NMR spectroscopy to study cyclodextrin-based host-guest assemblies with polynuclear clusters.

Natural cyclodextrin (CD) macrocycles are known to form diverse inclusion complexes with a wide variety of organic molecules, but recent work has revealed that inorganic clusters also form multicomponent supramolecular complexes and edifices. Such molecular assemblies exhibit a high degree of organization in solution governed by various chemical processes including molecular recognition, host-guest attraction, hydrophobic repulsion, or chaotropic effect. Nuclear magnetic resonance (NMR) spectroscopy is one of the most efficient and practical analytical techniques to characterize the nature, the strength and the mechanism of these interactions in solution. This review provides a brief overview on recent examples of the contribution of NMR to the characterization of hybrid systems in solution based on CD with polynuclear clusters, including polyoxometalates (POMs), metallic clusters and hydroborate clusters. The focus will be first on using 1H (and 13C) NMR of the host, i.e., CD, to identify the nature of the interactions and measure their strength. Then, 2D NMR methods will be illustrated by DOSY as a means of highlighting the clustering phenomena, and by NOESY/ROESY to evidence the spatial proximity and contact within the supramolecular assemblies. Finally, other NMR nuclei will be selected to probe the inorganic part as a guest molecule. Attention will be paid to classical host-guest complexes Cluster@CD, but also to hierarchical multi-scale, multi-component assemblies such as Cluster@CD@Cluster.

A new series of bioactive Mo(V)2O2S2-based thiosemicarbazone complexes: Solution and DFT studies, and antifungal and antioxidant activities.

This paper deals with the synthesis, characterization, and studies of biological properties of a series of 5 coordination compounds based on binuclear core [Mo(V)2O2S2]2+ with thiosemicarbazones ligands bearing different substituents on the R1 position of the ligand. The complexes are first studied using MALDI-TOF mass spectrometry and NMR spectroscopy to determine their structures in solution in relation to single-crystal X-Ray diffraction data. In a second part, the antifungal and antioxidative activities are explored and the high potential of these coordination compounds compared to the uncoordinated ligands is demonstrated for these properties. Finally, DFT calculation provides important support to the solution studies by identifying the most stable isomers in each [Mo2O2S2]2+/Ligand system, while the determination of HUMO and LUMO levels is performed to explain the antioxidative properties of these systems.

Tuning the nuclearity of [Mo2O2S2]2+-based assemblies by playing with the degree of flexibility of bis-thiosemicarbazone ligands.

[MoV2O2S2]2+-based thiosemicarbazone complexes appear as very promising molecules for biological applications due to the intrinsic properties of their components. This paper deals with the synthesis and characterization of six coordination complexes obtained by the reaction of [MoV2O2S2]2+ clusters with bis-thiosemicarbazone ligands that contain flexible or rigid spacers between the two thiosemicarbazone units. Interestingly, structural characterization by single-crystal X-ray diffraction, MALDI-TOF MS technique and NMR spectroscopy revealed that the nuclearity of the complex is controlled by the nature of the spacer between the thiosemicarbazone units. Binuclear complexes, namely [MoV2O2S2(L1-3)], are isolated with flexible spacers while tetranuclear complexes [(MoV2O2S2)2(L4-6)2] are formed when the bis-thiosemicarbazone ligands are built on rigid spacers.

Perfluoroalkyl Selenoxides, Selenones and Selenoximines: General Preparation and Properties.

A selective access to perfluoroalkyl selenoxides, via Oxone® as oxidant or to selenones by using a Polyoxometalate-based Ionic Liquid (POM-IL) as a catalyst for the oxidation step is described. The reaction works with various perfluoralkyl chains and substituents with satisfactory to excellent yields. A two-step one-pot reaction from selenocyanates was performed to gain access to perfluoroalkyl selenoxides. The previously unknown perfluoroalkyl selenoximines family was also prepared with good yields. Having unlocked two strategies for the synthesis of fluoroalkylated SeIV and SeVI compounds, we then evaluated the Hansch-Leo lipophilicity parameters of these groups. Finally, asymmetric aryl perfluoroalkyl selenoximines were resolved to determine their absolute configurations.

Grafting of Anionic Decahydro-Closo-Decaborate Clusters on Keggin and Dawson-Type Polyoxometalates: Syntheses, Studies in Solution, DFT Calculations and Electrochemical Properties.

Herein we report the synthesis of a new class of compounds associating Keggin and Dawson-type Polyoxometalates (POMs) with a derivative of the anionic decahydro-closo-decaborate cluster [B10H10]2- through aminopropylsilyl ligand (APTES) acting as both a linker and a spacer between the two negatively charged species. Three new adducts were isolated and fully characterized by various NMR techniques and MALDI-TOF mass spectrometry, notably revealing the isolation of an unprecedented monofunctionalized SiW10 derivative stabilized through intramolecular H-H dihydrogen contacts. DFT as well as electrochemical studies allowed studying the electronic effect of grafting the decaborate cluster on the POM moiety and its consequences on the hydrogen evolution reaction (HER) properties.

Synthesis, Structures, and Solution Studies of a New Class of [Mo2O2S2]-Based Thiosemicarbazone Coordination Complexes.

This paper deals with the synthesis, structural studies, and behavior in solution of unprecedented coordination complexes built by the association of a panel of 14 representative thiosemicarbazone ligands with the cluster [Mo2O2S2]2+. These complexes have been thoroughly characterized both in the solid state and in solution by XRD and by NMR, respectively. In particular, HMBC 1H{15N} and 1H DOSY NMR experiments bring important elements for understanding the complexes' behavior in solution. These studies demonstrate that playing on the nature and the position of various substituents on the ligands strongly influences the coordination modes of the ligands as well as the numbers of isomers in solution, mainly 2 products for the majority of complexes and up to 5 for some of them.

Screening of biological properties of MoV2O2S2- and MoV2O4-based coordination complexes: Investigation of antibacterial, antifungal, antioxidative and antitumoral activities versus growing of Spirulina platensis biomass.

This paper deals with the biological potential of coordination compounds based on binuclear core [MoV2O2E2]2+ (E = O or S) coordinated with commercially available ligands such as oxalates (Ox2-), L-cysteine (L-cys2-), L-histidine (L-his-), Iminodiacetate (IDA2-), Nitrilotriacetate (HNTA2- or NTA3-) or ethylenediamine tetraacetate (EDTA4-) by means of various in vitro assays in a screening approach. Results suggest that the obtained complexes show weak antibacterial and antifungal properties while not being cytotoxic on cancerous and mammalian cells. In contrast, [Mo2O2E2(L-cys)2]2- complexes stand out as powerful antioxidant, whereas [Mo2O2E2(EDTA)]2- associating tetraphenylphosphonium counter-cations display strong antibiotic activity. Finally, some complexes have evidenced a positive activity towards the growing of spirulina platensis together with a modification of the proportions of biological components inside the cells. These findings reveal promising bioactivity of the bridged binuclear Mo(+V) cores inside complexes and encourage further research for new highly active yet non-toxic molecules for biological and biomedical applications.

Synthesis, Physical Properties and Application of a Series of New Polyoxometalate-Based Ionic Liquids.

This paper deals with the preparation and the characterization of four new ionic liquids resulting from the pairing of various polyoxotungstates or polyoxomolybdates with the cation trihexyltetradecylphosphonium. The physical properties measured by different techniques evidence that the viscosity and the rheological behaviors of such POM-based ionic liquids, POM-ILs, strongly depend on the nature of the POM, especially its charge. Playing on the nature of the POM, we can indeed obtain Newtonian liquids or some much more viscous materials exhibiting characteristics of resins or pseudo-plastics. In a second part of this study, the potentialities of using such materials both as solvent and catalyst for the oxidation of a series of alcohols are presented as proof of concept. This part highlights great differences in strength and selectivity as a function of the POM-IL used. Furthermore, a very simple way to recycle the catalyst is also presented.

Unprecedented coupling reaction between two anionic species of a closo-decahydrodecaborate cluster and an Anderson-type polyoxometalate.

A novel decahydrodecaborate-functionalized Anderson type polyoxometalate has been synthesized and characterized in solution by ESI-MS, various NMR techniques and electrochemical methods. DFT studies provide strong support to understand the properties of this hybrid system.

Probing Dynamic Library of Metal-Oxo Building Blocks with γ-Cyclodextrin.

Formation of one of the most representative polyoxometalate (POM) archetypes, namely the Lindqvist anion M6O192- with M = MoVI or WVI, was considered as elusive in aqueous solution. Herein, we report on the host-guest stabilization of the M6O192- species with M = Mo or W from aqueous solution using γ-cyclodextrin as trapping agent. The adduct {M6O19@γ-CD}2- reveals remarkable hydrolytic stability that offers new opportunities for exploring potentialities of molybdates or tungstates in the field of biology and medicine, when combined to CDs as efficient drug carrier/delivery agents.

Host-Guest Binding Hierarchy within Redox- and Luminescence-Responsive Supramolecular Self-Assembly Based on Chalcogenide Clusters and γ-Cyclodextrin.

Water-soluble salts of anionic [Re6 Q8 (CN)6 ]4- (Q=S, Se, Te) chalcogenide octahedral rhenium clusters react with γ-cyclodextrin (γ-CD) producing a new type of inclusion compounds. Crystal structures determined through single-crystal X-ray diffraction analysis revealed supramolecular host-guest assemblies resulting from close encapsulations of the octahedral cluster within two γ-CDs. Interestingly, nature of the inner Q ligands influences strongly the host-guest conformation. The cluster [Re6 S8 (CN)6 ]4- interacts preferentially with the primary faces of the γ-CD while the bulkier clusters [Re6 Se8 (CN)6 ]4- and [Re6 Te8 (CN)6 ]4- exhibit specific interactions with the secondary faces of the cyclic host. Furthermore, analysis of the crystal packing reveals additional supramolecular interactions that lead to 2D infinite arrangements with [Re6 S8 (CN)6 ]4- or to 1D "bamboo-like" columns with [Re6 Se8 (CN)6 ]4- and [Re6 Te8 (CN)6 ]4- species. Solution studies, using multinuclear NMR methods, ESI-MS and Isothermal titration calorimetry (ITC) corroborates nicely the solid-state investigations showing that supramolecular pre-organization is retained in aqueous solution even in diluted conditions. Furthermore, ITC analysis showed that host-guest stability increases significantly ongoing from S to Te. At last, we report herein that deep inclusion alters significantly the intrinsic physical-chemical properties of the octahedral clusters, allowing redox tuning and near IR luminescence enhancement.

pH-Controlled One Pot Syntheses of Giant Mo2O2S2-Containing Seleno-Tungstate Architectures.

Self-assembly reactions of Na2WO4, SeO2/SeO32-, and [Mo2O2(µ-S)2(H2O)6]2+ give, depending on the pH value, two new large polyoxometalate complexes: i.e., [(γ-Se2W14O52)3(Mo2O2(µ-S)2(H2O)2)6]24- (1) isolated at pH 3.5 and [(γ-Se2W14O52)4(WO3(H2O))8(W2O5)2(W4O12)2(Mo2O2(µ-S)2(H2O)2)4(Mo2O2(µ-S)2(SeO3))4]36- (2) isolated at pH 2. Both compounds were characterized in the solid state by single-crystal X-ray diffraction, EDX, elemental analysis, TGA, and FT-IR and in solution by ESI-MS and NMR.

Nonconventional Three-Component Hierarchical Host-Guest Assembly Based on Mo-Blue Ring-Shaped Giant Anion, γ-Cyclodextrin, and Dawson-type Polyoxometalate.

In this communication, we report on a noteworthy hybrid supramolecular assembly built from three functional components hierarchically organized through noncovalent interactions. The one-pot synthesis procedure leads to the formation of large Mo-blue ring-shaped anion {Mo154}, which contains the supramolecular adduct based on the symmetric encapsulation of the Dawson-type [P2W18O62]6- anion by two γ-cyclodextrin units. Such a nanoscopic onion-like structure, noted [P2W18O62]@2γ-CD@{Mo154} has been characterized by single-crystal X-ray diffraction, thus demonstrating the capability of the giant inorganic torus to develop relevant supramolecular chemistry, probing the strong affinity of the inner and outer faces of the γ-CD for the polyoxometalate surfaces. Furthermore, interactions and behavior in solution have been studied by multinuclear NMR spectroscopy, which supports specific interactions between γ-CD and POM units. Finally, the formation of this three-component hybrid assembly from one-pot procedure, in water and using nearly stoichiometric conditions, is discussed in terms of the driving forces orchestrating this highly efficient multilevel recognition process.

Polyoxometalate, Cationic Cluster, and γ-Cyclodextrin: From Primary Interactions to Supramolecular Hybrid Materials.

Herein, we report on a three-component supramolecular hybrid system built from specific recognition processes involving a Dawson-type polyoxometalate (POM), [P2W18O62]6-, a cationic electron-rich cluster [Ta6Br12(H2O)6]2+, and γ-cyclodextrin (γ-CD). Such materials have been investigated using a bottom-up approach by studying the specific interactions between γ-CD and both types of inorganic units. Their ability to interact has been investigated in the solid state by single-crystal X-ray diffraction (XRD) and in solution using multinuclear NMR methods (including DOSY, EXSY, and COSY), electrospray ionization mass and UV-vis spectroscopies, electrochemistry, and isothermal titration calorimetry experiments. Single-crystal XRD analysis reveals that POM:γ-CD constitutes a highly versatile system which gives aggregates with 1:1, 1:2, and 1:3 stoichiometry. Surprisingly, these arrangements exhibit a common feature wherein the γ-CD moiety interacts with the Dawson-type POMs through its primary face. We present also the first structural model involving an octahedral-type metallic cluster with γ-CD. XRD study reveals that the cationic [Ta6Br12(H2O)6]2+ ion is closely embedded within two γ-CD units to give a supramolecular ditopic cation, suitable to be used as a linker within extended structure. Solution study demonstrates clearly that pre-associations exist in solution, for which binding constants and thermodynamic parameters have been determined, giving preliminary arguments about the chaotropic nature of the inorganic ions. Finally, both building blocks, i.e., the ditopic supramolecular cation {[Ta6Br12(H2O)6]@2CD}2+ and the Dawson-type anion, react together to give a three-component, well-ordered hybrid material derived either as a supramolecular hydrogel or single crystals. The solid-state structure shows an unprecedented helicoidal tubular chain resulting from the periodic alternation of POM and supramolecular cation, featuring short hydrogen-bonding contacts between the electron-poor POM and electron-rich cluster. The 1D tubular ionic polymer observed in the single crystals should make it possible to understand the long-range ordering observed within the hydrogel hybrid material. The supramolecular chemical complementarities between the γ-CD-based ditopic cation and POM open a wide scope for the design of hybrid materials that accumulate synergistic functionalities.

Two Compartmentalized Inner Receptors for the Tetramethylammonium Guest within a Keplerate-Type Capsule.

The host-guest interactions between the spherical porous Keplerate anion, [Mo132O372(CH3CO2)30(H2O)72](42-) (abbreviated {Mo132}) and the tetramethylammonium cation have been investigated extensively by one- and two-dimensional (EXSY, ROESY, and DOSY) and variable-temperature NMR. Evidence of two inner receptor sites specific for a NMe4(+) guest appears consistent with a quite striking compartmentalization phenomenon. ROESY NMR analyses showed that both sites exhibit a close spatial proximity with the hanging inner acetate groups, while a quantitative EXSY study revealed that these two sites are differentiated by their exchange rates. These NMR data support the hypothesis that these two inner sites could be delimited by the hanging inner acetate groups forming triangular (S1) or pentagonal (S2) hydrophobic pockets on the inner side of the capsule wall. Furthermore, the stability constants associated with the trapping process of the NMe4(+) guest on both the S1 and S2 sites have been determined, showing that the stability constant of the S1 sites decreases significantly as the concentration of the capsule increases gradually, while that of the S2 sites remains nearly unaffected. Such an observation has been interpreted as a result of the plugging process of the {Mo9O9} pores by the counterions NH4(+), which causes unfavorable electrostatic interactions for the NMe4(+) coordination on the proximal S1 site. Finally, the thermodynamic parameters of the NMe4(+) transfer from the solvated situation to the interior of the capsule were estimated from variable-temperature NMR experiments that provide the split of the global process into two successive events corresponding to the plugging and transfer across the inorganic shell.

Chemical Programming of the Domain of Existence of Liquid Crystals.

This work illustrates how enthalpy and entropy changes responsible for successive phase transitions of cyanobiphenyl-based liquid crystals can be combined to give cohesive free energy densities. These new parameters are able to rationalize and quantify the demixing of the melting and clearing processes that occur in thermotropic liquid crystals. Minor structural variations at the molecular level can be understood as pressure increments that alter either the melting or clearing temperatures in a predictable way. This assessment of microsegregation operating in amphiphilic molecules paves the way for the chemical programming of the domain of existence of liquid-crystalline phases.

Tunable Keplerate Type-Cluster "Mo132 " Cavity with Dicarboxylate Anions.

The internal functionalization of the Keplerate-type capsule Mo132 has been carried out by ligand exchange leading to the formation of glutarate and succinate containing species isolated as ammonium or dimethylammonium salts. Solution NMR analysis is consistent with asymmetric inner dicarboxylate ions containing one carboxylato group grafted onto the inner side of the spheroidal inorganic shell while the second hangs toward the center of the cavity. Such a disposition has been confirmed by the single-crystal X-ray diffraction analysis of the glutarate containing {Mo132 } species. A detailed NMR solution study of the ligand-exchange process allowed determining the binding constant KL of acetate (AcO(-) ), succinate (HSucc(-) ) or glutarate (HGlu(-) ) ligands at the 30 inner coordinating sites, which vary such as K AcO -

Hydrophobic effect as a driving force for host-guest chemistry of a multi-receptor Keplerate-type capsule.

The effectiveness of the interactions between various alkylammonium cations and the well-defined spherical Keplerate-type {Mo132} capsule has been tracked by (1)H DOSY NMR methodology, revealing a strong dependence on the self-diffusion coefficient of the cationic guests balanced between the solvated and the plugging situations. Analysis of the data is fully consistent with a two-site exchange regime involving the 20 independent {Mo9O9} receptors of the capsule. Furthermore, quantitative analysis allowed us to determine the stability constants associated with the plugging process of the pores. Surprisingly, the affinity of the capsule for a series of cationic guests increases continuously with its apolar character, as shown by the significant change of the stability constant from 370 to 6500 for NH4(+) and NEt4(+), respectively. Such observations, supported by the thermodynamic parameters, evidence that the major factor dictating selectivity in the trapping process is the so-called "hydrophobic effect". Computational studies, using molecular dynamics simulations, have been carried out in conjunction with the experiments. Analysis of the radial distribution functions g(r) reveals that NH4(+) and NMe4(+) ions behave differently in the vicinity of the capsule. The NH4(+) ions do not exhibit well-defined distributions when in close vicinity. In contrast, the NMe4(+) ions displayed sharp distributions related to different scenarios, such as firmly trapped or labile guest facing the {Mo9O9} pores. Together, these experimental and theoretical insights should aid in the exploitation of these giant polyoxometalates in solution for various applications.

Synthesis, characterization, and tuning of the liquid crystal properties of ionic materials based on the cyclic polyoxothiometalate [{Mo4O4S4(H2O)3(OH)2}2(P8W48O184)](36-).

A series of compounds resulting from the ionic association of a nanoscopic inorganic cluster of formula [K2NaxLiy{Mo4O4S4(OH)2(H2O)3}2(HzP8W48O184)]((34-x-y-z)-), 1, with several organic cations such as dimethyldioctadecylammonium DODA(+), trimethylhexadecylammonium TMAC16(+), alkylmethylimidazoliums mimCn(+) (n = 12-20) and alkyl-dimethylimidazoliums dmimCn(+) (n = 12 and 16) was prepared and characterized in the solid state by FT-IR, EDX, Elemental analysis, TGA and solid state NMR. The solid state NMR experiments performed on (1)H, (13)C and (31)P nuclei evidenced the interactions between the cations and 1 as well as the organization of the alkyl chains of the cations within the solid. Polarized optical microscopy, DSC and SA-XRD experiments implicated mesomorphic phases for DODA(+) and mimCn(+) salts of 1. The crystallographic parameters were determined and demonstrated that the inter-lamellar spacing could be controlled upon changing the length of the alkyl chain, a very interesting result if we consider the huge size of the inorganic cluster 1 and the simple nature of the cations.

Tracking "apolar" NMe4+ ions within two polyoxothiomolybdates that have the same pores: smaller clathrate and larger highly porous clusters in action.

Two nanosized polyoxothiometalates were synthesized based on linking oxomolybdate building blocks with {Mo2O2S2}(2+) groups. Remarkably, both compounds are formed selectively primarily upon changing the related concentrations in a logical way; they exhibit common structural features based on the same {Mo9O6S3}-type pores, which result in connections between {Mo6O21} pentagons and {Mo2O2S2}(2+) linkers. Whereas the much larger spherical Mo132-type Keplerate contains twenty pores, the smaller Mo63 -type cluster remarkably contains only two. The two compounds and a similar Keplerate exhibit interesting supramolecular properties related to interactions with the unusual predominantly apolar NMe4(+) cations. Structural characterization of the Mo63 -type compound reveals in the solid state a clathrate-like species that contains four NMe4(+) cations embedded in two types of structurally well-adapted pockets. Related NMR spectroscopic investigations in solution using NMe4(+) as the NMR spectroscopic probe are in agreement with the solid-state description. (1)H NMR spectroscopic experiments (1D variable-temperature, 2D total correlation spectroscopy (TOCSY), exchange spectroscopy (EXSY), and diffusion-ordered spectroscopy (DOSY)) feature firmly immobilized and mobile NMe4(+) ions in relationship with the type of host-guest arrangements. The use of the (1)H NMR DOSY spectroscopic methodology has been successfully applied to track the interactions of the NMe4(+) cations with the {Mo9O6S3} pores of a sulfurated Keplerate, thereby allowing the first quantitative analysis of this type of plugging process. The stability constant K=(210±20) mol(-1) L is discussed related to the character of the process.