Electronic structure, bonding and reactivity of zintl clusters

Thesis

This thesis presents two aspects of computational studies on the endohedral transition metal-based Zintl clusters, polyanionic clusters formed by p-block metal/metalloid in combination with alkali or alkaline earth metals. The first focuses mainly on the fundamental questions of electronic structure, metal-metal bonding and geometry in Sn-containing Zintl ions. We consider how the interactions between the Sn and the transition metal vary as a function of metal and also of composition of the cage, and probe the interplay of metal-metal, metal-Sn and Sn-Sn bonding. The late transition metals typically have relatively inert d¹⁰ cores, and interactions with the cage are minimal. However, as the d orbitals become higher in energy in the middle of the periodic table, back-bonding causes a weakening and then, ultimately, cleavage of some of the Sn-Sn bonds in the cluster. All the work reported in this first aspect has been done in collaboration with experimental colleagues from Professor Zhong-Ming Sun’s group at Nankai University in China.

The second component of the thesis is a study of the potential role of these endohedral clusters in catalysis. The [Ru@Sn₉]^6– cluster has been shown to be an active catalyst for the reverse water-gas shift (rWGS) reaction when supported on CeO₂, but details of the exact structure of the supported species, or the mechanism, remain uncertain. The cluster appears not to survive intact on absorption - instead, it forms a RuSn9 monolayer, maximising the number of Sn-O and Ru-O bonds. The junction between the Ru and Sn atoms in the monolayer proves to be highly oxophilic, and oxide ions in these bridging sites can be transferred to the bound CO in a low-barrier step, leading to effective catalysis. It seems, therefore, that the composition of the cluster is more important than its geometry in the context of catalysis.

Authors: Mondal, S

• DOI: 10.5287/ora-az62pa1xq
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English