Crystal structure of N-butyl-2,3-bis(dicyclohexylamino)cyclopropeniminium chloride benzene monosolvate

The structure of the acid chloride salt of the superbase N-butyl-2,3-bis(dicyclohexylamino)cyclopropenimine is reported.


Chemical context
Pentasubstituted diaminopropenimines are a relatively new class of superbases that operate via the establishment of a stable aromatic electronic delocalization upon protonation. Originally reported as four-electron Lewis donors , a more recently exploited application for the use of pentasubstituion is that of a superbase, with one of the six nitrogen coordination sites available for protonation, making these molecules facile initiators of stereoselective Michael (Bandar & Lambert, 2012) and Mannich reactions (Bandar & Lambert, 2013), hydroaminations (Mirabdolbaghi & Dudding, 2015), and ring-opening polymerization (Stukenbroeker et al., 2015;Xu et al., 2018). A number of examples of acid salts of these species have been structurally characterized, permitting direct observation of the aromatized cyclopropeniminium structures (Stukenbroeker et al., 2015;Bruns et al., 2010;Bandar et al., 2015;Belding & Dudding, 2014;Guest et al., 2020;Kozma et al., 2015;Belding et al., 2016;Bandar & Lambert, 2012, 2013Mirabdolbaghi & Dudding, 2015). Examples of free-base pentasubstituted diaminopropenimines are uncommon, and these are typically only obtained with aromatic substituents at the imine position, which decreases the basicity of the imine by the delocalization of the nitrogen lone pair p-orbital into the aromatic group, facilitating isolation Kozma et al., 2015;Bruns et al., 2010). Some of these Kozma et al., 2015;Belding & Dudding, 2014) are bis(cylopropenimine) variants of the famous 'proton sponge ', 1,8-bis(dimethylamino)naphthalene and related classes of bifunctional Lewis superbases (Alder et al., 1968). The only other example, to our knowledge, is an Naminosubstituted example, which also decreases the basicity of the nitrogen lone pair by induction, a minor resonance structure delocalizing the double bond into the N-N contact, and, in the crystal structure, a nearby hydrogen bond with a water proton .
Compound 1 can mediate the polymerization of lactic acid to 99% completion in 10 minutes or less. However, no X-ray crystal structure of the free base, nor an acid salt of this superbase has been reported. In this report we provide the first X-ray crystallographic structure of a benzene solvate of the hydrochloride salt [1H]ClÁC 6 H 6 .

Structural commentary
[1H]Cl crystallizes in the P2 1 /n space group on a general position as a closely associated ion pair, with the protonation site at the n-butyl imine as expected, and one formula unit in the asymmetric unit, as well as one benzene molecule, also on a general position (Fig. 2). The organic salt and the benzene molecule are generally well ordered, except for the methyl carbon of the n-butyl group, which shows a mild wagging disorder. This disorder was treated with a two-site disorder model.
Free-base 1 would be expected to have localized double bonds at the n-butylimine C N position, and at the opposing cyclopropene position (see scheme). In the isolated free base of 1-mesityl-2,3-bis(diisopropylamino)cyclopropenimine , the unprotonated C N imine bond is 1.2951 (14) Å in length, while the C-N bonds to the tertiary amines are longer, at an average of 1.3494 (10) Å . The localized cyclopropene double bond is shorter, at 1.3712 (14) Å , than the single bonded C-C cyclopropene contacts at an average of 1.4155 (10) Å . Protonation of the n-butylimine position during crystal growth results in all nitrogen atoms being three-coordinate, leading to delocalization of the imine Figure 1 Catalytic ring-opening polymerization mediated by 1.

Figure 2
Displacement ellipsoid plot of the asymmetric unit of [1H]ClÁC 6 H 6 with ellipsoids set at the 50% probability level. Hydrogen atoms shown as small spheres.
double-bond character across all three C-N contacts. Correspondingly, the cyclopropene double bond is delocalized around the ring, giving a three-membered aromatic system. In [1H]Cl, the central C 3 N 3 triangle is thus highly planar, with the six atoms exhibiting an r.m.s. deviation of only 0.0052 Å from the best-fit plane of the six atoms. The three C-N bonds are approximately equal in length, with the two tertiary cyclohexylamine positions having C-N lengths of 1.3279 (13) Å on average. The C-N bond to the protonated butyl nitrogen is only slightly shorter at 1.319 (2) Å . The three cyclopropene C-C bonds exhibit lengths consistent with aromaticity; the unique C-C bond opposite the n-butyl group is 1.388 (2) Å , while the other two C-C bonds are similar or slightly shorter at 1.377 (2) and 1.383 (2) Å . Though these latter two bonds are equivalent under molecular point symmetry, their differences are attributed to the asymmetric crystal packing environment of the P2 1 /n space group. The comparable nature of the bond metrics of the three C-N bonds and the three cyclopropenyl C-C bonds is consistent with aromatization, and an analogous aromatization of the C 3 N 3 core of 1-mesityl-2,3-bis(diisopropylamino)cyclopropeniminium tetrafluoroborate was observed in the crystal structure of this salt . See Table 1 for C 3 N 3 bond metrics.
The comparison between free-base forms of cyclopropenimine  and the protonated forms demonstrate aromatization upon protonation, and explain the behavior of 1 as a superbase. While alkylimines are typically weak bases (pK a of conjugate acid ranges from about 2-5 (Fraser et al., 1983), the pK a of 1H + is a staggering 27 (Bandar & Lambert, 2012), more on the scale of a C-H bond. The drastic difference in basicity between typical alkylimines and 1 can be explained by the observed aromatization upon protonation. As a result, the 1 H resonance of the N-H hydrogen in [1H]Cl is a sharp singlet at 7.4 ppm in deuterated chloroform, suggesting little to no exchange like that typically observed for broad N-H resonances. The stabilization of a molecule by aromatization is quantified by the Dewar Resonance Energy (DRE), which ranges from about 6-25 kJ mol À1 per electron (Slayden & Liebman, 2001). Thus in the case of 1, aromatic stabilization between 12 and 50 kJ mol À1 upon protonation explains the large reported basicity.

Supramolecular features
Interionic/molecular interactions were examined using packing diagrams, and by the determination of partial atomic charge from Hirshfeld analysis. In the following discussion Hirshfeld charges are presented in parenthesis. The proton of the butylimine group (+0.121) interacts strongly with the chloride ion (À0.666) at a short HÁ Á ÁCl distance of 2.26 (2) Å . The chloride is positioned in a pocket surrounded by hydrogen atoms. In addition to the strong interaction with the acidic N-H proton, the chloride resides 2.8152 (7) Å from a benzene proton, H6AA (+0.046), and 2.7169 (6) Å from an intramolecular axial cyclohexyl proton, H29A (+0.050). The crystal packing demonstrates that the C 3 N 3 planes of all molecules pack parallel to each other (as required by the space-group symmetry), with a normal slightly oblique to the (101) plane (see Fig. 3). The molecular planes stack in a staggered fashion via intervening inversion centers at the origin (Fig. 3, red) and at the center of the a edge (Fig. 3, teal). One face of the benzene solvent molecule interacts distally with the cyclohexyl group of one 1H + ion [closest atomic CÁ Á ÁC distance: 3.829 (3) Table 1 Comparative bond lengths (Å ) for 1-mesityl-2,3-bis(diisopropylamino)cyclopropenimine, 1-mesityl-2,3-bis(diisopropylamino)cyclopropeniminium , and N-n-butyl-2,3-bis(dicyclohexyl)cyclopropeniminium.
way along the b axis ( Fig. 4, top). Viewed from 90 along the [101] direction, the benzene solvent molecules sit along a second channel, with the chloride ions sitting at the intersections of both channels, providing ionic bonds to the surrounding 1H + cations (Fig. 4,bottom). In this latter view, it is apparent that along the [101] direction, the chloride ions are positioned between the axial protons H26 (+0.058) and H29B (+0.059) of the flanking cyclohexyl groups. In summary, the 1H + cations interact with each other and through the benzene solvent molecule via their alkyl groups, and the chloride counter-ion is situated in a pocket of cyclohexyl and benzene C-H contacts, with the proximal N-H interaction on one side.

Database survey
In

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. A disordered methyl group was treated with a two-site disorder model, with atom positions freely refined, and relative occupancies refined using Free Variable 2 with a final ratio of 0.71 (3): 0.29 (3). RIGU/SIMU restraints were applied to the wagging methyl group. C-H hydrogen atoms were treated using a standard riding model. The imine proton was located as a peak in the Fourier difference map and was freely refined.
Hirshfeld charge was determined at the 3-21G/B3LYP level of theory by iterative computation of electronic structure of [1H]ClÁC 6 H 6 using ORCA (Neese, 2018) followed by rerefinement of the structure using non-spherical form factors computed using NoSpherA2 (Kleemiss et al., 2021), and repeating the process until the structure converged. Hirshfeld charges resulting from this approach are given in Table 3   SHELXL (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).  (7) Special details Experimental. Single-crystal X-ray crystallographic data were obtained on a Bruker D8 Quest PHOTON 100 diffractometer with an Oxford Cryostream 700 low-temperature device. The radiation was from a sealed-tube molybdenum Kα source with a TRIUMPH monochromator. Crystals were typically multiple, and a single piece was cut away with a razor blade, mounted on a MiTeGen loop with paratone-N oil, and collected at 100K in ω/φ scansets. Integration was performed using SAINT, and data were reduced and absorption-corrected using SADABS (Bruker, 2008). Space group determination was performed using XPREP (Sheldrick, 2008), and the structure was solved using intrinsic phasing using SHELXT (Sheldrick,The structural model of [1H]Cl·C 6 H 6 was refined using the least-squares approach with the ShelX package (Sheldrick, 2015a) with Olex2 as a GUI (Dolomanov et al., 2009). 2015b). 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.