Cyclic hexamer

A quite different structural motif is found in the curious cyclic hexamer [(BNMc2)6] which can be obtained as orange-red crystals by distilling the initial product formed by dehalogenation of (Me2N)2BCl with Na/K alloy ( >... 

These results, together with infrared and electron diffraction studies, indicate that gaseous HF comprises an equilibrium mixture of monomers and cyclic hexamers, though chain dimers may also occur under some conditions of temperature and pressure ... 

Temp., °C Time, hr Cyclic dimer Yield 5, % Cyclic tetramer Polymer + cyclic hexamer Mnc... 

It is expected, therefore, that the cyclic hexamer also exhibits a characteristic tendency to complex with cations. In fact, the addition of an acetonitrile solution of metal thiocyanates to a solution of the cyclic hexamer in the same solvent shifted the carbonyl absorption to a lower wave number46,52 The shift values depended upon the kind of metal ions present, and the largest shift value of 40 cm-1 was observed for barium thiocyanate (molar ratio of Ba2+/hexamer = 10). In addition to the shift of the carbonyl absorption, the intensities of the C—O-C stretching vibrations around 1200 cm-1 varied appreciably. 

Proton noise decoupled 13C-NMR spectra of equimolar mixtures of the cyclic hexamer and metal thiocyanates showed that the signals of the carbonyl carbon and two methine carbons gave downfield shifts upon the addition of metal thiocyanates, while those of the three methylene carbons of the tetrahydropyran ring gave upfield... 

Table 6.1H-NMR chemical shift changes of the cyclic hexamer of 6,8-dioxabicyclo[ 3.2.1 J-octan-7-one upon interaction with thiocyanate salts in deuterkjchloroforma...

Cyclic hexamer thiocyanate salt =1 1 (mol) concn., 3% 100 MHz room temp. positive and negative values refer to downfield and upfiekl shifts, respectively. 

The conversion of the cyclic hexamer of fiiran and acetone into naphthafiirophane was reported <96TL6201>. Stepwise synthesis of core-modified, zncso-substituted porphyrins (e.g., 19) was achieved by acid catalyzed condensations <96TL197>. 

A dimer made up of two zinc porphyrins bearing a 7-azabicy-clo[2.2.1]heptadiene fused at the C2-C3 /3-positions was reported by Knapp (61). The compound was designed to dimerize with a pyrrole-over-pyrrole geometry similar to that found in the photosynthetic special pair. Dimerization at KT3 M was confirmed by VPO and JH NMR spectroscopy. Dilution to 10-5 m or addition of DMAP caused disaggregation of the complex. In the solid state, this compound assembles as a cyclic hexamer with the vicinal porphyrin planes almost perpendicular. 

Various polyphosphates are effective sequestering agents under appropriate conditions. The best known of these is sodium hexametaphosphate (10.14), the cyclic hexamer of sodium orthophosphate. Further examples are the cyclic trimer sodium trimetaphosphate (10.15), as well as the dimeric pyrophosphate (10.16), the trimeric tripolyphosphate (10.17) and other linear polyphosphates (10.18). All of these polyanions function by withdrawing the troublesome metal cation into an innocuous and water-soluble complex anion by a process of ion exchange as shown in Scheme 10.7 for sodium hexametaphosphate. Hence these compounds are sometimes referred to as ion-exchange agents. 

Me2SnCl2 or Ph2SnCl2, treated with LLAIH4, in the presence of Et20 and Et3N, provide the cyclic hexamers [Me2Sn]6 or [Ph2Sn]698. 

Two of the water clusters that appear to exhibit particular thermodynamic stability under ambient conditions of the liquid phase are shown in Fig. 5.29 a cyclic pentamer (Wsc) and a cyclic hexamer (W6C). The characteristic factors (F1)-(F3) that distinguish these clusters from other possible isomeric forms can be identified as follows ... 

The cyclic hexamer 9 (named rosette ) presents 18 hydrogen bonds48,49 9 is the first step in the formation of the cyanuric acid/melamine lattice formed by a sheet of non-covalent aggregates50. 

Evaporation of the xylene filtrate and trituration of the residue with methylene chloride yields ca. 11 g of solid which consists mainly of cyclic hexamer and cyclic tetramer. The methylene chloride contains inter alia, cyclic pentamer, cyclic heptamer, and bishomo compound, and these can be obtained as pure samples in low yield by fractional crystallization procedures. The compositon of the reaction mixtures can be qualitatively established by TLC by means of the following values in 9 1 petroleum... 

Incomplete removal of water and/or too slow removal of water can result in the formation of a significant amount of cyclic octamer, which is difficult to separate from the cyclic hexamer. It is important, therefore, to bring the reaction mixture to reflux (10-15 min) and to remove all the water. In addition to the ca. 85 ml of H2O/HCHO removed in the first phase of the reaction, an additional ca. 10 ml (as a cloudy, lower layer in the Dean-Stark trap) is removed in the second phase. 

HPLC analysis of this product usually shows it to contain less than 1% of other calixarenes. If these are present in greater amounts (usually the cyclic tetramer, cyclic heptamer or cyclic octamer), their content can be reduced to less than 1% by triturating the product with hot acetone. The purity of the product can be qualitatively checked with TLC using a petroleum ether (30-60°C)/methylene chloride mixture (1 1) as eluant. The Rf of the cyclic hexamer is ca. 0.65, and the appearance of any spots with greater than 0.1 indicates the presence of other calixarenes. 

Figure 13.3 Proton ordering in hydrogen bonding, illustrating favorable (Grotthuss-like) ordering in a chain (above) or cyclic hexamer (a), contrasted with the unfavorable isomer (b), one of many similar that cannot survive thermodynamically. All clusters are ice rule-compliant (cf. Sidebar 5.18).

The complex (269) is hexameric in the solid state and in solution, and the same structure presumably occurs in the complexes Ni(SR)2 obtained from reaction (197). In the cyclic hexamer the six nickel atoms form a regular hexagon with twelve symmetrically bridging SEt-groups which are situated above and below the plane of the nickel atoms.1944 A corresponding cyclic tetramer has been obtained with jV-methylpiperidine-4-thiolate anion, SCsHsNMeJ.1946 Also in the complex [Ni(SC5H9NMe)2]4 (270) the nickel atoms lie in a plane. 

Homometallic cyclic hexamer iridium complex, preparation,... 

Cyclic methylpolysilanes very readily undergo rearrangement reaction. By refluxing pure decamethylcyclopentasilane in THF in the presence of naphthalene radical anions, 10% of the cyclic hexamer is formed after 1 h. 

For the large-scale synthesis of 125, the crude reaction product was a mixture of cyclic hexamer and cyclic pentamer (9 1) and the compounds were separated by column chromatography. 

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