Steric Congestion Research Articles

Efficient Selective Sorting of Semiconducting Carbon Nanotubes Using Ultra-Narrow-Band-Gap Polymers.

Conjugated polymers with narrow band gaps are particularly useful for sorting and discriminating semiconducting single-walled carbon nanotubes (s-SWCNT) due to the low charge carrier injection barrier for transport. In this paper, we report two newly synthesized narrow-band-gap conjugated polymers (PNDITEG-TVT and PNDIC8TEG-TVT) based on naphthalene diimide (NDI) and thienylennevinylene (TVT) building blocks, decorated with different polar side chains that can be used for dispersing and discriminating s-SWCNT. Compared with the mid-band-gap conjugated polymer PNDITEG-AH, which is composed of naphthalene diimide (NDI) and head-to-head bithiophene building blocks, the addition of a vinylene linker eliminates the steric congestion present in head-to-head bithiophene, which promotes backbone planarity, extending the π-conjugation length and narrowing the band gap. Cyclic voltammetry (CV) and density functional theory (DFT) calculations suggest that inserting a vinylene group in a head-to-head bithiophene efficiently lifts the highest occupied molecular orbital (HOMO) level (−5.60 eV for PNDITEG-AH, −5.02 eV for PNDITEG-TVT, and −5.09 eV for PNDIC8TEG-TVT). All three polymers are able to select for s-SWCNT, as evidenced by the sharp transitions in the absorption spectra. Field-effect transistors (FETs) fabricated with the polymer:SWCNT inks display p-dominant properties, with higher hole mobilities when using the NDI-TVT polymers as compared with PNDITEG-AH (0.6 cm2 V–1 s–1 for HiPCO:PNDITEG-AH, 1.5 cm2 V–1 s–1 for HiPCO:PNDITEG-TVT, and 2.3 cm2 V–1 s–1 for HiPCO:PNDIC8TEG-TVT). This improvement is due to the better alignment of the HOMO level of PNDITEG-TVT and PNDIC8TEG-TVT with that of the dominant SWCNT specie.

Study of Pd-catalyzed Selective Mono- and Di-C(sp3)-H Bond Activation: A Bi-ligand Model.

Controlling the number of C-H bond activation is a long-standing challenge in organic synthesis. Recently, Yu's group demonstrated that in Pd-catalyzed alanine's arylation, pyridine-type ligands favor a mono-C-H bond activation, while quinoline-type ligands favor a di-C-H bond activation. To disclose the underlying principles, a theoretical study (density functional theory (DFT)) has been carried out. Our study indicates that a mono-ligand model, which is generally adopted in the community, does not reproduce the experimentally observed mono-/di-selectivity, while a bi-ligand model can rationalize the experimental observations well, including the observed diastereoselectivity in diarylation. The electron-rich pyridine-type ligands with less steric congestion can promote the C-H bond activation reaction of alanine derivatives. The quinoline-type ligands have a better π back-donation interaction with the metal, which makes a more active C-H bond activation than the pyridine-type ligands for this reaction. This bi-ligand model, which is a necessity, allows the understanding and future design of a dual ligand effect in C-H bond activation.

Ortho-substituent effect on metal coordination geometry, chain microstructure and polymer properties in α-diimine nickel-catalyzed long chain α-Olefin polymerization

The tuning of polyolefin microstructure, including the branching density and branching distribution, through the ligand modification in transition metal-based catalysts has received considerable interest. In this work, we present the synthesis and characterization of a series of α-diimine Ni(II) catalysts containing bulky diarylmethyl moiety and varied steric substituents, and a systematic investigation on the catalytic behavior of these catalyst for 1-octene and 1-decene polymerization. Ni3 with ortho-diisopropyl substituents adopts a common distorted tetrahedral geometry, while a rare square planar geometry is observed for Ni4 with ortho-phenyl substituents. The ortho-substituent effect on the preference of insertion fashion (1,2 or 2,1-insertion) and chain-walking in long chain α-olefin polymerization is also established. Highly branched amorphous polyolefin can be obtained by the unsymmetrical catalyst with ortho-alkyl substituents (H, Me, iPr) in which the major 1,2-insetion pathway was observed, whereas moderate branched semicrystalline polyolefin can be produced by the unsymmetrical catalyst with ortho-phenyl substituents, which showed a significant increase in 2,1-insertion. Most interestingly, the branching distribution analysis through 13C NMR indicated that the polymer obtained by Ni3 gave the higher percentage of long-chain branches relative to methyl branches, while Ni4 afforded the polymer with methyl branches being predominant. These significant differences in coordination geometry and catalytic behavior between Ni3 and Ni4 may be associated with the unique steric congestion of the ortho-phenyl substituents. In particular, the comparative investigation of the mechanical properties of the polymers obtained by Ni3 and Ni4 was also performed.

Helically Twisted Nanoribbons Based on Emissive Near-Infrared Responsive Quaterrylene Bisimides.

Graphene nanoribbons (GNRs) have the potential for next-generation functional devices. So far, GNRs with defined stereochemistry are rarely reported in literature and their optical response is usually bound to the ultraviolet or visible spectral region, while covering the near-infrared (NIR) regime is still challenging. Herein, we report two novel quaterrylene bisimides with either one- or twofold-twisted π-backbones enabled by the steric congestion of a fourfold bay arylation leading to an end-to-end twist of up to 76°. The strong interlocking effect of the π-stacked aryl substituents introduces a rigidification of the chromophore unambiguously proven by single-crystal X-ray analysis. This leads to unexpectedly strong NIR emissions at 862 and 903 nm with quantum yields of 1.5 and 0.9%, respectively, further ensuring high solubility as well as resolvable and highly stable atropo-enantiomers. Circular dichroism spectroscopy of these enantiopure chiral compounds reveals a strong Cotton effect Δε of up to 67 M-1 cm-1 centered far in the NIR region at 849 nm.

C–H⋯S Hydrogen Bond Assisted Supramolecular Encapsulation of Fullerenes with Nanobelts

C–H⋯S Hydrogen Bond Assisted Supramolecular Encapsulation of Fullerenes with Nanobelts

Confinement‐Induced Selectivities in Gold(I) Catalysis—The Benefit of Using Bulky Tri‐(ortho‐biaryl)phosphine Ligands

The confinement of a catalytic site is an efficient strategy to control a reaction and modulate its selectivity. In the present work, a new class of structurally simple and easily accessible bulky tri-(ortho-biaryl)phosphine ligands were accessed, and their gold(I) complexes synthesized. Their X-ray diffraction analysis and the comparative evaluation of their VBur % and G steric parameters against a series of gold complexes commonly employed in catalysis demonstrated their confined nature. Despite their notable steric congestion, these complexes exhibited remarkable catalytic activities and unusual selectivities, both in nature and level, that make them unique in the field of synthetic homogeneous gold catalysis.

Confinement‐Induced Selectivities in Gold(I) Catalysis—The Benefit of Using Bulky Tri‐(ortho‐biaryl)phosphine Ligands

AbstractThe confinement of a catalytic site is an efficient strategy to control a reaction and modulate its selectivity. In the present work, a new class of structurally simple and easily accessible bulky tri‐(ortho‐biaryl)phosphine ligands were accessed, and their gold(I) complexes synthesized. Their X‐ray diffraction analysis and the comparative evaluation of their VBur% and G steric parameters against a series of gold complexes commonly employed in catalysis demonstrated their confined nature. Despite their notable steric congestion, these complexes exhibited remarkable catalytic activities and unusual selectivities, both in nature and level, that make them unique in the field of synthetic homogeneous gold catalysis.

Diastereoselective Indole-Dearomative Cope Rearrangements by Compounding Minor Driving Forces.

Reported herein is the discovery of a diastereoselective indole-dearomative Cope rearrangement. A suite of minor driving forces promote dearomatization: (i) steric congestion in the starting material, (ii) alkylidene malononitrile and stilbene conjugation events in the product, and (iii) an unexpected intramolecular π-π* stack on the product side of the equilibrium. The key substrates are rapidly assembled from simple starting materials, resulting in many successful examples. The products are structurally complex and bear vicinal stereocenters generated by the dearomative Cope rearrangement. They also contain a variety of functional groups for interconversion to complex architectures.

Titanium Complexes of Salicylbenzoxazole and Salicylbenzothiazole Ligands for the Ring-Opening Polymerization of ε-Caprolactone and Substituted ε-Caprolactones and Their Copolymerizations.

Two series of titanium complexes, including salicylbenzoxazole titanium complexes (1-4) and salicylbenzothiazole titanium complexes (5-8), were successfully synthesized and characterized by NMR spectroscopy, elemental analysis, and X-ray diffraction crystallography (for 2 and 5). The 1H NMR spectra of complexes 7 and 8 reveal fluxional behavior in solution at room temperature, and the activation parameters were determined by lineshape analysis of variable-temperature (VT) NMR spectra in toluene-d8: for 7, ΔH⧧ = 73.0 ± 1.8 kJ mol-1, ΔS⧧ = 22.1 ± 5.5 J mol-1 K-1; for 8, ΔH⧧ = 73.7 ± 1.2 kJ mol-1, ΔS⧧ = 20.3 ± 3.8 J mol-1 K-1. The positive values of ΔS⧧ suggested that the isomerization occurred via a dissociative mechanism. All complexes were active initiators for the ring-opening polymerization of ε-caprolactone (ε-CL) and three substituted ε-CLs: γ-methyl-ε-caprolactone (γMeCL), γ-ethyl-ε-caprolactone (γEtCL), and γ-phenyl-ε-caprolactone (γPhCL). Of all complexes, complex 5 was found to be the most active initiator in this study. The copolymerizations between ε-CL and three substituted ε-CLs produced completely random copolymers. The polymerization was proposed to proceed via a dissociative coordination-insertion mechanism. The catalytic activity of the salicylbenzoxazole titanium complex was lower than that of its closely related salicylbenzothiazole titanium congener. Additionally, DFT calculations unveiled that the ligand decoordination step and the less steric congestion at the titanium center in the salicylbenzothiazole titanium complexes were the key factors in enhancing the catalytic rate.

Radical-Mediated Functionalization of Internal Alkenes: Synthesis of Multisubstituted Allylic and Homoallylic Azides.

Radical-mediated functionalization of alkenes provides a powerful tool for transformation of simple alkenes into numerous value-added products. The precedent radical functionalization of alkenes is mainly restricted to terminal alkenes, while the conversion of internal alkenes generally remains challenging, as the increased steric congestion on alkenes significantly conflicts with the intermolecular addition of radicals. Herein, we describe an efficient photoredox catalytic functionalization of internal trisubstituted alkenes, leading to a plethora of valuable multifunctionalized allylic and homoallylic azides, which are otherwise difficult to obtain. The azide products serve as versatile feedstock for construction of useful heterocycles. Allylic or homoallylic azides are selectively generated in the transformation, regulated by the regioselective deprotonation process. This method also features mild reaction conditions and high product diversity.

A High-Throughput Approach to Repurposing Olefin Polymerization Catalysts for Polymer Upcycling.

Efficient and economical plastic waste upcycling relies on the development of catalysts capable of polymer degradation. A systematic high-throughput screening of twenty-eight polymerization catalyst precursors, belonging to the catalyst families of metallocenes, ansa-metallocenes, and hemi- and post-metallocenes, in cis-1,4-polybutadiene (PB) degradation reveals, for the first time, important structure-activity correlations. The upcycling conditions involve activation of the catalysts (at 0.18 % catalyst loading) with tri-iso-butyl aluminum at 50 °C in toluene. The data indicate the ability to degrade PB is a general reactivity profile of neutral group 4 metal hydrides. A simple quantitative-structure activity relationship (QSAR) model utilizing two descriptors for the distribution of steric bulk in the active pocket and one measuring the metal ion electrophilicity reveals the degradation ability improves with increased but not overbearing steric congestion and lower electrophilicity.

A High‐Throughput Approach to Repurposing Olefin Polymerization Catalysts for Polymer Upcycling

AbstractEfficient and economical plastic waste upcycling relies on the development of catalysts capable of polymer degradation. A systematic high‐throughput screening of twenty‐eight polymerization catalyst precursors, belonging to the catalyst families of metallocenes, ansa‐metallocenes, and hemi‐ and post‐metallocenes, in cis‐1,4‐polybutadiene (PB) degradation reveals, for the first time, important structure–activity correlations. The upcycling conditions involve activation of the catalysts (at 0.18 % catalyst loading) with tri‐iso‐butyl aluminum at 50 °C in toluene. The data indicate the ability to degrade PB is a general reactivity profile of neutral group 4 metal hydrides. A simple quantitative‐structure activity relationship (QSAR) model utilizing two descriptors for the distribution of steric bulk in the active pocket and one measuring the metal ion electrophilicity reveals the degradation ability improves with increased but not overbearing steric congestion and lower electrophilicity.

Chemistry of the Bis(imine)-Based Tetradentate Ligand Stabilized Group 14 E(II) Cations (E=Ge and Sn).

The ambiphilic Ge(II) and Sn(II) cationic species have been reported to be isolated through kinetic or thermodynamic stabilizations. Nonetheless, steric congestion or excessive coordination of donor atoms to the cationic center concurrently disfavors its prompt reactivity. Our research in this field revolves around the utilization of structurally non-rigid bis(imine) based tetradentate supporting ligands for the stabilization of Ge(II) and Sn(II) cationic species. Such E(II) cationic systems have been advantaged due to inherent flexibility present at the ligand backbone allowing disposal of E(II) orbitals through geometric rearrangements for further reactivity. The bifunctionality present in the ligand enables the first examples of Ge(II) bis-monocations. Furthermore, the redox-active nature of the ligand encourages participation in chemical transformations. In this personal account we have provided a detailed discussion of our published work in this direction in the last five years.

Mechanistically Guided Workflow for Relating Complex Reactive Site Topologies to Catalyst Performance in C-H Functionalization Reactions.

Leveraging congested catalyst scaffolds has emerged as a key strategy for altering innate substrate site-selectivity profiles in C-H functionalization reactions. Similar to enzyme active sites, optimal small molecule catalysts often feature reactive cavities tailored for controlling substrate approach trajectories. However, relating three-dimensional catalyst shape to reaction output remains a formidable challenge, in part due to the lack of molecular features capable of succinctly describing complex reactive site topologies in terms of numerical inputs for machine learning applications. Herein, we present a new set of descriptors, "Spatial Molding for Approachable Rigid Targets" (SMART), which we have applied to quantify reactive site spatial constraints for an expansive library of dirhodium catalysts and to predict site-selectivity for C-H functionalization of 1-bromo-4-pentylbenzene via donor/acceptor carbene intermediates. Optimal site-selectivity for the terminal methylene position was obtained with Rh2(S-2-Cl-5-MesTPCP)4 (30.9:1 rr, 14:1 dr, 87% ee), while C-H functionalization at the electronically activated benzylic site was increasingly favored for Rh2(TPCP)4 catalysts lacking an ortho-Cl, Rh2(S-PTAD)4, and Rh2(S-TCPTAD)4, respectively. Intuitive global site-selectivity models for 25 disparate dirhodium catalysts were developed via multivariate linear regression to explicitly assess the contributing roles of steric congestion and dirhodium-carbene electrophilicity in controlling the site of C-H functionalization. The workflow utilizes spatial classification to extract descriptors only for reactive catalyst conformers, a nuance that may be widely applicable for establishing close correspondence between ground-state model systems and transition states. Broader still, SMART descriptors are amenable for delineating salient reactive site features to predict reactivity in other chemical and biological contexts.

A Mechanistic Study on the Non‐enzymatic Hydrolysis of Kdn Glycosides

AbstractSialic acids are biologically important carbohydrates that are prevalent throughout nature. We are interested in their intrinsic reactivity in aqueous solution and how such reactivity affects the design of substrates for investigation of enzymes that process these sugars. To probe the reactivity differences between two sialic acid family members N‐acetylneuraminic acid and Kdn we measured the rate constants for hydrolysis of 4‐nitrophenyl 3‐deoxy‐d‐glycero‐α‐d‐galacto‐non‐2‐ulosonide in aqueous solution. The kinetic data is consistent with glycosidic C−O bond cleavage occurring via four mechanistic pathways, and these are: (i) hydronium ion‐catalyzed hydrolysis of the neutral sugar; (ii) hydronium ion‐catalyzed hydrolysis of the glycosidic carboxylate; (iii) water‐catalyzed hydrolysis of the anionic glycoside; and (iv) base‐promoted reaction of the anionic glycoside. To study the effects of C‐5 substitution on the Kdn glycoside we made 4‐nitrophenyl 5‐O‐methyl‐α‐Kdn glycoside and determined its rate constants for hydrolysis. All hydrolytic rate constants for both Kdn glycosides were larger than those reported for the parent N‐acetyl‐α‐neuraminide. The water‐catalyzed reaction (pathway iii) exhibited a βlg value of −1.3±0.1. We conclude that the larger rate constants associated with C5‐oxygen containing sialosides results from less steric congestion at the hydrolytic transition states than for the parent C‐5 acetamido glycoside.

Synthesis of γ-Sultam-Annelated δ-Lactams via the Castagnoli-Cushman Reaction of Sultam-Based Dicarboxylic Acids.

An unusual type of highly reactive sultam-based dicarboxylic acids and correscponding anhydrides was employed in the Castagnoli-Cushman reaction delivering diastereomerically pure adducts at room temperature. Due to steric congestion, the initial adducts were prone to decarboxylation affording diastereomeric mixtures of bicyclic sultam lactams, separable by HPLC. The choice of a protecting group on the sultam nitrogen atom allows liberation of the NH-sultam, which is not only suitable for further modification but represents a known pharmacophore for carbonic anhydrase inhibition.

Cooperativity of steric bulk and H-bonding in coordination sphere engineering: heteroleptic PdII cages and bowls by design.

Recently developed self-assembly strategies allow to rationally reduce the symmetry of metallosupramolecular architectures. In addition, the combination of multiple ligand types without creating compound mixtures has become possible. Among several approaches to realize non-statistical heteroleptic assembly, Coordination Sphere Engineering (CSE) makes use of secondary repulsive or attractive interactions in direct vicinity of the metal nodes. Previously, we used steric congestion to turn dinuclear [Pd2L4] cages with fourfold symmetry into [Pd2L3X2] (X = solvent, halide) bowl structures. Here, we introduce a new subtype of this strategy based on balancing hydrogen bonding and repulsive interactions between ligands carrying quinoline (LQu) and 1,8-naphthyridine (LNa) donors to generate trans-[Pd2L2] and [Pd2L3L′] cages, assisted by templation of encapsulated fullerenes. Combined with steric congestion caused by acridine (LAc) donors, we further report the first example of a heteroleptic [Pd2L2L′X2] bowl. Formation, structure and fullerene binding ability of these metallo-supramolecular hosts were studied by NMR, mass spectrometry and single crystal X-ray diffraction.

Sugar decorated star-shaped (co)polymers with resveratrol-based core \u2013 physicochemical and biological properties

Star-shaped glycopolymers due to the attractive combination of the physicochemical, morphological, self-assembly properties along with biological activity have gained increased attention as innovative agents in novel cancer therapies. Unfortunately, the production of these highly desirable biomaterials remains a challenge in modern macromolecular chemistry. The main reason for that is the low polymerizability of ionic glycomonomers originated from their steric congestion and the occurrence of ionic interactions that generally negatively influence the polymerization progress and hinder controllable reaction pathway. In this work, the new ionic sugar monomer was (co)polymerized for the first time via Activator Generated by Electron Transfer Atom Transfer Radical Polymerization (AGET ATRP) using a three-arm resveratrol-based core to obtain star-like (co)polymers. The obtained products were examined in terms of their physicochemical properties and morphology. Aside from the synthesis of these new glycopolymers, also a thorough description of their thermal properties, ability to self-assembly, the formation of stable superstructures was studied in detail. It was found that examined (co)polymers did not show any heterogeneities and phase separation, while their variation of glass transition temperature (Tg) was strictly related to the change in the number of glycomonomer. Also, the stability and shapes of formed superstructures strictly depend on their composition and topology. Finally, we have shown that synthesized carbohydrate-based polymers revealed high antiproliferative activity against several cancer cell lines (i.e., breast, colon, glioma, and lung cancer). The cytotoxic activity was particularly observed for star-shaped polymers that were systematically enhanced with the growing concentration of amine moieties and molecular weight. The results presented herein suggest that synthesized star-shaped glyco(co)polymers are promising as drug or gene carriers in anticancer therapies or anti-tumor agents, depending on their cytotoxicity.Graphical abstract

Two 3D Cd(II) coordination polymers pillared by linear ligand: synthesis, structure and luminescent properties

Two layered-pillared Cd(II)-containing coordination polymers, [Cd2(BTC)(bpdh)2(HCOO)(H2O)·H2O]n (1) and [CdNa(BTC)(bpdh)(H2O)2·H2O]n (2) (H3BTC = 1,3,5-benzenetricarboxylic acid, bpdh = 2,5-bis-(3-pyrazinyl)-3,4-diaza-2,4-hexadiene) have been controlly synthesized under hydrothermal condition. In both complexes, the BTC3− ligands bridge metal ions to build 2D layers, and the linear bpdh ligands link the layers into 3D motifs. Compared with our earlier reports, it showed that the Cd2+ ion with larger radius can effectively reduce the steric overcrowding, which is very favorable for the construction of layered-pillared structure. Furthermore, the solid state luminescent properties of 1 and 2 have also been investigated.

Non-covalent interactions in aminoalcohols: Quantification of the intra-molecular hydrogen bond strength from the enthalpy of vaporization and quantum chemistry

Qualitatively, the intramolecular hydrogen bonding (intra-HB) in aminoalcohols a well-known phenomenon that is observed by spectroscopic methods. Quantitatively, the strength of this bonding cannot be measured directly. In this work, two thermodynamic tools related to the energetics of hydrogen bonding were developed to indirectly assess the intra-HB strength. The first one is the standard molar vaporization enthalpy at 298 K. We have compiled experimental data for 2-aminoethanol and 3-amino-1-propanol derivatives and evaluated the intra-HB strength in series of structurally various aminoalcohols. Starting with 2-aminoethanol and 3-amino-1-propanol as archetypal molecules for each series of derivatives, we then placed alkyl substituents directly near OH and (or) NH2 to understand the changes in strength due to increasing non-covalent steric overcrowding of the space between OH and NH2. It was also interesting to follow changes in intra-HB strength when we replaced the hydrogens in the NH2–group with the methyl groups. The HB-strength derived in this way was successfully validated with help of G4 quantum-chemical calculations. In the latter approach the intra-HB strength was defined as the difference between the energetics of the H-bonded and the non-bonded conformers of the appropriate aminoalcohol.