Hydroboration of Alkynes: η-Alkene-Borane versus η--Boratabutadiene.

A series of hydroborated η-σ,π-alkene-borane complexes have been synthesized from the reaction of ruthenium-bis(σ)borate complex [Cp*Ru(μ-H)BH(S-CH═S)] () and terminal as well as internal alkynes. Likewise, the reactions of manganese-bis(σ)borate complexes [Mn(CO)(μ-H)BHNCSCH(NL)] (L = NCSCH or H) were explored with terminal alkynes that yielded boratabutadiene complexes [Mn(CO){(NCSCH)N}{(RMeC)═B(HC═CHR)}] [R = phenyl () or -tolyl ()] via triple hydroboration of alkynes. These complexes feature a boratabutadiene ligand that is coordinated to a metal through the η-CBCC mode. To the best of our knowledge, these are the first examples of η--boratabutadiene-coordinated manganese complexes generated by the -hydroboration of alkynes. The steric and electronic effects of the borate ligands have been demonstrated using a less sterically hindered manganese-bis(σ)borate complex, [Mn(CO)(μ-H)BH(HNCSCH)], that generated monohydroborated complexes [(CO)Mn(μ-H)(HNCSCH)B(RC═CHR)] (for , R = Ph and R = H; for , R = -Tol and R = H; for , R = R = Ph). Theoretical studies using density functional theory methods and chemical bonding analyses established the bonding and stability of these species.