Coordination chemistry of alkali and alkaline earth cations: Synthesis and X-ray crystal structure of cesium (picrate) (benzo-15-crown-5) Cs⁺C₆H₂N₃O₇^-(C₁₄H₂₀O₅)

Crystals of the Cs⁺ Pic^- (B15C5) complex (Pic=picrate; B15C5=benzo-15-crown-5) (M_r=629.3) are yellow prisms which belong to the triclinic space group Pī with a=7.377(4), b=11.372(2), c=14.806(2) Å, α=90.31(1), β=91.06(2), γ=108.32(2)⁰, Z=2, D_x=1.77, and D_m=1.77 g cm^-3. Final R=0.055 for 3575 observed reflections out of a total of 4004 measured reflections. Cesium forms a 1:1 anion paired complex with B15C5 like sodium rather than a charge separated sandwich structure as found for potassium and expected for cesium in view of the ion-cavity radius concept. The Cs cation is 9-coordinate involving the five crown oxygens (Cs...O, 3.00(1) to 3.24(1) Å), the phenoxide (Cs...O^-, 3.03(1) Å) and an ortho nitro group oxygen (Cs...O, 3.01(1) Å) of the picrate counteranion and, uniquely, with two additional oxygens (Cs...O, 3.17(1) and 3.40(1) Å) from a para nitro group of the picrate belonging to the adjacent molecule in the lattice. The Cs⁺ ion lies 2.07 Å out of the mean plane formed by the crown oxygens. This system provides the first structural evidence that the interaction stoichiometry of an alkali cation with a cyclic multidentate ligand is not a function of the ion and cavity size alone but also of its Lewis acid strength as modified by the charge neutralizing anion.