Sodium bis(ethylenediamine)copper(II) tetracyanidocuprate(I)

The title compound, Na[Cu(en)2][Cu(CN)4], where en represents ethylenediamine, NH2CH2CH2NH2, crystallizes as a salt with two distinct cations, Na+ and [CuIIen2]2+, and discrete [CuI(CN)4]3− anions. The anion geometry is tetrahedral, with angles at the copper atom ranging from 105.0 (1) to 115.4 (1)°. The Cu—C distances are in the range 1.976 (3) to 1.993 (3) Å. The divalent copper atom is coordinated by four N atoms of the two bidentate en ligands in a slightly distorted square-planar geometry. In the crystal, each sodium ion interacts with cyanide N atoms of four different anions, with Na—N distances lying in the narrow range of 2.344 (3) to 2.367 (3) Å, and an approximately tetrahedral arrangement around the sodium ions. The interacting sodium ions and [CuI(CN)4]3− anions form a three-dimensional network with channels which contain the [Cu(en)2]2+ cations. One of the chelate rings in the cation shows partial disorder between two different conformations and the C atoms were refined with occupancies in the ratio 0.817 (15):0.183 (15).

supplementary materials Acta Cryst. (2013). E69, m307-m308 [doi:10.1107/S1600536813012075] Sodium bis(ethylenediamine)copper(II) tetracyanidocuprate(I) Peter W. R. Corfield, Robert K. Dobbs and Brian Bell Comment There has been widespread interest in structures of cyanide-bridged copper compounds, which can show self-assembly into polymeric structures. See, for example, Colacio et al. (2002) and Kim et al. (2005). The structure determination of the title compound was undertaken as part of a series of synthetic and structural studies of mixed-valence copper cyanide complexes containing chelating amine bases (Corfield et al., 2012). The coordinated amines stabilize the divalent copper atoms against reduction by the cyanide groups. The title compound was crystallized from systems with the base ethylenediamine. The structures of two other compounds from this system have been described (Williams et al., 1972;Weiss et al., 2006).
There are significant interactions between the sodium ions, Na + , and the nitrogen atoms of CN groups from four different [Cu(CN) 4 ] 3anions, with an average Na--N distance of 2.358 (2) Å. The arrangement of cyanide N atoms around the sodium ions is roughly tetrahedral, with angles ranging from 100.8 (1)° to 129.8 (1)°. The C-N-Na angles average 140.8°, with wide variation from 113.3 (2)° to 165.3 (2)°, unlike the linear arrangement that would be expected for covalently bridging cyanide groups.
Three of the four Na-N interactions link the Na + cations and the [Cu(CN) 4 ] 3anions into ribbons extending along the direction of the a axis, (Fig. 2) while a fourth Na-N interaction links the ribbons into an open three-dimensional structure with channels along the a axis. The [Cuen 2 ] 2+ cations reside in these channels (Fig. 3). There appear to be significant interactions between the Cu atoms of the cation and cyanide N atoms from two neighboring [Cu(CN) 4 ] 3anions, with N1 and N2 atoms of the anions lying approximately in the axial positions for the square planar cationic copper atoms, at distances of 3.09 Å and 2.69 Å respectively. We have not described the latter interaction as a long covalent bond, because the putative Cu-N-C angle is 99.0°, far different from the linear angle expected for a bonded cyanide group. Further, the Cu atom is displaced very little from the N 4 plane of the coordinated ethylenediamine ligands. Three possible weak N-H···N hydrogen bonds link N8 and N9 atoms from the ethylenediamine ligands with neighboring cyanide N atoms, with N···N distances ranging from 3.30 to 3.53 Å, N···H distances from 2.64 to 2.67 Å, and angles at the hydrogen atoms ranging from 132° to 165°. Other intermolecular contacts appear normal, with the shortest H···H intermolecular distance found at 2.58 Å, and the shortest H···C distance at 2.41 Å, for H5B-C3(1 -x, -1/2 + y, 1/2 -z).

Experimental
The compound was prepared by addition of ~13 ml of a solution containing 8.0 g (0.133 mol) of ethylenediamine to a solution containing 8.34 g (0.17 mol) sodium cyanide and 8.17 g (0.09 mol)copper(I) cyanide in 33 ml water. Crystals usually appear after several days. Often, the first compound filtered off is Cu 3 en 2 (CN) 4  Sodium was analyzed by atomic absorption spectroscopy, with a Perkin-Elmer 303 instrument. Found: 6.0 (2)%; calculated 6.15%. Total copper was analyzed iodometrically: found 33.8 (1)% from 13 measurements; calculated 34.0%.
Cyanide was analyzed by titration with AgNO 3 : found 26.7(1.7)% from two measurements; calculated 27.8%. The infrared spectrum was taken with a Perkin-Elmer 710B spectrophotometer. There is one CN stretching frequency, at 2095 cm -1 . Conductance measurements with a classical conductance bridge and glass cell gave a molar conductance of 293 (2) ohm -1 .cm 2 .mol -1 , a reasonable value for a three ion electrolyte (Angelici, 1977).

Refinement
Refinements with anisotropic temperature factors for Cu, N and C atoms and isotropic factors for the constrained hydrogen atom parameters converged smoothly. H atoms were placed in calculated positions with N-H = 0.90Å and C -H = 0.97Å. They were included in the refinment in a riding motion approximation with U iso (H) = 1.2U eq (C,N) or 1.5U eq (C) for disordered atoms. At this point, a difference Fourier synthesis showed peaks of 0.6 e/A 3 that indicated partial occupancy of an alternative conformation δ for the ethylenediamine carbon atoms C6 and C7, which have a λ conformation. Further refinements allowed for disorder of this ethylenediamine group. The extra atoms were labeled C6A and C7A and were refined isotropically, with hydrogen atoms assigned in constrained positions. The nitrogen atoms N5 and N8 were assumed to be common to both conformations. The main conformation, involving C6 and C7, refined to an occupancy of 81.7 (15)%, while the alternative conformation with C6A and C7A has an occupancy of 18.3 (15)%.
Inclusion of the disordered atoms in the refinement lowered R1 from 0.0339 to 0.0322.
A difference Fourier synthesis at this point showed several holes of depth -0.6 e/A 3 within 1 A of the copper atoms. It was felt that these features might reflect small anisotropic errors in the diffracted intensities due to the size of the crystal relative to the size of the fine focus X-ray beam, as well as a slight crystal movement noted towards the end of the data collection. The anisotropy was modeled by using the program XABS2, (Parkin et al., 1995). Twelve parameters were used to modify the observed structure factors. Subsequent refinements lowered R1 from 0.0322 to 0.0300 for all reflections, and reduced the maximum height or depth of features in the final difference Fourier synthesis to 0.32 e/A 3 or less.

Special details
Experimental. Diffraction data were collected with Cu Kα radiation. At a later stage, Mo Kα radiation was used in measurements made to obtain improved unit-cell dimensions. Each of the 25 reflections used for cell measurement was determined in four different orientations using the CAD4 SET4 command. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. 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 R-factors(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.