O‐Alkyldithiophosphate Nickel Complexes with dcpf Ligand as Efficient Electrocatalysts for Hydrogen Evolution

Abstract

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.

Heteroleptic nickel complexes bearing O‐methyldithiophosphate and aminodiphosphine monosulfide ligands as robust molecular electrocatalysts for hydrogen evolution

“…À ligand, while the bands presented at around 699 cm À1 are connected with the P-S asymmetric vibration for CH 3 O{O}PS 2 À ligand, which is in good accordance with those observed for O-alkyldithiophosphate nickel complexes with diphosphine ligands. [32,33,40] In addition, the strong bonds at about 1436, 893, and 755 cm À1 can be ascribed to the P Ph, P N P, and P S stretching vibrations for the aminodiphosphine monodulfide ligand, respectively. [43,44] The 1 H NMR spectra of 1-5 display three mutiplets in the region of 7.28-7.90 ppm assigned to the phenyl protons of PPh 2 and {S}PPh 2 moieties of the aminodiphosphine monosulfide ligand.…”

“…Based on the previously reported researches, [33,40,47] two possible different pathways for the formation of 1-5 can be proposed. For the synthetic method I, as depicted in Scheme 3, the mechanism is believed to proceed firstly by the nucleophilic attack of the negative charged sulfur atom of the (CH 3 O) 2 P{S}S À anion on the nickel atom of the nickel complex [RN(PPh 2 )({S}PPh 2 )]NiCl 2 to give an tetra-coordinated intermediate I A .…”

“…In II A , the RN(PPh 2 ) ({S}PPh 2 ) only as a monodentate ligand coordinates to the nickel center by the stronger phosphorus atom from the Ph 2 P moiety instead of the sulfur atom in the Ph 2 P S one. [43,45] One (CH 3 O) 2 P{S}S À is monodentate ligand in II A while another (CH 3 O) 2 P{S}S À serves as bidentate one, [47] which is significantly different from the pentacoordinated intermediate containing the symmetric diphorsphine [33] or the aminodiphosphine ligand. [40] Subsequently, the intermediate II A is rapidly isomerized into ionic intermediate II B by the nucleophilic attack of the sulfur atom of the monodentate RN(PPh 2 )({S}PPh 2 ) ligand and the synergistic elimination of the monodentate (RO) 2 P{S}S À ligand, which is most likely due to the sterically crowded hindrance among the ligands in the intermediate II A.…”

“…[31] Recently, Xie and group have designed a series of molecular electrocatalysrs, O-alkyldithiophosphate nickel complexes containing dppf/dcpf (dcpf = 1,1-bis [dicyclohexylphosphino]ferrocene) diphosphine ligands, for the proton reduction to hydrogen using trifluoroacetic acid (TFA) as the proton source. [32,33] The TOF values of [(dcpf)Ni(S 2 P{O}OR)] are from 1047 to 2545 s À1 , and a possible ECEC catalytic mechanism has also been proposed. [33] Likewise, aminodiphosphine, RN(PPh 2 ) 2 , as an excellent short-bite diphosphine ligand due to the structure extremely close to bis(diphenylphosphanyl)methane (dppm) and has much more advantages than dppm ligand, [34] offers a diversity of coordination modes, such as monodentate, bidentate chelating and bridging modes, which is reflected in the rich coordination chemistry.…”

“…[32,33] The TOF values of [(dcpf)Ni(S 2 P{O}OR)] are from 1047 to 2545 s À1 , and a possible ECEC catalytic mechanism has also been proposed. [33] Likewise, aminodiphosphine, RN(PPh 2 ) 2 , as an excellent short-bite diphosphine ligand due to the structure extremely close to bis(diphenylphosphanyl)methane (dppm) and has much more advantages than dppm ligand, [34] offers a diversity of coordination modes, such as monodentate, bidentate chelating and bridging modes, which is reflected in the rich coordination chemistry. Furthermore, various N-substituents can be readily introduced to the RN(PPh 2 ) 2 ligand, which may facilitate the formation of polymetallic complexes [35][36][37] or the anchoring of the coordination compounds into mesoporous materials.…”