Cyanuric Acid and Cyanurates

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Russian Journal of Coordination Chemistry, Vol. 28, No. 5, 2002, pp. 301–324. Translated from Koordinatsionnaya Khimiya, Vol. 28, No. 5, 2002, pp. 323–347.Original Russian Text Copyright © 2002 by Seifer.

The organic derivatives of cyanuric acid find wideindustrial application today, which, unfortunately, doesnot apply to its inorganic salts due to the lack of system-atic studies on metal cyanurates. This fact, undoubt-edly, hampers their wide use.

The composition of cyanurates includes the

S

-triaz-ine ring formed as a result of trimerization of thecyanato groups. Therefore, before we consider cyanuricacid and its salts, we shall briefly discuss some ques-tions in the chemistry of cyanides that are common tomany classes of cyanogen anions.

The free hydrocyanic acid HCN gives two types ofderivatives [1] since it has two tautomeric forms:

The synthesis conditions specify the particular formthat enters into a reaction. The acid itself mainly con-sists of the nitrile form (

~99%

) with 1% of the isonitrileform as an admixture.

Cyanides or nitriles are the most studied derivatives ofthese two forms. The first name usually refers to the saltsof inorganic cations, while the latter name is applied to thederivatives with organic radicals. As to the derivatives ofthe isonitrile form, the best studied of them are the organicisonitriles, whereas the salts are only poorly studied.

The electronic structure of hydrocyanic acid

enables two types of reactions, namely, dissociationresulting in salt formation and addition reactions occur-ring at the triple bond

C

≡

N

. The reactions of the first

type are commonly known; therefore, we shall discussthe reactions of the second type.

In the presence of strong acids, hydrocyanic acidundergoes trimerization

to give a ring similar to the benzene ring [2]. Theobtained compound was called, in organic chemistry,symmetric 1,3,5-triazine or

S

-triazine. Thus, the pres-ence of a triple bond in the cyano group predeterminesits capability of polymerizing [3].

The cyano group is contained in different cyanateanions [4] that can also involve chalcogen atoms: cyan-ate (

OCN

–

), isocyanate (

NCO

–

), fulminate (

CNO

–

),thiocyanate (

SCN

–

), isothiocyanate (

NCS

–

), selenocy-anate (

SeCN

–

), and tellurocyanate (

TeCN

–

). The poly-merization of HOCN gives cyanuric acid, while that ofHSCN yields thiocyanuric acid.

The free cyanic acid HOCN has low stability. Likehydrocyanic acid, it readily polymerizes in an anhy-drous state to give a mixture of cyanuric acid andcyamelide at room temperature [5–7]. The ratio of thecomponents in the mixture greatly depends on the tem-perature. Thus, below

0°C

, cyanic acid spontaneouslytransforms into cyamelide, whereas above

150°ë

, onlycyanuric acid is formed. This can be explained by thefact that cyanic acid has two tautomeric forms [8, 9]:

.

H–C≡N H–N=C:→

nitrile form isonitrile form

H–C≡N: H–C=N: H+ :C≡N:[ ] ––+

:

3H–C≡N ,N

HCN

CH

N

HC

H–N=C=O H–O–C≡N 0°C 150°C N

CN

C

NC

O

OHHO

H

O C O C O

NH NH

cyamelide isocyanic acid cyanic acid cyanuric acid

Cyanuric Acid and Cyanurates

G. B. Seifer

Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 117907 Russia

Received April 17, 2001

Abstract

—A review of studies concerned with an interesting group of compounds of cyanuric acid, which isan intermediate between inorganic and organic compounds, is given. A first attempt is made to generalize andsystematize all the known compounds of this acid. The syntheses, IR studies, thermal decomposition, and themechanism of thermal conversion of the cyanuric acid salts are considered. This review may prove of interestfor the researchers working in different fields, chemical engineers, students, post-graduates, and teachers ofhigher schools.

302

RUSSIAN JOURNAL OF COORDINATION CHEMISTRY

Vol. 28

No. 5

2002

SEIFER

At low temperatures, the polymerization of cyanicacid occurs due to the cleavage of the double bondC=O. As the temperature is increased, this processoccurs through the rupture of the triple bond

C

≡

N

. Inthe latter case, a

S

-triazine ring is formed that is incor-porated in the composition of many cyanuric com-pounds of the

C

3

N

3

X

3

type (where X = OH, H, Hal, R,OR, SR, SH,

NH

2

, N

3

, CN,

NH–NH

2

). Thus, cyanuricacid

C

3

N

3

(OH)

3

(or

H

3

C

3

N

3

O

3

) appears to be the ances-tor of this class of compounds that can be treated as itsderivatives. For example, cyanuramide

C

3

N

3

(NH

2

)

3

(also called melamine in industry) is the triamide of cyan-uric acid, while cyanuric chloride

C

3

N

3

Cl

3

is its acidchloride. The complete hydrolysis of these compoundsalways gives cyanuric acid. At the same time, the deriva-tives of cyanuric acid are mainly produced not from cyan-uric acid but via the polymerization of the nitrile groupsdue to the cleavage of their triple bond

C

≡

N

.The cyanuric acid derivatives containing the

S

-triaz-ine ring (

C

3

N

3

) are considered to be promising com-pounds for the synthesis of complexes. In terms of theircapability of forming complex compounds, the cyanateanions can be arranged in the following order [10–15](atoms bonded to the central atom of the complex areunderlined):

SeCN

–

< SCN

–

< OCN

–

< H

2

O < NCO

–

<NCS

–

< NCSe

–

< CH

3

CN < NC

–

< NH

3

< RNC <

CNO

–

<CN

–

. The ligand in the cyanuric acid complex is bondedto the central atom through the nitrogen atom of the

S

-triazine ring. Therefore, one can suppose that, accord-ing to their field strength, the cyanuric acid derivativesand its anions will be arranged in this series to the rightof

H

2

O

but to the left of

NH

3

; i.e., they are supposed tobe moderate-field ligands.

The introduction of cyanuric cycles into complexcompounds seems to be a promising direction of inves-tigations, since the complexation can noticeably changethe ligand properties. Unfortunately, this questionremains open. Only one paper [16] is available todaythat is devoted to the complexing properties of herbi-cides of the S-triazine series.

There are several methods of preparation of the cya-nuric compounds

C

3

N

3

X

3

, but the most frequently usedtechnique is the polymerization of the XCN nitriles.Depending on the nature of the X atom at the cyanogroup, the polymerization reaction can occur eitherspontaneously or with heating or even with a catalyst.Cyanurhydride or the

S

-triazine

C

3

N

3

H

3

forms as aresult of the hydrocyanic acid polymerization catalyzedby hydrogen halides (HCl, HBr, HI). The polymeriza-tion of HCN in the presence of HCl occurs in solutionseven in the cold [17] to give sesquihalides with theempirical formula 2HCN · 3HHal [18].

It was established in [19–21] that sesquihalides con-tain a

S

-triazine ring that forms upon the removal ofHHal as follows:

2 C3N3H6Cl3[ ] 3HCl⋅2 C3N3H3[ ] 3HCl⋅ 2C3N3H3.

–3HCl –3HCl

The entropy changes and the heat effect of the HCNpolymerization are calculated in [22], while the mag-netic anisotropy and the charge delocalization in

S

-tri-azine are considered in [23]. The IR spectrum of thepolymerized HCN is given in [3] (

ν

, cm–1): 3450, 3370,3314, 3260, 3219, 3184 ν(NH2); 2222, 2172 ν(C≡N);1648, 1611 δ(NH2); 1624 ν(C=N); 1249 δ(NH2).

Cyanurcyanide or hexacyanogen C3N3(CN)3 formsduring the thermal decomposition of the substances(such as AgCN or Hg(CN)2) that proceeds with the evo-lution of large quantities of free cyanogen. In this case,the major portion of cyanogen rapidly polymerizes intobrown and thermally stable paracyanogen (CN)x, whilethe remaining portion removed as (CN)2 polymerizeson cooling into colorless monoclinic crystals of cyanur-cyanide that melt at 119°ë [24, 25]. The boiling pointof C3N3(CN)3 was found to be 262°C at 771 mmHg.This substance is isolated from benzene solution in theform of a solvate with two benzene molecules.

Cyanuric chloride or cyanuric acid trichlorideC3N3Cl3 [26–31] forms white monoclinic crystals witha pungent odor. Its vapors are very toxic and harmful tothe eyes and olfactory organs. Their maximum permis-sible concentration (MPC) in air is 0.1 mg/m3 [32]. Theboiling point of the compound is 190°ë at 720 mmHg;its density is 1.32 g/cm3. The authors of [30, 33]reported different melting points of C3N3Cl3. Today, theC3N3Cl3 crystals are believed to melt in the interval of146–146.5°C [34].

Cyanuric chloride dissolves poorly in water. How-ever, when its aqueous solution is allowed to stand or isheated, it undergoes hydrolysis to form cyanuric acid:

Thus, cyanuric chloride can be regarded as the oxy-chloride of this acid. On the contrary, cyanuric chloridedissolves readily in organic solvents (acetone, chloro-form, benzene). It crystallizes from benzene as thecrystal solvate C3N3Cl3 · 2C6H6.

This compound is one of the most important deriva-tives of S-triazine and is widely used in nucleophilicsubstitution reactions to produce a great variety of sub-stances containing cyanuric rings. The chlorine atomsin cyanuric chloride are very mobile and are replaced insuccession, which makes it possible to synthesizemono-, di-, or trisubstituted derivatives. However, thereplacement of the chlorine atoms by other atoms orgroups is gradually hampered such that the third chlo-rine atom is replaced with difficulty. Cyanuric chloridereacts with different nucleophiles: alcohols, phenols,naphthols [35, 36], ammonia, and organic amines [37].The products of the partial replacement of organicamines were used to obtain amidohydrides [37].

Cyanuric chloride that has lost one chlorine atomcan enter into the composition of polymers. Thus, theorganotin compound {Me2SnH(C3N3Cl2}3 has thestructure of a polymer, which was confirmed by X-raydiffraction analysis in [38].

C3N3Cl3 3H2O+ C3N3 OH( )3 3HCl.+=

RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 5 2002

CYANURIC ACID AND CYANURATES 303

The bromine derivative, which is analogous to cya-nuric chloride, is obtained by reacting HBr with cyano-gen bromide (BrCN) in benzene or via polymerizationof BrCN in the presence of small quantities of Br2 [39].The bromine atoms in C3N3Br3 are sufficiently mobile,and therefore, this compound can be used in many syn-theses of cyanuric acid derivatives.

The chlorine atoms in C3N3Cl3 can be replaced bythe azide group by carefully adding a cooled NaN3solution to cyanuric chloride [40] or by mixing the ace-tone solutions of these components. The cyanuric azideC3N3(N3)3 forms colorless crystals that melt at 94°C.This compound is very sensitive to impact and shaking.It detonates spontaneously or on heating. Themonoazide C3N3Cl2(N3) and diazide C3N3Cl(N3)2 areless sensitive to mechanical action and, thus, are morefrequently used in industry. The structure of cyanuricazide is considered in [40]. The N–N distance in theazide group of C3N3(N3)3 is determined in [41].

The replacement of the halogen atoms in C3N3Hal3by hydrazine and its derivatives was used to synthesizethe cyanuric derivatives C3N3(NH–NH2)3 andC3N3(NH–NHC6H5)3 [42]. This process occurs in stepsand, depending on the ratio of the initial reagents, cangive the products of complete or partial substitution ofthe halogen atoms. Cyanuric hydrazine can also be pro-duced from trimethyl cyanurate C3N3(CH3)3 [39].

Cyanuric hydrazine C3N3(NH–NH2)3 forms finewhite crystals (mp 287°C). When it was mixed withbenzaldehyde in the presence of HCl and shaken, thetribenzylidene derivative of cyanuric acid was formed.

The sodium salt of cyanurtricyanamide(C3N3)(CN2H)3 was synthesized in [43]. This com-pound contains the (C3N3)(CN2)3– anion, which evi-dently can be used as the binding unit in the productionof various polymers.

The reaction of C3N3Cl3 with HI gives an amor-phous brown compound (CNI)x [8] that decomposes onheating into paracyanogen (CN)x and I2. However, thecyanurate structure of (CNI)x is confirmed by the factthat when treated with hot water, it is hydrolyzed togive cyanuric acid. Therefore, this compound can beassigned the formula C3N3I3, the more so since one ofthe intermediate products formed during its synthesis isC3N3ClI2.

Similar to other cyanuric halides, cyanuric fluoridewas obtained only by the indirect method during thereaction of trifluoroacetonitrile F3C–CN with NF3 at514°ë [44]. The cyanuric trifluoride C3N3F3 forms inthe mixture with other compounds. At the same time,cyanuric fluoride can be synthesized by the reaction ofcyanogen chloride with NF3 at 500°C, by electrolysis ofan aqueous solution of NaCN with F2, or via distillationof cyanogen iodide ICN over AgF [8]. According to[36], C3N3F3 is usually obtained by reacting C3N3Cl3with SF4 or HF at –78°ë and further increasing the tem-perature of the mixture to 0°C. When C3N3F3 is heated

in vacuum, it dissociates to give cyanogen fluoride [45]:C3N3F3 = 3FCN.

Cyanuric chloride and dicyanuric fluoride werestudied by IR and Raman spectroscopies and by inelas-tic neutron scattering [46].

As was noted above, in the reaction of cyanuricchloride with ammonia, the chlorine atoms are replacedby the amido group to give cyanuramide, calledmelamine in industry [47–49]. Melamine can also beobtained by many other methods [49–56].

The melamine structure was discussed in [57, 58].Although it has four hypothetical isomers, only two ofthem are known in practice:

.

Melamine crystallizes as colorless monoclinicprisms that sublime on slow heating [55–60]. Accord-ing to the data of [61], it melts with decomposition at354°ë. At room temperature, melamine dissolvespoorly in water, while its solubility increases with tem-perature.

The optical properties of melamine are consideredin [62]. The characteristic bands in its IR spectrum lieat 3333, 3125, 1660, 1560, and 810 cm–1 [63].

The strong interaction of the π-electrons of the cyan-uric ring with the unshared electron pair of the aminenitrogen atom imparts basic properties to the melamineamino groups. The dissociation constants of melaminein aqueous solutions were found to be K1 = 1.26 × 10–9;K2 = 1.58 × 10–14, and K3 = 1 × 10–17 [63].

Although melamine is a weak base, it neverthelesscan form salts [53, 63–70]. However, it almost alwaysacts as a monoacidic base.

The yellow needles of melaminium picrate areformed when melamine reacts with the picric acid(NO2)3C6H2OH [55, 71]. This compound is poorly sol-uble in water and decomposes at 268°ë without melt-ing. The high thermal stability and the low solubility ofmelaminium picrate allow one to use it in the chemicalanalysis. For example, it is used for both qualitative andquantitative determination of melamine in industrialproduction [72]. The C3N3(NH3)3 · HOC6H2(NO2)3 ·2H2O crystals are dried at 100°ë and weighed.Melamine can be also determined by the titrimetricmethod [73].

With AgNO3, melamine forms the adduct AgNO3 ·C3N3(NH2)3 [63]. When it is heated in an aqueousammonia solution, C3N3(NH2)2NAg2 is obtained:

HN

CNH

C

NHC

NH

NHHNN

CN

C

NC

NH2

NH2H2N

isomelamine melamine

C3N3 NH2( )3 2AgNO3+

= C3N3 NH2( )2NAg2 2HNO3,+

304


SEIFER

where two hydrogen atoms of the amido group arereplaced by silver cations.

According to the data of [74–76], the thermaldecomposition of melamine proceeds in stages and isaccompanied by the detachment of ammonia and grad-ual linking of cyanuric rings through the imide bridges:

The melam [C3N3(NH2)2]2NH that forms during themelamine decomposition was first produced by theauthors of [49] by heating NH4CNS to 300°C. The NH2groups in melam are sufficiently mobile and can bereplaced by other atoms or groups. For example, theirreplacement by the halogen atoms gives derivatives ofthe following types:

.

The compound was syn-

thesized by the replacement of the amido groups inmelam by HS [41]. The synthesized compound retainstwo cyanuric rings linked by an imide bridge. Themelam structure was established in [76].

The melem [C3N3(NH2)]2(NH)2 is obtained whenmelam is heated for a long time. The melem heatingresults in its gradual decomposition and the formationof melon.

The melon (C3N3)2(NH)3 is a yellow powder insolu-ble in water or diluted acids. When melon is heated inan inert atmosphere, it is decomposed with the evolu-tion of ammonia,

and the formation of carbonic nitride (or cyanuricnitride) [77]. The melon structure was reported in [77]and was shown to include two cyanuric rings linked viaC–NH–C bonds.

The multistep hydrolysis occurs in an aqueous solu-tion of melamine containing an acid or alkali on heating[49, 77–79]:

Since cyanuric acid is produced in the hydrolysis ofmelamine, the latter can be regarded as its triamide,whereas ammeline is its diamide and ammelide is itsmonoamide.

Ammeline can be synthesized by the methodsdescribed in [57, 74, 80, 81]. It crystallizes as fine whiteneedles poorly soluble in water, alcohol, or ether but sol-uble with heating in mineral acids, NH4OH, or strongalkalis. When ammeline is boiled with diluted solutionsof HNO3 or KOH, it is also hydrolyzed to give cyanuricacid [82]. Like melamine, ammeline also has two tauto-meric forms, namely, isoammeline and ammeline:

.

Ammeline can form salts with strong acids andAgNO3 [8].

Ammelide or monoamide of cyanuric acid can beobtained by different methods [83–88]. It is a whitepowder that is moderately soluble in hot water butinsoluble in organic solvents. Like the above deriva-tives of cyanuric acid, it also has two tautomeric forms:

Similar to melamine and ammeline, it also formssalt-like products of addition with acids and bases [71].

The interest recently shown in melamine and itsderivatives is dictated by the fact that their reaction withformaldehyde yields rubbery products with the cyan-uric rings being linked through the bridges:

.

2C3N3 NH2( )3 C3N3( )2 NH2( )4NH

C3N3( )2 NH2( )2 NH( )2 C3N3( )2 NH( )3

2 C3N3( )N 3 CN( )2 N2.+

–NH3

–NH3 –NH3

800°C–NH3

melamine melam

melem melon

cyanuric nitride(carbonic nitride)

C3N3 NH C3N3

NH2

NH2

Hal

HalC3N3 NH C3N3

NH2

Hal

Hal

NH2

or

C3N3 NH C3N3

NH2

NH2

HS

HS

C3N3( )2 NH( )3 2C3N4 NH3,+=

C3N3( ) NH2( )3 C3N3( ) NH2( )2OH

C3N3( ) NH2( ) OH( )2 C3N3( ) OH( )3.

H2O

H2O H2O

cyanuric acid

melamine ammeline

ammelide

HN

CNH

C

NHC

NH

OHN

N

CN

C

NC

NH2

OHH2N

isoammeline ammeline

HN

CNH

C

NHC

NH

OON

CN

C

NC

NH2

OHHO

isoammelide ammelide

C

NC

N

CN

NH2H2N

NH2

C

NC

N

CN

C

NC

N

CN

NH NHCH2

NH NHCH2

NH NHCH2

+ 3CH2O2



Such products are used in various industrial fields.Moreover, the amido group of melamine and its deriv-atives can be replaced by other groups or atoms toafford different substances which also have wide appli-cation.

Thioammeline C3N3(NH2)2HS can be produced byboiling dicyandiamide or cyanamide with an acidifiedKCNS solution or alcoholic HCNS solution [89]. Itsstructure was determined in [90]. According to [8],dithioammeline is formed when thioammeline istreated with bromine water. It is a white crystallinepowder soluble in alkalis. Its structure is supposed tocontain a bridge of sulfur atoms. The synthesis of thio-ammelide was reported in [8]. The sulfur-containingcompound C3N3(N2S2)3 was obtained in [91].

The cyanuric acid H3C3N3O3 was synthesized forthe first time at the end of the XVIII century by heatingurea until ammonia ceased to evolve:

However, the composition of the product formedwas established later [93] and, since that time, cyanuricacid has been synthesized in many works [94–104].The salts of cyanuric acid are formed also in the courseof polymerization of metal cyanates in an alkalinemedium [105].

Cyanuric acid forms white crystals that precipitatefrom a solution with two hydration water molecules. Itis stable and dissolves in mineral acids without decom-position, but when heated with strong concentratedacids, it slowly decomposes with the evolution of NH3and CO2. Cyanuric acid is poorly soluble in water. At25°ë, its solubility is ~2 × 10–2 mol/l [106]. It is alsopoorly soluble in alcohol but dissolves in cold concen-trated sulfuric acid without decomposition. Cyanuricacid is not poisonous and is odorless. On heating to400°ë, it transforms into cyanic acid (HNCO) withoutmelting [107, 108]. The density of cyanuric acid (d0 =1.768 g/cm3) was determined in [98]. It is a weak acid(K1 = 1 × 10–7, K2 = 5 × 10–12, and K3 = 3 × 10–15) and itstwo first dissociation constants are close to the respectiveconstants of H2CO3 (K1 = 4 × 10–7 and K2 = 5 × 10–11),while the third dissociation constant slightly exceedsthe dissociation constant of water (K = 1 × 10–16).

Nevertheless, cyanuric acid gives three types ofsalts: monosubstituted MIH2C3N3O3, disubstituted

HC3N3O3, and trisubstituted C3N3O3. However,it mainly forms mono- and disubstituted salts. The thirdhydrogen atom in aqueous solutions of this acid isreplaced with difficulty and requires a considerableexcess of a strong alkali.

The fact that cyanuric acid gives two types of saltswas first established in [109]. When Hg(CH3COO)2reacts with a free cyanuric acid or with sodium cyanu-rate, it gives products that have different properties dueto the different types of the mercury cation addition to

3CO NH2( )2 H3C3N3O3 3NH3.+=

M2I M3

I

the cyanurate anion. It was further shown that cyanuricacid exhibits the keto-enol tautomerism [5, 35, 110, 111]:

.

Free cyanuric acid exists in the crystals as isocyan-uric acid [112]. It contains strong NH···O hydrogenbonds. The interatomic distances and the electron den-sity distribution in the H3C3N3O3 molecule are esti-mated in [113–116]. The H3C3N3O3 crystals are mono-clinic: a = 7.749 Å; b = 6.736 Å; c = 11.912 Å, β =130.69°; Z = 4; space group C2/n. The molecules in thecrystal are arranged in parallel layers [109]. The C–Ndistance is 1.372 Å, the C–O distance is 1.220 Å, andthe NCN and CNC angles are 115.3° and 124.7°,respectively. Within the limits of one layer, the mole-cules are linked through a NH···O hydrogen bond 2.778and 2.798 Å in length.

The oxygen atoms in the C–O bonds of the isocyan-uric acid have lone electron pairs (located at an angle of120° to the ring plane). The electron density maxima inthe direction of the π-components of the C–N, C–O,and N–H bonds are equal to 0.40, 0.24, and 0.25 e/Å3,respectively. These charges were further verified in[115], and it was found that the electron density peaksfor the C–O and N–H bonds on the theoretical crosssections are 0.1 e/Å3 lower than on the experimentalcross sections, while for the C–N bonds, this value is0.2–0.3 e/Å3 and, in the region of the lone electronpairs, these peaks are 0.1–0.2 e/Å3 higher.

The molecular refraction of the isocyanuric cyclewas studied using organoelement allylisocyanurates in[117]. The experimental values were found to be lowerthan the theoretical values calculated from the additivescheme using the tabular bond refractions. The reasonfor this lies in the mutual influence of the allyl groupsand the cyanurate cycle. The correction of the isocyan-urate cycle for refraction of compounds of this classwas assumed to be –1.20 cm3 (2σ = 0.15 cm3).

The vibrational spectra of cyanuric acid have beenconsidered in a number of papers [118–121]. The vibra-tional spectra of cyanuric, monothiocyanuric, andtrithiocyanuric acids were calculated in [103, 119,120]. The assignment of the absorption bands observedin the IR spectra was performed in [120]. As shown in[121–126], the frequencies 1535–1560 cm–1 and 784–810 cm–1 are characteristic of the S-triazine ring withthe benzene structure. The cyanuric acid spectra alsoexhibit bands due to the stretching vibrations of the car-bonyl groups ν(C=O) in the range of 1695–1720 cm–1

and of the imido group of the ring ν(NH) in the range of2828–2907 cm–1.

HN

CNH

C

NHC

O

OON

CN

C

NC

OH

OHHO

→

isocyanuric acid cyanuric acid

306


SEIFER

The consecutive replacement of the hydrogen atomsin cyanuric acid by the amido group yields ammelide,ammeline, and melamine, respectively. Melamine canform salts with different acids [127], including cyanuricacid proper. The crystal structure of the saltC3N3(NH2)3 · H3C3N3O3 · 3HCl is considered in [128].The salts of this type are formed when a proton is fullytransferred from an acid to the amide nitrogen atom.

The reaction of the alkali-metal cyanurates withbromine was used to synthesize the mono- and dibro-mocyanurates of potassium, sodium, and lithium.When the excess bromine was further reacted with thelithium salts, the dibromocyanuric acid HC3N3O3Br2formed as extended rectangular plates [129]. The IRspectral studies revealed N–Br bonds in theHC3N3O3Br2 molecule. This acid is soluble in acetone,methyl ethyl ketone, dimethylformamide, and acetoni-trile at room temperature, while it is poorly soluble inwater. In an atmosphere of dry nitrogen, HC3N3O3Br2decomposes already at 307–309°ë. When treated withfree chlorine at 150°C, it gives dichlorocyanuric acidHC3N3O3Cl2. The reaction of its potassium salt with N :Cl2 [130] yielded a compound that was assigned the for-mula of the binary salt [Ni(H2O)6](C3N3O3Cl2)2 ·2KC3N3O3Cl2. The other derivatives of cyanuricacid can be synthesized by chlorinating the respectivesalts [131].

The interaction of cyanuric acid with the alkali-metal halides was studied using samples produced bypressing under 38 t/cm2 [132]. In the IR spectra of the1 : 1 samples, the characteristic bands of cyanuric acid(see above) shifted by ~25 cm–1 toward the long-waveregion due to the complex formation. The tendency toform complexes with the alkali-metal halides increasesin the series Cl < Br < I. However, the compounds thusformed are very unstable and decompose when treatedwith water into the starting reagents.

Cyanuric acid is qualitatively determined in solu-tions by adding an ammonia solution of copper sulfateto give copper cyanurate of an amethyst color [133].

The microcrystalloscopic analysis for the C3N3

cyanurate anion is performed by heating the aqueoussolution of the acid with NaOH on a slide [134]. As themixture becomes more and more concentrated, fineneedles of sodium cyanurate precipitate that can beidentified using a microscope. The method suggested in[135] for the quantitative determination of cyanuricacid is based on the precipitation of a poorly solublemelaminium cyanurate.

The organic derivatives of cyanuric acid can be syn-thesized by a number of the methods described in [136–145], and, depending on the starting reagent, one canobtain both the cyanuric and isocyanuric acid deriva-tives. These compounds differ not only in the structureof the S-triazine ring but also in their physical proper-ties.

O33–

The reaction between cyanuric acid and organicbases was used to obtain the adducts, which were thenstudied by IR spectroscopy [146].

The synthesis of cyanurtriurea (CONH2)3C3N3O3can be performed at 200°C according to the followingreactions [147]:

The compound obtained is an amorphous powderweakly soluble in water.

The structure of the trimethylcyanurate crystals(CH3)3C3N3O3 was studied in [148]. The crystals areorthorhombic: a = 8.474 Å, b = 6.719 Å, c = 14.409 Å,space group Pnma. The structure of this compound isplanar due to the conjugation of the π-electrons of theS-triazine ring and the lone electron pairs of the oxygenatom. All three methoxy groups are rotated in the samedirection such that the molecule has 3/m symmetry. Thelength of the N–C bonds is 1.311–1.344 Å, while theCNC and NCN angles are 113.3°C and 126.8°, respec-tively. The molecules in the crystal are arranged in lay-ers perpendicular to the b axis. The distance betweenthe layers is 3.36 Å.

The products of addition of the cyanogen bromide(CNBr) to the triethylcyanurate (C2H5)3C3N3O3 ·2CNBr are also described in the literature [5]. Suchcomplexes are formed due to the addition of a ligandmolecule to the cyanurate triazine ring.

It has already been noted that at room temperature,free cyanuric acid occurs in the ketone form. Therefore,to produce its salts, this form should be first convertedto the enol form. This is accomplished by treating theacid with excess alkali. The unreacted alkali is thenremoved by extraction with an alcohol. The cyanuratesof the most active alkali metals produced in this waycan be further used as starting reagents for the synthesisof cyanurates of other cations by the ionic exchangemethod.

The sodium salt Na2HC3N3O3 · H2O precipitates inthe form of white needles when cyanuric acid is treatedwith excess NaOH [148]. The monosubstituted potas-sium salt KH2C3N3O3 is formed in the reaction of aceticacid with concentrated potassium cyanate [6]. Whenthis salt is dissolved in concentrated KOH and theobtained product is salted out with an alcohol, whiteneedles of the disubstituted K2HC3N3O3 are precipi-tated that are hydrolyzed in water to give the sameKH2C3N3O3. The synthesis of the alkali-metal cyanu-rates is also described in [149, 150].

The TlOH hydrate is also a strong base and thus cangive both the mono- and disubstituted derivatives of

cyanuric acid. The strength of the Tl+ field (Z/ =

0.45) is close to that of the Rb+ cation (Z/ = 0.45);

3CO NH2( )2 H3C3N3O3 3NH3,+=

H3C3N3O3 3CO NH2( )2+

= CONH2( )3C3N3O3 3NH3.+

rTl+2

rRb+2



however, the attempt made to obtain its trisubstitutedcyanurate in solution failed.

The ammonium salt (NH4)H2C3N3O3 crystallizes asfluorescent prisms [6]. It has a low stability and decom-poses already at 130°ë with the evolution of ammoniaand the formation of pure cyanuric acid.

All cyanurates with monovalent cations are finelycrystalline powders that were studied by X-ray diffrac-tion analysis [151]. The comparison of the X-ray dif-fraction patterns of the pairs NaH2C3N3O3–K3C3N3O3,K2HC3N3O3–Rb2HC3N3O3, Li2HC3N3O3–LiH2C3N3O3and K2HC3N3O3–KH2C3N3O5 for the average reflectionangles reveals a noticeable similarity. This similarity

suggests that the atoms in the lattice of the alkali-metalcyanurates have close motifs of their arrangement.

The structure of KH2C3N3O3 · H2O is studied in[152]. The crystals of this salt are monoclinic: a =11.044 Å; b = 16.390 Å; c = 7.199 Å, and β = 103.80°,Z = 8; space group Cm. The structure consists of K+ cat-

ions, cyanuric acid anions H2C3N3 , and crystalliza-tion water molecules. The anionic layers alternate withinorganic hydrate layers of the water molecules and K+

ions.The solubilities of the alkali-metal cyanurates in

water at 20°C (mol/l) are as follows:

Thus, as the hydrogen atoms of cyanuric acid areconsecutively replaced by the alkali-metal cations, thesolubilities of the obtained salts increase.

The salt AgH2C3N3O3 was first synthesized by add-ing a silver nitrate solution to a H3C3N3O3 solutionacidified with acetic acid [70]. With an excess of Ag+,trisubstituted salt Ag3C3N3O3 was obtained that couldbe dried at 105°ë without decomposition. Its boilingwith KOH yielded the mixed salt KAg2C3N3O3.

When studying interactions in the AgNO3–NaxH3 − xC3N3O3–H2O systems, the following silver cya-nurate derivatives were produced [153]: AgH2C3N3O3 ·2H2O, NaAgHC3N3O3 · H2O, Ag2HC3N3O3 · H2O,NaAg2C3N3O3 · 3H2O, Ag3C3N3O3 · H2O,Na[Ag(H2C3N3O3)2] · H2O.

It can be seen from the above list that the 18-electronsilver cation forms, in addition to the simple cyan-urates, two mixed salts with alkali-metal cations andthe complex compound [154]. The formation of the lat-ter complex suggests that in the salts with multielectroncentral atoms, the cyanuric acid anion can really act asthe ligand.

The magnesium derivative of cyanuric acidMg(H2C3N3O3)2 · 14H2O and its calcium saltCa(H2C3N3O3)2 · 8H2O are discussed in [148]. Thesecompounds are moderately soluble in water. The cal-cium salt crystals are triclinic. The disubstituted saltCaHC3N3O3 · 3H2O was also isolated.

The barium compounds Ba(H2C3N3O3)2 · 2H2Owere synthesized by adding barium hydroxide to a hotsolution of cyanuric acid. Ba(OH)2 was added until theinitially formed precipitate dissolved. When this solu-tion was cooled, prismatic white crystals precipitated[155]. The finely crystalline BaHC3N3O3 · H2O salt wassynthesized by reacting a hot cyanuric acid with excessBa(OH)2. The radium salt is similar to barium salt [156].

The alkali-metal salts were synthesized in [157] byreacting hot saturated solutions of the hydroxides of

these metals with cyanuric acid. In all cases, the disub-stituted salts EHC3N3O3 · H2O (where E = Ca2+, Sr2+,Ba2+) were isolated. The trisubstituted derivatives ofthese cations were obtained by mixing equivalentamounts of the reagents and further evaporation of thesolution to dryness or by thermal decomposition of thedisubstituted salts.

The magnesium cyanurates were prepared by twomethods [158], namely, by heating magnesium hydrox-ide with a cyanuric acid solution or by precipitatingfrom hot solutions of magnesium salts with titratedsolutions of NaH2C3N3O3 and Na3C3N3O3. In the firstcase, the normal salt Mg3(C3N3O3)2 · 8H2O precipitatesfrom the hot solution, while on cooling, the disubsti-tuted salt MgHC3N3O3 · 5H2O forms.

It was noted in [5, 149] that the PbHC3N3O3 · 3H2Osalt can be produced from basic lead acetate, whereasits boiling with excess AgNO3 results in the binary saltAg4Pb(C3N3O3)2 · 2H2O. The studies of the Pb(NO3)2–NaxH3 – xC3N3O3–H2O systems performed in [159]revealed that in solutions, lead, like silver, can give theprecipitate of poorly soluble trisubstituted cyanurate.This precipitate is formed in all cases when the solutioncontains excess lead ions. With excess cyanurate ions,hydrolysis takes place that yields OH– ions and thusdrastically increases the pH of the mixture. It is knownfrom [160] that at pH greater than 7.8, lead hydroxideis formed and, therefore, the OH– ions gradually pene-trate into the Pb3(C3N3O3)2 precipitate and the basiccyanurate (PbOH)2HC3N3O3 is obtained. As for the di-substituted cyanurate PbHC3N3O3, this salt is only pre-cipitated from weakly acidic solutions and, thus, can besynthesized only from monosubstituted alkali-metalcyanurates.

Copper cyanurates have been well studied by theauthors of [155, 161–168]. A number of Cu(II) saltswith specific color were synthesized: the blue-coloredmixed salt LiCuC3N3O3 · 2H2O and the red-violet com-plexes Li[Cu(HC3N3O3)(H2C3N3O3)] · 2H2O,

O3–

Compound CsH2C3N3O3 RbH2C3N3O3 KH2C3N3O3 NaH2C3N3O3 Na2HC3N3O3 Na3C3N3O3

Solubility 2 × 10–2 2 × 10–2 1 × 10–1 5 × 10–2 1.5 × 10–1 5 × 10–1

308


SEIFER

Na2[Cu(H2C3N3O3)4] · 6H2O, and K2[Cu(H2C3N3O3)4] ·6H2O. In the course of dehydration, their colors changeto light green (LiCuC3N3O3), blue(Na2[Cu(H2C3N3O3)4]), and violet(K2[Cu(H2C3N3O3)4]). The monosubstituted saltCu(H2C3N3O3)2 and polymeric disubstituted(CuHC3N3O3)n were obtained in [169]. Moreover, cop-per forms typical mixed cyanurate complexes withammonia and pyridine [164, 166, 170–175]. Accordingto [169], their structure can be described as follows:two cyanuric rings in the lactam form are linkedthrough the copper atom that also coordinates twoammonia molecules.

The structure of the cooper cyanurate complexes isconsidered in [176]. Such complexes are stable anddecompose only in concentrated acids or on boilingwith alkalis. The IR spectroscopic study revealed that inthe copper cyanurate complexes, the nitrogen atoms aredonors, the cyanuric acid anions having the lactam form.The crystal structure of these salts is studied in [166].

The trisubstituted copper cyanurate was obtained byprecipitating from a hot solution of disubstituted potas-sium cyanurate with a copper acetate solution at a ratio

of C3N3 : Cu2+ equal to 2. This salt is formed alsofrom the H3C3N3O3 and Cu(CH3COO)2 solutions takenat the ratio of 3 : 1. When such mixtures are evaporated,the monosubstituted cyanurate Cu(H2C3N3O3)2 · 2H2Ois first crystallized. After it is isolated and the evapora-tion is further continued, the blue crystals ofCu3(C3N3O3)2 · 5H2O precipitate from the solution.

The evaporation of the aqueous solutions of themanganese salts with free cyanuric acid taken at ratiosof 1 : 2 and 1 : 4 yields light pink crystals of MnX2 ·2H3C3N3O3 · yH2O, where X = Cl–, NCS–, CH3COO–,

and 1/2S [177–179]. However, it was noted thatsuch syntheses with alkali-metal cyanurates gave darkprecipitates containing manganese ions in the higheroxidation states. The IR studies of these complexesshowed that their acido groups are arranged in the innersphere and are directly bonded to the manganese atoms.

When the monosubstituted sodium or potassiumcyanurates were used for the precipitation from thesalts of Mn2+, Co2+, and Ni2+ (E2+) at 60°ë, the cyanu-rates of the respective cations were obtained [180, 181].

Mixing of solutions at ratios of H2C3N3 : E2+ equalto 1 : 1, 2 : 1, and 4 : 1 resulted in the formation of thebasic salt (EOH)H2C3N3O3 · xH2O, which was filteredoff in the hot state. On cooling of the solution, the crys-tals of the monosubstituted cyanurate E(H2C3N3O3)2slowly precipitated and, finally, the å2[E(H2C3N3O3)4]crystals salted out from the remaining liquid with M =Na+, K+. This procedure was used to obtain the lightpink crystals of Mn(H2C3N3O3)2 · 4H2O andK2[Mn(H2C3N3O3)4] · 4H2O, the pink crystals ofCo(H2C3N3O3)2 · 6H2O and K2[Co(H2C3N3O3)4] ·

O33–

O42–

O3–

4H2O, the (NiOH)H2C3N3O3 · 2H2O andNi(H2C3N3O3)2 · 4H2O crystals with green color, andthe light blue crystals of Na2[Ni(H2C3N3O3)4] · 6H2O.Their IR spectroscopic studies showed that in all thehydrated compounds, cation E has an almost octahedralcoordination, with some water molecules entering theinner coordination sphere of the metal. The cyanurateanion is coordinated through the nitrogen atom.

The author of [169] synthesized the Co2+, Ni2+, andZn2+ cyanurates via the following reaction:

The synthesis of the Cu2+ and Co2+ compounds withthe organosubstituted derivatives of cyanuric acid isdescribed in [168] and is performed according to thereaction

where H4L is 1,3-diallyl-5,2-hydroxy-3-phenoxypro-pyl isocyanurate.

The M2[E(H2C3N3O3)4] · xH2O complexes with E =Cu2+, Mn2+, Ni2+, Co2+, Zn2+, or Cd2+ and M = Na+ or K+

were described in [161–166, 169, 180–182]. Anions

H2C3N3 are coordinated through the nitrogen atom[165]; in their complexing properties, they are interme-diate between ammonia and water, being, however,closer to water.

The structure of Co(H2C3N3O3)2 · 7H2O is discussedin [183]. Its crystals are monoclinic: a = 14.028 Å; b =6.614 Å; c = 17.067 Å, β = 98.78°, space group P21/n. Thestructure consists of the complex cations

[Co(H2C3N3O3)(H2O)5]+, anions H2C3N3 , and crys-tallization water molecules. The cobalt atoms coordi-nate the nitrogen atom of only one H2C3N3O3 group andthe oxygen atoms of five water molecules. The second

H2C3N3 anion is in the outer sphere of the complex.

The reaction of the heavy-metal acetates withK2HC3N3O3 first yields disubstituted cyanurates withthe general formula EHC3N3O3 · xH2O. With excess

K2HC3N3O3 (HC3N3 : E2+ = 2 : 1), mixed com-plexes K[E(HC3N3O3)(H2C3N3O3)] · 2H2O are formedthat contain two different cyanurate anions. The com-position of these derivatives is determined by thehydrolysis of the K2HC3N3O3 excess occurring in the

solutions. The obtained H2C3N3 ions react with thedisubstituted salt and produce the mixed complex. It isnoteworthy that one of the hydrogen atoms in the

H2C3N3 anion of these complexes is sufficientlymobile and, under specific conditions, particularly,when treated with an excess heavy-metal salt, can bereplaced to give the salt K2E[E(HC3N3O3)2]2 · xH2O.

The studies of the interaction in the system AlCl3–Na3C3N3O3–H2O [184] revealed the formation of sev-

2NaH2C3N3O3 ECl2+ E H2C3N3O3( )2 2NaCl.+=

H4L ECl2+ E H3L( )2 2HCl,+=

O3–

O3–

O3–

O32–

O3–

O3–



eral white poorly soluble compounds, with only some

of them simultaneously containing Al3+ and C3N3 .For instance, with a sufficient sodium cyanurate excess,the soluble aluminate NaAlO2 is formed instead of thealuminum cyanurates. The salt that formally has theformula of the trisubstituted aluminum cyanurateAlC3N3O3 · 3H2O only precipitates in a narrow range of

the reagent ratios C3N3 : Al3+, 0.75 < n ≤ 1.25. Thevalue of n = 1 remains constant up to n = 1.5 in the solu-tion, but the composition of the solid phase changes inthis case due to the penetration of the Na+ ions into thepreviously formed precipitate. Further studies of thesolid phases showed that the AlC3N3O3 · 3H2O phase isin fact the complex acid H[AlO(HC3N3O3)] · 2H2Owhose hydrogen can be replaced by sodium or potas-sium ions to form the M[AlO(HC3N3O3)] · H2O salts(M = Na+, K+).

The similar system ScCl3–Na3C3N3O3–H2O [185]exhibits the same interaction, the only difference beingthat with the sodium cyanurate excess, no soluble scan-date is formed and, even in the narrower range of theratios 0.9 < n ≤ 1.1, the ScO[ScO(HC3N3O3)]· 2H2Oprecipitate is produced. In the range of 1.1 < n ≤ 1.5,this precipitate absorbs the Na+ ions and transformsinto the salt Na[ScO(HC3N3O3)] · 2H2O. Thus, in thecase of the Al3+ and Sc3+ cations forming the ampho-teric hydroxides, the interaction with the alkali-metalcyanurates gives only oxy complexes containing cya-nurate ions.

A similar sodium salt was produced from InCl3 and

Na3C3N3O3 taken at a ratio of C3N3 : In3+ equal to1.5. The Na[InO[HC3N3O3)] · 2H2O salt forms a whiteweakly soluble powder similar in its properties to theAl3+ and Sc3+ salts. The formation of such complexescan be explained by the fact that, as a result of theintense hydrolysis occurring in the aqueous solutions ofthe trisubstituted alkali-metal cyanurates, both OH– and

H2C3N3 are present in the solution. These anionsgive a weakly soluble compound E(OH)2H2C3N3O3 ·H2O with three-charge cations; one of the hydrogen

atoms in H2C3N3 anion is mobile and can bereplaced by the alkali metal. The migration of this atomto the outer sphere of the complex is accompanied bysimultaneous rearrangement of the hydroxy salt intothe oxo salt, as a result of which the compound turnsinto the complex acid H[EO(HC3N3O3)] · 2H2O.

In the ECl3–Na3C3N3O3–H2O systems with E = Y3+,La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+,Ho3+, Tm3+, Yb3+, Lu3+, only one cyanurate of theabove-mentioned elements is formed, i.e., EC3N3O3 ·H2O. At 1.5 < n ≤ 2.0, the poorly soluble monosubsti-tuted sodium cyanurate, which forms due to the hydrol-ysis of the starting reagent, penetrates into the precipi-tation.

O33–

O33–

O33–

O3–

O3–

The monosubstituted yttrium cyanurate wasobtained in [186] by concentrating a solution contain-ing yttrium acetate and monosubstituted potassiumcyanurate until crystallization. The obtained crystalshave the formula Y(H2C3N3O3)3 · 6H2O; i.e., the com-pound thus formed is monosubstituted yttrium cyan-urate.

The Fe3+, In3+, and Bi3+ cyanurates were prepared byheating stoichiometric amounts of the respectivewashed hydroxides with cyanuric acid [187]. The reac-tions that occur in this case can be written as follows:

By analogy with the compounds described above,the isolated compounds can be regarded as complexacids: H[FeO(HC3N3O3)] · 2H2O, H[InO(HC3N3O3)] ·2H2O, and H[BiO(HC3N3O3)] · 5H2O. The mobility oftheir hydrogen atoms is confirmed by the possibility oftheir replacement by alkali-metal ions to give salts.

Out of the tetravalent cation cyanurates, only the zir-conyl derivative was obtained [188]. It was synthesizedfrom zirconyl hydroxide and cyanuric acid. The synthe-sis was carried out in an acetic acid medium. This acidwas removed, first, through evaporation on a water bathand, then, through heating to 125°C in a thermostat. Theresidue formed was the white oxo salt ZrO(HC3N3O3) ·4H2O, which is disubstituted zirconyl cyanurate.

Given in Table 1 are the types of inorganic deriva-tives of cyanuric acid. It can be seen that cyanuric acidforms a sufficiently large number of salts. Their varietylies within the limits known for the other acids, the onlyexception being the M2E[E(HC3N3O3)2]2 · 6H2O salts.On the one hand, these compounds contain the N: Ebond, which is confirmed by the bands of the stretchingvibrations of these bonds in the range of 500–520 cm–1

in their IR spectra. At the same time, when treated withhot water for a long period of time, these compoundsdecompose into two simple salts, M2HC3N3O3 andEHC3N3O3, which makes these compounds similar tothe binary salts such as alums or schoenites. Com-pounds with a weakly stable coordination sphere areknown to be referred to as binary salts [189]; therefore,it would be more correct to consider theM2E[E(HC3N3O3)2]2 · 6H2O compounds to be thebinary salts M2HC3N3O3 · 3EHC3N3O3 · 6H2O.

A number of metal cyanurates have been character-ized by IR spectroscopy [190]. The authors synthesizedthe salts by evaporating mixtures of H3C3N3O3 withmetal hydroxides taken in a 1.5-fold excess at 100°C.Neither the chemical analysis of the obtained com-pounds nor the assignment of the observed IR absorp-tion bands was performed. Therefore, the authors of[190] could only conclude that all salts of cyanuric acidhad ionic bonds.

E OH( )3 EO OH( ) H2O;+=

EO OH( ) H3C3N3O3+

= EO H2C3N3O3( ) H2O.+

310


SEIFER

The IR spectra of sodium cyanurates are discussedin Table 2 in [191]. The authors studied how the vibra-tion frequencies of the C=O bond changed with theextent of replacement of the hydrogen atoms in cyan-uric acid by the sodium cation. The ν(C=O) frequenciesfully disappear from the spectrum of the trisubstitutedsalt Na3C3N3O3 when the anion ring fully rearrangesinto a “benzene” ring. All the salt samples were dried ina thermostat at 125°ë. However, no complete dehydra-tion was achieved under these conditions, since cyan-urates contain not only crystallization but also zeolitewater. The composition of the compounds under studywas established by chemical analysis. The frequencyassignment was performed using data from [192–197].

It was noted in [182] that the IR spectra of the tran-sition-metal cyanurate complexes contain a band at505–510 cm–1. As seen from Table 2, this band is absentfrom the spectra of the simple salts, which allows oneto assign it to the stretching vibration of the N: En+

coordination bond. This assignment agrees with thedata of [163, 181], which also suggest that the cyanu-rate groups are coordinated through the nitrogen atom.

As far as the bands from the stretching vibrations ofthe cyano groups ν(C≡N) in the IR spectra of the cyan-uric acid salts (Table 2) are concerned, their intensityincreases with an increase in the polarizing action of the

cations from Cd2+(Z/ = 1.88) to Fe3+ (Z/ =

6.67). This gives evidence of the cation having an

rCd2+2 r

Fe3+2

increasing effect on the S-triazine ring of the anion.According to the data of [188], the band ν(C≡N) in theIR spectra of cyanurates should be assigned to a strongpolarization of the cyanurate anion by cations. As fol-lows from [193], the benzene-type ring has two intenseabsorption bands corresponding to the vibrations ν(C–N) + ν(C=N) of the conjugated systems. In the case ofthe cyanuric ring, the absorption bands correspondingto these bonds are at 1450–1500 and 1530–1600 cm–1,respectively [3, 198]. The band ν(C≡N) that appears inthe spectra of the salts is likely due to the strong distor-tion of the cyanuric ring as a result of the polarizingeffect of the cation:

As can be seen from the above scheme, the electronsare drawn off toward the highly charged cation which isfollowed by the electron density redistribution in thecyanuric ring and appears as the respective change inthe frequency of the stretching vibrations of the cyan-uric ring.

Table 3 gives for comparison the frequencies ofstretching vibrations of separate bonds in the cyan-urates of the slightly and highly polarizing cations[126, 199, 200]. One can see that in the spectra of the

C

NC

N

CN

KO OK

OK

C

NC

N

CNH

O

O

OZrO

Table 1. Inorganic derivatives of cyanuric acid

Compound Formula (examples) Cations

Monosubstituted salt EIH2C3N3O3 · xH2O* Li+, Na+, K+, Rb+, Cs+, Tl+, Ag+, Mg2+, Ca2+, Sr2+, Ba2+, Co2+, Ni2+, Y3+

Disubstituted salt HC3N3O3 · xH2O Li+, Na+, K+, Tl+, Rb+, Ag+, Mg2+, Ca2+, Sr2+, Ba2+, Pb2+, Ni2+, Co2+, Mn2+, Zn2+, Cd2+

Trisubstituted saltC3N3O3 · xH2O

Na+, K+, Ag+, Mg2+, Ca2+, Sr2+, Ba2+, Pb2+, Cu2+, Ln3+, Y3+

Mixed salt MEHC3N3O3 · xH2O M = Na+, K+; E = Ag+

ME2C3N3O3 · 2H2O M = Na+, K+; E = Ag+

Basic salt (EOH)H2C3N3O3 · 2H2O Pb2+, Co2+, Ni2+, Mn2+, Zn2+, Cd2+, Cu2+

(EOH)2HC3N3O3 · 2H2O Pb2+, Co2+, Ni2+, Zn2+

Oxo salt H[EO(HC3N3O3)] · xH2O Fe3+, In3+, Bi3+

M[EO(HC3N3O3)] · 2H2O M = Na+, K+; E = Al3+, Sc3+, In3+

EO(HC3N3O3)] · 4H2O Zr4+

Monosubstituted complex salts M[E(H2C3N3O3)2] · H2O M = Na+, K+; E = Ag+

M2[E(H2C3N3O3)4] · 6H2O M = Na+, K+, Cs+

E = Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+

Mixed complex salts M[E(HC3N3O3)(H2C3N3O3)] · 2H2O M = Li+, Na+, K+

E = Cu2+, Mn2+, Ni2+, Co2+

Disubstituted complex salts (binary)

M2E[E(HC3N3O3)2]2 · 6H2O or M = K+

M2HC3N3O3 · 3EHC3N3O3 · 6H2O E = Ni2+, Co2+, Mn2+

* Hereinafter, EI is the metal equivalent, E is the heavy metal, M is the alkali metal.

E2I

E3I



highly polarizing cations, the band in the range 1590–1600 cm–1 corresponding to the stretching vibrationsν(C=N) of the ring either disappears or becomes weak.Simultaneously, two new bands appear, namely, theband at 2125–2130 cm–1 that lies in the range of vibra-tions of ν(C≡N) of the cyanate anion and the bandν(C=O) at 1780 cm–1, whose position suggests somestrengthening of the bond in the carbonyl group. Thebands ν(NH) are either absent from the spectra of thesalts with highly polarizing cations or appear as aninflection. The bond in the oxo group becomes alsoweaker, apparently, due to the strengthening of bondingbetween carbon and nitrogen atoms in the neighboringcyano group.

The above data indicate that as the polarizing actionof the cation increases, the separate bonds in the S-tri-azine ring become weaker. Thus, an increase in thepolarizing action of cations produces the same effect onthe S-triazine ring as the heating process and causes a

one-sided deformation, thus facilitating salt decompo-sition upon heating. It also becomes clear that the ther-mal decomposition on the mono- and disubstituted cyan-urates should occur at lower temperatures than that ofthe trisubstituted salts because the symmetry of theanion S-triazine ring in the latter salts does not changewith increasing the strength of the cation field.

The studies on the thermal stability of cyanurateswere undertaken in the second half of the XX centuryafter it was reported in [136] that these salts firmly holdtheir crystallization water and that some of them trans-form into cyanates during decomposition [136]. Thethermolysis of cyanurates was studied in [201]. It wasfound that when heated, the mono- and disubstitutedsalts of Na+, Ca2+, Co2+, Zn2+, Mn2+, and Pb2+ subse-quently transform into trisubstituted salts:

2EIH2C3N3O3 E2I HC3N3O3 H3C3N3O3,+=

Table 2. Vibration frequencies (cm–1) in the IR spectra of the metal cyanurates. The field strength of the given ion (Z/r2)is given in parentheses

Monosubstituted Disubstituted Trisubstituted

AssignmentCs+ Li+ BiO+ FeO+ Rb+ Mn2+ Zn2+ ZrO2+ K+ Ca2+ Pr3+ Mg2+

(0.37) (1.64) (2.08) (6.67) (0.45) (2.44) (2.90) (5.26) (0.56) (1.64) (2.22) (3.28)

420–480 410–480 412–470 412–475 455–480 470–480 δ(NCO)

550–570 535–590 554–595 551–560 552–595 550–580 ν(MO), δ(C=O)

634–695 605–640 613–695 600–610 600–684 δ(CNC), δ(NCO)

710–790 711–795 710–795 720–790 708–794 770–780 ω(H2O), π(CO)

835–875 800–850 800–870 800–870 800–880 820–880 δ(C3N3), γ(H2O)

960–990 930–963 960–990 914–990 965–990 960–995 δ(C3N3), ρ(H2O)

1030–1090 1015–1080 1068–1087 1037–1084 1035–1090 1020–1060 ν(C–O)

1130–1150 1120–1172 1125–1155 1150–1155 ν(C–N)

1221–1260 1210–1280 1230–1290 1234–1240 δ(NH)

1351–1390 1345–1350 1340–1390 1315–1392 1340–1390 1350–1390 ν(C–N)

1420–1498 1400–1480 1400–1490 1400–1485 1410–1480 1440–1480 ν(C3N3)

1500–1590 1520–1590 1500–1578 1500–1592 1500–1590 1515–1570 ν(C3N3)

1600–1680 1615–1680 1600–1697 1641–1690 1605–1690 1600–1682 ν(C=N), δ(NH), δ(H2O)

1710–1780 1703–1780 1720–1791 1720–1790 1720–1730 sh ν(C=O)

2120–2140 w 2160–2130 w ν(C≡N)

2700–2730 2780–2785 2700–2770 2700–2705 sh Hydrogen bond

2830–2840 2830–2860 Hydrogen bond

3050–3070 3055–3080 3060–3080 ν(NH)

3143–3170 3126–3180 3100–3150 3100–3180 ν(NH)

3205–3280 3200–3281 3200–3281 3200–3240 3230–3240 ν(H2O), ν(NH)

3300–3350 3300–3380 3300–3390 3330–3354 ν(H2O), ν(NH)

3420–3490 3420–3463 3440–3460 3460–3480 3450–3480 3432–3450 ν(H2O), ν(NH)

3512–3570 3525–3590 3525–3565 ν(H2O), ν(NH)

3630–3685 3640–3700 ν(OH)

312


SEIFER

where EI is the metal equivalent. The anhydrous trisub-stituted salts decompose to give the respective metalcyanate. As the temperature is further increased, thecyanates of bivalent cations transform fully or partiallyinto cyanamides.

The decomposition of a free cyanuric acid on heat-ing was studied in [107] using DTA, TGA, X-ray pow-der diffraction analysis, and the electroconductivitymethod. The cyanuric acid was found to decompose at400°ë and to give highly volatile cyanic acid with bp23.5°ë [6].

The thermolysis of nickel cyanurates was investi-gated in [180] to show that the dehydration of thesesalts occurs in one stage in the temperature interval of190–200°ë. The cyanuric anion decays at 420–425°Cto produce nickel and alkali-metal cyanates. Thedecomposition terminates in the formation of nickeloxide at 520°C and in the oxidation of sodium cyanatewith oxygen to Na2CO3 at 680°C. It is interesting tonote that when the Na2[Ni(H2C3N3O3)4] · 6H2O com-plex is heated to 270°ë, it transforms without changingits composition into a pink diamagnetic compound. Theauthor of [202] explains that this change in the color ofthe complex salts occurs, as a rule, due to the change inthe mode of coordination of the cyanurate ligand,namely, the formation of the coordination bond O: En+ instead of N: En+.

The thermal decomposition of Cu(H2C3N3O3)2 ·2NH3 begins with the loss of two ammonia molecules[167]. Further heating is accompanied by stepwiseelimination of the cyanic acid HNCO and results in the

E2I HC3N3O3 2E3

I C3N3O3 H3C3N3O3,+= formation of the salt CuHC3N3O3. At the same time, thedecomposition of a similar pyridine derivative is moreinvolved since this compound is a polymer.

The thermal decomposition of cyanurates can beconventionally divided into two stages, i.e., a stage thatis common for all salts and a stage that is specific toeach separate salt. The common stage includes thedecomposition of the mono- and disubstituted cyan-urates and always starts with the evolution of a free cya-nic acid that precipitates on the cold walls of the gastubes and polymerizes again to give cyanuric acid. Inthe presence of water vapors, cyanuric acid undergoeshydrolysis as follows:

The depolymerization of the trisubstituted cyanu-rate

proceeds, as a rule, at temperatures higher than in thecase of “acid” salts, the only exception being the saltsof slightly polarizing cations of heavy alkali metals andthallium (Cs+, Rb+, Tl+, K+) [203]. Thus, both acid andthe trisubstituted cyanurates of the heavy alkali metalsand thallium exhibit the typical one-stage decomposi-tion of their anhydrous forms. The obtained residuecontains a melt of the alkali-metal cyanate whose melt-ing point is lower than the thermal stability temperatureof the respective cyanurate.

Figure 1 shows how the temperature of the thermaldissociation of the anion S-triazine ring in the trisubsti-tuted alkali-metal cyanurates depends on the ionicradius of the cation. One can see that the thermal stabilityof these compounds decreases in the series Li+ Cs+

HNCO 2H2O+ NH4HCO3.=

M3C3N3O3 3MOCN

Table 3. Vibration frequencies in the IR spectra of the metal cyanurates (cm–1). The field strength of the given ion (Z/r2)is given in parentheses

Assignment

EH2C3N3O3 E2HC3N3O3 H[EO(HC3N3O3)] EO(HC3N3O3)

Cs+

(0.37)Rb+

(0.45)K+

(0.56)Na+

(1.04)Rb+

(0.45)K+

(0.56)Na+

(1.04)Bi3+

(2.08)In3+

(3.53)Fe3+

(6.67)Zr4+

(5.26)

ν(C≡N) 2130 2125 2130 2130

ν(C=N) + ν(C–N) 1480 1480 1485 1440 1450 1455 1460 1460 1450

1500 1500 1490 1520 1520

1590 1585 1600 1610 1620 1625 1540 1600 sh 1590 sh 1590 sh

ν(C=O) 1710 1730 1710 1710 1720 1720 1730 1710 1720 1710

1740 1740 1720

1780 1780 1780 1780

ν(NH) of the ring 2930 2930 2940 2950 w 2960 2930 w

2980 2980 2980 3000

δ(NH) of the ring 1680 1680 1650 1660 1680 1690 1685 1690 sh 1690 sh 1690 sh

ν(C–O) 1080 1080 1090 1080 1070 1063 1015 1055 1050 1055

1070 1090

1090 1050 1060 1060 1065



due to the growth in the intrinsic deformability of thebulky single-charged cations [204].

In some cases, the thermogram patterns of therelated compounds make it possible to establish boththe course of decomposition and the structure of thesecompounds. In this connection, of special interest canbe the cyanurates of the 18-electron silver cation [154].Thus, all the cyanurates of this cation are dehydrated inthe same way without decomposition in the interval of210–230°ë. The small single-charged silver cation pro-

duces a sufficiently slight polarizing effect (Z/ =

0.78). The thermal decomposition of the monosubsti-tuted silver cyanurate proceeds at 380–420°ë with theevolution of cyanic acid:

i.e., at the temperature close to the thermal stability ofthe alkali-metal cyanurates. The silver cyanate AgOCNthus formed is decomposed further only at 510°C.

However, in the case of the disubstituted saltAg2HC3N3O3 · H2O, a weak exothermic effect appearson the thermogram at 360°C before the evolution ofcyanic acid, this effect being further developed for thetrisubstituted salt (exothermic effect at 380°ë). Thecomparison of this effect with the decomposition of sil-ver isocyanate used as the reference shows that in therange of 360–380°ë, the exothermic effect correspondsto the decomposition according to the scheme

This fact allows one to assign the effect at 360–380°ë to the depolymerization of the cyanuric ring,

rAg+2

AgH2C3N3O3 AgOCN 2HNCO,+=

AgNCO Ag CO 1/2N2.+ +=

which is accompanied by the decomposition of the iso-cyanate part of the obtained compounds. The formationof isocyanate in the course of Ag3C3N3O3 · H2O decom-position suggests that one silver equivalent is bound tothe anion S-triazine ring in a somewhat different waythan the other two equivalents. The formation of such abond can be represented by the scheme

N

CN

C

N HC

OO AgAg

O

HOAg N

CN

C

NC

OO AgAg

O

AgT°C+ H2O,

and indicates that a new Ag–N bond arises during dehy-dration. The thermal dissociation of Ag3C3N3O3 (indi-cated by dotted lines in the scheme) is accompanied bythe reaction

and yields two silver cyanate isomers, namely, the low-stable AgNCO isocyanate and the thermally more sta-ble cyanate AgOCN. The decomposition of the lattercyanate

occurs in the temperature interval of 480–510°C andgives a black substance Ag2CN whose composition is

Ag3C3N3O3 AgNCO 2AgOCN+=

Ag CO 1/2N2,+ +

2AgOCN Ag2CN CO2 1/2N2+ +=

close to Ag2C2 acetylide, with one carbon atom beingreplaced by the nitrogen atom.

All silver cyanurates decompose completely in therange of 680–720°ë with the formation of the metal:

Thus, it was found for the first time that the 18-elec-tron silver can form trisubstituted salts due to the addi-tion of a third metal equivalent to the cyanurate anionthrough the nitrogen atom [109] rather than through theoxygen atom.

The processes of heating of the magnesium cyan-urates [158] only differ in the values of the second andthird effects. As was noted above, the IR spectrum ofthe trisubstituted salt contains the ν(OH) bands at 3640and 3700 cm–1, which allows one to assign the formulaof a basic salt to this compound, with two molecules of

Ag2CN 2Ag 1/2 CN( )2.+=

500

400

300

2000 1.0 1.4 1.8

Ionic radius, Å

T, °C

Li+Na+

K+

Rb+

Cs+

1

2

Fig. 1. The change in the temperature of (1) cyanurate ringdestruction and (2) cyanate melting in the series of thealkali-metal salts M3C3N3O3.

314


SEIFER

the disubstituted cyanurate being linked by the hydro-gen bond (2700 cm–1) to the magnesium hydroxide

molecule. The dehydration of this compound (at270°ë) results in the formation of a trisubstituted salt:

.

The above scheme shows that the loss of water isaccompanied by the binding of two S-triazine rings ofthe cyanurate anion through the magnesium cation. Asa result, one metal of the trisubstituted salt differs in itsposition from the other two metal atoms.

The monosubstituted magnesium cyanurateMg(H2C3N3O3)2 · 3H2O is dehydrated without decom-position. The decomposition of this salt differs fromthat of the trisubstituted salt only in the effect on thethermogram that corresponds to the evaporation of cya-nic acid at 310–330°C and follows the reaction

The thermal dissociation of the trisubstituted mag-nesium cyanurate occurs in the same temperature inter-val and gives two cyanates, namely, Mg(NCO)2 isocy-anate and Mg(OCN)2 cyanate. Mg(OCN)2 decaysalmost immediately after it is formed (400–410°ë)according to the equation

This transformation proceeds easily since the cyan-ate already has a Mg–O bond. At the same time, the IRspectrum of the residue still contains the bands ν(NCO)at 2205, 2180 cm–1 and δ(NCO) at 680 cm–1 corre-sponding to magnesium isocyanate. The latter com-pound decomposes at higher temperatures since theformation of the oxide from it requires that the anion bepreliminarily rotated through 180°. Only after such arotation of the cyanate group does the thermolysis ter-minate (490–500°ë) in the formation of magnesiumoxide,

and the IR spectrum of the residue no longer containsany bands in addition to 550 and 450 cm–1 for ν(Mg–O).

The processes observed in the thermolysis of theEHC3N3O3 salts (E = Ca2+, Sr2+, Ba2+) are similar,although they slightly differ in their temperature inter-

vals, which, most likely, is due to the weakening of thepolarizing effect of the ions in the series: Ca2+ (Z/r2 =1.78) > Sr2+ (Z/r2 = 1.24) > Ba2+ (Z/r2 = 0.98). Accord-ing to the data of [157], the processes taking place ontheir heating are presented in Table 4.

The decay of the S-triazine ring proper in the trisub-stituted cyanurates E3(C3N3O3)2 leads to the formationof two equivalents of the E(OCN)2 cyanate and oneequivalent of the E(NCO)2 isocyanate. One can seefrom Table 4 that already this stage of decompositionexhibits some differences in the course of the furthertransformation of the decomposition products. Therapid formation of the potassium and strontium cyan-amides from their isocyanates is explained by the factthat the E(NCO)2 salts already has E–N bonds. Thedecomposition of the E(OCN)2 cyanates into the samecyanamides requires additional energy for the rotationof the cyanato group through 180°. According to [205],the activation energy of this transformation is estimatedas 96 kcal/mol. Apparently, it is exactly for this reasonthat the transformation of two molecules of potassiumand strontium cyanate is delayed to higher tempera-tures.

The heating of lead cyanurate follows a scheme sim-ilar to the above processes, but the temperature inter-vals are somewhat different. This is associated with thedifferent ratio of the decomposition products formed.The processes were established using the procedure in[159]. Data in Table 5 indicate that the dehydration oflead cyanurates proceeds without their noticeabledecomposition. The decay of the cyanuric ring startsabove 300°ë and is accompanied by the simultaneousformation of different cyanates of this metal; thedecomposition of the lead isocyanate yields its cyan-amide. The éëN– cyanate ions that form during depo-lymerization above 300°ë are oxidizing agents (thatcan even oxidize the Cr3+ ions to Cr6+). Therefore, in thetemperature interval of 380–490°C, three processesoccur simultaneously, i.e., the evaporation of cyanicacid, the oxidation of the metal by its own cyanatogroup, and the thermal decomposition of lead isocyan-

T°C+ 8H2O

C

N

C N

C

N

O

H

O

O

(H2O)3Mg

HO Mg OH

C

N

C N

C

N

O

HO

O

(H2O)3Mg

C

N

C N

C

N

O

O

O

Mg

Mg

C

N

C N

C

N

O

O

O

Mg

3Mg H2C3N3O3( )2

= Mg3 C3N3O3( )2 12HNCO.+

2Mg OCN( )2 2MgO 2CO N2 CN( )2.+ + +=

Mg NCO( )2 MgO CO 1/2N2 1/2 CN( )2+ + +=



ate [206, 207]. Thus, the example with lead illustrated,for the first time, the possibility of the self-oxidationand reduction of the products of the cyanurate thermol-ysis. The thermal stability of Pb3O4 is low and, alreadyabove 550°ë, the compound loses its oxygen and trans-forms into PbO; this process corresponds to the effectat 530–580°ë. The decomposition of lead cyanuratesterminates at 650–680°ë in the oxidation–reductiondecomposition to give metallic lead.

The thermal decomposition of the monosubstitutedcyanurate complexes K2[E(H2C3N3O3)4] · 4H2O (E =

Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+) is almost identical,and only at the high-temperature stage does it dependon the nature of the heavy metal in the composition ofthe starting complex [182] (Table 6). All these cyan-urates are dehydrated below 300°ë. The complexdecomposition starts with the decomposition of thecyanurate ring near 400°ë. This process is accompa-nied by the evolution of cyanic acid and the formationof potassium and heavy-metal cyanates. The followingscheme of decomposition is determined by the thermalstability of the heavy-metal cyanate since potassium

Table 4. Process occurring during the thermolysis of the alkali-metal cyanurates

Temperature of the effect, °CProcess

Ca2+ Sr2+ Ba2+

110–150 200–250 260–280 3EHC3N3O3 · H2O 3EHC3N3O3 + 3H2O

240–280 310–380 380–470 3EHC3N3O3 E3(C3N3O3)2 + 3HNCO

340–450 430–460 525–575 E3C3N3O3 ECN2 + 2E(OCN)2 + CO2

580–670 600–680 – 2E(OCN)2 2ECN2 + 2CO2

700–740 Ba(OCN)2 + SiO2 BaSiO3 + CO + N2 + C

~760 BaCN2 + C Ba(CN)2

Table 5. Processes occurring during the thermolysis of lead cyanurates

Compound Temperatureof the effect, °C Process

Pb3(C3N3O3)2 · 2H2O 230–250 Pb3(C3N3O3)2 · 2H2O Pb3(C3N3O3)2 + 2H2O

340–370

400–470 4Pb(OCN)2 + 2PbCN2 Pb3O4 + 3Pb(CN)2 + 3N2 + 4CO

550–570 Pb3O4 3PbO + 1/2O2

650–670 3PbCN2 + 3PbO 6Pb + 3CO + 3/2(CN)2 + 3/2N2

700–730 Caking

Pb(H2C3N3O3)2 · 2H2O 200–240 Pb(H2C3N3O3)2 · 2H2O Pb(H2C3N3O3)2 + 2H2O

300–360

380–450 4Pb(OCN)2 + 2PbCN2 + 8H3C3N3O3 Pb3O4 + 3Pb(CN)2 + 24HNCO + 3N2 + 4CO

530–580 Pb3O4 3PbO + 1/2O2

630–680 3Pb(CN)2 + 3PbO 6Pb + 3CO + 3/2(CN)2 + 3/2N2

690–720 Caking

(PbOH)2HC3N3O3 250–280 (PbOH)2HC3N3O3 Pb2O(HC3N3O3) + H2O

330–350 2Pb2O(HC3N3O3) 2Pb2O(OCN)2 + 2HNCO

400–490 2Pb2O(OCN)2 Pb3O4 + Pb(CN)2 + 2CO + N2

550–570 Pb3O4 3PbO + 1/2O2

650–670 Pb(CN)2 + 3PbO 2Pb + 2PbO + CO + 1/2(CN)2 + 1/2N2

2Pb3 C3N3O3( )2 4Pb OCN( )2 2Pb NCO( )2+

2PbCN2 + 2CO2

6Pb3 H2C3N3O3( )2 4Pb OCN( )2 2Pb NCO( )2 8H3C3N3O3+ +

2PbCN2 + 2CO2

316


SEIFER

cyanate decomposes in an argon atmosphere at above750°ë [208].

The formation of free metals as a result of thedecomposition can be explained by the fact that, for theindicated transition metals (except for manganese), thedecomposition of isocyanates occurs through the inter-mediate formation of nitrides including sufficiently sta-ble (Co, Ni, Zn) and poorly stable (Cu, Cd) nitrides.

The thermal decomposition of theH[AlO(HC3N3O3)] · 2H2O acid begins with the loss ofone water molecule at 150°ë [209]. The second watermolecule is lost at 280–330°ë, which is followed by thecomplete decomposition of the compound and the evo-lution of cyanic acid and the formation of aluminiumoxide:

2 H AlO HC3N3O3( )[ ] H2O⋅{ }= Al2O3 6HNCO H2O.+ +

This course of decomposition confirms that anincrease in the temperature is followed by completedecomposition.

The disubstituted cyanurates Na[EO(HC3N3O3)] ·2H2O (E = Al3+, Sc3+, In3+) exhibit an endothermiceffect (150–180°ë) due to the loss of one water mole-cule, which is followed by another endothermic effect(380–410°ë) due to compound decomposition. Thiseffect is rather complicated and includes the hydrolysisof the compound by its own water

and the dehydration

Na EO HC3N3O3( )[ ] H2O⋅= NaH2C3N3O3 EO OH( )+

2EO OH( ) E2O3 H2O.+=

Table 6. Thermolysis of monosubstituted cyanurate complexes of heavy metals

E2+ Temperature of the effect, °C Process

Mn2+ 260–280 K2[Mn(H2C3N3O3)4] · 4H2O K2[Mn(H2C3N3O3)4] + 4H2O

370–400

740–820

Co2+ 250–290 K2[Co(H2C3N3O3)4] · 4H2O K2[Co(H2C3N3O3)4] + 4H2O

360–410

780–820 Co3N 3Co + 1/2N2

Ni2+ 260–300 K2[Ni(H2C3N3O3)4] · 4H2O K2[Ni(H2C3N3O3)4] + 4H2O

360–400

750–760 Ni3N 3Ni + 1/2N2

Cu2+ 230–300 K2[Cu(H2C3N3O3)4] · 4H2O K2[Cu(H2C3N3O3)4] + 4H2O

370–410

540–570

Zn2+ 230–300 K2[Zn(H2C3N3O3)4] · 4H2O K2[Zn(H2C3N3O3)4] + 4H2O

370–420

685–730 Zn3N2 3Zn + N2

Cd2+ 240–300 K2[Cd(H2C3N3O3)4] · 4H2O K2[Cd(H2C3N3O3)4] + 4H2O

380–420

K2 Mn H2C3N3O3( )4[ ] 2KNCO Mn OCN( )2 8HNCO+ +

MnO + CO + 1/2(CN)2 + 1/2N2

3K2 Co H2C3N3O3( )4[ ] 6KNCO 3Co NCO( )2 24HNCO+ +

Co3N + 6CO + 5/2N2

3K2 Ni H2C3N3O3( )4[ ] 6KNCO 3Ni NCO( )2 24HNCO+ +

Ni3N + 6CO + 5/2N2

K2 Cu H2C3N3O3( )4[ ] 2KNCO Cu NCO( )2 8HNCO+ +

Cu + 2CO + N2

3K2 Zn H2C3N3O3( )4[ ] 6KNCO 3Zn NCO( )2 24HNCO+ +

Zn3N2 + 6CO + 2N2

K2 Cd H2C3N3O3( )4[ ] 2KNCO Cd NCO( )2 8HNCO+ +

Cd + 2CO + N2



The sodium cyanurate thus formed decomposesalready at 410–430°ë to give sodium cyanate and cya-nic acid:

The sodium cyanate melting occurs in the range of510–540°ë. In addition to the above effects, the indiumsalt exhibits one more endothermic effect at 820°ë dueto the conversion of In2O3 into In3O4, which occursalmost at 850°ë.

The thermolysis of H[EO(HC3N3O3)] · 2H2O (E =Fe3+, In3+) also begins with the dehydration of the sub-stance, which simultaneously loses two water mole-cules [187]. The thermal dissociation of the cyanuricring for these metals follows the same scheme:

The only differences observed are in the tempera-tures at which dissociations begin: 410°C (Fe3+) and360°C (In3+). Their dissociation products decomposefurther in different ways. Thus, the indium oxocyanateInO(OCN) decomposes already at 500°ë with the for-mation of In2O3 and the evolution of the gas mixtureaccording to the equation

Then, In2O3 decays at 820°ë with the detachment ofoxygen:

As for the iron oxocyanate FeO(OCN), it decom-poses already at 570°ë to form black FeO. The thermaleffect accompanying the decomposition of the ironoxocyanate (produced from its salts and KNCO) almostcoincides with this effect in both temperature and sign.

The processes that occur during theH[BiO(HC3N3O3)] · 5H2O thermolysis almost overlap[187]. By analogy with the above-said, one can supposethat the compound dehydration (near 300°ë) will pro-ceed simultaneously with the dissociation of the S-tri-azine cyanurate anion. However, unlike the oxocyan-ates of trivalent metals considered above, theBiO(OCN) molecule contains both an oxidizing agent(cyanate ion) and a reducing agent (cation Bi3+) whoseproperties intensify on heating. Therefore, the forma-

NaH2C3N3O3 NaOCN 2HNCO.+=

H EO HC3N3O3( )[ ] EO OCN( ) 2HNCO.+=

2InO OCN( ) In2O3 CO 1/2 CN( )2 1/2N2.+ + +=

3In2O3 2In3O4 1/2O2.+=

tion of bismuth oxocyanate is immediately followed bythe oxidation–reduction process (330°ë) according tothe scheme

This process is accompanied by the evolution ofcyanic acid (370°C).

Bi(V) in Bi2O5 is known to be unstable above 357°ë[106]. Therefore, the next exothermic effect at 430°ë isassociated with the following decomposition:

i.e., it is caused by the reduction of Bi5+ to Bi3+ at theexpense of the cyanato group electrons.

The thermal processes occurring during heating ofyttrium cyanurates are listed in Table 7, which is bor-rowed from [187]. The thermal decomposition ofY(H2C3N3O3)3 · 6H2O is featured by the partial hydrol-ysis of the salt by the remaining water in the tempera-ture interval of 360−450°ë. In this case, oxygenbridges =Y–O–Y= are formed that link the cyanurateanions into polymeric networks.

The trisubstituted yttrium cyanurate YC3N3O3 · H2Oundergoes thermal decomposition and, at first, loseshydration water. This water is of the zeolite type and isreversibly adsorbed again when the compound is keptin humid air. The thermolysis of this salt is distin-guished by the exothermic effect at 280°C, which isassociated neither with gas evolution nor with massloss. This effect is followed by the coagulation of amor-phous particles apparently due to polymerization. Afragment of the model of this network is shown inFig. 2.

One can see that the cyanurate anion is symmetri-cally surrounded by three yttrium cations. It was shownin [188] that a violation in the symmetry of the sur-rounding results in weakening of the bonds inside the S-triazine ring of the anion. This leads, first of all, to areduction in the cyanurate stability. In the case of

H BiO HC3N3O3( )[ ] 5H2O⋅= BiO OCN( ) 2HNCO 5H2O.+ +

BiO2 CN( )

2BiO2 CN( ) Bi2O3 CO 1/2 CN( )2 1/2N2,+ + +=

Table 7. Thermal decomposition of yttrium cyanurates

Compound Temperatureof the effect, °C Process

Y(H2C3N3O3)3 · 6H2O 200–275 Y(H2C3N3O3)3 · 6H2O Y(H2C3N3O3)3 · 3H2O + 3H2O

360–450 6[Y(H2C3N3O3)3 · 3H2O] (Y2O)3(C3N3O3)4 + 15H2O + 24HNCO

670–750 (Y2O)3(C3N3O3)4 3Y2O3 + 6CO + 3(CN)2 + 3N2

YC3N3O3 · H2O 100–160 YC3N3O3 · H2O YC3N3O3 + H2O

280–360 Crystal enlargement

630–750 2YC3N3O3 Y2O3 + 3CO + 3/2(CN)2 + 3/2N2

318


SEIFER

YC3N3O3, the symmetric polarization of the anion bythe cation makes the whole system thermally stable.

The decomposition processes occurring on heatingof the rare-earth metal cyanurates are similar to thosefor yttrium. The loss of the hydration water byPrC3N3O3 · H2O is not accompanied by any significanteffect, which suggests its zeolite nature [210]. This lossis reversible and is characteristic, in general, of zeolitemoisture. The destruction of the cyanurate anion insuch a structure proceeds in the temperature interval of520–620°ë and is followed by the dissociation of thecyanurate anion according to the reaction

Further destruction of the residue (the exothermiceffect at 690°ë) is likely to occur due to the interactionof the decomposition products:

As was noted in [188], the thermolysis of zirconylcyanurate starts with the loss of three molecules ofhydration water at 160°ë and is accompanied by partialhydrolysis of the substance:

Further heating to 320°ë results in the formation ofthe monosubstituted dizirconyl cyanurate

The thermal dissociation of the cyanurate anion giv-ing the dizirconyl cyanate Zr2O3(OCN)2 occurs at410°ë after the evaporation of cyanic acid:

PrC3N3O3 Pr OCN( )3 PrO OCN( )+ C CO N2.+ +

PrO OCN( ) C+ PrO CN( ) CO.+=

ZrO HC3N3O3( ) 4H2O⋅= ZrO OH( ) H2C3N3O3( ) 3H2O.+

2ZrO OH( ) H2C3N3O3( )= Zr2O3 H2C3N3O3( ) H2O.+

The complete destruction of the dizirconyl cyanatetakes place only near 860°ë and proceeds in stages,thus confirming the polymeric structure of the com-pound. The remaining residue is zirconium oxide:

The polymerization occurring on heating can berepresented as the binding of the Zr2O3(OCN)2 mole-cules through the bridges according to the followingscheme:

As a result of this process, the thermal stability ofdizirconyl cyanate seems to increase.

While summarizing the consideration of the ther-molysis of the metal cyanurates, one can conclude thattheir thermal stability greatly depends on the chemicalnature of the cation bonded to the cyanuric acid residue[211]. As the strength of the cation field is increased,the temperature of destruction of the S-triazine ring ofthe anion is first increased from cesium (Z/r2 = 0.37) toyttrium (Z/r2 = 2.68) and then sharply drops from zinc(Z/r2 = 2.90) to aluminium (Z/r2 = 9.38) (see Fig. 3).

Such a pattern of the curve indicates that for highlypolarizing cations, the strength of bonds in the S-triaz-ine ring of the anion decreases with increasing thestrength of the cation field. Evidently, this is caused bythe drawing of the electrons of the ring toward the cat-ion. The resulting electron density redistribution in themolecule makes the bonds in the ring weaker and thesalt appears to be more “heated.”

Similarly, the growth in Z/r2 is also followed bychanges in the frequencies ν(C=N) of the S-triazinering in the IR spectra of the di- and trisubstituted cyan-

Zr2O3 H2C3N3O3( ) Zr2O3 OCN( )2 4HNCO.+=

Zr2O3 OCN( )2 2ZrO2 CO 1/2 CN( )2 1/2N2.+ + +=

:N C O

O C N:

O Zr O Zr OOZrO

:N C O

O C N:

O Zr O

O

CN

CN

C

N

O

CN

CN

C

N

O OY

O OY

O OY

O O O

C C CN

CN

C

N N

CN

C

N N

CN

C

N

O OY

OY

O O

O OO

Fig. 2. A fragment of the structure of YC3N3O3 · H2O.



urates (see Fig. 4). Some explanation of the change inthe ν(C=N) frequencies can be found from analogywith thiocyanates. It is known [197] that the EI–N=C=Ssystem can be described by either the covalent formEI−N=C=S or by the polar form EI–N≡C–S. As thefraction of the polar form increases, the ν(CN) frequen-cies of thiocyanates increase, and, conversely, with anincrease in the covalent fraction, they decrease. Theinflections on the curves in Fig. 4 are, most likely,caused by the change in the nature of the bond betweenthe cyanurate anion and the metal atom. For the weaklypolarizing cations, this bond has an ionic nature, whilefor the highly polarizing cations, it is covalent.

This fact seems to be responsible for the change inthe composition of the products of the thermal destruc-tion of the anion S-triazine ring observed in cyanurateswith a change in the bond nature. Thus, the decomposi-tion of the cyanurates of the bulky 8-electron alkali-metal cations gives only the EIOCN cyanates, whilewith increasing the strength of the cation field, the frac-tion of the covalent bond increases and the dissociationof such compounds yields residues with an increasingamount of isocyanate EINCO. This makes it possible toconclude that as the cation field strength and its deform-ability increase, the metal atom and the cyanurateanions are first bonded by the ionic bond through theoxygen atom and, then, this bond becomes more andmore covalent and is realized through the nitrogenatom.

The cyanuric acid and its derivatives are used in theproduction of pesticides, optical bleaches, and disinfec-tants [32, 212]. For instance, the sodium salt of dichlo-rocyanuric acid is used to disinfect spaces, fabrics, anddishwear [213]. Its solution is a strong antiseptic anddestroys the pathogens of tuberculosis, skin diseases,and infections caused by Bacillus pyocyaneus, staphy-lococcus, and Escherichia coli.

Cyanuric chloride and melamine have found impor-tant industrial application. Thus, the products of com-plete or partial replacement of the chlorine ions in

C3N3Cl3 by the NH2 groups or R are widely used inagriculture as herbicides to kill weeds [72, 214, 215].

Cyanuric chloride is also used in the production ofpigments. The stepwise replacement of the chlorineions in C3N3Cl3 extends the possibility of synthesizingdifferent derivatives of great importance in the pigmentproduction. It can be used to introduce the S-triazinering into two different pigments having different colors[36]. For example, when the blue and yellow pigmentsare bound through the S-triazine, the green pigment isproduced. Moreover, the introduction of the cyanuricring into the composition of the pigments increasestheir affinity to the cellulose fiber, which improves theirdyeing properties.

The other derivative of cyanuric acid, i.e., cyanur-amide or melamine C3N3(NH2)3, is used in the produc-tion of valuable plastics prepared from it and formalde-hyde by crosslinking the melamine molecules throughthe methylene bridges:

C

N

C N

C

NH2N

H2N

NH2 + CH2O

C

N

C N

C

NH2N

H2N

NH CH2OH + H2N C

N C

N

CNNH2

NH2

C

N

C N

C

NH2N

H2N

NH CH2 C

N C

N

CNNH2

NH2

NH

Melamine–formaldehyde resins are used in the production of plastics, carbamide glue, layered plastics, and

500

300

2 4 6 8 10Field strength of Mn+(Z/r2)

T, °C

CsRbK

Na

SrCa

Li

BaPr

Y

Mg

Sc

Al

Fig. 3. The dependence of the temperature of the cyanuricanion destruction on the strength of the cation field (Z/r2) inthe trisubstituted cyanurates.

320


SEIFER

varnishes. Such varnishes exhibit perfect insulating,anticorrosion, and decorative properties. They are usedto coat automobile parts, while enamels made on theirbasis are applied to the finish of the vehicle’s body.

The products of the triazine series are also used inthe pharmaceutical industry for the production of tri-panazide used in medicine to treat sleeping sickness[216]. Cyanurtriazide is used in the production ofexplosives. Although the mono- and diazides of cyan-uric acid are less sensitive to impact, they are used morefrequently in the production of detonators.

The polymerization of KNCO isocyanates is used inindustry to produce polyisocyanurate resins that arethermally stable (up to 300–350°C) and have improvedstrength and optical properties [217]. Such resins areapplied in the production of aviation fiber glasses. Themost essential fact is that the stability of isocyanurateresins is almost two times as high as that of polyure-thane resins (stable up to 150–200°ë) [218].

Melamine is used also for recovery in cyanate bathsintended for quenching machine tools and parts [219].This quenching method, called the carbonitration tech-nique, makes it possible to increase the article’sstrength by 2.5–3 times. However, the quenching pro-cess is accompanied by the formation of K2CO3, whichdecreases the bath output. With the introduction ofmelamine into the melt, the reaction

makes the carbonitration process continuous due to therecovery of the potassium cyanate.

2K2CO3 C3N3 NH2( )3+

= 4KNCO CO2 2NH3,+ +

We hope that this review will be useful for chemistsspecializing in different fields and facilitate the use ofcyanuric acid derivatives in practice.

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1530

1490

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Field strength of Mn+(Z/r2)

ν(C = N), cm–1

K Ba

Na

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Fig. 4. The dependence of the frequency ν(C=N) in the IRspectra of (1) disubstituted and (2) trisubstituted cyanurateson the cation field strength (Z/r2).



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Cyanuric Acid and Cyanurates

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