FIELD OF INVENTION 
This invention relates to tetrabenzimidazole, and more particularly to 
cyclic tetrabenzimidazole, a yellowish substance useful as a chelating 
agent, catalyst and an electrooptic component. 
BACKGROUND OF INVENTION 
In U.S. Pat. No. 3,481,945, tetrabenzimidazole, tetrabenzothiazole and 
tetrabenzoxazole were described and claimed. A single example of a method 
of producing the compound was detailed. In that method, 2,3-diaminobenzoic 
acid was refluxed with meta-cresol and a copper compound to form a dark 
substance having a melting point above 400.degree. C. The substance was 
described as copper tetrabenzimidazole, a chelate. The properties of the 
isolated compound were described in depth; an acidic solution was strongly 
colored, and both acidic and neutral solutions exhibited strong 
fluorescence when excited by visible light. 
However, it was recently determined that the substance produced by 
following the method disclosed in the patent was not copper 
tetrabenzimidazole, but rather the well known compound fluorindine 
(5,12-dihydroquinoxalo[2,3-b]phenazine). The error was discovered when 
mass spectrometry work on the isolated substance indicated that the 
substance had the empirical formula of fluorindine. A sample of 
fluorindine was then prepared by an established synthetic process; that 
sample had properties identical to those of the compound isolated by the 
method disclosed in the patent. Thus, the substance isolated and described 
in the patent is fluorindine, not copper tetrabenzimidazole. 
SUMMARY OF INVENTION 
It is therefore an object of this invention to provide a novel cyclic 
tetrabenzimidazole. 
It is a further object of this invention to provide an aromatic compound 
having a cyclic structure. 
It is a further object of this invention to provide a compound useful as a 
chelating agent, a catalyst and an electrooptic component. 
It is a further object of this invention to provide a novel synthetic 
approach to yield a compound with the structure of tetrabenzimidazole but 
with unexpected properties. 
This invention may be accomplished in a cyclic tetrabenzimidazole, a 
yellowish, visually non-fluorescent solid having a molecular weight of 
approximately 464, a melting point above 350.degree. C., slight solubility 
in a 1:1 mixture of ethanol and chloroform, and IR absorption in potassium 
bromide at 1620, 1550, 1450, 1400 and 1260 cm.sup.-1, and the tautomers, 
oxidation, ionization and chelation forms thereof. 
DISCLOSURE OF PREFERRED EMBODIMENT 
Other objects, features and advantages will occur from the following 
description of a preferred embodiment and the accompanying drawings, in 
which:

Cyclic tetrabenzimidazole is an aromatic compound which from a structural 
standpoint appears to be similar to phthalocyanines and porphyrins. 
Because of its four external and four internal imidazole nitrogens, 
tetrabenzimidazole can have a number of tautomers, with the protons in the 
center or on the periphery of the ring. These forms can chelate zero, 
mono, di, tri and tetravalent atoms, and complex with atoms in its various 
states of oxidation and ionization. Cyclic tetrabenzimidazole possesses 
two additional oxidized forms, with two and zero imidazole protons. The 
partially oxidized form has its own amphoteric and tautomeric properties. 
There is shown in FIG. 1 the cyclic tetrabenzimidazole according to this 
invention. The compound is characterized by four internal and four 
external nitrogens. The four protons attached to the nitrogen atoms can be 
in any of the possible tautomeric forms, i.e. internal or external, so 
long as it is sterically possible. Furthermore, as a result of the 
amphoteric properties of benzimidazoles, any or all of the four protons 
can be removed to form negatively charged ions, or alternatively up to 
four protons can be added to the imide nitrogens to form positively 
charged ions (so long as steric hindrance does not prevent this proton 
addition). The molecule can form up to four coordinate bonds with atoms 
such as metals; it can chelate zero to tetravalent atoms. 
The ionization, chelation, tautomeric and oxidation forms of 
tetrabenzimidazole are identical to tetrabenzimidazole itself except for 
the removal or addition of the hydrogen atoms or nitrogen atoms or the 
attachment of a complexing atom to the four internal nitrogen atoms. 
The compound exhibits additional oxidation states with two and zero 
imidazole protons. These oxidation states have the same empirical formulas 
as anions but with fewer available electrons. The chelates can form both 
anions and cations in acidic or basic solutions respectively. 
The pale color of tetrabenzimidazole is quite surprising and unexpected: 
previously, a visually fluorescent compound had been expected. Even though 
tetrabenzimidazole has the same internal cruciform structure as 
porphorins, it does not have the same high electron mobility and 
delocalization within the large ring structure and therefore it does not 
have the intense color and longer wave length color characteristic of 
porphorins. Furthermore, its spectral properties are very different from 
those of the phthalocyanine family of compounds even though its structure 
has many similarities--in particular the large similarly spaced ring 
structure which also includes a number of aromatic rings and eight 
nitrogen atoms. Molecular models of tetrabenzimidazole indicate it could 
be planar but the pale yellow to colorless character of the compound 
indicates that there is minimal electron delocalization beyond individual 
benzimidazole rings and therefore the compound would not be required to be 
in planar form. Molecular orbital calculations also indicate that there is 
not macrocyclic electron delocalization. 
The cyclic tetrabenzimidazole may be prepared from 2,3-diaminobenzoic acid, 
a monomer having a single carboxy group and two amino groups on a benzene 
ring: 
##STR1## 
The monomer may be synthesized by traditional methods, for example as 
disclosed by Chapman and Stephen, J. Chem. Soc., 1925, 1791. The overall 
synthetic approach for the cyclic tetrabenzimidazole contemplates the 
production from the monomer of linear dimers, coupled to form a linear 
tetramer, which is cyclized as a last step. 
45 grams of 2,3-diaminobenzoic acid was suspended in 500 ml of chloroform, 
to which was slowly added 15 ml of triethylamine. After cooling with an 
ice water bath, 150 ml of thionyl chloride was added dropwise. The 
reddish-brown solution was refluxed for one hour. After cooling, the 
reaction mixture was poured into water to destroy excess thionyl chloride, 
and washed with water and a saturated sodium chloride solution (brine). 
The organic layer was dried over magnesium sulfate, and evaporated. 
Sublimation afforded in 75% yield the acid chloride as pale yellow 
crystals, mp 117.degree. C.: 
##STR2## 
The acid chloride was then hydrolyzed with base to form the thiadiazole 
acid. 39.7 9 of the acid chloride was suspended in 500 ml of 10% potassium 
hydroxide and refluxed for 30 minutes. After total dissolution, the 
reaction mixture was filtered, cooled and acidified with 10% hydrochloric 
acid. The precipitate was centrifuged, washed with water and dried in a 
vacuum oven to give in 78% yield off-white crystals of the thiadiazole 
acid, mp 175.degree.-177.degree. C.: 
##STR3## 
The second element for production of the dimer, the ester of 
2,3-diaminobenzoic acid, was formed by suspending 45 g of the 
diaminobenzoic acid in 500 ml of absolute ethanol. After addition of 20 ml 
of concentrated sulfuric acid, the reaction mixture was refluxed for 72 
hours. The ethanol was stripped off and the residual oil dissolved in 
water and neutralized with concentrated ammonium hydroxide to pH 8. The 
white suspension was stirred with hot methylene chloride for 30 minutes, 
filtered through celite, and washed several times with hot methylene 
chloride. The methylene chloride fraction was twice washed with water, and 
brine, dried over anhydrous magnesium sulfate, and evaporated under 
reduced pressure to afford in 78% yield the ester as a crystalline solid, 
mp 60.degree.-70.degree. C.: 
##STR4## 
The ester and acid were then coupled to form the benzimidazole ester dimer 
by cyclodehydration. A solution of 120 g of 
N-diphenylphosphinyl-N'-methylpiperazine in 500 ml of methylene chloride 
was chilled to 0.degree. C. A solution of 33.64 ml of triflic anhydride in 
200 ml of methylene chloride was added dropwise. After 15 minutes, a 
mixture of 18 g of the thiadiazole acid and 18 g of the diamino ester was 
added, and the mixture was stirred at room temperature for twenty minutes. 
After filtration and solvent evaporation, the semi-solid residue was 
washed with a copious amount of water and 10% potassium bicarbonate, again 
washed with water, dried, and recrystallized from heptane to give in 
approximately 55% yield the benzimidazole ester dimer as yellow crystals, 
mp 172.degree.-173.degree. C.: 
##STR5## 
After several days the mother liquor yielded another approximately 30% of 
the dimer for a total yield of 85%. 
The next steps in the synthesis comprised the production of the acid and 
diamino ester forms of the dimer, which were then coupled into the linear 
tetramer. The benzimidazole acid dimer was prepared by suspending 6.48 g 
of the ester dimer in 200 ml of 10% potassium hydroxide with a few drops 
of ethanol, and refluxing for an hour. After cooling, the solution was 
made slightly acidic by dropwise addition of concentrated hydrochloric 
acid. The bright yellow precipitate was centrifuged, washed with water, 
filtered, and dried in a vacuum oven to give in 88% yield the acid dimer 
as bright yellow crystals, mp over 300.degree. C. 
##STR6## 
The diamino ester dimer was prepared by suspending 6.48g of the ester dimer 
and 130 g of anhydrous stannous chloride in 250 ml of absolute ethanol, 
and passing anhydrous hydrogen chloride gas through to make a clear 
solution. After four hours of reflux, the resulting green solution was 
distilled under vacuum to remove the solvent. The solid residue was 
dissolved in water, made basic with concentrated ammonium hydroxide, and 
the resulting yellowish-white slurry was filtered through celite and 
washed with water. The residue was washed repeatedly with methylene 
chloride, and the light green solution was twice washed with water and 
then brine, and dried over anhydrous magnesium sulfate. After evaporation, 
light yellow crystals were left. Thin layer chromatographic analysis on 
silica (2:1 hexane:ethyl acetate) left a residue at the origin, with a 
single compound of lower Rf. A solution of the crystals in methylene 
chloride was passed through a column of silica (2:1 hexane:ethyl acetate) 
and on evaporation left in 85% yield the diamino ester dimer as clean 
yellow crystals which darkened on standing in air, mp 
106.degree.-107.degree. C.: 
##STR7## 
The next step was to couple by cyclodehydration the diamino ester and acid 
dimers to form a protected linear tetramer. A solution of 13.5 g of 
N-diphenylphosphinyl-N'-methylpiperazine in 100 ml of methylene chloride 
at 0.degree. C. was prepared, to which was added dropwise a solution of 
16.82 ml of triflic anhydride in 100 ml of methylene chloride. After 15 
minutes a mixture of 2.96 g of the benzimidazole acid dimer and 2.96 g of 
the diamino ester dimer was added and stirred for two hours at room 
temperature. After filtration and solvent evaporation, the semi-solid 
residue was thoroughly washed with water and 10% potassium hydroxide 
solution, water again, and then toluene. The residue was then 
recrystallized from hot ethanol leaving in 60% yield the diamino protected 
tetramer ester as bright yellow crystals, mp 240.degree.-244.degree. C.: 
##STR8## 
The next step was to deprotect the diamino functions by reduction of the 
thiadiazole. This was accomplished as before, by suspending 1.11 g of the 
protected tetramer ester and 22.2 g of anhydrous stannous chloride in 100 
ml of ethanol, and passing through enough anhydrous hydrogen chloride gas 
to make a clear solution. The resulting off-red solution was refluxed for 
four hours and distilled under reduced pressure to remove the solvent. The 
solid residue was suspended in water, made basic by the addition of 
concentrated ammonium hydroxide, and the resulting white slurry was 
filtered through celite and washed with water. The residue was washed 
repeatedly with a 1:1 mixture of chloroform and methanol until no color 
came through. The light yellow solution was concentrated under reduced 
pressure and held in a freezer overnight. After the addition of a small 
amount of methanol, the white tin hydroxide precipitate was filtered off. 
On evaporation, the solution left in 84% yield the tetramer diamino ester 
as bright yellow crystals, mp 269.degree.-270.degree. C.: 
##STR9## 
The crystals darken on standing in air; they are best kept in the freezer 
under nitrogen. 
The final step in synthesis of the cyclic product was accomplished by 
heating 264 mg of the tetramer diamino ester neat at 300.degree. C. in a 
sand bath for 15 minutes under nitrogen. Chromatographic analysis on 
reverse phase silica, with 3:1 acetonitrile:ethyl acetate, revealed 
disappearance of the starting material. The resulting dark brown solid was 
refluxed for one hour with 150 ml of a 1:1 mixture of chloroform and 
ethanol, filtered hot through celite, and washed with the same solvent. On 
evaporation, there was left 195 mg of fine light yellow crystals, 
mp&gt;350.degree. C. The crystals were extremely insoluble, with only slight 
solubility in a 1:1 mixture of chloroform and ethanol, and exhibited no 
ester carbonyl in the IR spectrum. The mass spectrum showed the expected 
peak at m/e 464. Although the crystals were lightly colored, they 
exhibited no fluorescence when excited by visible light. 
The .sup.1 H NMR (DMSO-d.sub.6) showed four multiple peaks at: 8.5-8.2, 
8.1-7.7, 7.6-7.2 and 6.8-6.5. The integrated areas were in the ratio 
3:8:4:3, respectively. The .sup.13 C NMR(DMSO-d.sub.6) showed six broad 
peaks at : 153.2, 136.5, 132.3, 128.3, 124.3, and 115.4. The crystals were 
then sublimed in a sand bath above 350.degree. C. and under a high vacuum 
to provide a 7% yield cyclic tetrabenzimidazole as yellowish crystals. The 
IR spectrum (in potassium bromide) remained as before 
sublimation:absorption at 1620, 1550, 1450, 1400 and 1260 cm.sup.-1, 
although the subliminal product was generally cleaner in appearance. 
Although the cyclic tetrabenzimidazole was synthesized from the 
2,3-diaminobenzoic acid monomer in a stepwise fashion, the cyclic tetramer 
may likely be produced in a single self-condensation reaction employing a 
dilute solution of the monomer or related compounds. 
FIGS. 2A and 2B depict tautomers of a partially oxidized form of the cyclic 
tetrabenzimidazole according to this invention. These forms show 
delocalization throughout the macrocyclic structure and therefore should 
be highly colored. FIG. 3 depicts the fully oxidized form which has no 
tautomers because of the lack of hydrogens on the nitrogens, but is 
structurally similar to the tetranegative ion of the unoxidized form with 
the exception of having four less electrons in its structure. These forms 
should be intensely colored. A copper chelate is shown in FIG. 4. The 
chelates can form cationic and anionic species in acidic and basic 
solutions. 
A copper complex of the cyclic tetrabenzimidazole product was prepared from 
a mixture of 116 mg of cyclic tetrabenzimidazole and 1 g of copper (II) 
acetate monohydrate, heated in a sand bath at 300.degree. C. for 30 
minutes. The brown solid was washed with 10% hydrochloric acid and water, 
and dried in a vacuum oven to give 150 mg of pale yellow crystals, mp 
greater than 350.degree. C. The IR spectrum of the crystals was similar to 
the starting cyclic tetrabenzimidazole; absorption at 1620, 1450, 1260, 
1320, 1275, 1050 and 750 cm.sup.-1. After sublimation above 350.degree. C. 
under high vacuum, the pale yellow crystals afforded in 15% yield the 
copper complex as colorless crystalline plates. The IR spectrum yielded 
peaks at 1430, 1410, 1330, 1300, 1125, 940, 760 and 700 cm.sup.-1; the 
1620 cm.sup.-1 peak was lacking. 
Although specific features of the invention are shown in some drawings and 
not others, this is for convenience only as each feature may be combined 
with any or all of the other features in accordance with the invention. 
Other embodiments will occur to those skilled in the art and are within the 
following claims: