Crystal structures of two solvates of (18-crown-6)potassium acetate

Hydrogen bonding plays an important role in the structures of two solvated forms of [K(18c6)]OAc (18c6 = 18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane and OAc = acetate).


Chemical context
As a result of the macrocyclic ether 1,4,7,10,13,16-hexaoxacyclooctadecane ('18-crown-6') being a hexadentate ligand that is highly specific for the potassium cation, it is frequently used to manipulate the properties of various potassium compounds. On the one hand, the [K(18c6)] + cation (18c6 = 18-crown-6) is a powerful tool to crystallize large anions with the objective to make them accessible for single-crystal structure determination. Thus, the crystal structures of numerous anionic complex compounds have been observed from their [K(18c6)] + salts, e.g. [HPMo 12 O 40 ] 4- (Neier et al., 1995) and [HgR f 2 X] À (R f = CF 3 , C 6 F 5 , X = Br, I; Schulz et al., 2003) to mention just two examples among many. The same applies to a broad ensemble of unusual non-metal anions such as I 3 À (Sievert et al., 1996) and the radical species C 2 N 4 S 2 À (Makarov et al., 2005). Moreover, since the early days of crown-ether chemistry, 18-crown-6 has been used to enhance the solubility of reactive potassium salts in organic media, e.g. KMnO 4 (Doheny & Ganem, 1980). [K(18c6)]OAc (OAc = acetate) has been shown to be useful as an acetylation agent for alkyl halides (Liotta et al., 1974), and over the past few years 'CECILs' (crown ether complex cation ionic liquids) such as [K(18c6)]OAc and [K(18c6)]OH gained in importance as basic catalysts for various organic transformations (e.g. Song et al., 2011;Abaszadeh & Seifi, 2015). In view of the broad application of 18-crown-6-complexed potassium acetate, [K(18c6)]OAc, it is surprising that the crystal structure of this simple compound has never been determined. In this paper we present the structures of two ISSN 2056-9890 solvated forms thereof, namely the dihydrate, [K(18c6)]OAcÁ2 H 2 O (1), and the acetic acid hydrate, [K(18c6)]OAcÁHOAcÁ-H 2 O (2).

Structural commentary
Both title compounds crystallize in the monoclinic space group P2 1 /c with one formula unit of [K(18c6)]OAc and two solvent molecules in the asymmetric unit. In the dihydrate 1 ( Fig. 1), the potassium atom is coordinated by the crown ether ligand in a slightly unsymmetrical hexadentate mode with K-O distances (Table 1) ranging from 2.8248 (13) to 2.9684 (11) Å and a median value of 2.922 Å . The acetate counter-ion is attached to potassium in a chelating coordination mode where the K-O distances are significantly different with 2.6992 (11) (K-O7) and 2.8861 (11) Å (K-O8). As a result of the additional coordination of the acetate ion, the potassium ion is slightly displaced from the crown ether O 6 plane. The two water molecules do not coordinate to the potassium ion and interact via O-HÁ Á ÁO hydrogen bonds (see Supramolecular features section).
By contast, in the acetic acid hydrate 2 (Fig. 2) the coordination number of the potassium atom is raised to nine by a coordinating water molecule and consequently the K-O bonds (Table 2) to the acetate ligand are significantly elongated to 2.9562 (16) (K-O8) and 3.0303 (19) Å (K-O7).
Moreover, in compound 2 the coordination of the 18c6 ligand is more unsymmetrical than in compound 1 [K-O = 2.7855 (14)-3.0337 (13) Å ], but the average K-O distance is virtually identical at 2.920 Å . In general, the geometry of the [K(18c6)]OAc ion pair is not fundamentally influenced by the additional water ligand. Thus, the angle between the KO 2 C(acetate) plane and the O 6 plane of the 18c6 ligand is similar in both compounds [1: 68 (1), 2 64 (1) ]. The same applies to the displacement of the potassium ion from the O 6 centroid, which is 0.080 Å in 1 and 0.082 Å in 2.

Figure 2
The molecular structure of compound 2, with dispalcement ellipsoids drawn at the 50% probability level. C-bound H atoms have been omitted for clarity.  Schulz et al., 2003). Herein the cation-anion interactions are weak in general, but the KÁ Á ÁX interaction is a little stronger than the KÁ Á ÁF interaction on the opposite side of the K(18c6)] + cation and the potassium ion is therefore moved slightly out of the macrocycle [e.g. X = I: K-O = 2.768 (6)-2.895 (6) Å , K-centroid(O 6 ) = 0.020 Å ].

Synthesis and crystallization
Single crystals of the title compounds were obtained by slow evaporation of a solution of commercial available potassium acetate in the presence of an equimolar amount of 18-crown-6 in water (1) or in diluted acetic acid (2).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. All H atoms were fixed geome-trically and refined using a riding model with U iso (H) = 1.2U eq (C). C-H distances in CH 3 groups were constrained to 0.98 Å and those in CH 2 groups to 0.99 Å . Methyl H atoms were allowed to rotate around the C-C vector (AFIX 137 in SHELXL). O-H distances within H 2 O molecules were restrained to 0.96 Å (DFIX restraint in SHELXL; the s.u. applied was 0.01 Å ), while the coordinates of the HOAc hydrogen atom H3 in compound 2 was refined freely.
( Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.