Ligand-Centered Hydrogen Evolution with Ni(II) and Pd(II)DMTH

Abstract

In this study, we report a pair of electrocatalysts for the hydrogen evolution reaction (HER) based on the noninnocent ligand diacetyl-2-(4-methyl-3-thiosemicarbazone)-3-(2-pyridinehydrazone) (H 2 DMTH, H 2 L 1 ). The neutral complexes NiL 1 and PdL 1 were synthesized and characterized by spectroscopic and electrochemical methods. The complexes contain a non-coordinating, basic hydrazino nitrogen that is protonated during the HER. The pK a of this nitrogen was determined by spectrophotometric titration in acetonitrile to be 12.71 for NiL 1 and 13.03 for PdL 1 . Cyclic voltammograms of both NiL 1 and PdL 1 in acetonitrile exhibit diffusion-controlled, reversible ligand-centered events at −1.83 and −1.79 V (vs ferrocenium/ferrocene) for NiL 1 and PdL 1 , respectively. A quasireversible, ligand-centered event is observed at −2.43 and −2.34 V for NiL 1 and PdL 1 , respectively. The HER activity in acetonitrile was evaluated using a series of neutral and cationic acids for each catalyst. Kinetic isotope effect (KIE) studies suggest that the precatalytic event observed is associated with a proton-coupled electron transfer step. The highest turnover frequency values observed were 6150 s −1 at an overpotential of 0.74 V for NiL 1 and 8280 s −1 at an overpotential of 0.44 V for PdL 1 . Density functional theory (DFT) computations suggest both complexes follow a ligand-centered HER mechanism where the metals remain in the +2 oxidation state.

Metal–Ligand Cooperativity Promotes Reversible Capture of Dilute CO2 as a Zn(II)-Methylcarbonate

“…All solvents and materials were purchased as reagent grade from VWR or Sigma-Aldrich. H 2 L 1 (diacetyl-2-(4-methyl-thiosemicarbazone)-3-(2-hydrazinopyridine)) was isolated in high yields from the procedure published by Cowley et al and the NiL 1 , PdL 1 , CuL 1 (MeOH), and ZnL 1 (MeOH) complexes were synthesized using the previously reported methods. − For the other ligands, the thiosemicarbazide precursors were synthesized as described previously for 4-benzyl-3-thiosemicarbazide . All complexes and ligands in this study are stable in air and water and did not require protection from the atmosphere.…”

Synthesis of cobalt(ii) phenolate selenoether complexes to mimic hydrogenase-like activity for hydrogen gas production

“…1–3 To date, Pt serves as the best catalyst for an electrocatalytic hydrogen evolution reaction (HER), but its scarcity and high cost limit practical large-scale application and inspire the search for economically viable and Earth-abundant catalysts. 4–7 Recently, a number of Earth-abundant 3d-transition metal electrocatalysts, mainly comprised of iron, 8–14 cobalt, 15–21 and nickel, 22–28 have been reported to electrocatalyze the HER. Chief among these transition metal electrocatalysts is cobalt due to its energetically affordable conversion from 3d 6 Co( iii ) to 3d 7 Co( ii ) and then to nucleophilic 3d 8 Co( i ) species, and it has been extensively used.…”

“…There is emerging interest in the use of redox-active ligands that open an alternative HER route, involving ligand-assisted metal-centered reactivity where ligands act as electron reservoirs and assist the metal center in the formation of products (Chart 1). 25,28,30–32 In this context, the elements of the 4 th period, especially selenium, can be useful for electrocatalytic activity as these elements provide the architecture of soft donor ligands, which stabilize the low-valent oxidation states of 3d-metal ions. 33–35…”

“…[1][2][3] To date, Pt serves as the best catalyst for an electrocatalytic hydrogen evolution reaction (HER), but its scarcity and high cost limit practical large-scale application and inspire the search for economically viable and Earth-abundant catalysts. [4][5][6][7] Recently, a number of Earth-abundant 3d-transition metal electrocatalysts, mainly comprised of iron, [8][9][10][11][12][13][14] cobalt, [15][16][17][18][19][20][21] and nickel, [22][23][24][25][26][27][28] have been reported to electrocatalyze the HER.…”

“…There is emerging interest in the use of redox-active ligands that open an alternative HER route, involving ligand-assisted metal-centered reactivity where ligands act as electron reservoirs and assist the metal center in the formation of products (Chart 1). 25,28,[30][31][32] In this context, the elements of the 4 th period, especially selenium, can be useful for electrocatalytic activity as these elements provide the architecture of soft donor ligands, which stabilize the low-valent oxidation states of 3d-metal ions. [33][34][35] Furthermore, the presence of selenium in the ligand core provides additional stability to the catalyst in an oxidative environment due to the weak selenium-oxygen bond, [36][37][38][39][40] and consequently, selenocysteine has been utilized in the [NiFe]hydrogenase enzyme.…”