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

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 = 1/2, 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 rho(r) at the bond critical point (bcp) between Ru and the carbene carbon atoms. The energetic contribution of sigma-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 sigma-donor. Among the studied Ru-NHC complexes, the POM derivative 1 shows the largest pi-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.