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The title compound, K+·[(C8H5O4)2H]-·2H2O or K+·C16H11O8-·2H2O, was prepared by slow evaporation of an aqueous solution of potassium hydrogen phthalate. The molecular complex consists of a potassium cation coordinated to a proton-bound hydrogen phthalate dimer and two water mol­ecules. The potassium cation resides on a twofold axis in a distorted square-antiprism coordination geometry. The compound is isomorphous with the ammonium analogue, previously misidentified. As potassium hydrogen phthalate is frequently used in the manufacture of buffers, organic carbon standards, acidimetric standards and various other products, the crystallization of a compound with a different stoichiom­etery from a solution containing the acidimetric standard has important practical implications.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270104019043/sq1160sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270104019043/sq1160Isup2.hkl
Contains datablock I

CCDC reference: 256985

Comment top

The hydrated potassium salt of the proton-bound hydrogen o-phthalate dimer [space group C 2/c, Z = 4, a = 13.5830 (3) Å, b = 21.1270 (5) Å, c = 6.8170 (2) Å and β = 116.552 (1)°] was obtained in an attempt to grow crystals of potassium hydrogen o-phthalate (commonly abbreviated to KAP for potassium acid phthalate) from an aqueous solution at room temperature. Instead of precipitating the usual six-sided {010} plates characteristic of KAP, heavily etched elongated prisms were observed. As these irregular crystals lacked the perfect basal cleavage planes characteristic of KAP, the sample was indexed and found to belong to the C-centered monoclinic crystal system, space group C 2/c, as opposed to the primitive orthorhombic system of KAP.

The crystal habit is a combination of the pinacoids {010} and {110} elongated along [001], lacking other identifiable faces. The molecular formula was determined to be [K+ H+(o-C6H4COOHCOO)2] 2H2O, a molecular complex that contains a potassium cation, a proton-bound hydrogen phthalate dimer and two water molecules (Fig. 1). The hydrogen phthalate dimer is bridged by two symmetry-related H atoms, each with half-site occupancies. The conformation of the hydrogen phthalate ion is characterized by the nearly orthogonal carboxylate groups making an angle of 84.2° to one another.

The eight-coordinate potassium cation occupies a site on the twofold axis in a distorted square-antiprism coordination geometry, approximately 82 symmetry (Fig. 2). The coordination sphere is composed of two water molecules and the hydrogen phthalate dimer in the molecular complex, and additionally two hydrogen phthalate ions from adjacent molecular complexes along the C-centered [001] staggered-cation channels (Fig. 3). The potassium ions are enveloped by spiraling proton-bound hydrogen phthalate dimers and water molecules. The dominant interchannel interactions are of the van der Waals type between aryl rings.

It is surprising that this substance has not been reported until now, given the abundant research conducted on KAP and other phthalic acid salts (Okaya, 1965; Gougoutas et al., 1980; Smith, 1975; Eremina et al., 1993; Kariuki, 1995). As KAP is frequently used in the manufacture of buffer solutions, acidimetric standards and various other products, the possible contamination arising from the crystallization of a compound with a different stoichiometry has obvious practical implications for any analytical applications utilizing KAP.

Smith (1977) reported a structure that bears a remarkable resemblance to our compound (Fig. 4), but is described under the unusual banner 'ammonium hydrogen phthalate hemihydrate(?)' and is characterized as NH4(C8H5O4)2·0.5H2O(?) [monoclinic, C 2/c, Z = 8, a = 13.564 (8) Å, b = 21.17 (1) Å, c = 6.840 (5) Å and β = 112.8 (8)°]. It was immediately evident that this was the ammonium isomorph of our potassium salt; however, it had not been described as such.

There are several questionable features contained in the report of the ammonium structure, the first and foremost being the question mark in the title. The author used flotation methods to determine the density of the crystal, and imposed a water/ammonium disorder model to account for the observed density. While the author does well to narrate the derivation of his complex solution, the disorder model employed is forced. It seems that the author's primary emphasis was to publish what he viewed as a noteworthy conformation of the phthalate anion accepting that 'the proposed formula may be wrong'. Several data sets were collected on poorly scattering crystals of the ammonium compound prepared according to the method of Smith (1977). While our proposed model was a correct solution, R1 was only 0.10.

Experimental top

Crystals of the title compound were obtained by slow evaporation of a solution containing acidimetric grade potassium hydrogen phthalate (Aldrich, 22 g) and deionized water (Barnsted NANOpure, 18.2 M W−1 cm−1, 200 ml) in a 250 ml crystallization dish at room temperature (approx. 296 K). After approximately four days, crystals of both KAP and the title compound were produced in the same dish. Currently, the dominance of one crystal over the other appears random, though the formation of KAP is strongly favored as the title compound is only observed in approximately 1% of growth experiments conducted. Formation of the title compound does not appear to depend on temperature, exposure to light or rate of evaporation.

Refinement top

All H atoms were initially located in a difference Fourier map. H atoms were then placed in idealized positions and constrained to ride on their parent atoms, with C—H distances in the range 0.95–1.00 Å. Uiso(H) values were fixed at 1.2Ueq of the parent atoms for CH groups and 1.5Ueq of the parent atoms for methyl groups.

Computing details top

Data collection: KappaCCD Software (Nonius, 1997); cell refinement: HKL SCALEPACK (Otwinowski & Minor 1997); data reduction: HKL-2000 (Otwinowski & Minor 1997); program(s) used to solve structure: SIR97 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: maXus (MacKat et al. 1998); software used to prepare material for publication: SHELXL97 (Sheldrick, 1997).

Figures top
[Figure 1] Fig. 1. A view of the molecular complex of the title compound, with displacement ellipsoids at the 50% probability level.
[Figure 2] Fig. 2. The coordination sphere of the potassium cation, illustrating the distorted square-antiprism coordination geometry.
[Figure 3] Fig. 3. A view along [010], perpendicular to the [001] potassium ion channel. Hydrogen bonds are indicated as dotted lines.
[Figure 4] Fig. 4. Packing diagrams of the title compound (left) and the ammonium analogue (right), illustrating the isomorphism. The spheres (right) indicate the position of the proposed disorder in the ammonium structure. A water molecule occupies this position in the structure of the title compound (left).
Potassium hydrogen diphthalate dihydrate top
Crystal data top
K+·C16H11O8·2H2OF(000) = 840
Mr = 406.38Dx = 1.542 Mg m3
Monoclinic, C2/cMo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2ycCell parameters from 239 reflections
a = 13.5830 (3) Åθ = 2.4–24.4°
b = 21.1270 (5) ŵ = 0.36 mm1
c = 6.8170 (2) ÅT = 273 K
β = 116.5521 (11)°Cut-block, colourless
V = 1749.93 (8) Å30.48 × 0.31 × 0.29 mm
Z = 4
Data collection top
Nonius KappaCCD
diffractometer
2041 independent reflections
Radiation source: fine-focus sealed tube1682 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.024
ϕ and ω scansθmax = 29.9°, θmin = 3.2°
Absorption correction: multi-scan
HKL-2000 (Otwinowski & Minor 1997)
h = 1919
Tmin = 0.847, Tmax = 0.903k = 2929
3680 measured reflectionsl = 99
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.031Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.082H-atom parameters constrained
S = 1.00 w = 1/[σ2(Fo2) + (0.0522P)2]
where P = (Fo2 + 2Fc2)/3
2041 reflections(Δ/σ)max = 0.001
131 parametersΔρmax = 0.25 e Å3
0 restraintsΔρmin = 0.26 e Å3
Crystal data top
K+·C16H11O8·2H2OV = 1749.93 (8) Å3
Mr = 406.38Z = 4
Monoclinic, C2/cMo Kα radiation
a = 13.5830 (3) ŵ = 0.36 mm1
b = 21.1270 (5) ÅT = 273 K
c = 6.8170 (2) Å0.48 × 0.31 × 0.29 mm
β = 116.5521 (11)°
Data collection top
Nonius KappaCCD
diffractometer
2041 independent reflections
Absorption correction: multi-scan
HKL-2000 (Otwinowski & Minor 1997)
1682 reflections with I > 2σ(I)
Tmin = 0.847, Tmax = 0.903Rint = 0.024
3680 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0310 restraints
wR(F2) = 0.082H-atom parameters constrained
S = 1.00Δρmax = 0.25 e Å3
2041 reflectionsΔρmin = 0.26 e Å3
131 parameters
Special details top

Experimental. Data was collected with ω and ϕ scans in 2º increments with 30 second exposures per degree. Crystal-to-detector distance was 30 mm. 13910 full and partial reflection were integrated.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
K10.50000.550188 (16)0.25000.01852 (12)
O10.34638 (7)0.47208 (4)0.30945 (14)0.0208 (2)
O20.18625 (7)0.52109 (4)0.11436 (15)0.0230 (2)
H20.22500.55270.15960.035*
O30.41424 (7)0.34097 (4)0.50179 (14)0.0186 (2)
O40.39982 (7)0.34408 (4)0.16310 (14)0.0209 (2)
H4A0.46720.34590.22870.031*0.50
O50.70096 (8)0.62456 (4)0.27164 (16)0.0234 (2)
H5A0.6687 (14)0.6395 (7)0.339 (3)0.028*
H5B0.6704 (14)0.6380 (7)0.143 (3)0.028*
C10.24635 (10)0.47017 (5)0.19816 (19)0.0170 (3)
C20.18190 (9)0.41029 (5)0.14561 (19)0.0167 (3)
C30.06716 (10)0.41098 (6)0.0489 (2)0.0225 (3)
H30.03010.44950.01040.027*
C40.00777 (11)0.35547 (6)0.0094 (2)0.0250 (3)
H40.06870.35670.05430.030*
C50.06204 (11)0.29794 (6)0.0646 (2)0.0240 (3)
H50.02210.26050.03850.029*
C60.17629 (10)0.29612 (6)0.1593 (2)0.0211 (3)
H60.21270.25740.19540.025*
C70.23649 (10)0.35203 (5)0.20022 (19)0.0158 (3)
C80.35957 (10)0.34651 (5)0.30218 (19)0.0154 (2)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
K10.01609 (19)0.0186 (2)0.0189 (2)0.0000.00607 (15)0.000
O10.0149 (4)0.0192 (5)0.0235 (5)0.0011 (3)0.0043 (4)0.0016 (3)
O20.0166 (4)0.0143 (4)0.0332 (5)0.0003 (3)0.0066 (4)0.0020 (3)
O30.0173 (4)0.0204 (4)0.0164 (4)0.0023 (3)0.0060 (4)0.0007 (3)
O40.0140 (4)0.0325 (5)0.0166 (4)0.0011 (3)0.0071 (4)0.0003 (3)
O50.0299 (5)0.0232 (5)0.0200 (5)0.0095 (4)0.0137 (4)0.0038 (3)
C10.0155 (6)0.0193 (6)0.0160 (6)0.0010 (4)0.0070 (5)0.0009 (4)
C20.0155 (6)0.0187 (7)0.0158 (6)0.0001 (4)0.0069 (5)0.0006 (4)
C30.0156 (6)0.0213 (7)0.0283 (7)0.0030 (4)0.0078 (5)0.0005 (5)
C40.0138 (6)0.0269 (8)0.0319 (7)0.0032 (5)0.0080 (6)0.0041 (5)
C50.0217 (6)0.0199 (7)0.0311 (7)0.0064 (5)0.0123 (6)0.0056 (5)
C60.0211 (6)0.0158 (6)0.0263 (7)0.0007 (4)0.0105 (5)0.0010 (5)
C70.0149 (6)0.0186 (6)0.0146 (6)0.0001 (4)0.0072 (5)0.0009 (4)
C80.0176 (6)0.0094 (6)0.0187 (6)0.0008 (4)0.0077 (5)0.0005 (4)
Geometric parameters (Å, º) top
K1—O3i2.7831 (8)O3—K1ii2.7832 (9)
K1—O3ii2.7831 (9)O4—C81.2899 (14)
K1—O1ii2.8270 (9)O4—H4A0.8200
K1—O1i2.8270 (9)O5—H5A0.827 (17)
K1—O12.8279 (8)O5—H5B0.833 (17)
K1—O1iii2.8280 (8)C1—C21.4882 (16)
K1—O53.0954 (10)C2—C31.3947 (17)
K1—O5iii3.0954 (10)C2—C71.3986 (16)
K1—C8i3.5377 (12)C3—C41.3800 (17)
K1—C8ii3.5377 (12)C3—H30.9300
K1—K1ii4.0143 (4)C4—C51.3836 (18)
K1—K1iv4.0144 (4)C4—H40.9300
K1—H5A2.816 (15)C5—C61.3895 (18)
O1—C11.2255 (15)C5—H50.9300
O1—K1ii2.8271 (9)C6—C71.3921 (17)
O2—C11.3166 (14)C6—H60.9300
O2—H20.8200C7—C81.5010 (17)
O3—C81.2301 (15)C8—K1ii3.5376 (12)
O3i—K1—O3ii68.57 (3)O1ii—K1—K1iv122.27 (2)
O3i—K1—O1ii133.86 (3)O1i—K1—K1iv44.784 (17)
O3ii—K1—O1ii65.29 (2)O1—K1—K1iv95.36 (2)
O3i—K1—O1i65.29 (2)O1iii—K1—K1iv44.766 (18)
O3ii—K1—O1i133.86 (3)O5—K1—K1iv88.504 (18)
O1ii—K1—O1i160.84 (4)O5iii—K1—K1iv124.221 (18)
O3i—K1—O1115.73 (2)C8i—K1—K1iv78.338 (19)
O3ii—K1—O1122.01 (3)C8ii—K1—K1iv148.645 (19)
O1ii—K1—O189.55 (2)K1ii—K1—K1iv116.223 (17)
O1i—K1—O179.26 (2)O3i—K1—H5A69.9 (3)
O3i—K1—O1iii122.01 (3)O3ii—K1—H5A40.2 (4)
O3ii—K1—O1iii115.73 (2)O1ii—K1—H5A74.9 (3)
O1ii—K1—O1iii79.26 (2)O1i—K1—H5A118.9 (3)
O1i—K1—O1iii89.55 (2)O1—K1—H5A160.2 (3)
O1—K1—O1iii108.60 (4)O1iii—K1—H5A80.8 (3)
O3i—K1—O574.34 (2)O5—K1—H5A15.2 (3)
O3ii—K1—O555.33 (3)O5iii—K1—H5A106.6 (3)
O1ii—K1—O579.48 (2)C8i—K1—H5A81.1 (3)
O1i—K1—O5110.58 (3)C8ii—K1—H5A47.8 (3)
O1—K1—O5168.76 (2)K1ii—K1—H5A118.6 (3)
O1iii—K1—O567.14 (2)K1iv—K1—H5A103.2 (4)
O3i—K1—O5iii55.33 (3)C1—O1—K1ii137.71 (8)
O3ii—K1—O5iii74.34 (2)C1—O1—K1127.99 (7)
O1ii—K1—O5iii110.58 (3)K1ii—O1—K190.45 (2)
O1i—K1—O5iii79.48 (2)C1—O2—H2109.5
O1—K1—O5iii67.14 (2)C8—O3—K1ii118.39 (7)
O1iii—K1—O5iii168.76 (2)C8—O4—H4A109.5
O5—K1—O5iii118.99 (3)K1—O5—H5A62.9 (10)
O3i—K1—C8i17.81 (2)K1—O5—H5B96.3 (11)
O3ii—K1—C8i86.13 (3)H5A—O5—H5B108.4 (16)
O1ii—K1—C8i151.23 (3)O1—C1—O2122.85 (10)
O1i—K1—C8i47.84 (3)O1—C1—C2123.17 (10)
O1—K1—C8i109.63 (3)O2—C1—C2113.98 (10)
O1iii—K1—C8i112.59 (3)C3—C2—C7118.85 (11)
O5—K1—C8i81.48 (3)C3—C2—C1121.17 (11)
O5iii—K1—C8i61.44 (3)C7—C2—C1119.94 (11)
O3i—K1—C8ii86.13 (3)C4—C3—C2121.03 (11)
O3ii—K1—C8ii17.81 (2)C4—C3—H3119.5
O1ii—K1—C8ii47.84 (3)C2—C3—H3119.5
O1i—K1—C8ii151.23 (3)C3—C4—C5119.97 (12)
O1—K1—C8ii112.59 (3)C3—C4—H4120.0
O1iii—K1—C8ii109.63 (3)C5—C4—H4120.0
O5—K1—C8ii61.44 (3)C4—C5—C6119.95 (11)
O5iii—K1—C8ii81.48 (3)C4—C5—H5120.0
C8i—K1—C8ii103.81 (4)C6—C5—H5120.0
O3i—K1—K1ii142.547 (19)C5—C6—C7120.23 (11)
O3ii—K1—K1ii94.538 (18)C5—C6—H6119.9
O1ii—K1—K1ii44.785 (17)C7—C6—H6119.9
O1i—K1—K1ii122.27 (2)C6—C7—C2119.95 (11)
O1—K1—K1ii44.766 (18)C6—C7—C8117.37 (10)
O1iii—K1—K1ii95.36 (2)C2—C7—C8122.68 (10)
O5—K1—K1ii124.220 (18)O3—C8—O4124.38 (11)
O5iii—K1—K1ii88.505 (18)O3—C8—C7121.08 (10)
C8i—K1—K1ii148.646 (19)O4—C8—C7114.39 (10)
C8ii—K1—K1ii78.338 (19)O3—C8—K1ii43.80 (5)
O3i—K1—K1iv94.538 (18)O4—C8—K1ii114.62 (7)
O3ii—K1—K1iv142.545 (19)C7—C8—K1ii114.26 (7)
O3i—K1—O1—C120.91 (10)K1—O1—C1—C2141.61 (9)
O3ii—K1—O1—C1100.72 (10)O1—C1—C2—C3169.65 (12)
O1ii—K1—O1—C1160.86 (11)O2—C1—C2—C310.33 (16)
O1i—K1—O1—C134.78 (9)O1—C1—C2—C78.13 (18)
O1iii—K1—O1—C1120.63 (10)O2—C1—C2—C7171.89 (11)
O5—K1—O1—C1173.50 (12)C7—C2—C3—C40.75 (19)
O5iii—K1—O1—C148.23 (9)C1—C2—C3—C4177.05 (12)
C8i—K1—O1—C12.75 (10)C2—C3—C4—C50.4 (2)
C8ii—K1—O1—C1117.77 (9)C3—C4—C5—C60.2 (2)
K1ii—K1—O1—C1160.86 (11)C4—C5—C6—C70.5 (2)
K1iv—K1—O1—C176.76 (10)C5—C6—C7—C20.13 (19)
O3i—K1—O1—K1ii139.95 (3)C5—C6—C7—C8179.84 (11)
O3ii—K1—O1—K1ii60.14 (3)C3—C2—C7—C60.47 (18)
O1ii—K1—O1—K1ii0.0C1—C2—C7—C6177.37 (11)
O1i—K1—O1—K1ii164.36 (3)C3—C2—C7—C8179.23 (11)
O1iii—K1—O1—K1ii78.50 (2)C1—C2—C7—C82.94 (17)
O5—K1—O1—K1ii12.64 (14)K1ii—O3—C8—O490.91 (11)
O5iii—K1—O1—K1ii112.63 (3)K1ii—O3—C8—C793.71 (11)
C8i—K1—O1—K1ii158.11 (3)C6—C7—C8—O383.48 (14)
C8ii—K1—O1—K1ii43.09 (3)C2—C7—C8—O396.81 (14)
K1iv—K1—O1—K1ii122.38 (2)C6—C7—C8—O492.33 (13)
K1ii—O1—C1—O2112.43 (12)C2—C7—C8—O487.37 (14)
K1—O1—C1—O238.41 (16)C6—C7—C8—K1ii132.74 (9)
K1ii—O1—C1—C267.55 (16)C2—C7—C8—K1ii47.56 (13)
Symmetry codes: (i) x, y+1, z1/2; (ii) x+1, y+1, z+1; (iii) x+1, y, z+1/2; (iv) x+1, y+1, z.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O2—H2···O5iii0.821.762.5815 (12)174
O4—H4A···O4iii0.821.622.4347 (17)173
O5—H5A···O3ii0.827 (17)1.925 (18)2.7433 (13)169.9 (14)
O5—H5B···O4iv0.833 (17)1.907 (17)2.7330 (13)171.4 (15)
Symmetry codes: (ii) x+1, y+1, z+1; (iii) x+1, y, z+1/2; (iv) x+1, y+1, z.

Experimental details

Crystal data
Chemical formulaK+·C16H11O8·2H2O
Mr406.38
Crystal system, space groupMonoclinic, C2/c
Temperature (K)273
a, b, c (Å)13.5830 (3), 21.1270 (5), 6.8170 (2)
β (°) 116.5521 (11)
V3)1749.93 (8)
Z4
Radiation typeMo Kα
µ (mm1)0.36
Crystal size (mm)0.48 × 0.31 × 0.29
Data collection
DiffractometerNonius KappaCCD
diffractometer
Absorption correctionMulti-scan
HKL-2000 (Otwinowski & Minor 1997)
Tmin, Tmax0.847, 0.903
No. of measured, independent and
observed [I > 2σ(I)] reflections
3680, 2041, 1682
Rint0.024
(sin θ/λ)max1)0.701
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.031, 0.082, 1.00
No. of reflections2041
No. of parameters131
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.25, 0.26

Computer programs: KappaCCD Software (Nonius, 1997), HKL SCALEPACK (Otwinowski & Minor 1997), HKL-2000 (Otwinowski & Minor 1997), SIR97 (Altomare et al., 1994), SHELXL97 (Sheldrick, 1997), maXus (MacKat et al. 1998).

Selected bond lengths (Å) top
K1—O3i2.7831 (8)K1—O12.8279 (8)
K1—O1i2.8270 (9)K1—O53.0954 (10)
Symmetry code: (i) x, y+1, z1/2.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O2—H2···O5ii0.821.762.5815 (12)174
O4—H4A···O4ii0.821.622.4347 (17)173
O5—H5A···O3iii0.827 (17)1.925 (18)2.7433 (13)169.9 (14)
O5—H5B···O4iv0.833 (17)1.907 (17)2.7330 (13)171.4 (15)
Symmetry codes: (ii) x+1, y, z+1/2; (iii) x+1, y+1, z+1; (iv) x+1, y+1, z.
 

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