First X-ray Characterization and Theoretical Study of π-Alkyne, Alkynyl-Hydride, and Vinylidene Isomers for the Same Transition Metal Fragment [Cp*Ru(PEt3)2]+

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Abstract
The reaction of the chloro-complex [Cp*RuCl(PEt3)2] with acetylene gas in methanol gave the π-alkyne complex [Cp*Ru(η2-HC⋮CH)(PEt3)2][BPh4] (1), which has been structurally characterized by X-ray analysis. The alkyne complex undergoes spontaneous isomerization even at low temperature, yielding the metastable alkynyl-hydride complex [Cp*Ru(H)(C⋮CH)(PEt3)2][BPh4] (2), as the result of the oxidative addition of the alkyne C−H bond. This compound has also been structurally characterized despite it tautomerizes spontaneously into the stable primary vinylidene [Cp*Ru(CCH2)(PEt3)2][BPh4] (3). This species has been alternatively prepared by a two-step deprotonation/protonation synthesis from the π-alkyne complex. Moreover, the reaction of the initial chloro-complex with monosubstituted alkynes HC⋮CR (R = SiMe3, Ph, COOMe, tBu) has been studied without detection of π-alkyne intermediates. Instead of this, alkynyl-hydride complexes were obtained in good yields. They also rearrange to the corresponding substituted vinylidenes. In the case of R = SiMe3, the isomerization takes place followed by desilylation, yielding the primary vinylidene complex. X-ray crystal structures of the vinylidene complexes [Cp*Ru(CCH2)(PEt3)2][BPh4] (3) and [Cp*Ru(CCHCOOMe)(PEt3)2][BPh4] (10) have also been determined. Both, full ab initio and quantum mechanics/molecular mechanics (QM/MM) calculations were carried out, respectively, on the model system [CpRu(C2H2)(PH3)2]+ (A) and the real complex [Cp*Ru(C2H2)(PEt3)2]+ (B) to analyze the steric and electronic influence of ligands on the structures and relative energies of the three C2H2 isomers. QM/MM calculations have been employed to evaluate the role of the steric effects of real ligands, whereas full ab initio energy calculations on the optimized QM/MM model have allowed recovering the electronic effects of ligands. Additional pure quantum mechanics calculations on [Cp*Ru(C2H2)(PH3)2]+ (C) and [CpRu(C2H2)(PMe3)2]+ (D) model systems have been performed to analyze in more detail the effects of different ligands. Calculations have shown that the steric effects induced by the presence of bulky substituents in phosphine ligand are responsible for experimentally observed alkyne distortion and for relative destabilization of the alkyne isomer. Moreover, increasing the phosphine basicity and σ donor capabilities of ligands causes a relative stabilization of an alkynyl-hydride isomer. The combination of both steric and electronic effects, makes alkyne and alkynyl-hydride isomers to be close in energy, leading to the isolation of both complexes.

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