Synthesis and Properties of the Indenyl Ruthenium(II) Complex [Ru{(E)-η1-C(C⋮CPh)CHPh}(η5-C9H7)(κ2-P-dppm)] (dppm = bis(diphenylphosphino)methane). An Organometallic Intermediate in the Catalytic Dimerization of Phenylacetylene

The reaction of the hydride complex [RuH(η 5 -C 9 H 7 )( κ 2 - P- dppm)] ( 1 ) with an excess of 1,4-diphenyl-1,3-butadiyne, PhC⋮C−C⋮CPh, yields complex [Ru{( E )-η 1 -C(C⋮CPh) CHPh}(η 5 -C 9 H 7 )( κ 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: Δ H ⧧ = 11 kcal mol - 1; Δ S ⧧ = −44 cal mol - 1 K - 1 . The σ-enynyl complex 3 is protonated with an equimolar amount of HBF 4 ·Et 2 O to give the cationic alkynylalkylidene complex [Ru{ C(C⋮CPh)CH 2 Ph}(η 5 -C 9 H 7 )( κ 2 - P- dppm)][BF 4 ] ( 4 ), which in turn is deprotonated by t BuOK 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(η 5 -C 9 H 7 )(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.