A R T I C L E S
Zakzeski et al.
washed thoroughly with water and acetone and then dried in a
vacuum oven. The reaction temperature was monitored using
a thermocouple located inside a Hastelloy C thermowell. During a
typical reaction, 3.46 g of anhydrous toluene (99.8%, Aldrich), 13
mmol of either CF
Aldrich), C COOH (97%, Aldrich), CCl
Aesar), CCl HCOOH (99%, Alfa Aesar), CClH
Aesar), CH COOH (99.7%, EMD), CF SO H (98%, Aldrich), or
CH SO H (g99.5%, Aldrich), 3.8 mmol of either (CF CO)
g99%, Aldrich), (CF ClCO) O (98%, Aldrich), (C CO) O (99%,
Aldrich), (CCl CO) O (96%, Alfa Aesar), (CCl HCO) O (Aldrich,
6%), (CClH CO) O (Alfa Aesar, 96%), (CH CO) O (99%, EM
Science), (CF SO O (99+%, Aldrich), or (CH SO O (97%,
Aldrich), respectively, 0.0039 g of Rh(acac) (97%, Aldrich), and
.0604 g of NH VO (99+%, Aldrich) were placed into the
3
COOH (99%, Aldrich), CF
COOH (99%, Alfa
COOH (99%, Alfa
2
ClCOOH (98%,
2
F
5
3
2
2
3
3
3
3
3
3
2
O
(
2
2
F
2 5
2
3
2
2
2
9
2
2
3
2
3
2
)
2
3
2 2
)
3
0
4
3
Figure 1. Turnover number for Rh-catalyzed oxidative carbonylation of
autoclave, which was then sealed and purged four times with
nitrogen. Next, the reactor was pressurized at 293 K with 0.345
toluene to toluic acid as a function of acid pK
mmol (3.46 g) of toluene, 13.0 mmol of the designated acid, 3.8 mmol of
the corresponding anhydride, 10 µmol (0.0039 g) of Rh(acac) , 0.5163 mmol
(0.0604 g) of NH VO , P ) 0.345 MPa, PO2 ) 0.345 MPa, T ) 353 K.
a
. Reaction conditions: 37.5
2
MPa O (99.993%, Praxair) and 0.345 MPa CO (99.5%, Praxair).
3
The reactor was then heated to 353 K in ∼10 min and held at this
temperature for 4 h. Upon completion of the reaction, the reactor
was quenched with ice water to 308 K and vented. The quantity of
toluic acid was determined by gas chromatography using an Agilent
Technologies 6890N gas chromatograph equipped with an FID
detector and an HP-1 capillary column coated with cross-linked
methylsiloxane.
4
3
CO
the Gibbs free energy of reaction at the reaction temperature.
Rate coefficients were computed from transition state theory,
as described by eq 2:
Calculations. Electronic structures and energies of reactants,
k T
q
q
B
∆S
R
-∆H
RT
products, and transition states were determined using density
k )
exp
(
)
exp
(
)
(2)
2
5
h
functional theory (DFT) as implemented in Q-Chem. The
B3LYP functional was used to describe electron exchange and
correlation, and the 6-31G* basis set was used to locate
optimized ground-state and transition-state structures. The
LANL2DZ effective core potential was used to describe the Rh
atom. After a particular molecular structure was optimized to a
stationary point (transition-state or minimum-energy structure),
its energy was further refined at a higher level of theory using
the 6-311G**/LANL2DZ basis set (also see the Supporting
Information). All of the stationary and transition-state points were
found on the basis of gas-phase calculations. The growing string
method (GSM) was used to locate the transition state (TS)
where k is the elementary rate coefficient, k
B
is Boltzmann’s
q
q
constant, h is Planck’s constant, and ∆S and ∆H are the entropy
and enthalpy of activation, respectively, at the reaction temper-
ature. Energy decomposition analysis (EDA) was performed
using the method of Khaliullin et al., as implemented in
Q-Chem.
2
7
Results and Discussion
a
Figure 1 shows the effect of acid pK on the number of
turnovers obtained for Rh(III) at various reaction times. The
reactions for t ) 4 h were repeated in triplicate. Comparison of
the turnover numbers for reactions at different times indicates
that the relative order of catalytic activity with respect to acid
pK remained constant. In each case, the total reaction time
a
includes the time required to heat and cool the autoclave (see
the Supporting Information). As indicated by Figure 1, only
2
6
connecting two minimum-energy structures. In this method, a
minimum-energy path connecting the reactant and product is
estimated without making an initial guess for the reaction path.
The geometry at the point with the highest energy on this
minimum-energy path is taken as an estimate of the transition-
state geometry, which is then converged to the exact saddle point
by the transition-state-finding algorithm implemented in Q-Chem.
Vibrational analyses were performed to verify the nature of the
energy minima and transition states as well as to generate
thermochemical data. All of the reported energies and free
energies are for the gas phase at 353 K, the experimental reaction
temperature. Vibrational, rotational, and translational entropy
values were computed using standard statistical-mechanical
methods within the rigid-rotor-harmonic-oscillator approxima-
tion. The energetics of toluene binding were corrected for basis-
set superposition error using the counterpoise method. Equilib-
rium constants were calculated at a standard state of 1 bar and
adjusted to molar concentration units according to eq 1:
acids with pK
a
’s between -2 and 3 were effective in producing
’s
COOH (pK
toluic acid, and the most effective acids were those with pK
between 0 and 0.5.
a
2
8-30
Within this range, CClF
2
a
) 0.35) yielded the largest number of turnovers. Reduced
catalytic activity was observed for acids with pK values greater
a
than that of CClF
CClH COOH, and CH
smaller than that of CClF
CH SO OH). The data shown in Figure 1 also confirm that both
2
COOH (such as CCl
COOH) as well as acids with pK
COOH (such as C COOH and
3
COOH, CCl
2
HCOOH,
2
3
a
’s
2
2 5
F
3
2
very strong and very weak acids are ineffective in promoting
the oxidative carbonylation of toluene to toluic acid, consistent
with the observations of Fujiwara and co-workers for the
o
-∆ν
o
RTc
-∆G
RT
6
K )
exp
(1)
oxidative carbonylation of alkanes.
(
)
(
o
)
P
The oxidative carbonylation of toluene to toluic acid involves
several important steps in which ligands play an integral role.
These include coordination of toluene to the Rh complex and
where K is the equilibrium constant, R is the gas constant, T is
the absolute temperature, c° is the standard-state concentration
in the liquid phases (1 mol/L), P° is the standard-state pressure
(
1 bar), ∆ν is the difference between the sums of the stoichio-
(27) Khaliullin, R. Z.; Cobar, E. A.; Lochan, R. C.; Bell, A. T.; Head-
Gordon, M. J. Phys. Chem. A 2007, 111, 8753–8765.
metric coefficients of the products and reactants, and ∆G° is
(
(
28) Kurz, J. L.; Farrar, J. M. J. Am. Chem. Soc. 1969, 91, 6057–6062.
29) Gelb, R. I.; Alper, J. S. J. Chem. Eng. Data 1998, 43, 1068–1071.
(
26) Peters, B.; Heyden, A.; Bell, A. T.; Chakraborty, A. J. Chem. Phys.
004, 120, 7877–7886.
(30) Moroi, Y.; Yano, H.; Shibata, O.; Yonemitsu, T. Bull. Chem. Soc.
Jpn. 2001, 74, 667–672.
2
1
1100 J. AM. CHEM. SOC. 9 VOL. 131, NO. 31, 2009