Chimica Inorganic Chemistry

Scientiﬁca Acta 3, Special issue, 61 – 69 (2009)

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 properties 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

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)

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 properties 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]

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.

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 formation 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. Both types of processes are strictly metal-centered and require the formation of metal-peroxo or metal-hydroperoxo species as primary intermediates. The group’s research in the copper protein field involves tyrosinase, the enzyme responsible for melaniza- tion processes, hemocyanin, the oxygen carrier protein in many invertebrates, and multicopper oxidases such as laccase, ascorbate oxidase and ceruloplasmin. The basic aspects of the dioxygen activation processes mediated by these enzymes are still quite obscure. The structural differences among their active sites indicate that the molecular pathways for the dioxygen reduction steps and the protein-substrate interactions are different for the different enzymes. In the case of heme peroxidases, the mechanism of hydrogen peroxide activation is clearer, because the enzyme intermediates are well characterized. The focus of the research is then to understand the origin of stereoselectivity effects exhibited by the enzymes towards various types of substrates. In addition, it is possible to introduce or modify in a programmed way the enzymatic activities of proteins like myoglobin or cytochrome c by genetic modification of protein residues or replacement of the native heme cofactor with synthetic heme derivatives. The knowledge of the structure reactivity relationship in metalloenzymes is exploited for the synthesis of model complexes with desired reactivity. These compounds are also used to trap very active intermediates, by working at low temperatures in non-polar solvents, thus allowing the spectroscopic characterisation of elusive intermediates.[15−17]

4.2 Involvement in physiological relevant reactions In the presence of hydrogen peroxide, heme proteins form active intermediates which are able to oxidize exogenous molecules. Often these products are not stable compounds but reactive species on their own, such as organic radicals, and they can both diffuse to the bulk of the solution or react with the protein that

a b

Fig. 7: (a) Modiﬁed iron porphyrin with enhanced catalytic activity. (b) Modiﬁcation sites in myoglobin upon exposi- tion to reactive nitrogen species.

generated them. In particular, the bioinorganic chemistry group interest focuses on the self-modification ex- perienced by heme proteins with globin type fold (such as myoglobin, hemoglobin and neuroglobin) when treated with nitrite or catechols in the presence of hydrogen peroxide. The reactive nitrogen species generated by nitrite give rise to nitration, oxidation and/or cross linking reactions between the proteins or their subunits. The quinones formed upon reaction with catechols easily modify cysteine and histidine residues, and eventually cause protein aggregation which induces precipitation. The pattern of modifications expe- rienced by the protein strongly depends on the nature of the protein and the reaction conditions.[18−21] A further research project deals with modification induced in proteins when reactive quinone species are produced by the action of copper enzymes, such as tyrosinase, or copper containing neuromelanin pig- ments on dopamine and norepinephrine, i.e. neurotransmitter cathecols present in neurons. Elucidation of these reactions will help understanding the mechanisms of the induction of neurologic pathologies such as Parkinson’s disease.

Components of the research group: Luigi Casella, Enrico Monzani, Stefania Nicolis, Sara Palavicini

5 Characterization of materials by their content of trace elements through neutron activation analysis Neutron activation analysis is a nuclear analytical method especially useful in the trace determination of elements in different materials (metal, stone, glass, ceramic, organic, plastics, biological, etc.) with excellent sensitivity, precision and accuracy. [22−23] This method is made possible by the availability at the University of Pavia of a research nuclear re- actor TRIGA Mk II for neutron irradiation (at the LENA Laboratorio Energia Nucleare Applicata) and a

Studi di spettroscopia EPR e di chimica dei radicali

Meccanismi di radiolisi γ Radicali nei materiali (danno da Materia Ioni radicali radiazioni) e in sistemi Irraggiata Stati elettronici eccitati di interesse biologico e- accelerati (basi chimiche della radiolisi)

Sintesi di nuovi materiali per l’industria e la produzione di energia

Fig. 8: Basic and practical targets of Radiation Chemistry radiochemistry area at the Dipartimento di Chimica Generale, with specially equipped laboratories for the handling of irradiated materials and the measurement of induced radioactivity. It is well known that the composition of a material, in particular that of its trace elements, influences the characteristics and properties of the material itself. A first application is in the characterization and certification of ultrapure materials for use in electronics, biomedical devices, standard reference materials. The amount of trace elements and their inter-relationship, evaluated through multivariate statistical methods, constitute a real “fingerprint” of the material and/or the raw materials used in its production. The fingerprint is retained even when materials undergo different technological processes in the production of artefacts. This can be exploited in the field of environmental studies and archaeometry. [24−30] In the latter case, the fingerprint based on trace elements is especially useful in the studies on the origin of the raw materials employed in the production of artefacts of archaelogical and historical interest, or the certification of the authenticity of artworks.

(Massimo Oddone)

6 High energy radiation chemistry

High energy radiations, both electromagnetic (gamma and X-rays) and particles (electron and accelerated ion beams), interact in a non selective way with atoms and molecules of the medium, causing ionizations, excitations, breaking and forming of chemical bonds leading to irreversible chemical transformations. Ra- diation chemistry deals with the study of these reactions with both basic (analysis of mechanisms, characterization of labile radiolytic intermediates) and practical targets (synthesis and modiﬁcation of new materials, analysis of radiation induced damage, applications in industry, farming, environment). [31−39] The research work carried out by the Radiation Chemistry group in the Dipartimento di Chimica Gen- erale of the University of Pavia (afﬁliated to the School of Engineering) covers the following topics:

6.1 Radiolytic Mechanisms Radiation chemistry investigations were carried out on biologically and technologically important poly- mers, such as perﬂuorinated polyethers, aromatic and aliphatic polyesters (as matrixes for the controlled release of drugs), polyoleﬁnes. The main problems concerned the role of the ionic, radical and excitation

components in the radiolysis mechanism, the analysis of the energy transfer mechanism and the spectroscopic and structural characterization of the labile paramagnetic intermediates (neutral radicals and ion radicals) through EPR and UV-vis spectroscopy in matrixes at low temperature.

6.2 Synthesis of new materials 6.2.1 FIRB Project “Use of ionizing radiations, nanochemistry and photochemistry in the materials technology of the tyre industry” This research is carried out in cooperation with the Universities of Genova, Milano Bicocca, the Polytech- nic of Torino, ISMAC-CNR laboratories in Genova and Pirelli Tyres as industrial partner. The goals are (i) the production of inorganic fillers based on silica and lamellar silicates modified through grafting of unsa- tureted polyolefine oligomers; the chemical modification of surfaces (on silica) and between the lamelles (in the silicates) is aimed to enhance the strengthening mechanism in the elastomer filler composites by improving the degree of dispersion of the filler (quenching of the differences in polarity and exfoliation of phyllosilicates) and enhancing the mechanism of formation of “chemically bound rubber” in the vulcanization step. (ii) the analysis of the dynamomechanic properties of the composites obtained with modified silicas through vulcanization by radiation and conventional techniques. (iii) study of the chemical and physical properties of the new materials, including the relationship between structure and properties. (iv) ESR study of the reaction mechanisms (grafting and vulcanization) and molecular dynamic studies of the filler/elastomer interface.

6.2.2 Functionalization of polyoleﬁnc membranes for use as solid electrolytes in fuel cells (cooperation with Pirelli Labs) Protonic conduction membranes have been synthesized, based on polyethylene styrene sulfonate. The research dealt with kinetic studies of the reactions, the analysis of the functional groups distribution ac- cording to membrane depth, the analysis of the electrical properties of the membranes in CH3OH/O2 fuel cells .

6.3 Financed projects in progress 6.3.1 Project INFM “Excalibur”: EPR studies of the damage caused by irradiation to cryocon- served cells The research is part of the INFM project “Excalibur”, coordinated by INFM laboratories of Legnaro. Ob- jectives: studies on the biological effects of radiations in the low doses ﬁeld; transmission of the damage and effects induced by radiation LET; studies of cell responses to radiation damage during cryoconserva- tion. The research conduced by the group in Pavia deals with the characterization of the radical component of the damage caused by radiations, by means of EPR spectroscopy in bulk.

6.3.2 Project INFM “Dossier”: Research on new dosimetric methods for retrospective dosime- try by EPR spectroscopy The research, part of INFM project coordinated by the “Istituto Superiore della Sanità”, is focused on the identification of organic and inorganic compounds which can be used as radiation dosimeters, through the production of stable radicals that can be identified and quantified by EPR.

Components of the research group: Armando Buttafava, Daniele Dondi, Antonio Faucitano, Claudia Palamini, Fabio Piepori

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