Poly[diethylammonium [tetra-μ2-cyanido-κ8 C:N-tricuprate(I)]], a two-dimensional network solid

The title compound crystallizes as a CuCN network solid, with diethylammonium cations sandwiched between two-dimensional, planar CuCN sheets comprised of trigonally and digonally coordinated Cu atoms in 24-membered rings.


Structure description
There has been continuing interest in the synthesis and structures of CuCN network solids containing protonated nitrogen bases, with at least 40 such structures listed in the CSD (Groom et al., 2016). For instance, a recent paper reports optical memory effects for two tetramethylammonium CuCN structures (Nicholas et al., 2019) while Grifasi et al. (2016) is one of several papers reporting on the interesting topologies and photoluminescence of many CuCN networks. The present compound was prepared as part of our own ongoing structural studies in this area.
Of the two independent Cu atoms, Cu1 is linearly coordinated to two CN groups and lies on the crystallographic twofold rotation axis [0, y, 0], while trigonally coordinated Cu2 is in a general position, Fig. 1. Each of the two independent CN groups bridges two copper(I) atoms to build a two-dimensional CuCN network perpendicular to the a axis. Four such sheets cross the unit cell, as shown in the packing diagram, Fig. 2. The network is made up of 24-membered rings, which are almost planar, with an r.m.s. deviation from the 24-atom plane of 0.128 (5) Å , where the e.s.d. given is the average of the 24 individual data reports e.s.d.'s. Most such networks in the literature are honeycomb structures made up of 18-membered hexagonal rings, although a network similar to that described here was reported by Ferlay et al. (2013). The three-coordinated Cu2 atom has a geometry far from ideal trigonal planar, with C/N-Cu-C/N angles of 114.7 (3), 116.4 (2), and 128.3 (3) and bond lengths Cu-C/N ranging from 1.889 (8) to 1.960 (7) Å .
The ammonium cation lies on the crystallographic twofold axis [0, y, 1 2 ] and assumes a gauche conformation, with the torsion angle C32-C31-N3-C31(Àx, y, 1 À z) = À62.1 (6) . Each cation forms two N-HÁ Á ÁN hydrogen bonds to N2 of the bridging C2 N2 group of two adjacent sheets, which ties adjacent sheets into a three-dimensional network, as shown in Fig. 2. Table 1 gives details of the single independent hydrogen bond, while the lower part of Fig. 2

Synthesis and crystallization
A mixture of 0.359 g (4.01 mmol) of CuCN and 0.330 g of NaCN (6.73 mmol) with 25 ml of H 2 O was stirred and the light remaining precipitate was filtered off. 1.55 g (21.2 mmol) of diethylamine dissolved in 10 ml of H 2 O were added, and the stirred mixture was left open to air. Crystals began to form after one week and were harvested as conglomerates of thick, yellow-green plates several weeks later. The intent had been to prepare a mixed-valence compound similar to those prepared from bidentate amines (Corfield & Michalski, 2014;Corfield & Sabatino, 2017) and to use the fivefold excess of base to stabilize any Cu II formed by air-oxidation. However, no crystalline mixed-valence compounds containing the base were obtained in this and similar preparations with diethylamine. The IR spectrum, obtained with a Thermo Scientific Nicolet iS50 FT-IR instrument, showed strong stretching bands at 2111 cm À1 and 2136 cm À1 for CN, and at 3118 cm À1 and 3186 cm À1 for N-H. The N-H frequencies for the protonated base may be compared with the band at 3281 cm À1 (w) found for the free base diethylamine.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Towards the end of the refinements, each of the two CN groups was refined as a superposition of NC and CN groups, whose occupancies were varied. For C1 N1, the occupancy factor refined to close to 50%, so this occupancy was fixed at 50%, while the occupancy for C2 N2 favors one orientation over the other by 78 (8)%. This preferred orientation is doubtless due to the hydrogenbonding interactions with the cation discussed above. The Flack x factor (Parsons et al., 2013) is 0.096 (25), which implies Table 1 Hydrogen-bond geometry (Å , ). (4) 3.230 (6) 149 (6)

Figure 2
Top: View along the a axis of one sheet. Hydrogen bonds from cations above and below the sheet are shown. Bottom: Projection down the b axis, showing the sheets stacked perpendicular to the a axis, and the hydrogen bonds linking the sheets together. (Scale is slightly larger than in the top diagram.) The chains of hydrogen bonds along the [102] direction are evident. Cu atoms are colored red, N atoms blue, and C and NH atoms black. H atoms of the C 2H 5 gropu are not shown, and only one orientation for each of the disordered CN groups is drawn.

Figure 1
The asymmetric unit of the title compound is emboldened. Different crystallographic twofold axes pass through Cu1 and N3. Displacement ellipsoids are drawn at the 50% probability level, while H atoms are depicted as arbitrary spheres. Cu atoms are colored red, N atoms blue, and C and H atoms black.
that the crystal exhibits minor twinning about the (010) plane; more pronounced twinning was seen in a different crystal not used in this work. The final refinement uses the SHELXL BASF and TWIN commands, with no noticeable changes in the structure.

Data collection
Enraf-Nonius KappaCCD diffractometer Radiation source: fine-focus sealed tube Graphite monochromator Detector resolution: 9 pixels mm -1 combination of ω and φ scans Absorption correction: multi-scan (Otwinowski & Minor,1997 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. Refinement. Hydrogen atoms on the C atoms were constrained, with C-H distances of 0.97 Å for the methylene group and 0.96 Å for the methyl group. The N-H atom was refined, with a restraint on the N-H bond length but not on the temperature factor.  (6)