Scrutinizing the substituent effect on Mo-based electrocatalysts for molecular hydrogen release through axial-equatorial decomposition: a DFT study

Based on the experimental precedent discovered by Kuranadasa and coworkers [H. I. Karunadasa et al., Nature, 2010, 464, 1329] to produce dihydrogen from water electrocatalyzed by 2,6-bis[1,1-bis(2-pyridyl)ethyl]-pyridine oxo-molybdenum complexes, we performed an extensive analysis to study the substituent group effect of derivatised compounds coming from the before mentioned Mo-based metal-organic cations in terms of two kinds of substitutions: axial and equatorial at the para-position of pyridine rings; several conceptual tools were used to back up our conclusions. We found that each type of substituent group (electron-withdrawing and electron-donating ones) exerts an independent influence on energetic parameters (energy barrier and overall energy). This opens the chance to search for a synergistic effect by combining these opposite behaviours of these substituents located in the equatorial and axial para-positions of pyridine rings to computationally modulate the aforementioned energetic parameters. This procedure will make easier the proposal of new catalysts to favour either kinetically or thermodynamically or in both ways the production of dihydrogen from water. Additionally, we encompassed a key point: the number of solvent molecules, so that including their presence in further investigations is mandatory.