Packing polymorphism in the structure of trans-aqua[N,N′-bis(salicylidene)ethane-1,2-diamine-κ4 O,N,N′,O′]chloridomanganese(III) monohydrate

The crystal structure of a second phase, which results from packing polymorphism, is described for a previously reported MnIII complex.

. Polyhedron, 22, 1191Polyhedron, 22, -1198. We obtained the same hydrated complex through an alternative synthesis, and crystallized a new polymorph, in the space group P2 1 . The molecular conformation of the complex is virtually unmodified, but the absence of the glide plane in the new polymorph halves the unit-cell parameter c, affording a non-centrosymmetric crystal structure with Z = 2, while the previously reported crystal is centrosymmetric with Z = 4. Both phases represent a case of packing polymorphism, similar to other dimorphic crystal structures retrieved from the Cambridge Structural Database.

Chemical context
Schiff base organic compounds are widely employed ligands in modern coordination chemistry because they are easily accessible and display high versatility (Zarei et al., 2015). Within this field, the coordination chemistry of H 2 salen [N,N 0bis(salicylidene)ethane-1,2-diamine] has been studied with virtually all transition metals. The chelating character of the dianionic ligand salen is known to stabilize not only M 2+ cations, but also higher oxidation states, providing that ancillary anions such as Cl À are present. In the case of manganese, this strategy may be used to stabilize Mn III and Mn IV oxidation states, generally in octahedral ligand fields. The resulting complexes are potentially of interest in various interdisciplinary fields such as structural chemistry, catalytic processes involving metalloproteins or enzymes (Sarkar et al., 2017), magnetochemistry (Blinov, 2017), and NLO materials.
Regarding the sub-family of Mn III -salen derivatives, they have been used mainly as models for biological systems involving this metal cation. For our part, we focus on salen-based materials, which can display non-linear optical response, for example with Co III as the metal centre (Quintero-Té llez et al., 2016). While extending our work to Mn III , we prepared the title complex, for which a synthesis was previously reported (Panja et al., 2003). These authors synthesized the complex using a Mn III compound as starting material, namely [Mn(salen)-OAc]ÁH 2 O, which was reacted with MnCl 2 Á4H 2 O in water. Crystallization at room temperature afforded brownish black ISSN 2056-9890 microcrystals, and the authors characterized the complex in space group P2 1 /n, with Z = 4. We obtained the same compound through a more straightforward synthetic route, using a one-pot reaction between salicylaldehyde, diethylenetriamine, and MnCl 2 , in MeOH. In contrast to the previous synthesis, crystallization was carried out at low temperature (283 K) in methanol, affording brown crystals. The structure determination shows that this phase crystallizes in space group P2 1 , with Z = 2.
Although we have no strong experimental evidences regarding the mechanism triggering the polymorphism for this complex, we believe that the temperature and the solvent of crystallization could be the key parameters. We report here the structure of the P2 1 polymorph, along with its characterization in solution by means of UV-Vis spectroscopy.

Structural commentary
The asymmetric unit of the P2 1 phase contains one [Mn(salen)(OH 2 )Cl] neutral complex and one lattice water molecule, both in general positions (Fig. 1). As expected, the Mn III centre displays a slightly distorted octahedral geometry, with the four donor sites of ligand salen in the equatorial plane (N1/N2/O1/O2). The metal deviates by only 0.056 Å from the equatorial plane, and axial sites are occupied by a water molecule (O3W) and the chloride ion (Cl) at normal distances. Deviations from an ideal octahedral geometry result from the bite angles of the chelating salen ligand.
The relative position of the lattice water molecule and the complex molecule is very similar in both polymorphs: a fit between the asymmetric units of each phase, carried out using all non-H atoms in the complex, shows that the unique significant differences are for the phenol rings C1-C6 and C11-C16, which are rotated about their bonds C7-C6 and C10-C11, by ca 6.4 and 13.9 , respectively. However, such a limited change in the conformation of the complex is unlikely to promote the polymorphism. On the other hand, each phase gives a clearly different simulated powder diffraction pattern (Fig. 2).
The crystal structure reported by Panja et al. is based on a primitive monoclinic unit cell with parameters a = 6.6470 (2), b = 7.3330 (2), c = 33.8260 (10) Å and = 95.1650 (17) . The cell volume V is 1642.07 (8) Å 3 , corresponding to a P2 1 /n structure with four formulas per unit cell. An obvious relation is observed with the parameters of our phase (Table 3): the cell symmetry is retained, with very similar a, b and parameters, while the c parameter is almost exactly halved. The resulting cell volume is then V = 838.67 (10) Å 3 . Therefore, the unit-cell content is also halved to Z = 2, and a marginal difference of 2% for the calculated densities is observed between the two polymorphs. It is worth mentioning that after the data collection was completed, we checked the correctness of the short c parameter for the P2 1 polymorph, by re-building the reciprocal space: no extra diffraction spots with indices (h k l/2) for a potential supercell are observed in the 0kl and h0l layers. This can be quantitatively assessed by integrating the collected frames after doubling the c parameter: the statistics for intensities over the whole (hkl) pattern are then hI/(I)i = 4.70 if l is even (10251 reflections) and hI/(I)i = 0.16 if l is odd (10053 reflections). The previously reported P2 1 /n polymorph gives much more balanced statistics, hI/(I)i = 84.74 for l = 2n and hI/(I)i = 85.67 for l = 2n + 1 [given that, Figure 1 The structure of the title solvate, with displacement ellipsoids for non-H atoms at the 50% probability level.

Figure 2
Simulated powder diffraction patterns for the P2 1 /n form of the title compound (Panja et al., 2003; blue spectrum) and the P2 1 form (this work; red spectrum). A fit between the molecules constituting the asymmetric units in both phases is also displayed, using the same colour scheme (Macrae et al., 2008). apparently, original structure factors are not available anymore for this crystal, intensities F o 2 and standard deviations (F o 2 ) were generated using the dedicated tool in PLATON (Spek, 2009)]. These statistics support the correctness of the unit cells for both polymorphs.
A comparison of unit cells shows that molecules related by the screw axis parallel to [010] remain in the same relative orientation (Fig. 3), including the water molecules. Each pair of molecules is inverted in the P2 1 /n polymorph, while the lack of a glide plane in the new phase restrains the cell contents to this pair of molecules, which is extended in the crystal through lattice translations. The key point is then that the new phase crystallizes in a non-centrosymmetric space group, P2 1 , while doubling the c parameter gives a centrosymmetric space group, P2 1 /n. The presence or absence of an inversion centre affords two phases related by packing polymorphism (Brog et al., 2013) The electronic spectrum of the title compound in DMSO shows one band at 264 nm assigned to the ligand ! * transition, and a broad band at 598 nm, which corresponds to d-d transitions (Fig. 4). The d-d band is satisfactorily fitted with two Gaussian functions (Fig. 4, inset;OriginLab Corp., 2017), and can be assigned to the 5 E g ! 5 T 2g transition, consistent with the distorted octahedral ligand field observed for the metal centre in the solid state. If no conformational flexibility is possible for this complex, the polymorphism is then due to different packing structures, rather than geometric modifications.

Supramolecular features
The presence of both a coordinated and a lattice water molecules favours the formation of O-HÁ Á ÁO hydrogen bonds in the crystal (Table 1). The coordinated molecule O3W serves as donor, forming bonds with the lattice water O4W and the chloride atom of a neighbouring complex in position (x, y À 1, z). The lattice molecule O4W serves both as donor and acceptor, forming bonds with the chloride and phenolate atom O2 of two symmetry-related complexes. The resulting supramolecular structure is a 3D framework based essentially on discrete chains extended to large ring motifs. The comparison between the Hirshfeld surfaces for the asymmetric units in the two phases (Fig. 5;Turner et al., 2017) is consistent with the observed crystal symmetries and provides some clues about the factor causing the packing polymorphism. For the P2 1 /n phase, the inversion centre allows the formation ofcontacts between symmetry-related C11-C16 benzene rings (symmetry code: 2 À x, Ày, 1 À z). Such weak interactions are reflected in the red spots on the Hirshfeld surface, marked with arrows in Fig. 5. The main consequence of the absence of an inversion centre in the P2 1 crystal is the removal of these contacts (Fig. 5, bottom), in connection with the small rotation of 13.9 observed for this part of the Schiff base (see previous section and Fig. 2).
The crystal structure of the non-hydrated complex has been reported (Martínez et al., 2002), in space group P2 1 , but the A comparison between the cell content for the P2 1 /n and P2 1 forms (top and bottom, respectively). Dashed lines relate molecules with identical orientation in both crystals, and symmetry elements are displayed (top: screw axes, glide planes and inversion centres; bottom: screw axes).

Figure 4
UV-vis spectrum of the title polymorph dissolved in DMSO. The experimental spectrum (red line) is fitted with Gaussian functions for which maxima are indicated. The sum of these Gaussian functions affords the theoretical spectrum (dotted blue line). The visible range of the spectrum is displayed in the inset, using a scale allowing the d-d transitions to be assessed, fitted with two Gaussian functions, giving a maximum at = 598 nm. Table 1 Hydrogen-bond geometry (Å , ).  (3) 178 (5) packing structure is then modified, since the array of hydrogen bonds is different.

Database survey
Retrieving cases of packing polymorphism by mining the Cambridge Structural Database is not a straightforward task, since no dedicated tools have been designed for such a search (CSD, version 5.39, updated May 2018;Groom et al., 2016). It is thus difficult to estimate whether or not this phenomenon is common. Restraining the search to the symmetry class 2/m, we however found some cases very similar to that observed for the title compound, with packing dimorphism in space groups P2 1 /n and P2 1 (or any alternative settings for these groups), some of which are listed in Table 2. For each pair, the ratio between the unit-cell volumes for the P2 1 /n and P2 1 phases is very close to 2, because of the loss of the glide plane and the halving of the cell parameter c. Very simple molecules are found, such as glycine (DOLBIR; Arul Asir Abraham et al., 2015) and also more complex molecules (YURVAI; van den Hende et al., 1995). Using simulated powder diffraction patterns in order to ensure that a pair of crystal structures forms a genuine case of packing dimorphism, false positive occurrences may also be detected. For example, the reported crystal structures for 4-cyano-4 0 -ethyl-bipbenyl, referenced KUSVID and KUSVID01 (space groups P2 1 /c and P2 1 , respectively; Haase et al., 1992) almost certainly represent the same crystal structure rather than two packing polymorphs resulting from a reversible distortive phase transition, as was reported.

Synthesis and crystallization
Equimolar amounts (1 mmol) of MnCl 2 (0.125 g), salicylaldehyde (108 ml) and diethylenetriamine (106 ml) in MeOH (5 ml) were placed in a beaker and the mixture was kept under magnetic stirring for 30 minutes at room temperature. As the Schiff base ligand was formed in situ, the condensation reaction between the aldehyde and the amine afforded water, which participates as a reagent. The mixture was left at room temperature for one day, filtered, and then cooled to 283 K, affording brown single crystals of the title compound after eight days ( N). The UV-Vis spectrum (Fig. 4) was measured in a DMSO solution (' 1.3Â10 À2 mM) using a Cary 50 spectrophotometer ( max /", nm/10 À3 M À1 cm À1 ): 264/114.5, 598/ 1.16.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms for water molecules O3W and O4W were found in a difference map, and freely refined.     (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010).