Eric Saint-Aman

Intramolecularly hydrogen-bonded versus copper(II) coordinated mono- and bis-phenoxyl radicals.

Ligands bearing two salicylidene imine moieties substituted in ortho and para positions by tert-butyl groups have been electrochemically oxidized into mono- and bis-phenoxyl radicals. The process involves an intramolecular proton coupled to electron transfer and affords a radical in which the oxygen atom is hydrogen-bonded to a protonated ammonium or iminium group. A weak intramolecular dipolar interaction exists between the two phenoxyl moieties in the bis-radical species. The copper(II) complexes of these ligands have been characterized and electrochemically oxidized. The mono-phenoxyl radical species are X-band EPR silent. The bis-phenoxyl radical species exhibits a (S= 3/2) ground state: it arises from a ferromagnetic exchange coupling between the two spins of the radicals and that of the copper(II) when the spacer is rigid enough; a flexible spacer such as ethylidene induces decomplexation of at least one phenoxyl group. Metal coordination is more efficient than hydrogen-bonding to enhance the chemical stability of the mono-phenoxyl radicals.

Dinuclear Zinc(II)−Iron(III) and Iron(II)−Iron(III) Complexes as Models for Purple Acid Phosphatases

The heterodinuclear ZnIIFeIII complex 1 and the isostructural FeIIFeIII complex 2 with the dinucleating ligand from 2,6-bis[{bis(2-pyridylmethyl)amino}methyl]-4-methoxyphenol (HBPMOP, 3) were prepared and characterized by X-ray crystallography. Solution studies (UV/Vis spectroscopy; electrochemistry) are described. A pH-induced change in the coordination spheres of the metal centers is seen. These complexes serve as models for the mixed-valence oxidation state in purple acid phosphatases. The cleavage acceleration of the activated phosphodiester 2-hydroxypropyl p-nitrophenyl phosphate (HPNP) was investigated in acetonitrile/water (1:1) in the presence of complexes of the ligand BPMOP and its methyl analogue BPMP with regards to its dependence on the pH value. At the optimum pH value (8.5 ± 0.2), the ZnIIFeIII complex from BPMOP shows a 2-fold higher rate acceleration compared with that of the complex containing BPMP. The diiron complex from BPMOP is 4-fold more reactive than the homologous complex from BPMP. The heterodinuclear ZnIIFeIII catalysts are at least 10-fold more reactive than the homonuclear FeIIFeIII catalysts.

Viologen-based redox-switchable anion-binding receptors

A series of viologen-based receptors have been synthesized and their anion-binding properties have been investigated by-NMR, UV-visible spectroscopy, electrochemistry and X-ray diffraction analyses. Linking two positively charged viologens through a propyl chain promotes a remarkable chelate-like binding of chlorides revealed by 1H-NMR spectroscopy. Of all the anionic species investigated, only fluoride is detectable by the naked eye and by electrochemical methods. The reduction-triggered formation of a π-dimer from two viologen-based cation radicals was also investigated by electrochemical and spectroelectrochemical methods and by theoretical calculations.

Electrochemical recognition of metal cations by poly(crown ether ferrocene) films investigated by cyclic voltammetry and electrochemical impedance spectroscopy

Abstract The electrochemical behavior of the pyrrole-substituted crown ether ferrocene 1 was investigated in detail in 0.1 M TBAP+CH 3 CN by cyclic voltammetry (CV) and chronoamperometry. In the potential range 0–0.7 V, a reversible diffusion-controlled process was observed with a formal potential of 0.53 V corresponding to the ferrocene/ferricinium redox couple. Electro-oxidative polymerization of 1 was accomplished either by repeated CV scans, or by controlled potential electrolysis. The resulting polymer films were tested in acetonitrile electrolyte for the amperometric recognition of alkali and alkaline earth metal cations. Their remarkable recognition properties towards Ca 2+ and Ba 2+ cations have been confirmed by electrochemical impedance spectroscopy (EIS). EIS allowed construction of a calibration curve based on the variation of the low frequency capacity of the film as a function of the guest cation concentration, reflecting the variation of the formal potential between the free and the complexed immobilized redox couple. Capacity measurements using EIS appear to be an attractive method for cation sensing with films based on molecular redox receptors.

Complexation and electrochemical sensing of anions by amide-substituted ferrocenyl ligands

New amide-containing ferrocenyl ligands, L1–5, were prepared and the voltammetric and 1H NMR investigations of anion binding were carried out in organic media. The electrochemical recognition ability of L1–5 towards F−, HSO4−, H2PO4− and ATP2− is based on the synergy between H-bonding to amide protons in anion complexation to reduced, neutral ligands and ion-pairing interactions developed with the oxidized, cationic form of the ligands. The strength of the anion–ligand interactions depends on the number of ferrocene centers and amide groups in the receptor, and on the accessibility of the binding sites. Clear two-wave cyclic voltammetry features allowed the amperometric titration of H2PO4− and ATP2− by ligands L4 and L5 built from a cyclotriveratrylene structural unit, and containing a combination of three ferrocene centers with three (L4) or six (L5) amide groups.

Catecholase activity of a μ-hydroxodicopper(II) macrocyclic complex: structures, intermediates and reaction mechanism

The monohydroxo-bridged dicopper(II) complex (1), its reduced dicopper(I) analogue (2) and the trans-μ-1,2-peroxo-dicopper(II) adduct (3) with the macrocyclic N-donor ligand [22]py4pz (9,22-bis(pyridin-2′-ylmethyl)-1,4,9,14,17,22,27,28,29,30- decaazapentacyclo -[22.2.114,7.111,14.117,20]triacontane-5,7(28),11(29),12,18,20(30), 24(27),25-octaene), have been prepared and characterized, including a 3D structure of 1 and 2. These compounds represent models of the three states of the catechol oxidase active site: met, deoxy (reduced) and oxy. The dicopper(II) complex 1 catalyzes the oxidation of catechol model substrates in aerobic conditions, while in the absence of dioxygen a stoichiometric oxidation takes place, leading to the formation of quinone and the respective dicopper(I) complex. The catalytic reaction follows a Michaelis–Menten behavior. The dicopper(I) complex binds molecular dioxygen at low temperature, forming a trans-μ-1,2-peroxo-dicopper adduct, which was characterized by UV–Vis and resonance Raman spectroscopy and electrochemically. This peroxo complex stoichiometrically oxidizes a second molecule of catechol in the absence of dioxygen. A catalytic mechanism of catechol oxidation by 1 has been proposed, and its relevance to the mechanisms earlier proposed for the natural enzyme and other copper complexes is discussed.

(Ferrocenylmethyl)trimethylammonium cation: a very simple probe for the electro-chemical sensing of dihydrogen phosphate and ATP anionsElectronic supplementary information (ESI) available: cyclic voltammograms of 1 in the presence of increasing amounts of ATP2– or HSO4–. See http://www.rsc.org/suppdata/nj/b1/b107713a/

By virtue of strong ion-pairing interactions that are reinforced following its oxidation to the ferrocinium form, (ferrocenylmethyl)trimethylammonium cation is able to electrochemically sense dihydrogen phosphate and ATP anions in organic electrolytes; clear two-wave voltammetry features allow their amperometric titration by this very simple derivative of ferrocene.

Soluble heterometallic coordination polymers based on a bis-terpyridine-functionalized dioxocyclam ligand.

Soluble homo- and heterometallic coordination polymers containing transition metal cations (Cu(2+), Fe(2+), Co(2+), and Ni(2+) ions) were prepared in a two-step procedure using a polytopic bis(terpyridine)dioxocyclam ligand 1H(2) (dioxocyclam = 1,4,8,11-tetraazacyclotetradecane-5,7-dione). These supramolecular systems incorporate two different metal complexes, the metal cations being located both between two terpyridine units and in the macrocyclic framework. The characterization of these soluble architectures was investigated by cyclic voltammetry, mass spectrometry, viscosimetry, and UV-vis absorption and electron paramagnetic resonance (EPR) spectroscopies. Our results clearly indicate the formation of well-organized heterometallic polymers in which two different metal ions alternate in the self-assembled structure. These investigations furthermore brought to light an original acid-controlled disassembling process of the homometallic copper(II) polymer into dinuclear complexes.

Anion recognition and redox sensing by a metalloporphyrin–ferrocene–alkylammonium conjugate

The synthesis and characterization of a novel redox molecular receptor is reported. This chemosensor is structured on a ferrocene fragment whose cyclopentadienyl moieties have been connected to distinct and complementary zinc porphyrin and alkylammonium binding sites, enabling multipoint recognition and detection of anionic species. Cumulative effects of multiple anchoring points on this ammonium–ferrocene–metalloporphyrin chemosensor allowed the unprecedented ferrocene-based voltammetric sensing of halide anions.

Coordination of Ferrocenyl Ligands Bearing bipy Subunits: Electrochemical, Structural and Spectroscopic Studies

The coordinative properties of mono(bipy) and bis(bipy) (bipy refers to 6or 6,6 -substituted 2,2 -bipyridine) carbonyloxyand carboxamido-bridged derivatives of ferrocene L1−5 towards a number of metal cations (CuI, CuII, NiII, FeII, CoII, HgII, PbII, AgI) have been investigated by the interplay of voltammetry, mass spectrometry and X-ray diffraction studies. Particular attention was paid to the electrochemical recognition properties of these redox-active ligands, as monitored by modulation of the ferrocene/ferricinium redox couple upon complexation. It was found that the bis(bipy)bridged ligands exhibit a sandwich effect that gives rise to a