Caesium propano­ate monohydrate

Caesium propano­ate monohydrate, Cs⁺·C₃H₅O₂⁻·H₂O, is composed of two symmetry-independent Cs⁺ cations, which are situated on the special position 4e of space group P2₁m, one symmetry-independent propano­ate mol­ecule in a general position and a pair of water mol­ecules also situated on special position 4e. Two pairs of these symmetry-independent cations, four propano­ate mol­ecules and two pairs of symmetry-independent water mol­ecules form a repeat unit. These units form columns that are directed along the c axis and possess symmetry mm2. There are four such columns passing through each unit cell. Each column is inter­connected to its neighbours by four bifurcated three-centred O_w—H⋯O_p (w = water, p = propano­ate) hydrogen bonds of moderate strength. There are also four intra­molecular O_w—H⋯O_p hydrogen bonds of moderate strength within each column. One Cs⁺ cation is coordinated by six oxygen atoms (two water and four carboxyl­ate) in a trigonal–prismatic geometry, while the other Cs⁺ cation is coordinated by four water and four carboxyl­ate O atoms in a tetra­gonal–prismatic arrangement.

1. Chemical context

No structure of a simple hydrated alkali propano­ate has been determined until now. (`Simple' means a structure where the constituting cation belongs just to one chemical species.) This is in contrast to alkali formates and acetates where water-free alkali salts and complexes with parent acids as well as hydrates are known. These structures show different structural motifs: Some of them are layered, such as lithium acetate dihydrate, LiC₂H₃O₂·2H₂O [refcode LIACET06 (Kearley et al., 1996) in the Cambridge Structural Database, version 5.41, update of November 2019 (Groom et al., 2016)], some are columnar including sodium di­hydrogen tri­acetate, NaC₂H₃O₂·2C₂H₄O₂ (NADHAC; Perotti & Tazzoli, 1981) while the cations and anions surround each other in the structure of catena-[bis­(μ₄-acetato)­tetra­kis­(μ₃-acetato)­bis­(μ₂-acetato)­octa­aqua­octa­lith­ium] (UVELAJ; Martínez Casado et al., 2011).

In the series of carb­oxy­lic acids with an increasing number of carbon atoms, it is propionic acid where the hydro­phobic properties start to be prominent. Formates and acetates are definitely distinct from propano­ates and other carboxyl­ates related to the acids C_nH_2n+1CO₂ where n > 2. This is due to the longer and more voluminous organic chains in the latter compounds, which need space [cf. Duruz & Ubbelohde (1972) and Dumbleton & Lomer, 1965]. Cohesion is provided by van der Waals forces. Occasionally, positional disorder of these groups may take place. Dicalcium barium hexa­kis­(propano­ate) (Stadnicka & Glazer, 1980) can serve as an example.

The structural differences between alkali formates and acetates on one hand and simple alkali propano­ates such as Li(C₃H₅O₂) (Martínez Casado et al., 2009) and M(C₃H₅O₂); M = Na, K, Rb, Cs (Fábry & Samolová, 2020) on the other reflect the chemical differences between these two groups of compounds. The latter structures are characterized by stacking of layers that are composed of a metal–oxygen bilayer surrounded by hydro­phobic layers comprising the ethyl groups. The cohesion forces between the hydro­phobic layers hold these structures together. The structure of the chemically related compound Tl(C₃H₅O₂) (refcode WEWKAM; Martínez Casado et al., 2010) is also a layered structure with three symmetry-independent cations.

Mirnaya et al. (1991) pointed out the tendency for various alkanoates to form hydrates. Such a case is reported in this study – see the Synthesis and crystallization section. It is of inter­est how strikingly different the title structure Cs(C₃H₅O₂)·H₂O is from Cs(C₃H₅O₂) (Fábry & Samolová, 2020), despite the chemical similarity.

2. Structural commentary

The title structure confirms the tendency for various alkanoates to form hydrates, as noted by Mirnaya et al. (1991). Caesium propano­ate monohydrate is composed of two symmetry-independent Cs⁺ cations, which are situated on the special position 4e, i.e. on a symmetry plane, one symmetry-independent propano­ate mol­ecule in a general position and a pair of water mol­ecules also situated on special position 4e of space group P2₁m. Two pairs of these symmetry-independent cations, four propano­ate mol­ecules and two pairs of symmetry-independent water mol­ecules form a repeat unit. These units form columns along the c-axis direction (Fig. 1a). The length of the repeat unit along the c axis corresponds to this unit-axis length. Each column has mm2 symmetry (Fig. 1b). There are four such columns passing through each unit cell (Fig. 2). The columns are inter­connected by bifurcated three-centered O_w—H⋯O_p (w = water, p = propano­ate) hydrogen bonds (Jeffrey, 1995), whose lengths and angles are quite different, but which are still of moderate strength (Gilli & Gilli, 2009); the donor is O4 and the donated hydrogen is H1o4 (Table 1). Each column thus donates four three-centered bifurcated hydrogen bonds (Jeffrey, 1995) to its neighbours (Fig. 2). There are also intra­molecular two-centered O_w—H⋯O_p hydrogen bonds of moderate strength within each column in the structure; the donor is O3 and the donated atom is H1o3.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O3—H1o3⋯O2^i	0.820 (9)	1.988 (7)	2.783 (2)	163.3 (19)
O4—H1o4⋯O1^ii	0.820 (7)	1.941 (6)	2.748 (2)	168.0 (5)
O4—H1o4⋯O2^ii	0.820 (7)	2.646 (5)	3.293 (2)	137.0 (5)
O3—H1o3^iii⋯O2^iv	0.820 (9)	1.988 (7)	2.783 (2)	163.3 (19)
O4—H1o4^iii⋯O1^v	0.820 (7)	1.941 (6)	2.748 (2)	168.0 (5)
O4—H1o4^iii⋯O2^v	0.820 (7)	2.646 (5)	3.293 (2)	137.0 (5)
Symmetry codes: (i) -y+1, x, -z+2; (ii) x, y, z-1; (iii) ; (iv) ; (v) .

Figure 1 View (DIAMOND; Brandenburg, 2005) of the title motif along (a) the c axis and (b) the b axis. Displacement ellipsoids are shown at the 30% probability level: the cations and the O and C atoms are shown in green, red and grey, respectively. H atoms are shown as small light-grey spheres. The covalent bonds are represented by solid lines, Cs—O bonds by dashed black lines and hydrogen bonds by yellow dashed lines.

Figure 2 Packing of the title mol­ecules in the unit cell (DIAMOND; Brandenburg, 2005). Displacement ellipsoids are shown at the 30% probability level: the cations and the O and C atoms are shown in green, red and grey, respectively. H atoms are shown as small light-grey spheres. The covalent bonds are represented by solid lines, Cs—O bonds by dashed black lines and hydrogen bonds by yellow dashed lines.

Cs1 is coordinated by six oxygen atoms (two of them are water O atoms and four are carboxyl­ate O atoms) in a trigonal–prismatic geometry, while Cs2 is in a less regular tetra­gonal–prismatic coordination environment (by four water and four carboxyl­ate oxygen atoms) (Fig. 2). The bond-valence sums (Brese & O'Keeffe, 1991) of Cs1 and Cs2 are 0.902 (3) and 0.997 (2) v.u., respectively. The fact that the cation with a smaller number of ligands that exhibits a regular coordination environment has a smaller bond-valence sum than that with the larger number of surrounding cations seems to be a peculiarity of the present structure, for example compared to β-K₂SO₄ compounds with two symmetry-independent cations (Fábry & Pérez-Mato, 1994). One has eleven ligands while the other has nine. The former has a more irregular coordination compared to the latter and its bond-valence sum is also lower than that of the latter cation. This example is a specific case that has been considered by Brown (1992): The larger coordination number usually results in the formation of a larger cavity around the cation. Stabilization of the cation causes the cation to shift towards some ligand. Such a shift contributes to irregularity of the coordination polyhedron with large numbers of ligands. Despite this stabilization, the bonding of a cation with a high coordination number tends to be lower than that of a cation with a low coordination number.

In contrast to the alkali propano­ates, M(C_nH_2n+1COO), the methyl­ene–methyl­ene, meth­yl–methyl carbon atoms are not in close contact in the title structure. The closest contact C2⋯C3^vi, i.e. a methyl­ene–methyl contact is 3.961 (4) Å; symmetry code: (vi) −y + 1, x, −z + 1. This is related to the fact that no disorder of the ethyl groups is observed in the studied structure. At the same time, there are elongated voids in the c-axis direction that run parallel through the 4d positions and which are surrounded by the ethyl groups. The radius of the void is 1.381 Å while its height nearly corresponds to the c axis. The voids were calculated and depicted (see the supporting information) using Mercury 4.0 (Macrae et al., 2020).

3. Synthesis and crystallization

The crystals formed spontaneously in a droplet from dissolved deliquescent crystals of Cs(C₃H₅O₂) that otherwise have been grown from an aqueous solution of Cs₂CO₃ with a little excess of propionic acid (Fábry & Samolová, 2020).

4. Structure determination and refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. The non-hydrogen atoms were determined by a charge-flipping method (Palatinus & Chapuis, 2007). The positions of the methyl­ene hydrogen atoms were calculated and refined under the following constraints: C—H = 0.97 Å with U_iso(H) = 1.2U_eq(C). The methyl hydrogen atoms were found in the difference electron-density maps and refined under the constraints C—H = 0.96 Å and U_iso(H) = 1.5U_eq(C). The water hydrogen atoms were also found in the difference electron density maps. The O—H distances were restrained to 0.820 (1) Å with U_iso(H) = 1.5U_eq(O). When the water H atoms were refined, the O—H_water distances converged to values of ∼0.78 Å. The structure was treated as an inversion twin. The Flack (1983) parameter is 0.03 (3).

Table 2
Experimental details

Crystal data
Chemical formula	Cs^+·C3H5O2^−·H2O
Mr	224
Crystal system, space group	Tetragonal, P21m
Temperature (K)	230
a, c (Å)	17.7764 (3), 4.2223 (1)
V (Å^3)	1334.25 (4)
Z	8
Radiation type	Mo Kα
μ (mm^−1)	5.47
Crystal size (mm)	0.46 × 0.04 × 0.03
Data collection
Diffractometer	Bruker D8 VENTURE Kappa Duo PHOTON 100 CMOS
Absorption correction	Multi-scan (SADABS; Bruker, 2017)
Tmin, Tmax	0.190, 0.866
No. of measured, independent and observed [I > 3σ(I)] reflections	12844, 2019, 1872
Rint	0.021
(sin θ/λ)max (Å^−1)	0.703
Refinement
R[F > 3σ(F)], wR(F), S	0.014, 0.037, 1.10
No. of reflections	2019
No. of parameters	78
No. of restraints	4
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å^−3)	0.24, −0.33
Absolute structure	Flack (1983), 831 Friedel pairs
Absolute structure parameter	0.03 (3)
Computer programs: Instrument Service and SAINT (Bruker, 2017), SUPERFLIP (Palatinus & Chapuis, 2007) and JANA2006 (Petříček et al., 2014), DIAMOND (Brandenburg, 2005); extinction correction according to Becker & Coppens (1974).

Supporting information

Computing details

Data collection: Instrument Service (Bruker, 2017); cell refinement: SAINT (Bruker, 2017); data reduction: SAINT (Bruker, 2017); program(s) used to solve structure: SUPERFLIP (Palatinus & Chapuis, 2007); program(s) used to refine structure: JANA2006 (Petříček et al., 2014); molecular graphics: DIAMOND (Brandenburg, 2005); software used to prepare material for publication: JANA2006 (Petříček et al., 2014).

Caesium propanoate monohydrate

Crystal data

Cs^+·C3H5O2^−·H2O	There have been used diffractions with I/σ(I)>20 for the unit cell determination.
Mr = 224	Dx = 2.230 Mg m^−3
Tetragonal, P421m	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P -4 2ab	Cell parameters from 9815 reflections
a = 17.7764 (3) Å	θ = 2.3–30.0°
c = 4.2223 (1) Å	µ = 5.47 mm^−1
V = 1334.25 (4) Å^3	T = 230 K
Z = 8	Needle, colourless
F(000) = 832	0.46 × 0.04 × 0.03 mm

Data collection

Bruker D8 VENTURE Kappa Duo PHOTON 100 CMOS diffractometer	2019 independent reflections
Radiation source: IµS micro-focus sealed tube	1872 reflections with I > 3σ(I)
Quazar Mo multilayer optic monochromator	Rint = 0.021
φ and ω scans	θmax = 30.0°, θmin = 2.3°
Absorption correction: multi-scan (SADABS; Bruker, 2017)	h = −24→24
Tmin = 0.190, Tmax = 0.866	k = −19→24
12844 measured reflections	l = −5→4

Refinement

Refinement on F^2	H atoms treated by a mixture of independent and constrained refinement
R[F > 3σ(F)] = 0.014	Weighting scheme based on measured s.u.'s w = 1/(σ^2(I) + 0.0004I^2)
wR(F) = 0.037	(Δ/σ)max = 0.042
S = 1.10	Δρmax = 0.24 e Å^−3
2019 reflections	Δρmin = −0.33 e Å^−3
78 parameters	Extinction correction: B-C type 1 Lorentzian isotropic (Becker & Coppens, 1974)
4 restraints	Extinction coefficient: 330 (110)
22 constraints	Absolute structure: Flack (1983), 831 Friedel pairs
Primary atom site location: charge flipping	Absolute structure parameter: 0.03 (3)

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cs1	0.695062 (6)	0.195062 (6)	0.48412 (6)	0.03299 (4)
Cs2	0.420270 (6)	0.079730 (6)	0.51015 (5)	0.03461 (4)
O1	0.56778 (8)	0.21997 (9)	0.9877 (8)	0.0493 (5)
O2	0.44454 (7)	0.20549 (7)	1.0090 (6)	0.0395 (4)
O3	0.78820 (8)	0.28820 (8)	0.9682 (10)	0.0442 (7)
O4	0.56624 (9)	0.06624 (9)	0.0554 (10)	0.0534 (9)
C1	0.50192 (12)	0.24285 (12)	0.9444 (5)	0.0310 (6)
C2	0.49413 (16)	0.32018 (16)	0.7976 (7)	0.0549 (9)
C3	0.41676 (19)	0.34209 (18)	0.6915 (8)	0.0631 (11)
H1c2	0.513302	0.35775	0.943391	0.0658*
H2c2	0.528887	0.324875	0.621629	0.0658*
H1c3	0.383944	0.344695	0.871774	0.0947*
H2c3	0.418684	0.390343	0.589772	0.0947*
H3c3	0.398058	0.305262	0.545034	0.0947*
H1o3	0.7817 (4)	0.3335 (4)	0.994 (9)	0.0662*
H1o4	0.5599 (3)	0.1117 (3)	0.035 (8)	0.0801*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cs1	0.03383 (7)	0.03383 (7)	0.03130 (9)	−0.00310 (6)	−0.00090 (7)	−0.00090 (7)
Cs2	0.03802 (7)	0.03802 (7)	0.02778 (8)	−0.00027 (6)	0.00236 (8)	−0.00236 (8)
O1	0.0321 (7)	0.0484 (8)	0.0675 (11)	0.0054 (6)	0.0002 (13)	0.0072 (13)
O2	0.0324 (6)	0.0340 (7)	0.0520 (9)	−0.0018 (5)	0.0050 (10)	0.0042 (11)
O3	0.0348 (6)	0.0348 (6)	0.0629 (18)	−0.0038 (8)	−0.0063 (12)	−0.0063 (12)
O4	0.0372 (7)	0.0372 (7)	0.086 (2)	−0.0007 (9)	0.0092 (11)	0.0092 (11)
C1	0.0314 (9)	0.0300 (9)	0.0316 (12)	0.0010 (8)	0.0005 (9)	0.0018 (9)
C2	0.0470 (15)	0.0427 (15)	0.0749 (18)	−0.0065 (12)	−0.0065 (14)	0.0230 (14)
C3	0.070 (2)	0.0518 (18)	0.067 (2)	0.0210 (16)	−0.0106 (17)	0.0148 (15)

Geometric parameters (Å, º)

O1—C1	1.253 (3)	Cs1—O3	3.108 (3)
O2—C1	1.247 (3)	Cs2—O2	3.102 (2)
C1—C2	1.514 (4)	Cs2—O2^iv	3.108 (2)
C2—C3	1.498 (4)	Cs2—O2^v	3.102 (2)
C2—H1c2	0.97	Cs2—O3^vi	3.886 (3)
C2—H2c2	0.97	Cs2—O3^vii	3.984 (3)
C3—H1c3	0.96	Cs2—O4	3.237 (3)
C3—H2c3	0.96	Cs2—O4^viii	3.477 (3)
C3—H3c3	0.96	Cs2—O4^ix	3.237 (3)
Cs1—O1^i	3.116 (2)	Cs2—O4^x	3.477 (3)
Cs1—O1	3.136 (2)	O3—H1o3	0.820 (9)
Cs1—O1^ii	3.116 (2)	O3—H1o3^iii	0.820 (9)
Cs1—O1^iii	3.136 (2)	O4—H1o4	0.820 (7)
Cs1—O3^i	3.198 (3)	O4—H1o4^iii	0.820 (7)
O1^i—Cs1—O1	84.96 (7)	O2^v—Cs2—O4^x	59.75 (5)
O1^i—Cs1—O1^ii	75.75 (6)	O3^vi—Cs2—O3^vii	64.87 (8)
O1^i—Cs1—O1^iii	131.38 (5)	O3^vi—Cs2—O4	102.75 (7)
O1^i—Cs1—O3^i	81.03 (6)	O3^vi—Cs2—O4^viii	146.78 (4)
O1^i—Cs1—O3	138.89 (5)	O3^vi—Cs2—O4^ix	102.75 (7)
O1^i—Cs1—O4	46.47 (5)	O3^vi—Cs2—O4^x	146.78 (4)
O1—Cs1—O1^ii	131.38 (5)	O3^vii—Cs2—O4	147.56 (4)
O1—Cs1—O1^iii	75.17 (6)	O3^vii—Cs2—O4^viii	96.57 (7)
O1—Cs1—O3^i	139.66 (4)	O3^vii—Cs2—O4^ix	147.56 (4)
O1—Cs1—O3	82.14 (7)	O3^vii—Cs2—O4^x	96.57 (7)
O1—Cs1—O4	88.43 (6)	O4—Cs2—O4^viii	77.84 (8)
O1^ii—Cs1—O1^iii	84.96 (7)	O4—Cs2—O4^ix	61.92 (5)
O1^ii—Cs1—O3^i	81.03 (6)	O4—Cs2—O4^x	106.39 (6)
O1^ii—Cs1—O3	138.89 (5)	O4^viii—Cs2—O4^ix	106.39 (6)
O1^ii—Cs1—O4	46.47 (5)	O4^viii—Cs2—O4^x	57.23 (5)
O1^iii—Cs1—O3^i	139.66 (4)	O4^ix—Cs2—O4^x	77.84 (8)
O1^iii—Cs1—O3	82.14 (7)	Cs1—O1—Cs1^viii	84.96 (4)
O1^iii—Cs1—O4	88.43 (6)	Cs2—O2—Cs2^viii	85.68 (3)
O3^i—Cs1—O3	84.05 (8)	Cs1—O3—Cs1^viii	84.05 (4)
O3^i—Cs1—O4	107.86 (8)	Cs1—O3—Cs2^xi	107.57 (11)
O3—Cs1—O4	168.08 (8)	Cs1—O3—Cs2^xii	172.44 (11)
O2^i—Cs2—O2	85.68 (6)	Cs1^viii—O3—Cs2^xi	168.38 (11)
O2^i—Cs2—O2^iv	74.70 (5)	Cs1^viii—O3—Cs2^xii	103.51 (11)
O2^i—Cs2—O2^v	131.48 (4)	H1o3—O3—H1o3^iii	105.0 (10)
O2^i—Cs2—O3^vi	45.21 (4)	Cs1—O4—Cs2^i	137.82 (6)
O2^i—Cs2—O3^vii	91.97 (5)	Cs1—O4—Cs2	99.19 (9)
O2^i—Cs2—O4	62.50 (5)	Cs1—O4—Cs2^xiii	137.82 (6)
O2^i—Cs2—O4^viii	113.38 (5)	Cs1—O4—Cs2^ix	99.19 (9)
O2^i—Cs2—O4^ix	99.35 (7)	Cs2^i—O4—Cs2	77.84 (4)
O2^i—Cs2—O4^x	167.93 (4)	Cs2^i—O4—Cs2^xiii	70.40 (6)
O2—Cs2—O2^iv	131.48 (4)	Cs2^i—O4—Cs2^ix	120.22 (5)
O2—Cs2—O2^v	74.89 (5)	Cs2—O4—Cs2^xiii	120.22 (5)
O2—Cs2—O3^vi	90.09 (6)	Cs2—O4—Cs2^ix	76.52 (7)
O2—Cs2—O3^vii	44.10 (4)	Cs2^xiii—O4—Cs2^ix	77.84 (4)
O2—Cs2—O4	110.17 (5)	H1o4—O4—H1o4^iii	105.0 (7)
O2—Cs2—O4^viii	59.75 (5)	O1—C1—O2	124.0 (2)
O2—Cs2—O4^ix	166.01 (5)	O1—C1—C2	116.1 (2)
O2—Cs2—O4^x	94.51 (7)	O2—C1—C2	119.9 (2)
O2^iv—Cs2—O2^v	85.68 (6)	C1—C2—C3	116.3 (2)
O2^iv—Cs2—O3^vi	45.21 (4)	C1—C2—H1c2	109.47
O2^iv—Cs2—O3^vii	91.97 (5)	C1—C2—H2c2	109.47
O2^iv—Cs2—O4	99.35 (7)	C3—C2—H1c2	109.47
O2^iv—Cs2—O4^viii	167.93 (4)	C3—C2—H2c2	109.47
O2^iv—Cs2—O4^ix	62.50 (5)	H1c2—C2—H2c2	101.72
O2^iv—Cs2—O4^x	113.38 (5)	C2—C3—H1c3	109.47
O2^v—Cs2—O3^vi	90.09 (6)	C2—C3—H2c3	109.47
O2^v—Cs2—O3^vii	44.10 (4)	C2—C3—H3c3	109.47
O2^v—Cs2—O4	166.01 (5)	H1c3—C3—H2c3	109.47
O2^v—Cs2—O4^viii	94.51 (7)	H1c3—C3—H3c3	109.47
O2^v—Cs2—O4^ix	110.17 (5)	H2c3—C3—H3c3	109.47

Symmetry codes: (i) x, y, z−1; (ii) y+1/2, x−1/2, z−1; (iii) y+1/2, x−1/2, z; (iv) −y+1/2, −x+1/2, z−1; (v) −y+1/2, −x+1/2, z; (vi) y, −x+1, −z+1; (vii) y, −x+1, −z+2; (viii) x, y, z+1; (ix) −x+1, −y, z; (x) −x+1, −y, z+1; (xi) −y+1, x, −z+1; (xii) −y+1, x, −z+2; (xiii) −x+1, −y, z−1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H1o3···O2^xii	0.820 (9)	1.988 (7)	2.783 (2)	163.3 (19)
O4—H1o4···O1^i	0.820 (7)	1.941 (6)	2.748 (2)	168.0 (5)
O4—H1o4···O2^i	0.820 (7)	2.646 (5)	3.293 (2)	137.0 (5)
O3—H1o3^iii···O2^xiv	0.820 (9)	1.988 (7)	2.783 (2)	163.3 (19)
O4—H1o4^iii···O1^ii	0.820 (7)	1.941 (6)	2.748 (2)	168.0 (5)
O4—H1o4^iii···O2^ii	0.820 (7)	2.646 (5)	3.293 (2)	137.0 (5)

Symmetry codes: (i) x, y, z−1; (ii) y+1/2, x−1/2, z−1; (iii) y+1/2, x−1/2, z; (xii) −y+1, x, −z+2; (xiv) x+1/2, −y+1/2, −z+2.