Multi-Tier Electronic Structure Analysis of Sita's Mo and W Complexes Capable of Thermal or Photochemical N2 Splitting

Abstract

An emerging approach for the activation of the nitrogen molecule is the light-driven splitting of the N–N bond. Less than ten examples for complexes capable of N2 photoactivation are currently known, and the underlying photophysical and photochemical processes after light absorption are largely unresolved. All complexes have a central [M(µ-η1:η1-N2)M] unit with equivalent ligand spheres around each metal. For several of these complexes, small modifications of the ligand sphere result in thermal rather than photochemical activity. Herein, we analyse the electronic structures and computed UV/Vis spectra of four complexes: two thermally and two photochemically active complexes, each either involving molybdenum or tungsten. The analysis of electronic structures and spectra is based on the molecular orbitals, difference densities and the charge-transfer numbers provided by TheoDORE. We find that the spectra of the photochemically active complexes contain excitations with more ligand-to-metal charge-transfer character and higher intensity, providing a plausible explanation for light-induced nitrogen splitting.