Novel 1,3-diethyl-2-thiobarbiturates of 2,2′-bipyridine and 1,10-phenanthroline: Synthesis, crystal structure and thermal stability

Article abstract of DOI:10.1016/j.molstruc.2018.06.035

Co-crystallization of 1,3-diethyl-2-thiobarbituric acid (HDetba) with 2,2′-bipyridine (Bipy) and 1,10-phenanthroline (Phen) results in preparing a salt co-crystal, BipyH(Detba)(HDetba) (1), and the salt, PhenH(Detba)·H₂O (2). The compounds are characterized by single–crystal and powder X–ray diffraction and TG-DSC. The nitrogen atoms of BipyH⁺ adopt a cis conformation and the N–C–C–N torsion angle is ?17.3(1)o. There are six intermolecular hydrogen bonds O–H?O, N–H?O, C–H?O and C–H?S in (1) which form a 2D plane network. One Detba^– ion and one HDetba molecule form a pair by means of O–H?O hydrogen bonds. Detba^? anions in (2) do not form dimers, they are connected by N–H?O, C–H?S, and C–H?O hydrogen bonds only with PhenH⁺ cations and water molecules which form a 3D net. Different π?π interactions between the rings are found in (1)?(2).

Full text of DOI:10.1016/j.molstruc.2018.06.035

Accepted Manuscript
Novel 1,3-diethyl-2-thiobarbiturates of 2,2′-bipyridine and 1,10-phenanthroline:
Synthesis, crystal structure and thermal stability
Nicolay N. Golovnev, Maxim S. Molokeev, Irina V. Sterkhova, Maxim K. Lesnikov
PII:
S0022-2860(18)30736-1
10.1016/j.molstruc.2018.06.035
MOLSTR 25322
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 1 March 2018
Revised Date: 7 June 2018
Accepted Date: 8 June 2018
Please cite this article as: N.N. Golovnev, M.S. Molokeev, I.V. Sterkhova, M.K. Lesnikov, Novel 1,3-
diethyl-2-thiobarbiturates of 2,2′-bipyridine and 1,10-phenanthroline: Synthesis, crystal structure and
thermal stability, Journal of Molecular Structure (2018), doi: 10.1016/j.molstruc.2018.06.035.
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Novel 1,3-diethyl-2-thiobarbiturates of 2,2’-bipyridine and 1,10-phenanthroline: synthesis,
ACCEPTED MANUSCRIPT
crystal structure and thermal stability
Nicolay N. Golovneva, Maxim S. Molokeevb,a,c*, Irina V. Sterkhovad, Maxim K. Lesnikova
a Siberian Federal University, 79 Svobodny Ave., Krasnoyarsk 660041, Russia
b
Laboratory of Crystal Physics, Kirensky Institute of Physics, Federal Research Center KSC SB
RAS, bld. 38 Akademgorodok 50, Krasnoyarsk 660036, Russia
c Department of Physics, Far Eastern State Transport University, 47 Seryshev Str., Khabarovsk
680021, Russia
d Favorsky Institute of Chemistry, Siberian Branch, Russian Academy of Sciences, 1 Favorsky,
Irkutsk 664033, Russia
* Corresponding author:
Maxim Molokeev
E-mail: msmolokeev@gmail.com
Laboratory of Crystal Physics, Kirensky Institute of Physics, Federal Research Center KSC SB RAS,
bld. 38 Akademgorodok 50, Krasnoyarsk 660036, Russia
Tel.: +7-391-249-45-07
†Electronic supplementary information (ESI) available. See DOI:xxx
Abstract
Co-crystallization of 1,3-diethyl-2-thiobarbituric acid (HDetba) with 2,2’-bipyridine (Bipy) and
1,10-phenanthroline (Phen) results in preparing a salt co-crystal, BipyH(Detba)(HDetba) (1),
and the salt, PhenH(Detba)ꢀH2O (2). The compounds are characterized by single–crystal and
powder X–ray diffraction and TG-DSC. The nitrogen atoms of BipyH+ adopt an cis
conformation and the N—C—C—N torsion angle is −17.3(1)º. There are six intermolecular
hydrogen bonds O—HꢀꢀꢀO, N—HꢀꢀꢀO, C—HꢀꢀꢀO and C—HꢀꢀꢀS in (1) which form a 2D plane
network. One Detba– ion and one HDetba molecule form a pair by means of O—HꢀꢀꢀO hydrogen
bonds. Detba− anions in (2) do not form dimers, they are connected by N—HꢀꢀꢀO, C—HꢀꢀꢀS and
C—HꢀꢀꢀO hydrogen bonds only with PhenH+ cations and water molecules which form a 3D net.
Different π−π interactions between the rings are found in (1)−(2).
Keywords: 1,3-diethyl-2-thiobarbituric acid; 1,10-phenanthroline; 2,2’-bipyridine; salt; salt co-
crystal; X-ray diffraction; infrared spectroscopy; thermal stability
1. Introduction
Derivatives of barbituric acid have anesthetic, sedative, anticonvulsive, antimicrobial,
antifungal, antiviral and anti-cancer properties [1, 2]. Moreover, the organic salts of barbituric
acids and their metal complexes possess potentially useful properties, for instance, biological
activities [3, 4], solvatochromism, molecular recognition, photoluminescence, catalytic activity
1

etc. [2]. They are promising materials for nonlinear optics (NLO) [5, 6] and organic solar cells
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[7]. While the structures of metal complexes with barbituric (H2ba) and 2-thiobarbituric acids
(H2tba) and some of their derivatives have been studied [8, 9] organic salts and co-crystalls of
barbituric acids received less attention. For example, there are no structural data on the
compounds of 1,3-diethyl-2-thiobarbituric acid (C8H12N2O2S, HDetba) (Fig. 1), although it and
its compounds can be applied in various fields. For example, HDetba has been proposed as an
alternative to acidic photopolymerizable dental materials [10] and for determining nicotine and
twelve metabolites in human urine and animals using high-performance liquid chromatography
[11]. Its compounds can be used as NLO materials [12] and for the manufacture of field-effect
transistors [13]. Similar to the H2tba, HDetba has five potential hydrogen bonding acceptors: (i)
two carbonyl O atoms, (ii) one S atom and (iii) two N atoms. Each of the N atoms is linked to
one -C2H5 group and therefore can only act as a weak hydrogen bonding acceptor. In the absence
of strong hydrogen bonding donors in HDetba, they can be replaced by C atoms, forming weak
C—HꢀꢀꢀY hydrogen bonds (Y = F, O, N) [14]. Therefore, further exploration of organic systems
of 1,3-diethyl-2-thiobarbituric acid can provide abundant supramolecular architectures.
The free bases of 2,2’-bipyridine (Bipy) and 1,10-phenanthroline (Phen) are used as
complexing agents for iron and other metal ions in chemical and biological systems. A classical
use of Phen is that of the Fe(II)-complex (ferroin) as an indicator in redox titrations [15]. Bipy is
an interesting ligand because of its potential application in preparing of nonlinear optical
materials (NLO), sensor objects, coatings and catalysis [16], single-molecule magnets [17]. As a
part of our continuing investigations in the structures and properties of 1,3-diethyl-2-
thiobarbiturate compounds [18-26], a salt co-crystal 2,2’-bipyridin-1-ium 1,3-diethyl-2-
thiobarbiturate 1,3-diethyl-2-thiobarbituric acid, BipyH(Detba)(HDetba) (1), and a salt
phenanthrolin-1-ium 1,3-diethyl-2-thiobarbiturate monohydrate, PhenH(Detba)ꢀH2O (2), were
synthesized and characterized by single-crystal and powder X-ray diffraction methods. The
thermal decomposition of compounds was also studied. To the best of our knowledge, no
information on the structure of the solid and the properties of Bipy or Phen compounds with
HDetba exists in literature [8]
2. Experimental section
2.1. Reagents and synthesis
1,3-diethyl-2-thiobarbituric acid, 1,10-phenanthrolinium chloride monohydrate and 2,2’-
bipyridine were purchased from Sigma-Aldrich and used without further purification. Sodium
hydroxide and ethanol (95%, CAS 64-17-5) as the reagents of analytical grade (Acros) were used
without additional purification.
2

Compound BipyH(Detba)(HDetba) (1) was prepared by crystallization from ethanol. 2,2'-
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bipyridine (0.039 g, 2.5 mmol) and 1,3-diethyl-2-thiobarbituric acid (0.1 g, 5.0 mmol) were
dissolved in ethanol (1.5 cm3) at room temperature. A light yellow transparent solution was left
at a temperature of 3 °C. After 2 days, the formation of yellow rhombic single crystals was
observed. The crystalline product was filtered off and dried between the sheets of filter paper in
air at room temperature (yield 67%). Elemental analysis for C26H32N6O4S2 (1): Calc.: C, 56.1%;
H, 5.79%; N, 15.1%; S, 11.5%. Found: C, 56.7%; H, 5.58%; N, 14.9%; S, 11.2%. Single crystals
suitable for the structural analysis were selected directly from the total mass of precipitate (1).
Compound (1) was crystallized from the aqueous solution at pH = 3.1 (a multitest IPL-103 pH
meter, Semico, Russia) with yield 40-60%. Repeated experiments showed poor reproducibility
of this method.
The salt, PhenH(Detba)ꢀH2O (2), was crystallized from water. 1,10-phenanthroline
hydrochloride monohydrate (0.117 g, 5.0 mmol)) was dissolved in 5 ml of water and then an
equimolar amount of solid NaOH was added. A white precipitate of Phen was formed. Then, 1,3-
diethyl-2-thiobarbituric acid (0.1 g , 5.0 mmol) was added to this mixture. The sediment partially
dissolved, changing color from white to yellow. The mixture was heated to 80 °C and kept at this
temperature for one hour until the yellow precipitate completely dissolved. The light yellow
solution was slowly cooled and allowed to evaporate at room temperature (pH = 3.9). A week
later, a little amount of orange oil was formed in the solution, its amount increased as the water
evaporated. The solid residue after complete removal of water was a mixture of orange crystals
and a glassy substance. Single orange crystals of (2) were taken directly from the resulting
mixture. Some of these crystals were used for elemental and structural analysis, as well as for
thermal measurements. Elemental analysis for C20H22N4O3S (2): Calc.: C, 60.3%; H, 5.56%; N,
14.1%; S, 8.05%. Found: C, 59.8%; H, 5.68%; N, 14.4%; S, 7.87%. However, we could not get
compound (2) at a volume sufficient for powder XRD measurements. The attempts to obtain
other compounds of HDetba with Bipy and Phen by the crystallization from the aqueous solution
were unsuccessful.
2.2. X-ray diffraction analysis
The intensity patterns were collected from single crystals (1) and (2) at 100 K using a D8
Venture X-ray single crystal diffractometer (Bruker AXS) equipped with a CCD-detector,
graphite monochromator and a Mo Kα radiation source. The absorption corrections were applied
using the SADABS program. The structures were solved by direct methods using package
SHELXS and refined in the anisotropic approach for non-hydrogen atoms using the SHELXL
program [27]. All hydrogen atoms were found via Fourier difference maps. Further, the
hydrogen atoms were linked with C, N atoms in the HDetba molecule, the Detba–, BipyH+,
3

PhenH+ ions, were positioned geometrically as riding on their parent atoms with d(C—H) =
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0.93−0.98 Å, d(N—H) = 0.86−0.89 Å depending on geometry and Uiso(H) = 1.2Ueq(C, N). All
hydrogen atoms of the H2O molecules were refined with bond length restraint d(O—H) = 0.9 Å
and Uiso(H) = 1.2Ueq(O). The structure test for the presence of missing symmetry elements and
possible voids was produced using the PLATON program [28]. The DIAMOND program was
used for crystal structure plotting [29].
Powder X-ray diffraction data of (1) was obtained using a D8 ADVANCE diffractometer
(Bruker) equipped with a VANTEC detector with a Ni filter. The measurements were made
using Cu Kα radiation. The structural parameters defined by a single crystal analysis were used
as base in the powder pattern Rietveld refinement. The refinement was produced using the
TOPAS 4.2 program [30]. Low R-factor and good refinement results (Table 1S, Fig. 1S) indicate
the phase purity of the powder sample (1).
2.3. Physical measurements
The chemical analysis was carried out with an HCNS-0 EA 1112 Flash Elemental Analyser.
TGA was carried out on a simultaneous SDT-Q600 thermal analyzer (TA Instruments, USA)
under dynamic air atmosphere (50 ml/min flow rate) within 25–350 °C at the scan rate of 10
°C/min. The compound weight was 4.516 mg for (1) and 4.590 mg for (2). Platinum crucibles
with perforated lids were used as containers.
3. Results and discussion
3.1. Theoretical consideration
2,2'-bipyridine and 1,10-phenanthroline behave as weak bases, often forming
monoprotonated species of BipyH+ and PhenH+, respectively, as a result of protonation of one N
atom. In the aqueous solution, stepwise acidity constants are pKa1 = 1.51, pKa2 = 4.48 for the
BipyH22+, and pKa1 = 1.92, pKa2 = 5.01 for the PhenH22+ (30 ºC, 0.1M KNO3) [31]. The acidity
constant is pKa1 = 2.78 for HDetba (25 ºC, 0.25M KCl) [32]. The HDetba speciation (Fig. 2)
shows the predominance of an anionic species at pH > 4 (α`1 ≈ 1), and a neutral species at pH <
1.5 (α`2 ≈ 1) in the aqueous solution. A neutral molecule Bipy species (Fig. 2) is dominated at pH
> 6 (α0 ≈ 1), a cationic species BipyH+ between pH 2 and pH 4 (α1 ≈ 0.7−0.8). A diprotonated
cationic species BipyH22+ (α2) forms at pH < 3, and it is dominated at pH < 1. The speciation of
Phen − HDetba system (Fig. 2S) shows the predominance of a neutral Phen species at pH > 6.5,
a cationic species PhenH+ between pH 2.5 and pH 4.5, and a cationic species PhenH22+ at pH < 1.
4

According to the species distribution diagrams, pH values of the aqueous solution were chosen
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for the synthesis of (1) [pH ≈ 3] and (2) [pH ≈ 4].
It is generally accepted that the reaction of an acid with base is expected to form a salt if
ꢁpKa= pKa(base) − pKa(acid) > 2 or 3 [33]. Indeed, when the equimolar amounts of HDetba
interact with Phen (ꢁpKa = 2.23), salt (2) was obtained. The difference in the values of Ka of
BipyH+ и HDetba is less (ꢁpKa = 1.70), which promoted the formation of (1) salt co-crystals at
pH 3.
3.1. Crystal structures of (1)
The unit cells of (1) correspond to triclinic symmetry. Space group P-1 was determined from
the statistical analysis of reflection intensities. The main crystal data are enumerated in Table 1.
Compound (1) is a salt co-crystal [34]. The asymmetric unit contains one BipyH+ ion, one
Detba– ion and one HDetba molecule (Fig. 3a). The main bond lengths C—N, С—С and valence
angles (Table 2S) practically coincide with those found earlier in other related compounds of
HDetba [18−26], but С—S and O1—C4 bonds are slightly (0.01-0.02 Å) shorter. However, the
O2A—C6A bond is noticeably (0.03−0.04 Å) longer than the others (Table 2S), since the O2A
atom in the HDetba molecule is bonded to the hydrogen atom, that is, the HDetba acid is in the
enol form. The conformations of Bipy and its protonated species constitute an important aspect
in understanding the properties of this popular ligand in coordination chemistry. Bipy exists in a
trans conformation in solid state [35]. The cis conformer of the cation is by far more stable than
trans, according to ab initio self consistent-field (SCF) calculation [36]. As expected, 2,2’-
bipyridine offers interesting motifs by virtue of the predominantly cis dispositions of the pairs of
nitrogen atoms in its monoprotonated salts [8]. The monoprotonated cis species of BipyH+ could
be stabilized by cationic hydrogen bond N
However, there are also compounds in which BipyH+ is in the form of a trans conformer [8, 37].
In (1), the bipyridinium cation has a cis conformation and the N N torsion angle is
−17.3(1)º. It clearly indicates that BipyH+ does not behave as a proton sponge. As expected [38
a weak intramolecular hydrogen bond is found in the BipyH+ cation with an N2ꢀꢀꢀN1 distance of
2.654(1) Å and N2 HꢀꢀꢀN1 angle of 106.52(7)º. The main bond lengths C—N, С—С and the
—HꢀꢀꢀN, as in numerous proton sponges [14].
—C—C—
],
—
angles in BipyH+ practically coincide with those found earlier in other related compounds [8].
For instance, the bond lengths C7C—C8C (1.4790(16) Å), N1C—C8C 1.3470(15) Å) and
N2C—C7C (1.3514(16) Å) coincide with those found in [BipyH]ClO4 [C7C—C8C (1.481 Å),
N1C—C8C 1.347 Å) and N2C—C7C (1.352 Å)] (CSD refcode PYPYPC02) [39]. In (1), the
BipyH+ ion is planar, the standard deviation of atoms from this plane is 0.183 Å. As expected
[40], the effect of protonation at N2 site in BipyH+ is manifested in the C—N—C angles being
much wider than those at an unprotonated N1 atom [123.4(1)° versus 117.2(1)°]. Six
5

intermolecular hydrogen bonds O—HꢀꢀꢀO, N—HꢀꢀꢀO, C—HꢀꢀꢀO and C—HꢀꢀꢀS in the structure
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1
1
8
(Table 2) form a 2D plane network with supramolecular motifs R2 (6), R2 (7) and R8 (39)
(Figure 4a). One Detba– ion and one HDetba molecule join to form pairs by means of O2A—
H1AꢀꢀꢀO2B and C5A—H5AꢀꢀꢀO2B hydrogen bonds. In the salt co-crystal (1), a strong hydrogen
bond O2A—H1AꢀꢀꢀO2B is characterized by a small distance d(O···O) = 2.440 (1) Å, a large
value of the angle O2A—H1A···O2B (178(2) º) and a long bond O2A—H1A (d(O2A—H1A) =
1.32(2) Å). Close geometric parameters of the O—H···O hydrogen bond were previously
established in the salt co-crystal of the piperidine (Pipe) with 2-thiobarbituric acid,
PipeH(Htba)(H2tba) (3) [41]. In the formation of strong hydrogen bonds, an increase in the
distance d(O—H) reaches 0.2Å [42] as in compound (1). In (3), the Htba− ion and the H2tba
molecule also form a pair, but this happens due to two hydrogen bonds N
are combined into infinite chains by the intermolecular hydrogen bonds O
Replacing the hydrogen atoms of the NH groups in H2tba by the C2H5 groups leads to weakening
of N atoms donor capacity. In this case, weak C HꢀꢀꢀO hydrogen-bonds in (1) play an important
role in the formation of the supramolecular structure. The C HꢀꢀꢀO hydrogen-bond distances in
—
HꢀꢀꢀO. These pairs
—
HꢀꢀꢀO and N HꢀꢀꢀO.
—
—
—
(1) are of the same order of magnitude as those reported previously [37].
Topological analysis of the net, using simplification and excluding all 0-, 1- and 2-
coordinated nodes revealed that this is a uninodal 4-c net with point symbol (43.63) which is
known as SP (6,3)Ia [30]. It should be noted that H-bond net of NH4Detba [18] forms a relative
2D net with point symbol (44.62) named sql. Compounds LiDetba and NaDetba [19] have
relative 2D nets (42.64) named SP (6,3)Id. Compounds M(Detba)2(H2O)2 (M = Ca, Sr, Mn) [23,
26] also have 2D nets (44.62) sql, compound Eu(Detba)3 [24] has 2D net (63) hcb, and
Pb(Detba)2 [21] has 2D net (43)2(44.66.83) kgd. A common property of all these nets is that they
form only 2D nets and this can be associated with a small amount of hydrogen bond donors in
HDetba ligands. Further analysis of interactions in (1) reveals π−π interactions between HDetba,
Detba– and BipyH+ rings (Table 3S, Fig. 3Sa).
3.2. Crystal structures of (2)
The unit cell of (2) also corresponds to triclinic symmetry. The main crystal data are shown in
Table 1. The asymmetric unit contains two Detba– ions, two PhenH+ ions and two water
molecules. (Fig. 3b). The structure test for the presence of missing symmetry elements revealed
nonspacegroup translation A (0, 1/2, 1/2). As groups of atoms (O1C, O1D); (C9C, C9D), (O1W,
O2W) violated this translation, the crystal has pseudosymmetry and no obvious spacegroup
change was needed. The main bond lengths C—N, С—С (Table 2S) and the valence angles in
HDetba− ion coincide with those found earlier in other related compounds [18−26]. Similar to (1)
С—S and O1—C4 bonds are slightly (0.01−0.02 Å) shorter. In PhenH+, the C1A—C2A
6

(1.434(2) Å), C1B—C2B 1.436(2) Å), N1A—C2A (1.363(2) Å), N1B—C2B (1.363(2) Å),
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N2A—C1A (1.357(2) Å), and N2B—C1B (1.359(2) Å) distances coincided with those found
earlier (CSD refcode CUZDIK) [43]. PhenH+ ions are practically flat, so the standard deviation
of atoms from planes for ions A and B is only 0.048 and 0.052 Å, respectively. As in compound
(1), the effect of protonation at the N2A and N2B sites in A and B respectively, is manifested in
the C—N—C angles being much wider than those for the unprotonated nitrogen atoms
[123.1(1), 123.0(1)° versus 116.5(1), 116.4(1)°
bond is found in A and B of PhenH+ cation with an N2ꢀꢀꢀN1 distance of 2.7476(2) Å and
2.7517(2) Å, and N HꢀꢀꢀN angle of 104.121(9)º, 104.31(1)º, respectively. The torsion angle
]. Similar to (1), a weak intramolecular hydrogen
—
C8—C7—C9—C10 of Detba− ions in (1)−(2) is small (<4 deg) therefore the Detba- ions are
present in conformational state (A) [18].
Nine N—HꢀꢀꢀO, O—HꢀꢀꢀO, C—HꢀꢀꢀO and C—HꢀꢀꢀS intermolecular hydrogen bonds in the
structure (Table 2) form a 3D net. Some 2D nets can be marked (Fig. 4b) and the shortest ring
4
4
6
motifs in them are R4 (18), R4 (20), R6 (20). Water molecules participate in moderately strong
O—HꢀꢀꢀO interaction with anions (d(OꢀꢀꢀO) = 2.809 and 2.876 Å). There are also π−π
interactions between Detba– and PhenH+ rings (Table 3S, Fig. 3Sb). The self-assembly and
crystal structure for 1,10-phenanthrolinium barbiturate monohydrate, PhenH(Hba)ꢀH2O (CSD
refcode MUYVET), and 1,10-phenanthrolinium 2-thiobarbiturate, PhenH(Htba) (CSD refcode
MUYVET) have been reported earlier [5]. The study demonstrated how two barbiturate or
thiobarbiturate moieties join to form dimers by means of N—HꢀꢀꢀO hydrogen bonds (d(NꢀꢀꢀO) =
2.805 and 2.823 Å). These dimers were stabilized by the cation−anion interactions of N—HꢀꢀꢀO
and N—HꢀꢀꢀN types. As in (2), the water molecules in PhenH(Hba)ꢀH2O participate in the
formation of moderately strong hydrogen bonds with Hba− anions. Similar to (1)−(2), in
PhenH(Hba)ꢀH2O and PhenH(Htba) the C—N—C angles for protonated N atoms were larger
than for the unprotonated N atoms, and the charge in O1=С4—С5—С6=O2 group of Hba− and
Htba− was delocalized.
Topological analysis of (2) using ToposPro program [30] showed that this 3D net is a 6-
nodal
(3-c)(4-c)(4-c)(5-c)(5-c)(5-c)
net
with
the
point
symbol
(3.4.5.63)(3.4.65.72.8)(3.42.5.62)(4.6.8)(42.65.72.8)(43.66.7), which is our novel contribution.
3.4. Thermal decomposition
The TG-DSC curves of BipyH(Detba) (1) and(PhenH(Detba)ꢀH2O (2) in the range of 25−300 °С
are shown in Fig. 4S and Fig. 5S, respectively. The TG curve clearly indicates that thermal
decomposition/oxidation of (1) occurs in one step (Fig. 4S). An endothermic effect for (1) at
110.3 °С corresponds to the compound melting. This value of melting point is higher than that
7

for Bipy (70.5 °С) and less than that for HDetba (112°С, [19]), which confirms the individuality
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of the compound (1). The decrease in the mass of the substance at T > 112 °C corresponds to the
oxidation of compound and evaporation of HDetba and Bipy. In the range of 150−230 °C, the
mass of the sample decreased rapidly in accordance to the mean TG curve (Fig. 4S). The
transformations at 150−230 °С are accompanied by the exo effect at ~160 °С and endo effect at
216 °C. The appearance of a weak endothermic peak at 216 °C can be explained by the thermal
effects superposition of oxidative decomposition and evaporation of thermolysis products. In the
temperature range of 150−230 °C, almost all the bipyridine is removed, which is confirmed by
the absence of an endothermic effect at the boiling point of Bipy (273 °C). At 250 °C, the total
mass loss reaches ~95%, and then the mass decreases only by ~ 1% when heated to 300 °C. The
carbonaceous residue (~5%) is probably formed as a result of incomplete oxidation of the
HDetba molecules and Detba− ions with the release of CO2. As seen from the TG curve (Fig.
5S), the thermal decomposition process for the (2) can be divided into three stages. The first
stage starts in the range of 70—135 °С with mass loss of 4.60%, which corresponds to the loss of
H2O molecule with the theoretical mass loss of 4.52%. The Phen and HDetba have their melting
points of 117 °С and 112 °С [19], respectively, these data confirm the individuality of compound
(2). The dehydration of (2) is accompanied by endo effect at 93.6 °C. In the second stage of
transformation at 140−250 °C, evaporation and oxidation of 1,3-diethyl-2-thiobarbituric acid and
phenanthroline occur simultaneously. With the temperature over 250 °C, the endothermic
evaporation processes of HDetba and Phen are likely to become increasingly important, as
evidenced by the endothermic peak at 275 °C on the DSC curve. The carbonaceous residue of
thermolysis (2) at 310 ° C was an oily black substance.
4. Conclusions
For the first time, the organic salt co-crystal (1) and salt (2) of 1,3-dialkyl-2-thiobarbituric acid
were prepared and structurally characterized. The compounds were obtained by the reaction in
the aqueous medium of an acid with the Bipy and Phen. At Bipy : HDetba = 1: 2 molar ratio, an
anhydrous salt co-crystal (1) was formed, containing BipyH+ ion, Detba− ion, and HDetba
molecule. The salt (2) was obtained by the reaction of an acid with the 1,10-phenanthroline at a
Phen : HDetba = 1 : 1 molar ratio. It contains one PhenH+ cation, one Detba− anion and one
water molecule. The formation of the salt co-crystal (1) and the salt (2) is consistent with the
"ꢁpKa rule" proposed earlier for two-component organic crystals [33].
The protonation of the N atom in (1) and (2) resulted in a slight deformation of the ring and
consequently a slightly larger C—N—C bond angle (>120°). In (1), the O—HꢀꢀꢀO and C—HꢀꢀꢀO
8

intermolecular hydrogen bonds connect the anion and neutral acid in a pair. The O
hydrogen bond between an acid (HDetba) and its conjugate base (Detba−) is very strong, with
—
HꢀꢀꢀO
ACCEPTED MANUSCRIPT
d(OꢀꢀꢀO) = 2.440(1) Å. The Detba− anion is bound to one BipyH+ ion by two N
HꢀꢀꢀO intermolecular hydrogen bonds (Fig. 4a). HDetba is bound by two C
intermolecular hydrogen bonds with one BipyH+ cation and C
—
HꢀꢀꢀO and C
—
—
HꢀꢀꢀO
—
HꢀꢀꢀS intermolecular hydrogen
bond with another cation. BipyH+ cations in (1), and PhenH+ in (2), are not directly connected to
each other (Fig. 2). The Detba− anions in (2) do not form dimers, unlike barbiturate and
thiobarbiturate ions in corresponding phenanthrolinium salts [5]. Each chain of two water
molecules is bound by the OW
turn are connected with to the PhenH+ cations by the N
HꢀꢀꢀX (X = O, S) weak hydrogen bonds exert a significant influence on the supramolecular
—
HWꢀꢀꢀO hydrogen bonds with three Detba− anions, which in
HꢀꢀꢀO, C HꢀꢀꢀS hydrogen bonds. The
—
—
C
—
architecture of the compounds. The compound (2) has a new topology of a 3D net.
Several types of π−π interactions between the rings were found in the (1)-(2) (Fig. 2S). Co-
crystal of (1) melts at 110.3 ° C and salt (2) is dehydrated in the range of 70-135 ° C, a further
increase in temperature leads to their oxidative decomposition.
Supplementary data
The crystallographic data (excluding structure factors) for the structural analysis have been
deposited with Cambridge Crystallographic Data Centre ((1) - CCDC # 1826542; (2) -
CCDC # 1826544). The information may be obtained free of charge from The Director, CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44(1223)336-033, E-mail:
deposit@ccdc.cam.ac.uk, or www: www.ccdc.cam.ac.uk).
Acknowledgements
The study was carried out within the public task of the Ministry of Education and Science of the
Russian Federation to the Siberian Federal University (4.7666.2017/BP) in 2017-2019 and
RFBR according to research projects 17-52-53031. X-ray data from single crystals were obtained
with use the analytical equipment of Baikal Center of collective use of SB RAS and with use the
analytical equipment of Krasnoyarsk Center of collective use of SB RAS.
9

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Graphical Abstract
12

Table 1. Crystal structure parameters of (1) and (2)
ACCEPTED MANUSCRIPT
BipyH(Detba)(HDetba) (1)
C26H32N6O4S2
0.40×0.31×0.18
Yellow
PhenH(Detba)ꢀH2O (2)
C20H22N4O3S
0.34×0.31×0.15
Yellow
Single crystal
Moiety formula
Dimension (mm)
Color
556.69
398.48
Molecular weight
Temperature (K)
Space group, Z
a (Å)
100
100
P-1, 2
P-1, 4
10.1936 (6)
11.0122 (6)
13.4505 (8)
68.875 (2)
87.388 (2)
72.767 (2)
1342.00 (14)
1.378
10.9194 (9)
13.351 (1)
13.8901 (11)
104.723 (3)
101.020 (3)
99.626 (3)
1872.1 (3)
1.414
b (Å)
c (Å)
α (º)
β (º)
γ (º)
V (Å3)
ρcalc (g/cm3)
ꢂ (mm-1)
0.243
0.203
Reflections
measured
Reflections
independent
Reflections with F
> 4σ(F)
57298
7863
90030
10998
6564
7730
60.15
60.16
2θmax (°)
h, k, l - limits
Rint
-14 ≤ h ≤ 14;
-15 ≤ k ≤ 15;
-18 ≤ l ≤ 18
0.0381
-15 ≤ h ≤ 15;
-18 ≤ k ≤ 19;
-19 ≤ l ≤ 19
0.0775
2
The weighed
refinement of F2
Number of
w=1/[σ2(Fo )+(0.0385P)2+0.6
w=1/[σ2(Fo )+(0.056P)2+0.744P]
2
513P]
350
521
refinement
parameters
0.0344
0.0891
1.017
0.382
-0.359
0.001
0.0493
0.1241
1.036
0.893
-0.406
0.404
R1 [Fo > 4σ(Fo)]
wR2
Goof
∆ρmax (e/Å3)
∆ρmin (e/Å3)
(∆/σ)max
13

Table 2. Hydrogen-bond geometry in (1) and (2) structures (Å, °).
ACCEPTED MANUSCRIPT
D—
Transformation
for A atom
D—H
d(D—H) d(H···A)
D···A
A
H···A
BipyH(Detba)(HDetba) (1)
N2C—H2C
C6C—H6C
C9C—H9B
C4C—H4C
C5A—H5A
O2A—H1A
0.86
0.93
0.93
0.93
1.91
142
163
168
171
2.6395 (14)
O1B
O1A
O1A
S1
x, y, z
x, y, 1+z
x, y, 1+z
1+x, -1+y, 1+z
x, y, z
2.39
2.53
2.76
3.2923 (17)
3.4411 (15)
3.6822 (14)
3.233 (1)
0.93 2.6486(7) 121.47 (8)
1.12(2) 1.32 (2) 178 (2)
O2B
O2B
2.440 (1)
x, y, z
PhenH(Detba)ꢀH2O (2)
N1A—H1A
N1B—H1B
0.86
0.86
1.94
1.96
142
139
2.671 (2)
O2D
O2C
O2C
O1D
O1W
O2D
O1C
S2
x, y, z
x, y, z
2.674 (2)
2.809 (2)
2.876 (2)
2.854 (2)
2.827 (2)
2.992 (2)
3.639 (2)
3.030 (2)
O1W—H1WA 0.90 (2) 1.94 (2)
O1W—H1WB 0.83 (2) 2.11 (2)
O2W—H2WA 1.05 (2) 1.82 (2)
O2W—H2WB 1.05 (2) 1.79 (2)
163 (2)
154 (2)
168 (2)
170 (2)
142
x, y, z
1-x, 1-y, 1-z
x, y, z
x, y, z
C12B—H12B
C10A—H10A
C12A—H12A
0.93
0.93
0.93
2.20
2.84
2.39
1-x, 2-y, 2-z
x, 1+y, z
1-x, 1-y, 1-z
145
126
O1D
14

ACCEPTED MANUSCRIPT
Fig. 1. Graphic formulas of 1,3-Diethyl-2-thiobarbituric acid (HDetba)
Fig. 2. Bipy and HDetba species distribution diagram as a function of solution pH: α0 − Bipy, α1
− BipyH+, α2 − BipyH22+ ; α`2 − HDetba, α`1− Detba−. Here αi, αi’ are the molar fractions of
corresponding species.
15

ACCEPTED MANUSCRIPT
a)
b)
Fig. 3. The asymmetric unit of the (1) (a), (2) (b) unit cell. Symmetry independent, different
molecules are marked by A, B, C, D labels. All atoms in the asymmetric unit are labeled. The
neighboring symmetry-generated atoms are represented by principal ellipses with an individual
color. The intermolecular hydrogen bonds are represented by dashed lines. The ellipsoids are
drawn at the 50% probability level, except for the hydrogen atoms represented by spheres.
16

ACCEPTED MANUSCRIPT
a)
b)
Fig. 4. Hydrogen bonding in (1) (a) and (2) (b). The H-bonds are marked by dashed lines, the H-
bond motifs are marked by circles. Different ions/molecules are marked by A, B, C, D labels.
Some H atoms were deleted to simplify the figure.
17

ACCEPTED MANUSCRIPT
- The Bipy and the Phen with HDetba form salt co-crystal (1) and salt (2), respectively
- In (1), H-bonds connect Detba– and HDetba in a pair, in (2) the Detba– ions do not
- The O—H···O hydrogen bond between an acid HDetba and Detba− is very strong
- The compound PhenH(Detba)·H2O has a new topology of a 3D net
- Co-crystal of (1) melts at 110.3 °C and salt (2) starts to dehydrate at 70 °C