Pentapotassium Bis(hydrogenphospate) Dihydrogenphospate Monohydrate

: The structure of K 5 (HPO 4 ) 2 (H 2 PO 4 ) · H 2 O was determined via single crystal diffraction. The crystal structures of phosphate salts of potassium have been known since the early days of crystallography. Here, we present a new monohydrate adduct between K 2 HPO 4 and KH 2 PO 4 .


Introduction
Potassium phosphate finds its use as a food additive and fertilizer. The latter mostly contains monopotassium phosphate (MKP) (also known as potassium dihydrogenphosphate, KDP, or monobasic potassium phosphate) and dipotassium phosphate (also known as dipotassium hydrogen phosphate or potassium phosphate dibasic). The same combination is often used as a buffer for enzyme studies, where it is referred to as the Gomori buffer [1].
Structures of potassium phosphate have been known since the early days of crystallography, with the structure of KH 2 PO 4 by Hassel in 1925 [2]. It is a known ferroelectric material showing a phase transition below 122 K [3]. For K 2 HPO 4 , orthorhombic crystal structures of the trihydrate [4] and anhydrate are known [5], as well as a monoclinic form [6] of the latter. Here, we report the structure of a monohydrate adduct of K 2 HPO 4 and KH 2 PO 4 in a 2:1 ratio. It can be seen as a double salt with two different anions and can be named pentapotassium bis(hydrogenphospate) dihydrogenphospate monohydrate. Given the common nature of the starting compounds and their general use, it is quite surprising that new phases can still be found. The moiety formula of the structure can be written as K 5

Results and Discussion
The title compound was obtained from a cocrystallization experiment between (R)-(+)-1-(4-bromophenyl)ethylamine and L-Lactic acid in a potassium phosphate buffer (0.5 M, pH 7) in an attempt to crystallize L-Lactic acid in an environment suitable for enzy-

Results and Discussion
The title compound was obtained from a cocrystallization experiment between (R)-(+)-1-(4-bromophenyl)ethylamine and L-Lactic acid in a potassium phosphate buffer (0.5 M, pH 7) in an attempt to crystallize L-Lactic acid in an environment suitable for enzymatic activity.
Rather than cocrystals with L-Lactic acid, the obtained crystals proved to be the 2:1 adduct of K 2 HPO 4 and KH 2 PO 4 .
The obtained structure resolved from single crystal data showed the structure crystallizing in the monoclinic system with the space group P2 1 /m.
The unit cell is filled with 6 PO 4 tetrahedra surrounded by 10 potassium cations. The PO 4 tetrahedra belonging to H 2 PO 4 anions and 1 K + are found on a mirror plane, giving a total of 2 formula units per unit cell (Z = 0.5). Every phosphate oxygen not carrying a hydrogen is coordinated to at least 3 potassium cations, and for those with a hydrogen atom to 2 potassium cations, the hydrogens are involved in hydrogen bonds with neighbouring phosphate oxygens. Potassium is known for its high coordination number (CN), with reported coordination numbers up to 12 [7]. A CN of 8 is found for K1 and K11, involving phosphate oxygens and the water molecule. K2 is coordinated to 9 oxygen atoms all from PO 4 units. The water molecule, also completely located on the mirror plane, interacts with 3 potassium atoms and its hydrogens bridge 2 H 2 PO 4 anions.
K2 sits in a distorted capped square antiprismatic geometry ( Figure 2B) made from four edge-and face-coordinating PO 4 units, while K1 only interacts with the vertices of seven PO 4 units and the water molecule, giving a bicapped trigonal prismatic geometry ( Figure 2A). K11, on the other hand, is more regularly surrounded in a trigonal dodecahedron from three edge-coordinating PO4 tetrahedra, a vertex of another, and the water molecule ( Figure 2C).

Results and Discussion
The title compound was obtained from a cocrystallization experiment between (R)-(+)-1-(4-bromophenyl)ethylamine and L-Lactic acid in a potassium phosphate buffer (0.5 M, pH 7) in an attempt to crystallize L-Lactic acid in an environment suitable for enzymatic activity.
Rather than cocrystals with L-Lactic acid, the obtained crystals proved to be the 2:1 adduct of K2HPO4 and KH2PO4.
The obtained structure resolved from single crystal data showed the structure crystallizing in the monoclinic system with the space group P21/m.
The unit cell is filled with 6 PO4 tetrahedra surrounded by 10 potassium cations. The PO4 tetrahedra belonging to H2PO4 anions and 1 K + are found on a mirror plane, giving a total of 2 formula units per unit cell (Z' = 0.5). Every phosphate oxygen not carrying a hydrogen is coordinated to at least 3 potassium cations, and for those with a hydrogen atom to 2 potassium cations, the hydrogens are involved in hydrogen bonds with neighbouring phosphate oxygens. Potassium is known for its high coordination number (CN), with reported coordination numbers up to 12 [7]. A CN of 8 is found for K1 and K11, involving phosphate oxygens and the water molecule. K2 is coordinated to 9 oxygen atoms all from PO4 units. The water molecule, also completely located on the mirror plane, interacts with 3 potassium atoms and its hydrogens bridge 2 H2PO4 anions.
K2 sits in a distorted capped square antiprismatic geometry ( Figure 2B) made from four edge-and face-coordinating PO4 units, while K1 only interacts with the vertices of seven PO4 units and the water molecule, giving a bicapped trigonal prismatic geometry ( Figure 2A). K11, on the other hand, is more regularly surrounded in a trigonal dodecahedron from three edge-coordinating PO4 tetrahedra, a vertex of another, and the water molecule ( Figure 2C).
The diffraction data were collected on a MAR345 image plate detector using monochromated MoKα radiation (Incoatec IµS microfocus, montel mirrors). CrysAlis PRO was used for integration and data reduction, and the implemented absorption correction was used [8]. The structure was solved via SHELXT [9] and refined against |F 2 | using SHELXL-2018/3 [10]. No restraints were applied, apart from a similarity restraint on the O-H bond lengths of the crystal water. All hydrogen atoms were located in the difference maps and refined freely. Table 1 gives the crystallographic and refinement details of K 5 (HPO 4 ) 2 (H 2 PO 4 )·H 2 O. CCDC 2262315 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures. Author Contributions: Conceptualization, K.R. and T.L.; methodology, C.C.G., T.L. and K.R.; formal analysis, C.C.G.; data curation, K.R.; writing-original draft preparation, K.R.; writing-review and editing, T.L. and C.C.G.; supervision, T.L.; funding acquisition, T.L. and K.R. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Fonds de la Recherche Scientifique-FNRS (CDR J.0168.22).

Conflicts of Interest:
The authors declare no conflict of interest.