Probing the electronic structure of N-heterocyclic carbenes through gas-phase photoelectron spectroscopy and density functional calculations.
The development of stable N-heterocyclic carbenes (NHC) has brought about a renaissance in organometallic catalysis. In the Grubb's Cl2(PCy3)2Ru=X2 type ruthenium catalysts, NHC ligands have effectively replaced the more commonly used PR3 ligands, and afford an increased yield in ring opening metathesis and transfer hydrogenation reactions. Photoelectron spectroscopy and d. functional calcns. are used to elucidate the electronic structure of these transition-metal carbene catalysts, as well as the free NHC ligands (NHC=H2IMes, Pri2Me2Imid, IMes). Direct comparisons of the Mo(CO)5NHC photoelectron spectra to the spectra of the Mo(CO)5PR3 analog affords a clearer understanding of the bonding between the NHC ligand and the transition metal. We have found that the NHC ligands are stronger σ donors than the phosphine analogs, while lacking significant π-type interactions. Theor. modeling also indicates the lack of low lying π-type orbitals with correct symmetry available for π-back-bonding to the metal center . [on SciFinder(R)]