Reactions of low-valent dirhodium complexes with alkynes. Synthesis and reactivity of alkyne and vinylidene complexes

Article abstract of DOI:10.1021/om00151a029

Treatment of the low-valent binuclear complex Rh₂(CO)₃(dppm)₂ (1, dppm = bis(diphenylphosphino)methane) with excess phenylacetylene in benzene yields the A-frame bridging vinylidene complex Rh₂(CO)₂(dppm)₂(C=CHPh) (2), while treatment of 1 with a stoichiometric amount of phenylacetylene in refluxing acetone produces the non-A-frame bridging acetylene complex Rh₂(CO)₂(dppm)₂(PhC≡CH) (3). The bridging diphenylacetylene complex Rh₂(CO)₂(dppm)₂(PhC≡CPh) (4) has also been prepared. At 80°C, solutions of 2 catalyze the disproportionation of PhC≡CH to 1,4-diphenylbutadiyne and styrene. Compound 2 also catalyzes the hydrogenation of PhC≡CH to styrene. Deuterium labeling studies indicate that the vinylidene species is involved in the catalysis, resulting primarily in a novel "geminal" hydrogenation of PhC≡CH to PhCH=CD₂. The molecular structures of 2 and 4 have been determined by single-crystal X-ray diffraction. (2: P1, a = 14.684 (4) A?, b = 14.818 (4) A?, c = 13.527 (2) A?, α = 102.56 (2)°, β = 101.56 (2)°, γ = 73.13 (2)°, Z = 2, R = 0.048, R_w = 0.069. 4: P2₁/c, a = 12.602 (2) A?, b = 21.093 (2) A?, c = 27.119 (5) A?, β = 90.77 (1)°, Z = 4, R = 0.051, R_w = 0.058.) The A-frame structure of 2 shows distorted square-planar coordination including trans P donors at each Rh and a Rh?Rh separation of 3.011 (1) A?. The structure of 4 reveals that the PhCCPh moiety bridges the two Rh atoms in the perpendicular or μ,η² mode with each Rh possessing a distorted tetrahedral geometry. The Rh-Rh bond length in 4 is 2.644 (1) A?.