Crystal structure of 1-butyl-3-{2-[(indan-5-yl)amino]-2-oxoethyl}-1H-imidazol-3-ium chloride

A new amide moiety bearing an imidazolium salt as precursor to an N-heterocyclic carbene was synthesized. The synthetic procedure, the compound’s characterization and its crystal structure, including a comparison of geometrical parameters with related compounds, are reported and discussed.


Chemical context
N-Heterocyclic carbenes (NHCs) are neutral compounds in which a 6e À -containing divalent carbon atom is placed between two hetero atoms. They are typically derived from their parent imidazolium salts by deprotonation of the carbon atom located in between the two nitrogen atoms (Bhatia et al., 2013). The high reactivity in the case of carbenes can be attributed to the presence of an incomplete octet resulting in a strong electron-donating ability (Hopkinson et al., 2014). Arduengo was the first to successfully isolate a free carbene and characterize it by obtaining a single crystal X-ray structure for the same. This study opened a new era in organic chemistry allowing the investigation of the so-called NHCs as ligands (Arduengo et al., 1991). To date, a tremendous amount of research on NHCs has enhanced the popularity of these carbene compounds in organic synthesis, organometallic chemistry, organocatalysis, medicinal and pharmaceutical applications and essentially every discipline of modern day science. Over the past two decades, N-heterocyclic carbene (NHC) ligands have been among the most exploited in organic synthesis. They can be considered superior to phosphine ligands as their electronic and steric properties can be easily fine-tuned by simple variations in their structures (Díez-Gonzá lez et al., 2009;Hermann, 2002;Froese et al., 2017). Attempts have been made to tune or modify the electronic and steric properties of NHCs by changing the substituent at one or both nitrogen centres. These changes in electronics and steric properties may further provide subtle information about the mechanism of catalytic transformations (Huynh, 2018;Peris, 2018). Although the term hemilability for a coordinated ligand was first introduced in 1979 (Jeffrey & Rauchfuss, 1979), the first hemilabile NHC ligand was developed some twenty years later (McGuinness & Cavell, 2000). The presence of hemilabile coordination sites in a ligand system plays a crucial role in catalysis as well as in biological sciences (cytotoxicity). The modular electronic and steric properties of the hemilabile ligand systems provide extra stability to transition metal complexes (Peris, 2018;Normand & Cavell, 2008). Herein we present the synthesis and crystal structure of the chloride salt of the potentially hemilabile amido-functionalized NHC ligand precursor, 1-butyl-3-{2-[(indan-5-yl)amino]-2-oxoethyl}-1H-imidazol-3-ium.

Structural commentary
The title compound, consists of a chloride anion and an Nsubstituted imidazolium cation, combining the NHC precursor moiety with a amide (-NH-C(O)-CH 2 -) moiety. The amide group is linked to one nitrogen of the imidazolium ring, N2, by a methylene group, and bears on its opposite side a indanyl substituent bound to the amide nitrogen atom N1. The other (non-amidic) substituent on the second nitrogen atom, N3, of the imidazolium ring is an extended n-butyl chain, whose mean plane (C15-C18) is inclined to the plane of the imidazolium ring (N2/N3/C12-C14) by 73.2 (6) . The central -CH 2 -C atom, C1, of the indanyl substituent is disordered over two positions with a refined occupancy ratio of C1:C1 0 = 0.84 (2):0.16 (2). Atom C1 resides 0.393 (12) Å below the plane (on the opposite side of the imidazolium moiety) of the four planar C atoms of the pentene ring (C2-C5), while atom C1 0 is 0.40 (4) Å above this plane (i.e. on the same side as the imidazolium moiety).
Crystallographic data of NHC precursor cations substituted by CH 2 -C(O)-NH functional groups (amides) are relatively scarce. A search of the Cambridge Structural Database (CSD, version 3.59, August 2018; Groom et al., 2016) yielded only 16 hits. Compared to published values of imidazolium salts with amide substituents, the geometrical parameters of the title compound are decidedly unexceptional, falling within the reported ranges. Only the C-C bond between methylene atom C11 and the carbonyl carbon C10 is relatively short [C10-C11 = 1.506 (6) Å ] and thereby close to the shortest such bond reported to date, i.e. 1.502 Å for a related compound with no substituent on the amide and a dodecyl chain on the other side of the imidazolium cation (Lee et al., 2003a). In general, all bond lengths of the two moieties and the methylene linker are in rather close ranges with the largest differences observed being those which lead to further substituents. These are the N-C bond of the amide to its substituent on N ranging from ca 1.409 Å for a phenyl (Samantaray et al., 2007) to 1.482 Å for a t-butyl (Ray et al., 2007), and the N-C bond of the imidazolium ring to the second substituent ranging from ca 1.422 Å for a pyrimidyl (Lee et al., 2009) to 1.483 Å for a rather bulky 3,5-di-tert-butyl-2-hydroxybenzyl (Wan & Zhang, 2016). Given the variety of the substituents on both sides in published structures, this observation is not surprising as the potential extension of the system beyond the imidazolium and amide moieties would be expected to have a considerable influence on these bond lengths. Strictly within the imidazolium and amide moieties, the strongest deviation is found for the amide C(O)-N bond [here C10-N1 = 1.339 (6) Å ] ranging from ca 1.301 Å for an unsubstituted amide, i.e. -C(O)-NH 2 , (Lee et al., 2003b) to 1.355 Å for a phenyl-substituted amide (Lee & Zeng, 2012). A shorter C10-N1 bond is indicative of a strong tautomeric effect, i.e. C O double-bond delocalization towards a C N double bond. In the title compound, the nitrogen atom of the amide is bound to an indanyl group and the C10-N1 bond length of 1.339 (6) Å comprises a rather average value for a -C(O)-NH-bond. A value that often varies in such compounds is the angle at which the plane of the amide moiety [-CH 2 -C(O)-NH, calculated without H-atom positions] is arranged with respect to the imidazolium ring plane (C 3 N 2 ). Here the dihedral angles range from ca 42.64 (Lee et al., 2003b) to 85.95 . In the title compound, this dihedral angle is 71.9 (3) . At this angle, resonance between the two moieties (amide and imidazolium) can clearly be excluded. In contrast, the angle between the amide moiety and the aromatic ring of the indanyl substituent is only 18.1 (2) , suggesting together with the N1-C8 bond length of only 1.427 (6) Å , that the resonance of the aromatic ring extends to the amide moiety and/or vice versa. This relative orientation of the two aromatic systems is probably supported by a weak intramolecular C-HÁ Á ÁO hydrogen bond, between the amide oxygen atom (O1) and the aromatic carbon atom C9 (Table 1 and Fig. 1).

Supramolecular features
The comparably large unit cell of the crystal structure with Z = 8 is rather thin with a short b axis of 5.3986 (11) Å , and the eight imidazolium cations are arranged in a single layer within the cell. The chloride anions and imidazolium cations form symmetric pairs, two-by-two, supported by C-HÁ Á ÁCl hydrogen bonds involving hydrogen atoms of an n-butyl methylene C atom (C15-H15B), an imidazolium C atom (C12-H12), the amide N atom (N1-H1) and a C atom of the methylene linker (C11-H11A), and the chloride anion Cl1 (Table 1 and Fig. 2). In the crystal, these two-by-two units are linked by C11-H11BÁ Á ÁCl1 ii hydrogen bonds, forming slablike structures propagating along the b-axis direction (Table 1 and Fig. 3). Weak C13-H13Á Á ÁO1 iii interactions link the slabs to form layers lying parallel to the bc plane (Table 1 and Fig. 3)

Synthesis and crystallization
The title compound, was synthesized by the simple reaction of n-butyl imidazole with 2-chloro-N-(indan-5-yl)acetamide in dry acetonitrile as solvent. All reagents and solvents required for the synthesis were purchased commercially and used without any further purification.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The N-bound hydrogen atom (H1) was located in a difference-Fourier map and freely refined. The C-bound H atoms were placed in calculated positions and treated as riding: C-H = 0.95-0.99 Å with U iso (H) = 1.5U eq (Cmethyl) and 1.2U eq (C) for other H atoms.
The methylene carbon atom C1 of the indanyl substituent is disordered over two positions with a refined occupancy ratio of 0.84 (2):0.16 (2). This disorder was modelled with constraints (SADI for all C-C bonds involving C1, SIMU and DELU). The crystal studied was refined as a twin with matrix [1 0 0.9, 0 1 0, 0 0 1]; the resulting BASF value is 0.30.

Figure 2
A view of the two-by-two hydrogen-bonded unit (dashed lines; see Table 1 for details). Only the H atoms (grey balls) involved in the intra-and intermolecular interactions have been included. The unlabelled atoms are related to the labelled atoms by the symmetry operation Àx + 1 2 , Ày + 3 2 , Àz.

1-Butyl-3-{2-[(indan-5-yl)amino]-2-oxoethyl}-1H-imidazol-3-ium chloride
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. Refinement. Refined as a 2-component twin.