Synergistic effect in bimetallic Ni-Al clusters. Application to efficient catalytic reductive dehalogenation of polychlorinated arenes

Article abstract of DOI:10.1016/S0040-4020(00)00383-5

Subnanometric Ni-Al clusters, in situ prepared by reduction of equimolar amounts of Ni(OAc)₂ and Al(Acac)₃ induced by alkoxide activated sodium hydride, exhibit a high catalytic activity for efficient reductive dehalogenation of aliphatic and aromatic halides and polychlorinated arenes. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4020(00)00383-5

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 4765±4768
Synergistic Effect in Bimetallic Ni±Al Clusters. Application
to Ef®cient Catalytic Reductive Dehalogenation of
Polychlorinated Arenes
a
Fabien Massicot, Raphael Schneider, Yves Fort, Sandra Illy-Cherrey and Olivier Tillement
a
a,
b
b
*
È
a
Á Â Â Â Â
Synthese Organique et Reactivite, UMR 7565, Universite Henri Poincare-Nancy 1, BP 239, 54506 Vandoeuvre les Nancy Cedex, France
  Â
Laboratoire de Science et Genie des Materiaux Metalliques, UMR 7584, Ecole des Mines de Nancy, I.N.P.L, 54502 Nancy Cedex, France
b
Received 24 February 2000; accepted 8 May 2000
AbstractÐSubnanometric Ni±Al clusters, in situ prepared by reduction of equimolar amounts of Ni(OAc)2 and Al(Acac)3 induced by
alkoxide activated sodium hydride, exhibit a high catalytic activity for ef®cient reductive dehalogenation of aliphatic and aromatic halides
and polychlorinated arenes. q 2000 Elsevier Science Ltd. All rights reserved.
Introduction
Results and Discussion
Organic halides, especially polychlorinated aromatic ones
(PCBs), are important industrial chemicals but represent
also a major environmental problem because they are
quite dif®cult to degrade either by reductive or oxidative
enzymatic pathways.1 Therefore a variety of dehalogenation
reagents have been developed during the last few years
including transition metal catalysed hydrogenation2 or
reduction,3 metal hydride reduction,4,5 hydride or Grignard
reagents combined with transition metal salts6,7 or, as
reported recently, by activation of Ni(II) using an arene-
catalysed lithiation process.8 In recent years, many reports
have been made concerning the combination of organo-
metallic reagents with Ni(II) salts for performing useful
organic reactions.6a,b,8
Transmission electron microscopy (TEM) of the non-pyro-
phoric colloidal Ni±Al black suspensions, obtained from in
situ reduction of a [1:1] Ni(OAc)2±Al(Acac)3 mixture by
t-BuONa-activated NaH, revealed a homogeneous distribu-
tion of uniformly sized and amorphous very small particles
(,0.5 nm). In addition, Energy-Dispersive-X-ray-Spectro-
scopy (EDXS) showed a good preservation (up to 96%) of
the initial metal ratios in the Ni±Al materials. The amount
of hydrogen evolved indicated that, at the end of the
preparation, both salts were totally reduced by alkoxide
activated NaH. However, because aluminium is a very
oxophilic metal, it was not possible to prove unambiguously
the existence of oxide-free Ni±Al nano-intermetallics in
these experiments. In fact, by Electron-Energy-Loss-
Spectroscopy (EELS), we could identify the materials in
the Ni(0) and Al(III) oxidation states. The selected area
electron diffraction patterns showed only a very broad
diffraction halo indicating the absence of conventional
crystalline phases. Ni atoms appeared to be aggregated in
very small isolated clusters of a few tenths the size of atoms.
In comparison to our previously described nanometric Ni
clusters,10,11 it may be assumed that a nanometric aluminium
oxide matrix is spontaneously formed in the presence of
traces of oxygen contained in the reaction solvent indicating
that a high dispersion of Ni species in this matrix is obtained
for our Ni±Al clusters. Therefore, we thought that such a
dispersion may have an in¯uence on the catalytic activity
and we decided to investigate this in the dehalogenation of
representative (poly)halogenated compounds.
However, many of the proposed methods suffered from
some limitations and the development of new and ef®cient
dehalogenation procedures continues to be of current inter-
est. In the course of our studies dealing with low tempera-
ture synthesis of new polymetallic nanomaterials, we found
that subnanometrical Ni±Al clusters can be easily prepared
by simultaneous reduction of Al(III) and Ni(II) salts by
activated NaH.9 In this paper, we would like to report a
remarkable synergistic effect on the catalytic properties of
these materials allowing reductive dehalogenation of alkyl
and aryl (poly)halides.
We ®rst studied the Ni±Al catalysed reduction of represen-
tative alkyl or aryl halides. The results reported in Table 1
Keywords: reductive dehalogenation; nickel catalyst; bimetallic clusters.
* Corresponding author. E-mail: yves.fort@sor.uhp-nancy.fr
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)00383-5

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F. Massicot et al. / Tetrahedron 56 (2000) 4765±4768
Table 1. Ni±Al clusters catalysed dehalogenation of monohalogenated compounds (typical reaction conditions: t-BuOH (8 mmol), Ni(OAc)2 (4 mmol),
Al(Acac)3 (4 mmol, for Ni±Al Catalyst), NaH (108 mmol), organic halide (40 mmol), THF, re¯ux)
Entry
Starting material
Ni catalyst
Conv.a (%)
Ni±Al catalyst
Conva (%)
Timea (h)
Yielda (%)
Timea (h)
Yielda (%)
1
2
3
4
5
6
1-Bromodecane
1-Chlorodecane
1-Chloroadamantane
Bromobenzene
Chlorobenzene
1-Chloro-4-(tri¯uoro
methyl) benzene
4-Chloroanisole
1-Chloronaphthalene
4-Chlorophenol
2
5
48
1
60
±
100
100
95
100
100
±
Quant.
98
95
Quant.
Quant.
±
0.25
1.25
1.0
0.5
0.75
1
100
100
100
100
100
100
Quant.
Quant.
Quant.
Quant.
Quant.
98b
7
8
9
10
±
90
±
±
100
±
±
2
2.5
0.25
4
95
98
100
100
91
90d
Quant.
80
92c
±
Octyl 4-methyl-1-
benzene sulfonate
4-Tolyl benzenesulfonate
±
±
±
11
8
95
92
2
100
97
a Unless otherwise noted, determined by GC.
b Isolated yield.
c 1,10-binaphthyl was isolated in 5% yield.
d 1,10-binaphthyl was isolated in 4% yield.
were compared with those obtained for Ni-catalysed
reactions.12
4-methyl-1-benzenesulfonate, a substrate more sensitive to
elimination, with Ni±Al clusters (entry 10). Octane (80%)
was obtained as the major product besides octene (20%)
resulting from a fast elimination observed at the beginning
of the reaction. Note that the amount of octene remained
constant during all the experiment. In addition, the high
yield of octane obtained after 4 h reaction time allowed us
to reject a classical free-radical mechanism based on a pure
single electron transfer (SET) pathway.
We found that the reduction of 1-bromo, 1-chlorodecane
and 1-chloroadamantane in the presence of 10 mol%
Ni±Al clusters in re¯uxing THF led to the corresponding
alkanes as the sole reaction products (Table 1, entries 1±3).
Ni±Al clusters were found to be much more ef®cient than
the monometallic Ni ones (Table 1) since reaction times
were strongly decreased. The same behaviour was observed
for the reduction of haloarenes. Bromo and chlorobenzene
were quantitatively converted into benzene in less than 1 h
under Ni±Al catalysis (entries 4 and 5). As expected, the
presence of electron donating groups on the aromatic ring
decreased the reduction rate (compare entries 5 and 7) while
electron-withdrawing ones had a weak effect (entry 6).
Chloronaphthalene gave the expected reduction product
(naphthalene) in 90% yield. In this case, the reduction was
accompanied by the homocoupling of the starting material
leading to 1,10-binaphthyl as already observed with Ni
containing reducing reagent.13 On the other hand, the
presence of a hydroxyl group on the aromatic ring markedly
enhanced the reaction rate (entry 9). This result may be
interpreted as a consequence of the chelating properties of
the in situ generated phenoxide. Entries 10 and 11 showed
that tosylates could also be reduced by our reagent. Finally,
it must be noted that all these reactions may be conducted in
the presence of 5 mol% of Ni±Al for an extended reaction
time (2±12 h).
Short reaction times associated with the high ef®ciency of
our method led us to believe that a mechanism based on
carbon halide hydrogenolysis appeared to be more probable.
Indeed, molecular hydrogen (evolved during the preparation
of our reagent and/or its regeneration by NaH) could be
adsorbed on the surface of the Ni±Al catalyst yielding a
nickel hydride species. Subsequent oxidative addition of
the organic halide and reductive elimination led to the
hydrocarbon and initial Ni species were regenerated by
sodium hydride reduction. A hydrogenolysis based mechan-
ism was supported by the fact that the addition of a stoichio-
metric amount relative to the aryl halide of a reactive
ethylenic compound, such as styrene, induced a strong
decrease in the reaction rate. For example, 1,4-dichloro-
benzene was reduced in 18 h in the presence of styrene.
Styrene was assumed in this example to act as a Ni(0)
scavenger leading to a classical hydrogenation process.
The enhanced catalytic activity of Ni±Al clusters compared
to Ni ones may then be attributed to an increased dispersion
of Ni species in a subnanometric aluminium matrix. Surpris-
ingly, it appeared that the active site accessibility was not
decreased.
The interpretation of the synergistic effect obtained in
Ni±Al induced reactions remains unclear however, the
obtained results may provide some clues. At ®rst, a pure
SN2 mechanism as well as an elimination-reduction path-
way is ruled out since 1-chloroadamantane is rapidly and
quantitatively reduced into adamantane. An elimination-
reduction pathway may also be excluded for 1-bromo-
decane. Indeed, no elimination product was detected by
GC during the Ni±Al catalysed reduction when 1-decene
slowly produced decane under the same reaction con-
ditions.14 This result was also con®rmed by reacting octyl
We next studied the dehalogenation of polychlorinated
benzenes. The results obtained using Ni±Al and Ni clusters
as catalyst are reported in Table 2. A catalyst loading of
10 mol% Ni relative to each carbon±chlorine unit was
used in these experiments.
In all cases the dechlorination of polychlorobenzenes was
complete leading to benzene as the sole reaction product.

F. Massicot et al. / Tetrahedron 56 (2000) 4765±4768
4767
Table 2. Ni±Al clusters catalysed dehalogenation of polyhalogenated arenes (typical reaction conditions: a catalyst loading 10 mol% was used per C±CI unit.
Reactions on aryl di-, tri-, and tetrachlorides were performed on respectively 20, 13.3 and 10 mmol starting material using t-BuOH (8 mmol), Ni(OAc)2
(4 mmol), Al(Acac)3 (4 mmol), NaH (108 mmol). Dehalogenation of pentachlorophenol was performed on 8 mmol starting material, t-BuOH (8 mmol),
Ni(OAc)2 (4 mmol), Al(Acac)3 (4 mmol) and NaH (116 mmol))
Entry
Starting material
Ni catalyst
Conv.a (%)
Ni±Al catalyst
Conva (%)
Timea (h)
11
Yielda (%)
Quant.
Timea (h)
1.5
Yielda (%)
Quant.
1
2
100
±
100
100
±
±
±
±
1.5
1.5
Quant.
Quant.
3
±
100
4
5
6
25
±
100
Quant.
1.75
1.75
8.0
100
100
100
Quant.
Quant.
Quant.
±
±
27
100
Quant.
7
8
±
±
±
0.75
6.0
100
100
Quant.
110
93
Quant.
95b
a Unless otherwise noted, determined by GC.
b Isolated yield by ¯ash chromatography.
The higher ef®ciency of Ni±Al clusters compared to Ni ones
was con®rmed since completion of Ni±Al induced reactions
were obtained in less than 8 h while Ni-catalysed ones
needed 11 to 110 h. It is worthy to note that mono- or
dihalogenated intermediates were only observed as traces
during the reduction of di- and trihalogenated substrates
respectively. In contrast, 1,2,4,5-tetrachlorobenzene was
®rstly and selectively reduced into 1,2,4-trichlorobenzene
(90% yield after 1 h) which underwent a classical reductive
dehalogenation into benzene. Compared to the methods
reported in the literature,15 these results are interesting
since all dehalogenations were completed in short reaction
times and under mild conditions. In addition, and as
previously observed with 4-chlorophenol (Run 9, Table
1), the presence of a hydroxyl group on the aromatic ring
allowed a rapid and ef®cient reduction process.
biphenyl (Table 2, entry 8), a well-known representative
substrate of the PCB family.16 We found that dehalogena-
tion can be quantitatively obtained in 6 h leading to
biphenyl as the sole reaction product. No trace of 4-chloro-
biphenyl was detected. This last result underlines the
particular ef®ciency of our catalyst.
Conclusion
The results obtained from the above experiments show that
the non-pyrophoric bimetallic Ni±Al catalyst, obtained by
simultaneous NaH induced reduction of Ni(II) and Al(III)
salts under mild conditions, exhibit a remarkable synergistic
effect for the reductive dehalogenation of aromatic and
aliphatic (poly)chlorides and bromides. Work is in progress
in order to de®ne scope and limitations with functional
halides.
Finally, we examined the dehalogenation of 4,40-dichloro-

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F. Massicot et al. / Tetrahedron 56 (2000) 4765±4768
Experimental
residual hydride insured a safe hydrolysis step for
completed reactions.) Except for benzene, products were
then extracted by diethyl ether and puri®ed either by
distillation or by ¯ash chromatography on silica gel. Their
spectroscopic data are in accordance with those of authentic
samples.
Materials
Tetrahydrofuran (THF) was distilled from benzophenone-
sodium adduct and stored over sodium wire. Tertiary butyl
alcohol (t-BuOH) (Aldrich) was distilled from sodium.
Sodium hydride (NaH) (65% in mineral oil, Fluka) was
used after three washings with 20 mL of THF under N2
atmosphere. Nickel acetate [Ni(CH3COO)2] (Fluka) was
dried under vacuum (20 mmHg) at 1108C during 12 h.
Water content after drying was lower than 0.5 mol%.
Aluminium acetylacetonate [Al(CH2COCH2COCH3)3]
(97%, Acros) was use without further puri®cation.
Acknowledgements
The authors wish to thank M. Bernard Reibel for his perfect
technical assistance.
References
Preparation of Ni±Al clusters
1. (a) Hudlicky, M. In Comprehensive Organic Synthesis; Trost,
B. M., Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 8, p 895.
(b) Pinder, A. R. Synthesis, 1980, 425.
t-BuOH (8.0 mmol) in THF (5 mL) was added dropwise at
638C to a suspension of degreased NaH (108.0 mmol) in
THF (35 mL) followed by Al(Acac)3 (4.0 mmol) and
Ni(OAc)2 (4.0 mmol) and the mixture was stirred at re¯ux
for 5 h under a nitrogen atmosphere. Gaseous hydrogen
evolved and a non-pyrophoric black colloidal suspension
was formed. Note that the same procedure was used without
Al(acac)3 for Ni clusters. The measure of the hydrogen
evolution allowed us to verify the complete preparation of
the catalysts; 18 mmol or 12 mmol of H2 must be obtained
for the preparation of Ni±Al or Ni catalyst, respectively.
2. Cucullu, M. E.; Nolan, S. P.; Belderrain, T. R.; Grubbs, R. H.
Organometallics 1999, 18, 1299 and references therein.
3. Wei, B.; Hor, T. S. A. J. Mol. Catal. A 1998, 132, 223 and
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4. (a) Tabaei, S.-M. H.; Pittman, Jr., C. U.; Mead, K. T.
Tetrahedron Lett. 1991, 32, 2727. (b) Tabaei, S.-M. H.; Pittman,
Jr., C. U. Tetrahedron Lett. 1993, 34, 3263. (c) Yang, C. Y.;
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Characterization of Ni±Al nanoparticles
6. (a) Tabaei, S.-M. H.; Pittman, Jr., C. U.; Mead, K. T. J. Org.
Chem. 1992, 57, 6669. (b) Yoon, N. M.; Lee, H. J.; Ahn, J. H.;
Choi, J. J. Org. Chem. 1994, 59, 4687. (c) Liu, Y.; Schwartz, J.
Tetrahedron 1995, 51, 4471.
Characterization of Ni±Al clusters was performed using
Transmission Electron Microscopy (TEM). TEM specimens
were prepared by placing a drop of the colloidal solution
onto a holey-carbon-coated TEM grid and were studied
using a Philips CM20 with an unsaturated LaB6 cathode
operating at 200 kV. EDX spectra were recorded by
means of an EDXS spectrometer equipped with an ultrathin
window X-ray detector. The analysis was carried out in
nanoprobe mode with a diameter of the probe of 10 nm.
The presence of the Al Ka peak at 1.486 keV and the Ni
Ka peak at 7.47 keV can be observed.
7. (a) Hara, R.; Sato, K.; Sun, W.-H.; Takahashi, T. Chem.
Commun. 1999, 845. (b) Hara, R.; Sun, W.-H.; Nishihara, Y.;
Takahashi, T. Chem. Lett. 1997, 1251.
8. Alonso, F.; Radivoy, G.; Yus, M. Tetrahedron 1999, 55, 4441.
9. Dubois, J. M.; Fort, Y.; Tillement, O. Pat. WO 98-FR; Chem.
Abstr. 1999, 130, 254063.
10. Gallezot, P.; Leclercq, C.; Fort, Y.; Caubere, P. J. Mol. Catal.
1994, 93, 79.
Reductive dehalogenation procedure
11. Illy, S.; Tillement, O.; Dubois, J. M.; Massicot, F.; Fort, Y.;
Ghanbaja J. Phil. Mag. A. 1999, 79, 1021.
A typical dehalogenation procedure is as follows: The alkyl
or aryl monohalide (40 mmol) in THF (10 mL) was added to
the obtained dark grey suspension of the Ni±Al clusters and
the products were detected by GC. Reductions of di-, tri-
and tetrachlorobenzenes were performed on respectively 20,
13.3 and 10 mmol using a 10 mol% catalyst loading relative
to each chlorine function [t-BuOH (8 mmol), Ni(OAc)2
(4 mmol), Al(Acac)3 (4 mmol) and NaH (108 mmol)]. The
reactions were monitored by GC analyses (HP1 column,
20 m£0.32 mm ID£0.25 m) and GC-MS (EI) analyses of
small aliquots using hydrocarbons (C8±C14) as internal
standard. At the end of the reaction, 10 mL of water were
added dropwise at 08C. (Note that the low amount of
12. Previous catalytic use of NaH containing Ni reducing agents
(called NiCRA) were brie¯y described: Vanderesse, R.; Brunet,
J. J.; Caubere, P. J. Org. Chem. 1981, 46, 1270. Note that the
results presented in this paper for the Ni-catalysed reactions were
obtained under the same conditions as those used for Ni±Al ones.
13. Brunet, J. J.; Vanderesse, R.; Caubere, P. J. Organomet. Chem.
1978, 157, 125.
14. Control experiments performed with 1-decene showed that a
mixture of various decenes (resulting from double bond isomerisa-
tion) was produced besides 5±10% of decane after 18 h.
15. For dehalogenation of polychlorobenzenes, see Refs.
2,4b,4d,7a,8.
16. For dehalogenation of chlorobiphenyls, see Refs. 4a,4c,6a.