Subject Chemistry

Paper No and Title Paper 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No and Module 11: Crown Ethers, Complexes and Title Cryptands Module Tag CHE_P1_M11

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 11: Crown Ethers, Complexes and Cryptands

TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Summary

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 11: Crown Ethers, Complexes and Cryptands

1. Learning Outcomes After studying this module, you shall be able to  Learn what are ‘crown ethers’ and ‘cryptands’.  Understand the structural details of crypts and crown ethers  Analyze the size to cavity ratio for encapsulation of appropriate metal atom in the cavities of both crown ethers and cryptands.  Learn about the synthesis, applications and uses of crown ethers and cryptands. 2. Introduction 2.1 Crypts and Crown Ethers: Introduction Crypts and crown ethers constitute an important and an interesting class of complexing ligands. Apart from metal complexes, a variety of unusual species among which 'alkalides' and 'electrides' was made possible through them and they require special mention. The crowns and crypts are enormously studied due to their increasing applications varied chemical and physical processes. Their use as biochemical models further draws greater interest towards them.

2.2 Structure of Crown ethers and Crypts: The study of crown ethers grew enormously after Pedersen’s finding of crown ethers and their abilities to bind strongly with metal ions in 1967. When the Nobel was conferred upon the three chemists Pederson, Cram and Lehn in 1987, the advances in host guest and supramolecular chemistry gained special attention. The development of this field was because of the remarkable properties of crown ethers. Crypts and crown ethers have an interesting property to form complexes even with many slowly reacting alkali metals. This property made possible things including pulling ionic substances into organic solvents, which is very difficult as such. Ions of all sizes fit into cavity sizes which are apt for them. Since the ionic radius of group 1 metals increase down the group, thus, lithium, sodium and potassium are said to be compatible with 12-, 15- and 18- membered rings respectively. Biological systems employ crowns and crypts widely, especially to understand the perplexing and very efficient selectivity between Na+ and K+ ions.

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 11: Crown Ethers, Complexes and Cryptands

Crown ethers (or crowns) are known as a group of macrocyclic polyethers. The structure has

ethereal oxygen atoms are separated by two methylene (CH2-) groups. A classical example of a crown ether is 2, 3, 11, 12-dibenzo 1, 4, 7, 10, 13, 16-hexaoxacyclooctadeca-2, single 11- diene which is designated as dibenzo-18-Crown-6 (or simply 18-C-6 or Crown-6 as handy notations) has the structure which is depicted given in Figure 1. The crown ether’s name dibenzo-18-Crown-6 shows that there are two benzene rings and 18 atoms form the crown- shaped ring. The ring consists of 6 are oxygen atoms. Thus, we can say that, when n depicts the ring size, m is the number of ethereal oxygen atoms, the crown ether can thus be abbreviated as nC-m. Crown ethers which possess 3-20 ethereal oxygen atoms are now known. Apart from the cavity which encapsulates the metal ion in it, the remaining part of the molecule I is puckered to give a crown-like arrangement and hence the name ‘crown ethers’.

Many macropolycyclic ligands which are related to each other are also known to us and are called as ‘cryptates’ (or cryptands or simply, crypts). They are more potent, stronger and selective complexing agents for alkali metals. Crypts contain nitrogen, oxygen and sometimes phosphorus and sulfur atoms in their core structure. They are for the same reason considered as the 3D equivalents of crown ethers. The molecules are cross-linked appropriately with donor atoms correctly positioned in the bridging group in order encapsulate metal ions in structures having cage-like formations.

A typical crypt is the molecule called Crypt-222 (222 denotes the no. of ethereal oxygen atoms in each N-N bridge) having the basic structure given in Figure 1. Cryptates (meaning hidden) are so called because they wrap around and hide the cation. It is because of the polyether bridges formed between the two N atoms which resemble the seams of a football – that this class of crypts is really known as 'football ligands'. Entire class of these ligands form complexes with very high formation constants.

A B CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 11: Crown Ethers, Complexes and Cryptands

Fig. 1: A: Graphic structure of 18-Crown-6, a macrocyclic polyether; the cavity [•] size depends on the size of the ring. Metal cation which is appropriately size is trapped in this cavity to forming a stable complex. B. Graphic structure depiction of a cryptand; in this structure, when oxygen atoms occupy [•] positions it is Crypt-222; Mixed donor cryptates result when S atoms or others occupy these. The ligands encapsulate metal ions in cage-like structures and form highly stable complexes.

(I) Novel Crown Ethers and Important Properties The crown ethers having other varied and uniquely interesting functions are at the crucial stages of development. An example is the compound in Figure 2 changes its colour when it captures an ion like sodium. There’s a possibility that it can be employed to make ion detectors.

Fig. 2: Coloured crown ether

Efforts in research areas have also been made to enhance the ion-capturing ability of crown ethers. The lariat crown developed by Professor Seiji Shinkai of Kyushu University in Japan has a long chain as its name suggests (Figure 3). It uses this chain to wrap around the ion, “fixing” the complex so that the ion won’t escape.

Fig. 4: Lariat crown

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 11: Crown Ethers, Complexes and Cryptands

Taking the idea a level ahead, Professor Jean Mary Lehn in France synthesized the double- cyclic crown ether. Professor Jean Mary Lehn and his team gave the molecule cryptand its name after the Greek term crypto, which means “to hide” (Figure 5). The two rings of cryptand provide extra strength to hold the ion. In case a regular crown ether “surrounds” an ion, a cryptand “locks it up”. This ion-capturing capability of a cryptand can reach upto a hundred thousand times more than that of 18-crown-6.

Fig. 5: Cryptand capturing an ion and picture of another cryptand

2.3 Synthesis of Crown Ethers and Cryptands In an attempt to prepare an alkyl phenolic ether, the first synthesis of crowns was achieved. Instead of the expected product, the contaminating catechol present in the reaction mixture, formation of a different product was observed by the chemists. The isolated product was found to be dibenzo-18-Crown-6. Numerous other crowns were subsequently prepared in Pederson's laboratory. Later on Pederson followed the synthesis and characterisation of a variety of crowns and crypts as well as their metal complexes, e.g. sepulchrates, anionic crypts and inorganic crypts.

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 11: Crown Ethers, Complexes and Cryptands

A short note on synthesis of sepulchrates, anionic crypts and inorganic crypts: Sepulchrates: They are a group of multicyclic encapsulating ligands called 'sepulchrates' which are related closely to the football ligands. These are synthesised around a metal ion which cannot be released. Condensation of formaldehyde and ammonia on to the N atoms of 3+ Co(en)3 (en = ethylene diamine) gives rise to a sephulchrate containing tris(methylene) amino caps on opposite faces of the coordination octahedron. Sepulchrates are stable over large pH ranges. This makes the investigation of solution chemistry of elements like Mo, W which are prone to hydrolysis and polymerisation by changes in pH, more simpler. Anionic Crypts: Although cationic crypts has dominated in its complexes with alkali and few other metal cations for a long time- recently. Anionic crypts viz., - [CIN(CH2CH2NHCH2CH2NHCH2CH2)3N] have also come into existence. Inorganic Crypts: Inorganic crypts may also surround a metal cation completely. They are of organic origin, just like the crowns and crypts; one such compound reported by Vogtle and Weber is

(NH4)17Na[NaW21Sb9O86].14H2O where the heteropolytungstate, an inorganic crypt, completely surrounds the Na+ ion. This compound is reported to have antiviral activity.

2.4 Application and Uses of Crown Ethers and Crypts Crowns and crypts find many important applications and uses. These include preparative organic chemistry, solvent extraction, phase transfer catalysis, stabilisation of uncommon or reactive oxidation states and the promotion of other improbable reactions. Few classical examples are listed below: i] The enhancement of solubility of alkali metal salts in organic media can be assisted using crowns and crypts. This is because they contain large hydrophobic organic ring in the ligand;

e.g. KOH and KMnO4 can be used as an alkali and an oxidising agent, respectively in organic media also. 2- 3- ii] Various zintl saIts containing alkali metal cation as well as zintl anions viz. Te3 , Sb7 , 2- 2- 2- 4- Bi4 , Sn5 , Pb5 and Sn9 have been prepared with the help of crowns and crypts in anhydrous solvent. iii] Mixed metal complexes of the alkalis can be prepared by the exploitation of selectivity and differential stabilities of alkali des and metal complexes. For example, a K-Na alloy reacts with the crown 18-C-6 to give [K(8-C-6)]+Na- as the product which contains both the alkali metals, one as cation and other as anion.

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 11: Crown Ethers, Complexes and Cryptands

2+ iv] Crown ethers can also act as second coordination sphere ligands e.g. [Pt(bipy)(NH3)2] is 2+ known to produce a crystalline compound [Pt(bipy)(NH3)(18-C-6)] with 18-C-6.

3. Summary In this module, we have taught you that:  Crypts and crown ethers constitute an important and an interesting class of complexing ligands.  When the Nobel was conferred upon the three chemists Pederson, Cram and Lehn in 1987, the advances in host guest and supramolecular chemistry gained special attention.  The crowns and crypts are enormously studied due to their increasing applications varied chemical and physical processes. Their use as biochemical models further draws greater interest towards them.  Crown ethers (or crowns) are known as a group of macrocyclic polyethers. Many macropolycyclic ligands which are related to each other are also known to us and are called as ‘cryptates’ (or cryptands or simply, crypts).  The two rings of cryptand provide extra strength to hold the ion. In case a regular crown ether “surrounds” an ion, a cryptand “locks it up”. This ion-capturing capability of a cryptand can reach upto a hundred thousand times more than that of 18-crown-6.  Crowns and crypts find many important applications and uses. These include preparative organic chemistry, solvent extraction, phase transfer catalysis, stabilisation of uncommon or reactive oxidation states and the promotion of other improbable reactions.

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 11: Crown Ethers, Complexes and Cryptands