B. Wang, C. Hu et al.
and consequently generates active vanadium centers for the
aerobic oxidation of alcohols.
Specifically, under un-optimized conditions, compound 3 ex-
hibits extraordinary efficiency in converting salicylol to sali-
cylaldehyde with high conversion and selectivity (conv.=
100%, sele.=96.7%). Importantly these new vanadium clus-
ters can be easily recycled and reused with unchanged cata-
lytic activity. This opens a new route to functionalize POVs
toward catalytic applications and may shed light on discov-
ering more new polynuclear metal clusters. The catalytic
cycle and mechanism on such polynuclear mixed-valent va-
nadium clusters are rather complicated. Indeed, the investi-
gation on other new organo-POV structures by using this
approach and the study of the detailed catalytic mechanism
are in process, which will be reported in due course.
Based on our results and precedent work, a plausible cat-
alytic mechanism can be proposed and is shown in Figure 5.
First, species A reacts with both alcohol and molecular
oxygen and consequently strips down two imidazole mole-
cules to form the vanadium(V) species B. Species B further
reacts with alcohol and affords a vanadium alcoholate, C.
Then, the subsequent dehydration of C by oxidation repro-
duces a vanadium(IV) species D and the target molecules.
Finally, species D reacts with two imidazole molecules to
form species A again.
Experimental Section
4 3
Synthesis of 1: NH VO (0.234 g, 2 mmol) in 1-methylimidazole (10 mL)
was sealed in a 50 mL Teflon-lined stainless steel container, which was
heated to 908C under autogenously pressure for 72 h. Yield: 50% (based
on V); IR (KBr): n˜ = 3136 (m), 1531 (m), 1513(s), 1420 (s), 1277(m),
1
4
228(m), 1098(s), 924 (s), 862(m), 788(s), 726(s), 652(s), 608(m),
À1
90(m) cm ; elemental analysis calcd (%) for C32
H
48
N
16
O
15
V
6
: C, 31.98;
H, 4.02, N, 18.64 (18.34); found: C 31.71; H, 4.61; N, 18.34.
Synthesis of 2: NH VO (0.234 g, 2 mmol) and [VO(acac)
4
3
A
H
U
T
E
N
N
2
] (0.265 g,
1
mmol) in 1-methaimidazole (10 mL) was sealed in a 50 mL Teflon-lined
stainless-steel container, which was heated to 1008C under autogenously
pressure for 72 h. Yield: 81% (based on V); IR (KBr): n˜ =3125 (m),
1
8
534 (m), 1519(m), 1421 (m), 1281(m), 1230(m), 1095(s), 935(s), 935(s),
90(m), 816(m), 775(s), 662(m), 615(s) cm ; elemental analysis calcd
À1
(
%) for C32
H
48 16 14 6
N O V : C 32.41, H 4.08, N 18.89; found: C 31.97, H
4
.45, N 18.39;
Synthesis of 3: The synthetic procedure was the same as for compound 2
except that 1-ethylimidazole (10 mL) was used instead of 1-methaimida-
zole. Yield: 90% (based on V). IR (KBr): n˜ =3115 (m), 1524 (m), 1446
(
6
m), 1318(m), 1240 (m), 1096(s), 946(m), 925(s), 879(m), 775(m), 750(s),
67(m), 626(w), 527(m) cm ; elemental analysis calcd (%) for (found)
À1
64 16 14 6
for C40H N O V : C 37.01, H 4.97, N 17.26; found: 37.38; H 4.89, N
1
6.91.
X-ray crystallography: The crystallographic data have been deposited
with the Cambridge Crystallographic Data Centre (CCDC) as CCDC-
8
88578 (1), CCDC-888579 (2), and 888580 (3). These data can be ob-
Figure 5. The proposed reaction pathway of oxidation of salicylol.
Acknowledgements
This work was financially supported by the National Natural Science
Foundationof China (21231002, 21276026, and 21173021), the 111 Project
In summary, we have successfully prepared three new
neutral hexanuclear vanadium clusters, named compounds
(
B07012), the Program of Cooperation of the Beijing Education Commis-
sion (20091739006).
1
, 2, and 3, in imidazoles, in which these versatile organic
compounds serve as the coordination ligands, the structural-
directing agents, and simultaneously, the reaction media.
Compound 1 can be precisely converted to mixed-valent 2
Keywords: alcohols · imidazoles · ligands · oxidation ·
polyoxometalates · vanadium
IV
and 3 upon introducing a V source in corresponding imida-
zoles, whereas 2 can undergo single-crystal-to-single-crystal
transformation to 3. These materials are chemically and
structurally stable, yet have the long-sought-after character-
istic as highly efficient and selective oxidation catalysts of
benzyl alcohols by using molecular oxygen as the oxidant.
[
1] a) Chem. Rev. 1998, 98, 1–390 (Special Issue on Polyoxometalates;
4412
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Chem. Eur. J. 2013, 19, 4408 – 4413