Chimica Inorganic Chemistry
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Scientifica Acta 3, Special issue, 61 – 69 (2009) Chimica Inorganic Chemistry Dipartimento di Chimica Generale, Università di Pavia, Viale Taramelli 12, 27100 Pavia, Italy Presenta: Antonio Poggi, [email protected] Le ricerche nel campo della Chimica Inorganica svolte presso il Dipartimento di Chimica Generale dell’U- niversità di Pavia coprono un vasto ambito di temi, che includono: sintesi, proprietà coordinanti e reattività di composti di coordinazione; chimica supramolecolare di metalli di transizione; chimica bioinorganica; applicazioni dell’analisi di attivazione neutronica a studi geochimici, ambientali, medico-legali, biomedici e archeologici; studi cinetici EPR su reazioni radicaliche indotte da radiazioni in sistemi organici e metal- lorganici. The inorganic chemistry studies carried out at the Dipartimento di Chimica Generale of the University of Pavia cover a wide range of topics, spanning from classical Inorganic Chemistry to Radiochemistry and Radiation Chemistry. 1 Introduction The main subjects of the research carried out at the Dipartimento di Chimica Generale of the University of Pavia are inorganic chemistry and analytical chemistry. This paper gives a brief overview of the current research activities in inorganic chemistry. The main themes of research include synthesis, binding proper- ties and reactivity of coordination compounds, supramolecular chemistry of transition metals, bioinorganic chemistry, applications of neutron activation analysis to geochemical, environmental, forensic, biomedical and archaeological studies, EPR kinetic studies on radiation-induced radical reactions of organic and met- allorganic systems. Many of these studies are carried out in cooperation with universities, institutions and industries, both in Italy and abroad, in the framework of national and international research projects. The names of the researchers involved in each theme can be found at the end of each section. 2 Receptors for Anions based on Supramolecular Chemistry Multi-component supramolecular chemistry is a recent discipline; its goal is the design of molecular level devices composed by several subunits. Each subunit can exist independently, with its own properties and is bound to other subunits by non-covalent interactions. In the supramolecular structure, each subunit retains its original specific properties but, thanks to cooperative effects, the new system displays different properties with respect to its components. Several supramolecular devices behave as receptors toward ionic or neutral substrates and are therefore suited for use in separation technology as well as in the recognition and sensing of chemical species. Recent studies from our laboratory dealt with the design of receptors for anions, whose activity can be controlled through coordination to a metal centre. [1¡5] 2.1 Cage Receptors for Anions Most anion receptors contain N-H fragments which behave as hydrogen-bond donors toward the anion. However, the C-H fragment, when polarized by a proximate positive charge, can also act as a hydrogen- bond donor toward anions. The tripod molecule L1 can coordinate a cation (Fe2+, Co2+) through the bipyridyl groups at the end of its arms, thus forming a cage. (Figures 1 and 2) Each arm also carries a positively charged imidazolium fragment, which acts as a hydrogen-bond donor toward anions, which are © 2009 Università degli Studi di Pavia 62 Scientifica Acta 3, Special issue (2009) Fig. 1: Molecular model of ligand L1; asterisks indicate the imidazolium fragments. Light blue: Carbon, blue: Nitrogen; hydrogen atoms have been omitted for clarity. a b II 4¡ II 4¡ ¡ Fig. 2: Molecular structures of complexes [Fe L1Br] (a) and [Fe L1N3] (b): both spherical (Br , a) and linear ¡ anions (N3 , b) are hosted inside the cage, in the space between the cation and the imidazolium fragments. Light blue: Carbon, blue: Nitrogen, grey: Hydrogen, red: Fe2+. included in the cage in aqueous solution: halides are selectively included with respect to other spherical ¡ ¡ ¡ ¡ [6] anions (H2PO4 and HSO4 ), with decreasing affinity going from Cl to I . 2.2 Anion receptors assembled on metal centres Urea can also act as a hydrogen-bond donor, especially when activated by electron-withdrawing sub- stituents: ligand L2 can bind a cation through its phenathroline moiety while hydrogen bonds are formed between the anion and the substituted urea. (Figures 3 and 4) © 2009 Università degli Studi di Pavia Scientifica Acta 3, Special issue (2009) 63 Fig. 3: Molecular model of ligand L2. Light blue: Carbon, blue: Nitrogen, grey: Hydrogen, red: Oxygen; yellow: Fluorine. F3C CF3 CF3 N H H 2 x H O N N NH H O H [CuI(CH CN) ]+ N N O 3 4 N N N Cu N N N a b I + I Fig. 4: (a):Scheme of the synthesis of receptor [Cu (L2)2] . (b): Structural model of [Cu (L2)2]Cl calculated by semiempirical methods: tetrahedral coordination to CuIof the phenanthroline moieties leads to the formation of a binding site suitable for spherical anions such as Cl¡, held by hydrogen bonds to the urea fragments. Light blue: Carbon, blue: Nitrogen, grey: Hydrogen, red: Oxygen; yellow: Fluorine, dark red: Cu+ ion, green: Chloride. Coordination of several ligands to different metal centres yields different anion receptors, whose prop- erties depend on the number of ligands bound around the cation. [7¡9] Components of the research group: Valeria Amendola, Greta Bergamaschi, Michela Di Casa, Luigi Fabbrizzi, Maurizio Licchelli, Lorenzo Mosca, Antonio Poggi 3 Self-assembled molecular devices: micelles and monolayers 3.1 Self-assembled micellar molecular devices The research activity of this group is aimed at the study of self-assembled molecular devices inside micellar nanocontainers. In aqueous solution, the use of micelles of traditional or polymeric surfactants allows the concentration in a nanovolume of different molecular components, provided that they are hydrophobic in nature. The inclusion in micellar containers causes a huge increase of the local concentration of molecular components, promotes their interaction (e.g. coordinative or donor-acceptor) because of poor solvation and permits control of their acid-base and coordinative properties. This approach allowed us to obtain several systems for the selective fluorescent sensing of cations (Cu2+, Hg2+, Ni2+).[10;11] © 2009 Università degli Studi di Pavia 64 Scientifica Acta 3, Special issue (2009) Fig. 5: “off-on-off” pH-window fluorescent sensor. Fluorescent emission from the chromophore inside the micelle is controlled by the protonation of the basic components. Fig. 6: Fluorescent lipophilicity meter. Only lipophilic substrates can enter the micelle and displace the chromophore from the adduct, thus quenching its fluorescence. Moreover, the same approach yielded an “off-on-off” pH-window fluorescent sensor, especially inter- esting for the chemistry of biological systems and in medicine. (see Fig. 5) [12] The range of signalled pH can be moved at will along the pH scale, through suitable modification of the molecular components. In the same field, we prepared fluorescent lipophilicity meters, which allow the evaluation, through an optical signal, of the lipophilicity of a molecule, and, for instance, of its ability to penetrate through the cell membrane. (see Fig. 6) [13] Lipophilicity meters based on the micellar approach have been tested on simple organic systems and more complex molecules of pharmaceutical interest, such as antibiotics and anti-inflammatory drugs. © 2009 Università degli Studi di Pavia Scientifica Acta 3, Special issue (2009) 65 3.2 coordination chemistry in molecular SelfAssembledMonolayers grafted on Si and SiO2 surfaces and SAM of metal nanoparticles on glass surfaces In the last years our group has also been engaged in the field of the modification of surfaces through the for- mation of self-assembled monolayers, with the goal of establishing a coordination chemistry on surfaces. + Classical complexes such as ferrocene and [Cu(bipy)2] were deposited on Silicon surfaces through the formation of stable covalent bonds. Their properties were studied by means of XPS, FTIR-ATR and electro- chemical techniques.[14] More recently, our investigations shifted to glass surfaces (or, generally, surfaces covered in SiO2): we are currently working to both coordination chemistry, with ligands/complexes bound to the surface, and the permanent or reversible binding of nanoparticles of noble metals. Also in this latter case, the goal is the preparation of materials and devices useful in the biomedical field, for instance through the controlled release of exact quantities of metal ions or nanoparticles, which can act as antibacterials. Components of the research group: Giacomo Dacarro, Yuri Antonio Diaz Fernandez, Carlo Mangano, Piersandro Pallavicini, Luca Pasotti, Angelo Taglietti 4 Metalloproteins containing copper and heme cofactors 4.1 Spectroscopic and chemical characterization The bioinorganic chemistry group interest focuses on metalloproteins involved in oxidative processes and containing copper or heme as cofactors and on their model complexes. The study has various objectives, covering mechanistic, structural and applied aspects, which are pursued through the application of various approaches including classical biochemical techniques, site-directed mutagenesis, chemical modification, cofactor reconstitution, and kinetic and spectroscopic characterization of the proteins. The enzymatic activities considered are centered on dioxygen activation by copper oxidases and copper oxygenases, and peroxide activation by heme peroxidases.