Computational investigation of the reaction of nickel-bis(dithiolene) and nickel-bis(diselenolene) complexes with OH

In the present study, the reactivity of OH with Ni(X2C2H2)2 and Ni(X2C2H2)2 – (where X = S or Se) was investigated. From the thermodynamics, it found that the OH radical attacks a backbone C atom of the Ni(S2C2H2)2 complex. For the Ni(Se2C2H2)2 complex, the OH is predicted to target the ligating chalcogen atom. The significance of this is that with the attack of OH to a backbone C atom, the thermodynamic cost to lose a proton or hydrogen atom ranges from exergonic to marginally endergonic depending on the oxidation state of the complex. Notably, such a process results in a rearrangement of the complex, likely leading to deactivation of the catalyst. Where OH has attacked a ligating chalcogenide atom, the thermodynamic cost to lose a proton or hydrogen is endergonic regardless of oxidation state of the complex. Where OH attacks a coordinating chalcogenide atom, the thermodynamics for the addition of a proton was considered. At the present level of theory, it was found that for the dithiolene and diselenolene monoanionic complexes, the addition of a proton is marginally endergonic. However, following protonation, the loss of water is significantly exergonic and results in the regeneration of the neutral non-oxidized Ni complex. Given the greater tendency for OH to attack Se versus S, it may be speculated that the use of diselenolene ligands may offer a means to protect the Ni complex from damaging OH radicals due to the thermodynamic tendency for OH to attack Se atom of the diselenolene complexes not seen in the dithiolene complexes.

Computational Investigation into the Ni(SeNHC2(CN)2)2 and Ni(SNHC2(CN)2)2 Complexes as Potential Catalysts for Hydrogen Production

To reduce our carbon footprint, we must look at alternative non-carbon-containing fuels to prevent continued global climate change. One environmentally friendly alternative fuel is molecular hydrogen. Herein the Ni(SeNHC₂(CN)₂)₂ complex was studied using DFT to determine the thermodynamics associated with the electrocatalytic formation of H₂(g). From the calculated thermodynamics, it appears that the Ni(SeNHC₂(CN)₂)₂ complex is predicted to catalyze the production of H₂ gas under mildly reducing conditions relative to the SHE. Notably, the thermodynamics are better than the values calculated for the analogous Ni(SNHC₂(CN)₂)₂ complex which has been shown experimentally to catalyze the formation of H₂ gas in aqueous solution. Regarding possible kinetic reactivity, the HOMO-LUMO gap energies were calculated. From the gap energies, it is expected that the Se-containing compounds would be more reactive to electron transfer in the third reduction step, meaning therefore that a smaller overpotential would be needed to drive the reduction of Red2-H₂ relative to ^SRed2-H₂ in agreement with past experimental work. Thus, the use of Se in such compounds may offer a means to improve the catalysts for H₂ production.

A computational investigation into nickel-bis(diselenolene) complexes as potential catalysts for reduction of H+ to H2

As a result of burning fossil fuels, levels of greenhouse gases in our atmosphere are increasing at an alarming rate. Such an increase in greenhouse gases threatens our planet due to global climate change. To reduce the production of greenhouse gases, we must switch from fossil fuels to alternative fuels for energy. The most viable alternative energy source involves the conversion of solar energy into chemical energy via the photocatalytic splitting of water to form molecular hydrogen. In the present work, the Ni-bis(1,2-diamine-diselenolene) and Ni-bis(1,2-dicyano-diselenolene) complexes were studied using density functional theory (DFT). From the results, it was found that the 1,2-diamine-diselenolene and 1,2-dicyano-diselenolene nickel complexes catalyze the formation of H2(g) with overall reaction Gibbs energies of +8.7 kJ mol–1 and +8.4 kJ mol–1, respectively, in a dilute aqueous environment versus the standard hydrogen electrode (SHE). Although both are able to catalyze the HER through a marginally endergonic reaction, the most thermodynamically favourable pathways differed between the complexes. In particular, the most thermodynamically favourable pathway for the formation of H2 by CNOx involves an EECC mechanism, whereas for NH2Ox, the most thermodynamically favourable pathway occurs via an ECCE mechanism. From the results presented, the choice of substituent on the alkene backbone significantly affects the reduction potential and reaction Gibbs energies of protonation. The considerably more positive reduction potential for the CN complexes may offer a solution to the problems experimentally observed for the production of H2.