Juggling Optoelectronics and Catalysis: The Dual Talents of Bench Stable 1,4-Azaborinines.

Boron- and nitrogen-doped polycyclic aromatic hydrocarbons (B-PAHs) have established a strong foothold in the realm of organic electronics. However, their catalytic potential remains largely untapped. In this study, we synthesise and characterise two bench stable B,N-doped PAH derivatives based on a 1,4-azaborinine motif. Most importantly, the anthracene derived structure is an efficient catalyst in the reduction of various carbonyls and imines. These results underscore the potential of B,N-PAHs in catalytic transformations, setting the stage for deeper exploration in this chemical space.

A Reversibly Porous Supramolecular Peptide Framework.

The ability to use bio-inspired building blocks in the assembly of novel supramolecular frameworks is at the forefront of an exciting research field. Herein, we present the first polyproline helix to self-assemble into a reversibly porous, crystalline, supramolecular peptide framework (SPF). This framework is assembled from a short oligoproline, adopting the polyproline II conformation, driven by hydrogen-bonding and dispersion interactions. Thermal activation, guest-induced dynamic porosity and enantioselective guest inclusion have been demonstrated for this novel system. The principles of the self-assembly associated with this SPF will be used as a blueprint allowing for the further development of helical peptide linkers in the rational design of SPFs and metal-peptide frameworks.

Chemical synthesis of 4'-thio and 4'-sulfinyl pyrimidine nucleoside analogues.

Analogues of the canonical nucleosides required for nucleic acid synthesis have a longstanding presence and proven capability within antiviral and anticancer research. 4'-Thionucleosides, that incorporate bioisosteric replacement of furanose oxygen with sulfur, represent an important chemotype within this field. Established herein is synthetic capability towards a common 4-thioribose building block that enables access to thio-ribo and thio-arabino pyrimidine nucleosides, alongside their 4'-sulfinyl derivatives. In addition, this building block methodology is templated to deliver 4'-thio and 4'-sulfinyl analogues of the established anticancer drug gemcitabine. Cytotoxic capability of these new analogues is evaluated against human pancreatic cancer and human primary glioblastoma cell lines, with observed activities ranging from low µM to >200 µM; explanation for this reduced activity, compared to established nucleoside analogues, is yet unclear. Access to these chemotypes, with thiohemiaminal linkages, will enable a wider exploration of purine and triphosphate analogues and the application of such materials for potential resistance towards relevant hydrolytic enzymes within nucleic acid biochemistries.

Synthesis of C6-modified mannose 1-phosphates and evaluation of derived sugar nucleotides against GDP-mannose dehydrogenase.

Sufferers of cystic fibrosis are at significant risk of contracting chronic bacterial lung infections. The dominant pathogen in these cases is mucoid Pseudomonas aeruginosa. Such infections are characterised by overproduction of the exopolysaccharide alginate. We present herein the design and chemoenzymatic synthesis of sugar nucleotide tools to probe a critical enzyme within alginate biosynthesis, GDP-mannose dehydrogenase (GMD). We first synthesise C6-modified glycosyl 1-phosphates, incorporating 6-amino, 6-chloro and 6-sulfhydryl groups, followed by their evaluation as substrates for enzymatic pyrophosphorylative coupling. The development of this methodology enables access to GDP 6-chloro-6-deoxy-ᴅ-mannose and its evaluation against GMD.

Fluorescent supramolecular hierarchical self-assemblies from glycosylated 4-amino- and 4-bromo-1,8-naphthalimides.

An investigation into the self-assembly of two 4-amino- and a 4-bromo-1,8-naphthalimide (Nap) based structures (1-3) possessing an appended glycan unit, from protic polar media, is presented. The results demonstrate the formation of complex hierarchical luminescent aggregates, wherein the morphologies, sizes and spherical structures were highly dependent on both the media and the Nap structure. Upon cleaving the native glycosidic bond, using an enzyme, the structure/morphology of the self-assembly of 3 in buffered solution was significantly transformed.

Correction: Glycosylated naphthalimides and naphthalimide Tröger's bases as fluorescent aggregation probes for Con A.

Correction for 'Glycosylated naphthalimides and naphthalimide Tröger's bases as fluorescent aggregation probes for Con A' by Elena Calatrava-Pérez et al., Org. Biomol. Chem., 2019, DOI: 10.1039/c8ob02980f.

Glycosylated naphthalimides and naphthalimide Tröger's bases as fluorescent aggregation probes for Con A.

Herein we report the synthesis of fluorescent, glycosylated 4-amino-1,8-naphthalimide (Nap) 1, and the related 1,8-naphthalimides Tröger's bases (TBNap) 2 and 3, from 1,8-naphthalic anhydride precursors, the α-mannosides being introduced through the use of CuAAC mediated 'click' chemistry. We investigate the photophysical properties of these probes in buffered solution and demonstrate their ability to function as fluorescent probes for Concanavalin A (Con A) lectin. We show that both the Nap and TBNap structures self-assemble in solution. The formation of the resulting supramolecular structures is driven by head-to-tail π-π stacking and extended hydrogen bonding interactions of the Nap and the triazole moieties. These interactions give rise to spherical nano-structures (ca. 260 nm and 100 nm, for 1 and 3, respectively), which interact with the Con-A protein, the interaction being probed by using both luminescent and Scanning Electron Microscopy imaging as well as dynamic light scattering measurements. Finally, we show that these supramolecular assembles can be used as luminescent imaging agents, through confocal fluorescence imaging of HeLa cells of the per-acetylated version 2.

Anisotropic Thermal and Guest-Induced Responses of an Ultramicroporous Framework with Rigid Linkers.

The interdependent effects of temperature and guest uptake on the structure of the ultramicroporous metal-organic framework [Cu3 (cdm)4 ] (cdm=C(CN)2 (CONH2 )- ) were explored in detail by using in situ neutron scattering and density functional theory calculations. The tetragonal lattice displays an anisotropic thermal response related to a hinged "lattice-fence" mechanism, unusual for this topology, which is facilitated by pivoting of the rigid cdm anion about the Cu nodes. Calculated pore-size metrics clearly illustrate the potential for temperature-mediated adsorption in ultramicroporous frameworks due to thermal fluctuations of the pore diameter near the value of the target guest kinetic diameter, though in [Cu3 (cdm)4 ] this is counteracted by a competing contraction of the pore with increasing temperature as a result of the anisotropic lattice response.

Solution-State Anion Recognition, and Structural Studies, of a Series of Electron-Rich meta-Phenylene Bis(phenylurea) Receptors and Their Self-Assembled Structures.

meta-Phenylene bis(phenylurea) receptors 1-4 were designed and synthesized to investigate the association between receptor shape, anion-selective binding and anion-directed self-assembly processes. Solution studies, performed through 1H NMR titrations with a variety of tetra- N-butylammonium salts, demonstrated strong binding of 2 equiv of H2PO4-, AcO-, BzO- anions and comparatively weak binding of Cl-, HSO4-, and SO42- anions. Binding modes and stability constants (log ß) were determined by regression analysis of the obtained 1H NMR titration data in DMSO- d6, and the cooperativities of the binding interactions were probed. Host-guest complexes of receptors 1 and 2 were studied in the crystalline phase to further probe the anion-binding behavior of this motif. This included a triple-stranded helicate consisting of three strands of receptor 2 arranged around a mixed-phosphate anionic core, which was characterized by using X-ray crystallography.

A Lanthanide Luminescent Cation Exchange Material Derived from a Flexible Tricarboxylic Acid 2,6-Bis(1,2,3-triazol-4-yl)pyridine (btp) Tecton.

The synthesis of the three-dimensional metal-organic framework material, [Zn7L6]·(H2NMe2)4·(H2O)45 (1), derived from a flexible tricarboxylic acid 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) ligand, is presented. The btp ligand, H3L, adopts a three-dimensional hydrogen bonding network in the crystalline state through a combination of carboxylic acid dimer and syn-anti-btp/carboxylic acid hydrogen bonding synthons. The Zn(II) species 1 exhibits a three-dimensional framework structure with the rare crs topology and contains linear and undulated solvent channels extending in three dimensions. The guest exchange and gas adsorption properties of 1 were investigated; herein we demonstrate the exchange of dimethylammonium cations from the as-synthesized material with cationic guest molecules in the form of dyes and luminescent Ln(III) ions. Sensitization of Eu(III) and Tb(III) inside the porous network of 1 was achieved upon cation exchange, with a view toward developing functional luminescent materials.

One-pot facile synthesis of 4-amino-1,8-naphthalimide derived Tröger's bases via a nucleophilic displacement approach.

We report here a novel one-pot synthetic strategy for the synthesis of a family of N-alkyl-1,8-naphthalimide based Tröger's bases via a nucleophilic substitution reaction of a common 'precursor' (or a 'synthon') N-aryl-1,8-naphthalimide Tröger's base heated at 80 °C in neat aliphatic primary amine, in overall yield of 65-96%. This methodology provides an efficient and one-step facile route to design 1,8-naphthalimide derived Tröger's base structures in analytically pure form without the use of column chromatography purification, that can be used in medicinal chemistry and as supramolecular scaffolds. We also report the formation of the corresponding anhydride, and the crystallographic analysis of two of the resulting products, that of the N-phenyl-4-amino-1,8-naphthalimide and the anhydride derived Tröger's bases.

Flexible Porous Coordination Polymers from Divergent Photoluminescent 4-Oxo-1,8-naphthalimide Ligands.

Two new luminescent ditopic naphthalimide-derived ligands, N-(4-cyanophenylmethylene)-4-(4-cyanophenoxy)-1,8-naphthalimide (L3) and N-(4-carboxyphenylmethylene)-4-(4-carboxyphenoxy)-1,8-naphthalimide (H2L4), have been prepared, and their coordination chemistry has been explored in the synthesis of three new coordination polymer materials. Complex poly-[Ag(L3)2]BF4·4.5H2O·0.5THF (1) is a 3-fold 2D → 2D parallel interpenetrated coordination polymer in which three interwoven sheets define inter- and intralayer channels containing anions and solvent molecules. Molecules of L3 interact in 1 through dominant head-to-head π-π stacking interactions, in an opposite aggregation mode to that observed in the free ligand in the crystalline phase. Complexes poly-[Cu(L4)(OH2)]·2DMF·0.5H2O (2) and poly-[Cd2(L4)2(OH2)2]·1.5DMF·3H2O (3) are related noninterpenetrated two-dimensional coordination polymers defined by one-dimensional metal-carboxylate chains, forming layers that interdigitate with adjacent networks through naphthalimide π-π interactions. Both materials undergo structural rearrangements on solvent exchange with acetonitrile; in the case of 3, this transformation can be followed by single-crystal X-ray diffraction, revealing the structure of the acetonitrile solvate poly-[Cd2(OH2)2(L4)2]·2MeCN (4), which shows a significant compression of the primary channels to accommodate the solvent guest molecules. Both materials display modest CO2 adsorption after complete evacuation, and the original expanded phases can be regenerated by reimmersion in DMF. The photophysical properties of each ligand and complex were also explored, which revealed variations in emission wavelength, based on solid-state interactions, including a notable shift in the fluorescence emission band of 3 upon structural rearrangement to 4.

Coordination Chemistry and Structural Dynamics of a Long and Flexible Piperazine-Derived Ligand.

A long and highly flexible internally functionalized dipyridyl ligand α,α'-p-xylylenebis(1-(4-pyridylmethylene)-piper-4-azine), L, has been employed in the synthesis of a series of coordination polymer materials with Co(II), Cd(II), and Ag(I) ions. In poly-[Cd(L)(TPA)] 1 and poly-[Co(L)(IPA)], 2, (TPA = terephthalate, IPA = isophthalate) the ligand adopts a similar linear conformation to that seen in the structure of the unbound molecule and provides a long (2.6 nm) metal-metal bridging distance. Due to the mismatch of edge lengths with that provided by the carboxylate coligands, geometric distortions from the regular dia and (4,4) network geometries for 1 and 2, respectively, are observed. In poly-[Ag2(CF3SO3)2(L)], 3, the ligand coordinates through both pyridine groups and two of the four piperazine nitrogen donors, forming a high-connectivity 2-dimensional network. The compound poly-[Ag2(L)](BF4)2·2MeCN, 4, a porous 3-dimensional cds network, undergoes a fascinating and rapid single-crystal-to-single-crystal rearrangement on exchange of the acetonitrile guests for water in ambient air, forming a nonporous hydrated network poly-[Ag2(L)](BF4)2·2H2O, 5, in which the well-ordered guest water molecules mediate the rearrangement of the tetrafluoroborate anions and the framework itself through hydrogen bonding. The dynamics of the system are examined in greater detail through the preparation of a kinetic product, the dioxane-solvated species poly-[Ag2(L)](BF4)2·2C4H8O2, 6, which undergoes a slow conversion to 5 over the course of approximately 16 h, a transition which can be monitored in real time. The reverse transformation can also be observed on immersing the hydrate 5 in dioxane. The structural features and physical properties of each of the materials can be rationalized based on the flexible and multifunctional nature of the ligand molecule, as well as the coordination behavior of the chosen metal ions.

Porous Polyrotaxane Coordination Networks Containing Two Distinct Conformers of a Discontinuously Flexible Ligand.

A new divergent homopiperazine-derived ligand N,N'-bis(4-carboxyphenyl)-1,4-diazacycloheptane H2L has been prepared, containing a semirigid saturated heterocyclic core which is capable of providing multiple distinct bridging geometries. Reaction of H2L with zinc acetate in DMSO gives a two-dimensional parallel interpenetrated polyrotaxane structure 1 in which the loops and rods are formed by the bent cis-(eq,ax) twist boat and trans-(ax,ax) twist chair conformers, respectively. By matching the distances between the solvated metal sites in the structure of 1, a related material 2 can be prepared incorporating the pillaring ligand trans-1,2-bis(4-pyridyl)ethylene bpe. Compound 2 displays a similar polyrotaxane interpenetration mode, permitted by the presence of both cis and trans ligand conformers, but displays a three-dimensional 2.69 topology related to the dia diamondoid network. The guest exchange and gas adsorption properties of both materials were investigated; while compound 1 displays poor stability to guest exchange and negligible gas uptake, the higher connectivity microporous compound 2 shows facile guest exchange and a surprisingly high CO2 capacity of 12 wt % at 1 bar and 273 K, and a zero-loading enthalpy of adsorption of -32 kJ mol-1. High-pressure adsorption isotherms also show moderate physisorption of H2 and CH4 within the material.

Liquid-phase enantioselective chromatographic resolution using interpenetrated, homochiral framework materials.

Effective separation of mixtures of enantiomers is of continuing interest in analytical and preparative chromatography, with new materials frequently designed and tested. We report two new enantiomerically pure 2D→3D interpenetrated materials used as stationary liquid chromatographic (LC) phases that are shown to resolve selected racemic mixtures with enantiomeric and chemical selectivity. Dicarboxylate ligands derived from amino acids on naphthalene and perylene cores form 2D frameworks that interpenetrate to give 3D structures. Selectivity is initially tested by uptake from solution; subsequent LC methods show that the materials exhibit resolution of racemic analytes in 'micro-columns' and that the two closely related materials show markedly different selectivity for different analytes with much greater activity than the ligands alone. Comparison with a close-packed analogue suggests that the separation activity is largely due to surface effects.

Ligand isomerism fine-tunes structure and stability in zinc complexes of fused pyrazolopyridines.

Fused-ring pyrazoles offer a versatile platform for derivitization to give finely tuned and functional ligands in coordination assemblies. Here, we explore the pyrazolo[4,3-b]pyridine (HL1) and pyrazolo[3,4-c]pyridine (HL2) backbones and their N-substituted derivatives, using their coordination chemistry with zinc(II) in the solid state and in solution to examine the steric and electronic effects of varying their substitution pattern. The parent heterocycles HL1 and HL2 both generate robust and permanently porous isomeric MOFs on reaction with zinc and a dicarboxylate co-ligand. The subtle geometric change offered by the position of the backbone pyridyl nitrogen atom leads to substantial changes in the pore size and total pore volume, which is reflected in both their surface areas and CO2 uptake performance. Both materials are also unusually resilient to atmospheric water vapour by virtue of the strong metal-azolate bonding. The isomeric chelating ligands L3-L6, generated by N-arylation of the parent heterocycles with a 2-pyridyl group, each coordinate to zinc to give either mononuclear or polymeric coordination compounds depending on the involvement of the backbone pyridine nitrogen atom. While crystal packing influences based on the steric preferences of the ligands are dominant in the crystalline phase, fluorescence spectroscopy is used to show that the 2H isomers L4 and L6 show distinct coordination behaviour to the 1H isomers L3 and L5, forming competing [ML] and [ML2] species in soution. The first stability constant for L6 with zinc(II) is an order of magnitude larger than for the other three ligands, suggesting an improved binding strength based on the electron configuration in this isomer. These results show that careful control of remote substitution on fused pyrazole ligands can lead to substantial improvements in the stability of the resulting complexes, with consequences for the design of stable coordination assemblies containining labile metal ions.

Coordination sphere hydrogen bonding as a structural element in metal-organic Frameworks.

In the design of new metal-organic frameworks, the constant challenges of framework stability and structural predictability continue to influence ligand choice in favour of well-studied dicarboxylates and similar ligands. However, a small subset of known MOF ligands contains suitable functionality for coordination sphere hydrogen bonding which can provide new opportunities in ligand design. Such interactions may serve to support and rigidity the coordination geometry of mononuclear coordination spheres, as well as providing extra thermodynamic and kinetic stabilisation to meet the challenge of hydrolytic stability in these materials. In this perspective, a collection of pyrazole, amine, amide and carboxylic acid containing species are examined through the lens of (primarily) inner-sphere hydrogen bonding. The influence of these interactions is then related to the overall structure, stability and function of these materials, to provide starting points for harnessing these interactions in future materials design.

Squaramide-Based Self-Associating Amphiphiles for Anion Recognition.

The synthesis and characterisation of two novel self-assembled amphiphiles (SSAs) SQS-1 and SQS-2 are reported. Both compounds, based on the squaramide motif, were fully soluble in a range of solvents and were shown to undergo self-assembly through a range of physical techniques. Self-assembly was shown to favour the formation of crystalline domains on the nanoscale but also fibrillar film formation, as suggested by SEM analysis. Moreover, both SQS-1 and SQS-2 were capable of anion recognition in DMSO solution as demonstrated using 1 H NMR and UV/Vis absorption spectroscopy, but displayed lower binding affinities for various anions when compared against other squaramide based receptors. In more competitive solvent mixtures SQS-1 gave rise to a colourimetric response in the presence of HPO42- that was clearly visible to the naked eye. We anticipate that the observed response is due to the basic nature of the HPO42- anion when compared against other biologically relevant anions.

Cyclic Aliphatic Hydrocarbons as Linkers in Metal-Organic Frameworks: New Frontiers for Ligand Design.

In this Minireview we outline the development of cyclic aliphatic moieties as ligands in metal-organic frameworks (MOFs), with a focus on the relationship between ligand design and synthesis and the properties of the subsequent materials. Aliphatic ligands have received considerably less attention than aromatic analogues in MOF chemistry but offer advantages in their unique combinations of geometric and electronic properties which are unattainable from conventional ligands. Here, we focus on rigid and semi-rigid backbone moieties derived from monocyclic and fused polycyclic aliphatic backbones, including cyclohexane and adamantane, cubane and bicyclo[2.2.2]octane, and discuss the synthetic chemistry of these species along with their potential importance as the next generation of building blocks for microporous materials.

A Series of Manganese(III) Salen Complexes as a Result of Team-Based Inquiry in a Transnational Education Programme.

The development of a team-based approach to research-led transnational practical chemistry teaching is described in which a team of five Chinese students on an articulated transnational degree programme, supported by a team of academic and technical staff, carried out a study examining the structural chemistry of a series of manganese(III) salen complexes. A series of four crystallographically characterized manganese(III) salen complexes with ancillary carboxylate ligands are reported here. The carboxylate coordination modes range from the bridging syn-anti µ2 -κO : κO' mode observed in the predominant cyclohexanoate and isobutyrate species, to a capping terminal monodentate mode for the adamantanoate species, and an unusual mixture of bridging and terminal coordination modes observed in a second minor phase of the cyclohexanoate species. The variation on extended structures based on the weakly interacting aliphatic backbones may provide a useful basis for further structural studies.