Hemispherands

The same procedure can be applied for the preparation of hemispherands with modified terphenyl units (other substituents) and hemispherands in which the central aromatic ring is a pyridine ring.14,15 

Caution Carry out all procedures in a well-ventilated hood, and wear disposable vinyl or latex gloves and chemical-resistant safety goggles. 

Dry all glassware in an electric oven (110°C, 2 h). Assemble the triplenecked flask, argon balloon via the adaptor, septum, stopper, stirrer bar and magnetic stirrer. Heat all glassware with the electric heat gun and allow to cool under an argon atmosphere. 

Place 2.0 g of teranisyl 7 in the triple-necked flask as quickly as possible to retain the argon atmosphere. 

Assemble the teflon syringe and the needle and flush the syringe with argon. Fill the syringe via the septum with 50 mL of diethyl ether and add this to the flask containing 7 via the septum. Dissolve 7 in the diethyl ether. 

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Other molecules include cryptophanes (e.g., 23), ° hemispherands (an example is 24 ° ), andpodands The lastnamed are host compounds in which two or more arms come out of a central structure. Examples are 2S °° and 26. ° Compound 26,... 

The hemispherands, spherands, calixarenes, and related derivatives. A number of hosts for which the pre-organization criterion is half met (the hemispherands) (Cram et al., 1982) or fully met (the spherands) (Cram, Kaneda, Helgeson Lein, 1979) have been synthesized. An example of each of these is given by (251) and (252), respectively. In (251), the three methoxyl groups are conformationally constrained whereas the remaining ether donors are not fixed but can either point in or out of the ring. This system binds well to alkali metal ions such as sodium and potassium as well as to alkylammonium ions. The crystal structure of the 1 1 adduct with the f-butyl ammonium cation indicates that two linear +N-H - 0... 

Scheme 1 Typical examples of phosphorylated cryptands A [12, 13], B [14, 15], C [16], macrocycle [17], hemispherand [18], and cavitand [19]...

As stated above, systematic names of macrocyclic host molecules were absurdly complicated for routine discussions [22]. Therefore Vogtle proposed the name coronand for crown ethers, and that of coronates for their complexes while cryptand complexes were called cryptates . The corresponding noncyclic analogues are podands such as 64 [23] and podates, respectively. The cumbersome name podando-coronands (and correspondingly podando-coronates ) was proposed for lariat ethers [24] having at least one sidearm like 65. Examples of hemispherands 66 [25], cavitands 25 [26] and those of some other hosts are discussed in Chapter 7 in some detail, whilst the exceptional stability of fragile guests 4 [2a] and 67 [27] in the hemicarcerand 5 cavity are discussed in Chapters 1 and Section 7.3. 

Spherands, hemispherands, and other similar macrocycles capable of inclusion complex formation [33]... 

Hemispherands like 66 [37] and 239 [38] are macrocycles in which at least halfofthe ligating heteroatoms is preorganized prior to complexation (another part being crown ether) while in cryptahemispherands like 240 [39] there are two crown type bridges. The latter are very strong binders of alkali metal ions. 

It should be stressed that calixarenes, hemispherands and spherands, carcerands and hemicarcerands, and some other molecules discussed elsewhere in this book, belong to the cyclophane group of compounds. 

Hemispherands have a partial preorganization of the ligand. The X-ray structure of... 

Na(BPh4) H20 shows the metal coordinated by the hemispherand donors and by a water molecule.438 The related complex (99)NaNCS MeOH shows the metal to be coordinated by all six donors of the crown, which is in a boat configuration and the terphenyl is folded away. The metal geometry is a biapical square pyramid.439... 

Spherands (10) and hemispherands (11) are macrocyclic ligands which consist of arrangements of phenyl groups. 

Hemispherands (11) are by nature only partially preorganized for complexation, depending on their structural characteristics.30 129 132 Addition of hydrocarbon-based bridges, which essentially converts the monocycles to bicyclic systems, serves to provide a more rigid structural framework as in the cryptands.132... 

Cross-fertilisation between the crown ethers (or, more generally, corands), cryptands, spherands and podands has produced an enormous range of hybrid hosts such as cryptaspherands and hemispherands, many exhibiting all the useful features of the parent materials (Figure 3.16). 

Why do you think corand 4.12 ([24]aneN602) is such an effective ATPase mimic given the discussion of the relatively low affinity for guest species such as alkali metal cations and halide anions set out in Chapters 3 and 4 Similarly, why was a hemispherand chosen as the central cation-binding core of enzyme mimic 12.16 as opposed to a spherand ... 

The photophysical properties of terphenyl-based acyclic (22a) and cyclic hemispherands (22c, 22d, see fig. 27) have been investigated in organic solvents and in KBr pellets (1 wt%) with the purpose of introducing them later in optical amplifiers (Sloofif et al., 1998). Absorption cross sections for the 1.54 pm emission, which is quite broad with fwhh = 70 nm, amount to 0.62, 1.1, and 0.93 x 10 20 cm2 for [Er(22a)], [Er(22c)] and [Er(22d)], respectively. The best photoluminescence intensities are obtained with the cyclic ligands. The optical gain of the complexes doped into a polymer channel waveguide is on the order of 1.7 dB cm-1, while the threshold power is as low as 1.4 mW. 

Different materials for the hydrophobic membrane in which the receptor is incorporated, have been investigated. Polysiloxanes that have the required glass transition temperature and dielectric constant provide a stable chemical system that transduces the complexation of cationic species into electronic signals. The material properties can be optimized by copolymerization of three building blocks viz. dimethyl-, (3-cyanopropyl)methyl-, and methacryloxypropylmethyl siloxane. CHEMFETs made with this terpolymer have fast response times (<. 1 sec.). With valinomycin and hemispherands (2) and (3) linear responses to changing K+ concentrations are obtained in the range 10"5 - 1.0M (55-58 mV/decade) in a solution of 0.1M NaCl. Similar devices specific for Na+ and Ca2+ have been obtained with other ionophores. 

For the synthesis of a photopolymerizable receptor molecule that has a high selectivity for K+, we have modified the hemispherand synthesis according to Scheme 4. The final polymer membrane is represented by Figure 8. The response to a variation in K+ concentration (10 s - 10"1 M) in the presence of 0.1 M NaCl is given in Figure 9. The potentiometric selectivities (log K ) determined by the mixed solution method are -3.0 (Na+), -3.5 (Ca2+), -4.0 (Mg2+), and -0.7 (NH/). This renders this CHEMFET an excellent K+-sensor with a longterm stability t> 100 days). 

For the characterization of the polysiloxane membranes we have measured the impedance spectra in the presence of different concentrations of cations. Both valinomycin (1) and the two different hemispherands (2 and 3) have been incorporated in these membranes. Buck et al (24, 25) have shown that this technique allows to separate the surface exchange rates and the rates of bulk transport. The copolymer described above containing valinomycin initially has a very high resistance but after several days conditioning in a 0.1 M KC1 solution the resistance is the same as that of a similar membrane to which KB(4-C1C6H4)4 was added. 

Also the final dielectric constants of both membranes are similar. This indicates that the final amount of charge carriers hardly differs and the contributions of the lipophilic anion to the membrane conductance is of the same order as of the chloride anion. Membranes with the hemispherand, either free or covalently attached, have about the same resistance. Because at the low frequency part of the impedance spectra a slight distortion was observed with the hemispherands, kinetic measurements were carried out with the valinomycin containing membrane. 

In addition to the spherands, Cram synthesized compounds in which at least half of the binding sites may be considered to be preorganized, the so-called hemispherands.6 In the parent molecule 2, the molecular cavity is composed of a rigid m-teranisyl moiety in which the oxygen binding sites are conformationally organized prior to complexation, and a flexible polyether... 

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