Hydrogen-bond-supported 3D networks: Two different polymeric structures featuring chlorine atoms as ligands and as anions and investigations as epoxide catalysts

Article abstract of DOI:10.1246/cl.2008.1144

Two novel hydrogen-bonded network polymers, [MoO₂Cl ₂-(H₂O)₂]₂[4,4′-H ₂bipy]²⁺·2Cl^- (1) and [MoO ₂Cl₄]^2-[4,4′-H₂-bipy]

Full text of DOI:10.1246/cl.2008.1144

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
1144
Chemistry Letters Vol.37, No.11 (2008)
Hydrogen-bond-supported 3D Networks: Two Different Polymeric Structures
Featuring Chlorine Atoms as Ligands and as Anions and Investigations as Epoxide Catalysts
Yi Luan,1 Ge Wang,ꢀ1 Rudy L. Luck,2 Yingnan Wang,1 Han Xiao,1 and Hangjun Ding1
1School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China
2Department of Chemistry, Michigan Technological University, Houghton, MI 49931, USA
(Received July 2, 2008; CL-080652; E-mail: gewang@mater.ustb.edu.cn)
Two novel hydrogen-bonded network polymers, [MoO2Cl2-
chlorine atoms. The Mo–O(oxo) and Mo–Cl distances,
1.668(3) and 1.680(3), and the cis-MoO(oxo)2 and trans-MoCl2
angles 103.60(18) and 156.44(4), are comparable to those report-
ed for other related MoO2Cl2L2 complexes. The 3D hydrogen-
bonded framework of compound 1 consists of interactions
between [MoO2Cl2(H2O)2], [4,40-H2bipy]2þ, and Clꢁ (Figure 1
left). Each Clꢁ counter ion is stabilized by strong hydrogen
bonds with one [4,40-H2bipy]2þ and three other adjacent
MoO2Cl2(H2O)2 centers resulting overall in a 2 D network
undulating sheet pattern. Adjacent layers of [4,40-H2bipy]2þ
cations are arranged in a herringbone pattern when viewed
perpendicular to the plane. The ClꢂꢂꢂN and ClꢂꢂꢂO distances
ꢁ
(H2O)2]2[4,40-H2bipy]2þ 2Cl (1) and [MoO2Cl4]2ꢁ[4,40-H2-
.
bipy]2þ (2) [bipy = bipyridine] contained different three-di-
mensional NHꢂꢂꢂCl hydrogen-bonded arrangements resulting in
different polymeric structures; complex 1 or 2 as catalytic pre-
cursor, H2O2 as a source of oxygen and NaHCO3 as cocatalyst
showed great efficiency for the epoxidation of olefinic com-
pounds under ambient conditions.
Organometallic crystal engineering of two- and three-
dimensional organic–inorganic hybrid polymers has recently
attracted significant attention, owing to the synthesis of a large
number of novel supramolecular architectures with a wide vari-
ety of physical and chemical properties.1 Recently, new synthet-
ic strategies2 utilizing different kinds of hydrogen bonds, such as
NHꢂꢂꢂCl interactions, have been reported.3 Two main approaches
for network structures have been employed: (i) anion-directed
assembly connected network via NHꢂꢂꢂClꢁ hydrogen bonds,4
and (ii) metal-assisted, self-assembly processes to form a NHꢂꢂꢂ
ClMClnꢁ1 hydrogen-bond network.5 Slight modifications in the
syntheses, such as changing the counter ions varying the struc-
tural characteristics of the polydentate organic ligand and in
the metal–ligand ratios, result in supramolecular conformational
isomerization. Based upon the strategy outlined as (i) above, the
complex 1 has been synthesized. In contrast strategy (ii) allows
for the formation of another dioxo–MoVI polymeric complex 2
using different metal–ligand ratios.15
Dioxo–MoVI complexes have been utilized as catalysts for
a variety of oxidation reactions such as epoxidation and the
oxidation of alcohols and sulfides.6 Our interest in dioxo–MoVI
hydrogen-bond networks stems from previous studies on di-
oxomolybdenum(VI) complexes, that were demonstrated to be
efficient epoxidation catalysts with tert-butyl hydroperoxide
(TBHP) as the oxidant.7 From both the economic and environ-
mental viewpoints, employing H2O2 as the stoichiometric oxi-
dant is preferable to use of organic oxidants such as TBHP
and PhIO for catalytic oxidations,8 because the by-product
H2O is environmentally friendly and H2O2 is cheap and readily
available.9 The two dioxo–MoVI network polymers were
obtained by crystal engineering design experiments, and their
potential as catalytic precursors for epoxidation using H2O2 as
the oxidant have been investigated.
˚
˚
(N(1)–H(1)ꢂꢂꢂCl(3) 3.123(3) A, O(3)–H(3A)ꢂꢂꢂCl(3) 3.080(3) A,
˚
˚
O(4)–H(4B)ꢂꢂꢂCl(3) 3.080(3) A, O(3)–H(3B)ꢂꢂꢂCl(3) 3.068(3) A)
˚
are within the accepted range (2.91–3.53 A), and the hydrogen
bonds do not deviate much from linearity (162.2, 166(6),
175(7), and 167(6)ꢃ, respectively).
Interestingly enough in the structure of [MoO2Cl4]2ꢁ[4,40-
H2bipy]2þ (2), which consists of the biprotonated bipyridinium
molecule and the molybdenum complex [MoO2Cl4]2ꢁ, the Mo
atom has a distorted octahedral geometry with cis-oxo groups
and four chlorine atoms. The 4,40-H2bipy molecules form linear
chains in parallel layers connected to one another by N–HꢂꢂꢂCl
bonded interactions to two opposite [MoO2Cl4]2ꢁ anions
resulting in the 3D framework (Figure 1 right). Therefore,
[MoO2Cl4]2ꢁ[4,40-H2bipy]2þ (2) contains only N–HꢂꢂꢂCl inter-
actions as its structure-determining factor instead of the two
kinds of interaction mentioned for 1. The ClꢂꢂꢂN distances in 2
are longer than those in compound 1, N(1)–H(1)ꢂꢂꢂCl(1)
3.235(2), N(1)–H(1)ꢂꢂꢂCl(1) 3.227(2), and the angle of hydrogen
bonds is different and deviates from linearity (141.1 and 128.2ꢃ,
respectively), suggesting perhaps a more electrostatic nature to
the bonding. It is noteworthy that in complex 2, O atoms, bonded
to Mo atoms located intra to the planes generated by the [4,40-
H2bipy]2þ cations, situated at distances of 2.787 A are closer
˚
than the sum of their van der Waals radii and perhaps suggestive
of H bonds. This would imply that an OH or OH2 formulation
Despite the involvement of the [4,40-H2bipy]2þ cation in
both hydrogen-bonding networks, 1 and 2 have completely dif-
ferent extended supramolecular structures. Complex 1 consists
of the uncharged molybdenum complex [MoO2Cl2(H2O)2]
accompanied by the diprotonated bipyridinium molecule and
two chloride anions. The Mo atom has a distorted octahedral
coordination with cis-oxo groups, cis-H2O ligands and trans-
Figure 1. Structure of the NHꢂꢂꢂCl hydrogen-bonded layer in
crystalline 1 and 2.
Copyright Ó 2008 The Chemical Society of Japan

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Chemistry Letters Vol.37, No.11 (2008)
1145
Table 1. Complexe (1)-catalyzed epoxidation of selected ole-
fins using H2O2 as oxidant, bicarbonate as cocatalysta
er. The high yields (as listed in Table 1) were only obtained
when catalyst and bicarbonate were applied in concert.
Dioxo–MoVI complexes convert to oxodiperoxo–MoVI in
the presence of hydrogen peroxide.12 Under our reaction condi-
tions, it was assumed that the 4,40-bipy ligand would coordinate
to the resulting MoO(O2)2 centers under alkaline conditions.
This reassembled LMoO(O2)2 complex could then act as cata-
lyst with NaHCO3 as the cocatalyst using H2O2 as the oxygen
source.13 Bhattacharyya et al. have previously demonstrated
that MnSO4 and [MoO(O2)2(saloxH)] are efficient catalysts for
epoxidation reactions in the presence of NaHCO3/H2O2.14
Yao and Richardson have shown that a key aspect of these
Yieldb/%
1
2
Entry
Substrate
Time/h
1
Product
Selectivity/%
O
1
2
3
99 99
96 96
92 93
99
99
99
O
1.5
1
reactions is that hydrogen peroxide andꢁbicarbonaꢁte combine
O
11
to produce peroxymonocarbonate, HCO4
.
HCO4 would be
O
more reactive than hydrogen peroxide and can be expected to
attack the metal center or other more potent activated groups
at a much faster rate.
4
4
86 87
90 89
4
5
99
99
In summary, we have synthesized the first structurally
distinct 3D hydrogen-bond networks 1 and 2 which consists of
the dioxo–MoVI and 4,40-bipyridinium cation. We demonstrate
that in the construction of a hydrogen-bonded network, com-
plexes based on two different types of NHꢂꢂꢂCl interactions can
be obtained under slightly different conditions. The epoxidation
of alkenes has also been examined in the presence of hydrogen
peroxide and NaHCO3 using complex 1 or 2. This reaction pro-
vides an environmentally constructive route for the conversion
of a variety of alkenes to their respective epoxides.
O
aReaction conditions: a given substrate (10 mmol), catalyst 1
(0.1 mmol, 1 mol %), NaHCO3 (2.5 mmol, 25 mol %) and 30%
aqueous H2O2 (40 mmol) were dissolved in acetonitrile
(10 mL) at 25 ꢃC. bDetermined by GC using an internal standard
technique.
˚
would be required. The Mo=O distances at 1.680(3) A argues
strongly against such assignments.
The process by which the two complexes formed was differ-
ent. The polymer 1, which was composed of the neutral
MoO2Cl2(H2O)2 molecule and the bipyridinium cation, em-
ployed chloride anions to attain neutrality and to direct the 3D
polymerization. In contrast in polymer 20 the protonated ring
nitrogen –NHþ and [MoO2Cl4]2ꢁ were used as hydrogen-
bond donors and acceptors, respectively. [MoO2Cl4]2ꢁ was
presumably obtained from MoO2Cl2 when more bipyridinium
cation is generated by the addition of more 4,40-bipy ligand to
the reaction.
The catalytic epoxidation properties of compounds 1 and
2 were then assessed. We found that with complex 1 or 2 as cat-
alytic precursor, NaHCO3 as a cocatalyst, and H2O2 as an oxy-
gen source, we were able to catalyze the epoxidation of olefinic
compounds. Cyclooctene was selected as a standard substrate for
the optimization process with H2O2 in acetonitrile. A good yield
(99%) of the epoxide was achieved using a catalytic amount of 1
(1 mol %) and NaHCO3 as cocatalyst (25 mol %) for 1 h at room
temperature (25 ꢃC) (Table 1, Entry 1). To evaluate the scope of
this procedure, the oxidation of other alkenes was also studied
(Table 1). The catalytic attributes of complex 2 were almost
identical to those for complex 1 under similar conditions.
Dioxo–MoVI complexes have been explored for and demon-
strated the potential to be epoxidation catalysts. Kuhn et al. have
reported that dioxo–MoVI derivatives are able to catalyze the
olefin epoxidation reaction with TBHP but not with H2O2.10
In this catalytic system, dioxo–MoVI complexes 1 and 2 were
successfully applied as epoxidation catalysts using H2O2 as
the oxidant, NaHCO3 as the cocatalyst. The epoxidation of al-
kenes in the presence of bicarbonate is known.11 Interestingly
in our system, the use of the dioxo MoVI complexes 1 and 2 or
bicarbonate alone gave much lower yields of epoxides (less than
10% or 30%, respectively) than when they were applied togeth-
We thank the ‘‘973’’ program (No. 2007CB613301) and
the ‘‘863’’ program (No. 2006AA03Z459) for support.
References and Notes
D. Braga, F. Grepioni, G. R. Desiraju, Chem. Rev. 1998, 98,
1375.
1
2
3
G. R. Desiraju, J. Chem. Soc., Dalton Trans. 2000, 3745.
a) C. J. Adams, A. Angeloni, A. G. Orpen, T. J. Podesta,
B. Shore, Cryst. Growth Des. 2006, 6, 411. b) T. Dorn, C.
Janiak, K. Abu-Shandi, CrystEngComm 2005, 7, 633.
J. Keegan, P. E. Kruger, M. Nieuwenhuyzen, J. O’Brien,
N. Martin, Chem. Commun. 2001, 2192.
4
5
6
A. Angeloni, A. G. Orpen, Chem. Commun. 2001, 343.
F. E. Kuhn, A. M. Santos, M. Abrantes, Chem. Rev. 2006,
106, 2455.
7
G. Wang, G. Chen, R. L. Luck, Z. Wang, Z. Mu, D. G.
Evans, X. Duan, Inorg. Chim. Acta 2004, 357, 3223.
J.-M. Bregeault, Dalton Trans. 2003, 3289.
8
9
R. Noyori, M. Aoki, K. Sato, Chem. Commun. 2003, 1977.
10 F. E. Kuhn, A. M. Santos, A. D. Lopes, I. S. Goncalves, E.
Herdtweck, C. C. Ramao, J. Mol. Catal. A: Chem. 2000,
164, 25.
11 H. Yao, D. E. Richardson, J. Am. Chem. Soc. 2000, 122,
3220.
12 A. Jimtaisong, R. L. Luck, Inorg. Chem. 2006, 45, 10391.
13 N. Gharah, S. Chakraborty, A. K. Mukherjee, R.
Bhattacharyya, Chem. Commun. 2004, 2630.
14 S. K. Maiti, S. Dinda, N. Gharah, R. Bhattacharyya, New. J.
Chem. 2006, 30, 479.
15 Supporting Information is also available electronically
on the CSJ-Journal Web site, http://www.csj.jp/journals/
chem-lett/index.html.
Published on the web (Advance View) October 18, 2008; doi:10.1246/cl.2008.1144