Highly selective photo-catalytic dimerization of α-methylstyrene by a novel palladium complex with photosensitizing ruthenium(II) polypyridyl moiety

Article abstract of DOI:10.1039/b508013d

A novel dinuclear complex containing the photo-sensitizing Ru unit and a Pd center is effective toward selective catalytic dimerization of α-methylstyrene leading to 2,4-diphenyl-4-methyl-1-pentene under visible-light irradiated conditions. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b508013d

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Highly selective photo-catalytic dimerization of a-methylstyrene by a
novel palladium complex with photosensitizing ruthenium(II)
polypyridyl moiety{
Akiko Inagaki,* Shinichi Edure, Shinichi Yatsuda and Munetaka Akita*
Received (in Cambridge, UK) 10th June 2005, Accepted 6th September 2005
First published as an Advance Article on the web 4th October 2005
DOI: 10.1039/b508013d
4
[(bpy)2Ru(bpm)Pd(Me)Cl](PF6)2 with AgBF4 in acetone and
acetonitrile, respectively (eqn (1)).
A novel dinuclear complex containing the photo-sensitizing
Ru unit and a Pd center is effective toward selective
catalytic dimerization of a-methylstyrene leading to 2,4-
diphenyl-4-methyl-1-pentene under visible-light irradiated
conditions.
The challenge to highly efficient conversion of abundant solar
energy into chemical energy has received much attention in
recent years.1 In the natural photosynthetic processes, two types of
photochemical molecular devices play key roles for the conversion:
(1) an antenna device to collect solar light and (2) a reaction
center where the excitation energy is used to perform the charge-
separation process converting the electronic energy into redox
chemical energy. Thus far, Ru(II) polypyridyl complexes have
been widely used as a building block for redox-active, luminescent
supramolecules because of their attractive photophysical
properties such as absorption of visible light and long excited-
state lifetime.2 However, studies of the polynuclear Ru(II)
polypyridyl complexes combined with a catalytic system are
limited.3 One of the reasons may be ascribed to the properties
of the second metal fragment introduced as a reaction center,
which should possess a certain stability against photo-decomposi-
tion as well as a high reactivity toward outer substrates to
construct an effective catalytic system for utilizing solar energy.
Based on the strategies, we designed a dinuclear system,
(bpy)2Ru(BL)Pd(R)(L) (bpy 5 2,29-bipyridyl, BL 5 bridging
ligand, R 5 alkyl), in expectation of an excited energy transfer
from the Ru center to the Pd center, from which a highly reactive
species is easily generated by dissociation of a weakly bound
solvent ligand (L). In this communication, we report selective
photo-catalytic dimerization of a-methylstyrene to yield 2,4-
ð1Þ
Complexes 1 and 2 were unambiguously identified on the basis
of their 1H and 13C NMR spectra as well as ESI-MS spectral data.
The NMR signals for the methyl group of 1 appeared at dH 1.30
(s, 3H) and dC 5.2. The methyl signals for the coordinated acetone
(dH 2.23, dC 32.0) significantly broadened at low temperature
suggesting a fast dissociation process. The solid-state structure of 2
(Fig. 1), as determined by X-ray crystallography, is consistent with
the solution structure.5 Structural features of 2 are comparable
to those of [(bpy)2Ru(dpp)PdCl2](PF6)2 (dpp 5 2,3-bis(2-pyridyl)-
pyrazine) reported by Yam3 and related mononuclear complexes.6
The Pd center was slightly distorted from a normal square planar
geometry with the N–Pd–N bite angle (N(6)–Pd(1)–N(8)) of
79.9(3)u, showing a subtle strain brought about by linking with
[(bpy)2Ru(bpm)]2+.
Complexes 1 and 2 are photochemically stable in CH3NO2
solution, and no deterioration was observed after irradiation for
over 50 h.
The absorption spectrum of 2 shown in Fig. 2 is apparently a
superposition of those of the related mononuclear complexes,
[(bpy)2Ru(bpm)](PF6)2 and [(bpy)PdMe(MeCN)](BF4). Complex 2
…
diphenyl-4-methyl-1-pentene by novel dinuclear Ru Pd
complexes, [(bpy)2Ru(bpm)PdMe(solv)](PF6)2(BF4) (bpm 5 2,29-
bipyrimidine, solv 5 Me2CO (1), CH3CN (2)), under visible-light
irradiated condition.
Polypyridyl ruthenium complexes containing the solvated
Pd center, [(bpy)2Ru(bpm)Pd(Me)(Me2CO)](PF6)2(BF4) (1)
and [(bpy)2Ru(bpm)Pd(Me)(CH3CN)](PF6)2(BF4) (2), were
prepared in quantitative yields by Cl2 abstraction from
Chemical Resources Laboratory, Tokyo Institute of Technology, R1-27,
4259 Nagatsuta Midori-ku Yokohama, 226-8503, Japan.
E-mail: akiko_inagaki@res.titech.ac.jp; makita@res.titech.ac.jp;
Fax: +81 45 924 5230; Tel: +81 45 924 5230
{ Electronic supplementary information (ESI) available: Experimental
section. See http://dx.doi.org/10.1039/b508013d
Fig. 1 Molecular structure of the cationic part of [(bpy)2Ru(bpm)-
Pd(Me)(MeCN)]3+ (2), drawn with thermal ellipsoids at the 30%
probability level.
5468 | Chem. Commun., 2005, 5468–5470
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Fig. 3 Time–product distribution curves for the reaction of 1 with
a-methylstyrene 3: (a) without switching off and (b) light was switched off
during the ‘‘dark’’ periods.
A dinuclear structure with the light-harvesting Ru(II) unit and
the Pd center in close proximity was essential for the catalysis. Not
only complexes 1 and 2 but also an intermediate C (vide infra) were
effective for the dimerization, while analogues for the mononuclear
components of 1, [(bpy)3Ru](PF6)2 and [(bpy)PdMe(Me2CO)]BF4
(5), or a 1:1 mixture of them were totally ineffective. Thus far, any
precedent work of selective dimerization of 3 to give 4 mediated by
transition metal catalysts has not been reported.10
Fig. 2 Absorption spectra of 2, [(bpy)2Ru(bpm)](PF6)2 and
[(bpy)PdMe(MeCN)]BF4 in deaerated CH3CN solution.
exhibited a low-energy absorption band at 420 nm (lmax 5 424 nm)
due to the incorporation of Ru(II) polypyridyl moiety. Thus,
Ru(II) polypyridyl moiety should act as a visible light-harvesting
unit to collect light energy to the Pd center. If the Ru(II) and the Pd
center are properly channeled in this system, excitation of the Ru
unit would result in population of the excited state Pd center which
may promote insertion/dissociation processes in the intermolecular
reactions. Based on this scenario, photochemical reactions by
dinuclear complexes 1 and 2 were investigated.
A plausible reaction mechanism for the catalytic dimerization
is outlined in Scheme 1. The reaction sequence consists of two
processes, i.e. the initial step to form the hydride intermediate B
and the catalytic cycle.
The first process involves insertion of 3 into the Pd–CH3 bond
in 1 to form A (step (i)) and subsequent b-elimination gives
2-phenyl-2-butene (6) and B (step (ii)). In the second process,
successive insertion of 3 into B (step (iii)) and C/C* (steps (iv)–(v))
During the reactivity study toward various unsaturated hydro-
carbons, complexes 1 and 2 were found to be catalytically active
toward dimerization of terminal olefins such as styrene, 1-hexene,
methyl acrylate and a-methylstyrene (3).7 Above all, a-methyl-
styrene was most suitable for the mechanistic study (vide infra) and
thus further details of the reaction were investigated as follows. A
nitromethane solution of a-methylstyrene 3 containing a catalytic
amount of 1 (1 mol%) was irradiated by visible light (150 W Xe
lamp, l . 420 nm) at room temperature under an Ar atmosphere.8
Under these conditions, a-methylstyrene (3) was selectively
dimerized to give a head-to-tail dimer, 4-methyl-2,4-diphenyl-1-
pentene (4), over 90% yield together with small amounts of trimers
(ca. 3%)9 (eqn (2)). The dimer 4 was characterized by comparison
with a commercially available authentic sample. Reactions in
acetone and acetonitrile were sluggish compared with those in
nitromethane, presumably because the reaction was suppressed by
solvent coordination.
ð2Þ
The reaction was monitored by GC and the time–product
distribution curves are shown in Fig. 3(a) and (b). Within 4 h of
irradiation, 100 equivalents of 3 was completely consumed to give
the dimer 4 for over 90% yield (Fig. 3(a)). Repetitive addition of
the monomer 3 (100 equiv.) up to three times also resulted in the
complete consumption of 3 to give 4. Within 30 h, the reaction did
not proceed in the dark nor in the absence of the catalyst. In order
to verify the effect of light, the lamp was switched off at an
appropriate time intervals, and the reaction did not proceed during
the ‘‘dark’’ periods (Fig. 3(b)).
Scheme 1
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4 [(bpy)2Ru(bpm)Pd(Me)Cl](PF6)2 was quantitatively synthesized by the
reaction of [(bpy)2Ru(bpm)](PF6)2 and (1,5-cod)PdMeCl in CH3NO2.
Details of the synthetic procedure and spectroscopic data are given
as ESI.
gives D, which undergoes b–H elimination to yield 4 and B
(step (vi)).
A series of control experiments has been done to establish the
proposed reaction mechanism. Addition of 5 equivalents of 3 to 1
caused immediate formation of an equimolar mixture of A and
(E)-6.11 Further irradiation caused a gradual conversion of A into
C preceding the formation of 4. Both intermediates A and C were
characterized by 1H, 13C NMR (for C) and ESI-MS spectro-
scopy.12,13 Complex C was stable enough to be isolated by treating
1 with an excess amount of 3 for 1 h. Its intermediacy was
confirmed by the addition of 3 to an isolated sample of C, which
resulted in the catalytic formation of 4 under the same irradiated
condition. Notably, steps (i), (ii) and (iii) could be undergone
without light to give C,14 although the rate of formation from A
was substantially smaller comparing to that under the irradiated
condition.15 Judging from these results, photo-excitation of C
seems to be essential for the second insertion step which leads to
the progress of the catalytic cycle.
5 X-Ray diffraction measurements were made on a Rigaku RAXIS IV
imaging plate area detector with graphite-monochromated Mo-Ka
radiation at 260 uC. Crystal data for 2?3CH3CN: C37H37B2F14N12-
¯
PPdRu, M 5 1175.85, triclinic, space group P1, a 5 12.147(11),
˚
b 5 12.263(11), c 5 16.624(16) A, a 5 88.05(4), b 5 71.97(4), c 5
,
89.15(4)u, V 5 2353(4) A , Z 5 2, Dc 5 1.691 g cm23, m 5 8.75 cm21
3
˚
R1 (wR2) 5 0.076 (0.180) for the 9577 unique data with I . 2s(I) and
593 parameters. In the solid state, counter anions of complex 2 was
composed of one PF6 and two BF4 anions which was different from that
of the solution (two PF6 and one BF4) CCDC 275767. See http://
dx.doi.org/10.1039/b508013d for crystallographic data in CIF or other
electronic format.
6 P. K. Byers, A. J. Canty, B. W. Skelton and A. H. White, J. Organomet.
Chem., 1987, 336, C55; P. K. Byers, A. J. Canty, B. W. Skelton and
A. H. White, J. Organomet. Chem., 1990, 393, 299; B. A. Markies,
A. J. Canty, W. de Graaf, J. Boersma, M. D. Janssen, M. P. Hogerheide,
W. J. J. Smeets, A. L. Spek and G. van Koten, J. Organomet. Chem.,
1994, 482, 191; A. Bayler, A. J. Canty, P. G. Edwards, B. W. Skelton
and A. H. White, J. Chem. Soc., Dalton Trans., 2000, 3325.
7 Reaction of 1 with styrene and 1-hexene gave the corresponding head-
to-tail dimers (a mixture of cis- and trans-RCHLCHCRCH3, R 5 Ph,
nBu), however, that with methyl acrylate gave a head-to-head dimers (a
mixture of D2- and D3-dimethyldihydromuconate). When the formation
rate of the dimers were compared under the same reaction condition,
that of styrene, 1-hexene and methyl acrylate was much smaller
than that of a-methylstyrene (a-methylstyrene . 1-hexene, methyl
acrylate . styrene).
…
This work demonstrates that novel dinuclear Ru Pd com-
plexes containing a photo-sensitizing Ru(II) unit and a reactive Pd
center catalyze selective dimerization of a-methylstyrene 3 to give 4
under visible-light irradiation. On the basis of the mechanistic
study, generation of the photoexcitated species [C]* by visible-light
by which the second insertion step can proceed is most likely the
key step in the catalytic cycle. Further studies to elucidate the
effect of light as well as applications to various substrates are now
in progress.
8 Typical reaction procedures: To a CD3NO2 solution (0.37 mL) of 1
(11.3 mg, 1.00 mmol, 1 mol%) was added a-methylstyrene 3 (130 mL,
1.00 mmol, substr./cat. 5 100) and adamantane (10.0 mg, an internal
standard). The solution was divided into two 5 mm i.d. glass tubes; one
for the irradiation and the other for the dark reaction which was covered
with aluminium foil. The two samples were placed at a distance of 10 cm
from an Xe lamp (150 W, with a L42 cut-off filter (l . 420 nm)). The
We are grateful to the Ministry of Education, Culture, Sports,
Science and Technology of the Japanese Government and the
Japan Society for Promotion of Science and Technology (Grant-
in-Aid for Young Scientists (B): No. 16750046) for financial
support of this research.
1
reaction was followed by H NMR and/or GC after appropriate time
intervals.
9 Formation of the trimers was indicated by the GC-MS peak at m/z 354
(C27H30).
10 Dimerization of methyl methacrylate catalyzed by alkylcobaloxime:
M. Kijima, K. Miyamori and T. Sato, J. Org. Chem., 1987, 52, 706;
dimerization of vinylarenes by Pd catalysts in the presence of In(OTf)3
as an acid co-catalyst: T. Tsuchimoto, S. Kamiyama, R. Negoro,
E. Shirakawa and Y. Kawakami, Chem. Commun., 2003, 852;
dimerization of a-methylstyrene by Pd catalyst giving 1,3,3-trimethyl-
1-phenylindan as a product: Z. Jiang and A. Sen, Organometallics, 1993,
12, 1406.
Notes and references
1 T. Hasobe, H. Imahori, P. V. Kamat, T. K. Ahn, S. K. Kim, D. Kim,
A. Fujimoto, T. Hirakawa and S. Fukuzumi, J. Am. Chem. Soc., 2005,
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M. Gorka, S.-K. Lee, J. van Heyst and F. Vo¨gtle, J. Am. Chem. Soc.,
2004, 126, 16466; D. M. Guldi, I. Zilbermann, G. Anderson,
N. A. Kotov, N. Tagmatarchis and M. Prato, J. Am. Chem. Soc.,
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F. M. MacDonnell, J. Am. Chem. Soc., 2004, 126, 11621; D. Gust,
T. A. Moore and A. L. Moore, Acc. Chem. Res., 2001, 34, 40; F. Fungo,
A. Luis, L. A. Otero, L. Sereno, J. J. Silber and E. N. Durantini,
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W. Xia, Q. Zhuang, Y. Zhao and Y. Huang, J. Phys. Chem., 1996, 100,
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K. Kalyanasundararam, Photochemistry of Polypyridine and Porphyrin
Complexes, Academic Press, London, 1992.
2 See, for example: V. Balzani, A. Juris, M. Venturi, S. Campagna and
S. Serroni, Chem. Rev., 1996, 96, 759, and references therein
J.-P. Sauvage, J.-P. Collin, J.-C. Chambron, S. Guillerez, V. Balzani,
F. Barigelletti, L. De Cola and L. Flamigni, Chem. Rev., 1994, 94, 993,
and references therein; V. Balzani and F. Scandola, Supramolecular
Photochemistry; Horwood, Chichester, England, 1990; G. Denti,
S. Serroni, S. Campagna, V. Ricevuto and V. Balzani, Coord. Chem.
Rev., 1991, 111, 227; F. Denti, S. Campagna, L. Sabatino, S. Lerroni,
M. Ciano and V. Balzani, Inorg. Chem., 1990, 29, 4750.
1
11 Spectral data for 6: H NMR (200 MHz, CD3NO2, RT): d 1.79 (dq,
J 5 6.84, 0.98 Hz, 3H, –CH3), 2.02 (m, 3H, –CH3), 5.91 (qq, J 5 6.84,
1.38 Hz, 1H, LC(Me)H). Ph protons were not fully characterized
because they overlap with other aromatic protons. m/z (GCMS) 132 (M,
59%), 117 (M 2 5, 100). 6 was characterized according to the reported
spectroscopic data in the following references: E. Vedejs, J. Cabaj and
M. J. Peterson, J. Org. Chem., 1993, 58, 6509; P. Fristrup, D. Tanner
and P.-O. Norrby, Chirality, 2003, 15, 360.
12 Spectral data for A: Two sets of the two methyl signals (dH 1.57, 1.62
for –CH3, dH 1.24, 1.27 for –CH2CH3) were attributed to the two
diastereomers of A with the chiral carbon atom attached to Pd. 1H
NMR (200 MHz, CD3NO2, RT): d 1.20 (t, J 5 7.4 Hz, 3H,
PdCCH2CH3), 1.27 (t, J 5 7.4 Hz, 3H, PdCCH2CH3), 1.57 (s, 3H,
PdCMe), 1.62 (s, 3H, PdCMe), 1.8–2.0 (m, 2H, PdCCH2CH3), 7.5–9.2
(m, 27H, Ph). ESI-MS spectrum: m/z 1101: {(PdCMeEtPh)(PF6)2}+,
1043: {(PdCMeEtPh)(PF6)(BF4)}+, 985: {(PdCMeEtPh)(BF4)2}+.
Pd 5 {(bpy)2102Ru(bpm)106Pd}.
13 Spectral data for C: The two signals for the diastereotopic methyl groups
1
were located at dH 1.50 and 1.56. Additional H, 13C NMR, and ESI-
MS spectral data are given as Electronic supplementary information.
14 Although the formation of C was observed under the dark condition,
the dimer 4 did not form over a prolonged reaction time.
15 When the formation rate of C from A was compared between the
irradiated and dark conditions, the rate under irradiation was nearly as
twice as fast compared to that under the dark condition.
3 Stoichiometric reaction: V. W. Yam, V. W. Lee and K.-K. Cheung,
Organometallics, 1997, 16, 2833; catalytic olefin isomerization:
M. Osawa, M. Hoshino and Y. Wakatsuki, Angew. Chem., Int. Ed.,
2001, 40, 3472.
5470 | Chem. Commun., 2005, 5468–5470
This journal is ß The Royal Society of Chemistry 2005