Mechanistic Insights into Homogeneous Electrocatalytic and Photocatalytic Hydrogen Evolution Catalyzed by High-Spin Ni(II) Complexes with S₂N₂-Type Tetradentate Ligands

Abstract

We report homogeneous electrocatalytic and photocatalytic H evolution using two Ni(II) complexes with SN-type tetradentate ligands bearing two different sizes of chelate rings as catalysts. A Ni(II) complex with a five-membered SCS-Ni chelate ring (1) exhibited higher activity than that with a six-membered SCS-Ni chelate ring (2) in both electrocatalytic and photocatalytic H evolution despite both complexes showing the same reduction potentials. A stepwise reduction of the Ni center from Ni(II) to Ni(0) was observed in the electrochemical measurements; the first reduction is a pure electron transfer reaction to form a Ni(I) complex as confirmed by electron spin resonance measurements, and the second is a 1e/1H proton-coupled electron transfer reaction to afford a putative Ni(II)-hydrido (Ni-H) species. We also clarified that Ni(II) complexes can act as homogeneous catalysts in the electrocatalytic H evolution, in which complex 1 exhibited higher reactivity than that of 2. In the photocatalytic system using [Ru(bpy)] as a photosensitizer and sodium ascorbate as a reductant, complex 1 with the five-membered chelate ring also showed higher catalytic activity than that of 2 with the six-membered chelate ring, although the rates of photoinduced electron-transfer processes were comparable. The Ni-H bond cleavage in the putative Ni-H intermediate should be involved in the rate-limiting step as evidenced by kinetic isotope effects observed in both photocatalytic and electrocatalytic H evolution. Kinetic analysis and density functional theory calculations indicated that the difference in H evolution activity between the two complexes was derived from that of activation barriers of the reactions between the Ni-H intermediates and proton, which is consistent with the fact that increase of proton concentration accelerates the H evolution.

Spectroscopic Characterisation of a Bio‐Inspired Ni‐Based Proton Reduction Catalyst Bearing a Pentadentate N2S3 Ligand with Improved Photocatalytic Activity

“…This difference is similarly attributed to the higher thermodynamic driving force for charge recombination between Ni I and Asc + for catalyst 2 (Δ G =−760 meV) compared with catalyst 1 (Δ G =−580 meV). The decay associated spectra (DAS) indicate that the Ni I species for this class of catalysts shows positive absorption from 440 to 550 nm, in agreement with the DFT‐calculated Ni I spectrum of catalyst 1 (Figure A, inset), and the reported Ni I spectrum of chemically reduced catalyst 2 …”

“…The photocatalytic activities of catalysts 1 and 2 were investigated by coupling them with the typical [Ru(bpy) 3 ] 2 + as photosensitiser and ascorbate as electron donor in an optimised solvent mixture [20] consisting of 1:1( v/v) dimethylacetamide (DMA)a nd aqueous ascorbate/ascorbic acid buffer (pH 4, 0.1 m). The solution was irradiated with ab lue LED light (centred at 463 nm) to selectively excite the photosensitiser.U nder these conditions, catalytic proton reduction was observed for both catalysts, as evidencedb yt he detectiono fh ydrogen gas by gas chromatography ( Figure 5).…”

“…The decay associated spectra (DAS) indicate that the Ni I speciesf or this class of catalysts showsp ositive absorption from 440 to 550 nm, in agreement with the DFT-calculated Ni I spectrum of catalyst 1 (Figure 7A, inset), and the reported Ni I spectrum of chemically reduced catalyst 2. [20]…”

“…[17,18] Thisr ole was attributed to aSatom in the photocatalytically active nickel complexes with bidentate dithiolenel igands (Ni-S 4 type) reported by Fan et al [19] More recently,K ojimae tal. demonstrated that Ni com- [ plexes bearing thioethers in the form of tetradentateN 2 S 2 ligands show photocatalytic activity towards protonr eduction when coupled with aR uP S. [20] Electrocatalytic and photocatalytic experiments in combination with DFT analysisf avoured a scenarioi nw hich cleavage of the NiÀHb ond in the key reactive Ni II Hi ntermediate is the rate-limiting step of the catalytic reaction.…”

Electrocatalytic property, anticancer activity, and density functional theory calculation of [NiCl(P^N^P)]Cl.EtOH

“…The proposed electrocatalytic mechanism involves the stepwise reduction of Ni(II) by two electrons to form [LNi 0 ] 0 in acetonitrile. [ 72 ] A mechanism for the hydrogen ion reduction using the PCP pincer complex has been reported by Luca et al [ 42 ] A similar mechanism has been considered for the hydrogen ion reduction using the synthesized complex in this work with some modifications (see Figure 10). The optimized structures of the complex at each step of the cycle are also shown in Figure 10.…”