A One-Dimensional Coordination Polymer Constructed from Cadmium(II) Cations and Sparfloxacinate Anions

1. Introduction

Figure 1. Chemical scheme for sparfloxacin (C₁₉H₂₂N₄O₃F₂).

2. Results and Discussion

2.1. Crystal Structure of [Cd(spar)₂]_n·nH₂O (1)

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)°.

Figure 2. The asymmetric unit of 1 (50% displacement ellipsoids), expanded to show the complete Cd²⁺ 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. See Table 1 for symmetry codes.

Figure 3. Detail of 1 showing the coordination geometry of the Cd²⁺ 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. See Table 1 for symmetry codes.

Table 1. Selected geometrical data (Å,°) for 1.

Table 1. Selected geometrical data (Å,°) for 1.

Cd1–O4	2.264(2)	Cd1–O1 i	2.269(2)
Cd1–O5 ii	2.280(2)	Cd1–O2	2.292(2)
Cd1–O6 ii	2.304(2)	Cd1–O3	2.3466(19)
O4–Cd1–O1 i	87.58(8)	O4–Cd1–O5 ii	90.75(8)
O1 i–Cd1–O5 ii	102.81(8)	O4–Cd1–O2	104.12(8)
O1 i–Cd1–O2	98.16(7)	O5 ii–Cd1–O2	154.76(7)
O4–Cd1–O6 ii	164.30(7)	O1 i–Cd1–O6 ii	100.15(8)
O5 ii–Cd1–O6 ii	74.28(8)	O2–Cd1–O6 ii	88.40(8)
O4–Cd1–O3	92.27(7)	O1 i–Cd1–O3	170.82(8)
O5 ii–Cd1–O3	86.37(7)	O2–Cd1–O3	72.97(7)
O6 ii–Cd1–O3	82.26(8)

Symmetry codes: ⁱ 2 − x, 1 − y, 1 − z; ⁱⁱ 1 − x, 1 − y, 1 − z.

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.

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 Table 1). The bonds of the C1 and C20 spar⁻ anions are colored mint and plum, respectively.

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.

Table 2. Hydrogen-Bond geometries for 1.

Table 2. Hydrogen-Bond geometries for 1.

N1–H1A···O1 iii	0.86	2.33	3.021(3)	138
N1–H1B···O3	0.86	1.96	2.598(3)	130
N1–H1B···O5	0.86	2.35	3.061(3)	141
N5–H5A···O4 iii	0.86	2.59	3.221(3)	131
N5–H5B···O6	0.86	1.98	2.609(3)	129

The four columns specify the D–H, H···A and D···A separations (Å) and the D–H···A angle (°), respectively. Symmetry code: ⁱⁱⁱ x − 1, y, z.

3. Experimental Section

3.2. 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 = 0.93–0.98 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.

4. Conclusions

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