Ammonia and borane activation by Tantalum Carbide cluster anion Ta2C4−: A theoretical approach

Abstract

Activation of C–H, B–H as well N–H sigma bonds has been a subject of fundamental interest due to potential feedstock in chemical industry. The co-operative effect of the metal centers in a di-nuclear carbide cluster Ta2C4− for methane C–H activation and dissociation has recently been revealed, in which a molecule of hydrogen is evolved. Based on that, we have explored reaction pathways for ‘E−H’ sigma bond activation and dissociation processes of gaseous Ammonia (E: N) and Borane (E: B) using the stable form of the aforesaid complex at DFT levels. The geometries and energetics associated with the reactions are found to be method insensitive. The course of the reaction is initiated by a 1:1 precursor complex formed between the cluster and N (/B)H3. This complex formation is found to be more exothermic than that of its methane counterpart. In case of BH3, considerable lengthening of two B–H bonds in the first step is observed which implies that the two B–H bonds are activated simultaneously under the influence of Ta2C4−. But for NH3, N–H bond lengthening as well as activation is observed to be insignificant in this precursor complex. The chemical nature of the participating hydrogen atoms is inspected by NPA analysis (‘protic’ type in NH3 and ‘hydride’ type in BH3).The overall dehydrogenation procedures for both the molecules are found to be multi-step with high exothermicity. Highly negative net energy change in terms of Gibbs Free Energy (−31.03 kcal/mol for ammonia and −31.36 kcal/mol for borane) as well as Enthalpy (−33.68 kcal/mol for ammonia and −34.13 kcal/mol for borane), in forming a H2 molecule with stable Ta2C4N(/B)H− complex, with respect to the reactant pair govern the thermodynamic feasibility of the overall process. In a nutshell, this computational study provides a detail understanding of the activation and dissociation processes of the concerned gaseous molecules, which will be beneficial for further experimental studies.