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

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.

Sasol disclosed a catalyst system composed of a Cr source, iPrN(PPh 2 ) 2 ligand, and methylaluminoxane (MAO), which produces mainly 1-octene through ethylene tetramerization [22][23][24]. CrCl 3 (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, iBu 3 Al, 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 CrCl 3 (thf) 3 used in the preparation was the cause of this problem. Although the structure of CrCl 3 (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 CH 2 Cl 2 [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 CrCl 3 (thf) 3 . In fact, it was reported as an "irregular violet solid" which may mean that it is not a pure CrCl 3 (thf) 3 but a mixture containing other forms of THF adduct of CrCl 3 (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. CrCl 3 (thf) 3 is paramagnetic; thus, it cannot be subjected to structural analysis using 1 H and 13 C NMR spectroscopy. We also previously found that the composition (as well as the structure) of the commercial source of (2-ethylhexanoate) 3 Cr, 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) 2 CrOH, the catalytic performance was consistent and, moreover, significantly improved.
Molecules 2021, 26, x FOR PEER REVIEW 2 of 11 (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 1 H and 13 C 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.

Results and Discussion
CrCl3(thf)3 was commercially available from Aldrich (St. Louis, MO, United States), 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 methylcyclohexaneinsoluble 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 CrCl 3 ·6H 2 O (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 CrCl 2 (OH)(H 2 O) 2 ; 34% weight loss is expected for the transformation of CrCl 3 ·6H 2 O to CrCl 2 (OH)(H 2 O) 2 (Scheme 2). The structure of CrCl 3 ·6H 2 O was reported to be [CrCl 2 (H 2 O) 4 ]Cl·2H 2 O, and the action of heat under vacuum could liberate outer-sphere Cl − (as HCl with a proton in an innersphere water) and three water molecules (two from the outer sphere and one from the inner sphere), consequently leaving CrCl 2 (OH)(H 2 O) 2 (possibly, [CrCl 2 (H 2 O) 2 (µ-OH)] 2 adopting an octahedral structure) in the reaction pot. 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 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 1 H-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 Clbridged 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 Treatment of CrCl 2 (OH)(H 2 O) 2 , after dissolving in THF, with Me 3 SiCl resulted in the precipitation of a violet solid, which was confirmed by infrared spectroscopy to be completely anhydrous. Because a large portion of H 2 O in CrCl 3 ·6H 2 O was removed at the evacuation step, 6.0 equiv. Me 3 SiCl (equivalent amount that needed for the transformation of CrCl 2 (OH)(H 2 O) 2 to CrCl 3 (thf) x , Scheme 2) was added instead of 60 equiv./Cr, and accordingly, the amount of THF could be minimized (36 mL/10 g-CrCl 3 ·6H 2 O). The color and shape of the precipitated solid (irregular violet solid) were almost identical to that of the commercial source of CrCl 3 (thf) 3 and the one prepared without the evacuation step. However, as aforementioned, the catalyst performance was significantly better when using CrCl 3 (thf) 3 prepared via the evacuation step. The product of the reaction between [(CH 3 CN) 4 Ag] + [B(C 6 F 5 ) 4 ] − and CrCl 3 (thf) 3 (i.e., [(CH 3 CN) 4 CrCl 2 ] + [B(C 6 F 5 ) 4 ] − ) differed depending on the source of CrCl 3 (thf) 3 . Although [(CH 3 CN) 4 CrCl 2 ] + [B(C 6 F 5 ) 4 ] − was isolated by precipitation in CH 3 CN 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 [(CH 3 CN) 4 CrCl 2 ] + [B(C 6 F 5 ) 4 ] − by counting the number of CH 3 CN molecules per each Cr atom through the analysis of integration values in the 1 H-NMR spectra recorded using THF-d 8 with 9-methylanthracene as an internal standard. For the product obtained using CrCl 3 (thf) 3 prepared via the evacuation step, the number of CH 3 CN 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 CrCl 3 (thf) 3 or the one prepared without the evacuation step was used.

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, United States) 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).

[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

General Remarks
All manipulations were performed in an inert atmosphere using a standard glove box and Schlenk techniques. CH 2 Cl 2 and CH 3 CN were stirred over CaH 2 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).

[CrCl 2 (µ-Cl)(thf) 2 ] 2
A Schlenk flask containing crystalline solid CrCl 3 ·6H 2 O (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, Me 3 SiCl (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 CH 2 Cl 2 (53 g) was added to dissolve the solid. When the vial was placed inside a closed chamber (~250 mL size) containing methylcyclohexane (~15 mL), CH 2 Cl 2 was slowly evaporated and purple crystals were deposited within 24 h, which were isolated by decantation (4.55 g, 69%).

[(CH 3 CN) 4 CrCl 2 ] + [B(C 6 F 5 ) 4 ] −
A solution of [(CH 3 CN) 4 Ag] + [B(C 6 F 5 ) 4 ] − (7.789 g, 8.189 mmol) in acetonitrile (17.3 g) was added to a suspension of [CrCl 2 (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 9methylanthracene (20.0 mg) were dissolved in THF-d 8 4 ] − (7.55 g, 7.781 mmol) in CH 2 Cl 2 (46 g). Upon addition, the color of the solution changed immediately from olivegreen 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 CH 3 CN and CH 2 Cl 2 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.

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 Å). 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.

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).

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