Methylaluminoxane-Free Chromium Catalytic System for Ethylene Tetramerization.

Ethylene tetramerization catalyst systems comprising a Cr(III) complex containing PNP ligands and methylaluminoxane (MAO) are useful for the production of 1-octene. However, a concern with these systems is the use of expensive MAO in excess. Herein, we report a catalytic system that avoids the use of MAO. Metathesis of CrCl3(THF)3 and [(CH3CN)4Ag]+[B(C6F5)4]- afforded [L4CrIIICl2]+[B(C6F5)4]- (L = CH3CN or tetrahydrofuran (THF)), which was converted to [(PNP)CrCl2L2]+[B(C6F5)4]-, where PNP is iPrN(PPh2)2 (1) or [CH3(CH2)16]2CHN(PPh2)2 (2). The molecular structures of [(THF)4CrIIICl2]+[B(C6F5)4]- and [1-CrCl2(THF)2]+[B(C6F5)4]- were unambiguously determined by X-ray crystallography. The cationic (PNP)CrIII complexes paired with [B(C6F5)4]- anions, that is, [(PNP)CrCl2(CH3CN)2]+[B(C6F5)4]-, exhibited high activity in chlorobenzene when activated with common trialkylaluminum species (Me3Al, Et3Al, and iBu3Al). The activities and selectivity were comparable to those of the original MAO-based Sasol system (1-CrCl3/MAO). When activated with Et3Al or iBu3Al, the Cr complex, [2-CrCl2(CH3CN)2]+[B(C6F5)4]-, which bears long alkyl chains, showed high activity in the more desirable methylcyclohexane solvent (89 kg/g-Cr/h) and much higher activity in cyclohexene (168 kg/g-Cr/h). Other advantages of the [2-CrCl2(CH3CN)2]+[B(C6F5)4]-/Et3Al system in cyclohexene were negligible catalyst deactivation, formation of only a negligible amount of polyethylene side product (0.3%), and formation of fewer unwanted side products above C10. The [B(C6F5)4]- anion is compatible with trialkylaluminum species once it is not paired with a trityl cation. Hence, [(PNP)CrCl2(CH3CN)2]+[B(C6F5)4]-/Et3Al exhibited a significantly higher activity than that of a previously reported system composed of [Ph3C]+[B(C6F5)4]-, that is, 1/CrCl3(THF)3/[Ph3C]+[B(C6F5)4]-/Et3Al.

Introduction

Production of linear α-olefins (LAOs) through n class="Chemical">ethylene oligomerization is an important issue in both industry and academia,[1−6] and development of efficient catalysts for this process is an active research area.[7−22] LAOs are used in the polyolefin industry as comonomers, and demand for them has increased with the increase in polyolefin production using the homogeneous metallocene-type catalysts. Recently, LAOs have also been used in the production of poly(α-olefin) lubricant base-stocks.[23] LAOs can be separated from wide-distribution mixtures of 1-alkenes that are produced using nickel catalysts via the Shell higher olefin process.[24,25] Catalysts that can selectively generate 1-hexene or 1-octene from ethylene have also been discovered. For example, a trimerization catalyst composed of pyrrole, (2-ethylhexanoate)3Cr(III), and an alkylaluminum reagent (Et3Al + Et2AlCl) was discovered at Phillips in the early 1990s,[26−28] and a tetramerization catalyst composed of iPrN(PPh2)2 (1), Cr(acac)3, and methylaluminoxane (MAO) was discovered at Sasol in the early 2000s.[29−33] There is some controversy concerning the active species in these selective oligomerization catalysts. Electron paramagnetic resonance (EPR) studies showed that the majority of CrIII precursors were converted to EPR-silent CrII by the action of the alkylaluminum cocatalyst.[34−36] On the basis of this observation, a catalytic cycle involving the CrII/IV species was proposed,[37,38] but a catalytic cycle involving the cationic CrI/III species paired with a MAO-derived noncoordinating anion is currently more generally accepted (Scheme ).[34,35,38−40] We recently prepared various zwitterionic CrII complexes, but none of them were active in ethylene oligomerization, convincing us of the validity of the catalytic cycle involving cationic CrI/III species.[41]

Scheme 1

Active Species Proposed for Ethylene Tetramerization, and the CrIII Complexes Targeted in This Work

Although the Phillips system requires inexn class="Chemical">pensive alkylaluminum Et3Al and Et2AlCl as an activator, the use of expensive MAO in excess (Al/Cr, 300–500) is a critical concern when considering the commercial application of the Sasol system. A catalytic system containing [Ph3C]+[B(C6F5)4]− instead of MAO (i.e., 1-CrIIICl3/[Ph3C]+[B(C6F5)4]−/Et3Al) was proposed on the basis of the aforementioned catalytic cycle involving cationic CrI/III species, but its activity was significantly inferior to that of the MAO-derived system and resulted in the generation of a large amount of polyethylene (PE).[42] Its failure was attributed to the instability of [B(C6F5)4]− anions, and efforts were made to develop other types of noncoordinating anions, leading to the development of catalytic systems containing [Al(OC(CF3)3)4]− anions (e.g., CrCl3(THF)3/1/AlEt3/[Ph3C]+[Al(OC(CF3)3)4]− and [1-CrI(CO)4]+[Al(OC(CF3)3)4]−/AlEt3) that exhibited reasonably high activity.[43,44] Herein, avoiding the use of highly reactive [Ph3C]+[B(C6F5)4]−, we prepared cationic CrIII complexes paired with common [B(C6F5)4]− anions, which showed high activity even when activated with a common trialkylaluminum agent (Scheme ).[45] There have been several previous reports of cationic CrIII complexes paired with [B(C6F5)4]− anions. However, their preparations were not targeted toward ethylene oligomerization.[46−49]

Results and Discussion

Preparation of Cationic CrIII Complexes

Our synthetic strategy for the targeted complexes ([(PNP)CrIIICl2]+n class="Chemical">[B(C6F5)4]−) was to react [L4CrCl2]+[B(C6F5)4]− (L = CH3CN or tetrahydrofuran (THF)) with PNP ligands (Scheme a). The key complex in this synthetic scheme, [(CH3CN)4CrIIICl2]+[B(C6F5)4]−, was prepared by the metathesis of CrCl3(THF)3 and [(CH3CN)4Ag]+[B(C6F5)4]− in acetonitrile. The silver complex, [(CH3CN)4Ag]+[B(C6F5)4]−, was easily prepared by reacting commercially available K+[B(C6F5)4]− with AgNO3 in acetonitrile.[50,51] In this work, the molecular structure of the silver complex was unambiguously confirmed by X-ray crystallography (Figure a).[52] The key complex [(CH3CN)4CrIIICl2]+[B(C6F5)4]− was isolated as green thin needle-shaped crystals, but the quality of the crystals was unsatisfactory for X-ray crystallography. After replacing the acetonitrile ligand with THF, pale pink-colored and needle-shaped single crystals suitable for X-ray crystallography were obtained, and its structure was elucidated to be that of the desired [(THF)4CrCl2]+[B(C6F5)4]− (Figure b). Wass et al. also reported the synthesis of the related complex [(THF)4CrCl2]+[Al(OC(CF3)3)4]−, which was prepared by the reaction of CrCl2(THF)2 and [Cp2Fe]+[Al(OC(CF3)3)4]−.[53]

Scheme 2

Synthetic Scheme for the Target Complexes

Figure 1

Thermal ellipsoid plots (30% probability level) of [(CH3CN)4Ag]+[B(C6F5)4]− (a), [(THF)4CrCl2]+[B(C6F5)4]− (b), and (o-Me2NC6H4CH2)3Cr (c). Selected bond distances (Å) and angles (deg) are as follows: In (a), Ag–N(1), 2.311(4); Ag–N(2), 2.293(4); Ag–N(3), 2.247(3); Ag–N(4), 2.323(3); N(1)–Ag–N(2), 120.99(11); N(3)–Ag–N(4), 135.82(10); N(2)–Ag–N(3), 102.70(11); N(1)–Ag–N(3), 106.56(10); C–N(1)–Ag, 170.1(3); C–N(2)–Ag, 172.1(3); C–N(3)–Ag, 177.9(2); C–N(4)–Ag, 155.7(3); angle between the N(1)–Ag–N(2) and N(3)–Ag–N(4) planes, 85.04(6). In (b), Cr–O(1), 2.002(3); Cr–O(2), 2.004(3); Cr–Cl(1), 2.2978(10); Cl(1)–Cr–Cl(1)′, 180; O(1)–Cr–O(1)′, 180; O(2)–Cr–O(2)′, 180; Cl(1)–Cr–O(1), 90; Cl(1)–Cr–O(2), 90; O (1)–Cr–O(2), 90. In (c), Cr–N(1), 2.4006(15); Cr–N(2), 2.4177(15); Cr–N(3), 2.3847(15); Cr–C(1), 2.1096(18); Cr–C(10), 2.1186(18); Cr–C(19), 2.1173(18); C(1)–Cr–N(2), 168.03(6); C(19)–Cr–N(1), 167.95(6); C(10)–Cr–N(3), 166.72(6).

Thermal ellipsoid plots (30% probability level) of n class="Chemical">[(CH3CN)4Ag]+[B(C6F5)4]− (a), [(THF)4CrCl2]+[B(C6F5)4]− (b), and (o-Me2NC6H4CH2)3Cr (c). Selected bond distances (Å) and angles (deg) are as follows: In (a), Ag–N(1), 2.311(4); Ag–N(2), 2.293(4); Ag–N(3), 2.247(3); Ag–N(4), 2.323(3); N(1)–Ag–N(2), 120.99(11); N(3)–Ag–N(4), 135.82(10); N(2)–Ag–N(3), 102.70(11); N(1)–Ag–N(3), 106.56(10); C–N(1)–Ag, 170.1(3); C–N(2)–Ag, 172.1(3); C–N(3)–Ag, 177.9(2); C–N(4)–Ag, 155.7(3); angle between the N(1)–Ag–N(2) and N(3)–Ag–N(4) planes, 85.04(6). In (b), Cr–O(1), 2.002(3); Cr–O(2), 2.004(3); Cr–Cl(1), 2.2978(10); Cl(1)–Cr–Cl(1)′, 180; O(1)–Cr–O(1)′, 180; O(2)–Cr–O(2)′, 180; Cl(1)–Cr–O(1), 90; Cl(1)–Cr–O(2), 90; O (1)–Cr–O(2), 90. In (c), Cr–N(1), 2.4006(15); Cr–N(2), 2.4177(15); Cr–N(3), 2.3847(15); Cr–C(1), 2.1096(18); Cr–C(10), 2.1186(18); Cr–C(19), 2.1173(18); C(1)–Cr–N(2), 168.03(6); C(19)–Cr–N(1), 167.95(6); C(10)–Cr–N(3), 166.72(6).

When iPrN(PPh2)2 (1) was added to a solution of n class="Chemical">[(CH3CN)4CrIIICl2]+[B(C6F5)4]− in CH2Cl2, the color of the solution immediately changed from olive green to brown and eventually became bluish-green after several hours. Bluish-green solids were isolated almost quantitatively by trituration in hexane, but various attempts to grow single crystals for X-ray crystallography were unsuccessful; bluish-green oils were deposited in most cases. After replacing the coordinating acetonitrile with THF, pale blue-colored and plate-shaped single crystals suitable for X-ray crystallography were obtained, the structure of which was elucidated to be that of the desired [1-CrCl2(THF)2]+[B(C6F5)4]− (Figure a). Reacting 1 with THF adduct [(THF)4CrIIICl2]+[B(C6F5)4]−, even in a 1:1 mole ratio, with the aim of directly obtaining greenish [1-CrCl2(THF)2]+[B(C6F5)4]− resulted in the generation of a brown solution. X-ray crystallographic studies of the isolated yellow, needle-shaped single crystals revealed their structure to be that of the bis(PNP)CrIII complex [12-CrCl2]+[B(C6F5)4]− (Figure b). The THF adduct, [(THF)4CrIIICl2]+[B(C6F5)4]−, is sparingly soluble in CH2Cl2, which is in contrast with the high solubility of the acetonitrile adduct, [(CH3CN)4CrIIICl2]+[B(C6F5)4]−, in CH2Cl2, and the reaction of 1 with [(THF)4CrIIICl2]+[B(C6F5)4]−, even in a 1:1 mole ratio, afforded mainly the bis(PNP)CrIII complex, [12-CrCl2]+[B(C6F5)4]−, leaving half of the feed reactant, [(THF)4CrIIICl2]+[B(C6F5)4]−, in the solid phase. Similarly, it has been reported that reacting [(THF)4CrCl2]+[Al(OC(CF3)3)4]− with the PNP ligand does not afford the mono(PNP)CrIII complex, but instead affords the bis(PNP)CrIII complex, [(PNP)2CrCl2]+[Al(OC(CF3)3)4]−.[53] Whereas the bis(PNP)CrIII complex, [12-CrCl2]+[B(C6F5)4]−, is soluble in hydrocarbon solvents such as toluene, the mono(PNP)CrIII complex, [1-CrCl2L2]+[B(C6F5)4]− (L = THF or CH3CN), is insoluble in hydrocarbon solvents. However, it is soluble in more polar chlorobenzene or CH2Cl2.

Figure 2

Thermal ellipsoid plots (30% probability level) of [1-CrCl2(THF)2]+[B(C6F5)4]− (a) and [12-CrCl2]+[B(C6F5)4]− (b). Selected bond distances (Å) and angles (deg) are as follows. In (a), Cr–Cl(1), 2.2799(15); Cr–Cl(2), 2.2869(15); Cr–P(1), 2.4413(14); Cr–P(2), 2.4844(15); Cr–O(1), 2.027(3); Cr–O(2), 2.057(3); Cl(1)–Cr–Cl(2), 179.59(7); P(1)–Cr–P(2), 66.85(5); P(1)–N–P(2), 106.0(2). In (b), Cr–Cl(1), 2.287(2); Cr–Cl(2), 2.273(2); Cr–P(1), 2.485(2); Cr–P(2), 2.454(2); Cr–P(3), 2.477(2); Cr–P(4), 2.464(2); Cl(1)–Cr–Cl(2), 171.97(9); P(1)–Cr–P(2), 66.90(8); P(1)–N(1)–P(2), 105.6(4).

Thermal ellipsoid plots (30% probability level) of n class="Chemical">[1-CrCl2(THF)2]+[B(C6F5)4]− (a) and [12-CrCl2]+[B(C6F5)4]− (b). Selected bond distances (Å) and angles (deg) are as follows. In (a), Cr–Cl(1), 2.2799(15); Cr–Cl(2), 2.2869(15); Cr–P(1), 2.4413(14); Cr–P(2), 2.4844(15); Cr–O(1), 2.027(3); Cr–O(2), 2.057(3); Cl(1)–Cr–Cl(2), 179.59(7); P(1)–Cr–P(2), 66.85(5); P(1)–N–P(2), 106.0(2). In (b), Cr–Cl(1), 2.287(2); Cr–Cl(2), 2.273(2); Cr–P(1), 2.485(2); Cr–P(2), 2.454(2); Cr–P(3), 2.477(2); Cr–P(4), 2.464(2); Cl(1)–Cr–Cl(2), 171.97(9); P(1)–Cr–P(2), 66.90(8); P(1)–N(1)–P(2), 105.6(4).

Aliphatic hydrocarbon solvents (e.g., n class="Chemical">methylcyclohexane) are the best options for commercial ethylene tetramerization. Hence, complexes soluble in methylcyclohexane are desirable. A PNP ligand bearing long alkyl chains ([CH3(CH2)16]2CHN(PPh2)2, 2 in Scheme ) was prepared in good yield (70%) by the conventional synthetic method, that is, reacting Ph2PCl with a long-chain alkyl amine [CH3(CH2)16]2CHNH2 in the presence of excess triethylamine.[29] The alkyl amine, [CH3(CH2)16]2CHNH2, was prepared from inexpensive 18-pentatricontanone ([CH3(CH2)16]2C=O).[54,55] The long-chain PNP ligand is highly soluble in hexane. PNP ligand 2 was reacted with [(CH3CN)4CrIIICl2]+[B(C6F5)4]− in CH2Cl2. To minimize (or avoid) generation of undesired bis(PNP)CrIII species, the mole ratio of the reactants was set at 1:2. A green species was extracted from the reaction mixture using hot methylcyclohexane (ca. 50 °C) while leaving [(CH3CN)4CrIIICl2]+[B(C6F5)4]−, which remained owing to its addition in excess, in the solid phase. The extracted species was assigned as the desired mono(PNP)CrIII complex [2-CrCl2(CH3CN)2]+[B(C6F5)4]− on the basis of its green color and elemental analysis data and was used for oligomerization studies. The complex is clearly soluble in hot methylcyclohexane (ca. 50 °C), but the solution becomes turbid by the formation of a dispersion of fine particles upon cooling to room temperature. It is freely soluble in hydrocarbon solvents bearing π-electrons such as toluene or cyclohexene, even at room temperature.

Preparation of the cationic (PNP)n class="Chemical">CrIII complex containing an ortho-dimethylaminobenzyl ligand instead of chloride was attempted (Scheme b). The starting Cr-precursor, (o-Me2NC6H4CH2)3Cr, was prepared by reacting CrCl3(THF)3 with o-Me2NC6H4CH2Li.[56] The pure complex was isolated by recrystallization in CH2Cl2 at −30 °C, and its molecular structure was unambiguously confirmed by X-ray crystallography (Figure c). When (o-Me2NC6H4CH2)3Cr was treated with [H(OEt2)2]+[B(C6F5)4]− in CH2Cl2, the color of the solution immediately changed from red to violet. In the 1H NMR spectrum, signals assignable to o-Me2NC6H4CH3 are present, indicating the protonation of the ortho-dimethylaminobenzyl ligand by the action of [H(OEt2)2]+[B(C6F5)4]− (Figure S7 in Supporting information). The diethyl ether signals, which are clearly observed in the 1H NMR spectrum of [H(OEt2)2]+[B(C6F5)4]− in CD2Cl2 at 3.55 and 1.42 ppm, are absent, indicating that diethyl ether coordinates to the paramagnetic CrIII center. On the basis of these observations, the generated complex was assigned as [(o-Me2NC6H4CH2)2CrIII(OEt2)2]+[B(C6F5)4]−. All attempts to grow single crystals of this complex were unsuccessful and, furthermore, the color of the solution changed during the recrystallization process, indicating that it is unstable. Thus, PNP ligand 1 was subsequently added to the CD2Cl2 solution containing the unpurified [(o-Me2NC6H4CH2)2CrIII(OEt2)2]+[B(C6F5)4]−. In the 1H NMR spectrum, the signals for 1 are absent, indicating the coordination of the PNP ligand to the paramagnetic CrIII center (Figure S7). Broad diethyl ether signals are observed at 3.78 and 1.54 ppm after overnight reaction. These observations positively indicate the generation of the desired complex [1-Cr(CH2C6H4NMe2)2]+[B(C6F5)4]−. Attempts to grow single crystals of this species were also unsuccessful. Using the long-chain PNP ligand (2), a methylcyclohexane-soluble complex was prepared by the same procedure, which was tentatively assigned as [2-Cr(CH2C6H4NMe2)2]+[B(C6F5)4]− and used for the oligomerization studies.

X-ray Crystallographic Studies

Single crystals of [(CH3CN)4Ag]+[B(C6F5)4]− suitable for X-ray crystallography were grown in acetonitrile solution at −30 °C. Tetrahedral coordination of the CH3CN ligands to silver is observed with no direct chemical bonding between [(CH3CN)4Ag]+ and [B(C6F5)4]− (Figure a). One Ag–N–C bond angle deviates significantly from linearity (155.7(3)°), whereas the other three are almost linear (170.1(3), 172.1(3), and 177.9(2)°). Single crystals of [(THF)4CrCl2]+[B(C6F5)4]− were grown in THF at −30 °C after the acetonitrile ligands in [(CH3CN)4CrIIICl2]+[B(C6F5)4]− were replaced with THF. The CrIII center adopts perfect octahedral geometry surrounded by two chloride and four THF ligands (Figure b). The four THF ligands form a plane with the Cr center, whereas the two chloride ligands are situated at the axial sites in trans configuration. The oxygen atoms in THF adopt sp2 hybridization, that is, the sum of bond angles around O is 360°, indicating π-donation from O to the CrIII center. The structure of (o-Me2NC6H4CH2)3Cr is shown in Figure c. The Cr center adopts distorted octahedral geometry with facial arrangement of three benzylic CH2 groups and three amine ligands. The plain formed by the three carbon ligands is parallel to the plane formed by the three nitrogen ligands (the angle between the C(1)–C(10)–C(19) and N(1)–N(2)–N(3) planes, 0.90(6)°), and the benzylic CH2 ligand is widely exposed, making it very accessible for the attack by [H(OEt2)2]+[B(C6F5)4]−.

Figure shows the structures of the mono(PNP)CrIII complex, n class="Chemical">[1-CrCl2(THF)2]+[B(C6F5)4]−, and bis(PNP)CrIII complex, [12-CrCl2]+[B(C6F5)4]−. Both complexes exhibit distorted octahedral coordination. The PNP and the two THF ligands form a plane with the CrIII center (the angle between the P–Cr–P and O–Cr–O planes, 4.02(9)°) in the former case, and the two PNP ligands form a rather distorted plane with the CrIII center in the latter case (the angle between the P(1)–Cr–P(2) and P(3)–Cr–P(4) planes, 8.52(7)°). The two chlorides occupy the axial sites almost linearly (the Cl–Cr–Cl angles, 179.59(7) and 171.97(9)°). In both cases, the sum of the bond angles around the nitrogen atom in the PNP ligand is 360°, which indicates the delocalization of the nitrogen lone pair through phosphorous atoms by adopting sp2-hybridization. The sum of the bond angles around the oxygen atom in THF is 360°, indicating π-donation from the O to the CrIII center by adopting sp2-hybridization. The Cr–OTHF distances are elongated in [1-CrCl2(THF)2]+[B(C6F5)4]− when compared to those in [(THF)4CrCl2]+[B(C6F5)4]− (average 2.042 vs 2.003 Å)

Ethylene Tetramerization Studies

As shown in Table , the [(PNP)CrCl2(CH3CN)2]+[B(C6F5)4]− complexes were sn class="Chemical">creened for ethylene oligomerization after activation with trialkylaluminum agents (Me3Al, Et3Al, or iBu3Al) with the expectation that the trialkylaluminum would replace the chloride ligands with alkyl groups (i.e., Me, Et, or iBu) and abstract the acetonitrile ligands to generate vacant sites for ethylene coordination, thus generating the active species necessary for the catalytic cycle involving cationic CrI/III species. As expected, the [1-CrCl2(CH3CN)2]+[B(C6F5)4]−/Me3Al (Al/Cr, 300) catalytic system shows high activity (155 kg/g-Cr/h) in chlorobenzene, in which [1-CrCl2(CH3CN)2]+[B(C6F5)4]− is soluble, with reasonable selectivity (1-hexene 42%, 1-octene 44%, PE 2.2%; entry 1). As anticipated, the catalytic system based on the bis(PNP)Cr complex, that is, [12-CrCl2(CH3CN)2]+[B(C6F5)4]−/Me3Al, shows low activity (50 kg/g-Cr/h, entry 2). The use of Me3Al is not preferable to the use of MAO because the price of MAO is elevated mainly by the high cost of Me3Al. The use of Et3Al or iBu3Al instead of Me3Al lowers the activity (74 and 126 kg/g-Cr/h, respectively) and resulted in the formation of a significant amount of PE (ca. 10%, entries 3 and 4). We attributed these unsatisfactory results to incomplete alkylation, owing to the lower reactivity of Et3Al and iBu3Al compared to that of Me3Al. When a small proportion of Me3Al (30 μmol, Al/Cr = 20) is admixed with Et3Al (420 μmol), the activity becomes even higher (288 kg/g-Cr/h) than that observed for [1-CrCl2(CH3CN)2]+[B(C6F5)4]−/Me3Al (entry 5). The selectivity of this improved system is also satisfactory, with formation of only a negligible amount of PE (0.7%). Similar improvement is also achieved avoiding the use of any Me3Al using Et3Al or iBu3Al alone and allowing 1 h activation time. Thus, the activities (224 and 221 kg/g-Cr/h, entries 6 and 7) become comparable to those of the system containing MAO, that is, [1-CrCl3]/MAO (203 kg/g-Cr/h, entry 8) when [1-CrCl2(CH3CN)2]+[B(C6F5)4]− is used after activation with Et3Al or iBu3Al for 1 h. The selectivity is also similar to that observed with [1-CrCl3]/MAO, generating only a small amount of PE (1%).

Table 1

Ethylene Oligomerization Resultsa

entry	solvent	catalyst	activator	activity (kg/g-Cr/h)	1-C6 (wt %)	cy-C6 (wt %)	1-C8 (wt %)	>C10 (wt %)	PE (wt %)
1	C6H5Cl	[1-CrCl2]+[B(C6F5)4]−	Me3Al	155	41.6	2.4	43.7	9.4	2.2
2	C6H5Cl	[12-CrCl2]+[B(C6F5)4]−	Me3Al	50	30.4	3.4	61.3	3.2	1.1
3	C6H5Cl	[1-CrCl2]+[B(C6F5)4]−	Et3Al	74	30.7	2.2	47.2	8.2	11.7
4	C6H5Cl	[1-CrCl2]+[B(C6F5)4]−	iBu3Al	126	31.3	2.5	45.8	9.7	10.6
5	C6H5Cl	[1-CrCl2]+[B(C6F5)4]−	iBu3Al (+Me3Al, 6.6%)	288	44.3	2.4	40.1	12.2	0.7
6	C6H5Cl	[1-CrCl2]+[B(C6F5)4]−	Et3Al (1 h activation)	224	44.3	2.3	40.6	11.4	1.3
7	C6H5Cl	[1-CrCl2]+[B(C6F5)4]−	iBu3Al (1 h activation)	221	37.5	2.1	46.6	12.6	1.0
8	C6H5Cl	1-CrCl3	MMAOb	203	44.9	2.2	39.6	11.3	1.2
9	C6H5Cl	[2-CrCl2]+[B(C6F5)4]−	Me3Al	140	50.2	2.1	37.3	8.7	1.6
10	MeC6H11	[1-CrCl2]+[B(C6F5)4]−	Et3Al	13	8.4	3.1	53.5	30.6	3.7
11	MeC6H11	[1-CrCl2]+[B(C6F5)4]−	iBu3Al	26	10.5	4.5	68.0	14.9	1.3
12	MeC6H11	[2-CrCl2]+[B(C6F5)4]−	Et3Al	77	13.6	4.9	71.3	8.2	1.4
13	MeC6H11	[2-CrCl2]+[B(C6F5)4]−	iBu3Al	84	14.3	4.7	70.3	9.4	1.0
14	MeC6H11	[2-CrCl2]+[B(C6F5)4]−	iBu3Al	89 (1.5 h)	13.8	4.8	70.9	9.4	0.4
15	MeC6H11	[2-Cr(CH2C6H4NMe2)2]+ [B(C6F5)4]−	(iBu2Al)2O	29	20.8	3.2	44.5	11.2	8.0
16	MeC6H11	1 + Cr(acac)3	MMAOb	163	7.9	4.6	72.7	13.1	1.3
17	MeC6H5	[2-CrCl2]+[B(C6F5)4]−	iBu3Al	65	8.1	3.1	58.4	8.5	0.8
18	C6H10	[2-CrCl2]+[B(C6F5)4]−	iBu3Al	168	15.2	4.9	74.1	5.3	0.3
19	C6H10	[2-CrCl2]+[B(C6F5)4]−	Et3Al	168	14.2	4.9	71.7	7.5	0.1
20	C6H10	[2-CrCl2]+[B(C6F5)4]−	Et3Al	160 (1 h)	14.2	4.9	71.6	8.0	0.3
21	C6H10	1 + Cr(acac)3	MMAOb	257	10.1	4.7	73.1	8.3	0.9

Oligomerization conditions: Cr complex: 1.5 μmol; Al/Cr: 300; solvent: 20 mL; temperature: 75 °C for entries 1–9, 45 °C for entries 10–21; ethylene: 30 bar for entries 1–9, 45 bar for entries 10–21, 30 min.

Modified MAO sourced from Akzo Nobel (MMAO-3A 7.0 Al wt % in heptane).

Oligomerization conditions: Cr complex: 1.5 μmol; n class="Chemical">Al/Cr: 300; solvent: 20 mL; temperature: 75 °C for entries 1–9, 45 °C for entries 10–21; ethylene: 30 bar for entries 1–9, 45 bar for entries 10–21, 30 min.

Modified MAO sourced from Akzo Nobel (Mn class="Chemical">MAO-3A 7.0 Al wt % in heptane).

Aliphatic hydrocarbon solvents, for example, n class="Chemical">methylcyclohexane, are reported to be the best choice for the ethylene oligomerization reaction considering their boiling point and safety. Upon changing the solvent from chlorobenzene to methylcyclohexane, in which [1-CrCl2(CH3CN)2]+[B(C6F5)4]− is insoluble, the activity of [1-CrCl2(CH3CN)2]+[B(C6F5)4]−/Et3Al (or iBu3Al) becomes unsatisfactorily low (13 and 26 kg/g-Cr/h, entries 10 and 11). However, [2-CrCl2(CH3CN)2]+[B(C6F5)4]−, which bears long alkyl chains on the PNP ligand and hence is soluble in methylcyclohexane, exhibits satisfactory activity when activated with Et3Al or iBu3Al (77 and 84 kg/g-Cr/h, entries 12 and 13). The activity achieved is ca. 50% that is attained with the original MAO-based Sasol system (1/Cr(acac)3/MAO) under otherwise identical conditions (163 kg/g-Cr/h, entry 16). Furthermore, the performance of this catalytic system exhibits negligible deactivation over the rather long reaction time of 1.5 h; the ethylene consumption and hence the productivity increase almost linearly with time and reach 134 kg/g-Cr by running the oligomerization for 1.5 h (entry 14, Figure a). Another advantage of this system is that unwanted side products above C10 (labeled as “>C10” labeled in Table ) were lowered (ca. 9 vs 13%) by the use of [2-CrCl2(CH3CN)2]+[B(C6F5)4]−/iBu3Al (or Et3Al) instead of 1/Cr(acac)3/MAO, even though the 1-hexene/1-octene ratio becomes higher (14/70 vs 8/73). Coproduction of 1-hexene and 1-octene is an acceptable option in industry, and the higher 1-hexene/1-octene ratio is not a critical disadvantage. Another type of cationic Cr complex paired with the [B(C6F5)4]− anion, [2-Cr(CH2C6H4NMe2)2]+[B(C6F5)4]−, shows negligible activity when activated with Et3Al or iBu3Al. It shows low activity when activated with more Lewis acidic tetraisobutylaluminoxane (iBu2Al)2O (29 kg/g-Cr/h, entry 15).

Figure 3

Ethylene consumption vs time monitored by a mass-flow controller (MFC) in the tetramerization reaction performed in methylcyclohexane (a) or cyclohexene (b) using [2-CrCl2(CH3CN)2]+[B(C6F5)4]−/iBu3Al or Et3Al.

Ethylene consumption vs time monitored by a mass-flow controller (MFC) in the tetramerization reaction n class="Chemical">performed in methylcyclohexane (a) or cyclohexene (b) using [2-CrCl2(CH3CN)2]+[B(C6F5)4]−/iBu3Al or Et3Al.

The performance of n class="Chemical">[2-CrCl2(CH3CN)2]+[B(C6F5)4]− was also studied in π-electron-bearing solvents such as toluene and cyclohexene, in which it is freely soluble. By changing the methylcyclohexane solvent to toluene, the activity of [2-CrCl2(CH3CN)2]+[B(C6F5)4]−/iBu3Al is somewhat lowered to 65 kg/g-Cr/h (entry 17). The same trend was also observed in the original MAO-based Sasol system; the activity drops by half upon changing the methylcyclohexane solvent to toluene.[29] However, the activity of [2-CrCl2(CH3CN)2]+[B(C6F5)4]−/iBu3Al almost doubles by changing the methylcyclohexane solvent to cyclohexene (84 vs 168 kg/g-Cr/h, entries 13 and 18). The activity achieved in cyclohexene is almost identical to that of the original MAO-based Sasol system (1/Cr(acac)3 MAO) in methylcyclohexane (168 vs 163 kg/g-Cr/h). The same high activity is attained when activator iBu3Al is replaced with the more common and cheaper Et3Al (entry 19) and, moreover, the catalyst deactivation is negligible over the course of 1 h oligomerization (entry 20, Figure b). The π bond in cyclohexene does not participate in product formation; no additional signals are observed in the gas chromatography (GC) chart with the use of cyclohexene instead of methylcyclohexane (Figures S2 and S3). The formation of slightly fewer unwanted side products >C10 (5.3–8.0%) and formation of negligible amounts of PE (0.1–0.3%) are other advantages observed for the use of cyclohexene. Cyclohexene is an inexpensive and abundant chemical currently used in industry on a large scale, and its boiling point is different from those of the products 1-hexene and 1-octene (83, 63, and 121 °C, respectively), facilitating separation of products and solvent. Hence, it may be possible to use cyclohexene as the solvent in the ethylene tetramerization process. Interestingly, the MAO-based original Sasol system also showed 1.6 times higher activity in cyclohexene than in methylcyclohexane, which was originally reported to be the best choice of solvent (257 vs 163 kg/g-Cr/h, entry 21).

The activities of [2-CrCl2(CH3CN)2]+[B(C6F5)4]−/n class="Chemical">Et3Al (or iBu3Al) achieved in this work (89 and 168 kg/g-Cr/h in methylcyclohexane and cyclohexene, respectively) are significantly higher than that previously reported for 1/CrCl3(THF)3/[Ph3C]+[B(C6F5)4]−/Et3Al (2.5 kg/g-Cr/h).[42] The low activity in the latter case was attributed to the destruction of [B(C6F5)4]− anions by the action of Et3Al; [Ph3C]+[B(C6F5)4]− was reported to react with trialkylaluminum, with the [B(C6F5)4]− anion being fragmented. We believe that this instability is triggered by the reaction of the trialkylaluminum reagent with the trityl cation ([Ph3C]+), not by the reaction with the [B(C6F5)4]− anion itself.[42,57] The 19F NMR signals were monitored to investigate the stability of the [B(C6F5)4]− anion (Figure ). In the 19F NMR spectra of [(CH3CN)4Ag]+[B(C6F5)4]−, [(CH3CN)4CrCl2]+[B(C6F5)4]−, and [1-CrCl2(CH3CN)2]+[B(C6F5)4]−, a set of −C6F5 signals are clearly observed at −132 (ortho), −161 to −163 (para), and −164 to −167 (meta) ppm (Figure a,b). In the 19F NMR spectrum of [1-CrCl2(CH3CN)2]+[B(C6F5)4]−/iBu3Al (Al/Cr, 20) in chlorobenzene-d5, only the ortho-signal at −133 ppm is observed (Figure c); the signals for the para- and meta-fluorine are absent, possibly due to interaction with the paramagnetic CrIII center. The stability of the [B(C6F5)4]− anion itself in the presence of iBu3Al is further indicated by the clear presence of a set of −C6F5 signals alone at −134, −157, and −162 ppm for [Li]+[B(C6F5)4]−/iBu3Al (Al/Li, 20) in chlorobenzene-d5 (Figure d). In this study, [Ph3C]+[B(C6F5)4]− is not used with the trialkylaluminum reagent, but cationic Cr complexes paired with [B(C6F5)4]− anions were prepared by an alternative route. By avoiding the use of [Ph3C]+[B(C6F5)4]−, an efficient catalytic system containing [B(C6F5)4]− anions may be realized.

Figure 4

19F NMR spectra of [(CH3CN)4CrCl2]+[B(C6F5)4]− (a), [1-CrCl2(CH3CN)2]+[B(C6F5)4]− (b), [1-CrCl2(CH3CN)2]+[B(C6F5)4]−/iBu3Al (Al/Cr, 20) (c), and [Li]+[B(C6F5)4]−/iBu3Al (Al/Li, 20) (d).

19F NMR spectra of n class="Chemical">[(CH3CN)4CrCl2]+[B(C6F5)4]− (a), [1-CrCl2(CH3CN)2]+[B(C6F5)4]− (b), [1-CrCl2(CH3CN)2]+[B(C6F5)4]−/iBu3Al (Al/Cr, 20) (c), and [Li]+[B(C6F5)4]−/iBu3Al (Al/Li, 20) (d).

Conclusions

Cationic (PNP)n class="Chemical">CrIII complexes paired with [B(C6F5)4]− anion ([(PNP)CrCl2(CH3CN)2]+[B(C6F5)4]−, where PNP is iPrN(PPh2)2 (1) or [CH3(CH2)16]2CHN(PPh2)2 (2)) were prepared using [(CH3CN)4CrIIICl2]+[B(C6F5)4]−. The cationic (PNP)CrIII complexes paired with [B(C6F5)4]− anions [(PNP)CrCl2(CH3CN)2]+[B(C6F5)4]− showed high activities in ethylene tetramerization reactions performed in chlorobenzene, even when activated with common trialkylaluminum reagents (Et3Al and iBu3Al), thus avoiding the use of expensive MAO. Both the activity and selectivity were comparable to those of the original MAO-based Sasol system (1-CrCl3/MAO). When the PNP ligand is modified with long alkyl chains, the cationic (PNP)CrIII complex paired with [B(C6F5)4]− anions, that is, [2-CrCl2(CH3CN)2]+[B(C6F5)4]−, activated with Et3Al or iBu3Al showed high activity in the more desirable aliphatic hydrocarbon solvent methylcyclohexane (89 kg/g-Cr/h) and, moreover, showed much higher (2 times) activity in cyclohexene (168 kg/g-Cr/h), which is as high as that exhibited by the original MAO-based Sasol system (1/Cr(acac)3/MAO) in methylcyclohexane (163 kg/g-Cr/h). Further advantages exhibited by the catalytic system, [2-CrCl2(CH3CN)2]+[B(C6F5)4]−/Et3Al, in cyclohexene were negligible catalyst deactivation, the formation of a negligible amount of PE (0.3%), and the formation of lower amounts of unwanted side products above C10. The [B(C6F5)4]− anion is destroyed when [Ph3C]+[B(C6F5)4]− reacts with common trialkylaluminum. Consequently, catalytic system 1/CrCl3(THF)3/[Ph3C]+[B(C6F5)4]−/Et3Al failed. However, the [B(C6F5)4]− anion in itself is compatible with trialkylaluminum once it is not paired with a trityl cation; thus, [(PNP)CrCl2(CH3CN)2]+[B(C6F5)4]−/Et3Al (or iB3Al) exhibited high activity.

Experimental Section

General Remarks

All manipulations were n class="Chemical">performed under an inert atmosphere using a standard glove box and Schlenk techniques. Methylene chloride, acetonitrile, chlorobenzene, and CD2Cl2 were stirred over CaH2 and transferred to the reservoir under vacuum. Toluene, hexane, THF, and C6D6 were distilled from benzophenone ketyl. Methylcyclohexane (anhydrous grade), toluene, and cyclohexene used for the oligomerization reactions were purchased from Aldrich and purified over a Na/K alloy. Ethylene was purified by contact with molecular sieves and copper for more than 12 h under 40 bar pressure. The 1H NMR (400 MHz), 13C NMR (100 MHz), and 31P NMR (162 MHz) spectra were recorded on a Varian Mercury plus 400 spectrometer. Elemental analyses were carried out at the Analytical Center, Ajou University. GC-flame ionization detection (GC-FID) analysis was performed on an YL instrument 6500GC system equipped with a HP-PONA (50 m × 0.200 mm × 0.50 μm) column. CrCl3(THF)3,[58] [Ag(CH3CN)4]+[B(C6F5)4]−,[51] [H(OEt2)2]+[B(C6F5)4]−,[59] and 18-aminopentatriacontane ([CH3(CH2)16]2CH–NH2)[54,55] were prepared by literature methods. Modified MMAO-3A was sourced from Akzo Nobel (7.0 Al wt % in heptane).

[L4CrCl2]+[B(C6F5)4]− (L = CH3CN, THF)

A solution of Ag(CH3CN)4[B(C6F5)4] (2.54 g, 2.67 mmol) in n class="Chemical">acetonitrile (15 mL) was added to a solution of CrCl3(THF)3 (1.00 g, 2.67 mmol) in acetonitrile (15 mL). AgCl immediately precipitated as gray solids. After stirring overnight, the precipitates were removed by filtration. The filtrate was concentrated in vacuo to give a residue that was redissolved in acetonitrile (ca. 4 mL). Green thin needle-shaped microcrystals were deposited when the solution was kept in a freezer at −30 °C. The deposited green microcrystals were isolated by decantation (1.55 g, 60%). Anal. Calcd for [(CH3CN)4CrCl2]+[B(C6F5)4]− (C32H12BCl2CrF20N4, 966.15 g mol–1): C, 39.78; H, 1.25; N, 5.80%. Found: C, 39.55; H, 1.65; N, 6.17%. The isolated solid (1.00 g, 1.04 mmol) was dissolved in THF (10 mL). The solution was concentrated in vacuo to give a residue that was redissolved in THF (ca. 3 mL). Crystals suitable for X-ray crystallography were deposited when the solution was kept in a freezer at −30 °C. The deposited pale pink needle-shaped crystals were isolated by decantation (0.430 g, 38%). Anal. Calcd for [(THF)4CrCl2]+[B(C6F5)4]− (C40H32BCl2CrF20O4, 1090.37 g mol–1): C, 44.06; H, 2.96; O, 5.88%. Found: C, 43.82; H, 3.22; O, 5.88%.

[1-CrCl2(CH3CN)2]+[B(C6F5)4]− (1 = iPrN(PPh2)2)

A solution of iPrN(PPh2)2 (0.089 g, 0.21 mmol) in mn class="Chemical">ethylene chloride (0.5 mL) was added dropwise to a solution of [(CH3CN)4CrCl2]+[B(C6F5)4]− (0.200 g, 0.207 mmol) in methylene chloride (1.5 mL). Upon addition, the color of the solution changed immediately from olive green to brown and finally became bluish-green after 3 h. The solvent was removed under vacuum to obtain an oily residue, which was redissolved in methylene chloride (ca. 1.0 mL). The solvent was then completely removed under vacuum. The residue was triturated in hexane to isolate a bluish-green solid (0.254 g, 94%). Anal. Calcd for C55H33BCl2CrF20N3P2 (1311.51 g mol–1): C, 50.37; H, 2.54; N, 3.20%. Found: C, 50.09; H, 2.93; N, 2.76%.

[12-CrCl2]+[B(C6F5)4]−

A solution of iPrN(PPh2)2 (0.079 g, 0.185 mmol) in mn class="Chemical">ethylene chloride (2 mL) was added dropwise to a stirred suspension of [CrCl2(THF)4]+[B(C6F5)4]− (0.200 g, 0.183 mmol) in methylene chloride (2 mL). The resulting solution was stirred overnight. After filtration, the volatiles in the filtrate were removed under vacuum to obtain a brown oily residue, which was redissolved in methylene chloride. After all volatiles were completely removed under vacuum, the residue was dissolved in methylene chloride (0.4 mL). Layer diffusion of hexane (1 mL) afforded yellow needle-shaped crystals that were suitable for X-ray crystallography (0.116 g, 38%). Anal. Calcd for C78H54BCl2CrF20N2P4 (1656.87 g mol–1): C, 56.54; H, 3.29; N, 1.69%. Found: C, 56.23; H, 3.64; N, 1.73%.

Compound 2 ([CH3(CH2)16]2CHN(PPh2)2)

A solution of [CH3(CH2)16]2CHNH2 (0.505 g, 0.994 mmol) in n class="Chemical">methylene chloride (5 mL) was added dropwise at 0 °C to a solution of Ph2PCl (0.482 g, 2.19 mmol) and triethylamine (1.00 g, 9.94 mmol) in methylene chloride (5 mL). After stirring for 30 min at 0 °C, the ice bath was removed. After stirring at room temperature overnight, the solution was filtered to remove the generated [Et3NH]+Cl–. Hexane (10 mL) was added to dissolve the product. After the insoluble impurities were removed by filtration, the solvent was removed under vacuum. The residue was dissolved in methylene chloride (10 mL) and acetonitrile (10 mL) was added to precipitate solids. The precipitated white solids were redissolved in methylene chloride (10 mL), and acetonitrile (10 mL) was again added to precipitate solids, which were isolated by filtration (0.593 g, 68%). 1H NMR (400 MHz, C6D6): δ 7.45–7.79 (br, 8H, Ph), 7.08–7.16 (m, 12H, Ph), 3.51–3.64 (m, 1H, NCH), 2.11–2.12 (m, 2H, NCHCH2), 1.79–1.86 (m, 2H, NCHCH2), 1.20–1.36 (m, 60H, CH2), 0.93 (t, 6H, J = 6.4 Hz, CH3) ppm. 13C NMR (100 MHz, CD2Cl2): δ 140.0 (br, Cipso), 132.8 (d, 2JP–C = 10 Hz, Cortho), 128.3 (Cpara), 127.6 (d, 3JP–C = 3 Hz, Cmeta), 61.8 (t, 3JP–C = 7.5 Hz, −CHN), 39.1 (t, 3JP–C = 6.1 Hz, −CH2CHN), 32.6, 30.7, 30.5, 30.4, 30.3, 30.2, 30.1, 28.1, 23.5, and 14.8(CH3) ppm. 31P NMR (162 MHz, CD2Cl2): δ 52.24 ppm. Anal. Calcd for C59H91NP2 (875.66 g mol–1): C, 80.87; H, 10.47; N, 1.60%. Found: C, 81.25; H, 10.26; N, 1.55%.

[2-CrCl2(CH3CN)2]+[B(C6F5)4]−

A solution of [CH3(CH2)16]2CHN(PPh2)2 (0.100 g, 0.114 mmol) in n class="Chemical">methylene chloride (2 mL) was added dropwise to a solution of [(CH3CN)4CrCl2]+[B(C6F5)4]− (0.220 g, 0.228 mmol) in methylene chloride (3 mL). Upon addition, the color of the solution changed immediately from olive green to bluish-green and finally became green after 3 h. After stirring for 3 h, the volatiles were removed under vacuum. The residue was redissolved in methylene chloride (3 mL) and all volatiles were completely removed to obtain a bluish-green residue. Methylcyclohexane (10 mL) was added, and the product was dissolved by heating to ca. 50 °C using a heat gun. The solution was filtered through Celite while it was hot. The solvent was then removed under vacuum, and the residue was redissolved in hot methylcyclohexane (10 mL). The solution was again filtered through Celite at ca. 50 °C to remove any insoluble impurities. Removing the solvent under vacuum afforded a greenish waxy solid (0.147 g, 73%). Anal. Calcd for C87H97BCl2CrF20N3P2 (1760.38 g mol–1): C, 59.36; H, 5.55; N, 2.39%. Found: C, 59.69; H, 6.06; N, 1.94%.

(o-Me2NC6H4CH2)3Cr[56]

n-BuLi (2.5 M in n class="Chemical">hexane, 10.3 g, 37.2 mmol) was added to a solution of N,N-dimethyl-o-toluidine (5.00 g, 37.0 mmol) in a mixture of hexane (20 mL) and diethyl ether (10 mL). After stirring overnight at 45 °C, the generated solids were isolated by filtration. The isolated pale yellow solid (2-dimethylaminobenzyllithium, 4.23 g, 30.0 mmol, 81%) was dissolved in THF (30 mL), and the resulting solution was added to a stirred suspension of CrCl3(THF)3 (3.74 g, 9.98 mmol) in THF (30 mL) at −78 °C. Upon stirring at −78 °C for 1 h, the color of the solution changed from bright purple to dark orange. The reaction mixture was then warmed to room temperature and stirred overnight. The color of the solution changed from dark orange to dark red. The solvent was removed under vacuum to obtain the solids that were washed with diethyl ether (60 mL). Methylene chloride (100 mL) was added to dissolve the product, and insoluble byproducts were removed by filtration. The solvent was removed under vacuum to obtain a red solid (3.31 g, 73%). The product was further purified by recrystallization from methylene chloride at −30 °C.

[2-Cr(CH2C6H4NMe2)2]+[B(C6F5)4]−

A solution of [H(OEt2)2]+[B(C6F5)4]− (0.183 g, 0.221 mmol) in n class="Chemical">methylene chloride (0.5 mL) was added dropwise to a solution of (o-Me2NC6H4CH2)3Cr (0.100 g, 0.220 mmol) in methylene chloride (3 mL). Upon addition, the color of the solution changed immediately from dark red to violet. After stirring for 1 h, a solution of [CH3(CH2)16]2CHN(PPh2)2 (0.193 g, 0.220 mmol) in methylene chloride (2 mL) was added dropwise. Upon addition, the color of the solution changed immediately from violet to dark orange and became green after stirring overnight. All volatiles were removed under vacuum. Methylcyclohexane (10 mL) was added, and the product was dissolved by heating to 50 °C with a heat gun. The solution was filtered through Celite while hot. Removing the solvent under vacuum afforded a greenish waxy solid (0.280 g, 68%), which was used for the oligomerization studies without further purification and characterization.

Typical Procedure for Ethylene Oligomerization (Entry 19 in Table )

In a glove box, a dried 75 mL bomb reactor was charged with cyclohexene (19 mL) and n class="Chemical">Et3Al (0.450 mmol). The reactor was assembled and removed from the glove box. The reactor was then heated to 35 °C using an oil bath. The chromium complex (1.5 μmol) dissolved in cyclohexene (1 mL) was injected into the reactor using a syringe. Ethylene gas (45 bar) was immediately fed into the reactor. The temperature immediately increased to 45 °C owing to the generated heat. Ethylene consumption was monitored using a MFC under a constant pressure of 45 bar. After conducting the oligomerization at 45 °C for 30 min, the reactor was cooled with an ice bath and the ethylene gas was vented off. Ethanol (2 mL), aqueous HCl (10%, 2 mL), and nonane (0.700 g) as an internal standard for GC analysis were successively added. The organic upper layer was taken for GC analysis.

X-ray Crystallography

Reflection data were collected at 100 K on a Bruker APEX II CCD area diffractometer using n class="Chemical">graphite-monochromated Mo Kα radiation (λ = 0.7107 Å). Specimens of suitable quality and size were selected, mounted, and centered in the X-ray beam with the aid of a video camera. The hemisphere of the reflection data was collected as φ and ω scan frames at 0.5°/frame and an exposure time of 10 s/frame. The cell parameters were determined and refined using the SMART program. Data reduction was performed using SAINT software. The data were corrected for Lorentz and polarization effects. An empirical absorption correction was applied using the SADABS program. The structures of the compounds were solved by direct methods and refined by full matrix least-squares methods using the SHELXTL program package with anisotropic thermal parameters for all nonhydrogen atoms. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre. Crystallographic data for [(CH3CN)4Ag]+[B(C6F5)4]− (CCDC #, 1515442; Supporting Information): C36H12AgBF20N4, M = 951.14, monoclinic, a = 10.9963(5), b = 17.0100(5), c = 19.1408(9) Å, β = 105.158(2)°, V = 3456.7(3) Å3, T = 100(2) K, space group P21/c, Z = 4, 6607 unique (R(int) = 0.0449), which were used in all calculations. The final w2 was 0.0979 (I > 2σ(I)). Crystallographic data for [(THF)4CrCl2]+[B(C6F5)4]− (CCDC #, 1515443; Supporting Information): C40H32BCl2CrF20O4, M = 1090.36, tetragonal, a = b = 17.0405(3), c = 28.3198(10) Å, α = β = γ = 90°, V = 8223.5(4) Å3, T = 100(2) K, space group I4̅c2, Z = 8, 4962 unique (R(int) = 0.0540), which were used in all calculations. The final w2 was 0.0654 (I > 2σ(I)). Crystallographic data for (o-Me2NC6H4CH2)3Cr (CCDC #, 1515444; Supporting Information): C27H36CrN3, M = 454.59, monoclinic, a = 7.0261(3), b = 28.8953(11), c = 11.9390(5)Å, β = 100.211(2)°, V = 2385.48(17) Å3, T = 100(2) K, space group P21/n, Z = 4, 4055 unique (R(int) = 0.0150), which were used in all calculations. The final wR2 was 0.0923 (I > 2σ(I)). Crystallographic data for [1-CrCl2(THF)2]+[B(C6F5)4]− (CCDC #, 1515445; Supporting Information): C59H43BCl2CrF20NO2P2, M = 1373.59, monoclinic, a = 20.9402(14), b = 17.8800(12), c = 17.0227(12) Å, β = 113.683(4)°, V = 5836.7(7) Å3, T = 100(2) K, space group P21/c, Z = 4, 10574 unique (R(int) = 0.1041), which were used in all calculations. The final wR2 was 0.1433 (I > 2σ(I)). Crystallographic data for [12-CrCl2]+[B(C6F5)4]− (CCDC #, 1515446; Supporting Information): C78.5H55BCl3CrF20N2P4, M = 1699.28, monoclinic, a = 10.5859(4), b = 27.5961(11), c = 25.9776(11) Å, β = 97.937(3)°, V = 7516.1(5) Å3, T = 100(2) K, space group P21/c, Z = 4, 13893 unique (R(int) = 0.1169), which were used in all calculations. The final w2 was 0.2752 (I > 2σ(I)).