Epoxide Coupling and the ROP of ε-Caprolactone: Mg and Al

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

CO2/Epoxide Coupling and the ROP of ε‑Caprolactone: Mg and Al Complexes of γ‑Phosphino-ketiminates as Dual-Purpose Catalysts Beesam Raghavendra, P. V. S. Shashank, Madhusudan K. Pandey, and N. Dastagiri Reddy* Department of Chemistry, Pondicherry University, Pondicherry 605 014, India S Supporting Information *

ABSTRACT: γ-Phosphino-ketimines Ph2PC[C(Me)O][C(Me)NAr] (Ar = 2,6-diisopropylphenyl, L1H; Ar = 2,6-difluorophenyl, L2H) were synthesized by treating deprotonated ketimines with PPh2Cl. Their Mg complexes [(L1)2(THF)Mg] (1) and [(L2)2(THF)2Mg] (2) were obtained in excellent yields from a reaction between LH and di-nbutylmagnesium in THF. Addition of a slight excess of trimethylaluminum to LH in toluene yielded the Al complexes [L1AlMe2] (3) and [L2AlMe2] (4). Complexes 1 and 3 displayed high catalytic activity in the synthesis of cyclic carbonates from CO2 and epoxides and also in the ring-opening polymerization (ROP) of ε-caprolactone. However, complexes 2 and 4, which contain fluoro substituents, showed poor activity in the synthesis of cyclic carbonates and could not initiate the ROP reaction. The pentacoordinated Mg complex 1 was found to be a better catalyst than the aluminum complex 3. It was also observed that the complexes 1 and 3 were more efficient than the unsubstituted ketiminate complexes reported in the literature.

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INTRODUCTION Carbon dioxide is abundant, inexpensive, renewable, and nontoxic. Its utilization in making industrially important cyclic carbonates and polycarbonates has been a theme of interest for the past several years.1 Cyclic carbonates are well-known as polar solvents due to their high boiling points and polar nature. They also find applications in the lithium ion battery industry and in the manufacture of thermosetting coatings and fine chemicals.2 Cyclic carbonates can be prepared from CO2 and epoxides, which is a 100% atom economical route. Due to its high C−O bond energy, CO2 is relatively inert and a catalyst is required to activate it. Quite a good number of metal complexes, especially those of Mg, Al, Fe, Co, Zn, and Ti, have been reported for catalyzing the reaction.3 Polycaprolactone (PCL) is a widely explored synthetic biodegradable and biocompatible polymer because of its broad spectrum of applications in biomedical and pharmaceutical industries.4,5 Metal-based catalysis of the ring-opening polymerization (ROP) of ε-caprolactone, with its advantages over a polycondensation reaction, has emerged as a promising tool for the preparation of PCL.6 Complexes of a diverse line of metals such as Al, Mg, Ti, Zn, La, and Y have been employed successfully for producing PCL with good molar mass and narrow dispersity.7 There are many metal complexes reported so far which can efficiently catalyze either the ROP of ε-caprolactone or CO2/ epoxide coupling. Only a handful of reports on catalysts capable of performing both functions, which can be called dual-purpose catalysts, exist in the literature.8 For instance, a bimetallic salphen chromium complex has been used as a catalyst for carrying out the ROP of β-butyrolactone and the copolymerization of CO2/ epoxide.8b © XXXX American Chemical Society

The binding ability of the substrate to the metal center depends on the acidity of the metal center, which can be controlled by tuning the supporting ligand. A few ketiminatebased Mg and Al complexes have been reported to show catalytic activity in the ROP of ε-caprolactone.9,10 Recently, we have demonstrated that the Al complexes of N-benzoyl-N′arylbenzamidinate ligands are much more active than the structurally analogous ketiminate Al complexes (Figure 1).11

Figure 1. Structural resemblance between ketiminate and N-acylamidinate ligands.

The increase in the activity has been achieved by replacing CH by the more electronegative N in the mainframe of the ketiminate ligand, thereby making the metal center more acidic. Very recently, we have described Mg complexes of these ligands as dual-purpose catalysts, which showed much higher activity than their ketiminate counterparts.12 Prompted by these results, we became interested in altering the donor ability of ketiminate ligands by incorporating electronically important groups such as −PPh2 (Figure 1) and probing the catalytic activity of their Mg and Al complexes. In this work, we have synthesized γ-PPh2ketiminate Mg and Al complexes and explored their catalytic Received: January 10, 2018

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DOI: 10.1021/acs.organomet.8b00017 Organometallics XXXX, XXX, XXX−XXX

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Organometallics activity toward the ROP of ε-caprolactone and the synthesis of cyclic carbonates from epoxides and CO2.

proligands, indicating deprotonation. Additionally, complexes 1 and 2 showed resonances for coordinated THF molecules, and 3 and 4 showed a singlet around −0.85 ppm for AlMe2. It was observed that the 31P NMR resonances shifted downfield and appeared in the range of δ −5.26 to −10.61 ppm. This spectral evidence corroborates the formation of complexes 1−4. Further, the solid-state structural analysis of complexes 1−3 was carried out using single-crystal X-ray diffraction studies. Single crystals of complexes 1 and 2 suitable for diffraction studies were grown from saturated solutions of THF at room temperature, whereas the crystals of complex 3 were obtained from a solution of toluene at 0 °C. The ORTEP diagrams of the complexes are depicted in Figures 3−5, and their important bond parameters are given in Table S2. The molecular structure of 1 displays a distorted-trigonal-bipyramidal geometry at the metal center. The distortion is calculated in terms of τ parameter (τ = (β − α)/60), which is equal to 1 for a perfect trigonalbipyramidal geometry and 0 for a perfect square-pyramidal geometry.14 In 1, τ is found to be 0.61 (Figure 3b). O atoms of the ketiminate ligands occupy axial positions with an O1−Mg1− O1′ bond angle (β) of 175.93(14)°. N atoms assume equatorial positions. The N1−Mg1−N1′ bond angle (α), which is the largest among the bond angles in the equatorial plane, is 139.48(13)°. Mg1−N1 and Mg1−O1 distances are comparable to those of the corresponding Mg ketiminate complexes reported in the literature.15 The molecular structure of 2 shows a slightly distorted octahedral geometry with the chelating ketiminate ligands occupying trans positions. The bite angles of the ligands L1 and L2 in 1 and 2 (N1−Mg1−O1) are 84 and 82° respectively. In the Al complex 3, the metal center is in a distorted-tetrahedral geometry with the bond angles at Al ranging from 91.13 to 120.1°. The chelating atoms form an angle of 91° at Al. The Al− N and Al−O distances are similar to those found in a related ketiminate aluminum complex reported in the literature.10a It is noteworthy that the ligands preferred the N,O coordination mode over the N,P and O,P coordination modes, presumably due to the hard nature of the metals. CO2/Epoxide Coupling. The coupling reactions of cyclohexene oxide (CHO) with CO2 using Mg and Al complexes 1−4 as catalysts were carried out at 100 °C under an atmosphere of CO 2 at 140 psi in the presence of the cocatalyst ntetrabutylammonium bromide (TBAB) (Scheme 3). The results are summarized in Table 1. All of the reactions yielded cyclic carbonates exclusively. The Mg complex 1 catalyzed the reaction with 84% conversion of 1000 equiv of CHO and exhibited a TOF of 65 h−1 (Table 1, entry 2), while the Al complex 3 performed the catalysis with 57% conversion and a TOF of 44 h−1 (entry 4). Complexes 2 and 4, which have fluoro substituents on the aryl ring, afforded low conversions of 54% and 16%, respectively, even with 500 equiv of CHO (entries 3 and 6). In our recent work,12 we noticed a dramatic increase in the activity of Mg complexes when the coordinated THF was replaced with the less basic CH3CN. Hence, the Mg complex [(L2)2(CH3CN)2Mg] (5)an acetonitrile analogue of 2was synthesized and examined for its catalytic activity. 5 has been characterized by NMR and single-crystal X-ray techniques. The molecular structure along with significant bond parameters are given in Figure 6. The molecular structure of 5 shows an octahedral geometry around the Mg center. The bond parameters are almost similar to those found in 2 and also a structurally analogous octahedral Mg complex reported in our recent communication.12 The catalytic experiments using 5 revealed only a marginal improvement in the conversion in comparison to

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RESULTS AND DISCUSSION Synthesis of γ-PPh2-ketimines. The γ-phosphino-ketimines Ph2PC[C(Me)O][C(Me)NAr] (Ar = 2,6-diisopropylphenyl, L1H; Ar = 2,6-difluorophenyl, L2H) were synthesized by adopting a procedure reported in the literature for the synthesis of a γ-PPh2-β-diketimine.13 The corresponding ketimines were treated with n-BuLi for 6 h followed by the addition of PPh2Cl to obtain the PPh2-substituted ketimines in excellent yields (Scheme 1). Formation of L1H and L2H was determined by Scheme 1. Synthesis of the Proligands

using standard analytical techniques. The disappearance of a resonance corresponding to γ protons around δ 5.0 ppm and the presence of additional aromatic resonances in their 1H NMR spectra indicated the replacement of γ protons by PPh2. The presence of a singlet in the 31P NMR spectrum (δ −14.27 ppm for L1H and at δ −14.92 ppm for L2H) confirmed the PPh2 substitution. Further, the solid-state structures of L1H and L2H (Figure 2) were determined by single-crystal X-ray diffraction studies, and the important bond parameters are provided in Table S1 in the Supporting Information.

Figure 2. Single-crystal X-ray structures of L1H and L2H. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 40% probability level.

Synthesis and Structural Characterization of γ-PPh2ketiminate-Mg and -Al Complexes. Treating the ketimines L1H and L2H with di-n-butylmagnesium in a 2:1 molar ratio in THF for 4 h afforded the penta- and hexacoordinated Mg complexes [(L1)2(THF)Mg] (1) and [(L2)2(THF)2Mg] (2), respectively, in good yields (Scheme 2). Similar reactions with a slight excess of trimethylaluminum in toluene for 4 h under reflux conditions produced aluminum complexes [L1AlMe2] (3) and [L2AlMe2] (4) (Scheme 2). Complexes 1−4 are unstable toward air and moisture. The 1H NMR spectra of the complexes showed no resonance for −NH, which appears around δ 14.0 ppm in the B

DOI: 10.1021/acs.organomet.8b00017 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 2. Synthesis of Mg and Al Complexes 1−4

Figure 3. (a) Single-crystal X-ray structure of 1. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 40% probability level. (b) Geometry around Mg: α = 139.48(13)°; β = 175.93(14)°; τ = 0.61.

Figure 4. Single-crystal X-ray structure of 2. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 40% probability level.

2 (Table 1, entry 7). Under similar conditions, the synthesis of propylene carbonate (PC) from propylene oxide (PO) and CO2 using complexes 1−5 as catalysts was also studied. 1 showed 100% conversion within 4 h when 1000 equiv of propylene oxide was used, which resulted in a very good TOF of 250 h−1 (entry 8). With the same catalyst loading, 2, 3, and 5 exhibited TOFs of 118, 188, and 148 h−1, respectively (entries 11, 13, and 16). When the PO/1 ratio was increased to 2000, a drastic reduction in the conversion to 60% was observed (entry 9). Without the cocatalyst the Al complex 3 produced the polyether poly(CHO) exclusively (entry 17). The pentacoordinated Mg complex 1 showed better catalytic activity than the aluminum complex 3 under similar conditions in the coupling reactions of both CHO and PO. In general, the Mg complexes have been found to be better catalysts than the corresponding Al catalysts.

Figure 5. Single-crystal X-ray structure of 3. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 40% probability level.

Ring-Opening Polymerization of ε-Caprolactone. The ROP of ε-caprolactone using complexes 1−5 was performed at 70 °C in toluene as solvent (Scheme 4). No cocatalyst was used. The resultant poly(caprolactone) was characterized by NMR spectroscopy. Mn and Mw values of the polymer were obtained by gel permeation chromatography (GPC). The catalytic activities reported here have been termed as per Redshaw’s classification.16 The results of the ROP reactions reflecting the performance of C

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Organometallics Scheme 3. Synthesis of Cyclic Carbonates from CO2/Epoxide

Figure 6. Single-crystal X-ray structure of 5. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 40% probability level. Selected distances (Å): Mg1−O1 1.9746(10), Mg1−N1 2.1597(13), Mg1−N2 2.2559(16), O1−C3 1.2756(17), N1−C1 1.3144(18), C1−C2 1.465(2), C2−C3 1.408(2), C2−P1 1.800(15). Selected bond angles (deg): N1−Mg1−O1 83.61(4), O1−Mg1−N2 90.46(5), N1−Mg1−N2 89.50(5), O1−Mg1−O1′ 180.0, N1−Mg1− N1′ 180.0, N2−Mg1−N2′ 179.999(1).

the catalysts are furnished in Table 2. The Mg complex 1 displayed high activity and completed the ROP of 200 molar equiv of CL within 15 min (Table 2, entry 1). At high monomer concentrations (Table 2, entry 5; [CL]/[Mg] = 1000/1) 1 displayed a TOF of 3520 h−1 and produced the polymer with high molar mass (∼10 × 103). A linear relationship between [CL]/[Mg] ratio and the molar mass of the polymer (Mn), as shown in Figure 7a, indicates a well-controlled polymerization. Complex 2, which is coordinated by the fluoro-substituted ligand L2, was found to be catalytically inactive under these conditions (Table 2, entry 6). A similar observation was also made in our recent work with structurally identical N-benzoyl-N′-arylbenzamidinate-Mg complexes. It was hypothesized that due to the increased acidity of the metal center the THF molecules were tightly held, thereby making the complex inert.17 However, these fluoro-substituted N-benzoyl-N′-arylbenzamidinate-Mg complexes showed good activity when THF was replaced with the less basic acetonitrile. Moreover, the acetonitrile analogue of even the nonfluorinated Mg complex showed double the activity of the THF complex in the ROP of CL as well as in the CO2/ epoxide coupling (in terms of TOF).12 Hence, the acetonitrileMg complex of L2 (5) was explored for its catalytic activity.

Scheme 4. Ring-Opening Polymerization of ε-Caprolactone

However, the catalyst was found to be inactive and the ROP did not take place even after 300 min (Table 2, entry 13). Al complex 3 showed good activity and converted 80% of 1000 molar equiv of CL into poly(caprolactone) within 60 min, and a TOF of 810 h−1 could be achieved in this reaction (Table 2, entry 11). A wellcontrolled polymerization was confirmed by establishing a linear relationship between [CL]/[Al] ratio and the molar mass (Mn) (Figure 7b). The fluoro-substituted Al complex 4 is found to be

Table 1. Synthesis of Cyclic Carbonate from Epoxide and CO2 Catalyzed by Complexes 1−5a entry

complex

epoxide

[epoxide]/[cat.] /[TBAB]

time (h)

1e 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17e

1 1 2 3 3 4 5 1 1 2 2 3 3 4 5 5 3

CHO CHO CHO CHO CHO CHO CHO PO PO PO PO PO PO PO PO PO CHO

1000/1/0 1000/1/25 500/1/25 1000/1/25 500/1/25 500/1/25 500/1/25 1000/1/25 2000/1/25 500/1/25 1000/1/25 500/1/25 1000/1/25 500/1/25 500/1/25 1000/1/25 500/1/0

13 13 13 13 13 13 13 4 4 4 4 4 4 4 4 4

conversn (%)b

TONc

TOFd (h−1)

84 54 57 72 16 61 100 60 83 47 99 75 42 89 59 polyethers

840 270 570 360 80 305 1000 1200 415 470 495 750 210 445 590

64.6 20.7 43.8 27.6 6.1 23.4 250 300 103.7 117.5 123.7 187.5 52.5 111.2 147.5

Reaction conditions: 0.02 mmol of initiator, CO2 (140 psi), 100 °C. bDetermined by comparison of the integrals of resonances arising from the methylene protons for CHO and methyl protons for PO in the 1H NMR spectra. cTON = (epoxide/catalyst) × (conversion)/100. dIn moles of epoxide consumed per mole of initiator per hour (TON/reaction time in hours). eWithout cocatalyst. a

D

DOI: 10.1021/acs.organomet.8b00017 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 2. ROP of ε-Caprolactone Initiated by 1−5a entry

complex

[CL]/[cat.]

time (min)

yieldb (%)

Mn(GPC)c

Mn(calcd)d

TOF (h−1)e

PDI

1 2 3 4 5 6 7 8 9 10 11 12 13

1 1 1 1 1 2 3 3 3 3 3 4 5

200/1 400/1 600/1 800/1 1000/1 200/1 200/1 400/1 600/1 800/1 1000/1 200/1 200/1

15 15 15 15 15 300 60 60 60 60 60 300 300

99 97 94 92 88

23581 42216 63393 87634 96834

22572 44232 64296 83904 100320

792 1552 2256 2944 3520

1.81 1.98 2.09 1.80 1.86

94 91 89 85 81

21570 38657 62933 74112 89774

21432 41496 60876 77520 92340

188 364 534 680 810

1.68 1.74 2.01 2.20 1.76

a Reactions used 0.02 mmol of initiator. All polymerization reactions were carried out at 70 °C in 2 mL of toluene. bIsolated yield. cObtained from GPC analysis using a column calibrated by polystyrene standard, multiplied by a correcting factor of 0.56.18 dTheoretical Mn = (monomer/initiator) × (isolated yield) × (Mw of ε-CL). eIn moles of ε-CL consumed per mole of initiator per hour (TON/reaction time in hours).

Figure 8. Plot of ln([CL]0/[CL]t) vs time for the polymerization of εCL catalyzed by 1. Conditions: [CL]/[Mg] = 200/1 in 2 mL of toluene at 70 °C.

Figure 7. (a) Plot of molar mass (Mn) and dispersity (PDI) vs [CL]/ [Mg] for the polymerization of ε-CL using complex 1 at 70 °C (entries 1−5 in Table 2). (b) Plot of number-averaged molar mass (Mn) and dispersity (PDI) vs [CL]/[Al] for the polymerization of ε-CL using complex 3 at 70 °C (entries 7−11 in Table 2). In both plots, red squares (■) represent Mn (corrected) values and blue triangles (▲) represent PDI values.

Figure 9. Plot of ln([CL]0/[CL]t) vs time for the polymerization of εCL catalyzed by 3. Conditions: [CL]/[Al] = 200/1 in 2 mL of toluene at 70 °C.

inactive and did not produce the polymer even after 5 h (Table 2, entry 12). Kinetic experiments were carried out using 1 and 3, and Figures 8 and 9 display the semilogarithmic plots of ln([CL]0/[CL]t) versus time, respectively. The plots indicate that the reactions proceed with first-order dependence on the monomer with induction periods of ∼3 and ∼12 min, respectively. The induction period can be attributed to the insertion of the first CL molecule into the M−O(ketiminate) bond, which is part of the chelate ring system stabilized by resonance. Further insertions are relatively easier, as they involve

M−O(polymer) bonds. The different induction periods displayed by 1 and 3 correlate well with the difference in the Mg−O and Al−O bond energies. An overall comparison of the γPPh2-ketiminate-Mg and -Al complexes (1 and 3) synthesized in this work with the unsubstituted ketiminate complexes reported in the literature reveals that the former are more efficient than the latter, when the reactions are conducted in the absence of a cocatalyst (Chart 1).9b,10a However, it was recently reported that the addition of BnOH improved the activity of ketiminate-Al E

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Organometallics

Chart 1. Comparison of Catalytic Efficiencies of Ketiminate and N-Acyl-N′-substituted Amidinate Mg/Al Complexes with the −PPh2-Substituted Ketiminate Complexes 1 and 3 in the ROP of ε-CL

Figure 10. 1H NMR spectra of a reaction between 3 and ε-CL in a 1/5 ratio in benzene-d6: (A) complex 3 at room temperature. (B) ε-CL and 3 at room temperature; (C, D) ε-CL and 3 at 70 °C after 30 and 60 min, respectively.

complexes manifold.7a Hence, a reaction between 200 equiv of CL and 1 equiv of 3 was probed in the presence of 2 equiv of BnOH. Surprisingly, the reaction produced the polymer in comparatively low yields. Our earlier experiments with Al complexes had proved that the Al-Me groups were not involved in the ROP.11a To ascertain if in the case of 3 also the Al-Me groups have no role in the ROP, a reaction between 3 and ε-caprolactone in 1/5 molar ratio was monitored by NMR. The 1H NMR spectra are shown in Figure 10. It is clear from the spectra that there is no change in the chemical shift of the Al-Me resonance, which overrules their involvement in the ring opening. On the basis of these findings, a

coordination−insertion mechanism, as shown in Scheme 5, has been proposed.10a,11a

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CONCLUSION In this account of work, magnesium and aluminum complexes of two γ-PPh2-ketiminate ligands with different steric and electronic environments were synthesized and characterized. The catalytic activity of the complexes was explored toward the ring-opening polymerization of ε-caprolactone and the synthesis of cyclic carbonates via the insertion of CO2 into cyclohexene oxide and propylene oxide. In the ROP of ε-caprolactone, complexes 1 and 3, which contain isopropyl groups in the ortho positions of the Nphenyl group of the ketimnate ligand, were found to be F

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Organometallics

atoms were refined anisotropically. The contribution of the hydrogen atoms, in their calculated positions, was included in the refinement using a riding model. Upon convergence, the final Fourier difference map of the X-ray structures showed no significant peaks. In the molecular structures of complexes 2 and 5 two molecules of THF and one molecule of CH3CN were found, respectively, but they could not be refined because of serious disorder. Therefore, a new data set corresponding to omission of the missing solvent was generated with the SQUEEZE algorithm and the structure was refined to convergence. Relevant data concerning crystallographic data, data collection, and refinement details are summarized in Table S3 in the Supporting Information. Crystallographic information files (CIF) for the structures reported in this paper have also been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. 1543917 (L1H), 1543918 (L2H), 1543920 (1), 1543923 (2), 1543928 (3), and 1812549 (5). General Procedure for the Preparation of Proligands L1H and L2H. To a stirred solution of the corresponding ketimine19 in toluene was added n-BuLi at −78 °C. The mixture was warmed to room temperature and stirred for 6 h. The reaction mixture was again cooled to −78 °C, and a solution of chlorodiphenylphosphine in toluene was added. The reaction mixture was warmed to room temperature, stirred for 20 h, and filtered. All volatiles were removed from the filtrate under vacuum, and the residue was recrystallized from a mixture of hexane and toluene. L1H: N-(2,6-diisopropylphenyl)ketimine (1.20 g, 4.62 mmol), toluene (30 mL), n-BuLi (3.47 mL, 1.6 M in hexane, 5.55 mmol), chlorodiphenylphosphine (1.12 g, 5.09 mmol, in 5 mL of toluene). Yield: 92% (1.88 g). Mp: 144−146 °C. 1H NMR (CDCl3, 400 MHz): δ 14.27 (br, 1H, NH), 7.48−7.40 (m, 4H, Ar H), 7.29−7.25 (m, 4H, Ar H), 7.21−7.16 (m, 3H, Ar H), 7.09−7.07 (d, 2H, Ar H), 2.93−2.86 (m, 2H, CH(CH3)2), 2.14−2.13 (d, 3H, COCH3), 1.55 (d, 3H, CNCH3), 1.13−1.11 (d, 6H, CH(CH3)2), 1.08−1.06 (d, 6H, CH(CH3)2). 31P NMR (161 MHz, CDCl3): δ −14.27 (s). 13C NMR (100 MHz, CDCl3): δ 201.74, 201.49, 173.51, 173.33, 145.32, 138.13, 138.00, 133.91, 130.96, 130.78, 129.18, 128.55, 128.50, 127.36, 123.90, 96.44, 96.35, 31.08, 30.95, 28.86, 24.59, 22.83, 20.64, 20.47. HRMS (ESI): m/z calcd for C29H35NOP [M + H]+ 444.2456, found 444.2463. Anal. Calcd for C29H34NOP: C, 78.53; H, 7.73; N, 3.16. Found: C, 78.20; H, 7.78; N, 3.18. L2H. N-(2,6-difluorophenyl)ketimine (0.80 g, 3.79 mmol), toluene (20 mL), n-BuLi (2.84 mL, 1.6 M in hexane, 4.54 mmol), chlorodiphenylphosphine (0.92 g, 4.17 mmol, in 3 mL of toluene). Yield: 83% (1.24 g). Mp: 174−176 °C. 1H NMR (400 MHz, CDCl3): δ 14.20 (br, 1H, Ar H), 7.51−7.47 (t, 4H, Ar H), 7.38−7.34 (m, 4H, Ar H), 7.30−7.21 (m, 3H, Ar H), 7.00−6.96 (m, 2H, Ar H), 2.04 (d, 3H, CH3), 2.02 (s, 3H, CH3). 31P NMR (161 MHz, CDCl3): δ (ppm) −14.92 (s). 13C NMR (100 MHz, CDCl3): 202.65, 202.48, 172.38, 172.11, 159.38, 159.34, 156.89, 156.85, 137.43, 137.31, 131.15, 130.97, 128.63, 128.58, 128.37, 128.28, 128.18, 127.55, 116.63, 116.47, 116.30, 112.22, 112.16, 112.03, 111.98, 99.37, 99.26, 31.16, 31.07, 20.39, 20.16. HRMS (ESI): m/z calcd for C23H21F2NOP [M + H]+ 396.1328, found 396.1335. Anal. Calcd for C23H20F2NOP: C, 69.87; H, 5.10; N, 3.54. Found: C, 69.76; H, 5.17; N, 3.56. Synthesis of γ-PPh2-ketiminate-Mg and -Al Complexes. General Procedure for the Preparation of Magnesium Complexes. To a stirred solution of L1H or L2H in THF or CH3CN was added di-nbutylmagnesium at 0 °C. The mixture was warmed to room temperature and stirred for 4 h. The reaction mixture was concentrated by applying vacuum until a white precipitate was obtained. The precipitate was filtered and washed with hexane followed by drying under high vacuum. [(L1)2(THF)Mg] (1). L1H (0.40 g, 0.90 mmol), THF (20 mL), MgBu2 (0.45 mL, 1 M in heptane, 0.45 mmol). Yield: 83% (0.40 g). Mp: 201− 203 °C. Dissolving the precipitate in hot THF and keeping the solution at room temperature overnight afforded pale yellow crystals. 1H NMR (400 MHz, CDCl3): δ 7.42−7.38 (t, 8H, Ar H), 7.26−7.22 (m, 8H, Ar H), 7.18−7.14 (t, 4H, Ar H), 7.02 (s, 6H, Ar H), 3.84−3.83 (d, 4H, OCH2CH2), 2.96−2.89 (m, 4H, CH(CH3)2), 1.99−1.96 (t, 4H, OCH2CH2), 1.60 (d, 6H, COCH3), 1.42 (s, 6H, CNCH3), 1.23 (d, 12H, CH(CH3)2), 1.00 (d, 12H, (CH(CH3)2). 31P NMR (161 MHz,

Scheme 5. Plausible Mechanism for the Ring-Opening Polymerization of ε-Caprolactone Initiated by 3.

catalytically highly active. The complexes containing fluoro substituents (2, 4, and 5), on the other hand, are inactive. These fluoro-substituted complexes also showed relatively poor activity in comparison to 1 and 3 in the synthesis of cyclic carbonates via the insertion of CO2 into epoxides. This work and our earlier work suggest that the fluoro substitution on the ligand does not enhance the catalytic activity of the complex in these reactions but rather decreases it.12 A comparison of these results with the literature reports revealed that catalysts 1 and 3 were much more efficient than the structurally analogous ketiminate Mg and Al complexes in initiating the ROP of ε-caprolactone in the absence of an alcohol cocatalyst. Contrary to a literature report,7a wherein a ketiminate-Al complex showed very high activity in the presence of 2 equiv of BnOH, complex 3 exhibited lower activity in the presence of either BnOH or iPrOH. Complexes 1, 3, and 5 were also found to be catalytically active toward the synthesis of cyclic carbonates via the insertion of CO2 into epoxides in the presence of tetrabutylammonium bromide as a cocatalyst. To the best of our knowledge, these are the first examples of ketiminate complexes used as catalysts in the synthesis of cyclic carbonates.

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EXPERIMENTAL SECTION

General Procedures. All manipulations were carried out using standard Schlenk line and glovebox techniques under an atmosphere of dry nitrogen. Trimethylaluminum, acetylacetone, aniline, 2,6-difluoroaniline, 2,6-diisopropylaniline, chlorodiphenylphosphine, and di-nbutylmagnesium were procured from Aldrich and used as received. εCaprolactone, cyclohexene oxide, and propylene oxide were purchased from Acros Organics, stirred over CaH2 for 24 h, and distilled. Ketimines were prepared by following the literature procedures.19 Tetrahydrofuran, toluene, and hexane were distilled fresh from Na/benzophenone ketyl before using. CDCl3 was distilled from calcium hydride. C6D6 was dried over sodium metal. NMR spectra were recorded on a Bruker 400 MHz (1H NMR, 400 MHz; 13C NMR, 100 MHz; 31P NMR, 161 MHz) NMR spectrometer. No reference was used for 31P NMR spectra. The molar mass and the dispersity of the polymers were measured against polystyrene standards by GPC (gel permeation chromatography) equipped with two 7.5 mm × 300 mm Agilent PLGel columns (5 μm pore size) at 20 °C using tetrahydrofuran as eluent. HRMS data were recorded on an Agilent 6540 UHD Q-TOF mass spectrometer. Elemental analyses were performed using a Thermo Scientific Flash 2000 CHNS analyzer. The purity of complexes 1−5 has been established by assigning all the peaks in 1H and 13C NMR. X-ray Crystallographic Studies. Single crystals of L1H, L2H, 1−3, and 5 were mounted on glass fibers in paraffin oil and then brought into the cold nitrogen stream of a low-temperature device so that the oil solidified. Data collection was performed on an OXFORD XCALIBUR diffractometer, equipped with a CCD area detector, using graphitemonochromated Mo Kα (λ = 0.71073 Å) radiation and a lowtemperature device. All calculations were performed using SHELXS-97 and SHELXL-97.20 The structures were solved by direct methods and successive interpretation of the difference Fourier maps, followed by full-matrix least-squares refinement (against F2). All non-hydrogen G

DOI: 10.1021/acs.organomet.8b00017 Organometallics XXXX, XXX, XXX−XXX

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Organometallics CDCl3): δ (ppm) −5.66 (s). 13C NMR (100 MHz, CDCl3): δ (ppm) 191.99, 191.59, 178.86, 178.78, 147.41, 140.01, 139.82, 139.67, 130.90, 130.72, 128.56, 128.14, 128.09, 126.66, 124.17, 123.27, 97.04, 96.98, 68.94, 29.24, 29.02, 27.96, 26.39, 26.29, 25.76, 24.74, 24.46. [(L2)2(THF)2Mg] (2). L2H (0.55 g, 1.39 mmol), THF (25 mL), MgBu2 (0.69 mL, 1 M in heptane, 0.69 mmol). Yield: 85% (0.56 g). Mp: 162− 164 °C. Dissolving the precipitate in hot THF and keeping the solution at room temperature overnight afforded pale yellow crystals. 1H NMR (400 MHz, CDCl3): δ (ppm) 7.58−7.52 (m, 4H, Ar H), 7.41−7.26 (m, 6H, Ar H), 7.26−7.19 (m, 7H, Ar H), 7.16−7.08 (m, 3H, Ar H), 7.00− 6.88 (m, 4H, Ar H), 6.73−6.67 (m, 2H, Ar H), 3.67−3.64 (t, 10H, OCH2CH2), 1.77−1.74 (m, 10H, OCH2CH2), 1.60 (d, 3H, CH3), 1.35 (d, 6H, CH3), 1.33 (d, 3H, CH3). 31P NMR (161 MHz, CDCl3): δ (ppm) −5.26 (s), −8.39 (d). 13C NMR (100 MHz, CDCl3): δ (ppm) 193.11, 192.70, 183.78, 183.29, 182.70, 180.45, 180.38, 157.57, 157.23, 155.15, 154.79, 153.58, 139.70, 139.57, 138.69, 138.57, 138.33, 138.16, 135.99, 135.81, 132.30, 132.13, 131.41, 131.23, 131.14, 131.00, 130.82, 130.65, 130.60, 130.36, 130.18, 128.56, 128.51 128.46, 128.27, 128.22, 128.17, 127.84, 127.62, 127.22, 126.98, 126.67, 124.77, 112.08, 111.87, 111.65, 111.02, 104.99, 104.86, 97.29, 97.23, 68.13, 29.10, 28.88, 25.90, 25.74, 25.63, 25.39, 25.01, 24.92. [(L2)2(CH3CN)2Mg] (5). L2H (0.60 g, 1.51 mmol), CH3CN (30 mL), MgBu2 (0.83 mL, 1 M in heptane, 0.83 mmol). Yield: 88% (0.59 g). Mp: 202−204 °C. Dissolving the precipitate in hot acetonitrile and keeping the solution at room temperature overnight afforded pale yellow crystals. 1H NMR (400 MHz, CDCl3): δ (ppm) 7.66−7.60 (m, 4H, Ar H), 7.41−7.31 (m, 5H, Ar H), 7.29−7.17 (m, 11H, Ar H), 7.08−6.97 (m, 4H, Ar H), 6.81−6.75(m, 2H, Ar H), 2.00 (s, 3H, CH3CN), 1.67− 1.66 (s, 3H, CH3), 1.42 (s, 3H, CH3), 1.40 (s, 3H, CH3), 1.31 (s, 3H, CH3), 31P NMR (161 MHz, CDCl3): δ (ppm) −5.27 (s), −8.40 (d). 13C NMR (100 MHz, CDCl3): δ (ppm) 193.11, 192.70, 183.29, 139.69, 139.56, 138.57, 138.16, 132.30, 132.12, 131.41, 130.84, 130.74, 130.65, 130.60, 130.36, 130.18, 128.56, 128.51, 128.47, 128.27, 128.22, 127.63, 127.22, 126.99, 126.67, 116.52, 112.08, 111.87, 104.99, 104.86, 97.23, 29.10, 28.88, 25.91, 25.64, 25.39, 25.01, 24.92, 22.79, 21.14, 2.05. General Procedure for the Synthesis of Aluminum Complexes. A solution of L1H or L2H in toluene was cooled to −78 °C in a 100 mL Schlenk flask, and AlMe3 was added. The reaction mixture was warmed to room temperature, and the mixture was refluxed for 4 h. The solvent was removed under vacuum to obtain a solid residue. [L1AlMe2] (3). L1H (0.30 g, 0.67 mmol), toluene (20 mL), AlMe3 (0.37 mL, 2 M in toluene, 0.74 mmol). Yield: 87% (0.293 g). Mp: 194 °C dec. Dissolving the solid residue in minimum amount of toluene and keeping the solution at 0 °C for 2 days afforded yellow crystals. 1H NMR (400 MHz, CDCl3): δ (ppm) 7.51−7.47 (t, 4H, Ar H), 7.38−7.34 (m, 4H, Ar H), 7.30−7.26 (m, 2H, Ar H), 7.20−7.18 (d, 1H, Ar H), 7.13− 7.11 (t, 2H, Ar H), 2.86−2.76 (m, 2H, CH(CH3)2), 2.27 (d, 3H, −COCH3), 1.51 (s, 3H, −CNCH3), 1.20 (d, 6H, CH(CH3)2), 0.98 (d, 6H, CH(CH3)2), −0.86 (s, 6H, AlMe2). 31P NMR (161 MHz, CDCl3): δ (ppm) −10.61(s). 13C NMR (100 MHz, CDCl3): δ (ppm) 189.77, 189.44, 180.34, 180.23, 141.90, 139.79, 136.64, 136.50, 131.03, 130.86, 128.79, 128.74, 127.85, 127.34, 124.44, 103.88, 103.76, 28.22, 27.56, 27.38, 24.63, 24.32, 24.21, 24.10, −11.70. HRMS (ESI): m/z calcd for C31H40AlNOP [M + H]+ 500.2662, found 500.2659. [L2AlMe2] (4). L2H (0.45 g, 1.13 mmol), toluene (30 mL), AlMe3 (0.62 mL, 2 M in toluene, 1.25 mmol). Yield: 76% (0.39 g). Mp: 129 °C. The solid residue was purified by washing it with n-hexane (6 mL). 1H NMR (400 MHz, CDCl3): δ (ppm) 7.50−7.40 (m, 4H, Ar H), 7.39− 7.36 (m, 4H, Ar H), 7.32−7.28 (m, 2H, Ar H), 7.22−7.18 (d, 1H, Ar H), 6.98−6.94 (m, 2H, Ar H), 2.20 (d, 3H, −COCH3), 1.78 (s, 3H, −CNCH3), −0.85 (d, 6H, AlMe). 31P NMR (161 MHz, CDCl3): δ (ppm) −9.46 (s). 13C NMR (100 MHz, CDCl3): δ (ppm) 192.29, 192.00, 182.75, 182.59, 157.72, 157.68, 155.24, 155.20, 136.49, 136.35, 131.02, 130.84, 128.84, 128.79, 127.92, 127.88, 121.59, 112.40, 112.34, 112.22, 112.16, 103.45, 103.33, 28.36, 28.19, 23.95, 23.80, −11.89. HRMS (ESI): m/z calcd for C25H26AlF2NOP [M + H]+ 452.1535, found 452.1512. ROP of ε-Caprolactone Catalyzed by 1−5. A mixture of the catalyst and ε-caprolatone in toluene (2 mL) was stirred at 70 °C for the specified duration. To terminate the polymerization, several drops of

glacial acetic acid (∼2 mL) were added to the reaction mixture. The resultant viscous solution was dissolved in dichloromethane and was transferred to a flask containing cold methanol (50 mL) with stirring. The polymer, which was precipitated, was collected by filtration, washed with cold methanol, and dried under vacuum to obtain a white solid. Insertion of CO2 into Epoxides Catalyzed by 1−5 in the Presence of Quaternary Ammonium Salt (n-Bu4NBr). In a typical procedure for the cycloaddition of epoxide with CO2, the catalyst, epoxide, and quaternary ammonium salt were taken in a 50 mL highpressure reactor inside the glovebox. The reactor was brought out, pressurized with CO2, and placed in an oil bath maintained at 100 °C, and the contents were stirred using a magnetic stirrer. After the completion of the stipulated time, the reactor was cooled in an ice bath before the excess pressure was slowly released. The resultant mixture was analyzed by using 1H NMR.

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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.8b00017. Crystallographic data for L1H, L2H, 1−3, and 5 and 1 H/13C and 31P NMR spectra of all compounds (PDF) Accession Codes

CCDC 1543917−1543918, 1543920, 1543923, 1543928, and 1812549 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 data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for N.D.R.: [email protected]. ORCID

N. Dastagiri Reddy: 0000-0003-2290-9445 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Department of Science and Technology (DST), New Delhi, India, for financial support and the Alexander von Humboldt Stiftung of Germany for the donation of a glovebox. B.R. thanks the UGC for a fellowship. We thank the Central Instrumentation Facility, Pondicherry University, for NMR spectra and DST-FIST for the singlecrystal X-ray diffraction facility.

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REFERENCES

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DOI: 10.1021/acs.organomet.8b00017 Organometallics XXXX, XXX, XXX−XXX