A pentanuclear yttrium hydroxo cluster as an oxidation catalyst. Catalytic oxidation of aldehydes in the presence of air

Article abstract of DOI:10.1039/b315218a

The air- and moisture-stable pentanuclear yttrium cluster H ₅[Y₅(μ₄-O)(μ₃-O) ₄(μ-η²-Ph₂acac) ₄(η²-Ph₂acac)₆] (Ph ₂acac = dibenzoylmethanide) has been used as a homogenous catalyst for the oxidation of aldehydes to the corresponding carboxylic acids in the presence of air.

Full text of DOI:10.1039/b315218a

A pentanuclear yttrium hydroxo cluster as an oxidation catalyst.
Catalytic oxidation of aldehydes in the presence of air
Peter W. Roesky,* Graciela Canseco-Melchor and Agustino Zulys
Institut für Chemie, Freie Universität Berlin, Fabeckstrasse 34–36, 14195 Berlin, Germany.
E-mail: roesky@chemie.fu-berlin.de
Received (in Cambridge, UK) 24th November 2003, Accepted 28th January 2004
First published as an Advance Article on the web 23rd February 2004
2 6 2
composition [Y(H O) Cl
]+Cl2 in which the six water molecules
The air- and moisture-stable pentanuclear yttrium cluster
2
2
H
5
[Y
5
(m
4
-O)(m
3
-O)
4
(m-h -Ph
2
acac)
4
(h -Ph
2
acac)
6
] (Ph
2
acac =
are coordinated via the oxygen atom onto the metal centre. Addition
of base just leads to deprotonation of the coordinated water
molecules. The pentanuclear arrangement of 1 is a common
dibenzoylmethanide) has been used as a homogenous catalyst
for the oxidation of aldehydes to the corresponding carboxylic
acids in the presence of air.
1
0,13,14
structural motif for lanthanide oxo/hydroxo clusters.
We
chose this common structural motif so that we would have easy
access to nanostructural lanthanide catalysts.
In recent years, lanthanide clusters have attracted increasing
interest as a result of their novel structural characteristics and
1
fascinating properties, as well as their many potential applications
in fields such as in magnetic and optical materials and catalytic
processes, and their use as precursors for lanthanide-based oxide
(
1)
2
materials. The chemistry of lanthanide clusters is not as developed
3
,4
as the cluster chemistry of the transition metals.
This is
Our intention was to use 1 as catalyst for an aldol reaction.
Therefore, cyclohexylcarbaldehyde, acetone, small amounts of
water and catalytic amounts (0.5 mol%) of 1 were mixed together
particularly surprising given that oligomeric lanthanide structures
are proposed to play a key role in some catalytic transformations.5
Therefore, we set out to synthesise an easily accessible air-stable
lanthanide cluster in order to investigate its properties in homoge-
nous catalysis. It is well known that the most stable lanthanide
clusters are oxo/hydroxo clusters. Recently, we and others have
synthesised new tetradeca- and pentadecanuclear lanthanide oxo/
hydroxo clusters.7 Basically, two different approaches towards the
preparation of oxo/hydroxo clusters were investigated. One way to
obtain these compounds is the hydrolysis of a moisture-sensitive
starting material, whereas the other approach is hydrolytic, wherein
the water molecules are deprotonated in a controlled manner.3
Most of the oxo/hydroxo clusters are stabilised by ligands such as
,6
2 2
in CH Cl . To our surprise, no aldol condensation product was
formed. Instead, the aldehyde was oxidised quantitatively to the
corresponding carboxylic acid. Under these conditions, the reaction
was complete after 12 h. No isopropanol byproduct was detected by
GC/MS. Later, it turned out that the reaction also runs without
acetone and water. However, when leaving out 1 or performing the
reaction under a nitrogen atmosphere, no oxidation reaction was
observed. Therefore, we propose that the aldehyde is oxidised by
oxygen from the air (eqn. 2).§ Besides the carboxylic acid, traces of
cyclohexanol and cyclohexanone were also detected by GC/MS. It
was established previously that air oxidation of aliphatic aldehydes
,8
,5
alkoxides,9 phenoxides, b-diketones
5
10,11
and amino acids.3 In
15
leads to these byproducts via CO extrusion.
contrast to the well-established solid-state structures of lanthanide
oxo/hydroxo clusters, almost nothing is known about their potential
in catalysis. In this paper, we report the synthesis and structural
characterisation of an air- and moisture-stable pentanuclear yttrium
(2)
2
2
cluster,
2
H
5
[Y
5
(m
4
-O)(m
3
-O)
4
(m-h -Ph
2
acac)
4
(h -Ph
2
acac)
6
]
(1;
In the next step, the scope and the limitations of the reaction were
investigated. A variety of aliphatic, olefinic and aromatic aldehydes
were used as starting materials (Table 1). It turned out that aliphatic
aldehydes are converted in high yields under the conditions
described above (entry 1–4). Thus, turnover numbers up to 200
have been achieved in 12 h. On the other hand, aromatic systems are
not oxidised (entry 7 and 8). A CNC double bond has a negative
Ph acac = dibenzoylmethanide), and its application in homoge-
nous catalysis.
In order to have easy access to reasonable amounts of the
proposed cluster catalyst, we followed a synthetic route which is
1
2,13
comparable to a procedure described previously.
one equivalent of YCl ·6H O with two equivalents dibenzoyl-
methane in presence of triethylamine yields 1 as yellow crystals
eqn. 1).† Cluster 1 crystallises in the tetragonal space group P4/n,‡
Reaction of
3
2
(
the unit cell contains two discrete pentayttrium cores (Fig. 1) in a
square-based pyramidal arrangement. This structure is similar to
1
3
that of [Eu
5
(OH)
5
(Ph
2
acac)10]. Each triangular face of the square
8
-O moiety, similar to [Y (OH) -
pyramid is capped by one m
3
9
O
2
2
10
2
(
(
[
aacac)16
BA
2 6 4
Ln14(OH)18(o-O NC H
]
(aacac = allylacetoacetate), [Ln
benzoylacetone, Ln Sm, Eu, Gd, Dy, Er),
O)24] (Ln = Er, Yb) and related com-
9 2 8
O (OH) (BA)16]
12
=
=
7
1
1,13
pounds.
In the square-base face, four yttrium atoms are linked
by one m
4
-O atom. In 1, each yttrium ion is coordinated to eight
oxygen atoms displaying a square antiprismatic arrangement. The
core is surrounded by ten peripheral ligands. The ligand shows
Y
5
two types of coordination behaviour. Six ligands are terminal
chelates and four are bridging chelates bonding to two metal ions
that belong to the base of the polyhedra. The apical yttrium, which
lies on a fourfold symmetry axis, is bonded to two chelate ligands,
which are disordered in the solid-state structure. The disorder in the
structure may also be a result of a twinning problem. The formation
of the hydroxy bridges can be explained by a closer look at the
starting material. YCl
3
·6H
2
O is, in fact, an ionic species of
Fig. 1 Solid-state structure of 1, with hydrogen atoms omitted.
7
38
C h e m . C o m m u n . , 2 0 0 4 , 7 3 8 – 7 3 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Table 1 Air oxidation of aldehydes catalysed by 1a
(nOH), 2059 m, 2677 m, 1599 s (nCNO), 1537 s sh, 1467 s sh, 1430 s, 1297
1
m sh, 1223 m, 1070 w, 1024 w (nC–O), 720 m sh, 609 w, 525 m. H NMR
(CDCl
, 25 °C): d 2.13 (s, CH), 6.40–7.96 (m, Ph). 13C{H} NMR (CDCl
25 °C): d 31.1 (CH), 127.1 (m-C ), 128.1 (p-C ), 128.7 (o-C ).
Crystal data for 1: C150 , M = 2761.92, tetragonal space group
Reaction
3
3
,
b
Entry
Aldehyde
time/h
Yield (%)
H
6 5
H
6 5
6 5
H
‡
115 25 5
H O Y
6
3
1
1
2
2
> 99
P4/n, a = 1944.6(4), c = 1853.0(4) pm, V = 7021(3) 3 10 pm , T =
293(2) K, Z = 2, m = 2.111 mm , 30 524 reflections collected, R1 =
1
2
3
4
> 99c
21
0
.0643 for 3378 F > 2(F), wR2 = 0.1867 for all 3378 data, 300 parameters,
8
2
78
> 99
all non-hydrogen atoms calculated anisotropic (except C11–C15 and C26–
C38); the positions of the H atoms were calculated for idealised positions.
1
1
1
8
2
74
87
The structure was solved and refined using SHELXS-97 and SHELXL-
18
9
7. CCDC 224900. See http://www.rsc.org/suppdata/cc/b3/b315218a/ for
d
8
2
71
99
crystallographic data in CIF or other electronic format.
2 2
To a stirred solution of 0.5 mol% of 1 in 4 mL of CH Cl at 25 °C was
e
§
added 149 mL of cyclohexylcarbaldehyde (1.23 mmol). Then, the solution
d f
was stirred at 40 °C for 12 h. The mixture was quenched by addition of 1 N
5
6
12
24
45 ,
2 2
HCl (1 mL), CH Cl was removed using a rotary evaporator and the
aqueous layer was extracted with diethyl ether (2 3 10 mL). The combined
d f g
7.5 , ,
organic phase was washed and dried over Na
2 4
SO . The solvent was
removed under reduced pressure.
7
8
12
12
0
0
1
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2
3
a
. b Determined
Reaction conditions: 0.5 mol% of 1, 40 °C, solvent: CH
2
Cl
2
4
1
c
d
e
by H NMR spectroscopy. CDCl
3
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f
g
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5
6
7
8
9
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influence if it is conjugated to an aromatic group (entry 5 and 6).15
Attempts to recover the catalyst have, so far, been unsuccessful.
We suggest that oxygen from the air may insert into the RC(NO)–
H bond. Thus, a peroxyacid may initially be formed, which then
rearranges to the carboxylic acid. To the best of our knowledge,
other lanthanide catalysts do not catalyse this type of reaction.
Thus, Ln(OTf)
give aldol reaction products, whereas [Ln{N(SiMe
Tishchenko or an aldol-Tishchenko reaction.
We thank the Deutsche Forschungsgemeinschaft, the Fonds der
Chemischen Industrie, the DAAD (fellowship for A. Z.) and the
CONACyT (postdoctoral fellowship for G. C.-M.) for financial
support.
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1
1
287–1303; L. G. Hubert-Pfalzgraf, Coord. Chem. Rev., 1998,
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16
17
3
3 9
and lanthanide alkoxides such as [La (OtBu) ]
5
3
)
2
}
3
] leads to a
10 L. G. Hubert-Pfalzgraf, N. Miele-Pajot, R. Papernik and J. Vaissermann,
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1
1
1
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Notes and references
3
213–3220.
†
Preparation of 1: 200 mg (0.66 mmol) of YCl
3
·6H
2
O and 263 mg (1.17
15 D. R. Lakin, J. Org. Chem., 1990, 55, 1563–1568.
16 S. Kobayashi, Synlett, 1994, 545–546.
mmol) of Ph acacH (dibenzoylmethane) were dissolved in 10 mL of
2
methanol and 147 mg (2.46 mmol) of triethylamine were added to the
solution. The mixture was stirred at room temperature for 18 h. A yellow
precipitate was formed, which was filtered off and washed with 5 mL of
hexane. Yellow crystals were obtained by diffusion after three days
17 M. Shibasaki, H. Sasai and T. Arai, Angew. Chem., Int. Ed. Engl., 1997,
36, 1237–1256.
18 G. M. Sheldrick, SHELXS-97, Program for Solution of Crystal
Structures, University of Göttingen, Germany, 1997; G. M. Sheldrick,
SHELXL-97, Program for Refinement of Crystal Structures, University
of Göttingen, Germany, 1997.
[
2 2
CH Cl –hexane, (1 : 6)]. Yield: 213 mg (33%). Found: C, 65.40; H, 4.22;
150 115 25 5
H O Y
requires: C, 65.22; H, 4.19%. IR (KBr): n/cm2 3421 m
1
C
C h e m . C o m m u n . , 2 0 0 4 , 7 3 8 – 7 3 9
739