Next Article in Journal
Preparation and Thermal Properties of Magnetic PW@CaCO3@Fe3O4 Phase-Change Microcapsules and Their Application to Textile Fabrics
Next Article in Special Issue
Ethylene/Styrene Copolymerization by (Me3SiC5H4)TiCl2(O-2,6-iPr2-4-RC6H2) (R = H, SiEt3)-MAO Catalysts: Effect of SiMe3 Group on Cp for Efficient Styrene Incorporation
Previous Article in Journal
Design, Synthesis, and Acaricidal Activity of 2,5-Diphenyl-1,3-oxazoline Compounds
Previous Article in Special Issue
Influence of Polytetrafluoroethylene Content, Compaction Pressure, and Annealing Treatment on the Magnetic Properties of Iron-Based Soft Magnetic Composites
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Molecules
Volume 29
Issue 17
10.3390/molecules29174150
molecules-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Intensive Cycloalkyl-Fused Pyridines for Aminopyridyl–Zinc–Heteroimidazoles Achieving High Efficiency toward the Ring-Opening Polymerization of Lactides

by
Yun Wang
1,2,
Wenjuan Zhang
1,*,
Pengjiang Zhu
1,2,
Wei You
2,
Xiaopan Xue
1,2,
Rui Wang
1,
Yanping Ma
2 and
Wen-Hua Sun
2,*
1
Beijing Key Laboratory of Clothing Materials R&D and Assessment, Beijing Engineering Research Center of Textile Nanofiber, School of Materials Science and Engineering, Beijing Institute of Fashion Technology, Beijing 100029, China
2
Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
*
Authors to whom correspondence should be addressed.
Molecules 2024, 29(17), 4150; https://doi.org/10.3390/molecules29174150
Submission received: 28 July 2024 / Revised: 23 August 2024 / Accepted: 30 August 2024 / Published: 31 August 2024
(This article belongs to the Special Issue Organometallic Compounds: Design, Synthesis and Application)
Browse Figures
Versions Notes

Abstract

:
The model precatalyst sp3- and sp2-N dinitrogen-coordinated zinc–heteroimidazole has been used as an efficient catalyst for the ring-opening polymerization of cyclic esters. Subsequent to our exceptional active 5,6,7-trihydroquinolin-8-amine-zinc catalysts for the ring-opening polymerization (ROP) of ε-caprolactone, various pyridine-fused cycloalkanones (ring size from five to eight) are developed for the correspondent fused amine–pyridine derivatives and their zinc–heteroimidazole chloride complexes Zn1–Zn8 (LZnCl2) bearing N-diphenylphosphinoethyl pendants. Activated with two equivalents of LiN(SiMe3)2, the title zinc complexes efficiently promote the ROP of L-lactide (L-LA) in situ; among them, Zn4/2Li(NSiMe3)2 catalyzed 500 equivalent L-LA at 80 °C with 92% conversion in 5 min (TOF: 5520 h−1). Under the same conditions, the catalytic efficiency for the ROP of rac-LA by Zn1–Zn8/2Li(NSiMe3)2 was slightly lower than that for L-LA (highest TOF: 4440 h−1). In both cases, cyclooctyl-fused pyridyl–zinc complexes exhibited higher activity than others, while the cycloheptyl-fused zinc complexes showed the lowest activity. The microstructure analysis of the polymers showed they possessed a linear structure capped with CH3O as major and cyclic structure as minor. In this work, all the ligands and zinc complexes were well characterized by 1H/13C/31P NMR, FT-IR spectroscopy as well as elemental analysis.

1. Introduction

Aliphatic polyesters, such as polylactide (PLA) and polycaprolactone (PCL), being environment-friendly materials with good biocompatibility and biodegradability, have attracted considerable attention in the past decades [1,2]. The polycondensation of lactic acid and the ROP of lactides are two major processes in the formation of PLAs [3]. The molecular weights of the PLAs obtained through condensation are generally low, so ROP is extensively investigated for PLA production in order to take advantage of their useful properties [4]. Metal-complex precatalysts lead to the ROP of lactides(LA) with not only high efficiency but also the formation of well-defined PLAs [5,6,7]. Currently, tin(II) 2-ethylhexanoate is the most popular precatalyst for the industrial production of PLAs via ROP [8]; however, due to the potential cytotoxicity of tin, there are concerns about its use for PLA production, which is a limitation [9,10]. Therefore, the development of new metal complex catalysts for the ROP of LA based on non-toxic and environmental-friendly metals is in high demand [11,12,13,14]. As an essential element in the human body, as well as having low toxicity for animals and a low cost [15,16], zinc complexes have become a promising candidate for the ROP of cyclic esters. Targeting efficient zinc catalysts for the ROP of LA, various ligands have been explored such as β-diketiminate [17,18,19,20], phenolate [21,22,23,24,25,26], Schiff bases [27,28,29,30,31], bis(pyrazolyl)amide [32,33], N-heterocyclic carbenes [34,35,36] and guanidines [37,38,39]. The stereochemical attributes of the ligands are useful factors in controlling the micro-structure and properties of the resultant PLAs [40].
As described above, the ligands of zinc-complex precatalysts commonly contain nitrogen and/or oxygen donors [17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40]. In addition, there have been a few reports on the ROP of LA with zinc complexes containing a phosphorus group. For example, zinc complex 1 (Scheme 1), including a bis(phosphinimino)methyl ligand, catalyzed 100 equivalent rac-LA with a > 95% conversion rate within 2 h in toluene at 60 °C [41]. A series of cationic zinc complexes (2 and 3, Scheme 1) supported by bis(phosphinimine) ligands performed well in the ROP of rac-LA [42,43,44,45]. Zinc complex 2 (Scheme 1) achieved the ROP of rac-LA with 90% conversion in 50 min at ambient temperature at the molar ratio of [rac-LA]/[2] = 200:1, characterized by a slightly hetero-enriched microstructure [polymer tacticity (Pr) = 0.63] [42]. Meanwhile, the cationic alkyl zinc complexes rac-3 and meso-3 (Scheme 1), as isolatable stereoisomers, individually catalyzed 1000 equivalent rac-LA monomers at 40 °C, resulting in PLAs with 89.6% conversion by rac-3 in 4 h and 95.3% conversion by meso-3 in 3.8 h, more importantly observing a higher molecular weight in PLA of 3.90 × 105 g/mol [45]. Moreover, zinc complex 4 (Scheme 1), bearing a diarylphosphido-diphosphine, also efficiently initiated the ROP of L-LA with a 91% yield in the presence of isopropyl alcohol in toluene within 20 min with [L-LA]/[4] = 200 [46]. The extensive copolymerization of L-LA and ε-caprolactone (ε-CL) proceeded smoothly at 110 °C for random copolymers [47]. Another P^O-phosphinophenolate zinc complex 5 (Scheme 1) polymerized 100 equivalents of rac-LA with a 97% conversion rate in CH2Cl2 after 2 h and showed a good capability of catalyzing the homo(co)-polymerization of L-LA or ε-CL and trimethylene carbonate (TMC), achieving PCL/PTMC-co-PLLA co-polymers and PTMC-co-PCL-co-PLLA terpolymers [48].
Using the soft donor abilities of phosphorus-based ligands [49,50], a series of zinc complexes containing N^N^P ligands (6, Scheme 1) were evaluated for the ROP of ε-CL [51], achieving 90% conversion at high molar ratio of [ε-CL]/[Zn] = 5000:1 within 2 min, indicating a remarkable TOF of 1.35 × 105 h−1; meanwhile, their iron analogues also performed well for the ROP of ε-CL (TOF up to 8.82 × 103 h−1), producing high-molecular-weight PCL (Mn up to 2.43 × 105 g/mol), demonstrating an exceptional model of iron-complex precatalysts [52]. Subsequently, a series of zinc complexes bearing cycloalkyl-fused pyridines with ring sizes tuned from five to eight within their backbones are prepared and achieve good activity for the ROP of lactides (L-LA and rac-LA) under different parameters (time, temperature, solvent or molar ratio of monomer), indicating the significant influence of the ring size of fused cycloalkyls and of the R group on the ortho-position of pyridine on catalytic performance.

2. Results and Discussion

2.1. Synthesis and Characterization of L1–L8 and Their Zn (II) Complexes Zn1–Zn8

A series of ligands L1–L8 of monocycloalkyl (ring size from five to eight)-fused pyridine-bearing pendant N-diphenylphoshinoethyl groups, N-(Ph2PCH2CH2)-2-RC7H7N-8-NH (R = H L1, R = Ph L2, R = Me L3), N-(Ph2PCH2CH2)-2-RC9H9N-8-NH (R = H L4, R = Ph L5), N-(Ph2PCH2CH2)-2-RC10H11N-8-NH (R = H L6, R = Ph L7), N-(Ph2PCH2CH2)-2-RC11H13N-8-NH (R = H L8), were prepared according to the literature (as shown in Scheme 2) [53,54,55]. The Zn(II) complexes Zn1–Zn8 were obtained by treating the ligands L1–L8 with zinc(II) chloride in ethanol at room temperature for 12 h, respectively. All novel compounds were confirmed by 1H, 13C and 31P NMR (shown in Figures S1–S36), FT-IR spectroscopy and elemental analysis. The 31P NMR spectrum of ligands showed one signal around −20 ppm, while their corresponding zinc complexes showed a shift around −22 ppm. These little shifts changes in the 31P NMR spectra indicated little environmental change around phosphine, and also suggesting the possible dissociation of phosphine to zinc. Subsequently, the structures of Zn2, Zn3, Zn7 and Zn8 were further determined by single-crystal X-ray diffraction, which agreed well with their 31P NMR spectra.
The single crystals of Zn2, Zn3, Zn7 and Zn8 suitable for X-ray determination were obtained by diffusing diethyl ether into a dichloromethane solution. Their crystal structures are depicted in Figure 1, Figure 2, Figure 3 and Figure 4, respectively, and the selected bond lengths and angles are summarized in Table 1. The molecular structures of Zn2, Zn3, Zn7 and Zn8 are similar, in which the zinc is four-coordinated by two nitrogen atoms and two chlorine atoms to form a distorted tetrahedral geometry. Similar to our previous reports [51], the phosphorus donor did not coordinate to zinc, which agreed well with the little shift in its 31P NMR spectra.
The bond distances between the Zn (II) and Cl atoms vary between 2.2018(5) and 2.230(2) Å and the Zn-N bond lengths vary between 2.0442(13) and 2.1162(16) Å. Notably, the bond lengths of the Zn-Cl bond are longer than those of the Zn-N bond, which is in accordance with previous reports [56,57]. As shown in Table 1, the bond length of the Zn-NH bond is longer than that of the Zn-Nquinoline bond in both the Zn2 and Zn8 structures [2.1091(15) vs. 2.0777(15); 2.0920(12) vs. 2.0442(13) Å], indicating more effective coordination by Nquinoline than NH; On the contrary, the bond length of Zn-NH in Zn3 and Zn7 is shorter than that of Zn–Nquinoline. The different bond lengths between Zn-NH and Zn-Nquinoline indicate that the size of substituents and rings influence the molecular structure of the zinc complexes.
The R substituent and fused ring size also greatly affect the structure. For Zn3, Zn5 and Zn7, all bearing the same R substituent of phenyl, the dihedral angle between the phenyl ring and pyridine plane is 26.49° in Zn3 (five-ring), which is much smaller than that (43.07°) in Zn7 (seven-ring) and that (48.74°) in Zn5 (six-ring) [51], suggesting the great influence of cycloalkyl ring size. Another interesting observation is that, due to the torsion of different cycloalkyls, the distance from N(H) and P to the pyridine plane varied, as shown by 0.440 Å and 0.537 Å in Zn3, 0.703 Å and 1.365 Å in Zn5 and 0.471 Å and −3.918 Å in Zn7, respectively, suggesting the increased distortion of the Zn7 structure. In addition, regarding the distortion of the rings, the carbon C6 atoms of the cycloalkyl deviate from the pyridyl-based plane with a distance of 0.408 Å for Zn2, 0.348 Å for Zn3, 1.340 Å for Zn7 and 1.400 Å for Zn8. These different distortions will have an effect on their catalytic activity.

2.2. Ring-Opening Polymerization (ROP) of L-LA by Zn1-Zn8/2LiN(SiMe3)2

In the beginning, Zn6 was tested for the ROP of L-LA without any additional initiator, but there was no activity observed, considered the result of the stability of the Zn-Cl bond. According to our previous work [51], a binary system consisting of zinc complexes and two equivalents of LiN(SiMe3)2 exhibited extremely high activity toward the ring-opening polymerization of ɛ-CL in situ; here, a similar system was explored for the ROP of L-LA. Firstly, the effect of LiN(SiMe3)2 on the ROP of L-LA was investigated by using Zn6 in toluene at a molar ratio of [LA]:[Zn] = 250:1 and within 10 min at 30 °C. The results showed that no polymer was obtained when using one equivalent LiN(SiMe3)2. However, when increasing the LiN(SiMe3)2 from one to two equivalents, the monomer conversion could reach 36% under the same conditions. In addition, the blank experiments showed 10% conversion of monomers was achieved using only two equivalents of LiN(SiMe3)2 as catalysts. Therefore, a catalyst system of Zn6/2LiN(SiMe3)2 was employed for the ROP of L-LA, and the polymerization results are collected in Table 2.
In the first instance, the effect of benzyl alcohol (BnOH) on polymerization was investigated with the molar ratio of [L-LA]:[Zn] = 250:1 at a temperature kept at 30 °C (runs 1–3, Table 2). The results indicated that the monomer conversion decreased from 36% to 11% when one equivalent benzyl alcohol was added. Then, the amount of benzyl alcohol was further increased to three equivalents, and the monomer conversion continued to decrease to only 8%. These results revealed that benzyl alcohol could not improve the monomer conversion in this catalytic system. In order to enhance the monomer conversion, the polymerization was screened from 10 min to 60 min under the same conditions (runs 1 and 4–6, Table 2). The results showed that monomer conversion greatly increased from 36% in 10 min to 86% in 60 min with the extension of time, but the molecular weight decreased from 3.99 × 104 to 1.65 × 104 g/mol and the polydispersity became broader, suggesting the increased side reaction of transesterification during longer times. The comparison of the Mn values with the calculated values [Mn(calcd)] shows large differences, which indicates that the polymerizations are not well controlled [59]. The kinetics of polymerization by Zn6, shown as a semilogarithmic plot, slightly deviates from the first order with the polymerization rate constant kapp as 0.03177 min−1 (LA/Zn = 250:1) or 0.02294 min−1 (LA/Zn = 250:0.5) (Figure S37 in Supplementary Materials).
Secondly, the molar ratio of [L-LA]:[Zn] was fixed at 250:1 and the run time at 10 min, and the influence of temperature on polymerization was also investigated (runs 1 and 7–10, Table 2). As the reaction temperature increased from 30 °C to 80 °C, the monomer conversion rate also increased gradually from 36% to 100%, indicating that higher temperatures favored the increase in the polymerization rate. At the same time, the polymer obtained from 50 °C and 80 °C possessed similar Mn values (Mn = 2.66–2.83 × 104 g/mol) and molecular weight distributions (Mw/Mn = 1.87–2.08); within this temperature range, the Mn values were all lower than the Mn(calcd) values. This may be explained by the efficiency of active species and the effect of transesterification during polymerization [60]. Moreover, the molecular weight distribution of all obtained polymers was broad, suggesting more side reactions of transesterification at higher temperatures [61,62,63].
Subsequently, in order to explore the role of various solvents for the ROP of L-LA, the polymerizations were carried out in dichloromethane at 30 °C with THF and 60 °C with n-hexane (runs 11–13, Table 2). When THF was used, high monomer conversion of up to 92% was achieved at [L-LA]:[Zn] = 250:1 within 10 min. However, when dichloromethane and n-hexane were used, the monomer conversion was extremely low, at 0 and 8%, respectively. The reason for this may be related to the coordination competition between the monomer and solvent for the active zinc centers [64,65].
Fixing the temperature at 80 °C and run time at 10 min, increasing the molar ratio of [L-LA]:[Zn] from 250:1 to 1000:1 led to a slightly decrease in monomer conversion from 100% to 89% (runs 10, 14, 15, Table 2), but the molecular weights of the polymer were very similar (from 2.46 to 2.66 × 104 g/mol), which were much lower than Mn(calcd). Furthermore, the effect of the amount of toluene was also explored at a high molar ratio of [L-LA]:[Zn] = 1000:1 in 10 min at 80 °C (runs 15–17, Table 2). When increasing the toluene volume from 1 mL to 5 mL, the monomer conversion gradually decreased from 89% to 65% and the molecular weight of the polymer slightly decreased from 2.61 × 104 g/mol to 2.02 × 104 g/mol. This may be attributed to the deactivation of some active species caused by more impurities in toluene [66]. Moreover, the reaction time was reduced from 10 min to 5 min at 80 °C with the molar ratio of [L-LA]:[Zn] = 500:1, and the monomer conversion also decreased from 96% to 63% (runs 15 vs. 18, Table 2). The average molecular weight of the obtained polymer decreased from 2.61 × 104 g/mol to 1.86 × 104 g/mol.
With the [L-LA]:[Zn] ratio kept at 500:1 and temperature and time at 80 °C and 5 min, respectively, all zinc complexes Zn1–Zn8 were evaluated for the ROP of L-LA by using pre-treatment with two equivalents of LiN(SiMe3)2, and the results are shown in runs 18, 19–25 (Table 2). The data revealed that the catalytic system Zn1–Zn8/2LiN(SiMe3)2 can efficiently catalyze the ROP of L-LA, in which the catalytic efficiency is related with the size of the substituent group (R) and the size of the pyridyl-fused ring (shown in Figure 5).
According to Figure 5, complexes Zn4 (six-ring) and Zn8 (eight-ring) show higher monomer conversion rates than the other complexes, both exceeding 90% (TOF = 5.52 × 103 h−1), much higher than those of the other zinc complexes. The R also greatly affected the activity, as shown by the following activity order: Zn1 (75%, H) > Zn2 (70%, Me) > Zn3 (44%, Ph); Zn4 (92%, H) > Zn5 (53%, Ph); Zn6 (63%, H) > Zn7 (36%, Ph). This indicated that the smaller substituents favored the polymerization. The fused ring size also greatly affected the activity, as shown by the following activity order: R = H, Zn4 (six-ring) = Zn8 (eight-ring) > Zn1 (75%, five-ring) > Zn6 (63%, seven-ring); R = Ph, Zn5 (53%, six-ring) > Zn3 (44%, five-ring) > Zn7 (36%, seven-ring). Thus, Zn7 and Zn6 displayed the lowest activity, and the reason is probably due to the more distorted fused seven-ring structures in Zn7 and Zn6. The polymer obtained by using Zn4 possessed the highest molecular weight (Mn = 3.9 × 104 g/mol).
In order to gather more information about the mechanism of the ROP of L-LA, the obtained PLLA (run 18, Table 2) was characterized by a 1H NMR and MALDI-TOF spectrum. The MALDI-TOF mass spectrum revealed two families of peaks (as shown in Figure 6): both A/A* and B/B* were separated by an m/z of 72, in which the major peaks A/A* can be ascribed to CH3OH + (C3H4O2)n + Na+/K+ and the minor peaks B/B* are assignable to (C3H4O2)n + Na+/K+. The 1H NMR spectrum clearly showed the presence of CH3O protons around δ 3.75 ppm (shown in Figure 7), which may come from the methanolysis of the polymer chain during the quenching process [60,67,68]. In addition, there was no signal of -N(SiMe3)2 in either the 1H NMR or MALDI-TOF spectra, which was consistent with a previous report in the literature [51].
Based on the above results, the mechanism of the ROP of L-LA catalyzed by Zn1–Zn8/2LiN(SiMe3)2 was proposed, as shown in Scheme 3. It is assumed that a Lewis pair will form during the ROP process and can be ‘loosened up’ at higher temperatures to form a zwitterionic intermediate. Subsequently, this ‘loose’ pair can initiate the polymerization, causing the ring to expand to form the cyclic macrolactone metal ring; this mechanism has also been reported in the literature [51,69,70]. Finally, methanol was added to terminate the polymerization reaction, and a linear polymer capped with a methoxyl group was obtained [71]. Furthermore, its minor cyclic structure may be due to intramolecular transesterification.

2.3. Ring-Opening Polymerization of rac-LA by Zn1–Zn8/2LiN(SiMe3)2

Under the conditions of the molar ratio of the monomer to the catalyst of 500:1, polymerization temperature of 80 °C and reaction time of 5 min, the catalytic performance of Zn1–Zn8/2LiN(SiMe3)2 for the ROP of rac-LA was also investigated. The polymerization results are shown in Table 3.
The results showed that Zn1–Zn8/2LiN(SiMe3)2 also displayed good efficiency for the ROP of rac-LA but without stereoselectivity and all the resultant polymers were amorphous. Both the substituent and ring size of the ligand greatly influenced the activity, and the order of activity is as follows: Zn1 (59%, R = H) > Zn2 (53%, R = Me) > Zn3 (52%, R = Ph); Zn4 (71%, R = H) > Zn5 (53%, R = Ph); Zn6 (46%, R = H) > Zn5 (18%, R = Ph). Zn8, bearing a small substituent group with an eight-membered ring, exhibited the highest activity (TOF = 4.44 × 103 h−1), higher than Zn1–Zn7 (TOF range: 1.08–4.26 × 103 h−1). Additionally, the catalytic efficiency of the Zn/2LiN(SiMe3)2 catalytic system for the ROP of rac-LA was generally lower than that for the ROP of L-LA (Figure 8).
The microstructure of poly(rac-LA) (run 1, Table 3) was also studied by MALDI-TOF and 1H NMR spectra. The MALDI-TOF mass spectrometry showed two families of A/A* and B/B* (as shown in Figure S38), in which the major family peaks A/A* were attributed to CH3OH + (C3H4O2)n + Na+/K+, as the visible linear structure with a methoxy end group, while the minor family peaks B/B* were assigned as the cyclic structure (C3H4O2)n + Na+/K+. According to the 1H NMR spectrum (as shown in Figure S39), the signal at around 3.75 ppm of the CH3O protons appeared to further confirm the MALDI-TOF analysis. Therefore, the mechanism is similar to that for the ROP of L-LA. All the obtained polymers were sticky. In order to exemplify their stereoselectivity, they were further characterized by using decoupling 1H NMR (shown in Figure S40) and results showed that their Pm values varied approximately between 0.30 and 0.35.

3. Materials and Methods

3.1. General Considerations

All operations of moisture- and air-sensitive chemicals were carried out under an atmosphere of high-purity nitrogen using standard Schlenk techniques and a N2-filled glovebox. LiN(SiMe3)2 (1.0 M in THF) was purchased from Aldrich (San Diego, CA, USA) and used as received. Toluene, n-hexane, tetrahydrofuran and diethyl ether were dried by refluxing over sodium benzophenone, distilled under N2 and stored over activated molecular sieves (4 Å) for 24 h in a glovebox prior to use. Dichloromethane was dried over CaH2 for 48 h, distilled under N2 and stored over activated molecular sieves (4 Å) in a glovebox prior to use. L-lactide and rac-lactide were purchased from TCI and used as received without further purification. And other reagents were obtained from common commercial suppliers.
All NMR spectra were recorded with Bruker Avance III 400 or Bruker Avance III 400 HD spectrometers (400 MHz for 1H NMR, 100 MHz for 13C NMR) at room temperature. The chemical shifts were related to the position of the NMR signal using tetramethylsilane (TMS) as a reference. And 31P NMR spectra were measured on a Bruker Avance II + 400 instrument. Elemental analyses were performed with a Flash EA 1112 microanalyzer, while IR spectra were conducted using a Perkin-Elmer System 2000 FT-IR spectrometer (Thermo Flash Smart, Beijing, China). MALDI-TOF MS analysis was performed with a Bruker Ultraflex mass spectrometer. For MALDI MS analysis, mass spectra were acquired with a SmartBeam laser (355 nm, Bruker, Bremen, Germany) operating at 200 Hz with a laser focus of 50 μm. The device parameters for MALDI MS were chosen as follows: plate offset voltage, 19 kV; deflector detector voltage, 20 kV. Data were processed using DateAnalysis 3.0 (Bruker Daltonics,). GPC analysis was performed using a system composed of a 390-LC multidetector (MDS), a 209-LC pump injection module (PIM) and a PL-GPC 50 plus instrument. THF was used as the eluent (flow rate: 1 mL/min, at 40 °C) and the molecular weights and molecular weight distributions were calculated using polystyrene as the standard.

3.2. Synthesis and Characterization of Ligands and Zinc Complexes

3.2.1. 2-(Diphenylphosphino)ethanamine, Ligands (L1–L8) and Zinc Complexes (Zn4, Zn5) Were Synthesized as Previously Reported [51,53,54,55]

2-(diphenylphosphino)ethanamine. In a 250 mL double-mouth round-bottom flask filled with nitrogen, tBuOK (12.03 g, 105 mmol) and diphenylphosphine (9.63 g, 52.6 mmol) were dissolved in 100 mL dry THF and the solution was stirred for 1 h at ambient temperature. Then, 2-chloroethylamine hydrochloride (6.58 g, 55.6 mmol) was added to the mixture and heated to reflux for 20 h at 80 °C. During the reaction. After cooling to room temperature, THF was removed under reduced pressure. First, 10% HCl was added to make the solution strongly acidic, and then the solution was washed three times with toluene. Secondly, the mixture was extracted three times with toluene after beingneutralized with 10% NaOH. Finally, the resulting toluene solution was washed three times with saturated NaCl and dried with MgSO4 to give a yellow oily liquid (9.4 g, 77% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 7.47–7.38 (m, 4H), 7.35–7.28 (m, 6H), 2.88–2.80 (m, 2H), 2.24 (t, J = 8.0 Hz), 1.57 (s, 2H). 13C NMR (100 MHz, CDCl3, TMS): δ 137.16, 13.64, 132.81, 132.62, 130.78, 130.66, 128.96, 128.84, 128.77, 128.58, 128.51, 38.81, 38.59, 30.91, 30.78. 31P NMR (162 MHz, CDCl3, TMS): δ 22.03.
N-(2-(diphenylphosphaneyl)ethyl)-6,7-dihydro-5H-cyclopenta[b]pyridin-7-amine (L1). 5,6-dihydro-7H-cyclopenta[b]pyridin-7-one (0.5 g, 3.85 mmol), 2-(diphenylphosphino)ethanamine (1.15 g, 5 mmol) and sodium triacetoxyborohydride (1.7 g, 8.05 mmol) were dissolved in 50 mL 1,2-dichloroethane (DCE). The rection mixture was stirred for 6 h at ambient temperature under a nitrogen atmosphere. After the reaction, the mixture was quenched with saturated NaHCO3 and the organic layer was extracted three times with ethyl acetate. Then, the mixture was dried with MgSO4 and the solvent was removed with a rotary evaporator. The crude product was further purified by chromatography on silica gel with petroleum ether/ethyl acetate (5:1, v/v) to yield the product as a green oily liquid (0.7 g, 52% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 8.37 (d, J = 4.9 Hz, 1H), 7.50 (d, J = 7.5 Hz, 1H), 7.48–7.39 (m, 4H), 7.38–7.28 (m, 6H), 7.09–7.06 (m. 1H), 4.18 (t, J = 7.1 Hz), 3.02–2.74 (m, 4H), 2.43–2.33 (m, 3H), 1.91 –1.79 (m, 1H). 13C NMR (100 MHz, CDCl3, TMS): δ 163.41, 145.81, 137. 49, 137.41, 137.37, 137.29, 135.54, 131.88, 131.70, 131.69, 131.65, 131.52, 127.63, 127.51, 127.47, 127.44, 127.40, 127.38, 121.19, 61.73, 43.97, 30.48, 28.19, 28.07, 27.12. 31P NMR (162 MHz, CDCl3, TMS): δ 20.74. FT-IR (cm−1): 3049 (m), 2934 (m), 1671 (w), 1583 (m), 1480 (m), 1432 (m), 1423 (m), 1330 (m), 1264 (m), 1182 (w), 1092 (m), 1023 (m), 867 (w), 790 (m), 736 (s), 693.18 (s). Anal. Calcd for C22H23N2P (1/2H2O): C, 74.35; H, 6.81; N, 7.88. Found: C, 73.91; H, 6.86; N, 7.57.
N-(2-(diphenylphosphaneyl)ethyl)-2-methyl-6,7-dihydro-5H-cyclopenta[b]pyridine-7-amine (L2). The synthesis of L2 followed the same procedure as that of L1, except that 2-methyl-5,6-dihydro-7H-cyclopenta[b]pyridin-7-one (1.13 g, 7,7 mmol) was used to yield the product as a green oily liquid (0.94 g, 34% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 7.45–7.37 (m. 5H), 7.33–7.26 (m, 6H), 6.92 (d, J = 7.7 Hz, 1H), 4.13 (t, J = 6.8 Hz, 1H), 2.98–2.69 (m, 4H), 2.50 (s, 3H), 2.38–2.29 (m, 3H), 2.24 (s, 1H), 1.91–1.76 (m, 1H). 13C NMR (100 MHz, CDCl3, TMS): δ 162.88, 155.54, 137.59, 137.46, 137.33, 132.27, 131.91, 131.85, 131.73, 131.65, 131.47, 127.63, 127.46, 127.42, 127.39, 127.36, 120.74, 61.87, 43.98, 43.76, 30.44, 28.27, 28.15, 26.80, 23.05. 31P NMR (162 MHz, CDCl3, TMS): δ 18.57. FT-IR (cm−1): 3049 (m), 2929 (m), 1669 (w), 1584 (m), 1480 (w), 1431 (s), 1327 (m), 1260 (m), 1223 (m), 1181 (m), 1151 (m), 1094 (m), 1023 (m), 833 (w), 798 (m), 737 (s), 691 (s). Anal. Calcd for C23H25N2P (2/3 H2O): C, 74.17; H, 7.13; N, 7.52. Found: C, 74.36; H, 7.06; N, 7.30.
N-(2-(diphenylphosphaneyl)ethyl)-2-phenyl-6,7-dihydro-5H-cyclopenta[b]pyridin-7-amine (L3). The synthesis of L3 followed the same procedure as that for L1, except that 2-phenyl-5,6-dihydro-7H-cyclopenta[b]pyridin-7-one (2.10 g, 10 mmol) yielded the product as a green oily liquid (1.34 g, 32% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 7.95 (d, J = 7.4 Hz, 2H), 7.57–7.43 (m, 9H), 7.34–7.29 (m, 6H), 4.23 (t, J = 7.2 Hz, 1H), 3.09–2.78 (m, 4H), 2.47–2.38 (m, 3H), 2.20 (s, 1H), 1.95–1.85 (m, 1H). 13C NMR (100 MHz, CDCl3, TMS): δ 163.65, 155.08, 138.69, 137.56, 137.50, 137.44, 137.37, 133.95, 132.20, 131.89, 131.70, 131.52, 127.60, 127.50, 127.47, 127.45, 127.41, 127.38, 125.90, 118.29, 61.80, 44.09, 43.87, 30.77, 28.35, 28.23, 26.89. 31P NMR (162 MHz, CDCl3, TMS): δ 20.35. FT-IR (cm−1): 3053 (m), 2932 (m), 1584 (m), 1480 (w), 1431 (s), 1327 (m), 1260 (m), 1223 (m), 1181 (m), 1151 (m), 1094 (m), 1023 (m), 833 (w), 798 (m), 737 (s), 691 (s). Anal. Calcd for C28H27N2P (1/2 H2O): C, 77.94; H, 6.54; N, 6.49. Found: C, 77.78; H, 6.53; N, 6.50.
N-(2-(diphenylphosphaneyl)ethyl)-5,6,7,8-tetrahydro-quinolin-8-amine (L4). The synthesis of L4 followed the same procedure as that for L1, except that 6,7-dihydroquinolin-8(5H)-one (1.13 g, 7.7 mmol) yielded the product as a yellow oily liquid (1.66 g, 60% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 8.42 (d, J = 4.2 Hz, 1H), 7.53–7.45 (m, 4H), 7.41–7.29 (m, 7H), 7.11–7.04 (m, 1H), 3.82 (t, J = 5.8 Hz, 1H), 3.03–2.75 (m, 4H), 2.73 (s, 1H), 2.42 (t, J = 7.8 Hz, 2H), 2.15–1.94 (m, 2H), 1.81–1.66 (m, 2H). 13C NMR (100 MHz, CDCl3, TMS): δ 157.26, 146.81, 138.63, 138.53, 138.50, 138.41, 136.85, 132.92, 132.73, 132.66, 132.48, 132.37, 128.59, 128.43, 128.40, 128.37, 128.34, 121.79, 57.79, 44.52, 44.30, 29.24, 29.12, 28.80, 28.52, 19.57. 31P NMR (162 MHz, CDCl3, TMS): δ 20.38.
N-(2-(diphenylphosphaneyl)ethyl)-2-phenyl-5,6,7,8-tetra-hydroquinolin-8-amine (L5). The synthesis of L5 followed the same procedure as that for L1, except that 2-phenyl-6,7-dihydroquinolin-8(5H)-one (1.71 g, 7.7 mmol) yielded the product as a yellow oily liquid (1.98 g, 60% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 7.99–7.91 (m, 2H), 7.49–7.21 (m, 15H), 3.78 (t, J = 6.5 Hz, 1H), 2.99–2.64 (m, 5H), 2.42–2.33 (m, 2H), 2.14–1.60 (m, 2H), 1.73–1.60 (m, 2H). 13C NMR (100 MHz, CDCl3, TMS): δ 155.97, 153.27, 138.47, 137.73, 137.60, 137.49, 136.58, 131.84, 131.77, 131.65, 131.58, 129.72, 127.57, 127.51, 127.42, 127.35, 125.71, 117.47, 57.07, 43.54, 43.33, 28.51, 28.39, 28.07, 27.58, 19.02. 31P NMR (162 MHz, CDCl3, TMS): δ 20.44.
N-(2-(diphenylphosphaneyl)ethyl)-6,7,8,9-tetrahydro-5H-cyclohepta[b]pyridin-9-amine (L6). The synthesis of L6 followed the same procedure as that for L1, except that 5,6,7,8-tetrahydro-9H-cyclohepta[b]pyridin-9-one (1.23 g, 7.7 mmol) yielded the product as a yellow oily liquid (1.51 g, 52% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 8.29 (d, J = 4.6 Hz, 1H), 7.44–7.34 (m, 4H), 7.32–7.20 (m, 7H), 6.98 (t, J = 6.0 Hz, 1H), 3.84 (d, J = 9.2 Hz, 1H), 2.89–2.58 (m, 4H), 2.39–2.27 (m, 3H), 1.95–1.88 (m, 2H), 1.74–1.63 (m, 2H), 1.53–1.34 (m, 2H). 13C NMR (100 MHz, CDCl3, TMS): δ 161.12, 144.75, 135.79, 131.81, 131.59, 127.45, 127.39, 127.38, 127.32, 127.31, 120.46, 62.27, 44.14, 43.93, 33.20, 33.53, 28.34, 28.23, 27.62, 26.34. 31P NMR (162 MHz, CDCl3, TMS): δ 20.40. FT-IR (cm−1): 3049 (w), 2921 (m), 1572 (m), 1476 (w), 1430 (m), 1337 (w), 1260 (m), 1090 (m), 1019 (m), 792 (m), 737 (s), 693 (s). Anal. Calcd for C24H27N2P (2/3 H2O): C, 74.59; H, 7.39; N, 7.25. Found: C, 74.43; H, 7.48; N, 7.08.
N-(2-(diphenylphosphaneyl)ethyl)-2-phenyl-6,7,8,9-tetra-hydro-5H-cyclohepta[b]pyridin-9-amine (L7). The synthesis of L7 followed the same procedure as that for L1, except that 2-phenyl-5,6,7,8-tetrahydro-9H-cyclohepta[b]pyridin-9-one (1.74 g, 7.7 mmol) yielded the product as a yellow solid (2.16 g, 62% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 8.02 (d, J = 7.4 Hz, 2H), 7.53–7.32 (m, 9H), 7.28–7.19 (m, 6H), 3.91 (d, J = 9.3 Hz, 1H), 2.90–2.68 (m, 4H), 2.50–2.31 (m, 2H), 2.09–1.94 (m, 2H), 1.91–1.69 (m, 2H), 1.56–1.32 (m, 2H). 13C NMR (100 MHz, CDCl3, TMS): δ 161.54, 153.16, 139.48, 138.75, 138.68, 138.63, 138.55, 137.76, 135.35, 132.80, 132.62, 132.59, 128.60, 128.48, 128.45, 128.35, 126.71, 117.91, 63.07, 45.50, 45.28, 34.29, 33.90, 29.45, 29.35, 29.20, 27.41. 31P NMR (162 MHz, CDCl3, TMS): δ 20.02. FT-IR (cm−1): 3282 (w), 2928 (m), 1583 (w), 1562 (w), 1455 (w), 1431 (m), 1386 (w), 1332 (w), 1305 (w), 1260 (m), 1090 (m), 1025 (m), 969 (m), 938 (w), 913 (w), 859 (w), 827 (w), 795 (m), 738 (s), 691 (s). Anal. Calcd for C30H31N2P (1/2 H2O): C, 78.41; H,7.02; N, 6.10. Found: C, 78.10; H, 6.88; N, 5.95.
N-(2-(diphenylphosphaneyl)ethyl)-5,6,7,8,9,10-hexahydrocycloocta[b]pyridin-10-amine (L8). The synthesis of L8 followed the same procedure as that for L1, except that 6,7,8,9-tetrahydrocycloocta[b]pyridin-10(5H)-one (1.35 g, 7.7 mmol) yielded the product as a yellow oily liquid (0.93 g, 31% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 8.42 (d, J = 4.6 Hz, 1H), 7.41–7.30 (m, 5H), 7.28–7.21 (m, 6H), 7.07–7.01 (m, 1H), 4.11–4.05 (m, 1H), 2.81–2.50 (m, 4H), 2.36–2.17 (m, 2H), 2.04–1.97 (m, 2H), 1.68–1.52 (m, 2H), 1.43–1.23 (m, 4H). 13C NMR (100 MHz, CDCl3, TMS): δ 159.70, 146.28, 137.78, 137.69, 137.66, 137.56, 135.92, 135.48, 131.88, 131.69, 131.61, 131.43, 127.47, 127.34, 127.27, 120.62, 56.26, 44.07, 43.85, 37.68, 31.67, 31.53, 28.49, 28.37, 26.11, 23.40. 31P NMR (162 MHz, CDCl3, TMS): δ 20.51. FT-IR (cm−1): 3048 (w), 2923 (m), 1577 (m), 1476 (m), 1430 (s), 1368 (w), 1261 (m), 1184 (w), 1102 (m), 1068 (m), 1023 (m), 965 (w), 924 (w), 844 (w), 800 (m), 737 (s), 693 (s). Anal. Calcd for C25H29N2P(1/2 H2O): C, 75.54; H, 7.61; N, 7.05. Found: C, 75.40; H, 7.50; N, 6.94.
N-(2-(diphenylphosphaneyl)ethyl)-6,7-dihydro-5H-cyclopenta[b]pyridin-7-aminozinc dichloride (Zn1). L1 (0.21 g, 0.60 mmol) and ZnCl2 (0.081 g, 0.60 mmol) were dissolved in 5 mL ethanol, respectively. Then, the ethanol solution of zinc chloride was added dropwise to the ligand solution and stirred for 12 h at room temperature under an atmosphere of nitrogen. The resulting precipitate was collected by direct filtration to yield Zn1 as a green powder (0.19 g, 68% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 8.80 (d, J = 5.0 Hz, 1H), 7.78–7.59 (m, 5H), 7.42–7.33 (m, 7H), 4.41–4.36 (m, 1H), 3.21–2.84 (m, 4H), 2.74–2.59 (m, 2H), 2.54–2.38 (m, 2H), 1.91–1.78 (m, 1H). 13C NMR (100 MHz, CDCl3, TMS): δ 162.00, 146.91, 137.48, 136.55, 133.32, 133.17, 133.10, 132.94, 130.24, 130.09, 129.07, 129.03, 128.99, 128.95, 124.66, 62.26, 28.11. 31P NMR (162 MHz, CDCl3, TMS): δ 25.86. FT-IR (cm−1): 3200 (m), 3073 (w), 2959 (w), 1606 (m), 1479 (w), 1436 (m), 1346 (w), 1321 (m), 1262 (m), 1204 (m), 1149 (w), 1096 (m), 1073 (w), 1034 (m), 1015 (m), 941 (m), 872 (w), 800 (m), 778 (w), 745 (s), 697 (s). Anal. Calcd for C22H23Cl2N2PZn (1/2H2O): C, 53.74; H, 4.92; N, 5.70. Found: C, 53.81; H, 4.82; N, 5.45.
N-(2-(diphenylphosphaneyl)ethyl)-2-methyl-6,7-dihydro-5H-cyclopenta[b]pyridine-7-aminozinc dichloride (Zn2). The synthesis of Zn2 followed the same procedure as that for Zn1, except that L2 (0.28 g, 0.77 mmol) was used to yield Zn2 as a green powder (0.25 g, 65% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 7.63 (d, J = 7.8 Hz, 1H), 7.57–7.50 (m, 2H), 7.44–7.37 (m, 5H), 7.35–7.28 (m, 3H), 7.16 (d, J = 7.8 Hz, 1H), 4.37–4.28 (m, 1H), 3.24 (s, 1H), 3.12–2.81 (m, 4H), 2.71 (s, 3H), 2.41–2.28 (m, 1H), 1.86–1.76 (m, 1H). 13C NMR (100 MHz, CDCl3, TMS): δ 160.85, 156.68, 136.97, 133.85, 133.42, 133.22, 132.54, 132.36, 129.45, 128.87, 128.80, 128.69, 128.62, 125.14, 63.15, 28.26, 23.09. 31P NMR (162 MHz, CDCl3, TMS): δ 22.19. FT-IR (cm−1): 3196 (m), 2955 (w), 1611 (m), 1587 (w), 1472 (m), 1434 (m), 1382 (w), 1348 (w), 1312 (w), 1265 (w), 1204 (w), 1144 (m), 1096 (m), 1074 (m), 1015 (m), 973 (w), 942 (w), 859 (m), 834 (m), 798 (m), 743 (s), 698 (s). Anal. Calcd for C23H25Cl2N2PZn: C, 55.62; H, 5.07; N, 5.64. Found: C, 55.29; H, 5.08; N, 5.42.
N-(2-(diphenylphosphaneyl)ethyl)-2-phenyl-6,7-dihydro-5H-cyclopenta[b]pyridin-7-aminozinc chloride (Zn3). The synthesis of Zn3 followed the same procedure as that for Zn1, except that L3 (0.21 g, 0.49 mmol) was used to yield Zn3 as a green powder (0.20 g, 73% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 7.91–7.86 (m, 2H), 7.73 (d, J = 7.9 Hz, 1H), 7.60–7.49 (m, 6H), 7.44–7.35 (m, 5H), 7.33–7.28 (m, 3H), 4.48–4.37 (m, 1H), 3.24 (s, 1H), 3.12–2.87 (m, 4H), 2.78–2.67 (m, 2H), 2.25–2.14 (m, 1H), 1.89–1.77 (m, 1H). 13C NMR (100 MHz, CDCl3, TMS): δ 161.93, 157.37, 137.20, 136.73, 135.32, 133.38, 133.19, 132.62, 132.44, 130.40, 129.38, 128.84, 128.76, 128.66, 128.59, 128.08, 123.99, 63.15, 45.99, 32.64, 28.31, 27.65. 31P NMR (162 MHz, CDCl3, TMS): δ 22.20. FT-IR (cm−1): 3193 (m), 1597 (w), 1576 (w), 1457 (m), 1433 (m), 1339 (w), 1309 (w), 1230 (m), 1206 (w), 1157 (m), 1100 (m), 1072 (m), 1043 (w), 1009 (m), 971 (m), 945 (m), 842 (m), 791 (w), 736 (s), 694 (s). Anal. Calcd for C28H27Cl2N2PZn: C, 60.18; H, 4.87; N, 5.01. Found: C, 59.99; H, 4.84; N, 4.86.
N-(2-(diphenylphosphaneyl)ethyl)-5,6,7,8-tetrahydroquinolin-8-aminozinc dichloride (Zn4) [51]. The synthesis of Zn4 followed the same procedure as that for Zn1, except that L4 (0.37 g, 1.03 mmol) was used to yield Zn4 as a white powder (0.38 g, 74% yield).
N-(2-(diphenylphosphaneyl)ethyl)-2-phenyl-5,6,7,8-tetra-hydroquinolin-8-aminozinc dichloride (Zn5) [40]. The synthesis of Zn5 followed the same procedure as that for Zn1, except that L5 (0.28 g, 0.64 mmol) was used to yield Zn5 as a white powder (0.32 g, 87% yield).
N-(2-(diphenylphosphaneyl)ethyl)-6,7,8,9-tetrahydro-5H-cyclohepta[b]pyridin-9-aminozinc dichloride (Zn6). The synthesis of Zn6 followed the same procedure as that for Zn1, except that L6 (0.39 g, 1.0 mmol) was used to yield Zn6 as a white powder (0.42 g, 78% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 8.56 (d, J = 4.3 Hz, 1H), 7.68 (d, J = 7.4 Hz, 1H), 7.59–7.47 (m, 4H), 7.40–7.31 (m, 7H), 4.06 (d, J = 8.1 Hz, 1H), 3.13–2.66 (m, 6H), 2.53–2.48 (m, 1H), 2.07–1.73 (m, 5H). 13C NMR (100 MHz, CDCl3, TMS): δ 156.82, 144.32, 139.75, 137.35, 132.14, 131.95, 131.90, 131.72, 128.42, 128.35, 127.87, 127.85, 127.80, 127.77, 123.37, 32.33, 28.33, 26.29, 24.67. 31P NMR (162 MHz, CDCl3, TMS): δ 23.01. FT-IR (cm−1): 3201 (m), 3070 (w), 2951 (w), 1591 (m), 1478 (m), 1450 (m), 1431 (s), 1357 (w), 1333 (w), 1261 (m), 1204 (m), 1165 (w), 1094 (m), 1028 (m), 944 (m), 800 (m), 769 (w), 740 (s), 695 (s). Anal. Calcd for C24H27Cl2N2PZn (1/2H2O): C, 55.46; H, 5.43; N, 5.39. Found: C, 55.22; H, 5.45; N, 5.01.
N-(2-(diphenylphosphaneyl)ethyl)-2-phenyl-6,7,8,9-tetrahydro-5H-cyclohepta[b]pyridin-9-aminozinc dichloride (Zn7). The synthesis of Zn7 followed the same procedure as that for Zn1, except that L7 (0.22 g, 0.46 mmol) was used to yield Zn7 as a white powder (0.21 g, 78% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 7.83 (d, J = 6.2 Hz, 2H), 7.73 (d, J = 6.9 Hz, 1H), 7.58–7.45 (m, 8H), 7.39–7.31 (m, 6H), 4.18 (s, 1H), 3.27 (s, 1H), 2.94–2.81 (m, 4H), 2.77–2.64 (m, 2H), 2.56–2.42 (m, 1H), 2.18–1.92 (m, 4H), 1.91–1.79 (m, 1H). 13C NMR (100 MHz, CDCl3, TMS): δ 161.54, 153.16, 139.48, 138.75, 138.68, 138.63, 138.55, 137.76, 135.35, 132.80, 132.78, 132.62, 132.59, 128.60, 128.48, 128.45, 128.42, 128.35, 126.71, 117.91, 63.07, 45.50, 45.28, 34.29, 33.90, 29.46, 29.35, 29.20, 27.41. 31P NMR (162 MHz, CDCl3, TMS): δ 20.02. FT-IR (cm−1): 3282 (m), 3057 (w), 2937 (m), 1588 (m), 1566 (m), 1458 (m), 1433 (m), 1388 (m), 1362 (m), 1305 (w), 1234 (m), 1203 (m), 1073 (m), 1045 (m), 987 (m), 931 (m), 910 (m), 848 (m), 749 (s), 700 (s). Anal. Calcd for C30H31Cl2N2PZn: C, 61.40; H, 5.32; N, 4.77. Found: C, 61.28; H, 5.33; N, 4.58.
N-(2-(diphenylphosphaneyl)ethyl)-5,6,7,8,9,10-hexahydrocycloocta[b]pyridin-10-aminozinc chloride (Zn8). The synthesis of Zn8 followed the same procedure as that for Zn1, except that L8 (0.19 g, 0.49 mmol) was used to yield Zn8 as a white powder (0.20 g, 78% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 8.66 (d, J = 4.4 Hz, 1H), 7.70 (d, J = 7.5 Hz, 1H), 7.54–7.39 (m, 6H), 7.37–7.32 (m, 5H), 4.38–4.27 (m, 1H), 3.29 (s, 1H), 3.03–2.88 (m, 1H), 2.85–2.60 (m, 4H), 2.55–2.42 (m, 1H), 2.28–2.17 (m, 1H), 2.08–1.92 (m, 2H), 1.66–1.51 (m, 3H), 1.43–1.33 (m, 2H). 13C NMR (100 MHz, CDCl3, TMS): δ 155.60, 146.90, 141.03, 138.84, 135.83, 132.95, 132.77, 129.37, 129.26, 128.88, 128.81, 128.74, 124.79, 60.69, 48.20, 37.26, 32.00, 26.32, 23.92. 31P NMR (162 MHz, CDCl3, TMS): δ 22.29. FT-IR (cm−1): 3235. (m), 3059.60 (w), 2925 (m), 1598 (m), 1452 (w), 1436 (m), 1357 (m), 1260 (m), 1197 (m), 1094 (m), 1056 (m), 1015 (s), 948 (m), 868 (w), 801 (m), 745 (w), 736 (s), 696 (s). Anal. Calcd for C25H29Cl2N2PZn (1/2 EtOH): C, 57.01; H, 5.89; N, 5.11 Found: C, 56.70; H, 5.60; N, 5.05.

3.2.2. General Procedure for Ring-Opening Polymerization of L-lactide or rac-LA

A typical polymerization procedure is outlined as follows using Zn6 as the representative precatalyst. In a Schlenk flask, Zn6 (0.0051 g, 0.010 mmol) and LiN(SiMe3)2 (0.02 mmol, 2 equiv) were dissolved in 1 mL toluene and the mixture was stirred at room temperature for 30 min under a nitrogen atmosphere. Then, the Zn6/LiN(SiMe3)2 was immediately injected into the monomer (L-LA, 0.36 g, 2.5 mmol) to react for the designated time at a specified temperature. After the reaction, the resulting viscous solution was transferred to a flask containing 100 mL cold methanol and stirred. The ring-opening polymerization procedure of the rac-lactide is similar to that of the L-lactide above.

3.2.3. X-ray Crystallographic Analyses

Single-crystal X-ray diffraction data for Zn2, Zn3, Zn7 and Zn8 were conducted on an XtaLAB Synergy-R HyPix diffractometer with graphite-monochromated Cu-Kα radiation (λ = 1.54184 (Å)) at 169.98(10) K. The cell parameters were obtained through global optimization of the positions of all the reflected signals collected. Intensities were corrected for Lorentz and polarization effects and empirical absorption. The structures were solved by direct methods and refined by full-matrix least squares on F2. All hydrogen atoms were placed in calculated positions. Structure solution and refinement were performed using the SHELXTL-97 package [72,73,74]. Crystal data and structure refinements for Zn2, Zn3, Zn7 and Zn8 are summarized in Table 4.

4. Conclusions

In summary, the finely tuned ring sizes of the cycloalkyl-fused pyridine-bearing pendant N-diphenylphosphinoethyl groups are developed as ligands L1–L8 for their zinc chloride complexes Zn1–Zn8. The catalytic systems, Zn1–Zn8/2Li(NSiMe3)2, display good activities toward the ROP of L-LA and rac-LA in situ without BnOH. In general, the cyclohexyl- or cyclooctyl-fused pyridyl–zinc catalysts demonstrate competitively higher activities. For L-LA, 92% monomer conversion was achieved with the [L-LA]: [Zn] molar ratio of 500:1 in only 5 min and at 80 °C. By comparison, the activity for the ROP of rac-LA were generally observed to be much lower than that seen for L-LA, in which the highest monomer conversion could only reach 74% under the same polymerization conditions and all of the obtained poly(rac-LA) product was amorphous. The microstructures of PLAs indicate the linear structures capped with a CH3O- end group were the major and the zwitterionic species was proposed as the intermediate in the polymerization.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/molecules29174150/s1. Figures S1–S36: 1H/13C/3lP NMR spectra of ligand and zinc complexes. Figure S37: Kinetics plot of polymerization of rac-LA by Zn6/2LiN(SiMe3)2. Figures S38 and S39: MALDI-TOF and 1H NMR spectra of PLAs. Figure S40: Decoupling 1H NMR spectrum of the poly(rac-LA) obtained using Zn1-Zn8/2LiN(SiMe3)2. CCDC 2330187-2330190 for Zn2, Zn3, Zn7 and Zn8. For crystallographic data in CIF or another electronic format, see Supplementary Materials.

Author Contributions

Conceptualization, W.Z. and W.-H.S.; methodology, Y.W. and P.Z.; investigation, Y.W., W.Y. and X.X.; data curation, Y.W., Y.M. and R.W.; writing—original draft preparation, Y.W.; writing—review and editing, W.Z. and W.-H.S.; supervision, W.Z. and W.-H.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (51973005) and Beijing Scholar Program (Project No. RCQJ20303).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Materials, further inquiries can be directed to the corresponding authors.

Acknowledgments

For their support, W.Z. also thanks the Classified Development of Municipal Colleges and Universities—the Project of Constructing the Emerging Interdisciplinary Platform Based on “Clothing Science” of Beijing Institute of Fashion Technology.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Geyer, R.; Jambeck, J.R.; Law, K.L. Production, use, and fate of all plastics ever made. Sci. Adv. 2017, 3, e1700782. [Google Scholar] [CrossRef] [PubMed]
  2. Yang, J.; Sun, Z.; Duan, R.; Li, L.; Pang, X.; Chen, X. Copolymer of lactide and ε-caprolactone catalyzed by bimetallic Schiff base aluminum complexes. Sci. China Chem. 2016, 59, 1384–1389. [Google Scholar] [CrossRef]
  3. Hartmann, M.H. Biopolymers from Renewable Resources; Kaplan, D.H., Ed.; Springer: Berlin, Germany, 1998; pp. 367–411. [Google Scholar]
  4. Reddy, M.M.; Vivekanandhan, S.; Misra, M.; Bhatia, S.K.; Mohanty, A.K. Biobased plastics and bionanocomposites: Current status and future opportunities. Prog. Polym. Sci. 2013, 38, 1653–1689. [Google Scholar]
  5. Duan, R.; Hu, C.; Li, X.; Pang, X.; Sun, Z.; Wang, X. Air-stable salen–iron complexes: Stereoselective catalysts for lactide and ε-caprolactone polymerization through in situ initiation. Macromolecules 2017, 50, 9188–9195. [Google Scholar] [CrossRef]
  6. Zhang, W.; Sun, W.-H. Chapter 39. In Synthesis and Applications in Chemistry and Materials; Pombeiro, A.J.L., Mahmudov, K.T., Da Silva, M.D.F.C.G., Eds.; World Scientific: Singapore, 2024; pp. 217–313. [Google Scholar]
  7. Thomas, C.M. Stereocontrolled ring-opening polymerization of cyclic esters: Synthesis of new polyester microstructures. Chem. Soc. Rev. 2010, 39, 165–173. [Google Scholar] [CrossRef]
  8. Parwe, S.P.; Warkad, S.D.; Mane, M.V.; Shedage, P.S.; Garnaik, B. Investigation of the biocompatibility and cytotoxicity associated with ROP initiator and its role in bulk polymerization of l-lactide. Polymer 2017, 111, 244–251. [Google Scholar]
  9. Conn, R.E.; Kolstad, J.J.; Borzelleca, J.F.; Dixler, D.S.; Filer, L.J.; Ladu, B.N.; Pariza, M.W. Safety assessment of polylactide (PLA) for use as a food-contact polymer. Food Chem. Toxicol. 1995, 33, 273–283. [Google Scholar] [CrossRef]
  10. Tanzi, M.C.; Verderio, P.; Lampugnani, M.G.; Resnati, M.; Dejana, E.; Sturani, E. Cytotoxicity of some catalysts commonly used in the synthesis of copolymers for biomedical use. J. Mater. Sci. Mater. Med. 1994, 5, 393–396. [Google Scholar] [CrossRef]
  11. Gao, J.; Zhang, W.; Wang, X.; Wang, R.; Han, M.; Cao, F.; Hao, X. Isoselective Ring-Opening Polymerization of rac-Lactide Catalyzed by Simple Potassium Amidate Complexes Containing Polycyclic Aryl Group. Catalysts 2023, 13, 770. [Google Scholar] [CrossRef]
  12. Gao, J.; Zhang, W.; Cao, F.; Solan, G.A.; Zhang, X.; Jiang, Y.; Hao, X.; Sun, W.-H. Potassium N-arylbenzimidates as readily accessible and benign (pre) catalysts for the ring opening polymerization of ε-CL and L-LA. Mol. Catal. 2020, 498, 111280. [Google Scholar] [CrossRef]
  13. Wang, Y.; Wang, X.; Zhang, W.; Sun, W.-H. Progress of ring-opening polymerization of cyclic esters catalyzed by iron compounds. Organometallics 2023, 42, 1680–1692. [Google Scholar] [CrossRef]
  14. Gao, J.; Zhu, D.; Zhang, W.; Solan, G.A.; Ma, Y.; Sun, W.-H. Recent progress in the application of group 1, 2 & 13 metal complexes as catalysts for the ring opening polymerization of cyclic esters. Inorg. Chem. Front. 2019, 6, 2619–2652. [Google Scholar]
  15. Porchia, M.; Pellei, M.; Del Bello, F.; Santini, C. Zinc complexes with nitrogen donor ligands as anticancer agents. Molecules 2020, 25, 5814. [Google Scholar] [CrossRef] [PubMed]
  16. Platel, R.H.; Hodgson, L.M.; Williams, C.K. Biocompatible initiators for lactide polymerization. Polym. Rev. 2008, 48, 11–63. [Google Scholar] [CrossRef]
  17. Cheng, M.; Attygalle, A.B.; Lobkovsky, E.B.; Coates, G.W. Single-site catalysts for ring-opening polymerization: Synthesis of heterotactic poly (lactic acid) from rac-lactide. Chem. Soc. 1999, 121, 11583–11584. [Google Scholar] [CrossRef]
  18. Chisholm, M.H.; Gallucci, J.; Phomphrai, K. Coordination chemistry and reactivity of monomeric alkoxides and amides of magnesium and zinc supported by the diiminato ligand CH (CMeNC6H3-2, 6-iPr2)2. A comparative study. Inorg. Chem. 2002, 41, 2785–2794. [Google Scholar] [CrossRef]
  19. Keram, M.; Ma, H. Ring-opening polymerization of lactide, ε-caprolactone and their copolymerization catalyzed by β-diketiminate zinc complexes. Appl. Organomet. Chem. 2017, 31, e3893. [Google Scholar] [CrossRef]
  20. Köhler, M.; Rinke, P.; Fiederling, K.; Görls, H.; Ueberschaar, N.; Schacher, F.H.; Kretschmer, R. Catalytic Activity of Various β-Diketiminate Zinc Complexes toward the Ring-Opening Polymerization of Caprolactone and Derivatives. Macromol. Chem. Phys. 2021, 222, 2100187. [Google Scholar] [CrossRef]
  21. Huang, M.; Pan, C.; Ma, H. Ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate catalyzed by magnesium and zinc complexes derived from binaphthyl-based iminophenolate ligands. Dalton Trans. 2015, 44, 12420–12431. [Google Scholar] [CrossRef]
  22. Gong, Y.; Ma, H. High performance benzoimidazolyl-based aminophenolate zinc complexes for isoselective polymerization of rac-lactide. Chem. Commun. 2019, 55, 10112–10115. [Google Scholar] [CrossRef]
  23. Jędrzkiewicz, D.; Ejfler, J.; Gulia, N.; Szafert, S. Designing ancillary ligands for heteroleptic/homoleptic zinc complex formation: Synthesis, structures and application in ROP of lactides. Dalton Trans. 2015, 44, 13700–13715. [Google Scholar] [CrossRef] [PubMed]
  24. Kan, C.; Hu, J.; Huang, Y.; Wang, H.; Ma, H. Highly isoselective and active zinc catalysts for rac-lactide polymerization: Effect of pendant groups of aminophenolate ligands. Macromolecules 2017, 50, 7911–7919. [Google Scholar] [CrossRef]
  25. Chotard, F.; Lapenta, R.; Bolley, A.; Trommenschlager, A.; Balan, C.; Bayardon, J.; Malacea-Kabbara, R.; Bonnin, Q.; Bodio, E.; Cattey, H.; et al. Phenoxyamidine Zn and Al complexes: Synthesis, characterization, and use in the ring-opening polymerization of lactide. Organometallics 2019, 38, 4147–4157. [Google Scholar] [CrossRef]
  26. Li, M.; Behzadi, S.; Chen, M.; Pang, W.; Wang, F.; Tan, C. Phenoxyimine ligands bearing nitrogen-containing second coordination spheres for zinc catalyzed stereoselective ring-opening polymerization of rac-lactide. Organometallics 2019, 38, 461–468. [Google Scholar] [CrossRef]
  27. Fuchs, M.; Schmitz, S.; Schäfer, P.M.; Secker, T.; Metz, A.; Ksiazkiewicz, A.N.; Pich, A.; Kögerler, P.; Monakhov, K.Y.; Herres-Pawlis, S. Mononuclear zinc (II) Schiff base complexes as catalysts for the ring-opening polymerization of lactide. Eur. Polym. J. 2020, 122, 109302. [Google Scholar] [CrossRef]
  28. Soobrattee, S.; Zhai, X.; Nyamayaro, K.; Diaz, C.; Kelley, P.; Ebrahimi, T.; Mehrkhodavandi, P. Dinucleating amino-phenolate platform for zinc catalysts: Impact on lactide polymerization. Inorg. Chem. 2020, 59, 5546–5557. [Google Scholar] [CrossRef] [PubMed]
  29. Lai, F.J.; Chiu, L.L.; Lee, C.L.; Lu, W.Y.; Lai, Y.C.; Ding, S.; Chen, H.Y.; Wu, K.H. Improvement in zinc complexes bearing Schiff base in ring-opening polymerization of ε-caprolactone: A five-membered ring system. Polymer 2019, 182, 121812. [Google Scholar]
  30. Santulli, F.; Bruno, F.; Mazzeo, M.; Lamberti, M. Zinc Complexes Bearing Dinucleating Bis (imino-pyridine) binaphthol Ligands: Highly Active and Robust Catalysts for the Lactide Polymerization. ChemCatChem 2023, 15, e202300498. [Google Scholar] [CrossRef]
  31. Zhang, X.; Chen, K.; Chicoma, M.; Goins, K.; Prior, T.J.; Nile, T.A.; Redshaw, C. Alkoxy-Functionalized Schiff-Base Ligation at Aluminum and Zinc: Synthesis, Structures and ROP Capability. Catalysts 2021, 11, 1090. [Google Scholar] [CrossRef]
  32. Lian, B.; Thomas, C.M.; Casagrande, O.L.; Lehmann, C.W.; Roisnel, T.; Carpentier, J.F. Aluminum and zinc complexes based on an amino-bis (pyrazolyl) ligand: Synthesis, structures, and use in MMA and lactide polymerization. Inorg. Chem. 2007, 46, 328–340. [Google Scholar] [CrossRef]
  33. Zikode, M.; Ojwach, S.O.; Akerman, M.P. Structurally rigid bis (pyrazolyl) pyridine Zn (II) and Cu (II) complexes: Structures and kinetic studies in ring-opening polymerization of ε-caprolactone. Appl. Organomet. Chem. 2017, 31, e3556. [Google Scholar] [CrossRef]
  34. Fliedel, C.; Mameri, S.; Dagorne, S.; Avilés, T. Controlled ring-opening polymerization of trimethylene carbonate and access to PTMC-PLA block copolymers mediated by well-defined N-heterocyclic carbene zinc alkoxides. Appl. Organomet. Chem. 2014, 28, 504–511. [Google Scholar] [CrossRef]
  35. Schnee, G.; Fliedel, C.; Avilés, T.; Dagorne, S. Neutral and Cationic N-Heterocyclic Carbene Zinc Adducts and the BnOH/Zn (C6F5)2 Binary Mixture–Characterization and Use in the Ring-Opening Polymerization of β-Butyrolactone, Lactide, and Trimethylene Carbonate. Eur. J. Inorg. Chem. 2013, 21, 3699–3709. [Google Scholar] [CrossRef]
  36. Tufano, F.; Santulli, F.; Grisi, F.; Lamberti, M. N-Heterocyclic Carbene-Based Zinc Complexes: Same Precursors for Different Lactide Ring-Opening Polymerization Mechanisms. ChemCatChem 2022, 14, e202200962. [Google Scholar] [CrossRef]
  37. Hermann, A.; Hill, S.; Metz, A.; Heck, J.; Hoffmann, A.; Hartmann, L.; Herres-Pawlis, S. Next generation of zinc bisguanidine polymerization catalysts towards highly crystalline, biodegradable polyesters. Angew. Chem. Int. Ed. 2020, 59, 21778–21784. [Google Scholar] [CrossRef] [PubMed]
  38. Schäfer, P.M.; Fuchs, M.; Ohligschläger, A.; Rittinghaus, R.; McKeown, P.; Akin, E.; Schmidt, M.; Hoffmann, A.; Liauw, M.A.; Jones, M.D.; et al. Highly Active N, O Zinc Guanidine Catalysts for the Ring-Opening Polymerization of Lactide. ChemSusChem 2017, 10, 3547–3556. [Google Scholar] [CrossRef] [PubMed]
  39. Hermann, A.; Becker, T.; Schäfer, M.A.; Hoffmann, A.; Herres-Pawlis, S. Effective Ligand Design: Zinc Complexes with Guanidine Hydroquinoline Ligands for Fast Lactide Polymerization and Chemical Recycling. ChemSusChem 2022, 15, e202201075. [Google Scholar] [CrossRef] [PubMed]
  40. Ghosh, S.; Schäfer, P.M.; Dittrich, D.; Scheiper, C.; Steiniger, P.; Fink, G.; Ksiazkiewicz, A.N.; Tjaberings, A.; Wölper, C.; Gröschel, A.H.; et al. Heterolepic β-Ketoiminate Zinc Phenoxide Complexes as Efficient Catalysts for the Ring Opening Polymerization of Lactide. ChemistryOpen 2019, 8, 951–960. [Google Scholar] [CrossRef]
  41. Hill, M.S.; Hitchcock, P.B. Synthesis of C2 and Cs symmetric zinc complexes supported by bis(phosphinimino)methyl ligands and their use in ring opening polymerisation catalysis. J. Chem. Soc. Dalton Trans. 2002, 4694–4702. [Google Scholar] [CrossRef]
  42. Wheaton, C.A.; Hayes, P.G. Cationic zinc complexes: A new class of catalyst for living lactide polymerization at ambient temperature. Chem. Commun. 2010, 46, 8404–8406. [Google Scholar] [CrossRef]
  43. Wheaton, C.A.; Hayes, P.G. Cationic organozinc complexes of a bis (phosphinimine) pincer ligand: Synthesis, structural and polymerization studies. Dalton Trans. 2010, 39, 3861–3869. [Google Scholar] [CrossRef] [PubMed]
  44. Wheaton, C.A.; Hayes, P.G. Exploring the versatility of a bis (phosphinimine) pincer ligand: Effect of sterics on structure and lactide polymerization activity of cationic zinc complexes. Catal. Sci. Technol. 2012, 2, 125–138. [Google Scholar] [CrossRef]
  45. Sun, H.; Ritch, J.S.; Hayes, P.G. Ring-opening polymerisation of rac-lactide mediated by cationic zinc complexes featuring P-stereogenic bisphosphinimine ligands. Dalton Trans. 2012, 41, 3701–3713. [Google Scholar] [CrossRef]
  46. D’Auria, I.; Lamberti, M.; Mazzeo, M.; Milione, S.; Roviello, G.; Pellecchia, C. Coordination chemistry and reactivity of zinc complexes supported by a phosphido pincer ligand. Chem. Eur. J. 2012, 18, 2349–2360. [Google Scholar] [CrossRef] [PubMed]
  47. D’Auria, I.; Lamberti, M.; Rescigno, R.; Venditto, V.; Mazzeo, M. Copolymerization of L-Lactide and ε-Caprolactone promoted by zinc complexes with phosphorus based ligands. Heliyon 2021, 7, e07630. [Google Scholar] [CrossRef] [PubMed]
  48. Fliedel, C.; Rosa, V.; Alves, F.M.; Martins, A.M.; Avilés, T.; Dagorne, S. P, O-Phosphinophenolate zinc (II) species: Synthesis, structure and use in the ring-opening polymerization (ROP) of lactide, ε-caprolactone and trimethylene carbonate. Dalton Trans. 2015, 44, 12376–12387. [Google Scholar] [CrossRef]
  49. Goel, R.G.; Henry, W.P.; Jha, N.K. Reinvestigation of tricyclohexylphosphine complexes of zinc (II) and cadmium (II) halides. Preparation, characterization, and phosphorus-31 nuclear magnetic resonance and vibrational spectroscopic studies. Inorg. Chem. 1982, 21, 2551–2555. [Google Scholar] [CrossRef]
  50. Goel, R.G.; Ogini, W.O. Preparation, characterization, and spectral studies of neutral tri-tert-butylphosphine complexes of zinc (II) and cadmium (II). Inorg. Chem. 1977, 16, 1968–1972. [Google Scholar] [CrossRef]
  51. Cao, F.; Wang, Y.; Wang, X.; Zhang, W.; Solan, G.A.; Wang, R.; Ma, Y.; Hao, X.; Sun, W.-H. Zinc 8-aminotrihydroquinolines appended with pendant N-diphenylphosphinoethyl arms as exceptionally active catalysts for the ROP of ε-CL. Catal. Sci. Technol. 2022, 12, 6687–6703. [Google Scholar] [CrossRef]
  52. Wang, Y.; Zhang, W.; Wang, X.; Zuo, W.; Xue, X.; Ma, Y.; Sun, W.-H. N-(2-(Diphenylphosphino) ethyl)-2-alkyl-5, 6, 7, 8-tetrahydro-quinolin-8-amines iron (ii) complexes: Structural diversity and the ring opening polymerization of ε-caprolactone. RSC Adv. 2023, 13, 29866–29878. [Google Scholar] [CrossRef]
  53. Hayashi, T.; Konishi, M.; Fukushima, M.; Kanehira, K.; Hioki, T.; Kumada, M. Chiral (beta-aminoalkyl) phosphines. Highly efficient phosphine ligands for catalytic asymmetric Grignard cross-coupling. J. Org. Chem. 1983, 48, 2195–2202. [Google Scholar] [CrossRef]
  54. Wang, Z.; Pan, B.; Liu, Q.; Yue, E.; Solan, G.A.; Ma, Y.; Sun, W.-H. Efficient acceptorless dehydrogenation of secondary alcohols to ketones mediated by a PNN-Ru (II) catalyst. Catal. Sci. Technol. 2017, 7, 1654–1661. [Google Scholar] [CrossRef]
  55. Wang, Z.; Li, Y.; Liu, Q.B.; Solan, G.A.; Ma, Y.; Sun, W.-H. Direct Hydrogenation of a Broad Range of Amides under Base-free Conditions using an Efficient and Selective Ruthenium (II) Pincer Catalyst. ChemCatChem 2017, 9, 4275–4281. [Google Scholar] [CrossRef]
  56. Kang, M.S.; Cho, J.; Nayab, S.; Jeong, J.H. Synthesis and characterization of Zn (II) and Cu (II) complexes bearing (chiral substituent)(diethyl)-ethanediamine derivatives as precatalysts for rac-lactide polymerisation. Polyhedron 2019, 158, 135–143. [Google Scholar] [CrossRef]
  57. Kwon, K.S.; Nayab, S.; Lee, H.; Jeong, J.H. Synthesis and structural characterisation of zinc complexes bearing furanylmethyl and thiophenylmethyl derivatives of (R, R)-1, 2-diaminocyclohexanes for stereoselective polymerisation of poly (rac-lactide). Polyhedron 2014, 77, 32–38. [Google Scholar] [CrossRef]
  58. Save, M.; Schappacher, M.; Soum, A. Controlled ring-opening polymerization of lactones and lactides initiated by lanthanum isopropoxide, 1. General aspects and kinetics. Macromol. Chem. Phys. 2002, 203, 889–899. [Google Scholar] [CrossRef]
  59. Zhu, D.; Guo, L.; Zhang, W.; Hu, X.; Nomura, K.; Vignesh, A.; Hao, X.; Zhang, Q.; Sun, W.-H. Dialkylaluminum 2-substituted 6, 6-dimethylcyclopentylpyridin-7-oxylates toward structural-differentia-tion of the ring-opening polymerization of ε-caprolactone and L-lactides. Dalton Trans. 2019, 48, 4157–4167. [Google Scholar] [CrossRef]
  60. Jiang, Y.; Zhang, W.; Han, M.; Wang, X.; Solan, G.A.; Wang, R.; Ma, Y.; Sun, W.-H. Phenoxy-imine/-amide aluminum complexes with pendant or coordinated pyridine moieties: Solvent effects on structural type and catalytic capability for the ROP of cyclic esters. Polymer 2022, 242, 124602. [Google Scholar] [CrossRef]
  61. Duda, A.; Penczek, S. Polymerization of epsilon-Caprolactone Initiated by aluminum isopropoxide trimer and/or tetramer. Macromolecules 1995, 28, 5981–5992. [Google Scholar] [CrossRef]
  62. Cayuela, J.; Bounor-Legaré, V.; Cassagnau, P.; Michel, A. Ring-opening polymerization of ε-caprolactone initiated with titanium n-propoxide or titanium phenoxide. Macromolecules 2006, 39, 1338–1346. [Google Scholar] [CrossRef]
  63. Jhurry, D.; Bhaw-Luximon, A.; Spassky, N. October. Synthesis of polylactides by new aluminium Schoff’s base complexes. Macromol. Symp. 2001, 175, 67–79. [Google Scholar] [CrossRef]
  64. Ropson, N.; Dubois, P.; Jérôme, R.; Teyssié, P. Macromolecular engineering of polylactones and polylactides. 20. Effect of monomer, solvent, and initiator on the ring-opening polymerization as initiated with aluminum alkoxides. Macromolecules 1995, 28, 7589–7598. [Google Scholar] [CrossRef]
  65. Zhang, Q.; Zhang, W.; Wang, S.; Solan, G.A.; Liang, T.; Rajendran, N.M.; Sun, W.-H. Sodium iminoquinolates with cubic and hexagonal prismatic motifs: Synthesis, characterization and their catalytic behavior toward the ROP of rac-lactide. Inorg. Chem. Front. 2016, 3, 1178–1189. [Google Scholar] [CrossRef]
  66. Dou, J.; Zhu, D.; Zhang, W.; Wang, R.; Wang, S.; Zhang, Q.; Zhang, X.; Sun, W.-H. Highly efficient iron (II) catalysts toward ring opening polymerization of ε-caprolactone through in situ initiation. Inorg. Chim. Acta. 2019, 488, 299–303. [Google Scholar] [CrossRef]
  67. Montaudo, G.; Montaudo, M.S.; Puglisi, C.; Samperi, F.; Spassky, N.; LeBorgne, A.; Wisniewski, M. Evidence for ester-exchange reactions and cyclic oligomer formation in the ring-opening polymerization of lactide with aluminum complex initiators. Macromolecules 1996, 29, 6461–6465. [Google Scholar] [CrossRef]
  68. Spassky, N.; Simic, V.; Montaudo, M.S.; Hubert-Pfalzgraf, L.G. Inter-and intramolecular ester exchange reactions in the ring-opening polymerization of (D, L)-lactide using lanthanide alkoxide initiators. Macromol. Chem. Phys. 2000, 201, 2432–2440. [Google Scholar] [CrossRef]
  69. Castro-Osma, J.A.; Alonso-Moreno, C.; García-Martinez, J.C.; Fernandez-Baeza, J.; Sanchez-Barba, L.F.; Lara-Sanchez, A.; Otero, A. Ring-opening (ROP) versus ring-expansion (REP) polymerization of ε-caprolactone to give linear or cyclic polycaprolactones. Macromolecules 2013, 46, 6388–6394. [Google Scholar] [CrossRef]
  70. Shaik, M.; Peterson, J.; Du, G. Cyclic and linear polyhydroxylbutyrates from ring-opening polymerization of β-butyrolactone with amido-oxazolinate zinc catalysts. Macromolecules 2019, 52, 157–166. [Google Scholar] [CrossRef]
  71. Bhunia, M.; Vijaykumar, G.; Adhikari, D.; Mandal, S.K. Highly active carbene potassium complexes for the ring-opening polymerization of ε-caprolactone. Inorg. Chem. 2017, 56, 14459–14466. [Google Scholar] [CrossRef]
  72. Dolomanov, O.V.; Bourhis, L.J.; Gildea, R.J.; Howard, J.A.; Puschmann, H. OLEX2: A complete structure solution, refinement and analysis program. J. Appl. Cryst. 2009, 42, 339–341. [Google Scholar] [CrossRef]
  73. Sheldrick, G.M. Crystal structure refinement with SHELXL. Acta Cryst. 2015, C71, 3–8. [Google Scholar]
  74. Sheldrick, G.M. SHELXT-Integrated space-group and crystal-structure determination. Acta Cryst. 2015, A71, 3–8. [Google Scholar] [CrossRef] [PubMed]
Molecules 29 04150 sch001
Scheme 1. Zinc complexes containing phosphorus atoms for the ROP of lactides.
Scheme 1. Zinc complexes containing phosphorus atoms for the ROP of lactides.
Molecules 29 04150 sch001
Molecules 29 04150 sch002
Scheme 2. Synthesis of L1–L8 and Zn (II) complexes Zn1–Zn8.
Scheme 2. Synthesis of L1–L8 and Zn (II) complexes Zn1–Zn8.
Molecules 29 04150 sch002
Molecules 29 04150 g001
Figure 1. An ORTEP drawing of Zn2 with thermal ellipsoids of 30% probability. All hydrogen atoms are omitted for clarity.
Figure 1. An ORTEP drawing of Zn2 with thermal ellipsoids of 30% probability. All hydrogen atoms are omitted for clarity.
Molecules 29 04150 g001
Molecules 29 04150 g002
Figure 2. An ORTEP drawing of Zn3 with thermal ellipsoids of 30% probability. All hydrogen atoms are omitted for clarity.
Figure 2. An ORTEP drawing of Zn3 with thermal ellipsoids of 30% probability. All hydrogen atoms are omitted for clarity.
Molecules 29 04150 g002
Molecules 29 04150 g003
Figure 3. An ORTEP drawing of Zn7 with thermal ellipsoids of 30% probability. All hydrogen atoms are omitted for clarity.
Figure 3. An ORTEP drawing of Zn7 with thermal ellipsoids of 30% probability. All hydrogen atoms are omitted for clarity.
Molecules 29 04150 g003
Molecules 29 04150 g004
Figure 4. An ORTEP drawing of Zn8 with thermal ellipsoids of 30% probability. All hydrogen atoms are omitted for clarity.
Figure 4. An ORTEP drawing of Zn8 with thermal ellipsoids of 30% probability. All hydrogen atoms are omitted for clarity.
Molecules 29 04150 g004
Molecules 29 04150 g005
Figure 5. Comparison of the monomer conversion (L-LA) achieved using Zn1–Zn8/2LiN(SiMe3)2 (runs 18, 19–25, Table 2).
Figure 5. Comparison of the monomer conversion (L-LA) achieved using Zn1–Zn8/2LiN(SiMe3)2 (runs 18, 19–25, Table 2).
Molecules 29 04150 g005
Molecules 29 04150 g006
Figure 6. MALDI–TOF mass spectrum of the PLLA obtained using Zn6/2LiN(SiMe3)2 (run 18, Table 2).
Figure 6. MALDI–TOF mass spectrum of the PLLA obtained using Zn6/2LiN(SiMe3)2 (run 18, Table 2).
Molecules 29 04150 g006
Molecules 29 04150 g007
Figure 7. 1H NMR spectrum of the PLLA obtained using Zn6/2LiN(SiMe3)2 (run 18, Table 2).
Figure 7. 1H NMR spectrum of the PLLA obtained using Zn6/2LiN(SiMe3)2 (run 18, Table 2).
Molecules 29 04150 g007
Molecules 29 04150 sch003
Scheme 3. Possible mechanistic pathways for the ROP of L-LA using Zn6/2LiN(SiMe3)2.
Scheme 3. Possible mechanistic pathways for the ROP of L-LA using Zn6/2LiN(SiMe3)2.
Molecules 29 04150 sch003
Molecules 29 04150 g008
Figure 8. Comparison of the monomer conversions (L-LA and rac-LA) achieved using Zn1–Zn8/2LiN(SiMe3)2.
Figure 8. Comparison of the monomer conversions (L-LA and rac-LA) achieved using Zn1–Zn8/2LiN(SiMe3)2.
Molecules 29 04150 g008
Table 1. Selected bond lengths and angles for Zn2, Zn3, Zn7 and Zn8.
Table 1. Selected bond lengths and angles for Zn2, Zn3, Zn7 and Zn8.
Zn2Zn3Zn7Zn8
Bond lengths (Å)
Zn1–Cl12.2018(5)2.2220(6)2.230(2)2.2214(4)
Zn1–Cl22.2287(5)2.2053(6)2.220(2)2.2081(4)
Zn1–N12.0777(15)2.1162(16)2.085(7)2.0442(13)
Zn1–N22.1091(15)2.0846(17)2.080(7)2.0920(12)
Bond angles (°)
Cl2–Zn1–Cl1115.75(2)118.29(2)116.84(10)117.623(18)
N1–Zn1–Cl1114.14(5)106.87(5)111.9(2)117.20(4)
N1–Zn1–Cl2111.37(5)118.83(5)118.9(2)105.86(4)
N1–Zn1–N285.08(6)85.86(7)82.4(3)82.74(5)
N2–Zn1–Cl1113.51(5)107.26(5)112.7(2)109.14(4)
N2–Zn1–Cl2113.23(5)114.83(5)109.0(2)119.54(4)
Table 2. Ring-opening polymerization of L-LA by Zn1–Zn8/2LiN(SiMe3)2 a.
Table 2. Ring-opening polymerization of L-LA by Zn1–Zn8/2LiN(SiMe3)2 a.
RunCatL-LA:ZnSolventT/°Ct/minConv./% bTOF/h−1Mn(calcd) cMn dMw/Mn d
1Zn6250:1toluene3010365401.303.991.74
2 eZn6250:1toluene3010111650.390.591.17
3 fZn6250:1toluene301081200.280.511.15
4Zn6250:1toluene3020483601.734.191.62
5Zn6250:1toluene3040652442.343.061.72
6Zn6250:1toluene3060862153.101.652.03
7Zn6250:1toluene50106810202.452.721.87
8Zn6250:1toluene60108512753.062.832.06
9Zn6250:1toluene70109614403.462.771.87
10Zn6250:1toluene801010015003.602.662.08
11Zn6250:1CH2Cl2301000–––
12Zn6250:1THF60109213803.311.582.12
13Zn6250:1hexane601081200.292.521.97
14Zn6500:1toluene80109628806.912.492.13
15Zn61000:1toluene801089534012.812.612.13
16 gZn61000:1toluene801079474011.382.381.79
17 hZn61000:1toluene80106539009.362.022.33
18Zn6500:1toluene8056337804.541.861.97
19Zn1500:1toluene8057545005.401.961.90
20Zn2500:1toluene8057042005.041.682.04
21Zn3500:1toluene8054426403.172.682.01
22Zn4500:1toluene8059255206.623.901.75
23Zn5500:1toluene8055331803.821.512.17
24Zn7500:1toluene8053621602.591.921.99
25Zn8500:1toluene8059255206.621.842.27
a Reaction conditions: 1.0 mL toluene, 10 μmol [Zn] + 20 μmol LiN(SiMe3)2; b determined by 1H NMR spectroscopy; c 104 g/mol, Mn(calcd) = molar ratio of [L-LA]/[Zn] × 144 × conv.%; d 104 g/mol, GPC data were recorded from THF vs. polystyrene standards using a correcting factor of 0.58 [58]; e 1 equiv. BnOH; f 3 equiv. BnOH; g 3 mL toluene; h 5 mL toluene.
Table 3. Ring-opening polymerization of rac-LA by Zn1–Zn8/2LiN(SiMe3)2 a.
Table 3. Ring-opening polymerization of rac-LA by Zn1–Zn8/2LiN(SiMe3)2 a.
RunCatrac-LA: ZnT/°Ct/minConv./% bTOF/h−1
1Zn1500:1805593540
2Zn2500:1805533180
3Zn3500:1805523120
4Zn4500:1805714260
5Zn5500:1805533180
6Zn6500:1805462760
7Zn7500:1805181080
8Zn8500:1805744440
a Reaction conditions: 1.0 mL toluene, 10 μmol [Zn] + 20 μmol LiN(SiMe3)2; b determined by 1H NMR spectroscopy.
Table 4. Crystal data and structure refinements for Zn2, Zn3, Zn7 and Zn8.
Table 4. Crystal data and structure refinements for Zn2, Zn3, Zn7 and Zn8.
Zn2Zn3Zn7Zn8
empirical formulaC23H25Cl2N2PZnC28H27Cl2N2PZnC30H31Cl2N2PZnC25H29Cl2N2PZn
formula weight496.69558.75586.81524.74
temperature/K170.00(10)169.98(10)169.98(10)169.98(10)
crystal systemtriclinictriclinicmonoclinictriclinic
space groupP-1P-1P21/cP-1
a/Å9.2670(2)8.8906(3)12.9499(5)9.5988(3)
b/Å10.2665(3)10.7637(3)16.8402(6)10.9347(4)
c/Å12.7846(3)15.1833(5)13.0719(5)13.0072(4)
α/˚84.143(2)108.512(3)9084.306(3)
β/˚75.169(2)105.259(3)106.844(4)75.326(3)
γ/˚87.433(2)94.871(2)9074.958(3)
volume/Å31169.44(5)1306.49(8)2728.40(19)1274.64(8)
Z2242
ρcalc/g cm31.4111.4201.4291.367
μ/mm−14.2893.9083.7703.964
F (000)512.0576.01216.0544.0
crystal size/mm30.25 × 0.25 × 0.20.3 × 0.3 × 0.250.15 × 0.1 × 0.020.33 × 0.3 × 0.28
radiationCuKα (λ = 1.54184)CuKα (λ = 1.54184)CuKα (λ = 1.54184)CuKα (λ = 1.54184)
2θ range for data collection/°7.184 to 154.4786.45 to 153.5567.132 to 154.4468.378 to 154.348
index ranges−11 ≤ h ≤ 11,
−12 ≤ k ≤ 12,
−16 ≤ l ≤ 14		−11 ≤ h ≤ 11,
−13 ≤ k ≤ 13,
−18 ≤ l ≤ 19		−16 ≤ h ≤ 16,
−20 ≤ k ≤ 21,
−16 ≤ l ≤ 16		−12 ≤ h ≤ 12,
−13 ≤ k ≤ 13,
−16 ≤ l ≤ 14
reflections collected13,88115,34619,54615,774
independent reflections4716 [Rint = 0.0236, Rsigma = 0.0229]5274 [Rint = 0.0245, Rsigma = 0.0314]5574 [Rint = 0.0669, Rsigma = 0.0560]5156 [Rint = 0.0218, Rsigma = 0.0196]
data/restraints/parameters4716/0/2635274/0/3075574/6/3255156/0/281
goodness-of-fit on F21.0501.0321.1001.024
final R indices [I ≥ 2σ (I)]R1 = 0.0319, wR2 = 0.0838R1 = 0.0367, wR2 = 0.0969R1 = 0.1142, wR2 = 0.3324R1 = 0.0272, wR2 = 0.0722
final R indices [all data]R1 = 0.0333, wR2 = 0.0849R1 = 0.0388, wR2 = 0.0989R1 = 0.1221, wR2 = 0.3364R1 = 0.0281, wR2 = 0.0730
largest diff. peak/hole/e Å−31.13/−0.431.53/−0.392.67/−0.931.04/−0.35
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Wang, Y.; Zhang, W.; Zhu, P.; You, W.; Xue, X.; Wang, R.; Ma, Y.; Sun, W.-H. Intensive Cycloalkyl-Fused Pyridines for Aminopyridyl–Zinc–Heteroimidazoles Achieving High Efficiency toward the Ring-Opening Polymerization of Lactides. Molecules 2024, 29, 4150. https://doi.org/10.3390/molecules29174150

AMA Style

Wang Y, Zhang W, Zhu P, You W, Xue X, Wang R, Ma Y, Sun W-H. Intensive Cycloalkyl-Fused Pyridines for Aminopyridyl–Zinc–Heteroimidazoles Achieving High Efficiency toward the Ring-Opening Polymerization of Lactides. Molecules. 2024; 29(17):4150. https://doi.org/10.3390/molecules29174150

Chicago/Turabian Style

Wang, Yun, Wenjuan Zhang, Pengjiang Zhu, Wei You, Xiaopan Xue, Rui Wang, Yanping Ma, and Wen-Hua Sun. 2024. "Intensive Cycloalkyl-Fused Pyridines for Aminopyridyl–Zinc–Heteroimidazoles Achieving High Efficiency toward the Ring-Opening Polymerization of Lactides" Molecules 29, no. 17: 4150. https://doi.org/10.3390/molecules29174150

APA Style

Wang, Y., Zhang, W., Zhu, P., You, W., Xue, X., Wang, R., Ma, Y., & Sun, W. -H. (2024). Intensive Cycloalkyl-Fused Pyridines for Aminopyridyl–Zinc–Heteroimidazoles Achieving High Efficiency toward the Ring-Opening Polymerization of Lactides. Molecules, 29(17), 4150. https://doi.org/10.3390/molecules29174150

Article Metrics

Back to TopTop