Dibenzo-30-crown alkali metal complexes

It has been proposed that the first stage of alkali metal complex formation by dibenzo-30-crown-10 in methanol involves a fast ligand conformational change which is then followed by a stepwise substitution of the coordinated solvent by the ligand (Chock, 1972) ... 

Figure 4.11 Molecular structures of typical crown-ether complexes with alkali metal cations (a) sodium-water-benzo-I5-crown-5 showing pentagonal-pyramidal coordination of Na by 6 oxygen atoms (b) 18-crown-6-potassium-ethyl acetoacetate enolate showing unsymmelrical coordination of K by 8 oxygen atoms and (c) the RbNCS ion pair coordinated by dibenzo-I8-crown-6 to give seven-fold coordination about Rb.

Polymeric pseudocrown ether networks have been generated in situ by the photopolymerization of poly(ethylene glycol) diacrylate transition metal complexes <00CM633>, and the effect of metal ion templation was evaluated. The 1,6,13,18-tetraoxa[6.6]paracyclophane-3,15-diyne (termed pyxophanes) was prepared from hydroquinone and l,4-dichlorobut-2-yne it forms size-selective 7i-complexes with alkali metal cations <00CC2377>. Dibenzo[ ]crown-m have been used in numerous elegant studies in which they were the needles that were threaded by diverse reagents the resultant... 

Many polypodands having terminal donor groups are also available, e.g. (39)-(41).33 These also exhibit cation complexation and phase transfer properties. KMn04 and aqueous alkali metal picrates are much more readily taken into organic phases in the presence of these podands than with dibenzo-18-crown-6.239 The KSCN complex of (39) exhibits a novel coordination geometry as all 10 donor atoms participate in coordination of the metal cation, and in order to do this the three arms wrap around the cation in a propeller-like fashion.240... 

Figure 25 Plots of log K, (in MeOH at 25 °C) for complex formation between alkali metal cations and several cyclohexano- and dibenzo-crown ethers (reproduced with permission from reference 31)...

In 1967, DuPont chemist Charles J. Pedersen (21) discovered a class of ligands capable of complexing alkah metal cations, a discovery which led to the Nobel Prize in Chemistry in 1987. These compounds, known as crown ethers or cryptands, allow gready enhanced solubiHty of sodium and other alkali metals in amines and ethers. About 50 crown ethers having between 9—60 membered ohgoether rings were described (22). Two such structures, dibenzo-18-crown-6 (1) and benzo-9-crown-3 (2), are shown. 

Bright, D. and Truter, M. R. Crystal structure of complexes between alkali-metal salts and cyclic polyethers. Part 1. Complex formed between rubidium sodium isothiocyanate and 2,3,11,12-dibenzo-l,4,7,10,13,16-hexaoxocyclo-octadeca-2,11-diene ( dibenzo-18-crown-6 ). J. Chem. Soc. (B) 1544-1550 (1970). 

Macrocycllc compounds (some crown ethers and cryptands) are selective reagents for extractive separation of alkali metals [22-27]. These ligands form cationic complexes with alkali metal ions, and these can be extracted as ion-pairs with suitable counter-ions e.g., picrate) [28], most often into chloroform. For potassium, p-nitrophenoxide was used as counter-ion [29]. In cases, where a coloured anionic complex is a counter-ion [30], the extract may serve as a basis for determining the alkali metal. The effect of the structure of the dibenzo-crown ether rings upon the selectivity and effectiveness of isolation of alkali metals has been studied in detail [31]. Chromogenic macrocyclic reagents applied for the isolation and separation of alkali metals have been discussed [32]. 

The distribution of alkali-metal picrate 1 1 complexes of dibenzo-18-crown-6 between water and benzene has been investigated. The formation of 2 1 complexes was recognized for Rb and Cs ions in a large excess of polyether. The extractability of complex cation-picrate ion pairs decreased in the order K > Rb > Cs>Na>Li. This order was confirmed in a parallel investigation, and bis-(3,5-di-t-butylbenzo)-18-crown-6 also has good extraction ability for alkali-metal ions. ... 

In the context of crown ether hosts, non-covalent bonds of pole-pole, pole-dipole, and dipole-dipole types can all be employed [3-6] in the formation of host-guest complexes. Where the guest species is an alkali metal (i.e. Li, Na", K", Rb, Cs ), alkaline earth metal (i.e. Mg, Ca, Sr, Ba ), or harder transition or post-transition metal (e.g. Ag", TT, Hg, Pb, La, Ce ) cation [3-6,14], an electrostatic (M" O) pole-dipole interaction binds the guest to the host whilst the (M" X ) pole-pole interaction with the counterion (X ) is often retained. The features are exemplified by the X-ray crystal structure [15] shown in Fig. la for the 1 2 complex (1) (NaPF jj formed between dibenzo-36-crown-12 (1) and NaPF. Molecular complexes involving metal cations have considerable strengths even in aqueous solution and a template effect involving the metal cation is often observed during the synthesis of crown ether derivatives. 

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