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).