Polyhedron 49 (2013) 151–157

Contents lists available at SciVerse ScienceDirect

Polyhedron

journal homepage: www.elsevier .com/locate /poly

Simple is best: Diamine zinc complexes as unexpected catalysts inlactide polymerisation

Prisca K. Eckert a, Ines dos Santos Vieira a, Viktoria H. Gessner b, Janna Börner c, Carsten Strohmann a,⇑,Sonja Herres-Pawlis a,⇑a Fakultät Chemie, Anorganische Chemie, Technische Universität Dortmund, 44227 Dortmund, Germanyb Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germanyc Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA

a r t i c l e i n f o

Article history:Received 14 March 2012Accepted 3 September 2012Available online 24 September 2012

Keywords:Biodegradable PolymersAminesZincRing-opening polymerisationCoordination compounds

0277-5387/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.poly.2012.09.012

⇑ Corresponding authors. Present address: FakultätDepartment Chemie, Ludwig-Maximilians-Universitä13, 81377 München, Germany (S. Herres-Pawlis).Strohmann), +49 89 218077867 (S. Herres-Pawlis).

E-mail addresses: carsten.strohmann@tu-dortmusonja.herres-pawlis@tu-dortmund.de (S. Herres-Pawl

a b s t r a c t

A series of monomeric coordination compounds containing zinc(II) chloride or zinc(II) bromide and the1,2-diamine ligands TMEDA, TEEDA, (R,R)-TMCDA, (R,R)-TECDA and cis-TMCDA have been synthesisedand structurally characterised using X-ray diffraction analysis. All but one complex could be successfullyused as initiators for the solvent-free ring-opening polymerisation of D,L-lactide. In particular, the polylac-tide (PLA) prepared with compounds containing (R,R)-TECDA is obtainable in high yields with molecularweights suitable for industrial applications. Thus a class of stable, easy to synthesise and efficient PLA cat-alysts with neutral ligands is introduced.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

As fossil resources are being depleted inexorably and non-degradable waste is spread throughout the environment, the forceto establish sustainable solutions has led to rapid progress in thefield of biodegradable polymers which have gained increasinginterest as an alternative to synthetic petrochemical-based poly-mers. Polylactide (PLA) is particularly attractive as it can be pro-duced from lactic acid, a product of inexpensive renewable rawmaterials like corn, sugar beets or even agricultural waste [1].PLA is typically obtained via a catalytic ring-opening polymerisa-tion (ROP) mediated by metal-containing initiators, with tin(II)ethylhexanoate (SnOct2) as the most common example [1]. Besideshomoleptic catalysts like tin(II) ethylhexanoate, zinc(II) lactate andaluminium(III) isopropoxide in combination with alcohols as initi-ators, single-site catalysts that combine the catalytic metal centreand the initiating moiety have been developed by coordinationchemistry [1].

It has been shown that a multitude of complexes with differentmetals and ligand classes can be used as active initiators for the

ll rights reserved.

für Chemie und Pharmazie,t München, Butenandtstr. 5-Fax: +49 231 7555048 (C.

nd.de (C. Strohmann),is).

ROP of lactide [1], but many of them are toxic and/or suffer fromlimited stability towards humidity and oxygen, which is a majordrawback for industrial applications [2]. The wide application ofPLA in all fields of life, such as food packaging, medicinal sutures,and composite materials, has raised the demand for alternativecatalysts. In recent years the interest – particularly with respectto food packaging – has focused on non-toxic metals, especiallytitanium [3], magnesium [4] and zinc [5]. Zinc complexes with an-ionic N-donor ligands like b-diketiminates [6], trispyrazolylborates[7], or Schiff bases [5f] are active catalysts in the ROP of cyclic es-ters, but are generally sensitive towards air and moisture. How-ever, for industrial purposes, there is an exigent need forinitiators which tolerate air, moisture and acidic impurities in themonomer [8]. The problematic sensitivity can be ascribed to theanionic nature of the coordinating ligand systems which stabilisealmost all of these complexes. Up to now, only a few systems usingneutral ligands have been described [9]. These make use of strongdonors such as guanidines [10], carbenes [5e,11], phosphinimines[12], trispyrazolylmethanes [13], and substituted amines [14] orpyridines [15]. Davidson and co-workers reported sodium com-plexes with simple peralkylated bidentate, tridentate and tetra-dentate tertiary amines that showed good activities in thesolution ROP of lactide at room temperature [16].

Here we present the use of zinc halide complexes with neutraldiamine ligands, a novel but rather simple approach to PLA cata-lysts. The utilised amines are easily prepared and the startingmaterial inexpensive. The tetramethylcyclohexane-1,2-diamine

152 P.K. Eckert et al. / Polyhedron 49 (2013) 151–157

2ligands (TMCDA), for example, can be produced starting from abyproduct of the nylon-6,6 manufacturing process [17]. We dem-onstrate that these amine adducts are efficient catalysts in lactidepolymerisation. Until now, such simple amines have not been usedwithout co-catalysts for the ring-opening polymerisation of cyclicesters, as a general paradigm in lactide chemistry favours the pres-ence of anionic ligands or co-catalysts [9]. The zinc amine com-plexes presented herein impress with their high stability,simplicity and facile synthesis, and their unexpected efficiency ascatalysts.

2. Materials and methods

2.1. Materials

N,N,N0,N0-Tetramethylethylenediamine (TMEDA, L1), N,N,N0,N0-Tetraethylethylenediamine (TEEDA, L2), zinc(II) bromide and zin-c(II) chloride were obtained from Aldrich and used without furtherpurification. The solvents for synthesising the diamine zinc com-plexes, were used without prior drying or purification. (R,R)-TMCDA (L3) [18], (R,R)-TECDA (L4) [19] and cis-TMCDA (L5) [20]were synthesised according to literature procedures. Enantiomericpurity of L3 was also determined according to a literature method[21]. D,L-Lactide (3,6-dimethyl-1,4-dioxane-2,5-dione) was ob-tained from Purac and used without further purification.

2.2. Physical measurements

1H NMR [internal standard CHCl3 (d = 7.26) for CDCl3, externalstandard tms]: Bruker DPX 300 (300.1 MHz), Bruker DRX 400(400.1 MHz), Bruker DRX 500 (500.1 MHz), Varian Inova 500(499.8 MHz). 13C NMR [internal standard CDCl3 (d = 77.16), exter-nal standard tms]: Bruker DPX 300 (75.5 MHz), Bruker DRX 400(100.6 MHz), Bruker DRX 500 (125.8 MHz), Varian Inova 500(100.6 MHz). Assignment of all signals was supported by DEPTand HSQC experiments. Homonuclear decoupled 1H NMR experi-ments were performed on a Varian NOVA 600 (599.8 MHz) accord-ing to literature procedures [5a]. Elemental analysis: LecoInstrument CHNS-932. DSC: TA Instruments, Q 2000. The DSC mea-surements are shown in the Supporting information.

2.3. Gel permeation chromatography

Average molecular weights and the weight distribution of theobtained polylactide samples were determined by gel permeationchromatography (GPC) in THF as mobile phase at a flow rate of1 mL/min. The utilised GPCmax VE-2001 from Viscotek is a combi-nation of a HPLC pump, a PSS SDV column with a porosity of 500 Åand a refractive index detector (VE-3580). Universal calibrationwas applied to evaluate the chromatographic results. Kuhn–Mark–Houwink (KMH) parameters for the polystyrene standards(KPS = 0.011 mL/g, aPS = 0.725) were taken from the literature[22]. Previous GPC measurements utilising online viscosimetrydetection revealed the KMH parameters for polylactide(KPLa = 0.053 mL/g, aPLa = 0.610) [10a].

2.4. Crystallographic data

Data was collected on an Oxford Diffraction XcaliburS using thePrograms CRYSALIS (Oxford, 2008) and CRYSALIS RED (Oxford, 2008). Thestructures were solved using direct methods (SHELXS-90) [23] andstructural refinement was done with SHELXL-97 [24]. Table 1 givesfurther details of the data collection and structure refinement ofcompounds C2–C4 and C6–C8. Further information, including ORTEP

plots and atomic coordinates, are available in the Supportinginformation.

2.5. Preparation of compounds

2.5.1. Synthesis of [ZnBr2(TMEDA)] (C1)300 mg (2.58 mmol) TMEDA (L1) and 582 mg (2.58 mmol) zin-

c(II) bromide were dissolved in acetone (10 mL). After slow evapo-ration of the solvent the adduct was obtained as colourless crystals,which were washed with cool acetone (yield: 731 mg, 2.14 mmol;83%). This compound has already been reported by Citeau [25]. 1HNMR: (400.1 MHz, CDCl3): d = 2.57 [bs, 12H; N(CH3)2], 2.72 (bs, 2H;CH2N). {1H}13C NMR: (100.6 MHz, CDCl3, CDCl3): d = 48.1 [N(CH3)2],56.6 (NCH2). CHN analysis: observed: C, 21.2; H, 4.6; N, 8.1; calcu-lated: C, 21.1; H, 4.7; N, 8.2.

2.5.2. Synthesis of of [ZnBr2(TEEDA)] (C2)313 mg (1.82 mmol) TEEDA (L2) and 409 mg (1.82 mmol) zin-

c(II) bromide were dissolved in acetone (5 mL) and dichlorometh-ane (5 mL). After slow evaporation of the solvent the adduct wasobtained as colourless crystals, which were washed with cool ace-tone (yield: 612 mg, 1.54 mmol; 85%). 1H NMR: (400.1 MHz,CDCl3): d = 1.12 (t, 3JHH = 7.05 Hz, 12H; CH3), 2.74–2.81 [m, 8H;N(CH2CH3)2], 3.07–3.16 (m, 4H; CH2N). {1H}13C NMR:(100.6 MHz, CDCl3): d = 8.7 (CH3), 46.3 [N(CH3)2], 50.4 (NCH2).CHN analysis: observed: C, 30.3; H, 6.2; N, 7.1; calculated: C,30.2; H, 6.1; N, 7.1.

2.5.3. Synthesis of [ZnBr2{(R,R)-TMCDA}] (C3)132 mg (0.59 mmol) zinc(II) bromide and 100 mg (0.59 mmol)

(R,R)-TMCDA (L3) were dissolved in acetone (5 mL). After slowevaporation of the solvent the adduct was obtained as colourlesscrystals, which were washed with cool acetone (yield: 183 mg;0.46 mmol, 78%). 1H NMR: (500.1 MHz, CDCl3): d = 1.16–1.34 (m,4H, CH2-cyclohexyl), 1.84–1.89 (m, 2H; CH2-cyclohexyl), 2.01–2.05 (m, 2H; CH2-cyclohexyl), 2.42 [s, 6H; N(CH3)2], 2.57–2.60(m, 2H; CHN), 2.63 [s, 6H; N(CH3)2]. {1H}13C NMR: (125.8 MHz,CDCl3): d = 22.3 + 24.2 (CH2-cyclohexyl) 41.1 + 47.1 [N(CH3)2],64.5 (CHN). CHN analysis: observed: C, 30.5; H, 5.6; N, 7.1; calcu-lated: C, 30.4; H, 5.6; N, 7.1.

2.5.4. Synthesis of [ZnBr2{(R,R)-TECDA}] (C4)99 mg (0.44 mmol) zinc(II) bromide and 100 mg (0.44 mmol)

(R,R)-TECDA (L4) were dissolved in acetone (4 mL) and dichloro-methane (1 mL). After slow evaporation of the solvent the adductwas obtained as colourless crystals, which were washed with coolacetone (yield: 185 mg, 0.41 mmol; 93%). 1H NMR: (400.1 MHz,CDCl3): d = 1.19–1.31 (m, 2H; CH2), 1.27 (t, 6H, 3JHH = 7.0 Hz;CH3), 1.27 (t, 6H, 3JHH = 7.0 Hz; CH3), 1.38–1.51 (m, 2H; CH2),1.81–1.87 (m, 2H; CH2), 2.11–2.20 (m, 2H; CH2), 2.82–2.93 (m,4H; CH2N), 2.93–2.98 (m, 2H; CHN), 3.20–3.30 (m, 4H; CH2N).{1H}13C NMR: (100.6 MHz, CDCl3): d = 10.7 + 12.5 (CH3),25.3 + 28.7 (CH2 cyclohexyl), 41.2 + 48.5 (CH2N), 66.6 (CHN). CHNanalysis: observed: C, 37.4; H, 6.6; N, 6.3; calculated: C, 37.2; H,6.7; N, 6.2.

2.5.5. Synthesis of [ZnCl2{(R,R)-TMCDA}] (C5)100 mg (0.59 mmol) (R,R)-TMCDA (L3) and 80 mg (0.59 mmol)

zinc(II) chloride were dissolved in acetone (4 mL) and diethylether(1 mL). After slow evaporation of the solvent the adduct was ob-tained as colourless crystals, which were washed with cool acetone(yield: 153 mg; 0.50 mmol, 85%). This compound has already beenreported by Lee et al. [26]. 1H NMR: (400.1 MHz, CDCl3): d = 1.14–1.32 (m, 4H; CH2 cyclohexyl), 1.82–1.88 (m, 2H; CH2 cyclohexyl),2.00–2.07 [m, 2H; CH2 cyclohexyl], 2.40 [s, 6H; N(CH3)2], 2.56–2.59 (m, 2H; CHN), 2.63 [s, 6H; N(CH3)2]. {1H}13C NMR:

Table 1Data collection and structure refinement details for coordination compounds C2–C4 and C6–C8.

Compound [ZnBr2(TEEDA)](C2)

[ZnBr2{(R,R)-TMCDA}](C3)

[ZnBr2{(R,R)-TECDA}](C4)

[ZnCl2{(R,R)-TECDA}](C6)

[ZnBr2(cis-TMCDA)](C7)

[ZnCl2(cis-TMCDA)](C8)

Formula C10H24Br2N2Zn C10H22Br2N2Zn C14H30Br2N2Zn C14H30Cl2N2Zn C10H22Br2N2Zn C40H88Cl8N8Zn4

Formula weight (g mol�1) 397.50 395.49 451.59 362.67 395.49 1226.26T (K) 173(2) 173(2) 173(2) 173(2) 173(2) 173(2)k (Å) 0.71073 0.71073 0.71073 0.71073 0.71073 0.71073Crystal system orthorhombic orthorhombic monoclinic monoclinic monoclinic monoclinicSpace group Pna21 P212121 P21 P21 Pc P21/na (Å) 14.7220(7) 9.9744(3) 7.7979(4) 10.7451(3) 7.8745(2) 16.5047(6)b (Å) 12.6946(5) 11.5500(3) 14.2499(6) 15.0309(4) 15.2200(4) 12.2012(4)c (Å) 7.9144(4) 12.5813(3) 16.4858(7) 11.1830(3) 12.3293(4) 27.3607(9)b (�) 90 90 101.980(5) 103.615(3) 99.452(3) 95.658(4)V (Å3) 1479.12(11) 1449.42(7) 1791.99(14) 1755.40(8) 1457.60(7) 5483.0(3)Z 4 4 4 4 4 4Dcalc (Mg m�3) 1.785 1.812 1.674 1.372 1.802 1.486l (Mo Ka) (mm�1) 7.039 7.183 5.821 1.694 7.143 2.155F(000) 792 784 912 768 784 2560Crystal dimensions (mm) 0.40 � 0.40 � 0.20 0.40 � 0.40 � 0.10 0.40 � 0.30 � 0.10 0.40 � 0.40 � 0.20 0.50 � 0.40 � 0.20 0.60 � 0.30 � 0.20h range (�) 2.77–27.00 2.39–25.00 2.53–25.00 2.31–25.00 2.14–26.99 2.17–27.00Index ranges �18 6 h 6 18 �11 6 h 6 11 �9 6 h 6 9 �12 6 h 6 12 �10 6 h 6 9 �20 6 h 6 20

�16 6 k 6 16 �13 6 k 6 13 �16 6 k 6 16 �17 6 k 6 17 �19 6 k 6 19 �15 6 k 6 15�10 6 l 6 10 �14 6 l 6 14 �19 6 l 6 19 �13 6 l 6 13 �15 6 l 6 15 �34 6 l 6 34

Reflections collected 20439 28142 18202 26181 25166 44258Independent reflections 3213 (Rint = 0.0443) 2549 (Rint = 0.0438) 6291 (Rint = 0.0556) 6172 (Rint = 0.0338) 6268 (Rint = 0.0428) 11883 (Rint = 0.0505)Data/restraints/

parameter3213/1/140 2549/0/141 6291/1/351 6172/1/351 6268/2/279 11883/0/557

Goodness-of-fit (GOF) onF2

1.009 1.006 1.003 1.045 1.031 0.996

Final R indices [2r(I)] R1 = 0.0498,wR2 = 0.1149

R1 = 0.0166,wR2 = 0.0341

R1 = 0.0334,wR2 = 0.0543

R1 = 0.0219,wR2 = 0.0416

R1 = 0.0248,wR2 = 0.0339

R1 = 0.0304,wR2 = 0.0371

R indices (all data) R1 = 0.0558,wR2 = 0.1166

R1 = 0.0191,wR2 = 0.0343

R1 = 0.0486,wR2 = 0.0553

R1 = 0.0261,wR2 = 0.0419

R1 = 0.0337,wR2 = 0.0343

R1 = 0.0642,wR2 = 0.0384

Absolute structureparameter

0.11(2) �0.004(10) �0.033(12) �0.008(8) 0.005(6)

P.K. Eckert et al. / Polyhedron 49 (2013) 151–157 153

(100.6 MHz, CDCl3): d = 22.2 (CH2 cyclohexyl), 24.2 (CH2 cyclohexyl),40.25 + 46.5 [N(CH3)2], 64.6 (CHN). CHN analysis: observed: C, 39.0;H, 7.2; N, 9.3; calculated: C, 39.2; H, 7.3; N, 9.1.

2.5.6. Synthesis of [ZnCl2{(R,R)-TECDA}] (C6)300 mg (1.32 mmol) (R,R)-TECDA (L4) and 180 mg (1.32 mmol)

zinc(II) chloride were dissolved in acetone (5 mL) and dichloro-methane (5 mL). After slow evaporation of the solvent the adductwas obtained as colourless crystals, which were washed with coolacetone (321 mg, 0.88 mmol; 67%). 1H NMR: (400.1 MHz, CDCl3):d = 1.22 (t, 6H, 3JHH = 7.4 Hz; CH3), 1.23 (t, 6H, 3JHH = 7.05 Hz;CH3), 1.26–1.41 (m, 4H; CH2), 1.79–1.81 (m, 2H; CH2), 2.09–2.11(m, 2H; CH2), 2.77–2.85 (m, 4H; CH2N), 2.86–3.05 (m, 2H; CHN),3.07–3.16 (m, 4H; CH2N). {1H}13C NMR: (100.6 MHz, CDCl3):d = 10.7 + 12.4 (CH3), 25.1 + 28.3 (CH2 cyclohexyl), 41.0 + 48.3(CH2N), 66.4 (CHN). CHN analysis: observed: C, 46.0; H, 8.0; N,7.5; calculated: C, 46.4; H, 8.3; N, 7.7.

2.5.7. Synthesis of [ZnBr2(cis-TMCDA)] (C7)52 mg (0.23 mmol) zinc(II) bromide and 41.6 mg (0.24 mmol)

cis-TMCDA (L5) were dissolved in acetone (2 mL). After slow evap-oration of the solvent the adduct was obtained as colourless crystals(88 mg, 0.22 mmol, 97%). 1H NMR: (300.1 MHz, CDCl3): d = 1.41–1.56 (m, 2H; CH2 cyclohexyl), 1.71–1.87 (m, 4H; CH2 cyclohexyl),2.04–2.16 (m, 2H; CH2 cyclohexyl), 2.68, 2.68 [s, 6H; N(CH3)2],2.78–2.87 (m, 2H; CHN). {1H}13C NMR: (100.6 MHz, CDCl3):d = 22.5, 22.8 (CH2), 46.0, 50.5 [N(CH3)2], 65.0 (CHN). CHN analysis:observed: C, 30.4; H, 5.5; N, 7.0; calculated: C, 30.4; H, 5.6; N 7.1.

2.5.8. Synthesis of [ZnCl2(cis-TMCDA)] (C8)54 mg (0.40 mmol) zinc(II) bromide and 68 mg (0.40 mmol) cis-

TMCDA (L5) were dissolved in acetone (2 mL). After slow evapora-

tion of the solvent the adduct was obtained as colourless crystals(97 mg, 0.32 mmol, 80%). 1H NMR: (400.1 MHz, CDCl3): d = 1.41–1.54 (m, 2H; CH2 cyclohexyl), 1.71–1.87 (m, 4H; CH2 cyclohexyl),2.01–2.13 (m, 2H; CH2 cyclohexyl), 2.65 + 2.67 [s, 6H; N(CH3)2],2.75–2.81 (m, 2H; CHN). {1H}13C NMR: (100.6 MHz, CDCl3):d = 22.7 + 23.0 (CH2), 45.6 + 50.0 [N(CH3)2], 65.1 (CHN). CHN analy-sis: observed: C, 30.4; H, 5.5; N, 7.0; calculated: C, 30.4; H, 5.6; N, 7.1.

2.6. General procedure for D,L-lactide polymerisation

3.603 g D,L-Lactide (3,6-dimethyl-1,4-dioxane-2,5-dione,25 mmol, used as purchased) and the initiator (I/M ratio 1/500 or1/1000) were weighed into a 50 mL flask, which was flushed withargon and closed with a glass stopper. The reaction vessel was thenheated at the designated temperature. After the reaction time thepolymer melt was allowed to cool to room temperature and dis-solved in dichloromethane (25 mL). The PLA was precipitated inice-cooled ethanol (350 mL) and dried under vacuum at 50 �C.

3. Results and discussion

3.1. Synthesis of diamine-zinc adducts

For the preparation of the zinc coordination compounds, wechose a set of simple 1,2-diamine ligands (Fig. 1). Two ligands,N,N,N0,N0-tetramethylethylenediamine (TMEDA, L1) and N,N,N0,N0-tetraethylethylenediamine (TEEDA, L2) are commerciallyavailable; the first is commonly used in the vast field ofcoordination chemistry [27] and the latter has already been ap-plied to substantially accelerate anionic ethylene polymerisationin comparison to L1 [28]. Additionally, we employed the ligands

R2N NR2

NR2

NR2R= Me: TMEDA (L1)R= Et: TEEDA (L2)

R = Me: (R,R)-TMCDA (L3)R = Et: (R,R)-TECDA (L4)

NR2

NR2

R = Me: cis-TMCDA (L5)

Fig. 1. Ligands used for the synthesis of zinc complexes.

Fig. 3. Molecular structures of complexes C2 and C3.

Fig. 4. Molecular structures of complexes C4 and C6. The asymmetric units containmore than one molecule of the zinc complex, of which only one is displayed here.

154 P.K. Eckert et al. / Polyhedron 49 (2013) 151–157

(R,R)-tetramethylcyclohexane-1,2-diamine [(R,R)-TMCDA (L3)], thetetraethyl substituted (R,R)-TECDA (L4) and, for a change in geom-etry, the ligand cis-TMCDA (L5). For the synthesis of the ligands L3and L4, (R,R)-cyclohexane-1,2-diamine was extracted from a mix-ture of isomers via racemic resolution with L-(+)-tartaric acid[29]. Subsequent Eschweiler-Clarke methylation or alkylation withdiethyl sulphate gave L3 and L4, respectively [18,19,21]. The dia-stereomerically pure ligand L5 was synthesised by separating cis-cyclohexane-1,2-diamine from a mixture of isomers with nickel(II)chloride and subsequent Eschweiler–Clarke methylation [20].

Coordination compounds of all five ligands were obtained byadding one equivalent of ligand to a solution of one equivalent ofzinc bromide in acetone (or mixtures of acetone and other sol-vents). Upon slow removal of the solvents, colourless single crys-tals of the complexes C1, C2, C3, C4 and C7 were formed. Toobtain information about the effect of the halide, the complexesC5, C6 and C8 were synthesised with zinc chloride (Fig. 2).

X-ray diffraction analysis showed that all coordination com-pounds C1-C8 form monomers. The crystal structures of the com-pounds C1 and C5 have already been reported [25,26]. Themolecular structures of the compounds C2–C4 and C6–C8 areshown in Figs. 3–5. CHN analysis and NMR spectroscopy confirmedthe formation of only one type of product, which is stable insolution.

Table 2 summarises selected bond lengths and angles surround-ing the zinc atom which displays a distorted tetrahedral coordina-tion sphere. The Zn–Br and Zn–N distances of all compounds varyonly slightly [2.337(1)–2.371(1) and 2.053(2)–2.122(2) Å]. Thebond lengths responsible for the coordination of the diamineligand to the metal centre are similar to those of coordinationcompounds with zinc halides and (�)-a-isosparteine (2.093 Å),(�)-sparteine (2.078, 2.086 Å), N-isopropyl-N,N0,N0-trimethylethyl-ene-1,2-diamine (2.113, 2.078 Å) [30] or 9-oxabispidine (2.074 Å)[14e]. The angles surrounding the central zinc atom range from85.84(7)� in C1 to 120.30(17)� in C3, with N–Zn–N, the angle com-prising the coordination to the diamine ligand, being the smallest.A similar distorted tetrahedral coordination sphere with a smallN–Zn–N angle is seen in coordination compounds with relateddiamine ligands [14e,30].

As can be seen in Fig. 6 and Table 3, the overall geometries ofthe complexes with a cyclohexane backbone (C3–C8) differ signif-icantly depending on the ligand. The coordination compounds C3

R2N NR2

Zn

Br Br

C3 R = Me,C4 R = Et,C5 R = Me,C6 R = Et,

C1 R = MeC2 R = Et

C7 R = Me, X = BrC8 R = Me, X = Cl

R2N NR2

Zn

X X

R2N NR2

Zn

X X

X = BrX = BrX = ClX = Cl

Fig. 2. Molecular structures of the zinc(II) halide complexes C1–C8 applied inlactide ROP.

Fig. 5. Molecular structures of complexes C7 and C8. The asymmetric units containmore than one molecule of the zinc complex, of which only one is displayed here.

and C5 containing the ligand (R,R)-TMCDA (L4) are straight mole-cules. In contrast, the complexes C4 and C6 containing the ligand(R,R)-TECDA (L4) display a different geometry. Here, the zinc halidemoiety is bent away from the ligand plane by 28.5(3)� and 24.2(1)�,respectively. To our knowledge this bent geometry has not yetbeen observed in similar amine coordination compounds [30]and might lead to interesting catalytic activity in the lactide poly-merisation. Within the compounds C7 and C8 containing the ligandcis-TMCDA, the coordination reveals another interesting feature:

Table 2Selected bond lengths (Å) and angles (�) of complexes C1-C8 (X = Br, Cl).

Zn–N Zn–X N–Zn–N/X–Zn–X N–Zn–X

C1 [25]a 2.072(2)2.113(2)

2.337(1)2.347(1)

87.2(1)/119.4(1) 112.7(1), 111.6(1)111.9(1), 109.5(1)

C2 2.087(5)2.094(5)

2.349(1)2.350(1)

88.8(2)/118.5(1) 108.9(2), 114.8(2)113.0(2), 109.2(2)

C3 2.083(2)2.085(2)

2.358(1)2.359(1)

87.3(1)/118.9(1) 110.8(1), 112.5(1)112.0(1), 111.1(1)

C4a 2.112(6)2.083(5)

2.371(1)2.339(1)

86.8(2)/115.6(1) 120.3(2), 112.2(2)106.0(1), 112.6(2)

C5 [26]a 2.057(3)2.071(3)

2.213(1)2.201(1)

87.6(1)/116.7(1) 114.9(1), 115.0(1)108.5(1), 110.5(1)

C6a 2.088(2)2.103(2)

2.120(1)2.247(1)

87.0(1)/119.1(1) 117.7(1), 112.2(1)105.4(1), 110.9(1)

C7a 2.114(3)2.074(3)

2.354(1)2.360(1)

86.5(1)/113.7(1) 110.7(1), 116.5(1)114.7(1), 112.1(1)

C8a 2.122(2)2.053(2)

2.195(1)2.223(1)

85.8(1)/113.5(1) 113.7(1), 116.8(1)113.0(1), 111.2(1)

a The asymmetric unit contains more than one molecule of the zinc complex butthe geometry of only one is displayed here.

Fig. 6. Lateral view of the coordination compounds C3, C4 and C7 in the crystalsand the angles between the planes N–Zn–N and the plane comprising thecyclohexane ring (see Table 2).

Table 3Angles (�) between the plane comprising the N donor atoms and Zn and the planescontaining atoms of the ligands. The first column shows the angle between the N–Zn–N plane and the plane containing the two nitrogen atoms and the two stereogeniccarbon centres. The second column shows the angle between the N–Zn–N plane andthe plane containing the six carbon atoms of the cyclohexane ring.

N–Zn–N/N–C–C–N N–Zn–N/C–C–C–C–C–C

C1a 10.3(3) –C2 9.0(5) –C3 6.9(1) 8.7(1)C4a 27.1(4) 28.5(3)C5a 10.1(1) 12.2(2)C6a 25.5(1) 24.2(1)C7a 15.7(2) 71.2(1)C8a 20.7(1) 76.4(1)

a The asymmetric unit contains more than one molecule of the zinc complex butthe geometry of only the first molecule is discussed in this table.

1 For the gel permeation chromatography, see Ref. [10b].

P.K. Eckert et al. / Polyhedron 49 (2013) 151–157 155

the complex molecules are not bent at the zinc halide moiety butalready at the stereogenic carbon centres due to the cis-configura-tion. Hence, the geometry looks slightly crooked in comparison tothe other complexes.

All zinc halide adducts (C1–C8) are highly stable on the benchand non-sensitive towards humidity. They can be stored withoutany precautions for months. This is typical for neutral zinc com-plexes and is a great advantage compared to most of the investi-gated single-site PLA catalysts, which are often highly sensitivetowards humidity [1]. DSC measurements showed that no decom-position of the adducts takes place below 250 �C.

3.2. Polymerisation activity

The crystalline compounds C1–C8 were applied to lactide poly-merisation. Therefore, the monomer D,L-lactide and the initiator(I/M ratio 1:500) were heated at 150 �C. To meet industrial condi-tions, the lactide was used without any previous purification, suchas sublimation or recrystallisation. After a reaction time of 24 or48 h, the melt was dissolved in dichloromethane at room temper-ature, and the PLA was precipitated in cold ethanol, isolated, anddried under vacuum at 50 �C. In order to assess the catalytic activ-ity of the complexes, the polymer yield was defined and the molec-ular weights as well as the polydispersity (PD) of the obtained PLAsamples were determined by gel permeation chromatography1

(see Table 4).The simple TMEDA complex C1 does not show any activity, even

after a reaction time of 48 h. However, we found that the com-plexes C2–C8 initiate the ring-opening polymerisation of D,L-lactideand that the influence of the halogen is small. The most active cat-alysts in the ROP of D,L-lactide are the TECDA-containing complexesC4 and C6. They combine quantitative yields with high molecularweights up to 133000 g mol�1, which lie in the range suitable forindustrial applications [31].

For the most active system [ZnBr2{(R,R)-TECDA}] (C4) a kineticstudy was performed that shows a first order reaction for the poly-merisation up to a reaction time of three hours and 80% conversion(determined by 1H NMR, see Supporting information and Fig. 7).Afterwards the polymerisation proceeds slower, probably due toside reactions and increasing polymer viscosity.

Interestingly, the cis-TMCDA complexes C7 and C8 lead to in-creased yields but considerably smaller molecular weights thantheir (R,R)-TMCDA analogues. With regard to the theoretically ex-pected molecular weights of 53000 and 68000 g mol�1 for ROPwith C7 and C8 under these conditions, respectively, the obtainedmolecular weights are too small.

For the complexes C3 and C7 different polymerisation condi-tions were investigated. The catalyst load was lowered from 0.02to 0.01 mol% and the polymerisation temperature was lowered to135 �C and raised to 160 �C. With C3 the polymerisation at 135 �Cdid not yield any polymer within the reaction time of 48 h, whileat 165 �C a yield of 95% PLA was obtained with a molecular weightof 33000 g mol�1 and a PD of 2.1. Compared to the polymerisationtemperature of 150 �C the yield is much higher but the molecularweight does not increase significantly which means that morechains have been started for polymerisation. The higher PD revealsthat the polymerisation is less controlled at the higher temperature.With an I/M ratio of 1:1000 at 150 �C the yield of 18% is nearly thesame as with an I/M ratio of 1:500. The molecular weight increasesto 32000 g�mol�1 and the PD of 1.8. Complex C7 shows activity at135 �C as well as at 165 �C. At 135 �C the yield is 55% after 48 hand hence lower than at 150 �C (70%) and 165 �C (92%). The PDhas a very small value of 1.2 at 135 �C indicating a well-controlledpolymerisation at that temperature. The obtained Mw is10000 g mol�1. At 165 �C the PD is higher (1.9) and the molecularweight increases to 15000 g mol�1 compared to the polymerisationat 150 �C. With an I/M ratio of 1:1000 the polymerisation at 150 �Cshows nearly the same yield (70%) as with the ratio of 1:500. TheMw is higher (14000 g mol�1) but not twice as high as could be ex-pected by comparison to the higher catalyst load. The PD with a va-lue of 1.5 is the same for both catalyst loads at 150 �C. The results athigher temperatures together with the DSC measurements showthat the complexes do not decompose at these temperatures. Thisis highly advantageous for industrial use.

Table 4Polymerisation results of D,L-lactide initiated by C1–C8 in the lactide melt.

OO

O

O

initiator0.5 n O

O

n

lactide PLA

ΔT

No. Time (h) Temp. (�C) I/M ratio Yield (%) Mw (g mol�1) PDa Prb

C1 24 150 1:500 0 – – –C1 48 150 1:500 0 – – –C2 48 150 1:500 56 58000 1.9 n.d.C3 48 150 1:500 20 25000 1.5 0.54C3 48 150 1:1000 18 32000 1.8 n.d.C3 48 135 1:500 0 – – –C3 48 165 1:500 95 33000 2.1 n.d.C4 24 150 1:500 92 133000 1.9 0.54C4 48 150 1:500 91 113000 1.9 n.d.C5 48 150 1:500 27 38000 1.6 0.55C6 24 150 1:500 89 99000 2.1 n.d.C6 48 150 1:500 93 84000 1.9 n.d.C7 48 150 1:500 73 10000 1.5 0.50C7 48 150 1:1000 70 14000 1.5 n.d.C7 48 135 1:500 55 10000 1.2 n.d.C7 48 165 1:500 92 15000 1.9 n.d.C8 48 150 1:500 94 6000 1.8 0.50

a PD = Mw/Mn where Mn is the number-average molar mass.b Pr: probability of racemic enchainment calculated by analysis of the homonuclear decoupled 1H NMR spectra [5].

0 100 200 300 400 5000.0

0.5

1.0

1.5

2.0

2.5

ln([

LA] 0

/[LA

] t)

time / min

Fig. 7. First order plot of ln([LA]0/[LA]t) vs. time at 150 �C for the polymerisation ofD,L-lactide with C4.

156 P.K. Eckert et al. / Polyhedron 49 (2013) 151–157

To investigate the influence of the catalyst structure on the tac-ticity of the resulting polymer, Pr values were determined byhomonuclear decoupled 1H NMR spectroscopy of the polymerssynthesised using the TMCDA complexes C3, C5, C7 and C8. A valueof 0.5 indicates an atactic microstructure whereas a Pr between 0.5and 1 reveals heterotactic enrichment. The values in Table 3 showthat the resulting polymers are atactic with a very small heterotac-tic bias for the (R,R)-TMCDA complexes C3 and C5.

The observed efficiency of the simple diamine complexes israther surprising as complexes with neutral ligands often lackactivity. Up to now, besides a 9-oxabispidine complex [14e], onlycomplexes with strong N donor ligands or highly nucleophilicco-ligands have shown significant ROP activity [1,9]. The activityof complexes containing ligands L1–L5 increases in the following

order: TMEDA (L1), cis-TMCDA (L5), (R,R)-TMCDA (L3), TEEDA(L2), (R,R)-TECDA (L4). This may be due to different effects: onthe one hand, different solvation properties of the complexes inthe lactide melt may be attributed to the better solubility of(R,R)-TECDA complexes. On the other hand, from a structural pointof view, it is remarkable that the most active complex [ZnBr2(TEC-DA)] (C4) also exhibits the smallest N–Zn–N and X–Zn–X angles inthe trans (R,R) configured complexes (see Table 3). Additionally, aconsiderable bending of the zinc(II) halide from the ligand planeis evident in C4 and C6; this is not observed in the complexes withligands L1 (C1), L2 (C2) and L3 (C3 and C5) (see Fig. 6). Thisarrangement in the TECDA complexes C4 and C6 hints at an open-ing of the complex for the coordination of the lactide substrate. Theunexpected activity of the complexes in ROP may thus be ascribedto an interplay between the spatial overload – causing the bendingof the zinc(II) halide – and the accessibility of the active site for theattack of lactide as the initial step of the polymerisation. In the cis-TMCDA complexes C7 and C8, the bending is more distant from thezinc centre, which again leads to a more enclosed zinc environ-ment. This correlates with considerably smaller molecular weights.We are currently working on detailed mechanistic studies tounderstand and further improve the activity of the initiators.

4. Conclusion

We herein reported the synthesis and structural characterisationof a series of robust diamine zinc complexes and their application ascatalysts in lactide polymerisation. In particular, the (R,R)-TECDAcomplexes C4 and C6 show an unexpected activity with high yieldsand molecular weights. With regard to structural features, a correla-tion between accessibility at the zinc centre and ROP performancecould be found. The presented catalysts stand out because of theircombination of reactivity and stability, which are often in conflictwith each other. Moreover, they are derived from low-priced start-ing materials, which make their large-scale industrial use feasible.In general, the importance of neutral ligands for the ring-openingpolymerisation of lactide cannot be underestimated. With regard

P.K. Eckert et al. / Polyhedron 49 (2013) 151–157 157

to the major breakthrough of bioplastics as a replacement for petro-chemical plastics in the commodity market, every robust catalystsystem represents a step towards greater sustainability of oursociety.

Acknowledgments

Support by the FCI (financial support and a fellowship forS.H.-P.), the Bundesministerium für Bildung und Forschung(MoSGrid, 01IG09006), and the DFG is gratefully acknowledged.Moreover, we gratefully acknowledge financial support of theMERCUR Research Center Ruhr. J.B. thanks the Stiftung derDeutschen Wirtschaft (sdw) for granting a doctoral fellowship.S.H.-P. thanks Prof. K. Jurkschat for his valuable support. Theauthors thank Dr. Wolf Hiller (TU Dortmund) for the determinationof the probability of the heterotactic enchainment.

Appendix A. Supplementary material

CCDC 745415, 745416, 745418, 858793, 858794 and 858795contains the supplementary crystallographic data for C4, C6, C3,C2, C7 and C8, respectively. These data can be obtained free ofcharge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, orfrom the Cambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: de-posit@ccdc.cam.ac.uk. Supplementary data associated with thisarticle can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2012.09.012.

References

[1] (a) For review articles, see: O. Dechy-Cabaret, B. Martin-Vaca, D. Bourissou,Chem. Rev. 104 (2004) 6147;(b) J. Wu, T.-L. Yu, C.-T. Chen, C.-C. Lin, Coord. Chem. Rev. 250 (2006) 602;(c) R.W. Drumwright, P.R. Gruber, D.E. Henton, Adv. Mater. 12 (2000) 1841;(d) H.R. Kricheldorf, Chemosphere 43 (2001) 49;(e) P.J. Dijkstra, H. Dum, J. Feijen, Polym. Chem. 2 (2011) 520;(f) C.A. Wheaton, P.G. Hayes, B.J. Ireland, Dalton Trans. 25 (2009) 4832;(g) M.J. Stanford, A.P. Dove, Chem. Soc. Rev. 39 (2010) 486. and referencestherein.

[2] (a) For applications, see: T. Hayashi, Prog. Polym. Sci. 19 (1994) 663;(b) R.G. Sinclair, Pure Appl. Chem. A33 (1996) 585;(c) E. Chiellini, R. Solaro, Adv. Mater. 8 (1996) 305;(d) W. Amass, A. Amass, B. Tighe, Polym. Int. 47 (1998) 89;(e) R. Langer, Acc. Chem. Res. 33 (2000) 94.

[3] (a) C.J. Chuck, G. Davidson, M.D. Jones, G. Kociok-Köhn, M.D. Lunn, S. Wu,Inorg. Chem. 45 (2006) 6595;(b) M. Frediani, D. Sémeril, A. Mariotti, L. Rosi, P. Frediani, L. Rosi, D. Matt, L.Toupet, Macromol. Rapid Commun. 29 (2008) 1554.

[4] (a) L.F. Sánchez-Barba, A. Garcés, M. Fajardo, C. Alonso-Moreno, J. Fernández-Baeza, A. Otero, A. Antinolo, J. Tejeda, A. Lara-Sanchéz, M.I. López-Solera,Organometallics 26 (2007) 6403;(b) M.-Y. Shen, Y.-L. Peng, W.-C. Hung, C.-C. Lin, Dalton Trans. (2009) 9906;(c) L. Wang, H. Ma, Macromolecules 43 (2010) 6535.

[5] (a) B.M. Chamberlain, M. Cheng, D.R. Moore, T.M. Ovitt, E.B. Lobkovsky, G.W.Coates, J. Am. Chem. Soc. 123 (2001) 3229;(b) C.K. Williams, L.E. Breyfogle, S.K. Choi, W. Nam, V.G. Young, M.A. Hillmyer,W.B. Tolman, J. Am. Chem. Soc. 125 (2003) 11350;(c) J. Ejfler, S. Szafert, K. Mierzwicki, L.B. Jerzykiewicz, P. Sobota, Dalton Trans.(2008) 6556;(d) J.-C. Wu, B.-H. Huang, M.-L. Hsueh, S.-L. Lai, C.-C. Lin, Polymer 46 (2005)9784;(e) T.R. Jensen, C.P. Schaller, M.A. Hillmyer, W.B. Tolman, J. Organomet. Chem.690 (2005) 5881;

(f) M.D. Jones, M.G. Davidson, C.G. Keir, L.M. Hughes, M.F. Mahon, D.C.Apperley, Eur. J. Inorg. Chem. (2009) 635.

[6] (a) M. Cheng, A.B. Attygalle, E.B. Lobkovsky, G.W. Coates, J. Am. Chem. Soc. 121(1999) 11583;(b) M.H. Chisholm, J.C. Huffman, K. Phomphrai, J. Chem. Soc., Dalton Trans.(2001) 222;(c) M.H. Chisholm, J. Gallucci, K. Phomphrai, Inorg. Chem. 41 (2002) 2785;(d) A.P. Dove, V.C. Gibson, E.L. Marshall, A.J.P. White, D.J. Williams, J. Chem.Soc., Dalton Trans. (2004) 570;(e) M.H. Chisholm, J.C. Gallucci, K. Phomphrai, Inorg. Chem. 44 (2005) 8004.

[7] (a) M.H. Chisholm, N.W. Eilerts, Chem. Commun. 1996, 853;(b) M.H. Chisholm, N.W. Eilerts, J.C. Huffman, S.S. Iyer, M. Pacold, K.Phomphrai, J. Am. Chem. Soc. 122 (2000) (1854) 11845;(c) M.H. Chisholm, J. Gallucci, K. Phomphrai, Chem. Commun. (2003) 48.

[8] R.H. Platel, L.M. Hodgson, C.K. Williams, Polym. Rev. 48 (2008) 11.[9] I. dos Santos Vieira, S. Herres-Pawlis, Eur. J. Inorg. Chem. (2012) 765.

[10] (a) J. Börner, S. Herres-Pawlis, U. Flörke, K. Huber, Eur. J. Inorg. Chem. (2007)5645;(b) J. Börner, U. Flörke, K. Huber, A. Döring, D. Kuckling, S. Herres-Pawlis,Chem. Eur. J. 15 (2009) 2362.

[11] T.R. Jensen, L.E. Breyfogle, M.A. Hillmyer, W.B. Tolman, Chem. Commun. (2004)2504.

[12] (a) H. Sun, J.S. Ritch, P.G. Hayes, Inorg. Chem. 50 (2011) 8063;(b) C.A. Wheaton, P.G. Hayes, Dalton Trans. 39 (2010) 3861;(c) C.A. Wheaton, B.J. Ireland, P.G. Hayes, Organometallics 28 (2009) 1282.

[13] M.G. Cushion, P. Mountford, Chem. Commun. 47 (2011) 2276.[14] (a) J.H. Jeong, Y.H. An, Y.K. Kang, Q.T. Nguyen, H. Lee, B.M. Novak, Polyhedron

27 (2008) 319;(b) S. Nayab, H. Lee, J.H. Jeong, Polyhedron 30 (2011) 405;(c) Y.K. Kang, Q.T. Nguyen, R.-E. Lee, H. Lee, J.H. Jeong, Bull. Korean Chem. Soc.30 (2009) 257;(d) Y.K. Kang, J.H. Jeong, N.Y. Lee, Y.T. Lee, H. Lee, Polyhedron 29 (2010)2404;(e) J. Börner, U. Flörke, A. Döring, D. Kuckling, M.D. Jones, M. Steiner, M.Breuning, S. Herres-Pawlis, Inorg. Chem. Commun. 13 (2010) 369.

[15] J. Börner, U. Flörke, A. Döring, D. Kuckling, M.D. Jones, S. Herres-Pawlis,Sustainability 1 (2009) 1226.

[16] B. Calvo, M.G. Davidson, D. García-Vivó, Inorg. Chem. 50 (2011) 3589.[17] T.A. Whitney, J. Org. Chem. 45 (1980) 4214.[18] J.-C. Kizirian, N. Cabello, L. Pinchard, J.-C. Caille, A. Alexakis, Tetrahedron 61

(2005) 8939.[19] A.P. Cole, V. Mahadevan, L.M. Mirica, X. Ottenwaelder, T.D.P. Stack, Inorg.

Chem. 44 (2005) 7345.[20] (a) P.K. Eckert, V. Schill, C. Strohmann, Inorg. Chim. Acta 376 (2011) 634;

(b) R. Saito, Y. Kidani, Chem. Lett. (1976) 123;(c) R. Saito, Y. Kidani, Chem. Lett. 10 (1977) 1141.

[21] C. Strohmann, V.H. Gessner, J. Am. Chem. Soc. 130 (2008) 11719.[22] J.E. Mark, Polymer Data Handbook, Oxford University Press, New York,

1999.[23] G.M. Sheldrick, SHELXS-90, University of Göttingen, 1990.[24] G.M. Sheldrick, SHELXL-97, University of Göttingen, 1997.[25] H. Citeau, O. Conrad, D.M. Giolando, Acta Crystallogr., Sect. E 57 (2001) m5.[26] N.Y. Lee, J.U. Yoon, J.H. Jeong, Acta Crystallogr., Sect. E 63 (2007) m2471.[27] (a) For examples, see V.H. Gessner, C. Strohmann, J. Am. Chem. Soc. 130 (2008)

14412;(b) S.P. Green, C. Jones, A. Stasch, Chem. Commun. 47 (2008) 6285;(c) C. Strohmann, V.H. Gessner, A. Damme, Chem. Commun. 29 (2008) 3381;(d) E. Hevia, D.J. Gallagher, A.R. Kennedy, R.E. Mulvey, C.T. O’Hara, C. Talmard,Chem. Commun. (2004) 2422;(e) A.S. Perucha, J. Heilmann-Brohl, M. Bolte, H.-W.M. Wagner,Organometallics 27 (2008) 6170;(f) J.T. York, E.C. Brown, W.B. Tolman, Angew. Chem., Int. Ed. 44 (2005) 7923;(g) J.T. York, E.C. Brown, W.B. Tolman, Angew. Chem., Int. Ed. 44 (2005) 7745–7748.

[28] (a) G. Crassous, M. Abadie, F. Schue, Eur. Polym. J. 15 (1979) 747;(b) C.B. Collum, Acc. Chem. Res. 25 (1992) 448.

[29] J.F. Larrox, E.N. Jacobsen, J. Org. Chem. 59 (1994) 1939.[30] (a) J.L. Alcántara-Flores, S. Bernès, Y. Reyes-Ortega, R. Zamorano-Ulloa, Acta

Crystallogr., Sect. E 59 (2003) m988;(b) Y.-M. Lee, S.K. Kang, Y.-I. Kim, S.-N. Choi, Acta Crystallogr., Sect. C 58 (2002)m453;(c) A. Johannsson, E. Wingstrand, M. Håkansson, Inorg. Chim. Acta 358 (2005)3293.

[31] I. Bechthold, PhD Thesis, Berlin, 2003.