Further evidence for ‘extended’ cumulene complexes : derivatives from reactions with halide anions and water

Reactions of [Ru{C=C(H)‐1,4‐C6H4CºCH}(PPh3)2Cp]BF4 ([1a]BF4) with HX, give [Ru{CºC‐1,4‐C6H4‐C(X)=CH2}(PPh3)2Cp] (X = Cl, Br) 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‐CºCH}(LL)Cp ]BF4 [M(LL)Cp´ = Ru(PPh3)2Cp ([1a]BF4); Ru(dppe)Cp* ([1b]BF4); Fe(dppe)Cp ([1c]BF4); Fe(dppe)Cp* ([1d]BF4)] react with H+/H2O to give the acyl‐functionalised complexes [M{CºC‐1,4‐C6H4‐C(=O)CH3}(LL)Cp´]. The Markovnikov addition of the nucleophile to the remote alkyne is difficult to rationalise from the vinylidene form of the precursor and is much more satisfactorily explained from initial isomerisation to the quinoidal cumulene [M(=C=C=C6H4=C=CH2)(LL)Cp´]+ prior to attack at the more exposed, remote quaternary carbon. Thus, whilst representative acetylide complexes [Ru(CºC‐1,4‐C6H4‐CºCH)(PPh3)2Cp] and [Ru(CºC‐1,4‐C6H4‐CºCH)(dppe)Cp*] reacted with the small electrophiles [CN]+ and [C7H7]+ at the β‐carbon to give vinylidene complexes, trityl cation ([CPh3]+) reacted with [M(CºC‐1,4‐C6H4‐CºCH)(LL)Cp´] at the exposed 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{CºC‐1,4‐C6H4‐C(=O)CH2CPh3}(LL)Cp´]. Evincing the scope of the reaction, the (5‐ethynyl‐2‐thienyl)vinylidene complexes [M{C=C(H)‐2,5‐cC4H2S‐CºCH}(LL)Cp´]BF4 add water readily to give [M{CºC‐2,5‐cC4H2S‐C(=O)CH3}(LL)Cp´]