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Graphical abstract Six new aromatic dithiolate nickel complexes [RN(PPh 2) 2 Ni(bdt)] and [RN(PPh 2) 2 Ni(tdt)] have been synthesized and fully characterized. The nickel atom adopts a slightly distorted square-planar coordination by RN(PPh 2) 2 and dithiolate ligands. All complexes can electrocatalyze protons reduction to H 2 in the presence of FTA. Abstract Six aromatic dithiolate nickel complexes bearing N - substituted bis(diphenylphosphanyl)amine (RN(PPh 2) 2) ligands, [RN(PPh 2) 2 Ni(bdt)] (bdt = 1,2-benzenedithiolate; R = (CH 2) 3 OCH 3 (1), (CH 2) 3 SCH 3 (2), CH(CH 3)C 6 H 5 (3)) and [RN(PPh 2) 2 Ni(tdt)] (tdt = 3,4-toluenedithiolate; R = (CH 2) 3 OCH 3 (4), (CH 2) 3 SCH 3 (5), CH(CH 3)C 6 H 5 (6)), have been synthesized in moderated yields by condensations of RN(PPh 2) 2 NiCl 2 with 1,2-benzenedithiol or 3,4-toluenedithiol in the presence of Et 3 N at room temperature. All the complexes were fully characterized by elemental analysis, FTIR, NMR (1H, 13C, and 31P) spectroscopy, UV–Vis spectrum, and thermo-gravimetric analysis. The crystal structures of complexes 1 , 2 , 3 , and 5 have been investigated by single crystal X-ray diffraction. The nickel atom adopts a slightly distorted square-planar coordination by two phosphorus atoms of RN(PPh 2) 2 and two sulfur atoms of aromatic dithiolate ligands in their structures. Furthermore, the electrochemical behaviors of complexes 1 – 6 and their catalytic activities for electrochemical hydrogen evolution in CH 2 Cl 2 solution with trifluoroacetic acid (TFA) as a proton source have been studied by cyclic voltammetry. [ABSTRACT FROM AUTHOR]

Six new O‐alkyldithiophosphate nickel complexes with dcpf ligand, [(dcpf)Ni(S2P{O}OR)] (dcpf = 1,1′‐bis (dicyclohexylphosphino)ferrocene, R = CH3 (1), CH3CH2 (2), Ph (3), 4‐MeC6H4 (4), PhCH2 (5) and PhCH2CH2 (6)), have been synthesized by the treatment of dcpf with ((RO)2PS2)2Ni in satisfactory yields. These complexes were characterized by elemental analysis, spectroscopy (FTIR, UV–vis, 1H, 13C, and 31P NMR), thermogravimetric analysis and single crystal X‐ray diffraction. The nickel atom in 1, 2·CH2Cl2, 3·CH2Cl2, 4·2CH2Cl2·THF, and 2(5)·hexane adopts a slightly distorted square‐planar coordination environment finished by two phosphorus atoms of dcpf ligand and two sulfur atoms of O‐alkyldithiophosphate ligand. Furthermore, the electrochemical properties for complexes 1–6 were also investigated by cyclic voltammetry. With the addition of 120 mM trifluoroacetic acid (TFA), the turnover frequency (TOF) values for 1–6 are estimated to be 1243.83, 1046.54, 1331.71, 2545.29, 1899.03, and 1191.37 s−1, with the overpotential (η) values of 0.62, 0.58, 0.71, 0.67, 0.60, and 0.56 V, respectively. The result of electrochemical studies indicates that all complexes can be used as efficient molecular eletrocatalysts for the reduction of protons to hydrogen in the presence of TFA in MeCN. [ABSTRACT FROM AUTHOR]

Five heteroleptic maleonitriledithiolate nickel complexes bearing bis(diphenyl-phosphanyl)amine ligands, RN(PPh 2) 2 Ni(mnt), have been synthesized and structurally characterized. The coordination geometry around the nickel atom is a slightly distorted square-planar configuration. All complexes can electrocatalyze reduction of protons to hydrogen in the presence of trifluoroacetic acid in MeCN. • The nickel atom in the crystal structure of the complexes adopts a slightly distorted square-planar coordination environment. • The complexes are stable under acidic and electrochemical conditions. • The complexes can be used as low-cost robust molecular eletrocatalysts for hydrogen evolution. • ECEC mechanism for electrocatalytic hydrogen evolution has been proposed. Five neutral heteroleptic maleonitriledithiolate nickel complexes [RN(PPh 2) 2 Ni(mnt)] bearing bis(diphenylphosphanyl)amine ligands (where mnt2− = maleonitriledithiolate; R = CH 3 O(CH 2) 3 (1), CH 3 S(CH 2) 3 (2), (S)-CH 3 CHPh (3), (CH 3) 2 CH(CH 2) 2 (4), and p- CH 3 C 6 H 4 (5)) have been synthesized and fully characterized by elemental analysis, FTIR, NMR (1H, 13C, and 31P) spectroscopy, UV–Vis spectrum, single crystal X-ray diffraction, thermogravimetric analysis, and density functional theory (DFT) calculation. The nickel atom in 1 , 2 , 4 , and 5 ⋅1.5CH 2 Cl 2 adopts a slightly distorted square-planar coordination environment with two phosphorus atoms of RN(PPh 2) 2 and two sulfur atoms of mnt2−, respectively, while the nickel atom in 3 exhibits a pseudo square pyramid with HP 2 S 2 chromophore. Furthermore, the electrochemical properties for 1 – 5 were also investigated by cyclic voltammetry. With the addition of 120 mM trifluoroacetic acid (TFA) to 1 – 5 in MeCN, the turnover frequency (TOF) values are estimated to be 368.59–585.17 s−1 with the corresponding overpotential (η) values of 0.59–0.73 V. The electrochemical studies indicate that all complexes can be used as low-cost robust molecular eletrocatalysts for the reduction protons to hydrogen in the presence of TFA and a possible ECEC mechanism for electrocatalytic hydrogen evolution has been also proposed. [ABSTRACT FROM AUTHOR]