Borane Group Research Articles

A Systematic Study of Nonionic Di- and Multiborane Catalysts for the Oligomerization and Polymerization of Epoxides.

Borane catalysis has emerged as a powerful technology in epoxide polymerization. Still, the structure-activity correlations for these catalysts are not fully understood to date, especially regarding compounds with nonionic backbones. Thus, in this work, 13 different borane catalysts of this respective type are described and investigated for their epoxide oligomerization and polymerization performance, using propylene oxide (PO), 1-butylene oxide (BO) and allyl glycidyl ether (AGE) as monomers. Structurally, special emphasis is put on catalysts with different linker lengths and linker flexibilities as well as the introduction of more than two borane functionalities. Importantly, this screening is conducted both under typical polymerization conditions as well as under the chain transfer agent (CTA)-rich conditions relevant for large-scale production. It is found that suitable preorganization of the borane groups, such as present in biphenyl derivatives, offers a simple route to high-performing catalysts and quantitative monomer conversion of the investigated epoxides. Furthermore, it is demonstrated that a diborane-catalyzed oligomerization can be kept active over weeks, whereby repeated addition of monomer batches (14 steps) constantly results in full conversion and well-defined oligoethers, underlining the practical potential of this method. The absence of co-initiating counter ions is suggested as an inherent advantage of nonionic catalysts.

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.

Borotropy: The Mechanism of Boron Transfer Between Adjacent N‐Atoms of Pyrazol‐1‐yl Rings

AbstractThis work examines theoretically the problem of the migration of borane groups between the nitrogen atoms of pyrazolyl rings. We have studied as a model the BH2 pyrazol‐1‐yl derivatives to determine the effects of substituents on the pyrazole ring; also 1‐(9‐borabicyclo[3.3.1]nonan‐9‐yl)‐1H‐pyrazoles to compare the calculated barriers with the experimental ones as well as to determine steric effects, i. e., how the buttressing effect increases the rate. The interacting quantum atoms methodology has been used for partitioning the energy of the systems in their stationary points into the contribution of pyrazole and BH2 group and their mutual interaction.

Thermochemical and kinetic investigation of the hydrazine borane as an alternative to hydrazine: The N2H4BH3 + H reactional system

The properties of hydrazine borane (N2H4BH3) were studied to assess its potential as an alternative fuel for use in space missions. The reactions of N2H4BH3 with atomic hydrogen were explored through three different elementary steps, and the thermochemical properties were determined using ab initio methods and density functional theory. The hydrogen abstraction from the borane group had the lowest adiabatic barrier height and all reactions were exothermic. The rate coefficient for hydrogen abstraction from the borane group was 1.4×10−13cm3molecule−1s−1 at 350 K, and N2H4BH3 was found to have better kinetic stability than hydrazine at intermediate and low temperatures.

Structure determination of a bis[4-(di-n-butylamino)phenyl](pyridin-3-yl)borane tetramer highlighting a unique geometric conformation of the core 16-membered ring.

The tetramer of bis(4-di-n-butylaminophenyl)(pyridin-3-yl)borane [systematic name: 2λ4,4λ4,6λ4,8λ4-tetrabora-1,3,5,7(1,3)-tetrapyridinacyclooctaphane-11,31,51,71-tetrakis(ylium)], C132H192B4N12, was synthesized unexpectedly and crystallized. Its structure contains an unusual 16-membered ring core made up of four (pyridin-3-yl)borane groups. The ring adopts a conformation with pseudo-S4 symmetry that is very different from the two other reported examples of this ring system. Density functional theory (DFT) computations indicate that the stability of the three reported ring conformations is dependent on the substituents on the B atoms, and that the pseudo-S4 geometry observed in the bis(4-dibutylaminophenyl)(pyridin-3-yl)borane tetramer becomes significantly more stable when phenyl or 2,6-dimethylphenyl groups are attached to the boron centers.

Reactions of nickel boranyl compounds with pnictogen-carbon triple bonds.

The catalytic conversion of unsaturated small molecules such as nitriles into reduced products is of interest for the production of fine chemicals. In this vein, metal-ligand cooperativity has been leveraged to promote such reactivity, often conferring stability to bound substrate - a balancing act that may offer activation at the cost of turnover efficiency. This report describes the reactivity of a [(diphosphine)Ni] compound with pnictogen carbon triple bonds (R-C[triple bond, length as m-dash]E; E = N, P), where the diphosphine contains two pendant borane groups. For E = N, cooperative nitrile coordination is observed to afford {Ni}2 complexes displaying B-N interactions, whereas for E = P, B-P interactions are absent. This work additionally outlines a structure-activity relationship that uses nitrile dihydroboration as a model reaction to unveil the effect of SCS stabilization, employing [(diphosphine)Ni] where the diphosphine contains 0, 1, or 2 pendant Lewis acid groups.

Fluoride ion sensing with an acridinium borane

With our continuing interest in the chemistry of cationic boranes, we have synthesized the tetrafluoroborate salt of 1-dimesitylboron-4-( N-methyl-9-acridinium)-phenylene, which acts as a turn-on fluoride anion sensor, visibly changing from yellow to orange upon binding fluoride. To understand this reactivity, we spectroscopically and computationally analyzed the cation and fluoroborate adduct. Ultraviolet–visible spectroscopy and time-dependent density functional theory calculations revealed the basis of the color change to be a low energy redshift in a intramolecular charge transfer band. Electrochemical studies were undertaken to further probe this system. Cyclic voltammetry indicated a reversible one-electron reduction for the cation and a cathodic shift of −0.12 V in the first reduction wave upon fluoride binding. Chemical reduction of the cation yielded the acridine borane radical, which was verified by electron paramagnetic resonance spectroscopy.

Two‐Dimensional Materials and their Hetero‐Superlattices for Photocatalytic Hydrogen Evolution Reaction

AbstractSince the discovery of graphene, there has been intensive research on other two‐dimensional (2D) analogues. Some of them are elemental 2D materials such as phosphorene and bismuthene. Others are binary compounds fascinating in terms of energy applications are transition metal dichalcogenides (TMDCs: MoS2, MoSe2) and MXenes (Ti3C2, Nb2C3). 1T‐forms of MoS2 and MoSe2 are probably the best materials reported for dye‐sensitized photocatalytic hydrogen evolution reaction (HER). Here, we first highlight the phase engineering in 2D‐TMDCs to enhance hydrogen evolution activity. Generation of hetero‐superlattices of TMDCs with other HER active materials such as graphene, carbon nitride and borocarbonitrides, by utilizing coupling and electrostatic restacking strategies, appears to be advantageous for photocatalytic application. These hetero‐superlattices provide a counterpoint to the van der Waals heterostructures by means of covalent bonds. Ladder‐like networks of heterolayers generated due to cross‐linking enhance the interfacial area and charge‐transfer interactions, thereby improving HER activity. The use of 2D phosphorene as a photocatalytic material is limited by its ambient instability. Covalent functionalization of phosphorene surface with tris(pentafluorophenyl) borane and benzyl group increases ambient stability and dispersibility. The functionalized surface can be further utilized to cross‐link with amine or acid‐modified TMDCs. The HER activity obtained with phosphorene‐MoS2 superlattices is the highest reported among the phosphorene‐based systems, thus setting a new example for metal‐free catalysis. The ambient instability of MXenes and other related 2d materials under photocatalytic conditions can be resolved either by functionalization or by forming hetero‐superlattices.

Nucleoside boranephosphonates as potential therapeutic agents

Nucleoside boranephosphonates are nucleotide analogues in which one of the non-bridging oxygen atom of the phosphate part has been replaced by a borane group (-BH3). This modification imparts a wide spectrum of biological activity, e.g., activation of ribonuclease H, resistance to endo- and exonucleases, and their respective triphosphates are good substrates for DNA and RNA polymerases. Nucleoside boranephosphonate derivatives are used in antisense therapy, silencing gene expression using siRNA strategies, and as potential antiviral and anti-cancer prodrugs. Boranephosphonates find also applications as aptamers and as substrates in a new method of DNA sequencing. This review briefly presents potential biological applications of nucleoside boranephosphonates.

Boranephosphonates. Unraveling chemistry of the P-BH3 functional group

Nucleoside boranephosphonates are an important class of nucleotide analogues in which one of the non-bridging oxygen atoms has been replaced by a borane moiety (-BH3). In the presence of iodine nucleoside boranephosphonates react with 1° and 2° amines to afford the corresponding phosphoramidates, while with 3° and heteroaromatic amines, the borane group is modified forming new derivatives with the P-B-N bond system. We investigated mechanistic aspects of these reactions and showed that both of them followed the same initial steps that involved most likely formation of λ3-boranephosphonate as a key, electrophilic intermediate. It reacts further, eventually leading to either the corresponding phosphoramidate diester or the B-ammoniumboranephosphonate derivative, depending on the type of amine used.

Copper-Catalyzed Ring-Opening/Borylation of Cyclopropenes

Copper-Catalyzed Ring-Opening/Borylation of Cyclopropenes

Observation of Borane-Olefin Proximity Interaction Governing the Structure and Reactivity of Boron-Containing Macrocycles.

While attractive interactions between borane and olefin have been postulated to trigger various boron-mediated organic transformations, proximity structures of these functional groups, other than the formation of weak van der Waals complexes, have never been directly observed. Here we show that a close intramolecular borane-olefin interaction operates in macrocyclic systems containing borane and olefinic groups obtained by multi-step 1,2-carboboration between a strained alkyne and 9-borafluorene derivatives. Depending on Lewis acidity of the borane moiety and the size of the macrocycles, the magnitude of interaction changes, resulting in different reaction modes. The whole picture of the multi-step reactions has been revealed experimentally with theoretical supports. The present finding may not only provide a deeper understanding of the fundamental boron-mediated interaction but also lead to the development of new organic transformations involving molecular activation by boranes.

Observation of Borane–Olefin Proximity Interaction Governing the Structure and Reactivity of Boron‐Containing Macrocycles

AbstractWhile attractive interactions between borane and olefin have been postulated to trigger various boron‐mediated organic transformations, proximity structures of these functional groups, other than the formation of weak van der Waals complexes, have never been directly observed. Here we show that a close intramolecular borane–olefin interaction operates in macrocyclic systems containing borane and olefinic groups obtained by multi‐step 1,2‐carboboration between a strained alkyne and 9‐borafluorene derivatives. Depending on Lewis acidity of the borane moiety and the size of the macrocycles, the magnitude of interaction changes, resulting in different reaction modes. The whole picture of the multi‐step reactions has been revealed experimentally with theoretical supports. The present finding may not only provide a deeper understanding of the fundamental boron‐mediated interaction but also lead to the development of new organic transformations involving molecular activation by boranes.

Synthesis and Structure of an o-Carboranyl-Substituted Three-Coordinate Borane Radical Anion.

Bis(1‐(4‐tolyl)‐carboran‐2‐yl)‐(4‐tolyl)‐borane [(1‐(4‐MeC6H4)‐closo‐1,2‐C2B10H10‐2‐)2(4‐MeC6H4)B] (1), a new bis(o‐carboranyl)‐(R)‐borane was synthesised by lithiation of the o‐carboranyl precursor and subsequent salt metathesis reaction with (4‐tolyl)BBr2. Cyclic voltammetry experiments on 1 show multiple distinct reduction events with a one‐electron first reduction. In a selective reduction experiment the corresponding paramagnetic radical anion 1.− was isolated and characterized. Single‐crystal structure analyses allow an in‐depth comparison of 1, 1.−, their calculated geometries, and the S1 excited state of 1. Photophysical studies of 1 show a charge transfer (CT) emission with low quantum yield in solution but a strong increase in the solid state. TD‐DFT calculations were used to identify transition‐relevant orbitals.

Cover Feature: Octaboraneyl Complexes of Nickel: Monomers for Redox‐Active Coordination Polymers (Chem. Eur. J. 49/2020)

Eight pendant boranes are better than one for the activation of Lewis bases. A new diphosphine ligand that features four pendant borane groups has been prepared. The coordination chemistry of this new scaffold has been established using nickel—giving an octaboraneyl Ni0 complex; this species reacts readily with Lewis bases such as 4-(dimethylamino)pyridine and 4,4’-bipyridine. In the latter case, an unusual air-sensitive open-shell Ni-coordination polymer is obtained, which has been characterized in the solid state. More information can be found in the Full Paper by M. W. Drover et al. on page 11180).

Controlled Radical Polymerization toward Ultra-High Molecular Weight by Rationally Designed Borane Radical Initiators

Summary Ultra-high-molecular-weight (UHMW) polymers can display outstanding properties; however, it is challenging to synthesize UHMW polymers by using external free radical initiators. To address this, here, we report the ration design and adoption of B-alkylcatecholborane (RBCat) as a radical initiator in controlled radical polymerization in the presence of a chain-transfer agent. The autoxidation of RBCat with one alkyl substituent generates only one alkyl radical. The oxygen is incorporated and stored in the oxidized form (ROOB-Cat) that slowly releases the alkyl radical. The rationally designed alkyl radical is proposed to preferentially activate the chain-transfer agent (CTA) and grow the polymer chain from CTA, so limited free polymer chains are formed from the initiating alkyl radicals. Polymerizations of (meth)acrylates are achieved with narrow molecular weight distribution (Ð = 1.10) and predetermined molecular weights even in a UHMW range (up to 3 million). The successful chain extension of UHMW indicates the high degree of control of the polymerization.

Merging New Organoborane Chemistry with Living Radical Polymerization

In this issue of Cell Reports Physical Science, Pan and co-workers synthesize alkylcatecholboranes that can be oxidized by oxygen to drive the release of carbon-centered radicals for initiating reversible addition-fragmentation chain transfer (RAFT) polymerization. These organoboranes can slowly decompose to radicals that enable RAFT polymerization with exceptional control yielding ultra-high molecular weight (UHMW) polymers.

Monodispersed bimetallic nanoparticles anchored on TiO2-decorated titanium carbide MXene for efficient hydrogen production from hydrazine in aqueous solution

Selective catalytic decomposition of hydrazine (N2H4) to provide a clean energy carrier hydrogen (H2) is a promising alternative to fossil fuels for the future energy economy. NiPt nanoparticles (NPs) dispersed on delamination of TiO2-decorated Ti3C2Tx (denoted as DT-Ti3C2Tx) nanosheets are prepared via a simple wet chemical reduction method and applied as an efficient catalyst for dehydrogenation of N2H4 in aqueous solution. The rich oxygen-containing functional groups on the surface of DT-Ti3C2Tx not only facilitate the formation and immobilization of monodisperse NiPt NPs but also enhance the synergistic effect between metal NPs and MXene support. Among all of the tested samples, the optimized Ni0.8Pt0.2/DT-Ti3C2Tx nanocatalyst exhibits the 100% H2 selectivity and best catalytic performance with a TOF value of 1220 h−1 for the selective decomposition of N2H4 at 323 K. In addition, this catalyst also shows excellent catalytic performance for hydrogen production from hydrazine borane (N2H4BH3) via the hydrolysis of borane group and selective decomposition of hydrazine moiety. The TiO2-decorated Ti3C2Tx MXene can be applied as an excellent support to obtain well-dispersed and ultrafine metal NPs for various applications.

Hydrodecyanation of Secondary Alkyl Nitriles and Malononitriles to Alkanes using DiMeImd-BH3.

The decyanation of secondary aliphatic nitriles and the 2-fold decyanation of malononitriles leading to alkanes in the presence of 1,3-dimethylimidazol-2-ylidene borane (diMeImd-BH3) are reported. These reactions proceed via a radical mechanism that involves the addition of a borane radical to the nitrile to form an iminyl radical, followed by cleavage of a carbon-carbon bond. Theoretical calculations suggest that the β-cleavage of these iminyl radicals, which affords NHC-BH2CN and the corresponding alkyl radicals, is the rate-determining step in this reaction.

Coordination and Dehydrogenation of Diphosphine-Borane Ph2PCH2PPh2·BH3 at a Heterometallic MoRe Center to Give an Agostic Boryl-Bridged Derivative.

The coordination chemistry of the title diphosphine-borane adduct at heterometallic MoRe centers was examined through its reactions with the hydride complex [MoReCp(μ-H)(μ-PCy2)(CO)5(NCMe)] (Cp = η5-C5H5). The latter reacted rapidly with stoichiometric amounts of dppm·BH3 (dppm = Ph2PCH2PPh2) in refluxing toluene solution, with displacement of the nitrile ligand, to give [MoReCp(μ-H)(μ-PCy2)(CO)5(κ1P-dppm·BH3)], with a P-bound diphosphine-borane ligand arranged trans to the PCy2 group. Decarbonylation of the latter complex was accomplished rapidly upon irradiation with visible-UV light in toluene solution at 263 K, to give the agostic derivative [MoReCp(μ-H)(μ-PCy2)(CO)4(κ1P,η2-dppm·BH3)] as major product (Mo-Re = 3.2075(5) Å), along with small amounts of the diphosphine-bridged complex [MoReCp(μ-H)(μ-PCy2)(CO)4(μ-dppm)]. Extended photolysis of the agostic complex at 288 K promoted an unprecedented dehydrogenation process involving the borane group and the hydride ligand, to give the diphosphine-boryl complex [MoReCp(μ-η2:κ2P,B-H2B·dppm)(μ-PCy2)(CO)4] (Mo-Re = 3.075(1) Å). The latter displayed a boryl ligand in a novel bridging coordination mode, it being σ-bound to one of the metal atoms (B-Re = 2.38(2) Å) while interacting with the second metal atom via a strong side-on tricentric B-H-M interaction (B-Mo = 2.31(1); H-Mo = 1.9(1) Å). The overall dehydrogenation process was endergonic by 43 kJ/mol, according to density functional theory calculations.