Antonio Poggi

Anion recognition by dimetallic cryptates

Abstract Bis-tren cryptands (i.e. octamine cages consisting of two tripodal tetramine subunits covalently linked by given spacers) are able to incorporate first two metal ions, then an ambidentate anion, according to a cascade mechanism. In particular, dicopper(II) cryptates behave as effective receptors for anions, which fill the empty cavity of the cage and place their donor atoms in the two axial sites left available by each Cu(II) centre (which adopts a trigonal bipyramidal stereochemistry). Anion encapsulation by dicopper(II) cryptates often induces the development of a rather intense anion-to-metal charge transfer absorption band in the visible region, so that the recognition process is signalled by the appearance of a bright colour. Two examples are considered in detail: (i) that of a rigid bis-tren cryptate containing 1,3-xylyl spacers, which does not recognise the shape, but the bite of the polyatomic anion (i.e. the distance between two consecutive donor atoms); and (ii) that of the flexible cryptate containing 2,5-furanyl spacers, which is able to include also monoatomic anions, in particular halides, displaying peak selectivity in favour of Cl − .

Molecular events switched by transition metals

Transition metals can typically give rise to two (or more) distinct states of comparable stability (two consecutive oxidation states; two different stereochemical arrangements). In a multicomponent system, the conversion of one state to the other can modify a given property of a nearby subunit or can induce drastic changes in the system topology. In this sense, the metal behaves as a switch, which can be operated through an external input (the variation of the pH or of the redox potential). Recent examples of molecular switching by transition metals are reviewed: (i) the quenching/enhancement of the emission of a luminescent fragment effected by a nearby metal centred redox couple (e.g. NiII/NiIII); (ii) the pH driven motion of an aminoalkyl side chain in a NiII scorpionate complex, which is signalled by the variation of the light emission intensity of an appended anthracenyl fragment; (iii) the pH controlled translocation of a NiII ion within a multidentate ligand containing two compartments of different coordinating tendencies and (iv) the intramolecular translocation of a Cl− anion between two pre-positioned metal centres CuII and NiII, within a ditopic receptor, electrochemically driven through the NiII/NiIII redox change.

Transition Metals in Supramolecular Chemistry

Preface. Supramolecular chemistry and photophysics. Energy conversion and information-processing devices based on transition metal complexes V. Balzani, A. Credi, F. Scandola. Transition metal redox active ligand systems for recognising cationic and anionic guest species P.D. Beer. Ligand design for enhanced molecular organization -- selectivity and specific sequencing in multiple receptor ligands, and orderly molecular entanglements D.H. Busch. Metal-ions: a self-assembly motif in supramolecular oligomers E.C. Constable. Biomedical targeting: a role for supramolecular chemistry K.A. Miller, E.A. Deutsch, S.R. Cooper. Redox chemistry of metal ion complexes: preparation of new materials L. Echegoyen, E. Perez-Cordero. pH and redox switches based on metal centres G. de Santis, M. di Casa, L. Fabbrizzi, M. Licchelli, C. Mangano, P. Pallavicini, A. Perotti, A. Poggi, D. Sacchi, A. Taglietti. Schiff base macrocycles and metallo-biosite modelling D.E. Fenton. Self-assembly of mono and dinuclear metal complexes oxidation catalysis and metalloenzyme modelling B.L. Feringa, O.-J. Gelling, M.T. Rispens, M. Lubben. Artificial porphyrins containing cyclopropane units functioning as electron shuttles C. Floriani. Homo and heterobinuclear metal complexes with bis-macrocyclic ligands Th.A. Kaden. Ferrocene as a building block for supramolecular systems A.E. Kaifer. Macrocyclic polyamine complexes beyond metalloenzyme models E. Kimura, M. Shionoya. Towards molecular wires and switches: exploiting coordination chemistry for nonlinear optics and molecular electronics J.A. McCleverty. Molecular interactions between metalloproteins involved in electron transfer processes: tetrahaeme cytochrome c3 flavodoxin. 1H NMR and molecular modelling studies P. Nuno Palma, J. le Gall, J.A. Wampler, J.J.G. Moura. Supramolecular models of metalloproteins P.A. Gosling, R.J.M. Klein-Gebbink, A.P.H.J. Schenning, M.C. Feiters, R.J.M. Nolte. The role of macrocyclic receptors in the organization of metal centres A. Bencini, A. Bianchi, P. Paoletti, E. Garcia-Espana. Metallomacrocycles and clefts, receptors for neutral molecules and anions D.M. Rudlevich, W.T.S. Huck, F.C.J.M. Van Veggel, D.N. Reinhoudt. Chiral recognition by functionalized cyclodextrin metal complexes G. Maccarrone, E. Rizzarelli, G. Vecchio. Transition metal-directed threading and knotting processes J.-C. Chambron, C.-O. Dietrich Buchecker, V. Heitz, J.-F. Nierengarten, J.-P. Sauvage. Expanded porphyrins. Receptors for cationic, anionic and neutral substrates J.L. Sessler, A.K. Burrell, H. Furuta, G.W. Hammi, B.L. Iverson, V. Kral, D.J. Magda, T.D. Mody, K. Shreder, D. Smith, S.J. Weghorn. Following the self-assembly process in solution A.F. Williams, C. Piguet, R.F. Carina. Author Index. Subject Index.

The Molecular Design of Fluorescent Sensors for Ionic Analytes

AbstractMolecular fluorescent sensors can be synthesized by covalently linking a photoactive fragment (e.g., anthracene) to a receptor subunit displaying affinity toward the envisaged substrate. The electron transfer process is the privileged signal transduction mechanism: redox active substrates (e.g., transition metals) typically release/uptake an electron to/from the proximate photoexcited fluorophore, the recognition being signaled through fluorescence quenching; redox inactive substrates (d0 and d10 metals, H+) deactivate an existing quenching relay (e.g., a tertiary nitrogen atom close to the fluorophore) and their recognition is signaled through fluorescence enhancement. An-ionic substrates can be conveniently recognized on the basis of the metal–ligand interaction: polyamine receptors containing the photophysically inactive ZnIIion bind the carboxylate group. In the case of amino acids,

Metal-induced assembling/disassembling of fluorescent naphthalenediimide derivatives signalled by excimer emission

{\text{NH}}_3^ + {\text{ - CH(R) - COO}}^ -

Molecular Devices Based on Metallocyclam Subunits

, selectivity is improved when the receptor platform bears additional groups capable to interact specifically with the R substituent. If R is capable of transferring an electron to the nearby photoexcited fluorophore, the recognition is signaled through fluorescence quenching.

Excimer emission induced by metal ion coordination in 1,8-naphthalimide-tethered iminopyridine ligands

Bicyclam [6,6′-bi(1,4,8,11-tetra-azacyclotetradecane)](L2) was prepared by reducing 6,6′-bi(1,4,8,11-tetra-azacyclotetradecan-5,7-dione)(L1) with B2H6 in bis(2-methoxyethyl) ether. Ligand L2 forms homo- and hetero-binuclear complexes with Cu2+ and Ni2+, the spectral properties and the electrochemistry of which have been studied. Whereas the visible spectra are very similar to those of the mononuclear complexes with cyclam (1,4,8,11-tetra-azacyclotetradecane), the electrochemistry indicates a weak interaction between the metal ions co-ordinated by bicyclam, as shown by a shift of the second oxidation potential M2+→ M3+ to more positive values.

Redox switchable ligands suitable for transition metal ions: Protonation, complexation and electrochemical properties of a ferrocene-modified tetraamine diketone and its saturated analogue

The new quadridentate bischelating ligands 2 and 3 display in solution the typical absorption and emission properties expected for naphthalenediimide derivatives. Spectrophotometric studies show that systems 2 and 3 interact with Zn(II), Cd(II) and Cu(I) in CHCl3 or MeCN according to an apparent 1:1 stoichiometry. Molecular modelling, ESI-MS and 1H NMR experiments indicate that the complex species formed in the presence of stoichiometric amounts of metal ion are nonhelical [2 + 2] adducts. The metal-induced self-assembling process is signalled by an intense excimer-type emission caused by the intramolecular interaction of two naphthalenediimide subunits that face each other in the [2 + 2] adduct, as shown by molecular modelling studies. In the presence of excess metal ion, a disassembling process takes place, leading to a dinuclear complex with a 2:1 metal/ligand stoichiometry, in which the intramolecular excimer is no longer allowed to form because the interaction between the naphthalenediimide subunits has been lost. Thus, the overall metal-induced assembling/disassembling process is signalled by the appearance and disappearance of the excimer band in the emission spectrum.