Crystal structure of the deuterated heptahydrate of potassium phosphate, K3PO4·7D2O

An intricate network of medium-strong O—D⋯O hydrogen bonds consolidates the crystal structure of water-rich K3PO4·7D2O.


Chemical context
Following projects devoted to studying the formation and crystal chemistry of hydrous arsenate and phosphate phases of monovalent metals, viz. NaH 2 AsO 4 (Ring et al., 2017), K 2 HAsO 4 (H 2 O) 2.5 and K 2 HAsO 4 (H 2 O) 6 (Stö ger et al., 2012), M 2 HXO 4 Á2H 2 O (M = Rb, Cs; X = P, As; Stö ger & Weil, 2014), and several acidic thallium(I) arsenate phases (Schroffenegger et al., 2019), we became interested in the system K 3 PO 4 /H 2 O. Although hydrate phases of potassium orthophosphate have been known for a very long time to exist for the 3-hydrate and the 7-hydrate (Gmelin, 1938), crystal-structure determinations of these two phases or of any other hydrate of K 3 PO 4 have not been reported so far. Previous investigations on the trihydrate revealed that the crystal structure of K 3 PO 4 Á3H 2 O is incommensurately modulated below 300 K (Stö ger, 2020). To better elucidate the role of hydrogen bonding in this structure with the aid of single-crystal neutron diffraction, we started crystalgrowth experiments to obtain the deuterium analogue K 3 PO 4 Á3D 2 O. The title compound, K 3 PO 4 Á7D 2 O, was the unexpected product of such a crystallization attempt at temperatures below the freezing point of pure water, and its crystal structure is reported here.

Structural commentary
Taking 3.1 Å as the upper limit of K-O bond lengths in the first coordination sphere, each of the three crystallographically independent potassium cations is surrounded by six water molecules and one oxygen atom of the phosphate group (Fig. 1). The highly irregular coordination polyhedra show K-O bond lengths ranging between 2.6665 (9) and 3.0151 Å ( Table 1). The overall mean of 2.821 Å for the 21 bonds is in good agreement with the value of 2.861 Å calculated from 469 individual K-O bonds in crystal structures with coordination ISSN 2056-9890 numbers of 7 for the potassium cation (Gagné & Hawthorne, 2016). The [K(D 2 O) 6 O] polyhedra share corners and edges to build up a three-dimensional network (Fig. 2). Each water molecule is a donor group of two slightly bent O-DÁ Á ÁO hydrogen bonds, but only two of the water molecules (O3w, O6w) also serve as acceptor groups for one hydrogen bond. All other hydrogen bonds are directed towards the O atoms of the phosphate group, with O1 being twofold, O2 threefold, O3 fourfold and O4 threefold acceptor atoms, respectively (Fig. 3). Judging from the OÁ Á ÁO distances [range 2.6931 (12)-2.9025 (13) Å ; Table 2], hydrogen bonds of medium strength are formed in the crystal structure. The PO 4 tetrahedron shows almost equal P-O bond lengths typical of a fully deprotonated orthophosphate group (mean 1.546 Å ), with marginal angular distortions.
A bond-valence analysis (Brown, 2002), using the parameters of Brese & O'Keeffe (1991) Network of corner-and edge-sharing [KO 7 ] polyhedra in the crystal structure of K 3 PO 4 Á7D 2 O, viewed along [001]. Displacement ellipsoids are displayed at the 90% probability level. For clarity, D atoms are not shown.

Figure 1
The expanded asymmetric unit of K 3 PO 4 Á7D 2 O showing the complete potassium coordination polyhedra. Displacement ellipsoids are displayed at the 74% probability level; O-DÁ Á ÁO hydrogen bonds are indicated by green lines; symmetry codes refer to Table 1.    (Mootz & Wunderlich, 1970). As a result of the different size of the Na + cation compared to K + , the role of NH 4 + as an active species in hydrogen bonding, and the different amounts of water molecules in these three crystal structures, there is no evident structural relation to K 3 PO 4 Á7D 2 O.

Synthesis and crystallization
Commercial anhydrous K 3 PO 4 (Sigma-Aldrich) was dissolved in a small amount of warm D 2 O. Cooling to 255 K afforded rod-like crystals of the title heptahydrate that grew over night, with maximum edge lengths in the millimetre range.

Potassium phosphate heptahydrate
Crystal data