Role of Anation on the Mechanism of Proton Reduction Involving a Pentapyridine Cobalt Complex: A Theoretical Study

Kinetic and thermodynamic aspects of proton reduction involving pentapyridine cobalt­(II) complex were investigated with the help of quantum chemical calculations. Free energy profile of all possible mechanistic routes for proton reduction was constructed with the consideration of both anation and solvent bound pathways. The computed free energy profile shows that acetate ion plays a significant role in modulating the kinetic aspects of Co­(III)–hydride formation which is found to be the key intermediate for proton reduction. Upon replacing solvent by acetate ion, one electron reduction and protonation of Co^I species become more rapid along with slow displacement reaction. Most favorable pathways for hydrogen evolution from Co­(III)–hydride species is also investigated. Among the four possible pathways, reduction followed by protonation of Co­(III)–hydride (RPP) is found to be the most feasible pathway. On the basis of QTAIM and NBO analyses, the electronic origin of most favorable pathway is explained. The basicity of cobalt center along with thermodynamic stability of putative Co^III/II–H species is essentially a prime factor in deciding the most favorable pathway for hydrogen evolution. Our computed results are in good agreement with experimental observations and also provided adequate information to design cobalt-based molecular electrocatalysts for proton reduction in future

Quantum mechanical study on complexation phenomenon of pillar[5]arene towards neutral dicyanobutane

Based on density functional theory calculations, we have addressed the electronic structure, binding and nature of non-covalent interactions between alkylated pillar[5]arene (P[5]A) and 1,4-dicyanobutane (DCB)-based host-guest macrocycles. Neutral 1,4-dicyanobutane-based alkylated DCB_ProP[5]A is found to show higher binding energy when compared to the other three host-guest macrocycles. These complexes are largely stabilised by non-covalent interactions, which are ascertained through NCI and QTAIM analyses. Furthermore, the second-order perturbation energy of NBO analysis show that LP (N) – σ*(C-H) interactions predominate in DCB_ProP[5]A complex. Particularly, alkyl substituents (-methyl, -ethyl and -propyl) are playing a vital role in stabilising the host-guest complexes. In summary, the present work not only exhibits an efficient strategy to build a new family of alkylated P[5]A inclusion complexes but also providing deeper understanding on various non-covalent interactions towards 1,4-dicyanobutane (DCB) guest molecules inside the host environment.</p