Aromatic 1,4,2,3-Diazadiborole Featuring an Unsymmetrical B=B Entity: A Versatile Synthon for Unusual Boron Heterocycles.

The replacement of a CC unit with an isoelectronic BN unit in aromatic systems can give rise to molecules and materials with fascinating properties. We report here the synthesis, characterization, and reactivity of a 1,4,2,3-diazadiborole species, 2, featuring an unprecedented 6π-aromatic BN-heterocyclic moiety that is isoelectronic to cyclopentadienide (Cp-). Bearing an unsymmetrical B=B entity, 2 exhibits reactivity toward oxidants, protic reagents, electrophiles, and unsaturated substrates. This reactivity facilitates the synthesis of a variety of novel mono- and bicyclic organoboron derivatives through mechanisms including ring retention, cleavage/recombination, annulation, and expansion. These findings reveal innovative synthetic routes to BN-embedded aromatic compounds via desymmetrization, affording unique building blocks for synthetic chemistry.

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 B3 H4 + analogues.

Bond-Strengthening Backdonation in Aminoborylene-Stabilized Aminoborylenes: At the Intersection of Borylenes and Diborenes.

Singly NHC-coordinated (aminoboryl)aminoborenium salts react with Na2 [Fe(CO)4 ] to yield stable coordination complexes of aminoborylene-stabilized aminoborylenes, which exhibit exceptional σ-donor properties. Upon photolytic CO extrusion from the metal center, the diboron ligand adopts a novel η3 -BBN coordination mode, where bond-strengthening backdonation from the metal center into the vacant B-B π-orbital is observed. This bonding situation can be alternatively described as a Fe-diaminodiborene complex. In a related reduction of CAAC-stabilized (aminoboryl)aminoborenium with KC8 , the reduced species can be captured with nucleophiles to form three-coordinate (diaminoboryl)borylenes, where both amino groups have migrated to the distal boron atom. Collectively, these reactions illustrate the isomeric flexibility imparted by amino groups on this reduced diboron system, thus opening multiple avenues of novel reactivity.

Coordination of Asymmetric Diborenes towards Cationic Coinage Metals (Au, Ag, Cu).

Synthesis of a series of cationic coinage metal (Au: 2-4, Ag: 5, and Cu: 6) complexes of asymmetric diborenes (1 a, 1 b) is reported. X-ray diffraction analysis reveals that the metal center interacts unsymmetrically with the B2 moiety and the terminal boron atom exhibits pronounced pyramidal geometry. A computational study shows that in complexes 2-6, the bonding between the B2 units and the metal center is dominated by electrostatic interactions concomitant with non-negligible covalent contributions.

Increasing the Reactivity of Diborenes: Derivatization of NHC-Supported Dithienyldiborenes with Electron-Donor Groups.

A series of NHC-supported 1,2-dithienyldiborenes was synthesized from the corresponding (dihalo)thienylborane NHC precursors. NMR and UV/Vis spectroscopic data, as well as X-ray crystallographic analyses, were used to assess the electronic and steric influences on the B=B double bond of various NHCs and electron-donating substituents on the thienyl ligands. Crystallographic data showed that the degree of coplanarity of the diborene core and thienyl groups is highly dependent on the sterics of the substituents. Furthermore, any increase in the electron-donating ability of the substituents resulted in the destabilization of the HOMO and greater instability of the resulting diborenes.

Diboryldiborenes: π-Conjugated B4 Chains Isoelectronic to the Butadiene Dication.

B(sp2 )-B(sp3 ) diborane species based on bis(catecholato)diboron and N-heterocyclic carbenes (NHCs) underwent catechol/bromide exchange selectively at the sp3 -hybridized boron atom. The reduction of the resulting 1,1-dibromodiborane adducts led to reductive coupling and isolation of doubly NHC-stabilized 1,2-diboryldiborenes. These compounds are the first examples of molecules exhibiting π-electron delocalization over an all-boron chain.

CO2 Binding and Splitting by Boron-Boron Multiple Bonds.

The room-temperature, ambient-pressure reactions of CO2 with two species containing boron-boron multiple bonds led to the incorporation of either one or two CO2 molecules. The structural characterization of a thermally unstable intermediate in one case indicates that an initial [2+2] cycloaddition is the key step in the reaction mechanism.

Crystalline Neutral Allenic Diborene.

A zwitterionic boraalkenyl boronium 3 was synthesized by reduction of cyclic (alkyl)(amino)carbene (cAAC) and trimethylphosphine (PMe3 )-coordinated tetrabromodiborane 2 with KC8 in the presence of PMe3 . Further reduction of 3 led to the formation of neutral allenic diborene 4. X-ray diffraction and computational studies revealed that 4 features the cumulated C=B and B=B double bonds. The reaction of 4 with four isonitrile molecules afforded a heterocycle 5 with the B2 C3 five-membered ring, via a complete scission of the B=B bond of 4.

Reactivity of a Dihydrodiborene with CO: Coordination, Insertion, Cleavage, and Spontaneous Formation of a Cyclic Alkyne.

Under a CO atmosphere, the dihydrodiborene [(cAAC)HB=BH(cAAC)] underwent coordination of CO concomitant with reversible hydrogen migration from boron to the carbene carbon atom, as well as reversible CO insertion into the B=B bond. Heating the CO adduct resulted in two unusual cAAC ring-expansion products, one presenting a B=C bond to a six-membered 1,2-azaborinane-3-ylidene, the other an unprecedented nine-membered cyclic alkyne resulting from reductive cleavage of CO and spontaneous formation of a C≡C bond.

Engineering a Small HOMO-LUMO Gap and Intramolecular C-H Borylation by Diborene/Anthracene Orbital Intercalation.

The diborene 1 was synthesized by reduction of a mixture of 1,2-di-9-anthryl-1,2-dibromodiborane(4) (6) and trimethylphosphine with potassium graphite. The X-ray structure of 1 shows the two anthryl rings to be parallel and their π(C14 ) systems perpendicular to the diborene π(B=B) system. This twisted conformation allows for intercalation of the relatively high-lying π(B=B) orbital and the low-lying π* orbital of the anthryl moiety with no significant conjugation, resulting in a small HOMO-LUMO gap (HLG) and ultimately a C-H borylation of the anthryl unit. The HLG of 1 was estimated to be 1.57 eV from the onset of the long wavelength band in its UV/Vis absorption spectrum (THF, λonset =788 nm). The oxidation of 1 with elemental selenium afforded diboraselenirane 8 in quantitative yield. By oxidative abstraction of one phosphine ligand by another equivalent of elemental selenium, the B-B and C1 -H bonds of 8 were cleaved to give the cyclic 1,9-diborylanthracene 9.

Dibora[2]ferrocenophane: A Carbene-Stabilized Diborene in a Strained cis-Configuration.

Unsaturated bridges that link the two cyclopentadienyl ligands together in strained ansa metallocenes are rare and limited to carbon-carbon double bonds. The synthesis and isolation of a strained ferrocenophane containing an unsaturated two-boron bridge, isoelectronic with a C=C double bond, was achieved by reduction of a carbene-stabilized 1,1'-bis(dihaloboryl)ferrocene. A combination of spectroscopic and electrochemical measurements as well as density functional theory (DFT) calculations was used to assess the influence of the unprecedented strained cis configuration on the optical and electrochemical properties of the carbene-stabilized diborene unit. Initial reactivity studies show that the dibora[2]ferrocenophane is prone to boron-boron double bond cleavage reactions.

Uncatalyzed Hydrogenation of First-Row Main Group Multiple Bonds.

Room temperature hydrogenation of an SIDep-stabilized diboryne (SIDep=1,3-bis(diethylphenyl)-4,5-dihydroimidazol-2-ylidene) and a cAAC-supported diboracumulene (cAAC=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) provided the first selective route to the corresponding 1,2-dihydrodiborenes. DFT calculations showed an overall exothermic (ΔG=19.4 kcal mol-1 ) two-step asynchronous H2 addition mechanism proceeding via a bridging hydride.

From an electron-rich bis(boraketenimine) to an electron-poor diborene.

The reaction of the bisboracumulene (CAAC)2 B2 (CAAC=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) with excess tert-butylisocyanide resulted in complexation of the isocyanide at boron. Though this compound might be formally drawn with a lone pair on boron, these electrons are highly delocalized throughout a conjugated π-network consisting of the π-acidic CAAC and isocyanide ligands. Heating this compound to 110 °C liberated the organic periphery of both isocyanide ligands, yielding the first example of a dicyanodiborene. Cyclic voltammetry conducted on this diborene indicated the presence of reduction waves, making this compound unique among diborenes, which are otherwise highly reducing.

Unexpected luminescence behavior of coinage metal π-diborene complexes.

A number of unprecedented photophysical phenomena were observed in the study of luminescent π-diborene complexes of Cu and Ag. These observations included unusually high fluorescence quantum yields (up to 100%) in solution for complexes of these metals. This result indicates that very little or no intersystem crossing between S1 and T(n) occurs in the complexes, despite the strong spin-orbit coupling of the metal atoms. The replacement of carbon with boron thus yields luminescent isolobal analogues of otherwise non-emissive olefin complexes of Cu and Ag.

Direct hydroboration of B=B bonds: a mild strategy for the proliferation of B-B bonds.

Synthetic access to electron-precise boron chains is hampered by the preferential formation of nonclassical structures. The few existing strategies for this involve either strongly reducing reagents or transition-metal catalysts, both with distinct disadvantages. The synthesis of new furyl- and thienyl-substituted diborenes is presented, along with their direct hydroboration with catecholborane (CatBH) to form a new electron-precise B-B bond and a B3 chain. The reaction is diastereoselective and proceeds under mild conditions without the use of strong reducing agents or transition-metal catalysts commonly used in B-B coupling reactions.