Pinched cone conformer

In the presence of excess AgNOs, 5 forms an octanuclear complex [(en)Pt(UH)Ag]4(N03)8 4H2O (11), which from its partially resolved X-ray structure has been assigned a pinched cone conformation where four Ag centers bind to the uracil 0(2) donor atoms of the four uracil monoanions. On addition of NaCl, all four Ag dissociate to give 6. 

For Ag+, complexation via the calixarene aromatic rings has been firmly established for both the partial cone and cone conformations of the immobile tetra/j-propyl ethers by X-ray crystallography.60 In both cases, the Ag+ cation is sandwiched between two distal (opposite sides of the molecule, as opposed to proximal, meaning adjacent to one another) calixarene rings, which are nearly mutually perpendicular giving a pinched cone conformation. The exposed face of the cation is coordinated to a triflate anion in both cases. For the partial cone case, the Ag+ ion also interacts with one of the anisole oxygen atoms, which is inverted with respect to the other three at the lower rim (Figure 3.85). 

A single-crystal X-ray structure for a tetraloop compound shows the molecules (not unexpectedly) in a pinched cone conformation (see Figure 5.10a), although the time-averaged conformation deduced from the1H NMR-spectra is C4v-symmetrical (Figure 5.10b). 

The nitrate salt of the analogous [ (en)PtnAg(UrH, A (7),A (5),(9(2), 0(4)) 4]8+ cation exhibits a different structure, which can be formally derived from that of the pinched-cone conformer of Fig. 15,a, by coordination of silver ions to the uracilate oxygens. In all the octanuclear complexes, two metal units, Pt(en) and M, represent each corner of the boxes. These results demonstrate that, irrespective of the 120° angle between the Pt-N(l)... 

The regioselective incorporation of the phenolic units (apparent from the simple NMR spectrum) was confirmed by X-ray analysis for several derivatives and for the parent 97a itself. This shows the molecules in a strongly pinched cone conformation in which two opposite phenolic units are nearly parallel (effective C2 symmetry), a situation that would not be favorable for guest inclusion. This pinched cone conformation might be the energy minimum in solution but it could not be detected by NMR spectroscopy down to the lowest available temperature (150 K in CDFC12).187... 

The connection of adjacent phenolic units by directional bridges may be mentioned as an additional possibility. The C2-symmetrical lactone 102a is obtained in 36% yield from the tetrachloromethylated calix[4]arene tetrapropyl ether by reaction with salicylic acid. The formation of the Cs-symmetrical isomer is not observed in this case, probably since the most nucleophilic phenolate attacks at opposite rings, due to a pinched cone conformation of the tetrapropyl ether.196 In contrast, the similar cyclization with 3-hydroxymethyl-2-naphthol gave mainly the Cs isomer (Cs/C2 = 95/5). However, the C2 isomer 102b could be isolated and has been resolved by enantioselective HPLC (Chiralpak AD, a = 3.17).145... 

MM-calculations lead to an energy minimum with C2v -symmetry ( pinched cone conformation) forcalix[4]arenes (a) Harada, T. Ohseto, F. Shinkai, S. Tetrahedron 1994,50,13377-13394. (b) Harada, T. Shinkai, S. J. Chem. Soc., Perkin Trans. 2 1995,2231-2242. 

Around the same time as this, Reinhoudt developed a calix[4]arene system with only two urea or thiourea functionalities attached on opposite faces 70-72 [167]. These less substituted systems display both inter- and intramolecular hydrogen bonding as a result of the calixarene adopting a pinched cone conformation (demonstrated by the use of NOESY NMR). In some spectra it is impossible for the connectivities to be made within a single molecule, so the only possibility left is that dimerisation occurs. As with the initial experiments of Rebek and Bohmer, the extent of hydrogen bonding was observed to be solvent dependent. Concentration dependant FTIR was also used, to observe the effects on the NH stretching vibrations, but no concentration dependence was observed. Of the three urea derivatives used, only 72 showed no evidence of dimerisation... 

Fig. 12a, b a The structure of C4-acetone. b Possible orientation of the acetone molecule with respect to C6 in pinched cone conformation. T is ca. 120° and is equally divided into four angles of 30°. The angles between the twofold axes of calixarene and the CD3 vector averaged about the twofold axis of acetone are a, pi and p2 for the A, and B2 orientations. (Adopted from [48] by permission of the PCCP Owner Societies)... 

Figure 4.12 X-Ray crystallographic structures of 6 in (a) pinched cone conformation and (b) 1,2,3-alternate conformation Taken from Wolfgong et...

Capsule-shaped Ir(l) and Rh(l) cationic complexes with a triphosphinocaltx[6]arene as a multidentate ligand were recently synthesized (Figure 33). These organometallic bis-caltxarene complexes showed dynamic behavior with size-selective molecular encapsulation, which was confirmed by variable-temperature P H NMR measurements in the presence of various molecules. X-ray crystal analysis showed that the calix[6]arene moiety adopted the same pinched-cone conformation as the non-coordinated caltxarene. Molecules such as CFlj,Cly or CICH2CH2CI are too small to fit the cavity of the iridium and rhodium bis-calixarene complexes and cannot restrict the dynamic behavior at 25 °C. On the contrary, molecules such as X2CHCHX2 (X = C1 or Br), benzene, toluene, o- or w-xylene just fit in the cavity and show the dynamic behavior. Finally, large molecules (p-xylene, cumene, mesitylene, etc.) could not enter the cavity. 

The ultramicroporous 3-D MOF [Cu(F-pymo)2(H20)j 25] (F-pymo = 5-fluoro-pyrimidin-2-oIate) reported by Navarro, Parra, Sironi, and co-workers [212] displays a zeolitic gismondine-like topology with hehcal channels of ca. 2.9 A diameter. These channels result from the interconnection of two different cyclic motifs, namely metallacahx[8]arenes and metallacalix[4]arenes exhibiting either 1,3-altemate or a pinched cone conformation (Fig. 24a). These are filled with water... 

The crystal structme of 28 showed that the compound adopts a pinched-cone conformation with the ethylene glycol substituents properly disposed to fold in and bind a metal cation at the base of the cahxarene and that the two thiourea units are effectively oriented to provide an anion-binding site through hydrogen bonding. NMR titrations in CD3CN of 28 with the TBA... 

Fig. 3.4 The X-ray structure of 116 showing the pinched cone conformation of this cmnpound...

Among the solid state structures of the larger calix[ ]arenes, Raston [96] and coworkers reported the first inclusion complex of p-tert-butylcalix[9]arene C[9] with o-carborane (Fig. 7.30). In this structure, the calix[9]arene macrocycle adopts a conformation composed by three 3/4-cone clefts and a pleated-loop portion. Perrin et al. [97] showed that p-tert-butylcalix[10]arene C[10] adopts in the solid state a pinched cone conformation stabilized by a circular H-bond between OH groups and very similar to that found for calix[6]arene macrocycle [98]. 

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