Elsevier

Inorganica Chimica Acta

Volume 350, 4 July 2003, Pages 121-125
Inorganica Chimica Acta

Conformational effects in β-diiminate ligated magnesium and zinc amides. Solution dynamics and lactide polymerization

Dedicated to Professor Pierre Braunstein
https://doi.org/10.1016/S0020-1693(02)01510-4Get rights and content

Abstract

The preparation of the compounds LMg(NiPr2)(THF) (1); and LZnNiPr2 (2), are reported for L=the bulky β-diiminate ligand, CH(CMeN-2-tBuC6H4)2. In solution compound 2 is shown to exist as a mixture of syn- and anti-rotamers that do not interconvert significantly. Compound 1 readily and reversibly dissociates THF in benzene-d6 or toluene-d8 from a site where THF is syn to the tBu group of L. Both 1 and 2 are catalyst precursors for the ring-opening polymerization of lactides and it is shown that the syn-conformer of 2 reacts much faster than the anti. Polymerization of rac-lactide employing 2 in benzene or CH2Cl2 or 1 in THF yield approximately 90% heterotactic PLA (isi+sis). These results are compared with related work by Coates [J. Am. Chem. Soc. 123 (2001) 3229]; and us [J. Chem. Soc., Dalton Trans. (2001) 222] and us employing the symmetric β-diiminate ligand CH(CMeN-2,6-iPr2C6H3)2.

The compounds LM(NiPr2)(THF)n where L=CH(CMeN-2-tBuC6H4)2, M=Mg, Zn and n=0 or 1 have been prepared and shown to be active for lactide polymerization. The magnesium complex exists exclusively in a syn-conformer while the zinc complex exists in a mixture of syn and anti. The syn-conformer is principally responsible for lactide polymerization.

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Introduction

Polylactide (PLA) has been gaining attention in both academic and industrial circles due to its biocompatibility and biodegradability [1], [2], [3]. It can be used in applications such as biodegradable packaging materials, artificial tissue matrices, surgical sutures, and drug delivery systems [4]. In order to obtain desired mechanical properties, PLA is often copolymerized with other monomers such as glycolide and caprolactone [5], [6], [7], [8], [9]. Recently, single-site catalysts have been employed in lactide polymerization providing an entry point to new PLA microstructures [10], [11]. For example, syndiotactic PLA was synthesized from meso-lactide using a chiral aluminum isopropoxide catalyst by Coates et al. [12] and, in their subsequent work, they were able to make heterotactic PLA from rac-lactide using a β-diiminato zinc complex [13], [14]. Another challenge in PLA microstructure is to make a 1:1 mixture of l-PLA and d-PLA, called a stereocomplex, that has a high melting temperature (Tm) ∼230 °C compared with Tm 180 °C of pure l-PLA [15], [16], [17]. This higher melting temperature gives rise to a broader range of applications. However, the normal synthesis of the stereocomplex requires a chiral separation of the enantiomers l- and d-lactide which adds significantly to the cost of production. Baker et al. tried to circumvent this problem by using a racemic aluminum isopropoxide catalyst to polymerize rac-lactide [11]. The idea was that one catalyst enantiomer would polymerize d-lactide and the other catalyst enantiomer would polymerize l-lactide in parallel thereby giving a mixture of pure d-PLA and l-PLA chains. However, this result was later challenged by Coates et al. who, upon detailed examination of the polymer microstructure, showed that the resulting polymer was not the stereocomplex but rather a stereo block [(d-PLA)m(l-PLA)m]n where m∼11 [18]. Therefore, the synthesis of the stereocomplex from commercially available rac-lactide remains to be achieved [19]. Prompted by these considerations and the success of single-site catalyst systems employing zinc and magnesium with β-diiminato ligands [13], [14], [20], [21], we undertook the following study employing the 2-((2-tert-butylphenyl)amino)-4-((2-tert-butylphenyl)imino)-2-pentene ligand (LH). The premise was a simple one, namely that the bulky tBu groups would prefer to adopt the anti conformation, in preference to the syn as represented by the drawings shown in A and B, respectively. The anti-confirmation has virtual C2 symmetry and thus the metal center is chiral and exists as a 50:50 enantiomeric mixture.

Section snippets

LMg(NiPr2)(THF) (1)

The reaction of Mg(NiPr2)2 with 1 equiv. of the free β-diiminato ligand LH in refluxing THF gives compound 1 as a green–yellow solid.

LZn(NiPr2) (2)

Our best preparation of compound 2 is from the reaction between LiNiPr2 (2 equiv.), LH (1 equiv.) and a solution of ZnCl2 (1 equiv.) in THF. Subsequent extraction of the dried reaction residue with hexanes gave compound 2 as a yellow solid. Elemental analyses and full details of the preparation are given in the Section 3 together with spectroscopic data.

Solution characterization of compounds 1 and 2

With our

General considerations

The manipulation of air-sensitive compounds involved the use of anhydrous solvents and dry and oxygen-free nitrogen employing standard Schlenk line and drybox techniques. rac-Lactide was purchased from Aldrich and was sublimed three times prior to use. Tetrahydrofuran, dichloromethane, and hexanes were distilled under nitrogen from sodium/benzophenone, calcium hydride, and potassium metal, respectively. The β-diiminato ligand CH(CMeN-2-tBuC6H4)2 [23] and Mg(NiPr2)2 [24] were prepared according

Acknowledgements

We thank the Department of Energy, Office of Basic Energy Sciences, Chemistry Division for financial support of this work. K.P. acknowledges the Institute for the Promotion of Teaching Science and Technology (IPST), Thailand, for an opportunity to work on this project.

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