Polyoxometalate-catalyzed insertion of oxygen from O2 into tin-alkyl bonds

Article abstract of DOI:10.1021/ja409559h

The polyoxometalate H₅PV₂Mo₁₀O ₄₀ mediates the insertion of an oxygen atom from H₅PV ₂Mo₁₀O₄₀ into the tin-carbon bond of n-Bu ₄Sn through its activation by

Full text of DOI:10.1021/ja409559h

Article
pubs.acs.org/JACS
Polyoxometalate-Catalyzed Insertion of Oxygen from O2 into
Tin−Alkyl Bonds
Alexander M. Khenkin, Irena Efremenko, Jan M. L. Martin, and Ronny Neumann*
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel 76100
S
* Supporting Information
ABSTRACT: The polyoxometalate H5PV2Mo10O40 mediates
the insertion of an oxygen atom from H5PV2Mo10O40 into the
tin−carbon bond of n-Bu4Sn through its activation by electron
transfer to yield 1-butanol and (n-Bu3Sn)2O. The reaction is
initiated by electron transfer from n-Bu4Sn to H5PVV Mo10O40
2
to yield the ion pair n-Bu4Sn•+−H5PVIVVVMo10O40. The
H5PVIVVVMo10O40 moiety was identified by UV−vis and EPR.
DFT calculations show that n-Bu4Sn•+−H5PVIVVVMo10O40 is
relatively unstable and forms more stable Bu+ and Bu3Sn+ cations coordinated to the polyoxometalate, which were also identified
by ESI-MS. Products are released at higher temperatures. In the presence of molecular oxygen as the terminal oxidant the reac-
tion is catalytic.
transfer type mechanism.10 Advantageously, such reactions can
INTRODUCTION
■
be catalytic with molecular oxygen as terminal oxidant. Now we
The selective oxygenation of hydrocarbons, especially using
report that H5PV2Mo10O40 can insert oxygen atoms into the
ground-state O2 as the oxygen donor, remains an important and
Sn−C bond of tin−alkyl compounds such as n-Bu4Sn and that
significant research challenge. The ubiquitous reaction of O2
this reaction can be catalytic with O2 as terminal oxidant to
with hydrocarbons occurs by autoxidation pathways with free
yield primary alcohols (Scheme 1).
radical intermediates that often preclude the selective formation
of mono-oxygenated products.1 As pertains to the selective
Scheme 1. Aerobic Oxygenation of Tetrabutyltin Catalyzed
by H5PV2Mo10O40
oxygenation of alkanes, for example hydroxylation, both C−H
bond activation and C−O bond formation at a transition-metal
center are required. Enzymes such as methane monooxygenase,
cytochrome P450, and others typically form high-valent and
highly reactive iron− or copper−oxo intermediates from O2
under reducing conditions that lead to insertion of oxygen into
C−H bonds.2 An alternative strategy was suggested quite long
RESULTS AND DISCUSSION
■
ago by Shilov and co-workers, where a C−H bond activation,
e.g. of methane, takes place at a PtII catalytic site and reported
nucleophilic attack of water after formation of a PtIV inter-
mediate leads to oxygenation.3 There are, however, only a few
examples where this reaction was carried out with O2 to form
the PtIV intermediate.4 Recently, Gunnoe, Groves, and co-workers
have outlined a pathway where an insertion of an oxygen atom
into a M−C bond leads to formation of an alkoxide complex
that then can activate a C−H bond for 1,2-addition across the
M−OR bond to form a metal hydrocarbyl complex and a free
alcohol.5 There are only a few examples of oxygen insertion
into M−C bonds,6 and it would appear that the Baeyer−Villiger
type oxidation of methylrhenium trioxide via formation of η2-
peroxo species is the most studied reaction.7 Recently, a flavin-
catalyzed version of this reaction was reported with H2O2 as the
oxidant.5
Anaerobic Reactions. The oxygenation of n-Bu4Sn by
oxygen transfer from H5PV2Mo10O40 was first studied under
anaerobic reaction conditions. Thus, 0.029 mmol n-Bu4Sn and
0.01 mmol of H5PV2Mo10O40 in 1 mL of acetonitrile reacted at
70 °C under Ar for 2 h to quantitatively yield 1-BuOH (∼0.005
mmol), a mixture of 1-butene, 2-butene, and 2-methylpropene
(∼0.005 mmol, 1:1:1 ratio), and ∼0.005 mmol of (n-Bu3Sn)2O,
as analyzed by GC-FID and GC-MS. The result that on the one
hand only the primary alcohol was formed while all isomeric
butene compounds were formed is notable, as is the anaerobic
reaction stoichiometry that shows that each equivalent of
H5PV2Mo10O40 yielded a combined 1 equiv of 1-butanol and
isomeric butenes. Other solvents gave different 1-butanol/butene
ratios, while some solvents were not suitable (Table S1, Supporting
Information). The acid form of the polyoxometalate was required
or a relatively strong acid such as trifluoroacetic acid needed to
be added to the reaction (Figure S1, Supporting Information).
We have shown that phosphovanadomolybdates, notably the
H5PV2Mo10O40 polyoxometalate of the α-Keggin structure, can
insert oxygen into C−H bonds of arenes and alkylarenes8 and
primary and vicinal alcohols9 by an electron transfer−oxygen
Received: September 29, 2013
Published: December 6, 2013
© 2013 American Chemical Society
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H4PVMo11O40 was equally reactive, and H3PMo12O40 showed
less reactivity that was increased by ∼55% in the presence of
VO(O-i-Pr)3, likely due to the in situ formation of a phos-
phovanadomolybdate. Analogous tungstates were not reactive,
nor was a monomeric vanadate such as VO(O-i-Pr)3 or a de-
cavanadate with or without added acid. These latter compounds
have never been observed to be active in electron transfer−
oxygen transfer type reactions.10
During the anaerobic reaction H5PV2Mo10O40 was reduced,
as evidenced by the formation of a heteropoly blue compound
(λmax 750 nm).11 Furthermore, very similar to what was
reported in the past,12 the typical EPR spectrum for the V(IV)
containing polyoxometalate (Figure 1) shows the presence of
Figure 2. 13C NMR spectra of (n-Bu)4Sn in CD3CN (bottom) and of
a 1/1 (n-Bu)4Sn/H5PV2Mo10O40 mixture after 2 h at 70 °C.
The reaction between H5PV2Mo10O40 and n-Bu4Sn was
further studied by 13C and 1H NMR. In Figure 2 (bottom), the
spectrum of n-Bu4Sn shows peaks at 29.0, 27.1, 13.0, and 8.4
ppm assigned to the β, γ, δ, and α carbon atoms, respectively.
After n-Bu4Sn was heated for 2 h at 70 °C under Ar in the
presence of 1 equiv of H5PV2Mo10O40 (Figure 2, top), a new
spectrum was measured with four peaks at 27.4, 26.5, 18.7, and
12.9 ppm attributable to an intermediate species that is only
slightly different than the spectrum of (n-Bu3Sn)2O. Notably no
1-butanol, butene isomers, or (n-Bu3Sn)2O was observed in the
NMR spectra.14 A similar H NMR experiment (Figure S2,
1
Supporting Information) also revealed such an intermediate,
which by integration indicated formation of 1 equiv of inter-
mediate per 2 equiv of n-Bu4Sn using 1,2-dichloroethane as
internal standard. The 119Sn and 51V NMR spectra were very
broadened and featureless, apparently due to the presence of
paramagnetic vanadium(IV) and possibly spin on the tin atom.
The 31P NMR spectrum showed the residual presence (very
low intensity) of some nonreduced polyoxometalate.
Figure 1. EPR spectrum resulting from the reaction of n-Bu4Sn and
H5PV2Mo10O40: (black) experimental; (gray) simulation. The
spectrum was obtained by heating 29 mM n-Bu4Sn and 1.5 mM
H5PV2Mo10O40 in acetonitrile under argon at 50 °C for 20 min in a
quartz capillary tube (3 mm, 100 μL). Spectra were recorded at 100 K:
microwave frequency, 9.424 GHz; microwave power, 12 mW; field
modulation, 1 G; receiver gain, 60; time constant, 1310 ms; scan time,
200 s. The simulation reveals that the spectrum contains two type of
vanadium atoms in a ∼65/35 ratio. Species 1 (65%): g⊥= 1.980, g∥ =
1.937, A⊥ = 165 G, A∥ = 459 G. Species 2 (35%): g⊥ = 1.977, g∥ =
1.936, A⊥ = 150 G, A∥ = 550 G.
Clearly, the final reaction products, 1-butanol and butenes,
previously observed as noted above were released upon injec-
tion into the GC due to the elevated temperature, 220 °C, at
the injection port. The final products including (n-Bu3Sn)2O
can, however, be isolated by using the following procedure.
n-Bu4Sn (10 μmol) and H5PV2Mo10O40 (10 μmol) in 10 mL of
acetonitrile were reacted under Ar to produce a blue solution.
The acetonitrile was evaporated off, and the solid residue was
heated at 120 °C for 1 h in a pressure tube. After cooling, a
GC-MS and GC analysis of the gas phase was carried out and
the three isomeric butenes (1-butene, 2-butene, and isobutene
in the ratio 1/1/1) were found. Then 1-butanol and (n-Bu3Sn)2O
were extracted from the solid slurry by CDCl3. This solution
shows 13C NMR and 1H NMR spectra characteristic of 1-butanol
and (n-Bu3Sn)2O in CDCl3.14 The amount of 1-butanol deter-
mined from this solution and was approximately 5 μmol. The
two species related to reduced H5PV2Mo10O40: one where
VO2+ is supported on the polyoxometalate (major species, ∼65%)
and one where the vanadium is incorporated within the poly-
oxometalate (minor species, ∼35%). Together the UV−vis and
EPR spectra indicate an electron transfer oxidation of n-Bu4Sn.
The importance of the initiation of the reaction by electron
transfer from the alkyltin substrate to H5PV2Mo10O40 is also
observable by comparing the reactivity of n-Bu4Sn to that of
Me4Sn (0.2 mmol of substrate, 0.1 mmol of H5PV2Mo10O40,
10 mL of acetonitrile, 70 °C, Ar, 2 h). Analysis was by GC-FID
and GC-MS. Under identical conditions n-Bu4Sn (ionization
potential 8.76 eV)13 reacted 25 times faster, yielding 100 μmol
(∼100% yield) of products, than Me4Sn (ionization potential
9.69 eV),13 which yielded only 8 μmol (8% yield) of MeOH. It
should be noted that these literature values represent vertical
gas phase ionization potentials. Our DFT calculations (vide
infra) yielded a vertical ionization potential for n-Bu4Sn of 8.48 eV,
in good agreement with experiment. The calculated adiabatic
ionization potentials for n-Bu4Sn and Me4Sn are 7.8 and 9.1 eV
in the gas phase and 5.7 and 6.7 eV in acetonitrile, respectively.
1
ratio, determined by H NMR, of 1-butanol to (n-Bu3Sn)2O is
1/0.9, which is only slightly different than the theoretical ratio,
which should be 1/1 assuming equal amounts of 1-butanol and
butene isomers are formed. This experiment confirms the
reaction stoichiometry shown in Scheme 2, where the oxidant,
[O], is H5PV2Mo10O40, as will be elaborated upon below.
Further information on possible intermediates was obtained
by electrospray ionization mass spectrometry (ESI-MS; Figure 3).15
Although the negative ion spectrum is dominated by peaks at
m/z ≤882 attributable to the fragmentation of H5PVMo11O40,
peaks at m/z ≥1056 can be assigned to the fragmentation of
(n-Bu4Sn)x-H4PVMo11O40 (x = 1, 2) intermediates, as sum-
marized in Scheme 3. Importantly, in the positive ion mode, in
addition to the typical peaks associated with the fragmentation
of n-Bu4Sn, we were able to identify a fragment centered at m/z
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1.5 × 10−2 M−1 s−1, which is the same order of magnitude
found for the oxidation of n-Bu4Sn to a cation radical by
hexachloroiridate(IV) that has a redox potential similar to that
of H5PV2Mo10O40.13 Eyring plots of the rate constants as a
function of temperature (Figure S4, Supporting Information)
yielded the following values for the activation energy, enthalpy,
entropy, and free energy of activation: ΔEa = 14.6 kcal/mol,
ΔH⧧ = 14.0 kcal/mol, ΔS⧧ = −29.6 eu, ΔG⧧ = 23.6
Scheme 2. Stoichiometry of the Reaction between (n-Bu)4Sn
and H5PV2Mo10O40
323
kcal/mol.
323
323
Oxygen insertion into the Sn−C bond was studied by
reacting 10 mM H5PV2Mo10
O
40
(40−50% 18O) with 29 mM
18
n-Bu4Sn in CH3CN at 70 °C under Ar. The reaction yielded
1-butanol (43%18O) and (n-Bu3Sn)2O (45% 18O), as deter-
mined by mass spectrometry. Clearly, there is quantitative oxygen
transfer from the polyoxometalate to both oxygenated products.
Aerobic Reactions. Catalytic aerobic oxidation (n-Bu4Sn
(2.9 mmol) and H5PV2Mo10O40 (0.10 mmol) in 10 mL of
CH3CN, 70 °C, 1 bar of O2, 7 h) yielded both 1-butanol and
butenes in a ∼9/1 molar ratio (1.2 mmol, 12 turnovers) as well
as ∼0.6 mmol of (n-Bu3Sn)2O. The same reaction using 18O2
(95% 18O) yielded 1-BuOH (84% 18O) and (n-Bu3Sn)2O (16%
18O). Therefore, under aerobic conditions, the oxygen atom in
1-butanol comes mostly from O2, whereas the oxygen atom in
(n-Bu3Sn)2O is mostly from the polyoxometalate. Some
labeling of the latter occurs, possibly because under turnover
conditions defect oxygen structures of the polyoxometalate
after oxygen transfer are “replenished” by 18O2. Support for this
scenario comes also by carrying out a reaction in the presence
of S8. Thus, n-Bu4Sn (2.9 mmol), H5PV2Mo10O40 (0.10 mmol),
and S8 (4.0 mmol) in 10 mL of CH3CN at 70 °C under argon
were reacted for 7 h and then analyzed by GC-FID and
GC-MS. Di- and trisulfides, (n-Bu3)2Sx (x = 2, 3), were formed
and also (n-Bu3Sn)2O was obtained but (n-Bu3Sn)2S was not.
Calculations on the Anaerobic Reaction. Mechanistic
insight into the oxidation of n-Bu4Sn with H5PV2Mo10O40 using
the 1,2-isomer as an exemplary polyoxometalate was obtained
using density functional theory (DFT) calculations.16 Thermo-
dynamic considerations show that the overall oxygen transfer
reaction according to eq 2 is exergonic by 16.6−18.8 kcal/mol
depending on the butene isomer formed at 70 °C in acetonitrile.
Figure 3. Negative ion ESI-MS of a 1/1 (n-Bu)4Sn/H4PVMo11O40
mixture.
Scheme 3. Assignment of the ESI-MS Spectrum
2n‐Bu4Sn + 2H5PVV Mo10O → O(n‐Bu3Sn)2 + BuOH
2
40
+ C4H8 + 2H5PVIV2Mo10O39
665.17 with an isotope distribution fitting for a [(Bu3Sn)2-
OVOH2]+ species. This is an indication of binding of the
incipient reaction product, (n-Bu3Sn)2O (Scheme 1), to the
polyoxometalate. Notably, also a butyl-polyoxometalate frag-
ment was observed (Scheme 3).
The kinetics of the reaction of H5PV2Mo10O40 with n-Bu4Sn
was studied by following the reduction of H5PV2Mo10O40 by
UV−vis spectroscopy. Thus, 15 mM n-Bu4Sn and 0.25−2 mM
H5PV2Mo10O40 in acetonitrile were reacted at 50 °C. A log kobs
versus log [H5PV2Mo10O40] plot (Figure S3, Supporting
Information) shows that the reaction is first order (slope
1.02) in H5PV2Mo10O40. Similarly, changing the concentration
of n-Bu4Sn (1.7−29 mM) at a constant concentration of
H5PV2Mo10O40 (1 mM) shows that the reaction is first order in
n-Bu4Sn (slope 1.003). Therefore, the reactions are second
order overall (eq 1), with a second-order rate constant of
(2)
However, since H5PV2Mo10O40 is hydrated with ∼35 mole-
cules of water, explicit consideration of a water molecule in
close proximity to the reaction site of H5PV2Mo10O40 shows
that it will readily occupy the oxygen defect site of the de-
oxygenated compound, H5PV2Mo10O39, with an energy gain of
14.4 kcal/mol, thus rendering the oxygenation reaction even more
exergonic (eq 3). ΔG343 ranges from −45.4 to −47.6 kcal/mol
depending on the butene isomers formed.
H5PVIV2Mo10O39 + H2O → H7PVIV2Mo10O
(3)
40
As previously shown, oxidation reactions with H5PV2Mo10O40
are inclined to outer-sphere reactions of proton transfer (PT),
electron transfer (ET), and coupled proton and electron transfer
(CPET) as the first step in the reaction pathway.17 In this case the
calculations demonstrate that ET is exergonic in a polar solution
while other outer-sphere reactions are endergonic (Scheme 4).18
d[H5PV Mo10O (red)]/dt = k[H5PV Mo10O ][Bu4Sn]
2
40
2
40
(1)
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Scheme 4. Calculated ΔG343 in kcal/mol for Outer-Sphere
Scheme 5. Scenarios for the Transformation of Complex 6
V
Interactions of H5PV2 Mo10O40 with n-Bu4Sn in Acetonitrile
The ESI-MS data showing species where n-Bu4Sn is co-
ordinated to the polyoxometalate prompted a more detailed in-
vestigation of the ET process. Indeed, we found that formation
of complex 5 (Figure 4), in which the tin atom is bound, with a
Figure 4. Optimized geometry of n-Bu4Sn coordinated to the bridging
oxygen atom of H5PV2Mo10O40 before (complex 5) and after ET
(complex 6).
bond energy of 8 kcal/mol, to a bridging oxygen atom of the
polyoxometalate (Figure 4), precedes ET. In this complex there
is a strong distortion of the coordination sphere around the tin
atom with an increase of ∠C−Sn−C from 109.7° in n-Bu4Sn
to 166.8° in 5. This deformation makes complex 5 less ener-
getically favorable by 13.0 kcal/mol and determines the activa-
tion barrier for reduction of the polyoxometalate.19 Such a
deformation is more favorable for n-Bu4Sn in the triplet state
than in the singlet state. Thus, the subsequent formation of the
triplet complex 6 (Figure 4) proceeds with an energy gain of
2.6 kcal/mol and results in the ET from n-Bu4Sn to H5PV2Mo10O40.
The highest occupied orbital in n-Bu4Sn is mostly localized
along the Sn−C bond (Figure S5, Supporting Information);
thus, both coordination to the strongly electronegative O atom
in 5 and the ET in 6 lead to a considerable lengthening of one
Sn−C bond, from 2.20 Å in n-Bu4Sn to 2.30 Å in 5 and 2.31 Å
in 6. Moreover, the ET causes significant elongation of the
O−Sn bond, from 2.26 Å in 5 to 2.35 Å in 6. The bonding
pattern in 6 allows for three further possible transformations, as
detailed below.
Figure 5. Geometry and localization of the highest spin density in
complexes 7 and 8.
is slightly uphill, but this is compensated by their interaction with
the polyoxometalate to yield complexes 7 and 8.
(iii) The least favorable transformation involves cleavage of a
Sn−C bond (Scheme 5(iii)) with formation of 8 (Figure 5) and
•
release of SnBu3 , which in turn is stabilized by reaction with
H5PV2Mo10O40.
Thus, all three transformations of complex 6 lead to the same
reaction products 7 and 8 with a total energy gain of 64.8 kcal/mol.
The Bu+ cation in 8 is strongly bound to bridging oxygen atoms
+
(WBI = 0.84), while the SnBu3 cation in 7 is only weakly
bonded to the bridging oxygen atom (WBI = 0.25). The spin
density in these complexes is mainly localized on the V and Mo
atoms adjacent to the coordination sites (Figure 5). It is there-
1
fore reasonable that 7 and 8 are not observable by 13C and H
NMR.
The release of 1-butanol from 8 requires the cleavage of
adjacent V−O and Mo−O bonds. Although these bonds are
weakened, this reaction was found to be thermodynamically
unfavorable and kinetically forbidden at 70 °C in acetonitrile.
The lowest energy pathways for the transformation of an ion-
pair complex involves traces of water that facilitate the forma-
tion of defect structures with coordinatively unsaturated sites
(CUS).17 In this way 9 (Figure 6) is formed from 8, where the
1-butanol fragment is weakly bound to the vanadium atom of
the defect site (WBI = 0.32). Release of 1-butanol with sub-
sequent closing of the defect site (Figure 6) results in formation
of H7PVIV2Mo10O40, where a bridging oxygen atom has been
replaced by an oxygen atom from water. This mechanistic
(i) The energetically most favorable transformation of 6
(Scheme 5(i)) involves the breaking of a Sn−C bond with for-
mation of [SnBu3 -H5PVIVVVMo10O40)] (complex 7) and the
+
formation of a butyl radical, Bu• (Figure 5). The latter is
quickly trapped by another molecule of H5PV2Mo10O40 to form
[Bu+-(H5PVIVVVMo10O40)] (complex 8) (Figure 5).
(ii) Cleavage of the Sn−O bond yielding [H5PVIVVVMo10O40]−
(complex 3) and SnBu4•+ is energetically less favorable (Scheme 5(ii)).
One of the Sn−C bonds in SnBu4•+ is further lengthened from
2.31 Å in 6 to 2.48 Å in SnBu4•+, with a corresponding change
in the Wiberg bond index (WBI) from 0.67 to 0.26.20 The sub-
sequent cleavage of the Sn−C bond to yield Bu• and (n-Bu)3Sn+
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stable product in the reaction mixture that is likely to be
1
observable by 13C and H NMR, as found experimentally.
Release of (n-Bu3Sn)2O at higher temperature would result
in an 18O-labeled product, as observed, and formation of
H7PVIV2Mo10O40, where a bridging oxygen atom has been
replaced by an oxygen atom from water.
CONCLUSIONS
■
The reaction of phosphomolybdates, H3+xPVxMo12‑xO40, where
x = 1, 2, with n-Bu4Sn under anaerobic reaction conditions
proceeds by electron transfer from n-Bu4Sn to the poly-
oxometalate, as observed experimentally by UV−vis and EPR
spectrometry. At 70 °C under argon the oxygenated or
dehydrogenated products are not released but the formation of
the incipient products can be observed by ESI-MS. Thus, there
is inner-sphere formation of the coordinated oxygenated prod-
ucts after the initial electron transfer. At higher temperatures of
120 °C and above the products are released with the expected
reaction stoichiometry, as delineated in Scheme 2. We have
used DFT to gain a mechanistic understanding; Figure 8 sum-
Figure 6. Water-assisted formation of the defect complex 9 showing
that the O atom in the weakly bound 1-butanol fragment originates
from the polyoxometalate lattice while the water molecule saturates
the CUS.
scenario is in agreement with the observed formation of
1-butanol as the only alcohol product and with the oxygen
atom in 1-butanol originating from the polyoxometalate lattice.
The coordinated butyl can undergo dehydrogenation with
formation of n-butene. This reaction is more favorable than
oxygenation by 3.6 kcal/mol at 70 °C in acetonitrile.
In the similar defect structure 7-H2O formed in the presence
of an inner-sphere water molecule, a coordinated (n-Bu)3SnOH
moiety remains strongly bonded to the V atom of the defect
site (WBI = 0.45) and its subsequent dissociation is not a
feasible step toward the oxygenated product. On the other
hand, extremely high negative charge on the oxygen atom
bound to SnBu3+ favors bonding of (n-Bu)3Sn• (Scheme 5(iii))
to yield coordinated (n-Bu3Sn)2O.21 This bulky moiety is
especially stable in the presence of two inner-sphere water
molecules that mediate the very open defect complex 10
(Figure 7), in which a V atom bound to (n-Bu3Sn)2O is con-
Figure 8. Relative energies of the key intermediates in the anaerobic
oxidation of n-Bu4Sn at 70 °C in acetonitrile.
marizes the energy profile obtained from the calculations.22
The computations show that an initial interaction between
n-Bu4Sn and H5PV2Mo10O40 leads to the ion pair n-Bu4Sn•+−
H5VIVVVMo10O40 (6). At this point three exergonic scenarios
were identified, through cleavage of Sn−C or Sn−O bonds
yielding butyl and tributyltin radicals or cations that react with
the polyoxometalate leading to two paramagnetic intermediates,
Bu+−H5VIVVVMo10O40 (7) and n-Bu3Sn+−H5VIVVVMo10O40
(8).23 Likely these open-shell intermediates cannot be observed
by NMR. Bu+−H5VIVVVMo10O40 yields 1-butanol and butenes
at higher temperatures, but n-Bu3Sn−H5VIVVVMo10O40 appa-
rently reacts with a tributyltin radical to yield a closed-shell
intermediate, (n-Bu3Sn)O− H5VIVVVMo10O39, with a defect
structure that was identified by ESI-MS and NMR. Release of
1-BuOH, butenes, and (n-Bu3Sn)O is possible at higher tem-
peratures in the presence of water through coordinatively
unsaturated species in a reaction that is in part thermodynami-
cally driven by the formation of the two-electron-reduced
H7PVIV2Mo10O40.24 The isomerization of n-butene is known to
be polyoxometalate acid catalyzed at >100 °C.25
Figure 7. Water-assisted formation of defect complex 10.
nected to the remainder of the polyoxometalate structure by
only one bond: i.e., it closely resembles the [(Bu3Sn)2-OVOH2]+
species observed by ESI-MS. Moreover, the distance between
the displaced V atom and the central P in this complex is
5.55 Å, much longer than the typical V−P distance in the POM
(∼3.55 Å). This complex is the major V(IV)-containing species
observed in the EPR spectrum (Figure 1), as also discussed
previously for other oxidation reactions.12,16a The calculations
show that 10 has a closed-shell electronic configuration in the
ground state, whereas the lowest energy diradical form is 5.8
kcal/mol higher in energy. Complex 10 seems to be the only
Despite the fact that products are not released under anaer-
obic reaction conditions at 70 °C, under O2 the reactions do proceed
catalytically with formation of products. Here H4PVMo11O40 was
found to be less effective. Experiments indicate that, under
these conditions, the Sn−O bond in the (n-Bu3Sn)2O product
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two f exponents given by Martin and Sundermann.32 This
combination is of double-ζ plus polarization quality. The cc-pVDZ-
PP basis set−RECP combination was used for tin.33 As this is well-
known to speed up “pure” DFT calculations on large systems by 1−2
orders of magnitude, we applied the density fitting (DF)
approximation34 for calculating the two-electron (Coulomb) integrals.
The automatic algorithm as implemented in Gaussian 09 generated the
auxiliary basis sets required for this. Thus, the geometries of all the
complexes were optimized at the PBE-DF/PC1 level of theory without
symmetry constraints. Subsequently, single-point calculations at the
optimized geometries were performed using the empirical hybrid
meta-GGA functional M0635 combined with the same basis set (M06/
PC1), as well as using the PBE functional but combined with a larger
AUG-PC1 basis set. The AUG-PC1 basis set represents a combination
of the aug-pc-1 basis set on the main-group elements,36 the aug-cc-
pVDZ-PP basis set on tin,31 and SDD basis set with added f basis
functions on molybdenum and vanadium. The final level of theory
applied in the present work is thus an additive approximation:
Ee[M06/AUG-PC1] ≈ E[PBE-DF/AUG-PC1] − E[PBE-DF/PC1] +
E[M06/PC1]. Bulk solvent effects of the acetonitrile medium have
been calculated at the M06/PC1 level via the self-consistent reaction
field (SCRF) method,37 using the continuum solvation model
COSMO as it is implemented in Gaussian 09.38 Dispersion interac-
tions within the computed structures were approximately taken into
account by adding the D2 empirical dispersion correction term,39 with
an s6 value of 0.25, as suggested by Karton et al. for the M06 func-
tional.40 For qualitative interpretation of the computational results, the
M06/PC1 electron density of the structures in optimized geometries
was analyzed using natural bond orbital (NBO) analysis.41
The performance of the applied computational method in the
prediction of optimized geometries and in energies of protonated
polyoxometalates in the absence and in the presence of additional
coordinated species was discussed in our previous papers.16 The ability
of the computational method to accurately predict relative energies of
species with different multiplicity states, usually not a trivial task for
DFT methods, is especially important for the present study. Using
small molybdenum and vanadium oxide model complexes, relative to
basis set limit CCSD(T) benchmark data, we found RMSDs of 3.49
and 7.26 kcal/mol, respectively, for electron transfer (ET) and coupled
proton and electron transfer (CPET) reactions. This is close to the
performance of an accurate double-hybrid method such as B2GP-
PLYP. These latter results will be discussed in detail in a forthcoming
full paper.
is formed mostly via oxygen transfer from the polyoxometalate.
The computational and 18O labeling studies with 18O2 indicate
that following the oxygen transfer a reduced complex with an
intact Keggin structure, H7PVIV2Mo10O40, is formed by reaction
with water and then oxidized with O2. On the other hand, the
same experiments and those with S8 show that the source of
oxygen in 1-butanol is at least mostly from O2. The mode of
reaction of O2 with the reduced polyoxometalate−substrate
intermediate species R−H5PVIVVVMo10O40 remains an im-
portant topic to be studied by experiment and computation,
and both inner-sphere and outer-sphere reactions should be
considered.26,27 Another comment to make is that cleavage of
the initially formed n-Bu4Sn•+ could also form butyl radicals, as
suggested in stoichiometric oxidations with Ir(IV) and Fe(III),13a
which could be trapped by O2 to yield an intermediate peroxo
species that should yield equimolar amounts of 1-butanol and
n-butanal.13a The absence of an aldehyde product and the ab-
sence of oxygenates from the more stable secondary carbon
radical, as well as the known inhibition of free radical reactions
by H5PV2Mo10O40,27 all argue against such a reaction pathway.
In a more general context, the insertion of an oxygen atom
from O2 into a metal−carbon bond is an important step in the
development of a catalytic oxidation of alkanes to alcohols
through a combination of C−H bond activation and oxygen
atom insertion.
EXPERIMENTAL SECTION
■
Oxidation Reactions. Reactions under anaerobic conditions were
carried out in 15 mL Ace glass pressure tubes. The tubes were charged
with the appropriate amounts of substrate, catalyst, and solvent and
were gently flushed with argon for 5 min. The tubes were then frozen
with liquid N2, pumped down, filled with argon, and then thawed. This
procedure was repeated three times until final pressurization to 2 bar
of argon. Thus, in a typical anaerobic experiment, H5PV2Mo10O40·
34H2O (23 mg, 10 μmol) was dissolved in acetonitrile (1 mL), and
then n-Bu4Sn (10 μL, 29 μmol) was placed in a 15 mL Ace glass
pressure tube. The resulting solution was taken through several gas/
degas cycles as noted above. The glass tube was placed in an oil bath at
70 °C. After 2 h the solution turned dark green. The reaction mixture
was cooled, and the solution was analyzed for n-Bu4Sn conversion and
product formation by GC and GC-MS. GC and GC-MS analyses were
preformed on aliquots directly withdrawn from the reaction mixture
after dilution with dichloromethane and addition of n-decane as an
ASSOCIATED CONTENT
external standard.
■
18
Insertion of 18O into products from H5PV2Mo10
O
(40−50%
18
40
S
* Supporting Information
18O) was measured by reacting n-Bu4Sn and H5PV2Mo10
O
40
(10 mM) in acetonitrile at 70 °C. 18O incorporation was calculated
from the mass spectra by comparing peak intensities. For 1-butanol peaks
at m/z 73 and 75 were compared. 18O incorporation for (Bu3Sn)2O was
determined by ESI-MS by comparing peak intensities at m/z 597 and
599, respectively. Kinetic profiles of the interaction of n-Bu4Sn with
H5PV2Mo10O40 were obtained by purging a solution of 2 μmol of
H5PV2Mo10O40 in 2 mL of acetonitrile with argon for 20 min in a
quartz cuvette and then heating to 50 °C. The reaction was initiated by
the addition of 29 μmol of n-Bu4Sn. The reaction was followed by
reduction of the polyoxometalate at λmax 750 nm. Pseudo-first-order
rate constants (r2 = 0.99) were computed from the absorption data.
Computational Methods. All computations were done using a
locally modified version of the Gaussian 09 package. Geometry op-
timizations and harmonic frequency evaluations have been performed
using the nonempirical GGA density functional28 PBE29 in
conjunction with the PC1 basis set. The latter is a combination of
Jensen’s polarization consistent pc-1 basis set30 for the main-group
elements with the Stuttgart−Dresden−Dolg (SDD) basis set−RECP
(relativistic energy-consistent pseudopotential) combination31 for
molybdenum and vanadium. To the latter wa added a single f-type
polarization function, the exponent being the geometric average of the
Text, tables, and figures giving additional experimental information
and data and details of the calculations. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
Ronny.Neumann@weizmann.ac.il
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Israel Science Foundation
Grant No. 1073/10 and the Helen and Martin Kimmel Center
for Molecular Design. Dr. Lev Weiner is thanked for the EPR
measurements. J.M.L.M. holds the Baroness Thatcher
Professorial Chair of Chemistry. R.N. is the Rebecca and Israel
Sieff Professor of Chemistry.
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dx.doi.org/10.1021/ja409559h | J. Am. Chem. Soc. 2013, 135, 19304−19310

Journal of the American Chemical Society
Article
mechanism. However, the most energetically favorable CPET is by
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