A monoclinic polymorph of 4,4′-dichloro-2,2′-(piperazine-1,4-diyldimethylene)diphenol

The titile compound, C18H20Cl2N2O2, crystallizes as a monoclinic form in the space group P21/n, with Z′ = 1/2. It is polymorphic with the previously reported orthorhombic form [Kubono, Tsuno, Tani & Yokoi (2008). Acta Cryst. E64, o2309]. In the present polymorph, the molecule lies on a crystallographic inversion centre at the piperazine ring centroid. An intramolecular O—H⋯N hydrogen bond forms an S(6) ring motif. Intermolecular C—H⋯O hydrogen bonding generates a C(5) chain motif propagating along the b axis, forming sheets parallel to (02) with a first-level graph set S(6)C(5)R 6 6(34).


Comment
We have recently reported the crystal structure of 4,4'-dichloro-2,2'-(piperazine-1,4-diyldimethylene)diphenol, (I) (Kubono et al., 2008), which crystallizes as an orthorhombic form in space group Pbca with Z' = 1. (I) can act as complexing reagents (Bharathi et al., 2006), so trying to synthesize the zinc complex with this ligand. The crystalline metal complexes were not given, however, a small amount of a new polymorph of the title compound, (II), was obtained from the reaction solution.
We report here the molecular and crystal structure of (II).
The molecule of (II) crystallizes in the centrosymmetric space group P2 1 /n with Z' = 1/2 in the asymmetric unit. The molecular structure of (II) is shown in Fig. 1. The molecule lies on a crystallographic inversion centre at the piperazine ring centroid. It is interesting to note that in the orthorhombic form (I) (Kubono et al., 2008) the molecule has a pseudo-inversion centre. The piperazine ring adopts a chair conformation. The bond lengths and angles in (II) are normal and comparable with those in the orthorhombic form (I) (Kubono et al., 2008), in the monoclinic and orthorhombic polymorph of dichlorophenol derivative (Kubono & Yokoi, 2007) and in the p-cresol one (Kuppayee et al., 1999). The molecular structures of (I) and (II) are closely similar, so the only slight differences were observed. The maximum differences of bond distance and angle between the two polymorphs are less than 0.02 Å [C1-C2: (I) 1.377 (4) (Table 1), forming a S(6) ring motif (Bernstein et al., 1995).
In the crystal structure of (II), there is intermolecular C-H···O hydrogen bond, involving a aromatic H atom (Table 1).
Atom C3 in the molecule at (x, y, z) acts as hydrogen bond donor to atom O1 in the molecule at (3/2 -x,1/2 + y,5/2 -z), so forming a C(5) chain running parallel to the [010] direction and generated by the n-glide plane at y = 1/4. The molecules are linked by the combination of the S(6) ring and the C(5) chain into a sheet parallel to (202) with a first level graph set S(6)C(5)R 6 6 (34) (Fig. 2). In the crystal structure of (I), intermolecular C-H···O hydrogen bonds involving methylene H atoms generate C(5) chain motifs to form a sheet with a first level graph set S(6)C(5)R 6 6 (26) (Kubono et al., 2008). Each polymorph is characterized by the hydrogen bonding network structure.