On the electronic structure of second generation Hoveyda–Grubbs alkene metathesis precursors

Hoveyda–Grubbs complexes are commonly used catalysts in alkene metathesis. They are precursors of the catalytic species and need to be activated before entering into the real catalytic cycle. This activation can determine in a large extent the performance of the catalyst and thus, it is important to understand the factors that influence their activation and stability. In the present work, the electronic structure of 15 different Hoveyda–Grubbs precatalysts is analyzed by means of DFT quantum chemical calculations. Electronic delocalization measures and aromaticity indices have been used to study the influence of the Hoveyda ligand substituents on the Ru⋯O interaction and the metal carbene bond, and to analyze whether or not the 5-membered metal containing ring exhibits metalloaromaticity. Results show that these complexes do not exhibit any metalloaromaticity, although delocalization measures indicate that there is a certain π electron delocalization on the Hoveyda ligand. On the other hand, present study shows that the Ru⋯O interaction is mainly electrostatic and that the influence of the substituents not only depends on the changes occurred at the O atom but also on the metal site. Substituents para to the alkoxy group (meta to the carbene) mainly tune the electronic structure of the chelating ligand in such a way that electron donor substituents strengthen the Ru⋯O interaction whereas electron withdrawing ones induce the reverse effect. However, substituents in para to the carbene (meta to the alkoxy) mainly tune the electronic structure of the ruthenium carbene bond modifying the Ru δ - C ene δ + bond polarization and producing the opposite effect.