Giancarlo De Santis

A FLUORESCENT CHEMOSENSOR FOR THE COPPER(II) ION

Abstract The two-component system anthracene-9-carboxylic acid 1,4,8,11-tetrathia-cyclotetradecan-6-ylester ( 1 ) has been synthesized, structurally characterized and investigated as a fluorescent chemosensor for the Cu 11 ion. Crystallographic details for 1 : space group P 2 1 / c with α = 9.894(2), b = 24.608(2), c = 10.759(2) A , α = 90, β = 105.88, γ = 90°; V = 2519.7 A 3 , Z = 4 (R = 0.088, R w = 0.066) . In ethanolic solution 1 selectively incorporates Cu 11 into its tetrathia crown component in the presence of other 3d metal ions and signals the recognition through the quenching of the fluorescence of the anthracene fragment. Quenching of the photo-excited state (which has a charge transfer nature) takes place through a photo-induced electron transfer from the fluorophore to the metal center and involves the Cu II /Cu I couple. The d 10 cation Ag 1 competes successfully with Cu 11 for the tetrathia cavity of 1 in an ethanolic solution (not in an acetonitrile solution), but the occurrence of the metal/receptor interaction cannot be signalled, due to the poor redox activity of the non-transition cation Ag 1 .

Controlling the acidity of the carboxylic group by a ferrocene based redox switch

Abstract The pH dependent oxidation behaviour of the ferrocene subunit in ferrocenylcarboxylic acids of formula Fc(CH 2 ) n COOH ( n = 0, 1, 2) has been investigated through differential pulse voltammetry experiments in 4:1 acetonitrile/water solutions. Different voltammetric responses have been obtained depending whether the pH has been varied through a buffer (a single peak, whose E 1/2 value decreases with the increasing pH) or through an acid-base titration (two distinct peaks, the less anodic corresponding to the oxidation of the FcCOO − species, the more anodic corresponding to the oxidation of the FcCOOH species, whose relative intensities vary with the pH). The electrochemically determined pK a values for the Fc(CH 2 ) n COOH and [Fc + ](CH 2 ) n COOH forms indicate that the Fc-to-Fc + oxidation process makes the acidity of the proximate carboxylic group increase. Such a switching effect, electrostatic in nature, decreases as n increases.

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

Abstract The new ferrocene-containing water-soluble ligands 1 and 2 were synthesized and their protonation and complexation properties toward NiII and CuII studied as a function of pH, by means of potentiometric titration experiments. Electrochemical measurements were performed in aqueous solution on pure 1 and 2 and in the presence of NiII and CuII cations, in the pH range 2–12, allowing us to determine the redox potential values relative to the ferrocene oxidation in the free ligands and in their NiII and CuII complexes. 1 and 2 behave as redox switchable ligands, the former enhancing, the latter decreasing its binding ability upon oxidation of the appended ferrocene function. Besides, the CuII complex of ligand 1 and the NiII complex of ligand 2 behave as two-centre two-electron redox systems, the complexed metal cation being subject to further oxidation to MIII.

Appending two non-equivalent ferrocene fragments to a metallocyclam core

Abstract A three-component system in which two non-equivalent ferrocene fragments are appended to a metallocyclam subunit has been prepared through an Ni 11 template synthesis. The system undergoes two distinct and consecutive reversible one- electron oxidation processes centred on the two ferrocene fragments, but decomposes on oxidation of the Ni 11 centre.

Supramolecular assemblies containing metallocyclam subunits

Abstract The synthetic approach to supramolecular systems containing one or more metallocyclam subunits is reviewed. In particular, the template synthesis of azacyclam complexes has been used to design supramolecular coordination compounds displaying multi-electron redox activity. Moreover, it is shown that whenever the supramolecular design requires a component able to provide a fast and reversible redox change, a metallocyclam subunit should be used. Finally, the design of supramolecular systems in which a light-emitting fragment (anthracene) has been linked to a metallocyclam subunit has been considered. In particular, anthracene fluorescence can be switched on/off through the CuI/CuII redox couple inside a tetra-thia-macrocyclic environment, via an electron transfer mechanism.

Control of the redox activity of anthraquinone through an appended nickel(II)-cyclam side chain. A two-site three-electron redox system

Abstract The conjugate system 1 has been prepared through reaction of 2-chloromethylanthraquinone with a five-fold excess of cyclam in chloroform at room temperature. Reaction of 1 with NiX 2 ·6H 2 O (XCl, ClO 4 ) gave the corresponding Ni( 1 )X 2 complexes, in which the metal centre has been incorporated by the tetraaza-macrocyclic subunit. The redox behaviour of the nickel(II) complexes has been investigated in a CH 2 Cl 2 solution of 0.1 M Bu 4 NX, by voltammetric techniques. Two-phase reduction of the quinone subunit of the Ni( 1 )X 2 complex in CH 2 Cl 2 by aqueous Cr II takes place at a rate much higher than that observed for the simple anthraquinone molecule, but the reduced conjugate system partitions between CH 2 Cl 2 and water.

Electrons and Ions Moving Across Liquid Membranes

Abstract Oxidation and reduction reactions can be carried out by interfacing the aqueous solution containing the reducing agent and the aqueous solution containing the oxidizing agent by a layer of a water immiscible solvent (e.g. CH2Cl2, the liquid membrane): in the membrane a lipophilic redox system C has to be present, which transports electrons from the aqueous reducing phase to the aqueous oxidizing phase and, in its oxidized form C+, X− anions in the opposite direction. Metal complexes have been tested as carriers for the transport of electrons across liquid membranes. In particular, transition metal complexes of lipophilic versions of cyclam and 2,2′-bipyridine have been investigated. The three-phase redox processes can be controlled by varying the potential of the C/C+ couple in the CH2Cl2 solution. Further selectivity effects derive from the kinetics of the electron transfer process at the membrane/aqueous phase interface. The possibility to perform light driven electron transport processes media...

Template synthesis of a ferrocene-metallocyclam conjugate

Abstract Arylsulfonamides act as efficient locking fragments in the Ni II template cyclization of open-chain tetramines, in the presence of formaldehyde, to give cyclam-like azamacrocycles. In particular, if ferrocenesulfonamide is used, a ferrocene-metallocyclam conjugate is obtained, which releases two electrons according to two distinct and consecutive one- electron steps (Fc/Fc + and Ni II /Ni III ).

Amides and sulfonamides : efficient molecular padlocks for the template synthesis of azacyclam (1,3,5,8,12-pentaazacyclotetradecane) macrocycles

Amides and sulfonamides, both aliphatic and aromatic, acted as efficient locking fragments, in the presence of formaldehyde and base (triethylamine), in closing the open-chain tetramine 1,9-diamino-3,7-diazanonane around labile metal centres prone to a square type of co-ordination, i.e. NiII and CuII, to give a pentaazamacrocyclic complex of the azacyclam family. The product of the copper(II) template reaction involving methanesulfonamide as a locking fragment (3-methanesulfonyl-1,3,5,8,12-pentaazacyclo-tetradecane)dinitratocopper(II), was obtained in crystalline form and its crystal and molecular structure determined from single-crystal X-ray diffraction data, collected with the use of Cu-Kα radiation: trigonal, space group R3c, a=b=c= 14.997(3)A, α=β=γ= 98.48(2)°, Z= 6. Only the four secondary amine nitrogen atoms of the macrocycle are bound to the CuII, giving a regular square stereochemistry. The tertiary nitrogen atom N(1), which presents distinct sp2 structural features, is not involved in the co-ordination. The axial positions of the elongated octahedron are occupied by oxygen atoms of the nitrate ions. A kinetic investigation was carried out on the copper(II) template reactions involving diprotic acids as locking fragments, including amides, amines and carbon acids, such as nitroethane and diethyl malonate: for the systems investigated the rate of the template reaction seems to be related to the strength of the diprotic acid. This suggests that the monodeprotonated form of the acid is present in the rate-determining step of the cyclisation process.

Redox-active liquid membranes: transport of electrons across a dichloromethane layer mediated by the [CoIIIL1X2]+–[CoIIL1X2] redox system (L1= 1-hexadecyl-1,4,8,11-tetra-azacyclotetradecane, X = Cl or ClO4)

The lipophilic complex [CoIIIL1Cl2]Cl (L1= 1-hexadecyl-1,4,8,11-tetra-azacyclotetradecane) can transport electrons, through the CoIII–CoII redox change, from an aqueous phase containing CrII to an aqueous phase containing the mild oxidizing agents FeIII or [NiIIIL3]3+(L3= 1,4,8,11-tetra-azacyclotetradecane), in aqueous HCl, across a bulk CH2Cl2 membrane. The transport of electrons, to which a counter transport of Cl– ions is coupled, implies that the potential associated with the [CoIIIL1Cl2]Cl + e–→[CoIIL1Cl2]+ Cl– half-reaction (–0.60 V vs. ferrocenium–ferrocene, measured in a CH2Cl2 solution, 0.1 mol dm–3 in NBu4Cl) should lie between that associated with the aqueous CrIII–CrII couple and that associated with the aqueous ox–ox– couple (ox = FeIII or [NiIIIL3]3+). Replacement of Cl– by ClO4– stops the electron transport from CrII to FeIII or [NiIIIL3]3+. This is ascribed to the especially high potential associated with the [CoIIIL1(ClO4)2]ClO4+ e–→[CoIIL1(ClO4)2]+ ClO4– half-reaction. In particular, the [CoIIL1(ClO4)2] complex in a CH2Cl2 solution, covered by a layer of aqueous HClO4, is not oxidized even by bubbling dioxygen. This complex can transport electrons, across the bulk CH2Cl2 membrane, from aqueous CrII only to the very strong oxidizing agent CeIV in aqueous HClO4.