Synthesis and structures of mono- and di-nuclear aluminium and zinc complexes bearing α-diimine and related ligands, and their use in the ring opening polymerization of cyclic esters

: A series of organoaluminium imino-amido complexes of the type {[ArNC(Me 2 )C(Me)=NAr]AlMe 2 } (Ar = 2,6- i Pr 2 C 6 H 3 ( 1 ), Ar = 2,6-Et 2 C 6 H 3 ( 2 ); Ar = 2,6-Me 2 C 6 H 3 ( 3 ) have been prepared via reaction of AlR 3 and the respective  -diimine. Similar reaction of the bis(  -diimine) [ArN=C(Me)C(Me)=N-] 2 (Ar = 2,6- i Pr 2 C 6 H 3 ) with AlMe 3 afforded the bimetallic complex [ArN  C(Me) 2 C(Me)=NAlMe 2 ] 2 ( 4 ), whilst reaction of the acetyl-imino compound [O=C(Me)C(Me)=NAr] (Ar = 2,6-Et 2 C 6 H 3 ) with AlMe 3 afforded the bimetallic complex {[OCMe 2 CH(Me)=NAr]AlMe 2 } 2 ( 5 ). In related organozinc chemistry, we have isolated {[ArNC(Me)(Et)C(Me)=NAr]ZnEt} (Ar = 2,6- i Pr 2 C 6 H 3 , 6 ) and the trinuclear complex {[ArN=C(Me)COCHCO(Me)C(Me)=NAr][OCH(Me)C(Me)=NAr](ZnEt) 3 } (Ar = 2,6- 140.3, 142.1 (Ar and C=C), 190.0 (N= C CH 3 ). IR (Nujol, cm −1 ): 2926 s, 2713 w, 1644 m, 1445 s, 1369 s, 1186 w, 1124 m, 1018 m, 956 m, 911 m, 850 w, 789 m, 727 s, 590 m, 498 m. Elemental analysis calcd. for C 54 H 83 Zn 3 N 3 O 3 •0.5toluenen (1018.34): C 64.88; H 8.24; N 3.95. Found: C 64.40, H 8.70, N 4.01%. Multi-metallic complexes (of Al, Zn or Ga) derived from imine-based ligands can effectively operate as catalysts for the ring opening polymerization (ROP) of  -caprolactone (  -CL) and  -valerolactone (  -VL) and the co-polymerization thereof.


Introduction
The global issues associated with the use of single use plastics and their impact on the environment have stimulated further interest in the development of more environmentally friendly polymers.One possible route for accessing such materials is via the use of ring opening polymerization (ROP) of cyclic esters using metal-based catalysts.[1] The main advantages of this route is that by manipulating the coordination environment at the metal, it is possible to control both the catalytic activity of the system and the properties of the resultant products.The choice of metal centre is dictated by a number of factors including cost, abundance, toxicity and performance.Given this, catalysts employing the metals aluminium and zinc continue to attract much attention; main group metal-based ROP systems have been recently reviewed.[2] The use of chelating ligands in many areas of polymerization catalysis has proved beneficial both in terms of catalyst stability and as an aid in the crystallization of the metal species involved.In particular, this has proved highly successful in olefin polymerization, where the use of the N,N-bi-dentate -diimines has opened up new avenues in nickel-based catalysis.[3] Furthermore, such -diimines are known to react with dialkylzinc or trialkylaluminium reagents under reflux, which results in the transfer of an alkyl group to the imine backbone.[4] The resulting imino-amide and pyridyl-amide complexes offer the opportunity of further investigations of possible cooperative effects.
There is also much interest in frameworks capable of binding simultaneously multiple metal centers, which stems from the possibility of utilizing beneficial cooperative effects.[5] Following on from the early nickel work, numerous frameworks capable of binding simultaneously multiple metal centers have been designed using simple condensation chemistry, for example in nickel-based chemistry, those shown in Chart S1 (see ESI) have been reported.
[6] Given this extensive use of imine-based ligation in -olefin oligo-/polymerization, we were somewhat surprised at the rather limited use of such ligation for the metal-catalyzed ROP of cyclic esters.[7] In particular, systems have been reported bearing bipyridyl or phenanroline, guanidine-pyridine, -diiminate and more recently a number of -diimines and amidates, as shown in chart 1 (zinc) and chart 2 (aluminium).In the case of the amidinate aluminium complexes, the bimetallic complexes out performed their monometallic counterparts for both the ROP of -CL, which suggested the presence of beneficial cooperative effects.Also relevant to the work herein is the report by Bochmann et al,[7b] who reported that zinc cations bearing the -diimine (diazadiene) ligand (MeC=NC6H3Pr i 2-2,6)2, are active for the ROP of -caprolactone under mild conditions (60 o C, < 1h) but with low conversions (< 13%).
Finally, for this type of -diimine ligand set, we and others have found that such ligation can, in the presence of alkali metals, aid in the stabilization of metalmetal bonded species.[8] Moreover, low-valent Al II -Al II species bearing -diimine ligation have been shown to be active catalysts for -caprolactone polymerization, and were found to be highly active, which was proposed to be due to the cooperative role between the two Al(II)

centers. [8j]
The molecular structures and ROP capability towards the cyclic esters -caprolactone (-Cl), -valerolactone (-VL) and rac-lactide (r-LA) of the complexes 19 (Chart 3), which are prepared from the pre-ligands L iPr , L Et , L Me , L Et-NO , L iPr-NO , L iPr-N4 and L iPr-N2-ArCH2Ar-N2 , are reported herein.The effect of the presence of these reduced -diimines on the ROP process has also been evaluated herein.The interest in the area is stimulated by the application of poly(caprolactone)/poly(lactide) type biodegradable polymers in the packaging and medical arenas.[9]

Aluminium complexes
The -diimines L iPr , L Et and L Me were prepared by standard condensation route as reported in the literature.Herein, we have also structurally characterized the related complexes 13, which are shown in Figure 1 and Figure S3; selected bond lengths and angles, as well as the values of the four-coordinate geometry index 4, [11a] are given in Table 1.In each case, the distorted tetrahedral aluminium centre is bound by a chelating imino-amido ligand.The N1C1C2N2 portion of the imino-amido ligand is almost planar, with an average torsional angle of 3.43º (for 1), 0.55º (for 2) and 5.44º (for 3).The aluminum atom lies within this plane with the greatest deviation observed for 3 (Al atom  As noted previously, the 1 H NMR spectra for 13 are consistent with the transfer of one methyl to an imino carbon atom with one singlet for the imino-methyl group at 1.95 (for 1), 1.93 (for 2) and 2.03 ppm (for 3) integrating for three protons and another singlet for the two amino-methyl groups at 1.27 (for 1), 1.30 (for 2) and 1.49 ppm (for 3) integrating for six protons.Characteristic high-field resonances for the aluminum methyl groups are observed at 0.93 (for 1), 0.95 (for 2) and 0.78 ppm (for 3, s, 6H).The presence of the asymmetric amido-imino ligands, causes non-equivalence of the protons of the four isopropyl, ethyl and methyl substituents in each ligand of compounds 1, 2 and 3 respectively.In complex 1, the methine protons give rise to two septets (δ = 2.97 and 3.67 ppm) and the methyl groups appear as four doublets (δ = 1.03, 1.07, 1.21 and 1.24 ppm).In complex 2, the methylene protons give rise to three multiplets (δ = 2.45, 2.65 and 3.08 ppm) and the methyl groups appear as two triplet (δ = 1.18 and 1.24 ppm).In complex 3, the methyl groups appear as two singlets (δ = 2.31 and 2.50 ppm) (Figures S13S18).These structures were further investigated by 13 C NMR spectroscopy.In particular, the resonances for the aluminum methyl groups (at 7.5, 7.8 and 7.1 ppm for 13), amido carbon (67.5, 68.9 and 69.6 ppm for 13) and imino carbon (198.6, 197.8
View Article Online DOI: 10.1039/C9DT04332B and 197.5 ppm for 13) were clearly detected.Furthermore, the presence of the amido-imino chelating fragment in 1, 2 and 3 is supported by absorptions at 1629, 1659 and 1622 cm 1 in their respective IR spectra, which correspond to C=N bonds in their imino-amido skeletons.The reaction of diacetyl (2,3-butanedione) with 2,6-diisopropylaniline formed 3-(2,6-diisopropylphenylimino)butan-2-one (L ipr-NO ), which was further reacted with hydrazine to form the bis(α-diimino) ligand (L iPr-N4 ) bearing two bidentate sites (Scheme 2) and possessing zigzag -N=CC=NN=CC=N-bridging spacers.The molecular structure of ligand L iPr-N4 is shown in Figure S1 of the Supporting Information.The bis(α-diimino) compound (L iPr-N4 ) was reacted with two equivalents of AlMe3 to form the corresponding asymmetric bi-nuclear aluminium complex [ArNC(Me)2C(Me)=NAlMe2]2 (4) as shown in Scheme 2. From the literature, it is known that the reaction of -diimine compounds with AlR3 can readily afford imino-amido aluminium compounds or enamine aluminium compounds, resulting from alkyl transfer from aluminium to either the imine carbon atom (C-alkylation) or the imine nitrogen atom (N-alkylation) respectively.[12] Furthermore, the regioselective R-group
View Article Online DOI: 10.1039/C9DT04332B transfer step occurring in these reactions is highly dependent on both the metal and the type of R group present in the organometallic reagent.[13] Interestingly, in compound 4, although both methyl groups attack at the imine carbon, one is found adjacent the Ar group, whilst the other resides on the most distant from Ar.It is thought that the formation of this asymmetric addition product 4 maybe dictated by steric strain imposed by the two amino-methyl groups or isopropyl.Moreover, the compound 4 is ca.18.0 kJ mol 1 more stable than the imino-amido isomer derived by the addition of both methyl groups to the imine carbon, that reside on the most distant from Ar, and 30.3 kJ mol 1 more stable than the isomer obtained from the addition of both methyls to the imine carbon, that reside adjacent the Ar group (Figure S4, Table S1).
View Article Online DOI: 10.1039/C9DT04332B The reaction of L iPr with ZnEt2 affords 6 via ethyl transfer to an imine carbon of the dpp-dad ligand (Scheme 4).The attachment of an ethyl group to the imine carbon of the chelate ligand generates a chiral center at C2 in molecules of 6 and its symmetry is thus distorted (Figure 4).This situation is reminiscent of chiral amido-imine complexes of zinc and magnesium, with a unit cell containing both isomers (R and S). [16] The non-symmetric nature of structure 6 is manifested in the different C-N bond distances for the chelate fragments.
). [16a] In structure 6, a five-membered ring ZnN2C2 adopts a distorted envelope conformation with the Zn atom displaced ca.0.18 Å out of the ring plane.There is a distorted triangle planar coordination around the Zn center formed by an ethyl group and two nitrogens of the chelating amido-imino.
We have previously reported an AlAl-bonded compound (dialumane) with an -diimine ligand, namely [L(THF)Al−Al(THF)L] ( 9), which contains sub-valent Al II centers and dianionic α-diimine ligands (L 2 , (2,6-iPr2C6H3)NC(CH3)]2 2 ).[8a] Complex 9 can act as a multi-electron donor in the reaction with small molecules, [8a, 20] for example, reaction of 9 with azobenzene derivatives proceeded through a four-electron reduction pathway that involved both the Al II centers and the L 2 ligands.[8a] Reactions involving multielectron transfers between metal centers and substrates are at the core of many important transformations in biology and chemistry.[21] In addition, multimetallic catalysis is based on the combined action of metals in a chemical transformation.It has witnessed rapidly increasing developments during the past decades in numerous areas of chemistry.Close proximity between the metal centers thus appears to provide favourable conditions for the occurrence of enhanced catalytic properties, and this proximity can result from the existence of direct metal−metal interactions.
[22] These species have attracted great interest not only because of the novel bonding nature of the low-valent, low-coordinate metal centers, but also because they display fascinating reactivity toward a variety of small molecules as well as potential applications in catalysis.
[23, 8j] Encouraged by this multi-electron-reduction property of dialumane 9, and the recent results reported by Fedushkin, Dagorne et al on related AlAl bonded complexes bearing acenaphthenequinonediamido ligation, [8j] we also included 9 as a potential catalyst in our studies on the ring opening polymerization (ROP) of the cyclic esters -caprolactone (-Cl), -valerolactone (-VL) and rac-lactide (r-LA), see next section.

-Caprolactone (-CL)
The Al-and Zn-based complexes prepared herein were tested as catalysts for the ROP of -CL (Table 2).At 30 °C, good conversions were achieved in the presence of complexes R= Et (2) system in terms of both conversion and control.The bimetallic system 4 (which is an i-Pr derivative) afforded only slightly higher conversion than 1 (91 vs 89%, cf.runs 1 and 4), but with far less control (2.10 vs 1.20).On the other hand, longer reaction times were required by the bimetallic species 5 (an ethyl derivative) in order to obtain complete conversion (480 min versus 60 min, runs 5 and 6).In the case of the Zn-based catalysts 6 and 7 (runs 7 and 8), mono-metallic 6 (an i-Pr derivative) afforded 84% conversion (run 7) with good control (1.20), whilst tri-metallic 7 (also an i-Pr derivative) afforded near quantitative conversion (99%) but with slightly less control (1.70).Interestingly, almost no activity was observed in the presence of the Zn species 8 (run 9).We ascribe this inactivity to the inefficient formation of the required catalytically active alkoxide species from this chloride pre-catalyst.Indeed, we note that in reports by other groups, the formation of M-OR species from parent a chloride complex required salt metathesis via the use of Na (or K) alkoxides, rather than by direct reaction with alcohols.[24] Concerning the effect of the metal center (zinc versus aluminium), slightly lower conversions and polymer Mn were observed in the presence of the Zn-species 6 at 30 ºC, compared to the values obtained when using the Al-derivative 1 (cf.runs 7 and 1).
Notably, the Al-Al bonded complex 9 outperformed all the other systems tested herein, the Mn of the isolated polymers was lower than the calculated values albeit with narrow polydispersities.Broader Mw/Mn (spanning from 1.7 to 2.6) were obtained in the case of the multimetallic species 4, 5 and 7, suggesting the occurrence of transesterification reactions between the two (or three, in the case of 7) metal centers.In spite of the broader polydispersities, conversions and Mn values achieved by using 9 were found to be higher than those obtained in the presence of the dialumane complex reported by Fedushkin,

Dagorne et al under the same reaction conditions. [8j]
By increasing the temperature to 80 °C, all complexes exhibited increased activity with high conversions achieved in most cases within minutes.In particular, only 1 minute was required in the presence of complex 9 (run 27) to achieve quantitative conversion.In all cases, Mn values lower than the expected were obtained.Compared to the experiments carried out at 30 °C, a broadening of molecular weight distribution, thought to be due to increased transesterification, was observed.These results highlighted several differences in the catalytic behavior of the complexes, for example, the bimetallic complex 4 allowed for significantly higher molecular weight albeit with less control (cf.runs 15 and 18) versus 1.However, in terms of catalytic conversion, the appeared to be little benefit from the presence of the second metal.Noteworthy, comparable results for zinc versus aluminium were observed at higher temperature (cf.runs 15 and 25).Regardless of the reaction temperature, narrower polydispersities were achieved in the presence of the Zn catalyst.

-valerolactone (-VL)
The ROP of δ-VL was next investigated (Table 3).In the presence of the Al-based complexes 14 and 9, high conversions spanning from 88 to > 99% were achieved in 4 h at 30 °C (runs 14 and 8).As for -CL, the R = Et derivative (2) was less active here than the i-Pr (1) and Me (3) derivatives.The mono-Al species 13 and the bimetallic compound 4 were found to be equally performing in terms of monomer conversion.
Nevertheless, the polymer molecular weight obtained in the presence of 4 (iPr) was found to be ca.2-fold higher than that of the material isolated with the monometallic complex 1 (iPr) at 30 ºC (23 kDa vs 11kDa, runs 4 and 1, respectively).Low monomer conversion (35%) was observed when compound N,O-chelate bimetallic complex 5 was employed (run 5).Both Zn-based species 6 and 7 exhibited good activity, allowing for 80 and 99% conversion, respectively (runs 6 and 7), suggesting that, unlike for Al (5), the presence of the N,O-chelate is not detrimental.However, direct comparisons are difficult given 7 is trimetallic and an i-Pr derivative versus bimetallic 5 (an Et derivative).Similarly to the case of -CL, shortened reaction times were required when performing the reaction at 80 °C instead of 30 °C (runs 914 and 1722).Indeed, almost complete conversion was achieved with complexes 14 and 9 and 67 within 15 minutes.Compared to the other catalysts, complex 5 proved to be less active, requiring longer reaction time for complete
View Article Online DOI: 10.1039/C9DT04332B monomer conversion even at higher temperature (runs 1517).In all cases, Mn values lower than the expected were observed, and polydispersities spanned the range 1.1 to ca.
2. Notably, complex 5 afforded oligomeric species (Mn = 500) both at 30 and 80 °C (runs 5 and 17, respectively), which suggested inefficient catalyst activation in the former case and early deactivation in the latter.The reactivity trend of the catalysts was found to be similar to that observed in the case of the ROP of -CL.
On increasing the temperature to 80 ºC, monometallic 1 -3 exhibited similar catalytic performances to the bimetallic system 4 and afforded polymers with comparable Mn values.In general, slightly better control was exhibited by the monometallic compounds (Mw/Mn 1.40 vs 1.70, runs 9 and 14, respectively).The N,O-chelate bimetallic complex 5 required longer reaction times to achieve high conversion, with slightly better results achieved in the presence of BnOH.The mono-Zn species 6 performed better in the presence of excess BnOH (two equivalents), albeit with slightly worse control.In the presence of one equivalent of BnOH, the tri-metallic compound 7 afforded a higher conversion versus 6, but with slightly poorer control.In turn, complex 6 was found to be slightly less active than the Al-derivative 1 at 80 o C. When the reaction was performed at room temperature, the system 1 allowed for better conversions and polymer Mn, as well as for narrower polydispersity (cf.runs 1 and 6).By increasing the temperature, comparable molecular weights were obtained, although higher conversion was achieved in the presence of the Al-based complex.Interestingly, the polydispersity was shown to be dependent on the amount of co-catalyst employed.
Also for -CL, the MM bonded complex 9 outperformed the other catalysts, particularly at 30 o C.

rac-Lactide (r-LA)
The ROP of rac-lactide (r-LA) promoted by the Al-based catalysts was then undertaken (Table 4).Moderate conversions were obtained in the presence of all complexes (runs 110) at both 30 and 80 o C.However, liquid oligomers whose Mn could not be detected by GPC were obtained in all cases, regardless of the reaction conditions employed.There was little variation in activity for 1 -3, and the bimetallic system 4 performed no better.A slightly enhanced conversion was achieved using the N,O-chelate bimetallic complex 5, whilst activity similar to that of 1  4 was displayed by the Al-Al bonded complex 9. Use of the Zn-based complexes (Table 5) led in general to better performances.In the presence of monometallic 6, 50% monomer conversion was achieved at 30 ºC, affording low molecular weight oligomers (run 1).An improvement was observed by increasing the temperature to 80 ºC (run 2).Indeed, 66% conversion was obtained in 10 min., affording PLA with Mn of ca. 10 kDa; trimetallic complex 7 displayed similar activity at 30 o C over

Co-polymerization of -CL and -VL
Finally, the co-polymerization of -CL with -VL was examined (Table 6).In the presence of complex 1, moderate conversion was observed by conducting the reaction at 30 ºC, while an enhancement was obtained on increasing the temperature to 50 ºC (54 to 85 %, runs 1 and 2, respectively).The formation of low molecular weight oligomers was achieved by using 2 at 30 ºC (run 3), while a co-polymer with Mn >7,800 was isolated in the reaction performed at higher temperature (run 4).Similar behaviour was exhibited also by complex 3 (runs 5 and 6).Notably, these three catalysts revealed a slight preference for the incorporation of -CL over the other co-monomer.Complete
View Article Online DOI: 10.1039/C9DT04332B conversion of both monomers was observed by using the bimetallic compound 4 (run 7), while liquid oligomers were isolated using complex 5 (run 8).Similarly to 1  3, complex 5 displayed a higher propensity towards the incorporation of -CL over -VL, while a co-polymer with a 1:1 CL/VL ratio was isolated in the presence of 4, as observed by 1 H NMR spectroscopy (70:30 versus 50:50, runs 8 versus 7).The bimetallic species 4 (iPr) was shown to be better performing than its mono-Al congener 1 (iPr), allowing for better conversion and higher polymer Mn (cf.runs 1 and 7).Moreover, the amount of -VL incorporated in the co-polymer was found to be higher than that of the product isolated in the presence of 1 (50% and 40%, respectively).In the case of the Zn-based catalysts, low molecular weight products were isolated in the presence of monometallic 6 at 30 ºC (run 9).Nevertheless, by increasing the temperature, improvements of monomer conversion and polymer molecular weight were achieved (run 10).In the case of trimetallic 7, high conversions were obtained both at 30 and 50 ºC (runs 11 and 12, respectively).
Monometallic complex 6 proved to preferentially incorporate -CL, regardless of the reaction temperature.Notably, in the presence of trimetallic catalyst 7, the tendency to incorporate -VL improved on increasing the temperature.Concerning to the effect of the metal center, the -CL incorporation was found to be higher in the co-polymers synthesized with the Zn-based complex 6 than in those obtained in the presence of the Al system 1 (50 vs 40%, respectively).Finally, full consumption of both monomers was achieved in the presence of the dialumane system 9, regardless of the reaction conditions (runs 13 and 14).

Kinetic studies
A kinetic study of the ROP of -VL using 1, 4 and 6 highlighted that the polymerization rate exhibited a first order dependence on the monomer concentration (Figure 7, left), and the conversion of monomer achieved over 60 min was > 75% (90% for 4).The activity trend was found to be 4 ≈ 1 > 6. outperformed its analog 1.The bimetallic Al species 5 was shown to be far less active.In fact, only low molecular weight oligomers were obtained under the optimized reaction conditions.Compared to the monometallic Zn species 6, the trimetallic complex 7 allowed for higher monomer conversions albeit with less control.The Cl-bearing complex 8 was found to be almost inactive in the ROP of caprolactone, which was thought to be due to activation problems.By contrast, the low-valent Al(II)Al(II) system 9 proved to be the best catalyst amongst those tested herein, and allowed for the complete conversion of the monomer at lower temperatures and/or shorter reaction times than required by the other systems herein.Notably, the activity of this complex in the ROP of -CL was found to be comparable to that of a recently disclosed species having the same type of MM bond.
[8j] This work and that of Fedushkin,Dagorne et al [8j] suggests that for this type of ligation, the presence of the M-M bond is highly beneficial in terms of activity and cooperation between the metal centres.A similar activity trend was observed in the CL/VL co-polymerization and all catalysts proved to preferentially incorporate CL over the other co-monomer.Concerning the polymerization of r-LA, oligomers were isolated when using the aluminum-based complexes, while isotactic PLAs were obtained in the presence of the zinc catalysts 6 and 7.

Experimental
General: All manipulations were carried out under an atmosphere of dry nitrogen using conventional Schlenk and cannula techniques or in a conventional nitrogen-filled glove box.Hexane and toluene were refluxed over sodium.All solvents were distilled and degassed prior to use.The α-diimine ligand L ipr , L Et , L Me and L ipr-NO , L Et-NO , L iPr-N4 and L iPr-N2-ArCH2Ar-N2 were prepared according to literature procedures.[6a, 10, 27] Trimethylaluminium (AlMe3) and diethyl zinc (ZnEt2) and hydrazine (H2NNH2) were purchased from Alfa Aesar.NMR spectra were recorded on a Mercury Plus-400 spectrometer.Elemental analyses were performed with an Elementar VarioEL III instrument.IR spectra were recorded using a Nicolet AVATAR 360 FT-IR spectrometer.
View Article Online DOI: 10.1039/C9DT04332B In the glovebox, a Schlenk tube was charged with the stock solutions of the catalyst and with the required amount of a toluene solution of benzyl alcohol.The mixture was stirred for 2 min at room temperature and then the monomer (2.5 mmol) along with 1.5 mL toluene were added.The reaction mixture was then placed into an oil bath pre-heated to the required temperature, and the solution was stirred for the required time.The polymerization mixture was then quenched by addition of an excess of glacial acetic acid (0.2 mL); the solution was then poured into methanol (200 mL) and the resultant polymer was then collected on filter paper and dried in vacuo.

Figure 1 .
Figure 1.The molecular structure of 1 (top left), 2 (top right) and 3 (bottom) (thermal ellipsoids are set at the 20% probability level; H atoms are omitted for clarity).

Figure 5 .
Figure 5.The molecular structure of 7 (thermal ellipsoids are set at the 20% probability level; most of H atoms, iPr groups of L are omitted for clarity; the C atoms in Ph are drawn as smaller spheres).
) Å).The N1-C1 and N2C6 bonds are1.286(6)and1.282(6)Å and correspond to a C=N double bond.It is also of note that attachment of H to the carbon atom bound to oxygen generates the second chiral center at C9, and the complex molecule adopts a homochiral configuration and the unit cell (Z = 4) consists of two pairs of enantiomers (SC5,SC9 and RC5,RC9).Complex 7 consists of a six-membered Zn3O3 ring with alternating zinc and oxygen atoms, with the Zn atoms each adopting tetrahedral coordination spheres.The conformation of the Zn3O3 cycle is a distorted boat, with the Zn2 and O3 atoms at the apices, which is assembled via μ2-bridging oxygen atoms of the L iPr-NO ligand.The ZnC bond lengths (mean value 1.972 Å) are comparable and are in the typical range reported for Zn-Me groups.[17]The ZnO bond lengths vary from1.967(3)  to 2.095(3) Å and compare well with values for ZnO single bonds found in the other cyclic zinc oxides compounds, such as [MeZn(bdmap)]2MeZnOOMe and [MeZn(bdmap)]2MeZnOH (Hbdmap = 1,3-bis(dimethylamino)propan-2-ol).[18] The coordination bond ZnN1 in Dalton Transactions Accepted Manuscript Published on 30 December 2019.Downloaded by University of Hull on 1/3/2020 12:57:55 PM.
allowing for complete monomer conversion within 5 minutes (run 10).No drop in activity was observed on progressively increasing the monomer/catalyst ratio from 100 to 2000 in the presence of different amounts of co-catalysts (runs 11-14).Whilst the modus operandi of this catalysts is not clear, work by Fedushkin, Dagorne et al, supported by DFT studies, suggests the Al(II)Al(II) bond is not cleaved during the catalytic process and the alcohol coordinates one of the metal centres leading, via proton transfer to a nitrogen atom of the ligand, to an Al(II)-alkoxide species.[8j] Preliminary ROP studies conducted in the absence of BnOH reveal a clear reduction in activity (conversion 44% over 15 min.),suggesting the alcohol is indeed playing a role here.[25] In the case of catalysts 13 and 6,

Scheme 2. Synthesis of complex 4.
The IR spectrum exhibits an intense absorption band at 1644 cm 1 , which corresponds to stretching vibrations of the C=N groups, whilst those associated with C=O double-bond character were lost.

Table 4 .
ROP of r-LA promoted by Al compounds 15 and 9.

Table 5 .
ROP of r-LA promoted by Zn compounds 6 and 7.