Disorder of the dimeric TCNQ–TCNQ unit in the crystal structure of [Ni(bpy)3]2(TCNQ–TCNQ)(TCNQ)2·6H2O (TCNQ is 7,7,8,8-tetracyanoquinodimethane)

The first example of an NiII complex containing an σ-dimerized TCNQ–TCNQ unit is presented, with a C—C bond length of 1.653 (11) Å. In addition, the σ-dimerized TCNQ–TCNQ unit (refined 75% occupancy) is disordered, forming also a less populated pair of TCNQ molecules (25% occupancy) with tightly π-stacked dicyanomethanide groups.


Chemical context
In the quest for new promising molecular magnetic materials besides the complexes of 3d and 4f elements, organic radicals have been explored (Nafady et al., 2014;Kubota et al., 2014;. Among these,7,7,8, in its anion radical form responds to magnetic probing. Its combination with 3d or 4f metal atoms may lead to interesting magnetic properties (Nishijo & Enomoto, 2015;Madalan et al., 2002;Ballester et al., 2002). In addition, materials containing TCNQ have been studied for their electric conductivity (Ballesteros-Rivas et al., 2011;. TCNQ (including its reduced forms), when combined with 3d metals, can be present as an non-coordinating species (in the neutral or anion radical form) or it can form a -bond with the metal atom (Ballester et al., 1999). We note that TCNQ .À anion radicals tend to dimerize, usually via stacking of their -clouds, but, in some cases, the dimerization tendency leads to the formation of -dimerized (TCNQ-TCNQ) 2À dianions (Dong et al., 1977;Hoffmann et al., 1983;Shimomura et al., 2010;Zhao et al., 1996). Within our search for new heterospin materials based on 3d metals and organic radicals, we have undertaken a study of the aqueous methanol system containing Ni II , 2,2 0 -bipyridine (bpy) and TCNQ. Several complexes of Ni IIcontaining TCNQ species have been reported previously, e.g.

Structural commentary
The unit cell of the title complex, 1, comprises two [Ni(bpy) 3 ] 2+ complex cations, a centrosymmetric TCNQ-TCNQ dimeric unit, two centrosymmetric crystallographically independent TCNQ ÁÀ anion radicals, and three crystal-lographically independent disordered solvent water molecules . The complex cation is optically active, but due to the centrosymmetric character of the space group, both Á and Ã enantiomers are present in the structure. The Ni-N bond lengths range from 2.078 (2) to 2.109 (2) Å . Similar values of 2.0895 (2) and 2.1023 (2) Å for Ni-N bonds were found in [Ni(bpy) 3 ] 2 [W(CN) 8 ]Á6H 2 O (Korzeniak et al., 2008). An outstanding feature of the structure of 1 is the presence of a -dimerized dianion TCNQA (Figs. 2 and 3), which is, to our knowledge, the first reported case of such a unit among Ni II complexes with TCNQ. This dianion is disordered with a less prevalent pair of anion radicals for which the exocyclic groups interact solely via tight -stacking, but are not -bonded; the refined site-occupation factors are 0.753 (9):0.247 (9) (Fig. 2). The simultaneous presence of both a -dimerized dianion and a pair of anion radicals can be considered as a manifestation of a not completed dimerization reaction. The C37A-C37A iii [symmetry code: (iii) 1 À x, 1 À y, 2 À z] dimerization bond length is 1.653 (11) Å and this value is within the usual range (see Database survey section). At the same time, this value is longer than a usual single C-C bond and, consequently, the corresponding bond angles around the C37A atom range from 105.6 (4) to 113.6 (3) , displaying significant deviations from the ideal tetrahedral angle. In the less populated pair of anion radicals within TCNQA, the distance between the C37B atom A view of the molecular components of the title compound, 1, showing the labelling and with displacement ellipsoids drawn at the 30% probability level. For the dimerized (TCNQ) 2 unit, only the more populated position is shown. [Symmetry codes: (i) 1 À x, 2 À y, 1 À z; (ii) 1 À x, 1 À y, 1 À z; (iii) 1 À x, 1 À y, 2 À z.]

Figure 3
A view of the packing of the title compound, 1, approximatively along the a axis. The complex cations, H atoms and O3 water molecules have been omitted for clarity. Possible hydrogen bonds are shown as orange dashed lines. [Symmetry codes: (ii) 1 À x, 1 À y, 1 À z; (iii) 1 À x, 1 À y, 2 À z; (vii) x, 1 + y, z; (x) 1 + x, y, z.] and its symmetry-related counterpart C37B iii is 3.06 (2) Å ; the interplanar distance between the least-squares plane P1 formed by atoms C31B, C37B, C38B and C39B and the leastsquares plane P2 formed by their symmetry-related counterparts through a centre of symmetry at (1 À x, 1 À y, 2 À z) is 3.03 Å . The distance of the C37 iii atom from the plane P1 is 2.90 Å and the slippage between atoms C37B and C37B iii is 0.98 Å . These geometric parameters suggest a very stronginteraction between the less populated pair of anion radicals in TCNQA, and they are pre-positioned for -dimerization with little structural rearrangement required upon formation of the covalent bond. This could be seen as an indication ofbond formation in the solid state upon crystallization rather than pre-formation of the -dimers in solution.
In addition to the TCNQA site, there are two crystallographically independent centrosymmetric TCNQ ÁÀ anion radicals, TCNQB and TCNQC, in the crystal structure of 1 (Fig. 3). The two anion radicals are neighbours and stack in a -stacked 'external bond over external bond' fashion (see Ballester et al., 1999). The exocyclic groups in these TCNQ units are almost in plane with the quinoide ring; the greatest deviation from planarity is represented by the torsion angle C45-C43-C46-C48 of 175.9 (2) in TCNQB.

Supramolecular features
A view of the packing of the structure of 1 is displayed in Fig. 3. The TCNQ units are arranged in a chain-like manner along the b axis; one chain-like arrangement is formed only by the TCNQA dimeric units, while a second one is built up of alternating TCNQB and TCNQC anion radicals. In both chain-like arrangements, the exocyclic groups are -stacked with each other. Ballester et al. (1999) defined four different stacking modes of TCNQ units, with typical intradimer distances between 3.09 and 3.45 Å . For TNCQA, the site with disordered -dimerized and radical anions, molecules are arranged in infinite channels along a string of inversion centres on both sides of each crystallographically independent unit. On one side there is the case of the less populated un-- Symmetry codes: (i) Àx þ 1; Ày þ 1; Àz þ 1; (ii) x; y þ 1; z; (iii) Àx; Ày þ 1; Àz þ 2; (iv) x À 1; y þ 1; z; (v) Àx; Ày þ 1; Àz þ 1; (vi) x À 1; y; z.
These observed distances are at the upper border for stacking arrangements reported for similar compounds (Ballester et al., 1999).
There are three crystallographically independent positionally disordered water solvent molecules in the structure which, through the formation of O-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds, play an important role in the formation of the supramolecular structure of 1 (Figs. 3, 4 and 5, and Table 1). Water molecules O1A and O2A are linked via NÁ Á ÁH-O-HÁ Á ÁN (the N atoms are from the nitrile groups of the TCNQ units) hydrogen-bonded bridges involving TCNQA dianions and TCNQC anion radicals, yielding a supramolecular layer within the bc plane (Figs. 3 and 4). In addition, these supramolecular layers are interconnected by O2AÁ Á ÁH-O3A-HÁ Á ÁO1A hydrogen-bonded bridges, resulting in a three-dimensional hydrogen-bonded supramolecular structure. We note that atoms O1A, O2A and O3A are only partially occupied due to the observed disorder. The alternatively positioned O1 and O3 water molecules (disordered positions O1B and O3B) form an additional hydrogen-bonded bridging path, NÁ Á ÁH-O2AÁ Á Á H-O3B-HÁ Á ÁO1B-HÁ Á ÁN, between the supramolecular layers. On the other hand, the least-occupied position (O2B) of water molecule O2 seems to be hydrogen bonded only to the nitrile N atom and so partially occupies the void in the structure in alternation with its symmetry-related atom O2B xi [symmetry code: (xi) Àx, 2 À y, 1 À z] (Fig. 5). Additional weak hydrogen-bonding interactions of the C-HÁ Á ÁN and C-HÁ Á ÁO types (Table 1) contribute to the stability of the structure.

Database survey
A search of the CSD (Groom et al., 2016) revealed 16 compounds with -dimerized TCNQ-TCNQ units. Among the hits in the CSD with -dimerized TCNQ-TCNQ dianions, there is no example containing an Ni II ion as the central atom, hence compound 1 is the first such example. The reported values of the C-C bond linking the two TCNQ units are slightly longer than a normal single bond; the reported values range from 1.612 Å , found in catena-[Zn(TCNQ-TCNQ)(bipy)]Áp-xy (bipy is 4,4 0 -bipyridine and p-xy is pxylene; Shimomura et al., 2010), to 1.673 Å , found in [Pt(bpy) 2 )(TCNQ-TCNQ)] (Dong et al., 1977). In 1, the corresponding value is 1.653 (11) Å , which is in line with the observed range in the published crystal structures.

Synthesis and crystallization
A solution of LiTCNQ (0.150 mmol, 31.6 mg) in methanol (2 ml) heated to 323 K was added dropwise to a mixture of Ni(NO 3 ) 2 Á6H 2 O (0.075 mmol, 21.8 mg) and bpy (0.225 mmol, 35.1 mg) in methanol (2 ml) at the same temperature. The dark-green solution that resulted was immediately enclosed in a 5 ml vial and cooled to room temperature (8.75 K h À1 ) in a programmable drying oven. The dark-green crystalline solid that resulted was filtered off, washed with a small amount of methanol and ether, and dried in air. The solid was mainly of research communications

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms bound to C atoms were positioned in calculated positions, with their U iso values set at 1.2 times the U eq value of the parent C atom. During refinement it became apparent that what initially was considered as only a -dimerized (TCNQ-TCNQ) 2À dianion is positionally disordered (see Fig. 2); it consists mostly of a -dimerized dianion disordered with a less abundant dimeric unit having closly -stacked dicyanomethanide groups. The effort to resolve this disorder yielded refined site-occupation factors of 0.753 (9):0.247 (9). The observed disorder involves the dicyanomethanide group involved in dimerization, as well as the quinoide ring atoms with the exception of atom C34. In order to control the geometric parameters, the disordered quinoide ring atoms, as well as the C37 atoms of each disordered moiety, were restrained to be coplanar (FLAT command) and equivalent bond lengths of disordered atoms were restrained to be similar (SADI commands). The refinement process concerning the solvent water molecules was carried out using an iterative approach which showed that there are three crystallographically independent water molecules in the asymmetric unit and that all of them are positionally disordered; some of the disorder is symmetry imposed, with atoms related through a centre of symmetry being mutually exclusive due to close contacts, and the site-occupation factors for these atoms (O1A, O1B, O3A and O3B) were considered to be exactly one half, while the refined site-occupation factors for atoms O2A and O2B are 0.908 (3) (Rigaku OD, 2015); cell refinement: CrysAlis PRO (Rigaku OD, 2015); data reduction: CrysAlis PRO (Rigaku OD, 2015); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2007); software used to prepare material for publication: publCIF (Westrip, 2010). Hydrogen site location: mixed H atoms treated by a mixture of independent and constrained refinement w = 1/[σ 2 (F o 2 ) + (0.050P) 2 + 0.5977P] where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.34 e Å −3 Δρ min = −0.24 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.