1. 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; Gómez et al., 1999).

Metal complexes containing monodentate, N-donor monooxazoline ligands are reasonably frequent in the solid state (Huang et al., 2014; Del Río & Gossage, 2009; Barclay et al., 2003; Valk et al., 1994; Michelin et al., 1991). In the case of 2-phenyl­oxazolines (Phox), which are of inter­est to this work, the chelating behaviour is more common and involves a second donor atom, generally N, O, S or Se, in the ortho position of the aromatic ring (Volpe et al., 2010; Bottini et al., 2010). Phox complexes of d-block metals have been studied as catalysts (Abu-Elfotoh, 2017; Bagherzadeh et al., 2008; Kandasamy et al., 2004).

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(SiR₃)₂ bis­(tri­alkyl­sil­yl)amides (M = alkali metal; R = Me, Et, ⁱPr, ^tBu, 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⁻ in synthetic procedures (Ojeda-Amador et al., 2016).

In this paper, we report the synthesis, crystal and mol­ecular structures of the mononuclear, three-coordinate lithium(I) complex [Li{N(Si(CH₃)₃)₂}(Phox)₂] (Phox = 4,4-dimethyl-2-phenyl-2-oxazoline). The product crystallized directly from the reaction mixture at 253 K, following an attempt to deprotonate the oxazoline by reaction between [LiN(Si(CH₃)₃)₂] and Phox in hexane.

2. Structural commentary

The [Li{N(Si(CH₃)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(CH₃)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 −x + 1, y, −z + ) 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 (α) of 35.81 (5)°. At the same time, the distance between the approximately face-to-face phenyl substituents equals 3.908 (5) Å, Fig. 2.

Figure 1 View of the mol­ecular structure of [Li{N(Si(CH3)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) −x + 1, y, −z + .

Figure 2 Distance between the phenyl rings (red dashed line) and dihedral angle (α) 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.

The Li—N bonds are 2.126 (2) and 1.942 (3) Å 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 0.1–0.2 Å 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 an amide (N{SiR₃}₂⁻, NArH⁻) and a neutral ligand such as sparteine, N,N,N′,N′-tetra­methyl­ethylenedi­amine, and N′,N′,N′′,N′′,N′′′-tetra­methyl­ethylenedi­amine (Clark et al., 2009; Henderson et al., 1997; Fjeldberg et al., 1984). The Li—N_oxazoline bond is longer than analogous bonds reported for dimeric lithium(I) complexes with bidentate [Jantzi et al., 2006; average 2.044 (3) Å] and tridentate oxazoline ligands [Stol et al., 2005; average 1.996 (3) Å]; the difference can be ascribed to distinct Li⁺ coordination numbers and ligands' denticities and bulk. To our knowledge, the title compound is the first example of a monodentate oxazoline complex of lithium(I) structurally characterized in the solid state.

In [Li{N(Si(CH₃)₃)₂}(Phox)₂], the methyl groups of the hexa­methyl­disilyl­amide moieties are nearly eclipsed (Fig. 3), with a dihedral angle of 5.80 (7)° between the C3—Si—N1 and the C3ⁱ—Siⁱ—N1 planes. Additionally, if one defines a vector connecting the symmetry-related Si and Siⁱ centres (Randazzo et al., 2006), the relative positions of the C3/C3ⁱ, C2/C1ⁱ and C1/C2ⁱ pairs across this vector produce C—Si⋯Siⁱ—Cⁱ torsion angles of −8.846 (11), −2.52 (7) and −2.52 (7)°, respectively (average value −4.63°). These figures are close to the 0° value attributed to the eclipsed conformation (Eliel & Wilen, 1994). Small torsion angles also appear in the amide ligand of [Cs{N(SiMe₃)₂}(tmeea)] (tmeea = tris­[2-(2-meth­oxy­eth­oxy)eth­yl]amine)], [{K(N{SiMe₃}₂)(^tBu-C₆H₅)}₂], and [{K(N{SiMe₃}₂)(Me₃C₆H₃)}₂], with average values of −3.81 (4), 4.70 and 0.75° respectively (Ojeda-Amador et al., 2016; Randazzo et al., 2006). A larger deviation from 0° appears in [K(18-crown-6)][Li{N(SiMe₃)₂}₂] (mean torsion angle −11.40°; Davison et al., 2023) and in the polymeric [{(Me₃Si)₂NLi}{(Me₃Si)₂NK}]_∞ (–11.94°; Morris et al., 2007). On the other hand, the methyl groups of unsolvated K(N{SiMe₃}₂) adopt an inter­mediate mean torsion angle of 38.29° (Tesh et al., 1990), while in [Li{N(SiMe₃)₂}(Me₆Tren)] (Me₆Tren = tris­[2-(di­methyl­amino)­eth­yl]amine) the average angle is 59.9°, almost exactly the ideal value of 60° for a staggered conformation (Cousins et al., 2010). In the present work, eclipsing implies less repulsion between the bulky methyl groups of both the hexa­methyl­disilyl­amide and Phox ligands than in the staggered arrangement.

Figure 3 Left: Mol­ecular structure of [Li{N(Si(CH3)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− and Phox ligands.

3. 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(CH₃)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(CH₃)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 Å. Besides those, there is only a weak intra­molecular π–π stacking inter­action between the aromatic rings of the Phox ligands, as already depicted in Fig. 2, which are 3.908 (5) Å far from one another. There is no classic intra or inter­molecular hydrogen bonding in the mol­ecule.

Figure 4 (a) Representation of the Hirshfeld surface for [Li{N(Si(CH3)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 dnorm function, generated by mapping, for each di, the region between 0.4 and 2.6 Å from the surface to the nearest inter­nal (di) and external (de) atoms.

4. 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.

5. 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⁻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 N₂(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(CH₃)₃)₂}(Phox)₂] formulation.

6. Refinement

Table 1 summarizes crystal data, data collection, and structure refinement details. The hydrogen atoms were located in difference-Fourier maps and were refined freely, except for the hydrogen atoms attached to C1, C6, C7, C11, C14, and C15 atoms, for which distance restraints (DFIX) were applied.

Table 1 Experimental details Crystal data Chemical formula [Li(C6H18NSi2)(C11H13NO)2] Mr 517.78 Crystal system, space group Monoclinic, C2/c Temperature (K) 100 a, b, c (Å) 15.612 (2), 12.7649 (17), 17.180 (4) β (°) 116.243 (5) V (Å3) 3070.9 (9) Z 4 Radiation type Mo Kα μ (mm−1) 0.14 Crystal size (mm) 0.50 × 0.44 × 0.28   Data collection Diffractometer Bruker D8 Venture/Photon 100 CMOS Absorption correction Multi-scan (SADABS; Krause et al., 2015) Tmin, Tmax 0.709, 0.746 No. of measured, independent and observed [I > 2σ(I)] reflections 44358, 3522, 2888 Rint 0.078 (sin θ/λ)max (Å−1) 0.650   Refinement R[F2 > 2σ(F2)], wR(F2), S 0.035, 0.092, 1.04 No. of reflections 3522 No. of parameters 252 No. of restraints 7 H-atom treatment All H-atom parameters refined Δρmax, Δρmin (e Å−3) 0.29, −0.24 Computer programs: APEX3 and SAINT (Bruker, 2015), SHELXS2008 (Sheldrick 2008), SHELXL2018/3 (Sheldrick, 2015), DIAMOND (Brandenburg & Putz, 1999), and Mercury (Macrae et al., 2020) and WinGX (Farrugia, 2012).