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Crystals 2012, 2(3), 1084-1091; doi:10.3390/cryst2031084
Published: 3 August 2012
Abstract: The synthesis, structure and properties of the bimetallic layered coordination polymer, [KDy(C8H3NO6)3(C8H5NO6)]n·2n(C10H9N2)·5n(H2O) = [KDy(Hptc)3(H3ptc)]n·2n(Hbipy)·5n(H2O), are described. The Dy3+ ion is coordinated by three O,N,O-tridentate doubly-deprotonated pyridine tri-carboxylate (Hptc) ligands to generate a fairly regular DyO6N3 tri-capped trigonal prism, with the N atoms acting as the caps. The potassium ion is coordinated by an O,N,O-tridentate H3ptc molecule as well as monodentate and bidentate Hptc ligands to result in an irregular KNO9 coordination geometry. The ligands bridge the metal-atom nodes into a bimetallic, layered, coordination polymer, which extends as corrugated layers in the (010) plane, with the mono-protonated bipyridine cations and water molecules occupying the inter-layer regions: Unlike related structures, there are no dysprosium–water bonds. Many O–H…O and N–H…O hydrogen bonds consolidate the structure. Characterization and bioactivity data are described. Crystal data: C52H42DyKN8O29, Mr = 1444.54, triclinic, (No. 2), Z = 2, a = 9.188(2) Å, b = 15.7332(17) Å, c = 19.1664(19) Å, α = 92.797(6)°, β = 92.319(7)°, γ = 91.273(9)°, V = 2764.3(7) Å3, R(F) = 0.029, wR(F2) = 0.084.
Coordination polymers, in which bridging ligands connect metal atoms into an extended network , have been intensively studied for the past 100 years. This area of chemistry is so vast that a journal solely devoted to reviewing the field—Coordination Chemistry Reviews—publishes some 3000 pages per year.
In this paper we describe the synthesis and structure of the new bimetallic coordination polymer [KDy(C8H3NO6)3(C8H5NO6)]n·2n(C10H9N2)·5n(H2O): (1) Related structures containing Dy3+ ions in combination with pyridine 2,4,6-tricarboxylate (ptc) ligands include α-[Dy(C8H2NO6)(H2O)3]n·nH2O , [Dy(C8H2NO6)(H2O)]n·nH2O , β-[Dy(C8H2NO6)(H2O)3]n·nH2O  and [Dy2(C8H2NO6)2−(H2O)5]n·nH2O . In all these compounds, the tri-anionic ptc ligand forms an O,N,O-tridentate link to one metal ion and also bridges to other Dy3+ ions to form a coordination network. One or more water molecules are also directly coordinated to the dysprosium ion.
The crystal structures of [K(VO2)(C8H3NO6)]n·nH2O , [KCaEr(C8H2NO6)2(H2O)]n  and [KBa(C8H2NO6)(H2O)2]n  have been reported but there are no mixed-metal K/Dy/ptc coordination polymers yet known.
2. Results and Discussion
2.1. Crystal Structure of [KDy(C8H3NO6)3(C8H5NO6)]n·2n(C10H9N2)·5n(H2O) (1)
Compound 1 is a bimetallic, layered coordination polymer: The complex asymmetric unit contains one K+ cation, one Dy3+ cation, three doubly-deprotonated (C8H3NO6)2− (Hptc2−) dianions, one neutral C8H5NO6 (H3ptc) molecule, two singly protonated bipyridinium (C10H9N2)+ cations and five water molecules (Figure 1).
The dysprosium ion in 1 is coordinated by the three O,N,O-tridentate C8H3NO6 dianions (in which two carboxylate protons have been lost from the C8H5NO6 neutral molecule) to generate a near-regular tri-capped trigonal prismatic DyO6N3 coordination geometry with the N atoms serving as the caps protruding through the prismatic side-faces (Figure 2). Each ligand forms one Dy–O bond to the “top” triangular face and one to the bottom. The dihedral angle between the O1/O7/O13 and O5/O11/O17 triangular faces is 4.30(4)° and the metal ion is displaced by −1.6580(13) Å from the first triplet of O atoms and by 1.6133(13) Å from the second. The Dy–N bond lengths (Table 1) (mean = 2.501 Å) are longer than all but one of the Dy–O bonds (mean = 2.413 Å). The Dy3+ bond-valence sum (BVS), calculated using the Brown–Altermatt  formalism, is 3.18, compared to an expected value of 3.00.
|Table 1. Selected bond-distances (Å) in 1.|
|K1–O13 ii||3.025(2)||K1–O11 iii||3.073(2)|
|K1–O12 iii||3.388(3)||K1–O1 ii||3.432(3)|
Symmetry codes: (i) x, y, z−1; (ii) 1−x, 1−y, 1−z; (iii) −x, 1−y, 1−z.
The geometrical parameters for the three Dy-bonded ligands (containing atoms N1, N2 and N3) are unexceptional, and all the carboxylate groups are close to coplanar with their attached rings. In each case, the protonated (carboxylic acid –CO2H) grouping is the one not bonded to the rare-earth ion at the para position with respect to the pyridine N atom.
The potassium ion in 1 is coordinated by the O,N,O-tridentate C8H5NO6 neutral molecule (containing N4), as well as monodentate and bidentate dianions also bonded to the dysprosium ion to generate a KNO9 coordination polyhedron (mean K–O = 3.095 Å) that can only be described as irregular (Figure 3). It is also decidedly asymmetric with the potassium ion displaced by 0.481 Å from the geometric centroid of its attached atoms. The next-nearest O atom is over 4.0 Å from the K+ ion and the BVS for potassium is 0.87 (expected value = 1.00). The existence of a protonated carboxylic acid forming a coordinate bond to a potassium ion from its OH moiety is uncommon, but some examples such as [K(C5H4N2O4)(C5H3N2O4)]n  and [K(C8H8O2S)(C8H7O4S)]n  have been structurally characterised previously.
A notable feature of this phase is the sharing of some of the ligand O atoms (i.e., as μ2 bridges) by the dysprosium and potassium ions: In particular, the three oxygen atoms of the O1/O7/O13 triangular face of the Dy-trigonal prism also bond to the K+ ion. This leads to polymeric chains of alternating DyO6N3 and KNO9 units, which propagate in the  direction (Figure 4).
When ligand bridging via the aromatic rings is also considered, a layered anionic network of stoichiometry [KDy(C8H3NO6)3(C8H5NO6)]2n− results, which propagates in the (010) plane (Figure 5). The layer is corrugated and the (C10H9N2)+ bipyridinium cations and water molecules occupy the inter-layer regions. The dihedral angles between the aromatic rings of the bipyridinium cations are 5.23(17)° and 3.39(18)° for the N5- and N7-containing ions, respectively. To complete the structure of 1, numerous O–H…O and N–H…O hydrogen bonds occur (Table 2). The acceptor atoms are either carboxylate O atoms or water O atoms.
|Table 2. Hydrogen-bond geometries for 1.|
The four columns specify the D–H, H…A and D…A separations (Å) and the D–H…A angle (°), respectively. Symmetry codes: (ii) 1−x, 1−y, 1−z; (iii) −x, 1−y, 1−z; (iv) x, y−1, z; (v) −x, 1−y, 2−z; (vi) −x, 2−y, 1−z; (vii) x−1, y, z; (viii) 1−x, 2−y, 1−z; (ix) 1−x, 1−y, 2−z.
2.2. Bacteriological Tests
Seven bacterial strains including two Gram positive Staphylococcus aureus and Micrococcus luteus and five Gram negative, Escherichia coli, Salmonella setubal, Salmonella typhimurium, Enterobacter aerogenes and Bordetella bronchiseptica were used. Roxithromycin (R) and Cefixixme (C) drugs were used as positive controls, which had shown maximum growth inhibition at 1 mg·mL−1 concentrations and DMSO was used as a negative control. Compound 1 shows significant antibacterial activity against all these bacterial strains (Table 3) except Salmonella typhimurium.
|Table 3. Bacteriological data.|
|Minimum Inhibitory Concenteration (MIC)|
|M. leuteus||S. aureus||Ent. bac||B. step||E. coli||S.typhi|
|30 ppm||150 ppm||200 ppm||70 ppm||110 ppm||Nil|
3. Experimental Section
The nominal pyridine 2,4,6-tricarboxylic acid (H3ptc) starting material was prepared by oxidizing 2,4,6-trimethyl pyridine with KMnO4 solution, as described by Syper et al. . This evidently generated a potassium salt: A 0.5-mmol solution was prepared by heating 0.11 g of the solid product in 10.0 mL water in a 50-mL round bottom flask. Then, 0.0063 g of 2,2-bipyridine in 5 mL methanol was added, and the mixture was refluxed for 25 min. Then, a 0.125 mmol solution of DyCl3·6H2O was prepared by dissolving 0.040 g of the metal chloride in 10 mL distilled water and this solution was added to the flask. The Dy:H3ptc:bipy molar ratio was 1:4:4. The mixture was refluxed for four hours and transferred to a vial for crystallization. After ten days, pale pinkish needle like crystals of the title compound were obtained. IR (cm−1): 3410 [ν(O–H)], 1631[νasym(O–C–O)], 1373 [ν(Car–C)], 1265 [νsym(O–C–O)], 539 [ν(M–O, M–N)]. TGA showed an initial weight loss of about 5% between 70 °C and 120 °C, presumably attributable to the loss of the water molecules of crystallization (calc. = 6%). From 205 °C to 300 °C (the limit of the experiment), a continuous weight loss of 55% occurred.
The single-crystal data for 1 (pale pink block 0.30 × 0.25 × 0.25 mm) were collected using a Bruker Kappa APEX II CCD diffractometer (graphite monochromated Mo Kα radiation, λ = 0.71073 Å) at room temperature. Data reduction with SAINT then proceeded and the structure was solved by direct methods with SHELXS. The resulting atomic model was developed and refined against |F|2 with SHELXL  and the “observed data” threshold for calculating the R(F) residuals was set as I > 2σ(I). The model was analyzed and validated with PLATON .
The C-bound H atoms were placed in idealised locations (C–H = 0.96–0.97 Å) and refined as riding atoms. The ligand N-bond and O-bound and water H atoms were located in difference maps and refined as riding atoms in their as-found relative locations. The constraint Uiso(H) = 1.2Ueq(carrier) was applied in all cases. The difference maps in the regions around the water molecules O2w and O4w were not very well defined, and the possibility of disorder of the H atoms attached to these O atoms cannot be ruled out. Full refinement details and software references are given in the deposited cif.
Crystal data for 1: C52H42DyKN8O29, Mr = 1444.54, triclinic, (No. 2), Z = 2, a = 9.188(2) Å, b = 15.7332(17) Å, c = 19.1664(19) Å, α = 92.797(6)°, β = 92.319(7)°, γ = 91.273(9)°, V = 2764.3(7) Å3, F(000) = 1454, T = 296(2) K, ρcalc = 1.736 g·cm−3, μ = 1.532 mm−1, 37880 reflections measured (−7 ≤ h ≤ 12, −20 ≤ k ≤ 20, −25 ≤ l ≤ 25; 4.88° ≤ 2θ ≤ 56.74°), RInt = 0.025, 13445 merged reflections, 12100 with I > 2σ(I), 825 variable parameters, R(F) = 0.029, wR(F2) = 0.084, w = 1/[σ2(Fo2) + (0.0387P)2 + 3.7247P], where P = (Fo2 + 2Fc2)/3, min./max. Dρ = −0.74, +1.12 e Å−3. Cambridge Structural Database deposition number: CCDC-879106.
The synthesis and crystal structure of [KDy(Hptc)3(H3ptc)]n·2n(Hbipy)·5n(H2O) has been described. The coordination of the dysprosium ion by three tridentate Hptc ligands in this compound is distinctly different to those of the compounds noted in the introduction, in which only one ptc ligand bonds in this way, as well as monodentate bridging ligands and water molecules.
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