A Study of Wurster Blue Radical Cation Compounds


My interest in Wurster Blue compounds was renewed recently , as I was writing another Chemexplore web page about indigo chemistry . I've alluded to them in several other web pages ; their extensive singlet electron delocalization and deep blue colors continue to fascinate me . In the indigo study I wondered what the effect would be of converting a compound that was already dark blue into a Wurster compound . Would it appear black and become appreciably electrically conductive ? That caused me to think about other blue-colored compounds , and how they might be modified into Wurster Blues .

Wurster Blue compounds are aromatic amines whose nitrogen atom lone pair has been oxidized by a one-electron oxidizer , making it an aminium radical cation :

The first Wurster Blue compound was that of N,N,N',N'-tetramethyl-p-phenylene diamine (sketch) , which was partially oxidized by bromine ; it was described by the German chemist Casimir Wurster (1854-1913) in 1879 [the references are presented at the end of the text , below] . Michaelis and co-workers (1939) studied a wide variety of Wurster Blue compounds related to this original one , with substitutions on both the nitrogen atoms and on the aromatic ring . They found that all of the Wurster compounds were unstable to a greater or lesser degree ; the more stable ones , like that of N,N,N',N'-tetramethyl-p-phenylene diamine , were protected against decomposition or solvolysis by the N-methyl groups , and by resonance stabilization with the aromatic ring and by supporting substitution groups .

The above Wurster Blue perchlorate compound is a good example of a molecule with an internal mixed-valent electronic state ; its two nitrogen atoms have four and five valence shell electrons , respectively . In effect , the extra fifth valence electron is very rapidly resonating between the two nitrogen kernels through the pi electron cloud surrounding the skeleton (sigma covalent bonds) of the molecule . The presence of singlet electrons in molecules , and in particular those in which it resonates throughout an extended pi cloud , causes them to strongly absorb incident light and appear intensely colored , usually dark blue .

The compound tris[tris(4-bromophenyl)aminium hexachloroantimonate] is another example of a stable radical cation molecule :

It has a deep blue color , and is a mildly oxidizing material . The singlet electron may be resonating throughout the molecule between the nitrogen and bromine atoms ; transient Br radical cations could be involved in the resonance . Tris can be prepared in a quantitative yield by the simple addition of antimony pentachloride (in a methylene chloride solution) to the tris(4-bromophenyl)amine substrate . Addition of ethyl ether to the mixture causes tris to precipitate as “fine , blue needle-shaped crystals” (Bell and co-workers , 1969) :

tris-amine + 1 SbCl5 /CH2Cl2 ------ (2) ethyl ether -------> tris-aminium+ SbCl6(c) + SbCl3 .

Bell and co-workers found that the hexachloantimonate salt of tris was more stable than that of the corresponding perchlorate compound , possibly because of the residual oxidizing power (~ 0.8 V) of the accompanying SbCl6 anion . This simple preparation method (addition of SbCl5 /CH2Cl2) will be recommended in the following proposed syntheses of novel Wurster Blue radical cation compounds from a variety of substrates having bis(dimethylamino) substituents . The objective in all these cases is to try to design new compounds with an exceptionally strong singlet electron resonance throughout the molecule , to the point where it might have an appreciable electrical conductivity in the solid state . I'll also outline the design and possible synthesis of Wurster Blues based on copper(II) coordinate covalent compounds ; these latter materials might even be superconductors at lower temperatures .


An Indigo Wurster Blue Compound


In the Indigo web page I suggested that an indigo analogue , 5,5'-bis(dimethylamino)indigo , might be converted into its corresponding Wurster Blue compound [underlined blue hyperlinks can be clicked when online to download the referenced document , which will open in a new window] :

The above example illustrates the general approach taken in this study : (1) select , if possible , a highly colored compound – preferably blue – as the starting material ; (2) re-design the molecule so as to have a dimethylamino group at its antipodes [extreme ends] , if chemically feasible ; and (3) treat the bis(dimethylamino) compound with SbCl5 /CH2Cl2 to produce its Wurster Blue compound . The resulting radical cation compound should have highly delocalized electrons in its pi cloud . If these singlet electrons are also delocalized throughout a crystal of the solid state material , it could have an appreciable electrical conductivity and exhibit metallic properties .

The strong one-electron oxidizer nitrosonium cation , NO+, should also be able to produce a radical cation on the tertiary nitrogen atom as shown in the above sketch . Its stable , crystalline BF4and PF6 salts are commercially available .


An Azulene Wurster Blue Compound


The partially-aromatic compound azulene has a deep blue color and a strong electron resonance in the pi cloud over its planar sigma skeleton . It should thus be an excellent candidate for conversion into a Wurster Blue compound . Several azulene syntheses have been devised since its first preparation by St. Pfau and Plattner in 1939 . I think the most practical of them is that one , combining pyridine and cyclopentadiene , by Hafner and Meinhardt (in Organic Syntheses) ; and Hafner's route is the one most suited for modification with bis(dimethylamino) groups on the azulene antipodes , as sketched below :


A Galvinoxyl Wurster Blue Compound


The neutral free radical galvinoxyl was first reported by Galvin M. Coppinger (guess where its common name originated) in 1957 , and again later that year by Kharasch and Joshi :

The molecule-wide delocalization of its singlet electron causes the crystalline material to have a dark blue color . The bulky tert-butyl groups at the C2,6 positions are essential for protecting the phenol oxygens , which are very sensitive toward oxidation . The dimethylamino free radicals appear to be less sensitive to oxidation , provided the singlet electron is delocalized throughout the molecule's pi cloud . A hybrid galvinoxyl-Wurster Blue compound might be prepared as follows :

The phenolic intermediate would be oxidized to the neutral free radical by alkaline ferricyanide solution , as described by Kharasch and Joshi for their preparation of galvinoxyl .

A simplified version of a galvinoxyl Wurster Blue neutral free radical might be synthesized as outlined in the following sketch :

In this latter compound the phenolic oxygen atom (flanked by its bulky tert-butyl bodyguards) would replace one of the dimethylamino groups in the original N,N,N',N'-tetramethyl-p-phenylene diamine Wurster Blue .


Crystal Violet , Methylene Blue , and Acridine Orange Wurster Compounds


The two well-known dyes , Crystal Violet (Gentian Violet) and Methylene Blue (methylthioninium chloride) , each have pairs of diametrically-opposed dimethylamino groups that when treated with SbCl5 /CHCl3 might be converted into their corresponding radical cation derivatives :

The hydrochloride salt of 3,6-bis-(dimethylamino)acridine is another dye , Acridine Orange . It might similarly be converted into its radical cation Wurster Blue derivative with SbCl5 /CH2Cl2 :


Naphthalene , Anthracene , Tetracene , and Pentacene Wurster Blues


Numerous Wurster Blue derivatives could be devised for the four common acenes : naphthalene (colorless , as in household mothballs) ; anthracene (colorless when pure , pale yellow when slightly impure) ; tetracene (also called napthacene ; it's orange) ; and pentacene , described in the Merck Index (8th edition , 1968 , p. 792) as deep blue needles with violet luster from hot nitrobenzene.

I outlined a suggested route to the 2,9-bis(dimethylaminium)pentacene hexachloroantimonate Wurster Blue in the Solar web page . Pentacene itself is a p-type of semiconductor , and there has been considerable research interest in recent years concerning its use as a solar cell component . I proposed in the Solar web page trying bilayer films of 2,9-bis(dimethylamino)pentacene (on top , exposed to the sunlight) and 2,9-bis(dimethylaminium)pentacene hexachloroantimonate underneath . The latter layer would provide “holes” in the pi cloud MO for the photoexcited electrons from the top layer to “fall into”, thereby creating a potential difference [voltage] between the upper layer of 2,9-bis(dimethylaminium)pentacene (positive) and lower Wurster Blue layer (negative) as long as the composite film is illuminated .

Given the multitude of isomers of bis(dimethylamino) compounds possible for these four acenes , the synthesis of their many Wurster Blue derivatives would represent a huge amount of chemistry , far more than the space limitation of this web page would permit . Instead , I'll restrict my comments to two established and synthetically valuable routes to the acenes and their derivatives .

The first useful synthetic route is that of the annelation of phthalic anhydride onto an available acene , via the Friedel-Crafts acylation reaction , followed by a dehydrating ring closure (generally using concentrated sulfuric acid or even oleum as the condensing agent) . The late Professor Louis F. Fieser (1899-1977) of Harvard University extensively studied quinones , among his many other research interests . In the early 1940s he devised a simple , efficient , multi-stage synthesis of quinones (I believe in connection with the development of new antimalaria drugs during World War II) . His quinone route later became a semi-micro scale project for undergraduate organic chemistry students , described in his excellent laboratory manual , Organic Experiments :

The phthalic anhydride annelation thus adds two aromatic rings to the original acene substrate , and provides an intermediate quinone with reactive carbonyl sites that can be utilized to derivatize , eg. with dimethylamino groups , the new acene product .

Actually , Fieser's annelation route is unnecessary in the case of the naphthalene Wursters , because two organic intermediates required for them (Proton Sponge and napthoquinone) are commercially available at a modest cost . The proposed preparations of the 1,4- and 1,8-bis(dimethylaminium)naphthalene Wursters are outlined in the sketch below :

Leonard and Paukstelis carried out an extensive study (1963) of the preparation of iminium salts . They obtained high yields of crystalline iminium perchlorate salts by combining secondary ammonium perchlorates with aldehydes and ketones . As expected , the aldehydes reacted faster than the ketones . The following is a precautionary tale from my experience trying to adapt their procedure to my own preparation of an iminium perchlorate :

“In general the fluoborate salts function less efficiently than the perchlorates but are probably handled with greater assurance of safety” (Leonard and Paukstelis , p. 3022) .

Speaking of handling perchlorate salts safely , some time ago I prepared one of the iminium perchlorate salts described by Leonard and Paukstelis . It was formed by the condensation of morpholine perchlorate and cyclohexanone in refluxing benzene , with azeotropic distillation of the water by-product . They obtained the iminium salt (m.p. 239-241 C) in 94% yield by this method . This proved to be a very lengthy process – something not mentioned by Leonard and Paukstelis – and required an overnight reaction period . Unfortunately , during the night the maintenance staff turned off the water in the Chemistry Building without notifying the researchers . The benzene apparently distilled off through the warm condenser , and the dry perchlorate salt overheated and detonated . When I returned to the laboratory in the morning , I was shocked and dismayed to see the apparatus disintegrated in a pile of shattered glass in the fume hood ! Undeterred I repeated the experiment , but this time substituting xylene (b.p. 138 C) for the benzene (b.p. 80 C) . Under my watchful eye this time , the reaction proceeded smoothly and was completed in about an hour and a half . After workup , a nearly quantitative yield of the tertiary iminium perchlorate salt was obtained .

The above section was copied and pasted from an earlier Chemexplore web page about the electrically conductive polymer polyaniline .

If it was successful , the proposed conversion of 1,4-napthoquinone to 1,4-bis(dimethylamino)naphthalene via its bis-iminium perchlorate salt would be a simple technique to carry out , but one not without its own peculiar hazards and risks . In the second step of the sequence I've indicated a two-electron reduction of the bis-iminium perchlorate to the bis-amine . Various reducing agents could be tried for this reaction . For example , reducing metals and cations might accomplish it :

2 [R=N(CH3)2+ (ClO4)] + Fe0 + 2 HCl (aq) -----------> 2 R–NH+(CH3)2 (Cl)2 + Fe(ClO4)2 ;

2 [R=N(CH3)2+ (ClO4)] + 2 CrCl2 (aq) -----------> 2 R–N(CH3)2 + 2 Cr(ClO4)(Cl)2 .

In the latter case , Cr(II) is a natural reducing agent (Eox0 = 0.407 V to Cr3+) , and has been used in the free radical reduction of olefins . Possibly the best reagent for this transformation might be a reducing anion , which would be electrostatically attracted to the cationic iminium nitrogen atom . Several such reducers are readily available . Dithionite (as Na2S2O4) , hypophosphite (as NaH2PO2 . H2O) , and borohydride (as NaBH4) are surprisingly strong reducing agents . They are all active in an alkaline medium :

S2O42- + 4 OH – 2e -----------> 2 SO32- + 2 H2O ; Eox0 = 1.12 V ;

Chemically : 2 [R=N(CH3)2+ (ClO4)] + Na2S2O4 + 4 NaOH ----- (H2O solvent) -------> 2 R–N(CH3)2 + 2 Na2SO3 + 2 NaClO4 + 2 H2O .

H2PO2- + 3 OH – 2e -----------> HPO32- + 2 H2O ; Eox0 = 1.65 V ;

BH4 + 8 OH – 8e -----------> H2BO3- + 5 H2O ; Eox0 = 1.24 V .

Anthraquinone is a readily available starting material , both commercially and via synthesis by the researcher , if desired (see Fieser's preparation of anthraquinone , above) . It could serve as the starting material for the 5,10-bis(dimethylaminium)anthracene Wurster Blue :

Fieser's phthalic anhydride annelation method could be used for the preparation of the 1,4-bis(dimethylaminium)anthracene Wurster Blue :

In this case maleic anhydride would be used to add a single aromatic ring onto the naphthalene substrate , in order to obtain the 1,4-anthraquinone intermediate .

Similarly , the 5,12-bis(dimethylaminium)tetracene Wurster Blue might be synthesized from naphthalene as the substrate acene , with a phthalic anhydride annelation :

An analogous route could be used for the preparation of the 5,14-bis(dimethylaminium)pentacene Wurster Blue (and also of pentacene itself , by Fieser's reduction methods) :

A very simple , clever synthesis of the pentacene system was devised by Bruckner and Tomasz in 1961 . They carried out a double Knoevenagel condensation of two equivalents of phthaladehyde with one equivalent of 1,4-cyclohexanedione , obtaining a nearly quantitative yield of pentacene-6,13-dione :

Half a century later Pramanik and Miller discovered a simple method to reduce pentacene-6,13-dione to pentacene in a high (~ 80%) overall yield . The two starting materials , phthaladehyde and 1,4-cyclohexanedione , are commercially available at a moderate cost , so pentacene-6,13-dione is now readily available for conversion into 6,13-bis(dimethylamino)pentacene and its Wurster Blue derivative .

As mentioned at the beginning of this section about the acene Wurster Blues , a synthesis route to the 2,9-bis(dimethylaminium)pentacene Wurster Blue was proposed in the Solar web page , to which the interested reader is referred . Although many more isomers of these acene Wurster Blues are conceivable , the limited available length of this web page obliges me to proceed to the next (and final) section about copper(II) coordinate covalent compounds that might be modified to form their corresponding Wurster Blues .


Wurster Blue Copper(II) Coordinate Covalent Compounds


In his review of metal acetylacetonate complexes , Fackler noted ,

“Copper(II) acetylacetonate crystallizes from benzene and other noncoordinating organic solvents as a sky-blue material in which the copper(II) is coordinated by four ligand oxygen atoms in a nearly square planar arrangement” (p. 384) .

The four oxygens form a “symmetric chelate ring”, with Cu–O = 1.92 . Cu(AcAc)2 molecules form stacks in the crystalline solid , with the intermolecular distance ~ 3.1 . The compound's magnetic susceptibility meff = 1.90 BM at room temperature . This value is typical of a paramagnetic material with a single unpaired electron per molecule [the spin-only magnetic moment m for an ion or a molecule with n unpaired electrons is m = [n (n+2)] , so the Cu(II) 3d9 singlet electron in Cu(AcAc)2 should produce in it a magnetic moment of 1.73 BM at room temperature] . Figgis and Lewis state that the magnetic susceptibility of copper(II) acetylacetonate is 1.91 BM at 292 K , and decreases slightly to 1.80 BM at 10 K ; “......... the compound is certainly antiferromagnetic in the low-temperature range” (p. 213) .

Fackler commented in his review that “more than sixty copper(II) complexes with b-diketone ligands are known” (p. 384) . The ligand of interest to us with respect to the Wurster Blues is 3-(dimethylamino)-2,4-pentanedione . Fackler and Cotton studied a dozen copper(II) acetylacetonate complexes with various g-substituents , that is , on C3 . One of them was 3-acetamido-2,4-pentanedione ; and they mention that “......... since isonitrosoacetylacetone , (CH3CO)2CNOH , is known to produce (CH3CO)2CHNH3Cl upon reduction with zinc ......” (p. 103) . The ligand 3-(dimethylamino)-2,4-pentanedione thus seems to be a reasonable target molecule and should be accessible via an organic synthesis procedure . The intermediate in this scheme , 3-chloro-2,4-pentanedione , is well-known ; it was one of the g-substituted b-diketone ligands studied by Fackler and Cotton , and is commercially available (eg. Aldrich) at a modest cost . A possible synthesis route to copper(II) bis-(3-dimethylaminium)-2,4-acetylacetonate Wurster Blue is presented in the following sketch :

The aminium singlet electron will resonate over the entire molecule through the pi cloud covering its sigma skeleton . In order to do so , it must cross over the copper(II) cation with its unpaired 3d9 singlet electron . The question is : could it magnetically couple with the copper electron , if their spins are compatible , to form a Cooper pair ? If so , the Wurster Blue derivative might become superconducting at a lower temperature .

The Valence Bond picture of the electronic condition of the Wurster Blue copper(II) bis(acetylacetonate) is shown in the sketch immediately above . In the VB analysis the copper(II) cation's 3d9 valence electron is relocated to an energetically-accessible frontier orbital when the strong , lower energy CuO covalent bonds are formed ; they take its 3dx2-y2 native orbital for use in the dsp2 hybrid orbital for those bonds . The most suitable frontier orbital is the 4pz , since in copper the 4s , 3d , and 4p energy levels are fairly close together , and relatively little energy would be required to promote the 3d9 electron to the 4pz orbital . This latter orbital has the correct shape , symmetry , and orientation to overlap with the molecule's pi cloud . That would allow the copper(II) 3d9 electron to conceivably couple with the resonating aminium singlet electron , if they have an antiparallel orientation with respect to each other .

There is an alternate picture of this copper(II) complex , based on Molecular Orbital Theory considerations . In this competing view , all of the valence electrons of the atoms involved are located in bonding molecular orbitals (BMOs) , with corresponding empty antibonding MOs at higher energy levels , covering the entire molecule . There is no distinct copper 3d9 electron ; it's blended into one of the BMOs . A one-electron oxidizer , if sufficiently strong , will remove an electron from the higher energy BMO , not specifically from the copper atom . If we do an electron head-count in the molecule , we can predict that it might actually be a chemical reducing agent :

In this alternate picture we see that it would be more energetically favorable for the oxidizer to remove the copper 3d9 electron” ; the two six-atom rings would then each have six electrons in six conjugated p orbitals (the copper 4pz orbital would be empty , but available for use) . They could then have an energetically-stabilizing 4n+2 p electron stabilization . The resulting molecular cation would be electronically stable ; it would not be a Wurster Blue cation , and it should be diamagnetic . In fact , the parent compound could well have mildly reducing properties as a consequence of this aromatic stabilization in the diamagnetic cation product .

In this latter analysis we conclude that the two dimethylamino side groups are irrelevant ; copper(II) bis(acetylacetonate) and its many analogues should behave in a similar electronic manner . In this respect the copper(II) bis-AcAc complexes should generally resemble the dithiolenemetal cation complexes , in which electrons can be added or removed , more or less readily , from the molecule's pi cloud , not specifically from the central metal cation .

Note that normally it's very difficult to oxidize copper(II) to copper(III) : Cu3+ + e- --------> Cu2+ ; E0red = 2.4 V ; conversely , Cu2+ e- --------> Cu3+ ; E0ox = – 2.4 V . This standard redox potential is the highest of that of all of the metal cations . The dimethylamino nitrogen atom presents a “soft target” for the SbCl5 oxidizer . What would happen when the parent compound , copper(II) bis-(3-dimethylamino)-2,4-acetylacetonate , is treated with SbCl5 /CH2Cl2 ? Would the unpaired electron continue to remain associated with the copper , perhaps in its 4pz orbital , as suggested by the Valence Bond Theory ? Would a genuine Wurster Blue compound be formed , with a one-electron resonance between the dimethylamino/dimethylaminium groups ? This hypothetical compound , now with two singlet electrons , should have a magnetic susceptibility meff ~ 2.83 BM (from m = [n (n+2)] ; n = 2 ; m = 8 = 2.83 BM) at room temperature .

In the alternate Molecular Orbital picture the SbCl5 oxidizer removes an electron from one of the pi cloud BMOs , ignoring the dimethylamino groups , to form a diamagnetic molecular cation . Of course , I much prefer the Valence Bond” scenario , since the Wurster Blue adduct might have a significant electrical conductivity and would have a chance of being a superconductor at lower temperatures . The “Molecular Orbital” diamagnetic cation compound , while doubtless possessing its own particular charms , would almost certainly lack these more interesting electrical features .

N,N,N',N'-tetramethylmalonamide (TMMA) , which has two dimethylamino side-groups , can also form bidentate , square planar copper(II) complexes . It's a rather obscure reagent ; so obscure , in fact , that the Aldrich Chemical Company , supplier of vast numbers of obscure compounds , doesn't offer it . While commercially available (from TCI America) , it's very expensive ! The conventional preparation of TMMA , from cooking diethyl malonate with a large excess of dimethylamine and an acid catalyst in an autoclave at 150 C for 8 hours (~ 80% yield , Lawson and Croom , 1963) seems rather inconvenient to me . Perhaps a simpler procedure might be the transamidation of diethyl malonate with two equivalents of dimethyl formamide (DMF) :

The volatile by-product , ethyl formate (b.p. 54 C) , would be distilled into a Dean Stark trap , thereby driving the reaction to completion . The high-boiling residue in the reaction flask would be fractionally distilled under a very high vacuum (oil pump required , protected with a dry ice /acetone or liquid nitrogen trap) to yield pure TMMA , b.p. 132 C (2.5 mm) .

Bull and Ziegler prepared a series of nine metal cation complexes with TMMA , one of which was the bidentate copper(II) compound . However , these were the simple adducts , without any base treatment ; they were the outer complexes (with external spectator anions) , rather than the inner complexes (internally zwitterionic , without external anions) that are more desireable for preparing the Wurster Blue derivative :

The bis(TMMA)copper(II) perchlorate complex prepared by Bull and Ziegler had a magnetic susceptibility of meff = 1.96 BM at 23 C , which is close to that for copper(II) bis(acetylacetonate) . The inner Cu(II) TMMA complex shown in the sketch would likely have a similar magnetic moment .

The readily accessible copper(II) bis(salicylaldehyde) complex can be treated with two equivalents of a primary amine to produce the corresponding bis-imine complex (Holm , Everett , and Chacravorty , p. 103) . This facile reaction might be used to prepare a Wurster Blue derivative of this well-known copper(II) coordinate covalent compound :

The bis-imine-copper(II) complex is quite interesting , as two of the nitrogen atoms directly involved in the resonance of the aminium singlet electrons are now bridged by the copper(II) cation . In a resonance structure the aminium singlet electron (nitrogen 2pz) and the copper(II) 4pz electron are situated side-by-side on neighboring atoms . Such a side-by-side proximity of two Cooper pair electrons would result in them having a very short coherence length , producing a strong magnetic coupling and a relatively stable Cooper pair ; these are the hallmarks of high temperature superconductors (HTS) such as YBCO (Tc = 93 K) and BSCCO-2212 (Tc ~ 110 K) .

On the other hand , the copper(II) bis-(salicylaldehyde imine) complex has the same sort of electronic structure as the copper(II) bis(acetylacetonate) complexes discussed above , and upon one-electron oxidation might form only a diamagnetic cation product in which the two six-atom rings have a stabilizing aromatic resonance . This latter material would probably have uninteresting electrical conductivity properties .

Amino acid ligands should also provide inner complexes of copper(II) which could be converted into Wurster Blue derivatives , as illustrated in the following sketches for glycine and b-alanine ligands modified with N,N-dimethyl-p-phenylenediamine :

These amino acid complexes also have the very interesting NCuN feature that could induce a HTS effect in them . And , unlike the copper(II) bis(acetylacetonate) and copper(II) bis(salicylaldehyde imine) complexes outlined above , their five- and six-atom rings with the copper cations lack any sort of electronic stabilization , aromatic or otherwise .

The critical NCuN grouping might not require the bulky 4-dimethylamino-p-phenylene groups in order to produce a Cooper pair . It could conceivably be incorporated in a macrocyclic molecule in which the copper(II) cation is bonded in a square planar symmetry :

A somewhat simpler square planar copper(II) complex designed along the same lines as above might be synthesized using cyanamide , HN2CN , as the ligand :

If the Wurster Blue molecules can stack reasonably closely in the crystalline solid , they have a good chance of superconducting at a lower temperature . Cooper pairs have the remarkable ability of being able to tunnel through insulating layers ; this is the Josephson effect , and the molecular gaps would be the Josephson junctions in the crystals . The aminium singlet electron resonance over the copper cation might produce the Cooper pair at a low temperature ; then , under an applied potential difference , all the Cooper pairs in the crystal simultaneously move together downfield , hopping from molecule to molecule through the Josephson gaps ......... superconductivity !

I hope this web page will draw attention to the broad research potential of new Wurster Blue radical cation compounds . The Wurster Blues offer an exciting new frontier of strongly resonant electronic systems to explore and possibly develop into practical applications .


References and Notes


Casimir Wurster : J. Andraos , “Named Reagents , Catalysts , and Compounds (G–Z)”, Dept. of Chemistry , York University , Toronto , Ontario , Canada [PDF , 346 KB] ; p. 45 .

Michaelis and co-workers : L. Michaelis , M.P. Schubert , and S. Granick , “The Free Radicals of the Type of Wurster's Salts”, J. Amer. Chem. Soc. 61 (8) , pp. 1981-1992 (1939) .

needle-shaped : The addition of a small quantity of a surfactant (detergent) to a crystallization solvent can result in the formation of product crystals with a pronounced acicular morphology , ie. as long , thin , needle-like crystals : J.M. Sugihara and S.R. Newman , “Recrystallization of Organic Compounds From Detergent–Water Systems”, J. Org. Chem. 21 (12) , pp. 1445-1447 (1956) . Such surfactants are known to act as crystal habit modifiers in the crystallization of solids from solutions . Their molecules are adsorbed on the sides of the growing crystals , inhibiting their sideways growth . The crystals then propagate lengthwise , resulting in the growth of long , slim needles . They are very pure ; the crystallization process resembles zone refining in certain aspects .These acicular crystals are especially desireable for electrical conductivity testing . In the preparation of the Wurster Blue compounds (as hexachloroantimonate salts) , a small quantity of a nonionic surfactant , such as a polyethoxylated fatty alcohol (eg. Brij 30) , could be dissolved in the methylene chloride with the Wurster Blue product . A small quantity of ethyl ether (perhaps ~ 10% by volume of the CH2Cl2) or cyclohexane would be added to the solution to decrease its polarity . Cooling the solution in a refrigerator or freezer would then hopefully induce a crystallization of the Wurster Blue SbCl6 salt in the form of long , thin needles . See also the electrical conductivity note below .

Bell and co-workers : SbCl5 has been used to form the stable Wurster Blue aminium salt , tris-(4-bromophenyl)aminium hexachloroantimonate , in a quantitative yield : F.A. Bell , A. Ledwith , and D.C. Sherrington , “Cation-radicals : Tris-(p-bromophenyl)amminium Perchlorate and Hexachloroantimonate”, J. Chem. Soc. C 1969 (19) , pp. 2719-2720 ; see also G.W. Cowell , A. Ledwith , A.C. White , and H.J. Woods , “Electron-Transfer Oxidation of Organic Compounds with Hexachloroantimonate [SbCl6] Ion”, J. Chem. Soc. B 1970 (2) , pp. 227-231 . The standard reduction potential of SbCl5 in a halide environment is ~ 0.8 V , indicating that it is a moderately strong oxidizing agent . The stoichiometry for the one-electron oxidation of a tertiary amine by SbCl5  is :

substrateN..(CH3)2 + 1 SbCl5 ---------> substrateN.+(CH3)2 [SbCl6] + SbCl3 .

electrical conductivity : The usual method of measuring the electrical conductivity (or resistivity) of metallic solids is to solder contacts onto an elongated or acicular crystal of the material , and then measure its conductivity or resistivity over a range of temperatures . The soldering technique is a delicate , specialized operation best left to researchers expert with it . There are apparently non-contact methods of determining the electrical conductivity of a material that may be of interest to researchers unfamiliar with the soldering technique . Three articles which explain their theory and describe their practice in some detail : “ASTM E1004 - 09 Standard Test Method for Determining Electrical Conductivity Using the Electromagnetic (Eddy-Current) Method” [web page ; the ASTM standard refers to “commercially available direct reading electrical conductivity instruments”] ; J. Heller and J.R. Feldkamp , “Auto-Tuned Induction Coil Conductivity Sensor for In-Vivo Human Tissue Measurements”, Measure. Sci. Rev. 9 (6) , pp. 162-168 (2009) [PDF , 352 KB] ; Y.J. Jiang ,et al. , “A New Method for Measuring the Graphite Content of Anthracite Coals and Soots”, Energy & Fuels 16 (5) , pp. 1296-1300 (2002) [PDF , 287 KB] . My impression in reading the latter two articles is that the researcher must first calibrate the apparatus with a series of standard materials with known properties , more or less similar to the test sample , in order to draw a conductivity/resistivity calibration curve in the anticipated range of the test sample's value .

nitrosonium cation : W.J. Plieth , Nitrogen, Ch. 5 , pp. 321-479 in Encyclopedia of Electrochemistry of the Elements , Vol. 8 , A.J. Bard (ed.) , Marcel Dekker , New York , 1978 ; p. 325 . The redox half-reaction (reduction) of the nitrosonium cation is :

NO+ + e-  -------------->   NO (g)  ;  E0red  =  1.45 V .

This standard reduction potential indicates that NO+ is a rather powerful oxidizing agent . The stoichiometry for the one-electron oxidation of a tertiary amine by a nitrosonium salt is :

substrateN..(CH3)2 + NO+ X ---------> substrateN.+(CH3)2 [X] + NO (g) .

Nitrosonium hexafluorophosphate has been used in organic reactions to abstract hydride from hydrocarbons , thereby forming their corresponding carbocation salts :

G.A. Olah , G. Salem , J.S. Staral , and T.-L. Ho , “Preparative Carbocation Chemistry . 13 . Preparation of Carbocations from Hydrocarbons via Hydrogen Abstraction with Nitrosonium Hexafluorophosphate and Sodium Nitrite – Trifluoromethanesulfonic Acid”, J. Org. Chem. 43 (1) , pp. 173-175 (1978) .

Two nitrosonium salts (X = BF4and PF6 ) are commercially available (Alfa-Aesar , Aldrich) .

azulene syntheses : A. St. Pfau and Pl. A. Plattner , “Zur Kenntnis der Flchtigen Pflanzenstoffe VIII. Synthese des Vetivazulens”, Helv. Chim. Acta 22 (1) , pp. 202-208 (1939) ; W. von E. Doering , J.R. Mayer , and C.H. DuPuy , “Two-Step Synthesis of Azulene”, J. Amer. Chem. Soc. 75 (10) , p. 2386 (1953) ; L.T. Scott , M.A. Minton , and M.A. Kerms , “A Short New Azulene Synthesis”, J. Amer. Chem. Soc. 102 (20) , p. 6311-6314 (1980) ; D.M. Lemal and G.D. Goldman , “Synthesis of Azulene , a Blue Hydrocarbon”, J. Chem. Educ. 65 (10) , pp. 923-925 (1988) ; S. Carret et al. , “Approach to the Blues : A Highly Flexible Route to the Azulenes”, Angew. Chem. Internat. Ed. Engl. 44 (32) , pp. 5130-5133 (2005) ; anon. (Web article) , Chem. 465 Organic , Experiment 16 , “Am I Blue ? – A Synthesis of Azulene”.

Hafner and Meinhardt : K. Hafner and K.-P. Meinhardt , “Azulene”, Org. Syn. Coll. Vol. 7 , pp. 15-18 (1990) [PDF , 133 KB] . Review : “Ziegler-Hafner Azulene Synthesis”, Comprehensive Organic Name Reactions and Reagents 2010 , pp. 3139-3143 .

galvinoxyl : G.M. Coppinger , A Stable Phenoxy Radical Inert to Oxygen”, J. Amer. Chem. Soc. 79 (2) , pp. 501-502 (1957) .

Kharasch and Joshi : M.S. Kharasch and B.S. Joshi , “Reactions of Hindered Phenols. I. Reactions of 4,4'-Dihydroxy-3,5,3',5',-tetra-tert-butyl Diphenylmethane”, J. Org. Chem. 22 (11) , pp. 1435-1438 (1957) .

acenes : J.E. Anthony , “The Larger Acenes : Versatile Organic Semiconductors”, Angew. Chem. Internat. Ed. Engl. 47 (3) , pp. 452-483 (2008) .

Organic Experiments : L.F. Fieser and K.L. Williamson , Organic Experiments , 7th edition , D.C. Heath , Lexington (MA) , 1992 ; the preparation of anthraquinone and anthracene are described in Ch. 53 , pp. 457-463 , and in earlier editions of Fieser's popular laboratory textbook .

Leonard and Paukstelis : N.J. Leonard and J.V. Paukstelis , Direct Synthesis of Ternary Iminium Salts by Combination of Aldehydes or Ketones with Secondary Amine Salts, J. Org. Chem. 28 (11) , pp. 3021-3024 (1963) .

free radical reduction : for example , see : H. Driguez , J.M. Paton , and J. Lessard , “The Chromous Chloride Promoted Addition of N-Haloamides to Olefins . III. Scope and Limitations for the Synthesis of N-(2-Haloalkyl)amides”, Can. J. Chem. 55 (4) , pp. 700-719 (1977) [PDF , 1109 KB . Note : Can. J. Chem. PDFs can be opened only with Acrobat Reader v. 6 or later . If desired , this application can be downloaded for free from Oldversion.com] ; H. Driguez and J. Lessard , “The Chromous Chloride Promoted Addition of N-Haloamides to Olefins. IV. Mechanistic Aspects , Can. J. Chem. 55 (4) , pp. 720-732 (1977) [PDF , 770 KB] ; H. Driguez , J.-P. Vermes , and J. Lessard , “The Chromous Chloride Promoted Addition of N-Haloamides to Olefins. V. The Addition of N-Chloroamides to Enol Ethers : Synthesis of Acyloxy and Acyl Derivatives of a-Amino Acetals and Ketals (Aldehydes and Ketones) and of 2-Amino Sugars” , Can. J. Chem. 56 (1) , pp. 119-130 (1978) [PDF , 747 KB] .

Pramanik and Miller : C. Pramanik and G.P. Miller , “An Improved Synthesis of Pentacene : Rapid Access to a Benchmark Organic Semiconductor”, Molecules 17 (4) , pp. 4625-4633 (2012) [PDF , 286 KB . Note : this file can be opened only with Acrobat Reader v. 6 or later] . These authors also outline several historical syntheses of pentacene .

Fackler : J.P. Fackler Jr. , “Metal b-Ketoenolate Complexes”, Prog. Inorg. Chem. 7 , pp. 361-425 , F.A. Cotton (ed.) , Interscience Publishers , New York , 1966 . Copper(II) bis(acetylacetonate) is mentioned on p. 384 . Fackler commented ,

“The number of b-diketones known to form metal complexes certainly exceeds one hundred and more potential ligands are prepared each year” (p. 363) .

More than sixty copper(II) complexes with b-diketone ligands were then (1966) known (p. 384) .

Of related interest : L.E. Marchi , “Metal Derivatives of 1,3-Diketones”, Inorg. Synth. 2 , pp. 10-17 , W.C. Fernelius et al. (eds.) , McGraw-Hill , New York , 1946 ; W.C. Fernelius and B.E. Bryant , “Preparation of Metal Derivatives of 1,3-Diketones”, Inorg. Synth. 5 , pp. 105-113 , T. Moeller et al. (eds.) , McGraw-Hill , New York , 1957 . This latter article provides a useful review of the various methods of preparing metal cation–b-diketone complexes .

Figgis and Lewis : B.N. Figgis and J. Lewis , “The Magnetic Properties of Transition Metal Complexes”, Prog. Inorg. Chem. 6 , pp. 37-239 , F.A. Cotton (ed.) , Interscience Publishers , New York , 1964 . Copper(II) bis(acetylacetonate) is mentioned on p. 213 .

Fackler and Cotton : J.P. Fackler Jr. and F.A. Cotton , “Electronic Spectra of b-Diketone Complexes . IV. g-Substituted Acetylacetonates of Copper(II)”, Inorg. Chem. 2 (1) , pp. 102-106 (1963) .

3-acetamido-2,4-pentanedione : This compound is now a well-known ligand (Hamac) for many metal cations ; for example , see : J. Hirsch , H. Paulus , and H. Elias , “Kinetics and Mechanism of the Formation and Acid Dissociation of Cobalt(II) , Nickel(II) , and Copper(II) Complexes with the Highly Enolized b-Diketone 3-(N-Acetylamido)pentane-2,4-dione (Hamac) in Aqueous Solution”, Inorg. Chem. 35 (8) , pp. 2343-2351 (1996) .

should be accessible : 3-(dimethylamino)-2,4-pentanedione might be a known chemical compound . Unfortunately I no longer have access to Chemical Abstracts or SciFinder Scholar at our local Science Library , so I was unable to conduct a proper literature search for it . Readers should , of course , conduct their own careful and thorough literature review of any topic of interest in these Chemexplore web pages .

bonding molecular orbitals : A molecular orbital energy level diagram of metal acetylacetonates [including that of copper(II)] is presented by Fackler (ref. above , on his p. 373) . The two low energy BMOs are p1 and p2 ; the corresponding p ABMOs are at a much higher energy level .

mildly reducing properties : The hypothesis that copper(II) bis(acetylacetonate) might be a mild reducing agent could be tested by the preparation of its charge transfer compounds with a variety of electron acceptors . Examples of these are shown in the sketch below :

The degree of charge transfer from the copper compound to the acceptor would vary from slight (the trinitro aromatics) to substantial (TCNQ and TCNE) . In the two latter cases the resulting charge transfer compounds might appear as shiny , black crystals with an appreciable electrical conductivity (but probably only as semiconductors , though , not as a molecular metal like TTFTCNQ) . These 1 : 1 adducts , if slowly and carefully crystallized from a suitable solvent , and with a small amount of a crystal habit modifiying surfactant present [see needle-shaped above] , might form the long , slender , acicular crystals required for electrical conductivity testing . The exploratory study of a possible interaction between copper(II) bis(acetylacetonate) and maybe several other AcAc and copper(II) salicylaldehyde complexes with various electron acceptor compounds would be a nice research project for advanced undergraduate chemistry students . Copper(II) bis(acetylacetonate) – called “cupric acetylacetonate” – picric acid , trinitrofluorenone , p-chloranil , TCNQ , and TCNE are all commercially available from the Aldrich Chemical Company .

Lawson and Croom : J.K. Lawson Jr. and J.A.T. Croom , “Dimethylamides from Alkali Carboxylates and Dimethylcarbamoyl Chloride”, J. Org. Chem. 28 (1) , pp. 232-233 (1963) . Their reaction of disodium malonate and dimethylcarbamoyl chloride , (CH3)2NCOCl , gave only a low (9.5%) yield of TMMA . It was then prepared by the more conventional amidation of diethyl malonate with a large excess of anhydrous dimethylamine , with (CH3)2NH2+Cl as a catalyst , in an autoclave at 150 C for 8 hours . This latter method gave a satisfactory yield (79.5%) of TMMA after distillation .

Bull and Ziegler : W.E. Bull and R.G. Ziegler , “Metal Complexes of N,N,N',N'-Tetramethylmalonamide”, Inorg. Chem. 5 (4) , pp. 689-692 (1966) .

Holm , Everett , and Chacravorty : R.H. Holm , G.W. Everett Jr. , and A. Chracravorty , “Metal Complexes of Schiff Bases and b-Ketoamines”, Prog. Inorg. Chem. 7 , pp. 83-214 , F.A. Cotton (ed.) , Interscience Publishers , New York , 1966 . Preparation of the salicylaldimine complexes was mentioned on p. 103 . Various copper(II) complexes with salicylaldimines are listed in Table II , pp. 100-101 .



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