Dinuclear molybdenum cluster-catalyzed radical addition and polymerization reactions by tuning the redox potential of a quadruple bonded Mo2 core

Article abstract of DOI:10.1039/c1dt11129a

We developed dinuclear molybdenum cluster-catalyzed radical addition and polymerization reactions by tuning the redox potential of the Mo₂ core. A 2,4,6-triisopropylbenzoate-supported Mo₂ complex acts as a catalyst for radical addition reactions of polyhaloalkanes to 1-alkenes and cyclopentene, while amidinate- and guanidinate-supported Mo₂ clusters are effective catalysts for the radical polymerization reaction of methyl methacrylate.

Full text of DOI:10.1039/c1dt11129a

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Dinuclear molybdenum cluster-catalyzed radical addition and polymerization
reactions by tuning the redox potential of a quadruple bonded Mo2 core†
Hayato Tsurugi, Kohei Yamada, Moumita Majumdar, Yoshitaka Sugino, Akio Hayakawa and
Kazushi Mashima*
Received 16th June 2011, Accepted 18th July 2011
DOI: 10.1039/c1dt11129a
We developed dinuclear molybdenum cluster-catalyzed radi-
cal addition and polymerization reactions by tuning the redox
potential of the Mo2 core. A 2,4,6-triisopropylbenzoate-
supported Mo2 complex acts as a catalyst for radical addition
reactions of polyhaloalkanes to 1-alkenes and cyclopentene,
while amidinate- and guanidinate-supported Mo2 clusters are
effective catalysts for the radical polymerization reaction of
methyl methacrylate.
amidinate or guanidinate ligands.7 Thus, taking into account
the advantageous compatibility of the reversible redox behavior
of the M–M bonded cores between [M2]4+ and [M2]5+, whose
redox potential can be controlled by changing the supporting
ligands (Fig. 1),5 we hypothesized that dinuclear metal clusters
can serve as versatile catalysts for activating various alkyl halides
to produce carbon-centered radicals, which initiate both radical
addition and radical polymerization reactions. We herein report
an application of dinuclear group 6 metal clusters as catalysts
for radical addition or radical polymerization reactions that
are highly dependent on the electron-donating characteristics of
the ligand; the combination of the benzoate-based ligand and
Mo2 core results in the formation of active catalysts for radical
addition of CCl4 to various alkenes, while radical polymeriza-
tion of methyl methacrylate is accomplished by Mo2 clusters
bearing more electron-donating amidinate and guanidinate-based
ligands.
Radical addition of halogenated compounds to alkenes and
alkynes is one of the most fundamental and useful reactions in
organic chemistry.1 Single electron transfer, activation of halo-
genated substrates to generate radicals, and bond recombination
are considered important reaction processes for realizing various
transition metal-catalyzed radical coupling reactions.2 Among
these steps, activation of alkyl halides proceeds by oxidation of
the transition metal center to generate carbon-centered radicals,
while bond recombination leading to the coupling products is ac-
companied by reduction of the metal center for Kharash reactions.
Thus, tuning the redox potential of the metal center is essential for
generating active radical species. Since the discovery of transition
metal-catalyzed radical addition3 and radical polymerization4
reactions, there has been extensive development of a wide variety
of transition metal complexes, such as Cu, Fe, Ru, and Ni.
Although there are numerous studies of mononuclear catalysts
of low-valent metal complexes, multinuclear metal clusters with
metal–metal multiple bonds have the advantage of activating a
wide variety of alkyl and aryl halides via precise control of the
redox potential.5–9 Notable previous examples are stoichiometric
activation of carbon–halogen bonds of alkyl and aryl halides
by dinuclear Mo2 and W2 clusters supported by four cyclic
guanidinate ligands6 and dinuclear Rh2 clusters that have four
Fig. 1 Dinuclear molybdenum complexes 1–3 and their redox potential
of dinuclear molybdenum core, [Mo2]4+/[Mo2]5+
.
We performed a radical addition reaction, i.e., Kharash addi-
tion, of CCl4 to 1-hexene catalyzed by dimolybdenum complexes
1–3 and the results are listed in Table 1. The solvent, i.e.,
chloroform or THF, influenced the reaction. The chemical yields in
CDCl3 were rather low. Reactions catalyzed by complexes 1 and
2 afforded a coupling product, 1,1,1-trichloro-3-chloroheptane,
in 19% and 22% yields (runs 1 and 2), respectively, whereas a
triphenylguanidinate-ligated complex 3, which was easily oxidized
compared with 1 and 2, resulted in 4% yield (run 3), probably
due to difficulties in the regeneration of [Mo2]4+ species (vide
infra). In THF-d8, the radical addition reaction catalyzed by 1 was
significantly accelerated, and the coupling product was obtained
in 84% yield (run 4), though complexes 2 and 3 did not show
Department of Chemistry, Graduate School of Engineering Science,
Osaka University, and CREST, JST, Toyonaka, Osaka, 560-8531, Japan.
E-mail: mashima@chem.es.osaka-u.ac.jp; Fax: +81-6-6850-6245; Tel: +81-
6-6850-6245
† Electronic supplementary information (ESI) available: General proce-
dures of radical addition and radical polymerization reactions, charac-
terization of the coupling products, terminal group analysis of PMMA,
molecular structure of 6, and UV-vis spectrum of the reaction sequence
for the formation of 6. CCDC reference number 809907. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt11129a
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Table 1 Radical addition of CCl4 to 1-hexene catalyzed by dinuclear
Table 2 Radical addition of CCl4 to various alkenesa
group 6 metal complexesa
Run
Substrate
Product
Yield (%)
1b
2
3
77
59
55
Run
Cat.
Solvent
Yield (%)b
4
61
1
2
3
4
1
CDCl3
CDCl3
CDCl3
19
22
4
84
29
2
92
23
46
35
2
3
1
THF-d8
5c
50d
5
2
THF-d8
6
3
THF-d8
7c
8c
9
1
CDCl3 : THF = 9 : 1
CDCl3 : 1,4-dioxane = 9 : 1
THF-d8
1
a [1] : [alkene] : [CCl4] = 0.03 : 1.0 : 1.3 (in mmol) in CDCl3 (0.50 mL) for
24 h at 80 ◦C under Ar. b CDCl3–THF (v/v = 9 : 1) was used as the solvent.
c [alkene] : [CCl4] = 1.2 : 1.0 (in mmol). d Styrene oligomer is formed.
4d,e
5e,f
10
THF-d8
a [cat.] : [alkene] : [CCl4] = 0.03 : 1.0 : 1.3 (in mmol) in solvent (total volume,
1 mL) for 3 h at 80 ◦C under Ar. b Determined by 1H NMR. c Reaction for
2 h. d 4: Cr2L1 . e Reaction for 24 h. f 5: W2L1
4
4
Table 3 Radical polymerization of MMA catalyzed by Mo2 complexesa
adequate solvent effects. Notably, the addition of a small amount
of THF was sufficient to accelerate the reaction (run 7), but a
non-polar cyclic ether such as 1,4-dioxane was not effective (run 8).
Compared with 1, the corresponding 2,4,6-triisopropylbenzoate-
ligated chromium and tungsten complexes, Cr2L1 (4) and W2L1
Run
Cat.
Temp. (◦C)
Conv. (%)b
Mn (¥ 104)c
PDIc
4
4
1
2
3
4
5
6
1
2
3
1
2
3
100
100
100
50
50
50
76
89
82
7
43
40
1.9
0.5
2.1
6.5
1.2
5.4
1.7
1.7
2.2
3.2
1.4
2.0
(5), were less effective catalysts for the radical addition reaction in
THF-d8, giving the coupling product in 46% and 35% yields after
24 h (runs 9 and 10).
Under optimal conditions using 1, we checked the applica-
bility of some donor molecules to 1-hexene. The addition of
ethyl trichloroacetate and propyl trichlorothioacetate to 1-hexene
produced the corresponding coupling products (Scheme 1, a).
Coupling of a dichloromethyl radical with 1-hexene proceeded
effectively upon treatment with bromodichloromethane as the
radical donor compared with chloroform (Scheme 1, b) due to
the facile activation of CHBrCl2 compared to chloroform.
a [cat.] : [BrC(CH3)2CO2CH2CH3] : [MMA] = 0.005 : 0.10 : 1.0 (in mmol) in
toluene-d8 (0.50 mL) for 12 h. b Determined by 1H NMR. c Determined by
GPC.
methyl methacrylate (MMA) in the presence of excess ethyl 2-
bromoisobutyrate, which is a typical initiator for living radical
polymerization (Table 3).10 All the Mo2 complexes 1–3 afforded
PMMA at 100 ◦C. The catalyst 3 afforded a polymer with a
rather broad PDI value (run 3) compared with those observed
for complexes 1 and 2 (runs 1 and 2). At 50 ◦C, among the
Mo2 complexes, the amidinato-supported Mo2 complex 2 gave
the best results in terms of the catalytic activity and polydispersity
(run 5).
Scheme 1 Radical addition of trihaloalkanes to 1-hexene.
The oxidation reaction of the Mo2 core by alkyl halides was
investigated under controlled conditions to study the radical
reaction mechanism. Treatment of 3 with benzyl bromide in THF
under refluxing conditions gave complex 6 as red crystals suitable
for X-ray diffraction study (eqn (1)).
With catalyst 1 for the radical addition reaction in hand, some
other alkenes were used, and mono-addition products of Cl–CCl3
across carbon–carbon double bonds were isolated in moderate
yield (Table 2). In contrast to the radical addition to 1-hexene, the
reactions in the absence of THF showed better results except for
the reaction to vinylcyclohexane. Catalyst 1 was tolerant to the
ethereal substrate, resulting in the moderate yields of 1 : 1 adducts
in the reactions with 3-ethoxy-1-propene (run 3). When styrene
was used as a substrate, 1 : 1 adduct and styrene oligomers were
formed (run 5).
(1)
Styrene oligomers were obtained in the radical addition reaction
of CCl4 to styrene by 1; therefore, we examined the catalytic
activity of complexes 1–3 for the radical polymerization of
The structure of 6 reveals the formation of the ionic compound,
where a bromide anion is found as a well-separated anion11
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(Figure S3†), in contrast to the Mo2 complexes with carboxylate-
or cyclic guanidinate-based ligands,6,12 where the halide anion or
the halogen atom of BF4 or PF6 coordinates to the axial site of
the Mo2 core. The cationic part of 6 is essentially the same as the
previously reported [3+][BF4],13 indicating that benzyl bromide acts
as an oxidant of 3. UV-vis spectral monitoring of the oxidation
reaction revealed that compound 3 was cleanly converted to com-
pound 6 with an isosbestic point.14 The activation of the carbon–
halogen bond by 3 proceeds through a single electron-transfer
mechanism.15 Recent studies on the mechanism of carbon–halogen
bond reductive cleavage of alkyl halides revealed that electron
transfer to alkyl halides did not produce anion-radical species
via an outer-sphere electron transfer mechanism.15d In addition,
the 2,4,6-triisopropylbenzoate-ligated cluster 1 possesses enough
space for coordination of the halogen atom to the axial site.6,12 We
thus presume that the carbon–halogen bond activation proceeds
through weak interaction of alkyl halides at the axial position
of the Mo2 core in solution. In the case of bulky amidinate
or guanidinate ligands, coordination of halide anions to the
axial site is unfavorable due to the steric repulsion. Accordingly,
the bromide anion was found as a counter anion for the
complex 6.
Based on the oxidation reaction of 3 by alkyl halides via a single
electron transfer, and the commonly accepted mechanism for the
low-valent metal catalyzed radical addition reaction,3 we propose
the catalytic cycle for the radical addition reaction as shown in
Scheme 2. At the first stage, the Mo2 complex 3 interacts with
alkyl halides to transfer a single electron from the Mo2(II,II) core
to yield the carbon radicals, ∑CR3, the halide anion, and Mo2(II,III)
species. The halide anion might weakly interact with the Mo2 core
or be positioned as a separated anion, and both species are in
equilibrium in solution. In the case of 6, formation of the ionic
species is more favorable due to the steric repulsion between the
bromide anion and the guanidinate ligand. Subsequent addition
of the carbon radical to alkenes generates Mo2(II,III) and new
radical species. Finally, one electron reduction of Mo2(II,III) by the
radical species results in the formation of a coupling product and
regeneration of the catalytically active species, Mo2(II,II). The low
catalytic activity of 3 for the radical addition reaction may be due
to the difficulty of the one-electron reduction of Mo2(II,III) species
by the radical, leading to deactivation by a recombination of the
carbon radicals.
a metal–metal multiple bond, where the choice of supporting
ligands plays an important role in the catalytic behavior. This is,
to the best of our knowledge, the first example of dinuclear group
6 metal complexes having metal–metal multiple bonds for radical
addition reactions via a single electron transfer process. Further
extension of the metal cluster chemistry having metal–metal
multiple bonds in the area of various catalytic transformations
is ongoing in our laboratory.
This work was supported by the Core Research for Evolutional
Science and Technology (CREST) program of the Japan Science
and Technology Agency (JST), Japan.
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Scheme 2 Plausible reaction mechanism.
10 Incorporation of alkyl halides to the polymer was confirmed by the
1H NMR spectra. The 1H NMR spectra of the resulting polymers are
shown in Supporting Information (Fig. S1 and S2†).
In summary, we developed radical addition and polymerization
reactions catalyzed by dinuclear group 6 metal complexes bearing
11 X-Ray diffraction data for 6 (CCDC 809907, see Figure S2†).
C88H88BrMo2N12O13, orthorhombic, Pna21, T = 113(2)K, No. of
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reflections measured total/unique = 24978/9098, Rint = 0.0896, a =
14 The oxidation reaction of 3 with CCl4 was monitored by UV-vis
measurement. The UV-vis spectra are shown in Fig. S4.†.
3
˚
˚
˚
˚
26.340(8) A, b = 15.955(4) A, c = 18.695(5) A, V = 7857(4) A , Z =
4, R1 [I > 2s(I)] = 0.0809, wR2 [I > 2s(I)] = 0.1661, R1 [all data] =
0.1336, wR2 [all data] = 0.2089.
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The Royal Society of Chemistry 2011
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©