Formation of an air-stable diborane via a stepwise BH3 addition of pyrido[1,2-a]isoindole with H2 evolution.

A novel and air-stable organo(hydro)diborane featuring a five-membered aryl ring supported bridging B-C-B three-centre-two-electron (3c-2e) bond has been reported. Pyrido[1,2-a]isoindole was found to undergo a stepwise BH3 addition reaction, during which a mono-BH3 adduct was formed from a electrophilic addition at the Cγ in pyrido[1,2-a]isoindole. A molecule of hydrogen was eliminated throughout the second step of addition reaction. DFT calculations indicate that the H2 evolution is concerted to the second BH3 addition rather than forming BC before the second BH3 attack.

Functionalization of N2 via Formal 1,3-Haloboration of a Tungsten(0) σ-Dinitrogen Complex.

Boron tribromide and aryldihaloboranes were found to undergo 1,3-haloboration across one W-N≡N moiety of a group 6 end-on dinitrogen complex (i.e. trans-[W(N2 )2 (dppe)2 ]). The N-borylated products consist of a reduced diazenido unit sandwiched between a WII center and a trivalent boron substituent (W-N=N-BXAr), and have all been fully characterized by NMR and IR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. Both the terminal N atom and boron center in the W-N=N-BXAr unit can be further derivatized using electrophiles and nucleophiles/Lewis bases, respectively. This mild reduction and functionalization of a weakly activated N2 ligand with boron halides is unprecedented, and hints at the possibility of generating value-added nitrogen compounds directly from molecular dinitrogen.

Silicon Forms a Rich Diversity of Aliphatic Polyol Complexes in Aqueous Solution.

A detailed examination of aqueous Si complexation by alditols and aldonic acids was conducted using high-sensitivity 29Si NMR spectroscopy of isotopically enriched solutions combined with theoretical modeling. Contrary to previous thinking, we have established that aliphatic polyols do not require a threo pair of hydroxy groups to form hypercoordinated Si complexes, although formation constants may be orders of magnitude higher if they are present. Thirteen distinctly different molecular assemblages containing 4-, 5-, or 6-coordinate Si centers have been identified, with significant concentrations of 5-coordinate Si bis-ligand complex being detected even under biologically relevant solution conditions.

Divergent and Multi-Stage Photoisomerization of Four-Coordinated Boron Compounds with a Naphthyl-Pyridyl/Thiazolyl Backbone.

Examination of the photoreactivity of a new class of N,C-chelate organoboron compounds, including a series of unsymmetrically substituted boron molecules, B(naph-pyridyl)(Ar1 )(Ar2 ) and B(naph-thiazolyl)(Ar1 )(Ar2 ), led to the discovery of new and divergent photothermal isomerization phenomena. These include the clean and regioselective photoisomerization by unsymmetrical boron, forming borepin isomers, some of which further isomerize to the corresponding boratanorcaradiene diastereomer pairs as a result of the generation of two chiral centers. Significantly, the boratanorcaradienes involving a 3-thienyl substituent on boron were found to thermally convert to BN-fluoranthene annulated borapentalene via an unprecedented reversible boratacyclopropane-boratacyclopentene rearrangement. Changing the pyridyl donor to a thiazolyl donor on the boron was found to provide the B(naph-thiazolyl)(Mes)2 compounds with a distinct new photoisomerization pathway-instead of borepin, forming new blue fluorescent polycyclic azaborinine species. This work illustrates the richness and complexity of boron photochemistry.

Lewis-Base Stabilization of the Parent Al(I) Hydride under Ambient Conditions.

Aluminum(III) is inherently electron deficient and therefore acts as a prototypical Lewis acid. Conversely, Al(I) is a rare, nucleophilic variant of aluminum that is thermodynamically unstable under ambient conditions. While attempts to stabilize and isolate Al(I) species have become increasingly successful, the parent Al(I) (i.e, Al-H) remains accessible only under extreme temperatures/pressures or matrix conditions. Here, we report the isolation of the parent Al(I) hydride under ambient conditions via the reduction of a Lewis-base-stabilized alkyldihaloalane. Computational and spectroscopic analyses indicate that the ground-state electronic configuration of this monomeric aluminum species is best described as an Al(I) hydride with non-negligible open-shell Al(III) singlet diradical character. These findings are also supported by reactivity studies, which reveal both the p-centered lone pair donating ability and the hydridic nature of the parent aluminene.

B-B Cleavage and Ring-Expansion of a 1,4,2,3-Diazadiborinine with N-Heterocyclic Carbenes.

A 1,4,2,3-diazadiborinine derivative was found to form Lewis adducts with strong two-electron donors such as N-heterocyclic and cyclic (alkyl)(amino)carbenes. Depending on the donor, some of these Lewis pairs are thermally unstable, converting to sole B,N-embedded products upon gentle heating. The products of these reactions, which have been fully characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction, were identified as B,N-heterocycles with fused 1,5,2,4-diazadiborepine and 1,4,2-diazaborinine rings. Computational modelling of the reaction mechanism provides insight into the formation of these unique structures, suggesting that a series of B-H, C-N, and B-B bond activation steps are responsible for these "intercalation" reactions between the 1,4,2,3-diazadiborinine and NHCs.

Boron-based stimuli responsive materials.

Boron-based stimuli responsive systems represent an emerging class of useful materials with a wide variety of applications. Functions within these boron-doped molecules are derived from external stimuli such as light, heat, and force, which alter their intra- and/or intermolecular interactions, yielding unique electronic/photophysical or mechanical properties that can be exploited as optical probes or switchable materials. In this review, the various state-switching mechanisms of these boron-based materials will be introduced, followed by a detailed account of recent advances in the field. Emphasis will be placed on structure-property relationships and the potential applications of the stimuli-responsive boron compounds.

Correction: Boron-based stimuli responsive materials.

Correction for 'Boron-based stimuli responsive materials' by Soren K. Mellerup et al., Chem. Soc. Rev., 2019, DOI: 10.1039/c9cs00153k.

D-π-A Triarylboranes as Reversible Fluorescent Probes for CO2 and Temperature.

Alkylamino-functionalized donor-π-acceptor (D-π-A) triarylboranes have been found to be able to capture CO2, forming the carbamic acid derivative. Due to the close proximity of the amine substituent to the boron center, CO2 binding greatly influences the intramolecular charge transfer fluorescence, causing distinct and reversible emission spectral/color change, with a rapid response time, and a detection limit as low as 100 ppm in methanol. In addition, this system is also effective as fluorescence "turn-on" temperature probes due to the dynamic B ← N bond dissociation/association.

Reversible Photoisomerization from Borepin to Boratanorcaradiene and Double Aryl Migration from Boron to Carbon.

B(npy)Ar2 (npy=2-(naphthalen-1-yl)pyridine) compounds bearing various nonbulky aryl groups undergo a clean and sequential two-step photoisomerization in which two aryl substituents on boron migrate to a carbon atom of the naphthyl moiety. The second isomerization step is the first example of a reversible photoisoermization between a borepin and a borirane. Both steric and electronic factors have been found to have a great impact on this photoreactivity. Furthermore, the borirane isomer reacts with oxygen, forming a rare oxaborepin dimer.

Experimental Evidence for a Triplet Biradical Excited-State Mechanism in the Photoreactivity of N,C-Chelate Organoboron Compounds.

N,C-chelate organoborates represent an emerging class of photoresponsive materials due to their photochromic switching at a boron center. Despite the promising applicability of such systems, little is known about the excited-state processes that lead to their unique photoreactivity, which is detrimental to the design of next-generation smart materials based on boron. As part of our ongoing effort to understand and improve the utility of these organoboron compounds, we report some of the first experimental evidence to support an excited-state mechanism for N,C-chelate organoborates. Femtosecond transient absorption spectroscopy combined with steady-state UV/vis and fluorescence measurements gives direct insight into their underlying photochemical processes, such as the formation of a common triplet charge-transfer state which either relaxes radiatively or undergoes the desired photoisomerization through a biradical intermediate. With this information, a complete mechanistic picture of the excited-state reactivity of N,C-chelate organoborates has been established, which is anticipated to lead to new smart materials with improved performance.

Stimuli-Responsive B/N Lewis Pairs Based on the Modulation of B-N Bond Strength.

A series of robust organoboranes with electronically tunable functionality of B/N Lewis pairs has been achieved. These compounds feature a B/N-containing core in which the interactions between the B and N atoms are modulated as a result of the structural flexibility of the nonconjugated backbone. Examination of the substituent effects of the Lewis base moiety reveals that bulky or aryl substituents favor the dynamic switching of the B-N bond in response to external stimuli, such as heat or mechanical pressure, leading to emission color modulation. This work provides a new, straightforward proof of concept toward new switchable materials design based on tunable electronic interactions.

Controlling Isomerization Selectivity in Chiral, Photochromic N,C-Chelate Organoboron Systems with Extended π-Conjugation.

Alkyne-functionalized chelate boron compounds with extended π-conjugation on one of the aryl groups attached to boron display thermally reversible and regioselective isomerization on the more delocalized substituent, forming base-stabilized boriranes with an intense color. Linking two of such boron chromophores through a 1,4-phenylene spacer via ethynyl moieties leads to photochemically inert molecules, while connecting them by a nonconjugated silicon bridge yields photochromic systems capable of switching at a single boron center.

Lanthanide Complexes with Photochromic Organoboron Ligand: Synthesis and Luminescence Study.

A novel, photochromic N^C-chelate organoboron functionalized dipicolinic acid (H2L) has been designed and synthesized. Lanthanide(III) complexes based on this ligand (L) with the general formula [NBu4]3[LnL3] (Ln = Eu or Tb) were prepared. The new ligand was found to be effective in both sensitizing and photomodulating the emission of a Eu(III) ion. The photoisomerization conversion of the boryl chromophore attached to the ligand of the lanthanide complex was determined to be quantitative by NMR analysis of the La(III) analogue.

Transforming benzylideneamine N,C-chelate boron compounds to BN-cycloocta-/cyclohepta-trienes bearing a tetrasubstituted B[double bond, length as m-dash]N unit via photoisomerization.

A series of benzylideneamine N,C-chelate boron compounds have been found to undergo rapid multistructural transformations with UV irradiation, yielding new BN-cycloocta-1,3,6-triene ((4Z,6Z)-1,2,3,8-tetrahydro-1,2-azaborocine) and BN-cyclohepta-1,3,5-triene (2,7-dihydro-1H-1,2-azaborepine) derivatives with a tetrasubstituted B[double bond, length as m-dash]N unit quantitatively. The simple imine donor also lends itself to achieving photoreactivity in compounds with two phenyl substituents on boron, which is the first example for this type of organoboron photochemistry.

Stabilising fleeting intermediates of stilbene photocyclization with amino-borane functionalisation: the rare isolation of persistent dihydrophenanthrenes and their [1,5] H-shift isomers.

The key intermediate, 4a,4b-dihydrophenanthrene (DPH), involved in the photocyclization of stilbene and derivatives is known to be unstable, and is therefore poorly characterized/understood. We have found that functionalising stilbenes with NMe2 and BMes2 groups can greatly enhance the stability of 4a,4b-DPHs, allowing quantitative isolation and full characterization of these rare species. Furthermore, we discovered that the new amino-borane decorated 4a,4b-DPHs can undergo thermal [1,5] H sigmatropic shift, forming isomers 4a,10a-DPHs. Both 4a,4b-DHPs and 4a,10a-DHPs are stable towards air and moisture, while only the former were found to undergo oxidative dehydrogenation upon irradiation at 365 nm under air, yielding brightly blue/green fluorescent NMe2 and BMes2 functionalised phenanthrene analogues. Control studies established that the trans-Mes2B-Ph-NMe2 unit is responsible for the stability of these isolated 4a,4b-DHPs and their [1,5]-H shift isomers.

Aryl Insertion vs Aryl-Aryl Coupling in C,C-Chelated Organoborates: The "Missing Link" of Tetraarylborate Photochemistry.

The photoreactivity of 9-borafluorene-based, C,C-chelated organoborates was investigated. Unlike the related tetraarylborates, the charge-transfer transitions imparted by the biphenyl chelate lead to selective insertion of one aryl substituent into the endocyclic B-C bond of the 9-borafluorene moiety, resulting in the formation of boratanorcaradienes. This photoreaction likely proceeds according to a Zimmerman rearrangement, which is analogous to one of the initially proposed mechanisms for tetraarylborates and provides additional insight into these long-debated photochemical reactions.

Identifying (BN)2-pyrenes as a New Class of Singlet Fission Chromophores: Significance of Azaborine Substitution.

Singlet fission converts one photoexcited singlet state to two triplet excited states and raises photoelectric conversion efficiency in photovoltaic devices. However, only a handful of chromophores have been known to undergo this process, which greatly limits the application of singlet fission in photovoltaics. We hereby identify a recently synthesized diazadiborine-pyrene ((BN)2-pyrene) as a singlet fission chromophore. Theoretical calculations indicate that it satisfies the thermodynamics criteria for singlet fission. More importantly, the calculations provide a physical chemistry insight into how the BN substitution makes this happen. Both calculation and transient absorption spectroscopy experiments indicate that the chromophore has a better absorption than pentacene. The convenient synthesis pathway of the (BN)2-pyrene suggests an in situ chromophore generation in photovoltaic devices. Two more (BN)2-pyrene isomers are proposed as singlet fission chromophores. This study sets a step forward in the cross-link of singlet fission and azaborine chemistry.

A simple multi-responsive system based on aldehyde functionalized amino-boranes.

A simple aldehyde functionalized amino-triarylborane donor-acceptor system (BO-1) was found to display distinct responses toward multiple external stimuli including solvent, temperature and pressure, with emission colours changing from blue to red. The operating mechanism most likely involves reversible switching between closed and open structures based on an intramolecular B ← O bond. Imbedded donor-acceptor charge transfer transitions played a key role in the multi-coloured fluorescent response of this new boron system to external stimuli. Multi-coloured and erasable fluorescent images on solid substrates based BO-1's "turn-on" response toward solvents, particularly water, are demonstrated.

Photochemical Generation of Chiral N,B,X-Heterocycles by Heteroaromatic C-X Bond Scission (X=S, O) and Boron Insertion.

Chiral organoboron compounds with a chelate backbone and mesityl/heterocycle substituents (thienyl, furyl, and derivatives thereof) undergo a quantitative phototransformation that yields rare, chiral N,B,X-containing heterocycles, such as base-stabilized 1,2-thiaborinines and 1,2-oxaborinines. Boriranes were observed as intermediates in some of these transformations. The oxaborinines display further reactivity, generating 4a,12b-dihydrobenzo[h][1,2]oxaborinino[4,3-f]quinolines through a sequential conrotatory electrocyclization and a [1,5]-H shift. The N,B,X-containing heterocycles display strong blue-green to orange-red emission in the solid state. Combined DFT//CASP2T calculations suggest that a common biradical intermediate is responsible for the formation of these compounds as well as their interconversion.