Pendant Amine Groups Research Articles

The Key Role of Proton‐Responsive Groups in Electrochemical Hydrogen Evolution Reaction

AbstractThe much‐needed global shift from fossil fuels to sustainable energy is driving significant attention towards hydrogen (H2) as a promising alternative. Proton reduction, a process central to H2 production, is a key area of research for this transition. Naturally‐occurring [FeFe] and [NiFe]‐hydrogenase enzymes play vital roles in the reversible production and oxidation of H2. These enzymes feature a proton‐relay unit comprising of pendant amine and thiol groups in the secondary coordination sphere at the active site. This unit accelerates the rate of H2 production/oxidation, making it a focal point for scientific exploration. Efforts are concentrated on mimicking the active sites of these enzymes both structurally and functionally. In this pursuit, many synthetic transition metal complexes with proton‐responsive units at the secondary coordination sphere of the active site mimic the enzyme's behavior. These units facilitate intramolecular metal‐hydride (M−H) generation and H2‐elimination via H+/H−s coupling, leveraging the proton from the pendant functional group and the hydride from the M−H intermediate. This review delves into electrocatalysts featuring pendant proton‐responsive units and their roles in the electrochemical hydrogen evolution reaction (eHER).

Cellulose nanofibers modified with pendant amine groups as potential reinforcement in bioepoxy for a mechanically tough biodegradable anticorrosive nanocomposite coating

Cellulose nanofibers modified with pendant amine groups as potential reinforcement in bioepoxy for a mechanically tough biodegradable anticorrosive nanocomposite coating

Effect of Variable Amine Pendants in the Secondary Coordination Sphere of Manganese Bipyridine Complexes on the Electrochemical CO2 Reduction

AbstractThe increasing concentration of CO2 in the atmosphere and its impact on the climate are matters of significant concern. Extensive research is being conducted on molecular catalysts to electrochemically reduce CO2 into valuable products to disrupt the unidirectional carbon flow. This study compares two manganese bipyridine catalysts, tailored with four or two benzylic diethylamine groups in the secondary coordination sphere. Either of these amine‐bearing scaffolds positioned close to the Mn center serves as effective proton relays to facilitate the formation of the corresponding Mn hydride intermediate. Alongside competitive H2 evolution, the reaction of this crucial intermediate with CO2 leads to formate. Our findings underscore the pronounced influence of external Brønsted acids on product selectivity. Notably, when employing the catalyst bearing four amine groups, the HCOO−/H2 ratio varies from 81 : 3 with 1.0 M iPrOH to 16 : 64 with 1.0 M PhOH, while the Mn complex adorned with two amine pendant groups consistently favors HCOO−, irrespective of the utilized proton sources. Infrared spectroelectrochemistry and density‐functional theory calculations unveil distinct disparities in the reactivity of the Mn hydrides toward CO2 due to the change of ligand bulkiness in the two cases. This work substantiates the importance of modulating spatial accessibility while modifying the second sphere encompassing molecular catalysts.

Exploring the effect of a pendent amine group poised over the secondary coordination sphere of a cobalt complex on the electrocatalytic hydrogen evolution reaction.

A CoIII complex (2) of a bispyridine-dioxime ligand (H2LNMe2) containing a tertiary amine group in the proximity of the Co center is synthesized and characterized. One of the oxime protons of the ligand is deprotonated, and the amine group remains protonated in the solid-state structure of the CoII complex (2a). The acid-base properties of 2 showed pKa values of 5.9, 8.4, and 9.6, which are assigned to the dissociation of two consecutive oxime protons and amine protons, respectively. The electrocatalytic proton reduction of 2 was investigated in an aqueous phosphate buffer solution (PBS), revealing a catalytic hydrogen evolution reaction (HER) at an Ecat/2 of -1.01 V vs. the SHE, with an overpotential of 673 mV and a kobs value of 2.6 × 103 s-1 at pH 7. For comparison, the HER of the Co complex (1) lacking the tert-amine group at the secondary sphere was investigated in PBS, which showed a kobs of 1.3 × 103 s-1 and an overpotential of 577 mV. At pH 4, however, 2 revealed a ∼3 times higher kobs value than 1, which suggests that the protonated amine group likely works as a proton relay site. Notably, no significant change in the reaction rate was observed at different pH values for 1, implying that oxime protons may not be involved in the intramolecular proton-coupled electron transfer reaction in the HER. The kobs values for Co complexes at pH 7.0 are significantly higher than those of the [Co(dmgH)2(pyridine)(Cl)] complex, implying that the primary coordination sphere around 1 or 2 enhances the HER and offers better catalyst stability in acidic buffer solutions.

Electrocatalytic Hydrogen Evolution using a Nickel-based Calixpyrrole Complex: Controlling the Secondary Coordination Sphere on an Electrode Surface.

Incorporating design elements from homogeneous catalysts to construct well defined active sites on electrode surfaces is a promising approach for developing next generation electrocatalysts for energy conversion reactions. Furthermore, if functionalities that control the electrode microenvironment could be integrated into these active sites it would be particularly appealing. In this context, a square planar nickel calixpyrrole complex, Ni(DPMDA) (DPMDA=2,2'-((diphenylmethylene)bis(1H-pyrrole-5,2-diyl))bis(methaneylylidene))bis(azaneylylidene))dianiline) with pendant amine groups is reported that forms a heterogeneous hydrogen evolution catalyst using anilinium tetrafluoroborate as the proton source. The supported Ni(DPMDA) catalyst was surprisingly stable and displayed fast reaction kinetics with turnover frequencies (TOF) up to 25,900 s-1 or 366,000 s-1 cm-2 . Kinetic isotope effect (KIE) studies revealed a KIE of 5.7, and this data, combined with Tafel slope analysis, suggested that a proton-coupled electron transfer (PCET) process involving the pendant amine groups was rate-limiting. While evidence of an outer-sphere reduction of the Ni(DPMDA) catalyst was observed, it is hypothesized that the control over the secondary coordination sphere provided by the pendant amines facilitated such high TOFs and enabled the PCET mechanism. The results reported herein provide insight into heterogeneous catalyst design and approaches for controlling the secondary coordination sphere on electrode surfaces.

Designing functionalized polymers through post-polymerization modification of side chain leucine-based polymer

The opportunity to modify the chemical functionality of the incorporated amino acid moiety in side chain amino acid-based polymers provides access to tailor their optical, mechanical, and biological properties. Methacrylate monomers from C-terminus modified amino acids and their subsequent polymerization leads to the formation of polymethacrylate polymers with pendant amine (-NH2) groups. To this end, the post-polymerization reactions on the amine groups offer a facile route to prepare functionalized side chain amino acid-based polymeric materials. Herein, we synthesized side chain L-leucine pendant homopolymer using reversible addition-fragmentation chain transfer (RAFT) polymerization of Boc-L-leucine methacryloyloxyethyl ester (Boc-leu-HEMA) and subsequently the Boc group is deprotected to prepare P(H3N+-leu-HEMA). Post-polymerization modifications on the pendant amine groups were performed to incorporate fluorescent probe, propargyl group, methacrylic moiety, hydrophobic fatty acid, anionic pendant, and hydrophilic polyethylene glycol (PEG) units; which demonstrates an efficient platform to build a library of functionalized polymers from a single polymeric backbone. We have studied the solvatochromic behavior of fluorescence probe bearing polymers using fluorescence spectroscopy. The ability of the pendent methacrylic unit containing polymers to undergo crosslinking and the mechanical properties of the synthesized gel are investigated by the rheological study.

In-situ growth of poly(m-phenylenediamine) in polyethylene (PE) matrix for nanofiltration membranes enabling outstanding selectivity of both mono-/di-valent ions and dyes/salt mixtures with enhanced water flux

Membrane-based nanofiltration is a crucial technique for treating dyestuff-salts wastewater, owing to its excellent selectivity towards mono/multivalent ions and organic matter. In this study, a facile and highly practical method was employed to enhance the hydrophilicity of the PE matrix through in-situ growth of poly(m-phenylenediamine) on the PE fibrils via the chemical oxidative polymerization of MPD. The inclusion of the poly(m-phenylenediamine) on PE enables the formation of homogenous and defect-free PA layer showing a unique honeycomb-like Turing morphology with characteristic crater-belt structure. Furthermore, the pendant amine groups from the poly(m-phenylenediamine) chain promotes the formation of a PA layer with a smaller effective mean pore size and a sharper pore size distribution, resulting from the formation of an extra polyamide network (reaction of PMPD with TMC) within the PA layer. The as-prepared PE-based NF membrane exhibits a high pure water permeance of 17.7 LMH bar−1, exceptional Na2SO4 rejection of 99.3%, and a long-term stability for cross-flow filtration for over 10 days. The PE-based NF membrane exhibits the water flux 2.2 times higher than that of the commercially available DK and DL membranes while maintaining a high salt rejection. Additionally, the PE-based NF membrane demonstrates exceptional selectivity, with a value of 115 for the separation of NaCl/Na2SO4 and 339.8 for Cl-/SO42- in a binary salt mixture. Furthermore, the PE-based NF membrane shows ultra-high selectivity values of 990 for CR/NaCl and 1039 for MB/NaCl in the treatment of dye/salt mixtures, respectively, which are significantly higher than those of the DK and DL membranes. The findings of this study highlight the potential of the in-situ growth of poly(m-phenylenediamine) on PE fibrils as a simple and cost-effective method for the modulation of hydrophilicity to enhance the NF membrane performance.

Cisplatin-Conjugated Polyurethane Capsule for Dual Drug Delivery to a Cancer Cell.

This paper describes the synthesis of a polymer-prodrug conjugate, its aqueous self-assembly, noncovalent encapsulation of a second drug, and stimuli-responsive intracellular dual drug delivery. Condensation polymerization between a functionalized diol and a commercially available diisocyanate in the presence of poly(ethylene glycol) hydroxide (PEG-OH) as the chain stopper produces an ABA-type amphiphilic block copolymer (PU-1) in one pot, with the middle hydrophobic block being a polyurethane containing a pendant tert-butyloxycarbonyl (Boc)-protected amine in every repeating unit. Deprotection of the Boc group, followed by covalent attachment of the Pt(IV) prodrug using the pendant amine groups, produces the polymer-prodrug conjugate PU-Pt-1, which aggregates to nanocapsule-like structures in water with a hydrophilic interior. In the presence of sodium ascorbate, the Pt(IV) prodrug can be detached from the polymer backbone, producing the active Pt(II) drug. Cell culture studies show appreciable cell viability by the parent polymer. However, the polymer-prodrug conjugate nanocapsules exhibit cellular uptake and intracellular release of the active drug under a reducing environment. The capsule-like aggregates of the polymer-prodrug conjugate were used for noncovalent encapsulation of a second drug, doxorubicin (Dox), and Dox-loaded PU-Pt-1 aggregate showed a significantly superior cell killing efficiency compared to either of the individual drugs, highlighting the promising application of such a dual-drug-delivery approach.

Synthesis of Thermoresponsive, Catechol-Rich Poly(ethylene glycol) Brush Polymers for Attenuating Cellular Oxidative Stress.

Herein, we report a platform to integrate customizable quantities of catechol units into polymers by reacting caffeic acid carbonic anhydride with polymers having pendant amine groups. Brush poly(ethylene glycol)-caffeamide (PEG-CAF) copolymers based on oligo(ethylene glycol)methyl ether methacrylate (OEGMA500) were obtained with a catechol content of approximately 30, 40, and 50 mol % (vs OEGMA content). Owing to the hydrophobicity of the introduced CAF groups, the catechol copolymers exhibited cloud points in the range of 23-46 °C and were used to fabricate thermoresponsive FeIII metal-phenolic network capsules. Polymers with the highest CAF content (50 mol %) proved most effective for attenuating reactive oxygen species levels in vitro, in co-cultured fibroblasts, and breast cancer cells, even in the presence of an exogenous oxidant source. The reported approach to synthesize customizable catechol materials could be generalized to other amine-functional polymers, with potential biomedical applications such as adhesives or stimuli-responsive drug delivery systems.

Bifunctional hybrid organosiliceous catalysts for aldol condensation – hydrogenation tandem reactions of furfural in continuous-flow reactor

A series of organic-inorganic hybrid bifunctional organosiliceous catalysts with accessible pendant amine groups as single basic sites (such as propylamine, diethylamine, pyrrolidine) and Pd nanoparticles was prepared from suitable synthesis processes. Pd/MCM-41 silica decorated with propylamine groups was highly active and selective for the single-reactor tandem aldol condensation/crotonization reaction between furfural with methyl isobutyl ketone, followed by hydrogenation, to access renewable biosolvent and biofuel precursors in the branched alkane range at mild temperature (80–100 ºC). The catalyst was characterized in detail using XRD, C and N elemental analysis, ICP-OES, TGA/DTA, HR-TEM, N2 adsorption/desorption at 77 K, solid-state 13C and 29Si MAS NMR and FT-IR spectroscopy CO2 as acid probe. The catalyst was robust when operated in a dual fixed-bed reactor achieving steady 20 % furfural conversion for 12 h on stream with preferential formation of 1-(furan-2-yl)− 5-methylhexan-3-one.

Rational preparation of cocoon-like g-C3N4/COF hybrids: Accelerated intramolecular charge delivery for photocatalytic hydrogen evolution

In virtue of the pliable structure and ample pendent amine groups, the metal-free graphic carbon nitride (g-C3N4, abbreviated as CN) in two-dimensional (2D) morphology could be covalently modified by organic materials to modulate its photoelectrical characteristics. In this work, CN is covalently wrapped by a 2D Schiff-base covalent organic framework (COF, denoted as TMP), which is composed of 2,4,6-tris(4-aminophenyl)− 1,3,5-triazine (TAPT), melem and 1,3,5-triformyl phloroglucinol (TP), by means of “one-pot” solvothermal method with a cocoon-like morphology presented (CN/TMP). Given the significant distinction of charge densities between the two constituent units, viz., melem and TAPT, the resultant acceleration of the intramolecular charge transfer (ICT) within the TMP shroud endows the formed integrate with a significantly raised charge delivery and lowered band gap energy (Eg) in comparison to a structure-analogous COF (TM) incorporated hybrid, CN/TM, in which the TAPT monomer is not involved in the TM COF construction. The photocatalytic water-splitting evaluation indicates CN/TMP conducted photocatalysis could provide the hydrogen (H2) production of 102.88 μmol·h−1, about 5.6 and 11.6 times greater than that of CN/TM and CN, respectively. Our modulation strategy at molecular level proffers a new opportunity for the construction of the carbon nitride based hybrids and the regulation in their photoelectric behaviors.

L‐glutamic acid as a versatile platform for rapid synthesis of functional polyesters via facile Passerini multicomponent polymerization

AbstractIn this work, a series of polyesters with different functional side groups were successfully prepared by Passerini three‐component polymerization (P‐3CP) in “one‐pot” from biorenewable l‐glutamic acid (l‐GA)‐based monomers. First, the polymerization conditions including monomer feed ratios, solvents and comonomer were systematically studied using Boc‐protected l‐GA with adipaldehyde and tert‐butyl isocyanide. Under the optimal polymerization conditions, a variety of N‐substituted l‐GA‐based monomers as well as functional isocyanides were attempted in the P‐3CP to produce polyesters with different functionalities. Moreover, it was found that after removal of pendant protecting groups, the obtained l‐GA‐based polyester degraded spontaneously into small molecules via 1,5‐intramolecular cyclization between the pendant amine groups and the ester groups in polymer backbone. The structures and thermal properties of obtained polymers were determined by 1H NMR, IR, SEC, MALDI‐ToF‐MS, DSC, and TGA measurements. Starting from biorenewable l‐GA, this approach will provide a facile and straightforward route to produce functional and biodegradable polyesters, thus effectively expanding the range of biodegradable polymers.

Thermoresponsive properties of star-shaped amphiphilic block copolymers with a cholic acid core and functional amine groups

Block copolymers having stimuli-responsive properties are gaining impetus in the design of drug and gene delivery systems for therapeutic purposes. Thermoresponsive polymers are particularly interesting due to the ease of control over their solution behavior in aqueous media. The fine tuning of the lower critical solution temperature (LCST) of such polymers is necessary to achieve a successful drug delivery. In this work, the thermoresponsive properties of block copolymers based on a bile acid are studied in aqueous media to determine the factors affecting the LCST. Poly(allyl glycidyl ether) (PAGE) and poly(ethylene glycol) (PEG) blocks were grafted from a cholic acid (CA) core to yield star-shaped block copolymers with 4 arms. The PAGE block was further functionalized to bear pendant amine groups. The addition of amine groups may be expected to improve the hydrophilicty of the polymers, but the functionalized polymers showed pronounced thermoresponsive properties and aggregation under physiological conditions. It is thus necessary to understand the changes of their solution properties for a better design of such polymers for drug delivery applications. In this work, we studied the effects of PEG length and of the amine groups on the LCST behavior in water at various salt concentrations. A shorter PEG chain of 21 repeat units shows a LCST of 16 °C, whereas a longer PEG of 41 repeat units failed to show thermoresponsiveness up to 80 °C and at concentrations up to 50 mg/mL. Functionalizing the allyl groups increased the LCST of the polymers. A salting-out effect was observed by a decrease in LCST and the presence of salt also promoted precipitation of the nanoparticles.

PH Changes That Induce an Axial Ligand Effect on Nonheme Iron(IV) Oxo Complexes with an Appended Aminopropyl Functionality

Nonheme iron enzymes often utilize a high-valent iron(IV) oxo species for the biosynthesis of natural products, but their high reactivity often precludes structural and functional studies of these complexes. In this work, a combined experimental and computational study is presented on a biomimetic nonheme iron(IV) oxo complex bearing an aminopyridine macrocyclic ligand and its reactivity toward olefin epoxidation upon changes in the identity and coordination ability of the axial ligand. Herein, we show a dramatic effect of the pH on the oxygen-atom-transfer (OAT) reaction with substrates. In particular, these changes have occurred because of protonation of the axial-bound pendant amine group, where its coordination to iron is replaced by a solvent molecule or anionic ligand. This axial ligand effect influences the catalysis, and we observe enhanced cyclooctene epoxidation yields and turnover numbers in the presence of the unbound protonated pendant amine group. Density functional theory studies were performed to support the experiments and highlight that replacement of the pendant amine with a neutral or anionic ligand dramatically lowers the rate-determining barriers of cyclooctene epoxidation. The computational work further establishes that the change in OAT is due to electrostatic interactions of the pendant amine cation that favorably affect the barrier heights.

Proton Relay in Iron Porphyrins for Hydrogen Evolution Reaction.

The efficiency of the hydrogen evolution reaction (HER) can be facilitated by the presence of proton-transfer groups in the vicinity of the catalyst. A systematic investigation of the nature of the proton-transfer groups present and their interplay with bulk proton sources is warranted. The HERs electrocatalyzed by a series of iron porphyrins that vary in the nature and number of pendant amine groups are investigated using proton sources whose pKa values vary from ∼9 to 15 in acetonitrile. Electrochemical data indicate that a simple iron porphyrin (FeTPP) can catalyze the HER at this FeI state where the rate-determining step is the intermolecular protonation of a FeIII-H- species produced upon protonation of the iron(I) porphyrin and does not need to be reduced to its formal Fe0 state. A linear free-energy correlation of the observed rate with pKa of the acid source used suggests that the rate of the HER becomes almost independent of pKa of the external acid used in the presence of the protonated distal residues. Protonation to the FeIII-H- species during the HER changes from intermolecular in FeTPP to intramolecular in FeTPP derivatives with pendant basic groups. However, the inclusion of too many pendant groups leads to a decrease in HER activity because the higher proton binding affinity of these residues slows proton transfer for the HER. These results enrich the existing understanding of how second-sphere proton-transfer residues alter both the kinetics and thermodynamics of transition-metal-catalyzed HER.

Synthesis of pH-sensitive nanocarriers based on polyacrylamide grafted nanocrystalline cellulose for targeted drug delivery to folate receptor in breast cancer cells

In this study, nanocrystalline cellulose (NCC) was prepared for the synthesis of new nanocarriers from microcrystalline cellulose to the targeted drug delivery of doxorubicin (DOX) and then a chlorinated derivative of the NCC was prepared as well. The chlorinated NCC was used as a macroinitiator in the atom transfer radical polymerization (ATRP) technique to graft polyacrylamide on NCC chains (PA-g-NCC). Then PA-g-NCC copolymer was further reacted to form the copolymer with amine pendant groups by the transamidation method. Finally, two pathways were followed for the synthesis of two nanocarriers. In the first pathway, carboxymethyl-β-cyclodextrin (CM-β-CD) was grafted to the aminated PA-g-NCC copolymer and then the synthesized nanocarrier was targeted by folic acid (FA) which was embedded into the cyclodextrin (CD) cavity via host–guest interaction. In the second pathway, the FA was directly grafted on the aminated PA-g-NCC copolymer without CD mediation. DOX as an anticancer drug was loaded into the nanocarriers spheres. The characterization of synthesized materials was performed by FT-IR, SEM, EDX, and TGA. Loading capacity was measured in-vitro drug release by using UV spectroscopy. To investigate the effect of drug-loaded nanocarriers on breast cancer cells, we cultured and treated MCF7 cells with drug-nanocarriers. The cell death rate was determined by the MTT assay and IC50 on culture medium.

Hyperbranched Bisphosphonate-Functional Polymers via Self-Condensing Vinyl Polymerization and Postpolymerization Multicomponent Reactions.

The synthesis of hyperbranched aminobisphosphonic acid polymers via reversible addition-fragmentation chain transfer (RAFT) self-condensing vinyl polymerization is reported. A novel acrylamide-functional chain transfer monomer is synthesized and characterized by 1 H and 13 C NMR spectroscopy, elemental analysis, and mass spectrometry. The monomer is subsequently copolymerized with an acrylamide monomer bearing a pendent amine group to create hyperbranched amine-functional polymers with degrees of branching dictated by changing the reaction stoichiometry. The aminobisphosphonate functional group is introduced via a 3-component Kabachnik-Fields reaction. An alternate functionalization of the amine polymers to create acid-degradable imine hydrogels is also employed. This work demonstrates the application of multicomponent reactions to RAFT-derived hyperbranched polymers and provides a new route to previously inaccessible polymers.

Investigation of Immobilization Effects on Ni(P2N2)2 Electrocatalysts.

A new synthetic route to complexes of the type Ni(P2N2)22+ with highly functionalized phosphine substituents and the investigation of immobilization effects on these catalysts is reported. Ni(P2N2)22+ complexes have been extensively studied as homogeneous and surface-attached molecular electrocatalysts for the hydrogen evolution reaction (HER). A synthesis based on postsynthetic modification of PArBr2NPh2 was developed and is described here. Phosphonate-modified ligands and their corresponding nickel complexes were isolated and characterized. Subsequent deprotection of the phosphonic ester derivatives provided the first Ni(P2N2)22+ catalyst that can be covalently attached via pendent phosphonate groups to an electrode without involvement of the important pendent amine groups. Mesoporous TiO2 electrodes were surface modified by attachment of the new phosphonate functionalized Ni(P2N2)22+ complexes, and these provided electrocatalytic materials that proved to be competent and stable for sustained HER in aqueous solution at mild pH and low overpotential. We directly compared the new ligand to a previously reported complex that utilized the amine moiety for surface attachment. Using HER as the benchmark reaction, the P-attached catalyst showed a marginally (9-14%) higher turnover number than its N-attached counterpart.

Amino [13]-macrodilactones: Synthesis, derivatization, and structural motifs

Macrocycles draw their usefulness from an ability to balance size, shape, and functionality according to general principles. These structural parameters manifest as the conformations of macrocycles and ultimately their functions. Reported here are results of an investigation on [13]-macrodilactones that take up unique folded conformations based on the interplay of multi-atom planar units and sterics. The defining feature of this group of [13]-macrodilactones is the pendant amine group attached to the ring. Amidation of the ring generated analogs that were designed to imitate features of the macrocyclic natural product Teixobactin. We also show that intramolecular interactions, and the A-value of an acylated amine can explain the conformational preferences in macrocycles.

Self-Assembly of Lanthanide-Covalent Organic Polyhedra: Chameleonic Luminescence and Efficient Catalysis.

A series of multinuclear lanthanide-covalent organic polyhedra (LnCOPs), including pillar-typed triangular prisms 1-Ln3 and tetrahedra 2-Ln4 (Ln = LaIII, SmIII, EuIII), have been constructed for the first time, through either one-pot subcomponent self-assembly or postassembly metalation. In contrast to the known tetrahedral cages based on transition metals, the pillar-typed polyhedra were favored from the same organic components in the presence of lanthanides. Besides this, facile transmetalations between the 1-Ln3 polyhedra endow cascade chameleonic luminescence. Meanwhile, the open metal sites and pendent amine groups on 1-Ln3 enable these polyhedra to catalyze the Henry reaction efficiently.