Origin of Binding of Ammonia–Borane to Transition-Metal-Based Catalysts: An Insight from the Charge and Energy Decomposition Method ETS-NOCV

Cellulose Regeneration in Imidazolium-Based Ionic Liquids and Antisolvent Mixtures: A Density Functional Theory Study

Cellulose can be dissolved in ionic liquids (ILs), and it can be recovered by adding antisolvent such as water or alcohol. In addition, the regenerated cellulose can be used for textiles, degradable membranes, hydrogels/aerogels, etc. However, the regenerated mechanism of cellulose remains ambiguous. In this work, density functional theory (DFT) calculation is reported for the cellulose regeneration from a cellulose/1-n-butyl-3-methylimidazolium acetate (BmimOAc)/water mixture. To investigate the microscopic effects of the antisolvents, we analyzed the structures and H-bonds of BmimOAc-nH₂O and cellobiose-ILs-nH₂O (n = 0-6) clusters. It can be found that when n ≥ 5 in the BmimOAc-nH₂O clusters, the solvent-separated ion pairs (SIPs) play a dominant position in the system. With the increasing numbers of water molecules, the cation-anion interaction can be separated by water to reduce the effects of ILs on cellulose dissolution. Furthermore, the BmimOAc-nH₂O and cellobiose-ILs (n = 0-6) clusters tend to be a more stable structure with high hydration in an aqueous solution. When the water molecules were added to the system, H-bonds can be formed among H₂O, the hydroxyl of cellulose, and the oxygen of OAc. Therefore, the interactions between cellulose and ILs will be decreased to promote cellulose regeneration. This work would provide some help to understand the mechanism of cellulose regeneration from the view of theoretical calculation.

Theoretical study on the microscopic mechanism of lignin solubilization in Keggin-type polyoxometalate ionic liquids

Keggin-type polyoxometalate derived ionic liquids (POM-ILs) have recently been presented as effective solvent systems for biomass delignification. To investigate the mechanism of lignin dissolution in POM-ILs, the system involving POM-IL ([C4C1Im]₃[PW₁₂O₄₀]) and guaiacyl glycerol-β-guaiacyl ether (GGE), which contains a β-O-4 bond (the most dominant bond moiety in lignin), was studied using quantum mechanical calculations and molecular dynamics simulations. These studies show that more stable POM-IL structures are formed when [C4C1Im]⁺ is anchored in the connecting four terminal oxygen region of the [PW₁₂O₄₀]^3- surface. The cations in POM-ILs appear to stabilize the geometry by offering strong and positively charged sites, and the POM anion is a good H-bond acceptor. Calculations of POM-IL interacting with GGE show the POM anion interacts strongly with GGE through many H-bonds and π-π interactions which are the main interactions between the POM-IL anion and GGE and are strong enough to force GGE into highly bent conformations. These simulations provide fundamental models of the dissolution mechanism of lignin by POM-IL, which is promoted by strong interactions of the POM-IL anion with lignin.

Theoretical study on the charge transport and metallic conducting properties in organic complexes

The charge transfer process between substrate molecular and dopant always appears in doped organic semiconductors, so that molecular doping is a common method to improve the electrical properties by combining appropriate complexes of electron acceptor and donor molecules. At the interface of the doped complexes, the amount of charge-transfer based on the charge analysis method could be affected by various factors, including the stacking structure, the HOMO_D-LUMO_A gaps, the offset defined by the donor ionization potential and the acceptor electron affinity IP_D-|EA_A|, and the strength of the intermolecular orbital interaction. To better understand the charge transport properties in complex crystals, reasonable mobility values were calculated by combining semi-classical Marcus-Hush theory with molecular dynamics simulation, in which the mobility values were on the same order of magnitude as experimental values. The largest and average room-temperature mobility were 4.59 and 0.21 cm² V^-1 s^-1 for TTF-TCNQ based on the anisotropic transport properties and random-walk schemes of the charge diffusion coefficient. The interface of the TTF-TCNQ crystal possesses metallic conducting properties with a predicted resistance of 4.43 kΩ. Charge-transfer complexes exhibit larger mobility and higher conductivity compared to the constituent donor and acceptor molecules.

Interpretation of Tolman electronic parameters in the light of natural orbitals for chemical valence

Understanding the nature and the strength of metal-ligand interactions in d- and f-block metal complexes has always been a central issue for both synthetic and theoretical chemists. These interactions are usually described according to the well accepted Dewar-Chatt-Duncanson model, and thus over the years numerous research groups directed their efforts to shed light on the role of σ- and π-contributions. Among others, the electronic parameter introduced by Tolman in the 1970s represents a milestone in this field. Herein we present a quantitative description of the nickel-phosphine bond in Tolman's nickel(0) carbonyl complexes. The combination of Natural Orbitals for Chemical Valence with Energy Decomposition Analysis resulted in the definition of a new parameter (T^phos) which comprises all the energetic contributions needed to describe the nickel-phosphine bond and thus stands as a reliable descriptor of the electronic properties of phosphines. Moreover, steric effects of phosphines (i.e. Tolman's cone angles) have been considered too, and a linear relation including Ni-P bond distances, T^phos and cone angle has been found.

SE2 reaction in noncarbon system: Metal-halide catalysis for dehydrogenation of ammonia borane

An electrophilic substitution (SE) reaction of BN isosteres has been investigated for the dehydrogenation of ammonia borane (AB) by metal chlorides (MCl2) using various ab initio calculations. In contrast to the typical SE reaction occurring at the carbon atom, the nitrogen atom in AB serves as the reaction center for the SE reaction with the boron moiety as the leaving group when the MCl2 approaches the AB. The SE2 backside reaction is favored as a trigger step for the dehydrogenation of AB by the MCl2 The SE2 reaction is found for 3d-transition-metal chlorides (e.g., FeCl2, CoCl2, NiCl2, CuCl2, and ZnCl2), while PdCl2 leads to the dehydrogenation of AB by a direct B-H σ-bond activation, similar to most organometallic catalysts. Interestingly, the polymerization of AB promoted by MCl2 can be explained with the similar SE2 mechanism, and the dehydrogenation of the BN derivative 3-methyl-1,2-BN-cyclopentane (CBN) bearing a carbon backbone ring also follows the SE2 reaction. In particular, the experimental observation that the use of metal-chloride catalysis decreases the by-products obtained during the hydrogenation of AB can be explained by our mechanism involving the SE2 reaction. This work is helpful for the development of novel metal-halide catalysts for practical hydrogen storage materials, including the BN moiety.

B-Hb⋯π interactions in benzene–borazine sandwich and multidecker complexes: a DFT study

Multidecker complexes of benzene/borazine can be formed through B-H_b⋯π interactions.

On the Stability of Cis- and Trans-2-Butene Isomers. An Insight Based on the FAMSEC, IQA, and ETS-NOCV Schemes

In the present account, the real space fragment attributed molecular system energy change (FAMSEC) approach, interacting quantum atoms energy decomposition scheme as well as molecular orbitals based the extended transition state scheme coupled with natural orbitals for chemical valence (ETS-NOCV) have been, for the first time, successfully used to delineate factors of importance for stability of the 2-butene conformers (cis-eq, cis-TS, trans-eq, trans-TS). Our results demonstrate that atoms of the controversial H-H contact in cis-eq (i) are involved in attractive interaction dominated by the exchange-correlation term, (ii) are weekly stabilized, (iii) show trends in several descriptors found in other typical H-bonds, and (iv) are part of most stabilized CH-HC fragment (loc-FAMSEC = -3.6 kcal/mol) with most favourably changed intrafragment interactions on trans-eq→cis-eq. Moreover, lower stability of cis-eq vs. trans-eq is linked with the entire HCCH (ethylenic) fragment which destabilized cis-eq (mol-FAMSEC, +3.9 kcal/mol) the most. Although the H-H contact can be linked with smaller, relative to trans-, rotational energy barrier in cis-2-butene, we have proven that to rationalize this phenomenon one must account for changes in interactions between various fragments that constitute the entire molecule. Importantly, we discovered a number of comparable trends in fundamental properties of equivalent molecular fragments on a methyl group rotation; for example, interaction between BP-free H-atoms in trans-eq (involving CH bonds of the methyl and ethylenic units) and BP-linked H-atoms in cis-eq. Clearly, rotational energy barrier cannot be entirely (i) rationalized by the properties of or (ii) attributed to the H-H contact in cis-eq. © 2016 Wiley Periodicals, Inc.

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-C4 H10 (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 (η(2) η(2) -H2 BNR2 C4 H8 )][BAr(F) 4 ] (NR2 =NH2 , NMeH) and [Rh(iPr2 PCH2 CH2 CH2 PiPr2 )(η(2) η(2) -H2 BNR2 C4 H8 )][BAr(F) 4 ] (NR2 =NH2 , NMeH, NMe2 ). For NR2 =NH2 subsequent metal-promoted, dehydrocoupling shows the eventual formation of the cyclic tricyclic borazine [BNC4 H8 ]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 HBNMeC4 H8 . The mechanism of dehydrogenation of 2,2-H,Me-1,2-B,N-C4 H10 using the {Rh(iPr2 PCH2 CH2 CH2 PiPr2 )}(+) catalyst has been probed. Catalytic experiments indicate the rapid formation of a dimeric species, [Rh2 (iPr2 PCH2 CH2 CH2 PiPr2 )2 H5 ][BAr(F) 4 ]. 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.

Metal-interacted histidine dimer: an ETS-NOCV and XANES study

We have analyzed the metal coordination in a histidine dimer, hydrated with a water molecule, based on the extended transition state scheme with the theory of natural orbitals for chemical valence (ETS-NOCV).

The performance of methallyl nickel complexes and boron adducts in the catalytic activation of ethylene: a conceptual DFT perspective

In this work, global and local descriptors of chemical reactivity and selectivity are used to explain the differences in reactivities toward ethylene of methallyl nickel complexes and their B(C6F5)3 and BF3 adducts. DFT calculations were used to explain why nickel complexes alone are inactive in ethylene polymerization while their boron adducts can activate it. It is shown that chemical potential, hardness, electrophilicity and molecular electrostatic potential surfaces describe fairly well the reactivity and selectivity of these organometallic systems toward ethylene. Experimental data indicates that addition of a borane molecule to nickel complexes changes dramatically their reactivity-behavior that is confirmed computationally. Our results show that bare complexes are unable to activate ethylene-a Lewis base-because they also behave as Lewis bases. The addition of the co-catalyst-a Lewis acid-turns the adducts into Lewis acids, making them active towards ethylene.

Combined activation strain model and energy decomposition analysis methods: a new way to understand pericyclic reactions

The recently introduced activation strain model (ASM) has allowed us to gain more insight into the intimacies of different fundamental processes in chemistry. In combination with the energy decomposition analysis (EDA) method, we have nowadays a very useful tool to quantitatively understand the physical factors that govern the activation barriers of reactions within organic and organometallic chemistry. In this Perspective article, we present selected illustrative examples of the application of this method to pericyclic reactions (Diels-Alder and double group transfer reactions) to show that this methodology nicely complements other more traditional, widely used theoretical methods.

Intramolecular hydrogen bond in biologically active o-carbonyl hydroquinones

Intramolecular hydrogen bonds (IHBs) play a central role in the molecular structure, chemical reactivity and interactions of biologically active molecules. Here, we study the IHBs of seven related o-carbonyl hydroquinones and one structurally-related aromatic lactone, some of which have shown anticancer and antioxidant activity. Experimental NMR data were correlated with theoretical calculations at the DFT and ab initio levels. Natural bond orbital (NBO) and molecular electrostatic potential (MEP) calculations were used to study the electronic characteristics of these IHB. As expected, our results show that NBO calculations are better than MEP to describe the strength of the IHBs. NBO energies (∆Eij(2)) show that the main contributions to energy stabilization correspond to LP-->σ* interactions for IHBs, O1…O2-H2 and the delocalization LP-->π* for O2-C2=Cα(β). For the O1…O2-H2 interaction, the values of ∆Eij(2) can be attributed to the difference in the overlap ability between orbitals i and j (Fij), instead of the energy difference between them. The large energy for the LP O2-->π* C2=Cα(β) interaction in the compounds 9-Hydroxy-5-oxo-4,8, 8-trimethyl-l,9(8H)-anthracenecarbolactone (VIII) and 9,10-dihydroxy-4,4-dimethylanthracen-1(4H)-one (VII) (55.49 and 60.70 kcal/mol, respectively) when compared with the remaining molecules (all less than 50 kcal/mol), suggests that the IHBs in VIII and VII are strongly resonance assisted.

On the origin of internal rotation in ammonia borane

The internal rotation in ammonia borane (AB) was studied on the basis of natural orbitals for chemical valence (NOCV) and eigenvectors for Pauli repulsion (NOPR). We found that the total hyperconjugation stabilization (ca. 5 kcal mol⁻¹), based on the charge transfer from the occupied σ (B–H) orbitals into the empty σ*(N–H), slightly favors the staggered conformation over the eclipsed one; however, the barrier to internal rotation in ammonia borane can be understood predominantly in a ‘classical’ way, as originating from the steric (Pauli) repulsion contributions (of the kinetic origin) that act solely between N–H and B–H bonds. Repulsion between the lone pair of ammonia and the adjacent B–H bonds was found to be dominant in absolute terms; however, it does not determine the rotational barrier. Similar conclusions on the role of CH↔HC repulsion appeared to be valid for isoelectronic ethane.

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