Chiral (1,2)-diphenylethylene-salen complexes of triel metals: coordination patterns and mechanistic considerations in the isoselective ROP of lactide.

DOI: 10.1002/chem.201304788

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& Ring-Opening Polymerisation

Chiral (1,2)-Diphenylethylene-Salen Complexes of Triel Metals: Coordination Patterns and Mechanistic Considerations in the Isoselective ROP of Lactide** Nicolas Maudoux,[a] Thierry Roisnel,[b] Vincent Dorcet,[b] Jean-FranÅois Carpentier,*[a] and Yann Sarazin*[a]

Abstract: The synthesis of enantiomerically pure aluminium, gallium and indium complexes supported by chiral (R,R)(HHONNOHH) (1), (R,R)-(MeHONNOHMe) (2), (R,R)-(tButBuONNOtButBu) (3), (R,R)-(MeNO2ONNOMeNO2) (4), (R,R)-(HOMeONNOHOMe) (5) and (R,R)-(ClClONNOClCl) (6) (1,2)-diphenylethylene-salen ligands is described. Several of these complexes have been crystallographically authenticated, which highlights a diversity of coordination patterns. Whereas all Ga complexes form [Ga2(CH2SiMe3)4(ONNO)] bimetallic species (ONNO = 1–3), aluminium [AlR(ONNO)] (R = Me, CH2SiMe3) and indium [In(CH2SiMe3)(ONNO)] derivatives are monometallic for ONNO = 1, 2 and 4–6, and only form the bimetallic complexes [Al2R4(ONNO)] and [In2(CH2SiMe3)4(ONNO)] for the most sterically crowded ligand 3. The [AlMe(ONNO)] complexes react with iPrOH to give [AlOiPr(ONNO)] complexes that are robust towards further iPrOH. The [In(CH2SiMe3)(ONNO)] congeners are inert towards excess alcohol, whereas the Ga com-

Introduction Isotactic poly(lactide) (PLA) and related stereoblock copolymers and stereocomplexes are biosourced, crystalline materials that exhibit high melting temperatures and other attractive properties for a variety of applications.[1] The search for rational [a] N. Maudoux, Prof. Dr. J.-F. Carpentier, Dr. Y. Sarazin Organometallics: Materials and Catalysis Dept. Institut des Sciences Chimiques de Rennes UMR 6226 CNRS–Universit de Rennes 1 35042 Rennes Cedex (France) E-mail: [email protected] [email protected] [b] Dr. T. Roisnel, Dr. V. Dorcet Centre de Diffractomtrie des Rayons X Institut des Sciences Chimiques de Rennes UMR 6226 CNRS - Universit de Rennes 1 35042 Cedex, Rennes (France) [**] ROP = Ring-opening polymerisation. Supporting information for this article (including details for the synthesis, characterisation and X-ray structures of all proteo-ligands (R,R)-1-H2–(R,R)6-H2 ; table of crystallographic data; complete tables of polymerisation data; all experimental plots for kinetic analyses and kinetic curve-fitting plots) is available on the WWW under http://dx.doi.org/10.1002/ chem.201304788. Chem. Eur. J. 2014, 20, 6131 – 6147

pounds decompose easily. All these alkyl complexes, as well as the [AlOiPr(ONNO)] derivatives, catalyse the ring-opening polymerisation (ROP) of racemic lactide (rac-LA). The [AlMe(ONNO)] complexes require additional alcohol to afford controlled reactions, but [AlOiPr(ONNO)] complexes are single-component catalysts for the isoselective ROP of rac-LA, with values of Pm in the range 0.80–0.90. Experimental evidence unexpectedly shows that chain-end control leads to the isoselectivity of these aluminium catalysts; also, the more crowded the coordination sphere, the higher the isoselectivity. The bimetallic Ga complexes do not afford controlled reactions, but the binary [In(ONNO)(CH2SiMe3)/(PhCH2OH)] systems competently mediate nonstereoselective ROP; evidence is given that an activated monomer mechanism is at work. Kinetic studies show that catalytic activity decreases when electronic density and steric congestion at the metal atom increase.

routes towards such materials through stereocontrolled ringopening polymerisation (ROP) of racemic lactide (rac-LA) has continued to attract sustained interest ever since the seminal discoveries by the groups of Spassky and Coates that enantiomerically pure aluminium-salen complexes[2] afforded highly stereoregular PLA architectures.[3, 4] Aluminium has actually proved in a class of its own, as Al complexes supported by dianionic tetradentate salen,[3–5] salan[6]and structurally related salalen and dialkoxy-diimino[7] ligands reliably enable the production of genuinely isotactic PLA (0.80 < Pm < 1.00; see below).[8] Otherwise, mitigated success in the isoselective ROP of LA has sporadically been achieved in the past 15 years with complexes of various metals (Ti, In, Y, Zn)[8, 9, 10] typically supported by a monophenolate ligand, affording PLA with a more moderate isotactic bias (Pm 0.65–0.80).[11] In spite of unique performances in term of isoselectivity,[8, 12] the aluminium-catalysed polymerisation of LA suffers from low activity, with the conversion of a few hundred equivalents of monomer typically requiring hours, if not days, at temperatures usually above 100 8C. To overcome this drawback, attempts have been made to substitute indium for aluminium,[13] with the hope that the larger metal (effective ionic radii for coordination number (CN) = 4: Al3 + , 0.32; In3 + , 0.62 )[14] would

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Full Paper cause emanating from chiral complexes), chain-end mechanism is provided.

Results and Discussion Syntheses and characterisation of Al, Ga and In complexes The enantiomerically pure proteo-ligands (R,R)-1-H2–(R,R)-6-H2 and (S,S)-2-H2 were obtained as yellow solids in good yields (87–95 %) by condensation of (1S,2S)- or (1R,2R)-1,2-diphenylethane-1,2-diamine with the relevant salicaldehyde in ethanol heated at reflux (Scheme 1). Experimental details for these compounds, including X-ray structure determination for (R,R)3-H2 and (R,R)-6-H2, are given in the Supporting Information. Synthesis of aluminium complexes The stoichiometric reaction of (R,R)-1-H2, (R,R)-2-H2 and (R,R)-4H2 with [Al2Me6] proceeded through elimination of two equivalents of methane to afford the desired aluminium-alkyl complexes [1-AlMe], [2-AlMe] and [4-AlMe] (Scheme 2).[22] The identity of the complexes was established by 1D and 2D NMR spectroscopy and X-ray diffraction crystallography (for [1AlMe]), and their purity was confirmed by combustion analyafford higher reaction rates, as often observed with alkalineand rare-earths. Initial results by the groups of Mehrkhodavandi[15] and Tolman and Hillmyer[16] offered much promise, but it has Scheme 1. Synthesis of enantiomerically pure proteo-ligands (R,R)-1-H2-(R,R)–6-H2. since proved impossible to produce highly isotactic PLA with indium catalysts.[7d, 17] The larger size of indium and different ses. The 1H NMR spectra were consistent with the presence of operative mechanisms are invoked as the main reasons for the a single metal atom per ancillary ligand and pointed at unsymobserved loss of isoselectivity. Such surmises could be chalmetrical structures in C6D6 solution. For instance, diagnostic resonances in the 1H NMR spectrum of [1-AlMe] include lenged by using gallium (Ga3 + , 0.47 for CN = 4)[14] catalysts, but surprisingly little has been reported concerning such sysa sharp singlet at d = 0.08 ppm assigned to AlCH3, a doublet tems.[13, 18] Horeglad and co-workers have reported that mixed and a doublet of doublets at d = 4.21 and 5.13 ppm, respecNHC-stabilised GaIII alkoxide species afforded isotactic-enriched tively, corresponding to the two non-equivalent NCH(Ph) hyPLA,[18b] and Williams and co-workers have used 8-quinolinoladrogens, and a doublet and singlet (d = 7.54 and 7.75 ppm, reto gallium complexes to the same effect,[19] but a main breakspectively) for the CH=N moieties. Similar patterns were obthrough remains elusive. served for the two other complexes. On the other hand, the Our group maintains a keen interest in the implementation known[21] bimetallic [3-Al2Me4] was repeatedly isolated (someof phenolate main-group metal catalysts for controlled ROP retimes with small amounts of the monometallic [3-AlMe]) upon actions, and in the comprehension of the associated mechareaction of [Al2Me6] (0.5 or 1.0 equiv) and the bulkier proteonisms.[20] It has occurred to us that a systematic study of familigand (R,R)-3-H2 (Scheme 2). This complex was crystallographilies of salen ROP catalysts constructed across triel elements has cally characterised. The molecule is C2-symmetric in the solidnot been reported to date. We describe here the synthesis, state; each Al centre is 4-coordinated and rests in a tetrahedral structural features and ROP catalytic activity of Al, Ga and In environment. Consistently with this, the solution NMR data feacomplexes incorporating an enantiomerically pure salen ture one set of resonances for all characteristic hydrogens. ligand, based on the less classical (1R,2R)-diphenylethylene The reactions of (R,R)-2-H2 and that of the bulkier (R,R)-3-H2 backbone, that we have previously employed in the enantiosewith [Al(CH2SiMe3)3] in C6D6 (80 8C) afford [2-Al(CH2SiMe3)] and lective catalysis of ketone cyanosilylation.[21] The diverging ROP [3-Al2(CH2SiMe3)4], respectively, upon release of SiMe4. The size mechanisms at work in the cases of chiral Al and In catalysts of the alkyl substituents (CH3 vs. CH2SiMe3) on the metal atom are discussed on the basis of experimental data, and evidence hence does not direct the formation of mono- or bimetallic for a highly isoselective Al-mediated (yet counterintuitive bespecies, which solely depends on the identity of the ortho subChem. Eur. J. 2014, 20, 6131 – 6147

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Scheme 2. Synthesis of enantiopure carbyl- and isopropoxide aluminium–salen complexes.[22]

stituents on the phenolate ligands. These two compounds were characterised spectroscopically (and crystallographically for the former), but were not isolated as a bulk material. All Almethyl complexes are soluble in ethers and aromatic hydrocarbons. Epimerisation of optically active centres in the ligand framework was never detected upon binding to the metal.[22] The equimolar reaction of [Al(OiPr)3] with (R,R)-1-H2–(R,R)-6H2 in toluene or benzene afforded mitigated results (Scheme 2). Complexes [1-AlOiPr]–[3-AlOiPr] and [5-AlOiPr] and [6-AlOiPr] were prepared in good yields (60–95 %) as yellow crystalline solids following reaction in toluene at 80 8C; with the exception of the poorly soluble [6-AlOiPr], all the compounds can be dissolved in common organic solvents. On the other hand, attempts to obtain [4-AlOiPr] on a preparative scale following this protocol only returned mixtures of compounds that could not be unambiguously identified nor separated. Although [4-AlOiPr] can be synthesised neatly in this way on an NMR scale, removal of the volatiles under vacuum leads to decomposition of the product. The reaction of [4AlMe] with iPrOH at 50–70 8C is also clean, but again it failed to give isolable [4-AlOiPr], as the desired complex was contaminated by large amounts on an unidentified species after evaporation of the volatile fraction. Although it could not be isolated, [4-AlOiPr] can finally be synthesised quantitatively and in spectroscopically pure fashion by treatment of (R,R)-4H2 with [AlMe2(OiPr)] in [D6]benzene at 70 8C; heating is required, because pure [4-AlMe] is otherwise unexpectedly generated upon release of one equiv of methane and one equiv of iPrOH if the reaction is instead carried out at 25 8C. Tolman, Hillmyer and co-workers have reported before similar difficulties in synthesising amino-bis(phenolate) aluminium complexes bearing electron-withdrawing groups on the aromatic rings.[23] All complexes [1-AlOiPr]–[5-AlOiPr] are monometallic on the basis of spectroscopic data, including [3-AlOiPr] in which the encumbered ligand could have been expected to lead to a bimetallic species as seen for [3-Al2Me4]. Chem. Eur. J. 2014, 20, 6131 – 6147

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Synthesis of gallium and indium complexes [Ga(CH2SiMe3)3] and [In(CH2SiMe3)3] were used instead of their methyl analogues, which are less convenient from a synthetic perspective.[17] The reaction of (R,R)-1-H2–(R,R)-3-H2 with the gallium precursor (1 or 2 equiv) gives the three corresponding bimetallic complexes [1-Ga2(CH2SiMe3)4]–[3-Ga2(CH2SiMe3)4] after work-up, independently of the nature of the substituents on the ligand framework (Scheme 3). Similar phenomena have been reported before for related salen-Ga complexes, and was attributed to the remarkable thermodynamic stability of the chelated Ga(alkyl)2 fragment.[24] These Ga complexes are soluble in common organic solvents, and have been characterised by NMR spectroscopy and elemental analysis, which are consistent with the proposed formulations. The molecular solid-state structure of [2-Ga2(CH2SiMe3)4] has been determined. The synthetic scenario with indium is similar to that seen for aluminium (Scheme 3). Treatment of [In(CH2SiMe3)3] with (R,R)1-H2, (R,R)-2-H2 or (R,R)-4-H2 at high temperature afforded the corresponding monometallic complexes [1-In(CH2SiMe3)], [2In(CH2SiMe3)] and [4-In(CH2SiMe3)], respectively, in high yields upon release of two equiv of SiMe4. The NMR data for these complexes is consistent with the presence of a single metal atom per salen ligand. As seen for aluminium (and gallium), the sterically encumbered (R,R)-3-H2 reacts with one equiv of metallic precursor to give a mixture of species from which only the bimetallic [3-In2(CH2SiMe3)4] (independently synthesised by treating the proteo-ligand with 2 equiv of [In(CH2SiMe3)3]) could be identified. Compound (R,R)-5-H2 was not utilised on account of its poor performance in polymerisation catalysis (see below), whereas (R,R)-6-H2 led to insoluble materials. All isolated indium complexes are fairly soluble in usual organic solvents and have been fully characterised. Attempts to prepare indium-isopropoxide species by treatment of these Inalkyl complexes with iPrOH proved unsuccessful as no reaction was observed (see below); comparable lack of reactivity of In-

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Scheme 3. Synthesis of gallium– and indium–salen complexes.[22]

alkyl bonds towards alcohol has already been reported.[7d, 17c] Salt metathesis routes using [2-InCl] as a precursor have not been thoroughly explored (initial attempts gave a mixture of unidentifiable species) as they are often erratic and lead to unpredictable outcomes during reactions with alkaline alkoxides;[17c–d] the chloro complex can itself be readily obtained through elimination of KCl in the salt metathesis reaction between the potassium salt 2-K2 (quantitatively synthesised by treatment of (R,R)-2-H2 with 2 equiv of KN(SiMe3)2) and [InCl3].

The arrangement in [1-In(CH2SiMe3)] resemble that in [2-Al(CH2SiMe3)]. The 5-coordinated indium atom sits in a slightly distorted SP geometry (t = 0.17) with the C(33) atom at the apical position and the metal resting 0.82 above the O(32)N(24)-N(9)-O(1) mean plane (Figure 3). The distances to C, N and O atoms are typical for such indium complexes.[7d, 17d] The addition of o-Me substituents in [2-In(CH2SiMe3)] bears no influence of the coordination sphere around the 5-coordinated metal (Figure 4), as the geometry (distorted SP with t = 0.13), arrangement (C(42)-C(41)-C(21)-C(22) torsion angle =

X-ray crystallographic studies Crystals of [1-AlMe], [1-Al(CH2SiMe3)], [1-In(CH2SiMe3)], [2-In(CH2SiMe3)], [2-Ga2(CH2SiMe3)4] and [3-In2(CH2SiMe3)4] suitable for X-ray diffraction studies were grown from the purified compounds and their structures were determined (Figure 1–6). The structure of [3-Al2Me4] matched that reported elsewhere,[21] confirming the formation of a bi-aluminium species. The aluminium atom in [1-AlMe] is 5-coordinated (Figure 1); the distorted geometry about the metal is halfway between trigonal bipyramidal (TBP) and square pyramidal (SP) (t = 0.55).[25] All bond lengths and angles fall in the range expected for Al-salen complexes.[2b,c] Binding of the ligand onto the metal entails a narrow torsion angle C(61)-C(60)-C(80)-C(81) of 64.18. The geometry around the metal in [2-Al(CH2SiMe3)] approaches SP (t = 0.13), with the bulky alkyl group (C(35)) located at the apex (Figure 2). All pertinent bond distances around the metal are very close to those in [1-AlMe], but the C(Ph)C(27)-C(28)-C(Ph) torsion angle is now a little larger (74.0 8). Chem. Eur. J. 2014, 20, 6131 – 6147

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Figure 1. Molecular solid-state structure of [1-AlMe]. Only one of the two independent but crystallographically equivalent molecules is depicted. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn at the 50 % probability level. Selected bond lengths [] and bond angles [8]: Al(2)O(51) 1.796(2), Al(2)O(71) 1.825(2), Al(2)C(91) 1.981(2), Al(2)N(79) 2.025(2), Al(2)N(59) 2.036(2); O(51)-Al(2)-O(71) 88.08(7), O(51)-Al(2)-C(91) 117.66(9), O(71)-Al(2)C(91) 103.32(9), O(51)-Al(2)-N(79) 127.98(7), O(71)-Al(2)-N(79) 89.61(7), C(91)Al(2)-N(79) 113.40(8), O(51)-Al(2)-N(59) 88.40(7), O(71)-Al(2)-N(59) 161.12(8), C(91)-Al(2)-N(59) N(79)-Al(2)-N(59) 78.02(7).

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Figure 2. Molecular solid-state structure of [2-Al(CH2SiMe3)]. Only one of the four independent but crystallographically equivalent molecules is depicted. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn at the 50 % probability level. Selected bond lengths [] and bond angles [8]: Al(1)O(1) 1.802(2), Al(1)O(2) = 1.810(2), Al(1)C(35) 1.976(3), Al(1)N(2) 2.022(3), Al(1)N(1) 2.040(2); O(1)-Al(1)-O(2) 87.91(10), O(1)-Al(1)-C(35) 108.60(12), O(2)-Al(1)-C(35) 113.50(12), O(1)-Al(1)-N(2) 150.54(11), O(2)-Al(1)-N(2) 88.02(10), C(35)-Al(1)-N(2) 99.76(12), O(1)-Al(1)-N(1) 88.21(10), O(2)-Al(1)-N(1) 142.62(11), C(35)-Al(1)-N(1) 102.94(11), N(2)-Al(1)-N(1) 77.63(10).

Figure 3. Molecular solid-state structure of [1-In(CH2SiMe3)]. Only one of the two independent but crystallographically equivalent molecules is depicted. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn at the 50 % probability level. Selected bond lengths [] and bond angles [8]: In(1)O(32) 2.094(4), In(1)O(1) 2.108(5), In(1)C(33) 2.144(8), In(1)N(9) 2.263(6), In(1) N(24) 2.267(6); O(32)-In(1)-O(1) 88.48(19), O(32)-In(1)-C(33) 113.3(3), O(1)In(1)-C(33) 116.2(3), O(32)-In(1)-N(9) 129.3(2), O(1)-In(1)-N(9) 82.5(2), C(33)In(1)-N(9) 115.4(3), O(32)-In(1)-N(24) 83.12(19), O(1)-In(1)-N(24) 139.6(2), C(33)-In(1)-N(24) 103.3(3), N(9)-In(1)-N(24) 73.01(19).

56.78; distance to ONNO mean plane = 0.83 ) and bond lengths are essentially identical to those in [1-In(CH2SiMe3)]. The structure of [2-In(CH2SiMe3)] mimics very closely that of its aluminium relative [2-Al(CH2SiMe3)]. With the more-hindered ligand bearing o-tBu groups, the solid-state structure of the bimetallic [3-In2(CH2SiMe3)4] depicted in Figure 5 shows that each of the two equivalent indium atoms lie in a distorted tetrahedral geometry. The molecule contains a crystallographic C2-symmetry axis, and this is consistent with the 1H NMR spectrum of this complex. The geometric features of the ligand in [3-In2(CH2SiMe3)4] match very well Chem. Eur. J. 2014, 20, 6131 – 6147

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Figure 4. Molecular solid-state structure of [2-In(CH2SiMe3)]. Only one of the two independent but crystallographically equivalent molecules is depicted. Hydrogen atoms omitted for clarity. Ellipsoids are drawn at the 50 % probability level. Selected bond lengths [] and bond angles [8]: In(1)O(31) 2.0967(17), In(1)O(11) 2.0997(19), In(1)C(1) 2.145(3), In(1)N(40) 2.260(2), In(1)N(20) 2.275(2); O(31)-In(1)-O(11) 88.46(7), O(31)-In(1)-C(1) 117.86(9), O(11)-In(1)-C(1) 112.64(10), O(31)-In(1)-N(40) 83.57(8), O(11)-In(1)-N(40) 137.85(8), C(1)-In(1)-N(40) 107.61(10), O(31)-In(1)-N(20) 129.96(7), O(11)-In(1)N(20) 82.05(8), C(1)-In(1)-N(20) 111.09(10), N(40)-In(1)-N(20) 72.18(8).

Figure 5. Molecular solid-state structure of [3-In2(CH2SiMe3)4], with disordered CH2SiMe3 fragments. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn at the 50 % probability level. Selected bond lengths [] and bond angles [8]: In(1)O(24) 2.104(2), In(1)C(28) 2.131(4), In(1)C(24) 2.172(4), In(1)N(8) 2.263(3); O(24)-In(1)-C(28) 109.19(16), O(24)-In(1)-C(24) 104.57(13), C(28)-In(1)-C(24) 134.99(17), O(24)-In(1)-N(8) 83.54(10), C(28)-In(1)N(8) 100.59(14), C(24)-In(1)-N(8) 112.13(15).

those found in (R,R)-3-H2 (Supporting Information), and indeed the disposition of the O and N atoms in the proteo-ligand strongly disfavours the formation of a monometallic species upon complexation. The arrangement in [2-Ga2(CH2SiMe3)4] is nearly identical to that in [3-In2(CH2SiMe3)4] (Figure 6). Each 4-coordinated galli-

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Figure 6. Molecular solid-state structure of [2-Ga2(CH2SiMe3)4]. Non-interacting solvent molecule (benzene) and hydrogen atoms are omitted for clarity. Ellipsoids are drawn at the 50 % probability level. Selected bond lengths [] and bond angles [8]: Ga(1)O(17) 1.890(3), Ga(1)C(18) 1.964(4), Ga(1)C(22) 1.977(4), Ga(1)N(8) 2.045(3), Ga(2)-O(47) 1.890(3), Ga(2)C(52) 1.950(4), Ga(2)C(48) 1.969(4), Ga(2)N(38) 2.054(3); O(17)-Ga(1)-C(18) 108.39(16), O(17)-Ga(1)-C(22) 107.55(15), C(18)-Ga(1)-C(22) 120.21(18), O(17)-Ga(1)-N(8) 92.61(13), C(18)-Ga(1)-N(8) 113.41(16), C(22)-Ga(1)-N(8) 110.87(15), O(47)Ga(2)-C(52) 111.44(17), O(47)-Ga(2)-C(48) 105.45(16), C(52)-Ga(2)-C(48) 124.1(2), O(47)-Ga(2)-N(38) 92.29(12), C(48)-Ga(2)-N(38) 112.29(16).

um atom sits in a distorted tetrahedral geometry. The GaC, GaO and GaN bond lengths are all fairly usual,[19] and a C(Ph)-C(H)-C(H)-C(Ph) torsion angle of 50.3 8 is measured. Reactivity with alcohol As mentioned above, the treatment of [4-AlMe] with iPrOH at 50–70 8C yields [4-AlOiPr], even if this complex could not be isolated. On the other hand, the attempted synthesis of [In-OiPr] species akin to [1-AlOiPr]–[5-AlOiPr] failed since our [In-CH2(SiMe3)2] complexes show no reactivity towards iPrOH. These observations are pertaining to ROP studies, since:[12] 1) Regular, effective single-component ROP catalysts affording complete and rapid initiation of the living polymerisation include an alkoxide as a reactive nucleophile; 2) rather than on their own, [metal–alkyl] species customarily afford more efficient (i.e., more controlled) living ROP catalysts when paired with one (or more) equiv of exogenous alcohol such as iPrOH or BnOH, and 3) the association of a metal precatalyst with a large excess of alcohol as an external transfer-agent generates binary catalysts for “immortal” ROP reactions.[12c] The reactivity of the new Al/In-alkyl complexes was therefore further interrogated. The alcoholysis of [1-AlMe] with 1 equiv of iPrOH at room temperature proceeds extremely slowly and requires 8 h at 70 8C to yield [1-AlOiPr] quantitatively. With 10 equiv of Chem. Eur. J. 2014, 20, 6131 – 6147

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alcohol at 70 8C, that is, under conditions relevant to “immortal” ROP catalysis, the quantitative production of [1-AlOiPr] still takes 5 h; once formed, the complex is robust in presence of excess alcohol and neither release of free proteo-ligand nor decomposition of the complex is observed even after prolonged exposure (19 h) at 70 8C. The slow formation of [1-AlOiPr] from the methyl precursor even at 70 8C argues against the use of [1-AlMe] as a good precatalyst for controlled ROP catalysis, and alkoxo complexes such as the pre-synthesised [1-AlOiPr] should be favoured instead. Complex [2-Ga2(CH2SiMe3)4] is sensitive towards alcohol. Rapid decomposition leading to the formation of a mixture of several species (including the starting material and free (R,R)-2-H2) is observed in the presence of 1–10 equiv of BnOH (Bn = PhCH2), at 100 8C but also at room temperature. More than the sheer impossibility to obtain [InOiPr] complexes on a preparative scale through alcoholysis with iPrOH, complexes [1-In(CH2SiMe3)2] (and its bulkier derivatives) remarkably show complete absence of reactivity towards BnOH, even after exposure to a 10-fold excess of alcohol at 100 8C for up to 68 h. The complex remains intact, as release of SiMe4 and/or free proteo-ligand is not detected by 1H NMR spectroscopy; this suggests that these indium complexes constitute candidates to catalyse ROP reactions according to an activated monomer mechanism.[7d, 12, 13]

Ring-opening polymerisation studies Preliminary screening The aluminium-alkyl complexes [1-AlMe] and [2-AlMe] mediate the polymerisation of racemic lactide (rac-LA) in the absence of any co-activator, partly converting 100 equiv of monomer in 24 h at 70 8C to give isotactic-biased PLA (Pm 0.60–0.65)[11] but without control over the molecular features (Mn,exp > Mn,theo). End-group analysis (NMR spectroscopy, MALDI-TOF MS) was inconclusive as to the nature of the initiating group. Under otherwise identical conditions, the reactions are somewhat faster and the control over the ROP parameters is enhanced (Mn,exp Mn,theo) upon addition of 1.0 equiv of alcohol (BnOH or iPrOH), whereas the resulting PLAs are significantly isotactic-enriched (0.70 < Pm < 0.80) and feature good endgroup fidelity. Yet, because alcoholysis of [Al]-Me with ROH is slow at 70 8C, it is likely that more than one type of catalytically active species (and possibly different mechanisms) are at work under these conditions, which is tentatively reflected by the relatively larger molecular weight distribution observed even at partial conversion (1.20 < Mw/Mn < 1.30). Details for these experiments can be found in Table 1 and in the Supporting Information. The use of alkoxide complexes [1-AlOiPr]–[6-AlOiPr] allows us to circumvent the question of the active species in the [AlMe/ROH] binary systems, and several general features can be extracted from the data collected in Table 1. The precatalyst [4AlOiPr] was prepared in situ prior to addition of the monomer by reaction of [4-AlMe] with 1.0 equiv of iPrOH at 70 8C for 6 h.[26] The four complexes were found to catalyse the isoselective ROP of rac-LA in a fairly controlled fashion at 50–130 8C.

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Full Paper 85 8C and above (Table 2 and the Supporting Information). In the absence of alcohol, rac-LA is polymerMn,NMR[c] Mn,SEC[d] Mw/ Pm[e] Entry Al T t Conv. Mn,theo[b] ised very slowly and/or without any control. Howev[g mol1] [g mol1] [g mol1] Mn [8C] [h] [%] er, the addition of 1.0 equiv of BnOH versus In affords an effective binary catalyst that gives narrowly dis1 [1-AlMe] 70 24 33 4800 n/d 38500 1.13 0.64 2 [1-AlMe][f] 70 24 95 13800 9400 9900 1.29 0.72 persed PLAs featuring Mw/Mn characteristically below 3 [1-AlOiPr] 50 24 63 9100 8300 8500 1.12 0.81 1.10. Both the reaction rates and control over the 4 [1-AlOiPr] 50 48 86 12500 9600 12200 1.34 0.82 ROP parameters compare well with those disclosed 5 [1-AlOiPr] 70 24 90 13000 12700 11800 1.45 0.80 by other phenolate–indium complexes.[13–17] End6 [2-AlOiPr] 50 72 96 13900 7500 8800 1.22 0.86 7 [3-AlOiPr] 70 24 15 2200 n/d n/d n/d n/d group analysis (MALDI-TOF MS, 1H NMR spectrosco8 [3-AlOiPr] 90 144 38 5500 5000 5300 1.05 0.90 py) confirmed the presence of BnOC(=O) and CH9 71 10300 2600 5000 1.09 0.75 9 [4-AlOiPr][g] 50 (CH3)OH termini. Under comparable conditions, 24 82 11900 5100 6000 1.33 0.77 10 [4-AlOiPr][g] 50 higher rates constant are observed with these indium 11 [5-AlOiPr] 50 24 49 7100 3100 4400 1.08 0.83 12 [6-AlOiPr] 50 9 83 12000 11200 11500 1.27 0.80 systems than with their aluminium-based derivatives (Tables 1 and 2 and the Supporting Information). Un[a] Polymerisation conditions: [rac-LA]0 = 0.8 m in toluene, [rac-LA]0/[Al]0 = 100:1. [b] Mn,theo = [rac-LA]0/[Al]0 conversion 144.13 + 60.10; n/d = not determined because fortunately, they do not afford any stereocontrol, as of experimental constraints. [c] Determined by 1H NMR spectroscopic end-group analthe resulting PLAs are at best slightly isotactic-enysis. [d] Determined by SEC analysis versus polystyrene standards and corrected by riched.[11] Note that, in the same way as that seen for [27] 1 [11] a factor of 0.58. [e] Determined by homodecoupled H NMR spectroscopy. [f] With [g] [2-AlOiPr] and [4-AlOiPr], a loss of stereocontrol is 1.0 equiv of iPrOH versus Al. Precatalyst prepared in situ. observed on replacing the p-H in [2-In(CH2SiMe3)2] by p-NO2 in [4-In(CH2SiMe3)2], whereas this substituWith the exception of [4-AlOiPr], for which no obvious extion was simply expected (on steric grounds) to lead to the obplanation has been found (other than the uncertainty on the served rate enhancement (Table 2, entries 4 and 5). amount of iPrOH added to generate in situ this complex), The bimetallic complexes [1-Ga2(CH2SiMe3)4]–[3-Ga2there is generally a fairly good agreement between theoretical (CH2SiMe3)4], [3-Al2Me4] and [3-In2(CH2SiMe3)4] all catalyse the ROP of rac-LA at 70–100 8C, but disappointingly they do not (calculated on the basis of monomer conversion and [rac-LA]0/ afford any stereocontrol (Pm < 0.60). This is so even in the case [Al]0) and experimental molecular weights; also, the polymers are monodisperse, with Mw/Mn between 1.05 and 1.34. Typicalof aluminium, which therefore differentiates the bimetallic [3Al2Me4] from the monometallic [3-AlOiPr]; one can surmise ly for aluminium,[3, 4b, 6–9] the catalysts are sluggish, requiring 1– 3 days to convert 100 equiv of monomer under mild condithat both a small metal atom and a crowded coordination tions (50–70 8C), although the precatalyst [3-AlOiPr] with bulky sphere are required to enable efficient stereocontrol with o-tBu substituents is inactive below 70 8C (Table 1, entries 7 these systems, although such simplification leaves aside the and 8). Expectedly, the rate decreases as steric hindrance inquestion of the ROP mechanism (coordination-insertion vs. accreases around the metal atom (Table 1, entries 4–7), whereas tivated monomer, see below). The Ga precatalysts polymerise the introduction of the electron-withdrawing groups leads to rac-LA with or without exogenous alcohol, but the reactions faster rates (Table 1, entries 9–10 and 12) and conversely the are not controlled in either case. Complexes [3-Al2Me4] and [3electron-donating p-OMe moiety in [5-AlOiPr] gives slightly In2(CH2SiMe3)4] both require addition of alcohol (1–4 equiv), lower conversions (Table 1, entry 11).[5e–f] and the rates are comparable to those seen for their monomeThese catalysts produce highly isotactic PLAs, typically with tallic congeners: for instance, the ROP of 100 equiv of rac-LA 0.80 < Pm < 0.90, which places them amongst the best systems catalysed by [3-Al2Me4]/BnOH (rac-[LA]0/[precat]0/[BnOH]0 = for the isoselective polymerisation of rac-LA.[8] The level of iso200:1:2; [rac-LA]0 = 0.8 m in toluene) proceeds with 87 % contactic stereocontrol increases with the steric congestion in the version after 24 h at 70 8C to give an atactic PLA in a controlled direct vicinity of the metal, compare entries 3–5 (o-R = H), 6 (ofashion (Mn,theo = 12 600 g mol1, Mn,SEC = 8600 g mol1, Mw/Mn = R = Me) and 8 (o-R = tBu) in Table 1. The reaction rates can be improved by increasing the Table 2. Polymerisation of racemic lactide mediated by indium complexes.[a] temperature, but this comes at Mn,NMR[c] Mn,SEC[d] Mw/ Pm[e] Entry In [LA]0/[In]0/ T t Conv. Mn,theo[b] the expense of stereoselectivity. [g mol1] [g mol1] [g mol1] Mn [PhCH2OH]0 [8C] [h] [%] In all cases, the presence of the 1 [1-In(CH2SiMe3)2] 100:1:0 85 7 1) and in ref. [28] for “living” reactions ([BnOH]0/[In]0 1).

experiment showed that the reaction is extremely slow without addition of exogenous alcohol (Table 4, entry 4). With 1.0 equiv of BnOH (Table 4, entries 1, 5 and 6), the rate decreases according to [2-In(CH2SiMe3)] < [1-In(CH2SiMe3)] ! [4-In(CH2SiMe3)], that is, the order is the same as that determined previously with aluminium precatalysts. The influence of alcohol concentration is minimal, as the rates are little modified when alcohol content increases 5-fold (Table 4, entries 1–3); this suggests that the kinetic rate law exhibits zeroth-order dependence in alcohol concentration. Note that the use of excess BnOH (5 equiv versus In) relates to so-called “immortal” ROP,[12c] and a zeroth-order dependence in [alcohol] has been reported for such reactions catalysed by indium/alcohol binary systems.[7d] The rates observed with the binary indium/BnOH are greater than those measured for aluminium systems, but the comparison is ill-advised not only because the reactions were not carried out systematically at the same temperatures, but also because the two types of precatalysts follow different operative mechanisms. The question of operative mechanisms with these aluminium and indium ROP precatalysts was considered. In the case of [1-AlOiPr]–[6-AlOiPr], which do not require addition of alcohol to mediate controlled ROP reactions and afford PLAs with predictable molecular features and end-groups through acyl cleavage by nucleophilic attack of isoproxide (see above), there can be little doubt that the polymerisation proceeds according to a classical coordination-insertion mechanism. The scenario is very different with the In-based systems [1-In(CH2SiMe3)] and [2-In(CH2SiMe3)], because 1) they are totally inert towards BnOH, and 2) they constitute very poor ROP precatalysts if no external alcohol is used. In these reactions (see Table 2, entry 1), we have failed to detect the polymer endgroups (in particular, the potential CH3 group resulting from initiation by the nucleophile [Me3SiCH2] followed by cleavage during the work-up of the polymer after quenching of the reaction was not detected) and we surmise that initiation occurs through the presence of residual protic impurities.[30] These two observations strongly point at an activated monomer mechanism in which the metal essentially acts as a Lewis acidic activator, as often suggested or demonstrated for indium ROP catalytic systems[13–17] and also recently demonstrated for a range of hard, oxophilic metals;[31] in particular, Chem. Eur. J. 2014, 20, 6131 – 6147

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Mountford, Junk and their co-workers have elegantly demonstrated that kinetic saturation upon increase of the concentration in metal precatalyst was to be expected in the case of an activated monomer mechanism.[31a] Accordingly, the effect of concentration in indium complex at a fixed [rac-LA]0/[BnOH]0 ratio (= 50:5, with [rac-LA]0 = 0.6 m because of experimental constraints) was assessed, by using [1-In(CH2SiMe3)] as the precursor and varying its concentration in a 10fold range (Figure 9 and Table 5). As long as [1-In(CH2SiMe3)]0 is maintained below or equal to the [BnOH]0 (i.e., with [rac-LA]0/[BnOH]0/[1-In(CH2SiMe3)]

Figure 9. Plots of monomer conversion versus reaction time for the ROP of rac-LA catalysed by [1-In(CH2SiMe3)] + BnOH at various indium contents. Reaction conditions: [rac-LA]0 = 0.6 m in [D8]toluene, [rac-LA]0/[BnOH]0 = 50:5, T = 100 8C, with [BnOH]0/[In]0 = 5:1 (&), 5:3 (~), 5:5 (^), 5:7 (&) and 5:10 (~).

Table 5. Kinetic rate constants for the ROP of rac-LA catalysed by BnOH/[1-In(CH2SiMe3)].[a] Entry

[BnOH]0/[In]0

104 kobs[b] [s1]

103 ki[c] [L mol1 s1]

103 kp[c] [L mol1 s1]

1 2 3 4 5

5:1 5:3 5:5 5:7 5:10

10.68 23.21 32.29 32.60 31.78

5.15 0.55 7.28 1.27 4.03 0.82 4.89 1.18 8.61 0.92

45.0 2.35 70.2 7.54 190 25.5 182 28.0 135 6.67

[a] Reaction conditions: [rac-LA]0 = 0.6 m in [D8]toluene, [racLA]0/[BnOH]0 = 50:5, T = 100 8C. [b] From the semi-logarithmic plot of monomer conversion versus time. [c] According to non-linear regression, as described in ref. [29] for “immortal” ROP reactions ([BnOH]0/[In]0) > 1) and in ref. [28] for “living” reactions ([BnOH]0/[In]0 1).

between 50:5:1 and 50:5:5), enhancement of the reaction rate is observed with increasing concentration in indium. When [1In(CH2SiMe3)]0 is greater than [BnOH]0, no further increase in reaction rate is observed: kinetic saturation is reached, as indeed the rate of propagation is determined by the number of growing polymer chains (i.e., the number of molecules of (macro)alcohol) and not by the number of potential catalytically active sites (i.e., the number of molecules of indium complex).

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Full Paper On the nature of stereocontrol

Table 6. Rates for the ROP of rac-LA catalysed by [(R,R)-2-AlOiPr] and/or [(S,S)-2-AlOiPr].[a]

Coates and co-workers have postulated that isoselectivity in the ROP of lactide catalysed by aluminium–SalBINAP alkoxides featuring axial chirality occurs through highly efficient enantiomorphic-site control.[4] Because the ligand chirality and the geometry of the polymer chain-end bound to the metal (and even the solvent) can potentially all influence the stereocontrol in the polymerisation of LA,[8] the origin of the isoselectivity with the chiral [1-AlOiPr]–[6-AlOiPr] was tackled. Even if the chirality of the complexes if often regarded as a source of site control,[8] both enantiomorphic and chain-end controls were here considered as equally plausible alternatives. In the case of an enantiomorphic-site control, using the pure (R,R) or (S,S) enantiomer of the precatalyst (as done so far in this study, where only the (R,R)-precatalysts have been used),[22] should first lead to the fast consumption of the matching enantiomer of LA; when all matching LA has been consumed, consumption of the “mismatching” enantiomer of LA should begin with an observable drop of the reaction rate. Each of these phases should be characterised by its specific first-order kinetic regime, with kobs, matching @ kobs, mismatching and an observable change of regime occurring at about 50 % conversion. On the other hand, the use of the racemic precatalyst mixture should alleviate this phenomenon with a single kinetic regime and should bring an overall better activity, as both enantiomers of LA should now be polymerised rapidly at equal rates. Figure 10 and Table 6 show that the rates (both kobs and kp) of polymerisation of rac-LA catalysed by the enantiomerically pure [(R,R)-2-AlOiPr] and [(S,S)-2-AlOiPr] are essentially identical.[22, 32] However, the conversion versus time plots do not exhibit any change in kinetic regime as the one anticipated for an enantiomorphic-site control. Also, using the racemic mixture of [(R,R)-2-AlOiPr] and [(S,S)-2-AlOiPr] (0.5 equiv of each, all other conditions remaining otherwise unchanged) did not lead to an increase of the rates. We therefore concluded that, unlike early expectations, chain-end control and not enantiomorphism is responsible for the high isoselectivity exerted by [1-AlOiPr]–[6-AlOiPr].

Figure 10. Plots of monomer conversion versus reaction time for the ROP of rac-LA catalysed by [(R,R)-2-AlOiPr] (1.0 equiv, &), [(S,S)-2-AlOiPr] (1.0 equiv, ^) and [(R,R)-2-AlOiPr] + [(S,S)-2-AlOiPr] (0.5 + 0.5 equiv, ~). Reaction conditions: [rac-LA]0 = 0.8 m in [D8]toluene, [rac-LA]0/[Al]0 = 50:1, T = 70 8C. Chem. Eur. J. 2014, 20, 6131 – 6147

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Entry

[2-AlOiPr] [equiv]

104 kobs[b] [s1]

103 ki[c] [L mol1 s1]

103 kp[c] [L mol1 s1]

1 2 3

(R,R) (1.0) (S,S) (1.0) (R,R) (0.5) + (S,S) (0.5)

0.80 0.92 1.02

10.0 3.44 n/d[d] n/d[d]

4.35 0.09 4.98 0.06 5.92 0.15

[a] Reaction conditions: [rac-LA]0 = 0.8 m in [D8]toluene, [rac-LA]0/[Al]0 = 50:1, T = 70 8C. [b] From the semi-logarithmic plot of monomer conversion versus time. [c] According to non-linear regression as described in ref. [28]; standard deviations apply to curve-fitting of a given set of data, not to duplicated experiments. [d] No initiation period detected.

This claim is further substantiated by analysis of the homodecoupled 1H NMR spectrum of a selected PLA sample of relatively low isotacticity (Pm = 0.75, Pr = 1Pm = 0.25) deliberately prepared at 70 8C with [2-AlOiPr].[33] The distribution of tetrads ([mmm] = 0.66; [rmr] = 0.03; [mmr] = [rmm] = 0.09; [mrm] = 0.12)[34] calculated according to the Bernoullian statistics that apply to chain-end controlled stereoselective mechanisms and those determined after deconvolution of the homodecoupled 1 H NMR spectrum (Figure 11; [mmm] = 0.66; [rmr] = 0.04; [mmr] = [rmm] = 0.08; [mrm] = 0.14) are in excellent agreement. On the other hand, statistics for an enantiomorphic-site control indicate that a distribution of stereo-errors mmr/mrm/rmr/ rmm = 1:2:1:1 is to be expected,[4b] and this did not match the experimental data (mmr/mrm/rmr/rmm = 2:4:1:2, Figure 11). Finally, it has been argued that, unlike for a chain-end controlled mechanism, stereoselectivity varies with monomer conversion in the case of site-control stereoselectivity;[15c] in our case, the value of Pm remains constant through the whole ROP reaction, which again is indicative of chain-end control. The same analyses and conclusions hold for other PLA samples synthesised herein.

Figure 11. Deconvolution of the methine region (d = 5.12–5.23 ppm) of the homodecoupled 1H NMR spectrum (500.13 MHz, CDCl3, 298 K) of a PLA prepared with (R,R)-2-AlOiPr (Pm = 0.75), showing the respective integrations for each tetrad.[33] Black, recorded homodecoupled 1H NMR spectrum; grey, deconvolution of the spectrum.

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Full Paper Conclusion A new range of monometallic chiral salen–aluminium complexes based on a less-classical chiral backbone has been developed. These constitute efficient precatalysts for the isoselective ring-opening polymerisation of racemic lactide. They afford PLA featuring high isotacticity (Pm 0.80–0.90) in the upper range reported to date. The combination of kinetic measurements for the two enantiomers (and their racemic mixtures) of the precatalyst and analysis of tetrad distribution (Bernoullian statistics vs. experimental data) has revealed that, unexpectedly, a chain-end controlled mechanism is at work instead of the enantiomorphic site control that could have been expected since the complexes are chiral.

If the synthesis of related monometallic gallium complexes with these salen ligands has unsurprisingly[24] proved elusive, three monometallic indium-alkyl analogues have been obtained. These complexes catalyse the controlled ROP of racemic lactide according to an activated monomer mechanism (based on the reactivity study with BnOH, the kinetic saturation experiments and polymer end-group analysis) to produce atactic polymer; the difference in the tacticity observed between isoselective aluminium precatalysts and non-stereoselective indium ones probably can be linked to two independent features: the coordination sphere between the small aluminium and larger indium atoms is very different in these two families of salen complexes, and the ROP mechanisms at work (experimental data agree with coordination-insertion for aluminium as seen elsewhere)[3–8] are not the same. Note that all our bimetallic Al, Ga and In complexes only afford atactic PLA, highlighting the necessity even in the case of aluminium to

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have a constrained arrangement about a 5-coordinated metal with these ligands if stereoselectivity is sought. Kinetic analysis of the ROP of lactide catalysed by these Al and In complexes show that the rates of polymerisation increase if electron-withdrawing groups are added to the aromatic rings in the ligand backbone; this is consistent with observations made before for related aluminium-salen complexes by Hillmyer, Tolman and co-workers,[35] Gibson and co-workers,[5e] and Nomura and co-workers.[5f] Yet, this is the opposite of the trend detected for Al-salan complexes.[23, 36] If it comes as no surprise that the introduction of bulky substituents in ortho position of the phenolates reduces substantially the reactions rates, the fact that it overall leads to increased isoselectivity could not have been surely anticipated: for instance, Gibson and co-workers have instead observed that their aluminium–salan systems switched from isoselective to non-stereoselective or even heteroselective upon replacement of o-H groups by o-Me, o-tBu and o-Cl.[6a] On the other hand, we recently reported that with achiral 4-coordinate aluminium complexes supported by bidentate phenoxy-imine ligands, the presence of a bulky o-SiPh3 proved mandatory (but not necessarily sufficient) to achieve significant isoselectivities.[7d] These considerations highlight the current difficulty in designing at will highly active and stereospecific ROP catalysts, even if some key features of ROP reactions are now better understood.[20d, 29, 31a, 35, 36] Future efforts must focus on increasing the isoselectivity and catalytic activity of these ROP precatalysts through the tuning of steric and electronic properties of the chiral ancillary ligands, to prepare high molecular PLAs that will lend themselves well to the examination of their physical properties.

Experimental Section General procedures All manipulations were performed under inert atmosphere using standard Schlenk techniques or in a dry, solvent-free glovebox (Jacomex; O2 < 1 ppm, H2O < 5 ppm) for catalyst loading. Racemic lactide (Acros) was purified by recrystallization from a hot (80 8C), concentrated iPrOH solution, followed by two subsequent recrystallizations in hot (105 8C) toluene; after purification, it was stored at 30 8C under the inert atmosphere of the glovebox. Benzyl alcohol and isopropanol (Aldrich) were dried and distilled over magnesium turnings and stored over 3 molecular sieves. [InCl3] (Strem), [GaCl3] (Strem), [AlBr3] (Strem), [AlMe3] (2.0 m solution in toluene;

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Full Paper Aldrich), [AlMe2(OiPr)] (Strem) and [Al(OiPr)3] (Aldrich) were used as received. Solvents (THF, Et2O, CH2Cl2, pentane and toluene) were purified and dried (water contents below 8 ppm) over columns alumina (MBraun SPS). THF was further distilled under argon from sodium/benzophenone ketyl. All deuterated solvents (Eurisotop, Saclay, France) were stored in sealed ampoules over activated 3 molecular sieves and were thoroughly degassed by several freezethaw-vacuum cycles. [Al(CH2SiMe3)3], [Ga(CH2SiMe3)3] and [In(CH2SiMe3)3] were synthesised according to literature procedures.[37] All proteo-ligands (R,R)-1-H2–(R,R)-6-H2 were prepared by condensation of enantiomerically pure R,R-1,2-diphenylethane-1,2-diamine (Aldrich) with the appropriately substituted 2-hydroxy-benzaldehydes; details are available from the Supporting Information. NMR spectra were recorded on Bruker AM-400 and AM-500 spectrometers. All 1H and 13C{1H} chemicals shifts were determined using residual signals of the deuterated solvents and were calibrated versus SiMe4. Assignment of the signals was carried out using 1D (1H, 13C{1H}) and 2D (COSY, HMBC, HMQC) NMR experiments. Elemental analyses were performed on a Carlo Erba 1108 Elemental Analyser instrument at the London Metropolitan University by Stephen Boyer and were the average of a minimum of two independent measurements. Size exclusion chromatography (SEC) measurements were performed on an Agilent PL-GPC50 equipped with two PL gel 5 MIXED-C columns and a refractive index detector. The column was eluted with THF at 30 8C at 1.0 mL min1 and was calibrated by using 11 monodisperse polystyrene standards in the range of 580 to 380 000 g mol1. The molecular weights of all PLAs were corrected by a factor of 0.58.[27] [1-AlMe]: A solution of (R,R)-1-H2 (300 mg, 0.72 mmol) in toluene (4 mL) was added dropwise to a solution of [AlMe3] (0.36 mL of a 2.0 m solution in toluene, 0.72 mmol) in toluene (2.0 mL). The reaction mixture was then stirred at 100 8C for 3.5 h. The reaction medium was allowed to cool down to room temperature, and the volatiles were pumped off under vacuum to give a deeply yellow powder. This solid was recrystallised from hot toluene to afford the expected compound as deep-yellow crystals. Yield: 190 mg, 58 % (non-optimised). 1H NMR (C6D6, 298 K, 500.13 MHz): d = 7.75 (s, 1 H, N = CH), 7.54 (d, 1 H, 4JHH = 1.6 Hz, N = CH), 7.34 (d, 1 H, 3JHH = 8.5 Hz, CaromH), 7.30 (d, 1 H, 3JHH = 8.5 Hz, CaromH), 7.13–7.09 (m, 2 H, CaromH), 7.00–6.95 (m, 3 H, CaromH) 6.94–6.85 (m, 5 H, CaromH), 6.65 (d, 2 H, 3 JHH = 6.4 Hz, CaromH), 6.47–6.35 (m, 4 H, CaromH), 5.13 (dd, 1 H, 3JHH = 11.3, 4JHH = 1.6 Hz, N-CH), 4.21 (d, 1 H, 3JHH = 11.3 Hz, N-CH), 0.08 ppm (s, 3 H, AlCH3); 13C{1H} NMR (C6D6, 298 K, 125.75 MHz): d = 172.0 (N = CH), 168.0 (C-OAl), 167.1 (C-OAl), 165.4 (N=CH), 139.3 (i-C6H5), 137.0 (CaromH), 136.0 (CaromH), 134.2 (i-C6H5), 134.0 (CaromH), 133.7 (CaromH), 130.4 (CaromH), 129.3 (CaromH), 129.2 (CaromH) 129.1 (CaromH), 128.6 (CaromH),128.4 (CaromH), 123.1 (CaromH), 123.0 (CaromH), 119.5 (Carom-CH=N), 119.2 (Carom-CH=N), 116.5 (CaromH), 116.0 (CaromH), 72.5 (N-CH), 71.3 (N-CH), 6.7 ppm (br, AlCH3); elemental analysis calcd (%) for C29H25AlN2O2 (460.50 g mol1): C 75.6; H 5.5; N 6.1; found: C 75.5; H 5.4, N 6.2. [2-AlMe]: A solution of (R,R)-2-H2 (100 mg, 0.22 mmol) in toluene (2 mL) was added dropwise to a solution of [AlMe3] (0.11 mL of a 2.0 m solution in toluene, 0.22 mmol) in toluene (1.0 mL). The reaction mixture was stirred at 100 8C for 4 h. The volatiles were then removed in vacuo at room temperature. Complex [2-AlMe] was isolated as an intense yellow powder after washing with cold pentane (2 1 mL). Yield: 90 mg, 83 %. 1H NMR (C6D6, 298 K, 500.13 MHz): d = 7.83 (d, 1H 4JHH = 1.6 Hz, N=CH), 7.58 (d, 1H 4JHH = 2.0 Hz, N=CH), 7.22 (m, 2 H, CaromH), 6.96 (m, 3 H, CaromH), 6.89 (m, 5 H, CaromH), 6.59 (m, 2 H, CaromH), 6.48 (m, 4 H, CaromH), 5.18 (dd, 1 H, 3 JHH = 11.1, 4JHH = 2.0 Hz, N-CH), 4.17 (dd, 1 H, 3JHH = 11.1 Hz, 4JHH = Chem. Eur. J. 2014, 20, 6131 – 6147

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1.6 Hz, N-CH), 2.63 (s, 3 H, o-CH3), 2.62 (s, 3 H, o-CH3), 0.05 ppm (s, 3 H, AlCH3); 13C{1H} NMR (C6D6, 298 K, 100.62 MHz): d = 172.23 (N= CH), 166.6 (C-OAl), 165.4 (C-OAl), 165.3 (N=CH), 139.8 (i-C6H5), 137.0 (CaromH), 136.3 (CaromH), 134.3 (i-C6H5), 131.7 (CaromH), 131.4 (CaromH), 131.3 (CaromH), 131.2 (Carom-CH3), 129.3 (CaromH), 129.2 (CaromH), 129.1 (CaromH), 129.0 (CaromH), 128.2 (CaromH), 127.9 (Carom-CH3), 118.6 (CaromCH = N), 118.2 (Carom-CH=N), 116.2 (CaromH), 115.7 (CaromH), 72.6 (NCH), 71.3 (N-CH), 16.6 (o-CH3), 6.3 ppm (br, AlCH3); elemental analysis calcd (%) for C31H29AlN2O2 (488.56 g mol1): C 76.2; H 6.0; N 5.7; found: C 76.1; H 6.2; N 5.7. [3-Al2Me4]: The reaction of [AlMe3] (0.31 mL of a 2.0 m solution in toluene, 0.62 mmol) and (R,R)-3-H2 (200 mg, 0.31 mmol) at 110 8C and work-up in the way described for [1-AlMe] afforded deepyellow crystals of the bimetallic [3-Al2Me4]. Yield: 170 mg, 72 %. 1 H NMR (C6D6, 298 K, 500.13 MHz): d = 8.40 (s, 2 H, N = CH), 7.53 (d, 2 H, 4JHH = 2.5 Hz, CaromH) 7.04–6.95 (m, 6 H, CaromH), 6.92–6.87 (m, 6 H, CaromH), 5.45 (s, 2 H, N-CH), 1.52 (s, 18 H, C(CH3)3), 1.18 (s, 18 H, C(CH3)3), 0.13 (br, 6 H, Al(CH3)(CH3)), 0.98 ppm (br, 6 H, Al(CH3)(CH3)); 13C{1H} NMR (C6D6, 298 K, 125.75 MHz): d = 173.9 (br, N = CH), 162.7 (C-OAl), 140.5 (Carom-tBu), 139.5 (Carom-tBu), 134.2 (i-C6H5), 133.2 (CaromH), 129.9 (CaromH), 129.5 (CaromH), 129.2 (CaromH), 128.4 (CaromH), 128.2 (CaromH), 128.0 (CaromH), 118.2 (Carom-CH=N), 74.4 (br, N-CH), 35.4 (C(CH3)3), 34.0 (C(CH3)3), 31.3 (C(CH3)3), 29.6 (C(CH3)3), 6.0 (Al(CH3)(CH3)), 9.8 ppm (Al(CH3)(CH3)); elemental analysis calcd (%) for C48H66Al2N2O2 (757.01 g mol1): C 76.2; H 8.8; N 3.7; found: C 75.9; H 8.8; N 3.6. [4-AlMe]: The reaction of [AlMe3] (0.28 mL of a 2.0 m solution in toluene, 0.56 mmol) and (R,R)-4-H2 (300 mg, 0.56 mmol) at 80 8C for 3 h and work-up in the way described for 1-AlMe afforded [4AlMe] as a brown powder. Yield: 295 mg (92 %). 1H NMR (C6D6, 298 K, 500.13 MHz): d = 8.16 (d, 1 H, 4JHH = 1.8 Hz, N = CH), 8.17 (m, 2 H, CaromH), 7.93 (m, 2 H„ CaromH), 7.83 (d, 1 H, 4JHH = 2.1 Hz, N=CH), 7.47 (m, 3 H, CaromH), 7.36 (m, 3 H, CaromH), 7.22 (m, 4 H, CaromH), 5.38 (dd, 1 H, 3JHH = 11.6 Hz, 4JHH = 2.1, N-CH), 4.96 (dd, 1 H, 3JHH = 11.6, 4 JHH = 1.8 Hz, N-CH), 2.38 (s, 3 H, o-CH3), 2.36 (s, 3 H, o-CH3), 0.70 ppm (s, 3 H, AlCH3); 13C{1H} NMR (C6D6, 298 K, 100.62 MHz): d = 172.3 (N=CH), 170.2 (C-OAl), 169.6 (C-OAl), 165.6 (N=CH), 137.8 (Carom-CH3), 137.7 (Carom-CH3), 137.4 (i-C6H5), 132.5 (i-C6H5), 132.4 (Carom-NO2), 132.3 (br, Carom-NO2 + CaromH) 130.5 (CaromH), 130.4 (CaromH), 130.3 (CaromH), 130.2 (CaromH), 130.1 (CaromH), 129.9 (CaromH), 129.7 (CaromH), 129.5 (CaromH), 129.2 (CaromH), 117.1 (Carom-CH=N), 116.7 (Carom-CH=N), 72.8 (N-CH), 71.2 (N-CH), 16.3 (o-CH3), 16.2 ppm (o-CH3); the resonance for AlCH3 could not be detected despite several modifications to the acquisition parameters. Elemental analysis calcd (%) for C31H27AlN4O6 (578.55 g mol1): C 64.4; H 4.7; N 9.7; found: C 64.3; H 4.8; N 9.6. [2-Al(CH2SiMe3)]: [Al(CH2SiMe3)3] (16 mg, 0.06 mmol) and (R,R)-2-H2 (25 mg, 0.06 mmol) were treated in C6D6 in a J-Young NMR tube at 80 8C for 2 h 30 min and afforded [2-Al(CH2SiMe3)]. This complex was not isolated and only characterised by 1H NMR spectroscopic and X-ray diffraction crystallography. 1H NMR (C6D6, 298 K, 500.13 MHz): d = 7.80 (d, 1 H, 4JHH = 1.6 Hz, N-CH), 7.59 (d, 1 H, 4JHH = 2.0 Hz, N=CH), 7.25–7.21 (m, 2 H, CaromH), 6.98–6.84 (m, 8 H, CaromH), 6.63–6.58 (m, 2 H, CaromH), 6.51–6.40 (m, 4 H, CaromH), 5.26 (dd, 1 H, 3 JHH = 11.2, 4JHH = 2.0 Hz, N-CH), 4.13 (dd, 1 H, 3JHH = 11.2, 4JHH = 1.6 Hz, N-CH), 2.65 (s, 3 H, Carom-CH3), 2.63 (s, 3 H, Carom-CH3), 0.35 (s, 9 H, Si(CH3)3), 0.30 to 0.66 ppm (AB system, 2 H, 2JHH = 12.3 Hz, AlCH2Si); 13C{1H} NMR (C6D6, 298 K, 100.62 MHz): d = 172.0 (N = CH), 166.4 (C-OAl), 165.4 (N = CH), 165.3 (C-OAl), 139.8 (i-C6H5), 137.1 (CaromH), 136.4 (CaromH), 134.1 (i-C6H5), 131.7 (CaromH), 131.4 (CaromH), 131.2 (Carom-CH3), 131.1 (Carom-CH3) 130.3 (CaromH), 129.3 (CaromH), 129.2 (CaromH), 129.1 (CaromH), 128.6 (CaromH), 128.4 (CaromH) 118.5 (Carom-CH = N), 118.2 (Carom-CH = N), 116.3 (CaromH), 115.8 (CaromH),

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C33H33Al2N2O3 (532.61 g mol1): C 74.4; H 6.3; N 5.3; found: C 73.8; H 5.9; N 5.6.

[3-Al2(CH2SiMe3)4]: The reaction of [Al(CH2SiMe3)3] (51 mg, 0.18 mmol) and (R,R)-3-H2 (50 mg, 0.08 mmol) in a J-Young NMR tube at 80 8C for 1.5 h in C6D6 afforded the bimetallic [3-Al2(CH2SiMe3)4] complex. This complex was not isolated and only characterised by 1H NMR spectroscopy. 1H NMR (C6D6, 298 K, 500.13 MHz): d = 8.61 (br, 2 H, N = CH), 7.56 (d, 2 H, 3JHH = 2.6 Hz, CaromH), 7.25 (br, 2 H, CaromH), 6.94–6.90 (m, 4 H, CaromH), 6.86–6.82 (m, 6 H, CaromH), 5.60 (s, 2 H, N-CH), 1.50 (s, 18 H, C(CH3)3), 1.21 (s, 18 H, C(CH3)3), 0.18 (s, 18 H, Si(CH3)3), 0.15 (s, 18 H, Si(CH3)3), 0.63 (br, 2 H, AlCH2Si), 0.97 to 1.19 ppm (br m, 6 H, AlCH2Si); 13 1 C{ H} NMR (C6D6, 298 K, 100.62 MHz): d = 172.9 (br, N=CH), 162.3 (C-OAl), 140.6 (Carom-tBu), 140.1 140.6 (Carom-tBu), 135.2 (i-C6H5), 133.6 (CaromH), 129.7 (CaromH), 129.6 (CaromH), 129.2 (CaromH), 118.9 (Carom-CH=N), 73.1 (br, N-CH), 35.5 (C(CH3)3), 34.2 (C(CH3)3), 31.4 (C(CH3)3), 30.1 (C(CH3)3), 3.1 (Si(CH3)3), 3.0 (Si(CH3)3), 1.1 (AlCH2Si), 1.7 ppm (AlCH2Si).

[3-AlOiPr]: [Al(OiPr)3] (120 mg, 0.59 mmol) and (R,R)-3-H2 (379 mg, 0.59 mmol) were dissolved in toluene (10 mL) and reacted at 80 8C for 3.5 days, and the volatiles were then pumped off under vacuum. The resulting solid was dissolved in hot hexane, and slow decrease of the temperature resulted in the formation of a deep yellow precipitate which was isolated by filtration, washed twice with cold hexane (2 2 mL) and dried under vacuum to yield [3AlOiPr] as a deep-yellow powder. Yield: 295 mg, 69 %. 1H NMR (C6D6, 298 K, 500.13 MHz): d = 8.03 (d, 1 H, 4JHH = 1.2 Hz, N = CH), 7.88 (d, 1 H, 4JHH = 1.7 Hz, N = CH), 7.81 (d, 2 H, 4JHH = 2.2 Hz, CaromH), 7.14 (d, 2 H, 4JHH = 2.2 Hz, CaromH), 6.98–6.90 (m, 5 H, CaromH), 6.86– 6.83 (m, 2 H, CaromH), 6.80–6.75 (m, 3 H, CaromH), 5.55 (dd, 1H 3JHH = 10.9 Hz, 4JHH = 1.6 Hz, N-CH), 4.45 (hept, 1 H, 3JHH = 5.9 Hz, CH(CH3)2), 4.29 (dd, 1 H, 3JHH = 10.9, 4JHH = 0.9 Hz, N-CH), 1.97 (s, 18 H, C(CH3)3), 1.37 (d, 6 H, 3JHH = 5.9 Hz, CH(CH3)2), 1.18 ppm (s, 18 H, C(CH3)3); 13 1 C{ H} NMR (C6D6, 298 K, 125.75 MHz): d = 172.0 (N=CH), 166.9 (N= CH), 165.3 (C-OAl), 163.9 (C-OAl), 141.5 (Carom-tBu), 141.4 (Carom-tBu), 139.6 (i-C6H5), 138.5 (Carom-tBu), 138.0 (Carom-tBu), 135.4 (i-C6H5), 131.7 (CaromH), 130.6 (CaromH), 130.2 (CaromH), 129.5 (CaromH), 129.3 (CaromH), 129.2 (CaromH), 129.0 (CaromH), 128.9 (CaromH), 128.2 (CaromH), 128.0 (CaromH), 119.4 (Carom-CH=N), 119.2 (Carom-CH=N), 73.02 (N-CH), 72.0 (N-CH), 63.4 (CH(CH3)2), 36.3 (C(CH3)3), 36.2 (C(CH3)3), 34.1 (C(CH3)3), 34.0 (C(CH3)3), 31.5 (C(CH3)3), 31.4 (C(CH3)3), 30.7 (C(CH3)3), 30.4 (C(CH3)3), 28.9 (CH(CH3)(CH3)), 28.8 ppm (CH(CH3)(CH3)); elemental analysis calcd (%) for C47H61AlN2O3 (728.98 g mol1): C 77.4; H 8.4; N 3.8; found: C 77.7; H 8.9; N 3.2.

[1-AlOiPr]: The proteo-ligand (R,R)-1-H2 (231 mg, 0.55 mmol) and [Al(OiPr)3] (112 mg, 0.55 mmol) were dissolved in toluene (10 mL), and the reaction mixture was stirred at 80 8C for 3 days. The volatiles were then pumped off under high vacuum to afford [1AlOiPr] as a pale-yellow solid which was washed with pentane (3 4 mL) and dried in vacuo to constant weight. Yield: 260 mg, 93 %. 1 H NMR (C6D6, 298 K, 500.13 MHz): d = 7.80 (d, 1 H, 4JHH = 1.2 Hz, N= CH), 7.57 (d, 1 H, 4JHH = 1.8 Hz, N=CH), 7.37–7.33 (m, 2 H, Carom-H), 7.15–7.12 (m, 4 H, Carom-H), 6.98–6.90 (m, 5 H, Carom-H), 6.88 (t, 1 H, 3 JHH = 7.5 Hz, Carom-H), 6.69 (dd, 2 H, 3JHH = 7.5, 4JHH = 1.7 Hz, Carom-H), 6.47–6.37 (m, 4 H, Carom-H), 5.57 (dd, 1 H, 3JHH = 11.0, 4JHH = 1.8 Hz, NCH), 4.57 (hept, 1 H, 3JHH = 6.0 Hz, CH(CH3)(CH3)), 4.18 (dd, 1 H, 3JHH = 11.0, 4JHH = 1.2 Hz, N-CH), 1.48 (d, 3 H, 3JHH = 6.0 Hz, CH(CH3)(CH3)), 1.45 ppm (d, 3 H, 3JHH = 6.0 Hz, CH(CH3)(CH3)); 13C{1H} NMR (C6D6, 298 K, 100.62 MHz): d = 171.7 (N=CH), 167.9 (C-OAl), 166.9 (C-OAl), 165.7 (N=CH), 139.8 (i-C6H5), 136.9 (CaromH), 135.9 (CaromH), 134.6 (iC6H5), 133.9 (CaromH), 133.6 (CaromH), 130.4 (CaromH), 129.4 (CaromH), 129.3 (CaromH), 129.2 (CaromH), 129.0 (CaromH), 128.7 (CaromH), 122.9 (CaromH), 122.8 (CaromH), 119.5 (Carom-CH=N), 119.2 (Carom-CH=N), 116.7 (CaromH), 116.2 (CaromH), 72.5 (N-CH), 71.6 (N-CH), 63.8 (CH(CH3)2), 28.8 (CH(CH3)(CH3)), 28.7 (CH(CH3)(CH3)); elemental analysis calcd (%) for C31H29AlN2O3 (504.56 g mol1): C 73.8; H 5.8; N 5.6; found: C 73.2; H 5.6; N 5.8. [2-AlOiPr]: [Al(OiPr)3] (68.3 mg, 0.33 mmol) and (R,R)-2-H2 (150 mg, 0.33 mmol) were dissolved in toluene (10 mL) and the reaction mixture was stirred at 80 8C for 3 days. The volatiles were then removed under vacuum. The resulting solid was dissolved in a toluene (2.5 mL), and reprecipitation was induced by slow addition of pentane (15 mL). The resulting powder was then washed with pentane (2 5 mL) and was dried to constant weight to give [2-AlOiPr] as a yellow powder. Yield: 110 mg, 62 %. 1H NMR (C6D6, 298 K, 500.13 MHz): d = 7.85 (s, 1 H, N=CH), 7.61 (s, 1 H, N=CH), 7.24 (m, 2 H, CaromH), 7.18 (m, 2 H, CaromH), 7.00–6.86 (m, 6 H, CaromH), 6.66 (m, 2 H, CaromH), 6.49–6.44 (m, 4 H, CaromH), 5.56 (d, 1 H, 3JHH = 10.7 Hz, NCH), 4.54 (hept, 1 H, 3JHH = 6.0 Hz, CH(CH3)(CH3)), 4.21 (d, 1 H, 3JHH = 10.7 Hz, N-CH), 2.67 (s, 3 H, o-CH3), 2.65 (s, 3 H, o-CH3), 1.49 (d, 3 H, 3 JHH = 6.0 Hz, CH(CH3)(CH3)), 1.46 ppm (d, 3 H, 3JHH = 6.0 Hz, CH(CH3)(CH3)); 13C{1H} NMR (C6D6, 298 K, 125.75 MHz): d = 171.9 (N = CH), 166.5 (C-OAl), 166.0 (N=CH), 165.3 (C-OAl), 140.3 (i-C6H5), 136.8 (CaromH), 136.1 (CaromH), 134.9 (i-C6H5), 131.6 (CaromH), 131.3 (CaromH), 131.1 (Carom-CH3), 131.0 (Carom-CH3), 130.3 (CaromH), 129.3 (CaromH), 129.2 (CaromH), 129.1 (CaromH), 128.4 (CaromH), 128.0 (CaromH) 118.6 (Carom-CH=N), 118.3 (Carom-CH=N), 116.3 (CaromH), 115.9 (CaromH), 72.6 (N-CH), 71.7 (N-CH), 63.9 (CH(CH3)(CH3)), 28.7 (CH(CH3)(CH3)), 28.6 (CH(CH3)(CH3)), 16.6 ppm (o-CH3); elemental analysis calcd (%) for Chem. Eur. J. 2014, 20, 6131 – 6147

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NMR-scale generation of [4-AlOiPr]: Although the reaction of [AlMe2(OiPr)] with (R,R)-4-H2 proceeds very cleanly as judged from the in situ NMR monitoring of the reaction in C6D6, all attempts to isolate [4-AlOiPr] failed as decomposition systematically occurred upon removal of the volatile fraction, for instance after the reaction of (R,R)-4-H2 (130 mg, 0.24 mmol) and [AlMe2(OiPr)] (28 mg, 0.24 mmol) in benzene. 1H NMR (C6D6, 298 K, 500.13 MHz): d = 8.03–7.98 (m, 2 H, CaromH), 7.66–7.63 (m, 1 H, N = CH), 7.50–7.46 (m, 2 H, CaromH), 7.43 (d, 1 H, 4JHH = 2.8 Hz, N = CH), 7.20–7.12 (m, 2 H, CaromH) 7.05–6.99 (m, 5 H, CaromH), 6.96–6.93 (m, 1 H, CaromH), 6.83 (dd, 2 H, 3JHH = 7.7, 4JHH = 1.6 Hz, CaromH), 5.45 (dd, 1 H, 3JHH = 10.5 Hz, 4 JHH = 2.0 Hz, N-CH), 4.34 (hept, 1 H, 3JHH = 5.7 Hz, CH(CH3)2), 4.19 (d, 1 H, 3JHH = 10.5 Hz, N-CH), 2.30 (s, 3 H, o-CH3), 2.27 (s, 3 H, o-CH3), 1.36 ppm (m, 6 H, CH(CH3)2); 13C{1H} NMR (C6D6, 298 K, 125.75 MHz): d = 171.6 (N = CH), 169.7 (C-OAl), 169.0 (C-OAl), 165.4 (N=CH), 139.0 (i-C6H5), 138.1 (i-C6H5), 137.6 (Carom-NO2), 133.3 (Carom-NO2), 132.1 (Carom-CH3), 132.0 (Carom-CH3), 130.4 (CaromH), 130.1 (CaromH), 129.8 (CaromH), 129.7 (CaromH), 129.5 (CaromH), 129.3 (CaromH), 129.1 (CaromH), 128.4 (CaromH), 128.2 (CaromH), 116.8 (Carom-CH = N), 116.3 (Carom-CH= N), 72.7 (N-CH), 71.8 (N-CH), 64.1 (CH(CH3)2), 28.4 (CH(CH3)(CH3)), 16.1(Carom-CH3). Satisfactory elemental analysis could not be obtained for this complex. [5-AlOiPr]: [Al(OiPr)3] (63.8 mg, 0.31 mmol) and (R,R)-5-H2 (150 mg, 0.31 mmol) were dissolved in toluene (3.5 mL) and reacted at 80 8C overnight. After removal the volatiles, the solid obtain was washed with pentane (2 2 mL) and dry under high vacuum to constant weight afforded [5-AlOiPr] as a deep-yellow powder. Yield: 162 mg, 92 %. 1H NMR (C6D6, 298 K, 500.13 MHz): d = 7.87 (d, 1 H, 4 JHH = 1.3 Hz, N = CH), 7.67 (d, 1 H, 4JHH = 1.9 Hz, N = CH), 7.25 (m, 4 H, CaromH), 6.94 (m, 8 H, CaromH), 6.80 (m, 2 H, CaromH), 6.18 (d, 1 H, 4 JHH = 3.2 Hz, CaromH), 6.14 (d, 1 H, 4JHH = 3.2 Hz, CaromH), 5.62 (dd, 1 H, 3 JHH = 10.9, 4JHH = 1.9 Hz, N-CH), 4.63 (hept, 1 H, 3JHH = 6.0 Hz, CH(CH3)2), 4.32 (dd, 1 H, 3JHH = 9.4, 4JHH = 1.3 Hz, N-CH), 3.14 (s, 3 H, O-CH3), 3.13 (s, 3 H, O-CH3), 1.51 (d, 3 H, 3JHH = 6.0 Hz, CH(CH3)(CH3)), 1.49 (d, 3 H, 3JHH = 6.0 Hz, CH(CH3)(CH3)); 13C{1H} NMR (C6D6, 298 K,

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Full Paper 125.75 MHz): d = 171.0 (N=CH), 165.4 (N=CH), 163.5 (C-OAl), 162.1 (C-OAl), 150.9 (Carom-OCH3), 150.6 (Carom-OCH3), 140.0 (i-C6H5), 135.0 (i-C6H5), 130.3 (CaromH), 129.5 (CaromH), 129.3 (CaromH), 129.1 (CaromH), 128.7 (CaromH), 128.6 (CaromH), 128.0 (CaromH), 126.9 (CaromH), 125.3 (CaromH), 123.9 (CaromH), 118.3 (Carom-CH = N), 117.9 (Carom-CH = N), 114.5 (CaromH), 113.9 (CaromH), 72.8 (N-CH), 71.9 (N-CH), 63.81 (CH(CH3)2), 55.4 (O-CH3), 55.3 (O-CH3), 28.9 (CH(CH3)(CH3)), 28.8 ppm (CH(CH3)(CH3); elemental analysis calcd (%) for C33H33AlN2O5 (564.61 g mol1): C 70.2; H 5.9; N 5.0; found: C 70.0; H 5.9; N 5.1. [6-AlOiPr]: [AlMe2(OiPr)] (21 mg, 0.18 mmol) and (R,R)-6-H2 (100 mg, 0.18 mmol) were heated in toluene (6 mL) at 80 8C for 4 h. A precipitate formed rapidly. It was isolated by filtration, washed with pentane (3 2 mL) and dried to constant weight in vacuum to give a product assumed to be [6-AlOiPr]. Yield: 106 mg (92 % on the basis of the proposed formulation). Elemental analysis calcd (%) for C31H25AlCl4N2O2 (642.34 g mol1): C 58.0; H 3.9; N 4.4; found: C 57.8; H 3.8; N 4.5. This compound was insoluble in common organic solvents used for NMR spectroscopy (THF, CH2Cl2, chloroform, benzene or toluene), and could therefore not be characterised. Note that the addition of lactide to a suspension of [6AlOiPr] in toluene at 50–100 8C (i.e., under polymerisation conditions) however yields a fully homogeneous solution. [1-Ga2(CH2SiMe3)4]: The title complex was isolated as a deepyellow powder following reaction of (R,R)-1-H2 (200 mg, 0.48 mmol) and Ga(CH2SiMe3)3 (316 mg, 0.96 mmol) at 80 8C (5 h) and work-up in the way described above for [4-AlMe]. Yield: 400 mg, 93 %. 1H NMR (C6D6, 298 K, 500.13 MHz): d = 8.30 (s, 2 H, N=CH), 7.05 (dd, 2 H, 3JHH = 7.9, 4JHH = 1.7 Hz, CaromH), 6.96–6.92 (m, 6 H, CaromH), 6.87–6.80 (m, 6 H, CaromH), 6.78–6.75 (m, 2 H, CaromH), 6.29 (m, 2 H, CaromH), 5.35 (s, 2 H, N-CH), 0.31 (s, 18 H, Si(CH3)3), 0.07 (s, 18 H, Si(CH3)3), 0.21 (br, 4 H, GaCH2Si), 0.92 (d, 2 H, 2JHH = 13.0 Hz, GaCH2Si), 1.30 ppm (d, 2 H, 2JHH = 12.3 Hz, GaCH2Si); 13 1 C{ H} NMR (C6D6, 298 K, 125.75 MHz): d = 171.1 (N=CH), 168.0 (COGa), 137.7 (CaromH), 135.9 (CaromH), 134.8 (i-C6H5), 129.9 (CaromH), 129.3 (CaromH), 129.2 (CaromH), 122.9 (CaromH), 117.9 (Carom-CH = N), 116.7 (CaromH), 74.4 (br, N-CH), 3.88 (GaCH2Si), 2.34 (Si(CH3)3), 1.98 (Si(CH3)3), 0.93 ppm (GaCH2Si); elemental analysis calcd (%) for C44H66Ga2N2O2Si4 (906.80 g mol1): C 58.3; H 7.3; N 3.1; found: C 58.2; H 7.4; N 3.2. [2-Ga2(CH2SiMe3)4]: Reaction of (R,R)-2-H2 (200 mg, 0.45 mmol) and Ga(CH2SiMe3)3 (295 mg, 0.90 mmol) in toluene (8 mL) at 70 8C overnight followed by identical work-up to that described above for [4AlMe] afforded the bimetallic [2-Ga2(CH2SiMe3)4] as a deep-yellow powder. Yield: 390 mg, 93 %. 1H NMR (C6D6, 298 K, 500.13 MHz): d = 8.36 (s, 2 H, N=CH), 7.01 (d, 2 H, 3JHH = 7.4 Hz, CaromH), 6.98–6.95 (m, 2 H, CaromH), 6.94–6.91 (m, 4 H, CaromH), 6.89–6.78 (m, 6 H, CaromH), 6.33 (t, 2 H, 3JHH = 7.5 Hz, CaromH), 5.36 (s, 2 H, N-CH), 2.26 (s, 6 H, oCH3), 0.32 (s, 18 H, Si(CH3)3), 0.07 (s, 18 H, Si(CH3)3), 0.22 (s, 4 H, GaCH2Si), 0.89 (d, 2 H, 3JHH = 12.7 Hz, GaC(H)(H)Si), 1.21 ppm (br d, 2 H, 3JHH = 11.7 Hz, GaC(H)(H)Si)); 13C{1H} NMR (C6D6, 298 K, 100.62 MHz): d = 171.0 (N=CH), 166.4 (C-OGa), 138.0 (CaromH), 135.0 (i-C6H5), 133.8 (CaromH), 130.8 (Carom-CH = N), 129.9 (CaromH), 129.2 (CaromH), 129.1 (CaromH), 117.1 (Carom-CH3), 116.4 (CaromH), 74.1 (br, NCH), 16.9 (o-CH3), 3.7 (GaCH2Si), 3.2 (Si(CH3)3), 2.0 (Si(CH3)3), 1.1 (GaCH2Si); elemental analysis calcd (%) forC46H70Ga2N2O2Si4 (934.85 g mol1): C 59.1; H 7.6; N 3.0; found: C 58.9; H 7.5; N 3.2. [3-Ga2(CH2SiMe3)4]: [Ga(CH2SiMe3)3] (82 mg, 0.24 mmol) and (R,R)-3H2 (80 mg, 0.12 mmol) were treated in benzene (2 mL) at 80 8C for 4 h. Removal the volatiles and drying under vacuum afforded the bimetallic [3-Ga2(CH2SiMe3)4] as a deep-yellow powder. Yield: 135 mg, 96 %. 1H NMR (C6D6, 298 K, 500.13 MHz): d = 8.39 (s, 2 H, N = CH), 7.56 (d, 2 H, 3JHH = 2.2 Hz, CaromH), 7.11 (d, 2 H, 3JHH = 2.2 Hz, CaromH), 6.95 (m, 4 H, CaromH), 6.84 (m, 6 H, CaromH), 5.40 (s, 2 H, NChem. Eur. J. 2014, 20, 6131 – 6147

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CH), 1.58 (s, 18 H, C(CH3)3), 1.23 (s, 18 H, C(CH3)3), 0.40 (s, 18 H, Si(CH3)3), 0.06 (s, 18 H, Si(CH3)3), 0.10 to 0.15 (AB system, 4 H, 2 JHH = 12.4 Hz, GaCH2Si), 0.90 to 1.18 ppm (AB system, 4 H, 2JHH = 12.4 Hz, GaCH2Si); 13C{1H} NMR (C6D6, 298 K, 100.62 MHz): d = 172.4 (N = CH), 165.4 (C-OGa), 141.2 (Carom-tBu), 138.3(Carom-tBu), 134.9 (iC6H5), 132.6 (CaromH), 130.3 (CaromH), 130.2 (CaromH), 129.3(CaromH), 129.0 (CaromH), 117.9 (Carom-CH=N), 76.0 (br, N-CH), 35.6 (C(CH3)3), 34.1 (C(CH3)3), 31.5 (C(CH3)3), 30.2 (C(CH3)3), 4.1 (GaCH2Si), 2.8 (Si(CH3)3), 2.4 (Si(CH3)3), 0.2 ppm (GaCH2Si); elemental analysis calcd (%) for C60H98Ga2N2O2Si4 (1131.22 g mol1): C 63.7; H 8.7; N 2.5; satisfactory elemental analysis could not be obtained after 3 attempts, but the proposed formulation and NMR assignment were corroborated by X-ray crystallography structural determination. [1-In(CH2SiMe3)]: [In(CH2SiMe3)3] (269 mg, 0.71 mmol) and (R,R)-1H2 (300 mg, 0.71 mmol) were treated in toluene (8 mL) at room temperature for 15 min and then at 40 8C for 20 h, leading to the formation of a yellowish precipitate. After cooling down to room temperature, the solution was filtered out and the resulting solid was washed with pentane (3 3 mL) and dried under vacuum to yield [1-(InCH2SiMe3)] as a deep-yellow powder. Yield: 320 mg, 72 %. 1H NMR (C6D6, 298 K, 500.13 MHz): d = 7.72 (d, 1 H, 4JHH = 1.5 Hz, N = CH), 7.52 (s, 1 H, N = CH), 7.23 (dd, 1 H, 3JHH = 5.8, 4JHH = 1.1 Hz, CaromH), 7.21 (dd, 1 H, 3JHH = 5.8, 4JHH = 1.1 Hz, CaromH), 7.12 (m, 2 H, CaromH), 7.03–6.90 (m, 10 H, CaromH), 6.55 (dd, 1 H, 3JHH = 7.9, 4 JHH = 1.9 Hz, CaromH), 6.52 (dd, 1 H, 3JHH = 7.9, 4JHH = 1.9 Hz, CaromH), 6.42–6.37 (m, 2 H, CaromH), 4.97 (dd, 1 H, 3JHH = 6.5, 4JHH = 1.4 Hz, NCH), 4.15 (d, 1 H, 3JHH = 6.5 Hz, N-CH), 0.19 (s, 9 H, Si(CH3)3), 0.42 ppm (m, 2 H, InCH2); 13C{1H} NMR (C6D6, 298 K, 125.75 MHz): d = 172.2 (C-OIn), 172.0 (N=CH), 171.7 (C-OIn), 170.8 (N=CH), 139.0 (i-C6H5), 137.9 (i-C6H5), 135.8 (CaromH) 135.7 (CaromH), 135.6 (CaromH), 135.5 (CaromH), 129.3 (CaromH) 129.2 (CaromH), 129.1 (CaromH), 128.9 (CaromH), 128.6 (CaromH), 128.4 (CaromH), 124.8 (CaromH), 124.7 (CaromH), 119.5 (Carom-CH=N), 119.2 (Carom-CH=N), 115.2 (CaromH), 115.1 (CaromH), 73.6 (N-CH), 71.4 (N-CH), 1.97 (Si(CH3)3), 3.0 ppm (InCH2Si); elemental analysis calcd (%) for C32H33InN2O2Si (620.52 g mol1): C 61.9; H 5.4; N 4.5; found: C 61.8; H 5.5; N 4.6. 2-K2 : Diethyl ether (20 mL) were added to a mixture of (R,R)-2-H2 (159 mg, 0.35 mmol) and KN(SiMe3)2 (141 mg, 0.71 mmol) and the reaction mixture was stirred overnight at room temperature. An insoluble precipitate formed; it was isolated by filtration and washed with pentane (3 5 mL) and dried under dynamic vacuum overnight at 50 8C to afford 2-K2 as a pale-yellow solid. Yield: 135 mg (74 %). 1H NMR ([D8]THF, 298 K, 500.13 MHz): d = 8.16 (s, 2 H, N = CH), 7.33 (d, 4 H, 3JHH = 7.5 Hz, CaromH), 7.18 (t, 4 H, 3JHH = 7.5 Hz, CaromH), 7.08 (t, 2 H, 3JHH = 7.5 Hz, CaromH), 6.83 (d, 2 H, 3JHH = 7.0 Hz, CaromH), 6.73 (d, 2 H, 3JHH = 7.0 Hz, CaromH), 5.93 (t, 2 H, 3JHH = 7.0 Hz, CaromH), 4.85 (s, 2 H, N-CH), 2.05 ppm (s, 6 H, Carom-CH3); 13C{1H} NMR ([D8]THF, 298 K, 100.62 MHz): d = 172.5 (Carom-OK), 168.7 (N = CH), 145.3 (i-C6H5), 133.9 (CaromH), 131.8 (CaromH), 130.4 (Carom-CH3), 128.9 (CaromH), 128.2 (CaromH), 126.8 (CaromH), 122.0 (Carom-CH=N), 108.0 (NCH), 18.4 ppm (Carom-CH3). [2-InCl]: [In(CH2SiMe3)3] (41 mg, 0.19 mmol) and (R,R)-2-K2 (102 mg, 0.19 mmol) were treated overnight in THF (12 mL) at room temperature. Stirring was then stopped, the precipitate of KCl was eliminated by filtration, the volatiles pumped off and the resulting solid was washed with pentane (3 2 mL) and dried under dynamic vacuum to afford [2-InCl] as a yellowish powder. Yield: 98 mg (88 %). 1H NMR ([D8]THF, 298 K, 500.13 MHz): d = 7.97 (br, 1 H, N = CH), 7.79 (br, 1 H, N = CH), 7.35–7.17 (m, 12 H, CaromH), 6.72 (d, 2 H, 3 JHH = 7.4 Hz, CaromH), 6.42 (t, 3JHH = 7.4 Hz, CaromH), 5.44 (br, 1 H, NCH), 5.02 (br, 1 H, N-CH), 2.25 ppm (s, 6 H, Carom-CH3); 13C{1H} NMR ([D8]THF, 298 K, 125.75 MHz): d = 173.4 (br, N = CH), 170.6 (br, N= CH),169.8 (Carom-OIn), 139.4 (i-C6H5), 136.1 (i-C6H5), 135.9 (CaromH),

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Full Paper 135.8 (CaromH), 134.9 (CaromH), 132.5 (Carom-CH3), 132.4 (Carom-CH3), 131.0 (CaromH), 130.5 (CaromH), 129.9 (CaromH), 129.7 (CaromH), 129.2 (CaromH), 118.8 (Carom-CH=N), 118.6 (Carom-CH=N), 115.5 (CaromH), 72.3 (N-CH), 71.9 (N-CH), 17.2 ppm (Carom-CH3); elemental analysis calcd (%) for C30H26InClN2O2 (596.81 g mol1): C 60.4; H 4.4; N 4.7; found: C 59.9; H 4.3; N 4.8.

(Si(CH3)3), 2.5 ppm (br, InCH2Si); elemental analysis calcd (%) for C34H35In2N4O6Si (738.57 g mol1): C 55.3; H 4.8; N 7.6; found: C 55.2; H 4.9; N 7.8.

[2-In(CH2SiMe3)]: [In(CH2SiMe3)3] (126 mg, 0.33 mmol) and (R,R)-2H2 (150 mg, 0.33 mmol) in toluene (8 mL) were treated at 70 8C for 18 h. Identical procedure and work-up to that described for [1-In(CH2SiMe3)2] then afforded [2-In(CH2SiMe3)] as a yellow powder. Yield: 200 mg, 93 %. 1H NMR (C6D6, 298 K, 500.13 MHz): d = 7.78 (d, 1 H, 4JHH = 1.3 Hz, N=CH), 7.54 (s, 1 H, N=CH), 7.21 (m, 2 H, CaromH), 7.05–6.94 (m, 5 H, CaromH), 6.91–6.84 (m, 5 H, CaromH), 6.57–6.52 (m, 2 H, CaromH), 6.47–6.43 (m, 2 H, CaromH), 5.00 (dd, 1 H, 3JHH = 5.5, 4JHH = 1.3 Hz, N-CH), 4.14 (d, 1 H, 3JHH = 5.5 Hz, 1 H, N-CH), 2.59 (s, 3 H, Carom-CH3), 2.56 (s, 3 H, Carom-CH3), 0.19 (s, 9 H, Si(CH3)3), 0.47 to 0.59 ppm (AB system, 2H 2JHH = 12.5 Hz, InCH2Si); 13C{1H} NMR (C6D6, 298 K, 125.75 MHz): d = 172.0 (N=CH), 171.4 (N=CH), 170.7 (C-OIn), 170.3 (C-OIn), 139.1 (i-C6H5), 139.0 (i-C6H5), 135.9 (CaromH), 135.8 (CaromH), 133.5 (CaromH), 133.4 (CaromH), 132.8 (Carom-CH3), 132.6 (Carom-CH3), 129.2 (CaromH), 129.1 (CaromH) 128.8 (CaromH), 128.6 (CaromH) 128.5 (CaromH), 128.4 (CaromH) 118.5 (Carom-CH=N), 118.0 (Carom-CH=N), 114.8 (CaromH), 114.8 (CaromH), 74.2 (N-CH), 71.0 (N-CH), 17.5 (Carom-CH3), 17.4 (Carom-CH3), 1.91 (Si(CH3)3), 2.9 ppm (br, InCH2Si); elemental analysis calcd (%) for C34H37InN2O2Si (648.57 g mol1): C 63.0; H 5.8; N 4.3; found: C 62.9; H 6.0; N 4.5.

In a glovebox, the metal catalyst (ca. 5.0–15.0 mg) was placed in a Schlenk flask together with the monomer (ca. 0.1–1.0 g). The Schlenk flask was sealed and removed from the glovebox. All subsequent operations were carried out on a vacuum manifold using Schlenk techniques. The required amount of solvent (toluene) was added with a syringe to the catalyst and the monomer, followed when necessary by addition of alcohol (iPrOH or BnOH, 3–10 mL). The resulting mixture was immerged in an oil bath pre-set at the desired temperature and the polymerisation time was measured from this point. The reaction was terminated by addition of MeOH and the polymer was precipitated in methanol or a methanol/pentane mixture. It was washed thoroughly and reprecipitated from dichloromethane/pentane. The polymer was then dried to constant weight in a vacuum oven at 55 8C under dynamic vacuum (< 5 102 mbar).

[3-In2(CH2SiMe3)4]: [In(CH2SiMe3)3] (234 mg, 0.62 mmol) and (R,R)-3H2 (200 mg, 0.31 mmol) were reacted in toluene (3.5 mL) at 70 8C for 14 h. Identical procedure to that described for [1-In(CH2SiMe3)2] gave the bimetallic [3-In2(CH2SiMe3)4] as a yellow solid. Yield: 365 mg, 96 %. 1H NMR (C6D6, 298 K, 500.13 MHz): d = 8.24 (s, 2 H, N=CH), 7.54 (d, 2 H, 3JHH = 2.7 Hz, CaromH), 6.99 (d, 2 H, 3JHH = 2.7 Hz, CaromH), 6.97–6.92 (m, 4 H, CaromH), 6.85–6.77 (m, 6 H, CaromH), 5.08 (s, 2 H, N-CH), 1.61 (s, 18 H, C(CH3)3), 1.23 (s, 18 H, C(CH3)3), 0.52 (s, 18 H, Si(CH3)3), 0.31–0.16 (AB system, 4 H, 2JHH = 12.7 Hz, InC(H)(H)) 0.01 (s, 18 H, Si(CH3)3), 0.85 ppm (AB system, 4 H, 2JHH = 12.7 Hz, InC(H)(H)); 13C{1H} NMR (C6D6, 298 K, 125.75 MHz): d = 174.4 (N=CH), 168.4 (C-OIn), 141.5 (Carom-tBu), 136.8 (Carom-tBu), 135.8 (i-C6H5), 131.7 (CaromH), 131.4 (CaromH), 129.9 (CaromH), 129.8 (CaromH), 129.2 (CaromH), 118.2 (Carom-CH = N), 78.2 (N-CH), 35.7 (C(CH3)3), 34.0 (C(CH3)3), 31.6 (C(CH3)3), 30.3 (C(CH3)3), 5.4 (InCH2Si), 3.1 (Si(CH3)3), 2.4 (Si(CH3)3), 0.2 ppm (InCH2Si); elemental analysis calcd (%) for C60H98In2N2O2Si4 (1221.41 g mol1): C 59.0; H 8.1; N, 2.3; found: C 58.9; H 8.2; N 2.4. [4-In(CH2SiMe3)]: [In(CH2SiMe3)3] (72 mg, 0.19 mmol) and (R,R)-4-H2 (100 mg, 0.19 mmol) were reacted in benzene (2.8 mL) at 80 8C for 3 h. After cooling down to room temperature, the volatiles were removed under vacuum and the powder was dried in vacuo to afford [4-In(CH2SiMe3)] as a yellow powder. Yield: 105 mg, 77 %. 1 H NMR (C6D6, 298 K, 500.13 MHz): d = 8.05 (dd, 1 H, 4JHH = 3.0, 4 JHH = 0.9 Hz, CaromH), 8.03 (dd, 1 H, 4JHH = 3.0, 4JHH = 0.9 Hz, CaromH), 7.62 (d, 1 H, 4JHH = 3.0 Hz, CaromH), 7.57 (d, 1 H, 4JHH = 3.0 Hz, CaromH), 7.49 (d, 1 H, 4JHH = 1.0 Hz, N=CH), 7.15–6.95 (m, 6 H, CaromH + N=CH), 6.91–6.81 (m, 5 H, CaromH), 5.06 (d, 1 H, 4JHH = 3.5 Hz, N-CH), 4.19 (d, 1 H, 4JHH = 3.5 Hz, N-CH), 2.24 (s, 3 H, Carom-CH3), 2.19 (s, 3 H, CaromCH3), 0.05 (s, 9 H, Si(CH3)3), 0.76 ppm (AB system, 2 H, 2JHH = 12.6 Hz, InCH2Si); 13C{1H} NMR (C6D6, 298 K, 125.75 MHz): d = 174.4 (C-OIn), 173.9 (C-OIn), 172.7 (CH=N), 170.7 (CH=N), 139.5 (i-C6H5), 137.2 (i-C6H5), 136.9 (Carom-NO2), 136.7 (Carom-NO2), 133.6 (Carom-CH3), 133.5 (Carom-CH3), 131.1 (CaromH), 130.8 (CaromH), 129.7 (CaromH), 129.5 (CaromH), 129.4 (CaromH), 129.3 (CaromH), 129.1 (CaromH), 128.2 (CaromH), 128.0 (CaromH),127.9 (CaromH), 116.5 (Carom-CH=N), 116.0 (Carom-CH=N), 74.6 (N-CH), 70.2 (N-CH), 17.0 (Carom-CH3), 16.9 (Carom-CH3), 1.6 Chem. Eur. J. 2014, 20, 6131 – 6147

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Typical Schlenk-scale polymerisation procedure

Typical NMR-scale polymerisation procedure All NMR-scale ROP reactions were systematically conducted in [D8]toluene. In a typical experiment, the catalyst and the monomer were loaded in an NMR tube in the glovebox. The NMR tube was placed in a Schlenk flask, which was then removed from the glovebox and connected to the vacuum manifold. All subsequent operations were performed using Schlenk techniques. The appropriate amounts of [D8]toluene and alcohol when required (iPrOH or BnOH) were added to the NMR tube in this order at room temperature. The NMR tube was then sealed, and gently heated to ensure complete dissolution of the monomer and introduced in the spectrometer pre-set at the desired temperature. Time measurement started at this point. Data points were collected at regular intervals (typically 30–480 s, with D1 = 0.5 s and NS = 8–16 scans) until conversion of the monomer stopped (this usually coincided with nearfull conversion). The conversion was reliably determined by integrating the methine region of PLLA (ca., d1H = 5.00 ppm in [D8]toluene) versus that of the monomer (ca., d1H = 4.08 ppm in [D8]toluene).

Curve-fitting methods For kinetic experiments, the values of ki and kp were determined with the software Origin and Datafit by curve-fitting of experimental data as described in references [28] and [29].

X-ray diffraction crystallography Crystals of (R,R)-3-H2, (R,R)-6-H2, [1-AlMe], [2-Al(CH2SiMe3)], [1-In(CH2SiMe3)], [2-In(CH2SiMe3)], [3-In2(CH2SiMe3)4] and [2-Ga2(CH2SiMe3)4] suitable for X-ray diffraction analysis were obtained by recrystallization of the purified compound. Diffraction data were collected at 150(2) K using a Bruker APEX CCD diffractometer with graphite-monochromated MoKa radiation (l = 0.71073 ). A combination of w and F scans was carried out to obtain at least a unique data set. The crystal structures were solved by direct methods, remaining atoms were located from difference Fourier synthesis followed by full-matrix least-squares refinement based on F2 (programs SIR97 and SHELXL-97).[38] Carbon- and oxygen-bound hydrogen atoms were placed at calculated positions and forced to ride on the attached atom. All non-hydrogen atoms were refined

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Full Paper with anisotropic displacement parameters. The locations of the largest peaks in the final difference Fourier map calculation as well as the magnitude of the residual electron densities were of no chemical significance. Relevant collection and refinement data are given in the Supporting Information. Crystal data, details of data collection and structure refinement for all compounds structurally characterised (CCDC 972232–972236 and 973341) CCDC-972232 ([1-AlMe]), CCDC-972233 ([1-In(CH2SiMe3)]), CCDC-972234 ([2-Ga2(CH2SiMe3)4]), CCDC-972235 ([2-In(CH2SiMe3)]) CCDC-972236 ([3In2(CH2SiMe3)4]) and 973341 ([2-Al(CH2SiMe3)]) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

[8] [9]

[10] [11]

Acknowledgements The authors acknowledge financial support from Total Raffinage-Petrochimie (grant to N.M.). We are grateful to Dr. Olivier Miserque, Dr. Martine Slawinski, Dr. Aurlien Vantomme, Dr. Alexandre Welle (Total Research & Technology Feluy, Belgium), and Dr. Jean-Michel Brusson (Total Corporate Research) for valuable discussions. We thank Stephen Boyer (London Metropolitan University) for combustion analyses. The contribution of Maelle Gouessan towards the preparation of complex [2-Ga2(CH2SiMe3)4] is much appreciated.

[12]

[13]

Keywords: aluminium · indium · ligands · reaction mechanisms · ring-opening polymerisation · stereoselectivity

[14] [15]

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Inorg. Chem. 2011, 50, 6775. For a fluorinated dialkoxide ligand, see: f) M. Bouyahyi, E. Grunova, N. Marquet, E. Kirillov, C. M. Thomas, T. Roisnel, J.-F. Carpentier, Organometallics 2008, 27, 5815. For a mixed fluorinated alkoxide/iminophenoxide ligand, see: g) A. Alaaeddine, C. M. Thomas, T. Roisnel, J.-F. Carpentier, Organometallics 2009, 28, 1469. For reviews, see: a) M. J. Stanford, A. P. Dove, Chem. Soc. Rev. 2010, 39, 486; b) P. J. Dijkstra, H. Du, J. Feijen, Polym. Chem. 2011, 2, 520. Y-mediated isoselective ROP of LA: C. Bakewell, T.-P.-A. Cao, N. Long, X. F. Le Goff, A. Auffrant, C. K. Williams, J. Am. Chem. Soc. 2012, 134, 20577. Zn-mediated isoselective ROP of LA: H. Wang, H. Ma, Chem. Commun. 2013, 49, 8686. Pm, the probability of a meso enchainment between two consecutive LA units, is the measure of isotacticity. For a given PLA sample, Pm is customarily determined by examination of the methyl region of the deconvoluted homodecoupled 1H NMR spectrum. See: a) K. A. M. Thakur, R. T. Kean, E. S. Hall, J. J. Kolstad, T. A. Lindgren, M. A. Doscotch, J. I. Siepmann, E. J. Munson, Macromolecules 1997, 30, 2422; b) M. H. Chisholm, S. S. Iyer, M. E. Matison, D. G. McCollum, M. Pagel, Chem. Commun. 1997, 1999. In the present study, we distinguish isotactic-enriched PLA (0.65 < Pm < 0.80) from isotactic PLA (Pm > 0.80) by arbitrarily setting the boundary at Pm = 0.80. For (0.50

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