Construction of one dimensional Co(II) and Zn(II) coordination polymers based on expanded N,N′-donor ligands

Article abstract of DOI:10.1016/j.ica.2017.09.023

Four new 1-D coordination polymers [Zn(acac)2(L1)] _n (1), [Co(acac)₂(L1)]_n (2), [Co(acac)₂(L2)]_n (3) and [Co(acac)₂(L3)]_n (4) were afforded by the complexation reaction of appropriate zinc and cobalt metal salts, acetylacetone co-ligand as well as three linear electron rich and bi-functional N,N′-bipyridyl-base ligands of N,N′-bis(pyridin-4-ylmethylene)naphthalene-1,5-diamine (L1), N,N′-bis(pyridin-4-ylmethylene) phenylene-1,4-diamine (L2) and N,N′-bis(pyridin-4-ylmethylene)hydrazine (L3). The structures of these compounds were characterized by FT-IR spectroscopy, elemental analysis, X-ray powder and single crystal X-ray diffractions. X-ray crystallography analyses revealed that these compounds have 1-D linear chain structures a containing {N₂O₄} metal coordination environment in which the N-donor Lx (x = 1–3) bridges occupy trans positions. The acetylacetone (acac) ancillary ligands control the coordination number of the metal cation and adopt chelating binding mode on octahedral metal center. Furthermore, 1-D chains are held together with their neighboring ones by C–H?O, C–H?π and π-π stacking intermolecular interactions to stabilize 2-D supramolecular networks. The two former cases 1 and 2, containing same L1 spacer ligand generate isomorphous structures. Theoretical calculations invoking electronic properties, frontier molecular orbital description and the strength of interactions between metal ion and coordinated atoms via second order perturbation energies were carried out using natural bond orbital analysis (NBO). Finally, thermal stability of compound 2–4 was examined by thermogravimetric (TGA) analysis.

Full text of DOI:10.1016/j.ica.2017.09.023

Accepted Manuscript
Research paper
Construction of one dimensional Co(II) and Zn (II) coordination polymers based
on expanded N,N'-donor ligands
Mansoureh Zahedi, Behrouz Shaabani, Muhittin Aygün, Canan Kazak
PII:
S0020-1693(17)30782-X
http://dx.doi.org/10.1016/j.ica.2017.09.023
ICA 17879
DOI:
Reference:
Inorganica Chimica Acta
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17 August 2017
8 September 2017
Please cite this article as: M. Zahedi, B. Shaabani, M. Aygün, C. Kazak, Construction of one dimensional Co(II)
and Zn (II) coordination polymers based on expanded N,N'-donor ligands, Inorganica Chimica Acta (2017), doi:
http://dx.doi.org/10.1016/j.ica.2017.09.023
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Construction of one dimensional Co(II) and Zn (II) coordination
polymers based on expanded N,N'-donor ligands
Mansoureh Zahedia, Behrouz Shaabani*, a, Muhittin Aygünb and Canan Kazakc
aDepartment of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz , Iran
*Email: shaabani.b@gmail.com (shaabani@tabrizu.ac.ir)
bDepartment of Physic, Faculty of Arts & Sciences Dokuz Eylul University, Izmir, Turkey
cDepartment of Physics, Faculty of Arts and Sciences, Ondokuz Mayıs University, Kurupelit, Samsun , Turkey
Abstract
Four new 1-D coordination polymers [Zn(acac)2(L1)]n (1), [Co(acac)2(L1)]n (2), [Co(acac)2(L2)]n
(3) and [Co(acac)2(L3)]n (4) were afforded by the complexation reaction of appropriate zinc and
cobalt metal salts, acetylacetone co-ligand as well as three linear electron rich and bi-functional
N,N'-bipyridyl-base ligands of N,N'-bis(pyridin-4-ylmethylene)naphthalene-1,5-diamine (L1),
N,N'-bis(pyridin-4-ylmethylene) phenylene-1,4-diamine (L2) and N,N'-bis(pyridin-4-
ylmethylene)hydrazine (L3). The structures of these compounds were characterized by FT-IR
spectroscopy, elemental analysis, X-ray powder and single crystal X-ray diffractions. X-ray
crystallography analyses revealed that these compounds have 1-D linear chain structures a
containing {N2O4} metal coordination environment in which the N-donor Lx (x=1–3) bridges
occupy trans positions. The acetylacetone (acac) ancillary ligands control the coordination
number of the metal cation and adopt chelating binding mode on octahedral metal center.
Furthermore, 1-D chains are held together with their neighboring ones by C–H···O, C−H···π and
π-π stacking intermolecular interactions to stabilize 2-D supramolecular networks. The two
former cases 1 and 2, containing same L1 spacer ligand generate isomorphous structures.
Theoretical calculations invoking electronic properties, frontier molecular orbital description and
the strength of interactions between metal ion and coordinated atoms via second order
perturbation energies were carried out using natural bond orbital analysis (NBO). Finally,
thermal stability of compound 2–4 was examined by thermogravimetric (TGA) analysis.

Keyword: Coordination polymers, Expanded N,N'-donor ligand, Crystal structure, NBO study,
Thermogravimetric analyses (TGA).
1. Introduction
In recent decades, coordination polymers have been a popular research topic in crystal
engineering, solid-state chemistry and supramolecular chemistry due to their intriguing diverse
architectures and increasing demand of functional materials with potentially controllable
properties. These materials can be used in various fields such as catalysts, optical, magnetic and
electronic devices and biological applications [1–7]. Structural properties of the organic ligands
are one of the main factors which can dramatically influence the topology and composition of the
coordination polymers [7–10]. Tailored derivatives of N,N'-bipyridyl-base ligands are good
candidates for designing and constructing of metal organic networks based on their significant
structural properties such as versatility in spacers between the pyridine rings, rigidity or
flexibility, rod-like shape and angular orientation of the donor atoms. These properties can
exhibit several particular features upon coordination to metal cations: a) Appropriate bridging
ability; two pyridyl N-donor atoms can coordinate to metal centers as a bidentate ligand. b) The
dihedral angles between the pyridyl rings can vary and adapt to meet the requirements for
constructing diverse frameworks. c) The ability to engage in intermolecular interactions like π-π
stacking due to the completely or partly participation of aromatic groups, which can also affect
crystal packing. These structural features can allow to construct diverse coordination polymers
with interesting one, two and three-dimensional structures [11–13]. Among the vast number of
bidentate N-donor ligands, N,N'-bis(pyridin-4-ylmethylene)naphthalene-1,5-diamine (L1), N,N'-
bis(pyridin-4-ylmethylene)
phenylene-1,4-diamine
(L2)
and
N,N'-bis(pyridin-4-
ylmethylene)hydrazine (L3) (Scheme 1) have attracted our attention, since they also can easily be
synthesized and have not been used very frequently for the design of extended structures. A
search in CSD data base [14] revealed that 18 structurally characterized complexes derivatives of
L1 have been identified [15–21]. 1-D ladder-shaped coordination polymer [Co2(L1)3(NO3)4]n
[15], two and three dimensional structure [Co(L1)1.5(NO3)2] and [Co(L1) (bpdc)]·(EtOH)
(H2bpdc = biphenyl-4,4'-dicarboxylic acid, H2ndc= 2,6-naphthalene dicarboxylic acid) [21] can
be mentioned as ones which involve Co(II) cation metal center and L1 linker ligand.

Investigations concerning the coordination polymers generated from L2 displayed that only 5
complexes have been characterized [22–27] and none of them contain cobalt cation as a metal
center. Constructing of metal-organic frameworks including L3 in their structures exhibit more
interests
[28–32];
extended
structures
of
{[Co(L3)(tp)(H2O)2]·3H2O}n
and
{[Co(L3)(ip)]·(L3)0.5}n (H2tp = terphetalic acid, H2ip = isophethalic acid) can be addressed as
examples [29].
Scheme 1. Expanded bipyridyl-base linkers used in this work.
Furthermore, coordination modes of β-diketone and its diversities are well known to rely on the
pH of the environment and the nature of the substituents and their relative positions to other
moieties in the β-diketones. Moreover, the –CH2– group readily deprotonates to form anionic
tautomeric species (Scheme 2) with new coordination properties. Different types of β-diketones
can be very attractive because of their considerable applications in variable fields such as metal-
extracting agents [33], design of luminescent compounds [34] and manufacturing of red OLED

devices [35]. According to the hot topic of metal-organic hybrid materials and applicable
properties of compounds containing β-diketones, special attention has been devoted to the
design, synthesis and topological investigation of coordination polymers containing β-diketones
in their structures [36–40].
Scheme 2. Schematic of acetylacetone ligand
In addition to the fact that covalent bonds are impetus in the self-assembly of metal centers and
organic ligands for construction of robust coordination polymers, weak intermolecular
interactions such as classical hydrogen bonds as well as C–H···O/···π and π-π stacking have
significant effects on topology and architecture of supramolcular networks. As a continuation of
our work in synthesis of coordination polymers based on the expanded N,N'-donor linker ligands
[19, 27, 41], we carried out the synthesis and detailed crystallographic characterization of four
new coordination polymers [M(acac)2(L1)]n (M = Zn (1), Co (2) and L1=(4-py)-CH=N-C10H6-
N=CH-(4-py)),
[Co(acac)2(L2)]n
(3)
(L2=(4-py)-CH=N-C6H4-N=CH-(4-py))
and
[Co(acac)2(L3)]n (4) (L3=(4-py)-CH=N-N=CH-(4-py)), respectively. Compounds 1–4 were
structurally characterized by elemental analysis, X-ray powder diffraction, FT-IR spectroscopy
and single crystal X-ray diffraction. In addition, thermal stability and NBO analyses using the
DFT method were investigated.
2. Experimental
2.1. Materials and Physical Measurements
Co(acac)2·2H2O was prepared according to the literature method [42] . All other reagents and
solvents were commercially available and used without further purification. NMR spectra were
recorded on a Bruker Avance II Ultrashield Plus 400. Elemental analyses were performed with a

Heraeus CHNO-Rapid VarioEL. FT-IR spectra were recorded on a Bruker Tensor 27 Perkin-
Elmer FT-IR. TG analyses were measured in air with a heating rate of 5 K.min−1 in the
temperature range 25–800 °C on a Mettler Toledo TGA/SDTA 851e instrument. X-ray powder
diffraction patterns were obtained at ambient temperature on flat samples with a Stoe Image plate
Detector IP-PSD with Cu Kα1 radiation.
2.2. Synthesis of the Spacer Ligands L1, L2 and L3
Synthesis of L1. We used a modified synthesis adapted from Lee et al. [16] : An ethanol (10 mL)
solution of 4-pyridinecarboxaldehyde (1.88 mL, 20.0 mmol) was added dropwise to an ethanol
solution (10 mL) of 1,5-diaminonaphthalene (1.58 g, 10.0 mmol) and the mixture was refluxed
for 3 h. The resulting yellow crystalline solid was washed with ethanol (3×5 mL) and hexane
1
(3×5 mL) and crystallized from CH2Cl2–hexane to give the pure ligand. Yield: 83%. H NMR
(400 MHz, CDCl3) δ: 8.84 (d, 4H), 8.61 (s, 2H), 8.29 (d, 2H), 7.90(d, 4H), 7.56 (m, 2H), 7.18 (d,
̶
2H). FT-IR (KBr, cm1): 3025(w), 1622 (s), 1590 (s), 1405 (s), 1316 (m), 1229 (m), 977 (m), 923
(m), 788 (s), 648 (w). Anal. Calc. for C22H16N4: C, 78.5; H, 4.7; N, 16.6. Found: C, 78.7; H, 4.8;
N, 16.5%.
Synthesis of L2. A modified synthesis producer adapted from ref [43] was used: An
acetonitrile solution (5mL) of 4-pyridinecarboxaldehyde (1.88 mL, 20.0 mmol) was added drop
wise to a solution of 1, 4-phenylendiamine (1.08 g, 10.0 mmol) dissolved in the same solvent and
the mixture was stirred for three hours at room temperature. The resulting yellow solid was
filtered off and washed with acetonitrile (3×5 mL) and hexane (3×5 mL) to give the pure ligand
L2. Yield: 78%. 1H NMR (400 MHz, CDCl3) δ: 8.80 (d, 4H), 8.54 (s, 2H), 7.79 (d, 4H), 7.36(s,
4H). FT-IR (KBr, cm–1): 3028 (w), 2921 (w), 2888 (w), 2855 (w), 1627 (s), 1595 (s), 1551 (s),
1486 (s), 1412 (s), 1323 (s), 1221 (m), 1191(s), 1100 (m), 984 (w), 891 (m), 815 (s), 603 (m).
Anal. Calc. for C18H14N4: C, 75.50; H, 4.93; N, 19.57. Found: C, 75.66; H, 4.89; N, 19.53%.
Synthesis of L3. According to the synthetic procedure of zur Loye et al. [44]. 1 mL (11
mmol) of hydrazine hydrate was added dropwise to a solution of 4-pyridinecarboxaldehyde (2.2

mL, 22 mmol) dissolved in ethanol (15 mL). Two drops of formic acid were added and the
mixture was stirred at room temperature for 24 h. The yellow solid product was filtered and
washed several times with ethanol/ether (1:1). Yield: 86%. 1H NMR (400 MHz, CDCl3) δ: 8.75
(d, 4H), 8.54 (s, 2H), 7.69 (d, 4H). FT-IR (KBr, cm–1): 2941 (w), 1623 (s), 1595 (s), 1552 (m),
1417(s), 1309 (s), 1207 (m), 1082 (m), 1058 (w), 975 (s), 814 (s), 678 (s), 508 (s). Anal. Calc.
for C12H10N4: C, 68.56; H, 4.79; N, 26.65. Found: C, 68.53; H, 4.81; N, 26.61%.
2.3. Preparation of Coordination Compounds
[Zn(acac)2(L1)]n (1). A mixture of dichloromethane and methanol (1:1, 2 ml) was gently layered
on the top of a dichloromethane solution (6 ml) of L1 (33.6 mg, 0.10 mmol) in a test tube. A
solution of Zn(OAc)2·2H2O (21.95 mg, 0.10 mmol) and acetylacetone (20.0 mg, 0.20 mmol) in
methanol (6 ml) was added carefully as a third layer. Yellow block crystals formed within two
days, yield~8%. FT-IR (KBr, cm–1): 3037(w), 2923(w), 2854(w), 1585(s), 1518(s), 1463 (s),
1403 (s),1361(m), 1313(w), 1228(w), 1060(w), 1007(m), 923(m), 835(m), 786 (m), 655 (w),
510(m).
We note that in contrast to 2–4 only few isolated crystals and no appreciable quantity of the
target compound could be obtained; no data from elemental and thermal analysis are available.
[Co(acac)2(L1)]n (2). A dichloromethane solution (6 mL) of L1 (33.6 mg, 0.10 mmol) was
allowed to diffuse slowly into a methanol solution (6 mL) of Co(acac)2·2H2O (29.5 mg, 0.10
mmol) through an intermediate layer of methanol/dichloromethane (1:1, 4 ml). Orange block
crystals formed within 2 days, yield 63%. IR (KBr, cm–1): 3038(w), 2954(w), 2916(w), 1582(s),
1516(s), 1461(s), 1407(s), 1362(m), 1315(w), 1230 (w), 1060(w), 1008(m), 923(m), 834 (m),
786 (m), 656(w), 509(m). Anal. Calc. for C32H30N4O4Co: C, 64.75; H, 5.09; N, 9.44. Found: C,
64.63; H, 5.12; N, 9.53%.
[Co(acac)2(L2)]n (3). To a tetrahydrofuran (THF) solution (5 mL) of L2 (28.6 mg, 0.10 mmol)
was carefully introduced a methanol solution (5 mL) of Co(acac)2·2H2O (29.5 mg, 0.10 mmol )
as a second layer. Orange block crystals formed within 4 days, yield 57%. FT-IR (KBr, cm–1)

3043(w), 2922(w), 2854(w), 1587(s), 1521(s), 1457(m), 1404(s), 1364(m), 1326(m), 1228(w),
992(w), 926(w), 785(m), 652(s), 510(m). Anal. Calc. for C28H28N4O4Co: C, 61.80; H, 5.19; N,
10.31. Found: C, 61.72; H, 5.26; N, 10.47%.
[Co(acac)2(L3)]n (4). This compound was prepared following a similar procedure to that used
for the synthesis of 2, except that L3 (21 mg, 0.10 mmol) was added instead of L1. Orange block
crystals formed within 1 day, yield 68%. FT-IR (KBr, cm–1): 3040(w), 2946 (w), 2871 (w), 1583
(s), 1514 (s), 1459(s), 1408 (s), 1371 (s), 1319 (m), 1251 (m), 1228(w), 1202 (w), 1055 (w),
1001 (m), 930 (m), 838 (m), 788 (m), 650(w), 508(m). Anal. Calc. for C22H24N4O4Co: C, 56.50;
H, 5.18; N, 11.99. Found: C, 56.27; H, 5.28; N, 11.78%.
2.4. X-ray Data Collection and Structure Determinations
Intensity data for all compounds were collected at 293 K with Mo-Kɑ radiation (λ = 0.71073 Å)
on an Xcalibur four-circle diffractometer equipped with an EOS CCD detector. The CrysAlisPro
software [45] was used for data collection, cell refinement and data reduction. The structures
were solved by direct methods (SHELXS-97) [46] and refined by full matrix least squares
procedures based on F2 as implemented in SHELXL-97 [47]. Moreover, for compound 3, the
OLEX2 Solvent Mask routine was used to remove the contribution of the disordered solvent
molecule, which can be attributed to THF solvent with a total void of 159 ų(37 electrons) per
unit cell as a filled space. Anisotropic displacement parameters were applied to all non-hydrogen
atoms and hydrogen atoms bonded to C were positioned geometrically and refined with a riding
model with isotropic displacement parameters set at 1.2 times Ueq of appropriate carrier atoms.
The crystal data and refinement parameters are presented in Table 1. Selected bond lengths and
angles are given in Tables S1 and S2.
CCDC 1550484, 1550482, 1550483 and 1550481 contains the supplementary crystallographic
data for 1–4, respectively. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/contsretrieving.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.

Table 1. X-ray data collection and structure refinement detail.
Compound
Chemical formula
Mr
1
2
3
4
C32H30N4O4Zn
599.97
C32H30CoN4O4
593.53
C28H28CoN4O4
543.47
C22H24CoN4O4
467.38
Crystal system, space
group
Triclinic, P-1
Triclinic, P-1
Triclinic, P-1
Triclinic, P-1
Temperature (K)
a, b, c (Å)
293
293
295
293
8.5618 (5), 8.9827 (5),
10.5511 (5)
8.5565 (6), 8.9574 (7),
10.5780 (7)
8.6267 (7), 11.5515 (13),
14.6400 (17)
6.3638 (7), 9.1119 (10),
10.3267 (13)
α, β, γ (°)
80.711 (4), 75.387 (5),
64.918 (5)
80.924 (6), 75.696 (6),
65.030 (7)
102.644 (10), 94.647 (8),
94.940 (8)
95.379 (10), 107.630 (11),
90.588 (9)
V (Å3)
Z
709.79 (6)
1
710.85 (9)
1
1410.7 (3)
2
567.70 (12)
1
µ (mm−1)
0.91
0.65
0.65
0.79
Crystal size (mm)
Absorption correction
Diffractometer
0.34 × 0.22 × 0.12
Analytical
Xcalibur, Eos
0.29 × 0.23 × 0.16
Analytical
Xcalibur, Eos
0.32 × 0.15 × 0.11
Analytical
Xcalibur, Eos
0.42 × 0.22 × 0.13
Analytical
Xcalibur, Eos
No. of measured,
independent and observed
[I > 2σ(I)] reflections
5477, 2902, 2505
5035, 2900, 2537
8856, 5710, 2803
2020, 1596, 1363
Rint
0.045
0.625
0.019
0.625
0.048
0.625
0.022
0.625
(sin θ/λ)max (Å−1)
R1, wR2, S
0.041, 0.148, 1.23
2902
0.038, 0.092, 1.06
2900
0.072, 0.167, 0.92
5710
0.040, 0.082, 1.07
1596
No. of reflections
No. of parameters
H-atom treatment
189
189
338
144
H-atom parameters
constrained
H-atom parameters
constrained
H-atom parameters
constrained
H-atom parameters
constrained
∆ρmax, ∆ρmin (e Å−3)
0.81, −0.98
0.26, −0.23
0.34, −0.34
0.37, −0.20
2.5. Theoretical Calculation Method

The density functional theory (DFT) according to the Kohn–Sham scheme was applied to study
the natural atomic charges, wiberg bond index, frontier molecular orbitals and second-order
perturbation energies from natural bond orbital (NBO) analysis. The starting atomic coordinates
and molecular structure parameters for single point calculations were those from the crystal
structures data of the compounds 1–4. All the computational results reported in this work were
obtained using the GAUSSIAN 03 package [48] at hybrid functional B3LYP level [49]. Basis set
of 6-31G* [50] was employed for all non-metal atoms and the effective core potential basis set
Lanl2dz [51] was used for metal atoms.
3. Result and Discussion
3.1. Synthesis and characterization
Relatively long and rigid conjugated nitrogen-donor Schiff-base linker ligands L1–L3 were
prepared by condensation of 4-pyridinecarboxaldehyde and required diamine reagents such as
1,5-naphtalinediamine, 1,4-phenylenediamine or hydrazine hydrate, in the mole ratio of 1:2.
Single crystals of [Zn(acac)2(L1)]n (1) were synthesized by the reaction of L1, Zn(OAc)2·2H2O
and acetylacetone in a methanol/dichloromethane layer-separation diffusion approach with a
molar ratio of 1:1:2. In summary, Metanol/dichloromethane and Metanol/tetrahydrofuran solvent
diffusion of Co(acac)2·2H2O and the appropriate biprydyl-base spacer ligands, in 1:1 molar ratio,
easily led to the formation of [Co(acac)2(Lx)]n (x= 1, 2 and 3) 2–4 coordination polymers.
The Fourier Transform Infrared spectrum of the free Schiff base ligands display the absorption
peaks of imine ν(C=N) in 1622, 1623 and 1627 cm–1 for L1, L2 and L3, respectively. Consistent
with this assignment, vibrations of the imine ν(C=N) groups are encountered in the range 1600-
1680 cm–1 [52]. These absorption bands in coordination polymers appear in lower frequency
range of 1587–1582 cm-1. In all compounds, sharp and strong characteristic bands of 1587–1514
cm–1 can be attributed to ν(C=O) or ν(C=C) stretching of the acetylactone ligand [53]. It has to be
mentioned that the azomethine (C=N) stretching frequencies overlap with νC=O in acac.
The results of our single crystal diffraction experiments are also supported by microanalytical
data, except for compound 1. Powder diffraction confirms that our samples are phase-pure (Figs.
S1-S4).

3.2. Description of the crystal structures
[M(acac)2(L1)]n (M = Zn (1), Co (2)). Isomorphous compounds 1 and 2 crystallized in the
triclinic P-1 space group. Zinc and cobalt metal centers in 1 and 2 are located in an octahedral
sphere consisting of two bidentate acetylacetone in the equatorial position and L1 bridging
ligand in the axial position, bound through terminal 4-pyridyl groups. This kind of octahedral
coordination environment is frequently observed for Zn(II) and Co(II) complexes [54–56]. The
average values of metal-ligand distance are 2.05 Å, 2.25 Å, 2.04 Å and 2.20 Å for Zn–O, Zn–N,
Co–O, Co–N, respectively. Each bridging ligand connects two M(acac)2 centers, resulting in the
formation of infinite 1D chains as given in Fig. 1a and Fig. S5. The observed octahedral
coordination geometry and bond lengths around the Zn(II) and Co(II) centers are close to the
reported 1-D structures of [Zn(lig)(ttfa)2]n, [Zn(lig)(btfa)2]n and [Co(lig)2(pyz)]n (Httfa = 2-
thenoyltrifluoroacetone, Hbtfa = benzoyltrifluoroacetone and lig = pyrazine) [57, 58]. The
separation between next-neighbour metal(II) cations connected by a spacer ligand L1 amounts to
20.1 Å. The aromatic rings in the L1 spacer ligand are subtend with interplanar angles of ca. 40°.
The adjacent chains are arranged in an alternate mode through the π-π stacking interactions
between L1 ligands with the C···C distance of 3.2 Å and a non-classical inter strand C–H···O
hydrogen bonds of 2.6 Å among the H atoms of L1 ligand and the oxygen atom of acetylacetone
from neighbor chains, which are significant and likely contribute to the assembly process and
stabilization of the 2-D supramolecular polymeric structures (Fig. 1b).

Figure 1. (a) 1-D linear chain structure 2 resulting from L1 ligand and Co(acac)2. Symmetry operators i= −x, −y+1,
−z+1. (b) Packing in polymer direction along [1-1-1]. H atoms have been omitted for clarity.
[Co(acac)2(L2)]n (3), [Co(acac)2(L3)]n (4). The Co(II) atoms are located in a distorted
octahedral coordination geometry in 3 and the octahedral one in 4. They have a coordination
sphere similar to that of compound 2. Thus N-donor atoms of two L2 and L3 bridges occupy
trans positions around the metal center in 3 and 4, respectively and oxygen atoms of two
acetylacetone co-ligands are coordinated equatorially. The 4,4'-bipyridyl-base ligands and Co(II)
cations are linked alternately to form a one-dimensional (1-D) linear chain running parallel to ab
plane and along [11-1] direction (body diagonal) in 3 and 4, respectively (Fig. 2a and Fig. S6a).

The average bond lengths of 2.20 Å for Co ̶ N and 2.03 Å for Co ̶ O lie in the normal range
observed in structures such as a 1-D zigzag chain [Co(dbm)2(4ptz)]n·nTHF (4ptz =2,4,6-tris-(4-
pyridyl)-1,3,5-triazine, dbm=1,3-diphenylpropane-1,3-dione) and a molecular complex
[Co(acac)2(lig)2]·2CH3CN (lig
=
ethylenedioxotetrathiafulvalene-pyridine). The related
compounds demonstrate same coordination environment of {N2O4} as in compounds 3 and 4,
either [59, 60].
The Co···Co distance subtended by a single spacer L2 amounts to 19.92 Å. This linker ligand
has arranged in a tilt conformation in which two pyridine rings display dihedral angles of ca. 30°
and 148˚ around phenyl ring. Meanwhile, presence of shorter spacer ligand L3, in which adopts a
C(6)=N(2)−N(2)i=C(6)i torsion angle of 180˚ and is exactly coplanar, led to the significantly
shorter separation of next-neighbour Co cations (15.92 Å) in 4.
Finally, in 3 two non-classic C
̶
H···O hydrogen bonds interaction [2.66 and 2.69 Å] between
H···π [2.83 and
carbon atoms of L2 spacer ligand and oxygen atoms of the acetylacetone with C
̶
2.88 Å] interaction of adjacent linker ligands and interaction of methylene group of
acetylacetone with aromatic rings of the spacer ligand are found in the structure and play an
important role in stabilizing of 2D structure (Fig. 2b). Whereas, non-classic C ̶ H···O hydrogen
bonds of pyridine ring with acetylacetone co-ligand [2.69 Å] is the only intermolecular
interaction which assembled compound 4 in two dimensional network (Fig. S6b).

Figure 2. (a) 1-D linear chain structure 3 resulting from L2 ligand and Co(acac)2. Symmetry operators i= x+2, y+1,
z; (c) Packing in polymer direction along [1-1-1] in 3.
3.3. Natural Bond Orbital (NBO) Analysis
The NBO analysis was used to investigate the electronic properties of compounds 1–4. The
selected natural atomic charges, electron configuration and wiberg bond index for compounds 1–
4 are shown in Tables S3-S6. Base on the electron configuration results, it can be concluded that
Zn2+ and Co2+ ions are coordinated mainly via 4s, 3d and 4p orbitals. All O- and N-donor atoms
supply electrons of 2s and 2p to form the coordination bonds with metal cations. The affinity of
charge transfer from ligand to metal ion can be deduced from natural charge of metal ions (1.30
for Zn2+, 0.77 for Co2+ in 2,4 and 0.75 for Co2+ in 3), which are much smaller than +2. The
differences of the NBO wiberg bond index among M–O and M–N bonds, that expresses the bond
order, make their bond lengths be different and are in good agreement with the X-ray crystal
structural data of reported compounds [61], in which the corresponding index shows a shorter
bond distance.

All possible interactions between donor (Lewis-type NBOs) and acceptor (non-Lewis NBOs) are
investigated by NBO analysis. The stabilization energy ∆Eij related to this delocalization is given
by second-order perturbative correction [62]. These interactions are named as ‘delocalization’
corrections to the zeroth-order natural Lewis structure. The most important values that have
resulted from n-n* interactions between lone-pair of oxygen and nitrogen orbitals with anti-lone-
pair of metal orbitals are summarized in table S3-S6. The larger ∆Eij is due to the more intensive
interaction between metal cations and coordinated atoms. This interaction is one of the most
important factors in the self-assembly of coordination polymers. Generally, the bond lengths of
donor and acceptor atoms are in good agreement with stabilization energies, which means
decreasing in bond distance is caused by increasing in stabilization energy.
As depicted in figure 3, both lowest unoccupied and highest occupied molecular orbital of
structure 1 are composed of the acetylacetone ligand, but they are not belong to the same co-
ligand; so, the charge transfer of ligand-to-ligand may be inferred from some contours of
molecular orbital. Highest occupied molecular orbitals (HOMO) of 2 consists of π orbitals of
naphthalene rings, meanwhile, α and β lowest unoccupied molecular orbitals (LUMO) are
centralized on metal center and contrary naphthalene ring, respectively (Fig. S7). Hence, metal-
to-ligand and ligand-to-metal charge transfer can be supposed for this coordination polymer. The
3D plots in structure 3 show that metal cation has the main contribution in HOMO_α orbitals,
while pyridine ring in spacer ligand constructs the LUMO orbitals, which means metal-to-ligand
charge transfer is expected (Fig. S8). The β frontiers orbitals of 3 are mainly include acetylactone
ancillary ligand in which intra-ligand charge transfer can be possible. The lowest α unoccupied
molecular orbital (LUMO) of 4 is localized on acetylacetone ligand and cobalt(II) constructs its
LUMO orbital, which means ligand-to-metal charge transfer (LMCT) is expected. For HOMO_β
and LUMO_β orbitals, the metal center is the main contributor to these orbitals (Fig. S9).

Figure3. Graphical presentation of calculated frontier orbitals [Zn(acac)2L1]n (1)
3.4 Thermal stabilities
To examine the stability of the coordination polymer 2–4, thermogravimetric analyses (TGA) of
these compounds were performed on polycrystalline samples at a heating rate of 5 °C·min-1 in
the temperature range of 25–800 °C in air (Fig. S10-S12). The TGA curves show no weight loss
up to about 190 °C demonstrating that the frameworks of 2–4 are retained up to this temperature
and the solvent molecules are absent. Coordination polymer 2 displays two weight loss steps.
The first discrete weight loss occurs between 190 and 315˚C, presumably due to the
decomposition of naphthalene spacer of L1 ligand (found 24.53%, calc 25.87%). Acetylacetone

co-ligands and pyridine rings of L1 has been lost in a continuous fashion in the range of 316–
540˚C (found 64.26%, calc 65.05%). The solid residue composed after 540˚C can be attributed to
CoO (found 11.66%, calc 12.79%). Framework 3 collapses in three steps. The first weight loss
step in the temperature range of 190–306˚C is probably due to the removal of phenyl fragment of
L2 ligand (found 18.15%, calc 19.03%). This decomposition is continued by the elimination of
one acetylacetone co-ligand up to 395˚C (found 18.40%, calc 18.50%). The last step followed by
release of residual part of L2 and the second co-ligand (found 48.86%, calc 49.62%). The final
remnant mass in 500˚C can be assigned to CoO (found 13.97%, calc 13.78%). The TGA curve
compound 4 exhibits consecutive two stages of thermal decomposition. The first obvious and
sharp weight loss can be associated to destruction of two acetylacetone and also the spacer part
of pyridine rings in L3 (found 49.63%, calc 48.74%). Continuously, a less sharp region is
observed over 275–420°C accompanying the complete decomposition of linker ligand (found
38.76%, calc 39.85%). These data indicate that the synthesized compounds not only have
acceptable stability but also display the same thermal stability, which can be attributed to the
presence of the same metal cation and analogous coordination environment and 1-D linear
structures in all. While, Differences in decomposition modes appear to depend, at least partially,
on the variation of intermolecular interactions among the adjacent 1-D linear chains and usage of
different expanded N,N'-donor linker ligands contain various spacers.
4. Conclusion
In the present paper, four new coordination polymers, [Zn(acac)2(L1)]n (1),[Co(acac)2(L1)]n (2),
[Co(acac)2(L2)]n (3) and [Co(acac)2(L3)]n (4) were synthesized and their structural
characterization has been done by single crystal X-ray diffraction. It gave evidence that the rigid
and long N,N'-bipyridyl-base linker ligands are able to join the Zn(acac)2 and/or Co(acac)2 nodes
through their 4-pyridyl nitrogen-donor atoms to create infinite 1-D linear chain coordination
polymers. The ability of N,N'–donor ligands to participate in intermolecular interactions via the
completely or partly engagement of aromatic groups may help stabilizing of 2-D supramolcular
networks through π···π stacking and C
̶ H···O/π non-classic hydrogen bond between adjacent
chains. Different Co···Co distances subtended by a single linker ligand and various intraligand

geometry among these synthesized compounds declare the remarkable effect of spacers between
pyridine rings on the extended structures. Thermogravimetric analysis has shown common
features with a few differences which reflect the architecture of each individual compound;
analysis showed high thermal stability and same starting decomposition temperature in all
coordination polymers which can be attributed to the structural similarity, but display different
weight loss steps that can probably be due to the influence of the different spacers in the linker
ligands and also variations in the intermolecular interactions. At the end, theoretical calculation
of natural charge, Wiberg bond index and delocalization of charge from lone-pair of donor
orbitals to anti-lone-pair of metal orbitals confirm the stabilization of these compounds and show
the obvious covalent interaction between the coordinated atoms and metal(II) ions.
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★ four new Zn(II) and Co(II) MOCPs have been prepared by reacting of various electron rich
and expanded N,N’-bipyridyl-base spacer ligands with M(acac)2, where acac= acetylacetone,
yielding novel supramolecular networks with 2-D structures and divers architectures.
★ thermogravimetric analysis displayed that the synthesized compounds have similar thermally
stable and exhibit no decomposition up to 190˚C, approximately.
★ Electronic properties, frontier molecular orbital description and the strength of interactions
between metal ion and coordinated atoms were done using NBO analysis.