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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Insights into the Formation Process of Yttrium−Aluminum Bimetallic Alkyl Complexes Supported by a Bulky Phosphazene Ligand Weifeng Rong,†,‡ Meiyan Wang,§ Shihui Li,† Jianhua Cheng,† Dongtao Liu,*,† and Dongmei Cui*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ‡ Shandong Institute of non-metallic materials, Jinan 250031, People’s Republic of China § Institute of Theoretical Chemistry, Jilin University, Changchun 130012, People’s Republic of China S Supporting Information *

ABSTRACT: The reaction of the yttrium dialkyl precursor [N[Ph2PNC6H3(iPr)2]2Y(CH2SiMe3)2] (1) with more than 3 equiv of trimethylaluminum or triethylaluminum afforded the dinuclear heterometallic complexes N[Ph2PNC6H3(iPr)2]2Y{(μ-R)2AlR2}R (R = Me (2), Et (3)). When trimethylaluminum loading was reduced to 2 equiv, the same reaction gave the product N[Ph2PNC6H3(iPr)2]2Y{(μ-Me)2(μ-CH2SiMe3)AlMe}Me (4), with quite a different bonding mode. All of these complexes were characterized by NMR spectra, X-ray crystallography, and elemental analysis. We elucidated the step-reaction mechanism between the yttrium dialkyl complex based on bulky phosphazene ligand and trialkylaluminum, which might shed new light on the formation process of active species in organoaluminum-dependent Ziegler−Natta catalytic systems.

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INTRODUCTION Organoaluminum compounds (AlR3) have become the most widely used cocatalysts in the polymerization of olefins, since the epoch-making discovery of the Ziegler−Natta catalytic system (TiCl4/AlEt3 or TiCl3/AlEt3) for the polymerizations of ethylene or propylene in the 1950s.1,2 With the innovation of the single-site metallocene precursors (1970s), methylaluminoxane (MAO), a hydrolysis product of AlMe3, was the best activator to replace the organoaluminum compounds. In the meantime, the excellent performances of the catalytic combination of organometallic precursors with organoborates and AlR3 drew researchers’ attention back to AlR3, which turned out to be the essential component of the catalytic system again to scavenge impurities of the polymerization system and, more importantly, to affect the activity and selectivity. Major achievements have revealed that AlR3 is involved in every stage of the polymerization, including olefin coordination (initiation)3 and insertion (propagation),4,5 as well as β-hydrogen elimination4,5 and β-alkyl elimination (termination).6 Therefore, AlR3 has occupied an irreplaceable position in the development of olefin polymerization, and investigations of the interaction of AlR3 with the catalytic precursors to find the truly active species to govern the activity and selectivity of the resultant catalytic systems have never stopped.2 The successful synthesis of the Tebbe reagent [Cp2Ti{(μ-CH2)(μ-Cl)Al(CH3)2}], although it was only used in the methylenation of carbonyl compounds,7a aroused scientists’ research interest because it gave some hints to the real active species of Ziegler−Natta catalysts for olefin polymerizations.7b,c In this regard, Pearce and Lappert did much pioneering work on the alkyl-bridged aluminum and the © XXXX American Chemical Society

d-/f-block metal complexes bearing a cyclopentadienyl ancillary ligand (Chart 1A),8 which rendered the lanthanide alkyl Chart 1. Typical Examples of Methylaluminate Rare-EarthMetal Complexes

complexes to be excellent models for clarifying the active sites.9 Evans and Anwander investigated the homo-10 and heteroleptic11 tetramethylaluminate rare-earth-metal complexes (Chart 1B,C) and the innovative chemistry of their methylidene and methine species,12 which significantly fueled the development of Ln−Al heterobimetallic complexes in olefin polymerization catalysis9f,11f,13 as well as in specially designed organic reactions.14 It is notable that most of the related model complexes (Chart 1C and Chart 2D−F)12a,c,d,i were synthesized by treatment of a permethylated lanthanide−aluminum precursor (Chart 1B)2,15 with ligand precursors. Moreover, these complexes tended to assemble to polylanthanide− polyaluminum [LnnAln]12a,13h,22a or monolanthanide−polyaluminum [LnAln]10c,11,12f,13,14b along with a few monolanthaReceived: December 28, 2017

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DOI: 10.1021/acs.organomet.7b00911 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

precursor and the cocatalysts as well as the design of catalytic combinations.

Chart 2. Selected Structural Models Originating from Ln(AlMe4)3 as Precursors

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RESULTS AND DISCUSSION Reaction of [N(Ph2PNC6H4iPr2)2]Y(CH2SiMe3)2 with an Excess of AlMe3. Considering that excess amounts of organoaluminum were always employed during the polymerization of conjugated dienes, we initially treated the yttrium dialkyl complex [N(Ph2PNC6H4iPr2)2]Y(CH2SiMe3)2 (1) with excess amounts of AlMe3 (3−10 equiv) in toluene at ambient temperature to give complex 2 as a white solid in a good yield (Scheme 1). The NMR spectral data of 2 in benzene-d6 were intriguing and inconsistent with what we expected for the usual bis(tetramethylaluminate) species LLn(AlMe4)2 (L = monoanionic ligand).10−15 For example, the 1H NMR spectrum in C6D6 (Figure S3) of 2 shows two sets of peaks for isopropyl groups with a 3JHH coupling constant of 6.8 Hz. The presence of more than one peak for a Y−Al fragment is indicative of an asymmetric structure. A doublet at δ 0.07 ppm with an integration of 9 is observed with the low coupling constant JYH = 0.8 Hz, indicating a 4JYH coupling, not a 2JYH coupling (around 2.4 Hz),13d which should be assigned to the three same methyl groups. Moreover, two different signals with an approximate integral ratio of 3:3 indicate clearly that there are two kinds of Y−CH3, which further excludes the formation of the bis(tetramethylaluminate) species LLn(AlMe4)2. Thus, a Y(η1-μ-CH3)Al(CH3)3-bonded linkage could be confirmed in C6D6 solution. However, an X-ray diffraction study shows a different structure. In the solid state, complex 2 adopts a distortedpentagonal-pyramidal coordination geometry (Figure 1). The terminal CH3 group (C49) is in the apical position, and N1, N2, and N3 from the ligand and the bridging methyl groups (C50 and C51) of the Al(CH3)4 occupy the equatorial plane. The Al(CH3)4 moiety shows a planar η2 coordination to Y3+ ion (Figure 4A), instead of η1-methyl coordination. This suggested a very fast exchange of bridging and terminal methyl groups in Al(CH3)4, where η1 coordination is preferred in solution and η2 coordination is preferred in the solid state. The difference proves the high coordinational flexibility of the tetramethylaluminate ligand and its ability to adapt perfectly to the given stereoelectronic requirements of the rare-earth-metal center. The molecular structure of complex 2 is very similar to that of the documented aluminate yttrium complex with a bulky hydrotris(pyrazolyl)borate ligand ((TptBu,Me)Y{η1-μ-(CH3)Al(CH3)3}(CH3)) (Chart 2E),12c,d bearing the similar monoaluminate core [Ln1Al1] (Figure 4D). However, the pincer-type ligand coordinates to the Y3+ ion in a κ3 fashion with Y−N bond lengths ranging from 2.422(3) to 2.468(3) Å and the

nide−monoaluminum complexes.9a,f,11i,16 However, in the polymerization procedure, the active species is generated by a direct reaction between the catalytic precursors bearing the ancillary ligands and organoaluminum. Therefore, the isolation of lanthanide−aluminum binuclear complexes is rather challenging but obviously important. Organolanthanide bis(alkyl) complexes bearing a monoanionic ancillary ligand with the activation of trialkylaluminum have proved to be extremely successful in olefin17 and conjugated diene polymerizations.18 Of these, rare-earth-metal bis(alkyl) complexes supported by phosphazene ligands have been documented not only in the polymerization of dienes19 but also in the synthesis of hydride20 and terminal imido complexes, owing to the versatile steric hindrance around the metal centers.21 We reported previously that the monoanionic phosphazene ligand stabilized the rare-earth-metal complexes [N(Ph2PNPh)2]Ln(CH2SiMe3)2,19 upon activation of AlR3 and organoborate, and showed a high activity toward the trans-1,4polymerization of isoprene. However, the analogous complex [N(Ph2PNAr)2]Ln(CH2SiMe3)2 (Ar = 2,6-diisopropylphenyl) was totally inert under the same conditions, which might be ascribed to the more crowded environment around the active metal center hindering the coordination of the monomer. This sterically bulky active metal center, on the other hand, might form a stable adduct with mono-AlR3, the structural model that mimics the real active species and is ubiquitous in the monoanionic ligand attached rare-earth-metal catalyst systems;11f,13 this adduct, however, has not been obtained to date. Herein we present the reaction of the yttrium complex 1, [N(Ph2PNC6H4iPr2)2]Y(CH2SiMe3)2, with AlR3 to afford monoanionic phosphazene-ligated Y−Al bimetallic complexes. The structures vary with the loading and type of AlR3. Hence, a fundamental understanding of the precursor−cocatalyst interaction and structure−reactivity relationship was established. This work sheds some new light on the mechanism of coordination polymerization and the choice of catalytic Scheme 1. Synthesis of Complex 2

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Figure 1. X-ray structure of 2 with 35% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Y1−C49 2.374(5), Y1−C50 2.612(5), Y1−C51 2.697(5), Y1−N3 2.422(3), Y1−N2 2.424(3), Y1−N1 2.468(3), Al1− C53 1.975(5), Al1−C52 1.982(6), Al1−C51 2.028(5), Al1−C50 2.034(6); C49−Y1−N3 104.26(13), C49−Y1−N2 106.64(14), N3− Y1−N2 116.36(11), C49−Y1−N1 97.72(14), N3−Y1−N1 60.88(11), N2−Y1−N1 60.87(11).

Figure 2. X-ray structure of 3 with 35% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Y1−C1 2.395(4), Y1−N2 2.420(4), Y1−N3 2.494(4), Y1−N1 2.475(4), Y1−C9 2.615(5), Y1−C3 2.702(6), Al1−C7 1.980(12), Al1−C3 2.031(5), Al1−C9 2.049(6), Al1−C5 2.098(12); C1−Y1−N2 108.65(15), C1−Y1−N3 103.09(17), N2− Y1−N3 114.34(13), C1−Y1−N1 95.09(16), N2−Y1−N1 61.03(14), N3−Y1−N1 60.20(13).

distance between yttrium atom and terminal carbon being 2.374(5) Å, close to the reported values.12c The bridging Y−C bonds are relatively longer (2.612(5) and 2.697(5) Å, respectively). The formation of such a constitution of mono(tetramethylaluminate) and a yttrium terminal methyl moiety instead of bis(tetramethylaluminate) clearly demonstrates that the sterically saturated environment imposed on the central yttrium by the bulky phosphazene ligand tends to produce metal complexes with low coordination numbers.12d,22b Reaction of [N(Ph2PNC6H4iPr2)2]Y(CH2SiMe3)2 with an Excess of AlEt3. Triethylaluminum is also a widely used cocatalyst in Ziegler−Natta catalyst systems.23 To date, only a few ethylaluminate rare-earth complexes have been authenticated by X-ray diffraction: for example, divalent ethylaluminate rare-earth-metal complexes,16,24 homoleptic Ln(III) complexes,25 and heteroleptic Ln(III) metallocene complexes.9a,f,13b,26 Following the procedure described for the preparation of 2, treatment of 1 with excess amounts of AlEt3 afforded complex 3 as colorless crystals (Scheme 2). The 1H NMR spectrum showed that 3 might feature the same η1 coordination in Y−Al fragment as that for 2 in solution (Figure S5). X-ray single-crystal diffraction showed that 3 and 2 are almost isostructural, crystallizing in the monoclinic space group P21/n for complex 3 in comparison to P21/c for complex 2 (Figure 2). Complex 3 contains η2-Y-(μ-Et)2-Al coordination (Figure 4F) and one terminal ethyl group, instead of the η1ethyl coordination depicted above. The change from methyl to

ethyl groups seems to slightly elongate all tof he bonds (Ln−C and Ln−N), as AlEt3 is more bulky than AlMe3, consistent with Anwander’s observation.25 Investigation of the Reaction Process between Complex 1 and Trimethylaluminum. The successful isolation of complexes 2 and 3 stimulated us to carefully explore the detailed reaction process. When 1 equiv of AlMe3 was added to a toluene solution of complex 1, a precipitate was generated instantly, which was a mixture of intractable components with poor solubility, inhibiting further characterization. Nevertheless, we eventually obtained the pure product 4 after another 1 equiv of AlMe3 was added (Scheme 3). Complex 4 was characterized by elemental analysis and the NMR spectrum. In the 1H NMR spectrum (Figure S7), similar to that of 2, two sets of peaks for isopropyl groups were observed at δ 3.71 and 3.91 ppm for −CH− groups, obviously different from those in the dialkyl precursor 1. The methylene and trimethylsilane protons showed two singlets at δ −0.67 and 0.20 ppm with an integral ratio of 2:9, which showed 4 to be a partially methylated compound. A slightly broad signal was assigned to the Y−CH3 group at δ 0.70 ppm. Colorless single crystals of 4 were obtained by recrystallization from a toluene/ hexane mixture at −30 °C. Complex 4 is a rather uncommon seven-coordinate monomer where the yttrium center displays a heavily distorted pentagonal bipyramidal coordination geometry, where C1 and C3 occupy the two apical positons while the

Scheme 2. Synthesis of Complex 3

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Organometallics Scheme 3. Synthesis of Complex 4

minate complex [La{(μ 2 -Me) 2 AlMe 2 } 2 {(μ 2 -Me) 3 AlMe}] (2.772(3), 2.980(3), 2.892(3) Å, average 2.881(3) Å).13d Taking the variances of metal radii into consideration, 4 features a true η3 coordination. To our knowledge this is the first well-defined mononuclear tetramethylaluminate complex [LnAl] with a hetero-η3-coordination mode in rare-earth chemistry. As expected, 2 was generated again by addition of more AlMe3 to 4. Accordingly, a reaction pathway has been deduced. The coordination of two AlMe3 groups to 1 generates 4, with release of one molecule of AlMe2(CH2SiMe3). Upon activation of another AlMe3, 4 releases a fragment of AlMe2(CH2SiMe3) to form 2, which can isomerize to complex 2′ by methyl exchange.

three N atoms, Y1, C2, and C4 form the equatorial plane (Figure 3). It is worthwhile to note that 4 features a η3-

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CONCLUSIONS In summary, we have demonstrated that the sterically crowded, monoanionic phosphazene ligand provides a unique environment for the isolation of yttrium−aluminum complexes N[Ph2PNC6H3(iPr)2]2Y{(μ-R)2AlR2}R (R = Me, Et) through the reaction of the corresponding dialkyl precursor [N(Ph2PNC6H4iPr2)2]Y(CH2SiMe3)2 with an excess of AlMe3 or AlEt3. By reduction of the loading of AlMe3, the first welldefined aluminate complex N[Ph 2 PNC 6 H 3 ( i Pr) 2 ] 2 Y{(μMe)2(μ-CH2SiMe3)AlMe}Me with a hetero-η3-coordination mode was isolated. Accordingly, a reaction pathway has been deduced to provide us with a full understanding of the detailed mechanism. Thus, the chemistry reported herein provides new insight into the active species of Ziegler−Natta catalysts and may cause some consideration of the foundation of new catalyst models.

Figure 3. X-ray structure of 4 with 35% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): C1−Y1 2.462(4), C2−Al1 2.032(5), C2−Y1 2.796(5), C3−Al1 2.041(5), C3−Y1 2.767(5), C4−Al1 2.018(5), C4−Y1 2.832(5), C5−Al1 1.963(5); Si1−C2−Al1 113.9(2), Si1−C2− Y1 173.4(2), Al1−C2−Y1 72.50(14), Al1−C3−Y1 73.05(14), Al1− C4−Y1 71.82(14), N3−Y1−N2 121.27(10), N3−Y1−N1 60.86(10), N2−Y1−N1 60.92(10), N3−Y1−C1 107.26(12), N2−Y1−C1 94.60(12), N1−Y1−C1 105.44(11).

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EXPERIMENTAL SECTION

General Methods and Materials. All reactions were carried out under a dry and oxygen-free argon atmosphere by using Schlenk techniques or under a nitrogen atmosphere in an MBraun glovebox. All solvents were purified with an MBraun SPS system. Organometallic samples for NMR spectroscopic measurements were prepared in the glovebox by use of NMR tubes sealed with paraffin film. 1H and 13 C{1H} NMR spectra were recorded on a Bruker AV400 or AV600 (FT, 400 MHz for 1H, 100 MHz for 13C; 600 MHz for 1H, 150 MHz for 13C) spectrometer. Elemental analyses were performed at the National Analytical Research Centre of the Changchun Institute of Applied Chemistry (CIAC). AlMe3 and AlEt3 were purchased as pure products from Aldrich. The yttrium dialkyl complex 1 was prepared according to literature procedures.19 X-ray Crystallographic Studies. Crystals for X-ray analysis were obtained as described in the preparations. The crystals were manipulated in a glovebox. Data collections were performed at −88.5 °C on a Bruker SMART APEX diffractometer with a CCD area detector, using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The determination of crystal class and unit cell parameters was carried out with the SMART program package. The raw frame data were processed using SAINT and SADABS to yield the reflection data file. The structures were solved by using the SHELXTL program.

Figure 4. Structurally characterized Ln−Al coordination modes ([LnAl]; A,22a B,22a C,13d,22a D,12c,d E,16a F,16b G,9f,16c H).

coordinated aluminate group (Figure 4H), which has three similar Y−C bond lengths (2.796(5), 2.767(5), 2.832(5) Å, average 2.798(5) Å) and a short Y···Al separation of 2.920(14) Å. These η3-coordinated Y−C bonds are significantly longer than the normal terminal Y−C1 bond (2.462(4) Å) of complex 4 and the η2-coordinated Y−C (μ2-CH3) bonds (average 2.655(5) Å) of complex 2 but are very close to those in the pentanuclear neodymium cluster [Cp* 5 -η 3 -Nd 5 {(μ 2 Me)3AlMe}(μ4-Cl)(μ3-Cl)2(μ2-Cl)6] (2.878(18), 2.875(16), 2.779(16) Å, average 2.844(16) Å)22a and the tetramethylaluD


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Organometallics Refinement was performed on F2 anisotropically for all non-hydrogen atoms by the full-matrix least-squares method. All non-hydrogen atoms were found from Fourier syntheses of electron density and were refined anisotropically and isotropically for hydrogens. All of the hydrogen atoms were placed in calculated positions and were refined in the riding model. More details on the crystallographic measurements can be found in Table S1. CCDC 1046172 (4), 1046173 (2), and 1046174 (3) contain supplementary crystallographic data for this paper. Synthesis of Complex 2. Under a nitrogen atmosphere, to a toluene (10 mL) solution of [N(PPh2NPh(iPr)2)2]Y(CH2SiMe3)2 (0.3 g, 0.3 mmol) was slowly added 3 equiv of AlMe3 (0.064 g, 0.9 mmol, in 2 mL of toluene). The solution was stirred for 24 h at ambient temperature. The yellow solution was concentrated to 2 mL under reduced pressure, and several drops of diethyl ether were added after filtration. Colorless crystals of 2 could be obtained after standing at ambient temperature for several days (0.172 g, 62%). 1H NMR (400 MHz, C6D6, 7.16 ppm, 25 °C): δ −0.39, −0.27 (s s, 3H, YCH3Al), 0.06 (d, 4JYH = 0.8 Hz, 9H, Y(CH3)Al(CH3)3), 0.40 (d, 3JHH = 6.8 Hz, 6H, CH(CH3)2), 0.62 (d, 3JHH = 6.8 Hz, 6H, CH(CH3)2), 0.70 (d, 2 JYH = 2.4 Hz, 3H, YCH3), 1.35 (d, 3JHH = 6.8 Hz, 6H, CH(CH3)2), 1.42 (d, 3JHH = 6.8 Hz, 6H, CH(CH3)2), 3.70 (hept, 3JHH = 6.8 Hz, 2H, CH(CH3)2), 3.90 (hept, 3JHH = 6.8 Hz, 2H, CH(CH3)2), 6.54 (m, 4H, Ar-H), 6.71 (m, 2H, Ar-H), 6.90 (m, 4H, Ar-H), 6.97 (m, 2H, ArH), 7.07 (m, 4H, Ar-H), 7.12 (s, 6H, Ar-H), 7.51 ppm (m, 4H, Ar-H). 13 C{1H} NMR (100 MHz, C6D6, 128.06 ppm, 25 °C) δ −0.30, 3.02 (s s, Y(CH3)Al(CH3)3), 21.43 (s, Y(CH3)Al(CH3)3), 23.41 (s, C6H3CH(CH3)2), 23.59 (s, C6H3CH(CH3)2), 27.00 (s, C6H3CH(CH3)2), 27.18 (s, C6H3CH(CH3)2), 28.83 (s, C6H3CH(CH3)2), 29.11(s, C6H3CH(CH3)2), 30.26 (d, JYC = 55 Hz, YCH3), 124.32 (s, aromatic CH), 124.82 (s, aromatic CH), 125.28 (s, aromatic CH), 125.70 (s, aromatic CH), 129.33 (s, aromatic CH), 131.37 (s, aromatic C), 131.89 (s, aromatic C), 132.23 (m, aromatic C), 132.44 (d, aromatic ipso-C), 132.68 (m, aromatic C), 133.28 (m, aromatic C), 133.41 (d, aromatic ipso-C), 134.43 (s, aromatic C), 137.90 (s, aromatic C), 140.91 (s, aromatic C), 145.15 (s, aromatic C), 146.58 ppm (s, aromatic C). Anal. Calcd for C53H69N3P2AlY: C, 68.75; H, 7.51; N, 4.54. Found: C, 69.07; H, 7.26; N, 4.36. Synthesis of Complex 3. By a procedure similar to that described for the preparation of 2, treatment of [N(PPh2NPh(iPr)2)2]Y(CH2SiMe3)2 (0.3 g, 0.3 mmol) with 3 equiv of AlEt3 (0.103 g, 0.90 mmol) gave 3 as a white solid. Colorless crystals could be obtained from a mixture of toluene and hexane after standing at ambient temperature for 2 days (0.164 g, 55%). 1H NMR (400 MHz, C6D6, 7.16 ppm, 25 °C): δ 0.20−0.27 (m, 2H, Y(μ-CH2CH3)Al(CH2CH3)3), 0.32 (m, 6H, Y(μ-CH2CH3)Al(CH2CH3)3), 0.41 (d, 3 JHH = 6.8 Hz, 6H, CH(CH3)2), 0.47 (quar, 3JHH = 8.0 Hz, 2H, YCH2CH3), 0.74 (d, 3JHH = 6.8 Hz, 6H, CH(CH3)2), 1.31−1.48 (m, 24H, Y(μ-CH2CH3)Al(CH2CH3)3 + CH(CH3)2), 1.99 (t, 3JHH = 8.0 Hz, 3H, YCH2CH3), 3.62 (hept, 3JHH = 6.6 Hz, 2H, CH(CH3)2), 3.70 (hept, 3JHH = 6.6 Hz, 2H, CH(CH3)2), 6.44 (m, 4H, Ar-H), 6.61 (m, 2H, Ar-H), 6.94−7.26 (m, 16H, Ar-H), 7.63 ppm (m, 4H, Ar-H). 13 C{1H} NMR (100 MHz, C6D6, 128.06 ppm, 25 °C): δ 12.52 (s, Y(μ-CH2CH3)Al(CH2CH3)3), 16.55 (s, Y(μ-CH2CH3)Al), 24.18 (s, C6H3CH(CH3)2)), 25.51(s, C6H3CH(CH3)2), 26.37 (s, C6H3CH(CH3)2), 29.07 (s, Y(μ-CH2CH3)Al(CH2CH3)3(CH2CH3)), 29.19 (s, Y(μ-CH2CH3)Al(CH2CH3)3(CH2CH3)), 45.12 (d, JYC = 58 Hz, YCH2CH3), 124.71 (s, aromatic CH), 124.92 (s, aromatic CH), 125.70 (s, aromatic CH), 131.16 (s, aromatic CH), 131.97 (s, aromatic C), 132.11 (s, aromatic C), 132.27 (s, aromatic C), 132.54 (s, aromatic C), 133.18 (m, aromatic ipso-C), 134.30 (s, aromatic ipso-C), 141.70 (s, aromatic C), 144.26 (s, aromatic C), 146.03 ppm (s, aromatic C). Anal. Calcd for C58H79N3P2AlY: C, 69.93; H, 7.99; N, 4.22. Found: C, 69.67; H, 8.12; N, 4.31. Synthesis of Complex 4. Under a nitrogen atmosphere, to a toluene (10 mL) solution of [N(PPh2NPh(iPr)2)2]Y(CH2SiMe3)2 (0.3 g, 0.3 mmol) was slowly added 2 equiv of AlMe3 (0.043 g, 0.6 mmol, in 2 mL of toluene). The solution was stirred for 12 h at ambient temperature. The yellow solution was concentrated to 2 mL under

reduced pressure, and several drops of hexane were added after filtration. Colorless crystals of 4 could be obtained after standing at −30 °C for 2 days (0.18 g, 60%). 1H NMR: δ −0.67 (s, 2H, YCH2), 0.20 (s, 9H, SiMe3), 0.21 (s, 6H, Al((μ-CH3)2Y), 0.25 (s, 3H, AlCH3), 0.40 (d, 3JHH = 6.6 Hz, 6H, CH(CH3)2), 0.62 (d, 3JHH = 6.6 Hz, 6H, CH(CH3)2), 0.70 (s, 3H, YCH3), 1.39 (d, 3JHH = 6.6 Hz, 6H, CH(CH3)2), 1.44 (d, 3JHH = 6.6 Hz, 6H, CH(CH3)2), 3.70 (hept, 3JHH = 6.6 Hz, 2H, CH(CH3)2), 3.91 (hept, 3JHH = 6.6 Hz, 2H, CH(CH3)2), 6.54 (m, 4H, Ar-H), 6.72 (m, 2H, Ar-H), 6.90 (m, 4H, Ar-H), 6.96 (m, 2H, Ar-H), 7.07 (m, 10H, Ar-H), 7.50 ppm (m, 4H, Ar-H). 13C{1H} NMR (100 MHz, C6D6, 128.06 ppm, 25 °C): δ 2.71 (s, CH2Si(CH3)3), 3.03 (s, CH2SiMe3), 3.46 (s, AlCH3), 23.42 (s, C6H3CH(CH3)2), 23.58 (s, C6H3CH(CH3)2), 27.00 (s, Y(μ-CH3)2Al), 27.14 (s, C6H3CH(CH3)2), 27.25 (s, C6H3CH(CH3)2), 28.83 (s, C6H3CH(CH3)2), 29.09 (s, C6H3CH(CH3)2), 30.30 (d, JYC = 54 Hz, YCH3), 124.34 (s, aromatic CH), 124.87 (s, aromatic CH), 125.32 (s, aromatic CH), 131.24 (s, aromatic C), 131.41 (s, aromatic C), 131.70 (s, aromatic C), 131.91 (m, aromatic C), 132.21 (m, aromatic C), 132.41 (d, aromatic ipso-C), 132.68 (m, aromatic C), 133.22 (s, aromatic C), 133.44 (d, aromatic ipso-C), 134.37 (s, aromatic C), 140.90 (s, aromatic C), 145.12 (s, aromatic C), 146.58 ppm (s, aromatic C). Anal. Calcd for C56H77N3P2SiAlY: C, 67.38; H, 7.78; N, 4.21. Found: C, 67.02; H, 7.95; N, 4.59.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00911. 1 H and 13C{1H} NMR spectra of complexes 1−4 and crystallographic data for complexes 2−4 (PDF) Accession Codes

CCDC 1046172−1046174 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*D.L.: e-mail, [email protected]. *D.C.: e-mail, [email protected]; fax, (+86) 431 85262774; tel, +86 431 85262773. ORCID

Dongmei Cui: 0000-0001-8372-5987 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the NSFC (Project Nos. 21634007, and 21674108), the MST “973” project (No. 2015CB654702), and the department of science and technology of Jilin province (Project No. 20160520001JH). The authors are grateful to the Computing Center of Jilin Province for essential support.

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