Six tris(bipyridyl)iron(II) complexes with 2-substituted 1,1,3,3-tetracyanopropenide, perchlorate and tetrafluoridoborate anions; order versus disorder, hydrogen bonding and C—N⋯π interactions

The structures are reported of six racemic tris(bipyridyl)iron salts with a range of 2-substituted-1,1,3,3-tetracyanopropenide anions, mostly also containing either perchlorate or tetrafluoridoborate as co-anions. In three of the compounds the polynitrile anions are fully ordered, and in three others they are disordered, while the co-anion is also ordered in three compounds, but disordered in two others. Supramolecular assemblies range from no continuous aggregation up to a three-dimensional hydrogen-bonded framework structure.

The polynitrile anions all have the constitution 1,1,3,3tetracyano-2-X-propenide (tcnX), and it will be convenient to use abbreviations as follows: X = OMe, tcnome; X = OEt, tcnoet; X = OPr, tcnopr; X = SEt, tcnset; X = SPr, tcnspr (cf Scheme). The compounds were all prepared using solvothermal reactions between mixtures of iron(II) salts, a 2,2 0bipyridine and polynitrile salts of the type K(tcnX), where the substituent X is as defined above. The independent ionic components in compound (IV), showing the atomlabelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Figure 5
The independent ionic components in compound (V), showing the atomlabelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Figure 6
The independent ionic components in compound (VI), showing the atomlabelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Figure 3
The independent ionic components in compound (III), showing the atomlabelling scheme. Displacement ellipsoids are drawn at the 30% probability level.
2,2 0 -bipyridine, and in compounds (III)-(VI), it is 5,5 0 -dimethyl-2,2 0 -bipyridine. In compound (I) there are two propenide anions, along with a water molecule having occupancy 0.776 (6); in compound (II), there is a single propenide anion and a perchlorate ion, while in each of (III)-(VI) there is a single propenide anion and a tetrafluoridoborate ion. All of the compounds crystallize in centrosymmetric space groups (Table 3), so that they contain equal numbers of cations having the Á and Ã configurations: in each case the reference cation was selected to be the one having the Á configuration.
In several of the compounds, the anions exhibit disorder. One of the propenide anions in compound (I), that containing atom O721 (Fig. 1) exhibits disorder of one of the C(CN) 2 units over two orientations with occupancies which refined to values which are equal within experimental uncertainly, 0.501 (7) and 0.499 (7), while the other anion, containing atom O821, exhibits whole anion disorder, again over two sets of atomic sites with refined occupancies 0.502 (2) and 0.498 (2): all of these occupancies were therefore set to 0.5. In compound (II), the propenide anion exhibits whole anion disorder over two sets of atomic sites with occupancies 0.754 (2) and 0.246 (2), while the disorder of the perchlorate anion was modelled using three sets of sites having occupancies 0.439 (3), 0.377 (3) and 0.184 (3).
The propenide anion of compound (III) is fully ordered, but the tetrafluoridoborate anion is disordered over two sets of atomic sites with occupancies 0.671 (4) and 0.329 (4): there is also an ethanol molecule present in the structure of (III) with occupancy 0.926 (5). There is no detectable disorder in the isostructural compounds (IV) and (V), but in compound (VI) the propenide anion is disordered over two sets of atomic sites with occupancies 0.508 (6) and 0.492 (6).
It is interesting to note that the polynitrile anions in compounds (II)-(V) are fully ordered while those in compounds (I), (II) and (VI) are disordered, and it is tempting to look to the direction-specific interionic interactions involving these ions for clues to the differences in behaviour. However, in (III)-(V) each of the ordered polynitrile anions only participates in a single hydrogen bond (Table 1), as is the case also for the disordered anion in (VI), whereas in both (I) and (II) the polynitrile anion participates in a large number of hydrogen bonds: in (I), also one of the C(CN 2 ) units in each orientation is involved, but in (II) both C(CN 2 ) units in both orientations are involved in hydrogen bonds, thus tethering these anions at both ends. Hence, no plausible explanation of polynitrile order versus disorder can be gleaned from hydrogen bonding: nor do the C-NÁ Á Á contacts provide any explanation, as there are more of these in (II) than in (III), while such short contacts are absent from the structures of (I) and (IV)-(VI).
The Fe-N distances in compounds (I)-(VI) all lie within a narrow range of less than 0.03 Å , with extreme values of 1.9579 (12) Å in (V) and 1.985 (3) Å in (III). These values indicate, in each compound, the presence of low-spin Fe II ; in comparable high-spin complexes, the Fe-N distances are always around 2.15 Å (Orpen et al., 1989).

Supramolecular features
With the exception of the isostructural pair of compounds (IV) and (V), the analysis of the supramolecular assembly is generally complicated by the various forms of anion disorder.
The supramolecular aggregation in compounds (I)-(VI) depends upon hydrogen bonds of a number of different types (Table 1); nearly all of the hydrogen bonds involve a donor from the cation and an acceptor from one of the anions, and so these may be regarded as charge-assisted hydrogen bonds (Gilli et al., 1994). The links between the cations and the polynitrile anions are based on C-HÁ Á ÁN hydrogen bonds, augmented in compounds (II) and (III) by C-NÁ Á Á interactions (Table 2). C-HÁ Á ÁO hydrogen bonds are present in the perchlorate salt (II) and C-HÁ Á ÁF hydrogen bonds in the salts (III)-(VI). In addition, the partial hydrate (I) contains a C-HÁ Á ÁO hydrogen bond together with O-HÁ Á ÁN hydrogen bonds involving just one of the two independent polynitrile anions; by contrast the partial ethanol solvate (III) contains just one O-HÁ Á ÁN hydrogen bond linking the ethanol component to the ordered polynitrile anion.
In compound (I), the independent components are linked by a substantial number of hydrogen bonds, six of which lie within the selected asymmetric unit (Fig. 1, Table 1), to form a three-dimensional framework structure, whose formation can be readily analysed in terms of three simpler sub-structures (Ferguson et al., 1998a,b;Gregson et al., 2000): it will be convenient to refer to the anions containing atoms O721 and O821 as anions 1 and 2 respectively. Aggregates consisting of the cation, anion 2 and the water component, which are related by the 2 1 screw axis along ( 1 2 , y, 1 4 ) are linked to form a complex chain running parallel to the [010] direction (Fig. 7), while similar aggregates which are related by the c-glide plane at y = 1 form a second, equally complex chain running parallel to the [001] direction (Fig. 8). The combination of these two chain motifs gives rise to a sheet structure lying parallel to (100) and adjacent sheets are linked by a centrosymmetric motif involving only the cations and the type 2 anions (Fig. 9). Part of the crystal structure of compound (I) showing the formation of a hydrogen-bonded chain running parallel to the [010] direction. For the sake of clarity, the type 1 anion and the H atoms not involved in the motif shown have been omitted.

Figure 10
Part of the crystal structure of compound (III) showing the formation of a hydrogen-bonded C 2 2 (12) chain running parallel to the [001] direction. For the sake of clarity, the tcnome anion, the ethanol component and the H atoms not involved in the motif shown have been omitted.

Figure 9
Part of the crystal structure of compound (I) showing the formation of the hydrogen-bonded ring motif, which links the (100) sheets. For the sake of clarity, the type 1 anion and the water molecule, the H atoms not involved in the motif shown, and the unit-cell outline have all been omitted. The Fe atom marked with an asterisk (*) is at the symmetry position (1 À x, 1 À y, 1 À z).

Figure 8
Part of the crystal structure of compound (I) showing the formation of a hydrogen-bonded chain running parallel to the [001] direction. For the sake of clarity, the type 1 anion and the H atoms not involved in the motif shown have been omitted.
Despite the disorder, the cooperative action of the hydrogen bonds leads to a coherent three-dimensional structure.
In compound (II), the occupancies of the tcnspr anion, 0.754 (2) and 0.246 (2), mean that interactions involving only the minor component can probably be ignored from the point of view of the supramolecular aggregation; in any event, of the C-HÁ Á ÁN contacts, only that within the selected asymmetric unit has a D-HÁ Á ÁA angle greater than 140 , so that the others can probably be discounted as structurally unimportant (Wood et al., 2009). All of the disorder components of the perchlorate anion have occupancies significantly less than 0.5, and the interactions involving these do not lead to any continuous aggregation.
The partial-occupancy ethanol component in compound (III) is linked to the tcnome anion by an O-HÁ Á ÁN hydrogen bond, but these two components play no further role in the supramolecular assembly: it seems likely that the ethanol component is present primarily in a space-filling role. The cation and the major disorder component of the tetrafluoridoborate anion are linked by a C-HÁ Á ÁF hydrogen bond within the selected asymmetric unit and bimolecular aggregates of this type which are related by translation are linked to form a C 2 2 (12) (Bernstein et al., 1995) chain running parallel to the [001] direction ( Fig. 10): this will be an interrupted chain because of the disorder exhibited by the tetrafluoridoborate anion.
A similar type of C 2 2 (12) chain is formed in each of compounds (IV) and (V), but now the cationtetrafluoridoborate aggregates are related by the 2 1 screw axis along ( 1 4 , y, 3 4 ) ( Fig. 11): the tcnoet anion in (IV) and the tcnset anion in (V) are pendent from this type of chain but play no other part in the aggregation. The cation-tetrafluoridoborate chain in compound (VI) is of the C 2 2 (13) type, built from aggregates related by translation along the [001] direction ( Fig. 12): again the polycyano anion is simply pendent from this chain.
Cg1, Cg2 and Cg3 represent the centroids of the rings (N11, C12-C16), (N61, C62-C66) and (N31, C32-C36) respectively. group (incorrectly described in the original report as 2-hydroxyethyl), one of the cyano groups forms contacts with two different pyridyl rings within the selected asymmetric unit, with NÁ Á Ácentroid distances of 3.212 (2) and 3.418 (2) Å (Setifi, Domasevitch et al., 2013). Here we have limited our attention to tncnXÁ Á Ácentroid contacts (where X represents an alkoxy or alkylsulfanyl group) of less than 3.4 Å ( Table 2). On this basis there are significant anionÁ Á Á interactions only in compounds (II) and (III): in (II), two such interactions link the cations and the major disorder component of the tcnX anion into a centrosymmetric four-ion aggregate, while in compound (III), the sole interaction of this type does not lead to any continuous aggregation as there are no hydrogen bonds between the cation and the polycyano anion (Table 1).

Database survey
The structures of compounds containing tcnX anions have been reported in recent years for a variety of systems, including complexes of cadmium (Setifi, Morgenstern et al., 2017), copper (Setifi, Setifi, El-Ammari et al., 2014;Addala et al., 2015) and iron (Setifi et al., 2010;Setifi, Domasevitch et al., 2013;Setifi, Setifi et al., 2013;Setifi, Setifi, Boughzala et al., 2014), as well as salts of purely organic cations mostly based on polypyridines (Setifi, Lehchili et al., 2014;Setifi et al., 2015Setifi et al., , 2016. Only in the complexes do the tcnX units acts as ligands, while the occur as free anions in all of the cadmium, iron and polypyridinium salts. In all of these salts, as in compounds (I)-(VI) reported here, the bond distances in the anions indicate delocalization of the negative charge over the whole of the tetracyanopropenide skeleton of the anion. Part of the crystal structure of compound (IV) showing the formation of a hydrogen-bonded C 2 2 (12) chain running parallel to the [010] direction. For the sake of clarity, the tcnoet anion and the H atoms not involved in the motif shown have been omitted.

Figure 12
Part of the crystal structure of compound (VI) showing the formation of a hydrogen-bonded C 2 2 (12) chain running parallel to the [001] direction. For the sake of clarity, the tcnopr anion and the H atoms not involved in the motif shown have been omitted.

Synthesis and crystallization
All chemical reagents and solvents are commercially available and were used without further purification. For the synthesis of compounds (III)-(VI), mixtures of 5,5 0 -dimethyl-2,2 0 -bipyridine (18.4 mg, 0.1 mmol), iron(II) tetrafluoridoborate hexahydrate (33.8 mg, 0.1 mmol), and 0.2 mmol of the appropriate polynitrile salt: [K(tcnome) for (III), K(tcnoet) for (IV), K(tcnset) for (V) or K(tcnopr) for (VI)] in waterethanol (4:1 v/v, 20 cm 3 ) were heated at 423 K for 3 d in a sealed Teflon-lined stainless steel vessel under autogenous pressure and then cooled gradually to room temperature at a rate of 10 K h À1 . After the reaction vessels had cooled to ambient temperature, crystals suitable for single-crystal X-ray diffraction were collected by filtration and dried in air. For the synthesis of compounds (I) and (II), a similar procedure was employed using 0.1 mmol of 2,2 0 -bipyridine, 0.1 mmol of iron(II) perchlorate hexahydrate and either 0.2 mmol of tcnome, for (I), or tcnspr, for (II).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Apart from the isostructural pair of compounds (IV) and (V), it was apparent at an early stage in the refinements that there was extensive disorder in the anionic components, although the cations were all fully ordered: in each of (I)-(VI), the asymmetric unit was selected such that the reference cation was the one having the Á configuration. Several low-angle reflections which had been attenuated by the beam stop were omitted from the final refinements: (101) (628) for (IV). In compound (I), one of the tcnome anions, that containing atom O721, exhibits orientational disorder of one of the C(CN) 2 units over two sets of atomic sites, while the other anion exhibits disorder of the whole anion, again over two sets of atomic sites. The tcnspr anion in compound (II) is disordered over two sets of atomic sites, while the perchlorate anion was found to be disordered over three sets of sites. In compound (III), the tcnome anion is fully ordered but the tetrafluoridoborate anion is disordered over two sets of sites, whereas in (VI), the tetrafluoridoborate anion is fully ordered but the tcnopr anion is disordered over two sets of sites. For compounds (IV) and (V), all H atoms were located in difference maps and then treated as riding atoms in geometrically idealized positions with C-H distances of 0.95 Å (pyridyl), 0.98 Å (CH 3 ) or 0.99 Å (CH 2 ) and with U iso (H) = kU eq (C), where k = 1.5 for the methyl groups, which were permitted to rotate but not to tilt, and 1.2 for all other H atoms. The H atoms bonded to C atoms in compounds (I)-(III) and (VI) were included in the calculations on the same basis. For the H atoms in the water component of compound (I), the atomic coordinates were refined, with U iso (H) = 1.5U eq (O), giving O-H distances of 0.96 (2) Å . For each of the disordered components, the bonded distances and the (1,3) non-bonded distances of the minor components were restrained to be equal to those of the corresponding major components, subject to s.u. values of 0.005 and 0.01 Å , respectively. In addition, the anisotropic displacement parameters of corresponding pairs of atoms were constrained to be identical. On this basis, the refined occupancies for the two anions in (I) were 0.500 (7) and 0.500 (7) in one anion and 0.502 (2) and 0.498 (2) in the other, so that thereafter these occupancies were all fixed at 0.5: the refined occupancy for the water component in the crystal selected for data collection was 0.776 (6). The refined tcnspr occupancies in (II) were 0.754 (2) and 0.246 (2) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Tris(5,5′-dimethyl-2,2′-bipyridine)iron(II) 1,1,3,3-tetracyano-2-ethoxypropenide tetrafluoridoborate (IV)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 0.40 e Å −3 Δρ min = −0.35 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )