Calix pyrrole complexes

Di-rerr-butylsodium pyrrolate serves as a source of the complexes of lanthanides [93CB2657 95JOM(495)C12]. Thus, with cyclooctadienyl chlorides of samarium, thulium, and lutetium, it affords species 89 [96JOM(507)287]. The N-coordinated samarium(II) calix-pyrrole complex is known [99AG(E)1432]. 

Calix[4]pyrroles offer an alternative receptor site for the recognition of anions. Miyaji et al. [400] prepared a series of calixpyrrole-anthracene receptors that possess conjugated (73) and unconjugated bond pathways of different lengths (74 and 75). The stability constants (103—105 M-1 determined by NMR titration) for the anion complexes of each member of the series follows the trend... 

That solvents can have a profound and until now often little understood effect on selectivity and affinity of macrocylic hosts is illustrated with the complexation of halide anions with calix[4]pyrrole (Table 1).[31]... 

TABLE 1. Dependence of complex stabilities of a calix[4]pyrrole with halide anions on solvent ... 

Gale and coworkers [60] have discovered a very simple displacement method for a calix[4]pyrrole-based anion receptor. Addition of calix[4]pyrrole 73 to a solution of tetrabutylammonium 4-nitrophenolate in CH2C12 caused a decrease in intensity of a UV band at 432 nm, and calix[4]pyrrole-4-nitrophe-nolate complex 74 was formed. Upon addition of F, 4-nitrophenolate was displaced from the complex, and the color of the solution changed from colorless to yellow due to the presence of the free nitrophenolate anion (Fig. 14). 

Shao and coworkers [61] have demonstrated that a noncovalent charge-transfer complex of calix[4]pyrrole-chloranil can be used as a chromogenic sensor for F and H2POj. Upon addition of calix[4]pyrrole 75 to the solution of chloranil 76 in chloroform, the color changed from pale yellow to blue. This indicated the formation of a charge-transfer complex between calix[4] pyrrole and chloranil in chloroform. The Job s plot analysis showed a 1 1 complex with... 

The medium effect of the complexation of calix[4]pyrrole and its derivatives with... 

The affinity of the various calix[4]pyrrole receptors (1,2, 8-aa/ / ) for the fluoride anion in acetonitrile at 298.15 K is reflected in the conductometric titration curves of fluoride against the ligand/anion ratio. The data shown in Fig. 4b are in accord with 1H NMR studies which show (Table 1) that the most significant chemical shift changes in the resonances of pyrrole NH and / -CH protons of receptors 1 and 2 were observed upon complexation with the fluoride anion in acetonitrile. 

Like calix[4]arenes, calix[4]pyrroles are versatile ligands to the extent that the composition of the anion receptor complex is solvent dependent. A representative example is that involving 8 and the fluoride anion. As shown in Fig. 4b, the well-defined change in curvature observed at 1 1 ligand fluoride mole ratio indicates that in acetonitrile one fluoride anion interacts per unit of receptor 8. However, in moving from acetonitrile to A/,A/-dimethylformamide, the noticeable changes in curvature observed at a ligand/anion mole ratio of 0.5 and 1, indicate respectively the formation of a 1 2 and 1 1 anion complexes, respectively, in this solvent. 

The variations observed in the thermodynamics of complexation of calix[4]pyrrole and its derivatives with the fluoride anion (Table 2) as a result of the medium effect are controlled by the solvation changes that reactants and product undergo in moving from one solvent to another. These differences in solvation are reflected in the thermodynamic parameters of transfer, AtP° for the reactants (fluoride and receptor) and the product (the anion complex) from a reference solvent, s-i to another solvent, s2. The relationship between these parameters and the thermodynamics of complexation, ACP°, in these solvents is shown in Equation (9). 

The above discussion highlights the importance of the effect of the medium on the complexation of calix[4]pyrroles and fluoride. Indeed, these data are a... 

Calix[4]pyrroles are versatile ligands to the extent that the composition of the anion receptor complex is solvent dependent. This chapter has been concerned with the affinity of calix[4]pyrrole for the fluoride anion. It was therefore considered of relevant to focus attention on the steps required for the derivation of reliable thermodynamic data. It is indeed the ratio between stability constants which defines quantitatively the selectivity factor. Thus, representative examples are given to demonstrate selectivity in terms of anion, receptor and solvent. The key role played by solvation in the complexation of these receptors with the fluoride anion is unambiguously demonstrated in the variations observed in the stability constants, enthalpies and entropies of complexation of these systems in the various solvents (Table 2). One convenient measure to assess solvation is through the thermodynamics of transfer of product and reactants from one... 

Other spectroscopic studies have been focused on the UV-Vis absorption properties of rhodium and iridium complexes of A -(2 -hydroxyphenyl)pyrrole-2-aldimine <2005MI167>, and of l,l, 5,5 -tetraaryl-2,2 -bipyrroles <2005H(66)319>. The spectrophotometric properties of calix[4]pyrroles as potential sensors and receptors are also the focus of much ongoing work <1998PAC2401, 2004JA16296, 2005JA8270>. 

Chromogenic octamethyl calix[4]pyrrole-based sensors (e.g., 165-167) for antipyretic carboxylates such as naproxen, ibuprofen, and salicylate, without bias by bicarbonate or carboxy termini of blood plasma proteins, have been described <2005JA8270>. The formation of a sensor-anion complex results in partial charge transfer and a dramatic change in color. 

Recent literature in the chemistry of calixpyrroles comprises the syntheses, characterization and complexation reactions of a series of binucleating Schiff-base calixpyrrole macrocycles <2007CEJ(A)3707> a study of the dynamics of calix[4]pyrrole and octafluorocalix[4]pyrrole as a function of the solvent and fluorine substitution <2007CEJ(A)1108> and oxygen reduction at dicobalt complexes of a Schiff base calixpyrrole ligand <2007AGE584>. 

Examples of ( 7 -C4N) coordination in pyrrole-derived macrocyles may be found in the reaction products of uranium halides with the tetraanion of the macrocycle [(-CH2-)5]4-calix[4]tetrapyr-role. " " As described in Equation (30), the reaction of Ul3(THF)4 with the potassium salt of the tetrapyrrolide in THF generates a dinuclear complex, [ [(-CH2-)5]4-calix[4]tetrapyrrole -UK(THF)3]2(/4-0) 2THF the 0x0 group is proposed to come from deoxygenation of a THF molecule ... 

A highly reactive uranium complex supported by the Et8-calix-[4]-tetrapyrrole tetraanion ligand has been reported. The compound was found to effect dinitrogen cleavage, solvent desoxygenation, and polysilanol depolymerization. These compounds are mentioned here because they involve 7r-bonding interactions between the pyrrole units and the uranium centers.142,142a The synthesis of a series of mono- and bis(tmp)uranium (tmp = tetramethylphospholyl) complexes has been reported. The synthetic procedures are illustrated in Scheme 72.132... 

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