Four Novel Oxomolybdenum-Organodiphosphonate Hybrids in the

Article pubs.acs.org/crystal

Four Novel Oxomolybdenum-Organodiphosphonate Hybrids in the Presence of Cu(II)−Organonitrogen Building Blocks: Synthesis, Crystal Structures, and Surface Photovoltage Properties Shou-Hui Sun, Zhen-Gang Sun,* Yan-Yu Zhu, Da-Peng Dong, Cheng-Qi Jiao, Jiang Zhu, Jing Li, Wei Chu, Hui Tian, Ming-Jing Zheng, Wan-Yue Shao, and Yan-Fei Lu School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, P. R. China S Supporting Information *

ABSTRACT: Four novel organic−inorganic hybrid materials of the copper(II)−molybdophosphonate family, [Cu(1,10-phen)Mo2O5(H 2 O)(hedp)] (1), [{Cu(1,10-phen)}{Cu(1,10-phen)(H 2 O)}MoO2(hedpH)2]·H2O (2), [Cu(1,10-phen)Mo0.5O(hedpH)] (3), and [{Cu(2,2′-bipy)(H2O)}MoO2(hedp)]·2H2O (4) (hedpH4 = 1-hydroxyethylidenediphosphonate, 2,2′-bipy = 2,2′-bipyridine, 1,10-phen = 1,10-phenanthroline), have been synthesized under hydrothermal conditions. In compound 1, the interconnection of {MoO6}, {CuN2O2}, and {CPO3} polyhedra leads to a 2D layer through edge-sharing and cornersharing, and such layers are further assembled into a 3D supramolecular structure by π−π stacking interactions. Compound 2 contains a 1D chain built up from {MoO6}, {CuN2O3}, and {CPO3} polyhedra, and then these 1D chains are further extended into 2D supramolecular layer by hydrogen bonding interactions. Such neighboring layers are further assembled into a 3D supramolecular structure via the π−π stacking interactions. For compound 3, the [{Cu(1,10-phen)}2MoO2(hedpH)2] units are interlinked by hydrogen bonds interactions to give a hydrogen bonded double chains. The neighboring chains are further assembled through π−π stacking interactions to form a 3D supramolecular structure. In compound 4, the interconnection of {MoO6}, {CuN2O3} and {CPO3} polyhedra via corner-sharing forms a 1D chain. These 1D chains are interlinked through hydrogen bonding interactions to form a 2D supramolecular layer, which are further assembled into a 3D supramolecular structure via the π−π stacking interactions. Surface photovoltage properties of the four compounds have also been studied.

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INTRODUCTION Polyoxometalates, as an important family of metal oxides, have been receiving considerable attention in solid state materials chemistry, owing to their fascinating properties and great potential applications in many fields such as catalysis, medicine, and material science.1 It has been recognized for a long time that the ability to functionalize polyoxometalate anions would extend their versatility and lead to new and more selective applications.2 In particular, the chemistry of coordination compounds of polyoxometalates with organic ligands provides knowledge about the interaction of small organic molecules with polyoxometalate surfaces.3 During the past few years, an important advance in polyoxometalate chemistry is the modification of polymolybdates with various organic ligands and transition metal complexes or fragments.4 As was indicated by Zubieta, polyoxomolybdate anions can be linked through secondary metal coordination groups forming extended structures under hydrothermal conditions.5 This strategy has provided a variety of novel coordination compounds of polyoxomolybdates with one-, two-, and three-dimensional cationic frameworks observed for the molybdate family of composites.6 In recent years, many research activities have focused on the synthesis of oxovanadium− and oxomolybdenum−organophosphonates, © XXXX American Chemical Society

which exhibit a remarkable range of structure type: complex molecular anion, 1D chain, 2D layered materials, and network.7 When organodiphosphonate ligands are exploited in designing the oxomolybdate component of the bimetallic oxide, a series of bimetallic organophosphonate hybrid materials of the copper(II)−molybdophosphonate family have been obtained by hydrothermal synthesis.8 With the aim of exploring new oxomolybdenum−organophosphonates with interesting structures and properties, we pay our attention to a flexible and multifunctional bisphosphonic acid, 1-hydroxyethylidenediphosphonic acid (hedpH4), which has been widely used as a strong chelating agent in the preparation of functional metal diphosphonates.9 By employing 1-hydroxyethylidenediphosphonic acid (hedpH4) as the phosphonate ligand, we have successfully obtained four novel organic−inorganic hybrid materials of the copper(II)−molybdophosphonate family, [Cu(1,10-phen)Mo2O5(H2O)(hedp)] (1), [{Cu(1,10-phen)}{Cu(1,10-phen)(H2O)}MoO2(hedpH)2]·H2O (2), [Cu(1,10-phen)Mo0.5O(hedpH)] (3), and[{Cu(2,2′-bipy)Received: September 24, 2012 Revised: November 19, 2012

A

dx.doi.org/10.1021/cg301392p | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Table 1. Crystal Data and Structure Refinement for 1−4

a

compounds

1

2

3

4

empirical formula fw crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalcd (g cm−3) μ (mm−1) goodness-of-fit on F2 R1, Rw (I > 2σ (I))a R1, Rw (all data)a

C14H14N2O13P2CuMo2 735.63 monoclinic P2(1)/c 14.8561(9) 16.4350(10) 8.8988(6) 90 93.3990(10) 90 2168.9(2) 4 2.253 2.330 1.044 0.0300, 0.0715 0.0384, 0.0757

C28H30N4O18P4Cu2Mo 1057.46 monoclinic P2(1)/c 17.2577(17) 11.8391(12) 19.1020(19) 90 105.8110(10) 90 3755.2(6) 4 1.870 1.705 1.043 0.0563, 0.1723 0.0773, 0.1898

C14H13N2O8P2CuMo0.5 510.71 monoclinic C2/c 10.425(2) 24.705(5) 13.031(2) 90 94.2050(10) 90 3346.9(12) 8 2.027 1.905 1.066 0.0531, 0.1336 0.0751, 0.1461

C12H18N2O12P2CuMo 603.70 monoclinic P2(1)/c 9.5237(4) 9.8364(4) 20.8937(8) 90 96.1810(10) 90 1945.92(14) 4 2.061 1.969 1.029 0.0268, 0.0759 0.0300, 0.0778

R1 = Σ(|F0| − |FC|)/Σ|F0|, wR2 = [Σw(|F0| − |FC|)2/ΣwF02]1/2.

Table 2. Selected Bond Distances (Å) and Angles (deg) for 1a Distances (Å) Mo(1)−O(9) Mo(1)−O(8) Mo(1)−O(13) Mo(1)−O(2) Mo(1)−O(3)#1 Mo(1)−O(5) Mo(2)−O(11) Mo(2)−O(12) Mo(2)−O(13) Mo(2)−O(6)#2 Mo(2)−O(10) Mo(2)−O(5)

1.686(3) 1.704(3) 1.900(3) 1.990(3) 2.255(2) 2.323(2) 1.686(3) 1.702(3) 1.894(2) 1.973(2) 2.272(3) 2.308(2)

Cu(1)−O(1) Cu(1)−O(4) Cu(1)−N(2) Cu(1)−N(1) P(1)−O(1) P(1)−O(3) P(1)−O(2) P(1)−C(1) P(2)−O(4) P(2)−O(6) P(2)−O(5) P(2)−C(1)

1.932(3) 1.941(3) 1.997(3) 2.004(3) 1.505(3) 1.518(3) 1.532(3) 1.834(4) 1.506(3) 1.527(2) 1.538(2) 1.836(4)

Angles (deg) O(9)−Mo(1)−O(8) O(9)−Mo(1)−O(13) O(8)−Mo(1)−O(13) O(9)−Mo(1)−O(2) O(8)−Mo(1)−O(2) O(13)−Mo(1)−O(2) O(9)−Mo(1)−O(3)#1 O(8)−Mo(1)−O(3)#1 O(13)−Mo(1)−O(3)#1 O(2)−Mo(1)−O(3)#1 O(9)−Mo(1)−O(5) O(8)−Mo(1)−O(5) O(13)−Mo(1)−O(5) O(2)−Mo(1)−O(5) O(3)#1−Mo(1)−O(5) O(11)−Mo(2)−O(12) O(11)−Mo(2)−O(13) O(12)−Mo(2)−O(13) a

104.75(15) 100.19(13) 99.44(13) 98.88(13) 91.67(13) 154.64(10) 95.04(13) 158.51(12) 84.95(11) 76.84(10) 166.32(12) 88.20(11) 72.86(9) 84.87(9) 72.90(8) 103.68(16) 100.71(13) 99.87(13)

O(11)−Mo(2)−O(6)#2 O(12)−Mo(2)−O(6)#2 O(13)−Mo(2)−O(6)#2 O(11)−Mo(2)−O(10) O(12)−Mo(2)−O(10) O(13)−Mo(2)−O(10) O(6)#2−Mo(2)−O(10) O(11)−Mo(2)−O(5) O(12)−Mo(2)−O(5) O(13)−Mo(2)−O(5) O(6)#2−Mo(2)−O(5) O(10)−Mo(2)−O(5) O(1)−Cu(1)−O(4) O(1)−Cu(1)−N(2) O(4)−Cu(1)−N(2) O(1)−Cu(1)−N(1) O(4)−Cu(1)−N(1) N(2)−Cu(1)−N(1)

96.13(13) 98.97(12) 150.89(11) 168.45(14) 87.51(15) 79.83(12) 79.04(12) 90.79(12) 165.05(13) 73.31(10) 82.94(9) 78.26(10) 95.30(11) 172.50(14) 92.14(13) 89.75(13) 174.04(12) 82.87(14)

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

are seldom reports about the photoelectric property of these materials.7b,8a To our knowledge, this is the first example of the studies on the surface photovoltage properties of oxomolybdenum−organophosphonates. Surface photovoltage spectroscopy (SPS) is a useful tool for surveying the charge change of the

(H2O)}MoO2(hedp)]·2H2O (4) (2,2′-bipy = 2,2′-bipyridine, 1,10-phen = 1,10-phenanthroline), and surface photovoltage properties of the four compounds have also been studied. At present, research on the properties of oxomolybdenum−organophosphonates is mainly focused on the magnetism; however, there B



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Table 3. Selected Bond Distances (Å) and Angles (deg) for 2a Distances (Å) Mo(1)−O(16) Mo(1)−O(15) Mo(1)−O(10) Mo(1)−O(6) Mo(1)−O(3) Mo(1)−O(7) Cu(2)−O(9) Cu(2)−O(11) Cu(2)−N(4) Cu(2)−N(3) Cu(2)−O(17) Cu(1)−O(1) Cu(1)−O(4) Cu(1)−N(1) Cu(1)−N(2) Cu(1)−O(12)#1

1.691(5) 1.692(5) 1.969(5) 2.034(5) 2.128(5) 2.199(5) 1.926(5) 1.952(5) 2.020(6) 2.021(7) 2.309(7) 1.930(5) 1.941(5) 2.003(6) 2.016(7) 2.248(5)

O(16)−Mo(1)−O(15) O(16)−Mo(1)−O(10) O(15)−Mo(1)−O(10) O(16)−Mo(1)−O(6) O(15)−Mo(1)−O(6) O(10)−Mo(1)−O(6) O(16)−Mo(1)−O(3) O(15)−Mo(1)−O(3) O(10)−Mo(1)−O(3) O(6)−Mo(1)−O(3) O(16)−Mo(1)−O(7) O(15)−Mo(1)−O(7) O(10)−Mo(1)−O(7) O(6)−Mo(1)−O(7) O(3)−Mo(1)−O(7) O(9)−Cu(2)−O(11) O(9)−Cu(2)−N(4) O(11)−Cu(2)−N(4)

102.6(3) 96.5(2) 96.5(2) 96.2(2) 91.2(2) 163.4(2) 91.4(2) 165.7(2) 85.1(2) 83.89(19) 168.6(2) 88.6(2) 84.0(2) 81.46(19) 77.3(2) 95.8(2) 166.5(3) 92.4(2)

P(3)−O(9) P(3)−O(7) P(3)−O(8) P(3)−C(3) P(1)−O(1) P(1)−O(3) P(1)−O(2) P(1)−C(1) P(2)−O(4) P(2)−O(5) P(2)−O(6) P(2)−C(1) P(4)−O(12) P(4)−O(11) P(4)−O(10) P(4)−C(3)

1.505(6) 1.520(5) 1.533(5) 1.820(8) 1.514(5) 1.526(5) 1.530(5) 1.833(8) 1.499(5) 1.532(5) 1.533(5) 1.820(8) 1.501(5) 1.502(5) 1.562(5) 1.831(8)

Angles (deg)

a

O(9)−Cu(2)−N(3) O(11)−Cu(2)−N(3) N(4)−Cu(2)−N(3) O(9)−Cu(2)−O(17) O(11)−Cu(2)−O(17) N(4)−Cu(2)−O(17) N(3)−Cu(2)−O(17) O(1)−Cu(1)−O(4) O(1)−Cu(1)−N(1) O(4)−Cu(1)−N(1) O(1)−Cu(1)−N(2) O(4)−Cu(1)−N(2) N(1)−Cu(1)−N(2) O(1)−Cu(1)−O(12)#1 O(4)−Cu(1)−O(12)#1 N(1)−Cu(1)−O(12)#1 N(2)−Cu(1)−O(12)#1

90.7(3) 173.4(3) 81.0(3) 87.7(3) 92.7(3) 102.7(3) 89.1(3) 96.4(2) 89.0(2) 170.9(3) 154.5(2) 89.9(2) 82.2(3) 112.0(2) 86.1(2) 98.7(2) 93.1(2)

Symmetry transformations used to generate equivalent atoms: For 2: #1, x, −y + 1/2, z + 1/2; #2, x, −y + 1/2, z − 1/2. were collected on a Bruker AXS D8 Advance diffractometer using Cu− Kα radiation (λ = 1.5418 Å) in the 2θ range of 5−60° with a step size of 0.02° and a scanning rate of 3°/min. Surface photovoltage spectroscopy (SPS) and field-induced surface photovoltage spectroscopy (FISPS) measurements were conducted with the sample in a sandwich cell (ITO/sample/ITO) with the light source-monochromator-lock-in detection technique in the range of 300−600 nm. Synthesis of [Cu(1,10-phen)Mo2O5(H2O)(hedp)] (1). A mixture of MoO3 (0.06 g, 0.45 mmol), CuCl2·2H2O (0.12 g, 0.70 mmol), 1,10phenanthroline (0.08 g, 0.43 mmol), and hedpH4 solution (0.12 mL, 0.30 mmol) was dissolved in 8 mL of distilled water. The resulting solution was stirred for about 1 h at room temperature, sealed in a 20 mL Teflon-lined stainless steel autoclave, and heated at 160 °C for 48 h under autogenous pressure. After the mixture was cooled slowly to room temperature, the blue block crystals were obtained. Yield: 52.5% (based on Mo). Anal. Calcd for C14H14N2O13P2CuMo2: C, 22.86; H, 1.92; N, 3.80; P, 8.42; Cu, 8.64; Mo, 26.08. Found: C, 22.81; H, 1.96; N, 3.85; P, 8.37; Cu, 8.70; Mo, 26.12. IR (KBr pellet, cm−1): 3445(m), 3063(w), 2932(w), 2848(w), 1621(m), 1512(w), 1429(m), 1131(s), 1083(s), 1036(s), 905(s), 810(w), 715(m), 560(m), 488(w). Synthesis of [{Cu(1,10-phen)}{Cu(1,10-phen)(H 2 O)}MoO2(hedpH)2]·H2O (2). The procedure was the same as that for 1 except that the reaction condition 160 °C for 48 h was replaced by 80 °C for 72 h. After the mixture was cooled slowly to room temperature, the blue block crystals were obtained. Yield: 45.6% (based on Mo). Anal. Calcd for C28H30N4O18P4Cu2Mo: C, 31.80; H, 2.86; N, 5.30; P,

solid surface, and it can be used to investigate the photophysics of the excited states and the surface charge behavior of the sample.10 The sensitivity of the method is about 108 q/cm2, which exceeds that of conventional spectroscopies such as XPS and Auger spectroscopy by many orders of magnitude.11 Recently, only one investigation on the surface photovoltage property of metal phosphonates has also been reported by our group.12 Herein we report the syntheses, crystal structures, and surface photovoltage properties of four title compounds.

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EXPERIMENTAL SECTION

Materials and Methods. All chemicals were used as obtained without further purification. 1-Hydroxyethylidenediphosphonic acid solution was obtained from Taihe Chemical Factory (60.0 wt %) and used as received. C, H and N were determined by using a PE-2400 elemental analyzer. Mo, Cu and P were determined by using an inductively coupled plasma (ICP) atomic absorption spectrometer. IR spectra were recorded on a Bruker AXS TENSOR-27 FT-IR spectrometer with KBr pellets in the range 4000−400 cm−1. The UV−Vis diffuse reflectance spectra were recorded with a JASCO V-570 UV− Vis−NIR spectrophotometer in the 200−600 nm. Thermogravimetric (TG) analyses were performed on a Perkin−Elmer Pyris Diamond TG−DTA thermal analyses system in static air with a heating rate of 10 K min−1 from 50 to 1100 °C. The X-ray powder diffraction data C



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Table 4. Selected Bond Distances (Å) and Angles (deg) for 3a Distances (Å) Mo(1)−O(8)#1 Mo(1)−O(8) Mo(1)−O(1)#1 Mo(1)−O(1) Mo(1)−O(4) Mo(1)−O(4)#1 Cu(1)−O(2) Cu(1)−O(5) Cu(1)−N(2) Cu(1)−N(1)

1.703(13) 1.703(13) 1.989(12) 1.989(12) 2.139(12) 2.139(12) 1.917(12) 1.932(13) 2.008(17) 2.026(16)

Cu(1)−O(8)#1 P(1)−O(3) P(1)−O(2) P(1)−O(1) P(1)−C(1) P(2)−O(6) P(2)−O(4) P(2)−O(5) P(2)−C(1)

2.319(14) 1.441(13) 1.527(13) 1.561(13) 1.842(17) 1.489(13) 1.507(13) 1.521(13) 1.828(18)

Angles (deg) O(8)#1−Mo(1)−O(8) O(8)#1−Mo(1)−O(1)#1 O(8)−Mo(1)−O(1)#1 O(8)#1−Mo(1)−O(1) O(8)−Mo(1)−O(1) O(1)#1−Mo(1)−O(1) O(8)#1−Mo(1)−O(4) O(8)−Mo(1)−O(4) O(1)#1−Mo(1)−O(4) O(1)−Mo(1)−O(4) O(8)#1−Mo(1)−O(4)#1 O(8)−Mo(1)−O(4)#1 O(1)#1−Mo(1)−O(4)#1 a

102.0(9) 94.9(6) 94.4(6) 94.4(6) 94.9(6) 165.3(7) 89.9(6) 168.0(6) 84.9(5) 83.7(5) 168.0(6) 89.9(6) 83.7(5)

O(1)−Mo(1)−O(4)#1 O(4)−Mo(1)−O(4)#1 O(2)−Cu(1)−O(5) O(2)−Cu(1)−N(2) O(5)−Cu(1)−N(2) O(2)−Cu(1)−N(1) O(5)−Cu(1)−N(1) N(2)−Cu(1)−N(1) O(2)−Cu(1)−O(8)#1 O(5)−Cu(1)−O(8)#1 N(2)−Cu(1)−O(8)#1 N(1)−Cu(1)−O(8)#1

84.9(5) 78.2(7) 95.5(5) 170.4(6) 90.5(7) 91.2(6) 171.8(6) 82.3(7) 90.5(5) 97.2(5) 96.2(6) 87.4(6)

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

Table 5. Selected Bond Distances (Å) and Angles (deg) for 4a Distances (Å) Mo(1)−O(9) Mo(1)−O(10) Mo(1)−O(7) Mo(1)−O(5)#1 Mo(1)−O(3) Mo(1)−O(6) Cu(1)−O(4) Cu(1)−N(1) Cu(1)−O(1) Cu(1)−N(2)

1.695(2) 1.699(2) 1.954(2) 1.9897(19) 2.2264(19) 2.296(2) 1.926(2) 1.982(2) 1.995(2) 2.007(2)

Cu(1)−O(8) P(1)−O(1) P(1)−O(3) P(1)−O(2) P(1)−C(1) P(2)−O(6) P(2)−O(4) P(2)−O(5) P(2)−C(1)

2.293(2) 1.507(2) 1.524(2) 1.562(2) 1.839(3) 1.515(2) 1.518(2) 1.5387(19) 1.828(3)

Angles (deg) O(9)−Mo(1)−O(10) O(9)−Mo(1)−O(7) O(10)−Mo(1)−O(7) O(9)−Mo(1)−O(5)#1 O(10)−Mo(1)−O(5)#1 O(7)−Mo(1)−O(5)#1 O(9)−Mo(1)−O(3) O(10)−Mo(1)−O(3) O(7)−Mo(1)−O(3) O(5)#1−Mo(1)−O(3) O(9)−Mo(1)−O(6) O(10)−Mo(1)−O(6) O(7)−Mo(1)−O(6) a

102.24(12) 99.17(10) 100.36(10) 99.97(10) 99.13(10) 149.01(8) 92.38(10) 165.34(10) 78.15(8) 76.92(8) 170.59(10) 87.14(10) 78.10(7)

O(5)#1−Mo(1)−O(6) O(3)−Mo(1)−O(6) O(4)−Cu(1)−N(1) O(4)−Cu(1)−O(1) N(1)−Cu(1)−O(1) O(4)−Cu(1)−N(2) N(1)−Cu(1)−N(2) O(1)−Cu(1)−N(2) O(4)−Cu(1)−O(8) N(1)−Cu(1)−O(8) O(1)−Cu(1)−O(8) N(2)−Cu(1)−O(8)

79.06(8) 78.27(7) 170.85(10) 93.87(9) 93.44(9) 90.12(10) 80.88(11) 152.93(10) 91.84(9) 92.75(10) 96.91(9) 109.72(10)

Symmetry transformations used to generate equivalent atoms: For 4: #1, −x + 1, y + 1/2, −z + 1/2; #2, −x + 1, y − 1/2, −z + 1/2.

11.72; Cu, 12.02; Mo, 9.07. Found: C, 31.75; H, 2.82; N, 5.36; P, 11.68; Cu, 12.07; Mo, 9.02. IR (KBr pellet, cm−1): 3467(m), 3074(w), 2918(w), 1621(m), 1512(w), 1417(m), 1131(s), 1083(s), 894(s), 799(m), 727(m), 679(m), 548(m), 476(w). Synthesis of [Cu(1,10-phen)Mo0.5O(hedpH)] (3). A mixture of MoO3 (0.06 g, 0.45 mmol), CuCl2·2H2O (0.04 g, 0.22 mmol), 1,10-

phenanthroline (0.05 g, 0.26 mmol), and hedpH4 solution (0.14 mL, 0.35 mmol) was dissolved in 8 mL of distilled water. The resulting solution was stirred for about 1 h at room temperature, sealed in a 20 mL Teflon-lined stainless steel autoclave, and heated at 120 °C for 72 h under autogenous pressure. After the mixture was cooled slowly to room temperature, the blue block crystals were obtained. Yield: 33.3% D



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(based on Mo). Anal. Calcd for C14H13N2O8P2CuMo0.5: C, 32.92; H, 2.57; N, 5.49; P, 12.13; Cu, 12.44; Mo, 9.39. Found: C, 32.87; H, 2.61; N, 5.45; P, 12.18; Cu, 12.38; Mo, 9.33. IR (KBr pellet, cm−1): 3063(w), 2979(w), 2930(w), 1639(m), 1582(w), 1517(w), 1426(m), 1327(m), 1147(s), 1088(s), 966(m), 907(s), 883(s), 792(m), 718(m), 661(w), 553(m), 481(w) Synthesis of [{Cu(2,2′-bipy)(H2O)}MoO2(hedp)]·2H2O (4). A mixture of MoO3 (0.06 g, 0.45 mmol), CuCl2·2H2O (0.12 g, 0.70 mmol), 2,2′-bipyridine (0.07 g, 0.43 mmol), and hedpH4 solution (0.12 mL, 0.30 mmol) was dissolved in 8 mL of distilled water. The resulting solution was stirred for about 1 h at room temperature, sealed in a 20 mL Teflon-lined stainless steel autoclave, and heated at 140 °C for 48 h under autogenous pressure. After the mixture was cooled slowly to room temperature, the blue block crystals were obtained. Yield: 55.8% (based on Mo). Anal. Calcd for C12H18N2O12P2CuMo: C, 23.87; H, 3.01; N, 4.64; P, 10.26; Cu, 10.53; Mo, 15.89. Found: C, 23.82; H, 3.05; N, 4.68; P, 10.21; Cu, 10.58; Mo, 15.83. IR (KBr pellet, cm−1): 3456(m), 3074(w), 2918(w), 2848(w), 1596(m), 1442(m), 1370(w), 1311(w), 1167(s), 1097(m), 1036(s), 941(s), 894(s), 822(w), 763(m), 727(w), 668(w), 632(w), 571(w), 535(w), 465(w). X-ray Crystallography. Data collections for compounds 1−4 were performed on the Bruker AXS Smart APEX II CCD X-diffractometer equipped with graphite monochromated MoKα radiation (λ = 0.71073 Å) at 293 ± 2 K. An empirical absorption correction was applied using the SADABS program. The structures were solved by direct methods and refined by full matrix least-squares on F2 by using the programs SHELXS-97.13 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms except those for water molecules were generated geometrically with fixed isotropic thermal parameters and included in the structure factor calculations. Hydrogen atoms for water molecules were not included in the refinement. Details of crystallographic data of compounds 1−4 are summarized in Table 1. Selected bond distances and angles of compounds 1−4 are listed in Tables 2−5. Hydrogen bonds for compounds 2−4 are listed in Table 6.

blue powders were obtained, which was indicated by X-ray powder diffraction analysis. Finally, compound 3 was observed in regions with the molar ratio MoO3/CuCl2·2H2O/1,10-phen/ hedpH4 = 4:2:2:3. In addition, the temperature also has a strong effect on the formation of the title compounds. Compound 1 was obtained as a good sample for X-ray diffraction studies at the used reaction temperature of 160 °C, while compounds 2 and 3 were obtained at 80 and 120 °C, respectively. When the reaction was carried out at 140 °C using 2,2′-bipy in place of 1,10-phen, compound 4 was isolated. The powder X-ray diffraction patterns (PXRD) of compounds 1−4 all match those simulated from single-crystal X-ray data (Figures S9−S12, Supporting Information). The diffraction peaks on the patterns correspond well in position, confirming that the four compounds are all pure phase. The differences in reflection intensity are probably due to preferred orientation in the powder samples. Crystal Structure of [Cu(1,10-phen)Mo2O5(H2O)(hedp)] (1). The asymmetric unit of compound 1 is shown in Figure 1. The

Table 6. Hydrogen Bonds for Compounds 2−4

Figure 1. ORTEP representation of a selected unit of compound 1. The thermal ellipsoids are drawn at the 50% probability level. All H atoms and lattice water molecules are omitted for clarity. Symmetry codes: A: x, −y + 1/2, z − 1/2; B: −x, −y, −z.

Compound 2 D−H···A

D(D−H)/Å

O2−H2D···O8

0.85

D−H···A

D(D−H)/Å

O6−H6A···O3

0.85

d(H···A)/Å

1.70 Compound 3 d(H···A)/Å

1.85 Compound 4

D−H···A

D(D−H)/Å

O1W−H1WA···O10 O1W−H1WB···O2

0.85 0.85

D−H−A/°

d(D···A)/Å

140.6

2.422(7)

D−H−A/°

d(D···A)/Å

129.0

2.472(5)

d(H···A)/Å D−H−A/° 2.03 1.98

157.4 120.0

two molybdenum environments of the binuclear subunit are quite distinct. The Mo(1) is six-coordinated by three oxygen atoms (O2, O5 and O3A) from two separate hedp4− anions, two terminal oxygen atoms (O8, O9) and one oxygen atom (O13) shared by two molybdenum atoms. The Mo(2) is also six-coordinated by two oxygen atoms (O5, O6B) from two separate hedp4− anions, two terminal oxygen atoms (O11, O12), a bridging oxygen atom (O13) to Mo(1), and one oxygen atom (O10) from the coordinated water molecule. Meanwhile, two kinds of molybdenum are coordinated by terminal oxygen atoms and bridging oxygen atoms to form a [Mo2O5]2+ cluster. The molybdenum−oxygen distances in compound 1 can be grouped into three types: the short terminal MoO bonds [1.686(3)−1.704(3) Å], the long terminal bonds trans to the MoO groups [1.973(2)−2.272(3) Å], and the Mo−O−Mo bridges [1.894(2)−2.323(2) Å] (Table 2). The Cu(1) ion is fourcoordinated by two nitrogen atoms (N1, N2) from the 1,10phenanthroline ligand [Cu(1)−N(1) = 2.004(3), Cu(1)−N(2) = 1.997(3) Å] and two oxygen atoms (O1, O4) from the hedp4− anion [Cu(1)−O(1) = 1.932(3), Cu(1)−O(4) = 1.941(3) Å]. In compound 1, all of the phosphonate oxygen atoms of hedp4− anion are involved in bonding. Each hedp4− anion links four molybdate centers and one copper subunit (Scheme 1a). The overall structure of compound 1 can be described as a 3D supramolecular structure. As shown in Figure 2, two {MoO6} octahedra and one {CuN2O2} planar square are linked by

d(D···A)/Å 2.837(3) 2.517(3)

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RESULTS AND DISCUSSION Syntheses. The copper−molybdophosphonates 1−4 were prepared by hydrothermal methods, which have been demonstrated to be particularly effective in the preparation of metal oxides and organic−inorganic hybrid materials. Generally, many factors can affect the formation and crystal growth of products, such as the type of initial reactants, molar ratio, pH value, reaction time and temperature in the process of hydrothermal synthesis. In an attempt to obtain good samples for X-ray diffraction studies, the optimal conditions of synthesizing were explored. It was found that the molar ratio of the starting materials plays an important role in the formation of four compounds. The best crystals of compounds 1, 2 and 4 were observed for the molar ratio MoO3/CuCl2·2H2O/1,10-phen or 2,2′-bipy/hedpH4 = 3:5:3:2. However, we try to prepare compound 3 following the same molar ratio to that of compounds 1, 2 and 4. The amorphous E



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about 3.3−3.8 Å.14 In compound 1, the 1,10-phen rings between the neighboring asymmetric unit are parallel to each other, and the face-to-face distance (3.65 Å) between adjacent 1,10-phen rings is in the normal range for such interactions; hence there are π−π stacking interactions (Figure 3). Crystal Structure of [{Cu(1,10-phen)}{Cu(1,10-phen)(H2O)}MoO2(hedpH)2]·H2O (2). As shown in Figure 4, Mo(1)

Scheme 1. Coordination Modes of Phophonate Ligands in Compounds 1 (a), Compounds 2 (b) and (c), Compounds 3 (c) and Compounds 4 (c)

{CPO3} tetrahedra through phosphonate oxygen atoms to form a [Cu(1,10-phen)Mo2O5(H2O)(hedp)] unit via edge-sharing and corner-sharing. Two so-built units are further linked into a {[Cu(1,10-phen)Mo2O5(H2O)(hedp)]2} cluster through phosphonate oxygen atoms. Neighboring {[Cu(1,10-phen)Mo2O5(H2O)(hedp)]2} clusters are cross-linked by hedp4− anions through phosphonate oxygen atoms into a 2D layer in the bcplane. The connectivity pattern results in a 24-membered {Mo− O−P−O−Mo−O−P−C−P−O−Mo−O−}2 heterocycle. Then the adjacent layers are further assembled into a 3D supramolecular structure through π−π stacking interactions. The π−π stacking interactions can play an important role in controlling the packing or assembly of compounds. The usual π interaction is an offset or slipped stacking of the aromatic nitrogen heterocycles or benzene rings. In the two interactions, the effective distance is

Figure 4. ORTEP representation of a selected unit of compound 2. The thermal ellipsoids are drawn at the 50% probability level. All H atoms and lattice water molecules are omitted for clarity. Symmetry codes: A: x, −y + 1/2.

adopts six-coordinated with four phosphonate oxygen atoms (O3, O6, O7 and O10) from two separate hedpH3− anions and two terminal oxygen atoms (O15, O16) to form [MoO2]2+ cluster.

Figure 2. Polyhedral view of the two-dimensional network of compound 1 in the bc-plane. All C atoms of 1,10-phen ligands are omitted for clarity. The connectivity pattern results in a 24-membered heterocycle.

Figure 3. A view of the three-dimensional supramolecular structure via the π−π stacking interactions. The π−π stacking interactions between the adjacent 1,10-phen rings with the face-to-face distance of 3.65 Å. F



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Figure 5. (a) View of the hydrogen bonded (green dotted lines) double layers of compound 2. (b) The connectivity of hydrogen bonds for compound 2. (c) The 3D supramolecular structure of compound 2. (d) The π−π stacking interaction between the adjacent 1,10-phen rings with the face-to-face distance of 3.49 Å.

The Mo−O distances fall into two types: the short terminal MoO bonds [1.691(5)−1.692(5) Å] and the long terminal bonds trans to the MoO groups [1.969(5)−2.199(5) Å] (Table 3). Each asymmetric unit contains two kinds of crystallographically independent Cu(II) ions. The Cu(1) is fivecoordinated by two nitrogen atoms (N1, N2) from the 1,10phenanthroline ligand [Cu(1)−N(1) = 2.003(6), Cu(1)−N(2) = 2.016(7) Å] and three oxygen atoms (O1, O4, O12A) from two separate hedpH3− anions [Cu(1)−O(1) = 1.930(5), Cu(1)− O(4) = 1.941(5), Cu(1)−O(12A) = 2.248(5) Å]. The Cu(2) ion is also five-coordinated by two nitrogen atoms (N3, N4) from the 1,10-phenanthroline ligand [Cu(2)−N(3) = 2.021(7), Cu(2)−N(4) = 2.020(6) Å], two oxygen atoms (O9, O11) from the hedpH3− anion [Cu(2)−O(9) = 1.926(5), Cu(2)− O(11) = 1.952(5) Å] and one oxygen atom (O17) from the coordinated water molecule [Cu(2)−O(17) = 2.309(7) Å]. In compound 2, the two hedpH3− ligands display different coordination modes. The first hedpH3− ligand links one molybdate center and two copper subunits (Scheme 1b). The second hedpH3− ligand links one molybdate center and one copper subunit (Scheme 1c). Compound 2 also features a complicated 3D supramolecular structure (Figure 5c). The interconnection of {MoO6}, {CuN2O3} and {CPO3} polyhedra via corner-sharing forms a 1D chain. There are hydrogen bonds between uncoordinated phosphonate oxygen atoms (O2) and (O8) from the phosphonate ligands with the distance of 2.422(7) Å (O2−H2D···O8) and the corresponding angle of 140.6° (Figure 5b and Table 6). In compound 2, these 1D chains are further connected through hydrogen bonding interactions to form a 2D supramolecular layer (Figure 5a). Such neighboring layers are further assembled into a 3D supramolecular structure by π−π stacking interactions between the adjacent 1,10-phen rings with the face-to-face distance of 3.49 Å (Figure 5d). Crystal Structure of [Cu(1,10-phen)Mo0.5O(hedpH)] (3). The asymmetric unit of compound 3 is shown in Figure 6. The Mo(1) is six-coordinated by four phosphonate oxygen atoms (O1, O4, O1A and O4A) from two separate hedpH3− anions

Figure 6. ORTEP representation of a selected unit of compound 3. The thermal ellipsoids are drawn at the 50% probability level. All H atoms and lattice water molecules are omitted for clarity. Symmetry codes: A: −x + 1, y, −z + 1/2.

and two terminal oxygen atoms (O8, O8A) to form a [MoO2]2+ cluster. Two pairs of Mo−O distances can be identified: the short terminal MoO bonds [1.703(13) Å] and the long terminal bonds trans to the MoO groups [1.989(12)−2.139(12) Å] (Table 4). The occupancy of molybdenum atom is 0.5. The Cu(1) ion is five-coordinated by two nitrogen atoms (N1, N2) from the 1,10-phenanthroline ligand [Cu(1)−N(1) = 2.026(16), Cu(1)−N(2) = 2.008(17) Å], two oxygen atoms (O2, O5) from the hedpH3− anion [Cu(1)−O(2) = 1.917(12), Cu(1)−O(5) = 1.932(13) Å] and one oxygen atom (O8A) to Mo(1) [Cu(1)− O(8A) = 2.319(14) Å]. Each hedpH3− ligand links one molybdate center and one copper subunit (Scheme 1c). The overall structure of compound 3 can be described as a 3D supramolecular structure type. As shown in Figure 7c, one {MoO6} octahedra and two {CuN2O3} square pyramids are linked by {CPO3} tetrahedra through phosphonate oxygen atoms to form a [{Cu(1,10-phen)}2MoO2(hedpH)2] unit via corner-sharing. There are hydrogen bonds between uncoordinated phosphonate oxygen atoms (O3) and (O6) from the phosphonate ligands with the distance of 2.472(5) Å (O6−H6A···O3) G



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Figure 7. (a) A hydrogen bonded double chains of compound 3. (b) The connectivity of hydrogen bonds. Hydrogen bonds are drawn as dotted green lines. (c) A view of the structure of compound 3. (d) The π−π stacking interaction between the adjacent 1,10-phen rings with the face-to-face distance of 3.74 Å.

Crystal Structure of [{Cu(2,2′-bipy)(H2O)}MoO2(hedp)]·2H2O (4). The asymmetric unit of compound 4 is shown in Figure 9.

and the corresponding angle of 129.0° (Figure 7b and Table 6). These [{Cu(1,10-phen)}2MoO2(hedpH)2] units are cross-linked by hydrogen bonding interactions to give hydrogen-bonded double chains (Figure 7a). The neighboring double chains are further connected through π−π stacking interactions to result in a 2D supramolecular layer. The average face-to-face distance of the aromatic units (phen) is about 3.74 Å (Figure 7d). Meanwhile, the neighboring layers are further assembled into a 3D supramolecular structure through π−π stacking interactions between the adjacent 1,10-phen rings from adjacent layers with the face-to-face distance of 3.49 Å (Figure 8).

Figure 9. ORTEP representation of a selected unit of compound 4. The thermal ellipsoids are drawn at the 50% probability level. All H atoms and lattice water molecules are omitted for clarity. Symmetry codes: A: −x + 1, y + 1/2, −z + 1/2.

The Mo(1) is six-coordinated by three phosphonate oxygen atoms (O3, O6 and O5A) and one hydroxyl oxygen atom (O7) from two separate hedp4− anions and two terminal oxygen atoms (O9, O10) to form a [MoO2]2+ cluster. The Mo−O distances can be grouped into two types: the short terminal MoO bonds [1.695(2)−1.699(2) Å] and the long terminal bonds trans to the MoO groups [1.954(2)−2.296(2) Å] (Table 5). The Cu(1) ion is five-coordinated by two nitrogen atoms (N1, N2) from the 2,2′-bipyridine ligand [Cu(1)−N(1) = 1.982(2), Cu(1)−N(2) = 2.007(2) Å], two oxygen atoms (O1, O4) from the hedp4− anion [Cu(1)−O(1) = 1.995(2), Cu(1)− O(4) = 1.926(2) Å] and one oxygen atom (O8) from the coordinated water molecule [Cu(1)−O(8) = 2.293(2) Å]. Each hedp4− ligand links one molybdate center and one copper subunit (Scheme 1c). Compound 4 also exhibits a 3D supramolecular structure type. The {MoO6} octahedra and the {CuN2O3} square pyramids

Figure 8. Polyhedral view of the 2D supramolecular structure in the ac-plane via the π−π stacking interactions in compound 3. H



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to stretching vibrations of the tetrahedral CPO3 groups, as expected.17 Two medium-to-strong bands of compounds 1−4 in the 880−940 cm−1 range can be assigned to the symmetric and antisymmetric v(MoO) of the cis {MoO2} groups of the molybdate units. The medium-to-strong bands observed in the 650−770 cm−1 region are attributed to the v(Mo−O−Mo).8a,18 Thermal Analysis. Thermogravimetric analyses (TGA) are conducted to examine the stabilities of these compounds. Thermogravimetric analyses of compounds 1−4 have been performed in the temperature range of 50−1100 °C in static air atmosphere. For compound 1, the first step started at 50 °C and was completed at 299 °C, corresponding to the release of one coordinated water molecule. The observed weight loss of 2.9% is basically close to the calculated value (2.4%). Then it exhibits a complicated weight loss between 299 and 636 °C, corresponding to elimination of the organic moieties and the collapse of the structures. Above 865 °C, the further decomposition of the organic groups results in an amorphous phase that was not characterized (Figure S5).19 Compound 2 exhibits a complex TGA profiles, which corresponds to the release of water molecules and the pyrolysis of the organic moieties in the temperature range 50−652 °C (Figure S6). There is a sharp weight loss starting from 885 °C, which can be attributed to the further decomposition of the organic groups. Compound 3 indicates two complicated overlapping steps of weight losses (Figure S7). The first step from 50 to 639 °C is due to the decomposition of the organic groups to give an amorphous gray powder. Following these complex processes, the further decomposition of the organic groups occurs above 788 °C, and the final product was not characterized. For compound 4, the weight loss between 50 and 171 °C corresponds to the release of two lattice water molecules (Figure S8). The weight loss of 5.8% is consistent with the calculated value (5.9%). Above the temperature of 229 °C, a continuous weight loss was observed up to 640 °C, corresponding to the release of one coordinated water molecule and the decomposition of organic groups. The product of the thermal decomposition is amorphous and was not further characterized. UV−Vis Spectra and Surface Photovoltage Properties. Seen from the UV−Vis absorption spectra of compounds 1−4 (Figures S13−S16), there are some wide and strong absorption bands. These absorption bands nearly cover the entire UV−Vis region, and the energies of the bands are in the range of band gap energy of semiconductor, so they can be seen as broad semiconductors.20 The surface photovoltage (SPV) method is a contactless and nondestructive technique. This technique has been used to investigate the photoelectric processes such as charge transfer and photocatalysis.21 On the basis of the principle

are linked by {CPO3} tetrahedra through phosphonate oxygen atoms leading to a 1D chain via corner-sharing. There are hydrogen bonds among the uncoordinated phosphonate oxygen atoms (O2), the terminal oxygen atoms (O10) from [MoO2]2+ cluster and lattice water with the distances of 2.517(3) Å (O1W−H1WB···O2), 2.837(3) Å (O1W−H1WA···O10) and the corresponding angles of 120.0°, 157.4° (Table 6). As shown in Figure 10, these one-

Figure 10. Polyhedral view of the 2D network of compound 4 in the ab-plane. Hydrogen bonds are drawn as dotted green lines. All H atoms are omitted for clarity.

dimensional chains are further connected through hydrogen bonding interactions to form a 2D supramolecular layer in the ab plane. The neighboring layers are further assembled into a 3D supramolecular structure through π−π stacking interactions between the 2,2′-bipy rings from adjacent layers with the faceto-face distance of 3.51 Å (Figure 11). IR Spectroscopy. The IR spectra for compounds 1−4 are recorded in the region 4000−400 cm−1. The absorption band at 3445 cm−1 for 1, 3467 cm−1 for 2, and 3456 cm−1 for 4 can be assigned to the O−H stretching vibrations of water molecules.15 The C−H stretching vibrations are observed as sharp, weak bands close to 3000 cm−1 for compounds 1−4. The bands at 1621, 1512, 1429 cm−1 for 1, 1621, 1512, 1417 cm−1 for 2, 1639, 1582, 1517, 1426 cm−1 for 3, and 1596, 1442, 1370, 1311 cm−1 for 4 can be assigned to the stretching bands of the pyridylrings of 1,10-phenanthroline ligands or 2,2′-bipy ligands.8a,16 Strong bands between 1200 and 900 cm−1 for four compounds are due

Figure 11. A view of the 3D supramolecular structure of compound 4. The π−π stacking interactions between the adjacent 2,2′-bipy rings with the face-to-face distance of 3.51 Å. I



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Figure 12. The SPS of compounds 1 (a), 2 (b), 3 (c) and 4 (d). Dotted lines are treated peak.

of SPS, Wang et al. have developed a field-induced surface photovoltage spectroscopy (FISPS) technique, which has been used to investigate the photoelectric properties of semiconductors under the effect of an external electric field.22 The detected signal by SPS is equivalent to the change in the surface potential barrier on illumination (δVs), which is given by the equation: δVs = V′s − Vso, where V′s and Vso are the surface potential barriers before and after illumination, respectively. As far as band-to-band transitions are concerned, a positive response of SPV (δVs > 0) means that the sample is characterized as a p-typed semiconductor, whereas a negative response is an n-typed semiconductor.23 In this paper, the UV−Vis absorption spectra and the surface photovoltage spectroscopy of compounds 1−4 are combined to analyze and assign. The SPS of compounds 1−4 are shown in Figure 12. They all appear as positive SPV response bands between 300 and 600 nm. It can be seen that the signal detected by SPS at 300−400 nm is a wide peak. The signal is actually the result of overlap of several SPV response bands. To make the assignment of each SPV response band clear, we separated them by the Origin 7.0 program. The SPS of compound 1 shows two positive SPV responses within 300−600 nm (Figure 12a). The SPV response at λmax = 324 nm is attributed to the π → π* transition of {Cu(phen)}2+, while the response at λmax = 349 nm can be assigned to band-to-band (O → Mo) transition arising from ligand-to-metal charge transfer transition (LMCT). There are two absorption bands in the UV−Vis absorption spectrum of compound 1 (Figure S13). The band (λmax = 260 nm) is assigned to π → π* transition of {Cu(phen)}2+. The band (λmax = 308 nm) is assigned to the LMCT (O → Mo). So there is a good corresponding relationship between SPS and UV− Vis spectra. In compound 2, the coordination environment is

similar to the compound 1 (Figure 12b). The former response at λmax = 322 nm may be assigned to the π → π* transition of {Cu(phen)}2+, and the latter response at λmax = 343 nm is assigned to the band-to-band transition based on LMCT. The LMCT response can be assigned to the O → Mo transition. There are two absorption bands in the UV−Vis absorption spectrum of compound 2 (Figure S14). The band (λmax = 258 nm) is assigned to π → π* transition of {Cu(phen)}2+, and the band (λmax = 307 nm) is assigned to the LMCT (O → Mo). By careful differentiation, the response band of compound 3 contains two filial bands at 320 and 345 nm (Figure 12c). The two SPV response bands are respectively assigned to the π → π* transition of {Cu(phen)}2+ and the band-to-band transition caused by LMCT (O → Mo). There are two absorption bands in the UV−Vis absorption spectrum of compound 3 (Figure S15) that are similar to those of compounds 1 and 2. The band (λmax = 261 nm) is assigned to π → π* transition of {Cu(phen)}2+, while the band (λmax = 310 nm) is assigned to the LMCT (O → Mo). The SPS of compound 4 also shows two positive SPV within 300−600 nm (Figure 12d). The SPV response at λmax = 325 nm is attributed to the π → π* transition of {Cu(phen)}2+, and the response at λmax = 335 nm is assigned to the band-to-band (O → Mo) transition based on LMCT. There are two absorption bands in the UV−Vis absorption spectrum of compound 4 at 256 and 306 nm (Figure S16). In compound 4, the absorption bands are also similar to those of compounds 1−3. The band (λmax = 256 nm) is assigned to the π → π* transition of {Cu(bpy)}2+. The band (λmax = 306 nm) is assigned to the LMCT (O → Mo). The UV−Vis absorption spectra and surface photovoltage spectra of compounds 1−4 indicate that not only do semiconductors possess photovoltage J



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1D chain which is built by coordination bonds. These 1D chains, which are of benefit to increasing of the SPV response intensities, are assembled into a 3D structure by hydrogen bonds and π−π stacking interactions. In compound 3, the individual cluster affects the transport of electrons or holes; therefore the SPV response’s intensity is the weakest. In addition, the types of bonds in the crystal have different effects when it comes to transferring electrons or holes. In compounds 1−4, hydrogen bonds and π−π stacking interactions can play an important role in transferring electrons or holes. As hydrogen bonds and π−π stacking interactions are weak, their ability to transfer electrons or holes is much lower than that of coordination bonds; therefore; the intensity of the SPV of compound 1 is the strongest and compound 3 is the weakest. Field-induced surface photovoltage spectroscopy (FISPS) can be measured by applying an external electric field to the sample with a transparent electrode. For a p-type semiconductor, when a positive electric field is applied on the semiconductor surface, the SPV response increases since the external field is consistent with the built-in field. On the contrary, when a negative electric field is applied, the SPV response is weakened. In contrast to p-type semiconductors, the SPV response intensity of n-type semiconductors increases as a negative field is applied and decreases as a positive electric field is applied. Figure 14 shows the FISPS of compounds 1−4 in the range of 300−600 nm when the external electric fields are −0.2, 0, and +0.2 V, respectively. The SPV response intensities of the four compounds all increase when the positive fields increase, while they decrease when the external negative fields increase. This is attributed to the positive electric field being beneficial to the separation of photoexcited electron−hole pairs, which in turn results in an increase of response intensity; however, the negative

characteristics, oxomolybdenum−organophosphonate hybrid materials also can exhibit the photovoltage property. Comparing the SPV responses of compounds 1−4 (Figure 13), it can be seen that without the external electric field the values of

Figure 13. A comparative SPS of compounds 1−4.

the four compounds’ response bands are the same, although their intensities are obviously different; compound 1 is the strongest and compound 3 is the weakest. This intensity difference is mainly due to the differences in their structures. Compounds 1−4 all possess infinite 3D structures. Among them, compound 1 contains a 2D layer formed by coordination bonds, and the 2D layer which is formed by two perpendicularly 1D infinite chains is beneficial to the conduction of electrons or holes, which means that more electrons diffuse to the surface, resulting in an increase of the SPV response intensities. The 2D layer is further connected into a 3D structure by π−π stacking interactions. Compounds 2 and 4 respectively exhibit an infinite

Figure 14. The FISPS of compounds 1 (a), 2 (b), 3 (c) and 4 (d). K



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electric field has just the opposite effect. The FISPS confirm the p-type characteristics of compounds 1−4.

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CONCLUSIONS In this study, four novel organic−inorganic hybrid materials of the copper(II)−molybdophosphonate family have been synthesized by the hydrothermal technique. In compound 1, the interconnection of {MoO6}, {CuN2O2}, and {CPO3} polyhedra leads to a 2D layer through edge-sharing and corner-sharing, and such layers are further assembled into a 3D supramolecular structure by π−π stacking interactions. Compound 2 contains a 1D chain built up from {MoO6}, {CuN2O3}, and {CPO3} polyhedra, and then these 1D chains are further extended into a 2D supramolecular layer by hydrogen bonding interactions. Such neighboring layers are further assembled into a 3D supramolecular structure via the π−π stacking interactions. For compound 3, the [{Cu(1,10-phen)}2MoO2(hedpH)2] units are interlinked by hydrogen bonds interactions to give hydrogenbonded double chains. The neighboring chains are further assembled through π−π stacking interactions to form a 3D supramolecular structure. In compound 4, the interconnection of {MoO6}, {CuN2O3}, and {CPO3} polyhedra via corner-sharing forms a 1D chain. These 1D chains are interlinked through hydrogen bonding interactions to form a 2D supramolecular layer, which are further assembled into a 3D supramolecular structure via the π−π stacking interactions. The SPS and FISPS of compounds 1−4 indicate that they exhibit surface photovoltage properties and show p-type semiconductor characteristics. Therefore, oxomolybdenum−organophosphonate hybrids can be regarded as a kind of novel photoelectric material.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files in CIF format for compounds 1−4, IR spectra of compounds 1−4, TG curves of compounds 1−4, XRD patterns of the experiments compared to those simulated from X-ray single-crystal data for compounds 1−4. UV−Vis absorption spectra of compounds 1−4. This information is available free of charge via the Internet at http://pubs.acs.org/. CCDC 883352−883355 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Center, 12, Union Road, Cambridge CB21EZ, UK; fax: (+44) 1223−336−033; e-mail: [email protected]).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 21071072) and the Program for Liaoning Excellent Talents in University (Grant No. LR2011030).

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REFERENCES

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Four Novel Oxomolybdenum-Organodiphosphonate Hybrids in the

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