The crystal structures of the potassium hydrogen monofluorophosphates, KHPO 3 F and K 3 [H(PO 3 F) 2 ], and the a modification of RbHPO 3 F Hillary A. Prescott, Sergej I. Troyanov and Erhard Kemnitz * Institut fu¨r Chemie, Humboldt-Universita¨t zu Berlin, Brook-Taylor-Str. 2, D-12489 Berlin, Germany Received October 9, 2002; accepted March 6, 2003 Abstract. Three new hydrogen monofluorophosphates, KHPO 3 F(I), K 3 [H(PO 3 F) 2 ](II), and a-RbHPO 3 F(III), were crystallographically characterized with single crystal X-ray analysis at 180 K. The compound I is monoclinic, space group P2 1 with a ¼ 7.273(1), b ¼ 14.086(3), c ¼ 7.655(2)  A, b ¼ 90.13(3), Z ¼ 8, and R 1 and wR 2 fac- tors of 0.0214 and 0.0395, respectively. II crystallizes in the monoclinic space group C2/c with a ¼ 7.973(3), b ¼ 11.635(4), c ¼ 9.688(4)  A, b ¼ 113.52(4), Z ¼ 4, and R 1 and wR 2 factors of 0.0581 and 0.1615, respectively. III is monoclinic with the P2 1 /n space group, a ¼ 7.465(2), b ¼ 15.551(8), c ¼ 7.563(4)  A, b ¼ 105.38(5), Z ¼ 8, and R 1 and wR 2 factors of 0.0365 and 0.0694, respectively. I and III include O–HO hydrogen bonding between the HPO 3 F tetrahedra to form branched spiral chains (I) or tetramers (III), respectively. In the structure of II, dimeric units are formed by a symmetrically disordered hydrogen bond between the PO 3 F tetrahedra. Both O and F atoms participate in cation coordination in all three structures, whereas only O atoms are involved in hydrogen bonding. Introduction Systematic studies of acid salts have shown that hydrogen sulfates, such as CsHSO 4 , RbHSO 4 , and NH 4 HSO 4 , under- go phase transitions and feature proton conductivity par- tially based on disorder in their crystal structures [1], [2]. The hydrogen sulfate and monofluorophosphate ions are isoelectronic and could, therefore, have similar chemical and physical properties. Yet until recently, the study of the hydrogen monofluorophosphates was quite limited. This is based on the hydrolysis of the monofluorophosphoric acid at lower pH that makes it particularly difficult to obtain crystalline, phosphate-free acid salts. With the help of ca- tion exchange and freeze-drying, we have been able to synthesize and crystallize numerous hydrogen monofluoro- phosphates; some of which have even been obtained in high purity [3]. Along with synthesis work, these studies [4] have led to the structural determination of the following hydrogen mono-fluorophosphates: MHPO 3 F with M ¼ NH 4 [5], Na [6], Cs [7], Rb (b-modification) [8], and organic cations [3]. A comparison with the hydrogen sulfates [9], [10] shows that these two classes of compounds differ not only in their thermal behavior but also structurally [3, 4, 11]. Nontheless, the crystal structures of hydrogen monofluorophosphates do have structural aspects observed before in acid salts. They also offer new insights into the general role of covalently bonded fluorine atoms in the hydrogen bonding of inorganic compounds. The basic sodium monofluorophosphate deca- hydrate, Na 2 PO 3 F  10 H 2 O [6] like K 2 PO 3 F [12], is isoty- pic to the corresponding sulfate, yet a general “non-isotyp- ism” is observed in the hydrogen monofluorophosphates and corresponding hydrogen sulfates. This can be explained by the substitution of oxygen by fluorine and the conse- quent absence of hydrogen bonds to fluorine as an H accep- tor. Accordingly, O–HF hydrogen bonds were only found in Na 2 PO 3 F  10 H 2 O. The synthesis of KHPO 3 F and the thermal behavior thereof have been reported earlier [13], [14]. Phase analysis was only carried out with powder XRD at that time. Until now, despite the successful synthesis of other hydrogen monofluorophosphates mentioned above, attempts to obtain single crystals of KHPO 3 F for structural characterizations have failed. Along with the absence of KHPO 3 F crystals large enough for single crystal X-ray structural analysis, a 3 : 1 (M/H ratio) composition has not been found for a hy- drogen monofluorophosphate, whereas this type of acid salt, M 3 [H(SO 4 ) 2 ], is quite common for the hydrogen sul- fates and has been obtained with the following cations: NH 4 [15], Na [16], K [17], [18], and Rb [19]–[21]. Experimental section Sample preparation The compounds presented here were prepared as described before [3] by reaction of metal carbonates with H 2 PO 3 F. The latter was prepared by cation exchange of Na 2 PO 3 F (Giulini Chemie GmbH) through a chromatography col- 604 Z. Kristallogr. 218 (2003) 604–611 # by Oldenbourg Wissenschaftsverlag, Mu¨nchen * Correspondence author (e-mail: erhard.kemnitz@chemie.hu-berlin.de) Brought to you by | California Institute of Technology Authenticated Download Date | 12/29/16 11:15 AM