Homoleptic borates and aluminates containing the difluorophosphato ligand - [M(O2PF2)X]Y- - synthesis and characterization

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 Li-3[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 center dot 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 Bronsted 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).