Synthesis of Substituted Pentafluorosulfanylpyrazoles Under Flow Conditions.

The development of flow conditions for the synthesis of pentafluorosulfanylpyrazoles is reported. A range of alkyl- and aryl-substituted SF5-alkynes were used in combination with different diazoacetates for this transformation. The corresponding substituted SF5-pyrazoles were obtained in up to 90% yield (average of 74% for 21 examples) as a mixture of isomers (up to 73:27 ratio). Synthetic transformations starting from an SF5-containing pyrazole were also demonstrated.

Aromatic 1,2-Azaborinin-1-yls as Electron-Withdrawing Anionic Nitrogen Ligands for Main Group Elements.

The 2-aryl-3,4,5,6-tetraphenyl-1,2-azaborinines 1-EMe3 and 2-EMe3 (E=Si, Sn; aryl=Ph (1), Mes (=2,4,6-trimethylphenyl, 2)) were synthesized by ring-expansion of borole precursors with N3 EMe3 -derived nitrenes. Desilylative hydrolysis of 1- and 2-SiMe3 yielded the corresponding N-protonated azaborinines, which were deprotonated with nBuLi or MN(SiMe3 )2 (M=Na, K) to the corresponding group 1 salts, 1-M and 2-M. While the lithium salts crystallized as monomeric Lewis base adducts, the potassium salts formed coordination polymers or oligomers via intramolecular K⋅⋅⋅aryl π interactions. The reaction of 1-M or 2-M with CO2 yielded N-carboxylate salts, which were derivatized by salt metathesis to methyl and silyl esters. Salt metathesis of 1-M or 2-M with methyl triflate, [Cp*BeCl] (Cp*=C5 Me5 ), BBr2 Ar (Ar=Ph, Mes, 2-thienyl), ECl3 (E=B, Al, Ga) and PX3 (X=Cl, Br) afforded the respective group 2, 13 and 15 1,2-azaborinin-2-yl complexes. Salt metathesis of 1-K with BBr3 resulted not only in N-borylation but also Ph-Br exchange between the endocyclic and exocyclic boron atoms. Solution 11 B NMR data suggest that the 1,2-azaborinin-2-yl ligand is similarly electron-withdrawing to a bromide. In the solid state the endocyclic bond length alternation and the twisting of the C4 BN ring increase with the sterics of the substituents at the boron and nitrogen atoms, respectively. Regression analyses revealed that the downfield shift of the endocyclic 11 B NMR resonances is linearly correlated to both the degree of twisting of the C4 BN ring and the tilt angle of the N-substituent. Calculations indicate that the 1,2-azaborinin-1-yl ligand has no sizeable π-donor ability and that the aromaticity of the ring can be subtly tuned by the electronics of the N-substituent.

Azidoborolate anions and azidoborole adducts: isolable forms of an unstable borole azide.

Boroles are well known to undergo ring expansion reactions with organic azides to yield 1,2-azaborinines. A synthon featuring both azide and borole moieties within the same molecule, 1-azido-2,3,4,5-tetraphenylborole, was found to be much less stable than the related, previously-reported azidoborafluorene and decomposed to intractable mixtures well below room temperature. It could, however, be trapped at -75 °C through the formation of Lewis base adducts, even in the form of the "azide-stabilized azidoborole" complex anion diazidoborolate. DFT calculations provide a rationale for the low stability of the azidoborole under study.

Rethinking Borole Cycloaddition Reactivity.

Boroles are attracting broad interest for their myriad and diverse applications, including in synthesis, small molecule activation and functional materials. Their properties and reactivity are closely linked to the cyclic conjugated diene system, which has been shown to participate in cycloaddition reactions, such as the Diels-Alder reaction with alkynes. The reaction steps leading to boranorbornadienes, borepins and tricyclic boracyclohexenes from the thermal reaction of boroles with alkynes are seemingly well understood as judged from the literature. Herein, we question the long-established mechanistic picture of pericyclic rearrangements by demonstrating that seven-membered borepins (i. e., heptaphenylborepin and two derivatives substituted with a thienyl and chloride substituent on boron) exist in a dynamic equilibrium with the corresponding bicyclic boranorbornadienes, the direct Diels-Alder products, but are not isolable products from the reactions. Heating gradually converts the isomeric mixtures into fluorescent tricyclic boracyclohexenes, the most stable isomers in the series. Results from mechanistic DFT calculations reveal that the tricyclic compounds derive from the boranorbornadienes and not the borepins, which were previously believed to be intermediates in purely pericyclic processes.

One-pot, room-temperature conversion of dinitrogen to ammonium chloride at a main-group element.

The industrial reduction of dinitrogen (N2) to ammonia is an energy-intensive process that consumes a considerable proportion of the global energy supply. As a consequence, species that can bind N2 and cleave its strong N-N bond under mild conditions have been sought for decades. Until recently, the only species known to support N2 fixation and functionalization were based on a handful of metals of the s and d blocks of the periodic table. Here we present one-pot binding, cleavage and reduction of N2 to ammonium by a main-group species. The reaction-a complex multiple reduction-protonation sequence-proceeds at room temperature in a single synthetic step through the use of solid-phase reductant and acid reagents. A simple acid quench of the mixture then provides ammonium, the protonated form of ammonia present in fertilizer. The elementary reaction steps in the process are elucidated, including the crucial N-N bond cleavage process, and all of the intermediates of the reaction are isolated.

Diborane(4) Azides: Surprisingly Stable Sources of Transient Iminoboranes.

Herein we describe the first examples of isolable electron-precise diboranes(4) that bear azide moieties: the acyclic 1,2-diazido-1,2-bis(dimethylamino)diborane(4) and the cyclic 1,4-diaryl-2,3-diazido-1,4-diaza-2,3-diborinines (aryl=mesityl, 2,6-xylyl, 4-tolyl). The reported examples are not only stable enough to be observed and isolated (putative transient diborane(4) azides previously reported by our group spontaneously decompose even below room temperature), but some of them are even robust enough to undergo controlled pyrolysis without explosive decomposition at temperatures well above 100 °C. In two cases, the controlled pyrolysis allows the isolation of complex diazaboretidines, which are the apparent dimerization products of endocyclic boryl-iminoboranes.

Synthesis of Complex Boron-Nitrogen Heterocycles Comprising Borylated Triazenes and Tetrazenes Under Mild Conditions.

The reactions of organic azides with diaryl(dihalo)diboranes(4) were explored, resulting in the observation of a number of surprising reactivity patterns. The reaction of phenyl azide with 1,2-diaryl-1,2-dihalodiboranes(4) resulted in the formation of five-membered rings comprising diboryl-triazenes with retention of the boron-boron bond, while the reaction of the peculiar 1,1-di(9-anthryl)-2,2-difluorodiborane(4) with phenyl azide yielded a six-membered ring bearing a diboryl-triazene, whereby the B-B bond was ruptured by the insertion of an arylnitrene-like reactive intermediate. Both types of heterocycles feature unprecedented connectivity patterns and are very rare examples of boryl triazenes beyond the more common 1,2,3-triazolatoboranes. They are also the product of a unique type of aryl migration from a boron center to the phenyl azide γ-nitrogen center. Lastly, the substitution of 1,2-diaryl-1,2-dihalodiboranes(4) with azide groups, using trimethylsilyl azide as the transfer reagent, yielded boryl-tetrazaboroles and diboryldiazadiboretidines (as side-products), invoking the intermediacy of the first N-boryl-substituted iminoboranes, which are BN isosteres of monoborylated alkynes. The synthetic results are complemented with mechanistic proposals derived from quantum-chemical calculations.

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.

Hexahalodiborate Dianions: A New Family of Binary Boron Halides.

The electron-precise binary boron subhalide species [B2 X6 ]2- X=F, Br, I) were synthesized and their structures confirmed by X-ray crystallography. The existence of the previously claimed [B2 Cl6 ]2- , which had been questioned, was also confirmed by X-ray crystallography. The dianions are isoelectronic to hexahaloethanes, are subhalide analogues of the well-known tetrahaloborate anions (BX4 - ), and are rare examples of molecular electron-precise binary boron species beyond B2 X4 , BX3 , and [BX4 ]- .

The reductive coupling of dinitrogen.

The coupling of two or more molecules of dinitrogen (N2) occurs naturally under the radiative conditions present in the ionosphere and may be achieved synthetically under ultrahigh pressure or plasma conditions. However, the comparatively low N-N single-bond enthalpy generally renders the catenation of the strongly triple-bonded N2 diatomic unfavorable and the decomposition of nitrogen chains a common reaction motif. Here, we report the surprising organoboron-mediated catenation of two N2 molecules under near-ambient conditions to form a complex in which a [N4]2- chain bridges two boron centers. The reaction entails reductive coupling of two hypovalent-boron-bound N2 units in a single step. Both this complex and a derivative protonated at both ends of the chain were characterized crystallographically.

Intriguing migrations in transient iminoborane adducts: two new pathways to aminoboranes.

Two unusual reactions were demonstrated with iminoboranes, both leading to aminoboranes of the type R2B[double bond, length as m-dash]NR2. In one case, a carboboration of di(tert-butyl)iminoborane with B(C6F5)3 led to (tBu)(C6F5)B-N(tBu)(B(C6F5)2). In the other, a transient IDipp (1,3-di(2,6-diisopropylphenyl)imidazol-2-ylidene) adduct of MesBNtBu undergoes a shift of one carbene-bound Dipp substituent to the iminoborane nitrogen atom, yielding (1-Dipp-imidazol-2-yl)(Mes)B-N(Dipp)(tBu). Mechanistic DFT studies indicate the intermediacy of a borane·iminoborane adduct in the carboboration reaction.

Cleavage of BN triple bonds by main group reagents.

We report two rare instances of an insertion into the strong (ca. 170 kcal mol-1) BN triple bond of iminoboranes. In the first, a silylene inserts into di-tert-butyliminoborane to form an iminosilane. In the second, the highly crowded iminoborane Ter-NB-TMP (TMP = 2,2,6,6-tetramethylpiperidyl, Ter = 2,6-(diphenylmethyl)-4-tert-butylphenyl) can be forced to react with Pip-CC-Pip (Pip = piperidyl) at 60 °C. The reaction product is the apparent result of Pip-CC insertion into the iminoborane BN bond.

Influence of the catalyst structure in the cycloaddition of isocyanates to oxiranes promoted by tetraarylstibonium cations.

In the context of our work on electron deficient group 15 cations as Lewis acid catalysts, we have synthesized the triflate salts of a series of tetraarylstibonium cations of general formula [ArSbPh3]+ with Ar = Mes (4+), o-(dimethylamino)phenyl (5+), and o-((dimethylamino)methyl)phenyl (6+). These new cationic antimony derivatives, along with the known [Ph4Sb]+ (1+), 1-naphthyltriphenylstibonium (2+), and [(Ant)SbPh3]+ (3+), have been evaluated as catalysts for the cycloaddition of oxiranes and isocyanates under mild conditions. While all stibonium cations favor the 3,4-oxazolidinone products, the reactivities of 5+ and 6+ are hindered by the ancillary amino donor which quenches the Lewis acidity of the antimony center. A comparison of the other stibonium cations shows that 4+ is the most selective catalyst.

Nitrogen fixation and reduction at boron.

Currently, the only compounds known to support fixation and functionalization of dinitrogen (N2) under nonmatrix conditions are based on metals. Here we present the observation of N2 binding and reduction by a nonmetal, specifically a dicoordinate borylene. Depending on the reaction conditions under which potassium graphite is introduced as a reductant, N2 binding to two borylene units results in either neutral (B2N2) or dianionic ([B2N2]2-) products that can be interconverted by respective exposure to further reductant or to air. The 15N isotopologues of the neutral and dianionic molecules were prepared with 15N-labeled dinitrogen, allowing observation of the nitrogen nuclei by 15N nuclear magnetic resonance spectroscopy. Protonation of the dianionic compound with distilled water furnishes a diradical product with a central hydrazido B2N2H2 unit. All three products were characterized spectroscopically and crystallographically.

Dinitramidoborates: A Fascinating Case of Competing Oxygen and Nitrogen Donors and Tautomerism.

Reactions of the BH4- anion with equimolar amounts of HN(NO2 )2 or of BH3 ⋅THF with K[N(NO2 )2 ]- produced a mono-substituted [BH3 N(NO2 )2 ]- anion, which contains a B-N connected dinitramido ligand. The reaction of BH4- with two equivalents of HN(NO2 )2 afforded the di-substituted borate anion consisting of two isomers, one with both nitramido ligands attached to B through N and the other one with one ligand attached through N and the other one through O. The disubstituted dinitramidoborates are marginally stable under ambient conditions, and the isomer with two N-connected ligands was characterized by its crystal structure. A tri-substituted borate was tentatively identified by NMR in the reaction of BH4- with a large excess of HN(NO2 )2 . All of the anions are highly energetic. Theoretical calculations show that the energy differences between the B-N and B-O tautomers are small, explaining the formation of both.

On the Reaction of Naphthalene Diimides with Fluoride Ions: Acid/Base versus Redox Reactions.

The characterization of a naphthalene diimide (NDI) radical anion is presented. Its properties allow a reliable comparison point for other systems involving the NDI radical anion, such as systems claimed to perform the oxidation of the fluoride anion. In addition to reiterating the obvious thermodynamic objections to such an unlikely oxidation, we present several observations that show that the fluoride anion does not act as a one-electron reducing agent toward the NDI investigated in this work.

Synthesis and Characterization of Nitro-, Trinitromethyl-, and Fluorodinitromethyl-Substituted Triazolyl- and Tetrazolyl-trihydridoborate Anions.

The problem of preparing energetic, exclusively mono-azolyl substituted hydridoborate anions in high yield and purity from [BH4 ]- and nitroazoles by hydrogen elimination was overcome by reacting the corresponding nitroazolate anions with the BH3 adducts BH3 ⋅S(CH3 )2 or BH3 ⋅THF. The highly-energetic, nitro-, trinitromethyl-, and fluorodinitromethyl- substituted triazolyl- and tetrazolyl-trihydridoborate anions were synthesized in this manner and characterized by vibrational and multinuclear NMR spectroscopy and their crystal structures. The use of excess BH3 resulted in some cases in the addition of a second BH3 molecule bound more-weakly to one of the nitrogen atoms of the azole ring. All monoazolyl-trihydridoborates were thermally less stable than the parent azolate anions. A decomposition product of tetraphenylphosphonium (5-(trinitromethyl)-5H-2λ4 -tetrazol-2-yl)trihydridoborate, the tetraphenyl-phosphonium (dinitro-1H-tetrazol-5-yl)methanide monohydrate, was also structurally characterized, providing some insight into the decomposition pathways of the nitromethyl-substituted azolyltrihydridoborate anions.

Ammonia-(Dinitramido)boranes: High-Energy-Density Materials.

Two ammonia-(dinitramido)boranes were synthesized by the reaction of dinitroamine with ammonia-borane. These compounds are the first reported examples of (dinitramido)boranes. Ammonia-mono(dinitramido)borane is a perfectly oxygen-balanced high-energy-density material (HEDM) composed of an ammonia-BH2 fuel group and a strongly oxidizing dinitramido ligand. Although it is thermally not stable enough for practical applications, its predicted specific impulse as a solid rocket propellant would be 333 s. Its predicted performance as an explosive matches that of pentaerythtritol tetranitrate (PETN) and significantly exceeds that of trinitrotoluene (TNT). Its structure was established by X-ray crystallography and vibrational and multinuclear NMR spectroscopy. Additionally, the over-oxidized ammoniabis(dinitramido)borane was detected by NMR spectroscopy.

Synthesis and characterization of fluorodinitroamine, FN(NO2)2.

NF3 and N(NO2)3 are known compounds, whereas the mixed fluoronitroamines, FN(NO2)2 and F2NNO2, have been unknown thus far. One of these, FN(NO2)2, has now been prepared and characterized by multinuclear NMR and Raman spectroscopy. FN(NO2)2 is the first known example of an inorganic fluoronitroamine. It is a thermally unstable, highly energetic material formed by the fluorination of the dinitramide anion using NF4(+) salts as the preferred fluorinating agent.

Nitryl cyanide, NCNO2.

The elusive nitryl cyanide, NCNO2, has been synthesized and characterized. It was prepared in good yield, isolated by fractional condensation, characterized by NMR and vibrational spectroscopy, and studied by theoretical calculations. Nitryl cyanide holds promise as a high energy density material (HEDM) and might also prove useful as a HEDM building block. The simplicity and inherent stability of nitryl cyanide, together with the known multitude of nitriles in interstellar space, suggest that the compound might also be a potential candidate for observations in atmospheric and interstellar chemistry.