Crystal structure of fac-bis[bis(pyridin-2-yl)methanamine]iron(II) 1,1,3,3-tetracyano-2-(dicyanomethylidene)propane-1,3-diide, [Fe(dipa)2](tcpd)

In the title compound, the FeII cation is octahedrally coordinated by two dipa ligands and has crystallographic twofold symmetry. The nonplanar polynitrile tcpd2− anion is disordered about an inversion center.

In the title compound, [Fe(C 11 H 11 N 3 ) 2 ](C 10 N 6 ), the Fe II cation is coordinated by two bis(pyridin-2-yl)methanamine (dipa) ligands and has crystallographic twofold symmetry. There are deviations from ideal octahedral geometry due to the steric requirements of the ligands. The polynitrile 1,1,3,3-tetracyano-2-(dicyanomethylidene)propane-1,3-diide (tcpd 2À ) dianion is disordered about an inversion center and is not coordinated to the Fe atom. The anion is not planar but has a propeller shape. In the crystal, weak N-HÁ Á ÁN interactions between the amine H atoms of the dipa ligands and two nitrile groups of the anion form an alternating chain of cations and anions related by the C-centering of the unit cell.

Chemical context
Polynitrile anions have recently received considerable attention in the fields of coordination chemistry and molecular materials (Benmansour et al., 2010). These organic anions are of interest for their ability to act towards metal centers with various coordination modes and for their high degree of electronic delocalization (Miyazaki et al., 2003;Yuste et al., 2009;Atmani et al., 2008;Karpov et al., 2018). We are interested in using these anionic ligands in combination with other neutral bridging coligands to explore their structural features and properties relevant to the field of molecular materials exhibiting the spin-crossover (SCO) phenomenon (Setifi et al., 2014;Dupouy et al., 2009). In an attempt to prepare such an iron(II) complex using solvothermal synthesis, we obtained instead the title compound facbis[bis(pyridin-2-yl)methanamine]iron(II) 1,1,3,3-tetracyano-2-(dicyanomethylidene)propane-1,3-diide, [Fe(dipa) 2 ](tcpd).

Structural commentary
The structure is built from Fe II cations coordinated by two bis(pyridin-2-yl)methanamine (C 11 H 11 N 3 ; dipa) ligands, and 1,1,3,3-tetracyano-2-(dicyanomethylidene)propane-1,3-diide (C 10 N 6 2À ; tcpd 2À ) anions. The Fe atom lies on a twofold axis, with its coordinated dipa ligands related by the twofold axis (Fig. 1). The anion lies on an inversion center and is disordered. Detailed geometry of the anion was extracted as described below. The dipa ligand coordinates the Fe atom through the central amino N atom and the two pyridinium N atoms in a fac arrangement. The dipa ligand assumes the butterfly conformation found previously (Setifi et al., 2017), with an approximate mirror plane bisecting the ligand, and the pyridine rings are at an angle of 56.66 (6) to each other. Fe-N distances to the pyridine N atoms average 1.959 (1) Å , slightly shorter than the Fe-N distance of 2.004 (2) Å to the amine group. The five-membered chelate ring angles at the Fe atom are 80.10 (6) and 81.55 (6) , while the butterfly angle N1-Fe-N3 is 90.10 (6) . The two independent trans angles at Fe in the octahedrally coordinated Fe atom are 172.75 (9) and 174.61 (6) . Otherwise, bond lengths and angles within the ligand are as expected. The tcpd 2À anion, which is disordered about a crystallographic inversion center, is propeller-shaped, with approximate C 3 symmetry, and a geometry similar to that described previously by Setifi et al. (2015). The C(CN) 2 planes are tilted by 31.1 (5), 24.7 (4), and 30.0 (5) with respect to the C21-C24 central plane. The C-C distances average 1.414 (18) Å , indicative of the sp 2 hybridization of all the C atoms. The average C-N distance in the CN groups is 1.154 (11) Å . Fig. 2 shows the packing of the crystal structure. The cations stack along the b-axis direction in columns related by the glide planes and the C-centering. The N3/C7-C11 pyridine ring lies almost perpendicular to the b axis and partially overlaps the parallel ring related by the center of symmetry at ( 1 4 , 1 4 , 1 2 ). The planes of these pyridine rings are 3.442 (1) Å apart. The anion is disordered about a center of symmetry displaced by b/2 from the center of these pyridine rings. Fig. 2 shows the interactions between the amine H atoms and the CN groups. There is an N2-H12Á Á ÁN25 hydrogen bond, with NÁ Á ÁN = 2.95 (1) Å and N-H Á Á Á N = 157.1 (2) ( Table 1). The weaker interactions N2-H13Á Á ÁN27(x À 1 2 , y + 1 2 , z) and N2-H13Á Á ÁN28(x À 1 2 , y + 1 2 , z) link the cations and anions into alternating chains along [110]. Possible interactions between CN groups and atom H6 are also listed in Table 1. Other short interactions are C6Á Á ÁN28(x À 1 2 , 1 2 + y, z) of 2.99 Å and C5Á Á ÁN28( 3 2 À x, 1 2 À y, 1 À z) of 3.11 Å . Projection of the structure down the b axis, with weak N-HÁ Á ÁN interactions shown as dashed orange bonds. Only one disorder mate for the anion is shown. Cations have more solid bonds. The asymmetric unit is shown in bold.

Figure 1
The [Fe(dipa 2 )] 2+ cation and tcpd 2À anion in the title compound. Atoms in the cation with the # suffix are related to the asymmetric unit by (1 À x, y, 3

Database survey
A search for the tcpd 2À anion in the Cambridge Structural Database (CSD, Version of 2017; Groom et al., 2016) produced 49 hits for structures with atomic coordinates available. We selected 20 of these hits for analysis, not using 23 variabletemperature studies and six with disordered tcpd 2À anions for which detailed parameters were not available. In nine of the 20 studies, the anion was present in an uncoordinated form, and in the rest, it was coordinated to a first-row transition metal. The bond lengths in the 20 structures analyzed were quite consistent, with sample deviations of 0.013 Å . The two types of C-C distances have the same average shortened distance of 1.417 (1) Å , and the C N bond lengths average 1.147 (1) Å , showing the same trends as in the present structure. In all cases, the anion as a whole was nonplanar, with each C(CN) 2 group twisted in the same direction relative to the central fouratom plane, with an average twist angle of 24.4 (7) . Presumably, this minimizes repulsion between the N atoms, which carry a partial negative charge. The average twist angle is the same, regardless of whether the anion is coordinated. In an individual structure, the twist angles were invariably scattered, with the average minimum twist angle some 67% of the average maximum twist angle. The twist angles for tcpd 2À in the present structure average 28.6 (3) , higher than the average twist angle in any of the nine free anions reviewed in the CSD. A search for the dipa ligand yielded nine hits, with one, two, or three dipa molecules coordinated to transition-metal atoms in all cases. There were only three instances of dipa coordinated to an Fe atom, including Setifi et al. (2017).

Synthesis and crystallization
The title compound was synthesized solvothermally under autogenous pressure from a mixture of FeSO 4 Á7H 2 O (28 mg, 0.1 mmol), dipa (19 mg, 0.1 mmol) and K 2 tcpd (28 mg, 0.1 mmol) in water-ethanol (4:1 v/v, 20 ml). This mixture was sealed in a Teflon-lined autoclave and held at 423 K for 3 d, and then cooled to room temperature at a rate of 10 K h À1 (yield 23%). Red blocks of the title compound suitable for single-crystal X-ray diffraction were selected directly from the synthesized product.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The tcpd 2À anion lies on an inversion center, which requires the anion to be disordered. The three C atoms bonded to the central C21 atom are easily resolved from their centrosymmetric equivalents, but the six CN groups are close enough to a centric array that the disorder mates are not resolved in a difference map. Indeed, a preliminary refinement that constrained the cyanide C and N atoms to centrically related positions converged successfully. This treatment led to unreasonable C-C N bond angles, however, and hindered a detailed analysis of the anion geometry. Scrutiny of a model indicated that while the cyanide C atoms might be very close to their centric counterparts, the cyanide N atoms ought to lie far enough apart to refine separately. Accordingly, the positions for the N atoms were calculated manually, assuming linear C-C N bonding and typical C-C and C N distances. The complete anion could then be refined by tightly restricting differences from threefold symmetry in chemically equivalent C-C distances and C-C-C angles, and refining the C and N atoms first isotropically and then anisotropically. At this point, cyanide C atoms had moved an average of 0.4 Å from their centric images, and cyanide N atoms were at an average distance of 0.5 Å from their images (Fig. 3). The restraints could now be removed for the final refinements, except that displacement parameters for nitrile groups 25-27 were constrained to be the same as those for nitrile groups 28-30, and a restraint on the anisotropy of C atoms 25-30 was added via an ISOR instruction. In a separate refinement, the coordinates of the cyanide C atoms were arbitrarily moved small amounts before the restrained refinements, but the same final structure was obtained. Cbound H atoms were constrained to idealized positions, with C-H distances of 1.00 Å for the CH group and 0.95 Å for aromatic H atoms, and with U values set at 1.2 times the U iso value of their bonded atoms. The positions of the amino H atoms were refined independently, although their final positions are very close to what would have been predicted. Their U values were set at 1.5 times the U iso value for N2. We also explored refinements in the space group Cc, which would not force disorder on the anion model, nor impose twofold symmetry on the cation. Although the noncentric model refined smoothly, it was abandoned because some of the displacement ellipsoids were unreasonable and convergence occurred at R 1 = 0.043 and wR = 0.109, values higher than in our final centric model, even though many more variables were refined in the noncentric model. The tcpd 2À anion structure superimposed on its disorder mate related by ( 3 2 À x, 1 2 À y, 1 À z). Atom C21 lies on the center of symmetry at ( 3 4 , 1 4 , 1 2 ). Computer programs: APEX2 and SAINT (Bruker, 2015), SHELXS97 (Sheldrick, 2008), SHELXL2017 (Sheldrick, 2015), ORTEPIII (Burnett & Johnson, 1996) and publCIF (Westrip, 2010 II) 1,1,3,3- Data collection: APEX2 (Bruker, 2015); cell refinement: SAINT (Bruker, 2015); data reduction: SAINT (Bruker, 2015); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2017 (Sheldrick, 2015); molecular graphics: ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: publCIF (Westrip, 2010).

fac-Bis[bis(pyridin-2-yl)methanamine]iron(II) 1,1,3,3-tetracyano-2-(dicyanomethylene)propane-1,3-diide, [Fe(dipa) 2 ](tcpd)
Crystal data [Fe(C 11  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.