Bis[μ-1,2-diphenyl-N,N′-bis(di-2-pyridylmethyleneamino)ethane-1,2-diimine]disilver(I) bis(hexafluoridophosphate) acetonitrile disolvate

In the centrosymmetric dinuclear title compound, [Ag2(C36H26N8)2](PF6)2·2C2H3N, the Ag+ ion is bound to four N atoms from two 1,2-diphenyl-N,N′-bis(di-2-pyridylmethyleneamino)ethane-1,2-diimine ligands in a distorted tetrahedral geometry. The ligand adopts a twist conformation, coordinating two metal centers by three pyridyl N atoms and one imine N atom and spanning two Ag+ ions, resulting in the formation of a helical dimeric structure.

In the centrosymmetric dinuclear title compound, [Ag 2 (C 36 H 26 N 8 ) 2 ](PF 6 ) 2 Á2C 2 H 3 N, the Ag + ion is bound to four N atoms from two 1,2-diphenyl-N,N 0 -bis(di-2-pyridylmethyleneamino)ethane-1,2-diimine ligands in a distorted tetrahedral geometry. The ligand adopts a twist conformation, coordinating two metal centers by three pyridyl N atoms and one imine N atom and spanning two Ag + ions, resulting in the formation of a helical dimeric structure.

Related literature
For the role of helicity in self-assembly processes in supramolecular chemistry, see: Stefankiewicz et al. (2008). For examples of single-and double-stranded architectures, see: Chowdhury et al. (2003); Stefankiewicz et al. (2008). The basic features to give predictable products have been established, see: Constable et al. (1997). We have previously reported the spontaneous resolution of silver double helicates (Sun et al., 2006) and entanglemental coordination polymers of silver helicates (Sun et al., 2007). For a related structure, see: He et al. (2000). For related literature, see: Beckmann & Brooker (2003).

S1. Comment
Helicity continues to receive considerable attention as it allows for a greater understanding of the self-assembly processes involved in supramolecular chemistry (Stefankiewicz et al., 2008). Many examples of both single-and double-stranded architectures have been reported (Chowdhury et al., 2003;Stefankiewicz et al., 2008). The basic features to give predictable products are established (Constable et al., 1997). We have previously reported the spontaneous resolution of silver double helicates (Sun et al., 2006) and entanglemental coordination polymers of silver helicates (Sun et al., 2007).
The title complex is a double helical silver(I) coordination compound with the similar ligands, benzil dihydrazone-N, N′-(di-2-pyridyl-ketimine) (Fig. 1). In the dimeric double helicate, each silver(I) centre coordinates to one imine nitrogen atoms, two pyridyl N atoms from one ligand and one pyridyl N atom from symmetry related ligand, forming a distorted tetrahedral geometry. Meanwhile, we notice that unlike the structure we reported before (Sun et al., 2007), each ligand coordinates with metal ions by using three pyridyl N atoms and one imine N atom, leaving one pyridyl ring of 2-pyridylketimine uncoordinated. After coordination with Ag ions, the two sets of pyridine rings distorted differently with one set showing the dihedral angles of cca. 70°, and the other displaying the angles with ca. 100°. It is the twisting angle in the latter set that displaces one of the pyridyl groups unfavorable for coordination. An interesting feature of the dication is that the ligand spans both silver ions, but does not wrap around the metal-metal axis as demonstrated by the bis(pyridylmine) Schiff base ligands (He et al., 2000). One of the two ligands pass above the Ag-Ag axis and the other goes beneath, with the [Ag 2 L 2 ] 2+ cation appearing more like a box than a double helix. The close distance of the silver(I) cations (Ag···Ag distance: cca. 5.02Å) seem unfavourable for helicate formation. However, it should be noted that coordination to the metal centers forces helical twisting of the ligand with the torsion angle of cca. 98° about the bond N(1)-C(7)-C(30)-N(8). Two di-2-pyridyl-ketimine moieties are found on the opposite sides of the N(1)-C(7)-C(30)-N(8) fragment, giving rise to a double helix.

S2. Experimental
Preparation of ligand L: An ethanolic solution (5 mL) of di-2-pyridyl-ketone (1.27 g, 8.2 mmol) was added slowly to a ethanolic solution (20 mL) of benzildihydrazone (0.98 g, 4.1 mmol) and the resulting solution was refluxed for four hours. The reaction mixture was condensed and cooled to room temperature. Preparation of the title complex: The ligand L (0.1 mmol, 0.057 g) and AgNO 3 (0.15 mmol, 0.027 g) were mixed in methanol and refluxed for two hours, then added 5 mL acetonitrile solution of KPF 6 , the yellow solution was filtered and evaporated at room temperature. A few days later orange block crystals were obtained.

S3. Refinement
All of the non-hydrogen atoms were refined with anisotropic thermal displacement coefficients. H atoms were placed at calculated positions with C-H = 0.93-0.96Å and included in a riding-model approximation with U iso (H) = 1.2U eq (C).
The order HADD was used to restraint the H atoms.

Figure 1
The double helical title compound showing the atom-numbering for the non-H and non-C atoms only, with the H atoms omitted for clarity, too. Displacement ellipsoids are drawn at the 50% probability level.   (9)