Evolution of aluminum aminophenolate complexes in the ring-opening polymerization of ε-caprolactone: electronic and amino-chelating effects

Abstract

A series of aluminum complexes bearing phenolate (O–Al and O₂–Al), biphenolate (OO–Al type), aminophenolate (ON–Al), aminobiphenolate (ONO–Al), bis(phenolato)bis(amine) (NNOO–Al), and Salan (ONNO–Al) type ligands were synthesized. ε-Caprolactone (CL) polymerization using these aluminum complexes as catalysts was investigated. The overall polymerization rates of Al catalysts with different ligands were found to be in the following order (k_obs values): ON^Br–Al (0.124 min⁻¹) ≥ O^Br₂–Al (0.121 min⁻¹) > ONNO^Br–Al (0.054 min⁻¹) > NNO^Br–Al (0.044 min⁻¹) ≥ ONO^Br–Al (0.043 min⁻¹) > O^Br–Al (0.033 min⁻¹) > NNOO^Br–Al (0.015 min⁻¹) ≥ ^BuONNO^Bu–Al (0.001 min⁻¹) = OO^Br–Al (0.001 min⁻¹). In addition, Al complexes with electron-donating substituents on ligands exhibited higher catalytic activity than those with bromo substituents. Density functional theory (DFT) calculations revealed that a dinuclear Al complex with two bridging methoxides had to rearrange to a phenolate bridged dinuclear Al complex with terminal methoxides. This is due to the low initiating ability of two bridging benzyl alkoxides. Combining the polymerization data and DFT results, it was concluded that the electron-donating substituents on the phenolate ring and chelating amino group enhance the electron density of the Al center. This may prevent the formation of a less active dinuclear Al complex with two bridging alkoxides (initiators) or facilitate its structural rearrangement. O^OMe–Al has been established as a powerful candidate with a high polymerization rate and it exhibits well-controlled polymerization for synthesizing the mPEG-b-PCL copolymer.

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Introduction

Poly-ε-caprolactone (PCL) is an advantageous biomaterial in various fields.¹ This is mainly due to its biodegradability,² biocompatibility,³ and permeability.⁴ Given the increasing demand for PCL, synthesizing PCL effectively remains a topic of concern. Compared to the traditional polycondensation,⁵ ring-opening polymerization using metal complexes⁶ as catalysts exhibited greater catalytic activity and controllability. Aluminum complexes⁷ proved to be effective catalysts for cyclic ester ROP combining both high conversion and ease of synthesis. Ligand design is closely related to the catalytic activity of Al catalysts due to the influence of ligands through inductive, steric, and chelating effects. The investigation of the inductive effect on the catalytic activity of cyclic ester polymerization by Al catalysts is illustrated in Fig. 1, and lots of Al complexes, including enolic Salen,⁸ Schiff base,⁹ 8-quinolinolate,¹⁰ ketiminate,¹¹ β-diketiminate,¹² amidinate,¹³ β-quinolyl-enaminate,¹⁴ and phenoxy-thiother¹⁵ ligands, showed higher activity with electron-withdrawing groups. However, Tolman's group¹⁶ reported a series of five-coordinate Al complexes bearing bis(phenolato)bis(amine) ligands (NNOO type) and their application in CL polymerization (Fig. 2A), and the polymerization results revealed that Al complexes with electron-donating groups, such as tert-butyl and methoxy groups, on the aromatic position para to the phenoxide donor oxygen exhibited higher catalytic activity compared to Al complexes with electron-withdrawing groups, such as bromo substituents. The same catalytic behavior of five-coordinate Al complexes bearing bis(phenolato)bis(amine) ligands for rac-lactide (rac-LA) polymerization was also reported by Gibson's group¹⁷ (Fig. 2B). These results attracted the attention of numerous researchers because many electron-deficient Al complexes^{9c–e,10,12,13,18} with electron-withdrawing substituents of ligands exhibited higher catalytic activity for cyclic ester ROP compared to electron-rich Al complexes with ligands holding electron-donating substituents.

Fig. 1 Classification of the catalytic activity of cyclic esters using Al complexes by the electronic tendency.

Fig. 2 Five-coordinate Al complexes bearing bis(phenolato)bis(amine) ligands and their CL polymerization results reported by (A) Tolman and (B) Gibson groups.

According to the kinetic data,^16b Tolman's group reported a plausible mechanism (Fig. 3) to explain why Al complexes with electron-donating groups exhibited higher catalytic activity for CL polymerization. Electron-rich Al complexes could create a vacant coordination site for CL coordination through the amine arm dissociation. However, the amine arm dissociation for electron-deficient Al complexes is hampered because of the enhanced Lewis acidity of the Al center by the electron-withdrawing groups of the ligands, and the sterically crowded Al center is not conducive to CL coordination to form a less stable octahedral intermediate.

Fig. 3 Mechanism of five-coordinate Al complexes bearing different bis(phenolato)bis(amine) ligands for CL ROP.

Tolman's study proposed a mechanism and the related kinetic data to explain why electron-rich Al complexes bearing bis(phenolato)bis(amine) ligands showed higher catalytic activity and polymerization properties different from other Al catalysts.^{9c–e,10,12,13,18} Inspired by Tolman's explanation, we sought to determine whether electron-rich Al complexes still have higher catalytic activity than electron-deficient Al complexes with four-coordinate Al structures, such as ONO–Al, NO–Al, and OO–Al complexes (Fig. 4) for CL ROP. How does the amine arm affect the polymerization rate of the catalyst? What role did the amine arm play at this time? Herein, a series of ONO–Al complexes (Scheme 1A), NO–Al complexes (Scheme 1B), OO–Al complexes (Scheme 1D), O₂–Al complexes (Scheme 1E), O–Al complexes (Scheme 1F), NNOO–Al complexes (Scheme 1G), and NNO–Al complexes (Scheme 1H) were synthesized, and their CL polymerization was investigated.

Fig. 4 Three- and four-coordinate Al complexes were studied for CL polymerization.

Scheme 1 Synthesis of ligands and associated Al complexes in this study.

In addition, ONNO–Salan Al complexes^18c,h,19 were reported with cyclic ester polymerization, as shown in Fig. 5. Gibson's, Feijen's, and Hormnirun's results of rac-LA ROP using ONNO–Salan Al complexes as catalysts showed that the Al complex with chloro substituents on the Salan ligand exhibited higher catalytic activity compared to Al complexes with methyl substituents. However, the steric effects between the chloro and methyl groups on the aromatic position ortho to the phenoxide donor oxygen are different,²⁰ and the catalytic comparison of the electronic effect is unfair. To confirm the catalytic properties of ONNO–Salan Al complexes for CL ROP, a series of ONNO–Salan Al complexes (Scheme 1C) were synthesized, and their CL polymerization was studied.

Fig. 5 rac-LA ROP using ONNO–Salan Al complexes as catalysts.

Results and discussion

Synthesis and characterization of Al complexes

For the synthesis of ONO-type ligands (Scheme 1A), a solution of two equivalents of 6-tert-butyl-4-substituted-phenol, one equivalent of aqueous formaldehyde, and one equivalent of benzylamine in ethanol was refluxed for 2 days. For the synthesis of ON-type ligands (Scheme 1B), para-formaldehyde, pyrrolidine, and 6-tert-butyl-4-substituted-phenol (1 : 1 : 1) were set in ethanol and stirred for 2 days. For the synthesis of ONNO-type ligands (Scheme 1C), two equivalents of 2-hydroxy-5-substituted-benzaldehyde were reacted with one equivalent of ethylenediamine to form the Salen ligands first, and sodium borohydride was used to reduce the Salen ligands to form Salan ligands. The Salan ligands reacted with formaldehyde (37% aqueous solution) in ethanol, and the solution was refluxed for 4 h. Sodium borohydride was transferred to the solution and refluxed for one day. ^BuONNO^OMe–OH and ^BuONNO^Br–OH were synthesized by following the literature procedure.²¹ For the synthesis of OO type ligands²² (Scheme 1D), 4-chlorobenzaldehyde and 6-tert-butyl-4-substituted-phenol (1 : 1) were combined in hexane with para-tolyl sulfonic acid as a catalyst and refluxed for two days. The ligand “OO^H–H” reacted with bromine to form OO^Br–H. NNOO^OMe–OH and NNOO^Br–OH (Scheme 1G) were synthesized following the literature procedure.^16aNNO^OMe–OH and NNO^Br–OH (Scheme 1H) were synthesized by the reaction of para-formaldehyde, N¹-benzyl-N²,N²-dimethylethane-1,2-diamine, and 6-tert-butyl-4-substituted-phenol (1 : 1 : 1) in ethanol and stirred for 1 day. All Al complexes were synthesized from ligands with a stoichiometric amount of AlMe₃ in toluene in quantitative yield. The ¹H NMR peak of the proton on phenolate disappeared after reacting with AlMe₃, and the presence of methyl groups on Al atoms was observed. Since L₂AlMe type complexes bearing 2,6-di-tert-butyl-4-methoxyphenolate (Scheme 1E) could not be isolated, 2,6-di-tert-butyl-4-methylphenol was used to synthesize O^Me₂–Al. The crystals of dinuclear ONO^OMe–AlOH (CCDC 2247652†) were obtained from the NMR tube in which ONO^OMe–Al was dissolved in CDCl₃ solution, and ONO^OMe–AlOH was formed by the reaction of ONO^OMe–Al and moisture. The Al atoms revealed a distorted trigonal bipyramidal geometry (τ₅ = 0.84)²³ with one nitrogen and two oxygen atoms of the ligand and two hydroxyl groups (Fig. 6). The structure of dinuclear ONO^OMe–AlOH implies that ONO^OMe–AlMe may react with benzyl alcohol (BnOH) to form the dinuclear Al complex with bridging benzyl alkoxide.

Fig. 6 Molecular structure of ONO^OMe–AlOH depicted as 50% probability ellipsoids (all hydrogen atoms are omitted for clarity).

The high concentration of ON^OMe–Al in toluene was placed in the freezer (−20 °C) for one week, and then the crystals of mononuclear ON^OMe–Al (CCDC 2247645†) were obtained. The Al atoms revealed a distorted tetrahedral geometry (τ₄ = 0.92)²⁴ with one nitrogen and one oxygen atom of the ON^OMe ligand and two methyl groups (Fig. 7).

Fig. 7 Molecular structure of ON^OMe–Al depicted as 50% probability ellipsoids (all hydrogen atoms are omitted for clarity).

A high concentration of ONNO^OMe–Al in THF was placed in the freezer (4 °C) for one month, and then the crystals of mononuclear ONNO^OMe–Al (CCDC 2236052†) were obtained. The Al atom revealed a distorted quadrangular pyramidal geometry (τ₅ = 0.75)²³ with two nitrogen and two oxygen atoms of Salan ligand and one methyl group (Fig. 8).

Fig. 8 Molecular structure of ONNO^OMe–Al depicted as 50% probability ellipsoids (all hydrogen atoms are omitted for clarity).

The method for obtaining the crystals of mononuclear ONNO^Br–Al (CCDC 2247643,†Fig. 9) was similar to that for ONNO^OMe–Al. The Al atom revealed the distorted quadrangular pyramidal geometry (τ₅ = 0.79)²³ that was similar to ONNO^OMe–Al (Fig. 8). The crystal data revealed that the electron-withdrawing groups, such as the bromo substituent, reduce the electron density of the Al atom and increase the bond strength between the Al and the methyl group.

Fig. 9 Molecular structure of ONNO^Br–Al depicted as 50% probability ellipsoids (all hydrogen atoms are omitted for clarity).

A high concentration of OO^Bu–Al in tetrahydrofuran (THF) was placed in the freezer (−20 °C) for three days, and then the crystals of mononuclear OO^Bu–Al (CCDC 2247640†) were obtained. The Al atoms revealed a distorted tetrahedral geometry (τ₄ = 0.92)²⁴ with two oxygen atoms of the OO^Bu ligand, one methyl group, and one oxygen atom of THF (Fig. 10).

Fig. 10 Molecular structure of OO^Bu–Al depicted as 50% probability ellipsoids (all hydrogen atoms are omitted for clarity).

Polymerization of ε-caprolactone

Table 1 presents the catalytic activity results for CL polymerization at 25 °C in toluene in the presence of benzyl alcohol (BnOH). The polymerization results revealed that Al complexes with an electron-donating group exhibited higher catalytic activity than Al complexes with an electron-withdrawing group.

Table 1 ε-Caprolactone polymerization with Al complexes as catalysts^a

Entry	Cat [CL] : [Cat] : [BnOH]	Time (min)	Conv.^b (%)	M nCal	M nNMR	M nGPC	Đ	k obs × 10^−3 (error) min^−1
a In general, the reaction was carried out in toluene with [CL] = 2.00 M. b Calculated from the molecular weight of Mw(CL) × [CL]0/[BnOH]0 × conversion yield + Mw(BnOH). c Data were obtained through ^1H NMR analysis. MnNMR was calculated as follows: molecular weight of CL × (integration of the peak at 4.0 ppm/integration of the peak at 7.3 ppm) × 5/2 + Mw(BnOH). d Obtained through gel permeation chromatography (GPC). Values of MnGPC were measured by the GPC and multiplied by 0.56.
1	ONO^OMe–Al	85	94	10 800	7900	17 300	1.19	48.5 (1)
	100 : 1 : 1
2	ONO^Br–Al	70	93	10 900	13 500	18 400	1.13	43.2 (1)
	100 : 1 : 1
3	ON^OMe–Al	20	>99	5800	9900	7800	1.32	152.0 (3)
	100 : 1 : 2
4	ON^OMe–Al	40	90	10 400	14 900	15 500	1.33	57.6 (16)
	100 : 1 : 1
5	ON^Br–Al	20	94	5500	10 500	7500	1.37	124.0 (1)
	100 : 1 : 2
6	ON^Br–Al	140	92	10 600	9900	10 200	1.19	17.3 (7)
	100 : 1 : 1
7	ONNO^OMe–Al	75	95	10 900	11 700	17 000	1.41	92.4 (2)
	100 : 1 : 1
8	ONNO^Br–Al	70	92	10 600	10 400	11 200	1.34	53.8 (32)
	100 : 1 : 1
9	^BuONNO^OMe–Al	1500	90	10 400	9300	6300	1.05	1.6 (1)
	100 : 1 : 1
10	^BuONNO^Br–Al	1680	90	10 400	8800	6700	1.04	1.4 (1)
	100 : 1 : 1
11	OO^OMe–Al	1620	92	10 600	11 900	18 100	1.17	1.4 (1)
	100 : 1 : 1
12	OO^Br–Al	2460	90	10 400	14 100	10 400	1.10	1.0 (1)
	100 : 1 : 1
13	OO^Bu–Al	660	93	10 600	18 000	11 800	1.21	4.3 (1)
	100 : 1 : 1
14	O^Me2–Al	13	90	10 400	14 100	19 800	1.70	244.5 (4)
	100 : 1 : 1
15	O^Br2–Al	40	95	10 900	12 800	17 300	1.36	121.1 (50)
	100 : 1 : 1
16	O^OMe–Al	18	90	5200	4700	6300	1.30	143.2 (35)
	100 : 1 : 2
17	O^OMe–Al	50	90	10 400	11 100	12 100	1.18	48.6 (9)
	100 : 1 : 1
18	O^Br–Al	70	90	5200	5400	3100	1.15	33.9 (4)
	100 : 1 : 2
19	O^Br–Al	160	92	10 600	8800	8900	1.06	16.9 (5)
	100 : 1 : 1
20	NNOO^OMe–Al	480	90	10 400	10 800	11 400	1.08	4.8 (1)
	100 : 1 : 1
21	NNOO^Br–Al	1620	90	10 400	10 800	11 400	1.06	1.5 (1)
	100 : 1 : 1
22	NNO^OMe–Al	40	92	5400	5400	4800	1.09	65.6 (23)
	100 : 1 : 2
23	NNO^OMe–Al	140	92	10 600	10 500	10 300	1.13	18.8 (9)
	100 : 1 : 1
24	NNO^Br–Al	60	92	5400	5900	4800	1.12	43.9 (9)
	100 : 1 : 2
25	NNO^Br–Al	220	93	10 700	12 800	8500	1.22	12.3 (5)
	100 : 1 : 1

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The overall polymerization rates of Al catalysts with different ligands (ranked by their k_obs values, Fig. 11) are in the following order: ON^Br–Al > O^Br₂–Al > ONNO^Br–Al > NNO^Br–Al > ONO^Br–Al > O^Br–Al > NNOO^Br–Al > ^BuONNO^Br–Al > OO^Br–Al. O^Br₂–Al exhibited higher catalytic activity than OO^Br–Al because the flexibility²⁵ of the coordination from phenolate to Al atom allows the Al atom to adjust its electron density to facilitate CL coordination to the Al center and alkoxide initiation. According to Tolman^16b results, the order of the catalytic activity of Al complexes should be O^Br–Al > ON^Br–Al > NNO^Br–Al and OO^Br–Al > ONO^Br–Al; however, in fact, OO^Br–Al and O^Br–Al exhibited lower catalytic activity compared with aminophenolate Al complexes. Although the amino group of ONO^Br–Al still occupies a coordination site and competes with CL coordination compared to OO^Br–Al, ONO^Br–Al exhibited higher catalytic activity than OO^Br–Al, and the amino group of ONO^Br–Al seems to have other functions in the catalytic process to enhance the catalytic activity. The phenomenon of NO^Br–Al exhibiting higher catalytic activity than O^Br–Al and O^Br₂–Al supports the earlier speculation regarding the role played by the amino group. In addition, ONNO^Br–Al exhibited higher catalytic activity than ^BuONNO^Br–Al, implying that two bulky t-butyl groups prevent CL from approaching the Al center. The Al complexes with electron-donating substituents on ligands exhibited higher catalytic activity than those with bromo substituents. In addition, the catalytic trend of these Al complexes is similar to those with bromo substituents, except that O^Me₂–Al exhibited higher catalytic activity than ON^OMe–Al, and the catalytic activity of O^OMe–Al is higher than those of NNO^OMe–Al and ONO^OMe–Al. This phenomenon is unexplained because the functional groups between O^Me₂–Al (with the methyl group in the phenolate ring) and ON^OMe–Al (with the methoxy group in the phenolate ring) are different. The catalytic activity of ONO^OMe–Al is higher than that of NNOO^OMe–Al because the dimethyl amino group^16b of NNOO^OMe–Al occupies a coordination site to compete against CL coordination and reduces the polymerization rate.

Fig. 11 Comparison of the catalytic activity of CL polymerization by using Al complexes bearing various substituted ligands.

The Đ values of producing PCL become broader when Al complexes exhibit higher catalytic activity. Despite the low coordination number of Al complexes, such as O–Al, O₂–Al, and ON–Al type complexes, they exhibited higher catalytic activity, and a reduced control over CL polymerization was observed when using these Al complexes. It may be attributed to the fact that when there are more exposed spaces around the uncrowded Al center, the chance of CL coordination increases along with the polymerization rate. However, it also increases the possibility of the coordination of the polymer to the metal center and then accelerates the occurrence of transesterification.²⁶

Catalytic polymerization ability test of the mPEG-b-PCL copolymer

The advantage of alkyl Al catalysts is that the initiators (alcohol precursors) can be easily exchanged with the alkyl group on the Al atom, and this advantage is very important for synthesizing PCL copolymers for drug delivery systems. Therefore, these Al complexes with mPEG 1900 as an initiator were tested for catalytic polymerization of the mPEG-b-PCL copolymer, and the optimal controllability of producing the mPEG-b-PCL copolymer is shown in Table 2.

Table 2 ε-Caprolactone polymerization with Al complexes as catalysts with the mPEG initiator ([CL] : [Cat] : [mPEG] = 45 : 1 : 1)^a

Entry	[CL] : [Cat] : [mPEG]	Time (h)	Conv.^b (%)	M nCal	M nNMR	M nGPC	Đ
a Generally, the reaction was carried out in toluene with [CL] = 0.90 M at 25 °C. b Calculated from the molecular weight of Mw(CL) × [CL]0/[mPEG]0 × conversion yield + Mw(mPEG). c Data were obtained through ^1H NMR analysis. MnNMR was calculated as follows: molecular weight of CL × (integration of the peak at 4.0 ppm/integration of the peak at 3.62 ppm) × 86 + Mw(mPEG) (an example in Fig. S60†). d Obtained through gel permeation chromatography (GPC). e [CL] = 1.55 M in toluene at 25 °C. f [CL] = 3.11 M in toluene at 25 °C. g [CL] = 3.51 M in toluene at 25 °C. h [CL] = 4.47 M in toluene at 25 °C.
1	ONO^OMe–Al	6	92	6600	6500	5700	1.16
2	ON^OMe–Al	25 min	95	6800	6700	6100	1.46
3	^BuONNO^OMe–Al	24	94	6700	6600	4800	1.06
4	OO^OMe–Al	2	90	6500	6300	5900	1.13
5	O^Me2–Al	2	94	6700	6800	6200	1.10
6	O^OMe–Al	40 min	95	6800	6200	7000	1.19
7	NNOO^OMe	8	90	6500	6200	4700	1.06
8^e	O^OMe–Al	40 min	90	5100	5500	4800	1.13
	31 : 1 : 1
9^f	O^OMe–Al	1.5	90	9900	9200	12 500	1.18
	78 : 1 : 1
10^g	O^OMe–Al	2.5	90	10 900	10 400	16 400	1.19
	88 : 1 : 1
11^h	O^OMe–Al	5	85	12 700	14 600	19 300	1.19
	112 : 1 : 1

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Among these Al complexes, ON^OMe–Al exhibited the highest polymerization rate (25 min, conv. = 95%) but lower control over polymerization (Đ = 1.46), and ^BuONNO^OMe–Al still exhibited the lowest catalytic activity (24 h, conv. = 94%). Because the catalytic activity of O^OMe–Al ranked second with greater controllability, O^OMe–Al was used as a catalyst to test its controllability, as shown in entries 8–11 of Table 2. When the linear relation M_nGPC and ([monomer]₀ × conv.)/[BnOH] (entries 6 and 8–11 in Table 2), as presented in Fig. 12, was considered, O^OMe–Al as a catalyst was believed to exhibit well-controlled polymerization with narrow Đ values (1.13–1.20) for CL polymerization. The polymerization results revealed that O^OMe–Al is a good candidate for synthesizing the mPEG-b-PCL copolymer.

Fig. 12 Linear plots of M_nGPCversus ([CL]₀ × conv.)/[mPEG], with Đ indicated by red solid dots (entries 6 and 8–11) in Table 2.

CL polymerization mechanism using OO^Bu–Al as a catalyst

Although the literature^22c,27 reported that Al complexes bearing biphenolates formed mononuclear Al complexes, dinuclear Al complexes bridged by benzyl alkoxide^27a,b were also reported (such as the structure of (OO^Bu–AlOBn)₂ in Scheme 2) and have maintained dinuclear geometric structures during the CL polymerization process. To determine whether the dimeric structure in the solution exists, the diffusion-order NMR spectroscopy (DOSY) analysis of OO^Bu–Al and (OO^Bu–AlOBn)₂ in CDCl₃ was carried out. The DOSY data (Fig. S61 and S62†) revealed that the diffusion coefficient (D) of OO^Bu–Al was 10.84 × 10⁻¹⁰ m² s⁻¹, and its hydrodynamic radius (r) was 3.66 Å. However, the D value of (OO^Bu–AlOBn)₂ was 6.01 × 10⁻¹⁰ m² s⁻¹, and its hydrodynamic radius (r) was 6.71 Å, larger than that of OO^Bu–Al. The DOSY data exhibited the dimeric structure of (OO^Bu–AlOBn)₂ in solution. To understand the difference in the catalytic activity between dinuclear Al complexes bridged by benzyl alkoxide and the in situ reaction of mononuclear Al complexes with BnOH, (OO^Bu–AlOBn)₂ was obtained through the reaction of OO^Bu–Al and BnOH (1 : 1 ratio, Scheme 2) and used to study the CL polymerization compared with the CL polymerization by using the in situ reaction of mononuclear OO^Bu–Al with BnOH. The polymerization results revealed that (OO^Bu–AlOBn)₂ exhibited lower catalytic activity for CL polymerization (2880 min, conv. = 90%, k_obs = 0.0008 min⁻¹) compared to the in situ reaction of mononuclear OO^Bu–Al with BnOH (660 min, conv. = 93%, k_obs = 0.0043 min⁻¹). These results implicated that Al catalysts with bridging benzyl alkoxide exhibited low catalytic activity because only one lone pair can initiate CL polymerization, and the bridging oxygen atom between two aluminum atoms fixes the orientation of the lone pair. This makes the initiation of CL polymerization by alkoxide difficult. Therefore, the electronic effect and amino chelating effect may affect the structures of Al catalysts with bridging benzyl alkoxide, thereby converting them into a more active structural state. Many Al complexes such as these bearing (2,6-bis(hydroxyalkyl)pyridine),²⁸ 2,2′-oxybis(N-cyclopentylanilinate),²⁹ amine-pyridine-bis(phenolate),³⁰ β-diketonate,³¹ biphenolate phosphine,³² β-quinolyl enolato,³³ catalen,³⁴ catam,^7e,35 ketiminate,^11,36 NHC bis-phenolate,³⁷N-phenylpicolinimidate,³⁸ phenolate,³⁹ pyrrolyl amide,⁴⁰ Salen,⁴¹ and triphenolate^22c,42 were reported with bridging alkoxide structures that are inactive^11,22c,43 during the polymerization process. Herein, biphenolate-associated Al catalysts with bridging benzyl alkoxide were used as a model, and the CL polymerization mechanism was investigated by density functional theory (DFT) calculations carried out using the Gaussian 09E program package.⁴⁴

Scheme 2 Reaction of OO^Bu–Al with BnOH to form (OO^Bu–AlOBn)₂.

Confirming the active species of the Al catalyst is the first necessary investigation. For dimer–tetramer interconversions (Scheme 3), the OO^OMe ligand structures in the calculation are simplified to reduce the calculation loading using methoxide ligands to replace benzyl alkoxides. All the calculations were carried out at the B3LYP level with the 6-31G(d) basis sets. All DFT data regarding optimizing the most suitable active species of Al catalysts have been presented in the ESI.† They revealed that the syn-Dimer-B is likely the catalytic site to accept CL coordination to form the intermediate I (the CL coordination on the inner site of syn-Dimer-B as shown in Fig. 13) for the ring-opening reaction.

Scheme 3 The interconversion from Dimer-A to Dimer-B through tetranuclear species T.

Fig. 13 Structures of intermediate I (the syn-Dimer-B with CL coordination).

In the polymerization catalytic cycle (Fig. 14), the catalyst (syn-Dimer-B) first accepts one CL to form intermediate I. The Al(1) of intermediate I (Fig. 13) has a twisted trigonal bipyramidal structure with CL and bridging methoxide on the axial sites. The bond angle between two axial ligands (O^car–Al(1)–O^μ-Me) is 162.06°. The bond length between O^car and Al(1) is 1.989 Å, slightly longer than those reported in the literature (1.8–1.9 Å),⁴⁵ indicating that the Al center weakly binds the CL. The terminal methoxide's oxygen atom shows a hydrogen bond-like interaction with the α-hydrogen of CL with a H^α⋯O^tm-Me distance of 2.174 Å, which is shorter than the sum of equilibrium van der Waals radii (eVDWR) of hydrogen and oxygen (∼2.6 Å),⁴⁶ indicating that a strong hydrogen bond-like interaction acts as a second attractive force to bind CL to the catalyst.

Fig. 14 The catalytic cycle of ring-opening polymerization catalyzed by syn-Dimer-B.

Intermediate I converts to intermediate II through transition structure TS-1 (Fig. 15), and the terminal OMe ligand is added to the CL molecule. The Al(1) center maintains a trigonal bipyramidal structure with the bridged phenoxide and terminal OMe as the axial ligands. The CL leads to the more Lewis-acidic equatorial site being further activated. The carbonyl group of CL rotates to face the terminal methoxide ligand with a C^carbonyl⋯O^OMe distance of 2.005 Å, which is between the sum (oxygen and carbon atoms) of eVDWR (3.75 Å) and single-bond covalent radii (1.38 Å),⁴⁷ indicating that the C^carbonyl–O^carbonyl bond is forming. Moreover, the addition occurs at the space between the two ortho-^tBu groups on the bidentate ligand. This demonstrates that the catalyst possesses a protected open site for the catalytic reaction to prevent transesterification.

Fig. 15 The transition structures of the catalytic reaction.

After forming the CL–OMe adduct, the complex transforms into intermediate II; the adduct chelates on Al(1) through its O^carbonyl and O^OMe atoms with a small bit angle of 67.67°. The axial coordinating atoms lead to O^μ-Ph and O^OMe. The O^carbonyl-Al(1) length is 1.788 Å, slightly longer than the O^OMe–Al(1) bond of I (1.784 Å). The ether oxygen of the adduct is weakly coordinated to Al with an O^OMe–Al length of 2.112 Å, which can easily dissociate from the Al center. The O^OMe atom dissociates from Al(1) to form intermediate III. The Al(1) center, due to losing one coordination ligand, converts to a tetrahedral geometry. Interestingly, the central H of the chelating ligand moves closer to Al(1), and the H⋯Al(1) distance is 2.760 Å, which is shorter than the sum of van der Waals radii of H and Al (2.94 Å), offering an agnostic-like interaction. The O^carbonyl–Al(1) bond is 1.736 Å, which elongates slightly compared to intermediate II (1.788 Å), due to the decrease of coordination points of the adduct. The O^carbonyl–C^carbonyl bond rotates, and the O^est atoms (the oxygen atom of the seven rings) of the adduct coordinate with Al(1) to form intermediate IV. The O^est–C^carbonyl bond length is elongated from 1.429 Å (III) to 1.456 Å (IV) because the coordination of the O^est atom on Al helps weaken the bond. Moreover, the structure of the adduct, due to the assistance of the complex, can dissociate the O^est–C^carb bond. The complex then undergoes a CL ring opening through transition structure TS-2 (Fig. 15). The C^carbonyl⋯O^est distance is further elongated to 1.981 Å, which is much longer than the C–O bond of IV. Moreover, the C^carbonyl is nearly converted to sp² hybridization (the sum of the three bond angles around it is 352.89°). This indicates that the structure of the adduct on TS-2 is close to the ring-opened oligomer. Finally, the CL ring opens to form intermediate V. The newly generated carbonyl group is located at an equatorial position of the Al(1) center, and the newly generated alkoxide ligand is in an axial position. Intermediate V contrasts with intermediate I, where the CL carbonyl oxygen is on an axial site, and the terminal methoxide is on an equatorial site.

The coordination of CL on syn-Dimer-B only requires 4.11 kcal mol⁻¹ (free energy), and this process can release 8.84 kcal mol⁻¹ heat (enthalpy change), indicating that the coordination site is readily accessible for CL. To consider the free energy changes of the mechanism (Fig. 16), after forming I, the reaction goes through TS-1, overcoming the highest energy barrier (16.65 kcal mol⁻¹) of the catalytic reaction. This is a moderate energy barrier caused by the bulky ortho-^tBu group on the phenol moieties, partially disturbing the addition of the terminal alkoxide to CL. After the addition to form II, the isomerization between II, III, and IV only shows slight energy differences due to the small structural differences. Then, the CL ring opens to form V through TS-2, facing a relatively small energy barrier of 11.92 kcal mol⁻¹. Finally, because of the large chelating ring of the oligomer, V is the intermediate with the highest free energy. It can revert to structure I easily by accepting one more CL molecule. Compared with the Br-substituted OO^Br–Al catalyst, the highest energy barrier from I to TS-1 is about 1 kcal mol⁻¹ lower than that from I–Br to TS-1–Br (0 to 17.625 kcal mol⁻¹), indicating that this mechanism also can explain the relatively high reactivity of OO^OMe–Al well, as shown in Table 1.

Fig. 16 The energy profile of the catalytic reaction.

According to the DFT results, Al complexes bearing phenolate ligands can easily form bridging benzyl alkoxide-associated dinuclear complexes with low catalytic activity. Thus, they undergo rearrangement to form active terminal benzyl alkoxide-associated dinuclear complexes for the ring-opening polymerization process. Many studies^{7e,f,11,16b,22c,28–30,32–38,42,48} have also reported that Al complexes (especially those containing phenolate ligands) easily form bridging benzyl alkoxide-associated dinuclear complexes. The bridging benzyl alkoxide-associated dinuclear Al complexes with electron-donating groups have relatively weak bonds between bridging benzyl alkoxide and aluminum atoms. Their bonds can be easily broken to rearrange and form the active terminal benzyl alkoxide-associated dinuclear complexes and increase their catalytic activity. However, the phenolate ligands, with electron-donating groups, may provide more electrons to the Al atoms to increase their electron density. Al catalysts with high electron density on Al atoms are expected to reduce the chance of benzyl alkoxide bridging to form the terminal benzyl alkoxide-associated Al complexes. Like the electron-donating groups in the phenolate ligands, the chelating amino group also provides electrons to the Al atoms. Using this concept to examine the catalytic activity of the Al complexes used in this study, the following conclusions can be drawn:

1. O^Br–Al and O^Br₂–Al exhibit a higher ability to rearrange from the bridging benzyl alkoxide-associated dinuclear complexes to active terminal benzyl alkoxide-associated dinuclear complexes compared to OO^Br–Al because OO^Br–Al with a rigid metal–ligand eight-membered ring has a poor ability²⁵ to enhance the electron density of the Al center through the oxygen pi-donation of the OO^Br ligand; therefore, OO^Br–Al exhibits the lowest catalytic activity [Fig. 17(1)].

Fig. 17 Comparison of the catalytic activity of CL polymerization by using Al complexes.

2. From four-coordinate O^Br–Al to four-coordinate NO^Br–Al and five-coordinate NNO^Br–Al [Fig. 17(2)], the catalytic activity of NO^Br–Al and NNO^Br–Al is higher compared to O^Br–Al since the chelating amino group may increase the electron density of the Al center to carry out the rearrangement. However, five-coordinate NNO^Br–Al revealed a more crowded environment around the Al center than four-coordinate NO^Br–Al and reduced the ability of the CL coordination to the Al center.

3. Three-coordinate O^Br₂–Al can rearrange to form terminal benzyl alkoxide-associated Al complexes because of the oxygen pi-donation of two terminal O^Br ligands.²⁵ In the case of ONO^Br–Al, adding an amino group to link two phenolates did not improve ONO^Br–Al's ability to rearrange because of the formation of a rigid seven-membered ring of ONO^Br–Al. As the number of amino groups in phenolate increases, the ability of the CL coordination to the Al center decreases [Fig. 17(3)]. In addition, five-coordinate NNOO^Br–Al and ^BuONOO^Br–Al with the high coordination number of Al complexes prevent bridging benzyl alkoxide structure, and the dimethyl amino group of less bulky NNOO^Br–Al is more flexible compared with ^BuONOO^Br–Al to dissociate from the Al center that easily accepts the CL coordination. Moreover, the electron donating group in the phenolate ring may increase the electron density of the Al center to afford a facile dimethyl amino group dissociation and improve the catalytic activity (Tolman's research).^16b

Conclusions

A series of Al complexes bearing phenolates and aminophenolates with different coordination numbers were synthesized, and their CL polymerization was investigated. Most Al complexes with low coordination numbers, such as ON–Al, O₂–Al, and O–Al type complexes, exhibited high catalytic activity in CL polymerization, except OO–Al and ONO–Al type complexes due to the rigid ring between Al–ligand structures that reduces the ability of the phenolates to control the electron density of the Al center. The ON–Al system exhibited a higher catalytic activity (1.1–3.9-fold for k_obs) than the O–Al type complex, implying that the amino group could improve the catalytic activity of three-coordinate Al complexes. Five-coordinate Al complexes, such as NNOO–Al and ONNO–Al type complexes, exhibited lower catalytic activity than four-coordinate ones because of the crowded Al center, which is not conducive to CL coordination. In addition, ONO–Al type complexes exhibited higher catalytic activity compared to NNOO–Al and ONNO–Al type complexes (9.9 and 37.3-fold for k_obs). Removing the amino group from high-coordinate Al complexes can enhance catalytic activity by reducing the Al center's repulsion to accept CL coordination. Moreover, Al complexes with an electron-donating substituent on the phenolate ring exhibited higher catalytic activity, contrasting with those with an electron-withdrawing one. DFT calculations revealed that the dinuclear Al complex with two bridging alkoxides exhibited low activity for CL polymerization because of the low initiating ability of two bridging methoxides. The dinuclear Al complex with two bridging alkoxides had to rearrange to a phenolate bridged dinuclear Al complex with terminal alkoxide. Then, CL coordinated to one Al center with terminal alkoxide. The electron-donating substituents on the phenolate ring can increase the bridging ability of the phenolate group and then increase the CL polymerization rate. In addition, the amino group can also make the Al center electron-rich by chelating to the Al center, facilitating the structural rearrangement of the dinuclear Al complex with two bridging alkoxides. For synthesizing the mPEG-b-PCL copolymer, O^OMe–Al is an efficient candidate with a high polymerization rate and controllability.

Experimental section

Standard Schlenk techniques and an N₂-filled glovebox were used throughout the isolation and handling of compounds. Solvents, ε-caprolactone, and deuterated solvents were purified before use. ε-Caprolactone was purified by distillation with magnesium sulfate anhydrous. The organic solvent was purified by distillation in the presence of sodium and benzophenone. Deuterated chloroform was purified by distillation in the presence of calcium hydride. Deuterated chloroform, ε-caprolactone, and trimethyl aluminum were purchased from Aldrich. Methoxy poly(ethylene glycol) 1900, ethane-1,2-diamine, N,N′-dimethylethane-1,2-diamine, 2-tert-buty-4-bromo-phenol, 3-tert-butyl-4-hydroxyanisole, benzylamine, paraformaldehyde, pyrrolidine, triethylamine, 5-bromosalicylaldehyde, ethylenediamine, sodium borohydride, 2-tert-butylphenol, 2,4-di-tert-butylphenol, 4-chlorobenzaldehyde, methyl magnesium bromide solution (1.0 M) in dibutyl ether, bromine, and 4-bromo-2,6-di-tert-butylphenol were purchased from Sigma-Aldrich. 2-Hydroxy-5-methoxybenzaldehyde was purchased from Nova-Matls. ¹H and ¹³C NMR spectra were recorded on a JEOL JNM-ECS400 (400 MHz for ¹H and 100 MHz for ¹³C) spectrometer with chemical shifts in ppm from the internal TMS or center line of CDCl₃. For ¹H PFGNMR diffusion measurement, all experiments were performed on a JEOL 400 MHz liquid state NMR spectrometer operating at 9.4 Tesla (with a proton resonance frequency of 400.052 MHz). Each freshly synthesized sample was tightly sealed in a 7 mm liquid state NMR tube so that the humidity of the sample did not change throughout the experiment (about 10 minutes each). The sample lengths were all longer than 40 mm to minimize the magnetic susceptibility effect from the sample. The sample was static (zero spinning speed) and with ²H lock used during the diffusion measurement to maintain the homogeneity of the magnetic field within 2 Hz inside the sample region and less than 1 Hz of fluctuation overnight. The 90° pulse width for ¹H was 9.5 μs. The spectral widths were 15 ppm for all samples. Standard NMR reference TMS was used and all experiments were performed at room temperature (25 °C). The spectrometer is equipped with a magnetic field gradient coil that enables the generation of a magnetic field gradient up to 30 G cm⁻¹ (300 mT m⁻¹). The pulse sequence used for measuring diffusion coefficient is DOSY with a diffusion time (Δ) of 60 ms and a gradient interval (δ) of 5 ms, respectively. The pulsed field gradients (mT m⁻¹) were 0, 60, 120, 240, and 300. The scan number was 8 and the recycle delay was 10 s for all samples. Microanalyses were performed using a Heraeus CHN-O-RAPID instrument. GPC measurements were performed on a Jasco PU-2080 PLUS HPLC pump system equipped with a differential Jasco RI-2031 PLUS refractive index detector using THF (HPLC grade) as an eluent (flow rate 1.0 mL min⁻¹, at 40 °C). The chromatographic column was JORDI Gel DVB 10³ Å, and primary polystyrene standards made the calibration curve to calculate M_nGPC. All X-ray diffraction data were accumulated using Rigaku Oxford Diffraction single crystal X-ray diffractometers with Mo Kα radiation (λ = 0.71073 Å). Data collection was executed using the CrysAlisPro 1.171.41.56a program. Cell refinement and data reduction were achieved using the CrysAlisPro 1.171.41.56a program. The structure was determined using the Olex2/ShelXL program and refined using full-matrix least squares. All non-hydrogen atoms were refined anisotropically, whereas hydrogen atoms were placed at calculated positions and included in the final refinement stage with fixed parameters. NNOO^OMe–OH,^16aNNOO^Br–OH,^16aONNO^OMe–OH,⁴⁹ONNO^Br–OH,^49bBuONNO^OMe–OH,²¹O^Me₂–Al,⁵⁰O^Br₂–Al,⁵¹NNOO^OMe–Al,^16a and NNOO^Br–Al ^16a were synthesized by following the literature procedure.

Synthesis of ONO^OMe–OH

In a 100 mL round-bottom flask, 3-tert-butyl-4-hydroxyanisole (3.60 g, 20.0 mmol), paraformaldehyde (37 wt% in water, 2 mL), and benzylamine (1.07 g, 10.0 mmol) were added and dissolved in 60 mL of EtOH. The mixture was refluxed, and the reaction was monitored by TLC, which revealed that the reaction was complete after 4 h. The mixture was then warmed to room temperature, and the volatiles were removed under vacuum. The pure product was obtained after purification via silica gel column chromatography (10/1 hexane/ethyl acetate). Yield: 3.92 g (80%). ¹H NMR (400 MHz, CDCl₃) δ 7.42–7.30 (m, 3H, meta-H + para-H of PhCH₂), 7.05 (br, 2H, ortho-H of PhCH₂), 6.83 (d, 2H, J = 2 Hz, para-H of Ph^tBu), 6.54 (d, 2H, J = 2 Hz, ortho-H of Ph^tBu), 3.76 (s, 6H, OCH₃), 3.64 (s, 4H, ArCH₂N), 3.58 (s, 2H, PhCH₂N), 1.41 (s, 18H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃, δ): 152.37 (MeO–C of Ar), 148.57 (HO–C of Ar), 138.51 (^tBu–C of Ar^tBu), 137.45 (NCH₂–C of Ph), 129.62 (ortho-C of NCH₂Ph), 129.12 (meta-C of NCH₂Ph), 128.10 (para-C of NCH₂Ph), 122.90 (NCH₂–C of Ar), 113.52 (ortho-C of NCH₂Ar), 112.82 (para-C of NCH₂Ar), 58.54 (NCH₂Ph), 56.78 (OCH₃), 55.77 (NCH₂Ar), 34.99 (C(CH₃)₃), 29.54 (C(CH₃)₃). Anal. calc. (found) for C₃₁H₄₁NO₄: C 75.73 (75.52), H 8.41 (8.25), N 2.85 (2.68).

Synthesis of ONO^Br–OH

This compound was prepared following the same procedure described for ONO^OMe–H by using 2-(tert-butyl)-4-bromo-phenol in place of 3-tert-butyl-4-hydroxyanisole. Yield: 4.57 g (78%). ¹H NMR (400 MHz, CDCl₃) δ 7.25–7.13 (br, 7H, ortho-H–ArCH₂ + C₆H₅CH₂), 6.99 (br, 2H, ortho-H–Ar^tBu), 3.51 (s, 4H, ArCH₂N), 3.46 (s, 2H, PhCH₂N), 1.29 (s, 18H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃, δ): 156.62 (HO–C of Ar), 139.50 (^tBu–C of Ar), 136.85 (NCH₂–C of Ph), 130.82 (ortho-C of Ar^tBu), 129.97 (meta-C of NCH₂Ph), 129.46 (ortho-C of NCH₂Ph), 129.27 (NCH₂–C of Ar), 128.35 (para-C of NCH₂Ph), 124.15 (ortho-C of NCH₂Ar), 111.71 (Br–C of Ar), 58.49 (NCH₂Ph), 55.94 (NCH₂Ar), 34.96 (C(CH₃)₃), 29.32 (C(CH₃)₃). Anal. calc. (found) for C₂₉H₃₅Br₂NO₂: C 59.10 (59.22), H 5.99 (5.79), N 2.38 (2.22).

Synthesis of ON^OMe–OH

To a 100 mL round-bottom flask, 3-tert-butyl-4-hydroxyanisole (3.60 g, 20.0 mmol), paraformaldehyde (37 wt% in water, 2 mL), and pyrrolidine (1.85 g, 26.0 mmol) were added and dissolved in 50 mL of EtOH. The mixture was refluxed, and the reaction was monitored by TLC, revealing that the reaction was complete after 4 h. The mixture was then warmed to room temperature, and the volatiles were removed under vacuum. The pure product was obtained after purification via silica gel column chromatography using a 10/1 hexane/ethyl acetate mixture as an eluent system. Yield: 4.31 g (82%). ¹H NMR (400 MHz, CDCl₃) δ 6.80 (d, 1H, J = 2 Hz, para-H of Ph^tBu), 6.43 (d, 1H, J = 2 Hz, ortho-H of Ph^tBu), 3.78 (s, 2H, ArCH₂N), 3.76 (s, 3H, OCH₃), 2.62 (br, 4H, N(CH₂CH₂)₂), 1.86 (br, 4H, N(CH₂CH₂)₂), 1.43 (s, 9H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃) δ 151.56 (MeO–C of Ar), 150.92 (HO–C of Ar), 137.53 (^tBu–C of Ar), 123.10 (NCH₂–C of Ar), 112.40 (ortho-C of NCH₂Ar), 110.59 (para-C of NCH₂Ar), 59.54 (OCH₃), 55.73 (NCH₂Ar), 53.33 (N(CH₂CH₂)₂), 34.90 (C(CH₃)₃), 29.48 (C(CH₃)₃), 23.83 (N(CH₂CH₂)₂). Anal. calc. (found) for C₁₆H₂₅NO₂: C 72.97 (72.66), H 9.57 (9.49), N 5.32 (5.21).

Synthesis of ON^Br–OH

This compound was prepared following the same procedure described for ON^OMe–H by using 2-(tert-butyl)-4-bromo-phenol was used in place of 3-tert-butyl-4-hydroxyanisole. Yield: 5.10 g (82%). ¹H NMR (400 MHz, CDCl₃) δ 7.25 (d, 1H, J = 2 Hz, para-H of Ph^tBu), 6.96 (d, 1H, J = 2 Hz, ortho-H of Ph^tBu), 3.76 (s, 2H, ArCH₂N), 2.61 (br, 4H, N(CH₂CH₂)₂), 1.85 (br, 4H, N(CH₂CH₂)₂), 1.39 (s, 9H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃, δ): 156.56 (HO–C of Ar), 138.71 (^tBu–C of Ar), 128.61 (para-C of NCH₂Ar), 128.35 (ortho-C of NCH₂Ar), 124.52 (NCH₂–C of Ar), 110.11 (Br–C of Ar), 58.67 (NCH₂Ar), 53.18 (N(CH₂CH₂)₂), 34.84 (C(CH₃)₃), 29.21 (C(CH₃)₃), 23.72 (N(CH₂CH₂)₂). C₁₅H₂₂BrNO: C 57.70 (57.58), H 7.10 (6.88), N 4.49 (4.19).

Synthesis of ^BuONNO^Br–OH

In a 100 mL round-bottom flask, 4-bromo-2-tert-butylphenol (4.56 g, 20.0 mmol), paraformaldehyde (37 wt% in water, 2 mL), and N,N′-dimethylethane-1,2-diamine (0.88 g, 10.0 mmol) were added and dissolved in 30 mL of EtOH. The mixture was refluxed, and the reaction was monitored by TLC, which revealed that the reaction was complete after 8 h. The mixture was then warmed to room temperature, and the volatiles were removed under vacuum. The white powder can be obtained by recrystallization in hexane. Yield: 3.40 g (60%). ¹H NMR (400 MHz, CDCl₃) δ 10.92 (s, 2H, OH), 7.25 (d, 1H, J = 2 Hz, para-H of Ph^tBu), 6.93 (br, 1H, ortho-H–Ar^tBu), 3.62 (s, 4H, PhCH₂N), 2.58 (br, 4H, NCH₂CH₂N), 2.23 (s, 6H, NCH₃), 1.34 (s, 18H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃) δ 155.91 (HO–C of Ar), 139.15 (C of Ar–C–^tBu), 129.12 (ortho-C of Ar^tBu), 129.02 (ortho-C of NCH₂Ph), 123.69 (NCH₂–C of Ar), 110.50 (Br–C of Ar), 61.81 (NCH₂Ar), 53.30 (NCH₂CH₂N), 41.31 (NCH₃), 34.88 (C(CH₃)₃), 29.19 (C(CH₃)₃). Anal. calc. (found) for C₂₆H₃₈Br₂N₂O₂: C 54.75 (54.61), H 6.72 (6.57), N 4.91 (4.84).

Synthesis of OO^H–OH

A stirred solution of 2-(tert-butyl) phenol (4.5 g, 30 mmol) in 60 mL of diethyl ether under nitrogen was cooled to 0 °C. To this solution, methyl magnesium bromide (10.0 mL, 3.0 M in diethyl ether) was slowly added. After gas evolution had ceased, the solvent was removed in vacuo. The resulting residue was dissolved in 150 mL of toluene. 4-Chlorobenzaldehyde (2.1 g, 15 mmol) was added, and the reaction mixture was refluxed for 12 h. After cooling to room temperature, the reaction mixture was washed with saturated NH₄Cl. The layers were separated, and the aqueous layer was extracted using diethyl ether. The combined organic extracts were dried with MgSO₄, and the volatile materials were removed at reduced pressure. Yielded: 1.0 g (42%) of OO^H–H. ¹H NMR (400 MHz, CDCl₃) δ 7.32 (d, 2H, J = 8 Hz, ortho-H of PhCl), 7.28 (d, 2H, J = 2 Hz, para-H of Ph^tBu), 7.12 (d, 2H, J = 8 Hz, meta-H of PhCl), 6.84 (t, 2H, J = 8 Hz, para-H of Ar–OH), 6.67 (d, 2H, J = 2 Hz, ortho-H of Ph^tBu), 5.71 (s, 1H, CH(ArOH)₂), 4.94 (s, 2H, OH), 1.40 (s, 18H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃) δ 152.89 (HO–C of Ar), 139.61 (para-C of ArCl), 137.62 (^tBu–C of Ar^tBu), 133.28 (Cl–C), 130.93 (meta-C of ArCl), 129.20 (ortho-C of ArCl), 128.36 (CH–C of ArOH), 127.61 (para-C of Ar^tBu), 126.59 (para-C of ArOH), 120.87 (para-C of CHAr), 45.66 (CH(ArOH)₂), 34.73 (C(CH₃)₃), 29.95 (C(CH₃)₃). Anal. calc. (found) for C₂₇H₃₁ClO₂: C 76.67 (76.56), H 7.39 (7.22).

Synthesis of OO^Br–OH

A solution of OO^H–H (4.2 g, 10 mmol) in methanol (50 mL) was stirred at 0 °C and treated dropwise with neat Br₂ (3.2 g, 20 mmol). After 30 min, the cooling bath was removed, and the mixture was kept at 25 °C for 12 h. The mixture was dissolved in ethyl acetate and washed with water and brine, and volatiles were removed from the organic phase by evaporation under reduced pressure. Flash chromatography (silica, hexane) of the residue provided an orange crystalline solid of OO^Br–H (2.95 g, 51%). ¹H NMR (400 MHz, CDCl₃) δ 7.36 (d, 2H, J = 2 Hz, para-H of Ph^tBu), 7.34 (d, 2H, J = 8 Hz, ortho-H of PhCl), 7.07 (d, 2H, J = 8 Hz, meta-H of PhCl), 6.72 (d, 2H, J = 2 Hz, para-H of PhCH), 5.66 (s, 1H, CH(ArOH)₂), 4.87 (s, 2H, OH), 1.38 (s, 18H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃, δ): 151.62 (HO–C of Ar), 139.76 (^tBu–C of Ar), 138.20 (para-C of ArCl), 133.86 (Cl–C), 130.37 (meta-C of ArCl), 129.98 (ortho-C of ArCl), 129.83 (CH–C of ArOH), 129.54 (para-C of Ar^tBu), 113.75 (Br–C), 44.78 (CH(ArOH)₂), 34.86 (C(CH₃)₃), 29.78 (C(CH₃)₃). Anal. calc. (found) for C₂₇H₂₉Br₂ClO₂: C 55.84 (55.51), H 5.03 (4.77).

Synthesis of OO^OMe–OH

A method similar to that for OOH–H was used except that 2-(tert-butyl)-4-methoxyphenyl was used in place of 2-(tert-butyl) phenol. Yield: 5.42 g (75%). ¹H NMR (400 MHz, CDCl₃) δ 7.31 (d, 2H, J = 8 Hz, ortho-H of PhCl), 7.12 (d, 2H, J = 8 Hz, meta-H of PhCl), 6.84 (d, 2H, J = 2 Hz, para-H of Ph^tBu), 6.72 (d, 2H, J = 2 Hz, para-H of PhCH), 5.65 (s, 1H, CH(ArOH)₂), 4.47 (s, 2H, OH), 3.62 (s, 6H, OCH₃), 1.37 (s, 18H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃) δ 153.47 (MeO–C of Ar), 146.69 (HO–C of Ar), 139.46 (^tBu–C of Ar^tBu), 139.26 (para-C of ArCl), 133.39 (Cl–C), 130.89 (meta-C of ArCl), 129.38 (ortho-C of ArCl), 129.24 (CH–C of ArOH), 112.93 (para-C of Ar^tBu), 111.85 (ortho-C of ArOMe, ArOH), 55.56 (OCH₃), 46.29 (CH(ArOH)₂), 34.97 (C(CH₃)₃), 29.82 (C(CH₃)₃). Anal. calc. (found) for C₂₉H₃₅ClO₄: C 72.11 (71.88), H 7.30 (7.05).

Synthesis of OO^Bu–OH

A method similar to that for OO^H–H was used except that 2-(tert-butyl)-4-methoxyphenol was used instead of 2,4-di-tert-butylphenol. Yield: 5.42 g (73%). ¹H NMR (400 MHz, CDCl₃) δ 7.33 (d, 2H, J = 8 Hz, ortho-H of PhCl), 7.30 (d, 2H, J = 8 Hz, meta-H of PhCl), 7.11 (d, 2H, J = 2 Hz, para-H of PhCH), 6.63 (d, 2H, J = 2 Hz, para-H of Ph^tBu), 5.63 (s, 1H, CH(ArOH)₂), 4.75 (s, 2H, OH), 1.38, 1.16 (s, 36H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃) δ 150.45 (HO–C of Ar), 143.05, 136.74 (^tBu–C of Ar^tBu), 139.86 (para-C of ArCl), 136.74 (C of Ar^tBu, ortho-C of ArOH), 133.08 (Cl–C), 130.93 (meta-C of ArCl), 129.05 (ortho-C of ArCl), 127.61 (CH–C of ArOH), 124.39 (para-C of Ar^tBu, meta-C of ArOH), 123.26 (ortho-C of Ar^tBu, meta-C of ArOH), 46.48 (CH(ArOH)₂), 34.95, 34.44 (C(CH₃)₃), 31.55, 29.99 (C(CH₃)₃). Anal. calc. (found) for C₃₅H₄₇ClO₂: C 78.55 (78.23), H 8.85 (8.49).

Synthesis of NNO^OMe–OH

One equivalent of benzaldehyde and one equivalent of N¹,N¹-dimethylethane-1,2-diamine were dissolved in ethanol and refluxed to form an imine compound, and sodium borohydride was used to reduce the imine compound to form an N-benzyl-N′,N′-dimethylethane-1,2-diamine ligand. The stirred solution of formaldehyde, 2-(tert-butyl)-4-methoxyphenol, and N-benzyl-N′,N′-dimethylethane-1,2-diamine in ethanol was refluxed for 1 h, and sodium borohydride was transferred and refluxed for one day. After the reaction was completed, the resulting mixture was cooled at room temperature, and water was added dropwise to quench the reaction and DCM. After stirring for 1 h, more water was added, and the organic layer was separated. The aqueous layer was extracted by DCM twice. The organic layer was dried over MgSO₄ and the solvent was removed by evaporation to produce the colorless gel product. The pure product was obtained using column chromatography (10/1 hexane/ethyl acetate). Yield: 1.30 g (65%). ¹H NMR (400 MHz, CDCl₃) δ 7.30–7.21 (m, 5H, H–Ph), 6.79 (d, J = 2.0 Hz, 1H, ortho-H of ^tBuAr), 6.45 (d, J = 2.0 Hz, 1H, para-H of ^tBuAr), 3.73 (s, 3H, OCH₃), 3.71 (s, 2H, ArCH₂N), 3.56 (s, 2H, PhCH₂N), 2.56 (t, J = 8.0 Hz, 2H, NCH₂CH₂NMe₂), 2.43 (t, J = 8.0 Hz, 2H, NCH₂CH₂NMe₂), 2.10 (s, 6H, N(CH₃)₂), 1.43 (s, 9H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃) δ 151.63 (C of MeO–Ar), 150.49 (C of HO–Ar), 138.01 (ipso-C of CH₂Ph), 137.83 (C of ^tBuAr), 129.64 (ortho-C of CH₂Ph), 128.40 (meta-C of CH₂Ph), 127.40 (para-C of CH₂Ar), 123.30 (C of CH₂Ar), 112.81 (ortho-C of ^tBuAr), 111.98 (para-C of ^tBuAr), 58.45 (NCH₂Ph), 57.95 (NCH₂Ar), 56.53 (OCH₃–Ar), 55.82 (NCH₂CH₂N(CH₃)₂), 50.46 (NCH₂CH₂N(CH₃)₂), 45.42 (N(CH₃)₂), 35.01 (C(CH₃)₃), 29.49 (C(CH₃)₃). Anal. calc. (found) for C₂₃H₃₄N₂O₂: C 74.55 (74.31), H 9.25 (9.21), N 7.56 (7.50).

Synthesis of NNO^Br–OH

A method similar to that used for NNOOMe–OH, except that 2-tert-buty-4-bromo-phenol was used instead of 2-tert-butyl-4-methoxyphenol. Yield: 1.10 g (60%). ¹H NMR (400 MHz, CDCl₃) δ 7.28–7.20 (m, 5H, H–PhCH₂), 7.22 (d, J = 2.0 Hz, 1H, ortho-H of ^tBuAr), 7.01 (d, J = 2.0 Hz, 1H, para-H of ^tBuAr), 3.66 (s, 2H, ArCH₂N), 3.53 (s, 2H, PhCH₂N), 2.54 (t, J = 8.0 Hz, 2H NCH₂CH₂N(CH₃)₂), 2.44 (t, J = 8.0 Hz, 2H, NCH₂CH₂N(CH₃)₂), 2.10 (s, 6H, N(CH₃)₂), 1.42 (s, 9H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃) δ 155.76 (C of HO–Ar), 139.16 (ipso-C of Ph), 137.67 (C of ^tBuAr), 128.96 (ortho-C of ^tBuAr), 129.48 (ortho-C of CH₂Ph), 128.96 (para-C of ^tBuAr), 128.30 (meta-C of CH₂Ph), 127.35 (para-C of CH₂Ph), 127.35 (C of CH₂Ar), 110.06 (Br–C of Ar), 58.30 (NCH₂Ph), 56.46 (NCH₂Ar), 56.11 (NCH₂CH₂N(CH₃)₂), 49.96 (NCH₂CH₂N(CH₃)₂), 45.10 (N(CH₃)₂), 35.00 (C(CH₃)₃), 29.25 (C(CH₃)₃). Anal. calc. (found) for C₂₂H₃₁BrN₂O: C 63.00 (62.89), H 7.45 (7.41), N 6.68 (6.50).

Synthesis of ONO^OMe–Al

A mixture of ONO^OMe–OH (2.45 g, 5.0 mmol) and trimethylaluminum (2.5 mL, 2 M in toluene) in toluene (30 mL) was stirred for 5 h at room temperature. After ONO^OMe–OH disappeared, as monitored by ¹H NMR spectroscopy, volatile materials were removed under vacuum to obtain the yellow mud. Hexane (40 mL) was used to wash the yellow mud to produce a white powder. Yield: 2.12 g (80%). ¹H NMR (400 MHz, CDCl₃) δ 7.50–7.41 (m, 5H, meta-H + para-H + ortho-H of Ph), 6.88 (d, 2H, J = 2 Hz, para-H of Ph^tBu), 6.39 (d, 2H, J = 2 Hz, ortho-H of Ph^tBu), 4.02 (s, 2H, PhCH₂N), 3.72 (s, 6H, OCH₃), 3.67 (s, 4H, ArCH₂N), 1.41 (s, 18H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃) δ 151.95 (MeO–C of Ar), 151.24 (HO–C of Ar), 140.84 (^tBu–C of Ar^tBu), 132.86 (NCH₂–C of Ar), 129.67 (NCH₂–C of Ph), 128.95 (ortho-C of NCH₂Ph), 128.73 (meta-C of NCH₂Ph), 121.40 (para-C of NCH₂Ph), 114.92 (ortho-C of NCH₂Ar), 112.87 (para-C of NCH₂Ar), 56.14 (OCH₃), 56.10 (NCH₂Ph), 55.02 (NCH₂Ar), 34.41 (C(CH₃)₃), 29.72 (C(CH₃)₃). Anal. calc. (found) for C₃₂H₄₂AlNO₄: C 72.29 (75.52), H 7.96 (8.25), N 2.63 (2.68).

Synthesis of ONO^Br–Al

ONO^Br–Al was prepared following the same procedure as described for ONO^OMe–Al, except that ONO^Br–OH was used instead of ONO^OMe–OH. Yield: 2.50 g (80%). ¹H NMR (400 MHz, CDCl₃) δ 7.53–7.50 (br, 3H, para-H + ortho-H of Ph), 7.35–7.34 (br, 4H, meta-H of PhC + ortho-H–Ar^tBu), 6.95 (d, 2H, J = 2 Hz, ortho-H–Ar^tBu), 3.99 (s, 2H, PhCH₂N), 3.65 (br, 4H, ArCH₂N), 1.38 (s, 18H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃) δ 157.00 (HO–C of Ar), 142.22 (C of Ar–C–^tBu), 132.64 (ortho-C of NCH₂Ph), 131.19 (NCH₂–C of Ar), 130.75 (meta-C of NCH₂Ph), 130.00 (NCH₂–C of Ph), 129.19 (para-C of NCH₂Ar), 128.03 (para-C of NCH₂Ph), 123.19 (ortho-C of NCH₂Ar), 109.96 (Br–C of Ar), 56.17 (NCH₂Ph), 54.26 (NCH₂Ar), 35.43 (C(CH₃)₃), 29.57 (C(CH₃)₃). Anal. calc. (found) for C₃₀H₃₆AlBr₂NO₂: C 57.25 (56.89), H 5.77 (5.56), N 2.23 (2.01).

Synthesis of ON^OMe–Al

A mixture of ON^OMe–OH (2.63 g, 5.0 mmol) and trimethylaluminum (2.5 mL, 2 M in toluene) in toluene (15 mL) was stirred for 3 h at room temperature. After ON^OMe–OH disappeared, as monitored by ¹H NMR spectroscopy, volatile materials were removed under a vacuum to obtain the white powder. Yield: 3.10 g (97%). ¹H NMR (400 MHz, CDCl₃) δ 6.87 (d, 1H, J = 2 Hz, para-H of Ph^tBu), 6.41 (d, 1H, J = 2 Hz, ortho-H of Ph^tBu), 3.77 (s, 2H, ArCH₂N), 3.74 (s, 3H, OCH₃), 3.15, 2.62 (br, 4H, N(CH₂CH₂)₂), 1.95 (br, 4H, N(CH₂CH₂)₂), 1.39 (s, 9H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃) δ 153.31 (HO–C of Ar), 150.08 (MeO–C of Ar), 140.08 (NCH₂–C of Ar), 122.19 (^tBu–C of Ar^tBu), 114.74 (ortho-C of NCH₂Ar), 112.48 (para-C of NCH₂Ar), 60.11 (OCH₃), 56.57 (NCH₂Ar), 54.78 (N(CH₂CH₂)₂), 35.31 (C(CH₃)₃), 29.72 (C(CH₃)₃), 22.96 (N(CH₂CH₂)₂). Anal. calc. (found) for C₁₈H₃₀AlNO₂: C 67.68 (67.41), H 9.47 (9.25), N 4.39 (4.11).

Synthesis of ON^Br–Al

ON^Br–Al was prepared following the same procedure described for ON^OMe–Al, except that ON^Br–OH was used instead of ON^OMe–OH. Yield: 3.42 g (93%). ¹H NMR (400 MHz, CDCl₃) δ 7.30 (d, 1H, J = 2 Hz, para-H of Ph^tBu), 6.95 (d, 1H, J = 2 Hz, ortho-H of Ph^tBu), 3.74 (s, 2H, ArCH₂N), 3.13, 2.62 (br, 4H, N(CH₂CH₂)₂), 1.96 (br, 4H, N(CH₂CH₂)₂), 1.37 (s, 9H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃) δ 158.26 (HO–C of Ar), 141.71 (^tBu–C of Ar^tBu), 130.52 (para-C of NCH₂Ar), 129.88 (ortho-C of NCH₂Ar), 124.24 (NCH₂–C of Ar), 108.11 (Br–C of Ar), 59.32 (NCH₂Ar), 54.85 (N(CH₂CH₂)₂), 35.33 (C(CH₃)₃), 29.57 (C(CH₃)₃), 22.97 (N(CH₂CH₂)₂). C₁₇H₂₇AlBrNO: C 55.44 (55.11), H 7.39 (7.21), N 3.80 (3.57).

Synthesis of ONNO^OMe–Al

A mixture of ONNO^OMe–OH (1.80 g, 5.0 mmol) and trimethylaluminum (2.5 mL, 2 M in toluene) in toluene (30 mL) was stirred for 3 h at 45 °C. After ONNO^OMe–OH disappeared, as monitored by ¹H NMR spectroscopy, volatile materials were removed under vacuum to obtain the white mud. Hexane (40 mL) was used to wash the white mud to produce white powder. Yield: 1.50 g (75%). ¹H NMR (400 MHz, CDCl₃) δ 6.81–6.73 (m, 4H, para-H + meta-H of NCH₂Ar), 6.51 (d, 2H, J = 2 Hz, ortho-H of NCH₂Ar), 4.26 (br, 2H, ArCH₂N), 3.73 (s, 6H, OCH₃), 3.25–2.70 (br, 4H, ArCH₂N + N(CH₂CH₂)₂), 2.78–2.68 (br, 2H, N(CH₂CH₂)₂), −0.84 (s, 3H, CH₃). ¹³C NMR (100 MHz, CDCl₃) δ 153.96 (AlO–C of Ar), 150.84 (MeO–C of Ar), 121.97 (NCH₂–C of Ar), 120.24 (ortho-C of ArO), 115.17 (ortho-C of NCH₂Ar), 114.72 (para-C of NCH₂Ar, ortho-C of ArOMe), 63.48 (OCH₃), 61.67 (NCH₂Ar), 56.10, 55.18 (N(CH₂CH₂)₂), 44.44 (NCH₃), −11.22 (CH₃). Anal. calc. (found) for C₂₁H₂₉AlN₂O₄: C 62.99 (62.67), H 7.30 (7.21), N 7.00 (6.61).

Synthesis of ONNO^Br–Al

ONNO^Br–Al was prepared following the same procedure described for ONNO^OMe–Al, except that ONNO^Br–OH was used instead of ONNO^OMe–OH. Yield: 1.98 g (80%). ¹H NMR (400 MHz, CDCl₃) δ 7.26 (d, 2H, J = 8 Hz, ortho-H of NCH₂Ar), 7.02 (d, 2H, J = 2 Hz, ortho-H of NCH₂Ar), 6.76 (br, 2H, meta-H of BrAr), 4.24, 3.20 (d, 4H, J = 14 Hz, ArCH₂N), 3.45, 3.05, 2.83, 2.65 (br, 4H, NCH₂CH₂N), 2.32 (s, 6H, (NCH₃)₂), −0.83 (s, 3H, CH₃). ¹³C NMR (100 MHz, CDCl₃) δ 158.98 (HO–C of Ar), 132.89 (ortho-C of ArO), 131.29 (NCH₂–C of Ar), 123.50 (ortho-C of OAr, meta-C of NCH₂Ar), 121.89 (para-C of NCH₂Ar, ortho-C of BrAr), 108.21 (Br–C of Ar), 62.82, 60.98 (N(CH₂CH₂)₂), 55.09 (NCH₂Ar), 44.63 (NCH₃), −11.24 (CH₃). Anal. calc. (found) for C₁₉H₂₃AlBr₂N₂O₂: C 45.81 (45.49), H 4.65 (4.19), N 5.62 (5.31).

Synthesis of ^BuONNO^OMe–Al

^BuONNO^OMe–Al was prepared following the same procedure described for ONNO^OMe–Al, except that ^BuONNO^OMe–OH was used instead of ONNO^OMe–OH. Yield: 2.18 g (85%). ¹H NMR (400 MHz, CDCl₃) δ 6.89 (d, 2H, J = 2 Hz, ortho-H of ^tBuAr), 6.42 (d, 2H, J = 2 Hz, ortho-H of ArCH₂), 4.23, 3.03 (d, 4H, J = 12 Hz, ArCH₂N), 3.73 (s, 6H, OCH₃), 3.03, 2.81 (m, 4H, NCH₂CH₂N), 2.23 (s, 6H, (NCH₃)₂), 1.45 (s, 18H, (CH₃)₃C), −0.89 (s, 3H, CH₃). ¹³C NMR (100 MHz, CDCl₃) δ 152.99 (MeO–C of Ar), 149.68 (AlO–C of Ar), 140.09 ((CH₃)₃C–C of ArO), 121.76 (NCH₂–C of Ar), 123.50 (para-C of (CH₃)₃–CAr), 121.89 (para-C of NCH₂Ar), 61.96 (CH₃O), 55.93 (NCH₂Ar), 55.32 (N(CH₂CH₂)₂), 43.93 (NCH₃), 35.16 ((CH₃)₃C), 29.79 ((CH₃)₃C), −11.56 (AlCH₃). Anal. calc. (found) for C₂₉H₄₅AlN₂O₄: C 67.94 (67.66), H 8.85 (8.54), N 5.46 (5.12).

Synthesis of ^BuONNO^Br–Al

^BuONNO^Br–Al was prepared following the same procedure described for ONNO^OMe–Al, except that ^BuONNO^Br–OH was used in place of ONNO^OMe–OH. Yield: 1.07 g (84%). ¹H NMR (400 MHz, CDCl₃) δ 7.31 (d, 2H, J = 2 Hz, ortho-H of ^tBuAr), 6.94 (d, 2H, J = 2 Hz, ortho-H of ArCH₂), 4.35, 3.05 (d, 4H, J = 12 Hz, ArCH₂N), 3.02, 2.85 (m, 4H, NCH₂CH₂N), 2.23 (s, 6H, (NCH₃)₂), 1.41 (s, 18H, (CH₃)₃C), −0.87 (s, 3H, CH₃). ¹³C NMR (100 MHz, CDCl₃) δ 157.91 (O–C of Ar), 141.63 ((CH₃)₃C–C of ArO), 130.12 (NCH₂–C of Ar), 129.42 (para-C of (CH₃)₃–CAr), 123.66 (para-C of NCH₂Ar), 107.46 (Br–C of ArO), 61.13 (NCH₂Ar), 55.18 (N(CH₂CH₂)₂), 44.10 (NCH₃), 35.18 ((CH₃)₃C), 29.62 ((CH₃)₃C), −11.54 (AlCH₃). Anal. calc. (found) for C₂₇H₃₉AlBr₂N₂O₂: C 53.13 (52.97), H 6.44 (6.42), N 4.59 (4.56).

Synthesis of OO^OMe–Al

A mixture of OO^OMe–OH (2.41 g, 5.0 mmol) and trimethylaluminum (2.5 mL, 2 M in toluene) in THF (20 mL) was stirred for 1 h at 0 °C and overnight at room temperature. After OO^OMe–OH disappeared, as monitored by ¹H NMR spectroscopy, volatile materials were removed under a vacuum to obtain the white powder. Yield: 2.43 g (82%). ¹H NMR (400 MHz, CDCl₃) δ 7.07 (d, 2H, J = 6 Hz, ortho-H of PhCl), 7.02 (d, 2H, J = 6 Hz, meta-H of PhCl), 6.62 (d, 2H, J = 2 Hz, para-H of Ph^tBu), 6.49 (d, 2H, J = 2 Hz, para-H of PhCH), 5.84 (s, 1H, CH(ArO)₂), 4.07 (t, 4H, J = 4 Hz, O(CH₂CH₂)₂), 3.55 (s, 6H, OCH₃), 1.62 (br, 4H, O(CH₂CH₂)₂), 1.27 (s, 18H, C(CH₃)₃) −0.70 (s, 3H, CH₃). ¹³C NMR (100 MHz, CDCl₃) δ 151.15 (MeO–C of Ar), 150.13 (AlO–C of Ar), 143.42 (para-C of ArCl), 139.46 (^tBu–C of Ar^tBu), 132.63 (CH–C of ArO), 131.21 (Cl–C), 130.72 (ortho-C of ArCl), 127.75 (meta-C of ArCl), 111.91 (para-C of Ar^tBu), 111.14 (ortho-C of ArOMe, ArOAl), 71.63 (O(CH₂CH₂)₂), 55.77 (OCH₃), 41.32 (CH(ArO)₂), 35.12 (C(CH₃)₃), 29.75 (C(CH₃)₃), 25.11 (O(CH₂CH₂)₂), −14.04 (AlCH₃). Anal. calc. (found) for C₃₄H₄₄AlClO₅: C 68.62 (68.52), H 7.45 (7.63).

Synthesis of OO^Bu–Al

A method similar to that for OO^OMe–Al was used, except that OO^Bu–OH was used instead of OO^OMe–OH. Yield: 3.10 g (96%). ¹H NMR (400 MHz, CDCl₃) δ 7.18 (d, 2H, J = 8 Hz, ortho-H of PhCl), 7.14–7.11 (m, 4H, meta-H of PhCl + meta-H of AlOAr), 5.89 (s, 1H, CH(ArO)₂), 4.24 (br, 4H, O(CH₂CH₂)₂), 1.91 (br, 4H, O(CH₂CH₂)₂) 1.41, 1.22 (s, 36H, C(CH₃)₃), −0.59 (s, 3H, AlCH₃). ¹³C NMR (100 MHz, CDCl₃) δ 153.50 (AlO–C of Ar), 144.77 (para-C of ArCl), 139.91, (^tBu–C of Ar^tBu), 132.19 (C of Ar^tBu, ortho-C of ArOH), 137.17 (Cl–C), 130.97 (meta-C of ArCl), 130.85 (ortho-C of ArCl), 127.65 (CH–C of ArOAl), 124.26 (para-C of Ar^tBu, meta-C of ArO), 121.37 (ortho-C of Ar^tBu, meta-C of ArO), 71.61 (O(CH₂CH₂)₂), 41.13 (CH(ArO)₂), 35.29, 34.32 (C(CH₃)₃), 31.77, 30.13 (C(CH₃)₃), −13.95 (AlCH₃). Anal. calc. (found) for C₄₀H₅₆AlClO₃: C 74.22 (74.00), H 8.72 (8.65).

Synthesis of OO^Br–Al

A method similar to that for OO^OMe–Al was used, except that OO^Br–OH was used in place of OO^OMe–OH. Hexane (40 mL) was used to wash the white mud to give the product as a white powder. Yield: 2.76 g (80%). ¹H NMR (400 MHz, CDCl₃) δ 7.23 (d, 2H, J = 8 Hz, ortho-H of ClPh), 7.21 (d, 2H, J = 2 Hz, para-H of ^tBuAr), 7.11 (d, 2H, J = 2 Hz, para-H of CHAr), 7.07 (d, 2H, J = 8 Hz, meta-H of ClPh), 5.86 (s, 1H, CH(AlOAr)₂), 4.16 (t, 4H, J = 8 Hz, O(CH₂CH₂)₂), 1.98 (br, 4H, O(CH₂CH₂)₂), 1.37 (s, 18H, C(CH₃)₃), −0.56 (s, 3H, AlCH₃). ¹³C NMR (100 MHz, CDCl₃) δ 154.87 (AlO–C of Ar), 141.12 (^tBu–C of ^tBuAr), 142.16 (para-C of ClAr), 131.79 (Cl–C), 131.79 (meta-C of ClAr), 130.38 (ortho-C of ClAr), 134.40 (CH–C of ArOAl), 128.16 (para-C of CHAr), 128.13 (para-C of ^tBuAr), 110.73 (Br–C of Ar), 71.72 (O(CH₂CH₂)₂), 40.71 (CH(ArO)₂), 35.16 (C(CH₃)₃), 29.64 (C(CH₃)₃), −14.11 (AlCH₃). Anal. calc. (found) for C₃₂H₃₈AlBr₂ClO₃: C 55.47 (55.67), H 5.53 (5.70).

Synthesis of O^OMe–Al

A mixture of O^OMe–OH (1.18 g, 5.0 mmol) and trimethylaluminum (2.5 mL, 2 M in toluene) in THF (50 mL) was stirred for 1 h at 0 °C and overnight at room temperature. After O^OMe–OH disappeared, as monitored by ¹H NMR spectroscopy, volatile materials were removed under vacuum to obtain the white powder. Yield: 1.46 g (80%). ¹H NMR (400 MHz, CDCl₃, δ): 6.81 (s, 2H, ortho-H of MeOAr), 4.20 (m, 4H, O(CH₂CH₂)₂), 3.77 (s, 3H, OCH₃), 2.08 (m, 4H, O(CH₂CH₂)₂), 1.41 (s, 18H, C(CH₃)₃), −0.67 (s, 6H, CH₃). ¹³C NMR (100 MHz, CDCl₃) δ 151.43 (MeO–C of Ar), 149.87 (AlO–C of Ar), 139.31 (^tBu–C of Ar^tBu), 110.82 (ortho-C of ArOMe), 71.24 (O(CH₂CH₂)₂), 55.72 (OCH₃), 35.17 (O(CH₂CH₂)₂), 30.59 (C(CH₃)₃), 25.20 (C(CH₃)₃), −6.97 (AlCH₃). Anal. calc. (found) for C₂₁H₃₇AlO₃: C 69.20 (68.79), H 10.23 (9.75).

Synthesis of O^Br–Al

A method similar to that for O^OMe–Al was used, except that O^Br–OH was used instead of O^OMe–OH. Yield: 1.81 g (85%). ¹H NMR (400 MHz, CDCl₃) δ 7.26 (s, 2H, ortho-H of BrAr), 4.22 (m, 4H, O(CH₂CH₂)₂), 2.10 (m, 4H, O(CH₂CH₂)₂), 1.38 (s, 18H, C(CH₃)₃), −0.67 (s, 6H, CH₃). ¹³C NMR (100 MHz, CDCl₃) δ 156.44 (O–C of Ar), 141.12 (Br–C of Ar), 127.83 (^tBu–C of Ar^tBu), 109.49 (ortho-C of BrAr), 71.13 (O(CH₂CH₂)₂), 34.97 (O(CH₂CH₂)₂), 30.36 (C(CH₃)₃), 25.13 (C(CH₃)₃), −7.13 (AlCH₃). Anal. calc. (found) for C₂₀H₃₄AlBrO₂: C 58.11 (57.93), H 8.29 (8.08).

Synthesis of NNO^OMe–Al

NNO^OMe–Al was prepared following the same procedure described for ON^OMe–Al, except that NNO^OMe–OH was used instead of ON^OMe–OH. Yield: 1.15 g (78%). ¹H NMR (400 MHz, CDCl₃) δ 7.36–7.23 (m, 5H, Ph), 6.89 (d, J = 8.0 Hz, 1H, para-H of ^tBuAr), 6.32 (d, J = 2.0 Hz, 1H, meta-H of ^tBuAr), 3.92 (s, 2H, NCH₂Ph), 3.78–3.66 (m, 5H, NCH₂Ar + OCH₃), 2.78 (br, 4H, NCH₂CH₂N), 2.25 (s, 6H, N(CH₃)₂), 1.40 (s, 9H, C(CH₃)₃), −0.70 (s, 6H, Al(CH₃)₂). ¹³C NMR (100 MHz, CDCl₃) δ 153.30 (CH₃O–C of Ar), 150.11 (HO–C of Ar), 140.22 (ipso-C of CH₂Ar), 132.10 (ortho-C of ^tBuAr), 129.95 (ortho-C of CH₂Ar), 129.30 (meta-C of CH₂Ar), 128.65 (para-C of CH₂Ar), 119.13 (ortho-C of ^tBuAr), 114.79 (meta-C of ^tBuAr), 112.34 (meta-C of ^tBuAr), 58.70 (OCH₃), 56.11 (NCH₂Ar), 53.20 (NCH₂Ph), 52.88 (NCH₂CH₂N), 49.56 (NCH₂CH₂N), 45.71 (NCH₃), 35.11 C(CH₃)₃, 29.33 C(CH₃)₃, −9.67 (Al(CH₃)₂). Anal. calc. (found) for C₂₅H₃₉AlN₂O₂: C 70.39 (70.12), H 9.22 (8.96), N 6.57 (6.26).

Synthesis of NNO^Br–Al

NNO^Br–Al was prepared following the same procedure described for NNO^OMe–Al, except that NNO^Br–Al was used instead of NNO^OMe–OH. Yield: 1.13 g (79%). ¹H NMR (400 MHz, CDCl₃) δ 7.39–7.37 (m, 3H, Ph), 7.30 (d, J = 2.0 Hz, 1H, meta-H of ^tBuAr), 7.24–7.18 (m, 2H, Ph), 6.86 (d, J = 2.0 Hz, 1H, para-H of ^tBuAr), 3.90 (s, 2H, PhCH₂N), 3.69 (s, 2H, ArCH₂N), 2.76 (br, 4H, (NCH₂CH₂N)), 2.24 (s, 6H, N(CH₃)₂), 1.37 (s, 9H, C(CH₃)₃), −0.71 (s, 6H Al(CH₃)₂). ¹³C NMR (100 MHz, CDCl₃) δ 158.34 (O–C of ^tBuAr), 141.53 (ipso-C of CH₂Ph), 132.00 (C of ^tBuAr), 130.55 (ortho-C of ^tBuAr), 130.20 (meta-C of ^tBuAr), 129.69 (ortho-C of CH₂Ph), 129.36 (meta-C of CH₂Ph), 128.74 (para-C of CH₂Ph), 121.69 (NCH₂–C of Ar), 107.73 (Br–C of Ar), 57.86 (NCH₂Ar), 53.69 (NCH₂Ph), 52.79 (NCH₂CH₂N), 50.43 (NCH₂CH₂N), 46.16 (N(CH₃)₂), 35.13 C(CH₃)₃, 29.19 C(CH₃)₃, −9.76 (Al(CH₃)₂). Anal. calc. (found) for C₂₄H₃₆AlBrN₂O: C 60.63 (60.45), H 7.63 (7.54), N 5.89 (5.63).

Synthesis of (OO^Bu–AlOBn)₂

BnOH (0.22 g, 0.2 mmol) was added to the solution of OO^Bu–Al (1.29 g, 2.0 mmol) in toluene (20 mL) at 0 °C for one day. The volatile materials were removed under vacuum to obtain the yellow powder. The powder was washed with hexane (30 mL) to obtain the pale yellow powder. Yield: 1.00 g (48%). ¹H NMR (400 MHz, CDCl₃) δ 7.27 (m, 4H, ortho-H of PhCl), 7.18 (d, 4H, J = 8 Hz meta-H of PhCl), 7.10, 7.01 (br, 10H, H–Bn), 6.94, 6.65 (br, 8H, H of ArO), 5.23 (s, 4H, CH₂Ph), 5.11 (s, 2H, CH(ArO)₂), 1.39, 1.17 (s, 36H, C(CH₃)₃). ¹³C NMR (100 MHz, CDCl₃) δ 151.86 (AlO–C of Ar), 142.29 (para-C of ArCl), 141.23, 137.24, 131.31, 130.88, 130.29, 129.13, 128.32, 127.36, 125.50, 124.28, 121.80, 68.46 (OCH₂Ph), 42.85 (CH(ArO)₂), 35.38, 34.30 (C(CH₃)₃), 31.59, 30.44 (C(CH₃)₃). Anal. calc. (found) for C₈₄H₁₀₄Al₂Cl₂O₆: C 75.60 (75.52), H 7.85 (7.53).

General procedures for the polymerization of ε-caprolactone

Synthesis of entry 1 (Table 2) illustrates a representative polymerization procedure using complex ONO^OMe–Al as a catalyst. The polymerization conversion was examined by ¹H NMR spectroscopy studies. Toluene (5.0 mL) was transferred to a mixture of complex ONO^OMe–Al (0.1 mmol) and ε-caprolactone (10 mmol) at 25 °C. At indicated time intervals, 0.05 mL aliquots were removed, trapped with CDCl₃ (1.0 mL), and analyzed by ¹H NMR. After the solution was stirred for 85 min, the reaction was quenched by adding ethanol (10.0 mL), and the polymer precipitated as a white solid when poured into n-hexane (60.0 mL). The isolated white solid was dissolved in CH₂Cl₂ (10.0 mL), and water (10.0 mL) was used to wash the organic solution. Volatile materials were removed under vacuum to give a purified crystalline solid with a yield of 1.05 g (92%).

Author contributions

P. K. G. and F. H. performed the experiments; T. B. H. performed the additional edits and proofreading; R. K. prepared the graphical abstract; Y.-T. C. performed all crystallographic measurements; H.-C. T. performed the NMR experiments; S. D. performed additional edits and proofreading; K.-H. W. performed the DFT experiments; H.-Y. C. conceptualized the project and wrote and edited the manuscript. All authors have read and agreed to the published version of the manuscript.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This study was supported by the National Science and Technology Council of Taiwan (Grant NSTC 113-2113-M-037-001, 111-2314-B-037-094-MY3, and 113-2113-M-037-009) and the Kaohsiung Medical University “NSYSU-KMU JOINT RESEARCH PROJECT” (NSYSU-KMU-113-P24 and KMU-DK109004). The authors thank the Center for Research Resources and Development at Kaohsiung Medical University for instrumentation and equipment support and the National Center for High-performance Computing (NCHC) of National Applied Research Laboratories (NARLabs) in Taiwan for providing computational and storage resources. The authors gratefully acknowledge the use of EA000600 of MOST 110-2731-M006-001, which belongs to the Core Facility Center of National Cheng Kung University.

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Footnotes

† Electronic supplementary information (ESI) available: Al complexes and polymer characterization data and details of the kinetic study. CCDC 2236052, 2247640, 2247643, 2247645 and 2247652. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt02923b

‡ These authors contributed equally.

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