The dicopper(II) complex with the ligand N,N,N',N',N"-pentakis[(1-methyl-2-benzimidazolyl)methyl]dipropylenetriamine (LB5) has been synthesized and structurally characterized. The small size and the quality of the single crystal required that data be collected using synchrotron radiation at 276 K. [Cu(2)(LB5)(H(2)O)(2)][ClO(4)](4): platelet shaped, P&onemacr;, a = 11.028 Å, b = 17.915 Å, c = 20.745 Å, alpha = 107.44 degrees, beta = 101.56 degrees, gamma = 104.89 degrees, V = 3603.7 Å(3), Z = 2; number of unique data, I >/= 2sigma(I) = 3447; number of refined parameters = 428; R = 0.12. The ligand binds the two coppers nonsymmetrically; Cu1 is coordinated through five N donors and Cu2 through the remaining three N donors, while two water molecules complete the coordination sphere. Cu1 has distorted TBP geometry, while Cu2 has distorted SP geometry. Voltammetric experiments show quasireversible reductions at the two copper centers, with redox potential higher for the CuN(3) center (0.40 V) and lower for the CuN(5) center (0.17 V). The complex binds azide in the terminal mode at the CuN(3) center with affinity lower than that exhibited by related dinuclear polyaminobenzimidazole complexes where this ligand is bound in the bridging mode. The catechol oxidase activity of [Cu(2)(LB5)](4+) has been examined in comparison with that exhibited by [Cu(2)(L-55)](4+) (L-55 = alpha,alpha'-bis{bis[(1-methyl-2-benzimidazolyl)methyl]amino}-m-xylene) and [Cu(2)(L-66)](4+) (L-66 = alpha,alpha'-bis{bis[2-(1-methyl-2-benzimidazolyl)ethyl]amino}-m-xylene) by studying the catalytic oxidation of 3,5-di-tert-butylcatechol in methanol/aqueous buffer pH 5.1. Kinetic experiments show that [Cu(2)(L-55)](4+) is the most efficient catalyst (rate constant 140 M(-1) s(-1)), followed by [Cu(2)(LB5)](4+) (60 M(-1) s(-1)), in this oxidation, while [Cu(2)(L-66)](4+) undergoes an extremely fast stoichiometric phase followed by a slow and substrate-concentration-independent catalytic phase. The catalytic activity of [Cu(2)(L-66)](4+), however, is strongly promoted by hydrogen peroxide, because this oxidant allows a fast reoxidation of the dicopper(I) complex during turnover. The activity of [Cu(2)(LB5)](4+) is also promoted by hydrogen peroxide, while that of [Cu(2)(L-55)](4+) is little affected. The phenol monooxygenase activity of [Cu(2)(LB5)](2+) has been compared with that of [Cu(2)(L-55)](2+) and [Cu(2)(L-66)](2+) by studying the ortho hydroxylation of methyl 4-hydroxybenzoate to give methyl 3,4-dihydroxybenzoate. The LB5 complex is much more selective than the other complexes since its reaction produces only catechol, while the main product obtained with the other complexes is an addition product containing a phenol residue condensed at ring position 2 of the catechol.

Fluorescent sensors for 3 d divalent metal ions have been designed by means of a supramolecular approach: an anthracene fragment (the signalling subunit) has been linked to either a cyclic or a noncyclic quadridentate ligand (the receptor). occurrence of the metal-receptor interaction is signalled through the quenching of anthracene fluorescence. When the receptor (i.e., the dioxotetramine subunit of sensors 2 and 3) is able to promote the one-electron oxidation of the metal, quenching takes place through a photoinduced metal-to-fluorophore electron-transfer mechanism. In the case of sensors containing a tetraamine binding subunit (4 and s), quenching proceeds copper complexes electron trattsferenergy transfer fluoreseaE. ~~1 1 8 0~) * by an energy-transfer process. Selective metal binding and recognition can be achieved by varying the pH, and metal ions can be distinguished (e.g., Cu" from Ni") by spectrofluorimetric titration experiments in buffered solutions. Whereas systems 2, 3 and 5 show reversible metal binding behaviour, the cyclam-containing system 4 irreversibly incorporates transition metals (due to the kinetic macrocyclic effect) and cannot work properly as a senIn the supramolecular world, a sensor is a two-component system in which the specific receptor for the intended substrate is connected to a subunit capable of signalling the occurrence of the receptor-substrate interaction. The signal is given by a drastic change of a property: thus sensor eficiency is related to the ease of detecting such a property and measuring its intensity over a substantial concentration range, possibly down to trace level, as well as to receptor specificity. In this context, j7uores-cence is a convenient property to investigate. Fluorescence is visible, can be determined in real time without excessively sophisticated and expensive instrumentation and, if the appropriate fluorophore is chosen, can be safely monitored at a concentration level as low as ~O -' M . Efficient sensing should involve variation of the investigated property by at least two orders of magnitude: in spectrofluorimetric measurements, such a situation would correspond to full quenching or to complete revival of the emission intensity.During the last decade, a number of fluorescent sensors has been designed for s-block metal ions.['] Most of them operate by a photoinduced electron-transfer (PET) mechanism.['] In a classic example from the de Silva group, the binding component of the sensor is an NO, crown, which is linked through the amine nitrogen atom to the powerful light-emitting fragment anthracene by a methylene group.131 The uncomplexed sensor 1 is not fluorescent, as the photoexcited fluorophore is deactivated by a nonradiative mode through the transfer of an electron from the highly re-(e.g., of a K' ion), the metal-ligand interaction decreases the amine oxidation potential drastically and prevents the electron transfer. As a consequence, the intense and characteristic anthracene emission is largely restored.We were interested in dev...

Dinuclear copper(II) complexes with the new ligand 1,6-bis[[bis(1-methyl-2-benzimidazolyl)methyl]amino]-n-hexane (EBA) have been synthesized, and their reactivity as models for tyrosinase has been investigated in comparison with that of previously reported dinuclear complexes containing similar aminobis(benzimidazole) donor groups. The complex [Cu2(EBA)(H2O)4]4+, five-coordinated SPY, with three nitrogen donors from the ligand and two water molecules per copper, can be reversibly converted into the bis(hydroxo) complex [Cu2(EBA)(OH)2]2+ by addition of base (pK a1 = 7.77, pK a2 = 9.01). The latter complex can also be obtained by air oxidation of [Cu2(EBA)]2+ in methanol. The X-ray structural characterization of [Cu2(EBA)(OH)2]2+ shows that a double μ-hydroxo bridge is established between the two Cu(II) centers in this complex. The coordination geometry of the coppers is distorted square planar, with two benzimidazole donors and two hydroxo groups in the equatorial plane, and an additional, lengthened and severely distorted axial interaction (∼2.5 Å) with the tertiary amine donor. The small size and the quality of the single crystal as well as the fair loss of crystallinity during data collection required the use of synchrotron radiation at 100 K. [Cu2(EBA)(OH)2][PF6]2:  orthorhombic Pca21 space group, a = 22.458(2) Å, b = 10.728(1) Å, c = 19.843(2) Å, R = 0.089. Besides OH-, the [Cu2(EBA)(H2O)4]4+ complex binds azide as a bridging ligand, with the μ-1,3 mode. Azide can also displace μ-OH in [Cu2(EBA)(OH)2]2+ as a bridging ligand. In general, the binding constants indicate that the long alkyl chain of EBA is less easily folded in the structures containing bridging ligands than the m-xylyl residue present in the previously reported dicopper(II) complexes. Electrochemical experiments show that [Cu2(EBA)(H2O)4]4+ undergoes a single, partially chemically reversible, two-electron reduction to the corresponding dicopper(I) congener at positive potential values (E 0‘ = 0.22 V, vs SCE). Interestingly, however, coordination to azide ion makes the reduction process proceed through two separated one-electron steps. The catalytic activity of [Cu2(EBA)(H2O)4]4+ in the oxidation of 3,5-di-tert-butylcatechol has been examined in methanol/aqueous buffer, pH 5.1. The mechanism of the catalytic cycle parallels that of tyrosinase, where no hydrogen peroxide is released and dioxygen is reduced to water. Low-temperature (−80 °C) spectroscopic experiments show that oxygenation of the reduced complex [Cu2(EBA)]2+ does not produce a stable dioxygen adduct and leads to a μ-oxodicopper(II) species in a fast reaction.

298ChemInform Abstract The novel functionalized macrocycle (III) is prepared from (I) and (II). A pH-dependent equilibrium is observed between the blue high-spin nickel(II) complex (IV) and the yellow low-spin complex (V). The nickel(II) complexes undergo a reversible one-electron process to give an authentic nickel(III) species in which the axial binding of the side arm is again controlled by the acidity.

The dizinc(II) complex of an octamine containing the anthracene subunit binds both the imidazolate anion and the imidazolate moiety of L-histidine, and signals the binding through the fluorescence quenching of the fluorophore.The design of multicomponent fluorescent systems able to detect the presence and monitor concentration changes of biologically active small molecules, in particular natural amino acids, is highly desirable. A fluorescent sensor of g-aminobuyric acid has recently been reported. 1 We have recently developed a receptor capable of recognising histidine in the presence of any other natural amino acid. 2 The receptor contains two Cu II ions prepositioned within a polyaza macrocycle. Recognition is based on the fact that the imidazole residue of histidine, in an aqueous solution adjusted to pH 9, deprotonates and bridges the metal centres. This situation is in some ways reminiscent of Cu-Zn superoxide dismutase (CuZn-SOD), in which a Cu II and a Zn II ion are bridged by the imidazolate residue of a histidine fragment.We considered that the octamine 1 † could provide a convenient framework for the construction of a fluorosensor for histidine, as (i) it offers two quadridentate binding sites for Cu II ions, leaving each metal centre coordinatively unsaturated and an open position for a further ligand (i.e. one of the two nitrogen atoms of an imidazolate subunit) and (ii) the anthracene fragment linking the tetraamine subunits gives an intense and characteristically structured fluorescent emission, suitable for signalling the occurrence of the receptor-substrate interaction.The pH titration experiments were carried out on an aqueous solution containing 1 (1 equiv.) and Cu 2+ (2 equiv.), in the absence and in the presence of 1 equiv. of imidazole (imH). Non-linear fitting of the titration curve in the absence of imH indicated the formation of the dimetallic species [Cu II 2 L] 4+ at pH 4. This is present as the major species in the 5-7 pH interval; at higher pH, hydroxide-containing species form. In the presence of imH, an imidazolate-containing dimetallic species [Cu II L(im)] 3+ formed as a major species between pH 7-11. However, whereas the [Cu II 2 L] 4+ system appears to be an excellent receptor for imidazole, it cannot function as a fluorosensor since the Cu II ions fully quench anthracene fluorescence in both the [Cu II 2 L] 4+ and the [Cu II L(im)] 3+ complexes, and thus any monitoring of the recognition process through the variation of the fluorescent emission is prevented. Thus, we considered the use of a pair of Zn II ions as binding sites for imidazole in the octamine receptor 1. Zn II is photophysically inactive and is expected to display some affinity towards a bridging imidazolate fragment, as it shows in the CuZn-SOD enzyme.Further pH titration experiments were carried out on an aqueous solution containing 1 (1 equiv.) and Zn 2+ (2 equiv.), in the absence and in the presence of 1 equiv. of imH. Non-linear fitting of the titration curve in the absence of imH indicated the formation of st...