Phenoxyimine Ligands Bearing Nitrogen-Containing Second

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Phenoxyimine Ligands Bearing Nitrogen-Containing Second Coordination Spheres for Zinc Catalyzed Stereoselective RingOpening Polymerization of rac-Lactide Min Li,†,∥ Shabnam Behzadi,†,∥ Min Chen,† Wenmin Pang,‡ Fuzhou Wang,*,§ and Chen Tan*,†

Organometallics Downloaded from pubs.acs.org by UNIV OF GOTHENBURG on 01/16/19. For personal use only.

†

Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, People’s Republic of China ‡ Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, People’s Republic of China § Institute of Physical Science and Information Technology, Anhui University, Hefei 230026, People’s Republic of China S Supporting Information *

ABSTRACT: A series of zinc (Zn1−Zn6) and aluminum (Al6) complexes supported by phenoxyimine ligands (L1− L6) bearing nitrogen-containing second coordination spheres were synthesized and investigated for ring-opening polymerization (ROP) of rac-lactide (rac-LA). The ligands L1−L4 and L6 bearing sterically opened nitrogen donors resulted in dynamic NNO-chelates associated with facile nitrogen−metal coordination interactions. The bulky ligand L5 resulted in L2Zn-type complex Zn5 by disproportionation. The zinc complexes exhibited catalytic activities in ROP of cyclic monomers, including rac-LA, ε-caprolactone (CL), δ-valerolactone (VL), and trimethylene carbonate (TMC). The zinc complex Zn6 bearing a bulky triphenylsilyl substituent at the 3-position of the phenoxy moiety exhibited good stereochemical control during the ROP of rac-LA to produce highly heterotactic PLAs at ambient temperature (Pr = 0.90) as well as low temperature (−30 °C, Pr = 0.94) with narrow molecular weight distributions (Mw/Mn < 1.2). The aluminum complex Al6 catalyzed not only homopolymerization of the above cyclic monomers but also copolymerization of rac-LA with CL/VL at high temperature.

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INTRODUCTION

complexes might have significant influence on catalytic performance. Recently, increasing attention has been directed to model metal complexes that mimic the properties of metalloenzymes. The model compounds generally exhibit much lower catalytic efficiencies than those of corresponding metalloenzymes, attributed to the lack of intermolecular interactions derived from the second coordination sphere.6 Recently, Chen and Mecking investigated the imine-based ligands bearing various second coordination spheres to modulate Pd and Ni catalyzed olefin (co)polymerization (Scheme 1, A−C).7 Notably, facile metal−nitrogen second coordination in the Brookhart’s Pd catalysts resulted in enhanced thermal stabilities and a wider range of polar monomer substrates.7d Consequently, it is highly fascinating to explore the influence of ligand second coordination spheres for ROP reactions. In this contribution, a series of zinc (Zn1−Zn6) and aluminum (Al6) complexes supported by phenoxyimine ligands (L1−L6) (Scheme 2) bearing different nitrogen-

Metal catalyzed ring-opening polymerization (ROP) has proven to be a powerful tool for the synthesis of biorenewable and biodegradable aliphatic polyesters especially polylactides (PLAs).1 Precise control of stereochemistry and molecular weight distribution (MWD) during ROP is important for the synthesis of PLAs with high thermal stability and well-defined microstructures.2 Recently, as a biocompatible metal element,3 zinc-based complexes have drawn increasing attention in the field of ROP.4 Zinc catalyzed ROP of rac-lactide (rac-LA) can generate heterotactic PLAs as well as isotactic PLAs. Markedly, some LZnR-type complexes in which R = active alkyloxide supported by NNO-chelate ligands bearing strong nitrogen donors display high heteroselectivity in the ROP of rac-LA2g,g,4 and inhibit the formation of the L2Zn-type complex by disproportionation.4i Mehrkhodavandi et al. investigated the role of the central nitrogen donors in zinc catalysts supported by NNO-chelate ligands for ROP of cyclic esters and suggested that the electronic effects of the central nitrogen donors influence the activity of zinc catalysts in the ROP of lactide.5 Similarly, the nature of marginal nitrogen donors in zinc © XXXX American Chemical Society

Received: October 27, 2018

A

DOI: 10.1021/acs.organomet.8b00788 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Metal Catalysts Supported by Imine-Based Ligands Bearing Second Coordination Spheres

Scheme 4. Synthesis of A4 and A5

(Scheme 2) in the presence of a formic acid catalyst (yield, 53−88%), respectively. The 3-triphenylsilyl-substituted ligand L6 was synthesized at a high temperature in toluene using ptoluenesulfonic acid (PTSA) as catalyst (yield, 81%). The zinc complexes were synthesized from the reactions of the corresponding ligand with 1.2 equiv of Zn[N(TMS)2]2. The LZnR-type complexes Zn1−Zn4 in which R = N(TMS)2 were formed. A L2Zn-type complex Zn5 was formed by using the bulky ligand L5, indicating L5 is a bidentate ligand, since the bulky substituent protects the nitrogen donor from the zinc center and consequently results in the L2Zn-type complex by disproportionation.4i In contrast, the ligand L6, in the absence of isopropyl, resulted in LZnR-type complex Zn6, indicating the zinc−nitrogen coordination is strong enough to inhibit the formation of the L2Zn-type complex by disproportionation. The 13C NMR spectra of Zn1, Zn2, Zn4, and Zn6 (Figures S22, S24, S28, S32) exhibited single peaks derived from the carbon atoms bonded to the marginal nitrogen atoms, indicating a facile and dynamic coordination between zinc and two equivalent nitrogen atoms. In contrast, in the case of Zn3 bearing morpholinyl groups, the 1H NMR and 13C NMR spectra showed different signals for the two morpholinyl groups (Figures S25, S26), indicating these two nitrogen atoms are strong donors. The complex Al6 was synthesized from the reaction of L6 with 1.2 equiv of AlMe3 and characterized by 1H NMR, 13C NMR, mass spectroscopy and elemental analysis. X-ray Crystallographic Studies. Suitable crystals of complexes Zn2, Zn5, and Al6 were obtained in n-hexane and dichloromethane. The molecular structures of Zn2, Zn5, and Al6 were confirmed by single-crystal X-ray diffraction, and the corresponding ORTEP diagrams with selected bond distances and angles are shown in Figures 1−3, respectively. Crystal data, data collection, and refinement parameters are listed in Table S1 (see the Supporting Information). The crystal structure analysis of Zn2 shows the ligand bonds to the central zinc in a NNO-tridentate manner (Figure 1). The central zinc exhibits distorted tetrahedral coordination geometry. The Zn1−N1 distance of 2.007(4) Å and the Zn1− O1 distance of 2.052(2) Å are shorter than that of Zn1−N2 [2.297(4) Å], suggesting the two nitrogen atoms of the second coordination sphere form a relatively weak coordination bond with the central zinc atom, indicating a potential coordinating effect by the nitrogen donors during ROP. The crystal structure of Zn5 shows an L2Zn-type structure (Figure 2). The Zn1 atom is four-coordinated by O1, O2, N1, and N4 atoms, forming a distorted tetrahedral geometry. The Zn1−N1 distance of 2.038(2) Å is slightly longer than that in Zn2 [2.007(4) Å], while the Zn1−O1 distance of 1.9362(19) Å is shorter than that in Zn2 [2.052(2) Å]. No coordination bonds between the zinc center and the nitrogen side arms were observed, indicating the bulky substituent kept the nitrogen donors from the zinc center and resulted in a bidentate coordination.

Scheme 2. Synthesis of the Ligands L1−L6 and the Complexes Zn1−Zn6 and Al6

containing second coordination spheres were designed, synthesized, and investigated for ROP of cyclic esters (Scheme 3), including rac-LA, ε-caprolactone (CL), δ-valerolactone Scheme 3. Cyclic Esters: rac-LA, CL, VL, and TMC

(VL), and trimethylene carbonate (TMC). The stereoselectivity of the above zinc and aluminum catalysts in the ROP of rac-LA and the aluminum catalyzed copolymerization of rac-LA with CL and VL have been investigated.

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RESULTS AND DISCUSSION Synthesis and Characterization of Complexes Zn1− Zn6 and Al6. The aniline derivatives A4 and A5 are newly reported compounds synthesized by nucleophilic substitution of 2,6-dichloronitrobenzene with anilines, subsequent methylation, and final reduction reaction (Scheme 4). The ligands L1−L5 were prepared by the condensation of the 2-hydroxy-3tert-butylbenzaldehyde with anilines using methanol as solvent B

DOI: 10.1021/acs.organomet.8b00788 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 1. Molecular structure of Zn2 with 30% probability level, and H atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Zn1−N4 = 1.911(4), Zn1−O1 = 1.949(3), Zn1−N1 = 2.007(4), Zn1−N2 = 2.297(4), N1−C17 = 1.292(6), O1−C19 = 1.302(5), C17−C18 = 1.428(6), C18−C19 = 1.426(6), N4−Zn1− O1= 118.37(16), N4−Zn1−N1 = 133.85(17), O1−Zn1−N1 = 91.30(15), N4−Zn1−N2 = 112.28(16), O1−Zn1−N2 = 119.39(15), N1−Zn1−N2 = 75.34(15), O1−C19−C18 = 123.2(4), C19−C18− C17 = 122.8(4), N1−C17−C18 = 127.0(4).

Figure 3. Molecular structure of Al6 with 30% probability level, and H atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Al1−O1 = 1.789(2), Al1−C47 = 1.943(4), Al1−C46 = 1.952(3), Al1−N1 = 1.964(3), N1−C1 = 1.299(3), O1−C3 = 1.325(3), C1−C2 = 1.424(4), C2−C3 = 1.422(4), O1−Al1−C47 = 107.22(15), O1−Al1−C46 = 111.29(15), C47−Al1−C46 = 120.3(2), O1−Al1−N1 = 91.57(11), C47−Al1−N1 = 114.98(17), C46−Al1−N1 = 107.73(14), C1−N1−Al1 = 119.4(2), C3−O1−Al1 = 123.5(2), N1−C1−C2 = 124.8(3), C3−C2−C1 = 121.5(3), O1− C3−C2 = 120.7(3).

< 1.2) remained (Figure 5), indicating a controlled ROP of rac-LA.9 The complexes Zn1−Zn4 produced atactic PLAs in the ROP of rac-LA (Table 1, entries 1−4). The Pr of the PLAs were calculated by the analysis of the methine region of the homonuclear decoupled 1H NMR spectra.10 The bulky complexes Zn5 and Zn6 exhibit stereoselectivity in the ROP of rac-LA (Table 1, entries 5−7). Markedly, Zn6 exhibited good stereochemical control during the ROP of rac-LA to produce highly heterotactic PLAs at 25 °C (Pr = 0.90) as well as low temperature (−30 °C, Pr = 0.94) with narrow molecular weight distributions (Mw/Mn < 1.2). The stereoselectivity of Zn6 catalyzed ROP is comparable to that of previously reported NNO-type zinc catalysts.2g,4 The stereoselectivity of Zn6 in the ROP of rac-LA decreased when THF and toluene were used as solvents (Table 1, entries 8 and 9). When toluene was used, the reaction temperature was increased to 80 °C to enhance the solubility of rac-LA in toluene. The complex Zn6 can also catalyze the ROP of other cyclic esters including CL, VL, and TMC. These ROPs generated polyesters with narrow molecular weight distributions (Table 1, entries 10−12). Al Catalyzed ROP. The complex Al6 can catalyze ROP of rac-LA, CL, VL, and TMC in the presence of BnOH at high temperature (Table 1, entries 13−17). An atactic PLA with a narrow molecular weight distribution was generated at 140 °C, indicating the high thermodynamic stability of Al6. The complex Al6 can catalyze the copolymerizations of racLA and CL/VL at high temperature (Table 2, entries 1−3). The copolymerization of equimolar rac-LA/CL (100/100) feed ratio at 96% conversion of rac-LA and 31% conversion of CL resulted in a copolymer containing 76 mol % of the lactide moiety (Table 2, entry 1). Moreover, it was indicated that the rac-LA/CL feed ratio has no notable influence on mole percent of the lactide moiety in the final copolymer (Table 2, entry 2). Therefore, the copolymer products are random copolymers, since the insertion rate ratio of two monomers is nearly constant during ROP.

Figure 2. Molecular structure of Zn5 with 30% probability level, and H atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Zn1−O1 = 1.9362(19), Zn1−O2 = 1.9394(18), Zn1− N1 = 2.038(2), Zn1−N4 = 2.0464(19), O1−C7 = 1.305(3), O2− C50 = 1.306(3), N1−C1 = 1.301(3), N4−C44 = 1.291(3), C45−C50 = 1.428(4), C45−C44 = 1.440(3), C2−C7 = 1.426(4), C2−C1 = 1.438(4), O1−Zn1−O2 = 104.60(8), O1−Zn1−N1 = 94.30(8), O2− Zn1−N1 = 118.80(7), O1−Zn1−N4 = 107.75(8), O2−Zn1−N4 = 92.27(8), N1−Zn1−N4 = 136.07(8), C7−O1−Zn1 = 122.26(16), C50−O2−Zn1 = 118.88(16), C1−N1−Zn1 = 116.82(16), C44− N4−Zn1 = 112.73(15), N4−C44−C45 = 129.3(2), N1−C1−C2 = 129.3(2), C50−C45−C44 = 123.0(2), C7−C2−C1 = 124.3(2), O2− C50−C45 = 121.4(2), O1−C7−C2 = 121.6(2).

The crystal structure of the aluminum complex Al6 shows the 3-triphenylsilyl-substituted ligand adopts a bidentate coordination mode by the N,O framework, with two coordinating methyl anions (Figure 3). The Al1−O1 distance of 1.789(2) Å is shorter than that of Al1−N1 [1.964(3)Å]. Zn Catalyzed ROP. All the zinc precatalysts Zn1−Zn6 can catalyze the ROP of rac-LA (Table 1, entries 1−6). The complex Zn3 exhibits lower catalytic activity than those of Zn1, Zn2, Zn4, and Zn6, probably due to the strong Zn−N coordination in Zn3. The L2Zn-type complex Zn5 exhibits the highest activity that is comparable to one of the most active NNO-type zinc catalysts reported by Tolman.8 The bulky complex Zn6 exhibits lower activity than that of Zn4, indicating the bulky SiPh3 substituent hinders the coordination between rac-LA and the zinc center. The plots of ln([LA]0/[LA]) versus time using these catalysts exhibit a significant trend of linear correlation (Figure 4), indicating first order ROP of rac-LA. The Mn of resulting PLA increased linearly with increasing monomer conversion (r close to 1) and narrow molecular weight distribution (Mw/Mn C

DOI: 10.1021/acs.organomet.8b00788 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. ROP of rac-LA, CL, VL, and TMC Catalyzed by Zn1−Zn6 and Al6 entrya

cat.

mon.

time (h)

temp. (°C)

conv.b (%)

Kobs (h−1)

104 Mnc (GPC)

104 Mnd (calcd.)

Mw/Mnc

Pre

1 2 3 4 5f 6 7 8h 9i 10 11 12 13f,j 14f,j 15f,j 16f,j 17f,j

Zn1 Zn2 Zn3 Zn4 Zn5 Zn6 Zn6 Zn6 Zn6 Zn6 Zn6 Zn6 Al6 Al6 Al6 Al6 Al6

rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA CL VL TMC rac-LA rac-LA CL VL TMC

1 1 24 1 1 5 96 5 0.17 5 5 5 1 0.17 0.17 0.17 0.17

25 25 25 25 25 25 −30 25 80 25 25 25 140 160 140 140 140

67 69 82 58 95 94 33 87 84 48 15 80 59 82 87 90 96

1.2(1) 1.1(2) 0.071(6) 0.87(9) 2.4(3) 0.53(6) 0.0042(5) 0.41(5) 10(2) 0.14(2) 0.032(4) 0.33(5) 0.89(7) 9.8(9) 11(2) 13(2) 17(3)

1.38 1.51 1.99 1.88 2.65 2.50 0.71 2.19 2.49 1.19 0.38 2.98 3.62 2.73 6.57 6.28 6.18

1.95 2.01 2.38 1.69 2.75g 2.73 0.97 2.53 2.44 1.11 0.32 1.65 1.71f 2.37f 2.00f 2.07f 1.97f

1.51 1.51 1.65 1.38 1.36 1.16 1.17 1.08 1.18 1.03 1.07 1.23 1.12 1.27 1.62 1.49 2.07

0.64 0.52 0.62 0.68 0.86 0.90 0.94 0.86 0.72

0.58

a Conditions: 10 μmol of cat., [cat]o/[M]o = 1:200, 5 mL of CH2Cl2. bMeasured by 1H NMR spectra. cGPC data in THF vs polystyrene standard. The Mn of LA was multiplied by 0.58; the Mn of CL was multiplied by 0.53. dMn (calcd.) = 200 × Mmonomer × conv. % + 166.11. eThe Pr of the PLAs was calculated by the analysis of the methine region of the homonuclear decoupled 1H NMR spectra. f[cat]o/[M]o/[BnOH]o = 1:200:1. gMn (calcd.) = 200 × Mmonomer × conv. % + 108.14. hTHF was used as solvent. iSolvent: toluene. jNeat.

ligands (L1−L6) bearing nitrogen-containing second coordination spheres for ROP of cyclic esters, including rac-LA, CL, VL, and TMC. The ligands L1−L4 and L6 bearing a sterically opened nitrogen donor resulted in dynamic NNO-chelates. In contrast, the bulky ligand L5 resulted in L2Zn-type complex Zn5 by disproportionation, since the bulky substituent protects the nitrogen donor from the zinc acceptor. The zinc complexes Zn1−Zn4 generated atactic PLA. The Zn6 bearing a bulky triphenylsilyl substituent at the 3-position of the phenoxy moiety exhibited good stereochemical control during ROP of rac-LA and generated highly heterotactic PLAs at ambient temperature (Pr = 0.90) as well as low temperature (−30 °C, Pr = 0.94) with narrow molecular weight distributions (Mw/Mn < 1.2). The aluminum complex Al6 can catalyze not only the homopolymerization of the above cyclic monomers but also the copolymerization of rac-LA with CL/VL at high temperature. It should be noted that anilines and the derived imine-based ligands have been widely used in various transition metal and main group metal catalysts, with notable examples including salicylaldimine, pyridine-imine, and α-diimine ligands.11 The anilines in this work are specially designed with various electronic and steric effects and correspondingly varied coordinating abilities. It is expected that these anilines will find further applications in the synthesis of various imine-based ligands and metal catalysts.

Figure 4. Plots of ln([M]0/[M]) versus time (min) for the ROP of rac-LA catalyzed by Zn1−Zn6 (Conditions: 5 mmol of Zn, [Zn]/ [LA]0 = 1:50; solvent, 0.6 mL of CDCl3; 25 °C).

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EXPERIMENTAL SECTION

General. All manipulations, unless otherwise mentioned, were carried out under an atmosphere of N2 in a glovebox. Nuclear magnetic resonance (1H, 13C NMR) spectra were recorded on a Bruker 400 MHz instrument at room temperature. Molecular weights and molecular weight distributions were determined by gel permeation chromatography (GPC) employing a series of two linear Styragel columns (HR2 and HR4) at an oven temperature of 45 °C. Single crystals of the zinc complexes were obtained by slow diffusion of n-hexane into CH2Cl2. X-ray crystallography analysis was performed on an Oxford Diffraction Gemini S Ultra CCD diffraction meter equipped with mirror Cu Kα (λ = 1.54184 Å) radiation at room

Figure 5. Plots of Mn (■) and PDI (⧫) versus conversion (%) in the ROP of rac-LA catalyzed by Zn6 (Conditions: 5 mmol of Zn6; [Zn6]0:[LA]0 = 1:100; solvent, 5 mL of CH2Cl2; 25 °C).

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CONCLUSIONS In summary, we have explored a series of zinc (Zn1−Zn6) and aluminum (Al6) complexes supported by phenoxyimine D

DOI: 10.1021/acs.organomet.8b00788 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 2. Copolymerization of rac-LA and CL/VL Catalyzed by Al6 entrya

cat.

time (h)

co-mon.

conv. ratiob

Kobs (h−1) ratioc

lactide (mol %) in copolymerc

104 Mnd (GPC)

Mw/Mnd

1 2e 3

Al6 Al6 Al6

5 5 0.17

CL CL VL

96/31 50/16 50/13

0.60(5)/0.075(7) 0.12(2)/0.033(5) 4.1(5)/0.72(9)

76 76 79

1.46 1.04 1.66

1.53 1.14 1.08

Conditions: 10 mmol of Al6, [cat]o/[A/B]o/[BnOH]o = 1:(100/100):1, neat, 140 °C. bConv. (rac-LA)/conv. (comon.). cMeasured by 1H NMR spectra. dGPC data in THF vs polystyrene standard. e[cat]o/[A/B]o/[BnOH]o = 1: (50/200):1. a

hydroxy-3-tert-butylbenzaldehyde with o-amine-substituted anilines using methanol as solvent, usually in the presence of a formic acid catalyst. A typical synthetic procedure of L1 is as follows: Formic acid (0.25 mL) was added to a stirred solution of 2-hydroxy-3-tert-butylbenzaldehyde (1.06 g, 6 mmol), and A1 (1.16 g, 5 mmol) in methanol (20 mL). The mixture was refluxed for 24 h, then cooled, and the precipitate was separated by filtration. The crude product was washed with cold ethanol and dried under vacuum to give the desired product (1.32 g, 67% yield). 1H NMR (400 MHz, CDCl3) δ 14.40 (s, 1H, OH), 9.02 (s, 1H, CHN), 7.40 (dd, J = 7.7, 1.7 Hz, 1H, Ar-H), 7.20 (dd, J = 7.6, 1.7 Hz, 1H, Ar-H), 7.12 (t, J = 8.1 Hz, 1H, Ar-H), 6.91− 6.81 (m, 3H, Ar-H), 3.79−3.68 (m, 8H, NCH2), 2.98−2.88 (m, 8H), 1.49 (s, 9H, CCH3).13C NMR (100 MHz, CDCl3) δ 168.24 (CH N), 161.08 (COH), 145.47, 137.91, 136.51, 130.35, 130.33, 126.48, 119.00, 118.19, 114.77, 67.23 (NCH2), 51.87, 35.02 (CCH3), 29.46 (CCH3). Elemental analysis: calcd. (%) for C25H33N3O: C, 76.69; H, 8.49; N, 10.73. Found: C, 76.65; H, 8.51; N, 10.69. HRMS (m/z): calcd. for C25H33N3O: 391.2624, found: 392.2692 [M + H]+. L2 was obtained as a yellow solid (1.10 g, 53%). 1H NMR (400 MHz, CDCl3) δ 14.61 (s, 1H, OH), 9.00 (s, 1H, CHN), 7.40−7.34 (m, 1H, Ar-H), 7.20−7.15 (m, 1H, Ar-H), 7.06 (t, J = 8.0 Hz, 1H, ArH), 6.81 (dd, J = 11.7, 7.8 Hz, 3H, Ar-H), 2.85 (t, J = 5.2 Hz, 8H, NCH2), 1.59 (t, J = 5.6 Hz, 8H), 1.47 (s, 13H, NCH2CH2CH2, CCH3). 13C NMR (100 MHz, CDCl3) δ 167.49 (CHN), 161.51, 147.11, 137.81, 136.49, 130.34, 129.70, 126.06, 119.43, 117.67, 114.54, 53.15, 35.05 (CCH3), 29.51 (CCH3), 26.44, 24.47 (CCH3). Elemental analysis: calcd. (%) for C27H37N3O: C, 77.28; H, 8.89; N, 10.01. Found: C, 77.26; H, 8.90; N, 10.03. HRMS (m/z): calcd. for C27H37N3O: 419.2937, found: 420.3002 [M + H]+. L3 was obtained as a yellow solid (1.78 g, 84%). 1H NMR (400 MHz, CDCl3) δ 14.42 (s, 1H, OH), 9.01 (s, 1H, CHN), 7.39 (d, J = 7.3 Hz, 1H, Ar-H), 7.19 (d, J = 6.9 Hz, 1H, Ar-H), 7.10 (t, J = 8.0 Hz, 1H, Ar-H), 6.85 (dd, J = 15.9, 7.9 Hz, 3H, Ar-H), 3.77−3.66 (m, 8H, OCH2), 2.97−2.87 (m, 8H, NCH2), 1.49 (s, 9H, CCH3). 13C NMR (100 MHz, CDCl3) δ 168.14 (CHN), 161.00, 145.40, 137.79, 136.39, 130.27, 130.24, 126.42, 118.93, 118.14, 114.69, 67.11, 51.79, 34.92 (CCH3), 29.40 (CCH3). Elemental analysis: calcd. (%) for C25H33N3O3: C, 70.89; H, 7.85; N, 9.92. Found: C, 70.86; H, 7.89; N, 9.95. HRMS (m/z): calcd. for C25H33N3O3: 423.2522, found: 424.2584 [M + H]+. L4 was obtained as a yellow solid (1.84 g, 79%). 1H NMR (400 MHz, CDCl3) δ 13.03 (s, 1H, OH), 8.09 (s, 1H, CHN), 7.25−7.10 (m, 8H, Ar-H), 6.74−6.60 (m, 8H, Ar-H), 3.15 (s, 6H, NCH3), 1.34 (s, 9H, CCH3). 13C NMR (100 MHz, CDCl3) δ 167.40 (CHN), 160.60, 149.53, 144.81, 141.71, 137.37, 130.77, 130.29, 129.03, 126.97, 126.46, 118.69, 118.01, 117.75, 114.21, 39.42 (NCH3), 34.89 (CCH3), 29.37 (CCH3). Elemental analysis: calcd. (%) for C31H33N3O: C, 80.31; H, 7.17; N, 9.06. Found: C, 80.35; H, 7.15; N, 7.15. HRMS (m/z): calcd. for C31H33N3O: 463.2624, found: 464.2694 [M + H]+. L5 was obtained as a yellow solid (2.78 g, 88% yield). 1H NMR (400 MHz, C2Cl4D2) δ 13.47 (s, 1H, OH), 8.35 (s, 1H, CHN), 7.35 (d, J = 7.6 Hz, 1H, Ar-H), 7.20 (t, J = 7.6 Hz, 2H, Ar-H), 7.13− 7.06 (m, 5H, Ar-H), 6.83 (t, J = 7.6 Hz, 1H, Ar-H), 6.57 (t, J = 8.0 Hz, 1H, Ar-H), 5.87 (d, J = 7.5 Hz, 2H), 3.23−3.07 (m, 10H, NCH3, CHCH3), 1.45 (s, 9H, CCH3), 1.11 (s, 24H). 13C NMR (100 MHz, C2Cl4D2) δ 168.59 (CHN), 160.54, 147.69, 144.38, 143.92, 143.44, 137.84, 129.96, 129.77, 129.32, 127.09, 124.43, 124.28, 119.07, 117.90, 109.25, 43.44 (NCH3), 34.58 (CCH3), 29.37 (CCH3), 27.88,

temperature. Mass spectra were recorded on a P-SIMS-Gly of Bruker Daltonics Inc. (EI+). HRMS spectra were measured on a GCT premier CAB048 mass spectrometer operating in MALDI-TOF (matrix-assisted laser desorption/ionization time-of-flight) mode. Toluene, CDCl3, tetrahydrofuran, n-hexane, and CH2Cl2 were purified over 4 Å molecular sieves. Lactide was recrystallized from CH2Cl2 and n-hexane. Other chemicals were purchased and used as received. The compounds A1−A3,7d 3-tert-butylsalicylaldehyde,12 3triphenylsilylsalicylaldehyde,13 and bis(trimethylsilylamide)zinc14 were prepared according to reported procedures. 2,6-Di(N-methylanilino)nitrobenzene. After drying the 500 mL reactor under a N2 atmosphere, KOH (10.10 g, 180 mmol), 2,6dianilinonitrobenzene (6.06 g, 20 mmol), and DMF (200 mL) were added to the reactor. Then iodomethane (17.03 mL, 120 mmol) was added to the toluene, and the mixture was stirred for 10 min. The reaction was stirred for 24 h at room temperature, quenched with water (200 mL) and extracted with ethyl acetate, dried over Na2SO4, filtered, and concentrated. The crude product was washed with methanol and hexane to afford the desired product as yellow solid (6.00 g, 90% yield). 1H NMR (400 MHz, CDCl3) δ 7.45 (t, J = 8.1 Hz, 1H, Ar-H), 7.26−7.12 (m, 6H, Ar-H), 6.84 (t, J = 7.4 Hz, 2H, ArH), 6.72 (d, J = 7.9 Hz, 4H, Ar-H), 3.21 (s, 6H, NCH3). 13C NMR (100 MHz, CDCl3) δ 149.61, 148.61, 142.67, 132.41, 129.03, 127.07, 119.67, 115.56, 40.53 (NCH3). Elemental analysis: calcd. (%) for C20H19N3O2: C, 72.05; H, 5.74; N, 12.60. Found: C, 72.09; H, 5.71; N, 12.62. HRMS (m/z): calcd. for C20H19N3O2: 333.1477, found: 334.1551 [M + H]+. 2,6-Di(N-methylanilino)aniline (A4). Acetic acid glacial (2.5 mL, 134 mmol) was added dropwise to a stirred solution of 2,6-di(Nmethylanilino)nitrobenzene (3.33 g, 10 mmol) in methanol (30 mL) with zinc (3.19 g, 50 mmol) in methanol (30 mL) at 0 °C. The reaction was monitored by thin layer chromatography (TLC). After the disappearance of 2,6-di(N-methylanilino)nitrobenzene, the solution obtained was filtered over silica gel and concentrated by evaporation. The crude product was washed with methanol and dried under vacuum to afford a white solid (2.88 g, 95% yield). 1H NMR (400 MHz, CDCl3) δ 7.20 (dd, J = 8.7, 7.1 Hz, 4H, Ar-H), 6.97 (d, J = 7.8 Hz, 2H, Ar-H), 6.78−6.66 (m, 7H, Ar-H), 3.95 (s, 2H, NH2), 3.22 (s, 6H, NCH3). 13C NMR (100 MHz, CDCl3) δ 148.82, 141.77, 135.18, 129.17, 125.91, 118.66, 117.89, 113.71, 38.87 (NCH3). Elemental analysis: calcd. (%) for C20H21N3: C, 79.17; H, 6.98; N, 13.85. Found: C, 79.20; H, 6.95; N, 13.82. HRMS (m/z): calcd. for C20H21N3: 303.1735, found: 304.1806 [M + H]+. 2,6-Di(N-methyl-2,6-diisopropylanilino)aniline (A5). The synthesis of A5 followed the same procedure as that for A4. The reaction of 2,6-di(N-methyl-2,6-diisopropylanilino)nitrobenzene (5.78 g, 11.53 mmol) and zinc (3.69 g, 57.65 mmol) in methanol (28.8 mL) with acetic acid glacial (2.88 mL, 155 mmol) gave A5 (3.70 g, 68% yield). 1H NMR (400 MHz, CDCl3) δ 7.21 (dd, J = 8.6, 6.6 Hz, 2H, Ar-H), 7.14 (d, J = 7.6 Hz, 4H, Ar-H), 6.57 (d, J = 7.9 Hz, 1H, Ar-H), 6.55−6.31 (m, 2H, Ar-H), 3.52−3.04 (m, 12H, NH2, CHCH3, NCH3), 1.22 (d, J = 7.1 Hz, 12H, CHCH3), 1.06 (d, J = 7.0 Hz, 12H, CHCH3). 13C NMR (100 MHz, CDCl3) δ 147.84, 144.17, 139.07, 130.05, 126.93, 124.58, 118.09, 112.59, 42.58 (NCH3), 28.25 (CHCH3), 25.12 (CHCH3), 23.66 (CHCH3). Elemental analysis: calcd. (%) for C32H45N3: C, 81.48; H, 9.62; N, 8.91. Found: C, 81.51; H, 9.60; N, 8.93. HRMS (m/z): calcd. for C32H45N3: 471.3613, found: 472.3685 [M + H]+. Synthesis of the Ligands L1−L5. The ligands L1−L5 were prepared in a similar manner by the reaction of the suitable 2E

DOI: 10.1021/acs.organomet.8b00788 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

calcd. (%) for C31H50N4O3Si2Zn: C, 57.43; H, 7.77; N, 8.64. Found: C, 57.40; H, 7.68; N, 8.51. The synthesis of Zn4 followed the same procedure as that for Zn1, and L4 (0.23 g, 0.25 mmol) was used to afford the desired product (0.16 g, 91% yield). 1H NMR (400 MHz, CDCl3) δ 7.97 (s, 1H, CHN), 7.34−7.28 (m, 1H, Ar-H), 7.25−7.16 (m, 7H, Ar-H), 6.88 (t, J = 7.3 Hz, 2H, Ar-H), 6.82 (d, J = 8.2 Hz, 4H, Ar-H), 6.30 (t, J = 7.5 Hz, 1H, Ar-H), 6.20 (dd, J = 7.8, 1.9 Hz, 1H, Ar-H), 3.23 (s, 6H, NCH3), 1.40 (s, 9H, CCH3), −0.06 (s, 18H, SiCH3). 13C NMR (100 MHz, CDCl3) δ 172.42 (CHN), 170.97, 150.12, 143.05, 142.32, 142.01, 134.22, 132.51, 129.31, 128.14, 125.38, 121.05, 118.57, 117.44, 113.94, 41.33 (NCH3), 35.36 (CCH3), 29.46 (CCH3), 5.35 (SiCH3). MALDI-TOF-MS (m/z): calcd. For C37H50N4O2Si2Zn: 686.2815. Found: 526.1803 [M − N(TMS)2]. Elemental analysis: calcd. (%) for C37H50N4O2Si2Zn: C, 64.56; H, 7.32; N, 8.14. Found: C, 64.63; H, 7.36; N, 8.10. The synthesis of Zn5 followed the similar procedure as that for Zn1, and L5 (0.60 g, 0.80 mmol) was used to afford the desired product (0.50 g, 94% yield). 1H NMR (400 MHz, CDCl3) δ8.43 (s, 2H, CHN), 7.30−7.15 (m, 14H, Ar-H), 7.06 (dd, J = 7.5, 1.4 Hz, 2H, Ar-H), 6.97 (dd, J = 7.9, 1.4 Hz, 2H, Ar-H), 6.54 (dd, J = 14.6, 7.8 Hz, 4H, Ar-H), 5.76−5.70 (m, 2H), 5.62 (dd, J = 8.3, 0.8 Hz, 2H), 3.61 (s, 6H, NCH3), 3.51 (s, 6H, NCH3), 3.23 (dt, J = 13.8, 6.9 Hz, 2H, CHCH3), 3.02 (dt, J = 13.8, 6.9 Hz, 2H, CHCH3), 2.86 (dt, J = 13.6, 6.8 Hz, 2H, CHCH3), 2.74 (dt, J = 13.4, 6.7 Hz, 2H, CHCH3), 1.24 (dd, J = 14.8, 6.8 Hz, 12H, CHCH3), 1.12 (s, 18H, CCH3), 1.01 (dd, J = 15.1, 6.8 Hz, 12H, CHCH3), 0.94 (d, J = 6.8 Hz, 12H, CHCH3), 0.89 (d, J = 6.8 Hz, 6H, CHCH3), 0.77 (d, J = 6.9 Hz, 6H, CHCH3). 13C NMR (100 MHz, CDCl3) δ 178.96 (CHN), 172.52, 149.12, 148.70, 147.12, 147.00, 146.46, 144.82, 144.76, 144.59, 142.42, 134.37, 132.74, 131.65, 127.57, 127.33, 126.67, 125.17, 125.01, 124.66, 124.23, 118.59, 114.03, 110.88, 110.60, 45.92 (NCH3), 44.63 (NCH3), 35.01 (CCH3), 29.42 (CCH3), 28.69, 28.43, 28.31, 28.11, 25.41, 25.24, 24.82, 24.69, 24.42, 24.35, 23.60, 23.30. MALDI-TOF-MS (m/z): calcd. For C86H112N6O2Zn: 1324.8138. Found: 695.3103 [M − L5]. Elemental analysis: calcd. (%) for C86H112N6O2Zn: C, 77.83; H, 8.51; N, 6.33. Found: C, 77.69; H, 8.45; N, 6.37. The synthesis of Zn6 followed the similar procedure as that for Zn1, and L6 (0.60 g, 0.80 mmol) was used to afford the desired product (0.26 g, 87% yield). 1H NMR (400 MHz, CDCl3) δ 8.70 (s, 1H, CHN), 8.58 (dd, J = 7.7, 1.5 Hz, 6H, Ar-H), 8.36−8.28 (m, 10H, Ar-H), 8.25−8.17 (m, 6H, Ar-H), 8.12 (d, J = 7.7 Hz, 1H, ArH), 7.88−7.86 (m, 2H, Ar-H), 7.73 (d, J = 7.8 Hz, 4H, Ar-H), 7.29− 7.25 (m, 2H, Ar-H), 4.20 (s, 6H, NCH3), 0.62 (s, 18H, SiCH3). 13C NMR (100 MHz, CDCl3) δ 175.70 (CHN), 172.60, 150.92, 146.41, 142.80, 141.81, 138.76, 136.52, 136.01, 129.35, 128.99, 128.39, 127.61, 124.53, 121.22, 117.82, 117.49, 115.10, 41.79 (NCH3), 4.93 (SiCH3). Elemental analysis: calcd. (%) for C51H56N4OSi3Zn: C, 68.77; H, 6.34; N, 6.29. Found: C, 68.59; H, 6.47; N, 6.20. Synthesis of the Aluminum Complex Al6. AlMe3 (0.6 mL, 2 M in toluene, 1.2 mmol) was added dropwise to a stirred solution of L6 (0.665 g, 1 mmol) in toluene (10 mL) at room temperature. The mixture was stirred for 1 h at room temperature and heated to 100 °C for 12 h. The resulting solution was cooled to room temperature and filtered. Solvents were removed under vacuum. Recrystallization of the residue from diethyl ether formed yellow crystals of Al6 (0.50 g, 70% yield). 1H NMR (400 MHz, CDCl3) δ 7.87 (s, 1H, CHN), 7.58 (br, 6H, Ar-H), 7.36 (br, 12H, Ar-H), 7.18 (br, 7H, Ar-H), 6.81 (br, 2H, Ar-H), 6.67 (br, 5H, Ar-H), 6.52 (s, 1H, Ar-H), 3.01 (s, 6H, NCH3), −1.32 (s, 6H, AlCH3). 13C NMR (100 MHz, CDCl3) δ 174.17, 170.36, 149.13, 147.14, 144.21, 143.24, 137.56, 136.54, 134.97, 129.30, 129.19, 129.06, 127.62, 125.94, 119.32, 118.91, 117.39, 115.11, 39.59 (NCH3), −10.83 (AlCH3). MALDI-TOF-MS (m/z): calcd. for C47H44AlN3OSi: 721.3069. Found: 706.2202 [M − Me]. Elemental analysis: calcd. (%) for C47H44AlN3OSi: C, 78.19; H, 6.14; N, 5.82. Found: C, 78.25; H, 6.11; N, 5.85. X-ray Crystallography. Suitable crystals of complexes Zn2, Zn5, and Al6 were obtained by layering n-hexane onto their dichloro-

23.94. Elemental analysis: calcd. (%) for C43H57N3O: C, 81.73; H, 9.09; N, 6.65. Found: C, 81.75; H, 9.12; N, 6.67. HRMS (m/z): calcd. for C43H57N3O: 631.4502, found: 632.4569 [M + H]+. Synthesis of the Ligand L6. p-Toluenesulfonic acid (PTSA) (10 mg, 0.06 mmol) was added to a stirred solution of 2-hydroxy-3triphenylsilylbenzaldehyde (0.38 g, 1 mmol) and A4 (0.30 g, 1 mmol) in toluene (20 mL). The mixture was refluxed for 24 h, and then the solvent was removed. The crude product was purified by chromatography on silica gel with petroleum ether/ethyl acetate/ triethylamine) (v/v/v = 500:10:3) to afford a yellow solid (0.54 g, 81% yield). 1H NMR (400 MHz, CDCl3) δ 12.98 (s, 1H, OH), 8.00 (s, 1H, CHN), 7.56 (d, J = 6.8 Hz, 6H, Ar-H), 7.41 (t, J = 7.2 Hz, 3H, Ar-H), 7.34 (t, J = 7.2 Hz, 6H, Ar-H), 7.27−7.06 (m, 9H, Ar-H), 6.92−6.87 (m, 1H, Ar-H), 6.66 (q, J = 7.1 Hz, 3H, Ar-H), 6.59 (d, J = 8.2 Hz, 4H, Ar-H), 3.09 (s, 6H, NCH3). 13C NMR (100 MHz, CDCl3) δ 166.84 (CHN), 166.19, 149.53, 144.56, 142.25, 141.46, 136.54, 134.84, 134.74, 129.37, 129.00, 127.72, 127.07, 126.25, 121.81, 118.59, 118.09, 118.04, 114.27, 39.43 (NCH3). Elemental analysis: calcd. (%) for C45H39N3OSi: C, 81.17; H, 5.90; N, 6.31. Found: C, 81.15; H, 5.93; N, 6.35. HRMS (m/z): calcd. for: C45H39N3OSi: 665.2862, found: 666.2935 [M + H]+. Synthesis of the Zinc Complexes Zn1−Zn6. All zinc complexes were prepared in a similar manner by the reactions of the corresponding ligand with 1.2 equiv of the Zn[N(SiMe3)2]2 using CH2Cl2 as solvent at room temperature. A typical synthetic procedure of Zn1 is as follows: From the reaction between Zn[N(SiMe3)2]2 (0.116 g, 0.30 mmol) and L1 (0.098 g, 0.25 mmol) in CH2Cl2 (15 mL) at room temperature for 24 h, the resulting solution was filtered. Solvents were removed under vacuum. The crude product was washed with n-hexane and then dried under vacuum to give the desired product Zn1 (0.13 g, 86% yield). 1 H NMR (400 MHz, CDCl3) δ 9.20 (s, 1H, CHN), 7.40 (dd, J = 7.3, 1.5 Hz, 1H, Ar-H), 7.27 (t, J = 8.1 Hz, 1H, Ar-H), 7.09 (d, J = 8.2 Hz, 2H, Ar-H), 7.04 (dd, J = 7.7, 1.5 Hz, 1H, Ar-H), 6.59 (t, J = 7.6 Hz, 1H, Ar-H), 3.94 (m, 8H, NCH2), 3.05 (m, 4H, NCH2CH2), 2.98 (m, 4H, NCH2CH2), 1.48 (s, 9H, CCH3), −0.05 (s, 18H, SiCH3). 13 C NMR (100 MHz, CDCl3) δ 171.48 (CHN), 171.01, 144.54, 142.85, 135.92, 133.99, 132.83, 127.73, 119.17, 117.21, 114.41, 66.91, 52.43, 35.55 (CCH3), 29.60 (CCH3), 6.02 (SiCH3). MALDI-TOFMS (m/z): calcd. for C31H50N4OSi2Zn: 614.2815; 455.1368 [M − N(TMS)2]. Elemental analysis: calcd. (%) for C31H50N4OSi2Zn: C, 60.41; H, 8.18; N, 9.09. Found: C, 60.22; H, 8.15; N, 9.20. The synthesis of Zn2 followed the same procedure as that for Zn1, and L2 (0.63 g, 1.5 mmol) was used to afford the desired product (0.74 g, 77% yield). 1H NMR (400 MHz, CDCl3) δ 9.30 (s, 1H, CHN), 7.40 (d, J = 6.9 Hz, 1H, Ar-H), 7.21 (t, J = 8.0 Hz, 1H, ArH), 7.07 (d, J = 8.0 Hz, 3H, Ar-H), 6.57 (t, J = 7.5 Hz, 1H, Ar-H), 2.94 (m, J = 16.8 Hz, 8H, NCH2), 1.85 (m, J = 19.6 Hz, 8H, NCH2CH2), 1.61 (m, 4H), 1.52 (s, 9H, CCH3), −0.03 (s, 18H, SiCH3). 13C NMR (100 MHz, CDCl3) δ 170.93 (CHN), 170.79, 145.89, 142.62, 135.70, 134.17, 132.11, 127.14, 119.44, 116.90, 113.67, 53.89, 35.55 (CCH3), 29.70 (CCH3), 26.15, 23.66, 5.89 (SiCH3).MALDI-TOF-MS (m/z): calcd. for C33H54N4OSi2Zn: 642.3128. Found: 482.2171 [M − N(TMS)2]. Elemental analysis: calcd. (%) for C33H54N4OSi2Zn: C, 61.51; H, 8.45; N, 8.69. Found: C, 61.34; H, 8.40; N, 8.45. The synthesis of Zn3 followed the same procedure as that for Zn1, and L3 (0.64 g, 1.5 mmol) was used to afford the desired product (0.81 g, 84% yield). 1H NMR (400 MHz, CDCl3) δ 7.28 (d, J = 7.3 Hz, 1H), 7.14 (t, J = 8.0 Hz, 1H, Ar-H), 6.92 (d, J = 6.8 Hz, 1H, ArH), 6.76 (d, J = 8.2 Hz, 1H, Ar-H), 6.56 (d, J = 7.6 Hz, 1H, Ar-H), 6.48 (t, J = 7.4 Hz, 1H, Ar-H), 4.99 (s, 1H), 4.49 (m, J = 12.3 Hz, 1H), 3.64 (m, J = 12.1 Hz, 3H), 3.52 (m, 2H), 3.36 (m, J = 3.4 Hz, 1H), 3.22 (m, J = 6.9 Hz, 1H), 2.13 (m, J = 16.0, 6.7 Hz, 4H), 1.74 (m, 4H), 1.48 (s, 9H, CCH3), −0.07 (d, J = 13.9 Hz, 18H, SiCH3). 13 C NMR (100 MHz, CDCl3) δ 166.36, 148.32, 142.73, 139.24, 137.76, 129.45, 129.14, 127.82, 121.34, 114.05, 112.97, 109.22, 89.83, 65.25, 58.95, 50.58, 35.18 (CCH3), 30.15 (CCH3), 24.73, 23.01, 6.19 (SiCH3).MALDI-TOF-MS (m/z): calcd. For C31H50N4O3Si2Zn: 646.2713. Found: 487.1290 [M − N(TMS)2]. Elemental analysis: F

DOI: 10.1021/acs.organomet.8b00788 Organometallics XXXX, XXX, XXX−XXX

Organometallics

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methane solution at room temperature under nitrogen, respectively. Data collections were performed on a Bruker SMART APEX diffractometer with a CCD area detector, using graphite monochromated Mo Kα radiation (λ = 0.71073 Å) (for Zn2 and Zn5) and mirror-monochromated Cu KR radiation (λ = 1.54184 Å) (for Al6). The determination of crystal class and unit cell parameters was carried out by 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. Refinement was performed on F2 anisotropically for all non-hydrogen atoms by the full-matrix least-squares method. The hydrogen atoms were placed at the calculated positions and were included in the structure calculation without further refinement of the parameters. Details of the crystal data and structure refinements for complexes Zn2, Zn5, and Al6 are listed in Table S1. Polymerizations. Polymerizations were carried out in a 15 mL Schlenk tube equipped with a magnetic stirrer. A typical kinetic study procedure was exemplified by the polymerization of 200 equiv of monomer using zinc and aluminum complex as a catalyst. After drying the Schlenk tube under a N2 atmosphere, monomer and solvent were added successively into the Schlenk tube. Then the Schlenk tube was placed in an oil bath, the temperature of which was preset at specific polymerization temperatures. The catalyst solution was injected into a stirred monomer solution in solvent. The solution was stirred for a specific time, and the reaction was quenched by adding 1.5 equiv of PhCO2H into the reaction mixture. A small amount of sample was taken from the mixture to measure the conversion of monomer by 1H NMR analyses. The remaining sample was dropped into n-hexane to form white precipitates. The precipitates were collected by filtration and dried under vacuum to give a white solid for GPC analysis.

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REFERENCES

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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.8b00788. Experimental details and crystallographic and spectroscopic data (PDF) Accession Codes

CCDC 1559583, 1860424, and 1877347 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]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.W.). *E-mail: [email protected] (C.T.). ORCID

Min Chen: 0000-0003-3992-5759 Fuzhou Wang: 0000-0003-0470-7110 Chen Tan: 0000-0002-5398-2445 Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC, 21801002). G

DOI: 10.1021/acs.organomet.8b00788 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

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DOI: 10.1021/acs.organomet.8b00788 Organometallics XXXX, XXX, XXX−XXX