Multi‐Tier Electronic Structure Analysis of Sita's Mo and W Complexes Capable of Thermal or Photochemical N2Splitting

Severine Rupp,Felix Plasser,Vera Krewald

doi:10.1002/ejic.201901304

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 N2photoactivation 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.

Highlights

Computational chemists are often tasked with the analysis of electronic transitions in UV/Vis spectra computed with timedependent density functional theory (TD-DFT) or other quantum chemistry methods
In previous work we found that the thermodynamics of the isomerization path from the linear [M(μ-η1:η1-N2)M] core to the diamond-shaped [M(μ-N)2M] core is less favorable for the bulkier system.[16b]. At the same time, dispersion interactions were found to stabilize the bis-μ-N molybdenum dimer product of the thermal path to such an extent that without them the reaction would be endergonic.[16b]. It is worth noting that for the product of the photochemical path it was not even possible to find a stable structure when dispersion corrections were not included
Previous work showed that the structures of the two molybdenum complexes that are capable of thermal and photochemical N2 splitting, Motherm and Mophoto, are nearly identical.[16b]. Overall, the tungsten complexes have similar structural characteristics

Summary

Introduction

Computational chemists are often tasked with the analysis of electronic transitions in UV/Vis spectra computed with timedependent density functional theory (TD-DFT) or other quantum chemistry methods. For a more objective interpretation of transitions, the TheoDORE program makes use of the one-electron transition density matrix.[1] The user chooses a fragmentation pattern for the molecule, e.g. based on functional groups or ligands, and the character of the excitations is interpreted in terms of Frenkel excitonic states or charge separated states, both of which can be localized or delocalized with respect to the chosen fragments In this contribution, we use these three tiers of electronic structure analysis, i.e. MO analysis, difference density analysis, and TheoDORE analysis, to obtain a better understanding of molybdenum and tungsten complexes with linear [M(μ-η1:η1-N2)M] cores. We identify electronic structure characteristics associated with either photochemical or thermochemical reactivity of these complexes

Results

Discussion

Conclusion