Size Of Substituent Research Articles

Size and Electronic Effects on the Performance of (Corrolato)cobalt-Modified Electrodes for Oxygen Reduction Reaction Catalysis.

Considering the worldwide efforts for designing catalysts that are not based on platinum group metals while still reserving the many advantages thereof, this study focused on the many variables that dictate the performance of cathodes used for fuel cells, regarding the efficient and selective reduction of oxygen to water. This was done by investigating two kinds of porous carbon electrodes, modified by molecular cobalt(III) complexes chelated by corroles that differ very much in size and electron-withdrawing capability. Examination of the electronic effect uncovered shifts in the CoII/CoIII redox potentials and also large differences in the affinity of the cobalt center to external ligands. Spontaneous absorption of the catalysts was found to depend on the size of the corrole's substituents (C6F5 ≫ CF3 ≫ H) and the metal's axial ligands (PPh3 versus pyridine), as well as on the porosity of the carbon electrodes (BP2000 > Vulcan). The better-performing cobalt-based catalysts were almost as active and selective as 20% platinum on Vulcan in terms of the onset potential and the only 2-10% undesirable formation of hydrogen peroxide. Durability was also addressed by using the best-performing modified cathode in a proper anion-exchange membrane fuel cell setup, revealing very little voltage change during 12 h of operation.

Diastereofacial selectivity of α-aminonitrile formation from substituted cyclohexanones

The formation of α-aminonitriles from substituted cyclohexanones exhibits diastereofacial selectivity resulting in unequal amounts of the two possible geometric isomers. The degree of selectivity increases with the nature of the substitution on the cyclohexyl ring indicating that this likely results from the degree to which the substitution can ‘lock’ the conformation of the cyclohexyl ring into the most stable conformation. The ratio of trans:cis isomers seen for the reaction of 4-substituted cyclohexanones with dimethylamine hydrochloride and potassium cyanide in water were 92:8 for 4-phenyl, 81:19 for 4-t-butyl, and 62:38 for 2-methyl. Varying the size of the amino group substituents did not have a dramatic effect on the selectivity that was observed.

Pteridine reductase (PTR1): initial structure-activity relationships studies of potential leishmanicidal arylindole derivatives compounds

ABSTRACT Leishmaniasis is a public health concern, especially in Brazil and India. The drugs available for therapy are old, cause toxicity and have reports of resistance. Therefore, this paper aimed to carry out initial structure-activity relationships (applying molecular docking and dynamic simulations) of arylindole scaffolds against the pteridine reductase (PTR1), which is essential target for the survival of the parasite. Thus, we used a series of 43 arylindole derivatives as a privileged skeleton, which have been evaluated previously for different biological actions. Compound 7 stood out among its analogues presenting the best results of average number of interactions with binding site (2.00) and catalytic triad (1.00). Additionally, the same compound presented the best binding free energy (−32.33 kcal/mol) in dynamic simulations. Furthermore, with computational studies, it was possible to comprehend and discuss the influences of the substituent sizes, positions of substitutions in the aromatic ring and electronic influences. Therefore, this study can be a starting point for the structural improvements needed to obtain a good leishmanicidal drug.

Peculiarities of some Fmoc-dipeptides gelation in DMSO/water medium

The possibility of the gels formation with a high-water content of 90% v/v using Fmoc-dipeptides without mechanical actions or pH-switching was demonstrated. For the first time, the formation of a colloid gel based on Fmoc-LL dipeptide was discovered. The effect of side substituents in the amino acid residues of Fmoc-dipeptides in the range of GG, AA, LL, and FF on their ability to form gels in DMSO/water systems was thoroughly studied. Non-linear dependence of gel formation kinetics in dipeptide/DMSO/water systems, gel properties, and supramolecular organization in xerogels on the size of side substituents was observed. The results obtained give a new insight into the mechanism of gelation with low-molecular-weight gelators based on Fmoc-dipeptides and may be useful for the preparation of new supramolecular or colloidal gels.

N,N-Bis(2,4-Dibenzhydryl-6-cycloalkylphenyl)butane-2,3-diimine-Nickel Complexes as Tunable and Effective Catalysts for High-Molecular-Weight PE Elastomers.

Four examples of N,N-bis(aryl)butane-2,3-diimine-nickel(II) bromide complexes, [ArN=C(Me)-C(Me)=NAr]NiBr2 (where Ar = 2-(C5H9)-4,6-(CHPh2)2C6H2 (Ni1), Ar = 2-(C6H11)-4,6-(CHPh2)2C6H2 (Ni2), 2-(C8H15)-4,6-(CHPh2)2C6H2 (Ni3) and 2-(C12H23)-4,6-(CHPh2)2C6H2 (Ni4)), disparate in the ring size of the ortho-cycloalkyl substituents, were prepared using a straightforward one-pot synthetic method. The molecular structures of Ni2 and Ni4 highlight the variation in the steric hindrance of the ortho-cyclohexyl and -cyclododecyl rings exerted on the nickel center, respectively. By employing EtAlCl2, Et2AlCl or MAO as activators, Ni1-Ni4 displayed moderate to high activity as catalysts for ethylene polymerization, with levels falling in the order Ni2 (cyclohexyl) > Ni1 (cyclopentyl) > Ni4 (cyclododecyl) > Ni3 (cyclooctyl). Notably, cyclohexyl-containing Ni2/MAO reached a peak level of 13.2 × 106 g(PE) of (mol of Ni)-1 h-1 at 40 °C, yielding high-molecular-weight (ca. 1 million g mol-1) and highly branched polyethylene elastomers with generally narrow dispersity. The analysis of polyethylenes with 13C NMR spectroscopy revealed branching density between 73 and 104 per 1000 carbon atoms, with the run temperature and the nature of the aluminum activator being influential; selectivity for short-chain methyl branches (81.8% (EtAlCl2); 81.1% (Et2AlCl); 82.9% (MAO)) was a notable feature. The mechanical properties of these polyethylene samples measured at either 30 °C or 60 °C were also evaluated and confirmed that crystallinity (Xc) and molecular weight (Mw) were the main factors affecting tensile strength and strain at break (εb = 353-861%). In addition, the stress-strain recovery tests indicated that these polyethylenes possessed good elastic recovery (47.4-71.2%), properties that align with thermoplastic elastomers (TPEs).

Substituent Effects on Torsional Strain in Cyclopentene Derivatives: A Computational Study.

Density functional theory calculations were used to create a library of ring strain energies (RSEs) for 73 cyclopentene derivatives with potential use as monomers for ring-opening metathesis polymerization (ROMP). An overarching goal was to probe how substituent choice may influence torsional strain, which is the driving force for ROMP and one of the most understudied types of RSEs. Potential trends investigated include substituent location, size, electronegativity, hybridization, and steric bulk. Using traditional and recently developed homodesmotic equations, our results show that the size and substitution (bulk) of the atom directly bonded to the ring have the greatest influence on torsional RSE. A complex interplay between bond length, bond angle, and dihedral angle dictates the relative eclipsed conformations between the substituent and its neighboring hydrogens and was found to be responsible for the notable differences in RSEs. Furthermore, substituents placed on the homoallylic position resulted in higher RSEs than the same substituent placed on the allylic position due to increased eclipsing interactions. Different levels of theory were also assessed, and it was determined that consideration of electron correlation in calculations increased RSEs by ∼2-5 kcal mol-1. Further increasing the level of theory did not significantly change RSEs, indicating that the increased computational cost and time may not be necessary for improved accuracy.

Insights into substitution strategy towards thermodynamic and property regulation of chemically recyclable polymers

The development of chemically recyclable polymers serves as an attractive approach to address the global plastic pollution crisis. Monomer design principle is the key to achieving chemical recycling to monomer. Herein, we provide a systematic investigation to evaluate a range of substitution effects and structure−property relationships in the ɛ-caprolactone (CL) system. Thermodynamic and recyclability studies reveal that the substituent size and position could regulate their ceiling temperatures (Tc). Impressively, M4 equipped with a tert-butyl group displays a Tc of 241 °C. A series of spirocyclic acetal-functionalized CLs prepared by a facile two-step reaction undergo efficient ring-opening polymerization and subsequent depolymerization. The resulting polymers demonstrate various thermal properties and a transformation of the mechanical performance from brittleness to ductility. Notably, the toughness and ductility of P(M13) is comparable to the commodity plastic isotactic polypropylene. This comprehensive study is aimed to provide a guideline to the future monomer design towards chemically recyclable polymers.

Expanding the substrate scope of pyrrolysyl-transfer RNA synthetase enzymes to include non-α-amino acids in vitro and in vivo

The absence of orthogonal aminoacyl-transfer RNA (tRNA) synthetases that accept non-l-α-amino acids is a primary bottleneck hindering the in vivo translation of sequence-defined hetero-oligomers and biomaterials. Here we report that pyrrolysyl-tRNA synthetase (PylRS) and certain PylRS variants accept α-hydroxy, α-thio and N-formyl-l-α-amino acids, as well as α-carboxy acid monomers that are precursors to polyketide natural products. These monomers are accommodated and accepted by the translation apparatus in vitro; those with reactive nucleophiles are incorporated into proteins in vivo. High-resolution structural analysis of the complex formed between one PylRS enzyme and a m-substituted 2-benzylmalonic acid derivative revealed an active site that discriminates prochiral carboxylates and accommodates the large size and distinct electrostatics of an α-carboxy substituent. This work emphasizes the potential of PylRS-derived enzymes for acylating tRNA with monomers whose α-substituent diverges substantially from the α-amine of proteinogenic amino acids. These enzymes or derivatives thereof could synergize with natural or evolved ribosomes and/or translation factors to generate diverse sequence-defined non-protein heteropolymers.

Construction of 2D metal-organic precursor networks with tunable porosity: A Monte Carlo simulation approach

Two-dimensional network structures comprising functional organic building blocks have been recently recognized as promising platforms for various physio-chemical processes running in confined spaces. In this study, we present theoretical results of the self-assembly of metal-organic precursor networks on a crystalline surface and provide quantitative characteristics of these planar architectures. To that end, a coarse-grained model of the adsorbed overlayer comprising halogenated polyaromatic aromatic hydrocarbon (PAH) molecules and bivalent metal atoms was used in combination with the equilibrium Monte Carlo (MC) simulation method. Our calculations focused on the effect of molecular features, such as size, shape and distribution of halogen substituents in the PAH monomer, on the quality and morphology of the networks formed on a model (111) crystalline surface. For the simulated porous assemblies, representing the precursor stage of the surface-assisted Ullmann coupling reaction, such descriptors as the unit cell parameters, radial distribution functions, pore size distributions and structure factors were determined. The obtained results demonstrated that fine-tuning of the monomeric PAH units enables the creation of periodic and aperiodic networks having diverse porous properties, ranging from monoporous to heteroporous with bi- and multimodal pore-size distributions. The findings of our theoretical investigations, apart from the potential application in the optimization of 2D polymeric networks, can also be helpful in designing real confined nanoenvironments for selective capture and immobilization/transformation of guest species.

Structural Quantification of the Surface-Confined Metal-Organic Precursors Simulated with the Lattice Monte Carlo Method.

The diversity of surface-confined metal-organic precursor structures, which recently have been observed experimentally, poses a question of how the individual properties of a molecular building block determine those of the resulting superstructure. To answer this question, we use the Monte Carlo simulation technique to model the self-assembly of metal-organic precursors that precede the covalent polymerization of halogenated PAH isomers. For this purpose, a few representative examples of low-dimensional constructs were studied, and their basic structural features were quantified using such descriptors as the orientational order parameter, radial distribution function, and one- and two-dimensional structure factors. The obtained results demonstrated that the morphology of the precursor (and thus the subsequent polymer) could be effectively tuned by a suitable choice of molecular parameters, including size, shape, and intramolecular distribution of halogen substituents. Moreover, our theoretical investigations showed the effect of the main structural features of the precursors on the related indirect characteristics of these constructs. The results reported herein can be helpful in the custom designing and characterization of low-dimensional polymers with adjustable properties.

Rhodium-Catalyzed Alkenylation of Arenes with Multi-Substituted Olefins: Comparison of Selectivity and Reaction Rate as a Function of Olefin Identity

Rhodium-catalyzed arene alkenylation using Cu(II) carboxylates as the in situ oxidant and mono-substituted olefins has been previously reported (e.g., J. Am. Chem. Soc. 2019, 139, 5474; J. Am. Chem. Soc. 2018, 140, 17007; Organometallics 2019, 38, 3860; J. Am. Chem. Soc. 2020, 142, 10534). Herein, studies are extended to multi-substituted olefins with the goal of evaluating the effect of olefin substitution pattern and substituent identity on selectivity and turnover frequency. The influence of olefin substitution is probed by comparing the conversion of benzene to alkenyl arenes with ethylene, propylene, 1-butene, cis-2-butene, trans-2-butene, isobutene, 2-methyl-2-butene, and tetramethylethylene as well as the phenyl-substituted olefins and isomers of propenylbenzene. The rate of oxidative hydrophenylation for multi-substituted olefins follows the trend monosubstituted > disubstituted > trisubstituted, and tetrasubstituted olefins are unreactive. To probe the effect of substituent size on Markovnikov/anti-Markovnikov regioselectivity, cyclohexyl, tert-butyl, isopropyl, ethyl, and methyl substituted α-olefins are compared. Selectivity for anti-Markovnikov products generally increases as substituent steric bulk is increased. Tolerance for some functionalized olefins is demonstrated. The ortho/meta/para regioselectivity with mono-substituted arenes reveals that arene and olefin identity influences selectivity. Further mechanistic studies provide evidence for Curtin–Hammet control of ortho/meta/para regioselectivity with monosubstituted arenes.

Neuro-Evolutive Modeling of Transition Temperatures for Five-Ring Bent-Core Molecules Derived from Resorcinol

Determining the phase transition temperature of different types of liquid crystals based on their structural parameters is a complex problem. The experimental work might be eliminated or reduced if prediction strategies could effectively anticipate the behavior of liquid crystalline systems. Neuro-evolutive modeling based on artificial neural networks (ANN) and a differential evolution (DE) algorithm was applied to predict the phase transition temperatures of bent-core molecules based on their resorcinol core. By these means, structural parameters such as the nature of the linking groups, the position, size and number of lateral substituents on the central core or calamitic wings and the length of the terminal chains were taken into account as factors that influence the liquid crystalline properties. A number of 172 bent-core compounds with symmetrical calamitic wings were selected from the literature. All corresponding structures were fully optimized using the DFT, and the molecular descriptors were calculated afterward. In the first step, the ANN-DE approach predicted the mesophase presence for the analyzed compounds. Next, ANN models were determined to predict the transition temperatures and whether or not the bent-core compounds were mesogenic. Simple structural, thermophysical and electronic structure descriptors were considered as inputs in the dataset. As a result, the models determined for each individual temperature have an R2 that varied from 0.89 to 0.98, indicating their capability to estimate the transition temperatures for the selected compounds. Moreover, the impact analysis of the inputs on the predicted temperatures showed that, in most cases, the presence or not of liquid crystalline properties represents the most influential feature.

The role of lateral substituent size in azobenzene chromophores on photo- and chiro-optical properties of photoactive polymethacrylates

Properties of functional liquid crystalline (LC) macromolecular materials, especially those showing self-organizing behaviour and targeted for specific purposes, strongly rely on the molecular structure of the mesogens used as side-chains. In order to get inside this relationship, new series of chiral LC polymethacrylates (PMA6Azo-R) with azobenzene groups having different lateral substituents (R), namely the methyl (-Me) and ethyl (-Et) groups on the central benzene ring, were synthesized and their comparative study was carried out. The synthesized polymers consist of photochromic azobenzene fragments that differ in the size of the lateral substituent (R = -H, -Me, -Et). All polymers exhibit the cholesteric phase, whereas the polymer without lateral alkyl group in addition forms the low-temperature SmĈ* phase. Investigations of the photoorientation process under linearly polarised visible light irradiation reveal that an increase in the size of side substituent considerably reduces the values of maximum achievable photoinduced dichroism (from 0.89 for PMA6Azo-H to 0.33 for PMA6Azo-Et that is associated with a decrease in the degree of mesogen anisometry). The effect of the lateral substituent size in azobenzene photochrome on the chiro-optical properties and mesomorphic behaviour of studied polymers was established. The presence of the lateral substituent in the photochrome/mesogen fragment leads to a significant decrease in isotropization temperature. Chiro-optical properties of the polymers and their photoswitching were studied, and it was found, that the introduction of the lateral substituent in the azobenzene fragment prevents the formation of exciton couples, that leads to a decrease in the circular dichroism (CD) values. UV irradiation and E-Z isomerisation of the azobenzene chromophores results in complete disappearance of CD signal; subsequent visible light irradiation does not lead to its full recovery. On the other hand, annealing of the films at temperatures higher than the glass transition almost fully recovers the initial CD values. The designed polymethacrylates can be considered as promising materials for various photonic applications.

Regio- and Enantioselective Allylic Cyanomethylation by Synergistic Rhodium and Silane Catalysis.

Rh/silane-cocatalyzed regio- and enantioselctive allylic cyanomethylation with inert acetonitrile directly has been developed. Addition of a catalytic amount neutral silane reagent as an acetonitrile anion carrier is essential for the success of this reaction. The synthesis of mono- and bis-allylation products can be switched by adjusting the size of substituents on the silane, ligands, and temperature. Chiral homoallylic nitriles could be synthesized in above 20:1 branch/linear ratio, up to 98% yield and >99% ee.

Unraveling abnormal buried interface anion defect passivation mechanisms depending on cation-induced steric hindrance for efficient and stable perovskite solar cells

Although ionic liquids (ILs) have been widely employed to heal the defects in perovskite solar cells (PSCs), the corresponding defect passivation mechanisms are not thoroughly understood up to now. Herein, we first reveal an abnormal buried interface anion defect passivation mechanism depending on cation-induced steric hindrance. The IL molecules containing the same anion ([BF4]-) and different sizes of imidazolium cations induced by substituent size are used to manipulate buried interface. It was revealed what passivated interfacial defects is mainly anions instead of cations. Theoretical and experimental results demonstrate that the large-sized cations can weaken the ionic bond strength between anions and cations, and facilitate the interaction between anions and SnO2 as well as perovskites, which is conducive to interfacial defect passivation and ameliorating interfacial contact. It can be concluded that interfacial chemical interaction strength and defect passivation effect are positively correlated with the size of cations. The discovery breaks conventional thinking that large-sized modification molecules would weaken their chemical interaction with perovskite. Compared with the control device (21.54%), the device based on 1,3-Bis(1-adamantyl)-imidazolium tetrafluoroborate (BAIMBF4) with maximum size cations achieves a significantly enhanced efficiency of 23.61% along with much increased moisture, thermal and light stabilities.

Nematic liquid crystalline polymer films for gas separation

ABSTRACT The gas separation performances of free-standing planar aligned nematic LC polymer films were investigated for gas separations of He, CO2, CH4 and Xe. The films consist of derivatives of 1,4-phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate)s with respective cyano, chloro, methyl and phenyl substituents on the central aromatic cores. Two new LC derivatives of 1,4-phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate)s were successfully synthesised and fully characterised. Single gas permeation and sorption data show increasing gas permeabilities with increasing steric size of the substituents while the ideal gas selectivity of He over CH4 and He over CO2 decreases. The sorption coefficient of all films is independent of the LC substituents, while the subsequently extracted diffusion coefficient for the films with a phenyl substituent is three times higher compared to the films with a cyano substituent, demonstrating that the steric size of the LC substituents mainly affects the diffusion of gasses rather than the solubility of the gases. Irrespective of a methyl or a phenyl substituent, a larger kinetic diameter of Xe gives a 20 times lower diffusion coefficient compared to the smaller species (CO2).

Exploring the Limits of Intramolecular London Dispersion Stabilization with Bulky Dispersion Energy Donors in Alkane Solution

We present an experimental study of a cyclooctatetraene-based molecular balance disubstituted with increasingly bulky tert-butyl (tBu), adamantyl (Ad), and diamantyl (Dia) substituents in the 1,4-/1,6-positions for which we determined the valence-bond shift equilibrium in n-hexane (hex), n-octane (oct), and n-dodecane (dod). Computations including implicit and explicit solvation support our temperature-dependent NMR equilibrium measurements indicating that the more sterically crowded 1,6-isomer is always favored, irrespective of solvent, and that the free energy is quite insensitive to substituent size.

Reactivity of triphosphinoboranes towards H3B·SMe2: access to derivatives of boraphosphacycloalkanes with diverse substituents.

Triphosphinoboranes activated the B-H bond in the BH3 molecule without any catalysts at room temperature. Hydroboration reactions led to boraphosphacyloalkanes with diverse structures. The outcomes of reactions depend on the size of the phosphanyl substituent on the boron atom of the parent triphosphinoborane, where derivatives of boraphosphacyclobutane and boraphosphacyclohexane were obtained. Furthermore, the precursor of triphosphinoboranes, namely bromodiphosphinoborane, also exhibited high reactivity towards H3B·SMe2, yielding bromo-substituted boraphosphacyclobutane. The obtained products were characterized by heteronuclear NMR spectroscopy, single crystal X-ray diffraction, and elemental analysis.

Fullerene Complexation in a Hydrogen-Bonded Porphyrin Receptor via Induced-Fit: Cooperative Action of Tautomerization and C-H···π Interactions.

A supramolecular chiral hydrogen-bonded tetrameric aggregate possessing a large cavity and tetraarylporphyrin substituents was assembled using alternating 4H- and 2H-bonds between ureidopyrimidinone and isocytosine units, respectively. The aggregation mode was rationally shifted from social to narcissistic self-sorting by changing urea substituent size only. The H-bonded tetramer forms a strong complex with C60 guest, at the same time undergoing remarkable structural changes. Namely, the cavity adjusts to the guest via keto-to-enol tautomerization of the ureidopyrimidinone unit and as a result, porphyrin substituents move apart from each other in a scissor blade-like opening fashion. The rearrangement is accompanied by C-H···π interaction between the alkyl solubilizing groups and the nearby placed porphyrin π-systems. The latter interaction was found to be crucial for the guest complexation event, providing energetic compensation for otherwise costly tautomerization. We showed that only the systems possessing sufficiently long alkyl chains capable of interacting with a porphyrin ring are able to form a complex with C60. The structural rearrangement of the tetramer was quantitatively characterized by electron paramagnetic resonance pulsed dipolar spectroscopy measurements using photogenerated triplets of porphyrin and C60 as spin probes. Further exploring the C-H···π interaction as a decisive element for the C60 recognition, we investigated the guest-induced self-sorting phenomenon using scrambled tetramer assemblies composed of two types of monomers possessing alkyl chains of different lengths. The presence of the fullerene guest has enabled the selective scavenging of monomers capable of C-H···π interaction to form homo-tetrameric aggregates.

The Phosphinate Group in the Formation of 2D Coordination Polymer with Sm(III) Nodes: X-ray Structural, Electrochemical and Mössbauer Study

A coordination polymer has been synthesized using ferrocene-based ligand-bearing phosphinic groups of 1,1'-ferrocene-diyl-bis(H-phosphinic acid)), and samarium (III). The coordination polymer's structure was studied by both single-crystal and powder XRD, TG, IR, and Raman analyses. For the first time, the Mössbauer effect studies were performed on ferrocenyl phosphinate and the polymer based on it. Additionally, the obtained polymer was studied by the method of cyclic and differential pulse voltammetry. It is shown that it has the most positive potential known among ferrocenyl phosphinate-based coordination polymers and metal-organic frameworks. Using the values of the oxidation potential, the polymer was oxidized and the ESR method verified the oxidized Fe(III) form in the solid state. Additionally, the effect of the size of the phosphorus atom substituent of the phosphinate group on the dimension of the resulting coordination compounds is shown.