Pincer ligands combined with the right metals: towards green chemistry

Abstract

The research of pincer ligands of PCP type has increased during the last years. These ligands can form complexes with metals and participate in the catalytic acceptorless dehydrogenation reactions through metal-ligand cooperation (MLC). This type of reactions are not dangerous for the environment and uses a catalyst to carry them out, according to the principles of green chemistry. In this work we have studied pyridine-based PCP-Ruthenium complexes. One of them complexes is capable of catalysing dehydrogenative reactions of alcohols without an agent acceptor, through the dearomatization / aromatization of the complex by MLC. Its structure, stability, through the relative energy of Gibbs, and aromaticity of each complex have been studied using calculations within the framework of density functional theory (DFT). It has been established, from calculations of relative energy of Gibbs, which complex is more stable and less favoured thermodynamically as a function of the proposed substituents of the phosphorus atoms of its structure. In addition, changes have been discovered in the Lewis structure proposed by Milstein and collaborators of one of the complexes, through the results of Mayer Bond Orders (MBO). And, structurally the steric maps have confirmed the changes that the substituents produce on the coordination sphere around the metal. The aromaticity of the complexes has also been checked through different aromaticity indexes based on the electronic, magnetic and geometric structure, and it has been seen that the NICS, HOMA and FLU indices give better results than the rest. No differences in aromaticity have been observed with respect to the substituents of the phosphorus atoms, but a reduction in the aromaticity between complexes has generally been seen, and one of the complexes has been established as non-aromatic. Finally, the metal used experimentally by Milstein and collaborators has been replaced by iron and osmium to study also the structure, stability and aromaticity of this. It has been seen that the complex is more stable, the strength of bond increases, and it is more aromatic with iron in its structure