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A. Behn et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 91–97
the rate of toluic acid formation becomes limited by the reversible
coordination of toluene and the activation of a C H bond on the
benzene ring. The overall rate of toluic acid formation is very sen-
sitive to the basicity of the anionic ligand and passes through an
optimum as the pKa of the conjugate acid of the anionic ligand
is varied from −1 to 5. This trend is explained by a theoretical
analysis of the effects of ligand composition on the equilibrium con-
stant for toluene coordination and the rate coefficient for activation
of a C H bond on the benzene ring. Consistent with experimen-
tal observation, the predicted apparent first-order rate coefficient
passes through a well-defined maximum vs conjugate acid pKa‡’s.
Only minor numerical differences are seen in ꢂGcooord and ꢂGact
for toluene coordinated in the para, meta, and ortho positions, in
agreement with the experimentally observed lack of selectivity for
any specific isomer of toluic acid. Finally, when compared against
previous studies of the analogous Rh(III) system, the present com-
putations predict a higher activity for Pd(II) complexes relative
to Rh(III) complexes, and a shift in the peak activity away from
CClF2COO− (the optimal ligand for Rh(III)) toward less basic ligands.
The calculated curves for ꢂGapp vs pKa of the conjugate acids of the
anionic ligand for activation in the para, meta, and ortho positions
are more similar for Pd than for the Rh-based system in agreement
with the observed low selectivity of Pd(II) complexes relative to
the high selectivity to para-toluic acid seen for comparable Rh(III)
complexes.
Fig. 8. Computed values of ꢂGcooord for toluene binding in various 2 configurations,
and model toluene structure to indicate positions of the different carbons.
which the rate limiting step involves toluene coordination and C
H
activation. A similar volcano plot of overall activity vs pKa was
found; however, the highest activity in this case was for CClF2COO−
anions. Theoretical analysis of that system revealed a minimum in
the computed ꢂGapp vs pKa for CClF2COO− anions, with a value
of 26.9 kcal/mol. The experimentally observed p/m ratios ranged
between 1.75 and 3.75, with the higher values occurring for ligands
with a conjugate acid pKa near zero. The production of o-toluic acid
was minimal.
Acknowledgments
Under analogous reaction conditions, the activity of the Pd(II)
complexes reported here is significantly higher than those for the
Rh(III) complexes reported earlier [18]. The peak in TON (1740) for
Pd(II) occurs for a less basic ligand, CF3COO−, and is more than three
times that (560) for Rh(III) with the optimal ligand, CClF2COO−.
The calculated values of ꢂGapp determined for Pd(II) and Rh(III)
complexes are consistent with both the higher activity of Pd(II)
relative to Rh(III) and the shift in the basicity of the optimal lig-
and. For Pd(II), the lowest value of ꢂGapp for para activation of
toluene is 18.5 kcal/mol with CF3CF2COO− ligands. This value is
nearly 10 kcal/mol lower than that for the minimum value of ꢂGapp
(26.9 kcal/mol) found for Rh(III) with CClF2COO− ligands, consistent
with experiment. Finally, the predicted selectivity for the Pd cat-
alysts is low, given the relatively similar values of ꢂGapp for para,
meta, and ortho activation.
This work was supported by the Methane Conversion Coop-
erative funded by BP. Calculations were performed on a cluster
provided to the University of California Berkeley, College of Chem-
istry through grant: NSF CHE-0840505.
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