A dinuclear nickel catalyst based on metal–metal cooperation for electrochemical hydrogen production

Journal Pre-proofs Research paper A dinuclear nickel catalyst based on metal–metal cooperation for electrochemical hydrogen production Tomohiko Hamaguchi, Keisuke Kai, Isao Ando, Ken Kawano, Kosei Yamauchi, Ken Sakai PII: DOI: Reference:

S0020-1693(19)31912-7 https://doi.org/10.1016/j.ica.2020.119498 ICA 119498

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Inorganica Chimica Acta

Received Date: Revised Date: Accepted Date:

6 December 2019 1 February 2020 2 February 2020

Please cite this article as: T. Hamaguchi, K. Kai, I. Ando, K. Kawano, K. Yamauchi, K. Sakai, A dinuclear nickel catalyst based on metal–metal cooperation for electrochemical hydrogen production, Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.119498

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© 2020 Published by Elsevier B.V.

A dinuclear nickel catalyst based on metal–metal cooperation for electrochemical hydrogen production

Tomohiko Hamaguchi*†, Keisuke Kai†, Isao Ando†, Ken Kawano‡,§, Kosei Yamauchi‡,§ and Ken Sakai‡,§,

†

Department of Chemistry, Faculty of Science, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku,

Fukuoka 814-0180, Japan ‡

Department of Chemistry, Faculty of Science, Kyushu University, Motooka 744, Nishi-ku, Fukuoka

819-0395, Japan §

International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University,

Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan  Center

for Molecular Systems (CMS), Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-

0395, Japan

Phone

+81-92-871-6631 (ex 6213)

Fax

+81-92-865-6030

E-mail address

[email protected]

Abstract In the quest for efficient metal catalysts for hydrogen production, a new dinuclear nickel complex of formula [{(-2-thiazolethiolate)Ni(1,3-bis(diphenylphosphino)propane)}2](BF4)2 (2) has been synthesized. Complex 2 shows electrochemical catalytic behavior for hydrogen production with a overpotential of 0.26 V, a current efficiency of 59%, and a turnover number of 6.4. The lack of catalytic behavior of a related mononuclear complex under the same conditions suggests that the cooperation between the two Ni centers plays a pivotal role in the catalytic behavior.

Keywords Ni complex; dinuclear complex; electrochemistry; catalyst; hydrogen

1. Introduction Today, renewable energy is attracting growing attention in terms of the energy issue, global warming, and environmental pollution.[1] Solar and wind power are among the most popular types of renewable energy sources, and their non-constant output is usually stored in a battery as electric power or in a variety of chemicals as chemical energy.[2] In this regard, the hydrogen molecule is an attractive system for chemical energy storage mainly because energy is produced with no harmful waste formation.[3] Consequently, the efficient catalytic production of hydrogen, especially using mononuclear[4] or dinuclear complexes,[5] constitutes an active area of research worldwide. Recently,

we

have

reported

the

mononuclear

complex

[Ni(1,3-

bis(diphenylphosphino)propane)(2-mercaptopyridinate-N,S)]BF4 (1, Fig. 1)[6] The mononuclear complex shows electrocatalytic activity for hydrogen production under trifluoroacetic acid condition; however, it does not show under acetic acid condition. In the catalytic behavior, weaker acid is a suitable proton source rather than stronger acid based on environmental problems.

Figure 1. Molecular structures of mononuclear complex 1 (left) and dinuclear complex 2 (right). dppp stands for 1,3-bis(diphenylphosphino)propane.

Herein, we report a new dinuclear complex of formula [{(-tzS)Ni(dppp)}2](BF4)2 (2), where tzS and dppp denote 2-thiazolethiolate and 1,3-bis(diphenylphosphino)propane, respectively (Fig. 1). The complex shows the electrocatalytic activity for hydrogen production under acetic acid condition. Furthermore, we compare its catalytic behavior with that of a similar mononuclear complex to evaluate the effect of dimerization.

2. Experimental All materials were purchased from commercial suppliers (Alfa Aesar, Wako Pure Chemical Industries, Ltd., Kanto Chemical Co., Inc., and Tokyo Chemical Industry Co., Ltd.) and were used without further purification. 2.1 Measurements Electrospray ionization mass (ESI-mass) spectral data were obtained on a JEOL JMST100CS spectrometer. Cyclic voltammetry was performed on a BAS BAS100B/W electrochemical workstation using three electrode cells (a glassy carbon electrode as the working electrode( = 3 mm), a Pt coil electrode as the counter electrode, and a homemade Ag+/Ag electrode as the reference electrode) in a 0.1 mol L−1 n-Bu4NPF6/CH3CN medium. For these experiments, 10 mL of a 1 × 10−3

mol L−1 solution of complex 2 was used. The stock solution of acetic acid was 10 mol L−1 acetic acid/0.1 mol L−1 n-Bu4NPF6/CH3CN. The E1/2 of the ferrocenium/ferrocene couple was 0.10 V vs Ag+/Ag. The scan rate was 100 mV s−1, unless otherwise specified. Controlled potential electrolysis was carried out as reported elsewhere,[7] except that an anionic exchange membrane (Selemion AMT, AGC Engineering) was used to separate the working compartment from the counter compartment. A solution of complex 2/acetic acid/n-Bu4NPF6/CH3CN (6 × 10−6 mol/9 × 10−4 mol/1.2 × 10−3 mol/12 mL) was used for the electrolysis. The electrolysis was carried out for 25 minutes with − 1.50 V vs Ag+/Ag. C, H, and N analysis was carried out by the Service Center of the Elementary Analysis of Organic Compounds of Kyushu University.

2.2 Synthesis [{(-2-thiazolethiolate)Ni(1,3-bis(diphenylphosphino)propane)}2](BF4)2 (2) A

mixture

of

[Ni(H2O)6](BF4)2

(250

mg,

7.34

×

10−4

mol)

and

1,3-

bis(diphenylphosphino)propane (303 mg, 7.34 × 10−4 mol) in CH3CN (20 mL) was stirred for 3 h. To this solution, 2-thiazolethiol (86.0 mg, 7.34 × 10−4 mol) and triethylamine (102 L, 7.34 × 10−4 mol) in CH3CN (20 mL) were added, and the mixture was stirred overnight. The red-orange solution was then evacuated under vacuum to dryness. The crude product was purified by column chromatography with silica gel ( = 2 cm, l = 6.5 cm, Wakogel C-200). A mixture of CH3OH/CH2Cl2 (1:100, v/v) was used as the eluent. The main first fraction was collected and evaporated to dryness. The residue was recrystallized by the vapor diffusion of diethyl ether into a CH3CN solution, affording red block crystals that were collected by filtration with 58.1% yield (287 mg). Anal. found: C, 52.49%; H, 4.30%; N, 2.08%; calcd. for [C60H56N2P4S4Ni2](BF4)2H2O: C, 52.78%; H, 4.28%; N, 2.05%. 2.3 X-ray crystallography

Single crystals of 2(CH3CN)2(diethyl ether) suitable for single-crystal X-ray analysis were obtained by the slow vapor diffusion of diethyl ether into a CH3CN solution of complex 2 at room temperature. The data were collected on a RIGAKU R-AXIS RAPID II IP diffractometer. A multiscan absorption correction was applied to the intensity data. The structure was solved by a direct method (SIR-2011)[8] and refined by the full-matrix least-squares method on F2 (SHELXL-2016/6)[9] using the Yadokari-XG software package.[10] All the non-hydrogen atoms were refined with anisotropic parameters. Hydrogen atoms were included in the calculated positions and refined by a riding model. Crystallographic diagrams were created with the ORTEP program[11] and the Mercury program.[12] CCDC: 1901835. Crystal data for 2(CH3CN)2(diethyl ether) : C68H72B2F8N4Ni2O1P4S4, M = 1504.45 g•mol−3, T = 120(2) K, monoclinic, P21/c, a = 25.2930(4) Å, b = 14.6432(3) Å, c = 20.5612(4) Å  = 113.0500(12)°, V = 7007.3(2) Å3, Dcalc = 1.426 g•cm−3, Crystal size 0.180 x 0.180 x 0.100 mm3, reflections collected 110234, independent reflections 15971, R(int) = 0.0468, Goodness-of-fit on F2 1.014, R1 = 0.0443, wR2 = 0.1237 (for I>2σ(I)), R1 = 0.0521, wR2 = 0.1287 (for all data) (R1 = ||Fo| − |Fc||/|Fo|. wR2 = [w(Fo2 − Fc2)2/w(Fo2)2]1/2). Selected bond distances and angles : Ni1–P1 = 2.2011(7) Å, Ni1–P2 = 2.2170(7) Å, Ni1–N1 = 1.921(2) Å, Ni1–S1 = 2.2157(7) Å, Ni2–P3 = 2.2156(7) Å, Ni2–P4 = 2.2304(7) Å, Ni2–N2 = 1.936(2) Å, Ni2–S2 = 2.2326(7) Å, P1–Ni1–P2 = 91.03(3) °, N1–Ni1–S1 = 92.96(6) °, P3–Ni2–P4 = 93.23(3) °, N2–Ni2–S2 = 93.31(7) °.

3. Results and discussion 3.1 Synthesis and characterization Complex 2 was synthesized under almost the same method as complex 1.[6] The complex was purified from column chromatography and recrystallization. The complex was air-stable in solid form and in solution, and was characterized by elemental analysis and ESI-mass (Fig. S1).

The X-ray diffraction structure of complex 2 indicates that the complex has two monoNiII(dppp) units bridged by two tzS ligands, as shown in Fig. 2. Each Ni center is coordinated by two P atoms of the dppp ligand, one S atom of the tzS ligand, and one N atom of the second tzS ligand. The largest deviations from the least-squares mean plane are 0.016(1) Å for Ni1 and 0.046(1) Å for Ni2, which is consistent with the square planar geometry of the Ni(II) ions. The two square planar planes exhibit a V-shape with a dihedral angle of 59.75(3)º (Fig. S2). This V-shape structure is similar to that of Ni–Fe hydrogenase.[13] The Ni...Ni distance of 3.869 Å is consistent with the absence of a metal– metal bond. 2-mercaptopyridinate of complex 1 acts as bidentate ligand and complex 1 is mononuclear. 2-thiazolethiolate of complex 2 acts as bridging ligand and complex 2 is dinuclear. 2-thiazolethiol has wider S–C–N angle rather than that of 2-mercaptopyridine; therefore, 2-thiazolethiol would be less bidentate rather than 2-mercaptopyridine. We think that the difference of S–C–N angle makes the difference of the structure.

Figure 2. Molecular structure of dinuclear complex 2. Counter anions, solvent molecules, and hydrogen atoms are omitted for clarity.

3.2 Electrochemical behavior

Fig. 3 shows a cyclic voltammogram of complex 2. With cathodic scan starting at 0.4 V, the complex exhibits three reduction peaks at Ep= –1.03, –1.47 and –1.60 V (all potentials are shown vs the ferrocene/ferrocenium (Fc/Fc+) redox couple). Redin et al. reported that the related complex [(SC3H6S){Ni(1,2-bis(diphenylphosphinoethane))}2](BF4)2 showed two Ni(II)/Ni(I) redox couples at E1/2 = –1.12 and –1.26 V and two Ni(I)/Ni(0) redox couples at E1/2 = –2.02 and –2.16 V.[14] By analogy, the reduction peak at Ep= –1.03 V can be assigned to Ni(II)–Ni(II)/Ni(I)–Ni(I), and the two reduction peaks at Ep = –1.47 and –1.60 V are attributable to Ni(I)–Ni(I)/Ni(0)–Ni(I) and Ni(0)–Ni(I)/Ni(0)– Ni(0), respectively. These peaks have less corresponding oxidation peaks, that may be a result of the rigid doubly bridged structure that disfavors a square planar Ni(I) or Ni(0) species. After reduction, the reduced complex shows three oxidation peaks Ep= –1.15, –0.91 and –0.22 V with anodic scan. The oxidation peak at Ep= –0.91 would be attributable to Ni(II)–Ni(II)/Ni(I)–Ni(I). Others could be attributable to the oxidation of decomposed complex. Next, the electrochemical catalytic hydrogen production by complex 2 was investigated using acetic acid as a proton source (Fig. 4). On adding acetic acid, the intensity of the cathodic peak at –1.6 V increased. The cathodic current increases with an increase in the amount of acid. Therefore, electrochemical product grew in as acetic acid was increased. This peak can be safely related to the potential of Ni(I)–Ni(0)/Ni(0)–Ni(0), because acetic acid does not produce such a peak in the absence of complex 2 (Fig. S3). Therefore, acetic acid triggers the electrochemical catalytic behavior of complex 2. The overpotential (Eop) was calculated from equation (1),[15] where EHA is the standard potential for the reduction of acetic acid in CH3CN (−1.23 V vs Fc+/Fc)[16] and Ecat/2 is the potential at which the current is half of icat,[15b] that is, 0.26 V. 𝐸op = |𝐸HA ― 𝐸cat/2| (1)

Figure 3. Cyclic voltammogram of 1 × 10 − 3 mol L−1 complex 2 in CH3CN containing 0.1 mol L−1 nBu4N•PF6. The scan rate was 100 mV s −1. The arrow indicates the initial potential and the direction of the scan.

Figure 4. Catalytic behavior of complex 2 for electrochemical hydrogen production with various amounts of acetic acid. The solution was 1 × 10−3 mol L−1 complex 2 in CH3CN containing 0.1 mol L−1 n-Bu4N•PF6 and various amount of acetic acid. The scan rate was 100 mV s −1. The arrow indicates the initial potential and the direction of the scan.

To investigate the catalytic behavior in detail, a controlled potential electrolysis was carried out. After 25 min of electrolysis at –1.60 V, a total amount of 3.22 × 10−5 F was passed, and 9.50 × 10−6 mol of hydrogen molecules was detected by gas chromatography. The current efficiency and turnover number of complex 2 were not so outstanding, reaching values of 59% and 6.4, respectively.

The Ni(II) ion of mononuclear complex 1 is coordinated by two diphenylphosphino P atoms, one aromatic N atom, and one thiolate S atom, forming a coordination system very similar to that of complex 2. This mononuclear complex shows two-step one-electron irreversible redox waves at –1.23 and –1.65 V that are attributed to Ni(II)/Ni(I) and Ni(I)/Ni(0), respectively. It is worth mentioning that although Ni(0) species can be generated under very similar potentials for both the mononuclear and the dinuclear complexes, the mononuclear complex shows no catalytic behavior under acetic acid conditions.[6] This suggests that the presence of two metal centers plays an important role in catalytic behavior. In general, the key process for hydrogen molecule production catalyzed by the mononuclear Ni complex would be the generation of a hydride derivative [NiII(H)]+ from [Ni0]0 and H+,[17] in which the proton would be doubly reduced by two redox processes: Ni(0)/Ni(I) and Ni(I)/Ni(II). In the case of the dinuclear complex, a hydride complex could be [NiI–NiI(H)]+, in which the proton would be doubly reduced by two Ni(0)/Ni(I) processes. Because the Ni(0)/Ni(I) redox process is more suitable for proton reduction than Ni(I)/Ni(II), only the dinuclear complex will show catalytic behavior. Therefore, the cooperation between the two Ni atoms seems to be essential to produce hydrogen.

4. Conclusions In conclusion, we have synthesized a new Ni dinuclear complex whose structure has been determined by an X-ray diffraction study to contain two mono-NiII(dppp) units bridged by two tzS ligands. In acetic acid/CH3CN solution, the complex catalyzes the electrochemical production of hydrogen. As a similar mononuclear complex does not show catalytic behavior under similar conditions, the cooperation between the two Ni atoms in the dinuclear counterpart seems to play a pivotal role in the catalytic behavior. This result provides new knowledge for designing efficient catalysis complexes.

Acknowledgments This work was partially supported by JSPS KAKENHI 17H05390 in Coordination Asymmetry and also supported in part by funds (No.:171011 and 197203) from the Central Research Institute of Fukuoka University.

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.ica.2017.xx.xxx.

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Tomohiko Hamaguchi: Conceptualization, Methodology, Resources, Investigation, Writing - Original Draft Keisuke Kai: Resources, Investigation, Writing - Original Draft Isao Ando : Writing - Review & Editing Ken Kawano: Investigation, Writing - Review & Editing Kosei Yamauchi : Resources, Writing - Review & Editing Ken Sakai : Resources, Writing - Review & Editing

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Graphical Abstract

Highlights • A dinuclear Ni complex produces hydrogen from acetic acid electrochemically. • However, a related mononuclear complex does not under the same conditions. • The cooperation between the two Ni centers plays a pivotal role in the production.

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