Compound 1 is a one-dimensional coordination polymer: The asymmetric unit contains a Cd²⁺ 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 CdO₆ 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 second spar⁻ anion (containing C20) has a broadly similar geometry: The C20–O4 and C20–O5 bond lengths are 1.273(3) Å and 1.250(3) Å, respectively, and the dihedral angle between C20/O4/O5 and the N6 ring (r.m.s. deviation = 0.037°) is 4.0(4)°. The dihedral angle between the N6 ring and the pendent three-membered ring is 73.5(2)°. The N6 and C24 rings (r.m.s. deviation for the latter = 0.041 Å), fused at the C23–C28 bond, are tilted by 4.38(16)°. The piperazine ring in the second anion shows the same type of positional disorder as the first, in a 0.908(8):0.092(8) ratio for the two orientations.

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.

To complete the structure of 1, several N–H···O hydrogen bonds occur of varying strengths, including a bifurcated N–H···(O,O) link (Table 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 p–p 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.

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 –CO₂ asymmetric and symmetric stretches, respectively.

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.

Elemental analysis: calc (%) for C₃₈H₄₄CdF₄N₈O₇: C 49.98, H 4.86, N 12.27; found (%): C 49.76, H 4.49, N 12.04. IR (cm⁻¹, KBr): 3450 (br, m), 1629 (s), 1570 (s), 1449 (s), 1364 (w), 1290 (s).

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 = 0.93–0.98 Å) 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.

Crystal data for 1: C₃₈H₄₄CdF₄N₈O₇, M_r = 913.21, triclinic, (No. 2), Z = 2, a = 9.2256(4) Å, b = 12.8767(5) Å, c = 17.4297(7) Å, α = 89.505(2)°, β = 85.062(2)°, γ = 70.757(2)°, V = 1947.20(14) ų, F(000) = 936, T = 296(2) K, ρ_calc = 1.558 g·cm⁻³, μ = 0.640 mm⁻¹, 27884 reflections recorded (3.4° ≤ 2θ ≤ 50.0°; −10 ≤ h ≤ 10, −15 ≤ k ≤ 15, −20 ≤ l ≤ 20), R_Int = 0.039, 6848 merged reflections, 6225 with I > 2σ(I), 541 variable parameters, R(F) = 0.036, wR(F²) = 0.082, min./max. ∆ρ = −0.58/0.46 e Å⁻³. Cambridge Structural Database deposition number: CCDC-888200.