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

Reactions of [Ru{C=C(H)‐1,4‐C 6 H 4 C≡CH}(PPh 3 ) 2 Cp]BF 4 ([ 1 a ]BF 4 ) with hydrohalic acids, HX, results in the formation of [Ru{C≡C‐1,4‐C 6 H 4 ‐C(X)=CH 2 }(PPh 3 ) 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=C 6 H 4 =C=CH 2 )(PPh 3 ) 2 Cp] + . Similarly, [M{C=C(H)‐1,4‐C 6 H 4 ‐C≡CH}(LL)Cp ]BF 4 [M(LL)Cp’=Ru(PPh 3 ) 2 Cp ([ 1 a ]BF 4 ); Ru(dppe)Cp* ([ 1 b ]BF 4 ); Fe(dppe)Cp ([ 1 c ]BF 4 ); Fe(dppe)Cp* ([ 1 d ]BF 4 )] react with H + /H 2 O to give the acyl‐functionalised phenylacetylide complexes [M{C≡C‐1,4‐C 6 H 4 ‐C(=O)CH 3 }(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=C 6 H 4 =C=CH 2 )(LL)Cp’] + prior to attack at the more exposed, remote quaternary carbon. Thus, whilst representative acetylide complexes [Ru(C≡C‐1,4‐C 6 H 4 ‐C≡CH)(PPh 3 ) 2 Cp] ( 4 a ) and [Ru(C≡C‐1,4‐C 6 H 4 ‐C≡CH)(dppe)Cp*] ( 4 b ) reacted with the relatively small electrophiles [CN] + and [C 7 H 7 ] + at the β‐carbon to give the expected vinylidene complexes, the bulky trityl ([CPh 3 ] + ) electrophile reacted with [M(C≡C‐1,4‐C 6 H 4 ‐C≡CH)(LL)Cp’] [M(LL)Cp’=Ru(PPh 3 ) 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=C 6 H 4 =C=C(H)CPh 3 }(LL)Cp’] + , which were isolated as the water adducts [M{C≡C‐1,4‐C 6 H 4 ‐C(=O)CH 2 CPh 3 }(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 C 4 H 2 S‐C≡CH}(LL)Cp’]BF 4 ([ 7 a – d ]BF 4 add water readily to give [M{C≡C‐2,5‐ c C 4 H 2 S‐C(=O)CH 3 }(LL)Cp’] ( 8 a – d )].