Ruthenium vinylidene and σ-acetylide complexes containing 1,4,7-trimethyl-1,4,7-triazacyclononane (Me₃tacn): Synthesis and alkyne-coupling reactivity

The ruthenium(II) complexes [Ru(Me₃tacn)(L)₂X]PF₆ (L = PMe₃, X = O₂CCF₃ (1a); L = PMe₃, X = Cl (1b); L = 1/2 dmpe, X = O₂CCF₃ (1c)) are prepared. Only 1a reacts with 1 equiv of RC≡CH (R = Ph and p-tolyl) to give the vinylidene complexes [Ru(Me₃tacn)-(PMe₃)(O₂CCF ₃){C=CH(R)}]PF₆ (R=Ph (2a) and p-tolyl (2b)) in refluxing 1,2-dichloroethane. Reaction of 2a and 2b with PMe₃ in methanolic KOH solution give the corresponding σ-acetylide complexes [Ru(Me₃tacn)(PMe₃)₂(C≡CR)]PF₆ (R=Ph (3a) and p-tolyl (3b)). Similarly, treatment of 2a with P(OMe₃)₃ affords [Ru(Me₃tacn)(PMe₃)(P(OMe) ₃)(C≡CPh)]-PF₆ (3c). Oxidative cleavage of the vinylidene ligand in 2a by oxygen gives [Ru(Me₃tacn)-(PMe₃)(O₂CCF ₃)(CO)]PF₆ (4) and benzaldehyde. Complex 1b reacts with 2.5 equiv of RC≡CH (R=Ph, p-tolyl) and 1.5 equiv of KOH in methanol to yield the η³-butenynyl species [Ru(Me₃-tacn)(PMe₃){η³-RC ₃=CH(R)}]PF₆ (R = Ph (5a) and p-tolyl (5b)). In addition, 2a and 2b react with RC≡CH (R = Ph, p-tolyl) and KOH in methanol to give 5a and 5b, respectively. Treatment of 2a with p-tolylC≡CH and KOH in methanol gives [(Me₃tacn)Ru(PMe₃){/73-PhC ₃=CH(p-tolyl)}]PF₆ (5c) and [(Me₃tacn)Ru(PMe₃){η ³-(p-tolyl)C3=CH(Ph)}]PF₆ (5c′) in a 1:1 ratio. Reacting 2b with PhC≡CH similarly gives 5c and 5c′ in equal amounts. Structures of 3c, 5a, and 5c/5c′ are established by X-ray crystallography. Mechanistic insights from the isolated complexes suggest that hydrogen shift between vinylidene and acetylide moieties is an important process in the coupling of alkynes.