Cryptate counterions

The values of k and of k found in THF at -30°C are listed in Table 1. The values of k may be considered to be the same within the experimental errors. The k values Increase rapidly with the size of the counterion. The most surprising fact is that 1 becomes higher than k with all cryptated counterions. A plot of log versus 1/a gives approximately a straight line (Fig. 2) and it may be seen that the experimental value for k (obtained from kinetics for Na+) is quite far from the extrapolated value for ji . 

Alkali metal anions have also been generated as a result of cryptand stabilization of the corresponding cation. Cryptands were found to enhance the solubility of zerovalent alkali metals in various organic solvents.156-157 Initially, the solutions apparently contain the cryptate cation and solvated electrons together with free ligand. When more metal is dissolved, metal anions, M , are formed.158 Dye and co-workers have isolated gold-colored crystals of [Na+ c 2.2.2]Na 159160 and the crystal structure has been determined.161,162 Anion clusters such as Sb] , Pb2 and Sn," have been isolated as crystalline salts of the [2.2.2] cryptate counterion [2.2.2].162,163... 

Various homo- and heteropolyatomic anions have been stabilized with cryptate counterions. (Na 2.2.21) forms stable complexes with Pb52 (139), Sn94 (140), and Sb73 (141) by inhibiting reversion to the initial sodium-metal alloy phase. In a similar vein, the crystal structures of (K[2.2.2])+(HgTe2) containing a linear anion (142) and (K[2.2.2]+)2Sn42 have been described (143). 

Modem work on these and related bare post-transition element clusters began in the 1960s after Corbett and coworkers found ways to obtain crystalline derivatives of these post-transition element clusters by the use of suitable counterions. Thus, crystalline derivatives of the cluster anions had cryptate or polyamine complexed alkali metals as countercations [8]. Similarly, crystalline derivatives of the cluster cations had counteractions, such as AlCLj, derived from metal halide strong Lewis acids [9]. With crystalhne derivatives of these clusters available, their structures could be determined definitively using X-ray diffraction methods. 

The crown ethers and the related cryptates (cryptates were first reported by J-M Lehn (J ) of France in 1969 (Figure 3)), have been used in a variety of synthetic procedures primarily because of their ability to solvate ionic materials in organic solvents (1, 14). Recently, it has been shown that crown ethers in the solid state form "sandwich" like complexes with most metals and that the counterion is also tightly bound (1 ). It is this evidence that suggested their use as ion traps in silicone RTV formulations. 

Propylene sulfide gives polymers with perfectly stable thiolate living ends in ethereal solvents (16). The propagation reaction has been studied in THF (7-9,17-2lT and in THP (8.9.21.22.23) with Na+,Cs+, - and several cryptatee as counterions, by dilatometry. It has been shown that even for non cryptated species, the associations of ion pairs do not generally occur in the range of concentrations examined ([C]<10 3 mole.l- in THF and < 2 10 mole.l - in THP). The main ionic species are ion pairs in equilibrium with free ions ... 

The results obtained with different counterions are shown In Table.II. Free alkoxlde Ions are about seventy times more reactive than cryptated Ion pairs. Cryptated Ion pairs are surprisingly slightly less or as reactive as the corresponding non complexed Ion pairs within the experimental errors though the... 

Figure 3. Linear dependence of the apparent bimolecular rate constant of living polyethylene oxide propagation on the fraction of free ions a with cryptates as counterions in THF at 20°C K + 222 ( ) with + [222] ( ) + [ ] ) with 4>kBCs + [ ] -...

In order to elucidate this point, viscosity measurements of living and deactivated PDMS solutions were performed In toluene, with Li+ + [211] as counterion. As no significant change was observed, It can be deduced that the fraction of aggregates Is negligible (< 1% for [C] 10 mole. " ). Moreover, conductance measurements made on model sllanolates In THF Indicate that the fraction of free Ions Is very low. In our system (benzene) we conclude therefore that the contribution to the reactivity from free ions can be neglected. Thus the main Ionic species are cryptated ion pairs, and... 

Of particular significance in this respect has been the ability to prepare, characterize and study most intriguing species, the alkalides [2.79, 2.80] and the electrides [2.80, 2.81] containing an alkali metal anion and an electron, respectively, as counterion of the complexed cation. Thus, cryptates are able to stabilize species such as the sodide [Na+ c 9]Na- and the electride [K+ c 9]e-. They have also allowed the isolation of anionic clusters of the heavy post-transition metals, as in ([K+ c cryp-tand]2 Pb52-) [2.82]. 

Jost, R, Galand, P.J.N., Schurhammer, R., Wipff, G. 2007. Supramolecular interactions of cryptates in concentrated solutions The effect of solvent and counterions investigated by MD simulations. Solvent Extr. Ion Exch. 25 (2) 257-271. 

The endo-endo conformation of cryptands can be internally protonated to form proton cryptates. With the small cryptands, e.g. [1.1.1]- and [2.1.1]-cryptand (15a and 15b), the two internal protons are so efficiently shielded from H2O and OH that deprotonation only very slowly occurs even in strong base (8UA6044). Alkali cation cryptates are able to stabilize unusual species as their counterions. Dye and coworkers have isolated several alkali metal anions by this method. The sodium species (Na [2.2.2]cryptand Na ) was obtained as gold metallic crystals and gave a Na NMR with a broad Na -cryptate resonance and a narrow, upheld Na resonance. The other alkali metals show similar behavior and an electride salt (Na [2.2.2]cryptand e l has even been isolated (B-79MI52105). Crystalline anionic clusters of the heavy post-transition metals (such as Sb7 , Pbs , Sng ) were first obtained with alkali metal cryptates as the counterions (75JA6267). 

Anionic Polymerization of Cyclosiloxanes with Cryptates as Counterions... 

The anionic polymerization of cyclosiloxanes was examined In benzene and toluene with lithium cryptates as counterions. Only one type of active species Is observed In the case of LI + [211] thus, the kinetics of the propagation and of the by-product cyclosiloxanes formation can be studied In detail for the first time. The reactivity of cryptated sllanolate Ion pairs toward the ring opening of D3 Is greatly enhanced compared to that of other systems. 

Thus, in the present paper we present our data concerning the anionic polymerization of cycloslloxanes, namely D3 and D4, with cryptates as counterions, In benzene or toluene solution. 

Table IV. Molecular Weight Distribution of PDMS Obtained from D3 using Lithium Cryptates as Counterions...

A similar inversion of reactivities was also reported for thiolate active centers with alkali metal counterions complexed with cryptates, in propylene sulfide (2-methylthiirane) polymerization [111]. 

In addition to cryptates [26-30] or crown ether complexes [32] (as noted above), bulky and soft, weakly interacting counterions, such as Me4N% Bu4Ph rare earth complexes [38] and phosphazenium cations [39-45] have also recently been studied. 

---