Search

While common in biological systems, building blocks with low symmetry and flexibility pose numerous problems for synthetic self-assembly such as the formation of isomers of assemblies that are usually difficult to distinguish and purify. We herein report a design and synthesis strategy relying on shape complementarity and conformational constraints in dual curvature ligands that effectively promotes high selectivity during self-assembly and self-sorting of metal-organic assemblies. Three aromatic amide-based ligands (L1-L3) with a central 1,8-diazatriptycene core were designed and used for self-assembly with Pd2+. While hundreds of stereoisomers based on the conformational flexibility around the amides and the unsymmetrical non-planar structure of the core are possible upon coordination with the metal, the constraints designed into the ligands direct the self-assembly towards only a single Pd2L4 cage (L1) or Pd4L8 double-walled metallomacrocycle (L2) structure, even in mixtures of the ligands. We further demonstrate that this structural approach and the modularity of the ligand synthesis affords ready access to the first deep endohedral functionalized double-walled Pd4L8 cavitands (L3). These results highlight the potential of this new design strategy and open the door to selectively functionalized cavity-based architectures for numerous applications.

The design and construction of colorimetric ammonia (NH3) sensors with high selectivity, great stability and short response times at room temperature are highly needed for human health and food safety. We report herein a chiral supramolecular FeII 4L4 metal-organic cage (MOC-1) able to detect NH3(g) at room temperature in less than 10 s, which we used to monitor food safety at 4 1C on fresh bacon bits. The detection is accompanied by a dramatic colour change from light brown to purple. In addition, MOC-1 exhibits superior selectivity among NH3(g) and twelve analytes including common solvents and amines. The reproducibility was studied by five continuous cyclic tests. Simple and low-cost smartphone-based and chemometrics analytical methods were used to study the sensing performance. The thermal stability is more than 200 1C. The sensing mechanism is associated with a high-spin to low-spin transition of the FeII ions, as expected for a ligand field strength that meets spin crossover conditions. These results show the high potential of MOC-1 as a convenient and inexpensive NH3 gas sensor at room temperature, which could be used in the field of food safety assessment.

A graphical abstract is available for this content

Reported here is the first FeII based supramolecular cage with pyridyl-hydrazone ligand scaffolds that exhibits temperature induced spin crossover behaviour. Density functional theory calculations were employed to investigate the cause of the occurrence of this phenomenon based on the ligand structure. These results indicate that the reported low-spin cages with pyridyl-imine sites could be reconsidered for spin crossover by carefully manipulating the functional groups in the ligand system.

This project seeks to develop a method to control Aromatic Oligoamide Foldamer (AOF) self-assembly in solution by providing a method to prevent multi-stranded helix formation using a reversible synthetic modification. To this end, a Diels-Alder strategy was designed by screening 1,8-diazaanthracene aromatic monomers, acting as dienes, with a range of dienophiles to that would break the extended conjugation in the monomer and could also add steric constraints to further disfavor multi-stranded helix formation [1]. By incorporating this unit into longer sequences, we showed, based on structural studies, that the Diels-Alder modification is sufficient to prevent multi-helix formation. Finally, by applying retro-Diels-Alder conditions to the oligomer, it is possible to remove the dienophile and induce multi-helix formation. Together, these reactions provide a new tool for controlling multi-helix formation in AOF and expand their potential use in the broader area of self-assembly.

Lytic polysaccharide monooxygenases (LPMO) are a family of copper containing metalloenzymes important for biofuel and biochemical production due to their ability to oxidatively cleave the strong glycosidic C-H bonds in cellulose. The active site of LPMO consists of a single copper atom bound in a T-shaped N3 coordination environment called the histidine brace. The same structure is also found in particulate methane monooxygenase suggesting that the histidine brace is responsible for the oxidative capabilities of the enzymes. However, the role of the histidine brace as well as many other structural features of the active site remain unclear. Synthesizing and studying synthetic models of the histidine brace can increase the understanding of the structural features important for reactivity and generate new powerful catalysts for the oxidation of organic substrates. Currently, the limited number of LPMO model complexes that have been described and the large structural variation between these makes comparison difficult, limiting the understanding of how different modifications affect reactivity. In this PhD project, two complimentary approaches will be used to generate the most accurate models of the LPMO active site to date. The first one will be based on traditional small molecule models, different structural parameters will be modified across similar compounds such as the chelate ring size, the position and the methylation of the imidazole or the imine versus the amine function. The second approach will be the developing of a new generation of accurate histidine brace mimics using foldamer scaffolds. Through systematic variation of both first and second coordination spheres, we aim to generate well-defined series of complexes that will allow us to develop a structure-reactivity relationship. In doing so, we aim to define the important features necessary in biomimetic copper complexes for the oxidation of C-H bonds with high bond dissociation energies.

Areas ranging from fine chemical synthesis to sustainable energy development rely on highly efficient metal-based catalysts to perform key chemical transformations [1]. Then metals such as Pd, Pt, Rh, Ir or Ru are used because they are proficient at mediating the multi-electron transfer steps required for these applications. In contrast to synthetic chemistry, biological systems do not use noble metals but rather employ cooperative reactivity between multiple redox sites [2]. These sites store and provide additional electrons to avoid unfavorable oxidation states at the reactive centers. However, these cooperative processes remain not well-known, specifically in multi-metallic systems [3]. Then, replicating this strategy of redox cooperativity in synthetic complexes could offer a valuable way to enhance the reactivity of first-row transition metals like iron, copper, etc. To address this challenge, our project seeks to develop and study the biomimetic strategy of incorporating metal-based electron reservoirs within the periphery of reactive metal centers. Thus, ditopic ligands able to interact with two metallic centers have been developed based on N,N,N’,N’-tetra(heterocyclic)propylene- and ethylene-diamines. Modifying the substituents on the heterocycles and the Bite angle may provide interesting properties to our bimetallic systems. For now, their monometallic counterparts have been synthesized and characterized through spectroscopic and electrochemical technics that brought promising results.

Orthogonal assembly in biological systems is essential for mediating the myriad complex interactions and biological processes necessary for life. Reaching this level of complexity with purely synthetic systems remains a valuable goal in chemistry but is generally limited by the handful of compatible interactions that can be included on small molecules. The use of folded oligomers that can selectively interact with each other could provide a nearly endless supply of directional building blocks for self-assembly that can be made orthogonal to each other through simple sequence modification. Specifically, exploiting the multiple helix forming properties of some aromatic foldamers for this purpose could lead to new abiotic tools for precisely controlling self-assembly over large size regimes. To this end, our group is interested in how aromatic oligoamides can be used as tools for orthogonal self-assembly of larger structures, Figure 1. This communication describes two approaches for controlling self-assembly with aromatic oligoamides based on A) the incorporation of specific metal binding sites into oligomers to allow formation of diverse metal-organic architectures and B) directing large-scale structural changes with a Diels-Alder/retro-Diels-Alder strategy that allows stimuli responsive hybridization. The properties of these systems and their potential as orthogonal building blocks for controlled self-assembly will be discussed.

Areas ranging from fine chemical synthesis to sustainable energy development rely on highly efficient metal-based catalysts to perform key chemical transformations [1]. Then metals such as Pd, Pt, Rh, Ir or Ru are used because they are proficient at mediating the multi-electron transfer steps required for these applications. In contrast to synthetic chemistry, biological systems do not use noble metals but rather employ cooperative reactivity between multiple redox sites [2]. These sites store and provide additional electrons to avoid unfavorable oxidation states at the reactive centers. Replicating this strategy of redox cooperativity in synthetic complexes could offer a valuable way to enhance the reactivity of base metal catalysts like iron, copper, etc. Such kind of systems have already been developed by Rauchfuss et al. to mimic Fe-Fe hydrogenase [3] and Bosnich and co-workers placed the basics for understanding bimetallic systems [4]. However, increase the comprehension of incorporating and controlling the additional redox centers is still a major challenge. Here, we planned to develop new bimetallic complexes where the two metals will be in a close proximity thanks to a ditopic ligand and then, to better understand the cooperativity between the metal centers and highlight the importance of the ligands design. References

This project seeks to develop a method to control Aromatic Oligoamide Foldamer (AOF) self-assembly in solution by providing a method to prevent multi-stranded helix formation using a reversible synthetic modification. To this end, a Diels-Alder strategy was designed by screening 1,8-diazaanthracene aromatic monomers, acting as dienes, with a range of dienophiles to that would break the extended conjugation in the monomer and could also add steric constraints to further disfavor multi-stranded helix formation [1]. By incorporating this unit into longer sequences, we showed, based on structural studies, that the Diels-Alder modification is sufficient to prevent multi-helix formation. Finally, by applying retro-Diels-Alder conditions to the oligomer, it is possible to remove the dienophile and induce multi-helix formation. Together, these reactions provide a new tool for controlling multi-helix formation in AOF and expand their potential use in the broader area of self-assembly.

Structure-activity relationships of Ni particles with different morphologies, crystallinities and compositions were investigated for glucose hydrogenation. Nanorods and nanospheres were obtained respectively by altering hydrogen pressure during synthesis, and nanobead chains were synthesized for analogy analysis following a known procedure. Glucose hydrogenation over Ni nanosphere catalysts exhibits the lowest reaction barrier (Ea = 32 kJ/mol) compared to nanorods and nanobead chains (98 kJ/mol), and a constant sorbitol selectivity of 100 % was obtained at a temperature of 140 °C. A comprehensive characterization with ATR-IR, Raman, XPS, ICP, P-XRD, HRTEM and SAED revealed that the combined factors of poly-crystallinity, amorphous structure and P- dopant make Ni nanospheres superior. This provides a valuable path in the process of designing better non-noble catalysts to definitely avoid the use of noble metals in the future in biomass valorization reactions.

Ni nanoparticles supported on carbon black (Ni/CB) with Ni particle sizes 6.9–23.5 nm were found to exhibit a size effect, with a significant impact on catalytic performance in the glucose hydrogenation reaction. A series of Ni nanoparticles with different sizes, as determined by statistical analysis of TEM images, were obtained by varying the synthesis temperature using a sol-gel method. The highest glucose conversion and sorbitol production in glucose hydrogenation was possible with Ni/CB catalysts with Ni particles size around 17 nm obtained at 550 °C. Similarly, graphene nanoplatelets (GNP) supported Ni with different particle sizes synthesized under different temperature conditions have an identical behaviour, that is, GNP-550 exhibits the highest catalytic performance. Therefore, it can be concluded that size effect appeared when the particle size is above 10 nm, which provides a reference for the design of subsequent catalysts for liquid phase hydrogenation reactions pertinent to biomass conversion.

The authors regret that one of the author’s names, Chieh-Kai Chan, was spelt incorrectly. The correct spelling is as given here and in the author list above.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to uthors and readers.

Efficient excited-state electron transfer between an iron(III) photosensitizer and organic electron donors was realized with green light irradiation. This advance was enabled by the use of the previously reported iron photosensitizer, [Fe(phtmeimb)2]+ (phtmeimb = {phenyl[tris(3-methyl-imidazolin-2-ylidene)]borate}, that exhibited long-lived and luminescent ligand-to-metal charge-transfer (LMCT) excited states. A benchmark dehalogenation reaction was investigated with yields that exceed 90% and an enhanced stability relative to the prototypical photosensitizer [Ru(bpy)3]2+. The initial catalytic step is electron transfer from an amine to the photoexcited iron sensitizer, which is shown to occur with a large cage-escape yield. For LMCT excited states, this reductive electron transfer is vectorial and may be a general advantage of Fe(III) photosensitizers. In-depth time-resolved spectroscopic methods, including transient absorption characterization from the ultraviolet to the infrared regions, provided a quantitative description of the catalytic mechanism with associated rate constants and yields.

The mechanism of a visible light-driven dehalogenation/cyclization reaction was investigated using ruthenium(II), iridium(III) and iron(III) photosensitizers by means of steady-state photoluminescence, time-resolved infrared spectroscopy, and nanosecond/femtosecond transient absorption spectroscopy. The nature of the photosensitizer was found to influence the product distribution such that the dehalogenated, non-cyclized products were only detected for the iron photosensitizer. Strikingly, with the iron photosensitizer, large catalytic yields required a low dielectric solvent such as dichloromethane, consistent with a previous publication. This low dielectric solvent allowed ultrafast charge-separation to outcompete geminate charge recombination and improved cage escape efficiency. Further, the identification of reaction mechanisms unique to the iron, ruthenium, and iridium photosensitizer represents progress towards the long-sought goal of utilizing earth-abundant, first-row transition metals for emerging energy and environmental applications.

Carbon black (CB) supported Ni, Fe, or Fe-Ni alloy catalysts were synthesized by sol-gel to elucidate the reaction pathways over each catalyst, as well as synergistic effects in glucose to sorbitol hydrogenation. The bimetallic materials presented small and alloyed nanoparticles that were richer in reduced metallic sites at the surface than their monometallic counterparts. Glucose isomerization to fructose was favoured over Fe/CB, while glucose hydrogenation to sorbitol is the dominating pathway over Ni/CB catalyst. By contrast, sorbitol production was promoted and undesired isomerization was suppressed when Fe and Ni formed a nanoalloy. In addition, the alloy catalyst presented better stability than the corresponding monometallic catalyst. A comparison with a mechanical mixture of Fe/CB and Ni/CB monometallic catalysts demonstrated the synergy at the nanoscale in the alloy. By comparing different Fe:Ni ratios, the 1:1 formulation was identified as the best compromise to achieve a high activity while maintaining high sorbitol selectivity.

Supramolecular chemistry concepts are used to design new spin crossover cages.

While water clusters play an essential role in the stability of biological structures, their ability to stabilize synthetic oligomers is less understood. We have synthesized a heptameric sheet-like aromatic oligoamide foldamer with ferrocene as turn unit. It shows strong interactions with water in the solid state and in solution. The water binding limits the fluxional processes resulting from the flexible ferrocene unit, highlighting the importance of such interactions for conformational studies on this class of molecule.

Diversification of the structures and applications possible for foldamers relies on expansion of the building block library available for their synthesis. In this work, we describe the synthesis of a range of three dimensional heteroaromatic monomers, based on iptycene scaffolds, that are suitable for the synthesis of aromatic oligoamide foldamers. These units can be obtained in gram quantities in up to 80% yield through [4+2] cycloaddition between diester, diamine, and amino acid derivatives of 1,8-diazaanthracenes and a variety of dienophiles. X-ray structural studies of the monomers and an oligomer show that the new motif orients the two heterocyclic rings and attached groups at an angle of approximately 120° to each other, opening new geometric considerations for the design of this class of foldamer.

Lytic polysaccharide monooxygenases (LPMO) are a family of copper containing metalloenzymes important for biofuel and biochemical production due to their ability to oxidatively cleave the strong glycosidic C-H bonds in cellulose. The active site of LPMO consists of a single copper atom bound in a T-shaped N3 coordination environment called the histidine brace. The same structure is also found in particulate methane monooxygenase suggesting that the histidine brace is responsible for the oxidative capabilities of the enzymes. However, the role of the histidine brace as well as many other structural features of the active site remain unclear. Synthesizing and studying synthetic models of the histidine brace can increase the understanding of the structural features important for reactivity and generate new powerful catalysts for the oxidation of organic substrates. Currently, the limited number of LPMO model complexes that have been described and the large structural variation between these makes comparison difficult, limiting the understanding of how different modifications affect reactivity. In this PhD project, two complimentary approaches, based on small molecule biomimetics and foldamer based scaffolds, will be used to generate the most accurate models of the LPMO active site to date. Through systematic variation of both first and second coordination spheres, we aim to generate well-defined series of complexes that will allow us to develop a structure-reactivity relationship. In doing so, we aim to define the important features necessary in biomimetic copper complexes for the oxidation of C-H bonds with high bond dissociation energies.

Self-assembly of secondary structural elements represent an intriguing approach for non-covalent synthesis that can potentially offer greater selectivity and orthogonality than existing methods. To this end, the ability of aromatic oligamide foldamers to hybridize into multiple helices can be seen as a powerful tool for the formation of discrete molecular cages or even networks. Still, the spontaneity of this process complicates synthetic aspects of the incorporation of multi-helical segments into larger scaffolds. In this poster we describe the synthesis of a series of diazaiptycene based aromatic monomers via Diels-Alder reaction and show that their incorporation into longer oligomers leads to structural distortions that prevent aggregation between multiple strands. Importantly the use of a TAD (triazoledione) group as dienophile opens a way to the control the oligomer topology via a Diels-Alder / retro Diels-Alder strategy and the use of thermal stimulus to trigger double helix formation. Perspectives for using this approach for the design and synthesis of larger molecular architectures formed via double helix driven self-assembly are also discussed.

This Ph.D. work was focusing on the development of sterically hindered Lewis bases, in particular triaryl phosphines and amines, which then were employed for the rational design of frustrated Lewis pair catalysts. In this context, we prepared new types of FLPs by combining strong boron Lewis acids with the group XV (N, P) derived 9-hetero-triptycene. Initially, several novel synthetic approaches enabling the access to the 9-azatriptycene framework were attempted. We then turned our attention to the preparation of sterically hindered 9-phosphatriptycene scaffold. Two methodologies were developed allowing to reach unprecedented ortho-substituted unsymmetrical 9-phosphatriptycene derivatives which include the first example of P-chirogenic 9-phosphatriptycene ever reported. By means of computational and experimental methods, the unique electronic and steric properties, and Brønsted and Lewis basicities of 9-phosphatriptycenes were quantified. The coordination of 9-phosphatriptycenes with transition metal [Rh(III) and Au(I)] was finally investigated and the resulting complexes were studied and characterized by IR, NMR, and X-ray diffraction crystallography. Lastly, the association of phosphatriptycenes with tris(pentafluorophenyl)borane was experimentally tested and applied as FLP catalyst, enabling the transition-metal-free hydrogenation reaction of unactivated alkenes under mild reaction condition., (SC - Sciences) -- UCL, 2020

As metalloproteins exemplify, the chemical and physical properties of metal centers depend not only on their first but also on their second coordination sphere. Installing arrays of functional groups around the first coordination sphere of synthetic metal complexes is thus highly desirable, but it remains a challenging objective. Here we introduce a novel approach to produce tailored second coordination spheres. We used bioinspired artificial architectures based on aromatic oligoamide foldamers to construct a rigid, modular and well-defined environment around a metal complex. Specifically, aza-aromatic monomers having a tethered [2Fe–2S] cluster have been synthesized and incorporated in conical helical foldamer sequences. Exploiting the modularity and predictability of aromatic oligoamide structures allowed for the straightforward design of a conical architecture able to sequester the metal complex in a confined environment. Even though no direct metal complex–foldamer interactions were purposely designed in this first generation model, crystallography, NMR and IR spectroscopy concurred to show that the aromatic oligoamide backbone alters the structure and fluxional processes of the metal cluster.