Barbara Seghi

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...

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.

Dicopper(II) and dicopper(III) complexes with a double-ring octa-aza macrocycle

The novel double-ring octa-aza molecule (2) incorporates two copper(II) ions in aqueous solution with simultaneous release of four protons; the resulting dicopper(II) complex shows a weak metal–metal interaction and is easily oxidized to the trivalent state by two consecutive one-electron steps separated by 110 mV.

Metal complexes that transport electrons across liquid membranes

Absrracr Lipophilic metal complexes, displaying one electron redox activity, are used as carriers for the transport of electrons across bulk liquid membranes from an aqueous reducing phase to an aqueous oxidizing phase. The [FeIII,lI(bpy)313+/2+ redox system (bpy = 4,4‘-di-rerr-butyl-2,2’dipyridyl, 1) transports electrons from both cationic and anionic aqueous reducing agents to aqueous Ce(1V) and counter-transports perchlorate ions in the opposite direction. Electron/chloride ion cross transport between aqueous persulfate and aqueous metal centred reducing agents, mediated by the membrane dissolved [Ni1r**11LC12]2+/3+ redox system (L =(Ncety1)-cyclam, 2) takes place at a much higher rate. The overall transport rate in the two types of experiments is controlled by that of the redox process occurring at the membrane/aqueous reducing phase interface and is to be related to the type of mechanism (inner or outer sphere) by which the two-phase electron transfer process takes place.

Solution chemistry in non-co-ordinating solvents (CH2Cl2, CHCl3) of nickel(II) and nickel(III) complexes of the lipophilic macrocycle 1-hexadecyl-1,4,8,11-tetra-azacyclotetradecane. The emphasized role of the anion co-ordination

Nickel(II) complexes of the lipophilic tetramine macrocycle N-cetylcyclam (1-hexadecyl-1,4,8,11-tetra-azacyclotetradecane) L1, dissolve in non-co-ordinating solvents such as CH2Cl2 and CHCl3 as intact Ni(L1)X2 species, whose spin state and colour depend on the co-ordinating tendencies of the apically bound X– anions: Cl–, and SCN–, blue, high-spin complexes; ClO4–, BF4–, and I–, yellow, low-spin complexes. The complex Ni(L1)Br2 exists in solution as an equilibrium mixture of the blue and yellow species and the blue-to-yellow conversion is exothermic; moreover, the equilibrium is displaced to the right on further addition of the background electrolyte, tetra-alkylammonium or tetraethylphosphonium bromide. A general model is proposed to explain the thermodynamic aspects of the high-spin/low-spin interconversion in both co-ordinating (e.g. water) and non-co-ordinating media. Finally, NiII(L1)X2 complexes in CH2Cl2 solution (0.1 mol dm–3 Bun4NX), undergo reversible one-electron oxidation processes to give a [NiIII(L1)X2]X species. The NiIII–NiII redox potential decreases dramatically with increasing co-ordinating tendency of the X– ions. This permits us to obtain a spectrochemical series for electrochemically inert inorganic anions. On this basis, it appears that ClO4– is a more strongly co-ordinating ligand than BF4–.

Metal complexes that transport electrons across liquid membranes: the [FeII,IIIL3]2+,3+ redox system (L = 4,4′-di-tert-butyl-2,2′-bipyridine)

The lipophilic complex [FeIIL3][ClO4]2(L = 4,4′-di-tert-butyl-2,2′-bipyridine) has been prepared to be used as a carrier for the transport of electrons from an aqueous oxidizing phase to an aqueous reducing phase, separated by a CH2Cl2 bulk liquid membrane. Two-phase redox experiments indicated that the CH2Cl2-dissolved [FeIIL3][ClO4]2 complex is oxidized by aqueous CeIV to give [FeIIIL3][ClO4]3. On the other hand, [FeIIIL3][ClO4]3 can be reduced, under two-phase conditions, by a series of aqueous reducing agents according to the rate sequence: NO2– > [FeII(CN)6]4– > FeII > SO32–. Three-phase experiments have been carried out in which electrons are transported by [FeIIL3][ClO4]2 from the aqueous reducing phase to the aqueous phase containing CeIV, across the bulk liquid membrane, and ClO4– ions are transported by [FeIIIL3][ClO4]3 in the opposite direction. The rate of the electron transport is controlled by the rate of the slowest of the two redox processes at the two sides of the membrane. By varying the concentrations of the aqueous reactants, it is possible to determine the two-phase rate-determining step of the overall three-phase process and to perform selective oxidation by CeIV of the investigated reducing agents.