Single-pot ethane carboxylation catalyzed by new oxorhenium(V) complexes with N,O ligands

Article abstract of DOI:10.1002/adsc.200505092

The oxorhenium(V) chelates [ReOCl-(N,O-L)(PPh₃] [N,O-L = (OCH₂CH₂)N(CH₂CH₂,OH)-(CH ₂COO) (2), (OCH₂CH₂)N(CH₂COO) (CH₂-COOCH₃) (3)] and [ReOCl₂(N,O-L)(PPh ₃] [N,O-L = C₅H₄N(COO-2) (4) C ₅H₃(COOCH₃-2)(COO-6) (5)] have been prepared by reaction of [ReOCl₃(PPh₃)₂] (1), in refluxing methanol, with N,N-bis(2-hydroxy-ethyl)glycine [bicine; N(CH₂CH ₂OH)₂(CH₂COOH)], N-(2-hydroxyethyl) iminodiacetic acid [N(CH₂CH₂-OH)(CH₂COOH) ₂], picolinic acid [NC₅H₄(COOH-2)] or 2,6-pyridinedicarboxylic acid [NC₅H₃(COOH-2,6) ₂], respectively, with ligand esterification in the cases of 3 and 5. All these complexes have been characterized by IR and multinuclear NMR spectroscopy, FAB⁺-MS, elemental and X-ray diffraction structural analyses. They act as catalysts, in a single-pot process, for the carboxylation of ethane by CO, in the presence of potassium peroxodisulfate K ₂S₂O₈, in trifluoroacetic acid (TFA), to give propionic and acetic acids, in a remarkable yield (up to ca. 30%) and under relatively mild conditions, with some advantages over the industrial processes. The picolinate complex 4 provides the most active catalyst and the carboxylation also occurs, although much less efficiently, by the TFA solvent in the absence of CO. The selectivity can be controlled by the ethane and CO pressures, propionic acid being the dominant product for pressures about ca. 7 and 4 atm, respectively (catalyst 4), whereas lower pressures lead mainly to acetic acid in lower yields. These reactions constitute an unprecedented use of Re complexes as catalysts in alkane functionalization.

Full text of DOI:10.1002/adsc.200505092

FULL PAPERS
Single-Pot Ethane Carboxylation Catalyzed by New
Oxorhenium(V) Complexes with N,O Ligands
Alexander M. Kirillov,a Matti Haukka,b Marina V. Kirillova,a
Armando J. L. Pombeiroa,*
a
´
Centro de Química Estrutural, Complexo I, Instituto Superior Tecnico, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
Fax: (þ351)-21-846-4455, e-mail: pombeiro@ist.utl.pt
b
University of Joensuu, Department of Chemistry, P. O. Box 111, FIN-80101 Joensuu, Finland
Received: February 18, 2005; Accepted: May 9, 2005
Abstract: The oxorhenium(V) chelates[ReOCl-
fluoroacetic acid (TFA), to give propionic and acetic
(N,O-L)(PPh3)] [N,O-L¼(OCH2CH2)N(CH2CH2OH)- acids, in a remarkable yield (up to ca. 30%) and under
(CH2COO) (2), (OCH2CH2)N(CH2COO)(CH2- relatively mild conditions, with some advantages over
COOCH3) (3)] and [ReOCl2(N,O-L)(PPh3)] [N,O-L¼ the industrial processes. The picolinate complex 4
C5H4N(COO-2) (4) C5H3N(COOCH3-2)(COO-6) (5)] provides the most active catalyst and the carboxyla-
have been prepared by reaction of [ReOCl3(PPh3)2] tion also occurs, although much less efficiently, by
(1), in refluxing methanol, with N,N-bis(2-hydroxy- the TFA solvent in the absence of CO. The selectivity
ethyl)glycine [bicine; N(CH2CH2OH)2(CH2COOH)], can be controlled by the ethane and CO pressures,
N-(2-hydroxyethyl)iminodiacetic acid [N(CH2CH2- propionic acid being the dominant product for pres-
OH)(CH2COOH)2], picolinic acid [NC5H4(COOH- sures about ca. 7 and 4 atm, respectively (catalyst 4),
2)] or 2,6-pyridinedicarboxylic acid [NC5H3(COOH- whereas lower pressures lead mainly to acetic acid
2,6)2], respectively, with ligand esterification in the in lower yields. These reactions constitute an unprece-
cases of 3 and 5. All these complexes have been char- dented use of Re complexes as catalysts in alkane
acterized by IR and multinuclear NMR spectroscopy, functionalization.
FAB þ-MS, elemental and X-ray diffraction structural
analyses. They act as catalysts, in a single-pot process,
for the carboxylation of ethane by CO, in the pres- Keywords: alkanes; carboxylation; C H activation;
ence of potassium peroxodisulfate K2S2O8, in tri- homogeneous catalysis; N,O ligands; rhenium
À
Introduction
more valuable oxidized productsisa field of currently
growing interest with industrial relevance.[6]
The oxo chemistry of rhenium is attracting much inter-
Recently, we have found that some rhenium[5] and va-
est, in particular due to the discovery that methyltrioxo- nadium[7,8] compoundscontaining amino alcohol and
rhenium [(Me)ReO3] and itsderivativesare among the
amino carboxylate ligandscatalyze the peroxidative hy-
most versatile oxidation catalysts known to date.[1,2] droxylation, oxygenation and halogenation of cycloal-
Nevertheless, the application of inorganic coordination kanes,[5,7] and the conversion of methane into acetic
compounds of rhenium in catalysis still remains an un- acid.[8] The latter reaction hasalso been noticeably ex-
derdeveloped field, namely in the functionalization of plored by others[9] following variousroutesand with dif-
unsaturated hydrocarbons and their derivatives,[3] and ferent typesof metal catalysts, but not with Re. Of par-
isalmots unexplored with repsect to alkane,s except
ticular significance to our work is the metal catalyst/po-
for (i) some alkane dehydrogenation reactions, com- tassium peroxodisulfate/trifluoroacetic acid system pio-
monly in rather low yields, catalyzed by rhenium polyhy- neered by Fujiwara et al.[9e–k, m] Thisactive research con-
dridesin the presence of a hydrogen acceptor, driven by
continuousremoval of H 2 or under irradiation,[4] and (ii) propionic acid[10–14] which hasbeen investigated rather
for a few peroxidative oxidationsof cycloalkanescata- scantly (usually limited to single experiments) in spite
trasts with the case of carboxylation of ethane to
lyzed, also in low yields, by rhenium benzoylhydrazido of (i) the wide application of thisacid for the preparation
[15]
and -diazenido complexes.[5] Alkanes(namely the gas-
eousones, methane, ethane and propane) are the most
of variousderivatives(broadly applied
asfood and
feed preservatives, herbicides, therapeutic agents, per-
prevalent and relatively cheap natural sources of organ- fumes, inhibitors, in liquid crystal mixtures, etc.) and
ic matter and their effective utilization by conversion to (ii) the difficultiesto itsindustrial manufacture, [15a] usu-
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Alexander M. Kirillov et al.
FULL PAPERS
ally requiring drastic conditions (high temperatures and acid), in refluxing methanol, led to the formation of
pressures), more than one synthetic stage and/or leading the neutral oxorhenium(V) complexes[ReOCl{( OCH2-
to a product mixture that requiresa further complex
work-up. A main objective of the current study was {(OCH2CH2)N(CH2COO)(CH2COOCH3)}(PPh3)] (3),
thusto esarch for Re complexesthat could catalyze [ReOCl2{C5H4N(COO-2)}(PPh3)] (4) or [ReOCl2-
CH2)N(CH2CH2OH)(CH2COO)}(PPh3)] (2), [ReOCl-
the functionalization of ethane into carboxylic acids, {C5H3N(COOCH3-2)(COO-6)}(PPh3)] (5), respective-
i.e., propionic and acetic acids, if possible in a process ly, in 87–42% isolated yields, as shown in Scheme 1 (re-
that could overcome some of these limitations.
It isalso known that iron(II and III) complexeswith
actions a–d). All these complexes are stable in the solid
state, 4 and 5 are also stable in Me2CO, CH2Cl2, MeCN
picolinic acid are components in efficient Gif systems[16] or CHCl3 solution, whereas 2 and 3 decompose slowly
which are active for the functionalization of liquid alka- within a few days in these solvents. All compounds
nes. In the past two decades Gif chemistry was widely ex- were characterized by IR, 1H, 31P{1H} and 13C{1H}
plored,[17] but Re catalysts were not described. More- NMR spectroscopy, FABþ-MS, elemental and single
over, such systems have not yet been applied to gaseous crystal X-ray diffraction structural analyses.
alkanes, except in a single case involving the oxidation of
In the formation of the purple complexes 2 and 3 (re-
n-butane and propane.[18] Thus, of particular interest to actionsa and b, Scheme 1), bicine and N-(2-hydroxy-
our study would be the use of picolinate and related li- ethyl)iminodiacetic acid, respectively, provided (upon
gands in our Re/ethane systems, although they are quite deprotonation of one carboxylic acid group and one hy-
different from the Gif ones.
As starting material we have selected the rhenium(V) which replace one triphenylphosphine and two chlor-
compound [ReOCl3(PPh3)2][19] (1) that isa common pre-
ides. In reactions c and d (Scheme 1), picolinic acid
droxyethyl arm) dianionic tridentate N,O,O ligands
cursor for a variety of oxorhenium complexes (some of and 2,6-pyridinedicarboxylic acid, respectively, formed
which are of pharmaceutical significance),[20] namely bidentate monoanionic N,O ligands(upon deprotona-
with polydentate N,O-donor ligands.[21–23] However, tion of one carboxylic acid group) which replace one tri-
since its use with amino alcohol or amino carboxylate phenylphosphine and one chloride in 1 to afford the cor-
N,O ligands is still scarce,[24] another aim of thiswork
wasto extend itsapplication to thistype of ligand,s
namely picolinic acid [NC5H4(COOH-2)], 2,6-pyridine-
responding reddish purple complex 4 and the greenish
grey compound 5, asmicrocrystalline materials.
In complexes 2, 3 and 5 the N,O,O or N,O ligand exhib-
dicarboxylic acid [NC5H3(COOH-2,6)2], the glycine itsone free O-arm. In 2 thisisa free hydroxyethyl group,
derivative N(CH2CH2OH)2(CH2COOH) and N-(2-hy- in accord with the detection, in the IR spectrum, of a
droxyethyl)iminodiacetic
(CH2COOH)2].
acid
[N(CH2CH2OH)- strong and broad n(OH) band at 3364 cmÀ1, whereas
in 3 and 5 it isa methyl acetate moiety formed by ester-
Hence, in the current study we have prepared new oxo- ification of one carboxylic acid by MeOH, presumably
rhenium(V) complexeswith thees chelating N,O li-
gandsand found that they can exhibit a remarkable cat-
under the influence of the metal centre since the ester
isnot formed in blank experimentswith N-(2-hydroxy-
alytic activity for the conversion of ethane, under rela- ethyl)iminodiacetic or 2,6-pyridinedicarboxylic acid
tively mild conditions, to a mixture of propionic and ace- and methanol, upon refluxing for 8 h in the absence of
tic acids(overall yield up to 30%, based on the alkane),
in a single-pot process with various advantages over the
industrial ones. This constitutes a very rare example of
the application of inorganic rhenium complexesascata-
lysts in the field of alkane functionalization and the most
active Re catalytic system (among either the inorganic
or the organometallic species), so far reported in this
field.
Results and Discussion
Synthesis and Spectroscopic Characterization
The reactionsof [ReOCl 3(PPh3)2] (1) with the aliphatic
or aromatic N,O-species N(CH2CH2OH)2(CH2COOH)
(bicine), N(CH2CH2OH)(CH2COOH)2 [N-(2-hydroxy-
ethyl)iminodiacetic acid], NC5H4(COOH-2) (picolinic
Scheme 1. (a) N(CH2CH2OH)2(CH2COOH), (b) N(CH2CH2
OH)(CH2COOH)2, (c) NC5H4(COOH-2), (d) NC5H3
acid) or NC5H3(COOH-2,6)2 (2,6-pyridinedicarboxylic (COOH-2,6)2.
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Ethane Carboxylation Catalyzed by New Oxorhenium(V) Complexes
FULL PAPERS
complex 1. The methyl ester group was confirmed by the phine coordination,[27] the corresponding signal is slight-
IR spectra of 3 and 5 which exhibit strong bands assigned ly shifted downfield from d¼ À5.47 ppm in the unco-
to nas(COOMe) [1744 (3) or 1743 (5) cmÀ1] and ns ordinated phosphine to 2.67 (2) and 3.75 (3), and
(COOMe) [1341 (3) or 1334 (5) cmÀ1].[25] Besides, the upfield to À10.80 (4) and À11.83 ppm (5). The 1H and
1
corresponding methyl signals were detected in the H 13C{1H} NMR spectra show resonances at the usual
[d¼3.84 (3) or 3.66 (5)] and 13C{1H} [d¼52.38 (3) or chemical shifts for the coordinated PPh3 and N,O,O or
53.39 (5)] NMR spectra. Despite the fact that related N,O ligands.[22a,22e,28] Elemental analyses are also consis-
complexeswith N-(2-hydroxyethyl)iminodiacetate and tent with the proposed formulations which are authenti-
2,6-pyridinedicarboxylate were previously synthesized cated by single crystal X-ray diffraction studies, as indi-
for different metals, the ester derivatives of these ligands cated below.
in compounds 3 and 5 {i.e., N-(2-oxyethyl)(2-meth-
oxycarbonylmethyl)iminoacetate [ÀOCH2CH2N(CH2-
COOÀ)CH2COOCH3] and 2-methoxycarbonyl-6-pyri- X-Ray Crystal Structures
dinecarboxylate [C5H3N(COOCH3-2)(COOÀ-6)], re-
spectively} constitute, to the best of our knowledge, The crystallographic data are summarized in Table 1
the first such examples in metal complexes.
and selected bond lengths and angles in Table 2. The mo-
The IR spectra of all complexes exhibit characteristic lecular structures of complexes 2–5 are shown in Fig-
¼
strong intensity n(Re O) bandsat 957 ( 2, 3), 988 (4) and ures1–4. All compounds 2–5 present a distorted octa-
1001 (5) cmÀ1, valuesin good agreement with those re-
hedral coordination geometry about rhenium, consist-
ported for other oxorhenium compounds.[22d,22e,26] Addi- ing of the N,O,O or N,O ligand, PPh3, the oxo and one
tional bandsin the regions1707–1650 and 1437–
1434 cmÀ1 are assigned to the nas(COO) and ns(COO),
respectively.[25]
or two chloride ligands.
In complex 2 (Fig. 1) bicine isdoubly deprotonated
and ligatesthe rhenium through the ethoxy oxygen O1
trans to chloride, the carboxylate oxygen O2 trans to
The molecular ionswere clearly observed with the ex-
pected isotopic patterns in the FABþ-MS of all com- the oxo group and the amino nitrogen N1 which is trans
pounds, except 3 for which the heaviest fragment was to the triphenylphosphine ligand. The binding of the bi-
(Mþ ÀCl). Other peaks correspond to the stepwise frag- cine
ligand
involvesthe
chelate
rings
À
À
À
À
À
À
À
À
mentationsby lossof Cl, PPh 3 and N,O ligand fragments. Re N1 C2 C3 O1 and Re N1 C4 C3 02 with bite
The 31P{1H} spectra are diagnostic of triphenylphos- anglesof N1
Re O1 82.4(3)8 and N1 Re O2
À
À
À
À
Table 1. Crystal data and structure refinement details for compounds 2–5.
2
3
4
5
empirical formula
Fw
C24H26ClNO5PRe
661.08
C25H26ClNO6PRe
689.09
C24H19Cl2NO3PRe
657.47
C26H21Cl2NO5PRe
715.51
temp (K)
l (Š)
cryst. syst.
Space group
a (Š)
b (Š)
c (Š)
a (deg)
100(2)
100(2)
293(2)
100(2)
0.71073
monoclinic
P21/n
10.4728(4)
9.1751(4)
24.8023(7)
90
0.71073
monoclinic
P21/n
15.6260(15)
10.6661(13)
15.9049(12)
90
0.71073
monoclinic
C c
7.5697(6)
39.306(5)
7.8335(8)
90
0.71073
monoclinic
P21/c
9.6672(14)
32.4691(15)
8.5137(10)
90
b (deg)
98.801(2)
90
2355.17(15)
4
1.864
5.378
111.500(8)
90
2466.4(4)
4
1.856
5.142
96.13(3)
90
2317.4(4)
4
1.884
5.570
111.162(10)
90
2492.1(5)
4
1.907
5.194
g (deg)
V (Š3)
Z
1calc (Mg/m3)
m(Mo Ka) (mmÀ1
)
no. of collected reflns23401
no. of unique reflns4217
RInt.
21887
5624
0.0602
0.0695, 0.1407
0.0824, 0.1457
1.306
3.473, À2.256
9039
4183
0.0443
0.0281, 0.0497
0.0487, 0.0549
1.088
1.933, À1.349
12653
5660
0.0250
0.0256
final R1[a], wR2[b] (Iꢀ2 s)
R1, wR2 (all data)
GOF on F2
0.0189, 0.0457
0.0205, 0.0463
0.897
0.770, À1.763
0.0249, 0.0520
0.0338, 0.0544
1.106
0.849, À1.739
À3
largest diff. peak and hole, e Š
[a]
R1¼Sj jFo j À jFc j j/SjFo j.
wR2¼[S[w(Fo ÀFc2)2]/S[w(Fo )2]]1/2.
[b]
2
2
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Alexander M. Kirillov et al.
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Table 2. Selected bond lengths(Š) and angles(deg) for compounds 2–5.
2
3
4
5
À
Re N(1)
2.235(10)
1.943(9)
2.052(9)
1.860(9)
2.361(3)
2.259(3)
1.908(3)
2.058(3)
1.729(3)
2.3831(11)
2.138(4)
2.067(3)
2.186(3)
2.037(2)
À
Re O(1)
À
Re O(2)
À
Re O(3)
1.663(3)
1.672(2)
À
Re Cl(1)
2.3294(10)
2.3650(14)
2.4551(11)
158.60(15)
96.57(9)
2.3227(9)
2.3645(9)
2.4685(9)
163.41(10)
93.81(7)
À
Re Cl(2)
À
Re P(1)
2.441(3)
2.4324(10)
166.85(12)
89.65(9)
À
À
O(1) Re O(3)
101.5(4)
163.8(3)
168.8(3)
82.3(2)
93.9(3)
91.8(3)
82.4(3)
77.9(3)
174.3(3)
92.0(3)
À
À
O(1) Re Cl(1)
O(2) Re O(3)
O(2) Re Cl(1)
O(3) Re Cl(1)
À
À
92.25(13)
172.78(8)
93.32(10)
89.27(12)
77.69(11)
79.87(11)
169.06(8)
95.61(9)
À
À
À
À
102.36(12)
87.42(17)
74.65(15)
101.35(8)
89.93(10)
75.16(10)
À
À
N(1) Re O(3)
À
À
N(1) Re O(1)
À
À
N(1) Re O(2)
À
À
N(1) Re P(1)
96.94(11)
169.29(13)
90.29(7)
168.63(7)
À
À
N(1) Re Cl(1)
The coordination geometry around the rhenium cen-
tre in 3 (Fig. 2) issimilar to that of 2 with the exception
that the chelated ethoxy arm is trans to the oxo ligand in-
stead of the carboxylate group. Most of the bonding
parametersin 3 (Table 2) are comparable with those of
À
complex 2 apart from the Re O3 bond, 1.729(3) Š,
which is significantly shorter and has a length that is in
the normal range for such a type of bond.[29]
The molecular structure of 4 (Fig. 3) exhibitsa triphe-
nylphosphine ligand trans to the chloride Cl2, and a pi-
colinate, chloride Cl1 and oxo atom in the equatorial
sites. In the picolinate NC5H4COOÀ ligand, the nitrogen
N1 is trans to the chloride Cl1 and the carboxylate oxy-
gen O1 trans to the oxo atom, forming an
Figure 1. An ORTEP-3 representation of 2. Ph hydrogen
atomshave been omitted for clarity.
À
79.87(11)8. The Re N1 bond length, 2.235(10) Š, issig-
À
À
nificantly longer than the Re O1 1.943(9) and Re O2
2.052(9) Š ones(Table 2), ascommonly observed for
N,O,O or N,O donor ligandsin oxorhenium complex-
es.[22d,22e] The Re O3 distance of 1.860(9) Š is much lon-
ger than the normal Re Ooxo bond length (ca. 1.67–
À
À
1.70 Š)[29] in six-coordinated monooxorhenium com-
plexes. This could point to a considerabletrans influence
of the carboxylate ligand, but isnot observed for com-
plexes 4 and 5. Alternatively, it could be accounted for
by considering the possible contamination of 2 by itsiso-
mer with the oxo-ligand trans to a ligand ethoxy arm (as
in 3) and thusthe chloro ligand (with a much longer co-
ordination bond length) trans to the carboxylate arm.
À
À
The O1 Re O3 angle of 101.5(4)8, i.e., between the
¼
À
Re O and the adjacent Re O bondsisremarkably larg-
er than 908. Other bond lengthsand anglesdo not differ
significantly from the expected values found for oxorhe-
nium compoundscontaining N, O, P and Cl donor
atoms.[22b–22g]
Figure 2. An ORTEP-3 representation of 3. Ph hydrogen
atomshave been omitted for clarity.
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Ethane Carboxylation Catalyzed by New Oxorhenium(V) Complexes
FULL PAPERS
the PPh3 and Cl2 ligands. In both dichloro complexes,
À
the Re Cl1 bond [2.3294(10) Š for 4; 2.3227(9) Š for
5] trans to N1 is slightly shorter than the Re Cl2 bond
À
[2.3650(14) Š for 4; 2.3645(9) Š for 5] trans to the triphe-
nylphosphine. Most of the binding parameters of the
N,O ligand in 5 compare favorably with those found in
[ReBr{NC5H3(COOMe-6)COOEt}(CO)3].[31]
À
The Re P1 distances in all compounds 2–5 (2.43–
2.47 Š) (Table 2) are in accordance with those repor-
ted[22h,24d,26b,28d,32] for other oxorhenium triphenylphos-
phine complexes.
Catalytic Activity of Rhenium Complexes towards
Ethane Carboxylation
Figure 3. An ORTEP-3 representation of 4. Ph hydrogen
atomshave been omitted for clarity.
All the rhenium complexes 1–5, in the presence of
K2S2O8 and in trifluoroacetic acid (TFA) at 808C, exhib-
Re N1 C2 C1 O1 ring with an N1 Re O1 bite angle it catalytic activity for the carboxylation of ethane with
of 74.65(15)8 which isreduced significantly from the ex- carbon monoxide to give propionic acid (Scheme 2
pected octahedral value of 908. The Re N1 and Re O3 and Table 3). Acetic acid is also formed as a result of
distances [2.138(4) and 1.663(3) Š, respectively] are the competitive Re-catalyzed partial oxidation of
shorter than the corresponding ones in 2 and 3 (Table 2), ethane by the peroxodisulfate. Complex 4, with a picoli-
À
À
À
À
À
À
À
À
À
whereasthe Re O1 bond length of 2.067(3) Š is nate ligand, provides the most active catalyst (overall
longer, and thiscan indicate a stronger interaction of
yieldsand TONsup to ca. 30% and 45, respectively, en-
the metal with the oxo ligand. Most of the bond distan- try 10), being followed by compounds 3 and 2. The other
cesof complex 4 are comparable to those of related complexes, 1 and 5, are much less active, catalyzing
oxo complexeswith picolinate or 3-hydroxypicolinate
mainly the oxidation of ethane to acetic acid. Complexes
such as [ReO(Me)(NC5H4COO)2], [ReOI2{NC5H3- 3 and 4 also exhibit better selectivity towards the forma-
(OH-3)COO}(PPh3)] and [ReOCl3{NC5H3(OH-3)- tion of propionic acid, the propionic-to-acetic acid mo-
COO}].[22j,30]
lar ratiosreaching the value of ca. 5:1.
Since it became apparent that the complex 4 isthe
In 5 the coordination geometry around the rhenium
centre (Fig. 4) and the metal-ligand distances are analo- most active and selective catalyst for the formation of
gousto those of 4, although with swapped positions of propionic acid, the effectsof CO and C 2H6 pressure on
the yield of productsand on the TON were investigated
for that catalyst, for the purpose of optimization of the
conditions. The amount of carbon monoxide has a rele-
vant effect on the yield of propionic acid (Fig. 5; entries
6–11, Table 3) which gradually increases with the CO
pressure up to 30 atm, for a constant pressure (10 atm)
of C2H6. However, further enhancement of pCO mark-
edly decreases that yield, conceivably due to the deacti-
vation of the catalyst as a result of formation of carbonyl
complexes. Hence, higher CO pressures should be
avoided. The TON and the selectivity (propionic-to-
acetic acid molar ratios) follow similar dependences
on the CO pressure.
In the absence of added CO, an appreciable amount of
propionic acid isstill formed (1.3% yield, entry 6), the
TFA behaving asthe carbonylating agent of ethane.
Hence, the previously recognized [8,9d] ability of TFA to
Figure 4. An ORTEP-3 representation of 5. Ph hydrogen
atomshave been omitted for clarity.
Scheme 2.
Adv. Synth. Catal. 2005, 347, 1435–1446
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Alexander M. Kirillov et al.
FULL PAPERS
Table 3. Catalytic activity of complexes 1 – 5 towardsthe carboxylation of ethane.
Entry Catalyst Amount of
catalyst [mmol]
pC2H6 [atm] pCO [atm] Yield[a] [%]
TON[c] Molar ratio
propionic/acetic
acid
Propionic Acetic Total[b]
acid
acid
1
2
3
4
5
6
7
8
1
2
3
4
5
4
4
4
4
4
4
4
4
4
1
1
4
4
4
4
0.01
10
10
10
10
10
10
10
10
10
10
10
5
15
10
10
10
10
10
10
10
15
15
15
15
15
0
10
15
20
30
40
30
30
30
20
20
30
30
30
30
0.2
2.0
4.5
7.7
0.0
1.3
10.9
15.4
19.1
23.8
10.4
2.5
0.5
23.6
4.2
4.7
–
1.9
3.5
2.6
4.3
1.5
5.0
5.2
5.2
5.2
5.2
3.9
5.6
1.1
5.4
2.0
2.2
0.3
0.8
1.0
–
2.1
5.5
7.1
12.0
1.5
6.3
16.1
20.6
24.3
29.0
14.3
8.1
1.6
29.0
6.2
6.4
17.0
22.0
37.0
2.3
0.1: 1
0.6: 1
1.7: 1
1.8: 1
–
0.3: 1
2.1: 1
3.0: 1
3.7: 1
4.6: 1
2.7: 1
0.4: 1
0.5: 1
4.4: 1
2.1: 1
2.1: 1
–
0.01
0.01
0.01
0.01
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
9.5
24.6
31.5
37.2
44.5
22.0
6.3
3.7
44.5
9.5
10.5
0.5
1.2
1.5
0.3
9
10
11
12
13
14[d]
15
16[d]
17[e]
18[f]
19[g]
20[h]
6.9
0.3
0.8
1.0
–
–
0.2
–
–
–
0.2
[a]
Molesof product/100 molesof ethane.
[b]
[c]
[d]
[e]
[f]
Propionic acidþacetic acid.
Molesof product (propionic acid þacetic acid)/mol of catalyst.
0.2 mmol of picolinic acid were added to the reaction mixture.
In the presence of CBrCl3 (1: 1, relatively to ethane).
In the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl (0.5: 1, relatively to ethane).
In the presence of Ph2NH (1: 1, relatively to ethane).
In the presence of 2,6-di-tert-butyl-4-methylphenol (1: 1 or 0.5: 1, relatively to ethane).
[g]
[h]
Re catalysts. The yield of acetic acid practically does not
depend on the CO pressure from 0 up to 30 atm, indicat-
ing that CO isnot involved in thisperoxidative oxidation
process. Moreover, since the propionic acid yield de-
creases with a decrease of pCO (below ca. 30 atm), the
selectivity towards this acid also decreases and, for pres-
sures below ca. 4 atm, acetic acid becomesthe main
product.
The pressure of ethane has also an important effect on
the yields of the products (Fig. 6, for a constant pressure,
30 atm, of CO). Thus, the yield of propionic acid increas-
esupon increasing the C H6 pressure to 10 atm, beyond
2
which a decrease occurs. The yield of acetic acid is less
dependent on the C2H6 pressure and inversion of the se-
lectivity occursfor pC2H6 below ca. 7 atm when that acid
becomesthe dominant product. Therefore, the selectiv-
ity can be controlled by both pC2H6 and (see above)
pCO: pressures above ca. 7 and ca. 4 atm, respectively,
lead to the predominance of propionic acid, relatively
to acetic acid, and, for the best yield of propionic acid
and the best selectivity, the optimum pC2H6 and pCO
Figure 5. Effect of the CO pressure on the yield of propionic
(a) and acetic (b) acid in the ethane carboxylation catalyzed
by complex 4 (pC2H6 ¼10 atm). Point numbersare those of
the Table 3 entries.
carboxylate methane, when using some vanadium cata- are 10 and 30 atm, respectively. In contrast, the forma-
lysts, has now been extended to ethane in the presence of tion ofacetic acid, which doesnot involve carbonylation,
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Ethane Carboxylation Catalyzed by New Oxorhenium(V) Complexes
FULL PAPERS
However, the ability of these Re complexes to either
À
abstract the hydrogen atom from an alkane C H bond
or to abstract RÀ from an alkane RH (mechanisms i or
ii, respectively), to the best of our knowledge, has not
yet been demonstrated. Although this cannot definitely
rule out the involvement of such complexes in the cata-
lytic process, it nevertheless points to the possibility of
an alternative mechanism based on the metal-assisted
thermolytic decomposition[9g] of peroxodisulfate (or
peroxodisulfuric acid, H2S2O8, formed from K2S2O8 in
the acidic CF3COOH medium) to the sulfate radical
À .
.
SO4 (or HSO4 in acidic medium) which could behave
asthe H atom abstractor from ethane to give the ethyl
radical. The capacity of the sulfate radical for hydrogen
abstraction reactions from alkanes, including ethane,
hasalready been clearly recognized and the rate con-
stants have even been determined.[35] Thisradical mech-
anism (i) should involve both carbon-centred and oxy-
gen-centred radicals and is supported by the observation
that when the reaction iscarried out in the presence of
either a C- or an O-centred radical trap, like CBrCl3,
Figure 6. Effect of the C2H6 pressure on the yield of propion-
ic (a) and acetic (b) acid in the ethane carboxylation cata-
lyzed by complex 4 (pCO¼30 atm). Point numbersare those
of the Table 3 entries.
predominatesover that of propionic acid for pC2H6 and TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), Ph2NH
pCO lower than ca. 7 and 4 atm, respectively.
Since complex 4, with a picolinate ligand, isthe most
or 2,6-di-tert-butyl-4-methylphenol (BHT),[36] the yields
of the acid productsbecome negligible (entries17–20,
active catalyst and picolinic acid (either incorporated Table 3). In the carboxylation process, the ethyl radical
.
in the catalyst[16] or added asan additive to the reaction
would yield the acyl radical (EtCO ) which, upon oxida-
tion and reaction with CF3COOH (TFA), would form
mixture)[17,33] isknown to enhance strongly the rate of
Gif reactions, we also tested in our systems the effect the mixed anhydride CF3COOCOEt that, on further re-
of addition of thisfree acid. However, no appreciable ef-
fect wasdetected on adding a ten-fold excess(relatively
to the catalyst) to the reaction mixture with either the COCF3). Thisfollowsa related proposal
action with CF3COOH, would generate propionic acid
(EtCOOH) and trifluoroacetic anhydride (CF3COO-
[14]
for methyl
most active complex 4 (compare entry 14 with entry 10, radical reactions.
Table 3) or one of the least active ones, compound 1
Alternatively to the carbonylation, the ethyl radical
(compare entry 16 with entry 15). Hence, the effect of can also be oxidized by the peroxodisulfate to yield ace-
picolinic acid isrecognized only when coordinated to a
metal centre asin 4. Although complex 5 containsan ar-
omatic N,O ligand similar to picolinate in 4, it isone of
tic acid,[13b] and the latter reaction predominatesover
the former for sufficiently low CO pressures.
It isalso noteworthy to mention that our reactionsare
the most inactive catalysts; the steric hindrance associat- not expected to involve, asintermediate,s either free
ed to the methyl ester group (Fig. 4) may play a deacti- propanol or free ethanol, or, asa carbonylating agent,
vating role.
The isolation and full characterization of intermedi- these alcohols are not converted significantly into the
atesin the reactionswere attempted but still remain elu- carboxylic acids(only ethanol gives, in very low yield,
CO2 (a possible product of TFA decomposition), since
sive and therefore the mechanisms are unclear. In prin- acetic acid) and the acid yieldsdo not increase in the
ciple, they can involve intermediate oxo and peroxo presence of that gas. The N,O chelating ligands, in our
complexesand, in thisrespect, it isnoteworthy to men-
systems, can probably be involved (upon decoordina-
tion that the peroxomethylrhenium(VII) complex tion of one N or O atom) in proton-transfer steps as
[(Me)ReO(O2)2] isthe active pseciesof the known
[(Me)ReO3]/H2O2 system in olefin and aromatic oxida-
postulated [8,16,17a,33,37] in other alkane oxidation systems.
Replacement of TFA, asthe solvent, by acetic acid or
tions.[1c, d] Other high valent peroxo-Re complexes trichloroacetic acid results in the loss of activity of the
are also known, such as [(Me)ReO(O2)2H2O],[34a] systems. These are also inactive for the carboxylation
[34b]
¼
[(Me)ReO(O2)2{O P(NMe2)3}],
[H4Re2O13][34c] and of methane to acetic acid.
[ReCl2(O2)(NNCOOMe)(PPh3)2].[34d] The reactions
The carboxylation of ethane into propionic and acetic
can proceed[4d,13,14] via either (i) hydrogen atom abstrac- acid isstill very little explored. [10–14] Most of the publish-
tion from ethane (formation of ethyl radical) or (ii) elec- ed worksreport only single experiments, with the excep-
trophilic attack of a highly oxidized Re centre to a C H tion of one study[13a] on the Pd(OAc)2/Cu(OAc)2 system
À
bond of this substrate with resulting heterolytic cleavage for which variousfactorsof the catalytic activity were in-
and formation of an ethylrhenium species.
vestigated. In order to compare our results with those
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Alexander M. Kirillov et al.
FULL PAPERS
Table 4. Comparison of activities of transition metal catalysts towards ethane carboxylation.[a]
Catalyst
pC2H6 [atm]
pCO [atm]
Yield[b]
TON[d]
Ref.
Propionic acid
Acetic acid
Total[c]
Co(OAc)2
10
30
1
30
20
20
30
0.4
2.1
17.7
23.8
2.7
–
–
3.1
2.1
17.7
29.0
2.3
25.3
11.0
44.5
[11,12]
[13a]
[13a]
Pd(OAc)2/Cu(OAc)2
Pd(OAc)2/Cu(OAc)2
Re complex 4
10
5.2
Thiswork
[a]
K2S2O8 and TFA were used as oxidant and solvent, respectively.
Molesof product/100 molesof ethane.
[b]
[c]
[d]
Propionic acidþacetic acid.
Molesof products(propionic acid þacetic acid)/mol of catalyst.
previously reported with other transition metal cata- complexes/aqueous H2O2/MeCN system at room tem-
lysts, we considered the best quoted yields and TONs, perature.[5]
when available (Table 4). Cobalt(II) acetate exhibitsa
The synthetic process we have achieved in this work
low activity with a total yield and TON of 3.1% and for propionic acid has some considerable advantages
2.3, respectively.[11,12] A better TON of 25.3 (based on over those normally used in industry[15a] for itsmanufac-
the total of both atoms) was achieved[13a] for the above ture, although using a more expensive oxidizing agent.
mixed Pd(II)/Cu(II) catalyst, but with a low yield In fact, it (i) either proceedsunder much milder condi-
(2.1%) which, in order to increase it considerably (to tions (e.g., the industrial ethylene carbonylation process
17.7%), required the marked reduction of ethane pres- normally requirestemperaturesabove 250 8C and pres-
sure to 1 atm. In view of the reported[10] activity of sures above 100 atm, and the air oxidation of naphtha
CaCl2 (although with a much lower TON, 1.3, and with operatesat ca. 1708C and above 40 atm) or (ii) ismore
a high yield also requiring the low atmospheric pressure direct and simple (e.g., than the two-stage process via
of C2H6), we have also tested that salt for ethane pres- the production of propanal by high pressure and high
sures of 1 and 10 atm (in both cases at a CO pressure temperature hydroformylation of ethylene, followed
of 30 atm) but obtained rather modest results, i.e., only by oxidation of that aldehyde; or than the production
tracesof propionic acid were detected with acetic acid
yieldsof 18.4% and 3.6%, and low TONsof 0.3 and
5.5, respectively.
of propionic acid asa secondary product in the three-
stage Monsanto or Cativa processes for acetic acid, the
first two stages operating under drastic conditions,
Hence, our system with the Re complex 4 catalyst, to whereasthe third one, i.e., the carboxylation of alcohol
the best of our knowledge, is the most active one so far requires an expensive Rh or Ir catalyst), or (iii) is
reported for ethane carboxylation.
more selective (e.g., the naphtha oxidation process leads
to a product mixture with a low proportion, ca. 10–15%,
of propionic acid).
Conclusion
We expect that the investigation will be extended to
other alkanesand reaction,s and to different rhenium
The study shows that (i) inorganic oxorhenium(V) com- catalysts, oxidizing agents and solvents, with attempts
plexeswith N,O ligandssuch aspicolinate and iminoace- to isolate intermediates, thus allowing the expansion of
tates, like those of the present study, can be conveniently the still underdeveloped rhenium-based catalysis and
prepared from the oxochloro starting material [ReOCl3- the establishment of a single-pot functionalization proc-
(PPh3)2] and (ii) are particularly promising sources of ac- ess of gaseous alkanes under mild or moderate condi-
tive catalysts for the single-pot ethane carboxylation un- tionswith potential commercial value.
der rather mild conditionsand with remarkable yieldsof
propionic acid. The picolinate complex provides, ac-
cording to our best knowledge, the most active system
so far reported not only for the carboxylation of ethane
Experimental Section
but also, in general, for the functionalization of any al-
kane by using an Re coordination compound as a cata-
lyst. In the unique previously reported[2i,5] cases of Re-
catalyzed alkane oxidations, i.e., the peroxidative oxida-
tion of cyclohexane, only rather low yieldswere ach-
ieved: 1.5% (although with a TON of 126) by the
General Materials and Procedures
All synthetic work was performed under a dinitrogen atmos-
phere using standard Schlenk techniques. The solvents were
dried and degassed by standard methods. Potassium perrhen-
ate (Merck), triphenylphosphine (Aldrich), N,N-bis(2-hydroxy-
ethyl)glycine (bicine) (Aldrich), N-(2-hydroxyethyl)iminodi-
acetic acid (Aldrich), picolinic acid (Aldrich) and 2,6-pyridine-
[(Me)ReO3]/pyrazine-2-carboxylic
acid/anhydrous
H2O2 system, in refluxing acetonitrile at 808C,[2i] and a
maximum of 2.4% by the benzoyldiazenido-rhenium dicarboxylic acid (Aldrich) were obtained from commercial
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Ethane Carboxylation Catalyzed by New Oxorhenium(V) Complexes
FULL PAPERS
sources and used as received. Oxotrichlorobis(triphenylphos-
phine)rhenium(V) [ReOCl3(PPh3)2] (1) wasprepared by a
published method.[19]
addition of MeOH (10 mL) led to the precipitation of a solid
which wasfiltered off, wahsed with MeOH (3 ꢁ5 mL) and
dried under vacuum, to give the second crop of 3; yield:
20 mg (8%, i.e., a total isolated yield of 80%). Product 3 issolu-
ble in MeOH, CH2Cl2, CHCl3 (slightly) and MeCN (slightly);
mp 1888C (dec.); anal. calcd. for C25H26ClNO6PRe (MW
689): C 43.57, H 3.80, N 2.03; found: C 43.51, H 4.01, N 2.05;
FABþ-MS: m/z¼654 [MÀCl]þ, 640 [MÀClÀMeþH]þ, 516
C, H and N elemental analyses were carried out by the Mi-
´
croanalytical Service of the Instituto Superior Tecnico. Melting
pointswere determined on a Kçfler table. Positive-ion FAB
mass spectra were obtained on a Trio 2000 instrument by bom-
barding 3-nitrobenzyl alcohol (NBA) matricesof the samples
with 8 keV (ca. 1.18·1015 J) Xe atoms. Mass calibration for [ReOCl(PPh3)þO]þ; IR (selected bands): n¼3059 w, 2952 w
data system acquisition was achieved using CsI. Infrared spec-
tra (4000–400 cmÀ1) were recorded on a Jasco FT/IR-430 in-
strument in KBr pellets. 1H, 31P{1H} and 13C{1H} NMR spectra
were measured on a Varian UNITY 300 spectrometer at ambi-
ent temperature. The 31P chemical shifts are relative to external
85% H3PO4 solution.
nas(CH), 2872 w ns(CH), 1743 s nas(COOMe), 1672 s nas
À
(COO), 1434 s ns(COO), 1341 s ns(COOMe), 1095 s n(C O),
1
957 s n(Re O) cmÀ1; H NMR (CDCl3): d¼7.78–7.45 (m,
¼
15H, PPh3), 5.05–4.51 (m, 4H, CH2), 4.13–3.93 (m, 2H,
CH2), 3.84 (s, 3H, CH3), 3.72–3.26 (m, 2H, CH2); 13C{1H}
¼
NMR (CDCl3): d¼191.08 (C O), 169.35 (COOMe), 135.07–
128.13 (m, PPh3), 72.43, 67.78, 63.74 and 62.32 (CH2), 52.38
(CH3); 31P{1H} NMR (CDCl3): d¼3.75 (s). X-ray quality crys-
talswere grown by lsow evaporation at 5 8C of a CH2Cl2-
MeOH solution of 3.
Preparation of [ReOCl{(OCH2CH2)N(CH2CH2OH)-
(CH2COO)}(PPh3)] (2)
To a suspension of 1 (300 mg, 0.36 mmol) in 30 mL of MeOH
an excess of bicine (176 mg, 1.08 mmol) was added and the re-
action mixture wasrefluxed for 10 min. The resulting purple
clear solution was taken to dryness under reduced pressure
to give a purple residue to which 10 mL MeOH were added.
The white solid that separated out from the resulting solution
wasfiltered off and discarded. The filtrate wasevaporated un-
der reduced pressure to give a purple residue which was treated
with 30 mL Et2O to yield a solid. It was filtered off, washed with
Et2O and dissolved in 20 mL CH2Cl2 to form a cloudy solution.
Filtration followed by concentration under reduced pressure
yielded a purple oil. The addition of Et2O (20 mL) led to the
precipitation of a solid which was filtered off, washed with
Et2O (3ꢁ10 mL) and dried under vacuum to afford complex
2 asa light purple powder; yield: 100 mg (42%, based on 1).
Product 2 issoluble in MeOH, Me 2CO, CH2Cl2, CHCl3 and
MeCN; mp 1588C (dec.); anal. calcd. for C24H26ClNO5PRe
(MW 661): C 43.60, H 3.96, N 2.12; found: C 43.80, H 4.28, N
1.91; FABþ-MS: m/z¼661 [M]þ, 641 [MÀHClþO]þ, 625
[MÀHCl]þ, 596 [MÀClÀEtOþO]þ, 580 [MÀClÀEtO]þ,
319 [MÀClÀEtOÀPPh3 þH]þ. IR (selected bands): n¼
3364 sbr n(OH), 3059 m, 2943 m nas(CH), 2868 m ns(CH),
Preparation of [ReOCl2{C5H4N(COO-2)}(PPh3)] (4)
To a suspension of 1 (300 mg, 0.36 mmol) in 30 mL of MeOH
an excess of picolinic acid (133 mg, 1.08 mmol) was added
and the reaction mixture wasrefluxed for 1 h. The resulting
deep red clear solution was concentrated under reduced pres-
sure to give a red oil which was treated with 15 mL CH2Cl2. The
solid that separated out from the resulting solution was filtered
off and discarded. The filtrate was evaporated under reduced
pressure giving a red oil which was dissolved in 30 mL Et2O
to form a red precipitate. It wasfiltered off, washed by Et 2O
(3ꢁ10 mL) and dissolved on a filter in 15 mL CH2Cl2 to
form a deep red solution which was taken to dryness under re-
duced pressure yielding a deep red oil. The addition of Et2O
(30 mL) led to the formation of a solid which was filtered off,
washed with Et2O (3ꢁ10 mL) and dried under vacuum to
give complex 4 as a reddish purple microcrystalline material;
yield: 206 mg (87% based on 1). Product 4 isosluble in
MeOH, Me2CO, CH2Cl2, CHCl3, C6H6 and MeCN; mp 2238C
(dec.); anal. calcd. for C24H19Cl2NO3PRe (MW 657.5): C
43.84, H 2.91, N, 2.13; found: C 44.02, H 2.96, N 2.03; FABþ-
MS m/z¼658 [MþH]þ, 622 [MÀCl]þ, 587 [MÀ2Cl]þ, 553
[MÀ{C5H4NCOO}þH2O]þ, 535 [MÀ{C5H4NCOO}]þ, 499
[MÀ{C5H4NCOO}ÀHCl]þ, 447 [Re(PPh3)À2H]þ; IR (se-
À
1650 sbr nas(COO), 1436 s ns(COO), 1096 s n(C O), 957 s
1
n(Re O) cmÀ1; H NMR (CDCl3): d¼7.73–7.37 (m, 15H,
¼
PPh3), 4.64 (q, J¼5.4 Hz, 2H, CH2), 4.36–3.95 (m, 6H, CH2),
3.39–2.92 (m, 2H, CH2); 13C{1H} NMR (CDCl3): d¼188.15
lected bands): n¼3056 m, 2971 w nas(CH), 2874 w ns(CH),
1
1704 s nas(COO), 1436 s ns(COO), 988 s n(Re O) cmÀ1; H
¼
¼
(C O), 134.75–128.54 (m, PPh3), 72.27, 69.11, 64.57, 63.23
and 59.35 (CH2); 31P{1H} NMR (CDCl3): d¼2.67 (s). X-ray
quality crystals were grown by slow evaporation at 58C of a
CH2Cl2-MeOH solution of 2.
NMR (CDCl3): d¼8.93 (t, J¼7.5 Hz, 1H, PyH), 8.73 (t, J¼
7.4 Hz, 1H, PyH), 8.54–7.10 (m, 15H, PPh3 þ2H, PyH);
13
1
¼
C{ H} NMR (CDCl3): d¼163.36 (C O), 148.10, 143.73,
137.63, 127.66 and 125.35 (PyC), 134.03–128.30 (m, PPh3);
31P{1H} NMR (CDCl3): d¼ À10.80 (s). X-ray quality crystals
were grown at 58C by vapor diffusion of n-C5H12 into a
CH2Cl2 or a C6H6-Me2CO solution of 4.
Preparation of [ReOCl{(OCH2CH2)N(CH2COO)-
(CH2COOCH3)}(PPh3)] (3)
A mixture of 1 (300 mg, 0.36 mmol) and N-(2-hydroxyethyl)-
iminodiacetic acid (191 mg, 1.08 mmol) wasrefluxed in
MeOH (30 mL) for 30 min to give a grayish purple suspension.
It wasfiltered off, washed with MeOH (3 ꢁ15 mL) and dried
under vacuum to afford complex 3 asa purple microcrystalline
solid; yield: 179 mg (72% based on 1). The filtrate wastaken to
dryness under reduced pressure to give a purple residue. The
Preparation of [ReOCl2{C5H3N(COOCH3-2)(COO-
6)}(PPh3)] (5)
A mixture of 1 (300 mg, 0.36 mmol) and 2,6-pyridinedicarbox-
ylic acid (180 mg, 1.08 mmol) wasrefluxed in MeOH (50 mL)
for about 6 h to produce a greenish brown suspension. It was
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Alexander M. Kirillov et al.
FULL PAPERS
filtered off, washed with MeOH (3ꢁ15 mL) and dried under
vacuum to afford complex 5 as a greenish grey microcrystalline
solid; yield: 136 mg (53% based on 1). Product 5 issoluble in
Me2CO, CH2Cl2, MeCN, CHCl3 (slightly) and C6H6 (slightly);
mp 1688C; anal. calcd. for C26H21Cl2NO5PRe (MW 715.5): C
43.64, H 2.96, N 1.96; found: C 43.96, H 3.14, N 1.79; FABþ-
MS: m/z¼715 [M]þ, 679 [MÀHCl]þ, 628 [MÀ2ClÀOH]þ,
613 [MÀClÀMeOH]þ; IR (selected bands): n¼3042 w,
2912 w n(CH), 1744 s nas(COOMe), 1707 s nas(COO),À11437 s
¼
5 mL of diethyl ether and 90 mL of n-butyric acid (asinternal
standard). The obtained mixture was stirred, then filtered off
and analyzed by gas chromatography using a Fisons Instru-
mentsGC 8000 esriesgaschromatograph with a DB WAX
fused silica capillary column (P/N 123–7032) and the JAS-
CO-BORWIN v.1.50 software. In some cases, products were
also identified by GC-MS, 1H and 13C{1H} NMR techniques.
Blank experimentswere performed in the absence of metal
complex catalyst or of K2S2O8. Propionic acid wasnever detect-
ed and only traces(below 0.5%) of acetic acid were revealed in
the presence of this peroxo reagent but without the metal com-
plex.
ns(COO), 1334 s ns(COOMe), 1001 s n(Re O) cm
NMR (CDCl3): d¼7.88–7.35 (m, 18H, PPh3 þPyH), 3.66 (s,
;
1H
13
1
¼
3H, CH3); C{ H} NMR (CDCl3): d¼187.86 (C O), 176.83
(COOMe), 142.58, 138.20, 127.88, 126.66 and 126.26 (PyC),
134.25–128.80 (m, PPh3), 53.39 (CH3); 31P{1H} NMR
(CDCl3): d¼ À11.83 (s). X-ray quality crystals were grown
by slow evaporation at 58C of a CH2Cl2 solution of 5.
Acknowledgements
This work has been partially supported by the FundaÅa˜o para a
ˆ
Ciencia e a Tecnologia (FCT), Portugal, and its POCTI
(FEDER funded) programme (project POCTI/QUI/43415/
2001). 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 gratefully acknowledge Prof. Y. Fuji-
wara (Kyushu University) for valuable discussions and advice
in the early stages of the alkane carboxylation studies, as well
as Prof. A. Palavra (IST) for the generous assistance and advice
in setting-up the original reactor system. They also thank Prof.
X-Ray Crystal Structure Determinations
The X-ray diffraction data were collected with a NoniusKappa
CCD diffractometer using Mo-Ka radiation. The Denzo-Sca-
lepack program package[38] wasused for cell refinementsand
data reductions. All structures were solved by direct methods
using the SIR97 or SIR2002 programs[39] with the WinGX
graphical user interface.[40] An empirical absorption correction
wasapplied to all data (T min/Tmax: 0.07136/0.11199, 0.5226/
0.8207, 0.2585/0.6058, 0.3878/0.6521 for 2–5, respectively) us-
´
´
J. J. R. Frausto da Silva and Dr. Jose A. L. da Silva (IST) for
ˆ
general support, Dr. Maria Candida Vaz (IST) for the direction
ing XPREP in SHELXTL v.6.12 program.[41] Structural refine-
´
of the elemental analysis service and Mr. Indalecio Marques
[42]
mentswere carried out with SHELXL-97.
All hydrogens
(IST) for running the MS and GC-MS spectra.
were placed in idealized positions and constrained to ride on
their parent atom.
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-239092 (2), CCDC-239093 (3),
CCDC-239094 (4), CCDC-239095 (5). Copiesof the data can
be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK [Fax: int. codeþ44(1223)336-
033; E-mail: deposit@ccdc.cam.ac.uk].
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