Coordination chemistry of alkali and alkaline earth cations: Synthesis and X-ray crystal structure of cesium (picrate) (benzo-15-crown-5) Cs+C6H2N3O 7 − (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 groupP witha=7.377(4),b=11.372(2),c=14.806(2) , =90.31(1), =91.06(2), =108.32(2)⁰,Z=2,D _x =1.77, andD _m =1.77 g cm^–3. FinalR=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) ), the phenoxide (Cs...O^–, 3.03(1) ) and anortho nitro group oxygen (Cs...O, 3.01(1) ) of the picrate counteranion and, uniquely, with two additional oxygens (Cs...O, 3.17(1) and 3.40(1) ) from apara nitro group of the picrate belonging to the adjacent molecule in the lattice. The Cs⁺ ion lies 2.07 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. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82027.