Molecularly Engineered Lithium-Chromium Alkoxide for Selective Synthesis of LiCrO2

Created by: Olusola Ojelere

Achieving control over 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(OtBu)4Cl(THF)2] (1) and [LiCr(OtBu)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, EPR studies revealed a monomeric structure in the solution. TEM analysis of the synthesized LiCrO2 nanoparticles showed nearly uniform particles size of approximately 20 nm. The sensitivity of LiCrO2 phase towards oxidation was investigated by X-ray diffraction revealing the formation of the stable Li2CrO4 after calcination in the air.

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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 Cr+3 exhibits antiferromagnetic nature with two-dimensional (2D) frustration and interesting structural and magnetic behaviour.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 due 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+.6-9 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 few chromium (III) alkoxides are known and their reactivity and structural patterns are not well established. 12-16 To the best of our knowledge, there is no report on the preparation of stoichiometric LiCrO2 nanomaterials from molecular single source precursors.17-19 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. 20-22 We demonstrate here, for the first time, the influence of molecular-level processing in 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 multisource precursor approach.

Results and Discussion

Synthesis of [Li2Cr(OtBu)4Cl(THF)2]n (1)

The heterometallic alkoxide [Li2Cr(OtBu)4Cl(THF)2] (1) was synthesized via a salt metathesis reaction between solvated CrCl3 activated in THF (CrCl3(THF)3) and four equivalents of LiOtBu in THF /n-heptane. The colour 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 in n-heptane. The obtained product was the chloride-containing compound [Li2Cr(OtBu)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 shows deviations of carbon (found: 50.93%, calc.: 53.58%), possibly due to partial hydrolysis during the sample preparation.

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 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 are 1.862 and 1.871 Å, respectively. The bond angles around the oxygen of the tert‑butoxy ligand and chromium centre are 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 are 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 centre and the tert‑butoxy ligand is shorter than 1.963 Å observed between the lithium centre and coordinating THF molecule, while Li-Cl distance is. 2.286 Å. The one-dimensional polymeric chain resulted from Li-Cl-Li connection between the trinuclear asymmetric units. [Li2Cr(OtBu)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-1:2κ4 O:O;1:3 κ4 O:O-bis(tetrahydrofuran)-2 κO,3 κO-chromium(III)- dilithium(I)]-µ-chloro].

Synthesis of [LiCr(OtBu)2(PyCH=COCF3)2(THF)2] (2).

Mixed-metal alkoxide are potential precursors to oxide ceramics due to their susceptibility towards 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 towards 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=COCF3) to the solution of compound 1. Upon the ligand addition, colour 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 chromium (III) centre in octahedral geometry with two β-heteroarylalkenol ligands and two tert butoxy groups shared between chromium and lithium centres. 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 centre.27 The elemental analysis of the bulk material shows small deviations and revealed the selective transformation to the heteroleptic lithium-chromium complex.

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

The UV-vis spectra show 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 LMCT and π-π* transition of the aromatic moiety in the ligand backbone. The bidentate ligand with the synergistic effect of the electron donating aromatic system and the electron pulling perfluoroalkyl-group stabilizes the compound by a pseudo push-pull effect, which provides more stability to the molecule by saturating the coordination sphere of the chromium centre. However, the compound (2) is not stable in the gas phase due to the weakly bonded THF molecules which is released before the sublimation point