Synthesis and characterization of one, two and three-dimensional Cu(I) polymers supported by bipyridylamide ligands

Article abstract of DOI:10.1016/j.poly.2018.07.032

Reaction of bipyridylamide ligands N-(3-pyridyl)nicotinamide (3-pna), N-(4-pyridyl)nicotinamide (4-pna) and N-(4-pyridyl)isonicotinamide (4-pina) with Cu(I) salts afforded four coordination polymers. X-ray crystallography analysis showed one-, two- and three-dimensional networks. The correlation between the pyridyl nitrogen donor disposition and dimensionality of the structures was discussed. Reflectance UV–Vis and fluorescence spectroscopic methods were used to characterize the MLCT features of these complexes. Thermal stability of the coordination polymers under nitrogen was also investigated.

Full text of DOI:10.1016/j.poly.2018.07.032

Accepted Manuscript
Synthesis and Characterization of One, Two and Three-dimensional Cu(I) Pol-
ymers Supported by Bipyridylamide Ligands
Clement Mugenzi, Douglas R. Powell, Nikolay N. Gerasimchuk, Lei Yang
PII:
S0277-5387(18)30426-1
https://doi.org/10.1016/j.poly.2018.07.032
POLY 13297
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
10 May 2018
3 July 2018
23 July 2018
Please cite this article as: C. Mugenzi, D.R. Powell, N.N. Gerasimchuk, L. Yang, Synthesis and Characterization
of One, Two and Three-dimensional Cu(I) Polymers Supported by Bipyridylamide Ligands, Polyhedron (2018),
doi: https://doi.org/10.1016/j.poly.2018.07.032
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis and Characterization of One, Two and Three-dimensional
Cu(I) Polymers Supported by Bipyridylamide Ligands
Clement Mugenzi,a Douglas R. Powell,b Nikolay N. Gerasimchuk,c Lei Yanga*
a Department of Chemistry, University of Central Arkansas, Conway, AR 72035, USA
d Department of Chemistry and Biochemistry, University of Oklahoma, Norman, OK, 73019,
USA
c Department of Chemistry, Missouri State University, Springfield, MO 65897, USA
*Corresponding author. E-mail: lyang@uca.edu. Fax: +1(501)4503623
Abstract:
Reaction of bipyridylamide ligands N-(3-pyridyl)nicotinamide (3-pna), N-(4-
pyridyl)nicotinamide (4-pna) and N-(4-pyridyl)isonicotinamide (4-pina) with Cu(I) salts afforded
four coordination polymers. X-ray crystallography analysis showed one-, two- and three-
dimensional networks. The correlation between the pyridyl nitrogen donor disposition and
dimensionality of the structures was discussed. Reflectance UV-vis and fluorescence
spectroscopic methods were used to characterize the MLCT features of these complexes.
Thermal stability of the coordination polymers under nitrogen was also investigated.
Keywords:
Keywords: Copper(I) coordination polymer, Crystal structure, n  π* interaction,
Spectroscopic characterization, TGA
1. Introduction
Transition metal coordination polymers supported by organic ligands with amide groups have
attracted significant attention due to their potential applications as absorbents [1], catalysts [2],
luminescent sensors [3], magnetic materials [4] and gas separation/storage materials [5]. Based
on these reported works, it has been known that ligand conformation, length, donor orientation
and synthetic conditions are key factors contributing to the structural diversity and functionality
of polymeric networks. For instance, by fine-tuning the ligand donor disposition, polymeric
architectures featuring different dimensionality and pore size were prepared [6]. Over the past
few years, we have focused on design and synthesis of coordination polymers containing
1

pyridylamide ligands. Our previous work has demonstrated that pyridylamide ligands can display
versatile coordination modes under different reaction conditions [7]. Meanwhile, the N–H and
C=O groups on pyridylamide ligands may introduce hydrogen bonding interactions, conferring
interesting supramolecular networks to the polymers. In addition, disposition of pyridyl nitrogen
donors and flexibility of the amide groups are quite essential for altering the architectural
topologies [8]. In this work, we report the synthesis of four Cu(I) coordination polymers with
bipyridyl amide ligands N-(3-pyridyl)nicotinamide (3-pna), N-(4-pyridyl)nicotinamide (4-pna)
and N-(4-pyridyl)isonicotinamide (4-pina) (Scheme 1). Cu(I) was chosen because of its strong
preference towards tetrahedral geometry, so the resulting coordination polymers can be better
compared and analyzed based on the ligating topology difference of the ligands. In our effort to
better understand the function of the amide groups, we also used 1,2-bis(4-pyridyl) ethane (4-bpe)
to construct a control polymer. These Cu(I) polymers with one-, two- and three dimensional
networks were characterized by X-ray crystallography, FT-IR, reflectance UV-vis,
photoluminescence and TGA.
Scheme 1. Synthesis of the bipyridylamide ligands used in this work.
2

2. Experimental
2.1. Physical measurements
Elemental analyses were carried out by Atlantic Microlabs, Norcross, GA. FT-IR spectra were
collected on a Nicolet Magna 560 FT-IR spectrometer with an ATR attachment. Reflectance
UV-vis spectra were recorded on a Cary 50 spectrometer. Photoluminescence measurement was
taken with Perkin-Elmer model LS 45 luminescence spectrometer. TGA analysis was performed
on TA Instrument STD Q600 in an alumina crucible with a nitrogen flow of 100  5 mL/min and
a heating rate of 10°C/min. The crucible prior to each experiment was calcined using a propane
torch. Data of TGA measurements were processed using the TA Universal Analysis software
package.
2.2. Chemicals
All reagents were obtained from commercial sources and used as received unless otherwise
noted. Ligands N-(3-pyridyl)nicotinamide (3-pna), N-(4-pyridyl)nicotinamide (4-pna) and N-(4-
pyridyl)isonicotinamide (4-pina) were synthesized according to literature [9]. 1,2-bis(4-pyridyl)
ethane (4-bpe) was purchased from Acros.
2.3. Synthesis of Cu(I) polymers
{[Cu(3-pna)2]SbF6}n (1) A solution of [Cu(NCCH3)4]SbF6 (0.116 g, 0.25 mmol) in 1 mL
DMF was added to a solution of 3-pna (0.050 g, 0.50 mmol) in 1 mL DMF. The yellow solution
was stirred for 4.0 h and 20 mL of Et2O was added to the mixture. The resulting yellow powder
was washed with Et2O (5 mL × 3). The product was dissolved in DMF and the solution was
filtered through a celite plug. Vapor diffusion of Et2O into the clear yellow filtrate at room
temperature led to the formation of yellow crystals suitable for X-ray crystallographic
characterization (0.106 g, 60.8% yield). Anal. Calc. for C22H18CuF6N6O2Sb: C, 37.87; H, 2.60; N,
12.05. Found: C, 37.61; H, 2.76; N, 11.87. FT-IR (cm-1): 1659, 1606, 1585, 1541, 1485, 1423,
1336, 1298, 1191, 1121, 1034, 808, 731, 717, 694, 654, 637.
{[Cu(4-pna)2]SbF6}n (2) Complex 2 was synthesized through similar procedures as complex 1.
After Et2O wash, the raw product was dissolved in DMF and the resulting solution was filtered
through a celite plug. Vapor diffusion of Et2O into the clear yellow solution at room temperature
led to the formation of yellow crystals suitable for X-ray crystallographic characterization (0.094
3

g, 54.0% yield). Anal. Calc. for C22H18CuF6N6O2Sb: C, 37.87; H, 2.60; N, 12.05. Found: C,
37.54; H, 2.88; N, 11.71. FT-IR (cm-1): 1683, 1673, 1596, 1506, 1477, 1428, 1417, 1333, 1293,
1265, 1212, 1115, 1085, 1031, 1009, 898, 825, 725, 711, 693, 622, 587, 633, 471, 437.
{[Cu(4-pina)(CH3CN)2]SbF6}n (3) A solution of [Cu(NCCH3)4]SbF6 (0.116 g, 0.25 mmol) in
1 mL CH3CN was added to a solution of 4-pina (0.025 g, 0.25 mmol) in 1 mL CH3CN. The
yellow solution was stirred for 2 h and 20 mL of Et2O was added to the mixture. The resulting
yellow powder was washed with Et2O (5 mL × 3). The raw product was dissolved in CH3CN and
the resulting solution was filtered through a celite plug. Vapor diffusion of Et2O into the clear
yellow filtrate at room temperature led to the formation of yellow crystals suitable for X-ray
crystallographic characterization (0.128 g, 82.4% yield). Anal. Calc. for C15H15CuF6N5OSb: C,
31.03; H, 2.60; N, 12.06. Found: C, 30.85; H, 2.50; N, 12.33. FT-IR (cm-1): 1701, 1618, 1593,
1553, 1514, 1434, 1340, 1296, 1271, 1216, 1123, 1066, 904, 837, 761, 715, 662, 635, 537.
{[Cu2(4-pina)4](SbF6)2(H2O)}n (4) Complex 4 was synthesized through similar procedures as
complex 1 in DMF. After Et2O wash, the raw product was dissolved in DMF and the resulting
solution was filtered through a celite plug. Vapor diffusion of Et2O into the clear yellow solution
at room temperature led to the formation of deep red crystals suitable for X-ray crystallographic
characterization (0.116 g, 64.8% yield). Anal. Calc. for C44H36Cu2F12N12O4Sb2: C, 37.87; H,
2.60; N, 12.05. Found: C, 38.20; H, 2.35; N, 12.14. FT-IR (cm-1): 1691, 1666, 1592, 1508, 1421,
1333, 1293, 1293, 1210, 1115, 1063, 1013, 900, 830, 754, 692, 655, 638, 535.
{[Cu(4-bpe)2](SbF6)}n (5) Complex 5 was synthesized through similar procedures as complex
1 in DMF. After Et2O wash, the raw product was dissolved in DMF and the resulting solution
was filtered through a celite plug. Vapor diffusion of Et2O into the clear orange solution at room
temperature led to the formation of deep orange crystals suitable for X-ray crystallographic
characterization (0.108 g, 64.7% yield). Anal. Calc. for C24H24CuF6N4Sb: C, 43.17; H, 3.62; N,
8.39. Found: C, 43.56; H, 3.55; N, 8.40. FT-IR (cm-1): 1608, 1560, 1496, 1419, 1336, 1211, 1068,
1012, 828, 651, 629, 539.
2.4. X-ray crystallography
X-ray crystallographic data were collected on crystals with dimensions of 0.44 × 0.26 × 0.24
mm for 2, 0.42 × 0.38 × 0.20 mm for 3, 0.08 × 0.07 × 0.03 mm for 4, 0.36 × 0.25 × 0.20 mm for
5. Data were collected at 100 K using a diffractometer with a Bruker APEX CCD area detector
4

[10] and graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). All three structures were
solved by direct methods and refined by full-matrix least-square methods on F2 [11]. The crystal
parameters, data collection and refinement are summarized in Table S1. Non-hydrogen atoms
were refined with anisotropic displacement parameters, and the positions of hydrogen bonded to
carbons and nitrogens were initially determined by geometry and were refined using a riding
-
model. For complex 5, the SbF6 anions have severe disorder problem and therefore were best
modeled by using the squeeze problem [12]. The mass, absorption coefficients are shown
-
without SbF6 anions as required by Squeeze and Checkcif programs.
2.5. Hirshfeld surface analysis
In order to investigate the important intra- and intermolecular interactions occurring within
and among the coordination networks, the analysis of Hirshfeld surface was performed by using
CrystalExplorer (version 3.0) software [13]. The calculation was based on the CIF files from the
X-ray crystal structures of metal complexes. The two-dimensional fingerprint diagrams were
plotted from the Hirshfeld surface calculation to highlight the types of interaction and the
corresponding areas on the Hirshfeld surface.
3. Results and discussion
3.1. Crystal structures
Complex 1 was synthesized through the reaction of ligand 3-pna with [Cu(NCCH3)4]SbF6 in
DMF. Although Et2O diffusion recrystallization of the product led to the formation of light
yellow crystals, unfortunately X-ray crystallographic analysis only afforded an unpublishable
-
structure due to the serious disorder problems of SbF6 anions and solvent molecules. However,
interesting one-dimensional chain structure was still observed (Fig. S1). In the structure, each
Cu(I) center is supported by four 3-pyridyl nitrogen donors, two of which come from the amine
side of two 3-pna ligands, and the other two come from the carbonyl side of two 3-pna ligands.
The τ4 structural index is 0.92 [14], suggesting a tetrahedral geometry of the Cu(I) center. Each
3-pna ligand adopts a non-planar syn conformation, which has been observed in previously
reported metal complexes [15]. Two adjacent Cu(I) centers are connected by one 3-pna ligand
with a distance of 9.411 Å and the resulting 18-member macrocycle ring with two head-to-tail
ligands and two Cu(I) centers has a size of 7.582 Å × 3.369 Å. Connection of these macrocycle
5

units by the Cu(I) centers as the joints leads to the formation of an infinite 1-D polymeric chain
on the crystallographic z-axis. Although the amide groups (dihedral angle of O=C–N–C = 1.82°)
are almost coplanar to the pyridyl ring on the amine side, the large torsion angle between two
pyridyl groups (55.45°) demonstrates how ligand backbone twists itself to adapt the tetrahedral
coordination geometry of Cu(I) centers. This type of flexibility from bipyridyl amide ligands has
been observed in our previous work [8].
Through similar synthesis as complex 1, yellow crystals of complex 2 were obtained from
vapor diffusion of Et2O into the DMF solution of the product. X-ray crystallographic analysis
showed a 2-D polymeric grid structure on the xy crystallography plane (Fig. 1). The dimensional
difference between complexes 1 and 2 is attributed to the fact that the pyridyl nitrogen donor on
the amine side of ligand 4-pna sits on the symmetric position, leading to an angular ligating
topology with less degree of bending than ligand 3-pna. Accordingly, the four 4-pna ligands on
each Cu(I) center extends to four different Cu(I) centers to form an architecture with a higher
dimensionality. For each Cu(I) center, the τ4 structural index is 0.88, suggesting a greater
deviation from a standard tetrahedral geometry. In order to adapt to this geometry, the two
pyridyl rings of ligand 4-pna has a better co-planar conformation (torsion angle = 29.02°). In our
previous reported 1-D divalent metal (Zn2+ and Cu2+) coordination polymers supported by 4-pna,
the ligands display larger torsion angles (33.97° and 48.82°) possibly due to a lower
dimensionality [8a]. The amide group itself shows a dihedral angle of 15.34° that is much larger
than complex 1. This greater twist could be a result of inter-sheet hydrogen bonding interactions
involving the amine protons and carbonyl oxygens. The shape of the pore is rhombic with an
approximate dimension of 12.2 × 12.2 Å.
6

Fig. 1. X-ray crystal structure of complex 2. Thermal ellipsoids are shown at the 30% level. Most
-
hydrogens and all of the SbF6 anions are omitted for clarity.
-
A noteworthy feature of complex 2 is the interactions between SbF6 anions and the
coordination framework. Three types of weak interactions can be observed from the X-ray
crystal structure (Fig. 2): (a) FH hydrogen bonding interactions between fluorine atoms and C–
H groups on pyridyl rings (black dashed lines); (b) Flone pairπaromatic interactions between
fluorine atoms and 3-pyridyl rings (blue dashed lines); (c) Flone pair π*C=O interactions between
fluorine atoms and amide carbonyl groups (red dashed lines). For type (a), the average FH
distance is 2.573 Å, suggesting weak hydrogen bonding interactions. The type (b) interaction is a
-
typical anion-π interaction between the SbF6 and the aromatic pyridyl ring. The measured F
πaromatic centroid distance (3.466 Å) is slightly longer than the average F πaromatic centroid
distance from previously reported examples (3.220 Å), but still shorter than the required distance
(4 Å) for anion-π interactions [16]. The type (c) interaction is particularly interesting because it is
a hybrid of anion-π interaction and n  π* interaction. In conventional anion-π interactions, the
donor is the lone pair(s) of an anion and the acceptor usually is an aromatic π system [17]. For
the n  π* interaction, it is a newly recognized non-covalent interaction between a neutral
7

nucleophile (donor) and the π-hole of a nearby carbonyl group (acceptor) [18]. According to the
-
crystal structure of complex 2, the donor fluorine atoms with lone pairs are from SbF6 anions,
and the acceptor carbons are from the neutral carbonyl groups of ligands. Therefore by definition,
the interactions should be anion  π* interactions. Since the lone pair(s) from anions essentially
are nucleophile, so even if the type (c) interaction is not a typical n  π* interaction, we think
it’s more appropriate to analyze its nature according to Bürgi-Dunitz trajectory model (Scheme 2)
[19]. First, this interaction is evidenced by the FCC=O distance (d) of 2.907 Å, which is smaller
than the sum of the van der Waals radii of carbon and fluorine (3.17 Å) [16]. In addition, the
Bürgi-Dunitz angle of FC=O (θ) is 100.37, which is within the 109 ± 10 requirement [20].
Although the pyramidalization of the carbonyl group from planarity (distance Δ and angle ) is
often a signature structure deviation in biomolecules and organic molecules such as proteins and
peptoids with n  π* interactions, the Δ and measured in complex 2 are only 0.004 Å and
5.18, respectively. Such a small deviation might be a result of the bridging function of ligand 4-
pna, in which the two Cu(I) centers stretch the ligand from the opposite directions, causing the
higher rigidity of the amide group than those in non-coordinated molecules.
Fig. 2. Three types of weak interactions observed from the X-ray crystal structure of complex 2:
(a) FH hydrogen bonding interactions (black dashed lines); (b) Flone pairπaromatic interactions
8

(blue dashed lines); (c) Flone pairπ*C=O (non-conventional n  π*) interactions (red dashed
lines).
Scheme 2. Geometric parameters in Bürgi-Dunitz trajectory interaction.  is the Nu-C=O
angle and  is the angle between R–R’–C plane and C=O. Both  and  indicate the extent of
pyramidalization.
The aforementioned Flone pairπ*C=O interaction in complex 2 is also supported by Hirshfeld
-
surface analysis. As shown in Fig. 3, the Hirshfeld surface of SbF6 anion mapped with dnorm has
a prominent red area (highlighted by a blue circle) between one of the fluorine atoms and the
carbonyl carbon, suggesting the presence of a close contact on the two sides of the surface. The
analysis of the corresponding 2-D fingerprint plot indicates the percentage of the surface taken
by F CC=O interaction is 18.7% (Fig. S2). Since the emerging of n  π* interactions, a few
examples involving small metal complexes have been reported [21]. But to our best knowledge,
there are still no reported examples of this interaction in coordination polymers.
9

-
Fig. 3. Hirshfeld surface of SbF6 anion mapped with dnorm function in complex 2. Hydrogens are
omitted for clarity.
Based on the X-ray crystal structures of complexes 1 and 2, the disposition of the pyridyl
nitrogen donors is the major reason for the dimensionality difference of the two polymers.
However, the conformation flexibility arising from the amide group also makes a significant
contribution. Fig. 4 is the superposed 3-pna (blue, complex 1) and 4-pna (red, complex 2) ligands
aligned with the 3-pyridyl rings. The two Cu–N1 bonds are nearly overlapped, but the amide
group twist starting from C4–C6 bond significantly changes the ligand conformations. Duo to the
rotation of C4–C6 bond, the Cu–N1 bond in complex 1 adapts a cis conformation with the N–H
group, and a trans conformation with the C=O group in 3-pna ligand. In contrast, the Cu–N1
bond in complex 2 adapts a trans conformation with the N–H group, and a cis conformation with
the C=O group in 4-pna ligand. As a result, the bent ligating angle of ligand 3-pna is much
smaller than that of ligand 4-pna, leading to the lower dimensionality of complex 1.
10

Fig. 4. Superposition of 3-pna in complex 1 (blue) and 4-pna in complex 2 (red).
In our effort to further probe the effect of pyridyl N disposition and ligand angular
coordination conformation on the resulting polymeric structures, ligand 4-pina was employed in
the preparation of Cu(I) polymers. When the reaction ratio of 4-pina and [Cu(NCCH3)4]SbF6 is
1:1, yellow crystals of complex 3 were obtained from the Et2O vapor diffusion into the CH3CN
solution of the product. X-ray crystallography revealed a 1-D polymeric chain structure depicted
in Fig. 5. The tetrahedral geometry of each Cu(I) center (τ4 = 0.90) is fulfilled by two N4-py
donors and two Nacetonitrile donors with an average Cu–N distance of 2.001 Å. Each 4-pina ligand
bridges two Cu(I) atoms in a linear fashion, resulting in the formation of a zigzag chain with a
neighboring CuCu distance of 13.23 Å. The torsion angle between the two 4-pyridyl rings is
17.3, suggesting a better coplanar conformation of the ligand backbone than 3-pna and 4-pna in
complexes 1 and 2. A similar coplanar conformation was also observed in our previous work, in
which the divalent Fe(II) and Zn(II) centers are connected by 4-pina ligands in a linear fashion to
form nearly straight 1-D polymers [8a]. In fact, a CCDC (Cambridge Crystallographic Data
Centre) search of reported coordination polymers involving 4-pina ligands shows that usually the
two pyridyl rings of 4-pina possess a coplanar arrangement with small torsion angles, but some
examples with large torsion angles were also observed [5d, 22]. The N–H group of the amide
linker interacts with one lattice CH3CN molecule through a strong hydrogen bond with a
-
HNacetonitrile distance of 2.251 Å (Fig. S3). Two SbF6 anions interact with ligands differently:
One has one of its F atoms interact with two C–H groups on a nearby pyridyl ring through two
-
hydrogen bonding interactions with an average HF distance of 2.68 Å, and the other SbF6
11

anion connects two neighboring zigzag polymeric chains by hydrogen bonds to form a ladder-
like supramolecular structure (Fig. S3).
Fig. 5. X-ray crystal structure of complex 3. Thermal ellipsoids are shown at the 30% level. Most
-
hydrogens and all of the SbF6 anions are omitted for clarity.
Inspired by the 1-D chain structure of complex 3, we further investigated the effect of reaction
ratio on the dimensionality of polymeric structures based on an assumption that the two labile
CH3CN ligands in complex 3 could be replaced if more 4-pina ligands are available. Indeed, the
addition of two equivalents 4-pina to one equivalent [Cu(NCCH3)4]SbF6 led to a distinctive red
color change. The recrystallization of the product generated deep red crystals and X-ray
crystallography analysis revealed a 3-D architecture (Fig. 6a). As we expected, the tetrahedral
geometry of each Cu(I) center is now occupied by four pyridyl nitrogen donors from four 4-pina
ligands (τ4 = 0.82) with an average Cu–N distance of 2.026 Å. The four bridging 4-pina ligands
then coordinate to four other Cu centers with a neighboring CuCu distance of 13.167 Å. For
each ligand, the two pyridyl rings show a co-planar conformation with a torsion angle of 5.323.
The rhombus-shaped open tunnels (19.759 Å × 8.580 Å) in the architecture are occupied by
-
SbF6 anions that interact with the ligands through C–HF hydrogen bonding. Further structural
analysis revealed a three-fold interpenetration phenomenon among three independent and
identical 3-D networks described above (Fig. 6b). This feature is attributed to the strong
hydrogen bonding interactions among the amide groups (green dashed lines in Fig. 6a). For the
4-pina ligand on each 3-D network, the N–H group interacts with the C=O group from the
12

second 3-D network through a HO distance of 2.017 Å, while the C=O groups interact with the
N–H groups from the third network on the opposite direction with the HO distance of 2.093 Å.
Fig. 6. (a) 3-D network of complex 4. Thermal ellipsoids are shown at the 30% level. Most
-
hydrogens and all of the SbF6 anions are omitted for clarity. Inter-network hydrogen bonding
interactions are shown as green dashed lines. (b) Three-fold interpenetration (red, blue and black)
of complex 4.
In order to further understand how the amide group of ligand 4-pina affects the topology of the
resulting structure, ligand 1,2-bis(4-pyridyl) ethane (4-bpe) was employed in our work. The two
4-pyridyl groups of 4-bpe ligand can afford a similar linear coordination conformation as 4-pina
ligand, but the ethylene group of 4-bpe ligand will eliminate the possibility of inter-network
hydrogen bonding observed in complex 4. We hope the structural comparison between the
networks supported by 4-bpe and complex 4 can shed some light on the function of organic
ligands in the controllable construction of coordination polymers. The reaction of two
equivalents 4-bpe and one equivalent [Cu(NCCH3)4]SbF6 led to the formation of a similar 3-D
network (complex 5) as observed in complex 4, in which the tetrahedral Cu(I) centers act as
nodes and 4-bpe ligands act as organic struts (Fig. 7). Each 4-bpe ligand connects two Cu(I)
centers in a linear fashion, leading to a CuCu distance of 13.256 Å, which is slightly longer
than the 13.167 Å in complex 4. However, several significant structural differences are also
observed in complex 5: (a) each Cu(I) center has a better tetrahedral geometry with a τ4 index of
0.98; (b) the open tunnel formed in complex 5 has a square shape with a larger size of 9.683 Å ×
9.683 Å; (c) the 4-bpe ligand has a stair like conformation with a trans ethylene group, and the
two 4-pyridyl rings perfectly parallel to each other without any torsion; (d) a five-fold
interpenetration topology in contrast to the three-fold in complex 4 (Fig. 7b). The differences
13

(a)-(c) could be at least partially attributed to the fact that the ethylene group with a trans
conformation is more compressible, so ligand 4-bpe has a greater flexibility to adjust its pyridyl
nitrogen donors to satisfy the Cu(I) centers preferring a tetrahedral geometry. For difference (d),
we believe the major reason is lack of hydrogen bond donors/acceptors on ligand 4-dpe. In order
to form the inter-network hydrogen bonding among the amide groups of 4-pina in complex 4, the
3-D networks have to arrange in a ABCABC fashion that significantly limited the degree of
interpenetration. While in complex 5, the lack of inter-network hydrogen bonding and the better
tetrahedral geometry of Cu(I) centers expands the tunnel size of each individual 3-D network,
allowing a higher degree of interpenetration. This is supported by a reported 3-D Cu(I) network
with 4,4’-bipyridine ligands, in which the τ4 index of Cu(I) center is 0.91 and a 4-fold
interpenetration topology is observed [23]. The structural analysis on complexes 4 and 5 clearly
demonstrated that the functional groups on organic ligands are capable of altering the structure
and topology of networks, and therefore could serve as an approach for the rational design and
synthesis of coordination polymers.
Fig. 7. (a) 3-D network of complex 5. Thermal ellipsoids are shown at the 30% level. Hydrogens
are omitted for clarity. (b) Five-fold interpenetration (red, blue, green, purple and black) of
complex 5.
14

Table 1. Selected bond length (Å) and bond angles () of complexes 2, 3, 4 and 5.
Complex 2
Cu1–N1
Cu1–N2
Cu1–N3
Cu1–N4
Complex 3
Cu1–N1
Cu1–N2
Cu1–N3
Cu1–N4
Complex 4
Cu1–N1
Cu1–N2
Cu1–N3
Cu1–N4
Complex 5
Cu1–N1
Cu1–N2
Cu1–N3
Cu1–N4
2.0309(11)
2.0309(11)
2.0956(12)
2.0956(12)
1.905(9)
1.967(9)
2.066(6)
2.066(6)
1.996(10)
2.015(10)
2.053(11)
2.069(10)
2.033(4)
2.033(4)
2.033(4)
2.033(4)
N1–Cu1–N2 118.56(7)
N1–Cu1–N3 117.69(5)
N1–Cu1–N4 103.76(5)
N2–Cu1–N3 103.76(5)
N2–Cu1–N4 117.69(5)
N3–Cu1–N4 93.12(7)
N1–Cu1–N2 116.2(4)
N1–Cu1–N3 116.6(2)
N1–Cu1–N4 105.3(2)
N2–Cu1–N3 116.6(2)
N2–Cu1–N4 105.3(2)
N3–Cu1–N4 94.0(3)
N1–Cu1–N2 132.4(5)
N1–Cu1–N3 106.4(5)
N1–Cu1–N4 99.8(4)
N2–Cu1–N3 98.8(4)
N2–Cu1–N4 105.2(5)
N3–Cu1–N4 114.9(5)
N1–Cu1–N2 106.3(2)
N1–Cu1–N3 111.07(11)
N1–Cu1–N4 111.07(11)
N2–Cu1–N3 111.07(11)
N2–Cu1–N4 111.07(11)
N3–Cu1–N4 106.3(2)
The superposed images of ligands 4-pina and 4-bpe from complexes 3-5 shown in Fig. 8
further show us how ligand coordination conformation is associated with its flexibility. From Fig.
8a, it can be clearly seen that the linear ligating direction difference of 4-pina ligands in
complexes 3 (black) and 4 (purple) starts from C3–C6 bond, leading to a more bending
conformation of 4-pina in 4 (Cu–N1 and Cu–N3 angle = 18) than 3 (Cu–N1 and Cu–N3 angle =
10). We believe this difference is a result of the different coordination environments/geometries
of the Cu(I) centers in the two polymers ((Npy)2(Nacetontrile)2 in 3 with a τ4 = 0.90 and (Npy)4 in 4
with a τ4 = 0.82). For the 4-bpe ligand in complex 5 (green), the spatial orientation of C6-C7
bond apparently differs from the C6-N2 bond of 4-pina ligand, resulting in a better linear
conformation of 4-bpe. The conformation difference of the three ligands can be better observed
from the side view of the superposed images (Fig. 8b). Although the trans fashion of Cu–N1 and
Cu–N3 bonds in 3 and 4 orientates very differently, the ligand 4-pina adapts a nearly co-planar
conformation in both polymers. In contrast, the 4-bpe ligand in 5 possesses a stair-shaped
arrangement with two parallel pyridyl groups (vertical distance = 1.430 Å), leading to a better
180 linear coordination conformation.
15

Fig. 8. Superposition of 4-pina and 4-bpe ligands in complexes 3 (black), 4 (purple), and 5
(green) with top view (a) and side view (b).
3.2. Reflectance UV-vis spectra
Fig. 9. Reflectance UV-vis spectra of complexes 1-5.
The reflectance UV-vis spectra of the five complexes were collected and shown in Fig. 9.
Complexes 1, 2 and 5 show the similar charge transfer (CT) transition bands around 335 nm,
16

leading to similar orange colors of the compounds. The CT band of complex 3 blue shifts to 315
nm, presumably due to the coordination of acetonitrile molecules to the Cu(I) centers. The
weaker π-acceptor property of acetonitrile could enlarge the energy gap between the Cu(dπ) and
ligand π* orbitals, contributing to the absorption at a lower wavelength [24]. Complex 4 has a
distinctive dark red color that corresponds to a broad CT band centered at ~400 nm. It has been
reported that [Cu(I)(4-Ac-py)4]+ gives a smaller MLCT (metal to ligand charge transfer) energy
due to the conjugated nature of Ac group that can interfere with the π and π* orbitals of pyridine
[24,25]. We believe the amide group between the two 4-pyridyl rings of 4-pina has a similar
effect when four 4-pina ligands coordinate to one Cu(I) center. This is supported by the
observation of the CT band of complex 5 at 335 nm, in which the ligand 4-bpe has a non-
conjugate ethylene group on the para position of the pyridyl nitrogen donor. The
photoluminescence properties of all five complexes were also investigated in the solid state at
room temperature. However, only complex 4 showed well-defined emission peaks at 527 and
585 nm upon photoexcitation at 390 nm (Fig. 10). The feature could be tentatively assigned to
MLCT luminescent emission, as ligand 4-pina didn’t show any features with the same excitation
wavelength.
Fig. 10. Solid state emission spectra of complex 4 at room temperature.
17

3.3. Thermal analysis
The thermal stability of five complexes was investigated by thermal gravimetric analysis
(TGA). Complexes 1-3 showed very similar decomposition curves from room temperature to
1400 C (Fig. 11). When temperature increases to ~275 C, the polymeric frameworks start
collapse. The dominant stage of the weight loss represents the loss of organic ligands. The final
decomposition residue of complex 1 contains copper metal droplets (Fig. S4), which is quite
different from the inorganic metal salts residue from the thermal decomposition of complexes 2-
3. This corresponds to the lower weight of complex 1 curve (12.2%) at the end of the heating
than both complexes 2 (25.3%) and 3 (27.9%). For complex 4, a two-stage weight loss pattern
was observed. The first stage starts between 137~159 C, suggesting the loss of lattice DMF
-
molecules. The sharp drop from 300 to 394 C represents loss of ligand 4-pina and SbF6 anions.
The decomposition of complex 5 starts at 260 C and the major mass loss (~76%) ends around
400 C. Although it is believed that a higher degree of interpenetration usually is a positive
contributor to the thermal stability of structures [26], the nearly identical decomposition behavior
of both complexes 4 and 5 suggest interpenetration is the not the only factor governing thermal
stability. Early research has indicated van der Waals interaction such as ππ could also
contribute to the stability and function of the framework [27]. In our case, we believe the inter-
network hydrogen bonding interactions lock the individual 3-D networks in complex 4, which
may increase the rigidity of the structure to resist the thermal decomposition.
18

Fig. 11. TGA curves of complexes 1–5.
4. Conclusions
Cu(I) coordination polymers supported by bipyridylamide ligands were synthesized and
characterized. The tuning of pyridyl nitrogen donor position verse the amide group led to the
significant change of dimensionality. The angular ligating conformation of 3-pna and 4-pna
resulted in the formation of one- and two-dimensional polymeric structures, while the linear
ligating conformation of 4-pina permits the formation of a 3-D polymer with inter-network
hydrogen bonds. The connection between the functional groups of ligands and the degree of
interpenetration was investigated by structural analysis and comparison of two three-dimensional
networks supported by 4-pina and 4-bpe ligands. The reflectance UV-vis and fluorescence
characterization presented the MLCT features arising from the Cu(I) center and conjugated
pyridyl rings. TGA analysis demonstrated that these polymers are stable up to ~250 C. It also
appears that the inter-network weak interactions might be one of the factors that can enhance the
stability of polymeric structures. In the process of revealing ligating mode directing trends, our
work illustrated that ligands’ conformational flexibility and subtle adjustment of nitrogen donor
disposition can instill significant structural diversity of resulting polymers. In addition,
utilization of functional groups on ligands to provide access to control the stability, rigidity and
even photoluminescence behavior seems quite intriguing based on this work, which may serve as
instructive example for our future study on function of coordination polymers.
19

Acknowledgements
This work was supported by the start-up fund provided by the College of Natural
Sciences and Mathematics (CNSM) and University Research Council (URC) from University of
Central Arkansas.
Appendix A. Supplementary material
CCDC 1822224-1822227 contains the supplementary crystallographic data for complexes 2–5.
These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html,
or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
References:
[1] (a) X. Wang, J. Zhao, M. Le, H. Lin, J. Luan, G. Liu, X. Wang, J. Inorg. Organomet. Polym.
Mate. 28 (2018) 800-804; (b) T. Kitao, Y. Tokunage, D. Umeyama, C.R. Kim, T. Uemura, S.
Kitagawa, Inorg. Chim. Acta 424 (2015) 221-225; (c) Y.Q. Chen, Y.K. Qu, G.R. Li, Z.Z.
Zhuang, Z. Chang, T.L. Hu, J. Xu, X.H. Bu, Inorg. Chem. 53 (2014) 8842-8844; (d) G.
Mukherjee, K. Biradha, Cryst. Growth Des. 13 (2013) 4100-4109; (e) L. Rajput, K. Biradha,
New. J. Chem. 34 (2010) 2415-2418.
[2] D. Deng, B. Ji, W. Wang, L. Ma, F. Luo, New J. Chem. 41 (2017) 12611-12616; (b) Y.
Xiong, G. Liu, X. Wang, J. Zhang, H. Lin, X. Sha, CrystEngComm 19(2017) 4561-4570; (c) X.
L. Wang, J. Sun, H. Lin, Z. Chang, X. Bai, X. Wang, Polyhedron 124 (2017) 30-40; (d) M. Garai,
K. Biradha, Chemistry-A European Journal 23 (2017) 273-277; (e) H.H. Lee, S.H. Jung, S. Park,
K.M. Park, J.H. Jung, New. J. Chem. 37 (2013) 2330-2335.
[3] Z. Hou, P. Li, H.Y. Wang, Z. Li, H. Li, Dyes and Pigments 147 (2017) 429-435; (b) X. Wang,
J. Zhao, M. Le, H. Lin, G. Liu, X. Wang, CrystEngComm 18 (2016) 9316-9324; (c) K. Banerjee,
S. Roy, M. Kotal, K. Biradha, Cryst. Growth Des.15 (2015) 5604-5613; (d) Q.L. Lu, J. Luan,
X.L. Wang, H.Y. Lin, M. Le, G.C. Liu, Polyhedron 83 (2014) 108-115.
20

[4] (a) J.L. Guillet, I. Bhowmick, M.P. Shores, C.J.A. Daley, M. Gembicky, J.A. Golen, A.L.
Rheingold, L.H. Doerrer, Inorg. Chem. 55 (2016) 8099-8109; (b) D. Zhao, W. Si, P. Wang, X.
Chen, J. Jiang, Inorg. Chem. 53 (2014) 3494-3502.
[5] (a) Y.C. Qin, X.F. Feng, F. Luo, G.M. Sun, Y.M. Song, X.Z. Tian, H.X. Huang, Y. Zhu, Z.J.
Yuan, M.B. Luo, S.J. Liu, W.Y. Xu, Dalton Trans. 42 (2013) 50-53; (b) C.H. Lee, H.Y. Huang,
Y.H. Liu, T.T. Luo, G.H. Lee, S.M. Peng, J.C. Jiang, I. Chao, K.L. Lu, Inorg. Chem. 52 (2013)
3962- 3968; (c) R. Yun, Z. Lu, Y. Pan, X. You, J. Bai, Angew. Chem. Int. Ed. 52 (2013) 11282-
11285; (d) S.S. Xiong, Y.B. He, R. Krishna, B.L. Chen, Z.Y. Wang, Cryst. Growth Des. 2013,
13, 2670-2674; (e) L.T. Du, S.L. Yang, L. Xu, H.H. Min, B.S. Zheng, CrystEngComm 16 (2014)
5520-5523.
[6] (a) J. Z. Travis, S.R. Pumford, B.L. Martinez, R.L. LaDuca, Polyhedron 142 (2018) 25-37; (b)
X.L. Wang, Y. Xiong, X. T. Sha, G.C. Liu, H.Y. Lin, Cryst. Growth Des. 17 (2017) 483-496; (c)
Y.C. Lo, Y,J. Chang, P.M. Chhetri, W.J. Huang, J.D. Chen, Polyhedron 117 (2016) 777-787.
[7] E.P. McMoran, J.A. Goodner, D.R. Powell, L. Yang, Inorg. Chim. Acta 421 (2014) 465-472.
[8] (a) E.P. McMoran, C. Mugenzi, K. Fournier, M. Draganjac, D. Tony, K. Jeong, D.R. Powell,
L. Yang, J. Coord. Chem. 69 (2016) 375-388; (b) E.P. McMoran, M.N. Dominguez, E.M. Erwin,
D.R. Powell, G.T. Rowe, L. Yang, Inorg. Chim. Acta 446 (2016) 150-160.
[9] (a) M. Kondo, A. Asami, H. Chang, S. Kitagawa, Crystal Engineering, 2 (1999) 115-122; (b)
T.S. Gardner, E. Wenis, J. Lee, J. Org. Chem. 19 (1954) 753-757; (c) D.K. Kumar, D.A. Jose, P.
Dastidar, A. Das, Langmuir 20 (2004) 10413-10418; (d) C.B. Aakeröy, S.V. Panikkattu, B.
DeGHaven, J. Desper, Cryst. Growth Des. 12 (2012) 2579-2587.
[10] (a) Data Collection: SMART Software Reference Manual 1998. Bruker-AXS, 5465 E.
Cheryl Parkway, Madison, WI 53711-5373 USA; (b) Data Reduction: SAINT Software
Reference Manual, 1998, Bruker-AXS, 5465 E. Cheryl Parkway, Madison, WI 53711-5373 USA.
[11] (a) G.M. Sheldrick 2000, SHELXTL Version 6.10 Reference Manual. Bruker-AXS, 5465 E.
Cheryl Parkway, Madison, WI 53711-5373 USA. (b) International Tables for Crystallography,
Vol C, Tables 6.1.1.4, 4.2.6.8, and 4.2.4.2, Kluwer: Boston, 1995.
[12] P. van der Sluis, A.L. Spek, Acta Cryst. A46 (1990) 194-201.
[13] S.K. Wolff, D.J. Grimwood, J.J. McKinnon, M.J. Turner, D. Jayatilaka, M.A. Spackman.
Univeristy of Western Australia, Available online at: www.hirshfeldsurface.net (accessed 09 July
2015).
21

[14] L. Yang, D.R. Powell, R.P. Houser, Dalton Trans. (2007) 955-964.
[15] (a) D.K. Kumar, A. Das, P. Dastidar, CrysEngComm, 9 (2007) 548-555; (b) C.J. LaRose,
R.L. LaDuca, Inorg. Chim. Acta, 461 (2017) 92-101; (c) C.L. White, R.L. LaDuca, Inorg. Chim.
Acta, 451 (2016) 171-176; (d) C.L. White, R.L. LaDuca, CrysEngComm, 18 (2016) 6789-6797;
(e) P.E. Kraft, L.E. Weingartz, R.L. LaDuca, Inorg. Chim. Acta, 432 (2015) 283-288; (f) M.D.
Torres Salgado, C.J. Bouchey, J.A. Wilson, R.L. LaDuca, J. Coord. Chem., 68 (2015) 2029-2040;
(g) J.S. Goldsworthy, R.J. Staples, R.L. LaDuca, J. Mol. Struc., 1062 (2014) 116-124; (h) P.E.
Kraft, J.E. Mizzi, R.L. LaDuca, Inorg. Chim. Acta, 409 (2014) 449-457; (i) J.W. Uebler, A.L.
Pochodylo, R.J. Staples, R.L. LaDuca, Cryst. Growth Des., 13 (2013) 2220-2232; (j) K. Uemura,
S. Kitagawa, K. Fukui, K. Saito, J. Am. Chem. Soc., 126 (2004) 3817-3828.
[16] T.J. Mooibroek, P. Gamez, J. Reedijk, CrystEngComm, 10 (2008) 1501-1515.
[17] (a) H.T. Chifotides, K.R. Dunbar, Acc. Chem. Res. 46 (2013) 894-906; (b) M. Giese, M.
Albrecht, K. Rissanen, Chem. Rev. 115 (2015) 8867-8895; (c) J. Zhao, D. Yang, X.J. Yang, B.
Wu, Coord. Chem. Rev. 2018, in press, https://doi.org/10.1016/j.ccr.2018.01.002.
[18] R.W. Newberry, R.T. Raines, Acc. Chem. Res. 50 (2017) 1838-1846.
[19] H.B. Burgi, J.D. Dunitz, E. Shefter, J. Am. Chem. Soc. 95 (1973) 5065-5067.
[20] S.K. Singh, A. Das, PCCP 17 (2015) 9596-9612.
[21] (a) S. Banerjee, A. Bauza, A. Frontera, A. Saha, RSC Advances 6 (2016) 39376-39386; (b)
C.Q. Wang, A.M. Li, S.A. Al-Thabiati, T.C.W. Mak, Crys. Growt. Des. 13 (2013) 1926-1936; (c)
C.Q. Wang, Z.W. Wang, G. Wang, A.M. Li, Q.H. Jin, Y.H. Deng, T.C.W. Mak, Euro. J. Inorg.
Chem. 2013 (2013) 2591-2600; (d) C.Q. Wang, Mak, T.C.W. Mak, Crys. Growt. Des. 11 (2011)
832-842; (e) M. Adler, M. Marsch, N.S. Nudelman, G. Boche, Angew. Chem. Int. Ed. 38 (1999)
1261-1263.
[22] (a) D. De, S. Neogi, E.C. Sanudo, P.K. Bharadwaj, Chem. Eur. J., 21 (2015) 17422-17429;
(b) R.S. Forgan, R.J. Marshall, M. Struckmann, A.B. Bleine, D.L. Long, M.C. Bernini, D.
Fairen-Jimenez, CrystEngComm, 17 (2015) 299-306; (c) Z.H. Xuan, D.S. Zhang, Z. Chang, T.L.
Hu, X.H. Bu, Inorg. Chem., 53 (2014), 8985-8990; (d) J.W. Uebler, J.A. Wilson, R.L. LaDuca,
CrystEngComm, 15 (2013) 1586-1596; C. (e) Hou, Q. Liu, T. Okamura, P. Wang, W.Y. Sun,
CrystEngComm, 14 (2012) 8569-8576; (f) D.K. Kumar, A. Das, P. Dastidar, CrystEngComm, 9
(2007) 895-901.
[23] J.H. Qin, X.L. Li, H. Guo, Z. Kristallogr. NCS, 222 (2007) 318-320.
22

[24] S. Kitagawa, M. Munakata, A. Higashie, Inorg. Chim. Acta, 59 (1982) 219-223.
[25] J.E. Del Bene., J. Am. Chem. Soc. 101 (1979) 6184-6189.
[26] R. Haldar, N. Sikdar, T.K. Maji, Materials Today 18 (2015) 97-116.
[27] M. Xue, S. Ma, Z. Jin, R.M. Schaffino, G.S. Zhu, E.B. Lobkovsky, S.L. Qiu, B.L. Chen,
Inorg. Chem. 47 (2008) 6825-6828.
23

Four Cu(I) coordination polymers supported by bipyridylamide ligands were synthesized and
characterized by X-ray crystallography, FT-IR, reflectance UV-vis and TGA. Detailed structural
analysis revealed how pyridyl nitrogen disposition and amide groups impact the dimensionality
and self-penetration of the constructed coordination networks.
24