Eliminating the need for independent counterions in the construction of metal-organic rotaxane frameworks (MORFs)

Article abstract of DOI:10.1039/b911889f

The combination of a disulfonated-dibenzo-24crown-8 ether wheel and a dicationic 1,2-bis(pyridinium)ethane axle yields a neutral [2]pseudorotaxane which in combination with neutral metallic units such as a Cu(ii)benzoate paddlewheel or Cu(i)Br unit yields

Full text of DOI:10.1039/b911889f

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Eliminating the need for independent counterions in the construction
of metal–organic rotaxane frameworks (MORFs)w
Lisa K. Knight, V. Nicholas Vukotic, Elizabeth Viljoen, Christopher B. Caputo
and Stephen J. Loeb*
Received (in Austin, TX, USA) 17th June 2009, Accepted 7th August 2009
First published as an Advance Article on the web 27th August 2009
DOI: 10.1039/b911889f
The combination of a disulfonated-dibenzo-24crown-8 ether
wheel and a dicationic 1,2-bis(pyridinium)ethane axle yields a
neutral [2]pseudorotaxane which in combination with neutral
metallic units such as a Cu(II)benzoate paddlewheel or Cu(I)Br
unit yields one-dimensional coordination polymers (linear and
bent) which are the first examples of charge neutral, metal–organic
rotaxane frameworks.
In particular, this new MORF material must have an
overall neutral framework to eliminate the need for mobile,
space-filling counterions that would interfere with any
rotational or flipping motion of the wheel component.
Herein, we present an approach to eliminating counterions
in MORFs which combines neutral linkers with neutral
metal nodes. We have recently demonstrated that neutral
[
2]pseudorotaxanes can be prepared from a combination
1
2
Since its inception, the field of metal–organic frameworks
of sulfonated macrocycles and dipyridinium axles. The
combination of a dipositive axle and a dinegative wheel not
only gives rise to an overall neutral linker but drastically
increases the interaction between axle and wheel due to
electrostatic interactions between the sulfonate and pyridinium
groups. When combined with a neutral metal complex capable
of propagating a polymeric framework, this design should
yield an overall neutral MORF.
(
MOFs) has developed into one of the most active areas
1
of research in materials chemistry. These compounds are
inorganic–organic hybrids consisting of metal ions or clusters
connected through organic bridging ligands into extended
2
one-, two-, or three-dimensional networks. The past decade
has seen the emergence of new MOFs which have shown
4
3
promise for applications such as gas storage and separation,
5
catalysis, chemical sensing and magnetism.
6
7
Two neutral [2]pseudorotaxane ligands, [1CDSDB24C8]
and [2CDSDB24C8], were created by employing the
anti isomer of disulfonated-dibenzo-24-crown-8 ether,
We are interested in developing and exploiting a specialized
type of MOF that incorporates a [2]rotaxane as one of the
2
ꢀ
organic linkers; i.e. metal–organic rotaxane frameworks
8
MORFs). MORFs provide a simple and modular way to
[DSDB24C8] , as the wheel component and either the well
0
2+
studied 1,2-bis(4,4 -bipyridinium)ethane dication, 1 , or a
(
2
new extended version, 2 , derived from 1,4-bis(4-pyridyl)-
3
benzene as the axle units; see Fig. 1.
+
create a crystalline lattice that contains dynamic molecular
components. This may provide a unique method for altering
bulk materials properties by manipulating motion at the
molecular level. For example, solid state molecular rotors
can be used as a polarization rotation unit for the construction
1
X-Ray crystals of {[1CDSDB24C8](MeOH)
(DMF)} were
grown from a mixture of [Me N] [DSDB24C8] and 1[BF ] in
4 2 4 2
7
MeOH–DMF.z Fig. 2 shows the structure of this formally
9
of ferroelectric materials. Nakamura recently demonstrated
zwitterionic ligand which is overall neutral since the two
that the 1801 flip-flop of a m-fluoroanilinium ring could be
used as a polarization rotation unit in a crystal lattice and that
1
0
this was controllable by an external electrical field.
In proof-of-principle experiments, we have established that
, 2 and 3D MORFs can be prepared and have verified their
1
1
1
solid state structures by single crystal X-ray diffraction.
Although these materials establish the concept of MORFs,
they do not have the requisite characteristics necessary for the
study of molecular dynamics as proposed. For these endeavours,
the MORF material must be a crystalline solid that is physically
and thermally robust and has sufficient porosity to allow the
dynamic components to ‘‘function’’ in a well-defined ‘‘space’’.
Department of Chemistry and Biochemistry, University of Windsor,
Windsor, Ontario, Canada N9B 3P4. E-mail: loeb@uwindsor.ca;
Fax: +1 519 973 7098; Tel: +1 519 253 3000 (3529)
w Electronic supplementary information (ESI) available: Details of
synthesis and crystallization. CCDC 736643 {[1CDSDB24C8]-
Fig.
1
The
[2]pseudorotaxanes,
[1CDSDB24C8]
and
2+
2+
[2CDSDB24C8], formed between the dications 1 and 2 and the
dianion [DSDB24C8] . Each can act as a neutral linker for bridging
(MeOH)
and CCDC 736645 ({[CuBr(2)](H
7
(DMF)}, CCDC 736644 {[Cu
2 4 2 x
(OBn) (1)](MeOH) (DMF)}
O)12 ). For ESI and crystallographic
2ꢀ
2
}
x
data in CIF or other electronic format see DOI: 10.1039/b911889f
metal fragments into metal–organic rotaxane frameworks.
This journal is ꢁc The Royal Society of Chemistry 2009
Chem. Commun., 2009, 5585–5587 | 5585

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Fig.
[Cu
4
Two parallel strands of the linear 1D MORF
{
2 4 x
(OBn) (1CDSDB24C8)]} showing the polymeric metal–ligand
Fig. 2 A ball-and-stick representation of the X-ray structure of the
neutral, [2]pseudorotaxane ligand, [1CDSDB24C8]. (Colour key:
S = yellow, O = red, N = blue, C = dark gray; axle = gold bonds,
wheel = silver bonds.)
framework (blue) in wire format and the individual DSDB24C8 axle
components (red) in space-filling mode. (Colour key: Cu = dark blue,
S = yellow.)
Adjacent strands of the 1D MORF are staggered with
respect to each other. Although this presumably minimizes
steric interactions between the relatively bulky paddlewheel
units, the likely driving force behind this orientation is
electrostatic. Due to the nature of the threading between axle
and wheel, the closest approach of a positively charged
pyridinium N-atom and a negatively charged sulfonate
positive charges from the pyridinium groups are offset by the
negative charges of the two sulfonate groups.
The arrangement of axle and wheel is almost identical
+
2
to that observed for the [2]pseudorotaxane [1CDB24C8]
1
4
involving the same axle and DB24C8. The electron-rich
aromatic units of the crown ether are p-stacked with the
electron-poor pyridinium groups of the axle which results in
a charge transfer absorption and yellow colour for this species.
In order to produce a neutral, metal–organic rotaxane
˚
O-atom within an individual rotaxane is 7.29 A. However,
when these 1D polymers are positioned next to each other in
˚
parallel rows, there is a much closer approach of 4.52 A
framework (MORF), the Cu paddlewheel [Cu
2
(BnO
4
)]
between pyridinium N-atoms and sulfonate O-atoms from
different rotaxanes. This distance is very similar to that
observed for intermolecular interactions between ligands in
unit was chosen as a neutral metal node capable of linear
1
propagation. Combining one equivalent of 1[BF ] with 2
5
4
2
˚
equivalents of [Me N] [DSDB24C8] and one equivalent of
4
the solid state structure of [1CDSDB24C8]; 4.76 A.
It can also be seen from Fig. 3 that the sulfonate groups of
2
[
Cu (BnO )(H O) ] in MeOH–DMF and allowing the solvent
2
4
2
2
to evaporate slowly yielded turquoise coloured crystals with
formula {[Cu (OBn) (1CDSDB24C8)](MeOH) (DMF)}
2 4 x
{[Cu (OBn) (1CDSDB24C8)]} are positioned fairly close to
2
4
2
x
.
one of the aromatic rings of the paddlewheel unit. The closest
˚
Fig. 3 shows the repeating unit of this new 1D MORF
in which the terminal pyridine groups of the neutral
pseudorotaxane ligand bond to the two open coordination
sites of the Cu(II) paddlewheel. This results in a linear
coordination polymer (MORF) which is completely neutral.
The orientation of the pyridine axle with respect to the crown
ether wheel is essentially identical to that of the free ligand
Oꢂ ꢂ ꢂC approach of 3.07 A could be construed as a weak
hydrogen bonding interaction and may also influence the
overall arrangement of the interlocked components. In order
2
+
to eliminate this interaction, the longer axle 2 was prepared
and the analogous [2]pseudorotaxane [2CDSDB24C8] used to
prepare MORFs.
Yellow single crystals of formula {[CuBr(2CDSDB24C8)]-
[
1CDSDB24C8].
The linear strands of {[Cu (OBn) (1CDSDB24C8)]}
x
(H
of 2[BF
equivalent of CuBr in MeOH–H
2
O)10
}
x
were obtained from the reaction of one equivalent
with 2 equivalents of [Me N] [DSDB24C8] and one
O after concentration of
]
2
4
2
2
4
4
are arranged in a parallel fashion as shown in Fig. 4. The
paddlewheel units are offset to optimize packing and eliminate
voids. By design there are no counterions and due to efficient
packing, only two molecules of MeOH and a single molecule
of DMF per repeating unit filling small voids in the lattice.
2
the solution. Fig. 5 shows that this material is a 1D
MORF comprising a trigonal planar Cu(I) centre bridging
[2CDSDB24C8] units and a single bromide ligand completing
the coordination sphere.
The bond angle at the Cu(I) centre between two rotaxane
ligands is 133.41 producing a wave-like coordination polymer
Fig. 3 A ball-and-stick representation of the repeating unit in the
X-ray structure of the 1D MORF {[Cu
2
(OBn)
4
(1CDSDB24C8)]}
x
Fig. 5 A ball-and-stick representation of the X-ray structure of the
showing the [2]pseudorotaxane, [1CDSDB24C8], bridging two Cu(II)
paddlewheel units. (Colour key: Cu = dark blue, S = yellow,
O = red, N = blue, C = dark gray; coordination polymer axle =
gold bonds, wheel = silver bonds.)
x
1D MORF {[CuBr(2CDSDB24C8)]} showing the [2]pseudorotax-
ane, [2CDSDB24C8], bridging two CuBr units. (Colour key: Cu =
dark blue, Br = green, S = yellow, O = red, N = blue, C = dark
gray; coordination polymer axle = gold bonds, wheel = silver bonds.)
5
586 | Chem. Commun., 2009, 5585–5587
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1
2
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2
´
2
3
For recent examples see: (a) M. Gallo and D. Glossman-Mitnik,
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x
Fig. 6 Two strands of the bent 1D MORF {[CuBr](2)} showing the
polymeric metal–ligand framework (blue) in wire format and the
individual DSDB24C8 axle components (red) in space-filling mode.
(Colour key: Cu = dark blue, S = yellow, Br = green.)
(
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(
MORF) which is charge neutral. Fig. 6 shows how these 1D
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[Cu (OBn) (1CDSDB24C8)]} , there are no direct inter-
1
1623.
a
4
For recent examples see: (a) B. Chen, C. Liang, J. Yang,
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{
2
4
x
actions between the strands as all the closest contacts to the
sulfonate groups and the bromide ligand are with water
molecules which form a polar hydration layer between the
3
8, 1477.
5
6
For a recent review see: J. Y. Lee, O. K. Farha, J. Roberts,
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and E. B. Lobkovsky, J. Am. Chem. Soc., 2008, 130, 6718;
1
6
strands.
The achievement of a neutral framework MORF free from
independent counterions is a major step towards reaching our
goal of creating robust materials with dynamic interlocked
components. Although the macrocyclic rings in these 1D
systems are static (up to the decomposition temperature of
(d) B. Chen, L. Wang, Y. Xiao, F. R. Fronczek, M. Xue, Y. Cui
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2
00 1C) these new MORF materials provide a potential
strategy for the creation of truly porous MORFs which will
need to be robust three-dimensional solids that maintain
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are exploring the possibility that replacing the simple benzoate
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7 For a recent example see: Z.-X. Wang, X.-L. Li, B.-L. Liu,
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8
(a) S. J. Loeb, Chem. Commun., 2005, 1511; (b) S. J. Loeb, Chem.
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4
x
2
dicarboxylate linker might lead to the creation of a robust
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3
D lattice; a 2D carboxylate grid pillared by a neutral
rotaxane linker. There is ample precedence for this type of
0
10 T. Akutagawa, H. Koshinaka, D. Sato, S. Takeda, S.-I. Noro,
H. Takahshi, R. Kumai, Y. Tokura and T. Nakamura, Nat.
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MOF with simple linkers such as DABCO, 4,4 -bipyridine as
1
7
well as more complex linkers. Results on these systems will
be reported in due course.
1
1 (a) G. J. E. Davidson and S. J. Loeb, Angew. Chem., Int. Ed., 2003,
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Notes and references
z X-Ray data were collected on a Bru
¨
ker APEX CCD diffractometer
Crystal
12 D. J. Hoffart, J. Tiburcio, A. de la Torre, L. K. Knight and
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18,19
following
standard
procedures.
(DMF)}: C56
data for
, M = 1244.41,
{
T = 173(2) K, triclinic, space group P1, a = 10.342(2), b = 12.310(2),
[1CDSDB24C8](MeOH)
7
H
85
N
5
O S
22 2
ꢀ
˚
c = 13.586(2) A, a = 73.597(2)1, b = 69.539(2)1, g = 76.445(2)1,
3
ꢀ3
ꢀ1
˚
V = 1536.7(5) A , rcalc = 1.345 g cm , m = 0.168 mm , Z = 1,
reflections collected = 14 583 (Rint = 0.0398), final R indices [I 4 2sI]:
R
1
2 1 2
= 0.0808 , wR = 0.2101, R indices (all data): R = 0.1078. wR =
0.2327, GoF = 1.034 with data/variables/restraints = 5386/412/5.
Crystal data for {[Cu
2
(OBn)
4
(1)](MeOH)
2
(DMF)}
x
:
C
, M = 1695.72, T = 173(2) K, triclinic, space group P1,
79
H
85Cu2-
ꢀ
N
5
O
S
25 2
˚
a = 10.756(3), b = 11.397(3), c = 17.365(5) A, a = 103.580(3)1, b =
¨
15 C. B. Aakeroy, N. Schultheiss and J. Desper, Dalton Trans., 2006,
1627.
3
ꢀ3
,
˚
9
8.407(3)1, g = 104.257(3)1, V = 1957.8(9) A , rcalc = 1.438 g cm
ꢀ1
m = 0.679 mm , Z = 1, reflections collected = 18 532 (Rint
.0663), final R indices [I 4 2sI]: R = 0.0593, wR = 0.1324, R
indices (all data): R = 0.0982, wR = 0.1497, GoF = 0.991
with data/variables/restraints 6873/533/0. Crystal data for
[CuBr(2)](H O)10 78BrCuN 1422.81,
T = 173(2) K, monoclinic, space group C
=
16 For an example of a porous coordination polymer with an internal
surface functionalized by anionic sulfonate groups see: S. Horike,
S. Bureekaew and S. Kitagawa, Chem. Commun., 2008, 471.
17 (a) A. Pichon, C. Mendicute Fierro, M. Nieuwenhuyzen and
S. L. James, CrystEngComm, 2007, 9, 449; (b) O. K. Farha,
K. L. Mulfort, A. M. Thorsness and J. T. Hupp, J. Am. Chem.
Soc., 2008, 130, 8598.
0
1
2
1
2
=
{
2
x
} :
C
58
H
4
24
O S
2
,
M
=
2
/c, a = 27.257(5),
3
˚
˚
b = 14.035(2), c = 21.840(4) A, b = 125.981(4), V = 6761(2) A ,
r
ꢀ3
ꢀ1
calc = 1.398 g cm , m = 1.054 mm , Z = 4, reflections collected =
¨
18 G. M. Sheldrick, SHELXTL 5.03 Program Library, Bruker
2
1 266 (Rint = 0.0741), final R indices [I 4 2sI]: R
1
= 0.1001,
= 0.3176,
Analytical Instrument Division, Madison, Wisconsin, USA, 2003.
19 All X-ray figures were prepared with DIAMOND 3.2-CRYSTAL
IMPACT, Postfach 1251, D-53002, Bonn, Germany 2007.
wR
GoF = 1.163 with data/variables/restraints = 3526/407/22.
2
= 0.2816, R indices (all data): R
1
= 0.1453. wR
2
This journal is ꢁc The Royal Society of Chemistry 2009
Chem. Commun., 2009, 5585–5587 | 5587