Dinuclear zinc(II) dithiocarbamate macrocycles: ditopic receptors for a variety of guest molecules

The synthesis of a series of dinuclear zinc(II) dithiocarbamate (dtc) macrocyclic receptors containing aryl spacer groups of different sizes is reported. As evidenced from 1H NMR titration investigations, these receptors have the ability to bind various neutral and anionic bidentate guests species, including 1,4-diazabicyclo[2.2.2]octane (DABCO), isonicotinate and terephthalate in a cooperative 1 : 1 intramolecular inclusion complex. Stability constant determinations reveal a correlation between the strength of complexation and complementary receptor cavity : guest molecule size. In particular, the X-ray structure of a 1 : 1 host-guest complex between a dinuclear zinc(II) dtc receptor and DABCO illustrates the cooperative nature in which the dinuclear receptor associates with the bidentate guest.     

Anion Sensing by Solution‐ and Surface‐Assembled Osmium(II) Bipyridyl Rotaxanes

We report the preparation of [2]rotaxanes containing an electrochemically and optically active osmium(II) bipyridyl macrocyclic component mechanically bonded with cationic pyridinium axles. Such interlocked host systems are demonstrated to recognise and sense anionic guest species as shown by ¹H NMR, luminescence and electrochemical studies. The rotaxanes can be surface assembled on to gold electrodes through anion templation under click copper(I)‐catalysed Huisgen cycloaddition conditions to form rotaxane molecular films, which, after template removal, respond electrochemically and selectively to chloride.     

Transition metal and organometallic anion complexation agents

Anions are ubiquitous species, and therefore, their sensing is of considerable interest. Anion receptors containing electrochemically active groups such as ferrocene or cobaltocenium, or optically active groups such as ruthenium(II) bipyridyl derivatives, allow the binding of anions to be detected by a physical response at the metal centre. These systems have been incorporated into various acyclic, macrocyclic and calix[4]arene frameworks, many of which include an amide hydrogen-bonding group. Anions may be recognised in a range of environmental conditions, with some receptors even being active in aqueous solution. The incorporation of new transition metal and organometallic systems has led to the development of several new strategies in anion recognition.     

Redox active macrocyclic receptors for neutral guests

A novel series of triazine-appended macrocyclic complexes has been investigated as potential hydrogen bonding receptors for complementarily disposed heterocycles. Cocrystallization of a melamine-appended azacyclam complex of Cu(II) has been achieved with barbitone, the barbiturate anion and thymine. In each case, a complementary DAD/ADA hydrogen bonding motif between the melamine group and the heterocycle has been identified by X-ray crystallography. Electrochemical studies of the copper macrocycles in both nonaqueous and aqueous solution show anodic shifts of the Cu(II/)(I) redox couple of more than 60 mV upon addition of guest molecules with matching H-bonding motifs. The Zn(II) analogues have been synthesized via transmetalation of the Cu(II) complex, and their guest binding properties investigated by NMR spectroscopy. (1)H NMR shifts of up to 0.8 ppm were observed upon addition of guest, and stability constants are similar to those obtained electrochemically.     

Redox‐Triggered Chirality Switching and Guest‐Capture/Release with a Pillar[6]arene‐Based Molecular Universal Joint

A chiral electrochemically responsive molecular universal joint (EMUJ) was synthesized by fusing a macrocyclic pillar[6]arene (P[6]) to a ferrocene‐based side ring. A single crystal of an enantiopure EMUJ was successfully obtained, which allowed, for the first time, the definitive correlation between the absolute configuration and the circular dichroism spectrum of a P[6] derivative to be determined. The self‐inclusion and self‐exclusion conformational change of the EMUJ led to a chiroptical inversion of the P[6] moiety, which could be manipulated by both solvents and changes in temperature. The EMUJ also displayed a unique redox‐triggered reversible in/out conformational switching, corresponding to an occupation/voidance switching of the P[6] cavity, respectively. This phenomenon is an unprecedented electrochemical manipulation of the capture and release of guest molecules by supramolecular hosts.     

Tetrapyrrole derivatives substituted with ferrocenylethynyl moieties. synthesis and electrochemical studies

Up to eight redox-active ferrocenyl units have been incorporated, through the unsaturated ethynyl linkers, on the periphery of a series of cyclic tetrapyrrole derivatives including zinc(II) phthalocyanine and 2,3-naphthalocyanine, and nickel(II) meso-diphenylporphyrin. The synthesis of the former two macrocycles 4 and 7 involves the Sonogashira coupling reaction of ferrocenylethyne with 4,5-dichlorophthalonitrile (1) or 6,7-dibromonaphthalonitrile (5), respectively, followed by a base-promoted cyclization. The meso-bis(ferrocenylethynyl)porphyrin 11 has been prepared from the dibromo analogue 10 also by a palladium-catalyzed coupling reaction. These novel macrocyclic compounds have been spectroscopically and electrochemically characterized. As revealed by cyclic voltammetry, the ferrocenyl moieties appear to be electrochemically independent in these complexes and there is no significant electronic coupling among the iron(II) centers.     

Synthesis and electrochemical studies of phenylazo substituted tetraaza macrocyclic complexes of Ni(II)

Synthesis and characterization of some phenyl azo substituted tetraaza macrocyclic complexes of Ni(II) are reported. Electrochemical behaviour of these macrocyclic complexes has been examined using polarographic, voltammetric and spectroscopic techniques. These studies show that both the ligand and the metal are electrochemically active. The electrochemical behaviour of the azo function has been shown to occur through a single four-electron process. When a nitro group is also present, the nitro function of the azo moiety is reduced in a six-electron cathodic wave.     

Electrochemistry of the bis(1,4,7-triazacyclodecane) cobalt(III) complex and its role in the catalytic reduction of hydrogen

The electrochemistry of the bis(1,4,7-triazacyclodecane) cobalt(III) complex at a mercury electrode, HMDE, in aqueous Britton–Robinson buffer solutions was investigated using cyclic voltammetry, double-potential-step chronoamperometry and chronocoulometry. The cyclic voltammetric data were analyzed by digital simulation to confirm and to measure the heterogeneous and homogeneous parameters for the suggested electrode mechanism. Generally, the complex is electrochemically reduced giving rise to two cyclic voltammetric waves. The first wave is a diffusion-controlled reversible wave. It is assigned to the stable Co(III)/Co(II) redox couple. The second one is found to be irreversible and corresponding to a reduction of Co(II) to Co(I) species. The monovalent cobalt, highly unstable, is rapidly protonated, and then forms cobalt hydride. The hydride decomposes to hydrogen molecules and regenerates Co(II) species following a disproportionation pathway. The overall reduction mechanism is concluded to be an EECC kinetics.     

Zinc oxide nanoparticles-based electrochemical sensor for the detection of nitrate ions in water with a low detection limit—a chemometric approach

We report for the first time a cyclic voltammetric nitrate sensor with a low detection limit based on the immobilization of zinc oxide nanoparticles on the surface of the platinum working electrode using chitosan membrane. Cyclic voltammetric data demonstrated that zinc oxide nanoparticles can electrochemically reduce nitrate ions to ammonium ions with high conductivity. In order to estimate electroanalytical parameters for each of the nitrate concentrations, Gaussian and Lorentzian curve fitting algorithms were performed on cyclic voltammetric data. Among them, the best analytical performance results were obtained with Gaussian calibration linear model. The zinc oxide modified platinum electrode showed a linear response to nitrate ions over a concentration range from 0.1 to 2.0 mM with a low detection limit and high sensitivity of 10 nM and 39.91 μA/cm² mM, respectively. The nitrate ion concentrations in drinking water samples were determined using Gaussian calibration linear model and the predicted, added nitrate ion concentration values showed good correlation.     

Voltammetric Characterization of Redox‐Inactive Guest Binding to LnIII[15‐Metallacrown‐5] Hosts Based on Competition with a Redox Probe

A novel competitive binding assay was implemented to monitor the binding of a redox inactive substrate to a redox inactive metallacrown host based on its competition with ferrocene carboxylate (FcC^?) using cyclic voltammetry (CV). First, the binding of FcC^? to Ln^III[15‐MC‐5] (LnMC) hosts was characterized by cyclic voltammetry. It was shown that the voltammetric half wave potentials, E_1/2, shifted to more positive potentials upon the addition of LnMC. The explicit dependence of E_1/2 with the concentration of LnMC was used to determine the association constants for the complex. The FcC^? binding strength decreased with larger central lanthanide metals in the LnMC hosts, and substantially weaker binding was observed with La^III. X‐ray crystallography revealed that the hydrophobic host cavity incompletely encapsulated FcC^? when the guest was bound to the nine‐coordinate La^III, suggesting the LnMC’s ligand side chains play a substantial role in guest recognition. With knowledge of the MC‐FcC^? solution thermodynamics, the binding affinity of a redox inactive guest was then assessed. Addition of sodium benzoate to a LnMC and FcC^? mixture resulted in E_1/2 shifting back to the value observed for FcC^? in the absence of LnMC. The association constants between benzoate and LnMC’s were calculated via the competitive binding approach. Comparison with literature values suggests this novel assay is a viable method for determining association constants for host–guest systems that exhibit the proper electrochemical behavior. Notably, this CV competitive binding approach does not require the preparation of a modified electrode or a tethered guest, and thus can be generalized to a number of host–guest systems.     

Electrocatalytic oxidation of some biologically important compounds at conducting polymer electrodes modified by metal complexes

Conducting poly(3-methylthiophene) electrodes were electrochemically prepared. The resulting polymer films were modified with an inorganic complex, ferrocene. The incorporation of the ferrocene/ferrocenium moiety into the polymer film resulted in enhanced charge transfer towards the oxidation of some organic molecules of biological interest. The electrochemical response of the complex-containing polymer electrode was compared to that of the unmodified polymer electrode and that of the substrate. Apparent diffusion coefficients of the redox species were estimated from the cyclic voltammetric data for different biological molecules at the ferrocene-containing polymer electrode. Infra-red spectroscopic measurements for the “as-grown” films revealed the presence of the inorganic complex within the polymer. The modified polymer electrode showed noticeable enhancement for the charge transfer across the film interface and can be used as an electrochemical sensor for biological compounds. Received: 3 June 1997 / Accepted: 7 July 1997     

Highly selective fluorescent recognition of phenyl amino alcohol based on ferrocenyl macrocyclic derivatives

Four ferrocenyl macrocyclic derivatives 3 and 4 containing anthracene fluorophores have been synthesized. The fluorescent properties of these receptors have been studied in three organic solvents, both in the absence and presence of phenyl amino alcohols. All receptors exhibit sensitive fluorescence response to l- or d-phenylglycinol, a strong emission band is produced due to the intermolecular exciplex between host and guest. These special phenomena were not observed when other species were used as the guests; such highly selective fluorescent response indicates that these receptors can easily discriminate phenylglycinol from other similar species. Solvent comparative experiments also indicate that acetonitrile is the most appropriate solvent to detect this fluorescent change. The intramolecular energy transfer between excited anthracene and ferrocene, and π–π stacking interaction between the aromatic rings play critical roles in this special fluorescence enhancement. Model calculations at DFT level further suggest the possible interaction modes, structures and relatively steric position between the host and guest also influence the optical response.     

Sulfate selective anion recognition by a novel tetra-imidazolium zinc metalloporphyrin receptor

Imidazolium groups have been successfully incorporated into the structure of a "picket fence" porphyrin molecule to produce a novel tetra-imidazolium zinc metalloporphyrin anion receptor. UV/visible spectroscopic studies reveal that this receptor is selective for sulfate anions, capable of strongly complexing sulfate in competitive water-DMSO (5 : 95) solvent mixtures. Cyclic and square wave voltammetric studies demonstrate the receptor's ability to sense a variety of anions electrochemically.     

Positive Allosteric Control of Guests Encapsulation by Metal Binding to Covalent Porphyrin Cages

The allosteric control of the receptor properties of two flexible covalent cages is reported. These receptors consist of two zinc(II) porphyrins connected by four linkers of two different sizes, each incorporating two 1,2,3-triazolyl ligands. Silver(I) ions act as effectors, responsible for an on/off encapsulation mechanism of neutral guest molecules. Binding silver(I) ions to the triazoles opens the cages and triggers the coordination of pyrazine or the encapsulation of N,N′-dibutyl-1,4,5,8-naphthalene diimide. The X-ray structure of the silver(I)-complexed receptor with short connectors is reported, revealing the hollow structure with a cavity well-defined by two eclipsed porphyrins. Rather unexpectedly, the crystallographic structure of this receptor with pyrazine as a guest molecule showed that the cavity is occupied by two pyrazines, each binding to the zinc(II) porphyrin in a monotopic fashion.     

A Vinylanthracene and Vinylferrocene‐Containing Copolymer: A New Dual pH/Sulfide Sensor

A single electroactive copolymer containing both anthracene and ferrocene units has been synthesized to form a dual pH/sulfide sensor. Under cyclic voltammetric conditions the oxidation of the ferrocene moiety undergoes an electrocatalytic reaction with sulfide, producing a linear increase in the oxidative peak current over the range 0.2–2.0 mM. Furthermore, square wave voltammetry was utilized to determine the pH of the solution by measuring the variation in the anthracene oxidation potential with respect to the ferrocene oxidation wave.     

Spectroscopic techniques and cyclic voltammetry with synthesis: manganese(II) coordination stability and its ligand field parameters effect on macrocyclic ligands

Manganese(II) macrocyclic complexes are prepared with different macrocyclic ligands, containing cyclic skeleton bearing organic components which have different chromospheres like N, O and S donor atoms and stereochemistry. Thus, six macrocyclic ligands, were prepared and their capacity to retain the manganese(II) ion in solid as well as in aqueous solution was determined and characterized by elemental analyses, molar conductance measurements, magnetic susceptibility measurements, mass, (1)H NMR, IR, electronic spectral and cyclic voltammetric studies. The electronic spectrum of this system showed a dependence that may be consistent with the formation of stable complexes and coordination behaviour of the ions. ESR spectra of all the complexes are recorded in solid as well as solution, which show the oxidation state of the manganese(II). Spin Hamiltonian manganese(II), which can be defined as the magnetic field vector (H): H = gBeta(e)HS + D[S(2)(z) - 35/12] + E[S(2)(z) - S(2)(y)] + ASI + (1/6)a [S(4)(x) + S(4)(y) + S(4)(z) - 707/16] + (1/180)F[(35S(2)(z) - 475)/(2S(2)(z) + 3255/10)] Significant distortion of the manganese(II) ion in observed geometry is evident from the angle subtended by the different membered chelate rings and the angles spanned by trans donor atoms octahedral geometry. Cyclic voltammetric studies indicate that complexes with all ligands undergoes one electron oxidation from manganese(II) to manganese(III) followed by a further oxidation to manganese(IV) at a significantly more positive potential.     

Macrocyclic versus acyclic preorganization in organoplatinum(II)-based host-guest complexes

Two host-guest systems have been constructed,by employing structurally similar terpyridine platinum (II) macrocycle and molecular tweezer as the synthetic receptors.The macrocycle/guest complex displays low-energy emission signal,reinforced non-covalent binding affinity,and enhanced photosensitization capability than those of the molecular tweezer/guest one.The discrepancy between macrocyclic and acyclic preorganization modes originates from the different numbers of Pt (II)...Pt (II) metal-metal bonds in host-guest complexation structures.     

New strategy and methods for constructing artificial macrocyclic anion receptors. Selective binding of tetrahedral oxoanions

A new rational strategy for assembling highly selective neutral macrocyclic anionic receptors proposed by the authors is considered. The strategy includes preliminary theoretical modeling of supramolecular complexes, analysis of synthetic paths for receptor preparation, selection and synthesis of building blocks followed by their cyclocondensation under thermo-dynamic control in the presence of acid corresponding to the target template anion, which provides anion-induced combinatorial selection of the macrocyclic ligand in the emerging dynamic combinatorial library. Analysis of the properties of the obtained anion receptors showed that the number of hydrogen bonds formed between the receptor and the guest anion mainly determines the binding energy. The receptor selectivity for a particular anion is determined by the nature of coordination sites, their geometry in the macrocyclic cavity, and the overall conformation rigidity of the macrocycle. Using the developed strategy and novel synthetic approaches, a large series of highly selective anion receptors with record binding constants (up to 10^p7 L mol^p-1 in both highly and weakly solvating media) was constructed and the structures of the host—guest complexes were studied in detail by both experimental and theoretical methods     

Electrochemically induced motion in copper complexes of tetraferrocene-cyclam

The tetra(ferrocene)-cyclam macrocycle 1,4,8,11-tetra(ferrocenylmethyl)-1,4,8,11-tetraazacyclotetradecane (L) and two corresponding Cu^I and Cu^II complexes have been prepared and characterized from spectroscopic and electrochemical studies (cyclic voltammetry and rotating disk electrode experiments). Both complexes exist under two energetically distinct geometries which differ mainly from the relative positionning of the ferrocenylmethyl groups above or below the macrocyclic plane and it is shown that a reversible change from one geometry to the other can be electrochemically induced following a copper-centered electron transfer. The equilibrium constants, mechanisms and kinetic parameters of these rearrangements have been evaluated using electrochemical simulation. Presented at the 3rd Chianti Electrochemistry Meeting, July 3.–9., 2004, Certosa di Pontignano, Italy     

Liquid‐Crystalline Mesogens Based on Cyclo[6]aramides: Distinctive Phase Transitions in Response to Macrocyclic Host–Guest Interactions

Producing macrocyclic mesogens that are responsive to guest encapsulation presents a significant challenge. Cyclo[6]aramides, a type of macrocycle with a hydrogen‐bond‐constrained backbone, exhibit thermotropic lamellar, discotic nematic, hexagonal, and rectangular columnar mesophases over a considerably wide temperature range, including at room temperature. Additionally, cyclo[6]aramides show unusual mesophase transitions from lamellar to hexagonal columnar phase mediated by macrocyclic host–guest (H–G) interactions between the macrocycles and alkylammonium salts. The phase transition, triggered by an organic guest engaging in H–G interactions with a macrocyclic cavity, provides a novel strategy for manipulating the properties of liquid‐crystalline materials. The crystal structure of a homologous cyclo[6]aramide reveals a disk‐shaped, near‐planar molecular backbone that facilitates intermolecular π–π stacking and leads to columnar assembly.