Hydrogen Release from Dialkylamine–Boranes Promoted by Mg and Ca Complexes: A DFT Analysis of the Reaction Mechanism

Mg and Ca β‐diketiminato silylamides [HC{(Me)CN(2,6‐iPr₂C₆H₃)}₂M(THF)_n{N(SiMe₃)₂}] (M=Mg, n=0; M=Ca, n=1) were studied as precatalysts for the dehydrogenation/dehydrocoupling of secondary amine–boranes R₂HNBH₃. By reaction with equimolar quantities of amine–boranes, the corresponding amidoborane derivatives are formed, which further react to yield dehydrogenation products such as the cyclic dimer [BH₂?NMe₂]₂. DFT was used here to explore the mechanistic alternatives proposed on the basis of the experimental findings for both Mg and Ca amidoboranes. The influence of the steric demand of amine–boranes on the course of the reaction was examined by performing calculations on the dehydrogenation of dimethylamine–borane (DMAB), pyrrolidine–borane (PB), and diisopropylamine–borane. In spite of the analogies in the catalytic activity of Mg‐ and Ca‐based complexes in the dehydrocoupling of amine–boranes, our theoretical analysis confirmed the experimentally observed lower reactivity of Ca complexes. Differences in catalytic activity of Mg‐ and Ca‐based complexes were examined and rationalized. As a consequence of the increase in ionic radius on going from Mg²⁺ to Ca²⁺, the dehydrogenation mechanism changes and formation of a key metal hydride intermediate becomes inaccessible. Dimerization is likely to occur off‐metal in solution for DMAB and PB, whereas steric hindrance of iPr₂NHBH₃ hampers formation of the cyclic dimer. The reported results are of particular interest because, although amine–borane dehydrogenation is now well established, mechanistic insight is still lacking for many systems.     

Do Rhodium Bis(σ‐amine‐borane) Complexes Play a Role as Intermediates in Dehydrocoupling Reactions of Amine‐boranes?

The recently synthesized rhodium complex [Rh{P(C₅H₉)₂(η²‐C₅H₇)}(Me₂HNBH₃)₂]BAr^F₄ ( 2 ), which incorporates two amine‐boranes coordinated to the rhodium center with two different binding modes, namely η¹ and η², has been used to probe whether bis(σ‐amine‐borane) motifs are important in determining the general course of amine‐boranes dehydrocoupling reactions. DFT calculations have been carried out to explore mechanistic alternatives that ultimately lead to the formation of the amine‐borane cyclic dimer [BH₂NMe₂]₂ ( A ) by hydrogen elimination. Sequential concerted, on‐ or off‐metal, intramolecular dehydrogenations provide two coordinated amine‐borane molecules. Subsequent dimerization is likely to occur off the metal in solution. In spite of the computationally confirmed presence of a BH???NH hydrogen bond between amine‐borane ligands, neither a simple intermolecular route for dehydrocoupling of complex 2 is operating, nor seems [Rh{P(C₅H₉)₂(η²‐C₅H₇)} B ]⁺ to be important for the whole dehydrocoupling process.     

A Study of the Reactivity of Secondary Phosphanes with Radical Sources: A New Dehydrocoupling Reaction

The reactions of secondary phosphanes with radical sources have been investigated. A stoichiometric dehydrocoupling of Ph₂PH with 1,1′‐azobis[cyclohexane‐1‐carbonitrile] (VAZO^® 88) affords tetraphenyldiphosphane in good yields, whilst reduction of the nitrosyl function was observed upon using 2,2,6,6‐tetramethylpiperidin‐1‐oxyl (TEMPO). Dialkylphosphane–borane adducts also undergo a dehydrocoupling reaction in the presence of VAZO^® 88 to form R₄P₂.     

“Spontaneous” Ambient Temperature Dehydrocoupling of Aromatic Amine–Boranes

The dehydrocoupling/dehydrogenation behavior of primary arylamine–borane adducts ArNH₂ ? BH₃ ( 3 a – c ; Ar= a : Ph, b : p‐MeOC₆H₄, c : p‐CF₃C₆H₄) has been studied in detail both in solution at ambient temperature as well as in the solid state at ambient or elevated temperatures. The presence of a metal catalyst was found to be unnecessary for the release of H₂. From reactions of 3 a , b in concentrated solutions in THF at 22 °C over 24 h cyclotriborazanes (ArNH‐BH₂)₃ ( 7 a , b ) were isolated as THF adducts, 7 a , b? THF, or solvent‐free 7 a , which could not be obtained via heating of 3 a – c in the melt. The μ‐(anilino)diborane [H₂B(μ‐PhNH)(μ‐H)BH₂] ( 4 a ) was observed in the reaction of 3 a with BH₃?THF and was characterized in situ. The reaction of 3 a with PhNH₂ ( 2 a ) was found to provide a new, convenient method for the preparation of dianilinoborane (PhNH)₂BH ( 5 a ), which has potential generality. This observation, together with further studies of reactions of 4 a , 5 a , and 7 a , b , provided insight into the mechanism of the catalyst‐free ambient temperature dehydrocoupling of 3 a – c in solution. For example, the reaction of 4 a with 5 a yields 6 a and 7 a . It was found that borazines (ArN‐BH)₃ ( 6 a – c ) are not simply formed via dehydrogenation of cyclotriborazanes 7 a – c in solution. The transformation of 7 a to 6 a is slowly induced by 5 a and proceeds via regeneration of 3 a . The adducts 3 a – c also underwent rapid dehydrocoupling in the solid state at elevated temperatures and even very slowly at ambient temperature. From aniline–borane derivative 3 c , the linear iminoborane oligomer (p‐CF₃C₆H₄)N[BH‐NH(p‐CF₃C₆H₄)]₂ ( 11 ) was obtained. The single‐crystal X‐ray structures of 3 a – c , 5 a , 7 a , 7 b? THF, and 11 are discussed.     

Stoichiometric reactivity of dialkylamine boranes with alkaline earth silylamides

Reactions of β-diketiminato group 2 silylamides, [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)M(THF)(n){N(SiMe(3))(2)}] (M = Mg, n = 0; M = Ca, Sr, n = 1), and an equimolar quantity of pyrrolidine borane, (CH(2))(4)NH·BH(3), were found to produce amidoborane derivatives of the form [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)MN(CH(2))(4)·BH(3)]. In reactivity reminiscent of analogous reactions performed with dimethylamine borane, addition of a second equivalent of (CH(2))(4)NH·BH(3) to the Mg derivative induced the formation of a species, [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)Mg{N(CH(2))(4) BH(2)NMe(2)BH(3)}], containing an anion in which two molecules of the amine borane substrate have been coupled together through the elimination of one molecule of H(2). Both this species and a calcium amidoborane derivative have been characterised by X-ray diffraction techniques and the coupled species is proposed as a key intermediate in catalytic amine borane dehydrocoupling, in reactivity dictated by the charge density of the group 2 centre involved. On the basis of further stoichiometric reactions of the homoleptic group 2 silylamides, [M{N(SiMe(3))(2)}(2)] (M = Mg, Ca, Sr, Ba), with (CH(3))(2)NH·BH(3) and (i)Pr(2)NH·BH(3) reactivity consistent with successive amidoborane β-hydride elimination and [R(2)N[double bond, length as m-dash]BH(2)] insertion is described as a means to induce the B-N dehydrocoupling between amine borane substrates.     

Efficient catalysis of ammonia borane dehydrogenation

In the presence of an iridium pincer complex, dehydrogenation of ammonia borane (H3NBH3) occurs rapidly at room temperature in tetrahydrofuran to generate 1.0 equivalent of H2 and [NH2BH2]5. A metal borohydride complex has been isolated as a dormant form of the catalyst which can be reactivated by reaction with H2.     

P?C‐Activated Bimetallic Rhodium Xantphos Complexes: Formation and Catalytic Dehydrocoupling of Amine–Boranes

{Rh(xantphos)}‐based phosphido dimers form by P C activation of xantphos (4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene) in the presence of amine–boranes. These dimers are active dehydrocoupling catalysts, forming polymeric [H₂BNMeH]_n from H₃B⋅NMeH₂ and dimeric [H₂BNMe₂]₂ from H₃B⋅NMe₂H at low catalyst loadings (0.1 mol %). Mechanistic investigations support a dimeric active species, suggesting that bimetallic catalysis may be possible in amine–borane dehydropolymerization.     

PC‐Activated Bimetallic Rhodium Xantphos Complexes: Formation and Catalytic Dehydrocoupling of Amine–Boranes

{Rh(xantphos)}‐based phosphido dimers form by P? C activation of xantphos (4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene) in the presence of amine–boranes. These dimers are active dehydrocoupling catalysts, forming polymeric [H₂BNMeH]_n from H₃B?NMeH₂ and dimeric [H₂BNMe₂]₂ from H₃B?NMe₂H at low catalyst loadings (0.1 mol %). Mechanistic investigations support a dimeric active species, suggesting that bimetallic catalysis may be possible in amine–borane dehydropolymerization.     

Towards the design of novel boron‐ and nitrogen‐substituted ammonia‐borane and bifunctional arene ruthenium catalysts for hydrogen storage

Electronic‐structure density functional theory calculations have been performed to construct the potential energy surface for H₂ release from ammonia‐borane, with a novel bifunctional cationic ruthenium catalyst based on the sterically bulky β‐diketiminato ligand (Schreiber et al., ACS Catal. 2012, 2, 2505). The focus is on identifying both a suitable substitution pattern for ammonia‐borane optimized for chemical hydrogen storage and allowing for low‐energy dehydrogenation. The interaction of ammonia‐borane, and related substituted ammonia‐boranes, with a bifunctional η⁶‐arene ruthenium catalyst and associated variants is investigated for dehydrogenation. Interestingly, in a number of cases, hydride‐proton transfer from the substituted ammonia‐borane to the catalyst undergoes a barrier‐less process in the gas phase, with rapid formation of hydrogenated catalyst in the gas phase. Amongst the catalysts considered, N,N‐difluoro ammonia‐borane and N‐phenyl ammonia‐borane systems resulted in negative activation energy barriers. However, these types of ammonia‐boranes are inherently thermodynamically unstable and undergo barrierless decay in the gas phase. Apart from N,N‐difluoro ammonia‐borane, the interaction between different types of catalyst and ammonia borane was modeled in the solvent phase, revealing free‐energy barriers slightly higher than those in the gas phase. Amongst the various potential candidate Ru‐complexes screened, few are found to differ in terms of efficiency for the dehydrogenation (rate‐limiting) step. To model dehydrogenation more accurately, a selection of explicit protic solvent molecules was considered, with the goal of lowering energy barriers for H‐H recombination. It was found that primary (1°), 2°, and 3° alcohols are the most suitable to enhance reaction rate. © 2014 Wiley Periodicals, Inc.     

Magnesium-Catalyzed Tandem Dehydrogenation-Dehydrocoupling: An Atom Economical Access to Alkynylboranes

Owing to the unusual reactivity of sterically hindered amine–borane complexes, a catalytic system based on magnesium salts was designed to perform a tandem dehydrogenation–dehydrocoupling between terminal alkynes and boranes. The reaction is providing pure alkynylboranes within few minutes at room temperature, with only two molecules of hydrogen as a byproduct.     

Heterogeneous or homogeneous catalysis? Mechanistic studies of the rhodium-catalyzed dehydrocoupling of amine-borane and phosphine-borane adducts

In depth, comparative studies on the catalytic dehydrocoupling of the amine-borane adduct Me(2)NH.BH(3) (to form [Me(2)N-BH(2)](2)) and the phosphine-borane adduct Ph(2)PH.BH(3) (to form Ph(2)PH-BH(2)-PPh(2)-BH(3)) with a variety of Rh (pre)catalysts such as [[Rh(1,5-cod)(micro-Cl)](2)], Rh/Al(2)O(3), Rh(colloid)/[Oct(4)N]Cl, and [Rh(1,5-cod)(2)]OTf have been performed in order to determine whether the dehydrocoupling proceeds by a homogeneous or heterogeneous mechanism. The results obtained suggest that the catalytic dehydrocoupling of Me(2)NH.BH(3) is heterogeneous in nature involving Rh(0) colloids, while that of Ph(2)PH.BH(3) proceeds by a homogeneous mechanism even when starting with Rh(0) precursors such as Rh/Al(2)O(3). The catalytic dehydrocoupling reactions are thought to proceed by different mechanisms due to a combination of factors such as (i) the greater reducing strength of amine-borane adducts, (ii) the increased ease of dissociation of phosphine-borane adducts, and (iii) phosphine ligation and/or poisoning of active catalytic sites on metal colloids.     

Effect of borane source on the enantioselectivity in the enantiopure oxazaborolidine‐catalyzed asymmetric borane reduction of ketones

The effect of borane source on enantioselectivity in the enantiopure oxazaborolidine‐catalyzed asymmetric borane reduction of ketones has been investigated by using (S)‐3,1,2‐oxazaborobicyclo[3.3.0]octane and (S)‐7,3,1,2‐thiaxazaborobicyclo[3.3.0]octane as catalysts. The results indicate that the enantioselective order of different borane sources is borane–dimethyl sulfide < borane–N,N‐diethylaniline < borane–THF for the asymmetric reduction of a ketone under the same conditions. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:740–746, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20370     

Synthesis and dehydrocoupling reactivity of iron and ruthenium phosphine-borane complexes

The Fe and Ru phosphine-borane complexes CpM(CO)2PPh2 x BH3 (1, M = Fe, 4, M = Ru) were synthesized utilizing the reaction of the phosphine-borane anion Li[PPh2 x BH3] with the iodo complexes CpM(CO)2I. The Fe complex 1 reacted with PMe3 to yield CpFe(CO)(PMe3)(PPh2 x BH3) (2) and CpFe(PMe3)2(PPh2 x BH3) (3) whereas the Ru species 4 gave only CpRu(CO)(PMe3)(PPh2 x BH3) (5). The complexes 1-5 were characterized by 1H, 11B, 13C and 31P NMR spectroscopy, MS, IR and X-ray crystallography for 1 to 4, and EA for 1, 2 and 4. The reactivity of 1 and 4 towards PPh2H x BH3 was explored. Although no stoichiometric reactions were detected under mild conditions, both 1 and 4 were found to function as dehydrocoupling catalysts to afford Ph2PH x BH2 x PPh2 x BH3 in the melt at elevated temperature (120 degrees C). The carbonyl Fe2(CO)9 also functioned as a dehydrocoupling catalyst under similar conditions. Complex 1 and Fe2(CO)9 represent the first reported active Fe complexes for the catalytic dehydrocoupling of phosphine-borane adducts.     

Ir(PCy3)2(H)2(H2B-NMe2)]+ as a latent source of aminoborane: probing the role of metal in the dehydrocoupling of H3B⋅NMe2H and retrodimerisation of [H2BNMe2]2

The Ir^III fragment {Ir(PCy₃)₂(H)₂}⁺ has been used to probe the role of the metal centre in the catalytic dehydrocoupling of H₃B?NMe₂H ( A ) to ultimately give dimeric aminoborane [H₂BNMe₂]₂ ( D ). Addition of A to [Ir(PCy₃)₂(H)₂(H₂)₂][BAr^F₄] ( 1 ; Ar^F=(C₆H₃(CF₃)₂), gives the amine‐borane complex [Ir(PCy₃)₂(H)₂(H₃B?NMe₂H)][BAr^F₄] ( 2 a ), which slowly dehydrogenates to afford the aminoborane complex [Ir(PCy₃)₂(H)₂(H₂B? NMe₂)][BAr^F₄] ( 3 ). DFT calculations have been used to probe the mechanism of dehydrogenation and show a pathway featuring sequential BH activation/H₂ loss/NH activation. Addition of D to 1 results in retrodimerisation of D to afford 3 . DFT calculations indicate that this involves metal trapping of the monomer–dimer equilibrium, 2 H₂BNMe₂ ? [H₂BNMe₂]₂. Ruthenium and rhodium analogues also promote this reaction. Addition of MeCN to 3 affords [Ir(PCy₃)₂(H)₂(NCMe)₂][BAr^F₄] ( 6 ) liberating H₂B? NMe₂ ( B ), which then dimerises to give D . This is shown to be a second‐order process. It also allows on‐ and off‐metal coupling processes to be probed. Addition of MeCN to 3 followed by A gives D with no amine‐borane intermediates observed. Addition of A to 3 results in the formation of significant amounts of oligomeric H₃B?NMe₂BH₂?NMe₂H ( C ), which ultimately was converted to D . These results indicate that the metal is involved in both the dehydrogenation of A , to give B , and the oligomerisation reaction to afford C . A mechanism is suggested for this latter process. The reactivity of oligomer C with the Ir complexes is also reported. Addition of excess C to 1 promotes its transformation into D , with 3 observed as the final organometallic product, suggesting a B? N bond cleavage mechanism. Complex 6 does not react with C , but in combination with B oligomer C is consumed to eventually give D , suggesting an additional role for free aminoborane in the formation of D from C .     

The Synthesis,Characterization and Dehydrogenation of Sigma‐Complexes of BN‐Cyclohexanes

The coordination chemistry of the 1,2‐BN‐cyclohexanes 2,2‐R₂‐1,2‐B,N‐C₄H₁₀ (R₂=HH, MeH, Me₂) with Ir and Rh metal fragments has been studied. This led to the solution (NMR spectroscopy) and solid‐state (X‐ray diffraction) characterization of [Ir(PCy₃)₂(H)₂(η²η²‐H₂BNR₂C₄H₈)][BAr^F₄] (NR₂=NH₂, NMeH) and [Rh(iPr₂PCH₂CH₂CH₂PiPr₂)(η²η²‐H₂BNR₂C₄H₈)][BAr^F₄] (NR₂=NH₂, NMeH, NMe₂). For NR₂=NH₂ subsequent metal‐promoted, dehydrocoupling shows the eventual formation of the cyclic tricyclic borazine [BNC₄H₈]₃, via amino‐borane and, tentatively characterized using DFT/GIAO chemical shift calculations, cycloborazane intermediates. For NR₂=NMeH the final product is the cyclic amino‐borane HBNMeC₄H₈. The mechanism of dehydrogenation of 2,2‐H,Me‐1,2‐B,N‐C₄H₁₀ using the {Rh(iPr₂PCH₂CH₂CH₂PiPr₂)}⁺ catalyst has been probed. Catalytic experiments indicate the rapid formation of a dimeric species, [Rh₂(iPr₂PCH₂CH₂CH₂PiPr₂)₂H₅][BAr^F₄]. Using the initial rate method starting from this dimer, a first‐order relationship to [amine‐borane], but half‐order to [Rh] is established, which is suggested to be due to a rapid dimer–monomer equilibrium operating.     

A robust, air-stable, reusable ruthenium catalyst for dehydrogenation of ammonia borane

We describe an efficient homogeneous ruthenium catalyst for the dehydrogenation of ammonia borane (AB). This catalyst liberates more than 2 equiv of H(2) and up to 4.6 system wt % H(2) from concentrated AB suspensions under air. Importantly, this catalyst is robust, delivering several cycles of dehydrogenation at high [AB] without loss of catalytic activity, even with exposure to air and water.     

Preparation and ultraviolet absorption spectra of the isomeric naphthobenzothiophenes and naphthobenzofurans

Naphtho[2,1-b]benzothiophene has been synthesized unequivocally by the ring closure of 2-phenylmercapto-1-tetralone, followed by dehydrogenation of the resulting dihydro derivative. The other isomeric naphthobenzothiophenes have been prepared by modifications of known methods and their ultraviolet absorption spectra have been compared. Condensation of α-naphthol with 2-chlorocyclohexanone leads to 7,8,9,10-tetrahydronaphtho-[1,2--6]benzofuran, which can be readily dehydrogenated with selenium to naphtho[1,2-b]-benzofuran. The ultraviolet spectra of the naphthobenzofurans are reported, and compared to their sulfur analogs.     

A rhodium(III) complex of the linear diborazine H3B·NMe2BH2·NMe2H: an intermediate in the dehydrocoupling of H3B·NMe2H

The solid‐state structure of the rhodium complex (dimethylamine–dimethylaminoborane–borane‐κ²H,H′)dihydridobis(triisopropylphosphane‐κP)rhodium(III) tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate, [RhH₂(C₄H₁₈B₂N₂)(C₉H₂₁P)₂](C₃₂H₁₂BF₂₄), is reported. The complex contains the linear diborazine H₃B·NMe₂BH₂·NMe₂H, a kinetically important intermediate in the transition‐metal‐mediated dehydrocoupling of H₃B·NMe₂H, ultimately affording the dimeric amino‐borane [H₂BNMe₂]₂. The structure of the title complex contains a distorted octahedral Rh^III centre, with mutually trans phosphane ligands and cis hydride ligands. The diborazine is bound through two Rh—H—B σ‐bonds and exhibits a gauche conformation with respect to the B—N—B—N backbone.     

Rationale on the Abnormal Effect of Temperature on the Enantioselectivity in the Asymmetric Borane Reduction of Ketones Catalyzed by L‐Prolinol

The effect of temperature on the enantioselectivity of the oxazaborolidine‐catalyzed asymmetric borane reduction of ketones was investigated in the presence of (5S)‐3‐oxa‐1‐aza‐2‐borabicyclo[3.3.0]octane (=(3aS)‐tetrahydro‐1H,3H‐pyrolo[1,2‐c][1,3,2]oxazaborole; 1a ) and its 2‐methoxy derivative ( 1b ) as catalysts, which were synthesized from L ‐prolinol with borane and trimethyl borate, respectively. The results indicate that the two catalysts induce a different temperature‐dependent enantioselectivity. The enantioselectivity of the B‐unsubstituted (5S)‐3‐oxa‐1‐aza‐borabicyclo[3.3.0]octane ( 1a ) increases with increasing temperature, while its B‐methoxy‐substituted derivative 1b shows the highest enantioselectivity at ca. 50°. (5S)‐3‐Oxa‐1‐aza‐2‐borabicyclo[3.3.0]octane ( 1a ) is more likely to dimerize than its 2‐methoxy derivative 1b . The conversion rates of L ‐proline to L ‐prolinol in the presence of different amounts of borane were also determined in this study.     

Incorporation of Boron into Uranium Metallacycles: Synthesis,Structure, and Reactivity of Boron-Containing Uranacycles Derived from Bis(alkynyl)boranes

The reaction of uranacyclopropene complex (C₅Me₅)₂U[η²-1,2-C₂(SiMe₃)₂] with B-aryl bis(alkynyl)borane PhB(C≡CPh)₂ led to the first six-membered uranium metallaboracycle, while the reaction with B-amino bis(alkynyl)borane (Me₃Si)₂NB(C≡CPh)₂ afforded an unexpected uranaborabicyclo[2.2.0] complex via [2+2] cycloaddition. The reaction with CuCl revealed the non-innocent property of the rearranged bis(alkynyl)boron species towards oxidant. The reactions with isocyanide DippNC: (Dipp=2,6-iPr₂-C₆H₃) and isocyanate tBuNCO afforded the novel uranaborabicyclo[3.2.0] complexes. All new complexes have been structurally characterized. DFT calculations were performed to provide more insights into the electronic structures and the reaction mechanism.