Temperature-Controlled Chiral and Achiral Copper Tetrazolate Metal–Organic Frameworks: Syntheses, Structures, and I₂ Adsorption

Abstract

Four tetrazole-based three-dimensional (3D) metal–organic frameworks (MOFs), {[Cu^II(btz)]·0.5H₂O}_<i>n</i> (<b>1</b>), [Cu^II(btz)]_<i>n</i> (<b>1′</b>), {[Cu^II(btz)]·0.5I₂}_<i>n</i> (<b>1′·0.5I₂</b>), and [Cu^IICu^I₂(btz)₂]_<i>n</i> (<b>2</b>) [H₂btz = 1,5-bis­(5-tetrazolo)-3-oxapentane)], have been successfully obtained and characterized by crystallography. Compound <b>1</b> features a chiral porous framework. The bulk crystallization of <b>1</b> is composed of enantiomers <b>1a</b> (<i>P</i>4₁) and <b>1b</b> (<i>P</i>4₃), which has been demonstrated by the crystal structure analyses of nine crystals of <b>1</b> randomly selected. The Cotton effect displayed in the solid-state circular dichroism spectrum of <b>1</b> is therefore attributed to enantiomeric excess rather than enantiopurity. The completely dehydrated phase of <b>1</b>, that is, <b>1′</b>, can adsorb 0.5 I₂ molecule per formula unit to yield compound <b>1′·0.5I₂</b>, which has been supported by single-crystal X-ray diffraction, elemental analysis, and thermogravimetric analysis. The locations of I₂ in the pores were unambiguously determined, and the interactions between I₂ molecules and the pore structures were investigated. Compound <b>2</b> crystallizes in an achiral <i>C</i>2/<i>c</i> space group. Interestingly, the formations of chiral <b>1</b> and achiral <b>2</b> significantly depend on the reaction temperature. Between 80 and 140 °C, we got compound <b>1</b> as the only product. At 150 °C, both <b>1</b> and <b>2</b> were in coexistence in the final product. From 160 to 180 °C, only compound <b>2</b> was obtained. More interestingly, treatment of the crystals of <b>1</b> or the mixture of <b>1</b> and <b>2</b> obtained at 150 °C in their mother liquor at 170 °C yielded the crystals of <b>2</b> in a single phase