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10.1002/ejic.201901152
European Journal of Inorganic Chemistry
catalysts.
Catalyst:
24
µmol.
Acetonitrile:
20
mL.
Ratio
catalyst/cyclooctene/H2O2= 1/250/250.
Catalysts
T(°C)
(%)
ref
(n-Bu4N)3[NaHAsW9O33{P(O)CH2CH2CO2H}2]
(2)
20
96
5
(n-Bu4N)3[NaHPW9O34{P(O)p-C6H4NH2}2] (3)
(n-Bu4N)4[NaAsW9O33{P(O)tBu}2]
20
20
97
5
5
7.5
(n-Bu4N)4[NaAsW9O33{P(O)tBu}2]
50
20
100
100
5
(n-Bu4N)3[NaHAsW9O33{P(O)tBu}2] (1)
This
work
Figure 3: Optimized structure of the [NaAsW9O33{P(O)CH3}2]4- (left) and
[NaHAsW9O33{P(O)CH3}2]3- (right) anions showing the location of the Na+
cation (violet) inside the lacuna of the anion and of the proton (white) on
one of nucleophilic WO. All the atoms - W (in blue), O (in red), As (in
prune), P (in orange), C (in grey), H (in white) - are represented in stick
mode, except for the lacuna (ball and sticks). Distances within the ball and
stick part are in Å.
1.2. Theoretical insight in the exact localization of
the proton in (n-Bu4N)3[NaHAsW9O33{P(O)R}2]
Having in mind these experimental results, we were
interested in determining the nature of the catalytic site and
especially the role of protons in the reactivity of the (n-
Bu4N)3[NaHAsW9O33{P(O)R}2]
compounds
(R
=
-
CH2CH2CO2H, -tBu). A DFT study was thus carried out (see
experimental part for computational details) aiming at
analyzing the energetic and structural factors associated to
the protonation of the phosphonyle polyoxoanions. This was
first carried out on a slightly simplified model of the anionic
part of the compound, namely [NaHAsW9O33{P(O)CH3}2]3-.
Working on such highly charged species can be justified by
the non-coordinating nature of the counter ions (n-Bu4N+).
In a general way, the [NaAsW9O33{P(O)R}2]4- anions are
obtained though the grafting of two phosphonyle groups onto
four oxygen atoms from the lacuna of the B,α−{AsW9O33}9-
unit. Consequently, the two free O atoms of the lacuna are
strongly nucleophilic. Furthermore, the two O atoms from the
{RP=O} functions present nucleophilic properties, as shown
before.11,16 For this reason, we have considered that the Na+
cation was located inside the lacuna and linked to these four
O atoms, a location which was found as a minimum (Figure 3)
by DFT calculations (see Computational details). Despite the
1.3. Theoretical examination of the interaction of
n-
H2O2
with
(
Bu4N)3[NaHAsW9O33{P(O)CH3}{P(O)CH2CH2COOH}]
Regarding the literature and the impact of acetic acid in
epoxidation processes,19 it was also interesting to determine
whether the presence of carboxylic acid functions found on
the hybrid of POM could affect the chemical process. To carry
out this theoretical study, one CH3
group of
[NaAsW9O33{P(O)CH3}2]4- was replaced by a CH2CH2CO2H
group. In this case, a similar structure was obtained for the
protonated
product
[NaAsW9O32(OH){P(O)CH3}{P(O)CH2CH2COOH}]3- (designed
as {W-OH}, see figure 4, top/left and figure S2 for complete
results), and no significant impact of the carboxylic acid
function was observed at this stage. In particular, as only one
carboxylic acid arm was added (see Figure 4), this
simplification obviously led to a dissymmetry. However, we
found that the coordination of the proton on one or the other
{W=O} group of the lacuna differed by less than 0.1 kcal.mol-1.
Furthermore, the most stable structure resulting from the
transfer of the carboxylic acid proton onto the second {W=O}
group of the lacuna was found to be more than 15 kcal.mol-1
higher in energy. Consequently, the possibility for
transprotonation from the CH2CH2CO2H chain to the POM
framework was definitively discarded.
absence of X-ray diffraction structural determination available
n-
for
the
family
of
compounds
(
Bu4N)3[NaHAsW9O33{P(O)R}2],17 the coordination of Na+ is in
line with the partial structure determined by X-Ray
crystallography obtained on (n-Bu4N)3[NaHPW9O34{As(O)p-
C6H4NH2}2] (3).16 In contrast, in the absence of experimental
elements, the position of the H+ counterion had to be
determined computationally. This was carried out through
geometry optimization of the regioisomers differing from the
proton position (see figure S1 for complete results). The
isomers where the proton is localized on one of the two
nucleophilic O atoms that delimit the lacuna (Figure 3) are the
most stable. This is in accordance with the experimental
results obtained by the group of Mizuno in the case of other
lacunary POMs such as [γ-{SiW10O32(H2O)2}2(µ-O)2]4-.18 The
presence of the proton is shown to significantly lengthen the
Na…O(H)W distance, compared to when the proton is absent
(Figure 3). In conclusion, it is probably more accurate to
consider one {W=O} group that delimit the lacuna of the
{XW9} scaffold as a {W-OH} group. Further insight within the
formation of the catalytically active site was sought through
examination of the interaction of H2O2 with the lacuna.
In a second step, the interaction of H2O2 with the
[NaAsW9O32(OH){P(O)CH3}{P(O)CH2CH2COOH}]3-
model
compound was examined. Thus, the formation of
a
hydroperoxide site {W-OOH} (associated to a free water
molecule), instead of the {W-OH} group, was found to be
quasi athermic. Moreover, it can be stabilized by more than
10 kcal.mol-1 through coordination of water to the Na+ center
(figure 4, top/right).
On the other hand, the formation of a peracid function
on the carboxylic acidic arm was slightly less favored than the
formation of the hydroperoxide (+1.4 kcal.mol-1, {CO-OOH}
figure 4, bottom/left). This preference did not change when
H2O was bound to Na+ (total: +2.7 kcal.mol-1, see Table S3).
Finally, the formation of
a
peroxo-hydroxo pair
(designed as {W-OO} figure 4, bottom/right, and figure S4 for
complete studies) after proton transfer from the
hydroperoxide to the other {W=O} site of the lacuna, was also
slightly disfavored (+2.5 or + 4.2 kcal.mol-1, without or with a
water molecule coordinated on the Na+ cations respectively).
Despite these small energy differences, and considering:
3
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Makrygenni, Ourania
