Relationship between structure, fluxionality and racemization activity in organometallic derivatives of polyoxometalates

Article abstract of DOI:10.1016/j.tetasy.2007.02.002

The catalytic activity of several organometallic derivatives of polyoxometalates, especially those of general formula [M₄O₁₆{M′(η⁶-arene)}₄] (M = Mo, W; M′ = Ru, Os), was evaluated in the racemization of secondary alcohols. A link between composition, fluxionality and catalytic activity was established.

Full text of DOI:10.1016/j.tetasy.2007.02.002

Tetrahedron: Asymmetry 18 (2007) 367–371
Relationship between structure, fluxionality and racemization
activity in organometallic derivatives of polyoxometalates
Danielle Laurencin,a Richard Villanneau,a Anna Proust,a,* Anne Brethon,b
Isabel W. C. E. Arendsb and Roger A. Sheldonb,*
aLaboratoire de Chimie Inorganique et Mate´riaux Mole´culaires, Universite´ Pierre et Marie Curie, Case courrier 42,
UMR 7071, 4, place Jussieu, 75252 Paris Cedex 05, France
bBiocatalysis and Organic Chemistry, Department of Biotechnology, Delft University of Technology, Julianalaan 136,
2628 BL Delft, The Netherlands
Received 23 November 2006; accepted 1 February 2007
Available online 2 March 2007
Abstract—The catalytic activity of several organometallic derivatives of polyoxometalates, especially those of general formula
[M4O16{M0(g6-arene)}4] (M = Mo, W; M0 = Ru, Os), was evaluated in the racemization of secondary alcohols. A link between compo-
sition, fluxionality and catalytic activity was established.
ꢀ 2007 Elsevier Ltd. All rights reserved.
1. Introduction
Ph
Ph
Ph
Ph
Ph
Due to their great importance in the pharmaceutical, agri-
cultural and other fine chemical industries, several teams
have focused over the past 10 years on the development
of methods for preparing enantiomerically pure secondary
alcohols.1 Among the different routes which were explored,
the Dynamic Kinetic Resolution (DKR) appears as a
powerful tool for converting a racemic alcohol into one
single enantiomer.2 It combines the action of an enzyme,
which catalyzes the enantioselective transformation of
one of the enantiomers, with that of a metal catalyst, which
catalyzes the isomerization of the other enantiomer.
Ph
O
Ru
Ru
Br
Cl
Ph
Ph
Ph
Ph
OC
OC
CO
CO
Compound A
Figure 1. Efficient Ru(II) racemization catalysts.3b,4e
Compound B
highest activity in the presence of 1 equiv of K3PO4 with
respect to the alcohol.4e,5a The search for readily accessible
and more sustainable racemization catalysts, which do
not require any additives, remains an important goal, on
which a few teams have recently started to work, via the
synthesis of heterogeneous ruthenium catalysts.6
Several RuII racemizing catalysts have thus been synthe-
sized, some of which appeared to be particularly active
and compatible with the enzymatic reaction (Fig. 1).3–5
Nevertheless, with regards to industrial applications, one
of the major drawbacks of these systems is that large quan-
tities of additives or a special control of the experimental
conditions are necessary, in order to obtain a high activity.
For instance, compound A reacts best under an argon
atmosphere and in the presence of 1 equiv of t-BuOK with
respect to the catalyst,3b whereas compound B displays its
Over the past 10 years, a great variety of RuII–arene deriv-
atives of polyoxometalates have been synthesized,7 in par-
ticular by our group. These compounds are constituted of
one or several {Ru(arene)}2+ fragments coordinated to
an oxometallic core. They are tunable in composition; in
charge (neutral or anionic species); in the mode of grafting
of the organometallic fragment(s), and in their behaviour
in solution (some compounds maintain their molecular
structure, whereas others isomerize). They are easy to
*
Corresponding authors. Tel.: +33 1 44 27 30 34 (A.P.); e-mail addresses:
proust@ccr.jussieu.fr; r.a.sheldon@tnw.tudelft.nl
0957-4166/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2007.02.002

368
D. Laurencin et al. / Tetrahedron: Asymmetry 18 (2007) 367–371
obtain in reasonable yields and are not sensitive to oxygen
or humidity.
meaning that these complexes, in the absence of additive,
have evolved towards a catalytically active species. It thus
appears that the grafting of {Ru(arene)}2+ fragments onto
a polyoxometalate framework leads to an activation of the
ruthenium centre, allowing it to evolve more easily towards
a catalytically active complex. Unfortunately, these cooper-
ative effects between the polyoxoanion and the organo-
metallic moiety are moderately efficient: the racemization
activity of the compounds remains low, insofar as after
22 h of reaction, the ee has not yet reached the value of 0%.
Given the special environment of the RuII–arene fragment
in these architectures and the robustness of the com-
pounds, they could be an alternative to the racemization
catalysts described so far.3–6 Herein, we report and discuss
the activity of some of these organometallic oxides in the
racemization of 1-phenylethanol.
Nevertheless, it is noteworthy that the catalytic activity dif-
fers from one organometallic oxide to the other, meaning
that all oxoanionic cores do not have the same efficiency
in activating the ruthenium. Apart from the complex (n-
Bu4N)xCs5ꢀx[PW11O39{Ru(p-cymene)(H2O)}] (entry 15),
the family of compounds [M4O16{Ru(arene)}4] (M = Mo,
W; entries 6–10) displayed the highest activity. However,
even within this family of complexes, different racemization
activities were observed. We thus proceeded to a more de-
tailed analysis of the catalytic tests, in order to rationalize
the differences observed, and with the prospect of under-
standing better as to which factors are likely to enhance
the racemization activity of organometallic derivatives of
polyoxometalates.
2. Results and discussion
2.1. Study of the catalytic activity of {Ru(g6-arene)}2+
derivatives of polyoxometalates
We concentrated our study on the racemization of enantio-
merically pure 1-phenylethanol, which is commonly
regarded as the gold standard in this area. The catalytic
activity of several different organometallic derivatives of
polyoxometalates in the racemization of (S)-1-phenyletha-
nol was tested. In addition, the low-nuclearity oxometa-
lates (n-Bu4N)2[WO4] and (n-Bu4N)2[Mo2O7], and the
[Ru(arene)Cl2]2 complexes (arene = toluene, p-cymene
and hexamethylbenzene) were tested for comparison with
the results reported in Table 1.
2.2. Comparison of the catalytic activity of the
As expected,8 in the absence of any additive, the ruthenium
precursors are inactive in the racemization of (S)-1-phenyl-
ethanol (entries 3–5). Similarly, the low-nuclearity molyb-
date and tungstate anions display no catalytic activity
(entries 1 and 2a), nor does the isopolyoxomolybdate
[Mo6O19]2ꢀ (entry 2b).
[M4O16{Ru(g6-arene)}4] (M = Mo, W) complexes
The family of complexes of general formula [M4O16-
{Ru(arene)}4] (M = Mo, W; arene = toluene, p-cymene,
hexamethylbenzene) has drawn our attention over the past
six years.7b–e In the solid state, two isomeric forms have
been evidenced, as shown in Figure 2. These differ by the
arrangement of the four {Ru(arene)}2+ fragments around
a central cubic [M4O16]8ꢀ core, and are referred to as the
‘Triple Cubane’ and ‘Windmill’ forms, a terminology ini-
Conversely, for most of the organometallic derivatives of
polyoxometalates tested, the ee values were below 100%,
tially proposed by Suss-Fink et al.7a In solution, some of
Table 1. Racemization of (S)-1-phenylethanol by ruthenium derivatives of
polyoxometalates
¨
the species maintain their solid state structure, whereas
others isomerize, a process during which coordination sites
on the ruthenium are temporarily released (Fig. 3).7b,e
Entry Catalysta
eeb,c (%)
1
(n-Bu4N)2[WO4]
100
100
100
96
2a
2b
3
(n-Bu4N)2[Mo2O7]
(n-Bu4N)2[Mo6O19
[Ru(toluene)Cl2]2
Based on our previous studies of the fluxionality of these
species in CHCl3 and CH2Cl2,7b,e we have determined the
behaviour of the different complexes in chlorobenzene by
various spectroscopic techniques (Table 2, entries 6–10).
]
4
5
6
[Ru(p-cymene)Cl2]2
[Ru(hmb)Cl2]2
[Mo4O16{Ru(toluene)}4]
100
100
48
d
7
8
[Mo4O16{Ru(p-cymene)}4]
[Mo4O16{Ru(hmb)}4]d
60
80
9
[W4O16{Ru(p-cymene)}4]
59
57
96
100
100
99
10
11
12
13
14
15
[W4O16{Ru(hmb)}4]d
[{Ru(hmb)}2W5O18{Ru(hmb)(H2O)}]d
[{Ru(hmb)}2Mo5O18{Ru(hmb)(H2O)}]d
[{Ru(p-cymene)}2Mo2O4{MeC(CH2O)3}2]
(n-Bu4N)xNa10ꢀx[Sb2W20O70{Ru(p-cymene)}2]
(n-Bu4N)xCs5ꢀx[PW11O39{Ru(p-cymene)(H2O)}]
67
a Reaction conditions: (S)-1-phenylethanol (0.50 mmol), catalyst: 5 mol %
(W) (entry 1), 5 mol % (Mo) (entries 2a and 2b), 5 mol % (Ru) (entries 3–
15) PhCl (5 mL), N2 atm, 70 ꢁC, 22 h.
b ee of the alcohol: (%) = 100 · ([S] ꢀ [R])/([S] + [R]).
c Measured by GC equipped with a chiral column, using an internal
standard technique. The main by-product was acetophenone (<3%).
d hmb = hexamethylbenzene.
[Mo4O16{Ru(hexamethylbenzene)}4] 7c
[Mo4O16{Ru(toluene)}4] 7e
Figure 2. Molecular structures of ‘Windmill’ (left) and ‘Triple Cubane’
(right) compounds.

D. Laurencin et al. / Tetrahedron: Asymmetry 18 (2007) 367–371
369
Figure 3. Schematic representation of the isomerization between the ‘Windmill’ (left) and ‘Triple Cubane’ (right) oxometallic structures.
Table 2. Influence of the isomerization process on the racemization activity of [M4O16{M0(arene)}4] (M = Mo, W; M0 = Ru, Os)
Entry
Catalysta
Isomer (in the solid state)
Isomerization in PhClb
ee (%)
6
7
8
9
10
16
[Mo4O16{Ru(toluene)}4]
[Mo4O16{Ru(p-cymene)}4]
[Mo4O16{Ru(hmb)}4]
[W4O16{Ru(p-cymene)}4]
[W4O16{Ru(hmb)}4]
Triple Cubane or Windmill
Windmill
Windmill
Windmill
Windmill
Yes
Yes
No
No
No
Yes
48
60
80
59
57
91
[Mo4O16{Os(p-cymene)}4]
Windmill
a Reaction conditions: (S)-1-phenylethanol (0.50 mmol), catalyst: 5 mol % (Ru) (entries 6–10), 5 mol % (Os) (entry 16), PhCl (5 mL), N2 atm, 70 ꢁC, 22 h.
b Determined by IR (entry 6), 95Mo NMR (entries 7, 8 and 16), 1H and 13C NMR (entries 9 and 10).
The following conclusions thus stem from the analysis of
their racemization activity:
tion in less than 30 min.3b,d We thus tried to improve the
catalytic activity of the most active species ([Mo4O16-
{Ru(toluene)}4], [Mo4O16{Ru(p-cymene)}4], [W4O16{Ru(p-
cymene)}4] and [W4O16{Ru(hexamethylbenzene)}4]), by
varying the experimental conditions.
1. The nature of the arene ligand coordinated to the
ruthenium hardly influences the catalytic activity of
the complexes, as can be shown by comparing the ee’s
for compounds [W4O16{Ru(hexamethylbenzene)}4] and
[W4O16{Ru(p-cymene)}4], which both maintain the
‘Windmill’ form in PhCl (entries 9 and 10).
2.3. Attempts to enhance the catalytic activity of
the [M4O16{Ru(g6-arene)}4] complexes (M = Mo,
arene = toluene, p-cymene; M = W, arene = p-cymene,
hexamethylbenzene)
2. The nature of the metal, which constitutes the [M4O16]8ꢀ
core has an effect on the racemization activity of the
complexes: compounds with tungsten are more active
than those with molybdenum, given the results obtained
for complexes [Mo4O16{Ru(hexamethylbenzene)}4] and
[W4O16{Ru(hexamethylbenzene)}4], which both main-
tain the ‘Windmill’ form in PhCl (entries 8 and 10).
3. The complexes which isomerize in solution give rise to
the highest activity; indeed, according to the first point
of this list, given the poor activity of [Mo4O16{Ru(hexa-
methylbenzene)}4], one would also have expected a
low racemization activity for [Mo4O16{Ru(toluene)}4]
and [Mo4O16{Ru(p-cymene)}4]; the higher activity of
the latter compounds could thus be attributed to the fact
that they isomerize in solution (entries 6 and 7).
2.3.1. Influence of the nature of the solvent. Previous stud-
ies have shown that the nature of the solvent often plays an
important role with regards to the efficiency of racemiza-
tion catalysts.4b,c,5a We thus tested the activity of the com-
plexes in two other solvents often used in racemization
reactions: toluene and t-amylalcohol. The results obtained
in the case of [W4O16{Ru(hexamethylbenzene)}4] and
[Mo4O16{Ru(p-cymene)}4] are reported in Table 3.
In both cases, the catalytic activity appeared to be much
lower in toluene and t-amylalcohol than in chlorobenzene.
In toluene, the lower solubility of the catalysts could ex-
plain the decrease in racemization activity. In t-amylalco-
hol, the competitive coordination of the solvent on the
ruthenium could account for the loss of efficiency in the
racemization of 1-phenylethanol.
Consequently, chemical composition and intrinsic behav-
iour in solution are two factors which shed light on the dif-
ferences observed in the catalytic activity of the compounds
of the [M4O16{Ru(arene)}4] family. The complexes which
evolve towards more active species are either those for
which tungsten is the metal constituent of the oxoanionic
core (entries 9 and 10), or those which isomerize in solution
and thus easily release coordination sites on the ruthenium
(entries 6 and 7). Nevertheless, their racemization activity
remains low compared to that reported in the literature
for other RuII complexes, which can achieve the racemiza-
Consequently, chlorobenzene appeared as the most appro-
priate solvent for these catalysts, and was thus used in the
majority of the following studies.
2.3.2. Influence of irradiation. Due to positive influence
of the Triple Cubane/Windmill isomerization in chloro-
benzene on the racemization activity of [Mo4O16{Ru(tolu-
ene)}4] and [Mo4O16{Ru(p-cymene)}4], we decided to

370
D. Laurencin et al. / Tetrahedron: Asymmetry 18 (2007) 367–371
Table 3. Influence of the solvent on the racemization activity of
[Mo4O16{Ru(p-cymene)}4] and [W4O16{Ru(hmb)}4]
Table 5. Influence of t-BuOK and TEMPO on the racemization activity
Catalysta
Additiveb
ee (%)
g(ACP) (%)
Catalysta
Solvent
ee (%)
[Mo4O16{Ru(p-cymene)}4]
—
t-BuOK
TEMPO
60
0.3
58
2.9
23.5
8.7
[Mo4O16{Ru(p-cymene)}4]
PhCl
Toluene
t-Amylalcohol
60
90
89
[W4O16{Ru(hmb)}4]
—
t-BuOK
TEMPO
57
96
52
2.9
6.8
2.6
[W4O16{Ru(hmb)}4]
PhCl
Toluene
t-Amylalcohol
57
88
85
a Reaction conditions: (S)-1-phenylethanol (0.50 mmol), catalyst: 5 mol %
(Ru), PhCl (5 mL), N2 atm, T = 70 ꢁC, 22 h.
a Reaction conditions: (S)-1-phenylethanol (0.50 mmol), catalyst: 5 mol %
(Ru), solvent (5 mL), N2 atm, 70 ꢁC, 22 h.
b Quantities of additive: 1 equiv/Ru.
favour even more the release of coordination sites on the
metal by irradiating the reaction medium,7g,9 in order to
decoordinate the arene ligand.
On the other hand, in the case of [W4O16{Ru(hexamethyl-
benzene)}4], adding TEMPO only slightly increases the
racemization activity, whereas the addition of t-BuOK
strongly decreases it.
Results were similar for the different compounds tested,
and those obtained in the case of [Mo4O16{Ru(toluene)}4]
are reported in Table 4. A clear enhancement of the racemi-
zation activity was observed under irradiation: the ee
reached a value close to 35% in only 5 h, even though the
irradiation had been carried out in toluene rather than
chlorobenzene.10 Consequently, the catalytic activity is
higher under irradiation, because of the efficient release
of coordination sites on the ruthenium.
Such differences observed in the effect of t-BuOK and
TEMPO can be related to the difference in behaviour of
these two complexes in chlorobenzene, and suggests that
they evolve towards different catalytically active species.
In the case of [Mo4O16{Ru(p-cymene)}4], it is unfortu-
nately not possible to exploit the great increase in the race-
mization activity by adding t-BuOK, given the substantial
quantity of acetophenone which forms simultaneously.
2.3.4. Racemization activity of [Mo4O16{Os(g6-p-cym-
ene)}4]. Since neither the solvent, nor the irradiation of
the medium, nor the addition of cocatalysts appeared to
lead to active racemization catalysts without increasing in
parallel the quantity of acetophenone formed, we tried to
obtain better activity by changing the nature of the metal
implied in the catalysis.
Nevertheless, a high percentage of acetophenone simulta-
neously formed in solution, in a way which appeared diffi-
cult to control, as shown by the difference in the figures
observed for two reactions performed under the same
conditions (7.9% vs 15%). This prohibitively high yield in
acetophenone and the problem of reproducibility of the
experiments under irradiation led us to look for other
means of activation of the racemization catalysts.
Having recently synthesized the OsII compound
[Mo4O16{Os(g6-p-cymene)}4], which, like its ruthenium
analogue, displays the windmill form in the solid state
and isomerizes in chlorobenzene - thus releasing coordina-
tion sites on the osmium,11 we tested its racemization activ-
ity. Unfortunately, as shown in Table 2 (entry 16), the
catalytic activity of the osmium derivative is much lower
than that of the ruthenium complex (entry 7). Conse-
quently, it appears that ruthenium is the most appropriate
metal for catalyzing these reactions.
2.3.3. Influence of additives. For an important number of
racemization catalysts, the addition of molecules such as t-
BuOK or TEMPO, favours the evolution of the RuII com-
plexes towards an active species.3b,4d,5a We thus tried to
evaluate if these molecules were also able to enhance the
catalytic activity of complexes such as [W4O16{Ru(hexa-
methylbenzene)}4] or [Mo4O16{Ru(p-cymene)}4]. The
results are given in Table 5.
The effect of TEMPO and t-BuOK varies from one catalyst
to the other. On one hand, in the case of [Mo4O16{Ru(p-
cymene)}4], adding t-BuOK not only considerably acceler-
ates the racemization process, but also strongly increases
the quantity of acetophenone formed, whereas adding
TEMPO only enhances the percentage of acetophenone.
Finally, it is noteworthy that when used as racemization
catalysts in the conditions of the DKR, that is, in the pres-
ence of a lipase (such as CALB) and of an acylating agent
(such as isopropenylacetate), the catalytic activity of the
different [M4O16{Ru(arene)}4] compounds appeared to be
even lower. The different tests performed seem to show that
they are deactivated by both the enzyme and the acylating
agent.
Table 4. Effect of the irradiation on the racemization activity
b
Irradiation Solvent
T (ꢁC) Timea (h) ee (%) g(ACP) (%)
No
Yes
Yes
PhCl
70
22
5
5
48
32
36
2.8
7.9
>15
3. Conclusion
Toluene ꢁ110
Toluene ꢁ110
The catalytic activity of several {Ru(arene)}2+ derivatives
of polyoxometalates in the racemization of 1-phenyletha-
nol was tested, with the prospect of finding new efficient
catalysts for the DKR of secondary alcohols.
a Reaction conditions: (S)-1-phenylethanol (0.50 mmol), [Mo4O16
-
{Ru(toluene)}4] (5 mol % (Ru)), solvent (5 mL), inert atmosphere (N2 or
Ar).
b Yield in acetophenone.

D. Laurencin et al. / Tetrahedron: Asymmetry 18 (2007) 367–371
371
The racemization activity of the compounds tested was
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Y.; Shinohara, A.; Hayashi, K.; Nomiya, K. Eur. J. Inorg.
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Despite their globally moderate catalytic activity, {Ru(ar-
ene)}2+ derivatives of polyoxometalates appear as a new
class of racemization catalysts in which the environment
of the RuII moiety is very different from that of the other
catalysts described so far. This suggests that by looking
at a greater variety of ruthenium complexes, better racemi-
zation catalysts are likely to be encountered.
8. Riermeier, T. H.; Gross, P.; Monsees, A.; Hoff, M.;
Trauthwein, H. Tetrahedron Lett. 2005, 46, 3403.
9. The lamp used for the irradiations was a 150 W Heraeus TQ
150 Z1 lamp doped with gallium-iodide.
10. Toluene was chosen as the solvent for the irradiations
because it is more inert under UV-light than chlorobenzene.
11. Laurencin, D.; Villanneau, R.; Boubekeur, K.; Thouvenot,
Acknowledgement
´
R.; Villain, F.; Benard, M.; Rohmer, M.-M.; Proust, A., in
preparation.
The authors are thankful to COST action D29-0016/04, for
financial support.
12. Takahashi, K.; Yamagushi, M.; Shido, T.; Ohtani, H.; Isobe,
K.; Ichikawa, M. J. Chem. Soc., Chem. Commun. 1995, 1301.