Persistent C-I...π halogen-bonded layer motifs involving 4-iodobenzoate paddlewheel units, Cu2(4-Ibz)4(L) 2

Article abstract of DOI:10.1039/c3ce26919a

The Cu₂(4-Ibz)₄ paddlewheel unit (4-Ibz = 4-iodobenzoate) has been synthesised with a series of monodentate axial ligands, L = acetone, DMSO, EtOH, DMF, giving Cu₂(4-Ibz)₄(L) ₂, or with linear ditopic ligands, L₂ = dabco, 4,4-bipyridine, giving Cu₂(4-Ibz)₄(L₂). The crystal structures of these compounds form persistent layer structures which are propagated in the four orthogonal directions of the iodobenzoate ligands around each paddlewheel unit via a combination of halogen bonds and offset π-stacking arrangements. Halogen bonds are either C-I...π or C-I...I interactions. The axial ligands are coordinated orthogonal to the layers and either fill spaces in the layers above and below (monodentate ligands, L) or link layers together (ditopic ligands, L₂). Only in the case of the longest ditopic ligand used, 4,4′-bipyridylethane, does the paddlewheel fail to form.

Full text of DOI:10.1039/c3ce26919a

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…
Persistent C–I p halogen-bonded layer motifs
involving 4-iodobenzoate paddlewheel units,
Cu2(4-Ibz)4(L)23
Cite this: DOI: 10.1039/c3ce26919a
´
Paul Smart, Angela Bejarano-Villafuerte, Rebecca M. Hendry and Lee Brammer*
The Cu2(4-Ibz)4 paddlewheel unit (4-Ibz = 4-iodobenzoate) has been synthesised with a series of
monodentate axial ligands, L = acetone, DMSO, EtOH, DMF, giving Cu2(4-Ibz)4(L)2, or with linear ditopic
ligands, L2 = dabco, 4,4-bipyridine, giving Cu2(4-Ibz)4(L2). The crystal structures of these compounds form
persistent layer structures which are propagated in the four orthogonal directions of the iodobenzoate
ligands around each paddlewheel unit via a combination of halogen bonds and offset p-stacking
Received 25th November 2012,
Accepted 11th January 2013
…
…
arrangements. Halogen bonds are either C–I p or C–I I interactions. The axial ligands are coordinated
orthogonal to the layers and either fill spaces in the layers above and below (monodentate ligands, L) or
link layers together (ditopic ligands, L2). Only in the case of the longest ditopic ligand used,
4,49-bipyridylethane, does the paddlewheel fail to form.
DOI: 10.1039/c3ce26919a
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ditopic ligands9 or assemblies of higher dimensionality via
multitopic ligands.10
Introduction
The identification of novel supramolecular synthons from
which new structures can be designed and reliably synthesised
is vital to advancing the field of crystal engineering.1 In order
to evaluate the propensity of a new synthon to direct the self-
assembly process it is important to assess how the proposed
synthon competes with different types of intermolecular
interaction in the solid state.2,3 The use of inorganic tectons
can introduce interaction geometries which are unavailable
with purely organic species.4 However, as the metal-containing
units often form in situ during the self-assembly process, this
introduces competition between formation of coordination
bonds and of the directional intermolecular interactions from
which the synthon is derived.5
The ‘‘paddlewheel’’ motif comprising two (divalent) metal
ions bridged by four carboxylate or other similar monoanionic
ligands has seen extensive use in the development of metal–
metal multiple bond chemistry6 and more recently is a
favoured secondary building unit (SBU)7 in the formation of
metal–organic framework materials (MOFs).8 Such paddle-
wheel units impose rigid geometrical constraints on the
carboxylate groups, maintaining a 90u angle between them,
and can be further coordinated by axial ligands, which enable
paddlewheel units to be linked into linear assemblies via
Previously we examined the use of paddlewheels containing
the halogenated carboxylate ligand 3-Ibz (3-Ibz = 3-iodobenzo-
ate). The Cu2(3-Ibz)4 units were combined with a set of neutral
axial ligands that were either capable of forming coordination
polymers through the linking of paddlewheel units, or of
accepting halogen bonds from the iodobenzoate ligands to
form extended halogen-bond networks.9a A subsequent study
introduced ligands with two different coordinating groups,
and ligands capable of hydrogen bonding.9b It was found that
for short linear linkers such as dioxane and diazabicyclcooc-
tane (dabco), and ligands with a sterically hindered coordinat-
ing group such as 2,6-dimethypyrazine, coordination polymers
did not form; instead networks were propagated via halogen
bonds between the axial ligands and the iodobenzoate ligands.
Axial ligands with hydrogen bonding capability such as
nicotinamide and methanol were found to propagate networks
formed through hydrogen bonds rather than halogen bonds.
3-Cyanopyridine and 4-cyanopyridine, ligands with one strong
and one weakly coordinating group, did not form coordination
polymers but instead paddlewheels were linked through
hydrogen bonds or via p–p interactions between axial ligands.
Longer linkers such as 4,49-bipyridine (bipy) and 1,2-bis(4-
pyridyl)ethane (bpa) were found to form coordination poly-
mers, although the halogen bonding of the 3-Ibz ligand was
strong enough to disrupt the formation of the paddlewheel
units leading to linked mononuclear Cu(3-Ibz)2 units. The
bent linker pyrimidine was the only ligand which formed a
coordination polymer comprised of linked Cu2(3-Ibz)4 paddle-
wheel units, as the 120u angle subtended by the nitrogen
Department of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK.
E-mail: lee.brammer@sheffield.ac.uk
Electronic supplementary information (ESI) available: Tables providing detail
3
of twin refinements of crystal structures of 2, 4?4DMF and 7?BnOH. CCDC
912204–912210. For ESI and crystallographic data in CIF and other electronic
format see DOI: 10.1039/c3ce26919a
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atoms resulted in a zig-zag chain which allowed the paddle-
wheel units to pack together efficiently.
Synthesis of [Cu2(4-Ibz)4(EtOH)2] (3) (EtOH = ethanol)
4-Iodobenzoic acid (99.2 mg, 0.4 mmol) and ammonium
thiocyanate (15.2 mg, 0.2 mmol) were dissolved in benzyl
alcohol (6 mL). Benzyl alcohol and EtOH were mixed in a 1 : 1
ratio and 1 mL of this mixture was layered on top of this
solution as a buffer layer. Copper(II) acetate monohydrate (39.9
mg, 0.2 mmol) was dissolved in EtOH (2 mL) and this was
carefully layered on top of the buffer layer. Small blue block
crystals of 3 started to form after 4 days along with a colourless
precipitate which was collected separately. At this point a
single crystal was removed and used for crystal structure
determination by X-ray diffraction. Yield 22 mg, 18.1%. Calc.:
C, 31.84; H, 2.34; N, 0.00; I, 42.05%. Found: C, 32.15; H, 2.02;
N, 0.00; I, 41.82%.
The halogen-bonded networks in the structures presented
in our previous work form a number of different topologies
and use a number of different halogen bond acceptor sites,
making structural predictions for related compounds difficult.
The geometry of the 3-iodobenzoate ligand in the paddlewheel
is such that rotation of the haloaryl ring relative to the
carboxylate group changes considerably the orientation of the
halogen relative to the coordination network direction. It is
this degree of freedom that allows the diverse halogen-bond
network topologies of the 3-iodobenzoate paddlewheel com-
pounds to form. Paddlewheels employing 4-iodobenzoate
ligands do not possess this degree of freedom as the torsional
flexibility of the ligand does not change the orientation of the
iodo group. Therefore, we reasoned that the type of halogen-
bonded networks formed may be more consistent. Only three
examples of coordination polymers comprising 4-iodobenzo-
ate paddlewheels have been previously reported.11 All three are
bridged by short linear linker molecules (pyrazine, 2-amino-
pyrazine and dabco). 2-Aminopyrazine and dabco were found
to produce crystal structures with solvent filled channels,
however they were found to collapse irreversibly upon
desolvation. In the present study we present a series of Cu(II)
coordination compounds incorporating 4-iodobenzoate
ligands, and including one of four monodentate coordinating
solvents, or three ditopic linker ligands of differing length.
Synthesis of [Cu2(4-Ibz)4(DMF)2]?4DMF (4?4DMF) (DMF =
dimethylformamide)
Copper(II) acetate monohydrate (78 mg, 0.39 mmol) and
4-iodobenzoic acid (248 mg, 1.0 mmol) were dissolved in
DMF (8 mL) and left to evaporate. Large blue blocks of 4?4DMF
started to form after 4 days. At this point a single crystal was
removed and used for crystal structure determination by X-ray
diffraction. Yield 246.1 mg, 81.1%. Calc.: C, 35.56; H, 3.76; N,
5.41; I, 32.67%. Found: C, 35.19; H, 3.48; N, 5.21; I, 31.79%.
Synthesis of [Cu2(4-Ibz)4(dabco)] (5) (dabco =
diazabicyclo[2.2.2]octane)
4-Iodobenzoic acid (96 mg, 0.39 mmol) and dabco (24 mg, 0.21
mmol) were dissolved in benzyl alcohol (1 mL). A buffer layer
of benzyl alcohol (0.25 mL) was layered on top of this solution.
Copper(II) acetate monohydrate (40 mg, 0.20 mmol) was
dissolved in methanol and this was carefully layered on top
of the buffer layer. A blue gel formed after 2 days which
contained small green crystals of 5. At this point a single
crystal was removed and used for crystal structure determina-
tion by X-ray diffraction. Yield 54.2 mg, 45.3%. Calc.: C, 33.27;
H, 2.30; N, 2.28; I, 41.36%. Found: C, 33.27; H, 2.18; N, 2.44; I,
41.30%.
Experimental
General
All reagents and solvents were purchased from Sigma Aldrich
or Alfa Aesar and used as received. Elemental analyses were
conducted by the Elemental Analysis Service, Department of
Chemistry, University of Sheffield.
Synthesis of [Cu2(4-Ibz)4(acetone)2] (1) (4-Ibz = 4-iodobenzoate)
Synthesis of [Cu2(4-Ibz)4(bipy)]?nMeOH (6?nMeOH) (bipy =
4,49-bipyridine, MeOH = methanol)
4-Iodobenzoic acid (149 mg, 0.6 mmol) was dissolved in
acetone (10 mL) and carefully layered on top of a solution of
copper(II) nitrate trihydrate (30 mg, 0.12 mmol) in ethyl acetate
(10 mL). Small green block-shaped crystals of 1 started to form
after 7 days. At this point a single crystal was removed and
used for crystal structure determination by X-ray diffraction.
Yield 13.0 mg, 17.6% Calc.: C, 33.17; H, 2.29; I, 41.23%. Found:
C, 34.20; H, 2.54; I, 41.52%.
4-Iodobenzoic acid (96 mg, 0.39 mmol) and bipy (18 mg, 0.11
mmol) were dissolved in benzyl alcohol (1 mL). A buffer layer
of benzyl alcohol (0.25 mL) was layered on top of this solution.
Copper(II) acetate monohydrate (40 mg, 0.20 mmol) was
dissolved in methanol and this was carefully layered on top
of the buffer layer. Small green needles of 6?nMeOH started to
form after 3 days. At this point a single crystal was removed
and used for crystal structure determination by X-ray diffrac-
tion. Yield (for n = 2) 11.2 mg, 8.6%. Calc. (for n = 0): C, 35.90;
H, 1.90; N, 2.20; I, 39.93%. Found: C, 34.44; H, 1.98; N, 2.05; I,
39.66%.
Synthesis of [Cu2(4-Ibz)4(DMSO)2] (2) (DMSO =
dimethylsulfoxide)
4-Iodobenzoic acid (149 mg, 0.6 mmol) was dissolved in DMSO
(6 mL) and carefully layered on top of a solution of copper(II)
nitrate trihydrate (30 mg, 0.12 mmol) in ethyl acetate (10 mL)
Green block-shaped crystals of 2 started to form after 5 days. At
this point a single crystal was removed and used for crystal
structure determination by X-ray diffraction. Yield 14.3 mg,
19.9%. Calc.: C, 30.23; H, 2.22; I, 39.93%. Found: C, 29.88; H,
2.09; I, 39.68%.
[Cu(4-Ibz)2(bpa)]?BnOH (7?BnOH) (bpa = bis(4-pyridyl)ethane,
BnOH = benzyl alcohol)
4-Iodobenzoic acid (96 mg, 0.39 mmol) and bpa (18 mg, 0.10
mmol) were dissolved in benzyl alcohol (1 mL). A buffer layer
of benzyl alcohol (0.25 mL) was layered on top of this solution.
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Table 1 Crystal data, data collection and structure refinement parameters for 1–7
1
2
3
4?4DMF
5
6?nMeOH
7?BnOH
Crystal colour
Crystal size/mm
Blue
0.18 6 0.13
6 0.05
Blue
0.23 6 0.15
6 0.06
Blue
0.34 6 0.21
6 0.15
Blue
Green
Green
Purple
0.03 6 0.02
6 0.02
0.20 6 0.20
6 0.16
Monoclinic
P21/n, 2
12.997(3)
12.621(3)
16.880(3)
90
96.15(3)
90
2753(1)
1.874
0.02 6 0.015 0.02 6 0.01
6 0.01
Tetragonal
I4/m, 2
13.4825(4)
13.4825(4)
9.8363(3)
90
6 0.01
Monoclinic
P21/c, 8
15.230(3)
22.664(5)
27.692(5)
90
102.821(10)
90
9320(3)
1.812
Crystal system
Space group, Z
a/Å
b/Å
c/Å
Triclinic
Triclinic
Triclinic
Triclinic
¯
¯
¯
¯
P1, 1
P1, 2
P1, 2
P1, 4
6.6678(5)
11.9530(9)
12.7015(10)
93.451(4)
103.761(4)
102.981(4)
951.20(13)
2.149
6.5847(5)
15.8533(13)
18.2578(15)
92.436(6)
90.283(6)
93.469(6)
1900.7(3)
2.221
12.196(2)
13.011(2)
13.015(2)
74.517(7)
68.073(7)
87.380(8)
1842.9(5)
2.176
8.2079(19)
16.029(4)
24.699(6)
102.764(17)
98.004(16)
101.353(14)
3049.9(12)
1.851
a/u
b/u
90
90
c/u
V/Å3
1788.02(9)
2.332
100
Density/Mg m23
Temperature/K
m/mm21
100
4.417
100
4.530
100
4.557
100
3.082
100
3.608
100
2.785
4.699
2h range/u
3.32 to 51.98 2.24 to 55.18 3.26 to 55.02 3.76 to 54.80 4.28 to 55.00 2.34 to 42.10
17 854 16 142 37 498 10 752 7097 25 328
3718 (0.0419) 5059 (0.0640) 8303 (0.0679) 2502 (0.0368) 1094 (0.0745) 10 006 (0.1888) 5627 (0.3200)
1.72 to 49.38
16 794
Reflns collecteda
Independent reflns, n, (Rint
Reflns used in refinementa
L.S. parameters, p
Restraints, r
R1(F)b, I . 2s(I)
wR2(F2)b, all data
S(F2)b, all data
a
)
3718
228
0
5059
455
12
8303
446
4
2502
342
0
1094
65
0
10 006
337
25
5627
448
0
0.1080
0.3417
1.030
0.0302
0.0660
1.029
0.0512
0.1300
1.060
0.0411
0.1091
1.057
0.0429
0.1149
1.031
0.0859
0.2756
1.057
0.1425
0.4112
0.906
a
The no. of reflections and Rint values for compounds 2, 4?4DMF and 7?BnOH correspond to the largest twin component. Further details of
the twin refinements can be found in Tables S1–S3, ESI. R1(F) = S (|Fo| 2 |Fc|)/S|Fo|; wR2(F2) = [Sw(Fo 2 Fc2)2/SwFo4 1/2; S(F2) = [Sw(Fo
] 2
b
2
2
3
Fc2)2/(n + r 2 p)]1/2
.
Copper(II) acetate monohydrate (40 mg, 0.20 mmol) was
dissolved in methanol and this was carefully layered on top
of the buffer layer. Small purple needles of 7?BnOH started to
form after 5 days. At this point a single crystal was removed
and used for crystal structure determination by X-ray diffrac-
tion. Yield 30.9 mg, 36.4%.
nent twin; one of the DMF molecules is disordered over two
orientations with occupancies 0.76(3) and 0.24(3). In com-
pound 5 the dabco molecule sits on a 4-fold rotation axis and
is therefore disordered over 4 orientations. In compound
6?nMeOH one of the iodo substituents of each independent
paddlewheel is disordered over two positions with occupancies
0.524(3) : 0.476(3) and 0.703(3) : 0.297(3), respectively. The
crystals of compounds 5, 6?nMeOH and 7?BnOH were very
small and weakly diffracting, leading to lower quality intensity
data; the latter is also a 2-component twin. The diffuse solvent
located in the pores of 6?nMeOH were accounted for using the
solvent masking function implemented in the refinement
program olex2.14 The pores account for 20.4% of the unit cell
volume; the radius of the largest pore is 2.8 Å. The total
electron count per unit cell is 352.8, corresponding to 2.32
MeOH molecules per formula unit (i.e. n = 2.32). Further
details of twin refinements for compounds 2, 4?4DMF and
Crystallography
For compounds 1–7 a suitable crystal was mounted in a stream
of cold N2 gas on a Bruker SMART or Kappa APEX-2 CCD
diffractometer, equipped with graphite-monochromated Mo
Ka radiation from a sealed-tube source. Details of the crystal,
data collection and refinement parameters for 1–7 are
summarized in Table 1. Data were corrected for absorption
using empirical methods (SADABS) based upon symmetry-
equivalent reflections combined with measurements at differ-
ent azimuthal angles.12 The structures were solved by direct
methods and refined by full-matrix least-squares based on
weighted F2 values for all reflections using the SHELXTL suite
of programs13 or using the program olex2.14 Non-hydrogen
atoms were refined anisotropically, except for 6?nMeOH where
only iodine atoms were refined anisotropically, and in 7?BnOH
where only a small subset of atoms were refined anisotropi-
cally. Hydrogen atoms were placed in calculated positions,
refined using idealized geometries (riding model), and
assigned fixed isotropic displacement parameters. The crystal
of compound 2 was a 2-component twin. In compound 3 the
EtOH molecules are disordered. One was modelled in two
orientations with occupancies 0.685(3) and 0.315(3), whereas
the other was best modelled in a single orientation with large
displacement ellipsoids. Compound 4?4DMF was a 3-compo-
7?BnOH are provided in the ESI.
3
Results
Reaction in suitable solvents of copper(II) acetate or copper(II)
nitrate with 4-iodobenzoic acid and a number of potential
axial ligands has been used to prepare a series of crystalline
coordination complexes and networks comprising copper(II)
ions coordinated by the 4-iodobenzoate ligands (Scheme 1).
The crystal structures of these compounds highlight the
propensity of the iodobenzoate ligands to form extended
halogen-bonded networks. The reliability and flexibility of the
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molecules at the axial sites of the paddlewheel, however the
paddlewheel is not located on a symmetry element. The
paddlewheels pack in an identical manner to 1, forming the
same extended halogen-bonded layer involving motifs 1 and 2
…
…
(Fig. 1b). The C–I p halogen bonds are slightly longer (I
C
3.476(10) Å and 3.443(10) Å) and the angle at the donor
deviates slightly more from linearity for one of the interactions
Scheme 1 The secondary building units (SBUs) which occur in compounds 1–7.
a) SBU-I: paddlewheel b) SBU-II: mononuclear. Arrows indicate the direction of
the coordination polymer chains.
…
…
(C–I C 172.7(4) and 177.8(4)u). Similarly, one of the C–I
halogen bonds formed is longer than in 1 with a correspond-
I
…
…
…
ingly smaller angle (C–I I 3.7825(10) Å, C–I I 164.7(3)u, I I–C
81.4(3)u) and the second of these interactions is much shorter
…
…
…
(I I 3.6448(11) Å, C–I I 172.5(3)u, I I–C 84.1(3)u). The
rectangular voids within the halogen-bonded layers are slightly
distorted to accommodate the larger DMSO molecules requir-
ing changes in the halogen-bond geometries relative to those
in 1 (vide supra). In addition to this, the halobenzoate groups
are bent slightly away from their ideal planar geometry, and
the range of ring-carboxylate torsion angles is greater (between
2 and 18u).
halogen-bonded networks has been investigated through the
use of three coordinating solvents with differing steric
demands. The possibility of using the halogen bond as a
synthon for the formation of porous materials has been
investigated through the use of three linear ditopic linker
ligands of increasing size. All of the compounds synthesized
form extended halogen-bonded networks using one or more of
the three motifs shown in Scheme 2.
Compound 3 also comprises copper(II) paddlewheels (SBU-I)
that do not possess inversion symmetry. Two of the iodo-
benzoate ligands are almost planar (ring-carboxylate torsion
angle 5u) whist the other two ligands have a larger ring-
carboxylate torsion angle (13–16u). The axial sites of the
paddlewheels are occupied by ethanol molecules, one of which
is disordered over two orientations. 3 forms the same halogen-
[Cu2(4-Ibz)4(acetone)2], 1, [Cu2(4-Ibz)4(DMSO)2], 2, [Cu2(4-
Ibz)4(EtOH)2], 3 and [Cu2(4-Ibz)4(DMF)2]?4DMF, 4?4DMF
The crystal structure of 1 comprises Cu(II) paddlewheels (SBU-
I, Scheme 1) with acetone solvent molecules coordinated at the
axial sites of the paddlewheel. The paddlewheel sits on an
inversion centre, and the aryl rings are slightly twisted with
respect to the carboxylate groups with C–C–C–O torsion angles
12.9(4) and 10.0(4)u. One of the independent iodobenzoate
…
bonded layer as 1 and 2 (Fig. 1). The C–I p halogen bonds are
…
the shortest of the three compounds (I C 3.400(5) and
…
3.424(5) Å, C–I C 176.35(15) and 176.33(16)u), whereas the
…
…
…
I
groups forms a C–I p halogen bond to the p system of a
C–I I halogen bonds are longer than those in 1 and 2 (I
…
…
…
neighbouring paddlewheel (I
C
3.432(4) Å, C–I
C
3.7578(8) and 3.7719(7) Å, C–I I 169.36(14) and 169.62(15)u,
…
176.64(13)u, cf. motif 1 in Scheme 2). This iodo group also
accepts a halogen bond from the second halobenzoate group
I
I–C 71.97(13) and 71.64(13)u). The halogen-bonded layers
pack in a similar manner to those in 1 and 2, allowing the
…
…
of
a
third paddlewheel unit (I
…
I
3.7142(5) Å, C–I
I
hydroxyl groups of the coordinated ethanol molecules to form
…
170.35(11)u; I I–C 80.04(11)u, cf. motif 2 in Scheme 2). These
halogen bonds are supported by anti-parallel p-stacking of the
halobenzene units. The resulting halogen-bonded layer shown
in Fig. 1a possesses rectangular voids, and the layers stack
such that the coordinated solvent molecules of one layer are
located in the rectangular voids of the adjacent layers resulting
in a close packed structure.
O–H O forms hydrogen bonds to the carboxylate oxygen of a
…
paddlewheel from a neighbouring layer (O O 2.836(5) and
3.189(5) Å for the ordered and the disordered ethanol ligands,
respectively).
As in compounds 1, 2 and 3, the axial sites of the Cu(II)
paddlewheels in 4?4DMF are occupied by coordinated solvent
molecules, in this case DMF. 4?4DMF also forms an extended
halogen-bonded layer, although in this case it is propagated
The crystal structure of 2 is very similar to that of 1,
comprising Cu(II) paddlewheels (SBU-I) with DMSO solvent
…
only through motif 1 type interactions (I C 3.589(3) Å and
…
3.387(3) Å, C–I C 173.9(1) and 176.5(1)u, respectively; see
Fig. 2). These halogen bonds are shorter than in 1 and 2, as the
resulting voids within the layer are square rather than
rectangular in cross-section and are not occupied by the
axially coordinated solvent molecules of adjacent layers.
Instead the layers stack such that the adjacent layers are
rotated by y45u relative to each other, thereby situating the
coordinated DMF solvent molecules above and below the
square voids in the halogen-bonded layer.
As a consequence of this less efficient layer packing, two
molecules of DMF per paddlewheel are required in order to fill
the additional space between the layers. The DMF molecules
Scheme 2 The halogen bonding motifs present in compounds 1–6. (a) Motif 1.
…
Pairs of C–I p halogen bonds between iodobenzoate ligands supported by
…
…
antiparallel p-stacking. (b) Motif 2. Involves pairs of C–I I and C–I p halogen
bonds. (c) Motif 3. Similar to motif 1, but involving SBU-II, and the halogen bond
acceptor is the p-system of the diimine linker ligand rather than the
halobenzoate.
…
form a number of weak C–H O hydrogen bonds.
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Fig. 1 The crystal structures of (a) 1 (b) 2 and (c) 3. The paddlewheel unit is shown at the top and the corresponding halogen-bonded layers (comprising motif 1 and
motif 2 halogen bonds) are shown at the bottom.
solutions. The structure of 5 is most similar to that of [Cu2(4-
Ibz)4(pyrazine)],9a which forms the same halogen-bonded layer
and is interpenetrated in the same manner as 5; it was
synthesized via a similar ambient temperature diffusion
method using the same solvent system as for 5.
[Cu2(4-Ibz)4(dabco)], 5, [Cu2(4-Ibz)4(bipy)]?nMeOH, 6?nMeOH
and [Cu(4-Ibz)2(bpa)]?BnOH, 7?BnOH
The crystal structure of 5 comprises Cu(II) paddlewheels (SBU-
I) linked through their axial sites by a dabco molecule to form
a 1D coordination polymer. The iodobenzene rings are very
nearly coplanar with the carboxylate groups (torsion angles
0.7(18) and 2.4(17)u). The iodobenzoate groups form an
extended 2D layer perpendicular to the coordination polymer
chains. This 2D layer closely resembles that found in 4?4DMF,
propagated by motif 1 type interactions (Fig. 3). The structure
is interpenetrated, with the square voids of the halogen-
bonded halobenzoate layer being filled by a dabco molecule of
the interpenetrated structure. This results in a large distortion
of the halogen-bonded layer, forcing the halobenzoate groups
to bend significantly (Fig. 3b), and also lengthening the
The crystal structure of compound 6?nMeOH comprises
Cu(II) paddlewheels linked through their axial sites by bipy
ligands to form a 1D coordination polymer which runs parallel
to the c-axis. Two crystallographically independent paddle-
wheels exist in the unit cell, here designated type A and type B.
Each type of paddlewheel forms a coordination polymer chain
composed exclusively of that type of paddlewheel (Fig. 4a). The
halogen bonding motif in 6 is similar to that of 1 and 2,
however the network extends infinitely in only one direction,
forming halogen-bonded tapes which are four paddlewheels
wide. The edges of the tapes are made up of type B
paddlewheels whilst the inside rows are formed by type A
paddlewheels. Similarly to 1 and 2, the halogen-bonding motif
results in rectangular voids in the tapes which in this case are
filled by bipy molecules from the other polymer chain
resulting in pseudo-interpenetration (Fig. 4b). Running along
the coordination polymer chain, paddlewheel units are tilted
in an alternating manner (Fig. 4a), which allows the halogen-
bonded tapes to stack in a herringbone pattern (Fig. 4c). In
this way efficient space filling is achieved with a minimum
number of solvent molecules being included in the structure.
Although coordination polymers comprising paddlewheel
SBUs linked by bipy have been previously synthesized in
crystalline form, e.g. [Cu2(O2CPh)4(bipy)],15 it proved very
difficult to prepare such material with 4-Ibz; many failed
synthesis attempts yielded uncharacterised amorphous pow-
…
…
halogen bonds (I C 3.749(17) and 3.746(16) Å, C–I C 159.7(7)
and 160.1(7)u). The structure of 5 differs considerably from the
Zn analogue [Zn2(4-Ibz)4(dabco)]?3.25DMF reported by
Burrows et al.11 In the Zn compound the iodobenzoate ligands
form halogen bonds to the DMF solvent molecules rather than
forming any of the motifs found in Scheme 2. This leads to
offset arrangement of the chains and leads to the formation of
solvent-filled channels within the structure. The difference
between the two compounds can be attributed to the different
synthesis methods used. [Zn2(4-Ibz)4(dabco)]?3.25DMF was
synthesized by heating a sealed tube containing the metal
salt and ligands dissolved in DMF, with crystals forming upon
cooling of a homogenous solution of the reactants. 5 was
formed through the ambient temperature diffusion of the
metal salt into a solution that contains the ligand molecules,
with the crystals forming upon mixing of the BnOH and MeOH
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Fig. 2 (a) The paddlewheel unit of 4?4DMF. (b) The corresponding halogen-
bonded layer, comprising type 1 motifs only.
ders. In practice only a few very small crystals of 6?nMeOH
were synthesized. Attempts to produce this type of compound
by Burrows et al. also failed, instead yielding coordination
polymers comprising mononuclear units,11 which reflects our
experience with the 3-IBz analogue9a and in the present work
when using the still longer ligand 4,49-bipyridylethane in
7?BnOH.
Fig. 3 The crystal structure of 5. (a) The paddlewheels of 5 interact through
motif 1 interactions. (b) The halogen-bonded layer is very similar to that formed
in 3. Only one copy of the interpenetrated network is shown for clarity. (c) The
halogen-bonded coordination polymer chains interpenetrate to form a close-
packed network (second copy of the network shown in grey).
The crystal structure of compound 7?BnOH (Fig. 5) com-
prises mononuclear copper centres chelated by two iodo-
benzoate ligands linked into a 1D coordination polymer
through the final coordination sites by bpa molecules (SBU-
II). The halobenzoate groups form halogen bonds to the p
…
layers (C O 3.49(3) Å), creating voids which are filled with
BnOH solvent molecules. The alcohol oxygen of the solvent
accepts a weak C–H O hydrogen bond from the second CH2
group of the bpa ligand (C O 3.23(2) Å), and also donates a
hydrogen bond to the carboxylate oxygen of the adjacent layer
(O O 2.793(19) Å).
In all of the structures presented herein a common
structural arrangement is present, the antiparallel p-stacking
of iodobenzoate groups combined with C–I p halogen bonds,
…
…
…
…
system of the bpa ligand (motif 3; I C 3.608(17) Å, C–I
C
169.0(6)u). This motif is similar to motif 1 and is also
supported by antiparallel p-stacking between iodobenzoate
groups. The halogen bonds link the coordination polymer
chains into a sheet (Fig. 5b), and these sheets stack through
…
…
…
weak C–H O hydrogen bonds between the CH2 groups of the
which is present in the three motifs represented in Scheme 2.
bpa linkers and the carboxylate oxygen atoms of the adjacent
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Fig. 5 The crystal structure of 7?BnOH. (a) The benzyl alcohol molecules form
hydrogen bonds to the carboxylate oxygen of the adjacent layers. (b) The
coordination polymer chains of 7 form motif 3 interactions resulting in pores
that are filled by benzyl alcohol solvent molecules (not shown).
This arrangement can therefore be regarded as a reliable
synthon for cross-linking of coordination polymers containing
4-iodobenzoate ligands. It is also a flexible arrangement, as
demonstrated by the range of values for the halogen bond
distances and angles. For these reasons it is reasonable to
hypothesise that a porous structure propagated in part by such
interactions may show some dynamic guest-responsive beha-
viour. Even though the synthon itself forms reliably, there are
a number of ways that the ligands can orient themselves which
satisfy the geometric demands of synthon formation. For this
reason attempts to build porous structures from the 4-iodo-
benzoate paddlewheel through the pillaring of the resulting
Fig. 4 The crystal structure of 6?nMeOH. (a) View of the independent
coordination polymer chains, each comprising only one type of paddlewheel.
Type A paddlewheels are shown in green, type B paddlewheels are shown in
yellow. (b) View of assembly of coordination polymer chains into halogen-
bonded tapes, which are four paddlewheel units wide. The halogen-bonded
tape is assembled via motifs 1 and 2. The rectangular voids are filled by bipy
molecules from other coordination polymers that thread through. Such
polymers are represented only by the (isolated) bipy molecules. (c) Scheme
showing the herringbone arrangement of stacked tapes viewed directly down
the direction of propagation of the tapes. Bipy molecules are represented by
blue lines. Haloaryl rings are represented by green or yellow sticks. The position
of one of the tapes is highlighted by a red box. The blue box highlights the
parallel coordination polymer chains shown in (a).
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halogen-bonded layers were unsuccessful, resulting in either
network interpenetration (compound 5), the breakdown of the
layer (6?nMeOH), or the breakdown of the paddlewheel SBU
completely (7?BnOH).
Acknowledgements
We thank the CCDC, STFC and EPSRC for support for PS. ABV
was supported by the Erasmus-Socrates exchange programme
and RMH was supported by the Sheffield Undergraduate
Research Experience (SURE) programme.
Conclusions
4-Iodobenzoate paddlewheels, here Cu2(4-Ibz)4, do not possess
the degree of conformational freedom available to their 3-Ibz
analogues9 and as such the halogen-bonded networks which
form in these compounds are more consistent. In the
compounds reported herein a common structural arrange-
ment defines the intermolecular interaction between iodo-
benzoate groups, whereby antiparallel p-stacking of
iodobenzoates is combined with C–I p halogen bonding to
an aromatic ring that is perpendicular to the p-stacked ligands
(motifs 1–3, Scheme 2).
Two different halogen-bonded layers form in this set of
compounds. One is built entirely from the aforementioned
interactions involving iodobenzoate groups (motifs 1 and 3),
whereas the other type of layer includes a similar motif
whereby the iodine atom involved in one motif 1 interaction
also acts as a halogen bond acceptor. The type of layer that
forms is determined by how efficiently the resulting layers
pack once the axial ligand (i.e. perpendicular to the layer) is
coordinated. Motif 2 interactions leave a larger void within the
layer, allowing interdigitation of the layers with the void filled
by the axial ligands of a neighbouring layer, leading to very
efficient stacking.
Attempts to prepare porous frameworks by use of axially
coordinated pillar ligands to covalently link the Cu2(4-Ibz)4
layers was only partially successful. Although a pillared layered
structure was obtained when using dabco or 4,49-bipy as the
pillar, these structures were doubly interpenetrated and
exhibited no porosity. Use of the still longer bipyridylethane
ligand to extend the separation between layers in a manner
analogous to many MOFs built from paddlewheel layers
comprising metal dicarboxylates instead resulted in break-
down of the halogen-bonded layer and paddlewheel SBU.
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