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• 0   Introduction

  Metal-organic coordination polymers have received intense interests, especially in the fast developed field on metal-organic frameworks (MOFs) with well defined pore architectures, due to their tunable structural features and high surface areas^[1-3]. The research for these materials with functional properties had led to intense studies because they possess advantageous cumulative attributes emerging from both metal and ligand components^[4-5]. The structural and functional diversity of these coordination polymers has brought these materials wide attention in recent years. They had the tenability, versatility and original flexibility, of which place themselves at the frontier between zeolites and enzymes^[6-8].

  Compounds containing azo groups are well known for their excellent photoisomerizable properties^[9-12]. The MOFs constructed with azo-based ligand also have shown interesting application on such as the photoswitchable CO₂ adsorption capacity^[13-18]. Among the azo-based ligands, azo-4, 4′-bipyridine, in which two pyridine rings linked by the azo moiety, has been used to construct several MOFs which exhibited a wide variety of architectures and attractive electric/magnetic properties due to its controllability on the coordination configuration of the metal centers^[19-20]. Herein, we extend the construction strategy of azopyridine by linking two 4, 4′-bipyridine groups by an azo linker. The syntheses of two silver and cobalt complexes were reported respectively, and the application of Co complex as a catalyst on the light driven hydrogen evolution is shown.

  1   Experimental

  1.1   Reagents and measurements

  All chemicals were of reagent grade quality obtained from commercial sources and used without further purification. The elemental analyses of C, H and N were performed on a Vario EL Ⅲ elemental analyzer. ¹H NMR spectra were measured on a Varian INOVA 400M spectrometer. Solution fluorescent spectra were measured on JASCO FP-6500. Both excitation and emission slit widths were 2 nm.

  Electrochemical measurements were carried under nitrogen at room temperature, performed on ZAHNER ENNIUM Electro-chemical Workstation with a conventional three-electrode system with a homemade Ag/AgCl electrode as a reference electrode, a platinum silk with 0.5 mm diameter as a counter electrode, and glassy carbon electrode as a working electrode. The solution concentrations in cyclic voltammograms were ca. 1.0 mmol·L^-1 for the cobalt-based compounds and 0.1 mol·L^-1 for the supporting electrolyte, (n-Bu₄N)PF₆. The addition of NEt₃HCl (0.1 mmol·L^-1 in DMF) was carried out with syringe.

  Photoinduced hydrogen evolution was performed in a 40 mL flask. The flask was sealed with a septum, pre-degassed by bubbling nitrogen for 15 min under atmospheric pressure at room temperature. In the experiment, the catalyst was solved in little amount of DMF, and the EtOH/H₂O solution (1:1, V/V) containing fluorescein(FL) and triethylamine were added with the total volume of 5.0 mL. The pH value of the solution was adjusted to a suitable pH value by adding HCl or NaOH and was measured by the pH value meter. Then the samples were irradiated by a 500 W Xenon lamp, and the reaction temperature was maintained at 293 K by using a water filter to absorb heat. The generated photoproduct of H₂ was characterized by GC 7890T instrument analysis using a 5A molecular sieve column (0.6 m×3 mm), thermal conductivity detector, and nitrogen used as carrier gas. The amount of H₂ generated was determined by the external standard method. Hydrogen in the resulting solution was not measured and the slight effect of the hydrogen gas generated on the pressure of the flask was neglected for calculation of the volume of hydrogen gas.

  1.2   Syntheses and characterizations

  1.3   Crystallography

  The intensities of the complexes were collected on a Bruker SMART APEX CCD diffractometer with graphite-monochromated Mo Kα (λ=0.071 073 nm) using the SMART and SAINT programs^[22]. The structure was solved by direct methods and refined on F² by full-matrix least-squares methods with SHELXTL program^[23]. In all of the structural refinements, the non-hydrogen atoms were refined anisotropically. Hydrogen atoms were fixed geometrically at calculated distances and allowed to ride on the parent non-hydrogen atoms. For the refinement of 2, one of the perchlorate ions was restrained with idealized distance on the Cl-O bond. Crystal data of the complexes were listed in Table 1.

  Table 1.  Crystal data of the complexes

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  	trans-L acetate	cis-L	1	2
  Formula	C28H30N6O8	C20H14N6	C28H26Ag2Cl2N10O8	C40H32Cl2CoN12O10
  Formula weight	578.58	338.37	917.23	970.61
  T/K	298(2)	298(2)	298(2)	298(2)
  Crystal system	Monoclinic	Monoclinic	Monoclinic	Orthorhombic
  Space group	P21/c	C2/c	C2/c	Fddd
  a/nm	0.876 16(2)	1.852 3(2)	1.986(4)	0.827 4(2)
  b/nm	2.227 95(5)	0.616 1(1)	1.176(3)	2.056 3(5)
  c/nm	0.779 60(2)	1.454 9(1)	1.630(4)	5.436 7(13)
  β/(°)	100.455(2)	107.16(1)	100.97(3)
  V/nm3	1.496 01(6)	1.586 3(3)	3.737(15)	9.250(4)
  Z	2	4	4	8
  Dc/(g·cm-3)	1.284	1.417	1.630	1.394
  u/mm-1	0.096	0.090	1.249	0.554
  F(000)	608	704	1824	3976
  Reflections	8 513	3768	8 493	12 000
  Unique reflections	2 637	1 400	3273	2 037
  Reflections[I > 2σ(I)]	1 531	1 275	2 245	951
  Rint	0.040 8	0.015 6	0.042 8	0.079 0
  R1[I > 2σ(I)]	0.053 1	0.037 1	0.054 0	0.093 8
  wR2(all data)	0.155 1	0.145 5	0.169 7	0.227 0
  Goodness of fit	1.025	1.073	1.056	1.094

  1.2.1   Synthesis of trans-1, 2-di((4, 4′-bipyridine)-3-yl)diazene acetate (trans-L acetate)

  3-amino-4, 4′-bipyridine (0.8 g, 5 mmol)^[21] was added to 40 mL water at 0 ℃, then NaOCl (80 mL) was added, and the reaction mixture was stirred for 24 h (Scheme 1). After filtering the reaction mixture, the filtrate was then reduced in volume in vacuo to afford the orange solid. The solid then recrystallization in glacial acetic acid to give the trans-L acetate. Yield: 0.65 g (46%). Anal. Calcd. for C₂₈H₃₀N₆O₈(%):C 58.13, H 5.23, N 14.53; Found(%): C 58.33, H 5.19, N 14.32. ¹H NMR(400 MHz, CDCl₃): δ 2.23 (s, 6H) 7.45 (d, 2H), 7.51 (d, 1H), 7.71(s, 1H), 7.76(d, 2H), 8.81 (d, 1H).

  Figure Scheme 1.  Synthesis of trans-and cis-1, 2-di((4, 4′-bipyridine)-3-yl)diazene

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  1.2.4   Synthesis of {[Co(trans-L)₂(H₂O)₂](ClO₄)₂}_n (2)

  Complex 2 was obtained from the mixture of trans-L acetate (86 mg, 0.15 mmol) in dichloromethane and Co(ClO₄)₂·6H₂O(27 mg, 0.075 mmol) in methanol after two days as orange red crystals. Yield: 29 mg (40%). Anal. Calcd. for C₄₀H₃₂Cl₂CoN₁₂O₁₀(%): C 49.50, H 3.32, N 17.32; Found(%): C 49.92, H 3.52, N 17.80.

  1.2.3   Synthesis of {[Ag₂(trans-L)(ClO₄)₂]·4CH₃CN}_n (1)

  Complex 1 was obtained from the mixture of trans-L acetate (43 mg, 0.075 mmol) in dichloromethane and AgClO₄ (31 mg, 0.15 mmol) in acetonitrile as orange crystals. Yield: 41 mg (60%). Anal. Calcd. for C₂₈H₂₆Ag₂Cl₂N₁₀O₈(%): C 36.67, H 2.86, N 15.27; Found(%): C 36.22, H 2.76, N 14.84.

  1.2.2   Synthesis of cis-1, 2-di((4, 4′-bipyridine)-3-yl) diazene (cis-L)

  When the aforementioned orange solid recrystallized in CH₂Cl₂, the pure cis-L was obtained. Yield: 0.34 g (40%). Anal. Calcd. for C₂₀H₁₄N₆(%): C 70.99, H 4.17, N 24.84; Found(%): C 58.33, H 5.19, N 14.32. ¹H NMR(400 MHz, CDCl₃): δ 7.35 (d, 1H), 7.38 (d, 2H), 7.60(s, 1H), 8.57(d, 2H), 8.71 (d, 1H).

  2   Results and discussion

  Ligand 1, 2-di((4, 4′-bipyridin)-3-yl)diazene was synthesized via the condensation of 3-amino-4, 4′-bipyridine, and both trans-and cis-forms of the compound were present in the product. The cis-form was separated directly, and the trans-form was recrystallized from ice acetic acid as an acetate compound. The cis-configuration ligand crystallized in a space group of C2/c, with the two bipyridine groups sitting in the same side of N=N bond. While the trans-form ligand crystallized as an acetate salt in a space group of P2₁/c, with the two bipyridine groups sitting in the opposite side of N=N bond. The N=N distances of the azo group in both form are same(0.125 6(3) nm).

  Both the trans-and cis-form ligand reacted with metal ion Ag and Co, however, only the trans-form gave the crystalline compound suitable for X-ray crystal analysis, perhaps due to the lower stretching force for the trans-configuration which is more suitable to form stable complex when coordinated with the metal ions. Complex 1 crystallized in C2/c space group. Single crystal analysis showed that there are one Ag ion, one half ligand, one perchlorate counter ion and two acetonitrile solvent molecules present in an asymmetric unit. It could be found that all the four N atom of the pyridine rings coordinated to the Ag ions, and each Ag ion was coordinated with two pyridine N atoms from two different ligands in a linear way with the Ag-N bond distance being 0.224 nm on average. The structure could be described as two infinite chain of alternating Ag(Ⅰ) separated by 1.176 nm linked by the azo group to form an infinite ladder chain. The closest introchain Ag…Ag distance is 0.672 nm. The pairs of ladder chains stack together by π…π interactions between the ligands and in which the ligands lie parallel to each other with a distance of 0.346 nm, and the weak interchain Ag…Ag interaction with a Ag…Ag distance of 0.365 nm between the different ladder chains, forming a stair up two-dimensional sheet.

  Complex 2 crystallized in Fddd space group. Single crystal analysis showed that the complex exists as a three-dimensional polymer in which each Co(Ⅱ) is coordinated in an octahedral geometry to four ligands and two water molecules occupied on the axial places. In this complex, there are only two opposite N atoms on the different pyridine rings (N1 and N1A as shown in Fig. 1) of the ligand participate in the coordination to metal centers, thus the ligand act as a μ₂-bridge ligand, the four ligand coordinated in one Co center extend in a stair-up way, and the whole 3D structure compose of several infinite chains. It could not be found a cycle rings in the structure, thus the structure of 2 could be described as a dendrimer. Nevertheless, viewed from a axle, the structure seems to have a one dimension channel (Fig. 3).

  Figure 1.  Structure of cis-form(left) and trans-form (right) of liagnd 1, 2-di((4, 4′-bipyridin)-3-yl)diazene

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  Figure 2.  (a) Coordination environment of Ag center in 1 with 30% probability displacement ellipsoids; Top(b) and side(c) view of complex 1

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  Figure 3.  (a) Coordination environment of Co center in 2; View of compound 2 from a (a), b(b) and c(c) axle

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  Solar energy conversion of water into the environmentally clean fuel hydrogen offers one of the best long-term solutions for meeting future energy demands. Current solar fuel research involves developing these molecular based systems containing a photosensitizer for light absorption, a catalyst for H₂ generation, an electron source for proton reduction and a means of electron transfer to the catalyst. The coordination atoms around the Co ions in this structure were quite similar with that of the normal used [Co(dmgBF₂)₂(OH₂)₂] photocatalyst^[24], and 2 is soluble in DMF solution, providing the possibility of using it in the homogeneous light driven hydrogen production system.

  Cyclic voltammogram of 2 in a DMF solution exhibited an irreversible Co²⁺/Co⁺ redox process at -0.67 V (vs Ag/AgCl). Photolysis of a solution of FL(photosensitizer, 3.0 mmol·L^-1) and 2 (20 μmol·L^-1) in a solvent mixture containing triethylamine (NEt₃, electron donor, 10%, V/V) in H₂O/EtOH(1:1, V/V) at 25 ℃ results the direct H₂ generation. The volume of H₂ was quantified at the end of the photolysis by GC analysis of the headspace gases^[25]. The amount of H₂ generation in 12 hours photolysis maximizes at pH 10.5(Fig. 4a). The increase of pH values really decreased the efficiency, due to the lower proton concentration in solution and the fact that the H₂ generation became more thermodynamically unfavourable with increasing pH value. Whereas at lower pH value, the potential protonation of NEt₃ diminishes its ability to function as an electron donor, and the NEt₃ decomposition becomes less facile^[26]. The light induced H₂ evolution depends on the concentration of sacrificial reagent NEt₃, the optimal concentration is 10% with a decrease in activity at both lower and higher concentration. The turnover number (TON) calculated was about 340 moles H₂ per mole of catalyst. Control experiment of this system without either the complex 2, the FL or the NEt₃ were carried out, the absence of any of them yielded unobservable amount of H₂, demonstrating that they are essential for H₂ generation. Moreover, these artificial photosynthetic systems could not work well in the absence of the light. With the fixed concentration of FL (2 mmol·L^-1), the TON reach the platform value when the concentration of 2 varied to 20 μmol·L^-1, further addition of 2 causes a very little hydrogen evolution enhancement corresponding to the catalyst(Fig. 4b). These results suggest FL were decomposed during the photolysis, similar to those related systems containing photosensitizer FL and NEt₃as sacrificial electron donor^[26].

  Figure 4.  (a) Hydrogen production of the systems containing 2 (20 μmol·L^-1), FL (3.0 mmol·L^-1) and NEt₃(10%, V/V) in EtOH/H₂O (1:1, V/V) with the pH value varied; (b)Hydrogen production of the systems containing FL(3.0 mmol·L^-1), NEt₃ (10%, V/V) in EtOH/H₂O(1:1, V/V) at pH=11.0 in 5 mL solution with different concentrations of the redox catalyst 2

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  3   Conclusions

  In a summary, we prepared two metal organic coordination polymer assembled by Ag ion or Co ion with the azo based ligand trans-L. The complex 1 forms a one dimension infinite ladder chain, while the structure of complex 2 is a 3D infinite chain model. The application of the Co complex(2) as a catalyst is showed on the light driven hydrogen evolution.

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• 

  Scheme 1  Synthesis of trans-and cis-1, 2-di((4, 4′-bipyridine)-3-yl)diazene

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  Figure 1  Structure of cis-form(left) and trans-form (right) of liagnd 1, 2-di((4, 4′-bipyridin)-3-yl)diazene

  30% probability displacement ellipsoids; Selected bond distances(nm): N(3)-N(3A) 0.125 5(2) in cis-L, 0.125 6(3) in trans-L; Symmetry codes: A:-x+1, y, 0.5-z (cis); A:-x-2, 1.5+y, 2.5-z (trans).

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  Figure 2  (a) Coordination environment of Ag center in 1 with 30% probability displacement ellipsoids; Top(b) and side(c) view of complex 1

  Selected bond distances(nm) and angle(°): Ag(1)-N(1) 0.224 2(7), Ag(1)-N(2) 0.225 5(7); N(2)-Ag(1)-N(1) 178.4(2)

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  Figure 3  (a) Coordination environment of Co center in 2; View of compound 2 from a (a), b(b) and c(c) axle

  Selected bond distances(nm): Co(1)-O(1W) 0.211 3(5), Co(1)-N(1) 0.216 7(3); Symmetry codes: A: 0.25-x, 0.25-y, z; B: 0.25-x, y, 1.25-z; C: x, 0.25-y, 1.25-z

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  Figure 4  (a) Hydrogen production of the systems containing 2 (20 μmol·L^-1), FL (3.0 mmol·L^-1) and NEt₃(10%, V/V) in EtOH/H₂O (1:1, V/V) with the pH value varied; (b)Hydrogen production of the systems containing FL(3.0 mmol·L^-1), NEt₃ (10%, V/V) in EtOH/H₂O(1:1, V/V) at pH=11.0 in 5 mL solution with different concentrations of the redox catalyst 2

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  Table 1.  Crystal data of the complexes

  	trans-L acetate	cis-L	1	2
  Formula	C28H30N6O8	C20H14N6	C28H26Ag2Cl2N10O8	C40H32Cl2CoN12O10
  Formula weight	578.58	338.37	917.23	970.61
  T/K	298(2)	298(2)	298(2)	298(2)
  Crystal system	Monoclinic	Monoclinic	Monoclinic	Orthorhombic
  Space group	P21/c	C2/c	C2/c	Fddd
  a/nm	0.876 16(2)	1.852 3(2)	1.986(4)	0.827 4(2)
  b/nm	2.227 95(5)	0.616 1(1)	1.176(3)	2.056 3(5)
  c/nm	0.779 60(2)	1.454 9(1)	1.630(4)	5.436 7(13)
  β/(°)	100.455(2)	107.16(1)	100.97(3)
  V/nm3	1.496 01(6)	1.586 3(3)	3.737(15)	9.250(4)
  Z	2	4	4	8
  Dc/(g·cm-3)	1.284	1.417	1.630	1.394
  u/mm-1	0.096	0.090	1.249	0.554
  F(000)	608	704	1824	3976
  Reflections	8 513	3768	8 493	12 000
  Unique reflections	2 637	1 400	3273	2 037
  Reflections[I > 2σ(I)]	1 531	1 275	2 245	951
  Rint	0.040 8	0.015 6	0.042 8	0.079 0
  R1[I > 2σ(I)]	0.053 1	0.037 1	0.054 0	0.093 8
  wR2(all data)	0.155 1	0.145 5	0.169 7	0.227 0
  Goodness of fit	1.025	1.073	1.056	1.094

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