Multi-Electron Visible Light Photoaccumulation on a Dipyridylamine Copper(II)-Polyoxometalate Conjugate Applied to Photocatalytic Generation of CF3 Radicals.

This article describes the synthesis and characterization of an organic-inorganic hybrid polyoxometalate functionalized by a short link with a tripodal N-based ligand and its copper complex. Upon visible light irradiation, the latter is able to store up to three reducing equivalents. The locus of the reduction is discussed based on physicochemical measurements and DFT calculations. In the presence of Togni's reagent, this complex allows for the photocatalytic generation of CF3 radicals, opening the road to valuable synthetic applications.

Interplay Between Hydrogen Bonding and Electron Transfer in Mixed Valence Assemblies of Triarylamine Trisamides.

Hydrogen-bonding interactions are assumed to play a critical role in the long-range transport of light or charge recently observed in supramolecular assemblies of C3 -symmetrical discotic molecules. Herein, the structure of mixed valence assemblies formed by irradiating triarylamine trisamide (TATA) molecules was determined by multifarious techniques under various conditions with the aim of probing the interplay between the hydrogen bonding network and the rate of electron transport in different states (solution, gel, film). Irradiation was performed under initial states that vary by the degree of association of TATA monomers through hydrogen bonds. Firstly, a significant shift of the N-H and C=O stretching frequencies was observed by FTIR upon irradiation thus revealing an overlooked signature of TATA⋅+ species and interacting mixed valence aggregates. Secondly, gels and films both mostly consist of hydrogen-bonded TATA polymers but their EPR spectra recorded at 293 K reveal very different behaviors: localized electrons in the gels versus fully delocalized electrons in the films. Hydrogen bonding thus appears as a necessary but not sufficient condition to get fast electron transfer rates and a packing of the TATA monomers particularly suitable for charge transport is assumed to exist in the solid state. Finally, defects in the hydrogen bonding network are detected upon increasing the number of radical species in the mixed valence assemblies present in the film state without impeding the delocalization of the unpaired electrons. A delicate balance between hydrogen bonds and packing is thus necessary to get supramolecular polarons in mixed valence TATA assemblies.

N-Heterocyclic Carbene Boranes: Dual Reagents for the Synthesis of Gold Nanoparticles.

N-Heterocyclic carbenes (NHCs) have drawn considerable interest in the field of nanomaterials chemistry as highly stabilizing ligands enabling the formation of strong and covalent carbon-metal bonds. Applied to gold nanoparticles synthesis, the most common strategy consists of the reduction of a preformed NHC-AuI complex with a large excess of a reducing agent that makes the particle size difficult to control. In this paper, we report the straightforward synthesis of NHC-coated gold nanoparticles (NHC-AuNPs) by treating a commercially available gold(I) precursor with an easy-to-synthesize NHC-BH3 reagent. The latter acts as both the reducing agent and the source of surface ligands operating under mild conditions. Mechanistic studies including NMR spectroscopy and mass spectrometry demonstrate that the reduction of gold(I) generates NHC-BH2 Cl as a by-product. This strategy gives efficient control over the nucleation and growth of gold particles by varying the NHC-borane/gold(I) ratio, allowing unparalleled particle size variation over the range of 4.9±0.9 to 10.0±2.7 nm. Our strategy also allows an unprecedented precise and controlled seeded growth of gold nanoparticles. In addition, the as-prepared NHC-AuNPs exhibit narrow size distributions without the need for extensive purification or size-selectivity techniques, and are stable over months.

When Identification of the Reduction Sites in Mixed Molybdenum/Tungsten Keggin-Type Polyoxometalate Hybrids Turns Out Tricky.

The mixed molybdenum/tungsten Keggin-type polyoxometalate (POM) hybrid (TBA)4[PW9Mo2O39{Sn(C6H4I)}] (TBA = tert-butylammonium) has been prepared by the reaction between [α-PW9Mo2O39]7- and [Cl3Sn(C6H4I)] in dried acetonitrile, in the presence of tetra-n-butylammonium bromide. A further coupling reaction affords the ferrocenyl derivative (TBA)4[PW9Mo2O39{Sn(C6H4)C≡C(C6H4)Fc}]. The POM hybrids have been thoroughly characterized by NMR and IR spectroscopies. Electrochemical analysis confirms their ease of reduction compared to the all-W analogue, albeit with a second reduction process occurring at a lower potential than in the all-Mo species. It is noteworthy that the second reduction is accompanied by an unusual red shift of the electronic absorption spectrum. Whereas there is no doubt that the first reduction deals with Mo, the location of the second electron in the bireduced species, on the second Mo or on W, has thus been the subject of a cross-investigation by spectroelectrochemistry, electron spin resonance, and theoretical calculations. Finally, it came out that the second reduction is also Mo-centered with two unpaired and antiferromagnetically coupled extra electrons.

Structure-Activity Relationship Studies of New Sinapic Acid Phenethyl Ester Analogues Targeting the Biosynthesis of 5-Lipoxygenase Products: The Role of Phenolic Moiety, Ester Function, and Bioisosterism.

Sinapic acid is found in many edible plants and fruits, such as rapeseed, where it is the predominant phenolic compound. New sinapic acid phenethyl ester (SAPE) analogues were synthesized and screened as inhibitors of the biosynthesis of 5-lipoxygenase (5-LO) in stimulated HEK293 cells and polymorphonuclear leukocytes (PMNL). Inhibition of leukotriene biosynthesis catalyzed by 5-LO is a validated therapeutic strategy against certain inflammatory diseases and allergies. Unfortunately, the only inhibitor approved to date has limited clinical use because of its poor pharmacokinetic profile and liver toxicity. With the new analogues synthesized in this study, the role of the phenolic moiety, ester function, and bioisosterism was investigated. Several of the 34 compounds inhibited the biosynthesis of 5-LO products, and 20 compounds were 2-11 times more potent than zileuton in PMNL, which are important producers of 5-LO products. Compounds 5i (IC50: 0.20 µM), 5l (IC50: 0.20 µM), and 5o (IC50: 0.21 µM) bearing 4-trifluoromethyl, methyl, or methoxy substituent at meta-position of the phenethyl moiety were 1.5 and 11.5 times more potent than SAPE (IC50: 0.30 µM) and zileuton (IC50: 2.31 µM), respectively. Additionally, compound 9 (IC50: 0.27 µM), which was obtained after acetylation of the 4-hydroxyl of SAPE, was equivalent to SAPE and 8 times more active than zileuton. Furthermore, compound 20b (IC50: 0.27 µM) obtained after the bioisosteric replacement of the ester function of SAPE by the 1,2,4-oxadiazole heterocycle was equivalent to SAPE and 8 times more active than zileuton. Thus, this study provides a basis for the rational design of new molecules that could be developed further as anti 5-LO therapeutics.

Tuning Photoinduced Electron Transfer in POM-Bodipy Hybrids by Controlling the Environment: Experiment and Theory.

The optical and electrochemical properties of a series of polyoxometalate (POM) oxoclusters decorated with two bodipy (boron-dipyrromethene) light-harvesting units were examined. Evaluated here in this polyanionic donor-acceptor system is the effect of the solvent and associated counterions on the intramolecular photoinduced electron transfer. The results show that both solvents and counterions have a major impact upon the energy of the charge-transfer state by modifying the solvation shell around the POMs. This modification leads to a significantly shorter charge separation time in the case of smaller counterion and slower charge recombination in a less polar solvent. These results were rationalized in terms of Marcus theory and show that solvent and counterion both affect the driving force for photoinduced electron transfer and the reorganization energy. This was corroborated with theoretical investigations combining DFT and molecular dynamics simulations.

Modeling the Oxygen Vacancy at a Molecular Vanadium(III) Silica-Supported Catalyst.

Here we report on the use of a silanol-decorated polyoxotungstate, [SbW9O33( tBuSiOH)3]3- (1), as a molecular support to describe the coordination of a vanadium atom at a single-site on silica surfaces. By reacting [V(Mes)3·thf] (Mes = 2,4,6-trimethylphenyl) with 1 in tetrahydrofuran, the vanadium(III) derivative [SbW9O33( tBuSiO)3V(thf)]3- (2) was obtained. Compound 2 displays the paramagnetic behavior expected for a d2-VIII high spin complex (SQUID measurements) with a triplet electronic ground state (ca. 30 kcal·mol-1 more stable than the singlet, from DFT calculations). Compound 2 proves to be a reliable model for reduced isolated-vanadium atom dispersed on silica surfaces [(≡Si-O)3VIII(OH2)], an intermediate that is often proposed in a Mars-van Krevelen type mechanism for partial oxidation of light alcohols. Oxidation of 2 under air produced the oxo-derivative [SbW9O33( tBuSiO)3VO]3- (3). In compound 2, the d2-electrons are localized in degenerated d(V) orbitals, whereas in the electronically analogous bireduced-[SbW9O33( tBuSiO)3VO]5-, 3·(2e), one electron is localized on d(V) orbital and the second one is delocalized on the polyoxotungstic framework, leading to a unique case of a bireduced heteropolyanion derivative with completely decoupled d1-V(IV) and d1-W(V). Our body of experimental results (EPR, magnetic measurements, spectroelectrochemical studies, Raman spectroscopy) and theoretical studies highlights (i) the role of the apical ligand coordination, i.e., thf (σ-donor) vs oxo (π-donor), in destabilizing or stabilizing the d(V) orbitals relative to the d(W) orbitals, and (ii) a geometrical distortion of the O3VO entity that causes a splitting of the degenerated orbitals and the stabilization of one d(V) orbital in the bireduced compound 3·(2e).

Copper-Catalyzed Aziridination with Redox-Active Ligands: Molecular Spin Catalysis.

Small-molecule catalysts as mimics of biological systems illustrate the chemists' attempts at emulating the tantalizing abilities displayed by nature's metalloenzymes. Among these innate behaviors, spin multistate reactivity is used by biological systems as it offers thermodynamic leverage towards challenging chemical reactivity but this concept is difficult to translate into the realm of synthetic organometallic catalysis. Here, we report a rare example of molecular spin catalysis involving multistate reactivity in a small-molecule biomimetic copper catalyst applied to aziridination. This behavior is supported by spin state flexibility enabled by the redox-active ligand.

Circumventing Intrinsic Metal Reactivity: Radical Generation with Redox-Active Ligands.

Nickel complexes have gained sustained attention as efficient catalysts in cross-coupling reactions and co-catalysts in dual systems due to their ability to react with radical species. Central to this reactivity is nickel's propensity to shuttle through several accessible redox states from Ni0 to NiIV . Here, we report the catalytic generation of trifluoromethyl radicals from a nickel complex bearing redox-active iminosemiquinone ligands. This unprecedented reactivity is enabled through ligand-based oxidation performing electron transfer to an electrophilic CF3+ source while the nickel oxidation state is preserved. Additionally, extension of this reactivity to a copper complex bearing a single redox equivalent is reported, thus providing a unified reactivity scheme. These results open new pathways in radical chemistry with redox-active ligands.

New Hydroxycinnamic Acid Esters as Novel 5-Lipoxygenase Inhibitors That Affect Leukotriene Biosynthesis.

Leukotrienes are inflammatory mediators that actively participate in the inflammatory response and host defense against pathogens. However, leukotrienes also participate in chronic inflammatory diseases. 5-lipoxygenase is a key enzyme in the biosynthesis of leukotrienes and is thus a validated therapeutic target. As of today, zileuton remains the only clinically approved 5-lipoxygenase inhibitor; however, its use has been limited due to severe side effects in some patients. Hence, the search for a better 5-lipoxygenase inhibitor continues. In this study, we investigated structural analogues of caffeic acid phenethyl ester, a naturally-occurring 5-lipoxygenase inhibitor, in an attempt to enhance the inhibitory activity against 5-lipoxygenase and determine structure-activity relationships. These compounds were investigated for their ability to attenuate the biosynthesis of leukotrienes. Compounds 13 and 19, phenpropyl and diphenylethyl esters, exhibited significantly enhanced inhibitory activity when compared to the reference molecules caffeic acid phenethyl ester and zileuton.

C-N Bond Formation from a Masked High-Valent Copper Complex Stabilized by Redox Non-Innocent Ligands.

The reactivity of a stable copper(II) complex bearing fully oxidized iminobenzoquinone redox ligands towards nucleophiles is described. In sharp contrast with its genuine low-valent counterpart bearing reduced ligands, this complex performs high-yielding C-N bond formations. Mechanistic studies suggest that this behavior could stem from a mechanism akin to reductive elimination occurring at the metal center but facilitated by the ligand: it is proposed that a masked high oxidation state of the metal can be stabilized as a lower copper(II) oxidation state by the redox ligands without forfeiting its ability to behave as a high-valent copper(III) center. These observations are substantiated by a combination of advanced EPR spectroscopy techniques with DFT studies. This work sheds light on the potential of redox ligands as promoters of unusual reactivities at metal centers and illustrates the concept of masked high-valent metallic species.

A Simple Synthesis of Triangular All-Metal Aromatics Allowing Access to Isolobal All-Metal Heteroaromatics.

A simple synthetic method allows the one-pot assembly of C3 -symmetric, 44-core-valence-electron, triangular Pd or Pt clusters and their heterobimetallic mixed Pd/Pt analogues. These mixed metal complexes are the first examples of stable triangular all-metal heteroaromatics. In contrast to traditional heteroaromatic molecules formed combining main-group elements, they actually retain structural and electronic features of their homonuclear analogues.

Tandem C-H activation/arylation catalyzed by low-valent iron complexes with bisiminopyridine ligands.

Tandem C-H activation/arylation between unactivated arenes and aryl halides catalyzed by iron complexes that bear redox-active non-innocent bisiminopyridine ligands is reported. Similar reactions catalyzed by first-row transition metals have been shown to involve substrate-based aryl radicals, whereas our catalytic system likely involves ligand-centered radicals. Preliminary mechanistic investigations based on spectroscopic and reactivity studies, in conjunction with DFT calculations, led us to propose that the reaction could proceed through an inner-sphere C-H activation pathway, which is rarely observed in the case of iron complexes. This bielectronic noble-metal-like behavior could be sustained by the redox-active non-innocent bisiminopyridine ligands.

Synthesis of triangular tripalladium cations as noble-metal analogues of the cyclopropenyl cation.

The first C3 -symmetric 44-core-valence-electron triangular palladium clusters, [{(SAr')(PAr3 )Pd}3 ](+) , have been synthesized by activation of the CS bond of isothioureas. Owing to delocalized metal-metal bonding, these stable complexes are the first noble-metal analogues of the π-aromatic cyclopropenyl cation [C3 H3 ](+) , with their all-metal aromaticity involving d-type atomic orbitals.

Solid-state hierarchical cyclodextrin-based supramolecular polymer constructed by primary, secondary, and tertiary azido interactions.

The crystallization of a di-azido-α-cyclodextrin revealed a polymeric self-assembly involving a variety of azido-type interactions. The crystal arrangement relies on the cooperativity of a primary azido inclusion, a secondary azido-azido interaction involving an unprecedented distribution of canonical forms, and a tertiary azido-groove interaction. The second azido group brings in a major contribution to the supramolecular structure illustrating the benefit of a difunctionalization for the generation of hierarchy.

Keggin-type polyoxometalates as Cu(II) chelators in the context of Alzheimer's disease.

Two Keggin polyoxometalates were used as new copper ligands to counteract the effects of CuII(Amyloid-ß) interaction. Their ability to remove CuII from CuII(Amyloid-ß), to stop CuII(Amyloid-ß) induced formation of reactive oxygen species and to restore apo-like self-assembly of CuII(Amyloid-ß) was shown.

Highly Luminescent and Photostable Core/Shell/Shell ZnSeS/Cu:ZnS/ZnS Quantum Dots Prepared via a Mild Aqueous Route.

An aqueous-phase synthesis of 3-mercaptopropionic acid (3-MPA)-capped core/shell/shell ZnSeS/Cu:ZnS/ZnS QDs was developed. The influence of the Cu-dopant location on the photoluminescence (PL) emission intensity was investigated, and the results show that the introduction of the Cu dopant in the first ZnS shell leads to QDs exhibiting the highest PL quantum yield (25%). The influence of the Cu-loading in the dots on the PL emission was also studied, and a shift from blue-green to green was observed with the increase of the Cu doping from 1.25 to 7.5%. ZnSeS/Cu:ZnS/ZnS QDs exhibit an average diameter of 2.1 ± 0.3 nm and are stable for weeks in aqueous solution. Moreover, the dots were found to be photostable under the continuous illumination of an Hg-Xe lamp and in the presence of oxygen, indicating their high potential for applications such as sensing or bio-imaging.

Addition of N-heterocyclic carbenes to a ruthenium(VI) nitrido polyoxometalate: a new route to cyclic guanidines.

The scope of N-atom transfer from the electrophilic ruthenium(VI) nitrido containing polyoxometalate [PW(11)O(39)Ru(VI)N](4-) has been extended to the N-heterocyclic carbene {CH(2)(Mes)N}(2)C and the coupling product {CH(2)(Mes)N}(2)CNH(2)(+) characterized by (1)H NMR and high-resolution mass spectrometry. Because guanidines display many fields of applications ranging from biology to supramolecular chemistry, this could afford an original route to the synthesis of cyclic guanidines. This also enlarges the potential of nitrido complexes in the synthesis of heterocycles, mainly illustrated in the literature through the formation of aziridines through N-atom transfer to alkenes. In the course of the reaction, the ruthenium(III)-containing polyoxometallic intermediate [PW(11)O(39)Ru(III){NC{N(Mes)CH(2)}(2)}](5-) has been thoroughly characterized by continuous-wave and pulsed electron paramagnetic resonance, which nicely confirms the presence of the organic moiety on the polyoxometallic framework, Ru K-edge X-ray absorption near-edge structure, and electrochemistry.

Mn-Doped Quinary Ag-In-Ga-Zn-S Quantum Dots for Dual-Modal Imaging.

Doping of transition metals within a semiconductor quantum dot (QD) has a high impact on the optical and magnetic properties of the QD. In this study, we report the synthesis of Mn2+-doped Ag-In-Ga-Zn-S (Mn:AIGZS) QDs via thermolysis of a dithiocarbamate complex of Ag+, In3+, Ga3+, and Zn2+ and of Mn(stearate)2 in oleylamine. The influence of the Mn2+ loading on the photoluminescence (PL) and magnetic properties of the dots are investigated. Mn:AIGZS QDs exhibit a diameter of ca. 2 nm, a high PL quantum yield (up to 41.3% for a 2.5% doping in Mn2+), and robust photo- and colloidal stabilities. The optical properties of Mn:AIGZS QDs are preserved upon transfer into water using the glutathione tetramethylammonium ligand. At the same time, Mn:AIGZS QDs exhibit high relaxivity (r 1 = 0.15 mM-1 s-1 and r 2 = 0.57 mM-1 s-1 at 298 K and 2.34 T), which shows their potential applicability for bimodal PL/magnetic resonance imaging (MRI) probes.

Electroactive benzothiazole hydrazones and their [Mo6O19]2- derivatives: promising building blocks for conducting molecular materials.

The electroactive benzothiazole hydrazone AMBTH-H(2), a new member of the 2,2'-azino-bis(N-alkylbenzothiazole) family, was synthesised in a five-step procedure and characterised by using X-ray diffraction along with two intermediates and the N-methylbenzothiazole hydrazone MBTH-H(2). Both AMBTH-H(2) and MBTH-H(2) were coupled to [Mo(6)O(19)](2-) in acetonitrile in the presence of dicyclohexylcarbodiimide and dimethylaminopyridine to give two new diazoalkane-hexamolybdates, which were isolated as tetrabutylammonium salts and characterised by using IR, UV/Vis and NMR spectroscopies, cyclic voltammetry and, for one of them, X-ray diffraction. The packing arrangement molecules in crystals of AMBTH-H(2), the redox features of the AMBTH-hexamolybdate hybrid together with a good electronic communication between the organic pi system and the molybdenum centres make these compounds very promising blocks for the synthesis of conducting molecular materials.