[rac-1,8-Bis(2-carbamoylethyl)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane]copper(II) diacetate tetrahydrate: crystal structure and Hirshfeld surface analysis

The metal ion of the title salt hydrate shows a 4 + 2 (N4O2) tetragonally elongated coordination geometry defined by four macrocyclic-N atoms and two weakly associated acetate-O atoms; the crystal features conventional hydrogen-bonding interactions.


Chemical context
Owing to the multifarious applications of different metal complexes of a wide variety of macrocyclic ligands (Ali et al., 2019;Bernhardt & Sharpe, 2000;Lamani et al., 2018;Vicente et al., 2003;Xu et al., 2020), studies on some N-pendent macrocyclic ligands and their metal complexes were described by us recently Dey, Rabi, Palit et al., 2021). In a continuation of this work, a new N-pendent carbamoyl-derived macrocyclic ligand, 'tet-am', C 22 H 46 N 6 O 2 , prepared from 'tet-a 0 (an isomeric ligand of the hexamethyl tetrazamacrocyclic ligand) and acrylamide has been synthesized, by employing the procedure described for the preparation of a related N-pendent ligand . Thereafter, the interaction of the new 'tet-am' ligand with copper(II) acetate monohydrate furnished violet crystals formulated as [Cu(tet-am)](O 2 CCH 3 ) 2 Á4H 2 O, hereafter (I). Herein, we describe the synthesis of (I), its analysis by single crystal X-ray diffraction and a detailed study of supramolecular association by an evaluation of the calculated Hirshfeld surfaces and two-dimensional fingerprint plots. ISSN 2056-9890

Structural commentary
The molecular structure diagram showing the complex dication and loosely associated anions is shown in Fig. 1. The Cu atom is located on a centre of inversion and is coordinated by tertiary and secondary N atoms with the bond length formed by the former, i.e. Cu-N2 = 2.0016 (12) Å , being approximately 0.1 Å shorter than the Cu-N1 bond of 2.1086 (11) Å . Whereas the conformation of the five-membered chelate ring is best described as being an envelope with the C4 atom being the flap atom, the six-membered chelate ring approximates a chair conformation. The acetate anions are weakly associated with the complex cation, forming relatively long CuÁ Á ÁO3 separations of 3.2048 (15) Å with extra stability to the threeion aggregate provided by intramolecular amine-N-HÁ Á Á(carboxylate) hydrogen bonds, Table 1. The coordination geometry for the Cu centre can therefore, be considered 4 + 2 N 4 O 2 tetragonally distorted. From symmetry, the N1-bound carbamoylethyl groups lie to opposite sides of the CuN 4 plane and the N1-C9-C10-C11 torsion angle of À178.52 (12) is consistent with an -anti-periplanar (-ap) configuration.

Figure 2
A view of the unit-cell contents of (I) shown in projection down the a-axis direction. The O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds are shown as orange and blue dashed lines, respectively.

Figure 1
The molecular structure of the complex dication in (I) along with the loosely associated anions, showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level. The molecule is disposed about an inversion centre with unlabelled atoms related by the symmetry operation 1 À x, 1 À y, 1 À z. The weak CuÁ Á ÁO3 interactions above and below the CuN 4 plane are shown as dashed lines.
carboxylate-O2 atom and to the second water molecule, i.e. water-O2W. The latter forms donor interactions with the amide-O and carboxylate-O3 atoms. As can be seen from the unit-cell diagram of Fig. 2, globally, the three-ion aggregates align in chains along the a axis direction with the prominent hydrogen bonds between the molecules in that direction being water-O-HÁ Á ÁO(water) and water-O-HÁ Á ÁO3(carboxylate).
The remaining hydrogen bonds extend laterally to consolidate the three-dimensional supramolecular network.

Analysis of the Hirshfeld surfaces
The Hirshfeld surface analysis for each constituent of (I) was performed to provide further information on the supramolecular connections in the crystal and to differentiate the modes of association of the water molecules. The calculated Hirshfeld surfaces were mapped over the normalized contact distance d norm (Spackman & Jayatilaka, 2009). These along with the associated two-dimensional fingerprint plots (Spackman & McKinnon, 2002) were calculated with Crystal Explorer 17 (Turner et al., 2017) following literature precedents (Tan et al., 2019). The colour for the d norm surface was scaled between À0.621 (blue) and 1.131 a.u. (red). Key interatomic parameters are listed in Table 2. As a hydrogen-bond donor, the two bright red spots on the d norm -Hirshfeld surface of the O1W-water molecule are due to the formation of conventional water-O-HÁ Á ÁO(water) and water-O-HÁ Á ÁO(carboxylate) hydrogen bonds, Fig. 3(a). The other bright-red spot appearing on the d norm -Hirshfeld surface is due to the formation of a conventional primary amide-N-HÁ Á ÁO(water) hydrogen bond, Fig. 3(b). Further, weak methylene/methyl-C-HÁ Á ÁO(water) interactions are also shown as faint red spots near atoms H5A, H7B and H8C in Fig. 3(b). Similar to the O1W-water molecule, the two O2W-H atoms participate in conventional water-O-HÁ Á ÁO(carboxylrboxylate) and water-O-HÁ Á ÁO(amide) hydrogen bonds. These hydrogen bonds are manifested as two bright-red spots on the d norm -Hirshfeld surface of the O2W molecule, Fig. 4(a). The third bright red spot, evident in Fig. 4(b), is due to the water-O-HÁ Á ÁO(water) hydrogen bond as discussed above.
For the carboxylate anion, the bright-red spots on its d norm -Hirshfeld surface correspond to the water-O-HÁ Á ÁO(carboxylate) hydrogen bonds, Fig. 5(a); the amide-N-HÁ Á ÁO(carboxylate) hydrogen bond, which also leads to a bright-red spot, is highlighted in Fig. 5 Two views of the Hirshfeld surface for the O1W-water molecule of (I) over d norm highlighting (a) O1W-HÁ Á ÁO(water/carboxylate) hydrogen bonds and (b) amide-N-HÁ Á ÁO1W hydrogen bonds as well as weak C-HÁ Á ÁO1W interactions.

Table 2
A summary of short interatomic contacts (Å ) for (I) a .

Contact
Distance Symmetry operation 2.14 Àx + 1 2 , y À 1 2 , Àz + 1 interactions, with separations of 0.38 and 0.47 Å shorter than the sum of van der Waals radii, respectively, are shown as faint red spots in Fig. 5 On the d norm -Hirshfeld surface calculated for the cation, the bright-red spots near the amide-O1, methyl-H7A, amine-H1N and amide-H2N atoms, Fig. 6, arise from interactions mentioned above.
The amide-N-HÁ Á ÁO(amide) hydrogen bond, which serves to link cations, is shown as bright-red spots near the amide-O1 and amide-H3N atoms in Fig. 7(a). Especially highlighted in Fig. 7(b) is a short H7CÁ Á ÁH10A contact, reflected as a faintred spot on the d norm -Hirshfeld surface, with a separation of 2.14 Å , which is 0.26 Å shorter than sum of the van der Waals radii.
CÁ Á ÁH and HÁ Á ÁN/NÁ Á ÁH contacts appear in the two-dimensional fingerprint plots of the cation, their contributions to the overall Hirshfeld surface are only 2.8 and 2.0%, respectively. As observed for the anion, the weak connection between the Cu II centre and the carboxylate ligand is reflected in a very low contribution of OÁ Á ÁCu/CuÁ Á ÁO contacts (0.1%) to the overall Hirshfeld surface of the cation.

Database survey
There are two relevant structures in the literature available for comparison having closely related 14-membered tetraaza macrocycles bearing two pendent N-bound CH 2 CH 2 CONH 2 arms (Kang et al., 2008). These structures present very different coordination geometries to each other and to that of (I). The common feature of the literature structures is the presence of perchlorate counter-anions, which do not coordinate the Cu II atom in either case. Rather, the amide-O atom of one side-arm folds over the molecule to form a Cu-O bond. In the C-rac-macrocyclic complex, a square-pyramidal geometry ensues with the amide-O atom [2.207 (4) Å ] occupying the apical position. While the trans-orientated Cu-N(tertiary) bond lengths of 2.083 (4) and 2.086 (4) Å are longer than Cu-N(secondary) bonds of 2.035 (4) and 2.045 (4) Å , the differences between the short and long bond lengths are not as great as noted above for (I). In the structure with the configurational C-meso isomer, the coordination geometry changes to trigonal-bipyramidal with the amide-O atom occupying an equatorial position, forming a significantly shorter Cu-O bond length [2.007 (4) Å ] compared to that in the racemic isomer. The tertiary-N atoms occupy axial positions and form Cu-N(tertiary) bond lengths of 2.063 (4) and 2.088 (4) Å which overlap with the Cu-N(secondary) bond lengths of 2.077 (4) and 2.090 (3) Å . The foregoing demonstrates a dependency of the Cu atom coordination geometry and the magnitudes of putative Cu to O interactions on the nature of the counter-anion and isomeric form of the ligand.

Synthesis and crystallization
Synthesis of N-carbamoylethyl pendent derivative (tet-am): The isomeric ligand, tet-a (0.320 g, 1.0 mmol), dissolved in hot methanol (50 ml), and acrylamide (0.28 g, 4.0 mmol), taken in a minimum amount of hot methanol, were mixed. The reaction mixture was refluxed for about 12 h, cooled to room temperature, filtered and allowed to stand for three days to evaporate slowly. The white product that formed, tet-am, was separated by filtration, washed with methanol followed by water and finally dried in a desiccator over silica gel; m.p. 458 K.
[Cu(tet-am)](O 2 CCH 3 ) 2 Á4H 2 O (I): The macrocycle, tet-am (0.426 g, 1.0 mmol) and copper(II) acetate monohydrate (0.199 g, 1.0 mmol) were dissolved separately in hot methanol (25 ml) and mixed while hot, resulting in an immediate colour change. The solution was heated on a steam-bath until the volume was reduced to less than 10 ml. After standing overnight, the sticky material that had formed was dissolved in a minimum amount of ethanol followed by the addition of excess diethylether. The liquid portion was decanted and the remaining violet precipitate, (I), was dried over silica gel and stored in a vacuum desiccator. Some violet crystals suitable for X-ray analysis were collected from the mother liquor (ethanol + diethylether) during the isolation of the complex; m.p. 378 K.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The carbon-bound H atoms were placed in calculated positions (C-H = 0.96-0.98 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set to 1.2-1.5U eq (C). The O-and N-bound H atoms were located in a difference-Fourier map and were refined with O-H = 0.82AE0.01 and N-H = 0.86AE0.01 Å distance restraints, and with U iso (H) set to 1.5U eq (O) and 1.2U eq (N), respectively.  Table 4 Experimental details.

Funding information
Crystal data Chemical formula [Cu(C 22 -1,8-Bis(2-carbamoylethyl)-5,5,7,12,12,14-hexamethyl-1,4,8,11- DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010). 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.