Diborynes are main-group compounds that contain a formal boron–boron triple bond (B≡B). These molecules are usually stabilized by strong σ-donor ligands, most often N-heterocyclic carbenes (NHCs). The support from these ligands allows the B≡B unit to exist as a discrete, isolable structure.
Boron–boron triple bond (B≡B) stabilized by N-heterocyclic carbenes (NHCs) | |
| Names | |
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| IUPAC name boranylidyneborane | |
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3D model (JSmol) |
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PubChem CID |
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| Properties | |
| B2 | |
| Molar mass | 21.63 g/mol |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references | |
Multiple bonds between heavier main-group elements are uncommon. They often bend away from linearity because their orbitals do not overlap well enough to maintain true triple-bond character. Diborynes stand out because their B≡B bond remains almost perfectly linear. This geometry closely resembles that of carbon–carbon alkynes and marks an important step forward in understanding how main-group atoms can form strong, directional π-bonds.
The first diboryne that could be handled under normal laboratory conditions was reported in 2012. Braunschweig and co-workers showed that reducing a carbene-stabilized diborane precursor produced a halide-free, linear B≡B triple bond. The resulting compound was stable enough for full spectroscopic and crystallographic study, which confirmed its unusual bonding pattern.
Beyond their structure, diborynes show a wide range of reactions at the B≡B unit. They can take part in multi-electron processes such as chalcogen insertion or diboration and cycloaddition reactions that build larger B–B frameworks. Their electron-poor boron centers also give these molecules adjustable Lewis acidity and allow them to activate small molecules. In addition, their excited-state features lead to recognizable optical signals, including clear absorption bands and, in some related complexes, room-temperature phosphorescence. Because of these properties, diborynes serve as useful models for main-group multiple bonding and as promising starting points for boron-based π-materials and proposed B≡B polymer chains.
Synthesis
First reported synthetic method
The first isolable diboryne was reported in 2012 by Braunschweig and co-workers. The synthesis started from a dibromodiborane(4) precursor supported by two N-heterocyclic carbenes (NHCs). These ligands donate electron density to boron and help stabilize the low-valent centers. In the first step, the precursor was treated with a strong reductant such as potassium graphite (KC8). This reaction removed the halide groups through a two-electron reduction and produced a neutral diboron species that still carried both NHC ligands. A second, carefully controlled reduction at low temperature then formed the B–B multiple bond. This step produced the first NHC→B≡B←NHC complex as a dark crystalline solid. The team confirmed the structure by multinuclear NMR spectroscopy, UV–vis absorption studies, and single-crystal X-ray diffraction. The B–B distance and the nearly linear geometry matched the expectations for a formal triple bond.
Synthetic developments
Diborynes have been known for less than fifteen years, so the synthetic routes to these compounds are still limited. Even so, several advances have expanded the small set of available B≡B-containing systems. One important step was the synthesis of a silylene-stabilized diboryne. This work showed that an ambiphilic silicon donor can also support a B≡B unit. It also provided the first crystal structure of a diboryne that is not based on a classical NHC ligand. Another development came from the use of mesoionic carbenes (MICs). These ligands have stronger σ-donor and π-acceptor character than common NHCs. They stabilize diborynes with different electronic features and give access to new bonding environments around the B≡B unit.
Several computational studies have suggested additional ways to stabilize diborynes. One idea is to use multitopic N-heterocyclic carbene frameworks. Calculations predict that these ligands could support larger diboryne-based assemblies and even form nanostructures with extended B≡B units. Other DFT studies have looked at metallaborocyclic systems. These models explore whether a metal center inside a ring can help stabilize a B–B triple bond. The results show clear differences between boron and carbon analogues in the same type of framework. Together, these studies show that new possibilities for diboryne chemistry are slowly appearing, although practical synthetic access remains narrow and continues to develop.
Reactivity
Lewis acid-base adducts
Diborynes show clear Lewis acidity at both boron atoms, even though the B≡B bond is formally a linear triple bond. Early experimental work noted that each boron center carries an open p-orbital that can accept electron density from neutral donors. This feature creates two separate coordination sites on the B≡B unit. Theory supported this observation. Calculations showed that the molecule also has low-lying empty orbitals derived from the σu+ and σg+ manifolds. These orbitals make the B≡B bond ready to form Lewis adducts without breaking the basic σ+2π bonding pattern.
These orbital features explain why diborynes bind donors such as pyridines, phosphines, and additional carbenes. The B≡B bond usually stays close to linear when such adducts form, and the B–B distance increases only slightly. The combination of accessible empty orbitals and a stable π-framework allows diborynes to act as Lewis acids while still keeping the defining structure of a main-group triple bond.
Bond strength and bonding analysis
Studies on the B≡B triple bond have helped clarify how stable this bond is and how it differs from a normal carbon–carbon triple bond. Thermochemical measurements and force-field analysis showed that the B≡B bond is much weaker than C≡C. This result matches bond-energy calculations carried out on carbene-stabilized diborynes. Even so, the B≡B unit is stable enough to handle under normal conditions. Strong σ-donation from N-heterocyclic carbenes (NHCs) and related ligands fills the π-bonding orbitals and prevents easy bond breaking.
Main-group surveys point out that the linear shape of the B≡B bond is unusual. Heavier group-14 and group-15 systems almost always bend, because their orbitals cannot overlap well enough to support full π-bonding. Diborynes behave differently. X-ray structures and computational studies both show that NHC→B≡B←NHC complexes contain three B–B bonding orbitals: a σ-bond and two delocalized π-bonds. These findings explain why the B≡B unit keeps a nearly straight geometry even though it is part of a low-valent main-group framework.
Chalcogen insertion reactions
The B≡B triple bond can take part in multi-electron addition reactions, including the reductive insertion of sulfur, selenium, and tellurium. In these reactions, the chalcogen atom inserts into the B–B bond to form a three-membered B–E–B ring. The reactions usually proceed smoothly when carried out under controlled reducing conditions. The resulting products keep a clear link to the original B≡B unit, because the system can undergo net 2- or 4-electron changes without fully breaking the multiple bond.
Chalcogen insertion is now used as a common way to test the reactivity of diborynes. The resulting B–E–B rings often retain some π-character between boron and the chalcogen atom. They also show new spectroscopic signals that reveal how electron density shifts after the insertion step. These features make the reaction a useful probe of the electronic structure of the B≡B core.
Hydnroboration and formation of boron clusters
Diborynes react cleanly with hydridoboranes and show a clear stepwise pattern during hydroboration. In a single hydroboration reaction, one equivalent of a monohydridoborane adds across one of the B–B π-bonds. This step gives a B–B(H)(R2B) product that still keeps one π-bond intact. The reaction shows that the B≡B unit can accept a controlled two-electron addition without losing its basic σ+2π bonding framework.
Stronger conditions or extra equivalents of the hydroborating reagent lead to double hydroboration. This process fully saturates the B≡B bond and forms B4-containing intermediates made from the formal addition of two boron and two hydrogen atoms to the original triple bond. Several of these intermediates can be converted to the same cationic tetraborane after hydride abstraction. This result shows that different hydroboration routes can feed into one common B4 cluster.
These convergent pathways demonstrate that diborynes are useful starting points for building small boron clusters. The B≡B triple bond acts as a modular unit that can be elaborated into larger polyboron frameworks through controlled addition reactions. Hydroboration therefore sits alongside chalcogen insertion and cycloaddition as one of the major bond-transforming reactions available to diborynes.
Carbon monoxide binding and coupling
Diborynes react with carbon monoxide in a way that gives a rare example of metal-free CO activation at a main-group multiple bond. In the first reported case, an NHC-stabilized diboryne reacted with CO at room temperature to form a four-membered B2C2O2 ring. The product came from stepwise CO binding followed by intramolecular coupling across the B≡B bond.
Work on the related compound B2(SIDip)2 later showed that the outcome depends on the electronic nature of the B–B unit. Species with stronger cumulene character (B=B=B) bind CO more easily and then undergo ring-forming coupling. Molecules with a more classical B≡B triple bond react more slowly or require harsher conditions. These results indicate that the amount of π-density along the B–B bond, which is controlled by the ligand, plays a major role in CO activation.
Computational studies support these observations. They show that the B≡B unit can activate CO through different σ-activation pathways depending on the polarity of the substrate. Polar molecules interact with boron-centered empty orbitals, while nonpolar substrates such as CO react through a π-limited pathway that remains accessible because of the highly polarized π-system of the B≡B bond. Taken together, these findings place CO reactivity within the broader set of small-molecule activation reactions available to diborynes and show that the B≡B unit can mediate multi-electron processes more commonly seen in transition-metal chemistry.
Photophysical Properties
Diborynes show distinct photophysical behavior because of the special electronic structure of the B≡B triple bond. Free NHC-stabilized diborynes display clear π→π* absorption bands. These bands come from the polarized B–B π-system, which carries high π-density along the bond and has low-lying empty orbitals at each boron atom. The electronic link between the two boron centers is also visible in 11B–11B spin–spin coupling measurements. These data support the presence of a delocalized multiple bond in the ground state.
The photophysical properties change when the diboryne coordinates to an electrophilic main-group or transition-metal center. π-Complexation with electron-poor main-group atoms alters the frontier orbitals of the B≡B unit and shifts the absorption features. The B≡B bond acts as a π-donor in these systems, similar to the way alkynes bind. The spectral changes are usually small and predictable. They reflect the redistribution of π-density that occurs during complex formation and show that the B≡B chromophore is sensitive to its environment. More striking effects appear in transition-metal π-complexes of diborynes. Several of these complexes show strong room-temperature phosphorescence, sometimes with high quantum yields. The emissions arise from increased metal–boron π-interaction and efficient population of low-lying triplet states. The emission bands are often shifted to lower energy compared to free diborynes. This trend matches the stabilization of the LUMO and the stronger spin–orbit coupling provided by the metal center. These observations show that the B≡B unit can take part in both metal-centered and ligand-centered excited states. As a result, diborynes and their complexes may serve as useful components in luminescent and optoelectronic materials.
Conclusion
Diborynes have broadened the scope of main-group chemistry by showing that a B≡B triple bond can support behaviors usually linked to transition-metal systems. The linear σ+2π framework gives the bond a stable but flexible electronic structure. This structure allows diborynes to show clear photophysical signals and to take part in many types of bond activation, including CO coupling, alkyne cleavage, chalcogen insertion, hydroboration, and π-complexation. These reactions demonstrate that the B≡B unit can serve as both a reactive center and a tunable π-chromophore. Broader studies of main-group bonding support this view, and they place diborynes among a growing set of low-valent p-block species that can mimic or complement the roles of transition metals. As a result, diborynes provide new ways to activate small molecules, form boron clusters, and design functional boron-based materials.
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