Synthesis, Crystal Structures, and Surface Photovoltage and

Feb 13, 2014 - Three novel metal carboxyphosphonates with a 3D pillared-layered ...... further cross-linked through the organic pillars of the carboxy...
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Synthesis, Crystal Structures, and Surface Photovoltage and Molecular Recognition Properties of Three Novel Metal Carboxyphosphonates with a 3D Pillared-Layered Structure Shao-Ping Shi, Yan-Yu Zhu, Zhen-Gang Sun,* Wei Zhou, Lu-Lu Dai, Ming-Xue Ma, Wen-Zhu Li, Hui Luo, and Tong Sun School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, People’s Republic of China S Supporting Information *

ABSTRACT: Three novel metal carboxyphosphonates with a 3D pillared-layered structure, namely, [Mn(HL)] (1), [Co(HL)]·H2O (2), and [Cd3(L2)(H2O)3] (3), (H3L = 4-HOOCC6H4CH2NHCH2PO3H2), have been hydrothermally synthesized. Compounds 1−3 all feature three-dimensional (3D) framework structures with two-dimensional (2D) inorganic layers pillared by H3L. In compounds 1 and 2, each {MO4} (M = Mn, Co) tetrahedron is linked via corner-sharing by three {CPO3} tetrahedra to form a two-dimensional (2D) inorganic layer. The adjacent layers are further pillared by the organic backbone {−C6H4CH2NHCH2−} of the carboxyphosphonate ligand, generating a 3D pillared-layered structure. For compound 3, interconnection of {Cd(1)O5N}, {Cd(2)O5N}, and {CPO3} polyhedra via edge- and corner-sharing forms a two-dimensional (2D) inorganic layer. Such neighboring 2D inorganic layers are further cross-linked via the organic backbone {−C6H4CH2NHCH2−} of the carboxyphosphonate ligands and {Cd(3)O6} polyhedra, generating a 3D pillared-layered structure. Surface photovoltage properties of the compounds 1 and 2 have been studied. An interesting feature of compound 2 is the presence of dehydration/ hydration properties. The molecular recognition properties of compound 3 in alkyl alcohol emulsions have also been studied.

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INTRODUCTION The chemistry of the metal phosphonates has been a research field of rapid expansion in recent years, mainly owing to their structural diversity and potential applications in proton conductivity, intercalation chemistry, photochemistry, ion exchange, catalysis, magnetism, and materials chemistry.1 Usually, the metal phosphonates exhibit a variety of open frameworks such as layered and microporous structures, and their structures could be controlled by ligand geometry and the metal center coordination mode.2 Many efforts have been devoted to the exploration of metal phosphonate materials with new structure types, especially open-framework structures. Much work on metal phosphonates has shown that the use of bi- and multifunctional phosphonic acids containing additional functional groups such as −NH2, −OH, and −COOH subfunctional groups has been found to be an effective method to prepare metal phosphonates with open-framework and microporous structures.3 During the past few years, a number of metal phosphonates with framework structures, using phosphonic acids with amine, hydroxyl, and carboxylate groups as ligands, have been isolated in our laboratory.4 Results from ours and other groups indicate that the carboxyphosphonic acids, such as HOOC−R−PO3H2, HOOC−RNHCH2PO3H2, or HOOC−RN−(CH2PO3H2)2 (R represent an organic group), have been proven to be very useful ligands for the © 2014 American Chemical Society

preparation of metal phosphonates with framework structures, in which the organic part plays a controllable spacer role and the −COOH and −PO3 groups chelate metal ions to form novel structure types.5 In a recent article, a carboxyphosphonic acid with rigid structure, 4-HOOCC6H4CH2NHCH2PO3H2 (H3L), was used as a ligand to synthesize metal phosphonates with a higher dimensionality. To the best of our knowledge, only one investigation on lanthanide carboxyphosphonates with two types of 3D network structures based on 4HOOCC6H4CH2NHCH2PO3H2 has recently been reported by Mao et al.6 More recently, zinc(II) and cadmium(II) carboxyphosphonates with a 3D pillared-layered structure and high thermal stability have also been obtained by our group.7 In this paper, by employing 4-HOOCC6H4CH2NHCH2PO3H2 (H3L) as the phosphonate ligand, we produced three novel metal carboxyphosphonates with a 3D pillared-layered structure, namely, [Mn(HL)] (1), [Co(HL)]·H2O (2), and [Cd3(L2)(H2O)3] (3). Herein we report their syntheses, crystal structures, and surface photovoltage and molecular recognition properties. To date, research on the properties of transition metal(II) phosphonates is mainly focused on the magnetism, Received: October 10, 2013 Revised: February 9, 2014 Published: February 13, 2014 1580

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1658(m), 1599(s), 1565(s), 1389(s), 1280(m), 1179(s), 1086(s), 1019(s), 868(w), 826(w), 767(s), 700(w), 641(w), 565(m). Synthesis of [Cd3(L2)(H2O)3] (3). A mixture of Cd(Ac)2·2H2O (0.16 g, 0.6 mmol) and H3L (0.06 g, 0.25 mmol) was dissolved in 8.0 mL of distilled water. The solution was then stirred for about 1 h before the resulting mixture was placed in a 20 mL Teflon lined stainless steel autoclave and then heated under autogenous pressure at 120 °C for 4 d. After slow cooling to RT, the colorless plate crystals were isolated. The pH value of the resultant solution was 5.0. Yield: 52.5% (based on Cd). Anal. Calcd for C18H22Cd3N2O13P2: C, 24.75; H, 2.54; N, 3.21; P, 7.09; Cd, 38.60. Found: C, 24.79; H, 2.51; N, 3.25; P, 7.04; Cd, 38.66%. IR (KBr, cm−1): 3440(m), 3229(m), 2918(w), 1607(m), 1540(s), 1414(m), 1120(s), 1053(s), 986(s), 868(w), 767(s), 700(w), 582(m), 523(m), 456(w). Crystallographic Studies. Crystallographic data for all compounds (Table 1) was collected on the Bruker AXS Smart APEX II

luminescence, proton conductivity, and ion exchange, etc.; there are few reports about photoelectric properties of these materials. Surface photovoltage spectroscopy (SPS) is an effective tool to investigate the charge change of the solid surface, which can be used to survey the photophysics of the excited states and the surface charge behavior of the sample. Recently, only a few investigations on the surface photovoltage properties of metal phosphonates have been reported by our group.8 Materials with open-framework and microporous structures are expected to find their use as hybrid composite materials in electro-optical and sensing applications. In fact, some microporous luminescent MOF materials have been realized for the sensing of ions and small molecules recently.9 Although great efforts have been made for the sensing applications of the luminescent MOF materials, to the best of our knowledge, there have been no reports about the molecular recognition properties of metal phosphonate hybrids; this is the first example of the studies on the molecular recognition properties of metal phosphonate materials.

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Table 1. Crystal Data and Structure Refinement for Compounds 1−3 compounds empirical formula FW cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalcd (g cm−3) μ (mm−1) GOF on F2 R1, Rw (I > 2σ(I))a R1, Rw (all data)a

EXPERIMENTAL SECTION

Materials and Methods. The 4-HOOCC6H4CH2NHCH2PO3H2 (H3L) was obtained as described in the literature.6,10 All other chemicals were obtained from commercial sources and used without further purification. C, H, and N were determined by using a PE-2400 elemental analyzer. Mn, Co, Cd, and P were determined by using an inductively coupled plasma (ICP) atomic absorption spectrometer. IR spectra were recorded in the range of 4000−400 cm−1 on a Bruker AXS TENSOR-27 FT-IR spectrometer, by using dry KBr pellets containing 2% of the sample. The X-ray powder diffraction data was collected on a Bruker AXS D8 Advance diffractometer. Thermogravimetric (TG) analyses were performed on an NETZSCH STA-449-F3 at a heating rate of 10 °C min−1 under an air gas flow from 50 to 1000 °C. The measurements of surface photovoltage spectroscopy (SPS) and field-induced surface photovoltage spectroscopy (FISPS) were carried out with the sample in a sandwich cell (ITO/sample/ITO) with the light source monochromator lock-in detection technique. The luminescence spectra were reported on a HITACHI F-7000 spectrofluorimeter (solid). The luminescent properties of compound 3 in solvent emulsions were investigated at room temperature. The emulsions were prepared by introducing the sample (1.5 mg) as a powder into different solvents (each 4.0 mL). The fluorescence spectra of the emulsions were measured after aging overnight. Synthesis of [Mn(HL)] (1). A mixture of MnCl2·4H2O (0.12 g, 0.6 mmol) and H3L (0.12 g, 0.5 mmol) was dissolved in 8.0 mL of distilled water, adjusted to a pH of 4.0 with 1 M NaOH solution dropwise. The solution was stirred at RT for about 1 h, and then heated at 180 °C for 4 d in a sealed 20 mL Teflon-lined stainless steel vessel under autogenous pressure. After slow cooling to RT, the pink plate crystals were isolated. The pH value of the resultant solution was 4.0. Yield: 40.4% (based on Mn). Anal. Calcd for C9H10MnNO5P: C, 36.32; H, 3.39; N, 4.71; P, 10.41; Mn, 18.46. Found: C, 36.37; H, 3.35; N, 4.68; P, 10.46; Mn, 18.41%. IR (KBr, cm−1): 3398(m), 2994(w), 2801(w), 1683(s), 1448(m), 1431(m), 1263(s), 1170(s), 1078(s), 910(s), 767(m), 708(w), 632(w), 523(m), 465(w). Synthesis of [Co(HL)]·H2O (2). A mixture of CoCl2·6H2O (0.14 g, 0.6 mmol) and H3L (0.06 g, 0.25 mmol) was dissolved in 8.0 mL of distilled water and adjusted to a pH of 5.0 with 1 M NaOH solution dropwise. The solution was then stirred for about 1 h before the resulting mixture was placed in a 20 mL Teflon lined stainless steel autoclave and then heated under autogenous pressure at 120 °C for 4 d, after which it was cooled to RT. The red block crystals were isolated as a monophasic product based on the powder XRD measurement. The pH value of the resultant solution was 5.0. Yield: 50.6% (based on Co). Anal. Calcd for C9H12CoNO6P: C, 33.75; H, 3.78; N, 4.38; P, 9.68; Co, 18.41. Found: C, 33.72; H, 3.75; N, 4.43; P, 9.63; Co, 18.46%. IR (KBr, cm−1): 3583 (m), 3490(s), 3036(w), 2776(w),

a

1 C9H10NO5PMn

2 C9H12NO6PCo

3 C18H22N2O13P2Cd3

298.09 monoclinic P2(1)/c 11.8907(7) 10.7607(7) 8.4193(5) 90 101.4600(10) 90 1055.79(11) 4 1.875

320.10 orthorhombic Pbca 8.6664(11) 9.7198(12) 27.196(3) 90 90 90 2290.9(5) 8 1.856

873.52 orthorhombic Pca2(1) 10.6837(9) 5.1842(4) 43.511(4) 90 90 90 2409.9(3) 4 2.408

1.410 1.013 0.0283, 0.0726

1.657 1.003 0.0378, 0.0870

2.829 1.037 0.0548, 0.1465

0.0368, 0.0779

0.0594, 0.0988

0.0642, 0.1561

R1 = ∑(|FO| − |FC|)/∑|F0|; wR2 = [∑w(|FO| − |FC|)2/∑wFO2]1/2.

CCD X-diffractometer equipped with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 293 ± 2 K. Absorption corrections were made using the SADABS program. The structures of compounds 1−3 were solved by direct methods and refined by full-matrix leastsquares fitting on F2 by SHELXS-97.11 Anisotropic thermal parameters were applied to all non-hydrogen atoms. Hydrogen atoms of organic ligands were generated geometrically with fixed isotropic thermal parameters and included in the structure factor calculations. Selected bond lengths and angles of compounds 1−3 are given in Tables 2−4.

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RESULTS AND DISCUSSION Syntheses. With the 4-HOOCC6H4CH2NHCH2PO3H2 (H 3 L) as the phosphonate ligand, three novel metal carboxyphosphonates with a 3D pillared-layered structure have been successfully prepared. In an attempt to produce high-quality single crystals, we tried to change different reaction parameters and found that the M2+/H3L ratio has great influence on the formation of compounds 1−3. Phase pure compounds 2 and 3 can be obtained with the M2+/H3L ratio (6:2.5). We tried our best to grow single crystals of compounds 2 and 3 with other molar ratios, but no good samples for X-ray diffraction study were obtained. However, we tried to isolate compound 1 with the M2+/H3L ratio (6:2.5). Amorphous pink 1581

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Table 2. Selected Bond Distances (Å) and Angles (deg) for 1a Distances (Å) Mn(1)−O(3)#1 Mn(1)−O(4) P(1)−O(2) P(1)−O(1)

2.0279(16) 2.0684(19) 1.5084(17) 1.5190(17)

Mn(1)−O(1)#2 Mn(1)−O(2)#3 P(1)−O(3) P(1)−C(1)

2.0612(16) 2.0925(18) 1.5163(16) 1.824(2)

Angles (deg) O(3)#1−Mn(1)−O(1)#2 O(1)#2−Mn(1)−O(4) O(1)#2−Mn(1)−O(2)#3 O(2)−P(1)−O(3) O(3)−P(1)−O(1) O(3)−P(1)−C(1) a

112.34(7) 111.35(8) 97.02(7) 114.56(10) 109.10(10) 105.05(11)

O(3)#1−Mn(1)−O(4) O(3)#1−Mn(1)−O(2)#3 O(4)−Mn(1)−O(2)#3 O(2)−P(1)−O(1) O(2)−P(1)−C(1) O(1)−P(1)−C(1)

122.71(8) 100.14(7) 109.34(8) 115.91(10) 105.84(11) 105.32(11)

Symmetry transformations used to generate equivalent atoms for 1: #1 x + 1, −y + 3/2, z + 1/2; #2 −x + 1, −y + 1, −z + 2; #3 x + 1, y, z.

Table 3. Selected Bond Distances (Å) and Angles (deg) for 2a Distances (Å) Co(1)−O(1) Co(1)−O(2)#2 P(1)−O(3) P(1)−O(1)

1.950(3) 1.980(2) 1.512(2) 1.518(3)

Co(1)−O(3)#1 Co(1)−O(5)#3 P(1)−O(2) P(1)−C(1)

1.951(2) 1.991(2) 1.516(3) 1.820(3)

Angles (deg) O(1)−Co(1)−O(3)#1 O(3)#1−Co(1)−O(2)#2 O(3)#1−Co(1)−O(5)#3 O(3)−P(1)−O(2) O(2)−P(1)−O(1) O(2)−P(1)−C(1) a

116.65(11) 108.20(10) 107.42(11) 112.16(14) 110.58(14) 106.89(17)

O(1)−Co(1)−O(2)#2 O(1)−Co(1)−O(5)#3 O(2)#2−Co(1)−O(5)#3 O(3)−P(1)−O(1) O(3)−P(1)−C(1) O(1)−P(1)−C(1)

101.85(10) 113.61(11) 108.67(10) 114.03(15) 105.87(15) 106.80(16)

Symmetry transformations used to generate equivalent atoms for 2: #1 −x + 2, −y, −z; #2 −x + 5/2, y − 1/2, z; #3 −x + 2, y − 1/2, −z + 1/2.

and one carboxylate oxygen atom in Figure 2a. The phosphonate oxygen atoms (O1, O2, and O3) and one carboxylate oxygen atom (O4) are all monodentate. The Mn− O distances range from 2.0279(16) to 2.0925(18) Å (see Table 2), which are similar to those reported for other Mn(II) carboxyphosphonates.12 Both phosphonate and carboxylate groups of the ligand are deprotonated, but the amine group is protonated based on the requirement of charge balance. Compound 1 exhibits a three-dimensional framework with a pillared-layered structure. Each {CPO3} tetrahedron connects three {MnO4} tetrahedra through three phosphonate oxygen atoms, and the {MnO4} tetrahedra are interconnected by {CPO3} tetrahedra via corner-sharing to feature a twodimensional (2D) inorganic layer in the bc-plane (Figure 3a). The result of connections in this manner is formation of regular windows made up of 16 atoms, which consist of four Mn, four P, and eight O atoms with the sequences Mn−O−P−O−Mn− O−P−O−Mn−O−P−O−Mn−O−P−O in the inorganic layer. Such neighboring 2D inorganic layers are further cross-linked via the organic backbone {−C6H4CH2NHCH2−} of the carboxyphosphonate ligands, generating a 3D pillared-layered structure with a 1D channel system along the b-axis (Figure 3b). The channels running along the b-axis are formed by 28membered rings composed of three Mn(II) cations, two HL2− ligands, and three phosphonate groups of the ligand (Figure 3c). The size of the channel is estimated to be 11.2 Å (P1−P1) × 4.6 Å (C7−C7) based on structure data. The adjacent benzyl rings of the carboxyphosphonate ligands are parallel, and the distance between them is 3.45 Å. It is within the range of π−π stacking interactions (3.3−3.8 Å); hence there exist π−π stacking interactions (Figure 3d).

powders were isolated, which can be seen from the X-ray powder diffraction analysis. When the M2+/H3L ratio is 6:5, the best crystals of compound 1 were obtained. In addition, the reaction temperature plays an important role in the growth of high-quality single crystals. Compound 1 was obtained as a good sample at 180 °C, while compounds 2 and 3 were synthesized at 120 °C. We also tried to prepare compounds 1− 3 under the same reaction environments at different temperatures, but impure crystals or powders were obtained. The pH value was found to be very important for the formation of the three compounds. NaOH was employed as the inorganic base to adjust the pH values of the above reaction system, the initial and final pH values of the resultant solution are about 4.0 and 4.5 for compound 1 and 5.0 and 5.5 for compounds 2 and 3, respectively. Besides, it was unnecessary to adjust the pH values of compound 3. Amorphous powders are obtained at other pH values under the same synthesis condition for compounds 1−3. The powder XRD experimental patterns and the simulated XRD patterns of the corresponding compounds 1−3 are shown in Figures S10−S12, Supporting Information. Crystal Structure of [Mn(HL)] (1). X-ray single crystal diffraction revealed that compound 1 crystallizes in the monoclinic space group P2(1)/c (see Table 1). The asymmetric unit of the structure for compound 1 is composed of one Mn(II) cation and one HL2− anion. As shown in Figure 1, the Mn(II) cation is four-coordinated by one carboxylate oxygen atom (O4C) from one HL2− anion and three phosphonate oxygen atoms (O2, O1A, and O3B) from three separate HL2− anions. On the other hand, the carboxyphosphonate ligand acts as a tetradentate metal linker, bridging with four Mn(II) cations through three phosphonate oxygen atoms 1582

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Table 4. Selected Bond Distances (Å) and Angles (deg) for 3a Distances (Å) Cd(1)−O(2)#1 Cd(1)−O(3)#2 Cd(1)−O(6) Cd(1)−O(8) Cd(2)−O(9) Cd(2)−N(2) Cd(3)−O(13) Cd(3)−O(11) Cd(3)−O(4)#4 O(1)−Cd(2)#5 O(3)−Cd(1)#5 O(5)−Cd(3)#7 O(8)−Cd(2)#5 P(1)−O(2) P(2)−O(8) P(2)−O(9) P(2)−C(10)

2.214(9) 2.274(8) 2.324(8) 2.327(8) 2.328(10) 2.413(11) 2.199(12) 2.272(11) 2.348(12) 2.226(9) 2.274(8) 2.323(10) 2.363(8) 1.519(9) 1.485(8) 1.516(10) 1.803(14)

Cd(1)−N(1) Cd(1)−O(1) Cd(2)−O(1)#2 Cd(2)−O(7)#3 Cd(2)−O(8)#2 Cd(2)−O(6) Cd(3)−O(12) Cd(3)−O(5)#4 Cd(3)−O(10) O(2)−Cd(1)#6 O(4)−Cd(3)#7 O(7)−Cd(2)#8 P(1)−O(3) P(1)−O(1) P(2)−O(7) P(1)−C(1)

2.397(10) 2.428(9) 2.226(9) 2.234(9) 2.363(8) 2.481(8) 2.210(13) 2.323(10) 2.367(11) 2.214(9) 2.348(12) 2.234(9) 1.512(9) 1.548(9) 1.502(9) 1.850(13)

Angles (deg) O(2)#1−Cd(1)−O(3)#2 O(2)#1−Cd(1)−O(6) O(3)#2−Cd(1)−O(8) O(2)#1−Cd(1)−N(1) O(6)−Cd(1)−N(1) O(2)#1−Cd(1)−O(1) O(6)−Cd(1)−O(1) N(1)−Cd(1)−O(1) O(1)#2−Cd(2)−O(9) O(1)#2−Cd(2)−O(8)#2 O(9)−Cd(2)−O(8)#2 O(7)#3−Cd(2)−N(2) O(8)#2−Cd(2)−N(2) O(7)#3−Cd(2)−O(6) O(8)#2−Cd(2)−O(6) O(13)−Cd(3)−O(12) O(12)−Cd(3)−O(11) O(12)−Cd(3)−O(5)#4 O(13)−Cd(3)−O(4)#4 O(5)#4−Cd(3)−O(4)#4 O(12)−Cd(3)−O(10) O(5)#4−Cd(3)−O(10)

100.9(3) 172.1(3) 122.1(3) 88.7(4) 95.3(3) 87.0(3) 87.6(3) 73.1(3) 96.1(3) 77.5(3) 165.8(3) 103.1(3) 103.4(3) 171.1(3) 81.3(3) 86.3(5) 101.2(5) 135.4(5) 122.3(5) 56.0(4) 127.2(5) 97.3(4)

O(3)#2−Cd(1)−O(6) O(2)#1−Cd(1)−O(8) O(6)−Cd(1)−O(8) O(3)#2−Cd(1)−N(1) O(8)−Cd(1)−N(1) O(3)#2−Cd(1)−O(1) O(8)−Cd(1)−O(1) O(1)#2−Cd(2)−O(7)#3 O(7)#3−Cd(2)−O(9) O(7)#3−Cd(2)−O(8)#2 O(1)#2−Cd(2)−N(2) O(9)−Cd(2)−N(2) O(1)#2−Cd(2)−O(6) O(9)−Cd(2)−O(6) N(2)−Cd(2)−O(6) O(13)−Cd(3)−O(11) O(13)−Cd(3)−O(5)#4 O(11)−Cd(3)−O(5)#4 O(12)−Cd(3)−O(4)#4 O(13)−Cd(3)−O(10) O(11)−Cd(3)−O(10) O(4)#4−Cd(3)−O(10)

85.9(3) 90.0(3) 83.0(3) 90.0(3) 147.5(3) 161.3(3) 74.4(3) 95.4(3) 101.2(4) 92.0(4) 161.5(3) 78.6(3) 77.5(3) 85.0(3) 84.3(3) 139.3(5) 99.7(5) 102.0(4) 83.3(4) 87.9(4) 55.6(4) 140.4(4)

Symmetry transformations used to generate equivalent atoms for 3: #1 x, y − 1, z; #2 x − 1/2, −y + 1, z; #3 x − 1/2, −y, z; #4 −x, −y + 1, z + 1/2; #5 x + 1/2, −y + 1, z; #6 x, y + 1, z; #7 −x, −y + 1, z − 1/2; #8 x + 1/2, −y, z. a

four-coordinated environment with one carboxylate oxygen atom (O5C) from one HL2− anion and three phosphonate oxygen atoms (O1, O2A, and O3B) from three separate HL2− anions. The carboxyphosphonate ligand functions as a tetradentate ligand, binding four Co(II) cations through three phosphonate oxygen atoms (O1, O2, and O3) and one carboxylate oxygen atom (O5) (Figure 2b). The Co−O distances range from 1.950(3) to 1.991(2) Å (see Table 3); these bond lengths are comparable to those reported for other Co(II) phosphonates.13 Both phosphonate and carboxylate groups of the ligand are deprotonated, but the amine group is protonated based on the requirement of charge balance. Compound 2 exhibits a three-dimensional framework with pillared-layered structure. Each {CPO3} tetrahedron connects three {CoO4} tetrahedra through three phosphonate oxygen atoms, and the {CoO4} tetrahedra are interconnected by {CPO3} tetrahedra via corner-sharing to form a two-dimen-

Figure 1. Structure unit of compound 1 showing the atom labeling. Thermal ellipsoids are shown at the 50% probability level. Symmetry code for the generated atoms: (A) x + 1, −y + 3/2, z + 1/2; (B) −x + 1, −y + 1, −z + 2; (C) x + 1, y, z.

Crystal Structure of [Co(HL)]·H2O (2). Compound 2 crystallizes in the orthorhombic space group Pbca (see Table 1). Each asymmetric unit contains one Co(II) cation and one H2L− anion. As shown in Figure 4, the Co(II) cation exhibits a 1583

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Figure 2. The coordination fashion of H3L in (a) compound 1 and (b) compound 2 and the coordination fashion of (c) H3L1 and (d) H3L2 in compound 3.

Figure 3. (a) View of the layer structure for compound 1 in the bc-plane. (b) The 3D framework structure of compound 1 along the b-axis. (c) The 28-atom ring in compound 1. (d) The π−π stacking interaction in compound 1 (a = 3.45 Å).

ligands, and three phosphonate groups of the ligand (Figure 5c). The size of the channel is estimated to be 13.5 Å (P1−O2) × 3.5 Å (C5−C7) based on structure data. In the structure of 2, the adjacent benzyl rings of the carboxyphosphonate ligands are almost parallel, and the distance between them is 3.40 Å, which implies π−π stacking interactions (Figure 5d). Crystal Structure of [Cd3(L2)(H2O)3] (3). Compound 3 crystallizes in the orthorhombic space group Pca2(1) (see Table 1). The asymmetric unit of the structure for compound 3 is composed of three Cd cations and two L3− anions. As shown in Figure 6, the Cd1 atoms are octahedrally coordinated by one oxygen atom (O6) from one coordinated water molecule, one nitrogen atom (N1) from one L(1)3− anion, and four phosphonate oxygen atoms (O1, O2A, O3B, O8) from four separate L3− anions. The Cd2 atom adopts an octahedrally coordinated state with one oxygen atom (O6) from one coordinated water molecule, one nitrogen atom (N2) from L(2)3− anion, and four phosphonate oxygen atoms (O1B, O7C, O8B, O9) from four separate separate L3− anions. The Cd3 atoms are also octahedrally coordinated by two oxygen atoms (O12, O13) from two coordinated water molecules and four carboxylate oxygen atoms (O10, O11, O4D, O5D) from two separate L3−. On the other hand, the carboxyphosphonate ligand H3L1 acts as a heptadentate metal linker, bridging with five Cd(II) cations through three phosphonate oxygen atoms (O1, O2, O3), two carboxylate oxygen atoms (O4, O5), and

Figure 4. Structure unit of compound 2 showing the atom labeling. Thermal ellipsoids are shown at the 50% probability level. Symmetry code for the generated atoms: (A) −x + 2, −y, −z; (B) −x + 5/2, y − 1/2, z; (C) −x + 2, y − 1/2, −z + 1/2.

sional (2D) inorganic layer in the ab-plane (Figure 5a). The result of connections in this manner is formation of regular windows made up of 16 atoms, which consist of four Co, four P, and eight O atoms with the sequences Co−O−P−O−Co− O−P−O−Co−O−P−O−Co−O−P−O in the inorganic layer. Such neighboring 2D inorganic layers are further cross-linked via the organic backbone {−C6H4CH2NHCH2−} of the carboxyphosphonate ligands, generating a 3D pillared-layered structure with a 1D channel system along the b-axis (Figure 5b). The channels running along the b-axis are formed by 28membered rings composed of three Co(II) cations, two HL2− 1584

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Figure 5. (a) The 2D inorganic layer structure of compound 2 in the ab-plane. (b) View of the 3D framework for compound 2 along the b-axis. (c) The 28-atom ring in compound 2. (d) The π−π stacking interaction in compound 2 (a = 3.40 Å).

five Cd(II) cations through three phosphonate oxygen atoms (O7, O8, O9), two carboxylate oxygen atoms (O10, O11), and one nitrogen atom (N2) in Figure 2d. The bond lengths of Cd−O and Cd−N are in the range of 2.201(11)−2.484(8) and 2.209(13)−2.398(10) Å (see Table 4), respectively. These values are in agreement with those reported for other cadmium(II) phosphonate compounds.14,15 Compound 3 can be described as a three-dimensional framework with a pillared-layered structure. The interconnection of Cd(1)O5N, Cd(2)O5N, and CPO3 polyhedra via edgeand corner-sharing forms a two-dimensional (2D) inorganic layer in the ab-plane (Figure 7a). Such neighboring 2D inorganic layers are further cross-linked via the organic backbone {−C6H4CH2NHCH2−} of the carboxyphosphonate ligands and Cd(3)O6 polyhedra, generating a 3D pillaredlayered structure with a 1D channel system along the b-axis

Figure 6. Structure unit of compound 3 showing the atom labeling. Thermal ellipsoids are shown at the 50% probability level. Symmetry code for the generated atoms: (A) x, y − 1, z; (B) x − 1/2, −y + 1, z; (C) x − 1/2, −y, z; (D) −x, −y + 1, z + 1/2.

one nitrogen atom (N1) (Figure 2c). The carboxyphosphonate ligand H3L2 acts as a heptadentate metal linker, bridging with

Figure 7. (a) View of the layer structure for compound 3 in the ab-plane. (b) The 3D framework structure of compound 3 along the b-axis. (c) The 42-atom ring in compound 3. 1585

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Figure 8. The SPS signals of (a) compound 1 and (b) compound 2.

(Figure 7b). The channels running along the b-axis are formed by 42-membered rings composed of four Cd(II) cations, four HL2− ligands, and two phosphonate groups of the ligand (Figure 7c), and the size of the channel is estimated to be 20.8 Å (Cd1−Cd2) × 11.2 Å (Cd3−Cd3) based on structure data. Differently from compounds 1 and 2, the adjacent benzyl rings of the carboxyphosphonate ligands are parallel and the distance between them is 4.08 Å. It is not within the range of π−π stacking interactions (3.3−3.8 Å), so there are no π−π stacking interactions. IR Spectroscopy. The IR spectra for compounds 1−3 were recorded in the region from 4000−400 cm−1 (Figures S1−S3, Supporting Information). The absorption bands at 3398 cm−1 for 1 and 3490 cm−1 for 2 are assigned to the stretching vibration of N−H.16 The absorption band at 3583 cm−1 for 2 and 3440 cm−1 for 3 can be attributed to the O−H stretching vibrations of water molecules.8b,17 The C−H stretching vibrations are observed as sharp, weak bands close to 3000 cm−1 for compounds 1−3. The bands at 1683, 1448, and 1431 cm−1 for 1, 1658, 1599, and 1565 cm−1 for 2, and 1607, 1540, 1414 cm−1 for 3 are observed, which are shifted from the excepted value of uncoordinated carboxylic acids [ν(C−O) typically around 1725−1700 cm−1]. These shifts are due to the carboxylate function coordinated to the metal atom, and these bands are due to the asymmetric and symmetric stretching vibrations of C−O groups when present as COO− moieties. The set of bands between 1200 and 900 cm−1 are assigned to stretching vibrations of the tetrahedral CPO3 groups for compounds 1−3.18 Additional weak bands at low energy for the title compounds are found. These bands are probably due to bending vibrations of the tetrahedral CPO3 groups. Thermal Analysis. In order to examine the thermal stabilities of compounds 1−3, thermogravimetric analyses were performed. Compound 1 reveals two steps of weight loss (Figure S4, Supporting Information). Between 182 and 295 °C, compound 1 completes the first step weight loss (28.9%), which corresponds to partial decomposition of organic groups. During the thermal decomposition, intermediate compounds are formed between 295 and 416 °C for compound 1. The intermediate phases contain Mn2P2O7 (JCPDS 00-007-0153) and black glassy carbon, which are indicated by XRD powder studies and the color (Figure S7, Supporting Information). The theoretic weight loss is 52.9%, which is calculated by the intermediate compound Mn2P2O7. The calculated weight loss is larger than the observed weight loss, because of an amorphous product (black glassy carbon)

existing at the same time. The second weight loss occurs between 416 and 655 °C, the total weigh loss at 655 °C is 70.3%, and the final residues are unidentified because they are amorphous. For compound 2, the TG curve also shows two steps of weight loss (Figure S5, Supporting Information). The first step, the weight loss of 5.4% from 50 to 145 °C, corresponds to the release of one lattice water molecule (5.8% theoretical). The second step occurred in the range of 360−768 °C, which can be attributed to the decomposition of the organic group. The final product is Co2P2O7 (JCPDS 00-0070153) based on XRD powder pattern (Figure S8, Supporting Information). The total weight loss (54.7%) is close to the theoretical value (52.7%). Compound 3 indicates two main steps (Figure S6, Supporting Information). The first step, in the temperature range 70−185 °C, can be due to release of three coordinated water molecules. The weight loss of 5.6% is close to the calculated value (6.2%). The second step occurred in the range of 312−843 °C, corresponding to the decomposition of the organic group. The final product is Cd3(PO4)2 (JCPDS 00031-0234) and black glassy carbon, which are indicated by XRD powder studies and the color (Figure S9, Supporting Information). The calculated weight loss (45.4%) is slightly larger than the observed weight loss (39.8%), because of an amorphous product (black glassy carbon) existing at the same time. In order to study the dehydration/hydration properties of compound 2, it was heated to 160 °C for 3 h, which lead to the release of lattice water, and then examined by powder diffraction. Subsequently, the same sample was suspended in water overnight, before again being examined by powder diffraction (Figure S13, Supporting Information). Interestingly, when the calcined sample at 160 °C was suspended in water overnight, and then the solid was filtered off and dried at room temperature, the same XRD pattern as that of the original crystal was regenerated. The results show that the dehydrated compound 2 can absorb water reversibly. Thus, the dehydrated solid of compound 2 may be a potential reversible adsorbent material for water molecules. Surface Photovoltage Properties. Surface photovoltage spectroscopy (SPS) of compounds 1 and 2 were measured in the range of 300−800 nm. The SPS not only relates to the electron transitions under light inducement but also reflects the separation and transfer of photogenerated charges as well as optical absorption characteristics of semiconductor samples.19 The method is used to calculate the detected SPS signal using the equation δVs = Vs′ − Vs°, where δVs is the change in the surface potential barrier on illumination and Vs′ and Vs° are the 1586

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Figure 9. The FISPS of (a) compound 1 and (b) compound 2.

Figure 10. The emission spectra of (a) alcohol solvents and (b) compound 3 emulsion. The transition intensities of (c) various alcohols solvent when excited at 392 nm and (d) compound 3 introduced into various pure alcohol solvents when excited at 297 nm.

surface potential barriers before and after illumination, respectively.20 As far as band to band transitions are concerned, δVs > 0 means that the sample is a p-type semiconductor, whereas δVs < 0 is an n-type semiconductor.21 The SPS signals of compounds 1 and 2 are depicted in Figure 8a,b. Some overlapped phenomena can be found at 300−600 nm for compound 1 and 300−500 nm for compound 2. But by treatment with origin 6.0, the surface photovoltage responses are separated distinctly. The response band of compounds 1 and 2 contains two filial bands. Surface photovoltage properties of the free H3L ligand were also investigated (Figure S14, Supporting Information), and the response at λmax = 396 nm is assigned to the π → π* transition of the ligands. The responses at λmax = 398 nm for 1 and 396 nm for 2 are assigned to the π → π* transition of the ligands. The responses at λmax = 465 nm for 1 and 438 nm for 2 can be attributed to the LMCT

transition (from ligand to metal charge transfer transition). The surface photovoltage spectra indicate that not only semiconductors can exhibit the photovoltage property but also some materials with the semiconductor characteristics (such as coordination polymers) also possess photovoltage characteristics. Comparison of compound 1 with other manganese compounds, such as [Mn(pdc)(H2O)]n, {[Mn(pdc)(phen)(H2O)]·3H2O}n and {[Mn(cyan)2(H2O)4]·2HCl·2(Hcyan)} by Niu et al. Reveals that the SPV response of compound 1 is stronger (Figure 8a and Figure S15, Supporting Information).19c The intensity difference is mainly attributed to the differences in their structures. Compound 1 features a 3D framework structure with coordination bonds. The 3D structure of compound 1 exhibits π−π stacking interactions. The coordination bonds and π−π stacking interactions can 1587

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Figure 11. The fluorescence properties of 3 emulsion in the presence of various amounts of (a) methanol, (b) ethanol, (c) 1-propanol, (d) 1butanol, and (e) 1-pentanol solvent (increasing in steps of 0.5 mL from 2.5 to 4.0 mL).

electric field and increases with application of a negative electric field. The FISPS of compounds 1 and 2 in the range of 300− 800 nm are depicted in Figure 9, when the external electric fields are +0.2, 0, and −0.2 V, respectively. The SPV responses of compounds 1 and 2 increase with application of a positive electric field and reduce with application of a negative electric field. The increase of response intensity is due to the positive electric field being beneficial to the separation of photoexcited electron−hole pairs; however, the negative electric field has just the opposite effect. The FISPS confirm that compounds 1 and 2 possess p-type characteristics. Molecular Recognition Properties. The solid-state luminescent properties of the free H3L ligand as well as compound 3 were investigated at room temperature and are depicted in Figure S16, Supporting Information. The free carboxyphosphonate ligand H3L displays a fluorescent emission band at 336 nm upon excitation at 310 nm. In contrast, compound 3 gives broad fluorescent emissions under the same experimental conditions. Compound 3 exhibits a broad

provide more transmission passages, which will increase the intensity of the responses. However, [Mn(pdc)(H2O)]n is just a 3D infinite structure with coordination bonds. So the SPV intensity of [Mn(pdc)(H2O)]n is lower than that in compound 1. {[Mn(pdc)(phen)(H2O)]·3H2O}n possesses a 2D infinite structure through hydrogen bonding interactions. {[Mn(cyan)2(H2O)4]·2HCl·2(Hcyan)} is a mononuclear structure, which is further assembled into a 3D supramolecular structure through hydrogen bonds and weak interactions. Because hydrogen bonds are weak, the ability of coordination bonds to transfer electrons or holes is larger than that of hydrogen bonds. Thus, the SPV response of compound 1 is the strongest. By applying an external electric field to the sample with a transparent electrode, we can measure the field-induced surface photovoltage spectroscopy (FISPS). If the SPV response of the sample increases with application of a positive electric field and reduces with application of a negative electric field, the sample is a p-type semiconductor. In contrast, the SPV response of an n-type semiconductor reduces with application of a positive 1588

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tetrahedra and {CPO3} tetrahedra is in turn linked by cornersharing to form a 2D inorganic layer. The adjacent 2D inorganic layers are further cross-linked through the organic pillars of the carboxyphosphonate ligand, generating a 3D pillared-layered structure. For compound 3, the interconnection of {CdO5N} polyhedra and {CPO3} tetrahedra via edgeand corner-sharing forms a two-dimensional (2D) inorganic layer. Such neighboring 2D inorganic layers are further crosslinked via the organic backbone {−C6H4CH2NHCH2−} of the carboxyphosphonate ligands and {CdO6} polyhedra, generating a 3D pillared-layered structure. Furthermore, the SPS and FISPS of compounds 1 and 2 indicate that they exhibit surface photovoltage properties and show p-type semiconductor characteristics. Therefore, it is important and interesting to study the surface photovoltage properties of metal phosphonates. The dehydration/hydration experiment shows that compound 2 may be a potential reversible adsorbent material for water molecules. The luminescent analysis indicates that compound 3 may be good candidates for blue-light luminescent materials, and compound 3 may be potentially used for the sensing of methanol, ethanol, 1-propanol, and 1-butanol and distinguishing 1-butanol from methanol, ethanol, 1-propanol, and 1-pentanol by a luminescent method.

emission band between 350 and 550 nm with one strong blue fluorescent emission peak at 432 nm (λex = 310 nm). The fluorescent spectrum of compound 3 shows a significant enhancement in intensity and a large shift to higher wavelength compared with that of the free carboxyphosphonate ligand (H3L), which is attributed to the metal-to-ligand charge transfer (MLCT).22 Unfortunately, the luminescent lifetime of compound 3 is not observed, since the lifetime of compound 3 is too short to be measured. The investigation of luminescent properties indicates that compound 3 may be a good blue-light luminescent material. We explore the potential of compound 3 for sensing of alkyl alcohol by a luminescent method. The luminescent properties of methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol emulsions at room temperature are shown in Figure 10a. The emulsions were prepared by introducing 1.50 mg of compound 3 powder into 4.0 mL of methanol, ethanol, 1-propanol, 1butanol, and 1-pentanol at room temperature. The luminescent properties of compound 3 in different alkyl alcohol emulsions were investigated (Figure 10b). Comparing the luminescent intensity of alkyl alcohol and compound 3 in alkyl alcohol emulsions (Figure 10a,b), we observed that the luminescent intensity of compound 3 in alkyl alcohol emulsions is obviously enhanced. In addition, it can be seen that the luminescent intensity of methanol and ethanol are almost the same (Figure 10c), while the luminescent intensity of compound 3 in methanol and ethanol emulsions are obviously different (Figure 10d). The results show that compound 3 may have a recognition effect for alkyl alcohol. Then we examined the sensing of alkyl alcohol molecules in detail. Compound 3 was dispersed in different kinds of alcohol solvents as the standard emulsion (1.50 mg), while the alcohol solvent content was gradually increased to monitor the emissive response. As shown in Figure 11a,b,c, the luminescent intensity of compound 3 emulsions gradually increased with the addition of methanol, ethanol, and 1-propanol solvent. While we increased the content of 1-butanol solvent in the emulsion of compound 3, the luminescent intensity was quenched (Figure 11d). The luminescent intensity of compound 3 emulsion did not show significant changes by the addition of 1-pentanol solvent (Figure 11e). Such behavior indicates that compound 3 could be used for the sensing of methanol, ethanol, 1-propanol, and 1-butanol. Upon increase of the content of alkyl alcohol, the changes of luminescent intensity of compound 3 in 1butanol emulsion were different from that of compound 3 in other alkyl alcohols. Therefore, compound 3 could be used for distinguishing 1-butanol from methanol, ethanol, 1-propanol, and 1-pentanol by a luminescent method.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files in CIF format for compounds 1−3, IR spectra of compounds 1−3, XRD pattern of the intermediate products in the thermal decomposition for compounds 1−3, and XRD patterns of the experiments compared with those simulated from X-ray single-crystal data for compounds 1−3. This material is available free of charge via the Internet at http://pubs.acs.org. CCDC 948673 (1), 948674 (2), and 948675 (3) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Center, 12, Union Road, Cambridge CB21EZ, U.K.; fax (+44) 1223-336-033; email [email protected]).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This work is supported by the National Natural Science Foundation of China (Grant No. 21371085). Notes

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The authors declare no competing financial interest.

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CONCLUSIONS In summary, three novel metal carboxyphosphonates with a 3D pillared-layered structure, namely, [Mn(HL)] (1), [Co(HL)]· H 2 O (2), and [Cd 3 (L 2 )(H 2 O) 3 ] (3) (H 3 L = 4HOOCC6H4CH2NHCH2PO3H2), have been synthesized under hydrothermal conditions. Compounds 1−3 all feature three-dimensional (3D) framework structures with two-dimensional (2D) inorganic layers pillared by H3L. In compound 1, {MnO4} tetrahedra and {CPO3} tetrahedra connect with each other to form a 2D inorganic layer by the mode of cornersharing. Such neighboring 2D inorganic layers are further supported by the organic pillars {−C6H4CH2NHCH2−} of the carboxyphosphonate ligand to form a 3D pillared-layered structure. For compound 2, the interconnection of the {CoO4}

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