Theoretical characterization of a Ru N-Heterocyclic Carbene derivative of a polyoxometalate. Enhanced π-interaction in oxide supported TM-organic linkages

Abstract

The electronic structure of the N-heterocyclic carbene (NHC) functionalized polyoxometalate [K(PW₉O₃₄)₂(cis-WO₂)(cis-RuLMe₂)]¹²⁻ (1) is studied computationally; and its Ru-NHC bonding situation is compared with other monometallic Ru-NHC complexes, as well as other representative carbene complexes. The DFT analysis further supports the low spin configuration, S = ½, as the ground state, as the unoccupied d ruthenium orbitals are high in energy and mixed with the d-type orbitals of W. We gauge the TM-carbene bonding nature by means of atoms in molecules (AIM) quantum theory and a modified version of energy decomposition analysis (EDA) based on orbital deletion. According to the calculations, the NCH ligand in POM derivative 1 is more tightly bonded than in its analogous neutral monometallic Ru-NHC complexes. This is manifested in a higher electron density ρ(r) at the bond critical point (bcp) between Ru and the carbene carbon atoms. The energetic contribution of σ-donation is similar for all Ru-NHC complexes, and significantly higher than for Fischer-type carbene complex, in agreement with the description of the N-heterocyclic carbene ligand as a strong σ-donor. Among the studied Ru-NHC complexes, the POM derivative 1 shows the largest π-back-donation ability, explaining the observed short Ru-C_carbene bond distance and the calculated high interaction energy. The POM framework acting as ligand makes the ruthenium d electrons more loose via orbital mixing and promotes the back-donation to the ligand.