Microwave synthesis and single-crystal-to-single-crystal transformation of magnesium coordination polymers exhibiting selective gas adsorption and luminescence properties

A series of new magnesium coordination polymers, [Mg2(BTEC)(H2O)4]·2H2O (1), [Mg2(BTEC)(H2O)6] (2), and [Mg2(BTEC)(H2O)8] (3) (BTEC = 1,2,4,5-benzenetetracarboxylate anion), were synthesized from magnesium nitrate and 1,2,4,5-benzenetetracarboxylic acid with variable ratios of organic base under microwave-assisted solvothermal reactions at the temperature of 180 (1) or 150 °C (2 and 3). The results from crystal structure analysis showed that 1 was a 3D metal–organic framework, 2 a 2D layer, and 3 a 1D ladder structure. In addition, heating 1 at the temperature of 150 °C would convert it to 1a which can be a novel single-crystal-to-single-crystal transformation of coordination polymer. Single crystal of 1a has a new 3D structure with formula [Mg15(BTEC)6(HBTEC)2(H2O)12]·4H2O, containing unsaturated magnesium sites. Further gas adsorption experiments revealed that 1a undergoes a highly selective sorption of hydrogen instead of nitrogen gases. Photoluminescence studies also displayed that 1a emits extra strong green luminescence around 510 nm under the excitation of ultraviolet light.