Zinc complexes supported by claw-type aminophenolate ligands: Synthesis, characterization and catalysis in the ring-opening polymerization of rac-lactide

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PAPER

Zinc complexes supported by claw-type aminophenolate ligands: synthesis,

characterization and catalysis in the ring-opening polymerization of

rac-lactide†

Shaodi Song, Xingyu Zhang, Haiyan Ma* and Yang Yang

Received 18th September 2011, Accepted 2nd December 2011

DOI: 10.1039/c2dt11767c

A series of zinc silylamido complexes bearing claw-type multidentate aminophenolate ligands,

[LZnN(SiMe₃)₂] (L = –OAr¹-CH₂N[(CH₂)_nNR₂]CH₂Ar², n = 2 or 3; R = Me or Et (1a–3a, 5a, 7a and

8a); L = –OC₆H₂-4,6-^tBu₂-2-CH₂N[(CH₂)₂OMe]₂(9a)), have been synthesized via the reaction of Zn

[N(SiMe₃)₂]₂and 1 equiv. of corresponding aminophenol. The reaction of Zn[N(SiMe₃)₂]₂with the

proligand L⁶H (2-{N-(2-methoxybenzyl)-N-[3-(N′,N′-dimethylamino)propyl]aminomethyl}-4-methyl-6-

tritylphenol) resulted in the formation of bisphenolate zinc complex 6 regardless of the stoichiometric

ratio of the two starting materials. Complex 4b with an initiating group of 3-tert-butyl-2-methoxy-5-

methylbenzyloxy was obtained and further studied via the X-ray diffraction method to be monomeric.

Zinc ethyl complex 2c was also prepared from the reaction of ZnEt₂and 1 equiv. of proligand L²H as the

representative complex with an alkyl initiating group. All zinc silylamido complexes efﬁciently initiated

the ring-opening polymerization of rac-lactide in the presence or absence of isopropanol at ambient

temperature. The steric and electronic characteristics of the ancillary ligands signiﬁcantly inﬂuenced the

polymerization performance of the corresponding zinc complexes. The introduction of bulky ortho-

substituents on the phenoxy moiety resulted in an apparent decrease of catalytic activity while a slightly

enhanced isotactic selectivity. Meanwhile, the elongation of the pendant amine arm to three-carbon-atom

linkage led to signiﬁcant decline of the catalytic activity in the absence of isopropanol. The zinc ethyl

complex 2c was not such an efﬁcient initiator as the silylamido ones, but the alkoxy complex 4b gave an

obviously faster and better controlled polymerization when compared to the zinc silylamido complexes.

electronegativity, and the steric hindrance and the electronic

effect brought by the ancillary ligand, contribute to the active

Introduction

Polylactides (PLAs) produced from renewable resources, have

been studied intensively due to their biocompatible, biodegrad-

able properties and potential as attractive alternatives for com-

mercial oleﬁnic materials.^1–6Among the methods nowadays

adopted to obtain PLAs with high molecular weight and speciﬁc

stereo-microstructure, the ring-opening polymerization (ROP) of

lactides initiated by single-site metal complexes is particularly

emphasized.^7–12,13For a well-deﬁned metal complex L_mMR (L_m

= multidentate ligand; M = central metal; R = initiating group),

both the character of the central metal such as radius and

site with speciﬁc interspace and electronic effect, which signiﬁ-

cantly inﬂuence the polymerization process.¹⁴The physical,

mechanical and thermal properties of polylactide are highly

dependent on the polymer’s tacticity,^8,9,15,16and the stereocom-

plexed PLA produced from a blend of poly-^L-LA and poly-D-LA

has a T_mvalue up to 230 °C.^17–19It is believed that forming a

stereocomplexed PLA from rac-LA at the molecular level via a

catalytic process will afford the material with superior processing

properties. Thus controlling the microstructure of PLA with the

aim to get stereocomplexed PLA from rac-LA has become an

attractive research spot.^13,20Researchers all over the world

endeavor to explore catalysts possessing good biocompatibility,

high activity and excellent stereoselectivity, especially isotactic

selectivity for the ROP of rac-LA.

The “non-properties” such as colorless, odorless and non-tox-

icity of zinc²¹make it a superior candidate in functioning as a

catalytic center,²²and its complexes are usually efﬁcient catalysts

for the ROP of lactides, as included in a recent review¹²and

other literatures.^20,23–51In terms of controlling the stereo-micro-

structure of the resultant PLA, some discrete zinc complexes

display high heterotactic or low to moderate isotactic selectivity

Shanghai Key laboratory of Functional Materials Chemistry, and

Laboratory of Organometallic Chemistry, East China University of

Science and Technology, 130 Meilong Road, Shanghai, 200237,

People’s Republic of China. E-mail: haiyanma@ecust.edu.cn;

Fax: +86 21 64253519; Tel: +86 21 64253519

†Electronic supplementary information (ESI) available: CCDC reference

1

number 845892 for 4b. CIF ﬁle for zinc complex 4b, H NMR spectra

of oligomers of rac-LA by 2a/ⁱPrOH, 4b, 4b/ⁱPrOH, the ESI-TOF

spectra of oligomer of rac-LA obtained by complex 2a, the methine

region of homonuclear decoupled ¹H NMR spectrum of poly(rac-LA)

prepared with 1a in the presence of ⁱPrOH in toluene. See DOI:

10.1039/c2dt11767c

3266 | Dalton Trans., 2012, 41, 3266–3277

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in the ROP of rac-lactide.^42,49–52The desired high isotactic

selectivity is only available for aluminum complexes bearing

Salen-type^53–56or similar ligands.^47,57,58Although versatile

multidentate ligands such as β-diketiminate,^59,60phenoxy-

amine,^33,43,61,62phenoxy-imine^{20,39,41,63–66}have been exten-

sively developed to complex with zinc, the factors either elec-

tronic or steric of the ancillary ligand which may induce high

isotactic selectivity in the ROP of rac-lactide is undiscovered.

Previously we reported that upon coordination with a zinc ion,

an unsymmetrical monoanionic aminophenolate ligand could

construct a scorpionate tripodal geometry around the metal

center, providing relatively easily adjustable steric hindrance and

electronic effect.⁴⁸The variation of the substituents on the

phenyl moieties led to moderate stereoselectivity switching from

heterotactic bias to isotactic bias, indicating that the steric protec-

tion is still not sufﬁcient for the stereoselective coordination/

insertion of lactide monomers. Herein we further modiﬁed the

ancillary ligands either by elongating the pendant amine arm or

by introducing a more bulky amino group. A series of zinc silyl-

amino complexes, representative zinc benzyloxy or zinc ethyl

complexes bearing such unsymmetrical monoanionic aminophe-

nolate ligands were obtained and evaluated for the ring-opening

polymerization of rac-lactide.

Results and discussion

Synthesis and characterization of zinc complexes

The claw-type multidentate amino-phenol proligands L^1–8

H

Scheme 1

with different pendant amine arms (–NCH₂CH₂NEt₂or

–NCH₂CH₂CH₂NMe₂) were synthesized according to our pre-

vious methods as illustrated in Scheme 1.⁴⁸Analytically pure

L^1–3H and L^5–8H could be obtained by column chromato-

graphy, while L⁴H was only obtained as a mixture contaminated

with 1/6 equiv. of 3-tert-butyl-2-methoxy-5-methylbenzyl

alcohol, a reduction product of the starting arylaldehyde, due to

their very similar polarities.

The obtained proligands L^1–3H and L^5–8H were then used to

complex with Zn[N(SiMe₃)₂]₂in the ratio of 1 : 1 at ambient

temperature, zinc silylamido complexes 1a–3a, 5a, 7a and 8a

could be isolated successfully as colorless, air/moisture sensitive

crystalline solids in moderate yields (Scheme 2). From the reac-

tion of L⁶H and Zn[N(SiMe₃)₂]₂, only the bis-ligated complex

6, without a silylamido group, could be isolated (Scheme 3),

which is hardly soluble in common hydrocarbon solvents. In

comparison, all the silylamido complexes were smoothly dissol-

vable in hexane or a mixture of hexane and toluene. Particularly,

complexes 2a and 3a with cumyl or chloro substituents also

possess enough solubility in hydrocarbon solvents, which is in

contrast to their analogues with a pendant –NCH₂CH₂NMe₂

group in the ligand framework.⁴⁸

With the expectation that the zinc silylamido complex bearing

ligand L⁴might be isolated from the reaction of Zn[N(SiMe₃)₂]₂

and the impure L⁴H (containing 1/6 equiv. of 3-tert-butyl-2-

methoxy-5-methylbenzyl alcohol), a similar amine elimination

reaction was carried out. After work-up, the target complex 4a

co-crystallized with another colorless substance 4b, which was

proved to be a product of 4a with 3-tert-butyl-2-methoxy-5-

methylbenzyl alcohol (Scheme 4). Tediously fractional

Scheme 2

Dalton Trans., 2012, 41, 3266–3277 | 3267

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Scheme 3

Fig. 1 ORTEP diagram of the molecular structure of complex 4b.

Thermal ellipsoids are drawn at the 20% probability level. Hydrogen

atoms are omitted for clarity. Selected bond distances: Zn–O2 1.863 Å,

Zn–O1 1.939 Å, Zn⋯O3 5.263 Å and Zn⋯O4 5.382 Å, Zn–N1

2.097 Å, Zn–N2 2.109 Å; selected angles: O2–Zn–O1 11.16°, O2–Zn–

N1 134.74°, O1–Zn–N1 97.43°, O2–Zn–N2 113.55°, O1–Zn–N2

110.65°, N1–Zn–N2 86.11°.

Scheme 4

crystallization failed to afford 4a in pure form, however, from

the mother liquor analytically pure complex 4b could be isolated

as cubic colorless crystals, which was further characterized via

X-ray diffraction study (Fig. 1, vide post).

Scheme 5

With the aim to own complexes that permit the comparison of

the effect of different initiating groups and ligand frameworks on

the polymerization process, we further synthesized zinc ethyl

complex 2c via the treatment of L²H with 1 equiv. of diethyl

zinc (Scheme 5), and zinc silylamido complex 9a bearing a less

bulk aminophenolate ligand {–OC₆H₂-4,6-^tBu₂-2-CH₂N

[(CH₂)₂OMe]₂} (L⁹H was prepared according to the literature⁶⁷)

via the amine elimination route (Scheme 6).

Scheme 6

The stoichiometric structures of all the complexes except for

1

complex 6 were conﬁrmed on the basis of H and ¹³C NMR

spectroscopy as well as elemental analysis. The very poor solubi-

lity of complex 6 either in C₆D₆or in CDCl₃hampered further

structural characterization. Similar to our previous report,⁴⁸in

the ¹H NMR spectra (C₆D₆) of all the zinc complexes with

exclusion of 9a, the two protons of the methylene group in each

Ar-CH₂-N unit are inequivalent and give rise to two doublets as

compared to one singlet for the proligand; the proton resonances

of the pendant amine arm (both N(CH₂)_nN and NCH₂CH₃moi-

eties) also exhibit unidentiﬁable coupling modes. All these

suggest the participation of all three nitrogen donors in the

coordination with the zinc center. While for 9a, one singlet

accounting for two protons of Ar-CH₂-N unit is displayed, and

four multiplets with each representing two protons of the two

NCH₂CH₂OMe groups were observed, which possibly indicates

some ﬂuxional coordination behavior of the two ether arms in

solution. The coordination of the aryl-methoxy group to the

metal center in complexes 1a–3a, 5a and 7a is excluded based

on the slight change of the corresponding chemical shifts of

3268 | Dalton Trans., 2012, 41, 3266–3277

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Ring-opening polymerization of rac-lactide

methoxy protons between complex and proligand. As for

complex 8a, the existence of a weak interaction between the

ﬂuorine atom and the zinc center was studied with ¹⁹F NMR

spectroscopy, however, a chemical-shift difference of 3.48 ppm

in ¹⁹F NMR (L⁸H, −118.49 ppm vs. complex 8a, −115.01 ppm)

might suggest nothing.

Polymerization with zinc silylamido complexes. The ring-

opening polymerizations of rac-LA employing zinc silylamido

complexes 1a–3a, 5a, 7a–9a as initiators were examined system-

atically. As shown in Table 1, all of the zinc silylamido com-

plexes alone are efﬁcient initiators for the ROP of rac-LA either

in toluene or in THF at ambient temperature. In each case, high

conversion could be reached within hours when the [rac-

LA]₀: [Zn]₀ratio is 200. PLAs with high molecular weighs and

Molecular structure of 4b

narrow to moderate molecular weight distributions (M_w/M_n

1.07–1.74) could be obtained.

=

Single crystals of complex 4b suitable for X-ray structural deter-

mination were obtained from a saturated solution of hexane–

toluene mixture at room temperature. An ORTEP drawing of the

molecular structure of complex 4b is given in Fig. 1. Although

bearing a substituted benzyloxy group, complex 4b possesses a

monomeric structure in the solid state. The zinc atom is coordi-

nated by three heteroatom donors of the tetradentate ligand and

one 3-tert-butyl-2-methoxy-5-methylbenzyloxy group adopting

a distorted tetrahedral geometry. Similar to the results from

Lin,²⁴Chisholm⁶⁸and our previous work,⁴⁸both the ether func-

tional groups have no coordination interaction with the zinc

center as evidenced by the long distances of Zn⋯O3 = 5.263 Å

and Zn⋯O4 = 5.382 Å. The bond distances of Zn–N1 and Zn–

N2 are 2.097 Å and 2.109 Å respectively, both falling into the

range of Zn–N coordinating bond lengths reported for common

zinc complexes^{29,48,51,69,70}(2.058–2.324 Å), which are however

shorter than their silylamido analogues.⁴⁸The angles of O2–Zn–

O1 = 111.16(14)°, O2–Zn–N1 = 134.74(15)° and O2–Zn–N2 =

113.55(16)° deviate signiﬁcantly from the normal value of

109.37°, obviously due to the constrains imposed by the multi-

coordination mode of the ligand.

Compared to our previous results,⁴⁸zinc silylamido com-

plexes 1a–3a, 5a, 7a and 8a, either with a pendant diethylamino

group or elongated three-carbon pendant amino arm, display sig-

niﬁcantly lower catalytic activities for the ROP of rac-LA. It

takes around 2–8 h for the monomer conversions up to 98%,

whereas their analogues with a pendant NCH₂CH₂NMe₂group

accomplish the polymerization within 1 h under the identical

conditions.⁴⁸For complexes with the same substituents on the

phenyl rings, complex 2a with a pendant NCH₂CH₂NEt₂group

is more active than complex 5a with a pendant NCH₂CH₂CH₂-

NMe₂group (Table 1, runs 5, 7 vs. runs 12, 14), suggesting the

prominent inﬂuence of the chain length. Such drastic inﬂuence

of one-carbon-elongation was also observed for other metal

complex systems, such as Salen–Al complexes,^71–73thioalkane-

diylbisphenolate rare earth metal complexes.^74–77

Besides, several structure–activity trends similar to our pre-

vious results⁴⁸can be drawn from the comparison of the

polymerization runs in Table 1. It is found that the catalytic

activity obviously decreases when the ^tBu groups on the

Table 1 ROP of rac-LA initiated by zinc silylamido complexes^a

[rac-LA]₀: [Zn]₀: [ⁱPrOH]₀

Solvent

T/°C

t/min

Conv.^b(%)

10⁻⁴M_n,calcd.

10⁻⁴M_n

PDI^d

P_m

c

d

e

Run

Cat.

1

2

3

4

5

6

7

8

1a

200 : 1 : 0

200 : 1 : 1

200 : 1 : 0

200 : 1 : 1

200 : 1 : 0

200 : 1 : 1

200 : 1 : 0

200 : 1 : 1

200 : 1 : 0

200 : 1 : 1

200 : 1 : 0

200 : 1 : 1

200 : 1 : 0

200 : 1 : 1

200 : 1 : 0

200 : 1 : 1

200 : 1 : 0

200 : 1 : 1

200 : 1 : 0

200 : 1 : 1

200 : 1 : 0

200 : 1 : 1

200 : 1 : 0

200 : 1 : 1

Tol.

THF

Tol.

THF

Tol.

THF

Tol.

THF

Tol.

THF

Tol.

25

24

25

28

25

27

25

24

25

150

45

120

45

180

60

120

75

120

30

430

45

300

45

480

45

330

45

180

45

120

45

90

96

92

78

95

98

89

91

95

98

99

82

93

96

83

98

99

96

62

83

99

83

98

99

2.59

2.77

2.65

2.25

2.74

2.82

2.56

2.62

2.74

2.82

2.85

2.36

2.68

2.77

2.39

2.82

2.85

2.77

1.78

2.39

2.85

2.39

2.82

2.85

9.43^f

4.60

6.38

1.89

9.34

4.74

—

11.25

6.15

2.85

1.69

5.13

2.49

2.16^f

1.98^f

2.65

2.45

1.19^f

1.10^f

4.78

2.73

6.95

2.98

9.37

2.55

1.34^f

1.44

1.39

1.41

1.55

1.46

—

1.42

1.57

1.47

1.43

1.73

2.05^f

1.69^f

1.43

1.42^f

1.36^f

1.48

1.54

1.55

1.32

1.07

0.61

0.60

0.58

0.65

—

2a

0.65

0.51

0.53

0.58

0.55

0.63

0.61

0.59

0.60

0.61

0.39

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

3a

5a

7a

8a

9a

20

10

1

c

^a[rac-LA]₀= 1.0 M, [Zn]₀= 0.005 M. ^bDetermined by H NMR spectroscopy. M_n,calcd= ([rac-LA]₀/[Zn]₀) × 144.13 × Conv.%. ^dDetermined by

GPC, Waters M515 pump, 25 °C, 1 mL min⁻¹, PS as standards. ^eP_mis the probability of forming a new m-dyad, determined by homonuclear

decoupled ¹H NMR spectroscopy. ^fDetermined by GPC, Waters 1515 pump, 35 °C, 1 mL min⁻¹, PS as standards.

This journal is © The Royal Society of Chemistry 2012

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phenoxy unit are replaced by cumyl groups with the rest of the

ancillary ligand remaining unchanged (1a vs. 2a, Table 1).

Namely, bulky substituents of the phenoxy moiety, especially at

the ortho-position, tend to hinder the approach of a monomer to

the central metal and are disadvantageous to the catalytic

activity. Introduction of halogens on the ancillary ligand was

reported to have an inconsistent inﬂuence on the polymerization

rate.^50,51The conclusion drawn from our study is that the cataly-

tic activity will increase when chlorine atoms are substituted at

the ortho- and para-positions of the phenoxy moiety (3a,

Table 1), which is in accordance with the results of tetradentate

Salan– or Salen–Al complexes,^78,79but conﬂicts with those of

tridentate Schiff-base zinc and magnesium complexes,^50,51bis

(aminophenolato) aluminum complexes,⁸⁰and aminophenolate

titanium, zirconium complexes.⁸¹Substituents on the phenyl

ether moiety have a slight inﬂuence on the catalytic activity

when compared with those on the phenoxy unit. Complex 7a

with ortho-^tBu group shows parallel activity to 5a without any

ortho-substitution on the phenyl ether moiety.

The particularity of the ﬂuorine atom usually causes com-

pounds to have unpredictable properties, it is also the case for

complex 8a with ﬂuoro instead of methoxy group on one phenyl

ring. In comparison with the methoxy analogue 1a, complex 8a

displays declined activity for the ROP of rac-LA either in

toluene or THF in the absence of isopropanol (Table 1, runs 1, 3

vs. runs 20, 22). Although the interaction of ﬂuoro to the central

metal is not conclusive from ¹⁹F NMR spectroscopy, ﬂuxional

coordination of the ﬂuoro to the zinc center might be suggested

to be responsible for the decrease of the activity.⁸²

indicative of the initiation with the N(SiMe₃)₂group and there-

fore a coordination–insertion polymerization by a single-site

catalyst. Still, the intra- and intermolecular transesteriﬁcations

are inevitable, as evident from the ESI-TOF mass spectrum.

This series of zinc silylamido complexes give overall elevated

isotactic stereoselectivities for the ROP of rac-lactide compared

to our previous work.⁴⁸Except for complexes 3a and 9a, all the

complexes produce polylactide with a slightly isotactic bias

(P_m= 0.55–0.65). Complex 2a with sterically demanding cumyl

groups shows the highest preference for isotactic dyad enchain-

ment, the P_mvalue of the polymer sample is up to 0.65 at

ambient temperature. Moreover, it is found that the polymeriz-

ation of ^L-LA by 2a proceeds faster than that of rac-LA, also

implying a sequential insertion preference of lactide monomer

with same chirality.

The slight increase in isotactic stereoselectivity promoted us to

study complex 2a in more detail. Polymerization conditions,

such as polymerization time, temperature, monomer to initiator

ratio, different solvents (toluene, THF, dichloromethane), and so

on, were varied to study the inﬂuence on the isotactic selectivity

of 2a (see Table S1†). However, no apparent improvement in

isotactic selectivity could be observed.

Polymerization with zinc alkoxy complexes. Upon addition of

isopropanol, the activities of all zinc silylamido complexes

increased signiﬁcantly. The inﬂuence of ligand framework and

substituents became less profound and the activities of all com-

plexes were somewhat close to one another, which promoted us

to study the real active species. The typical NMR-scale reaction

of complex 2a with isopropanol in 1 : 1 ratio indicated that the

zinc isopropoxide complex “L²ZnOⁱPr” was generated quanti-

tively (Fig. 3). To acquire some information about the ROP of

rac-lactide initiated by the in situ generated zinc isopropoxide,

the NMR-scale polymerization was also conducted with [rac-

LA]₀: [2a]₀: [ⁱPrOH]₀= 10 : 1 : 1. The polymerization started

instantaneously and the active oligomer could be identiﬁed

unambiguously (Fig. 4), which did not decompose even with the

addition of the second equiv. of isopropanol. Small amount of

free ligand L²H appeared until 7 equiv. of isopropanol was

added. The OⁱPr group end-capping on the obtained oligomer

Among these silylamido zinc complexes, complex 9a indi-

cates the highest activity, and a monomer conversion of up to

98% could be achieved within 10 min in toluene at 25 °C. It is

apparent that when less steric hindrance is imposed by the ligand

framework the catalytic activity is signiﬁcantly enhanced.

1

From the H NMR spectra of the resultant polymer samples

obtained by these zinc silylamido complexes, no clear end-

groups could be distinguished. Whereas, in the ESI-TOF mass

spectrum of an oligomer obtained by complex 2a with [rac-

LA]₀: [Zn]₀= 10 : 1, a series of signals end-capped with

N(SiMe₃)₂and a hydroxy group do dominant (see Fig. 2),

Fig. 3 ¹H NMR spectrum (C₆D₆, 400 MHz) of zinc isopropoxide

complex generated in situ from the reaction of 2a and isopropanol

([Zn]₀: [ⁱPrOH]₀= 10 : 1, at ambient temperature; *, free HN(SiMe₃)₂).

Fig. 2 ESI-TOF mass spectrum of the oligomer of rac-LA obtained

with 2a ([rac-LA]₀: [2a]₀= 10 : 1, in toluene).

3270 | Dalton Trans., 2012, 41, 3266–3277

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well-controlled polymerization process by 4b. The ¹H NMR

spectra of oligomer samples obtained by 4b show that the poly-

mers are systematically end-capped with 3-tert-butyl- 2-

methoxy-5-methylbenzyloxy group and a hydroxy group (See

ESI, Fig. S3†), giving evidence that the benzyloxy group in 4b

acts as an efﬁcient initiating group for the polymerization of

lactide. In the presence of isopropanol, the polymerization rate

by 4b keeps constant, and the polymerization affords PLA with

nearly half of the theoretical molecular weight. Both end groups

1

of OⁱPr and substituted benzyloxy could be identiﬁed in the H

NMR spectra of the polymer samples (See ESI, Fig. S3†).

Obviously, the excess isopropanol acts as a chain transfer

reagent instead.

The bis-ligated complex 6 shows neglectable catalytic activity

for the polymerization of rac-lactide even in the presence of iso-

propanol with the polymerization time extended to 4 days. As

general observed, the aroxy group is not nucleophilic enough to

initiate the ring-opening polymerization of cyclic esters.

From Tables 1 and 2, it is further concluded that the initiating

group has no essential inﬂuence on the stereoselectivity of the

corresponding zinc complex. In most of the cases, the isotacticity

of the resultant polymer is unchanged regardless of the presence

or absence of isopropanol. The slight deviation of P_mvalues in

some cases is mainly due to the transesteriﬁcation during the

polymerization process, which becomes signiﬁcant in the case of

complex 2c due to the quite low polymerization rate.

Fig. 4 ¹H NMR spectrum (400 MHz, C₆D₆) of active rac-lactide oli-

gomer by 2a/ⁱPrOH ([rac-LA]₀: [Zn]₀: [ⁱPrOH]₀= 10 : 1 : 1, at 25 °C; *,

monomer; **, free HN(SiMe₃)₂).

could be distinctly observed (See ESI, Fig. S1†). All these

suggest that the reaction of zinc silylamido complex with isopro-

panol in situ generates the well-deﬁned zinc isopropoxide

complex, which behaves as a single-site initiator for the coordi-

nation/insertion polymerization of rac-lactide.

Without the addition of isopropanol, zinc ethyl complex 2c

could hardly initiate the ROP of rac-LA at ambient temperature

(Table 2). Even in the presence of isopropanol, it still took more

than 48 h for complex 2c to gain high monomer conversion up

to 96%. From our previous study,⁴⁸it is known that at ambient

temperature the reaction of such zinc ethyl complex with alcohol

is unexpectedly slow. Thus, polymerization of rac-lactide at elev-

ated temperature of 60 °C was conducted. To our surprise, either

in the presence or in the absence of isopropanol, complex 2c

shows sufﬁcient catalytic activity for the ROP of rac-LA, which

is in contrast to our previous results. High monomer conversions

could be achieved within relatively short periods (Table 2, runs 3

and 4).

Benzyloxy complex 4b, although possessing bulky groups on

both phenyl moieties, displays higher activity than zinc silyla-

mido complex 2a with a similar ligand framework. As depicted

in Fig. 5, when the monomer conversions are lower than 89%, a

linear relationship of molecular weight versus monomer conver-

sion could be obtained by complex 4b alone, and the resultant

PLA samples have relatively low PDI = 1.14–1.31, suggesting a

Fig. 5 The relationship of M_n, PDI of PLA sample versus monomer

conversion catalyzed by complex 4b ([rac-LA]₀= 0.79 M, [rac-

LA]₀: [4b]₀= 158 : 1, 24 °C, in toluene; with P_mvalues in parentheses).

Table 2 ROP of rac-LA catalyzed by zinc complexes 2c and 4b^a

c

d

e

Run

Cat.

[rac-LA]₀: [Zn]₀: [ⁱPrOH]₀

T/°C

t/h

Conv.^b(%)

10⁻⁴M_n,calcd.

10⁻⁴M_n

PDI^d

P_m

1

2

3

4

5

6

2c

200 : 1 : 0

200 : 1 : 1

200 : 1 : 0

200 : 1 : 1

200 : 1 : 0

200 : 1 : 1

26

60

25

72

48

72

12

0.75

10

96

92

90

92

90

0.29

2.77

2.65

2.59

2.65

1.29

—

2.47

7.23^f

4.20

4.09

1.02

1.36

1.39^f

1.35

1.39

1.11

0.59

0.61

0.62

4b

c

^a[rac-LA]₀= 1 M, [Zn]₀= 0.005 M. ^bDetermined by ¹H NMR spectroscopy. M_n,calcd= ([rac-LA]₀/[Zn]₀) × 144.13 × Conv.%. ^dDetermined by

GPC, Waters M515 pump, 25 °C, 1 mL min⁻¹, PS as standards. ^eP_mis the probability of forming a new m-dyad, determined by homonuclear

decoupled ¹H NMR spectroscopy. ^fDetermined by GPC, Waters 1515 pump, 35 °C, 1 mL min⁻¹, PS as standards.

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diethylethane-1,2-diamine (2.32 g, 20.0 mmol) and 2-methoxy-

benzaldehyde (2.72 g, 20.0 mmol) in methanol (20 mL) were

heated to reﬂux for 24 h. After cooling to room temperature,

sodium borohydride (1.51 g, 40.0 mmol) was added slowly to

the above yellow solution and the resultant mixture was stirred

for another 3 h at 50 °C. The reaction mixture was extracted with

methylene dichloride, and the combined organic phase was dried

over anhydrous MgSO₄. After removal of the solvent by rotary

evaporation, a viscous yellow oil was obtained, to which was

added the solution of paraformaldehyde (0.72 g, 24 mmol) and

2,4-di-tert-butylphenol (4.13 g, 20.0 mmol) in methanol

(20 mL), and then stirred at 80 °C for 12 h. The mixture was

cooled and concentrated under vacuum to give an oil, which was

puriﬁed by column chromatography (silica gel 100 mesh, pet-

roleum ether/ethyl acetate = 4 : 1) to afford a light yellow oil

after removal of all the volatiles (4.78 g, 52.6%). Found: C,

76.51; H, 10.26; N, 6.10. Calc. for C₂₉H₄₆N₂O₂: C, 76.60; H,

Conclusions

A series of zinc silylamido complexes, zinc ethyl complex and

zinc benzyloxy complex, ligated with claw-type multidentate

aminophenolate ligand were obtained and evaluated for the ROP

of rac-lactide. All of the zinc silylamido complexes efﬁciently

initiated the ROP of rac-LA at ambient temperature, with the

initiating group N(SiMe₃)₂end-capping the polymer chain as

evidenced in the ESI-TOF mass spectrum. The introduction of

bulky ortho-substituents on the phenoxy ring resulted in an

apparent decrease of the catalytic activity while a slightly

enhanced isotactic selectivity (P_mup to 0.65 at ambient tempera-

ture). Replacement of the dimethylamino group with a diethyl-

amino group and introduction of a longer pendant amine arm

both led to a dramatic decline of the catalytic activity in the

absence of isopropanol. Benzyloxy complex 4b alone could

initiate a well-controlled polymerization (conv. < 89%), whereas

the zinc ethyl complex 2c was not so efﬁcient as the silylamido

or alkoxy analogues, showed low activity for lactide polymeriz-

ation even in the presence of isopropanol at room temperature,

but proved to be more active at elevated temperature.

1

10.20; N, 6.16%. H NMR (400 MHz, CDCl₃): δ 10.76 (br, 1H,

OH), 7.28 (d, 2H, J = 7.2 Hz, ArH), 7.21 (d, 1H, J = 2.4 Hz,

ArH), 6.96–6.87 (m, 3H, ArH), 3.90 (s, 3H, OCH₃), 3.79 (s, 2H,

Ar–CH₂N), 3.75 (s, 2H, NCH₂–Ar), 2.63–2.58 (m, 2H,

NCH₂CH₂N), 2.54–2.50 (m, 2H, NCH₂CH₂N), 2.39 (q, 4H, J =

7.2 Hz, NCH₂CH₃), 1.46 (s, 9H, C(CH₃)₃), 1.31 (s, 9H,

C(CH₃)₃), 0.91 (t, 6H, J = 7.2 Hz, NCH₂CH₃); ¹³C NMR

(100 MHz, CDCl₃): 158.3, 154.4, 140.2, 135.6, 131.6, 128.9,

126.0, 123.8, 122.8, 122.2, 120.4, 110.5 (all ArC), 59.0 (OCH₃),

55.3 (N–CH₂–Ar), 53.9 (Ar–CH₂–N), 50.8 (NCH₂CH₂N), 50.1

(NCH₂CH₂N), 47.4 (NCH₂CH₃), 35.0 (C(CH₃)₃), 34.2 (C

(CH₃)₃), 31.8 (C(CH₃)₃), 29.7 (C(CH₃)₃), 11.6 (NCH₂CH₃).

Experimental

General considerations and materials

All manipulations were carried out under a dry argon atmosphere

using standard Schlenk-line or glove-box techniques. Toluene,

petroleum ether and n-hexane were reﬂuxed over sodium benzo-

phenone ketyl prior to use. Benzene-d₆, chloroform-d and other

reagents were carefully dried and stored in glove-box. Zn

[N(SiMe₃)₂]₂,⁸³3-tert-butyl-5-methyl-2-methoxybenzaldehyde⁸⁴

and 4,6-di(tert-butyl)-2-[N,N-di(2-methoxyethyl)aminomethyl]

phenol (L⁹H)⁶⁷were prepared according to the literature

methods. rac-Lactide (Aldrich) was sublimed twice under

vacuum at 80 °C. Isopropanol was dried over calcium hydride

prior to distillation. All other chemicals were commercially avail-

able and used after appropriate puriﬁcation. Glassware and vials

used in the polymerization were dried in an oven at 120 °C over-

night and exposed to a vacuum–argon cycle three times.

4,6-Dicumyl-2-{N-(2-methoxybenzyl)-N-[2-(N′,N′-diethyl amino)

ethyl]aminomethyl}-phenol (L²H). The procedure was same as

that of L¹H, except that 2,4-dicumylphenol (6.61 g, 20.0 mmol)

was used to afford ligand L²H as a light yellow oil (6.14 g,

53.0%). Found: C, 80.91; H, 8.76; N, 4.75. Calc. for

C₃₉H₅₀N₂O₂: C, 80.93; H, 8.71; N, 4.84%. ¹H NMR (400 MHz,

CDCl₃): δ 10.36 (s, 1H, OH), 7.29 (m, 4H, ArH), 7.25–7.17 (m,

7H, ArH), 7.15–7.12 (m, 1H, ArH), 6.58 (d, 1H, J = 7.6 Hz,

ArH), 6.84–6.78 (m, 3H, ArH), 3.68 (s, 3H, OCH₃), 3.63 (s, 2H,

Ar–CH₂N), 3.59 (s, 2H, NCH₂–Ar), 2.45 (m, 2H, NCH₂CH₂N),

2.30 (m, 6H, NCH₂CH₂N, NCH₂CH₃), 1.71 (s, 6H,

C(CH₃)₂Ph), 1.70 (s, 6H, C(CH₃)₂Ph), 0.84 (t, 6H, J = 6.8 Hz,

NCH₂CH₃); ¹³C NMR (100 MHz, CDCl₃): δ 159.0, 155.1,

152.9, 152.6,140.4, 139.4, 132.6, 129.3, 128.6, 127.8, 127.3,

127.2, 126.9, 126.4, 125.5, 125.4, 124.0, 121.3, 111.1 (all ArC),

58.5 (CH₃O–Ar), 55.4 (Ar–CH₂N), 53.7 (NCH₂–Ar), 51.5

(NCH₂CH₂N), 51.0 (NCH₂CH₂N), 47.6 (NCH₂CH₃), 43.4 (C

(CH₃)₂Ph), 43.1 (C(CH₃)₂Ph), 32.1 (C(CH₃)₂Ph), 30.4

(C(CH₃)₂Ph), 12.5 (NCH₂CH₃).

NMR spectra were recorded on Bruker AVANCE-400 and

Bruker AVANCE-500 spectrometers at 25 °C (¹H: 400,

500 MHz; ¹³C: 100 MHz) unless otherwise stated. Chemical

1

shifts for H and ¹³C NMR spectra were referenced internally

using the residual solvent resonances and reported relative to

tetramethylsilane (TMS). Elemental analyses were performed on

an EA-1106 instrument. Spectroscopic analyses of polymers were

performed in CDCl₃. Gel permeation chromatography (GPC)

analyses were carried out on a Waters instrument (M515 pump,

Optilab Rex injector) in THF at 25 °C, or a Waters instrument

(1515 pump, Waters 2414 RI) in THF at 35 °C, at a ﬂow rate of

1 mL min⁻¹. Calibration standards were commercially available

narrowly distributed linear polystyrene samples that cover a

broad range of molar masses (10³< M < 2 × 10⁶g mol⁻¹).

4,6-Dichloro-2-{N-(2-methoxybenzyl)-N-[2-(N′,N′-diethyl amino)

ethyl]aminomethyl}-phenol (L³H). The procedure was same

as that of L¹H, except that 2,4-dichlorophenol (3.26 g,

20.0 mmol) was used to afford ligand L³H as a white solid

(3.32 g, 40.4%). Found: C, 61.34; H, 6.80; N, 6.71. Calc. for

C₂₁H₂₈Cl₂N₂O₂: C, 61.31; H, 6.86; N, 6.81%. ¹H NMR

(400 MHz, CDCl₃): δ 7.22–7.24 (m, 2H, ArH), 7.17 (d, 1H, J =

7.2 Hz, ArH), 6.86–6.84 (m, 2H, ArH), 6.83 (d,1H, J = 8.0 Hz,

ArH), 3.79 (s, 3H, OCH₃), 3.68 (s, 2H, Ar–CH₂N), 3.63 (s, 2H,

Synthesis of proligands

4,6-Di-tert-butyl-2-{N-(2-methoxybenzyl)-N-[2-(N′,N′-diethyl

amino)ethyl]aminomethyl}-phenol (L¹H). The solution of N,N-

3272 | Dalton Trans., 2012, 41, 3266–3277

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NCH₂–Ar), 2.58 (quasi s, 4H, NCH₂CH₂N), 2.30 (q, 4H, J =

7.2 Hz, NCH₂CH₃), 0.95 (t, 6H, J = 7.2 Hz, NCH₂CH₃); ¹³C

NMR (100 MHz, CDCl₃): δ 158.2, 153.1, 131.6, 129.2, 128.5,

127.5, 125.9, 125.1, 122.6, 121.7, 120.3, 110.5 (all ArC), 56.4

(CH₃O–Ar), 55.2 (Ar–CH₂N), 53.3 (NCH₂–Ar), 50.4

(NCH₂CH₂N), 49.7 (NCH₂CH₂N), 46.5(NCH₂CH₃), 11.1

(NCH₂CH₃).

NCH₂CH₂CH₂N), 1.47 (quintet, 2H, J = 6.0 Hz, NCH₂CH₂CH_2-

N).¹³C NMR (100 MHz, CDCl₃): δ 158.1, 154.3, 146.3, 133.6,

131.6, 131.3, 130.6, 128.8, 128.6, 127.0, 126.3, 125.3, 125.2,

122.5, 120.5, 110.3 (all ArC), 63.3 (CPh₃), 58.2 (Ar–OCH₃),

57.5 (Ar–CH₂), 55.1 (Ar–CH₂), 52.3 (NCH₂CH₂CH₂N), 51.0

(NCH₂CH₂CH₂N), 45.3 (NCH₃), 23.8 (NCH₂CH₂CH₂N), 21.0

(Ar–CH₃).

4,6-Dicumyl-2-{N-(3-tert-butyl-2-methoxy-5-methylbenzyl)-N-

[2-(N′,N′-diethylamino)ethyl]aminomethyl}-phenol (L⁴H). The

procedure was same as that of L¹H, except that 3-tert-butyl-2-

methoxy-5-methylbenzaldehyde (4.13 g, 20.0 mmol) was used

in the ﬁrst step and 2,4-dicumylphenol (6.61 g, 20.0 mmol) was

used in the last step to afford ligand L⁴H as a light yellow oil: in

pure form (∼20 mg), as a mixture of L⁴H and 1/6 equiv. of 3-

tert-butyl-2-methoxy-5-methylbenzyl alcohol (4.70 g, 36.2%).

For the pure L⁴H: ¹H NMR (400 MHz, CDCl₃): δ 10.18 (s, 1H,

OH), 7.26–7.24 (m, 4H, ArH), 7.23–7.20 (m, 4H, ArH), 7.18 (s,

1H, ArH), 7.16–7.14 (m, 1H, ArH), 7.07 (t, 1H, J = 6.8 Hz,

ArH), 6.96 (s, 1H, ArH), 6.81 (s, 1H, ArH), 6.78 (s, 1H, ArH),

3.57 (s, 3H, OCH₃), 3.53 (s, 2H, Ar–CH₂N), 3.50 (s, 2H,

NCH₂–Ar), 2.36 (br s, 4H, NCH₂CH₂N), 2.25 (q, 4H, J = 7.2

Hz, NCH₂CH₃), 2.18 (s, 3H, Ar–CH₃), 1.69 (s, 6H,

C(CH₃)₂Ph), 1.68 (s, 6H, C(CH₃)₂Ph), 1.35 (s, 9H, C(CH₃)₃),

0.81 (t, 6H, J = 7.2 Hz, NCH₂CH₃).

4,6-Dicumyl-2-{N-(3-tert-butyl-2-methoxy-5-methylbenzyl)-N-

[3-(N′,N′-dimethylamino)propyl]aminomethyl}-phenol (L⁷H).

The procedure was same as that of L¹H, except that 3-tert-butyl-

2-methoxy-5-methylbenzaldehyde (4.34 g, 20.0 mmol), 2,4-

dicumylphenol (6.61 g, 20.0 mmol) and N,N-dimethylpropane-

1,3-diamine (3.06 g, 30.0 mmol) were used to afford ligand

L⁷H as a light yellow oil (6.41 g, 48.2%). Found: C, 81.10; H,

9.76; N, 4.17. Calc. for C₄₃H₅₈N₂O₂: C, 81.34; H, 9.21;

1

N,4.41%. H NMR (400 MHz, CDCl₃): δ 7.25 (s, 2H, ArH), δ

7.24 (s, 2H, ArH), 7.19–7.17 (m, 5H, ArH), 7.15–7.12 (m, 1H,

ArH), 7.09–7.04 (m, 1H, ArH), 6.96 (d, 1H, J = 2.0 Hz, ArH),

6.75 (d, 1H, J = 2.0 Hz, ArH), 6.74 (d, 1H, J = 2.4 Hz, ArH),

3.59 (s, 2H, Ar–CH₂N), 3.58 (s, 3H, Ar–OCH₃) 3.51 (s, 2H,

Ar–CH₂N), 2.28 (t, 2H, J = 6.0 Hz, NCH₂CH₂CH₂N), 2.16 (s,

3H, Ar–CH₃), 2.08 (s, 6H, N(CH₃)₂), 2.02 (t, 2H, J = 6.0 Hz,

NCH₂CH₂CH₂N), 1.66 (s, 12H, C(CH₃)₂Ph), 1.52 (quintet, 2H,

J = 6.0 Hz, NCH₂CH₂CH₂N), 1.33 (s, 9H, C(CH₃)₃); ¹³C NMR

(100 MHz, CDCl₃): δ 156.8, 154.0, 151.6, 151.5,142.4, 139.8,

135.1, 132.8, 130.5, 129.8, 127.8, 128.0, 127.7, 126.9, 125.9,

125.8, 125.5, 125.0, 124.9, 122.0 (all ArC), 62.5 (ArOCH₃),

58.6 (Ar–CH₂N), 57.6 (Ar–CH₂N), 51.6 (NCH₂CH₂CH₂N),

51.1 (NCH₂CH₂CH₂N), 45.3 (NCH₃), 42.4 (C(CH₃)₂Ph), 42.1

(C(CH₃)₂Ph), 34.9 (C(CH₃)₃), 31.1 (C(CH₃)₃), 29.5

(C(CH₃)₂Ph), 23.7 (NCH₂CH₂CH₂N), 21.1 (Ar–CH₃).

4,6-Dicumyl-2-{N-(2-methoxybenzyl)-N-[3-(N′,N′-dimethyl

amino)propyl]aminomethyl}-phenol (L⁵H). The procedure was

same as that of L¹H, except that 2,4-dicumylphenol (6.61 g,

20.0 mmol) and N,N-dimethylpropane-1,3-diamine (3.06 g,

30.0 mmol) were used to afford ligand L⁵H as a light yellow oil

(3.61 g, 32.5%). Found: C, 81.04; H, 8.61; N, 4.95. Calc. for

C₃₈H₄₈N₂O₂: C, 80.81; H, 8.57; N, 4.96%. ¹H NMR (400 MHz,

CDCl₃): δ 7.27–7.26 (m, 4H, ArH), 7.22–7.15 (m, 7H, ArH),

7.12–7.09 (m, 1H, ArH), 6.91 (dd, 1H, J = 7.6, 1.6 Hz, ArH),

6.80–6.77 (m, 2H, ArH), 6.70 (d, 1H, J = 2.0 Hz, ArH), 3.65 (s,

3H, Ar–OCH₃), 3.59 (s, 2H, Ar–CH₂N), 3.54 (s, 2H, Ar–

CH₂N), 2.33 (t, 2H, J = 6.0 Hz, NCH₂CH₂CH₂N), 2.10 (s, 6H,

N(CH₃)₂), 2.04 (t, 2H, J = 6.0 Hz, NCH₂CH₂CH₂N), 1.67 (s,

6H, C(CH₃)₂Ph), 1.64 (s, 6H, C(CH₃)₂Ph), 1.50 (quintet, 2H, J

= 6.0 Hz, NCH₂CH₂CH₂N); ¹³C NMR (100 MHz, CDCl₃): δ

158.2, 154.0, 151.8, 151.6, 139.6, 135.1, 131.8, 128.9, 127.9,

127.7, 126.9, 125.8, 125.6, 125.4, 124.7, 122.0, 120.4, 110.4

(all ArC), 58.5 (CH₃O–Ar), 57.7 (Ar–CH₂N), 55.2 (NCH₂–Ar),

52.7 (NCH₂CH₂CH₂N), 51.2 (NCH₂CH₂CH₂N), 45.5

(N(CH₃)₂), 42.6 (C(CH₃)₂Ph), 42.1 (C(CH₃)₂Ph), 31.3

(C(CH₃)₂Ph), 29.6 (C(CH₃)₂Ph), 24.2 (NCH₂CH₂CH₂N).

4,6-Di-tert-butyl-2-{N-(2-ﬂuorobenzyl)-N-[2-(N′,N′-diethyl

amino)ethyl]aminomethyl}-phenol (L⁸H). The procedure was

same as that of L¹H, except that 2-ﬂuorobenzaldehyde (2.48 g,

20.0 mmol) was added in the ﬁrst step and 2,4-tert-butylphenol

(6.61 g, 20.0 mmol) was used in the last step to afford ligand

L⁸H as a light yellow oil (3.16 g, 35.7%). Found: C, 75.93; H,

9.70; N, 6.15. Calc. for C₂₈H₄₃FN₂O: C, 75.97; H, 9.79; N,

6.33%. ¹H NMR (400 MHz, CDCl₃): δ 10.41 (br, 1H, OH), 7.33

(t, 1H, J = 7.2 Hz, ArH), 7.26–7.20 (m, 2H, ArH), 7.09 (t, 1H, J

= 7.2 Hz, ArH), 7.02 (t, 1H, J = 9.2 Hz, ArH), 6.89 (d, 1H, J =

2.4 Hz, ArH), 3.77 (s, 2H, Ar-CH₂N), 3.69 (s, 2H, NCH₂-Ar),

2.59 (s, 4H, NCH₂CH₂N), 2.41 (q, 4H, J = 7.2 Hz, NCH₂CH₃),

1.46 (s, 9H, C(CH₃)₃), 1.30 (s, 9H, C(CH₃)₃), 0.95 (t, 6H, J =

7.2 Hz, NCH₂CH₃). ¹³C NMR (100 MHz, CDCl₃): 161.6 (d,

J_FC= 244.7 Hz), 154.1, 140.4, 135.8, 132.2 (d, J_FC= 4.2 Hz),

129.2 (d, J_FC= 8.2 Hz), 124.9 (d, J_FC= 4.2 Hz), 124.2, 124.1

(d, J_FC= 3.5 Hz), 123.1, 121.9, 115.3 (d, J_FC= 22.3 Hz) (all

ArC), 58.2 (Ar–CH₂N), 51.0 (NCH₂–Ar), 50.7 (NCH₂CH₂N),

50.2 (NCH₂CH₂N), 46.9 (NCH₂CH₃), 35.1 (C(CH₃)₃), 34.2

(C(CH₃)₃), 31.8 (C(CH₃)₃), 29.7 (C(CH₃)₃), 11.6 (NCH₂CH₃).

¹⁹F NMR (376 MHz, C₆D₆): −118.49.

2-{N-(2-Methoxybenzyl)-N-[3-(N′,N′-dimethylamino)propyl]

aminomethyl}-4-methyl-6-tritylphenol (L⁶H). The procedure

was same as that of L¹H, except that N,N-dimethylpropane-1,3-

diamine (3.06 g, 30.0 mmol) and 4-methyl-2-tritylphenol

(7.01 g, 20.0 mmol) were used to afford ligand L⁶H as a white

solid (5.27 g, 45.1%). Found: C, 81.85; H, 7.60; N, 4.73. Calc.

for C₄₀H₄₄N₂O₂: C, 82.15; H, 7.58; N, 4.79%. ¹H NMR

(400 MHz, CDCl₃): δ 7.20–7.18 (m, 9H, ArH), 7.16–7.14 (m,

4H, ArH), 7.12–7.10 (m, 3H, ArH), 6.85–6.73 (m, 5H, ArH),

3.63 (s, 2H, Ar–CH₂N), 3.54 (s, 3H, Ar–OCH₃), 3.50 (s, 2H,

Ar–CH₂N), 2.29 (t, 2H, J = 6.0 Hz, NCH₂CH₂CH₂N), 2.14 (s,

3H, Ar–CH₃), 2.12 (s, 6H, N(CH₃)₂), 2.02 (t, 2H, J = 6.0 Hz,

Synthesis of zinc complexes

[(L¹)ZnN(SiMe₃)₂] (1a). The ligand L¹H (0.455 g,

1.00 mmol) was added slowly to a solution of Zn[N(SiMe₃)₂]₂

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(0.385 g, 1.00 mmol) in light petroleum ether (20 mL). The sol-

ution was stirred at room temperature for 24 h and ﬁltered. All

the volatiles of the clear light yellow ﬁltrate were removed to

give a foam-like matter, which was further dried under vacuum

for half an hour and then dissolved with the proper amount of

hexane and kept at −40 °C to give colorless crystals (413 mg,

60.8%). Found: C, 61.94; H, 9.31; N, 6.20. Calc. for

C₃₅H₆₃N₃O₂Si₂Zn: C, 61.87; H, 9.35; N, 6.18%. ¹H NMR

(400 MHz, C₆D₆): δ 7.56 (s, 1H, ArH), 7.07 (t, 1H, J = 8.0 Hz,

ArH), 6.98 (d, 1H, J = 7.2 Hz, ArH), 6.77 (t, 1H, J = 7.2 Hz,

ArH), 6.73 (d, J = 2.0 Hz, 1H, ArH), 6.46 (d, 1H, J = 8.0 Hz,

ArH), 4.54 (d, 1H, J = 14.0 Hz, Ar–CH₂N), 4.32 (d, 1H, J =

12.0 Hz, Ar–CH₂N), 4.18 (d, 1H, J = 14.0 Hz, NCH₂–Ar), 3.43

(d, 1H, J = 12.0 Hz, NCH₂–Ar), 3.15 (s, 3H, OCH₃), 2.62–2.41

(m, 7H, NCH₂CH₂N, NCH₂CH₃), 2.12–2.08 (m, 1H,

NCH₂CH₃), 1.80 (s, 9H, C(CH₃)₃), 1.29 (s, 9H, C(CH₃)₃),

0.42–0.81 (br m, 6H, NCH₂CH₃), 0.59 (s, 18H, N(Si(CH₃)₃)₂);

¹³C NMR (100 MHz, C₆D₆): δ 165.4, 159.0, 138.0, 134.7,

134.4, 130.4, 126.6, 124.8, 121.0, 120.7, 111.4 (all ArC), 60.5

(CH₃OAr), 55.0 (Ar–CH₂N), 53.5 (NCH₂–Ar), 50.1

(NCH₂CH₂N), 45.6 (NCH₂CH₂N), 36.0 (NCH₂CH₃), 34.1 (C

(CH₃)₃), 32.2 (C(CH₃)₃), 30.6 (C(CH₃)₃), 23.1 (N(CH₂CH₃)₂),

7.5 (N(Si(CH₃)₃)₂).

colorless crystals (571 mg, 81.2%). Found: C, 73.25; H, 8.10; N,

4.17. Calc. for C₄₁H₅₄N₂O₂Zn·(3/8 C₆H₁₄): C, 73.72; H, 8.48;

N, 3.98%. ¹H NMR (400 MHz, C₆D₆): δ 7.63–7.60 (m, 3H,

ArH), 7.39 (d, 2H, J = 7.2 Hz, ArH), 7.21–7.18 (m, 4H, ArH),

7.07 (m, 3H, ArH), 6.97 (d, 1H, J = 7.2 Hz, ArH), 6.83 (s, 1H,

ArH), 6.74 (t, 1H, J = 7.2 Hz, ArH), 6.43 (d, 1H, J = 8.2 Hz,

ArH), 4.20 (d, 1H, J = 14.0 Hz, Ar–CH₂N), 3.81 (d, 1H, J =

14.0 Hz, Ar–CH₂N), 3.56 (d, 1H, J = 12.4 Hz, NCH₂–Ar), 3.29

(d, 1H, J = 12.4 Hz, NCH₂–Ar), 3.09 (s, 1H, OCH₃), 2.52–2.48

(m, 1H, NCH₂CH₂N), 2.23 (s, 3H, C(CH₃)₂Ph), 2.09–2.02 (m,

7H, NCH₂CH₃, NCH₂CH₂N), 1.88 (s, 3H, C(CH₃)₂Ph), 1.75 (s,

6H, C(CH₃)₂Ph), 1.69 (t, 3H, J = 8.0 Hz, ZnCH₂CH₃), 0.57 (t,

6H, J = 6.8 Hz, NCH₂CH₃), 0.47–0.34 (m, 2H, ZnCH₂CH₃);

¹³C NMR (100 MHz, C₆D₆): δ 165.6, 158.8, 153.2, 153.0,

137.8, 134.5, 133.3, 129.9, 128.8, 128.1, 127.6, 127.3, 127.1,

126.2, 125.5, 124.3, 122.2, 121.5, 120.6, 111.1 (all ArC), 57.4

(CH₃O–Ar), 54.8 (Ar–CH₂N), 52.5 (NCH₂–Ar), 50.2

(NCH₂CH₂N), 47.6 (NCH₂CH₂N), 42.8 (NCH₂CH₃), 42.6

(NCH₂CH₃), 32.1 (C(CH₃)₂Ph), 31.9 (C(CH₃)₂Ph), 31.7

(C(CH₃)₂Ph), 31.5 (C(CH₃)₂Ph), 27.6 (C(CH₃)₂Ph), 23.0

(C(CH₃)₂Ph), 14.3 (NCH₂CH₃), 14.0 (NCH₂CH₃), 8.6

(ZnCH₂CH₃), −1.1 (ZnCH₂CH₃).

[(L³)ZnN(SiMe₃)₂] (3a). An analogous method to that of 1a

was utilized, except that L³H (0. 411 g, 1.00 mmol) was used to

give colorless granular crystals (419 mg, 65.9%). Found: C,

50.49; H, 7.12; N, 6.74. Calc. for C₂₇H₄₅Cl₂N₃O₂Si₂Zn: C,

[(L²)ZnN(SiMe₃)₂] (2a). Following a procedure similar to that

described for 1a, L²H (0.579 g, 1.00 mmol) was treated with Zn

[N(SiMe₃)₂]₂(0.385 g, 1.00 mmol) in toluene (20 mL) at room

temperature to obtain a foam-like matter. Recrystallization with a

solvent mixture of hexane and toluene afforded colorless needle-

like crystals (455 mg, 56.6%). Found: C, 67.50; H, 8.26; N,

4.93. Calc. for C₄₅H₆₇N₃O₂Si₂Zn: C, 67.26; H, 8.40; N, 5.23%.

¹H NMR (400 MHz, C₆D₆): δ 7.58 (d, 2H, J = 7.6 Hz, ArH),

7.57 (s, 1H, ArH), 7.25 (dd, 2H, J = 7.6, 0.8 Hz, ArH),

7.20–7.16 (m, 2H, ArH), 7.14–7.12 (m, 2H, ArH), 7.04–6.98

(m, 3H, ArH), 6.84 (dd, 1H, J = 7.6, 0.8 Hz, ArH), 6.71–6.67

(m, 2H, ArH), 6.39 (d, 1H, J = 8.4 Hz, ArH), 4.47 (d, 1H, J =

14.0 Hz, Ar–CH₂N), 4.24 (d, 1H, J = 12.4 Hz, NCH₂–Ar), 4.08

(d, 1H, J = 14.0 Hz, Ar–CH₂N), 3.36 (d, 1H, J = 12.4 Hz,

NCH₂–Ar), 3.08 (s, 3H, OCH₃), 2.45–2.40 (m, 2H,

NCH₂CH₂N), 2.20 (s, 3H, C(CH₃)₂Ph), 2.31–2.16 (m, 2H,

NCH₂CH₂N), 2.10–1.80 (m, 2H, NCH₂CH₃), 1.79–1.71 (m, 5H,

C(CH₃)₂Ph, NCH₂CH₃), 1.66 (s, 3H, C(CH₃)₂Ph), 1.60 (s, 3H,

C(CH₃)₂Ph), 0.70–0.32 (br, 6H, NCH₂CH₃), 0.53 (s, 18H,

N(Si(CH₃)₃)₂, overlapped with the signal of NCH₂CH₃); ¹³C

NMR (100 MHz, C₆D₆): δ 165.4, 158.9, 153.0, 152.2, 137.5,

134.4, 133.4, 130.3, 129.0, 128.2, 127.9, 127.3, 127.1, 125.5,

124.5, 120.9, 120.6, 120.5, 111.3 (all ArC), 60.2 (CH₃O–Ar),

54.9 (Ar–CH₂N), 52.9 (NCH₂–Ar), 49.7 (NCH₂CH₂N), 44.2

(NCH₂CH₂N), 42.8 (N(CH₂CH₃)), 42.3 (C(CH₃)₂Ph), 34.1 (C

(CH₃)₂Ph), 31.4 (C(CH₃)₂Ph), 31.3 (C(CH₃)₂Ph), 26.6

(N(CH₂CH₃)), 7.5 (N(Si(CH₃)₃)₂).

1

50.98; H, 7.13; N, 6.61%. H NMR (400 MHz, C₆D₆): δ 7.45

(d, 1H, J = 2.4 Hz, ArH), 7.11 (t, 1H, J = 8.0 Hz, ArH), 6.94

(dd, 1H, J = 7.2, 1.2 Hz, ArH), 6.83 (t, 1H, J = 7.2 Hz, ArH),

6.49 (d, 1H, J = 8.0 Hz, ArH), 6.44 (d, 1H, J = 2.4 Hz, ArH),

4.35 (d, 1H, J = 14.0 Hz, NCH₂–Ar), 4.06 (d, 1H, J = 12.8 Hz,

Ar–CH₂N), 4.01 (d, 1H, J = 14.0 Hz, NCH₂–Ar), 3.17 (s, 3H,

OCH₃), 3.01 (d, 1H, J = 12.8 Hz, Ar–CH₂N), 2.52–2.46 (m, 3H,

NCH₂CH₂N), 2.28–2.22 (m, 1H, NCH₂CH₂N), 2.18–2.14 (m,

1H, NCH₂CH₃), 2.06–2.03 (m, 1H, NCH₂CH₃), 1.83–1.79 (m,

1H, NCH₂CH₃), 0.74 (t, 6H, J = 7.2 Hz, CH₂CH₃), 0.65–0.63

(m, 1H, NCH₂CH₃), 0.55 (s, 18H, N(Si(CH₃)₃)₂); ¹³C NMR

(100 MHz, C₆D₆): δ 162.0, 158.7, 134.0, 130.6, 130.2, 129.4,

125.6, 123.7, 120.9, 120.1, 116.6, 111.3 (all ArC), 58.5

(CH₃O–Ar), 55.0 (Ar–CH₂N), 53.9 (NCH₂–Ar), 51.8

(NCH₂CH₂N), 46.6 (NCH₂CH₂N), 32.0 (NCH₂CH₃), 23.1

(NCH₂CH₃), 7.2 (N(Si(CH₃)₃)₂).

{(L⁴)Zn[(3-tert-butyl-2-methoxy-5-methylbenzyloxy]}

(4b).

Method A: The impure ligand L⁴H [containing L⁴H (0.649 g,

1.00 mmol) and 3-tert-butyl-2-methoxy-5-methylbenzyl alcohol

(0.037 g, 1/6 mmol)] was added slowly to a solution of Zn

[N(SiMe₃)₂]₂(0.385 g, 1.00 mmol) in toluene (20 mL) and the

solution was stirred for 24 h at room temperature. The second

portion of 3-tert-butyl-2-methoxy-5-methylbenzyl alcohol

(0.187 g, 5/6 mmol) was added to the above solution and stirred

for another 24 h. After ﬁltration, the ﬁltrate was evaporated to

dryness to give foam-like solids, which was dried under vacuum

for half an hour. The resultant solids were dissolved with proper

amount of hexane and kept at −39 °C to give colorless crystals

(570 mg, 61.3%). Method B: The mixture of impure ligand L⁴H

[containing L⁴H (0.649 g, 1.00 mmol) and 3,5-di-tert-butyl-2-

methoxybenzyl alcohol (0.037 g, 1/6 mmol)] was added slowly

[(L²)ZnEt] (2c). The ligand L²H (0.579 g, 1.00 mmol) was

dissolved in toluene (20 mL), to this solution was added diethyl

zinc (1.00 mL, 1.00 mmol, 1 M in hexane). The resultant light

yellow mixture was stirred for 24 h at room temperature. Evapor-

ation of all the volatiles in vacuo afforded white solids, which

were further dried under vacuum for several hours. The solids

were then recrystallized with hexane and kept at −39 °C to give

3274 | Dalton Trans., 2012, 41, 3266–3277

View Article Online

[L⁶Zn] (6). The ligand L⁶H (0.585 g, 1.00 mmol) was added

to a solution of Zn[N(SiMe₃)₂]₂(0.385 g, 1.00 mmol) in toluene

(20 mL), and then 3,5-di-tert-butyl-2-methoxybenzyl alcohol

(0.187 g, 5/6 mmol) was added. The obtained solution was

stirred for 24 h at room temperature and ﬁltered. After a similar

work-up procedure, colorless crystals were obtained (277 mg,

30.1%). Found: C, 74.48; H, 8.66; N, 3.11. Calc. for

C₅₇H₇₈N₂O₄Zn: C, 74.36; H, 8.54; N, 3.04%. ¹H NMR

(400 MHz, C₆D₆): δ 7.92 (s, 1H, ArH), 7.61 (s, 1H, ArH), 7.60

(d, 2H, J = 7.2 Hz, ArH, partially overlapped), 7.46 (br s, 1H,

ArH), 7.35 (d, 2H, J = 7.2 Hz, ArH), 7.21–7.17 (m, 5H, ArH),

7.07–7.04 (m, 2H, ArH), 6.99 (t, 1H, J = 7.2 Hz, ArH), 6.81 (d,

1H, J = 0.8 Hz, ArH), 5.56 (s, 2H, Ar–CH₂O), 4.06 (d, 1H, J =

13.6 Hz, Ar–CH₂N), 3.91 (s, 3H, OCH₃), 3.74 (d, 1H, J = 12.4

Hz, NCH₂–Ar), 3.55 (d, 1H, J = 13.6 Hz, Ar–CH₂N), 3.27 (s,

3H, OCH₃), 3.13 (d, 1H, J = 12.4 Hz, NCH₂–Ar), 2.55–2.50 (m,

1H, NCH₂CH₂N), 2.36 (s, 3H, Ar–CH₃), 2.24 (s, 3H, Ar–CH₃),

2.21 (s, 3H, C(CH₃)₂Ph), 2.16–2.13 (m, 1H, NCH₂CH₂N),

2.04–1.84 (m, 1H, NCH₂CH₂N), 1.99–1.83 (m, 1H,

NCH₂CH₂N), 1.84 (s, 3H, C(CH₃)₂Ph), 1.74 (s, 3H,

C(CH₃)₂Ph), 1.72 (s, 3H, C(CH₃)₂Ph), 1.68–1.64 (m, 4H,

NCH₂CH₃), 1.52 (s, 9H, C(CH₃)₃), 1.32 (s, 9H, C(CH₃)₃),

0.76–0.74 (m, 3H, NCH₂CH₃), 0.29–0.28 (m, 3H, NCH₂CH₃);

¹³C NMR (100 MHz, C₆D₆): δ 165.5, 157.8, 155.8, 152.9,

152.4, 142.8, 141.2, 138.1, 134.3, 134.3, 133.4, 131.7, 129.8,

128.9, 128.8, 128.1, 127.9, 127.6, 127.2, 127.1, 126.8, 126.5,

125.5, 125.4, 124.4, 121.4 (all ArC), 65.2 (Ar–CH₂O), 62.8

(CH₃O–Ar), 62.1 (CH₃O–Ar), 60.1 (NCH₂–Ar), 53.9 (Ar–

CH₂N), 50.6 (NCH₂CH₂N), 46.7 (NCH₂CH₂N), 42.7 (C

(CH₃)₂Ph), 42.6 (C(CH₃)₂Ph), 35.2 (C(CH₃)₃), 35.0 (C(CH₃)₃),

32.0 (C(CH₃)₂Ph), 31.9 (C(CH₃)₂Ph), 31.5 (C(CH₃)₂Ph), 31.4

(C(CH₃)₃), 31.3 (C(CH₃)₃), 27.5 (C(CH₃)₂Ph), 23.0

(NCH₂CH₃), 21.6 (Ar–CH₃), 21.0 (Ar–CH₃).

2

slowly to a solution of Zn[N(SiMe₃)₂]₂(0.385 g, 1.00 mmol) in

light petroleum ether (20 mL). The solution was stirred for 24 h

at room temperature and plenty of white solids precipitated.

After ﬁltration and drying under vacuum, the white solids were

dissolved with toluene (more than 45 mL) and kept at −39 °C to

afford a white powder (0.468 g, 75.9%). Found: C, 78.31; H,

6.99; N, 4.51. Calc. for C₈₀H₈₆N₄O₄Zn·(C₇H₈): C, 77.93; H,

7.03; N, 4.54%. MS (m/z): 647 (40, [M − L]⁺); 632 (57, [M − L

−

CH₃]⁺); 584 (6, L⁺); 525 (74, {M

− L − [o-

MeOC₆H₄CH₂]}⁺); 463 (70, {L − [o-MeOC₆H₄CH₂]}⁺); 361

(100, {L − [o- MeOC₆H₄CH₂] − N(CH₂)₃N(CH₃)₂}⁺); 285 (61,

{M − L − [o-Ph₃C-o-CH₂-p-CH₃C₆H₂O]⁺}; 269 (46, {M − L −

[o-Ph₃C-o-CH₂-p-CH₃C₆H₂O] − CH₃}⁺).

[(L⁷)ZnN(SiMe₃)₂] (7a). An analogous method to that of 1a

was utilized, except that L⁷H (0. 762 g, 1.00 mmol) was used to

give a white crystalline solid (530 mg, 51.3%). Found: C, 67.68;

H, 8.80; N, 4.64. Calc. for C₄₉H₇₆N₃O₂Si₂Zn: C, 68.38; H,

1

8.90; N, 4.88%. H NMR (400 MHz, C₆D₆): δ 7.51 (d, 1H, J =

2.4 Hz, ArH), 7.41–7.39 (m, 2H, ArH), 7.30–7.27 (m, 2H,

ArH), 7.11–7.05 (m, 5H, ArH), 7.00–6.94 (m, 2H, ArH), 6.71

(d, 1H, J = 2.4 Hz, ArH), 6.64 (d, 1H, J = 1.6 Hz, ArH), 4.53

(d, 1H, J = 13.2 Hz, Ar–CH₂N), 4.20 (d, 1H, J = 14.0 Hz,

NCH₂–Ar), 4.03 (d, 1H, J = 13.2 Hz, Ar–CH₂N), 3.40 (s, 3H,

Ar–OCH₃), 3.40 (d, 1H, J = 14.0 Hz, NCH₂–Ar, partially over-

lapped), 2.53–2.45 (m, 1H, NCH₂CH₂CH₂N), 2.17 (s, 3H, Ar–

CH₃), 2.16–2.08 (m, 2H, NCH₂CH₂CH₂N), 2.08 (s, 3H,

C(CH₃)₂Ph), 1.80–1.66 (m, 6H, N(CH₃)₂), 1.61 (s, 3H,

C(CH₃)₂Ph), 1.59 (s, 3H, C(CH₃)₂Ph), 1.57 (s, 3H, C(CH₃)₂Ph),

1.41–1.38 (m, 1H, NCH₂CH₂CH₂N), 1.36 (s, 9H, C(CH₃)₃),

1.30–1.26 (dd, J = 12.8, 4.8 Hz, 1H, NCH₂CH₂CH₂N), 0.67

(dt,1H, J = 16, 4.8 Hz, NCH₂CH₂CH₂N), 0.44 (s, 18H,

N(Si(CH₃)₃)₂); ¹³C NMR (100 MHz, C₆D₆): δ 162.8, 158.6,

154.7, 152.5, 143.6, 137.5, 133.9, 133.4, 132.5, 129.5, 127.2,

126.3, 125.4, 124.6, 123.9, 120.4 (all ArC), 63.1 (CH₃O–Ar),

61.4 (Ar–CH₂N), 54.1 (NCH₂–Ar), 49.3 (NCH₂CH₂CH₂N),

48.3 (NCH₂CH₂CH₂N), 47.8 (NCH₃), 42.9 (C(CH₃)₂Ph), 42.4

(C(CH₃)₂Ph), 35.7 (C(CH₃)₃), 35.0 (C(CH₃)₂Ph), 31.5

(C(CH₃)₂Ph), 31.4 (C(CH₃)₂Ph), 31.2(C(CH₃)₃), 26.4

(C(CH₃)₂Ph), 22.1 (CH₂CH₂CH₂), 21.1 (CH₃–Ar), 7.5

(N(Si(CH₃)₃)₂).

[(L⁵)ZnN(SiMe₃)₂] (5a). An analogous method to that of 1a

was utilized, except that L⁵H (0.565 g, 1.00 mmol) was used to

give a white crystalline solid (453 mg, 56.4%). Found: C, 67.20;

H, 8.48; N, 5.24. Calc. for C₄₅H₆₈N₃O₂Si₂Zn: C, 67.17; H,

1

8.52; N, 5.22%. H NMR (400 MHz, C₆D₆): δ 7.55 (d, 1H, J =

1.8 Hz, ArH), 7.41 (d, 2H, J = 7.6 Hz, ArH), 7.31 (d, 2H, J =

7.6 Hz, ArH), 7.12 (d, 2H, J = 7.8 Hz, ArH), 7.08 (d, 2H, J =

7.3 Hz, ArH), 7.04–6.95 (m, 3H, ArH), 6.80 (d, 1H, J = 6.8 Hz,

ArH), 6.75 (d, 1H, J = 1.8 Hz, ArH), 6.70 (t, 1H, J = 7.3 Hz,

ArH), 6.41 (d, 1H, J = 8.0 Hz, ArH), 4.41 (br d, 1H, J = 13.0

Hz, Ar–CH₂N), 4.25 (d, 1H, J = 13.5 Hz, Ar–CH₂N), 4.07 (br

d, 1H, J = 13.0 Hz, Ar–CH₂N), 3.42 (d, 1H, J = 13.5 Hz, Ar–

CH₂N), 3.11 (s, 3H, Ar–OCH₃), 2.52 (t, 1H, J = 12.0 Hz,

NCH₂CH₂CH₂N), 2.30–2.20 (m, 1H, NCH₂CH₂CH₂N), 2.18 (s,

3H, C(CH₃)₂Ph), 1.82–1.67 (m, 6H, N(CH₃)₂), 1.63 (s, 3H,

C(CH₃)₂Ph), 1.61 (s, 6H, C(CH₃)₂Ph), 1.60–1.52 (m, 2H,

NCH₂CH₂CH₂N), 1.35 (dd, 1H, J = 12.0, 5.6 Hz, NCH₂CH₂-

CH₂N), 0.74 (br d, 1H, NCH₂CH₂CH₂N), 0.43 (s, 18H,

N(Si(CH₃)₃)₂); ¹³C NMR (100 MHz, C₆D₆): δ 163.0, 159.1,

154.7, 152.7, 137.5, 134.9, 133.7, 130.4, 128.1, 127.3, 127.2,

126.3, 125.5, 124.6, 120.7, 120.4, 119.6, 110.3 (all, ArC), 61.2

(CH₃O–Ar), 60.9 (Ar–CH₂N), 54.9 (NCH₂–Ar), 53.0(NCH₂-

CH₂CH₂N), 50.3 (NCH₂CH₂CH₂N), 43.0 (N(CH₃)₂), 42.4

(C(CH₃)₂Ph), 31.6 (C(CH₃)₂Ph), 31.5 (C(CH₃)₂Ph), 26.7

(C(CH₃)₂Ph), 22.1 (NCH₂CH₂CH₂N), 7.5 (N(Si(CH₃)₃)₂).

[(L⁸)ZnN(SiMe₃)₂] (8a). An analogous method to that of 2a

was utilized, except that L⁸H (0.443 g, 1.00 mmol) was used to

give colorless granular crystals (408 mg, 61.2%). Found: C,

61.25; H, 9.09; N, 6.28. Calc. for C₃₄H₆₀FN₃OSi₂Zn: C, 61.19;

H, 9.06; N, 6.30%. ¹H NMR (400 MHz, C₆D₆): δ 7.56 (d, 1H, J

= 2.0 Hz, ArH), 6.94 (t, 1H, J = 7.2 Hz, ArH), 6.86 (m, 1H,

ArH), 6.78–6.70 (m, 3H, ArH), 4.48 (d, 1H, J = 14.0 Hz, Ar–

CH₂N), 4.15 (d, 1H, J = 12.4 Hz, Ar–CH₂N), 4.07 (d, 1H, J =

14.0 Hz, NCH₂–Ar), 3.32 (d, 1H, J = 12.4 Hz, NCH₂–Ar),

2.73–2.13 (m, 6H, NCH₂CH₂N, NCH₂CH₃), 2.11–1.93 (m, 1H,

NCH₂CH₃), 1.78 (s, 9H, C(CH₃)₃), 1.46–1.35 (m, 1H,

NCH₂CH₃), 1.30 (s, 9H, C(CH₃)₃), 0.95–0.61 (br, 3H,

NCH₂CH₃), 0.60–0.10 (br, 3H, NCH₂CH₃), 0.55 (s, 18H,

N(Si(CH₃)₃)₂), overlapped with the signal of NCH₂CH₃). ¹³C

NMR (100 MHz, C₆D₆): δ 165.2, 162.3(d, J_FC= 243.8 Hz),

138.1, 134.9 (t, J_FC= 3.2 Hz), 134.8, 130.9 (d, J_FC= 8.4 Hz),

Dalton Trans., 2012, 41, 3266–3277 | 3275

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126.5, 125.0, 124.5(d, J_FC= 3.4 Hz), 120.2, 119.2(d, J_FC= 15.8

Hz), 115.8 (d, J_FC= 23.1 Hz) (all ArC), 60.3 (CH₃O–Ar), 52.9

(Ar–CH₂N), 49.7 (NCH₂CH₂N), 46.1 (NCH₂CH₂N), 36.0

(NCH₂CH₃), 34.1 (C(CH₃)₃), 32.2 (C(CH₃)₃), 30.6 (C(CH₃)₃),

7.5 (N(Si(CH₃)₃)₂).¹⁹F NMR (376 MHz, C₆D₆): −115.01.

was further dried in a vacuum oven at 50 °C for 16 h. Each reac-

tion was used as one data point. In the cases where isopropanol

was used, the solution of initiator was injected to the solution of

rac-lactide in toluene or THF to which isopropanol was added.

Otherwise the procedures were the same.

[(L⁹)ZnN(SiMe₃)₂] (9a). An analogous method to that of 1a

was utilized, except that L⁹H (0.352 g, 1.00 mmol) was used to

give granular colorless crystals (343 mg, 59.5%). Found: C,

56.48; H, 9.46; N, 4.92. Calc. for C₂₇H₅₄N₂O₃Si₂Zn: C, 56.27;

Acknowledgements

This work is subsidized by the National Natural Science Foun-

dation of China (NNSFC, 20604009 and 21074032), the

Program for New Century Excellent Talents in University (for

H. Ma, NCET-06-0413), Science and Technology Commission

of Shanghai Municipality (STCSM, contract no. 10dz2220500),

and the Fundamental Research Funds for the Central Universities

(WK0914042). All the ﬁnancial supports are gratefully

acknowledged. H. Ma also thanks the very kind donation of a

Braun glove-box by AvH foundation.

1

H, 9.44; N, 4.86%. H NMR (400 MHz, C₆D₆): δ 7.54 (s, 1H,

ArH), 6.77 (s, 1H, ArH), 3.41 (s, 2H, Ar-CH₂N), 3.00– 2.95 (m,

2H, NCH₂CH₂O), 2.89 (s, 6H, OCH₃), 2.89–2.85 (m, 2H,

NCH₂CH₂O), 2.66–2.59 (m, 2H, NCH₂CH₂O), 2.31–2.28 (m,

2H, NCH₂CH₂O), 1.76 (s, 9H, C(CH₃)₃), 1.42 (s, 9H, C(CH₃)₃),

0.45 (s, 18H, N(Si(CH₃)₃)₂); ¹³C NMR (100 MHz, C₆D₆): δ

164.5, 138.0, 135.6, 124.9, 124.5, 121.7 (All, Ar-C), 69.2

(NCH₂CH₂O), 62.3 (NCH₂CH₂O), 59.0 (CH₃O), 57.8 (CH₃O),

35.8 (NCH₂CH₂O), 34.1 (C(CH₃)₃), 32.3 (NCH₂CH₂O), 30.3

(C(CH₃)₃), 6.2 (N(Si(CH₃)₃)₂).

Notes and references

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Article Doi

Zinc complexes supported by claw-type aminophenolate ligands: Synthesis, characterization and catalysis in the ring-opening polymerization of rac-lactide

DOI: 10.1039/c2dt11767c

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