Cleavage of the C-S bond with the formation of a binuclear copper complex with 2-thiolato-3-phenyl-5-(pyridine-2-ylmethylene)-3,5-dihydro-4H-imidazole-4- one. A new mimic of the active site of N2O reductase

Article abstract of DOI:10.1039/c3dt50422k

The treatment of the ligands with copper(ii) chloride dihydrate led to the formation of a binuclear copper complex with a [Cu^+1.5Cu ^+1.5] redox state as a result of C-S bond cleavage in the course of the reaction. This complex catalyses the electrochemical reduction of nitrous oxide and triphenyl phosphine oxidation under N₂O action.

Full text of DOI:10.1039/c3dt50422k

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Cleavage of the C–S bond with the formation of a
binuclear copper complex with 2-thiolato-3-phenyl-
5-(pyridine-2-ylmethylene)-3,5-dihydro-4H-imidazole-
4-one. A new mimic of the active site of N2O reductase†
Alexander G. Majouga,a Elena K. Beloglazkina,*a Anna A. Moiseeva,a
Olga V. Shilova,a Eugeniy A. Manzheliy,a Maria A. Lebedeva,b E. Stephen Davies,b
Andrei N. Khlobystovb and Nikolay V. Zyka
Cite this: Dalton Trans., 2013, 42, 6290
Received 14th February 2013,
Accepted 4th March 2013
DOI: 10.1039/c3dt50422k
www.rsc.org/dalton
The treatment of the ligands with copper(II) chloride dihydrate led complexes with thiolate that act as synthetic models of the cat-
to the formation of
a
binuclear copper complex with
a
alytic site of N2OR have been reported so far.9
[Cu+1.5Cu+1.5] redox state as a result of C–S bond cleavage in the
In this communication we report the synthesis of a new
course of the reaction. This complex catalyses the electrochemical binuclear thiolate copper complex with the N2S-type organic
reduction of nitrous oxide and triphenyl phosphine oxidation ligand 2-thiolato-3-phenyl-5-(pyridine-2-ylmethylene)-3,5-
dihydro-4H-imidazole-4-one. The target complex was
under N2O action.
6
synthesized by the complexation reaction of ethoxycarbonylalk-
ylthio substituted 3-phenyl-5-(pyridine-2-ylmethylene)-3,5-
dihydro-4H-imidazole-4-ones 1–5 obtained in one step from
2-thioxo-3-phenyl-5-(pyridine-2-ylmethylene)-3,5-dihydro-4H-imi-
dazole-4-one by alkylation with the corresponding bromo
esters (Scheme 1) with CuCl2·2H2O. It is known that one of the
routes to polynuclear early transition metal complexes with
bridged thiolato ligands is available via C–S bond cleavage
reactions of coordinated sulfides.10 We supposed that ligands
1–5, as suitable substrates for nucleophilic substitution of a
thiolate fragment at a primary or a secondary carbon atom of
ω-thioester, can give a thiolate complex in the reactions with
copper salts.
Nitrous oxide (N2O) is an important intermediate of bacterial
denitrification,1 wherein N2O is produced via reductive dimeri-
sation of NO catalysed by a membrane-bound heme-contain-
ing NO-reductase.2 In the terminal step of denitrification N2O
is reduced to N2 in a two-electron reduction process catalyzed
by nitrous oxide reductase (N2OR).3 N2OR is a functional
homodimer with a molecular mass of 65 kDa per subunit.
According to the RSA each subunit consists of two domains.4
The first one contains a binuclear copper mixed valence
cluster Cu(1.5)–Cu(1.5) (also known as a Cu(I)–Cu(II)) center,
designated CuA;5 a similar center is also found in the cyto-
chrome c oxidases.6 The second, a CuZ site of N2OR as a
μ-sulfido-tetracopper cluster with a distorted tetrahedral shape
that cycles through tetracopper(I) and mixed-valent (with one
copper(I) and three copper(II)) states during catalysis.7 During
the last decade much interest has been focused on the binuc-
lear CuA electron-transfer sites,8 for which the significance of
highly covalent interactions between copper and thiolate has
been demonstrated through detailed structural, spectroscopic
and synthetic modeling studies. Only a few examples of copper
aDepartment of Chemistry, Lomonosov Moscow State University, Leninskie Gory,
Moscow, Russia. E-mail: bel@org.chem.msu.ru; Fax: +7 495 9394652;
Tel: +7 495 9394029
bSchool of Chemistry, University of Nottingham, Nottingham NG7 2RD, UK
†Electronic supplementary information (ESI) available: Synthesis, characteriz-
ation, crystallographic information, electrochemistry, electronic spectroscopy
and spectroelectrochemistry results. CCDC 723648. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3dt50422k
Scheme 1 Synthesis of complex 6.
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Scheme 2 Proposed scheme of the electrochemical reduction of 6.
Fig. 1 Crystal structure of complex 6. Selected bond lengths (Å): Cu1–Cu2
2.5620(10); Cu1–Cl1 2.4644(16); Cu2–Cl1 2.4613(18); Cu1–N11 1.939(5); Cu1–
N13 2.055(5); Cu1–S1 2.2524(19).
The crystal structure of complex 6 is illustrated in Fig. 1.
Both copper atoms of the binuclear cluster have almost equiv-
alent metal coordination geometries and are located in a tetra-
hedral environment (disregarding the neighboring Cu atom)
bound by one bridging chlorine atom. The Cu–Cu distance is
2.5620(10) Å. This distance is similar to those found for the
CuI–CuIV centre in N2OR (2.6 Å11), and thus complex 6 may be
considered as a conventional structural analogue of the active
site of N2O reductase and, as in the native enzyme7 N2O, binds
only at the CuI and CuIV centers in the reduction process.
The identical coordination sphere geometry of both copper
atoms in complex 6 suggests that it exists in the delocalized
mixed-valence form with a [Cu+1.5Cu+1.5] oxidation state, which
serves as a useful reference model for some native metallopro-
teins.12 EPR spectra recorded at room temperature for the poly-
crystalline powder of compound 6 show features typical for
such mixed valence copper complexes.13 The spectra contain
an isotropic signal centered at g = 2.1258. No hyperfine split-
ting is observed at 300 K. Apparently, ethanol acts as a redu-
cing agent in the process of complex 6 formation. Similar
reduction processes induced by alcohols or sodium dithio-
nite14 have been described in the literature.
X-Ray diffraction structural analysis of complex 6 indicated
C–S bond cleavage within the ligand molecules 1–5 with the
formation of the Cu–S bond during the metal complexation
with Cu(II). In addition, formation of complex 6 was not
observed if an anhydrous copper(II) chloride was used for the
metal complexation. Thus we propose a possible mechanism
for C–S-bond cleavage (Scheme 1S†). We suppose that the reac-
tion may be induced by coordination of CuCl2 as a soft Lewis
acid to a donor sulfur atom of the ligand which acts as a soft
Lewis base. This coordination activates the SN2 nucleophilic
substitution reaction at the CH2-group next to the sulfur atom
in which an H2O molecule acts as a nucleophile and the
leaving thiolate group is stabilized via the formation of a
strong covalent S–Cu bond. In this case the main organic
byproduct is ethyl hydroxyacetate (Scheme 2), the presence of
which in the reaction mixture was indicated by ESI-MS (m/z
104).
Fig. 2 The cyclic voltammogram of 6 in DMF with [NBu4+][BF4−] (0.2 M) as the
supporting electrolyte at a scan rate of 0.1 V s−1 (GC working electrode, Pt wire
secondary electrode, saturated calomel reference electrode, room temperature,
Ar atmosphere, E1/2 (Fc+/Fc) = +0.49 V).
redox process was assessed by monitoring the peak-to-peak
separation and the ratio of peak currents at a range of scan
rates between 0.02 and 0.3 V s−1. The CV curve of complex 6 in
the anodic region exhibits a quasi-reversible oxidation process
at a potential of E1/2 = +0.48 V corresponding, probably, to the
Cu+1.5Cu+1.5 → Cu+2Cu+2 oxidation.15 The complex displays
three one-electron reduction processes showing a quasi-revers-
ible reduction at +0.11 V and two electrochemically irreversible
reductions at −1.11 and −1.23 V, and the fourth reduction
wave at −1.67 V corresponding to the two-electron irreversible
process. Also a small additional peak with E1/2 = −0.84 V
(Fig. 2S,† peak b) appears in the CV before the second
reduction wave which can be explained by the adsorption of
the reduced species on the electrode surface.16 The first
reduction process is Cu based as confirmed by spectroelectro-
chemical data (Scheme 2) whereas the second and third
reductions at −1.11 and −1.22 V are more likely to be ligand
based16 and correspond to the sequential addition of an elec-
tron to each of the communicating ligand fragments in the
complex molecule.
The electronic spectra of complex 6 recorded in DMF solu-
tion together with its electrochemically generated one- and
two-electron reduced forms are shown in Fig. 3. The neutral
complex 6 (Fig. 3, blue line) displays a series of very intense
(>104 M−1 cm−1) overlapping bands at 300–450 nm, apparently
due to metal-to-ligand charge-transfer (MLCT), e.g. bands at
424 nm assigned to an S→Cu transition and 305 nm assigned
to an N→Cu17 transition, and intraligand transitions at 370
and 389 nm.16 In addition, a low-intensity d–d transition band
is observed in the 700–850 nm region. A broad band observed
in the CuA centres in the region of 700–850 nm and attributed
to the Cu–Cu intervalence transition associated with electron
delocalization in the mixed-valence metal core is not observed
Cyclic voltammetry of compound 6 was performed in DMF
and acetonitrile solutions in the presence of 0.2 M [Bu4N][BF4]
or 0.1 M [Bu4N][ClO4] as a supporting electrolyte (Fig. 2 and
2S† respectively). The nature of the electron transfer for each
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Thus, complex 6 is able to catalyze the reduction of nitrous
oxide to N2 under mild conditions at moderate potentials
(approximately −1.0 V). Nitrous oxide reductase type activity
was tested in the model reaction of phosphine oxidation. It
was found that in the presence of 5 mol% of complex 6 100%
conversion of PPh3 into the corresponding oxide occurs within
4 h at room temperature whereas the reactions of PPh3 with
N2O under identical conditions in the absence of complex 6
give only 8% of Ph3PO.
In conclusion, a new mixed-valence copper complex with a
tridentate N2S type organic ligand has been synthesised and
its capability to activate the oxidation activity of nitrous oxide
has been shown. Based on the structural data and the poten-
tial to activate N2O in chemical reactions this complex may be
considered as a conventional model of the nitrous oxide
reductases.
Fig. 3 UV/Vis absorption spectra of complex 6 showing (a) the decay of the
neutral species (blue) and growth of the one electron reduced species (red) elec-
trochemically generated at the potential of −0.50 V and (b) the decay of the
neutral species (blue) and growth of the two electron reduced species (green)
electrochemically generated at −1.18 V in DMF solution in the presence of 0.2
M [NBu4][BF4] as the supporting electrolyte. Grey lines represent intermediate
spectra.
in the compound 6 UV/Vis spectrum and is possibly shifted to
the NIR region as previously reported for the related
Cu+1.5Cu+1.5 complexes.18 For the one electron reduced species
generated by applying the potential of −0.50 V (Fig. 3a) the
absorption band at 424 nm drastically decreases, whereas a
new broad band at ∼530 nm appears. The latter has been
reported for homonuclear CuICuIN2S systems19 and thus con-
firms that the first reduction process is copper based. In
addition, the low intensity broad band at 700–850 nm related
to the d–d transitions in the CuIICuI system disappears which
also proves formation of the homonuclear CuICuI species.
Reoxidation by applying the potential of +0.20 V results in a
UV/Vis spectrum identical to the one of the neutral compound
which suggests that the first reduction process is chemically
reversible in nature. For the two-electron reduced species elec-
trochemically generated by applying the potential of −1.18 V
(Fig. 3b) two sharp intraligand transition peaks at 370 and
389 nm disappear which suggests the process is ligand based.
The MLCT band at 424 nm changes intensity and is red
shifted to 440 nm also confirming changes occurring in the
ligand. Furthermore, this change is irreversible under the
experimental conditions and is likely to be accompanied by
the decomposition of the dinuclear complex as the UV/Vis
spectrum of the re-oxidised species is drastically different from
the one of the neutral complexes.
We have studied the electrochemical behavior of complex 6
in the presence of N2O (Fig. 3S†). In the presence of nitrous
oxide at a slow scan rate (20 mV s−1) the first reduction
process is not shifted compared to the one in the absence of
N2O (Fig. 2S†), and remains quasi-reversible, indicating that
N2O does not bind to the neutral complex 6 or to its one elec-
tron reduced species in solution. At the potential of the
adsorption peak at −0.84 V, an increase in the current is
observed (compare peaks b and b′ in Fig. 2S and 3S† respect-
ively). This current increase is consistent with an electrocata-
lytic process at this potential. The electrocatalytical reduction
of N2O by the adsorbed catalyst has been reported previously
for complexes of NiII with macrocyclic polyamines where an
increase in the current at the adsorption peak was observed.20
The proposed mechanism of the N2O interactions with 6 in an
electrochemical cell is shown in Scheme 1S.†
We thank the Russian Foundation for Basic Research
(Project 10-03-00677) for financial support.
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