Scheme 1 Synthesis of coordination network 3.
We found that, under thermal and radical conditions, the
polymerization of guest 1 was completely suppressed within
network 3. We first exposed network 3 to standard polymerization
conditions for styrene monomers; crystals of 3 were heated to
80 1C in cyclohexane in the presence of AIBN (3 mol% to
monomer 1) for 1 d under an Ar atmosphere. Then crystals
were filtered, and intercalated triphenylene guests were extracted
with CH2Cl2 after decomposing crystals with THF/water. As
we expected from the crystal structure, 2-vinyltriphenylene (1)
was quantitatively recovered.12 We also conducted a similar
reaction with a large excess amount of AIBN (ca. 12 equiv. to 1),
Scheme 2 AIBN-initiated reactions of vinyl monomer 1 in network
crystal 3 and in solution.
The formation of oxygenated products 4 and 5 are attributable
to radical reactions with molecular oxygen. It is known that
thermolysis of AIBN in the presence of oxygen gives rise to
2-cyano-2-propylperoxyl and 2-cyano-2-propyloxyl radicals,
which can react with olefins to yield epoxide and carbonyl
compounds.15,16 Nevertheless, these oxidation pathways are
considerably slower than polymerization under relatively high
concentration of vinyl monomer 1 and cannot be major reaction
pathways. It is therefore noteworthy that coordination network
3 selectively suppressed the polymerization by spatial separation,
while making dominant the hidden radical oxidation pathways.
In conclusion, we have synthesized a porous coordination
network 3 in which radical polymerization of styrene derivative 1
is completely inhibited since the guest molecules are spatially
well-separated with regular intervals. Yet, guest 1 is still
capable of reacting with radical species in the pores, thus
predominantly forming oxygenated products 4 and 5. We
believe that our coordination network is a new crystalline-state
reaction container that makes it possible to analyze complicated
reaction pathways, to reveal unknown mechanisms, and, even
better, to find synthetically useful reactions.
1
yet H NMR and HPLC analyses revealed that no polymeric
products formed within the pores of 3 (Fig. 2a). Besides 1,
AIBN and its pyrolytic product, 2,2,3,3-tetramethylsuccinonitrile,
were found in the reaction mixture extracted from crystals,
which indicated the penetration of the radical initiator into the
pores.13 It is also notable that guest 1 in network 3 underwent
neither polymerization nor cyclodimerisation even under UV
irradiation by a Hg lamp.
Based on the above results, we next examined the AIBN-
initiated reaction of network-embedded 1 with molecular oxygen
(Scheme 2). When the crystals of 3 were treated with AIBN
solution under aerobic conditions for 1 d, 2-formyltriphenylene (4)
and epoxide 5 were obtained in 7 and 20% yield, respectively
(Fig. 2b and Fig. S1 in ESIw). This reaction was not accompanied
by polymerization and thus unreacted 1 was fully recovered.
The oxygenated products 4 and 5 can be detected even in the
reaction mixture after polymerization in solution, but their
yields are quite low (o1%) (Fig. 2c).14
This research was supported in part by KAKENHI
(20044006), JSPS, and Global COE program (Chemistry
Innovation through Cooperation of Science and Engineering),
MEXT, Japan.
Notes and references
z Crystal data for 3: C56H38I6N12Zn3, M = 1836.56, orthorhombic,
a = 27.574(3) A, b = 13.8942(17) A, c = 44.874(6) A, a = b = g =
901, V = 17192(4) A3, T = 90(2) K, space group Pbca, Z = 8, 171 751
reflections measured, 17 586 independent reflections (Rint = 0.0509).
The final R1 value is 0.0804 (I 4 2s(I)). The final wR(F2) value
is 0.2084 (all data), CCDC 838868. The SQUEEZE program in
PLATON was used for analysis to remove the disordered solvent
densities in the pores.
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Fig. 2 HPLC chromatograms of the products from the reactions
of styrene derivative 1 and AIBN: (a) in crystals 3 under Ar, (b) in
crystals 3 under aerobic conditions, and (c) in solution under aerobic
conditions. (*denotes an impurity signal which derives from epoxide 5
formed under the measurement conditions.)
c
12114 Chem. Commun., 2011, 47, 12113–12115
This journal is The Royal Society of Chemistry 2011