Further Evidence for ‘Extended’ Cumulene Complexes: Derivatives from Reactions with Halide Anions and Water

Reactions of Ru{C=C(H)-1,4-C₆H₄C≡CH}(PPh₃)₂Cp]BF₄ ( 1 a ]BF₄) with hydrohalic acids, HX, results in the formation of Ru{C≡C-1,4-C₆H₄-C(X)=CH₂}(PPh₃)₂Cp] X=Cl ( 2 a-Cl ), Br ( 2 a-Br )], arising from facile Markovnikov addition of halide anions to the putative quinoidal cumulene cation Ru(=C=C=C₆H₄=C=CH₂)(PPh₃)₂Cp]⁺. Similarly, M{C=C(H)-1,4-C₆H₄-C≡CH}(LL)Cp ]BF₄ M(LL)Cp’=Ru(PPh₃)₂Cp ( 1 a ]BF₄); Ru(dppe)Cp* ( 1 b ]BF₄); Fe(dppe)Cp ( 1 c ]BF₄); Fe(dppe)Cp* ( 1 d ]BF₄)] react with H⁺/H₂O to give the acyl-functionalised phenylacetylide complexes M{C≡C-1,4-C₆H₄-C(=O)CH₃}(LL)Cp’] ( 3 a – d ) after workup. The Markovnikov addition of the nucleophile to the remote alkyne in the cations 1 a–d ]⁺ is difficult to rationalise from the vinylidene form of the precursor and is much more satisfactorily explained from initial isomerisation to the quinoidal cumulene complexes M(=C=C=C₆H₄=C=CH₂)(LL)Cp’]⁺ prior to attack at the more exposed, remote quaternary carbon. Thus, whilst representative acetylide complexes Ru(C≡C-1,4-C₆H₄-C≡CH)(PPh₃)₂Cp] ( 4 a ) and Ru(C≡C-1,4-C₆H₄-C≡CH)(dppe)Cp*] ( 4 b ) reacted with the relatively small electrophiles CN]⁺ and C₇H₇]⁺ at the β-carbon to give the expected vinylidene complexes, the bulky trityl (CPh₃]⁺) electrophile reacted with M(C≡C-1,4-C₆H₄-C≡CH)(LL)Cp’] M(LL)Cp’=Ru(PPh₃)₂Cp ( 4 a ); Ru(dppe)Cp* ( 4 b ); Fe(dppe)Cp ( 4 c ); Fe(dppe)Cp* ( 4 d )] at the more exposed remote end of the carbon-rich ligand to give the putative quinoidal cumulene complexes M{C=C=C₆H₄=C=C(H)CPh₃}(LL)Cp’]⁺, which were isolated as the water adducts M{C≡C-1,4-C₆H₄-C(=O)CH₂CPh₃}(LL)Cp’] ( 6 a–d ). Evincing the scope of the formation of such extended cumulenes from ethynyl-substituted arylvinylene precursors, the rather reactive half-sandwich (5-ethynyl-2-thienyl)vinylidene complexes M{C=C(H)-2,5-^cC₄H₂S-C≡CH}(LL)Cp’]BF₄ ( 7 a – d ]BF₄ add water readily to give M{C≡C-2,5-^cC₄H₂S-C(=O)CH₃}(LL)Cp’] ( 8 a – d )].