Synthesis and crystal structure of bis(1-{[(quinolin-8-yl)imino]methyl}pyrene-κ2 N,N′)silver(I) trifluoromethanesulfonate

In the title salt, [Ag(qPyr)2]CF3SO3 where qPyr = 1-(quinoline-2-ylmethylene)aminopyrene, the AgI atom exhibits a distorted tetrahedral coordination by two chelating 1-(quinoline-2-ylmethylene)aminopyrene ligands.


Chemical context
Silver metal and its salts have been used for their well known antimicrobial properties since ancient times (Chernousova & Epple, 2013). In recent years, the use of silver has regained interest due to the emergence of multidrug-resistant organisms (MDROs) (Kresse et al., 2007;Liu et al., 2010;Thornton et al., 2016). Silver is primarily used topically to treat chronic infections in burn wounds (deBoer et al., 2015). The metal exerts its microbial toxicity by slowly releasing Ag I ions that inflict damage on cell walls, produce reactive oxygen species and bind to DNA base pairs as well as proteins, impeding normal cellular functions (Liu et al., 2010;Thornton et al., 2016). As silver ions tend to precipitate as AgCl in the presence of blood plasma chloride (Chernousova & Epple, 2013), there is a need for stable silver complexes that can slowly and sustainably release silver ions into biological matrices. Herein we report the synthesis and characterization of a novel silver complex, [Ag(qPyr) 2 ]CF 3 SO 3 [where qPyr = 1-(quinoline-2-ylmethylene)aminopyrene] which could serve as a stable complex for the delivery of silver. In the design of this compound, qPyr was included due to its characteristic absorption and emission profile, which could allow tracking of the ligand and silver within the cell membrane of the bacteria (Ray et al., 2006).

Structural commentary
The molecular structure of the cation in the title complex is shown in Fig. 1. The coordination environment of the Ag I atom in the cationic complex is distorted tetrahedral ( Table 1). The qPyr ligand binds to the metal in a bidentate fashion. In this complex, the chelate rings composed of atoms Ag1, N2, C8, C9, N1 and Ag1, N4, C34, C35, N3 are reasonably planar, ISSN 2056-9890 with mean deviations of 0.054 (3) and 0.059 (3) Å , respectively. The dihedral angle between these two chelate planes is 69.0 (4) . The two quinoline fragments within the qPyr ligand in the title complex are satisfactorily planar, with mean deviations of 0.031 (4) and 0.035 (4) Å . The dihedral angles between the quinoline moieties and the pyrene rings are quite similar [73.5 (4) and 73.8 (3) ].

Supramolecular features
The packing pattern exhibits the presence of both intra-and intermolecular offsetstacking interactions (Figs. 2 and 3). The extent of the intermolecularinteraction is found to be relatively stronger [3.543 (5) Å ] compared to the intramolecularstacking interactions [3.642 (5) and 3.617 (5) Å ]. In both cases, the angle between the ring normal and the vector between the ring centroids is close to 20 and centroid-to-centroid distances are within the upper limit of 3.8 Å (Janiak, 2000). The crystal packing of the complex reveals also a non-classical hydrogen-bonding interaction (Steiner, 1996)

Figure 1
The molecular structure of the cation in the title salt. Displacement ellipsoids correspond to the 50% probability level; the counter-anion is not shown.

Figure 2
Representation of intramolecularstacking within the title complex.

Figure 3
Representation of intermolecularstacking within the title complex.
the triflate anion (Table 2, Fig. 4). The arrangement of the two types of molecules along the c axis is shown in Fig. 5.

Figure 5
Packing diagram of the title salt along the c axis.

Synthesis and crystallization
Synthesis of the qPyr ligand A solution of 1-pyrenecarboxaldehyde (115 mg, 0.50 mmol) in 10 ml of dichloromethane was added drop wise to a solution of 8-aminoquinoline (72 mg, 0.50 mmol) in 10 ml of methanol. The mixture was heated to reflux for 16 h and then concentrated under reduced pressure. The precipitate thus formed was collected by vacuum filtration affording 162 mg (91% yield) of N-(1-pyrene)-1-quinolin-2-ylmethanimine (qPyr) as a light-brown powder.
Synthesis of the title complex Two equivalents of qPyr (100 mg, 0.28 mmol) were dissolved in 20 ml of 1:1 methanol:dichloromethane along with one equivalent of silver trifluoromethanesulfonate (36 mg, 0.14 mmol). The reaction mixture was then stirred for 12 h. After this time, the solution was concentrated under reduced pressure. The resulting precipitate was collected through vacuum filtration affording a light-yellow powder. This powder was recrystallized from methanol to obtain [Ag(qPyr) 2 ]CF 3 SO 3 as a light yellow-brown powder (124 mg, 91%). Single crystals were obtained by vapor diffusion of ethyl ether into a solution of [Ag(qPyr) 2 ]CF 3 SO 3 in methanol.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were included in calculated positions on the C atoms to which they are bonded, with C-H = 0.93 Å and U iso (H) = 1.2U eq (C).   program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: XP in SHELXTL (Sheldrick, 2008) and CrystalMaker (Palmer, 2014); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

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.