The synthesis and crystal structure of bis[3,3-diethyl-1-(phenylimino-κN)thiourea-κS]silver hexafluoridophosphate

The distorted title square-planar silver(I) complex was obtained in very good yield after gentle mixing of solutions of the N,N-diethylphenylazothioformamide (ATF) ligand with silver hexafluoridophosphate in tetrahydrofuran. In the crystal, one sulfur atom from an ATF ligand of a neighboring complex coordinates to the silver atom, with a bond distance of 2.9884 (14) Å. This creates a polymeric zigzag chain propagating along the c-axis direction.


Chemical context
The redox-active azothioformamide (ATF) ligand class was identified as a metal coordinative species over 40 years ago (Bechgaard, 1974(Bechgaard, , 1977. These ligands were found to coordinate and solvate late transition metal(0) species, particularly Cu, Pd, Pt, and Ni (Nielsen et al., 2007). Further investigations found that ATF ligands were capable of removing similar late transition metal (Cu or Pd) nanoparticles and catalysts from polymeric materials (Nielsen et al., 2005(Nielsen et al., , 2006. As these ligands are redox-active, it was suggested that, during coordination, the two ligands singly reduce as the metal oxidizes to (+2) and coordinates in a 2:1 fashion of ligands to metal. This observation was confirmed utilizing computational comparisons of crystal structures from the found species and a copper(I) complex (Johnson et al., 2017). Those comparisons led to the discovery that ATF ligands stay neutral when mixed with copper(I) salts behaving as 1:1 species in the presence of halide counter-ions and 2:1 species in the presence of noncoordinating counter-ions (such as BF 4 and PF 6 ). The copper(I) halide coordination complexes crystallize out of concentrated THF solution as dimers yet exhibit 1:1 coordination as observed in titration studies. The importance of understanding the variability in the binding phenomena of the various oxidation states in metals can help determine how and in which oxidation state these ligands can coordinate, solvate and remove metals from materials to allow for higher purity. While most trace-metal removal is accomplished with mineral acids, a mild ligand alternative could allow for the removal of metals from acid sensitive materials such as polymers, pharmaceuticals or APIs, or from metals found in electronic waste (e-waste). Silver(I) catalysts and co-catalysts have become increasingly common over the past twenty years, and with silver a precious metal, the potential value of its recycling following synthetic reactions is worthwhile. The investigation of monovalent metals led to this report describing the coordination complex formed when the N,N-diethylphenylazothioformamide (ATF) ligand is treated with an Ag(I) species containing the non-coordinative counter-ion hexafluoridophosphate in concentrated THF solution.

Structural commentary
The experiment described herein involved the mixing of AgPF 6 with a concentrated THF solution of the ATF ligand at room temperature which yielded the title complex in excellent yield (> 95%).
The molecular structure of the asymmetric unit of the title complex is shown in Fig. 1. Selected bond lengths and bond angles involving atom Ag1 are given in Table 1. The silver(I) atom has a distorted square-planar AgN 2 S 2 coordination geometry with a 4 fourfold parameter of 0.32 ( 4 = 1 for a perfect tetrahedral geometry and 0 for a perfect square-planar geometry. For intermediate structures, including trigonalpyramidal and seesaw, 4 falls within the range of 0 to 1; Yang et al., 2007). Such distorted square-planar silver complexes, once considered rare have become more common (Chowdhury et al., 2003;Ino et al., 2000;Suenaga et al., 2002;Young & Hanton, 2008;Pointillart et al., 2008;Hanton & Young, 2006). These compounds usually require strengthened bonds through polymeric networks and herein we try to rationalize our structure through a similar network.
The crystal structure of the ligand ATF has been reported by Johnson et al. (2017). The ATF ligand-bond distances in the title complex match more closely to the neutral species than the singly reduced ligand as the presence of a PF 6 counter-ion suggests monovalent oxidation of silver. Although the asymmetric unit suggests the 2:1 binding species with two S-Ag and two N-Ag bonds, the N4-Ag1 bond is lengthened in comparison with previously mentioned complexes (Johnson et al., 2017). This lengthening has influenced the packing structure of the crystal to allow for an adjacent ATF ligand to interact with the silver atom at a bond distance Ag1Á Á ÁS2 i of 2.9884 (14) Å , producing a polymeric zigzag chain ( Fig. 2 and Table 1). If atom Ag1 is now considered to be fivefold AgN 2 S 3 coordinate it has a perfect square-pyramidal geometry with a 5 fivefold parameter of 0.04 ( 5 = 1 for perfect trigonalpyramidal geometry and 0 for perfect square-pyramidal geometry; Addison et al., 1984). Sulfur atom S1 is involved in an intramolecular C-HÁ Á ÁS hydrogen bond ( Fig. 1  S1-Ag1-S2 156.32 (6) N1-Ag1-N4 159.01 (10) S1-Ag1-N1 71.07 (7) S2-Ag1-N4 71.38 (7) S1-  109.01 (7) S2-Ag1-N1 117.39 (7) Symmetry code: (i) x; Ày þ 1 2 ; z þ 1 2 .

Figure 1
A view of the molecular structure of the asymmetric unit of the title complex, with atom labeling. Displacement ellipsoids are drawn at the 30% probability level.

Table 2
Hydrogen-bond geometry (Å , ). The bond distances for ATF ligand complexes were compared to computationally modeled neutral and singly reduced ATF species as to ascertain the absolute oxidation state of the ligands (Johnson et al., 2017). The computationally compared neutral ligand necessitated rotation at 1.33 kcal mol À1 to give a transition state containing the planar 1,4-heterodiene motif while the computationally calculated singly reduced ATF ligand flattens to adopt the binding motif. Table 3 provides comparative bond distances for these species to known bis-bidentate ATF copper(I), copper(II), and palladium (II) species that are found as distorted tetrahedral conformations and square-planar nickel(II) and platinum (II) species (Nielsen et al., 2007;Johnson et al., 2017).
Also, to note, is that repeated attempts to create the silver(I) tetrafluoroborate variation were unsuccessful. UV-Vis absorbance in acetonitrile displayed no photophysical properties or effects. The melting point of the complex was found to occur at 329 K, which is similar to the melting point of 325 K for the ligand, further suggesting the weak binding interaction.

Supramolecular features
In the crystal, the polymeric zigzag chains that propagate along the c-axis direction, are linked by C-HÁ Á ÁF hydrogen bonds, forming slabs parallel to the ac plane (Table 3 and Fig. 3).
The two ligands in the title complex crystal are asymmetric in regard to their respective distances to the silver atom from the coordinating sulfur and nitrogen atoms of each ligand and asymmetric in the geometries of the two diethyl thioformamide units on each ligand (Figs. 1 and 2, and Table 1). It is proposed that the interaction between the adjacent sulfur atom to the bis-coordinated silver, as shown in Fig. 2, provides the asymmetry in the binding interaction as the sulfur of the second ATF (that does not conjugate to a bridging silver atom) is slightly closer to its silver atom than the ligand that Table 3 Bond lengths (Å ) and characteristic geometries of related ATF mono-and divalent metal complexes. CSD = Cambridge Structural Database (Groom et al., 2016); DSP = distorted square-planar; DT = distorted tetrahedral.

Figure 3
A view along the c axis of the crystal packing of the title complex. The C-HÁ Á ÁS and C-HÁ Á ÁF hydrogen bonds are shown as dashed lines. For clarity, only the H atoms involved in hydrogen bonding have been included.

Figure 2
A partial view along the b axis of the crystal packing of the title complex. For clarity, the PF 6 anions and the H atoms have been omitted.
contains the polymeric sulfur bridge. The packing structure also displays an alternating coordination throughout the crystalline lattice connecting silver atoms to sulfurs. The distorted square-planar structure is rare in silver(I) systems and it is suggested that the interconnecting sulfur atom ladderlike chain structure strengthens the framework (Shin et al., 2009). Secondly, the second bound ATF ligand displays both ethyl groups in the diethyl group of the thioformamide facing in the same direction instead of opposite directions as seen in the crystal structure of the ligand (Johnson et al., 2017), and thus a higher energy kinetic state (Shin et al., 2009). It is suggested that the large PF 6 counter-ions inhibit the rotation of the second ethyl group so as to allow for more space. Counter-anion influence for silver coordination complexes has been seen in other systems (Zhao et al., 2012;Huang et al., 2008).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The C-bound H-atoms were included in calculated positions and refined as riding on the parent C atom: C-H = 0.93-0.97 Å with U iso (H) = 1.5U eq (Cmethyl) and 1.2U eq (C) for other H-atoms.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq  (7)