Crystal structure of [2,13-bis(acetamido)-5,16-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.07,12]docosane-κ4 N]silver(II) dinitrate from synchrotron X-ray data

The title compound, [Ag(C24H46N6O2)](NO3)2, has a square-planar geometry with the nitrate anions on general sites. The macrocycle adopts the trans-III conformation. The crystal packing is stabilized by hydrogen-bonding interactions among the N–H groups of the macrocycle and its actetamide substituents, with the O atoms of the nitrate anions and of an acetamide group as the acceptor atoms.

The asymmetric unit of the title compound, [Ag(C 24 H 46 N 6 O 2 )](NO 3 ) 2 [C 24 H 46 N 6 O 2 is (5,16-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.0 7,12 ]docosane-2,13-diyl)diacetamide, L], consists of one independent half of the [Ag(C 24 H 46 N 6 O 2 )] 2+ cation and one nitrate anion. The Ag atom, lying on an inversion centre, has a square-planar geometry and the complex adopts a stable trans-III conformation. Interestingly, the two O atoms of the pendant acetamide groups are not coordinated to the Ag II ion. The longer distance of 2.227 (2) Å for Ag-N(tertiary) compared to 2.134 (2) Å for Ag-N(secondary) may be due to the effects of the attached acetamide group on the tertiary N atom. Two nitrate anions are very weakly bound to the Ag II ion in the axial sites and are further connected to the ligand of the cation by N-HÁ Á ÁO hydrogen bonds. The crystal packing is stabilized by hydrogen-bonding interactions among the N-H donor groups of the macrocycle and its actetamide substituents, and the O atoms of the nitrate anions and of an acetamide group as the acceptor atoms.

Chemical context
Macrocycles with N-substituted groups on the polyaza macrocyclic ring and their transition metal complexes have attracted considerable attention because of their structural and chemical properties, which are different from those of the corresponding unsubstituted macrocyclic systems. Recently, it has been shown that the cyclam (1,4,8,11-tetraazacyclotetradecane) derivatives and their metal complexes exhibit anti-HIV activity (Ronconi & Sadler, 2007;De Clercq, 2010;Ross et al., 2012). These cyclam-based macrocyclic ligands have a moderately flexible structure, and can adopt both planar (trans) and folded (cis) configurations. There are five conformational trans isomers for the cyclam moiety, which differ in the chirality of the sec-NH centers (Choi, 2009). The trans-I, trans-II and trans-V configurations can fold to form cis-I, cis-II and cis-V isomers, respectively (Subhan et al., 2011). The conformation of the macrocyclic ligand and the orientations of the N-H bonds are very important factors for co-receptor recognition. Therefore, knowledge of the conformation and crystal packing of transition metal complexes containing the cyclam ligand has become important in the development of new highly effective anti-HIV drugs that specially target alternative events in the HIV replicative cycle (De Clercq, 2010). Partially N-substituted tetraazamacrocycles and their complexes have been much less widely studied. This may be ISSN 2056-9890 due to the difficulty encountered in the attachment of only one or two pendant arms to the tetraaza macrocycle by several steps and in low yields. The presence of two methyl substituents on the macrocyclic ring carbon atoms next to the secondary amine groups facilitates syntheses, as N-substitution takes place only on the less sterically hindered nitrogen atoms.
The syntheses and crystal structures of transition metal complexes with the constrained cyclam ligand containing two acetamide groups on the nitrogen atoms have received much attention because of the effects of the functional groups on their chemical properties and coordination geometry (Choi et al., 2001a,b,c;Choi & Lee, 2007). The nitrate ion can also coordinate to the transition metal ions in a monodentate, chelating bidentate or bridging bidentate fashion. The oxidation state of the metal, the nature of other ligands and steric factors influence the mode of coordination.
In this communication, we report the synthesis and structural characterization a new silver(II) complex, [Ag(C 24 H 46 -N 6 O 2 )](NO 3 ) 2 , (I) to confirm the conformation and bonding modes of the macrocyclic ligand and the nitrate anions.

Structural commentary
The structural analysis showed the space group to be P1 with Z = 1. The asymmetric unit contains one independent half of the [Ag(C 24 H 46 N 6 O 2 )] 2+ cation and one nitrate anion. The silver(II) cation is situated on a center of inversion in the small triclinic cell, which contains a single silver(II) complex. An ellipsoid plot of the title compound is shown in Fig. 1 along with the atomic numbering scheme. The two methyl groups on the six-membered chelate rings and the two -(CH 2 ) 4 -parts of the cyclohexane backbones are anti with respect to the macrocyclic plane. Two pendant acetamide groups in the Ag II complex molecule are also trans to each other, and thus the macrocyclic skeleton adopts the most stable trans-III (RRSS) conformation. The five-membered chelate rings adopt a gauche, and the six-membered rings are in chair conformations. The Ag II cation is surrounded by a square-planar array of four nitrogen atoms from the secondary and tertiary amines in the macrocycle. Interestingly, the oxygen atoms of the acetamide substituents are not coordinated to the metal center. It is noteworthy that the Zn II , Ni II and Cu II complexes of the same ligand have a tetragonally distorted octahedral environment with the four N atoms of the macrocyclic ligand in equatorial positions and the O atoms of the pendant acetamide groups in axial positions (Choi et al., 2001a,b,c;Choi & Lee, 2007). The Ag-N bond lengths of 2.134 (2) and 2.227 (2) Å from the donor atoms of the macrocycle can be compared to those determined in [Ag(cyclam)](ClO 4 ) 2 [2.158 (2)-2.192 (2) Å ; Ito et al., 1981], [Ag(tmc)](ClO 4 ) 2 [2.194 (2)-2.196 (2) Å ; tmc = 1, 4,8,11-tetramethyl-1,4,8,11tetraazacyclotetradecane;Po et al., 1991] -meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane;Mertes, 1978] and [Ag(3,6,13,12 Moon et al., 2010]. The longer Ag-N(tertiary) bond distance, compared to the length of the Ag-N(secondary) bond may be due to the steric and inductive effects of the pendant acetamide group on the tertiary N atom. The Ag-O distance of 3.109 (2) Å is longer than the corresponding distances in [Ag(cyclam)](ClO 4 ) 2 [2.788 (2) Å ; Ito et al., 1981], [Ag(tmc)](ClO 4 ) 2 [2.889 (4) Å ; Po et al., 1991], [Ag(tet a)](NO 3 ) 2 [2.807 (4) Å ; Mertes, 1978] and [Ag(3,6,13,12 Moon et al., 2010]. The longest N1-C4 bond distance is also probably due to the effect of the acetamide group and the cyclohexane ring. The nitrate anion has a slightly distorted trigonal-planar geometry because of the hydrogen bonding interactions and the very weak interaction with the silver(II) ion. Two nitrate ions are located above and below the coordination planes, and each are linked to the cation via N-HÁ Á ÁO hydrogen bonds.

Supramolecular features
Extensive hydrogen-bonding interactions occur in the crystal structure ( Table 1). The nitrate ions are connected to the ligand of the cation via N-HÁ Á ÁO hydrogen bonds. The nitrate anions have slightly distorted trigonal-planar geometries because of these interactions and the very weak interaction with the silver(II) cation. The supramolecular architecture involves hydrogen bonds between the N-H groups of both the macrocycle and its pendant acetamide substituents as donors, and the O atoms of the nitrate anions and the acetamides as acceptors. An array of these contacts generate a two-dimensional sheet of molecules stacked along the b-axis direction (Fig. 2). This hydrogen-bonded network helps to stabilize the crystal structure.  (Choi et al., 2001c) and [Cu(C 24 H 46 N 6 O 2 )]-(ClO 4 ) 2 (Choi et al., 2001c) have been reported previously. In all of these structures, two O atoms of the acetamide substituents occupy the axial positions, giving rise to a tetragonally distorted octahedral geometry. This is quite unlike the squareplanar geometry of the title compound as the two O atoms of the acetamide substituents are not bound to the silver(II) cation in this case. Until now, no structure of the complex ion [Ag(C 24 H 46 N 6 O 2 )] 2+ with any anion has been reported.

Figure 2
The crystal packing in complex (I), viewed along the b-axis direction. Dashed lines represent N-HÁ Á ÁO hydrogen-bonding interactions.
In the synthesis of the title complex, two pertinent features are found. One is that the complex contains the silver in the unusually high oxidation state, Ag II . This is stabilized by the macrocycle L. The complex is the product of the disproportionation of the Ag I complex according to the following equation: 2Ag I + L ! Ag II L + Ag(s) # It is generally understood that macrocyclic ligands possess a suitable cavity size and hard nitrogen donor atoms that can form stable Ag II complexes in aqueous solution (Ali et al., 2004).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H distances of 0.98-1.00 Å and an N-H distance of 0.88-1.0 Å . All displacement parameters of H atoms U iso (H) were set to 1.2 or 1.5U eq of their respective parent atoms.   (Shin et al., 2016); cell refinement: HKL3000sm (Otwinowski & Minor, 1997); data reduction: HKL3000sm (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics:

Funding information
DIAMOND 4 (Putz & Brandenburg, 2014); software used to prepare material for publication: publCIF (Westrip, 2010). Extinction correction: SHELXL2018 (Sheldrick, 2015b), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.065 (6) 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 atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq  (5)