Synthesis and properties of the indenyl ruthenium(II) complex [Ru{(E)-η1-C(C=CPh)=CHPh}(η5C9H7)(κ2-P-dppm)] (dppm = bis(diphenylphosphino)methane).: An organometallic intermediate in the catalytic dimerization of phenylacetylene

The reaction of the hydride complex [RuH(eta(5)-C9H7)(kappa(2)-P-dppm)] (i) with an excess of 1,4-diphenyl-1,3-butadiyne, PhCdropC-CdropCPh, yields complex [Ru{(E)-eta(1)-C(CdropCPh)=CHPh}(eta(5)-C9H7)(kappa(2)-P-dppm)] (3), formed by regio- and stereoselective insertion of the alkyne into the Ru-H bond, in toluene or benzene-d(6). The reaction, about 4 times slower than with the terminal alkyne phenylacetylene, proceeds via an associative mechanism, characterized by the following activation parameters: DeltaH(double dagger) = 11 kcal mol(-1); DeltaS(double dagger) = -44 cal mol(-1) K-1. The sigma-enynyl complex 3 is protonated with an equimolar amount of HBF4.Et2O to give the cationic alkynylalkylidene complex [Ru{=C(CdropCPh)CH2Ph}(eta(5)-C9H7)(kappa(2)-P-dppm)][BF4] (4), which in turn is deprotonated by (BuOK)-Bu-t to regenerate quantitatively complex 3. Both complexes 3 and 4 have been characterized by X-ray structural analysis. Complex 3 catalyzes the dimerization of PhCdropCH to give (E)- and (Z)-1,4-diphenyl-1-buten-3-yne under milder conditions than analogous indenyl complexes [RuX(eta(5)-C9H7)(dppm)] (X = H, CdropCPh, (E)-CH=CHPh), while complex 4 is inactive. The a-metathesis reaction between complex 3 and PhCdropCH is not the rate-determining step in the catalytic cycle.