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

The conjugated diruthenium bisalkylidene complex [(Cy3P)(2)Cl2Ru](2)(mu-CHC H=CHCH) (1, an inseparable 10:1 mixture off and Z alkene isomers 1E and 1Z) and the separate E and Z isomers of the monoruthenium alkylidene complexes (Cy3P)(2)Cl2Ru=CHCH=CHCH=CH2 (2E and 2Z) and (Cy3P)(2)Cl2Ru=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 an d PCy3 are very different. Isomerization in 1 is the fastest, with Delta H- double dagger = 100 +/- 5 kJ/mol and Delta S-double dagger = 67 +/- 3 J/(mo l K) (1Z to 1E) or 49 +/- 3 J/(mol K) (1E to 1Z) as determined by dynamic N MR, giving t(1/2) = II s for the approach to equilibrium at 25 degrees C. T he 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 P Cy3, 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 per formed to help interpret these results and elucidate which of several possi ble isomerization mechanisms operates in each case.