Tertiary phosphane-modified Ni(II) 1,3-Benzothiazol-2-ylacetonitriledithiolates: Tuning heterogeneous OER electrocatalysis through phosphane denticity, steric modulation, and chelate ring-size variation

Developing robust and active transition metal-based electrocatalysts for the generation of oxygen is crucial for enhancing the performance of numerous energy conversion systems. In this study, syntheses and characterization of three new heteroleptic Ni(II) dithiolate complexes: [(Bzdt)Ni(PPh₃)₂] (NiBz1), [(Bzdt)Ni(dppe)] (NiBz2), and [(Bzdt)Ni(dppf)] (NiBz3), (where Bzdt^2-, PPh₃, dppe, and dppf represents 1,3-benzothiazol-2-ylacetonitriledithiolate, triphenylphosphine, 1,2-bis(diphenylphosphino)ethane, and 1,1ʹ-bis(diphenylphosphino)ferrocene, respectively) have been presented. These complexes have been characterized using several spectroscopic techniques, FESEM-EDX, and for NiBz2 using single crystal X-ray diffraction. Structural investigations reveal that Ni(II) center in NiBz2 is coordinated to two S atoms from dithiolate ligand and two P atoms from dppe ligand forming a distorted square planar geometry. Hirshfeld surface analysis indicates the presence of key non-covalent interactions, including C-H···S, C-H···C, C-H···Ni, and C-H···N, which support the stability of the supramolecular framework. Electrochemical investigations of these complexes as OER electrocatalysts in alkaline media reveal that all three show notable activity (j > 46.11 mA.cm^-2 at 10 mV·s^-1), with NiBz2 standing out due to its lowest overpotential (η = 354 mV at j = 10 mA.cm^-2) and Tafel slope (53 mV.dec^‑1). This performance places NiBz2 among the best heterogeneous Ni(II)-dithiolate based OER electrocatalysts so far reported. Its outstanding activity is attributed to a combination of favorable electronic properties, such as a reduced charge-transfer gap and charge transfer resistance, which facilitate efficient electron transfer during catalysis. Overall, this study showcases that phosphine-derived steric and electronic tuning enables charge-transfer gap engineering within Ni(II)-dithiolate frameworks, and establishes a clear correlation between ligand-dependent electronic structure and OER performance.