Ankan Paul

Catalytic dehydrogenation of ammonia borane at Ni monocarbene and dicarbene catalysts.

The development of ammonia borane (AB) as a promising hydrogen storage medium depends upon the ability to reversibly release H(2) from the system. We use density functional theory to investigate the mechanism of the catalytic dehydrogenation of AB by Ni N-heterocyclic carbene (NHC) complexes, which we show proceeds through Ni monocarbene and dicarbene species. Although Ni(NHC)(2) dehydrogenates AB, it competitively decomposes into a monocarbene species because AB readily displaces NHC from Ni(NHC)(2) and reaction of displaced NHC with abundant AB makes Ni monocarbene formation thermodynamically favored over the dicarbene catalyst. Prediction of NHC displacement by AB is consistent with the experimental observation of NHC-BH(3). The Ni monocarbene species Ni(NHC)(NH(2)BH(2)) competitively dehydrogenates AB with barriers consistent with the experimental temperature required to obtain reasonable reaction rates. The Ni monocarbene pathway also involves rate-limiting steps that exhibit both N-H and B-H kinetic isotope effects (KIEs), as observed experimentally. The predicted N-H and B-H KIEs are also in quantitative agreement with experiment. In contrast, AB dehydrogenation by Ni(NHC)(2) does not exhibit a B-H KIE. Activation of AB at both mono- and dicarbene catalysts proceeds through cis-carbene proton acceptance and involves transition states with significant electron delocalization over the pi-system of the carbene and its phenyl rings. NHC Ni catalysts involving carbenes with substituent groups containing steric factors that preclude planarity of the phenyl rings to the carbene aromatic system, such as the Imes and Idipp ligands, are predicted to have lower reactivity, in agreement with experiment. The addition of electron donating and withdrawing groups to the phenyl rings demonstrate the importance of pi-system electron delocalization by their influence on the barrier to cis-carbene proton acceptance.

The existence of secondary orbital interactions

B3LYP/6‐311+G** (and MP2/6‐311+G**) computations, performed for a series of Diels‐Alder (DA) reactions, confirm that the endo transition states (TS) and the related Cope‐TSs are favored energetically over the respective exo‐TSs. Likewise, the computed magnetic properties (nucleus‐independent chemical shifts and magnetic susceptibililties) of the endo‐ (as well as the Cope) TSs reveal their greater electron delocalization and greater aromaticity than the exo‐TSs. However, Woodward and Hoffmanns original example is an exception: their endo‐TS model, involving the DA reaction of a syn‐ with an anti‐butadiene (BD), actually is disfavored energetically over the corresponding exo‐TS; magnetic criteria also do not indicate the existence of SOI delocalization in either case. Instead, a strong energetic preference for endo‐TSs due to SOI is found when both BDs are in the syn conformations. This is in accord with Alder and Steins rule of “maximum accumulation of double bonds:” both the dienophile and the diene should have syn conformations. Plots along the IRCs show that the magnetic properties typically are most strongly exalted close to the energetic TS. Because of SOI, all the points along the endo reaction coordinates are more diatropic than along the corresponding exo pathways. We find weak SOI effects to be operative in the endo‐TSs involved in the cycloadditions of cyclic alkenes, cyclopropene, aziridine, cyclobutene, and cyclopentene, with cyclopentadiene. While the endo‐TSs are only slightly lower in energy than the respective exo‐TSs, the magnetic properties of the endo‐TSs are significantly exalted over those for the exo‐TSs and the Natural Bond Orbitals indicate small stabilizing interactions between the methylene cycloalkene hydrogen orbitals (and lone pairs in case of aziridine) with π‐character and the diene π MOs.

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).

Copper(II) and Nickel(II) Complexes of β-Aminoketoxime Ligand: Syntheses, Crystal Structures, Magnetism, and Nickel(II) Templated Coupling of Oxime with Nitrile

The syntheses, molecular structures, and magnetic properties of a dicopper(II) complex, [Cu(2)(HL(1))(2)](ClO(4))(2) (1), and its nickel(II) analog, [Ni(2)(HL(1))(2)](ClO(4))(2) (2), of a beta-amino ketoxime ligand (H(2)L(1) = 4,4,9,9-tetramethyl-5,8-diazadodecane-2,11-dione dioxime) are discussed. The metal centers in out-of-plane oximate bridged dinuclear complexes (1 and 2) display distorted trigonal bipyramidal geometry and form a six-membered M(2)(NO)(2) ring oriented in a boat conformation. The two copper(II) centers in 1 interact ferromagnetically giving rise to a triplet-spin ground state whereas the two nickel(II) centers in 2 interact antiferromagnetically to stabilize a singlet-spin state. Variable temperature magnetic susceptibility measurements establish the presence of a weak ferromagnetic coupling (J = 13 cm(-1)) in 1 and a weak anitiferromagnetic coupling (J = -12 cm(-1)) in 2. The exchange coupling constant derived from B3LYP computations in conjunction with broken symmetry spin-projection techniques for the oximate bridged dinuclear copper(II) complex shows excellent agreement with the corresponding experimental value. A square-planar mononuclear nickel(II) complex of the dioxime ligand, [Ni(H(2)L(1))](ClO(4))(2) (3), is reported along with its crystal structure, which reacts with acetonitrile to produce a six-coordinate mononuclear complex, [Ni(L(2))](ClO(4))(2) (4). The ligand (L(2)) in complex 4 is the iminoacyl derivative of oxime, where the coupling of oxime and acetonitrile takes place via a proton-assisted pathway. The iminoacylation of H(2)L(1) works with other nitriles like butyronitrile and benzonitrile. Computational studies support a proton-assisted coupling of oxime with nitrile. The critical transition states have been located for the iminoacylation reaction. Complex 4 can be converted back to complex 3 by reacting with sodium acetate in methanol.

The geometry and electronic topology of higher-order charged Möbius annulenes.

Higher-order aromatic charged Möbius-type annulenes have been L(k) realized computationally. These charged species are based on strips with more than one electronic half-twist, as defined by their linking numbers. The B3LYP/6-311+G(d,p) optimized structures and properties of annulene rings with such multiple half-twists (C(12)H(12)(2+), C(12)H(12)(2-), C(14)H(14), C(18)H(18)(2+), C(18)H(18)(2-), C(21)H(21)(+), C(24)H(24)(2-), C(28)H(28)(2+), and C(28)H(28)(2-)) have the nearly equal C-C bond lengths, small dihedral angles around the circuits, stabilization energies, and nucleus-independent chemical shift values associated with aromaticity. The topology and nature of Möbius annulene systems are analyzed in terms of the torus curves defined by electron density functions (rho(r)(pi), ELF(pi)) constructed using only the occupied pi-MOs. The pi-torus subdivides into a torus knot for annulenes defined by an odd linking number (L(k) = 1, 3pi) and a torus link for those with an even linking number (L(k) = 2, 4pi). The torus topology is shown to map onto single canonical pi-MOs only for even values of L(k). Incomplete and misleading descriptions of the topology of pi-electronic Möbius systems with an odd number of half twists result when only signed orbital diagrams are considered, as is often done for the iconic single half twist system.

Reactivity of Biomimetic Iron(II)-2-aminophenolate Complexes toward Dioxygen: Mechanistic Investigations on the Oxidative C–C Bond Cleavage of Substituted 2-Aminophenols

The isolation and characterization of a series of iron(II)-2-aminophenolate complexes [(6-Me3-TPA)Fe(II)(X)](+) (X = 2-amino-4-nitrophenolate (4-NO2-HAP), 1; X = 2-aminophenolate (2-HAP), 2; X = 2-amino-3-methylphenolate (3-Me-HAP), 3; X = 2-amino-4-methylphenolate (4-Me-HAP), 4; X = 2-amino-5-methylphenolate (5-Me-HAP), 5; X = 2-amino-4-tert-butylphenolate (4-(t)Bu-HAP), 6 and X = 2-amino-4,6-di-tert-butylphenolate (4,6-di-(t)Bu-HAP), 7) and an iron(III)-2-amidophenolate complex [(6-Me3-TPA)Fe(III)(4,6-di-(t)Bu-AP)](+) (7(Ox)) supported by a tripodal nitrogen ligand (6-Me3-TPA = tris(6-methyl-2-pyridylmethyl)amine) are reported. Substituted 2-aminophenols were used to prepare the biomimetic iron(II) complexes to understand the effect of electronic and structural properties of aminophenolate rings on the dioxygen reactivity and on the selectivity of C-C bond cleavage reactions. Crystal structures of the cationic parts of 5·ClO4 and 7·BPh4 show six-coordinate iron(II) centers ligated by a neutral tetradentate ligand and a monoanionic 2-aminophenolate in a bidentate fashion. While 1·BPh4 does not react with oxygen, other complexes undergo oxidative transformation in the presence of dioxygen. The reaction of 2·ClO4 with dioxygen affords 2-amino-3H-phenoxazin-3-one, an auto-oxidation product of 2-aminophenol, whereas complexes 3·BPh4, 4·BPh4, 5·ClO4 and 6·ClO4 react with O2 to exhibit C-C bond cleavage of the bound aminophenolates. Complexes 7·ClO4 and 7(Ox)·BPh4 produce a mixture of 4,6-di-tert-butyl-2H-pyran-2-imine and 4,6-di-tert-butyl-2-picolinic acid. Labeling experiments with (18)O2 show the incorporation of one oxygen atom from dioxygen into the cleavage products. The reactivity (and stability) of the intermediate, which directs the course of aromatic ring cleavage reaction, is found to be dependent on the nature of ring substituent. The presence of two tert-butyl groups on the aminophenolate ring in 7·ClO4 makes the complex slow to cleave the C-C bond of 4,6-di-(t)Bu-HAP, whereas 4·BPh4 containing 4-Me-HAP displays fastest reactivity. Density functional theory calculations were conducted on [(6-Me3-TPA)Fe(III)(4-(t)Bu-AP)](+) (6(Ox)) to gain a mechanistic insight into the regioselective C-C bond cleavage reaction. On the basis of the experimental and computational studies, an iron(II)-2-iminobenzosemiquinonate intermediate is proposed to react with dioxygen resulting in the oxidative C-C bond cleavage of the coordinated 2-aminophenolates.

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

The low-lying electronic states of nickel cyanide and isocyanide: A theoretical investigation

At different levels of coupled cluster theory optimum structures, energetics, and harmonic vibrational frequencies for several low-lying doublet and quartet electronic states of linear NiCN and NiNC were studied using four contracted Gaussian basis sets, ranging from Ni[6s5p4d2f], CN[4s3p2d] to Ni[8s7p5d3f2g1h], CN[5s4p3d2f1g]. The most reliable predictions were obtained with a relativistic Douglas-Kroll restricted open-shell-based coupled cluster method including singles, doubles, and perturbative triple excitations [DK-R/UCCSD(T)]. This level of theory was used in conjunction with correlation-consistent polarized valence Douglas-Kroll recontracted quadruple-zeta basis sets (cc-pVQZDK). The energetic ordering of the electronic states of NiCN is predicted to be 2delta < 2sigma+ < 2pi < 4delta < 4pi and that of NiNC is 2delta approximately 2sigma+ < 2pi < 4delta < 4pi < 4sigma-. Our theoretical investigation supports the assignment of the ground-state term symbol, the Ni-C stretching frequency, and the bending frequency for the ground electronic state of NiCN by Kingston et al. [J. Mol. Spectrosc. 215, 106 (2002)] and by Sheridan and Ziurys [J. Chem. Phys. 118, 6370 (2003)]. The predicted structure of the 2delta ground state of NiCN, r(e)(Ni-C) = 1.822 angstroms and r(e)(C-N) = 1.167 angstroms, at DK-R/UCCSD(T)/cc-pVQZDK shows excellent agreement with the experimentally determined Ni-C bond length of 1.826 A and less satisfactory agreement for the C-N bond length of 1.153 angstroms [J. Chem. Phys. 118, 6370 (2003)]. It is also concluded that the metal-to-ligand pi back donation is weak or negligible. Additionally, we found that on the 2delta surface the linear cyanide isomer lies lower in energy than the linear isocyanide isomer by 12.2 kcal mol(-1).

Theoretical Investigation on the Chemistry of Entrapment of the Elusive Aminoborane (H2NBH2) Molecule

Aminoborane (H2 N=BH2 ) is an elusive entity and is thought to be produced during dehydropolymerization of ammonia borane, a molecule of prime interest in the field of chemical hydrogen storage. The entrapment of H2 N=BH2 through hydroboration of exogenous cyclohexene has emerged as a routine technique to infer if free H2 N=BH2 is produced or not during metal-catalyzed ammonia borane dehydrogenation reactions. But to date, the underlying mechanism of this trapping reaction remains unexplored. Herein, by using DFT calculations, we have investigated the mechanism of trapping of H2 N=BH2 by cyclohexene. Contrary to conventional wisdom, our study revealed that the trapping of H2 N=BH2 does not occur through direct hydroboration of H2 N=BH2 on the double bond of cyclohexene. We found that autocatalysis by H2 N=BH2 is crucial for the entrapment of another H2 N=BH2 molecule by cyclohexene. Additionally, nucleophilic assistance from the solvent is also implicated for the entrapment reaction carried out in nucleophilic solvents. In THF, the rate-determining barrier for formation of the trapping product was predicted to be 16.7 kcal mol(-1) at M06 L(CPCM) level of theory.