Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270107036165/ga3059sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270107036165/ga3059Isup2.hkl |
CCDC reference: 627009
The title compound was prepared by dissolving KOH (0.05 mol, 2.8 g) and B(OH)3 (1 mol, 0.62 g) in H2O (20 ml). To this solution, a solution of maleic acid (0.02 mol, 2.32 g) in H2O (20 ml) was added dropwise at room temperature, producing a colourless solution. The reaction mixture was stirred at 323 K for 9 h, yielding a white solid. Single crystals of (I) suitable for X-ray diffraction study were obtained by slow evaporation of a saturated aqueous solution at 298 K (yield 2.1 g, 83% based on boric acid). Spectroscopic analysis: 1H NMR (D2O, 298 K, TMS, δ, p.p.m.): 6.02 (2H, s, HC═ CH); 13C NMR: (D2O, 298 K, TMS, δ, p.p.m.): 130.80 (COOH), 175.51 (HC═ CH). IR spectra were recorded from a KBr disc, using a Jasco 680 Plus FT–IR spectrometer. IR (cm-1): ν(br, O—H) 3366, ν(w, C═C) 1633, ν(s, C═Oasym) 1561, ν(s, C═Osym) 1399–1306.
The maleate H atoms were constrained to an ideal geometry, with C—H distances of 0.93 Å and Uiso(H) = 1.2Ueq(C). Atoms H4 and H5 of the boric acid were also constrained to an ideal geometry, with O—H distances of 0.98 Å and Uiso(H) = 1.5Ueq(C). Atom H3 was located in a difference Fourier synthesis and refined isotropically.
Data collection: X-AREA (Stoe & Cie, 2002); cell refinement: X-AREA; data reduction: X-RED32 (Stoe & Cie, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: Mercury (Version 1.4.1; Macrae et al., 2006); software used to prepare material for publication: enCIFer (Version 1.2; Allen et al., 2004).
[K2(C4H2O4)(H3BO3)] | F(000) = 512 |
Mr = 254.09 | Dx = 1.892 Mg m−3 |
Orthorhombic, P212121 | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: P 2ac 2ab | Cell parameters from 5260 reflections |
a = 6.4968 (5) Å | θ = 2.5–27.9° |
b = 10.2591 (11) Å | µ = 1.07 mm−1 |
c = 13.3852 (10) Å | T = 296 K |
V = 892.14 (14) Å3 | Prism, colourless |
Z = 4 | 0.53 × 0.52 × 0.40 mm |
Stoe IPDS II diffractometer | 5260 independent reflections |
Radiation source: sealed X-ray tube,long-fine focus | 6521 reflections with I > 2σ(I) |
Plane graphite monochromator | Rint = 0.065 |
Detector resolution: 6.67 pixels mm-1 | θmax = 26.0°, θmin = 2.5° |
ω scan rotation method | h = −6→8 |
Absorption correction: integration (X-RED32; Stoe & Cie, 2002) | k = −12→12 |
Tmin = 0.520, Tmax = 0.652 | l = −16→16 |
6497 measured reflections |
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.027 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.072 | w = 1/[σ2(Fo2) + (0.0411P)2 + 0.1611P] where P = (Fo2 + 2Fc2)/3 |
S = 1.05 | (Δ/σ)max < 0.001 |
1748 reflections | Δρmax = 0.25 e Å−3 |
148 parameters | Δρmin = −0.36 e Å−3 |
0 restraints | Absolute structure: Flack (1983), with how many Friedel pairs? |
Primary atom site location: structure-invariant direct methods | Absolute structure parameter: 0.01 (4) |
[K2(C4H2O4)(H3BO3)] | V = 892.14 (14) Å3 |
Mr = 254.09 | Z = 4 |
Orthorhombic, P212121 | Mo Kα radiation |
a = 6.4968 (5) Å | µ = 1.07 mm−1 |
b = 10.2591 (11) Å | T = 296 K |
c = 13.3852 (10) Å | 0.53 × 0.52 × 0.40 mm |
Stoe IPDS II diffractometer | 5260 independent reflections |
Absorption correction: integration (X-RED32; Stoe & Cie, 2002) | 6521 reflections with I > 2σ(I) |
Tmin = 0.520, Tmax = 0.652 | Rint = 0.065 |
6497 measured reflections |
R[F2 > 2σ(F2)] = 0.027 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.072 | Δρmax = 0.25 e Å−3 |
S = 1.05 | Δρmin = −0.36 e Å−3 |
1748 reflections | Absolute structure: Flack (1983), with how many Friedel pairs? |
148 parameters | Absolute structure parameter: 0.01 (4) |
0 restraints |
Experimental. no |
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·All H atoms were located in a difference Fourier map. |
x | y | z | Uiso*/Ueq | ||
K2 | 0.70097 (7) | 0.46219 (5) | 0.80259 (4) | 0.03035 (14) | |
O2 | 0.9029 (3) | 0.22194 (14) | 0.74675 (13) | 0.0302 (3) | |
H5 | 1.0125 | 0.1985 | 0.7937 | 0.036* | |
O3 | 0.6338 (2) | 0.12886 (16) | 0.65155 (13) | 0.0324 (4) | |
O1 | 0.8659 (2) | −0.00609 (14) | 0.73950 (12) | 0.0279 (3) | |
H4 | 0.7915 | −0.0686 | 0.6976 | 0.033* | |
O4 | 0.7902 (3) | 0.62339 (16) | 0.64157 (12) | 0.0349 (3) | |
O6 | 0.6924 (3) | 0.38549 (18) | 0.47184 (12) | 0.0414 (4) | |
O7 | 0.5149 (3) | 0.37034 (16) | 0.61327 (13) | 0.0402 (4) | |
O5 | 0.6328 (3) | 0.81487 (17) | 0.65806 (15) | 0.0401 (4) | |
C1 | 0.6500 (3) | 0.7029 (2) | 0.62152 (15) | 0.0253 (4) | |
C3 | 0.4556 (3) | 0.5506 (2) | 0.51034 (16) | 0.0281 (4) | |
H2 | 0.3504 | 0.5442 | 0.4634 | 0.034* | |
C4 | 0.5656 (3) | 0.42696 (19) | 0.53478 (15) | 0.0257 (4) | |
C2 | 0.4894 (4) | 0.6681 (2) | 0.54721 (17) | 0.0318 (5) | |
H1 | 0.4050 | 0.7352 | 0.5246 | 0.038* | |
B1 | 0.7992 (4) | 0.1155 (2) | 0.71346 (16) | 0.0245 (4) | |
K1 | 0.94261 (8) | −0.12703 (6) | 0.91944 (4) | 0.03950 (16) | |
H3 | 0.620 (6) | 0.204 (4) | 0.642 (3) | 0.050 (9)* |
U11 | U22 | U33 | U12 | U13 | U23 | |
K2 | 0.0268 (2) | 0.0300 (2) | 0.0342 (2) | −0.00049 (18) | 0.00256 (18) | −0.00061 (19) |
O2 | 0.0283 (7) | 0.0205 (7) | 0.0418 (9) | −0.0015 (6) | −0.0085 (6) | −0.0017 (6) |
O3 | 0.0296 (8) | 0.0213 (8) | 0.0462 (9) | 0.0018 (7) | −0.0135 (7) | −0.0007 (7) |
O1 | 0.0272 (7) | 0.0199 (7) | 0.0367 (7) | −0.0009 (6) | −0.0073 (6) | 0.0001 (6) |
O4 | 0.0340 (8) | 0.0278 (8) | 0.0429 (8) | 0.0009 (8) | −0.0129 (7) | 0.0018 (7) |
O6 | 0.0435 (9) | 0.0397 (9) | 0.0409 (9) | 0.0078 (9) | 0.0099 (8) | −0.0057 (8) |
O7 | 0.0497 (10) | 0.0285 (8) | 0.0423 (9) | 0.0080 (8) | 0.0093 (8) | 0.0090 (7) |
O5 | 0.0336 (9) | 0.0312 (9) | 0.0554 (10) | −0.0036 (7) | −0.0021 (8) | −0.0169 (8) |
C1 | 0.0252 (10) | 0.0228 (9) | 0.0279 (10) | −0.0048 (8) | 0.0010 (8) | 0.0013 (8) |
C3 | 0.0297 (10) | 0.0267 (10) | 0.0280 (9) | 0.0006 (9) | −0.0089 (8) | −0.0010 (8) |
C4 | 0.0274 (10) | 0.0221 (9) | 0.0275 (10) | −0.0021 (8) | −0.0036 (8) | −0.0032 (8) |
C2 | 0.0368 (12) | 0.0228 (9) | 0.0357 (10) | 0.0042 (9) | −0.0130 (10) | 0.0023 (9) |
B1 | 0.0234 (10) | 0.0218 (10) | 0.0282 (10) | 0.0011 (9) | 0.0002 (9) | −0.0001 (9) |
K1 | 0.0374 (3) | 0.0431 (3) | 0.0380 (3) | 0.0051 (2) | 0.0015 (2) | 0.0056 (2) |
K2—O5i | 2.6952 (19) | O7—C4 | 1.245 (3) |
K2—O4 | 2.7778 (18) | O5—C1 | 1.253 (3) |
K2—O6ii | 2.8380 (19) | C1—C2 | 1.485 (3) |
K2—O3iii | 2.8340 (18) | C3—C2 | 1.321 (3) |
K2—O1iv | 2.8879 (16) | C3—C4 | 1.492 (3) |
K2—O2 | 2.8905 (16) | C3—H2 | 0.9300 |
K2—O7 | 2.962 (2) | C2—H1 | 0.9300 |
O2—B1 | 1.359 (3) | K1—O6v | 2.785 (2) |
O2—H5 | 0.9800 | K1—O6vi | 2.880 (2) |
O3—B1 | 1.364 (3) | K1—O2v | 2.8907 (18) |
O3—H3 | 0.79 (4) | K1—O7i | 3.004 (2) |
O1—B1 | 1.366 (3) | K1—O3vi | 3.1462 (18) |
O1—H4 | 0.9800 | K1—O4v | 3.2062 (19) |
O4—C1 | 1.252 (3) | K1—C3i | 3.301 (2) |
O6—C4 | 1.253 (3) | K1—O1 | 2.7547 (16) |
O5i—K2—O4 | 130.80 (6) | O5—C1—C2 | 114.77 (19) |
O5i—K2—O6ii | 110.44 (6) | O4—C1—C2 | 119.9 (2) |
O4—K2—O6ii | 103.92 (5) | C2—C3—C4 | 127.89 (19) |
O5i—K2—O3iii | 71.25 (5) | C2—C3—K1iii | 121.86 (18) |
O4—K2—O3iii | 88.23 (5) | C2—C3—H2 | 116.1 |
O6ii—K2—O3iii | 71.47 (5) | C4—C3—H2 | 116.1 |
O5i—K2—O1iv | 152.28 (5) | K1iii—C3—H2 | 65.0 |
O4—K2—O1iv | 65.05 (5) | O7—C4—O6 | 125.7 (2) |
O6ii—K2—O1iv | 81.71 (5) | O7—C4—C3 | 117.05 (19) |
O3iii—K2—O1iv | 136.22 (5) | O6—C4—C3 | 117.14 (18) |
O5i—K2—O2 | 86.42 (5) | C3—C2—C1 | 125.9 (2) |
O4—K2—O2 | 102.26 (5) | C3—C2—H1 | 117.0 |
O6ii—K2—O2 | 124.41 (5) | C1—C2—H1 | 117.0 |
O3iii—K2—O2 | 156.76 (5) | O2—B1—O1 | 119.54 (18) |
O1iv—K2—O2 | 66.63 (4) | O2—B1—O3 | 120.60 (19) |
O5i—K2—O7 | 70.14 (6) | O1—B1—O3 | 119.83 (19) |
O4—K2—O7 | 67.09 (5) | O1—K1—O6v | 126.15 (5) |
O6ii—K2—O7 | 163.31 (5) | O1—K1—O6vi | 124.93 (5) |
O3iii—K2—O7 | 93.67 (5) | O6v—K1—O6vi | 100.05 (4) |
O1iv—K2—O7 | 105.47 (5) | O1—K1—O2v | 68.37 (5) |
O2—K2—O7 | 72.11 (5) | O6v—K1—O2v | 97.54 (5) |
B1—O2—K2 | 123.04 (13) | O6vi—K1—O2v | 78.44 (5) |
B1—O2—K1iv | 110.53 (13) | O1—K1—O7i | 72.40 (5) |
K2—O2—K1iv | 84.23 (4) | O6v—K1—O7i | 156.76 (5) |
B1—O2—H5 | 111.9 | O6vi—K1—O7i | 74.10 (5) |
K2—O2—H5 | 111.9 | O2v—K1—O7i | 103.09 (5) |
K1iv—O2—H5 | 111.9 | O1—K1—O3vi | 146.88 (5) |
B1—O3—K2i | 114.34 (13) | O6v—K1—O3vi | 67.59 (5) |
B1—O3—K1vii | 136.36 (13) | O6vi—K1—O3vi | 72.90 (5) |
K2i—O3—K1vii | 95.12 (5) | O2v—K1—O3vi | 144.24 (5) |
B1—O3—H3 | 107 (3) | O7i—K1—O3vi | 89.30 (5) |
K2i—O3—H3 | 123 (3) | O1—K1—O4v | 60.94 (4) |
K1vii—O3—H3 | 80 (2) | O6v—K1—O4v | 68.60 (5) |
B1—O1—K1 | 133.79 (13) | O6vi—K1—O4v | 164.74 (6) |
B1—O1—K2v | 111.25 (12) | O2v—K1—O4v | 92.61 (5) |
K1—O1—K2v | 86.77 (4) | O7i—K1—O4v | 120.37 (5) |
B1—O1—H4 | 107.2 | O3vi—K1—O4v | 109.98 (5) |
K1—O1—H4 | 107.2 | O1—K1—C3i | 81.87 (5) |
K2v—O1—H4 | 107.2 | O6v—K1—C3i | 119.54 (6) |
K2—O4—K1iv | 80.42 (4) | O6vi—K1—C3i | 101.56 (6) |
K1iv—O6—K2viii | 103.57 (6) | O2v—K1—C3i | 141.91 (5) |
K1iv—O6—K1vii | 110.13 (6) | O7i—K1—C3i | 43.21 (4) |
K2viii—O6—K1vii | 112.76 (6) | O3vi—K1—C3i | 66.33 (5) |
K2—O7—K1iii | 121.72 (6) | O4v—K1—C3i | 93.12 (5) |
O5—C1—O4 | 125.3 (2) | ||
C2—C3—C4—O7 | 79.8 (3) | O5—C1—C2—C3 | −168.8 (2) |
K1iii—C3—C4—O7 | −43.87 (18) | O4—C1—C2—C3 | 12.5 (4) |
C2—C3—C4—O6 | −104.1 (3) | K2—O2—B1—O1 | −143.73 (15) |
K1iii—C3—C4—O6 | 132.23 (18) | K1iv—O2—B1—O1 | 119.63 (17) |
C2—C3—C4—K1iv | −37.9 (3) | K2—O2—B1—O3 | 38.6 (3) |
K1iii—C3—C4—K1iv | −161.56 (13) | K1iv—O2—B1—O3 | −58.0 (2) |
C2—C3—C4—K1iii | 123.6 (3) | K1—O1—B1—O2 | 55.5 (3) |
C2—C3—C4—K1vii | −168.0 (2) | K2v—O1—B1—O2 | −51.3 (2) |
K1iii—C3—C4—K1vii | 68.36 (14) | K1—O1—B1—O3 | −126.77 (17) |
C4—C3—C2—C1 | 0.9 (4) | K2v—O1—B1—O3 | 126.43 (17) |
K1iii—C3—C2—C1 | 105.4 (3) | B1—O1—K1—O6v | −85.42 (18) |
Symmetry codes: (i) −x+1, y−1/2, −z+3/2; (ii) −x+3/2, −y+1, z+1/2; (iii) −x+1, y+1/2, −z+3/2; (iv) −x+2, y+1/2, −z+3/2; (v) −x+2, y−1/2, −z+3/2; (vi) −x+3/2, −y, z+1/2; (vii) −x+3/2, −y, z−1/2; (viii) −x+3/2, −y+1, z−1/2. |
D—H···A | D—H | H···A | D···A | D—H···A |
O2—H5···O4v | 0.98 (1) | 1.73 (1) | 2.689 (3) | 166 (1) |
O3—H3···O7 | 0.79 (4) | 1.88 (4) | 2.645 (2) | 165 (4) |
O1—H4···O5ix | 0.98 (1) | 1.66 (1) | 2.618 (1) | 163 (1) |
Symmetry codes: (v) −x+2, y−1/2, −z+3/2; (ix) x, y−1, z. |
Experimental details
Crystal data | |
Chemical formula | [K2(C4H2O4)(H3BO3)] |
Mr | 254.09 |
Crystal system, space group | Orthorhombic, P212121 |
Temperature (K) | 296 |
a, b, c (Å) | 6.4968 (5), 10.2591 (11), 13.3852 (10) |
V (Å3) | 892.14 (14) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 1.07 |
Crystal size (mm) | 0.53 × 0.52 × 0.40 |
Data collection | |
Diffractometer | Stoe IPDS II diffractometer |
Absorption correction | Integration (X-RED32; Stoe & Cie, 2002) |
Tmin, Tmax | 0.520, 0.652 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 6497, 5260, 6521 |
Rint | 0.065 |
(sin θ/λ)max (Å−1) | 0.617 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.027, 0.072, 1.05 |
No. of reflections | 1748 |
No. of parameters | 148 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.25, −0.36 |
Absolute structure | Flack (1983), with how many Friedel pairs? |
Absolute structure parameter | 0.01 (4) |
Computer programs: X-AREA (Stoe & Cie, 2002), X-AREA, X-RED32 (Stoe & Cie, 2002), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), Mercury (Version 1.4.1; Macrae et al., 2006), enCIFer (Version 1.2; Allen et al., 2004).
K2—O5i | 2.6952 (19) | C1—C2 | 1.485 (3) |
K2—O4 | 2.7778 (18) | C3—C2 | 1.321 (3) |
K2—O6ii | 2.8380 (19) | C3—C4 | 1.492 (3) |
K2—O3iii | 2.8340 (18) | K1—O6v | 2.785 (2) |
K2—O1iv | 2.8879 (16) | K1—O6vi | 2.880 (2) |
K2—O2 | 2.8905 (16) | K1—O2v | 2.8907 (18) |
K2—O7 | 2.962 (2) | K1—O7i | 3.004 (2) |
O3—B1 | 1.364 (3) | K1—O3vi | 3.1462 (18) |
O4—C1 | 1.252 (3) | K1—O4v | 3.2062 (19) |
O6—C4 | 1.253 (3) | K1—C3i | 3.301 (2) |
O7—C4 | 1.245 (3) | K1—O1 | 2.7547 (16) |
O5—C1 | 1.253 (3) | ||
O2—B1—O1 | 119.54 (18) | O1—B1—O3 | 119.83 (19) |
O2—B1—O3 | 120.60 (19) |
Symmetry codes: (i) −x+1, y−1/2, −z+3/2; (ii) −x+3/2, −y+1, z+1/2; (iii) −x+1, y+1/2, −z+3/2; (iv) −x+2, y+1/2, −z+3/2; (v) −x+2, y−1/2, −z+3/2; (vi) −x+3/2, −y, z+1/2. |
D—H···A | D—H | H···A | D···A | D—H···A |
O2—H5···O4v | 0.980 (0) | 1.730 (0) | 2.689 (3) | 165.9 (1) |
O3—H3···O7 | 0.79 (4) | 1.88 (4) | 2.645 (2) | 165 (4) |
O1—H4···O5vii | 0.980 (0) | 1.660 (0) | 2.618 (1) | 163.4 (1) |
Symmetry codes: (v) −x+2, y−1/2, −z+3/2; (vii) x, y−1, z. |
It has been well established that orthoboric acid, B(OH)3, is the archetype and primary source of oxo–boron compounds (Freyhard & Wiebcke, 1994; Li et al., 1999). In most cases, it does not behave as a Brønsted acid with the formation of the conjugate-base anion [BO(OH)2]-, but rather as a Lewis acid with the formation of the tetrahedral anion [B(OH)4]2- (Farmer, 1982; Coddington & Taylor, 1989). In dilute aqueous solutions, this monobasic acid exists almost solely as an equilibrated mixture of undissociated molecular B(OH)3 and the tetrahydroxyborate anion B(OH)4-. At higher concentrations, secondary equilibria involving condensation reactions of the two dominant monomeric species take place, giving oligomers such as the triborate monoanion [B3O3(OH)4]-, the triborate dianion [B3O3(OH)5]2-, the tetraborate [B4O5(OH)4]2- and the pentaborate [B5O6(OH)4]2-. Hence, several interesting supramolecular hydrogen-bonded architectures involving tetrahydroxy pentaborate and boric acid incorporating a tertiary or quarternary ammonium ion have been isolated and characterized in recent years (Turdybecov et al., 1992; Loboda et al., 1994; Freyhard et al., 1994; Li & Mak, 1997). However, hydrogen-bonded structures containing only monomeric borate species are rare. In (CH3)4N+·BO(OH)2-·2(NH2)2CO·H2O and (Et4N)2[BO(OH)2]·B(OH)3·5H2O, the BO(OH)2- unit is present in the host lattice (Li et al., 1999; Freyhard et al., 1994). The three structures [(C2H5)4N+]2·CO32-·(NH2)2CO·2B(OH)3·H2O, [(PPh3)2N+·Cl-]·B(OH)3 and the 1:2 adduct of melamine with boric acid (Li et al., 1999; Andrews et al., 1983; Roy et al., 2002) would appear to be the only structures known to date containing the undissociated B(OH)3 unit. Here, we report the synthesis and crystal structure of the title compound, (I), a crystalline solid in which, remarkably, both the B(OH)3 molecule and the dipotassium maleate salt coexist in an unusual coordination mode.
The basic fragment of (I), with asymmetric unit formula [K2(C4H2O4)·B(OH)3], is illustrated in Fig. 1, and geometric parameters are given in Table 1. The structure may be conveniently described as two crystallographically independent K atoms located between the two types of ligands, viz. boric acid molecules and maleate ligands, which form bonds to the K atoms via their O atoms. There are weak hydrogen bonds linking the boric acid and maleate dianions; each B(OH)3 unit interacts with three maleate ligands through O—H···O bonds. All hydrogen bonds linking adjacent boric acid and maleate dianions are donated exclusively by B(OH)3 molecules [O···O distances in the range 2.618 (1)–2.689 (3) Å and O—H···O angles in the rnage 163.4 (1)–165.9 (1)°] (Table 2). The K atoms reside between the layers, linked by bonds to the O atoms of the layers. This connectivity of hydrogen bonds and K—O bonds give rise to the three-dimensional structure (Fig. 2).
Three O atoms (O1, O2 and O3) are bonded to the B atom to form a BO3 triangle. The average value of the three O—B—O angles around B is 119.99 (19)°, indistinguishable from the expected trigonal value of 120.0° [Please check rephrasing]. The mean B—O distance of 1.363 (3) Å is in good agreement with the reported trigonal B—O distances in [(C2H5)4N+]2·CO32-·(NH2)2CO·2B(OH)3·H2O (1.362 Å), [(PPh3)2N+·Cl-]·B(OH)3 (1.36 Å) and the 1:2 adduct of melamine with boric acid (1.362 Å) (Li et al., 1999; Andrews et al., 1983; Roy et al., 2002).
The potassium centres in (I) display very different coordination geometries. Most of the K—O distances are slightly longer than the sum of the covalent radii (2.69 Å; Allen et al., 1987) but may be considered weak ionic bonds owing to their directionality; other K—O distances are considerably longer (Table 1). Atom K2 is seven-coordinate; the coordination polyhedron is a distorted monocapped trigonal prism with capping atom O5i (Fig. 1; symmetry code as in Fig. 1). The K2—O coordination distances are between 2.695 (2) and 2.962 (2) Å (mean 2.84 Å), in agreement with the average K—O distance for KO7 moieties [Cambridge Structural Database, Version 5.25; 2.80 (11) Å for 446 observations; Allen, 2002]. Atom K1 is eight-coordinate, through atoms O1, O2v and O3vi from three different boric acid molecules, O4v, O6v, O6vi and O7i from four different maleate ligands, and an η1-type coordination to C3 of the C═C bond (Fig. 1; symmetry codes as in Fig. 1). The K1—O coordination distances range from 2.754 (2) to 3.206 (2) Å (mean 2.95 Å). Organometallic complexes of potassium metal or K+ that exhibit both η1– and η2-coordination are well known (Kuhl et al., 1999; Chitsaz & Neumuller, 2001; Ganesan et al., 2002); the K—C distances are typically in the range 2.9–3.5 Å. Although potassium readily forms organometallic complexes with coordination numbers as high as η8 to a single ligand in the case of bicyclic aromatic species (Cloke et al., 2000) or the cyclooctatetraenyl anion (Xia et al. 1991), there are only two reported structures in the literature containing K+—C═C η1-type compounds (McPherson et al. 1978; Noordik et al. 1974). The closest distance between K atoms is K1v···K2 = 3.877 (8) Å (symmetry code as in Fig. 1).
The title compound has a unique structure, with no directly comparable compounds containing K atoms coordinated by both dicarboxylic acid and boric acid moieties. Previously reported related examples are found in the structures of potassium dimalatoborate monohydrate (Zviedre & Kolesnikova, 1983), potassium dicitratoborate dihydrate (Zviedre et al. 1984), potassium bis(salicylato)borate salicylic acid (Zviedre et al. 1992) and catena-[[µ6-bis(oxalato)borate]potassium] (Zavalij et al., 2003). In these examples however, the dicarboxylate moieties are bonded directly to tetrahedrally coordinated B atoms. The successful synthesis of the title compound under atmospheric pressure may help in the search for new ways of synthesizing borate crystals with interesting structural features.