Kinetics of the Diels-Alder reaction between C60 and acenes

Article abstract of DOI:10.1016/j.cplett.2004.09.005

The kinetics of the Diels-Alder reactions between C₆₀ and the linear acenes anthracene and tetracene are studied in toluene, in the temperature range 22-63 °C. It is observed that tetracene reacts much more readily with C₆₀ than does

Full text of DOI:10.1016/j.cplett.2004.09.005

Chemical Physics Letters 397 (2004) 402–407
www.elsevier.com/locate/cplett
Kinetics of the Diels–Alder reaction between C60 and acenes
*
´
Ginka H. Sarova, Mario N. Berberan-Santos
´ ´ ´
Centro de Quımica-Fısica Molecular, Instituto Superior Tecnico, 1049-001 Lisboa, Portugal
Received 27 July 2004; in final form 27 July 2004
Abstract
The kinetics of the Diels–Alder reactions between C60 and the linear acenes anthracene and tetracene are studied in toluene, in the
temperature range 22–63 ꢀC. It is observed that tetracene reacts much more readily with C60 than does anthracene. The different
reactivities of anthracene and tetracene towards C60 correlate with the respective aromaticity loss upon cycloaddition, as previously
predicted theoretically. The two monoadducts also display different kinetics as regards the dissociation back to the reagents. In the
studied temperature range, tetracene monoadduct decomposition by retro-Diels–Alder reaction is negligible, while the anthracene
monoadduct is unstable above room temperature.
ꢁ 2004 Elsevier B.V. All rights reserved.
1. Introduction
and aromaticity loss occur upon addition to a 6–6 bond
[6]. As for the diene, an aromaticity gain (e.g., quinodi-
methanes [6]), no aromaticity change (e.g., cyclopentad-
iene), or an aromaticity loss (e.g., anthracene and other
acenes [7]) can occur. The role of aromaticity loss in the
relative reactivity of linear acenes (naphthalene, anthra-
cene, tetracene, etc.) in Diels–Alder reactions was re-
cently studied theoretically with ab initio and DFT
quantum-chemical methods [7,8].
In this work, the kinetics of the Diels–Alder reactions
of C60 with anthracene and tetracene are investigated
experimentally, and the results compared to theoretical
predictions.
The electron-deficient character of C60 determines its
high reactivity as a dienophile in Diels–Alder ([4 + 2]
cycloaddition) reactions [1–3]. For this reason, mono-
and multiple C60 functionalization by the Diels–Alder
reaction is a common synthetic procedure in fullerene
chemistry [1–3]. Although numerous derivatives of C60
have been obtained by Diels–Alder cycloaddition
[1–3], almost no experimental kinetic data is available
for this important fullerene reaction. The only exception
is the reaction of C60 with cyclopentadiene, for which
the kinetics of both the direct [4] and the reverse
Diels–Alder [5] reactions were studied. The energetics
of the Diels–Alder cycloaddition of quinodimethanes
to C60 was recently addressed theoretically [6]. A factor
that can determine both the rates and the thermodynam-
ics of a Diels–Alder reaction is the overall gain or loss of
aromaticity when going from the reactants to the addi-
tion product [6]. These aromaticity changes may occur
in the dienophile and/or in the diene. In the case of
C60, both a strain release (owing to pyramidalization)
2. Experimental
C60 (99.5%) was purchased from Sigma–Aldrich and
used as received. Anthracene (>99.0%) and tetracene
(>97.0%) were from Fluka. Toluene (Merck) was of
spectroscopic grade. Absorption spectra were recorded
on a Shimadzu UV 3101PC UV–VIS–NIR spectropho-
tometer using 1.0 cm path length cells, and a thermo-
stated cell holder. The temperature was controlled to
within 0.5 ꢀC, and the reactions carried out in the dark.
*
Corresponding author. Fax: +351 218464455.
E-mail address: berberan@ist.utl.pt (M.N. Berberan-Santos).
0009-2614/$ - see front matter ꢁ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.09.005

G.H. Sarova, M.N. Berberan-Santos / Chemical Physics Letters 397 (2004) 402–407
403
All spectra were run against toluene as a blank. In order
where
¼ eFAl½FAꢁ
and the parameter b, with b 6 1, is
to avoid a competing Diels–Alder reaction (with singlet
oxygen as the dienophile) [9], the tetracene reactions
were carried out in sealed and degassed cells (4 freeze–
pump–thaw cycles of the C60 solution in toluene, prior
to mixing with tetracene under vacuum). No precaution
of this kind was necessary for the aerated anthracene
solutions, that for the concentration used were stable
at all temperatures in the absence of fullerene.
A
ð10Þ
1
e
a
Kð½FAꢁ ꢀ ½FAꢁ Þ
e
0
b ¼
¼
:
ð11Þ
1 þ a 1 þ Kð½Aꢁ þ ½F ꢁ þ ½FAꢁ ꢀ ½FAꢁ Þ
e
e
e
0
From a fit of Eq. (9) to experimental data, it is in
principle possible to obtain the parameters C, b and
A . Then [FA]e is computed from A provided the
1
1
absorption coefficient eFA is known. Next, the equilib-
rium constant is obtained from
3. Method of data analysis
½FAꢁ
e
K ¼
:
ð12Þ
ð½Aꢁ ꢀ ½FAꢁ Þð½F ꢁ ꢀ ½FAꢁ Þ
The Diels–Alder reaction between acenes and C60 is
described by a concerted mechanism [10],
0
e
0
e
On the other hand, the parameter C allows the calcu-
lation of k1,
k1
F þ A ¢ FA;
ð1Þ
k
ꢀ1
C
k1 ¼
ð13Þ
1
K
where F is the fullerene, A is the acene, and FA is the
monoadduct. The rate equation for the monoadduct is
½Aꢁ þ ½F ꢁ þ
and k is finally computed from K and k1. The param-
e
e
ꢀ1
d½FAꢁ
eter b is redundant, and not always obtainable with sat-
isfactory precision, as it can be much smaller than unity.
Furthermore, it is correlated with A , and it is prefera-
ble to write explicitly the dependence of parameter b on
[FA]e and perform the fit with only two parameters, C
and [FA]e. In the two systems studied, particular cases
of Eq. (9) apply, as will be discussed.
¼ k1½Aꢁ½F ꢁ ꢀ k ½FAꢁ
ð2Þ
ꢀ1
dt
1
If the deviation to equilibrium is defined as x =
[FA]e ꢀ [FA], Eq. (2) becomes
dx
dt
¼ ꢀCx ꢀ k1x2
ð3Þ
with
C ¼ ½k1ð½Aꢁ þ ½F ꢁ Þ þ kꢀ1ꢁ;
ð4Þ
e
e
4. Results
C being the inverse of the relaxation time of the system
[11]. Integration of Eq. (3) yields
4.1. Reaction with anthracene
x0
x ¼
;
ð5Þ
ð6Þ
ð7Þ
ð1 þ aÞeCt ꢀ a
where x0 = x(0) and
The reaction of C60 with anthracene in toluene was
studied in the temperature range 22–63 ꢀC. The initial
concentration of anthracene was 1.1 · 10ꢀ2 M while
the C60 concentration was kept fixed at 2.1 · 10ꢀ4 M.
After mixing C60 and anthracene, four prominent
absorption features appear at 436 nm (sharp band),
486 nm (broad band), 640 nm (shoulder) and 706 nm
(weak band), Fig. 1, their intensity progressively increas-
ing with time. These absorption bands are characteristic
of the mono-adduct of C60 and anthracene [12–14],
formed upon [4 + 2] cycloaddition, Scheme 1.
The addition occurs at one of the 30 equivalent 6–6
bonds of C60, and the reaction is regioselective from
the point of view of anthracene, taking place at the cen-
tral ring, across the 9, 10 positions. The existence of isos-
bestic points at 586 and 612 nm, see Fig. 1, confirms that
multiple addition [15] is negligible for the experimental
conditions and observation time window used, in spite
of the relatively large excess of anthracene. Identical re-
sults were obtained for all the other temperatures.
Since the acene was always in large excess in the
experiments, it results from Eq. (11) that
k1x0
C
Kð½FAꢁ ꢀ ½FAꢁ Þ
;
e
0
a ¼
¼
1 þ Kð½Aꢁ þ ½F ꢁ Þ
where K is the equilibrium constant,
e
e
k1
K ¼
:
k
ꢀ1
For small x0, a ꢂ 1 and Eq. (5) reduces to the well-
known relaxation equation [11]. In the described exper-
iments, however, [FA]0 = 0, and a is not always small.
If the absorbance A of the system is measured at a
wavelength, where only the addition product FA ab-
sorbs, then
A ¼ eFAlð½FAꢁ ꢀ xÞ;
ð8Þ
e
where eFA is the absorption coefficient and l is the opti-
cal pathlength. Finally,
1 ꢀ eꢀCt
AðtÞ ¼ A
;
ð9Þ
1 1 ꢀ beꢀCt

404
G.H. Sarova, M.N. Berberan-Santos / Chemical Physics Letters 397 (2004) 402–407
0.6
0.4
0.2
0
420
470
520
570
620
670
720
wavelength/nm
Fig. 1. Electronic absorption spectra of the reaction mixture C60–anthracene as a function of time, for a temperature of 63 ꢀC. The absorption
spectrum with the lowest 436 nm absorption corresponds to the starting C60–anthracene mixture. The remaining spectra correspond to reaction times
of 60, 130, 185, 295, 482, 1523 and 2343 min.
expected for a Diels–Alder reaction. Values of K, k1
and k were estimated as described, using the deter-
ꢀ1
mined A and C, and an absorption coefficient eFA for
1
the monoadduct of 324 Mꢀ1 cmꢀ1 at 706 nm [14]. The
results are summarized in Table 1.
4.2. Reaction with tetracene
Scheme 1.
The reaction of C60 with tetracene in toluene was
studied in the temperature range 25–59 ꢀC. Owing to
the faster rates for this reaction, and to the low solubility
of tetracene in toluene, tetracene was not in large excess
in the experiments, as was the case with anthracene. In-
stead, a molar ratio C60:tetracene of 10:13 was used. The
initial concentration of tetracene was 2.3 · 10ꢀ4 M while
the C60 concentration was kept fixed at 1.8 · 10ꢀ4 M.
After mixing C60 and tetracene, two prominent absorp-
tion features appear at 640 nm (shoulder) and 706 nm
(weak band), Fig. 3, their intensity progressively increas-
ing with time. The absorption features observed with
anthracene at shorter wavelengths are in this case
masked by the intrinsic absorption of tetracene, whose
onset occurs at approximately 500 nm. Nevertheless,
K½FAꢁ
K½F ꢁ
½F ꢁ
e
0
0
b ¼
<
<
¼ 0:02:
ð14Þ
ð15Þ
1 þ K½Aꢁ
Hence, Eq. (9) reduces to
1 þ K½Aꢁ
½Aꢁ
0
0
0
AðtÞ ¼ A ð1 ꢀ eꢀCt
Þ
1
The fitted values of C and A for several tempera-
1
tures are given in Table 1 (see also Fig. 2). It is seen from
Table 1 that there is a progressive decrease of A from
1
its maximum possible value (corresponding to an irre-
versible reaction) as the temperature is increased. This
shows that the reversibility of the reaction is non-
negligible for temperatures higher than ambient. The de-
crease of the computed equilibrium constant with tem-
perature shows that the reaction is exothermic, as
Table 1
Fitted parameters and calculated rate constants and equilibrium constant for the reaction between C60 and anthracene
T (ꢀC)
A
C (10ꢀ6 sꢀ1
)
K (Mꢀ1
)
k1 (10ꢀ4
M
ꢀ1 sꢀ1
)
k
ꢀ1
(10ꢀ6 sꢀ1
)
1
22
39
49
57
63
a
0.068a
0.066
0.062
0.057
0.053
1.8
8.9
20
14300a
3000
950
1.6
7.8
16
0.013a
0.26
1.7
40
53
480
330
31
38
6.4
12
Extrapolated.

G.H. Sarova, M.N. Berberan-Santos / Chemical Physics Letters 397 (2004) 402–407
405
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0
500
1000
1500
2000
2500
3000
3500
4000
4500
time/min
Fig. 2. Absorbance at 706 nm of the reaction mixture C60–anthracene as a function of time, for a temperature of 49 ꢀC. The solid line is the fit with
Eq. (15) (for the fitted parameters, see Table 1).
3
0.6
2.5
2
1.5
0
1
480
580
680
780
0.5
0
380
430
480
530
580
630
680
730
780
Wavelength/nm
Fig. 3. Electronic absorption spectra of the reaction mixture C60–tetracene as a function of time, for a temperature of 59 ꢀC. The absorption
spectrum with the lowest 436 nm absorption corresponds to the pure C60 solution. The other spectra correspond to the reaction mixture for reaction
times of 0, 36, 84, 139, 191, 267 and 408 min. Inset: pure C60 solution and reaction mixture for 0, 84, 139 and 408 min.
the 640 and 706 nm absorption bands suffice to identify
the reaction product as the mono-adduct of C60 and
tetracene, formed upon [4 + 2] cycloaddition, Scheme 2.
The addition occurs at one of the 30 equivalent 6–6
bonds of C60, and the reaction is regioselective from
the point of view of tetracene, taking place at one of
the inner rings. It is expected that, in the long wave-
length range, this compound should display an absorp-
tion spectrum identical to that of the anthracene
monoadduct. The existence of isosbestic points (as ob-
served for the anthracene mixture) at 409, 436, 586
and 612 nm, Fig. 3, confirms that multiple addition is
negligible for the experimental conditions used. Identical
results were obtained for all the other temperatures.
Scheme 2.

406
G.H. Sarova, M.N. Berberan-Santos / Chemical Physics Letters 397 (2004) 402–407
Table 2
Fitted parameter and calculated rate constant for the reaction between
C60 and tetracene
are given in Table 3. In the experimental temperature
range the reaction of C60 with anthracene is reversible,
while the reaction of C60 with tetracene is essentially
irreversible. This results at least in part from the fact
that the activation energy of the anthracene reaction is
ca. 10 kJ molꢀ1 higher than that of tetracene. Mainly
for this reason, the second order rate constant for the di-
rect reaction at room temperature is two orders of mag-
nitude higher for tetracene, 6.5 · 10ꢀ2 vs. 2.9 · 10ꢀ4
T (ꢀC)
C (10ꢀ6 sꢀ1
)
k1 (10ꢀ2 ꢀ1 sꢀ1
M )
25
33
40
46
54
59
2.0
3.9
6.9
11
3.1
9.6
16
26
32
14
30
47
M
ꢀ1 sꢀ1 for anthracene. The observed activation energy
difference of 10 kJ molꢀ1 is in close agreement with the
results of quantum-chemical calculations for two
Diels–Alder reactions involving these acenes: The com-
puted differences are 10 and 11 kJ molꢀ1 with ethene
[8], and acetylene [7] as the dienophile, respectively. The-
oretical calculations also predict a more negative reac-
tion enthalpy for the tetracene reaction [7,8], which
may also correlate with a higher activation energy for
the reverse reaction.
Unlike the situation with anthracene, it is observed
that the long time limit of the absorbance,A , is essen-
1
tially temperature independent and coincident with the
value expected for complete conversion of the fullerene
(0.058). The reaction is therefore irreversible at all tem-
peratures. In such a case the parameter b in Eq. (8)
becomes
½F ꢁ
The available activation and thermodynamic param-
eters for the reactions of C60 with dienes are collected in
Table 4. With respect to the direct reaction, it is seen
that the activation enthalpy is smallest for the cyclopent-
adiene reaction, where no aromaticity reduction occurs.
0
b ¼
ð16Þ
½Aꢁ
0
and is no longer a fitting parameter, since its value is
known. The data were fitted with Eq. (9) (with b fixed
at the value given by Eq. (16)). The values of C for sev-
eral temperatures are given in Table 2 (see also Fig. 4).
Values of k1 were computed from C, and are also pre-
sented in Table 2.
Table 3
Arrhenius parameters for the reactions of C60 with anthracene and
tetracene
Diels–Alder
Reverse Diels–Alder
log(A/Mꢀ1 sꢀ1
)
Ea
(kJ molꢀ1
log(A/sꢀ1
)
Ea
(kJ molꢀ1
)
5. Discussion and conclusions
)
Anthracene 6.8
Tetracene 7.4
59
49
16.9
–
140
–
The activation parameters of the reactions studied,
obtained from the Arrhenius plots of the rate constants,
0.06
0.05
0.04
0.03
0.02
0.01
0
0
500
1000
1500
2000
2500
3000
3500
4000
time/min
Fig. 4. Absorbance at 706 nm of the reaction mixture C60–tetracene as a function of time, for a temperature of 54 ꢀC. The solid line is the fit with Eq.
(9) (for the fitted parameter, see Table 2).

G.H. Sarova, M.N. Berberan-Santos / Chemical Physics Letters 397 (2004) 402–407
407
Table 4
Activation and thermodynamic parameters (1 M, 298 K) for the Diels–Alder reaction of C60 with several dienes
Diene
Direct Diels–Alder
Reverse Diels–Alder
Reaction
Reference
DàH
DàS
DàH
DàS
DàH
DàS
(kJ molꢀ1
)
(J Kꢀ1 molꢀ1
)
(kJ molꢀ1
)
(J Kꢀ1 molꢀ1
)
(kJ molꢀ1
)
(J Kꢀ1 molꢀ1
)
Anthracene
57
–
ꢀ123
–
138
–
71
–
ꢀ81
ꢀ96
–
ꢀ195
ꢀ256
–
This work
[16]
9,10-Dimethylanthracene
Tetracene
Cyclopentadiene
47
27
ꢀ111
ꢀ161
–
109
–
This work
[4,5]
ꢀ12
ꢀ82
ꢀ149
This aromaticity reduction is larger for anthracene than
for tetracene, and consequently the reaction with this
last compound has the lowest enthalpy of activation of
the two. It is expected that the activation enthalpy will
decrease smoothly along the acene series, being highest
for benzene and approaching the cyclohexadiene and
cyclopentadiene values when the number of rings is very
large. The activation entropy is always negative and
large, as expected for a Diels–Alder reaction [17], and
does not vary significantly with the diene. With respect
to the reverse reaction, the activation enthalpies for
the only two reactions available are high, and surpris-
ingly the highest value is that of anthracene. The two
activation entropies work however in the opposite direc-
tion, and the t1/2 at 90 ꢀC are 39 and 20 min (calculated),
for cyclopentadiene and anthracene adducts, respec-
tively. The origin of the relatively large positive value
of the activation entropy for the anthracene reaction re-
mains to be understood. The reaction enthalpies (Table
4) are large and negative, as expected for Diels–Alder
reactions [17]. The reaction entropies are also large
and negative. The highest value, observed for 9,10-
dimethylanthracene, may be directly related with a par-
ticularly facile retro-Diels–Alder reaction for this
compound [16]. Further studies are nevertheless neces-
sary to better understand all these aspects.
Acknowledgements
ˆ
This work was supported by Fundac¸a˜o para a Cien-
cia e a Tecnologia (FCT, Portugal) within Project POC-
TI/34400/QUI/2000. G.S. thanks the FCT for a post-
doctoral grant.
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