1. Introduction

People still paly a great deal of attention on dinuclear copper complexes with two metal ions in close proximity. These complexes are particularly of interest in relation to their potential uses as catalysts, biological processes and inorganic materials [1-3]. Generally, efforts are made to magnetic interactions between copper ions, and such studies have served for understanding the spin-exchange mechanism in antiferro-or ferromagnetic coupling [4]. However, their potentials as new materials in liquid have been ignored. Inspired by that hydrogenase enzymes (nickel or iron complexes) can efficiently catalyze both the production and the oxidation of dihydrogen [5], and several molecular catalysts for the production of hydrogen from acid or water based on transition metal complexes have been developed [6-9], we also designed some molecular electrocatalysts by the assemblies of complexes [10-12]. It is widely regarded that transition metals capable of forming metal-hydride intermediates are essential to the HER mechanism. Based on this consideration, much effort has been made to design coordinatively unsaturated complexes supported by tetra-and pentadentate ligands for proton or water reduction [13-16]. As an expanded work, here we present the synthesis, structure, characterization and magnetic properties of one new copper(Ⅱ) complex, [(HL)CuCl-CuCl(HL)]HCl 1, as well as its electro-catalytic properties for proton and water reduction to hydrogen.

2. Results and discussion 2.1. General characterization

Based on the literature procedure, 2-(pyridylmethyl)amino-N, N-bis(2-methylene-4, 6-difluorophenol) (H₂L) was prepared [17]. In the presence of triethylamine, the reaction of CuCl₂·2H₂O and H₂L at one molar ratio set of 1:1 provides a novel Cu(Ⅱ) complex 1, [(HL)CuCl-CuCl(HL)]HCl (Scheme 1). As shown in Fig. 1, in solid complex 1 consists of two copper units. Each of the metal centers is coordinated by two nitrogen atoms and one oxygen atom from one deprotonated ligand (HL^-). The observed Cu-N bond distances fall in the range 1.991(3)-2.078(4) Å and the bond distances of Cu—O fall in the range 1.946(3)-1.980(3) Å. Noteworthy is that the Cu—Cl bond distances are asymmetric at 2.2754(11) and 2.2844(12) Å, respectively. To test the state of complex 1 in liquid, we also did ESI-MS measurement in methanol. From Fig. S1 (Supporting information), Cl^- ligand dissociates from copper center in methanol, which is in agreement with the result from ESI-MS measurement which exhibits one ion at a mass-to-charge ratio (m/z) of 454.0386, with the mass and isotope distribution pattern corresponding to that of [HL-Cu]⁺ (calcd. m/z of 454.824).

2.2. Magnetic properties of complex 1

To explore its properties as one magnetic material in solid, the magnetic behavior of complex 1 was investigated in the temperature range 2-300 K. As shown in Fig. 2, the decrease of χ_MT from 1.122 cm³·K·mol^-1 at 300 K to 0.0126 cm³·K·mol^-1 at 2 K reveals a paramagnetic system with an antiferromagnetic interaction for 1. It is worthy to note that the χ_M values exhibit a complicated change when the temperature is lowered. The value increases to a maximum around 140 K, then decreases to a minimum around 28 K, finally increases abruptly cooling to 2 K (Fig. 2). The magnetic analysis was carried out using the a modified Bleaney-Bowers equation S₁ = S₂ = 1/2 spin coupled dimer model (H = -2JS₁S₂) [Eq. (1) [4, 18]

(1)

In which N, g, β, k and T have their usual meanings; J is magnetic exchange coupling; ρ is the molar fraction of noncoupled species; Nα shows the contributions of the magnetic impurity and the sum of diamagnetic and van Vleck paramagnetic parts, respectively. The least-squares fits to the magnetic susceptibility data (the red solid line of Fig. 2) to give the parameters: J =-160 cm^-1, ρ = 7.96 × 10^-2, Nα = 1.1 ×10^-4 cm³ mol^-1 and R = 5.4 ×10^-6 with g of 2.0 fixed. The J value indicates the moderately strong antiferromagnetic exchange coupling within the copper(Ⅱ) units. The magnetic behavior of complex 1 is well correlated with the previous work [19].

From ESI-MS analysis (Fig. S1 in Supporting information), the title complex provides the species of [HL-Cu]⁺ in methanol. To study its function as an electrocatalyst in liquid, the electrochemical behavior of 1 was investigated in a DMF solution with [(n-Bu)₄N]ClO₄ as the supporting electrolyte. As shown in Fig. 3a, the CV of complex 1 shows a quasi-reversible redox peak at -0.86 V versus Ag/AgNO₃, which is assigned to the couple of Cu^Ⅱ/Cu^Ⅰ.

To test possible electrocatalytic activity of this copper complex, CVs of complex 1 were measured in the presence of acetic acid (proton source). From Fig. S₂ (Supporting information) it can be seen that the catalytic current near the couple of Cu^Ⅱ/Ⅰ (-0.86 V versus Ag/AgNO₃) increased with increasing acetic acid concentration from 0.00 mmol/L to 1.40 mmol/L. This indicates that hydrogen evolution electrocatalyzed by complex 1 requires the reduction of Cu(Ⅱ) to Cu(Ⅰ), Cu(Ⅰ) to Cu(0) and protonation. Interestingly, upon addition of acetic acid, a new quasi-reversible redox peak and an irreversible reduction wave were observed near -1.55 (Cu^Ⅰ/Cu⁰) and -0.78 V versus Ag/AgNO₃, respectively. Further addition of acid from 0.0 to 1.4 mmol/L results in a positive shift from -0.65 V to -0.42 V versus Ag/AgNO₃, and an increase in the peak current, consistent with a catalytic process [20]. Based on the above observations, the couples of Cu^Ⅱ/Cu^Ⅰ and Cu^Ⅰ/Cu⁰ are devoted to proton reduction, which were observed in the reported copper complexes [10, 15].

On the basis of above analyses and literature precedents [15, 21], we also tried to propose the catalytic cycle depicted in Scheme 2 for the generation of hydrogen from acetic acid mediated by 1. Oneelectron reduction of [(HL)Cu^Ⅱ]⁺ provides a putative [(HL)Cu^Ⅰ] species. Addition of hydrogen proton to [(HL)Cu^Ⅰ]^- gives the Cu^Ⅲ-H species. Then one-electron reduction of the Cu^Ⅲ-H species affords H₂, and further regenerates the starting complex 1. Although the relative contributions are indistinguishable in this analysis, we suspect that these processes are complementary H₂ evolution pathways.

To further test the electro-catalytic activity of complex 1, bulk electrolysis of a DMF solution containing 3.18 μmol/L complex 1 was conducted in the presence of 1.4 mmol/L acetic acid at variable applied potential using a glassy carbon plate electrode in a doublecompartment cell. Fig. S3a (Supporting information) shows the total charge of bulk electrolysis of complex 1 in the presence of acid, when the applied potential was -1.45 V versus Ag/AgNO₃, the maximum charge reached 32 mC during 2 min of electrolysis, accompanying evolution of a gas, which was confirmed as H₂ by gas chromatography (GC). A controlled-potential electrolysis (CPE) experiment under the same potential without complex 1 only gave a charge of 11 mC (Fig. S3b, Supporting information), showing that this complex does serve an effective hydrogen production under such conditions. According to Fig. S4 (Supporting information), ~0.021 mL of H₂ was produced over an electrolysis period of 2 h. By using Eqs. (2) [22] and (3) [23], TOF was calculated to be 16.3 moles of hydrogen per mole of catalyst per hour at an OP of 941.6 mV (Eq. S1 and Fig. S5, Supporting information).

(2)

(3)

Where, ΔC is the charge from the catalyst solution during CPE minus the charge from solution without catalyst during CPE; F is Faraday's constant, n1 is the number of moles of electrons required to generate one mole of H₂, n2 is the number of moles of catalyst in solution, and t is the duration of electrolysis.

To study the electrochemical behavior of complex 1 in aqueous medium, CVs were measured in buffered aqueous solutions where pH = 3.0-7.0, which is the range associated with catalytic water reduction. As observed in Fig. 3b, in a pH 7.0 solution, an irreversible Cu^Ⅱ/Cu^Ⅰ wave was observed at -1.48 V versus Ag/AgCl. And the strength of peak current increased markedly with decreasing pH values, which reflects the shifts in the thermodynamic potential for electrocatalysis [24]. As shown in Fig. S6 (Supporting information), in the absence of complex 1, a catalytic current was not apparent until a potential of -1.60 V versus Ag/ AgCl was attained. With addition of 0.354 mmol/L complex 1, the onset of catalytic current was observed at about -0.96 V versus Ag/ AgCl, and the current strength increased significantly with increasing concentrations of complex 1 from 0.00 mmol/L to 0.354 mmol/L.

To further get the evidence for the electro-catalytic activity in aqueous media by complex 1, the bulk electrolysis of a 3.18 mmol/L complex 1 was conducted in a 0.25 mol/L buffer solution under variable applied potentials. When the applied potential was -1.45 V versus Ag/AgCl, the maximum charge was only 86 mC during 2 min of electrolysis in absence of complex 1 (Fig. 4a). Under the same conditions, the charge reached 1809 mC with addition of complex 1 (Fig. 4b), accompanying a large amount of gas bubble appeared, which was confirmed to be H₂ by GC analysis. The evolved H₂ was analyzed by gas chromatography, Fig. S7a (Supporting information), which gave 4.56 mL of H₂ over an electrolysis period of 1 h with a Faradaic efficiency of 99% for H₂ (Fig. S7b in Supporting information). According to Equations (2) and (4) [6, 24], we also calculated the TOF for the catalyst as reaching a maximum of 1415.6 moles of hydrogen per mole of catalyst per hour at an overpotential (OP) of 787.6 mV (pH 7.0) (Eq. S₂ and Fig. S8 in Supporting information), where

(4)

Based on the TOF values, under the similar conditions, the electro-catalytic activity of complex 1 is higher than several copper complexes, including a copper complex supported by a tetradentate amine phenol ligand with a TOF of 104.3 moles of H₂ per mole of catalyst per hour at an OP of 839 mV [15], a copper complex with dicyano acetic acid methyl ester ligand that shows 285 mol of hydrogen per mole of catalyst per hour at an OP of 787.6 mV [25], and a dinuclear copper complex with 6-(3-aminomethylpropanol)-2-tert-buty-4-methylphenol ligand that shows a TOF of 650 mol of hydrogen per mole of catalyst per hour at an OP of 836.7 mV [21].

3. Conclusion

The reaction of 2-(pyridylmethyl)amino-N, N-bis(2-methylene-4, 6-difluorophenol) (H₂L) and Cl₂·2H₂O affords the copper(Ⅱ) complex, [(HL)CuCl-CuCl(HL)]HCl, which has been characterized by physics-chemical and spectroscopic methods. In solid state, this complex exhibits a magnetic exchange interaction between copper (Ⅱ) ions. In liquid, it acts as an electrocatalyst for hydrogen generation both from acetic acid and aqueous buffer. This discovery affords a new method for designing material that shows different performance in different states.

4. Experimental

Synthesis of the complex [(HL)CuCl-CuCl(HL)]HCl 1: To a solution of H₂L (0.393 g, 1 mmol) and triethylamine (0.10 g, 1 mmol) in acetonitrile (20 mL), Cl₂·2H₂O (0.171 g, 1 mmol) in methanol was added and the mixture was stirred for 30 min. Single crystals were obtained from the filtrate which was allowed to stand at room temperature for several days, collected by filtration, and dried in vacuo (0.681 g, 67%). The elemental analysis results (Found C, 47.87; H, 3.08; N, 5.56. C₄₀H₃₁Cl₃Cu₂F₈N₄O₄ requires C, 47.24; H, 3.07; N, 5.51) were in agreement with the formula of the sample used for X-ray analysis.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (Nos. 20971045, 21271073).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2017.03.027.