Search

A novel family of redox-active dinuclear copper(II)-based macrocycles self-assembled from dithiocarbamate ligands has been synthesised; a naphthyl-spaced complex exhibits selective electrochemical recognition of perrhenate and dihydrogenphosphate anions.

Chemical oxidation of two preformed dinuclear copper(II) dithiocarbamate macrocycles leads to the formation of a novel mixed-valence tetranuclear copper(II)/copper(III) dithiocarbamate [2]catenane in near quantitative yield. Structural (see X-ray crystal structure), magnetic susceptibility, electrospray mass spectrometry, and electrochemical studies all support the formation of a CuII CuIII CuII CuIII tetranuclear catenane dication.

The Cu2+ complex of the ligand L1 (L1 = [Ru(tpycyclam)(mtpy)]2+, tpycyclam = 1-[4′-p-tolyl-(2,2′:6′,2′′-terpyridyl)]-1,4,8,11-tetraazacyclotetradecane and mtpy = 4′-methyl-2,2′:6′,2′′-terpyridine) is able to display a selective sensing response against anions such as ATP. The L1−Cu2+ system contains a Ru(tpy)22+ fluorescent core and a cyclam−Cu2+ subunit. The fluorescent response of the Ru(tpy)22+ core in an aqueous environment (acetonitrile/water, 70:30) is governed by the quenching produced by the Cu2+ cation. In the presence of anions the L1−Cu2+ system shows a rich emission fluorescent response. Electrochemical studies on the Cu(cyclam)2+ complex in propylene carbonate in the presence of certain anions allowed us to conclude that the interaction with the anions sulfate, phosphate, ATP, ADP and GMP is mainly electrostatic whereas chloride and bromide form axial covalent bonds with the copper atom in the Cu(cyclam)2+ complex. The addition of anions that coordinate at the axial positions of the square-planar Cu2+−cyclam subunit (such as Cl−, Br− and OH−) produced an enhancement of the emission intensity. This enhancement is attributed to the occupation of the apical positions of copper in the Cu2+−L1 complex that might reduce the probability of an interaction between the Cu2+ ions and the Ru(tpy)22+ groups reducing the quenching process and, consequently, resulting in an enhancement of the fluorescence. A prediction of the fluorescent behaviour with the anions sulfate, phosphate, ATP, ADP and GMP that give electrostatic interactions with the positively charged cyclam−Cu2+ and/or Ru(tpy)22+ centres is rather difficult. Potentiometric studies on the L1−Cu2+−H+−ATP system suggested that the [Cu(L1)(ATP)] complex is responsible for the quenching observed at basic pH in the presence of ATP. It is noticeable that the quenching of the intensity of the fluorescence due to ATP in the pH range 9 to 10 is selective and no other anion studied is able to quench the emission of the Ru(tpy)22+ core in this pH range.

The selective sensing of the ATP anion in an aqueous environment over phosphate, sulfate, and chloride using a fluorescence chemosensor based on Ru(tpy)22+ cores attached to polyammonium groups is reported.

A synthetic strategy has been devised for the preparation of new compounds in which terpyridyl fragments are linked to 1,4,8,11-tetraazacyclotetradecane (cyclam). Reaction of excess cyclam with 4′-[(4-bromomethyl)phenyl]-2,2′:2″,6′-terpyridine afforded the ligand 1-[4′-p-tolyl-(2,2′:6′,2″-terpyridyl)]-1,4,8,11-tetraazacyclotetradecane (L1) in which the tetraaza macrocycle was covalently attached to one benzyl-terpyridyl fragment. Under similar conditions reaction of cyclam with excess 4′-[(4-bromomethyl)phenyl]-2,2′:2″,6′-terpyridine gave the tetra substituted cyclam derivative 1,4,8,11-[4′-p-tolyl-(2,2′:6′,2″-terpyridyl)]-1,4,8,11-tetraazacyclotetradecane (L2). The multidentate ligand L2 was crystallographically characterised by single-crystal X-ray diffraction techniques. Reaction of L1 with [Ru(mtpy)Cl3] gave the heteroleptic ruthenium(II) complex [Ru(L1)(mtpy)][PF6]2 (mtpy = 4′-methyl-2,2′:6′2″-terpyridine). The fluorescent intensity of the metallo-receptor [Ru(L1)(mtpy)][PF6]2 was quenched selectively in the presence of copper(II) in an aqueous environment. To gain insight into the nature of this interaction, potentiometric titrations on [Ru(L1)(mtpy)]2+ in the presence of Cu2+ were carried out. The quenching of the emission intensity was associated with the presence of the copper {Cu[Ru(L1)(mtpy)]}4+ complex in solution. The receptor 1-[4′-methyl-(2,2′:6′,2″-terpyridyl)]-1,4,8,11-tetraazacyclotetradecane (L3), in which the cyclam fragment is separated from the terpyriyl by a methylene group, has also been synthesised by reaction of 4′-bromomethyl-2,2′:2″,6′-terpyridine and cyclam. With L3 as starting material, the ruthenium complex [Ru(L3)(mtpy)]2+ was prepared in order to evaluate the effect that the nature of the spacer has on the quenching of the fluorescence upon addition of Cu2+.

The redox-functionalised polyazacycloalkane receptor 1,4,7,10,13,16-hexa(ferrocenylmethyl)-1,4,7,10,13,16-hexaazacyclooctadecane (L1) has been designed and synthesized and its binding, electrochemical and metal extraction properties studied. The results have been compared to those obtained for the parent redox-active polyazacycloalkane 1,4,8,11-tetra(ferrocenylmethyl)-1,4,8,11-tetraazacyclotetradecane, L3. The cadmium complex of L1 and the free receptor L3 have been crystallographically characterised. The structure of [CdL1][PF6]2·4CH3CN·4H2O consists of cationic [CdL1]2+ units in which the cadmium ion exhibits a distorted octahedral environment. Solution studies by potentiometric methods have also been carried out in the presence of H+ and Cu2+ in 1,4-dioxane–water (70∶30 v/v, 25 °C, 0.1 mol dm–3 KNO3) for L1 and thf–water (70∶30 v/v, 25 °C, 0.1 mol dm–3 [NBun4][ClO4]) for L3. Electrochemical experiments as a function of the pH in the presence of Ni2+, Cu2+, Zn2+, Cd2+ or Pb2+ have been carried out for L1 and L3. The solvent extraction properties of L1 have also been studied.

Reactions of 4′-[4-(bromomethyl)phenyl]-2,2′:6′,2′′-terpyridine with 4,10-diaza-15-crown-5 and 1-aza-12-crown-4 in dichloromethane yielded the ligands L1 and L3, respectively. Reaction of an excess of 4′-[4-(bromomethyl)phenyl]-2,2′:6′,2′′-terpyridine with 4,10-diaza-15-crown-5 yielded L2, while treatment of the same terpyridine ligand with 1,4,7,10,13-pentaazacyclopentadecane afforded L4. Reactions of L1, L3, and L4 with Ru(mtpy)Cl3 (mtpy = 4′-methyl-2,2′:6′,2′′-terpyridine) in methanol yielded the metallo receptors [Ru(L1)(mtpy)][PF6]2, [Ru(L3)(mtpy)][PF6]2, and [Ru(L4)(mtpy)][PF6]2 after precipitation with ammonium hexafluorophosphate and column chromatography. On treating L3 with RuCl3, the homoleptic ruthenium complex [Ru(L3)2][PF6]2 was obtained. The synthesized metallo receptors contain oxa-aza crown or polyazacycloalkane moieties as recognition sites and [Ru(tpy)2]2+ cores as the signal-generating centre. The electronic spectra of the complexes are as expected for an Ru(tpy)22+ chromophore, with the main Ru[d(π)] tpy(π*) MLCT transition at ca. 484 nm and intense ligand-centred transitions in the UV region. One of the most interesting aspects of these ruthenium complexes is their multicomponent nature, as they contain both coordination sites and fluorescent Ru(tpy)22+ cores. The cations [Ru(L1)(mtpy)]2+, [Ru(L3)(mtpy)]2+, and [Ru(L3)2]2+ display an emission maximum at ca. 650 nm, the intensity of which is pH dependent, showing an enhancement upon protonation. The metallo receptor [Ru(L1)(mtpy)]2+ selectively senses Hg2+ in preference to Cu2+, Cd2+, and Pb2+. The emission intensity vs. pH curve for [Ru(L2)(mtpy)]2+ in the presence of Cu2+ and Hg2+ ions is close to that of the free receptor, but the presence of Cd2+ or Pb2+ enhances the emission intensity in the range pH 4−6. For the [Ru(L3)2]2+ complex, Cd2+, Pb2+, and Hg2+ induce an enhancement of the fluorescence of the Ru(tpy)22+ core in the range pH 3.5−7.5. These results are compared with those obtained for the metallo receptor [Ru(L4)(mtpy)]2+ containing a polyazacycloalkane moiety as the binding domain.