Electronic and Steric Tuning of Catalytic H₂ Evolution by Cobalt Complexes with Pentadentate Polypyridyl-Amine Ligands

Abstract

Structural modifications of molecular cobalt catalysts have provided important insights into the structure-function relationship for the hydrogen evolution reaction. We have shown that replacement of equatorial pyridines with more basic and conjugate isoquinoline groups of a pentadentate ligand results in lower overpotential and higher catalytic activity for electro- and photolytic H production in aqueous solutions. To fully understand the electronic and steric effects of the axial group that lies trans to the proposed cobalt hydride intermediate, isoquinoline groups were introduced in two new pentadentate ligands, N, N-bis(2-pyridinylmethyl)[3-(2-pyridinyl)isoquinoline)]-1-methanamine (DPA-1-MPI) and N, N-bis(2-pyridinylmethyl)[1-(2-pyridinyl)-isoquinoline)]-3-methanamine (DPA-3-MPI). Despite a slight structural difference of the introduced isoquinoline group, the resulting cobalt complexes display drastic changes in their electro- and photochemical properties. There are positive shifts of 290 and 260 mV, respectively, for the Co/Co and Co-H/Co-H couples from [Co(DPA-1-MPI)(HO)](PF) to [Co(DPA-3-MPI)(HO)](PF), with the former being ∼32 times as active as the latter in photocatalytic H production. Density functional theory (DFT) calculations show that the protonation of Co to yield the Co-H species is energetically more favorable for [Co(DPA-1-MPI)(HO)](PF) than that of [Co(DPA-3-MPI)(HO)](PF). Both experimental results and DFT computations suggest that the presence of a planar conjugate bipyridyl unit or its isoquinoline derivative is a key feature for stabilizing low valent Co species toward proton binding. The incorporation of an electron-donating group trans to the proposed Co-H species also facilitates proton binding and H-H bond formation, which is proposed to occur by the heterolytic coupling of Co-H species. The overall catalytic H evolution is presented as the modified electron transfer (E)-proton transfer (C)-electron transfer (E)-proton transfer (C) (mod-ECEC) pathway. This study provides important new insight into the electronic and steric factors controlling catalytic H production by Co complexes with pentadentate ligands.

The Effect of Guest Metal Ions on the Reduction Potentials of Uranium(VI) Complexes: Experimental and Theoretical Investigations

“…1 HNMR spectra were recorded in CD 3 CN solvent for complexes 1-6 with aB ruker 400 MHz NMR instrument, and tetramethylsilane was used as an internal standard. 1 HNMR spectra were recorded in CD 3 CN solvent for complexes 1-6 with aB ruker 400 MHz NMR instrument, and tetramethylsilane was used as an internal standard.…”

“…Elemental analyses (C, H, and N) were performed by using a PerkinElmer 2400 series II CHN analyzer.I Rs pectra in KBr pellets (4000-400 cm À1 )w ere recorded by using aP erkinElmer RXI FTIR spectrophotometer.T he electronic absorption spectra (500-250 nm) in acetonitrile solution were collected with aH itachi U-3501 spectrophotometer.E SI-mass spectra were recorded with a WATERS Xevo G2-S QTof mass spectrometer in HRMS grade acetonitrile. 1 HNMR spectra were recorded in CD 3 CN solvent for complexes 1-6 with aB ruker 400 MHz NMR instrument, and tetramethylsilane was used as an internal standard.…”

“…[1][2][3][4] In many cases, the presence of as pecific redox-inactive metal ion is crucial for the functioning of variousn aturally occurring enzymes. [1][2][3][4] In many cases, the presence of as pecific redox-inactive metal ion is crucial for the functioning of variousn aturally occurring enzymes.…”

“…The redox behavior of transition-metal complexes is extremely importantf or their reactivity and efficiency towards various catalytic oxidation/reduction, polymerization, andc oupling reactions. [1][2][3][4] In many cases, the presence of as pecific redox-inactive metal ion is crucial for the functioning of variousn aturally occurring enzymes. [5][6][7] For example, the Ca 2 + ion is vital for the oxo-bridged tetra-manganese-basede nzymei nP hoto-systemI I( PS II) that oxidizesw ater to generate molecular oxygen.…”

Synthesis, Characterization, and Electrocatalytic Behavior for Hydrogen Evolution of a Dinuclear Copper(II) Complex of 1‐[(2‐Carboxymethyl) benzene]‐3‐[2‐carboxybenzene] triazene

“…[13] To mimic the behavior of this natural process, people focus their works on the design and development of catalysts based on transition metal complexes for electrochemical driven hydrogen production via water or proton reduction, and several transition metal compounds have been developed as electro-catalysts for H 2 evolution. [14][15][16][17] Based on that the deprotonation makes the triazenido species, […”