Development and understanding of cobaloxime activity through electrochemical molecular catalyst screening

Abstract

Electrochemical molecular catalyst screening (EMoCS) has been developed. This technique allows fast analysis and identification of homogeneous catalytic species through tandem catalyst assembly and electrochemistry. EMoCS has been used to study molecular proton reduction catalysts made from earth abundant materials to improve their viability for water splitting systems. The efficacy of EMoCS is proven through investigation of cobaloxime proton reduction activity with respect to the axial ligand in aqueous solution. Over 20 axial ligands were analysed, allowing rapid identification of the most active catalysts. Structure-activity relationships showed that more electron donating pyridine ligands result in enhanced catalytic currents due to the formation of a more basic Co-H species. The EMoCS results were validated by isolating and assaying the most electroactive cobaloximes identified during screening. The most active catalyst, [Co(III)Cl(dimethyl glyoximato)2(4-methoxypyridine)], showed high electro- and photoactivity in both anaerobic and aerobic conditions in pH neutral aqueous solution.

Computational, electrochemical, and spectroscopic studies of two mononuclear cobaloximes: the influence of an axial pyridine and solvent on the redox behaviour and evidence for pyridine coordination to cobalt(i) and cobalt(ii) metal centres

“…S26 and 27 † ). The inability of these sterically hindered substituted pyridines to coordinate was also observed for the [Co(dmgH) 2 (H 2 O) 2 ] system, 39 and indicated that the change(s) observed in the cyclic voltammogram of complex 1 in the presence of pyridine is not the result of Brønsted acid–base behaviour. On the other hand, the addition of 4-aminopyridine resulted in similar changes to what was observed with the addition of pyridine and further validates the use of cyclic voltammetry to assess the coordination of pyridine.…”

“…1), however, cobaloximes still continue to generate a lot of interest due to their high activity towards electrocatalytic proton reduction. 15,19,37–39 …”

“…59,60 Cobaloximes of the type [Co(dmgH) 2 (L)Cl] (where L = a nitrogen-containing Lewis base such as pyridine and its derivatives) have been studied to explore the influence of the axial ligand on the electroactivity of the cobaloxime. 39,59 However, in the case of the BF 2 capped system [Co(dmgBF 2 ) (solv) 2 ] (where solv = a solvent molecule such as MeOH, H 2 O, etc . ), the lability of the axial position of the Co(II) metal centre has resulted in an elusive species with pyridine in the axial position, and as such, this species has been primarily studied as a species generated in situ .…”

A Poly(cobaloxime)/Carbon Nanotube Electrode: Freestanding Buckypaper with Polymer‐Enhanced H2‐Evolution Performance

“…[10][11][12][13] Thea dvantages of cobaloximes are their facile synthesis and the tunability of their catalytic properties by modifying the substituents on the equatorial and/or axial ligands. [13][14][15][16] Moreover,t hey exhibit high proton-reduction activity at ar elatively low overpotential [17] and can operate in aqueous solutions [18,19] even under aerobic conditions. [20,21] Recently,v ersatile strategies were reported for their immobilization onto metal-oxide [13,22] and carbon-based [23][24][25] surfaces for applications in electro-and photoelectrocatalytic H 2 production.…”

“…TheG C/ MWCNT-PyCo electrodes displayed areversible redox wave at E 1/2 = 0.24 Vv ersus the standard hydrogen electrode (SHE), whereas ar eversible process was observed at E 1/2 = 0.08 Vv ersus SHE with GC/MWCNT-pPyCo.T hese redox processes are attributed to the cobaloximes reversible Co III / Co II redox couple. [16] Thes mall shift in redox potential between the monomer and polymer is presumably due to achange in the chemical environment surrounding the cobalt center upon incorporation into ap olymer chain. Thes mall redox waves observed in the case of GC/MWCNT-PyCo at E red = 0.13 Vand E ox = 0.08 Vm ay be the result of different adsorption interactions of the PyCo generating varying chemical environments on the GC/MWCNT surface.I n contrast, the single redox process observed with GC/ MWCNT-pPyCo results most likely from amore homogenous environment around the cobalt catalysts,p rovided by the polymeric matrix.…”