Predicting the Cis−Trans Dichloro Configuration of Group 15−16 Chelated Ruthenium Olefin Metathesis Complexes: A DFT and Experimental Study

Abstract

Gradient-corrected (BP86) and hybrid (M06-L) density functional calculations were used to study the relative stability of cis and trans-dichloro X-chelated benzylidene ruthenium complexes (X = O, S, Se, N, P). Calculations in the gas phase differed from experimental results, predicting the trans-dichloro configuration as being more stable in every case. The addition of Poisson-Boltzmann (PBF) continuum approximation (dichloromethane) corrected the disagreement and afforded energies consistent with experimental results. Novel N, Se, and P chelated ruthenium olefin metathesis complexes were synthesized to evaluate calculation predictions. These findings reinforce the importance of including solvent corrections in DFT calculations of ruthenium metathesis catalysts and predict that stronger sigma donors as chelating atoms tend to electronically promote the unusual and less active cis-dichloro configuration.

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

“…This result is in agreement with previous calculations, [11,29] which indicate that strong s donors stabilize the trans isomer. [11] Nevertheless, our more extended analysis surprisingly indicates that the energy barrier for the trans to cis isomerization, around 28 kcal mol À1 in systems 6-10, is scarcely affected by this strong change in the relative stability of the cis and trans isomers. We also explored the impact of electron-donating (OMe) or -withdrawing (NO 2 ) substituents on the phenoxy moiety.…”

“…Lemcoff and co-workers investigated the effect of stronger s donors as chelating atoms, which were shown to stabilize the cis isomer. [11] As for the mechanism of isomerization, which involves the exchange of two equatorial ligands in a square-pyramidal complex with the Ru=CHR bond in the axial position, the dissociative and the concerted pathways shown in Scheme 2 are usually invoked. Preliminary DFT calculations suggested that the dissociative pathway is viable, although the concerted pathway was not investigated.…”

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

“…In contrast, pre-initiators 6 and 7 gave poly9 with M n s of 62000 (PDI = 1.2) and 1.4). These results suggest that the trans isomers are in toluene energetically more favorable than in dichloromethane, which is in accordance with literature [26][27][28] and with calculations [15]. Further, it is quite obvious that the trans isomer is formed rapidly and is readily extracted from the chemical balance by entering the polymerization cycle.…”

“…As it is known, that the position of the cis-trans dichloro isomerization equilibrium is influenced by the nature of the solvent (high dielectric constant solvents favor the cis isomer) [26][27][28], we conducted a further test series in toluene, at that time at 80°C. Assumedly, 80°C is a temperature high enough to cancel out the influence of the energy for dissociation of the Ru-halogen bond and the initiation efficacy should be only dependent on the chemical balance of the isomerization reaction [13].…”

Olefin metathesis catalyst bearing a chelating phosphine ligand

“…For that purpose latent catalysts/initiators have been developed [6]. A successful design motif for latent catalysts/initiators is the use of a chelating carbene ligand similar to the one used in Hoveyda type complexes with strongly coordinating co-ligands using O [7], N [8], S [9] and Se [10] donor atoms as the second binding unit.…”