Phosphasalalen Rare-Earth Complexes for the Polymerization of rac-Lactide and rac-β-Butyrolactone

Lactide and rac -β -Butyrolactone

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Phosphasalalen Rare-Earth Complexes for the Polymerization of rac-

Hui Liu and Xiaochao Shi*

Cite This: Inorg. Chem. 2021, 60, 705 −717

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sı Supporting Information

ABSTRACT: A series of new phosphasalalen pro-ligands, analogues of salalen but with an iminophosphorane replacing the imine

functionality, and their corresponding rare-earth alkoxide and siloxide complexes were synthesized. The multinuclear NMR spectra

and X-ray diﬀraction analyses revealed that, for the tert-butoxide and ethoxide complexes, the resulting phosphasalalen rare-earth

product was composed of a mononuclear alkoxide and a binuclear complex containing bridged alkoxo and hydroxo groups, while an

analogous binuclear complex was isolated as the sole product for the siloxide complex. All the complexes could catalyze the

heteroselective ring-opening polymerization (ROP) of rac-lactide (P up to 0.77) with high catalytic activities and a controlled

polydispersity. Remarkably, the yttrium and lutetium phosphasalalen complexes could also eﬃciently catalyze the ROP of rac-β-

butyrolactone to produce syndiotactic polymers (P up to 0.73) while their salalen analogues were inert, revealing the special eﬀects

of the iminophosphorane moiety. Detailed end-group analyses and kinetic investigations suggested that the alkoxo−hydroxo-bridged

complexes maintained their binuclear structures in the polymerization.

INTRODUCTION

syndiospeciﬁc catalysts for the polymerization of rac-LA or rac-

BBL with outstanding activities and stereoselectivities to

■

The environmental issues regarding plastic waste have

attracted much attention to the development of biodegradable

and biocompatible polymers that can decompe quickly without

produce highly heterotactic PLA (P up to 0.99) and highlyy

syndiotactic PHBs (P up to 0.94). The Williams group has

also described a series of phosphasalen rare-earth complexes,

some of which exhibited high catalytic activities and a

tolerance for low catalyst loadings with excellent polymer-

harming the organisms and environment. As polymeric

alternatives, aliphatic polyesters, such as poly(lactides) (PLA)

and poly(3-hydroxybutyrate) (PHB), are among the most

promising ecologic materials and have excellent applications in

ization control. The impressive catalytic performance of rare-

the ﬁelds of packaging, agriculture, biomedical research, etc.

earth complexes for the ROP of rac-LA and rac-BBL

encouraged continuous eﬀorts in this ﬁeld for further

enhancement.

Salalens (Figure 1C), combining half salen (Figure 1A) and

half salan (Figure 1B) pro-ligands in a hybrid mold, are

The metal-catalyzed ring-opening polymerizations (ROP) of

rac-lactide (rac-LA) and rac-β-butyrolactone (rac-BBL)

represent a promising way to access these polymers, especially

involving the control of the stereotactivity. Nowadays, a wide

range of metal catalysts have been developed to produce

diﬀerent stereospeciﬁc PLAs and PHBs; the rare-earth (Ln)

catalysts are among those receiving most intensive inves-

tigations due to their high catalytic activities, high degrees of

stereocontrol, and the ability to control the microstructures of

polymers by reasonably modifying the ancillary ligands as well

[ONNO]-type tetradentate pro-ligands that include imine- and

Received: September 14, 2020

Published: January 6, 2021

−8

as choosing the metal centers.

phenolate)s pro-ligands were proven to be extremely eﬀective,

and their yttrium complexes have been widely investigated as

The amine-bridged bis-

(

2021 American Chemical Society

Inorg. Chem. 2021, 60, 705−717

Article

Scheme 1. Synthesis of Phosphasalalen Pro-Ligands

Figure 1. General structure of [ONNO]-type tetradentate pro-

ligands.

amine-neutral donors and two phenolate arms. Their

corresponding metal complexes have been used successfully

in the polymerization of diverse monomers, such as the

stereoselective ROP of rac-LA, the copolymerization of

epoxides with carbon dioxide, and the isospeciﬁc polymer-

−11

ization of α-oleﬁn.

Surprisingly, heteroatomic substitutions

namely phosphinophenols and aminopyrrolidinylphenols with

on the backbone of salalen pro-ligands have been less

investigated; only a few sulfur-subsituted pro-ligands were

reported, and in some the sulfur atom did not coordinate to

bulky substituents at the ortho-phenoxide positions, were

prepared following the literature procedures. The corre-

sponding phosphonium bromide salts, prepared by the

oxidation of phosphinophenol with liquid bromine in situ,

were reacted with equimolar amounds of aminopyrrolidinyl-

phenols via a Kirsanov reaction in the presence of 1,4-

diazabicyclo[2.2.2]octane (DABCO) to eﬃciently give the

desired phosphasalalen pro-ligands L1−L3 (Scheme 1, Ad =

adamantanyl and Cumyl = 2-phenylpropan-2-yl). The pro-

the metal centers. On the other hand, the iminophosphorane

moieties behave as strong σ- and π-donors due to the presence

of two lone pairs at the nitrogen atom and the lack of π-

accepting ability, respectively. With diﬀerent steric and

electronic properties imposed on the imine moieties, the

iminophosphorane moieties are of great interest and are well

established in coordination and catalytic chemistry. The

phosphasalen pro-ligands (Figure 1D), referring to the

iminophosphorane-substituted salen pro-ligands, have at-

tracted intensive research attentions, and their metal

complexes exhibited rather diﬀerent catalytic performances

ligand L1 was puriﬁed by recrystallization from its Et

solution, while L2 and L3 were puriﬁed by column

chromatography on silica gel (CH Cl /MeOH). The pure

phosphasalalen pro-ligands were obtained with around 50%

yields as white powders that were stable in the presence of

oxygen and moisture, and all the new pro-ligands were fully

,13a,14

when compared with their salen analogues.

The success

of iminophosphorane moieties in ligand designs prompted us

to modify the versatile salalen pro-ligands by introducing an

iminophosphorane moiety. We envisage that the incorporation

of an iminophosphorane moiety to the normal salalen

backbone will result in a new kind of pro-ligand, namely

phosphasalalen (Figure 1E), and the corresponding metal (e.g.,

rare-earth metal) complexes would exhibit interesting catalytic

properties in various applications (e.g., the ROP of rac-LA and

characterized by multinuclear NMR spectroscopy ( H, C and

P NMR, Figures S1 −S9).

The deprotonation of L1−L3 with 3 equiv of sodium

bis(trimethylsilyl)amide generated the corresponding sodium

salts; the reaction was monitored using P NMR spectroscopy,

which showed obvious chemical shifts as singlet signals, such as

shifts from δ 44.87 to 21.61 ppm for L1 (Figures S3 and S17).

The reaction of the sodium salts of ligands with LnCl (THF)

rac-BBL).

at a low temperature (−30 °C) to form the rare-earth

phosphasalalen chloride complexes (from δ 21.61 ppm for the

sodium salt of L1 to δ 34.33 ppm for its corresponding

Herein, we report a new family of phosphasalalen pro-

ligands containing a chiral rigid pyrrolidine framework and

their rare-earth complexes for the syndiospeciﬁc polymer-

ization of rac-LA and rac-BBL. The NMR spectra demon-

strated that the synthesized rare-earth mixtures contained

mainly the mononuclear alkoxide complex and a minor

binuclear complex with bridged alkoxo and hydroxo groups.

All the complexes were highly active catalysts for the

heterospeciﬁc ROP of rac-LA. Interestingly, the phosphasalalen

complexes could also serve as eﬃcient catalysts for the ROP of

rac-BBL while their salalen analogues were inert, revealing the

considerable role of the iminophosphorane functionality in the

structure−reactivity relationship.

phosphasalalen yttrium chloride in P{ H} NMR spectra,

Figure S19), led to formation of the phosphasalalen metal

alkoxide mixtures 1−6 in Scheme 2 following the addition of

potassium alkoxides (ethoxide and tert-butoxide). In the

P{ H} NMR spectra of 1−6, two peaks with the a relative

ratio of integrations around 9:1 (except around 8:2 for 5) were

observed, suggesting that each mixture contained two diﬀerent

complexes. To clarify the detailed structures of both

complexes, we took mixture 3 as an example to investigate

in detail. The peaks at δ 36.94 (major) and 33.32 ppm (minor)

in the P{ H} NMR spectrum had a ratio of 10:1 in

integration; in the H NMR spectrum, the peak assigned for

RESULTS AND DISCUSSION

■

the −O Bu group possessed equal integration to each of the

Synthesis and Characterization of Complexes. The

phosphasalalen pro-ligands L1−L3 with chiral pyrrolidine

backbones were handled as stable bromine salts, and their

synthetic routes are outlined in Scheme 1. The reagents,

four discrete peaks assigned for the tert-butyl substituents in

ligand L1. The C{ H} NMR spectrum showed a single set of

equivalent doublets at δ 167.4 ppm, which was assigned for the

quaternary phosphine-attached phenolate carbon atom (see

Inorg. Chem. 2021, 60, 705−717

Inorganic Chemistry

Article

Scheme 2. Synthesis of Phosphasalalen Rare-Earth Complexes 1−6

Figures S20 −S22 ). All the data suggested that the major

component, namely 3a, possessed a mononuclear structure

the multinuclear NMR spectra for its formation and the low

content of 3b.

with the coordination of one ligand and one −O Bu group

We failed to realize the transformation of 3a to 3b by

heating mixture 3 at 70 °C or adding a controlled amount of

(

Scheme 2). Furthermore, we fortunately obtained the

crystal structure of 3b, which was the minor component in

water (n(H O):n(Y ) = 0.5), implying that the formation of

, revealing the binuclear butoxo−hydroxo-bridged structure

the binuclear complex 3b proceeded in an unexpected way.

of 3b (Figure 2 ). The multinuclear NMR spectra of the

No evidence indicating the formation of a binuclear complex

with two bridged −O Bu groups has ever been observed, since

the H NMR spectra of 3 had no diﬀerences in a wide

operating temperature ranging from −80 to 60 °C in THF-d

(

Figures S42 −S43). When more bulky ligands L2 and L3 were

used, the resultant mixtures 5 and 6 could be also obtained,

and the mononuclear structure was identiﬁed as the major

component. However, P{ H} NMR spectra revealed higher

ratios of binuclear complexes compared to 3, which were

22.4% in 5 and 16.6% in 6 (Figures S32 and S35, respectively).

These suggested that the steric hindrance of the ligands played

an important role in the formation of the binuclear complexes.

Interestingly, when a bulkier and more easily decomposed

−

OSiMe₃group was used as a coordinated group, the

multinuclear NMR spectrum demonstrated that the binuclear

Figure 2. Molecular structure of complex 3b (thermal ellipsoids are

shown at the 30% probability level). Hydrogen atoms were omitted

for clarity. Selected bond distances (Å) and angles (°) are as follows:

Y1−O1, 2.159(2); Y1−O2, 2.135(2); Y1−O5, 2.297(2); Y1−O6,

complex 7b was the sole product (Scheme 3), giving evidence

Scheme 3. Synthesis of Phosphasalalen Rare-Earth Complex

.258(2); Y1−N1, 2.525(2); Y1−N2, 2.383(3); Y2−O3, 2.154(2);

Y2−O4, 2.159(2); Y2−O5, 2.304(2); Y2−O6, 2.243(2); Y2−N3,

.544(3); Y2−N4, 2.376(3); O1−Y1−N1, 76.45(8); O2−Y1−N2,

0.65(9); N2−Y1−N1, 71.83(10); O1−Y1−O5, 106.46(9); O2−

Y1−O6, 102.56(9); O5−Y1−O6, 71.23(7); O3−Y2−N3, 76.26(8);

O4−Y2−N4, 80.84(9); N3−Y2−N4, 70.45(9); O3−Y2−O5,

05.37(8); O4−Y2−O6, 100.39(9); and O5−Y2−O6, 71.34(8).

crystalline solid of 3b (δ 33.32 ppm in P{ H} NMR

spectrum) convinced us of the mononuclear structure of 3a

Figures S23 −S26 ). A solution DOSY NMR experiment was

also used to compare the hydrodynamic radii of complexes 3a

and 3b with those estimated from the DFT calculations and X-

ray crystal structure, respectively (see Figure S44 for details).

The solution hydrodynamic radii of complexes 3a and 3b were

determined to 15.09 and 20.52 Å, respectively (C D , 25 °C).

of another important factor to form the binuclear complexes.

On the basis of the above experimental ﬁndings, we reasoned

the formation of binuclear hydroxo-bridged complexes was

mainly due to the rigid inherence and steric congestion of the

ligands as well as the unusual O−C or O−Si bond cleavage of

the −OR groups, which might accompany the hydrogen

The radius determined from the crystal structure of complex

b was 21.250 Å, which was very close to that determined from

the DOSY NMR spectrum (20.52 Å). For complex 3a, the

DFT calculations (computed at B3LYP and 3-21g*) of the

optimized geometry showed a radius of 15.825 Å, which was in

agreement with that determined by the DOSY NMR analysis

abstraction of solvent.

Single crystals of the binuclear complexes 3b, 6b, and 7b

suitable for single-crystal X-ray diﬀractions were isolated from

their n-hexane solutions, and the molecular structures of the

complexes as well as the selected bond lengths and bond angles

are depicted in Figures 2−4, respectively. As shown in the

molecular structures of these complexes, the rigidity of the

(

15.09 Å). Thus, the DOSY NMR evidence also supported the

mononuclear structure of complex 3a. However, we could not

obtain the pure complex 3a despite having enough evidence in

Inorg. Chem. 2021, 60, 705−717

Article

the two metal centers are also similar. The ³¹P{ H} NMR

spectrum of complex 3b exhibits a singlet signal at room

temperature (δ 33.32 ppm), suggesting the similar coordina-

tion environments of the two phosphorus atoms. As expected,

the bond lengths of Y−N (pyrrolidine) are longer than those

of Y−N (iminophosphorane), and all the bond lengths of Y−N

4,15,18

are in accordance with the values from previous results.

The bond lengths of Y−O (O Bu) are 2.297(2) and 2.304(2)

Å, which are slightly longer than those of Y−O (OH)

(

2.258(2) and 2.243(2) Å, respectively; this could be

attributed to the steric hindrance of the bulky −O Bu ligand.

The bond angles of O(Ar)−Y−N (pyrrolidine) (76.45(8)°

and 76.26(8)°) are smaller than those of O(Ar)−Y−N

Figure 3. Molecular structure of complex 6b (thermal ellipsoids are

shown at the 30% probability level). Hydrogen atoms were omitted

for clarity. Selected bond distances (Å) and angles (°) are as follows:

Y1−O1, 2.159(3); Y1−O2, 2.197(3); Y1−O3, 2.247(3); Y1−O4,

(

iminophosphorane) (80.65(9)° and 80.84(9)°), which is

attributed to the rigidity of the iminophosphorane moiety. The

bond lengths and bond angles in complexes 6b and 7b have

the same tendency as those in complex 3b, and the

corresponding parameters in 6b and 7b are also comparable

to those of 3b.

.257(3); Y1−N1, 2.501(4); Y1−N2, 2.396(4); O1−Y1−N1,

8.49(13); O2−Y1−N2, 80.17(14); N2−Y1−N1, 69.53(13); O3−

Y1−O4, 71.83(14); O1−Y1−O3, 153.75(10); and O4−Y1−N2,

168.87(14).

Ring-Opening Polymerization of rac-LA and rac-BBL.

As our purpose for the design and synthesis of phosphasalalen

ligands, 1−6 were tested for the stereospeciﬁc polymerization

of rac-LA and rac-BBL to elucidate the structure−reactivity

relationship. The representative polymerization results are

summarized in Tables 1 and 2 for rac-LA and rac-BBL,

respectively. Due to the special electronic eﬀect of the

iminophosphorane moiety with a strong σ- and π-donating

capability, the phosphasalalen rare-earth precursors 1−6

showed high catalytic activities for the polymerization of rac-

LA (P up to 0.77). Remarkably, the yttrium and lutetium

phosphasalalen mixtures 2−6 could also eﬃciently catalyze the

syndioselective polymerization of rac-BBL (P up to 0.73),

whereas their corresponding salalen analogues were inactive.

Figure 4. Molecular structure of complex 7b (thermal ellipsoids are

shown at the 30% probability level). Hydrogen atoms were omitted

for clarity. Selected bond distances (Å) and angles (°) are as follows:

Y1−O1, 2.165(2); Y1−O2, 2.154(2); Y1−O5, 2.3123(19); Y1−O6,

All the complexes could catalyze the rac-LA polymerization

in the absence of other chain transfer agents or coactivators.

Prominent solvent eﬀects were observed in the polymerization

results, of which a polar solvent was beneniﬁcal for obtaining

the polymer with a higher heteroselectivity. The phosphasa-

lalen yttrium mixture 3 could serve as a highly eﬃcient catalyst

for the ROP of rac-LA, and converted 200 equiv of monomers

nearly quantitatively in 4 min (1 M rac-LA solution in THF) to

.247(2); Y1−N1, 2.533(3); Y1−N2, 2.373(2); Y2−O3, 2.154(2);

Y2−O4, 2.135(2); Y2−O5, 2.310(2); Y2−O6, 2.253(2); Y2−N3,

.509(3); Y2−N4, 2.375(3); O1−Y1−N1, 76.92(8); O2−Y1−N2,

0.91(8); N2−Y1−N1, 70.61(8); O1−Y1−O5, 103.42(8); O2−Y1−

O6, 99.97(8); O5−Y1−O6, 72.09(8); O3−Y2−N3, 77.31(8); O4−

Y2−N4, 80.76(9); N3−Y2−N4, 99.72(9); O3−Y2−O5, 103.67(8);

O4−Y2−O6, 103.10(8); and O5−Y2−O6, 72.03(7).

give a polylactide with a high molecular weight (M = 37.0

kDa, M /M = 1.07) and good heteroselectivity (P = 0.72).

pyrrolidine backbone has no inﬂuences on the coordination of

atoms; however, its inherent chirality induces the bulky amino-

iminophosphorane moiety to form a rather crowded

conﬁguration around the Ln−OH group. The phosphasalalen

ligands bind to the metal centers with two oxygen atoms, one

imine atom from the iminophosphorane, and one amine atom

from pyrrolidine, and the four coordinated atoms wrap

diastereospeciﬁcally in the fac-mer mode in both complexes.

Each metal center exhibits a distorted octahedral geometry

formed by the tetradentate phosphasalalen ligand and the

alkoxo and hydroxo bridges, revealing clearly that the

complexes adopt a binuclear structure in the solid state.

Moreover, the two related metal centers and the bridged

alkoxo and hydroxo groups construct a [Ln O ] four-

The high catalytic activity enabled 3 to catalyze 2000 equiv of

rac-LA with a conversion of 70% in 120 min (entry 9 in Table

1). These results showed that the yttrium phosphasalalen

complexes were among the most eﬃcient yttrium catalysts for

the polymerization of rac-LA. Compared to the salalen

yttrium analogue, 3 exhibited a comparable catalytic activity

but lower stereoselectivity (P

= 0.72 for 3 vs P = 0.85 for the

salalen yttrium complex under similar polymerization con-

ditions) in the ROP of rac-LA. When the central metal was a

scandium ion with a small radius, 1 showed a much lower

catalytic activity (entry 1 in Table 1). This could be partly

attribute to the bulky phosphasalalen ligand and the

subsequent compaction in space around the smaller Sc

center, which was unfavored for the coordination and insertion

of lactide monomers. The lutetium phosphasalalen mixture 4

showed a comparably catalytic performance to that of 3.

Surprisingly, the coordinated alkoxide anions had an

inconceivable inﬂuence on the catalytic activity; the yttrium

mixture 3 with tert-butoxide had a much higher catalytic

activity than 2 with an ethoxide, which diﬀers from the

membered ring, and the quaternion ring is in a nearly perfect

plane. The distances of the two metal centers to the

corresponding coordinated atoms are very close in the

complexes, e.g., the bond lengths of Y−O (pyrrolidine-side

phenolate) in complex 3b are 2.159(2) (Y1−O1) and

.154(2) Å (Y2−O3); the coordination geometries around

Inorg. Chem. 2021, 60, 705−717

Inorganic Chemistry

Article

Table 1. Data for the ROP of rac-LA Using the Phosphasalalen Rare-Earth Catalysts

entry

cat.

[I]:[LA]

t (min)

con. (%)

M (kDa)

M_n_,_c_a_l_c(kDa)

M /M_n

P_r

1:200

1:500

1:200

1:500

1:1000

1:2000

1:200

120

22.6

17.2

32.1

37.0

40.9

37.6

62.7

101.6

103.8

38.4

24.6

26.1

25.9

18.5

20.8

28.3

62.7

28.6

28.3

70.0

139.9

201.9

27.7

28.0

28.3

25.7

28.6

1.03

1.24

1.07

1.05

1.06

1.10

1.25

1.30

1.03

1.11

1.15

1.05

1.06

1.08

0.66

0.67

0.69

0.72

0.62

0.69

0.73

0.70

0.69

0.75

0.76

0.63

0.77

0.68

120

All reactions were carried out at room temperature (25 °C), [LA] = 1 M, and with tetrahydrofuran (THF) as the solvent. Conversion was

determined from the H NMR spectra (CDCl ) of crude polymers by integrating the methine resonance (rac-LA, δ 4.98−5.06 ppm; PLA, δ 5.08−

.22 ppm). Determined by SEC in THF using polystyrene as the standard. M

= 144.13 × [LA]/[I] × (polymer conversion) (%) + 73.06

n,calc

(

−O Bu)/45.06 (−OEt). Determined by the analysis of the tetrad signals in the methine region of the homodecoupling H NMR spectrum at 25

C. Dichloromethane (DCM) was use as the solvent. Toluene was used as the solvent.

Table 2. Data for the ROP of rac-BBL Using the Phosphasalalen Rare-Earth Catalysts

entry

cat.

[BBL]

2 M

1 M

2 M

4 M

2 M

2 M (0 °C)

2 M (50 °C)

2 M

[I]:[BBL]

t (h)

con. (%)

M (kDa)

M_n_,_c_a_l_c(kDa)

M /M_n

P_r

1:200

1:400

1:600

1:800

1:200

1:400

1:600

1:800

1:200

1:600

1:200

1:600

1:200

1:600

1:200

12.7

22.6

25.8

31.9

19.1

14.9

22.4

26.6

35.0

19.9

20.6

21.4

24.5

10.3

21.5

14.1

30.6

13.8

30.6

20.8

20.7

17.3

16.6

27.6

42.4

63.4

16.9

16.8

31.4

46.6

61.4

15.6

16.9

16.6

16.3

16.8

47.6

15.2

42.4

16.9

48.1

16.3

16.7

16.3

1.04

1.13

1.10

1.14

1.16

1.05

1.21

1.11

1.13

1.19

1.16

1.08

1.10

1.07

1.20

1.09

1.14

1.11

1.08

1.18

1.16

1.13

0.69

0.63

0.60

0.63

0.73

0.72

0.70

0.67

0.68

0.59

0.66

0.65

0.58

0.57

0.43

0.42

0.64

0.62

0.66

0.54

0.62

2 M

General polymerization conditions are as follows: toluene as the solvent, at room temperature (25 °C), and the polymerization time was not

optimized. Conversion of rac-BBL as determined by a H NMR analysis of the crude polymer by integrating the methine resonance (CDCl ; rac-

BBL, δ 1.50−1.54 ppm; PHB, δ 1.25−1.31 ppm). Determined by SEC in THF using polystyrene as the standard. M

= 86.09 × [BBL]/[I] ×

n,calc

(

polymer conversion) (%) + 45.06 (−OEt)/73.06 (−O Bu)/89.19 (−OSiMe ). P is the probability of racemic linkages between monomer units

f g

measured by C NMR spectroscopy at 25 °C in CDCl . THF was used as the solvent. DCM was used as the solvent.

previous observation that a smaller alkoxide group improvedd

ethoxide also showed lower activities than that of 3 but with a

the activity due to its higher initiation rate of the rac-LA

polymerization (entries 2−3, respectively, in Table1). This

might be attributed to the steric interaction between the

alkoxide group and the ortho-phenolate group, which renders

the bulky tert-butoxide more active than the ethoxide in regard

to initiating the polymerization. The mixtures 5 and 6 with

similar heteroselectivity. The binuclear complexes 3b and 6b

showed comparable catalytic activities to those of 3 and 6,

respectively. Due to the low content of the binuclear complex

in 1−6, it is reasonable to conclude that the monomeric

complex played a predominate role in the ROP of rac-LA.

Inorg. Chem. 2021, 60, 705−717

Article

Figure 5. (Left) Plot showing ﬁrst-order kinetic plots of ln([LA] /[LA] ) vs time (s) for the polymerization of rac-LA catalyzed by 3 in THF at 25

−3

−1

−3 −1

C and [LA] = 1 M. [I]:[LA] = 1:200, k = 10.84 × 10 s , R = 0.9927; [I]:[LA] = 1:300, k = 5.08 × 10 s , R = 0.9937; [I]:[LA] =

obs

−

−1

−3 −1

:400, k_o_b_s= 2.86 × 10 s , R = 0.9928; and [I]:[LA] = 1:500, k = 1.94 × 10 s , R = 0.9926. (Right) Plot of k vs [I] for the calculation of

the propagation rate constant k with polymerization conditions of THF, 25 °C, [LA] = 1 M, and [3] = 2−5 mM.

Complex 7b with a −OSiMe group could also quantitatively

eﬃciently and converted 200 equiv of monomers in high

conversions within 5 h; however, 20 h was needed for complex

7b, implying the lower catalytic initiation eﬃciency of the

trimethylsiloxide group. Moreover, the mixture 1 with central

convert 200 equiv of rac-LA in 120 min.

All the resultant PLAs catalyzed by phosphasalalen rare-

earth catalysts had high molecular weights and narrow

molecular weight distributions (M /M ≤ 1.30), suggesting

Sc ions was inactive for the ROP of rac-BBL (entry 24 in

that the polymerization proceeded in a controlled manner. In

the rac-LA ROP catalyzed by 3, a ﬁrst-order dependence of the

conversion rate on the lactide concentration was observed, as

Table 2). C{ H} NMR measurements were used to

investigate the tacticity of the resulting PHBs. The methyl

region displayed two peaks corresponding to the r and m diads

at δ 169.34 and 169.21 ppm, respectively, indicating all the

resultant PHBs were syndiotactic-enriched since the r diad was

more abundant in most of the samples (except 5, which gave

slightly isotactic-enriched PHB). However, the calculated

molecular weights of the resultant polymers (based on the

monomer feed ratio and conversion) did not show a clear

relationship with the experimental values for the number-

averaged molecular weight. Despite of the presence of chiral

pyrrolidine in the phosphasalalen rare-earth complexes,

statistical analysis by the Bernoulli model triad test conﬁ rmed

a chain-end control mechanism in the ROP of rac-BBL (Table

S1), that is, the stereochemistry in the polymerization was

controlled by the chirality of the last-inserted butyrolactone

unit. Considering the fact that the corresponding salalen rare-

earth complexes were inactive for the ROP of rac-BBL, we

ascribed the eﬃciently catalytic performances of 2−6 to the

crucial iminophosphorane moiety in phosphasalalen ligands.

NBO (natural bond orbital) analyses proved the signiﬁcant

diﬀerence between the iminophosphorane moieties and the

imine moieties in their divergent electronic properties (Figure

evidenced by the linear ﬁts to the plot of ln([LA] /[LA] ) vs

^tⁱ^m^e⁽^Fⁱ^g^u^r^e⁵⁾^F^r^o^m^t^h^e^kobs values determined over a range

of diﬀerent catalyst concentrations, the reaction also had a ﬁrst-

order dependence on the catalyst concentration. Thus, the

overall rate law is second-order with the propagation rate

−

−1 −1

constant at k = 3.02 × 10 mM s . The ﬁrst-order

dependence on both the lactide and catalyst concentrations

was also reported by the phosphasalen rare-earth complexes.

In addition to the excellent activity for the polymerization of

rac-LA, the phosphasalalen yttrium and lutetium complexes 2−

could also eﬀectively catalyze the ROP of rac-BBL at room

temperature; the summary of polymerization results is shown

in Table 2. Toluene was the optimal solvent for this

polymerization, and the monomer concentration slightly

inﬂuenced the catalytic property in our trials; a moderate 2

M concentration was selected for further investigations (entries

5, 6, and 10 in Table 2). Under these conditions, 3 could

nearly complete the polymerization of 200 equiv of rac-BBL in

h to give the polymer with a high molecular weight (M =

4.9 kDa), a narrow polydisperisity (M /M = 1.05), and a

moderate syndioselectivity (P = 0.73). The lutetium mixture 4

). The charge diﬀerence between the phosphorus and the

could also convert 200 equiv of monomer in a conversion of

nitrogen atoms in an iminophosphorane moiety (|Δq| = 2.94)

is much higher than that between the carbon and the nitrogen

atoms in an imine moiety (|Δ q | = 0.49). The Wiberg indexes of

6%; however, the tacticity of the resultant polymer (P =

.58) was much lower than that of the polymer produced by

the yttrium analogue (P = 0.73), implying the signiﬁcant

inﬂuence of metal centers in stereocontrol (entry 1 and 4,

respectively, in Table 2). Similar to the inﬂuence of the

alkoxide group in rac-LA polymerization, a bulky group was

also beneﬁcial to the catalytic activity in the polymerization of

rac-BBL. For example, 3 with a tert-butoxide group could

convert the monomer in a 91% conversion over 8 h, while

there was an 80% conversion dover 10 h for 2 containing an

ethoxide with the monomer-to-feed ratio of 400 (entries 2 and

Figure 6. Comparison of the iminophosphorane and imine in Wiberg

indexes and NBO charge calculated using Gaussian09 with the

B3PW91 functional and 6-31+G* basis set.

, respectively, in Table 2). The pure binuclear complexes 3b

and 6b could also promote the polymerization of rac-BBL

Inorg. Chem. 2021, 60, 705−717

Inorganic Chemistry

by multinuclear NMR spectroscopy (Figure 7 ) and MALDI-

Article

the P−N and C−N bonds are 1.28 and 1.84, respectively. All

a terminal hydroxybutyrate unit featuring peaks at δ 4.15 and

3.64 ppm in the H NMR spectrum. In the C{ H} NMR

of these indicate that the iminophosphorane adopts a dipolar

−

structure (P −N instead of the conventional PN) and is

spectrum, the end-group analysis conﬁrmed the existence of

much more electron-donating than the imine. It is well-

known that, in the ROP of lactone, the high catalytic activity

relies signiﬁcantly on the eﬃcient insertion of the lactone unit,

thus requiring a labile metal alkoxide bond in the insertion

one −O Bu end group with peaks at δ 28.05 and 65.83 ppm

and one terminal hydroxybutyrate unit with signals at δ 21.80,

4.07, and 172.01 ppm. No signals assigned to the −COOH

1 13 1

group were observed in the NMR spectra ( H and C{ H}),

implying the absence of a OH-derivated polymer. This is in

agreement with the inability of L−M−OH to initiate a

c,14a

step.

Compared to the salalen ligands, the more electron-

donating phosphasalalen ligand can decrease the Lewis acidity

of the central yttrium and lutetium ions in 2−6 more

signiﬁcantly, beneﬁting the labilization of the metal-alkoxide

bond and reducing the ring-opening energy of this β-

butyrolactone attacked by the metal alkoxide; this could also

explain the inactivity of 1 with a more Lewis acidic scandium

center.

polymeric chain, as shown in previous reports. It should be

noted that the PHB oligomer obtained using 3 has an identical

H NMR spectrum with that obtained by using 3b (Figure

S51). The multinuclear NMR analyses of the PLA oligomer

obtained by using 3 also revealed the sole −O Bu end group in

the PLA (Figure S50 ). The structures and end groups of the

PHB oligomers obtained by complex 3b and mixture 3 were

further investigated by the MALDI-TOF mass spectrometry, as

shown Figures 5 and S52 . The polymer chain was separated by

the mass of a β-butyrolatone unit (86.03 amu), and

monomodal molecular weight distributions were observed in

both cases with many of peaks having same mass weight,

Due to the presence of two diﬀerent bridged groups (−O Bu

and −OH) in the complex 3b, it is important to clarify the real

initiation group during the polymerization. The micro-

structures of the obtained oligomer of rac-BBL were evaluated

indicating the sole −O Bu initiation group (73.06 amu) in both

cases that was identical to the NMR analyses.

Then, a new question was raised based on the above

observations regarding the exact structure of active species of

b during the polymerization of rac-BBL: does it maintain the

binuclear structure or decompose to a mononuclear active

phosphasalalen−Y−O Bu(β-butyrolactone) and an inactive

phosphasalalen−-Y−OH(β-butyrolactone)? As previous re-

ported, the excess lactone monomers could easily decompose

the binuclear catalytic precursors. In our trials, when 2 equiv

of rac-BBL was added to the solution of pure complex 3b, the

signals in the P{ H} NMR spectrum shifted to the lower ﬁeld

from δ 33.32 to 32.33 ppm) but retained a singlet peak,

(

implying that the decomposition of 3b might not take place.

The kinetic investigations of the rac-BBL polymerization were

conducted by 3 and the binuclear complex 3b (Figure 9). For

, the reaction kinetic featured a ﬁrst-order dependence in the

rac -BBL concentration (also on the catalyst concentration, see

Figures S53 −S54 ), and the semilogarithmic plot of ln([BBL] /

−

[

BBL] ) vs time was linear with a slope of k = 7.00 × 10

−

s , a value that is among those of the eﬃcient catalysts for rac-

BBL polymerization when compared to previous reports.

,17b

For complex 3b, however, a zero-order dependence on the rac-

BBL concentration was adopted, and the slope of linear plot

involving [BBL] -[BBL] vs time was k = 6.87 × 10⁻s .

−1

obs

The completely distinct reaction kinetic characteristics

between the 3 and 3b demonstrated that the active species

a (as the main component of 3) was structurally diﬀerent

compared to 3b in the catalytic systems for the polymerization

of rac-BBL. Furthermore, the SEC curves obtained by 3 and 3b

were monomodal; however, when the mixture of 3 and 3b

Figure 7. ¹H and ¹³C{ H} NMR spectra (CDCl ) of the PHB

oligomer prepared from complex 3b. Conditions are as folows: [BBL]

2 M, [I]:[BBL] = 1:20, toluene, and 25 °C.

(

1:1) as catalysts was in the same proportion, the molecular

weight distribution of the resultant polymer became wider, and

a noticeable bimodal curve was observed (Figure 10). Based

on all the above observations, it is reasonable to conclude that

the binuclear structure of 3b was maintained during the

polymerization (Scheme S1 ). The narrow molecular weight

distribution of PHBs catalyzed by 3 was attributed to the low

content of 3b and the probable presence of a transesteriﬁcation

side reaction in the polymerization of rac-BBL.

TOF mass spectroscopy (Figure 8). The H NMR spectrum of

the PHB oligomer, which was prepared by the catalyst 3b with

the monomer in a 1:20 molar ratio, clearly demonstrated the

expected PHB main chain structure, with δ 1.24 and 5.20 ppm

assigned to the methyl and methine protons, respectively. The

peak at δ 1.39 ppm could be assigned to the −O Bu en group,

whereas the other end of the polymeric chain was found to be

Inorg. Chem. 2021, 60, 705−717

Article

Figure 8. MALDI-TOF mass spectrum of the PHB oligomer prepared from complex 3b ([BBL] = 2M, [I]:[BBL] = 1:20, toluene, and 25 °C).

Figure 10. SEC curves of PHB polymers prepared from (a) 3, (b) 3b,

and (c) 50% 3 + 50% 3b. Conditions are as follows: [BBL] = 2 M,

[BBL]:[I] = 200, toluene, and 25 °C.

rac-BBL. The new phosphasalalen pro-ligands L1−L3 with

chiral pyrrolidine backbones could be synthesized eﬃciently by

the Kirsanov reaction between phosphinophenol and amino-

pyrrolidinylphenols, and the multinuclear NMR and X-ray

diﬀraction analyses revealed that their corresponding rare-earth

mixtures 1−6 each contained a mononuclear complex and a

binuclear complex with bridged alkoxo and hydroxo groups.

However, when a trimethylsiloxide group was involved, the

pure binuclear complex 7b was formed. All the complexes

could catalyze the heteroselective ROP of rac-LA with high

catalytic activities. Remarkably, in contrast to the inactivity of

their salalen analogues, the yttrium and lutetium phosphasa-

lalen mixtures 2−6 could eﬃciently promote the ROP of rac-

BBL to produce syndiotactic polymers (P up to 0.73). The

high polymerization activities of 2−6 in rac-BBL ROP could be

rationalized by the better electron-donating ability of the

chelating phosphasalalen ligands compared to their salalen

analogues. Not surprisingly, 1 with a more Lewis acidic

scandium center was inactive. The detailed end-group analyses

and the kinetic investigations proved the absence of hydroxo-

derived PHBs and, interestingly, that the binuclear structures

of the alkoxo−hydroxo-bridged complexes did not decompose

during the polymerization.

Figure 9. (a) Plot of ln([BBL] /[BBL] ) vs time using 3 and (b) plot

of [BBL] − [ BBL] vs time using complex 3b for the ROP of rac-

BBL. Conditions are as follows: [BBL] = 2 M, [I]:[BBL] = 1:200,

toluene, and 25 °C.

EXPERIMENTAL SECTION

General Considerations. All reactions were conducted under an

atmosphere of dry nitrogen using standard Schlenk line and glovebox

Vigor SG1200/750TS-F) techniques. Solvents and reagents were

obtained from commercial sources. THF, toluene, and hexane were

distilled from sodium or benzophenone, and DCM was distilled from

■

(

CONCLUSIONS

■

In conclusion, a family of rare-earth complexes bearing new

phosphasalalen ligands, which were salalen analogues with an

iminophosphorane replacing the imine functionality, were

synthesized and tested for the syndiotactic ROP of rac-LA and

CaH . Commercial reagents, namely BuLi, N-bromosuccinimide,

chlorodiphenylphosphine, and other commercially available reagents,

Inorg. Chem. 2021, 60, 705−717

Inorganic Chemistry

Article

were purchased from TCI or Aladdin and used without further

puriﬁcation. rac-Lactide was recrystallized from ethyl acetate and

sublimated twice. The phosphinophenols and aminopyrrolidinylphe-

nols with bulky substituents at the ortho-phenoxide positions were

129.64, 129.50, 129.44, 129.31, 129.22, 129.11, 128.52, 128.30,

127.31, 126.05, 124.93, 123.90, 120.10, 102.63, 101.53, 68.19 (s, N−

CH−CH −N), 68.13 (s, N−CH−CH −N), 65.94 (s, N−CH −Ph),

59.26 (s, N−CH), 53.71 (s, N−CH ), 47.50 (s, Ph−Ad(C)), 40.27 (s,

prepared according to the methods reported in previous literature.

Ph−Ad(C)), 40.22 (s, Ph−Ad(C)), 37.16 (s, Ph−Ad(C)), 36.97 (s,

H (400 MHz), P{ H} (162 MHz), and C{ H} NMR (100 MHz)

Ph−CH ), 36.85 (s, Ph−CH ), 29.23 (s, Ph−Ad(C)), 28.85 (s, N−

and H DOSY (600 MHz) measurements were obtained on a JNM-

ECZ400S/L 400 or Bruker AV600 MR spectrometer, respectively, in

a CDCl , THF-d , or C D solution (25 °C). The molecular weights

−CH

), 28.51 (s, Ph−Ad(C)), 23.22 (s, Ph−Ad(C)), 20.87 (s,

Ph−Ad(C)). Anal. Calc. for C52H64BrN O P: C, 72.63; H, 7.50; N,

2 2

3.26. Found: C, 72.86; H, 7.64; N, 3.30.

and molecular weight distribution of the polymers were measured by

means of gel-permeation chromatography (SEC) on a Waters LC-16

Synthesis of Pro-Ligand L3. The synthesis of pro-ligand L3 was

carried out in the same way as that described for pro-ligand L1, but 2-

(diphenylphosphanyl)-4-methyl-6-(2-phenylpropan-2-yl)phenol (2.5

g, 6 mmol) and (S)-2-((2-(aminomethyl)pyrrolidin-1-yl)methyl)-4-

methyl-6-(2-phenylpropan-2-yl)phenol (2.1 g, 6.1 mmol) were used.

The pure product was obtained by column chromatography on silica

(

35 °C) instrument. The M values of the PLAs were corrected with a

.58 factor, and those of the PHBs were not corrected. Single crystals

suitable for X-ray analysis were obtained from n-hexane solutions. The

intensity data of the single crystals were collected on the Bruker D8

Venture system. All determinations of the unit cell and intensity data

were performed with graphite-monochromated Ga Kα radiation (λ =

gel (DCM/MeOH 30:1) as a yellow solid (2.3 g, 47.3%). H NMR

(400 MHz, CDCl

1H, N−CH −Ph), 3.86 (d, 1H, NH−CH

3.59 (m, 1H, N−CH −Ph), 2.92 (m, 1H, N−CH-CH

1H, N−CH ), 2.13 (s, 3H, Ph−CH ), 2.08 (s, 3H, Ph−CH

(m, 1H, N−CH −CH ), 1.72 (s, 3H, C−CH ), 1.62 (s, 3H, C−

CH ), 1.60 (s, 3H, C−CH ), 1.45 (s, 3H, C−CH

(162 MHz, CDCl , ppm): δ 42.67 (s, P). C{ H} NMR (100 MHz,

CDCl , ppm): δ 169.68, 152.07, 150.58, 149.43, 148.67, 147.93,

, ppm): δ 7.92−6.11 (m, 24H, Ph−H), 4.06 (d,

), 3.83 (d, 1H, NH−CH ),

), 2.33 (dd,

), 2.06

1.34138 Å). The data of the complexes were collected at 173 k using

the ω-scan technique. All the non-hydrogen atoms were reﬁned

anisotropically, and all the hydrogen atoms were included but not

reﬁned. Crystallographic data (excluding structure factors, Table S2)

for the structure analysis were deposited with the Cambridge

Crystallographic Data Centre as CCDC 2031008 (3b), 2031009

). P{ H} NMR

(6b), and 2031010 (7b).

Synthesis of Pro-Ligand L1. At −78 °C, bromine (6.1 mmol)

was added dropwise to a solution of 2,4-di-tert-butyl-6-

(

diphenylphosphanyl)phenol (2.3 g, 6.0 mmol) in dry dichloro-

methane (20 mL). The solution was still stirred at −78 °C for 1 h.

DABCO (3.6 mmol) and (S)-2-((2-(aminomethyl)pyrrolidin-1-yl)-

methyl)-4,6-di-tert-butylphenol (2.4 g, 6.1 mmol) were dissolved in

DCM and added to the solution dropwise. The solution was stirring

at −78 °C for another 4 h, then warmed to room temperature and

stirred overnight. The cloudy solution was quenched and washed with

water (2 × 100 mL). The organic solution was dried over anhydrous

Na SO and evaporated in a vacuum. After recrystallization from

Et O, the pure product was isolated as a white solid (2.4 g, 50.8%).

H NMR (400 MHz, CDCl , ppm): δ 7.86−6.58 (m, 14H, Ph−H),

.80 (d, 1H, NH−CH ), 4.59 (m, 1H, N−CH −Ph), 3.95 (d, 1H,

NH−CH ), 3.85 (m, 1H, N−CH −Ph), 3.20 (m, 1H, N−CH), 2.95

(

m, 2H, N−CH −CH ), 2.30 (m, 1H, N−CH −CH ), 1.89 (m, 1H,

N−CH −CH ), 1.62 (m, 2H, N−CH−CH ), 1.26 (s, 9H, C(CH ) ),

3 3

.17 (s, 9H, C(CH ) ), 1.14 (s, 9H, C(CH ) ), 1.02 (s, 9H,

3 3

C(CH ) ). P{ H} NMR (162 MHz, CDCl , ppm): δ 44.87 (s, P).

C{ H} NMR (100 MHz, CDCl , ppm): δ 167.95, 153.49, 141.69,

41.54, 139.07, 137.82, 137.64, 133.33, 133.21, 132.96, 132.86,

31.18, 131.92, 131.81, 129.61, 129.47, 129.30, 129.18, 129.13,

28.92, 128.12, 127.82, 127.16, 125.81, 125.37, 119.58, 68.29 (s, N−

and a white slurry was immediately formed. The cloudy solution was

stirred for another 2 h at 50 °C and centrifuged to remove all

remaining insoluble salts, and the ﬁltrate was evaporated to dryness

and washed with cold hexane. Crystallization in hexane at −30 °C

CH−CH −N), 68.23 (s, N−CH−CH −N), 59.54 (s, N−CH −Ph),

3.39 (s, N−CH), 47.58 (s, N−CH ), 35.35 (s, Ph−C−CH ), 35.34

gave the mixture 1 as a white solid in a 49.8% yield (44.8 mg). H

(s, Ph−C−CH ), 35.10 (s, Ph−C−CH ), 34.12 (s, Ph−C−CH ),

NMR (400 MHz, C

, ppm): δ 7.67−6.60 (m, 14H, Ph−H), 4.00

), 3.52 (m, 1H, PN−CH ; 1H, N−CH −Ph),

−Ph), 3.00 (m, 1H, N−CH), 2.74 (m, 1H, N−

), 2.15 (m, 2H, N−CH −CH ; 1H, N−

), 1.91 (m, 1H, N−CH−CH ), 1.85 (s, 9H, C(CH ), 1.84

(s, 9H, O−C(CH ), 1.43 (s, 9H, C(CH ), 1.19 (s, 9H, C(CH ),

1.10 (s, 9H, C(CH ). P{ H} NMR (162 MHz, C , ppm): δ

1.62 (s, N−CH-CH ), 31.26 (s, C−CH ), 30.01 (s, N−CH −CH ),

(m, 1H, PN−CH

9.92 (s, C−CH ), 28.61 (s, C−CH ), 23.32 (s, C−CH ). Anal. Calc.

3.10 (m, 1H, N−CH

for C H BrN O P: C, 70.12; H, 8.19; N, 3.56. Found: C, 70.18; H,

CH−CH

), 2.56 (m, 1H, N−CH

.14; N, 3.58.

Synthesis of Pro-Ligand L2. The synthesis of pro-ligand L2 was

)

3 3

)

3 3

carried out in the same way as that described for pro-ligand L1, but 2-

adamantanyl-6-(diphenylphosphanyl)-4-methylphenol (2.6 g, 6

mmol) and 2-adamantanyl-6-(((S)-2-(aminomethyl)pyrrolidin-1-yl)-

methyl)-4-methylphenol (2.2 g, 6.1 mmol) were used. The pure

)

6 6

37.15 (s, P), 34.97 (s, P). Due to the asymmetric inherence of both

mononuclear and binuclear components (the presence of a chiral

carbon) in the mixture, we could not unambiguously assign the

product was obtained by column chromatography on silica gel

C{ H} NMR signals. C{ H} NMR (100 MHz, C D , ppm): δ

6 6

(DCM/MeOH 30:1) as a yellow solid (2.5 g, 48.9%). H NMR (400

167.02, 160.77, 136.05, 135.38, 135.24, 134.15, 133.91, 132.56,

132.47, 132.23, 132.15, 131.55, 131.45, 130.77, 129.40, 129.12,

127.62, 127.42, 125.81, 125.68, 124.01, 123.81, 123.25, 122.84,

111.66, 110.44, 69.64, 68.05, 66.57, 58.36, 51.55, 34.94, 34.89, 34.71,

34.70, 34.42, 34.33, 32.99, 32.95, 32.88, 31.02, 30.67, 30.32, 30.21,

28.93, 26.60, 24.51, 21.76, 19.36.

MHz, CDCl , ppm): δ 7.88−6.11 (m, 14H, Ph−H), 4.80 (m, 1H,

NH−CH ), 4.56 (m, 1H, N−CH −Ph), 4.01 (d, 1H, NH−CH ),

.87 (m, 1H, N−CH −Ph), 3.14 (m, 1H, N−CH), 2.97 (m, 2H, N−

CH ), 2.11 (s, 3H, Ph−CH ), 2.03 (s, 3H, Ph−CH ), 1.99−1.65 (m,

0H, Ph−Ad(H) and pyrrolidine−ring(H)), 1.51 (m, 2H, Ph−

31 1

Ad(H)), 1.42 (m, 2H, Ph−Ad(H)). P{ H} NMR (162 MHz,

CDCl , ppm): δ 44.46 (s, P). C{ H} NMR (100 MHz, CDCl ,

Synthesis of Mixture 2. The mixture 2 was synthesized using the

same procedure as that for complex 1, but pro-ligand L1 (78.8 mg,

0.10 mmol), YCl (THF) (44.8 mg, 0.10 mmol), and KOEt (8.6 mg,

ppm): δ 168.89, 153.77, 142.01, 141.93, 140.11, 135.83, 133.30,

33.29, 133.00, 132.90, 132.44, 132.31, 131.75, 131.64, 131.19,

3.5

0.10 mmol) were used. Mixture 2 was obtained in a 49.4% yield as a

Inorg. Chem. 2021, 60, 705−717

Inorganic Chemistry

Article

white solid (45.4 mg). H NMR (400 MHz, C D , ppm): δ 7.72−6.41

mononuclear and binuclear components (the presence of a chiral

carbon) in the mixture, we could not unambiguously assign the

(

m, 14H, Ph−H), 4.75 (d, 1H, PN−CH ), 4.57 (d, 1H, PN−

CH ), 3.25−3.75 (m, 2H, N−CH −Ph; 2H, O−CH ; 1H, N−CH),

C{ H} NMR signals. C{ H} NMR (100 MHz, C D , ppm): δ

6 6

.08 (m, 1H, N−CH ), 2.84 (m, 1H, N−CH ), 1.89 (s, 9H,

167.99, 161.73, 140.20, 140.12, 137.02, 136.38, 136.24, 135.95,

133.54, 133.45, 132.51, 132.42, 131.76, 131.28, 130.36, 129.21,

128.81, 128.61, 128.50, 128.41, 124.78, 124.24, 123.82, 112.60,

111.44, 71.08, 69.26, 69.09, 59.13, 52.52, 48.50, 48.46, 35.69, 35.41,

34.20, 33.98, 33.93, 32.09, 32.01, 31.66, 31.40, 31.30, 30.15, 27.63,

20.31.

C(CH ) ), 1.51 (s, 9H, C(CH ) ), 1.46 (s, 9H, C(CH ) ), 1.35 (d,

3 3

H, O−CH −CH ) 1.13 (s, 9H, C(CH ) ). P{ H} NMR (162

MHz, C D , ppm): δ 36.72 (s. P), 33.05 (s, P). Due to the

asymmetric inherence of both mononuclear and binuclear compo-

nents (the presence of a chiral carbon) in the mixture, we could not

unambiguously assign the C{ H} NMR signals. C{ H} NMR (100

3 3

Synthesis of Mixture 5. The mixture 5 was synthesized using the

same procedure as that for mixture 1, but pro-ligand L2 (87.6 mg,

0.10 mmol), YCl (THF) (44.8 mg, 0.10 mmol), and KOEt (8.6 mg,

MHz, C D , ppm): δ 167.63, 162.54, 139.00, 136.54, 135.50, 134.43,

33.64, 133.56, 132.16, 132.07, 131.50, 130.75, 129.02, 128.61,

28.42, 128.31, 128.21, 127.27, 127.08, 125.38, 125.21, 123.78,

23.67, 112.82, 110.65, 59.92, 58.69, 51.56, 51.11, 35.97, 35.32, 35.23,

3.88, 33.83, 33.79, 32.09, 31.98, 31.40, 31.28, 30.75, 29.86, 29.65,

3.96, 21.84, 20.76.

Synthesis of Mixture 3. The mixture 3 was synthesized using the

same procedure as that for mixture 1, but pro-ligand L1 (78.8 mg,

.10 mmol), YCl (THF) (44.7 mg, 0.10 mmol), and KO Bu (11.2

mg, 0.10 mmol) were used. The mixture 3 was obtained in a 51.2%

yield as a white solid (48.5 mg). The crystal of complex 3b was slowly

3.5

0.10 mmol) were used. The mixture 5 was obtained in a 43.4% yield

as a yellow solid (59.7 mg). H NMR (400 MHz, C D , ppm): δ

7.55−6.41 (m, 14H, Ph−H), 4.67 (d, 1H, PN−CH ), 3.35 (m, 2H,

N−CH −Ph; 1H, N−CH), 2.91 (m, 2H, O−CH −CH ), 2.69 (d,

1H, PN−CH ), 2.56 (m, 2H, N−CH ), 2.45 (m, 10H, Ph−

Ad(H)), 2.35 (d, 3H, Ph−CH ), 2.13 (d, 3H, O−CH −CH ), 2.06

3 2 3

(s, 3H, Ph−CH ), 2.06−1.64 (m, 20H, Ph−Ad(H)). P{ H} NMR

3.5

(162 MHz, CDCl , ppm): δ 35.62 (s, P), 32.28 (s, P). Due to the

asymmetric inherence of both mononuclear and binuclear compo-

nents (the presence of a chiral carbon) in the mixture, we could not

grown from the concentrated solution of mixture 3 in hexane. H

NMR (400 MHz, C D , ppm): δ 7.73−6.62 (m, 14H, Ph−H), 4.01

unambiguously assign the C{ H} NMR signals. C{ H} NMR (100

(

m, 1H, PN−CH ), 3.59 (br, 1H, N−CH −Ph), 3.14 (t, 1H, P

MHz, C D , ppm): δ 167.96, 162.87, 139.86, 137.97, 137.26, 136.68,

N−CH ), 3.06 (br, 1H, N−CH −Ph), 2.76 (m, 1H, N−CH), 2.67 (t,

135.80, 134.57, 133.85, 132.66, 132.05, 131.38, 131.12, 129.42,

129.30, 129.19, 128.89, 128.66, 125.79, 124.76, 122.60, 122.44,

121.98, 115.94, 114.70, 63.77, 63.56, 61.25, 59.35, 59.22, 53.11, 52.67,

52.35, 41.34, 41.14, 38.24, 38.14, 37.91, 37.78, 37.69, 37.62, 37.54,

30.32, 30.21, 30.12, 30.05, 29.85, 25.04, 21.62, 21.55, 21.43, 21.16.

Synthesis of Mixture 6. The mixture 6 was synthesized using the

same procedure as that for mixture 1, but pro-ligand L3 (82.8 mg,

0.10 mmol), YCl (THF) (44.8 mg, 0.10 mmol), and KOEt (8.6 mg,

H, N−CH ), 2.20 (m, 1H, N−CH ), 2.09 (m, 2H, N−CH−CH ),

.89 (m, 1H, N−CH −CH ), 1.83 (m, 1H, N−CH −CH ), 1.81 (s,

H, C(CH ) ), 1.49 (s, 9H, O−C(CH ) ), 1.25 (s, 9H, C(CH ) ),

3 3

.19 (s, 9H, C(CH ) ), 1.13 (s, 9H, C(CH ) ). P{ H} NMR (162

3 3

MHz, C D , ppm): δ 36.94 (s, P), 33.32 (s, P). Due to the

asymmetric inherence of both mononuclear and binuclear compo-

nents (the presence of a chiral carbon) in the mixture, we could not

unambiguously assign the C{ H} NMR signals. C{ H} NMR (100

MHz, C D , ppm): δ 167.36, 161.41, 139.75, 139.68, 136.48, 136.01,

35.87, 135.60, 133.53, 133.44, 132.59, 132.49, 131.62, 129.01,

28.58, 128.47, 128.45, 128.36, 127.02, 126.89, 124.99, 124.30,

23.70, 113.19, 112.01, 70.21, 69.02, 68.84, 67.94, 59.44, 52.25, 48.77,

5.90, 35.62, 35.36, 34.31, 33.96, 32.10, 31.94, 31.43, 31.29, 30.98,

3.5

0.10 mmol) were used. The mixture 6 was obtained in a 41.4% yield

as a pale yellow solid (50.7 mg). The crystal of 6b was slowly grown

from the concentrated solution of mixture 6 in hexane. H NMR (400

MHz, C D , ppm): δ 7.53−6.54 (m, 24H, Ph−H), 3.79 (d, 2H, P

N−CH ), 3.24 (m, 2H, O−CH −CH ), 2.54−2.72 (m, 2H, N−

CH −Ph; 1H, N−CH; 2H, N−CH ), 2.31 (s, 3H, Ph−CH ), 2.28 (s,

0.07, 30.03, 27.22, 25.42, 20.65.

1H, N−CH−CH ), 2.26 (s, 1H, N−CH−CH ), 2.22 (d, 3H, Ph−

b H NMR (400 MHz, C D , ppm): δ 7.73−6.50 (m, 14H, Ph−

CH ), 2.18 (m, 1H, N−CH −CH ), 2.14 (d, 1H, N−CH −CH ),

H), 4.65 (d, 1H, PN−CH ), 3.48 (m, 1H, N−CH −Ph), 3.32 (d,

1.98 (s, 3H, C−CH ), 1.88 (s, 3H, C−CH ), 1.83 (s, 3H, C−CH ),

31 1

H, PN−CH ), 3.26 (m, 1H, N−CH −Ph), 2.72 (dt, 2H, N−CH ,

1.80 (s, 3H, C−CH ). P{ H} NMR (162 MHz, C D , ppm): δ

H, N−CH), 1.89 (s, 9H, C(CH ) ), 1.87 (s, 4.5H, O−C(CH ) ),

34.94 (s, P), 31.29 (s, P). Due to the asymmetric inherence of both

mononuclear and binuclear components (the presence of a chiral

carbon) in the mixture, we could not unambiguously assign the

3 3

.76 (m, 2H, N−CH −CH , 2H, N−CH−CH ), 1.66 (s, 9H,

C(CH ) ), 1.39 (s, 9H, C(CH ) ), 1.14 (s, 9H, C(CH ) ). P{ H}

3 3

NMR (162 MHz, C D , ppm): δ 33.32 (s, P). C{ H} NMR (100

C{ H} NMR signals. C{ H} NMR (100 MHz, C D , ppm): δ

6 6

MHz, C D , ppm): δ 167.25, 161.61, 139.79, 139.05, 136.51, 135.28,

167.84, 162.17, 153.07, 152.61, 139.16, 137.01, 134.46, 133.58,

132.55, 131.98, 131.44, 130.28, 129.59, 129.12, 127.82, 127.48,

127.31, 126.78, 125.96, 125.77, 125.53, 125.28, 122.95, 122.60,

121.98, 115.59, 114.3, 64.97, 59.55, 59.21, 53.35, 52.45, 51.75, 44.76,

43.90, 43.31, 43.05, 38.52, 33.72, 32.88, 31.02, 30.48, 28.84, 27.85,

24.84, 22.33, 21.60.

35.13, 134.86, 133.44, 133.29, 132.04, 131.95, 131.44, 130.68,

28.78, 128.58, 128.48, 125.06, 124.32, 123.69, 112.55, 112.09,

11.32, 70.73 (s, PN−CH ), 67.78 (s, NH−CH ), 64.50 (s, N−

CH −Ph), 59.44 (s, O−C−CH ), 52.23 (s, N−CH), 51.68 (s, N−

CH ), 35.85 (s, C−CH ), 35.67 (s, C−CH ), 35.29 (s, C−CH ),

3.87 (s, N−CH−CH ), 33.80 (s, N−CH −CH ), 32.00 (s, C−

6b H NMR (400 MHz, C D , ppm): δ 7.45−6.35 (m, 24H, Ph−

CH ), 31.32 (s, C−CH ), 31.00 (s, C−CH ), 30.04 (s, C−CH ),

C−CH ), 21.28 (s, C−CH ), 20.54 (s, C−CH ). Anal. Calc. for

C H N O P Y : C, 68.72; H, 7.93; N, 3.34. Found: C, 69.32; H,

H), 3.95 (m, 2H, PN−CH ), 3.25 (m, 2H, O−CH −CH ), 2.55−

7.33 (s, C−CH ), 25.35 (s, C−CH ), 24.47 (s, C−CH ), 22.74 (s,

2.68 (m, 1H, N−CH −Ph; 2H, N−CH ; 1H, N−CH), 2.37 (d, 1H,

N−CH −Ph), 2.27 (s, 3H, Ph−CH ), 2.24 (d, 1H, N−CH−CH ),

132

2.15 (d, 1H, N−CH−CH ), 2.09 (d, 2H, N−CH −CH ), 2.02 (s,

.23; N, 3.26.

Synthesis of Mixture 4. The mixture 4 was synthesized using the

same procedure as that for mixture 1, but pro-ligand L1 (78.8 mg,

3H, Ph−CH ), 1.94 (s, 3H, C−CH ), 1.84 (s, 3H, C−CH ), 1.79 (s,

31 1

3H, C−CH ), 1.76 (s, 3H, C−CH ). P{ H} NMR (162 MHz,

C D , ppm): δ 31.29 (s, P). C{ H} NMR (100 MHz, C D , ppm):

.10 mmol), LuCl (THF) , (49.8 mg, 0.10 mmol), and KO Bu (11.2

δ 170.21, 164.82, 156.60, 155.45, 154.15, 153.26, 139.34, 138.54,

135.27, 133.30, 133.25, 131.70, 131.45, 131.22, 130.62, 130.15,

127.04, 126.86, 126.62, 125.60, 125.17, 124.98, 123.98, 123.66,

mg, 0.10 mmol) were used. The mixture 5 was obtained in a 45.5%

yield as a white solid (47.4 mg). H NMR (400 MHz, C D , ppm): δ

.85−6.72 (m, 14H, Ph−H), 4.16 (d, 1H, PN−CH ), 3.21 (m, 1H,

123.51, 116.66, 116.28, 69.10 (s, O−C−CH ), 57.42 (s, PN−

N−CH −Ph), 3.12 (m, 1H, N−CH −Ph), 2.90 (m, 1H, N−CH),

CH ), 54.36 (s, N−CH −Ph), 53.65 (s, N−CH−CH ), 44.93 (s, N−

.63 (d, 1H, PN−CH ), 2.23 (m, 2H, N−CH ; 2H, N−CH−CH ),

CH ), 42.30 (s, N−CH ), 33.61 (s, Ph−C−CH ), 31.66 (s, Ph−C−

.98 (m, 2H, N−CH −CH ), 1.95 (s, 9H, C(CH ) ), 1.69 (s, 9H,

CH ), 30.62 (s, Ph−C−CH ), 30.10 (s, Ph−C−CH ), 28.51 (s, N−

O−C(CH ) ), 1.38 (s, 9H, C(CH ) ), 1.28 (s, 9H, C(CH ) ), 1.18

CH−CH ), 27.21 (s, Ph−CH ), 25.61 (s, C−CH ), 22.75 (s, Ph−

(s, 9H, C(CH ) ). P{ H} NMR (162 MHz, C D , ppm): δ 38.94 (s,

CH ), 20.94 (s, C−CH ). Anal. Calc. for C

N O P Y : C,

102 112

6 2 2

P), 34.31 (s, P). Due to the asymmetric inherence of both

70.82; H, 6.53; N, 3.24. Found: C, 71.69; H, 6.61; N, 3.17.

Inorg. Chem. 2021, 60, 705−717

Inorganic Chemistry

Shanghai 200444, China; orcid.org/0000-0001-5178-

Article

Synthesis of Complex 7b. The complex 7b was synthesized

using the same procedure as that for mixture 1, but pro-ligand L1

emailing data_request@ccdc.cam.ac.uk , or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

(

⁷⁸^.⁸^m^g^,⁰^.¹⁰^m^m^o^l⁾^,^Y^C^l⁽^T^H^F⁾3.5 (44.8 mg, 0.10 mmol), and

KOSiMe (13.0 mg, 0.10 mmol) were used. The complex 7b was

obtained in a 49.8% yield as a pale yellow solid (42.2 mg, 0.025

mmol). The crystal of 7b was slowly grown from the concentrated

AUTHOR INFORMATION

Corresponding Author

■

solution of hexane. H NMR (400 MHz, C D , ppm): δ 7.66−6.44

(

m, 14H, Ph−H), 4.55 (d, 1H, PN−CH ), 3.38 (m, 1H, N−CH −

Xiaochao Shi − Department of Polymer Materials, College of

Materials Science and Engineering, Shanghai University,

Ph), 3.28 (d, 1H, PN−CH ), 3.23 (m, 1H, N−CH −Ph), 2.68 (m,

H, N−CH; 2H, N−CH ), 1.83 (s, 9H, C(CH ) ), 1.74 (m, 2H, N−

3 3

CH−CH ), 1.64 (m, 1H, N−CH −CH ), 1.61 (s, 9H, C(CH ) ),

3 3

7972 ; Email: xcshi@shu.edu.cn

.35 (m, 1H, N−CH −CH ), 1.31 (s, 9H, C(CH ) ), 1.07 (s, 9H,

3 3

31 1

C(CH ) ), 0.59 (s, 4.5H, Si−(CH ) ). P{ H} NMR (162 MHz,

3 3

Author

C D , ppm): δ 33.13 (s, P). C{ H} NMR (100 MHz, C D , ppm):

Hui Liu − Department of Polymer Materials, College of

Materials Science and Engineering, Shanghai University,

Shanghai 200444, China

δ 167.02, 161.57, 139.14, 139.06, 136.52, 135.58, 135.43, 135.04,

34.69, 133.31, 133.22, 132.10, 132.00, 131.44, 130.67, 128.86,

28.63, 128.31, 128.91, 125.06, 123.97, 123.87, 112.79, 111.51, 64.28

(

s, PN−CH −CH), 64.09 (s, N−CH −Ph), 59.27 (s, N−CH),

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c02741

1.84 (s, N−CH ), 35.91 (s, Ph−C−CH ), 35.90 (s, Ph−C−CH ),

5.25 (s, Ph−C−CH ), 31.98 (s, N−CH−CH ), 31.65 (s, Ph−C−

CH ), 31.43 (s, N−CH −CH ), 31.31 (s, C−CH ), 31.17 (s, C−

Notes

CH ), 30.17 (s, C−CH ), 29.91 (s, C−CH ), 24.55 (s, C−CH ),

The authors declare no competing ﬁnancial interest.

2.74 (s, C−CH ), 21.29 (s, C−CH ). Anal. Calc. for

C H N O P SiY : C, 67.36; H, 7.85; N, 3.31. Found: C, 67.93;

H, 7.64; N, 3.34.

132

ACKNOWLEDGMENTS

■

General Polymerization of rac-Lactide. In a glovebox, rac-LA

This work was supported by the National Natural Science

Foundation of China (no. 21704060) and the Program for

Professor of Special Appointment (Eastern Scholar) at

Shanghai Institutions of Higher Learning.

(

288 mg, 2 mmol, 200 equiv) was dissolved in THF (1 mL). A

solution of the catalyst (10 μmol in 1 mL THF) was injected into the

reaction mixture (5 μmol catalyst was used when [LA]/[I] ≥ 500)

such that the overall concentration of rac-LA was 1 M. Aliquots were

taken from the reaction under a nitrogen atmosphere, quenched with

hexane, and poured into hexane to precipitate the polymer. The

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