Reactivity of the electron-rich allenylidene-ruthenium complexes [Cp*Ru{=C=C=C(R)Ph}(dippe)][BPh4] (R = H, Ph).: X-ray crystal structure of a novel dicationic ruthenium carbyne (CP* = C5Me5; dippe=1,2-bis(diisopropylphosphine)ethane)

The reactions of the allenylidene complexes [Cp*Ru{=C=C=C R)Ph}(dippe)][BPh4] (R = H (1), Ph (2)) with different substrates have been studied, providing a new form of allenylidene-ruthenium reactivity. The observed reactivity pattern depends strongly on the substituents on the gamma-carbon. The secondary allenylidene I undergoes addition of weakly nucleophilic reagents such as pyrazole, 3,5-dimethylpyrazole, or thiophenol to the C-beta-C-gamma double bond, yielding substituted vinylidene compounds [Cp*Ru{=C=CHCH(L)Ph}(dippe)]-[BPh4] (L = pyrazolyl (3), 3,5-dimethylpyrazolyl (4), phenylsulfanyl 5)). The reaction of I with pyrrole or 2-methylfuran to afford analogous complexes [Cp*Ru{=C=CH-CH(L)Ph}-(dippe)][BF4] (L = 2-pyrrolyl (6), 5-methyl-2-furanyl (7)) takes place only in the presence of acid. This suggests that an initial protonation at the beta-carbon of the allenylidene occurs, enhancing the electrophilic character of the gamma-carbon atom. This mechanism involves the formation of dicationic carbyne ruthenium complexes [Cp*Ru{dropC-CH=C(R)Ph}(dippe)](2+) (R = H (8), Ph (9)), which have been isolated and characterized as [B(Ar-F)(4)] (Ar-F = 3,5-(CF3)(2)C6H3) salts, by protonation of the cationic allenylidenes with [H(Et2O)(2)][B(Ar-F)(4)]. The X-ray crystal structure of the carbyne compound 9 is reported. A series of neutral functionalized alkynyl compounds [Cp*Ru{CdropCCR(L)Ph}(dippe)] (L = CH3COCH2, R = H (10), R = Ph (11); L = pyrazolyl, R = H (12); R = Ph (13)) have also been synthesized by regioselective addition of anionic nucleophiles such as potassium acetonate or potassium pyrazolate. The structures of 11 and 13 in the solid state have been determined by X-ray diffraction analysis. Protonation of 10 and 11 with HBF4.Et2O yields the vinylidene compounds [Cp*Ru{=C=CH-CR(CH2COCH3)Ph}(dippe)][BF4] (R = H (14), Ph (15)).