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

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Synthesis and structures of mono- and di-nuclear aluminium and

zinc complexes bearing α-diimine and related ligands, and their use

in the ring opening polymerization of cyclic esters

a

a*

a

b

a,b*

Lin Xiao, Yanxia Zhao, Sijie Qiao, Ziyue Sun, Orlando Santoro and Carl Redshaw

a

College of Chemistry and Material Science, Northwest University, 710069 Xi’an, China

b

Department of Chemistry & Biochemistry, The University of Hull, Cottingham Rd, Hull, HU6 7RX,

U.K.

Abstract: A series of organoaluminium imino-amido complexes of the type

{

[ArNC(Me

2

)C(Me)=NAr]AlMe

2

} (Ar = 2,6-iPr

2

C

6

H

3

(1), Ar = 2,6-Et

and the respective -diimine. Similar

(Ar = 2,6-iPr ) with AlMe

C(Me)=NAlMe (4), whilst reaction of the

acetyl-imino compound [O=C(Me)C(Me)=NAr] (Ar = 2,6-Et ) with AlMe afforded the

bimetallic complex {[OCMe CH(Me)=NAr]AlMe (5). In related organozinc chemistry, we

have isolated {[ArNC(Me)(Et)C(Me)=NAr]ZnEt} (Ar = 2,6-iPr , 6) and the trinuclear

complex {[ArN=C(Me)COCHCO(Me)C(Me)=NAr][OCH(Me)C(Me)=NAr](ZnEt) } (Ar =

,6-iPr 7) from reactions of ZnEt with ArN=C(Me)C(Me)=NAr or

O=C(Me)C(Me)=NAr], respectively. Reaction of the bis(-diimine), Lⁱ^P^r^-^N²^-^A^r^C^H²^A^r^-^N², derived

2

C

6

H (2); Ar =

3

2

,6-Me

2

C

6

H

3

(3) have been prepared via reaction of AlR

3

reaction of the bis(-diimine) [ArN=C(Me)C(Me)=N-]

afforded the bimetallic complex [ArNC(Me)

2

C

6

H

3

2

2 2

]

2

C

6

H

3

2

2 2

}

2

C

6

H

3

2

C

6

H

3

,

2

[

/

2 2 2

iPr-N -ArCH Ar-N

from 4,4 -methylenebis(2,6-diisopropylaniline), with ZnCl

8). The molecular structures of complexes 1–8 are reported. Preliminary results of the ability

of 1–8, along with the previously reported metalmetal bonded complex

[ArN=C(Me)C(Me)=NAr]Al(THF)} (9), to act as catalysts for the ring opening

2

affords [L

(ZnCl

2

)

2

]

(

{

2

polymerization (ROP) of the cyclic esters -caprolactone (-Cl), -valerolactone (-VL) and

rac-lactide (r-LA) are presented. For -CL and -VL, best results were obtained using the

metalmetal bonded complex 9. For r-LA, the Al-based systems exhibited moderate activity

affording only liquid oligomers, whilst the Zn-based systems performed better affording at 80

^oC isotactic PLA with M

ca. 10 KDa with conversions of up to 66%. The co-polymerization of

n



-CL with -VL was also examined, and differing preferences were noted for monomer

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incorporation.

Introduction

The global issues associated with the use of single use plastics and their impact on the

environment have stimulated further interest in the development of more environmentally

friendly polymers. One possible route for accessing such materials is via the use of ring

opening polymerization (ROP) of cyclic esters using metal-based catalysts. [1] The main

advantages of this route is that by manipulating the coordination environment at the metal, it is

possible to control both the catalytic activity of the system and the properties of the resultant

products. The choice of metal centre is dictated by a number of factors including cost,

abundance, toxicity and performance. Given this, catalysts employing the metals aluminium

and zinc continue to attract much attention; main group metal-based ROP systems have been

recently reviewed. [2] The use of chelating ligands in many areas of polymerization catalysis

has proved beneficial both in terms of catalyst stability and as an aid in the crystallization of the

metal species involved. In particular, this has proved highly successful in olefin polymerization,

where the use of the N,N-bi-dentate -diimines has opened up new avenues in nickel-based

catalysis. [3] Furthermore, such -diimines are known to react with dialkylzinc or

trialkylaluminium reagents under reflux, which results in the transfer of an alkyl group to the

imine backbone. [4] The resulting imino-amide and pyridyl-amide complexes offer the

opportunity of further investigations of possible cooperative effects.

There is also much interest in frameworks capable of binding simultaneously multiple metal

centers, which stems from the possibility of utilizing beneficial cooperative effects. [5]

Following on from the early nickel work, numerous frameworks capable of binding

simultaneously multiple metal centers have been designed using simple condensation chemistry,

for example in nickel-based chemistry, those shown in Chart S1 (see ESI) have been reported.

[

6] Given this extensive use of imine-based ligation in -olefin oligo-/polymerization, we were

somewhat surprised at the rather limited use of such ligation for the metal-catalyzed ROP of

cyclic esters. [7] In particular, systems have been reported bearing bipyridyl or phenanroline,

guanidine-pyridine, -diiminate and more recently a number of -diimines and amidates, as

shown in chart 1 (zinc) and chart 2 (aluminium). In the case of the amidinate aluminium

2

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complexes, the bimetallic complexes out performed their monometallic coun^D^Ote^I^:r¹p⁰^.a¹⁰r³ts⁹^/^Cf⁹o^Dr^T⁰b⁴o³³t²h^B

the ROP of -CL, which suggested the presence of beneficial cooperative effects. [7g]

Chart 1. Representative imine-type ligation employed in zinc-based ROP of cyclic esters.

Chart 2. Representative imine-type ligation employed in aluminium-based ROP of cyclic esters.

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Also relevant to the work herein is the report by Bochmann et al, [7b] who reported that

zinc cations bearing the -diimine (diazadiene) ligand (MeC=NC

6

H

3

Prⁱ

2

-2,6) , are active

2

o

for the ROP of -caprolactone under mild conditions (60 C, < 1h) but with low

conversions (< 13%).

Finally, for this type of -diimine ligand set, we and others have found that such ligation

can, in the presence of alkali metals, aid in the stabilization of metalmetal bonded

II

species. [8] Moreover, low-valent Al –Al species bearing -diimine ligation have been

shown to be active catalysts for -caprolactone polymerization, and were found to be

highly active, which was proposed to be due to the cooperative role between the two Al(II)

centers. [8j]

The molecular structures and ROP capability towards the cyclic esters -caprolactone

(-Cl), -valerolactone (-VL) and rac-lactide (r-LA) of the complexes 19 (Chart 3),

which are prepared from the pre-ligands L , L , L , L^E^t^-^N^O, Lⁱ^P^r^-^N^O, Lⁱ^P^r^-^N⁴and

iPr

Et

Me

Lⁱ^P^r^-^N²^-^A^r^C^H²^A^r^-^N², are reported herein. The effect of the presence of these reduced

-diimines on the ROP process has also been evaluated herein. The interest in the area is

stimulated by the application of poly(caprolactone)/poly(lactide) type biodegradable

polymers in the packaging and medical arenas. [9]

Results and Discussion

Aluminium complexes

The -diimines L , L and L^M^ewere prepared by standard condensation route as

reported in the literature. [10] Subsequent treatment with AlMe in refluxing toluene

afforded the complexes {AlMe [ArNCMe C(Me)=NAr]} (Ar = 2,6-iPr 1,

,6-Et 2, 2,6-Me 3, Scheme 1); the structure of {AlMe [ArNCH C(Me)=NAr]}

Ar = 2,6-iPr CCDC number: 709826) has previously been reported, and can be

iPr

Et

3

2

C

6

H

3

2

C

6

H

3

2

C

6

H

3

2

(

2

C

6

H

3

prepared either via direct treatment of the parent diamine with trimethylaluminum (TMA),

iPr

[

4b] or via the reaction of L CuCl

2

with AlMe . [4h]

3

4

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Chart 3. Ligands and complexes investigated herein.

Herein, we have also structurally characterized the related complexes 13, which are

shown in Figure 1 and Figure S3; selected bond lengths and angles, as well as the values

of the four-coordinate geometry index  , [11a] are given in Table 1. In each case, the

4

distorted tetrahedral aluminium centre is bound by a chelating imino-amido ligand. The

N1C1C2N2 portion of the imino-amido ligand is almost planar, with an average

torsional angle of 3.43º (for 1), 0.55º (for 2) and 5.44º (for 3). The aluminum atom lies

within this plane with the greatest deviation observed for 3 (Al atom displaced ca. 0.024 Å

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(

for 1), 0.033 (for 2) and 0.16 (for 3) out of the plane.), with each adopting a distorted

tetrahedral geometry (bound by two methyl groups and two nitrogens of the chelating

amidoimino ligand). The non-symmetry of structures 13 is manifested in the different

CN bond distances in the bidentate chelates, whilst the Al–C, Al–N1, AlN2 and CN

bond lengths found for each of 13 are similar. As expected, the AlN bond with the

formally negatively charged amido nitrogen N2 is significantly shorter at 1.839(2),

1

.845(2) and 1.844(2) Å) Å than the neutral imino nitrogen AlN1 at 1.986(2), 1.984(3)

and 1.988(2) Å for compounds 13 respectively. The C1N1 bonds in 13 (1.286(3),

1

.291(4) and 1.295(3) Å are double and are shorter than C2N2 (1.475(3), 1.467(4) and

.477(3) Å, respectively). The C1C2 distances in 13 (1.508(4), 1.518(4) and 1.502(3)

Å are comparable with CC single bond distances. The sum of the bond angles around the

backbone C1 and C2 centers (C1=360.0º, C

2

= 435.2º for 1, C1=360.0º, C

2

=

2

3

4

35.7º for 3, C1=360.0º, C

2

= 435.9º for 3) argue strongly for sp and sp

hybridized carbon centers at C1 and C2, respectively.

1

As noted previously, the H NMR spectra for 13 are consistent with the transfer of

one methyl to an imino carbon atom with one singlet for the imino-methyl group at 1.95

(for 1), 1.93 (for 2) and 2.03 ppm (for 3) integrating for three protons and another singlet

for the two amino-methyl groups at 1.27 (for 1), 1.30 (for 2) and 1.49 ppm (for 3)

integrating for six protons. Characteristic high-field resonances for the aluminum methyl

groups are observed at 0.93 (for 1), 0.95 (for 2) and 0.78 ppm (for 3, s, 6H). The

presence of the asymmetric amido-imino ligands, causes non-equivalence of the protons

of the four isopropyl, ethyl and methyl substituents in each ligand of compounds 1, 2 and

3

respectively. In complex 1, the methine protons give rise to two septets (δ = 2.97 and

.67 ppm) and the methyl groups appear as four doublets (δ = 1.03, 1.07, 1.21 and 1.24

3

ppm). In complex 2, the methylene protons give rise to three multiplets (δ = 2.45, 2.65

and 3.08 ppm) and the methyl groups appear as two triplet (δ = 1.18 and 1.24 ppm). In

complex 3, the methyl groups appear as two singlets (δ = 2.31 and 2.50 ppm) (Figures

S13S18). These structures were further investigated by ¹³C NMR spectroscopy. In

particular, the resonances for the aluminum methyl groups (at 7.5, 7.8 and 7.1 ppm for

1

3), amido carbon (67.5, 68.9 and 69.6 ppm for 13) and imino carbon (198.6, 197.8

6

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and 197.5 ppm for 13) were clearly detected. Furthermore, the presence of the

amido-imino chelating fragment in 1, 2 and 3 is supported by absorptions at 1629, 1659



1

and 1622 cm in their respective IR spectra, which correspond to C=N bonds in their

imino-amido skeletons.

Scheme 1. Synthesis of complexes 13.

Figure 1. The molecular structure of 1 (top left), 2 (top right) and 3 (bottom) (thermal ellipsoids are set

at the 20% probability level; H atoms are omitted for clarity).

Table 1. Selected bond lengths (Å) and angles (°) for compounds 13.

1

2

3

AlN1

1.986(2)

1.839(2)

1.967(3)

1.970(3)

1.286(3)

1.508(4)

1.505(4)

1.475(3)

1.542(4)

1.547(4)

84.01(1)

109.29(2)

117.4(2)

108.52(2)

0.94

1.984(3)

1.845(2)

1.966(4)

1.959(4)

1.291(4)

1.518(4)

1.497(5)

1.467(4)

1.541(5)

1.554(5)

84.26(1)

109.5(2)

117.7(3)

107.4(2)

0.94

1.988(2)

1.844(2)

1.967(3)

1.965(3)

1.295(3)

1.502(3)

1.506(3)

1.477(3)

1.554(4)

1.543(4)

84.68(8)

108.23(1)

117.53(2)

108.52(2)

0.95

AlN2

AlC6

AlC7

C1N1

C1C2

C1C3

C2N2

C2C4

C2C5

N1AlN2

C6AlC7

N1C1C2

N2C2C1



4 [10a]

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Scheme 2. Synthesis of complex 4.

Figure 2. The molecular structure of 4 (thermal ellipsoids are set at the 20% probability level; H atoms

are omitted for clarity). Selected bond lengths (Å) and angles (°): Al1N1 1.971(2), Al1N2 1.860(2),

Al1C6 1.952(3), Al1C7 1.956(3), C1N1 1.306(4), C1C2 1.516(4), C1C3 1.493(4), C2N2

1

.454(3), C2C4 1.549(5), C2C5 1.515(4), N2N3 1.430(3), Al2N3 1.971(2), Al2N4 1.847(2),

Al2C13 1.966(3), Al2C14 1.972(3), C8N3 1.304(3), C8C9 1.519(4), C8-C10 1.485(4), C9N4

.459(4), C9C11 1.534(4), C9-C12 1.563(4); N2Al1N1 82.17(9), C6Al1C7 109.47(15),

1

N1C1C2 116.9(2), C1C2N2 105.3(2), C5C2C4 110.1(3), N4Al2N3 84.58(9),

C13Al2C14 120.47(13), N3-C8-C9 116.5(2), C9C8C10 119.4(2), C8C9N4 108.3(2),

C11C9C12 107.2(2); Four coordinate geometry index ( ) 0.95 (Al1) and 0.85 (Al2). [11a]

4

The reaction of diacetyl (2,3-butanedione) with 2,6-diisopropylaniline formed

-(2,6-diisopropylphenylimino)butan-2-one (Lⁱ^p^r^-^N^O), which was further reacted with

hydrazine to form the bis(α-diimino) ligand (Lⁱ^P^r^-^N⁴) bearing two bidentate sites (Scheme

) and possessing zigzag –N=CC=NN=CC=N-bridging spacers. The molecular

structure of ligand Lⁱ^P^r^-^N⁴is shown in Figure S1 of the Supporting Information. The

3

2

bis(α-diimino) compound (Lⁱ^P^r^-^N⁴) was reacted with two equivalents of AlMe

corresponding asymmetric bi-nuclear aluminium

ArNC(Me) C(Me)=NAlMe

known that the reaction of -diimine compounds with AlR

to form the

complex

3

[

2

] (4) as shown in Scheme 2. From the literature, it is

2

3

can readily afford

imino-amido aluminium compounds or enamine aluminium compounds, resulting from

alkyl transfer from aluminium to either the imine carbon atom (C-alkylation) or the imine

nitrogen atom (N-alkylation) respectively. [12] Furthermore, the regioselective R-group

8

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transfer step occurring in these reactions is highly dependent on both the metal and the

type of R group present in the organometallic reagent. [13] Interestingly, in compound 4,

although both methyl groups attack at the imine carbon, one is found adjacent the Ar

group, whilst the other resides on the most distant from Ar. It is thought that the formation

of this asymmetric addition product 4 maybe dictated by steric strain imposed by the two



1

amino-methyl groups or isopropyl. Moreover, the compound 4 is ca. 18.0 kJ mol more

stable than the imino-amido isomer derived by the addition of both methyl groups to the



1

imine carbon, that reside on the most distant from Ar, and 30.3 kJ mol more stable than

the isomer obtained from the addition of both methyls to the imine carbon, that reside

1

adjacent the Ar group (Figure S4, Table S1). The H NMR spectrum of 4 is consistent

with the molecular structure: the methine protons give rise to two septets ( = 2.87 and

3

.65 ppm) the methyl groups appear as four doublets ( =1.06, 1.11, 1.21 and 1.27 ppm),

and the Al-(CH groups are observed as two singlets at 0.81 and 0.75 ppm,

3 2

)

respectively (Figures S19 and S20). Noticeably greater bonding asymmetry was also

observed, with bond lengths of AlN(imino) (Al1N1 1.971(2) and Al2N3 1.971(2) Å)

and AlN(amido) (Al1N2 1.860(2) and Al2N4 1.847(2) Å, Figure 2). The

five-membered metallacycle that contains the Al atom adopts a puckered conformation,

with the Al atoms residing about 0.406 and 0.323 Å out of the C

2

N planes. The angle

between the two C

2

N Al metallacycles is 68.70°. The Al2C13 (1.966(3) Å) and

Al2C14 (1.972(3) Å) are somewhat longer than those of Al1C6 (1.952(3) Å) and

III

3

Al1C7 (1.956(3) Å). These values are comparable to others reported for Al C(sp )

bond lengths in LAlMe type compounds (1.947(4)2.010(4) Å). [14] In the IR spectra of

, there is an intense absorption at 1613 cm associated with the C=N stretching mode.

2



1

4

Scheme 3. Synthesis of complex 5.

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Figure 3. The molecular structure of 5 (thermal ellipsoids are set at the 20% probability level; H atoms

are omitted for clarity). Selected bond lengths (Å) and angles (°): AlN 2.207(1), AlO 1.849(1),

AlOA 1.962(1), AlC6 1.984(2), AlC7 1.987(2), NC1 1.279(2), C1C2 1.523(2), C1C3 1.500(2),

C2O 1.425(2), C2C4 1.541(2), C2C5 1.520(2); NAlO 76.53(5), C7AlC6 119.44(8),



OAlOA 75.73(5), AlOAlA 104.27(5). Five-coordinate geometry index ( ) 0.73 (Al). [11b]

Reaction of Lⁱ^p^r^-^N^Owith AlMe

[OCMe CH(Me)=NAr]AlMe (5) (Scheme 3). According to the single crystal X-ray

diffraction study, complex 5 is a centrosymmetric dimer with a planar Al core (Figure

), that contains a 5,4,5-fused ring system where Al, N, C1, C2 and O atoms are nearly

in toluene produces the bimetallic complex

3

{

2

}

2 2

2

O

2

3

coplanar with the angle between planes AlNC1C2O and AlOAlAOA being 18.4°. Each

aluminium atom is five coordinate and adopts a distorted square-pyramid with a

five-coordinate geometry index ( ) of 0.73; [11b] the methyl groups C6 are located at the

5

axial positions, whilst C7, N, O and OA are situated in the basal plane. The bond lengths

for C(1)N of 1.279(2) Å and C(1)O of 1.425(2) Å indicate a localized structure with a

C=N double bond and a CO single bond. The C=N double bond character can be further

verified by a very long AlN distance (2.207(1) Å) indicating a dative bonding from a

neutral nitrogen atom to aluminium. The Al−N distance is much larger than in compounds

1

4 having a four-coordinate aluminium (av. 1.982 Å), but comparable with the related

compounds with a five-coordinate aluminium (2.136(2)2.260(3) Å). [15] The shorter

AlO bond (1.849(1) Å) corresponds to a covalent AlO interaction, whereas the longer

one AlOA (1.962(1) Å) reflects the donor–acceptor bonding, and they are similar to the

normal AlO distance observed for an Al

2

O core in the other five coordinate aluminium

compounds. [15] The newly formed AlC6 (1.984(2) Å) and AlC7 (1.987(2) Å) bonds

are almost identical in length. These values are comparable to the Al-C bonds in

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compounds 1-4 (av. 1.964 Å). Compared to 4, compound 5 has higher symmetry. The H

NMR spectra of 5 consists of only one triplet and two multiplets due to the Et groups and

only one singlet due to the Al-(CH

3

2

) group (Figures S21 and S22). Furthermore, the IR

spectrum of compound 5 supports the proposed structure, featuring an intense band due to



1

stretching vibration vC=N bonds (1644 cm ).

Zinc complexes

Scheme 4. Synthesis of mono- and tri-nuclear organozinc complexes 6 and 7.

Figure 4. The molecular structure of 6 (thermal ellipsoids are set at the 20% probability level; H atoms

are omitted for clarity). Selected bond lengths (Å) and angles (°): ZnN1 2.106(2), ZnN2 1.876(2),

ZnC7 1.952(3), N1C1 1.278(4), C1C2 1.531(4), C1C3 1.508(4), C2N2 1.468(3), C2C4

1

.551(4), C2C5 1.562(4), C5C6 1.525(5), C7C8 1.505(5); N1ZnN2 82.11(9), C1C2C4

07.3(2), C1C2C5 108.2(2), C3C1C2 118.6(2).

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The reaction of Lⁱ^P^rwith ZnEt

affords 6 via ethyl transfer to an imine carbon of the

DOI: 10.1039/C9DT04332B

2

dpp-dad ligand (Scheme 4). The attachment of an ethyl group to the imine carbon of the

chelate ligand generates a chiral center at C2 in molecules of 6 and its symmetry is thus

distorted (Figure 4). This situation is reminiscent of chiral amido-imine complexes of zinc

and magnesium, with a unit cell containing both isomers (R and S). [16]

The non-symmetric nature of structure 6 is manifested in the different C-N bond

distances for the chelate fragments. The N1C1 (1.278(4) Å) bond is double and is much

shorter than C2N2 (1.468(3) Å). The ZnN1 (2.106(2) Å) bond length is remarkably

longer than that of ZnN2 (1.876(2)), reflecting the imino/amido character of the

N,N-chelate and the different bonding situation with donor–acceptor versus polar covalent

Zn–N interactions. We note that the length of the Zn–C(Et) bond (1.952(3) Å) is

somewhat

shorter

1-n-butyl-2-(2,6-diisopropylphenyl)iminoacenaphthen-1-yl]-2,6-diisopropylphenylmide}

. [16a] In structure 6, a five-membered ring ZnN adopts a distorted envelope

than

that

observed

in

LZn-n-Bu

(where

L

=

[

)

2

C

2

conformation with the Zn atom displaced ca. 0.18 Å out of the ring plane. There is a

distorted triangle planar coordination around the Zn center formed by an ethyl group and

two nitrogens of the chelating amido-imino.

The reaction of Lⁱ^P^r^-^N^Owith an equimolar amount of the ZnEt

under similar conditions

2

afforded the trinuclear complex 7 (Scheme 4). A single crystal X-ray structure

determination revealed 7 to consist of three different mononuclear units, namely

Lⁱ^P^r^-^N^O⁺^HZn(Et), Lⁱ^P^r^-^N^O^-²^HZn(Et) and Lⁱ^P^r^-^N^OZn(Et), which result from the abstraction of

two hydrogens from one of the methyl groups on the imino unit. Coupling of the imino

carbon and the “ex-methyl” carbon of two Lⁱ^P^r^-^N^Ounits leads to the formation of the

C4C5 single bond (1.517(7) Å) and generates a chiral center at C5 (Figure 5). The

C2C4 single bond has changed to a C=C double bond (1.351(6) Å), and the mean angle

3

2

of 120 around the central C4 atom is indicative of a change from sp to sp hybridization

due to the elimination of H. The C2O1 (1.337(5) Å) is between a C=O single bond

length (1.43 Å) and a double bond length (1.20 Å), and remarkably shorter than that of

C5O2 (1.427(5) Å), which can be ascribed to an allylic-like delocalization of the

negative charge over the C4=C2O1 fragment. Correspondingly, the C1C2 bond length

12

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Figure 5. The molecular structure of 7 (thermal ellipsoids are set at the 20% probability level; most of

H atoms, iPr groups of L are omitted for clarity; the C atoms in Ph are drawn as smaller spheres).

Selected bond lengths (Å) and angles (°): Zn1N1 2.171(4), Zn1O1 2.010(3), Zn1O3 2.035(3),

Zn1C13 1.972(5), N1C1 1.286(6), C1C2 1.486(6), C1C3 1.501(7), C2C4 1.351(6), C4H4

0

.9500, C2O1 1.337(5), C4C5 1.517(7), C13C14 1.505(8), Zn2N2 2.131(4), Zn2O1 2.095(3),

Zn2O2 1.967(3), Zn2C15 1.965(7), N2C6 1.282(6), C5C6 1.528(7), C5C7 1.533(6), C6C8

.507(6), C5O2 1.427(5), C15C16 1.437(9), Zn3N3 2.127(4), Zn3O2 2.041(3), Zn3O3 2.028(3),

1

Zn3C17 1.977(6), N3C10 1.273(6), C9C10 1.503(6), C10C12 1.512(7), C9C11 1.536(7),

C9H9 1.0000, C9O3 1.404(5), C17C18 1.476(8); N1Zn1O1 77.09(14), O1Zn1O3 86.26(12),

N1Zn1O3 109.24(14), N2Zn2O2 81.21(14), O1Zn2O2 88.30(12), N2Zn2O2 81.21(14),

N3Zn3O3 81.21(14), O3Zn3O2 99.90(13), N3Zn3O2 99.02(13).

(1.486(6) Å) is also somewhat shorter than that of C5C6 1.528(7) Å). The N1–C1 and

N2C6 bonds are 1.286(6) and 1.282(6) Å and correspond to a C=N double bond. It is

also of note that attachment of H to the carbon atom bound to oxygen generates the

second chiral center at C9, and the complex molecule adopts a homochiral configuration

and the unit cell (Z = 4) consists of two pairs of enantiomers (SC5,SC9 and RC5,RC9).

Complex 7 consists of a six-membered Zn

atoms, with the Zn atoms each adopting tetrahedral coordination spheres. The

conformation of the Zn cycle is a distorted boat, with the Zn2 and O3 atoms at the

apices, which is assembled via μ

-bridging oxygen atoms of the Lⁱ^P^r^-^N^Oligand. The ZnC

3

O

3

ring with alternating zinc and oxygen

3

O

3

2

bond lengths (mean value 1.972 Å) are comparable and are in the typical range reported

for Zn－Me groups. [17] The ZnO bond lengths vary from 1.967(3) to 2.095(3) Å and

compare well with values for ZnO single bonds found in the other cyclic zinc oxides

compounds, such as [MeZn(bdmap)]

2

MeZnOOMe and [MeZn(bdmap)] MeZnOH

2

(Hbdmap = 1,3-bis(dimethylamino)propan-2-ol). [18] The coordination bond ZnN1 in

13

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compound 7 (2.171(4) Å) is substantially longer compared to polar covalent ZnN2

(1.876(2) Å) in compound 6, but similar with the donor–acceptor Zn–N1 interactions



1

(

2.106(2) Å) in 6. The IR spectrum exhibits an intense absorption band at 1644 cm ,

which corresponds to stretching vibrations of the C=N groups, whilst those associated

with C=O double-bond character were lost.

Scheme 5. Synthesis of complexes 8.

The same method was employed for the synthesis of Lⁱ^P^r^-^N²^-^A^r^C^H²^A^r^-^N²as described for the

synthesis of Lⁱ^P^r^-^N⁴, namely 3-(2,6-diisopropylphenylimino)butan-2-one (Lⁱ^p^r^-^N^O) was

/

reacted with 4,4 -methylenebis(2,6-diisopropylaniline) to form the bis(α-diimino) ligand

(

Lⁱ^P^r^-^N²^-^A^r^C^H²^A^r^-^N²), which bears two potential bidentate binding sites. [6a] The molecular

structure of ligand Lⁱ^P^r^-^N²^-^A^r^C^H²^A^r^-^N²is shown in the Supporting Information (Figure S2).

The molecule is composed of two equivalent parts connected by atom C3 with the bond

angle C7–C3–C7A 121.7(5)º. The C-C 1.505(4) and C=N (1.291(4) Å) distances

correspond to single and double bonds, respectively. The two phenyl ring planes,

connected through C3, are almost orthorhombic with a dihedral angle between the two

rings of 87.8º. This bis(α-diimino) compound Lⁱ^P^r^-^N²^-^A^r^C^H²^A^r^-^N²was reacted with two

equivalents of ZnCl

8) (Scheme 5).

2

to form the corresponding zinc complex [Lⁱ^P^r^-^N²^-^A^r^C^H²^A^r^-^N²(ZnCl

2

)

2

]

(

14

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Figure 6. The molecular structure of 8 (thermal ellipsoids are set at the 20% probability level; H atoms

are omitted for clarity). Selected bond lengths (Å) and angles (°): Zn1N1 2.068(4), Zn1N2 2.081(5),

Zn2N3 2.059(5), Zn2N4 2.095(6), Zn1Cl1 2.183(4), Zn1Cl2 2.194(3), Zn2Cl2 2.171(5),

Zn2-Cl4 2.188(4), C1C2 1.508(7), C5C6 1.507(8), C1N1 1.256(6), C2N2 1.254(6), C5N3

1.283(6), C6N4 1.269(7), N1Zn1N2 77.9(2), N3Zn2N4 78.93(18), Cl1Zn1Cl2 120.30(14),

Cl3-Zn2Cl4 117.04(8), C25C9C37 113.6(4). Four-coordinate geometry index  0.87 (Zn1). [11a]

4

The molecular structure of complex 8 is shown in Figure 6, with selected bond

lengths and angles listed in the caption. Here, one ligand coordinates to two zinc(II)

centers, and the molecule is composed of two similar parts connected by C9. Both zinc

centers point in the same direction with respect to the ligand framework, and the latter

adopts a more pinched structure (C25C9C37 at 113.6(4)º), which is somewhat smaller

than observed in Lⁱ^P^r^-^N²^-^A^r^C^H²^A^r^-^N²(121.7(5)º). Moreover, the dihedral angle between the

two aromatic rings connected by C9 in Lⁱ^P^r^-^N²^-^A^r^C^H²^A^r^-^N²(87.8º) is much larger than that

observed in 8 (78.6º). The metal centers all possess distorted-tetrahedral geometries (4 =

0

.87) and deviate slightly (by 0.093 and 0.297Å) from the C

2

N planes. The angle

between the two C Zn planes is 69.0º and the bond distances associated with the

2

N

2

Lⁱ^P^r^-^N²^-^A^r^C^H²^A^r^-^N²backbone in 8 are virtually identical to those found in the free ligand.

The CN distances, which average 1.265 Å, correspond to C=N bonds, while the C–C

separation of 1.508 Å falls in the C–C single bond range. The ZnN bond lengths (av.

2

.076 Å) reflects the donor–acceptor bonding, and is very similar with that in

1

(t-Butyl-BIAN)ZnCl

2

(2.081 Å) which bears a neutral ligand. [19] The H NMR spectrum

of 8 is consistent with the molecular structure, viz the methine protons give rise to one

septet (δ = 2.93 ppm), the methyl groups appear as two doublets (δ =1.13 and 1.35 ppm),

15

Dalton Transactions

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and the methylene is observed as a singlet at 3.98 ppm (Figures S27 and S28).^D^O^I^:¹⁰^.¹⁰³⁹^/^C⁹^D^T⁰⁴³³²^B

We have previously reported an AlAl-bonded compound (dialumane) with an



-diimine ligand, namely [L(THF)Al−Al(THF)L] (9), which contains sub-valent Al^I^I

2



²^). [8a] Complex

centers and dianionic α-diimine ligands (L , (2,6-iPr

can act as a multi-electron donor in the reaction with small molecules, [8a, 20] for

example, reaction of 9 with azobenzene derivatives proceeded through a four-electron

2

C

6

H

3

)NC(CH

3

)]

2

9

II

2

reduction pathway that involved both the Al centers and the L ligands. [8a]

Reactions involving multielectron transfers between metal centers and substrates are

at the core of many important transformations in biology and chemistry. [21] In addition,

multimetallic catalysis is based on the combined action of metals in a chemical

transformation. It has witnessed rapidly increasing developments during the past decades

in numerous areas of chemistry. Close proximity between the metal centers thus appears

to provide favourable conditions for the occurrence of enhanced catalytic properties, and

this proximity can result from the existence of direct metal−metal interactions. [22] These

species have attracted great interest not only because of the novel bonding nature of the

low-valent, low-coordinate metal centers, but also because they display fascinating

reactivity toward a variety of small molecules as well as potential applications in catalysis.

[

23, 8j] Encouraged by this multi-electron-reduction property of dialumane 9, and the

recent results reported by Fedushkin, Dagorne et al on related AlAl bonded complexes

bearing acenaphthenequinonediamido ligation, [8j] we also included 9 as a potential

catalyst in our studies on the ring opening polymerization (ROP) of the cyclic esters

-caprolactone (-Cl), -valerolactone (-VL) and rac-lactide (r-LA), see next section.

Ring Opening Polymerization (ROP) of Cyclic Esters

-Caprolactone (-CL)



The Al- and Zn-based complexes prepared herein were tested as catalysts for the ROP of

-CL (Table 2). At 30 °C, good conversions were achieved in the presence of complexes

3 over 60 min (runs 14), with the R = i-Pr (1) and Me (3) systems outperforming the



1

16

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R= Et (2) system in terms of both conversion and control. The bimetallic system 4 (which

is an i-Pr derivative) afforded only slightly higher conversion than 1 (91 vs 89%, cf. runs 1

and 4), but with far less control (2.10 vs 1.20). On the other hand, longer reaction times

were required by the bimetallic species 5 (an ethyl derivative) in order to obtain complete

conversion (480 min versus 60 min, runs 5 and 6). In the case of the Zn-based catalysts 6

and 7 (runs 7 and 8), mono-metallic 6 (an i-Pr derivative) afforded 84% conversion (run 7)

with good control (1.20), whilst tri-metallic 7 (also an i-Pr derivative) afforded near

quantitative conversion (99%) but with slightly less control (1.70). Interestingly, almost

no activity was observed in the presence of the Zn species 8 (run 9). We ascribe this

inactivity to the inefficient formation of the required catalytically active alkoxide species

from this chloride pre-catalyst. Indeed, we note that in reports by other groups, the

formation of M-OR species from parent a chloride complex required salt metathesis via

the use of Na (or K) alkoxides, rather than by direct reaction with alcohols. [24]

Concerning the effect of the metal center (zinc versus aluminium), slightly lower

conversions and polymer M were observed in the presence of the Zn-species 6 at 30 ºC,

n

compared to the values obtained when using the Al-derivative 1 (cf. runs 7 and 1).

Notably, the Al-Al bonded complex 9 outperformed all the other systems tested herein,

allowing for complete monomer conversion within 5 minutes (run 10). No drop in activity

was observed on progressively increasing the monomer/catalyst ratio from 100 to 2000 in

the presence of different amounts of co-catalysts (runs 11-14). Whilst the modus operandi

of this catalysts is not clear, work by Fedushkin, Dagorne et al, supported by DFT studies,

suggests the Al(II)Al(II) bond is not cleaved during the catalytic process and the alcohol

coordinates one of the metal centres leading, via proton transfer to a nitrogen atom of the

ligand, to an Al(II)-alkoxide species. [8j] Preliminary ROP studies conducted in the

absence of BnOH reveal a clear reduction in activity (conversion 44% over 15 min.),

suggesting the alcohol is indeed playing a role here. [25] In the case of catalysts 13 and 6,

the M

n

of the isolated polymers was lower than the calculated values albeit with narrow

polydispersities. Broader M

w

/M (spanning from 1.7 to 2.6) were obtained in the case of

n

the multimetallic species 4, 5 and 7, suggesting the occurrence of transesterification

reactions between the two (or three, in the case of 7) metal centers. In spite of the broader

17

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polydispersities, conversions and M values achieved by using 9 were found to be higher

n

than those obtained in the presence of the dialumane complex reported by Fedushkin,

Dagorne et al under the same reaction conditions. [8j]

By increasing the temperature to 80 °C, all complexes exhibited increased activity

with high conversions achieved in most cases within minutes. In particular, only 1 minute

was required in the presence of complex 9 (run 27) to achieve quantitative conversion. In

all cases, M values lower than the expected were obtained. Compared to the experiments

n

carried out at 30 °C, a broadening of molecular weight distribution, thought to be due to

increased transesterification, was observed. These results highlighted several differences

in the catalytic behavior of the complexes, for example, the bimetallic complex 4 allowed

for significantly higher molecular weight albeit with less control (cf. runs 15 and 18)

versus 1. However, in terms of catalytic conversion, the appeared to be little benefit from

the presence of the second metal. Noteworthy, comparable results for zinc versus

aluminium were observed at higher temperature (cf. runs 15 and 25). Regardless of the

reaction temperature, narrower polydispersities were achieved in the presence of the Zn

catalyst.

Table 2. ROP of ɛ-CL promoted by complexes 19.

b,c

d

T

t

Conv.

M

n

M

nCalcd

b

Run Cat. -CL:M:BnOH

PDI

a

°

C

min

60

(%)

(kDa)

13.0

19.2

19.1

12.8

0.4

(kDa)

25.6

22.2

25.4

26.0

8.40

28.6

24.1

28.5

nd

1

2

3

4

5

6

7

8

9

1

2

3

4

5

6

7

8

9

250:1:1

250:1:2

250:1:1

100:1:1

1000:1:10

1000:1:3

2000:1:3

30

89

78

89

91

29

>99

84

99

2

1.20

1.30

1.20

2.10

2.70

1.20

1.70

nd

30 480

17.3

11.2

15.1

nd

30

60

5

1

0

>99

96

92

93

87

28.1

22.5

16.9

55.5

56.0

29.6

11.1

10.6

35.3

66.2

1.30

1.10

1.20

1.80

1.60

1

2

3

4

15

10

1

18

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1

2

5

6

7

8

9

0

1

2

3

4

5

6

7

1

2

3

4

5

6

7

9

250:1:1

125:1:1

250:1:1

500:1:1

1000:1:1

250:1:1

80

50

80

5

94

11.7

23.4

20.4

21.0

8.10

16.3

20.3

16.4

20.4

23.7

11.4

9.40

27.0

28.1

27.0

14.4

28.6

50.2

23.7

28.5

8.80

27.0

28.5

1.70

1.30

5

98

1.50

2.70

1.90

2.20

2.10

1.20

2.70

1.10

2.00

1.20

5

94

30

80

5

>99

88

21

99

30

5

94

5

>99

1

10.6

28.5

a

1

b

c

Determined by H NMR spectroscopy on crude reaction mixture. From GPC. Values corrected

considering Mark–Houwink factor (0.56) from polystyrene standards in THF. ^dCalculated from

(

[Monomer] /[OH] ) × conv. (%) × Monomer molecular weight + Molecular weight of BnOH.

0

-valerolactone (-VL)

The ROP of δ-VL was next investigated (Table 3). In the presence of the Al-based

complexes 14 and 9, high conversions spanning from 88 to > 99% were achieved in 4 h

at 30 °C (runs 14 and 8). As for -CL, the R = Et derivative (2) was less active here than

the i-Pr (1) and Me (3) derivatives. The mono-Al species 13 and the bimetallic

compound 4 were found to be equally performing in terms of monomer conversion.

Nevertheless, the polymer molecular weight obtained in the presence of 4 (iPr) was found

to be ca. 2-fold higher than that of the material isolated with the monometallic complex 1

(iPr) at 30 ºC (23 kDa vs 11kDa, runs 4 and 1, respectively). Low monomer conversion

(35%) was observed when compound N,O-chelate bimetallic complex 5 was employed

(run 5). Both Zn-based species 6 and 7 exhibited good activity, allowing for 80 and 99%

conversion, respectively (runs 6 and 7), suggesting that, unlike for Al (5), the presence of

the N,O-chelate is not detrimental. However, direct comparisons are difficult given 7 is

trimetallic and an i-Pr derivative versus bimetallic 5 (an Et derivative). Similarly to the

case of -CL, shortened reaction times were required when performing the reaction at

8

0 °C instead of 30 °C (runs 914 and 1722). Indeed, almost complete conversion was

achieved with complexes 14 and 9 and 67 within 15 minutes. Compared to the other

catalysts, complex 5 proved to be less active, requiring longer reaction time for complete

19

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monomer conversion even at higher temperature (runs 1517). In all cases, M

n

values

lower than the expected were observed, and polydispersities spanned the range 1.1 to ca.

2

5

. Notably, complex 5 afforded oligomeric species (M

n

= 500) both at 30 and 80 °C (runs

and 17, respectively), which suggested inefficient catalyst activation in the former case

and early deactivation in the latter. The reactivity trend of the catalysts was found to be

similar to that observed in the case of the ROP of -CL.

On increasing the temperature to 80 ºC, monometallic 1 – 3 exhibited similar catalytic

performances to the bimetallic system 4 and afforded polymers with comparable M

values. In general, slightly better control was exhibited by the monometallic compounds

/M 1.40 vs 1.70, runs 9 and 14, respectively). The N,O-chelate bimetallic complex 5

n

(M

w

n

required longer reaction times to achieve high conversion, with slightly better results

achieved in the presence of BnOH. The mono-Zn species 6 performed better in the

presence of excess BnOH (two equivalents), albeit with slightly worse control. In the

presence of one equivalent of BnOH, the tri-metallic compound 7 afforded a higher

conversion versus 6, but with slightly poorer control. In turn, complex 6 was found to be

o

slightly less active than the Al-derivative 1 at 80 C. When the reaction was performed at

room temperature, the system 1 allowed for better conversions and higher polymer M

n

, as

well as for narrower polydispersity (cf. runs 1 and 6).

Table 3. ROP of δ-VL promoted by compounds 19 (not 8).

b

c

T/

t

Conv.

M

n

M

nCalcd

b

Run

Cat. -VL:M:BnOH

PDI

a

°C

min

240

15

(%)

(kDa)

11.0

14.9

16.2

22.8

0.50

6.50

14.8

23.2

15.6

12.8

13.4

11.8

(kDa)

24.6

20.8

24.4

24.9

8.70

22.1

24.9

25.1

23.5

24.6

23.7

1

2

3

4

5

6

7

8

9

1

2

3

4

5

6

7

9

1

2

250:1:1

250:1:2

250:1:1

30

80

98

83

97

99

35

81

99

>99

93

98

94

1.10

1.40

1.60

1.70

1.60

1.20

1.70

1.60

1.40

1.70

2.00

1.50

1

0

1

20

1

20

12

15

20

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1

2

3

4

5

6

7

8

9

0

1

2

3

4

5

6

7

9

250:1:1

250:1:0

250:1:1

250:1:2

250:1:1

250:1:2

80

50

80

15

95

94

93

>99

61

83

97

98

99

16.1

16.9

22.2

19.6

0.50

16.8

10.9

12.2

10.1

16.8

23.1DOI: 10.

1

10.539

0

/C9DT04332B

23.7

23.5

25.0

15.1

20.1

24.4

24.6

24.7

12.5

1.70

240

15

1.80

1.50

1.70

1.50

1.70

1.60

1.70

15

a

Determined by H NMR spectroscopy on crude reaction mixture. ^bFrom GPC. Calculated from

1

c

(

[Monomer] /[OH] ) × conv. (%) × Monomer molecular weight + Molecular weight of BnOH.

0

By increasing the temperature, comparable molecular weights were obtained, although

higher conversion was achieved in the presence of the Al-based complex. Interestingly,

the polydispersity was shown to be dependent on the amount of co-catalyst employed.

Also for -CL, the MM bonded complex 9 outperformed the other catalysts, particularly

o

at 30 C.

rac-Lactide (r-LA)

The ROP of rac-lactide (r-LA) promoted by the Al-based catalysts was then undertaken

(Table 4). Moderate conversions were obtained in the presence of all complexes (runs

o

1

10) at both 30 and 80 C. However, liquid oligomers whose M

n

could not be detected

by GPC were obtained in all cases, regardless of the reaction conditions employed. There

was little variation in activity for 1 – 3, and the bimetallic system 4 performed no better. A

slightly enhanced conversion was achieved using the N,O-chelate bimetallic complex 5,

whilst activity similar to that of 1  4 was displayed by the Al-Al bonded complex 9. Use

of the Zn-based complexes (Table 5) led in general to better performances. In the presence

of monometallic 6, 50% monomer conversion was achieved at 30 ºC, affording low

molecular weight oligomers (run 1). An improvement was observed by increasing the

temperature to 80 ºC (run 2). Indeed, 66% conversion was obtained in 10 min., affording

o

PLA with M

n

of ca. 10 kDa; trimetallic complex 7 displayed similar activity at 30 C over

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

b

t

Conv.

M

nCalcd

Run

Cat.

r-LA:M:BnOH T℃

Products

a

min

%

(KDa)

21

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1

2

3

4

5

6

7

8

9

1

2

3

4

5

9

250:1:1

30 240

44

52

48

45

51

49

50

47

64

52

15.8

18.8

17.4

16.3

18.5

17.7

18.1

17.0

23.0

18.8

oligo

D

m

OI

e

: 10.1039/C9DT04332B

r

s

80

30 240

80 10

30 240

80 10

30 240

10

oligomers

80

10

50

10

a

Determined by ¹H NMR spectroscopy on crude reaction mixture.

b

Calculated from

(

[Monomer] /[OH] ) × conv. (%) × Monomer molecular weight + Molecular weight of BnOH.

0

over 4 h, affording a polymer with molecular mass significantly lower than the calculated

value (see Table 5, Run 3). Nevertheless, by increasing the temperature to 80 ºC, higher

conversion (69%) and M

n

were obtained (run 4). The syndiotactic bias (P ) of the PLA

r

1

was determined by homonuclear 2D-J resolved H NMR spectroscopy. [26] Isotactic

polymers were obtained in all cases (P values spanning from 0.20 to 0.30).

r

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

b

c

T

℃

t

Conv.

M

n

M

nCalcd

b

Run

Cat. r-LA:M:BnOH

PDI

P

r

a

min

240

10

%

(kDa)

nd

(kDa)

18.2

23.1

23.6

24.8

1

2

3

4

6

7

250:1:1

30

80

30

80

50

66

65

69

nd

9.80

3.70

5.80

2.00

2.30

1.70

0.22

0.34

0.19

240

10

a

1

b

0 0

Determined by H NMR spectroscopy on crude reaction mixture. Calculated from ([Monomer] /[OH] ) ×

conv. (%) × Monomer molecular weight + Molecular weight of BnOH.

Co-polymerization of -CL and -VL

Finally, the co-polymerization of -CL with -VL was examined (Table 6). In the presence

of complex 1, moderate conversion was observed by conducting the reaction at 30 ºC,

while an enhancement was obtained on increasing the temperature to 50 ºC (54 to 85 %,

runs 1 and 2, respectively). The formation of low molecular weight oligomers was

achieved by using 2 at 30 ºC (run 3), while a co-polymer with M >7,800 was isolated in

n

the reaction performed at higher temperature (run 4). Similar behaviour was exhibited

also by complex 3 (runs 5 and 6). Notably, these three catalysts revealed a slight

preference for the incorporation of -CL over the other co-monomer. Complete

22

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conversion of both monomers was observed by using the bimetallic compound 4 (run 7),

while liquid oligomers were isolated using complex 5 (run 8). Similarly to 1  3, complex

5

displayed a higher propensity towards the incorporation of -CL over -VL, while a

1

co-polymer with a 1:1 CL/VL ratio was isolated in the presence of 4, as observed by H

NMR spectroscopy (70:30 versus 50:50, runs 8 versus 7). The bimetallic species 4 (iPr)

was shown to be better performing than its mono-Al congener 1 (iPr), allowing for better

conversion and higher polymer M (cf. runs 1 and 7). Moreover, the amount of -VL

n

incorporated in the co-polymer was found to be higher than that of the product isolated in

the presence of 1 (50% and 40%, respectively). In the case of the Zn-based catalysts, low

molecular weight products were isolated in the presence of monometallic 6 at 30 ºC (run

9

). Nevertheless, by increasing the temperature, improvements of monomer conversion

and polymer molecular weight were achieved (run 10). In the case of trimetallic 7, high

conversions were obtained both at 30 and 50 ºC (runs 11 and 12, respectively).

Monometallic complex 6 proved to preferentially incorporate -CL, regardless of the

reaction temperature. Notably, in the presence of trimetallic catalyst 7, the tendency to

incorporate -VL improved on increasing the temperature. Concerning to the effect of the

metal center, the -CL incorporation was found to be higher in the co-polymers

synthesized with the Zn-based complex 6 than in those obtained in the presence of the Al

system 1 (50 vs 40%, respectively). Finally, full consumption of both monomers was

achieved in the presence of the dialumane system 9, regardless of the reaction conditions

(runs 13 and 14).

Table 6. Co-ROP of ɛ-CL and δ-VL using compounds 1 - 9 (not 8).

b,c

T

t

Conv.

M

n

a

b

Run Cat. -CL:-VL:M:BnOH

-CL:-VL

PDI

a

ºC min

30 180

%

(kDa)

9.40

1

2

3

4

5

6

7

1

2

3

4

250:250:1:1

60:40

62:38

65:35

58:42

73:27

47:53

50:50

54

85

41

69

39

84

98

1.70

50

30 180

50 60

30 180

60

11.2

oligomers

7.80

1.50

oligomers

50

60

22.4

18.6

1.70

1.80

23

Dalton Transactions

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8

9

5

6

7

9

250:250:1:1

50

30 180

50 60

30 180

50 60

30 120

60

72:28

78:22

75:25

65:35

50:50

52

34

o

D

l

O

i

I

g

:

o

10

m

.10

e

39/C9DT04332B

r

s

oligomers

10

62

12.3

9.40

22.5

8.30

18.4

1.80

2.00

1

2

3

4

85

1

>99

99

1.90

1.80

50

10

>99

a

1

b

c

Determined by H NMR spectroscopy. From GPC. Values corrected considering the Mark-Houwink

n n

factor (M × 0.56 x %CL+ M x %VL) from polystyrene standards in THF.

Kinetic studies

A kinetic study of the ROP of -VL using 1, 4 and 6 highlighted that the polymerization

rate exhibited a first order dependence on the monomer concentration (Figure 7, left), and

the conversion of monomer achieved over 60 min was > 75% (90% for 4). The activity

trend was found to be 4 ≈ 1 > 6.

Figure 7. Left: Plot of ln[VL]

0

/[VL] vs. time using complex 1, 4 and 6; right: relationship between

t

conversion and time for the polymerization of VL.

In conclusion, we have isolated and structurally characterized a number of aluminum,

zinc and gallium species bearing -diimine and related ligand sets. The known alkyl

transfer chemistry of organoaluminium and zinc towards -diimines was exploited in

related aluminium bis(-diimine) chemistry, and then extended to related acetylimino zinc

and aluminium systems. In the case of zinc, an unusual trinuclear complex

{

[ArN=C(Me)COCHCO(Me)C(Me)=NAr][OCH(Me)C(Me)=NAr](ZnEt)

3

}

(Ar

=

2

,6-iPr , 7) was identified. All complexes, together with a previously reported Al-Al

2

C

6

H

3

bonded dialumane complex, were tested as catalysts in the ROP of cyclic esters.

Concerning the hompolymerization of -CL and -VL, complexes 1 (iPr) and 3 (Me)

proved to be better performing than 2 (Et), while the bimetallic compound 4 slightly

24

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DOI: 10.1039/C9DT04332B

outperformed its analog 1. The bimetallic Al species 5 was shown to be far less active. In

fact, only low molecular weight oligomers were obtained under the optimized reaction

conditions. Compared to the monometallic Zn species 6, the trimetallic complex 7

allowed for higher monomer conversions albeit with less control. The Cl-bearing complex

8

was found to be almost inactive in the ROP of caprolactone, which was thought to be

due to activation problems. By contrast, the low-valent Al(II)Al(II) system 9 proved to

be the best catalyst amongst those tested herein, and allowed for the complete conversion

of the monomer at lower temperatures and/or shorter reaction times than required by the

other systems herein. Notably, the activity of this complex in the ROP of -CL was found

to be comparable to that of a recently disclosed species having the same type of MM

bond. [8j] This work and that of Fedushkin, Dagorne et al [8j] suggests that for this type

of ligation, the presence of the M-M bond is highly beneficial in terms of activity and

cooperation between the metal centres. A similar activity trend was observed in the

CL/VL co-polymerization and all catalysts proved to preferentially incorporate CL over

the other co-monomer. Concerning the polymerization of r-LA, oligomers were isolated

when using the aluminum-based complexes, while isotactic PLAs were obtained in the

presence of the zinc catalysts 6 and 7.

Experimental

General: All manipulations were carried out under an atmosphere of dry nitrogen using

conventional Schlenk and cannula techniques or in a conventional nitrogen-filled glove

box. Hexane and toluene were refluxed over sodium. All solvents were distilled and

degassed prior to use. The α-diimine ligand L , L , L and Lⁱ^p^r^-^N^O, L^E^t^-^N^O, Lⁱ^P^r^-^N⁴and

ipr

Et

Me

Lⁱ^P^r^-^N²^-^A^r^C^H²^A^r^-^N²were prepared according to literature procedures. [6a, 10, 27]

Trimethylaluminium (AlMe

3

) and diethyl zinc (ZnEt

2

) and hydrazine (H

2

NNH ) were

2

purchased from Alfa Aesar. NMR spectra were recorded on a Mercury Plus-400

spectrometer. Elemental analyses were performed with an Elementar VarioEL III

instrument. IR spectra were recorded using a Nicolet AVATAR 360 FT-IR spectrometer.

Ring open polymerization (ROP) of cyclic esters – General procedure

25

Dalton Transactions

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DOI: 10.1039/C9DT04332B

In the glovebox, a Schlenk tube was charged with the stock solutions of the catalyst and

with the required amount of a toluene solution of benzyl alcohol. The mixture was stirred

for 2 min at room temperature and then the monomer (2.5 mmol) along with 1.5 mL

toluene were added. The reaction mixture was then placed into an oil bath pre-heated to

the required temperature, and the solution was stirred for the required time. The

polymerization mixture was then quenched by addition of an excess of glacial acetic acid

(0.2 mL); the solution was then poured into methanol (200 mL) and the resultant polymer

was then collected on filter paper and dried in vacuo.

Kinetic studies

The polymerizations were carried out at 80 °C in a glovebox. The monomer to initiator

ratio was fixed at 500:1. At appropriate time intervals, 0.5 mL aliquots were removed

(

under N

2

) and were quenched with wet CDCl . The percent conversion was determined

3

1

by H NMR spectroscopy.

Synthesis of [O=C(Me)C(Me)=NAr] (Ar = 2,6-iPr

2

C

6

H

3

and 2,6-Et C

2 6

H

) (Lⁱ^P^r^-^N^O

3

and L^E^t^-^N^O

)

To a stirred solution of diacetyl (2,3-butanedione) (0.12 mol, 10.5 mL) in n-hexane (80.0

mL), 2,6-diisopropylaniline (0.10 mol, 18.9 mL) or 2,6-diethylaniline (0.10 mol, 16.5 mL)

was added drop-wise in the presence of the catalytic amount of formic acid. The solution

was stirred for 24 h at room temperature. The reaction progress was checked by TLC. The

products were isolated as yellow liquids by vacuum distillation (Lⁱ^p^r^-^N^O140 ºC/0.3 mmHg,

9.60 g, 80%; L^E^t^-^N^O104 ºC/0.3 mmHg, 16.60 g, 78%). Lⁱ^P^r^-^N^O: H NMR (400 MHz,

1

CDCl

.82 (s, 3H CCH

¹³C NMR (100.6 MHz, CDCl

5.0 (O=C CH ), 28.4 (CH(CH

N=CCH ), 200.0 ppm (O=CCH

918 w, 1852 w, 1714 s, 1642 s, 1584 w, 1455 s, 1340 s, 1325 m, 1282 w, 1253 m, 1196 m,

3

): δ = 1.13 (d, J = 5.2 Hz, 6H; CH(CH

), 2.56 (m, 2H; CH(CH ), 2.58 (s, 3H CCH

): δ = 15.0 (N−CCH ), 22.7 (CH(CH

3

)

2

), 1.14 (d, J = 5.2 Hz, 6H; CH(CH

3

)

2

),

1

3

)

2

3

), 7.09−7.17 (m, 3H; Ar).

3

)

2

), 23.2 (CH(CH

3

2

) ),

2

3

2

) ), 123.1, 124.5, 128.3, 129.1, 134.6, 145.1 (Ar), 166.8

−

1

(

3

). IR (Nujol, cm ):  = 3384w, 3082 m, 2967 s, 2881 s,

1

1123 s, 1051 m, 994 w, 921 m, 821 m, 763 s, 735 m, 677 w, 604 w, 518 w, 446 w.

26

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CH ), 1.70 (s,

L^E^t^-^N^O: ¹H NMR (400 MHz, DMSO-d

): δ = 1.04 (t, J = 7.6 Hz, 6H; CH

6

2

3

1

3

H CCH

100.6 MHz, CDCl

28.2, 129.0, 130.0, 146.4 (Ar), 166.5 (N=CCH

3

), 2.24 (q, 4H; CH

2

CH

3

), 2.48 (s, 3H CCH

3

), 6.99−7.10 (m, 3H; Ar). C NMR

CH ), 24.8 (CH CH ), 124.2, 126.2,

). IR (Nujol,

(

3

): δ = 13.6 (N=CCH

3

), 14.7 (CH

2

3

2

3

1

3

), 200.0 ppm (O=CCH

3

−

1

cm ):  = 3384w, 3079 m, 2957 s, 2881 s, 1919 w, 1858 w, 1690 s, 1644 s, 1582 w, 1445

s, 1339 s, 1308 m, 1201 m, 1109 s, 1048 w, 987 w, 942 w, 865 m, 834 s, 758 s, 605 w, 529

m.

Synthesis of [ArN=C(Me)C(Me)=N-]

2

(Ar = 2,6-iPr

2

C

6

H )

) (Lⁱ^p^r^-^N⁴

3

To a solution of 3-(2,6-diisopropylphenylimino)butan-2-one (Lⁱ^p^r^-^N^O0.05 mol, 12.3 mL)

in ethanol (50 mL), a solution of hydrazine (0.030 mol, 1.0 mL) in ethanol (5 mL) was

added in the presence of the catalytic amount of formic acid. The mixture was stirred for

2

4 h at room temperature affording the title compound as a yellow formed that was

1

isolated by filtration, washed with ethanol (2×10 ml) and dried in vacuum. Yield 75%. H

NMR (400 MHz, CDCl

.26 (s, 6H; CCH ), 2.68 (sept, 4H; CH(CH

MHz, CDCl ): δ = 13.1 (CCH ), 16.5 (CCH)), 22.9 (CH(CH

), 123.1, 124.0, 135.4, 146.1 (Ar), 156.9 (N=CCH ), 166.2 ppm (N=CCH

Nujol, cm ):  = 3415 w, 3247 w, 2866 m, 2347 w, 1934 w, 1842 w, 1629 s, 1598 s,

582 w, 1476 s, 1369 s, 1308 m, 1247 m, 1201 s, 1109 s, 926 w, 834 w, 758 s, 697 m, 636

3

): δ = 1.18 (d, J = 2.4 Hz, 24H; CH(CH

3

)

2

),2.05 (s, 6H CCH ),

3

1

3

2

3

) ), 7.07−7.20 (m, 6H; Ar). C NMR (100.6

2

3

)

2

), 23.3 (CH(CH

3

)

2

), 28.5

). IR

(

CH(CH

3

)

2

3

−

1

(

1

w, 453 w.

Synthesis of {[ArNC(Me

2

)C(Me)=NAr]AlMe

2

} (Ar = 2,6-iPr

2

C

6

3

H , 1)

AlMe3 (3.0 mmol, 0.216 g) was added to a solution of LiPr (3.0 mmol, 1.214 g) in 30 mL

of toluene, and the mixture was stirred at room temperature for 12 h. The solution was

then filtered and concentrated to about 5 mL. Yellow crystals were obtained upon standing

1

at −20 °C (1.170 g, 82%). H NMR (400 MHz, CDCl

3

): δ = 0.93 (s, 6H; Al(CH

3

2

) ), 1.03

(

d, J = 4.4 Hz, 6H; CH(CH

3

)

2

), 1.07 (d, J = 4.4 Hz, 6H; CH(CH ), 1.21 (d, J = 4.4 Hz,

3 2

)

6

H; CH(CH

3

)

2

), 1.24 (d, J = 4.4 Hz, 6H; CH(CH

3

)

2

), 1.27 (s, 6H NC(CH

3

2

) ), 1.95 (s, 3H

27

Dalton Transactions

Page 28 of 38

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DOI: 10.1039/C9DT04332B

N=CCH

.04−7.29 (m, 6H ; Ar). ¹³C NMR (100.6 MHz, CDCl

N=CCH ), 24.0 (NC(CH ), 24.3 (CH(CH ), 24.9 (CH(CH

7.8 (CH(CH ), 27.9 (CH(CH ), 28.2 (CH(CH ), 67.5 (NC(CH

24.7, 127.9, 138.1, 141.2, 142.6, 151.3 (Ar), 198.6 ppm (N=CCH ). IR (Nujol, cm ): 

3415 w, 2895 s, 2697 w, 2361 w, 1629 m, 1445 s, 1353 s, 1232 m, 1171 m, 1124 m,

063 m, 865 m, 789 m, 727 s, 636 m, 468 w. Elemental analysis calcd. for C31

476.70): C 78.10; H 10.36; N 5.88. Found: C 77.70, H 10.95 N 5.89%.

3

), 2.97 (sept, J = 4.4 Hz, 2H; CH(CH

3

)

2

), 3.67 (sept, J = 4.4 Hz, 2H; CH(CH

3

)

2

),

7

3

): δ = 7.5 (Al(CH

3

)

2

), 18.6

(

3

)

2

3

)

2

3

)

2

), 27.6 (CH(CH

3

2

) ),

2

1

3

)

2

3

)

2

3

)

2

3

) ), 123.4, 124.0,

2

−

1

3

=

1

H

49AlN

2

(

Synthesis of {[ArNC(Me

2

)C(Me)=NAr]AlMe

2

} (Ar = 2,6-Et

2

C

6

3

H , 2).

Following the procedure described for 1, complex 2 was prepared by reacting AlMe (3.0

3

Et

mmol, 0.216 g) with L (3.0 mmol, 1.046) in 30 mL of toluene. Yellow crystals (1.002 g,

1

7

9%); H NMR (400 MHz, CDCl

3

): δ = 0.95 (s, 6H; Al(CH

), 1.24 (t, J = 7.6 Hz, 6H; CH CH ), 1.30 (s, 6H; NC(CH

), 2.45 (m, 2H CH CH ), 2.65 (m, 4H CH CH ), 3.08 (m, 2H CH

.05−7.29 (m, 6H ; Ar). ¹³C NMR (100.6 MHz, CDCl

): δ = 7.8 (Al(CH ), 14.1

CH CH ), 16.0 (CH CH ), 17.7 (N=CCH ) 23.4 (CH CH ), 25.0 (CH CH ), 27.3

NC(CH ), 68.9 (NC(CH ), 123.5, 125.7, 126.6, 127.3, 135.6, 139.7, 144.3, 146.6

Ar), 197.8 ppm (N=CCH ). IR (Nujol, cm ): 2926 s, 1659 s, 1582 w, 1445 w, 1369 m,

247 w, 1124 m, 1018 w, 727m, 682 w, 484 w. Elemental analysis calcd. for C27

420.60): C 77.10; H 9.83; N 6.66. Found: C 76.88, H 10.25 N 6.43%.

3

)

2

), 1.18 (t, J = 7.6 Hz, 6H;

CH

2

CH

3

2

3

2

) ), 1.93 (s, 3H;

N=CCH

3

2

3

2

3

2

CH ),

3

7

3

)

3 2

(

2

3

2

3

2

3

2

3

)

2

)

3 2

−

1

3

1

H41AlN

2

(

Synthesis of {[ArNC(Me

Following the procedure described for 1, complex 3 was prepared by reacting AlMe

mmo, 0.220 g) with L^M^e(3.0 mmol, 0.900 g) to afford yellow crystals (0.880 g, 80%). H

NMR (400 MHz, CDCl ): δ = 0.78 (s, 6H; Al(CH ), 1.49 (s, 6H; NC(CH ), 2.03 (s,

H; N=CCH ), 2.31 (s, 6H; Ar-CH ), 2.50 (s, 6H; Ar-CH ), 7.04−7.26 (m, 6H ; Ar). C

2

)C(Me)=NAr]AlMe

2

} (Ar = 2,6-Me

2

C

6

H

3

, 3).

3

(3.0

1

3

)

2

)

3 2

1

3

NMR (100.6 MHz, CDCl

3

): δ = 7.1 (Al(CH

), 27.4 (Ar-CH ), 69.6 (NC(CH

40.9, 141.0, 146.1 (Ar), 197.5 ppm (N=CCH ). IR (Nujol, cm ): 2911 s, 1934 w, 1858 w,

3

)

2

), 17.1 (N=CCH

3

), 18.0 (NC(CH

3

)

2

),

2

1

1.0 (Ar-CH

3

)

3 2

), 123.1, 126.9, 128.3, 129.2, 129.9,

−

1

3

28

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1

622 s, 1461 s, 1369 s, 1216 s, 1171 s, 1140 s, 1109 m, 1018 m, 880 m, 789 s, 713 s, 651

s, 590 s, 484 m, 622 m. Elemental analysis calcd. for C23

.13; N 7.69. Found: C 75.78, H 9.64 N 7.55%.

H

33AlN (364.49): C 75.79; H

2

9

Synthesis of [ArNC(Me)

AlMe

(6.0 mmol, 0.432 g) was added to a solution of Lⁱ^P^r^-^N⁴(3.0 mmol, 1.460 g) in 30

mL of toluene, and the mixture was stirred at room temperature for 12 h. A color changed

2

C(Me)=NAlMe

2

]

2

(Ar = 2,6-iPr

2

C

6

H

3

, 4).

3

(from yellow to green) was observed. The mixture was filtered and concentrated to about

1

5

mL. Light-green crystals were obtained upon standing at −20 °C (1.358 g, 72%). H

NMR (400 MHz, CDCl

6.8 Hz, 6H; CH(CH

.21 (d, J = 6.8 Hz, 6H; CH(CH

3

): δ = 0.87 (s, 6H; Al(CH

), 1.06 (d, J = 6.8 Hz, 6H; CH(CH

), 1.22 (d, J = 6.8 Hz, 6H; CH(CH

3

)

2

), 0.81 (s, 6H; Al(CH

), 1.20 (s, 6H NC(CH

), 1.56 (s, 6H

3

)

2

), 1.01 (d, J

=

3

)

2

3

)

2

3

) ),

2

1

3

)

2

)

3 2

NC(CH

3

)

2

), 1.96 (s, 3H N=CCH

3

), 2.24 (s, 3H N=CCH ), 2.81 (sept, J = 6.8 Hz, 2H;

3

1

3

CH(CH

3

)

2

), 3.60 (sept, J = 6.8 Hz, 2H; CH(CH ) ), 6.92−7.29 (m, 6H ; Ar). C NMR

3

2

(

100.6 MHz, CDCl

N=CCH ), 24.0 (NC(CH

7.3 (CH(CH ), 27.7 (CH(CH

3

): δ = 7.4 (Al(CH

), 24.3 (NC(CH

), 27.8 (CH(CH

3

)

2

), 5.5 (Al(CH

), 25.0 (CH(CH

), 28.2 (CH(CH

3

)

2

), 17.2 (N=CCH

3

), 18.4

),

(

3

)

2

3

)

2

3

)

2

), 26.3 (CH(CH

3

)

2

3

)

2

3

)

2

3

)

2

3

2

) ), 65.1

(

NC(CH

Ar), 191.2, 196.5 ppm (N=CCH

461 s, 1369 s, 1201 m, 1140 s, 942 m, 789 w, 743 m, 682 m, 590 w, 484 w. Elemental

analysis calcd. for C38 (630.89): C 72.34; H 10.22; N 8.88. Found: C 72.43, H

0.65, N 8.66%.

3

)

2

), 66.8 (NC(CH

3

)

2

), 123.2, 123.5, 125.0, 128.4, 137.0, 141.1, 143.8, 151.3

−

1

(

3

). IR (Nujol, cm ): 2911 s, 2866 s, 1613 m, 1552 m,

1

H

64Al

2

N

4

1

Synthesis of {[OCMe

AlMe

(3.0 mmol, 0.216 g) was added to a solution of L^E^t^-^N^O(3.0 mmol, 0.652 g) in 30

mL of toluene, and the mixture was stirred at room temperature for 12 h. A color change

from yellow to pale yellow) was observed. The reaction mixture was filtered and

2

CH(Me)=NAr]AlMe

2

}

2

(Ar = 2,6-Et

2

C

6

H

3

, 5).

3

(

concentrated to about 5 mL. Colourless crystals of 5 were obtained upon standing at

1

−

20 °C (0.667 g, 77%). H NMR (400 MHz, THF-d

8

): δ = 0.92 (s, 12H; Al(CH

), 1.59 (s, 12H; NC(CH ), 1.76 (s, 6H; N=CCH ), 2.31 (m,

), 2.55 (m, 4H CH CH ), 7.10−7.18 (m, 6H; Ar). C NMR (100.6 MHz,

3

)

2

), 1.16

(t, J = 4.8 Hz, 12H; CH

2

CH

3

)

2

3

1

3

4

H CH

2

CH

3

2

3

29

Article Doi

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

DOI: 10.1039/c9dt04332b

Source and publish data:

Authors:

Article abstract of DOI:10.1039/c9dt04332b

Full text of DOI:10.1039/c9dt04332b

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