Crystal structure of poly[N,N-diethyl-2-hydroxyethan-1-aminium [μ3-cyanido-κ3 C:C:N-di-μ-cyanido-κ4 C:N-dicuprate(I)]]

A cyanide-bridged anionic three-dimensional network solid is described, with molecular formula {Cu2(CN)3}−. Charge neutrality is provided by guest N-protonated N,N-diethylethanolamine molecules.


Chemical context
This structure determination was undertaken as part of our ongoing study of mixed-valence copper cyanide complexes, with the goal of directed synthesis of new polymeric structures. The intention is to build amine-coordinated Cu II atoms into Cu I cyanide-bridged networks by having two or more CN groups coordinating to the Cu II atoms as well as the amine N atoms. This has proved somewhat elusive, however. For example, in the classic mixed-valence complex Cu 3 (CN) 4 en 2 ÁH 2 O where en is ethylenediamine (Williams et al., 1972), there is a three-dimensional Cu I 2 (CN) 4 2À network, with coordinated Cu II cations situated in cavities with no covalent links to the network. One case where a CN-linked network incorporates both Cu I and Cu II is that of Cu 3 (CN) 4 oen 2 , where oen is ethanolamine (Corfield et al., 1991;Jin et al., 2006). Here, there are two CN groups coordinating in a trans configuration to Cu II atoms (the resulting coordination polyhedron is distorted octahedral), with incorporation of Cu II into the two-dimensional network. This led us to attempt a similar synthesis involving the substituted ligand diethyl(2-hydroxyethyl)amine, or N,N-diethylethanolamine, et 2 oen. Instead of the expected blue or black mixed-valence crystals, pale-yellow crystals of the title compound, (et 2 oenH)[Cu 2 (CN) 3 ], were formed, in which the amine base has been protonated and does not coordinate to any Cu atom. ISSN 2056-9890

Structural commentary
The title compound crystallizes as a three-dimensional anionic network, [Cu 2 (CN) 3 ] À , with the cationic protonated base occupying cavities in the network. Fig. 1 shows the structures for the asymmetric unit of the network and for the cation. The crystal structure may be considered to be built up from centrosymmetric Cu 2 (CN) 6 dimers linked together by Cu(CN) 3 units that are in rough trigonal-planar coordination (Fig. 2). The dimeric units are held together by two 3 -CN groups bonded to the dimer Cu2 atoms via the cyanide C atoms. There is a short Cu2Á Á ÁCu2 distance of 2.5171 (7) Å , similar to the distance in copper metal, 2.56 Å . While there is undoubtedly some form of interaction between the Cu2 atoms, the stereochemistry about the metal is easier to understand if the CuÁ Á ÁCu contacts are not considered. Then the Cu I atoms in the dimers are seen as bonded tetrahedrally to four cyanide groups, two pointing away from the dimer center, and the other two bridging the two Cu I atoms. Cu-C distances to the C atom of the bridging CN group are unequal, at 2.022 (3)  The asymmetric unit of the anionic network and of the guest cation for the title compound. Ellipsoids are drawn at the 40% probability level. Arbitrary temperature factors are used to show the H atoms, except for H13, which was refined.
vary from 110.73 (11) to 124.64 (11) , and the Cu1 atom is 0.088 (2) Å from the trigonal plane through its bonded atoms, N1, C2, and N3. Selected interatomic distances are given in Table 1. The cation forms a roughly spherical shape. There may be an intramolecular hydrogen bond between the N-H bond and the hydroxyl O atom. Possible disordering in the cation is discussed below. We were not able to locate the hydroxyl H atom. The hydroxyl O atom is 2.907 (4) Å from Cu1, lying above the trigonal coordination plane in an approximately axial position. We do not consider the O atom bonded to Cu1, however.
We interpret the structure as a Cu I complex, not the mixedvalence compound that was expected. In support of this, we cite the pale-yellow color of the compound, and also the silence in the electron spin resonance (esr) measurement (Bender, 2015). This interpretation requires the amine base to be protonated, for charge balance. There is indeed very clear evidence for protonation of the base N atom in the difference Fourier maps and in successful refinement of this as an unrestrained H atom. The syntheses were carried out at an initial pH of 12.4, higher than the pK a of the conjugate acid of the ethanolamine base, which we measured by titration at 9.9-10.2, depending on the ionic strength. The protonated base at this pH would be a minor component of the mixture, evidently selected by the need for charge balance as the solid polymer crystallizes.
Cu complex, {Cu II Cu I (-CN) 3 } n , which appears to be closely related to the present structure: it has similar unit-cell dimensions, the same space group, the same color, and the same CuCN network topology, with Cu positions close to those found here. These authors report a triethylamine solvent molecule in the network cavities. In light of the present work, we suggest that the triethylamine molecules in Jian et al.
(2012) might be protonated. Their complex would in that case be a Cu I anionic network complex similar to that reported here, rather than the mixed-valence complex they report.

Supramolecular features
The packing arrangement in the unit cell is shown in a projection down the a axis in Fig. 3, and down the c axis in Fig. 4. Atom Cu1 is trigonally coordinated by three CN groups, C1 N1, C2 N2, and C3 N3. C1 N1 also bonds with Cu2, one of the dimer Cu atoms, while C3 N3 coordinates to Cu2 atoms in both a dimer at (x, y, z) and at (x + 1, y, z), thus linking the dimers into a column along the a axis. C2 N2 forms a bridge to a Cu2 dimer atom related by the n glide plane, linking the columns into a three-dimensional network. Topology around Cu1 involves one 12-membered ring and two 18-membered rings.

Database survey
Searches of the Cambridge Structure Database (CSD, Version 5.35; Groom et al., 2016) yielded 35 structures containing the Cu(CN) 2 Cu fragment with two CN groups bridging the two Cu atoms via the C atom. To this list we added the structures of inorganic compounds CuCNÁNH 3 (Cromer et al., 1965), which contains the first example determined for this unit, and [CuCN] 3 ÁH 2 O (Kildea et al., 1985). CuÁ Á ÁCu distances averaged 2.53 Å , with a range of 2.31-2.69 Å . The corresponding distance in the present work is 2.5171 (7) Å , close to the observed mean. The Cu-C distances to the bridging C atom of the CN group are almost always significantly different. The shorter distance averages 2.00 Å with a limited range of 1.90-2.13 Å . The longer one ranges from 2.10 to 2.52 Å , with an average of 2.25 Å . The Cu-C distances of 2.022 (3) and 2.221 (3) Å in the present work again fall very close to these averages. There is a rough correlation between the CuÁ Á ÁCu distance and the longer Cu-C distance, as noted by Stocker et al. (1999).

Synthesis and crystallization
The compound studied was synthesized as follows: CuCN (23 mmol) and NaCN (39 mmol) were stirred in 8 ml of water until all solids dissolved. 40 mmol of N,N-(diethylamino)ethanol in 6 ml of water were added. The solution turned orange and slow evaporation yielded yellow crystals after several days (a green powder was also obtained in some preparations). We also prepared the compound by reduction of Cu II : 2 mmol CuSO 4 Á5H 2 O and 40 mmol N,N-(diethylamino)ethanol were dissolved in 15 ml of water, and 5 mmol of NaCN in 10 ml water were added. Needle-like crystals up to 2 mm long were yielded through slow evaporation.
Infra-red spectra obtained with both a Nicolet iS50 FT-IR and a Buck 550 machine showed three bands in the CN stretching region, with bands at 2072, 2099, and 2122 cm À1 . In addition, there is a strong, broad band at 3430 cm À1 , reflecting the presence of the OH group. This band is present also in the IR spectrum of neat N,N-diethylethanolamine, as well as in that of the corresponding hydrochloride salt.
A ground-up sample of the compound was shown to be esr silent (Bender, 2015), confirming the absence of Cu II species in the structure.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. Intensities of three standard reflections were measured every two h during the 114 h of data collection. A small overall decay of 2.1 (5)% in standard intensity was noted; no correction was made for this decay.
C-bound hydrogen atoms were constrained to idealized positions with C-H distances of 0.97 Å for CH 2 groups and 0.96 Å for CH 3 groups, and U eq values fixed at 1.2 times the U iso of their bonded C atoms. The methyl torsional angles were refined. The N-bound hydrogen atom was independently refined.
After convergence in initial refinements, we observed considerable anisotropy in the displacement ellipsoid for O10, in the substituted ethanolamine cation, indicating a possible disorder. This disorder hindered unambiguous detection of the hydroxyl H atom in difference Fourier maps. We have made extensive attempts to model the disorder without success. The models invariably led to poor geometry without improving the agreement between calculated and observed structure factors. If the geometry was restrained to reasonable values, the agreement became even poorer. Refinements of non-centric models were also carried out in light of the close approach between hydroxyl groups related by the center of symmetry at ( 1 2 , 0, 1 2 ). These were also unsuccessful. In an attempt to improve the electron density around the hydroxyl group, the intensity data were smoothed by a 12 parameter model with XABS2 (Parkin et al., 1995). The smoothing did improve the electron density and lowered the R-factor slightly, but did not improve refinements of the disordered models. The final model does not include any disorder in the cation.  Computer programs: CAD-4 (Enraf-Nonius, 1994), SHELXS97 and SHELXL97 (Sheldrick, 2008) and ORTEPIII (Burnett & Johnson, 1996). Data reduction followed procedures in Corfield et al. (1973); data were averaged with a local version of SORTAV (Blessing, 1989).
The cyanide groups are mainly ordered, as indicated by refinement of C and N occupancy factors. Results clearly indicated that C3 bridges the two Cu2 atoms, not N3, and C3 N3 was refined as ordered. Refined occupancies for the other cyanide groups were 77.8(1.4)% for C1 N1 and 89.7(1.4)% for C2 N2, indicating a favored orientation. Although these occupancies were significantly different from 100%, we chose to use ordered cyanide groups in our final model.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Geometric parameters (Å, º)