The search for molecular catalysts that efficiently activate or cleave the dinitrogen molecule is an active field of research. While many thermal dinitrogen cleavage catalysts are known, the photochemical activation of N2 has received considerably less attention. In this paper, the first computational study of the osmium dimer [Os(II,III)2 (μ-N2 )(NH3 )10 ]5+ , which was shown to be capable of dinitrogen photocleavage, is presented. Despite its deceptively simple geometry, it has a complex electronic structure with a valence-delocalized and electronically degenerate ground state. Using multiconfigurational methods, the electronic structure at the ground state geometry and along the dinitrogen cleavage coordinate was investigated. The results indicate that an unoccupied molecular orbital with σ-bonding character between osmium and μ-N atoms and σ-antibonding dinitrogen character is most affected by N-N distance elongation. This implies that a lower barrier for thermal or photochemical N2 activation in linear M-N-N-M complexes can be achieved by lowering the energetic separation between this unoccupied orbital and the HOMO, representing a specific target for future catalyst design.
Keywords: ab initio calculations; mixed-valent compounds; nitrogen fixation; osmium; transition metals.
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