Formation processes of high-dimensional MoO frameworks in tetrakis(2-hydroxypropane-1,3-diaminium) hexatriacontamolybdate hydrate (C3H12N2O)4[Mo36O112(H2O)16-m]·nH2O crystals: Solid-phase structural conversions under restricted dehydration conditions

We found that the hexatriacontamolybdate [Mo36O112(H2O)16]8− ({Mo36}) compound of 1,3-diamino-2-propanol (βOHC3-DA) forms seven structural phases with the formula (βOHC3-DA)4{Mo36}·nH2O. They showed a range of dimensionality: three zero-dimensional (0D), two 1D, and two 2D MoO framework structures consisting of {Mo36} anions. Two of the phases have 0D framework structures crystallized in the mother solution. The remaining five phases were obtained when the crystals of these two 0D phases were aged in resin. The dense 2D framework ({Mo36}-nanosheet) of the title compound was formed via solid-phase condensation reactions under restricted dehydration conditions such as in resin-coated crystals, unlike the loose {Mo36}-nanosheet of the (C3DA)4{Mo36}·nH2O. The formation processes of the related high-dimensional MoO frameworks were guided by hydrogen-bonding contacts initially formed between {Mo36} anions in the crystal. There were two different conversion routes: the one starting from the phase consisting of {Mo36} hydrogen-bonded at their head/foot parts lead to the dense 2D nanosheet, while the other originating from the phase consisting of {Mo36} hydrogen-bonded at their trunk parts, to 1D {Mo36}-nanochain with rare triple oxygen bridges. These routes had neither branching nor intercrossing.

Graphical abstractSeven structural phases with MoO framework structures of various dimensionalities were found. Two phases having 0D framework structures were formed in the mother solution. The remaining five phases were obtained when the two phases were aged in resin, indicating that the high-dimensional (1D and 2D) MoO frameworks of the compound were formed under restricted dehydration conditions such as in resin. There were two different conversion routes respectively originating from the two phases formed in the mother solution. The phase sequences in the routes showed snapshots concerning the formation processes of the high-dimensional frameworks.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Structural diversity of the compound (C3H12N2O)4[Mo36O112(H2O)16-m]·nH2O was revealed. ► Seven structural phases with MoO framework structures of various dimensionalities were found. ► Seven phases showed snapshots concerning formation processes of high-dimensional frameworks. ► Initially formed hydrogen bonds guided us to obtain the compounds with high-dimensional frameworks. ► Restricted dehydration enabled us to obtain the compounds with high-dimensional frameworks.