Density Functional Study of Butadiyne to Butatrienylidene Isomerization in [Ru(HC≡CC≡CH)(PMe₃)₂(Cp)]⁺

Abstract

The butadiyne to butatrienylidene isomerization in [Ru(HCCCCH)(PMe3)2(Cp)]+ has been investigated by density functional calculations. Several possible minima have been identified on the potential energy surface for the coordinated C4H2 moiety, and a few plausible isomerization mechanisms have been analyzed by a DFT approach. The butatrienylidene complex has been found to be more stable than the butadiyne adduct by −13.1 kcal mol−1 in enthalpy and is the thermodynamically most stable species on the potential energy surface. The energetically most favorable isomerization pathway has been found to initially follow the same pathway experimentally and theoretically characterized for the simpler alkyne rearrangement on a d6 metal fragment, i.e. a 1,2-hydrogen shift passing through an agostic intermediate, and leading to a ethynyl vinyl intermediate, for which an activation enthalpy of 23.1 kcal mol−1 (activation free energy of 20.8 kcal mol−1) was found. The isomerization then proceeds through a proton migration from the Cβ to the terminal Cδ atom occurring through deprotonation of the ethynyl vinylidene, leading to a butadiynyl complex which is then reprotonated to the final butatrienylidene product, with an overall activation energy of 17.4 kcal mol−1 (activation free energy of 19.6 kcal mol−1).

Further Evidence for ‘Extended’ Cumulene Complexes: Derivatives from Reactions with Halide Anions and Water

“…At present, the exact mechanistic role of acid in these reactions is unclear. Re and colleagues have examined mechanisms of proton migration from ethynylvinylidene to butatrientylidene supported by a {Ru(PMe 3 ) 2 Cp} + fragment, and proposed that the isomerization proceeds via a deprotonation/re‐protonation mechanism . In our case, it seems that the additional acid is important in promoting protonation at the terminal carbon to give the quinoidal cumulene, albeit as a transient intermediate that we have not yet succeeded in detecting spectroscopically in solution (Scheme ).…”

Recent Mechanistic and Synthetic Developments in the Chemistry of Transition‐Metal Vinylidene Complexes

“…A theoretical study (B3LYP/LACV3P + + **) into the rearrangement of the butadiyne complex 17) has revealed the presence of a similar mechanistic pathway. [24] The calculations indicate that a direct 1,4-hydrogen shift from 13 c to give 17 is unlikely and that a ethynylvinyli-…”

Counteranion and Solvent Assistance in Ruthenium-Mediated Alkyne to Vinylidene Isomerizations

“…24 In the case of d 8 metal complexes, a bimolecular mechanism has been proposed for the alkyne to vinylidene isomerization on the fragment {RhCl-(P i Pr 3 ) 2 } via formation of the dimeric d 6 11 However, other researchers have gathered theoretical and experimental evidence in support of an alternative mechanism for this process which does not involve dimeric species. 13 , and no H/D scrambling was observed.…”

“…7d, 24 On the other hand, the 1,3-H migration (pathway B) entails oxidative addition of the coordinated alkyne to give a hydrido−alkynyl complex, which then isomerizes via 1,3-H shift. The influence of the chloride anion in these routes has been considered, namely, pathways AI-Cl and AII-Cl (stable intermediates in pathway B were not found).…”