Copper-assisted desulfurization of 1-R-tetrazole-5-thiols under complexation

Article abstract of DOI:10.1016/j.inoche.2020.107827

The interaction of 1-R-tetrazole-5-thiols with copper(II) chloride in solution was found to yield crystalline tetrazole complexes [CuCl₂L₂]_n (L is 1-R-tetrazole, R = Me, Et, Bu) and [CuCl₂L₂]_n·nMeCN (L is 1-phenyltetrazole). The observed transformation of 1-R-tetrazole-5-thiols into 1-R-tetrazoles presents the first examples of desulfurization of tetrazole-5-thiols under complexation. A possible pathway of desulfurization of 1-R-tetrazole-5-thiols is proposed. It includes initial oxidation of tetrazole-5-thiols by copper(II) ions into bis(tetrazol-5-yl)disulfanes, followed by copper-assisted oxidation by air oxygen. Obtained complexes [CuCl₂L₂]_n and [CuCl₂L₂]_n·nMeCN were found to be coordination polymers, 2D and 1D, respectively. In all them, 1-R-tetrazoles act as monodentate ligands coordinated to the metal via the tetrazole ring N⁴ atom, whereas chlorine atoms provide bidentate bridging coordination to generate polymeric structure.

Full text of DOI:10.1016/j.inoche.2020.107827

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Copper-assisted desulfurization of 1-R-tetrazole-5-thiols under complexation
Sergei V. Voitekhovich, Alexander S. Lyakhov, Ludmila S. Ivashkevich,
Oleg A. Ivashkevich
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DOI:
https://doi.org/10.1016/j.inoche.2020.107827
INOCHE 107827
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Please cite this article as: S.V. Voitekhovich, A.S. Lyakhov, L.S. Ivashkevich, O.A. Ivashkevich, Copper-assisted
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Copper-assisted desulfurization of 1-R-tetrazole-5-thiols under complexation
Sergei V. Voitekhovicha,*, Alexander S. Lyakhova, Ludmila S. Ivashkevicha, Oleg A. Ivashkevichb
aResearch Institute for Physical Chemical Problems of Belarusian State University,
Leningradskaya Street 14, Minsk 220006, Belarus
bBelarusian State University, Nezalezhnasti avenue 4, 220050 Minsk, Belarus
H
N
N
CuCl2, air
EtOH, HCl
L =
N
R
[CuCl2L2]n
N
SH
N
R = Me, Et, Bu
N
N
R
N
CuCl2, air
MeCN
H
[CuCl2L2]n · nMeCN
N
N
L =
N
Ph
N
Abstract
The interaction of 1-R-tetrazole-5-thiols with copper(II) chloride in solution was found to yield
crystalline tetrazole complexes [CuCl2L2]n (L is 1-R-tetrazole, R = Me, Et, Bu) and
[CuCl2L2]n·nMeCN (L is 1-phenyltetrazole). The observed transformation of 1-R-tetrazole-5-
thiols into 1-R-tetrazoles presents the first examples of desulfurization of tetrazole-5-thiols under
complexation. A possible pathway of desulfurization of 1-R-tetrazole-5-thiols is proposed. It
includes initial oxidation of tetrazole-5-thiols by copper(II) ions into bis(tetrazol-5-yl)disulfanes,
followed by copper-assisted oxidation by air oxygen. Obtained complexes [CuCl2L2]n and
[CuCl2L2]n·nMeCN were found to be coordination polymers, 2D and 1D, respectively. In all
them, 1-R-tetrazoles act as monodentate ligands coordinated to the metal via the tetrazole ring N4
atom, whereas chlorine atoms provide bidentate bridging coordination to generate polymeric
structure.
Keywords: Tetrazoles; Thiols; Copper(II) complexes; Desulfurization; Oxidation
1-Substituted tetrazole-5-thiols attract considerable attention because of their interesting
properties and applications [1]. In particular, they are intensively studied as corrosion inhibitors
[2, 3] and capping agents for the stabilization of semiconductor and noble metal nanocrystals [4].
Observed high efficiency of these compounds in the processes of inhibition and stabilization is
* Corresponding author. Tel.: +375 17 2095198; fax: +375 17 2264696.
E-mail addresses: azole@tut.by (S.V. Voitekhovich).

due to their strong coordination ability towards various metal cations. As ligands, tetrazole-5-
thiols are able to generate a wide variety of coordination compounds [3, 5]. This is not only due
to the presence of several competing donor centers, but also due to thiol-thione tautomerism and
the ability to undergo deprotonation with the formation of stable tetrazole-5-thiolates. For this
reason, the structures and properties of coordination compounds derived from tetrazole-5-thiols
have been studied intensively. In continuation of our earlier synthetic and structural
investigations of complexes with tetrazole ligands [6–8], in the present work we aimed to study
the interaction of copper(II) chloride with 1-substituted tetrazole-5-thiols. Although several
copper complexes of these ligands were structurally characterized [9–19], only few of them
belong to copper(II) family [17–19].
In order to prevent deprotonation of ligands, the interaction of 1-R-tetrazole-5-thiols 1a–c
with copper(II) chloride was carried out in ethanol in the presence of hydrochloric acid (Scheme
1). Green crystalline materials were isolated after slow evaporation of the reaction mixture on air
[20]. Elemental analysis and single crystal X-ray analysis showed that not thiotetrazole
complexes, but 1-R-tetrazole ones 2a–c were formed. In the case of 1-phenyltetrazole 1d,
formation of white precipitate and colorless crystals was observed under the same conditions. By
using single crystal X-ray analysis, the isolated crystals were identified as 1,2-bis(1-phenyl-1H-
tetrazol-5-yl)disulfane because its crystal data corresponded to those reported earlier [21]. The
crystals were characterized by MS and IR spectroscpies [22]. Further, mixture of reaction
products was dissolved in acetonitrile, and evaporation of obtained solution on air for 5 days
gave green crystalline complex 2d. It should be noted that the interaction of thiols 1a and 1d
with copper(II) chloride in acetonitrile leads to formation of complexes 2a and 2d,
correspondingly [23]. Thus, transformation of 1-R-tetrazole-5-thiols into 1-R-tetrazoles takes
place under complexation of 1.
H
CuCl2, air
N
[CuCl2L2]n
L =
SH
N
N R
EtOH, HCl
N
N
2a c

N
N
R
H
N
CuCl2, air
MeCN
N
L =
1a d
[CuCl2L2]n · nMeCN

N Ph
N
N
2d
a
b
c
d
R = Me, R = Et, R = Bu, R = Ph
Scheme 1. Complexation of 1-R-tetrazole-5-thiols
The obtained results allow to propose possible pathway of the observed desulfurization
(Scheme 2). At the initial stage of the reported complexation, slow addition of a solution of a
copper(II) salt to a solution of tetrazole-5-thols leads to the discoloration of the added solution.
2

After complete mixing the reagents, the reaction mixture becomes black in EtOH or raspberry
colored in MeCN. The observed color change and formation of disulfane 3 indicate oxidation of
thiol by copper(II) with the formation of copper(I). Such reactions are known for thiols, for
example for glutathione and cysteine [24]. Notably, disulfane 3 with R = Ph was synthesized by
oxidation of 1d by Fe(NO3)3 [21]. The formation of disulfane 3 is a key stage of the reported
desulfurization of tetrazole-5-thiols. This is confirmed by the fact that other tetrazole derivatives
having C5–S bond, namely 1-methyl-5-methylthio- and 1-methyl-5-methylsulfonyltetrazoles, do
not undergo desulfurization under action of copper(II) chloride, generating complexes of initial
ligands [6]. Disulfide bond is known to be broken by metal ions, but the mechanism of S−S bond
cleavage is quite complicated and may follow many pathways, including formation of sulfinate
and sulfonate derivatives [25]. Formation of these derivatives is also possible in our case. In
electrospray ionization mass spectrum of reaction mixture after 1 h interaction of 1d and CuCl2
in MeCN, molecular peak of disulphane 3 with R = Ph was observed (m/z = 355 [M + H]+). This
peak was not observed in mass spectrum of reaction mixture after 5 days interaction, whereas
new peaks at 421 and 458 m/z were detected. These peaks can be assigned to more oxidized
species, PhCN4S(O)2S(O)N4CPh (m/z
=
421 [M
+
H
+
H2O]+) and
PhCN4S(O)(OH)OS(O2)N4CPh (m/z = 458 [M + Na]+). As for the transformation of the thiol
group, finally it is oxidized to sulfate anion, since copper(II) sulfate monohydrate was found
among reaction products by using X-ray analysis. Sulfate anion can be formed by hydrolysis of
tetrazole-5-sulfonate while generating 1-R-tetrazole ligand.
N
N
N
N
R
N
N
N
N
S
SH
Cu2+
N
N
N
S
Cu+
R
R
N
3
Cu2+/Cu+
air O2
R
N
N
S(O)n
N
N
N
N
N
S(O)n
R
N
Cu2+/Cu+
air O2
O
N
N
N
H2O
N
O
S
H
2-
N
N
N
-SO4
N
O
R
R
Scheme 2. Possible pathway of copper-assisted desulfurization of 1-R-tetrazole-5-thiols
3

Complexes 2a and 2b were identified by comparison of their crystallographic data,
obtained on single crystals, with reported structural data for copper(II) chloride complex of 1-
methyltetrazole [26] and copper(II) chloride complex of 1-ethyltetrazole [27, 28], respectively.
The compared data were found to be in a good agreement. Complexes 2c and 2d were hitherto
unknown, and in this regard their crystal structures were obtained in the present work [29]. Since
only powder X-ray structural data [26] have been reported for 2a, we reinvestigated this
compound in the present work by using single crystal X-ray analysis [29] to improve the data.
Single crystal X-ray data of complexes 2a, 2с, and 2d were collected on a SMART Apex II
diffractometer using graphite monochromated Mo Kα radiation (λ=0.71073 Å) at 100 K for 2a
and 2d, and at room temperature for 2c. The structures were solved by direct methods (SIR2014)
[30] and refined on F2 by the full-matrix least squares technique (SHELXL 2014) [31]. The
intensities were corrected for absorption. For all compounds, non-hydrogen atoms were refined
anisotropically. The hydrogen atoms were placed in calculated positions and refined in a “riding”
model, with Uiso(H) = 1.5Ueq(C) for the methyl H atoms and Uiso(H) = 1.2Ueq(C) for other H
atoms.
Complex 2c presents a layered coordination polymer (Fig. 1). The copper atom, lying on
inversion center, shows considerably elongated octahedral coordination, with two Cl atoms and
two tetrazole ring N4 atoms of two ligand molecules in the equatorial sites [Cu–Cleq = 2.2885(4)
Å, Cu–N = 1.9956(15) Å] and two Cl atoms in the axial positions [Cu–Clax = 2.9972(4) Å]. Long
axial bonds are mainly electrostatic in nature. The copper and chlorine atoms form a polymeric
net, showing (4,4) type grid topology, with the alternation of short and long Cu–Cl bonds.
Complex 2d was found to be 1D coordination polymer, with uncoordinated disordered
acetonitrile solvent molecules in the cavities (Fig. 2). The coordination polyhedron of the Cu
atom is a highly elongated octahedron. The equatorial sites are occupied by two Cl atoms in cis
position [Cu–Cleq = 2.2783(3) and 2.2800(3) Å] and two tetrazole ring N4 atoms of two ligands
[Cu–N = 2.0044(11) and 2.0236(11) Å]. The more distant axial positions are occupied by two Cl
atoms [Cu–Clax = 2.8526(4) and 3.0440(4) Å], and in this regard these coordination bonds can be
considered as mainly electrostatic in nature. Coordination polymeric chains, extending along the
a axis, are formed at the expense of double chlorido bridges between two neighboring copper
atoms. In the coordination chain, each axial Cl atoms of one Cu octahedron lies in the equatorial
plane of neighboring Cu octahedron. The chains are linked into three-dimensional network by
intermolecular C–H···N interactions between the phenyl H and thetrazole ring N atoms, by C–
H···Cl hydrogen bonds of the phenyl and the tetrazole ring H atoms, and also by weak face-to-
face – interactions between symmetry-related phenyl rings. Acetonitrile solvate molecules are
located in cavities of this 3D network.
4

Fig. 1. Layered crystal structure of 2c viewed along the b axis (left), and a coordination layer in
2c viewed approximately along the a axis (right). Hydrogen atoms are omitted for clarity. Color
scheme for atoms: Cu red, Cl green, N blue, C black.
Fig. 2. Crystal packing of 2d viewed along the a axis (left), and coordination chain extending
along the a axis (right). Hydrogen atoms are omitted for clarity. Color scheme for atoms
corresponds to that in Fig. 1.
5

Coordination motifs of complexes 2c and 2d correspond to previous observations
concerning structural motifs of complexes [CuX2(1-R1-tetrazole)2]n (X
Cl, Br). According to
=
these observations, the complexes present 2D coordination polymers (with trans-orientation of
tetrazole ligands) when R1 substituents are non-furcate in α position, but 1D coordination motif
(with cis-orientation of tetrazole ligands) is realized in the case of furcate substituents. Complex
2c belongs to the first group together with complexes with R1
= Me [26, 32], Et [27, 28],
ClCH2CH2 [33], HOCH2CH2 [34], N3CH2CH2 [35], CH2=CHCH2 [36], CH2=CH [37], n-C6H11
[38]. Complex 2d and complexes with R1 = t-Bu [39], Ph [40], 2,4,6-Me3C6H2 [41] fall to the
second group. One should note that complex with R1 = Ph [40] was obtained by usual reaction of
1-phenyl-1H-tetrazole with copper(II) chloride, and the obtained crystals included uncoordinated
1-phenyl-1H-tetrazole molecules. Similar situation was realized in crystals of complex 2d,
containing uncoordinated acetonitrile solvent molecules. Probably, bulky rigid phenyl substituent
causes the formation of crystal packing with rather large cavities, which are filled with
uncoordinated solvent or ligand molecules.
In summary, we have presented the first examples of copper-assisted desulfurization of 1-
R-tetrazole-5-thiols. Therefore we note that possible desulfurization should be taken into account
when designing coordination compounds based on thiotetrazoles and other heterocyclic thiols
also.
Appendix A. Supplementary material
CCDC 1968606, 1968608 and 1968607 contain the supplementary crystallographic data for 2a,
2c and 2d, respectively. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
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[20] Solution of copper(II) chloride dihydrate (2 mmol) in ethanol (5 ml) was added under
stirring to the solution of 1-R-tetrazole-5-thiol 1a-c (2 mmol) in mixture of concentrated
hydrochloric acid (5 ml) and ethanol (10 ml). Obtained reaction mixture was allowed to
stand at room temperature for 4–5 days, and thereafter green crystals of 2a-c suitable for
X-ray analysis were formed.
Complex 2a. Yield: 49 %. Anal. Calc. for C4H8Cl2CuN8 (302.61): C, 15.88; H, 2.66; N,
37.03. Found: C, 16.07; H, 2.40; N, 36.65. FT-IR (neat): 3139 vs, 3036 s, 3018 s, 2961 m,
2878 w, 2811 w, 1757 m, 1599 m, 1515 s, 1468 s, 1445 s, 1421 s, 1373 m, 1308 m, 1293 s,
1236 m, 1176 vs, 1099 vs, 1062 m, 1023 s, 998 s, 880 s, 715 m, 682 s, 653 s cm–1.
8

Complex 2b. Yield: 40 %. Anal. Calc. for C6H12Cl2CuN8 (330.67): C, 21.79; H, 3.66; N,
33.89. Found: C, 21.88; H, 3.50; N, 33.50. FT-IR (neat): 3134 vs, 2989 s, 2949 s, 1753 v,
1643 m, 1502 s, 1468 s, 1383 s, 1364 s, 1307 m, 1282 m, 1202 s, 1172 s, 1097 s, 1015 s,
997 m, 879 s, 800 m, 715 m, 669 s, 645 s cm–1.
Complex 2c. Yield: 50 %. Anal. Calc. for C10H20Cl2CuN8 (386.77): C, 31.05; H, 5.21; N,
28.97. Found: C, 31.17; H, 5.29; N, 28.58. FT-IR (neat): 3124 vs, 2963 vs, 2934 s, 2873 s,
1758 m, 1682 m, 1499 s, 1465 s, 1452 s, 1375 s, 1320 m, 1273 m, 1235 m, 1180 s, 1168 s,
1107 s, 1089 s, 1052 m, 1016 m, 940 m, 880 s, 810 w, 750 m, 717 m, 672 m, 656 m, 640
m, 580 w cm–1.
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[22] 1,2-bis(1-phenyl-1H-tetrazol-5-yl)disulfane. Mp 143-145 C. MS (ESI): m/z (%) = 355
(100) [M + H]+. FT-IR (neat): 3187 m, 3106 s, 3070 s, 3058 s, 3026 s, 1595 s, 1497 s, 1461
s, 1407 s, 1384 s, 1319 m, 1294 m, 1266 s, 1227 s, 1178 w, 1157 m, 1085 s, 1074 s, 1015
s, 976 s, 923 m, 912 m, 894 w, 840 w, 759 s, 691 s, 613 w, 559 m, 548 m, 518 w, 491 s,
464 m, 421 w cm–1.
[23] Solution of copper(II) chloride dihydrate (170 mg, 1 mmol) and 1-phenyltetrazole-5-thiol
1d (356 mg, 2 mmol) in acetonitrile (20 ml) was allowed to stand at room temperature for
5 days, and thereafter green crystals of 2d suitable for X-ray analysis were formed. Yield:
182 mg, 39 %. Anal. Calc. for C32H30Cl4Cu2N18 (935.62): C, 41.08; H, 3.23; N, 26.95.
Found: C, 41.19; H, 2.96; N, 26.60. FT-IR (neat): 3100 s, 3071 s, 2250 m, 1753 m, 1680
m, 1595 s, 1505 s, 1471 m, 1413 m, 1367 m, 1335 w, 1310 m, 1291 m, 1219 m, 1173 s,
1155 m, 1085 s, 1060 s, 1006 s, 919 m, 893 m, 864 m, 762 m, 712 w, 685 m, 664 w, 590
m, 569 m, 511 m cm–1. Complex 2a was obtained in a similar way in 43 % yield.
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11

Author Statement
Sergei Voitekhovich: Conceptualization, Investigation, Writing - Original Draft.
Alexander Lyakhov: Investigation. Ludmila Ivashkevich: Writing - Review & Editing.
Oleg Ivashkevich: Supervision
12

Declaration of interests
Х The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests:
13

Highlights
• First examples of desulfurization of tetrazole-5-thiols under complexation with
copper(II) are reported.
• Plausible mechanism of desulfurization includes initial oxidation of tetrazole-5-
thiols by copper(II) ions into bis(tetrazol-5-yl)disulfanes, followed by copper-
assisted oxidation by air oxygen.
14