Amavadin and other vanadium complexes as remarkably efficient catalysts for one-pot conversion of ethane to propionic and acetic acids

Article abstract of DOI:10.1002/chem.200700980

Synthetic amavadin Ca[V{ON[CH(CH₃)COO]₂}₂] and its models Ca[V{ON(CH₂COO)₂}₂] and [VO{N(CH₂CH₂O}₃)], in the presence of K ₂S₂O₈ in trifluoroacetic acid (TFA), exhibit remarkable catalytic activity for the one-pot carboxylation of ethane to propionic and acetic acids with the former as the main product (overall yields up to 93%, catalyst turnover numbers (TONs) up to 2.0×10⁴). The simpler V complexes [VO(CF₃SO₃)₂], [VO(acac)₂] and VOSO₄ are less active. The effects of various factors, namely, C₂H₆ and CO pressures, time, temperature, and amounts of catalyst, TFA and K₂S₂O ₈, have been investigated, and this allowed optimisation of the process and control of selectivity. ¹³C-labelling experiments indicated that the formation of acetic acid follows two pathways, the dominant one via oxidation of ethane with preservation of the C-C bond, and the other via rupture of this bond and carbonylation of the methyl group by CO; the C-C bond is retained in the formation of propionic acid upon carbonylation of ethane. The reactions proceed via both C- and Ocentred radicals, as shown by experiments with radical traps. On the basis of detailed DFT calculations, plausible reaction mechanisms are discussed. The carboxylation of ethane in the presence of CO follows the sequential formation of C₂H₅ ^., C₂H₅CO^., C₂H ₅COO^. and C₂H₅COOH. The C ₂H₅COO^. radical is easily formed on reaction of C₂H₅CO^. with a peroxo V catalyst via a V{η-OOC(O)C₂H₅) intermediate. In the absence of CO, carboxylation proceeds by reaction of C₂H₅^. with TFA. For the oxidation of ethane to acetic acid, either with preservation or cleavage of the C-C bond, metal-assisted and purely organic pathways are also proposed and discussed.

Full text of DOI:10.1002/chem.200700980

DOI: 10.1002/chem.200700980
Amavadin and Other Vanadium ComplexesasRemarkably Efficient
Catalysts for One-Pot Conversion of Ethane to Propionic and Acetic Acids
[
a]
[a]
[a]
Marina V. Kirillova, Maxim L. Kuznetsov, JosØ A. L. da Silva,
[a]
[
a, b]
Maria Fµtima C. Guedesda Silva,
Armando J. L. Pombeiro*
Jo¼o J. R. Frafflsto da Silva, and
[
a]
1
3
Abstract:
Ca[V{ON[CH
Synthetic
amavadin
and control of selectivity. C-labelling
experiments indicated that the forma-
tion of acetic acid follows two path-
ways, the dominant one via oxidation
of ethane with preservation of the CꢀC
ments with radical traps. On the basis
of detailed DFT calculations, plausible
reaction mechanisms are discussed.
The carboxylation of ethane in the
presence of CO follows the sequential
ACHTREUNG
(CH )COO] } ] and its
3
2 2
models Ca[V{ON
VO{N(CH CH O) }], in the presence
of K S O in trifluoroacetic acid (TFA),
A
C
H
T
R
E
U
N
G
(CH COO) } ] and
2 2 2
[
ACHTREUNG
2
2
3
2
2
8
exhibit remarkable catalytic activity for
the one-pot carboxylation of ethane to
propionic and acetic acids with the
former as the main product (overall
yields up to 93%, catalyst turnover
bond, and the other via rupture of this
bond and carbonylation of the methyl
group by CO; the CꢀC bond is re-
formation
of
C H C,
C H COC,
2
5
2 5
C H COOC and C H COOH. The
2
5
2
5
C H COOC radical is easily formed on
2
5
tained in the formation of propionic
acid upon carbonylation of ethane. The
reactions proceed via both C- and O-
centred radicals, as shown by experi-
reaction of C H COC with a peroxo V
2 5
1
catalyst via a V{h -OOC(O)C H } in-
A
H
R
U
G
2 5
4
numbers (TONs) up to 2.0”10 ). The
simpler V complexes [VO
termediate. In the absence of CO, car-
boxylation proceeds by reaction of
C H C with TFA. For the oxidation of
A
H
R
U
G
[
VO(acac) ] and VOSO are less active.
ACHTREUNG
2
4
2
5
The effects of various factors, namely,
C H and CO pressures, time, tempera-
ethane to acetic acid, either with pres-
Keywords: carboxylation · density
functional calculations · homogene-
ous catalysis · reaction mechanisms ·
vanadium
ervation or cleavage of the CꢀC bond,
2
6
ture, and amounts of catalyst, TFA and
K S O , have been investigated, and
metal-assisted and purely organic path-
ways are also proposed and discussed.
2
2
8
this allowed optimisation of the process
[
3]
Introduction
products, for example, acetic acid, but undeservedly less
[4]
attention has been paid to the carboxylation of ethane, the
least reactive hydrocarbon (after methane) and the second
most abundant alkane (in natural gas). Direct transforma-
tion of ethane into the synthetically (and commercially) im-
portant propionic and acetic acids could become an attrac-
tive way of functionalising alkanes. The current methods for
preparing C H COOH, such as carbonylation of ethylene,
The search for expedient processes for utilising light alkanes
in synthesis is a current challenge to chemistry. Considerable
[1,2]
efforts have been directed
towards the functionalization,
[5]
under mild conditions, of methane to give more valuable
2
5
[5a]
[
a] Dr. M. V. Kirillova, Dr. M. L. Kuznetsov, Dr. J. A. L. da Silva,
Dr. M. F. C. Guedes da Silva, Prof. Dr. J. J. R. Frafflsto da Silva,
Prof. Dr. A. J. L. Pombeiro
Centro de Química Estrutural, Complexo I
Instituto Superior TØcnico, TU Lisbon
Av. Rovisco Pais, 1049-001, Lisbon (Portugal)
Fax : (+351)2184-6445-5
oxidation of propanal and direct oxidation of naphtha,
normally exhibit low selectivities and require harsh condi-
tions, multistage processes, difficult work-up of the product
mixture and, in some cases, considerable costs for reagents.
The preparation of acetic acid by the Monsanto or Cativa
processes also has some limitations, for example, the re-
quirement for three separate stages (high-temperature
steam reforming, further conversion of the synthesis gas to
methanol and carbonylation of methanol with Rh or Ir cata-
lysts).
E-mail: pombeiro@ist.utl.pt
[
b] Dr. M. F. C. Guedes da Silva
Universidade Lusófona de Humanidades e Tecnologias
Av. do Campo Grande, 376, 1749-024, Lisbon (Portugal)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
1828
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 1828 – 1842

FULL PAPER
Homogeneous catalyst systems for the conversion of
ethane to propionic and acetic acids have been reported
only scantly. They exhibit low selectivities and yields, and
the mechanisms of the reactions have not been established.
One of the first examples of ethane carbonylation in aque-
another gaseous alkane, that is, ethane. A fourth objective is
to broaden the application of V compounds in catalysis,
which is still an underdeveloped field of research.
[10]
We now report highly effective carboxylation of ethane to
propionic and acetic acids in the presence of K S O and in
2
2
8
[4b]
ous medium uses K S O at 105–1158C to give acetic and
TFA, catalysed by amavadin and its model or other V com-
plexes with N,O or O,O ligands, with overall yields (based
on ethane) of carboxylic acids of up to 93% and TONs of
2
2
8
propionic acids (the latter only in the presence of CO) with
a maximum TON of 0.4 relative to K S O . The oxidative
2
2
8
4
functionalization of ethane by perfluorocarboxylic acid an-
up to 2.0”10 (the yield based on K S O reaches 64%).
2
2
8
[4c]
hydrides to propionic acid and its mixed anhydride exhibit
a low propionic acid yield (maximum of ca. 1% based on
the alkane) and forms various by-products.
This is, to our knowledge, the most efficient functionaliza-
tion system for a gaseous alkane, under mild conditions, so
far reported. Possible mechanisms are suggested on the
basis of some experiments, for example, with radical traps
The reaction of ethane with CO in trifluoroacetic acid
1
3
(
TFA)/K S O has been studied by Fujiwara et al., who pio-
and C-labelled reagents, and DFT calculations. These in-
clude the analysis of the most plausible pathways for the
radical-formation and carboxylation processes, the study of
the role of TFA in the carboxylation in the absence of CO,
the investigation of both metal-assisted and purely organic
routes for the oxidation of ethane to acetic acid with and
without CꢀC bond cleavage and the roles of catalyst, oxi-
2
2
8
[1f,i]
neered the use of this system in alkane functionalization,
using Pd and/or Cu acetates as catalysts. The maximum
yield of propionic acid was about 18% (based on ethane)
II
II
[4d]
and a TON (moles of product per mole of metal catalyst) of
II
1
1 was achieved (Co acetate led to only 0.4% yield with a
[4f]
V
TON of 0.3). Recently, some of us found that a Re oxo
complex containing an N,O-picolinate ligand, that is,
dant and solvent.
[
ReOCl
A
C
H
T
R
E
U
N
G
(C H NCOO)
A
H
R
U
G
In contrast to the extensive theoretical studies that have
been undertaken to elucidate the mechanisms of various
2
5
4
3
[4g]
propionic and acetic acids of 29% (TON of 45), whereas
Re complexes with pyrazole or tris(pyrazolyl)methane li-
gands catalyse the oxidation of ethane mainly to acetic acid
with a maximum yield of about 40% (carboxylation to pro-
pionic acid proceeds to a lesser extent, with ca. 8%
[11]
[12]
alkane reactions (hydroxylation, halogenation, dehydro-
[
13]
[14]
genation, CH oxidation in sulfuric acid and on an Fe-
4
[15]
[16]
[17]
exchanged zeolite,
Ti, lanthanides, and Rh complexes, by unsaturated Al
CꢀH bond activation by Re,
Pt,
[18]
[19]
[20]
[4h]
[21]
+[22]
[23]
yield).
Systems based on V-containing polyphosphomo-
ions,
Sc
or metal oxides,
etc.), only a few papers
[6a]
lybdates/H O , chromic acid/H O (or tBuOOH),
N,N’-dihydroxypyromellitimide/O
ated in situ) are also known to catalyse homogeneously
the oxidation of ethane to acetic acid, although with low
activities. Some heterogeneous systems have also been
studied, for example, mixed metal oxides such as
propose plausible mechanisms of alkane carboxy-
2
2
[3a,b,d,g,4b,c,d,24]
lation,
and only two, to our knowledge, report
[4a]
detailed theoretical studies, one of them for a quite different
[25]
system from ours, and the other one (studied by some of
[7c]
us) concerning methane.
Mo V0.25Nb0.12Pd0.0005O , at high temperature.
1
x
Other more effective catalyst systems for the conversion
of ethane to propionic and acetic acids should be searched
for, and the still-unknown mechanisms elucidated, and these
are two main aims of the current study. Vanadium com-
pounds appear promising since they are convenient catalyst
precursors for various reactions of alkanes and aromatic
Results and Discussion
Catalysis studies: The following vanadium compounds were
used as catalyst precursors: Ca[V{ON(CH COO) } ] (1; ON-
(CH COO) =basic form of 2,2’-(hydroxyimino)diacetic acid
AHCTREUNG
2
2 2
AHCTREUNG
2
2
3
ꢀ
(HIDA) ; amavadin model), Ca[V{ON[CH
(2; ON(CH(CH )COO) =basic form of 2,2’-(hydroxyimi-
no)dipropionic acid (HIDPA)
A
H
R
U
G
[3e,6a,e–j,7]
compounds,
amanita fungi, and its model (see below). They catalyse
) peroxidative halogenation, hydroxylation and oxygena-
tion of alkanes to the corresponding organic halides, alco-
in particular amavadin, present in some
ACHTREUNG
3
2
3ꢀ
[26]
;
synthetic amavadin)
1
[VO{N
(4),
A
H
E
N
(CH CH O) }] (3; Scheme 1), [VO
A
H
R
U
G
2
2
3
3
3
2
2
[VO
A
H
R
U
G
(5;
acac=acetylacetonate)
and
2
[
6e]
[7a,c]
hols and ketones, 2) carboxylation of methane,
prop-
VOSO ·5H O (6). Each of them, in the presence of K S O
4 2 2 2 8
[
7d]
ane
nes,
and of C and C alka-
5 6
3) the oxidation of cer-
[
7b]
[8]
tain thiols and 4) decomposi-
[
9]
tion of H O .
Use of
2
2
amavadin for alkane functional-
ization has thus been limited to
liquid alkanes, methane and
propane (carboxylation), and a
third main aim of the current
work is to extend its catalytic
activity to functionalization of Scheme 1.
Chem. Eur. J. 2008, 14, 1828 – 1842
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1829

A. J. L. Pombeiro et al.
and in TFA, exhibits high catalytic activity for carboxylation
of ethane with carbon monoxide to give propionic acid as
the main product (Scheme 2, Table 1 and Table S1 in the
The carboxylation of ethane to propionic acid proceeds
even in the absence of CO (albeit in low yields of ca. 3%,
Figure 2), whereby the carbonyl group is then derived from
[3b,7]
TFA, as observed
for methane carboxylation. The reac-
tion is promoted by increasing p(CO) (in accord with its
role, when present, as a carbonylating agent), up to about
2
0–30 atm, at which propionic acid yields are about 73–78%
Scheme 2.
(Figure 2, and Figure S1 in the Supporting Information).
The maximum yield of propionic acid corresponds to the
minimum yield of acetic acid, that is, there is competition
between the two processes, which is also consistent with the
predominant oxidation of ethane to acetic acid in the ab-
sence of CO.
Supporting Information). Acetic acid is also formed as a
result of partial oxidation of ethane by peroxodisulfate.
Blank tests were performed at different CO pressures and
indicated no formation of propionic acid unless the vanadi-
um catalyst was added.
Decreasing the amount of catalyst leads to an increase in
TON, albeit with a decrease in product yield; for example,
4
The most active systems are provided by amavadin (2)
and its models 1 and 3 which can lead to overall yields of
for amavadin and its model 1, TONs of 1.53”10 and 2.0”
4
3
10 are reached for an C H /catalyst molar ratio of 78”10
2
6
carboxylic acids of up to 93% and/or TONs of up to 2.0”
(Figure 3, and Table S2 in the Supporting Information).
Such high TONs, which still correspond to considerable acid
yields (ca. 26–20%), are quite remarkable in the field of
alkane functionalization under mild conditions and, to our
knowledge, have not been previously reached.
4
10 (Table 1 and Tables S1–S4 in the Supporting Informa-
tion). Under the same conditions, the simpler V complexes
–6 and oxides such as V O and V O are less active but
4
2
5
2
4
still provided yields of about 40–75% (Table 1, entries 9–
1). The impact of various factors on the activity of 1–3 was
1
Trifluoroacetic acid has been used as solvent in various
[
1f,i,3b–d,g,4a,d–g]
investigated in detail (see the Supporting Information) with
the aim of optimising conditions and gaining mechanistic in-
formation.
At low ethane pressure (up to 3 atm) oxidation to acetic
acid proceeds more effectively than carboxylation to pro-
pionic acid (Figure 1a), but the latter reaction becomes
dominant at higher pressures, and the best selectivity to-
wards propionic acid is achieved at about 10 atm at a reac-
tion time of about 20 h (Figure 1b), and at a temperature of
alkane functionalization reactions,
plete role still remains unclear. It can act as a carbonylating
but its com-
[3b,4c,7]
[3b]
agent
(with possible formation of CHF3 ) and is also
a relatively strong acid [K =0.59 (pK =0.2)].
[27b]
Its re-
a
a
placement by another strong acid such as trichloro- or tri-
bromoacetic acid, sulfuric or nitric acid, or by acetonitrile or
a fluorinated solvent like trifluorotoluene or 1,1,2-trichloro-
trifluoroethane results in suppression of the catalytic activi-
ty. An optimum amount of TFA (see Table S3 in the Sup-
porting Information) leads to the highest overall yield of
92.5%, as a balance between the solubilising and carbony-
lating roles of TFA on the one hand, and the decreased con-
centrations of reacting species on the other.
808C (Figure 1c), above which a considerable drop in yield
occurs, possibly due to further oxidation of the acid and
thermal decomposition of the oxidant K S O , which occurs
2
2
8
[27a]
at about 1008C.
[
a]
Table 1. Conversion of ethane into propionic and acetic acids (selected results).
[
c]
Yield [%]
[b]
[b]
[d]
Entry
Catalyst
n
A
H
R
N
(C
2
H
6
)/n
A
H
E
N
(catalyst)
p
A
H
R
U
G
2
H
6
) [atm]
p(CO) [atm]
Propionic
acid
Acetic
acid
Total
TON
1
2
3
1
1
1
1
2
2
3
3
4
5
6
306
153
78.3”10
53
153
78.3”10
153
77.7”10
153
153
5
15
25
25
20
30
30
22
30
22
25
25
47.0
74.5
22.7
63.5
78.4
17.0
45.5
18.1
68.6
59.6
64.2
25.8
4.6
2.9
29.0
4.8
2.5
6.5
2.2
6.6
7.5
5.2
72.8
79.1
25.6
92.5
83.2
19.5
52.0
20.3
75.2
67.1
69.4
55.7
121
20.0”10
46.0
127
15.3”10
79.6
15.8”10
115
103
10
10
5
10
10
10
10
10
10
10
3
3
[
e]
4
5
6
7
8
9
0
1
3
3
3
3
1
1
153
106
[
a] Reaction conditions (unless stated otherwise): metal-complex catalyst (0.04–20.00 mmol), K
2
S
2
O
8
3
(4.00 mmol), CF COOH (5.0 mL), 808C, 20 h, in an
ꢀ
1
autoclave (13 mL capacity). [b] Measured at 258C (amounts of C
2
H
6
or CO gases correspond to 0.306 mmolatm , except for run 4, for which amounts
of those gases correspond to 0.212 mmolatm ). [c] Molar yield [%] based on C , that is, moles of products per 100 moles of ethane (determined by
GC or GC-MS); molar yields [%] based on K , if required, can be estimated (except for runs 1 and 4) as total yield”0.765, for example, 64% for
run 5; for runs 1 and 4, molar yields [%] based on K are 28 and 23%, respectively. [d] Turnover number (moles of acetic and propionic acids per
mole of metal catalyst precursor) after 20 h. [e] CF COOH (7.3 mL).
ꢀ
1
2
H
6
2 2 8
S O
2 2 8
S O
3
1830
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 1828 – 1842

Vanadium Catalysts for Carboxylation of Ethane
FULL PAPER
Figure 3. Effect of the amount of catalyst on the total yield of carboxylic
acids and TON of catalyst in ethane carboxylation catalyzed by amavadin
(
A
H
R
U
G
2 6
2). p(C H )=10 atm; p(CO)=30 atm; 808C; 20 h. For full reaction con-
ditions see Table S2 in the Supporting Information.
K S O is essential, and the yield of propionic acid in-
2
2
8
creases with its amount up to a maximum, beyond which a
pronounced decrease occurs (see Table S4 and Figure S2 in
the Supporting Information), possibly due to the increasing
amount of solid (the oxidant has limited solubility) with re-
sulting poor stirring. The yields of carboxylic acids based on
K S O are usually rather high, in many cases exceed 50%
2
2
8
(
Table 1, and Tables S1–S4 in the Supporting Information)
and reach 64% under appropriate reaction conditions.
Mechanistic discussion: To clearly establish the origin of the
alkyl and carbonyl groups in the propionic and acetic acids,
2 6
Figure 1. Effects of a) C H pressure, b) reaction time and c) temperature
on the yields of acetic (curve A) and propionic (curve B) acids in ethane
carboxylation catalysed by amavadin 2 (b, c) or its model 1 (a). p(CO)=
1
3
13
13
C-labelling experiments were performed with CH3 CH3
13
13
1
and CO, and the products identified by C{ H} NMR spec-
2
5 (a, b) or 30 atm (c); p
A
H
R
U
G
2
H
6
)=10 atm (b, c); 808C (a, b); 20 h (a, c).
13
13
troscopy. Thus, from the reaction of CH3 CH and CO,
3
Point numbers correspond to entries in Table S1 in the Supporting Infor-
mation.
catalysed by complex 1, the main products (Scheme 3a) are
1
3
13
13
13
CH3 CH COOH (d, J( C, C)=34 Hz, at d=8.5 and
A
H
R
U
G
3
1
3
13
13
13
2
8.3 ppm) and CH3 COOH (d, J
A
H
R
U
G
2
0.4 and 182.8 ppm), that is, the ethyl group of propionic
acid and both the methyl and carbonyl moieties of acetic
acid are derived from ethane, which thus undergoes carbox-
ylation to propionic acid and oxidation to acetic acid, both
with preservation of the ethane CꢀC bond. However, a
smaller amount (ca. 6% of the total amount of acetic acid)
1
3
of singly labelled CH COOH (s, d=20.4 ppm) was also de-
3
tected, that is CꢀC cleavage of ethane also occurred as a
competitive process, and the resulting methyl group under-
went carboxylation.
These results were confirmed by performing the reaction
1
3
of CH CH with CO (Scheme 3b), which leads not only to
3
3
Figure 2. Effect of CO pressure on the yields of acetic (curve A) and pro-
pionic (curve B) acids in ethane carboxylation catalysed by amavadin
13
CH CH COOH but also to the singly labelled acetic acid
CH3 COOH (the CH3 COOH/propionic acid molar ratio is
3
2
1
3
13
model 1. p
0 h. Point numbers correspond to entries in Table S1 in the Supporting
Information.
A
C
H
T
R
E
U
N
G
2 6
(C H )=5 atm (open points), 10 atm (filled points); 808C;
ca. 0.06) in accord with the occurrence, to some extent, of
ethane CꢀC bond rupture. At the low CO pressure (2 atm)
2
1
3
Chem. Eur. J. 2008, 14, 1828 – 1842
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1831

A. J. L. Pombeiro et al.
the presence of K S O , the or-
2
2
8
ganic
halides
CH CH X,
3
2
CH CHX and CH CX3 (X=
3
2
3
Br, Cl) are formed. The in-
volvement of K S O as an oxi-
dant is suggested by the isola-
tion from the reaction solution
2
2
8
Scheme 3.
in high yield of KHSO as a col-
4
used, unlabelled CH COOH is formed in relatively large
ourless crystalline material (identified by IR, elemental and
single-crystal X-ray diffraction analyses, the last-named of
3
amounts (see also Figure 2; the CH COOH/propionic acid
3
[29]
molar ratio is 1.2) due to oxidation of ethane, whereas car-
bonylation by TFA to afford CH CH COOH is also ob-
which was in accord with that already reported ). This salt,
however, is also formed upon refluxing TFA/K S O .
3
2
2
2
8
served, although only to
a
slight extent (the
Nevertheless, no carboxylic acid is detected in the absence
of the V catalyst. We have not yet succeeded in isolating
and fully characterizing any V intermediate, but amavadin
1
3
CH CH COOH/CH CH COOH molar ratio is ca. 0.08).
3
2
3
2
Although under typical reaction conditions very high se-
lectivity towards propionic and acetic acids is observed, at
lower gas pressures (e.g., 2 atm) the ethyl and methyl esters
of trifluoroacetic acid are also detected as reaction products,
although in much lower yields than the acids. Ethane was
IV
and its model (V complexes 2 and 1, respectively) are oxi-
[
6e,9]
V
dised
by H O to the corresponding V compounds,
2 2
V
which are related to bis-peroxo V complexes, since the de-
2
protonated hydroxyimine(1ꢀ) group h -ON of the ligands
1
3
13
confirmed as their carbon source by using CH3 CH ,which
(Scheme 1) is comparable to peroxide(2ꢀ). Moreover, V
3
13
13
2
13
13
affords CF COO CH CH3 (d, J
A
H
R
U
G
species can readily undergo peroxidation to give peroxo or
3
1
3
[10,30]
1
3.5 and 67.3 ppm) and CF COO CH (s, d=55.9 ppm),
hydroperoxo complexes
have been proposed
and compounds of these types
as active species in peroxidations of
3
3
[6a,g]
again proving a considerable extent of alkane CꢀC cleavage.
In the absence of CO, the main products are acetic acid and
the ester CF COOCH CH , which do not involve carbonyla-
hydrocarbons. Hence, the involvement, in our system, of a
V
peroxo V complex such as [V(OO){N(CH CH O) }], the
A
H
R
U
G
3
2
3
2 2 3
tion of ethane, although propionic acid is also detected on
account of a carbonylating role of TFA.
peroxo analogue of oxo complex 3, or a related species is a
reasonable assumption.
Radical formation: The reactions leading to the carboxylic
acids are believed to proceed mainly via both carbon- and
oxygen-centred radicals, since the yields of these products
sharply drop (especially that of propionic acid, formation of
which can be fully suppressed; see Table S5 in the Support-
On the basis of the above, one can propose that the
ethane reactions are initiated by hydrogen-atom abstraction
ꢀ
from RH by the sulfate anion radical SO C (or its protonat-
4
[
7c,28c]
ed form HSO C)
formed upon thermolytic decomposi-
4
ꢀ
2
ꢀ
tion of S O
(or the protonated form HS O in acidic
2
8
2
8
[28a–c]
[3c,28d]
ing Information) in the presence of a C-radical trap,
like 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) or
TFA medium).
Quantum chemical calculations indicate
ꢀ
that exothermic interactions of SO C , and particularly
4
[28a–c]
CBrCl , or an O-radical trap
such as Ph NH. Butylated
HSO C radicals, with C H occur with a low activation barrier
3
2
4
2
6
°
s
hydroxytoluene (BHT), O , benzoyl peroxide and PbEt , the
(the activation enthalpy in solution DH is 7.61 and
0.12 kcalmol , respectively; see reactions 2 in Table 2 and
2
4
[4c]
ꢀ1
latter group known to be possible radical initiators, also
strongly inhibit propionic acid formation, but oxidation of
ethane to acetic acid can be significantly promoted by such
species.
Scheme 5; the complete version of Table 2 is given in the
Supporting Information as Table S6). Selected transition
states (TS) are presented in Figure 4, whereas a wider view
is given in Figure S4 in the Supporting Information.
The formation of ethyl radicals has been proved by the
1
detection, by H (see Figure S3 in the Supporting Informa-
In contrast, direct H abstraction from ethane (homolytic
CꢀH bond cleavage to yield the ethyl radical) by an oxo or
13
tion) and
C NMR spectroscopy, of CH CH Br and
3 2
CH CHBr as the main products (ca. 11 and 3% yield based
a peroxo vanadium complex, for example, [V(OO){N-
3
2
+
on ethane; Scheme 4) when the reaction is carried out in the
presence of CBrCl (CH CH Cl, CH CHCl , and propionic
A
H
R
N
(CH CH O) }] or [V
A
H
R
U
G
A
H
R
U
G
2
2
3
2
2
3
heterolytic CꢀH bond cleavage leading to an ethyl V species
3
3
2
3
2
and acetic acids are also detected in small amounts).
are thermodynamically highly unfavourable (reactions 3–6
in Table S6 in the Supporting Information).
In contrast to the V catalyst, K S O has a decisive role in
2
2
8
radical formation, since without it no products (in particular
no organic halide when the reaction is performed in the
Carboxylation: The mechanism of further conversion of
C H C to propionic acid includes carbonylation of the former
2
5
presence of CBrCl ) are observed, whereas when the reac-
by CO to form the acyl radical C H COC (reaction 3,
3
2 5
tion is carried out in the absence of the metal catalyst but in
Scheme 5). The latter may then be converted by three
routes (I–III) similar to those
described for methane carboxy-
[7c]
lation.
Route I (Scheme 5),
which is the most favourable
for formation of propionic acid
Scheme 4.
1832
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 1828 – 1842

Vanadium Catalysts for Carboxylation of Ethane
FULL PAPER
ꢀ
1
Table 2. Energetic characteristics [kcalmol ] of the reactions involved in the proposed mechanisms at the CPCM-B3LYP/6-31+G*//gas-B3LYP/6-31G*
[
a]
and CPCM-B3LYP/6-311+G**//gas-B3LYP/6-311+G** (in parentheses) levels of theory.
°
DHs
No. Reaction
DH
s
DG
s
Radical formation
8ꢀ
4
4
ꢀ
Cꢀ
+C
1
a
HS
SO
O
C +C
!HSO
C+SO
+24.86 (+24.31)
ꢀ2.31 (ꢀ6.87)
2
ꢀ
2
4
2
H
6
!HSO
4
2
H
5
C
(via
TS1a)
(via
9.82 (7.61)
ꢀ4.56 (ꢀ8.70)
2
b
HSO
4
C+C
2
H
6
!H SO
2
4
+C
2
H
5
C
ꢀ0.28 (+0.12)
ꢀ9.19 (ꢀ12.74) ꢀ10.18 (ꢀ13.76)
TS1b)
Carboxylation
3
a
b
C
2
H
5
C+CO!C
2
H
5
COC
O) }]+HS
COOH!H SO
SO
}]+H
O) }]+C
A
H
R
U
G
2.72 (3.35)
ꢀ14.12 (ꢀ12.43)
4
4
[VO{N
HS
HS
A
C
H
T
R
E
U
N
G
(CH
2
CH
2
3
2
O
8ꢀ
![V(OO){N
A
H
U
G
2
CH
2
O)
3
}]+HS
2
O
7ꢀ
+8.16
+7.88
ꢀ
ꢀ
2
O
O
8
+CF
3
2
5
+CF
3
C(O)OSO
+7.66 (+7.59)
+1.57 (ꢀ1.57)
+2.49
ꢀ57.32
+2.61 (+5.63)
+1.46 (ꢀ0.86)
+2.60
2
8ꢀ
+H
2
O!H
CH O)
(CH CH
2
5
+HSO
SO
4
ꢀ
4
c
4
5
d
a
[VO{N
A
C
H
T
R
E
U
N
G
(CH
2
2
3
2
5
![V(OO){N
A
T
E
N
(CH
2
O)
3
}]+H
2
SO
4
[V(OO){N
A
C
H
T
R
E
U
N
G
2
2
3
2
H
5
CO {OOC(O)C
C
![V
A
H
R
U
G
2
H
5
}{N(CH
A
H
R
U
G
2
CH
2
O)
3
}]
4.51
5.53
5
b
[V
A
C
H
T
R
E
U
N
G
{OOC(O)C
2
H
5
}{N
A
H
R
U
(CH
2
CH
2
O)
3
}]![V(O){N
2
CH
2
O)
3
}]+C
2
H
5
COOC
ꢀ15.42
6
7
8
9
0
1
C
C
[V
2
H
H
5
5
COOC+CF
COOC+C
(CH
3
COOH!C
!C
CH O)
2
H
5
COOH+CF
3
COOC
A
H
R
N
1.50 (1.10)
5.53 (4.93)
+9.42 (+9.56)
ꢀ3.82 (ꢀ7.15)
ꢀ13.66
+9.71 (+9.95)
ꢀ4.27 (ꢀ7.45)
ꢀ13.98
2
2
H
6
2
H
5
COOH+C
2
H
5
C
+
+
A
C
H
T
R
E
U
N
G
(OOH){N
A
C
H
T
R
E
U
N
G
2
2
3
}] +C
2
H
5
COC![V
C(O)OC(O)C
OH
C(O)OSO
A
H
R
U
G
A
H
R
N
2 2 3 2 5
(OOH){N(CH CH O) }]+C H CO
+
ꢀ
C
C
2
H
H
5
CO +CF
3
COO !CF
3
2
H
5
ꢀ36.71 (ꢀ34.16)
ꢀ32.96 (ꢀ29.54)
+
ꢀ
1
1
2
5
CO +HSO
4
!C
2 5 2
H C(O)OSO
ꢀ
ꢀ
HS
2
O
8
+C
2
H
5
COC!SO
4
C +C
+CF
SO +CF
COOC
C(OH)
2
H
5
2
OH
4.49 (4.18)
ꢀ56.80 (ꢀ57.18) ꢀ56.18 (ꢀ56.61)
1
1
1
1
1
1
1
1
2
3
4
5
6
7
8
9
C
2
H
5
C+CF
3
COOH!C
2
H
6
3
COOC
COOC
12.06 (17.99) +13.23 (+16.70) +13.96 (+17.40)
HSO
4
C+CF
3
COOH!H
2
4
3
13.32 (12.85)
+4.05 (+3.96)
ꢀ87.76 (ꢀ86.19)
+6.45 (+8.21)
ꢀ8.62 (ꢀ9.79)
ꢀ7.91 (ꢀ8.53)
+5.33 (+8.17)
+1.29 (+4.21)
+3.78 (+3.64)
C
2
2
H
H
5
5
C+CF
3
3
COOC!CF
3
2 5
H
Carboxylation by TFA
C
C+CF
COOH!CF
)OC!CF
!CF H+C
COOH!CF
SO H+HSO
!CF
3
A
H
R
U
G
2
H
5
)OC
9.66 (10.56)
6.53 (5.94)
CF
CF
CF
CF
3
3
3
3
C(OH)
C+C
C+CF
C+H
A
C
H
T
R
E
U
N
G
(C
2
H
5
3
C+C
2
H
5
H
6
3
2
H
5
C
8.36 (6.77)
ꢀ8.67 (ꢀ9.44)
+5.30 (+7.96)
+1.52 (+4.33)
2
3
H+CF
3
COOC
16.30 (17.31)
15.10 (16.25)
3
C
2
4
3
4
Oxidation
2
2
0
1
C
C
2
H
H
5
5
C+HSO
4
C!C
2
H
5
OSO
2
OH
ꢀ81.42 (ꢀ80.80)
+22.33 (+21.06)
2
OSO OH+CF
2
3
COOH!C
2
H
5
OH+CF
3
COOH+SO
3
A
H
R
U
G
7.09 (5.84)
+
ꢀ
2
2
2
2
3
4
[VO{N
[V(OH){N
[V(HOC
(CH CH
[V(OO){N
A
C
H
T
R
E
U
N
G
(CH
(CH
){N
}]+C
(CH CH
2
CH
2
O)
CH
(CH
3
}]+CF
3
COOH![V(OH){N
C![V
(HOC
}] +CF
COO ![V(OC(O)CF
A
T
E
G
2
CH
2
O)
CH
){N-
3
}] +CF
3
COO
+13.67
ꢀ44.26
ꢀ8.71
+12.91
ꢀ6.17
+
+
A
C
H
T
R
E
U
N
G
2
2
O)
3
}] +C
2
H
+
5
A
H
R
N
2
H
5
A
T
U
G
2
2
O)
3
}]
ꢀ
A
C
H
T
R
E
U
N
G
2
H
5
A
C
H
T
R
E
U
N
G
2
CH
2
O)
OH
3
3
3
A
C
H
T
R
E
U
N
G
2
2
O)
3
2
H
5
25
26
27
28
29
30
31
A
C
H
T
R
E
U
N
G
2
2
O)
3
}]+C
2
H
5
C![V
(OOC ){N(CH CH
}]![V(=O){N
CH SO
OC+H
A
H
R
U
G
2
H
5
A
H
U
G
2
2
O)
3
}]
A
T
U
7.21
ꢀ47.93
[V
A
C
H
T
R
E
U
N
G
(OOC
2
H
5
){N
A
C
H
T
R
E
U
N
G
(CH
2
CH
2
O)
3
A
H
R
U
(CH
2
CH
2
O)
3
}]+CH
3
CH
2
OC
8.00
ꢀ13.02
CH
CH
CH
CH
CH
3
3
3
3
3
CH
CH
CH
2
2
2
OH+HSO
4
C!CH
C!CH
CHO+H
OH+HSO
C!CH
C!CH
C!CH C=OC+H
CH O)
3
2
2
4
ꢀ1.28 (ꢀ5.30)
ꢀ6.69 (ꢀ7.04)
ꢀ6.39 (ꢀ6.68)
OC+HSO
SO
ꢀ7.46 (ꢀ4.52) ꢀ88.53 (ꢀ92.40) ꢀ87.74 (ꢀ91.36)
ꢀ7.55 (ꢀ6.42) ꢀ14.93 (ꢀ18.61) ꢀ15.30 (ꢀ18.82)
4
3
2
4
CHOHC+H
SO
SO
SO
4
3
2
4
CHOHC+HSO
CHO+H
4.88 (4.07)
ꢀ80.28 (ꢀ80.83) ꢀ78.83 (ꢀ79.24)
ꢀ6.68 (ꢀ4.51) ꢀ17.21 (ꢀ20.76) ꢀ17.57 (ꢀ21.01)
+5.93 +6.06
4
3
2
4
CHO+HSO
4
3
2
4
+
+
32
33
34
35
[V
A
C
H
T
R
E
U
N
G
(OOH){N
A
C
H
T
R
E
U
N
G
(CH
2
2
3
}] +C
+H
2
H
5
C![V
SO
+CF
![V(=O){N
A
T
E
G
A
H
R
U
G
2
CH
2
O)
3
}]+C
2
H
5
+
ꢀ
C
C
2
H
H
5
+HSO
+CF
(CH
4
!CH
2
=CH
=CH
2
2
4
ꢀ37.60 (ꢀ39.33) ꢀ37.01 (ꢀ38.14)
+
ꢀ
2
5
3
COO !CH
2
2
3
COOH
ꢀ44.31 (ꢀ46.96) ꢀ43.52 (ꢀ45.20)
[V(OO){N
A
C
H
T
R
E
U
N
G
2
CH
2
O)
3
}]+C
2
H
4
3
}]+CH
3
CHO
ꢀ82.76
ꢀ83.17
Chem. Eur. J. 2008, 14, 1828 – 1842
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1833

A. J. L. Pombeiro et al.
Table 2. (Continued)
No. Reaction
°
DHs
DH
s
DG
s
CꢀC bond cleavage
3
3
6
7
[VO{N
[V(OC
(CH CH
(OCH
A
C
H
T
R
E
U
N
G
(CH
){N
O) }]+CH
){N(CH CH
2
CH
2
O)
CH
OSO
O)
}]+CO![V
3
}]+C
2
H
5
C![V
}]+HSO
2
OH
A
T
E
G
2
H
5
){N
A
H
R
U
G
2
CH
2
O)
3
}]
ꢀ35.18
ꢀ18.89
A
C
H
T
R
E
U
N
G
2
H
5
A
C
H
T
R
E
U
N
G
(CH
2
2
O)
3
4
A
H
R
U
G
2
A
R
U
G
17.96
ꢀ18.55
A
C
H
T
R
E
U
N
G
2
2
3
3
38
39
40
[V
[V
A
C
H
T
R
E
U
N
G
2
A
C
H
T
R
E
U
N
G
2
2
3
A
H
R
N
2
CO){N
A
H
R
U
G
2
CH
2
O)
3
}]
A
H
R
U
G
15.18
+9.29
A
C
H
T
R
E
U
N
G
(OCH
2
CO){N
A
C
H
T
R
E
U
N
G
(CH
2
CH
2
O) }]![V(=O){N
COOH!CF C(O)OC(O)CH
C(O)OC(O)C +CF
COOH!CF
3
A
H
R
U
G
2
CH
2
3
2
=C=O
A
H
R
U
17.39
ꢀ18.60
2
CH =C=O+CF
3
3
3
A
T
E
G
11.77 (11.36)
ꢀ20.15 (ꢀ14.25)
4
4
1
2
CF
3
2
H
5
3
3
C(O)OC(O)CF
3
+C
2
H
5
COOH
A
T
E
N
+6.74 (+7.14)
+1.85 (ꢀ0.22)
+7.23 (+6.89)
CH
3
3
3
3
CH=O+C
2
H
5
3
CH(C
C!CH )OC
A
H
R
U
G
2
H
5
A
H
E
G
6.59 (7.24)
9.81 (12.74)
0.51 (ꢀ3.29)
4
4
4
4
4
4
4
5
5
5
5
5
3
4
5
6
7
8
9
0
1
2
3
4
CH
CH
CH
CH(C
A
C
H
T
R
E
U
N
G
2
H
5
)OC!C
CH=O+CH
CH=O+HSO
C!CH
OH)OC!CH
OCH=O!H SO +CO
OCH=O+CF
2
H
5
3
C
A
T
E
G
+1.96 (+4.02)
ꢀ8.76 (ꢀ7.35)
4
3
CH
A
H
R
U
G
2
OH)OC
OCH=O
ACHTREUNG
CH
A
C
H
T
R
E
U
N
G
(OSO
2
3
C+HOSO
2
A
T
E
N
17.50 (15.56) +10.56 (+8.00)
HOSO
HOSO
2
2
2
4
A
H
E
G
22.78 (20.00)
21.45 (16.91)
ꢀ1.18 (ꢀ5.16)
ꢀ1.18 (ꢀ5.16)
3
COOH!H
2
SO
4
+CO+CF
)OC
COOH+CH
3
COOH
ACHTREUNG
CH
CH
CH
CH
3
3
3
3
COOH+C
2
H
5
C!CH
)OC!C
COOH+HSO
3
C(OH)
A
H
R
U
G
2
H
5
A
H
E
G
16.21 (17.26) +13.82 (+15.62)
C(OH)
A
C
H
T
R
E
U
N
G
(C
2
H
5
2
H
5
3
C
A
H
E
N
6.38 (5.74)
ꢀ7.44 (ꢀ9.16)
C!CH
OH)OC!CH
OCOOH!H SO +CO
OCOOH+CF
C(OH)
A
H
R
U
G
OH)OC
OCOOH
ACHTREUNG
17.80 (20.18) +17.48 (+18.31)
4
3
2
C(OH)
A
C
H
T
R
E
U
N
G
(OSO
2
3
C+HOSO
2
A
H
E
N
7.82 (6.78)
10.39 (8.27)
13.67 (9.86)
27.09 (26.87)
ꢀ8.56 (ꢀ10.28)
ꢀ16.18 (ꢀ19.61)
ꢀ16.18 (ꢀ19.61)
+5.43 (+5.86)
HOSO
HOSO
2
2
2
4
2
ACHTREUNG
3
COOH!H
4
+CO
2
+CF
3
COOH
+CH
ACHTREUNG
CF
a] DG
3
C(O)OC(O)CH
3
+CF
3
COOH!CF
3
C(O)OC(O)CF
3
3
COOH
+5.51 (+5.56)
[
s
values are indicated only for the processes which preserve the total number of molecules during reaction.
and operates in the presence of the metal catalyst, involves
oxygenation of C H COC with [V(OO){N
(CH CH O) }] to
tions (reactions 12–14) leading to ethyl trifluoroacetate
CF COOC H , and formation of this by-product was con-
AHCTREUNG
2
5
2
2
3
3
2
5
give C H COOC via formation of the intermediate [V-
firmed experimentally (see above).
2
5
A
C
H
T
R
E
U
N
G
{OOC(O)C H }{N(CH CH O) }] (Figure 4). The peroxo V
A
C
H
T
R
E
U
N
G
2
5
2 2 3
ꢀ
species is derived from reaction of 3 with HS O8 (reac-
Carboxylating role of TFA: In the absence of CO, the main
2
tion 4a) or with H SO (peroxomonosulfuric acid) formed
products are acetic acid and the ester CF COOC H , which
2
5
ꢀ
3
2
5
upon reaction of HS O with TFA or hydrolysis with traces
do not involve carboxylation of ethane. However, propionic
acid was also detected on account of the carboxylating role
of TFA, which possibly involves attack by C H C on the elec-
2
8
[27c]
of water
(reactions 4b–d). C H COOC abstracts a hydro-
2 5
gen atom from, for example, excess CF COOH or C H (re-
3
2
6
2
5
[
4c,31]
actions 6 and 7) to afford propionic acid.
trophilic carboxyl C atom of CF COOH
to give an oxo
3
Alternative pathways II and III (Scheme 6) involve oxida-
radical intermediate (reaction 15, Scheme 7 and Scheme S2
V
tion of C H COC either by a protonated peroxo V complex
in the Supporting Information). This intermediate undergoes
2
5
+
[32]
such as [V
A
C
H
T
R
E
U
N
G
(OOH){N
A
H
R
U
G
a known b-bond cleavage reaction
to give CF C and
2
2
3
3
oxodisulfate (reaction 11). Mixed anhydrides such as
CF C(O)OC(O)C H and C H C(O)OSO OH are easily
C H COOH (reaction 16), and the former product is con-
2 5
verted to trifluoromethane (reactions 17–19). Another less
3
2
5
2
5
2
formed (reactions 9–11) but they are thermodynamically
more stable than C H COOH. Thus, formation of propionic
favourable mechanism, involving attack of HSO C on the car-
4
bonyl C atom of TFA and formation of CO (Scheme 8), is
2
5
2
acid via routes II and III is less favourable than via catalytic
route I. A more detailed discussion is given in the Support-
ing Information.
discussed in the Supporting Information.
Oxidation: Oxidation of ethane to acetic acid without CꢀC
bond cleavage may occur via ethanol and acetaldehyde as
intermediates. Indeed, alcohols are oxidised by our systems
The presence in the reaction mixture of species such as
C H C, HSO C and CF COOH provides for several side reac-
2
5
4
3
1834
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 1828 – 1842

Vanadium Catalysts for Carboxylation of Ethane
FULL PAPER
tion 20) followed by solvolysis
with CF COOH (reaction 21);
3
2
) interaction of C H C with OH
2 5
of the protonated form of 3,
that
is,
[V(OH){N-
+
A
H
R
U
G
2
2
3
ethanol (reactions 22 and 23,
Figure 4) which can be liberated
upon exothermic substitution by
ꢀ
CF COO (reaction 24); 3) in-
3
teraction of C H C with the
2
5
peroxo V complex to give an
ethylperoxo species (reac-
tion 25) which, upon OꢀO bond
cleavage, transforms into
C H OC and regenerates 3 (reac-
2
5
tion 26). (See Supporting Infor-
mation for details and discus-
sion of other less favourable
routes.)
The oxidation of ethanol (or
of the ethoxy radical) to acetal-
dehyde and further to acetic
acid can occur easily by mecha-
nisms involving abstraction of
H atoms by radicals to form the
Figure 4. Equilibrium structures of selected TSs and intermediates (H atoms are not labelled for clarity). For a
wider view see Figure S4 in the Supporting Information.
acetyl radical CH COC (reac-
3
[33]
tions 27–31).
Plausible mech-
anisms for transformation of
CH COC into CH COOH were
Other processes leading to
3
3
[7c]
under typical conditions to the corresponding carboxylic
acids, without appreciable carbonylation (Table 3), that is,
ethanol leads to acetic acid (22% yield) with only traces of
propionic acid, and propanol forms propionic acid, with a
smaller amount of acetic acid, without detection of any bu-
tyric acid. The 15% yield of propionic acid from propanol is
much lower than that of 78% obtained from ethane under
identical conditions, and thus free propanol conceivably is
not involved in the carboxylation of ethane to propionic
acid. However, since the yield of acetic acid from ethanol
described previously.
CH CHO (reactions 60–67 in Table S6 in the Supporting In-
3
formation) require significantly higher energy.
Another possible route to acetaldehyde involves oxidation
V
of C H C by a hydroperoxo V complex, for example, [V-
2
5
+
+
A
H
R
U
G
A
H
R
U
tion 32) which, being a protonated ethylene, readily under-
goes deprotonation (reactions 33 and 34). Recently, it was
[34]
+
shown
that cation-radical vanadium clusters (V O )
2 5 n
V
(n=1, 2) oxidize C H to CH CHO. A peroxo V complex
2
4
3
(
22%) is much higher than that from ethane (5%), we
such as [V(OO){N
A
H
R
U
G
cannot rule out the possible involvement of some free etha-
nol in the oxidation of ethane to this acid.
Moreover, acetaldehyde is oxidised to acetic acid in a
much higher yield (ca. 40%) than ethanol (22%) or ethane
ethylene oxidation. The DG value for the corresponding
s
ꢀ
1
overall reaction 35 is strongly negative (ꢀ83.17 kcalmol ).
Detailed investigations of the possibility of this reaction and
its mechanism are underway.
(
5%), and this suggests possible involvement of the alde-
hyde in the conversion of ethane to acetic acid. Hence, the
oxidative route to acetic acid with preservation of the
alkane CꢀC bond can involve the sequential catalytic oxida-
CꢀC bond cleavage: A point that deserves further analysis is
the path for oxidation of ethane to acetic acid via CꢀC bond
cleavage. A plausible mechanism (Scheme 10A) for this re-
tions C H !CH CH OH!CH CHO!CH COOH. Acetic
action is based on interaction of C H C with an oxo vanadi-
2
6
3
2
3
3
2
5
acid is almost unreactive under our typical reaction condi-
tions (98% of its initial amount still remains after 20 h).
For conversion of the C H C radical to ethanol and acetal-
um catalyst, for example, [VO{N(CH CH O) }] (3), to give
A
H
R
U
G
2 2 3
IV
the V species [V
A
H
R
U
G
A
H
R
U
G
2
5
The latter may react with HSO C to yield the triplet structure
2
5
4
dehyde, several purely organic and metal-assisted mecha-
nisms were theoretically investigated (Scheme 9). Among
the most favourable are 1) formation of the ester
C H OSO OH upon interaction of C H C with HSO C (reac-
[VC
with CO to give the parent
CH =C=O via [VC(OCH CC=O){N
(CH CH O) }] (reac-
A
H
R
U
G
A
H
R
U
G
2
3
and ketene
A
H
R
U
G
ACHTREUNG
2
2
2
3
tions 38 and 39, Figure 4). Reaction of ketene with solvent
2
5
2
2
5
4
Chem. Eur. J. 2008, 14, 1828 – 1842
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1835

A. J. L. Pombeiro et al.
A
H
R
U
G
2 2 3
Scheme 5. The most plausible proposed mechanism I (A) and energy profile (B) for carboxylation of alkanes. [V]=V{N(CH CH O) }. Reaction and TS
numbers are those of Table 2 and Table S6 in the Supporting Information, respectively.
forms the mixed anhydride CF C(O)OC(O)CH , which
transforms into acetic acid (reactions 40, 54). ).
result of b-bond cleavage (reactions 43, 45, 49, 51), CH C is
3
3
3
[7c]
formed and reacts with CO to give the singly labelled acetic
1
3
13
13
Cleavage of the CꢀC bond may also occur at later stages
of ethane oxidation (Scheme 10C), that is, after formation
of CH CHO or even CH COOH. Their electrophilic carbon-
acid CH COOH (if CH3 CH and CO were starting ma-
3 3
terials). HOSO OCHO and HOSO OCOOH, upon decom-
2
2
position into CO or CO2 and H SO , respectively (reac-
3
3
2
4
yl C atoms may undergo attack by C H C or HSO C to give
tions 46, 47, 52, 53), are additional sources of the carbonyl
2
5
4
oxo radical intermediates (reactions 42, 44, 48, 50). As a
A
T
E
N
2 2 3
Scheme 6. Alternative (less favourable) pathways for carboxylation of alkanes. [V]=V{N(CH CH O) }. Reaction numbers are those of Table 2.
1836
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Chem. Eur. J. 2008, 14, 1828 – 1842

Vanadium Catalysts for Carboxylation of Ethane
FULL PAPER
[a]
Table 3. Comparative carboxylation of different substrates.
[
d]
Yield [%]
Propionic Acetic Total TON
[
b]
[c]
[e]
Entry Substrate
p(CO) [atm]
acid
acid
[
f]
1
2
3
4
5
C
C
C
CH
CH
2
2
3
H
H
H
6
5
7
30
30
30
30
78.4
0.3
14.5
0.4
4.8
22.0
5.8
83.2
22.3
20.3
40.4
–
127
34
31
61
–
OH
OH
CHO
3
40
[
g]
3
COOH 30
0.0
–
[
a] Catalyst 2 (0.0200 mmol), K
of oxidant to metal catalyst), CF
clave (13 mL capacity). [b] 3.06 mmol. [c] Measured at 258C (amounts of
2
S
2
O
8
(4.00 mmol, i.e., 200:1 molar ratio
3
COOH (5.0 mL), 808C, 20 h, in an auto-
ꢀ
1
CO correspond to 0.306 mmolatm ). [d] Molar yield [%] based on the
substrate, that is, moles of product per 100 moles of the corresponding
substrate. [e] Turnover number (moles of acetic and propionic acids per
mole of metal catalyst). [f] 10 atm, measured at 258C (i.e., 3.06 mmol).
[
g] The amount of acetic acid at the end of the reaction is 97.8% of its in-
itial amount, that is, it is essentially unreactive.
Conclusions
Scheme 7. Proposed mechanism (A) and energy profile (B) for carboxy-
This work has extended to ethane the catalytic activity of
amavadin and related vanadium complexes in functionaliza-
tion of alkanes. It has shown that they can act as remarkably
active catalysts for the direct one-pot carboxylation of
ethane to give mainly propionic acid and, to a smaller
extent, acetic acid, under mild conditions (the overall yield,
based on the alkane, can reach 93%). The catalysts remain
active even at very low concentrations and unprecedented
lation of C
Table 2. X=CF
in the Supporting Information .
2
H
5
C by TFA. Reaction numbers correspond to those of
COO, C or HSO . TS numbers are those of Table S6
3
2
H
5
4
group. Other less favourable mechanisms of CꢀC bond
cleavage are discussed in the Supporting Information.
4
high TONs are obtained (up to 2.0”10 ), still with good
Role of the vanadium catalyst: The role of the V centre in
the carboxylation processes can be envisaged as promoting
any of the following possible reactions: 1) oxygenation of
the acyl radical EtCOC by a peroxo V complex, which pro-
vides the most preferable catalytic route for propionic acid
yields. The catalyst activities are much higher than those we
[7]
have previously observed for methane carboxylation to
acetic acid and appear to be the highest so far reported in
the field of alkane functionalization under mild conditions.
Optimisation and control of selectivity of the process
have been achieved by studying the effects of a variety of
factors that require adjustments within particular ranges,
namely the ethane pressure (optimal of ca. 10 atm for pro-
pionic acid or ca. 3 atm for acetic acid), the CO pressure
(ca. 15–20 atm for propionic acid and absence of CO for
acetic acid), temperature (808C for propionic acid, whereas
the effect, above ca. 508C, is small for acetic acid), time (ca.
20 h for propionic acid and ca. 5 h for acetic acid), and the
amounts of TFA and K S O (e.g., 4 mmol for propionic
V
+
(
reactions 5–7); 2) oxidation of EtCOC to EtCO (reac-
V
tion 8) by a hydroperoxo V species in the carboxylation
process; 3) oxidation processes forming acetic acid via etha-
nol and acetaldehyde without CꢀC bond cleavage, namely,
+
by oxidation of EtC to Et (reaction 32) by a hydroperoxo
V
V
V
species, by O transfer from a peroxo V species (with
V
[6f]
formation of the corresponding oxo V species) to give
EtC (reaction 25) or C H (reaction 35) or by HO transfer
2
4
V
from a hydroxo V species to give EtC (reaction 23); 4) car-
bonylation process leading to acetic acid with CꢀC bond
2
2
8
acid under the typical conditions used).
The mechanisms involve both C- and O-centred radicals,
as indicated by the inhibiting effect of radical traps, and for-
IV
IV
cleavage via V –OC H and V –OCH C intermediates (re-
2
5
2
actions 36–39).
Scheme 8. Alternative (less favourable) pathways for the carboxylating role of TFA. Reaction numbers are those of Table 2. X=CF
3 2 5
COO, C H or
HSO
4
.
Chem. Eur. J. 2008, 14, 1828 – 1842
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1837

A. J. L. Pombeiro et al.
Scheme 9. Proposed mechanisms (A) and energy profile (B) for oxidation of ethane to acetic acid (via ethanol and acetaldehyde). [V]=V{N-
A
C
H
T
R
E
U
N
G
2 2 3 4 3
(CH CH O) }. X=HSO or CF COO. Reaction and TS numbers are those of Table 2 and Table S6 in the Supporting Information, respectively.
mation of ethyl halides in the presence of CBrCl , and evi-
sence of a catalyst, two alternative routes are operative, but
both lead to thermodynamically stable mixed anhydrides in-
stead of C H COOH.
3
dence has been presented for a key role of the oxidizing
ꢀ
agent K S O as a source of radicals (SO C , HSO C) suitable
2
2
8
4
4
2
5
for H abstraction from the alkane. The thus-formed ethyl
radical reacts with CO, and the resulting acyl radical
C H COC interacts further with the catalyst to yield the car-
A role of the TFA solvent as a carbonylating agent has
also been recognised, by performing experiments in the ab-
sence of CO and theoretical calculations. Under such condi-
tions, TFA reacts with the ethyl radical to yield an oxo radi-
cal intermediate which, upon b-bond cleavage, directly af-
2
5
boxyl radical C H COOC which, upon H abstraction from
2
5
the alkane or TFA, is converted to propionic acid. In the ab-
1838
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Chem. Eur. J. 2008, 14, 1828 – 1842

Vanadium Catalysts for Carboxylation of Ethane
FULL PAPER
Scheme 10. Proposed mechanisms (A, C) and energy profile for mechanism A (B) for ethane CꢀC bond cleavage. [V]=V{N
A
H
R
U
G
2 2 3
(CH CH O) }. Reaction and
TS numbers are those of Table 2 and Table S6 in the Supporting Information, respectively.
fords C H COOH, albeit in comparatively low yield due to
sive one via cleavage of this bond with carbonylation of a
methyl group. The formation of acetic acid via the former
route conceivably involves sequential (both purely organic
and metal-assisted) oxidation of ethane to ethanol and/or
acetaldehyde and further to acetyl radical with low activa-
2
5
competitive side reactions.
1
3
Experiments with C-enriched gases confirmed that the
ethyl group of propionic acid originates from ethane and the
carbonyl group from CO, whereas the formation of acetic
acid follows two pathways: a dominant one via ethane oxi-
dation with preservation of the CꢀC bond, and a less exten-
tion barriers. The formed CH COC undergoes peroxo-V-cata-
3
lysed conversion to CH COOH. However, propanol is not
3
Chem. Eur. J. 2008, 14, 1828 – 1842
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1839

A. J. L. Pombeiro et al.
expected to be an intermediate to propionic acid. For the
steel autoclave equipped with a Teflon-coated magnetic stirring bar at
8
08C (20 h reaction time).
oxidation of ethane to acetic acid with CꢀC bond cleavage,
Computational details: Full geometry optimisation of all transition-state
structures was carried out at the DFT level of theory using Beckeꢁs
several metal-assisted and organic pathways were also con-
sidered.
[42]
three-parameter hybrid exchange functional
in combination with the
The studies also suggest that ethane can become a promis-
ing raw material for the synthesis of propionic and acetic
acids, in the presence and absence of CO, respectively,
under mild conditions. However, in view of the appreciable
cost of TFA, its recycling and the search for a cheaper sol-
vent (and cheaper oxidant) are advisable, as well as estab-
lishing an easy method of separating the acid products.
Although the biological role of amavadin has not yet
been ascertained, its ability to catalyse the formation of hy-
drophilic carboxylic acids and esters from hydrophobic hy-
drocarbons like alkanes could possibly be used by organisms
not only for the elimination of noxious hydrocarbons but
also for biosynthetic purposes, although under reaction con-
ditions different from ours, which are not found in nature.
[43]
gradient-corrected correlation functional of Lee, Yang and Parr
[
44]
(B3LYP) with the help of the Gaussian98 program package. Restricted
approximations were employed for the structures with closed electron
shells, and unrestricted methods for the structures with open electron
[45]
[46]
shells. For purely organic reactions, the 6-31G*
and 6-311+G**
basis sets were applied. For reactions involving vanadium species, a rela-
tivistic Stuttgart pseudopotential described 10 core electrons, and the ap-
[
47]
propriate contracted basis set (8s7p6d1f)/[6s5p3d1f] was used for the
A
T
U
G
vanadium atom, and the 6-31G* basis set for other atoms. This level is
also denoted as B3LYP/6-31G* despite the use of the other basis set on
the V atom. Symmetry operations were not applied for all structures.
The Hessian matrix was calculated analytically for all optimised struc-
tures in order to prove the location of correct minima (no imaginary fre-
quencies) or saddle points (only one negative eigenvalue), and to esti-
mate the thermodynamic parameters, which were calculated at 808C and
1
0 atm. The nature of all transition states was investigated by analysis of
vectors associated with the imaginary frequency and, in some cases, by
[
48]
intrinsic reaction coordinate (IRC)
calculations. The entropic terms
and therefore the Gibbs free energies of activation and reaction calculat-
ed by using the standard expressions for an ideal gas are overestimated
or underestimated for reactions occurring in solution and proceeding
Experimental Section
s
with a change in number of molecules. Hence, the DG values are indicat-
[
26b]
[26b] [40]
[41]
ed only for processes that retain the total number of the molecules
during reaction. For the same reason, the DGs values are indicated only
for unimolecular reactions.
Catalysts 1,
methods. [VO
Aldrich) and V
and used as received. Ethane (AlphaGaz), C-enriched ethane (Aldrich),
2,
(acac)
(8; Merck) were obtained from commercial sources
3
and 4 were prepared according to published
°
A
C
H
T
R
E
U
N
G
2
] (5; Aldrich), VOSO ·5H O (6; Merck), V (7;
4
2
2 5
O
2
O
4
1
3
Solvent effects were taken into account in the single-point calculations
on the basis of the gas-phase geometries at the CPCM-B3LYP/6-31+
G*//gas-B3LYP/6-31G* and CPCM-B3LYP/6-311+G**//gas-B3LYP/6–
1
3
carbon monoxide (Air Products), C-enriched carbon monoxide (Al-
drich) and dinitrogen gases (Air Liquid Portugal), potassium peroxodisul-
fate (Fluka), trifluoroacetic acid (Aldrich), CBrCl
methylpiperidine-1-oxyl (TEMPO, Aldrich), Ph NH (Fluka), 2,6-di-tert-
butyl-4-methylphenol (BHT, Aldrich), benzoyl peroxide (Fluka), tetrae-
thyllead (50% xylene solution, Aldrich), ethanol (Panreac), propanol
[
49]
3
(Fluka), 2,2,6,6-tetra-
311+G** levels of theory by using the polarizable continuum model
[
50]
2
in the CPCM version. This method has been recognised as quite appro-
priate for consideration of the solvent effects of neutral and charged spe-
cies. The trifluoroacetic acid solvent was approximated by values of di-
electric constant and solvent radius of 8.55 and 2.18 Š, respectively. The
[
51]
(
(
Panreac), acetic acid (Merck), acetaldehyde (Fluka) and n-butyric acid
Aldrich) were obtained from commercial sources and used as received.
enthalpies and Gibbs free energies in the solution (H
mated by addition of the solvation energy DGsolv to gas-phase enthalpies
and Gibbs free energies (H and G ).
s s
and G ) were esti-
The reaction mixtures were prepared as follows:
.00 mmol) and TFA (5.0–7.3 mL) were added to 0.04–20.00 mmol of cat-
K
2
S
2
O
8
(1.08 g,
4
g
g
alyst, either in solid form, as
(K S O :catalyst 100:1) or as a 0.0020m solution in TFA, in a 13.0 mL
2 2 8
a
fine solid mixture with
K
2
S
2
O
8
For some structures, several possible conformations or coordination
modes have been calculated, and only the most stable ones are discussed.
stainless steel autoclave, equipped with a Teflon-coated magnetic stirring
bar. Then the autoclave was closed and flushed with dinitrogen three
times to remove the air and finally pressurised with ethane (0–15 atm)
and carbon monoxide (0–40 atm). The reaction mixture was vigorously
stirred for 2–20 h at 808C using a magnetic stirrer and an oil bath. After
the selected reaction time, the autoclave was cooled in an ice bath, then
degassed and opened. Diethyl ether (5 mL) and n-butyric acid as internal
standard (90 mL) were added to 1 mL of the reaction mixture. The result-
ing mixture was stirred, filtered and analysed by gas chromatography (in-
ternal standard method) on a Fisons Instruments GC 8000 series gas
chromatograph with a DB WAX fused-silica capillary column and the
Jasco-Borwin v.1.50 software. In some cases products were also identified
Acknowledgements
This work has been partially supported by the Fundażo para a CiÞncia e
a Tecnologia (FCT) and its POCI 2010 (FEDER funded) Programme,
Portugal. M.V.K. and M.L.K. are grateful to FCT and the POCTI pro-
gram for fellowships (grants BD/12811/03 and BPD/16369/98//BPD/5558/
2
001). We also thank Prof. Yuzo Fujiwara (University of Kyushu, Japan)
for stimulating discussions at the earlier stage of the carboxylation work,
Prof. António Palavra (IST) for the help in setting up the original reac-
tion system and Mr. Indalecio Marques for the GC-MS analyses.
1
13
1
by GC-MS and H and C{ H} NMR techniques on a Trio 2000 Fisons
spectrometer with a coupled Carlo Erba (Auto/HRGC/MS) gas chroma-
tograph and a Varian UNITY 300 NMR spectrometer, respectively.
Blank experiments were performed in the absence of the metal complex
[
1] a) Catalytic Activation and Functionalization of Light Alkanes, Vol.
4 (Eds.: E. G. Derouane, J. Haber, F. Lemos, F. Ramôa Ribeiro, M.
catalyst or of K
2 2 8
S O . Propionic acid was never detected, and only traces
4
of acetic acid were observed without the metal complex.
Guinet), NATO ASI series, Kluwer Academic Publishers, Dor-
drecht, The Netherlands, 1998; b) Activation and Functionalization
of Alkanes (Ed.: C. L. Hill), Wiley, New York, 1989; c) Methane
Conversion by Oxidative Process (Ed.: E. E. Wolf), Van Nostrand
Reinhold, New York, 1992; d) R. A. Periana, G. Bhalla, W. J. Tenn,
K. J. H. Young, X. Y. Liu, O. Mironov, C. J. Jones, V. R. Ziatdinov, J.
Mol. Cat. A. 2004, 220, 7; e) R. H. Crabtree, Chem. Rev. 1995, 95,
987; f) R. H. Crabtree, Chem. Rev. 1995, 95, 2599; g) C. Jia, T. Kita-
1
3
13
ACHTREUNG
The experiments with C-enriched ethane were performed at p
2 6
H )=
1
2
2 2 8
S O
in CF
3
1
3
The experiments with C-enriched carbon monoxide were performed at
1
3
p
A
C
H
T
R
E
U
N
G
( CO)=2.0 atm and p
A
C
H
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R
E
U
N
G
(C
2 6
H )=10 atm with catalyst 1 (0.020 mmol) and
K
2
S
2
O
8
(4.0 mmol) in CF
3
COOH (5.0 mL) contained in a 13 mL stainless
1840
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Chem. Eur. J. 2008, 14, 1828 – 1842

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