Dehydrogenation Of Amine Boranes Research Articles

Aluminium in der Chemie der Frustrierten Lewis‐Paare

AbstractDieser Übersichtsartikel beschreibt die Entwicklung der Verwendung von Aluminiumverbindungen in der Chemie der frustrierten Lewis‐Paare (FLPs) in den letzten 14 Jahren. Es werden die Synthese, die Reaktivität und die katalytischen Anwendungen von intermolekularen, intramolekularen und sogenannten versteckten FLPs mit Phosphor‐, Stickstoff‐ und Kohlenstoff‐Lewis‐Basen erörtert. Die intrinsisch höhere Acidität von Aluminiumverbindungen im Vergleich zu ihren Bor‐Analoga eröffnet unterschiedliche Reaktionswege. Die Ergebnisse werden in einer mehr oder weniger chronologischen Reihenfolge präsentiert. Es wird gezeigt, dass Al‐FLPs mit einer Vielzahl von polaren und unpolaren Substraten reagieren und sowohl stabile Addukte bilden als auch reversibel Bindungen aktivieren. Folglich werden einige katalytische Anwendungen der Titelverbindungen vorgestellt, wie z. B. die Dimerisierung von Alkinen, die Hydrierung von tert‐butylethylen und Iminen, die Aktivierung von C−F‐Bindungen, die Reduktion von CO2, die Dehydrierung von Aminboran und der Transfer von Ammoniak. Darüber hinaus wurden verschiedene Al‐FLPs als Initiatoren in Polymerisationsreaktionen eingesetzt.

Aluminum in Frustrated Lewis Pair Chemistry.

This review article describes the development of the use of aluminum compounds in the chemistry of frustrated Lewis pairs (FLPs) over the last 14 years. It also discusses the synthesis, reactivity and catalytic applications of intermolecular, intramolecular and so-called hidden FLPs with phosphorus, nitrogen and carbon Lewis bases. The intrinsically higher acidity of aluminum compounds compared to their boron analogs opens up different reaction pathways. The results are presented in a more or less chronological order. It is shown that Al FLPs react with a variety of polar and non-polar substrates and form both stable adducts and reversibly activate bonds. Consequently, some catalytic applications of the title compounds were presented such as dimerization of alkynes, hydrogenation of tert-butyl ethylene and imines, C-F bond activation, reduction of CO2, dehydrogenation of amine borane and transfer of ammonia. In addition, various Al FLPs were used as initiators in polymerization reactions.

Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes via intermolecular bimetallic cooperation.

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed via intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (Mδ+⋯M'δ-) have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate via the synergetic action of Lewis acidic (M+) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br). It was also demonstrated that this system generated in situ from the air-stable Mn(i) complex fac-[(CO)3(bis(NHC))MnBr] and NaBPh4 shows high activity for H2 production from several substrates (Me2NHBH3, tBuNH2BH3, MeNH2BH3, NH3BH3) at low catalyst loading (0.1% to 50 ppm), providing outstanding efficiency for Me2NHBH3 (TON up to 18 200) that is largely superior to all known 3d-, s-, p-, f-block metal derivatives and frustrated Lewis pairs (FLPs). These results represent a step forward towards more extensive use of intermolecular bimetallic cooperation concepts in modern homogeneous catalysis.

Dehydropolymerization of H3B·NMeH2 Mediated by Cationic Iridium(III) Precatalysts Bearing κ3- i Pr-PN R P Pincer Ligands (R = H, Me): An Unexpected Inner-Sphere Mechanism.

The dehydropolymerization of H3B·NMeH2 to form N-methylpolyaminoborane using neutral and cationic catalysts based on the {Ir(iPr-PNHP)} fragment [iPr-PNHP = κ3-(CH2CH2PiPr2)2NH] is reported. Neutral Ir(iPr-PNHP)H3 or Ir(iPr-PNHP)H2Cl precatalysts show no, or poor and unselective, activity respectively at 298 K in 1,2-F2C6H4 solution. In contrast, addition of [NMeH3][BArF4] (ArF = 3,5-(CF3)2C6H3) to Ir(iPr-PNHP)H3 immediately starts catalysis, suggesting that a cationic catalytic manifold operates. Consistent with this, independently synthesized cationic precatalysts are active (tested between 0.5 and 2.0 mol % loading) producing poly(N-methylaminoborane) with Mn ∼ 40,000 g/mol, Đ ∼1.5, i.e., dihydrogen/dihydride, [Ir(iPr-PNHP)(H)2(H2)][BArF4]; σ-amine-borane [Ir(iPr-PNHP)(H)2(H3B·NMe3)][BArF4]; and [Ir(iPr-PNHP)(H)2(NMeH2)][BArF4]. Density functional theory (DFT) calculations probe hydride exchange processes in two of these complexes and also show that the barrier to amine-borane dehydrogenation is lower (22.5 kcal/mol) for the cationic system compared with the neutral system (24.3 kcal/mol). The calculations show that the dehydrogenation proceeds via an inner-sphere process without metal–ligand cooperativity, and this is supported experimentally by N–Me substituted [Ir(iPr-PNMeP)(H)2(H3B·NMe3)][BArF4] being an active catalyst. Key to the lower barrier calculated for the cationic system is the outer-sphere coordination of an additional H3B·NMeH2 with the N–H group of the ligand. Experimentally, kinetic studies indicate a complex reaction manifold that shows pronounced deceleratory temporal profiles. As supported by speciation and DFT studies, a key observation is that deprotonation of [Ir(iPr-NHP)(H)2(H2)][BArF4], formed upon amine-borane dehydrogenation, by the slow in situ formation of NMeH2 (via B–N bond cleavage), results in the formation of essentially inactive Ir(iPr-PNHP)H3, with a coproduct of [NMeH3]+/[H2B(NMeH2)2]+. While reprotonation of Ir(iPr-PNHP)H3 results in a return to the cationic cycle, it is proposed, supported by doping experiments, that reprotonation is attenuated by entrainment of the [NMeH3]+/[H2B(NMeH2)2]+/catalyst in insoluble polyaminoborane. The role of [NMeH3]+/[H2B(NMeH2)]+ as chain control agents is also noted.

Amine-boranes reactions promoted by lanthanide(II) ions.

The catalytic activity in amine-borane dehydrogenation is shown for the first time for Ln(II) species using complexes [{(p-tBu-C6H4)2CH}2M·L] (M = Yb, Sm, L = (DME)2, TMEDA). The protonation of M(II)-C bonds with HNR1R2BH3 affords amidoborane complexes [M(NR1R2BH3)2L], which under excess HNMe2BH3 transform to [NMe2BH2NMe2BH3]- derivatives, both serving as the dehydrocoupling intermediates.

Coordinatively Unsaturated Amidotitanocene Cations with Inverted σ and π Bond Strengths: Controlled Release of Aminyl Radicals and Hydrogenation/Dehydrogenation Catalysis.

Cationic amidotitanocene complexes [Cp2 Ti(NPhAr)][B(C6 F5 )4 ] (Cp=η5 -C5 H5 ; Ar=phenyl (1 a), p-tolyl (1 b), p-anisyl (1 c)) were isolated. The bonding situation was studied by DFT (Density Functional Theory) using EDA-NOCV (Energy Decomposition Analysis with Natural Orbitals for Chemical Valence). The polar Ti-N bond in 1 a-c features an unusual inversion of σ and π bond strengths responsible for the balance between stability and reactivity in these coordinatively unsaturated species. In solution, 1 a-c undergo photolytic Ti-N cleavage to release Ti(III) species and aminyl radicals ⋅NPhAr. Reaction of 1 b with H3 BNHMe2 results in fast homolytic Ti-N cleavage to give [Cp2 Ti(H3 BNHMe2 )][B(C6 F5 )4 ] (3). 1 a-c are highly active precatalysts in olefin hydrogenation and silanes/amines cross-dehydrogenative coupling, whilst 3 efficiently catalyzes amine-borane dehydrogenation. The mechanism of olefin hydrogenation was studied by DFT and the cooperative H2 activation key step was disclosed using the Activation Strain Model (ASM).

Frustrated Lewis Acid-Base-Pair-Catalyzed Amine-Borane Dehydrogenation.

Metal-free catalysis by sterically encumbered Lewis Acid-Base pairs, popularly known as frustrated Lewis pairs (FLPs), is gaining importance by the day due to its promise of providing a greener alternative to transition-metal-based catalysis. One of the stumbling blocks in achieving catalytic dehydrogenation of amine-boranes is catalyst deactivation by the reaction product. Herein, we have theoretically investigated the routes of a dimethylxanthene-derived B,P-FLP-catalyzed dehydrogenation of dimethylamine-borane (DMAB), a rare instance which avoids catalyst inhibition by the reaction product. Our computational findings reveal that the dehydrogenation proceeds via formation of the ion pair [FLP-H]- and [HMe2N-BH2-H-BH2-NMe2H]+. This step is followed by indirect B-H activation assisted by a second DMAB molecule and further H2 release via deprotonation by the PPh2 center. It is revealed that the binding of NMe2BH2 to the FLP is unfavorable which ensures smooth propagation of the catalytic cycle. Catalytic dehydrogenation by the same mechanistic pathway is somewhat inhibited in the case of ammonia-borane by the same FLP due to the latent stabilization provided by strong hydrogen bonding interaction to FLP-NH2BH2 adduct which renders partial deactivation of the catalyst.

Catalysis with Colloidal Ruthenium Nanoparticles.

This review provides a synthetic overview of the recent research advancements addressing the topic of catalysis with colloidal ruthenium metal nanoparticles through the last five years. The aim is to enlighten the interest of ruthenium metal at the nanoscale for a selection of catalytic reactions performed in solution condition. The recent progress in nanochemistry allowed providing well-controlled ruthenium nanoparticles which served as models and allowed study of how their characteristics influence their catalytic properties. Although this parameter is not enough often taken into consideration the surface chemistry of ruthenium nanoparticles starts to be better understood. This offers thus a strong basis to better apprehend catalytic processes on the metal surface and also explore how these can be affected by the stabilizing molecules as well as the ruthenium crystallographic structure. Ruthenium nanoparticles have been reported for their application as catalysts in solution for diverse reactions. The main ones are reduction, oxidation, Fischer-Tropsch, C-H activation, CO2 transformation, and hydrogen production through amine borane dehydrogenation or water-splitting reactions, which will be reviewed here. Results obtained showed that ruthenium nanoparticles can be highly performant in these reactions, but efforts are still required in order to be able to rationalize the results. Beside their catalytic performance, ruthenium nanocatalysts are very good models in order to investigate key parameters for a better controlled nanocatalysis. This is a challenging but fundamental task in order to develop more efficient catalytic systems, namely more active and more selective catalysts able to work in mild conditions.

Iron-SNS and -CNS Complexes: Selective Caryl–S Bond Cleavage and Amine-Borane Dehydrogenation Catalysis

The synthesis, structure, and reactivity of an electron-rich FeII thioether-imine-thiolate complex, [Fe(SMeNS)(PMe3)3](OTf) (1-SNS), prepared by reaction of Fe(OTf)2(PMe3)4 with the SMeNHS ligand i...

Effect of thiocyanate‐based ionic liquids on the dehydrogenation of amine boranes: Experimental and molecular modeling studies

AbstractThe present investigation reports the effect of thiocyanate‐based ionic liquid (IL) on the thermal dehydrogenation of amine boranes. Fifteen ILs based on thiocyanate anion comprising cations from imidazolium, pyrrolidinium, and ammonium family were screened by a priori prediction of infinite dilution activity coefficient using the quantum chemical‐based conductor‐like screening model segment activity coefficient method. The IL with the highest predicted solubility of amine boranes converged to 1‐ethyl‐3‐methylimidazolium thiocyanate ([EMIM][SCN]) that was then used for the dehydrogenation reaction. In the IL medium, ammonia borane (AB) and ethylene diamine bisborane (EDAB) were found to produce ~1.64 equivalent of hydrogen with an induction period of 60 min and ~2.37 equivalent of hydrogen having an induction period of 30 min, respectively. It was observed that the mixture of AB and EDAB in [EMIM][SCN] produced prolonged induction period of 210 min. After 240 min, the mixed amine boranes only produce 0.43 equivalent of H2. To elucidate this slow dynamics, Density Functional Theory (DFT) optimized geometries followed by the natural bond orbital analysis revealed the formation of hydrogen bond between IL and amine borane moiety. Further, the presence of the stronger dihydrogen bond among the protic and hydridic hydrogens was also observed in AB + EDAB/[EMIM][SCN] system, resulting in longer induction period for dehydrogenation.

Dehydrogenation of Amine-Boranes Using p-Block Compounds.

Amine–boranes have gained a lot of attention due to their potential as hydrogen storage materials and their capacity to act as precursors for transfer hydrogenation. Therefore, a lot of effort has gone into the development of suitable transition‐ and main‐group metal catalysts for the dehydrogenation of amine–boranes. During the past decade, new systems started to emerge solely based on p‐block elements that promote the dehydrogenation of amine–boranes through hydrogen‐transfer reactions, polymerization initiation, and main‐group catalysis. In this review, we highlight the development of these p‐block based systems for stoichiometric and catalytic amine–borane dehydrogenation and discuss the underlying mechanisms.

Amine-Borane Dehydrogenation and Transfer Hydrogenation Catalyzed by α-Diimine Cobaltates.

Anionic α-diimine cobalt complexes, such as [K(thf)1.5 {(Dipp BIAN)Co(η4 -cod)}] (1; Dipp=2,6-diisopropylphenyl, cod=1,5-cyclooctadiene), catalyze the dehydrogenation of several amine-boranes. Based on the excellent catalytic properties, an especially effective transfer hydrogenation protocol for challenging olefins, imines, and N-heteroarenes was developed. NH3 BH3 was used as a dihydrogen surrogate, which transferred up to two equivalents of H2 per NH3 BH3 . Detailed spectroscopic and mechanistic studies are presented, which document the rate determination by acidic protons in the amine-borane.

Magnesium promoted autocatalytic dehydrogenation of amine borane complexes: A reliable, non-cryogenic, scalable access to boronic acids

Owing to the unusual reactivity of dialkylamine-borane complexes, a methodology was developed to simply access boronic acids. The intrinsic instability of magnesium aminoborohydride was tweaked into a tandem dehydrogenation borylation sequence. Proceeding via an autocatalytic cycle, amineborane dehydrogenation was induced by a variety of Grignard reagents. Overall, addition of the organomagnesium species onto specially designed dialkylamine-borane complexes led to a variety of boronic acids in high yields. In addition, the reaction can be performed under Barbier conditions, on a large scale.

Mechanistic insights into tandem amine-borane dehydrogenation and alkene hydrogenation catalyzed by [Pd(NHC)(PCy3)

The mechanism of tandem dimethylamine-borane (NHMe2BH3, DMAB) dehydrogenation and alkene hydrogenation catalyzed by [Pd(NHC)(PMe3)] are investigated by density functional theory (DFT) calculations [NHC = N,N′-bis(2,6-diisopropylphenyl) imidazole-2-ylidene]. Four possible DMAB dehydrogenation mechanisms have been carefully investigated involving concerted BH/NH activation, sequential BH/NH activation, sequential NH/BH activation, and proton transfer mechanism. DFT studies show that the NH proton transfers to ligated carbene carbon and sequential CH/BH activation is the most kinetically favorable pathway with the lowest activation barrier of 23.8 kcal/mol. For hydrogenation, it was found that a trans-dihydride Pd(II) complex, [Pd(H)2(NHC)(PMe3)], formed in the dehydrogenation process, serves as an effective catalyst for reduction of trans-stilbene.

Experimental Evidence Supporting Related Mechanisms for Ru(II)-Catalyzed Dehydrocoupling and Hydrolysis of Amine-Boranes

A family of ruthenium(II) half-sandwich complexes was tested for the hydrolytic decomposition of amine-boranes. The analysis of the catalytic results, together with a multilateral approach based on 1H, 11B NMR, and ESI-MS were used to propose a plausible and conceptually unified mechanism for both the hydrolysis and competitive dehydrogenation of amine-boranes. We propose the intermediacy of solvent-stabilized borenium cations during the catalytic cycle, evolving toward dehydrogenation products in distilled THF or releasing amine-hydroxyboranes in aqueous media. Both reaction pathways would liberate up to 1 equivalent of hydrogen through a metal-catalyzed process, but an out-of-cycle low-barrier hydrolysis of amine-hydroxyboranes would produce the 2 additional equivalents of hydrogen in aqueous solutions. Metal-catalyzed deuteration of (non hydrogen-productive) trisubstituted amine-boranes by using D2O as deuterium source was observed, and included as part of the mechanism proposal.

Iron(II) Complexes of a Hemilabile SNS Amido Ligand: Synthesis, Characterization, and Reactivity

We report an easily prepared bis(thioether) amine ligand, SMeNHSMe, along with the synthesis, characterization, and reactivity of the paramagnetic iron(II) bis(amido) complex, [Fe(κ3-SMeNSMe)2] (1). Binding of the two different thioethers to Fe generates both five- and six-membered rings with Fe-S bonds in the five-membered rings (av 2.54 Å) being significantly shorter than those in the six-membered rings (av 2.71 Å), suggesting hemilability of the latter thioethers. Consistent with this hypothesis, magnetic circular dichroism (MCD) and computational (TD-DFT) studies indicate that 1 in solution contains a five-coordinate component [Fe(κ3-SMeNSMe)(κ2-SMeNSMe)] (2). This ligand hemilability was demonstrated further by reactivity studies of 1 with 2,2'-bipyridine, 1,2-bis(dimethylphosphino)ethane, and 2,6-dimethylphenyl isonitrile to afford iron(II) complexes [L2Fe(κ2-SMeNSMe)2] (3-5). Addition of a Brønsted acid, HNTf2, to 1 produces the paramagnetic, iron(II) amine-amido cation, [Fe(κ3-SMeNSMe)(κ3-SMeNHSMe)](NTf2) (6; Tf = SO2CF3). Cation 6 readily undergoes amine ligand substitution by triphos, affording the 16e- complex [Fe(κ2-SMeNSMe)(κ3-triphos)](NTf2) (7; triphos = bis(2-diphenylphosphinoethyl)phenylphosphine). These complexes are characterized by elemental analysis; 1H NMR, Mössbauer, IR, and UV-vis spectroscopy; and single-crystal X-ray diffraction. Preliminary results of amine-borane dehydrogenation catalysis show complex 7 to be a selective and particularly robust precatalyst.

Toward a quantitative evaluation of the strength of Cp2M···η 2–borate interactions

Amine-boranes might be a crucial material toward a successful energy transition. A precise description of metal–ligand interactions involved in the homogeneous catalysis of the dehydrogenation of amine-boranes would represent a major step toward a global understanding of the reaction process. η 2 interactions between borates and organometallic compounds were identified as key intermediates in the reaction pathways. Herein are proposed tailored topological descriptors to measure the efficiency of several metallocenes to activate B–H bonds in prototypical Cp2M···η 2–borates complexes. The combined use of QTAIM and ELF partitions were used to ascertain the 3C/2e interactions between borates and metallocenes. The strength of the B–H bond is affected by two different parameters: the nature of the ligands bonded to the borates and the effect of the interaction with the metallocene. The use of tailored descriptors allows to evaluate the activation of the B–H bond due to the sole effect of the interaction between the borates and the metallocenes. Herein, we suggest the concomitant use of the electron density ratios $$\frac{{\rho \left( {{\text{BCP}}\;{\text{B}} - {\text{H}}_{\text{complex}} } \right)}}{{\rho \left( {{\text{BCP}}\;{\text{B}} - {\text{H}}_{{{\text{free}}\;{\text{borate}}}} } \right)}}$$ and distance ratios $$\frac{{d\left( {{\text{B}} - {\text{H}}_{\text{complex}} } \right)}}{{d\left( {{\text{BCP}}\;{\text{B}} - {\text{H}}_{\text{free borate}} } \right)}}$$ of the B–H bonds to classify the {borates + metallocenes} systems as a function of the strength of the B–H interaction with the metallocene. We further suggest to complement this evaluation based on the QTAIM analysis by a quantization of the contribution of the metallic center on the protonated basin defined within the ELF framework.

Visible Light Photocatalysis Using a Commercially Available Iron Compound

[CpFe(CO)2]2 (1) (Cp = η5-C5H5) is an effective precatalyst for the hydrophosphination of alkenes with Ph2PH under visible light irradiation, which appears to be a unique way to promote metal-catalyzed hydrophosphination. Additionally, 1 is a photocatalyst for the dehydrogenation of amine boranes and formation of siloxanes from tertiary silanes. These reactions have similar, if not improved, reactivity over the same transformations using 1 or related CpFeMe(CO)2 under UV irradiation, consistent with the notion that hydrophosphination with 1 proceeds via formation of CpFe(CO)2•. These results demonstrate that catalyst selection can avail the use of commercially available LED bulbs as photon sources, potentially replacing mercury arc lamps or other energy intensive processes in known or new catalytic reactions.

Characterization of B-H agostic compounds involved in the dehydrogenation of amine-boranes by group 4 metallocenes.

For over a decade, amine-borane has been considered as a potential chemical hydrogen vector in the context of a search for cleaner energy sources. When catalyzed by organometallic complexes, the reaction mechanisms currently considered involve the formation of β-BH agostic intermediates. A thorough understanding of these intermediates may constitute a crucial step toward the identification of ideal catalysts. Topological approaches such as QTAIM and ELF revealed to be particularly suitable for the description of β-agostic interactions. When studying model catalysts, accurate theoretical calculations may be carried out. However, for a comparison with experimental data, calculations should also be carried out on large organo-metallic species, often including transition metals belonging to the second or the third row. In such a case, DFT methods are particularly attractive. Unfortunately, triple-ζ all electrons basis sets are not easily available for heavy transition metal elements. Thus, a subtle balance should be reached between the affordable level of calculations and the required accuracy of the electronic description of the systems. Herein we propose the use of B3LYP functional in combination with the LanL2DZ pseudopotential for the metal atom and 6-311++G(2d,2p) basis set for the other atoms, followed by a single point using the DKH2 relativistic Hamiltonian in combination with the B3LYP/DZP-DKH level, as a "minimum level of theory" leading to a consistent topological description of the interaction within the ELF and QTAIM framework, in the context of isolated (gas-phase) group 4 metallocene catalysts.

ChemInform Abstract: Mechanistic Insights and Computational Design of Transition‐Metal Catalysts for Hydrogenation and Dehydrogenation Reactions

AbstractReview: [60 refs.