Sumanene-carbazole conjugate with push-pull structure and its chemoreceptor application.

The first-of-its-kind tetra-substituted sumanene derivative, featuring the push-pull chromophore architecture, has been successfully designed. The inclusion of both strong electron-withdrawing (CF3) and electron-donating (carbazole) moieties in this buckybowl compound has enhanced the charge transfer characteristics of the molecule. This enhancement was supported by ultraviolet-visible (UV-Vis) and emission spectra analyses along with density functional theory (DFT) calculations. The application of the title sumanene-carbazole push-pull chromophore as a selective recognition material for cesium cations (Cs+) was also presented. The title compound exhibited effective and selective Cs+-trapping ability, characterized by a high apparent binding constant value (at the level of 105) and a low limit of detection (0.09-0.13 µM). Owing to the tuned optical properties of the title push-pull chromophore, this study marks the first time in sumanene-tethered chemoreceptor chemistry where efficient tracking of Cs+ binding was possible with both absorption and fluorescence spectroscopies. This work introduces a new approach toward tuning the structure of bowl-shaped optical chemoreceptors.

Exploiting thiol-functionalized benzosiloxaboroles for achieving diverse substitution patterns - synthesis, characterization and biological evaluation of promising antibacterial agents.

Benzosiloxaboroles are an emerging class of medicinal agents possessing promising antimicrobial activity. Herein, the expedient synthesis of two novel thiol-functionalized benzosiloxaboroles 1e and 2e is reported. The presence of the SH group allowed for diverse structural modifications involving the thiol-Michael addition, oxidation, as well as nucleophilic substitution giving rise to a series of 27 new benzosiloxaboroles containing various polar functional groups, e.g., carbonyl, ester, amide, imide, nitrile, sulfonyl and sulfonamide, and pendant heterocyclic rings. The activity of the obtained compounds against selected bacterial and yeast strains, including multidrug-resistant clinical strains, was investigated. Compounds 6, 12, 20 and 22-24 show high activity against Staphylococcus aureus, including both methicillin-sensitive (MSSA) and methicillin-resistant (MRSA) strains, with MIC values in the range of 1.56-12.5 µg mL-1, while their cytotoxicity is relatively low. The in vitro assay performed with 2-(phenylsulfonyl)ethylthio derivative 20 revealed that, in contrast to the majority of known antibacterial oxaboroles, the plausible mechanism of antibacterial action, involving inhibition of the leucyl-tRNA synthetase enzyme, is not responsible for the antibacterial activity. Structural bioinformatic analysis involving molecular dynamics simulations provided a possible explanation for this finding.

Synthesis and crystal structure of an iron triazole complex resulting from the unexpected ligand cleavage of a triazolium carbene precursor.

Typically reactions of N-heterocyclic carbenes with transition metals are straightforward and require a carbene salt, a base strong enough to deprotonate such a salt and a metal. Yet when carbene precursors are in the form of triazolium salts, reaction may not proceed as easily as expected. In our work, we intended to obtain a triazolylidene complex of iron(II) chloride, but due to the presence of small amounts of water in the tetrahydrofuran solvent used, bis(acetonitrile)tetrakis(1-benzyl-1H-1,2,4-triazole-κN4)iron(II) µ-oxido-bis[trichloridoferrate(III)] acetonitrile disolvate, [Fe(C9H9N3)4(CH3CN)2][Fe2Cl6O]·2CH3CN - an interesting anion with a linear geometry of the O atom - was formed instead of the iron carbene complex. Reaction proceeded via cleavage of the alkyl N-substituent of the triazolium salt. The formation of the product was confirmed by X-ray crystallography. The crystal structure and possible reaction pathways are discussed.

Expanding the library of sumanene molecular receptors for caesium-selective potentiometric sensors.

This paper reports the synthesis and characterization of eight sumanene molecular receptors for the selective recognition of caesium cations (Cs+). The sumanene derivatives differed in the number (from one to nine), type (electron donating or electron withdrawing) and method of the attachment (functionalization of sumanene at the benzylic or aromatic carbons) of substituents in the sumanene skeleton. The ultimate goal of this work was to investigate the prospective use of various sumanene derivatives in the design of Cs+-selective potentiometric sensors, thus, expanding the library of sumanene receptors for such applications. Spectroscopic fluorescence titration with caesium hexafluorophosphate revealed that the formation of sandwich complexes is highly favourable, but the steric hindrance of bulky substituents can disrupt this preference. In the case of triaryl-substituted sumanene derivatives, theoretical calculations show that, indeed, sandwich complexes are energetically more advantageous by 2.3 times than 1 : 1 complexes. Furthermore, such functionalization significantly increases receptor solubility in the polymeric membrane of the potentiometric sensors, which was quantitatively evaluated with the COSMO model.

Aromatic Diboronic Acids as Effective KPC/AmpC Inhibitors.

Over 30 compounds, including para-, meta-, and ortho-phenylenediboronic acids, ortho-substituted phenylboronic acids, benzenetriboronic acids, di- and triboronated thiophenes, and pyridine derivatives were investigated as potential ß-lactamase inhibitors. The highest activity against KPC-type carbapenemases was found for ortho-phenylenediboronic acid 3a, which at the concentration of 8/4 mg/L reduced carbapenems' MICs up to 16/8-fold, respectively. Checkerboard assays revealed strong synergy between carbapenems and 3a with the fractional inhibitory concentrations indices of 0.1-0.32. The nitrocefin hydrolysis test and the whole cell assay with E. coli DH5α transformant carrying blaKPC-3 proved KPC enzyme being its molecular target. para-Phenylenediboronic acids efficiently potentiated carbapenems against KPC-producers and ceftazidime against AmpC-producers, whereas meta-phenylenediboronic acids enhanced only ceftazidime activity against the latter ones. Finally, the statistical analysis confirmed that ortho-phenylenediboronic acids act synergistically with carbapenems significantly stronger than other groups. Since the obtained phenylenediboronic compounds are not toxic to MRC-5 human fibroblasts at the tested concentrations, they can be considered promising scaffolds for the future development of novel KPC/AmpC inhibitors. The complexation of KPC-2 with the most representative isomeric phenylenediboronic acids 1a, 2a, and 3a was modeled by quantum mechanics/molecular mechanics calculations. Compound 3a reached the most effective configuration enabling covalent binding to the catalytic Ser70 residue.

Highly electron-deficient 3,6-diaza-9-borafluorene scaffolds for the construction of luminescent chelate complexes.

The synthesis and characterization of two fluorinated 3,6-diaza-9-hydroxy-9-borafluorene oxonium acids featuring improved hydrolytic stability and the strong electron-deficient character of the diazaborafluorene core is reported. These boracycles served as precursors of fluorescent spiro-type complexes with (O,N)-chelating ligands which revealed specific properties such as delayed emission, white light emission in the solid state and photocatalytic performance in singlet oxygen-mediated oxidation reactions.

Bis[1]benzothieno[1,4]thiaborins as a Platform for BODIPY Singlet Oxygen Photosensitizers.

Invited for the cover of this issue are Krzysztof Durka and co-workers at Warsaw University of Technology, University of Warsaw, Silesian University of Technology and Heinrich-Heine-Universität. The image depicts the generation of singlet oxygen by the BODIPY photosensitizer. Read the full text of the article at 10.1002/chem.202300680.

Bis[1]benzothieno[1,4]thiaborins as a Platform for BODIPY Singlet Oxygen Photosensitizers.

A synthetic approach to two regioisomeric π-electron extended [1,4]thiaborins annulated with two benzothiophene units has been developed. The central thiaborin rings of the boracycles obtained exhibit different electronic properties; this is reflected in their different aromatic characters, boron Lewis acidity and UV-vis spectroscopic behavior. Thiaborins were converted to boron dipyrromethene (BODIPY) complexes. Their emission spectra exhibit two distinct bands resulting from 1 LE and 1 CT transitions. Strong near-infrared phosphorescence in Zeonex thin films at 77 K indicates efficient intersystem crossing and the formation of triplet states. Separation of HOMO and LUMO orbitals between boracyclic and BODIPY moieties facilitates electron transfer to a 1 CT state followed by a transition to the 3 LE triplet state located on the ligand. These unique properties of spiro thiaborin-BODIPY complexes were explored for their application as singlet-oxygen photosensitizers. They show excellent photocatalytic performance with singlet oxygen quantum yields reaching 77 % and full conversion of the model organic substrate achieved after 1.5 h with only 0.05 % mol catalyst load.

TADF Invariant of Host Polarity and Ultralong Fluorescence Lifetimes in a Donor-Acceptor Emitter Featuring a Hybrid Sulfone-Triarylboron Acceptor.

10H-Dibenzo[b,e][1,4]thiaborinine 5,5-dioxide (SO2B)-a high triplet (T1 =3.05 eV) strongly electron-accepting boracycle was successfully utilised in thermally activated delayed fluorescence (TADF) emitters PXZ-Dipp-SO2B and CZ-Dipp-SO2B. We demonstrate the near-complete separation of highest occupied and lowest unoccupied molecular orbitals leading to a low oscillator strength of the S1 →S0 CT transition, resulting in very long ca. 83 ns and 400 ns prompt fluorescence lifetimes for CZ-Dipp-SO2B and PXZ-Dipp-SO2B, respectively, but retaining near unity photoluminescence quantum yield. OLEDs using CZ-Dipp-SO2B as the luminescent dopant display high external quantum efficiency (EQE) of 23.3 % and maximum luminance of 18600 cd m-2 with low efficiency roll off at high brightness. For CZ-Dipp-SO2B, reverse intersystem crossing (rISC) is mediated through the vibronic coupling of two charge transfer (CT) states, without involving the triplet local excited state (3 LE), resulting in remarkable rISC rate invariance to environmental polarity and polarisability whilst giving high organic light-emitting diode (OLED) efficiency. This new form of rISC allows stable OLED performance to be achieved in different host environments.

Oxazoline scaffold in synthesis of benzosiloxaboroles and related ring-expanded heterocycles: diverse reactivity, structural peculiarities and antimicrobial activity.

Two isomeric benzosiloxaborole derivatives 3a and 5a bearing fluorine and 4,4-dimethyl-2-oxazolin-2-yl substituents attached to the aromatic rings were obtained. Both compounds were prone to hydrolytic cleavage of the oxazoline ring after initial protonation or methylation of the nitrogen atom. The derivative 3c featuring N-methylammoniumalkyl ester functionality was successfully subjected to N-sulfonylation and N-acylation reactions to give respective derivatives which demonstrates its potential for modular synthesis of structurally extended benzosiloxaboroles. Compound 5c bearing N-ammoniumalkyl ester underwent conversion to a unique macrocyclic dimer due to siloxaborole ring opening. Furthermore, an unexpected 4-electron reduction of the oxazoline ring occurred during an attempted synthesis of 5a. The reaction gave rise to an unprecedented 7-membered heterocyclic system 4a comprising a relatively stable B-O-B-O-Si linkage and stabilized by an intramolecular N-B coordination. It could be cleaved to derivative 4c bearing BOH and SiMe2OH groups which acts as a pseudo-diol as demonstrated by formation of an adduct with Tavaborole. Apart from the multinuclear NMR spectroscopy characterization, crystal structures of the obtained products were determined in many cases by X-ray diffraction. Investigation of biological activity of the obtained compounds revealed that derivatives 3e and 3f with pendant N-methyl arylsulfonamide groups exhibit high activity against Gram-positive cocci such as methicillin-sensitive Staphylococcus aureus ATCC 6538P, methicillin-resistant S. aureus (MRSA) ATCC 43300 as well as the MRSA clinical strains, with MIC values in the range of 3.12-6.25 mg L-1. These two compounds also showed activity against Enterococcus faecalis ATCC 29212 and Enterococcus faecium ATCC 6057 (with MICs of 25-50 mg L-1). The results of the antimicrobial activity and cytotoxicity studies indicate that 3e and 3f can be considered as potential antibacterial agents, especially against S. aureus MRSA.

Design of a D3h-symmetry prismatic tris-(ferrocene-1,1'-diyl) molecular cage bearing boronate ester linkages.

This paper presents a simple, highly selective, and efficient (isolated yield of 68%) synthesis of a novel D3h-symmetry prismatic tris-(ferrocene-1,1'-diyl) organic cage (FcB-cage) by incorporating a boronate ester as a linkage motif. 1,1'-Diboronated derivatives of ferrocene and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) were used as the starting materials. The synthesized cage was comprehensively characterized by spectroscopic and microscopic methods, powder X-ray diffraction, thermogravimetry and voltammetry. Cyclic voltammetry analysis revealed the electronic communication between the ferrocene units of the FcB-cage. In addition, to better understand the mechanism behind the synthesis of such a cage, as well as its geometric properties, we performed DFT calculations.

Expedient Synthesis of Oxaboracyclic Compounds Based on Naphthalene and Biphenyl Backbone and Phase-Dependent Luminescence of their Chelate Complexes.

The approach to a series of six- and seven-membered oxaboraheterocycles based on naphthalene or biphenyl backbones was developed. The key synthetic step involved Br/Li exchange in respective potassium (bromoaryl)trifluoroborates followed by quenching with selected electrophiles (CO2 , DMF, Me2 Si(H)Cl) and hydrolytic workup. Two ring-expanded benzoxaborole congeners were obtained by an additional reduction step with LiAlH4 or NaBH4 . The obtained boracyclic compounds were characterized in detail by NMR spectroscopy and single-crystal X-ray diffraction. Specifically, biphenyl-based systems show dynamic behaviour interpreted in terms of inversion of non-planar seven-membered boraheterocycles. The acidity of the obtained compounds varies very strongly (pKa ranges from 3.1-9.6) depending on their structure. Due to the enhanced boron Lewis acidity, selected compounds were used as a basis for luminescent complexes with 8-hydroxyquinoline. A strong phase-dependent variation of emission-band maximum (480-527 nm) and photoluminescence quantum yield (10-95 %) was observed, which was rationalized in terms of specific aggregation effects.

Heavy-Atom Free spiro Organoboron Complexes As Triplet Excited States Photosensitizers for Singlet Oxygen Activation.

Herein, we present a new strategy for the development of efficient heavy-atom free singlet oxygen photosensitizers based on rigid borafluorene scaffolds. Physicochemical properties of borafluorene complexes can be easily tuned through the choice of ligand, thus allowing exploration of numerous organoboron structures as potent 1O2 sensitizers. The singlet oxygen generation quantum yields of studied complexes vary in the range of 0.55-0.78. Theoretical calculations reveal that the introduction of the borafluorene moiety is crucial for the stabilization of a singlet charge transfer state, while intersystem crossing to a local triplet state is facilitated by orthogonal donor-acceptor molecular architecture. Our study shows that quantitative oxidation of selected organic substrates can be achieved in 20-120 min of irradiation with only 0.05 mol % loading of a photocatalyst.

(Trifluoromethoxy)Phenylboronic Acids: Structures, Properties, and Antibacterial Activity.

Three isomers of (trifluoromethoxy)phenylboronic acids were studied in the context of their physicochemical, structural, antimicrobial and spectroscopic properties. They were characterized by 1H, 13C, 11B and 19F NMR spectroscopy. The acidity of all the isomers was evaluated by both spectrophotometric and potentiometric titrations. The introduction of the -OCF3 group influences the acidity, depending, however, on the position of a substituent, with the ortho isomer being the least acidic. Molecular and crystal structures of ortho and para isomers were determined by the single crystal XRD method. Hydrogen bonded dimers are the basic structural motives of the investigated molecules in the solid state. In the case of the ortho isomer, intramolecular hydrogen bond with the -OCF3 group is additionally formed, weaker, however, than that in the analogous -OCH3 derivative, which has been determined by both X-Ray measurements as well as theoretical DFT calculations. Docking studies showed possible interactions of the investigated compounds with LeuRS of Escherichia coli. Finally, the antibacterial potency of studied boronic acids in vitro were evaluated against Escherichia coli and Bacillus cereus.

Boronate Covalent and Hybrid Organic Frameworks Featuring PIII and P=O Lewis Base Sites.

Invited for the cover of this issue is Sergiusz Lulinski and his co-workers from Warsaw University of Technology and University of Warsaw. The image depicts a journey through the interior of the porous Covalent Organic Framework containing phosphorus and boron centres with carbon dioxygen trapped inside an imagined cave. Read the full text of the article at 10.1002/chem.202001960.

Differential Sensing of Saccharides Based on an Array of Fluorinated Benzosiloxaborole Receptors.

Fluorinated benzosiloxaboroles-silicon congeners of benzoxaboroles, were synthesized and tested as molecular receptors for mono- and disaccharides. The receptors differed in the Lewis acidity of the boron center as well as in the number of potential binding sites. The calculated stability constants indicated different binding affinity of benzosiloxaborole derivatives towards selected saccharides, enabling their classification using a receptor array-based sensing. Unique fluorescence fingerprints were created on the basis of competitive interactions of the studied receptors with both Alizarin Red S (ARS) and tested saccharide molecules. Detailed chemometric analysis of the obtained fluorescence data (based on partial least squares-discriminant analysis and hierarchical clustering analysis) provided the differential sensing of common saccharides, in particular the differentiation between glucose and fructose. In addition, DFT calculations were carried out to shed light on the binding mechanism under different pH conditions.

Boronate Covalent and Hybrid Organic Frameworks Featuring PIII and P=O Lewis Base Sites.

Two covalent organic frameworks comprising Lewis basic PIII centers and Lewis acidic boron atoms were prepared by poly-condensation reactions of newly obtained tris(4-diisopropoxyborylphenyl)phosphine with 2,3,6,7,10,11-hexahydroxytriphenylene and 2,3,6,7-tetrahydroxy-9,10-dimethylanthracene. Obtained materials exhibit significant sorption of dihydrogen (100 cm3 g-1 at 1 bar at 77 K), methane (20 cm3 g-1 at 1 bar at 273 K) and carbon dioxide (50 cm3 g-1 at 1 bar at 273 K). They were exploited as solid-state ligands for coordination of Pd0 centers. Alternatively, in a bottom-up approach, boronated phosphine was treated with Pd2 dba3 and poly-condensated, yielding hybrid materials where the polymer networks are formed by means of covalent boronate linkages and coordination P-Pd bonds. In addition, the analogous materials based on phosphine oxide were synthesized. The DFT calculations on framework-guest interactions revealed that the behavior of adjacent boron and phosphorus/phosphine oxide centers is reminiscent of that found in Frustrated Lewis Pairs and may improve sorption of selected molecules.

Experimental and Theoretical Insights into Molecular and Solid-State Properties of Isomeric Bis(salicylaldehydes).

A series of five bis(salicylaldehydes), including four isomeric compounds based on a benzene scaffold and a closely related naphthalene derivative, were investigated in order to elucidate the impact of resonance effects and intramolecular hydrogen bonds (HBs) on the macroscopic properties of these systems. Density functional theory (DFT) computations revealed important differences between isomers on the molecular level, which was reflected in different charge distributions, aromatic C-C bond orders, and aromaticity characters. The consequences of these features were evidenced by the UV-vis absorption spectra: for 1,3-diformyl-4,6-dihydroxybenzene (2), the longest wavelength absorption band is observed at 285 nm, while its isomers 1,4-diformyl-2,5-dihydroxybenzene (1), 1,4-diformyl-2,3-dihydroxybenzene (3), and 1,2-diformyl-3,6-dihydroxybenzene (4) are characterized by absorption in the visible range (379-407 nm). The specificity of 2 results from simultaneous lowering and elevation of HOMO and LUMO energy levels, respectively. We have found that the HOMO/LUMO energy variations follow trends observed in isomeric dihydroxybenzenes (HOMO) and phthalaldehydes (LUMO), and these effects operate separately to some extent. Furthermore, theoretical calculations indicate that the UV-vis spectral properties of bis(salicylaldehydes) are directly transferable to the corresponding bis(salicylaldimines) and their boron complexes. Finally, the influence of structural and molecular stabilization effects was analyzed by means of X-ray structural analysis and periodic DFT computations.

Cationic and Betaine-Type Boronated Acridinium Dyes: Synthesis, Characterization, and Photocatalytic Activity.

A series of isomeric boronated acridinium dyes were obtained by reactions of 10-(4'-octyloxyphenyl) functionalized 9(10H)-acridanone derivative with lithiated phenylboronic azaesters followed by aromatization with perchloric acid. The effect of the position of boronic group attached at ortho, meta, and para positions of the 9-phenyl ring on the photophysical properties was investigated. Conversion to related betaine trifluoroborato-substituted compounds was successfully performed, and the effect of this structural change on UV-vis absorption and fluorescence spectroscopy characteristics was established. Furthermore, cyclic voltammetry studies revealed that electrochemical behavior of cationic versus betaine structures is different in terms of redox potential values as well as stability. The theoretical calculations revealed a different scheme for molecular excitation processes in B(OH)2 versus BF3 --substituted compounds as charge transfer to acridinium core is observed from N-aryl or B-aryl moiety, respectively. Obtained compounds were active as photocatalysts in selected visible-light-promoted addition reactions to unsaturated substrates.

Ionic Porous Organic Polymers Based on Functionalized Tetraarylborates.

Lithium tetrakis(4-boronatoaryl)borates were subjected to polycondensation reactions with selected polyhydroxyl monomers such as 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and 2,3,6,7-tetrahydroxy-9,10-dimethylanthracene (THDMA). Obtained boronate-type ionic porous polymers TAB1-4 were characterized by PXRD, 6Li and 11B magic-angle spinning nuclear magnetic resonance (MAS NMR), FT-IR, SEM, and TGA. They exhibit relatively good sorption of H2 (up to 75 cm3/g STP), whereas N2 uptake at 77 K for lower pressure range is relatively poor (up to 50 cm3/g STP below P/P0 = 0.8). In addition, the effect of elongation of aryl arms in the tetraarylborate core on the materials' properties was studied. Thus, it was found that replacement of the 4-boronatophenyl with 4-boronatobiphenylyl group has a negative impact on the sorption characteristics.