Topochemical Syntheses of Polyarylopeptides Involving Large Molecular Motions: Frustrated Monomer Packing Leads to the Formation of Polymer Blends.

We report the topochemical syntheses of three polyarylopeptides, wherein triazolylphenyl group is integrated into the backbone of peptide chains. We synthesized three different monomers having azide and arylacetylene as end-groups from glycine, L-alanine and L-valine. We crystallized these monomers and the crystal structures of two of them were determined by single-crystal X-ray diffractometry. Due to the steric constraints, both of these monomers crystallized with two molecules, viz. conformers A and B, in the asymmetric unit. Consistently, in both cases, the A-conformers are antiparallelly π-stacked and B-conformers are parallelly slip-stacked, exploiting weak interactions. Though the arrangements of molecules in the pristine crystals were unsuitable for topochemical reaction, upon heating, they undergo large motion inside the crystal lattice to reach a transient reactive orientation and thereby the self-sorted conformer stacks react to give a blend of triazole-linked polyarylopeptides having two different linkages. Due to the large molecular motion inside crystals, the product phase loses its crystallinity.

Identification of Crucial Intermediates in the Formation of Humins from Cellulose-Derived Platform Chemicals Under Brønsted Acid Catalyzed Reaction Conditions.

Humins are one of the undesirable products formed during the dehydration of sugars as well as the conversion of 5-hydroxymethylfurfural (HMF) to value-added products. Thus, reducing the formation of humins is an important strategy for improving the yield of the aforementioned reactions. Even after a plethora of studies, the mechanism of formation and the structure of humins are still elusive. In this regard, we have employed density functional theory-based mechanistic studies and microkinetic analysis to identify crucial intermediates formed from glucose, fructose, and HMF that can initiate the polymerization reactions resulting in humins under Brønsted acid-catalyzed reaction conditions. This study brings light into crucial elementary reaction steps that can be targeted for controlling humins formation. Moreover, this work provides a rationale for the experimentally observed aliphatic chains and HMF condensation products in the humins structure. Different possible polymerization routes that could contribute to the structure of humins are also suggested based on the results. Importantly, the findings of this work indicate that increasing the rate of isomerization of glucose to fructose and reducing the rate of reaction between HMF molecules could be an efficient strategy for reducing humins formation.

Topochemical Cycloaddition Reaction between an Azide and an Internal Alkyne.

Here we report the synthesis of a trisubstituted-1,2,3-triazole-linked polymer using a topochemical azide-alkyne cycloaddition (TAAC) reaction. A cyclitol-derived monomer having an azide and an internal alkyne group was designed. The four hydroxy groups present in this monomer dictate its crystal packing such that the monomer molecules are arranged head-to-tail, thereby placing the internal alkyne and the azide units of adjacent molecules proximally. Although the alignment of the reactive groups in the monomer crystal is not favourable for a topochemical reaction, a reactive orientation can be achieved by the rotation of the reactive groups. Upon heating the crystals, the monomer underwent topochemical polymerization to yield the trisubstituted-1,2,3-triazole-linked-polycyclitol. This study demonstrates a new synthetic strategy for cycloaddition reaction between non-polarized internal alkynes and azides to yield trisubstituted triazoles.

Secondary Structure Tuning of a Pseudoprotein Between ß-Meander and α-Helical Forms in the Solid-State.

Tuning the secondary structure of a protein or polymer in the solid-state is challenging. Here we report the topochemical synthesis of a pseudoprotein and its secondary structure tuning in the solid-state. We designed the dipeptide monomer N3 -Leu-Ala-NH-CH2 -C≡CH (1) for topochemical azide-alkyne cycloaddition (TAAC) polymerization. Dipeptide 1 adopts an anti-parallel ß-sheet-like stacked arrangement in its crystals. Upon heating, the dipeptide undergoes quantitative TAAC polymerization in a crystal-to-crystal fashion yielding large polymers. The reaction occurs between the adjacent monomers in the H-bonded anti-parallel stack, yielding pseudoprotein having a ß-meander structure. When dissolved in methanol, this pseudoprotein changes its secondary structure from ß-meander to α-helical form and it retains the new secondary structure upon desolvation. This work demonstrates a novel paradigm for tuning the secondary structure of a polymer in the solid-state.

A Proton-Responsive Pyridyl(benzamide)-Functionalized NHC Ligand on Ir Complex for Alkylation of Ketones and Secondary Alcohols.

A Cp*Ir(III) complex (1) of a newly designed ligand L1 featuring a proton-responsive pyridyl(benzamide) appended on N-heterocyclic carbene (NHC) has been synthesized. The molecular structure of 1 reveals a dearomatized form of the ligand. The protonation of 1 with HBF4 in tetrahydrofuran gives the corresponding aromatized complex [Cp*Ir(L1 H)Cl]BF4 (2). Both compounds are characterized spectroscopically and by X-ray crystallography. The protonation of 1 with acid is examined by 1 H NMR and UV-vis spectra. The proton-responsive character of 1 is exploited for catalyzing α-alkylation of ketones and ß-alkylation of secondary alcohols using primary alcohols as alkylating agents through hydrogen-borrowing methodology. Compound 1 is an effective catalyst for these reactions and exhibits a superior activity in comparison to a structurally similar iridium complex [Cp*Ir(L2 )Cl]PF6 (3) lacking a proton-responsive pendant amide moiety. The catalytic alkylation is characterized by a wide substrate scope, low catalyst and base loadings, and a short reaction time. The catalytic efficacy of 1 is also demonstrated for the syntheses of quinoline and lactone derivatives via acceptorless dehydrogenation, and selective alkylation of two steroids, pregnenolone and testosterone. Detailed mechanistic investigations and DFT calculations substantiate the role of the proton-responsive ligand in the hydrogen-borrowing process.

Mechanism of the Hydrolysis of Endosulfan Isomers.

The objective of this study was to elucidate the mechanism of abiotic hydrolysis of ES isomers, i.e., Endosulfan-1 (ES-1) and Endosulfan-2 (ES-2), using a combination of experiments and density functional theory (DFT) calculations. Hydrolysis of both ES-1 and ES-2 resulted in the formation of Endosulfan Alcohol (ES-A). The rate of hydrolysis was first order in all cases and increased with both pH and temperature. Rate expressions describing the hydrolysis rates of ES-1 and ES-2 as a function of pH and temperature were obtained and validated with independent data sets. DFT calculations were performed using three functionals (M06-2X, B3LYP, and MPW1K) and both IEFPCM-UFF and SMD to introduce solvent effects. The geometry optimization of molecules ES-1 and ES-2 showed that the free energy of ES-1 was larger, and therefore, ES-2 was the more thermodynamically stable isomer. DFT calculations also supported a hydrolysis mechanism involving two successive attacks by OH- ions on C-O bonds resulting in the attachment of OH- and the elimination of SO3- from the ES molecule, but only the first attack was rate limiting. Calculations with all functionals and solvent effect combinations supported the experimentally observed result of faster hydrolysis of ES-2 than of ES-1. The MPW1K functional along with IEFPCM-UFF for solvent effect simulated the free energy of activation to be the closest for both ES-1 and ES-2 with less than 3% error with respect to the values computed from the experimental observations. The kinetic rate expression for ES hydrolysis derived on the basis of the proposed mechanism was identical to the rate expression derived from experiments. It was deduced that the hydrolysis rates of both ES isomers may vary over 3 orders of magnitude depending on the prevalent pH and temperature.

Hydroxypalladation precedes the rate-determining step in the Wacker oxidation of ethene.

The complete reaction mechanism and kinetics of the Wacker oxidation of ethene in water under low [Cl(-)], [Pd(II)], and [Cu(II)] conditions are investigated in this work by using ab initio molecular dynamics. These extensive simulations shed light on the molecular details of the associated individual steps, along two different reaction routes, starting from a series of ligand-exchange processes in the catalyst precursor PdCl4(2-) to the final aldehyde-formation step and the reduction of Pd(II). Herein, we report that hydroxylpalladation is not the rate-determining step and is, in fact, in equilibrium. The newly proposed rate-determining step involves isomerization and follows the hydroxypalladation step. The mechanism proposed herein is shown to be in excellent agreement with the experimentally observed rate law and rate. Moreover, this mechanism is in consensus with the observed kinetic isotope effects. This report further confirms the outer-sphere (anti) hydroxypalladation mechanism. Our calculations also ratify that the final product formation proceeds through a reductive elimination, assisted by solvent molecules, rather than through ß-hydride elimination.

Enhancing the reaction rates of Morita-Baylis-Hillman reaction in heterocyclic aldehydes by substitutions.

The molecular origin of the experimentally observed pronounced difference in the rates of Morita-Baylis-Hillman (MBH) reaction in heterocyclic aldehydes, depending on the position of the formyl group, is investigated herein by using DFT-based mechanistic studies and free energy computations. These calculations are based on the 1,4-diazobicyclo[2.2.2]octane (DABCO)-catalyzed MBH reaction of methyl acrylate with substituted 4- and 5-isoxazolecarbaldehyde, which are slow- and fast-reacting substrates, respectively. As a result of this study, we propose that by tailoring ring substitutions the reactivity of the formyl group for MBH reactions may be enhanced in slow-reacting heterocyclic aldehydes. This proposition is demonstrated by enhancing the rate of the MBH reaction in 4-isoxazolecarbaldehyde more than 10(4) -fold by installing an ester substitution at the C-3 position. Similarly, the reactivity of the formyl group towards the MBH reaction in substituted 3-pyrazolecarbaldehyde and pyridinecarbaldehyde is shown to be increased several-fold by a halo substitution. We also confirm that the reasons for different reactivities of heterocyclic aldehydes and the proposed scheme for improving the reaction rates remains valid for all the three mechanisms proposed for the MBH reaction, namely, Hill-Isaacs, McQuade, and Aggarwal.

Enhancement of Structural, Electrochemical, and Thermal Properties of High-Energy Density Ni-Rich LiNi0.85Co0.1Mn0.05O2 Cathode Materials for Li-Ion Batteries by Niobium Doping.

Ni-rich layered oxide LiNi1 - x - yCoxMnyO2 (1 - x - y > 0.5) materials are favorable cathode materials in advanced Li-ion batteries for electromobility applications because of their high initial discharge capacity. However, they suffer from poor cycling stability because of the formation of cracks in their particles during operation. Here, we present improved structural stability, electrochemical performance, and thermal durability of LiNi0.85Co0.1Mn0.05O2(NCM85). The Nb-doped cathode material, Li(Ni0.85Co0.1Mn0.05)0.997Nb0.003O2, has enhanced cycling stability at different temperatures, outstanding capacity retention, improved performance at high discharge rates, and a better thermal stability compared to the undoped cathode material. The high electrochemical performance of the doped material is directly related to the structural stability of the cathode particles. We further propose that Nb-doping in NCM85 improves material stability because of partial reduction of the amount of Jahn-Teller active Ni3+ ions and formation of strong bonds between the dopant and the oxygen ions, based on density functional theory calculations. Structural studies of the cycled cathodes reveal that doping with niobium suppresses the formation of cracks during cycling, which are abundant in the undoped cycled material particles. The Nb-doped NCM85 cathode material also displayed superior thermal characteristics. The coherence between the improved electrochemical, structural, and thermal properties of the doped material is discussed and emphasized.

Studies of Nickel-Rich LiNi0.85Co0.10Mn0.05O2 Cathode Materials Doped with Molybdenum Ions for Lithium-Ion Batteries.

In this work, we continued our systematic investigations on synthesis, structural studies, and electrochemical behavior of Ni-rich materials Li[NixCoyMnz]O2 (x + y + z = 1; x ≥ 0.8) for advanced lithium-ion batteries (LIBs). We focused, herein, on LiNi0.85Co0.10Mn0.05O2 (NCM85) and demonstrated that doping this material with high-charge cation Mo6+ (1 at. %, by a minor nickel substitution) results in substantially stable cycling performance, increased rate capability, lowering of the voltage hysteresis, and impedance in Li-cells with EC-EMC/LiPF6 solutions. Incorporation of Mo-dopant into the NCM85 structure was carried out by in-situ approach, upon the synthesis using ammonium molybdate as the precursor. From X-ray diffraction studies and based on our previous investigation of Mo-doped NCM523 and Ni-rich NCM811 materials, it was revealed that Mo6+ preferably substitutes Ni residing either in 3a or 3b sites. We correlated the improved behavior of the doped NCM85 electrode materials in Li-cells with a partial Mo segregation at the surface and at the grain boundaries, a tendency established previously in our lab for the other members of the Li[NixCoyMnz]O2 family.

First principles study of electrocatalytic behavior of olivine phosphates with mixed alkali and mixed transition metal atoms.

Lithium transition metal olivine phosphates are well known Li-ion battery cathode materials, but these materials can also be used as electrocatalyst. Recent experimental studies showed that olivine phosphates with mixed alkali metals (Li and Na) and mixed transition metals (Ni and Fe) provide better electrocatalytic activity compared to single alkali and transition metal alternatives. In the current work, we analyzed the role of alkali metals, transition metals and vacancies on the reactivity of a series of olivine phosphates with different stoichiometries using first principles calculations. To this end, we investigated the adsorption of water at the surface of these materials. We found that water binds preferably at Ni surface sites for materials devoid of alkali ion vacancies. We further found correlation between the calculated adsorption energy with experimentally measured overpotentials for a series of olivine phosphates. Additionally, we found correlation between the adsorption energy of the systems with the total charge polarization of surface and adsorbate. To explain the computed trends, we analyzed the occupancies of the partial density of states of the Ni and Fe 3d states and Bader atomic charges.

Insights into the Mechanism and Kinetics of Thermo-Oxidative Degradation of HFPE High Performance Polymer.

The growing requisite for materials having high thermo-oxidative stability makes the design and development of high performance materials an active area of research. Fluorination of the polymer backbone is a widely applied strategy to improve various properties of the polymer, most importantly the thermo-oxidative stability. Many of these fluorinated polymers are known to have thermo-oxidative stability up to 700 K. However, for space and aerospace applications, it is important to improve its thermo-oxidative stability beyond 700 K. Molecular-level details of the thermo-oxidative degradation of such polymers can provide vital information to improve the polymer. In this spirit, we have applied quantum mechanical and microkinetic analysis to scrutinize the mechanism and kinetics of the thermo-oxidative degradation of a fluorinated polymer with phenylethenyl end-cap, HFPE. This study gives an insight into the thermo-oxidative degradation of HFPE and explains most of the experimental observations on the thermo-oxidative degradation of this polymer. Thermolysis of C-CF3 bond in the dianhydride component (6FDA) of HFPE is found to be the rate-determining step of the degradation. Reaction pathways that are responsible for the experimentally observed weight loss of the polymer is also scrutinized. On the basis of these results, we propose a modification of HFPE polymer to improve its thermo-oxidative stability.