Photoluminescent lead(II) coordination polymers stabilised by bifunctional organoarsonate ligands

Four lead(II) coordination polymers were isolated under hydro(solvo)thermal conditions. The applied synthetic methodology takes advantage of the coordination behaviour of a new bifunctional organoarsonate ligand, 4-(1, 2, 4-triazol-4-yl)phenylarsonic acid (H2TPAA) and involves the variation of lead(II) reactants, metal/ligand mole ratios, and solvents. The constitutional composition of the four lead(II) coordination polymers can be formulated as [Pb2(TPAA)(HTPAA)(NO3)]·6H2O (1), [Pb2(TPAA)(HTPAA)2]·DMF·0.5H2O (DMF = N, N-Dimethylformamide) (2), [Pb2Cl2(TPAA)H2O] (3), and [Pb3Cl(TPAA)(HTPAA)2H2O]Cl (4). The compounds were characterized by single-crystal and powder x-ray diffraction techniques, thermogravimetric analyses, infra-red spectroscopy, and elemental analyses. Single-crystal x-ray diffraction reveals that 1 and 2 represent two-dimensional (2D) layered structures whilst 3 and 4 form three-dimensional (3D) frameworks. The structures of 1, 2, and 4 contain one-dimensional (1D) {PbII/AsO3} substructures, while 3 is composed of 2D {PbII/AsO3} arrays. Besides their interesting topologies, 1–4 all exhibit photoluminescence properties in the solid state at room temperature.


Introduction
Over the past decades, the design and synthesis of coordination polymers (CPs) developed into an active research area due to the interesting physicochemical properties and potential applications of the materials in electro-optical devices, in catalysis, sensors, gas storage, or separation materials [1][2][3][4][5][6][7][8][9][10][11]. The hydro(solvo)thermal techniques provide powerful synthetic methodologies for the construction of CPs through the self-assembly of metal ions and designed organic ligands involving elevated temperatures and pressures. However, up until now, it still remains a considerable challenge to achieve controllable preparation conditions to produce crystalline CP materials with desired topological and chemical attributes. The reason for this stems from the complex formation conditions that influence the selfassembly and crystallization process; the applied synthetic approach is influenced simultaneously by numerous parameters, including the structural characteristics of the organic Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. ligands, the coordination geometry of central metal ions, the metal/ligand molar ratio, available counteranions, the solvent system, the pH value of the solution, temperature, pressure, and reaction time [12][13][14][15][16]. Importantly, the nature of the organic ligand often influences the topology, the stability, the formation, and the crystallization conditions of the resulting hybrid materials.
Organoarsonic acids (R-AsO 3 H 2 ) display geometrical characteristics that closely relate to those of analogous phosphonic acids (R-PO 3 H 2 ); thus metal arsonates are expected to show structural and topological attributes that are similar to those of the metal phosphonates. However, the larger As(V) ionic radius and longer As−O bonds can be expected to result in modified architectures and distinguishing features to the corresponding phosphonate system. Pronounced differences of the pK a values may further distinguish the coordination chemistry of arsonates and phosphonates. So far, reports on metal arsonates are rather scarce. Most of the reported metal organoarsonates are hybrid polyoxometalate (POM) clusters based on V, Mo, or W [41][42][43][44][45][46][47][48][49][50][51][52]. In particular, Zubieta's research group has extensively explored the formation of such hybrid POMs [45,47,48,50,51]. We have reported several capsular arsonate-stabilized polyoxovanadates that incorporate substituted R-phenylarsonate and investigated their 3D assembly [53][54][55][56]. In such POMs, each arsonate functionality bridges several metal centers and shows a coordination behaviour that is closely comparable to that of the corresponding phosphonate-stabilized POMs [33,[53][54][55][56]. Synthetic approaches to Sn-and Pd-based organoarsonate complexes have also been explored under hydro(solvo)thermal conditions [57][58][59]. Recently, Tian et al reported examples of uranyl arsonates that are stabilized by phenylarsonate ligands [60]. To date, arsonate-stabilized metal-organic CPs are significantly less developed and investigated compared to the corresponding phosphonate compounds. The reported CPs predominantly contain s-, d-, and f-block metal ions, while significantly less attention has been paid to the p-block-based metal ions. This is somewhat surprising, as these types of CPs lend themselves to important applications in electroluminescent and photovoltaic conversion devices or fluorescent sensors [61][62][63][64][65][66][67]. As a heavy pblock metal ion, lead(II) may provide a potential opportunity to construct novel extended inorganic hybrids with fascinating topologies and interesting optical properties. Its large radius, variable stereochemical activity, and flexible coordination environment make a Pb(II) system to be an interesting candidate to prepare organoarsonate-stabilized network structures. To date, only very few lead arsonates have been reported in the literature [68,69].

Materials and instrumentation
The H 2 TPAA ligand was synthesized from N, N-dimethylformamide azine dihydrochloride using a general procedure previously reported by us [77,78]. All reagents were purchased from Sigma-Aldrich and used as received without further purification. 1 H and 13 C nuclear magnetic resonance data were recorded on a Bruker DPX 400 spectrometer (400.13 MHz for 1 H, 100.63 MHz for 13 C). Fourier transform infrared spectroscopy (FTIR) data were collected on a Per-kinElmer Spectrum One FTIR Spectrometer. Thermogravimetric analyses (TGAs) were performed in air on a Perkin Elmer Pyrus 1 TGA from 30-800°C at a heating rate of 10°C/min. Powder x-ray diffraction (PXRD) data were recorded on a Siemens D500 x-ray diffractometer at 40 kV, 30 mA with Cu-Kα radiation (λ = 1.54 056 Å), with a scan speed of 3°/min and a step size of 0.05°in 2θ at room temperature. The simulated patterns were derived from the Mercury Version 1.4 software (http://www.ccdc.cam.ac.uk/ products/mercury/) using the data obtained from the singlecrystal x-ray diffraction experiments (cif files). Elemental analyses (C, H, and N) were obtained from the Microanalysis Laboratory, School of Chemistry and Chemical Biology, University College Dublin. Solid state fluorescence measurements were carried out using a Fluorolog®-3 Spectrofluorometer.

X-ray crystallography
The data collections of 1 and 2 were carried out on a Bruker APEX Duo CCD x-ray diffractometer at 100 K using graphite-monochromated Cu-Kα (λ = 1.54 178 Å) and Mo-Kα radiation (λ = 0.71 073 Å), respectively, while those of 3 and 4 were carried out on a Rigaku 724 CCD x-ray diffractometer using graphite-monochromated Mo-Kα radiation (λ = 0.71 073 Å). The data collection temperatures of 1-4 were 100, 123, 293, and 95 K, respectively. The structures of these four coordination compounds were solved by direct methods using SHELXS-97, integrated using the OLEX2 software [79] and refined with full-matrix least squares on F 2 using the SHELXL-97 program [80]. All the non-hydrogen atoms were refined anisotropically. All the hydrogen atoms of 1 and 3 were refined on calculated positions. For 2 and 4, the hydrogen atoms belonging to As1O 3 groups (H13 and H33 in 2, H13 and H23 in 4) were located from their respective difference maps; the other hydrogen atoms in 2 and 4 were all refined on calculated positions. The DMF guest molecule exhibits a positional disorder over two sites (C43, O41 and C43′, O41′). The site occupancies of C43, O41, C43′, O41′ are all 0.5. Details of x-ray analysis, including the crystal parameters, data collection, and refinement parameters for compounds 1-4 are summarized in table 1. Selected bond lengths, angles, and hydrogen-bond interactions are summarized in tables S1.1-S4.2 (SI). Further details of the crystal structure determination have been deposited at the Cambridge Crystallographic Data Centre (CCDC). CCDC numbers for 1-4: 999 421-999 424, respectively.

Syntheses
Compounds 1-4 were reproducibly obtained in moderate yields under hydro(solvo)thermal conditions upon combination of lead salts and H 2 TPAA in H 2 O or DMF. The products form depending on the nature of the metal salt, the relative metal:ligand mole ratios, or the solvent system. Compound 1 selectively forms when Pb(NO 3 ) 2 and H 2 TPAA are reacted in H 2 O. When the nitrate reactant is substituted by PbCl 2 , compounds 3 and 4 are obtained, depending on the relative metal:ligand mole ratios. Compound 3 forms when equimolar ratios of PbCl 2 and H 2 TPAA are reacted with each other in H 2 O. Higher yields can be obtained when a PbCl 2 :H 2 TPAA mole ratio of 2:1 is used. However, when the relative quantity of PbCl 2 in the reaction system is reduced to a mole ratio of 1:2, phase-pure crystals of compound 4 are obtained. Reactions of PbCl 2 and H 2 TPAA at a mole ratio of 1:3 lead to product mixtures of 4 and a white unidentified precipitate. Corresponding hydrothermal reactions of PbX 2 (X = OAc or F) and H 2 TPAA in water did not lead to phase-pure products or crystals suitable for single-crystal x-ray diffraction. Compound 2 requires PbBr 2 as a starting material and forms in DMF. Best yields are obtained when the metal salt and the ligand are reacted at 100°C for 4 d at stoichimetric ratios as given by the formula of 2. 1:1 mole ratios or reduced reaction time lead to the co-precipitation of a white unidentified product. If the reactions of PbX 2 (X = Cl, OAc, or F) and H 2 TPAA were carried out in a DMF solvent system, then either a colourless clear solution (for Pb(OAc) 2 ) or floccule/ microcrystalline solids (for PbF 2 or PbCl 2 ) are obtained.
[Pb 2 Cl 2 (TPAA)H 2 O] (3) The asymmetric unit of 3 contains two Pb(II) ions, two terminal coordinated chloride anions, one fully deprotonated TPAA 2ligand (denoted as L As1 ), and one μ 2 bridging water molecule (figure S3). The compound crystallizes in the orthorhombic space group Pccn. Both two Pb(II) ions are in hemidirected coordination spheres. The Pb1(II) ion is six-coordinate with three arsonate oxygen atoms and one nitrogen atom deriving from three different ligands, one chlorine atom, and the μ 2 -bridging water molecule. The Pb2(II) ion is five-coordinate, whereby two arsonate oxygen atoms and one nitrogen atom are provided by three different ligands, one chlorine atom, and the μ 2 -coordinated water molecule, completing the distorted coordination sphere of this ion. The bond length of Pb2-O4W is 2.906(9) Å, which may be regarded as a semi-coordination mode. Except for Pb2-O4W, the other Pb-O distances are in the expected range, varying between 2.272(8) and 2.783 (9) Å. The distances of Pb1-N1 ii and Pb2-N2 ii are 2.737(10) and 2.851(9) Å, respectively, and those of Pb1-Cl1 and Pb2-Cl2 are 2.6855(30) and 2.7211(33) Å, respectively. The distance of Pb2-N2 ii is a little longer than the discussed Pb-N bond in 1  In 3, the L As1 ligand adopts a (κO1, O1-κO2-κO3, O3-κN1-κN2)-μ 6 coordination mode ( figure 3(a)). The AsO 3 functionalities from L As1 link Pb(II) ions into 2D layers that  extend parallel to the ac-plane ( figure 3(b)) and which are connected to each other by the L As1 spacer to form a 3D framework ( figure 3(c)). Intramolecular hydrogen-bond interaction occurs in 3 between the coordinated water molecule and the {AsO 3 } group (O3) to give a O4W-H4WB···O3 v distance of 2.751(10) Å [Symmetry code: (v) x, y−1, z].
[Pb 3 Cl(TPAA)(HTPAA) 2 H 2 O]Cl (4) There are three Pb (II) ions, two HTPAAligands, one fully deprotonated TPAA 2ligand (denoted as L As11 , L As21 , and L As31 ), one terminal coordinated chlorine atom, one terminal coordinating water molecule, and one lattice chloride ion in the asymmetric unit of 4 ( figure S4(a)). The compound crystallizes in the triclinic space group P-1. The Pb1(II)-Pb2(II) ions can be considered to adopt hemidirected coordination spheres. However, if the long Pb3-O22 bond (2.888(6) Å) is taken into consideration, the coordination environment of Pb3(II) may be described as holodirected. The Pb1(II) ion is fivecoordinate with four arsonate oxygen atoms originating from three different ligands; the remaining binding site is occupied by a chloride ion. The Pb2(II) ion is five-coordinate, whereby four arsonate oxygen atoms and one nitrogen atom are provided by five different ligands. The six-coordinate binding environment of the Pb3(II) ion is composed of four oxygen and one nitrogen atom derived from five different ligands and one coordinated water molecule. The bond lengths of Pb-O range from 2.361(5) to 2.888(6) Å. The bond lengths of Pb2-N21 ii and Pb3-N31 iv are 2.521(7) and 2.737(7) Å, respectively, and that of Pb1-Cl41 is 2.753(2) Å. The valences of Pb1 and Pb2 calculated from all the existing bonds are 1.85 and 1.87, respectively, being smaller than the assigned oxidation state of +II. However, no other weak Pb-O interactions can be found around the Pb1(II) and Pb2(II) centers. If only the relatively strong Pb-O bonds (Pb-O < 2.88 Å) are taken into account, the valence of Pb3 is 1.88. Further calculation shows that the long Pb3-O22 interaction also makes a considerable contribution, increasing the valence to 2.004 for Pb3.

X-ray powder, FTIR spectral, and thermogravimetric analyses
The phase-purity of the here presented compounds was confirmed by PXRD. The 2 theta values of the major reflections of the experimentally recorded PXRD patterns of the bulk solids of 1-4 match well to those of the simulated patterns, which were calculated from respective single-crystal data (SI, figures S5(a)-(d)). The FTIR spectra of the four compounds show typical As-C stretching vibration bands at 1098 cm −1 for 1, 1100 cm −1 for 2, 1097 cm −1 for 3, and 1099 cm −1 for 4. The vibrations associated with the {AsO 3 } moiety are very strong and occur at 864 and 851 cm −1 for 1, 873 cm −1 for 2, 859 and 835 cm −1 for 3, and 864 and 844 cm −1 for 4 (SI, figures S6(a)-(d)) [76]. The thermal stability of 1-4 was examined by TGA in an air atmosphere between 30-800°C (SI, figure S7). The TGA curve of 1 reveals the removal of constitutional lattice water molecules and the coordinated water molecule in a range between 30-320°C. 2 loses its lattice water and DMF molecules between 30-280°C. After 280°C, the framework of 2 undergoes oxidative degradation. The coordinated water molecule of 3 is lost between 30-110°C, after which the framework architecture of 3 decomposes gradually due to the oxidation of the organic ligand. The coordinated water molecule of 4 is lost between 30-245°C. The remaining structure of 4 is stable up to ∼305°C, after which the framework collapses due to the combustion of the organoarsonate ligands.

Luminescence properties of 1-4
Hybrid coordination compounds containing Pb(II) ions may have interesting photochemical and photophysical properties [61][62][63][64][65][66][67]86]. However, in comparison to many transition metal or lanthanide systems, the photoluminescence properties of lead(II)-organic frameworks are less explored. To further characterize 1-4, their photoluminescence properties were investigated in the solid state at room temperature. As illustrated in figure 5, emission bands at 461 and 486 nm (λ ex = 380 nm) for 1, 438 nm (λ ex = 380 nm) for 2, 458, 478, and 531 nm (λ ex = 380 nm) for 3 and 458 and 550 nm (λ ex = 370 nm) for 4 are observed. For H 2 TPAA, an emission band maximum centered at 456 nm is apparent upon photoexcitation at 373 nm (SI, figure S8). The emission bands at 461 nm of 1, 438 nm of 2, 458 nm of 3, and 458 nm of 4 may be due to the π → π* transition, as an approximate emission peak (456 nm) also appears in the spectra of the H 2 TPAA ligand. The emission bands at 486 nm of 1 and 478 nm of 3 can be attributed to ligand-to-metal charge transfer (LMCT) transitions involving delocalized π bonds of the aromatic arsonate groups and the p orbitals of Pb(II) centers. The lowenergy emissions with large stokes shift, characteristic for the bands at 531 nm for 3 and 550 nm for 4, can be assigned to metal-centered transitions involving s and p orbitals, as proposed by Vogler [87,88].

Conclusions
In summary, four lead(II) coordination polymers with distinctively different structural motifs were successfully isolated using hydro(solvo)thermal preparative conditions. The applied preparative approach utilized different lead reactants that were reacted with various metal/ligand molar ratios with a bifunctional organoarsonate ligand in H 2 O or DMF. These applied reaction parameters, including the counterions, played a crucial role of the topology and composition of the resulting Pb(II) coordination polymers. The results demonstrate that the coordination modes of the H 2 TPAA ligand are highly flexible, adopting μ 1 −, μ 2 −, μ 3 −, μ 4 −, μ 5 −, and μ 6 − bridging modes. 1 and 2 represent 2D layered structures, whilst 3 and 4 form 3D frameworks. The structures of 1, 2, and 4 contain 1D {Pb-AsO 3 } substructures, while the framework of 3 is characterized by 2D {Pb-AsO 3 } sub-structural motifs. Compounds 1-4 all exhibit photoluminescence properties in the solid state at room temperature. The arsonic acid functionalities in the examined compounds have a high propensity to be partially protonated under the applied reaction conditions. Among these four lead(II) coordination polymers, only 3 is stabilized by fully deprotonated arsonate functionalities. This reaction behaviour and the consequent lower tendency to bridge metal centers than corresponding phosphonate ligands can be interpreted in light of their pK a values. According to the literature, the pK a values of phenylphosphonic acid are 1.86 and 7.51 [89], and the pK a values of phenylarsonic acid are 3.8 and 8.5 [90], suggesting that the 2nd deprotonation event occurs more readily for phosphonic acids than that for the arsonic acids. Despite the fact that arsonate anions can also adopt a variety of potential coordination modes, their ability as complexing agents has not yet been fully explored. Consequently, new synthetic methodologies are required to explore to further fully understand the coordination chemistry of organoarsonates.