Two structurally related, hydrogen-bonded polymorphs of the zwitterionic complex trichlorido- ((dimethylphosphoryl)methanaminium-κO)zinc(II)

Introduction

The first report on the syntheses and the structures of metal complex (Zn, Ni, Pd) containing the neutral (dimethylphosphoryl)methanamine (dpma) ligand dates back to the nineties of the last century (Dodoff et al., 1990). Since then, a series of related transition metal complexes have been characterized (Borisov et al., 1994; Kochel, 2009; Vornholt et al., 2014). It has also been shown that the N-protonated dpma ligand (dpmaH⁺) is able to form the transition metal complexes 1 (Reiss, 2013a; Richert et al., 2014) and 2 (Reiss, 2013b; Scheme 1). A search for structures in the Cambridge Structural Database (CSD) proved that the neutral complexes 3, which compromise three halogenido and one O-coordinated phosphoryl moiety, are well known for trivalent metals (Dunbar et al., 1990; Burford, 1992; Vijjulatha et al., 1997; Aizawa et al., 2013). Only a few homologous anionic complexes 4 of divalent metals (Beagley et al., 1988; Muñoz et al., 1997; Exarchos et al., 2001) have been structurally characterized. The analogous copper(II) complex 5 (Newton et al., 1974; Marsh, 1997) is besides 2 a very rare example of a structurally characterized zwitterionic complex in this class of compounds (Scheme 1). The structure of an analogous zinc(II)-based zwitterionic complex 6 is unknown to the best of my knowledge.

Scheme 1:

Schematic drawings of dpmaH⁺-containing complexes of Cu(II) (1), Co(II) (1) and Mn(II) (2) and some trihalogenido complexes which furthermore coordinate to a phosphoryl moiety (3–6).

Finally, it is worth mentioning that the dpmaH⁺ cation is an excellent tecton to construct hydrogen-bonded architectures. There are numerous examples of one-dimensional polymeric motifs in structures of simple dpmaH⁺ salts caused by the predominance of a head-to-tail connection of the aminium group and the phosphoryl moiety (Kessler and Reiss, 2014). Based on its conformational flexibility, the dpmaH⁺ tecton is able to fit with the needs of packing and hydrogen bonding even in complex structures (e.g. Reiss, 2013c). This contribution is part of my continuing interest on the construction of hydrogen-bonded architectures based on complex tectons.

Results and discussion

The reaction of ZnCl₂ with dpma in a hydrochloric acid solution results in the crystalline material trichlorido(dpmaH-κO)zinc(II) (6). Two types of crystals can be distinguished optically. The majority of them are monocrystalline blocks with a monoclinic habitus (6a). Additionally, a limited number of tetragonally shaped, significantly smaller crystals are present (6b). The structural characterization of both specimens reveals that the title compound shows at least dimorphism.

Crystal structures

In both crystal structures 6a and 6b, the cationic dpmaH⁺ ligand coordinates to a formal negatively charged ZnCl₃ moiety. Also, in each structure, an intramolecular NH···Cl hydrogen bond, which is typical for the dpmaH⁺ ligand (Reiss, 2013a; Richert et al., 2014), leads to an eight-membered cyclic unit (Figure 1). The zwitterionic nature of this complex is clearly visible, as its cationic partial charge is located at the aminium group and the negative partial charge is distributed over the chlorido ligands. Caused by the intramolecular hydrogen bonds in both structures, the structural parameters of all crystallographically independent complex moieties are similar but not identical. In detail, in the structure of 6a, the asymmetric unit contains one complete dpmaHZnCl₃ complex, whereas in 6b, two crystallographically independent complexes are present (Figures 2 and 3, Tables 1 and 2). In the concept of crystal engineering of hydrogen-bonded structures, the title complex is a semi-rigid tecton that preferably accepts intermolecular hydrogen bonds at the ZnCl₂ moiety and donates hydrogen bonds by two of the hydrogen atoms of the NH₃⁺ group (see blue arrows in Figure 1). We have already shown that the dpmaH⁺ tecton, which shows an analogous topology, tends to form structures with a head-to-tail connection as the predominant motif (Buhl et al., 2013; Kessler and Reiss, 2014).

Figure 1:

The title complex dpmaHZnCl₃ is shown with the intramolecular hydrogen bond. Arrows indicate the preferable directions for further hydrogen-bonding interactions of this supramolecular tecton. The graph-set descriptor for the intramolecular hydrogen bond is S¹₁(7) (Grell et al., 1999).

Figure 2:

Upper part: the crystal structure of the monoclinic polymorph 6a is shown along [00-1]. Hydrogen bonds are indicated by blue fragmented lines. Ellipsoids are drawn at the 40% probability level. Symmetry codes: ′=1-x, 0.5+y, 0.5-z; ″=1+x, y, z. Lower part: the constructor graph of the hydrogen-bonded connection of the title complex in 6a is shown; crystallographically related hydrogen bonds are coded by their colors.

Figure 3:

Upper part: the crystal structure of the tetragonal polymorph 6b is shown. Hydrogen bonds are indicated by blue fragmented lines. Ellipsoids are drawn at the 40% probability level; symmetry code: ′=1+x, y, z. Lower part: constructor graph of the hydrogen-bonded connection of the title complex in 6b; crystallographically related hydrogen bonds are coded by the colors.

The fine-tuning of the geometrical parameters of these dpmaHZnCl₃ tectons can be read from intramolecular hydrogen bond distances (6a: N···Cl=3.2635(16) Å; 6b: N···Cl=3.293(4) and 3.292(4) Å) and is depending on the structure they construct. According to the very similar values of the two crystallographically independent intramolecular hydrogen bonds, they are considered as identical for any further discussions. The intramolecular N···Cl as well as the intermolecular ones are significantly shorter than those found in related neutral ZnCl₂L₂ moieties, with ligands L able to donate hydrogen bonds (Vornholt et al., 2014; Kociok-Köhn et al., 2015). The coordination of the Zn metal centers in both structures is almost that of a distorted tetrahedron. In detail, in all moieties, the Zn-O distances are shorter than the Zn-Cl distances, and the O-Zn-Cl angles are smaller than the Cl-Zn-Cl angels (Table 1).

It should be noted here that there is also a specific biological interest on halogenido zinc complexes. Trihalogenido zinc(II) complexes with ligands such as N⁶-furfuryladenine have been reported to have biological activity (Hošek et al., 2013). In a similar N⁶-benzyl-adenine zinc(II) complex, an intramolecular halogenido···H–N hydrogen bond stabilizes the conformation of the complex in a manner analogous to that in the dpmaHZnCl₃ complex (Jennifer et al., 2014).

In the monoclinic crystal structure of 6a, one out of the two chlorido ligands and the two hydrogen atoms of the aminium group that are not involved in the intramolecular hydrogen bond form hydrogen bonds to four neighboring complexes (N1···Cl2′=3.1986; N1···Cl2″=3.2768(16) Å; Figure 2, upper part). These connections lead to a layered structure parallel to the ab plane.

In the tetragonal crystal structure of 6b, each of the two crystallographically independent zinc(II) complexes form hydrogen-bonded connections by two chlorido ligands and the two hydrogen atoms of the aminium group, which are not involved in the intramolecular hydrogen bond, to three adjacent complexes (N1···Cl3′=3.229(4); N1···Cl5=3.222(4); N2···Cl2=3.220(4); N2′···Cl6=3.213(4) Å; Figure 3, upper part). Obviously, the intramolecular NH···Cl hydrogen bonds in the polymorph 6b are with 3.293(4) and 3.292(4) Å, slightly longer than the one in 6a (3.2635(16) Å). A finding which is in excellent agreement with only one intermolecular N···Cl distance below 3.25 Å in 6a and two of them in 6b for each crystallographically independent complex. Summing up the hydrogen bonding interactions in both polymorphs, the interaction quality is roughly the same, which may be the reason for their simultaneous occurrence. In line with these findings, the density of the polymorph 6a is lower (Table 2).

Common to both structures is a head-to-tail connection of the zwitterionic complexes yielding a chain structure as the primary motif (Figure 1). In the case of the layered, monoclinic polymorph 6a, each hydrogen-bonded chain is connected to two neighboring ones. In the case of the tetragonal polymorph 6b, chains are connected pairwise to form a complex chain structure along the c direction. For a better illustration of these similarities and differences of these complex hydrogen-bonded structures, the constructor graphs are shown (Figures 2 and 3, lower parts; Grell et al., 2002). Apart from my group’s efforts, constructor graphs are rarely used (e.g. Guzei et al., 2010). Nonetheless, it is my considered view that they are useful tools to decode and compare complex hydrogen-bonded structures.

Spectroscopy

Usage of the attenuated total reflection (ATR) method allows the collection of infrared (IR) spectra from selected single crystals. Therefore, we sorted out some tetragonal and some monoclinic-shaped crystals for the IR experiments. For the Raman experiment, a larger amount of the compound was needed, representing a cross-section sample of both polymorphs. The IR absorption bands and Raman signals, which are typical for the dpmaH⁺ cation (Buhl et al., 2013), are present in the corresponding spectra (Figure 4). As discussed before, the geometric parameters of the zwitterionic complexes are very similar in both polymorphs. Consequently, the IR spectra of 6a and 6b are marginally different (Figure 4). The Raman spectrum does not allow an assignment of individual signals that are associated with -ZnCl₃ moiety. Summing up the number of crystallographically independent Zn-Cl distances equals to nine. For these bonds as well as for the three Zn-O bonds present in both polymorphs, stretching modes should be in the range between 350 and 200 cm^-1.

Figure 4:

Upper part: IR spectra of the monoclinic polymorph 6a (red) and of the tetragonal polymorph 6b (black). Lower part: Raman spectrum of a cross-section sample of both polymorphs.

Conclusion

In the zwitterionic title complex trichlorido((dimethylphosphoryl)methanaminium-κO)zinc(II), the cationic dpmaH⁺ ligand coordinates the zinc(II) metal center with the oxygen atom. One more feature of this complex is the existence of an intramolecular N-H···Cl hydrogen bond, which fixes the conformation about the central P-C bond of the dpmaH⁺ ligand. It is shown that the title complex shows at least dimorphism. The two structurally related polymorphs 6a and 6b differ by their intramolecular connection via N-H···Cl hydrogen bonds. Their structural relation is visualized by the so-called constructor graphs (Grell et al., 2002).

Experimental

Crystallography

All intensity data were collected with an Xcalibur CCD-diffractometer (Oxford Diffraction now a part of Rigaku) using Mo-Kα radiation at ambient temperature. Both structures were solved with direct methods using the SHELXS-97 program (Sheldrick, 2008), and all refinements were carried out using the SHELX-2014 program suite (Sheldrick, 2015). The C-H hydrogen atoms were positioned with idealized geometry and refined using a riding model. The hydrogen atoms at the aminium groups were included into the latest stages of the refinement using the AFIX 138 option of the SHELX program with their U_iso values refined freely. CCDC-1048499 and CCDC-1048505 contain the supplementary crystallographic data for this article. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. The crystal data and the results of the structure refinement for both crystal structures presented in this contribution are shown in Table 2.

Synthesis

In a typical reaction, 0.54 g (0.5 mmol) of dpma and 0.68 g (0.5 mmol) of zinc(II) chloride were mixed and then stirred in 4 mL of concentrated hydrochloric acid and slowly heated until a colorless solution was obtained. Upon slow cooling to room temperature, colorless crystals formed. Crystals suitable for single crystal diffraction studies were grown at room temperature by slow evaporation. The majority of the crystals show a monoclinic habitus, whereas a few crystals are obviously tetragonally shaped. Anal. calcd. for C₃H₁₁Cl₃NOPZn (%): C 12.88, H 3.96, N 5.01. Found: C 12.71, H 3.88, N 4.85.