Coordination compounds of s-and se-containing organic ligands as catalysts of oxidation reaction under N 2O action

Article abstract of DOI:10.1080/10426507.2012.727514

Nitrous oxide (N2O) would be preferable to many other oxidants because in the reactions of N2O with organic compounds the only byproduct is N2, which is perfect from the point of view of green chemistry. We recently reported a series of new sulfur-and sel

Full text of DOI:10.1080/10426507.2012.727514

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Phosphorus, Sulfur, and Silicon and the
Related Elements
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Coordination Compounds of S- and Se-
Containing Organic Ligands as Catalysts
of Oxidation Reaction Under N2O Action
Elena K. Beloglazkina a , Alexander G. Majouga a , Anna A. Moiseeva
a & Nikolai V. Zyk a
a Department of Chemistry , M.V. Lomonosov Moscow State
University , 119991 , Moscow , Russia
Accepted author version posted online: 12 Sep 2012.Published
online: 29 May 2013.
To cite this article: Elena K. Beloglazkina , Alexander G. Majouga , Anna A. Moiseeva & Nikolai V.
Zyk (2013) Coordination Compounds of S- and Se-Containing Organic Ligands as Catalysts of Oxidation
Reaction Under N2O Action, Phosphorus, Sulfur, and Silicon and the Related Elements, 188:4, 377-383,
DOI: 10.1080/10426507.2012.727514
To link to this article: http://dx.doi.org/10.1080/10426507.2012.727514
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Phosphorus, Sulfur, and Silicon, 188:377–383, 2013
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2012.727514
COORDINATION COMPOUNDS OF S-
AND SE-CONTAINING ORGANIC LIGANDS AS
CATALYSTS OF OXIDATION REACTION UNDER
N2O ACTION
Elena K. Beloglazkina, Alexander G. Majouga,
Anna A. Moiseeva, and Nikolai V. Zyk
Department of Chemistry, M.V. Lomonosov Moscow State University, 119991
Moscow, Russia
GRAPHICAL ABSTRACT
Abstract Nitrous oxide (N2O) would be preferable to many other oxidants because in the
reactions of N2O with organic compounds the only byproduct is N2, which is perfect from the
point of view of “green” chemistry. We recently reported a series of new sulfur- and selenoor-
ganic ligands, their coordination compound with Co(II) or Cu(I,II), and the possibility for these
complexes to activate nitrous oxide in the oxidation reactions with phosphines and alkenes.
The electrochemical data confirm the stability of reduced form of investigated complexes and
the capacity of these complexes to associate with nitrous oxide.
Keywords Nitrous oxide; oxidation; organic ligands; catalysis
INTRODUCTION
Nitrous oxide (N2O) is an underutilized byproduct of chemical processes involving
nitric acid. Because of the increased use of nitrate containing fertilizers in agriculture,
a large amount of nitrous oxide is produced by denitrifying bacteria living in soil and
seawater and subsequently released to the atmosphere. This way, nitrous oxide has become
the second most important greenhouse gas (after CO2). Hence, the properties of nitrous
oxide and its reactivity are not only of interest for bioinorganic chemistry, but also of global
importance.
Received 9 July 2012; accepted 9 July 2012.
Address correspondence to Elena K. Beloglazkina, Department of Chemistry, Moscow State University,
119991 Moscow, Russia. E-mail: bel@org.chem.msu.ru
377

378
E. K. BELOGLAZKINA ET AL.
Although the reduction of N2O to dinitrogen and water is highly exothermic
(ꢀG◦=−80 kcal/mol), this gas is metastable and unreactive due to a kinetic barrier to
its decomposition.1 In nature, conversion of N2O to N2 and H2O is catalyzed under ambient
conditions during microbial dentrification by the metalloenzyme nitrous oxide reductase
(N2O-reductaze). N2O reductase catalyzes the two-electron reduction of N2O (N2O + 2H+
+2e– → N2 + H2O). X-ray crystallographic and spectroscopic studies have identified the
active 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. A mechanism for N2O reduction involves μ-1,3-coordination and
bending of N2O between two of the copper ions with the sulfide acting to facilitate electron
delocalization during the redox process.2
Despite the biological and environmental significance of nitrous oxide, not much
is known about the coordination chemistry of this molecule. Previously reported N2O
complexes formed with a redox active metal can be rapidly converted to N2 and metal-
oxidized species, O=Mn+2, N2O complexes are usually detected only as transient species.3
In fact, little metal complexes containing the N2O ligand have been identified by X-ray
diffraction. In [Ru(NH3)5(N2O)] X2, N2O ligand has been found to coordinate to the RuII
ion in a linear end-on mode through the terminal N atom.4
So, while a nitrous oxide has to be thermodynamically powerful oxidant, the oxidizing
processes are very slow except at high temperatures. Alkenes can be oxidized by N2O, but
the reactions required temperatures of at least 300 ◦C.5 Some small secondary alcohols were
oxidized by N2O at room temperature in the presence of platinum “black.”6 It describes the
oxidation of phosphines by supercritical N2O at 100 ◦C without catalyst.7
A few catalytic reactions of N2O using metal-mediated catalysis had been found. The
oxidation of Ph3P into Ph3P=O by N2O at room temperature was performed in the presence
of phtalocyanyne complexes of Ru, but the yields of this reaction are not satisfactory.8
Another example is the oxidation of olefins into corresponding epoxides by N2O in the
presence of manganese-substituted polyoxometalates.9
Our scientific group recently reported a series of new sulfur- and selenoorganic ligands
and their coordination compounds with Co(II) or Cu(I,II), which can activate nitrous oxide
in the oxidation reactions with phosphines and alkenes. The types of investigated complexes
are given under Scheme 1.
Scheme 1

COORDINATION COMPOUNDS
379
Scheme 2
Organic ligands in the complexes of types I and II are the 5-pyridylmethylene-
substituted 2-thiohydantoins and 5-pyridylmethylene-3,5-dihydro-4Н-imidazol-4-ones,
respectively. Type III complexes have in the structure sulfur- or selenium-containing
imino-pyridine ligands.
The synthesis of the thiohydantoin derivatives is outlined in the Scheme 2.10 The initial
3-substituted 2-thiohydantoins were prepared by the reaction of isothiocyanate with amino
acid ester with the yields to 85%. 5-Pyridylmethylene-substituted derivatives were prepared
by Knoevenagel condensation between heteroaromatic aldehydes and the thiohydantoin
core under basic catalyses. The key step in the formation of the dihydro-imidazolone
backbone was achieved by S-alkylation reactions under basic conditions.
To obtain the target coordination compounds, the method of slow diffusion of ligand
solution in CH2Cl2 to the cobalt or copper chloride solution in EtOH or CH3CN was used. In
the case of cobalt salt, the isolated coordination compounds have the composition LCoCl2
(Scheme 3).11
As a result of the reaction of dihydro-imidazolone ligands with copper(II) chloride,
surprisingly, different types of complexes were isolated, depending on the character of
substituent at sulfur atom and the reaction condition (Scheme 3). In the reaction with
S-methylated derivatives, the complexes, analogous to these with CoCl2, were obtained.12
However, in the case of carbethoxy-substituted ligand, we have obtained two different
types of complexes: if the complexation reaction was carried out under stirring of ligand
with copper chloride, the fast precipitation of dark-brown solid of complex 1 with the
composition of LCuCl2 took place. However, if we obtained the complex by slow diffusion
of ligand solution to the copper chloride solution, we have isolated the different coordination
compound.
X-Ray analysis indicated the formation of unexpected mixed-valence thiolate copper
complexes 2 in which the anionic ligand behaves as tridentate bridge between two copper
centers. Both copper atoms in complexes 2 have the same coordination sphere geometry,

380
E. K. BELOGLAZKINA ET AL.
Scheme 3
namely, a tetragonal pyramidal. Such coordination geometry suggests that this complex has
a delocalized mixed-valence form with a [Cu+1.5Cu+1.5] redox state. Thus, in the case of
ligands of this type, S-desalkylation and reduction of copper atom occur during the complex
formation. The reducing agent is probably ethyl alcohol.
Complexes 2, containing Cu(I) and Cu(II) ions, may be regarded as a low molecular
model of N2O-reductase. Both natural enzyme and synthesized complexes contain copper(I)
and copper(II) atoms, nitrogen and bridged sulfur donors, and the distances between copper
atoms are practically identical for the N2O-reductase and complex 2.
We have investigated the possibility of the reaction of complex 2а (2; R = Ph) with
nitrous oxide. First, we have studied the electrochemical behavior of this complex itself
and in the presence of N2O. At scanning potential in cathodic region, three one-electronic
reversible peaks were observed on cyclic voltammetry (CVA) curve of a complex 2a. These
peaks are probably due to three consecutive reduction processes (Scheme 4):
In the presence of N2O, the first reduction peak does not change position and remains
reversible, thus pointing to the fact that substrates are not bonded by initial complex.
However, at the potential of second cathodic peak (ЕRed = −1.13 V), the increase of the
current and the decrease of next wave current were observed. It means that initial complex
2a does not take part in catalytic cycle, as also the natural N2O-reductaze, a catalytic active
form of which is the reduced “all-copper(I)” form. The current increase is consistent with
an electrocatalytic process at this potential. We suppose that intermediate B can coordinate
N2O as additional ligand at a vacant coordination position.
Scheme 4

COORDINATION COMPOUNDS
381
Apparently, only N2 (not NH3) forms under catalytic N2O reduction because the NH3
re-oxidation peak under Epa ∼ 1.0 В is not observed at CVA reverse scan in the anodic
region after carrying of cathodic potential Epc = −1.0 V.
Note that in the absence of a complex 2a, the electrochemical reduction of N2O
occurs only at very negative potentials (−2.12 V).
We have investigated the catalytic activity of compound 2a in the model reaction of
triphenylphosphine oxidation by N2O. The reactions were carried out both in the presence
of complex 2a and without it, and also in the presence of inorganic salts of copper(I)
or copper(II). Triphenylphosphine oxide was obtained with quantitative yield only in the
presence of complex 2a.
Thus, complex 2a can be regarded as actual catalyst of phosphines oxidation. This
catalysis mimics the behavior of the N2ORs enzymes, which reduce nitrous oxide by two
electrons, yielding dinitrogen and water.
We have also investigated by CVA the interaction of cobalt complex 1a (1; R = Ph, M
= Co) with N2O.13 Complex 1a itself reduces on first two-electron stage “to metal” giving
the complex zero-valent cobalt. At slow scan rate (20 mV/s), the potential of first reduction
peak in the presence of N2O shifts to less negative potential, whereas the potential of second
reduction peak shifts to more negative potential.13 These facts confirm the interaction of
both initial and reduced form of complex 1a with N2O. At one time, increase in first
wave current and decrease in next wave current were observed. The increase in current is
consistent with an electrocatalytic process at the potential of the Co(II)/Co(0) redox couple.
The presumable mechanism of the N2O interactions with 1a in the electrochemical
cell involves two parallel processes:13 (a) the initial reduction of complex 1a followed by
complexation with N2O and regeneration of 1a accompanied by N2 deliberation, and (b)
the complexation of 1a with N2O at first followed by reduction of N2O-containing complex
with N2 evolution.
We have attempted to use complex 1a for activation of nitrous oxide in chemical
reactions. We have found that if N2O is bubbled to a solution of complex and Ph3P at
room temperature, catalytic oxidation to O=PPh3 takes place.13 Second, we have found that
norbornene reacts with nitrous oxide in the presence of complex 1a at room temperature
forming 3-oxatricyclo[3.2.1.02,4]oct-6-ene with yields upto 25%.13 In conclusion, complex
1a is capable of catalyzing norbornene and Ph3P oxidation by nitrous oxide under ambient
conditions.
It is unlikely that the oxygen source was anything other than N2O. The reactions
of norbornene with air in the presence of 1a or with N2O in the presence of CoCl2 at
the same conditions gave 0% oxidation products. The N2 byproduct is assumed but not
experimentally confirmed.
The final type of studied complexes was the sulfur- or selenium-containing pyridylim-
ines. The target ligands 3 were obtained by the reactions of 2-chalcogen-substituted amines
with 2-pyridine carboxaldehyde at almost quantitative yield.14 Metal complexes 4 precipi-
tated after boiling of ligands with equimolar amount of cobalt(II) or copper(II) chloride14
(Scheme 5). Metal atoms in these complexes have a distorted tetrahedral ligand environ-
ment; they are coordinated by two chloride anions and two nitrogen atoms of pyridine and
imine fragments; selenium or sulfur atoms do not participate in metal coordination.
Ligands and complexes of this type were studied by CVA.14 For example, ligand
3a (3; E=Se, M=Co, R,R=(CH2)4) undergoes a two-step reduction at substantially neg-
ative potentials. The additional quasi-reversible peak on complexes 4a (4; E=Se, M=Co,
R,R=(CH2)4) CVA curves corresponds to two-electron reduction at metal.

382
E. K. BELOGLAZKINA ET AL.
Scheme 5
Under the reduction of complex 4a in the presence N2O at the slow scan rates, the
intensity of first cathodic peak increases, along with a concomitant loss of reversibility.
At the same time, the following peaks became low intensive. These facts are related to
an electrocatalytic process at the formal potential of CoII/Co0 reduction. Thus, complex 5
is capable of catalyzing the reduction of nitrous oxide to N2 under mild conditions at a
moderate potential (Epc < −1.0 В).
During tests on selenium-containing complexes 4a, 4b (4; E = Se, M = Co, R =
H) for activation of nitrous oxide in chemical reactions, we found that norbornene reacts
with nitrous oxide in the presence of complexes under normal pressure at room temperature
forming 3-oxatricyclo[3.2.1.02,4]oct-6-ene with yields up to 90%.
Also, it has been found that if N2O is bubbled to a solution of the complex and Ph3P
at room temperature, catalytic oxidation to OPPh3 takes place.
It may be specially noted that sulfur-containing analogs of complex 4a,b, based on
CVA data and model oxidation reaction results neither react with nitrous oxide nor catalyze
phosphine oxidation under N2O action.
In summary, all types of investigated complexes, namely, complexes with 5-
pyridylmethylene-substituted thiohydantoins, pyridylmethylene-dihydro-imidazolones,
and selenium-containing imino-pyridines, are capable of catalyzing oxidation reactions
of norbornene and Ph3P by nitrous oxide under ambient conditions. The best results for
norbornene oxidation were demonstrated by selenium-containing cobalt complexes, for
phosphine oxidation - by the mixed-valence copper thiohydantoin complex.
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