A biphasic displacement of [60]fullerene from fac-(dihapto-[60]fullerene) (dihapto-1,10-phenanthroline) tricarbonyl molybdenum (0)

Article abstract of DOI:10.1016/j.jorganchem.2005.04.018

The Lewis bases (=L) triphenylphosphine (PPh₃) and tricyclohexyl phosphine (P(Cy)₃) displace [60]fullerene (C₆₀) from the complex fac-(η²-C₆₀)(η²-phen)Mo(CO) ₃ (phen = 1,10-phenanthroline). The progress of the reactions was followed observing the decrease of the absorbance values at 440 nm and by monitoring the stretching carbonyl region from 1700 to 2100 cm^-1. The plots of absorbance vs. time were biexponential, indicative of a biphasic behavior, for reactions under flooding conditions where [L] ? [fac-(η²-C₆₀)(η²-phen)Mo(CO) ₃]. The plot of absorbance vs. time consisted of two consecutive segments: the first segment of the plot was a decrease of absorbance with time followed by a second segment where the absorbance increased with time. The first segment of the biphasic plot was ascribed to the solvent-assisted displacement of C₆₀ from fac-(η²-C₆₀)(η²- phen)Mo(CO)₃ and the second segment to decomposition of the complex fac-(η¹-L)(η²-phen)Mo(CO)₃ produced in the first of the two consecutive reactions. The rate constant values corresponding to the first segment of the biphasic plot are independent of the chemical nature of L, the molar concentration of L, and the molar concentration of C₆₀ but dependent on the chemical nature of the solvent.

Full text of DOI:10.1016/j.jorganchem.2005.04.018

Journal of Organometallic Chemistry 690 (2005) 3366–3372
www.elsevier.com/locate/jorganchem
A biphasic displacement of [60]fullerene from
fac-(dihapto-[60]fullerene)(dihapto-1,10-phenanthroline)
tricarbonyl molybdenum (0)
1
Yessenia Ocasio-Delgado, Jonathan de Jesu´s-Segarra , Jose E. Cortes-Figueroa
*
´
´
Organometallic Chemistry Research Laboratory, Department of Chemistry, University of Puerto Rico,
P.O. Box 9019, Mayagu¨ez 00681-9019, Puerto Rico
Received 7 February 2005; revised 7 April 2005; accepted 7 April 2005
Available online 2 June 2005
Abstract
The Lewis bases (=L) triphenylphosphine (PPh3) and tricyclohexyl phosphine (P(Cy)3) displace [60]fullerene (C60) from the com-
plex fac-(g2-C60)(g2-phen)Mo(CO)3 (phen = 1,10-phenanthroline). The progress of the reactions was followed observing the
decrease of the absorbance values at 440 nm and by monitoring the stretching carbonyl region from 1700 to 2100 cmꢀ1. The plots
of absorbance vs. time were biexponential, indicative of a biphasic behavior, for reactions under flooding conditions where
[L] ꢁ [fac-(g2-C60)(g2-phen)Mo(CO)3]. The plot of absorbance vs. time consisted of two consecutive segments: the first segment
of the plot was a decrease of absorbance with time followed by a second segment where the absorbance increased with time.
The first segment of the biphasic plot was ascribed to the solvent-assisted displacement of C60 from fac-(g2-C60)(g2-phen)Mo(CO)3
and the second segment to decomposition of the complex fac-(g1-L)(g2-phen)Mo(CO)3 produced in the first of the two consecutive
reactions. The rate constant values corresponding to the first segment of the biphasic plot are independent of the chemical nature of
L, the molar concentration of L, and the molar concentration of C60 but dependent on the chemical nature of the solvent.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Biphasic; [60]Fullerene; Kinetics; Metal carbonyls; Transition metals
1. Introduction
mol [1,2] and (iii) [60]fullerene can be a labilizing ligand
[4,5]. The p-acceptor capacity of [60]fullerene can be ex-
plained on the basis that [60]fullerene has a strong ten-
dency to accept electrons [6–12]. Six waves at
potentials ranging from ꢀ0.50 to ꢀ3.26 V (vs. ferro-
cene/ferrocinium) have been reported [12]. In fact, due
to this high electron affinity, its chemical and physical
properties are similar to those of electron-deficient ole-
fins [13,14]. The assertion that [60]fullerene can act as
a r-Lewis base is supported by the observation that it
can compete with Lewis bases such as triphenylphos-
phine, tricyclohexylphosphine, and triethyl phosphite
for coordination sites in electronically deficient transi-
tion metal carbonyl complexes [1–3]. The claim that
[60]fullerene can be a labilizing ligand comes from the
Studies of the ligand exchange reactions on the com-
plexes (g2-C60)W(CO)5 [1], fac-(g2-C60)(g2-phen)W-
(CO)3 [2], and mer-(g2-C60)(g2-dppe)W(CO)3 [3],
(dppe = 1,2-bis(diphenylphosphino)ethane,
phen = 1,
10-phenanthroline) have shown that (i) [60]fullerene is
a good p-acceptor and a good r-Lewis base, (ii) the
W–C60 bond enthalpy is in the vicinity of 105–109 kJ/
*
Corresponding author. Tel.: +7874644231; fax: +7872653849.
E-mail address: jose.cortes.figueroa@usa.net (J.E. Cortes-
´
Figueroa).
1
Participant of the University of Puerto Rico at Mayaguez
¨
Undergraduate Research Program.
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.04.018

Y. Ocasio-Delgado et al. / Journal of Organometallic Chemistry 690 (2005) 3366–3372
3367
observation that piperidine (pip) displaces dppe from
mer-(g2-C60)(g2-dppe)W(CO)3 to produce mer-(g2-C60)
(g1-pip)2W(CO)3 exclusively [3]. On the other hand,
the observation that PPh3 displaces C60 from fac-(g2-
C60)(g2-phen)W(CO)3 is consistent with the strong
cis-labilizing nature of phenanthroline [5,15]. We now
report the results of the mechanistic study of the C60
displacement from fac-(g2-C60)(g2-phen)Mo(CO)3 by
triphenylphosphine and tricyclohexyl phosphine in vari-
ous solvents.
contents of the chromatography column in 10 mL of
dichloromethane followed by suction filtration. Dichlo-
romethane was nitrogen-purged to yield ꢂ0.017 g (ca.
25% yield) of a yellowish-brown solid identified as
fac-(g2-C60)(g2-phen)Mo(CO)3 by its infrared spectrum
in the carbonyl stretching region and elemental analysis
(mCO) dichloromethane (mCO, cmꢀ1): 1971 (s),1896
(w),1829 (s). Anal. Calc. for C75H8O3N2Mo: C, 83.34;
H, 0.75. Found: C, 82.70; H, 1.56%.
2.3. Kinetics experiments and data analysis
2. Experimental
Kinetics experiments were carried out under nitrogen.
The absorbance values at time infinity (A ) for the sec-
1
2.1. General
ond segment of the biphasic plot were determined by the
Kezdy–Swibourne method [17–20]. The rate constant
values for sets of consecutive rate constants were deter-
mined using a graphing calculator as described in the
Journal of Chemical Education [21–23]. For extremely
slow reactions, the second segment of the biphasic plot
was ignored and the rate constant values for the first
segment were estimated using a time-lag method [17–
21]. To test the validity of the time-lag approximation,
the rate constant values obtained using this approxima-
tion were compared with the rate constant values ob-
tained when both consecutive rate constants were
analyzed rigorously as described in the Journal of
Chemical Education. In the cases tested, the rate con-
stant values remained within 5% of their expected
values.
Infrared spectra were obtained on a Bruker Vector 22
Fourier transform infrared spectrophotometer and UV–
Vis spectra on a Beckman Coulter DU 640 UV/Vis spec-
trophotometer. A Caron 2050W heating and refrigerat-
ing circulator and a K/J Fluke digital thermometer
equipped with a bead thermocouple were used for tem-
perature control. Midwest Microlab, Indiapolis, IN,
performed elemental analyses.
2.2. Preparation and purification of materials
Bromobenzene (Fisher) and chlorobenzene (Mal-
linckrodt) were dried over phosphorous pentoxide and
fractionally distilled under nitrogen. Benzene (Aldrich)
and toluene (Aldrich) were distilled over sodium and
fractionally distilled under nitrogen. Triphenylphos-
phine (Aldrich) was recrystallized from absolute ethanol
and dried under vacuum conditions. Tricyclohexyl phos-
phine (Strem) was used as purchased without further
purification.
The complex (g2-phen)Mo(CO)4 was prepared
according to a published procedure [16]. The complex
fac-(g2-C60)(g2-phen)Mo(CO)3 was prepared photo-
chemically from (g2-phen)Mo(CO)4 and C60 (Aldrich)
using a medium pressure mercury arc lamp. In a 25 mL
round bottom flask equipped with a magnetic stirring
bar, a condenser, and a nitrogen inlet 0.02428 g
(0.06255 mmol) of (g2-phen)Mo(CO)4 and 0.04358 g
(0.06049 mmol) of C60 were dissolved in 10 mL of nitro-
gen-purged and dried toluene and irradiated under nitro-
gen during ꢂ1.5 h. After the reaction was completed,
toluene was vacuum-distilled or nitrogen-purged from
the reaction mixture. The resulting brownish solid was
then dissolved in ꢂ10 mL of carbon disulfide (CS2). Thin
layer chromatography analysis showed two components.
The fractions were separated by column chromatogra-
phy using a 15 cm long (1 cm diameter) column packed
2.4. Data analysis
Data of the kinetic experiments were analyzed using
linear least-squares and a non-linear regression com-
puter programs. Error limits, given in parentheses as
the uncertainties of the last digit(s) of the cited value,
are within one standard deviation.
3. Results
The reactions of fac-(g2-C60)(g2-phen)Mo(CO)3 with
the Lewis bases (L) triphenylphosphine and tricyclohexyl
phosphine in various solvents producing fac-(g1-L)(g2-
phen)Mo(CO)3 are biphasic. The reactions were studied
at 74.4, 64.4, and 54.4 ꢁC under flooding conditions
where the concentration of L was at least 105 times the
concentration of fac-(g2-C60)(g2-phen)Mo(CO)3 (ca.
10ꢀ6 M). The rate of disappearance of fac-(g2-C60)(g2-
phen)Mo(CO)3 was monitored by observing the decrease
of the absorbance values at 440 nm. The progress was
also followed by monitoring the stretching carbonyl re-
gion (mCO) from 1700 to 2100 cmꢀ1. The results suggest
formation of fac-(g1-L)(g2-phen)Mo(CO)3 followed by
decomposition (Eq. (2)). Fig. 1 shows the C–O stretching
˚
with 62 grade, 60–2000 mesh, 150 A silica gel (Aldrich).
The first fraction, unreacted C60, was eluted using CS2.
The remaining fraction was obtained by dissolving the

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Y. Ocasio-Delgado et al. / Journal of Organometallic Chemistry 690 (2005) 3366–3372
1893
1971
1827
2000
1950
1900
1850
1800
Wavenumber cm-1
Fig. 1. Infrared spectrum in the stretching carbonyl region of fac-(g2-C60)(g2-phen)Mo(CO)3 in chlorobenzene.
spectrum of fac-(g2-C60)(g2-phen)Mo(CO)3 in chloro-
benzene. The corresponding spectrum of the reaction
product (for L = PPh3) in chlorobenzene is given in
Fig. 2
Fig. 3 shows the m(CO) spectra of a reaction mixture
([PPh3] = 0.1001 M, T ꢂ 80 ꢁC in chlorobenzene) at var-
ious stages of the reaction progress. The rapid decrease
of the bandsÕ intensities at 1971, 1893, and 1827 cmꢀ1
corresponds to the disappearance of the parent complex
fac-(g2-C60)(g2-phen)Mo(CO)3. The appearance and
growth of the bandsÕ intensities at 1917, 1826, and
1795 cmꢀ1 corresponds to the formation of the complex
fac-(g1-PPh3) (g2-phen)Mo(CO)3. At the later stages of
the reaction, the productÕs bands disappear suggesting
decomposition of fac-(g1-PPh3)(g2-phen)Mo(CO)3.
This thermal decomposition was also reported in the
complex cis-(g1-PPh3)2Mo(CO)4 producing (g1-PPh3)-
Mo(CO)5 [24].
fac-ðg2-C60Þðg2-phenÞMoðCOÞ þ L
3
kobsd
! fac-ðg1-LÞðg2-phenÞMoðCOÞ þ C60
3
ð2Þ
fac-ðg1-LÞðg2-phenÞMoðCOÞ
3
k0obsd
! decomposition products
This decomposition was also observed during the
purification of fac-(g2-C60)(g2-phen)Mo(CO)3 by col-
umn chromatography. Once the excess of C60 was eluted
using CS2, the remaining fraction, fac-(g2C60)(g2-phen)-
Mo(CO)3, decomposed to produce C60 and cis-(phen)-
Mo(CO)4. It also seems that the complex fac-(g1-
L)(g2-phen)Mo(CO)3 undergoes thermal decomposition
to produce enough CO which in turns displaces L from
another molecule of fac-(g1-L)(g2-phen)Mo(CO)3.
A plot of absorbance vs. time for the reaction
([PPh3] = 0.3010 M in chlorobenzene at 74.4 ꢁC) is given
in Fig. 4. The continuous trace in Fig. 3 corresponds to
the graph of the equation
At ¼ a expðꢀkobsdtÞ ꢀ b expðꢀk0 tÞ þ A ;
ð3Þ
1
obsd
1826
1795
1917
2000
1950
1900
1850
1800
1750
Wavenumber cm-1
Fig. 2. Infrared spectrum in the stretching carbonyl region of fac-(g1-PPh3)(g2-phen)Mo(CO)3 in chlorobenzene.

Y. Ocasio-Delgado et al. / Journal of Organometallic Chemistry 690 (2005) 3366–3372
3369
a
b
c
d
e
f
2000
1950
1900
1850
1800
1750
Wavenumber cm-1
Fig. 3. Infrared spectra in the stretching carbonyl region of a reaction mixture of (g2-C60)(g2-phen)Mo(CO)3 and PPh3 (ꢂ0.1 M) in chlorobenzene.
The mixture was heated at ꢂ80 ꢁC under nitrogen during (a) 0 min, (b) 10 min, (c) 50 min, (d) 80 min, (e) 180 min, and (f) 230 min.
where a = 0.1665, b = 0.1560, kobsd = 1.442(3) · 10ꢀ3 sꢀ1
,
determined for various L concentrations and tempera-
tures are presented in Tables 1 and 2.
k0obsd ¼ 1.043ð3Þ ꢃ 10ꢀ4 sꢀ1 and A = 0.2290.
1
A detailed description about the analysis of biexpo-
nential plots of biphasic reactions is available in the
chemical literature [22,25,26]. The nature of the reac-
tions product was established by comparison of the
m(CO) spectrum of the reaction product with the spectra
of an actual sample of fac-(g2-phen)(g1-PPh3)Mo(CO)3.
The values of the pseudo-first order rate constant kobsd
4. Discussion
4.1. Displacement of C60 from fac-(g2-C60)(g2-phen)Mo-
(CO)3
The species formed in the first of the two consecutive
reactions is fac-(g1-L)(g2-phen)Mo(CO)3 where
L
displaces C60 from fac-(g2-C60)(g2-phen)Mo(CO)3. The
0.24
0.22
0.20
0.18
0.16
0.14
0.12
0.10
Table 1
Values of kobsd for C60 displacement from fac-(g2-C60)(g2-phen)-
Mo(CO)3 by triphenylphosphine in chlorobenzene at various [PPh3]
and [C60]/[PPh3] ratios
Temperature (K) ( 0.1) [PPh3] (M) [C60]/[PPh3] kobsd (·10ꢀ3 sꢀ1
)
347.6
337.6
327.6
0.0102
0.0102
0.0715
0.1014
0.3010
0.5016
0.01012
0.01220
0.00106
0
1.552(5)
1.565(3)
1.545(4)
1.528(1)
1.442(3)
1.327(3)
1.7(1)
0
0
0
0
0
0.1015
0.5034
0.9528
1.42(1)
1.54(3)
-1000
0
1000 2000 3000 4000 5000 6000 7000 8000
Time / seconds
0.01003
0.01003
0.01003
0.1001
0.1001
0.1001
0.5000
0.5000
0.5000
0
0
0
0
0
0
0
0
0
0.471(1)
0.521(1)
0.59(1)
Fig. 4. Plot of absorbance (440 nm) vs. time (s) for C60 displacement
from fac-(g2-C60)(g2-phen)Mo(CO)3 by PPh3 in chlorobenzene to
form fac-(g1-PPh3)(g2-phen)Mo(CO)3 at 74.4 ꢁC under flooding con-
ditions where [PPh3] ꢁ [fac-(g2-C60)(g2-phen)Mo(CO)3]. The contin-
uous trace is for absorbance values obtained from the equation that
best describes the relation between absorbance and time. Equation:
0.492(1)
0.591(2)
0.602(4)
0.534(2)
0.555(8)
0.565(3)
At ¼ a expðꢀkobsdtÞ ꢀ b expðꢀk0 tÞ þ A
,
where a = 0.1665, b =
1
obsd
0.1560, kobsd = 1.442(3) · 10ꢀ3 sꢀ1, ko0 bsd ¼ 1.043ð3Þ ꢃ 10ꢀ4 sꢀ1 and
A
1
= 0.2290. The first segment of the plot is ascribed to formation
0.0214
0.0631
0.10011
0
0
0
0.125(4)
0.126(2)
0.126(2)
of fac-(g1-PPh3)(g2-phen)Mo(CO) (governed by kobsd). The second
segment of the plot is ascribed to decomposition of fac-(g1-PPh3)(g2-
phen)Mo(CO)3 (governed by k0obsd).

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Y. Ocasio-Delgado et al. / Journal of Organometallic Chemistry 690 (2005) 3366–3372
Table 2
Average values of kobsd and activation parameters for C60 displacement from fac-(g2-C60)(g2-phen)Mo(CO)3 by L in various solvents
¼
¼
Temperature (K)
Solvent
L
kobsd (·10ꢀ3 sꢀ1
)
DHobsd ðkJ=molÞ
112(8)
DSobsd ðJ=K molÞ
33(21)
347.6
337.6
327.6
Chlorobenzene
PPh3
1.51(3)
0.55(1)
0.0.126(5)
347.6
Chlorobenzene
Toluene
PCy3
PPh3
1.54(1)
347.6
337.6
327.6
0.835(6)
0.312(3)
0.111(3)
92.7(2)
107(3)
113(9)
ꢀ35.9(4)
8(8)
347.6
337.6
327.6
Bromobenzene
Benzene
PPh3
PPh3
1.283(2)
0.382(1)
0.127(2)
347.6
337.6
327.6
1.248(4)
0.480(7)
0.107(6)
29(25)
kobsd values are independent of the chemical nature of L
and [L]. These observations suggest that L is not in-
volved in the steps contributing to the kobsd values. It
is informative to compare the behavior of the system un-
der study with the behavior of the closely related W
analogous complex fac-(g2-C60)(g2-phen)W(CO)3 [2].
The proposed mechanism, depicted in Scheme 1, was
reported for the W analogous complex, and it is being
adopted here for the reactions under study. This
mechanisms involves a solvent-assisted [60]fullerene
associative displacement producing fac-(solvent)(g2-
phen)Mo(CO)3 as an intermediate species. Assuming
that the concentration of this intermediate species, I, is
at steady-state concentration, this mechanism predicts
the following rate-law (Eqs. (3) and (4))
d½Sꢄ
ꢀ
¼ kobsd½Sꢄ;
ð3Þ
dt
where S = substrate = fac-(g2-C60)(g2-phen)Mo(CO)3
k1k2½Lꢄ
kobsd
¼
.
ð4Þ
k
½C60ꢄ þ k2½Lꢄ
The observation that kobsd values are [L] independent
ꢀ1
suggests that kꢀ1[C60] ꢅ k2[L] and Eq. (4) becomes
k1k2½Lꢄ
kobsd
¼
ꢂ k1.
ð5Þ
k
½C60ꢄ þ k2½Lꢄ
ꢀ1
solvent
solvent
k1, solvent
k-1, C60
N
N
N
CO
CO
N
CO
CO
Mo
CO
Mo
CO
Mo
CO
N
CO
N
CO
TS1
k2, L
solvent
L
L
N
N
CO
CO
CO
Mo
CO
Mo
CO
N
N
CO
TS2
Scheme 1. Proposed mechanism for the solvent-assisted C60 displacement from fac-(g2-C60)(g2-phen)Mo(CO)3. TS1 and TS2 are plausible transition
states.

Y. Ocasio-Delgado et al. / Journal of Organometallic Chemistry 690 (2005) 3366–3372
3371
-10
This observation is not surprising since [L] ꢁ [C60]
(since the source of C60 in the reacting mixture is fac-
(g2-C60)(g2-phen)Mo(CO)3, i.e., [L] ꢁ [C60]). Fortu-
nately, the concentration of [C60] can be experimentally
controlled. Values of kobsd where determined under con-
ditions where 0 6 [C60]/[L] ꢂ 1. The observation that
kobsd values are independent of [C60] and of [C60]/[L] ra-
tios demonstrate that k2 ꢁ kꢀ1. This result is both sur-
prising and interesting because the expectation is that
the intermediate species (I) is either coordinatively and
electronically unsaturated (I = fac-(g2-phen)Mo(CO)3)
or weakly coordinated by a solvent molecule (I = fac-
(solvent)(g2-phen)Mo(CO)3) (vide infra). The ratio,
-12
-14
-16
-18
-20
k
ꢀ1/k2, has been used as a criterion to estimate the de-
gree of selectivity of intermediate species [1,2,27–31].
For example, the competition ratio values for the inter-
mediate species produced in the C60 displacement on
fac-(g2-C60)(g2-phen)W(CO)3 at 74.2 ꢁC in chloroben-
zene (kꢀ1/k2 = 9(3) for L = PPh3, kꢀ1/k2 = 12(3) for
L = P(Cy)3) suggest some degree of selectivity by I to-
ward C60 [2]. The corresponding value for W(CO)5
(kꢀ1/k2 = 1.09(1) at 39 ꢁC in CS2), for which no solvent
involvement was suggested [1], supports some degree of
solvent involvement. In these studies, the observation
that the ratio values are nearly independent of L, and
independent of the temperature suggested that indeed
the transition state (TS1) involves a larger extent of
W–C60 bond-breaking than solvent–W bond-making.
Thus, in those cases it seems that the TS should resemble
the electronically unsaturated intermediate. The kinetics
behavior of the present subject is at contrast with the
behavior just described. The highly selectivity of the
intermediate species (i.e., k2 ꢁ kꢀ1) and the estimated
activation parameters for the reactions in chlorobenzene
0.0028
0.0030
0.0032
0.0034
1/T
Fig. 5. Plots of ln(k/T) vs. 1/T showing the isokinetic region for the
solvent-assisted C60 displacement from fac-(g2-C60)(g2-phen)Mo(CO)3
in j chlorobenzene (1), d toluene (2), m bromobenzene (3), and r
benzene (4).
parameters and rate constant values, the solvent/C60 ex-
change takes place via a common mechanism. The con-
cern about the existence of a real isokinetic temperature
with chemical meaning has been addressed elsewhere
[32]. The ‘‘proof’’ of its existence is a common point
of intersection in the Eyring plots or Arrhenius plots.
Often the common intersection occurs at temperatures
experimentally inaccessible and the uncertainty of the
intersection is large. Fortunately, in the present study
the isokinetic temperature (Tiso ꢂ 50 ꢁC) was experimen-
tally accessible.
The role of the solvent on the ligand exchange reac-
tions of metal carbonyl complexes have been reported
extensively [33–39]. Aromatic solvents may interact with
the substrate and intermediate species through an ole-
finic linkage [33–35] or a lone pair (in halogenated sol-
vents) [36]. The coordinated solvent may undergo a
‘‘chain walk’’ isomerization to attain the most stable
mode of coordination [33,34]. A metal–H–C agostic
bond [37,38] has been proposed as the mode of coordi-
nation for alkanes [39]. The activation parameters may
provide an estimate about the Mo–C60 bond dissocia-
tion enthalpy. The goodness of the estimate depends
on the degree of solvent involvement in the step gov-
erned by k1 in Scheme 1. The solvent may affect the
TS stability [40–42]. For example, the difference between
the gas phase W–CO bond dissociation enthalpy
(=192(12) kJ/mol) [40] and the W–CO bond dissociation
enthalpy in decalin (=167(7) kJ/mol)[41] has been as-
cribed to a TS stabilization by decalin [42]. This TS sta-
bilization of ꢂ25 kJ/mol may be a result of a C–H–W
agostic interaction. Since this TS stabilization of ꢂ50%
of the reported W–H–C agostic bond strength
(=56(12) kJ/mol) [42], there should be a partial agostic
¼
¼
ðDHobsd ¼ 112ð8Þ kJ=mol; DSobsd ¼ 33ð21Þ J=K molÞ indi-
cate that the departure of C60 from fac-(g2-C60)(g2-
phen)Mo(CO)3 may be assisted by the solvent and that
the TS1 involves some extent of solvent–Mo bond mak-
ing and C60–Mo bond breaking. This inference
prompted the kinetics studies in various solvents. The
expectation was that the kobsd values should depend
on the basicity/nucleophilicity of the solvent and that
the activation parameters should reflect a variation from
solvent to solvent in such a way that an isokinetic tem-
perature should be observed. Fig. 5 depicts the plots of
ln(kobsd/T) vs. 1/T for the reactions in various solvents.
The activation parameters estimated from these plots
¼
are presented in Table 2. Notice that smaller DHobsd val-
¼
ues are associated with smaller (more negative) DSobsd
values. Fig. 5 shows a common region (or a point within
experimental error) of intersection for all the plots. This
common point of intersection, in the vicinity of 50 ꢁC,
corresponds to the isokinetic temperature or the temper-
ature where the rate constant values are the same for all
solvents [32]. The existence of an isokinetic temperature
suggests that regardless of the variation of the activation

3372
Y. Ocasio-Delgado et al. / Journal of Organometallic Chemistry 690 (2005) 3366–3372
[5] Ligands which are weaker p-acceptors than CO may labilize a
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bond formation in the TS. In the present study, the kobsd
values at a given temperature were solvent-dependent.
Since the isokinetic temperature is close to the experi-
mentally accessible temperatures, the variations of kobsd
(at a given temperature) among the solvents are small
but statistically significant.
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5. Conclusions
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The ligand/[60]fullerene exchange reactions on fac-
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segment of the biphasic plot corresponds to formation
of fac-(g1-L)(g2-phen)Mo(CO)3 and the second segment
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The activation parameter values, the high selectivity of
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dependence of kobsd values on the nature of the solvent
support some degree, depending on the solvent, of sol-
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of fac-(solvent)(g2-phen)Mo(CO)3.
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Donors of The Petroleum Research Fund, adminis-
tered by the American Chemical Society, (ACS-41267-
B3) and the National Science Foundation (Grant
CHE-0102167) are acknowledged for support of this
work. Helpful comments from Professor Deborah
A. Moore-Russo are gratefully acknowledged. The
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versity of Puerto Rico Undergraduate Research Pro-
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