Reductive cleavage of N,N′-di-tert-butylcarbodiimide generates tert-butylcyanamide ligands, (Me3CNCN)−, that bind potassium both end-on and side-on in the same single crystal

The crystal structure is reported of a single crystal containing potassium salts of two different tert-butylcyanamide anions that co-crystallized with one equivalent of 1,3-di-tert-butyl urea.


Chemical context
A crystal containing two different potassium 18-crown-6 salts of tert-butylcyanamide anions, (Me 3 CNCN) À , and one equivalent of 1,3-di-tert-butyl urea, Fig. 1, was isolated during the reduction of incompletely dried N,N 0 -di-tert-butylcarbodiimide with [K(18-crown-6) 2 ][Gd II (NR 2 ) 3 ]. A reductive N-C bond cleavage evidently occurred to remove a tert-butyl group from the starting carbodiimide forming an (Me 3 CNCN) À tert-butylcyanamide anion that has not previously been observed as a ligand. This reaction could be attributed to the presence of the highly reducing Gd II ion (Ryan et al., 2018(Ryan et al., , 2020. The urea component of the crystal is a formal hydrolysis product of di-tert-butylcarbodiimide. The presence of water in this reaction system is evident from the fact that one of the 18-crown-6 counter-cations is aquated. The reduction of carbodiimides with Sm II bis(trimethylsilyl)amides, which are not as reducing as Gd II , has been known to form oxalamidinates and amidinates (Deacon et al., 2007).
The presence of both end-on and side-on bound tertbutylcyanamide anions in the same single crystal suggests that these two forms of this ligand are similar in energy. Nature did not pick one over the other during the crystallization process. Hence, this could be a versatile ligand depending on the coordination environment of the cation. In addition, the presence of urea in the single crystal with its hydrogenbonding connections suggests that this could be a valuable addition to crystallizations to construct complicated assemblies, as found here.
Although the basic structure of the anions is similar, their interactions with the potassium counter-cations are different. The K1-N1 distance of 3.027 (2) Å in the component with an end-on bound anion is considerably longer than the 2.699 (2) Å K2-N3 distance of the side-on form. The 3.197 (3) Å K2-C30 distance in the side-on component is considerably longer than either of these K-N distances.
The co-crystallized di-tert-butyl urea has metrical parameters identical within experimental error to the three structures in the literature (Gel'bol'dt et al., 2003(Gel'bol'dt et al., , 2005So et al., 2014 ORTEP representation of tert-butylcyanamide anions bound side-on and end-on and 1,3-di-tert-butyl urea, with displacement ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity.  Figure 1 The reaction scheme.

Supramolecular features
As shown in Fig. 3, the three components of the crystal are hydrogen bonded (

Synthesis and crystallization
N,N 0 -Di-tert-butylcarbodiimide was added dropwise to a darkblue solution of [K(18-crown-6) 2 ][Gd II (NR 2 ) 3 ] (R = SiMe 3 ) (30 mg, 0.026 mmol) in diethyl ether (5 mL) at 238 K. The solution changed from dark blue to colorless after a few minutes. Methylcyclohexane was layered into the solution and the solution was kept at 238 K, but no crystals were obtained. Solvent was removed to produce a white solid that was dissolved in toluene and placed in a vapor diffusion set up with hexanes. After 5 days, small colorless crystals were collected.

Aqua(tert-butylcyanamidato)(1,4,7,10,13,16-hexaoxacyclooctadecane)potassium(I)-(tert-butylcyanamidato)
(1,4,7,10,13,16-hexaoxacyclooctadecane)potassium(I)-N,N′-di-tert-butylcarbodiimide (1/1/1)   (13) 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. A colorless crystal of approximate dimensions 0.140 x 0.150 x 0.474 mm was mounted on a glass fiber and transferred to a Bruker SMART APEX II diffractometer. The APEX2 program package was used to determine the unitcell parameters and for data collection (45 sec/frame scan time for a sphere of diffraction data). The raw frame data was processed using SAINT and SADABS to yield the reflection data file. Subsequent calculations were carried out using the SHELXTL4 program. The diffraction symmetry was mmm and the systematic absences were consistent with the orthorhombic space groups Pnma and Pna21. It was later determined that space group Pna21 was correct. The structure was solved by direct methods and refined on F2 by full-matrix least-squares techniques. The analytical scattering factors for neutral atoms were used throughout the analysis. Hydrogen atoms involved in hydrogen bonding were located from a difference-Fourier map and refined (x,y,z and Uiso). The remaining hydrogen atoms were included using a riding model. Least-squares analysis yielded wR2 = 0.0649 and Goof = 1.017 for 614 variables refined against 11018 data (0.80 Å), R1 = 0.0326 for those 9556 data with I > 2.0sigma(I). The absolute structure was assigned by refinement of the Flack parameter.