New Topologically Unique Metal-Organic Architectures Driven by a Pyridine-Tricarboxylate Building Block

Two new three-dimensional (3D) coordination compounds, namely a lead(II) coordination polymer (CP) {[Pb3(μ5-cpta)(μ6-cpta)(phen)2]·2H2O}n (1) and a zinc(II) metal-organic framework (MOF) {[Zn2(μ4-cpta)(μ-OH)(μ-4,4-bipy)]·6H2O}n (2), were hydrothermally assembled from 2-(5-carboxypyridin-2-yl)terephthalic acid (H3cpta) as an unexplored principal building block and aromatic N,N-donors as crystallization mediators. Both products were isolated as air-stable microcrystalline solids and were fully characterized by IR spectroscopy, elemental and thermogravimetric analysis, and powder and single-crystal X-ray diffraction. Structural and topological features of CP 1 and MOF 2 were analyzed in detail, allowing to identify a topologically unique 4,5,5,6-connected net in 1 or a very rare 4,4-connected net with the isx topology in 2. Thermal stability and solid-state luminescent behavior of 1 and 2 were also investigated. Apart from revealing a notable topological novelty, both compounds also represent the first structurally characterized Pb(II) and Zn(II) derivatives assembled from H3cpta, thus opening up the application of this unexplored pyridine-tricarboxylate block in the design of new metal-organic architectures.

More specifically, for the design of MOFs, it is interesting to explore different aromatic carboxylic acids as flexible and stable building blocks with modifiable backbones and coordination preferences, along with the metal nodes that can exhibit unusual coordination preferences [27][28][29][30].Although multicarboxylate building blocks are among the most common ones used in the synthesis of MOFs, different aromatic N,N-donors also play an important role and frequently act as ancillary ligands Crystals 2018, 8, 353 2 of 10 to adjust the coordination modes of carboxylate spacers, to provide reinforcement of metal-organic networks via additional supramolecular interactions, or to facilitate crystallization [16,26,[31][32][33][34].
Following our interest in the exploration of novel and poorly investigated multicarboxylic acids for the design of metal-organic architectures [21,22,[24][25][26][27], in the present study we selected 2-(5-carboxypyridin-2-yl)terephthalic acid (H 3 cpta) as a main pyridine-tricarboxylate building block.Its choice is governed by the following reasons: (A) H 3 cpta can potentially act as an excellent bridging ligand to construct MOFs, given the presence in its structure of three COOH groups along with an N-pyridyl functionality; (B) H 3 cpta features a flexibility wherein pyridyl and phenyl rings can rotate around the C-C single bond; and (C) H 3 cpta is thermally stable and remains largely unexplored for the construction of MOFs, as attested by a search of the Cambridge Structural Database.
Hence, we report herein the hydrothermal synthesis, full characterization, thermal behavior, structural features, topological analysis, and luminescent properties of two novel lead(II) and zinc(II) 3D coordination compounds with very complex and topologically unusual metal-organic architectures.The obtained compounds represent the first structurally characterized Pb(II) and Zn(II) derivatives assembled from 2-(5-carboxypyridin-2-yl)terephthalic acid.

Materials and Physical Measurements
All chemicals were of analytical reagent grade and used as received.H 3 cpta was obtained from Jinan Henghua Sci. and Tec.Co., Ltd., Jinan, China.IR spectra were recorded on a Bruker EQUINOX 55 spectrometer (Bruker Corporation, Billerica, MA, USA) using KBr pellets.Elemental (C, H, N) analyses were run on an Elementar Vario EL elemental analyzer (Elementar, Langenselbold, Germany).Thermogravimetric analyses (TGA) were performed under N 2 atmosphere using a LINSEIS STA PT1600 thermal analyzer (Linseis Messgeräte GmbH, Selb, Germary) with a heating rate of 10 • C/min.Powder X-ray diffraction patterns (PXRD) were measured on microcrystalline samples using a Rigaku-Dmax 2400 diffractometer (Rigaku Corporation, Tokyo, Japan) with a Cu-Kα radiation (λ = 1.54060Å).Solid-state excitation and emission spectra were measured on an Edinburgh FLS920 fluorescence spectrometer (Edinburgh Instruments, Edinburgh, England) at room temperature.

Synthesis of {[Pb
A mixture of PbCl 2 (113.7 mg, 0.3 mmol), H 3 cpta (57.2 mg, 0.2 mmol), phen (59.4 mg, 0.3 mmol), NaOH (24.0 mg, 0.6 mmol), and H 2 O (10 mL) was stirred at room temperature for 15 min.Then, it was sealed in a 25 mL Teflon-lined stainless steel vessel and heated at 160 • C for three days, followed by cooling to room temperature at a rate of 10

X-ray Crystallography
Single-crystal X-ray data for 1 and 2 were collected on a Bruker APEX-II CCD diffractometer (Bruker Corporation, Billerica, MA, USA), using a graphite-monochromated Mo Kα radiation (λ = 0.71073 Å).Semiempirical absorption corrections were applied using the SADABS program.Crystal structures were determined using direct methods and refined by full-matrix least-squares on F 2 with the SHELXS-97 and SHELXL-97 programs [35,36].All the non-H atoms were refined anisotropically by full-matrix least-squares methods on F 2 .All the H atoms (except those of H 2 O and OH moieties) were placed in calculated positions with fixed isotropic thermal parameters, and included in structure factor calculations at the final stage of full-matrix least-squares refinement.Hydrogen atoms of H 2 O and OH moieties were located by difference maps and constrained to ride on their parent oxygen atoms.Some lattice solvent molecules in 2 are highly disordered and were removed using the SQUEEZE routine in PLATON (University of Glasgow, Glasgow, UK) [37].The number of solvent H 2 O molecules was obtained on the basis of elemental and thermogravimetric analyses.Crystal data for 1 and 2 are given in Table 1.Selected bond lengths and hydrogen bonding details are given in Tables S1 and S2, respectively (Supplementary Material).Topological analysis of metal-organic networks was performed following the concept of the simplified underlying net [38].Such nets were obtained by eliminating the terminal ligands [38] and contracting the bridging ligands to centroids and maintaining their connectivity [39].CCDC-1840702 and 1840703 for 1 and 2 contain the supplementary crystallographic data.

Hydrothermal Self-Assembly Synthesis
Hydrothermal treatment of the aqueous mixtures composed of a metal(II) chloride (PbCl 2 or ZnCl 2 ), 2-(5-carboxypyridin-2-yl)terephthalic acid as a principal building block, sodium hydroxide as a deprotonating agent, and an aromatic N,N-donor as a crystallization mediator (1,10-phenanthroline or 4,4 -bipyridine) resulted in the generation of two novel coordination compounds formulated as (2).These were isolated as microcrystalline solids and analyzed by standard methods including single-crystal X-ray diffraction, which allowed the establishment of their intricate 3D metal-organic architectures.
Compound 1 features a very complex 3D coordination polymer structure (Figure 1).An asymmetric unit of 1 contains three distinct Pb(II) atoms, two different µ 5 -and µ 6 -cpta 3− blocks, three terminal phen ligands, and two lattice H 2 O molecules.Three Pb(II) centers adopt distinct coordination environments (Figure 1a and Figure S1).The Pb1 atom is seven-coordinate and has a distorted {PbN 2 O 5 } geometry, which is completed by a pair of phen N atoms and five carboxylate O donors from four distinct cpta 3− moieties.The Pb2 center is also seven-coordinate and possesses a distorted {PbO 7 } geometry, which is taken by seven O donor atoms from five different cpta 3−  In the cpta 3− moieties, the dihedral angles between the two aromatic rings are 20.66 and 50.14 • .Carboxylate groups of the µ 6 -and µ 5 -cpta 3− blocks interlink the Pb1 and Pb2 nodes into 2D layer motifs, which are further interconnected via the Pb3 centers (through additional Pb3-O carboxylate bonds) to give rise to a very complex 3D metal-organic architecture (Figure 1b).
Compound 1 features a very complex 3D coordination polymer structure (Figure 1).An asymmetric unit of 1 contains three distinct Pb(II) atoms, two different μ5-and μ6-cpta 3− blocks, three terminal phen ligands, and two lattice H2O molecules.Three Pb(II) centers adopt distinct coordination environments (Figures 1a and S1).The Pb1 atom is seven-coordinate and has a distorted {PbN2O5} geometry, which is completed by a pair of phen N atoms and five carboxylate O donors from four distinct cpta 3− moieties.The Pb2 center is also seven-coordinate and possesses a distorted {PbO7} geometry, which is taken by seven O donor atoms from five different cpta 3 blocks behave as two different μ6-and μ5-spacers (modes I and II, Scheme 1), in which the COO − groups exhibit the monodentate, bidentate, or bridging tridentate modes.Although the N atom of cpta 3− remains uncoordinated, there is a rather short Pb3…N1 interaction (3.193 Å).In the cpta 3− moieties, the dihedral angles between the two aromatic rings are 20.66 and 50.14°.Carboxylate groups of the μ6-and μ5-cpta 3− blocks interlink the Pb1 and Pb2 nodes into 2D layer motifs, which are further interconnected via the Pb3 centers (through additional Pb3-Ocarboxylate bonds) to give rise to a very complex 3D metal-organic architecture (Figure 1b).
From the topological perspective, the present 3D framework (Figure 2c) is built from the 4-connected Zn1 and Zn2 centers (topologically equivalent), the 4-connected µ 4 -cpta 3− blocks and the 2-connected µ-OH − and µ-4,4 -bipy linkers.Hence, this binodal 4,4-connected framework can be classified within the isx topological type and described by the point symbol of (4.5 2 .6 3 ) 2 (4 2 .5.6 3 ).Although the present topological type has been theoretically predicted and referenced in databases [38], compound 2 appears to represent the first synthesized and structurally characterized metal-organic framework with the isx topology.

Thermogravimetric and Powder X-ray Diffraction Analysis
Thermal behavior and stability of CP 1 and MOF 2 were studied by thermogravimetric analysis (TGA) in the 25-800 °C temperature range under N2 atmosphere (Figure S1).TGA curve of 1 shows a release of two lattice water molecules between 42 and 86 °C (exptl, 2.6%; calcd, 2.3%); a dehydrated solid remains stable on further heating up to 304 °C.In 2, a weight loss in the 32-94 °C range corresponds to a removal of six lattice water molecules (exptl, 15.3%; calcd, 15.1%) and the dehydrated material keeps its integrity on heating up to 308 °C.
Microcrystalline samples of 1 and 2 were also investigated by PXRD (powder X-ray diffraction) analysis.PXRD patterns of the bulk products are given in Figures S4 and S5.The experimental results match those simulated from the single-crystal X-ray diffraction data, thus confirming a phase purity of the bulk samples of 1 and 2.

Luminescent Properties
Solid-state emission spectra of compounds 1, 2, and H3cpta were recorded at room temperature using the microcrystalline samples (Figure 3).The emission spectrum of H3cpta displays a band with a maximum at 371 nm (λex = 320 nm).In contrast, CP 1 and MOF 2 show more intense emission peaks with maxima at 374 (λex = 318 nm) and 376 nm (λex = 348 nm), respectively.This observation suggests that the emission bands in 1 and 2 are similar to those of the free H3cpta ligand, allowing their assignment to the intraligand π-π* or n-π* transitions [16,24,40].

Thermogravimetric and Powder X-ray Diffraction Analysis
Thermal behavior and stability of CP 1 and MOF 2 were studied by thermogravimetric analysis (TGA) in the 25-800 • C temperature range under N 2 atmosphere (Figure S1).TGA curve of 1 shows a release of two lattice water molecules between 42 and 86 • C (exptl, 2.6%; calcd, 2.3%); a dehydrated solid remains stable on further heating up to 304 • C. In 2, a weight loss in the 32-94 • C range corresponds to a removal of six lattice water molecules (exptl, 15.3%; calcd, 15.1%) and the dehydrated material keeps its integrity on heating up to 308 • C.
Microcrystalline samples of 1 and 2 were also investigated by PXRD (powder X-ray diffraction) analysis.PXRD patterns of the bulk products are given in Figures S4 and S5.The experimental results match those simulated from the single-crystal X-ray diffraction data, thus confirming a phase purity of the bulk samples of 1 and 2.

Luminescent Properties
Solid-state emission spectra of compounds 1, 2, and H 3 cpta were recorded at room temperature using the microcrystalline samples (Figure 3).The emission spectrum of H 3 cpta displays a band with a maximum at 371 nm (λ ex = 320 nm).In contrast, CP 1 and MOF 2 show more intense emission peaks with maxima at 374 (λ ex = 318 nm) and 376 nm (λ ex = 348 nm), respectively.This observation suggests that the emission bands in 1 and 2 are similar to those of the free H 3 cpta ligand, allowing their assignment to the intraligand π-π* or n-π* transitions [16,24,40].

Conclusions
In the present study, we applied a versatile aqueous medium approach for the hydrothermal synthesis of two novel 3D metal-organic architectures derived from 2-(5-carboxypyridin-2yl)terephthalic acid (H3cpta) as an underexplored tricarboxylate building block with a phenylpyridine core.In fact, the obtained coordination polymer 1 and metal-organic framework 2 represent the first structurally characterized Pb(II) and Zn(II) coordination compounds assembled from H3cpta.
Additionally, structural and topological features of 1 and 2 were highlighted, namely by performing the analysis and classification of their intricate underlying 3D networks.As a result, a topologically unique 4,5,5,6-connected net was identified in the structure of 1, whereas a very rare 4,4-connected net with the isx topology was determined in the structure of 2. Hence, the current work also contributes to the identification of topologically rare and unprecedented nets in metal-organic architectures.Both compounds also show promising luminescent properties.
Further research on widening a still very limited family of CPs and MOFs driven by H3cpta and related pyridine-tricarboxylate building blocks, as well as on establishing their functional properties and applications is currently under way in our laboratories.

Conclusions
In the present study, we applied a versatile aqueous medium approach for the hydrothermal synthesis of two novel 3D metal-organic architectures derived from 2-(5-carboxypyridin-2-yl)terephthalic acid (H 3 cpta) as an underexplored tricarboxylate building block with a phenyl-pyridine core.In fact, the obtained coordination polymer 1 and metal-organic framework 2 represent the first structurally characterized Pb(II) and Zn(II) coordination compounds assembled from H 3 cpta.
Additionally, structural and topological features of 1 and 2 were highlighted, namely by performing the analysis and classification of their intricate underlying 3D networks.As a result, a topologically unique 4,5,5,6-connected net was identified in the structure of 1, whereas a very rare 4,4-connected net with the isx topology was determined in the structure of 2. Hence, the current work also contributes to the identification of topologically rare and unprecedented nets in metal-organic architectures.Both compounds also show promising luminescent properties.
Further research on widening a still very limited family of CPs and MOFs driven by H 3 cpta and related pyridine-tricarboxylate building blocks, as well as on establishing their functional properties and applications is currently under way in our laboratories.

Table 1 .
Crystal data for compounds 1 and 2.