Crystal structures of three N,N,N′-trisubstituted thioureas for reactivity-controlled nanocrystal synthesis

Crystal structures of three N,N,N′-trisubstituted thioureas, with varying substitution patterns, for reactivity-controlled nanocrystal synthesis are reported.


Chemical context
To control the size of colloidal nanocrystals, many traditional methods terminate the synthesis during the nanocrystal growth at the desired size. However, this leads to a lower yield, higher size dispersity, and it is difficult to get a good reproducibility (Owen et al., 2010;Abe et al., 2012Abe et al., , 2013. Therefore, Owen et al. suggest a new method that uses a library of substituted thioureas, whose substitution pattern tunes their conversion reactivity (Hendricks et al., 2015;Hamachi et al., 2017). By this, the nanocrystal concentration can be adjusted and the desired nanocrystal size can be obtained at full conversion, with a high degree of consistency. This control is obtained by varying the substitution pattern of the thiourea, and thus the conversion reactivity (Hens, 2015). This can be understood from the fact that the conversion reactivity is influenced by the number of substituents, and their electronic and steric properties. The conversion rate, i.e. reactivity, decreases as the number of substituents increases, or by replacing electron-withdrawing with electron-donating groups (e.g. substituting aryl for alkyl substituents). These thioureas are synthesized via a one-step click reaction between isothiocyanates and primary or secondary amines (Hendricks et al., 2015). In addition, they have a long shelf-life and are airstable after synthesis (Hendricks et al., 2015). An additional advantage of these precursors is that the starting reagents are relatively cheap and widely commercially available, in large quantities. When added to a hot solution of metal oleate, such as lead, cadmium, zinc, etc., this results in the formation of highly reproducible, monodisperse, homogeneously capped   Molecular structures of (a) 1, (b) 2 and (c) 3, showing thermal displacement ellipsoids drawn at the 50% probability level and the atom-labelling scheme for the non-hydrogen atoms. For 2, both molecules of the asymmetric unit are shown. N2/N22 is less pronounced towards the tertiary amine phenyl ring, with a C1-N2/C21-N22 distance of 1.352 (3)/ 1.345 (3) Å , because of the latter phenyl ring being almost perpendicular to the central N-C( S)-N plane. As a consequence of the improved delocalization of N1/N21 in 2, the C1 S1/C21 S21 bond length decreases slightly -although less significantly in the case of C1 S1 -to 1.6835 (17)/1.6798 (19) Å in comparison with 1.
The structure of 3 has very recently been deposited with the Cambridge Structural Database (CSD) (refcode OYOSIH; Rahman et al., 2021); however, the mentioned structure was determined at room temperature and showed disorder of both butyl substituents, as well as the presence of unknown solvent, which was treated by the SQUEEZE procedure in PLATON (Spek, 2015). Here, our reported structure was determined at 100 K and shows no signs of any kind of (solvent) disorder. The unknown solvate structure of Rahman et al. (2021) might be caused by the use of acetone as solvent and recrystallization by slow evaporation from EtOH, whereas we used toluene as solvent and recrystallized from a hot hexane:EtOH (10:1) mixture by slowly cooling down. Compound 3 crystallizes in the trigonal space group R3, with one N-phenyl-N 0 ,N 0di-n-butylthiourea molecule in the asymmetric unit. The phenyl substituent on the secondary amine is twisted with respect to the central N-C-S-N plane, with a C1-N1-C2-C7 torsion angle of 55.54 (16) , while the two butyl substituents are found completely staggered (Fig. 1c). The N1-C1 and C1-N2 bond distances are 1.3594 (15) and 1.3432 (15) Å , respectively, while the C1 S2 (double) bond distance is 1.7004 (11) Å . The delocalization of N1 towards the secondary amine phenyl substituent is also noticed here, comparable to 2, while there is minimal delocalization of N2 towards the butyl substituents, consequently showing the shortest C1-N2 and the longest C1 S1 distances.

Figure 5
Packing in the structure of 3, (a) viewed down the c axis, showing the hexamer ring assembly of molecules, around the threefold rotoinversion axes, with one hexamer highlighted (green). (b) Detail of one hydrogenbonded hexamer ring assembly. Hydrogen atoms (except involved in hydrogen bonds) are omitted for clarity.

Figure 4
Packing in the structure of 2, viewed down the a axis, showing the assembly of hydrogen-bonded pairs of molecules, with one pair highlighted (green). Hydrogen atoms (except involved in hydrogen bonds) are omitted for clarity.
As previously mentioned, OYOSIH (Rahman et al., 2021) represents the same structure as 3, although determined at room temperature and showed disorder of both butyl substituents, as well as the presence of unknown solvent, which was treated by the SQUEEZE procedure in PLATON (Spek, 2015)
Mass spectroscopy. Mass spectra (MS) were measured with an Agilent ESI single quadrupole detector type VL and an Agilent APCI single quadrupole detector type VL.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. For all structures, the amine N-H hydrogen atoms could be located from a difference-Fourier electron-density map, and were further refined with isotropic temperature factors fixed at 1.2 times U eq of the parent atoms. All other hydrogen atoms were refined in the riding mode with isotropic temperature factors fixed at 1.2 times U eq of the parent atoms (1.5 times for methyl groups).  (Dolomanov et al., 2009); software used to prepare material for publication: Mercury (Macrae et al., 2020).

N,N,N′-Tribenzylthiourea (1)
Crystal data 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.

N-methyl-N,N′-Diphenylthiourea (2)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.58 e Å −3 Δρ min = −0.25 e Å −3 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.