Mechanistic Insights into the cis–trans Isomerization of Ruthenium Complexes Relevant to Catalysis of Olefin Metathesis

Abstract

The mechanism of the trans to cis isomerization in Ru complexes with a chelating alkylidene group has been investigated by using a combined theoretical and experimental approach. Static DFT calculations suggest that a concerted single-step mechanism is slightly favored over a multistep mechanism, which would require dissociation of one of the ligands from the Ru center. This hypothesis is supported by analysis of the experimental kinetics of isomerization, as followed by (1)H NMR spectroscopy. DFT molecular dynamics simulations revealed that the variation of geometrical parameters around the Ru center in the concerted mechanism is highly uncorrelated; the mechanism actually begins with the transformation of the square-pyramidal trans isomer, with the Ru==CHR bond in the apical position, into a transition state that resembles a metastable square pyramidal complex with a Cl atom in the apical position. This high-energy structure collapses into the cis isomer. Then, the influence of the N-heterocyclic carbene ligand, the halogen, and the chelating alkylidene group on the relative stability of the cis and trans isomers, as well as on the energy barrier separating them, was investigated with static calculations. Finally, we investigated the interconversion between cis and trans isomers of the species involved in the catalytic cycle of olefin metathesis; we characterized an unprecedented square-pyramidal metallacycle with the N-heterocyclic carbene ligand in the apical position. Our analysis, which is relevant to the exchange of equatorial ligands in other square pyramidal complexes, presents evidence for a remarkable flexibility well beyond the simple cis-trans isomerization of these Ru complexes.

A Molecular modeling study of the changes of some steric properties of the precatalysts during the olefin metathesis reaction

“…During the last two decades, since the first ruthenium Grubbs‐type carbene precatalyst was reported, the field of olefin metathesis has grown tremendously . Computational studies have been used over the last decade to try to elucidate and gain a deeper insight into the effect of ligand coordination and the mechanism of the catalytic reaction . Several attempts have also been made to use theoretical studies to elucidate various steric effects that might play a role during the course of the metathesis reaction , .…”

On the isomerization of a trans-dichloro to a cis-dichloro amide-chelated ruthenium benzylidene complex and the catalytic scope of these species in olefin metathesis

“…The latency is believed to be reasoned in the general inability of such cis-dichloro species to perform olefin metathesis; hence, an isomerization step to the respective trans-dichloro isomer has to precede olefin metathesis [21,22]. Theoretical calculations confirm this hypothesis and suggest either a concerted mechanism or dissociation of a neutral ligand to start the isomerization process [23,24]. However, only recently we have disclosed experimental evidence that the cis-trans isomerization of ruthenium complexes bearing a chelating ester substituted benzylidene ligand proceeds via dissociation of a chloride ligand, yielding a cationic species.…”

Initiation efficacy of halo-chelated cis-dichloro-configured ruthenium-based second-generation benzylidene complexes in ring-opening metathesis polymerization

“…Table 1, entries 2 and 11), M n values decrease in case of the halo-chelated pre-initiators (cf. Table 1, entries [12][13][14][15][16][17][18][19]. Still 1 is the pre-initiator with the highest initiation efficacy but variations of the M n value of different poly9 became smaller than at room temperature as the decrease in M n is bigger in case of using electronically modified initiators 3, 5, 6, and 7 (decrease by the factor of 1.8-2.6) than in case of the unsubstituted congeners 1, 2, 4, and 8 (decrease by the factor of 1.4-1.5).…”

“…Consequently, the activation energy necessary for releasing the actual catalytically active species via a dissociative pathway (please note that also an interchange mechanism might operate in certain cases [10]) is increasing; (b) additionally the chelate effect plays a role (ring size and further geometric features); (c) the nature of the chelating carbene determines whether the thermodynamically more favored stereoisomer is bearing a cis-or a trans-dichloro configuration [11]. The latter point is of relevance because cis-dichloro-configured complexes are believed to be olefin metathesis inactive and have to isomerize to their trans-dichloro congeners before an olefin metathesis reaction can occur [12,13].…”