Electromediators based on the Ni(II) and Cr(III) complexes with the redox-active ligands in the synthesis of sulfur-containing organic compounds

Article abstract of DOI:10.1134/S1070328413010077

The chromium(III) tris-o-semiquinolate complex Cr(L^SQ) ₃ (L^SQ is 3,6-di-tert-butyl-o-semiquinone) and the monoanionic paramagnetic nickel(II) complex [n-Bu₄N][Ni(L _S ^SQ)(L _S ^DT)] (L _S ^SQ is o-thiosemiquinone, L _S ^DT is benzene-1,2-dithiolate) are considered as electromediators of hydrogen sulfide oxidation in the presence of various organic substrates (hex-1-ene, oct-1-ene, benzene, toluene, and benzoic acid). It is revealed that the electrolysis of hydrogen sulfide at the oxidation potential of the mediators in the presence of the substrates affords the corresponding aliphatic and aromatic thiols in a yield of 62-75%.

Full text of DOI:10.1134/S1070328413010077

ISSN 1070ꢀ3284, Russian Journal of Coordination Chemistry, 2013, Vol. 39, No. 1, pp. 33–36. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © A.O. Okhlobystin, I.V. Smolyaninov, A.V. Okhlobystina, N.T. Berberova, Yu.Yu. Koldaeva, V.F. Abdulaeva, 2013, published in Koordinatsionnaya
Khimiya, 2013, Vol. 39, No. 1, pp. 36–40.
Electromediators Based on the Ni(II) and Cr(III) Complexes
with the RedoxꢀActive Ligands in the Synthesis of SulfurꢀContaining
Organic Compounds
,
A. O. Okhlobystina, I. V. Smolyaninova, b, A. V. Okhlobystinaa *, N. T. Berberovaa, b,
Yu. Yu. Koldaevaa, and V. F. Abdulaevaa
a Astrakhan State Technical University, ul. Tatishcheva 16, Astrakhan, 414025 Russia
b Southern Research Center, Russian Academy of Sciences, proezd Chekhova 41, RostovꢀonꢀDon, 344006 Russia
*eꢀmail: ionradical@gmail.com
Received March 26, 2012
Abstract—The chromium(III) trisꢀ
o
ꢀsemiquinolate complex Cr(LSQ)3 (LSQ is 3,6ꢀdiꢀtertꢀbutylꢀ
o
ꢀsemiꢀ
SQ is
ꢀBu N][Ni oꢀthioꢀ
(LS )(LS ) (LS
SQ
DT
quinone) and the monoanionic paramagnetic nickel(II) complex
[n
]
4
DT
semiquinone,
is benzeneꢀ1,2ꢀdithiolate) are considered as electromediators of hydrogen sulfide oxidaꢀ
LS
tion in the presence of various organic substrates (hexꢀ1ꢀene, octꢀ1ꢀene, benzene, toluene, and benzoic
acid). It is revealed that the electrolysis of hydrogen sulfide at the oxidation potential of the mediators in the
presence of the substrates affords the corresponding aliphatic and aromatic thiols in a yield of 62–75%.
DOI: 10.1134/S1070328413010077
At present the search for new methods of the synꢀ [3, 4]. Nevertheless, the organic electromediators are
thesis of organic compounds is one of the promising
directions in organic chemistry. The involvement of
electrochemical methods in processes of the oxidative
(reductive) transformation of hydrogen sulfide, sulfur,
and lowꢀmolecularꢀweight thiols is an actively develꢀ
oped area of organic electrochemistry. Electrochemiꢀ
cal transformations of sulfurꢀcontaining organic comꢀ
pounds include either the direct generation of their
oxidized (reduced) form at the electrode and further
reactions in the solvent bulk or the mediator redox
process under the action of radical ion intermediates,
metal salts, and halogens, which can undergo cyclic
regeneration on the electrode. We have earlier shown
[1, 2] that the electrochemical oxidation of hydrogen
sulfide in the presence of unsaturated compounds
affords the sulfurꢀcontaining products in a quantitaꢀ
tive yield. However, the direct electrochemical
method proposed for the synthesis is energyꢀconsumꢀ
ing because of the high anodic potential of oxidation
of hydrogen sulfide and thiols. Organic mediators
based on substituted triarylamines and phosphines
recommended themselves as mediators of the oxidaꢀ
tion of hydrogen sulfide and thiols in nonaqueous
media. The use of these compounds made it possible
to decrease the potential of electrosynthesis of the sulꢀ
not universal and can be removed from the regeneraꢀ
tion cycle during the reaction because of the formation
of poorly soluble salts, which decreases the yield of the
target products. Chemical oxidants (sterically hinꢀ
dered
oꢀbenzoquinones, oꢀiminobenzoquinones, and
related transition metal complexes) can also particiꢀ
pate in the oxidation of hydrogen sulfide and thiols
due to the innerꢀ or outerꢀsphere electron transfer,
which, in turn, changes the oxidation state of the
organic ligands.
The purpose of this work is to study the transition
metal complexes as electromediators in the elecꢀ
trosynthesis of organic compounds. The objects
of the study were the neutral chromium(III) trisꢀ
semiquinolate complex, Cr(LSQ)3 ( , where LSQ
is 3,6ꢀdiꢀtertꢀbutylꢀ ꢀsemiquinone, and the monoanꢀ
ionic paramagnetic nickel(II) complex,
ꢀBu N][Ni , where is ꢀthioꢀ
oꢀ
I)
o
SQ
DT
[n
] (II
)
o
(LS )(LS )
LSSQ
4
semiquinone, and DT is benzeneꢀ1,2ꢀdithiolate, conꢀ
LS
taining in one molecule an organic redoxꢀactive ligand
of the oꢀquinoid type and a transition metal ion inert
furꢀcontaining organic compounds by 1.0–0.4 V towards hydrogen sulfide:
33

34
OKHLOBYSTIN et al.
But
O•
tBu
• O
O
tBu
O
S
S
S
Cr
But
[
n · Bu4N]
•
Ni
O
O
•
S
But
tBu
(II)
(I)
Hexꢀ1ꢀene, octꢀ1ꢀene, benzene, toluene, and benꢀ time of microelectrolysis depended on the working
zoic acid were used as substrates involved in the reacꢀ concentration of the substance under study ranging,
tions with the oxidized form of hydrogen on the average, from 2 to 6 h. After the end of electrolꢀ
sulfide formed during electrolysis in the presence of a ysis, the cell was purged with argon for 1–1.5 h for the
mediator.
maximum removal of hydrogen sulfide. The reaction
products were identified by the CV method using stanꢀ
dard substances. The obtained mixture was dissolved
in hexane to separate the supporting electrolyte. Then
the solvent was separated from the reaction products
using fractional distillation.
EXPERIMENTAL
The nickel(II) and chromium(III) complexes were
synthesized by described procedures [5, 6]. The oxidaꢀ
tion potentials were measured by cyclic voltammetry
(CV) in a threeꢀelectrode cell with an IPCꢀpro potenꢀ
tiostat under argon. The working electrode was a staꢀ
tionary glassy carbon (GC) electrode with a diameter
of 2 mm, and the auxiliary electrode was a platinum
IR spectra were recorded on an FSM 1201 FTIR
spectrometer in thin KBr films. Sulfurꢀcontaining
organic substances were identified by the tables of freꢀ
quencies in the IR spectra [9]. Hydrogen sulfide was
prepared by a described procedure [10]. Quantitative
analysis of the initial reaction mixtures and obtained
products was performed on a KristallꢀLyuks 2000 gas
chromatograph equipped with a flame photometric
detector. Separation was carried using a ZBꢀ1 capillary
column in the linear temperatureꢀprogrammed
wire (
S
= 18 mm2). The reference electrode
(Ag/AgCl/KCl) was equipped with a waterꢀproof
membrane. The number of electrons transferred durꢀ
ing the electrode process was estimated with respect to
ferrocene used as a standard. The potential sweep was
0.20 V s–1. The supporting electrolyte was 0.1 M
Bu4NClO4. Solvents were purified and dehydrated
according to known procedures [7, 8]. The concentraꢀ
regime: the beginning at 35
increase was C/min, and the end of the program was
280 . Nitrogen was used as a carrier gas. The sample
volume was 0.5 L. The obtained compounds were
°
С, the rate of temperature
4
°
°
С
tion of complexes I and II was 0.001–0.003 mol/L.
µ
Preparative electrolysis was carried out on stationꢀ
ary platinum electrodes (plates with a surface area
700 mm2) in an undivided threeꢀelectrode 100ꢀmL
cell. The reference electrode was a saturated
silver chloride electrode with the electroconducting
waterꢀproof membrane. The supporting electrolyte
identified using standards for chromatography.
RESULTS AND DISCUSSION
The study of the electrochemical activity of the
was
nꢀBu4NClO4, and methylene chloride was used as
studied complexes (Table 1) showed that complex
ꢀsemiquinone fragments has the
I
a solvent. Electrolysis was carried out on a PIꢀ50ꢀ1.1 containing three
o
potentiostat. The value of potential on the working completed sevenꢀmembered cascade joined by six
electrode was established by 0.10–0.30 V more posiꢀ redox transitions, which corresponds to the change in
tive than the oxidation potential of the mediator. The the oxidation state of the coordinated ligands
–e
+e
–e
+e
–e
+e
[Cr(LCAT)3]3–
[Cr(LSQ)(LCAT)2]2–
[Cr(LSQ)2(LCAT)]–
Cr(LSQ)3
+e –e
–e
–e
[Cr(LBQ)3]3+
[Cr(LSQ)(LBQ)2]2+
[Cr(LSQ)2(LBQ)]+
The redox transitions in the cathodic region tron. At the same time, only the first process at the
0/1+
E
1 2
observed for complex
I
are reversible and oneꢀelecꢀ potential
= 1.02 V is reversible in the anodic
E
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 39
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ELECTROMEDIATORS BASED ON THE Ni(II) AND Cr(III) COMPLEXES
35
region. The further electrochemical oxidation of the
The electrochemical behavior of the monoanionic
chromium complex is irreversible and accompanied by nickel complex is characterized by two reversible oneꢀ
the elimination of free
S
o
ꢀquinone.
electron steps that proceed via the ligands
•
S
•
S
–
2–
S
S
S
S
S
S
–e
+e
Ni
Ni
Ni
S
S
S
•
Unlike the earlier considered transition metal the current of the oxidation peak indicates the cataꢀ
complexes with the N,Nꢀ and N,Sꢀcoordinated
redoxꢀactive ligands, compounds and II are not
lytic effect of the complexes [11]. In the case of the
chromium compound, the outerꢀsphere electron
transfer from the hydrogen sulfide molecule to the
monocation occurs
I
involved in the oxidation of hydrogen sulfide. In the
case of the chromium complex, this is due to steric
hindrance of the metal center. In the case of the nickel
derivative, this is the monoanionic character. Howꢀ
ever, for the both complexes the first anodic stage is
reversible, which makes it possible to consider these
objects as mediators of oxidation. The introduction of
–e
Cr(LSQ)3
[Cr(LSQ)2(LBQ)]+
H S
2
•+
H S
2
hydrogen sulfide in a solution of compound I changes
the shape of the CV curve (figure): the threefold
increase in the current of the first oxidation process is
observed in the anodic potential range in the presence
of H2S, and the reversibility of the redox transition disꢀ
appears. A similar electrochemical pattern is also
observed for the nickel(II) derivative. An increase in
Probably, the innerꢀsphere mechanism of hydroꢀ
gen sulfide activation takes place for the oxidized form
of complex II, which is the coordination of H2S on the
metal center followed by the innerꢀsphere electron
transfer in the H S–Ni– SQ system
LS
2
•
•
S
–
S
S
S
S
S
–e
Ni
Ni
S
S
•
•+
H S
H S
2
H
H
2
S
•
S
S
Ni
S
S
•
For the studied complexes
the values of decreasing the energy barrier
ER EMed, where ER is the oxidation potential of
hydrogen sulfide (1.60 V), and EMed is the potential of
the first anodic process of the complexes) of the reacꢀ
I
and II, we calculated
It was shown for the reaction of the oxidized form
of the Cr(LSQ)3 complex with H2S in the presence of
hexꢀ1ꢀene that the yield of hexaneꢀ1ꢀthiol reached
22% within 1 h. An increase in the reaction time (3 h)
allows one to enhance the yield of the product to 63%,
and the further occurrence of the reaction leads to the
accumulation of the products of thiol oxidation (disulꢀ
fides) in the solution. Similar regularities were estabꢀ
lished for the most part of the substrates considered.
As follows from Table 2, the efficiency of the action of
the oxidized form of complex II in the reactions
with hydrogen sulfide upon the introduction of the
substrates into the solution is higher than that for comꢀ
Δ
E
( E =
Δ
–
tion with hydrogen sulfide: 0.58 (I) and 1.12 V (II).
To confirm the efficiency of the mediators, we conꢀ
sidered the electrochemical activation of hydrogen
sulfide by the oxidized forms of complexes I and II in
the presence of unsaturated (hexꢀ1ꢀene, octꢀ1ꢀene)
and aromatic (benzene, toluene, benzoic acid) comꢀ
pounds under the electrolysis conditions. The
values of the maintained electrolysis potential for
plex I, and the yield of the reaction products is 5–10%
complexes I and II were 1.1 and 0.6 V, respectively. As
higher in the case of complex II. The mechanism of
thiol formation using toluene as an example can be
presented as follows:
a result of electrolysis at the potential of mediator oxiꢀ
dation, the corresponding aliphatic and aromatic thiꢀ
ols were formed (Table 2).
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 39
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2013

36
OKHLOBYSTIN et al.
Table 1. Oxidation potentials (V) of complexes
cording to the CV data*
I
and II acꢀ Table 2. Yields of the reaction products upon the reactions
of the oxidized form of H2S with the unsaturated and aroꢀ
matic substrates in the presence of metal complex
mediators
Substrate
Hexꢀ1ꢀene
Octꢀ1ꢀene
Benzene
Toluene
I and II*
2–/3–
1/2
2–/1–
1/2
1–/0
1/2
0/1+
1/2
E1p+/2+ E2p+/3+
Comꢀ
plex
E
E
E
E
Total yield of isomeric thiols, %
I
II
I
–1.32 –0.86 –0.24 1.02
–0.46 0.48
1.24
1.52
62.0
63.0
71.0
70.3
70.0
73.0
68.0
74.0
75.0
74.3
II
*
GC is the glassy carbon electrode, CH Cl ,
V
= 0.2 V/s, 0.1 M
10 mol/L, Ar, vs. Ag/AgCl/KCl (satuꢀ
is the potential of the reduction and oxidation halfꢀ
Benzoic acid
2
2
–3
n
ꢀBu NClO ,
1/2
waves; and
c
= 3
×
4
4
* CH Cl , Pt anode, 0.1 M
n
ꢀBu NClO , Ag/AgCl/KCl, c
=
2
2
4
4
Med
rated);
E
0.005 mol/L, = 0.01 mol/L,
c
H2S
c
= 0.1 mol/L, and
τ
= 3 h.
E
is the oxidation peak potential.
substrate
p
Cr(LSQ)3
H2S
H2•S+
CH3
CH2SH
CH3
–e
+e
SH
•
HS
+
[Cr(LSQ)2(LBQ)]+
–H+
Thus, the principal possibility of using the Cr(LSQ)3
no. MKꢀ923.2012.3), and the Federal target program
“Scientific and Scientific–Pedagogical Personnel of
Innovative Russia for 2009–2013” (state contract
no. 16.740.11.0594).
SQ
DT
and [
n
ꢀBu N][Ni ] complexes as electromeꢀ
(LS )(LS )
4
diators of hydrogen sulfide oxidation was shown and
the reactions of the oxidized form of hydrogen sulfide
with a series of unsaturated and aromatic substrates
affording thiols were carried out.
REFERENCES
1. Berberova, N.T. and Shinkar, E.V., Russ. Chem. Bull.
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,
ACKNOWLEDGMENTS
2. Berberova, N.T. and Shinkar’, E.V., Abstracts of Papers
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pounds “EKhOSꢀ2006”), Novocherkassk, 2006,
p. 162.
This work was supported by the Russian Foundaꢀ
tion for Basic Research (project no. 12ꢀ03ꢀ00513a),
the President of the Russian Federation (grant
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0
0.5
1.0
1.5
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E
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Cyclic voltammogram of the oxidation of complex
the absence and in the presence ( ) of H S (1 : 1).
I (1) in
2
2
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2013