Exploring the Reactivity of Ru-Based Metathesis Catalysts with a π-Acid Ligand Trans to the Ru−Ylidene Bond

Abstract

In this work we explore the reactivity induced by coordination of a CO molecule trans to the Ru-ylidene bond of a prototype Ru-olefin metathesis catalyst bearing the N-heterocyclic carbene (NHC) ligand SIMes. Static DFT calculations indicate that CO binding to the Ru center promotes a cascade of reactions with very low energy barriers that lead to the final crystallographically characterized product, in which the original methylidene group has attacked the proximal aromatic ring of the SIMes ligand leading to a cycloheptatriene through a Buchner ring expansion. Analysis of the relevant molecular orbitals, supported by ab initio molecular dynamics simulations, illuminate the key role of the pi-acid CO coordinated trans to the Ru-methylidene bond to promote this reactivity. Based on this result, we investigated to which extent a large set of pi-acid groups could promote this deactivating reaction. Results clearly indicate that almost any sufficiently pi-acidic group that can approach the Ru center in the sterically crowded position trans to the Ru-methylidene bond can promote this deactivation route.

Influence of Carbon Support on Electronic Structure and Catalytic Activity of Pt Catalysts: Binding to the CO Molecule

“…Cavallo et al illuminated the key role of the π−acid CO ligand for promoting the reactivity of the Ru−olefin metathesis catalyst. 84 We also investigated the changes in the spin state and electronic structure of the PAH-supported Pt atom and dimer due to adsorption of the CO molecule. As shown in Figure 4, the spin ground state of the CO−Pt 2 /PAH complex is the singlet state, and the triplet state is higher in energy for all configurations.…”

Deactivation of Ru-benzylidene Grubbs catalysts active in olefin metathesis

“…In this respect, it is worth noting that the position trans to the Ru-ylidene bond has been scarcely considered during the years [24,25,[32][33][34]. For example, Grubbs and coworkers have suggested that one of the ortho-F atoms of the NHC A shown in Scheme 2 could be engaged in an interaction with the Ru center that results in increased metathesis activity [35].…”

“…In this framework, the deactivation reaction of Scheme 1 illustrates that the coordination position trans to the Ru-ylidene bond is not ''innocent'' and can trigger interesting reactivity. For this reason, we investigated the mechanism of deactivation shown in Scheme 1 in case of the Ru-methyidene bond [32].…”

“…Along this line, we recently demonstrated that for a series of r-basic/p-acid ligands coordinated trans to a Rumethylidene bond of the systems, a quite acidic ligand was enough to produce such a decomposition of the olefin metathesis catalyst [24,32]. To insert the methylidene moiety inside one of the aromatic rings of the SIMes ligand, the system must carry out a Buchner ring expansion reaction to allow the formation of a cycloheptatriene type ring.…”

“…To insert the methylidene moiety inside one of the aromatic rings of the SIMes ligand, the system must carry out a Buchner ring expansion reaction to allow the formation of a cycloheptatriene type ring. Calculations suggested that CO binding may weaken p-back-bonding between the Ru atom and the ylidene ligand [24,32], making the latter more electrophilic and disengaging it from the metal center [38]. Electron density is donated from filled d orbitals on the Ru to the empty p orbital of the benzylidene.…”