catena-Poly[[(nitrito-κ2 O,O′)silver(I)]-μ-1,2-bis[1-(pyridin-4-yl)ethylidene]hydrazine-κ2 N:N′]

The asymmetric unit of the title compound, [Ag(NO2)(C14H14N4)]n, contains half of the repeating formula unit (Z′ = 1/2). The AgI ion lies on a twofold rotation axis. The primary structure consists of a one-dimensional coordination polymer formed by the AgI ions and the bipyridyl azine ligand in which there is an inversion center at the mid-point of the N—N bond. The nitrite anion interacts with the AgI ion through a chelating μ2 interaction involving both O atoms. In the crystal, the coordination chains are parallel and interact through Ag⋯π [3.220 (2) Å] and π–π [3.489 (3) Å] interactions.


Comment
The formation of Ag(I) complexes of "off-axis rod" type bipyridyl ligands has attracted much interest. Partly this is due to the relative ease of crystal formation, as compared to similar systems with other metals, and partly because aggregation of the one-dimensional polymeric chains typically formed is thought to give insight into the formation of more complicated twodimensional or three-dimensional networks (Khlobystov et al., 2001). Previous work on such bipyridyl ligands containing azine chromophores showed that all displayed simple one-dimensional chains based on the coordination of two ligands to each Ag(I) centre in a trans manner (Kennedy et al., 2005). However, the stacking of these chains is not simple -with much variation seen in the interaction types observed. With better coordinating anions Ag···anion interactions were important but Ag···Ag, Ag···solvent, Ag···azine and Ag···π contacts were also observed with little apparent systematic variation. Here we utilize the nitrite anion to limit the number of interchain Ag···anion interactions possible.
[Ag(pyC(Me)N-NC(Me)py)(NO 2 )] n (I) has the expected primary chain structure with each Ag(I) centre forming two dative bonds to pyridyl fragments from two seperate ligands, see Fig 1. However, the nitrite anion also interacts with the Ag(I) centre. Its O,O' chelating geometry appears to be more sterically demanding than that of other anions used with such systems (e.g. NO 3 , ClO 4 , BF 4 and SbF 6 ) and thus the NAgN angle of 142.18 (8) ° is considerably more bent than previously seen (range 167.0 to 180 °, Kennedy et al., 2005).
Whilst the observed chelating nitrite bonding mode is the commonest found in related Ag(I) complexes (see for example Blake et al., 1999;Chen & Mak, 2005;Tong et al., 2002) nitrite can also bridge between Ag(I) centres either through O atom coordination only (Cingolani et al., 1999) or more rarely by also using the central N atom to bind (Flörke et al., 1998). However, in (I) no further interactions are formed by the nitrite anion. Instead the intermolecular network expands through Ag···π interactions. Pyridyl rings lie equidistant above and below the plane of primary coordination (Ag1···C1 iii and Ag1···C1 iv are both 3.220 (2) Å, where iii is 1 + x, y, z and iv is 0.5 -x, y, 0.5 -z). Additionally the coordination chains also form π-π contacts that are within the range normally treated as significant (C3···C5 v = 3.489 (3) Å where v is x -1, y, z) (see Fig. 2 for the crystal packing).

Experimental
The azine ligand and complex (I) were synthesized as described in Kennedy et al. (2005), and crystals were grown by the solvent layering method also described therein.

Refinement
H atoms were placed in calculated positions and refined in riding modes with C-H = 0.98 or 0.95 Å for the CH 3 and CH groups respectively. For the methyl group U iso (H) = 1.5U eq and for CH groups U iso (H) = 1.2U eq of the parent C atoms.  Fig. 1. The molecular structure of (I) extended to show coordination geometry about Ag1. Non-H atoms are drawn as 50% probability displacement ellipsoids. i = 1.5 -x, y, 0.5 -z, ii = -1 -x, -y, -z.

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The 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 > 2σ(F 2 ) is used only for calculating Rfactors(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