Crystal structure of N,N,N′,N′-tetramethylethanediamine

N,N,N′,N′-tetramethylethanediamine, C6H16N2, crystallizes in the monoclinic crystal system in the space group P21/c. For the investigation of the conformation, quantum chemical methods were used and for intermolecular interactions, a Hirshfeld surface analysis was performed.

The title compound N,N,N 0 ,N 0 -tetramethylethanediamine, C 6 H 16 N 2 , is a bidentate amine ligand commonly used in organolithium chemistry for deaggregation. Crystals were grown at 243 K from n-pentane solution. The complete molecule is generated by a crystallographic center of symmetry and the conformation of the diamine is antiperiplanar. To investigate the intermolecular interactions, a Hirshfeld surface analysis was performed. It showed that HÁ Á ÁH (van der Waals) interactions dominate with a contact percentage of 92.3%. N,N,N 0 ,N 0 -tetramethylethanediamine (TMEDA, C 6 H 16 N 2 , 1) consists of two tertiary amine groups linked by an ethylene bridge. It can be used in cross-coupling or in olefin polymerization reactions where, e.g., a complex between dimethylnickel and TMEDA is used as a catalyst (Gö ttker-Schnetmann & Mecking, 2020). However, TMEDA is most commonly used in the chemistry of organolithium compounds. The lithium-carbon bond is characterized by its high polarity, as it contains a cationic lithium and carbanionic residues. These organolithium compounds form unreactive aggregates in non-polar solvents, which can be deaggregated by adding Lewis-basic ligands (Gessner et al., 2009). Compound 1 can be used as such a ligand, which can either chelate the metal center to form commonly dimeric structures or bridge two or more metal centers to form coordination polymers. The dimeric structural motif can be obtained in the butyl lithium TMEDA complex (Nichols & Williard, 1993), the enolate structure (Nichols et al., 2007) and in the phenyl(ethynyl) lithium (Schubert & Weiss, 1983), whereas the lithium diisopropylamide forms a polymeric structure with 1 bridging the lithium amide groups (Bernstein et al., 1992) (see scheme). The main benefit of deaggregation is the increased reactivity of organolithium compounds. Accompanying with smaller aggregates, the carbanionic center is more accessible for substrates due to an available coordination site at the metal center (Gessner et al., 2009). In the case of sterically more demanding ligands, however, the reactivity can even be reduced, since the coordination site at the lithium center can be sterically shielded (Knauer et al., 2019). Quantum chemical considerations of aggregation and deaggregation require knowledge of the energetically most favorable conformer. With knowledge about this conformer, quantum chemical equilibria can be used to calculate reasonable energies, for example to predict the reactivity or the formation of certain aggregates.

Structural commentary
Compound 1 crystallizes from n-pentane solution at 243 K in the monoclinic crystal system in space group P2 1 /c. The asymmetric unit consists of half of the molecule, with the other half generated by crystallographic inversion symmetry. The molecular structure of 1 is presented in Fig. 1 and selected bond angles are given in Table 1. The bond lengths for 1 are typical for C-C and C-N bonds and show no irregularities. The ethylene fragment is arranged in a staggered conformation where the nitrogen atoms are arranged in an antiperiplanar arrangement with N1-C3-C3 i -N1 i [symmetry code: (i) Àx, 1 À y, 1 À z] = 180.0 by symmetry. The conformations of the C1-N1-C3-C3 i and C2-N1-C3-C3 i groupings are anti [torsion angle = 167.33 (6) ] and gauche [-71.17 (8) ], respectively.
A quantum chemical geometry optimization was performed at the M062X/6-31+G(d,p) (Walker et al., 2013) theory level using Gaussian 16 (Frisch et al., 2016). The calculated geometry is shown in Fig. 2 and the bond angles are shown in Table 1. The angles of C2-N1-C1 and C2-N1-C3 differ by about 1 between the crystal structure and the quantum chemical calculated structure. Since these are gas phase calculations, this difference can be neglected. However, the N1-C3-C3 bond angles are in the same range. Therefore, we may assume that the presented conformation is at a local energy minimum.

Supramolecular features
The crystal packing of compound 1 is shown in Fig Table 1 Comparison of the bond angles ( ) between the crystal structure and the quantum computed structure..

Figure 2
Structure of 1 obtained by geometry optimization at the M062X/6-31+g(d,p) theory level.

Figure 3
A view along the b-axis direction of the crystal packing of 1.
a Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) was carried out: Fig. 4 illustrates the Hirshfeld surface mapped over d norm in the range from À0.072 to 1.201 arbitrary units and the related fingerprint plots generated by CrystalExplorer (Spackman et al., 2021;McKinnon et al., 2007). Weak van der Waals HÁ Á ÁH contacts are the largest region (92.3%). The remaining 6.7% are generated by NÁ Á ÁH contacts, whereby CÁ Á ÁH contacts do not contribute to crystal packing. In addition, no red spots are visible, which leads to the conclusion that the packing of the crystal is caused only by van der Waals interactions. The absence of packing effects such as hydrogen bonds also suggests that the most energetically favorable conformer is present.
Since 1 plays a major role in organolithium chemistry, it also finds application in the group of Strohmann et al. Thus, some publications are included here: t-butyl lithium N,N,N 0 ,N 0tetramethylethanediamine      (1 ml) was prepared at 243 K and 1 crystallized in the form of colorless blocks.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. For both compounds, the H atoms were positioned geometrically (C-H = 0.95-1.00 Å ) and refined using a riding model, with U iso (H) = 1.2U eq (C) for CH 2 and CH hydrogen atoms and U iso (H) = 1.5U eq (C) for CH 3 hydrogen atoms.

Computing details
Data collection: APEX2 (Bruker, 2018); cell refinement: SAINT (Bruker, 2018); data reduction: SAINT (Bruker, 2018); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009). 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.