Crystal structure of a layered coordination polymer based on a 44 net containing Cd2+ ions and 1,5-bis(pyridin-4-yl)pentane linkers

[Cd(C15H18N2)2(H2O)2](ClO4)2·C15H18N2·C2H6O, is a layered coordination polymer containing highly squashed 44 nets. The polymeric sheets alternate with layers of counter ions, free ligands and solvent molecules.


Chemical context
The most popular linking ligands in metal-organic frameworks (MOFs) are probably multi-functional carboxylates (Batten et al., 2009) but other functional groups are also possible. As part of our ongoing studies of flexible bifunctional pyridyl ligands (Plater et al., 2008) as potential MOF linkers, we now describe the synthesis and structure of the title layered coordination polymer, (I), which combines Cd 2+ ions and the little-studied ligand 1,5-bis(pyridin-4-yl)pentane, C 15 H 18 N 2 . The neutral bridging ligand necessitates the presence of perchlorate counter-ions (from the starting metal salt), which exert an important influence on the structure.

Structural commentary
The asymmetric unit of (I) contains two Cd 2+ ions (both lying on crystallographic inversion centres), three 1,5-bis(pyridin-4yl)pentane (C 15 H 18 N 2 ; L) molecules, two perchlorate ions, two water molecules and one ethanol molecule (Fig. 1). The cadmium ions, water molecules and two of the L molecules ISSN 1600-5368 combine to generate an infinite cationic network of composition [Cd(H 2 n . Both cadmium ions adopt almost regular trans-CdO 2 N 4 octahedral coordination geometries (Table 1) arising from two water molecules and four ligands. The mean Cd-O and Cd-N bond lengths are 2.327 and 2.341 Å , respectively. Bondvalence sum (BVS) calculations (Brese & O'Keeffe, 1991) in valence units for Cd1 and Cd2 yield values of 2.11 and 2.02, respectively, in close agreement with the expected value of 2.00. The octahedral angular variances (Robinson et al., 1971) for Cd1 and Cd2 are 2.53 and 10.57 2 , respectively. Both ligands bridge the Cd1 and Cd2 atoms, resulting in a highly squashed and contorted 4 4 network (O'Keeffe & Hyde, 1996), which propagates in the (110) plane, as shown in Fig. 2: each Cd1 atom is linked to four different Cd2 atoms and vice versa. The shortest Cd1Á Á ÁCd2 separations (via ligands) are 14.4350 (6) and 14.7807 (6) Å . The shortest non-bonded Cd1Á Á ÁCd1 and Cd2Á Á ÁCd2 separations across a squashed 4 4 square are both 11.0921 (5) Å . It is interesting that the shortest metal-metal distances in (I) of 10.0618 (4) and 10.1653 (4) Å for both Cd1 and Cd2 are inter-sheet separations.
For the N11 ligand molecule, the dihedral angle between the N11 and N12 rings is 77.8 (4) and the alkyl chain adopts a gaaa (g = gauche, a = anti) conformation (reading from the N11 ring to the N12 ring). Cd1 is displaced by 0.69 (1) Å from the N11 ring plane and Cd2 is displaced by À0.26 (1) Å from the N12 plane. In the N21 ligand molecule, the dihedral angle between the pyridine rings is 75.2 (4) and the alkyl-chain conformation is aaag (in the sense of the N21 ring to the N22 ring). The displacement of Cd1 from the N21 ring is 0.42 (1) Å and the displacement of Cd2 from the N22 ring is À0.58 (1) Å . The shortest out-and-back pathway from any metal atom to itself encompasses no fewer than 56 atoms (4 metal atoms and 4 Â 13 ligand atoms).
The mean Cl-O bond lengths in the perchlorate ions in (I) are 1.446 Å for the Cl1 species and 1.436 Å for the Cl2 species. The third (N31) ligand molecule is not bonded to the metal ions: the dihedral angle between its N31 and N32 rings is 18.3 (5) and its alkyl chain conformation is ggaa (from N31 to N32; Fig. 3).

Figure 2
Part of an infinite 4 4 sheet propagating in (110) in the structure of (I). The Cd1 and Cd2 ions are represented by orange and fuchsia spheres, respectively.

Figure 3
Part of a layer of perchlorate ions, N31-ligands and ethanol molecules in the structure of (I).
The O e -HÁ Á ÁO (e = ethanol) hydrogen bond is shown as a yellow line.

Supramolecular features
In the crystal, the infinite [Cd(H 2 O) 2 L 2 ] n sheets propagate in the (110)

Refinement
The O-bound H atoms were located in difference maps and refined as riding atoms in their as-found relative positions. The C-bound H atoms were placed geometrically and refined as riding atoms. The H atoms of the methyl group were allowed to rotate, but not to tip, to best fit the electron density. The constraint U iso (H) = 1.2U eq (C,O) or 1.5U eq (methyl C) was applied in all cases. Crystal data, data collection and structure refinement details are summarized in Table 3.

Poly[[diaquabis[1,5-bis(pyridin-4-yl)pentane-κ 2 N:N′]cadmium] bis(perchlorate) 1,5-bis(pyridin-4-yl)pentane ethanol monosolvate]
Crystal data [Cd(C 15  Special details 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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.