Mechanistic Studies into the Catalytic Activity of Half-Sandwich Ruthenium Complexes

A mechanistic investigation into the catalytic activity of half-sandwich ruthenium complexes, [Ru(η5-C5H5)(L)3]+ (where L = phosphorus- or nitrogen- donating ligands ) for the alkenylation of pyridine is reported. Mechanistic studies have demonstrated that [Ru(η5-C5H5)(PPh3)(NC5H5)2][PF6] is an important intermediate in the formation of E-2-styrylpyridine derivatives. Collaboration between the reported experimental and additional theoretical studies (conducted by David Johnson) has allowed for a catalytic cycle for the alkenylation of pyridine to be proposed, highlighting the role of vinylidene and pyridylidene ligands.
A general synthetic procedure for a range of [Ru(η5-C5H5)(PR3)(L)2][PF6] complexes from [Ru(η5-C5H5)(NCMe)3][PF6] was performed, where R = Ph, Me, iPr, OPh; and L = pyridine, 3-methylpyridine, 4-methylpyridine, 4-dimethylaminopyridine, 1-methylimidazole, t-butylimidazole. The properties of these complexes have been investigated.
The reactivity of [Ru(η5-C5H5)(PPh3)(NC5H5)2][PF6] with terminal alkynes was investigated in two different reaction media (dichloromethane and pyridine). The stoichiometric reaction of [Ru(η5-C5H5)(PPh3)(NC5H5)2][PF6] with phenylacetylene in a dichloromethane solution generated a vinylidene-containing intermediate [Ru(η5-C5H5) (PPh3)(NC5H5)(=C=CHPh)][PF6]. This reacts further to produce a pyridylidene-containing complex [Ru(η5-C5H5)(PPh3)(κ3-C3-C5H4NCH=CHPh)][PF6]. Upon the addition of excess pyridine a 1-ruthanaindolizine species is generated. Similar observations have been made with alkynes HC≡CR (where R = 4-F-C6H4, 4-CF3-C6H4) and [Ru(η5-C5H5)(PPh3)(L)2][PF6] (where L = 3-methylpyridine, 4-methylpyridine, 4-dimethylaminopyridine) and the substituent effects and properties of the pyridylidene-containing complexes are compared to the literature.
The addition of phenylacetylene or 4-ethynyl-α,α,α-trifluorotoluene to [Ru(η5-C5H5) (PPh3)(NC5H5)2][PF6] in a pyridine solution at 50 °C produces 2-substituted E-styrylpyridine derivatives in a 100 % atom efficient manner. The formation of the 1-ruthanaindolizine complex results in catalyst deactivation. A range of reaction conditions have been investigated to identify the optimum catalyst performance.