Metal-metal multiple bonds. 14. Synthesis, stereochemistry, and hydrogenation of protonated alkyne adducts of Cp2Mo2(CO)4

Article abstract of DOI:10.1021/om50003a018

Cp₂Mo₂(CO)₄(Mo≡Mo) reacts with alkynes, RC≡CR′ (R, R′ = H, alkyl, SnMe₃, Ph), to form 1:1 adducts Cp₂Mo₂(CO)₄(RC≡CR′) in high yields. Protonation of these alkyne adducts by strong acids (such as CF₃COOH, HBF₄·Et₂O, and HCl) yields compounds with a vinyl group both η¹ and η² bonded to the dimetal fragment. The protonation has been shown to occur in an anti-Markovnikov manner to give the (E)-alkene fragment. X-ray structural analysis of the protonated compounds shows a Mo-Mo single bond with the conjugate base of the acid (in the CF₃COOH case) coordinated to one of the molybdenum atoms. For Cp₂Mo₂(CO)₄(PhC≡CH) plus HBF₄·Et₂O, a H₂O adduct is formed upon addition of water. The water molecule was found to be coordinated to one of the molybdenums upon solution of the crystal structure. Cp₂Mo₂(CO)₄(Ph(H)C≡CH)(H ₂O)BF₄: a = 14.095 (8) A?, b = 9.625 (6) A?, c = 18.176 (10) A?, β = 108.34 (4)°, V = 2341 (2) A?³, Z = 4, space group P2₁/a. In the CF₃COOH adduct, the coordinated trifluoroacetate group is labilized in the presence of excess CF₃COOH, suggesting the possibility of facile displacement of coordinated TFA^- by groups such as H₂. This lability has been utilized in the hydrogenation of 1-hexyne and 1-hexene. The hydrogenation is quantitative when stoichiometric quantities of Cp₂Mo₂(CO)₄ and 1-hexyne are employed. When catalytic quantities of Cp₂Mo₂(CO)₄ and acid are used, then polymerization of the 1-hexyne becomes the favored reaction pathway.