Structure of Ni(II) Inclusion Complex in Solid/Solution States and the Enhancement of Catalytic Behavior in Electrochemical Hydrogen Production

In this article, we investigate the encapsulation of K₂[Ni(maleonitriledithiolate)₂] (<b>1</b>) within a host molecule, β-cyclodextrin (β-CD), via single-crystal X-ray analysis. An inclusion complex, K₂{[Ni(maleonitriledithiolate)₂]@(β-CD)₂} (<b>2</b>), was constructed from <b>1</b> and two β-CDs. The anion guest Ni complex included a host cavity, constructed using two β-CDs, and the Ni atom of the anion was located between the two hydrophilic primary rims. Ultraviolet-visible absorption spectroscopy revealed that inclusion complex <b>2</b> exhibited a 2:1 (host:guest) stoichiometry in the solution, which is consistent with the result obtained from X-ray crystallography. The association of the host and guest occurred in two steps, and the association constants for the first and second steps were 1.1(7) × 10⁴ and 1.8(5) × 10⁴ mol^-1 dm³, respectively. The catalytic behavior of <b>1</b> and <b>2</b> was investigated for electrochemical hydrogen production in the aqueous solution of an acetate buffer (pH = 4.72). During the catalytic reaction, inclusion complex <b>2</b> was observed to have a better catalytic reaction rate than <b>1</b>. The study findings provide insights into the effects of the encapsulation of guest molecules within host structures.