[4-(2-Aminoethyl)morpholine-κ2 N,N′]dibromidocadmium(II): synthesis, crystal structure and Hirshfeld surface analysis

The title coordination compound was synthesized upon complexation of 4-(2-aminoethyl)morpholine and cadmium(II) bromide tetrahydrate at 303 K. It crystallizes as a centrosymmetric dimer, with one cadmium atom, two bromine atoms and one N,N′-bidentate 4-(2-aminoethyl)morpholine ligand in the asymmetric unit.


Chemical context
Inorganic-metal halides may be associated with functionalized organic molecules (for example carboxylic acids, amides or amines) to produce neutral or ionic coordination compounds that combine and change the properties of both components.Fine-tuning the stoichiometry, reaction conditions and geometry of the organic ligands allows control of the dimensionality and geometry of the final product, resulting in a wide range of systems (Constable, 2019).This has become the main focus of coordination chemistry and has allowed for the development of many research fields, such as medicinal chemistry of coordination compounds, homogenous catalysis, and metal-organic frameworks (Malinowski et al. 2020;Zecchina & Califano 2018;Yaghi et al. 2019;Jones & Thornback 2007).In this context, morpholine is a heterocyclic bidentate ligand frequently used in medicinal chemistry and a privileged structural component of bioactive molecules.The morpholine molecule has become one of the most promising moieties evaluated in structure-activity relationship (SAR) studies, as it induces biological activity, as well as an improved pharmacokinetic and metabolic profile to the biomolecules that contain it.Morpholine and its derivatives have long been known for various activities such as analgesic, anti-inflammatory, antioxidant, anticancer, anti-neurodegenerative, etc.As a result of its biological and pharmacological importance, the synthesis of morpholine compounds has been extensively studied by many researchers (Rekka & Kourounakis 2010;Wijtmans et al., 2004;Ilas ˇet al., 2005;Pal'chikov 2013).Herein, we report the synthesis of the coordination compound [4-(2aminoethyl)morpholine-� 2 -N,N 0 ]dibromidocadmium(II) and examined it using single crystal X-ray diffraction, FTIR, NMR, and Hirshfeld surface studies as a part of our ongoing interest in morpholine derivatives.

Structural commentary
The title compound crystallizes in the triclinic P1 space group.Fig. 1 depicts a perspective view of the mononuclear centrosymmetric complex, [(Cd)(L)(Br) 2 ], where L = 4-(2-aminoethyl)morpholine, with the atom-labeling scheme.The asymmetric unit contains half of the molecule, consisting of one cadmium cation, two bromine anions and one 4-(2-aminoethyl)morpholine ligand that are located on a general positions and the other half of the molecule is generated by inversion symmetry.Although the synthesis was carried out in water, the title compound is neither a hydrate nor is water present in the coordination sphere of the metal.If water enters the coordination sphere of cadmium, the resulting complex is usually ionic, as one Br À has to stay outside the coordination sphere leading to lower entropy for the system.In addition, the large Br À ion is a better bridging ligand than water and can link the components in a threedimensional network.Hence, ignoring water during crystallization is more advantageous than retaining it in the coordination sphere.
In the structure, one of the symmetry-independent bromine atoms (Br1) is terminal, while the other (Br2) bridges two cadmium atoms related by inversion (À x + 1, À y, À z + 1).The metal atom further coordinates the 4-(2-aminoethyl) morpholine in a N,N 0 bidentate fashion, forming a fivemembered chelate ring (Cd1-N1-C5-C6-N2), which is shown in Fig. 2. The last coordination site of the distorted octahedron around the cadmium atom is occupied by an oxygen atom from a different morpholine moiety (x, y À 1, z).The size of the chelate ring is a key component in metal ion selection, with five-membered chelate rings preferring metal ions with an ionic radius near 1.0 A ˚. Bazargan et al. (2019) reported that the optimal size for the N-M distance is 2.5 A ˚and the N-M-N angle is 69 � for five-membered N-C-C-N-M chelate rings.In five-membered chelate rings, the M-N bond lengths and the N-M-N bond angle are considered to be inversely linked (Hancock 1992;Hancock et al., 2007;Dean et al., 2008).The Cd1-N1 and Cd1-N2 distances are 2.504 (2) and 2.306 (3) A ˚, respectively, while the N1-Cd-N2 angle is 76.06 (8) � .This chelate ring pattern appears to be present in all reported structures of with a metal coordinated by 4-(2-aminoethyl)morpholine (Ikmal Hisham et al., 2010;Suleiman Gwaram et al., 2011).According to the structural data for the title compound, the torsion angles O1-C1-C2-N1 and N1-C3-C4-O1 of the morpholine Ellipsoid plot of the title compound with displacement ellipsoids drawn at the 50% probability level.

Supramolecular features
The morpholine molecule is potentially an ambidentate Nand O-donor ligand, where the binding of morpholine to the metal center is most commonly accomplished through the nitrogen atom (Cvrtila et al., 2012;Cindric et al., 2013), except in cases where the nitrogen atom is protonated (Li et al., 2010;Willett et al., 2005).This leaves the oxygen atom free to participate in supramolecular interconnections via the formation of additional coordination bonds, acting as an acceptor for a halogen bond (Lapadula et al., 2010) or participating in hydrogen bonding (Weinberger et al., 1998), which can result in many different supramolecular architectures.A packing diagram of the title compound along the b-axis is shown in Fig. 3, showing the intermolecular C-H� � �O, C-H� � �Br and N-H� � �Br interactions (Table 1).The Br1 anion links adjacent molecules along the b-axis direction via the H3B and H4B atoms of the morpholine ring.Similarly, the Br2 anion links adjacent molecules along the a-axis direction via the H2C atom.The corresponding interaction distances for H3B� � �Br1, H4B� � �Br1 (x, y + 1, z) and H2C� � �Br1 (x À 1, y, z) are 2.96, 2.91 and 2.95 (2) A ˚, respectively.Further C-H� � �Br and N-H� � �Br hydrogen bonds link the components into a three-dimensional network.Owing to the higher electronegativity of the N-H� � �Br hydrogen bonds, they are shorter than the C-H� � �Br ones and hence they will have a larger effect on the packing than the C-H� � �Br interactions.On the other hand, the O-Cd coordination bond contributes to the formation of the three-dimensional network more than the N-H� � �Br and C-H� � �Br hydrogen bonds.Fig. 4 shows the R 2 2 (6) ring motif formed between two molecules through C-H� � �O intermolecular interactions (Bernstein et al., 1995;Motherwell et al., 2000).
To examine the intermolecular interactions present in the title compound in more detail, a Hirshfeld surface analysis was performed and the two-dimensional fingerprint plots were generated with CrystalExplorer 21.5 (Spackman et al., 2021).The R 2 2 (6) motif formed by the intermolecular interactions.

Figure 5
View of the Hirshfeld surface of the title compound mapped over d norm .
contacts with distances shorter than the sum of the van der Waals radii, while blue areas indicate distances longer than the sum of the van der Waals radii (Venkatesan et al., 2016).This colored mapping of contacts allows the visual identification of regions susceptible to participating in interactions with other molecules.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes. Fractional

Figure 3
Figure 3Packing diagram of the title compound along the b-axis.

Figure 4
Figure 4 Fig. 5 shows the most prominent intermolecular interactions as red spots corresponding to the Cd-Br and Cd� � �O contacts.The two-dimensional fingerprint plots are shown in Fig. 6.Each point of the Hirshfeld surface is associated with two types of distances: d e is the distance from the point to the nearest-to-the-surface external nucleus and d i is the distance from the point to the nearest-to-the-surface internal nucleus.The normalized contact distance, d norm , is the sum of the van der Waals radii, d e + d i , of each atom (McKinnon et al., 2007; Hathwar et al., 2015).The largest contributions to the Hirshfeld surface are represented as a point at d e + d i �2.4A ˚due to H� � �H (46.1%), a pair of wings with the tip at d e + d i �2.85A due to H� � �Br/Br� � �H (38.9%), a pair of spikes at d e + d i �2.45A ˚due to H� � �O/O� � �H (4.7%), a tip of a scissor-like image at d e + d i �2.7 A ˚due to Cd� � �Br/Br� � �Cd (4.4%) and a feather-like image at d e + d i �2.7 A ˚due to O� � �Cd/Cd� � �O (3.5%) contacts.The other contributions are Br� � �Br (1.1%), Br� � �O/O� � �Br (0.3%) and O� � �N/N� � �O (0.1%).All these interactions play a crucial role in the overall stabilization of the crystal packing.

Figure 7
Figure 7Synthesis of the title compound.

Table 2
Experimental details.