The Ag(I) coordination networks, {[Ag(pyrazine)]NO₂}_∞ and {[Ag(4,4′-bipy)]NO₂}_∞ have been constructed in order to investigate the effect of anion upon network topology; an unusual sawhorse connection is observed in the structure of {[Ag(pyrazine)]NO₂}_∞ with the nitrite anion acting to ‘block’ cis coordination sites. The preparation of extended networks using inorganic coordination polymers has become an area of increasing study in recent years One of the reasons that this interest has arisen is because the synthetic procedure used to construct these materials allows a high degree of design. Ultimately this may lead to the development of materials with tuneable properties including structures with host–guest properties similar to those observed with zeolites and compounds with interesting electronic or magnetic properties. The high degree of design arises from the coupling of the well understood coordination properties of individual metal ions and highly-developed ligand syntheses with the newer areas of supramolecular chemistry and crystal engineering We have been studying the effect of individual building-blocks upon network structure. This has included the control of network topology and interpenetration in adamantoid networks via ligand desig and studies on the effect of variation of solvent of crystallisation upon network structure The nature of the counter-anion has also been shown to have a dramatic effect upon network topology and this is particularly noticeable in Ag(I) chemistry Recently we have demonstrated that replacement of AgBF₄ or AgPF₆ with AgNO₃ results in a fundamental change of the extended structure of a coordination polymer with the ligand 3,6-di-4-pyridyl-1,2,4,5-tetrazine due to interactions between the Ag(I) centre and the NO₃^- anion to give a ‘helical staircase’ structure We now report the extension of these investigations to the use of AgNO₂, and report an unusual example of a sawhorse connection within an extended network.

{[Ag(pyz)]NO₂}_∞ (pyz=pyrazine) and {[Ag(4,4′-bipy)] NO₂}_∞ (4,4′-bipy=4,4′-bipyridyl) were prepared as colourless microcrystalline samples by adding a solution of the appropriate ligand in EtOH to a solution of AgNO₂ in H₂O.† In order to assess the effect of the nitrite counter-anion upon network topology single crystals of both complexes were grown by slow diffusion between an aqueous solution of AgNO₂ and an ethanolic solution of the ligand. {[Ag(pyz)]NO₂}_∞‡ exists as a three-dimensional network in which each Ag(I) ion adopts a distorted octahedral environment (Fig. 1): each Ag(I) centre is coordinated by two pyrazine ligands, Ag–N 2.277(5) Å, which bridge adjacent Ag(I) ions, and by the chelating nitrite counter-anion, Ag–O 2.487(6) Å, in the equatorial sites. The two remaining axial coordination sites are occupied by weak Ag···Ag interactions (Fig. 1). The Ag···Ag separation of 3.2168(3) Åis a typical value for Ag···Ag interactions unsupported by ligands Ag···Ag interactions have been found to be significant in the extended structures of inorganic supramolecular networks. This is perhaps best illustrated by the formation of short interactions [Ag···Ag 2.970(2) ÅL in the extended three-dimensional network formed by {[Ag(4,4′-bipy)]NO₃}_∞ in which each Ag(I) centre is coordinated by one N-donor from each of two 4,4′-bipy ligands and participates in one Ag···Ag interaction to give a T-shaped motif

It can be seen that in {[Ag(pyz)]NO₂}_∞ each Ag(I) ion acts as a sawhorse junction in the network, with the nitrite blocking two cis sites of the junction (Fig. 2). To our knowledge this represents the first example of such a junction within a coordination polymer array. Sawhorse junctions are extremely rare in inorganic framework structures with the most notable examples being IrF₄, RhF₄ and PtF₄ Therefore the overall network topology (Fig. 2) can be thought of as being related to the solid-state structure of IrF₄ which has been described as ‘{IrF₆} octahedra which share 4 F atoms, each with one other {IrF₆} group leaving a pair of cis vertices unshared’ Similarly the network of {[Ag(pyz)]NO₂}_∞ is built from octahedra with cis vertices unshared.

The structure of {[Ag(pyz)]NO₂}_∞ contrasts with that observed in the corresponding NO₃^- salt, {[Ag(pyz)]NO₃}_∞ Extended chains of alternating Ag(I) ions and pyrazine ligands are observed in {[Ag(pyz)]NO₃}_∞¹² and significantly the nitrate anion is non-coordinating, in contrast to the behaviour of the nitrite anion observed in {[Ag(pyz)]NO₂}_∞.

Single crystal X-ray studies of {[Ag(4,4′-bipy)]NO₂}_∞‡ reveal that a different network structure is adopted to that observed for {[Ag(pyz)]NO₂}_∞. The structure consists of slightly distorted linear chains of alternating Ag(I) ions and 4,4′-bipy ligands, N–Ag–N 171.98(10)° (Fig. 3) with the NO₂^- anions sitting between adjacent {[Ag(4,4′-bipy)]⁺}_∞ chains so that each Ag(I) centre forms two weak Ag···O interactions of 2.667(2) Åand one weak Ag···N interaction of 2.978(3) Å. Significantly, no Ag···Ag interactions are observed in {[Ag(4,4′-bipy)]NO₂}_∞, in contrast to {[Ag(pyz)]NO₂}_∞. The bridging 4,4′-bipy ligands in {[Ag(4,4′-bipy)]NO₂}_∞ adopt a very twisted arrangement, with a dihedral angle between the pyridyl rings of 41.3°. This value compares with observed values of 4.3° in [{Cu(cnge)₂}₂(µ-4,4′-bipy)][BF₄]₂¹³ (cnge=2-cyanoguanidine), and 28.0, 30.0° observed in [Cu(4,4′-bipy)(MeCN)₂]BF₄ Both twisted and flat 4,4′-bipy molecules are incorporated in [Cu(µ-4,4′-bipy)- (H₂O)₂(FBF₃)₂]·4,4′-bipy, with a dihedral angle of 9.29° being observed for the coordinated 4,4′bipy ligands. In contrast the non-coordinated 4,4′-bipy ligands are constrained to be ideally planar by crystallographic symmetry

The IR spectra of the two complexes {[Ag(pyz)]NO₂}_∞ and {[Ag(4,4′-bipy)]NO₂}_∞ are consistent with the non-coordinating nitrite anion in {[Ag(4,4′-bipy)]NO₂}_∞ [ν_sym(NO₂)=1243 cm^-1] and the chelating mode of coordination for the anion in {[Ag(pyz)]NO₂}_∞ [ν_sym(NO₂)=1269 cm^-1] IR spectra of both the precipitated (microcrystalline) products and single crystals were found to be identical confirming the crystal structures to be representative of the bulk.

Of the few previously reported examples of structurally characterised AgNO₂ complexes both strongly coordinate and uncoordinated/weakly interactin NO₂^- have been reported. In the former Ag–O bond lengths are comparable to those observed here

Current work is aimed at studying the wider application of the nitrite anion as a fundamental building-block of extended coordination polymers and investigating the use of anions as a controlling factor in Ag(I) supramolecular networks. Acknowledgements