Homoleptic borates and aluminates containing the difluorophosphato ligand – [M(O2PF2)x]y− – synthesis and characterization

Abstract

Weakly coordinating anions (WCAs) with the difluorophosphato ligand (O₂PF₂) were the target of this study. Initial experiments were conducted towards the preparation of homoleptic aluminates of the well-studied [Al(OR)₄]⁻-type. The preparation of the initial target structure Li[Al(O₂PF₂)₄] failed due to the remaining Lewis acidic character of the central aluminum atom. Instead, the formation of Li₃[Al(O₂PF₂)₆] and Al(O₂PF₂)₃ was observed with hexacoordinate aluminum atoms and verified by NMR, IR and X-ray crystallography. A possible mechanism towards these compounds was postulated in the solvent induced dismutation of the tetracoordinate Li[Al(O₂PF₂)₄]. A singly charged WCA was realized by the exchange of the central aluminum atom for boron. The [B(O₂PF₂)₄]⁻ anion was prepared starting from BH₃·S(CH₃)₂ and boron tribromide leading to the protic room temperature Ionic Liquid (IL) [H(S(CH₃)₂)][B(O₂PF₂)₄] and the neat liquid Brønsted acid H[B(O₂PF₂)₄], respectively, representing a significantly improved synthesis with regard to the first experiments of Dove et al. The basicity of the [B(O₂PF₂)₄]⁻ anion and its WCA quality were investigated on the basis of the IR-spectroscopic NH-scale and the salt [H(N(Oct)₃)][B(O₂PF₂)₄] that places it better than all oxyanions and close to the carboranate based WCAs. A pathway to the solvent free pure Li[B(O₂PF₂)₄] salt was established on a multi-gram scale with excellent purities enabling electrochemical applications (verified by NMR, IR, X-ray crystallography and cyclovoltammetry).