Reactivity of Zinc Halide Complexes Containing Camphor-Derived Guanidine Ligands with Technical Rac-Lactide

inorganics

Article Reactivity of Zinc Halide Complexes Containing Camphor-Derived Guanidine Ligands with Technical rac-Lactide

Angela Metz 1, Joshua Heck 1, Clara Marie Gohlke 1, Konstantin Kröckert 1, Yannik Louven 1, Paul McKeown 2, Alexander Hoffmann 1, Matthew D. Jones 2 and Sonja Herres-Pawlis 1,* ID

1 Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany; [email protected] (A.M.); [email protected] (J.H.); [email protected] (C.M.G.); [email protected] (K.K.); [email protected] (Y.L.); [email protected] (A.H.) 2 Centre for Sustainable Chemical Technologies, Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK; [email protected] (P.M.); [email protected] (M.D.J.) * Correspondence: [email protected]; Tel.: +49-241-80-939-02

Received: 27 October 2017; Accepted: 23 November 2017; Published: 30 November 2017

Abstract: Three new zinc complexes with monoamine–guanidine hybridligands have been prepared, characterized by X-ray crystallography and NMR spectroscopy, and tested in the solvent-free ring-opening polymerization of rac-lactide. Initially the ligands were synthesized from camphoric acid to obtain TMGca and DMEGca and then reacted with zinc(II) halides to form zinc complexes. All complexes have a distorted tetrahedral coordination. They were utilized as catalysts in the solvent-free polymerization of technical rac-lactide at 150 ◦C. Colorless polylactide (PLA) can be produced and after 2 h conversion up to 60% was reached. Furthermore, one zinc chlorido complex was tested with different qualities of lactide (technical and recrystallized) and with/without the addition of benzyl alcohol as a co-initiator. The kinetics were monitored by in situ FT-IR or 1H NMR spectroscopy. All kinetic measurements show ﬁrst-order behavior with respect to lactide. The inﬂuence of the chiral complexes on the stereocontrol of PLA was examined. Moreover, with MALDI-ToF measurements the end-group of the obtained polymer was determined. DFT and NBO calculations give further insight into the coordination properties. All in all, these systems are robust against impurities and water in the lactide monomer and show great catalytic activity in the ROP of lactide.

Keywords: ring-opening polymerization; polylactide; zinc; guanidine

1. Introduction Due to increasing environmental awareness, plastics based on petrochemical resources have to be replaced by plastics based on renewable raw materials. Nowadays several polymers, such as starch plastics, polyhydroxyalkanoate (PHA), cellulose acetate, and polylactide (PLA), can be used as an alternative to petrochemical-based plastics [1–5]. PLA has the greatest potential. Due to its resemblance to petrochemical polymers, PLA is utilized in the medical field, packaging, and microelectronics [5–9]. It is a biodegradable and biocompatible aliphatic polyester that can be produced from renewable raw materials such as corn and sugar beets [3,4,6–16]. PLA is produced by the ring-opening polymerization (ROP) of lactide, the dimer of lactic acid, using a metal-based catalyst system. For a broad application field, high molar masses, low dispersity values, and a control over the tacticity of the resulting PLA are key features [6–9,13]. The tacticity can be influenced by three parameters: polymer chain end, ligand chirality, and solvent [17–28]. Most catalysts produce atactic or heterotactic PLA but with isotactic PLA

Inorganics 2017, 5, 85; doi:10.3390/inorganics5040085 www.mdpi.com/journal/inorganics Inorganics 2017, 5, 85 2 of 19 the mechanical strength, melting point, and physical properties can be enhanced [29]. It is desirable to synthesize new catalysts that satisfy these requirements. In recent decades, numerous catalysts based on tin, zinc, aluminium, rare earth metals, and titanium containing anionic ligand systems were investigated. These ligand systems were used: trispyrazolylborates [30,31], aminophenolates [32], β-ketiminates [33–41], salan/salalen ligands, and Schiff bases [23–28,42–46], as well as alkyl ligands derived from acids [47]. Jones and co-workers synthesized bipyrrolidine salan proligands containing as the metal center ZrIV, TiIV, HfIV, and AlIII, which show high activity and control on the tacticity [42–44]. The fastest systems could be obtained by Williams et al. These dizinc complex systems containing Schiff bases can polymerize lactide at room temperature within one minute and without the addition of a co-initiator [48,49]. The problem with these anionic ligand systems is that the polymerization has mostly be enperformed with purified lactide (several times recrystallized or sublimed) and in solution, which is not suitable for industrial applications. Moreover, the catalysts are often combined with a high sensitivity towards moisture, air, and lactide impurities. Due to these conditions, the industrial target is to synthesize stable systems that can polymerize technical lactide under bulk conditions. Nowadays, the commonly used catalyst is tin(II) 2-ethylhexanoate, with alcohols as the initiating species [50–52]. Here the problem is the toxicity of the tin that remains in the polymers and, after composting, accumulates in the environment [53]. Feijen et al. published some criteria that an industrially used catalyst should meet. In addition, the ligands should be cheap and commercially available, the complex must be synthesized in high yields, the activity in the ROP (bulk conditions) is important, and the obtained polymer must be colorless with high molar masses [19]. A promising alternative is zinc containing neutral ligands as it is non-toxic, cost-efficient, robust against lactide impurities and water, and shows high activity. A large variety of neutral ligand systems are well-known, e.g., phosphinimines [54–58], iminopyridines [59,60], trispyrazolylmethanes [61], carbenes [62,63], piperidinyl-benzyl-anilines [64], and guanidine systems [65–68]. Jeong and co-workers synthesized in situ diisopropoxide zinc complexes with camphorylimine and produced heterotactic-enriched PLA with a narrow dispersity and good control at 25 ◦C in THF [59,60]. A low dispersity and very fast polymerization in bulk or solution were achieved with carbene zinc complexes by Tolman et al. The problem with this is that only purified lactide was used [62,63]. Superior robustness for lactide polymerization can be achieved with zinc guanidine complexes. As an example, zinc hybridguanidine/bisguanidine complexes show high activity in bulk and with technical rac-lactide at 150 ◦C[69,70]. Moreover, great activity can be reached with robust zinc complexes containing N,O donor functionalities [71]. With quinoline–guanidine bis(chelate) triflato complexes high activity with technical lactide and good molar masses can be reached. However, the comparable zinc chlorido complexes did not show any activity [72–74]. Herein, we report the synthesis and the full characterization of three new chiral monoamine-guanidine hybrid zinc complexes. As starting material, camphoric acid was used. These complexes were tested in the solvent-free polymerization of rac-lactide at 140 ◦C or 150 ◦C with or without a co-initiator. All complexes show high activity in the ROP of lactide and produce a polymer in a very short reaction time. DFT and NBO calculations were performed to obtain insight into the coordination properties. These systems provide an excellent, sustainable alternative to commercially used tin complexes.

2. Results and Discussion

2.1. Zinc Halide Complexes One new monoamine-hybridguanidine ligand 3-((1,3-dimethylimidazolidine-2-ylidene)amino)-1, 2,2-trimethylcyclopentane-1-amine (DMEGca) and the recently published ligand 2-(3-amino-2,2,3-trimethylcyclopentyl)- 1,1,3,3-tetramethylguanidine (TMGca) [75] based on camphoric acid were prepared by the reaction of the corresponding Vilsmeier salt N,N0-dimethylethylenechl Inorganics 2017, 5, 85 3 of 19

0 0 InorganicsInorganicsoroformamidiniumchloride 2017 2017, ,5 5, ,85 85 or N,N,N ,N -tetramethyl-chloroformamidiniumchloride with33 of of the 19 19 diamine [76,77]. First the (1R, 3S)-camphor-bisamine has to be generated on the basis of via(1viaR a,a 3 CurtiusCurtiusS)-camphor rearrangementrearrangement acid via a Curtius [75].[75]. TheThe rearrangement additionaddition ofof onon [75ee equivalent].equivalent The addition ofof VilsmeierVilsmeier of one equivalent saltsalt andand thethe of Vilsmeierpresencepresence ofofsalt trimethylaminetrimethylamine and the presence leadsleads of trimethylamine toto thethe monoamine-guanidinemonoamine-guanidine leads to the monoamine-guanidine hybridligandshybridligands L1L1 (TMGca)(TMGca) hybridligands andand L2L2 L1 (DMEGca)(DMEGca)(TMGca) [76,77][76,77]and L2 (Figure(Figure(DMEGca) 1).1). [76,77] (Figure1).

NN NN

NN NN NN NN NHNH22 NHNH22

TMGca,TMGca,L1L1 DMEGca,DMEGca,L2L2

FigureFigure 1.1. OverviewOverview ofof thethe utilizedutilized monomono monoamine-guanidineamine-guanidineamine-guanidine hybridligandshybridligands L1L1 [75][[75]75] and and L2L2..

TheThe ligandsligands werewere usedused inin complexcomplex synthesissynthesis withwith anhydrousanhydrous zinczinc chloride/bromidechloride/bromide inin drydry THFTHF The ligands were used in complex synthesis with anhydrous zinc chloride/bromide in dry toto obtainobtain crystalscrystals suitablesuitable forfor X-rayX-ray crystallographycrystallography (Figure(Figure 2).2). ThreeThree zinczinc halidehalide complexes,complexes, C1C1 THF to obtain crystals suitable for X-ray crystallography (Figure2). Three zinc halide complexes, [Zn(TMGca)Cl[Zn(TMGca)Cl22],], C2C2 [Zn(DMEGca)Cl[Zn(DMEGca)Cl22],], andand C3C3 [Zn(DMEGca)Br[Zn(DMEGca)Br22],], werewere obtained.obtained. InIn allall complexcomplex C1 [Zn(TMGca)Cl2], C2 [Zn(DMEGca)Cl2], and C3 [Zn(DMEGca)Br2], were obtained. In all complex systems,systems, twotwo moleculesmolecules areare inin thethe asymmetricalasymmetrical unit.unit. SinceSince bothboth conformersconformers showshow similarsimilar bondbond systems, two molecules are in the asymmetrical unit. Since both conformers show similar bond lengths lengthslengths andand angles,angles, onlyonly oneone waswas usedused forfor moremore preciseprecise examinations.examinations. InIn TableTable 11 thethe bondbond lengthslengths and angles, only one was used for more precise examinations. In Table1 the bond lengths and angles andand angles angles of of these these complexes complexes are are summarized. summarized. The The zi zincnc atom atom is is fourfold fourfold coordinated coordinated by by one one primary primary of these complexes are summarized. The zinc atom is fourfold coordinated by one primary amine, amine,amine, oneone guanidineguanidine moiety,moiety, andand twotwo halidehalide atoms.atoms. DueDue toto thethe differentdifferent coordinationcoordination strength,strength, thethe one guanidine moiety, and two halide atoms. Due to the different coordination strength, the bond bondbond lengthslengths forfor Zn–NZn–Nguagua (1.986(3),(1.986(3),USV 2.013(4),2.013(4), Symbol 2.065(5)2.065(5) Macro(s) Å)Å) areare shortershorter thanthan forfor Zn–NZn–Namineamine (2.053(3),(2.053(3),Description lengths for Zn–Ngua (1.986(3), 2.013(4), 2.065(5) Å) are shorter than for Zn–Namine (2.053(3), 2.065(4), 2.065(4),2.065(4), 2.041(5) 2.041(5) Å). Å). This This trend trend can can be be seen seen in in recent recent studies studies [69,78]. [69,78]. The The angle angle of of ZnN ZnN22 is is around around◦ 100° 100° 2.041(5) Å). This trend can be seen2220 in recent∠ studies\textangle [69,78]. The angle of ZnN2 is around 100 andANGLE the andand thethe angleangle betweenbetween thethe coordinationcoordination planesplanes {{∡∡ (ZnX(ZnX22,, ZnNZnN22)}◦)} isis 80.3°–83.4°,80.3°–83.4°,◦ inin goodgood agreementagreement angle between the coordination2221 planes {∡ (ZnX\textmeasuredangle2, ZnN2)} is 80.3 –83.4 , in good agreement withMEASURED the ANGLE withwith thethe valuevalue expectedexpected forfor anan idealideal tetrahedraltetrahedral geometrygeometry ofof 90°.90°. AllAll complexescomplexes areare tetrahedrallytetrahedrally value expected for an ideal tetrahedral2222 geometry∢ \textsphericalangle of 90◦. All complexes are tetrahedrally distortedSPHERICAL ANGLE distorteddistorted coordinated,coordinated, whichwhich cancan2223 bebe shownshown∣ byby \textmidthethe structuralstructural parameterparameter ττ44.. AA valuevalue ofof 00 indicatesindicatesDIVIDES aa coordinated, which can be shown by the structural parameter τ4. A value of 0 indicates a square-planar square-planarsquare-planar coordination,coordination, whereaswhereas2224 aa valuevalue∤ closeclose\textnmid toto 11 indicatesindicates aa tetrahedraltetrahedral environmentenvironment [79].[79].DOES InIn NOT DIVIDE coordination, whereas a value close to 1 indicates a tetrahedral environment [79]. In all complexes allall complexescomplexes thethe ττ44-value-value isis betweenbetween2225 0.870.87∥ andand 0.89.0.89.\textparallel TheThe delocalizationdelocalization ofof thethe guanidineguanidine doubledouble bondbondPARALLEL TO the τ4-value is between 0.87 and 0.89. The delocalization of the guanidine double bond is best described isis bestbest describeddescribed byby thethe structuralstructural2226 parameterparameter∦ ρρ [80].\textnparallel[80]. AA valuevalue ofof 11 showsshows aa completelycompletely delocalization.delocalization.NOT PARALLEL TO by the structural parameter ρ [802227]. A value∧ of 1\textwedge shows a completely delocalization. The value couldLOGICAL AND TheThe valuevalue couldcould bebe determineddetermined byby thethe ratioratio ofof thethe CCguagua–N–Nguagua bondbond lengthlength toto thethe sumsum ofof thethe CCguagua–N–Namineamine be determined by the ratio of the2228 Cgua–N∨gua bond\textvee length to the sum of the Cgua–Namine bond. ForLOGICAL all OR bond.bond. ForFor allall complexescomplexes thethe valuevalue isis 0.96/0.97,0.96/0.97, whichwhich showsshows moderatemoderate delocalization.delocalization. TheThe complexes the value is 0.96/0.97,2229 which∩ shows\textcap moderate delocalization. The intraguanidine twistINTERSECTION intraguanidineintraguanidine twist twist is is defined defined as as the the angles angles between between the the planes planes of of N Nguagua–N–Namineamine–N–Namineamine and and C Cguagua–C–Camineamine–– is deﬁned as the angles between222A the planes∪ of\textcup Ngua–Namine–Namine and Cgua–Camine–Camine. InUNIONC1 CCamineamine.. InIn C1C1 thethe averagedaveraged angleangle lies lies inin thethe regionregion ofof 35.6°, 35.6°, whereaswhereas in in C2 C2 andand C3C3 thethe valuevalue is is 13.1° 13.1° and and the averaged angle lies in the region222B of 35.6∫ ◦, whereas\textint in C2 and C3 the value is 13.1◦ and 7.1◦. InINTEGRALC2 7.1°.7.1°. In In C2 C2 and and C3 C3 the the free free rotation rotation in in the the guanidine guanidine moiety moiety is is hindered, hindered, which which leads leads to to smaller smaller angles angles and C3 the free rotation in the guanidine222C ∬ moiety\textiint is hindered, which leads to smaller angles [69DOUBLE–71]. INTEGRAL [69–71].[69–71]. AllAll complexescomplexes werewere fullyfully identifiedidentified byby NMR,NMR, IRIR spectroscopy,spectroscopy, andand MSMS measurements.measurements. TheyThey All complexes were fully identiﬁed222D by NMR,∭ IR\textiiint spectroscopy, and MS measurements. They are stableTRIPLE INTEGRAL areare stablestable underunder airair andand dodo notnot hydrolyze.222Ehydrolyze.∮ \textoint CONTOUR INTEGRAL under air and do not hydrolyze. 222F ∯ \textoiint SURFACE INTEGRAL

2232 ∲ \textointclockwise CLOCKWISE CONTOUR INTEGRAL

2233 ∳ \textointctrclockwise ANTICLOCKWISE CONTOUR INTEGRAL

2234 ∴ \texttherefore THEREFORE 2235 ∵ \textbecause BECAUSE

2236 ∶ \textvdotdot RATIO 2237 ∷ \textsquaredots PROPORTION

2238 ∸ \textdotminus DOT MINUS 2239 ∹ \texteqcolon EXCESS

223C ∼ \textsim TILDE OPERATOR

223D ∽ \textbacksim REVERSED TILDE 2240 ≀ \textwr WREATH PRODUCT

2241 ≁ \textnsim NOT TILDE 2242 ≂ \texteqsim MINUS TILDE FigureFigure 2.2. MolecularMolecularMolecular structuresstructures structures inin thethe solidsolid statestate ofof C1C1––C3C3.. HH H atomsatoms atoms areare are omittedomitted omitted forfor clarity.clarity. 2243 ≃ \textsimeq ASYMPTOTICALLY EQUAL TO 2244 ≄ \textnsimeq NOT ASYMPTOTICALLY EQUAL TO

2245 ≅ \textcong APPROXIMATELY EQUAL TO 2247 ≇ \textncong NEITHER APPROXIMATELY NOR ACTUALLY EQUAL TO

2248 ≈ \textapprox ALMOST EQUAL TO 2249 ≉ \textnapprox NOT ALMOST EQUAL TO

224A ≊ \textapproxeq ALMOST EQUAL OR EQUAL TO 224B ≋ \texttriplesim TRIPLE TILDE

224C ≌ \textbackcong ALL EQUAL TO 224D ≍ \textasymp EQUIVALENT TO

224E ≎ \textBumpeq GEOMETRICALLY EQUIVALENT TO 224F ≏ \textbumpeq DIFFERENCE BETWEEN

2250 ≐ \textdoteq APPROACHES THE LIMIT 2251 ≑ \textdoteqdot GEOMETRICALLY EQUAL TO

2252 ≒ \textfallingdoteq APPROXIMATELY EQUAL TO OR THE IMAGE OF 2253 ≓ \textrisingdoteq IMAGE OF OR APPROXIMATELY EQUAL TO

2254 ≔ \textcolonequals COLON EQUALS 2255 ≕ \textequalscolon EQUALS COLON

2256 ≖ \texteqcirc RING IN EQUAL TO 2257 ≗ \textcirceq RING EQUAL TO

2259 ≙ \texthateq ESTIMATES 225C ≜ \texttriangleeq DELTA EQUAL TO

2260 ≠ \textneq NOT EQUAL TO \textne 2261 ≡ \textequiv IDENTICAL TO

2262 ≢ \textnequiv NOT IDENTICAL TO

39 Inorganics 2017, 5, 85 4 of 19

Table 1. Key bond lengths and angles of the zinc complexes C1 [Zn(TMGca)Cl2], C2 [Zn(DMEGca)Cl2], and C3 [Zn(DMEGca)Br2].

C1 C2 C3

Zn–Ngua [Å] 1.986(3) 2.013(4) 2.006(5) USV SymbolZn–N Macro(s)amine [Å] 2.053(3) 2.065(4)Description 2.041(5) Zn–X [Å] 2.246(2), 2.284(2) 2.261(2), 2.292(2) 2.395(2), 2.434(2) 2220 \textangle◦ ANGLE N∠ amine–Zn–Ngua [ ] 99.8(2) 100.3(2) 101.2(2) ◦ 2221 ∡ (ZnCl\textmeasuredangle2, ZnN2)[ ] 80.3 80.6MEASURED ANGLE 83.4 [a] 2222 ∢ τ\textsphericalangle4 0.89 0.88SPHERICAL ANGLE 0.87 [b] 2223 ∣ \textmidρ 0.97 0.96DIVIDES 0.97 [c] 2224 Guanidine∤ \textnmid twist 35.6 13.1DOES NOT DIVIDE 7.1 ◦ 2225 [a] ∥ = 360 \textparallel−(α+β) α β [b] PARALLEL= 2 TOa τ4 141◦ , = largest angles around metal centre [79]; ρ (b+c) with a = d(Cgua–Ngua) and b 2226 ∦ \textnparallel[c] NOT PARALLEL TO and c = d(Cgua–Namine)[80]; The dihedral angles between the planes represented by Ngua,Namine,Namine and Cgua, 2227 \textwedge LOGICAL AND CAlk,C∧ Alk. Two twist angles for each guanidine moiety. Average value of dihedral angles. 2228 ∨ \textvee LOGICAL OR 22292.2. Density∩ Functional\textcap Theory Calculations INTERSECTION 222A ∪ \textcup UNION

222B The∫ electronic\textint structures of the complexes can be modeledINTEGRAL by DFT calculations to obtain more 222Cprecise insights∬ \textiint into the donor properties of the ligands. A benchmarkDOUBLE INTEGRAL of similar complexes was 222Dperformed∭ in earlier\textiiint reports [69]. All studies were performed byTRIPLE DFT INTEGRAL with the functional TPSSh [81], 222Edef2-TZVP∮ [82]\textoint as basis set, and empirical dispersion correctionCONTOUR withINTEGRAL Becke–Johnson damping 222FGD3BJ [83∯–85] in\textoiint the solvent acetonitrile (as SMD model). The resultsSURFACE INTEGRAL are shown in Table2. The bond 2232lengths are∲ predicted\textointclockwise to be longer than the experimental values,CLOCKWISE although CONTOUR the INTEGRAL relative trend is well 2233 ∳ \textointctrclockwise ANTICLOCKWISE CONTOUR INTEGRAL reproduced. The bond angles and structural parameter τ4 and ρ are in good agreement with the values 2234 ∴ \texttherefore THEREFORE obtained from the crystal structures. The DFT calculations show as well as before that the guanidine 2235 ∵ \textbecause BECAUSE nitrogen atoms lead to shorter Zn–N bond lengths, which shows the stronger donor strength in 2236 ∶ \textvdotdot gua RATIO 2237comparison∷ to the\textsquaredots Namine atom. PROPORTION

2238 ∸ \textdotminus DOT MINUS ◦ 2239Table 2. Calculated∹ \texteqcolon bond lengths [Å] and angles [ ] of C1–C3 [Gaussian09,EXCESS TPSSh, def2-TZVP, GD3BJ, SMD:MeCN].

223C ∼ \textsim TILDE OPERATOR 223D ∽ \textbacksim C1 C2REVERSED TILDE C3 2240 ≀ \textwr WREATH PRODUCT Zn–Ngua 2.009 2.053 2.024 2241 ≁ \textnsim NOT TILDE Zn–Namine 2.080 2.105 2.093 2242 ≂ \texteqsimZn–X 2.302, 2.314 2.303, 2.345MINUS TILDE 2.425, 2.430 2243 ≃ \textsimeqNamineZnNgua 99.3 97.9ASYMPTOTICALLY EQUAL TO99.1 2244 ≄ \textnsimeq(ZnX2, ZnN2) 83.5 75.3NOT ASYMPTOTICALLY EQUAL 88.7 TO [a] 2245 ≅ \textcongτ4 0.86 0.81APPROXIMATELY EQUAL TO 0.89 [b] 2247 ≇ \textncongρ 0.97 0.95NEITHER APPROXIMATELY 0.96 NOR ACTUALLY EQUAL TO [c] 2248 ≈ Guanidine\textapprox twist 35.6 14.5ALMOST EQUAL TO 6.2 ◦ 2249 [a] ≉= 360 \textnapprox−(α+β) α β [b] NOT= ALMOST2a EQUAL TO τ4 141◦ , = largest angles around metal centre [79]; ρ (b+c) with a = d(Cgua–Ngua) and b 224A ≊ \textapproxeq[c] ALMOST EQUAL OR EQUAL TO and c = d(Cgua–Namine)[80]; The dihedral angles between the planes represented by Ngua,Namine,Namine and Cgua, 224B CAlk,C≋ Alk. Two\texttriplesim twist angles for each guanidine moiety. Average value ofTRIPLE dihedral TILDE angles. 224C ≌ \textbackcong ALL EQUAL TO 224D ≍ \textasymp EQUIVALENT TO To get better insights into the donor–acceptor interactions of the complexes, NBO (natural 224E ≎ \textBumpeq GEOMETRICALLY EQUIVALENT TO population analysis) calculations were performed. The optimized structures obtained were used 224F ≏ \textbumpeq DIFFERENCE BETWEEN

2250to calculate≐ the\textdoteq charge transfer energies (by second-order perturbationAPPROACHES THE LIMIT theory) and the NBO 2251charges [≑86–88].\textdoteqdot Normally, these two sets of computed valuesGEOMETRICALLY allow an EQUAL estimation TO of the relative 2252donor strength≒ \textfallingdoteq [78]. The inﬂuence of the electronic effects can beAPPROXIMATELY reﬂected EQUAL well TO OR by THE the IMAGE NBO OF charges, 2253but these≓ do not\textrisingdoteq represent absolute charges. In Table3 the NBOIMAGE charges OF OR APPROXIMATELY on Zn, EQUAL N amine TO , and Ngua 2254are depicted.≔ The\textcolonequals calculated charges on the zinc atoms lie in theCOLON range EQUALS of 1.49–1.56 and the donating 2255 ≕ \textequalscolon EQUALS COLON nitrogen atoms of the primary amines have very strong negative charges (−0.95), whereas the charges 2256 ≖ \texteqcirc RING IN EQUAL TO of the nitrogen atoms of the guanidine moiety are between (−0.76) and (−0.79). The Ngua atom appears 2257 ≗ \textcirceq RING EQUAL TO less basic than the N atoms. This is in contrast to previous studies, where the N donor is more 2259 ≙ \texthateqamine ESTIMATES gua 225C ≜ \texttriangleeq DELTA EQUAL TO

2260 ≠ \textneq NOT EQUAL TO \textne 2261 ≡ \textequiv IDENTICAL TO

2262 ≢ \textnequiv NOT IDENTICAL TO

39 Inorganics 2017, 5, 85 5 of 19 Inorganics 2017, 5, 85 5 of 19

of the nitrogen atoms of the guanidine moiety are between (−0.76) and (−0.79). The Ngua atom appears less basic than the Namine atoms. This is in contrast to previous studies, where the Ngua donor is more basic and the stronger donor when compared to amine donors [69]. In all complexes, the Zn–N bonds basic and the stronger donor when compared to amine donors [69]. In all complexes, the Zn–N bonds were identiﬁed by NBO as covalent bonds; hence, no donor–acceptor interaction energies could be were identified by NBO as covalent bonds; hence, no donor–acceptor interaction energies could be obtained. Together with the short Zn–Ngua bonds, the picture seems mixed: guanidines are strong obtained. Together with the short Zn–Ngua bonds, the picture seems mixed: guanidines are strong donors AND highly basic, but here, NBO predicts that amine will be more basic. Hence, in this donors AND highly basic, but here, NBO predicts that amine will be more basic. Hence, in this case, case, the DFT analysis does not help elucidate the question of the donor strength. Further studies on the DFT analysis does not help elucidate the question of the donor strength. Further studies on amine–guanidine complexes are needed in the future. amine–guanidine complexes are needed in the future.

Table 3. Natural charge on zinc, Ngua,Namine, and halide atoms for complexes C1–C3. Table 3. Natural charge on zinc, Ngua, Namine, and halide atoms for complexes C1–C3.

C1C1 C2 C2 C3 C3 ZnZn 1.561.56 1.561.56 1.491.49 NamineNamine −0.95−0.95 −0.95−0.95 −0.95−0.95 NguaNgua −0.78−0.78 −0.76−0.76 −0.79−0.79 Cl/BrCl/Br −0.85/−0.85/−0.85−0.85 −0.85/−0.85/−0.85−0.85 −0.81/−0.81/−0.81−0.81

2.3. Polymerization Experiments Initially,Initially, complexes C1C1––C3C3 werewere tested tested in in the the activity activity of of the the ring-opening ring-opening polymerization polymerization of oftechnical, technical, unsublimed unsublimed racrac-lactide-lactide under under bulk bulk conditions conditions wi withoutthout any any co-initiator co-initiator at at 150 ◦°CC (polymerization(polymerization methodmethod aa,, SchemeScheme1 1,, Table Table4 ).4). The The use use of of technical technical lactide lactide and and the the high high temperatures temperatures should reﬂectreflect thethe industrialindustrial relevantrelevant conditionsconditions [[99].]. TheThe startstart ofof the measurement isis whenwhen thethe lactide monomer was melted. With one chlorido complex, two more kinetickinetic measurements at 140140 ◦°CC under stirring (400 rpm) were carriedcarried outout (polymerization(polymerization method b and c)) (Table(Table4 4).). Benzyl Benzyl alcohol alcohol was was added to one polymerization to determine if it can open the ring of thethe lactidelactide monomermonomer and to see thethe impactimpact onon thethe molarmolar masses masses (polymerization (polymerization method methodc )c (Table) (Table4). 4). In recentIn recent reports reports [ 70 [70]] it could it could be shownbe shown that that the the quality quality of lactideof lactide (technical (technical or recrystallized)or recrystallized) has has no no major major impact impact on on the the activity activity of theof the polymerization. polymerization. As As a result, a result, in polymerizationin polymerization methods methodsb and b andc different c different qualities qualities of lactide of lactide are usedare used (Table (Table4). More 4). More details details for the for individual the individu measurementsal measurements are shown are shown below below (Table (Table4). After 4). eachAfter measurementeach measurement the conversion the conversion was determined was determined by 1H by NMR 1H NMR or FT-IR or FT-IR spectroscopy spectroscopy and the and molar the molar mass andmass dispersity and dispersity D have D been have measured been measured by GPC by (gel GP permeationC (gel permeation chromatography). chromatography). The reaction The constantreaction kconstantapp equals kapp the equals slope the of slope the linear of the ﬁt linear of the fit semilogarithmicof the semilogarithmic plot of plot the of concentration the concentration against against time. Thetime. technical The technical lactide lactide was stored was stored at −33 at◦ C−33 in °C a glove in a glove box to box guarantee to guarantee the same the conditionssame conditions at every at measurement.every measurement. Every Every measurement measurement has been has performed been performed at least at twice. least twice.

Scheme 1.1. ROP of lactide.

Table 4.4. Polymerization conditionsconditions forfor polymerizationpolymerization methodsmethods aa––cc.. Polymerization Ratio Reaction Stirring Quality Conversion PolymerizationMethod (Monomer:Catalyst:Initiator)Ratio TemperatureReaction (°C) Stirring(rpm) racQuality-Lactide DeterminationConversion Method (Monomer:Catalyst:Initiator) Temperature (◦C) (rpm) rac-Lactide Determination a 500:1:0 150 - technical 1H NMR 1 ba 500:1:01000:1:0 150140 400 - technical technical 1HH NMR 1 cb 1000:1:01000:1:10 140140 400400 recrystallized technical HFT-IR NMR c 1000:1:10 140 400 recrystallized FT-IR

Polymerization method a has been conducted in an oven at 150 °C. First 180–200 mg of finely crushed monomer:catalyst (500:1) were weighed in a 2-mL reaction vessel and sealed under nitrogen.

Inorganics 2017, 5, 85 6 of 19

Polymerization method a has been conducted in an oven at 150 ◦C. First 180–200 mg of finelyInorganics crushed 2017, 5, 85 monomer:catalyst (500:1) were weighed in a 2-mL reaction vessel and sealed6 of 19 under nitrogen. The reaction time was up to 6 h. Figure3 shows the first-order controlled polymerizationThe reaction time catalyzed was up by to the6 h. zinc Figure halide 3 show complexess the first-orderC1–C3. Allcontrolled complexes polymerization show high activity catalyzed at by the zinc halide complexes−5 −1 C1–C3. All complexes show high activity at kapp = (7.3–12.8) × 10−5 s−1. kapp = (7.3–12.8) × 10 s . Complex C3 with bromide as halide showed the fastest reactivity for Complex C3 with bromide as halide− showed5 −1 the fastest reactivity for the ROP of lactide (kapp = 12.8 × the ROP of lactide (kapp = 12.8 × 10 s ) (Figure3 and Table5). After 2 h a conversion of more −5 −1 than10 s 50%) (Figure can be 3, reached. Table 5). It After has to2 h be a conversion noted that of the mo conversionre than 50% for canC1 beand reached.C3 seems It has very to be similar noted afterthat 2the h, whichconversion is related for C1 tothe and non-zero C3 seems intercept very similar of the kinetics after 2 forh, whichC1. This is interceptrelated to may the be non-zero due to initiationintercept by of additionalthe kinetics water for C1 molecules. This intercept from the may technical be due lactide.to initiation The bridgingby additional unit in water the guanidine molecules groupfrom hasthe notechnical influence lactide. on the The polymerization bridging unit activity. in the Allguanidine in all, allgroup three has catalysts no influence polymerize on the in polymerization activity. All in all, all three catalysts polymerize in the similar range of kapp. They show the similar range of kapp. They show similar reaction times. The obtained colorless polymers have asimilar chain lengthreaction between times. The 5000 obtained and 20,000 colorless g/mol, po whichlymers ishave shorter a chain than length the calculated between 5000 molar and masses. 20,000 Theg/mol, use which of technical is shorter lactide than leads the tocalculated more short molar chains masses. due to The the use residual of technical water andlactide other leads impurities, to more whichshort leadschains to due chain to the transfer residual reactions. water Also,and other intra- im andpurities, intermolecular which leads transesterification to chain transfer events reactions. lead toAlso, shorter intra- chains and thanintermolecular expected [ 4transesterification]. events lead to shorter chains than expected [4].

FigureFigure 3. 3.Semilogarithmic Semilogarithmic plotplot ofof thethe polymerization of technical racrac-lactide-lactide [LA]:[catalyst] [LA]:[catalyst] = = 500:1, 500:1, T ◦ 1 T== 150 150 °C,C, conversion conversion determined determined via via 1HH NMR NMR spectroscopy. spectroscopy.

ToTable check 5. Polymerization the impact data of fromthe polymerizationchirality of the method complexesa for complexes on theC1 tacticity–C3 after of two the hours. polymer, homonuclear decoupled 1H NMR spectra were measured for the resulting polymers [31,33]. The [a] k Conversion Mn,exp M probability value ofapp the heterotactic enchainment Pr is betweenn,calcd. 0.54 and 0.58Đ [c] for all complexes,P [d] which −5 −1 (%) [b] [c] (g/mol) r shows an atactic(10 polymers ) with a very slight(g/mol) heterotactic bias (Table 5). One reason why the chiral complexesC1 have no9.9 influence on 64the stereocontrol 20,000 of the polymerization 46,000 is 1.70 that the stereo 0.58 information C2 is not transferred to7.3 the polymer 55 due to a lack 5100 of steric encumbrance 39,500 at high 1.72 temperatures. 0.56 C3 12.8 62 13,000 44,500 1.74 0.54 Conditions: two hours, [M]/[Cat] = 500/1, 150 ◦C, solvent free, technical rac-lactide. [a] Determined from the slope of Table 5. Polymerization data from polymerization method a for complexes C1–C3 after two hours. the plots of ln(1/(1 − C)) versus time; [b] Determined by integration of the methine region of the 1H NMR spectrum; [c] [d] Determined by gel[a] permeation−5 chromatography (GPC) in THF using a viscosimetry detector; Probability of kapp (10 Mn,calcd. racemic enchainment calculated byConversion analysis of (%) the homonuclear[b] Mn,exp decoupled(g/mol) [c]1 H NMR spectra [31,33].Đ [c] Pr [d] s−1) (g/mol) C1 9.9 64 20,000 46,000 1.70 0.58 To checkC2 the impact7.3 of the chirality55 of the complexes 5100 on the tacticity39,500 of the polymer,1.72 homonuclear0.56 decoupledC3 1H NMR12.8 spectra were measured 62 for the resulting 13,000 polymers [ 44,50031,33]. The probability 1.74 0.54 value of theConditions: heterotactic two enchainment hours, [M]/[Cat]Pr is = between500/1, 150 0.54°C, solvent and 0.58 free, for technical all complexes, rac-lactide. which [a] Determined shows an from atactic polymerthe withslope aof very the plots slight of heterotacticln(1/(1−C)) versus bias (Tabletime; [b]5 ).Determined One reason by why integration the chiral of the complexes methine region have no of the 1H NMR spectrum; [c] Determined by gel permeation chromatography (GPC) in THF using a viscosimetry detector; [d] Probability of racemic enchainment calculated by analysis of the homonuclear decoupled 1H NMR spectra [31,33]. Moreover, with C2 further investigations were performed in an oil bath (polymerization method b) and in a jacketed vessel (polymerization method c). The reason why we have chosen C2 for more

Inorganics 2017, 5, 85 7 of 19 inﬂuence on the stereocontrol of the polymerization is that the stereo information is not transferred to the polymer due to a lack of steric encumbrance at high temperatures. InorganicsMoreover, 2017, 5, 85 with C2 further investigations were performed in an oil bath (polymerization method7 of 19 b) and in a jacketed vessel (polymerization method c). The reason why we have chosen C2 for more investigations is that all catalysts show similar reaction constants in polymerization and a higher investigations is that all catalysts show similar reaction constants in polymerization and a higher yield yield can be obtained for C2 (which is important for industrial use). The ratio was 1000:1 can be obtained for C2 (which is important for industrial use). The ratio was 1000:1 (polymerization (polymerization method b) and 1000:1:10 (BzOH) (polymerization method c) to determine the impact method b) and 1000:1:10 (BzOH) (polymerization method c) to determine the impact of the addition of the addition of benzyl alcohol on the polymerization activity. The reaction temperature was 140 of benzyl alcohol on the polymerization activity. The reaction temperature was 140 ◦C, stirring °C, stirring speed 400 rpm, and in polymerization method c an IR spectra was recorded every 5 min. speed 400 rpm, and in polymerization method c an IR spectra was recorded every 5 min. The ﬁnal The final reaction time in b was 13 h and in c 7 h. In polymerization method b technical lactide was reaction time in b was 13 h and in c 7 h. In polymerization method b technical lactide was used used and in polymerization method c recrystallized lactide was used. In a recently published study and in polymerization method c recrystallized lactide was used. In a recently published study it was it was shown that the quality of recrystallized vs. technical lactide makes no difference for our shown that the quality of recrystallized vs. technical lactide makes no difference for our systems [70]. systems [70]. The conversion in polymerization method b was determined via 1H NMR spectroscopy The conversion in polymerization method b was determined via 1H NMR spectroscopy and in and in polymerization method c with FT-IR spectroscopy. Here, the area between 1260 cm−1 and 1160 polymerization method c with FT-IR spectroscopy. Here, the area between 1260 cm−1 and 1160 cm−1 cm−1 shows the C–O–C stretch of the lactide monomer and the C–O–C asymmetric vibrations in the shows the C–O–C stretch of the lactide monomer and the C–O–C asymmetric vibrations in the polymer polymer chain of the polylactide (Figure 4) [89]. After integration over these two regions, the chain of the polylactide (Figure4)[ 89]. After integration over these two regions, the conversion can conversion can be determined. In the recent study we could show that the two different setups (oil be determined. In the recent study we could show that the two different setups (oil bath: conversion bath: conversion determined by 1H NMR spectroscopy (polymerization method b) or jacketed vessel: determined by 1H NMR spectroscopy (polymerization method b) or jacketed vessel: conversion conversion determined by FT-IR spectroscopy (polymerization method c) make no difference to the determined by FT-IR spectroscopy (polymerization method c) make no difference to the polymerization polymerization activity and reaction constant [70]. activity and reaction constant [70].

FigureFigure 4.4. 2D time-resolved FT-IRFT-IR spectraspectra ofof thethe polymerizationpolymerization methodmethodc crac rac-lactide-lactide withwithC2 C2..

TheThe polymerizationpolymerization with with benzyl benzyl alcohol alcohol is is more more than than three three times times faster faster than than without without (Table (Table6 and 6, FigureFigure5 5).). After After 7 7 h h a a conversion, conversion, determined determined by by 11HH NMRNMR spectroscopyspectroscopy upup toto 65%65% inin polymerizationpolymerization methodmethodc ccan can be be obtained, obtained, whereas whereas in polymerization in polymerization method methodb after 13b hafter only 13 66% h canonly be 66% determined. can be Thedetermined. molar masses The molar were masses determined were after determined 7 h (polymerization after 7 h (polymerization method c, M methodn = 8000 c g/mol), Mn = 8000 and g/mol) are in goodand are accordance in good withaccordance the theoretical with the ones theoretical (9400 g/mol). ones The(9400 dispersity g/mol). ofThe 1.15 dispersity shows excellent of 1.15 controlshows forexcellent polymerization control for method polymerizationc. Kinetic investigations method c. Kinetic for polymerization investigations method for polymerizationb show it does method not pass b throughshow it does the origin. not pass The through reason the could origin. be the The additional reason could initiating be the systems additional (e.g., initiating water and systems impurities (e.g., inwater the lactideand impurities monomer), in the which lactide lead monomer), to faster polymerization which lead to faster in the polymerization beginning. The in molar the beginning. masses in The molar masses in polymerization method b of 56,000 g/mol are too low (expected Mn = 91,000 g/mol). One reason could be the additional water residues in the technical lactide monomer. Presumably the recrystallized lactide still has some water residues. The Đ value of 1.57 shows good reaction control. To obtain some insight into the mechanism of the polymerization, end-group analyses were performed with MALDI-ToF measurements. The MALDI-ToF measurements for the polymerization

Inorganics 2017, 5, 85 8 of 19

polymerization method b of 56,000 g/mol are too low (expected Mn = 91,000 g/mol). One reason could be the additional water residues in the technical lactide monomer. Presumably the recrystallized lactide still has some water residues. The Đ value of 1.57 shows good reaction control. Inorganics 2017, 5, 85 8 of 19 To obtain some insight into the mechanism of the polymerization, end-group analyses were performedwith the co-initiator with MALDI-ToF benzyl alcohol measurements. reveal benzyl The al MALDI-ToFcohol at the measurements end of the chain. for It the can polymerization be shown that within polymerization the co-initiator c a benzyl low degree alcohol of revealtransesterification benzyl alcohol is obtained. at the end Ethoxy of the chain.and OH It−can are bethe shown end group that − inof polymerizationthe polymerizationc a low without degree benz of transesteriﬁcationyl alcohol. The ethoxy is obtained. end group Ethoxy stems and from OH theare precipitation the end group of ofthe the polymer polymerization in ethanol without and the benzyl water residue alcohol. is The due ethoxy to the endtechnical group lactide stems monomer from the precipitation (see Figures S1 of theand polymer S2). The inexact ethanol mechanism and the of water these residue complex is duesyst toems the is technicalnot yet known. lactide The monomer complexes (see Figuresdo not act S1 andas a single-site S2). The exact catalyst mechanism since they of cannot these open complex the lactide systems monomer is not yet themselves. known. The As complexesinitiating systems, do not acta co-initiator, as a single-site which catalyst can be since benzyl they alcohol cannot or open water, the has lactide the monomertask of starting themselves. the polymerization As initiating systems,catalyzed a by co-initiator, the zinc whichguanidine can becomplexes. benzyl alcohol In comparison or water, has to other the task zinc of N starting,N-guanidine the polymerization complexes, N N catalyzedthese systems by the show zinc by guanidine far the highest complexes. activity In comparisonin polymerization to other [65,69,70,72]. zinc , -guanidine Ngua,O zinc complexes, chlorido thesecomplexes systems show show the by same far the reaction highest constants activity in bu polymerizationt yield polymers [65,69 with,70,72 molar]. Ngua masses,,O zinc chloridoin good complexesaccordance show with thethe same theoretical reaction ones, constants which but demonstrates yield polymers excellent with molarreaction masses, control in good[71]. accordance with the theoretical ones, which demonstrates excellent reaction control [71]. Table 6. Reaction constants for varying polymerization conditions with C2. Table 6. Reaction constants for varying polymerization conditions with C2. Polymerization Mn,calcd. kapp [a] (10−5 s−1) C (%) [b] Mn,exp. (g/mol) [c] Đ [c] PolymerizationMethod [a] −5 −1 [b] [c] M(g/mol)n,calcd. [c] k (10 s ) C (%) Mn (g/mol) Đ Methodb app 1.4 63 (after 13 h) 56,000,exp. (after 13 h) (g/mol) 91,000 1.57 (after 13 h) bc 1.44.8 6365 (after(after 13 7 h)h) 56,000 8000 (after (after 13 7 h) h) 91,000 9400 1.57 1.15 (after (after 13 7 h)h) [a] Determinedc from the4.8 slope of the 65plots (after of 7 h)ln[1/(1−C)] 8000 versus (after 7 h)time; [b] Determined 9400 by 1.15 1H (after NMR 7 h) [a] [b] 1 spectroscopy;Determined from [c] Determined the slope of by the gel plots permeation of ln(1/(1 − chromatographyC)) versus time; (GPC)Determined in THF by usingH NMR a viscosimetry spectroscopy; [c] Determined by gel permeation chromatography (GPC) in THF using a viscosimetry detector. detector.

Figure 5.5. Polymerization method b andand cc with C2 atat 140140 ◦°C,C, 400 rpm. b = 1000:1, 1000:1, technical technical racrac-lactide.-lactide. cc == 1000:1:10, recrystallized rac-lactide.-lactide.

3. Materials and Methods 3. Materials and Methods All steps were performed under nitrogen (99.996%) dried with P4O10 granulate using Schlenk All steps were performed under nitrogen (99.996%) dried with P4O10 granulate using Schlenk techniques. Solvents were purified according to literature procedures and also kept under nitrogen techniques. Solvents were purified according to literature procedures and also kept under nitrogen [90]. [90]. All chemicals were purchased from Sigma-Aldrich GmbH (Taufkirchen, Germany), TCI GmbH All chemicals were purchased from Sigma-Aldrich GmbH (Taufkirchen, Germany), TCI GmbH (Eschborn, (Eschborn, Germany) and ABCR GmbH (Karlsruhe, Germany) and were used as received without Germany) and ABCR GmbH (Karlsruhe, Germany) and were used as received without further purification. further purification. D,L-lactide (Total Corbion) was stored at −33 °C under an inert atmosphere in a glove box. The precursors of the ligands TMGca and DMEGca were synthesized from (1R, 3S)-(+)- camphoric acid [75]. Ligand L1 was synthesized according to the literature [75]. N,N′- dimethylethylenechloroformamidinium chloride (DMEG-VS) and N,N,N′,N′- tetramethylchloroformamidinium chloride (TMG-VS) were synthesized as described in the literature [76,77].

Inorganics 2017, 5, 85 9 of 19

◦ D,L-lactide (Total Corbion) was stored at −33 C under an inert atmosphere in a glove box. The precursors of the ligands TMGca and DMEGca were synthesized from (1R, 3S)-(+)-camphoric acid [75]. Ligand L1 was synthesized according to the literature [75]. N,N0-dimethylethylenechloroformamidinium chloride (DMEG-VS) and N,N,N0,N0-tetramethylchloroformamidinium chloride (TMG-VS) were synthesized as described in the literature [76,77].

3.1. Physical Methods For L2 mass spectrum was obtained with a ThermoFisher Scientific Finnigan MAT 95 mass spectrometer (Waltham, MA, USA) for HR-EI and for C1–C3 a ThermoFisher Scientific LTQ-Orbitrap XLSpectrometer for HR-ESI. The tube lens voltage lay between 100 and 130 V. FT-IR spectra were measured with a Thermo Scientific Nicolet Avatar 380 spectrometer as KBr pellets (C1–C3) or as film between NaCl plates (L2). NMR spectra were recorded on the following spectrometers: Bruker (Karlsruhe, Germany) Avance II (400 MHz) or Bruker Avance III (400 MHz) (L2, C1–C3). The NMR signals were calibrated to the residual signals of the deuterated solvent (δH(CDCl3) = 7.26 ppm). MALDI-ToF mass spectra were determined on a Bruker Autoflex speed instrument using DCTB (trans-2-[3-(4-tert-Butylphenyl)-2-methyl-2-propenylidene]malonitrile) as matrix and ionized with sodium trifluoroacetate. Spectra were measured in reflector-positive mode. Gel Permeation Chromatography (GPC): The average molar masses and the mass distributions of the obtained polylactide samples of polymerization a,b were determined by gel permeation chromatography (GPC) in THF as mobile phase at a flow rate of 1 mL/min at 35 ◦C. The utilized GPCmax VE-2001 from Viscotek (Waghausel-Kirrlach, Germany) is a combination of an HPLC pump, two Malvern Viscotek T columns (porous styrene divinylbenzene copolymer) with a maximum pore size of 500–5000 Å, a refractive indaex detector (VE-3580), and a viscometer (Viscotek 270 Dual Detector). Universal calibration was applied to evaluate the chromatographic results. GPC analysis of polymerization c was carried out on an Agilent 1260 GPC/SEC MDS (Santa Clara, CA, USA) equipped with two PL MixedD 300 × 7.5 mm columns and a guard column, all heated to 35 ◦C. THF was used as eluent at a flow rate of 1 mL/min. A refractive index (RI) detector was used and this was maintained at 35 ◦C and referenced to 11 narrow polystyrene standards.

3.2. X-ray Analyses The single crystal diffraction data for C1–C3 are presented in Table7. These data were collected on a Bruker D8 goniometer with APEX CCD detector. An Incoatec microsource with Mo-Kα radiation (λ = 0.71073 Å) was used and temperature control was achieved with an Oxford Cryostream 700. Crystals were mounted with grease on glass fibers and data were collected at 100 K in ω-scan mode. Data were integrated with SAINT [91] and corrected for absorption by multi-scan methods with SADABS [92]. The structure was solved by direct and conventional Fourier methods and all non-hydrogen atoms were refined anisotropically with full-matrix least-squares based on F2 (XPREP [93], SHELXS [94] and ShelXle [95]). Hydrogen atoms were derived from difference Fourier maps and placed at idealized positions, riding on their parent C atoms, with isotropic displacement parameters Uiso(H) = 1.2Ueq(C) and 1.5Ueq(C methyl). All methyl groups were allowed to rotate but not to tip. Full crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary no. CCDC–1579451 for C1, CCDC–1579452 for C2 and CCDC–1579453 for C3. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44)-1223-336-033; e-mail: [email protected]). Important crystallographic information on C1–C3 is shown in Table7. Inorganics 2017, 5, 85 10 of 19

Table 7. Crystallographic data and parameters for C1–C3.

C1 C2 C3

Empirical formula C13H28Cl2N4Zn C13H26Cl2N4Zn C13H26Br2N4Zn Formula mass (g·mol−1) 376.66 374.65 463.57 Crystal size (mm) 0.24 × 0.21 × 0.18 0.23 × 0.16 × 0.16 0.23 × 0.20 × 0.19 T (K) 100(2) 100(2) 100(2) Crystal system Orthorhombic Monoclinic Monoclinic Space group P212121 P21 P21 a (Å) 13.478(2) 9.472(2) 9.297(2) b (Å) 15.173(2) 18.095(3) 14.257(3) c (Å) 17.570(2) 10.133(2) 13.200(3) α (◦) 90 90 90 β (◦) 90 103.91(1) 101.84(1) γ (◦) 90 90 90 V (Å3) 3593.1(8) 1685.8(5) 1712.4(6) Z 8 4 4 −3 ρcalcd. (Mg·m ) 1.393 1.476 1.798 µ (mm−1) 1.661 1.770 6.098 λ (Å) 0.71073 0.71073 0.71073 F(000) 1584 784 928 −17 ≤ h ≤ 12, ±20, hkl range ±12, ±24, ±13 ±11, ±17, ±16 −23 ≤ l ≤ 22 Reflections collected 28,044 17,928 20,541 Independent reflections 8953 8244 7045 Rint. 0.0776 0.0556 0.0516 Number of parameters 391 387 385 R1 [I ≥ 2σ(I)] 0.0438 0.0434 0.0455 wR2 (all data) 0.0942 0.0942 0.1164 Goodness-of-fit 0.946 0.985 1.027 Largest diff. peak, hole [eÅ−3] 0.420, −0.413 0.449, −0.427 1.989, −1.153 Absolute structure parameter 0.013(10) 0.000(13) 0.009(13)

3.3. Computational Details Density functional theory (DFT) calculations were performed with the program suite Gaussian 09 rev. E.01 [96]. The starting geometries for all complexes were generated from the molecular structures from the X-ray crystallography data. The Gaussian 09 calculations are performed with the nonlocal hybrid meta GGA TPSSh functional [83–85] and with the Ahlrichs type basis set def2-TZVP [82]. As solvent model, we used the SMD Model (SMD, acetonitrile) [97] as implemented in Gaussian 09. As empirical dispersion correction, we used the D3 dispersion with Becke–Johnson damping, as implemented in Gaussian, Revision E.01 [84,85]. For TPSSh, the values of the original paper have been substituted by the corrected values kindly provided by S. Grimme as private communication [83]. NBO calculations were accomplished using the program suite NBO 6.0 [86–88]. Some of these calculations have been performed within the MoSGrid environment [98–100].

3.4. Polymerization All polymerizations were reproduced at least twice. For polymerization a, the reaction vessels were prepared in a glove box and the technical lactide was stored at −33 ◦C in a glove box. The lactide and the catalyst (500:1) were weighed separately and homogenized in an agate mortar. The reaction mixture was placed in 10 reaction vessels (180–200 mg), tightly sealed and heated at 150 ◦C outside the glove box. The same applies for polymerization b. The ratio was 1000:1. In seven Young ﬂasks 0.5 g of the mixture was added. The tubes, containing a stirring bar (stirring speed = 400 rpm), were heated in an oil bath to 140 ◦C. The polymerization started with the melting point. After stopping the reaction under cool water the conversion was determined by dissolving the sample in 1–2 mL DCM and a 1H NMR spectrum was taken. The PLA was precipitated in ethanol (150 mL) and dried at 50 ◦C. Inorganics 2017, 5, 85 11 of 19

Polymerization kinetics followed by FT-IR spectroscopy (polymerization c) were measured with a Bruker Matrix-MF FT-IR spectrometer equipped with a diamond ATR probe (IN350 T) suitable for Mid-IR in situ reaction monitoring. The kinetics were carried out in a jacketed vessel under Argon counterflow and stirring conditions (mechanical stirring system 400 rpm), and the vessel was connected to a Huber Petite Fleur-NR circulation thermostat to keep the temperature constant (adjusted temperature: 150 ◦C; reaction temperature: 140 ◦C). After heating the recrystallized rac-lactide (0.14 mol) to 140 ◦C, a background spectrum was recorded before the IR probe was placed in the lactide melt. Subsequently −4 −3 Inorganicsthe catalyst 2017, (1.39 5, 85 × 10 mol) and the benzyl alcohol (1.39 × 10 mol) were added under argon11 of to19 the lactide melt. A measurement was recorded every five minutes to see the decrease of the C–O–C vibrationslactide monomer of the peakpolylactide against in the the increase IR spectrum of the C–O–C[89]. The asymmetric evaluating vibrations software ofwas the OPUS polylactide (Bruker in Optikthe IR GmbH spectrum 2014). [89]. The evaluating software was OPUS (Bruker Optik GmbH 2014).

3.5. General Synthesis of Bisguanidine Ligands with Chloroformamidinium Chlorides The corresponding primary diamine (4.26 g, 30 mmol) and triethylamine (3.03 g, 30 mmol) were dissolved in acetonitrile (60 mL). After the addition of a solution of the Vilsmeier salt (30 mmol) in acetonitrile (60(60 mL)mL) dropwise dropwise at at 0 ◦ C,0 °C, the reactionthe reaction mixture mixture was stirredwas stirred at reﬂux at overnight.reflux overnight. The reaction The wasreaction cooled was down cooled and down NaOH and (1.20 NaOH g, 30 (1.20 mmol) g, was30 mmol) added. was The added. solvent The and solvent NEt3 were and evaporatedNEt3 were evaporatedunder a vacuum. under Toa vacuum. complete To the complete deprotonation the deprotonation of the guanidine of the unit,guanidine KOH (20unit, mL, KOH 50 wt(20 %)mL, was 50 wtadded %) was and added the guanidine and the guanidine ligand was ligand extracted was withextracted acetonitrile with acetonitrile (3 × 50 mL). (3 × The50 mL). collected The collected organic organiclayers were layers dried were with dried Na 2withSO4 Naand2SO activated4 and activated carbon andcarbon the solventand the wassolvent evaporated was evaporated under reduced under reducedpressure pressure [76,77]. Ligand [76,77].L1 Ligandwas synthesized L1 was synthesized according according to the literature to the literature [75]. Ligand [75].L2 Ligandis shown L2 inis shownScheme in2. Scheme 2. 3-((1,3-Dimethylimidazolidine-2-ylidene)amin3-((1,3-Dimethylimidazolidine-2-ylidene)amino)-1,2,2-trimethylcyclopentane-1-amineo)-1,2,2-trimethylcyclopentane-1-amineL2 (DMEGca,L2 (DMEGca,Scheme2 ). 1 ◦ Yield:Scheme 78% 2). (5.58Yield: g, 78% 23.40 (5.58 mmol). g, 23.40 Yellow mmol). oil. H Yellow NMR (400oil. 1 MHz,H NMR CDCl (4003, 25MHz,C): CDClδ = 3.703, 25 (dd, °C):J =δ 7.6,= 3.70 3.7 (dd,Hz, 1H),J = 7.6, 3.24 3.7 (m, Hz, 2H), 1H), 3.02 3.24 (m, (m, 2H), 2H), 2.76 3.02 (s, 6H), (m, 2.08 2H), (m, 2.76 1H), (s,1.78 6H), (m, 2.08 2H), (m, 1.56 1H), (m, 1.78 1H), (m, 1.02 2H), (s, 3H),1.56 0.93(m, 13 1 ◦ (s,1H), 3H), 1.02 0.78 (s, 3H), (s, 3H) 0.93 ppm. (s, 3H),C{ 0.78H} NMR(s, 3H) (100 ppm. MHz, 13C{1 CDClH} NMR3, 25 (100C): MHz,δ = 162.2, CDCl 65.3,3, 25 49.4, °C): 45.2,δ = 162.2, 39.9, 65.3, 31.6, 49.4,30.1, 45.2, 29.1, 39.9, 24.6, 17.9,31.6, 9.230.1, ppm. 29.1, IR 24 (NaCl.6, 17.9, plates): 9.2 ppm.νe = IR 2949 (NaCl {s [ νplates):(C–Haliph ν )]}, = 2949 2866 {s {s [ν [(C–Hν(C–Haliphaliph)]},)]}, 2866 2463 {s w,[ν(C–H 1701aliph {vs)]}, [ν (C=N2463 w,gua )]},1701 1560 {vs (m),[ν(C=N 1507gua (s),)]}, 1560 1442 (s),(m), 1396 1507 (s), (s), 1281 1442 (s), (s), 1200 1396 (m), (s), 11201281 (m),(s), 1200 1080 (m), (m), 10311120 (m), (s), 9571080 (m), (m), 931 1031 (m), (s), 906 957 (w), (m), 858931 (w),(m), 762906 (m),(w), 858 721 (w), (w), 762 649 (m), (m), 721 581 (w), (s), 649 473 (m), (vs) 581 cm −(s),1. 473 MS + + + (vs)EI(+): cmm−/1.z MS(%), EI(+): M = Cm13/zH (%),26N4 :M 238.6 = C (1)13H [C26N134:H 238.626N4 ](1), 222.5 [C13H (2)26N [C4]13+, H222.524N3 ](2), 206.5[C13H (2)24N [C3]+12, 206.5H21N 3(2)] , + + + + 181.5[C12H21 (6)N3 [C]+,10 181.5H19N (6)3] [C, 168.510H19N (4)3]+, [C 168.58H16 (4)N4 [C] ,8H 153.416N4] (8)+, 153.4 [C8H (8)15N [C3]8H,15 114.4N3]+, (100)114.4 [C(100)7H 16[CN]7H16,N] 113.4+, 113.4 (55) + + + (55)[C7H [C157N]H15N], 110.4+, 110.4 (9) [C(9)8 H[C148H] 14.] HR-MS+. HR-MS EI(+): EI(+):m /mz/(%),z (%), M M = C=13 CH1326HN26N4:4: calc.: calc.: 238.2157 238.2157 [C [C1313HH2626NN44]]+;; + found: 238.2149 238.2149 [C [C13HH2626NN4]4+]. .

Scheme 2. DMEGca L2.

3.6. General Synthesis of Zinc Halid Halidee Complexes with Guanidine Ligands A solution of zinc halide (1 mmol) was dissolved in 2 mL dry THF and a solution of dissolved ligand (1.2 (1.2 mmol mmol in in 2 2 mL mL THF) THF) was was added added to to the the metal metal salt salt solution. solution. Crystals Crystals could could be beobtained obtained by slowly diffusion of diethyl ether. Complex C1 is shown in Scheme 3, C2 in Scheme 4 and C3 in by slowly diffusion of diethyl ether. Complex C1 is shown in Scheme3, C2 in Scheme4 and C3 in Scheme 5. Scheme5.

2-(3-Amino-2,2,3-trimethylcyclopentyl)-1,1,3,3-tetramethylguanidinedichloridozinc(II) C1 ([Zn(TMGca)Cl2], Scheme 3). Yield: 40% (0.15 g, 0.40 mmol). Colorless crystals. 1H NMR (400 MHz, CDCl3, 25 °C): δ = 3.58 (dd, J = 8.3, 1.1 Hz, 1H), 2.97 (s, 6H), 2.84 (s, 6H), 2.38 (m, 1H), 2.15 (m, 2H), 1.86 (m, 1H), 1.23 (s, 3H), 1.16 (s, 3H), 0.96 (s, 3H) ppm. 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ = 165.0, 70.1, 65.3, 48.7, 40.8, 36.6, 32.3, 27.2, 25.4, 18.6 ppm. IR (KBr): ν = 3414 (m), 3283 (m), 3269 (m), 3233 (vs), 3155 (s), 3011 (m), 2977 (vs), 2953 (vs), 2929 (vs), 2877 (s), 2798 (m), 1594 {vs [ν(C=Ngua)]}, 1559 {vs [ν(C=Ngua)]}, 1524 (vs), 1478 (vs), 1466 (vs), 1408 (vs), 1392 (vs), 1378 (vs), 1371 (vs), 1318 (m), 1295 (m), 1242 (s), 1229 (s), 1168 (s), 1148 (s), 1101 (s), 1079 (s), 1060 (s), 1034 (s), 1023 (s), 1002 (m), 977 (m), 954 (m), 931 (s), 913 (m), 890 (m), 852 (m), 771 (m), 734 (m), 719 (w), 636 (m), 617 (m), 602 (m), 574 (m), 558 (w), 533 (m) cm−1. HR-MS ESI(+): MeCN, m/z (%), M = C13H28Cl2N4Zn: 344.1279 (4) [C1213CH2835ClN468Zn]+, 343.1256 (20) [C13H2837ClN466Zn; C13H2835ClN468Zn]+, 342.1295 (5) [C13H2835ClN467Zn; C1213CH2835ClN466Zn; C1213CH2837ClN464Zn]+, 341.1275 (25) [C13H2835ClN466Zn; C13H2837ClN464Zn]+,

Inorganics 2017, 5, 85 12 of 19

2-(3-Amino-2,2,3-trimethylcyclopentyl)-1,1,3,3-tetramethylguanidinedichloridozinc(II) C1 ([Zn(TMGca)Cl2], 1 ◦ Scheme3 ). Yield: 40% (0.15 g, 0.40 mmol). Colorless crystals. H NMR (400 MHz, CDCl3, 25 C): δ = 3.58 (dd, J = 8.3, 1.1 Hz, 1H), 2.97 (s, 6H), 2.84 (s, 6H), 2.38 (m, 1H), 2.15 (m, 2H), 1.86 (m, 1H), 13 1 ◦ 1.23 (s, 3H), 1.16 (s, 3H), 0.96 (s, 3H) ppm. C{ H} NMR (100 MHz, CDCl3, 25 C): δ = 165.0, 70.1, 65.3, 48.7, 40.8, 36.6, 32.3, 27.2, 25.4, 18.6 ppm. IR (KBr): νe = 3414 (m), 3283 (m), 3269 (m), 3233 (vs), 3155 (s), 3011 (m), 2977 (vs), 2953 (vs), 2929 (vs), 2877 (s), 2798 (m), 1594 {vs [ν(C=Ngua)]}, 1559 {vs [ν(C=Ngua)]}, 1524 (vs), 1478 (vs), 1466 (vs), 1408 (vs), 1392 (vs), 1378 (vs), 1371 (vs), 1318 (m), 1295 (m), 1242 (s), 1229 (s), 1168 (s), 1148 (s), 1101 (s), 1079 (s), 1060 (s), 1034 (s), 1023 (s), 1002 (m), 977 (m), 954 (m), 931 (s), 913 (m), 890 (m), 852 (m), 771 (m), 734 (m), 719 (w), 636 (m), 617 (m), −1 602 (m), 574 (m), 558 (w), 533 (m) cm . HR-MS ESI(+): MeCN, m/z (%), M = C13H28Cl2N4Zn: 13 35 68 + 37 66 35 68 + 344.1279 (4) [C12 CH28 ClN4 Zn] , 343.1256 (20) [C13H28 ClN4 Zn; C13H28 ClN4 Zn] , 342.1295 (5) Inorganics35 2017, 5, 6785 13 35 66 13 37 64 + 35 1266 of 19 [C13H28 ClN4 Zn; C12 CH28 ClN4 Zn; C12 CH28 ClN4 Zn] , 341.1275 (25) [C13H28 ClN4 Zn; C H 37ClN 64Zn]+, 340.1339 (4) [C 13CH 35ClN 64Zn]+, 339.1308 (30) [C H 35ClN 64Zn]+, 241.2402 Inorganics13 28 2017, 54, 85 13 35 64 +12 28 4 35 64 + 13 28 4 12 of 19+ 340.1339 (4) [C12+ CH28 ClN4 Zn] , 339.1308+ (30) [C13H28 ClN4 Zn] , 241.2402+ (100) [C13H29N4]+, (100) [C13H29N4] , 224.2135 (20) [C13H26N3] ; calc.: 339.1294 [C13H28ClN4Zn] , 241.2392 [C13H29N4] , 224.2135 (20) [C13H26N3]+; calc.: 339.1294 [C13H28ClN4Zn]+, 241.2392 [C13H29N4]+, 224.2127 [C13H26N3]+. 13 + 35 64 + 35 64 + + 340.1339224.2127 [C(4)13 H[C2612N3CH] . 28 ClN4 Zn] , 339.1308 (30) [C13H28 ClN4 Zn] , 241.2402 (100) [C13H29N4] , 224.2135 (20) [C13H26N3]+; calc.: 339.1294 [C13H28ClN4Zn]+, 241.2392 [C13H29N4]+, 224.2127 [C13H26N3]+.

Scheme 3. ([Zn(TMGca)Cl2 ])]) C1..

Dichlorido-3-((1,3-DimethylimidazolidSchemeine-2-ylidene)amino)-1,2,2-trimethylcyclopentane-1-aminezinc(II) 3. ([Zn(TMGca)Cl2]) C1. C2 Dichlorido-3-((1,3-Dimethylimidazolidine-2-ylidene)amino)-1,2,2-trimethylcyclopentane-1-aminezinc(II) C2 2 1 ([Zn(DMEGca)Cl ], Scheme 4). Yield: 65% (0.24 g, 0.65 mmol). Colorless crystals. H NMR (4001 MHz, Dichlorido-3-((1,3-Dimethylimidazolid([Zn(DMEGca)Cl2], Scheme4 ). Yield:ine-2-ylidene)amino)-1,2,2-trimethylcyclopentane-1-aminezinc(II) 65% (0.24 g, 0.65 mmol). Colorless crystals. H NMR C2 CDCl3, 25 °C): δ = 3.74 (t, J = 6.7 Hz, 1H), 3.50 (m, 2H), 3.35 (m, 2H), 3.10 (s, 6H), 2.50 (s, 2H), 2.22 (m, ◦ 1 ([Zn(DMEGca)Cl(400 MHz, CDCl2],3, Scheme 25 C): 4δ). =Yield: 3.74 65% (t, J (0.24= 6.7 g, Hz, 0.65 1H),mmol). 3.50 Colorless (m, 2H), crystals. 3.35 (m, H 2H), NMR 3.10 (400 (s, MHz, 6H), 4H), 1.23 (s, 3H), 1.16 (s, 3H), 0.95 (s, 3H) ppm. 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ = 163.7, 68.7, CDCl2.50 (s,3, 25 2H), °C): 2.22 δ = 3.74 (m, 4H),(t, J = 1.236.7 Hz, (s, 1H), 3H), 3.50 1.16 (m, (s, 2H), 3H), 3.35 0.95 (m, (s, 2H), 3H) ppm.3.10 (s, 136H),C{1 H}2.50 NMR (s, 2H), (100 2.22 MHz, (m, 65.5, 49.9, 48.4, 38.5, 36.7, 33.1, 26.4, 25.1, 18.4 ppm. IR (KBr): ν = 3442 (w), 3233 (m), 3155 (w), 2965 ◦ 13 1 4H),CDCl 1.233, 25 (s, 3H),C): δ1.16= 163.7,(s, 3H), 68.7, 0.95 65.5,(s, 3H) 49.9, ppm. 48.4, C{ 38.5,H} NMR 36.7, (100 33.1, MHz, 26.4, CDCl 25.1,3, 25 18.4 °C): ppm. δ = 163.7, IR (KBr): 68.7, (m), 2872 (w), 1583 {vs [ν(C=Ngua)]}, 1501 (m), 1466 (m), 1424 (m), 1405 (m), 1382 (w), 1288 (m), 1254 65.5,ν = 344249.9, (w),48.4, 323338.5, (m),36.7, 315533.1, (w),26.4, 296525.1, (m),18.4 ppm. 2872 (w),IR (KBr): 1583 {vsν = [3442ν(C=N (w), 3233)]}, 1501(m), 3155 (m), (w), 1466 2965 (m), (w),e 1168 (w), 1148 (w), 1123 (w), 1102 (w), 1081 (w), 1061 (w), 1040 (w), 992gua (vw), 963 (w), 942 (vw), (m),1424 2872 (m), (w), 1405 1583 (m), {vs 1382 [ν(C=N (w), 1288gua)]}, (m),15011254 (m), (w),1466 1168(m), (w),1424 1148(m), 1405 (w), 1123(m), 1382 (w), (w), 1102 1288 (w), (m), 1081 1254 (w), 868 (vw), 807 (vw), 726 (vw) cm−1. HR-MS ESI(+): MeCN, m/z (%), M = C13H26Cl2N4Zn: 365.1527 (25) (w),1061 1168 (w), (w), 1040 1148 (w), (w), 992 (vw),1123 (w), 963 1102 (w), 942(w), (vw),1081 (w), 868 (vw),1061 (w), 807 1040 (vw), (w), 726 992 (vw) (vw), cm− 9631. HR-MS (w), 942 ESI(+): (vw), [C13H2737ClN4Na66Zn; C13H2735ClN4Na68Zn]+, 363.1539 (35) [C13H2735ClN4Na66Zn; C13H2737ClN4Na64Zn]+, −1 37 66 35 68 + 868MeCN, (vw),m /807z (%), (vw), M 726 = C 13(vw)H26 Clcm2N.4 HR-MSZn: 365.1527 ESI(+): (25) MeCN, [C13H m27/z (%),ClN 4MNa = CZn;13H C26Cl13H2N274Zn:ClN 365.15274Na Zn] (25), 361.1571 (60) [C13H2735ClN4Na64Zn]+, 343.1056 (10) [C13H2637ClN468Zn]+, 341.1079 (40) [C13H2637ClN466Zn; 37 66 35 35 66 68 + 37 64 + 35 66 3537 6464 + [C363.153913H27 ClN (35)4Na [C13Zn;H27 C13ClNH274NaClN4Zn;Na CZn]13H,27 363.1539ClN4Na (35)Zn] [C13H, 361.157127 ClN4Na (60)Zn; [C13 CH13H2727 ClNClN44NaNa Zn] ,, C13H2635ClN468Zn]+, 339.1098 (60) [C13H2635ClN466Zn; C13H2637ClN464Zn]+, 337.1131 (65) 3537 6864 ++ 3737 68 66 + 35 68 37+ 66 361.1571343.1056 (60) (10) [C [C1313HH2726 ClNClN4Na4 Zn]Zn] , 343.1056 341.1079 (10) (40) [C [C1313HH2626 ClNClN4 4Zn]Zn;, 341.1079 C13H26 (40)ClN [C4 13Zn]H26 ClN, 339.10984 Zn; [C13H2635ClN464Zn]+, 239.2226 (100) [C13H27N4]+, 222.1961 (55) [C13H24N3]+; calc.: 361.1108 35 6835 + 66 37 64 +35 66 37 6435 + 64 + C(60)13H26 [CClN13H426 Zn]ClN, 4 339.1098Zn; C13 H26(60)ClN [C4 13Zn]H26 ,ClN 337.11314 Zn; (65)C13H26 [C13ClNH264 Zn]ClN, 4 Zn]337.1131, 239.2226 (65) [C13H27ClN4NaZn]+, 337.1132 [C13H26ClN4Zn]+, 239.2230 [C13H27N4]+, 222.1970 [C13H24N3]+. 35 64 + + + + + [C(100)13H26 [CClN13H427NZn]4] ,, 222.1961239.2226 (55)(100) [C 13[CH1324HN273N] 4;] , calc.:222.1961 361.1108 (55) [C[C13H13H2724ClNN3]4;NaZn] calc.: ,361.1108 337.1132 + + + + + + + [C[C13HH2726ClNClN4NaZn]4Zn] , 239.2230, 337.1132 [C [C1313HH2726NClN4] 4,Zn] 222.1970, 239.2230 [C13 H[C2413NH327]N. 4] , 222.1970 [C13H24N3] .

Scheme 4. ([Zn(DMEGca)Cl 2]) C2.

Scheme 4. ([Zn(DMEGca)Cl2]) C2. Dibromido-3-((1,3-DimethylimidazolidSchemeine-2-ylidene)amino)-1,2,2-trimethylcyclopentane-1-aminezinc(II) 4. ([Zn(DMEGca)Cl2]) C2. C3 ([Zn(DMEGca)Br2]). Yield: 43% (0.20 g, 0.43 mmol). Colorless crystals. 1H NMR (400 MHz, CDCl3, 25 Dibromido-3-((1,3-Dimethylimidazolidine-2-ylidene)amino)-1,2,2-trimethylcyclopentane-1-aminezinc(II) C3 °C): δ = 3.88 (d, J = 10.3 Hz, 1H), 3.54 (m, 2H), 3.35 (m, 2H), 3.13 (s, 6H), 2.36 (m, 2H), 2.09 (m, 1H), ([Zn(DMEGca)Br2]). Yield: 43% (0.20 g, 0.43 mmol). Colorless crystals. 1H NMR (400 MHz, CDCl3, 25 1.86 (m, 1H), 1.61 (b. s, 2H), 1.24 (s, 3H), 1.19 (s, 3H), 0.95 (s, 3H) ppm. 13C{1H} NMR (100 MHz, CDCl3, °C): δ = 3.88 (d, J = 10.3 Hz, 1H), 3.54 (m, 2H), 3.35 (m, 2H), 3.13 (s, 6H), 2.36 (m, 2H), 2.09 (m, 1H), 25°C): δ = 163.8, 68.9, 65.9, 49.9, 48.5, 39.0, 36.6, 33.1, 26.2, 25.1, 18.7 ppm. IR (KBr): ν = 3279 (m), 3226 1.86 (m, 1H), 1.61 (b. s, 2H), 1.24 (s, 3H), 1.19 (s, 3H), 0.95 (s, 3H) ppm. 13C{1H} NMR (100 MHz, CDCl3, (m), 3146 (m), 2960 (m), 2925 (m), 2871 (m), 1586 {vs [ν(C=Ngua)]}, 1505 (m), 1458 (m), 1421 (m), 1397 25°C): δ = 163.8, 68.9, 65.9, 49.9, 48.5, 39.0, 36.6, 33.1, 26.2, 25.1, 18.7 ppm. IR (KBr): ν = 3279 (m), 3226 (m), 1384 (m), 1291 (m), 1225 (m), 1167 (m), 1147 (m), 1120 (m), 1079 (m), 1034 (m), 966 (w), 945 (w), (m), 3146 (m), 2960 (m), 2925 (m), 2871 (m), 1586 {vs [ν(C=Ngua)]}, 1505 (m), 1458 (m), 1421 (m), 1397 859 (vw), 800 (vw), 703 (vw) cm−1. HR-MS ESI(+): MeOH, m/z (%), M = C13H26Cl2N4Zn: 386.1358 (<1) (m), 1384 (m), 1291 (m), 1225 (m), 1167 (m), 1147 (m), 1120 (m), 1079 (m), 1034 (m), 966 (w), 945 (w), [C13H2681BrN467Zn; C1213CH2681BrN466Zn; C1213CH2679BrN468Zn]+, 385.0566 (<1) [C13H2681BrN466Zn; 859 (vw), 800 (vw), 703 (vw) cm−1. HR-MS ESI(+): MeOH, m/z (%), M = C13H26Cl2N4Zn: 386.1358 (<1) C13H2679BrN468Zn]+, 384.0989 (<1) [C13H2679BrN467Zn; C1213CH2679BrN466Zn; C1213CH2681BrN464Zn]+, [C13H2681BrN467Zn; C1213CH2681BrN466Zn; C1213CH2679BrN468Zn]+, 385.0566 (<1) [C13H2681BrN466Zn; 383.0598 (<1) [C13H2679BrN466Zn; C13H2681BrN464Zn]+, 382.0663 (<1) [C1213CH2679BrN464Zn]+, 381.0622 (<1) C13H2679BrN468Zn]+, 384.0989 (<1) [C13H2679BrN467Zn; C1213CH2679BrN466Zn; C1213CH2681BrN464Zn]+, [C13H2679BrN464Zn]+, 239.2227 (100) [C13H27N4]+, 222.1963 (72) [C13H24N3]+; calc.: 381.0632 383.0598 (<1) [C13H2679BrN466Zn; C13H2681BrN464Zn]+, 382.0663 (<1) [C1213CH2679BrN464Zn]+, 381.0622 (<1) [C13H26BrN4Zn]+, 239.2236 [C13H27N4]+, 222.1970 [C13H24N3]+. [C13H2679BrN464Zn]+, 239.2227 (100) [C13H27N4]+, 222.1963 (72) [C13H24N3]+; calc.: 381.0632 [C13H26BrN4Zn]+, 239.2236 [C13H27N4]+, 222.1970 [C13H24N3]+.

Inorganics 2017, 5, 85 13 of 19

Dibromido-3-((1,3-Dimethylimidazolidine-2-ylidene)amino)-1,2,2-trimethylcyclopentane-1-aminezinc(II) C3 1 ([Zn(DMEGca)Br2]). Yield: 43% (0.20 g, 0.43 mmol). Colorless crystals. H NMR (400 MHz, CDCl3, 25 ◦C): δ = 3.88 (d, J = 10.3 Hz, 1H), 3.54 (m, 2H), 3.35 (m, 2H), 3.13 (s, 6H), 2.36 (m, 2H), 2.09 (m, 1H), 13 1 1.86 (m, 1H), 1.61 (b. s, 2H), 1.24 (s, 3H), 1.19 (s, 3H), 0.95 (s, 3H) ppm. C{ H} NMR (100 MHz, CDCl3, ◦ 25 C): δ = 163.8, 68.9, 65.9, 49.9, 48.5, 39.0, 36.6, 33.1, 26.2, 25.1, 18.7 ppm. IR (KBr): νe = 3279 (m), 3226 (m), 3146 (m), 2960 (m), 2925 (m), 2871 (m), 1586 {vs [ν(C=Ngua)]}, 1505 (m), 1458 (m), 1421 (m), 1397 (m), 1384 (m), 1291 (m), 1225 (m), 1167 (m), 1147 (m), 1120 (m), 1079 (m), 1034 (m), 966 (w), −1 945 (w), 859 (vw), 800 (vw), 703 (vw) cm . HR-MS ESI(+): MeOH, m/z (%), M = C13H26Cl2N4Zn: 81 67 13 81 66 13 79 68 + 386.1358 (<1) [C13H26 BrN4 Zn; C12 CH26 BrN4 Zn; C12 CH26 BrN4 Zn] , 385.0566 (<1) 81 66 79 68 + 79 67 13 79 66 [C13H26 BrN4 Zn; C13H26 BrN4 Zn] , 384.0989 (<1) [C13H26 BrN4 Zn; C12 CH26 BrN4 Zn; 13 81 64 + 79 66 81 64 + C12 CH26 BrN4 Zn] , 383.0598 (<1) [C13H26 BrN4 Zn; C13H26 BrN4 Zn] , 382.0663 (<1) 13 79 64 + 79 64 + + [C12 CH26 BrN4 Zn] , 381.0622 (<1) [C13H26 BrN4 Zn] , 239.2227 (100) [C13H27N4] , 222.1963 Inorganics 2017, 5, 85+ + + 13+ of 19 (72) [C13H24N3] ; calc.: 381.0632 [C13H26BrN4Zn] , 239.2236 [C13H27N4] , 222.1970 [C13H24N3] .

Scheme 5. ([Zn(DMEGca)Br([Zn(DMEGca)Br2])]) C3C3..

4.4. Conclusions Conclusions TwoTwo differentdifferent monoamine-guanidine monoamine-guanidine hybrid hybrid ligands ligands have have been been synthesized synthesized and with and zinc with halides zinc halidesthree new three chiral new complexes chiral complexes were obtained. were obtained. The obtained The obtained crystals crystals were fullywere characterized fully characterized by X-ray by X-raydiffraction, diffraction, NMR, NMR, IR spectroscopy, IR spectroscopy, and mass and spectrometry. mass spectrometry. DFT calculations DFT calculations were performed were performed and are andin good are in accordance good accordance with the with solid the state solid structures, state structures, whereas whereas the NBO the analysis NBO analysis showed ashowed larger negativea larger negativecharge than charge the than guanidine the guanidine nitrogen nitrogen atom. In atom. the solvent-free In the solvent-free polymerization polymerization of technical of technicalrac-lactide, rac- lactide,the robust the robust and chiral and complexeschiral complexes show show great grea activityt activity and produceand produce colorless colorless polymers polymers at 150 at 150◦C. °C.One One selected selected complex complex was was tested tested with with the the addition addition of of a a co-initiator co-initiator andand recrystallizedrecrystallized lactide to determinedetermine the the impact of the co-initiator. We We used two different setups: in in the the polymerization of of technicaltechnical lactidelactide we we used used Young Young tubes tubes in an oilin bathan oil (conversion bath (conversion determined determined via 1H NMR via spectroscopy) 1H NMR spectroscopy)and in the polymerization and in the polymerization with recrystallized with lactiderecrystallized and benzyl lactide alcohol and benzyl we used alcohol a jacketed we used vessel a jacketedand the vessel conversion and the was conversion determined was via determined FT-IR spectroscopy. via FT-IR spectroscopy. The reaction The with reaction benzyl with alcohol benzyl is alcoholseveral timesis several faster times than faster without. than End-group without. analysisEnd-group via analysis MALDI-ToF via MALDI-ToF measurements measurements showed that showedthe co-initiator that the has co-initiator opened the has lactide opened monomer the lact andide leads monomer to only and a small leads degree to only of transesteriﬁcations.a small degree of transesterifications.All complexes show All high complexes stability, show are robust, high stabil andity, generate are robust, colorless and generate polymers. colorless In comparison polymers. to Inother comparison zinc N,N -guanidineto other zinc complexes, N,N-guanidine these systemscomplexes, show these by farsystems the highest show activityby far the in highest polymerization. activity inThey polymerization. provide an excellent, They provide sustainable an excellent, alternative sustainable to the conventionally alternative usedto the catalyst, conventionally tin octanoate. used catalyst, tin octanoate. Supplementary Materials: The following are available online at www.mdpi.com/2304-6740/5/4/85/s1. Cif and cif-checked ﬁles. Figure S1: MALDI-ToF measurement for polymerization a after 1 h. Conditions: Supplementary Materials: The following◦ are available online at www.mdpi.com/2304-6740/5/4/85/s1. Cif and 500:1 ([Zn(TMGca)Cl2], C1), 150 C, technical rac-lactide. Mn = 10,000 g/mol, Mw = 17,000 g/mol, Đ = 1.72. cif-checked files. Figure S1: MALDI-ToF measurement for polymerization a after 1 h. Conditions: 500:1 Figure S2: MALDI-ToF measurement for polymerization c after 7 h. Conditions: 1000:1 ([Zn(DMEGca)Cl2], ◦ ([Zn(TMGca)ClC2):10 [BzOH],2 140], C1),C, 150 400 °C, rpm, technical recrystallized rac-lactide.rac-lactide. Mn = 10,000Mn = 8000g/mol, g/mol, Mw = M17,000w = 9500 g/mol, g/mol, Đ = Đ1.72.= 1.15. Figure S2: MALDI-ToF measurement for polymerization c after 7 h. Conditions: 1000:1 ([Zn(DMEGca)Cl2], C2):10 [BzOH], Acknowledgments: Angela Metz thanks the RWTH Aachen (Erasmus+ Praktikumsförderung) for funding. 140The °C, authors 400 rpm, thank recrystallized Total Corbion rac for-lactide. the generous Mn = 8000 donation g/mol, M ofwrac = -lactide9500 g/mol, and Đ A. = Ohligschläger 1.15. for help with the preparation of Figure4. The authors thanks the Regional Computing Center of the University of Cologne (RRZK) Acknowledgments: Angela Metz thanks the RWTH Aachen (Erasmus+ Praktikumsförderung) for funding. The for providing computing time on the DFG-funded High Performance Computing (HPC) system CHEOPS as well authorsas support, thank the Total MoSGrid Corbion project for andthe thegenerous Leibniz donation Rechenzentrum of rac-lactide München and forA. computingOhligschläger time. for help with the preparation of Figure 4. The authors thanks the Regional Computing Center of the University of Cologne (RRZK) for providing computing time on the DFG-funded High Performance Computing (HPC) system CHEOPS as well as support, the MoSGrid project and the Leibniz Rechenzentrum München for computing time.

Author Contributions: Angela Metz, Matthew D. Jones, and Sonja Herres-Pawlis conceived and designed the experiments. Angela Metz, Joshua Heck, Clara Marie Gohlke, Konstantin Kröckert, and Yannik Louven performed and analyzed the experiments and data. Paul McKeown did the MALDI-ToF measurements. Alexander Hoffmann finalized the X-ray data and corrected the manuscript. Angela Metz, Matthew D. Jones, and Sonja Herres-Pawlis wrote the paper.

Conflicts of Interest: The authors declare no conflict of interest.

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