Oxidative C-H and C-C bond cleavage by a (2,2′-bipyridine)copper(I) chloride complex

Article abstract of DOI:10.1021/ic800745g

Acetonitrile is easily oxygenated at ambient reaction conditions to copper(II) oxalate [Cu(bpy)(ox)]_n mediated by copper(I) chloride in the presence of 3,5-di-tert-butylcatechol and 2,2′-bipyridine. In the case of other nitriles (e.g., propionitrile), instead, the unusual and selective 1,4-extradiol cleavage of 3,5-di-tert-butylcatechol occurs to give copper(II) tert-butylmaleate [Cu(bma)(bpy)(H₂O)]_n in good yield.

Full text of DOI:10.1021/ic800745g

Inorg. Chem. 2008, 47, 6121-6123
Oxidative CsH and CsC Bond Cleavage by a (2,2′-Bipyridine)Copper(I)
Chloride Complex
Ro´bert Csonka,† Jo´zsef Kaizer,† Michel Giorgi,‡ Marius Re´glier,‡ La´szlo´ Hajba,§ Ja´nos Mink,|,⊥
and Ga´bor Speier*,†
Department of Chemistry, UniVersity of Pannonia, H-8200 Veszpre´m, Hungary, Spectropoˆle-RX,
UniVersite´ Paul Ce´zanne Aix-Marseille III, F. S. T. Saint-Je´roˆme, SerVice D11, AVenue Escadrille
Normandie-Niemen, 13397 Marseille cedex 20, France, Department of Earth and EnVironmental
Sciences, UniVersity of Pannonia, H-8200 Veszpre´m, Hungary, Chemical Research Center of the
Hungarian Academy of Sciences, H-1025 Budapest, Hungary, and Faculty of Information
Technology, Research Institute of Chemical and Process Engineering, UniVersity of Pannonia,
H-8200 Veszpre´m, Hungary
Scheme 1. The Formation of Complex 1
Acetonitrile is easily oxygenated at ambient reaction conditions to
copper(II) oxalate [Cu(bpy)(ox)]n mediated by copper(I) chloride
in the presence of 3,5-di-tert-butylcatechol and 2,2′-bipyridine. In
the case of other nitriles (e.g., propionitrile), instead, the unusual
and selective 1,4-extradiol cleavage of 3,5-di-tert-butylcatechol
occurs to give copper(II) tert-butylmaleate [Cu(bma)(bpy)(H2O)]n
in good yield.
Copper(I) chloride in the presence of amines constitutes
an efficient and also industrially important oxidizing/
oxygenating system with dioxygen.1 The stoichiometry of
the reaction of CuCl and amines with O2 is Cu/O2 ) 4:1,
leading to [CuCl(amine)]4O2.2 The industrially used oxidative
coupling of phenols to aromatic polyethers3 or dipheno-
quinone4 is an important process, and catalysts of this type
have biological importance too because they mimic tyrosi-
nase,5 catechol oxidase,6 and catechol dioxygenase.7
We and others have found previously that catechols can
be oxidized to o-quinones8 or cleaved to cis,cis-muconic
acids9 by dioxygen in the presence of copper catalysts. It is
also known that catechols can be aminated to give o-
aminophenols,10 and we looked for possible catalysts for this
reaction. During the course of these studies, we observed
surprisingly that the solvent acetonitrile used had been
oxygenated. When copper(I) chloride in the presence of 3,5-
di-tert-butylcatechol (dtbcH2) or 3,5-di-tert-butyl-1,2-ben-
zoquinone and 2,2′-bipyridine (bpy) in acetonitrile as the
solvent is reacted with dioxygen (Scheme 1), the pale-green
compound of [Cu(bpy)(ox)]n (1) (ox ) oxalate) is obtained
after recrystallization in 46% yield. Complex 1 is paramag-
netic, with electron paramagnetic resonance (EPR) param-
eters of g| ) 2.173 and g⊥) 2.070 at 77 K. The structure of
[Cu(bpy)(ox)]n was proved by spectroscopic data and X-ray
crystal structure determination (Figure 1), and the data found
were slightly different from those observed earlier.11 The
2,2′-bipyridine(µ-1,2,3,4-oxalato)copper complex is com-
posed of polymeric chains of [Cu(bpy)]2+ ions bridged by
* Author to whom correspondence should be addressed. E-mail: speier@
almos.vein.hu.
†
Department of Chemistry, University of Pannonia.
Universite´ Paul Ce´zanne Aix-Marseille III.
Department of Earth and Environmental Sciences, University of
‡
§
Pannonia.
|
Chemical Research Center of the Hungarian Academy of Sciences.
Research Institute of Chemical and Process Engineering, University
⊥
of Pannonia.
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10.1021/ic800745g CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 14, 2008 6121
Published on Web 06/26/2008

COMMUNICATION
Figure 1. The molecular structure of 1 with selected bond distances (Å)
and angles (deg): Cu(1)sO(6) 1.9556(7), Cu(1)sO(53) 1.993(8), Cu(1)s
N(13) 2.029(7), Cu(1)sN(20) 2.024(10), Cu(1)sO(30) 2.327(7), Cu-
(1)sO(56) 2.317(7), Cu(2)sN(35) 1.942(10), Cu(2)sN(11) 1.999(8),
Cu(2)sO(28) 1.980(9), Cu(2)sO(3) 2.005(7), Cu(2)sO(43) 2.310(8),
Cu(2)sO(4) 2.300(7), O(6)sCu(1)sO(53) 91.1(4), O(6)sCu(1)sO(30)
76.8(3), N(13)sCu(1)sN(20) 80.5(4), N(35)sCu(2)sN(11) 81.3(4),
O(28)sCu(2)sO(43) 77.8(3), O(3)sCu(2)sO(4) 79.1(3). See also the
Supporting Information.
Figure 3. Result of the spectral subtraction (3) of the unlabeled (1) and
18O-labeled (2) complexes. (*Since the bpy bands are exhibiting small
frequency shifts in different complexes, it was difficult to compensate them
fully.)
value of 0.12.12 Its UV-vis spectrum shows bands at λmax
) 260 (log ε ) 3.78), 302 (4.03), and 423 (1.61) nm and IR
absorptions of νCO2(as) ) 1645 and νCO2(s) ) 1374 cm-1,
proving monodentate carboxylate coordination. The two
monodentate carboxylato groups of the maleate are in basal
positions [CusO 1.952(2) and 1.9691(19) Å] together with
the two N atoms of bpy [CusN 1.992(2) and 2.003(2) Å].
The carboxylate groups are coordinated to another copper(II)
ion, forming a linear polymer. In the apical position is an O
atom of a water molecule with a CusO distance of 2.371(2)
Å. The half amount of the copper ions weighted in was
obtained as CuCl2(bpy), and its molecular structure was
proved by IR spectrum and X-ray analysis and found to be
identical with former literature data.13
In order to prove that acetonitrile has been oxygenated,
we carried out the reaction with 13CH3CN and 18O2 as well.
Analysis of the infrared and Raman spectra of the products
using 13C- and 18O-labeled compounds gave an answer to
this question.
The IR spectra in the CO2 stretching region are presented
in Figure 3. The most isotope-sensitive bands were the CO
stretching modes, which are summarized in Table 1.
DFT quantum chemical calculations were made for the
bidentate coordinated oxalate ligand to Cu2+ with a general
C2V point group common for both asymmetrically labeled
oxalate ions. The isotope-sensitive band positions were
determined from the difference spectra of 1-1a and 1-1b,
where the bands of different isotopes appeared in the opposite
directions (see, e.g., Figure 3). In all cases, the second
derivatives were also compared to obtain more accurate band
positions and frequency shifts.
Scheme 2. The Formation of Complex 2
oxalate anions. The neighboring two copper centers have
slightly different geometries. The results show that aceto-
nitrile has been oxygenated to oxalate under very mild
conditions by the copper dioxygen complex(es) formed,
which is unprecedented.
When, instead of acetonitrile, propionitrile, isobutironitrile,
or benzilnitrile was used as the solvent, no oxygenation of
the solvent was observed. Instead, the dtbcH2 was oxygenated
to the copper(II) tert-butylmaleate complex [Cu(bma)(bpy)-
(H2O)]n (2) (bmaH2 ) tert-butylmaleic acid) (Scheme 2) in
42% yield. It forms a polymer where the tert-butylmaleate
acts as a bridging ligand between the Cu(II) centers (Figure
2). The complex is paramagnetic with EPR parameters of g|
) 2.270, g⊥ ) 2.050, and CuA| ) 177 G at 77 K. The
spectrum is axial with a g value consistent with a distorted
square-pyramidal CuN2O2O′ chromophore present with a τ
The highest-frequency IR band (v7) at 1650, 1643, and
1644 cm-1 for the 1, 1a, and 1b complexes, respectively,
exhibited isotope shifts 7 (9) and 6 (11) cm-1 for 18O and
13C labeling where, in parentheses, the calculated band
displacements are given.
Figure 2. The molecular structure of 2 with selected bond distances (Å)
and angles (deg): Cu(1)sN(1) 2.003(2), Cu(1)sN(2) 1.992(2), Cu(1)sO(19)
1.9691(1), Cu(1)sO(22_4) 1.952(2), Cu(1)sO(23) 2.371(2), N(1)sCu-
(1)sO(19) 165.74(9), N(2)sCu(1)sO(22) 172.87(9), N(1)sCu(1)sO(23)
94.90(10), O(19)sCu(1)sO(23) 99.17(9). See also the Supporting Informa-
tion.
(12) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C.
J. Chem. Soc., Dalton Trans. 1984, 1349–1356.
(13) Herna´ndez-Molina, M.; Gonza´les-Platas, J.; Ruiz-Pe´rez, C.; Lloret,
F.; Julve, M. Inorg. Chim. Acta 1999, 284, 258–265.
6122 Inorganic Chemistry, Vol. 47, No. 14, 2008

COMMUNICATION
Table 1. Infrared CO Stretching Frequencies (cm-1) of Coordinated Oxalate Ion
[Cu(bpy)(ox)]n (1)
complex using 12CH3CN + 18O2 (1a)
observed calculated observed calculated
1650 1659 1643 (7) 1650 (9)
complex using 13CH3CN + 16O2 (1b)
observed calculated
1644 (6) 1648 (11)
no
assignmentsa
v7
v1
v2
v8
CO2 asym stretch
CO2 asym stretch
CO2 asym stretch
CO2 asym stretch
1621
1405
1343
1625
1427
1326
1606 (15)
1397 (8)
1333 (10)
1611 (11)
1414 (13)
1302 (24)
1591 (30)
1373 (32)
1329 (14)
1588 (37)
1413 (14)
1313 (13)
a Assignments based on DFT calculations simplified as Cu(16O212C12C16O2)Cu (1), Cu(16O212C12C18O2)Cu (1a), and Cu(16O213C12C16O2)Cu (1b) labeled
complexes. In parentheses, the isotope shifts are listed.
The next band (v1) around 1600 cm-1 has a complex
structure due to the solid-state splitting, and in addition, it
overlaps with bpy ligand vibration. The band showed a strong
(15 cm-1) isotope shift for 1a and an even stronger one (30
cm-1) for the 1b complex. The calculated shifts were 13 and
37 cm-1 for 18O- and 13C-labeled complexes, respectively.
The medium IR band at 1405 cm-1 showed the strongest
isotope shift for the 13C isotopic species (over 32 cm-1),
which is the v2 asymmetric CO2 stretching, while the
calculated isotope shift was only 14 cm-1. Due to the rather
complex nature of the spectra, we are still not sure about
the assignment of the band at 1373 cm-1 for the 13C isotope
species.
Following the acetonitrile oxidation reaction by UV-vis
spectroscopy (SFigure 2, Supporting Information), it can be
seen that a band at ∼400 nm evolves and after a certain
time decreases. 3,5-Di-tert-butyl-1,2-benzoquine is formed
and then reduced by the substrate back to the catechol again
just acting as an electron-transfer mediator. The mechanism
of the stoichiometric formation of complexes 1 and 2 is at
the moment not fully understood. Copper(I) chloride in the
presence of amines with O2 forms complexes of [CuCl-
(amine)]4O2.17 The methyl group is oxygenated to the formyl
group and then to the carboxylate group. The cyano group
is then hydrolyzed to the carboxylato group, resulting in
oxalate, which coordinates to the copper(II) centers.18 In the
case of propionitrile as the solvent, 3,5-di-tert-butylcatechol
undergoes a 1,4-oxidative cleavage of the aromatic ring,
giving the copper(II) maleate complex 2.21 The mild
oxygenation of the CsH bond in acetonitrile to oxalate
represents to our knowledge the first case for this type of
reaction under very mild conditions,22 and the 1,4-oxidative
cleavage of 3,5-di-tert-butylcatechol leading to tert-butyl-
maleate is the first straightforward and selective reaction of
catechol oxygenation of this kind. The role of 3,5-di-tert-
butylcatechol in the oxygenation of acetonitrile to oxalate is
to be an electron-transfer mediator in a coupled catalytic
system as found in many cases in the oxidation of organic
substrates by dioxygen or dihydrogen peroxide.23
Acknowledgment. Financial support from the Hungarian
Research Fund (OTKA #K67871) and Ce´lker Ltd. is thank-
fully acknowledged.
Supporting Information Available: Full synthetic details of all
of the compounds, CIF files, and O2-uptake experiments. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
IC800745G
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Inorganic Chemistry, Vol. 47, No. 14, 2008 6123