Synthesis and Properties of the Indenyl Ruthenium(II) Complex [Ru{(<i>E</i>)-η¹-C(C⋮CPh)CHPh}(η⁵-C₉H₇)(<i>κ</i>²-<i>P</i>-dppm)] (dppm = bis(diphenylphosphino)methane). An Organometallic Intermediate in the Catalytic Dimerization of Phenylacetylene

The reaction of the hydride complex [RuH(η5-C9H7)(κ2-P-dppm)] (1) with an excess of 1,4-diphenyl-1,3-butadiyne, PhC⋮C−C⋮CPh, yields complex [Ru{(E)-η1-C(C⋮CPh)CHPh}(η5-C9H7)(κ2-P-dppm)] (3), formed by regio- and stereoselective insertion of the alkyne into the Ru−H bond, in toluene or benzene-d6. The reaction, about 4 times slower than with the terminal alkyne phenylacetylene, proceeds via an associative mechanism, characterized by the following activation parameters: ΔH⧧ = 11 kcal mol-1; ΔS⧧ = −44 cal mol-1 K-1. The σ-enynyl complex 3 is protonated with an equimolar amount of HBF4·Et2O to give the cationic alkynylalkylidene complex [Ru{C(C⋮CPh)CH2Ph}(η5-C9H7)(κ2-P-dppm)][BF4] (4), which in turn is deprotonated by tBuOK to regenerate quantitatively complex 3. Both complexes 3 and 4 have been characterized by X-ray structural analysis. Complex 3 catalyzes the dimerization of PhC⋮CH to give (E)- and (Z)-1,4-diphenyl-1-buten-3-yne under milder conditions than analogous indenyl complexes [RuX(η5-C9H7)(dppm)] (X = H, C⋮CPh, (E)-CHCHPh), while complex 4 is inactive. The σ-metathesis reaction between complex 3 and PhC⋮CH is not the rate-determining step in the catalytic cycle.