Directed supramolecular assembly of Cu(II)-based "paddlewheels" into infinite 1-D chains using structurally bifunctional ligands

Article abstract of DOI:10.1039/b513765a

The construction of Cu(II)-containing supramolecular chains is achieved by combining suitable anionic ligands (for controlling the coordination geometry and for creating a neutral building block) with four new bifunctional ligands containing a metal-coordinating pyridyl site and a self-complementary hydrogen-bonding moiety. Seven crystal structures are presented and in each case, the copper(ii) complex displays a "paddlewheel" arrangement, with four carboxylate ligands occupying the equatorial sites, leaving room for the bifunctional ligand to coordinate in the axial positions. The supramolecular chemistry, which organizes the coordination-complexes into the desired infinite 1-D chains, is driven by a combination of N-H...N and N-H...O hydrogen-bonds in five of the seven structures. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b513765a

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Directed supramolecular assembly of Cu(II)-based “paddlewheels”
into infinite 1-D chains using structurally bifunctional ligands
Christer B. Aakero¨y,* Nate Schultheiss and John Desper
Received 28th September 2005, Accepted 23rd November 2005
First published as an Advance Article on the web 6th December 2005
DOI: 10.1039/b513765a
The construction of Cu(II)-containing supramolecular chains is achieved by combining suitable anionic
ligands (for controlling the coordination geometry and for creating a neutral building block) with four
new bifunctional ligands containing a metal-coordinating pyridyl site and a self-complementary
hydrogen-bonding moiety. Seven crystal structures are presented and in each case, the copper(II)
complex displays a “paddlewheel” arrangement, with four carboxylate ligands occupying the equatorial
sites, leaving room for the bifunctional ligand to coordinate in the axial positions. The supramolecular
chemistry, which organizes the coordination-complexes into the desired infinite 1-D chains, is driven by
a combination of N–H · · · N and N–H · · · O hydrogen-bonds in five of the seven structures.
thereby creating a neutral “paddlewheel” complex, Scheme 1. Each
Introduction
copper ion is in an arrangement where four of the equatorial
positions are coordinated by the anions, leaving a vacant axial
position accessible for the supramolecular ligand.
The design and synthesis of inorganic–organic hybrid materials
with desired topologies, through supramolecular synthesis, is a
rapidly emerging field within crystal engineering.1 The ability
to translate the inherent dimensionality and geometry of any
coordination complex into extended architectures with specific
(and tunable) metrics is critical in producing materials with
desired chemical and physical properties.2 The use of coordinate
covalent bonds for the assembly of coordination polymers is
well established,3 whereas supramolecular synthetic strategies that
utilize non-covalent (i.e. hydrogen bonds,4 p–p interactions,5 etc.)
and metal–ligand interactions within the same network are far less
developed.6 Composite hybrid materials are of crucial importance
in natural systems7 and they have also found numerous uses in
synthetic high-tech applications.8 The combination of organic
ligands and coordination chemistry also brings together synthetic
flexibility (from principles of organic chemistry) and properties
and reactivities inherent in many transition-metal ions. Thus,
the ability to reliably prepare new extended hybrid materials
with desirable structures from soluble precursors remains a very
important target in synthetic and materials chemistry.
We are particularly interested in identifying systems that
simplify the supramolecular assembly which means that we need
to (a) carefully control the geometry of the complex ion and, (b)
eliminate the need for potentially disruptive counterions. This is
particularly important when targeting metal ions with diverse and
unpredictable coordination modes such as Cu(II). With this in
mind, a variety of structurally bifunctional organic ligands con-
taining a metal-ion coordinating moiety and a hydrogen bonding
functionality were synthesized with the specific supramolecular
goal of organizing a variety of Cu(II)-based complex ions into
infinite 1-D chains. Dinuclear Cu(II) complexes incorporating
four carboxylate moieties (CuII2L4, where L = acetate or 2-
fluorobenzoate) provide a reasonable starting point since the
four monoanionic functional groups bridge two copper(II) centers
Scheme 1 Substituted “paddlewheel” complex with arrows showing the
two axial coordination sites.
A structurally bifunctional ligand (a supramolecular linker) for
this family of complex ions could combine a donor atom capable of
effective metal-ion coordination with a functional group capable
of forming self-complementary hydrogen bonds to a neighboring
complex, thus extending the network. Four different ligands
were designed and synthesized to fit these specific requirements,
Scheme 2.
Ligands 1 and 2 have slightly dissimilar hydrogen-bonding
moieties due to the differences in electronic and steric influences
provided by the methyl and methoxy substituents, respectively. In
contrast, ligands 3 and 6 contain an ethynyl linker between the
two heterocyclic rings, allowing for greater separation between
the metal-ions within each chain, which provides a handle for
tuning some potentially important metrics of the resulting archi-
tecture. Ligand 6 contains an amino-pyridine fragment instead
of an amino-pyrimidine unit but it is still capable of forming
self-complementary hydrogen-bond interactions with ligands of
neighboring complex ions.
In this study, we outline a specific and predetermined
supramolecular design strategy for the construction of 1-D chains
from suitable bifunctional organic ligands 1,2,3,6 and dinuclear
copper(II) acetate or copper(II) 2-fluororobenzoate complex ions.
Department of Chemistry, Kansas State University, Manhattan, Kansas,
66506, USA. E-mail: aakeroy@ksu.edu
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 1627–1635 | 1627
©

View Article Online
Scheme 2 A family of structurally bifunctional ligands; 3-(2-amino-4-methylpyrimidin-6-yl)pyridine 1, 3-(2-amino-4-methoxypyrimidin-6-yl)pyridine
2, 1-(2-amino-4-methylpyrimidin-6-yl)-2-(3-methoxypyridin-5-yl)ethyne 3, 1-(2-aminopyrid-5-yl)-2-(pyrid-3-yl)ethyne 6.
The reliability of the assembly process is evaluated by an
examination of seven crystal structures in the context of the
desired supramolecular target and the primary intermolecular
interactions.
2.2 Hz, 1H), 6.41 (d, J = 8.8 Hz, 1H), 4.52 (s, 2H), 0.23 (s, 9H);
13C NMR (dC; 400 MHz, CDCl3): 157.52, 151.90, 140.73, 109.79,
107.70, 102.74, 94.55, 0.008; IR (KBr): 3452, 3301, 3160, 2156,
1626, 1486, 1390; ESI-IT-MS m/z 192 ([4 + H]+).
2-Amino-5-ethynylpyridine, 5
Experimental
A mixture of 2-amino-5-trimethylsilanylethynylpyridine (2.04 g,
10.7 mmol) and potassium carbonate (1.50 g, 10.9 mmol) were
stirred in methanol (30 mL) at room temperature for 2 hours.
The solution was then diluted with ethyl ether (100 mL) and
washed with water (4 × 100 mL). The solvent was removed on
a rotary evaporator and the residue chromatographed on silica
with hexanes/ethyl acetate (10 : 1) as the eluant◦. 5 was isolated as
a light brown solid (1.13 g, 92%). Mp 127–129 C; 1H NMR (dH;
400 MHz, CDCl3): 8.23 (s, 1H), 7.52 (dd, J = 8.8 Hz, J = 2.2 Hz,
1H), 6.44 (d, J = 8.8 Hz, 1H), 4.64 (s, 2H), 3.06 (s, 1H); 13C NMR
(dC; 200 MHz, CDCl3)10: 157.77, 152.11, 140.84, 108.66, 107.79,
81.36; IR (KBr): 3455, 3269, 3155, 2104, 1628, 1500, 1394, 1145,
831; ESI-IT-MS m/z 121 ([5 + 2H]+).
All chemicals were purchased from Aldrich and used without
further purification. Melting points were determined on a Fisher-
1
Johns melting point apparatus and are uncorrected. H and 13C
NMR spectra were recorded on a Varian Unity plus 400 MHz
spectrometer in CDCl3. Compounds were prepared for infrared
spectroscopic (IR) analysis as a mixture in KBr. Elemental analysis
was carried out by Atlantic Microlab Inc. Electrospray ionization-
ion-trap mass spectrometry (ESI-IT-MS) was carried out on a
Bruker Daltonics Esquire 3000 plus. The syntheses and char-
acterizations of 3-(2-amino-4-methylpyrimidin-6-yl)pyridine 1, 3-
(2-amino-4-methoxypyrimidin-6-yl)pyridine 2, and 1-(2-amino-4-
methylpyrimidin-6-yl)-2-(3-methoxypyridin-5-yl)ethyne 3 are re-
ported elsewhere.9
2-Amino-5-trimethylsilanylethynylpyridine, 4
1-(2-Aminopyrid-5-yl)-2-(pyrid-3-yl)ethyne, 6
2-Amino-5-bromopyridine (5.00 g, 28.9 mmol), trimethylsily-
lacetylene (3.97 g, 40.4 mmol), copper(I) iodide (0.170 g, 0.890
mmol), triphenylphosphine (0.600 g, 2.29 mmol), bis(triphenyl-
phosphine)palladium(II) dichloride (0.600 g, 0.856 mmol) were
added to a round bottom flask. Tetrahydrofuran (65 mL) and
triethylamine (65 mL) were added and dinitrogen bubbled through
the resultant mixture for 10 minutes. A condenser was attached
A mixture of 2-amino-5-ethynylpyridine (1.00 g, 8.70 mmol), 3-
bromopyridine (1.60 g, 10.12 mmol), copper(I) iodide (0.050 g,
0.262 mmol), triphenylphosphine (0.220 g, 0.840 mmol),
bis(triphenylphosphine)palladium(II) dichloride (0.220 g, 0.314
mmol) were added to a round bottom flask. Tetrahydrofuran
(30 mL) and triethylamine (30 mL) were added and dinitrogen
bubbled through the resultant mixture for 10 minutes. A condenser
was attached and the mixture heated at 70 ◦C under a dinitrogen
atmosphere. The reaction was monitored by TLC and allowed
to cool to room temperature upon completion (36 hours). The
solution was then diluted with 100 mL of ethyl acetate, washed
with water (3 × 100 mL) then washed with saturated aqueous
sodium chloride (1 × 100 mL). The organic layer was separated
and dried over magnesium sulfate. The solvent was removed on
a rotary evaporator and the residue chromatographed on silica
with a hexane/ethyl acetate mixture (1 : 1) as the eluant. 6 was
isolated as a light brown solid. The product was recrystallized
from chloroform producing orange block shaped crystals (1.2 g,
◦
and the mixture heated at 75 C under a dinitrogen atmosphere.
The reaction was monitored by TLC and allowed to cool to room
temperature on completion (48 hours). The solution was then
diluted with 100 mL of ethyl acetate, washed with water (3 × 100
mL) then washed with saturated aqueous sodium chloride (1 × 100
mL). The organic layer was separated and dried over magnesium
sulfate. The solvent was removed on a rotary evaporator and the
residue chromatographed on silica with a mixture of hexane/ethyl
acetate (10 : 1) as the eluant. 4 was isolated as a light tan
solid, which was recrystallized from dichloromethane/hexanes as
◦
1
colorless-opaque plates (4.81 g, 87%). Mp 91–93 C; H NMR
◦
1
(dH; 200 MHz, CDCl3): 8.19 (s, 1H), 7.48 (dd, J = 8.8 Hz, J =
79%). Mp 131–133 C; H NMR (dH; 200 MHz, CDCl3): 8.73
1628 | Dalton Trans., 2006, 1627–1635
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
(d, J = 2.6 Hz, 1H), 8.52 (dd, J = 4.9 Hz, J = 1.7 Hz, 1H),
8.28 (d, J = 2 Hz, 1H), 7.77 (dt, J = 8 Hz, J = 2 Hz, 1H), 7.56
(dd, J = 8.6 Hz, J = 2.4 Hz, 1H), 7.27 (m, 1H), 6.49 (dd, J =
8.4 Hz, J = 0.8 Hz, 1H), 4.77 (s, 1H); 13C NMR (dC; 200 MHz,
CDCl3): 157.79, 152.02, 151.50, 148.29, 140.39, 138.09, 122.96,
120.60, 109.08, 107.96, 90.37, 86.53; IR (KBr): 3318, 3140, 2213,
1602, 1508, 1398; ESI-IT-MS m/z 197 ([6 + H]+).
0.08 mmol) were added to a screw cap vial along with a 1 :
1 : 1 mixture of ethanol/acetonitrile/nitromethane. The mixture
was heated gently until a clear homogeneous solution resulted.
Without solvent evaporation, green block-shaped crystals were
collected after 2 days. Mp 160 ◦C (decomp.). Anal. Calcd. for
C34H36N8O10Cu2: C, 48.45; H, 4.31; N, 13.30. Found: C, 48.11; H,
4.28; N, 13.28%.
Tetrakis(l-acetato-O,Oꢀ)-bis(3-(2-amino-4-methylpyrimidin-6-
yl)pyridine)-dicopper(II), 7
Tetrakis(l-2-fluorobenzoato-O,Oꢀ)-bis(1-(2-aminopyrid-5-yl)-2-
(pyrid-3-yl)ethyne)-dicopper(II), 12
3-(2-Amino-4-methylpyrimidin-6-yl)pyridine (9 mg, 0.05 mmol)
and copper(II) acetate (10 mg, 0.05 mmol) were added to a
screw cap vial along with a methanol/acetonitrile mixture (2 :
1 mL). The mixture was heated gently until a clear homogeneous
solution resulted. Without solvent evaporation, green block-
shaped crystals were collected after 1 day. Mp 210–212 ◦C. Anal.
Calcd. for C28H32N8O8Cu2: C, 45.77; H, 4.39; N, 15.26. Found: C,
45.72; H, 4.46; N, 15.09%.
1-(2-Aminopyridin-5-yl)-2-(pyridin-3-yl)ethyne (11 mg, 0.06 mmol)
and copper(II) 2-fluorobenzoate (19 mg, 0.03 mmol) were added to
a screw cap vial along with a mixture of methanol/acetonitrile (3 :
5 mL). The mixture was heated gently until a clear homogeneous
solution resulted. Without solvent evaporation, green plate-
shaped crystals were collected after 1 day. Mp 255 ◦C (decomp.).
Anal. Calcd. for C40H25N3O8F4Cu2: C, 54.73; H, 2.87; N, 4.79.
Found: C, 54.65; H, 2.89; N, 4.96%.
Tetrakis(l-2-fluorobenzoato-O,Oꢀ)-bis(3-(2-amino-4-
methylpyrimidin-6-yl)pyridine)-dicopper(II), 8
Tetrakis(l-acetato-O,Oꢀ)-bis(1-(2-aminopyrid-5-yl)-2-(pyrid-3-
yl)ethyne)-dicopper(II), 13
3-(2-Amino-4-methylpyrimidin-6-yl)pyridine (9 mg, 0.05 mmol)
and copper(II) 2-fluorobenzoate (16 mg, 0.02 mmol) were added to
a screw cap vial along with a mixture of methanol/acetonitrile (1 :
5 mL). The mixture was heated gently until a clear homogeneous
solution resulted. Without solvent evaporation, green plate-
shaped crystals were collected after 1 day. Mp 224–226 ◦C. Anal.
Calcd. for C48H36N8O8F4Cu2·H2O: C, 53.75; H, 3.57; N, 10.45.
Found: C, 53.69; H, 3.30; N, 10.45%.
1-(2-Aminopyrid-5-yl)-2-(pyridin-3-yl)ethyne (13 mg, 0.07 mmol)
and copper(II) acetate (26 mg, 0.13 mmol) were added to a
screw cap vial along with a mixture of methanol/acetonitrile (3 :
5 mL). The mixture was heated gently until a clear homogeneous
solution resulted. Without solvent evaporation, green block-
shaped crystals were collected after 1 day. Mp 210 ◦C (decomp.).
Anal. Calcd. for C20H21N3O8Cu2: C, 43.09; H, 3.80; N, 7.54.
Found: C, 42.92; H, 3.77; N, 7.54%.
Tetrakis(l-acetato-O,Oꢀ)-bis(3-(2-amino-4-methoxypyrimidin-6-
yl)pyridine-dicopper(II), 9
X-Ray crystallography
3-(2-Amino-4-methoxypyrimidin-6-yl)pyridine (10 mg, 0.05 mmol)
and copper(II) acetate (10 mg, 0.05 mmol) were added to a
screw cap vial along with a methanol/acetonitrile mixture (2 :
1 mL). The mixture was heated gently until a clear homogeneous
solution resulted. Without solvent evaporation, green block-
shaped crystals were harvested after 1 day. Mp 212 ◦C (decomp.).
Anal. Calcd. for C28H32N8O10Cu2: C, 43.86; H, 4.21; N, 14.62.
Found: C, 43.62; H, 4.19; N, 14.57%.
X-Ray data were collected on a Bruker SMART CCD diffrac-
tometer (7–13)11 using Mo-Ka radiation. Data were collected
using SMART.12 Initial cell constants were found by small widely
separated “matrix” runs. An entire hemisphere of reciprocal space
was collected. Scan speed and scan width were chosen based
on scattering power and peak rocking curves. All datasets were
collected at low temperature.
Unit cell constants and orientation matrix were improved by
least-squares refinement of reflections thresholded from the entire
dataset. Integration was performed with SAINT,13 using this
improved unit cell as a starting point. Precise unit cell constants
were calculated in SAINT from the final merged dataset. Lorentz
and polarization corrections were applied, and data were corrected
for absorption.
Data were reduced with SHELXTL.14 The structures were
solved in all cases by direct methods without incident. In general,
hydrogens were assigned to idealized positions and were allowed
to ride. Where possible, the coordinates of hydrogen-bonding
hydrogens were allowed to refine. Heavy atoms were refined with
anisotropic thermal parameters. Crystallographic data are given
in Table 1, hydrogen bond geometries in Table 2 and thermal
ellipsoids and labeling schemes are shown in Fig. 1, 3, 6, 8, 10, 12.
CCDC reference numbers 278015–278021.
Tetrakis(l-2-fluorobenzoato-O,Oꢀ)-bis(3-(2-amino-4-
methoxypyrimidin-6-yl)pyridine-dicopper(II), 10
3-(2-Amino-4-methoxypyrimidin-6-yl)pyridine (9 mg, 0.05 mmol)
and copper(II) 2-fluorobenzoate (17 mg, 0.03 mmol) were added to
a screw cap vial along with a mixture of methanol/acetonitrile (1 :
5 mL). The mixture was heated gently until a clear homogeneous
solution resulted. Without solvent evaporation, green plate-
shaped crystals were collected after 1 day. Mp 224–226 ◦C. Anal.
Calcd. for C48H36N8O10F4Cu2: C, 53.03; H, 3.34; N, 10.31. Found:
C, 52.92; H, 3.14; N, 10.22%.
Tetrakis(l-acetato-O,Oꢀ)-bis(1-(2-amino-4-methylpyrimidin-6-yl)-
2-(3-methoxypyridin-5-yl)ethyne)-dicopper(II), 11
1-(2-Amino-4-methylpyrimidin-6-yl)-2-(3-methoxypyridin-5-yl)-
ethyne (10 mg, 0.04 mmol) and copper(II) acetate (17 mg,
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b513765a
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 1627–1635 | 1629
©

View Article Online
1630 | Dalton Trans., 2006, 1627–1635
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
◦
˚
Table 2 Selected hydrogen bond lengths (A) and angles ( ) for compounds 7–13
D–H · · · A
D–H
H · · · A
D · · · A
<(DHA)
7
8
N(12)–H(12A) · · · N(11)#2a
N(12)–H(12B) · · · O(42)#3
N(22)–H(22A) · · · N(63)#2b
N(22)–H(22B) · · · O(78)
0.87(2)
0.89(2)
0.93(4)
0.95(4)
0.73(4)
0.83(4)
0.95(3)
0.91(3)
0.88
2.17(2)
2.10(2)
2.31(4)
2.26(4)
2.27(4)
2.27(4)
2.24(3)
2.29(3)
2.52
3.0434(17)
2.9748(16)
3.241(4)
3.206(4)
3.001(4)
3.055(4)
3.177(3)
3.185(3)
3.253(5)
2.982(3)
3.061(3)
3.295(8)
3.078(3)
175(2)
170.5(19)
174(3)
173(3)
171(4)
159(3)
171(2)
170(3)
141.2
N(62)–H(62A) · · · N(21)#2
N(62)–H(62B) · · · O(48)#3
N(12)–H(12A) · · · N(11)#2c
N(12)–H(12B) · · · O(41)#2
N(22)–H(22A) · · · N(23)#2d
N(12)–H(12A) · · · O(42)#2e
N(12)–H(12B) · · · N(11)#3
N(12)–H(12B) · · · O(38)#3f
N(16)–H(16B) · · · O(31)#3g
9
10
11
0.82(4)
0.72(3)
0.88
2.22(4)
2.34(4)
2.43
155(3)
173(3)
168.3
12
13
0.88
2.32
144.5
a #2 −x, −y + 1, −z + 1. #3 − x + 1, y − 1/2, −z + 1/2. b #2 −x + 1, −y + 1, −z + 1. #3 x − 1, y, z. c #2 −x + 1, −y, −z + 1. d #2 −x + 1, −y + 1,
−z. e #2 x, y − 1, z + 1. #3 −x, −y, −z + 2. f #3 x + 1, y, z. g #3 −x + 1, −y + 1, −z + 1.
Results
Ligand 6 was produced in good yields by reacting 2-amino-5-
ethynylpyridine with 3-bromopyridine under an inert atmosphere.
2-Amino-5-ethynylpyridine was synthesized in excellent yields by
means of Pd-catalyzed Sonogashira coupling conditions15 of 2-
amino-5-bromopyridine and trimethylsilylacetylene followed by
base-promoted deprotection, Scheme 3.
The complex-ion in the crystal structure of 7 is composed
of two pyridine-pyrimidine (py-pym) ligands 1 bound at the
axial positions of the dicopper tetraacetate complex, Fig. 1.
Fig. 1 Labeled thermal ellipsoids of 7 (50% probability level).
The symmetry-related Cu(II) ion displays a square-pyramidal
self-complementary N–H · · · N hydrogen bonds, produced from
˚
coordination geometry with Cu–N distances of 2.1743(11) A.
the syn-amino proton and a pyrimidine nitrogen atom, Fig. 4.
This feature is then extended into a 2-D sheet through N–H · · · O
hydrogen bonds from the anti-amino proton to an adjacent acetate
oxygen atom from an adjacent strand, Fig. 5.
The complex ions are assembled into infinite chains through
self-complementary N–H · · · N hydrogen bonds, between the syn-
amino proton and a pyrimidine nitrogen atom. Neighboring
chains are subsequently organized into a 2-D sheet via N–H · · · O
hydrogen bonds between the anti-amino proton and an acetate
oxygen atom from an adjacent strand, Fig. 2(a) and (b).
The crystal structure of 8 contains two crystallographically
unique Cu(II)-complexes, Fig. 3. Within each complex the copper
ions are arranged in the familiar “paddlewheel” motif in which
two copper ions are bridged by four acetate groups, thus leaving
two axial sites for further coordination. The two py-pym ligands
1 are coordinated axially through the nitrogen atom of the pyridyl
moiety to the Cu(II) ions, each with a square-pyramidal geometry
The complex-ion in the crystal structure of 9 contains two py-
pym ligands 2 bound axially through their pyridyl nitrogen atoms
to the dicopper tetraacetate dimer affording a five-coordinate
square-pyramidal geometry around each Cu(II) ion with a Cu–
˚
N distance of 2.169(2) A, Fig. 6. Discrete one-dimensional chains
are formed from self-complementary N–H · · · N hydrogen bonds
between the syn-amino proton and a pyrimidine nitrogen atom,
whilst the anti-amino proton hydrogen bonds with a neighboring
acetate oxygen atom through N–H · · · O interactions, Fig. 7. The
second nitrogen atom of the pyrimidine ring is blocked by the
adjacent methoxy substituent and does not act as a hydrogen-
bond acceptor.
˚
with Cu–N distances of 2.145(3) and 2.149(2) A respectively. The
two unique complexes are connected in a zig-zag manner through
Scheme 3 Synthesis of 5 and 6.
The Royal Society of Chemistry 2006
This journal is
Dalton Trans., 2006, 1627–1635 | 1631
©

View Article Online
Fig. 2 (a) One-dimensional strand in 7 formed by self-complementary
N–H · · · N hydrogen bonds (top); (b) two-dimensional sheet produced
in 7 from a combination of N–H · · · N and N–H · · · O hydrogen bonds
(bottom).
Fig. 5 Extended network of 8 through a series of N–H · · · N and
N–H · · · O hydrogen bonds (the fluoro-phenyl rings have been omitted
for clarity).
Fig. 6 Labeled thermal ellipsoids of 9 (50% probability level).
Fig. 7 Infinite chain in 9 formed from Cu(II)-py coordination bonds
and self-complementary N–H · · · N and heteromeric N–H · · · O hydrogen
bonds.
Fig. 3 Labeled thermal ellipsoids of the two unique complex ions (a and
b) in 8 (50% probability level).
Fig. 4 1-D zig-zag chain in 8 produced through self-complementary
N–H · · · N hydrogen bonds.
Fig. 8 Labeled thermal ellipsoids of 10 (50% probability level).
˚
The complex ion in the crystal structure of 10 contains two
py-pym ligands 2 bound axially via the pyridyl nitrogen atoms
to the “paddlewheel” copper dimer unit, furnishing each Cu(II)
ion with a square-pyramidal coordination geometry with Cu–N
distances of 2.149(3) A, Fig. 8. Similar to structure 9, discrete
1-D chains are produced from self-complementary N–H · · · N
hydrogen bonds from the syn-amino proton to a pyrimidine
nitrogen atom, and N–H · · · O interactions from the anti-amino
1632 | Dalton Trans., 2006, 1627–1635
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
˚
proton to an acetate oxygen atom of a neighboring strand,
Fig. 9. Additional stabilization of the network is provided by p–
p stacking between the fluoro-substituted phenyl rings and the
of 2.162(3) and 2.198 (14) A respectively. In both structures the
py-py ligand 6 is essentially located on an inversion center, with
NH2 and H randomly disordered with 50% occupancy. Thus, the
complex ions are shown with both ends of the ligand containing
an amino group, Fig. 12. Although the ligand contains a potential
self-complementary N–H · · · N hydrogen bonding moiety, no hy-
drogen bonds are formed between adjacent ligands. Instead, both
pyridyl moieties coordinate to two different “paddlewheel” units
which, in effect, creates 1-D coordination polymers within the
lattice of both 12 and 13.
˚
amino-pyrimidine rings with distances as short as 3.28 A.
Fig. 9 1-D stair-step architecture stabilized by a combination of hydrogen
bonds and p–p-stacking in 10.
The crystal structure of 11 contains two py-pym ligands 3 bound
in the axial positions of the two Cu(II) ions contained within
the tetraacetate “paddlewheel” unit, creating a square-pyramidal
coordination geometry around each copper ion with Cu–N
˚
distances of 2.181(18) A, Fig. 10. Discrete infinite 1-D chains are
produced from self-complementary N–H · · · N hydrogen bonds
between the syn-amino proton and a pyrimidine nitrogen atom,
Fig. 11. Neighboring chains are subsequently organized into a 2-
D sheet via N–H · · · O hydrogen bonds between the anti-amino
proton and an acetate oxygen atom from an adjacent strand,
similar to what was observed in the structure of 7.
Fig. 12 Labeled thermal ellipsoids of 12 (a) and 13 (b) at 50% probability
level.
Fig. 10 Labeled thermal ellipsoids of 11 (50% probability level).
Discussion
In order to direct the assembly of complex ions into extended
inorganic–organic networks of specific (and pre-determined)
dimensionality or geometry, it is necessary to achieve and main-
tain control over both the immediate coordination environment
around the central metal ion, and over the way in which neigh-
boring complex ions recognize and interact with each other. By
incorporating the metal ion within an overall electrically neutral
building block, the need for potentially disruptive counterions has
been eliminated. In this study, we opted to start with tetrakis(l-
carboxylato-O,Oꢀ)-dicopper(II) complexes as these architectures
are neutral and they offer two available binding sites (axial
positions) that are organized in such a way as to produce,
essentially, a linear metal-containing building block. In all seven
Fig. 11 1-D strand in 11 formed through self-complementary
amino-pyrimidine N–H · · · N hydrogen bonds.
In the crystal structures of 12 and 13 there are two pyridine-
pyridine (py-py) ligands 6 bound axially, through the pyridyl
nitrogen atoms to the “paddlewheel” copper dimer unit, creating
a square-pyramidal coordination geometry with Cu–N distances
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 1627–1635 | 1633
©

View Article Online
crystal structures obtained in this effort, the desired coordination
environment around each copper (II) ion was achieved resulting
in rigid neutral dicopper(II) tetracarboxylate complexes. The next
phase in the supramolecular synthesis requires some means for
aligning and connecting these complex ions in such a way that
the inherent geometry of the metal complex is propagated into ex-
tended Cu-containing networks. We therefore examined the ability
of four different bifunctional ligands to act as supramolecular
reagents for the construction of 1-D motifs. All four ligands are
potentially capable of forming the desired architectures by virtue
of having a pyridyl moiety as a binding site for the metal, coupled
with an amino-pyrimidine or amino-pyridine moiety proficient in
forming self-complementary N–H · · · N/N · · · H–N interactions.
In all seven structures 7–13, the pyridyl moiety provides a
robust and reliable binding site for attaching the supramolecular
bifunctional ligand to the open, axial coordination sites on the
Cu(II) ion. For the supramolecular chemistry, three of the ligands
utilize an aminopyrimidine moiety, whereas one ligand, 6, relies on
an amino-pyridine functionality. For the two structures, 12 and 13,
where the latter ligand was employed, the desired intermolecular
interactions failed to materialize. Surprisingly, the disordered
bipyridine-type ligands acted as ditopic linkers between adjacent
Cu(II)-ions resulting in 1-D coordination polymers. The reason for
the failure of 6 to act in the desired manner is not obvious, but
it is possible that the pyridyl moiety, which is an integral part of
the hydrogen-bonding functionality, is simply too good a ligand
to give up a Cu(II) ion in exchange for acting as an acceptor in
an N–H · · · N hydrogen bond. This interpretation is supported by
the fact that when a pyridyl moiety is replaced with a less effective
ligand, a pyrimidine heterocycle, a dimeric hydrogen-bond motif
is favored over a pyrimidine–Cu, ligand–metal, interaction.
Each time a bifunctional ligand with a pyrimidine component
was employed (all five of the structures), neighboring ligands were
connected via a dimeric N–H · · · N motif. The exact directional
and structural consequence of this interaction is, however, com-
plicated by the fact that the 2-aminopyrimidine moieties, employed
in this study, are asymmetric about the C1–C4 vector, Scheme 2.
Consequently, each bifunctional ligand, 1–3, has two different
ways of producing a self-complementary N–H · · · N/N · · · H–N
motif. If the hydrogen bonds between adjacent 2-aminopyrimidine
moieties serve to maximize the through-space separation between
metal ions, Scheme 4a, the result is a diverging assembly, whereas if
the N–H · · · N/N · · · H–N synthon brings the complex ions closer
together, Scheme 4b, the assembly is converging.
Scheme 5 Possible cis or trans isomers based on the arrangement of the
pyridine nitrogen atom and the 2-amino substituent.
Steric considerations dictate that only one of the two arrange-
ments is possible in any given structure but, from a supramolecular
synthetic viewpoint, it would be of interest to establish whether
or not a suitable molecular substitution can provide a tool for
guiding this particular assembly process in a rational manner.
The two crystal structures 9 and 10 display the same converging
motifs, Scheme 4b, due to the fact that the OMe substituent
blocks the adjacent cyclic nitrogen atom, and thus prevents it
from participating as an acceptor in a self-complementary N–
H · · · N/N · · · H–N motif.
The two crystal structures 7 and 11 posses diverging motifs,
Scheme 4a, and in both cases the 2-amino moiety is trans to the
pyridyl nitrogen. The methyl substituent present on the pyrimidine
ring does not seem large enough to control the relative interplanar
orientation in the bifunctional ligands and it is not clear if a specific
interaction is responsible for the observed orientation. It is likely
to be a combination of factors that are responsible for the details
of the direction of the N–H · · · N dimer.
The crystal structure of 8 provides a good example of the
difficulties that can be encountered when trying to establish
clear structural patterns and/or intermolecular preferences. In
this structure there are two crystallographically unique metal-
containing building blocks: one with a cis, and one with a trans
orientation of the 2-amino functionality with respect to the pyridyl
moiety. The former ligand gives rise to a converging assembly,
whereas the latter produces a divergent motif.
The principle supramolecular target for this study, Cu(II)-
containing 1-D assemblies was realized in
a reasonable
supramolecular yield using a combination of new bifunctional
ligands and the well-known l-O,Oꢀ coordinating mode of the car-
boxylate functionality. In each case, we successfully controlled the
coordination geometry around the often unpredictable Cu(II) ion.
We were quite successful, five of seven structures, in connecting
neighboring complex-ions into infinite 1-D chains using ligand–
ligand N–H · · · N/N · · · H-N hydrogen bonds. The exact nature of
the assembly, convergent or divergent, is determined by the relative
orientation of the two aromatic components of our bifunctional
Another variable that needs to be taken into consideration is the
fact that the pyridyl moiety and the 2-amino substituent can be
arranged either cis or trans with respect to each other, Scheme 5.
Scheme 4 Pictorial description of converging or diverging structural motifs.
1634 | Dalton Trans., 2006, 1627–1635 This journal is The Royal Society of Chemistry 2006
©

View Article Online
ligands, which, in turn, can be represented by the rather soft
potential energy curve typically associated with aryl–aryl torsion
angles.
In a sense, our supramolecular success mirrors quite closely the
relative strengths of the interactions that are responsible for the
different recognition and binding events that take place in the
course of these reactions: metal–ligand (100%), hydrogen-bonds
(71%) and van der Waals forces (50%).
We are beginning to gain a better understanding of how
to design supramolecular ligands and synthetic strategies in
such a way as to minimize unwanted structural interference.
However, in order to make more, and more rapid, progress we
will require further systematic studies that clearly represent the
overall supramolecular outcome, success and failure, with respect
to clearly delineated assembly strategies.
F. J. Martinez-Martinez, H. Hoepfl and I. I. Padilla-Martinez, Cryst.
Growth Des., 2003, 3, 35; (d) A. J. Blake, N. R. Champness, A. N.
Khlobystov, D. A. Lemenovskii, W.-S. Li and M. Schroder, Chem.
Commun., 1997, 15, 1339; (e) A. B. Oren, M. D. Curtis and J. W.
Kampf, Chem. Mater., 2000, 12, 1519.
6 (a) C. B. Aakero¨y, A. M. Beatty and K. R. Lorimer, J. Chem. Soc.,
Dalton Trans., 2000, 21, 3869; (b) C. B. Aakero¨y, A. M. Beatty, D. S.
Leinen and K. R. Lorimer, Chem. Commun., 2000, 11, 935; (c) C. B.
Aakero¨y, A. M. Beatty, J. Desper, M. O’Shea and J. Valde´s-Mart´ınez,
Dalton Trans., 2003, 20, 3956; (d) D. Bose, J. Banerjee, S. H. Rahaman,
G. Mostafa, H.-K. Fun, R. D. Bailey Walsh, M. J. Zaworotko and B. K.
Ghosh, Polyhedron, 2004, 23, 2045; (e) T. J. Burchell, D. J. Eisler and
R. J. Puddephatt, Inorg. Chem., 2004, 43, 5550.
7 (a) D. W. Thompson, On Growth and Form, Cambridge University
Press, Cambridge, 1942; (b) S. J. Mann, J. Chem. Soc., Dalton Trans.,
1997, 21, 3953; (c) M. Li and S. Mann, J. Mater. Chem., 2004, 14, 2260;
(d) S. Mann, Chem. Commun., 2004, 1, 1.
8 (a) P. Wu, M. Bhamidipati, M. Coles and D. V. G. L. N. Roa, Chem.
Phys. Lett., 2004, 400, 506; (b) B. J. Kooi, J. Th and M. De Hosson,
J. Appl. Phys., 2004, 95, 4714; (c) H. Okumura, D. J. Twisselmann,
R. D. McMichael, M. Q. Huang, Y. N. Hsu, D. E. Laughlin and M. E.
McHenry, J. Appl. Phys., 2003, 93, 6528.
Acknowledgements
9 C. B. Aakero¨y, N. Schultheiss and J. Desper, Inorg. Chem., 2005, 44,
4983.
Financial support from NSF (CHE-0316479) and Kansas State
University is gratefully acknowledged. We also thank Takeo
Iwamoto for the ESI-IT-MS data collections.
10 A single sp-carbon is missing from the 13C NMR spectra.
11 Compound 7: Data were collected on a SMART APEX at 100 K. The
copper complex sits on a crystallographic inversion center. Coordinates
for the amine hydrogens, H12A and H12B, were allowed to refine.
Compound 8: Data were collected on a SMART 1000 at 100 K. The
lattice contains two independent copper complexes, each sitting on a
crystallographic inversion center. Two of the four 2-fluorobenzoates
existed in two nearly superimposable rotamers, differing in the relative
placement of the fluorine substituent. Coordinates for the amine
hydrogens, H22A, H22B, H62A, and H62B, were allowed to refine.
Compound 9: Data were collected on a SMART 1000 at 100 K. The
copper complex sits on a crystallographic inversion center. Coordinates
for the amine hydrogens, H12A and H12B, were allowed to refine.
Compound 10: Data were collected on a SMART 1000 at 100 K. The
copper complex sits on a crystallographic inversion center. Both
2-fluorobenzoates existed in two nearly superimposable rotamers,
differing in the relative placement of the fluorine substituent. The two
amine hydrogens, H22A and H22B, were placed in idealized positions
and were allowed to ride. Compound 11: Data were collected on a
SMART APEX at 100 K. The copper complex sits on a crystallographic
inversion center. The asymmetric unit contains a nearly superimposed
disordered solvent pair of ethanol and acetonitrile. Occupancies for
these two species were allowed to refine during initial model refinement
and were fixed at 0.3 and 0.4 for final cycles. Coordinates for the amine
hydrogens, H12A and H12B, were allowed to refine. Compound 12:
Data were collected on a SMART 1000 at 100 K. The copper complex
sits on a crystallographic inversion center. Both 2-fluorobenzoates
existed in two nearly superimposable rotamers, differing in the relative
placement of the fluorine substituent. The (nonsymmetric) acetylene
molecule sits on a crystallographic inversion center, with the 2-amino
substituent having 50% site occupancy. The amino hydrogens were
located in calculated positions and were allowed to ride. Compound
13: Data were collected on a SMART APEX at 100 K. The copper
complex sits on a crystallographic inversion center. The (nonsymmetric)
acetylene molecule sits on a crystallographic inversion center, with the
2-amino substituent having 50% site occupancy. The amino hydrogens
were located incalculated positions and were allowed to ride.
12 SMART v5.060, Bruker Analytical X-ray Systems, Madison, WI, 1997–
1999.
References and notes
1 (a) K.-W. Chi, C. Addicott, A. M. Arif and P. J. Stang, J. Am. Chem.
Soc., 2004, 126, 16569; (b) B. Chatterjee, J. C. Noveron, M. J. E.
Resendiz, J. Liu, T. Yamamoto, D. Parker, M. Cinke, C. V. Nguyen,
A. M. Arif and P. J. Stang, J. Am. Chem. Soc., 2004, 126, 10645; (c) K.
Nakabayashi, M. Kawano, M. Yoshizawa, S. Ohkoshi and M. Fujita,
J. Am. Chem. Soc., 2004, 126, 16694.
2 (a) M. Yoshizawa, S. Miyagi, M. Kawano, K. Ishiguro and M. Fujita,
J. Am. Chem. Soc., 2004, 126, 9172; (b) M. Fujita, M. Tominaga, A.
Hori and B. Therrien, Acc. Chem. Res., 2005, 38, 369; (c) M. Yoshizawa,
Y. Takeyama, T. Okano and M. Fujita, J. Am. Chem. Soc., 2003, 125,
3243; (d) J. L. C. Rowsell and O. M. Yaghi, Microporous Mesoporous
Mater., 2004, 73, 3; (e) J. L. C. Rowsell, A. R. Millward, K. S. Park and
O. M. Yaghi, J. Am. Chem. Soc., 2004, 126, 5666; (f) J. L. Atwood, L. J.
Barbour, S. J. Dalgarno, M. J. Hardie, C. L. Raston and H. R. Webb,
J. Am. Chem. Soc., 2004, 126, 13170.
3 (a) R. J. Hill, D.-L. Long, N. R. Champness, P. Hubberstey and M.
Schroder, Acc. Chem. Res., 2005, 38, 335; (b) K. Campbell, C. J. Kuehl,
M. J. Ferguson, P. J. Stang and R. R. Tykwinski, J. Am. Chem. Soc.,
2002, 124, 7266; (c) D. Braga, S. L. Giaffreda, F. Grepioni and M.
Polito, CrystEngComm, 2004, 6, 458–462; (d) J. F. Berry, F. A. Cotton,
C. S. Fewox, T. Lu, C. A. Murillo and X. Wang, Dalton Trans., 2004, 15,
2297; (e) G. J. McManus, Z. Wang and M. J. Zaworotko, Cryst. Growth
Des., 2004, 4, 11; (f) D. Braga, L. Brammer and N. R. Champness,
CrystEngComm, 2005, 7, 1; (g) N. Schultheiss, C. L. Barnes and E.
Bosch, Cryst. Growth Des., 2003, 3, 573; (h) M. Oh, C. L. Stern and
C. A. Mirkin, Inorg. Chem., 2005, 44, 2647.
4 (a) Supramolecular Assembly via Hydrogen Bonds II. Structure and
Bonding, ed. D. M. P. Mingos, Springer-Verlag, Berlin, Heidelberg,
New York, 2004; (b) G. R. Desiraju, Acc. Chem. Res., 1996, 29, 441;
(c) C. B. Aakeroy, A. M. Beatty and B. A. Helfrich, J. Am. Chem. Soc.,
2002, 124, 14425; (d) F. M. Raymo, M. D. Bartberger, K. N. Houk and
J. F. Stoddart, J. Am. Chem. Soc., 2001, 123, 9264; (e) J.-H. Fournier,
T. Maris, J. D. Wuest, W. Guo and E. Galoppini, J. Am. Chem. Soc.,
2003, 125, 1002; (f) D. Braga, M. Polito, D. D’Addario and F. Grepioni,
Cryst. Growth Des., 2004, 4, 1109.
13 SAINT v6.02, Bruker Analytical X-ray Systems, Madison, WI, 1997–
1999.
14 SHELXTL v5.10, Bruker Analytical X-ray Systems, Madison, WI,
1997.
5 (a) O. Ohmori, M. Kawano and M. Fujita, CrystEngComm, 2005,
7, 255; (b) A. Sundararaman, L. N. Zakharov, A. L. Rheingold and
F. Jaekle, Chem. Commun., 2005, 13, 1708; (c) E. V. Garcia-Baez,
15 K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron Lett., 1975,
16, 4467.
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 1627–1635 | 1635
©