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

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, that 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 about 13 water molecules and a new crystalline phase (2) crystallizes that upon 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[HTiNb9O...