Synthesis and characterization of rare-earth metal guanidinates stabilized by amine-bridged bis(phenolate) ligands and their application in the controlled polymerization of rac-lactide and rac-β-butyrolactone

Abstract

Eight rare-earth metal guanidinates supported by a versatile family of chelating amine-bridged bis(phenolate) ligands were synthesized. Metathesis reactions of rare-earth metal chlorides [LnClL¹(THF)] stabilized by amine-bridged bis(phenolate) ligand L¹ with in situ generated lithium guanidinates in a 1 : 1 molar ratio in THF afforded ytterbium guanidinates YbL¹ [R₂NC(NR¹)₂] [R¹ = –ⁱPr, R₂N = –NⁱPr₂ (1), –N(CH₂)₅ (2)]. Insertion reactions of the yttrium amides bearing bridged bis(phenolate) ligands with 1 equiv of N,N′-diisopropylcarbodiimide (DIC) yielded six yttrium guanidinates YL¹ [(SiHMe₂)₂NC(NⁱPr)₂] (3), YL²[(SiHMe₂)₂NC(NⁱPr)₂](THF) (4), YL³[(SiHMe₂)₂NC(NⁱPr)₂] (5), YL⁴[(SiHMe₂)₂NC(NⁱPr)₂] (6), YL⁵ [(SiHMe₂)₂NC(NⁱPr)₂] (7), YL⁶[(SiHMe₂)₂NC(NⁱPr)₂] (8), respectively. The behaviors of complexes 1–8 in the polymerization of rac-lactide (LA) and rac-β-butyrolactone (BBL) were also explored. It was found that complexes 1–8 efficiently initiated the ring-opening polymerization (ROP) of rac-LA and rac-BBL in a controlled manner, providing highly heterotactic polylactide (P_r up to 0.99) and highly syndiotactic poly(3-hydroxybutyrate) (P_r up to 0.82). The framework of the bridge played a significant role in governing the stereoselectivity, while guanidinate groups work as initiating groups.

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Introduction

With the growing global concern over sustainability, the preparation of biodegradable polymers such as poly(lactic acid) (PLA) and poly(3-hydroxybutyrate) (PHB) has become increasingly important and opened commercial prospects for a range of applications in the pharmaceutical, biomedical, and environmental fields.^1–6 The most straightforward route to prepare PLA and PHB is the ring-opening polymerization (ROP) of the corresponding cyclic monomers, promoted by initiators that achieve high activity with controlled molecular weight, polydispersity, and microstructure of resulting polymers.^1,4,7 Among the variety of metal complexes that have been disclosed for these polymerizations, rare-earth metal complexes have been reported to be efficient initiators for the preparation of unique polyesters with controlled features.^8–28

Amine bridged bis(phenolate)s, as one type of dianionic chelating ligands, have received considerable attention in rare-earth metal chemistry due to their easy availability, highly tunable features and outstanding capacity to stabilize metal centers. Rare-earth metal complexes stabilized by amine bridged bis(phenolate) ligands have been reported to be efficient initiators for the ROP of cyclic esters, yielding polymers in high yields.^{9,11–18,21–27,29–32} Recently, our group has reported a series of rare-earth metal complexes supported by bridged bis(phenolate) ligands.^{21–26,29–36} Lanthanide aryloxides and alkoxides stabilized by amine-bridged bis(phenolate) ligands are found to be efficient initiators for the controlled polymerization of rac-lactide (LA) and rac-β-butyrolactone (BBL).^22–25 Moreover, group IV metal complexes bearing an amine-bridged bis(phenolato) ligand are highly efficient in catalyzing regioselective intermolecular hydroamination reactions.³⁷

To gain better understanding of the relationship between the bis(phenolate) ligands and the catalytic property of corresponding rare-earth metal complexes, we have designed and synthesized new guanidinato complexes stabilized by several amine bridged bis(phenolate) ligands. Although guanidinato ligands have already been reported to stabilize rare-earth metal complexes, they were not employed as initiating groups in ROP of cyclic esters.^8c,38–40 Complexes bearing both bis(phenolate) and guanidinato ligands are thus tested in initiating the ROP of rac-lactide and rac-β-butyrolactone, and their activities and capacities in stereocontrol are compared.

Results and discussion

Synthesis and characterization of complexes 1–8

Ligand precursors L¹H₂–L⁶H₂ bearing various bridges were synthesized by Mannich condensations of substituted phenols, formaldehydes, and appropriate amines.⁴¹ They were isolated in moderate to good yields as white powders (Chart 1).

Chart 1 Ligand precursors L¹H₂–L⁶H₂.

Two general methods for synthesizing rare-earth metal guanidinates include metathesis reactions between a suitable rare-earth metal precursor and an alkali guanidinate,^38,39 and the insertion of carbodiimides into a rare-earth metal–nitrogen bond.⁴⁰ Ytterbium guanidinato complexes 1–2 were synthesized from metathesis reactions between the easily available Yb(THF)ClL¹ and freshly prepared lithium guanidinates (Scheme 1). Yttrium complexes 3–8 were prepared via insertion reactions. In a two-step sequence, ligand precursors LH₂ first reacted with Y[N(SiHMe₂)₂]₃(THF)₂ to generate yttrium amides, which were treated with one equivalent of N,N′-diisopropylcarbodiimide (DIC) affording six yttrium guanidinates 3–8 (Scheme 2 and Chart 2).

Scheme 1 Synthesis of complexes 1–2.

Scheme 2 Synthesis of complexes 3–8.

Chart 2 Complexes 3–8.

Complexes 1 and 2 are well soluble in THF, moderately soluble in toluene, and slightly soluble in hexane. Complexes 3–8 are moderately soluble in hexane. All complexes are quite air- and moisture-sensitive, but both the crystals and solution are quite stable when stored under argon. These complexes were characterized by elemental analysis, IR spectroscopy, X-ray diffraction studies in the cases of complexes 1, 2, 4 and 5, and by NMR spectroscopy for diamagnetic complexes 3–8.

Crystal structure analyses

Crystals suitable for X-ray diffraction analysis of complexes 1 and 2 were obtained from toluene and THF solution at room temperature, respectively, whereas crystals of complexes 4 and 5 were obtained from hexane solution, respectively. X-ray diffraction analyses show that complexes 1, 2, 4 and 5 have monomeric structures. The ORTEP diagrams of all four structures are depicted in Fig. 1–4, respectively. Their crystallographic data are given in Table 1, and the selected bond lengths and angles are provided in Table 2.

Fig. 1 ORTEP diagram of complex 1 showing the atom numbering scheme. Thermal ellipsoids are drawn at the 20% probability level. Hydrogen atoms and solvent molecules are omitted for clarity.

Fig. 2 ORTEP diagram of complex 2 showing the atom numbering scheme. Thermal ellipsoids are drawn at the 20% probability level. Hydrogen atoms are omitted for clarity.

Fig. 3 ORTEP diagram of complex 4 showing the atom numbering scheme. Thermal ellipsoids are drawn at the 10% probability level. Hydrogen atoms and solvent molecules are omitted for clarity.

Fig. 4 ORTEP diagram of complex 5 showing the atom numbering scheme. Thermal ellipsoids are drawn at the 10% probability level. Hydrogen atoms and solvent molecules are omitted for clarity.

Table 1 Crystallographic data for complexes (2 × 1)·toluene, 2, 4·toluene and (2 × 5)·THF

Compound	(2 × 1)·toluene	2	4·toluene	(2 × 5)·THF
Formula	C101H172N10O4Yb2	C46H78N5O2Yb	C55H94N5O3Si2Y	C94H172N10O5Si4Y2
Formula weight	1936.57	906.17	1018.44	1812.60
T (K)	223(2)	223(2)	223(2)	223(2)
Crystal system	Monoclinic	Triclinic	Monoclinic	Monoclinic
Crystal size (mm)	0.80 × 0.40 × 0.30	0.40 × 0.40 × 0.20	0.70 × 0.60 × 0.40	1.40 × 1.00 × 0.20
Space group	P21/c	P1̄	P21/c	P21/c
a (Å)	10.8249(7)	11.2280(7)	12.0592(16)	20.91(5)
b (Å)	15.1480(10)	11.8377(7)	16.234(2)	9.786(18)
c (Å)	31.913(2)	20.1349(18)	30.021(4)	28.19(9)
α (^o)	90	86.230(7)	90	90
β (^o)	98.688(2)	82.991(7)	90.787(4)	102.70(7)
γ (^o)	90	64.118(4)	90	90
V (Å^3)	5172.9(6)	2389.5(3)	5876.6(13)	5627(25)
Z	2	2	4	2
Dcalc. (g cm^−3)	1.243	1.259	1.151	1.070
μ (mm^−1)	1.848	1.995	1.076	1.115
F (000)	2040	950	2200	1960
θmax (^o)	27.49	27.49	27.48	27.49
Collected	29 123	22 816	32 030	26 518
Unique reflns	11 753	10 772	13 342	12 769
Obsd reflns [I > 2.0σ(I)]	10 165	9609	8780	4141
No. of variables	532	500	581	546
Goodness-of-fit	1.132	1.087	1.099	1.045
R	0.0575	0.0529	0.1220	0.1471
wR	0.1242	0.0992	0.3109	0.3242

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Table 2 Selected bond lengths (Å) and bond angles (^o) of complexes 1, 2, 4 and 5

Bond lengths	1	2	Bond lengths	4	Bond lengths	5
Yb1–O1	2.116(3)	2.092(3)	Y1–O1	2.157(5)	Y1–O1	2.117(8)
Yb1–O2	2.132(3)	2.105(3)	Y1–O2	2.141(4)	Y1–O2	2.093(9)
Yb1–N1	2.495(4)	2.513(3)	Y1–O3	2.397(5)	Y1–N1	2.538(10)
Yb1–N2	2.554(4)	2.553(4)	Y1–N1	2.514(5)	Y1–N2	2.574(9)
Yb1–N3	2.293(4)	2.282(4)	Y1–N3	2.396(6)	Y1–N3	2.312(9)
Yb1–N4	2.321(4)	2.338(4)	Y1–N4	2.406(5)	Y1–N4	2.418(10)

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Bond angles	1	2	Bond angles	4	Bond angles	5
N2–Yb1–N3	165.4(15)	163.3(12)	O1–Y1–O2	107.0(18)	O1–Y1–O2	105.4(3)
O1–Yb1–N1	78.6(14)	78.8(11)	O1–Y1–O3	88.7(18)	O1–Y1–N1	79.2(3)
O1–Yb1–N4	101.9(15)	100.3(13)	O2–Y1–O3	95.0(19)	O1–Y1–N2	95.3(3)
O2–Yb1–N1	78.9(13)	78.1(11)	O1–Y1–N1	77.6(18)	O1–Y1–N3	96.7(3)
O2–Yb1–N4	100.7(15)	103.1(13)	O1–Y1–N4	154.3(19)	O1–Y1–N4	150.7(3)
O1–Yb1–O2	157.0(14)	155.8(11)	O2–Y1–N1	92.5(18)	O2–Y1–N1	147.4(3)
N1–Yb1–N4	179.2(15)	177.3(12)	O2–Y1–N4	98.7(18)	N1–Y1–N2	70.8(3)
O1–Yb1–N2	90.4(15)	88.2(13)	O3–Y1–N1	165.9(18)	N1–Y1–N3	100.3(4)
N3–Yb1–N4	58.0(15)	58.2(13)	N1–Y1–N3	88.2(19)	N1–Y1–N4	93.6(3)
			N3–Y1–N4	55.6(19)	N3–Y1–N4	56.2(3)

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In complexes 1 and 2, the coordinating environments around the ytterbium ions are similar. Same as other rare-earth metal complexes stabilized by the same ligand L¹,^{10,17,22,24,38} the amino side-arm was also found to bind to the metal center in the solid state. The ytterbium centre is six-coordinated by two oxygen atoms and two nitrogen atoms from one amine-bridged bis(phenolate) ligand (L¹), and two nitrogen atoms from one guanidinate group. The coordination geometry can be described as distorted octahedron, in which O1, O2, N1, and N4 atoms occupy equatorial positions, and N2 and N3 atoms occupy axial positions. The N2–Yb–N3 angles are found to be 163 and 165°, respectively.

The yttrium ion in complex 4 is six-coordinated by two oxygen atoms and one nitrogen atom from the imidazolidine-bridged bis(phenolate) ligand (L²), two nitrogen atoms from the guanidinate group and one oxygen atom from one THF molecule. It is noteworthy that only one nitrogen atoms from the imidazolidine ring binds to the metal center in the solid state, which is different from the yttrium amide bearing the same ligand L².³²

In complex 5, the yttrium center is hexacoordinated by two oxygen atoms and two nitrogen atoms from the bis(phenolate) ligand L³, and two nitrogen atoms from the guanidinate group.

The average Y–O(Ar) bond lengths in 5 is 2.105 Å, which is slightly shorter than those in 4 (2.149 Å) and L¹-ligated yttrium guanidinates (2.128–2.162 Å).³⁸ The bond angles of O1–Y–O2 in 4 (107.0(18)°) and 5 (105.4(3)°) are much smaller than the average angle of 153.2° in L¹-ligated yttrium guanidinates. An obvious difference is also found for the values of the dihedral angle between the phenyl rings of the bis(phenolate) ligands, which are 84.5° for 4, and 81.8° for 5. Significantly larger values of 142.6–161.3° were found for L¹-ligated yttrium guanidinates, which suggests that the N-heterocycle bridge in 4 and the ethylenediamine bridge in 5 result in a more distorted geometry of the complexes. Such a difference might contribute to the different activity and selectivity of these complexes in initiating ROP.

Polymerization of rac-LA initiated by complexes 1–8

In general, all complexes 1–8 have been found to be active initiators for the controlled ROP of rac-LA at room temperature in THF. More than 70% yields were obtained within 1 h with monomer-to-initiator ratios of up to 1000 at 25 °C. The results are summarized in Table 3.

Table 3 Polymerization of rac-LA initiated by complexes 1–8^a

Entry	Initiator	[M]0/[I]0	Yield^b (%)	Mc^c ( × 10^4)	Mn^d ( × 10^4)	Đ^d	Pr^e
a General polymerization conditions: THF as the solvent, [rac-LA] = 1 mol L^−1, at 25 °C, reaction time 1 h.b Yield: weight of polymer obtained/weight of monomer used.c Mc = (144.13) × [M]0/[I]0 × (polymer yield) (%).d Measured by GPC calibrated with standard polystyrene samples.e Pr is the probability of racemic linkages between monomer units and is determined from the methine region of the homonuclear decoupled ^1H NMR spectrum.f In CH2Cl2.g In toluene.
1	1	500	99	7.1	8.9	1.16	0.99
2^f	1	500	67	4.8	6.9	1.25	0.78
3^g	1	500	99	7.1	8.6	1.23	0.75
4	1	300	99	4.3	5.2	1.15	0.98
5	1	700	99	10.0	11.5	1.12	0.98
6	1	1000	98	14.1	15.1	1.13	0.99
7	1	1200	98	16.9	17.8	1.11	0.98
8	1	1500	84	18.2	18.8	1.15	0.97
9	2	1000	97	14.0	13.2	1.08	0.99
10	2	1500	75	16.2	15.1	1.08	0.98
11	3	500	97	7.0	8.8	1.23	0.98
12	3	1000	90	13.0	16.5	1.21	0.97
13	4	500	83	6.0	8.5	1.68	0.62
14	4	1000	80	11.5	14.6	1.65	0.62
15	5	500	76	5.5	3.4	1.36	0.69
16	5	1000	70	10.1	12.0	1.35	0.69
17	6	500	95	6.9	4.2	1.44	0.51
18	6	1000	92	13.3	14.8	1.41	0.52
19	7	500	95	6.8	3.1	1.66	0.82
20	7	1000	91	13.1	15.3	1.57	0.82
21	8	500	94	6.7	3.3	1.58	0.82
22	8	1000	90	13.0	15.9	1.63	0.82

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Using complex 1 as the initiator, the ROP of rac-LA proceeds faster in THF and toluene than in CH₂Cl₂, and with higher stereoselectivities in THF than in toluene and CH₂Cl₂ (Table 3, entries 1–3). Overall, THF is the optimal solvent, which is consistent with reported results.²⁴ Almost quantitative yields were obtained with the ratio of monomer to complex 1 increasing from 300 to 1200 (Table 3, entries 4–7). A good yield of 84% was still obtained when the ratio was increased to 1500. Moreover, highly heterotactic PLA (P_r ≥ 0.97) was obtained. Complexes 1, 2 and 3 bearing the same bis(phenolate) ligand L¹ showed similar activities (Table 3, entries 6, 9 and 12), while complex 1 worked slightly better than the other two in the case of high [M]₀/[I]₀ ratios (Table 3, entries 7, 8, 10, and 12). Moreover, P_r values of the polymers are essentially the same. These findings are similar to results from lanthanide alkyls, aryloxides and oxides stabilized by the same phenolate ligand.^{17–19,22,24} Thus, neither the metal center nor the guanidinate group plays significant roles in influencing the polymerization rates or tacticity.

A comparative study of complexes 4–8 on the activity in initiating the stereoselective ROP of rac-LA under identical conditions revealed substantial differences (Table 3). Using complex 4 as the initiator, the yield is 83% and P_r is 0.62 when the ratio of monomer to initiator is 500 (Table 3, entry 13), which is similar to that of yttrium amide bearing the same imidazolidine-bridged bis(phenolate) ligand.^21,32 Complex 5 initiated ROP of rac-LA and afforded PLA with P_r value of 0.69 (Table 3, entries 15 and 16). The alkyl analogue of 5 (ref. 17) was reported to initiate ROP of rac-LA and afforded PLA of almost the same P_r value. Complex 6 effectively initiated rac-LA polymerization and produced PLA with P_r value of 0.51 (Table 3, entries 17 and 18). The stereoselectivity of 6 is similar to that of homopiperazine-bridged bis(phenolate) Ti(IV) complex.⁴² The methoxy-amine-bis(phenolate) yttrium complex 7 showed the same level of activity and selectivity (P_r = 0.82) as furan-amine-bis(phenolate) yttrium complexes 8 (Table 3, entries 19–22). Based on the findings discussed above, it is conclusive that the bis(phenolate) ligands exert crucial roles in influencing the tacticity. Overall, complexes stabilized by ligands with coordinating arms (1, 2, 3, 7, and 8) show higher stereoselectivity (P_r values of 0.82–0.99), while those carrying ligands of longer bridges (4, 5, and 6) show lower stereoselectivity (P_r values of 0.51–0.69). This finding emphasizes again the importance of bis(phenolate) ligands on tacticity.^{8f,8u,9,23–26}

All PLA obtained in the presence of complexes 1–8 showed relatively narrow molecular weight distributions in the range of M_w/M_n = 1.08–1.68. It is noteworthy that the average-number molecular weights (M_n) values of PLA obtained with complexes 1–3 are generally in good agreement with theoretical M_n values, which indicate that the polymerization proceeded in a living fashion without other significant side reactions. To illustrate this living character, the relationship between the number-average molecular weight (M_n) and the molar ratio of monomer to initiator 1 ([M]₀/[I]₀) was plotted in Fig. 5. As the monomer/initiator ratio increases, the molecular weight of the resulting polymer increased linearly, while molecular weight distributions were kept almost unchanged (1.12–1.16), strongly suggesting that the polymerization process is controllable.

Fig. 5 Polymerization of rac-LA initiated by complex 1 in THF at 25 °C. Relationship between the number-averaged molecular weight (M_n) and the molar ratio of monomer to initiator.

To investigate the mechanism of ROP of rac-LA initiated by these guanidinate complexes, oligomerization of rac-LA initiated by complex 1 in a [M]₀/[I]₀ ratio of 15 was carried out. However, no guanidinate or bis(phenolate) group was identified from the ¹H NMR spectrum of the oligomer. Considering the instability of the oligomer with the guanidinate end cap,³⁸ a new oligomer was prepared by quenching the oligomerization described above with benzyl alcohol. End group analysis by ¹H NMR spectroscopy clearly shows the existence of a benzyloxy group and a HOCH(CH₃)CO– group according to the resonances at about 5.18, 7.33 ppm assignable to the former and at 1.23, 2.57 and 4.34 ppm assignable to the latter, as shown in Fig. 6. The benzyloxy group is believed to come from the exchange reaction of benzyl alcohol with the guanidinate group. Meanwhile, no resonances corresponding to the bis(phenolate) ligand L¹ was observed in the ¹H NMR spectrum, which ruled out the possibility that the amine-bridged bis(phenolate) group is the initiating group in the polymerization process. These results revealed that the guanidinate group in these complexes acted as the initiating group in the ROP of rac-LA, while the bis(phenolate) group works as the spectator ligand controlling the stereoselectivity of the polymerization process.^24,25

Fig. 6 ¹H NMR spectrum of rac-LA oligomer initiated by complex 1 after quenching with benzyl alcohol.

ROP of rac-BBL initiated by complexes 1–8

ROP of BBL is generally much more challenging than that of LA despite of their larger ring strains,⁴³ and most of the catalysts show low stereoselectivity and a high prevalence of chain termination events, such as transesterification and chain transfer.^41,44 Complexes 1–8 proved efficient in initiating the ROP of rac-BBL, and are in general capable of controlling stereoselectivity to produce highly syndiotactic PHB in toluene. Representative results are summarized in Table 4.

Table 4 Polymerization of rac-BBL initiated by complexes 1–8^a

Entry	Initiator	[M]0/[I]0	t	Yield^b (%)	Mc^c (× 10^4)	Mn^d (× 10^4)	Đ^d	Pr^e
a General polymerization conditions: toluene as the solvent, [rac-BBL] = 2 mol L^−1, at 25 °C.b Yield: weight of polymer obtained/weight of monomer used.c Mc = 86.09 × [M]0/[I]0 × (polymer yield) (%).d Measured by GPC calibrated with standard polystyrene samples.e Pr is the probability of racemic linkages between monomer units and is determined from the carbonyl region of the ^13C{^1H} NMR spectroscopy at 25 °C.f In THF.g [rac-BBL] = 1 mol L^−1.
1	1	200	2 min	100	1.7	2.5	1.35	0.82
2	1	400	10 min	98	3.4	3.5	1.33	0.82
3	1	600	10 min	98	5.1	6.5	1.31	0.82
4	1	800	10 min	98	6.7	8.2	1.36	0.82
5	1	1000	10 min	96	8.3	9.7	1.42	0.82
6	1	1500	30 min	88	11.4	13.5	1.37	0.82
7^f	1	400	30 min	60	2.1	3.0	1.35	0.67
8^g	1	400	10 min	83	2.9	3.3	1.30	0.82
9	2	400	10 min	100	3.4	4.2	1.29	0.82
10	2	1000	10 min	92	7.9	9.7	1.44	0.82
11	3	200	1 h	97	1.7	2.5	1.35	0.82
12	3	400	1 h	80	2.4	3.1	1.31	0.82
13	4	200	10 h	70	1.2	2.3	1.78	0.58
14	4	400	10 h	55	1.9	3.0	1.86	0.58
15	5	200	10 h	16	0.3	—	—	—
16	6	200	10 h	96	1.7	1.8	1.83	0.57
17	6	400	10 h	85	2.9	3.5	1.78	0.57
18	7	200	10 h	96	1.7	1.7	1.86	0.81
19	7	400	10 h	75	2.6	3.3	1.80	0.81
20	8	200	10 h	63	1.1	2.3	1.78	0.79
21	8	400	10 h	56	1.9	3.2	1.82	0.79

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A full conversion of rac-BBL was achieved within 2 min in the ratio of [M]₀/[I]₀ = 200 (Table 4, entry 1) in toluene at 25 °C. Moreover, highly syndiotactic PHB (P_r = 0.82) was obtained. Increasing the monomer to initiator ratio to 1000 still led to almost full conversion within 10 min (Table 4, entries 2–5). A lower of yield of 88% was obtained when the monomer to initiator ratio was raised to 1500 (Table 4, entry 6). The solvent plays a significant role in influencing the activity and stereoselectivity, as the ROP proceeded much slower in THF and afforded PHB of lower P_r value (Table 4, entries 2 and 7). Decreasing the monomer concentration also slowed down the polymerization rate (Table 4, entries 2 and 8). These findings are consistent with those observed in the amino-bridged bis(phenolate) lanthanide systems.^14,15,24

Complex 2 showed similar activities and stereoselective control in initiating the ROP of rac-BBL (Table 4, entries 9 and 10). Both ytterbium complexes 1 and 2 are more active than the yttrium complex 3 (Table 4, entries 11 and 12), which may be attributed to the rare-earth metal centres of different ionic radii. However, the stereoselectivity is not at all influenced, which is quite different from that observed in rac-LA polymerization (vide supra).

The activities and stereoselective control of complexes 4–8 in initiating the ROP of rac-BBL were examined and compared under identical conditions. Complex 4 bearing an imidazolidine-bridged bis(phenolate) ligand is less active than complex 3, and the P_r value of the resulting PHB is much lower (Table 4, entries 13 and 14). The activity of complex 5 (Table 4, entry 15) is the lowest among complexes 1–8, and is obviously lower than that of the analogous yttrium amide.⁹ Complex 6 bearing a homopiperazine-bridged bis(phenolate) ligand showed higher activity than that of 4, while resulting PHB have almost the same P_r values (Table 4, entries 16 and 17). The methoxy-amine-bis(phenolate) complex 7 was found to be more active than the furan-amine-bis(phenolate) complex 8, and these two complexes show similar stereoselective control (P_r = 0.79–0.81). The same influence exerted by bis(phenolate) ligands on stereoselectivity has been observed as that for polymerization of rac-lactide (vide supra).

It was found that the PHB obtained with complexes 1–3 had relatively narrow molecular weight distributions (1.13 < M_w/M_n < 1.44) and average-number molecular weights (M_n) are generally in good agreement with the theoretical M_n values. For the ROP of rac-BBL initiated by complex 1, the number-average molecular masses (M_n) values of the resultant PHB increases as the monomer-to-initiator ratio increases, while the values of M_w/M_n were kept almost unchanged, indicating that the polymerization process is also controllable (Table 4, entries 1–5). However, the PHB produced with 4–8 in toluene had relatively broad molecular weight distributions (M_w/M_n = 1.31–1.86), which may be mainly attributed to the transesterification reactions during the polymerization process or to the relatively slow initiation step.

The ROP mechanism of rac-BBL was elucidated by using the method similar to that of rac-LA. The oligomer of rac-BBL was prepared by the reaction of complex 1 with rac-BBL in a 1 : 20 molar ratio which is quenched by benzyl alcohol. In addition to the resonances characteristic for the HOCH(CH₃)CH₂CO– group at 4.19, 2.30 and 1.38 ppm, respectively, the ¹H NMR spectrum in CDCl₃ clearly showed the existence of the benzyloxy group according to the resonances at about 7.34 and 5.12 ppm (Fig. 7), which arises from the exchange reaction between benzyl alcohol with the guanidinate group. Meanwhile, additional resonances at 6.95 and 5.81 ppm were observed, which may correspond to trans-crotonate groups that may result from the elimination side reactions in the polymerization progress. A similar elimination reaction in rac-BBL polymerization has been observed using amino-bridged bis(phenolate)lanthanide alkoxo complexes as the initiators.²⁴

Fig. 7 ¹H NMR spectrum of rac-BBL oligomer initiated by complex 1 after quenching with benzyl alcohol.

Conclusion

In summary, eight rare-earth metal guanidinates supported by a series of amine-bridged bis(phenolate) ligands were synthesized according to either metathesis reactions, or insertion reactions of carbodiimides. All eight complexes have been fully characterized, and the solid state structures of four of them have been determined. A comparative study on these complexes in initiating the ROP of rac-LA and rac-BBL revealed that bis(phenolate) ligands play critical roles in governing the stereoselectivity. ROP initiated by complexes 1–3 bearing the amine-bridged bis(phenolate) ligand L¹ afforded highly heterotactic PLA (P_r values of more than 0.97) and syndiotactic PHB (P_r values of 0.82). End group analysis of the oligomer revealed that in the ROP of rac-LA and rac-BBL the guanidinate groups serve as initiating groups.

Experimental

Reagents and general procedures

All manipulations requiring dry atmosphere were performed under a purified argon atmosphere by use of standard Schlenk techniques or in a glovebox. Solvents were degassed and distilled from sodium benzophenone ketyl under argon prior to use. Ligands L¹H₂–L⁶H₂,³⁸ lanthanide precursors L¹YbCl(THF)^30,45 and L¹Y[N(SiHMe₂)₂](THF) to L⁶Y[N(SiHMe₂)₂](THF)⁹ were prepared by modifications of the methods reported in the literature. Benzyl alcohol were dried over 4 Å molecular sieves for 1 week and then distilled before use. rac-Lactide (rac-LA) was recrystallized twice from dry toluene and then sublimed under vacuum at 50 °C. rac-β-Butyrolactone (rac-BBL) was freshly distilled from CaH₂ under nitrogen and degassed thoroughly by freeze–pump–thaw cycles prior to use.

Lanthanide analyses were performed by ethylenediaminetetraacetic acid titration with a xylenol orange indicator and a hexamine buffer. Carbon, hydrogen, and nitrogen analyses were performed by direct combustion with a Carlo-Erba EA-1110 instrument. The IR spectra were recorded with a Nicolet-550 Fourier transform IR spectrometer as KBr pellets. The ¹H and ¹³C NMR spectra were recorded in a C₆D₆ solution for complexes 3–6 with a Unity Varian spectrometer. Molecular weights and molecular weight distributions were determined against polystyrene standards by gel permeation chromatography (GPC) on a PL 50 apparatus, and THF was used as an eluent at a flow rate of 1.0 mL min⁻¹ at 40 °C. The microstructures of PLAs and PHBs were measured by homodecoupling ¹H NMR spectroscopy at 20 °C in CDCl₃ and by ¹³C{¹H} NMR spectroscopy at 40 °C in CDCl₃, respectively, on a Unity Varian AC-400 spectrometer.

Synthesis of L¹Yb[ⁱPr₂NC(NⁱPr)₂] (1). A Schlenk flask was charged with HNⁱPr₂ (0.38 g, 3.8 mmol), THF (20 mL), and a stirring bar. The solution was cooled to 0 °C, and n-BuLi (1.9 mL, 3.8 mmol, 2 M in hexane) was added by syringe. The solution was stirred for 1 h at 0 °C, and then N,N′-diisopropylcarbodiimide (DIC) (0.48 g, 3.8 mmol) was added. The resulting solution was slowly warmed to room temperature, and stirred for 1 h, and then was added slowly to L¹YbCl(THF) (3.05 g, 3.8 mmol) in 20 mL of THF. The mixture was stirred overnight at room temperature, and the solvent was removed in a vacuum. The residue was extracted with toluene, and LiCl was removed by centrifugation. Yellow crystals were obtained from toluene solution at room temperature in a few days (1.79 g, 51%). Anal. calc. for C₄₇H₈₂N₅O₂Yb (922.23): C, 61.21; H, 8.96; N, 7.59; Yb, 18.76. Found: C, 60.98; H, 9.11; N, 7.77; Yb, 18.66. IR (KBr, cm⁻¹): 2961 (s), 2898 (s), 2867 (s), 2373 (w), 1629 (s), 1561 (m), 1478 (s), 1414 (m), 1360 (m), 1309 (s), 1255 (m), 1237 (m), 1202 (m), 1166 (m), 1134 (m), 1027 (m), 876 (m), 838 (m), 744 (m), 694 (m), 528 (m), 446 (m).

Synthesis of L¹Yb[(CH₂)₅NC(NⁱPr)₂] (2). Following the procedure for the synthesis of complex 1, Li[(CH₂)₅NC(NⁱPr)₂] (3.8 mmol), which was formed in situ by the reaction of LiN(CH₂)₅ with DIC, reacted with L¹YbCl(THF) (2.81 g, 3.5 mmol) in THF (40 mL) afforded the final product as yellow crystals after crystallization in toluene solution (1.84 g, 58%). Anal. calc. for C₄₆H₇₈N₅O₂Yb (906.18): C, 60.97; H, 8.68; N, 7.73; Yb, 19.10. Found: C, 60.72; H, 8.86; N, 7.88; Yb, 18.99. IR (KBr, cm⁻¹): 2952 (s), 2864 (s), 2361 (w), 1630 (s), 1599 (m), 1513(m), 1478 (s), 1414 (m), 1360 (m), 1308 (m), 1257 (m), 1237 (m), 1202 (m), 1166 (m), 1131 (m), 1085 (m), 1028 (m), 875 (m), 838 (m), 744 (m), 529 (m), 446 (m).

Synthesis of L¹Y[(SiHMe₂)₂NC(NⁱPr)₂] (3). Reatment of the homoleptic Y[N(SiHMe₂)₂]₃(THF)₂ with 1 equiv of L¹H₂ afforded the corresponding bridged bis(phenolate) yttrium amide L¹Y[N(SiHMe₂)₂](THF). A solution of DIC (0.15 g, 1.2 mmol) in hexane (5 mL) was added by syringe at room temperature to a stirred solution of L¹Y[N(SiHMe₂)₂](THF) (0.98 g, 1.2 mmol) in hexane (20 mL). The reaction mixture was stirred for 12 h at room temperature. The solvent was removed in a vacuum, and a hexane (4 mL)/toluene (1 mL) solution was added. The resulting solution was kept at room temperature for crystallization, and colorless crystals were obtained in a few days (0.63 g, 60%). Anal. calc. for C₄₅H₈₂N₅O₂Si₂Y (870.24): C, 62.11; H, 9.50; N, 8.05; Y, 10.22. Found: C, 61.75; H, 9.75; N, 7.86; Y, 10.05. IR (KBr, cm⁻¹): 2956 (s), 2906 (s), 2122 (w), 1622 (s), 1551 (m), 1471 (s), 1375 (m), 1307 (m), 1248 (m), 1166 (m), 1133 (m), 1050 (m), 1028 (m), 940 (m), 912 (m), 838 (m), 805 (m), 779 (m), 744 (m), 528 (m), 446 (m). ¹H NMR (400 MHz, C₆D₆, 25 °C): δ 7.60 (s, 2H, ArH), 7.11 (s, 2H, ArH), 4.89 (m, 2H, N(SiHMe₂)₂), 4.26 (br, 2H, ArCH₂N), 3.72 (m, 2H, CH(CH₃)₂), 2.99 (br, 2H, ArCH₂N), 2.39 (br, 2H, NCH₂CH₂N), 2.08 (br, 2H, NCH₂CH₂N), 1.84 (br, 6H, N(CH₃)₂), 1.73 (s, 18H, C(CH₃)₃), 1.47 (s, 18H, C(CH₃)₃), 1.31–1.26 (m 12H, CH(CH₃)₂), 0.35 (m, 12H, N(SiHMe₂)₂). ¹³C{¹H} NMR (100 MHz, C₆D₆, 25 °C): δ 167.6 (CN₃), 162.4, 136.4, 135.7, 125.8, 124.8, 124.4(Ar-C), 65.3 (ArCH₂N), 59.8 (N(CH₂)₂N), 48.4 (N(CH₃)₂), 46.5 (CH(CH₃)₂), 35.6 (C(CH₃)₃), 34.3 (C(CH₃)₃), 27.6 (CH(CH₃)₂), 23.7 (CH(CH₃)₂), 2.6 (NSiHMe₂).

Synthesis of L²Y[(SiHMe₂)₂NC(NⁱPr)₂](THF) (4). Following the procedure for the synthesis of complex 3, L²Y[N(SiHMe₂)₂](THF) (0.96 g, 1.2 mmol), which was formed by the reaction of L²H₂ with Y[N(SiHMe₂)₂]₃(THF)₂, reacted with DIC in hexane (20 mL), affording the final product as colorless crystals after crystallization in toluene/hexane solution (0.67 g, 65%). Anal. calc. for C₄₈H₈₆N₅O₃Si₂Y (926.30): C, 62.24; H, 9.36; N, 7.56; Y, 9.60. Found: C, 61.99; H, 9.55; N, 7.73; Y, 9.85. IR (KBr, cm⁻¹): 2956 (s), 2915 (m), 2869 (m), 2210 (m), 2109 (m), 1614 (s), 1473 (s), 1363 (m), 1305 (s), 1241 (s), 1165 (m), 1129 (m), 1016 (w), 915 (m), 835 (m), 792 (w), 646 (w), 527 (w), 407 (w). ¹H NMR (400 MHz, C₆D₆, 25 °C): δ 7.44 (s, 2H, ArH), 6.80 (s, 2H, ArH), 4.89 (m, 2H, N(SiHMe₂)₂), 4.23 (d, ²J_HH = 14.1 Hz, 2H, ArCH₂), 3.72 (m, 2H, CH(CH₃)₂), 3.70 (br. s, 4H, α-CH₂ THF), 3.61 (d, ²J_H,H = 7.5 Hz, 1H, NCH₂N), 3.54 (br. s, m, 2H, NCH₂CH₂N), 2.85 (d, ²J_HH = 14.1 Hz, 2H, ArCH₂), 2.1 (d, ²J_HH = 7.4 Hz, 1H, NCH₂N), 1.90 (br. m, 2H, NCH₂CH₂N), 1.56 (s, 18H, C(CH₃)₃), 1.41 (s, 18H, C(CH₃)₃), 1.35 (br. s, 4H, β-CH₂ THF), 1.31–1.26 (m 12H, CH(CH₃)₂), 0.35 (m, 12H, N(SiHMe₂)₂). ¹³C{¹H} NMR (100 MHz, C₆D₆, 25 °C): δ 168.3 (CN₃), 160.6, 137.2, 136.7, 124.3, 123.9, 121.8 (Ar-C), 71.3 (NCH₂N), 69.0 (α-CH₂ THF), 56.9 (N(CH₂)₂N), 51.8 (ArCH₂N), 48.4 (CH(CH₃)₂), 35.7 (C(CH₃)₃), 35.6 (C(CH₃)₃), 34.1 (C(CH₃)₃), 32.1 (C(CH₃)₃), 25.23 (β-CH₂ THF), 27.6 (CH(CH₃)₂), 23.6 (CH(CH₃)₂), 2.6 (NSiHMe₂).

Synthesis of L³Y[(SiHMe₂)₂NC(NⁱPr)₂] (5). Following the procedure for the synthesis of complex 3, L³Y[N(SiHMe₂)₂](THF) (0.98 g, 1.2 mmol), which was formed by the reaction of L³H₂ with Y[N(SiHMe₂)₂]₃(THF)₂, reacted with DIC in hexane (20 mL), affording the final product as colorless crystals after crystallization in hexane solution (0.63 g, 60%). Anal. calc. for C₄₅H₈₂N₅O₂Si₂Y (870.24): C, 62.11; H, 9.50; N, 8.05; Y, 10.22. Found: C, 61.86; H, 9.68; N, 8.33; Y, 10.12. IR (KBr, cm⁻¹): 2958 (s), 2906 (m), 2869 (m), 2124 (w), 1620 (s), 1554 (m), 1472 (s), 1373 (m), 1310 (s), 1247 (m), 1171 (w), 1128 (w), 1073 (w), 1014 (m), 913 (m), 836 (m), 802 (m), 740 (w), 529 (m), 444 (m). ¹H NMR (400 MHz, C₆D₆, 25 °C): δ 7.63 (s, 2H, ArH), 6.986.93 (m, 2H, ArH), 5.56, 5.22, 4.89 (m, 2H, N(SiHMe₂)₂), 4.70, 4.06 (m, 2H, NCH₂Ar), 3.45, 3.32 (m, 2H, CH(CH₃)₂), 3.25, 2.53 (m, 2H, NCH₂Ar), 2.99, 2.81 (t, 4H, NCH₂CH₂N), 2.15 (s, 3H, NCH₃), 1.93 (s, 3H, NCH₃), 1.90, 1.80 (s, 18H, C(CH₃)₃), 1.46, 1.41 (s, 18H, C(CH₃)₃), 1.11, 0.84, 0.70 (m, 12H, CH(CH₃)₂), 0.47–0.14 (m, 12H, N(SiHMe₂)₂). ¹³C{¹H} NMR (100 MHz, C₆D₆, 25 °C): δ 171.0 (CN₃), 161.5, 160.8, 158.5, 136.8, 126.3, 125.4, 125.3, 124.9, 124.1, 123.8, 123.5, 117.0 (Ar-C), 70.8 (α-CH₂ THF), 65.0 (ArCH₂N), 56.1 (ArCH₂N), 51.3 (N(CH₂)₂N), 46.6 (N(CH₂)₂N), 46.5 (CH(CH₃)₂), 42.3 (NCH₃), 35.6 (C(CH₃)₃), 34.2 (C(CH₃)₃), 32.2 (C(CH₃)₃), 32.0 (C(CH₃)₃), 30.9 (CH(CH₃)₂), 30.3 (CH(CH₃)₂), 26.5 (β-CH₂ THF), 2.5 (NSiHMe₂).

Synthesis of L⁴Y[(SiHMe₂)₂NC(NⁱPr)₂] (6). Following the procedure for the synthesis of complex 3, L⁴Y[N(SiHMe₂)₂](THF) (0.99 g, 1.2 mmol), which was formed by the reaction of L⁴H₂ with Y[N(SiHMe₂)₂]₃(THF)₂, reacted with DIC in hexane (20 mL), affording the final product as colorless crystals after crystallization in hexane solution (0.72 g, 68%). Anal. calc. for C₄₆H₈₂N₅O₂Si₂Y (882.25): C, 62.62; H, 9.37; N, 7.94; Y, 10.08. Found: C, 62.35; H, 9.58; N, 7.83; Y, 10.32. IR (KBr, cm⁻¹): 2957 (s), 2906 (m), 2872 (m), 2119 (w), 1623 (s), 1556 (m), 1472 (s), 1362 (m), 1300 (m), 1244 (s), 1167 (m), 1092 (m), 993 (m), 940 (m), 797 (s), 659 (w), 519 (w). ¹H NMR (400 MHz, C₆D₆, 25 °C): δ 7.54 (s, 2H, ArH), 6.88 (s, 2H, ArH), 4.89 (m, 2H, N(SiHMe₂)₂), 4.27 (d, ²J_HH = 12.6 Hz, 2H, ArCHHN), 3.92 (m, 2H, CH(CH₃)₂), 3.39, 2.86 (m, 4H, NCH₂CH₂N), 2.68 (d, ²J_HH = 12.6 Hz, 2H, ArCHHN), 1.85, (m, 4H, NCH₂CH₂CH₂N) 1.66 (s, 18H, C(CH₃)₃), 1.43 (s, 18H, C(CH₃)₃), 1.36–1.21 (m, 12H, CH(CH₃)₂), 1.11 (m, 2H, NCH₂CH₂CH₂N), 0.40–0.28 (m, 12H, N(SiHMe₂)₂). ¹³C{¹H} NMR (100 MHz, C₆D₆, 25 °C): δ 169.3 (CN₃), 157.6, 137.5, 127.9, 127.8, 122.3, 118.5 (Ar-C), 63.2 (ArCH₂N), 52.1 (NCH₂CH₂N), 49.4 (NCH₂CH₂CH₂N), 48.2 (CH(CH₃)₂), 40.0 (NCH₂CH₂CH₂N), 35.1 (C(CH₃)₃), 34.8 (C(CH₃)₃), 32.2 (C(CH₃)₃), 31.5 (C(CH₃)₃), 31.3 (CH(CH₃)₂), 29.8(CH(CH₃)₂), 2.6 (NSiHMe₂).

Synthesis of L⁵Y[(SiHMe₂)₂NC(NⁱPr)₂] (7). Following the procedure for the synthesis of complex 3, L⁵Y[N(SiHMe₂)₂](THF) (0.96 g, 1.2 mmol), which was formed by the reaction of L⁵H₂ with Y[N(SiHMe₂)₂]₃(THF)₂, reacted with DIC in hexane (20 mL), affording the final product as colorless crystals after crystallization in hexane solution (0.65 g, 63%). Anal. calc. for C₄₄H₇₉N₄O₃Si₂Y (857.2): C, 61.65; H, 9.29; N, 6.54; Y, 10.37. Found: C, 61.35; H, 9.38; N, 6.83; Y, 10.13. IR (KBr, cm⁻¹): 2956 (s), 2906 (m), 2863 (m), 2122 (w), 1622 (s), 1551 (m), 1471 (s) 1310 (s), 1248 (m), 1167 (m), 1077(s), 1028 (w), 913 (m), 834 (m), 803 (m), 742 (m), 528 (m), 452 (w). ¹H NMR (400 MHz, C₆D₆, 25 °C): δ 7.56 (s, 2H, ArH), 6.97 (m, 2H, ArH), 4.80 (m, 2H, N(SiHMe₂)₂), 3.84 (m, 2H, CH(CH₃)₂), 3.59, 3.26 (m, 4H, NCH₂Ar), 2.98 (s, 3H, OCH₃), 2.76, 2.37 (m, 4H, NCH₂CH₂O), 1.74 (s, 18H, C(CH₃)₃), 1.43 (s, 18H, C(CH₃)₃), 1.30 (m, 12H, CH(CH₃)₂), 0.35 (d, 12H, N(SiHMe₂)₂). ¹³C{¹H} NMR (100 MHz, C₆D₆, 25 °C): δ 168.5 (CN₃), 161.4, 136.5, 125.4, 124.1, 123.9, 121.5 (Ar-C), 73.1 (NCH₂CH₂O), 64.5 (ArCH₂N), 60.4 (OCH₃), 49.6 (NCH₂CH₂O), 48.4 (CH(CH₃)₂), 35.5 (C(CH₃)₃), 34.6 (C(CH₃)₃), 32.8 (C(CH₃)₃), 32.1 (C(CH₃)₃), 28.6 (CH(CH₃)₂), 27.6 (CH(CH₃)₂), 2.6 (NSiHMe₂).

Synthesis of L⁶Y[(SiHMe₂)₂NC(NⁱPr)₂] (8). Following the procedure for the synthesis of complex 3, L⁶Y[N(SiHMe₂)₂](THF) (0.99 g, 1.2 mmol), which was formed by the reaction of L⁶H₂ with Y[N(SiHMe₂)₂]₃(THF)₂, reacted with DIC in hexane (20 mL), affording the final product as colorless crystals after crystallization in hexane solution (0.70 g, 66%). Anal. calc. for C₄₆H₈₁N₄O₃Si₂Y (883.24): C, 62.55; H, 9.24; N, 6.34; Y, 10.07. Found: C, 61.33; H, 9.18; N, 6.37; Y, 10.32. IR (KBr, cm⁻¹): 2956 (s), 2902 (m), 2122 (m), 1624 (s), 1548 (m), 1473 (s), 1373 (m), 1310 (s), 1246 (m), 1168 (w), 1129 (w), 1022 (m), 912 (m), 837 (m), 743 (m), 526 (m), 447 (m). ¹H NMR (400 MHz, C₆D₆, 25 °C): δ 7.58, 7.54 (s, 2H, ArH), 7.04 (m, 2H, ArH), 4.86 (m, 2H, N(SiHMe₂)₂), 4.02 (m, 1H, CHO), 3.88 (m. 4H, NCH₂Ar), 3.72 (m, 2H, CH(CH₃)₂), 3.50 (m, 2H, OCH₂CH₂), 2.43, 2,08 (m, 2H, NCH₂CHO), 1.75 (s, 18H, C(CH₃)₃), 1.44 (s, 9H, C(CH₃)₃), 1.42 (s, 9H, C(CH₃)₃), 1.31–1.26 (m 12H, CH(CH₃)₂), 1.10 (m, 4H, OCH₂CH₂CH₂CH), 0.40 (d, 12H, N(SiHMe₂)₂). ¹³C{¹H} NMR (100 MHz, C₆D₆, 25 °C): δ 170.2 (CN₃), 162.1, 135.6, 131.6, 130.6, 124.5, 118.1 (Ar-C), 79.6 (CHO), 67.4 (CH₂O), 64.6 (NCH₂CHO), 56.6 (ArCH₂N), 54.5 (NCH₂CH₂N), 46.8 (CH(CH₃)₂), 35.3 (C(CH₃)₃), 31.3 (C(CH₃)₃), 30.2 (CH(CH₃)₂), 28.6 (CH(CH₃)₂), 28.1 (THF-CH₂), 25.2 (THF-CH₂), 2.5 (NSiHMe₂).

General polymerization procedure

The procedures for polymerization of rac-LA and rac-BBL were similar and given below:

A 20 mL vial, equipped with a magnetic stirring bar, was charged in a glovebox with the desired amount of monomer and solvent. After the monomer was dissolved, a solution of the initiator was added to this solution via a syringe. The mixture was immediately stirred at the desired temperature for the desired time. The reaction was quenched by the addition of ethanol and then poured into ethanol to precipitate the polymer, which was dried under vacuum to constant weight.

Oligomer preparation

Oligomerization of rac-LA and rac-BBL was carried out with complex 1 as the initiator in THF and toluene, respectively, at 25 °C under the condition of a molar ratio of [monomer]/[initiator] of 15 or 20. The polymerization mixture was stirred for 0.5 h and then quenched by adding benzyl alcohol. The precipitated oligomers were collected, dried under vacuum, and used for ¹H NMR measurement.

X-ray crystallographic structure determinations

Suitable single crystals of complexes 1, 2, 4 and 5 were sealed in a thin-walled glass capillary for determination of the single-crystal structures. Intensity data were collected with a Rigaku Mercury CCD area detector in ω scan mode using Mo Kα radiation (λ = 0.71070 Å). The diffracted intensities were corrected for Lorentz/polarization effects and empirical absorption corrections.

The structures were solved by direct methods and refined by full-matrix least-squares procedures based on |F|². The hydrogen atoms in these complexes were generated geometrically, assigned appropriate isotropic thermal parameters, and allowed to ride on their parent carbon atoms. All of the hydrogen atoms were held stationary and included in the structure factor calculation in the final stage of full matrix least-squares refinement. The structures were solved and refined using SHELXTL-97 programs.

Acknowledgements

Financial support from the National Natural Science Foundation of China (Grants 21174095, 21132002, 21172165, 21372172 and 21402135), PAPD, the Major Research Project of the Natural Science Foundation of the Jiangsu Higher Education Institutions (Project 14KJA150007),and the Qing Lan Project is gratefully acknowledged.

Notes and references

1. O. Dechy-Cabaret, B. Martin-Vaca and D. Bourissou, Chem. Rev., 2004, 104, 6147–6176.
2. S. Mecking, Angew. Chem., Int. Ed., 2004, 43, 1078–1085.
3. A. P. Dove, Chem. Commun., 2008, 48, 6446–6470.
4. R. H. Platel, L. M. Hodgson and C. K. Williams, Polym. Rev., 2008, 48, 11–63.
5. S. Inkinen, M. Hakkarainen, A.-C. Albertsson and A. Södergård, Biomacromolecules, 2011, 12, 523–532.
6. J. Yang, R. van Lith, K. Baler, R. A. Hoshi and G. A. Ameer, Biomacromolecules, 2014, 15, 3942–3952.
7. B. J. O'Keefe, M. A. Hillmyer and W. B. Tolman, J. Chem. Soc., Dalton Trans., 2001, 2215–2224.
8. (a) T. M. Ovitt and G. W. Coates, J. Am. Chem. Soc., 2002, 124, 1316–1326; (b) A. Amgoune, C. M. Thomas and J.-F. Carpentier, Macromol. Rapid Commun., 2007, 28, 693–697; (c) N. Ajellal, D. M. Lyubov, M. A. Sinenkov, G. K. Fukin, A. V. Cherkasov, C. M. Thomas, J.-F. Carpentier and A. A. Trifonov, Chem.–Eur. J., 2008, 14, 5440–5448; (d) M. Bouyahyi, T. Roisnel and J.-F. Carpentier, Organometallics, 2010, 29, 491–500; (e) E. Grunova, E. Kirillov, T. Roisnel and J.-F. Carpentier, Dalton Trans., 2010, 6739–6752; (f) M. Bouyahyi, N. Ajellal, E. Kirillov, C. M. Thomas and J.-F. Carpentier, Chem.–Eur. J., 2011, 17, 1872–1883; (g) Y. Sarazin, B. Liu, T. Roisnel, L. Maron and J.-F. Carpentier, J. Am. Chem. Soc., 2011, 133, 9069–9087; (h) H. Y. Ma and J. Okuda, Macromolecules, 2005, 38, 2665–2673; (i) H. Ma, T. P. Spaniol and J. Okuda, Angew. Chem., Int. Ed., 2006, 45, 7818–7821; (j) H. Ma, T. P. Spaniol and J. Okuda, Inorg. Chem., 2008, 47, 3328–3339; (k) B. Lian, H. Ma, T. P. Spaniol and J. Okuda, Dalton Trans., 2009, 9033–9042; (l) J. C. Buffet, A. Kapelski and J. Okuda, Macromolecules, 2010, 43, 10201–10203; (m) A. Stopper, J. Okuda and M. Kol, Macromolecules, 2012, 45, 698–704; (n) H. E. Dyer, S. Huijser, A. D. Schwarz, C. Wang, R. Duchateau and P. Mountford, Dalton Trans., 2008, 32–35; (o) L. Clark, M. G. Cushion, H. E. Dyer, A. D. Schwarz, R. Duchateau and P. Mountford, Chem. Commun., 2010, 46, 273–275; (p) H. E. Dyer, S. Huijser, N. Susperregui, F. Bonnet, A. D. Schwarz, R. Duchateau, L. Maron and P. Mountford, Organometallics, 2010, 29, 3602–3621; (q) T.-P.-A. Cao, A. Buchard, X. F. Le Goff, A. Auffrant and C. K. Williams, Inorg. Chem., 2012, 51, 2157–2169; (r) C. Bakewell, R. H. Platel, S. K. Cary, S. M. Hubbard, J. M. Roaf, A. C. Levine, A. J. P. White, N. J. Long, M. Haaf and C. K. Williams, Organometallics, 2012, 31, 4729–4736; (s) E. M. Broderick, P. S. Thuy-Boun, N. Guo, C. S. Vogel, J. Sutter, J. T. Miller, K. Meyer and P. L. Diaconescu, Inorg. Chem., 2011, 50, 2870–2877; (t) E. M. Broderick, N. Guo, C. S. Vogel, C. L. Xu, J. Sutter, J. T. Miller, K. Meyer, P. Mehrkhodavandi and P. L. Diaconescu, J. Am. Chem. Soc., 2011, 133, 9278–9281; (u) A. Amgoune, C. M. Thomas, T. Roisnel and J.-F. Carpentier, Chem.–Eur. J., 2006, 12, 169–179.
9. J. W. Kramer, D. S. Treitler, E. W. Dunn, P. M. Castro, T. Roisnel, C. M. Thomas and G. W. Coates, J. Am. Chem. Soc., 2009, 131, 16042–16044.
10. C. X. Cai, L. Toupet, C. W. Lehmann and J.-F. Carpentier, J. Organomet. Chem., 2003, 683, 131–136.
11. Y. Chapurina, J. Klitzke, O. D. L. Casagrande Jr, M. Awada, V. Dorcet, E. Kirillov and J.-F. Carpentier, Dalton Trans., 2014, 14322–14333.
12. J. S. Klitzke, T. Roisnel, E. Kirillov, O. D. L. Casagrande Jr and J.-F. Carpentier, Organometallics, 2014, 33, 309–321.
13. C. X. Cai, A. Amgoune, C. W. Lehmann and J.-F. Carpentier, Chem. Commun., 2004, 330–331.
14. A. Amgoune, C. M. Thomas, S. Ilinca, T. Roisnel and J.-F. Carpentier, Angew. Chem., Int. Ed., 2006, 45, 2782–2784.
15. N. Ajellal, M. Bouyahyi, A. Amgoune, C. M. Thomas, A. Bondon, I. Pillin, Y. Grohens and J.-F. Carpentier, Macromolecules, 2009, 42, 987–993.
16. F. Bonnet, A. R. Cowley and P. Mountford, Inorg. Chem., 2005, 44, 9046–9055.
17. X. Liu, X. Shang, T. Tang, N. Hu, F. Pei, D. Cui, X. Chen and X. Jing, Organometallics, 2007, 26, 2747–2757.
18. W. Zhao, C. Li, B. Liu, X. Wang, P. Li, Y. Wang, C. Wu, C. Yao, T. Tang, X. Liu and D. Cui, Macromolecules, 2014, 47, 5586–5594.
19. W. Zhao, D. Cui, X. Liu and X. Chen, Macromolecules, 2010, 43, 6678–6684.
20. W. Zhao, Y. Wang, X. Liu and D. Cui, Chem. Commun., 2012, 48, 4483–4485.
21. Y. Luo, W. Li, D. Lin, Y. Yao, Y. Zhang and Q. Shen, Organometallics, 2010, 29, 3507–3514.
22. K. Nie, X. Gu, Y. Yao, Y. Zhang and Q. Shen, Dalton Trans., 2010, 6832–6840.
23. W. Li, Z. Zhang, Y. Yao, Y. Zhang and Q. Shen, Organometallics, 2012, 31, 3499–3511.
24. K. Nie, L. Fang, Y. Yao, Y. Zhang, Q. Shen and Y. Wang, Inorg. Chem., 2012, 51, 11133–11143.
25. K. Nie, T. Feng, F. Song, Y. Zhang, H. Sun, D. Yuan, Y. Yao and Q. Shen, Sci. China: Chem., 2014, 57, 1106–1116.
26. (a) S. Yang, K. Nie, Y. Zhang, M. Xue, Y. Yao and Q. Shen, Inorg. Chem., 2013, 53, 105–115; (b) S. Yang, Z. Du, Y. Zhang and Q. Shen, Chem. Commun., 2012, 48, 9780–9782.
27. M. Zhang, X. Ni and Z. Shen, Organometallics, 2014, 33, 6861–6867.
28. (a) C. Bakewell, A. J. P. White, N. J. Long and C. K. Williams, Angew. Chem., Int. Ed., 2014, 53, 9226–9230; (b) M. Mazzeo, R. Tramontano, M. Lamberti, A. Pilone, S. Milione and C. Pellecchina, Dalton Trans., 2013, 9338–9351; (c) C. Bakewell, T.-P.-A. Cao, N. Long, X. F. Le Goff, A. Auffrant and C. K. Williams, J. Am. Chem. Soc., 2012, 134, 20577–20580; (d) Z. Mou, B. Liu, X. Liu, H. Xie, W. Rong, L. Li, S. Li and D. Cui, Macromolecules, 2014, 47, 2233–2241; (e) M. Sinenkov, E. Kirillov, T. Roisnel, G. Fukin, A. Trifonov and J.-F. Carpentier, Organometallics, 2011, 30, 5509–5523; (f) G. Li, M. Lamberti, M. Mazzeo, D. Pappalardo, G. Roviello and C. Pellecchia, Organometallics, 2012, 31, 1180–1188; (g) C. Bakewell, T.-P.-A. Cao, X. F. Le Goff, N. J. Long, A. Auffrant and C. K. Williams, Organometallics, 2013, 32, 1475–1483; (h) N. Maudoux, T. Roisnel, J.-F. Carpentier and Y. Sarazin, Organometallics, 2014, 33, 5740–5748.
29. Y. Yao, M. Ma, X. Xu, Y. Zhang, Q. Shen and W.-T. Wong, Organometallics, 2005, 24, 4014–4020.
30. Z. Zhang, X. Xu, W. Li, Y. Yao, Y. Zhang, Q. Shen and Y. Luo, Inorg. Chem., 2009, 48, 5715–5724.
31. E. E. Delbridge, D. T. Dugah, C. R. Nelson, B. W. Skelton and A. H. White, Dalton Trans., 2007, 143–153.
32. F. M. Kerton, A. C. Whitwood and C. E. Willans, Dalton Trans., 2004, 2237–2244.
33. Z. Zhang, X. Xu, S. Sun, Y. Yao, Y. Zhang and Q. Shen, Chem. Commun., 2009, 7414–7416.
34. Y. Yao, X. Xu, B. Liu, Y. Zhang, Q. Shen and W.-T. Wong, Inorg. Chem., 2005, 44, 5133–5140.
35. X. Xu, Y. Yao, M. Hu, Y. Zhang and Q. Shen, J. Polym. Sci., Part A: Polym. Chem., 2006, 44, 4409–4419.
36. L. Zhou, Y. Wang, Y. Yao, Y. Zhang and Q. Shen, J. Rare Earths, 2007, 25, 544–548.
37. Q. Sun, Y. Wang, D. Yuan, Y. Yao and Q. Shen, Organometallics, 2014, 33, 994–1001.
38. T. Zeng, Y. Wang, Q. Shen, Y. Yao, Y. Luo and D. Cui, Organometallics, 2014, 33, 6803–6811.
39. T. Zeng, Q. Qian, Y. Wang, Y. Yao and Q. Shen, J. Organomet. Chem., 2015, 779, 14–20.
40. L. Zhou, Y. Yao, Y. Zhang, M. Xue, J. Chen and Q. Shen, Eur. J. Inorg. Chem., 2004, 2167–2172.
41. J.-F. Carpentier, Macromol. Rapid Commun., 2010, 31, 1696–1705.
42. S. L. Hancock, M. F. Mahon and M. D. Jones, Dalton Trans., 2011, 2033–2037.
43. A. Duda, A. Kowalski, P. Dubois, O. Coulembier and J. M. Raquez, Handb. Ring-Opening Polym., 2009, 1–51.
44. C. M. Thomas, Chem. Soc. Rev., 2010, 39, 165–173.
45. C. E. Willans, M. A. Sinenkov, G. K. Fukin, K. Sheridan, J. M. Lynam, A. Trifonov and F. M. Kerton, Dalton Trans., 2008, 3592–3598.

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Footnote

† CCDC 1047907–1047909 and 1048302 (for complex 1, 2, 4 and 5). For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra10151d

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