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

Reactions of [Ru{C=C(H)-1,4-C₆ H₄ C≡CH}(PPh₃ )₂ Cp]BF₄ ([1 a]BF₄ ) with hydrohalic acids, HX, results in the formation of [Ru{C≡C-1,4-C₆ H₄ -C(X)=CH₂ }(PPh₃ )₂ Cp] [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=C₆ H₄ =C=CH₂ )(PPh₃ )₂ Cp]⁺ . Similarly, [M{C=C(H)-1,4-C₆ H₄ -C≡CH}(LL)Cp ]BF₄ [M(LL)Cp'=Ru(PPh₃ )₂ Cp ([1 a]BF₄ ); Ru(dppe)Cp* ([1 b]BF₄ ); Fe(dppe)Cp ([1 c]BF₄ ); Fe(dppe)Cp* ([1 d]BF₄ )] react with H⁺ /H₂ O to give the acyl-functionalised phenylacetylide complexes [M{C≡C-1,4-C₆ H₄ -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=C₆ H₄ =C=CH₂ )(LL)Cp']⁺ prior to attack at the more exposed, remote quaternary carbon. Thus, whilst representative acetylide complexes [Ru(C≡C-1,4-C₆ H₄ -C≡CH)(PPh₃ )₂ Cp] (4 a) and [Ru(C≡C-1,4-C₆ H₄ -C≡CH)(dppe)Cp*] (4 b) reacted with the relatively small electrophiles [CN]⁺ and [C₇ H₇ ]⁺ at the β-carbon to give the expected vinylidene complexes, the bulky trityl ([CPh₃ ]⁺ ) electrophile reacted with [M(C≡C-1,4-C₆ H₄ -C≡CH)(LL)Cp'] [M(LL)Cp'=Ru(PPh₃ )₂ Cp (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=C₆ H₄ =C=C(H)CPh₃ }(LL)Cp']⁺ , which were isolated as the water adducts [M{C≡C-1,4-C₆ H₄ -C(=O)CH₂ CPh₃ }(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-^c C₄ H₂ S-C≡CH}(LL)Cp']BF₄ ([7 a-d]BF₄ add water readily to give [M{C≡C-2,5-^c C₄ H₂ S-C(=O)CH₃ }(LL)Cp'] (8 a-d)].