Crystal structure of a helical silver(I) coordination polymer based on an unsymmetrical dipyridyl ligand: catena-poly[[silver(I)-μ-N-(pyridin-4-ylmethyl)pyridine-3-amine-κ 2 N:N′] tetrafluoridoborate methanol hemisolvate]

The reaction of AgBF4 with the unsymmetrical ligand N-(pyridin-4-ylmethyl)pyridine-3-amine afforded a helical coordination polymer. The AgI atom adopts a slightly distorted linear coordination geometry. The symmetry-related right- and left-handed helical chains are arranged alternately via Ag⋯Ag and Ag⋯F interactions and π–π stacking interactions, resulting in the formation of a two-dimensional supramolecular network.


Chemical context
In supramolecular chemistry and material science, infinite helical coordination polymers have attracted particular interest for the past two decades because of their fascinating architecture, their similarities to biological systems and their potential applications in catalysis and optical materials (Leong & Vittal, 2011;Wang et al., 2012;Zhang et al., 2009). Despite numerous examples of helical coordination polymers, the rational strategy of construction of helical coordination polymers is still constrained by our poor understanding of the role of the metal ions and spacer ligands. Nevertheless, the combination of a silver ion with a linear coordination geometry and flexible unsymmetrical dipyridyl ligands composed of two terminal pyridines with different substitutednitrogen positions is one of the most promising strategies for achieving helical coordination polymers. Our group and that of Gao have already reported helical coordination polymers obtained through the reactions of silver salts and some unsymmetrical dipyridyl ligands such as N-(pyridin-3-ylmethyl)pyridine-2-amine , N-(pyridin-2ylmethyl)pyridine-3-amine  and N-(pyridin-4-ylmethyl)pyridine-3-amine Lee et al., 2015;Zhang et al., 2013). Herein, we report the crystal structure of the title compound prepared by the reaction of silver tetrafluoridoborate with the unsymmetrical dipyridyl ligand, N-(pyridin-4-ylmethyl)pyridine-3-amine (L), synthesized according to the procedure described by Lee et al. (2013). The structure of the title compound is related to those ISSN 2056-9890 of the Ag I coordination polymers with three different counteranions such as nitrate, perchlorate and trifluoromethanesulfonate Lee et al., 2015;Zhang et al., 2013).

Structural commentary
The molecular components of the title structure are shown in Fig. 1. The asymmetric unit consists of one Ag I ion, one L ligand, one tetrafluoridoborate anion and one half of a methanol molecule. Each Ag I ion is coordinated by two pyridine N atoms from two symmetry-related ligands in a geometry which is slightly distorted from linear [N1-Ag1-N3 = 174.70 (19) ], forming an infinite helical coordination polymer. The helical chain propagates along [010] (Fig. 2) with a pitch length of 15.6485 (14) Å , shorter than that [16.7871 (8) Å ] of the nitrate-containing Ag I coordination polymer reported by . The two pyridine rings coordinating the Ag I ion are tilted by 13.8 (3) with respect to each other. The two pyridine rings in the L ligand are almost perpendicular, the dihedral angle between their mean planes being 89.34 (15) .

Supramolecular features
In the crystal structure, symmetry-related right-and lefthanded helical chains are arranged alternately through AgÁ Á ÁAg A view of the molecular structure of the title compound with the atom numbering. Displacement ellipsoids are drawn at the 30% probability level. The F atoms of the tetrafluoridoborate group are disordered over two sets of sites with refined site-occupancy factors of 0.669 (13) (part A) and 0.331 (13) (part B). The disordered methanol solvent molecule is omitted for clarity. [Symmetry codes: (i) À x + 3 2 , y À 1 2 , À z + 3 2 ; (ii) À x + 3 2 , y + 1 2 , À z + 3 2 .] Table 1 Hydrogen-bond geometry (Å , ).

Database survey
The non-solvated structures of the silver(I) nitrate and perchlorate complexes of the same ligand have been reported by Zhang et al. (2013). Our group has reported the solvated form of the silver nitrate complex with an L ligand . These complexes adopt single-stranded helical structures. Our group has also reported the silver trifluorido-methanesulfonate complex with an L ligand, which displays a double-stranded helical structure (Lee et al., 2015).

Synthesis and crystallization
The N-(pyridin-4-ylmethyl)pyridine-3-amine ligand was synthesized according to a literature method (Lee et al., 2013). X-ray quality single crystals of the title compound were obtained by slow evaporation of a methanol solution of the ligand with AgBF 4 in the molar ratio 1:1.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The methanol solvent molecule resides on an inversion centre. Therefore the C12/O12 atoms were refined at the same sites with site occupancy factors of 0.5 using EXYZ/EADP constraints. All H atoms were positioned geometrically and refined using a riding model, with

catena-Poly[[silver(I)-µ-N-(pyridine-4-ylmethyl)pyridine-3-amine-κ 2 N:N′] tetrafluoridoborate methanol hemisolvate]
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.18 e Å −3 Δρ min = −0.70 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

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