A 3-D Metal-Organic Framework Constructed with Manganese(ΙΙ), 4,4'-Oxybis(benzoic acid) and 2,2'-Biphenyl: Synthesis, Crystal Structure and Photoelectric Property Characterization

Assembly of 4,4'-oxybis(benzoic acid) (H2L) with manganese chloride in the presence of 2,2'-biphenyl (2,2'-bpy) affords a new coordination polymer [Mn3L3(2,2'-bpy)2]n (1), in which the [MnL2]n layers are extended by L bridges resulting in a three-dimensional (3-D) coordination framework. The network structure of 1 has unusual (2,6)-connectivity and represents a new type of (812·123)(8)(3) topology. These identical and complementary networks are entangled to generate a self-penetrating supramolecular lattice. Moreover, the fluorescence spectrum of 1 exhibits fluorescent emission in the solution of methanol at room temperature. Electrochemical investigation illustrates the electrochemical properties of the title compound. The structure (C62H40Mn3N4O15)n is monoclinic with a = 14.2304(18), b = 17.019(2), c = 25.805(3) Å, α = γ = 90, β = 92.932(2)° and space group C2/c.


Crystal Structure Descriptions
The self-assembly of MnCl 2 ·4H 2 O with 4,4'-oxybis(benzoic acid) and 2,2'-biphenyl in H 2 O-CH 3 OH mixed solution was performed under weak acid conditions. If NaOH was not added into the reaction system, only unknown ropy precipitate was obtained. If methanol was replaced by ethanol under the same conditions, we could not obtain single crystals. Single crystal X-ray analysis of 1 reveals that the asymmetric unit consists of one and a half Mn(II) ions, one 2,2'-biphenyl and three L ligands, The Mn 1 coordinate with two nitrogen atoms from one 2,2'-biphenyl (Mn 1 -N 2.261(4)-2.263(4) Å) and four oxygen atoms from three different bridging L ligands (Mn 1 -O 2.074(4)-2.403(4) Å), the Mn 2 coordinate with six oxygen atoms from six L ligands (Mn 2 -O 2.178(4)-2.245(4) Å), each L ligand connect with four Mn atoms. We can see the environment of Mn in Figure 1. There are many π-π stacking interactions in the crystal ( Figure 2). We can see the distance between two benzene rings from diffenent L ligands is 3.875(4) Å, and the dihedral angle is 5.4(4)°. The π-π stacking interactions also exist between one benzene ring and one pyridine ring (3.857(4) Å, 10.68(4)°).  [15]) are available to accommodate guest molecule, we can see the big cavity exist in title compound Figure 4. As a result, each core of SBU are connected by four L ligands to generate a coordination layer along the bc plane, as illustrated in Figure 5, in which each SBU center acts as a 4-connected node by linking to four 2-connected ligands, displaying a familiar net topology. The two SBU center binding around each ligand are spaced by the distances of 15.050(3), 15.456(3) Å, respectively. Besides, esch SBU center also linked by the other two L ligands. These two-dimensional (2-D) layers are further extended into a 3-D coordination framework via the other L ligands. Thus, a 3-D (2,6)-connected network with the Schläfli symbol of (8 12 ·12 3 )(8)(3) is generated, as depicted in Figure 6. The structural consistency and phase purity of 1 was confirmed by comparing the measured pattern calculated from single-crystal data with the experimental X-ray powder diffraction (XRPD) analysis at room temperature ( Figure 7).

Fluorescence Emission Properties
Compound 1 exhibits fluorescence emission around 328 nm upon excitation at 312 nm in the solution of methanol at room temperature (Figure 8), the emission peak of complex 1 is neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal transfer (LMCT), in nature, since the Mn(II) ions with 3d5 configuration, are difficult to oxidize or reduce, therefore, it may be assigned to the π-π* intraligand fluorescence.

Electrochemical Properties
The electrochemical behavior of complex 1 in methanol and water solution has been investigated by cyclic voltammetry in the potential range from −0.2 to 0.8 V. The resulting cyclic voltammogram (CV) is shown in Figure 9. Complex 1 displays a couple of quasi-reversible oxidation and reduction waves with the reduction potential ranging from −0.1 to 0.2 V and the oxidation potential ranging from 0.2 to 0.5 V. The peaks of the reduction and the oxidation were assigned to the Mn IV /Mn II couple which corresponds to one electro-transfer process.

Materials and Instrumentation
All reagents and solvents were used directly as supplied commercially without further purification. Elemental analysis for C, H, and N was carried out on a Perkin-Elmer 2400 II elemental analyzer (Waltham, MA, USA). Excitation and emission spectra were acquired on a Perkin-Elmer instruments LS55 spectrofluorometer (Waltham, MA, USA). The X-ray powder diffraction (XRPD) was recorded on a XD-3 diffractometer (Beijing, China) at 36 kV, 25 mA for a Cu-target tube, and a graphite monochromator. Simulation of the XRPD spectra was carried out by the single-crystal data and diffraction-crystal module of the Mercury (Hg) program available free of charge via the Internet at http://www.iucr.org. Cyclic voltammetry were performed on a CHI760D electrochemical workstation. The three-electrode system consisted of a platinum wire counter electrode, a silver/silver chloride electrode, and a glassy carbon electrode (3.0 mm diameter) as working electrode. All electrochemical experiments were carried out in a conventional electrochemical cell holding methanol and water solution at room temperature.

Synthesis of (3-D) Coordination Framework 1
H 2 L (129.1 mg, 0.5 mmol) and 2,2'-biphenyl (77 mg, 0.5 mmol) were dissolved in the mixture of methanol (5 mL) and H 2 O (15 mL). Then an aqueous solution of sodium hydroxide was added dropwise with stirring to adjust the pH value of the solution being 5. Finally, aqueous solution of MnCl 2 ·4H 2 O (10 mL, 97 mg, 0.5 mmol) was added. The mixture was kept heated at 130 °C for three days. After cooling to room temperature, the reaction solution was filtered to remove a small quantity of white precipitation. Kept slow evaporation of the filtrate at room temperature and three days later X-ray quality yellow block-shaped single crystals were obtained. The crystals were isolated, washed with carbinol and dried at room temperature (Yield: 65% based on Mn

X-ray Structure Determination
Crystallographic data of the complex was collected on a Bruker SMART CCD diffractometer with graphite monochromated Mo-Ka radiation (λ = 0.71073 Å) at T = 296 K. Absorption corrections were applied by using the multi-scan program [16]. The structure was solved by direct methods and successive Fourier difference syntheses (SHELXS-97), and anisotropic thermal parameters for all nonhydrogen atomswere refined by full-matrix least-squares procedure against F 2 (SHELXL-97) [17]. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were set in calculated positions and refined by a riding mode, with C-H = 0.93 Å and Uiso(H) = 1.2 Ueq(C) for aromatic H atoms. The potential space of the complex was created by the function of the PLATON. First, we use PLATON to open the cif file. Second, we use the SOLV function, the detail is click the SOLV button, which is just like a water tap, later, we hit RETURN of our keyboard to continue, then we can get the potential space. The crystallographic data and experimental details for the structure analysis are summarized in Table 1, and the selected bond lengths and angles are listed in Table 2 1.07 R indices (all data) R1 = 0.0288, wR2 = 0.0732 Table 2. The selected bond lenths (Å) and angles (°) for complex 1.