Intramolecular C–H Activation Reactions of Ru(NHC) Complexes Combined with H₂ Transfer to Alkenes: A Theoretical Elucidation of Mechanisms and Effects of Ligands on Reactivities

Abstract

Recent experimental studies have identified Ru(II) NHC complexes that are highly reactive in the tandem intramolecular C(sp 3 )−H activation of an N-alkyl substituent to form a metallacycle and transfer hydrogenation of alkenes. These complexes are promising candidates for tandem catalytic processes that depend on a reversible uptake of hydrogen ("borrowing hydrogen catalysis"). We have elucidated the reaction mechanisms by density functional theory calculations and investigated ligand effects on reactivity. The reaction of ruthenium dihydride complex [Ru-(H) 2 (NHC)(CO)(PPh 3 ) 2 ] (1) with ethylene occurs via dissociative ligand exchange to replace one of the phosphine ligands with ethylene, followed by hydride migration and reductive elimination to form a Ru(0) intermediate. Subsequent C−H activation occurs via an oxidative addition mechanism. Bulkier NHC and phosphine ligands facilitate the dissociation of phosphine, which leads to a lower overall barrier. In addition, the N-i Pr substituted NHC ligand promotes the C−H oxidative addition/ruthenacyclization due to the release of steric strain caused by the N-i Pr group and the substituents on the NHC backbone. The reaction with the monohydride monochloride complex [RuHCl(NHC)(PPh 3 ) 2 ] (2) occurs via ligand exchange and hydride migration to form an alkyl ruthenium(II) complex. The type of phosphine ligand determines whether the subsequent intramolecular C−H activation proceeds via an associative or a dissociative mechanism. In the associative pathway, C−H activation occurs via a concerted σ-bond metathesis mechanism, which directly transfers the hydrogen atom from the C−H bond of the N-alkyl group on the NHC to the alkyl ligand on ruthenium. In the dissociative pathway, C−H activation occurs via stepwise C−H oxidative addition to form a Ru(IV) intermediate followed by reductive elimination of the alkane product.

Rate-Limiting Steps in the Intramolecular C–H Activation of Ruthenium N-Heterocyclic Carbene Complexes

“…This step is followed by hydride migration from the complex to ethylene (olefin) in which 16-electron intermediate int6 via transition-state ts5 is formed. The energy required for C–H reductive elimination, being a difference between int4 and ts5 , is about 6–12 kcal/mol, which is consistent with the results obtained by Houk et al 34 The release of ethylene is preceded by the migration of the two hydrogen atoms from the ruthenium complex. One of the hydride ligands is transferred to the ethylene in the ts5 .…”

“…In the final step of the cycle, the phosphine ligand, which dissociated in the first step, can associate back to the complex after the reductive elimination step. 34 This leads to the creation of more stable complex int11 [four-coordinate Ru (0)], which ends the catalytic cycle.…”

“…33 Finally, a very thorough mechanistic study of the entire catalytic cycle of the Williams catalyst and its hydride chloride derivative has been performed recently by Houk and co-workers. 34 In this work, he explored the mechanism of a tandem alkene hydrogenation/intramolecular C−H activation and showed that the Williams catalyst reacts via a dissociative process with stepwise hydride migration, reductive elimination, and oxidative addition via a Ru(0) intermediate, as originally suggested by Whittlesey. On the other hand, the hydride chloride complex is activated through a reactive Ru(IV) intermediate (Figure 2).…”

“…The same group studied also H 2 loss and C–H activation in Edney complexes as well as performed comparative analysis of the influence of substituents in the aryl group on the activation of C­(aryl)–X bond activation . Finally, a very thorough mechanistic study of the entire catalytic cycle of the Williams catalyst and its hydride chloride derivative has been performed recently by Houk and co-workers . In this work, he explored the mechanism of a tandem alkene hydrogenation/intramolecular C–H activation and showed that the Williams catalyst reacts via a dissociative process with stepwise hydride migration, reductive elimination, and oxidative addition via a Ru(0) intermediate, as originally suggested by Whittlesey.…”

Sustainable Option for Hydrogen Production: Mechanistic Study of the Interaction between Cobalt Pincer Complexes and Ammonia Borane

“…Single point energy calculations were carried out for the B3LYP and M06 (only for Pathway I-B, Figure S3a,b) levels, together with a larger basis set, in which the basis set for metal centers were kept unchanged and that for the other atoms were enlarged to 6-311++G (d,p). The methodology is similar to other research groups to calculate single point energies in the solvent phase modeled by the solvation model SMD, using the gas phase optimized structures for reactions between organic substrates and organometallic catalysts [48][49][50][51][52][53][54].…”