Synthesis and structural characterization of three new strontium(II) coordination polymers based on 4-nitrobenzoate

Abstract

The reaction of the water-rich compound [Sr(H sub(2)O) sub(7)(4-nba)](4-nba).2H sub(2)O (4-nba=4-nitrobenzoate) with dimethylformamide (DMF) or N-methylformamide (NMF) or pyrazole (Pyr) resulted in the incorporation of the O- or N-donor ligand in the coordination sphere of Sr(II) forming the one-dimensional (1D) coordination polymers [Sr(H sub(2)O) sub(3)(DMF)(4-nba)](4-nba) (1), [Sr sub(2)(H sub(2)O) sub(2)(NMF) sub(3)(4-nba) sub(4)] (2) and [Sr(Pyr) sub(2)(4-nba) sub(2)] (3) respectively. In the case of Mg(II), a similar protocol afforded crystallization of [Mg(H sub(2)O) sub(6)](4-nba) sub(2)(DMF) sub(2) (4), [Mg(H sub(2)O) sub(6)](3-nba) sub(2)(Acet) sub(2) (5), (3-nba=3-nitrobenzoate; Acet=Acetamide) in which the amides function as solvates. The syntheses, crystal structures and properties of 1-5 are reported. In 1-3 Sr(II) exhibits eight coordination while Mg(II) displays hexa coordination in 4 and 5 forming the centrosymmetric octahedral [Mg(H sub(2)O) sub(6)] sup(2+) unit. The Mu sub(3)-Eta sup(2):Eta sup(2)-tetradentate bridging binding modes in 1 and 2 and Mu sub(2)-Eta sup(1):Eta sup(2)-tridentate bridging binding mode in 3 of the 4-nitrobenzoate result in the formation of 1D coordination polymer. The carboxylate ion does not coordinate to Mg(II) in 4 and 5, but functions as a charge balancing counter anion. The {SrO sub(8)} polyhedra in 1 and {SrN sub(2)O sub(6)} polyhedra in 3 share edges in the 1D chains, while in 2 the {SrO sub(8)} polyhedra are linked into a 1D chain via alternate edge and face sharing arrangement. Compounds 1 (CH...O, O-H...O) and 3 (C-H...O, N-H...O) exhibit two varieties of hydrogen bonding interactions while 2 and 5 are involved in three types of H-bonding interactions viz. C-H...O, O-H...O, N-H...O. A study of the structural features of several alkaline-earth 4-nitrobenzoates is described.