Zirconium vs Aluminum Salalen Initiators for the Production of

Nov 8, 2016 - Complex Zr(1)(OiPr)2 was able to produce isotactic polylactide (PLA) from rac-lactide (Pm up to 0.85) in solution at 50 °C; in the melt...
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Zirconium vs Aluminum Salalen Initiators for the Production of Biopolymers Sarah M. Kirk,†,‡ Gabriele Kociok-Köhn,‡ and Matthew D. Jones*,‡ †

Doctoral Training Centre in Sustainable Chemical Technologies and ‡Department of Chemistry, University of Bath, Bath BA2 7AY, U.K. S Supporting Information *

ABSTRACT: Herein we report the synthesis and full characterization (NMR and solid-state structures) for a series of Zr(IV), Hf(IV), and Al(III) salalen complexes, together with salen bimetallic counterparts. With the salalen ligand, 1H2, monometallic complexes were observed in solution and solid state. Complex Zr(1)(OiPr)2 was able to produce isotactic polylactide (PLA) from rac-lactide (Pm up to 0.85) in solution at 50 °C; in the melt (130 °C) this reduced to ca. 0.75. Al(1)Me was significantly less active and produced PLA with only a very modest isotactic enchainment (Pm ≈ 0.6). Zr(1)(OiPr)2 was also able to produce copolymers with lactide and ε-caprolactone, producing copolymers of a “blocky” nature.

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the mechanism of initiation varied with temperature. At 130 °C the classical coordination insertion mechanism was in operation, whereas at lower temperature an external initiator was required and polymerization proceeded via an “activated monomer” mechanism. When rac-LA was used, only very modest isotactic PLA was prepared (probability of isotactic linkages, Pm, up to 0.59). Al(III) salalen complexes have been used to excellent effect by Kol, Lamberti, and Mazzeo.10b,12 They have prepared an Al(III) complex of a chiral salalen ligand based on an aminoethylpyrrolidine backbone and have isolated gradient isotactic multiblock PLA. The mechanism is a combination of enantiomorphic-site control and chain-end control.

INTRODUCTION Research into polylactide (PLA) has exploded in recent years.1 The reason is the favorable properties of the final polymer (biodegradability and biocompatibility) coupled with the fact that the starting material lactide (LA) can be sourced from annually renewable raw materials. PLA is prepared by the ringopening polymerization (ROP) of the cyclic ester monomer, LA. The properties of the polymer (thermal transitions and degradation profiles) are correlated to the polymer’s tacticity (either atactic, heterotactic, or isotactic). The tacticity can be controlled by the judicious choice of ligand and metal combination; however, clear structure−activity relationships are lacking in the area, and there is an element of serendipity in initiator design. There are a multitude of metal centers that can be applied for the controlled ROP of rac-LA, for example Al(III),2 Zn(II),3 In(III),4 groups 1 and 2,5 group 3,6 group 4,7 and the lanthanides.6d,8 Further, the use of bimetallic complexes is promising to be an alternative research strategy.2j,9 The majority of these examples utilize salan or salen ligands. In recent years the use of salalen ligands has been a promising avenue of research.7r,8i,10 For example, we have prepared a series of salalen ligands with aliphatic linkers and have shown that the polymer microstructure can be changed from moderately isotactic to moderately heterotactic by subtle changes to the ligand backbone.7r,10a,e Using similar ligands to these, Yao was able to prepare a series of efficient initiators for the production of heterotactic PLA (Pr up to 0.85), and the tacticity was related to the ionic radii of the complexes, with the ligand’s steric bulk appearing not to have any effect on tacticity.8i Wu and co-workers11 have prepared binuclear magnesium and zinc initiators based on heptadentate salalen systems. The complexes were found to be efficient initiators for the controlled polymerization of L-lactide; they observed that © XXXX American Chemical Society

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RESULTS AND DISCUSSION As part of our continuing studies concerning the utilization of salalen ligands for the production of PLA, we have prepared a salalen ligand with a planar and rigid backbone, 1H2, Scheme 1.7r,10a,d,e,13 This ligand has been previously prepared by Nozaki and utilized for CO2/epoxide alternating copolymerizations. Surprisingly, given the relative ease and scale of synthesis, there are no characterized examples of solid-state structures with such a ligand system.14 We have also prepared a salen, 2H2, Scheme 2, as a comparison. 1H2 can be prepared on a gram scale from commercially available (or readily synthesizable) materials in a day. All complexes have been characterized by 1H and 13C{1H} NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. Zr(1)(OiPr)2 crystallizes in the monoclinic space group P21/c with the isopropoxides cis to one another, and the Received: September 10, 2016

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DOI: 10.1021/acs.organomet.6b00718 Organometallics XXXX, XXX, XXX−XXX

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However, this is in contrast to analogous ONSO ligands (with a phenylene backbone) reported by Kol and Okuda in 2014,7q in which a fac−fac coordination mode was observed and consequently a trans orientation of the phenoxides as opposed to cis in our case. The coordination around the Zr(IV) center is pseudo-octahedral, which is exemplified by O(2)−Zr(1)−O(4) = 163.92(6)°, N(1)−Zr(1)−O(1) = 175.36(6)°, and N(2)− Zr(1)−O(2) = 85.13(6)°. All other metric data are analogous to salalen complexes previously reported in the CCDC.7r,15 The solid-state structure is maintained in solution with two distinct isopropoxide resonances and discrete diastereotopic doublets for the −CH2−, indicating that the ligand is “locked” once complexed to the Zr(IV) center. The same is observed for Hf(1)(OiPr)2, with the metric data being similar to Zr(1)(OiPr)2.10c Al(1)Me (Figure 1b) crystallizes in the monoclinic space group P21/c with the Al(III) center being in a pseudotrigonal bipyramidal geometry. This is exemplified by the angles O(1)−Al(1)−N(2) = 161.56(4)° and O(2)−Al(1)−N(1) = 120.64(4)°. All other metric data are consistent with previously reported Al(III)-salalen complexes in the literature.10a,b,e,13c,16 When the salen 2H2 was utilized, bimetallic complexes were isolated for Zr(IV), Hf(IV), and Al(III), Figure 2. The structure

Scheme 1. 1H2 and Complexes Utilizing 1H2 Prepared in This Study

Scheme 2. 2H2 and Complexes Utilizing 2H2 Prepared in This Studya

Figure 2. Solid-state structure of (A) Zr2(2)(OiPr)6 and (B) Al2(2)Me4. Ellipsoids are shown at the 30% probability level, and any disorder, solvent of recrystallization, and hydrogen atoms have been removed for clarity. Selected distances (Å): Zr(1)−O(1) 2.0509(14), Zr(1)−O(2) 1.9360(16), Zr(1)−O(3) 1.9257(15), Zr(1)−O(4) 2.1570(14), Zr(1)−O(8) 2.1860(14). Al(1)−C(1) 1.9481(14), Al(1)−C(2) 1.9414(14), Al(1)−N(1) 1.9795(10), Al(1)−O(1) 1.7661(10).

a

The tBu groups have been removed from the Zr(IV) complex for clarity.

ligand wraps around the metal center in a fac−mer fashion (salan fac and salen mer). This and the metric data are in agreement with previous salalen Zr(IV) complexes, Figure 1a.7r

of Zr2(2)(OiPr)6 is analogous to a phenylene diaminophenolate-Zr(IV) complex previously reported by Kol17 and to Hf2(2)(OiPr)6. Both Zr(IV) centers are octahedral, which is highlighted by the following angles: O(3)−Zr(1)−N(1) 168.87(6)°; O(2)−Zr(1)−N(1) 84.87(6)°. The coordination sphere of the Zr(IV) centers is completed by two terminal and two bridging OiPr moieties. The 1H NMR spectrum is consistent with the solid-state structure being maintained in solution with 2 × 2H septets for the terminal OiPr methines and 2 × 1H septets for the bridging methines. The bridging isopropoxides are subtly different due to O(4) being trans to the phenoxides and O(3) cis to the phenoxides. DOSY NMR spectra were recorded on all Zr/Hf complexes (see Table SI2), affording similar diffusion constants ((6.3−6.9) × 10−10 m2 s−1), which is to be expected given the complexes have similar radii in the solid state (∼7−7.5 Å). For Al2(2)Me4 the Al(III) centers are in tetrahedral environments {C(1)−Al(1)−C(2) 118.37(7)° and N(1)− Al(1)−O(1) 93.92(4)°}, with the Al−X distances similar to other salens in the literature.18 The solid-state structure is maintained in solution, as evident by the 1 × 12H singlet for

Figure 1. Solid-state structure of (A) Zr(1)(OiPr)2 and (B) Al(1)Me. Ellipsoids are shown at the 30% probability level, and any disorder, solvent of recrystallization, and hydrogen atoms have been removed for clarity. Selected distances (Å): Zr(1)−O(1) 1.9563(15), Zr(1)− O(2) 1.9466(15), Zr(1)−O(3) 2.0174(14), Zr(1)−O(4) 2.0731(14), Zr(1)−N(1) 2.3905(18), Zr(1)−N(2) 2.4134(18). Al(1)−O(1) 1.8312(8), Al(1)−O(2) 1.7578(8), Al(1)−C(1) 1.9637(12), Al(1)− N(1) 1.9731(9), Al(1)−N(2) 2.3009(9). B

DOI: 10.1021/acs.organomet.6b00718 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Polymerization of rac-LAa initiator 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19f 20 21 22 23

i

Zr(1)(O Pr)2 Zr(1)(OiPr)2 Zr(1)(OiPr)2 Zr(1)(OiPr)2 Zr2(2)(OiPr)6 Zr2(2)(OiPr)6 Zr2(2)(OiPr)6 Hf(1)(OiPr)2 Al(1)Me Al(1)Me Al(1)Me Al(1)Me Al2(2)Me4 Al2(2)Me4 Al2(2)Me4 Al2(2)Me4 Zr(1)(OiPr)2 Zr(1)(OiPr)2 Zr(1)(OiPr)2 Zr2(2)(OiPr)6 Zr2(2)(OiPr)6 Hf(1)(OiPr)2 Hf2(2)(OiPr)6

[complex]:[I]:BnOH

time/h

temp/°C

con%b

calcd Mnc

Mnd

PDId

Pme

100:1:0 200:1:0 800:1:0 100:1:0 100:1:0 200:1:0 400:1:0 100:1:0 100:1:1 200:1:1 400:1:1 100:1:1 100:1:2 200:1:2 400:1:2 100:1:1 300:1:0 900:1:0 300:1:0 300:1:0 600:1:0 300:1:0 300:1:0

24 48 168 96 24 24 48 48 96 240 672 192 5 24 48 16 2 16 2 0.5 0.5 2 1

80 80 80 50 80 80 80 80 80 80 80 65 80 80 80 80 130 130 130 130 130 130 130

72 100 94 97 99 98 97 92 73 45 78 48 74 98 98 69 81 53 84 64 58 83 71

10 425 28 850 108 350 14 025 14 325 28 275 55 925 13 300 10 625 13 075 45 050 7025 5425 14 225 28 325 10 050 35 050 68 750 36 350 27 700 50 175 35 925 30 725

13 600 29 050 67 650 6650 11 250 21 300 35 350 8500 13 350 18 800 33 750 11 450 4450 17 850 33 850 9950 38 800 34 950 17 000 81 400 70 800 16 650 30 350

1.05 1.13 1.08 1.01 1.35 1.68 1.27 1.02 1.04 1.02 1.07 1.04 1.02 1.09 1.27 1.01 1.03 1.44 1.26 1.22 2.04 1.09 1.24

0.79 0.75 0.71 0.85 0.49 0.46 0.57 0.77 0.61 0.54 0.55 0.64 0.50 0.43 0.43 0.55 0.71 0.69 0.74 0.40 0.42 0.72 0.61

a

All polymerizations 1−16 have been conducted in toluene at the temperature and time stated, and 17−23 have been performed under melt conditions. bDetermined from analysis of the 1H NMR spectrum of the crude product. Isolated yields are typically within 10% of the NMR conversions. cTheoretical Mn {(144 × equiv LA)/number of initiators × (conc/100) + end groups}; for Zr/Hf2(2)(OiPr)6 the theoretical Mn assumes one chain per complex. dAs determined by gel permeation chromatography (GPC) (THF). eAs determined from 1H{1H} NMR. fUsing recrystallized lactide as opposed to doubly sublimed monomer.

the Al−Me at −0.26 ppm and 1 × 2H imine singlet at 7.99 ppm. Polymerization Studies. Initial attempts focused on the polymerization in solution (Table 1 entries 1−16) to ascertain the reactivity and selectivity trends with the complexes. Entries 1−3 indicate that the molecular weight with Zr(1)(OiPr)2 increases with higher equivalents of rac-LA, at 80 °C. With lower [LA], it appears that one chain is growing per metal center; however, at higher [LA] there appears to be two chains per metal center. Further, PLA with an isotactic bias was observed; this is enhanced at lower temperature (entry 4), although a longer time was required to achieve the same level of conversion. Analysis of the microstructure of the PLA showed a small contribution from the sis tetrad and the sii, iis, and isi are approximately 1:1:1, indicating that PLA with a “blocky” characteristic is formed,19 presumably via a chain-endcontrolled mechanism, although a site-controlled mechanism with polymeryl exchanges cannot be ruled out.12 At 50 °C a high isotactic bias was achieved with Pm = 0.85. This is one of the highest reported isoselectivities for a Zr(IV) complex.7g Interestingly, the analogous −CH2CH2− (in place of C6H4) Zrsalalen gave similar conversion after only 2 h, but produced slightly heterotactic PLA (Pr = 0.6).7r This illustrates that subtle changes to the ligand have a dramatic effect on stereoselectivity and indicates that rigidity and the nature of the backbone are important for controlling the selectivity in the polymerizations using salalen ligands. Analysis of this polymer via MALDI-ToF MS indicated the required end groups (H− and −OiPr) as expected from the classical coordination insertion mechanism. The repeat unit was seen to be 72 g/mol, which is indicative of a degree of intermolecular transesterification occurring during

the polymerization, although the polydispersity index (PDI) remains narrow. Differential scanning calorimetry (DSC) analysis of the polymer from entry 4 yields a melting point of 184 °C and a Tc = 100 °C. For this system the solution kinetics were investigated (see SI for plots); as expected, the polymerization was first order with respect to monomer. At a concentration of 0.58 mol dm−3 at 80 °C in C6D5CD3 (100:1 LA:Init) the following rate constants were observed: kapp(L-LA) = 0.0020 min−1, kapp(D-LA) = 0.0022 min−1, and kapp(rac-LA) = 0.0008 min−1, which is expected for an isoselective catalyst, with rac-LA being polymerized slower. The kinetics for Zr2(2)(OiPr)6 (under analogous conditions) afforded kapp(racLA) = 0.0011 min−1, indicating that the dimer is slightly faster than the monomeric species, but this is a marginal difference. Zr(1)(OiPr)2 was also active for the polymerization of εcaprolactone (100:1, 80 °C, toluene 24 h) to afford polycaprolactone (Mn = 8200, PDI = 1.36, 81% conversion). Hf(1)(OiPr)2 was analogous to the Zr(IV) counterpart with good control, and isotactic PLA was achieved, entry 8. The bimetallic complex Zr2(2)(OiPr)6 was also active for the polymerization (entries 5−7) in solution, with atactic PLA being formed. The polydispersity is slightly broader than the mononuclear complex, and it appears at 100:1 and 200:1 that one chain grows per complex, even with the potential for multiple initiation sites. There is poor agreement under melt conditions, which may be related to complex dissociation/ degradation under such conditions. The MALDI-ToF (Table 1 entry 5) indicates that H/OiPr end groups are present with a repeat unit of 72 g/mol. The Al(III) complexes were active and able to control the molecular weight, with the addition of benzyl alcohol. PLA with a slight isotactic bias was formed C

DOI: 10.1021/acs.organomet.6b00718 Organometallics XXXX, XXX, XXX−XXX

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ratio. Further, copolymers with more blocky domains are isolated, as seen by the relatively high homolinkages (LA−LA and CL−CL) compared to heterolinkages in the polymer. In all cases unimodal GPC data were observed, and DSC of the polymers with rac-LA did not afford any thermal transitions. The homonuclear decoupled NMR (see SI) of the polymer isolated from the 75:25 (LA:CL) polymerization maintained an isotactic bias with a Pm of 0.77. If L-LA is used as opposed to rac-LA (last entry in Table 2), then it is possible to observe Tm = 141 °C and Tc = 83 °C for the lactide block.

(entry 12). MALDI-ToF analysis on PLA produced with Al(1)Me/BnOH showed the −OCH2C6H5 and −H end groups as expected. At 80 °C (Table 1 entry 9) the repeat unit was 72 g/mol; however, at 65 °C (Table 1 entry 12) this was 144 g/mol. There is relatively good agreement between calculated and theoretical Mn values for the aluminum initiators (Table 1, entries 10−16). Although not the main thrust of this work, it is also noteworthy that the bimetallic Al(III) complexes appear to be more active than the monometallic salalen. Such an effect may well be due to the reduced steric demand of the tetrahedral Al(III) centers. Zr(1)(OiPr)2 was also active in the melt with a slight reduction in tacticity, compared to 50 °C (cf. entry 17 vs entry 4). The initiator was also able to tolerate impurities in the monomer (entry 17 vs 19); however there was a reduction in the molecular weight, which may be related to chain transfer agents present in the unsublimed monomer. The Mn, PDI, and Pm were monitored as a function of conversion, Figure 3, under

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CONCLUSIONS In conclusion novel Zr(IV), Hf(IV), and Al(III) complexes based on a salalen ligand have been prepared and characterized in solution and solid state. The Zr(IV)-salalen complex is able to produce PLA with a high isotactic enchainment, one of the highest in the literature for a group 4 complex. When this is compared to the aliphatic backbone7r (same coordination around the metal center and steric requirements of the ortho/ para substituents), there is a dramatic switch from slight heterotactic to strong isotactic PLA. This indicates that the linkage between the two nitrogen centers is potentially key in determining the tacticity for Zr(IV) salalen complexes, and this is one area where future work should be targeted.

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EXPERIMENTAL SECTION

The preparation and characterization of all metal complexes were carried out under an inert argon atmosphere using standard Schlenk or glovebox techniques. All chemicals used were purchased from Aldrich and used as received except for rac-LA, which was recrystallized from dry toluene and doubly sublimed prior to use. Dry solvents used in handling metal complexes were obtained via a solvent purification system. 1H and 13C{1H} NMR spectra were recorded on a Bruker 400 or 500 MHz instrument and referenced to residual solvent peaks. CDCl3/C6D6 were dried over CaH2 prior to use with metal complexes. Coupling constants are given in hertz. CHN microanalysis was performed by Mr. Stephen Boyer of London Metropolitan University. Synthesis of Ligands. The ligand was prepared via modified procedures from the literature. See the SI for full characterization of the intermediates: 1H NMR (400 MHz, CDCl3) δ ppm 1.32 (s, 9 H, t Bu), 1.36 (s, 9 H, tBu), 1.39 (s, 9 H, tBu), 1.49 (s, 9 H, tBu), 2.71 (s, 3 H, N−CH3), 4.32 (s, 2 H, CH2), 6.96 (d, J = 2.3 Hz, 1 H, Ar−H), 7.10 (dd, J = 7.8, 1.3 Hz, 1 H, Ar−H), 7.19−7.23 (m, 1 H, Ar−H), 7.24 (d, J = 2.5 Hz, 1 H, Ar−H), 7.26 (d, J = 2.3 Hz, 1 H, Ar−H), 7.28−7.34 (m, 2 H, Ar−H), 7.50 (d, J = 2.5 Hz, 1 H, Ar−H), 8.63 (s, 1 H, N CH), 10.07 (s, 1 H, OH), 13.00 (s, 1 H, OH); 13C{1H} NMR (400 MHz, CDCl3) δ ppm 29.5 (C(CH3)3), 29.7 (C(CH3)3), 31.5 (C(CH3)3), 31.7 (C(CH3)3), 34.2 (C(CH3)3), 34.2 (C(CH3)3), 34.9 (C(CH3)3), 35.2 (C(CH3)3), 42.8 (N−CH3), 59.8 (CH2), 118.5 (Ar), 120.4 (Ar−H), 120.8 (Ar−H), 121.0 (Ar), 123.1 (Ar−H), 123.7 (Ar− H), 125.5 (Ar−H), 127.0 (Ar−H), 128.3 (Ar−H), 135.9 (Ar), 137.2

Figure 3. Plot of Mn (left) and PDI (right) against conversion for the melt polymerization using Zr(1)(OiPr)2 at 300:1 [M]:[I]. The numbers by the circles represent the Pm values determined at each point.

melt conditions. There was a roughly linear increase in molecular weight with respect to conversion, and the PDI remained in a tight band (1.01−1.05), indicating that the polymerization is relatively well controlled. Hf2/Zr2(2)(OiPr)6 were also active in the melt, although broad dispersities were observed (entries 20, 21, and 23), with poor molecular weight control. Zr(1)(OiPr)2 was also screened for the copolymerization of rac-LA and ε-caprolactone, Table 2. It is clear that the rac-LA is polymerized preferentially over ε-caprolactone, with the amount of ε-caprolactone always being less than the theoretical Table 2. Copolymerization Data Using Zr(1)(OiPr)2a

ratio in polymerb M:I

LA:CL

time/h

con % LA

100:1 100:1 100:1 100:1 200:1 100:1d

50:50 50:50 75:25 25:75 50:50 75:25

24 48 48 48 96 96

89 95 97 94 96 99

b

con % CL 43 63 61 69 45 99

b

c

ratio of linkages in copolymerb

Mnc

PDI

[PLA]:PCL]

LA−LA

CL−CL

LA−CL

CL−LA

6400 5500 6650 6350 11 350 6100

1.04 1.03 1.06 1.09 1.05 1.18

80:20 73:27 90:10 36:64 72:28 80:20

0.72 0.61 0.84 0.22 0.60 0.77

0.11 0.15 0.04 0.48 0.15 0.11

0.09 0.12 0.06 0.15 0.12 0.06

0.09 0.12 0.06 0.15 0.12 0.06

All polymerizations have been conducted in toluene at 80 °C for the given time. bDetermined from analysis of the 1H NMR spectrum. cAs determined by GPC (THF). dUtilizing L-LA (all others are rac-LA).

a

D

DOI: 10.1021/acs.organomet.6b00718 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

vacuo and washed with copious amount of methanol to remove unreacted monomer. For the copolymerizations the appropriate amount of each monomer was simultaneously dissolved in toluene followed by initiator, and the polymerization proceeded for the time and temperature given in Table 2. 1H NMR spectroscopy (CDCl3) and GPC (THF) were used to determine tacticity and molecular weights (Mn and Mw) of the polymers produced; Pm values were determined by analysis of the methine region of the homonuclear decoupled 1H NMR spectra. GPC was recorded on an Agilent 1260 Infinity instrument and was calibrated using RI, viscometer, and light scattering detectors using the Universal calibration method via multidetection software.

(Ar), 140.5 (Ar), 140.5 (Ar), 145.1 (Ar), 145.4 (Ar), 154.2 (Ar−OH), 158.2 (Ar−OH), 165.3 (CHN); m/z [C37H52N2O2 + Na]+ calcd 579.3926 g mol−1, found 579.3957 g mol−1. Synthesis of Complexes. See the SI for full details. Below are the procedures and characterization for Zr(1)(OiPr)2 and Al(1)Me. Zr(1)(OiPr)2. 1H2 (0.83 g, 1.49 mmol) was dissolved in 40 mL of hexane and 10 mL of toluene with zirconium isopropoxide 2-propanol complex (0.578 g, 1.49 mmol). The solution was stirred at 60 °C for 6 h. The solution was concentrated by removal of solvent, and the resulting crystals were filtered (330.1 mg, 0.43 mmol, 29%). 1H NMR (400 MHz, CDCl3) δ ppm 0.69 (d, J = 6.0 Hz, 3 H, CH-CH3), 0.87 (d, J = 6.0 Hz, 3 H, CH-CH3), 1.09 (s, 9 H, tBu), 1.11 (s, 9 H, tBu), 1.29 (br s., 6 H, CH-CH3), 1.32 (s, 9 H, tBu), 1.55 (s, 9 H, tBu), 3.32 (s, 3 H, N−CH3), 3.78 (d, J = 12.0 Hz, 1 H, CH2), 3.92 (m, J = 6.0 Hz, 1 H, CH-CH3), 4.52 (m, J = 6.0 Hz, 1 H, CH-CH3), 4.69 (d, J = 12.0 Hz, 1 H, CH2), 6.51 (d, J = 2.0 Hz, 1 H, Ar−H), 6.96 (d, J = 2.3 Hz, 1 H, Ar−H), 7.11 (d, J = 2.3 Hz, 1 H, Ar−H), 7.14−7.21 (m, 1 H, Ar−H), 7.27 (t, J = 1.0 Hz, 1 H, Ar−H), 7.37 (d, J = 8.0 Hz, 1 H, Ar− H), 7.47 (d, J = 7.8 Hz, 1 H, Ar−H), 7.53 (d, J = 2.3 Hz, 1 H, Ar−H), 8.52 (s, 1 H, NCH); 13C{1H } NMR (400 MHz, CDCl3) δ ppm 26.5 (CH−CH3), 26.7 (CH−CH3), 27.3 (CH−CH3), 27.3 (CH− CH3), 29.5 (C(CH3)3), 29.6 (C(CH3)3), 31.4 (C(CH3)3), 31.6 (C(CH3)3), 33.7 (C(CH3)3), 34.1 (C(CH3)3), 34.6 (C(CH3)3), 35.3 (C(CH3)3), 48.8 (N−CH3), 67.1 (CH2), 69.8 (CH−CH3), 70.8 (CH−CH3), 116.3 (Ar−H), 122.0 (Ar), 122.7 (Ar), 123.2 (Ar−H), 123.4 (Ar−H), 124.7 (Ar−H), 127.6 (Ar−H), 127.9 (Ar−H), 129.7 (Ar−H), 131.0 (Ar−H), 135.8 (Ar), 136.5 (Ar), 138.7 (Ar), 139.1 (Ar), 144.5 (Ar), 144.8 (Ar), 160.1 (Ar−O), 161.2 (Ar−O), 161.9 (NCH). Anal. Calcd for C43H64N2O4Zr: C 67.58, H 8.44, N 3.67. Found: C 67.44, H 8.57, N 3.56. Al(1)Me: 1H2 (0.5 g, 0.90 mmol) was dissolved in 20 mL of toluene, and a 2 M trimethylaluminum solution (0.44 mL, 0.90 mmol) was added slowly. The solution was stirred for 2 h; then solvent was removed. The solid was redissolved in 20 mL of hexane. The resulting crystals were filtered to yield a yellow solid (292.2 mg, 0.49 mmol, 54%). 1H NMR (400 MHz, C6D6) δ ppm −0.30 (s, 3 H, Al−CH3), 1.37 (s, 9 H, tBu), 1.45 (s, 9 H, tBu), 1.76 (s, 9 H, tBu), 1.81 (s, 9 H, t Bu), 2.26 (s, 3 H, N−CH3), 3.00 (d, J = 12.1 Hz, 1 H, CH), 4.13 (d, J = 12.1 Hz, 1 H, CH), 6.36 (d, J = 8.0 Hz, 1 H, Ar−H), 6.79−6.85 (m, 2 H, Ar−H), 6.94 (d, J = 2.8 Hz, 3 H, Ar−H), 7.60 (d, J = 2.5 Hz, 1 H, Ar−H), 7.81 (d, J = 2.5 Hz, 1 H, Ar−H), 7.90 (s, 1 H, NCH); 13 C{1H} NMR (400 MHz, C6D6) δ ppm 30.7 (C(CH3)3), 30.8 (C(CH3)3), 31.8 (C(CH3)3), 32.5 (C(CH3)3), 34.6 (C(CH3)3), 34.7 (C(CH3)3), 36.0 (C(CH3)3), 36.2 (C(CH3)3), 41.1 (N−CH3), 66.1 (CH2), 119.3 (Ar−H), 119.8 (Ar), 121.0 (Ar−H), 123.3 (Ar), 124.7 (Ar−H), 124.7 (Ar−H), 127.5(Ar−H), 128.3 (Ar−H), 128.8 (Ar−H), 133.7 (Ar−H), 138.4 (Ar), 139.0 (Ar), 139.1 (Ar) 141.4 (Ar), 142.2 (Ar), 145.9 (Ar), 157.8 (Ar−OH), 166.4 (NCH), 168.3 (Ar−OH). Anal. Calcd for C38H53N2O2Al: C 76.47, H 8.95, N 4.69. Found: C 76.25, H 9.08, N 4.60. Crystallography. All data were collected on a SuperNova EOS detector diffractometer using Cu Kα radiation (λ = 1.541 84 Å) or Mo Kα radiation (λ = 0.710 73 Å) or a Nonius kappa diffractometer using Mo Kα radiation (λ = 0.710 73 Å), all recorded at 150(2) K. All structures were solved by direct methods and refined on all F2 data using the SHELXL-2014 suite of programs. All hydrogen atoms were included in idealized positions and refined using the riding model. All refinement details are given in the .cif file. Data were straightforward except the following: Zr(1)(OiPr)2, two tBu groups were disordered over two sites; Zr2(2)(OiPr)6, two molecules of toluene are present in the unit cell, one tBu group was disordered over two sites, the carbon atoms of one bridging OiPr {O(8)} was disordered over two sites; Hf(1)(OiPr)2, both isopropoxides were disordered over two positions as was one tBu moiety. CCDC numbers 1503465−1503470 contain supplementary crystallographic data for this paper. Ring-Opening Polymerization Studies. For polymerizations the required monomer:initiator(:BnOH) ratio was dissolved in toluene at 80 °C (10 mL). In all cases 1.0 g of rac-lactide was used. For melt polymerizations 1.0 g of rac-lactide was used in the absence of solvent and the reaction heated to 130 °C. The solvents were removed in

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00718. Summary of NMR, GPC, and kinetic data (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail (M. D. Jones): [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was provided by the EPRSC grant (EP/ G03768X/1) for the Centre for Doctoral Training. We thank the EPSRC National Mass Spectrometry Service Centre Swansea for MALDI-ToF analysis. We thank Dr. John Lowe for assistance with DOSY NMR analysis.

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REFERENCES

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