Crown Ether Complexation
Crown Ethers are cyclic polyethers discovered by Pederson in 1967. Structures of three typical ethers are given below. The common names of these ethers include a number as a prefix to designate the total number of atoms in the ring and a number as a suffix to designate the number of oxygen atoms in the ring. Thus, 15-crown-5 is comprised of 15 atoms in the ring, 5 of which are O and 10 of which are C. Pederson shared the Nobel Prize in Chemistry in 1987 with Cram and Lehn for work in this area.
II. Host-Guest Chemistry
The characteristic chemistry of crown ethers involves complexation of the ether oxygens with various ionic species. This is termed "host-guest" chemistry, with the ether as host and the ionic species as guest. [Ask students whether the preferred guest would be cationic or anionic and why]. Crown ethers may be used as phase-transfer catalysts and as agents to promote solubility of inorganic salts in organic solutions. For example, "purple benzene" is a solution of benzene, 18-crown-6, and potassium permanganate that finds utility as an oxidizing agent. The crown ether dissolves in benzene, the potassium ion complexes with the crown ether, and the permanganate is forced to dissolve in the benzene in order to ion-pair with the potassium ion.
This type of chemistry (host-guest) is found in nature with cyclodextrins and macrocyclic polyether antibiotics, and thus, should be of interest to a variety of students.
III. Modeling and Calculation Exercises
(NOTE: These exercises were developed using CAChe, but should be adaptable for other software also.)
The purpose of this activity is to determine specificity of complexation of monovalent ions (Li+, Na+, K+) with different sizes of crown ethers.
- In the modeling program, draw structures of each of the following species: 12-crown-4, 15-crown-5, and 18-crown-6. Optimize the geometry (molecular mechanics) and note the appearance. Record the heats given as output. If desired, perform a semi-empirical calculation as well and record the energy. (ZINDO is preferred, since MOPAC is not parameterized for all the metal ions that will be used in this exercise).
- Now draw structures of each of the crown ethers complexed with Li+. In the structure, connect each oxygen with the Li+ by a weak bond. Optimize the geometries as before and note the appearances. Record the heats given as output. If desired, perform a semi-empirical calculation using ZINDO.
- Repeat step two for each crown ether complexed with Na+ and then with K+.
- Compute heats of association by subtracting the heat of the ether from the heat of the complex. The smaller the result, the more stable the complex.
Typical structures and results from MM2 are shown below.
Figure 1. Top left: 12-crown-4, DH = 25.68 kcal/mole
Top right: 12-crown-4-Li+; DH = 38.13; DHassoc = 12.45
Bottom left: 12-crown-4-Na+; DH = 46.88; DHassoc = 21.20
Bottom right: 12-crown-4-K+; DH = 47.68; DHassoc = 22.00
Figure 2. Top left: 15-crown-5, DH = 22.56 kcal/mole
Top right: 15-crown-5-Li+; DH = 44.32; DHassoc = 21.76
Bottom left: 15-crown-5-Na+; DH = 40.85; DHassoc = 18.29
Bottom right: 15-crown-4-K+; DH = 40.97; DHassoc = 18.41
Figure 3. Top left: 18-crown-6, DH = 37.36 kcal/mole
Top right: 18-crown-6-Li+; DH = 57.28; DHassoc = 19.92
Bottom left: 18-crown-6-Na+; DH = 55.44; DHassoc = 18.08
Bottom right: 18-crown-6-K+; DH = 50.69; DHassoc = 13.33 IV. Modeling and Calculation Extensions
- Explore complexation of crown ethers with small ammonium ions (quaternary as well as protonated) as the guests.
- Explore different crown ethers (e.g., dibenzo-18-crown-6 with dicyclohexyl-18-crown-6), both complexed and uncomplexed with guests.
- Substitute one or more O atoms in the ring with N atoms. These are hosts called cryptands.
- Explore acyclic polyethers and polyamines.
V. Experimental Extensions
Prepare a standard solution containing Li+, Na+, and K+. Measure the concentrations of each ion using, for example, AA or ICP. Add a known amount of the individual crown ether (e.g., 12-crown-4) and stir. Remeasure the concentrations of each ion and determine which ion is preferentially complexed with the crown ether. Repeat with the other crown ethers as well. Filter the suspension prior to analysis. Wear gloves when handling the crown ethers - they are toxic.
VI. Background References
- Pederson, C. J. J. Am. Chem. Soc. 1967, (Discovery of crown ethers)
- Cram, D. J.; Cram, Accts. Chem. Res. 1978, 11, 8. (General host-guest chemistry)
- Lehn, J. Am. Chem. Soc. 1975, 97, 6700. (Cryptands)
- Martell and Smith Critical Stability Constants, vol 5, 1st Supplement, Plenum Press, 1982, pp 368-370.