Next Article in Journal
The Wound Healing Peptide, AES16-2M, Ameliorates Atopic Dermatitis In Vivo
Next Article in Special Issue
Meloxicam and Study of Their Antimicrobial Effects against Phyto- and Human Pathogens
Previous Article in Journal
Ferroelectric Particles in Nematic Liquid Crystals with Soft Anchoring
Previous Article in Special Issue
Efficient and Reusable Iron Catalyst to Convert CO2 into Valuable Cyclic Carbonates
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Molecules
Volume 26
Issue 4
10.3390/molecules26041167
molecules-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Replacement of the Common Chromium Source CrCl3(thf)3 with Well-Defined [CrCl2(μ-Cl)(thf)2]2

by
Dong Geun Lee
1,†,
Jun Won Baek
1,†,
Jung Hyun Lee
1,
Hyun Ju Lee
1,
Yeong Hyun Seo
1,
Junseong Lee
2,
Chong Gu Lee
3 and
Bun Yeoul Lee
1,*
1
Department of Molecular Science and Technology, Ajou University, 206 Worldcup-ro, Yeongtong-gu, Suwon 16499, Korea
2
Department of Chemistry, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Korea
3
Precious Catalysts Inc. 201, Duryu-gil, Angangeup, Gyeongju 38029, Korea
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Molecules 2021, 26(4), 1167; https://doi.org/10.3390/molecules26041167
Submission received: 29 January 2021 / Revised: 19 February 2021 / Accepted: 19 February 2021 / Published: 22 February 2021
(This article belongs to the Special Issue Exclusive Feature Papers in Organometallic Chemistry)
Browse Figures
Versions Notes

Abstract

:
CrCl3(thf)3 is a common starting material in the synthesis of organometallic and coordination compounds of Cr. Deposited as an irregular solid with no possibility of recrystallization, it is not a purity guaranteed chemical, causing problems in some cases. In this work, we disclose a well-defined form of the THF adduct of CrCl3 ([CrCl2(μ-Cl)(thf)2]2), a crystalline solid, that enables structure determination by X-ray crystallography. The EA data and XRD pattern of the bulk agreed with the revealed structure. Moreover, its preparation procedure is facile: evacuation of CrCl3·6H2O at 100 °C, treatment with 6 equivalents of Me3SiCl in a minimal amount of THF, and crystallization from CH2Cl2. The ethylene tetramerization catalyst [iPrN{P(C6H4-p-Si(nBu)3)2}2CrCl2]+[B(C6F5)4]− prepared using well-defined [CrCl2(μ-Cl)(thf)2]2 as a starting material exhibited a reliably high activity (6600 kg/g-Cr/h; 1-octene selectivity at 40 °C, 75%), while that of the one prepared using the impure CrCl3(thf)3 was inconsistent and relatively low (~3000 kg/g-Cr/h). By using well-defined [CrCl2(μ-Cl)(thf)2]2 as a Cr source, single crystals of [(CH3CN)4CrCl2]+[B(C6F5)4]− and [{Et(Cl)Al(N(iPr)2)2}Cr(μ-Cl)]2 were obtained, allowing structure determination by X-ray crystallography, which had been unsuccessful when the previously known CrCl3(thf)3 was used as the Cr source.

1. Introduction

CrCl3(thf)3 has been widely used as a starting material for the synthesis of organometallic and coordination compounds of Cr [1,2,3,4,5,6,7,8,9,10]. Its preparation method was reported 60 years ago; anhydrous CrCl3, which is insoluble in organic solvents as well as in water, was extracted with THF by the aid of Zn dust using Soxhlet apparatus [11,12,13]. However, anhydrous CrCl3 is prepared by hazardous and not-facile process (treatment of Cr2O3·xH2O with CCl4 at 630 °C leads to the generation of extremely toxic phosgene gas), and thus, is expensive [14,15]. Another method was reported in which CrCl3·6H2O was reacted with excess thionyl chloride (S(O)Cl2) to generate anhydrous CrCl3. In this process, a different form of hygroscopic CrCl3 was generated, from which CrCl3(thf)3 could be immediately obtained after adding THF [16,17,18]. The most attractive and convenient method is reacting CrCl3·6H2O with excess Me3SiCl in THF, resulting in the deposition of CrCl3(thf)3 along with the generation of byproducts, HCl and Me3SiOSiMe3 (CrCl3·6H2O + 12 Me3SiCl → CrCl3(thf)3 + 12 HCl + 6 Me3SiOSiMe3) [19]. Excess Me3SiCl (~60 equiv./Cr) needs to be added to obtain a completely anhydrous form of CrCl3(thf)3 [20]. Otherwise (e.g., when 25 equiv. of Me3SiCl/Cr was added according to the literature), hydrated CrCl3(thf)2(H2O) is obtained, which was erroneously sold as CrCl3(thf)3 in the past [20,21].
Sasol disclosed a catalyst system composed of a Cr source, iPrN(PPh2)2 ligand, and methylaluminoxane (MAO), which produces mainly 1-octene through ethylene tetramerization [22,23,24]. CrCl3(thf)3 has been actively used as a component of the ethylene tetramerization catalyst system [25,26,27,28,29,30,31,32,33,34]. We also developed a very efficient catalyst for ethylene tetramerization (Scheme 1) [35,36,37]. The catalyst shows extremely high activity (~5000 kg/g-Cr/h) and is advantageous over other reported systems in that it works with an inexpensive activator, iBu3Al, avoiding the use of expensive MAO in excess (A1/Cr = 300–500) [38,39,40,41,42,43]. During the course of the studies, we had a problem in reproducing such an extremely high activity and eventually found that the Cr source CrCl3(thf)3 used in the preparation was the cause of this problem. Although the structure of CrCl3(thf)3 was revealed by X-ray crystallography, using a single crystal selected from the batch in the Soxhlet extraction process, it does not redeposit as good-quality crystals when redissolved in CH2Cl2 [44,45]. Structure elucidated by X-ray crystallography does not guarantee the purity of the bulk and we questioned the purity of the common Cr source CrCl3(thf)3. In fact, it was reported as an “irregular violet solid” which may mean that it is not a pure CrCl3(thf)3 but a mixture containing other forms of THF adduct of CrCl3 (e.g., μ2-Cl bridged multinuclear species) [46]. Elemental analysis (EA) data for Cr and Cl have been reported, but common carbon and hydrogen data were missing. CrCl3(thf)3 is paramagnetic; thus, it cannot be subjected to structural analysis using 1H and 13C NMR spectroscopy. We also previously found that the composition (as well as the structure) of the commercial source of (2-ethylhexanoate)3Cr, another type of important paramagnetic Cr(III) complex that is commercially used as a component in the Phillips ethylene trimerization catalyst [47]. is erroneous [21,48]. By correcting the structure to (2-ethylhexanoate)2CrOH, the catalytic performance was consistent and, moreover, significantly improved.

2. Results and Discussion

CrCl3(thf)3 was commercially available from Aldrich (St. Louis, MO, USA), but we prepared it to ensure complete dryness by reacting CrCl3·6H2O with excess Me3SiCl (60 equiv.) [20]. We modified the procedure as follows: CrCl3·6H2O was evacuated for ~6 h before the treatment with Me3SiCl. The catalyst [iPrN{P(C6H4-p-Si(nBu)3)2}2CrCl2]+[B(C6F5)4]−, prepared according to Scheme 1 using CrCl3(thf)3, which was prepared via the evacuation step, provided extremely high activity (~5000 kg/g-Cr/h). In contrast, the catalyst prepared using either the commercial source of CrCl3(thf)3 or the one prepared according to the reported method without the evacuation step did not exhibit such a high activity (~3000 kg/g-Cr/h), and there was some amount of methylcyclohexane-insoluble fraction when ligand iPrN{P(C6H4-p-Si(nBu)3)2}2 was reacted with [(CH3CN)4CrCl2]+[B(C6F5)4]−. The insoluble fraction was negligible when CrCl3(thf)3, which was prepared via the evacuation step, was used as the starting material.
We investigated what happened in the evacuation step. When the crystalline form of CrCl3·6H2O (10 g, 37.5 mmol) was evacuated at 100 °C, the color gradually changed from dark green to light green, light gray, and finally light purple with gradual loss of weight. Finally, an amorphous powder was obtained, and the weight loss converged to 36% (6.39 g remaining). Further weight reduction was minimal when the evacuation time was increased. It was confirmed with litmus paper that the removed volatile component was not pure water but acidic, presumably containing HCl. From these observations, the remaining Cr compound was tentatively considered as CrCl2(OH)(H2O)2; 34% weight loss is expected for the transformation of CrCl3·6H2O to CrCl2(OH)(H2O)2 (Scheme 2). The structure of CrCl3·6H2O was reported to be [CrCl2(H2O)4]Cl·2H2O, and the action of heat under vacuum could liberate outer-sphere Cl− (as HCl with a proton in an inner-sphere water) and three water molecules (two from the outer sphere and one from the inner sphere), consequently leaving CrCl2(OH)(H2O)2 (possibly, [CrCl2(H2O)2(μ-OH)]2 adopting an octahedral structure) in the reaction pot.
Treatment of CrCl2(OH)(H2O)2, after dissolving in THF, with Me3SiCl resulted in the precipitation of a violet solid, which was confirmed by infrared spectroscopy to be completely anhydrous. Because a large portion of H2O in CrCl3·6H2O was removed at the evacuation step, 6.0 equiv. Me3SiCl (equivalent amount that needed for the transformation of CrCl2(OH)(H2O)2 to CrCl3(thf)x, Scheme 2) was added instead of 60 equiv./Cr, and accordingly, the amount of THF could be minimized (36 mL/10 g—CrCl3·6H2O). The color and shape of the precipitated solid (irregular violet solid) were almost identical to that of the commercial source of CrCl3(thf)3 and the one prepared without the evacuation step. However, as aforementioned, the catalyst performance was significantly better when using CrCl3(thf)3 prepared via the evacuation step. The product of the reaction between [(CH3CN)4Ag]+[B(C6F5)4]− and CrCl3(thf)3 (i.e., [(CH3CN)4CrCl2]+[B(C6F5)4]−) differed depending on the source of CrCl3(thf)3. Although [(CH3CN)4CrCl2]+[B(C6F5)4]− was isolated by precipitation in CH3CN at −30 °C, the precipitate was not composed of good-quality crystals; thus, X-ray crystallography could not be used for characterization. We crosschecked the tentatively assigned structure of [(CH3CN)4CrCl2]+[B(C6F5)4]− by counting the number of CH3CN molecules per each Cr atom through the analysis of integration values in the 1H-NMR spectra recorded using THF-d8 with 9-methylanthracene as an internal standard. For the product obtained using CrCl3(thf)3 prepared via the evacuation step, the number of CH3CN per Cr was in good agreement with the expected value of 4 (4.1, 3.8, or 4.0). In contrast, the number deviated from 4 (6.5, 6.1, 5.3, or 6.2) when either the commercial source of CrCl3(thf)3 or the one prepared without the evacuation step was used.
The most striking difference between the two samples was that good-quality crystals were deposited in a CH2Cl2 solution of CrCl3(thf)3 that was prepared via the evacuation step, whereas an irregular solid was deposited when either the commercial source of CrCl3(thf)3 or the one prepared without the evacuation step was dissolved in CH2Cl2 (Figure 1). X-ray crystallography studies revealed that the deposited crystals were a Cl-bridged dinuclear complex [CrCl2(μ-Cl)(thf)2]2 (Figure 2). The Cr atom adopted an octahedral structure, with two THFs being in the cis-position. The bond distances between Cr and terminal-Cl (2.277 and 2.289 Å) were shorter than that between Cr and bridge-Cl (2.362 Å) as well as the Cr–Cl distances (3.299, 3.318, and 3.352 Å) observed for mer-CrCl3(thf)3, whose structure has been previously revealed using a single crystal obtained from the reaction pot of Cr(CH2SiMe3)4 and HCl in THF (CCDC# 983659) [20]. The distance between Cr and OTHF situated trans to terminal-Cl was greater than that between Cr and OTHF situated trans to bridge-Cl (2.045 vs. 2.018 Å). The OTHF atom situated trans to terminal-Cl adopted perfect trigonal planar geometry (sum of bond angle around O1 atom, 360.0°), indicating π-donation from O to Cr through adopting sp2-hybridization. The other OTHF atom situated trans to bridge-Cl deviated from a perfect trigonal planar geometry (sum of bond angle around O1 atom, 352.8°).
X-ray crystallography data do not guarantee the purity of the bulk. The EA data of C were roughly in agreement with the values calculated for the revealed structure [CrCl2(μ-Cl)(thf)2]2 (31.8%), although the measured values were not always within the required range of 31.8% ± 0.4% (30.7%, 30.9%, 30.4%, 31.0%, 30.5%, 30.2%, 30.4%, 30.9%, and 31.2%). In contrast, the EA data of C measured for either the commercial source of CrCl3(thf)3 or the one prepared without the evacuation step deviated substantially from the value calculated for CrCl3(thf)3 (38.5%). The data fluctuated severely by variation in sampling points (32.4%, 26.5%, 34.1%, 38.1%, and 33.8% for a bottle of purchased sample; 34.2%, 36.7%, 37.1%, 35.3%, and 34.1% for another bottle of purchased sample; 31.3%, 30.5%, 37.0%, 33.6%, and 33.9% for the sample prepared without the evacuation step). Before recrystallization, the sample prepared via the evacuation step also showed fluctuating C content by varying the sampling point, and the C content agreed neither with the structure of CrCl3(thf)3 nor the structure of [CrCl2(μ-Cl)(thf)2]2 (35.3%, 34.3%, 32.5%, 34.2%, and 32.4%). Though the carbon contents and the XRD pattern are inconsistent with the formula of the commercial source CrCl3(THF)3, the Cr content measured by ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy) and the Cl content measured by gravimetric analysis after treatment of AgNO3 to aqueous solution of CrCl3(THF)3 were not severely deviated from the values calculated for the formula CrCl3(THF)3 (Calcd. Cr 13.88, Cl 28.38. Found: Cr 14.04, Cl 29.09%). The Cr content measured for the prepared [CrCl2(μ-Cl)(thf)2]2 agreed accurately with the calculated value while the measured Cl content was slightly deviated (Calcd. Cr 17.19, Cl 35.15. Found: Cr 17.18, Cl 34.58%).
The X-ray powder diffraction (XRD) pattern observed for bulk [CrCl2(μ-Cl)(thf)2]2 isolated via the recrystallization procedure agreed well with single-crystal X-ray crystallography data (cif file) (Figure 3a vs. Figure 3b), indicating that the majority of deposited solid had a structure of [CrCl2(μ-Cl)(thf)2]2. In contrast, the XRD pattern of the purchased CrCl3(thf)3 or the one prepared by the reported method without the evacuation step did not agree with single-crystal X-ray crystallography data of mer-CrCl3(thf)3 (Figure 3c vs. Figure 3e,f). This observation, along with the mismatched EA data, creates uncertainty about the structure and composition of the purchased CrCl3(thf)3 as well as of the one prepared by the reported method. The solid deposited in the reaction pot of CrCl2(OH)(H2O)2 and Me3SiCl (6 eq) showed a large signal at 2θ = 11.6° along with the signals observed for the CrCl3(thf)3 samples (Figure 3d). The electron paramagnetic resonance (EPR) spectrum of [CrCl2(μ-Cl)(thf)2]2 showed a strong signal at g = 1.98, while relatively weak signals were observed for the CrCl3(thf)3 samples; in addition, the signal patterns were completely different for the two samples (Figure 4). In thermogravimetry analysis (TGA) of CrCl3(thf)3, roughly four stages of the weight loss were observed (30%, 50%, 60%, and 84% at 170 °C, 215 °C, 300 °C, and 1000 °C, respectively), while three stages of the weight loss were roughly observed for [CrCl2(μ-Cl)(thf)2]2 (35%, 53%, and 73% at 220 °C, 310 °C, and 1000 °C, respectively).
Many attempts to grow single crystals of [(CH3CN)4CrCl2]+[B(C6F5)4]− have been unsuccessful when [(CH3CN)4Ag]+[B(C6F5)4]− was reacted with the purchased CrCl3(thf)3 or the one prepared through the reaction of CrCl3·6H2O and excess Me3SiCl (60 eq) without the evacuation step (Scheme 1) [38]. However, single crystals suitable for X-ray crystallography could be obtained when [CrCl2(μ-Cl)(thf)2]2 was reacted with [(CH3CN)4Ag]+[B(C6F5)4]−. The structure determined by X-ray crystallography is shown in Figure 5. By reacting well-defined [(CH3CN)4CrCl2]+[B(C6F5)4]− with a iPrN{P(C6H4-p-Si(nBu)3)2 ligand [37], a high-quality Cr complex [iPrN{P(C6H4-p-Si(nBu)3)2}2CrCl2]+[B(C6F5)4]− was produced, which showed extremely high activity in the ethylene tetramerization reaction (activity: 6600 kg/g-Cr/h; selectivity: 1-octene—74.7%, 1-hexene—7.5%, cyclic-C6—3.8%, C10+—13.8%, PE—0.2%; reaction conditions: 34 bar ethylene, 235 mL methylcyclohexane, 0.45 mmol Cr catalyst, 90 mmol (iBu)3Al, 40 °C, 30 min). The Cr complex [iPrN{P(C6H4-p-Si(nBu)3)2}2CrCl2]+[B(C6F5)4]− is highly soluble in methylcyclohexane, which is advantageous in handling and operating the tetramerization reaction, and there is no way for purification, necessitating a high-quality Cr precursor [(CH3CN)4CrCl2]+[B(C6F5)4]− as well as high-quality ligand iPrN{P(C6H4-p-Si(nBu)3)2}2 to ensure consistently high activity.
In previous studies, we encountered a similar problem possibly caused by the use of impure CrCl3(thf)3. In the reaction of Li+[Et2Al(N(iPr)2)2]− and CrCl3(thf)3, solids were isolated; however, their structure could not be determined by X-ray crystallography. The product was tentatively considered to be [Et2Al(N(iPr)2)2]CrCl2 [48]. However, single crystals were obtained when well-defined [CrCl2(μ-Cl)(thf)2]2 replaced the impure CrCl3(thf)3, enabling structure determination by X-ray crystallography. The complex isolated from the reaction of Li+[Et2Al(N(iPr)2)2]− and CrCl3(thf)3 was not a Cr(III) complex [Et2Al(N(iPr)2)2]CrCl2 but a Cr(II) complex [{Et(Cl)Al(N(iPr)2)2}Cr(μ-Cl)]2 (Figure 6).

3. Materials and Methods

3.1. General Remarks

All manipulations were performed in an inert atmosphere using a standard glove box and Schlenk techniques. CH2Cl2 and CH3CN were stirred over CaH2 and transferred to a reservoir under vacuum. Toluene, hexane, and THF were distilled from benzophenone ketyl. EAs were carried out at the Analytical Center, Ajou University. CW X-band EPR spectra were collected at room temperature on an EMX plus 6/1 spectrometer (Bruker, Billerica, MA, USA) with experimental parameters of 1 mW microwave power, 10 G modulation amplitude, and 3 scans at KBSI Western Seoul Center Korea. HP-XRD data were obtained on a D/max-2500V/PC (Rigaku, Akishima, Japan) equipped with a Cu Kα radiation source (λ = 0.15418 nm).

3.2. [CrCl2(μ-Cl)(thf)2]2

A Schlenk flask containing crystalline solid CrCl3·6H2O (10.0 g, 37.5 mmol) was immersed in an oil bath at a temperature of 40 °C and then evacuated under full vacuum for 1 h. Under evacuation, the bath temperature was raised to 100 °C over an hour and then maintained at 100 °C for 4 h. As volatiles were removed, the crystalline solid changed to an amorphous powder and the color of the solid gradually changed from dark green to light green, light gray, and finally light purple. The weight reduced from 10.0 to 6.39 g by the removal of volatiles. Cold THF (32 g, −30 °C) was added to dissolve the remaining solid. By dissolution, heat was generated, and a dark-purple solution was obtained. Some insoluble portion was filtered off, and then, Me3SiCl (24.5 g, 225 mmol) was added to the filtrate. Stirring the solution overnight led to the deposition of a purple solid. The solid was isolated via filtration and washed with THF (10 mL) and hexane (10 mL). The isolated solid (6.60 g) was placed in a large vial (~70 mL size) and CH2Cl2 (53 g) was added to dissolve the solid. When the vial was placed inside a closed chamber (~250 mL size) containing methylcyclohexane (~15 mL), CH2Cl2 was slowly evaporated and purple crystals were deposited within 24 h, which were isolated by decantation (4.55 g, 69%).

3.3. [(CH3CN)4CrCl2]+[B(C6F5)4]−

A solution of [(CH3CN)4Ag]+[B(C6F5)4]− (7.789 g, 8.189 mmol) in acetonitrile (17.3 g) was added to a suspension of [CrCl2(μ-Cl)(thf)2]2 (2.478 g, 8.189 mmol) in acetonitrile (22.6 g). After stirring overnight at 60 °C, precipitated AgCl was removed by filtration and an olive-green solution was obtained. The solvent was removed using a vacuum line to obtain an olive-green solid (7.71 g). The isolated solid (20.0 mg) and 9-methylanthracene (20.0 mg) were dissolved in THF-d8, and the 1H NMR spectrum of the solution was recorded to count the number of CH3CN molecules per each Cr atom, which was 4.1. The yield was 97% based on the formula [(CH3CN)4.1CrCl2]+[B(C6F5)4]−. Single crystals suitable for X-ray crystallography were grown in CH2Cl2; a vial containing [(CH3CN)4CrCl2]+[B(C6F5)4]− (100 mg) in CH2Cl2 (1.0 g) was placed inside a closed chamber containing methylcyclohexane to deposit crystals.

3.4. [iPrN{P(C6H4-p-Si(nBu)3)2}2CrCl2]+[B(C6F5)4]−

A solution of iPrN{P(C6H4-p-Si(nBu)3)2}2 (9.50 g, 7.78 mmol) in CH2Cl2 (120 g) was added dropwise to a solution of [(CH3CN)4.1CrCl2]+[B(C6F5)4]− (7.55 g, 7.781 mmol) in CH2Cl2 (46 g). Upon addition, the color of the solution changed immediately from olive-green to bluish-green. After stirring for 2.5 h, the solvent was completely removed using a vacuum line. The residue was dissolved in a minimal amount of methylcyclohexane (~5 mL), and the solvent was removed using a vacuum line. This procedure was repeated once more to remove any residual CH3CN and CH2Cl2 completely. The residue (15.7 g, 100%) was dissolved in methylcyclohexane (141.3 g) to obtain a 10 wt% solution, which was used for ethylene tetramerization.

3.5. [{Et(Cl)Al(N(iPr)2)2}Cr(μ-Cl)]2

[CrCl2(μ-Cl)(thf)2]2 (0.312 g, 1.03 mmol) was added to a solution of Li+[Et2Al(N(iPr)2)2]− (0.302 g, 1.03 mmol) in toluene (4.5 mL) at −30 °C. The mixture was stirred for 8 h at room temperature. The color of the solution changed from dark brown to deep greenish-blue, and LiCl precipitated. The solvent was removed using a vacuum line, and the residue was dissolved in hexane (30 mL). After removal of LiCl via filtration, the filtrate was concentrated to ~10 mL and allowed to stand at −30 °C for 2 days. Blue crystals were deposited (0.178 g, 45%). A second crop of blue crystals was obtained from the mother liquor (0.028 g, 7%).

3.6. X-ray Crystallography

Specimens of suitable quality and size were selected, mounted, and centered in the X-ray beam using a video camera. Reflection data were collected at 100 K on an APEX II CCD area diffractometer (Bruker, Billerica, MA, USA) using graphite-monochromated Mo K–α radiation (λ = 0.7107 Ǻ). The hemisphere of the reflection data was collected as φ and ω scan frames at 0.5° per frame and an exposure time of 10 s per frame. The cell parameters were determined and refined using the SMART program. Data reduction was performed using the SAINT software. The data were corrected for Lorentz and polarization effects. An empirical absorption correction was applied using the SADABS program. The structure was solved by direct methods and refined by the full matrix least-squares method using the SHELXTL package and olex2 program with anisotropic thermal parameters for all non-hydrogen atoms.
Crystallographic data for [CrCl2(μ-Cl)(thf)2]2 (CCDC# 2044805) that were used in all calculations are as follows: C8H15.5Cl3CrO2, M = 302.05, triclinic, a = 6.8070(8), b = 9.3170(11), c = 10.3192(13) Å, α = 108.070(5)°, β = 96.448(5)°, γ = 94.719(5), V = 613.47(13) Å3, space group P-1, Z = 2, and 2285 unique (R(int) = 0.0505). The final wR2 was 0.1251 (I > 2σ(I)).
Crystallographic data for [(CH3CN)4CrCl2]+[B(C6F5)4]−·2(CH2Cl2) (CCDC# 2044806) that were used in all calculations are as follows: C34.5H17BCl7CrF20N4, M = 1178.48, monoclinic, a = 12.9472(4), b = 8.2694(3), c = 21.7555(8) Å, β = 99.8523(18)°, V = 2294.91(14) Å3, space group P2/c, Z = 2, and 4409 unique (R(int) = 0.0285). The final wR2 was 0.1594 (I > 2σ(I)).
Crystallographic data for [{Et(Cl)Al(N(iPr)2)2}Cr(μ-Cl)]2 (CCDC# 2044807) that were used in all calculations are as follows: C28H66Al2Cl4Cr2N4, M = 758.60, monoclinic, a = 10.2937(2), b = 12.8707(3), c = 14.7815(3) Å, β = 96.5009(18)°, V = 1945.77(7) Å3, space group P21/n, Z = 2, and 3675 unique (R(int) = 0. 1527). The final wR2 was 0.0909 (I > 2σ(I)).

4. Conclusions

The commonly used CrCl3(thf)3 is impure obtaining as an irregular solid with no possibility of recrystallization. Another form of the THF adduct of CrCl3 (i.e., [CrCl2(μ-Cl)(thf)2]2) was facilely prepared in this work. The structure of [CrCl2(μ-Cl)(thf)2]2 determined by X-ray crystallography agreed well with the XRD patterns and the C content of the bulk. Using well-defined [CrCl2(μ-Cl)(thf)2]2 instead of the impure CrCl3(thf)3 as a starting material, the activity of the ethylene tetramerization catalyst [iPrN{P(C6H4-p-Si(nBu)3)2}2CrCl2]+[B(C6F5)4]− was consistent and extremely high (6600 kg/g-Cr/h), and some Cr complexes (e.g., [(CH3CN)4CrCl2]+[B(C6F5)4]− and [{Et(Cl)Al(N(iPr)2)2}Cr(μ-Cl)]2) whose structure determination had failed previously could be isolated as high-quality crystals, enabling structure determination by X-ray crystallography. We strongly suggest the use of well-defined [CrCl2(μ-Cl)(thf)2]2 instead of the impure CrCl3(thf)3 as a Cr source in the synthesis of organometallic and coordination compounds of Cr.

5. Patents

A patent was applied on this study (Ajou University, Chromium Compound and Method for Preparing the Same. Kr 10-2020-0052494, 29 April 2020).

Author Contributions

Investigation, D.G.L., J.W.B., J.H.L., H.J.L. and Y.H.S.; Formal analysis, J.L.; Validation, C.G.L.; Conceptualization, funding acquisition, supervision and writing, B.Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

C1 Gas Refinery Program (2019M3D3A1A01069100); Priority Research Centers Program (2019R1A6A1A11051471).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

Ajou University (inventors: D. G. Lee, J. W. Baek, and B. Y. Lee) have applied for a patent covering this research.

Sample Availability

Samples of [CrCl2(μ-Cl)(thf)2]2 are available from the Precious Catalysts Inc.; cglee@s-pci.com (C.G.L.).

References

  1. Ambrose, K.; Murphy, J.N.; Kozak, C.M. Chromium Diamino-bis(phenolate) Complexes as Catalysts for the Ring-Opening Copolymerization of Cyclohexene Oxide and Carbon Dioxide. Inorg. Chem. 2020, 59, 15375–15383. [Google Scholar] [CrossRef] [PubMed]
  2. Jiménez, J.-R.; Poncet, M.; Doistau, B.; Besnard, C.; Piguet, C. Luminescent polypyridyl heteroleptic CrIII complexes with high quantum yields and long excited state lifetimes. Dalton Trans. 2020, 49, 13528–13532. [Google Scholar] [CrossRef]
  3. Baek, J.W.; Hyun, Y.B.; Lee, H.J.; Lee, J.C.; Bae, S.M.; Seo, Y.H.; Lee, D.G.; Lee, B.Y. Selective Trimerization of α-Olefins with Immobilized Chromium Catalyst for Lubricant Base Oils. Catalysts 2020, 10, 990. [Google Scholar] [CrossRef]
  4. Song, T.; Tao, X.; Tong, X.; Liu, N.; Gao, W.; Mu, X.; Mu, Y. Synthesis and characterization of chromium complexes 2-Me4CpC6H4CH2(R)NHCrCl2 and their catalytic properties in ethylene homo- and co-polymerization. Dalton Trans. 2019, 48, 4912–4920. [Google Scholar] [CrossRef] [PubMed]
  5. Sydora, O.L.; Hart, R.T.; Eckert, N.A.; Martinez Baez, E.; Clark, A.E.; Benmore, C.J. A homoleptic chromium(iii) carboxylate. Dalton Trans. 2018, 47, 4790–4793. [Google Scholar] [CrossRef]
  6. Olafsen, B.E.; Crescenzo, G.V.; Moisey, L.P.; Patrick, B.O.; Smith, K.M. Photolytic Reactivity of Organometallic Chromium Bipyridine Complexes. Inorg. Chem. 2018, 57, 9611–9621. [Google Scholar] [CrossRef]
  7. Ojelere, O.; Graf, D.; Mathur, S. Molecularly Engineered Lithium–Chromium Alkoxide for Selective Synthesis of LiCrO2 and Li2CrO4 Nanomaterials. Inorganics 2019, 7, 22. [Google Scholar] [CrossRef] [Green Version]
  8. Förster, C.; Dorn, M.; Reuter, T.; Otto, S.; Davarci, G.; Reich, T.; Carrella, L.; Rentschler, E.; Heinze, K. Ddpd as Expanded Terpyridine: Dramatic Effects of Symmetry and Electronic Properties in First Row Transition Metal Complexes. Inorganics 2018, 6, 86. [Google Scholar] [CrossRef] [Green Version]
  9. Zhao, R.; Ma, J.; Zhang, H.; Huang, J. Synthesis and Characterization of Constrained Geometry Oxygen and Sulphur Functionalized Cyclopentadienylchromium Complexes and Their Use in Catalysis for Olefin Polymerization. Molecules 2017, 22, 856. [Google Scholar] [CrossRef] [Green Version]
  10. Pei, L.; Tang, Y.; Gao, H. Homo- and Copolymerization of Ethylene and Norbornene with Anilido–Imine Chromium Catalysts. Polymers 2016, 8, 69. [Google Scholar] [CrossRef] [Green Version]
  11. Herwig, W.; Zeiss, H. Notes: Chromium Trichloride Tetrahydrofuranate. J. Org. Chem. 1958, 23, 1404. [Google Scholar] [CrossRef]
  12. Mastalir, M.; Glatz, M.; Stöger, B.; Weil, M.; Pittenauer, E.; Allmaier, G.; Kirchner, K. Synthesis, characterization and reactivity of vanadium, chromium, and manganese PNP pincer complexes. Inorg. Chim. Acta 2017, 455, 707–714. [Google Scholar] [CrossRef]
  13. Köhn, R.D.; Coxon, A.G.N.; Hawkins, C.R.; Smith, D.; Mihan, S.; Schuhen, K.; Schiendorfer, M.; Kociok-Köhn, G. Selective trimerisation and polymerisation of ethylene: Halogenated chromium triazacyclohexane complexes as probes for an internal ‘halogen effect’. Polyhedron 2014, 84, 3–13. [Google Scholar] [CrossRef] [Green Version]
  14. Heisig, G.B.; Hedin, H. Anhydrous Chromium(III) Chloride. Inorg. Synth. 1946, 2, 193–196. [Google Scholar] [CrossRef]
  15. Vavoulis, A.; Austin, T.E.; Tyree, S.Y., Jr. Anhydrous Chromium(III) Chloride. Inorg. Synth. 1960, 6, 129–132. [Google Scholar] [CrossRef]
  16. Shamir, J. New synthesis of chromium trichloride tetrahydrofuranate. Inorg. Chim. Acta 1989, 156, 163–164. [Google Scholar] [CrossRef]
  17. Pray, A.R. Anhydrous metal chlorides. Inorg. Synth. 1957, 5, 153–156. [Google Scholar]
  18. House, D.A.; Wang, J.; Nieuwenhuyzen, M. The synthesis and X-ray structure of trans-[CrCl2(nPrNH2)4]BF4·H2O and the thermal and Hg2+-assisted chloride release kinetics from some trans-[CrCl2(N)4]+ complexes. Inorg. Chim. Acta 1995, 237, 37–46. [Google Scholar] [CrossRef]
  19. So, J.H.; Boudjouk, P. A convenient synthesis of solvated and unsolvated anhydrous metal chlorides via dehydration of metal chloride hydrates with trimethylchlorosilane. Inorg. Chem. 1990, 29, 1592–1593. [Google Scholar] [CrossRef]
  20. Jeon, J.Y.; Park, J.H.; Park, D.S.; Park, S.Y.; Lee, C.S.; Go, M.J.; Lee, J.; Lee, B.Y. Concerning the chromium precursor CrCl3(thf)3. Inorg. Chem. Commun. 2014, 44, 148–150. [Google Scholar] [CrossRef]
  21. Sydora, O.L.; Knudsen, R.D.; Baralt, E.J. Preparation of an Olefin Oligomerization Catalyst. U.S. Patent 0,150,642A1, 13 June 2013. [Google Scholar]
  22. Bollmann, A.; Blann, K.; Dixon, J.T.; Hess, F.M.; Killian, E.; Maumela, H.; McGuinness, D.S.; Morgan, D.H.; Neveling, A.; Otto, S.; et al. Ethylene Tetramerization:  A New Route to Produce 1-Octene in Exceptionally High Selectivities. J. Am. Chem. Soc. 2004, 126, 14712–14713. [Google Scholar] [CrossRef] [PubMed]
  23. Overett, M.J.; Blann, K.; Bollmann, A.; Dixon, J.T.; Haasbroek, D.; Killian, E.; Maumela, H.; McGuinness, D.S.; Morgan, D.H. Mechanistic Investigations of the Ethylene Tetramerisation Reaction. J. Am. Chem. Soc. 2005, 127, 10723–10730. [Google Scholar] [CrossRef]
  24. Kuhlmann, S.; Blann, K.; Bollmann, A.; Dixon, J.T.; Killian, E.; Maumela, M.C.; Maumela, H.; Morgan, D.H.; Prétorius, M.; Taccardi, N.; et al. N-substituted diphosphinoamines: Toward rational ligand design for the efficient tetramerization of ethylene. J. Catal. 2007, 245, 279–284. [Google Scholar] [CrossRef]
  25. Boelter, S.D.; Davies, D.R.; Margl, P.; Milbrandt, K.A.; Mort, D.; Vanchura, B.A.; Wilson, D.R.; Wiltzius, M.; Rosen, M.S.; Klosin, J. Phospholane-Based Ligands for Chromium-Catalyzed Ethylene Tri- and Tetramerization. Organometallics 2020, 39, 976–987. [Google Scholar] [CrossRef]
  26. Hirscher, N.A.; Perez Sierra, D.; Agapie, T. Robust Chromium Precursors for Catalysis: Isolation and Structure of a Single-Component Ethylene Tetramerization Precatalyst. J. Am. Chem. Soc. 2019, 141, 6022–6029. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  27. Lee, H.; Hong, S.H. Polyhedral oligomeric silsesquioxane-conjugated bis(diphenylphosphino)amine ligand for chromium(III) catalyzed ethylene trimerization and tetramerization. Appl. Catal. A. Gen. 2018, 560, 21–27. [Google Scholar] [CrossRef]
  28. Newland, R.J.; Smith, A.; Smith, D.M.; Fey, N.; Hanton, M.J.; Mansell, S.M. Accessing Alkyl- and Alkenylcyclopentanes from Cr-Catalyzed Ethylene Oligomerization Using 2-Phosphinophosphinine Ligands. Organometallics 2018, 37, 1062–1073. [Google Scholar] [CrossRef] [Green Version]
  29. Nobbs, J.D.; Tomov, A.K.; Young, C.T.; White, A.J.P.; Britovsek, G.J.P. From alternating to selective distributions in chromium-catalysed ethylene oligomerisation with asymmetric BIMA ligands. Catal. Sci. Technol. 2018, 8, 1314–1321. [Google Scholar] [CrossRef]
  30. Alam, F.; Zhang, L.; Wei, W.; Wang, J.; Chen, Y.; Dong, C.; Jiang, T. Catalytic Systems Based on Chromium(III) Silylated-Diphosphinoamines for Selective Ethylene Tri-/Tetramerization. ACS Catal. 2018, 8, 10836–10845. [Google Scholar] [CrossRef]
  31. Höhne, M.; Peulecke, N.; Konieczny, K.; Müller, B.H.; Rosenthal, U. Chromium-Catalyzed Highly Selective Oligomerization of Ethene to 1-Hexene with N,N-Bis[chloro(aryl)phosphino]amine Ligands. ChemCatChem 2017, 9, 2467–2472. [Google Scholar] [CrossRef]
  32. Lifschitz, A.M.; Hirscher, N.A.; Lee, H.B.; Buss, J.A.; Agapie, T. Ethylene Tetramerization Catalysis: Effects of Aluminum-Induced Isomerization of PNP to PPN Ligands. Organometallics 2017, 36, 1640–1648. [Google Scholar] [CrossRef] [Green Version]
  33. Tomov, A.K.; Nobbs, J.D.; Chirinos, J.J.; Saini, P.K.; Malinowski, R.; Ho, S.K.Y.; Young, C.T.; McGuinness, D.S.; White, A.J.P.; Elsegood, M.R.J.; et al. Alternating α-Olefin Distributions via Single and Double Insertions in Chromium-Catalyzed Ethylene Oligomerization. Organometallics 2017, 36, 510–522. [Google Scholar] [CrossRef]
  34. Zhang, L.; Meng, X.; Chen, Y.; Cao, C.; Jiang, T. Chromium-Based Ethylene Tetramerization Catalysts Supported by Silicon-Bridged Diphosphine Ligands: Further Combination of High Activity and Selectivity. ChemCatChem 2017, 9, 76–79. [Google Scholar] [CrossRef]
  35. Kim, E.H.; Lee, H.M.; Jeong, M.S.; Ryu, J.Y.; Lee, J.; Lee, B.Y. Methylaluminoxane-Free Chromium Catalytic System for Ethylene Tetramerization. ACS Omega 2017, 2, 765–773. [Google Scholar] [CrossRef] [Green Version]
  36. Kim, T.H.; Lee, H.M.; Park, H.S.; Kim, S.D.; Kwon, S.J.; Tahara, A.; Nagashima, H.; Lee, B.Y. MAO-free and extremely active catalytic system for ethylene tetramerization. Appl. Organomet. Chem. 2019, 33, e4829. [Google Scholar] [CrossRef]
  37. Park, H.S.; Kim, T.H.; Baek, J.W.; Lee, H.J.; Kim, T.J.; Ryu, J.Y.; Lee, J.; Lee, B.Y. Extremely Active Ethylene Tetramerization Catalyst Avoiding the Use of Methylaluminoxane: [iPrN{P(C6H4-p-SiR3)2}2CrCl2]+[B(C6F5)4]−. ChemCatChem 2019, 11, 4351–4359. [Google Scholar] [CrossRef]
  38. Ewart, S.W.; Kolthammer, B.W.S.; Smith, D.M.; Hanton, M.J.; Dixon, J.T.; Morgan, D.H.; Debod, H.; Gabrielli, W.F.; Evans, S.J. Oligomerization of Olefinic Compounds in the Presence of an Activated Oligomerization Catalyst. U.S. Patent WO 2,010,092,554A1, 19 August 2010. [Google Scholar]
  39. Stennett, T.E.; Haddow, M.F.; Wass, D.F. Avoiding MAO: Alternative Activation Methods in Selective Ethylene Oligomerization. Organometallics 2012, 31, 6960–6965. [Google Scholar] [CrossRef]
  40. McGuinness, D.S.; Rucklidge, A.J.; Tooze, R.P.; Slawin, A.M.Z. Cocatalyst Influence in Selective Oligomerization:  Effect on Activity, Catalyst Stability, and 1-Hexene/1-Octene Selectivity in the Ethylene Trimerization and Tetramerization Reaction. Organometallics 2007, 26, 2561–2569. [Google Scholar] [CrossRef]
  41. McGuinness, D.S.; Overett, M.; Tooze, R.P.; Blann, K.; Dixon, J.T.; Slawin, A.M.Z. Ethylene Tri- and Tetramerization with Borate Cocatalysts:  Effects on Activity, Selectivity, and Catalyst Degradation Pathways. Organometallics 2007, 26, 1108–1111. [Google Scholar] [CrossRef]
  42. Hirscher, N.A.; Labinger, J.A.; Agapie, T. Isotopic labelling in ethylene oligomerization: Addressing the issue of 1-octene vs. 1-hexene selectivity. Dalton Trans. 2019, 48, 40–44. [Google Scholar] [CrossRef] [Green Version]
  43. Hirscher, N.A.; Agapie, T. Stoichiometrically Activated Catalysts for Ethylene Tetramerization using Diphosphinoamine-Ligated Cr Tris(hydrocarbyl) Complexes. Organometallics 2017, 36, 4107–4110. [Google Scholar] [CrossRef] [Green Version]
  44. Albert Cotton, F.; Duraj, S.A.; Powell, G.L.; Roth, W.J. Comparative structural studies of the first row early transition metal(III) chloride tetrahydrofuran solvates. Inorg. Chim. Acta 1986, 113, 81–85. [Google Scholar] [CrossRef]
  45. Köhn, R.D.; Coxon, A.G.N.; Chunawat, S.; Heron, C.; Mihan, S.; Lyall, C.L.; Reeksting, S.B.; Kociok-Köhn, G. Triazacyclohexane chromium triflate complexes as precursors for the catalytic selective olefin trimerisation and its investigation by mass spectrometry. Polyhedron 2020, 185, 114572. [Google Scholar] [CrossRef]
  46. Kern, R.J. Tetrahydrofuran complexes of transition metal chlorides. J. Inorg. Nucl. Chem. 1962, 24, 1105–1109. [Google Scholar] [CrossRef]
  47. Dixon, J.T.; Green, M.J.; Hess, F.M.; Morgan, D.H. Advances in selective ethylene trimerization—A critical overview. J. Organomet. Chem. 2004, 689, 3641–3668. [Google Scholar] [CrossRef]
  48. Jeon, J.Y.; Park, D.S.; Lee, D.H.; Eo, S.C.; Park, S.Y.; Jeong, M.S.; Kang, Y.Y.; Lee, J.; Lee, B.Y. A chromium precursor for the Phillips ethylene trimerization catalyst: (2-ethylhexanoate)2CrOH. Dalton Trans. 2015, 44, 11004–11012. [Google Scholar] [CrossRef]
Molecules 26 01167 sch001 550
Scheme 1. Preparation of extremely active ethylene tetramerization catalyst.
Scheme 1. Preparation of extremely active ethylene tetramerization catalyst.
Molecules 26 01167 sch001
Molecules 26 01167 sch002 550
Scheme 2. Modified preparation method for THF adduct of CrCl3.
Scheme 2. Modified preparation method for THF adduct of CrCl3.
Molecules 26 01167 sch002
Molecules 26 01167 g001 550
Figure 1. Macroscopic views of crystals of [CrCl2(μ-Cl)(thf)2]2 (a) and irregular solid known as CrCl3(thf)3 (b).
Figure 1. Macroscopic views of crystals of [CrCl2(μ-Cl)(thf)2]2 (a) and irregular solid known as CrCl3(thf)3 (b).
Molecules 26 01167 g001
Molecules 26 01167 g002 550
Figure 2. X-ray structure of [CrCl2(μ-Cl)(thf)2]2. Selected bond distances (Å) and angles (°): Cr-Cl1, 2.2773(12); Cr-Cl2, 2.3622(12); Cr-Cl3, 2.2885(11); Cr-O1, 2.045(3); Cr-O2, 2.018(3); C-O1-C, 109.5(3); C-O1-Cr, 125.5(2); C-O1-Cr, 125.0(2); C-O2-C, 108.7(3); C-O2-Cr, 121.5(2); and C-O2-Cr, 122.6(2).
Figure 2. X-ray structure of [CrCl2(μ-Cl)(thf)2]2. Selected bond distances (Å) and angles (°): Cr-Cl1, 2.2773(12); Cr-Cl2, 2.3622(12); Cr-Cl3, 2.2885(11); Cr-O1, 2.045(3); Cr-O2, 2.018(3); C-O1-C, 109.5(3); C-O1-Cr, 125.5(2); C-O1-Cr, 125.0(2); C-O2-C, 108.7(3); C-O2-Cr, 121.5(2); and C-O2-Cr, 122.6(2).
Molecules 26 01167 g002
Molecules 26 01167 g003 550
Figure 3. X-ray powder diffraction (XRD) patterns of [CrCl2(μ-Cl)(thf)2]2 calculated from cif file (a) and measured (b) and those of CrCl3(thf)3 calculated from cif file (c), obtained from the reaction ‘CrCl2(OH)(H2O)2 + Me3SiCl (6 eq)’ (d), obtained from the reaction ‘CrCl3·6H2O + Me3SiCl (60 eq)’ (e), and purchased (f).
Figure 3. X-ray powder diffraction (XRD) patterns of [CrCl2(μ-Cl)(thf)2]2 calculated from cif file (a) and measured (b) and those of CrCl3(thf)3 calculated from cif file (c), obtained from the reaction ‘CrCl2(OH)(H2O)2 + Me3SiCl (6 eq)’ (d), obtained from the reaction ‘CrCl3·6H2O + Me3SiCl (60 eq)’ (e), and purchased (f).
Molecules 26 01167 g003
Molecules 26 01167 g004 550
Figure 4. EPR spectra of [CrCl2(μ-Cl)(thf)2]2 and the commercial CrCl3(thf)3.
Figure 4. EPR spectra of [CrCl2(μ-Cl)(thf)2]2 and the commercial CrCl3(thf)3.
Molecules 26 01167 g004
Molecules 26 01167 g005 550
Figure 5. X-ray structure of [(CH3CN)4CrCl2]+[B(C6F5)4]−. Selected bond distances (Å) and angles (°): Cr-N1, 2.025(3); Cr-N2, 2.018(3); Cr-Cl1, 2.2804(9); Cr-N2-C, 178.8(3); and Cr-N1-C, 175.5(3).
Figure 5. X-ray structure of [(CH3CN)4CrCl2]+[B(C6F5)4]−. Selected bond distances (Å) and angles (°): Cr-N1, 2.025(3); Cr-N2, 2.018(3); Cr-Cl1, 2.2804(9); Cr-N2-C, 178.8(3); and Cr-N1-C, 175.5(3).
Molecules 26 01167 g005
Molecules 26 01167 g006 550
Figure 6. X-ray structure of [{Et(Cl)Al(N(iPr)2)2}Cr(μ-Cl)]2. Selected bond distances (Å) and angles (°): Cr-Cl1, 2.4212; Cr-N1, 2.141(6); Cr-N2, 2.119(6); Al-N1, 1.925(6); Al-N2, 1.953(6); Al-Cl2, 2.163(3); Cr--Cr’, 3.578(2); N1-Cr-N2, 84.3(2); Cl-Cr-Cl’, 83.96(8); N1-Al-N2, 95.0(3); and angle between N1-Cr-N2 and Cl-Cr-Cl’ planes, 25.1(1).
Figure 6. X-ray structure of [{Et(Cl)Al(N(iPr)2)2}Cr(μ-Cl)]2. Selected bond distances (Å) and angles (°): Cr-Cl1, 2.4212; Cr-N1, 2.141(6); Cr-N2, 2.119(6); Al-N1, 1.925(6); Al-N2, 1.953(6); Al-Cl2, 2.163(3); Cr--Cr’, 3.578(2); N1-Cr-N2, 84.3(2); Cl-Cr-Cl’, 83.96(8); N1-Al-N2, 95.0(3); and angle between N1-Cr-N2 and Cl-Cr-Cl’ planes, 25.1(1).
Molecules 26 01167 g006
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Lee, D.G.; Baek, J.W.; Lee, J.H.; Lee, H.J.; Seo, Y.H.; Lee, J.; Lee, C.G.; Lee, B.Y. Replacement of the Common Chromium Source CrCl3(thf)3 with Well-Defined [CrCl2(μ-Cl)(thf)2]2. Molecules 2021, 26, 1167. https://doi.org/10.3390/molecules26041167

AMA Style

Lee DG, Baek JW, Lee JH, Lee HJ, Seo YH, Lee J, Lee CG, Lee BY. Replacement of the Common Chromium Source CrCl3(thf)3 with Well-Defined [CrCl2(μ-Cl)(thf)2]2. Molecules. 2021; 26(4):1167. https://doi.org/10.3390/molecules26041167

Chicago/Turabian Style

Lee, Dong Geun, Jun Won Baek, Jung Hyun Lee, Hyun Ju Lee, Yeong Hyun Seo, Junseong Lee, Chong Gu Lee, and Bun Yeoul Lee. 2021. "Replacement of the Common Chromium Source CrCl3(thf)3 with Well-Defined [CrCl2(μ-Cl)(thf)2]2" Molecules 26, no. 4: 1167. https://doi.org/10.3390/molecules26041167

Article Metrics

Back to TopTop