Function of triazenido compound for electrocatalytic hydrogen production catalyzed by platinum complex

Abstract A new type of electrocatalyst based on a triazenido-platinum complex, Pt(PPh3)2(L)Cl (1), is prepared by the reaction of 1-[(2-methoxy) benzene]-3-[2-pyridine] triazene (HL) and Pt(PPh3)2Cl2 in the presence of triethylamine. Electrochemical studies indicate that HL, Pt(PPh3)2Cl2 and 1 can catalyze hydrogen evolution from acetic acid or a neutral buffer. To show triazenido ligand, HL, plays a role in determining the catalytic activities of the platinum complex, we systematically study the electrocatalytic activities of HL, Pt(PPh3)2Cl2 and Pt(PPh3)2(L)Cl and provide a possible catalytic mechanism for hydrogen generation catalyzed by 1.


Introduction
The growing awareness of issues related to the increase in global energy demand and environmental problems has made the search for viable carbon-neutral sources of renewable energy one of the most important challenges in science today [1]. As an alternative clean energy source, hydrogen has attracted attention [2,3]. in nature, hydrogenases based on fe-Ni and fe-fe complexes can generate dihydrogen from aqueous media [4]. Structural studies suggest that a basic nitrogen or oxygen may be incorporated into the backbone of the dithiolate ligand [4][5][6][7], and the amine [8] or oxygen [9] cofactor is proposed to facilitate the formation of the H-H bond and the transfer of protons to and from the metal [9]. As enzymes are difficult to obtain in sufficient amounts to adapt for commercial applications [10,11], a number of model catalysts based on transition metal complexes have been reported [12][13][14][15][16][17][18][19]. To further study the mechanism of catalysis for hydrogen evolution by transition metal complexes, we choose triazenido ligands and their complexes, because the central nitrogen of the triazenido-metal compound imparts greater basicity on the [N⋯N⋯N] − relative to the neutral nitrogen, making the triazenido compound more electron donating, amenable to binding to H + and hydrogen production. our previous reports have shown that triazenido complexes can efficiently catalyze hydrogen evolution both from acetic acid and aqueous buffer [20][21][22][23][24]. As a continuation, here we present a molecular catalyst based on a platinum complex supported by 1-[(2-methoxy) benzene]-3-[2-pyridine] triazene ion that can electro-catalyze hydrogen evolution both from acetic acid and aqueous buffer.

Materials and physical measurements
All chemicals were purchased and used without purification. Pt(PPh 3 ) 2 Cl 2 was prepared by a literature procedure [25]. 1 H Nmr analysis was conducted using a Bruker Am-500 instrument in CdCl 3 solution. 31 P Nmr spectrum was measured on a Bruker AV 600 spectrometer in CdCl 3 ; chemical shifts were quoted to 85% H 3 Po 4 . Elemental analyses for C, H, and N were obtained on a Perkin-Elmer analyzer model 240. ESi-mS experiment was performed on a Bruker daltonics Esquire 3000 spectrometer. Electrochemical studies were conducted on a CHi-660E electrochemical analyzer under N 2 using a three-electrode cell in which a glassy carbon electrode (1 mm in diameter) was the working electrode, a saturated Ag/AgNo 3 or Ag/AgCl electrode was the reference electrode, and platinum wire was the auxiliary electrode. in organic media, the ferrocene/ferrocenium (1+) couple was used as an internal standard and a solution of 0.10 m [(n-Bu) 4 N]Clo 4 was used as the supporting electrolyte. Controlled-potential electrolysis (CPE) in organic and aqueous media was conducted using the literature procedure [20]. Gas chromatography (GC) experiments were carried out with an Agilent Technologies 7890A GC instrument (Column: 19091 J-413, No. uSC184265H; detector: TCd; injection volume: 1 mL).

Electrochemical studies
To study the electrochemical properties of HL, Pt(PPh 3 ) 2 Cl 2 , and 1 in organic media, cyclic voltammograms (CVs) were conducted in dmf with 0.10 m [(n-Bu) 4  To investigate the effect of temperature of media on the electrochemical behavior of the platinum complexes, CVs were measured at different temperatures. for 1, the current strength significantly increases near -1.60 V versus Ag/AgNo 3 with increasing temperature from 298 to 323 K ( figure S8(a)). The current strength slightly increases near -1.57 V versus Ag/AgNo 3 with increasing temperature from 298 to 323 K for Pt(PPh 3 ) 2 Cl 2 ) ( figure S8(b)).  the platinum center and nitrogen of triazenido compound improves catalytic activity. The result also suggests that platinum complexes with more positive Pt ii/i redox potentials show lower activity for hydrogen generation.
Based on the ESi-mS and electrochemical analysis, a possible mechanism for electro-catalysis of 1 is presented in scheme 2. from scheme 2, a putative [Pt i (PPh 3 ) 2 (L)Cl] − species is formed by one-electron reduction in 1, then, the introduction of H + gives the [Pt iii (PPh 3 ) 2 (H⋯L)Cl] species, a highly reactive intermediate. further, one-electron reduction in the [Pt iii (PPh 3 ) 2 (H⋯L)Cl] species affords H 2 and regenerates the starting complex 1. Although the relative contributions are indistinguishable in this analysis, we suspect that these processes are complementary H 2 evolution pathways. further mechanistic studies are under investigation.
Proposed mechanism for hydrogen generation catalyzed by 1.

Catalytic hydrogen evolution from aqueous buffer
To explore the electrochemical behaviors of Pt(PPh 3 ) 2 Cl 2 and 1 in aqueous media, a much more attractive medium for the sustainable generation of hydrogen, CVs were measured in buffered aqueous solution with different pH. As shown in figure 5(a), a catalytic current is not apparent until a potential of −1.22 V versus Ag/AgCl is attained without 1. upon addition of 1, the onset of catalytic current is observed at −0.86 V versus Ag/AgCl, showing that addition of 1 can efficiently reduce the potential, and the current strength increases significantly with increasing concentration of 1 from 0.16 to 0.82 μm ( figure 5(a)). According to figure 5(b), the onset of this catalytic current is influenced by the solution pH, and the applied potential declines with increasing pH, evidencing the involvement of a proton in the initial stage of electrochemical catalysis. it is worth noting that, when pH is 2.8, one new peak at −0.95 V versus Ag/ AgCl appears, suggesting that Pt(PPh 3 ) 2 (L)Cl decomposes to a new component under these conditions. from figure S11(a), the onset of catalytic current is observed at −0.98 V versus Ag/AgCl without Pt(PPh 3 ) 2 Cl 2 , and the current strength increases significantly with increasing concentrations of Pt(PPh 3 ) 2 Cl 2 from 0.27 to 1.35 μm. As shown in figure S11(b), Pt(PPh 3 ) 2 Cl 2 shows a pH-dependent peak at −1.65 V versus Ag/AgCl, which is responsible for catalytic water reduction. The onset of this catalytic current is influenced by the solution pH, and the applied potential declines with increasing pH. Similar to 1, when pH is 2.8, one new peak at −0.91 V versus Ag/AgCl appears for Pt(PPh 3 ) 2 Cl 2 , suggesting one new component is formed.
in the presence of HL, the onset of catalytic current is observed at −1.02 V versus Ag/AgCl, and the current strength also increases with increasing concentration of HL from 0.08 to 1.63 μm ( figure S12(a)). HL also shows a pH-dependent peak at −1.61 V versus Ag/AgCl, and the applied potential declines with increasing pH ( figure S12(b)). When pH is 2.8, one new redox peak appeared at −1.02 V versus Ag/AgCl, showing that a new component, such as [HL-H] + , is formed.
To further study catalytic properties in aqueous media, bulk electrolysis from 1 and HL were measured in 0.25 m aqueous buffer (pH 7.0). Table 1 shows catalytic data by HL, Pt(PPh 3 ) 2 Cl 2 , and Pt(PPh 3 ) 2 (L)Cl in aqueous media. from figure 6(a), when the applied potential is −1.45 V versus Ag/AgCl, the maximum charge is only 35 mC for 2 min of electrolysis in the absence of 1. However, the addition of 1 affords 1598 mC during 2 min of electrolysis (figure6(b)), accompanied by a large amount of gas bubbles, which is confirmed as H 2 by GC. from figure S13(a), ~2.74 mL of H 2 is afforded over an electrolysis period of 1 h with a faradaic efficiency of 89.12% for H 2 (figure S13(b)). from equations (1)  This value is higher than some reported molecular catalysts for electrochemical hydrogen production from neutral water. for example, a copper(ii) complex supported by a tetradentate amine phenol ligand that shows a Tof of 300 mol of H 2 per mole of catalyst per hour at an overpotential of 869 mV [40], a copper(ii) complex with 1,3-bis[(4-chloro)benzene]triazene ligand displaying a Tof of 82 mol of H 2 per mole of catalyst per hour at an overpotential of 837.6 mV [23], and a copper(i) complex with 1-[(2-methoxy) benzene]-3-[2-(chloro)benzene] triazene ligand (272 mol of hydrogen per mole of catalyst per hour at an overpotential of 839 mV) [21].
Similar to the procedure for 1, bulk electrolysis from Pt(PPh 3 ) 2 Cl 2 and HL was conducted in 0.25 m buffer (pH 7.0). According to figures 6(c) and (d), during 2 min of electrolysis, Pt(PPh 3 ) 2 Cl 2 and HL give 623 and 260 mC, respectively. from figures S14 and S15, Pt Similar to that in organic media, 1 shows higher efficiency for hydrogen production than Pt(PPh 3 ) 2 Cl 2 . from a 72 h CPE of a 0.25 m buffer solution (pH 7.0) in the presence of 1, Pt(PPh 3 ) 2 Cl 2 , or HL, we also find that 1 is more active than Pt(PPh 3 ) 2 Cl 2 or HL. As shown in figures S16(a) and S16(b), a total of 1158 or 660 C is accumulated during the electrolysis catalyzed by 1 and Pt(PPh 3 ) 2 Cl 2 , respectively. in the presence of HL, only 238 C is passed during a 72 h electrolysis (figure S16(c)). from the above analysis, both HL and Pt(PPh 3 ) 2 Cl 2 can act as electrocatalysts, but in low catalytic efficiency for hydrogen generation. in order to improve the catalytic efficiency, it is necessary to introduce triazenido ligand into platinum complexes.

Conclusion
We have described one platinum-triazenido complex, Pt(PPh 3 ) 2 (L)Cl, that is obtained from the reaction with triazenido ligand, which has been characterized by physico-chemical and spectroscopic methods. HL, Pt(PPh 3 ) 2 Cl 2 and Pt(PPh 3 ) 2 (L)Cl can catalyze hydrogen evolution from acetic acid or a neutral buffer. Electrochemical studies show that the introduction of triazenido ligand into Pt(PPh 3 ) 2 Cl 2 can improve the catalytic efficiency for hydrogen production, and the platinum complex with more positive Pt ii/i redox potential exhibits lower activity for hydrogen generation.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This work was supported by the National Science foundation of China [grant number 20971045, 21271073].