Activation of Terminal Alkynes at the Sulfur-Rich Bimetallic Site [MoIII2Cp2(μ-SMe)3]+: Alkyne−Vinylidene Conversion and C−S and C−C Couplings Promoted by Addition of Unsaturated Substrates (RC⋮CH, RN⋮C, SCS). Crystal Structures of μ-η1:η2-Vinylidene, μ-η1:η2-Acetylide, and μ-η1:η3-Vinyl−Thioether Compounds

Reactions of the bis(nitrile) compound [Mo 2 Cp 2 (MeCN) 2 (μ-SMe) 3 ](BF 4 ) ( 1 ) with terminal alkynes in a 1:1 ratio in dichloromethane at room temperature led to the alkyne adduct [Mo 2 Cp 2 (μ-SMe) 3 (RCCH)](BF 4 ) ( 2: R = Tol ( 2a ), Ph ( 2b ), CH 3 C CH 2 ( 2c ), n Pr ( 2d ), CO 2 Me ( 2e ), CF 3 ( 2f )). Compounds 2a − d were readily deprotonated with Et 3 N to give neutral acetylide derivatives [Mo 2 Cp 2 (μ-η 1:η 2 -C⋮CR)(μ-SMe) 3 ] ( 3 ). Protonation of 3 afforded exclusively the vinylidene complexes [Mo 2 Cp 2 (μ-η 1:η 2 -C CHR)(μ-SMe) 3 ](BF 3 ) ( 4 ). Reaction of 1 with an excess of terminal alkyne RCCH in dichloromethane gave either the six-membered metallacycle compounds [Mo 2 Cp 2 (μ-η 2:η 4 -CR CHCR CHSMe)(μ-SMe) 2 ](BF 4 ) ( 5 ) or the S -methylthiophenium derivatives [Mo 2 Cp 2 (μ-η 2:η 4 -C 4 H 2 R 2 SMe)(μ-SMe) 2 ](BF 4 ) ( 6 ), depending on the nature of the R groups. Further reactions of the alkyne adducts 2 with isocyanide and carbon disulfide led to the vinyl−thioether complexes [Mo 2 Cp 2 (μ-η 1:η 3 -CR CR‘-SMe)(μ-SMe) 2 (RNC)](BF 4 ) ( 7, 8 ) and to their CS 2 adducts [Mo 2 Cp 2 (S 2 C−CR‘ CRSMe)(μ-SMe) 2 ](BF 4 ) ( 9, 10 ), which arise from regioselective C−S coupling. A mechanism is proposed for the formation of the cyclic thiometalla compounds 5, 9, and 10 and of the thiophenium species 6 which assigns a key intermediate role to vinyl−thioether species. The molecular structures of 3a, 4b, and 7a have been established by X-ray diffraction studies.