Construction of Three Novel Coordination Complexes by 3-Nitrophthalic Acid Plus N-Donor Ligands: Synthesis, Structure, and Properties

In order to explore new coordination frameworks with novel designed 3-nitrophthalic acid and N–donor ancillary ligands, three novel coordination complexes, namely, [Co2(3-NPA)2(2,2′-bipy)2(H2O)2]•2H2O (1), [Mn2(3-NPA)2(4,4′-bipy)3(H2O)6]•(4,4′-bipy) (2), and [Pb2O(3-NPA)]n (3) (where 3-NPAH2 = 3-nitrophthalic acid, 2,2′-bipy = 2,2′-bipyridine, 4,4′-bipy = 4,4′-bipyridine), have been hydrothermally synthesized. X-ray structure analysis reveals that 1 and 2 are dinuclear structures, while 3 is a two-dimensional (2D) network polymer. And the hydrogen bonds and π–π stacking also play important roles in affecting the final structure where complexes 1-2 have 3D and 2D supramolecular architectures, respectively. These complexes have been characterized by powder X-ray diffractions (PXRD) and thermal gravimetric analyses (TGA). In addition, their photochemical properties have also been investigated.


Introduction
The design and synthesis of zero-, one-, two-, or three-dimensional (0D, 1D, 2D, or 3D) coordination assemblies by utilizing directional metal−ligand dative bonds has attracted considerable interest in recent years, which may bring both intriguing architectures and tailor-made applications in such fields as porosity, magnetics, optoelectronics, catalysis, and so on [1,2]. As it is known, it is still a big challenge to predict the final structures of desired crystalline products, since the self-assembly process of crystalline products is influenced by many factors, such as metal ions, organic ligands, counter ions, solvent system, temperature, and pH of reaction system [3][4][5]. The delicate balance between the adaptability of the organic ligands with the plentiful and versatile coordination modes of the central metals as well as the coparticipation of counteranions and solvent molecules leads to the formation of either discrete polynuclear complexes or infinite coordination polymers, affording great opportunities for the construction of novel and unusual metal−organic crystalline materials [6,7]. In order to obtain new coordination complexes with various topological structures, multicarboxylates are often selected as bridging ligands to construct coordination complexes not only due to their versatile coordination modes to metal centers but also their strong ability to act as hydrogen bonding acceptors and donors [8][9][10][11]. Especially, the 3-nitrophthalic acid can serve as excellent candidates for building highly connected or helical coordination frameworks due to their versatile bridging fashions [12,13]. Apart from the carboxylate linkers, N-donor ligands are frequently used as ancillary ligands to give multipodal anions acting as bridging, chelating, and charge balance ligands for synthesizing polynuclear species [14]. Moreover, the hybrid coordination complexes constructed by 3-nitrophthalic acid are rarely documented to date. With this understanding, 3-NPAH 2 (3-nitrophthalic acid) was chosen as the organic ligands, 2,2 -bipy and 4,4 -bipy were chosen as neutral co-ligands to construct new coordination complexes under the hydrothermal reaction. This paper presents the syntheses, structures, thermal stabilities, luminescent properties, and powder X-ray diffraction (PXRD) of three new coordination complexes [Co 2 (3-NPA) 2 (2,2 -bipy) 2 (H 2 O) 2 ]•2H 2 O (1), [Mn 2 (3-NPA) 2 (4,4 -bipy) 3 (H 2 O) 6 ]• (4,4 -bipy) (2), and [Pb 2 O(3-NPA)] n (3).

Experimental
All chemicals were commercial materials of analytical grade and used without purification. Elemental analysis for C, H, and N was carried out on a Perkin-Elmer 2400 II elemental analyzer. The FT-IR spectrum was obtained on a PE Spectrum One FT-IR Spectrometer Fourier transform infrared spectroscopy in the 4000-400 cm −1 regions, using KBr pellets. Perkin-Elmer Diamond TG/DTA thermal analyzer was used to record simultaneous TG and DTG curves in the static air atmosphere at a heating rate of 10 K min −1 in the temperature range 25 to 1000 • C using platinum crucibles. Fluorescence spectra were recorded with F-2500 FL Spectrophotometer analyzer. PXRD patterns were obtained using a pinhole camera (Anton Paar) operating with a point-focused Ni-filtered Cu Kα radiation in the 2θ range from 5 • to 50 • with a scan rate of 0.08 • per second.

Crystal Structure Determination
Suitable single crystal with approximate dimensions were mounted on a glass fiber and used for X-ray diffraction analyses. Data were collected at 293 (2) K on a Bruker Apex CCD diffractometer using the ω scan technique with Mo Kα radiation (λ = 0.71069 Å). Absorption corrections were applied using the multi-scan technique [15]. The structures were solved by the Direct Method and refined by full-matrix least-square techniques on F 2 using SHELXL-97 [16]. All nonhydrogen atoms were refined anisotropically. The crystal data and structure refinement details for three complexes are shown in Table 1. Selected bond lengths and angles of the complexes are listed in Table 2, and possible hydrogen bond geometries are given in Table 3.

Description of the Structure Description of the Structure
The single-crystal X-ray diffraction analysis reveals that complexes 1 and 2 crystallize in the triclinic P-1 space group, and 3 belongs to the orthorhombic system with space group Cmca.
As shown in Fig. 6, the asymmetric unit of 3 is composed of two Pb(II) ions, one 3-NPA 2− anion and a μ4-O. In complex 3, there are two different kinds of metal centers in which Pb1 is coordinated to three carboxylate oxygen atoms (O6, O6#1, and O3#6) from two 3-NPA 2− and two μ4-oxygen atoms (O5, O5#3), whereas Pb2 ion possesses a five-coordinated sphere but the difference in this is coordinated by three carboxylate oxygen atoms (O6, O6#2, and O4#5) from three different 3-NPA 2− . The average Pb1-O and Pb2-O distances are 2.404 (2) and 2.466 (6) Å, respectively, both of which are in the normal range. The four Pb(II) ions form a tetranuclear unit [Pb 4 (μ4-O) 2 ] 4+ through bonding from one bridging μ4-O. This irregular polyhedron Pb 4 O 2 cluster is rarely documented in the literature [18]. Each cluster is linked to two neighboring ones by two 3-NPA 2− ligands. The coordination modes of 3-NPA 2− are shown in Supplementary Fig. S1c, and the 3-NPA 2− ligand shows μ5-η 2 :η 2 :η 1 :η 1 coordination mode in bridging fashion. Bonding of the 3-NPA 2− ligands to the Pb 4 O 2 clusters leads to a 2D network structure (Fig. 7). Notably, the open framework in 3 is sustained exclusively by 1D metal cluster chain running along the ac-direction as shown in Fig. 8. This packing gives rise to rhombic-shaped channels with a dimension of ca. 2.3039 (36) × 2.3039 (36) Å 2 and 2.3012 (36) × 2.3012 (36) Å 2 . The weaker nonclassical hydrogen bonds were observed between C-H moieties and the coordinated carboxylate oxygen atom (O4) as well as the uncoordinated carboxylate

Comparison of the Structures
It is known that multicarboxylate ligands have been proved to be excellent structural constructors due to their various coordination modes [19]. The different structures of the complexes 1-3 indicate that the multicarboxylate ligand have great influence on the structures of the complexes due to their different coordination modes. Therefore, comparison and comprehension of the coordination modes of the carboxylate ligand are a good and feasible method to predesign the coordination complexes. There are three kinds of coordination modes of 3-NPA 2− in the complexes 1-3 described above (Supplementary Fig. S1). In complex 1, each 3-NPA 2− anions link two Co(II) cations in a μ2-η 1 :η 1 :η 1 :η 0 coordination mode ( Supplementary Fig. S1a) to gain 0D molecular rings. However, in complex 2, each 3-NPA 2− anion coordinates one Mn(II) cation, displaying a μ1-η 1 :η 0 :η 0 :η 0 coordination mode ( Supplementary Fig. S1ba), yielding a 0D molecular chains. Interestingly, in 3, each 3-NPA 2− anion links five Pb(II) cations in a μ5-η 2 :η 2 :η 1 :η 1 coordination mode; in the coordination mode, the 3-NPA 2− anions bridge the Pb(II) cations to form a 2D network structure. On the basis of the above description, it can be seen that the variations in the coordination modes of 3-NPA 2− anions can result in the structure difference of the compounds.
From the structural descriptions above, it can be seen that the N-donor ligands also have a significant effect on the construction of various structures. The introduction of the chelating 2,2 -bipy ligand generally results in drastic change of the structures. In 3, the 3-NPA 2− anions bridge divalent metal cations to gain a 2D layer. However, when 2,2 -bipy is introduced into the reaction system of 3, a 0D dimeric structure is obtained. The formation of the low dimensional structure of 1 may be attributable to the steric hindrance of the chelating 2,2 -bipy ligand. From the results above, we can see that the N-donor ligands play an important role in the formation of the final complex structures. In addition, the hydrogen  bonds and π -π stacking also play important roles in affecting the final structures where complexes 1 and 2 have 3D and 2D supramolecular architectures, respectively.

IR Spectrum
In the IR spectra of the complexes 1 and 2, the strong and broad bands at about 3500-3000 cm −1 region are attributed to the symmetric O-H stretching modes and O-H bending modes, respectively. υ as COO appears strong peaks at 1601 and 1576 cm −1 in complex 1, 1602 and 1555 cm −1 in 2, and 1611 and 1574 cm −1 in 3. υ s COO appears peaks at 1403 and 1383 cm −1 in 1, 1450 and 1381 cm −1 in 2, and 1461 and 1391 cm −1 in 3. For complex 1, the strong peak at 1542 cm −1 is attributed for υ as NO 2 and additional peaks at 1338 and 1522 cm −1 are attributed for υ s NO 2 and C N. For complex 2, the strong peak at 1530 is attributed for υ as NO 2 and additional peaks at 1343 and 1489 cm −1 are consistent with υ s NO 2 and C N. In addition, for complex 3, the strong peak at 1529 is attributed for υ as NO 2 and the additional peak at 1344 cm −1 is consistent with υ s NO 2 .

XRD Patterns
To confirm the phase purity of the bulk materials, PXRD experiments have been carried out for complexes 1-3. The XRPD experimental and computer-simulated patterns of the corresponding complexes are shown in Supplementary Fig. S2. Although the experimental patterns have a few un-indexed diffraction lines and some are slightly broadened in comparison with those simulated from the single-crystal models, it can be still well considered that the bulk synthesized materials and the crystals used for diffraction are homogeneous.

Fluorescence Properties
Luminescent complexes are of great interest due to their various applications in chemical sensors, photochemistry, and light-emitting diodes (LEDs) [20,21]. Hence, the solid state photoluminescence properties of 3-NPAH 2 ligand and complexes 1-3 were investigated at room temperature ( Fig. 9) under the same experimental conditions. In the solid state, the strongest emission peak for the free ligand 3-NPAH 2 is at 438 nm with the excitation peak at 233 nm, which is attributed to the π * -n transitions [22]. The strongest excitation peaks for 1-3 are at 285, 230, and 255 nm, emission spectra mainly show strong peaks at 474, 462, and 441 nm, respectively. The ligand chelation to the metal center may effectively increase the rigidity of the ligand and reduce the loss of energy by radiationless decay, thus causing the red shift in 1, 2, and 3. Therefore, the luminescence behavior of complexes is caused by metal ligand charge transfer (MLCT) [23].

Thermogravimetric (TG) Analyses
In order to study the framework stability of the title complexes, the thermogravimetric (TG) analysis was performed in N 2 atmosphere on polycrystalline samples of complexes 1-3, and the TG curves are shown in Fig. 10. The TG curve of 1 shows the first loss of 7.79% in the temperature range of 89 to 186 • C, which indicates the exclusion of watermolecules and coordinated water molecules (calcd., 7.83%); The second stage occurs between 187 and 251 • C, the anhydrous complex loses 33.89% of total weight, which is due to the decomposition of two 2,2 -bipy (calcd., 33.93%). The final weight loss of 44.08% (calcd., 45.43%) corresponds to the loss of two 3-NPA 2− in the temperature range of 252 to 991 • C. The remaining residue corresponds to the formation of CoO (obsd., 14.24%; calcd., 16.28%).
For 2, the weight loss attributed to the gradual release of coordinated water molecules is observed in the range of 66 to 159 • C (obsd., 8.63%; calcd., 8.57%). When the temperature holds on rising, the product lost 49.37% of the total weight in the temperature range of 157 to 341 • C, which is related to the loss of 4,4 -bipy (calcd., 49.54%). Beyond 341 • C, the 3-NPA 2− decomposes gradually, the products lose 32.62% of the total weight (calcd., 33.17%). The residual percentage weight at the end of the decomposition of the complex is observed as 12.04%. Then the remaining weight is assigned to MnO (obsd., 12.04%; calcd., 12.25%).
TG curve of the complex 3 is presented in Fig. 10, where one weight loss step exists and the decomposition mainly takes place in the temperature range of 67 to 602 • C. The weight loss stage may be related to the removal of 3-NPA 2− (found, 30.98%; calcd., 32.69%). The residual percentage weight at the end of the decomposition of the complex is observed 68.59%, it corresponds to the PbO (calcd., 69.80%).

Conclusion
In summary, three novel coordination complexes have been synthesized by employing the rigid 3-NPAH 2 as the main ligand and different N-donor ligands as auxiliary ligands under hydrothermal conditions. In complexes 1-3, the versatile 3-NPA 2− anions exhibit different coordination modes and coordination capacity, and connect the metal ions into different metal-carboxylates subunits with different 0D dinuclear units and 2D-layered network frameworks, which show a great effect on the formation of the final architectures. At the same time, the N-donor ancillary coligands are versatile in construction of coordination complexes. Also, the hydrogen bonds and π -π stacking also play important roles in affecting the final structure. The present different structures of complexes 1-2 are extended into 2D or 3D supramolecular frameworks via the hydrogen bonding and the π -π stacking interactions. In addition, the thermal decomposition process for the complexes proves that complex 3 has a good thermal stability.