Cyclam metal complexes

NO release is considerably slowed down when lrans- Ru(Cl) (NO) (cyclam)] is reduced (Scheme 5.8(b)). It has also been observed [204] that the reduction in blood pressure in hypertensive Wistar rats is more prolonged by the administration of cyclam-NO than when SNP is administered. This suggests that the metal cyclam NO complex could be used as a long lasting vasodilator [214]. 

The effect of several metal ions on the anti-HIV activity of bi-cyclams has been reported (371). Zinc may facilitate the binding of bicyclams to the virus, and bis-Zn(II)-bicyclam complexes exhibit activity comparable to that of the parent bicyclam, but activity is reduced for other metal complexes such as Ni(II), Cu(II), Co(II), and Pd(II). 

The 12-membered tetraamine cyclen and bicyclen have inferior anti-HIV activity and are more toxic compared to cyclams and bicyclams. However, Kimura et al. (372) have shown that complexation of the monomeric cyclen (84) to Ni(II), Cu(II), and Zn(II) reduces the toxicity and increases the anti-HIV activity. This is also true for the bicyclen (85), for which the combination of dimerization and metal complexation potentiates the inhibition against HIV-infected MOLT-4 cells (373). 

In most cases, metal ion coordination by a dendrimer takes place by units that are present along the dendrimer branches (e.g., amine, imine, or amide groups) or appended at the dendrimer periphery (e.g., terpyridine, cathecolamide ligands). When multiple identical coordinating units are present, dendrimers give rise to metal complexes of variable stoichiometry and unknown structures. Luminescent dendrimers with a well defined metal-coordinating site have been reported so far [16, 17], and the most used coordination site is 1,4,8,11-tetraazacyclotetradecane (cyclam). 

Cyclam, or 1,4,8,11-tetraazacyclotetradecane, was first synthesised, inadvertently, by van Alphen in 1937 through the reaction of 1,3 bis (2 aminoethyl)-aminopropane with 1,3 dibromopropane. The first major studies of the ligand by Bosnich et al. used X-ray crystallography to elucidate different conformations of the metal complexes of the macrocycle.27 Their synthesis was similar to van Alphen s procedure, but did not involve a template method and as a result yields were unimpressive. They showed that five out of the possible ten different metal-complex conformers exist in the solid-state (Figure 3.51). 

Cyclam, with its 14-membered ring, has been demonstrated to complex with a very wide range of metal ions (and especially d-block metal ions) and tends to generate metal complexes that are especially kinetically and thermodynamically stable [22], In particular instances it is also known to aid the stabilisation of less common oxidation states such as Ni(III), Cu(III), Ag(II) and Ag(III) [25,26],... 

Sequestering in a cyclic environment imparts to the metal novel properties and favours redox activity. For instance, [Nin(cyclam)]2+ in 1 M HC1 is oxidised to the indefinitely stable [Nira(cyclam)]3+ complex, at a moderately positive potential (0.72 V vs. NHE) [9]. The acidic medium is required to prevent intramolecular electron transfer processes, leading to decomposition [10]. Moreover, [Nin(cyclam)]2+ can be electrochemically reduced to NiVcyclam) + at a mercury electrode, where it catalyses the reduction of CO2 to CO and HCOO (in an aqueous solution buffered to pH 5) [11]. This is nothing especially new encircling by tetra-aza macrocycles (e.g. porphyrins) is a trick known to Nature for billions of years to favour and control the redox activity of metal ions. 

The structural characteristics of cyclen and cyclam and their carboxylic and amidic derivatives in variously protonated states as well as their metal complexes were excellently reviewed by Guilard and co-workers <1998CCR1313>. As those derivatives are frequently used in the complexation of transition metal or lanthanide ions, the space arrangement, exact protonation sites, and presence of intramolecular hydrogen bonds are of interest from the point of view of kinetics of complex formation/dissociation. Since the number of related structures is very high, but no new remarkable information appeared since Guilard s review, derivatives of these macrocycles are not discussed in detail. 

Fig. 12. Schematic representation of a metal complex containing a Zn ion coordinated by two cyclam-cored dendrimer 5, and the corresponding scheme (Fig. 2e).

According to the modular approach, components of the fluorosensor can be changed at will. For instance, it could be of some interest to replace the quad-ridentate receptors of systems 4 and 5 by their cyclic counterparts, to obtain 6 and 7 [8]. The reason of the interest is that, ceteris paribus, cyclic ligands form more stable metal complexes than their open-chain analogues (the thermodynamic macrocyclic effect [12]). The tetramine receptor in 6 has the skeleton of the classical 14-membered macrocycle cyclam, whereas the receptor subunit of 7 refers to the other well-known object of macrocyclic chemistry dioxocyclam. 

Among examples of the electrochemical reduction of CO2 by metal complexes having macrocyclic ligands, the case of [Ni(cyclam)] adsorbed on an Hg electrode of especial interest from the viewpoint of high current efficiency and CO generation (almost 100% current efficiency) in H2O under relatively low overpotential conditions (64). The selectivity of CO/H2 formation in the CO2 reduction is largely dependent on the substituents of the Ni macrocycle complexes. Recently,... 

In photochemical reduction of CO2 by metal complexes, [Ru(bpy)3] is widely used as a photosensitizer. The luminescent state of [Ru(bpy)3] is reductively quenched by various sacrificial electron donors to produce [Ru(bpy)3] . Metal complexes used as catalyst in the photochemical reduction of CO2 using [Ru(bpy)3] are prerequisites which are reduced at potentials more positive than that of the [Ru(bpy)3] " redox couple (-1.33 V vs SCE) (72). Irradiation with visible light of an aqueous solution containing [Co (Me4(14)-4,ll-dieneNJ], [Ru(bpy)3], and ascorbic acid at pH 4.0 produces CO and H2 with a mole ratio of 0.27 1 (73). Similarly, photochemical reduction of CO2 is catalyzed by the [Ru(bpy)3] /[Ni(cyclam)] system at pH 5.0 and also gives H2 and CO. However, the quantum efficiency of the latter is quite low (0.06% at X = 400 nm), and the catalytic activity for the CO2 reduction decreases to 25% after 4 h irradiation (64, 74, 75). This contrasts with the high activity for the electrochemical reduction of CO2 by [Ni(cyclam)] adsorbed on Hg. 

Such hydrido intermediates have been detected in the reactions of Cr2 +, 120,121 Cu +, 122 Fe2 +,123 and Ti3 +.124 The rates of formation of the hydrido species conform to the usual reaction patterns for Id substitution kinetics. An example of H-atom addition to metal complexes is provided by the reaction of [Ni(cyclam)]2 + and a Co (II) tetraazamacrocycle.82,125... 

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