Cyanuric Acid And Cyanurates

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.

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 is an intermediate between inorganic and organic compounds, is given. A first attempt is made to generalize and systematize all the known compounds of this acid. The syntheses, IR studies, thermal decomposition, and the mechanism of thermal conversion of the cyanuric acid salts are considered. This review may prove of interest for the researchers working in different fields, chemical engineers, students, post-graduates, and teachers of higher schools.

The organic derivatives of cyanuric acid find wide industrial application today, which, unfortunately, does not apply to its inorganic salts due to the lack of systematic studies on metal cyanurates. This fact, undoubtedly, hampers their wide use. The composition of cyanurates includes the S-triazine ring formed as a result of trimerization of the cyanato groups. Therefore, before we consider cyanuric acid and its salts, we shall briefly discuss some questions in the chemistry of cyanides that are common to many classes of cyanogen anions. The free hydrocyanic acid HCN gives two types of derivatives [1] since it has two tautomeric forms: H–C≡N → H–N=C: nitrile form

isonitrile form

The synthesis conditions specify the particular form that enters into a reaction. The acid itself mainly consists of the nitrile form (~99%) with 1% of the isonitrile form as an admixture. Cyanides or nitriles are the most studied derivatives of these two forms. The first name usually refers to the salts of inorganic cations, while the latter name is applied to the derivatives with organic radicals. As to the derivatives of the isonitrile form, the best studied of them are the organic isonitriles, whereas the salts are only poorly studied. The electronic structure of hydrocyanic acid +

–

H–C≡N: H–C=N: H [ :C≡N: ] enables two types of reactions, namely, dissociation resulting in salt formation and addition reactions occurring at the triple bond C≡N. The reactions of the first –

:

+

NH NH O C O C O

0°C

H–N=C=O

type are commonly known; therefore, we shall discuss the reactions of the second type. In the presence of strong acids, hydrocyanic acid undergoes trimerization

HC

isocyanic acid

N

N

,

CH

to give a ring similar to the benzene ring [2]. The obtained compound was called, in organic chemistry, symmetric 1,3,5-triazine or S-triazine. Thus, the presence of a triple bond in the cyano group predetermines its capability of polymerizing [3]. The cyano group is contained in different cyanate anions [4] that can also involve chalcogen atoms: cyanate (OCN–), isocyanate (NCO–), fulminate (CNO–), thiocyanate (SCN–), isothiocyanate (NCS–), selenocyanate (SeCN–), and tellurocyanate (TeCN–). The polymerization of HOCN gives cyanuric acid, while that of HSCN yields thiocyanuric acid. The free cyanic acid HOCN has low stability. Like hydrocyanic acid, it readily polymerizes in an anhydrous state to give a mixture of cyanuric acid and cyamelide at room temperature [5–7]. The ratio of the components in the mixture greatly depends on the temperature. Thus, below 0°C, cyanic acid spontaneously transforms into cyamelide, whereas above 150°ë, only cyanuric acid is formed. This can be explained by the fact that cyanic acid has two tautomeric forms [8, 9]:

H–O–C≡N

150°C

N HO C

cyamelide

N

3H–C≡N

H C

cyanic acid

H O C

N

N C OH

cyanuric acid

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At low temperatures, the polymerization of cyanic acid occurs due to the cleavage of the double bond C=O. As the temperature is increased, this process occurs through the rupture of the triple bond C≡N. In the latter case, a S-triazine ring is formed that is incorporated in the composition of many cyanuric compounds of the C3N3X3 type (where X = OH, H, Hal, R, OR, SR, SH, NH2, N3, CN, NH–NH2). Thus, cyanuric acid C3N3(OH)3 (or H3C3N3O3) appears to be the ancestor of this class of compounds that can be treated as its derivatives. For example, cyanuramide C3N3(NH2)3 (also called melamine in industry) is the triamide of cyanuric acid, while cyanuric chloride C3N3Cl3 is its acid chloride. The complete hydrolysis of these compounds always gives cyanuric acid. At the same time, the derivatives of cyanuric acid are mainly produced not from cyanuric acid but via the polymerization of the nitrile groups due to the cleavage of their triple bond C≡N. The cyanuric acid derivatives containing the S-triazine ring (C3N3) are considered to be promising compounds for the synthesis of complexes. In terms of their capability of forming complex compounds, the cyanate anions can be arranged in the following order [10–15] (atoms bonded to the central atom of the complex are underlined): SeCN– < SCN– < OCN– < H2O < NCO– < NCS– < NCSe– < CH3CN < NC– < NH3 < RNC < CNO– < CN–. The ligand in the cyanuric acid complex is bonded to the central atom through the nitrogen atom of the Striazine ring. Therefore, one can suppose that, according to their field strength, the cyanuric acid derivatives and its anions will be arranged in this series to the right of H2O but to the left of NH3; i.e., they are supposed to be moderate-field ligands. The introduction of cyanuric cycles into complex compounds seems to be a promising direction of investigations, since the complexation can noticeably change the ligand properties. Unfortunately, this question remains open. Only one paper [16] is available today that is devoted to the complexing properties of herbicides of the S-triazine series. There are several methods of preparation of the cyanuric compounds C3N3X3, but the most frequently used technique is the polymerization of the XCN nitriles. Depending on the nature of the X atom at the cyano group, the polymerization reaction can occur either spontaneously or with heating or even with a catalyst. Cyanurhydride or the S-triazine C3N3H3 forms as a result of the hydrocyanic acid polymerization catalyzed by hydrogen halides (HCl, HBr, HI). The polymerization of HCN in the presence of HCl occurs in solutions even in the cold [17] to give sesquihalides with the empirical formula 2HCN · 3HHal [18]. It was established in [19–21] that sesquihalides contain a S-triazine ring that forms upon the removal of HHal as follows: 2 [ C 3 N 3 H 6 Cl 3 ] ⋅ 3HCl –3HCl

2 [ C 3 N 3 H 3 ] ⋅ 3HCl

–3HCl

2C 3 N 3 H 3 .

The entropy changes and the heat effect of the HCN polymerization are calculated in [22], while the magnetic anisotropy and the charge delocalization in S-triazine are considered in [23]. The IR spectrum of the polymerized 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 forms during the thermal decomposition of the substances (such as AgCN or Hg(CN)2) that proceeds with the evolution of large quantities of free cyanogen. In this case, the major portion of cyanogen rapidly polymerizes into brown and thermally stable paracyanogen (CN)x, while the remaining portion removed as (CN)2 polymerizes on cooling into colorless monoclinic crystals of cyanurcyanide that melt at 119°ë [24, 25]. The boiling point of C3N3(CN)3 was found to be 262°C at 771 mmHg. This substance is isolated from benzene solution in the form of a solvate with two benzene molecules. Cyanuric chloride or cyanuric acid trichloride C3N3Cl3 [26–31] forms white monoclinic crystals with a pungent odor. Its vapors are very toxic and harmful to the eyes and olfactory organs. Their maximum permissible concentration (MPC) in air is 0.1 mg/m3 [32]. The boiling 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, the C3N3Cl3 crystals are believed to melt in the interval of 146–146.5°C [34]. Cyanuric chloride dissolves poorly in water. However, when its aqueous solution is allowed to stand or is heated, it undergoes hydrolysis to form cyanuric acid: C 3 N 3 Cl 3 + 3H 2 O = C 3 N 3 ( OH ) 3 + 3HCl. Thus, cyanuric chloride can be regarded as the oxychloride of this acid. On the contrary, cyanuric chloride dissolves readily in organic solvents (acetone, chloroform, benzene). It crystallizes from benzene as the crystal solvate C3N3Cl3 · 2C6H6. This compound is one of the most important derivatives of S-triazine and is widely used in nucleophilic substitution reactions to produce a great variety of substances containing cyanuric rings. The chlorine atoms in cyanuric chloride are very mobile and are replaced in succession, which makes it possible to synthesize mono-, di-, or trisubstituted derivatives. However, the replacement of the chlorine atoms by other atoms or groups is gradually hampered such that the third chlorine atom is replaced with difficulty. Cyanuric chloride reacts with different nucleophiles: alcohols, phenols, naphthols [35, 36], ammonia, and organic amines [37]. The products of the partial replacement of organic amines were used to obtain amidohydrides [37]. Cyanuric chloride that has lost one chlorine atom can enter into the composition of polymers. Thus, the organotin compound {Me2SnH(C3N3Cl2}3 has the structure of a polymer, which was confirmed by X-ray diffraction analysis in [38].

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The bromine derivative, which is analogous to cyanuric chloride, is obtained by reacting HBr with cyanogen bromide (BrCN) in benzene or via polymerization of 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 syntheses of cyanuric acid derivatives. The chlorine atoms in C3N3Cl3 can be replaced by the azide group by carefully adding a cooled NaN3 solution to cyanuric chloride [40] or by mixing the acetone solutions of these components. The cyanuric azide C3N3(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. The monoazide C3N3Cl2(N3) and diazide C3N3Cl(N3)2 are less sensitive to mechanical action and, thus, are more frequently used in industry. The structure of cyanuric azide is considered in [40]. The N–N distance in the azide group of C3N3(N3)3 is determined in [41]. The replacement of the halogen atoms in C3N3Hal3 by hydrazine and its derivatives was used to synthesize the cyanuric derivatives C3N3(NH–NH2)3 and C3N3(NH–NHC6H5)3 [42]. This process occurs in steps and, depending on the ratio of the initial reagents, can give the products of complete or partial substitution of the halogen atoms. Cyanuric hydrazine can also be produced from trimethyl cyanurate C3N3(CH3)3 [39]. Cyanuric hydrazine C3N3(NH–NH2)3 forms fine white crystals (mp 287°C). When it was mixed with benzaldehyde in the presence of HCl and shaken, the tribenzylidene derivative of cyanuric acid was formed. The sodium salt of cyanurtricyanamide (C3N3)(CN2H)3 was synthesized in [43]. This compound contains the (C3N3)(CN2)3– anion, which evidently can be used as the binding unit in the production of various polymers. The reaction of C3N3Cl3 with HI gives an amorphous brown compound (CNI)x [8] that decomposes on heating into paracyanogen (CN)x and I2. However, the cyanurate structure of (CNI)x is confirmed by the fact that when treated with hot water, it is hydrolyzed to give cyanuric acid. Therefore, this compound can be assigned the formula C3N3I3, the more so since one of the intermediate products formed during its synthesis is C3N3ClI2. Similar to other cyanuric halides, cyanuric fluoride was obtained only by the indirect method during the reaction of trifluoroacetonitrile F3C–CN with NF3 at 514°ë [44]. The cyanuric trifluoride C3N3F3 forms in the mixture with other compounds. At the same time, cyanuric fluoride can be synthesized by the reaction of cyanogen chloride with NF3 at 500°C, by electrolysis of an aqueous solution of NaCN with F2, or via distillation of cyanogen iodide ICN over AgF [8]. According to [36], C3N3F3 is usually obtained by reacting C3N3Cl3 with SF4 or HF at –78°ë and further increasing the temperature of the mixture to 0°C. When C3N3F3 is heated RUSSIAN JOURNAL OF COORDINATION CHEMISTRY

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in vacuum, it dissociates to give cyanogen fluoride [45]: C3N3F3 = 3FCN. Cyanuric chloride and dicyanuric fluoride were studied by IR and Raman spectroscopies and by inelastic neutron scattering [46]. As was noted above, in the reaction of cyanuric chloride with ammonia, the chlorine atoms are replaced by the amido group to give cyanuramide, called melamine in industry [47–49]. Melamine can also be obtained by many other methods [49–56]. The melamine structure was discussed in [57, 58]. Although it has four hypothetical isomers, only two of them are known in practice:

HN HN C

NH C

N H

NH C NH

isomelamine

NH2 C N N H2N C

N

.

C NH2

melamine

Melamine crystallizes as colorless monoclinic prisms that sublime on slow heating [55–60]. According to the data of [61], it melts with decomposition at 354°ë. At room temperature, melamine dissolves poorly in water, while its solubility increases with temperature. The optical properties of melamine are considered in [62]. The characteristic bands in its IR spectrum lie at 3333, 3125, 1660, 1560, and 810 cm–1 [63]. The strong interaction of the π-electrons of the cyanuric ring with the unshared electron pair of the amine nitrogen atom imparts basic properties to the melamine amino groups. The dissociation constants of melamine in 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 nevertheless can form salts [53, 63–70]. However, it almost always acts as a monoacidic base. The yellow needles of melaminium picrate are formed when melamine reacts with the picric acid (NO2)3C6H2OH [55, 71]. This compound is poorly soluble in water and decomposes at 268°ë without melting. The high thermal stability and the low solubility of melaminium picrate allow one to use it in the chemical analysis. For example, it is used for both qualitative and quantitative determination of melamine in industrial production [72]. The C3N3(NH3)3 · HOC6H2(NO2)3 · 2H2O crystals are dried at 100°ë and weighed. Melamine can be also determined by the titrimetric method [73]. With AgNO3, melamine forms the adduct AgNO3 · C3N3(NH2)3 [63]. When it is heated in an aqueous ammonia solution, C3N3(NH2)2NAg2 is obtained: C 3 N 3 ( NH 2 ) 3 + 2AgNO 3 = C 3 N 3 ( NH 2 ) 2 NAg 2 + 2HNO 3 , Vol. 28

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where two hydrogen atoms of the amido group are replaced by silver cations. According to the data of [74–76], the thermal decomposition of melamine proceeds in stages and is accompanied by the detachment of ammonia and gradual linking of cyanuric rings through the imide bridges: 2C 3 N 3 ( NH 2 ) 3

H2O

–NH3

melon

2 ( C 3 N 3 )N

800°C

3 ( CN ) 2 + N 2 .

cyanuric nitride (carbonic nitride)

The melam [C3N3(NH2)2]2NH that forms during the melamine decomposition was first produced by the authors of [49] by heating NH4CNS to 300°C. The NH2 groups in melam are sufficiently mobile and can be replaced by other atoms or groups. For example, their replacement by the halogen atoms gives derivatives of the following types:

ammeline

( C 3 N 3 ) ( NH 2 ) ( OH ) 2

H2O

HN HN C

compound was synHS

thesized by the replacement of the amido groups in melam by HS [41]. The synthesized compound retains two cyanuric rings linked by an imide bridge. The melam structure was established in [76]. The melem [C3N3(NH2)]2(NH)2 is obtained when melam is heated for a long time. The melem heating results in its gradual decomposition and the formation of melon. The melon (C3N3)2(NH)3 is a yellow powder insoluble in water or diluted acids. When melon is heated in an inert atmosphere, it is decomposed with the evolution of ammonia, ( C 3 N 3 ) 2 ( NH ) 3 = 2C 3 N 4 + NH 3 , and the formation of carbonic nitride (or cyanuric nitride) [77]. The melon structure was reported in [77] and was shown to include two cyanuric rings linked via C–NH–C bonds.

2

H2N C N

N

cyanuric acid

NH C

N H

NH

N

C O

H2N C

isoammeline

HS NH C3N3

C NH2 N

C NH2

( C 3 N 3 ) ( OH ) 3 .

Since cyanuric acid is produced in the hydrolysis of melamine, the latter can be regarded as its triamide, whereas ammeline is its diamide and ammelide is its monoamide. Ammeline can be synthesized by the methods described in [57, 74, 80, 81]. It crystallizes as fine white needles poorly soluble in water, alcohol, or ether but soluble with heating in mineral acids, NH4OH, or strong alkalis. When ammeline is boiled with diluted solutions of HNO3 or KOH, it is also hydrolyzed to give cyanuric acid [82]. Like melamine, ammeline also has two tautomeric forms, namely, isoammeline and ammeline:

NH2 Hal NH2 Hal C3N3 NH C3N3 C3N3 NH C3N3 or . NH2 Hal Hal NH2 NH2 The C3N3 NH2

( C 3 N 3 ) ( NH 2 ) 2 OH

ammelide

(C 3 N3 ) 2 ( NH ) 3

melem

H2O

melamine

melam

(C 3 N3 ) 2 ( NH2 ) 2 ( NH) 2 –NH3

( C 3 N 3 ) ( NH 2 ) 3

( C 3 N 3 ) 2 ( NH 2 ) 4 NH

–NH3

melamine –NH3

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

+ 3CH2O

NH2 C

N C OH

N

.

ammeline

Ammeline can form salts with strong acids and AgNO3 [8]. Ammelide or monoamide of cyanuric acid can be obtained by different methods [83–88]. It is a white powder that is moderately soluble in hot water but insoluble in organic solvents. Like the above derivatives of cyanuric acid, it also has two tautomeric forms:

HN O C

NH C

N H

NH

N

C O

NH2 C

HO C

isoammelide

N

N C OH

ammelide

Similar to melamine and ammeline, it also forms salt-like products of addition with acids and bases [71]. The interest recently shown in melamine and its derivatives is dictated by the fact that their reaction with formaldehyde yields rubbery products with the cyanuric rings being linked through the bridges:

C N

N

C

NH CH2 NH C NH CH2 NH C N

N NH CH2 NH

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N

C

C N

.

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Such products are used in various industrial fields. Moreover, the amido group of melamine and its derivatives can be replaced by other groups or atoms to afford different substances which also have wide application. Thioammeline C3N3(NH2)2HS can be produced by boiling dicyandiamide or cyanamide with an acidified KCNS solution or alcoholic HCNS solution [89]. Its structure was determined in [90]. According to [8], dithioammeline is formed when thioammeline is treated with bromine water. It is a white crystalline powder soluble in alkalis. Its structure is supposed to contain a bridge of sulfur atoms. The synthesis of thioammelide was reported in [8]. The sulfur-containing compound C3N3(N2S2)3 was obtained in [91]. The cyanuric acid H3C3N3O3 was synthesized for the first time at the end of the XVIII century by heating urea until ammonia ceased to evolve: 3CO ( NH 2 ) 2 = H 3 C 3 N 3 O 3 + 3NH 3 . However, the composition of the product formed was established later [93] and, since that time, cyanuric acid has been synthesized in many works [94–104]. The salts of cyanuric acid are formed also in the course of polymerization of metal cyanates in an alkaline medium [105]. Cyanuric acid forms white crystals that precipitate from a solution with two hydration water molecules. It is stable and dissolves in mineral acids without decomposition, but when heated with strong concentrated acids, it slowly decomposes with the evolution of NH3 and CO2. Cyanuric acid is poorly soluble in water. At 25°ë, its solubility is ~2 × 10–2 mol/l [106]. It is also poorly soluble in alcohol but dissolves in cold concentrated sulfuric acid without decomposition. Cyanuric acid is not poisonous and is odorless. On heating to 400°ë, it transforms into cyanic acid (HNCO) without melting [107, 108]. The density of cyanuric acid (d 0 = 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 its two first dissociation constants are close to the respective constants of H2CO3 (K1 = 4 × 10–7 and K2 = 5 × 10–11), while the third dissociation constant slightly exceeds the dissociation constant of water (K = 1 × 10–16). Nevertheless, cyanuric acid gives three types of salts: monosubstituted MIH2C3N3O3, disubstituted I

I

M 2 HC3N3O3, and trisubstituted M 3 C3N3O3. However, it mainly forms mono- and disubstituted salts. The third hydrogen atom in aqueous solutions of this acid is replaced with difficulty and requires a considerable excess of a strong alkali. The fact that cyanuric acid gives two types of salts was first established in [109]. When Hg(CH3COO)2 reacts with a free cyanuric acid or with sodium cyanurate, it gives products that have different properties due to the different types of the mercury cation addition to RUSSIAN JOURNAL OF COORDINATION CHEMISTRY

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

HN O C

O C

N H

NH C O

isocyanuric acid

→

N HO C

OH C

N

N

.

C OH

cyanuric acid

Free cyanuric acid exists in the crystals as isocyanuric acid [112]. It contains strong NH···O hydrogen bonds. The interatomic distances and the electron density distribution in the H3C3N3O3 molecule are estimated in [113–116]. The H3C3N3O3 crystals are monoclinic: a = 7.749 Å; b = 6.736 Å; c = 11.912 Å, β = 130.69°; Z = 4; space group C2/n. The molecules in the crystal are arranged in parallel layers [109]. The C–N distance is 1.372 Å, the C–O distance is 1.220 Å, and the NCN and CNC angles are 115.3° and 124.7°, respectively. Within the limits of one layer, the molecules are linked through a NH···O hydrogen bond 2.778 and 2.798 Å in length. The oxygen atoms in the C–O bonds of the isocyanuric acid have lone electron pairs (located at an angle of 120° to the ring plane). The electron density maxima in the 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 peaks for the C–O and N–H bonds on the theoretical cross sections are 0.1 e/Å3 lower than on the experimental cross sections, while for the C–N bonds, this value is 0.2–0.3 e/Å3 and, in the region of the lone electron pairs, these peaks are 0.1–0.2 e/Å3 higher. The molecular refraction of the isocyanuric cycle was studied using organoelement allylisocyanurates in [117]. The experimental values were found to be lower than the theoretical values calculated from the additive scheme using the tabular bond refractions. The reason for this lies in the mutual influence of the allyl groups and the cyanurate cycle. The correction of the isocyanurate cycle for refraction of compounds of this class was assumed to be –1.20 cm3 (2σ = 0.15 cm3). The vibrational spectra of cyanuric acid have been considered in a number of papers [118–121]. The vibrational spectra of cyanuric, monothiocyanuric, and trithiocyanuric acids were calculated in [103, 119, 120]. The assignment of the absorption bands observed in 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 with the benzene structure. The cyanuric acid spectra also exhibit bands due to the stretching vibrations of the carbonyl 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 of 2828–2907 cm–1. Vol. 28

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The consecutive replacement of the hydrogen atoms in cyanuric acid by the amido group yields ammelide, ammeline, and melamine, respectively. Melamine can form salts with different acids [127], including cyanuric acid proper. The crystal structure of the salt C3N3(NH2)3 · H3C3N3O3 · 3HCl is considered in [128]. The salts of this type are formed when a proton is fully transferred from an acid to the amide nitrogen atom.

The reaction between cyanuric acid and organic bases was used to obtain the adducts, which were then studied by IR spectroscopy [146]. The synthesis of cyanurtriurea (CONH2)3C3N3O3 can be performed at 200°C according to the following reactions [147]:

The reaction of the alkali-metal cyanurates with bromine was used to synthesize the mono- and dibromocyanurates of potassium, sodium, and lithium. When the excess bromine was further reacted with the lithium salts, the dibromocyanuric acid HC3N3O3Br2 formed as extended rectangular plates [129]. The IR spectral studies revealed N–Br bonds in the HC3N3O3Br2 molecule. This acid is soluble in acetone, methyl ethyl ketone, dimethylformamide, and acetonitrile at room temperature, while it is poorly soluble in water. In an atmosphere of dry nitrogen, HC3N3O3Br2 decomposes already at 307–309°ë. When treated with free chlorine at 150°C, it gives dichlorocyanuric acid HC3N3O3Cl2. The reaction of its potassium salt with N : Cl2 [130] yielded a compound that was assigned the formula of the binary salt [Ni(H2O)6](C3N3O3Cl2)2 · 2KC3N3O3Cl2. The other derivatives of cyanuric acid can be synthesized by chlorinating the respective salts [131].

H 3 C 3 N 3 O 3 + 3CO ( NH 2 ) 2

The interaction of cyanuric acid with the alkalimetal halides was studied using samples produced by pressing under 38 t/cm2 [132]. In the IR spectra of the 1 : 1 samples, the characteristic bands of cyanuric acid (see above) shifted by ~25 cm–1 toward the long-wave region due to the complex formation. The tendency to form complexes with the alkali-metal halides increases in the series Cl < Br < I. However, the compounds thus formed are very unstable and decompose when treated with water into the starting reagents. Cyanuric acid is qualitatively determined in solutions by adding an ammonia solution of copper sulfate to give copper cyanurate of an amethyst color [133]. 3– The microcrystalloscopic analysis for the C3N3 O 3 cyanurate anion is performed by heating the aqueous solution of the acid with NaOH on a slide [134]. As the mixture becomes more and more concentrated, fine needles of sodium cyanurate precipitate that can be identified using a microscope. The method suggested in [135] for the quantitative determination of cyanuric acid is based on the precipitation of a poorly soluble melaminium cyanurate. The organic derivatives of cyanuric acid can be synthesized by a number of the methods described in [136– 145], and, depending on the starting reagent, one can obtain both the cyanuric and isocyanuric acid derivatives. These compounds differ not only in the structure of the S-triazine ring but also in their physical properties.

3CO ( NH 2 ) 2 = H 3 C 3 N 3 O 3 + 3NH 3 , = ( CONH 2 ) 3 C 3 N 3 O 3 + 3NH 3 . The compound obtained is an amorphous powder weakly soluble in water. The structure of the trimethylcyanurate crystals (CH3)3C3N3O3 was studied in [148]. The crystals are orthorhombic: a = 8.474 Å, b = 6.719 Å, c = 14.409 Å, space group Pnma. The structure of this compound is planar due to the conjugation of the π-electrons of the S-triazine ring and the lone electron pairs of the oxygen atom. All three methoxy groups are rotated in the same direction such that the molecule has 3/m symmetry. The length of the N–C bonds is 1.311–1.344 Å, while the CNC and NCN angles are 113.3°C and 126.8°, respectively. The molecules in the crystal are arranged in layers perpendicular to the b axis. The distance between the 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]. Such complexes are formed due to the addition of a ligand molecule 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 converted to the enol form. This is accomplished by treating the acid with excess alkali. The unreacted alkali is then removed by extraction with an alcohol. The cyanurates of the most active alkali metals produced in this way can be further used as starting reagents for the synthesis of cyanurates of other cations by the ionic exchange method. The sodium salt Na2HC3N3O3 · H2O precipitates in the form of white needles when cyanuric acid is treated with excess NaOH [148]. The monosubstituted potassium salt KH2C3N3O3 is formed in the reaction of acetic acid with concentrated potassium cyanate [6]. When this salt is dissolved in concentrated KOH and the obtained product is salted out with an alcohol, white needles of the disubstituted K2HC3N3O3 are precipitated that are hydrolyzed in water to give the same KH2C3N3O3. The synthesis of the alkali-metal cyanurates is also described in [149, 150]. The TlOH hydrate is also a strong base and thus can give both the mono- and disubstituted derivatives of 2 cyanuric acid. The strength of the Tl+ field (Z/ r Tl+ = 2

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

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however, the attempt made to obtain its trisubstituted cyanurate in solution failed. The ammonium salt (NH4)H2C3N3O3 crystallizes as fluorescent prisms [6]. It has a low stability and decomposes already at 130°ë with the evolution of ammonia and the formation of pure cyanuric acid. All cyanurates with monovalent cations are finely crystalline powders that were studied by X-ray diffraction analysis [151]. The comparison of the X-ray diffraction patterns of the pairs NaH2C3N3O3–K3C3N3O3, K2HC3N3O3–Rb2HC3N3O3, Li2HC3N3O3–LiH2C3N3O3 and K2HC3N3O3–KH2C3N3O5 for the average reflection angles reveals a noticeable similarity. This similarity Compound Solubility

CsH2C3N3O3 2×

10–2

RbH2C3N3O3 2×

10–2

suggests that the atoms in the lattice of the alkali-metal cyanurates 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 O 3 , and crystallization water molecules. The anionic layers alternate with inorganic 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:

KH2C3N3O3 1×

Thus, as the hydrogen atoms of cyanuric acid are consecutively replaced by the alkali-metal cations, the solubilities of the obtained salts increase. The salt AgH2C3N3O3 was first synthesized by adding a silver nitrate solution to a H3C3N3O3 solution acidified with acetic acid [70]. With an excess of Ag+, trisubstituted salt Ag3C3N3O3 was obtained that could be dried at 105°ë without decomposition. Its boiling with KOH yielded the mixed salt KAg2C3N3O3. When studying interactions in the AgNO3– NaxH3 − xC3N3O3–H2O systems, the following silver cyanurate 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-electron silver cation forms, in addition to the simple cyanurates, two mixed salts with alkali-metal cations and the complex compound [154]. The formation of the latter complex suggests that in the salts with multielectron central atoms, the cyanuric acid anion can really act as the ligand. The magnesium derivative of cyanuric acid Mg(H2C3N3O3)2 · 14H2O and its calcium salt Ca(H2C3N3O3)2 · 8H2O are discussed in [148]. These compounds are moderately soluble in water. The calcium salt crystals are triclinic. The disubstituted salt CaHC3N3O3 · 3H2O was also isolated. The barium compounds Ba(H2C3N3O3)2 · 2H2O were synthesized by adding barium hydroxide to a hot solution of cyanuric acid. Ba(OH)2 was added until the initially formed precipitate dissolved. When this solution was cooled, prismatic white crystals precipitated [155]. The finely crystalline BaHC3N3O3 · H2O salt was synthesized by reacting a hot cyanuric acid with excess Ba(OH)2. The radium salt is similar to barium salt [156]. The alkali-metal salts were synthesized in [157] by reacting hot saturated solutions of the hydroxides of RUSSIAN JOURNAL OF COORDINATION CHEMISTRY

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10–1

NaH2C3N3O3 5×

10–2

Na2HC3N3O3 1.5 ×

10–1

Na3C3N3O3 5 × 10–1

these metals with cyanuric acid. In all cases, the disubstituted salts EHC3N3O3 · H2O (where E = Ca2+, Sr2+, Ba2+) were isolated. The trisubstituted derivatives of these cations were obtained by mixing equivalent amounts of the reagents and further evaporation of the solution to dryness or by thermal decomposition of the disubstituted salts. The magnesium cyanurates were prepared by two methods [158], namely, by heating magnesium hydroxide with a cyanuric acid solution or by precipitating from hot solutions of magnesium salts with titrated solutions of NaH2C3N3O3 and Na3C3N3O3. In the first case, the normal salt Mg3(C3N3O3)2 · 8H2O precipitates from the hot solution, while on cooling, the disubstituted salt MgHC3N3O3 · 5H2O forms. It was noted in [5, 149] that the PbHC3N3O3 · 3H2O salt can be produced from basic lead acetate, whereas its boiling with excess AgNO3 results in the binary salt Ag4Pb(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 the precipitate of poorly soluble trisubstituted cyanurate. This precipitate is formed in all cases when the solution contains excess lead ions. With excess cyanurate ions, hydrolysis takes place that yields OH– ions and thus drastically increases the pH of the mixture. It is known from [160] that at pH greater than 7.8, lead hydroxide is formed and, therefore, the OH– ions gradually penetrate into the Pb3(C3N3O3)2 precipitate and the basic cyanurate (PbOH)2HC3N3O3 is obtained. As for the disubstituted cyanurate PbHC3N3O3, this salt is only precipitated from weakly acidic solutions and, thus, can be synthesized only from monosubstituted alkali-metal cyanurates. Copper cyanurates have been well studied by the authors of [155, 161–168]. A number of Cu(II) salts with specific color were synthesized: the blue-colored mixed salt LiCuC3N3O3 · 2H2O and the red-violet complexes Li[Cu(HC3N3O3)(H2C3N3O3)] · 2H2O, Vol. 28

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Na2[Cu(H2C3N3O3)4] · 6H2O, and K2[Cu(H2C3N3O3)4] · 6H2O. In the course of dehydration, their colors change blue to light green (LiCuC3N3O3), and violet (Na2[Cu(H2C3N3O3)4]), (K2[Cu(H2C3N3O3)4]). The monosubstituted salt and polymeric disubstituted Cu(H2C3N3O3)2 (CuHC3N3O3)n were obtained in [169]. Moreover, copper forms typical mixed cyanurate complexes with ammonia and pyridine [164, 166, 170–175]. According to [169], their structure can be described as follows: two cyanuric rings in the lactam form are linked through the copper atom that also coordinates two ammonia molecules. The structure of the cooper cyanurate complexes is considered in [176]. Such complexes are stable and decompose only in concentrated acids or on boiling with alkalis. The IR spectroscopic study revealed that in the copper cyanurate complexes, the nitrogen atoms are donors, the cyanuric acid anions having the lactam form. The crystal structure of these salts is studied in [166]. The trisubstituted copper cyanurate was obtained by precipitating from a hot solution of disubstituted potassium cyanurate with a copper acetate solution at a ratio 3– of C3N3 O 3 : Cu2+ equal to 2. This salt is formed also from the H3C3N3O3 and Cu(CH3COO)2 solutions taken at the ratio of 3 : 1. When such mixtures are evaporated, the monosubstituted cyanurate Cu(H2C3N3O3)2 · 2H2O is first crystallized. After it is isolated and the evaporation is further continued, the blue crystals of Cu3(C3N3O3)2 · 5H2O precipitate from the solution. The evaporation of the aqueous solutions of the manganese salts with free cyanuric acid taken at ratios of 1 : 2 and 1 : 4 yields light pink crystals of MnX2 · 2H3C3N3O3 · yH2O, where X = Cl–, NCS–, CH3COO–, 2–

and 1/2S O 4 [177–179]. However, it was noted that such syntheses with alkali-metal cyanurates gave dark precipitates containing manganese ions in the higher oxidation states. The IR studies of these complexes showed that their acido groups are arranged in the inner sphere and are directly bonded to the manganese atoms. When the monosubstituted sodium or potassium cyanurates were used for the precipitation from the salts of Mn2+, Co2+, and Ni2+ (E2+) at 60°ë, the cyanurates of the respective cations were obtained [180, 181]. – Mixing of solutions at ratios of H2C3N3 O 3 : E2+ equal to 1 : 1, 2 : 1, and 4 : 1 resulted in the formation of the basic salt (EOH)H2C3N3O3 · xH2O, which was filtered off in the hot state. On cooling of the solution, the crystals of the monosubstituted cyanurate E(H2C3N3O3)2 slowly 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 light pink crystals of Mn(H2C3N3O3)2 · 4H2O and K2[Mn(H2C3N3O3)4] · 4H2O, the pink crystals of Co(H2C3N3O3)2 · 6H2O and K2[Co(H2C3N3O3)4] ·

4H2O, the (NiOH)H2C3N3O3 · 2H2O and Ni(H2C3N3O3)2 · 4H2O crystals with green color, and the light blue crystals of Na2[Ni(H2C3N3O3)4] · 6H2O. Their IR spectroscopic studies showed that in all the hydrated compounds, cation E has an almost octahedral coordination, with some water molecules entering the inner coordination sphere of the metal. The cyanurate anion is coordinated through the nitrogen atom. The author of [169] synthesized the Co2+, Ni2+, and 2+ Zn cyanurates via the following reaction: 2NaH 2 C 3 N 3 O 3 + ECl 2 = E ( H 2 C 3 N 3 O 3 ) 2 + 2NaCl. The synthesis of the Cu2+ and Co2+ compounds with the organosubstituted derivatives of cyanuric acid is described in [168] and is performed according to the reaction H 4 L + ECl 2 = E ( H 3 L ) 2 + 2HCl, where H4L is 1,3-diallyl-5,2-hydroxy-3-phenoxypropyl isocyanurate. The M2[E(H2C3N3O3)4] · xH2O complexes with E = 2+ Cu , Mn2+, Ni2+, Co2+, Zn2+, or Cd2+ and M = Na+ or K+ were described in [161–166, 169, 180–182]. Anions – H2C3N3 O 3 are coordinated through the nitrogen atom [165]; in their complexing properties, they are intermediate between ammonia and water, being, however, closer to water. The structure of Co(H2C3N3O3)2 · 7H2O is discussed in [183]. Its crystals are monoclinic: a = 14.028 Å; b = 6.614 Å; c = 17.067 Å, β = 98.78°, space group P21/n. The structure consists of the complex cations – [Co(H2C3N3O3)(H2O)5]+, anions H2C3N3 O 3 , and crystallization water molecules. The cobalt atoms coordinate the nitrogen atom of only one H2C3N3O3 group and the oxygen atoms of five water molecules. The second – H2C3N3 O 3 anion is in the outer sphere of the complex. The reaction of the heavy-metal acetates with K2HC3N3O3 first yields disubstituted cyanurates with the general formula EHC3N3O3 · xH2O. With excess 2–

K2HC3N3O3 (HC3N3 O 3 : E2+ = 2 : 1), mixed complexes K[E(HC3N3O3)(H2C3N3O3)] · 2H2O are formed that contain two different cyanurate anions. The composition of these derivatives is determined by the hydrolysis of the K2HC3N3O3 excess occurring in the –

solutions. The obtained H2C3N3 O 3 ions react with the disubstituted salt and produce the mixed complex. It is noteworthy that one of the hydrogen atoms in the – H2C3N3 O 3 anion of these complexes is sufficiently mobile and, under specific conditions, particularly, when treated with an excess heavy-metal salt, can be replaced 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-

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eral white poorly soluble compounds, with only some 3– of them simultaneously containing Al3+ and C3N3 O 3 . For instance, with a sufficient sodium cyanurate excess, the soluble aluminate NaAlO2 is formed instead of the aluminum cyanurates. The salt that formally has the formula of the trisubstituted aluminum cyanurate AlC3N3O3 · 3H2O only precipitates in a narrow range of the reagent ratios C3N3 O 3 : Al3+, 0.75 < n ≤ 1.25. The value of n = 1 remains constant up to n = 1.5 in the solution, but the composition of the solid phase changes in this case due to the penetration of the Na+ ions into the previously formed precipitate. Further studies of the solid phases showed that the AlC3N3O3 · 3H2O phase is in fact the complex acid H[AlO(HC3N3O3)] · 2H2O whose hydrogen can be replaced by sodium or potassium 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 being that with the sodium cyanurate excess, no soluble scandate is formed and, even in the narrower range of the ratios 0.9 < n ≤ 1.1, the ScO[ScO(HC3N3O3)]· 2H2O precipitate is produced. In the range of 1.1 < n ≤ 1.5, this precipitate absorbs the Na+ ions and transforms into the salt Na[ScO(HC3N3O3)] · 2H2O. Thus, in the case of the Al3+ and Sc3+ cations forming the amphoteric hydroxides, the interaction with the alkali-metal cyanurates gives only oxy complexes containing cyanurate ions. A similar sodium salt was produced from InCl3 and 3–

3–

Na3C3N3O3 taken at a ratio of C3N3 O 3 : In3+ equal to 1.5. The Na[InO[HC3N3O3)] · 2H2O salt forms a white weakly soluble powder similar in its properties to the Al3+ and Sc3+ salts. The formation of such complexes can be explained by the fact that, as a result of the intense hydrolysis occurring in the aqueous solutions of the trisubstituted alkali-metal cyanurates, both OH– and – H2C3N3 O 3 are present in the solution. These anions give a weakly soluble compound E(OH)2H2C3N3O3 · H2O with three-charge cations; one of the hydrogen –

atoms in H2C3N3 O 3 anion is mobile and can be replaced by the alkali metal. The migration of this atom to the outer sphere of the complex is accompanied by simultaneous rearrangement of the hydroxy salt into the oxo salt, as a result of which the compound turns into 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 the above-mentioned elements is formed, i.e., EC3N3O3 · H2O. At 1.5 < n ≤ 2.0, the poorly soluble monosubstituted sodium cyanurate, which forms due to the hydrolysis of the starting reagent, penetrates into the precipitation. RUSSIAN JOURNAL OF COORDINATION CHEMISTRY

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The monosubstituted yttrium cyanurate was obtained in [186] by concentrating a solution containing yttrium acetate and monosubstituted potassium cyanurate until crystallization. The obtained crystals have the formula Y(H2C3N3O3)3 · 6H2O; i.e., the compound thus formed is monosubstituted yttrium cyanurate. The Fe3+, In3+, and Bi3+ cyanurates were prepared by heating stoichiometric amounts of the respective washed hydroxides with cyanuric acid [187]. The reactions that occur in this case can be written as follows: E ( OH ) 3 = EO ( OH ) + H 2 O; EO ( OH ) + H 3 C 3 N 3 O 3 = EO ( H 2 C 3 N 3 O 3 ) + H 2 O. By analogy with the compounds described above, the isolated compounds can be regarded as complex acids: H[FeO(HC3N3O3)] · 2H2O, H[InO(HC3N3O3)] · 2H2O, and H[BiO(HC3N3O3)] · 5H2O. The mobility of their hydrogen atoms is confirmed by the possibility of their replacement by alkali-metal ions to give salts. Out of the tetravalent cation cyanurates, only the zirconyl derivative was obtained [188]. It was synthesized from zirconyl hydroxide and cyanuric acid. The synthesis was carried out in an acetic acid medium. This acid was removed, first, through evaporation on a water bath and, then, through heating to 125°C in a thermostat. The residue formed was the white oxo salt ZrO(HC3N3O3) · 4H2O, which is disubstituted zirconyl cyanurate. Given in Table 1 are the types of inorganic derivatives of cyanuric acid. It can be seen that cyanuric acid forms a sufficiently large number of salts. Their variety lies within the limits known for the other acids, the only exception being the M2E[E(HC3N3O3)2]2 · 6H2O salts. On the one hand, these compounds contain the N: E bond, which is confirmed by the bands of the stretching vibrations of these bonds in the range of 500–520 cm–1 in their IR spectra. At the same time, when treated with hot water for a long period of time, these compounds decompose into two simple salts, M2HC3N3O3 and EHC3N3O3, which makes these compounds similar to the binary salts such as alums or schoenites. Compounds with a weakly stable coordination sphere are known to be referred to as binary salts [189]; therefore, it would be more correct to consider the M2E[E(HC3N3O3)2]2 · 6H2O compounds to be the binary salts M2HC3N3O3 · 3EHC3N3O3 · 6H2O. A number of metal cyanurates have been characterized by IR spectroscopy [190]. The authors synthesized the salts by evaporating mixtures of H3C3N3O3 with metal hydroxides taken in a 1.5-fold excess at 100°C. Neither the chemical analysis of the obtained compounds nor the assignment of the observed IR absorption bands was performed. Therefore, the authors of [190] could only conclude that all salts of cyanuric acid had ionic bonds. Vol. 28

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Table 1. Inorganic derivatives of cyanuric acid Compound

Formula (examples)

Cations

Li+, Na+, K+, Rb+, Cs+, Tl+, Ag+, Mg2+, Ca2+, Sr2+, Ba2+, Co2+, Ni2+, Y3+ I Li+, Na+, K+, Tl+, Rb+, Ag+, Mg2+, Ca2+, Sr2+, Ba2+, E 2 HC3N3O3 · xH2O Disubstituted salt Pb2+, Ni2+, Co2+, Mn2+, Zn2+, Cd2+ Trisubstituted salt Na+, K+, Ag+, Mg2+, Ca2+, Sr2+, Ba2+, Pb2+, Cu2+, I E 3 C3N3O3 · xH2O 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 M2E[E(HC3N3O3)2]2 · 6H2O or M = K+ (binary) M2HC3N3O3 · 3EHC3N3O3 · 6H2O E = Ni2+, Co2+, Mn2+ Monosubstituted salt

EIH2C3N3O3 · xH2O*

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

The IR spectra of sodium cyanurates are discussed in Table 2 in [191]. The authors studied how the vibration frequencies of the C=O bond changed with the extent of replacement of the hydrogen atoms in cyanuric acid by the sodium cation. The ν(C=O) frequencies fully disappear from the spectrum of the trisubstituted salt Na3C3N3O3 when the anion ring fully rearranges into a “benzene” ring. All the salt samples were dried in a thermostat at 125°ë. However, no complete dehydration was achieved under these conditions, since cyanurates contain not only crystallization but also zeolite water. The composition of the compounds under study was established by chemical analysis. The frequency assignment was performed using data from [192–197]. It was noted in [182] that the IR spectra of the transition-metal cyanurate complexes contain a band at 505–510 cm–1. As seen from Table 2, this band is absent from the spectra of the simple salts, which allows one to assign it to the stretching vibration of the N: En+ coordination bond. This assignment agrees with the data of [163, 181], which also suggest that the cyanurate groups are coordinated through the nitrogen atom. As far as the bands from the stretching vibrations of the cyano groups ν(C≡N) in the IR spectra of the cyanuric acid salts (Table 2) are concerned, their intensity increases with an increase in the polarizing action of the 2 2 cations from Cd2+(Z/ r Cd2+ = 1.88) to Fe3+ (Z/ r Fe3+ = 6.67). This gives evidence of the cation having an

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

N

C OK

NH

C OK N

O C O Zr O

N

C O C

N

As can be seen from the above scheme, the electrons are drawn off toward the highly charged cation which is followed by the electron density redistribution in the cyanuric ring and appears as the respective change in the frequency of the stretching vibrations of the cyanuric ring. Table 3 gives for comparison the frequencies of stretching vibrations of separate bonds in the cyanurates of the slightly and highly polarizing cations [126, 199, 200]. One can see that in the spectra of the

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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 Cs+ (0.37)

Li+

BiO+

(1.64) (2.08)

420–480 550–570

Disubstituted FeO+

Rb+

(6.67) (0.45)

Mn2+

(2.44) (2.90)

710–790 835–875 960–990 1030–1090

410–480 535–590 634–695 711–795 800–850 930–963 1015–1080

1221–1260 1351–1390 1420–1498 1500–1590 1600–1680

1210–1280 1345–1350 1400–1480 1520–1590 1615–1680

1710–1780

1703–1780 2120–2140 w 2780–2785 2830–2860 3055–3080

2700–2770

3205–3280 3300–3350 3420–3463

3126–3180 3200–3281 3300–3380 3440–3460 3512–3570

2700–2730 2830–2840 3050–3070 3143–3170

3420–3490

Zn2+

Trisubstituted ZrO2+

K+

(5.26) (0.56)

412–470 554–595 605–640 710–795 800–870 960–990 1068–1087 1130–1150 1230–1290 1340–1390 1400–1490 1500–1578 1600–1697

412–475 551–560 613–695 720–790 800–870 914–990 1037–1084 1120–1172 1234–1240 1315–1392 1400–1485 1500–1592 1641–1690

1720–1791

1720–1790 2160–2130 w

1340–1390 1410–1480 1500–1590 1605–1690

3100–3180 3200–3240 3300–3390 3450–3480 3525–3565

3460–3480 3525–3590 3630–3685

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Pr3+

(1.64) (2.22)

455–480 552–595 600–610 708–794 800–880 965–990 1035–1090 1125–1155

3060–3080 3100–3150 3200–3281

highly polarizing cations, the band in the range 1590– 1600 cm–1 corresponding to the stretching vibrations ν(C=N) of the ring either disappears or becomes weak. Simultaneously, two new bands appear, namely, the band at 2125–2130 cm–1 that lies in the range of vibrations of ν(C≡N) of the cyanate anion and the band ν(C=O) at 1780 cm–1, whose position suggests some strengthening of the bond in the carbonyl group. The bands ν(NH) are either absent from the spectra of the salts with highly polarizing cations or appear as an inflection. The bond in the oxo group becomes also weaker, apparently, due to the strengthening of bonding between carbon and nitrogen atoms in the neighboring cyano group. The above data indicate that as the polarizing action of the cation increases, the separate bonds in the S-triazine ring become weaker. Thus, an increase in the polarizing action of cations produces the same effect on the S-triazine ring as the heating process and causes a

Ca2+

Mg2+

Assignment

(3.28)

δ(NCO) ν(MO), δ(C=O) δ(CNC), δ(NCO) ω(H2O), π(CO) δ(C3N3), γ(H2O) δ(C3N3), ρ(H2O) ν(C–O) ν(C–N) δ(NH) 1350–1390 ν(C–N) 1440–1480 ν(C3N3) 1515–1570 ν(C3N3) 1600–1682 ν(C=N), δ(NH), δ(H2O) 1720–1730 sh ν(C=O) ν(C≡N) 2700–2705 sh Hydrogen bond Hydrogen bond ν(NH) ν(NH) 3230–3240 ν(H2O), ν(NH) 3330–3354 ν(H2O), ν(NH) 3432–3450 ν(H2O), ν(NH) ν(H2O), ν(NH) 3640–3700 ν(OH) 470–480 550–580 600–684 770–780 820–880 960–995 1020–1060 1150–1155

one-sided deformation, thus facilitating salt decomposition upon heating. It also becomes clear that the thermal decomposition on the mono- and disubstituted cyanurates should occur at lower temperatures than that of the trisubstituted salts because the symmetry of the anion S-triazine ring in the latter salts does not change with increasing the strength of the cation field. The studies on the thermal stability of cyanurates were undertaken in the second half of the XX century after it was reported in [136] that these salts firmly hold their crystallization water and that some of them transform into cyanates during decomposition [136]. The thermolysis of cyanurates was studied in [201]. It was found that when heated, the mono- and disubstituted salts of Na+, Ca2+, Co2+, Zn2+, Mn2+, and Pb2+ subsequently transform into trisubstituted salts: I

I

2E H 2 C 3 N 3 O 3 = E 2 HC 3 N 3 O 3 + H 3 C 3 N 3 O 3 , Vol. 28

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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 EH2C3N3O3 Assignment ν(C≡N) ν(C=N) + ν(C–N)

ν(C=O)

Cs+

Rb+

K+

(0.37)

(0.45)

(0.56)

Rb+

1480

1480

1590 1710

1585 1730 1740 2930 2980 1680 1080

2980 1680 1080

1485 1600 1710 1740 2930 2980 1650 1090

1440 1500 1610 1710

1450 1500 1620 1720

1455 1490 1625 1720

Bi3+

In3+

Fe3+

(2.08)

(3.53)

(6.67)

1520 1520 1540 1730 1710

1780 2940 2950 w 2960 2930 w 3000 1660 1080

1680 1690 1070 1063 1070 1090

I

E 2 HC 3 N 3 O 3 = 2E 3 C 3 N 3 O 3 + H 3 C 3 N 3 O 3 , where EI is the metal equivalent. The anhydrous trisubstituted salts decompose to give the respective metal cyanate. As the temperature is further increased, the cyanates of bivalent cations transform fully or partially into cyanamides. The decomposition of a free cyanuric acid on heating was studied in [107] using DTA, TGA, X-ray powder diffraction analysis, and the electroconductivity method. The cyanuric acid was found to decompose at 400°ë and to give highly volatile cyanic acid with bp 23.5°ë [6]. The thermolysis of nickel cyanurates was investigated in [180] to show that the dehydration of these salts occurs in one stage in the temperature interval of 190–200°ë. The cyanuric anion decays at 420–425°C to produce nickel and alkali-metal cyanates. The decomposition terminates in the formation of nickel oxide at 520°C and in the oxidation of sodium cyanate with oxygen to Na2CO3 at 680°C. It is interesting to note that when the Na2[Ni(H2C3N3O3)4] · 6H2O complex is heated to 270°ë, it transforms without changing its composition into a pink diamagnetic compound. The author of [202] explains that this change in the color of the complex salts occurs, as a rule, due to the change in the mode of coordination of the cyanurate ligand, namely, the formation of the coordination bond O: En+. En+ instead of N: The thermal decomposition of Cu(H2C3N3O3)2 · 2NH3 begins with the loss of two ammonia molecules [167]. Further heating is accompanied by stepwise elimination of the cyanic acid HNCO and results in the

EO(HC3N3O3)

H[EO(HC3N3O3)] Na+

(1.04) (0.45) (0.56) (1.04)

1090

I

K+

2130

ν(NH) of the ring δ(NH) of the ring ν(C–O)

E2HC3N3O3 Na+

2125 1460

2130 1460

2130 1450

1600 sh 1590 sh 1720 1710 1720 1780 1780

1685 1690 sh 1015 1055 1050

Zr4+ (5.26)

1060

1590 sh

1780

1690 sh 1050

1690 sh 1055

1060

1065

formation of the salt CuHC3N3O3. At the same time, the decomposition of a similar pyridine derivative is more involved since this compound is a polymer. The thermal decomposition of cyanurates can be conventionally divided into two stages, i.e., a stage that is common for all salts and a stage that is specific to each separate salt. The common stage includes the decomposition of the mono- and disubstituted cyanurates and always starts with the evolution of a free cyanic acid that precipitates on the cold walls of the gas tubes and polymerizes again to give cyanuric acid. In the presence of water vapors, cyanuric acid undergoes hydrolysis as follows: HNCO + 2H 2 O = NH 4 HCO 3 . The depolymerization of the trisubstituted cyanurate M3 C3 N3 O3

3MOCN

proceeds, as a rule, at temperatures higher than in the case of “acid” salts, the only exception being the salts of slightly polarizing cations of heavy alkali metals and thallium (Cs+, Rb+, Tl+, K+) [203]. Thus, both acid and the trisubstituted cyanurates of the heavy alkali metals and thallium exhibit the typical one-stage decomposition of their anhydrous forms. The obtained residue contains a melt of the alkali-metal cyanate whose melting point is lower than the thermal stability temperature of the respective cyanurate. Figure 1 shows how the temperature of the thermal dissociation of the anion S-triazine ring in the trisubstituted alkali-metal cyanurates depends on the ionic radius of the cation. One can see that the thermal stability Cs+ of these compounds decreases in the series Li+

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due to the growth in the intrinsic deformability of the bulky single-charged cations [204]. In some cases, the thermogram patterns of the related compounds make it possible to establish both the course of decomposition and the structure of these compounds. In this connection, of special interest can be the cyanurates of the 18-electron silver cation [154]. Thus, all the cyanurates of this cation are dehydrated in the same way without decomposition in the interval of 210–230°ë. The small single-charged silver cation pro2 duces a sufficiently slight polarizing effect (Z/ r Ag+ = 0.78). The thermal decomposition of the monosubstituted silver cyanurate proceeds at 380–420°ë with the evolution of cyanic acid: AgH 2 C 3 N 3 O 3 = AgOCN + 2HNCO, i.e., at the temperature close to the thermal stability of the alkali-metal cyanurates. The silver cyanate AgOCN thus formed is decomposed further only at 510°C. However, in the case of the disubstituted salt Ag2HC3N3O3 · H2O, a weak exothermic effect appears on the thermogram at 360°C before the evolution of cyanic acid, this effect being further developed for the trisubstituted salt (exothermic effect at 380°ë). The comparison of this effect with the decomposition of silver isocyanate used as the reference shows that in the range of 360–380°ë, the exothermic effect corresponds to the decomposition according to the scheme AgNCO = Ag + CO + 1/2N 2 . This fact allows one to assign the effect at 360– 380°ë to the depolymerization of the cyanuric ring,

N Ag

O

C

O C

N

N H HOAg C

O

T, °C

and indicates that a new Ag–N bond arises during dehydration. The thermal dissociation of Ag3C3N3O3 (indicated by dotted lines in the scheme) is accompanied by the reaction Ag 3 C 3 N 3 O 3 = AgNCO + 2AgOCN Ag + CO + 1/2N 2 , and yields two silver cyanate isomers, namely, the lowstable AgNCO isocyanate and the thermally more stable cyanate AgOCN. The decomposition of the latter cyanate 2AgOCN = Ag 2 CN + CO 2 + 1/2N 2 occurs in the temperature interval of 480–510°C and gives a black substance Ag2CN whose composition is RUSSIAN JOURNAL OF COORDINATION CHEMISTRY

Na+ Li+

500

K+

400

Rb+ Cs+ 1 300 2 200

0

1.0

1.4 1.8 Ionic radius, Å

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

which is accompanied by the decomposition of the isocyanate part of the obtained compounds. The formation of isocyanate in the course of Ag3C3N3O3 · H2O decomposition suggests that one silver equivalent is bound to the anion S-triazine ring in a somewhat different way than the other two equivalents. The formation of such a bond can be represented by the scheme

T°C

Ag

313

N Ag

O

C

O C

N

N Ag C

O

+ H2O,

Ag

close to Ag2C2 acetylide, with one carbon atom being replaced by the nitrogen atom. All silver cyanurates decompose completely in the range of 680–720°ë with the formation of the metal: Ag 2 CN = 2Ag + 1/2 ( CN ) 2 . Thus, it was found for the first time that the 18-electron silver can form trisubstituted salts due to the addition of a third metal equivalent to the cyanurate anion through the nitrogen atom [109] rather than through the oxygen atom. The processes of heating of the magnesium cyanurates [158] only differ in the values of the second and third effects. As was noted above, the IR spectrum of the trisubstituted salt contains the ν(OH) bands at 3640 and 3700 cm–1, which allows one to assign the formula of a basic salt to this compound, with two molecules of Vol. 28

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molecule. The dehydration of this compound (at 270°ë) results in the formation of a trisubstituted salt:

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

O (H2O)3Mg

C

N

N O

O C

C

O

(H2O)3Mg

C

H N

N O

C

N

The above scheme shows that the loss of water is accompanied by the binding of two S-triazine rings of the cyanurate anion through the magnesium cation. As a result, one metal of the trisubstituted salt differs in its position from the other two metal atoms. The monosubstituted magnesium cyanurate Mg(H2C3N3O3)2 · 3H2O is dehydrated without decomposition. The decomposition of this salt differs from that of the trisubstituted salt only in the effect on the thermogram that corresponds to the evaporation of cyanic acid at 310–330°C and follows the reaction 3Mg ( H 2 C 3 N 3 O 3 ) 2 = Mg 3 ( C 3 N 3 O 3 ) 2 + 12HNCO. The thermal dissociation of the trisubstituted magnesium cyanurate occurs in the same temperature interval and gives two cyanates, namely, Mg(NCO)2 isocyanate and Mg(OCN)2 cyanate. Mg(OCN)2 decays almost immediately after it is formed (400–410°ë) according to the equation 2Mg ( OCN ) 2 = 2MgO + 2CO + N 2 + ( CN ) 2 . This transformation proceeds easily since the cyanate already has a Mg–O bond. At the same time, the IR spectrum of the residue still contains the bands ν(NCO) at 2205, 2180 cm–1 and δ(NCO) at 680 cm–1 corresponding to magnesium isocyanate. The latter compound decomposes at higher temperatures since the formation of the oxide from it requires that the anion be preliminarily rotated through 180°. Only after such a rotation of the cyanate group does the thermolysis terminate (490–500°ë) in the formation of magnesium oxide, Mg ( NCO ) 2 = MgO + CO + 1/2N 2 + 1/2 ( CN ) 2 and the IR spectrum of the residue no longer contains any bands in addition to 550 and 450 cm–1 for ν(Mg–O). The processes observed in the thermolysis of the EHC3N3O3 salts (E = Ca2+, Sr2+, Ba2+) are similar, although they slightly differ in their temperature inter-

C

N

C

T°C

O

Mg O

C O

N

N O

HO Mg OH O

Mg

N H

C

Mg

C

N

N O

+ 8H2O .

C O C

N

vals, which, most likely, is due to the weakening of the polarizing effect of the ions in the series: Ca2+ (Z/r2 = 1.78) > Sr2+ (Z/r2 = 1.24) > Ba2+ (Z/r2 = 0.98). According to the data of [157], the processes taking place on their heating are presented in Table 4. The decay of the S-triazine ring proper in the trisubstituted cyanurates E3(C3N3O3)2 leads to the formation of two equivalents of the E(OCN)2 cyanate and one equivalent of the E(NCO)2 isocyanate. One can see from Table 4 that already this stage of decomposition exhibits some differences in the course of the further transformation of the decomposition products. The rapid formation of the potassium and strontium cyanamides from their isocyanates is explained by the fact that the E(NCO)2 salts already has E–N bonds. The decomposition of the E(OCN)2 cyanates into the same cyanamides requires additional energy for the rotation of the cyanato group through 180°. According to [205], the activation energy of this transformation is estimated as 96 kcal/mol. Apparently, it is exactly for this reason that the transformation of two molecules of potassium and strontium cyanate is delayed to higher temperatures. The heating of lead cyanurate follows a scheme similar to the above processes, but the temperature intervals are somewhat different. This is associated with the different ratio of the decomposition products formed. The processes were established using the procedure in [159]. Data in Table 5 indicate that the dehydration of lead cyanurates proceeds without their noticeable decomposition. The decay of the cyanuric ring starts above 300°ë and is accompanied by the simultaneous formation of different cyanates of this metal; the decomposition of the lead isocyanate yields its cyanamide. The éëN– cyanate ions that form during depolymerization above 300°ë are oxidizing agents (that can even oxidize the Cr3+ ions to Cr6+). Therefore, in the temperature interval of 380–490°C, three processes occur simultaneously, i.e., the evaporation of cyanic acid, the oxidation of the metal by its own cyanato group, and the thermal decomposition of lead isocyan-

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Table 4. Process occurring during the thermolysis of the alkali-metal cyanurates Temperature of the effect, °C

Process

Ca2+

Sr2+

Ba2+

110–150 240–280 340–450 580–670

200–250 310–380 430–460 600–680

260–280 380–470 525–575 – 700–740 ~760

3EHC3N3O3 · H2O 3EHC3N3O3 + 3H2O 3EHC3N3O3 E3(C3N3O3)2 + 3HNCO E3C3N3O3 ECN2 + 2E(OCN)2 + CO2 2E(OCN)2 2ECN2 + 2CO2 Ba(OCN)2 + SiO2 BaSiO3 + CO + N2 + C BaCN2 + C Ba(CN)2

Table 5. Processes occurring during the thermolysis of lead cyanurates Compound Pb3(C3N3O3)2 · 2H2O

Temperature of the effect, °C 230–250 340–370

Process Pb3(C3N3O3)2 · 2H2O 2Pb 3 ( C 3 N 3 O 3 ) 2

Pb3(C3N3O3)2 + 2H2O 4Pb ( OCN ) 2 + 2Pb ( NCO ) 2 2PbCN2 + 2CO2

Pb(H2C3N3O3)2 · 2H2O

400–470 550–570 650–670 700–730 200–240 300–360

Pb3O4 + 3Pb(CN)2 + 3N2 + 4CO 4Pb(OCN)2 + 2PbCN2 Pb3O4 3PbO + 1/2O2 3PbCN2 + 3PbO 6Pb + 3CO + 3/2(CN)2 + 3/2N2 Caking Pb(H2C3N3O3)2 · 2H2O Pb(H2C3N3O3)2 + 2H2O

380–450

4Pb(OCN)2 + 2PbCN2 + 8H3C3N3O3 Pb3O4 + 3Pb(CN)2 + 24HNCO + 3N2 + 4CO Pb3O4 3PbO + 1/2O2 3Pb(CN)2 + 3PbO 6Pb + 3CO + 3/2(CN)2 + 3/2N2 Caking (PbOH)2HC3N3O3 Pb2O(HC3N3O3) + H2O 2Pb2O(HC3N3O3) 2Pb2O(OCN)2 + 2HNCO 2Pb2O(OCN)2 Pb3O4 + Pb(CN)2 + 2CO + N2 Pb3O4 3PbO + 1/2O2 Pb(CN)2 + 3PbO 2Pb + 2PbO + CO + 1/2(CN)2 + 1/2N2

6Pb 3 ( H 2 C 3 N 3 O 3 ) 2

4Pb ( OCN ) 2 + 2Pb ( NCO ) 2 + 8H 3 C 3 N 3 O 3 2PbCN2 + 2CO2

(PbOH)2HC3N3O3

530–580 630–680 690–720 250–280 330–350 400–490 550–570 650–670

ate [206, 207]. Thus, the example with lead illustrated, for the first time, the possibility of the self-oxidation and reduction of the products of the cyanurate thermolysis. The thermal stability of Pb3O4 is low and, already above 550°ë, the compound loses its oxygen and transforms into PbO; this process corresponds to the effect at 530–580°ë. The decomposition of lead cyanurates terminates at 650–680°ë in the oxidation–reduction decomposition to give metallic lead. The thermal decomposition of the monosubstituted cyanurate complexes K2[E(H2C3N3O3)4] · 4H2O (E = RUSSIAN JOURNAL OF COORDINATION CHEMISTRY

Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+) is almost identical, and only at the high-temperature stage does it depend on the nature of the heavy metal in the composition of the starting complex [182] (Table 6). All these cyanurates are dehydrated below 300°ë. The complex decomposition starts with the decomposition of the cyanurate ring near 400°ë. This process is accompanied by the evolution of cyanic acid and the formation of potassium and heavy-metal cyanates. The following scheme of decomposition is determined by the thermal stability of the heavy-metal cyanate since potassium Vol. 28

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Table 6. Thermolysis of monosubstituted cyanurate complexes of heavy metals E2+ Mn2+

Temperature of the effect, °C

Process

260–280

K2[Mn(H2C3N3O3)4] · 4H2O

370–400

K 2 [ Mn ( H 2 C 3 N 3 O 3 ) 4 ]

K2[Mn(H2C3N3O3)4] + 4H2O

2KNCO + Mn ( OCN ) 2 + 8HNCO MnO + CO + 1/2(CN)2 + 1/2N2

740–820 Co2+

250–290

K2[Co(H2C3N3O3)4] · 4H2O

360–410

3K 2 [ Co ( H 2 C 3 N 3 O 3 ) 4 ]

K2[Co(H2C3N3O3)4] + 4H2O 6KNCO + 3Co ( NCO ) 2 + 24HNCO Co3N + 6CO + 5/2N2

Ni2+

780–820 260–300

Co3N 3Co + 1/2N2 K2[Ni(H2C3N3O3)4] · 4H2O

360–400

3K 2 [ Ni ( H 2 C 3 N 3 O 3 ) 4 ]

K2[Ni(H2C3N3O3)4] + 4H2O 6KNCO + 3Ni ( NCO ) 2 + 24HNCO Ni3N + 6CO + 5/2N2

Cu2+

750–760 230–300

3Ni + 1/2N2 Ni3N K2[Cu(H2C3N3O3)4] · 4H2O

370–410

K 2 [ Cu ( H 2 C 3 N 3 O 3 ) 4 ]

K2[Cu(H2C3N3O3)4] + 4H2O

2KNCO + Cu ( NCO ) 2 + 8HNCO Cu + 2CO + N2

540–570 Zn2+

230–300

K2[Zn(H2C3N3O3)4] · 4H2O

370–420

3K 2 [ Zn ( H 2 C 3 N 3 O 3 ) 4 ]

K2[Zn(H2C3N3O3)4] + 4H2O 6KNCO + 3Zn ( NCO ) 2 + 24HNCO Zn3N2 + 6CO + 2N2

Cd2+

685–730 240–300

Zn3N2 3Zn + N2 K2[Cd(H2C3N3O3)4] · 4H2O

380–420

K 2 [ Cd ( H 2 C 3 N 3 O 3 ) 4 ]

K2[Cd(H2C3N3O3)4] + 4H2O

2KNCO + Cd ( NCO ) 2 + 8HNCO Cd + 2CO + N2

cyanate decomposes in an argon atmosphere at above 750°ë [208]. The formation of free metals as a result of the decomposition can be explained by the fact that, for the indicated transition metals (except for manganese), the decomposition of isocyanates occurs through the intermediate formation of nitrides including sufficiently stable (Co, Ni, Zn) and poorly stable (Cu, Cd) nitrides. The thermal decomposition of the H[AlO(HC3N3O3)] · 2H2O acid begins with the loss of one water molecule at 150°ë [209]. The second water molecule is lost at 280–330°ë, which is followed by the complete decomposition of the compound and the evolution of cyanic acid and the formation of aluminium oxide: 2 { H [ AlO ( HC 3 N 3 O 3 ) ] ⋅ H 2 O } = Al 2 O 3 + 6HNCO + H 2 O.

This course of decomposition confirms that an increase in the temperature is followed by complete decomposition. The disubstituted cyanurates Na[EO(HC3N3O3)] · 2H2O (E = Al3+, Sc3+, In3+) exhibit an endothermic effect (150–180°ë) due to the loss of one water molecule, which is followed by another endothermic effect (380–410°ë) due to compound decomposition. This effect is rather complicated and includes the hydrolysis of the compound by its own water Na [ EO ( HC 3 N 3 O 3 ) ] ⋅ H 2 O = NaH 2 C 3 N 3 O 3 + EO ( OH ) and the dehydration 2EO ( OH ) = E 2 O 3 + H 2 O.

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317

Table 7. Thermal decomposition of yttrium cyanurates Compound Y(H2C3N3O3)3 · 6H2O

YC3N3O3 · H2O

Temperature of the effect, °C 200–275 360–450 670–750 100–160 280–360 630–750

Process Y(H2C3N3O3)3 · 6H2O Y(H2C3N3O3)3 · 3H2O + 3H2O 6[Y(H2C3N3O3)3 · 3H2O] (Y2O)3(C3N3O3)4 + 15H2O + 24HNCO (Y2O)3(C3N3O3)4 3Y2O3 + 6CO + 3(CN)2 + 3N2 YC3N3O3 · H2O YC3N3O3 + H2O Crystal enlargement 2YC3N3O3 Y2O3 + 3CO + 3/2(CN)2 + 3/2N2

The sodium cyanurate thus formed decomposes already at 410–430°ë to give sodium cyanate and cyanic acid:

tion of bismuth oxocyanate is immediately followed by the oxidation–reduction process (330°ë) according to the scheme

NaH 2 C 3 N 3 O 3 = NaOCN + 2HNCO.

H [ BiO ( HC 3 N 3 O 3 ) ] ⋅ 5H 2 O

The sodium cyanate melting occurs in the range of 510–540°ë. In addition to the above effects, the indium salt exhibits one more endothermic effect at 820°ë due to the conversion of In2O3 into In3O4, which occurs almost at 850°ë. The thermolysis of H[EO(HC3N3O3)] · 2H2O (E = Fe3+, In3+) also begins with the dehydration of the substance, which simultaneously loses two water molecules [187]. The thermal dissociation of the cyanuric ring for these metals follows the same scheme:

= BiO ( OCN ) + 2HNCO + 5H 2 O.

This process is accompanied by the evolution of cyanic acid (370°C). Bi(V) in Bi2O5 is known to be unstable above 357°ë [106]. Therefore, the next exothermic effect at 430°ë is associated with the following decomposition:

H [ EO ( HC 3 N 3 O 3 ) ] = EO ( OCN ) + 2HNCO.

2BiO 2 ( CN ) = Bi 2 O 3 + CO + 1/2 ( CN ) 2 + 1/2N 2 ,

The only differences observed are in the temperatures at which dissociations begin: 410°C (Fe3+) and 360°C (In3+). Their dissociation products decompose further in different ways. Thus, the indium oxocyanate InO(OCN) decomposes already at 500°ë with the formation of In2O3 and the evolution of the gas mixture according to the equation

i.e., it is caused by the reduction of Bi5+ to Bi3+ at the expense of the cyanato group electrons. The thermal processes occurring during heating of yttrium cyanurates are listed in Table 7, which is borrowed from [187]. The thermal decomposition of Y(H2C3N3O3)3 · 6H2O is featured by the partial hydrolysis of the salt by the remaining water in the temperature interval of 360−450°ë. In this case, oxygen bridges =Y–O–Y= are formed that link the cyanurate anions into polymeric networks. The trisubstituted yttrium cyanurate YC3N3O3 · H2O undergoes thermal decomposition and, at first, loses hydration water. This water is of the zeolite type and is reversibly adsorbed again when the compound is kept in humid air. The thermolysis of this salt is distinguished by the exothermic effect at 280°C, which is associated neither with gas evolution nor with mass loss. This effect is followed by the coagulation of amorphous particles apparently due to polymerization. A fragment of the model of this network is shown in Fig. 2. One can see that the cyanurate anion is symmetrically surrounded by three yttrium cations. It was shown in [188] that a violation in the symmetry of the surrounding results in weakening of the bonds inside the Striazine ring of the anion. This leads, first of all, to a reduction in the cyanurate stability. In the case of

2InO ( OCN ) = In 2 O 3 + CO + 1/2 ( CN ) 2 + 1/2N 2 . Then, In2O3 decays at 820°ë with the detachment of oxygen: 3In 2 O 3 = 2In 3 O 4 + 1/2O 2 . As for the iron oxocyanate FeO(OCN), it decomposes already at 570°ë to form black FeO. The thermal effect accompanying the decomposition of the iron oxocyanate (produced from its salts and KNCO) almost coincides with this effect in both temperature and sign. The processes that occur during the H[BiO(HC3N3O3)] · 5H2O thermolysis almost overlap [187]. By analogy with the above-said, one can suppose that the compound dehydration (near 300°ë) will proceed simultaneously with the dissociation of the S-triazine cyanurate anion. However, unlike the oxocyanates of trivalent metals considered above, the BiO(OCN) molecule contains both an oxidizing agent (cyanate ion) and a reducing agent (cation Bi3+) whose properties intensify on heating. Therefore, the formaRUSSIAN JOURNAL OF COORDINATION CHEMISTRY

BiO 2 ( CN )

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N O

N O

Y O C

C

O

O C C

N C N

N C N

N O

N O

Y O

O

Y O C

C

O

C

N C N

N C N

O C

O

N O

Y O

O

Y O C

C

O

N C N

O

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

YC3N3O3, the symmetric polarization of the anion by the cation makes the whole system thermally stable. The decomposition processes occurring on heating of the rare-earth metal cyanurates are similar to those for yttrium. The loss of the hydration water by PrC3N3O3 · H2O is not accompanied by any significant effect, which suggests its zeolite nature [210]. This loss is reversible and is characteristic, in general, of zeolite moisture. The destruction of the cyanurate anion in such a structure proceeds in the temperature interval of 520–620°ë and is followed by the dissociation of the cyanurate anion according to the reaction PrC 3 N 3 O 3

Pr ( OCN ) 3

PrO ( OCN )

+ C + CO + N 2 . Further destruction of the residue (the exothermic effect at 690°ë) is likely to occur due to the interaction of the decomposition products: PrO ( OCN ) + C = PrO ( CN ) + CO. As was noted in [188], the thermolysis of zirconyl cyanurate starts with the loss of three molecules of hydration water at 160°ë and is accompanied by partial hydrolysis of the substance: ZrO ( HC 3 N 3 O 3 ) ⋅ 4H 2 O = ZrO ( OH ) ( H 2 C 3 N 3 O 3 ) + 3H 2 O. Further heating to 320°ë results in the formation of the monosubstituted dizirconyl cyanurate 2ZrO ( OH ) ( H 2 C 3 N 3 O 3 ) = Zr 2 O 3 ( H 2 C 3 N 3 O 3 ) + H 2 O. The thermal dissociation of the cyanurate anion giving the dizirconyl cyanate Zr2O3(OCN)2 occurs at 410°ë after the evaporation of cyanic acid:

Zr 2 O 3 ( H 2 C 3 N 3 O 3 ) = Zr 2 O 3 ( OCN ) 2 + 4HNCO. The complete destruction of the dizirconyl cyanate takes place only near 860°ë and proceeds in stages, thus confirming the polymeric structure of the compound. The remaining residue is zirconium oxide: Zr 2 O 3 ( OCN ) 2 = 2ZrO 2 + CO + 1/2 ( CN ) 2 + 1/2N 2 . The polymerization occurring on heating can be represented as the binding of the Zr2O3(OCN)2 molecules through the bridges according to the following scheme: :N C O

:N C O

O Zr O O Zr O Zr O O Zr O O C N: O C N:

As a result of this process, the thermal stability of dizirconyl cyanate seems to increase. While summarizing the consideration of the thermolysis of the metal cyanurates, one can conclude that their thermal stability greatly depends on the chemical nature 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 of the anion is first increased from cesium (Z/r2 = 0.37) to yttrium (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 highly polarizing cations, the strength of bonds in the S-triazine ring of the anion decreases with increasing the strength of the cation field. Evidently, this is caused by the drawing of the electrons of the ring toward the cation. The resulting electron density redistribution in the molecule makes the bonds in the ring weaker and the salt appears to be more “heated.” Similarly, the growth in Z/r2 is also followed by changes in the frequencies ν(C=N) of the S-triazine ring in the IR spectra of the di- and trisubstituted cyan-

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urates (see Fig. 4). Some explanation of the change in the ν(C=N) frequencies can be found from analogy with thiocyanates. It is known [197] that the EI–N=C=S system can be described by either the covalent form EI−N=C=S or by the polar form EI–N≡C–S. As the fraction of the polar form increases, the ν(CN) frequencies of thiocyanates increase, and, conversely, with an increase in the covalent fraction, they decrease. The inflections on the curves in Fig. 4 are, most likely, caused by the change in the nature of the bond between the cyanurate anion and the metal atom. For the weakly polarizing cations, this bond has an ionic nature, while for the highly polarizing cations, it is covalent. This fact seems to be responsible for the change in the composition of the products of the thermal destruction of the anion S-triazine ring observed in cyanurates with a change in the bond nature. Thus, the decomposition of the cyanurates of the bulky 8-electron alkalimetal cations gives only the EIOCN cyanates, while with increasing the strength of the cation field, the fraction of the covalent bond increases and the dissociation of such compounds yields residues with an increasing amount of isocyanate EINCO. This makes it possible to conclude that as the cation field strength and its deformability increase, the metal atom and the cyanurate anions are first bonded by the ionic bond through the oxygen atom and, then, this bond becomes more and more covalent and is realized through the nitrogen atom. The cyanuric acid and its derivatives are used in the production of pesticides, optical bleaches, and disinfectants [32, 212]. For instance, the sodium salt of dichlorocyanuric acid is used to disinfect spaces, fabrics, and dishwear [213]. Its solution is a strong antiseptic and destroys the pathogens of tuberculosis, skin diseases, and infections caused by Bacillus pyocyaneus, staphylococcus, and Escherichia coli. Cyanuric chloride and melamine have found important industrial application. Thus, the products of complete or partial replacement of the chlorine ions in H2N

C

H2N

N

N

C NH2 + CH2O C

C

Y Ba Pr Li Sr Ca Na K Rb Cs

500

300

2

N

N

C

N

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C NH2

C

C

NH2 N

N

H2N

Melamine–formaldehyde resins are used in the produc-

NH2 N

N

N C NH CH2 NH

C

4 6 8 10 Field strength of M n+(Z/r 2 )

N

N

N

Al

C NH CH2OH + H2N C C

C

Sc

C3N3Cl3 by the NH2 groups or R are widely used in agriculture as herbicides to kill weeds [72, 214, 215]. Cyanuric chloride is also used in the production of pigments. The stepwise replacement of the chlorine ions in C3N3Cl3 extends the possibility of synthesizing different derivatives of great importance in the pigment production. It can be used to introduce the S-triazine ring into two different pigments having different colors [36]. For example, when the blue and yellow pigments are bound through the S-triazine, the green pigment is produced. Moreover, the introduction of the cyanuric ring into the composition of the pigments increases their affinity to the cellulose fiber, which improves their dyeing properties. The other derivative of cyanuric acid, i.e., cyanuramide or melamine C3N3(NH2)3, is used in the production of valuable plastics prepared from it and formaldehyde by crosslinking the melamine molecules through the methylene bridges:

H2N

H2N

Mg

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

N

N

H2N

T, °C

319

C NH2

tion of plastics, carbamide glue, layered plastics, and Vol. 28

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ν(C = N), cm –1

We hope that this review will be useful for chemists specializing in different fields and facilitate the use of cyanuric acid derivatives in practice.

Ca Li

1530 Na Ba K

REFERENCES

Er Pr Sr La Nd Y Na Ca Sr

1490 Rb K Ba

Sc

1 Al

Mg

Al 2

Mg

1450

0

2

4 5 9 10 Field strength of M n+(Z/r 2 )

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

varnishes. Such varnishes exhibit perfect insulating, anticorrosion, and decorative properties. They are used to coat automobile parts, while enamels made on their basis are applied to the finish of the vehicle’s body. The products of the triazine series are also used in the pharmaceutical industry for the production of tripanazide used in medicine to treat sleeping sickness [216]. Cyanurtriazide is used in the production of explosives. Although the mono- and diazides of cyanuric acid are less sensitive to impact, they are used more frequently in the production of detonators. The polymerization of KNCO isocyanates is used in industry to produce polyisocyanurate resins that are thermally stable (up to 300–350°C) and have improved strength and optical properties [217]. Such resins are applied in the production of aviation fiber glasses. The most essential fact is that the stability of isocyanurate resins is almost two times as high as that of polyurethane resins (stable up to 150–200°ë) [218]. Melamine is used also for recovery in cyanate baths intended for quenching machine tools and parts [219]. This quenching method, called the carbonitration technique, makes it possible to increase the article’s strength by 2.5–3 times. However, the quenching process is accompanied by the formation of K2CO3, which decreases the bath output. With the introduction of melamine into the melt, the reaction 2K 2 CO 3 + C 3 N 3 ( NH 2 ) 3 = 4KNCO + CO 2 + 2NH 3 , makes the carbonitration process continuous due to the recovery of the potassium cyanate.

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