Mild peroxidative oxidation of cyclohexane catalyzed by mono-, di-, tri-, tetra- and polynuclear copper triethanolamine complexes

Article abstract of DOI:10.1002/adsc.200505216

The mono-, di-, tri-, tetra- and polynuclear copper(II) triethanolamine (H₃tea) complexes [Cu(H₂tea)(N₃)] (1), [Cu ₂(H₂tea)₂(XC₆H₄COO) ₂]· 2 H₂O

Full text of DOI:10.1002/adsc.200505216

FULL PAPERS
DOI: 10.1002/adsc.200505216
Mild Peroxidative Oxidation of Cyclohexane Catalyzed by
Mono-, Di-, Tri-, Tetra- and Polynuclear Copper Triethanolamine
Complexes
a
a
a
Alexander M. Kirillov, Maximilian N. Kopylovich, Marina V. Kirillova,
a
b
a, c
Evgeny Yu. Karabach, Matti Haukka, M. F a´ tima C. Guedes da Silva,
a,
Armando J. L. Pombeiro *
a
Centro de Química Estrutural, Complexo I, Instituto Superior T e´ cnico, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
Fax: (þ351)-21-846-4455, e-mail: pombeiro@ist.utl.pt
b
c
University of Joensuu, Department of Chemistry, P. O. Box 111, FIN-80101, Joensuu, Finland
Universidade Lus o´ fona de Humanidades e Tecnologias, Campo Grande 376, 1749-024 Lisbon, Portugal
Received: May 25, 2005; Accepted: October 7, 2005
Abstract: The mono-, di-, tri-, tetra- and polynuclear gen-centred radical traps (supporting mainly radical
copper(II) triethanolamine (H tea) complexes mechanisms) were investigated and allowed us to ach-
3
[
Cu(H tea)(N )] (1), [Cu (H tea) (XC H COO) ]· ieve yields and TONs up to ca. 39% and 380, respec-
2 3 2 2 2 6 4 2
2
H O (X¼4-H 2a, 4-CH 2b, 3-Cl 2c), [Cu (H tea) - tively, corresponding to the most active copper sys-
2
3
3
2
2
(
(
1
4-OC H COO) (H O)]·4H O (3), [OꢀCu (tea) - tems so far reported for the oxidation of cyclohexane
6
4
2
2
2
4
4
BOH) ][BF ] (4) and [Cu (H tea) {m-C H (COO) - under mild conditions. The catalysts can be reused for
4
4 2
2
2
2
6
4
2
,4} ]·2n H O (5), respectively, are highly active and recycling and, at least complex 4 maintains almost the
n
2
selective catalysts or catalyst precursors for the perox- same level of activity even after five reaction cycles.
idative oxidation of cyclohexane, in acetonitrile, to a The preparation of the new complexes 1, 2b and 2c
cyclohexanol and cyclohexanone mixture, by aqueous by self-assembly at room temperature, and their full
þ
hydrogen peroxide in acidic medium (liquid biphasic characterization by IR spectroscopy, FAB-MS , ele-
catalysis) at room temperature and atmospheric pres- mental and X-ray diffraction structural (for 1 and
sure. The effects on the catalytic activity of various 2c) analyses are also reported.
factors, e.g., the relative amounts of cyclohexane, ox-
idant, catalyst, solvent and nitric acid, reaction time, Keywords: biphasic catalysis; copper complexes;
catalyst recycling and impact of both carbon- and oxy- cyclohexane; N,O ligands; oxidation
Introduction
transition metals. Among them, copper-containing cata-
[
11–14]
lysts are of a high potential interest,
since copper is
Oxidation of cyclohexane is a rather important and a cheap and widespread metal in nature, the third most
much-studied process due to the large demand for its abundant essential trace metal in the human body, pres-
[
14–23]
oxidized products, i.e., cyclohexanol and cyclohexanone ent in the active sites of many enzymes,
e.g., partic-
which are commodity chemicals used as intermediates ulate methane monooxygenase (pMMO), an as yet in-
in the manufacture of adipic acid, nylon-6,6’, polya- completely characterized enzyme that is known to cata-
[
20–23]
mide-6, urethane foams, acidulant in baking powder lyze alkane hydroxylation and alkene epoxidation,
[
1–5]
[6–10]
and lubricating additives.
directed towards the development of more efficient nase and blue oxidases.
processes than the current commercial ones [e.g., the The previously reported
Attempts
have been ceruloplasmin, haemocyanines, catechol oxidase, tyrosi-
[
24–34]
examples of copper-cat-
Dupont process for cyclohexane oxidation appears to alyzed oxidation of cyclohexane under relatively mild
operate with only 4% conversion, 85% selectivity, at conditions and using various oxidizing agents like H O ,
2
2
above 1508C, using air at above 12 atm and Co(III) t-BuOOH, O , CH COOOH or their combinations,
2
3
[
6]
naphthenate as catalyst] by selective oxidation of cy- are based on different Cu(I and II) complexes with
clohexane under milder conditions using different oxi- N,O imino ligands, Cu-containing silicates and zeo-
dizing agents and catalytic systems comprising various lites , mono- and dinuclear Cu complexes with bis{2-
[
24]
[26]
[
29]
Adv. Synth. Catal. 2006, 348, 159 – 174
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
159

FULL PAPERS
Alexander M. Kirillov et al.
[
N,N-bis(2-pyridylethyl)amino]-1,1-dimethylethyl} di-
[
28]
sulfide and related ligands, a salen complex N,N’-
(
(
copper(II) nitrate, Cu phthalocyanines and substitut-
ed (chloro- and nitro-) phthalocyanines encapsulated in
zeolites X and Y, Cu(II) complexes containing N,N-
bis(2-pyridylmethyl)-b-alanineamide ligands,
per(II) hydroxide and various crown ether Cu-com-
[25]
1,2-ethylene)-bis(salicylaldamine)-copper(II),
trimethylacetate)copper(II) and bis(ethylenediamine)-
bis-
[
30]
[31]
[
32]
cop-
[
27]
[33]
[34]
plexes, Cu-Gif system, etc. Nevertheless, most of
these catalysts still provide very low yields (relative to
[
26–29,32–34]
[26,30,31]
cyclohexane)
and low selectivity,
require
and environmentally unfriendly
components, are active only in the presence of various
[31,33]
rather expensive
[34]
[
27,33,34]
additives,
ses,
and/or involve complicated synthe-
thus being inaccessible on a large scale.
[
25,26,28,31–33]
[
35]
Recently we have preliminary reported a simple
and convenient synthesis of several new multinuclear
copper triethanolamine (H tea) complexes using cheap,
3
commercially available and environmentally tolerable
reagents and opened up their potential application as
highly active and selective catalysts or catalyst precur-
sors for the mild peroxidative oxidation of alkanes to
the corresponding alcohols and ketones. Hence, the
main purpose of this work is to provide a detailed inves-
[
35]
tigation of cyclohexane oxidation catalyzed by known
and newly synthesized copper triethanolamine com-
plexes of diverse nuclearity (Scheme 1), thus extending
the catalytic activity studies to a variety of experimental
conditions, e.g., the oxidant-to-catalyst and substrate-to-
catalyst molar ratios, reaction time, amount of solvent,
multiple recycling of catalyst and the influence of vari-
Scheme 1.
À1
ous types of radical traps, aiming at the optimization 2859 cm . Two high intensity bands associated with
of this process and the identification of the general n and n vibrations of azide ligand are also observed
as
s
À1
type of mechanism in the search for an efficient way at 2048 and 1384 cm , respectively. For complexes 2b
for cyclohexane oxidation.
and 2c, the vibrations related to the H tea ligands
2
show common features and contain broad bands in the
À1
3
500–3100 cm range assigned to the stretching vibra-
Results and Discussion
tions of OH groups and H O molecules, and strong or
medium intensity n(CH) bands in the 3000–2850 cm
2
À1
range. Several high intensity bands associated with nas
Synthesis and Spectroscopic Characterization of
Complexes 1, 2band 2c
and n vibrations of carboxylate groups were observed
s
in the spectra of complexes 2b and 2c in the 1600–
À1
1
530 and 1420–1370 cm ranges, respectively.
Addition of sodium azide or aromatic carboxylates to
The molecular ions were clearly observed with the ex-
þ
the aqueous blue solution of Cu(NO ) , H tea and pected isotopic patterns in the FAB -MS of 2b and 2c.
3
2
3
NaOH provides the formation, by self-assembly at Other typical peaks correspond to the stepwise frag-
room temperature, of mono- and dinuclear Cu com- mentations by loss of tea, carboxylates, and Cu(tea)
plexes. Hence, by adding sodium azide, p-toluic acid or fragments or to dissociation of dimeric structures to
3
1
-chlorobenzoic acid one obtains 1, 2b or 2c (Scheme give monomeric fragments, e.g., [Cu(H tea)(C H -
2 6 4
þ
), respectively, isolated as crystalline solids in good ClCOO)] in 2c. Elemental analyses are also consistent
þ
yields (79–86%), and characterized by IR and FAB - with the proposed formulations which are authenticated
mass spectroscopy, elemental analysis and X-ray crystal- by single crystal X-ray diffraction studies (for 1 and 2c),
lography (for 1 and 2c).
as indicated below.
The IR-spectrum of 1 shows a set of vibrations due to
the H tea ligand, e.g., n(OH) bands at 3480 and
2
À1
3308 cm , and n(CH) bands at 2985, 2909 and
160
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Mild Peroxidative Oxidation of Cyclohexane
FULL PAPERS
X-Ray Crystal Structures of Complexes 1 and 2c
In complex 1 (Figure 1), triethanolamine is deprotonat-
ed at one oxygen atom (O3) and binds the pentacoordi-
nated metal via the amino-N and three oxygen atoms
which occupy the axial positions of a trigonal-bipyrami-
dal coordination mode. The geometry is completed by
an azide ligand trans to the amino nitrogen N1, the
N1ÀCuÀN2 angle of 177.57(5)8 being essentially linear.
The mononuclear units are held together by OÀH···O
intermolecular hydrogen bonds forming a polynuclear
network (Figure 2) with the shortest CuÀCu separation
of 4.435(2) Š, which is in the range of 4–5 Š found for
[
23]
the trinuclear clusters of some multicopper oxidases.
The CuÀO1 and CuÀO2 lengths of ca. 2.07–2.13 Š are Figure 2. Fragment of the crystal packing of complex 1 illus-
trating the intermolecular hydrogen bonding pattern.
unexceptional for protonated oxygen atoms bonded to a
[36,37]
copper centre,
2.0289(11) Š] with the deprotonated oxygen atom.
and are slightly longer than CuÀO3
[
The binding of the H tea ligand involves the chelate to those of related triethanolaminate complexes of cop-
2
[
36]
[36]
rings CuÀN1ÀC1ÀC2ÀO1, CuÀN1ÀC3ÀC4ÀO2 and per such as [Cu(H tea)Cl],
[Cu(H tea)NCS],
2
2
[
2
37]
CuÀN1ÀC5ÀC6ÀO3 with bite N1ÀCuÀO angles of [Cu (H tea) (4,4’-bipy)](ClO )
and [Cu(H tea)-
2
2
2
2
4
[
39]
8
1.67(4), 84.60(4) and 85.87(5)8, respectively. The (CF COCHCOCF )]
.
3
3
CuÀN1 bond length, 2.0186(12) Š, is slightly longer
The molecular structure of 2c consists of centrosym-
2
þ
than the CuÀN2 [1.9550(13) Š], though shorter than metric binuclear fragments [Cu (H tea) ] stabilized
2
2
2
the CuÀO bonds. The azide ligand is linear with the by two monodentate 3-chlorobenzoate ligands (Fig-
N2ÀN3ÀN4 angle of 176.62(15)8, the N2ÀN3 and ure 3), which are further involved in an intermolecular
N3ÀN4 bond lengths of 1.2059(19) and 1.1525(19) Š be- hydrogen bonding network with non-coordinated water
[
38]
ing within the normal values reported for such bonds.
molecules, thus forming parallel chains in neighbouring
It coordinates the metal with a CuÀN2ÀN3 angle of layers (Figure 4). Each Cu ion in the molecule has a dis-
1
18.45(10)8. For 1 most of the bond distances are similar torted tetragonal bipyramidal geometry, and each H tea
2
acts as a tetradentate ligand with one bridging alkoxo
group. The binding of the H tea ligands involves the
2
five-membered chelate rings Cu1ÀN1ÀC1ÀC2ÀO1,
CuÀN1ÀC3ÀC4ÀO2 and CuÀN1ÀC5ÀC6ÀO3A with
bite N1ÀCuÀO angles of 78.73(6), 76.62(6) and
8
1
2.88(6)8, respectively. The CuÀO3A bond length of
.9420(14) Š does not differ from the standard values
for m-OÀCu bonds and agrees with the lengths (ca.
[
35,40,41]
1
.91À1.95 Š) found in related compounds,
as the CuÀO1 and CuÀO2 distances [2.5640(18) and
.4445(18) Š, respectively] are remarkably longer than
the normal CuÀO
bond distances. Neverthe-
where-
2
amino alcohol
less, they lie in the 2.43–2.56 Š range reported for 2a,
[
35]
[41,42]
5
and other triethanolaminate complexes.
The
Cu1ÀO3ÀCu1AÀO3A bridging mode is planar. The m-
Figure 1. An ORTEP-3 representation of 1. Selected bond
lengths (Š) and bond angles (8): C(1)ÀN(1) 1.4882(18), O3 atoms of the triethanolamine ligands generate a cen-
C(2)ÀO(1) 1.4443(18), C(3)ÀN(1) 1.4903(18), C(4)ÀO(2) trosymmetric Cu O core with an inversion centre dis-
2
2
1
.4402(17), C(5)ÀN(1) 1.4852(19), C(6)ÀO(3) 1.4342(17), posed between the two copper atoms. The CuÀCu sepa-
N(1)ÀCu(1) 2.0186(12), N(2)ÀN(3) 1.2059(19), N(2)ÀCu(1)
ration of 2.9222(5) and most of other bonding param-
eters agree with those found in related dinuclear com-
1
.9550(13), N(3)ÀN(4) 1.1525(19), O(1)ÀCu(1) 2.1343(11),
O(1)ÀH(1) 0.9026(10), O(2)ÀCu(1) 2.0655(10), O(2)ÀH(2)
[35,40,41]
plexes.
0
.9529(10), O(3)ÀCu(1) 2.0289(11); N(3)ÀN(2)ÀCu(1)
1
18.45(10), N(4)ÀN(3)ÀN(2) 176.62(15), N(2)ÀCu(1)ÀN(1)
1
77.57(5), N(2)ÀCu(1)ÀO(3) 96.09(5), N(1)ÀCu(1)ÀO(3)
8
5.87(5), N(2)ÀCu(1)ÀO(2) 95.57(5), N(1)ÀCu(1)ÀO(2)
8
4.60(4), O(3)ÀCu(1)ÀO(2) 121.31(4), N(2)ÀCu(1)ÀO(1)
9
6.17(5), N(1)ÀCu(1)ÀO(1) 81.67(4), O(3)ÀCu(1)ÀO(1)
1
14.92(4), O(2)ÀCu(1)ÀO(1) 120.60(4).
Adv. Synth. Catal. 2006, 348, 159 – 174
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FULL PAPERS
Alexander M. Kirillov et al.
Figure 3. An ORTEP-3 representation of 2c. Selected bond lengths (Š) and bond angles (8): C(1)ÀN(1) 1.485(3), C(2)ÀO(1)
1
1
2
.426(3), C(3)ÀN(1) 1.493(3), C(4)ÀO(2) 1.426(3), C(5)ÀN(1) 1.499(3), C(6)ÀO(3A) 1.422(2), C(7)ÀO(5) 1.250(3), C(7)ÀO(4)
.265(3), N(1)ÀCu(1) 2.0431(18), O(1)ÀH(1) 0.8911(12), O(2)ÀH(2) 0.8856(17), O(1)ÀCu(1) 2.5640(18), O(2)ÀCu(1)
.4445(18), O(3A)ÀCu(1) 1.9420(14), O(4)ÀCu(1) 1.9672(14), O(99)ÀH(99A) 0.8601; C(1)ÀN(1)ÀCu(1) 109.92(13),
C(3)ÀN(1)ÀCu(1) 108.79(13), C(5)ÀN(1)ÀCu(1) 107.22(12), C(6A)ÀO(3)ÀCu(1) 128.81(12), Cu(1)ÀO(3)ÀCu(1A) 97.12(6),
C(7)ÀO(4)ÀCu(1) 133.28(14), O(3)ÀCu(1)ÀO(3A) 82.88(6), O(3)ÀCu(1)ÀO(4) 95.42(6), O(3A)ÀCu(1)ÀO(4) 177.40(6),
O(1)ÀCu(1)ÀN(1) 78.73(6), O(2)ÀCu(1)ÀN(1) 76.62(6), O(3)ÀCu(1)ÀN(1) 161.61(6), O(3A)ÀCu(1)ÀN(1) 84.95(6),
O(4)ÀCu(1)ÀN(1) 97.17(6), O(3)ÀCu(1)ÀCu(1A) 41.62(4), O(3A)ÀCu(1)ÀCu(1A) 41.26(4), O(4)ÀCu(1)ÀCu(1A) 137.01(5),
N(1)ÀCu(1)ÀCu(1A) 124.87(5).
Figure 4. Fragment of the crystal packing of complex 2c·2 H O illustrating the intermolecular hydrogen bonding pattern.
2
Catalytic Activity of Copper Triethanolamine
Complexes towards Peroxidative Oxidation of
Cyclohexane
All the copper complexes 1–5 (Scheme 1) act as cata-
lysts or catalyst precursors for the oxidation of cyclohex-
ane, in acetonitrile, to a cyclohexanol and cyclohexa- Scheme 2.
none mixture, by aqueous hydrogen peroxide in acidic
medium (liquid biphasic catalysis) at room temperature
(
Scheme 2). The influence of various factors like the rel- on the activity of catalysts were investigated in a system-
ative amounts of HNO , H O , catalyst and solvent, re- atic way, the results being shown in Tables 1–6 and dis-
3
2
2
action time, catalystrecycling and impact of radical traps cussed below.
162
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Mild Peroxidative Oxidation of Cyclohexane
FULL PAPERS
Effect of the Amount of Nitric Acid
[5,6,8]
It is known
that the peroxidative oxidation of cyclo-
hexane catalyzed by some derivatives of Mn, Cr, Pd, Pt,
Hg, etc. proceeds more efficiently in acidic medium. We
have also found that the addition of nitric acid to the re-
action mixtures strongly enhances the activity towards
alkane hydroxylation of various catalysts like iron-chro-
[
43]
mium hydroxides and hydroxo complexes, vanadi-
[44, 45]
[35]
um
and copper complexes with N,O ligands.
In the current study, the cyclohexane oxidation in the
presence of the copper catalysts 1–5 practically does not
proceed unless nitric acid is added (Table 1, Figure 5).
Thus, in the absence of acid a lack of activity (complexes
2a and 5) or a very weak activity of 0.3–2.0% (com-
plexes 1, 3, 4) was detected (Table 1, entries 1, 6, 11,
Figure 5. Effect of the nitric acid-to-catalyst molar ratio on
1
5, 19). The addition of ten equivalents of HNO relative the total yield of products in the cyclohexane oxidation cata-
3
to catalyst leads to a dramatic increase of activity, e.g., lyzed by complexes 1, 2a, 3, 4 and 5. Reaction conditions are
for catalyst 1 the total yield increases from 2.0 to those of Table 1.
27.7% on changing the acid-to-catalyst molar ratio
from 0 to 10 (Figure 5, curve 1). The total yield of cyclo-
hexanol and cyclohexanone remains practically con- ity is displayed by complexes 3 and 4 which exhibit the
stant in the 10–50 range of n(HNO )/n(catalyst), where- highest yields at n(HNO )/n(catalyst)¼10 (Figure 5,
3
3
as further enhancement of such a ratio up to 100 pro- curves 3, 4). Complexes 2a and 5 require a higher
vides a decrease of the yield to ca. 10% (Table 1, entries amount of acid to display a maximum activity, ca. 15%
4, 5). A related effect of nitric acid on the catalytic activ- yield at the acid-to-catalyst molar ratio of 100 (Table 1,
Table 1. Peroxidative oxidation of cyclohexane catalyzed by complexes 1–5. Effect of the nitric acid-to-catalyst molar ratio.
[a]
[b]
Entry
Catalyst
n(HNO )/n(catalyst)
Yield of products [%]
Molar ratio
3
C H O/C H OH
[c]
6
10
6
11
Cyclohexanol
Cyclohexanone
Total
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
1
1
1
1
1
2a
2a
2a
2a
2a
3
3
3
3
4
4
4
4
5
0
10
25
50
100
0
10
50
100
150
0
10
50
100
0
10
50
100
0
0.6
16.0
11.3
12.1
7.5
0.0
2.5
4.8
6.0
5.8
0.0
7.9
8.3
8.6
0.4
14.2
10.7
10.0
0.0
1.4
11.7
16.0
14.9
10.1
0.0
2.8
5.6
9.4
4.0
0.3
6.8
5.8
5.6
1.0
8.9
9.4
9.1
0.0
3.0
5.8
8.6
5.2
2.0
27.7
27.3
27.0
17.6
0.0
2.3
0.7
1.4
1.2
1.3
–
1.1
1.2
1.6
0.7
–
0.9
0.7
0.7
2.5
0.6
0.9
0.9
–
0.9
1.1
1.2
0.9
5.3
10.4
15.4
9.8
1
1
1
1
1
1
1
1
1
1
2
2
2
2
0.3
14.7
14.1
14.2
1.4
23.1
20.1
19.1
0.0
5
5
5
5
10
50
100
150
3.5
5.5
7.0
5.7
6.5
11.3
15.6
10.9
[
[
[
a]
Reaction conditions: catalyst (25 mmol), H O (10.00 mmol), C H (0.63 mmol), MeCN (3 mL), 6 h reaction time.
Moles of product/100 moles of cyclohexane.
Cyclohexanolþcyclohexanone.
2
2
6
12
b]
c]
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163

FULL PAPERS
Alexander M. Kirillov et al.
Figure 5). Thus, the catalysts that are pentacoordinate
and comprise evident labile sites like CuÀN , CuÀH O
3
2
and Cu-m -O (complexes 1, 3, 4, respectively) exhibit
4
the highest activity at a relatively low amount of acid,
whereas the other complexes, 2 and 5, that are hexacoor-
dinate require a higher amount of acid conceivably to
promote unsaturation at the metal upon ligand protona-
tion. Although the cyclohexanone-to-cyclohexanol mo-
lar ratio for complexes 1 and 4 is maximal at higher
amounts of nitric acid (Table 1, entries 2–4, 16–18),
such an excess of acid should be avoided due to the ten-
dency for lowering the overall reaction selectivity to-
wards the formation of cyclohexanol and cyclohexa-
none, i.e., the formation of side products can then be fav-
oured. Based on these data, a controlled amount of acid
was used in further experiments, i.e., n(HNO )/n(cata- Figure 6. Effect of the oxidant-to-catalyst molar ratio on the
3
lyst)¼10 (for 1, 3, 4) or 100 (for 2 and 5).
total yield of products in the cyclohexane oxidation catalyzed
Hence, the need of acid for the peroxidative oxidation by complexes 1, 2b, 3, 4 and 5. Reaction conditions are those
of Table 2.
of cyclohexane is evident and presumably can be associ-
ated with (i) its involvement in proton transfer steps, ac-
tivation of catalyst by unsaturation of the metal centres
upon ligand protonation and acceleration of the oxida- bly be accounted for by their lower coordination num-
[
5,6,8,9,35]
tion reaction,
properties of the metal complexes,
ing of decomposition of hydrogen peroxide to water bile ligand(H O, N ). Moreover, a very high selectivity
(ii) the enhancement of oxidative ber (they are pentacoordinate, whereas the others are
[5,46]
(iii) the hamper- hexacoordinate) (Scheme 1), and the presence of a la-
[
35]
2
3
[
6,47]
and oxygen,
and enhancement of the formation of towards the formation of cyclohexanol and cyclohexa-
[
5,48]
peroxo complexes,
and (iv) the increase of selectivity none is exhibited by systems comprising complexes 3
towards the formation of cyclohexanone (a main raw and 4 since no traces of any by-products were detected
material for production of polyamide-6) by dehydrogen- by GC or GC-MS analyses of the final reaction mixtures.
ation of cyclohexanol, which can efficiently be catalyzed In contrast, a simple copper salt like Cu(NO ) under the
3
2
by strong mineral acids and some heterogeneous copper same reaction conditions exhibits a much lower activity,
[
6]
catalysts.
i.e., total yield of 3.2 and 5.4% at the peroxide-to-cata-
lyst molar ratio of 200 and 400, respectively (Table 2, en-
tries 29, 30). Therefore, the relevance of polydentate
N,O triethanolaminate ligands is evident and probably
associated to their involvement (upon decoordination
Effect of the Oxidant-to-Catalyst Molar Ratio
The amount of hydrogen peroxide has a significant ef- of an N or O atom) in proton-transfer steps, e.g., among
[
8,44,49,50]
fect and the overall yields of cyclohexanol and cyclohex- H O , oxo and/or peroxo ligands, as suggested
for
2
2
anone are enhanced in all catalytic systems upon in- some vanadium catalysts that can even require particu-
[
8,50]
creasing the peroxide-to-catalyst molar ratio up to 200 lar N,O additives
Table 2, Figure 6). The highest activity with total yields taining systems which are active only in the presence
of 30.6 and 27.2% is displayed by complexes 3 and 4, re- of additives like pyridine and acetaldehyde were also re-
spectively (Table 2, entries 19, 23), followed by other ported.
complexes which exhibit 12.2–17.5% yields for the
above molar ratio of 200. Further enhancement of
as cocatalysts. Some copper-con-
(
[
27,33,34]
n(H O )/n(catalyst) up to 400 leads to the increase of ac- Effects of the Cyclohexane-to-Catalyst Molar Ratio
2
2
tivity of complexes 1, 2 and 5 up to 27.7, 17.0 (for 2b) and and the Reaction Time
5.6%, respectively (Figure 6, Table 2), while a yield
drop of ca. 15 and 5% was detected when using catalysts The oxidation of cyclohexane proceeds even if a very
and 4 (Figure 6, curves 3, 4). Such a decrease of activity small relative amount of metal complex catalyst is
1
3
can probably be associated to the decreased concentra- used leading to rather high turnover numbers (TONs)
tion of acetonitrile which, as shown below, has also a rel- (Table 3). They gradually increase on changing the cy-
evant effect.
clohexane-to-catalyst molar ratio from 800 to 6400 (Fig-
All the complexes 2a, 2b, 2c and 5 exhibit similar activ- ure 7, Table 3, first four entries for each catalyst), and
ities under the reaction conditions used (Table 2), which values of 111 and 121 are reached within 6 h reaction
are significantly lower than those of complexes 1, 3 and time for the most active catalysts 3 and 4, respectively
4. The higher activity of the latter complexes can possi- (Figure 7, curves 3, 4). Other catalysts exhibit lower ac-
164
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 159 – 174

Mild Peroxidative Oxidation of Cyclohexane
FULL PAPERS
tivity under these reaction conditions with maximal Effect of the Amount of Solvent
TONs of ca. 48–87 (Figure 7).
The activity of all catalytic systems is also dependent Acetonitrile was used as a solvent for the liquid biphasic
on the reaction time. TONs tend to increase on the ex- oxidation of cyclohexane due to (i) the solubilization of
tension of reaction time from 6 to 72 h (Figure 8, Ta- the alkane and products in this solvent, (ii) the high sta-
ble 3, last three entries for each catalyst), e.g., from 87 bility towards oxidation under the reaction conditions
to 359, respectively, for catalyst 2b. At 24 h reaction applied (in contrast with other solvents, e.g., methanol,
time a TON of 285 was displayed by complex 3, while ethanol or acetone), (iii) the similarity of its boiling
at 72 h a TON of 378 was observed for complex 5 with point with that of cyclohexane what allows an easy recir-
tendency for even further increases with time (Figure 8, culation of the cyclohexane and solvent mixture, (iv) its
curves 3, 5). The highest TON values found for all stud- coordination ability and also in view of the best results
[
2,6,43]
ied catalytic systems are within the range of ca. 215–380 previously obtained in this solvent.
Table 3). In contrast, copper nitrate under similar reac- The amount of acetonitrile in the reaction mixture has
(
tion conditions shows a much lower TON of 45, which is a significant effect on the yields of products (Table 4,
ca. 5- and 8.5-times lower than the least and the highest Figure 9). At a low content of this solvent, only negligi-
TON values (Table 3, entries 6, 42), respectively, exhib- ble yields of products are detected (Table 4, entries 1, 5).
ited by the copper complexes.
The increase of its amount leads to a marked growth of
activity (Figure 9) which reaches the maximum yield
values of 37.2 and 38.5% for complexes 3 and 4, respec-
Table 2. Peroxidative oxidation of cyclohexane catalyzed by complexes 1–5. Effect of the oxidant-to-catalyst molar ratio.
[a]
[b]
Entry
Catalyst
n(H O )/n(catalyst)
Yield of products [%]
Molar ratio
2
2
C H O/C H OH
[c]
6
10
6
11
Cyclohexanol
Cyclohexanone
Total
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
1
1
50
100
200
400
50
100
200
400
50
100
200
400
50
100
200
400
50
100
200
400
50
100
200
400
50
100
200
400
200
400
1.1
4.4
8.2
16.0
1.9
3.8
5.0
6.0
2.5
4.4
7.5
9.4
3.5
3.9
7.0
6.8
4.8
9.6
20.0
7.9
0.0
3.5
15.4
14.2
2.6
4.1
6.5
7.0
1.1
1.6
1.7
4.9
9.3
11.7
2.5
4.1
7.2
9.4
2.0
3.1
6.0
7.6
2.8
3.7
6.8
7.8
2.8
5.4
10.6
6.8
0.9
4.0
11.8
8.9
2.4
3.5
5.8
8.6
2.1
3.8
2.8
9.3
17.5
27.7
4.4
1.6
1.1
1.1
0.7
1.3
1.1
1.4
1.6
0.8
0.7
0.8
0.8
0.8
0.9
1.0
1.1
0.6
0.6
0.5
0.8
–
1.2
0.8
0.6
0.9
0.9
0.9
1.2
1.5
2.4
1
2a
2a
2a
2a
2b
2b
2b
2b
2c
2c
2c
2c
3
3
3
3
4
4
4
4
5
7.9
12.2
15.4
4.5
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
7.5
13.5
17.0
6.3
7.6
13.8
14.6
7.6
15.0
30.6
14.7
0.9
7.5
27.2
23.1
5.0
5
5
5
7.6
12.3
15.6
3.2
[
[
d]
d]
Cu(NO3)2
Cu(NO3)2
5.4
[
[
[
[
a]
Reaction conditions: catalyst (25 mmol), H O (1.25–10.00 mmol), C H (0.63 mmol), MeCN (3 mL), 6 h reaction time.
Moles of product/100 moles of cyclohexane.
2
2
6
12
b]
c]
Cyclohexanolþcyclohexanone.
d]
For comparative purposes.
Adv. Synth. Catal. 2006, 348, 159 – 174
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
165

FULL PAPERS
Alexander M. Kirillov et al.
Figure 7. Effect of the cyclohexane-to-catalyst molar ratio on
the total turnover number of catalyst in the cyclohexane ox-
idation catalyzed by complexes 1, 2b, 3, 4 and 5 (6 h reaction
time). For other reaction conditions, see Table 3.
Figure 9. Effect of the amount of solvent on the total yield of
products in the cyclohexane oxidation catalyzed by com-
plexes 3 and 4. Reaction conditions are those of Table 4.
and the catalyst was recovered by full evaporation of
the reaction mixture under vacuum. The subsequent cy-
cles were initiated upon addition of new standard por-
tions of all other reagents. For comparative purposes,
the reactions were run in duplicate, in the presence
and in the absence of nitric acid.
The first cycle was performed with acid as required for
the initial activation of the catalyst and affords cyclohex-
anol and cyclohexanone with an overall yield of 34.2%
under the reaction conditions used (Table 5, entry 1, Fig-
ure 10). Repeating the experiment without and with
added nitric acid gives only very slightly decreased
yields of 33.4 and 32.9%, respectively (Table 5, entries
2
, 3). A similar behaviour of catalyst 4 is observed for
further reaction cycles, i.e., only a slight yield drop of
.1 and 5.2% is detected after the fifth reaction stage
2
Figure 8. Effect of the reaction time on the total turnover
number of catalyst in the cyclohexane oxidation catalyzed
by complexes 1, 2b, 3, 4 and 5. Reaction conditions are those
of Table 3.
tively, at 5 mL of acetonitrile. Further enhancement of
the content of acetonitrile in the reaction mixture up
to 10 mL leads to a significant yield drop, which conceiv-
ably can be associated with the concomitant reduction of
the concentration of the oxidant.
Catalyst Recycling
Recycling reagents is a common practice in industrial
oxidation of cyclohexane. Such a type of methodology
has been tested for complex 4 (Table 5) due to its higher
activity and stability as accounted for by its isolation, af-
Figure 10. Effect of the catalyst recycling on the peroxidative
ter the reaction, in a pure crystalline form. On comple- oxidation of cyclohexane catalyzed by complex 4. Reaction
tion of each stage, the products were analyzed as usual conditions are those of Table 5.
166
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Adv. Synth. Catal. 2006, 348, 159 – 174

Mild Peroxidative Oxidation of Cyclohexane
FULL PAPERS
Table 3. Peroxidative oxidation of cyclohexane catalyzed by complexes 1–5. Effects of the cyclohexane-to-catalyst molar ratio
and the reaction time on the turnover number (TON).
[a]
[b]
Entry
Catalyst
n(C H )/(catalyst)
Reaction
time [h]
TON
Molar ratio
6
12
C H O/C H OH
[c]
6
10
6
11
Cyclohexanol
Cyclohexanone
Total
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
800
1600
3200
6400
6400
6400
800
1600
3200
6400
6400
6400
800
1600
3200
6400
6400
6400
800
1600
3200
6400
6400
6400
800
1600
3200
6400
6400
6400
800
1600
3200
6400
6400
6400
800
1600
3200
6400
6400
6400
6400
6
6
6
15
14
16
29
72
90
6
10
13
39
109
163
10
18
30
47
124
157
7
11
14
24
101
144
21
28
39
41
112
182
23
24
30
48
70
86
7
20
21
26
45
116
127
6
10
17
25
86
196
7
12
25
39
92
202
6
12
16
24
82
170
25
43
66
70
173
175
23
39
48
73
123
147
6
35
35
42
1.4
1.6
1.6
1.6
1.6
1.4
1.0
1.0
1.2
0.7
0.8
1.2
0.8
0.7
0.9
0.8
0.7
1.3
1.0
1.2
1.1
1.0
0.8
1.2
1.2
1.6
1.7
1.7
1.5
1.0
1.0
1.7
1.6
1.5
1.7
1.7
0.8
1.0
1.1
1.0
0.8
1.3
0.5
6
74
24
72
6
6
6
188
217
12
20
30
2a
2a
2a
2a
2a
2a
2b
2b
2b
2b
2b
2b
2c
2c
2c
2c
2c
2c
3
3
3
3
3
3
4
4
4
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
6
64
24
72
6
6
6
195
359
17
30
55
6
86
24
72
6
6
6
216
359
13
23
30
6
48
24
72
6
6
6
183
314
46
71
105
111
285
357
46
63
78
121
193
233
13
21
34
66
111
378
45
6
24
72
6
6
6
4
4
4
5
5
5
5
5
6
24
72
6
6
6
11
17
33
61
166
16
10
17
33
50
212
29
6
24
72
72
5
Cu(NO3)2
[
[
[
a]
Reaction conditions: catalyst (1.56–12.50 mmol), H O (10.00 mmol), C H (10.00 mmol), MeCN (3 mL).
Moles of product/mol of catalyst.
Cyclohexanolþcyclohexanone.
2
2
6
12
b]
c]
for the experiments performed, respectively, in the ab- responding lowering of catalyst concentration in the re-
sence or in the presence of nitric acid (Table 5, Fig- action mixture as a result of the losses of the samples tak-
ure 10). The better results obtained without added en for the GC tests. Thus, the performed experiments
acid are in agreement with the detected small loss of ac- open up the possibility of recycling catalyst 4 which
tivity of catalyst 4 (Table 1, entries 16–18) upon increas- maintains almost the original level of activity after sev-
ing the relative amount of acid in the reaction mixture. A eral reaction cycles providing ca. 32–34% of cyclohex-
slight activity decrease can also be explained by the cor- ane conversion after each stage, with a rather high selec-
Adv. Synth. Catal. 2006, 348, 159 – 174
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
167

FULL PAPERS
Alexander M. Kirillov et al.
Table 4. Effect of the amount of solvent on the peroxidative oxidation of cyclohexane catalyzed by complexes 3 and 4.
[a]
[b]
Entry
Catalyst
MeCN [mL]
Yield of products [%]
Molar ratio
C H O/C H OH
[c]
6
10
6
11
Cyclohexanol
Cyclohexanone
Total
1
2
3
4
5
6
7
8
3
3
3
3
4
4
4
4
1
3
5
10
1
3
0.1
7.9
22.9
7.0
0.4
6.8
14.3
8.6
0.7
8.9
0.5
14.7
37.2
15.6
1.0
23.1
38.5
13.0
4.0
0.9
0.6
1.2
2.3
0.6
0.6
1.3
0.3
14.2
24.3
5.7
5
10
14.2
7.3
[
a]
Reaction conditions: catalyst (25 mmol), H O (10.00 mmol; 1.02 mL of 30% aqueous H O ), C H (0.63 mmol), 6 h reac-
2
2
2
2
6
12
tion time.
[
[
b]
c]
Moles of product/100 moles of cyclohexane.
Cyclohexanolþcyclohexanone.
[a]
Table 5. Effect of the catalyst recycling on the peroxidative oxidation of cyclohexane catalyzed by complex 4.
[
b]
Entry
Cycle
n(HNO )/n(catalyst)
Yield of products [%]
Cyclohexanol
3
[c]
Cyclohexanone
Total
st
1
2
3
4
5
6
7
8
9
1
2
2
3
3
4
4
5
10
–
10
–
10
–
10
–
24.1
22.1
22.2
22.3
20.8
20.0
17.8
20.9
18.1
10.1
11.3
10.7
10.8
9.3
13.0
11.4
11.2
10.9
34.2
33.4
32.9
33.1
30.1
33.0
29.2
32.1
29.0
nd
nd
rd
rd
th
th
th
th
5
10
[
a]
Reaction conditions: catalyst (25 mmol), H O (6.30 mmol), C H (0.63 mmol), MeCN (3 mL), 6 h reaction time. The cat-
2
2
6
12
alyst was recovered after each reaction cycle by full evaporation of the reaction mixture under vacuum, followed by the
addition of new standard portions of all reagents to run a subsequent batch.
Moles of product/100 moles of cyclohexane.
Cyclohexanolþcyclohexanone.
[
[
b]
c]
tivity (possibly close to 100%) to cyclohexanol and cy- tetramethylpiperidine-1-oxyl), or an oxygen radical
[
51]
clohexanone (Figure 10).
trap such as Ph NH. The addition of CBrCl in a stoi-
2
3
chiometric amount relative to cyclohexane or hydrogen
peroxide leads to 88 and 98% suppression of the product
formation, respectively (entries 2, 3). TEMPO leads to
Effect of Radical Traps
60% suppression when used in a stoichiometric amount
In order to determinate the type of mechanism of the in relation to cyclohexane (entry 4). Ph NH has also a
2
peroxidative oxidation of cyclohexane, several reactions high effect, i.e., 71 and 89% of activity suppression is de-
were performed in the presence of various radical trap- tected for the equimolar amount or an excess relative to
ping agents. The obtained results for reactions catalyzed cyclohexane, correspondingly (entries 5, 6). BHT (2,6-
by complex 3 are given in Table 6, and the other copper di-tert-butyl-4-methylphenol) has a lower inhibition ef-
complexes exhibit similar behaviour upon addition of fect, commonly below 50% (entries 7–9), whereas qui-
radical traps. The cyclohexane oxidation appears to pro- none almost completely suppresses the formation of cy-
ceed mainly via radical mechanisms involving both car- clohexanol and cyclohexanone when used in a stoichio-
bon-centred radicals (upon homolysis of an alkane CÀH metric amount relative to H O (entry 11).
2
2
[35]
bond) and oxygen-centred radicals on account of the
However, competitive non-radical pathways can-
pronounced decrease of the catalytic activity when the not be ruled out. They can involve electrophilic attack
experiments are performed in the presence of either a of an oxidized metal-peroxo, metal-superoxo or metal-
[
51]
carbon radical trap like CBrCl or TEMPO (2,2,6,6- oxo centre to a CÀH bond of cyclohexane with hetero-
3
168
asc.wiley-vch.de
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 159 – 174

Mild Peroxidative Oxidation of Cyclohexane
FULL PAPERS
Table 6. Effect of radical traps on the peroxidative oxidation of cyclohexane catalyzed by complex 3.
[
b]
Entry
Radical trap
n(rad. trap)/
n(rad. trap)/
n(H O )
Yield of products [%]
Yield drop owing
to rad. trap [%]
[a]
[d]
n(C H )
6
12
2
2
[c]
Cyclohexanol
Cyclohexanone
Total
1
2
3
4
5
6
7
8
9
–
–
–
20.0
2.6
0.3
3.7
5.6
10.6
1.1
0.3
8.5
3.3
1.0
9.5
6.8
6.6
2.7
0.5
30.6
3.7
0.6
12.2
8.9
3.4
23.9
18.8
16.3
6.0
0
88
98
60
71
89
22
38
47
80
98
CBrCl3
CBrCl3
TEMPO
Ph NH
Ph NH
BHT
BHT
BHT
Quinone
Quinone
1: 1
8: 1
1: 1
1: 1
8: 1
0.5: 1
1: 1
8: 1
1: 1
8: 1
1: 8
1: 1
1: 8
1: 8
1: 1
1: 16
1: 8
1: 1
1: 8
1: 1
2
2.4
2
14.4
12.0
9.7
3.3
0.2
1
1
0
1
0.7
[
[
[
[
a]
Reaction conditions: complex 3 (25 mmol), H O (5.00 mmol), C H (0.63 mmol), MeCN (3 mL), 6 h reaction time.
Moles of product/100 moles of cyclohexane.
2
2
6
12
b]
c]
Cyclohexanolþcyclohexanone.
d]
(
1 – total yield with radical trap/total yield without radical trap)Â100.
lytic cleavage of this bond to form an organocopper in- ligands providing a maximum yield of 7.2% was stud-
[
2,52,53]
[32]
[26–
termediate as proposed for other metals.
Alterna- ied. Other reported catalysts (Figure 11, Table 7)
29,33,34]
tively, a concerted mechanism via a direct insertion of
exhibit overall maximum yields of cyclohexanol
an activated electrophilic oxygen atom, e.g., a bridging and cyclohexanone in the range of 2.0–4.9%, which
oxygen, into an alkane CÀH bond, or through a pairwise are even lower that that (5.4%) found for copper nitrate.
process with CÀH addition to two oxygen atoms, as sug- Thus, most of the previously reported copper-containing
[21]
gested
for the biological alkane hydroxylation by catalytic systems display a considerably low activity and
pMMO, can, in principle, also be involved.
selectivity, require expensive components, inaccessible
on a large scale, and therefore likely will not become in-
dustrially viable.
Comparison of Activities of Various Copper Catalysts
towards Mild Oxidation of Cyclohexane
Our systems with copper triethanolaminate catalysts,
to the best of our knowledge, are the most active ones
so far reported for copper-catalyzed cyclohexane oxida-
The known examples of mild copper-catalyzed oxida- tion under mild conditions.
tion of cyclohexane to the alcohol and ketone are sum-
marized in Table 7 and Figure 11. For comparative pur-
poses, unified yield values were recalculated (whenever Conclusion
required) relative to cyclohexane as moles of products/
100 moles of cyclohexane, and the best quoted yields We have synthesized, by easily accessible self-assembled
and selectivities were considered, when available. The routes, a number of copper(II) complexes with a varia-
highest reported activity, 18%, for a Cu complex differ- ble nuclearity, presenting the polydentate triethanol-
ent from those of this study is displayed by [Cu(salen)] at aminate ligand in the {Cu (H tea) } (n¼1, m¼1; n¼
n
2
m
[
25]
8
08C and using hydrogen peroxide as oxidant, fol- 2, m¼2; n¼3, m¼2) or {Cu (tea) } core, and found
4
4
lowed by [Cu(MeCN) ][BF ] which provides 12% yield that they act as remarkably active and selective catalysts
4
4
at 608C when using tert-butyl hydroperoxide or peroxy- or catalyst precursors for the liquid biphasic peroxida-
[
24]
acetic acid as oxidant. Identical levels of activity, 11.9 tive oxidation, under mild conditions, of cyclohexane
and 11.0%, are shown by [CuCl Pc] and [Cu(tma) ] cat- to cyclohexanol and cyclohexanone.
1
6
2
[31,30]
alysts,
although with a limited selectivity of 64–82
The effects of various parameters have been investi-
and 91%, respectively, due to the formation of various gated and allowed us to achieve, in a single batch, yields
[31]
side products like valeraldehyde, succinic and glutaric and TONs of ca. 39% and 380, respectively, correspond-
[
30]
acids, in significant yield, or of cyclohexene which is ing, as far as we are aware, to the most active copper sys-
further oxidized under the reaction conditions applied tems so far reported for that alkane oxidation reaction.
(
e.g., refluxing with t-BuOOH) to cyclohexen-3-one The catalysts can be reused for recycling and, at least for
and cyclohexen-3-ol. Oxidation of cyclohexane with hy- the tetranuclear one, its activity remains almost un-
drogen peroxide catalyzed by copper(II) complexes changed even after five cycles.
containing N,N-bis(2-pyridylmethyl)-b-alanineamide
Adv. Synth. Catal. 2006, 348, 159 – 174
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
169

FULL PAPERS
Alexander M. Kirillov et al.
Table 7. Comparison of activities of copper catalysts towards mild oxidation of cyclohexane into the cyclohexanol and cyclo-
hexanone mixture.
[a]
Entry Catalyst
Oxidant
Temp. Time Selectivity Max. total Ref.
[
b]
[c]
[
8C]
[h]
[%]
yield [%]
[
d]
1
2
3
4
5
6
7
8
9
0
1
2
3
[Cu(MeCN) ][BF ]
t-BuOOH or CH COOOH
60
25
80
70
70
r.t.
70
20
75
100
r.t.
r.t.
r.t.
5
0.5
5
24
8
–
–
–
12.0
3.7
18.0
11.0
11.9
7.2
4.4
4.3
4.4
4.9
[24]
[28]
[25]
[30]
[31]
[32]
[33]
[27]
[26]
[29]
[34]
this work
this work
4
4
3
[e]
2
[Cu (Py SSPy )](ClO )
H O or t-BuOOH
2
2
2
4
2
2
[
f]
[Cu(salen)]
[Cu(tma)2]
[CuCl Pc]
H O
2
2
[
g]
t-BuOOH
91
64–82
[
h]
O /t-BuOOH
16
2
[
i]
[Cu(bdpg)Cl]Cl
[(CuCl ) (18-crown-6) (H O) ]
H O
2
2
–
2
[
j]
O2
O2
t-BuOOH
24
24
24
12
6
96
96
84
–
2
4
2
2
2
[
j]
Cu(OH)2
Cu-SiO2
1
1
1
1
Cu(NC )Si-MCM-41
H O
2
3
2
[
k]
Cu-Gif system
Cu(NO3)2
H O or O
–
2.0
5.4
38.5
2
2
2
2
2
H O
6
6
90
100
2
Complex 4
H O
2
[
[
[
[
[
[
[
[
[
[
[
a]
The most active copper catalyst from each reference is considered.
Towards formation of cyclohexanol and cyclohexanone.
b]
c]
d]
e]
f]
Moles of products (cyclohexanolþcyclohexanone)/100 moles of cyclohexane.
Here and there selectivity data are not available.
Py SSPy ¼bis{2-[N,N-bis(2-pyridylethyl)amino]-1,1-dimethylethyl} disulfide.
2
2
N,N’-(1,2-ethylene)-bis(salicylaldamine)-copper(II).
tma¼trimethyl acetate.
g]
h]
i]
Pc¼phthalocyanines, used also as encapsulated in zeolites X and Y.
bdpg¼N,N-bis(2-pyridylmethyl)-b-alanineamide.
Active only in the presence of acetaldehyde additive.
Cu-Gif¼Cu-powder/pyridine/acetic acid.
j]
k]
ment of unsaturated dicopper(II) centres in the catalytic
reaction. In this respect, it is noteworthy to mention that
the crystal structure of pMMO (from the methanotroph
[23]
Methylococcus capsulatus) has just been established
and shows that the enzyme is a trimer, each subunit con-
taining one dinuclear copper centre (with two Cu ions in
close proximity) and one mononuclear copper site, apart
from a third metal centre with zinc that can come from
the buffer used to crystallize the protein. The attractive
possibility of the dicopper centre to behave as the active
site of this enzyme for the hydroxylation of alkanes has
[
21–23]
also been considered,
albeit not yet confirmed.
Supporting testimony for a main radical mechanism of
the cyclohexane oxidation by our copper catalysts, in-
volving both C-centred and O-centred radicals has
been provided by experiments with radical traps. Cyclo-
hexyl hydroperoxide is a plausible product of the perox-
Figure 11. Comparison of activities of copper catalysts to-
wards mild oxidation of cyclohexane into the cyclohexanol idative oxidation of cyclohexane (formed, e.g., from re-
and cyclohexanone mixture. For details see Table 7.
action of this alkane with a hydroxyl radical generated
from the reaction of H O with a metal centre), and it
2
2
can decompose to the final cyclohexanol and cyclohex-
[
8,50]
Other advantages of our systems, over most of the oth- anone species.
The amount of that peroxide can be
er reported copper catalysts, include their easy and estimated from the variations in the alcohol and ketone
cheap synthesis and/or the use of easily available and en- yields, determined by GC analyses, upon addition to the
vironmentally tolerable starting materials.
The most active catalysts for the optimized conditions method reported by Shulꢁpin et al.
final reaction solution of PPh or Na S O , according to a
3
2
2
3
[
8,47]
However, in our
(
Figures 6, 7) are the pentacoordinate tri- and tetranu- case, no significant effect (yields remain identical within
clear ones, 3 and 4, suggesting the preferable involve- the standard deviations) was detected by addition of any
170
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 159 – 174

Mild Peroxidative Oxidation of Cyclohexane
FULL PAPERS
of those reagents what suggests that, under the experi-
mental conditions used, the decomposition of cyclohex-
yl hydroperoxide, if formed, to the corresponding alco-
hol and ketone can be promoted by our copper catalysts.
Such a type of decomposition reaction has been report-
Synthesis of 2b·2 H
2
O: To an aqueous solution (10 mL) of
Cu(NO ) ·2.5 H O (232 mg, 1 mmol) in HNO (1 mmol) was
3
2
2
3
added dropwise triethanolamine (0.13 mL, 1 mmol) and then
an aqueous solution (3 mL) of NaOH (120 mg, 3 mmol) with
continuous stirring at room temperature. p-Toluic acid
136 mg, 1 mmol) was dissolved in an aqueous solution
1 mL) of NaOH (40 mg, 1 mmol) and added to the resulting
(
(
[
54,55]
ed to be catalyzed by some copper
tion metal (V, Cr, Mo, Mn, Fe, Co, etc.)
and other transi-
[
56–58]
compounds.
deep blue solution. The reaction mixture was stirred overnight,
filtered off in case a slight amount of precipitate formed, and
Attempts to isolate and fully characterize active inter-
mediates have so far been unsuccessful, but will be pur- then left to evaporate in a beaker at ambient temperature.
sued.
Blue crystals of 2b were formed in a few days, then collected
and dried in air; yield: 86% (based on copper nitrate). Product
2
b is soluble in H O (slightly), MeOH, EtOH, Me CO, MeCN,
2
2
CH Cl and THF; mp 1608C (dec.), 110–1258C (dehydration);
2
2
Experimental Section
anal. calcd. for C H Cu N O (MW 729.8): C 46.08, H 6.35, N
2
8
46
2
2
12
þ
3
2
.84; found: C 46.07, H 6.33, N 3.85; FAB -MS: m/z¼692 [MÀ
þ
þ
H Oþ2H] , 546 [MÀ2 H OÀ{tea}] , 423 [Cu {H tea} þ
General Materials and Procedures
2
2
2
2
2
þ
þ
þ
H] , 275 [Cu {H tea}þH] , 211 [Cu{H tea}] ; FTIR (selected
2
2
2
Cu(NO ) ·2.5 H O (Fluka), triethanolamine (H tea) (Fluka),
À1
3
2
2
3
bands, cm ): 3434 s br and 3202 s br n(OH)þn(H O), 2976 s
2
sodium hydroxide (Fluka), nitric acid (Fluka), sodium azide
Merck), benzoic acid (Merck), p-toluic acid (Aldrich), 3-
and 2948 m n (CH), 2904 s and 2871 s n (CH), 1590 s and
as
s
(
1
546 s n (COO), 1404 s n (COO), 1092 s n(CÀO).
as
s
chlorobenzoic acid (Fluka), p-hydroxybenzoic acid (Aldrich),
terephthalic acid (Fluka), sodium tetrafluoroborate (Aldrich),
cyclohexane (Aldrich), acetonitrile (Lab-Scan), hydrogen per-
Synthesis of 2c·2 H O: Complex 2c was obtained as light-
2
blue X-ray quality crystals by the same procedure as for 2b ex-
cept that benzoic acid was replaced by 3-chlorobenzoic acid
(157 mg, 1 mmol); yield: 79% (based on copper nitrate).
oxide (30% in H O) (Fluka), diethyl ether (Lab-Scan), cyclo-
2
heptanone (Aldrich), cyclohexanol (Aldrich), cyclohexanone
Product 2c is soluble in H O (slightly), MeOH, EtOH,
2
(
2
Aldrich), 2,6-di-tert-butyl-4-methylphenol (BHT) (Aldrich),
,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) (Aldrich), di-
Me CO, MeCN, CH Cl and THF; mp 1548C; anal. calcd. for
2
2
2
C H Cl Cu N O (MW 770.6): C 40.52, H 5.23, N 3.64; found:
2
6
40
2
2
2
12
phenylamine (Fluka), bromotrichloromethane (Fluka) and
quinone (Fluka) were obtained from commercial sources and
used as received. Complexes 2a, 3, 4 and 5 were prepared ac-
þ
C 40.14, H 5.24, N 3.62; FAB -MS: m/z¼739 [MÀ2 H Oþ
2
þ
þ
5
{
[
H] , 443 [MÀ2 H O À 2 (C H ClCOO)ÀH] , 421 [Cu -
2
6
4
2
þ
þ
H tea} ÀH] , 369 [Cu{H tea}(C H ClCOO)þ3H ] , 274
2
2
2
6
þ
4
[35]
cording to the procedures described earlier.
þ
Cu {H tea] , 212 [Cu{H tea}þH] ; FTIR (selected bands,
2
2
2
C, H and N elemental analyses were carried out by the Mi-
croanalytical Service of the Instituto Superior T e´ cnico. Melting
points were determined on a Kçfler table. Positive-ion FAB
mass spectra were obtained on a Trio 2000 instrument by bom-
barding 3-nitrobenzyl alcohol (NBA) matrices of the samples
À1
cm ): 3420 s br and 3173 s br n(OH)þn(H O), 2976 s n
2
as
(
1
CH), 2912 s and 2873 s n (CH), 1588 s and 1550 s n (COO),
s
as
385 s n (COO), 1092 s n(CÀO).
s
1
5
with 8 keV (ca. 1.18Â10 J) Xe atoms. Mass calibration for
data system acquisition was achieved using CsI. Infrared spec-
À1
tra (4000–400 cm ) were recorded on a Jasco FT/IR-430 in-
X-Ray Crystal Structure Determinations
strument in KBr pellets.
X-ray data were collected on a Nonius-Kappa CCD diffrac-
tometer using Mo-Ka radiation (l¼0.71073 Š) and the Col-
[59]
[60]
lect data collection program. The Denzo-Scalepack or
Synthesis and Characterization of New Copper
Complexes
[61]
EvalCCD program packages were used for cell refinements
and data reduction. The structures were solved by direct meth-
[62]
Synthesis of 1: To an aqueous solution (10 mL) of Cu(NO ) ·
ods using the SHELXL-97 program. An empirical absorp-
tion correction based on equivalent reflections was ap-
3
2
2
.5 H O (232 mg, 1 mmol) in HNO (1 mmol) (the acid was
2
3
[
63,64]
added to avoid a spontaneous hydrolysis of the metal salt)
were added dropwise triethanolamine (0.13 mL, 1 mmol) and
an aqueous solution (6 mL) of NaOH (120 mg, 3 mmol) with
continuous stirring at room temperature. To the resulting
deep-blue purple solution an excess of sodium azide (130 mg,
plied.[
Structures were refined with the SHELXL-97 pro-
62]
[65]
gram and the WinGX graphical user interface. H O and
2
OH hydrogens were located from the difference Fourier map
but were not refined. Other hydrogens were placed in idealized
positions and constrained to ride on their parent atom. The
crystal data and structure refinement details are summarized
in Table 8. The molecular structures of complexes 1 and 2c in-
cluding selected bond lengths and angles as well as hydrogen
bonding networks are shown in Figures 1–4.
2
mmol) was added. The system was stirred overnight and
then left to concentrate in a beaker at ambient temperature.
Green X-ray quality crystals of 1 were formed in a few days,
then collected and dried in air; yield: 80% (based on copper ni-
trate). Complex 1 is slightly soluble in H O, MeOH, EtOH,
Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with
the Cambridge Crystallographic Data Centre as supplementa-
ry publication no. CCDC-271523 (1) and CCDC-271524 (2c).
Copies of the data can be obtained free of charge on applica-
tion to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
2
MeCN and DMSO; anal. calcd. for C H CuN O (MW
6
14
4
3
2
2
53.7): C 28.40, H 5.56, N 22.08; found: C 28.41, H 5.82, N
1.72; FTIR (selected bands, cm ): 3480 m br and 3308 m
À1
n(OH), 2985 w n (CH), 2909 m and 2859 m n (CH), 2048 s
as
s
n (N ), 1384 s n (N ).
as
3
s
3
Adv. Synth. Catal. 2006, 348, 159 – 174
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
171

FULL PAPERS
Alexander M. Kirillov et al.
Table 8. Crystal data and structure refinement details for compounds 1 and 2c·2 H O.
2
1
2c·2 H O
2
Empirical formula
Formula weight
Crystal dimensions (mm )
Temp (K)
C H CuN O
C H Cl Cu N O
6
14
4
3
26 40
2
2
2
12
253.75
770.58
3
0.56Â0.33Â0.30
0.33Â0.11Â0.06
120(2)
100(2)
Wavelength (Š)
Crystal system, Space group
Colour
0.71073
Monoclinic, P2 /n
Green
0.71073
Triclinic, P1
¯
1
Light blue
a¼7.4754(2),
b¼8.0440(3),
c¼14.4204(6)
a¼97.420(2),
b¼97.481(2),
g¼112.014(2)
781.99(5)
Unit cell dimensions (Š; deg.)
a¼8.5995(2),
b¼7.94300(10),
c¼14.4653(7)
a¼90,
b¼107.090(2),
g¼90
3
Volume (Š )
944.44(5)
3
Z, calcd density (Mg/m )
4, 1.785
2.301
524
1, 1.636
1.595
398
À1
Abs coeff (mm
F(000)
)
Max/min transmission factors
q range for data collection (deg.)
No. of collected reflns
0.28815/0.23546
3.57–27.48
8646
0.28352/0.22765
3.00–27.45
12027
No. of unique reflns
No. of data/restraints/parameters
1999 [R(int)¼0.0190]
1999/0/127
1.061
0.0211, 0.0545
0.0220, 0.0550
0.354, À0.390
3529 [R(int)¼0.0373]
3529/0/199
2
Goodness-of-fit on F
1.044
[a]
[b]
Final R1, wR2 indices [I > 2s(I)]
0.0313, 0.0709
0.0429, 0.0757
0.730, À0.623
R1, wR2 indices (all data)
Largest diff. peak and hole (eŠ
À3
)
[
[
a]
b]
R1¼Sj jF j – jF j j/SjF j.
o
c
o
2
o
2
c
2
2
o
2
1/2
wR2¼[S[w(F – F ) ]/S[w(F ) ]] .
[
Fax: int. codeþ44(1223)336–033; E-mail: deposit@ccdc.cam. Acknowledgements
ac.uk].
This work has been partially supported by the FundaÅ a˜ o para a
Ci eˆ ncia e a Tecnologia (FCT), Portugal, its POCTI programme
(
FEDER funded) (project POCTI/QUI/43415/2001), and by a
Catalytic Activity Studies
Human Resources and Mobility Marie-Curie Research Train-
ing Network (AQUACHEM project, CMTN-CT-2003-
503864). A. M. K. and M. V. K. are grateful to the FCT and
the POCTI programme for fellowships (BD/6287/01 and BD/
12811/03). The authors thank Dr. Maria C aˆ ndida Vaz (IST)
for the direction of the elemental analysis service and Mr. Inda-
l e´ cio Marques (IST) for running the FAB-MS and GC-MS spec-
tra.
The reaction mixtures were prepared as follows: to 1.56–
25.00 mmol of metal complex catalyst (used either as a solid
or as a 0.02 M MeCN solution) contained in the reaction flask
were added 1.00–10.00 mL MeCN, 1.25–10.00 mmol H O
2
2
(
1
30% in H O), HNO (acid-to-catalyst molar ratio¼10:1 for
2
3
, 3 and 4; or 100:1 for 2 and 5), and 0.63–10.00 mmol C H ,
6 12
in this order. The reaction mixture was stirred for 6–72 h at
room temperature and normal pressure, then 90 mL of cyclo-
heptanone (as internal standard) and 10.00 mL diethyl ether
(
to extract the substrate and the products from the reaction
mixture) were added. The obtained mixture was stirred during
0 min and then a sample was taken from the organic phase and
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asc.wiley-vch.de
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 159 – 174

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FULL PAPERS
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