range of styrene monomers, initiating their polymerization. Initiation is observed in cases of both electron-rich and electron-deficient styrene monomers at cluster loadings as low as 0.005 mol%. Furthermore, photo-excitation of B12 (OCH2-C6F5)12 in the presence of a less activated olefin such as isobutylene results in the production of highly branched poly(isobutylene). This work introduces a new class of air-stable, metal-free photo-redox reagents capable of mediating chemical transformations.

Functional handles appended to polymer chain ends are important tools often used as spectroscopic probes for determining polymer structure, affinity labels, and as reactive handles for the conjugation of functional payloads. An easily tunable molecular handle able to carry out multiple functions simultaneously would be of significant use at the polymer, materials, and biology interface. Here, we report the development of carborane-containing chain transfer agents (CTAs, commonly referred to as RAFT agents) which are used in reversible addition-fragmentation chain transfer (RAFT) polymerization. These carbor-ane RAFT agents establish control over polymerization processes leading to monodisperse (Đ = 1.03-1.15) polymers made from N-isopropylacrylamide, styrene, 4-chlorostyrene, and methyl acrylate monomers. The tunable nature of the carborane-based scaffold appended on the polymer chain end serves as a general 1 H NMR spectroscopic handle, which can be used to elucidate polymer molecular weight via end-group analysis. Isothermal titration calorimetry (ITC) measurements show that synthesized carborane terminated polymers exhibit strong binding to β-cyclodextrin with an affinity (Ka) of 9.37 × 10^4 M^−1 , thereby demonstrating its potential use as an affinity label. Additionally, we show that the free B-H vertices on the carborane RAFT agents exhibit a Raman vibrational signal at ∼2549 cm^−1 , a Raman-silent region for biological milieu, indicating its potential utility as an innate Raman active probe. The reported carborane RAFT agents bolster the expanding toolbox of molecular probes and serve as tunable platforms for incorporating additional and complementary handles for tailoring chain-end functionality and facilitating polymer analysis.

Photoexcited intramolecular charge transfer (CT) states in N,N-diaryl dihydrophenazine photoredox catalysts are accessed through catalyst design and investigated through combined experimental studies and density functional theory (DFT) calculations. These CT states are reminiscent of the metal to ligand charge transfer (MLCT) states of ruthenium and iridium polypyridyl complexes. For cases where the polar CT state is the lowest energy excited state, we observe its population through significant solvatochromic shifts in emission wavelength across the visible spectrum by varying solvent polarity. We propose the importance of accessing CT states for photoredox catalysis of atom transfer radical polymerization lies in their ability to minimize fluorescence while enhancing electron transfer rates between the photoexcited photoredox catalyst and the substrate. Additionally, solvent polarity influences the deactivation pathway, greatly affecting the strength of ion pairing between the oxidized photocatalyst and the bromide anion and thus the ability to realize a controlled radical polymerization. Greater understanding of these photoredox catalysts with respect to CT and ion pairing enables their application toward the polymerization of methyl methacrylate for the synthesis of polymers with precisely tunable molecular weights and dispersities typically lower than 1.10.