Experimental and Computational Studies of Steric Factors in the Isomerization of Internal Alkynes within the Coordination Environment of Half-Sandwich Metal Complexes

The isomerization of internal alkynes Ar¹C≡CAr² within the coordination environment of low-valent half-sandwich [Ru(dppe)Cp]⁺ complexes via a 1,2-migration process affords vinylidene species [Ru{=C=C(Ar¹)Ar²}(dppe)Cp]⁺. The rearrangement reactions of symmetrically and asymmetrically substituted substrates featuring different electron-donating and -withdrawing groups and of varying steric bulk were modelled using density functional theory (DFT), and the conclusions supported by experimental observations. Examination of the reaction pathway and associated activation barriers reveal a high solvent dependency for the generation of the key intermediate species [Ru(dppe)Cp]⁺ from [RuCl(dppe)Cp] by halide dissociation in the presence of Na⁺ salts of weakly coordinating anions, with the lattice enthalpy of the NaCl by-product playing a critical role in the overall thermochemical balance of the reaction. The activation barriers associated with the reaction of [Ru(dppe)Cp]⁺ with Ar¹C≡CAr², and the relative energies of the alkyne complexes [Ru(η²-Ar¹C≡CAr²)(dppe)Cp]⁺, are sensitive to the electron density of the alkyne and conformational changes associated with the 'bend-back' of the substrate. The latter differs by up to 66.1 kJ/mol, which in turn impacts the barrier height of the subsequent 1,2-migration step involved in the rearrangement process and ultimately the overall thermochemical nature of the complete reaction. The relative importance of these factors is evinced by the successful rearrangement of the very sterically congested 1(9-anthryl)-2(9-phenanthryl) acetylene into the fully characterized diaryl vinylidene complex, which was isolated in 89 % yield.