Syntheses, crystal structures, spectroscopy, and catalytic properties of two nickel-based hexaazamacrocyclic complexes with carboxylate ligands

Article abstract of DOI:10.1016/j.inoche.2017.06.024

Two new hexaazamacrocyclic nickel(II) complexes with the formula [NiL¹(4-nba)₂] (1) and [NiL²(sal)₂] (2) (L¹?=?3,10-dioctyl-1,3,5,8,10,12-hexaazacyclotetradecane, L²?=?3,10-diisobutyl-1,3,5,8,10,12-hexaazacyclotetradecane, 4-nba?=?p-nitrobenzoate, and sal?=?salicylate) were synthesized at room temperature. These complexes were characterized by physico-chemical and spectroscopic methods as well as single-crystal X-ray diffraction analysis. The coordination geometry in complexes 1 and 2 exhibit a distorted octahedron around the nickel(II) ion with hexaazamacrocyclic unit in the equatorial positions and two p-nitrobenzoate (or salicylate) anions in the axial positions. The degradation of methyl orange by potassium persulfate (KPS) in the presence of complex 1 (or 2) oxidation system occurred to near completion in 60?min compared to only 55% with KPS alone under UV light irradiation. Thus, both complexes in cooperation with KPS could be an attractive choice for degradation of organic pollutants for environmental remediation.

Full text of DOI:10.1016/j.inoche.2017.06.024

Accepted Manuscript
Syntheses, crystal structures, spectroscopy, and catalytic
properties of two nickel-based hexaazamacrocyclic complexes
with carboxylate ligands
Surachai Kongchoo, Kittipong Chainok, Anob Kantacha, Sumpun
Wongnawa
PII:
DOI:
Reference:
S1387-7003(17)30264-2
doi: 10.1016/j.inoche.2017.06.024
INOCHE 6691
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
2 April 2017
17 June 2017
28 June 2017
Please cite this article as: Surachai Kongchoo, Kittipong Chainok, Anob Kantacha,
Sumpun Wongnawa , Syntheses, crystal structures, spectroscopy, and catalytic properties
of two nickel-based hexaazamacrocyclic complexes with carboxylate ligands, Inorganic
Chemistry Communications (2017), doi: 10.1016/j.inoche.2017.06.024
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.

ACCEPTED MANUSCRIPT
Syntheses, Crystal Structures, Spectroscopy, and Catalytic Properties of
Two Nickel-Based Hexaazamacrocyclic Complexes with Carboxylate
Ligands
Surachai Kongchooa, Kittipong Chainokb, Anob Kantachac, Sumpun Wongnawaa,*
a Department of Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai,
Songkhla, 90112, Thailand.
b Department of Physics, Faculty of Science and Technology, Thammasat University, Khlong
Luang, Pathum Thani,12120, Thailand.
c Department of Chemistry, Faculty of Science, Thaksin University, Papayom, Phatthalung,
93110, Thailand.
* To whom correspondence should be addressed:
E-mail address: sumpun.w@psu.ac.th
1

ACCEPTED MANUSCRIPT
GRAPHICAL ABSTRACT
2

ACCEPTED MANUSCRIPT
HIGHLIGHTS
 Straightforward synthesis with high yield
 Only small amount required in the degradation reaction
 Degradation reaction proceeded faster than without the catalyst
 Highly recyclable and robust
3

ACCEPTED MANUSCRIPT
GRAPHICAL ABSTRACT - SYNOPSIS
[NiL1](ClO4)2 and [NiL2](ClO4)2 complexes were synthesized and used as precursor to
synthesize [NiL1(4-nba)2] and [NiL2(sal)2] complexes which were characterized with various
techniques and X-ray diffraction analysis. Subsequently, the catalytic properties of both
complexes in the degradation of methyl orange were investigated.
4

ACCEPTED MANUSCRIPT
ABSTRACT:
Two new hexaazamacrocyclic nickel(II) complexes with the formula [NiL1(4-nba)2] (1)
and [NiL2(sal)2] (2) (L1 = 3,10-dioctyl-1,3,5,8,10,12-hexaazacyclotetradecane, L2 = 3,10-
diisobutyl-1,3,5,8,10,12-hexaazacyclotetradecane, 4-nba = p-nitrobenzoate, and sal =
salicylate) were synthesized at room temperature. These complexes were characterized by
physico-chemical and spectroscopic methods as well as single-crystal X-ray diffraction
analysis. The coordination geometry in complexes 1 and 2 exhibit a distorted octahedron
around the nickel(II) ion with hexaazamacrocyclic unit in the equatorial positions and two p-
nitrobenzoate (or salicylate) anions in the axial positions. The degradation of methyl orange
by potassium persulfate (KPS) in the presence of complex 1 (or 2) oxidation system occurred
to near completion in 60 min compared to only 55 % with KPS alone under UV light
irradiation. Thus, both complexes in cooperation with KPS could be an attractive choice for
degradation of organic pollutants for environmental remediation.
KEYWORDS: Nickel(II) complexes; Crystal structures; π-π stacking interactions; Persulfate
activation; Methyl orange degradation; Advanced oxidation process
5

ACCEPTED MANUSCRIPT
The increasing world population with growing industrial demands has led to a situation
where protection of the environment has become a major problem and an importance factor
for several industrial processes. Needless to say, the dye pollutants from textile industry are
among those important sources of environmental contaminations. The resulting colored
wastewater imposes a major problem for the industry as well as a serious threat to the
environment [1,2]. Many studies have been carried out in which chemical and physical
processes such as electrochemical process, ion-exchange, flocculation and coagulation
technique, biological treatment, adsorption on activated carbon have been applied for color
removal from textile wastewater [3-5].
Moreover, the advanced oxidation processes, or the so-called “AOPs”, have been
widely studied for the treatment of industrial wastewaters. In general, AOPs are mainly based
on the generation of the highly reactive oxidizing species, hydroxyl radical, which may be
generated by various methods such as UV photolysis of hydrogen peroxide/ozone (H2O2/O3)
[6], photochemical catalysis (UV/TiO2) [7], Fenton’s reaction [8], and persulfate oxidation
2-
[9,10]. The activated persulfate oxidation uses persulfate anion (S2O8 ) which is also known
as peroxydisulfate or peroxodisulfate. Persulfate salts of (NH4)2S2O8, Na2S2O8, and K2S2O8
dissociate in water to yield the persulfate anion which is one of the very strong oxidation
species with high potential (Eo = 2.01 V) compared with H2O2 (Eo = 1.76 V). It shows some
advantages over other oxidants as being a solid chemical at room temperature with the ease
of storage and transport, high stability, high aqueous solubility, and relatively low cost [11].
There are several methods to activate the persulfate anions, such as thermal activation [12],
UV irradiation [13], base activation [14], and activation with transition metal catalysts [15],
·-
to generate reactive sulfate radicals (SO4 ). Activation of persulfate by metal ion catalysis
occurs by an oxidation - reduction reaction in which low valent metal ions (Mn+) act as
electron donors. Heterogeneous catalysis of the nickel(II) complexes with N-donor atoms
from the hexaazamacrocyclic and carboxylate mixed ligands have been extensively used in
the preparation of coordination polymers (CPs) [16,17]. In the family of aromatic carboxylate
ligands, for example, 1,3,5-benzenetricarboxylate (BTC3-), 2,2′-biquinoline-4,4′-
dicarboxylate (BQDC2-), and isonicotinnate the coordination network through space in one-,
two- or three-dimensional architectures have been reported [18,19]. The dimensionality of the
resulting coordination network can be increased by non-covalent interactions, such as
hydrogen bonding, ··· stacking, C-H···, and other van der Waals interactions [20]. The
coordination polymers have been an active research area in recent years because it can be
6

ACCEPTED MANUSCRIPT
applied in many fields such as magnetism, electronics, luminescence, and catalytic activity
[21,22].
In this report, we describe the syntheses, characterizations, and crystal structures of two
nickel(II) complexes, that is, [NiL1(4-nba)2] (1) and [NiL2(sal)2] (2) (L1 = 3,10-dioctyl-
1,3,5,8,10,12-hexaazacyclotetradecane, L2 = 3,10-diisobutyl-1,3,5,8,10,12-hexaazacyclotetra-
decane, 4-nba = p-nitrobenzoate, and sal = salicylate). In addition, catalytic capabilities of
these complexes working in cooperation with persulfate (from KPS) for the degradation of
methyl orange (MO) were investigated. For the preparation, 1 and 2 were synthesized at room
temperature in mixed MeCN/H2O solvent with triethylamine added for adjusting the pH of
the solution (See supporting information). The carboxylate ligands replaced the perchlorate
anions in the axial positions of both complexes. It is worth mentioning that during the
reaction of complex 2, the acetoxy group of the aspirinate ligand underwent hydrolysis
process resulting in the cleavage of a covalent bond in a molecule to give the salicylate ligand
which was confirmed in the structure by X-ray diffraction technique.
Single crystal X-ray diffraction analysis (See supporting information) showed that 1
crystallizes in the monoclinic space group of C2/c and containing [NiL1]2+ cation and two p-
nitrobenzoate anions as shown in Fig. 1a and packing diagram in Fig. 1c. The nickel(II) ion
of 1 is in a distorted octahedral geometry by binding to the four secondary nitrogen atoms of
the hexaazamacrocycle (Ni(1)-N(1) = 2.0562(19) and Ni(1)-N(3) = 2.0720(2) Å) forming the
equatorial plane in a square planar fashion and the distances are comparable to other
hexaazamacrocycle Ni(II) complexes with Ni-N distances in the range 2.0535(2)-2.0729(13)
Å [23,24]. Two oxygen atoms from two p-nitrobenzoate anions are located at the axial
positions (Ni(1)-O(1) = 2.1407(15) and Ni(1)-O(1)i = 2.1407(15) Å). The six-membered
chelate ring (N(1)-Ni(1)-N(3) = 93.98(8)o and N(1)i-Ni(1)-N(3)i = 93.98(8)o) adopt chair
conformation and the five-membered chelate ring assume gauche conformation (N(1)-Ni(1)-
N(1)i = 85.96(11)o and N(3)-Ni(1)-N(3)i = 86.09(11)o) [30]. The C-N-C angles of the tertiary
nitrogen atoms (N3 and its equivalent atoms) are in the range of 113.63(3)-118.13(4)o which
are larger than the ideal tetrahedral angle (109.5o). Thus, the tertiary nitrogen atoms (N3)
could use the sp2-like hybridization similar to the other hexaazamacrocyclic complexes
containing uncoordinated tertiary amino groups [25,26].
The structure of 1 shows intramolecular interactions of N-H···O hydrogen bonds
between the secondary amine groups of the hexaazamacrocyclic ligand and the oxygen atoms
7

ACCEPTED MANUSCRIPT
of carbonyl group on the p-nitrobenzoate ion (N(1)-H(1)···O(2) = 2.070Å and
(N1)(H1)(O2) = 149o) and the intermolecular interactions of N-H···O hydrogen bonds
between the nitrogen atoms of secondary amine groups and the oxygen atoms of nitro group
on the p-nitrobenzoate anions (N(3)-H(3)···O(4)ii = 2.28Å and (N3)(H3)(O4)ii (-x+1/2, -
y+1/2, -z+1) = 145o). The average bond lengths and bond angles of 1 are very similar to those
of previously reported [Ni(C16H38N6)(nicotinate)2] complex [27]. In addition, there are face-
to-face π-π stacking interactions at the distance 3.894 Å. These hydrogen bonds and π-π
stacking interactions in 1 influence the stability of the structure which, finally, generating a
one-dimensional network structure as shown in Fig. 1b.
(a)
(b)
(c)
8

ACCEPTED MANUSCRIPT
Fig. 1. Molecular structure of 1, (a) Coordination environment of a mononuclear Ni(II) ion
with the numbering scheme. (b) The 1D supramolecular network via the H-bonding and π-π
stacking interactions. (c) The packing unit cell, viewed along the a axis.
From the X-ray single crystal diffraction analysis (See supporting information),
complex 2 crystallizes in the monoclinic system with space group P21/n. The structure
asymmetric unit of 2 consists of a basic [NiL2]2+ cationic part (L2 = 3,10-diisobutyl-
1,3,5,8,10,12-hexaazacyclotetradecane) and two salicylate anions as shown in Fig. 2a. The
molecular packing arrangement of 2 plotted along the a axis is shown in Fig. 2b. In the
structure of 2, each nickel(II) ion has a distorted octahedral geometry with the two secondary
and two tertiary amines of the hexaazamacrocycle and two salicylate anions. The Ni(1)-N(2)
(tertiary amines) distance of 3.301 Å is significantly longer than the average Ni(1)-N(1) and
Ni(1)-N(3) (secondary amines) distances of 2.055 Å indicating no participation in the
coordination. The average Ni-N distance (2.096 Å) in NiN4 plane is significantly longer than
in the square-planar geometry of [Ni(LR,R)](ClO4)2 (LR,R = 1,8-di((R)-α-methylnaphthyl)-
1,3,6,8,10,13-hexaazacyclotetradecane) complex (Ni-N = 1.941 Å) [28] but is similar to that
observed for the octahedral nickel(II) complexes with 14-membered hexaazamacrocyclic
ligands [29]. Moreover, C-N (tertiary amines) bond distances are much shorter than other C-
N single bond distances indicating possibility of the sp2-like hybridization of the
uncoordinated tertiary nitrogen. The bond angles of the six-membered rings (N(1)-Ni(1)-N(3)
= 94.10(7)o and N(1)i-Ni(1)-N(3)i = 94.11(7)o) are large than those of the five-membered
rings (N(3)-Ni(1)-N(1)i and N(3)i-Ni(1)-N(1) = 85.89(7)o). The axial Ni(1)-O(1) bonds are
not quite perpendicular to the NiN4 equatorial plane as the O(1)axial-Ni(1)-Nequatorial angles
ranging between 88.80(7)-93.09(6)o. Interestingly, the secondary amines of the
hexaazamacrocyclic ligand form intramolecular hydrogen bonds with carboxylate oxygen
atoms [N(3)-H(3A)···O(2)i = 1.96 Å and 151o; symmetry code: (i) -x+2, -y+1, -z+1] and
hydrogen atoms of the hydroxyl groups of the salicylate ligand [O(3)-H(3)···O(2) = 1.77 Å
and 147o] and weak interactions of hydroxyl oxygen atoms of the salicylate ligand with the
adjacent of the hydrogen atoms of the i-butylamine act as pendant arms [C(6)-H(6)···O(3)ii =
2.51 Å and 165o; symmetry code: (ii) -x+1, -y+1, -z+1], as shown in Fig. 2b.
9

ACCEPTED MANUSCRIPT
(a)
(b)
Fig. 2. Molecular structure of 2, (a) Coordination environment of a mononuclear Ni(II) ion
with the numbering scheme. (b) The packing unit cell, viewed along the a axis.
PXRD pattern was used to check the phase purity of 1 and 2 (Figs. S1 and S2). It can be
seen that all the peaks present in the measured patterns match the simulated patterns from the
single crystal diffraction data indicating the homogeneity of the bulk synthesized material and
the as-grown crystals. The FT-IR spectrum (Fig. S3) of 1 shows very strong bands at 3301
and 3186 cm-1 that are assigned to the coordinated secondary amine groups. The bands
around 2926-2853 cm-1 can be assigned to sp2 C-H stretching mode of hexaazamacrocyclic
ligands. The sharp vibrations at 1590 and 1340 cm-1 can be assigned to the C-O of a p-
nitrobenzoate ion [27]. The weak band at 723 cm-1 is assigned to δ(C-H) of long chain alkyl
group of the pendant arm [30]. The bands at 517 and 429 cm-1 can be assigned to the Ni-O
and Ni-N, respectively [31]. The FT-IR spectrum (Fig. S4) of 2 shows very strong band at
3193 cm-1 due to the coordination of the secondary amine groups of hexaazamacrocyclic
ligands. The C=O bands of the salicylate anions appear at 1597 and 1350 cm-1, respectively.
10

ACCEPTED MANUSCRIPT
The positions of these carboxyl group stretching vibrations and the separation between them
are in the range expected for the monodentate carboxyl groups [32, 33]. In both complexes,
no strong bands at 1068 and 621 cm-1 of the perchlorate anions from starting materials are
present indicating the perchlorate has been replaced by carboxylate anions [30].
The thermal stability of 1 and 2 were examined by thermogravimetric analysis (TGA)
in N2 atmosphere from 50 oC to 1,000 oC, as shown in Fig. S5. Decomposition of 1 composed
o
of the first weight loss 50.98 % in the range 230-323 C corresponding to the loss of the
macrocyclic ligand (calculated 51.92 %). On further heating, the second weight loss (40.61
o
%) was observed in the range 323-900 C with the loss of p-nitrobenzoate ligand (calculated
39.82 %). Final residue, presumably a nickel oxide, 8.41 % was obtained (calculated: 8.26
%). The two steps decomposition was also observed for 2 from the TGA curve with the first
o
weight loss of 47.93 % occurred within the range 249-355 C corresponded to the release of
the macrocyclic ligand (calculated 48.22%). The second weight loss of 41.87 % took place at
o
355-900 C ascribable to the loss of salicylate ligand (calculated 42.66 %). The remaining
weight corresponded to NiO was 10.20 % (calculated 9.12 %).
Persulfate oxidation, a member in the class of advance oxidation process, has been
applied to degrade organic dyes such as crystal violet [11] and methyl orange [34].
Nevertheless, the uncatalyzed reaction efficiency is very low. In the presence of catalyst,
persulfate oxidation proceeds via production of strongly oxidizing sulfate radicals. In this
regard, the degradation efficiency of the catalytic oxidation of methyl orange was studied
using KPS as an oxidant and complex 1 (or 2) as heterogeneous catalyst. The degradation of
methyl orange with KPS alone was slow with almost no change in color even up to 80 min of
reaction (Fig. S6). However, when 1 (or 2) was present the reaction took place almost
immediately as indicated by the color change of methyl orange dye (Figs. S7 and S8). Under
the same conditions, the photodegradation efficiency of 1 was higher than that of 2 (Fig. 3)
which could be due to the directionality of metal-ligand interactions and the supramolecular
interactions leading to formation of channels that could be accessed from the exterior
allowing electron transfer and the incorporation of substrates inside the crystal. The reaction
using 1 as catalyst could degrade 96 % of methyl orange in 60 min but only 83 % with 2.
With KPS alone, in the same time span, the degradation was only 55 %. This could be the
result of the presence of 1 (or 2) facilitated the generation of sulfate radicals from KPS to
proceed more effectively. Recent reports of complexes of the same type, i.e. with large
organic ligands, are those of Subramanian et al. 2015 [34] and Feng et al. 2014 [35]. The
11

ACCEPTED MANUSCRIPT
former reported the Ni(II) macrocyclic complex which could catalyse persulfate oxidative
degradation of methyl orange. The Ni(II) complex could behave as homogeneous catalyst as
well as heterogeneous catalyst by fixing it on adsorbents such as activated carbon or
Amberlite. The degradation mechanism was proposed to proceed via the Ni(III) intermediate.
The latter reported M(II) complexes (M = Zn, Cd) which behaved as heterogeneous catalysts
working with persulfate in degradationof methyl orange. The key step was proposed as via
M(III) intermediates. In the present work, complex 1 (and 2) behaved as heterogeneous
catalysts and we deemed that similar mechanism could be applied. The surface adsorbed
2-
S2O8 anion accepted electron from solid complex 1 (or 2) leading to the formation of
transient Ni3+ in the solid complex which was quickly reduced back to Ni2+ complex after the
completion of first cycle (Scheme 1) [15,36]. Meanwhile, the generated sulfate radicals went
on to degrade the organic dyes. The sulfate radical is non-selective and strongly reactive
oxidant. It has been shown that the sulfate radical could oxidize a wide variety of organic
dyes pollutants such as Congo red [37-39] and methylene blue [40].
Fig. 3. Degradation efficiency (%) of methyl orange versus reaction time under UV
irradiation.
12

ACCEPTED MANUSCRIPT
Scheme. 1. The proposed catalytic mechanism.
Both catalysts, 1 and 2, are very robust as they survived unscratched through 5 cycles
of recycling use. During the photodegradation tests, the catalyst could be simply isolated by
centrifugation and reused by adding it into the freshly-prepared methyl orange solution to
start the next round of photodegradation reaction. The efficiency of methyl orange
degradation remained almost unchanged throughout 5 cycles of tests (Fig. 4) and not
significantly lower than when it was first used (Fig. 3). The FT-IR spectra of the catalysts 1
and 2 isolated after the 5th cycle (Figs. S9 and S10) matched well with the initial spectra of
both complexes (Figs. S3 and S4). The color of 1 remained unchanged both before and after
the reaction while the color of 2 after used became almost undistinguishable paler than before
used (Fig. S11). All these results indicated that both 1 and 2 are attractive catalysts in terms
of efficiency and robustness, especially complex 1 which showed higher efficiency than 2
while all the other data remained the same.
13

ACCEPTED MANUSCRIPT
Fig. 4. The recyclability of complexes 1 and 2 with KPS as oxidant for the photodegradation
of methyl orange solution under UV irradiation.
In conclusion, two new hexaazamacrocyclic nickel(II) complexes were synthesized and
structurally characterized. The nickel(II) ions in 1 and 2 have a distorted octahedral geometry
with hexaazamacrocyclic ligands in a planar conformation and p-nitrobenzoate (for 1) and
salicylate (for 2) ions in the axial positions. The structure of 1 exhibiting a one-dimensional
network is further interconnected by intramolecular N-H···O interactions between secondary
amines and oxygen atoms of the p-nitrobenzoate ligand and intermolecular π-π stacking
interactions. The salicylate ligand in the structure of 2 was the result from the cleavage of the
aspirinate molecule during the synthesis reaction. These structures display intramolecular
interaction between secondary amines and oxygen atoms of the salicylate ligand. In addition,
the degradation of methyl orange with activation of persulfate in the presence of each
complex showed that complex 1 had higher efficiency for the degradation of methyl orange
dye than complex 2 within the same time span.
Acknowledgements
This work was supported by the Songklanagarind Scholarship for Graduate Studies
from the Prince of Songkla University. We also thank the Department of Chemistry, Faculty
of Science, Prince of Songkla University for supporting this research financially.
References
[1] N. Modirshahla, M.A. Behnajady, Dyes Pigm. 70 (2006) 54-59.
[2] L.M.D. Silva, W.F. Jardim, Quim. Nova 29 (2006) 310-317.
[3] B.R. Babu, A.K. Parande, S.A. Kumar, S.U. Bhanu, Open J. Safe. Sci. Technol. 1 (2011) 12-18.
[4] S.S. Moghaddam, M.R.A. Moghaddam, M. Arami, J. Hazard. Mater. 175 (2010) 651-657.
[5] K. Mahmoudi, N. Hamdi, A. Kriaa, E. Srasra, Russ. J. Phys. Chem. A 86 (2012) 1294-1300.
[6] J.-C. Geng, L. Qin, C.-H. He, G.-H. Cui, Transition Met. Chem. 37 (2012) 579-585.
14

ACCEPTED MANUSCRIPT
[7] I.K. Konstantinou, T.A. Albanis, Appl. Catal. B 49 (2004) 1-14.
[8] S.H. Bossmann, E. Oliveros, S. Göb, S. Siegwart, E.P Dahlen, L. Payawan, M. Straub, M.
Wörner, A.M. Braun, J. Phys. Chem. A 102 (1998) 5542-5550.
[9] X.-R. Xu, S. Li, Q. Hao, J.-L. Liu, Y.-Y. Yu, H.-B. Li, Int. J. Environ. Bioener. 1 (2012) 60-81.
[10] C. Qi, X. Liu, X. Zhang, J. Ma, H. Tan, W. Ye, Chem. Eur. J. 249 (2014) 6-14.
[11] S. Gokulakrishnan, P. Parakh, H. Prakash, J. Hazard. Mater. 213-214 (2012) 19-27.
[12] M. Nie, Y. Yang, Z. Zhang, C. Yan, X. Wang, H. Li, W. Dong, Chem. Eng. J. 246 (2014) 373-
382.
[13] Y.-Q. Gao, N.-Y. Gao, Y. Deng, Y.-Q. Yang, Y. Ma, Chem. Eng. J. 195-196 (2012) 248-253.
[14] O.S. Furman, A.L. Teel, R.J. Watts, Environ. Sci. Technol. 44 (2010) 6423-6428.
[15] C.-H. Jiao, C.-H. He, J.-C. Geng, G.-H. Cui, Transition Met. Chem. 37 (2012) 17-23.
[16] J.-W. Cui, S.-X. Hou, K.V. Hecke, G.-H. Cui, Dalton Trans. 46 (2017) 2892-2903.
[17] S.Y. Hao, S.-X. Hou, K.V. Hecke, G.-H. Cui, Dalton Trans. 46 (2017) 1951-1964.
[18] T.-B. Lu, H. Xiang, R.L. Luck, Z.-W. Mao, D. Wang, C. Chen, L.N. Ji, Cryst. Eng. Comm. 41
(2001) 1-2.
[19] K.S. Min, M.P. Suh, Eur. J. Inorg. Chem. 2001 (2001) 449-455.
[20] J.-C. Geng, L. Qin, X. Du, S.-L. Xiao, G.-H. Cui, Z. Anorg. Allg. Chem. 638 (2012) 1233-
1238.
[21] J.M. Hu, V.A. Baltov, B. Yu, K.V. Hecke, G.-H. Cui, Dalton Trans. 45 (2016) 2426-2429.
[22] G.P. Yang, L. Hou, X.-J. Luan, B. Wu, Y.-Y. Wang, Chem. Soc. Rev. 41 (2012) 6992-7000.
15

ACCEPTED MANUSCRIPT
[23] J.H. Park, A.R. Jeong, D.K.A.K. Hastuti, M.J. Jeong, K.S. Min, J. Incl. Phenom. Macrocycl.
Chem. 82 (2015) 153-162.
[24] J.H. Park, A.R. Jeong, J.W. Shin, M.J. Jeong, C.S. Cho, K.S. Min, Inorg. Chem. Commun. 57
(2015) 44-46.
[25] Y.-W. Li, H. Xiang, T.-B. Lu, S.W. Ng, Acta Crystallogr., Sect. E 60 (2004) 309-311.
[26] S. Han, T. Kim, A.J. Lough, J.C. Kim, Inorg. Chim. Acta 370 (2011) 170-174.
[27] J.W. Shin, D.-W. Kim, D. Moon, Polyhedron 105 (2016) 62-70.
[28] K.S. Min, M.J. Park, J.J. Ryoo, Chirality 25 (2013) 54-58.
[29] S.-G. Kang, K. Ryu, S.-K. Jung, J. Kim, Inorg. Chim. Acta 293 (1999) 140-146.
[30] S. Kongchoo, A. Kantacha, S. Saithong, S. Wongnawa, J. Chem. Crystallogr. 46 (2016) 222-
229.
[31] J.C. Kim, J.A. Kim, Y.C. Kang, Transition Met. Chem. 31 (2006) 829-833.
[32] A.L. Abuhijleh, C. Woods, Inorg. Chem. Commun. 4 (2001) 119-123.
[33] G.B. Deacon, R.J. Phillips, Coord. Chem. Rev. 33 (1980) 227-250.
[34] G. Subramanian, P. Nalawade, S.J. Hinder, S.C. Pillai, H. Prakash, RSC Adv. 5 (2015) 31716-
31724.
[35] Z.-Q. Feng, X.-L. Yang, Y.-F. Ye, J. Inorg. Organomet. Polym. 24 (2014) 684-693.
[36] L. Qin, S.L. Xiao, P.J. Ma, G.-H. Cui, Transition Met. Chem. 38 (2013) 627-633.
[37] J.-C. Geng, L.W. Liu, S.L. Xiao, G.-H. Cui, Transition Met. Chem. 38 (2013) 143-148.
[38] L. Qin, L.-W. Liu, X. Du, X., G.-H. Cui, Transition Met. Chem. 38 (2013) 85-91.
16

ACCEPTED MANUSCRIPT
[39] Z.-T. Zhang, Y.Y. Wang, R. Yang, R. Liu, Y. Zhao, G.-H. Cui, Transition Met. Chem. 41
(2016) 169-178.
[40] C.-C. Wang, H.-P. Jing, P. Wang, J. Mol. Struct. 1074 (2014) 92-99.
17