Can a pentamethylcyclopentadienyl ligand act as a proton-relay in f-element chemistry? Insights from a joint experimental/theoretical study

Abstract

Isomerisation of buta-1,2-diene to but-2-yne by (Me₅C₅)₂Yb is a thermodynamically favourable reaction, with the Δ_rG° estimated from experimental data at 298 K to be −3.0 kcal mol⁻¹. It proceeds in hydrocarbon solvents with a pseudo first-order rate constant of 6.4 × 10⁻⁶ s⁻¹ and 7.4 × 10⁻⁵ s⁻¹ in C₆D₁₂ and C₆D₆, respectively, at 20 °C. This 1,3-hydrogen shift is formally forbidden by symmetry and has to occur by an alternative pathway. The proposed mechanism for buta-1,2-diene to but-2-yne isomerisation by (Me₅C₅)₂Yb involves coordination of methylallene (buta-1,2-diene) to (Me₅C₅)₂Yb, and deprotonation of methylallene by one of the Me₅C₅ ligands followed by protonation of the terminal methylallenyl carbon to yield the known coordination compound (Me₅C₅)₂Yb(η²-MeCCMe). Computationally, this mechanism is not initiated by a single electron transfer step, and the ytterbium retains its oxidation state (II) throughout the reactivity. Experimentally, the influence of the metal centre is discussed by comparison with the reaction of (Me₅C₅)₂Ca towards buta-1,2-diene, and (Me₅C₅)₂Yb with ethylene. The mechanism by which the Me₅C₅ acts as a proton-relay within the coordination sphere of a metal also rationalises the reactivity of (i) (Me₅C₅)₂Eu(OEt₂) with phenylacetylene, (ii) (Me₅C₅)₂Yb(OEt₂) with phenylphosphine and (iii) (Me₅C₅)₂U(NPh)₂ with H₂ to yield (Me₅C₅)₂U(HNPh)₂. In the latter case, the computed mechanism is the heterolytic activation of H₂ by (Me₅C₅)₂U(NPh)₂ to yield (Me₅C₅)₂U(H)(HNPh)(NPh), followed by a hydrogen transfer from uranium back to the imido nitrogen atom using one Me₅C₅ ligand as a proton-relay. The overall mechanism by which hydrogen shifts using a pentamethylcyclopentadienyl ligand as a proton-relay is named Carambole in reference to carom billiards.