Structures and Spectroscopy Studies of TwoM ( II )-Phosphonate Coordination Polymers Based on Alkaline EarthMetals ( M = Ba , Mg )

e two examples of alkaline-earth M(II)-phosphonate coordination polymers, [Ba2(L)(H2O)9]⋅3H2O (1) and [Mg1.5(H2O)9]⋅(LH2)1.5⋅6H2O (2) (H4L = H2O3PCH2N(C4H8)NCH2PO3H2), N,N -piperazinebis(methylenephosphonic acid), (L-H2 = O3PH2CHN(C4H8)NHCH2PO3) have been hydrothermally synthesized and characterized by elemental analysis, FT-IR, PXRD, TG-DSC, and single-crystal X-ray diffraction. Compound 1 possesses a 2D inorganic-organic alternate arrangement layer structure built from 1D inorganic chains through the piperazine bridge, in which the ligand L shows two types of coordination modes reported rarely at the same time. In 1, both crystallographic distinct Ba(1) and Ba(2) ions adopt 8-coordination two caps and 9-coordination three caps triangular prism geometry structures, respectively. Compound 2 possesses a zero-dimensional mononuclear structure with two crystallographic distinct Mg(II) ions. Free metal cations [MgO6] 2+ nn and uncoordinated anions (L-H2) 2− nn are joined together by static electric force. Results of photoluminescent measurement indicate three main emission bands centered at 300 nm, 378.5 nm, and 433 nm for 1 and 302 nm, 378 nm, and 434.5 nm for 2 (λλex = 235 nm), respectively. e high energy emissions could be derived from the intraligand ππ-nn transition stations of H4L (310 nm and 382 nm, λλex = 235 nm), while the low energy emission (>400 nm) of 1-2may be due to the coordination effect with metal(II) ions.


Introduction
e design and synthesis of coordination polymers (CPs) based on phosphonates have always been the most important part of the work for the researchers.is is not only due to their complicated structural diversity but also their potential applications in optics, catalysis, magnetism, molecular sensing and separation, gas adsorption, and molecular recognition [1][2][3][4].e choice of functionalized organic skeleton is the �rst important task in the construction of phosphonate CPs.e ligand H 4 L, as a type of multidentate ligand (O-and/or N-donor), can be protonated and/or deprotonated to produce H 3 L − , H 2 L 2− , HL 3− , and L 4− with versatile metal-binding and hydrogen-bonding capabilities.Most of its associated works have focused on the assembly of the transition metals,  10 -block metals, and lanthanides metal-organic open frameworks [5][6][7][8][9][10][11][12][13][14].e other important task is the choice of metal ion in the formation of CPs.Alkaline-earth metals are reasonably good candidates due to their variable stereochemical activity, �exible coordination environment, cheaper prices, and low toxicity.A series of alkaline-earth coordination compounds with novel structures and properties have been reported [15][16][17][18].Despite many obvious advantages, some shortcomings of itself, such as the tendency of forming solvated species and their unpredictable coordination numbers, are keeping them weighted down.erefore, our research has focused on the synthesis and photoluminescence of phosphonate CPs based on alkaline-earth metals.We hope to get further information on structures and properties of the alkaline-earth phosphonate CPs.e study on them with the ligand H 4 L has rarely been reported up to now [8].We previously reported the work of two compounds of 2D [Pb 2 (HL)]⋅(NO 3 )⋅2(H 2 O) [13] and 3D [Ba 3 (btc) 2 (H 2 O) 4 ]⋅0.5H 2 O [17] (H 3 btc = 1,3,5benzenetricarboxylic acid) and that we have gained a great deal of skills and experiences from this work.Herein, we discuss the structures of the two alkaline-earth M(II) phosphonate CPs, namely, [Ba 2), along with their �uorescent properties.

Experimental
2.1.Materials and General Methods.N, � -piperazinebis(methylenephosphonic acid) (H 4 L) was synthesized according to the literature's methods [19,20].All other chemical reagents were obtained from commercial sources and used without further puri�cation.e elemental analysis was conducted on a Perkin-Elmer 2400 LC II elemental analyzer.IR spectrum was carried out on a Nicolet Impact 410 FI-IR spectrometer with KBr pellets in the 400 cm −1 -4000 cm −1 region.UV-Vis spectrum was carried out on a UV-3600 UV-Vis-NIR spectrophotometer by SHIMADZU with BaSO 4 pellets in the 200 nm-800 nm region.ermal analysis (TG-DSC) was performed on a STA 449 F3 Jupiter analyzer by NETZSCH Co., of Germany in N 2 environment at a heating rate of 10 K⋅min −1 .Emission and excitation spectra were recorded on a PerkinElmer LS 55 �uorescence spectrometer.e powder X-ray diffraction (PXRD) pattern was collected on an ARL X � TRA diffractometer using graphite-monochromated Cu K radiation ( = 1.5418Å) in the angular range 2 = 4 ∘ -40 ∘ with stepping size of 0.02 ∘ and counting time of 4 s per step.2.4.Crystallography.Single crystals of both 1 and 2 were mounted on a Siemens Smart CCD diffractometer equipped with graphite-monochromated Mo K radiation ( = 0.71073 Å) at 293 K using the -2 scan technique.eir structures were solved by direct methods and re�ned by fullmatrix least-squares �tting on  2 by SHELXL-97.All nonhydrogen atoms were re�ned with anisotropic thermal parameters.e positions of hydrogen atoms were either located by difference Fourier maps or calculated geometrically and their contributions in structural factor calculations were included.e crystallographic data and structural re�nements are summarized in Table 1.Selected bond lengths (Å) and angles ( ∘ ) of 1-2 are listed in Table S1 (see supplementary material available online at http://dx.doi.org/10.1155/2013/378379).e hydrogen bond lengths (Å) and angles ( ∘ ) of 1-2 are given in Table S2 (see Supplementary material).CCDC nos.852680 and 852681 contain the supplementary crystallographic data for this paper, respectively.e data can be obtained free of charge from e Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.  2) C( 4) C( 6) C( 5)

Results and Discussion
N( 1) N( 2) P( 1) Ba( 2) Ba( 1) O( 13) O( 2) O( 12) O( 8) O( 10) O( 4) O( 5) O( 14) O( 15) O( 9) O( 9) O( 9) O( 7) O( 4) Ba( 2) O( 11) O( 11) Ba( 1) Ba( 1) Ba( 2) Ba( 2) N( 1) N( 2) N( 2  � Å, which are in consonance with those of Ba(II) compounds [17,18].ere are two types of coordination patterns for the ligand L 4− in 1 (Scheme 1).One is the form of hexadentate ligand.Each phosphonate group of the ligand L 4− , as both a bi-and monodentate ligand, uses two of three oxygen atoms to coordinate with two Ba(II) atoms in  2 : 2  1  0 mode.is mode has only been found in the compound Nd 2 (H 2 L) 3 ⋅9H 2 O reported by Groves et al. [6].It is similar to those of the reported compounds KCeL 4 ⋅4H 2 O and La 2 L⋅H 2 L⋅4.5H 2 O [7].Most of the reported cases act as monodentate donors to bridge with metal centers [8][9][10][11].e other is only a bidentate ligand, in which one of three oxygen atoms is in service to bridge with two Ba(II) atoms in  2 : 2  0  0 mode.e pure coordination mode has never been observed in all the H 4 L-compounds; however, the complicated coordination modes containing this manner have been reported for a phosphonate group [5].e uncoordinated phosphonate oxygen atoms and nine coordinated water oxygen atoms are involved in the formation of hydrogen bonds.e highdimensional materials could be developed harmoniously with the multidirectional coordination strategy adopted.Regarding it as the secondary building unit (SBU), the combinatorial polyhedra [Ba(1)Ba( 2        Note: 3.2.FTIR Spectroscopy.e IR spectra for the title compounds and the ligand H 4 L were recorded in the region from 4000 to 400 cm −1 (Figure 3).e broad bands at 3239 cm −1 for 1 and 3288 cm −1 , 3233 cm −1 for 2 are due to the H-OH stretching vibration of the water molecule, while the band at 3428 cm −1 for the ligand H 4 L is attributed to P-OH stretching vibration.e weak band in 2 appeared at 3010 cm −1 can be due to N-H group stretching; however, in 1 and H 4 L it is absent as these contain no protonated nitrogen atoms.e bands, at 2960 cm −1 , 2927 cm −1 , 2848 cm −1 , and 2813 cm −1 for 1, 2981 cm −1 , 2912 cm −1 for 2, and 3002 cm −1 , 2941 cm −1 for H 4 L, can be attributed to the asymmetric and symmetric C-H stretching vibrations of the -CH 2 -groups, respectively.e bands in the region of 2700∼2200 cm −1 are (PO-H) for H 4 L, which are characteristic of hydrogen phosphonate groups.However, there are no peaks in this region for 1-2, which means the change of con�guration of H 4 L in 1-2.e peaks at 1673 cm −1 for 1, 1687 cm −1 for 2, and 1689 cm −1 for H 4 L may be due to an overtone or combination band of the C-PO 3 stretching vibrations, slightly blue-shiing than that of H 4 L. e weak bands at 1625 cm −1 for 1 and 1584 cm −1 for 2 are due to the (H-OH) vibration, but there is missing data in this region for H 4 L. e corresponding (-CH 2 -) deformation bands appear at 1459 cm −1 , 1421 cm −1 , and 1375 cm −1 for 1, 1456 cm −1 , 1436 cm −1 , and 1386 cm −1 for 2, and 1448 cm −1 , 1440 cm −1 , and 1382 cm −1 for H 4 L. e weak bands at 1261 cm −1 for 1, 1265 cm −1 for 2 and 1267 cm −1 for H 4 L can be assigned to the (-CH 2 -) vibration.e P=O stretching vibrations are observed at 1213 cm −1 , 1140 cm −1 for 1, 1211 cm −1 , 1147 cm −1 for 2 and 1213 cm −1 , 1160 cm −1 for H 4 L. e classic strong -PO 3 vibrations (typically 1200-900 cm −1 ) [21] are at 1099 (vs), 1060 (vs), and 972 cm −1 (vs) for 1, 1076 (vs), 1025 (s), and 921 cm −1 (ms) for 2, and 1103 (s), 1031 (s) and 923 cm −1 (s) for H 4 L. All of these peaks can be assigned to the stretching vibrations associated with the N-C bonds and the CPO 3 tetrahedra.Compared with H 4 L, the changes of the IR spectra in the (PO) region show the coordination of oxygen atoms in phosphonic group with barium atoms in 1. e broad band at 923 cm −1 (P-OH) disappeared, but a new sharp band were observed at 971 cm −1 that can be attributed to (P-O − ) for 1.In addition, the intensity differences and shis of peaks from infrared spectra can bring us to the conclusion that the surrounding environment of the H 4 L unit has been modi�ed when coordinating with metal ions.Additional middle strong bands at low energy are found at 569 cm −1 and 484 cm −1 for 1, 553 cm −1 and 505 cm −1 for 2, and 572 cm −1 and 509 cm −1 for H 4 L. ese bands are probably due to bending vibrations of the tetrahedral O 3 PC groups and Ba-O (for 1) stretching vibrations.

PXRD and ermal
Characteristics.e powder XRD patterns of 1-2 indicate that as-synthesized products are the new materials, and the patterns are entirely consistent with the simulated those from the single-crystal X-ray diffractions (Figure 4).TG-DSC measures were conducted to examine the stabilities of two compounds (Figure 5).e combined TG-DSC analysis of 1 shows three major weight losses in N 2 atmosphere.e �rst and the second mass losses of 28.35% from 50 ∘ C to 260 ∘ C, with an endothermic peak centered at 94 ∘ C, correspond to the losses of three lattice water molecules and nine aqua ligands (calc.28.38%).e third step of about 12.63% (calc.12.69%), in the range of 400-640 ∘ C, can be assigned to the pyrolysis of the organic moieties of the ligand H 4 L. Two exothermic peaks centered at 463 ∘ C and 555 ∘ C indicate structural changes.From 640 ∘ C, thermal decomposition is still continuing, and the �nal residue is mainly Ba 2 P 2 O 6 at 1000 ∘ C. e TGA curve of 2 consists of four weight losses in N 2 atmosphere.e �rst loss of 15.19% with an endothermic peak centered at 94 ∘ C starts from 55 ∘ C and completes at about 110 ∘ C, owing to the release of four lattice water molecules (calc.15.11%).e second weight loss of 22.56%, from 110 ∘ C to 350 ∘ C, can be attributed to nine coordinated water molecules (calc.22.66%).e third step from 350 ∘ C is the process of the decomposition of organic group until 820 ∘ C, corresponding two exothermic peaks centered at 367 ∘ C and 732 ∘ C. e last step with a small exothermic peak centered at 950 ∘ C covers a temperature range of 820 ∘ C-1000 ∘ C, which corresponds to the continuios pyrolysis of the organic group of the ligand H 4 L. e �nal product is MgP 2 O 6 and MgO (1 : 1.3).e total weight losses of 67.27% agree with the calculated value (67.13%).
3.4.Photophysical Properties.e solid-state UV-Vis spectra of both 1-2 and H 4 L are shown in Figure 6(a).e three absorption bands observed at 213 nm, 229, nm and 296 nm for the free ligand H 4 L, 228 nm, 272 nm, and 298 nm for 1 and 227 nm, 277 nm, and 304 nm for 2, respectively.ese bands may be the electron-releasing character of the N-substituent attached to the -PO 3 group and caused by the    * transition from the nitrogen atom to the P=O group.e absorption bands of 1-2 can manifest a little red shi in the UV spectra, compared with those of the free ligand H 4 L. is may be that the total energy of system decreases aer the coordination with Ba(II) ions for 1 and aer the formation of a salt with Mg(II) ions for 2 along with hydrogen bonds.
Photoluminescent properties of alkaline earth metal complexes are not well studied as compared with those of transition metal and rare earth complexes [22].e solid-state �uorescent properties of both 1-2 and H 4 L were investigated at room temperature (Figure 6(b)).e free ligand H 4 L emits two strong �uorescent bands (1%T attenuation) centered at 310 and 382 nm under the excitation of 235 nm, which derived from the photo-induced electron transfer (PET).e compound 1 with Ba(II) ion displays two main �uorescent bands centred at 378.5 and 433 nm with a shoulder peak at 300 nm, and the compound 2 with Mg(II) ion also exhibits two strong �uorescent bands centred at 378 and 434.5 nm with a weak peak at 302 nm under the excitation of 235 nm.ese �uorescent bands could come from the intraligand  *   transition state of H 4 L. e emission intensity of H 4 L is 1%T attenuation in the test and those of the other two are normal.Compared with those of the free ligand H 4 L, the high energy emission bands (<400 nm) of 1-2 are blue-shied slightly and show signi�cant decreases in intensity.Moreover, the �uorescent intensity of 2 is less than that of 1. ey are probably related to O-H⋯N hydrogen bond interactions, the protonation of nitrogen atoms of H 4 L as well as speci�c coordination environment around metal ions.e low energy emission bands (>400 nm) of 1-2 are most likely because of the coordination effect with metal (II) ions [23,24].Investigation and consideration of N-heterocyclic systems in future may yield effective path for the preparation of luminescent materials.Results of photoluminescence measurement indicate that the two compounds display three emission peaks, derived from the organic ligand and coordination effect.ere are slight blueshi� and the signi�cant reduction in intensity in the high energy region for 1-2 as compared with the free ligand H 4 L. e behavior may be attributed to speci�c coordination environment around metal ions and lots of hydrogen bond interactions involving nitrogen atoms in the structure.So the work on the application of optical functional materials including alkaline-earth metals and  10 -metals is now in progress.

F 1 :
(a) Molecular structure of 1 showing the atom-labeling scheme (50% thermal ellipsoids).(b) 2-D layer built from inorganic chains and organic linker in the bc plane (le), and the 3-D supramolecular network of 1 stacked by hydrogen bonds along the y-axis (right) (Green: Ba, Pink: P, Grey: C, Red: O, Blue: N, White: H).

F 2 :
)O 14 ]  , made of a two caps triangular prism [Ba(1)O 8 ] and a three caps triangular prism [Ba(2)O 9 ] via planar-shared three oxygen atoms (O(1), O(4), and O(7)), are connected with each other by edge-shared two oxygen atoms (O(9), O(11)) to result in a 1D inorganic single chain viewed along the b-axis.Under the action of piperazine moiety (N(1)) of H 4 L, a 1D ladder-chain is born out from the single chain.en the ladder-chains further expand to a 2D layer with inorganic-organic alternative arrangement through bridging piperazine moiety (N(2)) in the bc plane (Figure 1(b) le).Layer of this type is analogous to those in 3D metal-H 4 L frameworks such as [Sr(H 2 L)(H 2 O) 2 ]⋅3H 2 O [8] and [M(H 2 L)]⋅H 2 O (M = Mn II , Co II ) [14].All the lattice water molecules are located in the inter-layers.ere are many complicated hydrogen bonds in the structure, in which hydrogen bonds formed among the lattice water molecules.e hydrogen bonds play an Mg((a) ORTEP view of 2 showing the atom-labeling scheme (30% thermal ellipsoids).(b) e 2-D layer in the bc plane and the 3-D supramolecular network of 2 stacked by hydrogen bonds.(Green: Mg, Pink: P, Grey: C, Red: O, Blue: N, White: H).

F 4 :
Experimental and simulated powder X-ray diffraction patterns of 1 (a) and 2 (b).

F 6 :
e solid state ��-�is spectra (a) and �uorescent spectra (b) of the two compounds and the li�and � 4 L under the excitation of ��5 nm.�ote: the �uorescent intensity decay for � 4 L is 1% T attenuation and those of the other two are normal.