Electrochemical reduction of a cryptate yielded a neutral

species with the properties of an expanded atom

Maitra, Uday

English

Indian Academy of Sciences

FEATURE J ARTICLE 
Molecule of the Month 
Cryptatium, the First "Elementoid" 
Uday Maitra 
Elec t rochemica l  r educ t ion  ofa  cryptate y ie lded  a neutra l  
spec ies  with the proper t ies  of an expanded atom. 
Most of the 92 elements, excluding the transuranium elements, 
were discovered by the turn of the nineteenth century. The early 
part of the twentieth century witnessed the discovery of the 
transuranium elements, but most of them were largely of aca- 
demic interest because of their high radioactivity, and relatively 
short half-life. Thus, the search for the heaviest element (or 
metal?) became rather difficult to pursue. 
As a matter unrelated to the discovery of elements, towards the 
end of the sixties, a rather novel metal ion complexing agent was 
serendipitously discovered by Charles Pedersen. He found that 
cyclic polyethers were able to complex alkali metal ions, by 
providing a cavity in which the charged alkali metal cation can 
sit comfortably in a sea of electrons donated by the oxygen 
atoms! In a sense this is similar to the hydration of a metal ion in 
water, except that the stabilization is done by oxygen atoms made 
available in a cyclic molecule. These crown ethers show high 
selectivity in complexing alkali metals, since the size of the 
cavity (which is determined by how big the ring is, it can be 12, 
15, 18, 21.. membered) can be made just optimum to match the 
Uday Maitra is at the 
Department of Organic 
Chemistry, Indian 
Institute of Science, 
Bangalore 560 012, India 
RESONANCE J November 1996 
FEATURE J ARTICLE 
Cryptates can be 
regarded as 
expanded atoms, 
where the positive 
charge of the metal 
ion has spread 
over many atoms. 
ionic radius of the alkali metal ion. Thus, 12-crown-4, 15-crown- 
5 and 18-crown-6 have been found to be selective for binding 
Li +, Na + and K +, respectively. The ability of the crown ethers to 
complex alkali metal ions selectively has led to many applications 
- one of the most important being the solubilization of inorganic 
salts in organic solvents in the presence of crown ethers. One can, 
for example, dissolve KMnO 4 in benzene in the presence of a 
crown ether to give a purple solution! 
Just when Pedersen was investigating crown ethers, Jean-Marie 
Lehn was busy creating a three dimensional cage for metal ions. 
His idea was to construct a cryptand (Latin crypta means cavity; 
and Greek kryptos means hidden) for efficient complexation of 
alkali metal ions. If a crown ether could be considered as a disk 
like object, the cryptand is a spherical object, which should be 
capable of capturing a metal ion more effectively. Indeed, Lehn 
had found that cryptands formed cryptates, alkali metal ion 
complexes of exceptional stability. 
Cryptates can be regarded as expanded atoms, where the postive 
charge of the metal ion has spread over many atoms. One 
interesting property of such metal complexes is that they interact 
very weakly with anions or with solvent molecules, since the 
metal ion is well stabilized inside the cage having a number of 
oxygen and nitrogen atoms which can donate electron pairs to it! 
As a result, such (almost) spherical metal complexes can be 
84 RESONANCE J November 1996 
FEATURE J ARTICLE 
regarded as super-heavy alkali-metal ions (compare the ionic 
radius of Cs + (1.65A) with that of the cryptate shown in the 
previous page which is about 5A)! If that is so, then it is 
interesting to think what would happen if an electron is added to 
a cryptate. Where will it go? Such systems have actually been 
made by Lehn, and are considered to be salt-like species with the 
electron as the anion (an electride). Such a species can also be 
thought of as a large alkali metal with a very small ionization 
potential (more like the Rydberg state of an atom). We therefore 
see two extremes: a true metal atom where the valence electron is 
in a metal orbital, or an electride (electron added to a cryptate) 
where the electron is at a very large distance from the metal 
cation. Lehn has considered another possibility in between, in 
which the valence electron will be associated with a ligand 
orbital. The cryptand shown above does not have a vacant low 
energy orbital for adding an electron, but one can certainly 
design another cryptand capable of accommodating an electron 
on its surface.. 
The addition of an 
electron to a 
cryptate can 
produce a species 
behaving like the 
Rydberg state of 
an atom. 
The cryptand shown below essentially has three bipyridine (bpy) 
units arranged as the three blades of a propeller, with the two 
ends of each of the three units being linked to a pair of nitrogen 
atoms. Bipyridyl units have been used extensively for the 
complexation of metal ions (many of you have probably seen the 
blood-red color of the bpy complex of ferrous ion, Fe(bpy)3Z+). 
RESONANCE I November 1996 85 
FEATURE I ARTICLE 
The cryptatium 
represents a 
neutral species 
somewhere in 
between an atom, 
and an electride. It 
can also be 
regarded as a 
molecular element. 
The bipyridine unit also has a relatively low-energy vacant 
orbital in which an electron can be added. How does one add an 
electron to a cation? This is nothing but a reduction process. 
Normal reducing agents can often create problems, because of 
the byproducts formed from them. A method which is commonly 
used for the generation of reactive metals is electrochemical 
reduction (readers may recall that reactive metals such as lqa, K, 
Ca etc. are always manufactured this way). Indeed, this was the 
method employed by Lehn and coworkers for adding an electron 
to cryptate I. Upon electrochemical reduction of I, they observ- 
ed the formation of air-sensitive blue-violet crystals on the 
cathode. Using special techniques necessary for the handling of 
air-sensitive solids, they were able to get an X-ray crystal structure 
of this blue-violet crystal. Unlike the crystal structure of a 
related cryptate bromide salt, the crystals of this blue-violet so- 
lid did not show any anion in between the cryptate units - 
suggesting that this species is electroneutral. This would be 
possible if the negative charge, i.e., the electron is residing in the 
molecule itself, and naturally the most likely site for it is the 
ligand orbital. Support for this hypothesis comes from the 
observation that in the crystal structure, the Ha + ion is closer to 
one bipyridine unit. This is because the bipyridine unit  with the 
extra electron draws the counter ion closer to it. Lehn and 
coworkers termed this compound sodio-eryptatium, and have 
suggested that this species may be described as an expanded 
atom, or as a radical contact ion pair. The cryptatium therefore 
represents a neutral species somewhere in between an atom, and 
an electride. It can also be regarded as a molecular element. Lehn 
has proposed that it might also be possible to extend the list of 
such elements by incorporating a doubly or a triply charged 
cation in the cryptand, and reducing the cryptate by adding two 
or three electrons, respectively. 
It  is needless to say that such molecules are likely to be of great 
interest for studying their magnetic,  electrical and other 
properties. Detailed studies should help us understand how 
U RESONANCE J November 1996 
FEATURE J ARTICLE 
exactly the electron gets delocalized over the three bpy units. 
Lehn has also envisaged a fulleriurn species, which can be 
considered to be a fullerene, such as C60, containing a metal ion 
M "+ in its internal cavity, and n electrons delocalized on the 
carbon framework! 
We can perhaps conclude that the search for the heaviest 
(molecular) element is still on! 
Address for Correspondence 
Uday Maitra 
Department of Organic 
Chemistry 
Indian Institute of Science 
Bangalere 560 012, India 
email: 
maitra@orgchem.iisc.ernet.in 
;ax: (080) 334 1683/309 269q 
RESONANCE J November 1996 87 


Molecule of the month: Cryptatium, the first "Elementoid"

Publications of the IAS Fellows

FEATURE J ARTICLE 
Molecule of the Month 
Cryptatium, the First "Elementoid" 
Uday Maitra 
Electrochemical  reduct ion ofa cryptate yielded aneutral 
species with the properties of an expanded atom. 
Most of the 92 elements, excluding the transuranium elements, 
were discovered by the turn of the nineteenth century. The early 
part of the twentieth century witnessed the discovery of the 
transuranium elements, but most of them were largely of aca- 
demic interest because of their high radioactivity, and relatively 
short half-life. Thus, the search for the heaviest element (or 
metal?) became rather difficult to pursue. 
As a matter unrelated to the discovery of elements, towards the 
end of the sixties, a rather novel metal ion complexing agent was 
serendipitously discovered by Charles Pedersen. He found that 
cyclic polyethers were able to complex alkali metal ions, by 
providing a cavity in which the charged alkali metal cation can 
sit comfortably in a sea of electrons donated by the oxygen 
atoms! In a sense this is similar to the hydration of a metal ion in 
water, except that the stabilization isdone by oxygen atoms made 
available in a cyclic molecule. These crown ethers show high 
selectivity in complexing alkali metals, since the size of the 
cavity (which is determined by how big the ring is, it can be 12, 
15, 18, 21.. membered) can be made just optimum to match the 
Uday Maitra is at the 
Department of Organic 
Chemistry, Indian 
Institute of Science, 
Bangalore 560 012, India 
RESONANCE J November 1996 
FEATURE J ARTICLE 
Cryptates can be 
regarded as 
expanded atoms, 
where the positive 
charge of the metal 
ion has spread 
over many atoms. 
ionic radius of the alkali metal ion. Thus, 12-crown-4, 15-crown- 
5 and 18-crown-6 have been found to be selective for binding 
Li +, Na + and K +, respectively. The ability of the crown ethers to 
complex alkali metal ions selectively has led to many applications 
- one of the most important being the solubilization of inorganic 
salts in organic solvents in the presence of crown ethers. One can, 
for example, dissolve KMnO 4 in benzene in the presence of a 
crown ether to give a purple solution! 
Just when Pedersen was investigating crown ethers, Jean-Marie 
Lehn was busy creating a three dimensional cage for metal ions. 
His idea was to construct a cryptand (Latin crypta means cavity; 
and Greek kryptos means hidden) for efficient complexation of
alkali metal ions. If a crown ether could be considered as a disk 
like object, the cryptand is a spherical object, which should be 
capable of capturing a metal ion more effectively. Indeed, Lehn 
had found that cryptands formed cryptates, alkali metal ion 
complexes of exceptional stability. 
Cryptates can be regarded as expanded atoms, where the postive 
charge of the metal ion has spread over many atoms. One 
interesting property of such metal complexes is that they interact 
very weakly with anions or with solvent molecules, since the 
metal ion is well stabilized inside the cage having a number of 
oxygen and nitrogen atoms which can donate lectron pairs to it! 
As a result, such (almost) spherical metal complexes can be 
84 RESONANCE J November 1996 
FEATURE J ARTICLE 
regarded as super-heavy alkali-metal ions (compare the ionic 
radius of Cs + (1.65A) with that of the cryptate shown in the 
previous page which is about 5A)! If that is so, then it is 
interesting tothink what would happen ifan electron is added to 
a cryptate. Where will it go? Such systems have actually been 
made by Lehn, and are considered to be salt-like species with the 
electron as the anion (an electride). Such a species can also be 
thought of as a large alkali metal with a very small ionization 
potential (more like the Rydberg state of an atom). We therefore 
see two extremes: a true metal atom where the valence lectron is
in a metal orbital, or an electride (electron added to a cryptate) 
where the electron is at a very large distance from the metal 
cation. Lehn has considered another possibility in between, in 
which the valence electron will be associated with a ligand 
orbital. The cryptand shown above does not have a vacant low 
energy orbital for adding an electron, but one can certainly 
design another cryptand capable of accommodating an electron 
on its surface.. 
The addition of an 
electron to a 
cryptate can 
produce a species 
behaving like the 
Rydberg state of 
an atom. 
The cryptand shown below essentially has three bipyridine (bpy) 
units arranged as the three blades of a propeller, with the two 
ends of each of the three units being linked to a pair of nitrogen 
atoms. Bipyridyl units have been used extensively for the 
complexation fmetal ions (many of you have probably seen the 
blood-red color of the bpy complex of ferrous ion, Fe(bpy)3Z+). 
RESONANCE I November 1996 85 
FEATURE I ARTICLE 
The cryptatium 
represents a 
neutral species 
somewhere in 
between an atom, 
and an electride. It 
can also be 
regarded as a 
molecular element. 
The bipyridine unit also has a relatively low-energy vacant 
orbital in which an electron can be added. How does one add an 
electron to a cation? This is nothing but a reduction process. 
Normal reducing agents can often create problems, because of 
the byproducts formed from them. A method which is commonly 
used for the generation of reactive metals is electrochemical 
reduction (readers may recall that reactive metals uch as lqa, K, 
Ca etc. are always manufactured this way). Indeed, this was the 
method employed by Lehn and coworkers for adding an electron 
to cryptate I. Upon electrochemical reduction of I, they observ- 
ed the formation of air-sensitive blue-violet crystals on the 
cathode. Using special techniques necessary for the handling of 
air-sensitive solids, they were able to get an X-ray crystal structure 
of this blue-violet crystal. Unlike the crystal structure of a 
related cryptate bromide salt, the crystals of this blue-violet so- 
lid did not show any anion in between the cryptate units - 
suggesting that this species is electroneutral. This would be 
possible if the negative charge, i.e., the electron is residing in the 
molecule itself, and naturally the most likely site for it is the 
ligand orbital. Support for this hypothesis comes from the 
observation that in the crystal structure, the Ha + ion is closer to 
one bipyridine unit. This is because the bipyridine unit with the 
extra electron draws the counter ion closer to it. Lehn and 
coworkers termed this compound sodio-eryptatium, and have 
suggested that this species may be described as an expanded 
atom, or as a radical contact ion pair. The cryptatium therefore 
represents a neutral species omewhere in between an atom, and 
an electride. It can also be regarded as a molecular element. Lehn 
has proposed that it might also be possible to extend the list of 
such elements by incorporating a doubly or a triply charged 
cation in the cryptand, and reducing the cryptate by adding two 
or three electrons, respectively. 
It is needless to say that such molecules are likely to be of great 
interest for studying their magnetic, electrical and other 
properties. Detailed studies should help us understand how 
U RESONANCE J November 1996 
FEATURE J ARTICLE 
exactly the electron gets delocalized over the three bpy units. 
Lehn has also envisaged a fulleriurn species, which can be 
considered to be a fullerene, such as C60, containing a metal ion 
M "+ in its internal cavity, and n electrons delocalized on the 
carbon framework! 
We can perhaps conclude that the search for the heaviest 
(molecular) element is still on! 
Address for Correspondence 
Uday Maitra 
Department of Organic 
Chemistry 
Indian Institute of Science 
Bangalere 560 012, India 
email: 
maitra@orgchem.iisc.ernet.in 
;ax: (080) 334 1683/309 269q 
RESONANCE J November 1996 87 


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Molecule of the month: Cryptatium, the first "Elementoid"

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