Download 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
cyamelide
0°C
type are commonly known; therefore, we shall discuss
the reactions of the second type.
In the presence of strong acids, hydrocyanic acid
undergoes trimerization
N
3H–C≡N
HC
H
C
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–N=C=O
H–O–C≡N
isocyanic acid
cyanic acid
150°C
N
HO C
H
O
C
N
N
C OH
cyanuric acid
1070-3284/02/2805-0301$27.00 © 2002 åÄIä “Nauka /Interperiodica”
.
302
SEIFER
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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CYANURIC ACID AND CYANURATES
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
303
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 ,
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SEIFER
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
–NH3
H2O
–NH3
(C 3 N3 ) 2 ( NH ) 3
melem
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
compound was syn-
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
HN
HN C
NH
C
N
H
NH
N
C O
H2N C
isoammeline
HS
C NH2
N
C
NH2
+ 3CH2O
( 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:
HS
NH C3N3
( C 3 N 3 ) ( NH 2 ) 2 OH
ammelide
NH2
Hal NH2
Hal
C3N3 NH C3N3
C3N3 NH C3N3
or
.
NH2
Hal Hal
NH2
NH2
The
C3N3
NH2
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
melamine
–NH3
The multistep hydrolysis occurs in an aqueous solution of melamine containing an acid or alkali on heating
[49, 77–79]:
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
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY
N
C
C
N
.
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CYANURIC ACID AND CYANURATES
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
305
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.
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SEIFER
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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CYANURIC ACID AND CYANURATES
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
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307
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,
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SEIFER
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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CYANURIC ACID AND CYANURATES
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
3–
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–
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.
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309
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.
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SEIFER
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
C OK
N
O C
O Zr
O
N
NH
C
C O
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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311
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+
(6.67) (0.45)
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
2700–2730
2830–2840
3050–3070
3143–3170
3420–3490
Rb+
3205–3280
3300–3350
3420–3463
Mn2+
Zn2+
(2.44) (2.90)
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
Ca2+
(1.64) (2.22)
455–480
552–595
600–610
708–794
800–880
965–990
1035–1090
1125–1155
1340–1390
1410–1480
1500–1590
1605–1690
2700–2770
3126–3180
3200–3281
3300–3380
3440–3460
3512–3570
3060–3080
3100–3150
3200–3281
3100–3180
3200–3240
3300–3390
3450–3480
3525–3565
3460–3480
3525–3590
3630–3685
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
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY
Pr3+
Mg2+
Assignment
(3.28)
470–480
550–580
600–684
770–780
820–880
960–995
1020–1060
1150–1155
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)
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 ,
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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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CYANURIC ACID AND CYANURATES
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
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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
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SEIFER
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
O
HO Mg OH
O
(H2O)3Mg
C
O
N
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-
O
Mg
O
C O
C
C
T°C
H
N
N
N
N
Mg
N
H
C
C
N
N
Mg
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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315
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
ECN2 + 2E(OCN)2 + CO2
E3C3N3O3
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
4Pb(OCN)2 + 2PbCN2
Pb3O4 + 3Pb(CN)2 + 3N2 + 4CO
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
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SEIFER
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
Ni3N
3Ni + 1/2N2
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 )
Vol. 28
No. 5
2002
318
SEIFER
N
O
N
O
C
Y
O
C
N
O
C
O
C
N
N
C
N
C
N
O
N
O
Y
O
O
C
Y
O
C
N
O
C
O
C
N
N
C
N
C
O
N
O
Y
O
O
C
Y
O
C
N
O
N
C
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-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY
Vol. 28
No. 5
2002
CYANURIC ACID AND CYANURATES
T, °C
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
N
H2N
H2N
N
C NH2 + CH2O
C
N
C
H2N
300
2
Mg
Sc
Al
4
6
8
10
Field strength of M n+(Z/r 2 )
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.
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:
N
N
C
C NH CH2OH + H2N C
C
N
N
Melamine–formaldehyde resins are used in the producRUSSIAN JOURNAL OF COORDINATION CHEMISTRY
C
C
NH2
N
N
N
C NH CH2 NH
C
Pr
Li
Sr
Ca
Na
K
Rb
Cs
500
N
N
Y
Ba
N
H2N
H2N
C
319
C
NH2
NH2
N
N
C
NH2
tion of plastics, carbamide glue, layered plastics, and
Vol. 28
No. 5
2002
320
SEIFER
We hope that this review will be useful for chemists
specializing in different fields and facilitate the use of
cyanuric acid derivatives in practice.
ν(C = N), cm –1
Ca
Li
1530
Na
Ba
K
1490 Rb
K Ba
1450
0
REFERENCES
Er
Pr
Sr La Nd
Y
Na
Ca
Sr
Sc
1
Al
Mg
Al 2
Mg
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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