Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270106046038/bg3020sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270106046038/bg3020Isup2.hkl |
CCDC reference: 632927
A mixture of CdCl2 (0.184 g, 1 mmol), 1,4-cyclohexanedicarboxylic acid (0.087 g, 0.5 mmol), 4,4'-bipy (0.078 g, 0.5 mmol) and water (18 ml) was sealed in a 25 ml Teflon-lined stainless steel reactor and was heated at 433 K for 3 d. On completion of the reaction, the reactor was cooled slowly to room temperature and the mixture was filtered, giving colorless single crystals suitable for X-ray analysis.#
H atoms bonded to C atoms were positioned geometrically [aromatic C—H = 0.93 Å and aliphatic C—H = 0.97–0.98 Å; Uiso(H) = 1.2Ueq(C)]. H atoms bonded to O atoms were located in a difference Fourier map and refined freely; their displacement parameters were set to 1.2Ueq(O).
Data collection: SMART (Siemens, 1996); cell refinement: SAINT (Siemens, 1996); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Bergerhoff et al., 1996); software used to prepare material for publication: SHELXTL (Siemens, 1996).
[Cd(C8H11O4)2(C10H8N2)(H2O)]·H2O | F(000) = 1328 |
Mr = 646.96 | Dx = 1.603 Mg m−3 |
Orthorhombic, Pbcn | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2n 2ab | Cell parameters from 7205 reflections |
a = 11.6450 (2) Å | θ = 2.2–25.0° |
b = 15.1490 (2) Å | µ = 0.88 mm−1 |
c = 15.1940 (2) Å | T = 130 K |
V = 2680.38 (7) Å3 | Prism, yellow |
Z = 4 | 0.50 × 0.34 × 0.23 mm |
Siemens SMART 1K CCD area-detector diffractometer | 2353 independent reflections |
Radiation source: fine-focus sealed tube | 2319 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.015 |
ϕ and ω scans | θmax = 25.0°, θmin = 2.2° |
Absorption correction: multi-scan (SADABS; Sheldrick, 1996) | h = −13→13 |
Tmin = 0.59, Tmax = 0.82 | k = −18→18 |
15765 measured reflections | l = −10→18 |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.022 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.067 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.06 | w = 1/[σ2(Fo2) + (0.0389P)2 + 2.4163P] where P = (Fo2 + 2Fc2)/3 |
2353 reflections | (Δ/σ)max = 0.001 |
187 parameters | Δρmax = 0.31 e Å−3 |
0 restraints | Δρmin = −0.51 e Å−3 |
[Cd(C8H11O4)2(C10H8N2)(H2O)]·H2O | V = 2680.38 (7) Å3 |
Mr = 646.96 | Z = 4 |
Orthorhombic, Pbcn | Mo Kα radiation |
a = 11.6450 (2) Å | µ = 0.88 mm−1 |
b = 15.1490 (2) Å | T = 130 K |
c = 15.1940 (2) Å | 0.50 × 0.34 × 0.23 mm |
Siemens SMART 1K CCD area-detector diffractometer | 2353 independent reflections |
Absorption correction: multi-scan (SADABS; Sheldrick, 1996) | 2319 reflections with I > 2σ(I) |
Tmin = 0.59, Tmax = 0.82 | Rint = 0.015 |
15765 measured reflections |
R[F2 > 2σ(F2)] = 0.022 | 0 restraints |
wR(F2) = 0.067 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.06 | Δρmax = 0.31 e Å−3 |
2353 reflections | Δρmin = −0.51 e Å−3 |
187 parameters |
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. |
x | y | z | Uiso*/Ueq | ||
Cd1 | 0.5000 | 0.256617 (12) | 0.2500 | 0.01347 (10) | |
O1 | 0.45642 (11) | 0.21948 (8) | 0.39961 (8) | 0.0169 (3) | |
O1W | 0.5000 | 0.10611 (16) | 0.2500 | 0.0410 (7) | |
H1 | 0.522 (3) | 0.073 (2) | 0.207 (2) | 0.049* | |
O2 | 0.47473 (12) | 0.36168 (9) | 0.37437 (9) | 0.0186 (3) | |
O3 | 0.10817 (13) | 0.58653 (9) | 0.49414 (9) | 0.0242 (3) | |
H3O | 0.084 (2) | 0.6258 (17) | 0.4607 (16) | 0.029* | |
O4 | 0.09878 (15) | 0.50486 (10) | 0.37250 (10) | 0.0345 (4) | |
N1 | 0.30290 (17) | 0.26912 (12) | 0.23973 (10) | 0.0150 (4) | |
C1 | 0.24431 (16) | 0.34095 (12) | 0.21330 (11) | 0.0167 (4) | |
H1A | 0.2854 | 0.3890 | 0.1919 | 0.020* | |
C2 | 0.12610 (16) | 0.34671 (12) | 0.21650 (12) | 0.0169 (4) | |
H2A | 0.0888 | 0.3978 | 0.1981 | 0.020* | |
C3 | 0.06353 (19) | 0.27500 (15) | 0.24769 (10) | 0.0151 (4) | |
C4 | 0.12410 (17) | 0.19982 (13) | 0.27414 (13) | 0.0168 (4) | |
H4A | 0.0851 | 0.1504 | 0.2947 | 0.020* | |
C5 | 0.24177 (17) | 0.19979 (13) | 0.26945 (12) | 0.0166 (4) | |
H5A | 0.2811 | 0.1496 | 0.2876 | 0.020* | |
C6 | 0.38757 (16) | 0.31591 (12) | 0.51290 (11) | 0.0156 (4) | |
H6A | 0.4254 | 0.2769 | 0.5554 | 0.019* | |
C7 | 0.39910 (16) | 0.41027 (13) | 0.54771 (12) | 0.0183 (4) | |
H7A | 0.4784 | 0.4290 | 0.5415 | 0.022* | |
H7B | 0.3808 | 0.4106 | 0.6100 | 0.022* | |
C8 | 0.32157 (16) | 0.47705 (12) | 0.50065 (12) | 0.0167 (4) | |
H8A | 0.3484 | 0.4850 | 0.4407 | 0.020* | |
H8B | 0.3265 | 0.5336 | 0.5304 | 0.020* | |
C9 | 0.19544 (15) | 0.44625 (12) | 0.49916 (12) | 0.0150 (4) | |
H9A | 0.1666 | 0.4435 | 0.5597 | 0.018* | |
C10 | 0.18713 (15) | 0.35465 (11) | 0.45781 (12) | 0.0151 (4) | |
H10A | 0.2134 | 0.3571 | 0.3973 | 0.018* | |
H10B | 0.1077 | 0.3354 | 0.4577 | 0.018* | |
C11 | 0.26005 (16) | 0.28859 (12) | 0.50920 (12) | 0.0173 (4) | |
H11A | 0.2539 | 0.2310 | 0.4817 | 0.021* | |
H11B | 0.2305 | 0.2838 | 0.5687 | 0.021* | |
C12 | 0.44409 (14) | 0.29972 (12) | 0.42342 (11) | 0.0132 (4) | |
C13 | 0.12770 (15) | 0.51430 (12) | 0.44886 (12) | 0.0161 (4) | |
O2W | 0.5000 | 0.50188 (17) | 0.2500 | 0.0411 (7) | |
H2 | 0.492 (2) | 0.466 (2) | 0.296 (2) | 0.049* |
U11 | U22 | U33 | U12 | U13 | U23 | |
Cd1 | 0.01055 (15) | 0.01481 (14) | 0.01505 (15) | 0.000 | 0.00374 (6) | 0.000 |
O1 | 0.0181 (7) | 0.0146 (6) | 0.0180 (7) | 0.0013 (5) | 0.0005 (5) | −0.0015 (5) |
O1W | 0.070 (2) | 0.0157 (12) | 0.0372 (14) | 0.000 | 0.0385 (12) | 0.000 |
O2 | 0.0212 (6) | 0.0162 (7) | 0.0183 (7) | −0.0009 (5) | 0.0039 (6) | 0.0019 (6) |
O3 | 0.0363 (9) | 0.0160 (7) | 0.0203 (7) | 0.0095 (6) | −0.0004 (6) | −0.0006 (6) |
O4 | 0.0568 (10) | 0.0233 (7) | 0.0233 (8) | 0.0146 (7) | −0.0170 (7) | −0.0031 (6) |
N1 | 0.0134 (9) | 0.0194 (8) | 0.0121 (8) | −0.0010 (7) | 0.0005 (6) | −0.0023 (6) |
C1 | 0.0171 (9) | 0.0195 (9) | 0.0135 (9) | −0.0030 (7) | 0.0006 (7) | 0.0008 (7) |
C2 | 0.0196 (9) | 0.0173 (9) | 0.0138 (9) | 0.0015 (7) | −0.0026 (7) | 0.0002 (7) |
C3 | 0.0144 (11) | 0.0192 (10) | 0.0116 (9) | −0.0002 (9) | −0.0023 (6) | −0.0026 (7) |
C4 | 0.0156 (9) | 0.0161 (9) | 0.0187 (9) | −0.0012 (8) | −0.0020 (8) | 0.0010 (8) |
C5 | 0.0167 (10) | 0.0165 (9) | 0.0168 (8) | −0.0010 (8) | −0.0014 (8) | −0.0009 (7) |
C6 | 0.0171 (9) | 0.0178 (9) | 0.0118 (9) | 0.0033 (7) | −0.0008 (7) | 0.0021 (7) |
C7 | 0.0171 (9) | 0.0226 (10) | 0.0152 (9) | 0.0015 (7) | −0.0036 (7) | −0.0031 (8) |
C8 | 0.0178 (10) | 0.0148 (8) | 0.0177 (9) | −0.0005 (8) | −0.0005 (7) | −0.0029 (7) |
C9 | 0.0156 (9) | 0.0168 (9) | 0.0126 (8) | 0.0017 (7) | 0.0017 (7) | −0.0005 (7) |
C10 | 0.0135 (9) | 0.0158 (9) | 0.0161 (9) | −0.0013 (7) | 0.0021 (7) | 0.0004 (7) |
C11 | 0.0179 (10) | 0.0165 (9) | 0.0176 (9) | 0.0002 (7) | 0.0051 (8) | 0.0036 (7) |
C12 | 0.0072 (8) | 0.0184 (9) | 0.0139 (8) | 0.0007 (7) | −0.0032 (7) | 0.0009 (7) |
C13 | 0.0131 (8) | 0.0166 (9) | 0.0186 (9) | −0.0012 (7) | 0.0021 (7) | 0.0007 (7) |
O2W | 0.083 (2) | 0.0154 (11) | 0.0249 (13) | 0.000 | 0.0057 (11) | 0.000 |
Cd1—O1W | 2.280 (2) | C4—C5 | 1.372 (3) |
Cd1—N1 | 2.3083 (19) | C4—H4A | 0.9300 |
Cd1—N1i | 2.3083 (19) | C5—H5A | 0.9300 |
Cd1—O1i | 2.3961 (13) | C6—C7 | 1.530 (3) |
Cd1—O1 | 2.3961 (13) | C6—C12 | 1.530 (2) |
Cd1—O2 | 2.4880 (13) | C6—C11 | 1.543 (3) |
Cd1—O2i | 2.4880 (13) | C6—H6A | 0.9800 |
Cd1—C12 | 2.7916 (18) | C7—C8 | 1.533 (3) |
Cd1—C12i | 2.7916 (18) | C7—H7A | 0.9700 |
O1—C12 | 1.276 (2) | C7—H7B | 0.9700 |
O1W—H1 | 0.87 (3) | C8—C9 | 1.541 (3) |
O2—C12 | 1.251 (2) | C8—H8A | 0.9700 |
O3—C13 | 1.312 (2) | C8—H8B | 0.9700 |
O3—H3O | 0.83 (3) | C9—C13 | 1.506 (2) |
O4—C13 | 1.216 (2) | C9—C10 | 1.526 (2) |
N1—C1 | 1.346 (3) | C9—H9A | 0.9800 |
N1—C5 | 1.347 (3) | C10—C11 | 1.527 (2) |
C1—C2 | 1.380 (3) | C10—H10A | 0.9700 |
C1—H1A | 0.9300 | C10—H10B | 0.9700 |
C2—C3 | 1.391 (3) | C11—H11A | 0.9700 |
C2—H2A | 0.9300 | C11—H11B | 0.9700 |
C3—C4 | 1.399 (3) | O2W—H2 | 0.89 (3) |
C3—C3ii | 1.481 (4) | ||
O1W—Cd1—N1 | 94.71 (4) | C5—C4—C3 | 119.27 (19) |
O1W—Cd1—N1i | 94.71 (4) | C5—C4—H4A | 120.4 |
N1—Cd1—N1i | 170.59 (9) | C3—C4—H4A | 120.4 |
O1W—Cd1—O1i | 76.42 (3) | N1—C5—C4 | 123.03 (19) |
N1—Cd1—O1i | 99.54 (5) | N1—C5—H5A | 118.5 |
N1i—Cd1—O1i | 82.69 (5) | C4—C5—H5A | 118.5 |
O1W—Cd1—O1 | 76.42 (3) | C7—C6—C12 | 114.78 (15) |
N1—Cd1—O1 | 82.69 (5) | C7—C6—C11 | 110.34 (15) |
N1i—Cd1—O1 | 99.54 (5) | C12—C6—C11 | 109.80 (14) |
O1i—Cd1—O1 | 152.84 (6) | C7—C6—H6A | 107.2 |
O1W—Cd1—O2 | 129.77 (3) | C12—C6—H6A | 107.2 |
N1—Cd1—O2 | 83.18 (5) | C11—C6—H6A | 107.2 |
N1i—Cd1—O2 | 90.79 (5) | C6—C7—C8 | 113.81 (15) |
O1i—Cd1—O2 | 153.61 (4) | C6—C7—H7A | 108.8 |
O1—Cd1—O2 | 53.46 (4) | C8—C7—H7A | 108.8 |
O1W—Cd1—O2i | 129.77 (3) | C6—C7—H7B | 108.8 |
N1—Cd1—O2i | 90.79 (5) | C8—C7—H7B | 108.8 |
N1i—Cd1—O2i | 83.18 (5) | H7A—C7—H7B | 107.7 |
O1i—Cd1—O2i | 53.46 (4) | C7—C8—C9 | 111.61 (15) |
O1—Cd1—O2i | 153.61 (4) | C7—C8—H8A | 109.3 |
O2—Cd1—O2i | 100.46 (6) | C9—C8—H8A | 109.3 |
O1W—Cd1—C12 | 103.53 (4) | C7—C8—H8B | 109.3 |
N1—Cd1—C12 | 79.21 (5) | C9—C8—H8B | 109.3 |
N1i—Cd1—C12 | 98.56 (5) | H8A—C8—H8B | 108.0 |
O1i—Cd1—C12 | 178.74 (5) | C13—C9—C10 | 112.34 (15) |
O1—Cd1—C12 | 27.14 (5) | C13—C9—C8 | 107.42 (14) |
O2—Cd1—C12 | 26.61 (5) | C10—C9—C8 | 109.98 (15) |
O2i—Cd1—C12 | 126.50 (5) | C13—C9—H9A | 109.0 |
O1W—Cd1—C12i | 103.53 (4) | C10—C9—H9A | 109.0 |
N1—Cd1—C12i | 98.56 (5) | C8—C9—H9A | 109.0 |
N1i—Cd1—C12i | 79.21 (5) | C9—C10—C11 | 110.49 (15) |
O1i—Cd1—C12i | 27.14 (5) | C9—C10—H10A | 109.6 |
O1—Cd1—C12i | 178.74 (5) | C11—C10—H10A | 109.6 |
O2—Cd1—C12i | 126.50 (5) | C9—C10—H10B | 109.6 |
O2i—Cd1—C12i | 26.61 (5) | C11—C10—H10B | 109.6 |
C12—Cd1—C12i | 152.95 (7) | H10A—C10—H10B | 108.1 |
C12—O1—Cd1 | 93.96 (11) | C10—C11—C6 | 112.22 (15) |
Cd1—O1W—H1 | 126 (2) | C10—C11—H11A | 109.2 |
C12—O2—Cd1 | 90.36 (11) | C6—C11—H11A | 109.2 |
C13—O3—H3O | 109.6 (17) | C10—C11—H11B | 109.2 |
C1—N1—C5 | 117.56 (19) | C6—C11—H11B | 109.2 |
C1—N1—Cd1 | 126.20 (14) | H11A—C11—H11B | 107.9 |
C5—N1—Cd1 | 116.04 (14) | O2—C12—O1 | 120.92 (16) |
N1—C1—C2 | 123.11 (18) | O2—C12—C6 | 122.13 (16) |
N1—C1—H1A | 118.4 | O1—C12—C6 | 116.93 (15) |
C2—C1—H1A | 118.4 | O2—C12—Cd1 | 63.03 (9) |
C1—C2—C3 | 119.02 (18) | O1—C12—Cd1 | 58.90 (9) |
C1—C2—H2A | 120.5 | C6—C12—Cd1 | 167.52 (12) |
C3—C2—H2A | 120.5 | O4—C13—O3 | 123.36 (17) |
C2—C3—C4 | 118.01 (19) | O4—C13—C9 | 123.27 (17) |
C2—C3—C3ii | 122.63 (13) | O3—C13—C9 | 113.28 (16) |
C4—C3—C3ii | 119.35 (14) |
Symmetry codes: (i) −x+1, y, −z+1/2; (ii) −x, y, −z+1/2. |
D—H···A | D—H | H···A | D···A | D—H···A |
O1W—H1···O4iii | 0.87 (3) | 1.82 (3) | 2.672 (2) | 168 (3) |
O3—H3O···O1iv | 0.83 (3) | 1.76 (3) | 2.5855 (19) | 172 (2) |
O2W—H2···O2 | 0.89 (3) | 1.99 (3) | 2.858 (2) | 165 (3) |
Symmetry codes: (iii) x+1/2, y−1/2, −z+1/2; (iv) −x+1/2, y+1/2, z. |
Experimental details
Crystal data | |
Chemical formula | [Cd(C8H11O4)2(C10H8N2)(H2O)]·H2O |
Mr | 646.96 |
Crystal system, space group | Orthorhombic, Pbcn |
Temperature (K) | 130 |
a, b, c (Å) | 11.6450 (2), 15.1490 (2), 15.1940 (2) |
V (Å3) | 2680.38 (7) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.88 |
Crystal size (mm) | 0.50 × 0.34 × 0.23 |
Data collection | |
Diffractometer | Siemens SMART 1K CCD area-detector diffractometer |
Absorption correction | Multi-scan (SADABS; Sheldrick, 1996) |
Tmin, Tmax | 0.59, 0.82 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 15765, 2353, 2319 |
Rint | 0.015 |
(sin θ/λ)max (Å−1) | 0.595 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.022, 0.067, 1.06 |
No. of reflections | 2353 |
No. of parameters | 187 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.31, −0.51 |
Computer programs: SMART (Siemens, 1996), SAINT (Siemens, 1996), SAINT, SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), DIAMOND (Bergerhoff et al., 1996), SHELXTL (Siemens, 1996).
Cd1—O1W | 2.280 (2) | Cd1—O1 | 2.3961 (13) |
Cd1—N1 | 2.3083 (19) | Cd1—O2 | 2.4880 (13) |
Cd1—N1i | 2.3083 (19) | Cd1—O2i | 2.4880 (13) |
Cd1—O1i | 2.3961 (13) |
Symmetry code: (i) −x+1, y, −z+1/2. |
D—H···A | D—H | H···A | D···A | D—H···A |
O1W—H1···O4ii | 0.87 (3) | 1.82 (3) | 2.672 (2) | 168 (3) |
O3—H3O···O1iii | 0.83 (3) | 1.76 (3) | 2.5855 (19) | 172 (2) |
O2W—H2···O2 | 0.89 (3) | 1.99 (3) | 2.858 (2) | 165 (3) |
Symmetry codes: (ii) x+1/2, y−1/2, −z+1/2; (iii) −x+1/2, y+1/2, z. |
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Considerable progress has recently been achieved on the crystal engineering of supramolecular architectures organized and sustained by means of coordinate covalent, supramolecular contacts (such as hydrogen bonds, π–π interaction), aurophilicity interaction (Colacio et al., 2002; Roesky & Andruh, 2003; Guilera & Steed, 1999), etc. As an excellent building block in the design and construction of supramolecular polymers, 4,4'-bipyridine (4,4'-bipy) has been widely used. On the other hand, 1,4-cyclohexanedicarboxylic acid (H2chdc) possesses a chair-type structure with cis- and trans-conformations, which can connect metal ions in different directions; it has two carboxylate groups and it has always been reported to link metal centers in a bridging mode (Qi et al., 2003; Thirumurugan et al., 2006). In spite of its being a good candidate for the construction of hydrogen bonding networks, its supramolecular chemistry has rarely been explored and so far mixed metal–organic complexes derived from 4,4'-bipy and chdc ligands have not been reported, even though the combination of 4,4'-bipy and chdc is expected to construct an interesting variety of polymeric compounds. We report here the first occurrence of a one-dimensional Cd complex [Cd(Hchdc)2(4,4'-bipy)(H2O)]n.nH2O (I), with an unusual binding mode for the Hchdc ligand; it chelates one CdII ion, while its remaining (protonated) carboylate group just forms hydrogen-bonding interactions.
Complex (I) consists of linear chains formed through 4,4'-bipy ligands linking seven-coordinated CdII ions. As shown in Fig. 1, the CdII ion presents a distorted pentagonal–bipyramid geometry. The water molecules and the Cd atom on one side, and the 4,4'-bpy unit on the other, are bisected by two sets of twofold axes. Two trans-related N donors of two 4,4'-bipy ligands stand at the apical positions. Four O atoms of two carboxylate groups of two chdc ligands and one coordinated water molecule form the equatorial plane. The 4,4'-bipy ligand functions as a spacer between two CdII ions, the Cd···Cd separation being 11.645 (2) Å. The dihedral angle between two pyridine rings is 43.08 (3)°. As stated, the Hchdc ligand chelates one CdII ion through one carboxylate group, while the remaining (protonated) one does not participate in coordination, being only involved in hydrogen-bonding. This is the first reported occurrence of a chdc ligand exhibiting a non-bridging coordination mode.
To our knowledge, this is the first example in the coordination chemistry of the chdc ligand in which it does not act in a bridging mode. As shown in Fig. 2, the CdII ions are interlinked by 4,4'-bipy ligands to generate linear chains along the a axis. The crystalline water molecule forms two O—H···O hydrogen bonds with two coordinated carboxylate O atoms.
A noteworthy feature of complex (I) is the two-dimensional hydrogen-bonding network (Fig. 3 and Table 2). Two uncoordinated and two coordinated carboxylate groups, one aqua ligand and one crystalline water molecule are linked through hydrogen-bonding interactions to form a butterfly-like unit. From a topological point of view such six-membered butterfly-like units can be represented as a (3,6) net (Li et al., 2004).