Issue 4, 2024

The rigidity and chelation effect of ligands on the hydrogen evolution reaction catalyzed by Ni(ii) complexes

Abstract

With increasing interest in nickel-based electrocatalysts, three heteroleptic Ni(II) dithiolate complexes with the general formula [Ni(II)L(L′)2] (1–3), L = 2-(methylene-1,1′-dithiolato)-5,5′-dimethylcyclohexane-1,3-dione and L′ = triphenylphosphine (1), 1,1′-bis(diphenylphosphino)ferrocene (DPPF) (2), and 1,2-bis(diphenylphosphino)ethane (DPPE) (3), have been synthesized and characterized by various spectroscopic techniques (UV-vis, IR, 1H, and 31P{1H} NMR) as well as the electrochemical method. The molecular structure of complex 2 has also been determined by single-crystal X-ray crystallography. The crystal structure of complex 2 reveals a distorted square planar geometry around the nickel metal ion with a NiP2S2 core. The cyclic voltammograms reveal a small difference in the redox properties of complexes (ΔE° = 130 mV) while the difference in the catalytic half-wave potential becomes substantial (ΔEcat/2 = 670 mV) in the presence of 15 mM CF3COOH. The common S^S-dithiolate ligand provides stability, while the rigidity effect of other ligands (DPPE (3) > DPPF (2) > PPh3 (1)) regulates the formation of the transition state, resulting in the NiIII–H intermediate in the order of 1 > 2 > 3. The foot-of-the-wave analysis supports the widely accepted ECEC mechanism for Ni-based complexes with the first protonation step as a rate-determining step. The electrocatalytic proton reduction activity follows in the order of complex 1 > 2 > 3. The comparatively lower overpotential and higher turnover frequency of complex 1 are attributed to the flexibility of the PPh3 ligand, which favours the easy formation of a transition state.

Graphical abstract: The rigidity and chelation effect of ligands on the hydrogen evolution reaction catalyzed by Ni(ii) complexes

Supplementary files

Article information

Article type
Paper
Submitted
23 Nov 2023
Accepted
12 Dec 2023
First published
13 Dec 2023

Dalton Trans., 2024,53, 1680-1690

The rigidity and chelation effect of ligands on the hydrogen evolution reaction catalyzed by Ni(II) complexes

A. Mishra, G. K. Mishra, Anamika, N. Singh, R. Kant and K. Kumar, Dalton Trans., 2024, 53, 1680 DOI: 10.1039/D3DT03932C

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