Parallel Approaches to Mono- and Bis-Propargylic Activation via Co2(CO)8 and [Ru3(μ-Cl)(CO)10]

Butyne derivatives CH3C⋮CCH2X and XCH2C⋮CCH2X (X = hydroxy, acetoxy, tosyloxy) were reacted with Co2(CO)8 and [PPN][Ru3(μ-Cl)(CO)10]. Whereas dissociation of hydroxy and acetoxy groups from the cluster-bound mono-oxypropargylic ligands CH3C⋮CCH2X requires the assistance of an acid, spontaneous dissociation occurs with a tosyloxy group. For cobalt, this produces the ether complex {Co2(CO)6}2(μ-CH3C⋮CCH2OCH2C⋮CCH3). For ruthenium, stepwise propargylic activation affords the allenyl complex Ru3(μ-Cl)(μ-η3-CH3CCCH2)(CO)9. The elusive allenylium cobalt intermediate [Co2(μ-η3-CH3CCCH2)(CO)6]+ was optimized at the B3PW91/6-31G* level of theory. Analyses of frontier orbitals, Mulliken charges, and Fukui indices reveal that the ruthenium complex is less electrophilic than the cobalt complex and that soft nucleophiles should react at metal centers in both complexes. This provides a rationale for the known reactivity of Nicholas' cobalt complexes. It is also consistent with the observed reactivity of a hydride with the ruthenium complex, which takes place at the metal, followed by H transfer to the propargylic carbon. With the bis-oxypropargylic ligands, the complexes Co2(μ-XCH2C⋮CCH2X)(CO)6 and [Ru3(μ-Cl)(μ-XCH2C⋮CCH2X)(CO)9]- (X = OH, OAc) were also prepared. Protonation of the latter yields the neutral allenyl species Ru3(μ-Cl)(μ-η3-XCH2CCCH2)(CO)9. Again, the specific behavior of the bis(tosyloxy) ligand (X = OTs) was observed. While the cobalt complex Co2(μ-η2-TsOCH2CCCH2OTs)(CO)6 turned out to be relatively stable, a butatriene-type ruthenium complex was detected by in situ 2D 1H−13C NMR. The target cationic butatriene complex [Ru3(μ-Cl)(μ-η4-CH2CCCH2)(CO)9][BF4] was finally obtained by protonation of the hydroxyallenyl complex Ru3(μ-Cl)(μ-η3-HOCH2CCCH2)(CO)9.