Efficient chromium-based catalysts for ethylene tri-/tetramerization switched by silicon-bridged/N,P-based ancillary ligands: a structural, catalytic and DFT study

Le Zhang ab, Wei Wei a, Fakhre Alam a, Yanhui Chen a and Tao Jiang *a
aTianjin Key Laboratory of Marine Resources and Chemistry, College of Chemical Engineering and Materials Science, Tianjin University of Science and Technology, Tianjin 300457, China. E-mail: jiangtao@tust.edu.cn
bHandan Key Laboratory of Organic Small Molecule Materials, Handan University, Handan 056005, China

Received 2nd August 2017 , Accepted 26th September 2017

First published on 27th September 2017


High performance catalysts switched by a series of silicon-bridged/N,P-based ancillary ligands have been explored. The precatalyst supported by L1 possessed a large steric bulk and exhibited a high activity of 16.8 × 106 g (mol Cr)−1 h−1 as well as a total selectivity of 99% toward 1-hexene and 1-octene. The selectivity for 1-hexene was adjustable from 59% to 88%. The catalyst bearing an L2 ligand, facilitated by a smaller steric bulk, displayed an identical activity of 13.0 × 106 g (mol Cr)−1 h−1 and a superior selectivity of 75% towards 1-octene under the appropriate conditions. The DFT calculations elucidated the reason for these excellent and tunable activities and selectivities.


Introduction

The PNP catalytic system for ethylene tetramerization was discovered by Sasol in 2004.1 Because of its good commercial value and atom economy, the PNP system and, in particular, its ligands are being developed for selective ethylene oligomerization. Ligand selection is one of the most crucial and adjustable factors for selective ethylene tri- and tetramer switching.2–5 Electronic and steric factors on N-substituted groups have been extensively scrutinized and explored in diverse catalytic systems for selective ethylene oligomerization.1,6–15 Moreover, the composition and length of PNP ligands have also been modified in search of an ideal catalytic system that is capable of producing 1-octene with high activity and selectivity.16–24 However, for ethylene tetramerization, the performance obtained by well developed ethylene trimerization catalytic systems (e.g., the SNS catalytic system with an activity of 8.3 × 106 g (mol Cr)−1 h−1 and a selectivity of 98% towards 1-hexene),25 are difficult to realize. However, new N,P-based ligands have shown promise. Kim22 and Gambarotta21 reported chromium(III) complexes of chiral DPPDME and PN(C)nNP ligands that were capable of producing 1-octene with 77% and 88% selectivity, respectively, with similar activities to the PNP catalytic system.1,6 It appears that ligands with diphosphorus favor ethylene tetramerization over trimerization. Recently, we also reported a silicon-bridged diphosphine (SBDP) ligand system with 4.2 × 106 g (mol Cr)−1 h−1 activity, which could produce 1-octene with 78% selectivity.26 Compared to the phosphine-coordinated system, ligands with nitrogen linkers showed preferable activities.27–31 Based on our previous studies, we hypothesized that introducing nitrogen into the backbone of a SBDP ligand may promote greater activity while maintaining selectivity towards 1-octene.

Herein, we describe a series of silicon-bridged/N,P-based (SBNP) ancillary ligands and their application in ethylene tri-/tetramerization catalysis. Density functional theory (DFT) calculations were also used to illustrate the excellent activity and selectivity of these systems.

Results and discussion

The target SBNP ligands L1–L4 were first prepared by convenient salt metathesis (Fig. 1). Deprotonation by n-butyllithium and subsequent dropwise addition of chlorodiphenylphosphine converted primary amines to N,P-based units. Diphenylphosphanide was obtained from the diphenylphosphine and butyllithium treatment and was added to dichlorodimethylsilane or dichloro(methyl)(phenyl)silane to produce Si,P-based units. The designed ligands were obtained by reacting the N,P-based units with n-butyllithium, followed by dropwise addition of Si,P-based units. These ligands were coordinated with CrCl3(THF)3 to produce precatalysts C1–C4 as green or blue powders (Fig. 1).
image file: c7cy01561e-f1.tif
Fig. 1 Synthesis and structures of the ligands and complexes.

When activated by DMAO (dried methylaluminoxane, methylaluminoxane was dried in a vacuum at 60 °C for 2 h)/AlEt3 in methylcyclohexane, the precatalysts proved to be highly active and selective towards 1-hexene/1-octene (Table 1). C1 was tested for ethylene oligomerization under an ethylene pressure of 1 MPa (Table 1, entry 1) and showed 99% total selectivity (1-hexene + 1-octene) and 6.4 × 106 g (mol Cr)−1 h−1 activity. It was believed that the sterically bulky N-substituted groups might be responsible for making such an effective catalyst. To modify the catalyst for ethylene tetramerization, we changed the N-substituted group from 2,6-diisopropylphenyl to cyclopentyl and isopropyl, obtaining L2 and L3, respectively. When tested for ethylene oligomerization, the corresponding precatalysts C2 and C3 (Table 1, entry 2 and entry 3) exhibited good performance, showing 60% selectivities toward 1-octene and 2.0 × 106 g (mol Cr)−1 h−1 oligomerization activities. Increasing the steric bulk on Si with phenyl substitution (C4) reversed the selectivity from tetramerization to trimerization (Table 1, entry 4). Notably, profiting from the electronic effect, catalysts with aryl-substituted groups on both N and Si atoms show higher activities compared to those catalysts with only alkyl-substituted groups.

Table 1 Comparison of the precatalysts for ethylene oligomerization
Entry Precatalyst Activity (106 g (mol Cr)−1 h−1) Product selectivity (wt%)
C4 C6 1-C6= C6a C6′′b C8 1-C8= ≥1-C10=
Reaction conditions: reaction temperature: 45 °C; reaction pressure: 1.0 MPa; reaction time: 30 min; solvent: methylcyclohexane (20 mL); n(cat): 0.3 μmol; n(Al)/n(Cr) = 1000; DMAO[thin space (1/6-em)]:[thin space (1/6-em)]AlEt3 = 4[thin space (1/6-em)]:[thin space (1/6-em)]1. a Methylcyclopentane in all products. b Methenecyclopentane in all products. c n (cat): 0.6 μmol.
1 C1 6.4 <1 89 99 Trace Trace 10 98 <1
2 C2 2.2 1 38 72 4 6 61 98 Trace
3 C3 1.8 1 38 79 3 5 61 97 Trace
4 C4 4.9 <1 66 97 <1 1 33 97 Trace


C1 was further investigated for ethylene tri-/tetramerization under a set of reaction conditions that varied the temperature, ethylene pressure and precatalyst mass (Table 2). Higher temperature increased 1-hexene selectivity at the expense of 1-octene, but extremely high temperatures deactivated the catalyst and consequently lowered the activity (Table 2, entry 1–3). Reducing the precatalyst mass markedly promotes activity (Table 2, entry 2 and entry 4–5), which shows that large masses of the precatalyst prevent it from forming active species. This result is fully consistent with a spectral study reported by Do.32 Although a low precatalyst mass efficiently converted it into active species, when the mass was reduced to 0.1 μmol, no activity was observed for ethylene oligomerization. However, increasing the n(Al)/n(Cr) ratio from 500 to 10[thin space (1/6-em)]000 recovered its activity to 6.8 × 106 g (mol Cr)−1 h−1, with 78% selectivity towards 1-hexene (Table 2, entry 7).

Table 2 Ethylene tetramerization by complex C1 under various conditions
Entry Ethylene pressure (MPa) Precatalyst (μmol) T (°C) Activity (106 g (mol Cr)−1 h−1) Product selectivity (wt%)
C4 C6 1-C6= C6a C6′′b C8 1-C8= ≥1-C10=
Reaction conditions: reaction time, 30 min; solvent, methylcyclohexane (20 mL); n(Al)/n(Cr) = 500; DMAO[thin space (1/6-em)]:[thin space (1/6-em)]AlEt3 = 4[thin space (1/6-em)]:[thin space (1/6-em)]1. a Methylcyclopentane in all products. b Methenecyclopentane in all products. c n (Al)/n(Cr) = 10[thin space (1/6-em)]000. d Low activity.
1 1 4.8 0 0.5 <1 60 99 <1 <1 39 98 <1
2 1 4.8 45 1.9 Trace 86 99 Trace Trace 5 96 9
3 1 4.8 60 1.6 Trace 88 99 Trace Trace 4 94 8
4 1 1.2 45 1.8 Trace 86 >99 Trace Trace 11 96 3
5 1 0.3 45 5.5 Trace 88 >99 Trace Trace 12 94 Trace
6 1 0.1 45 LAd Trace
7 1c 0.1 45 6.8 Trace 78 >99 Trace Trace 22 95 Trace
8 2c 0.1 10 7.8 Trace 74 >99 Trace Trace 26 96 Trace
9 4c 0.1 10 16.8 Trace 75 >99 Trace Trace 25 97 Trace


Combining a higher ethylene pressure with a low temperature can elevate the activity up to 16.8 × 106 g (mol Cr)−1 h−1 with the selectivities toward 1-hexene (75%) and 1-octene (24%), respectively. Aside from the ethylene dimer (C4), it is notable that the cyclic C6 by-products, which were produced nearly in all ethylene tri-/tetramerization systems, were almost absent and the total selectivity for 1-hexene and 1-octene reached more than 99%. This high selectivity can be understood to occur via a plausible mechanism for trimerization (Fig. 2). The expanded metallacyclic mechanism considers metallacycloheptane as a starting material, which undergoes β-H transfer by two different pathways. In one pathway, a 6,2-H shift in (a), followed by reductive elimination, converts intermediate (m) to 1-hexene (pathway A). Along the second pathway (pathway B), C → M agostic H transfer allows two alternative pathways. After transferring β-H to the chromium center, a double bond forms between two carbon atoms to form a hexenyl chromium species (h), which eliminates 1-hexene via M → C agostic H transfer and performs decomplexation with the chromium center (pathway B′). Alternatively, β-H transfer to the chromium atom and 2,6-C bonding forms the key intermediate (d) for producing cyclic C6 by-products (pathway A′). Methyl and methylenecyclopentane formed according to our proposed LCrH(cyclopentylmethyl) mechanism26 or LCrEt(cyclopentylmethyl) mechanism.33,34 The large distance between 2nd and 6th C decreases the chance of 2,6-C bonding, resulting in limited cyclic C6 products, especially in ligands with a large steric bulk on N, such as the L1 ligand.


image file: c7cy01561e-f2.tif
Fig. 2 Mechanism of ethylene trimerization.

C2 was further investigated for ethylene tetramerization under various reaction conditions, and it was found that the results were consistent with the results of the C1 precatalyst (Table 3). Here, we also observed that the precatalyst mass played a significant role in the catalyst activity (Table 3, entry 1–2) and only a small portion of the precatalyst became active species when a high concentration of the precatalyst was present. With the co-catalyst concentration being kept constant, the catalytic activity increased significantly with decreasing precatalyst concentration and reached up to 8.9 × 106 g (mol Cr)−1 h−1 (Table 3, entry 3–4). Low temperature and high ethylene pressure are beneficial to C2 for ethylene tetramerization, and selectivity was only slightly affected by the precatalyst mass. Under optimized conditions, C2 produces 1-octene with 75% selectivity and has 13.0 × 106 g (mol Cr)−1 h−1 oligomerization activity. Moreover, the cyclic by-products dominated the C6 products when less sterically hindered groups were attached to ligands.

Table 3 Ethylene tetramerization by complex C2 under various conditions
Entry Ethylene pressure (MPa) Precatalyst (μmol) T (°C) Activity (106 g (mol Cr)−1 h−1) Product selectivity (wt%)
C4 C6 1-C6= C6a C6′′b C8 1-C8= ≥1-C10=
Reaction conditions: reaction time, 30 min; solvent, methylcyclohexane (20 mL); n(Al)/n(Cr) = 500; DMAO[thin space (1/6-em)]:[thin space (1/6-em)]AlEt3 = 4[thin space (1/6-em)]:[thin space (1/6-em)]1. a Methylcyclopentane in all products. b Methylenecyclopentane in all products. c n (Al)/n(Cr) = 1000. d n (Al)/n(Cr) = 6000. e n (Al)/n(Cr) = 2000.
1 1 4.8 45 0.7 1 41 80 3 5 58 97 Trace
2 1 1.2 45 1.5 1 42 77 4 6 57 99 Trace
3c 1 0.6 45 2.2 1 38 72 4 6 61 98 Trace
4d 1 0.1 45 8.9 2 37 74.0 3 6 61 99 Trace
5e 1 0.3 10 0.7 3 27 33 8 13 70 97 Trace
6e 1 0.3 45 4.1 1 38 76 4 6 61 99 Trace
7e 1 0.3 60 0.2 2 48 90 <1 3 50 96 Trace
8d 0.4 0.1 10 1.8 1 52 86 3 5 47 96 Trace
9d 4 0.1 10 13.0 2 21 52 4 6 77 98 Trace


The excellent activities and selectivities of these catalysts encouraged us to perform DFT calculations. C1 and C2 with optimized structures (Fig. 3) were selected as possible intermediates for a DFT study. The P–Cr–P bite angle is a very important factor in PNP and PCCP systems, and a small P–Cr–P bite angle is beneficial to 1-octene selectivity.35 DFT calculations suggested that the different selectivities for these two systems (C1 and C2) might be due to their different P–Cr–P bite angles.


image file: c7cy01561e-f3.tif
Fig. 3 Optimized structures and main parameters of the possible reaction intermediates (the unit of the bond length is Å; all of the spin multiplicities are quartets).

In a growing metallacycle, the P–Cr–P angle has to be decreased as much as possible to decrease the steric constraint around the metal. The decreasing bite angles in intermediates from C1a to C1c and from C2a to C2c demonstrated this phenomenon. During this process, both the Cr–P–N–Si–P and Cr–Cn (n = 4, 6 and 8) rings were distorted due to the difference in the two Cr–C and two Cr–P bond distances, which allowed ring-opening decomposition to occur.22 In the growing metallacycle, the two Cr–P bond distances changed very little from C1a to C1b and the Cr–P–N–Si–P ring remained stable enough to maintain the catalytic cycle. By contrast, the two P–Cr bond distances in C1c changed enough to decompose and deactivate the ring. Fortunately, the large steric bulk of the N-substituted group on C1 distorted the metallacycloheptane and blocked further ethylene insertion. Thus, the metallacycloheptane efficiently released 1-hexene by reductive elimination and entered into the next catalytic cycle. In the case of C2, the two Cr–P bond distances remained similar for all the cyclic intermediates, exhibiting good stability. The Cr–C bond distance in C2b was similar to that of C1b which was less stable. The smaller steric bulk of the N-substituted group on C2 facilitated further ethylene insertion, which allowed the formation of C2c and release of 1-octene by reductive elimination.

Kinetic studies suggested that the metallacycle growth is the rate-determining step16,36,37 in ethylene oligomerization, according to the extended metallacyclic mechanism.6 McGuinness et al.38 further explored that 1-hexene forms via ethylene coordination to metallacyclopentane and 1-octene forms via a dual pathway mechanism (via bis(ethylene) metallacyclopentane and via ethylene coordination to metallacycloheptane). Hence, the tri- and tetramerization activity can be better explained by the coordination ability of the intermediates (metallacyclopentane for 1-hexene and metallacycloheptane for 1-octene) with ethylene, which is further related to the energy gaps between the intermediates' LUMOs and ethylene's HOMO.

To study the energy gaps (HOMOs/LUMOs) of ethylene and the intermediates, C1a and C2b were further investigated with molecular orbital (MO) energy calculations (shown in Fig. 4). The HOMOs of both the C1a and C2b structures exhibit 3dz2 symmetry and thus have the capability to overlap with empty C2H4 π* orbitals to form π back-bonding with an incoming ethylene molecule. Additionally, the geometric symmetry of the empty 3d orbital preferentially accepts π electrons from the incoming ethylene. However, no well-matched orbitals (bottom row; Fig. 4) were recognized and we further considered the LUMO+1 and LUMO+2 orbitals of the C1a and C2b structures (Fig. 5). LUMO+1C1a and LUMO+1C2b appeared to be best suited for interaction with the corresponding HOMOs.


image file: c7cy01561e-f4.tif
Fig. 4 Frontier molecular orbitals of C1a and C2b (top row: HOMO; bottom row: LUMO).

image file: c7cy01561e-f5.tif
Fig. 5 LUMO+1 (top) and LUMO+2 (bottom) of C1a and C2b.

As shown in Table S1, LUMO+1C1a is lower in energy than LUMO+1C2b, and its corresponding energy gap (between LUMO+1C1a and HOMOethylene) is also less than that of LUMO+1C2b and HOMOethylene; therefore, C1a can coordinate with ethylene compared to C2b and give higher activity for ethylene oligomerization, which is consistent with our experimental results.

Conclusions

A series of silicon-bridged/N,P-based (SBNP) ancillary ligands were synthesized and characterized. The precatalysts obtained by the coordination of the ligands and CrCl3(THF)3 proved to be excellent catalysts for ethylene tri-/tetramerization when activated by DMAO/AlEt3. C1 and C2 were observed to be the best catalysts, showing 16.8 × 106 g (mol Cr)−1 h−1 and 13.0 × 106 g (mol Cr)−1 h−1 activities for ethylene tri-/tetramerization, respectively. C1 showed 99% total selectivity (75%, 1-hexene; 24%, 1-octene), while C2 produced 1-octene with 75% selectivity. The extremely low production of the cyclic C6 by-products was explained by a mechanism proposed for trimerization products. DFT calculations were also conducted to probe the theoretical basis for the excellent activities and selectivities of these systems.

Experimental

General Procedure

All air and/or water sensitive reactions were performed under a nitrogen atmosphere in oven-dried flasks using standard Schlenk techniques or under a nitrogen atmosphere in a glovebox. Anhydrous reaction solvents were obtained by means of a multiple column purification system. Bis(phenyl)phosphorus chloride, diphenylphosphine, dichlorodimethylsilane, n-butyllithium (2.4 mol L−1 in n-hexane) and CrCl3(THF)3 were purchased from Aldrich Chemical Co. (Milwaukee, WI, USA) and were used as received. Lithium diphenylphosphine was prepared using the method reported by Peterson.39 Polymerization grade ethylene was obtained from Tianjin Summit Specialty Gases (China). Triethylaluminum (TEA, 1.0 mol L−1 in methylcyclohexane) and methylaluminoxane (MAO, 1.4 mol L−1 in toluene) were purchased from Albemarle Corp (USA). DMAO was prepared by pumping off all the volatile compounds of MAO at 40 °C for 6 h. NMR spectroscopic data for the ligands were obtained using a Bruker Ultrashield 400 (400 MHz for 1H, 162 MHz for 31P and 100 MHz for 13C in CDCl3 or C6D6). Elemental analyses were performed by Chinese Academy of Sciences (Shanghai Institute of Organic Chemistry).

Ligand synthesis

N-(2,6-Diisopropylphenyl)-N-((diphenylphosphanyl)dimethylsilyl)-1,1-diphenylphosphanamine (L1). 8.3 mL of n-BuLi (2.4 mol L−1 in n-hexane, 19.92 mmol) was added dropwise at −20 °C to a solution of 2,6-diisopropylaniline (3.582 g, 20.00 mmol) in n-hexane (20 mL). The reaction mixture was stirred overnight at room temperature, followed by the addition of chlorodiphenylphosphane (4.413 g, 20.00 mmol). The mixture was again stirred overnight at room temperature, the precipitated LiCl was filtered out, and the faint yellow solution was concentrated to give a yellow residue, which was recrystallized in hexane to give N-(2,6-diisopropylphenyl)-1-diphenylphosphanamine (L1a) (4.92 g, 13.60 mmol) in 68% yield. n-BuLi (5.7 mL, 2.4 mol L−1 in n-hexane, 13.60 mmol) was added dropwise at −20 °C to a solution of the above product. The mixture was stirred at room temperature for 5 h. After filtration and washing twice with 2 mL of n-hexane, lithium (2,6-diisopropylphenyl)(diphenylphosphanyl)amide (L1b) (4.997 g, 13.60 mmol) was obtained in approximately 100% yield after drying in a vacuum. Lithium diphenylphosphanide (L1c) was prepared by a similar method to L1b, with diphenylphosphane (3.800 g, 20.00 mmol) and n-BuLi (8.3 mL, 19.92 mmol, 2.4 mol L−1 in n-hexane). A yellow solid (3.843 g, 20.00 mmol) was obtained in nearly 100% yield. The above yellow product was added portionwise to a solution of dichlorodimethylsilane (5.162 g, 40.00 mmol) in n-hexane (20 mL) at −20 °C and then the mixture was stirred at room temperature for 8 h. After filtration, the volatiles were removed in a vacuum, giving a yellow oil. The colorless oil (chlorodimethylsilyl)diphenylphosphane (L1d) was obtained by vacuum distillation (155–160 °C, 10 mmHg, 2.137 g, 7.67 mmol, 38% yield). A solution of L1d (0.558 g, 2.00 mmol) in toluene (20 mL) was added dropwise to a solution of L1c (0.735 g, 2.00 mmol) in toluene (5 mL) at −20 °C. After overnight stirring and filtration, a yellow oil was obtained by removing the volatiles in a vacuum. Recrystallization from n-hexane produced ligand L1 (0.869, 1.44 mmol) in 72% yield. 1H NMR (400 MHz, C6D6) δ 7.98 (t, J = 7.5 Hz, 4H), 7.58 (dd, J = 10.4, 4.9 Hz, 4H), 7.14–6.96 (m, 15H), 3.58–3.46 (m, 2H), 1.11 (d, J = 6.7 Hz, 6H), 0.43 (d, J = 6.7 Hz, 6H), 0.41 (d, J = 2.0 Hz, 6H). 31P NMR (162 MHz, C6D6) δ 52.22 (d), −43.26 (d, J = 25.9 Hz). 13C NMR (101 MHz, C6D6) δ 148.32, 142.44, 142.40, 139.06, 138.87, 137.71, 137.69, 137.53, 137.52, 136.13, 136.12, 136.09, 135.96, 135.91, 135.88, 135.85, 129.69, 128.90, 128.83, 128.61, 128.41, 128.16, 126.84, 125.12, 29.37, 29.34, 26.61, 26.60, 24.03, 2.71, 2.63, 2.56.
N-Cyclopentyl-N-((diphenylphosphanyl)dimethylsilyl)-1,1-diphenylphosphanamine (L2). Using the same method as that for L1a, the preparation of N-cyclopentyl-1,1-diphenylphosphanamine (L2a) was achieved by using cyclopentanamine (1.703 g, 20.00 mmol), n-BuLi (8.3 mL, 19.92 mmol, 2.4 mol L−1 in n-hexane) and chlorodiphenylphosphane (4.413 g, 20.00 mmol). Vacuum distillation at 10 mmHg gave the colorless oil L2a (150–160 °C, 2.811 g, 10.43 mmol, 52% yield). The preparation of L2b was conducted by the same method as described for L1b with n-BuLi (4.4 mL, 10.44 mmol, 2.4 mol L−1 in n-hexane) and L2a (0.551 g, 2.00 mmol) and was isolated in nearly 100% yield (2.871 g, 10.43 mmol). Similar to L1, 0.685 g L2 (1.34 mmol, 67% yield) was obtained by using L2b (0.551 g, 2.00 mmol) and L1d (0.558 g, 2.00 mmol). 1H NMR (400 MHz, C6D6) δ 7.69 (t, 4H), 7.56 (t, 4H), 7.08 (m, 12H), 3.82–3.68 (m, 1H), 1.86–1.68 (m, 2H), 1.46 (m, 4H), 1.26 (m, 2H), 0.53 (s, 6H). 31P NMR (162 MHz, C6D6) δ 51.96 (br), −53.72 (d, J = 53.8 Hz). 13C NMR (101 MHz, C6D6) δ 140.90, 140.88, 140.70, 140.69, 137.30, 137.27, 137.12, 137.09, 135.49, 135.48, 135.31, 135.31, 133.08, 132.88, 128.86, 128.80, 128.74, 128.69, 128.42, 128.18, 63.09, 34.57, 34.55, 32.21, 23.79, 3.20, 3.08, 2.96.
N-((Diphenylphosphanyl)dimethylsilyl)-N-isopropyl-1,1-diphenylphosphanamine (L3). N-Isopropyl-1,1-diphenylphosphanamine (L3a) was synthesized by the same method as L1a using isopropylamine (1.182 g, 20.00 mmol), n-BuLi (8.3 mL, 19.92 mmol, 2.4 mol L−1 in n-hexane) and chlorodiphenylphosphane (4.413 g, 20.00 mmol). Colorless oil L3a was obtained by vacuum distillation at 10 mmHg (140–150 °C, 2.628 g, 10.80 mmol, 54% yield). Similar to L1, 0.631 g L3 (1.30 mmol, 65% yield) was obtained by using L3a (0.487 g, 2.00 mmol), n-BuLi (0.8 mL, 1.92 mmol, 2.4 mol L−1 in n-hexane) and L1d (0.558 g, 2.00 mmol). 1H NMR (400 MHz, C6D6) δ 7.95–7.72 (m, 4H), 7.46–7.36 (m, 2H), 7.32–7.25 (m, 2H), 7.18 (d, J = 16.4 Hz, 4H), 7.14–7.02 (m, 10H), 6.84 (s, 3H), 3.77 (td, J = 13.4, 6.4 Hz, 1H), 0.92 (d, J = 6.7 Hz, 3H), 0.84 (s, 3H), 0.82 (s, 3H). 31P NMR (162 MHz, C6D6) δ 52.08 (br), −57.15 (d, J = 118.3 Hz). 13C NMR (101 MHz, C6D6) δ 140.77, 140.76, 140.58, 140.56, 140.55, 140.38, 140.35, 140.03, 139.99, 139.84, 139.79, 137.76, 137.58, 135.87, 135.82, 134.35, 134.20, 133.67, 133.46, 133.22, 133.02, 131.89, 131.69, 130.03, 129.00, 128.93, 128.85, 128.75, 128.68, 128.47, 128.42, 128.17, 127.13, 52.42, 25.84, 25.72, 0.27.
N-((Diphenylphosphanyl)(methyl)(phenyl)silyl)-N-isopropyl-1,1-diphenylphosphanamine (L4). (Chloro(methyl)(phenyl)silyl)diphenylphosphane (L4d) was synthesized according to the method described for L1d with L1c (3.843 g, 20 mmol) and dichloro(methyl)(phenyl)silane (7.645 g, 40 mmol). Colorless oil L4d was yielded by vacuum distillation at 10 mmHg (165–175 °C, 3.136 g, 9.20 mmol, 46% yield). Similar to L1, 0.767 g L4 (1.40 mmol, 70% yield) was obtained by using L3a (0.487 g, 2.00 mmol), n-BuLi (0.8 mL, 1.92 mmol, 2.4 mol L−1 in n-hexane) and L4d (0.558 g, 2.00 mmol). 1H NMR (400 MHz, C6D6) δ 7.73–7.64 (m, 4H), 7.59–7.49 (m, 4H), 7.15–7.00 (m, 12H), 3.75 (m, J = 13.4, 6.7 Hz, 1H), 1.08 (d, J = 6.7 Hz, 6H), 0.57–0.39 (m, 6H). 31P NMR (162 MHz, C6D6) δ 50.42 (br), −53.03 (d, J = 54.7 Hz). 13C NMR (101 MHz, C6D6) δ 140.71, 140.70, 140.52, 140.50, 137.28, 137.25, 137.10, 137.07, 135.55, 135.55, 135.38, 135.37, 133.22, 133.02, 128.84, 128.78, 128.70, 128.64, 128.42, 128.18, 52.27, 25.82, 25.79, 25.76, 3.49, 3.38, 3.26.

Precatalyst synthesis

[N-(2,6-Diisopropylphenyl)-N-((diphenylphosphanyl)dimethylsilyl)-1,1-diphenylphosphanamine](THF)CrCl3 (C1). A solution of L1 (0.664 g, 1.10 mmol) was added dropwise to a solution of CrCl3(THF)3 (0.375 g, 1.00 mmol). A blue solution was obtained by stirring for 1 h. The resulting solution was added to 20 mL of n-hexane and then cooled at −30 °C. Blue solid C1 (0.609 g, 0.73 mmol, 78% yield) was obtained by filtration and dried in a vacuum. Anal. calcd. for C42H51Cl3CrNOSiP2: C, 60.47; H, 6.16; N, 1.68. Found: C, 60.24; H, 6.21; N, 1.63.
[N-Cyclopentyl-N-((diphenylphosphanyl)dimethylsilyl)-1,1-diphenylphosphanamine](THF)CrCl3 (C2). C2 was prepared with a yield of 81% by using the method for C1 with L2 (0.563 g, 1.10 mmol) and CrCl3(THF)3 (0.375 g, 1.00 mmol). Anal. calcd. for C35H43Cl3CrNOSiP2: C, 56.65; H, 5.84; N, 1.89. Found: C, 56.92; H, 5.89; N, 1.82.
[N-((Diphenylphosphanyl)dimethylsilyl)-N-isopropyl-1,1-diphenylphosphanamine](THF)CrCl3 (C3). C3 was prepared with a yield of 73% by using the method for C1 with L3 (0.534 g, 1.10 mmol) and CrCl3(THF)3 (0.375 g, 1.00 mmol). Anal. calcd. for C33H41Cl3CrNOSiP2: C, 55.35; H, 5.77; N, 1.96. Found: C, 55.91; H, 5.67; N, 1.89.
[N-((Diphenylphosphanyl)(methyl)(phenyl)silyl)-N-isopropyl-1,1-diphenylphosphanamine](THF)CrCl3 (C4). C4 was prepared with a yield of 69% by using the method for C1 with L4 (0.632 g, 1.10 mmol) and CrCl3(THF)3 (0.375 g, 1.00 mmol). Anal. calcd. for C38H43Cl3CrNOSiP2: C, 58.65; H, 5.57; N, 1.80. Found: C, 58.87; H, 5.54; N, 1.78.

Ethylene Oligomerization

Ethylene oligomerization reactions were carried out in a 150 ml steel Büchi autoclave equipped with a mechanical stirrer and temperature probe. After catalyst injection into the autoclave under a stream of N2, the autoclave was immediately pressurized with ethylene. The reaction was allowed to run for 30 min. The reactor was then quenched by cooling to 0 °C using an ice bath and depressurized. The products of oligomerization were analyzed by an Agilent 6890 gas chromatograph with a flame ionization detector (GC-FID) and an HP-5 GC capillary column. Heptane was used as the internal standard.

Computational details

All of the theoretical calculations were performed using the Gaussian 09 program package.40 DFT was used to optimize the structures and to calculate the parameters using the Becke–Lee–Yang–Parr functional (B3LYP) method. 6-31G (d,p) was used for the C, H, N, Si and P atoms, while Cr was described with the LANL2DZ (ECP) basis set.41–43 The Gaussian View 5.0 program was used to build the structures and produce the LUMOs and HOMOs. Monovalent cations with quartet spin states were considered in theoretical models for the DFT study.29,41,44–46

Author contributions

The manuscript was written through contributions of all authors. All of the authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was sponsored by The National Key Research and Development Program of China (2017YFB0306700) and the Tianjin Application Foundation and Cutting-edge Technology Research Program (Grants: 16JCZDJC31600).

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Footnote

Electronic supplementary information (ESI) available: 1H NMR, 31P NMR and 13C NMR spectra of L1–L4 and absolute energies of all intermediates (PDF). Cartesian coordinates for all intermediates (XYZ). See DOI: 10.1039/c7cy01561e

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