Imidazo[1,2-a]pyridine and Imidazo[1,5-a]pyridine: Electron Donor Groups in the Design of D-π-A Dyes.

This work investigates the electron-donating capabilities of two 10-π electron nitrogen bridgehead bicyclic [5,6]-fused ring systems, imidazo[1,2-a]pyridine and imidazo[1,5-a]pyridine rings. Eight compounds with varying positions of electron-withdrawing moieties (TCF or DCI) coupled to the imidazopyridine ring were synthesized and studied. DCI-containing compounds (Ib-IVb) exhibited a purely dipolar nature with broad absorption bands, weak fluorescence, large Stokes shifts, and strong solvatochromism. In contrast, TCF-containing compounds (Ia-IVa) demonstrated diverse properties. Imidazo[1,2-a]pyridine derivatives Ia and IIa were purely dipolar, while imidazo[1,5-a]pyridine derivatives IIIa and IVa displayed a cyanine-like character with intense absorption and higher quantum yields of emission. The observed gradual red shift in optical properties with changing electron-donor groups (IIb < Ib < IIIb < IVb) and (IIa < Ia < IIIa < IVa) underscores the stronger electron-donor character of imidazo[1,5-a]pyridine compared to that of imidazo[1,2-a]pyridine. Furthermore, crystalline powders of imidazo[1,2-a]pyridine derivatives exhibited fluorescence despite minimal emission in solution. Two compounds (Ib and IVa) were successfully formulated into nanoparticles for potential in vivo imaging applications in zebrafish embryos.

π-Bonding of Group 11 Metals to a Tantalum Alkylidyne Alkyl Complex Promotes Unusual Tautomerism to Bis-alkylidene and CO2 to Ketenyl Transformation.

A salt metathesis synthetic strategy is used to access rare tantalum/coinage metal (Cu, Ag, Au) heterobimetallic complexes. Specifically, complex [Li(THF)2][Ta(CtBu)(CH2tBu)3], 1, reacts with (IPr)MCl (M = Cu, Ag, Au, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to afford the alkylidyne-bridged species [Ta(CH2tBu)3(µ-CtBu)M(IPr)] 2-M. Interestingly, π-bonding of group 11 metals to the Ta─C moiety promotes a rare alkylidyne alkyl to bis-alkylidene tautomerism, in which compounds 2-M are in equilibrium with [Ta(CHtBu)(CH2tBu)2(µ-CHtBu)M(IPr)] 3-M. This equilibrium was studied in detail using NMR spectroscopy and computational studies. This reveals that the equilibrium position is strongly dependent on the nature of the coinage metal going down the group 11 triad, thus offering a new valuable avenue for controlling this phenomenon. Furthermore, we show that these uncommon bimetallic couples could open attractive opportunities for synergistic reactivity. We notably report an uncommon deoxygenative carbyne transfer to CO2 resulting in rare examples of coinage metal ketenyl species, (tBuCCO)M(IPr), 4-M (M = Cu, Ag, Au). In the case of the Ta/Li analogue 1, the bis(alkylidene) tautomer is not detected, and the reaction with CO2 does not cleanly yield ketenyl species, which highlights the pivotal role played by the coinage metal partner in controlling these unconventional reactions.

N-Alkyl substituted triazenide-bridged homoleptic iron(II) dimers with an exceptionally short Fe-Fe bond.

Dinuclear transition metal complexes with direct metal-metal interactions have the potential to generate unique reactivities and properties. Using asymmetric triazine ligands HN3tBuR (R = Et, iPr, nBu) featuring different alkyl substituents at 1,3-N centers, we report here the first rational synthesis of 'tetragonal lantern' type Fe(II) triazenides [Fe2(N3tBuR)4] [R = Et (1), iPr (2), nBu (3)] having an exceptionally short Fe-Fe distance (2.167-2.174 Å). Unlike the previously reported lantern structures with related amidinate or guanidinate ligands, highly air-sensitive 1-3 show a lower spin ground state, as indicated by Mössbauer, 1H NMR and DFT studies.

Expanding the horizons of porphyrin metal-organic frameworks via catecholate coordination: exploring structural diversity, material stability and redox properties.

Porphyrin based Metal-Organic Frameworks (MOFs) have generated high interest because of their unique combination of light absorption, electron transfer and guest adsorption/desorption properties. In this study, we expand the range of available MOF materials by focusing on the seldom studied porphyrin ligand H10TcatPP, functionalized with tetracatecholate coordinating groups. A systematic evaluation of its reactivity with M(iii) cations (Al, Fe, and In) led to the synthesis and isolation of three novel MOF phases. Through a comprehensive characterization approach involving single crystal and powder synchrotron X-ray diffraction (XRD) in combination with the local information gained from spectroscopic techniques, we elucidated the structural features of the solids, which are all based on different inorganic secondary building units (SBUs). All the synthesized MOFs demonstrate an accessible porosity, with one of them presenting mesopores and the highest reported surface area to date for a porphyrin catecholate MOF (>2000 m2 g-1). Eventually, the redox activity of these solids was investigated in a half-cell vs. Li with the aim of evaluating their potential as electrode positive materials for electrochemical energy storage. One of the solids displayed reversibility during cycling at a rather high potential (∼3.4 V vs. Li+/Li), confirming the interest of redox active phenolate ligands for applications involving electron transfer. Our findings expand the library of porphyrin-based MOFs and highlight the potential of phenolate ligands for advancing the field of MOFs for energy storage materials.

Acid/Base-Triggered Photophysical and Chiroptical Switching in a Series of Helicenoid Compounds.

A series of molecules that possess two quinolines, benzoquinolines, or phenanthrolines connected in a chiral fashion by a biaryl junction along with their water-soluble derivatives was developed and characterized. The influence of the structure on the basicity of the nitrogen atoms in two heterocycles was examined and the photophysical and chiroptical switching activity of the compounds upon protonation was studied both experimentally and computationally. The results demonstrated that changes in the electronic structure of the protonated vs. neutral species, promoting a bathochromic shift of dominant electronic transitions and alternation of their character from π-to-π* to charge-transfer-type, when additionally accompanied by the high structural flexibility of a system, leading to changes in conformational preferences upon proton binding, produce particularly pronounced modifications of the spectral properties in acidic medium. The latter combined with reversibility of the read-out make some of the molecules in this series very promising multifunctional pH probes.

Synthesis and Chiroptical Properties of a Chiral Isotopologue of syn-Cryptophane-B.

We report the synthesis and absolute configuration (AC) of a chiral isotopologue of syn-cryptophane-B. Low chiral signatures were measured by polarimetry and electronic circular dichroism, whereas most significant chiroptical effects were observed by vibrational circular dichroism (VCD) and Raman optical activity (ROA). The comparison of experimental VCD and ROA spectra with those predicted by DFT calculations allows the determination of the AC of the two enantiomers as (-)589-MP-syn-2 and (+)589-PM-syn-2.

2-[(4-Chloro-phen-yl)imino]-1,2-di-phenyl-ethanone.

The title Schiff base, C20H14ClNO, obtained from the reaction of 4-chloro aniline with benzil, has an approximate T shape. The dihedral angle between the phenyl rings of the benzil unit is 74.14 (15)°. The extended structure features C-H⋯O hydrogen bonds.

Vinylic Trifluoromethylselenolation via Pd-Catalyzed C-H Activation.

Trifluorometylselenolation via C-H activation is barely described in literature. In particular, no such vinylic functionalization has been yet described. Herein, a palladium-catalyzed trifluoromethylselenolation of vinylic C-H bonds is described. The 5-methoxy-8-aminoquinoline has been used as auxiliary directing group to perform this reaction. The reaction gives excellent yields with α-substituted compounds whatever the substituents and a microwave activation can be used to accelerate the reaction. With ß-substituted substrates lower yields, but still satisfactory, are obtained. This methodology was also successfully extended to other fluoroalkylselenyl groups.

Rational Preparation of Well-Defined Multinuclear Iridium-Aluminum Polyhydride Clusters and Comparative Reactivity.

We report an original alkane elimination approach, entailing the protonolysis of triisobutylaluminum by the acidic hydrides from Cp*IrH4. This strategy allows access to a series of well-defined tri- and tetranuclear iridium aluminum polyhydride clusters, depending on the stoichiometry: [Cp*IrH3Al(iBu)2]2 (1), [Cp*IrH2Al(iBu)]2 (2), [(Cp*IrH3)2Al(iBu)] (3), and [(Cp*IrH3)3Al] (4). Contrary to most transition-metal aluminohydride complexes, which can be considered as [AlHx+3]x- aluminates and LnM+ moieties, the situation here is reversed: These complexes have original structures that are best described as [Cp*IrHx]n- iridate units surrounding cationic Al(III) fragments. This is corroborated by reactivity studies, which show that the hydrides are always retained at the iridium sites and that the [Cp*IrH3]- moieties are labile and can be transmetalated to yield potassium ([KIrCp*H3], 8) or silver (([AgIrCp*H3]n, 10) derivatives of potential synthetic interest. DFT calculations show that the bonding situation can vary in these systems, from 3-center 2-electron hydride-bridged Lewis adducts of the form Ir-H⇀Al to direct polarized metal-metal interaction from donation of d-electrons of Ir to the Al metal, and both types of interactions take place to some extent in each of these clusters.

Study of syn and anti Xenon-Cryptophanes Complexes Decorated with Aromatic Amine Groups: Chemical Platforms for Accessing New Cryptophanes.

We report the synthesis of C3-symmetric cryptophanes decorated with three aromatic amine groups on the same CTB cap and their interaction with xenon. The relative stereochemistry of these two stereoisomers syn and anti was assessed thanks to the determination of the X-ray structure of an intermediate compound. As previously observed with the tris-aza-cryptophanes analogs anti-1 and syn-2 (J. Org. Chem. 2021, 86, 11, 7648-7658), both compounds anti-5 and syn-6 show a slow in-out exchange dynamics of xenon at 11.7 T. Our work supports the idea that the presence of nitrogen atoms grafted directly onto the cryptophane backbone has a strong impact on the in-out exchange dynamics of xenon whatever their stereochemistry. This result contrasts with the case of other cryptophanes decorated solely with methoxy substituents. Finally, we demonstrate that these new derivatives can be used to design new anti/syn cryptophanes bearing suitable ligands in order to constitute potent 129Xe NMR-based sensors. An example is reported here with the synthesis of the tris-iodo derivatives anti-13 and syn-14 from compounds anti-5 and syn-6.

Impact of the Syn/Anti Relative Configuration of Cryptophane-222 on the Binding Affinity of Cesium and Thallium.

We report in this article the synthesis, the X-ray crystal structure of compound syn-2, and its binding properties with cesium and thallium in aqueous solution under basic conditions. Compound syn-2 is the diastereomeric compound of anti-1 that shows very high affinity for cesium and thallium in aqueous solution under the same conditions. Despite the close structural similarities that exist between the syn-2 and anti-1 compounds, they show large discrepancy in their ability to bind cesium and thallium cations in the same conditions. Indeed, the syn-2 derivative has a lower affinity for these two cationic species and the binding constants are several orders of magnitude lower than those found for its congener. The large differences in affinity observed with these two compounds can be explained by the relative position of the six hydroxyl groups to each other.

Effect of the Stereoselectivity of para-Menthane-3,8-diol Isomers on Repulsion toward Aedes albopictus.

Vector-borne diseases cause around 700,000 deaths every year. Insect repellents are one of the strategies to limit them. Para-menthane-3,8-diol (PMD), a natural compound, is one of the most promising alternatives to conventional synthetic repellents. This work describes a diastereodivergent method to synthesize each diastereoisomer of PMD from enantiopure citronellal and studies their repellence activity against Aedes albopictus. We found that cis-PMD is the kinetic control product of the cyclization of citronellal, while trans-PMD is the thermodynamic control product. X-ray diffraction analysis of crystals highlighted some differences in hydrogen-bond patterns between cis or trans isomers. The present paper demonstrates that (1R)-(+)-cis-PMD has the highest repellency index using a new evaluation system for 24 h. (1S)-(-)-cis-PMD has somewhat lower and (1S)-(+)-trans-PMD and (1R)-(-)-trans-PMD have a slight effect. Volunteer tests show that (1R)-(+)-cis-PMD is the most efficient. This effect could be ascribed to the interaction of PMD/insect odorant receptors and their physical properties, that is, the evaporation rate.

Aromatic Trifluoromethylselenolation via Pd-catalyzed C-H functionalization.

The synthesis of trifluoromethylselenolated aromatic molecules via an auxiliary-assisted, palladium catalyzed, C-H bonds functionalization with trifluoromethyl tolueneselenosulfonate as reagent is described. The mono- or bis-products can be preferentially formed. Some mechanistic investigations were realized to better understand the reaction. This methodology was also extended to fluoroalkylselenyl groups.

Coinage Metal Complexes with Di-tertiary-butyl Sulfide as Precursors with Ultra-Low Decomposition Temperature.

We report here the synthesis of [Cu2 (TFA)4 (t Bu2 S)2 ] (1), [Ag4 (TFA)4 (t Bu2 S)4 ] (2) and [AuCl(t Bu2 S)] (3) (TFA=trifluoroacetate), which decompose in solution medium at ultra-low temperature (e. g., in boiling toluene) to afford phase-pure and highly crystalline Cu9 S5 , Ag2 S and metallic Au nanoparticles, respectively. The low decomposition temperature of these precursors is attributed to the facile decomposition mechanism in the di-tertiary-butyl sulfide ligand. These results are a significant step in the direction of establishing a general low-temperature strategy spanning a range of systems including thermodynamically metastable materials and incorporate them in technologies that are sensitive to the harsh conditions.

Strongly Polarized Iridiumδ--Aluminumδ+ Pairs: Unconventional Reactivity Patterns Including CO2 Cooperative Reductive Cleavage.

The iridium tetrahydride complex Cp*IrH4 reacts with a range of isobutylaluminum derivatives of general formula Al(iBu)x(OAr)3-x (x = 1, 2) to give the unusual iridium aluminum species [Cp*IrH3Al(iBu)(OAr)] (1) via a reductive elimination route. The Lewis acidity of the Al atom in complex 1 is confirmed by the coordination of pyridine, leading to the adduct [Cp*IrH3Al(iBu)(OAr)(Py)] (2). Spectroscopic, crystallographic, and computational data support the description of these heterobimetallic complexes 1 and 2 as featuring strongly polarized Al(III)δ+-Ir(III)δ- interactions. Reactivity studies demonstrate that the binding of a Lewis base to Al does not quench the reactivity of the Ir-Al motif and that both species 1 and 2 promote the cooperative reductive cleavage of a range of heteroallenes. Specifically, complex 2 promotes the decarbonylation of CO2 and AdNCO, leading to CO (trapped as Cp*IrH2(CO)) and the alkylaluminum oxo ([(iBu)(OAr)Al(Py)]2(µ-O) (3)) and ureate ({Al(OAr)(iBu)[κ2-(N,O)AdNC(O)NHAd]} (4)) species, respectively. The bridged amidinate species Cp*IrH2(µ-CyNC(H)NCy)Al(iBu)(OAr) (5) is formed in the reaction of 2 with dicyclohexylcarbodiimine. Mechanistic investigations via DFT support cooperative heterobimetallic bond activation processes.

Designed sol-gel precursors for atomically dispersed Nb and Pb within TiO2 as catalysts for dihydroxyacetone transformation.

The 'bottom-up' synthesis of homogeneously doped metal oxide nanoparticles (NPs) with precision at the atomic/molecular level offers many advantages. We report here the synthesis, structural characterization and hydrolytic behavior of new N-methyldiethanolamine-modified precursors of Ti(iv), Nb(v) and Pb(ii) and their application as excellent sol-gel precursors for obtaining atomically dispersed Nb and Pb within TiO2 having high surface areas (253-355 m2 g-1) and tunable acidic properties. Preliminary results on the use of these doped TiO2 as water-tolerant catalysts in transforming dihydroxyacetone to lactic acid are presented.

Proton-detected fast-magic-angle spinning NMR of paramagnetic inorganic solids.

Fast (60 kHz) magic angle spinning solid-state NMR allows very sensitive proton detection in highly paramagnetic organometallic powders. We showcase this technique with the complete assignment of 1H and 13C resonances in a high-spin Fe(ii) polymerisation catalyst with less than 2 mg of sample at natural abundance.

(2R)- and (2S)- 2-hydroxy- hexanoyl and octanoyl-l-homoserine lactones: New highly potent Quorum Sensing modulators with opposite activities.

The synthesis and the QS modulation activity of diastereoisomerically pure 2-hydroxy-N-acyl-l-homoserine lactones (2-OH-AHLs) are unveiled. (2R)- and (2S)- 2-hydroxy-N-hexanoyl-l-homoserine lactone and 2-hydroxy-N-octanoyl-l-homoserine lactone have been identified as very potent QS agonists and antagonists on the Vibrio fischeri-quorum sensing system with opposite activities depending on the configuration of the carbon atom with the hydroxyl group. Flexible molecular docking showed that the (2R)-OH configuration in the antagonist isomer induces new hydrogen bonds with Tyr70 and Asp79, two importantly conserved residues in the LuxR protein family, while the (2S)-OH agonist configuration exhibits a binding mode comparable to the natural ligand 3-oxo-hexanoyl-l-homoserine lactone (OHHL). For the analogs with long alkyl chain 3a and 3b and aromatic analogs, all are antagonists with no effect of the configuration at C-2.

Molecules versus Nanoparticles: Identifying a Reactive Molecular Intermediate in the Synthesis of Ternary Coinage Metal Chalcogenides.

The identification of reactive intermediates during molecule-to-nanoparticle (NP) transformation has great significance in comprehending the mechanism of NP formation and, therefore, optimizing the synthetic conditions and properties of the formed products. We report here the room temperature (RT) synthesis of AgCuSe NPs from the reaction of di-tert-butyl selenide with trifluoroacetates (TFA) of silver(I) and copper(II). The isolation and characterization of a molecular species during the course of this reaction, [Ag2Cu(TFA)4(tBu2Se)4] (1), which shows extraordinary reactivity and interesting thermochromic behavior (blue at 0 °C and green at RT), confirmed that ternary metal selenide NPs are formed via this intermediate species. Similar reactions with related dialkyl chalcogenide R2E resulted in the isolation of molecular species of similar composition, [Ag2Cu(TFA)4(R2E)4] [R = tBu, E = S (2); R = Me, E = Se (3); R = Me, E = S (4)], which are stable at RT but can be converted to ternary metal chalcogenides at elevated temperature. Density functional theory calculations confirm the kinetic instability of 1 and throw light on its thermochromic properties.