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]⁺

Abstract

The reaction of the chloro-complex [CpRuCl(PEt(3))(2)] with acetylene gas in methanol gave the pi-alkyne complex [CpRu(eta(2)-HCtbd1;CH)(PEt(3))(2)][BPh(4)] (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 [CpRu(H)(Ctbd1;CH)(PEt(3))(2)][BPh(4)] (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 [CpRu(=C=CH(2))(PEt(3))(2)][BPh(4)] (3). This species has been alternatively prepared by a two-step deprotonation/protonation synthesis from the pi-alkyne complex. Moreover, the reaction of the initial chloro-complex with monosubstituted alkynes HCtbd1;CR (R = SiMe(3), Ph, COOMe, (t)Bu) has been studied without detection of pi-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 = SiMe(3), the isomerization takes place followed by desilylation, yielding the primary vinylidene complex. X-ray crystal structures of the vinylidene complexes [CpRu(=C=CH(2))(PEt(3))(2)][BPh(4)] (3) and [CpRu(=C=CHCOOMe)(PEt(3))(2)][BPh(4)] (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(C(2)H(2))(PH(3))(2)](+) (A) and the real complex [CpRu(C(2)H(2))(PEt(3))(2)](+) (B) to analyze the steric and electronic influence of ligands on the structures and relative energies of the three C(2)H(2) 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 [CpRu(C(2)H(2))(PH(3))(2)](+) (C) and [CpRu(C(2)H(2))(PMe(3))(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 sigma 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.

Experimental and Computational Studies of Hydrogen Bonding and Proton Transfer to [Cp*Fe(dppe)H]

“…[60,61] The interaction energy at the hydride site remains almost unchanged, showing that the electronic effect of the phenyl substituents is also small. The results at the metal site contrast with those at the hydride site.…”

Structure and Reactivity of Coordinatively Unsaturated Half‐Sandwich Iron, Ruthenium and Osmium Complexes

“…[10,29,82] With the exception of the reaction with HCl, the amine complexes [(C 5 system for computation using a combination of DFT and quantum mechanics/molecular mechanics calculations. [97] Whereas the solid-state structures determined by X-ray crystallography for several alkynyl hydrido complexes show a transoid disposition of alkynyl and hydrido ligands (Figure 18 SO 3 ]. This compound is stabilized by means of an agostic interaction with a hydrogen atom of one of the isopropyl groups of the TpiPr ligand, as revealed by X-ray crystallography [ Figure 19; Ru···C(H agostic ) 2.627(6) Å , Ru···H agostic 1.83(6) Å ].…”

Metal Vinylidenes and Allenylidenes in Catalysis: Applications in Anti‐Markovnikov Additions to Terminal Alkynes and Alkene Metathesis

“…[2,[20][21][22] To rationalize the alternative pathways, several theoretical studies have been carried out, [23][24][25][26] and recent experimental and theoretical studies have demonstrated that a bimolecular mode for the h 2 -alkyne-h 1 -vinylidene isomerization process was also possible. [27,28] Vinylidenes can also be produced from ruthenium hydride complexes by insertion of the triple bond to form a vinyl ruthenium species followed by a-H migration.…”