Unexpected reduction pathway of a Co2+ salt to [HCo(CO) 4] via [Co2(CO)8] in an ionic liquid

Article abstract of DOI:10.1039/b616169n

The reduction pathway of Co²⁺ salt under 5.5 MPa of H ₂Co in the 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide ionic liquid is discussed. The operating spectroscopic observations under catalytic conditions were considered to be a powerful method, since intermediate species can be evidence, and infrared spectroscopy appears as a very suitable tool for reactions involving carbonyl complexes. The generation of neutral carbonyl complexes can be achieved starting from almost all cobalt forms in their +II, +III oxidation states and even from metallic powder. Cobalt carboxylates were introduced as precursors and also formed during recycling steps, and the generation of cobalt carbonyl active species requires high pressures, as high as 20MPa. The hydroformylation of alkenes was performed at 10MPa in the presence of a cobalt-pyridine-ionic liquid. Deconvolution of the spectra at 130 -140°C confirms the presence of HCo¹(CO) ₄+py.

Full text of DOI:10.1039/b616169n

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Unexpected reduction pathway of a Co2+ salt to [HCo(CO)4] via [Co2(CO)8]
in an ionic liquid†‡
Fre´de´ric He´brard,a Philippe Kalck,*a Lucien Saussine,b Lionel Magnab and He´le`ne Olivier-Bourbigoub
Received 6th November 2006, Accepted 10th November 2006
First published as an Advance Article on the web 23rd November 2006
DOI: 10.1039/b616169n
In the 1-butyl-3-methylimidazolium bis(trifluoromethyl-
sulfonyl)amide ionic liquid ([BMI][NTf2]), [Co(NTf2)2] is
reduced under 5.5 MPa of H2–CO to [Co2(CO)8] prior to
[HCo(CO)4], provided a pyridine ligand is present in the
medium.
into [pyH][Co(CO)4] (3).4 The pyridinium salt is in equilibrium
with the hydride [HCo(CO)4] (4) and pyridine according to eqn (1).
[pyH][Co−I(CO)4] ꢀ [HCoI(CO)4] +py
(1)
3
4
We were particularly interested in the mode of reduction of
the [Co(py)6]2+ cobalt(II) species involved in these systems and in
the process generating 4. For that purpose, the reaction of 2 with
H2–CO was followed in heptane in a HP-IR cell5 at 5.5 MPa6 by
varying the temperature. Under these conditions, no particular
change could be detected for the broad and intense T2 mCO band
at 1889 cm−1, characteristic of the anion [Co(CO)4]−. Conversely,
the appearance of 4 and the pyridium cation [pyH]+ was observed
(band at 1633 cm−1). In addition, free pyridine is also present in
the organic phase as shown by the presence of two bands at 1595
(w) and 1578 (m) cm−1. The overall equation for this reaction is
consistent with the reduction of the cobalt(II) centre of 2 into the
cobalt(−I) species (3).
It is usually very difficult to clearly determine the exact mecha-
nisms occurring during a catalytic cycle, more especially in the case
of short-lived species. For this purpose, operating spectroscopic
observations under catalytic conditions is certainly a powerful
method, since intermediate species can be evidenced, and infrared
spectroscopy appears as a very suitable tool for reactions involving
carbonyl complexes. In the course of industrial hydroformylation
involving cobalt catalysts, it is known that the generation of neutral
carbonyl complexes can be achieved starting from almost all cobalt
forms in their +II, +III oxidation states and even from metallic
powder. Among these Co(II) salts, mainly cobalt carboxylates, play
an important role as they are introduced as precursors and are also
formed during recycling steps. In these processes, the generation
of cobalt carbonyl active species requires high pressures, as high
as 20 MPa, and 150 to 200 ◦C.1
H
−CO
2
−−−ꢀ
[Co(py)6][Co(CO)4]2
3 [pyH][Co(CO)4] + 3py
(2)
ꢁ−−−
2
3
In order to gain more insight into the reduction path of
cobalt(II) species, we examined, again using infrared spectroscopy
at 5.5 MPa, at variable temperatures, the fate of [CoII(NTf2)2] (5)
dissolved in the ionic liquid [BMI][NTf2] in the presence of heptane
under a 1 : 1 H2–CO mixture. Salt 5 was prepared by addition
of two equivalents of the acid HNTf2 to a suspension of cobalt
carbonate in water.7 The resulting pale pink solid was dissolved
at room temperature in [BMI][NTf2]. This solution and heptane
were introduced in the HP-IR cell, and the mixture was stirred
efficiently to avoid gas–liquid transfer limitations. The pressure
was maintained at 5.5 MPa in order to fit the stability range of
the neutral carbonyl cobalt complexes, described for the classical
hydroformylation process.1b Before recording the spectra, the
system was allowed to equilibrate for 30 min for each observation
temperature. Heating in the absence of pyridine at successively
40, 60, 80, 100, 120, 130 and 140 ◦C (total duration 3.5 h) did
not result in any mCO band detection and, even after maintaining
the temperature for a further 2 h, no carbonyl complex could be
detected. Conversely, when adding 2 molar equivalents of pyridine
to 5, and heating under the same conditions and in the same way
allowed us to observe mCO bands above 120 ◦C. Fig 1 shows the
spectrum at 120◦◦C (Fig. 1b) with only the basal line and that
recorded at 130 C (Fig. 1c) which reveals the typical mCO bands
of 1 at 2112 (w), 2066 (s), 2054 (sh), 2041 (s), 2031 (s), 2024 (s)
for the terminal CO ligands and at 1867 (m) and 1861 (m) cm−1
In order to combine the catalytic activity of cobalt with an
efficient recycling process, the hydroformylation of alkenes was re-
cently performed at 10 MPa in the presence of a {cobalt–pyridine–
ionic liquid} system. The ionic liquid which has been shown to de-
liver the most promising results2 was 1-butyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)amide, [BMI][NTf2] (Scheme 1).
Scheme 1 Formula of [BMI][NTf2].
It is already well established that pyridine (py) leads to the
disproportionation of [Co2(CO)8] (1) into the Co(II)/Co(−I) salt
[Co(py)6][Co(CO)4]2 (2).3 Earlier studies have evidenced that 2 is
slowly transformed under ambient conditions by a H2–CO mixture
aLaboratoire de Catalyse, Chimie Fine et Polyme`res, Ecole Nationale
Supe´rieure des Inge´nieurs en Arts Chimiques et Technologiques 118, route
de Narbonne, 31077, Toulouse cedex 4, France. E-mail: Philippe.Kalck@
ensiacet.fr
bInstitut Franc¸ais du Pe´trole, BP 3, 69390, Vernaison, France
† The HTML version of this article has been enhanced with colour images.
◦
for the bridging CO ligands.8 At 140 C (see Fig. 1d and Fig. 2)
‡ Electronic supplementary information (ESI) available: Spectra of
[HCo(CO)4] and [Co2(CO)8] (solutions in heptane), recorded in the HP-IR
cell. See DOI: 10.1039/b616169n
significant changes occurred, the concentration of 1 dramatically
decreased (2068, 2041 and 2023 cm−1 clearly identified) and small
190 | Dalton Trans., 2007, 190–191
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Thus, under H2–CO pressure, above 130 ◦C, the reduction of
[Co(NTf2)2] in the presence of pyridine occurs to give first the
cobalt(0) complex [Co2(CO)8] (and some [Co4(CO)12] as well).
Further heating produces [HCo(CO)4] and [PyH][Co(CO)4], due
to the equilibrium of eqn (1) governed by temperature.
When using a classical salt such as CoCl2, we observed that
in order to form [Co(CO)4]−, it is necessary to add pyridine to
the medium. Observations, by HP-IR, of the reduction of CoCl2
in [BMI][NTf2] were carried out in the presence of 6 equivalents
of pyridine. The progressive formation of the anion [Co(CO)4]−
is clearly evidenced above 120 ◦C, at 5.5 MPa, and the overall
reduction scheme is summarized in eqn (3).
H
−CO
2
CoCl2, 6 py
[pyH][Co(CO)4] + 2[pyH]Cl + 3 py
]
(3)
−−−−ꢀ
ꢁ−−−−
[BMI][NTf
2
3
In conclusion, cobalt(II) salts, at 5.5 MPa and 130 ◦C, can
be reduced in an ionic liquid by H2–CO to carbonyl species,
provided that pyridine is added. The two cobalt(0) complexes,
namely [Co2(CO)8] and [Co4(CO)12], appear first before being
transformed into [HCo(CO)4]. Such a Co(II)–Co(0) sequence,
which co-produces a pyridinium cation, should involve a het-
erolytic activation process of H2 favoured by the presence of the
ionic liquid medium. Indeed, in the previous work carried out by
Lee and Kochi,10 it has been shown that for the dismutation and
reformation of [Co2(CO)6(PPh3)2], the addition of a salt inhibits
the radical processes.
Fig. 1 IR spectra (mCO region) for the [Co(NTf2)2]–Py–[BMI][NTf2] sys-
tem evolution under H2–CO pressure, varying the temperature. (a) 1 MPa,
40 ◦C; (b) 5 MPa, 120 ◦C; (c) 5.5 MPa, 130 ◦C; (d) 5.5 MPa , 140 ◦C.
Notes and references
1 (a) H. Adkins and G. Krsek, J. Am. Chem. Soc., 1948, 70, 383–386;
(b) B. Cornils, in New Synthesis with Carbon Monoxide, ed. J. Falbe,
Springer-Verlag, Berlin, Heidelberg and New York, 1980.
2 L. Magna, H. Olivier-Bourbigou, L. Saussine and V. Kruger-Tissot,
Institut Franc¸ais du Pe´trole, EP 1 352 889 A1, 2003.
3 I. Wender, H. W. Sternberg and M. Orchin, J. Am. Chem. Soc., 1953,
Fig. 2 Deconvolution spectra for the produced carbonyl species (c and d,
Fig. 1) starting from [Co(NTf2)2]–Py–[BMI][NTf2]. (a) Treatment as in
Fig. 1c (5.5 MPa, 130 ◦C); (b) treatment as in Fig. 1d (5.5 MPa, 140 ◦C).
75, 3041–3042.
4 G. Fachinetti, T. Funaioli and M. Marcucci, J. Organomet. Chem.,
1988, 353, 393–404.
5 The HP-IR cell is built from a Top Industrie HB2 autoclave of 45 mL
volume, with mechanical stirring and equipped with a Rushton-type
turbine. The IR beam of a Perkin Elmer GX FT-IR spectrometer is
focused on a monocrystalline silicon rod and refocused beyond the
reactor on the detector through a Cassegrain lens.
6 [Co2(CO)8]: 6.3 mmol; Py: 2 eq per Co; heptane: 20 mL.
7 Transposed from: M. J. Earle, B. J. McAuley, A. Ramani, K. R. Sedden
and J. M. Thomson, Queen’s University of Belfast, WO 02/072 260 A2,
2001.
amounts (based on the relative intensities) of [Co(CO)4]− appeared
at 1889 cm−1; the two mCO bands at 2053 (s) and 2030 (vs) cm−1
can be ascribed to the formation of a significant amount of 4.
Deconvolution of the spectra at 130 and 140 ◦C (Fig. 2)9 evidenced
the presence of a band at 2115 cm−1, which confirms the presence
of 4. It is worth noting that some [Co4(CO)12] (6) is present in the
solution, presumably in equilibrium with 1 and characterized by
the two bands at 2063 (s) and 2053 (s) cm−1, and the shoulder at
1868 cm−1.
8 G. Bor and K. Noack, J. Organomet. Chem., 1974, 64, 367–372.
9 Deconvolution parameters: Spectrum 5.02 software (Perkin Elmer),
c = 1.000, smooth length = 0%.
10 K. Y. Lee and J. K. Kochi, Inorg. Chem., 1989, 28, 567–578.
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