Silver(I)-mediated isomerisation of trans-[RuCl2(P–P)2] (P–P = four-membered ring chelate diphosphine ligand) to cis-[{Ru(P–P)2(μ-Cl)2}nAg]+ species, and applications in stereoselective syntheses of cis- and trans-[RuCl(L)(P–P)2]+ (L = neutral ligand)

Abstract

When complexes trans-[RuCl₂(diphos)₂] (diphos = (Ph₂P)₂CCH₂, Ph₂PCH₂PPh₂ or Ph₂PNHPPh₂) were treated with silver salts of poorly coordinating anions (e.g. AgBF₄, AgO₃SCF₃) in 1,2-dichloroethane halide abstraction was slow, even at reflux, and the reaction proceeded by an interesting mechanism involving a trans to cis isomerisation. At room temperature, a complex, characterized as [RuCl(diphos)₂(μ-Cl)Ag]⁺, forms. When heated to reflux for 5–10 min this reacts to give a mixture, which includes cis-[RuCl₂(diphos)₂], and species of the type [{Ru(diphos)₂(μ-Cl)₂}_nAg]⁺ (n = 1 or 2) in which the exact coordination geometry at Ag^I is unknown. The reactions have been followed by ³¹P-{¹H} NMR spectroscopy, and electronic spectroscopy results also support the isomerisation. The ruthenium and silver K-edge EXAFS spectra of the solid isolated from the reaction of trans-[RuCl₂(dppm)₂] and AgO₃SCF₃ lend support to this formulation; in particular, the best fit for the silver(I) environment includes coordination to one oxygen (from the ⁻O₃SCF₃; Ru–O 2.43 Å), two Cl (2.79 Å) and between one and two Ru (3.94 Å). Over 30–40 min at reflux, AgCl precipitates, the signals due to cis-[RuCl₂(diphos)₂] disappear from the ³¹P-{¹H} NMR spectra, those due to the [{Ru(diphos)₂(μ-Cl)₂}_nAg]⁺ mixture diminish, and new resonances, attributed to five-coordinate [RuCl(diphos)₂]⁺, appear. This reaction has been investigated for stereoselective ligand substitution. Treatment of trans-[RuCl₂(diphos)₂] with AgBF₄ or AgO₃SCF₃ in DCE in the presence of CH₃CN or CO yields exclusively trans-[RuCl(L)(diphos)₂]⁺ (L = CH₃CN or CO), but treatment of trans-[RuCl₂(diphos)₂] with AgBF₄ or AgO₃SCF₃ in DCE, followed by treatment with L, usually gave exclusively cis-[RuCl(L)(diphos)₂]⁺ (L = CH₃CN or CO).