Reactions of aldehydes with carbon dioxide at nickel(0) centers. A detailed kinetic analysis

Article abstract of DOI:10.1021/om970584m

The reactions of the nickel(0) complex [Ni(bipy)(COD)] (COD = cyclooctadiene, bipy = 2,2′-bipyridine) with benzaldehyde, propionaldehyde, and carbon dioxide/propionaldehyde were investigated. A detailed kinetic analysis revealed that [Ni(bipy)(COD)] reacts reversible with aldehydes and activation parameters for the forward and back reactions were calculated from a temperature dependence study: forward reaction (benzaldehyde) ΔH? = 47.8 ± 0.4 kJ/mol, ΔS? = -47 ± 1 J/(mol K); back reaction (COD) ΔH? = 45 ± 6 kJ/mol, ΔS? = -61 ± 19 J/(mol K) and forward reaction (propionaldehyde) ΔH? = 55 ± 3 kJ/mol, ΔS? = -58 ± 9.0 J/(mol K); back reaction (COD) ΔH? = 39 ± 0.5 kJ/mol, ΔS? = -58 ± 2 J/(mol K). It could be shown that carbon dioxide did not react with [Ni(bipy)(COD)] directly but with the nickel propionaldehyde complex to form a five-membered cyclic nickel complex. The reaction proceeded according to an associative mechanism, during which the carbon dioxide was inserted into a Ni-O bond by an oxidative coupling step. The activation parameters for this process were ΔH? = 43 ± 6 kJ/mol and ΔS? = -129 ± 19 J/(mol K).

Full text of DOI:10.1021/om970584m

98
Organometallics 1998, 17, 98-103
Rea ction s of Ald eh yd es w ith Ca r bon Dioxid e a t Nick el(0)
Cen ter s. A Deta iled Kin etic An a lysis
Christian Geyer,† Eckhard Dinjus,‡ and Siegfried Schindler*,†
Institute for Inorganic Chemistry, University of Erlangen-Nu¨rnberg, Egerlandstrasse 1,
91058 Erlangen, Germany, and Forschungszentrum Karlsruhe, ITC-CPV, Postfach 3640,
76021 Karlsruhe, Germany
Received J uly 9, 1997
The reactions of the nickel(0) complex [Ni(bipy)(COD)] (COD ) cyclooctadiene, bipy )
2,2′-bipyridine) with benzaldehyde, propionaldehyde, and carbon dioxide/propionaldehyde
were investigated. A detailed kinetic analysis revealed that [Ni(bipy)(COD)] reacts reversible
with aldehydes and activation parameters for the forward and back reactions were calculated
from a temperature dependence study: forward reaction (benzaldehyde) ∆Hq ) 47.8 ( 0.4
kJ /mol, ∆Sq ) -47 ( 1 J /(mol K); back reaction (COD) ∆Hq ) 45 ( 6 kJ /mol, ∆Sq ) -61 (
19 J /(mol K) and forward reaction (propionaldehyde) ∆Hq ) 55 ( 3 kJ /mol, ∆Sq ) -58 ( 9.0
J /(mol K); back reaction (COD) ∆Hq ) 39 ( 0.5 kJ /mol, ∆Sq ) -58 ( 2 J /(mol K). It could
be shown that carbon dioxide did not react with [Ni(bipy)(COD)] directly but with the nickel
propionaldehyde complex to form a five-membered cyclic nickel complex. The reaction
proceeded according to an associative mechanism, during which the carbon dioxide was
inserted into a Ni-O bond by an oxidative coupling step. The activation parameters for
this process were ∆Hq ) 43 ( 6 kJ /mol and ∆Sq ) -129 ( 19 J /(mol K).
In tr od u ction
could be synthesized from alkenes or alkynes and carbon
dioxide mainly at Ni(0) centers, but complexes of other
metals such as palladium or rhodium have also been
used.1,2,9-11,14-21 Furthermore, the catalytic coupling
reactions of alkynes with carbon dioxide were studied
by generating a nickel(0) complex electrochemically.22
Interest in the use of carbon dioxide as a C1 building
block in industrial organic synthesis has increased
primarily due to international efforts at reducing the
greenhouse gas carbon dioxide in the atmosphere and
the diminishing supply of carbon-based fossil fuel
resources.1-4 Thus far only a relatively small number
of reactions with carbon dioxide have found use in
industrial synthetic processes,1,2 due mainly to the
thermodynamic stability and kinetic inertness of the
carbon dioxide molecule.1 To use this molecule in
organic synthesis it is therefore necessary to either
activate carbon dioxide itself or a substrate. Promising
are stoichiometric and catalytic reactions of carbon
dioxide taking place at low-valent transition metal
complexes with unsaturated cosubstrates or hydrogen,
which have been intensively investigated by different
groups.1,5-13 Carbonic acids, lactones, and pyrones
Stoichiometric reactions of carbon dioxide with alde-
hydes or imines and nickel(0) complexes lead to five-
membered cyclic complexes.9,19-21 Mechanistic studies
of such reactions are difficult, mainly because of the
extreme sensitivity of dilute solutions of nickel(0)
complexes toward traces of dioxygen. We have over-
come these difficulties and present here a detailed
kinetic investigation of the reaction of carbon dioxide
with propionaldehyde at a nickel(0) center according to
eq 1.
This reaction leads stoichiometrically to a five-
membered cyclic nickel complex, where Ni(0) is oxidized
to Ni(II) and CO2 is connected via a new C-O bond to
the aldehyde forming a ligand with a formal charge of
† University of Erlangen.
‡ Forschungszentrum Karlsruhe.
X Abstract published in Advance ACS Abstracts, December 15, 1997.
(1) Behr, A. Carbon Dioxide Activation by Metal Complexes; VCH:
Weinheim, New York, 1988.
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Chem. 1988, 345, C17.
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D. J . Organomet. Chem. 1982, 224, 81.
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S0276-7333(97)00584-0 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 01/05/1998

Reactions of Aldehydes with CO2 at Ni(0) Centers
Organometallics, Vol. 17, No. 1, 1998 99
-II.21 Crystal structures of the reactant and the
product are known.19,23
Exp er im en ta l Section
Ma ter ia ls a n d Meth od s. All chemicals used were of
analytical reagent grade quality. [Ni(bipy)(COD)] was syn-
thesized according to a previously published procedure.24
Benzaldehyde, propionaldehyde, and cyclooctadiene (COD)
were distilled and stored under an argon atmosphere in a
glovebox. UV-vis spectra were measured on a Hewlett-
Packard spectrophotometer 8452 A.
F igu r e 1. Time-resolved spectra recorded during the
reaction of benzaldehyde with [Ni(bipy)(COD)]: benzalde-
hyde concentration ) 20 mM, [Ni(bipy)(COD)] concentra-
tion ) 0.16 mM, COD concentration ) 2.0 mM, solvent )
THF, T ) 25 °C, ∆t ) 0.1 s.
Kin etic Mea su r em en ts. Analytical reagent grade quality
THF was distilled and stored in a glovebox (Braun, Garching,
Germany; dioxygen and water content was less than 1 ppm).
Solutions of [Ni(bipy)(COD)] for the kinetic measurements
were prepared in the glovebox and transferred in glass
syringes to the stopped-flow instrument. Carbon dioxide
saturated solutions were prepared by bubbling high-quality
carbon dioxide (Linde, Germany, 4.5) through a solution of
[Ni(bipy)(COD)] to remove traces of dioxygen and then through
the solvent (solubility of carbon dioxide in THF at 25 °C )
205 mmol/L25). Dilution was accomplished by mixing this
solution with an argon-saturated solvent. The reaction was
studied under pseudo-first-order conditions, and time-resolved
spectra were recorded on an Applied Photophysics stopped-
flow SX.17MV instrument (Leatherhead, UK) equipped with
a J &M detector connected to a TIDAS 16-416 spectrophotom-
eter (J &M, Aalen, Germany). Slower measurements were
performed using the rapid kinetics spectrometer accessory
change upon reaction with carbon dioxide, which indi-
cated that no reaction had occurred. This is consistent
with the finding that many transition metal promoted
reactions with carbon dioxide go through a process of
substrate rather than carbon dioxide activation as
discussed in a recent review.6
Reactions of carbonyl compounds with Ni(0) com-
plexes according to eq 2 are well-known.30,31
Ketones and aromatic aldehydes form stable com-
plexes with Ni(0) and for compounds with R ) R′ ) Ph,
L ) Et3P; R ) R′ ) CF3, L ) Ph3P; and R ) Ph, R′ ) H,
L-L ) bipy, X-ray structural characterization was
possible.32-34 Furthermore, the complex Ni(bipy)(ben-
zaldehyde) was not found to react with carbon dioxide
according to eq 1. No spectral change was observed
when bubbling carbon dioxide through a solution of this
complex in THF. This was different for aliphatic
aldehydes where it was found that [Ni(bipy)(COD)]
catalyzed an aldol reaction in the absence of carbon
dioxide.35 However, in the presence of carbon dioxide
the reaction proceeded according to eq 1.19,21
Rea ction of Ben za ld eh yd e w ith [Ni(bip y)(COD)].
To gain a more detailed insight into these reactions we
decided to first study the reaction of an aldehyde with
[Ni(bipy)(COD)] prior to the reaction with carbon diox-
ide. To avoid possible problems with the occurrence of
an aldol reaction, we chose benzaldehyde (eq 2, R ) Ph,
R′ ) H, L-L ) bipy) for our investigation. Spectral
changes for this reaction are shown in Figure 1. Isos-
bestic points were observed at 390 and 485 nm. It was
found that solutions of [Ni(bipy)(COD)] decompose after
Applied Photophysics RX-1000 (Leatherhead, U.K.) on
a
Hewlett-Packard Packard spectrophotometer 8452 A. In all
kinetic experiments the thermostatting fluid contained sodium
dithionite and was constantly bubbled with nitrogen to avoid
contamination by dioxygen. Data fitting was carried out using
the integrated J &M software, the program Specfit (Spectrum
Software Associates, Chapel Hill, NC), or the program OLIS
(KINFIT routines, On-line Instrument Systems, Bogart, GA).
Resu lts a n d Discu ssion
Since it is unlikely that the three reactants in eq 1
are reacting in one step, it was necessary to first study
the reaction of [Ni(bipy)(COD)] with either carbon
dioxide or the aldehyde. Literature studies have re-
vealed that Ni(0) complexes can react with carbon
dioxide,26-29 and a structurally characterized carbon
dioxide complex was synthesized from a Ni(0) com-
pound.26,29 In contrast to these results we found that
the UV-vis spectrum of [Ni(bipy)(COD)] in THF did not
(23) Dinjus, E.; Walther, D.; Kaiser, J .; Sieler, J .; Thanh, N. N. J .
Organomet. Chem. 1982, 236, 123.
(24) Dinjus, E.; Gorski, I.; Uhlig, E.; Walther, D. Z. Anorg. Allg.
Chem. 1976, 422, 75.
(25) Gennaro, A. I., A. A.; Vianello, E.; J . Electroanal. Chem. 1990,
289, 203.
(26) Aresta, M.; Nobile, C. F. J . Chem. Soc., Chem. Commun. 1975,
636.
(27) Aresta, M.; Nobile, C. F. J . Chem. Soc., Dalton Trans. 1977,
708.
(28) J olly, P. W.; J onas, K.; Kru¨ger, C.; Tsay, Y.-H. J . Organomet.
Chem. 1971, 33, 109.
(29) Do¨hring, A.; J olly, P. W.; Kru¨ger, C.; Romao, M. J . Z. Natur-
forsch. 1985, 40b, 484.
(30) Dinjus, E.; Gorski, I.; Matschiner, H.; Uhlig, E.; Walther, D. Z.
Anorg. Allg. Chem. 1977, 436, 39.
(31) Dinjus, E.; Langbein, H.; Walther, D. J . Organomet. Chem.
1978, 152, 229.
(32) Tsou, T. T.; Huffman, J . C.; Kochi, J . K. Inorg. Chem. 1979,
18, 2311.
(33) Countryman, R.; Penfold, B. R. J . Chem. Soc., Chem. Commun.
1971, 1598.
(34) Kaiser, J .; Sieler, J .; Walther, D.; Dinjus, E.; Golic, L. Acta
Crystallogr. B 1982, 38, 1584.
(35) Walther, D.; Dinjus Z. Anorg. Allg. Chem. 1978, 440, 22.

100 Organometallics, Vol. 17, No. 1, 1998
Geyer et al.
plexes can dissociate in solution, liberating the coordi-
nated olefin.37,38 This is consistent with our findings
that addition of COD to solutions of [Ni(bipy)(COD)]
stabilize the complex as described above. A kinetic
study related to this study described the reaction of
[Ni(PEt3)4] with benzophenone and substituted ben-
zophenones in benzene.32 The authors suggested that
here the reactive species is [Ni(PEt3)3] which forms in
a rapid preequilibrium according to eq 4.32,39,40
[Ni(PEt3)4] h [Ni(PEt3)3] + PEt3
(4)
The equilibrium constant for this reaction derived
from the kinetic measurements was concordant with
that obtained earlier from spectrophotometric titra-
tions.41 [Ni(PEt3)3], which formed during the reaction,
then reacted further with benzophenone to yield the
product complex. Additional support for the proposed
mechanism was the finding that an increase in the
concentration of PEt3 led to a decrease in the reaction
rate; i.e., an inverse dependence of kobs on PEt3 concen-
tration.32 However, no temperature dependence studies
were performed.
From the kinetic results the mechanism shown in
Scheme 1 for the reaction of [Ni(bipy)(COD)] (A) with
benzaldehyde may be postulated. In a fast preequilib-
rium one of the bonds to COD is broken transforming
the four-coordinated [Ni(bipy)(COD)] (A) into a reactive
three-coordinated species [Ni(bipy)(COD′)] (B). In a
reversible reaction benzaldehyde is bound (presumably
in a η1(O) manner) to [Ni(bipy)(COD′)] (B) before, in
another reversible reaction, the final product is formed.
During this reaction benzaldehyde undergoes a coordi-
nation change to η2(C, O) and the remaining bond to
COD is broken. The rate for the formation of [Ni(bipy)-
(benzaldehyde)] (D) is given by eq 5 (abbreviations for
nickel complexes according to Scheme 1; R ) C6H5).
F igu r e 2. Plot of observed rate constants kobs vs benzal-
dehyde concentration at different temperatures: [Ni(bipy)-
(COD)] concentration ) 0.25 mM, COD concentration )
2.8 mM.
some time, even under inert conditions. The decompo-
sition which arises from loss of the labile ligand COD
could be suppressed by adding an excess of COD to the
solution. Absorbance time traces of the reaction of
[Ni(bipy)(COD)] with benzaldehyde and COD in excess
could be fitted perfectly to a single one-exponential
function, and the plot of the observed rate constants kobs
vs benzaldehyde concentration is linear at different
temperatures as shown in Figure 2 and Table 1. The
intercept indicates that the reaction is reversible, and
it is possible to measure the back reaction of COD with
premixed [Ni(bipy)(COD)] and benzaldehyde. Here
again we observe a linear dependence with an intercept.
The corresponding rate constants for the forward and
backward reaction are consistent with those obtained
for the forward reaction (Table 1). Equation 3 for the
observed rate constant kobs describes the kinetic findings
where ka and kb are the second-order rate constants for
the forward and back reaction and an equilibrium
constant close to 1 was obtained.
d[D]
dt
) k2[C] - k-2[D][COD]
(5)
kobs ) ka[aldehyde] + kb[COD]
(3)
Because [Ni(bipy)(COD′)(benzaldehyde)] (C) is an
intermediate, the equation needs to be modified by
including the fast preequilibrium between [Ni(bipy)-
(COD)] (A) and [Ni(bipy)(COD′)] (B) and furthermore
using the steady state approximation for [Ni(bipy)-
(COD′)(benzaldehyde)] (C). This leads to eq 6 for kobs
(description of the derivation of such equations is given
in the literature42).
The temperature dependence of the second-order rate
constants calculated from the slopes of the plots of kobs
vs benzaldehyde concentration (Figure 2 and Table 1)
or kobs vs COD concentration (Table 1) allowed the
determination of the activation parameters for the
forward reaction (∆Hq ) 47.8 ( 0.4 kJ /mol and ∆Sq )
-47 ( 1 J /(mol K)) and the back reaction (∆Hq ) 45 (
6 kJ /mol and ∆Sq ) -61 ( 19 J /(mol K)).
The kinetic findings support an associative mecha-
nism for the reaction of [Ni(bipy)(COD)] with benzal-
dehyde as well as the back reaction of [Ni(bipy)(ben-
zaldehyde)] with COD; however it is very unlikely that
the reaction proceeds in this way. [Ni(bipy)(COD)] is a
diamagnetic tetrahedral complex where the Ni(0) center
is bound to the planar bipyridine ligand and sym-
metrically coordinated to the two π-bonds of COD.23 It
is well-known that tetrahedral Ni(0) complexes undergo
dissociative substitution reactions (with positive activa-
tion entropies), and therefore it would be difficult to
explain why [Ni(bipy)(COD)] would be an exception.36
Furthermore, it is known that [Ni(bipy)(olefin)] com-
K
1 + K
k1k2[benzaldehyde] + k-1k-2[COD]
(
)
kobs
)
k2 + k-1
(6)
K/(1 + K) represents the fraction of [Ni(bipy)(COD′)]
(B) species where one Ni-COD bond is broken. From
(37) Yamamoto, T.; Yamamoto, A.; Ikeda, S. J . Am. Chem. Soc. 1971,
93, 3350.
(38) Yamamoto, T.; Yamamoto, A.; Ikeda, S. J . Am. Chem. Soc. 1971,
93, 3360.
(39) Berman, R. S.; Kochi, J . K. Inorg. Chem. 1979, 19, 248.
(40) Tsou, T. T.; Huffman, J . C.; Kochi, J . K. J . Am. Chem. Soc. 1979,
101, 6319.
(41) Tolman, C. A.; Seidel, W. C.; Gosser, L. W. J . Am. Chem. Soc.
1974, 96, 53.
(36) J ohnston, R. D.; Basolo, F.; Pearson, R. G. Inorg. Chem. 1971,
10, 247.
(42) Espenson, J . H. Chemical Kinetics and Reaction Mechanisms;
2nd ed.; McGraw-Hill, Inc.: New York, 1995.

Reactions of Aldehydes with CO2 at Ni(0) Centers
Organometallics, Vol. 17, No. 1, 1998 101
Ta ble 1. Secon d -Or d er Ra te Con sta n ts for th e Rea ction of [Ni(bip y)(COD)] w ith Ben za ld eh yd e a n d
[Ni(bip y)(ben za ld eh yd e)] w ith COD As Obta in ed fr om Eq 3 ([Ni(bip y)(COD)] Con cen tr a tion ) 0.25 m M)
[benzaldehyde] dependence
[COD] dependence
slope,
intercept,
K
intercept,
slope,
K
T/°C
ka/M-1 s-1
kb/M-1 s-1
()ka/kb)
ka/M-1 s-1
kb/M-1 s-1
()ka/kb)
15.0
20.0
25.0
30.0
35.0
62 ( 5
32.6 ( 0.7
55.2 ( 0.4
1.9
2.2
1.8
71 ( 1
99 ( 1
51 ( 7
74 ( 5
1.4
1.3
1.3
1.4
121 ( 2
139 ( 1
192 ( 2
105 ( 4
140 ( 10
209 ( 21
42 ( 2
-63 ( 6
117 ( 3
45 ( 6
-61 ( 19
∆Hq (kJ /mol)
47.8 ( 0.4
-47 ( 1
48.3 ( 1.5
-47 ( 5
∆Sq (J /(mol K))
Sch em e 1
We also investigated the reaction in toluene where
behavior was similar to that of THF was found. Only
small solvent effects were observed for the reaction of
[Ni(PEt3)4] with benzophenone.32 Although a direct
comparison of the reaction of [Ni(PEt3)4] with benzophe-
none with the reaction of [Ni(bipy)(COD)] is difficult, it
is interesting to observe that these reactions proceed
at similar rates. Second-order rate constants for the
reaction of the different neutral benzophenone ligands
range from 22 to 900 M-1 s-1 (depending on the selected
substituents) and agree well with our value of 100 M-1
s-1 at 25 °C.
Rea ction of Ca r bon Dioxid e w ith P r op ion a ld e-
h yd e a n d [Ni(bip y)(COD)]. The situation is different
when the aliphatic propionaldehyde is reacted with
[Ni(bipy)(COD)]. Without carbon dioxide present an
aldol condensation to R-methyl-â-ethylacrolein is cata-
lyzed by the nickel complex.35 Under our experimental
conditions it is still possible to observe spectrophoto-
metrically the formation of the propionaldehyde nickel
complex [Ni(bipy)(propionaldehyde)]. The UV-vis spec-
trum is very similar to that of [Ni(bipy)(benzaldehyde)],
but a much larger excess of propionaldehyde is needed.
To exclude the possibility that the green solution
obtained during the reaction of propionaldehyde with
[Ni(bipy)(COD)] is not [Ni(bipy)(R-methyl-â-ethylac-
rolein)], formed during the aldol condensation,43 we
measured the UV-vis spectrum of this complex by
adding R-methyl-â-ethylacrolein to [Ni(bipy)(COD)]. The
spectrum is clearly different and shows absorbance
maxima at 404 and 628 nm compared to 419 and 662
nm for [Ni(bipy)(propionaldehyde)]. Analogous to the
reaction with benzaldehyde, we could study the same
reaction with propionaldehyde according to eq 2 (R )
C2H5, R′ ) H, L-L ) bipy). Here, addition of COD in a
10-fold excess to the complex solution was not possible
because it largely suppressed the reaction. Further-
more, it was necessary to use a higher excess of
propionaldehyde in order to follow the reaction to
completion. The obtained dependence of the observed
rate constant kobs on the concentration of propionalde-
hyde at different temperatures is again linear with an
intercept (Figure 3). Since an excess of COD could not
be used, the back reaction is not pseudo-first-order and
therefore no rate constants were calculated from the
intercept. The second-order rate constants for the
reaction of [Ni(bipy)(COD)] with propionaldehyde de-
rived from the slopes are shown in Table 2. The rate
constants are approximately a factor of 60 smaller than
that for the reaction with benzaldehyde, and the binding
our kinetic findings we know that the overall equilib-
rium constant of the reaction is close to 1: Koverall
KK1K2 ≈ 1. The equilibrium constant K must be small
and with K , 1 we obtain eq 7 for kobs, confirming the
kinetic data.
)
Kk1k2
k-1k-2
kobs
)
[benzaldehyde] +
[COD]
(
)
(
)
k2 + k-1
k2 + k-1
(7)
The back reaction was also studied, and its mecha-
nism is shown in Scheme 1. In this case premixed
[Ni(bipy)(benzaldehyde)] (D) is reacted with COD and
benzaldehyde is then substituted, forming the [Ni(bi-
py)(COD)] (A) complex. In both the back and forward
reaction the measured rate constants include the rate
constants of the different steps as shown in eq 7. This
means that the calculated activation parameters are
composite values for the mixture of different rate
constants and not for a single reaction step. The
associative contributions from k1 and k-2 must account
for the overall negative activation entropies.
Not included in our mechanism is the possibility of
solvent coordination according to eq 8.
(43) Dinjus, E.; Go¨rls, H.; Uhlig, E.; Walther, D.; Sieler, J .; Lindqvist,
O. J . Organomet. Chem. 1984, 276, 257.

102 Organometallics, Vol. 17, No. 1, 1998
Geyer et al.
F igu r e 4. Repetitive scan spectra recorded during the
reaction of propionaldehyde and carbon dioxide with
[Ni(bipy)(COD)]: propionaldehyde concentration ) 0.9 M,
[Ni(bipy)(COD)] ) 1 mM, carbon dioxide concentration )
102.5 mM, solvent ) THF, T ) 35 °C, ∆t1 ) 0.06 s, ∆t2 )
14 s.
F igu r e 3. Plot of observed rate constants kobs vs propi-
onaldehyde concentration at different temperatures: [Ni(bi-
py)(COD)] concentration ) 0.25 mM
Ta ble 2. Secon d -Or d er Ra te Con sta n ts for th e
Rea ction of [Ni(bip y)(COD)] w ith P r op ion a ld eh yd e
a n d [Ni(bip y)(p r op ion a ld eh yd e)] w ith COD As
Obta in ed fr om Eq 3 ([Ni(bip y)(COD)]
Ta ble 3. Secon d -Or d er Ra te Con sta n ts ka for th e
Rea ction of Ca r bon Dioxid e w ith P r op ion a ld eh yd e
a n d [Ni(bip y)(COD)] Accor d in g to Eq 1
Con cen tr a tion ) 0.25 m M)
[propionaldehyde]
([Ni(bip y)(COD)] con cen tr a tion ) 1.0 m M)
dependence
[COD] dependence
K
T/°C
slope, ka/M-1 s-1
slope, kb/M-1 s-1
()ka/kb)
[propionaldehyde]
dependence
[carbon dioxide]
dependence
15.0
25.0
35.0
0.71 ( 0.02
1.68 ( 0.06
3.3 ( 0.2
(0.56 ( 0.02) x 103 0.00126
(0.99 ( 0.05) x 103 0.0017
(1.72 ( 0.08) x 103 0.00194
39 ( 0.5
ka/10-3 M-1 s-1
ka/10-3 M-1 s-1
T/°C
(calculated from slope) (calculated from slope)
∆Hq (kJ /mol)
∆Sq (J /mol K)
55 ( 3
20.0
30.0
35.0
40.0
3.5 ( 0.3
5.5 ( 0.2
28 ( 4
56 ( 5
67 ( 5
-58 ( 9
-58 ( 2
11 ( 1.4
40 ( 5
-157 ( 19
of propionaldehyde must therefore be significantly
weaker since addition of COD largely suppressed the
reaction. The back reaction of COD with the complex
[Ni(bipy)(propionaldehyde)] gave rate constants (Table
2) that are approximately 16 times larger than the ones
obtained for the back reaction of the nickel benzalde-
hyde complex. The overall equilibrium constant K is a
factor of 1000 smaller compared to the benzaldehyde
reaction with [Ni(bipy)(COD)]. The activation param-
eters obtained from the temperature dependence of
these reactions (Table 2) are similar to those obtained
for the reaction of [Ni(bipy)(COD)] with benzaldehyde,
but again it is important to note that these activation
parameters do not describe a single-step reaction but
are composite values for the combination of several rate
constants as discussed above.
Introduction of carbon dioxide into the system allowed
us to observe first the formation of [Ni(bipy)(propional-
dehyde)], followed by a slower reaction of this complex
with carbon dioxide. Spectral changes occurring during
this reaction are shown in Figure 4. The final product
is characterized by an absorbance maximum at 475 nm,
whereas the absorbance band of [Ni(bipy)(COD)] at 575
nm disappears during the reaction. The measured rate
constant for the first step is the same as that found for
the reaction of [Ni(bipy)(COD)] with propionaldehyde
in the absence of carbon dioxide as described above.
∆Hq (kJ /mol)
∆Sq (J /mol K)
43 ( 6
-129 ( 19
bilizing the complex by addition of COD was not feasible
because this was found to suppress the reaction with
carbon dioxide completely. However, it was possible to
obtain good kinetic data for the reaction of [Ni(bipy)-
(COD)] with propionaldehyde and carbon dioxide ac-
cording to eq 1. Propionaldehyde and carbon dioxide
were always kept in excess over [Ni(bipy)(COD)] to
ensure pseudo-first-order conditions. The dependence
of kobs on both (a) carbon dioxide and (b) propionalde-
hyde concentration at different temperatures showed
linear dependence (data are listed in Table 3), and the
following formal third-order rate law was obtained
(abbreviations for nickel complexes according to Scheme
2):
-d[A]/dt ) k[A][carbon dioxide][propionaldehyde]
(9)
Even though the temperature dependence was mea-
sured, decomposition of [Ni(bipy)(COD)] only allowed
the use of a narrow temperature range and the errors
of the individual measurements were larger compared
to those of the reactions with aldehydes alone. The
calculated activation parameters therefore do not rep-
resent the true values but indicate the trend in these
parameters. The activation parameters were calculated
as ∆Hq ) 43 ( 6 kJ /mol and ∆Sq ) -129 ( 19 J /(mol
K) (carbon dioxide concentration dependence) and ∆Hq
) 41 ( 6 kJ /mol and ∆Sq ) -154 ( 20 J /(mol K)
(propionaldehyde concentration dependence). The ac-
tivation parameters for the two data sets are within the
The kinetic studies of the slower reaction, the oxida-
tive coupling of carbon dioxide to the [Ni(bipy)(propi-
onaldehyde)] complex, proved difficult. The reaction
with carbon dioxide is quite slow, and partial decom-
position of [Ni(bipy)(COD)] under these conditions was
observed. This slow decomposition could be observed
in solution without any other reactants present. Sta-

Reactions of Aldehydes with CO2 at Ni(0) Centers
Organometallics, Vol. 17, No. 1, 1998 103
Sch em e 2
F igu r e 5. Possible transition state during the reaction of
the complex [Ni(bipy)(COD)(propionaldehyde)] with carbon
dioxide.
py)(propionaldehyde)] and a four-center transition state
is formed as shown by Figure 5. Additional support for
the postulated mechanism comes from a theoretical
study on formation of the oxanickelacyclopentene com-
plex from nickel(0), carbon dioxide, and alkyne. Here
too it has been postulated that alkyne binds to the
nickel(0) complex prior to reaction with carbon diox-
ide.47,48
experimental error of each other and the large negative
activation entropy confirms the oxidative coupling step;
i. e., bond formation and charge creation in the transi-
tion state. Unfortunately, high-pressure measurements
for activation volumes were not possible because of the
extreme sensitivity of the solutions toward dioxygen.
The kinetic measurements confirmed the assumption
that no direct reaction of carbon dioxide with the metal
center occurs. Instead, the aldehyde is activated and
then a reaction with carbon dioxide can take place. The
reaction mechanism shown in Scheme 2 is in accordance
with the kinetic findings. The first reaction in Scheme
2 is again a fast preequilibrium which summarizes the
overall reaction in Scheme 1 using propionaldehyde
instead of benzaldehyde. The reaction with carbon
dioxide is irreversible, leading to the final product, the
five-membered cyclic nickel complex. Since the equi-
librium shown in Scheme 2 lies to the left, the rate
Con clu sion s
Even though the nickel(0) complexes used in this
study were very sensitive toward traces of dioxygen we
were able to successfully perform kinetic measurements.
The complex formation of the aldehydes benzaldehyde
and propionaldehyde with [Ni(bipy)(COD)] was inves-
tigated. It was found that the overall equilibrium
constant for the reaction with propionaldehyde is ap-
proximately a factor of 1000 smaller compared to
benzaldehyde. Our kinetic studies of the reaction of
propionaldehyde and carbon dioxide with [Ni(bipy-
)(COD)] clearly demonstrate that carbon dioxide is not
activated by binding with the Ni(0) center but instead
reacts with the activated propionaldehyde of the com-
plex [Ni(bipy)(propionaldehyde)].
Ack n ow led gm en t. The authors gratefully acknowl-
edge financial support from the Bundesministerium fu¨r
Bildung, Wissenschaft, Forschung und Technologie
(BMBF) and the Deutsche Forschungsgemeinschaft.
Furthermore, we thank Prof. Rudi van Eldik for helpful
discussions and his generous support of this work.
equation for this reaction reduces to eq 10 for kobs
.
k2K
kobs
)
[CO2][propionaldehyde]
(10)
[COD]
Inserting K and the values for the concentrations
allows us to calculate the rate constant k2 ≈ 0.1 M-1
s-1. This value does not differ significantly for carbon
dioxide insertion reactions, and more important, the
activation parameters for carbon dioxide insertions into
metal-carbon, -hydrogen, or -oxygen bonds are quite
similar.4,44-46 Presumeably these reactions go through
a very similar transition state. We assume that here
the carbon dioxide molecule approaches the [Ni(bi-
OM970584M
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1984, 1244.
(46) Sullivan, B. P.; Meyer, T. J . Organometallics 1986, 5, 1500.
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1993, 66, 3289.
(48) Sakaki, S.; Mine, K.; Hamad, T.; Arai, T. Bull. Chem. Soc. J pn.
1995, 68, 1873.