Submerging a Biomimetic Metallo-Receptor in Water for Molecular Recognition: Micellar Incorporation or Water Solubilization? A Case Study.

Molecular recognition in water is an important topic, but a challenging task due to the very competitive nature of the medium. The focus of this study is the comparison of two different strategies for the water solubilization of a biomimetic metallo-receptor based on a poly(imidazole) resorcinarene core. The first relies on a new synthetic path for the introduction of hydrophilic substituents on the receptor, at a remote distance from the coordination site. The second involves the incorporation of the organosoluble metallo-receptor into dodecylphosphocholine (DPC) micelles, which mimic the proteic surrounding of the active site of metallo-enzymes. The resorcinarene ligand can be transferred into water through both strategies, in which it binds ZnII over a wide pH window. Quite surprisingly, very similar metal ion affinities, pH responses, and recognition properties were observed with both strategies. The systems behave as remarkable receptors for small organic anions in water at near-physiological pH. These results show that, provided the biomimetic site is well structured and presents a recognition pocket, the micellar environment has very little impact on either metal ion binding or guest hosting. Hence, micellar incorporation represents an easy alternative to difficult synthetic work, even for the binding of charged species (metal cations or anions), which opens new perspectives for molecular recognition in water, whether for sensing, transport, or catalysis.

Photochromism and Dual-Color Fluorescence in a Polyoxometalate-Benzospiropyran Molecular Switch.

The photophysical properties of a Keggin-type polyoxometalate (POM) covalently bounded to a benzospiropyran (BSPR) unit have been investigated. These studies reveal that both closed and open forms are emissive with distinct spectral features (λem (closed form)=530 nm, λem (open form)=670 nm) and that the fluorescence of the BSPR unit of the hybrid is considerably enhanced compared to BSPR parent compounds. While the fluorescence excitation energy of the BSPR reference compounds (370 nm) is close to the intense absorption responsible of the photochromic character (350 nm), the fluorescence excitation of the hybrid is shifted to lower energy (400 nm), improving the population of the emissive state. Combined NOESY NMR and theoretical calculations of the closed form of the hybrid give an intimate understanding of the conformation adopted by the hybrid and show that the nitroaryl moieties of the BSPR is folded toward the POM, which should affect the electronic properties of the BSPR.

Supramolecular control of a mononuclear biomimetic copper(II) center: bowl complexes vs funnel complexes.

Modeling the mononuclear site of copper enzymes is important for a better understanding of the factors controlling the reactivity of the metal center. A major difficulty stems from the difficult control of the nuclearity while maintaining free sites open to coordination of exogenous ligands. A supramolecular approach consists in associating a hydrophobic cavity to a tripodal ligand that will define the coordination spheres as well as access to the metal ion. Here, we describe the synthesis of a bowl Cu(II) complex based on the resorcinarene scaffold. This study supplements a previous work on Cu(I) coordination. It provides a complete picture of the cavity-copper system in its two oxidation states. The first XRD structure of such a bowl complex was obtained, evidencing a 5-coordinate Cu(II) ion with the three imidazole donors bound to the metal (two in the base of the pyramid, one in the apical position) and with an acetate anion, completing the base of the pyramid, and deeply included in the bowl. Solution studies conducted by EPR and UV-vis absorption spectroscopies as well as cyclic voltammetry highlighted interaction with coordinating solvents, various carboxylates that can sit either in the endo or in the exo position depending on their size as well as possible stabilization of hydroxo species in a mononuclear state. A comparison of the binding and redox properties of the bowl complex with funnel complexes based on the calix[6]arene core further highlights the importance of supramolecular features defining the first, second, and third coordination sphere for control of the metal ion.

Photochromic properties of polyoxotungstates with grafted spiropyran molecules.

The first systems associating in a single molecule polyoxotungstates (POTs) and photochromic organic groups have been elaborated. Using the (TBA)4[PW11O39{Sn(C6H4I)}] precursor, two hybrid organic-inorganic species where a spiropyran derivative (SP) has been covalently grafted onto a {PW11Sn} fragment via a Sonogashira coupling have been successfully obtained. Alternatively, a complex containing a silicotungstate {PW11Si2} unit connected to two spiropyran entities has been characterized. The purity of these species has been assessed using several techniques, including (1)H and (31)P NMR spectroscopy, mass spectrometry, and electrochemical measurements. The optical properties of the hybrid materials have been investigated both in solution and in the solid state. These studies reveal that the grafting of SPs onto POTs does not significantly alter the photochromic behavior of the organic chromophore in solution. In contrast, these novel hybrid SP-POT materials display highly effective solid-state photochromism from neutral SP molecules initially nonphotochromic in the crystalline state. The photoresponses of the SP-POT systems in the solid state strongly depend on the nature and the number of grafted SP groups.

Generation and characterisation of a stable nickel(ii)-aminoxyl radical complex.

A stable nickel(ii)-aminoxyl radical complex was generated by the reaction of a nickel(ii) complex supported by a tren ligand (tris(2-aminoethyl)amine) having bulky m-terphenyl substituents (TIPT: 3,5-bis(2,6-diisopropylphenyl)phenyl) and m-CPBA (m-chloroperoxybenzoic acid). The formation mechanism of the nickel(ii)-aminoxyl radical complex was examined.