Bis(2,2′-bi-1H-imidazole)copper(II) bis(1,1,3,3-tetracyano-2-ethoxypropenide)

In the title compound, [Cu(C6H6N4)2](C9H5N4O)2, the Cu2+ ion (site symmetry ) is coordinated by two N,N′-bidentate 2,2′-biimidazole (H2biim) ligands, generating a square-planar CuN4 geometry. The dihedral angle between the aromatic rings in the ligand is 0.70 (9)°. In the polynitrile 1,1,3,3-tetracyano-2-ethoxypropenide (tcnoet) anion, the C—N, C—C and C—O bond lengths indicate extensive electronic delocalization. An alternative description for the metal-ion geometry is an extremely distorted CuN6 octahedron, with two N-bonded tcnoet anions completing the coordination. In the crystal, the components are linked by N—H⋯N and C—H⋯N interactions.


Comment
Polynitrile anions are known to be interesting ligands in coordination chemistry because of their high electronic delocalization and their cyano groups juxtaposed in such a way that they cannot all coordinate to the same metal ion. They adopt different bridging or nonbridging coordination modes wich afford discrete or extended molecular architectures (Atmani et al., 2008;Benmansour et al., 2007;Triki et al., 2005;Thétiot et al., 2003). Following these structural and electronic characteristics, several series of binary systems "polynitrile/M(II)" with only polynitrile bridges or ternary systems "polynitrile/ co-ligand/M(II)" (M(II), transition metal ion) involving an additional bridging or chelate co-ligand have been reported. Most of them display one-dimensional, two-dimensional and three-dimensional polymeric assemblies, in which the polynitrile anions act as µ2-, µ3-and/or µ4-bridging ligands and exhibit unusual magnetic properties (Batten & Murray, 2003;Jones et al., 2006;Yuste et al., 2007;Setifi et al., 2006;Setifi et al., 2007).
In our case we have chosen to investigate the ternary system including 2,2'-biimidazole (H 2 bim) selected as co-ligand because it is bifunctional: the imino moieties can be coordinated to a metal ion acting as the first coordination sphere and the amino N-H and C-H groups, as the second coordination sphere, may donate multifold hydrogen bonds to tcnoet anions, extending the structure into a high-dimensional network. In this contribution we report the synthesis and the crystal structure of a new copper(II) compound with neutral 2,2'-biimidazole, Cu(H 2 biim) 2 ] (tcnoet) 2 , (I).
The crystal of (I) is built of [Cu(H 2 biim) 2 ] 2+ cations and (tcnoet)anions interconnected by hydrogen bonds. As shown in Fig. 1, the Cu(II) ion has a square coordination geometry, it locates on a symmetry inversion center and relates four nitrogen atoms of two symmetry-related 2,2'-biimidazole molecules which bind bidentately arranged trans to each other in the square plane [Cu1-N1 = 1.973 (2) Å and Cu1-N2 = 2.040 (2) Å] and interacts with two nitrogen atoms belonging to tcnoet ligands occupying the apical coordination sites [Cu1-N7 = 2.821 (3) Å]. Selected interatomic distances and angles are listed in Table 1. The Cu-N bond distances to H 2 biim and inter-ring C1-C2 bond length in (I) present no unusual features and are consistent with the previous report in [Cu(H 2 biim) 2 ]Cl 2 [Bencini & Mani, 1988], [Cu(Me 4 biim)ONO 2 ]Cl [Bernarducci et al., 1983] and [VOCl(H 2 Biim) 2 ]Cl [Cancela et al., 2001] complexes. In our case the principal coordination is planar and the Cu atom lies within that plane. Both imidazole rings are planar, with no atoms deviating by more than 0.007 A° from the least-squares plane. The two rings of H 2 biim are nearly coplanar, making an angle of 0.70 (9)°. This value compares well with that found in the mononuclear copper(II) species [Cu(H 2 biim) 2 ] 2+ which are in a strictly planar environment (Bencini & Mani, 1988) and that observed in the free H 2 biim molecule (Cromer et al., 1987), but it is smaller than that found for the mononuclear oxovanadium(IV) species [VOCl(H 2 biim) 2 ]Cl (Cancela et al., 2001).
[Cu(H 2 biim) 2 ](tcnoet) 2 units are connected to each other via hydrogen bonds N-H···N resulting in a one-dimensional chains as shown in Fig. 2. Furthermore these chains are maintained through van der Waals interactions on the (ab) plane and supplementary materials sup-2 connect each other via C-H···N hydrogen bonds into a two-dimensional network (Fig. 3). Interestingly, each tcnoetanion help to sustain the one-dimensional assembly and at the same time the final two-dimensional array.
In this complex, the three central C atoms (C11, C12 and C13) of the anionic ligand present an sp 2 hybridization as indicated by the sum of the three angles around them (359.98° or 360.0°). Two additional facts support the idea of electron delocalization over the three central C atoms: (i) the six central C-C bond distances (1.389 A° -1.428 A°) are longer than a normal C═C double bond (1.340 A°) and close to those of benzene and (ii) the C11-O1 bond distance 1.352 A° is much shorter than tne normal C-O single bond, suggesting that the two central and the C11-O1 bond present a partial double character.
Experimental H 2 biim and Ktcnoet ligands were synthesized with the published procedures respectively (Bernarducci et al., 1983;Middleton et al., 1958). To a methanolic suspension of H 2 biim (0.025 g, 5 ml) was added drowpize a solution of CuCl 2 .2H 2 O (0.032 g, 5 ml) resulting in a green solution. Ktcnoet was dissolved in water (0.084 g, 10 ml) and was added quickly to the former solution. The final solution was filtred and allowed to evaporate for a week, giving green blocks of (I). X band EPR spectrum from a polycristalline powdred sample of (I) recorded at room temperature exhibits a well defined axial signal with g parallel = 2.27 > g perpendicular = 2.07 consistent with a Cu(II) monomer.

Refinement
All H atoms were placed in geometrical positions and refined using a riding model, with C-H distances in the range 0.95-0.99 Å and their displacement parameters were set to U iso (H) = 1.5U eq (C) for methyl group and U iso (H) = 1.2U eq (C) for all others, while N-H bond lengths were fixed to 0.88 Å with U iso (H) = 1.2U eq (carrier N atom). Fig. 1. The metal environnement in (I), with displacement ellipsoids drawn at the 50% probability level. Unlabelled atoms are related to labelled atoms by the symmetry operation 1 -x, 1 -y, 1 -z.   3. View of the hydrogen bonded maintaining the chains along the c axis. Symmetry codes i(1 -x,1 -y,1 -z), ii(1 -x,1 -y,-z), iii(x,y,1 + z), VIi(x,y,-1 +z), Vi(1 -x,1 -y,2 -z).

Figures
Bis(2,2'-bi-1H-imidazole)copper(II) bis(1,1,3,3-tetracyano-2-ethoxypropenide) Crystal data [Cu(C 6  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.