Synthesis, crystal structure and Hirshfeld surface analysis of bis[4-(2-aminoethyl)morpholine-κ2 N,N′]diaquanickel(II) dichloride

The title coordination complex, [Ni(C6H14N2O)2(H2O)2]Cl2, crystallizes in the P21/n space group. The metal ion displays a slightly distorted octahedral geometry. The crystal structure is consolidated by N—H⋯Cl, N—H⋯O, C—H⋯Cl, C—H⋯O and O—H⋯Cl hydrogen bonding.


Chemical context
Morpholine has been recognized as a convenient ligand for the design of supramolecular structures (Cvrtila et al., 2012) since it is a ditopic heterocyclic molecule that can coordinate the metal ion via one hetero atom, leaving the other free for linking to other molecules either by coordination to metal ions (Gá lvez Ruiz et al., 2008;Willett et al., 2005;Lapadula et al., 2010;Clegg et al., 2010) or by hydrogen bonding (Lian et al., 2008;Weinberger et al., 1998;Ivanov et al., 2001). The coordination of a morpholine molecule to a metal ion activates the morpholine molecule as a hydrogen-bond donor by increasing the partial positive charge of the morpholine amine hydrogen. Coordination of two morpholine molecules through nitrogen lone pairs on a single metal centre can lead to a good bonding site for negatively charged small species. A prerequisite for this is that the morpholine ligands are bonded to the central ion in a cis configuration, which may be achieved by the use of chelating co-ligands. These will induce the binding of morpholine in a convenient configuration, so that its N-H groups form a pincer, which can bind guest molecules in the second coordination sphere. Intriguingly, reports of morpholine-based metal-organic receptors are scarce (White et al., 1999). Transition-metal ions can be an important source of magnetic moments, and when connected through proper bridging ligands, superexchange interactions can take place (Konar et al., 2005). Parallel to the development of organic electro-optical (EO) and non-linear optical (NLO) materials [Lamshö ft et al., (2011)], a subject of great interest comprises metal-organic chromophores.
Metal-organic frameworks (MOFs) are crystalline hybrid materials with networks constructed from the self-assembly of metal ions with, at least, one organic linker. As a result of their availability from commercial sources and/or easy synthetic methodologies, organic ligands based on carboxylate or nitrogen compounds have been used extensively, mainly with transition-metal ions, to isolate new and improved MOF architectures, to be studied for a large range of practical applications (Horcajada et al., 2012;Kreno et al., 2012;He et al., 2012;Chughtai et al., 2015). Morpholine can be used as a ligand in metal complexes and it can also be a component of protective coatings on fresh fruits and used as an emulsifier in the preparation of pharmaceuticals and cosmetic products (Kuchowicz & Rydzyń ski, 1998). As a continuation of our recent work on compounds belonging to the morpholine family (Chidambaranathan et al., 2023) we are now using morpholine as ligand for coordination complexes. The current study describes the synthesis, crystal structure, Hirshfeld surface, and infrared spectroscopy of bis[4-(2-aminoethyl)morpholine-2 N:N 0 ]diaquanickel(II) dichloride.

Structural commentary
The title compound ( Fig. 1) crystallizes in the monoclinic space group P2 1 /n with two complexes in the unit cell. The asymmetric unit comprises one half of an Ni II cation, which is located on an inversion centre, one [(4-(2-aminoethyl) morpholine] ligand, one coordinated water molecule and one chloride ion outside the metal coordination sphere. The nickel ion is in an octahedral environment of the type N 4 O 2 ; the coordination sphere comprises two N,N 0 -bidentate morpholine ligands defining the equatorial plane, which form two fivemembered chelate rings with the metal centre (Suleiman Gwaram et al., 2011). The two remaining trans axial positions are occupied by the oxygen atoms from the water molecules. As a result of symmetry, the N2-Ni-N2 i , N1-Ni-N1 i and O2-Ni-O2 i angles are 180 [symmetry code: (i) Àx + 1, Ày + 1, Àz + 1] and the axes of the octahedron are almost perpendicular to each other [N2-Ni-O2 = 90.80 (12), N2-Ni-O2 i = 89.20 (12) and O2-Ni-N1 i = 91.86 (14) ]. The morpholine rings adopt a chair conformationÁThe Ni-N (amine) distances, Ni-N1 and Ni-N2, are of 2.249 (4) and 2.067 (3) Å , respectively, in good agreement with the values observed in the literature (Chiumia et al. 1999;Chattopadhyay et al. 2005).

Supramolecular features
Figs. 2 and 3 highlight the main supramolecular interactions formed by the title compound (see also Table 1), while Fig. 4 shows the overall crystal structure viewed down the b-axis direction. In the crystal, the oxygen atoms of the water and of the morpholine molecules (O1 and O2) act as acceptors for several intermolecular interactions of the types N-HÁ Á ÁO and C-HÁ Á ÁO, respectively. The uncoordinated chloride anions act as acceptors to C-H and N-H groups of the morpholine molecules and link the adjacent molecules via O-HÁ Á ÁCl interactions involving the water molecules.

Figure 4
Crystal structure of the title compound viewed down the b-axis direction. Dotted lines represent supramolecular interactions.

Figure 5
The Hirshfeld surface of the title compound mapped over d norm , showing the relevant close contacts.  which accounts for 63.1% of the total crystal packing and is shown in Fig. 6b by a pair of symmetrical blunt spikes with points at d e + d i $2.4 Å . The high contribution of these interactions suggests that van der Waals interactions play a major role in the crystal packing (Hathwar et al., 2015). The HÁ Á ÁCl interactions are shown by the presence of a pair of wings in the fingerprint plot shown in Fig. 6c with the tips at d e + d i $2.5 Å , contributing 25.5% to the HS. The pair of sharp symmetrical spikes in the fingerprint plot delineated into OÁ Á ÁH/HÁ Á ÁO contacts (11.3% contribution to the HS, Fig. 6d), shows a symmetric distribution of points with the tips at d e + d i $2.1 Å .

Synthesis and crystallization
According to the reaction shown in the scheme, the title compound was synthesized by mixing two moles of 4-(2aminoethyl)morpholine (4.34 g) and one mole of nickel chloride hexahydrate (3.96 g) in 100 ml of double-distilled water at 303 K. At room temperature, the solution was allowed to evaporate, yielding plate-like ultramarine blue crystals of the title compound. The FT-IR spectrum of the compound was recorded on a BRUKER FT-IR spectrometer. FT-IR (KBr, cm À1 ): 3455 (w, N-H), 2967 (w, CH 2 ), 1614 (s, H 2 O), 1307 (s, C-N).
atoms, H1W and H2W, were found in a difference-Fourier map and refined freely. The morpholine ligand was found disordered over two positions with a site occupancy ratio of 0.708 (8):0.292 (8). The positions of the disordered atoms were identified from difference electron-density peaks and refined using DFIX restraints to achieve the target bond distance of the corresponding atoms. Anisotropic displacement parameters of atoms in the group were restrained to be equal using SIMU restraints with an effective standard deviation of 0.02 Å 2 .

sup-1
Acta Cryst. Data collection: APEX3 (Bruker, 2016); cell refinement: APEX3/SAINT (Bruker, 2016); data reduction: SAINT/XPREP (Bruker, 2016); program(s) used to solve structure: SHELXT2018/2 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: PLATON (Spek, 2020). Extinction correction: SHELXL2018/3 (Sheldrick, 2015b), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0076 (14) 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. Refinement. Morpholine moiety is disordered over two positions with a site occupancy ratio of 70:30. The positions of disordered atoms were identified from difference electron density peaks and refined using DFIX restraints to achieve target bond distance of corresponding atoms. Anisotropic displacement parameters of atoms in the group were restrained to be equal using SIMU restraint with an effective standard deviation of 0.02 Å2