Oxygen atoms crown ether

Crown ether (Section 3.7B) Acyclic ether containing multiple oxygen atoms. Crown ethers bind specific cations depending on the size of their central cavity. 

Because of the Lewis base properties of ether oxygen atoms, crown ethers are excellent complexing agents for Na, K, and NH. What kind of molecule might serve as a complexing agent for Cl or Br ... 

The chemistry of crown ethers has developed rapidly over the last 10 years, a great number of new crown compounds have been synthesized, and their applications have been broadened. As a novel kind of crown ether, selenacrown ether, has been initiated in recent years. Since selenium is a softer base than the oxygen atom, selenacrown ether forms stable complexes with soft metal cations. Because of its complicated characteristic, selenacrown ethers become a part of crown ether that cannot be neglected. 

The replacement of the oxygen atoms present in classic Pederson-type crown ethers by S atoms results in the formation of thia-crown hgands. These latter hgands are characterized by a tendency to form complexes with softer ions, including those of the transition, lanthanide, and actinide series. This stands in contrast to what is true for oxygen-based crown ethers, for which a preference for hard cations, such as alkali metal ions, is well established. One of the earliest examples of ISEs based on a thia-crown ether was reported... 

The strength of this bonding depends on the kind of ether Simple ethers form relatively weak complexes with metal ions but Charles J Pedersen of Du Pont discovered that cer tain polyethers form much more stable complexes with metal ions than do simple ethers Pedersen prepared a series of macrocyclic polyethers cyclic compounds contain mg four or more oxygens m a ring of 12 or more atoms He called these compounds crown ethers, because their molecular models resemble crowns Systematic nomencla ture of crown ethers is somewhat cumbersome and so Pedersen devised a shorthand description whereby the word crown is preceded by the total number of atoms m the ring and is followed by the number of oxygen atoms... 

Another group of macrocyclic ligands that have been extensively studied are the cycHc polyethers, such as dibenzo-[18]-crown-6 (5), in which the donor atoms are ether oxygen functions separated by two or three carbon atoms. The name crown ethers has been proposed (2) for this class of compounds because of the resemblance of their molecular models to a crown. Sandwich stmctures are also known in which the metal atom is coordinated with the oxygen atoms of two crown molecules. 

Related to the crown ethers are compounds, such as hexamethyl-[14]-4,ll-diene (6), which differ by the replacement of one or more of the oxygen atoms by other kinds of donor atoms, particularly N or S. MacrocycHc amine and thioether compounds have been synthesized. Compounds having more than one kind of heteroatom in the ring are called mixed-donor macrocycles. The naturally occurring metaboUtes nonactin [6833-84-7] and monactin [7182-54-9] have both ether and ester groups incorporated in the macrocyclic stmcture. 

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.

Zollinger and coworkers (Nakazumi et al., 1983) therefore supposed that the diazonium ion and the crown ether are in a rapid equilibrium with two complexes as in Scheme 11-2. One of these is the charge-transfer complex (CT), whose stability is based on the interaction between the acceptor (ArNj) and donor components (Crown). The acceptor center of the diazonium ion is either the (3-nitrogen atom or the combined 7r-electron system of the aryl part and the diazonio group, while the donor centers are one or more of the ether oxygen atoms. The other partner in the equilibrium is the insertion complex (IC), as shown in structure 11.5. Scheme 11-2 is intended to leave the question open as to whether the CT and IC complexes are formed competitively or consecutively from the components. ... 

In [Tl 18-crown-6] [CIO4] (Fig. 2.1), the thallium(I) cation resides 75 pm above the plane defined by the oxygen atoms of the crown ether and 66 pm in [ri 18-crown-6] [TII4] [39] whereas, for example, in the analogous potassium compound [K 18-crown-6] [CIO4] [40] the cation is found right in the middle of the crown ether (Fig. 2.2). 

The crown ether here was named by its decoverer Pedersen51 dicyclohexyl-18-crown-6(18 = number of atoms in the heterocyclic ring, 6 = number of oxygen atoms in the ring) its membrane shows an appreciably higher K+ selectivity with respect to the other alkali metal ions. There is still much research being carried out on the synthesis and practical use of crown ethers. 

Crown ethers are named as x-crown-y where x is the total number of atoms in the ring andy is the number of oxygen atoms. 

Structural details in CpNa(15-crown-5) 8499 show the Cp and the crown ether to be arranged in an almost coplanar manner with the oxygen atoms coordinated in a slightly asymmetric manner to sodium (Na-O 2.42-2.55 A). The Cp ring displays an asymmetric metal coordination mode with two close (2.67 and 2.71 A), two medium (2.85 and 2.91 A), and one longer Na-C distance (2.99 A). The asymmetric metal coordination may be accredited to the small sodium... 

From the method of preparation of [BeCl(12-crown-4)]+ (179), it is known that the Cl ligand can be substituted by a solvent molecule. We applied our most common test solvents water and NH3 to a Be2+ cation, where most coordination sites are occupied by a chelating ligand, in this case the crown ether 12-crown-4. In contrast to the tetrahedral [Be(solvent)4]2+ solvated complexes, the precursor complexes [Be(solvent)(12-crown-4)]2+ are quadratic pyramidal, where four oxygen donor atoms of the crown ether form the quadratic basis, while Cl- or a coordinating solvent molecule occupies the apical position. Addition of one water or ammonia molecule to [Be(12-crown-4)]2+ is exothermic (see Table IX). 

---