The first noncoordinated phosphonium diylide, [Me₂P(C₁₃H₈)₂]-, and its ylidic and cationic counterparts: Synthesis, structural characterization, and interaction with the heavy group 2 metals

Treatment of potassium or lithium fluorenide with MePCl₂ generates the organophosphine MeP(C₁₃H₉)₂, which on reaction with methyl iodide produces the phosphonium species [Me₂P(C₁₃H₉)₂]I in 74% yield. In the solid state, H···I contacts of <3.3 Å help generate a layered structure m which the fluorenyl rings are nearly parallel. On subsequent reaction of [Me₂P(C₁₃H₉)₂]I with either KH or K[N(SiMe₃)₂] the corresponding neutral phosphoylide, Me₂P(C₁₃H₉)(C₁₃H₈), forms in 67% yield and was structurally characterized. The phosphonium iodide [Me₂P(C₁₃H₉)₂]I was allowed to react with Ae[N(SiMe₃)₂]₂ (Ae = Ca, Ba), and the product from the reaction with the calcium complex was structurally identified as the salt [CaI(thf)₅][Me₂P(C₁₃ H₈)₂]. The anion, which is outside the coordination sphere of the calcium, represents the first structurally authenticated example of a free phosphonium diylide. The P-C(ylidic) bond length of 1.748(4) Å reflects some partial multiple bond character. ¹H and ³¹P NMR spectra suggest that the barium analogue is similar. Density functional theory calculations were performed on representative phosphonium diylides as an aid to interpreting the bonding in this class of compounds. Despite the strong electrostatic attraction that usually drives metal-ligand binding in highly ionic systems, calcium and barium prefer to coordinate to a single iodide ion and several neutral oxygen donors rather than to the charged diylide.