Lisa Roy

Changing Lanes from Concerted to Stepwise Hydrogenation: The Reduction Mechanism of Frustrated Lewis Acid–Base Pair Trapped CO2 to Methanol by Ammonia–Borane

Unabated anthropogenic release of carbon dioxide (CO2) is contributing to global climate change and represents a colossal environmental predicament. Furthermore, increasing demand for fossil fuel resources has raised concerns about the stability of the global energy supply. These problems have led the scientific community to look for renewable fuel alternatives. In nature, plants and algae trap and utilize CO2 in photosynthesis, but this process is not sufficient to combat the rapid rise of CO2 concentration in the atmosphere. The trapping of CO2 and its subsequent reduction has surfaced as a chemical challenge of great interest because this transformation could be a viable route for renewable carbonbased fuels. However, the limited reactivity of CO2 has slowed progress in developing efficient reduction methods. Primarily, CO2 reduction can be achieved by using electrocatalysts or heterogeneous photocatalysts that involve transition-metal containing complexes and materials. Recently, Stephan and Menard demonstrated the trapping of CO2 by a frustrated Lewis acid base pair (FLP) along with the subsequent reduction of the trapped CO2 to methanol by ammonia–borane. This is a rare instance where CO2 reduction to a liquid fuel has been achieved without the use of a transition metal. Additionally, this reaction has unfolded a new dimension to FLP-facilitated chemistry. Although ammonia–borane is a popular chemical hydrogen storage material, it has been recently shown to function as a hydrogenating agent for imines in a concerted fashion through simultaneous proton and hydride transfer from ammonia–borane to imines. Earlier theoretical investigations by Paul and co-workers suggested that ammonia– borane releases hydrogen in a similar fashion to transitionmetal complexes and N-heterocyclic carbenes. However, dehydrogenation of ammonia–borane is also known to initiate through stepwise routes, via N H activation, and in some cases B H activation. Thus, the FLP-CO2 reduction involves two interesting aspects: a) the mechanism of reduction of CO2 to methanol at room temperature and atmospheric pressure and b) the hydrogenation pathway by ammonia–borane for this particular substrate. A detailed understanding of the mechanistic features of this remarkable sequence of chemical reactions would provide valuable insights for developing strategies of CO2 reduction. In our current endeavor, we have used hybrid density functional theory to unravel the molecular pathways for the reduction of FLP trapped CO2 to methanol by ammonia–borane. Our computational investigation characterizes the crucial transition states and intermediates that are encountered along the reaction path of this intriguing reaction. Furthermore, we show the chameleon-like nature of ammonia–borane as a reducing agent by showing that the hydrogenation pathways change with similar substrates in different electronic environments. In the current study we have focused on unfolding the mechanistic details of the reduction of FLP-trapped CO2. The optimized molecular geometries of PMes3–AlCl3 and the FLP–CO2 adduct exhibit overall satisfactory agreement with the molecular structures obtained from X-ray crystallographic studies by Stephan and Menard. We find the trapping of CO2 by FLP is energetically favorable by 31.0 kcalmol , which is in good agreement with the experimental finding that the FLP–CO2 complex is stable at 80 8C. Scheme 1 displays the predicted route for the multistep reduction process of FLP–CO2 by ammonia–borane. Our computations show FLP–CO2 binds ammonia–borane through a weak stabilizing interaction (in the solution phase this is predicted to be less than 1 kcalmol , without zeropoint correction) between a hydridic hydrogen on the borane of ammonia–borane and the carbon of the trapped [a] L. Roy, Prof. Dr. A. Paul Raman Centre for Atomic, Molecular and Optical Sciences Indian Association for the Cultivation of Science 2A & 2B Raja S. C. Mullick Road, Kolkata-700032 (India) E-mail : rcap@iacs.res.in [b] Dr. P. M. Zimmerman College of Chemistry, University of California at Berkeley Berkeley, CA 94720. (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem201002282.

The Role of Solvent and of Species Generated in Situ on the Kinetic Acceleration of Aminoborane Oligomerization

The unexpected role of nucleophilic assistance of solvents and intermediates generated in situ in catalyzing NH2BH2 oligomerization is revealed in a computational study. The rate-determining free-energy barrier E(A) that is due to solvent participation for conversion of NH2BH2 to cyclotriborazane (NH2BH2)3 is only 12.7 kcal  mol(-1), whereas without nucleophilic assistance it is as high as 29.0 kcal  mol(-1) in THF (see figure).

Breaking the Myth of the Recalcitrant Chemisorbed Hydrogens on Boron Nitride Nanotubes: A Theoretical Perspective

Hydrogen storage has emerged as one of the foremost challenges in the pursuit of a hydrogen-based renewable energy economy. Ammonia borane (AB) is being investigated intensely for its potential to develop into a chemical hydrogen storage media because of its high gravimetric capacity of hydrogen (19.6 wt%) and low molecular weight (30.7 gmol ). In the last few years several catalysts have been devised by experimentalists that are known to effectuate release of hydrogen from AB at controlled temperature. However, hydrogenation of the spent fuel generated from dehydrogenation of AB is burdened with many obstacles. In recent times there has been considerable progress in recovering AB from spent fuel but sustainability is still a question which plagues chemical hydrogen storage through AB. Other related materials, also based on B and N atoms, which have been implicated as a potential hydrogen storage media are boron nitride nanotubes (BNNTs). BNNTs are multi-walled or single-walled nanounits which have networks of BN hexagons layered in cylindrical geometry, analogous to carbon nanotubes (CNTs) and are isostructural and isoelectronic with graphite. BNNTS are profoundly interesting in terms of both physical and chemical properties which distinguish them from their carbon counterparts. It is significant that despite of their chemical and thermal stabilities, unlike CNTs BNNTs can chemisorb hydrogen under milder conditions. Ma et al. were the first to demonstrate that multi-walled bamboolike BNNT samples could store hydrogen up to 2.6 wt% at room temperature. Furthermore it was realized that hydrogen was retained in the BN nanostructures mostly in chemisorbed form. Subsequently, Tang et al. discovered that collapsed BNNTs store hydrogen upto 4.2 wt% at room temperature. Moreover, Chen et al. were able to chemisorb hydrogen on BNNTs through electrochemical routes. Theoretical studies have suggested the capability of BNNTs to chemically adsorb H atoms favorably up to 50% coverage in an exo-hydrogenated fashion in zigzag (8,0) and (10,0) BNNTs which correspond to 4 wt% storage. However, experiments showed that chemisorbed hydrogen is released on heating the hydrogenated BN nanotubes above 350–450 8C suggesting the existence of strong B H and N H bonds. The deep kinetic trap for the chemisorbed hydrogen atoms on BNNTs is also supported by periodic density functional studies. If the chemisorbed hydrogen can be released at ambient temperatures then BNNTs could become a viable media for hydrogen storage. Of late there has been renewed interest in use of carbon materials like graphene and single-walled nanotubes for hydrogen storage through chemisorption. Hydrogenation of graphene and CNTs have been achieved by using atomic hydrogen and Birch reduction. On heating the hydrogenated graphene or CNTs, desorption of dihydrogen initiates at temperatures above 500 8C for graphane and 350 8C for hydrogneated CNTs. X-ray absorption fine structure (XAFS) studies show that the parent structure of these materials are restored on desorption of hydrogen. The high desorption temperatures have established the notion of an energyintensive dehydrogenation process and have become a stumbling block in realization of viable hydrogen storage in carbonand BN-based nanomaterials. Though there are numerous theoretical studies on hydrogen chemisorption on BNNTS and BN fullerenes and corresponding metal-decorated analogs and changes in magnetic and electronic properties of hydrogenated BNNTs, none has focused on the feasibility of low-temperature removal or release of hydrogen atoms through dehydrocoupling/dehydrogenation from hydrogenated BNNTs, which indeed is a challenging prospect. Can the dihydrogen molecules be released at room temperature to moderately elevated temperatures from hydrogenated BNNTs? The secret lies in low-barrier dehydrocoupling processes, which are integral steps for catalytic release of dihydrogen from such materials. We propose here to explore the consequences of the hypothesis that hydrogenated BNNTs (HBNNTs) and hydrogenated BN fullerenes are likely to be chemically equivalent to ammonia borane or generally to amine boranes. Our quantum chemical studies demystify the chemical signatures of chemisorbed hydrogen atoms on BNNTs and BN fullerenes and discloses the unique trait that optimal proton and hydride acceptors can induce concerted dehydrocoupling/ dehydrogenation of chemisorbed hydrogen atoms on BN nanotubes and fullerenes at low activation barriers surmountable at room temperatures (Figure 1). Experimental and theoretical studies have shown that the hydrogen atoms on AB are distinctively bipolar, where the hydridic and protic characters are displayed by the B H and N H hydrogen atoms, respectively. This unique attribute is exploited to release H2 from AB or dehydrogenate AB by [*] L. Roy, Dr. A. Paul Raman Centre for Atomic Molecular and Optical Sciences Indian Association for the Cultivation of Science 2A & 2B, Raja S. C. Mullick Road, Kolkata 700032 (India) E-mail: rcap@iacs.res.in

A metal-free strategy to release chemisorbed H2 from hydrogenated boron nitride nanotubes.

Chemisorbed hydrogen on boron nitride nanotubes (BNNT) can only be released thermally at very high temperatures above 350 °C. However, no catalyst has been identified that could liberate H2 from hydrogenated BN nanotubes under moderate conditions. Using different density functional methods we predict that the desorption of chemisorbed hydrogen from hydrogenated BN nanotubes can be facilitated catalytically by triflic acid at low free-energy activation barriers and appreciable rates under metal free conditions and mildly elevated temperatures (40-50 °C). Our proposed mechanism shows that the acid is regenerated in the process and can further facilitate similar catalytic release of H2 , thus suggesting all the chemisorbed hydrogen on the surface of the hydrogenated nanotube can be released in the form of H2 . These findings essentially raise hope for the development of a sustainable chemical hydrogen storage strategy in BN nanomaterials.

A Serendipitous Rendezvous with a Four-Center Two-Electron Bonded Intermediate in the Aerial Oxidation of Hydrazine

Oxidation by dioxygen has a rich repertoire of mechanistic intricacies. Herein, we report a hitherto unknown paradigm of dioxygen activation reaction which propagates through a four center two electron (4c-2e) bound species. Using static DFT and ab initio quantum chemical techniques we have unraveled the oxidation pathway for hydrazine and its methylated analogues by dioxygen which involves formation of this unconventional 4c-2e bonded species en route to the oxidation products. Inconvertible evidence in favor of such an unprecedented dioxygen activation route is provided by capturing the events of formation of the 4c-2e species in aqueous phase for hydrazine and its congeners and the experimentally observed products from the respective 4c-2e species, like H2O2 and N2H2 , diazene in the case of hydrazine using Car-Parrinello molecular dynamics simulations.

Lewis Acid Promoted Hydrogenation of CO2 and HCOO– by Amine Boranes: Mechanistic Insight from a Computational Approach

We employ quantum chemical calculations to study the hydrogenation of carbon dioxide by amine boranes, NMe3BH3 (Me3AB) and NH3BH3 (AB) weakly bonded to a bulkier Lewis acid, Al(C6F5)3 (LA). Additionally, computations have also been conducted to elucidate the mechanism of hydrogenation of carbon dioxide by Me3AB while captured between one Lewis base (P(o-tol3), LB) and two Lewis acids, Al(C6F5)3. In agreement with the experiments, our computational study predicts that hydride transfer to conjugated HCO2-, generated in the reaction of Me3AB-LA with CO2, is not feasible. This is in contrast to the potential hydrogenation of bound HCO2H, developed in the reduction of CO2 with AB-LA, to further reduced species like H2C(OH)2. However, the FLP-trapped CO2 effortlessly undergoes three hydride (H-) transfers from Me3AB to produce a CH3O- derivative. DFT calculations reveal that the preference for a H- abstraction by an intrinsically anionic formate moiety is specifically dependent on the electrophilicity of the 2 e- reduced carbon center, which in particular is controlled by the electron-withdrawing capability of the associated substituents on the oxygen. These theoretical predictions are justified by frontier molecular orbitals and molecular electrostatic potential plots. The global electrophicility index, which is a balance of electron affinity and hardness, reveals that the electrophilicity of the formate species undergoing hydrogenation is twice the electrophilicity of the ones where hydrogenation is not feasible. The computed activation energies at M06-2X/6-31++G(d,p) closely predict the observed reactivity. In addition, the possibility of a dissociative channel of the frustrated Lewis pair trapped CO2 system has been ruled out on the basis of predominantly high endergonicity. Knowledge of the underlying principle of these reactions would be helpful in recruiting appropriate Lewis acids/amine boranes for effective reduction of CO2 and its hydrogenated forms in a catalytic fashion.