Crystal structures of 2,2′-bipyridin-1-ium 1,1,3,3-tetracyano-2-ethoxyprop-2-en-1-ide and bis(2,2′-bipyridin-1-ium) 1,1,3,3-tetracyano-2-(dicyanomethylene)propane-1,3-diide

In each of the title compounds, the anion shows evidence of extensive electronic delocalization. A combination of N—H⋯N and X—H⋯N hydrogen bonds links the ions in (I) into a ribbon of alternating centrosymmetric (18) and (26) rings, and those in (II) into simple (7) chains of alternating cations and anion with further cations pendent from the chain.


Chemical context
Polynitrile anions have received considerable attention recently because of their importance in both coordination chemistry and in molecular materials chemistry (Miyazaki et al., 2003;Batten & Murray, 2003;Benmansour et al., 2007;Setifi, Domasevitch et al., 2013;Setifi, Lehchili et al., 2014). These organic anions are interesting for their extensive electronic delocalization, and for their structural versatility, in particular the potential to utilize a variety of coordination modes, including their action as bridging ligands between metal centres in 2 -, 3 -or 4 -modes, so forming polymeric assemblies which can be one-, two-or three-dimensional. Thus such anions readily form binary complexes with transition-metal and ternary complexes in which a transition-metal centre is also coordinated by other ISSN 2056-9890 bridging or chelating ligands, and such materials exhibit interesting magnetic properties (Atmani et al., 2008;Benmansour et al., 2008Benmansour et al., , 2010Benmansour et al., , 2012Setifi et al., 2009).
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, Charles et al., 2014). During the course of attempts to prepare such complexes, using the anions 1,1,3,3-tetracyano-2-ethoxypropenide (tcnoet) and tris(dicyanomethylene)methanediide (tcpd), we isolated the two title compounds whose structures are described here.

Structural commentary
Compound (I) consists of a 2,2 0 -bipyridin-1-ium cation and a 1,1,3,3-tetracyano-2-ethoxypropenide anion in which the C atoms of the ethyl group are disordered over two sets of sites having occupancies 0.634 (9) and 0.366 (9). In the selected asymmetric unit for (I) (Fig. 1) the two ions are linked by an N-HÁ Á ÁN hydrogen bond (Table 1). For compound (II), which consists of two 2,2 0 -bipyridin-1-ium cations and a single tris(dicyanomethylene)methanediide dianion, it was possible to select an asymmetric unit (Fig. 2) in which the two cations are both linked to the anion by N-HÁ Á ÁN hydrogen bonds (Table 2), although an asymmetric unit selected in this way does not fit neatly into the reference unit cell. It will be convenient to refer to the cations of compound (II) containing the atoms N11 and N31 as cations of types 1 and 2 respectively.

Figure 1
The independent ionic components of compound (I) showing the atomlabelling scheme and the N-HÁ Á ÁN hydrogen bond within the selected asymmetric unit. Displacement ellipsoids are drawn at the 30% probability level.

Figure 2
The independent ionic components of compound (II) showing the atomlabelling scheme and the N-HÁ Á ÁN hydrogen bonds within the selected asymmetric unit. Displacement ellipsoids are drawn at the 30% probability level. Table 2 Hydrogen-bond geometry (Å , ) for (II). angles for the type 1 and 2 cations of compound (II) are 10.92 (17) and 7.7 (2) respectively. Although each cation contains a short intra-cation N-HÁ Á ÁN contact (Tables 1 and  2), the very small N-HÁ Á ÁN angles indicate that these contacts are unlikely to be of structural significance (cf. Wood et al., 2009). In the anion of compound (I), the central bonds C31-C32 and C32-C33 have lengths which are equal within experimental uncertainly (Table 3). In addition, the four C-C bonds linking the cyano substituents to the central propenide unit are not only similar in length, but all of them are short for their type [mean value (Allen et al., 1987) 1.431 Å , lower quartile value 1.425 Å ]; on the other hand, the C-N distances are all similar and long for their type (mean value 1.136 Å , upper quartile value 1.142 Å ). These observations point to extensive delocalization of the negative charge in the anion of (I) with the forms (A)-(F) (see scheme below) all playing a role in the overall electronic structure. Accordingly, the N-HÁ Á ÁN hydrogen bond linking the two ions within the selected asymmetric unit of (I) is a charge-assisted hydrogen bond (Gilli et al., 1994). The tetracyanopropenide fragment of this anion is not planar: the two C(CN) 2 units are twisted out of the plane of the central C 3 O core in a conrotatory fashion, and the dihedral angles between the planes of the C(CN) 2 units and that of the central core are 10.60 (6) and 12.44 (4) respectively for the two units containing atoms C31 and C33.
In the anion of compound (II), the geometry at the central atom C5 (Fig. 2) is planar, and the three C-C bonds involving atom C5 are similar in length ( Table 4). Each of the independent C(CN) 2 units is rotated out of the plane of the central four-atom core, with dihedral angles between the planes of these three units and that of the central core of 23.8 (3), 27.0 (3) and 27.4 (2) , respectively, for the C(CN) 2 units containing atoms C51, C52 and C53. These rotations are in a concerted sense, giving approximate molecular, but not crystallographic, symmetry of D 3 (32) type for the anion. Although the bond distances involving the cyano substituents show some variations (Table 4) the approximate overall D 3 symmetry is consistent with delocalization of the two negative charges over the whole anion, particularly into the cyano groups.

Supramolecular interactions
The supramolecular assembly in compound (I) is determined by the linkage of the ion pairs, themselves internally linked by an N-HÁ Á ÁN hydrogen bond (Fig. 1) Table 3 Selected geometric parameters (Å , ) for (I).
À156.0 (4) C53-C5-C52-C522 27.5 (6) protonated pyridyl ring (Table 1), and both of which therefore can be regarded as charge-assisted hydrogen bonds. The hydrogen bond having atom C13 as the donor links ion pairs related by translation, forming a C 2 2 (12) (Bernstein et al., 1995) chain running parallel to the [111] direction (Fig. 3). The hydrogen bond having atom C16 as the donor links ion pairs related by inversion, forming a centrosymmetric R 4 4 (18) motif (Fig. 3). The combination of these two interactions generates a ribbon running parallel to [111] in which R 4 4 (18) rings centred at (n À 1 2 , n, n À 1 2 ) alternate with R 4 4 (26) rings centred at (n, n + 1 2 , n), where n represents an integer in both cases (Fig. 3). A single ribbon of this type passes through each unit cell. The crystal structure of compound (I) contains no C-HÁ Á Á hydrogen bonds, but there is a single rather weakstacking interaction between components of adjacent ribbons. The planes of the protonated pyridyl ring of the reference cation and of the unprotonated ring of the cation at (Àx, 1 À y, 1 À z) make a dihedral angle of 2.11 (7) : the ring-centroid separation is 3.7395 (8) Å and the shortest perpendicular distance from the centroid of one ring to the plane of the other is 3.3413 (5) Å , corresponding to a ring-centroid offset of ca 1.65 Å , so that there is only a very modest overlap of the two rings in question (Fig. 4). If this interaction is regarded as structurally significant, its effect is to link the ribbons (Fig. 3) into a sheet parallel to (110).
Despite the presence of three independent ions in the structure of compound (II), the supramolecular assembly in (II) is somewhat simpler than that in (I). Ion triplets (Fig. 2) which are related by the c-glide plane at y = 0.75 are linked by a C-HÁ Á ÁN hydrogen bond (Table 2), forming a C 1 2 (7) chain running parallel to the [001] direction (Fig. 5). This chain comprises alternating anions and type 2 cations, while the type 1 cations are simply pendent from the chain. Two chains of this type, related to one another by inversion, pass through each unit cell but there are no direction-specific interactions between adjacent chains. Hydrogen bonds of the C-HÁ Á Á type are absent from the crystal structure of compound (II) and the onlystacking interaction lies within the hydrogen-bonded chain.

Database survey
We have recently reported the structures of several salts containing the 2-ethoxy-1,1,3,3-tetracyanopropenide anion, including salts with the bis(2,2 0 -bi-1H-imidazole)copper(II) cation ( Part of the crystal structure of compound (I) showing the overlap between pairs of inversion-related cations, viewed normal to the ring planes. For the sake of clarity, the unit-cell outline, the anions, and H atoms bonded to C atoms in the cations have all been omitted. Atoms marked with an asterisk (*) are at the symmetry position (Àx, 1 À y, 1 À z).

Figure 3
Part of the crystal structure of compound (I) showing the formation of a hydrogen-bonded ribbon parallel to [111] in which centrosymmetric R 4 4 (18) and R 4 iron(II) , with the 1,1 0 -diethyl-4,4 0bipyridine-1,1 0 -diium dication (Setifi, Lehchili et al., 2014) and with tris(2,2 0 -bipyridine)iron(II) . In each of these salts, the cyano substituents in the anion adopt a very similar conformation to that observed here in compound (I) with, in each case, a similar pattern of bond distances and hence of electronic delocalization. Despite the disparate nature of the counter-ions, the anion conformation is almost constant, suggesting that this is determined primarily by intraanion forces, rather than by inter-ion interactions. The structures of two organic salts containing the 2-dicyanomethylene-1,1,3,3-tetracyaopropenediide anion have been reported. In both the N,N 0 -dimethyl-4,4-bipyridindiium salt [CSD (Groom & Allen, 2014) refcode BELTER; Nakamura et al., 1981)] and the bis(quinolinium) salt (CSD refcode QUCNPR10; Sakanoue et al., 1971) the anion adopts a conformation having approximately D 3 symmetry, just as found in compound (II) reported here: indeed, the anion in QUCNPR10 lies across a twofold rotation axis in space group Pbcn, so that while two of the twofold rotation axes are only approximate, the third is a crystallographic axis. As in compound (II), the C-C and C-N distances in the anions in both BELTER and QUCNPR10 show a degree of variation, but again the approximate symmetry is consistent with extensive electronic delocalization. The structures of the isomorphous salts of this anion with the cations [Ca(H 2 O) 6 ] 2+ (CSD refcode CAHCYB; Bekoe et al., 1967) and [Ba(H 2 O) 6 ] 2+ (CSD refcode BACMCP; Bekoe et al., 1963) have been determined, but no atomic coordinates are deposited in the CSD. A number of salts containing the 2,2 0 -bipyridin-1-ium cation with a range of organic anions have been structurally analysed, but more relevant to the present study are three salts of this cation with simple inorganic anions. In the hydrated monobromide (Bowen et al., 2004), the bromide ions and the water molecules are linked by O-HÁ Á ÁBr hydrogen bonds, forming C 1 2 (4) chains to which the cations are linked by N-HÁ Á Á O hydrogen bonds. In the thiocyanate salt, in which the cations are disordered over two sets of atomic sites (Kavitha et al., 2006), the ions are linked by a combination of N-HÁ Á ÁN and C-HÁ Á ÁN hydrogen bonds, forming C 1 2 (6) chains, while in the hydrogensulfate salt a combination of five independent hydrogen bonds links the ions into complex sheets (Kavitha et al., 2006).

Synthesis and crystallization
The salts K(tcnoet) and K 2 (tcpd) were prepared using published methods . Compounds (I) and (II) were prepared under solvothermal conditions in Teflon-lined steel autoclaves (inner volume ca 30 cm 3 ). For the synthesis of salt (I), a mixture of iron(II) sulfate heptahydrate (28 mg, 0.1 mmol), 2,2 0 -bipyridine (16 mg, 0.1 mmol) and Ktcnoet (45 mg, 0.2 mmol) was dissolved in water-ethanol (4:1 v/v, 15 cm 3 ) and then held in the autoclave at 393 K for 3 d. After slowly cooling to room temperature, pale-orange crystals of (I) suitable for single-crystal X-ray diffraction were obtained (yield 15%). The synthesis of (II) was similar to that of (I), but using K 2 tcpd (50 mg, 0.2 mmol) instead of K(tcnoet), giving yellow crystals suitable for single-crystal X-ray diffraction (yield 40%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. All H atoms in the cations were located in difference maps. The H atoms bonded to C atoms in the cations were then treated as riding atoms in geometrically idealized positions with C-H distances 0.95 Å and U iso (H) = 1.2U eq (C): for H atoms bonded to N atoms, the atomic coordinates were refined with U iso (H) = 1.2U eq (N), giving the N-H distances shown in Tables 1 and 2. It was apparent from an early stage that the ethoxy substituent in the anion of compound (I) was disordered over two sets of atomic sites having unequal occupancy. For the minor occupancy component, atoms O341, C341 and C342 (see Fig. 1), the bonded distances and the one angle non-bonded distances were constrained to be identical to the corresponding distances in the major component, atoms O321, C321 and C322, subject to s.u. values of 0.005 and 0.01 Å respectively. In addition, the atomic coordinates and anisotropic displacement parameters of atoms O321 and O341 were constrained to be identical. Subject to these conditions, the site occupancies refined to values of 0.634 (9) and 0.366 (9). The H atoms in the disordered ethyl group of the anion in compound (I) were included in calculated positions with C-H distances of 0.98 Å with U iso (H) = 1.5U eq (C) for the methyl groups, which were permitted to rotate but not to tilt, and C-H distances of 0.99 Å with U iso (H) = 1.2U eq (C) for the CH 2 groups.   SHELXL2014 (Sheldrick, 2015); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015) and PLATON (Spek, 2009).