Water Superstructures within Organic Arrays; Hydrogen‐Bonded Water Sheets, Chains and Clusters

Abstract

A strategy for encouraging the formation of extended water arrays is presented, in which molecules that contain a 1,4-dihydroquinoxaline-2,3-dione core are used as supramolecular hosts for the accommodation of guest water molecules and arrays. These molecules were selected as they contain a hydrophilic oxalamide-based "terminus" that allows water molecules to hydrogen-bond to the host organic molecules as well as to each other. The host molecules also contain a hydrophobic "end" based upon an aromatic ring, which serves to encourage the formation of discrete water clusters in preference to three-dimensional networks, as the water molecules cannot form strong hydrogen bonds with this part of the molecule. A systematic study of several hydrated structures of four organic molecules based on 1,4-dihydroquinoxaline-2,3-dione (qd) is discussed. The organic molecules, qd, 6-methyl-1,4-dihydroquinoxaline-2,3-dione (mqd), 6,7-dimethyl-1,4-dihydroquinoxaline-2,3-dione (dmqd) and 1,4-dihydrobenzo[g]quinoxaline-2,3-dione (Phqd), act as supramolecular crystal hosts for the clusters of water, with zero-, one- and two-dimensional arrays of water being observed. The hydrogen bonding in the structures, both within the water clusters and between the clusters and organic molecules, is examined. In particular, the structure of dmqd6 H2O contains a two-dimensional water sheet composed of pentagonal and octagonal units. Phqd3 H2O forms a hydrophilic extended structure encouraging the formation of one-dimensional chains consisting entirely of water. Both qd2 H2O and dmqd2 H2O can be considered to form one-dimensional chains, but only by utilising bridging carbonyl groups of the oxalamide moieties to form the extended array; if only the water is considered, zero-dimensional water tetramers are observed. The remaining hydrated structures, [Na+dmqd-]dmqdH2O, dmqd1/3H2O and mqd1/2H2O, all contain discrete water molecules but do not form extended water structures.

Hydrogen bonded H3O+, H2O, HF, F− in fluoride metalates (Al, Cr, Fe, Zr, Ta) templated with tren (tris-(2-aminoethyl)amine)

“…Metal bonded water molecules are also found and one of these water molecules is disordered with one fluoride ion in a neutral distorted octahedron. It must be noted that pentagons and octagons of water molecules are not encountered in fluoride metalates while they are frequently found in clathrates [15,16], in organic crystals [17] or in ice structures [18].…”

X-ray and Neutron Diffraction in the Study of Organic Crystalline Hydrates

“…The β forms are less stable and transform to the monohydrates at lower temperatures. It is not always true that related compounds crystallise as related hydrates, as shown by Oxtoby et al [54] The authors investigated a set of four closely related 4,4-dihydroquinoxaline-2,3-diones in order to understand water cluster formation, which resulted in the observation of zero-, one-and two-dimensional arrays of water. The authors concluded that in the formation of these hydrated crystal forms not only the strong hydrogen bonds are involved but indeed at number of competing and relatively weak interactions control the formation of specific arrays.…”

Titanium(IV) Complexes of Hydrazones and Azines

“…[15][16][17] Hydrazones (where R = H) can form supramo-lecular arrays in the solid state due to the presence of the protic NH 2 group available for hydrogen bonding, a feature which is being exploited in the crystal engineering field. [18][19][20][21][22][23] The ligands chosen for this study are shown in Figure 2, with the expectation of conveying different steric demands on the titanium(IV) centre and facilitating the formation of different arrays in the solid state. …”