Synthesis, Structure, and Transformation Studies in a Family of Inorganic−Organic Hybrid Framework Structures Based on Indium

Eight new open-framework inorganic−organic hybrid compounds based on indium have been synthesized employing hydrothermal methods. All of the compounds have InO<sub>6</sub>, C<sub>2</sub>O<sub>4</sub>, and HPO<sub>3</sub>/HPO<sub>4</sub>/SO<sub>4</sub> units connected to form structures of different dimensionality. Thus, the compounds have zero- (<b>I</b>), two- (<b>II</b>, <b>III</b>, <b>IV</b>, <b>V</b>, <b>VII</b>, and <b>VIII</b>), and three-dimensionally (<b>VI</b>) extended networks. The formation of the first zero-dimensional hybrid compound is noteworthy. In addition, concomitant polymorphic structures have been observed in the present study. The molecular compound, <b>I</b>, was found to be reactive, and the transformation studies in the presence of a base (pyridine) give rise to the polymorphic structures of <b>II</b> and <b>III</b>, while the addition of an acid (H<sub>3</sub>PO<sub>3</sub>) gives rise to a new indium phosphite with a pillared layer structure (<b>T1</b>). Preliminary density functional theory calculations suggest that the stabilities of the polymorphs are different, with one of the forms (<b>II</b>) being preferred over the other, which is consistent with the observed experimental behavior. The oxalate units perform more than one role in the present structures. Thus, the oxalate units connect two In centers to satisfy the coordination requirements as well as to achieve charge balance in compounds <b>II</b>, <b>IV</b>, and <b>VI</b>. The terminal oxalate units observed in compounds <b>I</b>, <b>IV</b>, and <b>V</b> suggest the possibility of intermediate structures. Both <i>in-plane</i> and <i>out-of-plane</i> connectivity of the oxalate units were observed in compound <b>VI</b>. The compounds have been characterized by powder X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and <sup>31</sup>P NMR studies.