Crystal structure of a three-coordinate lithium complex with monodentate phenyloxazoline and hexamethyldisilylamide ligands

The reaction between lithium hexamethyldisilazane, [Li{N(Si(CH3)3)2}] (LiHMDS), with 4,4-dimethyl-2-phenyl-2-oxazoline (Phox) in hexane produced the title complex [Li{N(Si(CH3)3)2}(Phox)2]. The molecule, which crystallizes in the C2/c space group, lies on a twofold rotation axis and the lithium cation adopts a trigonal–planar coordination environment by the coordination, through nitrogen atoms, of one unit of hexamethyldisilazane and two units of Phox, both ligands in a monodentate mode.


Chemical context
Oxazolines are a family of cyclic aminoethers characterized by five-membered heterocyclic rings containing one unsaturation.They can be prepared using various methods that typically involve the cyclization of an aminoalcohol as a general process (Mulahmetovic & Hargaden, 2019).These compounds have been widely used in synthesis, catalysis, and as proligands in coordination chemistry (Connon et al., 2021;Liu et al., 2016;Rasappan et al., 2008;McManus & Guiry, 2004;Go ´mez et al., 1999).
Lithium hexamethyldisilylamide (LiHMDS), in turn, was first crystallographically characterized as a cyclic, trimeric compound with alternating nitrogen and lithium atoms in a planar six-membered ring (Mootz et al., 1969).This complex and other bulky MN(SiR 3 ) 2 bis(trialkylsilyl)amides (M = alkali metal; R = Me, Et, i Pr, t Bu, Ph, etc.) are widely used in organic synthesis as deprotonating agents due to their low nucleophilicity and strong Brønsted basicity (Neufeld et al., 2016;Tang et al., 2005;Beak et al., 1996), and in coordination chemistry as sterically demanding starting materials to impose low coordination numbers (Mohamed, 2010;Power, 2004).In both fields, the high solubility conferred by the trimethylsilyl substituents in a wide range of non-polar organic solvents is an advantage of working with HMDS À in synthetic procedures (Ojeda-Amador et al., 2016).

Structural commentary
The [Li{N(Si(CH 3 ) 3 ) 2 }(Phox) 2 ] complex crystallizes in the C2/c space group as a discrete molecular unit with no solvent in the unit cell.The asymmetric unit comprises half of the anionic hexamethyldisilylamide ligand, {N 0.5 (Si(CH 3 ) 3 )} 0.5-, and one neutral 4,4-dimethyl-2-phenyl-2-oxazoline molecule, both coordinated to the lithium centre solely by the nitrogen atoms.Proper rotation about a twofold axis passing through the Li-N1 bond (symmetry operation À x + 1, y, À z + 1 2 ) leads to the complete, neutral lithium complex molecule.The coordination sphere of lithium(I) presents a trigonal-planar geometry with no deviation from planarity, Fig. 1.In this environment, the two five-membered oxazoline rings (each with a planarity deviation of 0.082 � ) form a dihedral angle (�) of 35.81 (5) � .At the same time, the distance between the approximately face-to-face phenyl substituents equals 3.908 (5) A ˚, Fig. 2.
The Li-N bonds are 2.126 (2) and 1.942 (3) A ˚long for the oxazoline and amide ligands, respectively.Such a significant bond-length variation comes from the stronger interaction of the lithium centre with the anionic amide nitrogen (higher negative density charge) compared to the neutral oxazoline.Similar 0.1-0.2A ˚bond-distance differences have been used to distinguish 'ionic' from 'dative' Li-N bonds (Henderson et al., 1997;Gregory et al., 1991) and were reported earlier for other mononuclear, trigonal planar Li complexes containing View of the molecular structure of [Li{N(Si(CH 3 ) 3 ) 2 }(Phox) 2 ], with the atom-numbering scheme.Hydrogen atoms are omitted for clarity.Displacement ellipsoids are drawn at the 50% probability level.Symmetry code: (i) À x + 1, y, À z + 1 2 .

Figure 2
Distance between the phenyl rings (red dashed line) and dihedral angle (�) between the five-membered oxazoline rings.Grey: carbon; red: oxygen; blue: nitrogen; light blue: silicon; purple: lithium.Displacement ellipsoids are drawn at the 50% probability level.Hydrogen atoms are excluded for clarity.

Supramolecular features
Hirshfeld surface analysis performed with the CrystalExplorer 21.5 software (Spackman et al., 2021) allowed a comprehensive examination of the non-covalent bonds governing the solid-state structure of [Li{N(Si(CH 3 ) 3 ) 2 }(Phox) 2 ].A pivotal aspect of this analysis involves the generation of 2D fingerprint plots (FP), which offer two-dimensional projections of the Hirshfeld surface (Hirshfeld, 1977).This enables meticulous analysis of the non-covalent forces supporting the supramolecular structure by quantifying the percentage contribution of each interaction.For [Li{N(Si(CH 3 ) 3 ) 2 }-(Phox) 2 ], the percentages of the total surface area corresponding to the H� � �H, C� � �H, and O� � �H contacts account for 82.2%, 11.5%, and 6.2%, respectively (Fig. 4).The frail O� � �H contacts occur between the hydrogen atoms of the methylene groups in the oxazoline rings and the oxygen atom of the adjacent molecules at 2.843 A ˚.Besides those, there is only a weak intramolecular �-� stacking interaction between the aromatic rings of the Phox ligands, as already depicted in Fig. 2, which are 3.908 (5) A ˚far from one another.There is no classic intra or intermolecular hydrogen bonding in the molecule.

Database survey
To the best of our knowledge, this is the first example of a heteroleptic lithium(I)-oxazoline complex in which the Phox ligands are monodentate and the HMDS anion adopts a nearly eclipsed conformation of the methyl groups, giving rise to a trigonal planar coordination environment about the metal.The combination of ten methyl and two phenyl substituents in the ligands efficiently shields the central ion (Fig. 3, right) and prevents significant intermolecular contacts involving the metal.

Synthesis and crystallization
The reactions were carried out under dinitrogen (99.999%,Praxair or Air Liquide) using Schlenk techniques.Analytical grade 2-amino-2-methyl-1-propanol, ethyleneglycol, potassium carbonate, glycerol, benzonitrile, n-butyl lithium (2.5 mol L À 1 solution in hexanes) and hexamethyldisilazane were acquired from commercial sources (Sigma-Aldrich, Merck, Synth) and used without purification.Hexane (Honeywell) was dried by standard methods (Armarego & Perrin, 1997) and distilled under N 2 (g) immediately before use.Lithium hexamethyldisilylamide, LiHMDS (Tai et al., 2017), and Phox (Mulahmetovic & Hargaden, 2019) were prepared using procedures adapted from the literature; Phox was distilled twice under vacuum before use.A solution of 0.472 g (2.85 mmol) of LiHMDS in 10 mL of hexane was added slowly to a hexane solution of 0.500 g (2.85 mmol) of Phox at 273 K.The colourless reaction mixture was stirred at room temperature for about 5 h and filtered through Celite.The resulting colourless solution was cooled down to 253 K for two days, after which block-shaped colourless crystals were isolated by filtration and dried under vacuum.Total yield: 0.621g (85.0%) based on the [Li{N(Si(CH 3 ) 3 ) 2 }(Phox) 2 ] formulation.

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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )

Figure 3
Figure 3 Left: Molecular structure of [Li{N(Si(CH 3 ) 3 ) 2 }(Phox) 2 ], showing the eclipsed conformation of the hexamethyldisilylamide ligands.Hydrogen atoms are omitted for clarity.Displacement ellipsoids are drawn at the 50% probability level.Right: Space-filling representation of the title compound, emphasizing the efficient shielding of the lithium cation by the methyl and phenyl substituents of the HMDS À and Phox ligands.

Figure 4 (
Figure 4 (a) Representation of the Hirshfeld surface for [Li{N(Si(CH 3 ) 3 ) 2 }(Phox) 2 ], highlighting the intermolecular O� � �H contacts (green dashed lines) between the oxazoline's methylene moiety and the oxygen atom of the adjacent molecule.Grey: carbon; red: oxygen; blue: nitrogen; light blue: silicon; purple: lithium.Hydrogen atoms are omitted for clarity.(b) Two-dimensional fingerprint plots in the d norm function, generated by mapping, for each d i , the region between 0.4 and 2.6 A ˚from the surface to the nearest internal (d i ) and external (d e ) atoms.

Table 1
Experimental details.