Organometallics
Article
It should be pointed out that the entropy here is referred
specifically to the entropy of the solute; the entropy of the
solvent is implicitly included in the dielectric continuum
model. For the thermochemical calculation, the temperature is
set to 25 °C and the pressure to 0.1 MPa. Solvation energies of
species in [Eim][NTf2] (Table S5) are evaluated separately by
the self-consistent reaction field polarizable continuum model
(IEF-PCM).72−74 Based on the experimental results under the
conditions for CH4 activation in the [Eim][NTf2]/H2O2
system, no IL decomposition was observed, indicating that
neither cation nor anion of the IL participated in the catalytic
reaction. H2O is poorly miscible with [Eim][NTf2]. As
discussed in Section 3.2, the AuIII complex and CH4 were
preferably dissolved in the [Eim][NTf2] solvent and performed
CH4 activation in the IL solvent, leading to the formation of
CH3OH. Therefore, the dielectric constant of 12 for
[Eim][NTf2]75,76 was chosen to calculate the solvent effect
during the CH4 oxidation reaction. Here, free energies are used
to describe energy profiles during the reaction. The calculated
enthalpies are shown in the Supporting information (Tables
S5−S9). To make the energy calculations more reliable,
different functionals, basis sets, and methods were used to re-
evaluate the DFT calculations, and the results are analyzed and
shown in Table S9. The different functionals and basis sets did
not affect the trend and conclusions of the reaction profile.
Four different B’s with CH4 and OTf ligands on different
positions and conformations, and four different conformations
of INT F with CH3, OTf, and Cl ligands on different positions
were calculated. The structures shown in this work are the
most stable configuration and lowest computed energy after
the conformational searches. Other structures (INT C, TS D‡,
INT E, TS G‡, INT H, INT I) were all optimized based on the
conformation of the optimized structures B and INT F.
3.5.2. C−H Activation to Form AuIII-CH3 Species. AuIII ion
(d8) is isoelectronic and isostructural with the PtII ion, which
was reported by Shilov et al. to be active for C−H activation in
CH4. Two key mechanistic steps have been proposed in PtII-
based Shilov chemistry: oxidative addition of CH4 to a PtII
catalyst to form a PtIV alkyl hydride, followed by a proton loss
or a nonredox route, where the proton is lost directly from the
PtII δ-complex.6,8,77 Oxidative addition reactions are typical for
electron-rich, low valent metal complexes.8 However, this
mechanism would not apply to AuIII, which is at the highest
known valence state of Au. Therefore, we only considered the
electrophilic substitution mechanism and studied the most
likely pathways for C−H activation in CH4 by density
functional theory (DFT) calculations.
mol and binding free energy of 3.7 kcal/mol were obtained.
Then, entropic quasiharmonic correction based on the
M062X-D3/def2tzvp method was performed, resulting in
corrected binding free energy of 3.6 kcal/mol. We have also
calculated the binding electronic energy of CH4 using the
dispersion corrected double hybridized functional B2PLYPD3
with the genecp basis sets. We obtained the value of −4.6 kcal/
mol, which is very close to the −4.5 kcal/mol by the M062X-
D3/genecp method, suggesting that the M062X-D3 method is
reliable for evaluating the binding of CH4 in B. The
unexpected strong CH4 binding arises from the multiple C−
H...O, C−H...Cl, or C−H...F hydrogen bonds between CH4
and AuClOTf2(phen), as shown in Figure S3. Although each
C−H...O, C−H...Cl, or C−H...F H-bond seems weak, the
multiple H-bonds are proposed to collectively stabilize the
AuIII-CH4 H-bonding structure B under high CH4 pressure.
As illustrated in Scheme 2, without prior dissolved CH4 in
the reaction media before mixing AgOTf with AuCl3(phen), an
orange precipitate was observed in the reactor and the orange
precipitate did not exhibit catalyst activity for CH4 oxidation,
indicating that AuClOTf2(phen) is not the catalyst. Since
experimental evidence supports the existence of an AuIII
complex stabilized only under CH4 pressure, structure B
with slightly positive binding free energy of CH4 is reasonable
by taking into account the possible calculation errors and the
rather high CH4 pressure. The binding free energy of CH4 to
catalyst precursor AuCl3(phen) is 5.3 kcal/mol, which is higher
than the binding energy of 3.7 kcal/mol in B in the IL on the
same M062X-D3/def2tzvp level. H2O binding free energy to
AuClOTf2(phen) was calculated to be 4.0 kcal/mol in IL at the
same level. The binding free energy of H2O is similar to that of
CH4 due to the much larger solvent energy loss and entropic
penalty for H2O binding. In our experiments, the pressure of
CH4 is quite high, then CH4 binding is easier than H2O
binding. Experimentally, the presence of H2O cannot stabilize
the AuIII complex upon AgOTf treatment. Therefore, we
propose that with sufficiently dissolved CH4 in the IL, CH4
participated in the dechlorination process of the precursor and
formed structure B due to the driving force of AgCl
precipitation. Because AgCl precipitation is a highly
thermodynamically favorable process and no other lower
energy structure was found before AuIII-CH4 H-bonding
structure B, B is defined as the zero point for the energy
profile in Figure 3. The stabilization of structure B by CH4
appears unusual at first sight. However, the existence of
methane hydrate composed of CH4 and H2O through H-
bonding and van der Waals force under high pressure and low
temperature (e.g., 3.8 MPa and 4 °C) may be viewed as an
example of the importance of H-bonding involving CH4. The
transition state (TS D‡) bearing a C−H activated CH4 was
obtained by the QST3 method and found to have a six-
membered ring consisting of an activated CH4, the AuIII center,
The reaction profile with the corresponding calculated free
energies of the AuIII complex in key steps of C−H activation
starting from the catalyst precursor A is proposed in Figure 3.
The formation of AuIII-CH4 H-bonding structure B is
accompanied by a highly thermodynamically favorable
formation of the AgCl precipitate, which has a solubility
product Ksp(AgCl) of 1.8*10−10 with a standard molar
enthalpy of formation of −127.0 kJ/mol.78 The binding
electronic energy of −3.9 kcal/mol and binding free energy of
4.8 kcal/mol (at standard conditions) for CH4 to
AuClOTf2(phen) as in structure B were first obtained using
the M062X/genecp method. To obtain the more accurate CH4
binding energy to AuClOTf2(phen), the geometries, vibra-
tional calculations, and solvation effects of structure B, CH4,
and AuClOTf2(phen) were reoptimized using the M062X-D3/
def2tzvp method. A binding electronic energy of −2.9 kcal/
−
and two oxygen atoms in the CF3SO3 ligand. This transition
state has a free energy barrier of 26.7 kcal/mol from structure
B. Six-membered metal cycle with activated Csp3-H structures
have also been reported on metal atoms such as Pt,79 Ir,18 and
Ti21,80 with the CH3COO− or CF3COO− ligand. In this work,
AuIII-CH3 species is formed simultaneously by C−H bond
cleavage and H transfer to the CF3SO3− ligand to form INT E.
The release of CF3COOH follows to form the more stable
structure INT F. The H-bonding effect of [Eim]+ on both
structure B and INT E was calculated. The difference in free
energies from B to INT E with or without the [Eim]+ H-
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Organometallics 2021, 40, 370−382
Huang, Tingyu
