Sodium iminoquinolates with cubic and hexagonal prismatic motifs : synthesis , characterization and their catalytic behavior toward the ROP of rac-lactide

Please do not adjust margins a. Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China; *E-mail: whsun@iccas.ac.cn. b. School of Materials Science and Engineering, Beijing Institute of Fashion Technology, Beijing 100029, China c. Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, UK d. University of Chinese Academy of Sciences, Beijing 100049, China † Electronic Supplementary Information (ESI) available: CCDC 1482013-1482018 for C1-C5 and C7’ and crystallographic data in CIF. See DOI: 10.1039/x0xx00000x Received 00th January 20xx, Accepted 00th January 20xx


Introduction
Biodegradable polylactide (PLA) has been the focus of much attention in recent years due to its numerous applications in packaging, agricultural materials, drug delivery, medical devices, and so on. 1 Industrially, stannous octoate is used in its preparation, however, the presence of any trace amounts of toxic tin that may be incorporated into the polymer raises concerns as to its safe use in medical and food packaging applications. 2To overcome this, complexes containing a variety of other metal centers have been identified as catalysts for the ring-opening polymerization (ROP) of lactide to give PLA including aluminum, rare earth metals, zinc as well as titanium. 3In addition, complexes based on environmentallyfriendly alkali metals have also come to the fore as efficient initiators for the ROP of lactides. 4Indeed, the solid state structures of these group 1 compounds often reveal unusual multinuclear assemblies adopting a raft of geometrical forms including 6-membered rings (A, Chart 1) and cubic arrays (B, Chart 1), 5 in addition to binuclear and mononuclear compounds 6,7 such as crown ether stabilized sodium aryloxides (C, Chart 1). 7ith regard to catalytic performance, sodium phenolates can deliver high iso-selectivities (Pm up to 0.86) towards the ROP of rac-lactide under mild conditions. 7Elsewhere, the tertbutylphosphine-bridged bisphenolate complex [tBu-OPO]Na2(DME)2 8 has shown to be efficient for the ROP of Llactide as have sodium compounds bearing O,O,O-tridentate bis-phenolates. 9Likewise, sodium complexes of 2,2'ethylidene-bis (4,6-di-tert-butylphenol)  (D, Chart 1), first reported in 2006 and later revisited, exhibit very good activities towards the ROP of L-lactide. 10Notably, sodium iminophenolates (E, Chart 1) display extremely high activities for the ROP of rac-lactide, 11 while sodium aminophenolates (F, Chart 1), which can aggregate to form dinuclear 12 and multinuclear compounds, 13 display good activity in the controlled ROP of rac-lactide.Recently we reported a series of metal-containing derivatives based on deprotonated 2-aryliminoquinolin-8-ols; their aluminum and yttrium complexes exhibited high activities toward the ROP of ε-caprolactone, 14a,b while their titanium This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins counterparts displayed high activities towards ethylene polymerization.14c-e To further explore the use of 2aryliminoquinolin-8-ols, we now disclose a series of sodium 2arylimino-8-quinlolates (C1 -C8) that differ not only in the substitution patterns on the N-aryl group but also on the quinolate unit itself.Their solid state structures will be examined using single crystal X-ray diffraction and their role as catalysts for the ROP of rac-lactide thoroughly investigated.

Synthesis and characterization of C1 -C8
The Single crystals of C1 suitable for the X-ray determination were grown from a toluene/hexane solution of the complex that had been left to stand in the refrigerator compartment of a glove box.A view of C1 is shown in Figure 1; selected bond distances and angles are collected in Table 1.C1 is tetrametallic and adopts a distorted cubic form in which the edges of the cube are made up of Na-O bonds; such structural motifs are common to sodium compounds. 5,11,12,15All three donor atoms of the O,N,N ligand coordinate to a sodium and each sodium is coordinated by two nitrogen and three bridging oxygen atoms.The Na-O-Na bond angles [range: 85.51(9) -94.58( 9) o ] are close to 90 o consistent with the cubic arrangement.In addition, the N-aryl planes of all four ligands are almost perpendicular to their corresponding Na-O-N-N coordination planes.The Na-O bond lengths range from 2.269(2) to 2.543(3) Å, which fall within the normal range found in sodium aryloxide compounds. 5,11,12The Na-Nquino bond lengths range from 2.352(3) to 2.408(3) Å, and are noticeably shorter than that     2) Å] fall in the same range as that seen C1.Likewise, the Na-N bond distances within a ligand show some disparity with the Na-Nquino lengths shorter than the Na-Nimino distances.
distances observed in either highlighting the weakness of this interaction and consistent with the steric properties imparted by the N-substituted 4-methyl-2,6dibenzhydrylphenyl group.
The molecular structure of C5 is depicted in Figure 5; selected bond distances and angles are reported in Table 5.The structure resembles C3 comprising a hexametallic array that takes the form of a hexagonal prism assembled through Na-O bonds.Two N,N,O ligands are fully chelated to sodium [Na3-O2 2.403(4) Å, Na3-Nimino 2.700( 5) Å, Na3-Nquino 2.432( 4)  Single crystals of C7' were grown by prolonged standing of a toluene/hexane solution of C7 stored in the refrigerator compartment of a glovebox.During crystallization it is evident that partial decomposition of C7 occurred generating C7'.Multiple attempts were made to grow crystals of C7 itself from different solvents or over shorter time periods; in each case the crystals formed were not of suitable quality for an X-ray determination.A view of the molecular structure of C7' is shown in Figure 6; selected bond distances and angles are given in Table 6.The structure reveals an unusual tetrametallic species, distinct from C1, C2 and C4, in which four sodium atoms are coordinated by four deprotonated L7 ligands and four neutral L7 ligands to form a core reminiscent of a cyclooctatetraene tub-shape.The sodium atoms display a variety of coordination numbers and geometries.Na1 is coordinated by N1 and three bridging oxygen atoms, O1, O2, O7, to form a tetrahedral geometry.At Na2 a distorted octahedral geometry is adopted in which three ligands Nquino^O-chelate, with O2, O4, N5, N7 forming the equatorial belt and N3 and O3 the axial sites [N3-Na2-O3 is 158.47( 9) º].Na3 is coordinated by O5, N9, N10 from the same ligand and two other bridging oxygen atoms (O6, O3) to give a geometry best described as distorted pyramidal with O3 occupying the apex and O5, N9, N10, O6 the basal plane.A distorted  Please do not adjust margins Please do not adjust margins Complex C7 was used, in the first instance, as the test (pro-) initiator to allow an investigation of the optimal conditions for the ring opening polymerization conditions of rac-lactide; the results are compiled in Table 7.
Firstly, we investigated the effect of temperature on the polymerization as it has been reported that the efficiencies of polymerization are affected by the reaction temperature. 17he polymerization runs, using benzyl alcohol as the activator, were performed over the 20 to 100 o C temperature range (runs 1 -6, Table 7), and it was found that optimal temperature occurred at 80 o C with a conversion to PLA of 91% (run 4, Table 7).Analysis of the polymers obtained at 60 o C, 80 o C and 100 o C (runs 3, 4, and 6, Table 7) revealed the molecular weights and molecular weight distributions to be similar.With regard to the duration of the polymerization (runs 4, 7 -11, Table 7), the conversion steadily increases from 0.5 hours to 3 hours at which point it appears to plateau between 3 and 6 hours.These trends are generally reproduced in the molecular weights and molecular weight distributions which increase and level out after 3 hours; the exception being for the slightly lower molecular weight for the polymer obtained after 6 hours (run 11, Table 7).It is unclear as the origin of this drop in molecular weight but it would seem plausible that it is due to re-initiation of the active species occurring at this point.
11a However, using C7 alone only showed low conversions (run 12, Table 7) with low molecular polymer obtained.By contrast C7 in the presence of one equivalent of benzyl alcohol, under comparable conditions, gave 91% conversion (run 4, Table 7).On further increasing the amount of benzyl alcohol from two to four equivalents resulted in a lowering of the conversion (runs 13, 14, Table 7).Notably, the PLA obtained showed a narrower molecular distribution, indicating a more controlled active species under these conditions.11a On increasing the amount of rac-lactide, the conversion was found to decrease to 25% (runs 15, Table 7) from 91% (runs 4, Table 7) which can be attributed to a lower concentration of the catalytic species.a Conditions: 20 μmol Na; 3.0 ml toluene.b Determined by 1 H NMR spectroscopy.c GPC data were recorded in CHCl3 vs. polystyrene standards, using a correcting factor of 0.58. 19irdly, the effect of solvent on the polymerization was probed (runs 16-18, Table 7).In tetrahydrofuran, C7/BnOH was essentially inactive, in hexane reasonable conversion could be achieved (65%) while in dichloromethane the highest conversion of the all the runs was obtained (97%).It would seem likely that detrimental coordination of THF to sodium 13,18 is responsible for the poor catalytic activity while the low solubility of C7 in hexane may account for its performance characteristics.In addition, the bulk polymerization (without solvent) also showed a good activity (run 19, Table 7), demonstrating the general applicability of this sodium species.
We have also explored the probability of racemic enchainment (i.e., the probability of a forming a new racemic diad) for the PLA's obtained using C1 -C8.These were calculated from their deconvoluted homodecoupled 1 H NMR spectra; the values obtained are collected in Table 8. 20 For each run the values are close to 0.5 (range: 0.44 -0.48) consistent with atactic structures for all the PLA's.

Characterization of PLA
A coordination-insertion mechanism is generally considered to be operative for the ring opening polymerization involving sodium complexes, as has been reported for a variety of other metal species.18b,21 The 1 H NMR spectrum for the polymer obtained in the absence of BnOH (run 12, Table 7) suggests cyclic structures which is further supported by the MALDI-TOF spectrum (Figure 7).In the MALDI spectrum peaks separated by 144 mass units along with two major repeating masses are evident (viz., A and B).Within the A series the two most intense peaks at m/z = 6364.2(A) and 6380.3 (A') are assigned as cyclic polymers based on [C6H8O4]44•Na + and [C6H8O4]44•K + , which can be explained by an intra-chain transesterification mechanism.In addition, a less intense B series of peaks corresponds to cyclic polymers of the type [C6H8O4]n+[C3H4O2]•Na + /K + (e.g., [C6H8O4]44+[C3H4O2]•Na + /K + , m/z = 6148.1 (B) and 6165.7 (B')), the result of inter-and intrachain transesterification side reactions; similar processes having been identified for other sodium complexes. 6,13Unlike some literature reports, 13,22 there is no evidence for CH3O terminal groups that can derive from methanol.A/A': [C 6 H 8 O 4 ] 44 H • Na + /K + B/B': The MALDI-TOF spectrum of PLA obtained without BnOH (run 12, Table 7) For the PLA obtained in the presence of benzyl alcohol, the 1 H NMR spectra show signals at ca. 5.1 ppm corresponding to a benzyl alkoxide group which would suggest an active species involving Na-OCH2Ph.The MALDI-TOF spectra of the PLA obtained with one equivalent of benzyl alcohol show peaks separated by 72 mass units (see Fig. 8a), suggesting intra-chain transesterification side reactions in the polymerization.Closer inspection reveals two (i.e,A and B) major repeating masses and another less intense peak.The major intense peaks (A and B) are assigned as cyclic polymers of general composition , indicating the presence of both intraand inter-chain transesterification side reactions. 6,13On the other hand, the less intense peaks for example at m/z = 1712.9and 1729.3 are assigned as linear polymers corresponding to PhCH2O+[C6H8O4]12•Na (C)/K + (C') with benzyl end-groups, in accordance with the 1 H NMR spectra.The MALDI-TOF spectrum of the PLA obtained using two equivalents of BnOH gave slightly different results for the polymer fractions.In this case, it is now composed of four types of polymer (see Fig. 8b).The most intense peaks were assigned as )) and they are attributed to cyclic polymers; the less intense peaks were assigned as PhCH2O+ ).On comparison of the two spectra in Figure 8, the ratio of polymer containing the PhCH2O end-groups increased, suggesting that more BnOH introduced the faster initiation of the active NaOCH2Ph species.This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Figure 8 MALDI-TOF spectra of the PLA's obtained in the presence of (a) 1 eq. of BnOH (run 4, Table 7) and (b) 2 eq. of BnOH (run 13, Table 7) The above results demonstrate that the cyclic polymers are the main structural types obtained using sodium-based C1 -C8 in either the presence or absence of BnOH, consistent with inter-and intra-chain transesterification as the major mechanisms.Notably different to that reported elsewhere, inter-chain transesterification side reactions play a key role in these polymerizations.In common to all, the polymers display quite broad molecular weight distributions which can be attributed to undesirable back-biting and intra-chain and transesterification reactions occurring, leading to macrocycles.Alternatively, catalyst inhomogeneity may also contribute to broad or multimodal polymer molecular weight distributions.

Conclusions
Eight examples of sodium 2-arylimino-8-quinolates, C1 -C8, have been synthesized and fully characterized.The crystal structures of C1 -C5 and C7' reveal a diversity of structural types with C1, C2 and C4 adopting tetrametallic cubic cores, C3 and C5 hexametallic hexagonal prismatic cores and C7', a decomposition product of C7, a tetrametallic structure with a core best described as a cyclooctatetraene-type tub-shape.In the absence of benzyl alcohol, C7 showed low activity for the ROP of rac-lactide while in its presence, C7 exhibited very good activity.By varying the substituents of the N-aryl group and quinolate moiety, the polymerization of rac-LA was influenced; indeed the steric (and electronic properties) of the substituents appended to the iminoquinolate greatly affected their catalytic activity.All the resultant PLA display high molecular weight and broad molecular weight distributions.Microstructural analysis of the PLA's generated showed that without benzyl alcohol, the polymers were cyclic in nature and contained some inter-chain by-product.With benzyl alcohol, the major component of the polymers was again based on cyclic structures and inter-chain polymer by-product along with a minor component based on a linear polymer with OCH2Ph end-groups.

Experimental General considerations
All reactions were performed using standard Schlenk techniques in an atmosphere of high-purity nitrogen or using glovebox techniques.Toluene and THF were dried by refluxing over sodium and benzophenone, and then distilled under nitrogen and stored over activated molecular sieves (4 Å) for 24 h in a glovebox prior to use.CDCl3, C6D6 and C2D2Cl4 were dried over activated 4 Å molecular sieves.CH2Cl2 and n-hexane were dried over CaH2 for 48 h, distilled under nitrogen and stored over activated molecular sieves (4 Å) in a glovebox prior to use.The ligands (L1 -L7) were prepared by the condensation reaction of 8-hydroxyquinoline-2-carbaldehyde with the corresponding substituted aniline as described in our previous work. 14Elemental analyses were performed using a PE2400II Series analyser (Perkin-Elmer Co.). 1 H and 13 C NMR spectra were recorded on a BrukerDMX-400 (400 MHz for 1 H, 100 MHz for 13 C) or Bruker Frontier-300 (300 MHz for 1 H, 75 MHz for 13 C) spectrometer.All spectra were obtained in the solvent indicated at 25 o C unless otherwise stated and chemical shifts are given in ppm and are referenced to SiMe4 (δ 0.00, 1 H, 13 C).IR spectra were recorded on a Perkin-Elmer System 2000 FT-IR spectrometer.The GPC measurements were performed using an instrument composed of a Waters-1515 HPLC pump, a Waters 2414 refractive index detector and a combination of Styragel HT-2, HT-3 and HT-4 columns, the effective molar mass ranges of which are 100-10000, 500-30 000 and 5000-600 000, respectively.CHCl3 was used as the eluent (flow rate: 1 mL min -1 , at 35 ºC).Molecular weights and molecular weight distributions were calculated using polystyrene as standard.

Ring opening polymerization of rac-lactide
A representative polymerization procedure, in the presence of one equivalent of benzyl alcohol (Table 7, run 4), is as follows: a toluene solution of C7 (0.020 mmol, 1.0 mL toluene) and BnOH (0.020 mmol) were added to a Schlenk tube in the glovebox at room temperature.The solution was stirred for 2 min, and then rac-lactide (5.0 mmol) along with toluene (2.0 mL) introduced.The reaction mixture was then placed into an oil bath preheated at 80 °C, and the solution stirred for the prescribed time (2 h).The polymerization mixture was then quenched by the addition of an excess of glacial acetic acid (0.2 mL), and the contents poured into methanol (200 mL).The resultant polymer was collected on filter paper and dried under reduced pressure.
The bulk polymerization process (run 19, Table 7) is as follows: C7 (0.020 mmol), BnOH (0.020 mmol) and rac-lactide (5.0 mmol) were added to a Schlenk tube in the glovebox at room temperature.The tube was then placed in an oil bath preheated to 120 °C, and the contents stirred for 2 hours at this temperature.The reaction mixture was then quenched by the addition of an excess of glacial acetic acid (0.2 mL) and methanol (200 mL) added to precipitate the polymer.The polymer was collected on filter paper and dried under reduced pressure.

Crystal structure determinations
Single crystals of C1 -C5 and C7' suitable for X-ray diffraction determinations were obtained from chilled toluene/hexane or tetrahydrofuran/hexane solutions.With graphite monochromated Mo-Kα radiation (λ = 0.71073 Å), cell parameters were obtained by global refinement of the positions of all collected reflections.Intensities were corrected for Lorentz and polarization effects and empirical absorption.The structures were solved by direct methods and refined by full-matrix least squares on F 2 .All hydrogen atoms were placed in calculated positions.Structure solution and refinement were performed by using the SHELXTL-97 package. 24Details of the Xray structure determinations and refinements are provided in Table 9.

Fig. 1 (
Fig. 1 (a) ORTEP drawing of C1 with thermal ellipsoids drawn at the 30% probability level.Hydrogen atoms are omitted for clarity; (b) Core structure with simplified ligand binding.

Fig. 2
Fig. 2 ORTEP drawing of C2 with thermal ellipsoids drawn at the 30% probability level.Hydrogen atoms are omitted for clarity.

Fig. 3
Fig. 3 (a) ORTEP drawing of C3 with thermal ellipsoids drawn at the 30% probability level.Hydrogen atoms are omitted for clarity; (b) Core structure with simplified ligand binding.

Fig. 4
Fig.4ORTEP drawing of C4 with thermal ellipsoids drawn at the 30% probability level.Hydrogen atoms are omitted for clarity.

Fig. 5
Fig. 5 ORTEP drawing of C5 with thermal ellipsoids drawn at the 30% probability level.Hydrogen atoms are omitted for clarity.

Fig. 6
Fig. 6 (a) ORTEP drawing of C7' with thermal ellipsoids drawn at the 30% probability level.Hydrogen atoms are omitted for clarity; (b) Core structure with simplified ligand binding.

Table 1
Selected bond lengths and angles in C1 Nimino bonds [2.604(3) -2.687(3) Å], indicative of the superior donor capacity of the quinolate nitrogen.The molecular structure of C2 is displayed in Figure2, while selected bond lengths and angles are presented in Table2.The structure is similar to C1 with a cubic arrangement made up of Na-O bonds that form the framework of the cube with each of

Table 2
Selected bond lengths and angles for C2

Table 4
Selected bond lengths and angles for C4

Table 6
Selected bond lengths and angles for C7' This journal is © The Royal Society of Chemistry 20xx

Table 7
ROP of rac-LA using C7 in the presence and absence of BnOH a

Table 8
ROP of rac-LA using C1 -C8 in the presence of BnOH a

Table 9
Crystal data and refinement details for C1 -C5 and C7'