Exploiting Noncovalent Interactions for Room-Temperature

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Exploiting non-covalent interactions for room temperature heteroselective rac-lactide polymerization using aluminium catalysts Sami Gesslbauer, Risto Savela, Yijun Chen, Andrew J. P. White, and Charles Romain ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00875 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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ACS Catalysis

Exploiting Non-Covalent Interactions for Room Temperature Heteroselective rac-Lactide Polymerization Using Aluminium Catalysts. S. Gesslbauer,a R. Savela,b Y. Chen,a A. J. P. White,a C. Romaina* a

Department of Chemistry, Molecular Sciences Research Hub (MSRH), Imperial College

London, W12 0BZ, London, UK. b

Laboratory of Organic Chemistry, Åbo Akademi University, FI-20500 Åbo, Finland.

ABSTRACT Whereas harnessing non-covalent interactions (NCIs) have largely been applied to late-transition metal complexes and to the corresponding catalytic reactions, there are very few examples showing the importance of NCIs in early-transition metal and main group metal catalysis. Here, we report on the effects of hydrogen bond donors in the catalytic pocket to explain the high activity and stereoselectivity of a series of aluminium catam complexes in rac-lactide ring-opening polymerisation (ROP). Four original aluminium catam catalysts have been synthetized and fully characterized. Structure-activity relationships and isotope effect show the importance of the NH moieties of the ligand in rac-lactide ROP. Computational studies highlight beneficial hydrogen bonds between the ligand and the monomer. Overall, structural characterization of the catalysts, mechanistic, kinetic and computational studies support the benefits of non-covalent interactions in the catalytic pocket.

ACS Paragon Plus Environment

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N

Pm = 0.65

N Al

Cl

O

O

Cl Cl

Cl

N Al O O O

Non-Covalent Interactions R

tBu

N

N Al O X O

R

tBu N Al Cl

O Cl

O

Cl Cl

O tBu

Pr = 0.96

Ph

Ph

tBu

tBu

tBu

+ O

N Cl N Al

isotactic-enriched C 5H 9 + co-cat N CH2Ph O O Al Li(THF)2 O CH2Ph N C 5H 9

Ph

N

N

This work Pr = 0.9

highly heteroselective catalysts (Pr > 0.8)

H N Al O Cl tBu

[Ph3P=N=PPh3]+Cl-

X = Et, OiPr R = H, D, Me

Ph N

H N

tBu

Pr = 0.98 N

Pr = 0.87 Ph

Page 2 of 24

Ph

+ epoxide

Pr = 0.67

"room temeperature" active catalysts (T < 30 OC)

TOC graphic

KEYWORDS Ring-Opening Polymerisation, rac-Lactide, Aluminium Catalysts, Non-Covalent Interactions, Hydrogen bond. Non-Covalent Interactions (NCIs) encompass various types of interactions including ion pair interactions, hydrogen bonding, dipolar interactions,  interactions, hydrophobic interactions, and Van der Waals interactions.1 They are found to be of importance in biology, chemistry, and material science among others. Related to chemistry and catalysis, NCIs are ubiquitous in organocatalysis and the so-called hydrogen bond catalysis.2, 3 There has been a growing interest in harnessing NCIs in metal catalysis to increase catalyst activity and selectivity. As highlighted in recent reviews, different approaches have been reported exploiting ligand-ligand interactions, ligand-substrate interactions and more sophisticated scenarios involving multiple interactions with a third species.1, 3-5 However, most of these strategies have been applied to late-transition metal complexes and thus have been applied to organic reactions catalysed by such metals (i.e. hydrogenation, hydroformylation, allylation reactions to name a few) as well as in polymerisation catalysis. For example, in olefin polymerisation, non-covalent attractive interactions have been proposed to explain the livingness of some post-metallocene catalysts due to C-H⸱⸱⸱F-C interactions between the “fluorinated” ligand and the growing polymer chain.6, 7 Interestingly,


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ACS Catalysis

there are very few examples highlighting the importance of NCIs in early transition metal and main group metal catalysis. For example, Peters and co-workers reported cooperative Lewis Acid/Ion Pair catalysis. Thus, a series of Al complexes bearing a salen-type ligand with tethered ammonium salts were found to outperform the corresponding “untethered” catalysts in the asymmetric synthesis of -lactones and the carbocyanation of aldehydes.8, 9 With this in mind, we decided to focus on new main group metal complexes bearing ligand(s) capable of forming NCIs of interest in polymerisation catalysis. The “catechol-amine” ligand scaffold (referred to as “catam”) is a good candidate as it offers two rigid o-aminophenolate moieties for coordination to high oxidation state and oxophilic metals as well as two NH moieties near the metal centre as potential hydrogen bond donors (Figure 1).10, 11 Whereas salen-, salanand salalen-type ligands have been thoroughly studied in lactide ring-opening polymerisation (ROP), less investigations have been carried out on the catam analogues where the nitrogen atoms are directly connected to the aryl moieties.12, 13 To the best of our knowledge, only titanium and zirconium complexes bearing a catam-type ligand [i.e. a phenylenediamine bis(phenolate)] have been investigated in lactide ROP.12 We recently reported the first series of aluminium catam complexes which was found able to catalyse rac-lactide ROP at room temperature;13 a rather rare feature for aluminium-based catalysts.14-17 Here, we report the benefit of hydrogen bond donors in the catalytic pocket to explain the high activity and stereoselectivity of a new family of aluminium catam complexes in rac-lactide ROP. Structural characterization of the catalysts, mechanistic, kinetic (including isotope effect) and computational studies highlight the benefit of such noncovalent interactions.


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H

tBu H

tBu

N

N

O

N

Al

O

H

tBu

N

H

tBu

O

tBu

tBu

Previous work

O

tBu

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tBu

catam-type ligand scaffold

H

tBu

N

Al

O

N

H

tBu

O

tBu

tBu This work

Figure 1: Representative structures of catam-type ligand scaffold and corresponding Al complexes. Complex synthesis and characterization.

tBu

R

N

N

R

tBu

AlEt3, THF

tBu

R

tBu

N

R

-40 C to r.t.

O

tBu

tBu 1: R = H 3: R= Me

R=H (H2 L) R=Me (H2MeL) R=D (D2DL)

iPA (1 eq.)

tBu

R

N

N

R

tBu

Al O O O

r.t.

O

tBu

H

tBu

Al

O

OH HO

N

tBu

tBu 2: R = H 4: R = D

Figure 2: synthesis of complex 1-4. The tethered o-aminophenol H2HL was prepared according to standard literature procedure using 3,5-di-tert-butyl catechol and 2,2-dimethyl-1,3-propanediamine in acetonitrile.18 Subsequently, H2HL reacts with one equivalent of AlEt3 in THF to cleanly afford the corresponding aluminium ethyl complex HLAl(Et) 1 in good yield (56 %, Figure 2). The 1H NMR spectrum (C6D6, 298 K) shows a C2-symmetric structure in solution with, among others, a characteristic shielded triplet and quartet peaks for the Al ethyl chain (1H = 1.19 ppm and 1H = -0.05 ppm) along with signals at 2.98 ppm attributed to the NH groups (Figure S6). The molecular structure of 1 has been confirmed by X-Ray diffraction of a single crystal obtained by diffusion of pentane into a


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ACS Catalysis

concentrated THF solution. As illustrated in Figure 3, the pentacoordinated aluminium atom exhibits an almost perfect square pyramidal geometry (5 = 0.03)19 with a planar coordination of the tetradendate ligand and the ethyl chain on the apical position. The 6-membered ring formed by the (N’N’Al) chelate adopts a chair conformation with both NH bonds in axial position pointing out in the same direction. This offers two orientationally-defined H-bond donors spaced by 2.79(1) Å as observed in well-known squaramide-type organocatalysts.20 A similar ligand conformation was observed in a Pd(II) complex reported by Wieghardt and co-workers.18 Two molecules of THF were found forming a hydrogen bond with the NH groups of the ligands [H7⋅⋅⋅O40 = 2.231(4) Å and H11⋅⋅⋅O50 = 2.181(6) Å] unambiguously confirming the ability of the ligand to act as a Hbond donor.

Figure 3: The crystal structure of 1 (ellipsoid plot 50% probability, H omitted except on the NH moieties) showing the N–H···O hydrogen bonds to the included tetrahydrofuran solvent


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molecules. The hydrogen bonding geometries [N···O and H···O (Å), and N–H···O (°)] are 3.110(2), 2.231(4) and 165.6(13) for the N7···O40 contact, and 3.061(2), 2.181(4) and 165.4(12) for the N11···O50 contact. Distance H7··· H11 is 2.79(1). 1 was found to react with 1 equivalent of iso-propanol (iPA) in THF at room temperature to afford the corresponding Al isopropoxide derivative HLAl(OiPr) 2. 1H NMR spectrum at 298K shows a set of broad signals attributed to a main compound featuring a C2-symmetric structure in solution. The DOSY NMR spectrum in d8-THF (solvent used for room temperature polymerisation) exhibits a monomeric species (Figure S15) in accordance with the proposed structure (Figure 2). Similarly, DOSY NMR analysis carried out in C6D6 in the presence of 10 equivalent of iPA (conditions for “immortal” polymerisations) shows a mononuclear complex with potential coordination of iPA (Figure S18). However, in the absence of coordinative molecules (i.e. THF, iPA), the DOSY NMR spectrum in C6D6 suggests the existence of both mononuclear and dinuclear species in equilibrium (Figure S17). In order to assess the importance of the NH moiety, a N-methylated ligand was synthesised with the aim to increase the steric bulk around the metal centre and to potentially improve the stereocontrol during the polymerisation. Following an adapted procedure from literature (using nBuLi

and MeI), a methylated version H2MeL of the pro-ligand H2HL was obtained and

subsequently reacted with AlEt3 in THF overnight to afford the corresponding methylated complex MeLAl(Et)

3 (Figure 2). The 1H NMR spectrum of 3 shows a C2-symmetric structure in solution

with, amongst others, one singlet 1H = 2.58 ppm (C6D6, 24 C) for the two N-methyl groups (Figure S8). Unambiguously, the molecular structure of 3 has been determined by X-Ray diffraction of a single crystal (Figure 4) obtained from a cold THF/pentane mixture (-40 C). Contrary to 1, the molecular structure of 3 exhibits a pentacoordinated aluminium atom adopting


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ACS Catalysis

a distorted trigonal bipyramidal geometry (5 = 0.63)19 with the two phenolate moieties trans to each other [O-Al-O = 169.2(11) ] and with both Al–O vectors approximately orthogonal to the N2Al chelate plane. This results in longer Al-O bonds and shorter Al-N bonds in complex 3 [Al1O1 = 1.859(3) Å, Al1-O17 =1.854(3) Å, Al1-N7 = 2.034(3) Å and Al1-N11 = 2.054(3) Å] than in complex 1 [Al1-N7 = 2.1057(14) Å, Al1-N11 = 2.1142(14) Å, Al1-O1 = 1.8175(12) Å, Al1O17=1.8176(13) Å]. The 6-membered ring formed by the (N’N’Al) chelate adopts a distorted twisted boat conformation with both methyl groups in equatorial position.

Figure 4: The crystal structure of 3 (ellipsoid plot 50% probability, H omitted) showing a pentacoordinated aluminium atom adopting a distorted trigonal bipyramidal geometry (5 = 0.63).


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rac-lactide polymerisation. Complex 2 (either directly or generated in-situ from the addition of iPA

to 1) was found to be active for rac-lactide ROP at room temperature in THF (2/rac-LA =

1/100, 90 min., 91% conv.) to afford well-defined heterotactic PLA (Pr  0.9). The polymerisation is well-controlled (Table 1), i.e.: i) linear increase of molar masses with monomer conversion (Figure S21), ii) low dispersity (Ð) and iii) pseudo first order in monomer with kHobs = (4.75 ± 0.29) x 10-4 s-1 (Figure S20). Analysis of the polymers by MALDI-ToF mass spectrometry shows the presence of an isopropoxide end-group, in line with a polymerisation occurring via a wellknown coordination-insertion mechanism (Figure S30). Table 1: rac-lactide polymerisation using 1-4.

Entry

Cat.

Cat./ iPA/ LA

Solvent

( eq.)

T

Time

Conv.a

Mn(SEC)(Ð)b

(°C)

(min)

(%)

(kg/mol)

Mn(calc.)d Prc

(kg/mol )

1

1

1/1/100

THF

25

90

92.5

10.8 (1.3)

0.90

13.3

2e

1

1/1/100

THF

25

15 + 120

85.3

7.3 (1.2)

0.89

12.3

3

2

1/0/100

THF

25

90

91.4

9.5 (1.3)

0.91

13.2

4

3

1/1/100

THF

25 (50)

180 (240)

0

-

-

-

(0)

-

-

-

5

4

1/0/100

THF

25

90

83.9

16.5 (1.3)

0.91

11.8

6

Hsalan-Alf

1/1/100

Toluene

90

960

46.1

5.7 (1.3)

0.54

6.6

7

Salen-Alg,21

1/0/100

Toluene

70

300

94

21.2 (1.1)i

0.08

-

8

2

1/9/1000

Toluene

90

30

90.4

11.5 (1.1)

0.61

13.0

9

2

1//4/500

THF

50

60

55.8

7.0 (1.2)

0.85

8.0

10

2

1/0/250

Bulkh

130

3.5

40.8

4.3 (1.1)

0.61

5.9

11

Salen-Al22

1/0/300

Bulk

130

30

25

14.3 (1.1)i

0.09

-

Reaction conditions: [LA]0 = 1 mol.L-1; a) Determined by 1H NMR by relative integration of signals at 5.06 ppm (monomer) and 5.20 ppm (polymer); b) Determined by SEC calibrated with


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ACS Catalysis

polystyrene standards in THF and corrected by a factor of 0.58;23 c) Probability of racemic linkages determined by 1H NMR spectroscopy; d) Calculated using the conversion; e) 1 was generated insitu by stirring AlEt3 and H2HL1 in THF for 15 minutes prior to the addition of LA and iPA; f) salan derivative of 1, see ESI; g) salen derivative of 1 with BnO instead of OiPr,24 h) in molten monomer, i) Determined by SEC calibrated with polystyrene standards in CHCl3, value not corrected.22 Interestingly, examples of Al complexes leading to highly heterotactic PLA (Pr > 0.80) are scarce and formation of isotactic PLA usually prevails.25-32 Gibson and co-workers reported PLA with Pr up to 0.98 using an aluminium salan-type catalyst.33 More recently, Jones and Kol separately reported a series of aluminium pyrolidine Schiff base catalysts affording highly heterotactic PLA with Pr = 0.87 and Pr = 0.98, respectively.34, 35 The observed heterotacticity is likely due to a chain-end control (i.e. the last inserted monomer in the polymer chain drives the insertion of the next one) with the presence of NCIs in the catalytic pocket which reinforces the chiral environment. In our previous work, we observed that the catam aluminium complexes bearing a somewhat more rigid ligand (ethyl backbone) led to slightly isotactic PLAs.13 This change in selectivity can be attributed to the different ligand flexibility which can favor a different mechanism (e.g. site-control). Further systematic modifications of the ligand backbone will be explored to rationalize the influence of the ligand and improve catalyst design. The activity at room temperature (< 30 C) is particularly notable as aluminium complexes usually require elevated temperature (> 70 C) to be significantly active. In addition to our previous aluminium catam complex series, only two other types of aluminium-based catalytic systems with significant activity at room temperature were previously reported, i.e.: i) an heterobimetallic aluminium-lithium complex featuring an anionic aluminium supported by a NON-type diamido ether tridentate ligand, and ii) in-situ generated anionic aluminium complexes bearing salen- or porphyrinato-type ligands in the presence of epoxide (CHO, PO) and ammonium halide salt.14-17 However, 2 is the first example of a well-defined discrete monometallic aluminium catalyst exhibiting both significant


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heteroselectivity and activity at room temperature (< 30 C) for rac-lactide. In comparison, the analogous aluminium salen and Hsalan complexes featuring the same propyl backbone displayed lower activities at higher temperatures (>70 °C) than catalyst 1 and 2 at room temperature (Table 1, entry 6-7 and Figure S10).21,

22, 24

These results highlight key features of the catam ligand

scaffold, i.e.: i) the rigid o-aminophenolate moieties forming a 5-membered (O’N’Al) chelate which has previously been found to be of importance in -CL and lactide ROP;13, 36 ii) the NH group directly connected to the aryl moiety which can act as hydrogen-bond donor due to more polarized N-H bonds than in the corresponding Hsalan ligand. Exploring the potential of these new aluminium catalysts, 2 was found to be active at low loading (as low as 0.1 mol. %) and in the presence of alcohol acting as chain-transfer agent (conditions close to “immortal” conditions) while maintaining its high stereoselectivity at higher temperature (50 C in THF, Pr = 0.85) leading to a very active system able to polymerise  250 eq. of rac-LA in 1h (TOF = 250 h-1, Table 1, entry 9). 2 also shows high activity in toluene at 90 C (TOF = 1800 h-1) and in molten monomer at 130 °C (TOF = 1700 h-1) with well-controlled molar masses and low dispersities (Table 1, Entry 8 and 10 respectively). However, the polymers obtained under these conditions were atactic due to the high temperature which reduces monomer selectivity. The catalytic system can also be generated in-situ by initially reacting the pro-ligand H2HL and AlEt3 in THF for 15 minutes before addition to a rac-LA/iPA mixture in THF (Table 1). Heterotactic polymers were obtained without significant loss of activity and stereoselectivity compared to the isolated catalyst 2 (Table 1, entry 2 and 3). The pro-ligand H2HL has also been tested in rac-lactide polymerization in the presence of sparteine and isopropanol as per standard organocatalyzed-reaction conditions (Table S1).37 The resulting catalytic system was able to slowly polymerize rac-LA at room temperature (5 mol. %


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ACS Catalysis

of H2-HL, 27% rac-LA conv., 24h, r.t.) to afford atactic PLA. In the absence of either sparteine or isopropanol, the catalytic system was found inactive. These results highlight the ability of the ligand scaffold to form hydrogen-bonds to promote lactide ROP. In similar conditions, the H2Hsalan-based catalytic system was found to be inactive (Table S1). This supports the importance of the NH group directly connected to the aryl moieties in the catam ligand scaffold. Changing solvent from dichloromethane to tetrahydrofuran shows a decrease in activity in accordance with competitive hydrogen bonding with the substrates. Surprisingly, complex 3 was inactive for rac-lactide ROP as per conditions previously investigated for complex 1 and 2. No activity was observed, neither at room temperature nor at 50 C in THF in the presence of 1 equivalent of iPA and 100 eq. of rac-lactide (Table 1, entry 4). This confirms the importance of the NH moieties in complexes 1 and 2, either by enabling the complex to adopt a suitable geometry for polymerisation and/or by forming favourable non-covalent interactions (NCI), such as hydrogen bonds observed in the molecular structure of complex 1 (Figure 3). In the same vein, Merkhodavandi and co-workers reported indium complexes where substitution of a secondary amine by a tertiary amine in the ligand scaffold led to a decrease of the catalyst activity by two orders of magnitude for lactide ROP.38 Jones and co-workers observed different “wrappings” of salan ligands around an Al centre due to a weak interaction between a NH moiety of the ligand and an isopropoxide oxygen atom.29 Thus, to further highlight the importance of the NH moieties of the ligand, deuterated derivatives of H2HL and complex HLAl(OiPr) 2 were synthesised, i.e. D2DL and DLAl(OiPr) 4 (see details in ESI). 4 was found to be an active catalyst for rac-LA ROP at room temperature (kDobs = 2.52 x 10-4 s-1) as per conditions used for 2. As previously observed with 2, the polymerization is wellcontrolled and leads to heterotactic PLA (Table 1, entry 5). Kinetic studies show that the catalyst


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ACS Catalysis

2 is almost twice as fast as the deuterated catalyst 4 (i.e. kH / kD .= 4.75x10-4 / 2.52x10-4 ~ 1.9, Table 1 entry 3 and 5) confirming the importance of the NH moieties of the ligand in the catalyst activity (Figure 5). These results suggest a secondary kinetic isotope effect (KIE) with a remote effect rather than breaking of a NH/D bond. The fairly high value of 1.9 (for a secondary KIE only) is likely due to equilibrium or binding isotope effects (EIE/BIE) in agreement with binding of the monomer with the NH moieties in the catalytic pocket.39

3

tBu

R

N

N

catalyst 2

R

tBu

Al O O O

2

tBu

ln(LA0/LAt)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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tBu 2: R = H 4: R= D

catalyst 4 1

0 0

2000

4000

6000

Time (sec)

Figure 5: Plot showing the isotope effect observed between 2 and 4 for rac-lactide ROP. Computational studies. In order to get a better understanding of the mechanism, DFT calculations were carried out using B97xD/6-31G(d,p) which includes a second-generation dispersion correction and solvation model (see computational details in supporting information).40 For computational simplicity, the reaction between an aluminium methoxide complex I(c) whose structure has been deduced from the molecular structure of 1 (ethyl group replaced by a methoxide group) and one or two molecules of lactide has been studied for the initiation and propagation step, respectively. Different approaches and coordination of the L-lactide molecule onto the


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ACS Catalysis

catalyst as well as molecules of THF and alcohol have been considered and are summarized in the supporting information (Figures S32-S34). Only the most favorable pathway will be discussed. In line with the experimental results showing a well-controlled ROP, a standard coordinationinsertion mechanism was envisaged and divided in two processes, i.e. initiation (first monomer insertion) and propagation (second monomer insertion).15, 26, 29, 41-45 Initiation mechanism and non-covalent interactions (NCI). During the initiation step, it was found that the L-lactide can displace coordinated molecules of THF (G = -1.5 kcal/mol, Figure S40-41)46 and docks on the top of the catalyst due to favourable NCIs with the ligand and the metal centre, including a hydrogen bond between one oxygen atom of the lactide carbonyl and one NH bond of the ligand as highlighted in the NCI surfaces in Figure 6 (bright blue dot).47 This suitably orientates the monomer and possibly contributes to its activation by decreasing the electron density on the carbon atom of the carbonyl in a similar manner as H-bonding activation by ROP organocatalysts (e.g. urea-based catalytic systems).48-51 These steric and electronic factors favor the subsequent nucleophilic attack by the Al-OMe bond onto the lactide carbonyl via a transition state III(t)-TS with G298 = 6.9 kcal/mol on the PES (Figure 7). Similarly, intermediate V(t) shows a hydrogen bond between one oxygen atom of the hemiacetal and one NH bond sticking out of the catalytic pocket. This suitably orientates the hemiacetal for the subsequent ring-opening via the second transition state VI(c)-TS with G298 = 9.2 kcal/mol. Overall, the initiation step features a low energy barrier of 14.4 kcal/mol and shows the existence of hydrogen bonds between the “substrate” (reacting monomer) and the NH moieties of the ligand. The calculated KIE values (~1.1) are lower than the experimental value (~1.9) but in accordance with a normal isotope effect as experimentally observed (Figure S46-S48). Such a difference can be due to BIE/EIE.39


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Hydrogen bond

Figure 6: Intermediate II(c) showing docking of the L-lactide on the top of the initiator (left) and NCI surface of II(c) showing a hydrogen bond between an O atom of L-lactide and one NH bond of the ligand (right). Framework Distortion Energy (FDE). As recently studied by Tolman, Cramer and co-workers, energetically low cost ligand distortion has been found to be a key feature in rationalizing and predicting catalyst activity for cyclic ester ROP.43, 52-55 This can be accessed by calculating the Framework Distortion Energy (FDE) which estimates the energy penalty incurred when distorting the ligand geometry of the “catalyst” to the geometry adopted in the rate-determining or turnoverlimiting transition state (TOL TS). Thus, considering I(c) as the catalyst and VI(t)-TS as the TOL TS, we found a low FDE of 5.8 kcal/mol56 which features among the lowest FDE reported for similar tetradentate Al complexes investigated for cyclic ester ROP. This is in line with the low energy barriers calculated and the high activity observed at room temperature. It should be mentioned that in both transition states III(t)-TS and VI(t)-TS, the 6-membered (N’N’Al) chelate adopts a twisted conformation which was found to be slightly more favorable than the chair conformation adopted in III(c)-TS and VI(c)-TS (G ~ 3 kcal/mol and 6 kcal/mol, respectively). Interestingly, VI(c)-TS shows a significantly higher FDE than VI(t)-TS (8.0 kcal/mol and 4.5


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kcal/mol for VI(c)-TS and VI(t)-TS, respectively) suggesting that the twisted conformation adopted by the ligand in the TS is more favorable than the chair conformation. Topographic Steric Map (TSM). Among the various molecular descriptors used to depict catalytic reactions, Cavallo and co-workers introduced topographic steric maps to characterize the shape of a catalytic pocket.57 Based on the buried volume in a considered sphere centered on an active site (here, the metal center), these maps give an estimation of the accessible molecular surface along with a shape of the catalytic pocket and the interaction surface between the catalyst and the substrate. This has previously been successfully used to rationalize various catalytic reactions, including Al-catalyzed cyclic ester ROP. Thus, the steric maps for I(c) and II(C) show a relatively flat space with the two NH slightly sticking out from the surface and suitably orientated to form hydrogen bonds (Figure 8). This could explain the favorable “docking” of the L-lactide on the top of the catalyst and be at the origin of the binding isotope effect.39 Similarly, the steric map of VI(c)-TS shows that most of the free space (up to 80% of the free volume) is located in the “South-East quadrant” where the reaction happens and one of the NH is directly pointing toward this free space (Figure 8). This supports potential hydrogen bonds between the substrate (lactide) and the ligand, and highlights the importance of hydrogen bond donors in the catalytic pocket. We find these topographic steric maps a complementary tool that supports the previously discussed NCI surfaces.


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First L-lactide insertion (initiation)

G

TS in "chair conformation" higher than in "twisted conformation"

kcal/mol

O O

H HN

Al

N

H HN

Al

N

O

O

HH

9.2 VI(t)-TS

III(c)-TS

O O

H

IV O O

0 OMe H HN Al O O N

-2.6 O

O

I(c) O

O H H

HH

II(t)

-5.2

O O

II(c)

HN

O O

II(c) OMe

N Al N

H

O O

HH

H

HN

N

Al

N

O

O

HH

OMe Al

Al

N

3.4

1.2 I (c)

O

HN

III(t)-TS

O H

MeO

6.9

O O I (t)

I(t)

VI(c)-TS

9.7

III(t)-TS

OMe

+15.2 VI(c)-TS

III(c)-TS

OMe

O

H H O O H N Al N O O

HH

O H O H OMe Al N N O O

O O

O H

MeO

O O

2.6 V

MeO H O

HN

Al

N

O H O

O O

H

O O O

VI-TS O

0.9 VII

H

-2.52 O O

MeO

V

H

IV OMe

O

H

O

VIII

O H O

MeO

O H

H

N Al N O O VII

O O

O H

O

N Al N O O VIII

II(t) Al geometry: pink: square pyramidal (sqp) blue: trigonal bipyramidal (tbp) black: octahedral (oct)

Conformation: conformation of 6-membered ring formed by N'N'Al chelate c: "chair" conformation t: "twisted" conformation (ligand backbone) Unless otherwise notified, "twisted" conformation for all intermediates from IV

Figure 7: Potential energy surface (PES) corresponding to first L-lactide insertion (initiation step). Data available here, DOI: 10.14469/hpc/3779.


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I(c)

VI(t)-TS

Figure 8: Topographic steric map of I(c) (left) and VI(t) (right) showing the position of the NH bond donor in the catalytic pocket. Propagation and stereoselectivity. In order to perceive the observed stereoselectivity, the reaction of VIII with a second lactide molecule has been considered with both L- and D-lactide molecules alternatively (Figure 9). As previously observed in the initiation step, the lactide “docking” on the top of the catalyst has been found energetically favorable (G298 = -9.7 kcal/mol and G298 = -8.0 kcal/mol for both D- and L-lactide, respectively). As highlighted in the NCI surfaces, similar hydrogen bonds can be observed between the oxygen atom of the lactide carbonyl and the NH moieties (Figure S34-S35). For both D- and L-lactide, the rate determining step was found to be the ring opening (as previously observed during the initiation) with an energy barrier difference of 2 kcal/mol in favor of a D-lactide over L-lactide insertion. This is in accordance with the formation of a highly heterotactic PLA as experimentally observed (Pr  0.9).46 Interestingly, XIII(D)-TS also features a lower FDE than XIII(L)-TS (FDE = 3.1 kcal/mol for XIII(D)-TS vs FDE = 7.0 kcal/mol for XIII(L)-TS). A single point analysis of the TS has to be considered cautiously and the true origin of the activation energy will require analysis along the reaction pathway, for example using a distortion/interaction-activation strain model.58


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G kcal/mol

XIV(L) 16.7

XIII(L)-TS 18.0 XIII(D)-TS 14.3

L-lactide insertion D-lactide insertion

X(D)-TS

MeO

0 I

XII(L) 7.8

7.7

XII(D) 6.1

O O

O H

XIV(D) 12.2

O

X(L)-TS

4.1 XI(D) 2.4

N Al H N O O

Ea2(L) = 26.0

Ea2(D) = 24.0

VIII -0.7 XI(L)

-2.52 VIII IX(L) -8.0

Ea1(L) = 12.1

Ea1(D) = 17.3

-9.7 IX(D) first monomer insertion

monomer coordination

TS1: insertion

hemiacetal rotation

TS2: ring-opening

Figure 9: PES showing second insertion of a lactide monomer (red = L-lactide, blue = D-lactide) after initial insertion of L-lactide as per initiation step in Figure 7 (“twisted” conformation). Data available here, DOI: 10.14469/hpc/3799. Conclusion This new family of aluminium catam complexes combines both high activity at room temperature and high heteroselectivity for rac-latide ROP. Structural characterisations of these aluminium complexes show that the NH moieties of the ligand can act as hydrogen bond donors. Mechanistic investigations establish that methylation of the NH moieties inhibits the catalyst activity at room temperature. Preliminary kinetic studies indicate that substituting the NH moieties


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by ND moieties lead to a kinetic isotope effect which could be attributed to monomer binding via the NH moieties. Finally, computational studies reveal that the NH hydrogen bond donors are wellpositioned in the catalytic pocket to interact with the reactive species (lactide, growing polymer) and to form hydrogen bonds, as highlighted on the NCI surfaces. Overall, structural characterisation of the catalysts, mechanistic, kinetic and computational studies highlight the importance of the NH moieties in the ligand to act as hydrogen bond donors and form beneficial hydrogen bonds during the polymerisation. Ligand design is currently under investigation to further exploit these non-covalent interactions to afford highly isoselective and highly active aluminium catalysts, which still remains a challenge in the field.

Supporting Information. The following files are available free of charge. NMR spectra, kinetic data, details of DFT calculations included in a PDF file. FAIR Data59,

60

and web-enhanced tables are available from DOI: 10.14469/hpc/3716, as per

funding council guidelines. Corresponding Author * Charles Romain, [email protected] Author Contributions The manuscript was written through contributions of all authors. ACKNOWLEDGMENT The authors thank ICL HPC for the computing resources, Peter Haycock for the DOSY NMR spectra and Stephen Boyer (London Metropolitan University) for the elemental analysis. RS thanks


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Academy of Finland, Harry Elvings Legat and Svenska tekniska vetenskapsakademien i Finland for funding. CR thanks Imperial College London for his Junior Research Fellowship and Prof. George Britovsek for his mentorship. REFERENCES (1) Neel, A. J.; Hilton, M. J.; Sigman, M. S.; Toste, F. D., Exploiting Non-Covalent π Interactions for Catalyst Design. Nature 2017, 543, 637-646. (2) Nishikawa, Y., Recent Topics in Dual Hydrogen Bonding Catalysis. Tetrahedron Lett. 2018, 59, 216-223. (3) Raynal, M.; Ballester, P.; Vidal-Ferran, A.; van Leeuwen, P. W. N. M., Supramolecular Catalysis. Part 1: Non-Covalent Interactions as a Tool for Building and Modifying Homogeneous Catalysts. Chem. Soc. Rev. 2014, 43, 1660-1733. (4) Knowles, R. R.; Jacobsen, E. N., Attractive Noncovalent Interactions in Asymmetric Catalysis: Links between Enzymes and Small Molecule Catalysts. Proc. Nat. Ac. Sci. 2010, 107, 20678-20685. (5) Davis, H. J.; Phipps, R. J., Harnessing Non-Covalent Interactions to Exert Control over Regioselectivity and Site-Selectivity in Catalytic Reactions. Chem. Sci. 2017, 8, 864-877. (6) Chen, C., Designing Catalysts for Olefin Polymerization and Copolymerization: Beyond Electronic and Steric Tuning. Nat. Rev. Chem. 2018, 2, 6-14. (7) Iwashita, A.; Chan, M. C. W.; Makio, H.; Fujita, T., Attractive Interactions in Olefin Polymerization Mediated by Post-Metallocene Catalysts with Fluorine-Containing Ancillary Ligands. Catal. Sci. Tech. 2014, 4, 599-610. (8) Brodbeck, D.; Broghammer, F.; Meisner, J.; Klepp, J.; Garnier, D.; Frey, W.; Kästner, J.; Peters, R., An Aluminum Fluoride Complex with an Appended Ammonium Salt as an Exceptionally Active Cooperative Catalyst for the Asymmetric Carboxycyanation of Aldehydes. Angew. Chem. Int. Ed. 2017, 56, 4056-4060. (9) Kull, T.; Cabrera, J.; Peters, R., Catalytic Asymmetric Synthesis of trans-Configured βLactones: Cooperation of Lewis Acid and Ion Pair Catalysis. Chem. Eur. J. 2010, 16, 9132-9139. (10) Zhao, B.; Han, Z.; Ding, K., The N⸱H Functional Group in Organometallic Catalysis. Angew. Chem. Int. Ed. 2013, 52, 4744-4788. (11) Khusnutdinova, J. R.; Milstein, D., Metal–Ligand Cooperation. Angew. Chem. Int. Ed. 2015, 54, 12236-12273. (12) Zelikoff, A. L.; Kopilov, J.; Goldberg, I.; Coates, G. W.; Kol, M., New Facets of an Old Ligand: Titanium and Zirconium Complexes of Phenylenediamine Bis(phenolate) in Lactide Polymerisation Catalysis. Chem. Commun. 2009, 6804-6806. (13) Gesslbauer, S.; Cheek, H.; White, A. J. P.; Romain, C., Highly Active Aluminium Catalysts for Room Temperature Ring-Opening Polymerisation of rac-Lactide. Dalton Trans. 2018, 47, 10410-10414. (14) Hild, F.; Haquette, P.; Brelot, L.; Dagorne, S., Synthesis and Structural Characterization of Well-Defined Anionic Aluminium Alkoxide Complexes Supported by NON-Type Diamido Ether Tridentate Ligands and Their Use for the Controlled ROP of Lactide. Dalton Trans. 2010, 39, 533-540.


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(43) Miranda, M. O.; DePorre, Y.; Vazquez-Lima, H.; Johnson, M. A.; Marell, D. J.; Cramer, C. J.; Tolman, W. B., Understanding the Mechanism of Polymerization of ε-Caprolactone Catalyzed by Aluminum Salen Complexes. Inorg. Chem. 2013, 52, 13692-13701. (44) Marlier, E. E.; Macaranas, J. A.; Marell, D. J.; Dunbar, C. R.; Johnson, M. A.; DePorre, Y.; Miranda, M. O.; Neisen, B. D.; Cramer, C. J.; Hillmyer, M. A.; Tolman, W. B., Mechanistic Studies of ε-Caprolactone Polymerization by (salen)AlOR Complexes and a Predictive Model for Cyclic Ester Polymerizations. ACS Catal. 2016, 6, 1215-1224. (45) Vieira, I. d. S.; Whitelaw, E. L.; Jones, M. D.; Herres-Pawlis, S., Synergistic Empirical and Theoretical Study on the Stereoselective Mechanism for the Aluminum Salalen Complex Mediated Polymerization of rac-Lactide. Chem. Eur. J. 2013, 19, 4712-4716. (46) Marshall, E. L.; Gibson, V. C.; Rzepa, H. S., A Computational Analysis of the RingOpening Polymerization of rac-Lactide Initiated by Single-Site β-Diketiminate Metal Complexes: Defining the Mechanistic Pathway and the Origin of Stereocontrol. J. Am. Chem. Soc. 2005, 127, 6048-6051. (47) Johnson, E. R.; Keinan, S.; Mori-Sánchez, P.; Contreras-García, J.; Cohen, A. J.; Yang, W., Revealing Noncovalent Interactions. J. Am. Chem. Soc. 2010, 132, 6498-6506. (48) Thomas, C.; Bibal, B., Hydrogen-Bonding Organocatalysts for Ring-Opening Polymerization. Green Chem. 2014, 16, 1687-1699. (49) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R. C.; Lohmeijer, B. G. G.; Hedrick, J. L., Organocatalytic Ring-Opening Polymerization. Chem. Rev. 2007, 107, 5813-5840. (50) Coderre, D. N.; Fastnacht, K. V.; Wright, T. J.; Dharmaratne, N. U.; Kiesewetter, M. K., H-Bonding Organocatalysts for Ring-Opening Polymerization at Elevated Temperatures. Macromolecules 2018, 51, 10121-10126. (51) Lin, B.; Waymouth, R. M., Organic Ring-Opening Polymerization Catalysts: Reactivity Control by Balancing Acidity. Macromolecules 2018, 51, 2932-2938. (52) Stasiw, D. E.; Mandal, M.; Neisen, B. D.; Mitchell, L. A.; Cramer, C. J.; Tolman, W. B., Why So Slow? Mechanistic Insights from Studies of a Poor Catalyst for Polymerization of εCaprolactone. Inorg. Chem. 2017, 56, 725-728. (53) Stasiw, D. E.; Luke, A. M.; Rosen, T.; League, A. B.; Mandal, M.; Neisen, B. D.; Cramer, C. J.; Kol, M.; Tolman, W. B., Mechanism of the Polymerization of rac-Lactide by Fast Zinc Alkoxide Catalysts. Inorg. Chem. 2017, 56, 14366-14372. (54) Macaranas, J. A.; Luke, A. M.; Mandal, M.; Neisen, B. D.; Marell, D. J.; Cramer, C. J.; Tolman, W. B., Sterically Induced Ligand Framework Distortion Effects on Catalytic Cyclic Ester Polymerizations. Inorg. Chem. 2018, 57, 3451-3457. (55) Mandal, M.; Luke, A. M.; Dereli, B.; Elwell, C. E.; Reineke, T. M.; Tolman, W. B.; Cramer, C. J., Computational Prediction and Experimental Verification of ε-Caprolactone RingOpening Polymerization Activity by an Aluminum Complex of an Indolide/Schiff-Base Ligand. ACS Catal. 2018, 885-889. (56) Considering the geometry of the resting species II(c) instead of the geometry of the catalyst I(c) gives a similar FDE of 5.1kcal/mol (see details in ESI). (57) Falivene, L.; Credendino, R.; Poater, A.; Petta, A.; Serra, L.; Oliva, R.; Scarano, V.; Cavallo, L., SambVca 2. A Web Tool for Analyzing Catalytic Pockets with Topographic Steric Maps. Organometallics 2016, 35, 2286-2293. (58) Bickelhaupt, F. M.; Houk, K. N., Analyzing Reaction Rates with the Distortion/Interaction‐Activation Strain Model. Angew. Chem. Int. Ed. 2017, 56, 10070-10086.


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(59) Wilkinson, M. D.; Dumontier, M.; Aalbersberg, I. J.; Appleton, G.; Axton, M.; Baak, A.; Blomberg, N.; Boiten, J.-W.; da Silva Santos, L. B.; Bourne, P. E.; Bouwman, J.; Brookes, A. J.; Clark, T.; Crosas, M.; Dillo, I.; Dumon, O.; Edmunds, S.; Evelo, C. T.; Finkers, R.; Gonzalez-Beltran, A.; Gray, A. J. G.; Groth, P.; Goble, C.; Grethe, J. S.; Heringa, J.; ’t Hoen, P. A. C.; Hooft, R.; Kuhn, T.; Kok, R.; Kok, J.; Lusher, S. J.; Martone, M. E.; Mons, A.; Packer, A. L.; Persson, B.; Rocca-Serra, P.; Roos, M.; van Schaik, R.; Sansone, S.-A.; Schultes, E.; Sengstag, T.; Slater, T.; Strawn, G.; Swertz, M. A.; Thompson, M.; van der Lei, J.; van Mulligen, E.; Velterop, J.; Waagmeester, A.; Wittenburg, P.; Wolstencroft, K.; Zhao, J.; Mons, B., The FAIR Guiding Principles for Scientific Data Management and Stewardship. Sci. Data 2016, 3, 160018. (60) Barba, A.; Dominguez, S.; Cobas, C.; Martinsen, D. P.; Romain, C.; Rzepa, H. S.; Seoane, F., Workflows Allowing Creation of Journal Article Supporting Information and Findable, Accessible, Interoperable, and Reusable (FAIR)-Enabled Publication of Spectroscopic Data. ACS Omega 2019, 4, 3280-3286.


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Exploiting Noncovalent Interactions for Room-Temperature

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