Ruthenium anion receptors

Ion, L., Morales, D., Perez, J., etal., Ruthenium biimidazole complexes as anion receptors. Chem. Commun. 2006, 91-93. 

Another interesting and potentially very useful group of calixarene-based anion receptors is represented by systems with appended transition metal complexes of 2,2-bipyridine units. Technically, these systems utilise classical hydrogen bonding interactions of amidic/urea functions hence, from this point of view, they do not differ from many other receptors. On the other hand, the covalent attachment of bipyridine complexes of ruthenium(II) or rhe-... 

Another class of metal-employing anion receptors is represented by structure 24 [23]. Its function is based on the incorporation of positively charged transition metal complexes directly into the calixarene skeleton. Such calixarenes with enhanced electron deficiency of the aromatic walls provide well-preor-ganised cavities suitable for anion inclusion. The corresponding rhenium [24], ruthenium, rhodium or iridium complexes of this type were prepared and studied for anion recognition [25,26]. 

We recently incorporated the ruthenium(II) bipyridyl moiety into acyclic, macrocyclic, and lower rim caUx[4Jarene structural frameworks to produce a new class of anion receptor capable of optical and electrochemical sensing (226, 253. 254). Stability constant determinations in DMSO using H NMR titration techniques demonstrated that these acyclic receptors (131 and 132) form strong complexes with chloride and dihydrogen phosphate anions (stronger than with analogous monopositive cobaltocenium based receptors). The ruthenium ion is dipositive and hence the electrostatic interactions are particularly favorable. The 4,4 -substituted ruthenium bipyridyls were observed to bind anions more... 

In an effort to produce selective anion spectroelectrochemical reagents the Lewis acidic redox-active cobalticinium unit and the redox- and photo-active ruthenium(II) bipyridyl moiety have recently been incorporated into calixarene host structural frameworks to produce novel calix[4]arene anion receptors which are extremely rare. The calix[4]arene ditopic anion receptor (22) containing two cobalticinium moieties was prepared as shown in Scheme 4 and its crystal structure is illustrated in (Fig 7). Proton NMR titration experiments suggested (22) acts as a ditopic anion receptor forming (22) 2X"(X = Cl",Br",N03, HS04") stoichiometric solution complexes. Analogous titration... 

Exotic calix[4]arene monotopic (23) and ditopic (24) anion receptors containing one and two ruthenium(II) bipyridyl moieties have been prepared (Schemes 5 and 6) and shown by NMR and cyclic voltammetry to bind and electrochemically recognise halide, dihydrogen phosphate and hydrogen sulphate anions. 

The Perez group has also synthesized a range of d transition metal-derived anion receptors based on rhenium and ruthenium, for example, complexes 137 and 138. Compound 137 contains a bidentate pyrazolylamidino ligand which provides one NH hydrogen-bond donor and an additional pyrazolyl NH donor. The complex binds chloride particularly strongly because of its rigid nature, with... 

Beer PD, Timoshenko V, Maestri M et al (1999) Anion recognition and luminescent sensing by new ruthenium(II) and rhenium(I) bipyridyl calix[4]diquinone receptors. Chem Commun... 

The anion coordination properties of receptors such as compound [93] (Fig. 51) are currently under investigation. This molecule contains both a redox-active ruthenium bipyridyl moiety and also a cobaltocenium unit. This type of host has already been shown by 1H nmr and fluorescence emission spectroscopy to exhibit remarkable selectivity for the chloride anion in preference to dihydrogenphosphate (Beer and Szemes, 1995). 

Beer, P. D., Szemes, F., Remarkable chloride over dihydrogen phosphate anion selectivity exhibited by novel macrocyclic bis [ruthenium(II) bipyridyl] and ruthenium(II) bipyridyl-metallocene receptors. J. Chem. Soc., Chem. Commun. 1995, 2245-2247. 

This type of receptor is represented by compounds 16a,b bearing ruthenium ) bipyridine moieties. Both calixarenes [18] exhibit 1 1 binding of chloride and bromide anions (DMSO-d6), and they are especially suitable for the complexation of H2POj (X16a=2.8-104 M-1 K16b=5.2 103 M"1). On the other hand, if we compare these results with those for similar non-calixarene receptors, where the bipyridine unit is substituted by alkyl, aryl or ethylene glycol substituents, the introduction of calixarene does not bring any substantially new features into the complexation abilities of these derivatives. As shown by X-ray analysis, the anion is encapsulated within the cavity formed by amidic functions with the contributions of CH...anion interactions from the bipyridine unit. 

Several calixarenes 17-20 bridged on the lower rim with ruthenium(II) and rhenium(I) bipyridyl complexes have been prepared and studied for anion recognition [19]. When compared to acyclic receptor 21 having a similar active structure (ruthenium complex), all receptors show significantly better complexation of AcO" and chloride in DMSO-d6. Thus, calixarene 17 exhibits an almost 30 times higher association constant for AcO (Kl7=9,990 M 1 vs K21=... 

The luminescence spectra of all receptors in CH3CN were found to be dramatically affected by the addition of acetate or chloride. While compound 19 exhibits an emission decrease, the other receptors 17,18 and 20 show a remarkable intensity increase (up to 500%) with a slight concomitant blue shift of the emission maximum (660 nm for 17). The anion-induced enhancement of luminescence intensity in the case of 17 is clearly due to the decrease of the electron transfer between the ruthenium(II) bipyridyl centre and the quinone moieties. Alternatively, receptors bearing ruthenium or rhenium complexes on the upper rim were also described [20]. 

Introduction of another ruthenium bipyridine moiety or bridging metallocene (ferrocene, cobaltocenium) results in sensors that specifically bind chloride anions (Figure 16.15). The structural modification of the amide receptor results in a decease in cavity dimensions and significant rigidity of the macrocycle. Therefore it cannot accommodate hydrogen phosphate anions, but only much smaller CF [45, 46]. 

A method for the preparation of thin films of Fe4[Ru(CN)6]3 ( ruthenium purple ) involving electrochemical reduction of K3[Ru(CN)6] in a solution of Fe2(S04)3 has been developed.28 This ruthenium purple modified electrode is claimed to be one of the best catalysts for evolution of oxygen and chlorine. Electrochemical studies on polyammonium macrocyclic complexes of [Ru(CN)6]4 indicate a 1 1 stoichiometry with a monoelectronic, reversible, oxidation for these complexes this illustrates the control of redox potential of anions by complexation with appropriate receptor molecules.29 The kinetics of oxidation of [Ru(CN)6]4 by [Mn04] in HC104 have been investigated by stopped-flow techniques. It is found that [Ru(CN)6]4" is quantitatively oxidized to [Ru(CN)6]3 in accordance with equation (1) and that two protonated intermediates [RuH(CN)6]3 and [RuH2(CN)6]3 are involved in the oxidation process.30... 

Heteroditopic ruthenium(n) and rhenium(l) bipyridyl bis(benzo-15-crown-5) ion-pair receptors 19 and 20 display, upon K+ complexation, a remarkable switch in anion selectivity <1998CC825>. In the absence of K+, receptors 19 and 20 exhibit a selectivity preference for II2PO4 over CP. However, addition of KPF6 caused in both cases, the formation of a sandwich K+ complex, which shows a reversed CP over H2POP binding selectivity because of a more favorable electrostatic attraction between K+- and CP-bound ions (positive binding cooperativity). 

A novel Os and Ru bis(bipyridyl) containing an amide macrocyclic receptor has been shown to detect the presence of anions by both electrochemical and optical methodologies. Photophysical studies have clearly shown that the rate constants of the energy transfer process responsible for the quenching of the luminescent ruthenium excited state significantly decreases in the presence of chloride ions. ... 

We also prepared an acyclic mixed-ruthenium(II) bpy-ferrocene receptor 135 (257). The emission of the ruthenium center in the free receptor is quenched by the ferrocene units. However, on addition of dihydrogen phosphate anion the emission increases 20-fold, being switched on by the presence of the anion... 

Upper rim calix[4]arenes functionalized with two and four ruthenium(II) bi-pyridyl amide groups (136 and 137) have very recently been prepared (258). These receptors also sense the presence of dihydrogen phosphate anions selectively. 

It is noteworthy that few of the receptors discussed so far exhibit specific binding and sensing of the chloride anion, yet this substrate is crucial for a large number of biological processes (261). The novel macrocyclic bis[ruthenium(II)bipyridyl] and ruthenium(II) bipyridyl-metallocene receptors (140-142) have been prepared. The H NMR titrations indicated that each re-... 

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