Formation Of Borazine Research Articles

Kinetics of borazine formation from ammonia borane dehydrocoupling reaction through Ab initio analysis

The Solid-state decomposition of ammonia borane (AB) proceeds through various simultaneous dehydrocoupling reactions between ammine (-NH3) and borane (-BH3) centres, which collectively leads to the formation of hydrogen and borazine as by-products. Therefore, the kinetic study on borazine release from AB decomposition is challenging due to multiple consecutive reactions and the heterogeneity of the reactant product phases. Ammonia borane has hydrogen bonding in solid state and hence a solid-state decomposition can follow some different reaction pathways and can be studied by introducing smaller clusters of AB. In this work, we have adopted a DFT approach to understand the reaction kinetics of decomposition of AB in gas phase through different intermediates and transition states. The theoretical studies have been carried out using B3LYP/6-311++G(d,p), 6-31-G(d), to investigate the potential energy surface and the possible reaction mechanism and kinetics of AB decomposition through different pathways, and their rate constants have been estimated by using transition state theory. Additionally, the Arrhenius kinetic parameters [Pre-exponential factor (A), Activation energy (Ea), rate constant (k)] of the individual reactions are also calculated by transition state theory. The obtained Arrhenius kinetic parameters have been used in modified Avrami-Erofeev kinetic model to predict the borazine release trend, under dynamic heating conditions. The theoretical kinetics calculation indicates that owing to various parameters i.e. electronegativity, steric factor, etc., the reactivity and geometry of each intermediates varies, which carry a prominent role in determining their corresponding kinetic properties (A, Ea). The borazine release has been observed to be more facile through B-(cyclo triborazanyl) amine borane (BCTB) mediated pathway; due to a B-(μH)-B bridge bonded intermediate formation. Interestingly, the experimental correlation of the theoretically predicted borazine release, have been observed to be accurate and also demonstrated that the BCTB nucleation follows through two-dimensional (2-D) growth kinetics, whereas CTB proceeds through three dimensional (3-D) nuclei growth kinetics.

Cyclolinear Oligo- and Poly(iminoborane)s: The Missing Link in Inorganic Main-Group Macromolecular Chemistry.

The reaction of n-C8 H17 B[N(Me)SiMe3 ]2 (1) with n-C8 H17 BCl2 (2 a) yielded, instead of a linear poly(iminoborane), the aminoborane n-C8 H17 B(Cl)N(Me)SiMe3 (4) and after cyclotrimerization the borazine cyclo-(n-C8 H17 BNMe)3 (6). Side reactions that result in borazine formation were effectively suppressed if 1,3-bis(trimethylsilyl)-1,3,2-diazaborolidines 7 were employed as co-monomers in combination with dichloro- or dibromoboranes 2 or 8, respectively. Silicon/boron exchange polycondensation led to oligo(iminoborane)s 11 a,b,ac,d. Alternative synthetic routes to such species involve Sn/B exchange of 1,3-bis(trimethylstannyl)-2-n-octyl-1,3,2-diazaborolidine (16) and n-C8 H17 BBr2 (8 a), and the initiated polycondensation of the dormant monomer 14 in the presence of a Brønsted acid (HCl, HOTf, or HNTf2 ; Tf=trifluoromethylsulfonyl). Although an attempt to obtain an oligo-/poly(iminoborane) with phenyl side groups yielded only insoluble material, the incorporation of aryl groups was proven for a derivative with both phenyl and n-octyl boron substituents (11 ac), as well as for a derivative with 4-n-butylphenyl side groups (11 d). The highest-molecular-weight sample obtained was 11 ac. Featuring about 18 catenated BN units, on average, this is the closest approach to a poly(iminoborane) known.

Thermal dehydrochlorination in the 4-fluoroaniline-trichloroborane system: identification of reactive intermediates involved in the formation of B,B',B''-trichloro-N,N',N''-tri((4-fluoro)phenyl)borazine.

Borazines are used in chemical vapor deposition processes to produce hybrid graphene-boron nitride nanostructures. As the knowledge on the mechanism of borazine formation is scarce, we studied the mechanism of formation of B,B',B''-trichloro-N,N',N''-tri(p-fluorophenyl)borazine (3a) from p-fluoroaniline and boron trichloride employing NMR spectroscopy, X-ray single crystal structure analysis, trapping experiments, and computational chemistry methods up to the coupled cluster CCSD(T) level of theory. These studies suggest the initial formation of the 1 : 1 adduct 1a (ArNH2BCl3, Ar = 4-fluorophenyl) with a dative B-N bond that could be fully characterized including single crystal X-ray diffraction. Adduct 1a undergoes unimolecular hydrogen chloride elimination with a first-order rate constant of k1 = 3.03(7) × 10-2 min-1 in toluene at 100 °C. This rate constant is in very good agreement with the one derived (k1 = 3.18 × 10-2 min-1) from computed activation parameters (ΔH‡373.15 = 28.1 kcal mol-1, ΔS‡373.15 = 1.56 eu, ΔG‡373.15 = 27.6 kcal mol-1). The product of the first hydrogen chloride evolution is anilinodichloroborane ArNHBCl2 (2a). Compound 2a cannot be isolated in a pure form due to instability, but its presence as a transient reactive intermediate can be derived from NMR spectroscopy. Reactive intermediates other than anilinodichloroborane cannot be assigned by NMR spectroscopy. We propose that the mechanism of formation of borazine 3a involves the reaction of 2a with 4-fluoroaniline as the rate determining step.

Ammonia Borane Dehydrogenation Catalyzed by (κ4-EP3)Co(H) [EP3 = E(CH2CH2PPh2)3; E = N, P] and H2 Evolution from Their Interaction with NH Acids.

Two Co(I) hydrides containing the tripodal polyphosphine ligand EP3, (κ4-EP3)Co(H) [E(CH2CH2PPh2)3; E = N (1), P (2)], have been exploited as ammonia borane (NH3BH3, AB) dehydrogenation catalysts in THF solution at T = 55 °C. The reaction has been analyzed experimentally through multinuclear (11B, 31P{1H}, 1H) NMR and IR spectroscopy, kinetic rate measurements, and kinetic isotope effect (KIE) determination with deuterated AB isotopologues. Both complexes are active in AB dehydrogenation, albeit with different rates and efficiency. While 1 releases 2 equiv of H2 per equivalent of AB in ca. 48 h, with concomitant borazine formation as the final "spent fuel", 2 produces 1 equiv of H2 only per equivalent of AB in the same reaction time, along with long-chain poly(aminoboranes) as insoluble byproducts. A DFT modeling of the first AB dehydrogenation step has been performed, at the M06//6-311++G** level of theory. The combination of the kinetic and computational data reveals that a simultaneous B-H/N-H activation occurs in the presence of 1, after a preliminary AB coordination to the metal center. In 2, no substrate coordination takes place, and the process is better defined as a sequential BH3/NH3 insertion process on the initially formed [Co]-NH2BH3 amidoborane complex. Finally, the reaction of 1 and 2 with NH-acids [AB and Me2NHBH3 (DMAB)] has been followed via VT-FTIR spectroscopy (in the -80 to +50 °C temperature range), with the aim of gaining a deeper experimental understanding of the dihydrogen bonding interactions that are at the origin of the observed H2 evolution.

Poly(iminoborane)s: An Elusive Class of Main-Group Polymers?

The significance of inorganic main-group polymers is demonstrated most clearly by the commercial relevance of polysiloxanes (silicones). Organoboron-based materials such as π-conjugated organoborane polymers and BN-doped polycyclic aromatic hydrocarbons are currently attracting considerable attention. Surprisingly, poly(iminoborane)s (PIBs; [BRNR']n ), that is, the parent unsaturated BN polymers, which are formally isoelectronic to polyacetylene, have not been convincingly characterized thus far. Herein, we present the synthesis and comprehensive characterization of a linear oligo(iminoborane), which comprises a chain of 12-14 BN units on average. With our synthetic approach, unwanted side reactions that result in borazine formation are effectively suppressed. Supporting DFT and TD-DFT calculations provide deeper insight into the microstructure and the electronic structure of the oligomer.

Unraveling the Crucial Role of Metal-Free Catalysis in Borazine and Polyborazylene Formation in Transition-Metal-Catalyzed Ammonia–Borane Dehydrogenation

Though the recent scientific literature is rife with experimental and theoretical studies on transition-metal (TM)-catalyzed dehydrogenation of ammonia–borane (NH3·BH3) due to its relevance in chemical hydrogen storage, the mechanistic knowledge is mostly restricted to the formation of aminoborane (NH2BH2) after 1 equiv of H2 removal from NH3·BH3. Unfortunately, the chemistry behind the formation of borazine and polyborazylene, which happens only after more than 1 equiv of H2 is released from ammonia–borane in these TM-catalyzed homogeneous reactions, largely remains unknown. In this work we use density functional theory to unravel the curious function of “free NH2BH2”. Initially, free NH2BH2 molecules form oligomers such as cyclotriborazane and B-(cyclodiborazanyl)aminoborohydride. We show that, through a web of concerted proton and hydride transfer based dehydrogenations of oligomeric intermediates, cycloaddition reactions, and hydroboration steps facilitated by NH2BH2, the development of the polyborazylene framework occurs. The rate-determining free energy barrier for the formation of a polyborazylene template is predicted to be 25.7 kcal/mol at the M05-2X(SMD)/6-31++G(d,p)//M05-2X/6-31++G(d,p) level of theory. The dehydrogenation of BN oligomeric intermediates by NH2BH2 yields NH3·BH3, suggesting for certain catalytic systems that the role of the TM catalyst is limited to the dehydrogenation of NH3·BH3 to maintain optimal amounts of free NH2BH2 in the reaction medium to enable polyborazylene formation. TM catalysts that fail to produce borazine and polyborazylene falter because they rapidly consume NH2BH2 in TM-catalyzed polyaminoborane formation, thus preventing the chain of events triggered by NH2BH2.

Ammonia borane: A high capacity chemical hydrogen storage material

With the worlds energy, climate and environment problems becoming even serious, it is urgent to find suitable alternative energy sources. Hydrogen has attracted considerable attention because of its abundant reserves, pollution-free, high energy density, which can provide an efficient and stable clean energy source. How to store hydrogen safely and efficiently is a barrier for its applications in transportation. Boron-nitrogen-hydrogen (B-N-H) compounds have attracted tremendous academic attention due to their high hydrogen storage density and mild hydrogen release condition. Ammonia borane (NH3BH3, AB), as a representative of B-N-H compounds with 19.6 wt% hydrogen capacity, moderate thermal stability and low hydrogen release temperature, has been considered to be the most promising hydrogen storage materials. Metal amidoborane (MAB), prepared through replacing one of the hydrogen atoms bonded N in AB by a metal atom, can effectively inhibit the formation of borazine. A lot of theoretical and experimental research has been conducted to improve the performance of these compounds for lowering the hydrogen release temperature, reducing the induction time and decreasing volatile harmful gases borazine, ammonia and diborane. Based on the special dihydrogen bond existing in the ammonia borane, the synthetic methods and dehydrogenation properties of ammonia borane and metal amidoborane with the annexing agent are summarized in this review. In the last part, the regeneration and applications are prospected.

The Mechanism of Borane–Amine Dehydrocoupling with Bifunctional Ruthenium Catalysts

Borane-amine adducts have received considerable attention, both as vectors for chemical hydrogen storage and as precursors for the synthesis of inorganic materials. Transition metal-catalyzed ammonia-borane (H3N-BH3, AB) dehydrocoupling offers, in principle, the possibility of large gravimetric hydrogen release at high rates and the formation of B-N polymers with well-defined microstructure. Several different homogeneous catalysts were reported in the literature. The current mechanistic picture implies that the release of aminoborane (e.g., Ni carbenes and Shvo's catalyst) results in formation of borazine and 2 equiv of H2, while 1 equiv of H2 and polyaminoborane are obtained with catalysts that also couple the dehydroproducts (e.g., Ir and Rh diphosphine and pincer catalysts). However, in comparison with the rapidly growing number of catalysts, the amount of experimental studies that deal with mechanistic details is still limited. Here, we present a comprehensive experimental and theoretical study about the mechanism of AB dehydrocoupling to polyaminoborane with ruthenium amine/amido catalysts, which exhibit particularly high activity. On the basis of kinetics, trapping experiments, polymer characterization by (11)B MQMAS solid-state NMR, spectroscopic experiments with model substrates, and density functional theory (DFT) calculations, we propose for the amine catalyst [Ru(H)2PMe3{HN(CH2CH2PtBu2)2}] two mechanistically connected catalytic cycles that account for both metal-mediated substrate dehydrogenation to aminoborane and catalyzed polymer enchainment by formal aminoborane insertion into a H-NH2BH3 bond. Kinetic results and polymer characterization also indicate that amido catalyst [Ru(H)PMe3{N(CH2CH2PtBu2)2}] does not undergo the same mechanism as was previously proposed in a theoretical study.

Donor–Acceptor Complexation and Dehydrogenation Chemistry of Aminoboranes

A series of formal donor-acceptor adducts of aminoborane (H(2)BNH(2)) and its N-substituted analogues (H(2)BNRR') were prepared: LB-H(2)BNRR'(2)-BH(3) (LB = DMAP, IPr, IPrCH(2) and PCy(3); R and R' = H, Me or tBu; IPr = [(HCNDipp)(2)C:] and Dipp = 2,6-iPr(2)C(6)H(3)). To potentially access complexes of molecular boron nitride, LB-BN-LA (LA = Lewis acid), preliminary dehydrogenation chemistry involving the parent aminoborane adducts LB-H(2)BNH(2)-BH(3) was investigated using [Rh(COD)Cl](2), CuBr, and NiBr(2) as dehydrogenation catalysts. In place of isolating the intended dehydrogenated BN donor-acceptor complexes, the formation of borazine was noted as a major product. Attempts to prepare the fluoroarylborane-capped aminoborane complexes, LB-H(2)BNH(2)-B(C(6)F(5))(3), are also described.

Analytical pyrolysis of alkylammonium tetraphenylborates

Various alkylammonium, dialkylammonium, trialkylammonium and tetraalkylammonium tetraphenylborates were prepared. The thermal decomposition curves of RNH3BPh4, R2NH2BPh4, R3NHBPh4 and R4NBPh4 (whereR=Me, Et,n-Bu) in nitrogen atmosphere indicate that the elimination of volatile matter leads to the formation of both 1∶1 complex of trialkylamino triphenylborane and dialkylamino diphenylborene. Further elimination of volatile matter leads to the formation of borazine at 600–680°C. When borazine is further heated at 980–1090°C an exothermic change indicates the polycyclic condensation of the borazine leading to the formation of boron nitride. The volatile matter evolved in these reactions was measured quantitatively and reaction mechanisms were suggested.

The electronic structures and energies of borazine, boronimide and dimeric boronimide

The electronic energies and structures of two proposed models of borazine have been examined byab initio calculations using Gaussian-Type atomic orbitals. It is found that the planarD 3h model is energetically preferred. The electronic population analysis reveals that nitrogen and boron atoms in borazine possess a negative and positive charge, respectively. Calculations have also been performed on boronimide and dimeric boronimide. The trimerisation and dimerisation energy of boronimide is calculated to be −193.8 kcal/mole and −63.3 kcal/mole respectively. The reorganisation energy of boronimide in various configurations is obtained and used to discuss the energetics of formation of borazine and dimeric boronimide.

Routes to the formation of 1,3,5-triaryl- and 2,4,6-trichloro-1,3,5-triarylborazines and intermediate compounds

The effect of ortho-substitution has been investigated in routes to the formation of 1,3,5-triaryl- and 2,4,6-trichloro-1,3,5-triaryl-borazines. The presence of two substituents in the ortho-positions inhibits the formation of the equivalent borazine, both in reactions involving the heating of a mixture of arylamine and triethylamine–borane and in those which involve heating the 1 : 1 adduct of the arylamine and trichloroborane. Products isolated in these cases include diazaboranes (bisaminoboranes)[(ArNH)2BH], aminodichloroboranes [ArNHBCl2], and triazaboranes (boronamines)[ArNH·BX·NAr·BX·NArH](X = Cl or H). A possible reaction path to the formation of borazines involving these intermediates is discussed.