A One-dimensional Coordination Polymer Constructed from Cadmium(ii) Cations and Sparfloxacinate Anions

The synthesis and crystal structure of the one-dimensional coordination polymer, [Cd(spar) 2 ] n ·n(H 2 O), are described, where spar − is the sparfloxacinate anion, −. The Cd 2+ ion is bonded to four spar − ligands: Two O,O-chelate with their β-keto carboxylate groupings and two are monodentate-bound through a carboxylate O atom, to result in a distorted CdO 6 octahedral coordination geometry. The bridging ligands lead to [100] polymeric chains in the crystal and N–H···O hydrogen bonds and possible weak aromatic – stacking interactions help to consolidate the structure.


Crystal
Compound 1 is a one-dimensional coordination polymer: The asymmetric unit contains a Cd 2+ cation, two mono-anionic spar − ligands and a disordered water molecule of crystallisation (Figure 2).
The metal ion in 1 is coordinated by two bidentate spar − anions, with the ketone O-atom and its syn-carboxylate O atom (O3 and O2, respectively, in the C1-containing ion and O6 and O5, respectively, in the C20-ion) serving as the donors, which generates a six-membered chelate ring in each case.The metal coordination sphere is completed by two monodentate-O bonded spar − species: In each case the carboxylate O-atom anti to the ketone O-atom is involved.Together, these lead to a moderately distorted octahedral geometry for the CdO 6 polyhedron (Table 1, Figure 3), with the monodentate O-atoms is a cis disposition.The mean Cd-O separation is 2.293 Å, the angular variance [15] for the O-Cd-O bond angles is 105.2 and the bond-valence-sum (BVS) for the metal ion, calculated by the Brown-Altermatt formalism [16], is 2.11 (expected value = 2.00).The -O2-C1-C2-C3-O3-Cd1-six-membered chelate ring approximates to a distorted half-chair, with O2 and Cd1 displaced by 0.320(5) Å and −0.702(7) Å, respectively, from the plane of the other four atoms (r.m.s.deviation = 0.026 Å).The -O5-C20-C21-C22-O6-Cd1-ring can be described in the same way, with O5 and Cd1 displaced by 0.190(5) Å and −1.013(7) Å, respectively, from the other atoms (r.m.s.deviation = 0.008 Å).The dihedral angle between the near-planar segments of the chelate rings is 32.0(3).
The important geometrical features of the first spar − anion (containing C1) are as follows: The C1-O1 and C1-O2 bond lengths of 1.266(4) Å and 1.250(3) Å, respectively, are typical for a delocalised carboxylate group and the dihedral angle between C1/O1/O2 and the adjacent N2-containing ring (r.m.s.deviation = 0.051 Å) is 11.5(5).The dihedral angle between the cyclopropane ring and the N2 ring is 66. 4(2).The dihedral angle between the N2 ring and the C5 ring (r.m.s.deviation = 0.021 Å), which are fused at the C4-C9 bond, is 7.50(15), indicating a significant puckering to the quinolone system.The piperazine ring adopts a typical chair conformation with the N-C q (q = quinolone) bond in an equatorial orientation.Its geometry is complicated by disorder of the C atoms bearing the terminal methyl groups over two orientations, in a 0.766(10):0.234(10) ratio, but both of these maintain the (3R*,5S*) relative configurations of these stereogenic atoms.
The extended structure of 1 features polymeric chains in the [100] direction (Figure 4), such that each spar − anion links two cadmium metal ions.Adjacent metal ions are fused via eight-membered loops, generated by crystallographic inversion symmetry.2).All of these bonds are intra-chain interactions.It is notable that neither of the piperazine H atoms (attached to N4 and N8) participates in a hydrogen bond, perhaps due in part to the steric crowding of the two adjacent methyl groups.Weak aromatic - stacking (centroid-centroid separation = 3.7164(17) Å between the N6 and C24 rings) might also play some role in consolidating the structure of 1.

Spectroscopy
By analogy with data from related compounds [9], the 1629 cm −1 band is assigned as a C=O(pyridone) stretch and the 1570 cm −1 and 1364 cm −1 signals correspond to the carboxylate -CO 2 asymmetric and symmetric stretches, respectively.

Synthesis and Characterization
A mixture of cadmium acetate trihydrate (0.25 mmol), sparfloxacin (0.5 mmol), 1,4-benzenedicarboxylic acid (0.25 mmol), sodium hydroxide (1 mmol), and water (15 mL) was stirred for 30 min at room temperature.The mixture was then transferred to a 25-mL Teflon-lined reactor and heated to 423 K for 72 h under autogenous pressure.Upon cooling, colorless prisms of 1 were recovered from the reaction mixture by vacuum filtration.The role of the 1,4-benzenedicarboxylic acid in the synthesis is unknown, but it has not proved possible to prepare 1 if it is not present.

Single-Crystal Data Collection and Analysis
The single-crystal data for 1 (colorless prism 0.20 × 0.20 × 0.18 mm) were collected using a Bruker APEX II CCD diffractometer (graphite monochromated MoKα radiation, λ = 0.71073 Å) at room temperature.Data reduction with SAINT [17] then proceeded and the structure was solved by direct methods with SHELXS-97 [18].The resulting atomic model was developed and refined against F 2 with SHELXL-97 [18] and the "observed data" threshold for calculating the R(F) residuals was set as I > 2σ(I).The C-bound H atoms were placed in idealised locations (C-H = 0.93-0.98Å) and refined as riding atoms.The N-bound H atoms were located in difference maps: Those attached to N1 and N5 were relocated to idealised locations (N-H = 0.86 Å) and refined as riding and those attached to N4 and N8 were refined as riding in their as-found relative locations.Due to the disorder of the piperazine rings, the location of the N4 and N8 H atoms are perhaps less certain, although they appeared reasonably distinctly in difference maps.The constraint U iso (H) = 1.2U eq (carrier) or 1.5U eq (methyl carrier) was applied as appropriate.The H atoms associated with the disordered water molecule could not be located; based on geometrical considerations, one of the water molecules may form a hydrogen bond to N4.The structural model was analysed and validated with PLATON [19] and full refinement details are given in the deposited cif.

Conclusions
The synthesis and crystal structure of the title one-dimensional coordination polymer have been described, in which the metal atom adopts a distorted octahedral geometry arising from its coordination by two O,O-bidentate and two O-monodentate spar − anions.The linkages in the polymeric chain are centrosymmetric eight-membered rings.The structure of 1 is completely different to that of Cd 2 (enro) 4 (H 2 O) 2 •4H 2 O (Henro = enroflaxacin; C 19 H 22 N 3 O 3 F), in which isolated binuclear complexes occur and the metal coordination geometry is a CdO 6 trigonal prism [5].In

Figure 2 .
Figure 2. The asymmetric unit of 1 (50% displacement ellipsoids), expanded to show the complete Cd 2+ coordination sphere.Hydrogen bonds are shown as double-dashed lines and the minor disorder components of the piperazine rings of the ligands and the disordered, uncoordinated water molecule are omitted for clarity.SeeTable 1 for symmetry codes.

Figure 3 .
Figure 3. Detail of 1 showing the coordination geometry of the Cd 2+ ion (50% displacement ellipsoids for Cd and O).The octahedral edges are shown as open lines and the C atoms of the chelate rings are shown as spheres.SeeTable 1 for symmetry codes.

Figure 4 .
Figure 4. Fragment of a [100] polymeric chain in 1 showing only the O atoms and linking C atoms of the ligands (50% displacement ellipsoids; symmetry codes as in Table1).The bonds of the C1 and C20 spar − anions are colored mint and plum, respectively.

Table 1
for symmetry codes.