Structural Studies of Alkaline Earth Metal Picrates

Abstract

Room-temperature, single-crystal X-ray structure determinations are recorded for alkaline earth metal picrates , M( pic )₂.nH₂O(M = Mg, Sr , Ba ; Hpic = 2,4,6-trinitrophenol, HOC₆H₂(NO₂)₃), all crystallized from aqueous solution. Magnesium picrate ,shown to be the nonahydrate, Mg( pic )₂.9H₂O, forms monoclinic crystals, space group P2₁/c, a 15.023(3), b 6.718(4), c 26.516(2) Ǻ, β 109.55(1)°, Z = 4 f.u .,conventional R on |F| was 0.049 for N_o = 4062 'observed' (I > 3σ(I)) reflections. The structure, unusually among the main group metal picrates studied so far, has no coordinative interaction between the metal and the picrate anion; the complex may be formulated as [Mg(OH₂)₆] ( pic )₂.3H₂O, with the metal present as the hexaaqua species (Mg-O 2.043(2)-2.077(2)Ǻ). The picrate ions are stacked end to end, presumably as a result of charge-transfer interactions, parallel to b, with side-by-side stacks forming a sheet along ab, and successive sheets are interleaved by layers of cations and water molecules. The strontium salt is a pentahydrate , monoclinic, C2/c, a 24.515(6), b 10.142(6), c 17.932(4) Ǻ, β 98.76(2)°, Z = 8 f.u .; R was 0.046 for N_o 2731. The complex is a linear polymer, [(H₂O)₃Sr(O,O′- pic )₂(μ-OH₂)](∞|∞).H₂O, with eight-coordinate (SrO₈) strontium. The barium salt is a hexahydrate , also a linear polymer [(H₂O)₅Ba(O,O′-pic )( O,O′,μ-O″-pic )]∞|∞).H₂O, the ten-coordinate (BaO₁₀) barium environment comprising the five water molecules, a pair of picrates, bidentate /chelating via the phenolic oxygen and a 2-nitro group oxygen, with one of the picrates from an adjacent metal atom offering an additional 4-nitro group oxygen, this bridging successive barium atoms. The complex is triclinic, Pī , a 15.120(5), b 11.633(3), c 6.766(3) Ǻ, α 87.24(3), β 79.16(3), γ 84.88(2)°, Z = 2 f.u .; R was 0.027 for N_o 3904. Repeated attempts to obtain crystals of beryllium picrate suitable for an X-ray structure determination were unsuccessful.