Calix diquinone

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... 

Taking into account om calix[4]tube ligand design10, the novel bis-calix[4]diquinone receptors were prepared via the initial synthesis of various bis-calix[4]arene derivatives connected by alkylene chain units and subsequent oxidation of the remaining phenolic... 

Similar structures 37 and 38, bearing ureido functions on the lower rim of calix[4]diquinone or monoquinone moieties, were used for anion recognition with the aid of electrochemistry (cyclic voltammetry) [47,48]. Some of these compounds were found [49] to bind selectively HSOj or H2POj anions. 

Two ureido functions introduced into the upper rim of calix[4]arene [52] or the corresponding calix[4]diquinone derivative 41 exhibit good complexa-tion abilities for various anions. Even in highly competitive solvents such as DMSO-d6, quinone 41 shows [53] very strong complexation for H2POj (K4l= 13,900 M-1) or for benzoate (K41=2,430 M-1). The complexation process can be observed using electrochemical methods (cyclic voltammetry) where the addition of anions generates substantial cathodic shifts. 

The Ru(II)bipyridylcalix[4]diquinone receptor 43 selectively binds and senses acetate anions (from H NMR titrations in DMSO-d6 solution K= 9,990 M1) [37]. This receptor exhibited only weak luminescence because calix[4]diquinone is an electron acceptor, quenching the [Ru(bpy)3]2+ emission... 

A further example of steric control on electron transfer processes involving calixarene hosts has been provided by Ziessel and coworkers who appended one (56) or two [Ru (bpy)3] moieties and a/ac-[Re Cl(CO)3(bpy)] (57) to a calix[4]diquinone [95]. For all the complexes the luminescence quantum yield is very low because of... 

The X-ray structure of calix[4]arene tetraquinone " shows it to be in a flattened partial cone conformer. Similarly, the dialkyl ether diquinones 96a, 96b, and 96c all assume the partial cone conformation but differ with respect to the orientation of the four residues in the cyclic array in 96a and 96b the alkoxyaryl... 

The electrochemical behavior of calixquinones " has been studied in some detail, and the complexation characteristics of several diquinones [e.g. 228 (R = t-Bu Y = Me, CH2C02Et, and CH2CONEt2)] have been investigated. Complexation constants ranging from 10 M to 4.8 x 10 M are observed, the latter with the complex of the ester with Ba ". The calix[4]arene monoquinone 226 (R = r-Bu Y = Et) binds Na" and shows an enhancement in the complexation constant of ca. 10 when reduced to the radical anion... 

The Ru°bipyridylcalix[4]diquinone receptor 83 selectively binds and senses acetate anions (from NMR titrations in DMSO-de solution K = 9990 M ) [54]. This receptor is only weakly luminescent because the Ru (bpy)3 MLCT emission is partially quenched by oxidative electron transfer to the electron-poor calix[4]diquinone. Addition of acetate to acetonitrile solutions of 83 resulted in a five-fold increase in liuninescence intensity (60% for chloride) concomitant with a slight blue shift of the emission maximum. Anion binding causes this increase in emission intensity by interrupting the electron transfer pathway from the Ru°(bpy)3 to the calix[4]diquinone, thus reducing its quenching effect. 

Complexes of rheniiun(l)tricarbonylchloride with pyridyl ligands are lu-minophores and hence in anion sensing have principally been used as reporter groups. For instance calix[4]diquinone receptor 90 selectively binds and senses acetate in DMSO solution [54] (from H NMR titrations K = 1790 in DMSO-de solution). The receptor exhibits relatively weak lumi-... 

Webber. P.R.A. Chen. G.Z. Drew, M.G.B. Beer. P.D. Cesium and rubidium-selective redox-active /jzT(calix[4]-diquinone) ionophores. Angew. Chem. Int. Ed. 2001. 40,... 

When a metal complex like osmium(II) fris(bipyridyl) or a metallocene, as in compound 3, is appended to the ligand, the MLCT excited state is quenched by energy transfer to the adjacent metal center. The rate of this quenching decreases as an anionic substrate binds between the two metal eomplexes. A similar switching mechanism is seen in the calix[4]diquinone-appended complex, 4. In this case, the fluorescence emission is quenched via an intramolecular electron transfer to the quinone. Anion binding between the Ru complex and the quinone blocks the quenching process, and the emission intensity is significantly recovered. 

A successive X-ray study [64] showed that in the solid state calix[8]arene 1,5-diquinone 32a adopts a pseudo-chair-like conformation (Fig. 7.18). In this structure, two anti-oriented 3/4-c(Mie moieties can be seen (in magenta and green in Fig. 7.18) while the two quinone rings (in grey) are on parallel planes. Two quinone oxygens point towards the centre of the molecule with a short distance of 3.27 A [64]. 

For both (3) and (4) Na+ and Ba + cations form very stable complexes. It is noteworthy that the diamide calix[4]arenediquinone (4) exhibits a larger association constant for Bu NH than for NH4 whereas with the diester derivative (3) the reverse trend is observed. Preliminary electronic absorption titration data of (7) with K+ and Ba + cations gave association constants of 5 x 10" and 3 x 10 respectively. Interestingly, this K+ over Ba " selectivity preference is similar to that exhibited by the dimethyl ether calix[4]diquinone ligand (8) but is the reverse shown by (3) and (4). 

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