Molecularly Engineered Lithium–Chromium Alkoxide for Selective Synthesis of LiCrO₂ and Li₂CrO₄ Nanomaterials

Abstract

Achieving control over the phase-selective synthesis of mixed metal oxide materials remains a challenge to the synthetic chemist due to diffusion-driven growth, which necessitates the search for new compounds with pre-existent chemical bonds between the phase-forming elements. We report here a simple solvothermal process to fabricate LiCrO₂ and Li₂CrO₄ nanoparticles from bimetallic single-source precursors, demonstrating the distinctive influence of molecular design and calcination conditions on the resulting nanomaterials. The chemical identity of [Li₂Cr(O^tBu)₄Cl(THF)₂] (1) and [LiCr(O^tBu)₂(PyCH=COCF₃)₂(THF)₂] (2) was unambiguously established in the solid state by single-crystal X-ray diffraction, revealing the formation of a coordination polymeric chain in compound 1, whereas electron paramagnetic resonance spectroscopy (EPR) studies revealed a monomeric structure in solution. TEM analysis of synthesized LiCrO₂ nanoparticles showed nearly uniform particles size of approximately 20 nm. The sensitivity of the LiCrO₂ phase towards oxidation was investigated by X-ray diffraction, revealing the formation of the stable Li₂CrO₄ after calcination in air.

1. Introduction

The chemical synthesis of two-dimensional transition metal oxides has attracted considerable research attention due to their interesting physical, chemical, and electronic properties [1]. In particular, LiCrO₂ with a layered triangular arrangement of Cr³⁺ exhibits antiferromagnetic nature with two-dimensional (2D) frustration and interesting structural and magnetic behavior [2,3]. In general, geometrically frustrated magnetic materials are being widely studied, possibly due to their natural inclination towards unconventional ground states, which often leads to a suppression of long-range magnetic ordering and promotes short-range magnetic correlations owing to fluctuations between nearly or totally degenerated magnetic ground states [4,5]. Moreover, in lithium-ion batteries, the cathode materials are mostly layered compounds of lithium transition metal oxides (LiMO₂, where M = V, Cr, Mn, Fe, Co, Ni, etc.), among which Cr-based cathode materials have attracted significant attention due to possible multiple electron transfer pathways and stable cyclability, which results from the three-electron redox couple Cr³⁺/Cr⁶⁺ [6,7,8,9]. The growing interest in the synthesis of lower-valency chromium complexes is due to their potential applications as catalysts in ring-opening polymerization reactions and as precursors for corresponding metal oxides [10,11]. However, only a few chromium(III) alkoxides are known, and their reactivity and structural patterns are not well established [12,13,14,15,16]. To our best knowledge, there is no report on the preparation of stoichiometric LiCrO₂ nanomaterials from molecular single-source precursors [17,18,19]. The pre-existent chemical bonds between the phase-forming elements in molecular precursors can facilitate the crystallization of materials at relatively lower temperature when compared to solid-state synthesis [20,21,22].

We demonstrate here, for the first time, the influence of molecular-level processing in the controlled synthesis of LiCrO₂ nanomaterials with stoichiometry control between the heterometallic precursors and the resulting mixed oxide ceramics, which is rather difficult in preparation with the solid-state or multi-source precursor approach.

2. Results and Discussion

2.1. Synthesis of [Li₂Cr(O^tBu)₄Cl(THF)₂]_n (1)

The heterometallic alkoxide [Li₂Cr(O^tBu)₄Cl(THF)₂] (1) was synthesized via a salt metathesis reaction between solvated CrCl₃ activated in THF (CrCl₃(THF)₃) and four equivalents of LiO^tBu in THF/n-heptane (Scheme 1). The color of the reaction mixture changed from pale blue to purple after 12 h of stirring at room temperature. The excess solvent was removed under vacuum, and the resulting product was purified by washing with n-heptane. The obtained product was the chloride-containing compound [Li₂Cr(O^tBu)₄Cl(THF)₂]_n (1) that forms a one-dimensional polymeric chain in the solid-state as revealed by single-crystal X-ray structure analysis. Elemental analysis showed deviations of carbon (found: 50.93%, calc.: 53.58%), possibly due to partial hydrolysis during the sample preparation.

2.1.1. X-ray Crystallography of Compound 1

The bimetallic lithium–chromium(III) compound 1 was isolated in THF at −18 °C and crystallized in the monoclinic space group C2/c with four molecules per unit cell. The asymmetric unit of compound 1 as shown in Figure 1 consisted of a trinuclear framework with the central chromium(III) atom adopting a distorted tetrahedral coordination sphere with four tert-butoxy ligands bridged to two peripheral lithium atoms. The bond distances between chromium and oxygen observed in compound 1 were 1.862 and 1.871 Å, respectively. The bond angles around the oxygen of the tert-butoxy ligand and chromium center were 83.03° and 124.56°, which deviates from ideal tetrahedral geometry (109.5°) due to the steric demand of the tert-butoxy groups. The peripheral lithium atoms were coordinated in a tetrahedral fashion by an additional THF and a chloride ligand per lithium atom. Moreover, the observed lithium–oxygen (Li–O) distances of 1.924 and 1.947 Å between the lithium center and the tert-butoxy ligand were shorter than the 1.963 Å observed between the lithium center and coordinating THF molecule, while the Li–Cl distance was 2.286 Å. The one-dimensional polymeric chain resulted from the Li–Cl–Li connection between the trinuclear asymmetric units. [Li₂Cr(O^tBu)₄Cl(THF)₂] (1) is isostructural to the mixed iron(III)/lithium bromo alkoxide reported by Barley et al. [23] and Mantymaki et al [24]. The structure can be fully described with the formula catena-poly[[tetra-µ₂-tert-butoxo-1:2κ⁴O:O;1:3κ⁴O:O-bis(tetrahydrofuran)-2κO,3κO-chromium(III)-dilithium(I)]-µ-chloro].

2.1.2. Electron Paramagnetic Resonance Spectroscopy (EPR) of Compound 1

Electron paramagnetic resonance spectroscopy (EPR) of the synthesized complex was measured on the X-band using a 2,2-diphenyl-1-picrylhydrazyl (DPPH) reference. The EPR spectrum of compound 1 showed one broad resonance apparently resulting from the presence of a mononuclear chromium(III) species (S = 3/2, d³) in solution and exhibited a Landé factor g(iso) of 1.972. The little hyper-fine coupling observed in the spectra could be attributed to the less-abundant chromium isotope ⁵³Cr with I = 3/2 (9.5% nat. abundance, Figure 2 inset). The feature at around 3388 G is reflected in the smaller feature at ~3416 G for the outer lines of the quartet resonance signal, where the inner lines overlap with the main signal, giving an estimated hyper-fine coupling of about 7 G for the ⁵³Cr isotopomer. The g-values observed in this compound were in good relation to the isotropic values reported for chromium(III) which are between 1.960 and 2.000 [25]. The influence of the two connected lithium centers to the hyperfine coupling of the chromium cation and the alkoxo ligands were not observed in the spectra. The UV–Vis spectra (Supplementary Materials, Figure S1) showed three absorption maxima at 406, 802, and an intense one at 610 nm resulting from the d–d transitions of the tetrahedral coordinated chromium(III) center.

2.2. Synthesis of [LiCr(O^tBu)₂(PyCH=COCF₃)₂(THF)₂] (2)

Mixed metal alkoxides are potential precursors to oxide ceramics due to their susceptibility to water that facilitates their hydrolytic activation and conversion into chemically homogeneous amorphous ceramics [26]. However, the limited thermal stability and extreme sensitivity of heterometallic alkoxides to moisture and air make their handling and storage rather difficult. Therefore, in our effort to improve the stability of compound 1, a ligand-modified alkoxide with enhanced stability was synthesized by adding two equivalents of bidentate β-heteroarylalkenol ligand 3,3,3-trifluoro(pyridin-2-yl)propen-2-ol(PyCH=COHCF₃) to the solution of compound 1 (Scheme 2). Upon the ligand addition, the color of the reaction mixture changed from purple to light-brown. The resulted compound 2 could be recrystallized from THF at −18 °C. The single crystal X-ray diffraction showed a chromium(III) center in octahedral geometry with two β-heteroarylalkenol ligands and two tert butoxy groups shared between chromium and lithium centers. This compound showed higher stability in air due to good electron donating and accepting properties of the chelating ligands that provide both structural stability and electronic saturation around the chromium center [27]. The elemental analysis of the bulk material showed small deviations and revealed the selective transformation to the heteroleptic lithium–chromium complex.

2.2.1. X-ray Crystallography of Compound 2

Compound 2 in Figure 3 crystalized in the monoclinic space group P2₁/c with four molecules per unit cell. The nitrogen atoms of the bidentate ligands adopted the axial position with a steric distortion (170.74° against 180°), whereas the bond distances of Cr–N1 and Cr–N2 were 2.111 and 2.087 Å, respectively. The bond angles N1–Cr–O3 and N2–Cr–O1 of the two bidentate ligands were 85.18° and 84.31°, respectively, due to the constrained bite angle. Moreover, the equatorial Cr–O bond distances of the heteroarylalkenolate ligands were elongated (Cr1–O1: 1.971 Å) compared to the bridged alkoxo ligands (Cr1–O2: 1.950 Å). In the case of the tetrahedrally coordinated lithium center, the Li–O distances of the tert-butoxy ligand (Li1–O2: 1.912 Å) were shorter than the weakly coordinated THF molecule (Li1–O5: 2.007 Å).

The UV–Vis spectra showed an intense absorption maximum at 590 nm of the d–d transition and three weak maxima at 375, 306, and 270 nm resulting from the ligand-to-metal charge transfer (LMCT) and π–π* transition of the aromatic moiety in the ligand backbone (Supplementary Materials, Figure S1). The bidentate ligand with the synergistic effect of the electron-donating aromatic system and the electron-pulling perfluoroalkyl-group stabilize the compound by a pseudo push–pull effect, which provides more stability to the molecule by saturating the coordination sphere of the chromium(III) center. However, compound (2) is not stable in the gas phase due to the weakly bonded THF molecules which were released before the sublimation point.

2.2.2. Thermogravimetric Analysis (TGA) of Compounds 1

Thermogravimetric analysis (TGA) of compound 1 was conducted from room temperature to 800 °C under a constant flow of nitrogen, and displayed a two-step decomposition profile (Figure 4). In the first step from 70 to 310 °C of the thermal transformation, a slow mass loss of around 25% was observed and it could be attributed to the release of weakly bonded two THF molecules. The sharp mass loss at a temperature of approximately 310 °C down to 28% of the initial mass owing to decomposition of tert-butoxy group from the complex indicated the formation of LiCrO₂ and LiCl, since the theoretical value of the residual mass is 25%. The slightly higher mass of the residue (3%) could be possibly due to carbon or fluorine impurities originating from the ligands.

2.3. Material Synthesis

The fabrication of LiCrO₂ nanoparticles was performed via solvothermal process using compound 2 in toluene under inert atmosphere with polyvinylpyrrolidone (PVP) as surfactant to influence the morphology of the formed oxide materials. A similar synthetic procedure was followed to synthesize Li₂CrO₄ using compound 1 which contained Li–Cr in a 2:1 ratio. As already explained by TGA, compound one can also be utilized as a precursor to LiCrO₂ and LiCl, as the byproduct is soluble in water and can be removed during purification of the nanoparticle by washing with water before calcination.

The powder X-ray pattern of the solid product obtained from compound 2 revealed the formation of crystalline LiCrO₂ (JCPDS 24-0600) [28] with some unidentified (amorphous) component (Figure 5). The material obtained from compound 1 containing Li–Cr in a 2:1 ratio was found to be Li₂CrO₄ (JCPDS 31-0715) [29].

The structural features of the prepared LiCrO₂ nanoparticles were observed by transmission electron microscope (TEM), which showed nearly uniform grains with particles of average size 20 nm (Figure 5). However, some agglomeration was observed due to residual carbon or surfactant (PVP).

The previously reported multi-source synthesis of LiCrO₂ resulted in inhomogeneous elemental distribution and nonuniform morphologies, which could be detrimental to application of the material. For instance, the report of Tanaka et al. for the synthesis of lithium chromium oxide nanoparticles by induction thermal plasmas resulted in material with different oxide phases and inhomogeneous morphology [30]. Therefore, synthesizing LiCrO₂ nanomaterials from single-source precursors 1 and 2 is a promising strategy to impose a better phase control (Figure 6).

3. Materials and Methods

All syntheses were performed under an inert nitrogen atmosphere using a modified Stock glass vacuum line. The used solvents were dried and distilled over sodium and stored over molecular sieves. All chemicals were of analytical grade and used without further purification. Chromium(III) chloride (CrCl₃) was obtained from Acros Organics (97% purity, Morris Plains, NJ, USA) and activated in dried THF by stirring for one day at room temperature. X-band EPR spectra were recorded with a Bruker System ELEXSYS 500E instrument (Bruker, Karlsruhe, Germany equipped with a Bruker ER 4131VT variable-temperature unit. Single-crystal XRD analysis and structure elucidation was performed on a STOE IPDS II diffractometer (STOE, Darmstadt, Germany) with graphite monochromated Mo Kα radiation (0.71071 Å). A numerical absorption correction based on crystal shape optimization was applied for all data. The programs used in this work were STOE’s XAREA [31,32,33] and the WINGX suite of programs [34], including SIR-92 [35], SHELX, and SHELXTL [36], and PLATON [37] for structure solution and refinement. H atoms were calculated geometrically, and a riding model was applied during the refinement process. Powder X-ray diffraction measurements were performed on a STOE-STADI P using Mo Kα radiation (0.71071 Å). TEM measurement was carried out by a TEM LEO912 omega from Zeiss company (Oberkochen, Germany) operated at 120 kV.

3.1. Precursor Synthesis

[Li₂Cr(O^tBu)₄Cl(THF)₂]n (1): A solution of (1.00 g, 6.31 mmol) CrCl₃ activated in 50 mL THF was slowly added to a solution containing four equivalent of [Li(O^tBu)] (1.52 g, 18.94 mmol) in 20 ml n-heptane/THF to afford a pale blue solution which later changed to purple after stirring for 12 h at room temperature. Volatile solvent was removed under vacuum and the resulting product was extracted from an n-heptane solution to get rid of the LiCl. A single-crystal was obtained from a concentrated solution in THF/n-heptane. The obtained yield was 73% (2.48 g, 4.61 mmol) of a purple solid. Elemental analysis expected: C: 53.58, H: 9.74, found: C: 50.93, H: 10.02.

[LiCr(O^tBu)₂(PyCH=COCF₃)₂(THF)₂] (2): A solution of 3,3,3-trifluoro(pyridin-2-yl) propen-2-ol (PyCH=COHCF₃) (0.71 g, 3.76 mmol) in 10 ml THF was slowly added to a purple solution of [Li₂Cr(O^tBu)₄Cl(THF)₂] (1) (1.00 g, 1.86 mmol) in 10 mL THF and stirred for 2 h at room temperature. Volatile solvent was removed under vacuum, and the resulting product was extracted from THF/n-heptane solution. The obtained yield was 91% (1.23 g, 1.69 mmol) of a light brown solid. Elemental analysis expected: C: 52.97, H: 6.11 and N: 3.86, found: C: 52.02, H: 6.16 and N: 3.53.

Single-crystals were obtained from a mixture n-heptane and THF solution at −18 °C. Compound 2 crystallized as a merohedric twin and was solved using twin law.

3.2. Synthesis of LiCrO₂ Nanoparticles

LiCrO₂ nanoparticles were prepared by dissolving 1.08 g of the precursor [LiCr(O^tBu)₂(PyCH=COCF₃)₂(THF)₂] (2) in 20 ml toluene and stirring until a uniform solution was obtained, followed by the addition of 0.3 g of poly(vinylpyrrolidone) (PVP; MW = 1,300,000) in isopropanol and magnetically stirred for 2 h to obtain a homogeneous solution. The mixture was then transferred into a Teflon-lined stainless-steel autoclave and heated at 180 °C for 24 h. Finally, the obtained precipitates were centrifuged, collected, and rinsed with deionized water and ethanol five times, and then dried at 70 °C under vacuum overnight. The dried nanoparticles were then calcinated at 600 °C for 2 h in nitrogen atmosphere to produce LiCrO₂ nanoparticles.

3.3. Synthesis of Li₂CrO₄ Nanoparticles

Li₂CrO₄ nanoparticles were prepared in a similar procedure as described above for LiCrO₂ nanoparticles. However, the obtained material was calcinated in air for 5 h to remove the leftover PVP from the material during which the material was oxidized from (III) to (IV).

4. Conclusions

In this study, a facile and efficient solvothermal method was employed to fabricate LiCrO₂ and Li₂CrO₄ nanoparticles from new heterometallic chromium(III)-based alkoxide, which was successfully stabilized via ligand modification using 3,3,3-trifluoro(pyridin-2-yl) propen-2-ol (PyCH=COCF₃). The strategy behind the synthesis of the precursors was to provide precise control over material stoichiometry at the molecular level, which is challenging in traditional solid-state synthesis. Since chromium is an earth-abundant element, its complexes and oxide materials can serve as economical and sustainable materials for device applications.