Computational Studies on the Mechanism of Alkane Dehydrogenation on 2D layered TiS2 and the Electronic Structure of a Manganese Dimer

Abstract

Firstly, the possibility of dehydrogenating linear and cyclic alkanes catalyzed by 2D layered Transition Metal Dichalcogenide, 1T-TiS2, was investigated with Density Functional Theory. The dehydrogenation mechanisms for cyclohexane and n-butane as representative reactants, were explored. The most likely reaction mechanism for C–H was found to occur on an edge-based S–S couple for both C–H activation steps. The first C–H activation reaction exhibited a reaction energy of –14.2, –12.7 and –12.0 kcal/mol for cyclohexane and n-butane addition, respectively. In contrast, the second C–H activation on the neighboring C-atom, was found to be comparatively unfavorable, resulting in an increase in energy to 19.6, 22.5 and 19.0 kcal/mol for formation of cyclohexene, 1- butene and 2-butene, respectively. Since there have been no published studies on the dehydrogenation of linear and cyclic alkanes on TiS2 thus far, this study may provide a guideline to further computational or experimental investigations. Secondly, the electronic structure of a dimeric manganese hydride catalyst supported by β- diketiminate ligands, [(2,6-iPr2PhBDI)Mn(μ-H)]2, was investigated with Density Functional Theory. An apparent triple bond between two manganese centers was anticipated to compensate for the electron-deficient nature of each metal center. However, our calculations interestingly revealed the absence of a multiple bond between the metal centers. In accordance with experimentally determined Heisenberg exchange coupling constants of –10.2 cm–1, the calculated Jo value of –10.9 cm–1 confirmed that the ground state involves antiferromagnetic coupling between high spin d5 Mn(II) centers. The effect of steric bulk on the bond order was interrogated via a model study with the least sterically demanding version of the β-diketiminate ligand and was found to be negligible. Mixing between metal- and ligand-based orbitals was alternatively designated as the main cause of the absence of a metal–metal multiple bond. Moreover, the specificity of hydrides providing only s- orbitals affords a relatively close positioning of the metal centers, while bridging ligands including p- orbitals perturb a bonding orbital between metal centers, lengthening the Mn–Mn distance. The proximity of the metal centers in [(2,6-iPr2PhBDI)Mn(μ-H)]2 leads to an increase in Pauli repulsion, resulting in destabilization of the dimer. The accessibility of the monomeric species may be the origin of the catalytic activity that [(2,6-iPr2PhBDI)Mn(μ-H)]2 exhibits.