Nitrogen makes up 78% of the Earth’s atmosphere. Ammonia derived from N2 and fossil H2 in the Haber-Bosch process represents the main source of nitrogen compounds used in fertilizers as well as a base chemical for the synthesis of value-added products. In order to access a more diverse range of N-containing chemicals without relying on NH3 as an intermediate, it is of high interest to develop tailored nitrogen-activating catalysts.
The complete splitting of the dinitrogen triple bond can be achieved thermally, photochemically, or electrochemically. While a few transition metal complexes are capable of photochemical dinitrogen activation, the mechanistic understanding of the underlying processes is still in its early stages. All known complexes for N2 photoactivation have a linear core of the form M-N-N-M which undergoes geometric and electronic changes during the light excitation process, however the nature of the responsible excited state is presently ill-defined.
Here we will present our multi-tier approach to study the complex electronic structure of two types pf nitrogen photoactivation complex: The catalyst design based on pincer ligands of the Schneider group[2,3] and Sita’s series of homologous transition metal complexes[4-6]. We analyse the structures of two of Sita's molybdenum[4-6,7] and tungsten[4-6] complexes and their electronic spectra in terms of the molecular orbitals, difference densities and the charge-transfer numbers provided by the wavefunction analysis program TheoDORE[8,9]. We study in particular the charge transfer character of the individual excitations and find that the transitions of photochemically active complexes have more charge-transfer character and higher intensity. While density functional theory (DFT) has proven to be a powerful tool for first insights into the differences between photochemically and thermally active N2-splitting complexes, in some cases the complexity of the electronic structure requires the use of multiconfigurational methods. For the rhenium complex of Schneider and coworkers we present a first multireference electronic structure analysis with a (16,16) active space based on density matrix renormalisation group (DMRG) that permits an appropriate treatment of spin-orbit coupling effects.
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