Copper β-diketonate molecular squares and their host-guest reactions

Article abstract of DOI:10.1002/anie.200701252

(Figure Presented) All square: Treatment of m-phenylene-bis(β- diketones) with [Cu(NH₃)₄]²⁺ yields molecular squares rather than the expected hexagons. The squares react readily with guests such as C₆₀ (see stru

Full text of DOI:10.1002/anie.200701252

Angewandte
Chemie
DOI: 10.1002/anie.200701252
Supramolecular Chemistry
Copper b-Diketonate Molecular Squares and Their Host–Guest
Reactions**
Chandi Pariya, Christopher R. Sparrow, Chang-Keun Back, Giselle Sandí, Frank R. Fronczek,
and Andrew W. Maverick*
In the field of porous supramolecular metal complexes, both
molecular[1] and extended-solid[2] materials have been exten-
sively studied in recent years. These materials are attractive
due to their applications in gas storage[3] and host–guest
chemistry.[4] Among the most often studied of these species
are the metal-containing “molecular squares”, that is, square-
shaped porous tetrameric structures. These have been pre-
pared by several approaches, the most common being the
reaction of an organic bridging ligand with a metal complex
that has available cis coordination sites (Figure 1a). The
which bifunctional organic moieties serve as the “corners”
and join metal atoms in the centers of the “sides” (Fig-
ure 1b).[7] These molecular squares are formed in one step, by
reaction of bis(b-diketone) ligands (1) with [Cu(NH3)4]2+
(Scheme 1). The new squares (2) are capable of binding
guests in both a s (for example, 4,4’-bpy) and p fashion (for
example, C60), and they are also surprisingly effective for
hydrogen-gas storage, both at 77 K and at room temperature.
Ligands 1a and 1b were synthesized by reaction of
isophthalaldehyde with 2,2,2-trimethoxy-4,5-dimethyl-1,3,2-
dioxaphospholene and 2,2,2-trimethoxy-4,5-diethyl-1,3,2-
dioxaphospholene, respectively, at room temperature and
heating the intermediate in methanol under nitrogen.[8]
Mixing solutions of 1a or 1b in CH2Cl2 with aqueous
[Cu(NH3)4]2+ produces 2a or 2b, respectively, which can be
isolated as dark green solids in approximately 95% yield.
The angles between the two b-diketone moieties in the
new ligands 1 are about 1208. Thus, we anticipated that their
reaction with metal ions would yield hexamers, for example,
[M6(m-pba)6]. However, X-ray analysis of the crystalline
products 2, which are formed in nearly quantitative yield,
shows that they are actually molecular squares, with diame-
ters of about 14 Š (Figure 2).[9] (We explored a variety of
conditions for preparing 2, but found no evidence of other
oligomers.) Thus, 2a and 2b are unusual examples of
molecular squares in which the corners are organic bridging
groups, and the metal atoms are in the centers of the sides.
The angles between the b-diketone moieties in the
structures of 2a and 2b are still approximately 1208. To
accommodate this angle within an overall square shape, there
must be some distortion elsewhere in the molecule. The two
ligands are coplanar with the metal atoms in an undistorted
[Cu(b-diketonate)2] complexes,[10] while, in the present
squares, the ligands are all bent away from coplanarity.
The formation of squares, despite the “incorrect” angle
between the b-diketonate moieties, may be due to two factors.
For example, of the reactions shown in Equations (1) and (2)
Figure 1. Schematic illustration of metal-organic molecular squares,
assembledfrom: a) linear organic linkers and90 8 metal units, or
b) linear metal units andorganic “corners”.
bridging ligand is frequently a pyridine derivative (for
example, 4,4’-bipyridine (4,4’-bpy)),[5] although bis-chelating
ligands have also been used.[6] In these cases, the 908
“corners” in the molecular squares are provided by metal
complexes. The resulting metal centers are usually coordina-
tively saturated, which makes it difficult for guest molecules
to interact directly with the metal atoms. Herein we report
molecular squares prepared by an alternative approach, in
[*] Dr. C. Pariya, C. R. Sparrow, Dr. F. R. Fronczek,
Prof. Dr. A. W. Maverick
Department of Chemistry
Louisiana State University
Baton Rouge, LA 70803-1804 (USA)
Fax: (+1)225-578-3458
E-mail: maverick@lsu.edu
Homepage: http://chemistry.lsu.edu
4 Cu2þ þ 4 m-pba2À Ð ½Cu4ðm-pbaÞ4ꢀ
6 Cu2þ þ 6 m-pba2À Ð ½Cu6ðm-pbaÞ6ꢀ
ð1Þ
ð2Þ
Dr. C.-K. Back, Dr. G. Sandí
Chemistry Division
Argonne National Laboratory
9700 South Cass Avenue, Argonne IL 60439 (USA)
reaction (1) is expected to be favored on entropy grounds.
Although self-assembly of cis square-planar or octahedral
metal coordination units with linear linkers normally yields
molecular squares, several examples have been reported in
which significant quantities of trimeric products (that is,
molecular triangles) form.[11] Studies of supramolecular self-
assembly have measured the entropy change that favors the
[**] We thank the U.S. Department of Energy (DE-FG02-01ER15267, to
A.W.M.; DE-AC02-06CH11357, to G.S.) andthe ACS-PRF (37234-
AC3, to A.W.M.) for support. Additional support was provided by the
NSF (CMC-IGERT, DGE-9987603, to C.R.S.) andthe Louisiana
Boardof Regents (LEQSF(1999-2000)-ENH-TR-13, for the X-ray
diffractometer). We thank Dr. R. M. Strongin and Dr. K. K. Kim for
assistance with synthetic procedures.
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(bringing the endo-coordinated Cu
atoms closer together, and the exo-
coordinated ones farther apart), as
À
do the larger Cu N bond lengths for
the internally bound bpy guest.
Reaction of 2 with other pyridine
derivatives leads to similar color
À
changes, thus indicating Cu N coor-
dination.
We studied fullerenes as exam-
ples of p-binding guests. Numerous
hosts have been reported that bind
fullerenes, but only a few of these
function by wrapping completely
around the guest. One such example
is the cofacial diporphyrins reported
by Tashiro and Aida.[15] The present
molecular squares 2 readily bind
Scheme 1. Synthesis of m-phenylenebis(b-diketones) 1 andtheir copper(II) molecular squares 2.
Figure 2. Crystal structure of b-diketonate molecular squares. H atoms
andsolvent molecules omittedfor clarity; ellipsoids shown at 50%.
a) [Cu4(m-pba)4] (2a), Cu1···Cu1’ 14.317(1) Š; Cu2···Cu2’ 14.647(1) Š.
b) [Cu4(m-pbpr)4] (2b), Cu1···Cu1’ 14.012(1) Š; Cu2···Cu2’ 14.661(1) Š.
Figure 3. Crystal structure of [Cu4(m-pba)4(4,4’-bpy)2]n (3a). H atoms
andsolvent molecules omittedfor clarity; ellipsoids shown at 50%.
Cu1···Cu2 11.807(1), Cu3···Cu4 16.226(1), Cu1-N1 2.360(8), Cu2-N2
2.363(8), Cu3-N3 2.251(7), Cu4-N4 2.250(8) Š. (Inversion-relatedpor-
tions of the 4,4’-bpy molecules at N3 andN4 are shown in a lighter
shade, without ellipsoids.)
formation of smaller cyclic oligomers over larger ones.[12]
Second, the formation of smaller cyclic products can be
favored kinetically: Lehn and co-workers reported that the
initial reaction of CuI ions with a polypyridine ligand
produced a cyclic mononuclear species, followed by rear-
rangement to the more stable trinuclear supramolecular
product.[13] In their case, this was possible because a coordi-
nating solvent was chosen, which permits ligand dissociation/
reassociation. In contrast, the compounds reported here are
soluble only in noncoordinating solvents such as CH2Cl2 and
CHCl3. Dissociation in these solvents would lead to unstable
ionic intermediates, thus making it difficult for [Cu4(m-pba)4]
to rearrange into higher oligomers that might be more stable.
Squares 2 react with several types of guest molecules. For
example, 2a reacts with 4,4’-bpy to produce the turquoise
fullerenes C60 and C70 in solvents such as chlorobenzene, as
evident from UV/Vis spectral changes on mixing. The C60
adduct of 2b, that is, [Cu4(m-pbpr)4·C60] (4b), formed crystals
that were suitable for X-ray analysis. The structure
(Figure 4)[16] shows two interesting features: the ethyl
polymeric
complex
[Cu4(m-pba)4(4,4’-bpy)2]n
(3a,
Figure 3).[14] In this compound the 4,4’-bpy molecules are
intra- and intermolecularly bonded to the molecular square.
The Cu···Cu distances in the undistorted squares 2a are longer
than normally observed in 4,4’-bpy-bridged complexes
(ca. 10 Š). However, in 3a, the 4,4’-bpy guest is accommo-
dated by means of smaller distortions at the Cu1 and Cu2
atoms (as compared to those found in 2a and 2b) and larger
distortions at the Cu3 and Cu4 atoms. The square-pyramidal
environment around the Cu atoms favors both changes
Figure 4. Crystal structure of [Cu4(m-pbpr)4·C60] (4b): a) Front view and
b) side view. H atoms and solvent molecules omitted for clarity;
ellipsoids shown at 50%. Cu1···Cu1’ 13.955(1) Š; Cu2···Cu2’
14.060(1) Š.
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(5 mL) and toluene (10 mL) at À208C. After two days, green crystals
groups from the m-pbpr ligands are all oriented toward the
C60 guest; and the Cu···Cu distances (13.96 and 14.06 Š) are
slightly smaller than in the solvated host (2b, Figure 2). These
structural features suggest that weak attractions between the
appeared. The crystals lost solvent rapidly in air to give a light green
powder. Yield: 35 mg (84%). Elemental analysis calcd for
C84H80N4Cu4O16 (Mr = 1655.74): C 60.93, H 4.87, N 3.38; found: C
60.76, H 4.78, N 3.10. Crystals were attached to glass fibers and
quickly cooled to 110 K for X-ray analysis.
À
C60 molecule and the C H bonds and Cu-O-C p systems of
the host stabilize the host–guest adduct.
4b: A solution of 2b (25 mg, 0.015 mmol) in CHCl3 (2 mL) was
layered with a mixture of CHCl3 and 1,2-dichlorobenzene (1:1, 1 mL)
The crystal structures of 2a, 2b, and 4b are all similar,
with the Cu4L4 squares arranged in parallel so as to create
“channels” (see the crystallographic data in the Supporting
Information); in 2a and 2b, the channels are filled with
solvent. This similarity of the structures suggested that it
might be possible to remove the solvents from 2 and use the
“empty” crystals for gas-storage experiments. However,
heating 2 or placing it under vacuum to remove the solvents
results in a loss of crystallinity, thus making it impossible to do
this experiment directly. Still, even noncrystalline unsolvated
2 cannot pack efficiently and should therefore remain porous;
thus, it might serve as a host for gas adsorption. For example,
Sudik et al. have recently reported gas-storage properties of
“metal-organic polyhedra” as noncrystalline solids.[17]
Accordingly, samples of 2a and 2b that had been heated to
1008C under vacuum overnight were used for H2 adsorption
experiments.[18] The results obtained at room temperature and
and then with
a solution of C60 (15 mg, 0.021 mmol) in 1,2-
dichlorobenzene (5 mL). Dark brown crystals had formed after
several days. These crystals also lost solvent rapidly, but they could be
mounted quickly and cooled to 110 K for X-ray analysis. The overall
yield after drying was 21 mg of dark brown powder, but analytically
pure material could not be obtained by this procedure.
X-ray analyses were performed on
a Nonius KappaCCD
diffractometer (MoKa radiation, l = 0.71073 Š) equipped with an
Oxford Cryosystems Cryostream. CCDC-640918–640923 (com-
pounds 1a, 1b, 2a·2CH2Cl2, 2b·6CHCl3·3C6H6, 3a·17.26CHCl3·
1.74CH2Cl2·0.5H2O, and 4b·2CHCl3·3C6H4Cl2, respectively) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Received: March 21, 2007
Published online: July 19, 2007
Keywords: copper · fullerenes · host–guest systems ·
hydrogen storage · supramolecular chemistry
PH = 75 atm were 0.65% (2a) and 0.56% w/w (2b), which
correspond to approximately 4.3 and 4.4 molecules of H2 per
molecule of 2a and 2b, respectively. Greater adsorption was
2
.
observed at 77 K and PH = 43 atm: 4.3% (2a) and 4.2% w/w
2
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metal-organic compounds,[19] and they demonstrate that non-
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The present study reports the complexation behavior of
the new bis(b-diketone) ligands 1 with copper(II) ions to yield
molecular squares 2. The supramolecular product in these
reactions is obtained in high yield in a simple room-temper-
ature reaction. The molecular squares 2 function effectively
as hosts for guests that bind through s- (4,4’-bpy), p- (C60),
and van der Waals (H2) interactions. We are now exploring
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the assembly of supramolecular hosts from other multi-
dentate b-diketone ligands.
Experimental Section
The preparation of the new bis(b-diketones) 1a and 1b is described in
the Supporting Information.
2a: A solution of [Cu(NH3)4]2+ was prepared from CuSO4·5H2O
(0.35 g, 1.4 mmol) in H2O (10 mL) by slow addition of conc. aqueous
NH3. A solution of 1a (0.275 g, 1.00 mmol) in CH2Cl2 (20 mL), was
then added and the mixture was stirred for 6 h. More CH2Cl2 (20 mL)
was then added, and the organic layer was collected and dried over
MgSO4. The residue was washed with hexane (2 ” 10 mL) and dried in
air. Yield: 0.325 g (96%). Elemental analysis calcd for C64H64O16Cu4
(Mr = 1343.40): C 57.22, H 4.80; found: C 57.43, H 4.69. 2b was
prepared by a similar method; yield: 95%. Elemental analysis calcd
for C80H96O16Cu4 (Mr = 1567.80): C 61.29, H 6.17; found: C 61.08, H
6.00. Single crystals of these two compounds suitable for X-ray
analysis were obtained by layering solutions in CH2Cl2 and CHCl3
with hexane and benzene, respectively.
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[7] Metal-organic supramolecules have been reported previously in
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Angew. Chem. Int. Ed. 2007, 46, 6305 –6308
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Communications
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[14] Crystal data for 3a·17.26CHCl3·1.74CH2Cl2·0.5H2O (Mr =
¯
3872.89): Triclinic, space group P1, T= 110 K, a = 14.824(3),
b = 19.384(4), c = 29.616(6) Š; a = 99.043(8), b = 97.751(11), g =
101.782(9)8; V= 8104(3) Š3; Z = 2; 1calcd = 1.587 MgmÀ3; R =
0.088, wR = 0.253.
[15] K. Tashiro, T. Aida, Chem. Soc. Rev. 2007, 36, 189.
[16] Crystal data for 4b·2CHCl3·3C6H4Cl2 (Mr = 2968.04): Triclinic,
¯
space group P1, T= 110 K, a = 9.933(3), b = 18.397(7), c =
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¯
group P1, T= 105 K, a = 7.577(3), b = 16.419(6), c = 17.423(7) Š;
a = 110.17(3), b = 92.57(2), g = 92.063(15)8; V= 2029.5(14) Š3;
Z = 1; 1calcd = 1.251 MgmÀ3; R = 0.066, wR = 0.193. Crystal data
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¯
for 2b·6CHCl3·3C6H6 (Mr = 2518.26): Triclinic, space group P1,
T= 110 K, a = 9.316(2), b = 16.726(4), c = 18.743(5) Š; a =
78.619(13), b = 86.943(13), g = 83.472(9)8; V= 2843.1 (12) Š3;
Z = 1; 1calcd = 1.471 MgmÀ3; R = 0.062, wR = 0.183.
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