Bond Formation upon Water Removal in an Unusual “Pseudo”-Topotactic Reaction Investigated by Single-Crystal Structure and in Situ Synchrotron X‑ray Powder Diffraction Analysis

• Published in: Crystal growth & design Vol. 19; no. 10; pp. 5743 - 5750
• Main Authors: Dopta, Joanna, Hansen, Anna-Lena, Pienack, Nicole, Mahnke, Lisa K, Reinsch, Helge, Etter, Martin, Näther, Christian, Bensch, Wolfgang
• Format: Journal Article
• Language: English
• Published: American Chemical Society 02.10.2019
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/acs.cgd.9b00727

The new compound [Cu­(cyclam)­(H2O)]­{[Cu­(cyclam)]2[HTiNb9O28]}·26H2O (1) (cyclam = 1,4,8,11-tetraazacyclotetradecane) was obtained under solvothermal conditions. Its crystal structure contains a monotitano-nonaniobate anion in which one position is equally occupied by Nb­(V) and Ti­(IV). The anions are expanded by [Cu­(cyclam)]2+ cations via Nb–O–Cu bridges generating {[Cu­(cyclam)]2[HTiNb9O28]}2– cluster units, which are arranged into layers. Between these layers there are additionally isolated [Cu­(cyclam)­(H2O)2]2+ cations as well as hydrate water molecules. Storage of 1 at room temperature leads to loss of ∼13 water molecules, and a new crystalline phase (2) crystallizes that, with heating, transforms into the anhydrate. The reversibility of this reaction was investigated by thermogravimetry and X-ray powder diffraction (XRPD). Temperature-dependent in situ synchrotron XRPD investigations prove an abrupt phase transition, in which especially the a axis is dramatically shortened and the {[Cu­(cyclam)]2[HTiNb9O28]2–} cluster is rearranged. Single-crystal X-ray diffraction of 2 reveals that, despite the unusual large shrinking of the unit cell volume, the domains formed by water removal exhibit some preferred orientation close to that expected for a topotactic reaction, which allowed the performance of a structure analysis. In the structure of 2, the two water molecules of the isolated [Cu­(cyclam)­(H2O)2]2+ cation in 1 are replaced by two terminal cluster O atoms, leading to the formation of chains via Nb–O–Cu bonds, and this phase transition is accompanied by an ordering of one of the two cyclam ligands.