Crystal structure of μ-cyanido-1:2κ2 N:C-dicyanido-1κC,2κC-bis(quinolin-8-amine-1κ2 N,N′)-2-silver(I)-1-silver(II): rare occurrence of a mixed-valence AgI,II compound

The title compound, [Ag2(CN)3(C9H8N2)2], is a mixed-valence disilver molecular complex. The Ag+ ion has the expected linear coordination geometry, while the Ag2+ centre is six-coordinated with a distorted [AgN5C] octahedral geometry. This compound belongs to class 1 or class 2 complexes in the Robin–Day classification.


Chemical context
The coordination chemistry of silver is clearly dominated by Ag I complexes. The oxidation state Ag II , with a paramagnetic 4d 9 electronic configuration, is however present in inorganic species like AgF 2 , a compound which readily decomposes in water, and is even able to oxidize SiCl 4 (Grochala & Mazej, 2015). Ag II is also stable in bimetallic perfluorinated compounds Ag II M IV F 6 , with M = Pt, Pd, Ti, Rh, Sn and Pb. In these solids, the Ag II sites are bonded to six F atoms, in an octahedral coordination geometry distorted by the Jahn-Teller effect. In contrast, AgO, precipitated from Ag in presence of K 2 S 2 O 8 in a basic medium, is a diamagnetic mixedvalence Ag I,III oxide, rather than a Ag II compound (Housecroft & Sharpe, 2012). Some actual Ag II coordination complexes may be formed in solution, for example [Ag(bpy) 2 ] 2+ , which follows the Curie law with a magnetic moment close to the spin-only value expected for a d 9 system (Kandaiah et al., 2012).
Recently, polynitrile and cyanidometallate anions have received considerable attention because of their importance in both coordination chemistry and in molecular materials chemistry (Atmani et al., 2008;Benmansour et al., 2008Benmansour et al., , 2009Benmansour et al., , 2012Setifi et al., 2013;Setifi, Lehchili et al., 2014;. In view of the possible roles of these versatile anionic ligands, we have been interested in using them in combination with other chelating or bridging neutral co-ligands to explore their structural and electronic characteristics in the extensive field of molecular materials exhibiting the spin-crossover (SCO) phenomenon (Dupouy et al., 2008Setifi et al., 2009;Setifi, Milin et al., 2014). During the course of attempts to prepare such complexes, using the dicyanidoargentate(I) anion, we isolated the title compound, whose structure is described here.

Structural commentary
The title complex (Fig. 1) is a binuclear silver compound placed in a general position, in which metallic sites present contrasting coordination environments. Ag1 is six-coordinated by two quinolin-8-amine bidentate ligands, one terminal cyanide ligand, and one bridging cyanide ligand. The quinoline ring system N1-C8 is slightly twisted, with a r.m.s. deviation of 0.04 Å , while the other, N11-C18, may be considered as planar (rms deviation: 0.01 Å ). Quinoline ligands are arranged cis in the octahedral coordination polyhedron, and their mean planes make a dihedral angle of 58.71 (5) . The amino groups bonded to C8 and C18 are trans to the cyanide ligands. The octahedral geometry around Ag1 is distorted, mainly because of bite angles for quinoline ligands, N1-Ag1-N9 = 69.59 (7) and N11-Ag1-N19 = 71.29 (7) . The coordination of the terminal cyanide ligand, C20 N21 is through the C atom, as determined from the structure refinement (see Refinement section). This orientation seems to be favored by the availability of atom N21 as an acceptor for hydrogen bonding with symmetry-related molecules in the crystal (Table 1).
Metal site Ag2 has a linear coordination with two cyanide ligands. Both ligands are coordinated through their C atoms (C22 and C24), and the coordination angle C22-Ag2-C24 = 176.05 (11) , close to the ideal angle of 180 expected for an sp hybridization of the metal. Site Ag2 may thus be confidently assigned to a Ag I coordination site, and charge balance for the complex should then set the oxidation state for the octahedral metal as Ag II , with a formal hybridization sp 3 d 2 . The title complex is a mixed-valence compound, with valences localized on a single site. According to the Robin-Day classification (Day et al., 2008), this compound should thus be a class 1 or class 2 mixed-valence compound. The deep-red color of the crystals should be the result of the * 4d(Ag) metal-toligand charge transfer, rather than a consequence of an intervalence charge transfer of a class 2 complex. Indeed, porphyrinato-Ag II compounds are generally purple or red compounds (e.g. Xu et al., 2007).
Cyanide ligand C22 N23 bridges metal sites Ag1 and Ag2, with oxidation states II and I respectively. The best structure refinement shows that this ligand is not disordered: the C atom is bonded to Ag + , and the N atom to the Ag II atom. This orientation observed for the bridge is consistent with the Pearson's HSAB principle (Pearson, 2005). The cyanide Lewis base is considered as a soft ligand, which preferentially forms covalent bonds with soft Lewis acid, like Ag + . However, the heteronuclear nature of this ligand induces an asymmetric character for the softness: based on the absolute electronegativity criterion, the C side of the cyanide ligand is expected to be softer than the N side. On the other hand, regarding the acid component of the coordination bonds, Ag + is expected to be softer than Ag 2+ , due to the charge difference, which makes Ag + more polarizable than Ag 2+ . The most stable acid-base interactions for the bridging mode of ligand C22 N23 is thus Ag + -C N-Ag 2+ , as observed in the X-ray-based structure refinement. From the reactivity point of view, the dicyanidoargentate(I) anion, [Ag(CN) 2 ] À , used as starting material, preserves the C coordination mode for the cyanide groups in the product. This anion thus acts as a ligand to the oxidized Ag II atom formed during the reaction. The The molecular structure of the title complex, with displacement ellipsoids drawn at the 30% probability level. Table 1 Hydrogen-bond geometry (Å , ).

D-HÁ
same C coordination is observed for the terminal cyanide group bonded to Ag 2+ , indicating that this fragment [Ag(CN)] + is also produced from dicyanidoargentate, probably prior to aminoquinoline coordination.

Supramolecular features
As described in the previous section, both terminal cyanide ligands are bonded to Ag1 and Ag2 as C ligands, allowing the N terminus to act as acceptor sites for hydrogen bonding (Ramabhadran et al., 2014). Amino groups of aminoquinoline ligands are the donors for these contacts (Table 1), forming a two-dimensional supramolecular network parallel to (102) (Fig. 2). Molecules are aggregated through a centrosymmetric R 2 4 (8) ring, where the donor group is the terminal cyanide C20/ N21 bonded to Ag1. The same cyanide ligand is engaged in R 1 2 (6) rings, where donors are from two different amino groups. This basic pattern of fused rings propagates in the [010] direction, via larger R 2 2 (10) rings. Finally, these rows of molecules are connected in the crystal via the long arms Ag2-C24 N25, which take part in large R 3 3 (19) rings. The shortest metalÁ Á Ámetal distance is observed in these rings involving Ag + ions: Ag2Á Á ÁAg2 i = 3.9680 (3) Å [symmetry code (i): Àx + 2, y + 1 2 , Àz + 1 2 ]. Although the resulting supramolecular structure is compact, hydrogen bonds, with HÁ Á ÁN contacts in the range 2.19 (3)-2.48 (3) Å , should be considered as interactions of moderate strength. The crystallized compound is an authentic molecular complex, in which the terminal cyanide ligands are not engaged in polymeric bonds.

Database survey
Complexes characterized by X-ray diffraction which include at least one Ag 2+ ion are much less common than Ag + complexes. An estimation using the field 'NAME = silver(II)' or 'NAME = silver(I)' in the current release of the CSD (version 5.36 with all updates; Groom & Allen, 2014), affords 63 and more than 8000 hits, respectively. Within Ag I complexes, the occurrence of the dicyanidoargentate ion is significant. It has been used not only as a counter-ion (e.g. Stork et al., 2005) but also as a ligand for numerous transition-metal ions, including Ag + (Lin et al., 2005).
For non-polymeric compounds, the most common coordination for Ag 2+ is the square-planar [AgN 4 ] arrangement, found in porphyrin derivatives and tetra-aza cyclic ligands (e.g. Xu et al., 2007). However, a few cases of sixcoordinate Ag 2+ species have been characterized, with N-donor ligands (Clark et al., 2009) and S-donor ligands (Shaw et al., 2006). Compounds with both Ag + and Ag 2+ ions which have been X-ray characterized seem to be very scarce. A 1D polymeric mixed-valent Ag I /Ag II polymer was obtained by reacting AgNO 3 , Na 2 S 2 O 8 and pyrazine in a CH 3 CN/H 2 O mixture, and the presence of Ag 2+ was confirmed by ESR (Sun et al., 2010). The two other cases retrieved from the CSD are ionic compounds, in which tetraazacyclotetradecane derivatives coordinate the Ag 2+ ion in a square-planar geometry, while the Ag + ion is present in the anionic polymeric part (Wang & Mak, 2001) or in an anionic cluster (Wang et al., 2002). The title complex is, as far we can see, the first nonpolymeric and non-ionic mixed-valence Ag I,II compound characterized by X-ray diffraction.

Synthesis and crystallization
The title compound was obtained under solvothermal conditions from a mixture of iron(II) sulfate heptahydrate (28 mg, 0.1 mmol), quinolin-8-amine (30 mg, 0.2 mmol) and potassium dicyanidoargentate (40 mg, 0.2 mmol) in water-ethanol (4:1 v/v, 20 ml). The mixture was transferred to a Teflon-lined autoclave and heated at 423 K for 48 h. The autoclave was then allowed to cool to ambient temperature. Deep-red crystals of the title compound were collected by filtration, washed with water and dried in air (yield 30%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Special attention was paid to the accurate orientation for the three cyanide ligands in the asymmetric unit. For each C N group, two refinements were carried out with each possible orientation, and the best model was retained on the basis of R 1 and wR 2 factors, and ADP for the C and N sites. For example, wR 2 for all data rises from 8.78% to ca. 9.30% if one cyanide ligand bonded to Ag2 is inverted. No evidence for disordered cyanido groups was detected in the difference maps. All C-bonded H atoms were placed in calculated positions and refined as riding atoms, with C-H bond lengths fixed to 0.93 Å . Amino H atoms bonded to N9 and N19 were found in a difference map and refined freely. For all H atoms, isotropic displacement parameters were calculated as U iso (H) = 1.2U eq (carrier atom). Part of the crystal structure of the title complex, emphasizing the N-HÁ Á ÁN hydrogen bonds (dashed red lines) forming R rings. The green molecule corresponds to the asymmetric unit.  -cyanido-1:2κ 2 N:C-dicyanido-1κC,2κC-bis(quinolin-8-