The density polarization reveals directions of electron displacements due to the substituent effect: Analysis performed on a metal-organic Mo-Oxo catalyst

Some Mo-oxo complexes bearing pyridine rings have the capability for dihydrogen production from water. However, energy barrier and overall energy vary depending on the effect exerted by several substituent groups located at different positions around one or more pyridine rings which are ligands of these compounds. Based on the Karunadasa and coworkers investigation where the para-position was experimentally tested in compounds derivatised from the 2,6-bis[1,1-bis(2-pyridil)ethyl]-pyridine oxo-molybdenum complex synthesized (Karunadasa et al., Nature, 2010, 464, 1329), we tested the combined effect of electron-withdrawing and electron-donating groups simulated as perturbations represented by point-charges. Then, we used the density polarization concept, δρ(r), a local reactivity descriptor corresponding to the partially integrated linear response function, χ(r, r′) (a non-local reactivity descriptor), which is able to reveal different displacements of π-electrons on molecular structures. We perturbed the para-positions in the pentadentate ligand 2,6-bis[1,1-bis(2-pyridil)ethyl]-pyridine in the Mo-based complex by means of point-charges. They were located in three different configurations of the organic ligand (trans, geminal, and cis) which could help to explain energy barriers and overall energy of reactions catalyzed by this type of Mo-complexes. Our results indicate that the trans configuration of point-charges induces the most amount of fraction of electron shifted on the complex. A Mo-based complex bearing the same trans configuration for electron-withdrawing and electron-donating substituent groups (cyano and amino, respectively), leads to a kinetically more favorable H₂ release than the cis or geminal configuration of the substituent groups aforementioned.