Density Functional Theory Prediction of the Electrocatalytic Mechanism of Proton Reduction by a Dicobalt Tetrakis(Schiff Base) Macrocycle

Abstract

A dicobalt tetrakis(Schiff base) macrocycle has recently been reported to electrochemically catalyze the reduction of H + to H 2 in an acetonitrile solution. Density functional theory (DFT) calculations using the ωB97X-D functional are shown to produce structural and thermodynamic results in good agreement with the experimental data. A mechanistic model based on thermodynamics is developed that incorporates electrochemical and magnetic details of the complex, accounting for electron-spin reorganization of the metal center after redox steps. The model is validated through a comparison of the predicted electrochemical potentials with the irreversible cyclic voltammogram of [Co 2 LAc] + , which shows redox-coupled spin-crossover (RCSCO) behavior for the Co II/III transitions. Using our model, we predict the thermodynamically favored mechanism of H 2 evolution by [Co 2 L] 2+ to be one of heterolytic proton attack on a [Co II 2 L(μ-H)] + species. Understanding the electronic details and thermodynamically preferred mechanism of this catalyst will aid in improving its efficiency and the future design of bimetallic Co-based H + electrocatalysts. Also, this work will assist in the future DFT modeling of bimetallic RCSCO complexes.

Distinct Bimetallic Cooperativity Among Water Reduction Catalysts Containing [CoIIICoIII], [NiIINiII], and [ZnIIZnII] Cores

“…The calculated potential is in good agreement with the experimental value À 0.12 V Fc/Fc + (0.28 V Ag/AgCl ). Species B contains a high-spin 3d 7 Co II and a low spin Co III leading to local asymmetry imposed by a longer Co II -pyrazole bond. Further reduction of B by one electron at a calculated potential of À 0.67 V Fc/Fc + , yielding species C, in reasonable agreement (ΔE < 20 mV) with the experimental value of À 0.49 V Fc/Fc + (À 0.09 V Ag/AgCl ).…”

“…Further reduction of B by one electron at a calculated potential of À 0.67 V Fc/Fc + , yielding species C, in reasonable agreement (ΔE < 20 mV) with the experimental value of À 0.49 V Fc/Fc + (À 0.09 V Ag/AgCl ). Species C contains two high spin 3d 7 Co II centers that are considerably more labile than their initial LS 3d 6 counterparts. As previously discussed, the distance of the two Co centers in 1 was measured by X-ray crystallography at 3.183 Å, thus shorter than in other similar Co 2 species [21] and in the NiÀ Ni and ZnÀ Zn distance observed for 2 and 3.…”

“…As previously discussed, the distance of the two Co centers in 1 was measured by X-ray crystallography at 3.183 Å, thus shorter than in other similar Co 2 species [21] and in the NiÀ Ni and ZnÀ Zn distance observed for 2 and 3. In order to release the tension of the core one Co II -pyrazole bond breaks off with an energy of + 9.1 kcal/mol and triggers an electronic redistribution in which the resulting five coordinate Co II remains HS 3d 7 with S = 3/2, while the other Co II responds to the stronger six-coordination becoming a low-spin 3d 7 ion with S = 1/2. This isolated step is slightly unfavorable and makes sense from a global perspective of enabling other favorable steps towards H 2 release.…”

“…The first possible PCET requires À 1.40 V Fc/Fc + and involves the reduction of one pyridine ring forming a reactive pyridinium radical in species K, likely associated with demetallation and decomposition observed by SEM. The second possible process involves a 2e À :1H + PCET at À 1.55 V Fc/Fc + along with chloride loss to form the 3d 7 Ni II -H hydride species I that favorably reacts with H + (À 35 kcal/mol) to form species J with 5-coordinate and 6-coordinate Ni II centers upon elimination of H 2 . Upon PCET at À 1.33 V Fc/Fc + J yields species H that contains a 3d 6 Ni III -H hydride that can be converted to I, continuing the catalytic cycle.…”

“…for a binuclear Co II catalyst based on a [N 6 O 2 ] Robson-type macrocycle [6] with a CoÀ Co distance of 3.22 Å. They recently offered a mechanism [7] that involved the reduction and protonation of the [Co II Co II ] core yielding a [Co II Co III -H] hydride, which rearranges into a bridged [Co II (μ-H)Co II ] hydride core with a CoÀ Co distance of 3.08 Å. Similarly Qu et al [8] isolated and characterized a binuclear species supported by benzene-1,2dithiolate with a [Co(μ-H)Co] core and Co•••Co distance of 2.41 Å.…”