Experimental and theoretical investigation of Z-E alkene isomerization in [(Cy₃P)₂Cl₂Ru]₂(μ-CHCH=CHCH) and related vinylalkylidenes

The conjugated dinithenium bisalkylidene complex [(Cy₃P)₂Cl₂Ru]₂(μ-CHCH=CHCH) (1, an inseparable 10:1 mixture of E and Z alkene isomers 1E and 1Z) and the separate E and Z isomers of the monoruthenium alkylidene complexes (Cy₃P)₂Cl₂Ru=CHCH=CHCH=CH₂ (2E and 2Z) and (Cy₃P)₂Cl₂Ru=CHCH=CHCH₃ (3E and 3Z) have each been synthesized through the stoichiometric reaction of olefin metathesis catalysts. Each pair of isomers undergo Z-E isomerization, though the isomerization rates and their dependencies on excess alkene and PCy₃ are very different. Isomerization in 1 is the fastest, with ΔH^‡ = 100 ± 5 kJ/mol and ΔS^‡ = 67 ± 3 J/(mol K) (1Z to 1E) or 49 ± 3 J/(mol K) (1E to 1Z) as determined by dynamic NMR, giving t_1/2 = 11 s for the approach to equilibrium at 25°C. The rate of this isomerization shows no phosphine dependence. Isomerizations of 2Z and 3Z are slower (t_1/2 = 5-20 h), but the former is promoted by PCy₃, while the latter is inhibited. In the presence of (Z)-1,3,5-hexatriene, the isomerization of 2Z is much slower, while the isomerization of 3Z is accelerated by (Z)-1,3-pentadiene. Density functional calculations were performed to help interpret these results and elucidate which of several possible isomerization mechanisms operates in each case.