Aromatic dithiolate nickel complexes bearing N-substituted bis(diphenylphosphanyl)amine: Synthesis, characterization, and electrocatalytic performance for hydrogen evolution

Electrocatalytic Hydrogen Evolution Catalyzed by 3,4‐Toluenedithiolate Nickel Complexes of Bis(diphenylphosphine)amine Ligand Containing An Azahydrophilic Group

“…These redox events are similar to the previously published report, [23,25] the first reduction events can be assigned to the one electron reduction of Ni II to Ni I , the second reduction events can be attributed to the Ni I /Ni 0 one, and the oxidation events of 0.15–0.21 V are presumably attributable to the oxidation of Ni II to Ni III process. Remarkably, the two reduction potentials of 2 are less negative in comparison to 1 and the analogue nickel complexes [15,20,23,25] . DuBois and co‐workers reported that there is effective electronic communication between the nickel atom and the para ‐substituents X on the phenyl rings in [Ni(P Ph 2 N C6H4X 2 ) 2 ](BF 4 ) 2 , which are eight bonds away.…”

“…While a singlet at 4.27 ppm for 2 is assigned the methylene protons for benzyl group, which indicate the benzyl group is attached to nitrogen atom of the N + (CH 3 ) 2 group, [27] and the relevant proton signals for the CH 2 N + (CH 3 ) 2 moieties shift to lower field about 3.55 and 2.71 ppm compared with the CH 2 N(CH 3 ) 2 moieties for 1 . In addition, the 31 P NMR spectrum of 1 shows only one singlet at 60.37 ppm assigned to the two phosphorus atoms of the bis(diphenylphosphine)amine ligand coordinated to the nickel atom, which is consistent with those of the analogous nickel complexes of [CH 3 O(CH 2 ) 3 N(PPh 2 ) 2 Ni(tdt)] (62.82 ppm) and [CH 3 S(CH 2 ) 3 N(PPh 2 ) 2 Ni(tdt)] (63.08 ppm) [25] . While the two phosphorus atoms signal for 2 appears a singlet at 65.40 ppm.…”

“…Although the terminal quaternary ammonium group of the bis(diphenylphosphine)amine ligand is five bonds away from the nickel atom, its stronger electron‐withdrawing property leads to decrease electron densities of the phosphorus atoms and has a significant influence on the reduction of the nickel atom. Furthermore, as shown in Figures S9 and S10, all the peak currents of Ni II /Ni I event for 1 and 2 are proportional to the square root of the scan rates in MeCN, indicating these redox events are diffusion‐controlled process and the calculated diffusion coefficients are 9.95×10 −6 and 1.32×10 −5 cm 2 s −1 for 1 and 2 , respectively [23,25,26,41] …”

“…The UV‐vis absorption spectra of 1 and 2 were recorded in CH 2 Cl 2 at room temperature. As shown in Figure 1, three intense absorption bands around 300, 262, and 231 nm appear, which can be assigned the π ‐ π * inter‐ and intra‐ligand charge transfer transitions (ILCT/LLCT) of both bis(diphenylphosphine)amine and 3,4‐toluenedithiolate ligands [23,25] . In addition, one weak absorption band at 530 nm should be assigned to d ‐ d transition from the dz 2 orbital of nickel with some admixture of the orbitals of bis(diphenylphosphine)amine and/or 3,4‐toluenedithiolate ligands to nickel charge transfer transition (LMCT), [25] indicating the nickel atom locates a NiS 2 P 2 square‐planar coordination geometry.…”

“…As shown in Figure 1, three intense absorption bands around 300, 262, and 231 nm appear, which can be assigned the π ‐ π * inter‐ and intra‐ligand charge transfer transitions (ILCT/LLCT) of both bis(diphenylphosphine)amine and 3,4‐toluenedithiolate ligands [23,25] . In addition, one weak absorption band at 530 nm should be assigned to d ‐ d transition from the dz 2 orbital of nickel with some admixture of the orbitals of bis(diphenylphosphine)amine and/or 3,4‐toluenedithiolate ligands to nickel charge transfer transition (LMCT), [25] indicating the nickel atom locates a NiS 2 P 2 square‐planar coordination geometry. Remarkably, as shown in Figure 2, the absorption bands of 1 and 2 at 262 nm dropped and the shoulder absorption band at 231 nm raised in the presence of TFA, implying the potential protonation process prior to electrochemical reduction, might resulting from the sulfur atom of the tdt ligand as a protonation site (see below for DFT calculations) [20,39,40] …”