Dehalogenation and hydrogenation of aromatic compounds catalyzed by nanoparticles generated from rhodium bis(imino)pyridine complexes

Article abstract of DOI:10.1021/om1003072

Chloro[2,6-bis{1-(phenyl)iminoethyl}pyridine]rhodium(I) complexes (RhCl(N,N,N); 1-11) have been prepared by reaction of the dimer [Rh(μ-Cl)(η²-C₂H₄)₂] ₂ with the corresponding nitrogen donor ligand. These complexes afford nanoparticles with a mean diameter of 1.5 ± 0.2 nm stabilized by the partially hydrogenated ligand, under 1 atm of hydrogen, in 2-propanol as solvent, at 60 °C, and in the presence of K^tBuO. Under a constant atmospheric pressure of hydrogen, the nanoparticles catalyze the dehalogenation of the chlorobenzene 1,2-, 1,3-, and 1,4-dichlorobenzene, 1,2,4- trichlorobenzene, fluorobenzene, 2-, 3-, and 4-chlorobiphenyl, and 4,4′- and 3,5-dichlorobiphenyl and the hydrogenation of benzene, toluene, p-xylene, styrene, α-methylstyrene, biphenyl, aniline, phenol, and pyridine. A Hg(0) poisoning test reveals that homogeneous and heterogeneous catalysis coexist during the dehalogenation reactions, whereas the hydrogenation processes are heterogeneous. The nanoparticles can be also generated in the presence of basic aluminum oxide of 150 mesh, which at the same time acts as a support. When they are deposited on the alumina, the nanoparticles do not significantly modify their catalytic activity.

Full text of DOI:10.1021/om1003072

Organometallics 2010, 29, 4375–4383 4375
DOI: 10.1021/om1003072
Dehalogenation and Hydrogenation of Aromatic Compounds
Catalyzed by Nanoparticles Generated from Rhodium
Bis(imino)pyridine Complexes§
†
Marıa L. Buil, Miguel A. Esteruelas,* Sandra Niembro, Montserrat Olivan,
,†
‡
†
ꢀ
Lars Orzechowski,† Cristina Pelayo,† and Adelina Vallribera‡
†
´
ꢀ
ꢀ
Departamento de Quımica Inorganica-Instituto de Ciencia de Materiales de Aragon, Universidad de
Zaragoza-CSIC, 50009 Zaragoza, Spain, and Departamento de Quımica, Universitat Autonoma de
‡
´
ꢁ
Barcelona, Cerdanyola del Valleꢁs, 08193 Barcelona, Spain
Received April 15, 2010
Chloro[2,6-bis{1-(phenyl)iminoethyl}pyridine]rhodium(I) complexes (RhCl(N,N,N); 1-11) have
been prepared by reaction of the dimer [Rh(μ-Cl)(η2-C2H4)2]2 with the corresponding nitrogen donor
ligand. These complexes afford nanoparticles with a mean diameter of 1.5 ( 0.2 nm stabilized by the
partially hydrogenated ligand, under 1 atm of hydrogen, in 2-propanol as solvent, at 60 °C, and in the
presence of KtBuO. Under a constant atmospheric pressure of hydrogen, the nanoparticles catalyze
the dehalogenation of the chlorobenzene 1,2-, 1,3-, and 1,4-dichlorobenzene, 1,2,4-trichloro
benzene, fluorobenzene, 2-, 3-, and 4-chlorobiphenyl, and 4,40- and 3,5-dichlorobiphenyl and the
hydrogenation of benzene, toluene, p-xylene, styrene, R-methylstyrene, biphenyl, aniline, phenol,
and pyridine. A Hg(0) poisoning test reveals that homogeneous and heterogeneous catalysis coexist
during the dehalogenation reactions, whereas the hydrogenation processes are heterogeneous. The
nanoparticles can be also generated in the presence of basic aluminum oxide of 150 mesh, which at the
same time acts as a support. When they are deposited on the alumina, the nanoparticles do not
significantly modify their catalytic activity.
Introduction
labeling the corresponding position of the ring with this
isotope.3 The use of chlorine as a protecting group offers
the opportunity to alter the orientation rules of aromatic
electrophilic substitution.4 Catalytic hydrodechlorination is
usually mediated by transition metals, mainly Ru, Rh, Ni,
Pd, and Pt, and performed with molecular hydrogen,5 silanes,6
alcohols,7 cyclic amines,8 dimethylformamide,9 metal hyd-
rides,10 metal alkoxides,11 sodium borohydride,12 alkyl Grig-
nard reagents,13 and formic acid and its salts.14
The hydrogenolysis of C-Cl bonds of aromatic com-
pounds is a reaction of great interest from both environmental
and chemical points of view.1 The accumulation of chloro-
arenes in the environment, in particular polychlorobiphenyls
(PCBs),2 is a serious health hazard, and their dehalogenation
must be therefore a high-priority target. Hydrogenolysis of a
C-Cl bond with deuterium has been employed for selectively
The hydrodechlorination of chloroarenes is sometimes
followed by hydrodearomatization of the dehalogenated
aromatic compound.15 The hydrogenation of benzene and
§
Dedicated to Prof. Carmen Najera on the occasion of her 60th birthday.
ꢀ
*To whom correspondence should be addressed. E-mail: maester@
unizar.es.
(1) (a) Grushin, V. V.; Alper, H. Chem. Rev. 1994, 94, 1047.
(b) Grushin, V. V.; Alper, H. Top. Organomet. Chem. 1999, 3, 193.
(c) Alonso, F.; Beletskaya, I. P.; Yus, M. Chem. Rev. 2002, 102, 4009.
(2) See for example: (a) Roth, J. A.; Dakojl, S. R.; Hughes, R. C.;
(6) (a) Pri-Bar, I.; Buchman, O. J. Org. Chem. 1986, 51, 734.
(b) Boukherroub, R.; Chatgilialoglu, C.; Manuel, G. Organometallics
1996, 15, 1508. (c) Cho, Y. S.; Han, J. S.; Yoo, B. R.; Kang, S. O.; Jung,
ꢀ
Carmody, R. E. Environ. Sci. Technol. 1994, 28, 80. (b) Lassova, L.; Lee,
€
H. K.; Hor, T. S. A. J. Org. Chem. 1998, 63, 3538. (c) Pittman, C. U., Jr.;
Yang, C. J. Hazard. Mater. 2001, B82, 299. (d) Akzinnay, S.; Bisaro, F.;
Cazin, C. S. J. Chem. Commun. 2009, 5752.
(3) See for example: Narisada, M.; Horibe, I.; Watanabe, F.; Takeda,
K. J. Org. Chem. 1989, 54, 5308.
(4) See for example: (a) Huffman, J. W. J. Org. Chem. 1959, 24, 1759.
(b) Fox, B. A.; Threlfall, T. L. Org. Synth. 1973, 5, 346.
(5) (a) Love, C. J.; McQuillin, F. J. J. Chem. Soc., Perkin Trans. 1
I. N. Organometallics 1998, 17, 570. (d) Stohr, F.; Sturmayr, S.; Schubert, U.
Chem. Commun. 2002, 2222. (e) Rahaim, R. J., Jr.; Maleczka, R. E., Jr.
Tetrahedron Lett. 2002, 43, 8823. (f) Karshtedt, D.; Bell, A. T.; Tilley, T. D.
Organometallics 2006, 25, 4471. (g) Yang, J.; Brookhart, M. J. Am. Chem.
Soc. 2007, 129, 12656. (h) Yang, J.; Brookhart, M. Adv. Synth. Catal. 2009,
351, 175.
(7) (a) Cucullu, M. E.; Nolan, S. P.; Belderrain, T. R.; Grubbs, R. H.
Organometallics 1999, 18, 1299. (b) Jurvilliers, X.; Schneider, R.; Fort, Y.;
Ghanbaja, J. Appl. Organomet. Chem. 2001, 15, 744. (c) Fujita, K.; Owaki,
M.; Yamaguchi, R. Chem. Commun. 2002, 2964. (d) Bei, X.; Hagemeyer, A.;
Volpe, A.; Saxton, R.; Turner, H.; Guram, A. S. J. Org. Chem. 2004, 69,
8626. (e) Chen, J.; Zhang, Y.; Yang, L.; Zhang, X.; Liu, J.; Li, L.; Zhang, H.
Tetrahedron 2007, 63, 4266.
ꢀ
ꢀ
1973, 2509. (b) Kvintovics, P.; Heil, B.; Palagyi, J.; Marko, L. J. Organomet.
Chem. 1978, 148, 311. (c) Grushin, V. V.; Alper, H. Organometallics 1991,
ꢀ
10, 1620. (d) Ferrughelli, I. T.; Horvath, I. T. J. Chem. Soc., Chem.
Commun. 1992, 806. (e) Marques, C. A.; Selva, M.; Tundo, P. J. Org.
Chem. 1994, 59, 3830. (f) Zhang, Y.; Liao, S.; Xu, Y. Tetrahedron Lett.
1994, 35, 4599. (g) Young, R. J., Jr.; Grushin, V. V. Organometallics 1999,
18, 294. (h) Angeloff, A.; Brunet, J.-J.; Legars, P.; Neibecker, D.; Souyri, D.
Tetrahedron Lett. 2001, 42, 2301.
(8) Imai, H.; Nishiguchi, T.; Tanaka, M.; Fukuzumi, K. J. Org.
Chem. 1977, 42, 2309.
(9) Zawisza, A. M.; Muzart, J. Tetrahedron Lett. 2007, 48, 6738.
r
2010 American Chemical Society
Published on Web 09/02/2010
pubs.acs.org/Organometallics

4376 Organometallics, Vol. 29, No. 19, 2010
Buil et al.
substituted aromatic compounds to the corresponding cyclo-
hexyl derivatives is a goal of notable industrial interest.16
Arene hydrogenation has also generated environmental inte-
rest17 due to the demand for cleaner-burning low-aromatic-
content diesel fuel, which has been stimulated in part by the
discovery that diesel exhaust particles contain powerful
carcinogens, and in relation to the hydrogen storage.18 The
greater number of arene hydrogenation catalysts are based
on platinum-group metals.19 While the majority are hetero-
geneous, it is conceivable that all of the so-called homo-
geneous catalysts are actually precursors to heterogeneous
catalysts. Typical heterogeneous catalysts comprise metals
supported on fixed beds, generally operated under harsh
Chart 1
conditions,20 or metal colloids or nanoparticles, which can
be used under mild conditions.21 A number of molecular
compounds have been reported to be homogeneous cata-
lysts. However, many have since been shown to be precursors
to heterogeneous systems.22
For a number of years we have been interested in devel-
oping new methods for the dehalogenation of both poly-
chlorinated arenes and cycloalkanes.23 During the course of
these investigations we have found that rhodium complexes
bearing bis(imino)pyridine ligands generate rhodium nano-
particles, which catalyze the dehalogenation of aromatic
compounds, including PCBs, and the hydrogenation of
benzene and functionalized aromatic compounds. Related
rhodium complexes have previously proven to be catalyst
precursors for the homologation of aromatic aldehydes with
(trimethylsilyl)diazomethane24 and for hydrosilylation,
Mukaiyama aldol, and cyclopropanation reactions.25
(10) (a) Carfagna, C.; Musco, A.; Pontellini, R.; Terzoni, G. J. Mol.
Catal. 1989, 57, 23. (b) Carfagna, C.; Musco, A.; Pontellini, R.; Terzoni, G.
J. Mol. Catal. 1989, 54, L23. (c) Quian, C.; Zhu, D.; Gu, Y. J. Mol. Catal.
1990, 63, L1. (d) Wu, W.; Xu, J.; Zhao, H.; Zhang, Q.; Liao, S. Chemosphere
2005, 60, 944.
(11) (a) Zask, A.; Helquist, P. J. Org. Chem. 1978, 43, 1619. (b) Viciu,
M. S.; Grasa, G. A.; Nolan, S. P. Organometallics 2001, 20, 3607.
(c) Desmarets, C.; Kuhl, S.; Schneider, R.; Fort, Y. Organometallics 2002,
21, 1554. (d) Navarro, O.; Kaur, H.; Mahjoor, P.; Nolan, S. P. J. Org. Chem.
2004, 69, 3173. (e) Navarro, O.; Marion, N.; Oonishi, Y.; Kelly, R. A., III;
Nolan, S. P. J. Org. Chem. 2006, 71, 685.
(12) (a) Tabaei, S.-M. H.; Pittman, C. U., Jr. Tetrahedron Lett. 1993,
34, 3263. (b) Arai, T.; Kashitani, K.; Kondo, H.; Sakaki, S. Bull. Chem. Soc.
Jpn. 1994, 67, 705.
(13) (a) Yuan, K.; Scott, W. J. J. Org. Chem. 1990, 55, 6188. (b) Hara,
R.; Sato, K.; Sun, W.-H.; Takahashi, T. Chem. Commun. 1999, 845. (c) Guo,
H.; Kanno, K.; Takahashi, T. Chem. Lett. 2004, 33, 1356. (d) Shen, M.; Sun,
W. H. Appl. Organomet. Chem. 2009, 23, 51.
(14) (a) Cortese, N. A; Heck, R. F. J. Org. Chem. 1977, 42, 3491.
(b) Pandey, P. N.; Purkayastha, M. L. Synthesis 1982, 876. (c) Anwer, M. K.;
Sherman, D. B.; Roney, J. G.; Spatola, A. F. J. Org. Chem. 1989, 54, 1284.
(d) Wiener, H.; Blum, J.; Sasson, Y. J. Org. Chem. 1991, 56, 6145. (e) Ben-
David, Y.; Gozin, M.; Portnoy, M.; Milstein, D. J. Mol. Catal. 1992, 73, 173.
Results and Discussion
ꢀ
ꢀ
(f) Benyei, A. Cs.; Lehel, S.; Joo, F. J. Mol. Catal. A: Chem. 1997, 116, 349.
(g) Arcadi, A.; Cerichelli, G.; Chiarini, M.; Vico, R.; Zorzan, D. Eur. J. Org.
Chem. 2004, 3404. (h) Joshi, A. V.; Baidossi, M.; Taha, N.; Mukhopadhyay,
1. Preparation of the Rhodium Catalyst Precursors. Treat-
ment at room temperature of toluene solutions of the dimer
[Rh(μ-Cl)(η2-C2H4)2]2 with 2.0 equiv of the corresponding
2,6-bis{1-(phenyl)iminoethyl}pyridine26 ligand (N,N,N) for
6 h leads to the rhodium(I) derivatives RhCl(N,N,N) (1-11),
according to eq 1.
ꢀ
S.; Sasson, Y. Synth. Commun. 2005, 35, 2715. (i) Ohlin, C. A.; Beni, Z.;
ꢀ
Laurenczy, G.; Ruiz, N.; Masdeu-Bulto, A. M. Appl. Organomet. Chem.
2007, 21, 156.
(15) See for example: (a) Schuth, C.; Reinhard, M. Appl. Catal. B:
€
Environ. 1998, 18, 215. (b) Blum, J.; Rosenfeld, A.; Gelman, F.; Schumann,
H.; Avnir, D. J. Mol. Catal. A: Chem. 1999, 146, 117. (c) Felis, V.; De
Bellefon, C.; Fouilloux, P.; Schweich, D. Appl. Catal. B.: Environ. 1999, 20,
91. (d) Stanger, K. J.; Angelici, R. J. J. Mol. Catal. A: Chem. 2004, 207, 59.
ꢀ
(e) Leger, B.; Nowicki, A.; Roucoux, A.; Rolland, J.-P. J. Mol. Catal. A:
Chem. 2007, 266, 221. (f) Xia, C.; Liu, Y.; Zhou, S.; Yang, C.; Liu, S.; Xu, J.;
Yu, J.; Chen, J.; Liang, X. J. Hazard. Mat. 2009, 169, 1029. (g) Calvo, L.;
Gilarranz, M. A.; Casas, J. A.; Mohedano, A. F.; Rodríguez, J. J. J. Hazard.
Mat. 2009, 161, 842. (h) Liu, M.; Han, M.; Yu, W. W. Environ. Sci. Technol.
2009, 43, 2519.
(16) (a) James, B. R.; Wang, Y.; Hu, T. Q. Chem. Ind. 1996, 68, 423.
(17) (a) Stanislaus, A.; Cooper, B. H. Catal. Rev. Sci. Eng. 1994, 36,
75. (b) Corma, A.; Martínez, A.; Martínez-Soria, V. J. Catal. 1997, 169, 480.
(c) Enya, T.; Suzuki, H.; Watanabe, T.; Hirayama, T.; Hisamatsu, Y. Environ.
Sci. Technol. 1997, 31, 2772.
(18) Hydrogen Production and Storage-R&D Priorities and Gaps;
Organization for Economic Cooperation and Development/International
Energy Agency, Paris, 2006.
They were isolated as dark green solids in excellent yields
(80-98%) and were characterized by elemental analysis,
1
FAB mass spectrometry, and H and 13C{1H} NMR spec-
troscopy. On the basis of the substituted positions on the
phenyl groups of the bis(imino)pyridine ligands, they can be
classified in three different categories (Chart 1): (i) substi-
tuted in one ortho position (complexes 2-5), (ii) substituted
(19) (a) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003,
198, 317. (b) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003,
191, 187. (c) Dyson, P. J. Dalton Trans. 2003, 2964.
(20) Cooper, B. H.; Donnis, B. B. L. Appl. Catal. A: Gen. 1996, 137,
203.
ꢀ
(23) (a) Esteruelas, M. A.; Herrero, J.; Lopez, F. M.; Martın, M.;
(21) Roucoux, A.; Schulz, J.; Patin, H. Chem. Rev. 2002, 102, 3757.
(22) See for example: (a) Weddle, K. S.; Aiken, J. D., III; Finke, R. G.
J. Am. Chem. Soc. 1998, 120, 5653. (b) Yang, H.; Gao, H.; Angelici, R. J.
Organometallics 2000, 19, 622. (c) Widegren, J. A.; Finke, R. G. Inorg.
Chem. 2002, 41, 1558. (d) Jaska, C. A.; Manners, I. J. Am. Chem. Soc. 2004,
126, 9776. (e) Hagen, C. M.; Widegren, J. A.; Maitlis, P. M.; Finke, R. G.
Oro, L. A. Organometallics 1999, 18, 1110. (b) Díaz, J.; Esteruelas, M. A.;
Herrero, J.; Moralejo, L.; Olivan, M. J. Catal. 2000, 195, 187. (c) Atienza,
M. A.; Esteruelas, M. A.; Fernandez, M.; Herrero, J.; Olivan, M. New J.
Chem. 2001, 25, 775. (d) Esteruelas, M. A.; Herrero, J.; Olivan, M.
ꢀ
ꢀ
ꢀ
ꢀ
Organometallics 2004, 23, 3891.
(24) Dias, E. L.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 2001,
ꢀ
J. Am. Chem. Soc. 2005, 127, 4423. (f) Leger, B.; Denicourt-Nowicki, A.;
123, 2442.
ꢀ
Oliver-Bourgigou, H.; Roucoux, A. Inorg. Chem. 2008, 47, 9090. (g) Leger,
(25) Dias, E. L.; Brookhart, M.; White, P. S. Chem. Commun. 2001,
B.; Denicourt-Nowicki, A.; Roucoux, A.; Oliver-Bourbigou, H. Adv. Synth.
Catal. 2008, 350, 153. (h) Wang, D.-W.; Lu, S.-M.; Zhou, Y.-G. Tetrahedron
Lett. 2009, 50, 1282.
423.
(26) Gibson, V. C.; Redshaw, C.; Solan, G. A. Chem. Rev. 2007, 107,
1745.

Article
Organometallics, Vol. 29, No. 19, 2010 4377
Scheme 1
Table 1. Dehalogenation of Chloroarenes with KtBuO/H2 in the
Presence of RhCl(N,N,N)a
Figure 1. 1H NMR spectra of complexes 7 (a) and 3 (b) in C6D6.
The enlargements of the peaks of both aromatic and aliphatic
regions of the spectra show the presence of two isomers for
complex 3.
Chart 2
in both ortho positions (complexes 6-8), and (iii) substituted
at the para position (complexes 9-11). The ortho alkyl sub-
stituents seem to exercise a protective effect on the complex.
Thus, while compounds 2-8 are moderately stable in air,
complexes 1 and 9-11 are very air sensitive and need to be
handled under rigorous exclusion of air.
a Conditions: 60 °C; 0.024 mmol of catalyst precursor, 1.2 mmol of
chlorinated arene; 1.2x mmol of KtBuO (x being the number of chlorine
atoms of the halogenated substrate), 5 mL of 2-propanol, 1 atm of H2.
Yields were determined by GC analyses.
The 1H and 13C{1H} NMR spectra of 1 and 6-11 are con-
sistent witha square-planar C2v-symmetrical ligand. Figure 1a
shows the 1H NMR spectrum of 7. However, in the 1H and
13C{1H} NMR spectra of complexes 2-5, which derive from
bis(imino)pyridine ligands with both phenyl groups substi-
tuted at one ortho position, two sets of peaks for each reson-
ance are observed. Figure 1b shows the 1H NMR spectrum
of complex 3.
2,6-bis{1-(phenyl)iminoethyl}pyridine ligands with asym-
metrically substituted phenyl groups,27 the isomer inter-
conversion is slow on an NMR time scale, even at 80 °C, as
1
is demonstrated by the H NMR spectra performed at this
temperature, which are the same as those at room tempera-
ture. Using a molecular model, it is possible to see how the
rotation around the N-Cipso bonds is blocked by steric
hindrance between both phenyl groups and the chlorine
ligand.
2. Dehalogenation of Aromatic Compounds. Complexes
1-5 and 9-11 are efficient catalyst precursors for the
hydrogenolysis of the C-Cl bonds of chlorobenzene, 1,2-
dichlorobenzene, and 1,3-dichlorobenzene. The reactions
were carried out under a constant atmospheric pressure of
hydrogen, in 2-propanol as solvent, at 60 °C, using 2% mol
of catalyst precursor, and in the presence of the stoichio-
metric amount of KtBuO in order to neutralize the HCl
1
The H and 13C{1H} NMR spectra of the ortho-mono-
substituted complexes 2-5 can be explained in terms of the
existence in solution of the isomers a and b shown in Chart 2.
Isomer a has Cs symmetry, and both substituents are situated
in the same face of the plane defined by the pyridyl group, the
rhodium atom, and the chlorine ligand, while in isomer
b each substituent is situated on one face. In contrast to what
is observed for other transition-metal complexes containing
ꢀ
(27) Campora, J.; Cartes, M. A.; Rodrıguez-Delgado, A.; Naz,
ꢀ
A. M.; Palma, P.; Perez, C. M. Inorg. Chem. 2009, 48, 3679.

4378 Organometallics, Vol. 29, No. 19, 2010
Buil et al.
generated during the hydrogenolysis (Scheme 1). Under
these conditions, benzene is formed in high yield after short
reaction times (Table 1). Once the dehalogenation has taken
place, the reduction of benzene to cylohexane is observed at
rates significantly slower than those for the hydrogenolysis
(Table 1, runs 8-11 and 14), in agreement with other tandem
processes of this type.15a-d,f-h
The best catalyst precursor is complex 11 (runs 8, 12, and
15), containing a p-CF3 substituent at the phenyl groups of
the 2,6-bis{1-(phenyl)iminoethyl}pyridine ligand. With this
complex, the quantitative dehalogenation of chlorobenzene
occurs after 15 min (run 8), whereas those of 1,2- and 1,3-di-
chlorobenzene take place after 30 min (runs 12 and 15). The
catalyst precursor keeps whole its activity for at least four
loads of substrate.
The formation of a dark precipitate is observed during the
reactions. This, along with the hydrogenation of benzene to
cyclohexane, does not seem to be consistent with a homo-
geneous catalytic process. Therefore, in order to distinguish
homogeneous catalysis from heterogeneous catalysis, we
performed a Hg(0) poisoning test19a by adding 1 mL of
Hg(0) (2820 equiv/equiv of Rh) to a well-stirred solution.
Figure 2 shows the course of the dehalogenation-hydrogen-
ation of chlorobenzene in the presence of complex 11, both in
the absence of mercury (a) and with mercury in the medium
(b). As can be observed, the addition of Hg(0) produces a
decrease of the initial hydrogenolysis rate, which falls to zero
when about 60% of the substrate has been dehalogenated,
and inhibits the hydrogenation of benzene to cyclohexane.
This suggests the coexistence of homogeneous and hetero-
geneous catalysis during the dehalogenation reaction while,
as expected, the benzene hydrogenation appears to be a
heterogeneous process.19a,22 During the dehalogenation pro-
cess, the reduction of 11 takes place. Both the reduced metal,
in a heterogeneous manner, and the molecular complex or/
and the molecular intermediate species of the reduction pro-
cess, in a homogeneous manner, appear to catalyze the halogen
abstraction. According to Figure 2, the amount of substrate
dehalogenated in an homogeneous manner in the absence of
Hg(0) should not be higher than 15%.
In an effort to gain insight into the nature of the true
catalyst, complex 11 was stirred under catalytic conditions in
the absence of substrate. After 7 h, the formed suspension
was filtered off. The catalytic inactivity of the resulting solu-
tion was verified, and the solid was characterized by trans-
mission electron microscopy (TEM). The technique revealed
the presence of small and homogeneously dispersed nano-
particles (Figure 3) showing a quite narrow size distribution
around a mean diameter of 1.5 ( 0.2 nm. We note that some
rhodium organometallic precatalysts under the TEM beam
are shown to form nanoclusters.22d,e However, typically
those colloids are not as well dispersed as the nanoparticles
seen herein. The nanoparticles are partially soluble in chloro-
form-d. Their 1H NMR spectrum at room temperature
shows aromatic resonances between 7.6 and 6.3 ppm and
aliphatic signals between 2.3 and 0.5 ppm, whereas the
19F{1H} NMR spectrum contains a singlet at -61.1 ppm,
characteristic of a CF3 group. These spectra suggest that the
presence of a partially hydrogenated bis(imino)pyridine ligand
stabilizes the nanoparticles. Rhodium nanoparticles stabilized by
2,20-, 3,30-, and 4,40-bipyridines have been shown to be efficient
for arene catalytic hydrogenation in ionic liquids.22f,g
Table 2 collects the results obtained for the dehalogena-
tion of other halogenated aromatic compounds, using com-
plex 11 as a catalyst precursor. Under the same conditions as
those used for the dehalogenation of chlorobenzene and 1,2-
and 1,3-dichlorobenzene, the dehalogenation of 1,4-dichloro-
benzene and 1,2,4-trichlorobenzene also takes place. The
dehalogenation of 1,4-dichlorobenzene is slower than that of
the 1,2- and 1,3-isomers. Thus, after 45 min 19% of chloro-
benzene still remains in the catalytic solution (run 1). After
400 min, the chlorine abstraction is quantitative and 14% of
benzene is reduced to cyclohexane. The dehalogenation of
1,2,4-trichlorobenzene (run 2) occurs at a rate similar to that
Figure 2. Hydrogen consumption (mL) vs time (min) in the
catalytic dehalogenation-hydrogenation of chlorobenzene
(1.2 mmol) with catalyst precursor 11 (0.024 mmol) and KtBuO
(1.2 mmol) in 2-propanol (5 mL) at 60 °C and 1 atm of H2: (a) in
the absence of Hg; (b) in the presence of added Hg.
Figure 3. TEM image (left) and particle size distribution histogram (right) of the nanoparticles generated from complex 11.

Article
Organometallics, Vol. 29, No. 19, 2010 4379
Table 2. Dehalogenation of Polychloroarenes and Fluorobenzene with KtBuO/H2 in the Presence of Catalyst Precursor 11a
a Conditions: 60 °C, 0.024 mmol of catalyst precursor, 1.2 mmol of chlorinated arene, 1.2x mmol of KtBuO (x being the number of chlorine atoms of
the halogenated substrate), 5 mL of 2-propanol, 1 atm of H2. Yields were determined by GC analyses.
Figure 4. (a) Plots of the hydrogen consumption (mL) vs time (min) during six runs of hydrogenation of benzene to cyclohexane (1 atm
of H2, 2-propanol, 60 °C) catalyzed by 2 mol % of catalyst precursor 11 and KtBuO (10% mol). (b) Plots of the hydrogen consumption
(mL) vs time (min) during seven runs of hydrogenation of benzene to cyclohexane (1 atm of H2, 2-propanol, 60 °C) catalyzed by 2 mol
% of catalyst precursor 11 and Al2O3. (c) Comparison of the catalytic performance of the systems formed by catalyst precursor
11/KtBuO (blue) and 11/Al2O3 (red).
of the dehalogenation of 1,4-dichlorobenzene. Fluoroben-
zene is also efficiently dehalogenated (run 3). Carbon-fluor-
ine bonds are stronger than carbon-chlorine bonds.28 In
agreement with this, the hydrogenolysis of fluorobenzene is
slightly slower than that of chlorobenzene. The quantitative
formation of benzene from fluorobenzene requires 50 min.
Complex 11 is also an efficient catalyst precursor for the
dehalogenation of 2-, 3-, and 4-chlorobiphenyl and 4,40- and
3,5-dichlorobiphenyl. The monochlorinated substrates are
transformed into biphenyl in quantitative yield after 100 (run
4), 115 (run 5), and 55 min (run 6). After 400 min, 20% of
biphenyl is reduced to cyclohexylbenzene and dicyclohexyl is
also formed after longer reaction times. The dehalogenation
of the dichlorobiphenyls occurs by stages via the correspond-
ing chlorobiphenyl (runs 7 and 8). Thus, after 300 min
between 58% and 68% of the substrates were dehalogenated
to chlorobiphenyl (35-38%) and biphenyl (23-30%).
3. Hydrogenation of Aromatic Compounds. Figure 4a
summarizes the course of the hydrogenation of benzene to
cyclohexane at a constant atmospheric pressure of hydrogen,
at 60 °C, in the presence of a 2 mol % mixture of complex 11
and KtBuO in a 1:5 molar ratio, and using 2-propanol as
solvent. In agreement with a heterogeneous process, the
reaction shows an induction period corresponding to the
formation of the small nanoparticles, which act as a solid
catalyst. The transformation of the molecular complex into
nanoparticles is quantitative when the first load of substrate
(28) Smart, B. E. In The Chemistry of Functional Groups; Patai, S.,
Rappoport, Z., Eds.; Wiley: New York, 1983; Suppl. D, Chapter 14.

4380 Organometallics, Vol. 29, No. 19, 2010
Buil et al.
Table 3. Hydrogenation of Aromatic Substrates with Catalyst
Precursor 11/KtBuOa
Table 4. Hydrogenation of Aromatic Substrates with Catalyst
a
Precursor 11/Al2O3
a Reaction conditions: 0.024 mmol of RhCl{N,N,N-p-(CF3)2}, 122.4
mg of Al2O3, 5 mL of 2-propanol, 1 h of preactivation under a H2
atmosphere (1 atm), 60 °C. The aromatic substrate (1.2 mmol) was then
added, and the hydrogen consumption was measured. b Turnover
frequency defined as mol of H2/((mol of rhodium) h).
a Reaction conditions: 0.024 mmol of RhCl{N,N,N-p-(CF3)2}, 0.12
mmol of KtBuO, 5 mL of 2-propanol, 1 h of preactivation under a H2
atmosphere (1 atm), 60 °C. The aromatic substrate (1.2 mmol) was then
added, and the hydrogen consumption was measured. b Turnover
frequency defined as mol of H2/((mol of rhodium) h).
tautomerizes and cyclohexanone is formed in quantitative
yield after 300 min (run 8). The reduction of pyridine is faster
than those of aniline and phenol. Thus, after 65 min piper-
idine is obtained in quantitative yield (run 9).
The versatility of this catalytic system prompted us to
deposit the nanoparticles on a support. We selected basic
aluminum oxide of 150 mesh (Al2O3), since it is a Brønsted
base as KtBuO, is easily accessible, is cheap, and is the
usual support in heterogeneous catalysis. Figure 4b sum-
marizes the course of the hydrogenation of benzene to
cyclohexane at a constant atmospheric pressure of hydrogen,
at 60 °C, in the presence of a 2 mol % mixture of complex 11
and Al2O3 in a 1:8.7 weight ratio, and using 2-propanol as
solvent. As in the presence of KtBuO, the reduction shows an
induction period corresponding to the generation of the
nanoparticles, which are deposited on the alumina. After
the reduction of the first load of substrate, the deposition
process is finished. Thus, the reduction of the second load of
substrate is faster than the first one and, in a manner similar
to the reduction in the presence of KtBuO, the reached
catalytic activity is entirely kept at least during the following
five loads. Interestingly, although the hydrogenation of the
first load of substrate in the presence of Al2O3 is slower than
in the presence of KtBuO, the reached activity from the
second to the seventh load is the same in both cases
(Figure 4c).
has been reduced. Thus, the reduction process is faster dur-
ing the second load and the activity that is reached is entirely
kept at least during the following five loads.
The rhodium nanoparticles can be also generated in the
absence of an aromatic substrate, by means of the preactiva-
tion for 1 h of the catalytic mixture under constant atmospheric
hydrogen pressure at 60 °C. The suspension generated in this
way catalyzes the reduction of benzene, substituted ben-
zenes, functionalized benzenes such as aniline and phenol,
and pyridine (Table 3). Benzene is reduced to cyclohexane in
quantitative yield after 200 min (run 1), whereas toluene (run 2)
and p-xylene (run 3) give the corresponding cycloalkanes
in 93% and 31% yields, respectively, after 400 min. The
exocyclic carbon-carbon double bonds of styrene (run 4)
and R-methylstyrene (run 5) are hydrogenated more quickly
than the aromatic rings. Ethylbenzene and cumene are
formed in quantitative yield after 2 and 5 min, while ethyl-
cyclohexane and isopropylcyclohexane are generated in 67%
and 53% yields after 380 and 480 min, respectively. Biphenyl
is hydrogenated to cyclohexylbenzene and dicyclohexyl,
both in 25% yield, after 580 min (run 6). These results show
that the steric demand of the substituents of benzene pre-
vents the reduction of the aromatic ring, probably as a result
of the hindrance experienced by the substituents of the
aromatic ring and the nanoparticle stabilizer. Aniline is
reduced to cyclohexylamine in quantitative yield after
270 min (run 7). Phenol gives cyclohexen-1-ol as result of
the hydrogenation of two carbon-carbon double bonds of
the aromatic ring. In the basic reaction medium, the enol
Table 4 collects the results obtained for the reductions of
benzene, substituted benzenes, functionalized benzenes, and
pyridine catalyzed by the suspension generated in the ab-
sence of the substrates by means of the preactivation for 1 h
of a mixture of complex 11 and Al2O3 in a 1:8.7 weight ratio,
under a constant atmospheric pressure of hydrogen at 60 °C.
Although the hydrogenations are slightly slower than those
shown in Table 3, in agreement with a longer activation

Article
Organometallics, Vol. 29, No. 19, 2010 4381
period of the catalytic system, these results demonstrate that
(i) the nanoparticles can also be generated with Al2O3 instead
of KtBuO and (ii) the nanoparticles supported on Al2O3, in
addition to being more handy, do not significantly modify
their catalytic activity.
nanoparticulated material in ethanol for several minutes; then,
one drop of the finely divided suspension was placed on a
specially produced structureless carbon support film having a
thickness of 4-6 nm and dried before observation.
Preparation of RhCl[2,6-bis-{1-(phenyl)iminoethyl}pyridine]
(1). 2,6-Bis{1-(phenyl)iminoethyl}pyridine (241 mg, 0.77 mmol)
was added to a solution of [Rh(μ-Cl)(η2-C2H4)2]2 (150 mg, 0.38
mmol) in toluene, giving a dark green suspension. After the
mixture was stirred for 6 h, the solvent was removed, and
pentane was added to afford a dark green solid, which was
washed repeatedly with pentane and dried in vacuo. Yield: 314
mg (90%). Anal. Calcd for C21H19ClN3Rh: C, 55.83; H, 4.24; N,
Concluding Remarks
This study has revealed that the complex [Rh(μ-Cl)(η2-
C2H4)2]2 reacts with 2,6-bis{1-(phenyl)iminoethyl}pyridines
to give rhodium(I) derivatives, which afford small nanopar-
ticles with a mean diameter of 1.5 ( 0.2 nm under a hydrogen
atmosphere and in the presence of a Brønsted base. These
nanoparticles, which are stabilized by the partially hydro-
genated bis(imino)pyridine ligand, catalyze the dehalogena-
tion and hydrogenation of aromatic compounds under mild
conditions, at a atmospheric pressure of hydrogen, and at 60 °C.
The nanoparticles can be deposited on alumina. Under these
conditions, they do not significantly modify their catalytic
activity.
1
9.30. Found: C, 55.55; H, 4.12; N, 9.18. H NMR (C6D6, 300
MHz): δ 7.75 (t, JH-H = 7.9, 1H, 4-py), 7.42 (d, JH-H = 7.3,
4H, o-Ph), 7.18 (t, JH-H = 7.3, 4H, m-Ph), 7.05 (t, JH-H = 7.3,
2H, p-Ph), 6.76 (d, JH-H =7.9, 2H, 3,5-py), 0.99 (s, 6H, Nd
CCH3). The 13C{1H} NMR spectrum could not be recorded due
to the low solubility of the complex in C6D6. MS (FABþ): m/z
416 (Mþ - Cl).
Preparation of RhCl[2,6-bis-{1-(2-methylphenyl)iminoethyl}-
pyridine] (2). This complex was prepared analogously to com-
plex 1, starting from 2,6-bis{1-(2-methylphenyl)iminoethyl}-
pyridine (263 mg, 0.77 mmol) and [Rh(μ-Cl)(η2-C2H4)2]2 (150 mg,
0.38 mmol). Yield: 340 mg (92%). Anal. Calcd for C23H23-
ClN3Rh: C, 57.57; H, 4.83; N, 8.76. Found: C, 57.18; H, 4.68; N,
8.58. 1H and 13C{1H} NMR spectroscopy revealed the presence
Experimental Section
All reactions were done either in a drybox or under argon
using standard Schlenk techniques. Solvents were dried by
known procedures and distilled under argon prior to use. C,
H, and N analyses were measured on a Perkin-Elmer 2400
CHNS/O analyzer. 1H and 13C{1H} NMR spectra were re-
corded on a Varian Gemini 2000, Bruker ARX 300, or Bruker
Avance 400 spectrometer. Chemical shifts are referenced to
residual solvent peaks or external CFCl3 (19F). Coupling con-
stants are given in hertz. All peaks in the 13C{1H} NMR spectra
are singlets, unless otherwise stated. Mass spectral analyses were
performed with a VG Auto Spec instrument. [Rh(μ-Cl)(η2-
C2H4)2]2,29 6,30 8,31 9,32 and the bis(imino)pyridine ligands33
were prepared as previously reported.
Physical Measurements. The catalytic reactions were fol-
lowed, at constant pressure, by measuring the hydrogen con-
sumption as a function of time on a gas buret (Afora 516256)
connected to a hydrogen gas reservoir. The analyses of the
products of the catalytic reactions were carried out on a
HP5890 II series gas chromatograph with a flame ionization
detector, using a 100% cross-linked methylsilicone gum column
(25 m ꢀ 0.32 mm, with 0.17 μm film thickness) and n-octane as
internal standard. The oven conditions used are as follows:
50 °C (hold 4 min) to 250 at 25 °C/min (hold 3 min). The reaction
products were identified by comparison of their retention times
with those observed for pure samples and by GC-MS experi-
ments run on an Agilent 5973 mass selective detector interfaced
to an Agilent 6890 series gas chromatograph system. Samples
were injected into a 30 m ꢀ 250 μm HP-5MS 5% phenylmethyl-
siloxane column with a film thickness of 0.17 μm (Agilent). The
GC oven temperature was programmed as follows: 35 °C for
6 min, 35-280 °C at 25 °C/min, 280 °C for 4 min. The carrier gas
was helium at a flow of 1 mL/min.
1
of two isomers (ratio 1:1). H NMR (C6D6, 300 MHz): δ 7.80
and 7.75 (both t, JH-H = 7.9, 1H, 4-py), 7.19-6.98 (m, 8H, Ph),
6.85 and 6.83 (both d, JH-H = 7.9, 2H, 3,5-py), 2.25 and 2.23 (s,
6H, -CH3), 0.97 (s, 6H, NdCCH3). 13C{1H} NMR (C6D6, 75.4
MHz): δ 166.6, 166.5 (CdN), 156.4 and 156.3 (2,6-py), 150.7,
131.0, 130.9, 130.8, 129.3, 126.2, 126.1, 125.6, 124.5, 122.9, 121.9
(Ph and py), 19.1 and 18.9 (-CH3), 17.0 (NdCCH3). MS
(FABþ): m/z 444 (Mþ - Cl).
Preparation of RhCl[2,6-bis{1-(2-ethylphenyl)iminoethyl}pyr-
idine] (3). This complex was prepared analogously to complex 1,
starting from 2,6-bis{1-(2-ethylphenyl)iminoethyl}pyridine (284 mg,
0.77 mmol) and [Rh(μ-Cl)(η2-C2H4)2]2 (150 mg, 0.38 mmol).
Yield: 372 mg (95%). Anal. Calcd for C25H27ClN3Rh: C, 59.12;
1
H, 5.36; N, 8.27. Found: C, 59.31; H, 5.08; N, 8.16. H and
13C{1H} NMR spectroscopy revealed the presence of two iso-
mers (ratio 1:0.9). Major isomer: 1H NMR (C6D6, 300 MHz): δ
7.85 (t, JH-H = 7.9, 1H, 4-py), 7.20-7.06 (m, 8H, Ph), 6.91 (d,
JH-H = 7.9, 2H, 3,5-py), 2.86-2.70 (m, 4H, -CH2-), 1.05 (s,
6H, NdCCH3), 1.04 (t, JH-H = 7.5, 6H, -CH3). 13C{1H} NMR
(C6D6, 75.4 MHz): δ 166.8 (d, JRh-C = 2, CdN), 156.4 (2,6-py),
149.9, 137.0, 128.8, 126.3, 126.0, 124.8, 123.0, 122.4 (Ph and py),
25.3 (-CH2-), 17.4 (NdCCH3), 14.1 (-CH3). Minor isomer:
1H NMR (C6D6, 300 MHz): δ 7.86 (t, JH-H = 7.9, 1H, 4-py),
7.20-7.06 (m, 8H, Ph), 6.94 (d, JH-H = 7.9, 2H, 3,5-py), 2.86-
2.70 (m, 4H, -CH2-), 1.11 (t, JH-H = 7.5, 6H, -CH3), 1.05 (s,
6H, NdCCH3). 13C{1H} NMR (C6D6, 75.4 MHz): δ 166.7 (d,
JRh-C = 2, CdN), 156.2 (2,6-py), 149.9, 136.8, 128.8, 126.3,
126.1, 124.8, 123.0, 122.4 (Ph and py), 25.2 (-CH2-), 17.4
(NdCCH3), 14.2 (-CH3). MS (FABþ): m/z 472 (Mþ - Cl).
Preparation of RhCl[2,6-bis{1-(2-isopropylphenyl)iminoethyl}-
pyridine] (4). This complex was prepared analogously to com-
plex 1, starting from 2,6-bis{1-(2-isopropylphenyl)iminoethyl}-
pyridine (305 mg, 0.77 mmol) and [Rh(μ-Cl)(η2-C2H4)2]2 (150
mg, 0.38 mmol). Yield: 362 mg (88%). Anal. Calcd for C27H31-
ClN3Rh: C, 60.51; H, 6.25; N, 7.84. Found: C, 60.05; H,
6.47; N, 8.04. 1H and 13C{1H} NMR spectroscopy revealed
the presence of two isomers (ratio 1:0.3). Major isomer: 1H
NMR (C6D6, 300 MHz): δ 7.87 (t, JH-H = 7.5, 1H, 4-py), 7.40-
7.09 (m, 8H, Ph), 6.95 (d, JH-H = 7.5, 2H, 3,5-py), 3.59 (m, 2H,
CH(CH3)2), 1.50 (d, JH-H = 6.9, 6H, CH(CH3)2), 1.14 (s, 6H,
NdCCH3), 1.08 (d, JH-H = 6.9, 6H, CH(CH3)2). 13C{1H}
NMR (C6D6, 75.4 MHz): δ 166.5 (CdN), 156.4 (2,6-py),
149.0, 141.8, 126.7, 126.3, 125.9, 124.6, 122.9, 122.2 (Ph and
Transmission electron microscopy (TEM) analyses were per-
ꢁ
formed at the Servei de Microscopia of the Universitat
Autonoma de Barcelona, using a JEOL JEM-2010 model at
ꢁ
200 kV. The TEM measurements were made by sonication of the
(29) Cramer, R. Inorg. Synth. 1990, 28, 86.
€
(30) Nuckel, S.; Burger, P. Organometallics 2001, 20, 4345.
(31) Dias, E. L.; Brookhart, M.; White, P. S. Organometallics 2000,
19, 4995.
(32) Haarman, H. F.; Ernsting, J.-M.; Kranenburg, M.; Kooijman,
H.; Veldman, N.; Spek, A. L.; van Leeuwen, P. W. N. M.; Vrieze, K.
Organometallics 1997, 16, 887.
ꢀ
ꢀ
ꢀ
~
(33) Esteruelas, M. A.; Lopez, A. M.; Mendez, L.; Olivan, M.; Onate,
i
i
E. Organometallics 2003, 22, 395.
py), 28.5 (CH, Pr), 24.2, 23.0 (CH3, Pr), 17.4 (NdCCH3).

4382 Organometallics, Vol. 29, No. 19, 2010
Buil et al.
Minor isomer: 1H NMR (C6D6, 300 MHz): δ 7.86 (t, JH-H
=
of chlorine atoms of the halogenated substrate), and n-octane
(200 μL) in 2-propanol (5 mL) placed in a 25 mL flask attached
to a gas buret, which was in turn connected to a Schlenk
manifold that had been previously evacuated and refilled with
hydrogen. The flask was then immersed in a 60 °C bath, and the
mixture was vigorously shaken (500 rpm) during the run. The
reaction was monitored by measuring the hydrogen consump-
tion and by periodic GC analysis of samples removed via
syringe.
Hg(0) Poisoning Test. Hg(0) (1 mL, 67.8 mmol) was added,
under a hydrogen atmosphere, to a solution of chlorobenzene
(1.2 mmol), complex 11 (0.024 mmol), KtBuO (1.2 mmol), and
n-octane (200 μL) in 2-propanol (5 mL) placed in a 25 mL flask
attached to a gas buret, which was in turn connected to a
Schlenk manifold that had been previously evacuated and re-
filled with hydrogen. The flask was then immersed in a 60 °C
bath, and the mixture was vigorously shaken (500 rpm) during
the run. The reaction was monitored by measuring the hydrogen
consumption. The hydrogen consumption was compared to that
of the same experiment performed in the absence of added
mercury.
Preparation of the Nanoparticles. Complex 11 (28.2 mg, 0.048
mmol) and KtBuO (27 mg, 0.24 mmol) were dissolved in 10 mL
of 2-propanol and were transferred under a hydrogen atmo-
sphere to a 25 mL reaction flask. The flask was connected to the
hydrogen reservoir and then was immersed in a 60 °C bath, and
the mixture was shaken (500 rpm) for 7 h. After this time the
hydrogen atmosphere was replaced by an argon atmosphere; the
black residue that formed was left to decant, and the solution
was removed. 2-Propanol was added to wash the residue, which
was then dried. 1H NMR (CDCl3, 300 MHz): δ 7.57-6.37 (m,
Ph), 2.2-0.5 (m, aliphatic protons). 19F NMR (CDCl3, 235.4
MHz): δ -61.1 ppm (s, -CF3).
7.5, 1H, 4-py), 7.40-7.09 (m, 8H, Ph), 6.92 (d, JH-H = 7.5, 2H,
3,5-py), 3.59 (m, 2H, CH(CH3)2), 1.40 (d, JH-H = 6.9, 6H,
CH(CH3)2), 1.14 (s, 6H, NdCCH3), 1.04 (d, JH-H = 6.9, 6H,
CH(CH3)2). 13C{1H} NMR (C6D6, 75.4 MHz): δ 166.7 (CdN),
156.4 (2,6-py), 149.1, 141.9, 126.6, 126.3, 125.8, 124.7, 123.1,
i
i
122.1 (Ph and py), 28.6 (CH, Pr), 24.1, 23.0 (CH3, Pr), 17.6
(NdCCH3). MS (FABþ): m/z 535 (Mþ), 500 (Mþ - Cl).
Preparation of RhCl[2,6-bis{1-(2-tert-butylphenyl)iminoethyll}-
pyridine] (5). This complex was prepared analogously to com-
plex 1, starting from 2,6-bis{1-(2-tert-butylphenyl)iminoethyl}-
pyridine (328 mg, 0.77 mmol) and [Rh(μ-Cl)(η2-C2H4)2]2 (150
mg, 0.38 mmol). Yield: 374 mg (86%). Anal. Calcd for
C29H35ClN3Rh: C, 61.76; H, 6.25; N, 7.45. Found: C, 61.46;
H, 5.76; N, 7.44. 1H and 13C{1H} NMR spectroscopy revealed
1
the presence of two isomers (ratio 1:1). H NMR (C6D6, 300
MHz): δ 7.77 and 7.79 (both t, JH-H = 7.9, 1H, 4-py), 7.38-7.46
(m, 2H, Ph), 7.16-6.97 (m, 6H, Ph), 6.78 and 6.82 (both d, JH-H
=
7.9, 2H, 3,5-py), 1.40 and 1.43 (both s, 18H, -tBu), 1.03 and
1.04 (both s, 6H, NdCCH3). 13C{1H} NMR (C6D6, 75.4 MHz):
δ 166.3, 166.2 (both d, JRh-P = 2.8, CdN), 156.2 and 156.1
(both d, JRh-P = 2.8, 2,6-py), 150.3, 150.1, 140.9, 129.3, 129.1,
128.6, 128.5, 126.3, 126.2 125.1, 124.8, 124.7, 121.4 (Ph and
py), 19.1 and 18.9 (-CH3), 36.6 (-C(CH3)3), 33.0 and 32.8
(-C(CH3)3), 18.4 and 18.3 (both s, NdCCH3). MS (FABþ):
m/z 528 (Mþ - Cl).
Preparation of RhCl[2,6-bis{1-(2,6-diethylphenyl)iminoethyl}-
pyridine] (7). This complex was prepared analogously to com-
plex 1, starting from 2,6-bis{1-(2,6-diethylphenyl)iminoethyl}-
pyridine (328 mg, 0.77 mmol) and [Rh(μ-Cl)(η2-C2H4)2]2 (150
mg, 0.38 mmol). Yield: 426 mg (98%). Anal. Calcd for
C29H35ClN3Rh: C, 61.76; H, 6.25; N, 7.45. Found: C, 62.03;
H, 6.46; N, 7.05. 1H NMR (C6D6, 300 MHz): δ 7.82 (t, JH-H
7.9, 1H, 4-py), 7.13-7.01 (m, 6H, Ph), 6.86 (d, JH-H = 7.9, 2H,
3,5-py), 2.74 (dq, JH-H = 7.5, JH-H = 14.8, 4H, -CH2-), 2.50
(dq, JH-H = 7.5, JH-H = 14.8, 4H, -CH2-), 1.12 (t, JH-H
7.5, 12H, -CH3), 1.01 (s, 6H, NdCCH3). 13C{1H} NMR (C6D6,
75.4 MHz): δ 167.0 (d, JRh-C = 1.6, CdN), 156.3 (d, JRh-C
2.6, 2,6-py), 148.0, 135.6, 126.5, 126.1, 124.6, 122.0 (Ph and py),
25.1 (-CH2-), 17.6 (NdCCH3), 14.1 (-CH3). MS (FABþ): m/z
528 (Mþ - Cl).
=
Hydrogenation Reactions in the Presence of KtBuO. In a typi-
cal procedure, the catalyst precursor 11 (14.1 mg, 0.024 mmol)
and KtBuO (135 mg, 0.12 mmol) were dissolved in 5 mL of
2-propanol and the resulting solution was transferred under a
hydrogen atmosphere to a 25 mL reaction flask. The flask was
connected to the hydrogen reservoir and then was immersed in a
60 °C bath, and the mixture was shaken (500 rpm) for 1 h. After
this time, the substrate (1.2 mmol) was added to the flask via
syringe and hydrogen uptake was recorded. In the case of phenol
it was dissolved in 2 mL of 2-propanol and added via syringe to a
preactivated solution of 2-propanol (3 mL) containing the
catalyst precursor and KtBuO. The reactions were monitored
by measuring the hydrogen consumption and by periodic GC
analysis of samples removed via syringe.
Hydrogenation Reactions in the Presence of Al2O3. In a typical
procedure, the catalyst precursor 11 (14.1 mg, 0.024 mmol) was
dissolved in 5 mL of 2-propanol and was transferred under a
hydrogen atmosphere to a 25 mL reaction flask containing basic
Al2O3 (122.4 mg). The flask was connected to the hydrogen
reservoir and then was immersed in a 60 °C bath, and the mix-
ture was shaken (500 rpm) for 1 h. After this time, the substrate
(1.2 mmol) was added to the flask via syringe and hydrogen
uptake was recorded. In the case of phenol it was dissolved in
2 mL of 2-propanol and added via syringe to a preactivated
mixture containing the catalyst precursor and aluminum oxide
in 3 mL of 2-propanol. The reaction was monitored by measur-
ing the hydrogen consumption and by periodic GC analysis of
samples removed via syringe.
=
=
Preparation of RhCl[2,6-bis{1-(4-methylphenyl)iminoethyl}-
pyridine] (10). This complex was prepared analogously to com-
plex 1, starting from 2,6-bis{1-(4-methylphenyl)iminoethyl}-
pyridine (263 mg, 0.77 mmol) and [Rh(μ-Cl)(η2-C2H4)2]2 (150
mg, 0.38 mmol). Yield: 332 mg (90%). Anal. Calcd for
C23H23ClN3Rh: C, 57.57; H, 4.83; N, 8.76. Found: C, 57.46;
H, 4.46; N, 8.48. 1H NMR (C6D6, 300 MHz): δ 7.79 (t, JH-H
=
7.9, 1H, 4-py), 7.39 (d, JH-H = 8, 4H, Ph), 7.00 (d, JH-H = 8,
4H, Ph), 6.81 (d, JH-H = 7.9, 2H, 3,5-py), 2.07 (s, 6H, -CH3),
1.08 (s, 6H, NdCCH3). The 13C{1H} NMR spectrum could not
be recorded, due to the low solubility of the complex in C6D6.
MS (FABþ): m/z 444 (Mþ - Cl).
Preparation of RhCl[2,6-bis{1-(4-trifluoromethylphenyl)imin-
oethyl}pyridine] (11). This complex was prepared analogously to
complex 1, starting from 2,6-bis{1-(4-(trifluoromethyl)phenyl)-
iminoethyl}pyridine (346 mg, 0.77 mmol) and [Rh(μ-Cl)(η2-
C2H4)2]2 (150 mg, 0.38 mmol). Yield: 323 mg (71%). Anal.
Calcd for C23H17ClF6N3Rh: C, 47.00; H, 2.91; N, 2.38. Found:
C, 46.64; H, 3.08; N, 2.50. 1H NMR (acetone-d6, 300 MHz): δ
8.63 (t, JH-H = 8, 1H, p-py), 8.02 (d, JH-H = 8, 2H, m-py), 7.75
(d, JH-H = 8.4, 4H, Ph), 7.58 (d, JH-H = 8.4, 4H, Ph), 1.85 (s,
6H, NdCCH3). 19F NMR (acetone-d6, 282.33 MHz): δ -57.8
ppm (s, -CF3). The 13C{1H} NMR spectrum could not be
recorded, due to the low solubility of the complex in acetone-d6.
MS (FABþ): m/z 587 (Mþ), 552 (Mþ - Cl).
Cycles of Hydrogenation of Benzene in the Presence of KtBuO.
The catalyst precursor 11 (14.1 mg, 0.024 mmol) and KtBuO
(135 mg, 0.12 mmol) were dissolved in 5 mL of 2-propanol, the
resulting solution was transferred under a hydrogen atmosphere
to a 25 mL reaction flask connected to the hydrogen reservoir,
and benzene (1.2 mmol) was added to the flask via syringe. The
flask was then immersed in a 60 °C bath and shaken (500 rpm).
The reaction was monitored by measuring the hydrogen con-
sumption and by periodic GC analysis of samples removed via
Catalytic Reactions of Dehalogenation-Hydrogenation. In a
typical procedure, the halogenated arene (1.2 mmol) was added,
under a hydrogen atmosphere, to a solution of the catalyst
precursor (0.024 mmol), KtBuO (1.2x mmol, x being the number

Article
Organometallics, Vol. 29, No. 19, 2010 4383
syringe. After 160 min the first load of benzene was hydro-
genated to cyclohexane. The system was refilled with hydrogen,
and benzene (second load, 1.2 mmol) was added via syringe. The
procedure was repeated up to six loads of benzene.
Cycles of Hydrogenation of Benzene in the Presence of Al2O3.
The catalyst precursor 11 (14.1 mg, 0.024 mmol) was dissolved
in 5 mL of 2-propanol and was transferred under a hydrogen
atmosphere to a 25 mL reaction flask containing basic Al2O3
(122.4 mg). The flask was connected to the hydrogen reservoir,
and benzene (1.2 mmol) was added to the flask via syringe. The
flask was then immersed in a 60 °C bath and shaken (500 rpm).
The reaction was monitored by measuring the hydrogen
consumption and by periodic GC analysis of samples removed
via syringe. After 460 min the first load of benzene was
hydrogenated to cyclohexane. The system was refilled with
hydrogen, and benzene (second load, 1.2 mmol) was added via
syringe. The procedure was repeated up to seven loads of
benzene.
Acknowledgment. Financial support from the MICINN
of Spain (Project Nos. CTQ2008-00810, CTQ2008-05409-
C02-01 and Consolider Ingenio 2010 CSD2007-00006),
ꢀ
ꢀ
the Diputacion General de Aragon (E35), the DURSI-
Generalitat de Catalunya (SGR 2009-SGR1441), and
the European Social Fund is acknowledged.