Structural Studies of the Alkali Metal Picrates

Abstract

A comparative series of room-temperature, single-crystal X-ray studies of the alkali metal (and ammonium) picrate structures is presented, all being in some sense revisitations of earlier work of various vintages, except for that of the sodium salt. Lithium picrate monohydrate is triclinic, Pī, a 10.710(6), b 7.186(1), c 7.112(1) Ǻ, α 65.89(1), β 71.90(3), γ 84.45(2)°, Z = 2 f.u.; conventional R on |F| was 0.053 for N_o = 1792 'observed' (I > 3σ(I)) reflections. Sodium picrate monohydrate is monoclinic, C 2/m, a 13.074(6), b 20.080(6), c 3.690(3) Ǻ, β 90.67(3)°, Z = 4 f.u.; R was 0.056 for N_o 519. Potassium picrate (anhydrous) is orthorhombic, Ibca , a 13.316(3), b 19.107(5), c 7.138(2) Ǻ, Z = 8 f.u ., R 0.036 for N_o 1308; the ammonium salt is isomorphous, a 13.474(4), b 19.790(7), c 7.131 (4) Ǻ, R 0.040 for N_o 981. Rubidium picrate (anhydrous) is monoclinic, P 2₁/c, a 10.604(1), b 4.556(3), c 19.183(2) Ǻ, β 101.41(1)°, Z = 4 f.u ., R 0.039 for N_o 1878; the caesium salt is isomorphous, a 10.810(1), b 4.700(1), c 19.400(3) Ǻ, β 101.43(1)°, R 0.027 for N_o 2177. The lithium salt is binuclear, the phenolic oxygen atoms bridging the five-coordinate lithium atoms, while the sodium salt is a two-dimensional polymer parallel to the ab plane, the picrates cross-linking one-dimensional arrays of sodium atoms of two types, one eight-coordinate and linked by bridging phenolic oxygen atoms, and the other six-coordinate and linked by water molecules. The potassium and ammonium salts are also two-dimensional polymers parallel to the ab planes, linked in the second dimension by pairs of picrates opposed in polarity and linking the eight-coordinate potassium atoms by phenolic oxygen atoms on the one hand, and the 4-nitro group oxygen atoms on the other. The rubidium and caesium salts are two-dimensional polymers in the ac plane, eight-coordinate metal atoms being linked by a complex web of picrate groups; unlike the other compounds, they do not contain bridging phenolic oxygen atoms.