Ligand Substituents Govern the Efficiency and Mechanistic Path of Hydrogen Production with [Cp*Rh] Catalysts

Abstract

We demonstrate that [Cp*Rh] complexes bearing substituted 2,2'-bipyridyl ligands are effective hydrogen evolution catalysts (Cp*=η -pentamethylcyclopentadienyl). Disubstitution (at the 4 and 4' positions) of the bipyridyl ligand (namely -tBu, -H, and -CF ) modulates the catalytic overpotential, in part due to involvement of the reduced ligand character in formally rhodium(I) intermediates. These reduced species are synthesized and isolated here; protonation results in formation of complexes bearing the unusual η -pentamethylcyclopentadiene ligand, and the properties of these protonated intermediates further govern the catalytic performance. Electrochemical studies suggest that multiple mechanistic pathways are accessible, and that the operative pathway depends on the applied potential and solution conditions. Taken together, these results suggest synergy in metal-ligand cooperation that modulates the mechanisms of fuel-forming catalysis with organometallic compounds bearing multiple non-innocent ligands.

Computational Insights on the Electrocatalytic Behavior of [Cp*Rh] Molecular Catalysts Immobilized on Graphene for Heterogeneous Hydrogen Evolution Reaction

“…These DFT results indicate that the electrochemical molecular properties of the species considered in this study are slightly dependent on the types of the diamine ligands, namely phen and bpy. This, in turn in good agreement with the recently reported results for such effects 22 www.nature.com/scientificreports www.nature.com/scientificreports/ with −1.21 V for the unsubstituted one. However, notable decrease in the E red o to −0.97 V was recorded for the -CF 3 substituents at the same positions of substitution.…”

“…It must be mentioned that performing such evaluating protocols requires initially performing accurate DFT-based geometry optimizations for all species that are involved in the electrochemical reduction and catalytic cycle of interest. According to previously reported experimental results concerning the redox behavior of complexes of [Rh(Cp*) (L)Cl] + (L= bpy or phen), such complexes commonly undergo a metal-centered two-electron reduction (Rh III/I ) at approximately −1.19 V vs Fc +/0 ,with simultaneous dissociation of chloride ion (Cl − ) 18,21,22 ; whereas the heterogenized analogue undergoes a nonmetal-centered one-electron reduction at −1.29 V vs Fc +/0 with alike dissociation of Cl 18 . As such, we optimized the geometries of these species that are involved in the electrochemical reduction using DFT calculations.…”

“…This can be attributed to various reasons that could be either computational or even experimental such as scanning rate and type of electrodes and concentration of the supporting electrolytes. Nevertheless, the majority of halide-bound diimine complexes if [Cp*Rh III ] can generally undergo two-electron electrochemical reduction that is observed experimentally as a single process 18,21,22,36,41 . Further attempts were conducted to reduce the discrepancy between the calculated and the experimental value of the 1-electron reduction process.…”

“…However, notable decrease in the E red o to −0.97 V was recorded for the -CF 3 substituents at the same positions of substitution. Nevertheless, it is highly anticipated that extending the range of such substituents may help in tuning the n-electron reduction (n:1 ↔ 2) of the heterogenized analogue of the molecular electrocatalysts [22][23][24] .…”

“…In their work, they demonstrated experimentally that the homogeneous molecular catalysts [Rh III (cp*)(bpy)Cl] 1+ can exhibit different redox behavior compared to the heterogenized analogue in acetonitrile as they undergo two-electron metal-centered and one-electron non-metal-centered reduction pathways, respectively. However, although many studies have been reported since the early work of Grätzel and Kölle concerning the catalytic properties of the homogeneous analogues of Rh complexes [19][20][21][22][23][24][25][26] , no results have been yet reported concerning the assessments of the catalytic behaviors of the heterogenized molecular catalysts for the HER.…”