Betaines and Electrides : From Sugar Beets and Baby Shampoo to Superconductors

 

Various inorganic crystal systems designed to contain Drude electron gas in spacious vacancies in their lattice structures were explored in a series of previous web pages . Drude electron gas consists of free , unassociated electrons that can migrate through atomic vacancies and channels in the lattice under an applied potential difference . They were so-named after the German physicist , Paul Drude (1863-1906) , who first described electrons as the energy and charge carriers in the phenomenon of electrical conductivity in metallic solids , in 1900 .

Proposal : There are three fundamental types of electrical conductors , in which electrical charge and energy are transported through a material :

* Ionic conductors such as molten salts at high temperatures , fast ion conductors (“superionic conductors” like b-alumina and Cu2HgI4) , and solutions of ionic compounds in aqueous and non-aqueous solvents ;

* Electronic conductors in which the charge and energy carriers are electrons in the orbitals comprising the metallic bond–conduction band in the crystal (these are the conventional metallic solids) ; and

* Electronic conductors in which the charge and energy carriers are unassociated , free electrons in interatomic void spaces in the lattice (these electrons are the hypothesized Drude electron gas) .

The betaine–electride and quaternary nitrogen–electride compounds discussed in this web page would be members of the third type of electrical conductors containing Drude electron gas .

Such novel materials might be high temperature superconductors . They might exhibit this remarkable and highly sought-after property because of the unassociated nature of their free electrons . The orbitals are in effect acting as chemical bonds , binding the electrons tightly to their respective kernels . Even the metallic bonds in the common metallurgical metals , while permitting the conduction electrons to circulate in the crystal lattice , retain a firm grip on them . The metallic bond mobile electrons are organized by the Fermi-Dirac distribution in a vast number of energy levels . Such an ordering scatters the conduction electrons across the physical dimensions of the lattice , thus preventing them from forming the Cooper pairs required for the appearance of superconductivity .

The Drude gas electrons are by contrast perfectly free as they circulate through channels of the large void spaces provided for them . Because they aren't associated with their original parent kernels in orbitals , they are unaffected by the Fermi-Dirac distribution . Pairs of such free electrons can traverse the physical dimensions of their crystal , under an applied potential difference , without interacting in any way with the atoms surrounding the void channels . They should be able to flow in a resistance-free manner in the solid , thereby meeting the prime requirement of superconductivity .

The concept of the crystal container for the Drude electron gas was introduced : the “bottle” that will confine this new electronic genie . A variety of common , familiar crystal structures were adopted for this purpose :

* zinc blendes and wurtzites , described in the web page “A New Picture of Superconductivity : Lightning Bolt Electrons in a Crystal” ;

* ReO3 and perovskites , proposed in the web page “Perovskites Designed as Drude Metals and Ambient Superconductors” ;

* rocksalts , described in “Rocksalts Designed as Super-electrides , Drude Metals , and as Possible High Temperature Superconductors” ;

* ilmenites , in Chromium as the Guest Atom in Super-electride Drude Metals” ;

* litharge , in “Lead , Tin , and Bismuth as the Guest Atoms in Super-electride Drude Metals” ; and ,

* rutile , in “Drude Electron Materials Having Rutile and Layered Structures”.

The chemistry of the inorganic solid state materials discussed in the above web pages was based on that of the well-known electrides . A brief review of electrides , with pertinent references , was presented in the Electrons web page . The common perception is that electrides can be formed primarily from the Alkali metal elements dissolved in anhydrous liquid ammonia . The Alkaline Earth elements (except for beryllium , a tough , refractory metal) and two of the Lanthanide elements , europium and ytterbium , can also produce electrides in ammonia solution . However , as pointed out in the above web pages , many other metal elements should also be able to generate electrides if they are dissolved in a receptive host matrix .

For example , I suggested that one gram-atom equivalent of mercury metal (6s2) might dissolve in two gram formula weights of a silicon disulfide host lattice . The resulting mercury electride would have the theoretical formula Hg2+ [**] Si2S4 , and is predicted to have the zinc blende or wurtzite crystal structure [GIF image , 54 KB] . The sulfur atoms would tetrahedrally coordinate the mercury atoms , forming very strong coordinate covalent bonds to the underlying Hg2+ kernels . The 6s2 valence electrons [**] should be “popped” into the tetrahedral cation vacancies [ ] in the lattice , similar to those in the well-known zinc blende compound Cu2[ ]HgI4 , ie. Hg2+[ ]Cu2I4 .

It might thus be possible to obtain the electride even of a less-reactive element like mercury , which is a Noble Metal in redox terms , if the zerovalent atom can be separated into two parts : its electronically stable cationic kernel and its valence electron(s) . Then , both components must be accomodated in suitable sites in the host crystal in the most energy-stabilizing manner possible . In the case of Hg0 its Hg2+ kernel will be stabilized by the strong S–>Hg coordinate covalent bonds , while its 6s2 valence electrons can reside in the very large cation vacancies in the lattice .

Recently various organic molecules have been considered as possible crystal containers for the Drude electron gas . The challenge is compounded by the powerfully reducing nature of the Alkali and Alkaline Earth atoms in the conventional electrides which tend to degrade any molecular host they are combined with . Nevertheless , reasonably stable (at room temperature) electrides in organic molecular hosts have been successfully synthesized . For example , see :

M.Y. Redko , J.E. Jackson , R.H. Huang , and J.L. Dye , “Design and Synthesis of a Thermally Stable Organic Electride”, J. Amer. Chem. Soc. 127 (35) , pp. 12416-12422 (2005) [DOI] .

An intriguing new design concept , with respect to using organic molecular crystal containers for the Drude electron gas , is to combine a betaine with the electride , dissolved in liquid ammonia , to produce a stabilized organic complex with it :

In the above example , when the sodium betaine electride is recovered from the liquid ammonia solvent in a crystalline state its sodium cations will be strongly bonded to the anionic carboxylates , while its highly reactive free electrons will be electrostatically bonded to the quaternary amine's cationic nitrogen atoms :

Hopefully both components of the original sodium electride would be sufficiently stabilized in their new molecular crystal container so that it could be isolated at room temperature and maybe it would even be able to survive intact at higher temperatures .

I suspect that in actual practice the sodium betaine electride sketched above wouldn't be very stable , and when warmed it would undergo an internal electronic rearrangement . The free electron would attack the methylene group of the acetate portion of the molecule and cause the electron pair in the CN bond to move onto the trimethylammonium group , thus reducing it to trimethylamine (a gas a room temperature) . Two of the remaining sodium acetate free radicals would then dimerize to form a residue of disodium succinate . Also , the free electrons in the crystalline electride are rather exposed and would be very sensitive to impacting oxygen and water molecules . Can we design other betaine electrides using more protective and stabilizing molecular hosts ? Let's review some betaine chemistry in the search for such improved crystal containers for the electrides .

 

A Brief Survey of Betaine Chemistry

 

Betaines are a sub-class of the zwitterions , which are molecules simultaneously having both one or more anionic and cationic centers . The amino acids are probably the best-known type of zwitterion , but they are protic in nature , with mobile acidic hydrogen atoms . By comparison betaines are non-protic zwitterions with no accompanying spectator ions . Zwitterions in general (including betaines) are amphoteric , and can absorb equimolar quantities of either an acid or a base :

The compound trimethylglycine (glycine betaine , or simply “betaine”) was first discovered in , and isolated from sugar beets , from whose Latin taxonomic name (beta vulgaris) the word is derived . It's a white , crystalline solid which is very soluble in water and protic organic solvents , and it has a sweet taste (like its cousin glycine , whose name was derived from the Greek word for “sweet”) . Betaine has no discrete melting point ; it rearranges above 300 C to its corresponding methyl ester :

Very likely this thermal instability is common to all betaines , and would set an upper temperature limit to their utility as electride complexants .

The anionic group in betaines is most often carboxy , RCOO , but sulfobetaines (with the sulfonate group , RSO3 ) , are frequently encountered as surfactants (surface active agents , “detergents”) . The antiprotozoal drug miltefosine is an example of a phosphobetaine :

Trimethylglycine betaine apparently is marketed as a health food supplement for humans and as a feed supplement for farm animals ; a number of Internet suppliers offer it for these purposes . However , the principal use of betaines is probably in many different forms of surfactants . In fact I have some betaine surfactant here at home in my bathroom , in the bottle of Johnson's baby shampoo shown below :

One of the components of this shampoo is the surfactant cocamidopropyl betaine (CAPB) , which provides foaming , moisturizing , antistatic , and “silkiness feeling” properties to the shampoo . CAPB also is relatively non-irritating to the eyes , unlike the commonly encountered anionic surfactants , which are more “salt-like” than betaines .

A long time ago in my younger days I prepared a series of betaine surfactants for use in an industrial chemistry project . As I was familar with these compounds I was curious as to what CAPB was chemically and how it might be manufactured from simple precursors . The following sketch outlines what I think is the most likely route to its synthesis :

This is a fairly straightforward surfactant synthesis . The 3-dimethylaminopropylamine intermediate can be readily prepared by the hydrogenation of 3-dimethylaminopropionitrile , which in turn is synthesized in the aza-Michael addition [PDF , 234 KB] of dimethylamine to acrylonitrile :

The analogous cocamidoethyl betaine might similarly be prepared via 2-dimethylaminoethylamine , available from hydrogenated dimethylaminoacetonitrile (synthesized from the Mannich reaction of dimethylamine , formaldehyde , and hydrogen cyanide) :

My own syntheses of sulfobetaine surfactants involved the simple combination of a a series of N,N-dimethyl fatty alkyl amines (I used the Armeen DM products , for example Armeen DM 16D , as the tertiary amine substrate) with the alkylating agent 1,3-propane sultone :

The resulting waxy , tan solid was obtained in a quantitative yield . Such sulfobetaines are known for their excellent chelating ability with “lime hardness” cations such as calcium and magnesium :

I was unaware of this research [DOI] by Linfield's group at the time , by the way . Actually , the use of 1,3-propane sultone and b-propiolactone in betaine syntheses was well established by then . For example , see the following literature on the subject :

T.L. Gresham et al. , “b-Propiolactone . XI. Reactions with Ammonia and Amines”, J. Amer. Chem. Soc. 73 (7) , pp. 3168-3171 (1951) [DOI] ; F.T. Fiedorek , “Preparation of Quaternary Amines from Tertiary Amines and Beta-Lactones”, U.S. Patent 2548428 , April 10th, 1951 [PDF , 248 KB] ; and R. Ernst , “Process for Producing Acyclic Surfactant Sulfobetaines”, U.S. Patent 3280179 , October 18th, 1966 [PDF , 1116 KB] . Note : these PDF files (U.S. Patents) can be opened only with Adobe Acrobat Reader v. 6 or later . If desired , a suitable version of this application can be downloaded for free from Oldversion.com .

I was also blissfully unaware then of the high toxicity of 1,3-propane sultone [DOI] (and of b-propiolactone , which at the time was a cheap industrial chemical , but is now quite costly) :

A satisfactory alternative now exists for b-propiolactone , with respect to betaine synthesis . Two French chemists , Le Berre and Delacroix , discovered in 1973 that acrylic acid (CH2=CH–COOH , b.p. 139 C) can alkylate tertiary amines to form N-ethylenecarboxybetaines in good yields :

Propiolates can also alkylate tertiary amines to form betaines : A.W. McCulloch and A.G. McInnes , “The Reaction of Propiolic Acid Esters with Tertiary Amines , Formation of Betaines”, Can. J. Chem. 52 (21) , pp. 3569-3576 (1974) [PDF , 334 KB ; this PDF file can be opened only with Adobe Acrobat Reader v. 6 or later] .

The electrophilic vinyl sulfonic acid might be a satisfactory , less toxic alternative to 1,3-propane sultone for betaine syntheses . It's commercially available as the sodium salt in a 25 wt.% water solution . Presumably it could be “cooked” with equimolar quantities of a fatty (or other) tertiary amine and acetic acid , the latter reagent regenerating the free sulfonic acid from its sodium salt .

 

Several Proposed Betaines for Chelating Electrides

 

The design concept for electride-chelating betaines is to synthesize bis-betaine compounds which will form cyclic chelates (like EDTA does with polyvalent cations) with the metal cation component of the electride . The free electron will accompany its cation companion bound by the chelate . If the chelate ring is large enough , the free electron may actually reside inside it , electrostatically bonded to the cationic quaternary nitrogen atom . This idea is illustrated in the following sketch by the hypothetical example of TMEDA (which I pronounce as “temeda” ; it's the common abbreviation for the reagent's full name , N,N,N',N'-tetramethylethylene diamine) :

TMEDA is a well known reagent in organic synthesis ; it's a “specialty solvent” with an excellent solvating power for strong bases and is itself a useful basic catalyst , for example in the Henry nitroaldol reaction : A. Majhi , S.T. Kadam , and S.S. Kim , “TMEDA Catalyzed Henry (Nitroaldol) Reaction under Metal and Solvent-free Conditions”, Bull. Korean Chem. Soc. 30 (8) , pp. 1767-1770 (2009) [PDF , 151 KB] . TMEDA is commercially available at a moderate cost .

The proposed chelating agent , TMEDA-bis-methylenecarboxy betaine , might also be prepared in a more simple , one-pot reaction using chloroacetic acid (m.p. 63 C) as the alkylating agent . The adhering HCl could probably be expelled from the intermediate bis-quaternary chloride carboxylic acid by refluxing the reaction mixture in a suitable low polarity chlorocarbon solvent such as chloroform (b.p. 61 C) :

The expelled HCl gas , which is toxic and very corrosive , should of course be scrubbed from the effluent stream in an alkali train (wash bottles) .

A somewhat milder version of this carboxybetaine formation was described by Marumo and Saitoh . Their alkylating agent was sodium chloroacetate formed in situ in the reaction vessel : H. Marumo and M. Saitoh , “Preparation of Amphoteric Surface Active Agents”, U.S. Patent 3555079 , January 12th, 1971 [PDF , 271 KB ; this PDF file can be opened only with Adobe Acrobat Reader v. 6 or later] .

A molecular model of the macrocyclic chelate (in the above sketch , shown using Ca2+ as the electride cation ; it should strongly bond to the carboxy groups) presents a more realistic picture of the complex , with the pair of free electrons located in the side of the ring halfway between the two quaternary nitrogen cations , where the positive coulombic attraction from both quaternized nitrogen atoms is the strongest :

The somewhat floppy ring folds in its mid-section to form two approximately hexagonal sections , the cationic one with the electron pair and the anionic one with the carboxy-calcium complex .

The electron pair in the rather flat TMEDA betaine macrocycle would still be vulnerable to attack by water and oxygen molecules , thereby reducing its chemical stability at higher temperatures . If the free electrons could be “hidden” inside the central cavity of a three dimensional polycyclic molecular container , the resulting betaine electride should be more stable , at least at room temperature and maybe even warmer than that . The two bicyclic tertiary amines quinuclidine (with one nitrogen) and DABCO (1,4-diazabicyclo[2.2.2]octane or triethylene diamine , with two nitrogens) offer such internal cavities in which the electride free electron(s) might reside . Various proposed betaine derivatives of quinuclidine and DABCO are shown in the following sketch :

The syntheses of a number of these betaine derivatives of quinuclidine and DABCO were described by Brack over forty years ago :

K. Brack , “Lactone and Sultone Adducts of Bicyclic Tertiary Amines”, U.S. Patent 3796714 , March 12th, 1974 [PDF , 339 KB ; this PDF file can be opened only with Adobe Acrobat Reader v. 6 or later] .

Alkylation of quinuclidine and DABCO by methyl chloroacetate , chloroacetic acid , or sodium chloroacetate could yield their mono- and bis-methylenecarboxy betaine derivatives , as mentioned above . Quinuclidine is a rather costly organic reagent , while DABCO is an industrial bulk chemical used primarily as a strongly basic catalyst in the manufacture of polyurethane polymers for coatings (paints and varnishes) and foams . DABCO has an additional advantage over quinuclidine in that it might be able to contain two free electrons from an electride , while quinuclidine could retain only one .

 

Quaternary Nitrogen Organic Electrides

 

When considering the betaineelectride compounds outlined above the question naturally arises : can we dispense with the metal cation and its accompanying anionic chelating group , since these are just electronically inert spectators ? That would leave the large quaternary nitrogen organic cation container and the singlet electron (or bis-cation and free electron pair) sheltered inside it . These quaternary nitrogen organic electrides might be synthesized by combining together a quaternary nitrogen salt with an Alkali or Alkaline Earth electride , as in the following metathesis reaction :

(CH3)4N+ Cl + Na+ / e -------- [liquid ammonia in Dewar] ---------> (CH3)4N+ / e + NaCl (ppt)

A brief digression would be in order at this point . Ammonium cations with associated free electrons “free radical ammonium cations”, NH4+ / e (and even hydrazinium cations) can seemingly form amalgams in mercury metal . Ammonium amalgam having the approximate formula NH4Hg was first reported in 1808 , and because of its unusual soft , “mushy” consistency was called “butter of ammonium” back then . Tetra-alkyl ammonium amalgams have also been prepared and studied , eg. by J.D. Littlehailes and B.J. Woodhall , “Quaternary Ammonium Amalgams”, Discuss. Faraday Soc. 45 , pp. 187-192 (1968) [DOI] :

Electrolysis of tetramethylammonium borofluoride in a dry polar aprotic solvent produced at a mercury cathode a grey solid amalgam which floated on the surface of the mercury and which could be isolated and purified . The amalgam was fairly stable in the absence of air and water . It was shown by X-ray powder photographs to be crystalline in nature , being similar to powder photographs of solid crystalline mercury but with a distorted lattice , and to the naked eye appeared in the form of many faceted dendrites” (p. 189) .

Although at first glance it might seem that this compound described by Littlehailes and Woodhall is the sought-after tetramethylammonium electride dissolved in a mercury host , it's more likely to be tetramethylammonium mercuride , (CH3)4N+ Hg ; similarly , “butter of ammonium” is probably ammonium mercuride , NH4+Hg . The difference between the electride and the mercuride is that in the former compounds the electron formally associated with the cation is free and unassociated (with any atomic orbitals) , while in the mercurides the cation's “free” electron has been transferred to the mercury atom's 6px,y,z frontier orbitals , making it an anion . Professor J.L. Dye and his co-workers have identified and studied the alkalides (Alkali metal anions) , which are electronically somewhat analogous to the mercurides :

Two of Professor Dye's reviews of the alkalides : J.L. Dye and M.G. deBacker , Physical and Chemical Properties of Alkalides and Electrides, Ann. Rev. Phys. Chem. 38 (1) , pp. 271-299 (1987) [DOI] ; M.J. Wagner and J.L. Dye , Alkalides, Electrides, and Expanded Metals, Ann. Rev. Mater. Sci. 23 , pp. 223-253 (1993) [DOI] .

The intermetallic compound sodium thallide , NaTl (a Zintl compound , or phase) has a well-defined cubic crystal structure similar to that of sodium chloride :

Undoubtedly sodium amalgams are electronically similar to NaTl , having analogous mercuride anions . In NaTl the thallide anions (6s2 6p2 electronic configuration) have a tetrahedral sp3 hybridization and have polymerized into a three-dimensional diamond-structure lattice with covalent TlTl bonds , throughout which the electronically inert sodium cations are periodically nested . However , none of the mercuride compounds such as “butter of ammonium” or even Littlehailes and Woodhall's tetramethylammonium mercuride have been as well characterized as sodium thallide .

Mercury metal is clearly too reactive to act as a solvent for any sort of electride , being readily reduced to mercuride anions by the free electrons . Anhydrous liquid ammonia , if free from contaminants like rust from its steel tank container (which catalyzes the conversion of sodium electride , for example , to sodamide and hydrogen gas) , is a stable , inert solvent for Alkali and Alkaline Earth electrides . The nitrogen atoms' lone pairs of electrons form coordinate covalent bonds to the electrophilic sodium cations , while the positive dipoles on ammonia's hydrogen atoms are thought to form van der Waals dipolar bonds to the free electrons , thereby stabilizing them energetically to a certain extent . Electrostatic free electrons probably also form van der Waals dipolar bonds to the hydrogen atoms on water molecules in clouds , resulting in a substantial build-up of electric charge in those clouds prior to its spectacular discharge to the ground as lightning . Storm clouds would thus be Nature's electrides !

Ammonia electrides are unstable at higher temperatures and can be reduced by the free electrons to amide anions (NH2 ) and hydrogen gas , particularly in the presence of suitable catalysts . The hypothetical (CH3)4N+ / e electride mentioned above probably wouldn't be very stable at ambient temperatures . The chemical stability of the hypothetical quaternary nitrogen electrides might be considerably enhanced by enclosing the free electron (or electrons) inside cavities within the organic molecule container . Such a sequestering of the powerfully reducing electrons should help to protect them from common reactive molecules in the environment such as water and oxygen molecules . We should have no illusions about their thermal stability , though . The free electrons undoubtedly will reduce CN and CC covalent bonds via free radical pathways at elevated temperatures . Hopefully the quaternary nitrogen electrides proposed below would be thermally stable at room temperature , making them readily accessible for study and possible practical development .

Several hypothetical quaternary nitrogen electride systems are presented in the following sketch . They include the familiar TMEDA and quinuclidine , now quaternized with N-methyl groups :

The flexible molecular substrates might “curl up” around the free electrons in an electrostatic embrace . Hopefully the free electron associated with the quinuclidinium cation would be able to nest inside the bicyclic structure as shown in the sketch , rather than outside it , underneath the pyramidal space formed in part by the angular N-methyl group .

Conversion of the quaternized salt to its sulfate form is shown in the sketch , since the calcium sulfate by-product (using calciumammonia electride) would probably be as insoluble in liquid ammonia as it is in water . However , as no metal cations would be present in the quaternary nitrogen electride product it would make good economic sense to use as cheap and atom-efficient a metal source as possible , providing its salt by-product is insoluble in liquid ammonia . Magnesium is a common , inexpensive , and very reactive Alkaline Earth metal . Presumably (with no data at hand to the contrary) magnesium chloride is sufficiently insoluble in liquid ammonia . Methyl chloride (chloromethane , b.p. 24 C) in a suitable solvent (ethyl ether) could then be used as the methylating agent , being “cooked” with the tertiary amine substrate in a pressure vessel such as an autoclave to produce the quaternary nitrogen chloride [the Menshutkin reaction (alkylation of tertiary amines , 1890) named after its inventor , the Russian chemist N.A. Menshutkin (18421907)] .

It would very convenient if the N,N-dimethyl amine sulfate salt could be obtained directly by combining together one equivalent of the tertiary amine with one equivalent of dimethyl sulfate (b.p. 188 C ; caution : a very hazardous , highly toxic reagent) , in an inert , high boiling diluent such as p-xylene (b.p. 138 C) in an autoclave . I'm not sure that both methyl groups of the dimethyl sulfate are available for the N-methylation , though . When combined with one equivalent of magnesium electride in liquid ammonia the by-product magnesium sulfate would likely be quite insoluble in the ammonia , and could be filtered from the solution of the organic electride .

DABCO might similarly be bis-N-methylated , then reacted with one equivalent of magnesium electride or two equivalents of sodium electride in liquid ammonia to produce the bis-quaternary ammonium electride containing two free electrons per DABCO molecule :

The large violet sphere in the center of the molecule is the hypothetical pair of Drude free electrons . This position in the center of the DABCO molecule , exactly halfway between the two cationic nitrogen atoms , is the optimum location for both electride electrons , as they will experience the greatest coulombic force from both positively charged nitrogens simultaneously .

I made a screensaver featuring this beautiful – and strange – molecule for both my desktop and laptop computers . Then , using a screen capture program I created an MP4 video (playable in all modern media players) of the screensaver image in motion . If interested , you can download this video , "dabconium-bipolaron-electride" [MP4 , 7427 KB / 7.25 MB] from Chemexplore .

This last compound could be referred to as “N,N'-dimethyldabconium 2e electride”. The two free electrons in the sketch above are shown associated together as a Cooper pair . However , they could also be two singlet electrons with parallel spins , located outside the DABCO molecule , each one underneath the pyramidal space formed by the angular N-methyl group . The two physical tests of electron spin resonance (ESR , or electron paramagnetic resonance , EPR , PDF , 1044 KB) and magnetic susceptibility (magnetochemistry) could help to resolve this question . In the latter test if the free electrons are singlets the electride should have a magnetic susceptibility of around 2.53 BM (Bohr magetons) , from the equation

meff = [n (n+2)]1/2 [PDF , 23 KB] ; if n = 2 (high spin electrons) , meff = 2.83 BM .

If the two electrons are spin-paired , the magnetic susceptibility measurement would show the electride is diamagnetic (meff would have some negative value) .

If the electride is an electrical conductor , a measurement of its Hall effect [PDF , 163 KB] should reveal whether singlets or pairs of electrons are the charge carriers in the conduction .

The dabconium electride shown above could provide researchers with a moderately stable chemical compound containing unassociated free electrons . We can confidently state they are indeed unassociated because the nitrogen and carbon atoms in the DABCO molecule have completed octets in their valence shells . They are fully saturated with valence electrons and can't accept any more from outside sources . As 2s,p elements their empty 3s,p frontier orbitals are at a much higher energy level , inaccesible to added electrons (GIF image , 16 KB) . This is probably true also for the various CC and CN antibonding molecular orbitals (ABMOs) . When the dabconium electride is heated the two free electrons probably would be promoted , energy-wise , up into those ABMOs , which would result in the fissioning of some of the CC and CN covalent bonds . The resulting free radical components would then recombine into various stable , lower energy decomposition products . Caution : if the quaternary ammonium electrides are heated strongly or are melted , such C–C and C–N bond fissioning could occur in a few milliseconds , resulting in an explosive decomposition of the material .

Another question concerning the free electrons is : would they be too strongly bound electrostatically by the cationic nitrogens to be pulled off them by an applied voltage across the ends of a dabconium electride crystal ? Perhaps , like the fast-ion conductors , eg. b-alumina (with sodium cations mobile in channels in the aluminum oxide lattice) and Cu2[ ]HgI4 (with Cu(I) cations mobile in the cation vacancies [ ] ) , the electride would become an electrical conductor only when heated , assuming it's thermally stable at higher temperatures .

Update added on March 30th, 2016 : I now strongly suspect that the pairs of Drude free electrons in all of these quaternary ammonium organic electrides will likely be bipolarons (electrically non-conducting) and not Cooper pairs (superconducting) . See the review article by :

J.L. Brdas and G.B. Street , “Polarons , Bipolarons , and Solitons in Conducting Polymers”, Accts. Chem. Res. 18 (10) , pp. 309-315 (1985) [DOI] [PDF , 1798 KB] .

Bipolarons are pairs of electrons associated together as a composite particle , like Cooper pairs , but are bonded to the lattice they are resident in by dipolar or coulombic bonds :

“A bipolaron is defined as a pair of like charges ......... associated with a strong local lattice distortion . The bipolaron can be thought of as analogous to the Cooper pair in the BCS theory of superconductivity , which consists of two electrons coupled together through a lattice vibration , ie. a phonon . The formation of a bipolaron implies that the energy gained by the interaction with the lattice is larger than the Coulomb repulsion between the two charges of same sign confined in the same location” (p. 311) .

The formation of pairs of electrons having antiparallel spins can be readily understood by comparing the ratio of the magnetic attractive force (north poles to south poles orientation) to the electric repulsive force between the two electrons . This Fm/Fe ratio can be calculated using simple algebra as approximately 1200 [GIF image , 34 KB] , or maybe even 2400 [GIF image , 12 KB] . So there should be no problem in experimentally synthesizing the proposed bis-quaternary ammonium electrides with pairs of Drude free electrons suspended between the two positive nitrogen atoms .

The electron pairs , however , will be acting as a dianion , for example like the sulfate anion , and the bis-quaternary ammonium electrides will behave essentially like a typical ionic salt . In simple terms the electron pairs are being very strongly coulombically bonded to their molecular containers , like true anions are ionically bonded to the cations in common salts . The electrides will be electrically non-conductive unless melted ; but melting , or even heating them isn't recommended as they would probably explode . The bis-quaternary ammonium bipolaron electrides would undoubtedly be a fascinating new type of chemical compound to synthesize and study , but they almost certainly wouldn't even be electrically conducting , let alone superconducting .

A polymeric version of the organic electride , polyethylene dabconium electride , might be prepared by the reaction of equimolar quantities of DABCO and 1,2-dibromoethane (ethylene dibromide) to first form the polyquat dibromide salt . The polar , aprotic acetonitrile , b.p. 82 C , is recommended as the reaction solvent to assist the Menshutkin quaternization of the DABCO . The polyquat would be a condensation type of polymer and probably wouldn't have a very high molecular weight .

In the second step , the crystalline polyquat would be dissolved (or suspended) in anhydrous liquid ammonia and added to two equivalents of sodium electride in liquid ammonia contained in a Dewar flask (or maybe in a conventional round-bottom flask cooled in a refrigerating bath of dry ice/acetone) . The polymeric electride should be “salted out” of solution by the by-product sodium bromide which is very soluble in liquid NH3 remaining dissolved in the ammonia :

This hypothetical polyethylene dabconium electride might be more convenient to use in “real-life applications” than the molecular N,N'-dimethyldabconium electride discussed above .

The bis amine oxide of DABCO , dabconium-N,N'-dioxide , might be prepared by reacting the tert-amine with various peroxide reagents . The “combo reagent” H2O2 CH3ReO3 in particular could be quite effective for this task : R. Huang and J.H. Espenson , “A Convenient Preparation of Sulfines (R2C=S=O) from Thioketones”, J. Org Chem. 64 (18) , pp. 6935-6936 (1999) [DOI] . The dabconium-N,N'-dioxide intermediate would then be dissolved in a suitable solvent (perhaps anhydrous THF) and added to a Dewar flask containg two equivalents of sodium electride dissolved in liquid ammonia . The sodium cations would bond to the amine oxides , while the two electrons would enter the molecule center , coulombically attracted to both positively charged ammonium nitrogens simultaneously :

Analogous sulfonium cations might also be used to contain free electron pairs , such as in the compound triethylene bis-sulfonium electride , whose imagined synthesis is sketched below :

The tricyclic cage compound methenamine (also known as hexamethylenetetramine , hexamine , and urotropine the last one a pharmaceutical compound prescribed as a urinary disinfectant) has been mono-N-alkylated by b-propiolactone to produce a betaine :

Methenamine is readily prepared by the reaction of ammonia and formaldehyde . By analogy , if methylamine hydrochloride is combined with formaldehyde the quaternary nitrogen compound tetra-N-methylmetheniminium tetrachloride might be produced . This salt could be dissolved in a suitable polar , nonaqueous solvent given the strong reducing properties of the electrides such as acetonitrile [based on the attempted preparation of the tetramethylammonium free radical in mercury by Littlehailes and Woodhall – see the discussion above] and electrolyzed . The cationic tetra-N-methylmetheniminium molecules will migrate to the cathode and hopefully be reduced on its surface to the corresponding bis-electride . The chloride anions will drift to the anode , where they will presumably be oxidized to chlorine atoms , which may combine with the acetonitrile solvent :

A molecular model of this latter bis-electride is presented in the following sketch . Both free electron pairs have been placed inside the molecule , but there actually might not be enough space in its trigonal pyramidal interior volume for both of them :

In conclusion , if ........

* these organic electrides can be successfully prepared and are stable at room temperature ; and ,

* their free electrons are associated together as Drude electron pairs ; and ,

* those electron pairs are mobile through the crystal lattice under an applied voltage ,

the proposed compounds would have a good chance of being high temperature superconductors . In any case they would be a remarkable new class of electronic materials for researchers to synthesize and study .

 

Related web pages in this series about Drude electron materials :

A New Picture of Superconductivity : Lightning Bolt Electrons in a Crystal” ;

Perovskites Designed as Drude Metals and Ambient Superconductors” ;

Rocksalts Designed as Super-electrides , Drude Metals , and as Possible High Temperature Superconductors” ;

Chromium as the Guest Atom in Super-electride Drude Metals” ;

Lead , Tin , and Bismuth as the Guest Atoms in Super-electride Drude Metals” ;

* Drude Electron Materials Having Rutile and Layered Structures” ; and ,

* Electride Chemistry and Unassociated , Free Electrons : A Possible New Approach to High Temperature Superconductivity (PDF , 674 KB)

 

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