Syntheses, Structures, and Photoluminescence of Four d10

CRYSTAL GROWTH & DESIGN

Syntheses, Structures, and Photoluminescence of Four d10 Metal-Organic Frameworks Constructed from 3,5-Bis-oxyacetate-benzoic Acid

2008 VOL. 8, NO. 10 3586–3594

Yin-Hua He, Yun-Long Feng,* You-Zhao Lan, and Yi-Hang Wen Zhejiang Key Laboratory for ReactiVe Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal UniVersity, Jinhua, Zhejiang 321004, P. R. China ReceiVed January 12, 2008; ReVised Manuscript ReceiVed May 20, 2008

ABSTRACT: A new multicarboxylate ligand, 3,5-bis-oxyacetate-benzoic acid (H3BOABA), was designed and introduced to construct novel metal-organic framework materials with potential luminescent properties. Four complexes, Zn2(BOABA)(OH) · H2O (1), Zn4(BOABA)2(4,4′-bipy)2(OH)2 · H2O · 0.5CH3CH2OH (2), Cd3(BOABA)2(H2O)6 · 6H2O (3), and Cd4(BOABA)2(phen)4(OH)2 · H2O (4), were synthesized hydrothermally and characterized. 1 features an unusual 5-connected 46.64 uninodal net with both the dinuclear Zn(II) unit and the BOABA ligand being the square pyramidal nodes. 2 presents an interesting three-dimensional (3,8)-connected network constructed from pillared undulate layers, which shows large one-dimensional channels with dimension of 11.97 × 13.82 Å along the a axis. The self-assembly between the Cd(II) ions and the BOABA ligand leads to a bilayer motif 3, whereas the presence of the auxiliary phen ligand results in a new compound 4 with monolayer construction based on tetranuclear [Cd4(OH)2]6+ clusters. Photoluminescence studies reveal that these four complexes exhibit strong fluorescent emission bands in the solid state at room temperature. The theoretic calculations show that four complexes are indirect band gap semiconductors and the fluorescent emission peaks could be attributed to the ligand-ligand charge transfer. The thermal stabilities of these complexes were also examined. Introduction Recently, the building of infinite frameworks based on multifunctional ligands and metal centers1 has had much interest due to their particular topologies2 and some potential applications such as catalysis, molecular recognition, and photochemistry.3 It is also well-known that the multiple coordination sites of ligands can form the structures of higher dimensions and the high symmetry of ligands may result in novel structures.4 For example, multibenzenecarboxylate ligands, such as terephthalic acid,5 1,3,5-benzenetricarboxylic acid,6 and 1,2,4,5benzenetetracarboxylicacid,7 have been extensively employed to prepare the metal-organic frameworks (MOFs) and a large number of significant ones have been reported.8 On the basis of the rigidity of 1,3,5-benzenetricarboxylic acid, we successfully designed a new multicarboxylate ligand, 3,5-bis-carboxymethoxy-benzoic acid (H3BOABA), which possesses one rigid carboxyl group and two flexible carboxyl groups (Scheme 1). This new ligand has the characteristic coordination chemistry of the rigid and flexible carboxylate system. In addition, it should be noted that three COOH groups of H3BOABA can be fully or partially deprotonated to be hydrogen bond donors or acceptors, which will possibly lead to complexes with higher dimension based on the intermolecular or intramolecular hydrogen bonding interactions, and the oxyacetate groups of H3BOABA with high plasticity are expected to have versatile coordination behavior to metal ions. On the other hand, polynuclear d10 metal (ZnII, CdII) complexes have attracted extensive interest in recent years because of their high transparency in the UV region and photoluminescent properties.9 However, to our best knowledge, there have been no related reports of the polymer constructed by d10 metal and H3BOABA ligand so far. With the aim of understanding the coordination chemistry of H3BOABA and preparing new materials with interesting structural topologies * To whom correspondence should be addressed. E-mail: [email protected].

Scheme 1. Structure of H3BOABA Ligand

and excellent physical properties, we have recently engaged in the research of these kinds of polymer complexes with such ligands. In this paper, we report the synthesis and structural characterization of a new family of luminescent MOFs: Zn2(BOABA)(OH) · H2O (1), Zn4(BOABA)2(4,4′-bipy)2(OH)2 · H2O · 0.5CH3CH2OH (2), Cd3(BOABA)2(H2O)6 · 6H2O (3), and Cd4(BOABA)2(phen)4(OH)2 · H2O (4). These four complexes are all based on the assembly of 3,5-bis-oxyacetate-benzoic acid and diimine ligands with Zn(II) and Cd(II) under hydrothermal conditions (Scheme 3). The results show that the BOABA ligand not only has versatile traits of coordination chemistry (Scheme 2) but also can form novel framework topologies, such as an unknown 5-connected 46.64 uninodal net and (3,8)-connected (43)2(46.617.85) net. Experimental Section Materials and Measurements. All reagents were purchased commercially and used without further purification. All complexes were obtained under a hydrothermal reaction in a 25 mL Teflon-lined stainless steel Parr bomb. Data collection was performed with Mo KR radiation (λ ) 0.71073 Å) on a Bruker APEX II area-detector diffractometer. Elemental analyses were carried out using a Perkin-Elmer 2400II elemental analyzer. IR spectra were obtained from KBr pellets on a Nicolet 5DX FT-IR spectrometer. The thermogravimetric measurements were performed on preweighed samples in an oxygen stream using a Netzsch STA449C apparatus with a heating rate of 10 °C/min. Fluorescent data were collected on a FS920 spectrofluorometer with a Xe-CW-source (450 W) and RR928P photomultiplier for signal

10.1021/cg8000398 CCC: $40.75  2008 American Chemical Society Published on Web 08/20/2008

MOFs from 3,5-Bis-oxyacetate-benzoic Acid

Crystal Growth & Design, Vol. 8, No. 10, 2008 3587

Scheme 2. The BOABA Ligand Employs Four Different Coordination Modes in These Four New Complexesa

a

A, B, C, and D represent the coordination modes of the ligand in 1, 2, 3, and 4, respectively.

Scheme 3. Hydrothermal Syntheses of 1-4

Table 1. Crystal Data and Structure Refinement for 1-4 compound

1

2

3

4

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z Dc µ/mm-1 F(000) crystal size θ range reflections collected/unique parameters refined goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) max, min ∆F/e Å3

C11H10O10Zn2 432.93 monoclinic P21/c 14.906(3) 10.362(2) 8.946(2) 90 105.42(3) 90 1332.1(5) 4 2.159 3.657 864 0.21 × 0.20 × 0.18 1.42-27.41 14396/3033 [Rint ) 0.0256] 218 1.007 R ) 0.0243, wR ) 0.0762 R ) 0.0289, wR ) 0.0801 0.542, -0.606

C43H39N4O19.5Zn4 1185.26 triclinic P1j 9.820(2) 11.824(2) 12.050(2) 116.65(3) 102.36(3) 96.21(3) 1187.6(4) 1 1.657 2.077 601 0.24 × 0.20 × 0.08 1.98-27.47 21157/5308 [Rint ) 0.0265] 351 1.085 R ) 0.0326, wR ) 0.1061 R ) 0.0406, wR ) 0.1119 0.739, -0.458

C22H38Cd3O28 1087.72 triclinic P1j 7.8386(5) 11.5023(3) 11.6566(3) 117.161(1) 103.249(2) 94.302(2) 890.4(1) 1 2.029 1.879 538 0.27 × 0.16 × 0.11 2.03-27.0 10126/3451 [Rint ) 0.0147] 241 1.016 R ) 0.0234, wR ) 0.0716 R ) 0.0256, wR ) 0.0731 0.516, -0.983

C70H52Cd4N8O20 1774.80 triclinic P1j 12.242(2) 12.558(3) 13.037(3) 65.30(1) 89.32(2) 68.86(2) 1675.4(4) 1 1.759 1.335 880 0.20 × 0.18 × 0.08 3.49-26.5 19328/6878 [Rint ) 0.0324] 462 1.002 R ) 0.0374, wR ) 0.1071 R ) 0.0504, wR ) 0.1155 0.994, -0.688

detection. XRPD patterns were collected on a Philips PW3040/60 automated powder diffractometer, using Cu KR radiation (λ ) 0.1542 nm) with a 2θ range of 5-50°. Synthesis of H3BOABA Ligand. The synthesis is according to the literature.10 A mixture of chloroacetic acid and 3,5-dihydroxybenzoic acid was stirred under basic conditions and refluxed at 90 °C for 3 h. The pH value was adjusted to around 11 by sodium hydroxide solution in the entire process. Then the pH value was

adjusted to 3-4 via concentrated hydrochloric acid as soon as the reaction was completed; meanwhile, a large quantity of light yellow powder was deposited. After the sample was filtrated, washed by water, and dried, the target H3BOABA ligand, yield of 80%, was obtained. Anal. Calcd (%) for C11H10O8: C, 48.88; H, 3.73. Found: C, 48.85; H, 3.40. Synthesis of Zn2(BOABA)(OH) · H2O (1). Zn(CH3CO2)2 · 2H2O 1.0 mmol (0.214 g), 3,5-bis-oxyacetate-benzoic acid 1.0 mmol (0.202 g),

3588 Crystal Growth & Design, Vol. 8, No. 10, 2008

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Table 2. Selected Bond Distances (Å) and Bond Angles (°) for 1a Zn(1)-O(9) Zn(1)-O(5)#1 Zn(1)-O(4)#2 Zn(1)-O(1) O(9)-Zn(1)-O(5)#1 O(9)-Zn(1)-O(4)#2 O(9)-Zn(1)-O(1) O(5)#1-Zn(1)-O(1) O(4)#2-Zn(1)-O(1)

1.903(3) 1.963(2) 1.975(2) 1.989(3) 115.49(8) 127.22(7) 105.87(7) 94.27(7) 105.81(7)

Zn(2)-O(9) Zn(2)-O(2) Zn(2)-O(7)#3 Zn(2)-O(8)#4 O(9)-Zn(2)-O(2) O(9)-Zn(2)-O(7)#3 O(2)-Zn(2)-O(7)#3 O(9)-Zn(2)-O(8)#4 O(2)-Zn(2)-O(8)#4

1.910(3) 1.958(2) 1.974(2) 1.979(3) 109.57(7) 121.91(7) 113.12(7) 106.37(7) 104.54(7)

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

Table 3. Selected Bond Distances (Å) and Bond Angles (°) for 2a Zn(1)-O(5)#1 Zn(1)-O(9) Zn(1)-O(1) Zn(1)-N(1) Zn(1)-O(9)#2 Zn(1)-O(7)#3 O(9)-Zn(1)-N(1) O(9)-Zn(1)-O(7)#3 O(5)#1-Zn(1)-O(1) N(1)-Zn(1)-O(9)#2

2.047(2) 2.070(2) 2.125(2) 2.126(2) 2.139(2) 2.164(2) 98.97(8) 168.17(7) 170.54(8) 179.66(8)

Zn(1) · · · Zn(2) Zn(2)-O(9)#2 Zn(2)-O(2) Zn(2)-O(8)#3 Zn(2)-N(2)#4 O(9)#2-Zn(2)-O(2) O(2)-Zn(2)-O(8)#3 O(2)-Zn(2)-N(2)#4 O(5)#1-Zn(1)-O(9)

3.117(1) 1.936(2) 1.948(3) 1.974(2) 2.020(2) 119.31(8) 110.26(9) 107.37(9) 91.42(8)

a Symmetry transformations used to generate equivalent atoms: #1 x, y + 1, z + 1; #2 -x, -y + 1, -z + 1; #3 -x + 1, -y + 1, -z + 1; #4 x - 1, y, z - 1; #5 x, y - 1, z - 1; #6 x + 1, y, z + 1.

imidazole 1.0 mmol (0.035 g), NaOH 2.5 mmol (0.095 g) were mixed in 15 mL of H2O/ethanol. Then the mixture was placed in a Parr Teflonlined stainless steel vessel (25 mL3) and heated to 160 °C for 72 h. Colorless crystals of 3 were obtained and collected by filtration, washed with water and ethanol, then dried in air, 20% yield (based on H3BOABA). IR (KBr pellet): V ) 3463s, 2924w, 1609s, 1558vs, 1465m, 1415s, 1329m, 1280w, 1174m, 1087m, 953m, 924m, 864m, 758m. Anal. Calcd(%) for C11H10O10Zn2: C,30.52; H,2.33. Found: C,31.02; H, 2.01. Synthesis of Zn4(BOABA)2(4,4′-bpy)2(OH)2 · H2O · 0.5CH3CH2OH (2). Zn(CH3CO2)2 · 2H2O 1.0 mmol (0.212 g), 3,5-bis-oxyacetatebenzoic acid 1.0 mmol (0.202 g), 4,4′-bipyridine 0.50 mmol (0.085 g) and NaOH 3 mmol (0.120 g) were mixed in 15 mL of H2O/ethanol. Then the mixture was placed in a Parr Teflon-lined stainless steel vessel (25 mL3) and heated to 160 °C for 72 h. Colorless crystals of 4 were obtained and collected by filtration, washed with water and ethanol, then dried in air, 28% yield (based on H3BOABA). IR (KBr pellet): V ) 3242mb, 2914w, 1648vs, 1610vs, 1578s, 1409s,1326s, 1292m, 1222m, 1164s, 1070m, 809m, 776m, 726m. Anal. Calcd (%) for C43H39N4O19.5Zn4: C,43.32; H,3.32; N,4.73. Found: C,44.98; H,3.27; N,2.60. Synthesis of Cd3(BOABA)2(H2O)6 · 6H2O (3). Cd(CH3COO)2 · 2H2O 1.0 mmol (0.136 g), 3,5-bis-oxyacetate-benzoic acid 1.0 mmol (0.202 g), imidazole 1.0 mmol (0.035 g) and NaOH 2.5 mmol (0.096 g) were mixed in 15 mL of H2O/ethanol. Then the mixture was placed in a Parr Teflon-lined stainless steel vessel (25 mL3) and heated to 160 °C for 72 h. Colorless crystals of 1 were obtained and collected by filtration, washed with water and ethanol, then dried in air, 20% yield (based on H3BOABA). IR (KBr pellet): V ) 3405sb, 3075w, 2924w, 1605vs 1552s,1429s, 1397s, 1320m, 1284w, 1175m, 1080m, 805w, 783w. Anal. Calcd(%) for C22H38Cd3O28: C, 25.29; H, 3.52. Found: C, 25.20; H, 3.88. Synthesis of Cd4(BOABA)2(phen)4(OH)2 · H2O (4). Cd(CH3COO)2 · 2H2O 1.0 mmol (0.136 g), 3,5-bis-oxyacetate-benzoic acid 1.0 mmol (0.202 g), phenanthroline 0.50mmol (0.100 g) and NaOH 2.5mmol (0.095 g) were mixed in 15 mL H2O/ethanol. Then the mixture was placed in a Parr Teflon-lined stainless steel vessel (25 mL3) and heated to 160 °C for 72 h. Colorless crystals of 2 were obtained and collected by filtration, washed with water and ethanol, then dried in air, 38% yield (based on H3BOABA). IR (KBr pellet): V ) 3418m, 3074w, 2922w, 1609vs, 1558s, 1514m, 1430s, 1381vs, 1287m, 1165m, 1085m, 853m, 725m. Anal. Calcd(%) for C70H52Cd4N8O20: C,47.37; H,2.95; N,6.32. Found: C,48.05; H, 2.60; N, 6.55.

Table 4. Selected Bond Distances (Å) and Bond Angles (°) for 3a Cd(1)-O(3W) 2.271(3) Cd(1)-O(4)#1 2.254(2) Cd(1)-O(7) 2.307(2) Cd(2)-O(1) 2.246(2) Cd(2)-O(2) 2.607(2) O(1)-Cd(2)-O(5)#4 133.09(8) O(1)-Cd(2)-O(7)#5 131.44(7) O(1)-Cd(2)-O(8)#5 79.21(7) O(1)-Cd(2)-O(1W) 92.05(9) O(1)-Cd(2)-O(2W) 93.08(8) O(5)#4-Cd(2)-O(1W) 88.37(9) O(5)#4-Cd(2)-O(2W) 83.79(8) O(7)#5-Cd(2)-O(2) 173.82(8) O(7)#5-Cd(2)-O(1W) 90.27(9)

Cd(2)-O(5)#4 2.263(2) Cd(2)-O(7)#5 2.309(2) Cd(2)-O(8)#5 2.574(2) Cd(2)-O(2W) 2.328(2) Cd(2)-O(1W) 2.336(3) O(7)#5-Cd(2)-O(2W) 90.77(8) O(1W)-Cd(2)-O(8)#5 100.45(9) O(2W)-Cd(2)-O(8)#5 86.35(9) O(2W)-Cd(2)-O(1W) 172.15(8) O(3W)#3-Cd(1)-O(7) 92.98(10) O(4)#1-Cd(1)-O(4)#2 180.00(1) O(4)#1-Cd(1)-O(7) 83.33(7) O(4)#1-Cd(1)-O(3W) 88.96(11)

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

Table 5. Selected Bond Distances (Å) and Bond Angles (°) for 4a Cd(1)-O(9) Cd(1)-O(9)#1 Cd(1)-N(1) Cd(1)-O(3) Cd(1)-O(5)#2 Cd(1)-N(2) O(9)-Cd(1)-O(9)#1 O(9)-Cd(1)-O(3) O(9)-Cd(1)-N(1) O(9)-Cd(1)-N(2) N(1)-Cd(1)-O(3) N(1)-Cd(1)-O(5)#2 O(3)-Cd(1)-O(5)#2 O(3)-Cd(1)-N(2)

2.239(3) 2.297(3) 2.324(4) 2.332(3) 2.360(3) 2.400(4) 84.04(10) 98.56(12) 164.46(15) 94.66(14) 96.98(17) 97.16(13) 77.94(10) 159.40(14)

Cd(2)-O(9) Cd(2)-O(6)#2 Cd(2)-O(1)#3 Cd(2)-N(4) Cd(2)-N(3) Cd(2)-O(2)#3 O(3)-Cd(1)-N(2) O(1)#3-Cd(2)-N(4) O(9)-Cd(2)-O(1)#3 O(9)-Cd(2)-O(2)#3 O(9)-Cd(2)-O(6)#2 O(9)-Cd(2)-N(3) O(9)-Cd(2)-N(4) N(4)-Cd(2)-O(2)#3

2.205(3) 2.291(3) 2.312(3) 2.357(4) 2.411(4) 2.570(3) 159.40(14) 120.16(13) 144.72(12) 94.32(10) 107.11(10) 101.02(13) 94.90(12) 152.40(13)

a Symmetry transformations used to generate equivalent atoms: #1 -x + 1, -y + 1, -z + 1; #2 -x + 1, -y + 1, -z; #3 x - 1, y + 1, z; #4 x + 1, y - 1, z.

Figure 1. Ball-and-stick representation of the asymmetric unit of 1. H atoms and water molecules were omitted for clarity (symmetry codes, #1 -x + 1, -y + 1, -z + 1; #2 -x + 1, y - 1/2, -z + 3/2; #3 -x, -y + 1, -z + 1; #4 -x, y - 1/2, -z + 3/2). Single-Crystal Structure Determination. The diffraction data for 1-4 were collected on a Bruker APXE II diffractometer equipped with a graphite-monochromatized MoKR radiation (λ ) 0.71073 Å) up to a 2θ limit of 55.0° at 296(2) K. Data intensity was corrected by Lorentzpolarization factors and empirical absorption. The structure was solved by direct methods and expanded with difference Fourier techniques. All non-hydrogen atoms were refined anisotropically. Except the hydrogen atoms on oxygen atoms, the other hydrogen atoms were generated geometrically. All calculations were performed with SHELXTL-97 package.11 Basic information pertaining to crystal parameters



Figure 2. (a) Two dimensional layer structure of 1. (BOABA ligands in black orientate upward while the gray orientate downward.) H atoms, and lattice water molecules were omitted for clarity. (b) Toplogically schematic representation of 3D structure. and structure refinement is summarized in Table 1, and the selected bond lengths and bond angles are listed in Tables 2 –5.

Computational Section We calculated the energy band structure of 1-4 based on their crystallographic data determined by X-ray experiments. The calculations were performed using density functional theory (DFT) in the generalized gradient approximation (GGA) of Perdew, Burke and Ernzerhof (PBE),12a as implemented in the DMol3 program Materials Studio 4.0.12b The core treatment of all electron and all electron relativistic was separately used for Zn- and Cd-complexes with double numerical plus d-functions (DND) atomic basis set.12c The other parameters and convergent criterions were set by the default values of DMol3 program. The partial density of states (PDOS) of the ligands and centermetals were calculated to analyze the origin of electronic transition of the photoluminescence emission spectra. Results and Discussion General Remarks. In most cases of synthesizing multicarboxylate complexes, the direct solution reactions at room temperature give rise to microcrystalline precipitates. Therefore, we employed the hydrothermal method to obtain suitable single crystal for X-ray diffraction. All complexes have a metal/main ligand ratio of 1:1, although most of them were tentatively prepared with a molar ratio of 1:2 or 2:1. As a matter of fact, the final products depend not on this metal-to-main ligand ratio of the reactants but on the auxiliary ligand and the pH value. For 1 and 3, the auxiliary ligands including imidazole, pyridine, and a small amount of 4,4′-bipy are necessary, whereas they do not join in coordination. So these auxiliary ligands possibly act as structure-directing reagents in this process. For 2 and 4, however, it is necessary to keep a strict ratio of 4,4′-bipy and phen; otherwise, the reactions will lead to other complexes, which have been validated by X-ray diffraction. The pH value also plays a vital role in these reactions. When the amount of sodium hydroxide is not between 2.5 and 3.0 mmol, the reactions will yield the precipitates whose IR spectra are inconsistent with the target complexes. Crystal Structures of 1. A single-crystal X-ray diffraction study of 1 reveals unique three-dimensional (3D) open frameworks consisting of two Zn(II) ions, one BOABA ligand, one µ2-OH, and one water molecule, as shown in Figure 1. The two metal centers adopt the same configuration. Each Zn(II) is primarily coordinated to two oxygen atoms from flexible carboxylic groups of two individual BOABA ligands (Zn-O 1.965-1.980 Å), one oxygen atom belonging to rigid carboxylic groups of another BOABA ligand (Zn-O 1.956 Å) and one hydroxyl oxygen atom (Zn-O 1.902 Å) in a slightly distorted tetrahedral geometry. The bridging Zn(1)-O(9)-Zn(2) angle of 113° is in the range of reported complexes2 and the Zn · · · Zn separation is 3.195 Å. It is worthwhile to note that the

Figure 3. ORTEP representation of the asymmetric unit in 2 (symmetry codes, #1 x, y + 1, z + 1; #2 -x, -y + 1, -z + 1; #3 -x + 1, -y + 1, -z + 1; #4 x - 1, y, z - 1).

Zn(1) · · · O(5)#2 distance of 2.794 Å and Zn(2) · · · O(8)#3 distance of 2.676 Å suggest the nonnegligible interaction between Zn(II) and µ2 oxygen atom of bidentate carboxylic group, which may be described as a semichelating coordination mode. Hence, Zn(II) can also be regarded as being in a pseudosquare pyramid environment.13 The BOABA ligand acts as hexa-connector to link six independent Zn(II) centers. Pairs of zinc ions are joined by the flexible carboxylic groups of bidentate fashion to result in one-dimensional (1D) chains (Zn · · · Zn separation is 4.523 Å). These two neighboring chains are further bridged via the hydroxyl oxygen atoms to generate double zigzag chains consisting of 12-membered circles. Then these double chains are interlinked by two oxyacetate groups of BOABA ligand in an antisyn fashion to form two--dimensional (2D) networks. The remaining rigid carboxylic groups adopt different coordination directions, as illustrated in Figure 2; that is, one points upward and the other points downward. The 2D networks coordinate to the opposite tropismatic carboxylic groups from neighboring BOABA ligands to complete the construction of the 3D framework. The water molecules are filled effectively in the interspace of the complex to provide additional hydrogen bonds to stabilize the crystal structure. Two Zn(II) joined by the hydroxyl oxygen atom act as a secondary building unit (SBU) and can be considered as a single 5-connected node. Each BOABA ligand can also be regarded as a five-connected vertex that links five SBUs. The linkage of five-connected vertices builds up a 3D network, as shown in Figure 2. Topology analysis indicates it is a new uninodal 5-connected net with a Schla¨fli symbol of 46.64. Both types of vertices reduced from the dinuclear Zn unit and the BOABA ligand show the same long Schla¨fli symbol of 4.4.4.4.4.4.62.62.84.84. Structures containing 5-connected metal centers are very rare, and only several examples of frameworks with 3D 5-connected networks have been reported to date14. The net reported here is based on the square pyramidal nodes,


He et al.

Figure 4. View of the three-dimensional architecture and its topological structure of 2.

Figure 7. Ball-and stick representation of 4. H atoms and C atoms of phen were omitted for clarity.

Figure 5. Ball-and-stick representation of 3. H atoms and lattice water molecules were omitted for clarity (symmetry codes, #1 x - 1, y - 1, z; #3 -x, -y, -z + 1; #4 x - 1, y - 1, z - 1; #8 x, y, z + 1; #9 -x, -y, -z).

Figure 6. A view along the c axis of the two-dimensional bilayer structure in 3.

which is obviously different from the above ones and other known 5-connected nets, such as BN and CaB6. Crystal Structures of 2. Single-crystal X-ray diffraction analysis reveals that 2 is a 3D network consisting of [Zn4(µ3OH)2]6+ clusters and two types of organic ligands, BOABA and 4,4′-bipy. The [Zn4(µ3-OH)2]6+ cluster center contains two crystallographically independent Zn(II) ions assuming to two different coordination geometries, as shown in Figure 3. Zn(1) is bounded by one N atom of a 4,4′-bipy ligand (Zn-N 2.128 Å), three O atoms of three individual BOABA ligands, and two O atoms of µ3-hydroxyl ligands (Zn-O 2.069 and 2.141 Å) in a distorted octahedral geometry. Zn(2) is four coordinated by two O atoms of two individual BOABA ligands (Zn-O 1.949 and 1.971 Å), one O atom of a µ3 hydroxyl ligand (Zn-O 1.940 Å), and one N atom of 4,4′-bipy ligand (Zn-N 2.024 Å) in a slightly distorted tetrahedron environment. As listed in Table

3, the average bond length of Zn(2) is shorter than that of Zn(1), which is consistent with the fact that the bond lengths in a tetrahedral geometry are generally shorter than those in other geometries. In the basic [Zn4(µ3-OH)2]6+ cluster, four Zn(II) ions lie on an absolute plane to form an approximate parallelogram (the adjacent Zn · · · Zn separation 3.12 × 3.52 Å) and other complexes containing such clusters almost have the same configuration.15 The bidentate carboxylic groups of the BOABA ligand bridge the clusters in an axis, and then the undentate flexible carboxylic groups further join the clusters along the c-axis to form an infinite 2D sheet. Furthermore, the additional 4,4′-bipy ligands employ a µ2-bidentate coordination mode to extend the 2D layers to be the final 3D architecture with large rectangular windows (11.97 × 13.82 Å). Besides, the solvent ethanol molecules and water molecules occupy the interspace to afford hydrogen bonds to stabilize its construction. From the view of topology, the framework of 2 can be represented as a new binodal (3,8)-connected net. The [Zn4(µ3OH)2]6+ cluster is surrounded by six BOABA ligands and four 4,4′-bipy ligands, but two 4,4′-bipy ligands doubly connect two [Zn4(µ3-OH)2]6+ clusters. So the [Zn4(µ3-OH)2]6+ cluster is only reduced into a 8-connected node and the BOABA ligand is reduced into a 3-connected node. The short Schla¨fli symbol for the 8-connected node is 46.617.85 and the short Schla¨fli symbol for the 3-connected node is 43. Thus, this new (3,8)-connected network can be topologically represented as a (43)2(46.617.85) net (Figure 4). Crystal Structures of 3. 3 is a unique 2D MOF containing a [Cd3(BOABA)2(H2O)6]n bilayer structure. In the asymmetric structure, there are two crystallographically different Cd(II) ions, with hexacoordinated distorted octahedral coordination fashion for Cd(1) and slightly distorted pentagonal bipyramid geometry for Cd(2) (Figure 5). Cd(1) is coordinated by four carboxylate



Figure 8. (a) Two dimensional layer structure of 4. H atoms, lattice water molecules, and phen ligands were omitted for clarity. (b) Threedimensional π-stacked pillared layer structure (phen ligands are shown as space filling plot).

Figure 9. Simulated and experimental XRPD spectra of complex 1 (B and A).

Figure 10. Simulated and experimental XRPD spectra of complex 2 (B and A).

oxygen atoms from two pairs of symmetrical independent BOABA ligands (Cd-O 2.255-2.308 Å) and two symmetrical

Figure 11. Photoluminescent spectra of 1-4 in solid state at room temperature.

aqua molecules (Cd-O 2.274 Å) in the axial site. Cd(2) is defined by five carboxylate oxygen atoms belonging to three different BOABA ligands (Cd-O 2.246-2.607 Å) and two aqua molecules (Cd-O 2.329-2.337 Å) occupying the vertexes. It is notable that three carboxylate groups of the BOABA ligand display three different functionalities (shown in Scheme 2C), one flexible carboxylate group bidentately bridging two different Cd(II) centers in syn-syn fashion, the other flexible one acting as chelate-bridge mode to link two unique Cd(II) ions, and the rigid one coordinating to the Cd(2) in chelate fashion. So the whole BOABA ligand acts as penta-connector to link five Cd(II) centers forming a novel 2D [Cd3(BOABA)2(H2O)6]n bilayer structure which contains trinuclear cadmium clusters with the Cd · · · Cd distance and Cd(2)-Cd(1)-Cd(2) angle being 4.07 Å and 180.00° according to the reported complexes.16 Taking the trinuclear cadmium clusters linked by flexible carboxylate group in “8” shape as a node, we can find that one flexible carboxylate group and the rigid carboxylate group join the neighboring nodes to one dimension double-chain structure and then the other flexible carboxylate group extend these doublechains to the 2D bilayer construction. It is interesting that the BOABA ligands involved in the bilayer structure adopt the same


He et al.

Figure 12. Energy band structure (1a and 2a) and density of states (1b and 2b) of 1 and 2.

coordination mode but apply the opposite tropismatic in uplayer and down-layer, and that benzene rings of the BOABA ligands are nearly parallel with the average separation of 3.40 Å as shown in Figure 6. Furthermore, there are hydrogen bonds between the aqua ligand and interstitial water molecule, aqua ligand and carboxylate oxygen atoms, interstitial water molecule and carboxylate oxygen atoms to increase the structural stability of the complex. It is well-known that π · · · π, C-H · · · π, and hydrogen bond interactions are necessary in the formation and stability of lowdimensional structures, especially those architectures containing a phen ligand and water molecules.2a In this complex, the adjacent 2D bilayers are further connected to each other via hydrogen bonds to complete the final 3D architecture. Crystal Structures of 4. 4 presents a 2D layer framework decorated by the phen ligands on the surface. As illustrated in Figure 7, the structure of 4 is constructed from [Cd4(µ3-OH)2]6+ clusters similar to that of 2, and also contains a crystallographic inversion center located at the center of the [Cd4(µ3-OH)2]6+ cluster, but its coordination environment around the Cd(II) is somewhat different from that of the Zn centers in 2. Cd(1) is in a distorted octahedron geometry with two nitrogen atoms from a chelate phen (Cd-N 2.324-2.401 Å), two oxygen atoms from one rigid and one flexible carboxyl groups of two individual BOABA ligands (Cd-O 2.333-2.357 Å) and two aqua ligands (Cd-O 2.241-2.297 Å). Cd(2) is six-coordinated by the chelate phen (Cd-N 2.357-2.409 Å), four oxygen atoms from one flexible chetating and one flexible bidentate carboxyl groups of two individual BOABA ligands (Cd-O 2.295-2.573 Å) and an OH oxygen atom (Cd-O 2.207 Å) in a distorted octahedron. As shown in Figure 7, three carboxylate groups of the ligand are all deprotonated and coordinate to four Cd atoms in three different fashions: unidentate, chelate and µ2-bidentate fashion link three clusters in three different ways to produce three different subrings, A, B and C, which are two 24-membered rings and one 20-membered ring and the lattice water molecules are located in ring A for its bigger diameter. It is worth noting

that two phen ligands belonging to one [Cd4(µ3-OH)2]6+ cluster on the same side of the 2D layer produce a cleft with a mean dimension of 4.193 Å, whereas two adjacent phen ligands from different clusters form a cleft of 11.385 Å. The moieties of neighboring layers are interimmersed into the bigger clefts of each other in an alternate fashion, which effectively fills in the cleft void (Figure 8). In the packing arrangement of 4, the adjacent 2D layers are parallel to each other with face-to-face π-π interactions between aromatic groups of phen ligands for the plane-to-plane distances of 3.372(3) and 3.623(4) Å and the centroid-centroid distance of 4.192(2) and 3.851(3) Å, respectively. The simulated and experimental XRPD patterns of the four complexes are shown in Figures 9 and 10 and Figures S1-S2, Supporting Information. Their peak positions are in good agreement with each other, indicating the phase purity of the products. The differences in intensity may be due to the preferred orientation of the powder samples. Thermogravimetric Analyses. To study the stability of these polymers, thermogravimetric analytical (TGA) studies were performed (Figures S3-S6, Supporting Information). The TGA curve of 1 displays two distinct weight losses. The first one from 75 to 280 °C corresponds to the loss of the lattice water molecule. The observed weight loss of 3.94% is in agreement with the calculated value of 4.15%. The dehydrated compound is stable up to 361 °C. The second one of 56.7% (calculated 58.2%) from 381 to 620 °C corresponds to the burning of the organic groups. The final residuals are ZnO. TGA curves for 2 exhibit two steps of weight losses. The weight loss before 150 °C corresponds to the release of the lattice water and solvent ethanol molecules. The observed weight loss of 5.91% is slightly less than the calculated value of 6.33%. The second step is from 260 to 550 °C, where the organic groups were burnt. The final residuals are ZnO. The observed total weight loss of 74.20% is slightly larger than the calculated one of 72.54%. The TGA diagram of 3 indicates two main weight losses. The first one before 200 °C corresponds to the loss of six lattice


water molecules and six aqua ligands. The observed weight loss of 20.3% is in agreement with the calculated one of 19.8%. The dehydrated compound is stable up to 320 °C. The second one of 33.67% (calculated 32.66%) from 295 to 550 °C corresponds to the burning of the organic groups. The final residuals are CdCO3. The TGA curve of 4 shows one main weight loss. This complex has no clear weight losses of water molecules (calculated 2.96%) and is stable up to 329 °C. The weight loss from 329 to 637 °C corresponds to the burning of the organic ligands. The final residuals are CdCO3. The observed total weight loss of 60.52% matches calculated one of 60.74%. Photoluminescent Properties. The solid-state luminescent properties of 1-4 were investigated at room temperature and their emission spectra are given in Figure 11. 3 displays strong fluorescent emission with a maximum at 357 nm upon photoexcitation at 266 nm, while 1 exhibits stronger purple and green fluorescent emission with a maximum at 367 and 513 nm, respectively. The fluorescent spectrum of 2 is not structureless with an emission maximum at 494 nm, which is mainly due to the contribution of the metal-hydroxy(or oxy) cluster.17a Similar to 2, 4 exhibits a blue photoluminescent emission peak at 465 nm. As reported in the literature,17b the enhancement of luminescence in d10 complexes may be attributed to the ligation of the ligand to the metal center. The coordination enhances the “rigidity” of the ligand and thus reduces the loss of energy through a radiationless pathway. And two transition types of the electronic excited state, namely, ligand-to-ligand change transfer (LLCT) and ligand-to-metal change transfer (LMCT), may be possible for such Cd(II) and Zn(II) coordination complexes.17 In terms of the calculated PDOS of four complexes (discussion below), the transition type of the electronic excitedstate may be identified as LLCT. Besides, it is apparent that 2 and 4 have a red shift compared with 1 and 3, which is due to the influences of the second ligands (discussion below). Since these condensed materials are highly thermally stable, colorless, and insoluble in common polar and nonpolar solvents, four complexes may be good candidates for potential photoactive materials. Energy Band Structure and Density of States. To further characterize the optical properties of four complexes, we calculated their energy band structures and density of states (DOS) using the DFT. The band structures of 1 and 2 are shown in Figure 12 (1a) and (2a), respectively. Both 1 and 2 are indirect band gap semiconductors. The optical energy gaps (Eg) are 3.21 eV (385 nm) and 1.89 eV (656 nm) and the corresponding k points of the top of valence band (TVB) and the bottom of conduction band (BCB) are B f E and Z f F for 1 and 2, respectively. By comparison of the total and partial DOS of 1 and 2 [shown in Figure 12 (1b) and (2b)], we attribute the decrease of Eg to the addition of second ligand (i.e., 4,4′-bipy). As shown in Figure 12 (2b), the states between 0.05 and 0.10 Ha are composed of the 2s and 2p electrons of N and C of 4,4′-bipy and lead the BCB of 2 to move down about 0.05 Ha relative to that of 1. At the same time, we can identify the photoluminescent emission peaks as the LLCT because the composition of the energy states around the conduction and valence band edges of 1 and 2 are mostly composed of the atomic orbitals of ligand. The optical properties of 3 and 4 are very similar to those of 1 and 2 and their band structures and DOS are given in Figure S7, Supporting Information. Both 3 and 4 are also indirect band gap semiconductors. The optical energy gaps (Eg) are 3.33 eV (372 nm) and 1.71 eV (725 nm) and the corresponding k points of TVB and BCB are Z f F, and Q f Z for 3 and 4, respectively. The total and partial DOS


of 3 and 4 indicate that the addition of second ligand (i.e., phen) leads to the decrease of Eg and the photoluminescent emission peaks can also be attributed to the LLCT. Conclusions A new multicarboxylate ligand, H3BOABA, was designed and four unique 2D and 3D MOFs were further obtained by hydrothermal reactions. 2 and 4 have the basic [M4(µ3OH)2]6+ clusters. The N-donor ligands play an important role in directing the final different structures. 1 is constructed from two opposite tropismatic BOABA ligands to be a 5-connected 46.64 uninodal net. 3 is a unique 2D MOF containing [Cd3(BOABA)2(H2O)6]n bilayer structure. These four MOFs display structure-related photoluminescence properties in the solid state. It is found that auxiliary ligands and the pH value play a very important role in controlling the coordination polymers. Four complexes are indirect band gap semiconductors and their fluorescent emission peaks could be attributed to the LLCT based on the calculated energy band structures and DOS. The addition of N-donor ligands will decrease the optical energy gaps, which guides one to design the materials with low energy gaps. Acknowledgment. The authors are grateful to the Natural Sciences Foundation of Zhejiang Province for financial support of the project (No. Y406355). Supporting Information Available: TG curves and X-ray crystallographic information files (CIF) are available for 1-4. XRPD spectra, Energy band structure and DOS of 3 and 4. This material is available free of charge via the Internet at http://pubs.acs.org.

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CG8000398

Syntheses, Structures, and Photoluminescence of Four d10

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