homoleptic zinc complex formation: synthesis, structures and application in ROP of lactides.

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Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/

Designing of ancillary ligand for heteroleptic/homoleptic zinc complexes formation: synthesis, structures and application in ROP of lactides D. Jędrzkiewicz,a J. Ejfler,a* N. Gulia,a Ł. Johna and S. Szaferta Synthesis and characterization of a series of new amino-phenol/naphthol ligands (L1,2-H), have been developed and their respective zinc complexes (1, 2-Zn) have been synthesized. The molecular structure of L1-H and 1, 2-Zn explored in detail by NMR, single-crystal X-ray studies and DFT calculations, confirmed existence of complexes as stabile dimers both in a solution and in a solid state. All complexes mediate the ring - opening polymerization (ROP) of lactide highly efficiently, at room temperature, in a controlled fashion. The influence of architecture of the ligand on the desired homo/heteroleptic complex formation, as well as the relationship between the initiator design and the catalytic activity have been investigated.

Introduction The coordination chemistry of zinc compounds, with functionalized aminophenolate ligands has been intensively studied over recent years, due to the fact that they form excellent initiators for ring opening polymerization (ROP) of 1 cyclic esters. Polylactide (PLA) constitute one of the most prominent products of ROP processes, among the numerous polyesters studied so far, mostly due to their broad range of applications as a commodity polymer for packaging (bottles, thin films), fibres (tissue, clothes), and for biomedical applications as bioresorbable sutures, screws, orthopedic implants, drug delivery carriers, or scaffolds for tissue 2 engineering. The most attractive zinc compounds for ROP of cyclic esters are “single-site” initiators based on heteroleptic 1,3-5 complexes of the general formula L-M-OR. The ancillary ligands (L) constitute the key component, yet they are very hard to match, as they should be large enough to stabilize and secure the metal center against reactions leading to deactivation of an active compound. On the other hand, they should be small enough to enable easy coordination of monomer molecule for further reactions. The recognition of an ideal couple ligand-metal center, according to these requirements, provides the possibility to synthesize “by design” new highly active, well-behaved initiators. An excellent work has been done in synthesis of monoanioning

ligand families, such as β-diketiminate, tris(pirazolyl)borate, 3-5 and related N-, O-donor ligands. Over the last years, the aminophenolate ligands have worked most favorably as a “scaffolding” for a wide variety of metal complexes, used as 5 initiators in ROP of cyclic esters. The application of ancillary aminophenolate ligands allows easy control of their coordination properties by backbone variation, introducing substituents on phenol rings and constructing hemilabile or bulky arms on nitrogen atom. All examples have indicated that the presence of appropriate, sizable substituents on nitrogen atoms and simultaneously in ortho position of phenols, improve steric protection of the metal center. Well-defined single-site initiators, supported by zinc aminophenolates confirmed by X-ray analysis usually present dimeric and 5e, 5l, 6 considerably rarer monomeric structures. Most of them are generated in situ in a direct reaction between [LZnR]2 and 5g-l alcohols. But this promising synthetic strategy for zinc aminophenolates may be damaged by Schlenk-type equilibria, 3k,7 redistribution or bis-chelatation reactions. We have been working on synthesis of the main group of 8 metal complexes containing N-, O-donor ligands. This report presents examinations of the reciprocal influence of substituents located on phenol and amine arm, with a view to building an appropriate structural motif of zinc complexes, which will act as active catalysts in ROP of lactides. Our study extensively correlates the experimental outcomes with DFT calculations in rationalization of the synthesis design. Herein, we have described the synthesis and characterization of zinc complexes with the aminophenolate ligands and their application as initiators for lactide polymerization.

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Experimental

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All reactions and operations were performed under inert atmosphere of N2, while using a glove-box (MBraun) or standard Schlenk techniques. Reagents were purified by standard methods: toluene, distilled from Na; CH2Cl2, distilled from P2O5; hexanes, distilled from Na; methanol, distilled from Mg; benzyl alcohol (Aldrich; >99%), distilled prior to use; C6D6, distilled from CaH2. L-LA ((3S)-cis-3,6-dimethyl-1,4-dioxane2,5-dione) (98%; Aldrich) was sublimed and recrystallized from toluene prior to its use. ZnEt2 (1.0 M solution in hexanes), Nmethylcyclohexylamine, formaldehyde (37% solution in H2O), 1-naphtol, 4-tert-butylphenol were purchased from Aldrich 1 13 and used as received. H and C NMR spectra were detected at 298 K, while using Bruker ESP 300E or 500 MHz spectrometers. Chemical shifts are reported in parts per million and referenced to residual protons in deuterated solvents. The number-average molar mass (Mn) and the molecular weight distribution index of the samples were determined by gel-permeation chromatography (GPC). The system was composed of a Viscotek VE 1122 solvent delivery system and a Shodex SE-61 refractive index detector. The analyses were performed in chloroform at 35 °C at a flow rate of 1 mL/min with the aid of a PLgel 3 µm MIXED-E (Polymers Laboratories) high-efficiency column (300mm × 7.5mm). The injection volume was 100 µL of the sample in chloroform (0.3% w/v). Polystyrene standards (Polymer Laboratories) with narrow molar mass distributions were used to generate a calibration curve and to compute the average molar masses and dispersities of the sample. Microanalyses were conducted with an ARL Model 3410 + ICP spectrometer (Fisons Instruments) and a VarioEL III CHNS (in-house). Theoretical diffusion coefficients (D) have been calculated for solid-state and DFT optimized structures of 1, 2-Zn, following Einstain-Stokes equation, corrected by a factor derived from 15 microfrictional theory and semi-empirically improved by 16 Chen eq. (1), where k is Boltzmann’s constant, T is the 17 temperature, η is the viscosity of deutered benzene . Spherical equivalent radii (Req) have been calculated from vdW volumes limited by van der Waals surface (V ) and volumes SES limited by Connolly surface (V , SES – solvent excluded surface) following eq. (2), where M is molecular weight of solute and eq. (3) respectively. Van der Waals and Connolly 18 volumes have been calculated by using Jmol software. Connolly surfaces have been generated with benzene probe 19 radius 2.7Ǻ. Corrected by shape factor theoretical diffusion coefficients (Df) were derived by means of using eq. (4) where f(p) is the shape factor and p is the geometrical factor for molecules regarded as ellipsoids ( = ⁄ , a – semi-major 20 axis, b – semi-minor axis). Semi axes have been measured with GaussView 3.09 program. For all structures prolate shape (with two short and one long axes) was assumed and calculated by eq. (5).

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General materials, methods and procedures


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Syntheses N-[methyl(2-hydroxy-5-tert-butylphenyl)]-N-methyl-N1 cyclohexylamine (L -H). To a solution of 1.15 g (7.67 mmol) of 4-tert-butylphenol and 1.00 mL (7.67 mmol) of Ncyclohexylmethylamine in MeOH (50 mL), 0.80 mL (10.64 mmol) of formaldehyde (37% solution in H2O) was added. The solution was stirred and heated under reflux for 24 h, until a crude product precipitated as a white solid. It was collected by filtration, washed with cold methanol and dried in vacuo to 1 give L -H. Yield 89% (1.88 g, 6.82 mmol). Anal. Calcd. (Found) for C18H29NO: C, 78.49 (78.39); H, 10.61 (10.93); N, 5.09 (5.05) + 1 %; ESI/MS: 276.3 [M+1] ; H NMR (500 MHz, C6D6, RT): δ = 11.28 (br, s, 1H, OH), 7.18 (dd, JHH = 8.4, 2.5 Hz, 1H, ArH), 7.11 (d, JHH = 8.4 Hz, 1H, ArH), 7.02 (d, JHH = 2.4 Hz, 1H, ArH), 3.51 (s, 2H, N-CH2-Ar), 2.21 (tt, JHH = 11.3, 3.3 Hz, 1H, N-CH), 1.91 (s, 3H, N-CH3), 1.60 – 1.49 (m, 4H, CH2), 1.45 – 1.36 (m, 1H, CH2), 13 1.31 (s, 9H, C(CH3)3), 1.02 – 0.78 (m, 5H, CH2); C NMR (75 MHz, C6D6, RT): δ = 157.2 (ArC-OH, 1C), 141.3 (ArC-C, 1C), 125.8 (ArCH, 1C), 125.2 (ArCH, 1C), 121.8 (ArC-CH2, 1C), 116.3 (ArCH, 1C), 62.1 (N-CH, 1C), 58.1 (N-CH2, 1C), 36.3 (N-CH3, 1C), 34.1 (C(CH3)3, 1C), 31.9 (C(CH3)3, 3C), 28.1 (CH2, 2C), 26.3 (CH2, 1C), 25.9 (CH2, 2C). 2 N-[methyl(1-naphtol)]-N-methyl-N-cyclohexylamine (L -H). 1.11 g (7.67 mmol) of 1-naphthol was dissolved in 50 mL of methanol. Then 1.0 mL (7.67 mmol) of Nmethylcyclohexylamine and 0.80 mL (10.64 mmol) of 37% aqueous solution of formaldehyde were added. After 15 minutes a crude product precipitated as a white solid. It was collected by filtration, washed with cold methanol and dried in 2 vacuo to give L -H. Yield 72% (1.49 g, 5.52 mmol). Anal. Calcd. (Found) for C18H23NO: C, 80.26 (80.18); H, 8.61 (8.76); N, 5.20 + 1 (5.17) %; ESI/MS: 270.2 [M+1] ; H NMR (500 MHz, C6D6, RT): δ = 12.03 (br, s, 1H, OH), 8.74 (d, JHH = 8.4 Hz, 1H, ArH), 7.71 (d, JHH = 8.2 Hz, 1H, ArH), 7.40 – 7.35 (m, 1H, ArH), 7.34 – 7.30 (m, 2H, ArH), 6.96 (d, JHH = 8.3 Hz, 1H, ArH), 3.54 (s, 2H, CH2N), 2.19 (tt, JHH = 11.2, 3.4 Hz, 1H, N-CH), 1.90 (s, 3H, N-CH3), 1.59 – 1.49 (m, 4H, CH2), 1.44 – 1.35 (m, 1H, CH2), 0.99 – 0.76 (m, 13 5H, CH2); C NMR (126 MHz, C6D6, RT) δ = 155.0 (s, 1C, ArCOH), 134.6 (s, 1C, ArC), 127.7 (s, 1C, ArCH), 126.9 (s, 1C, ArCH), 126.2 (s, 1C, ArCH), 126.0 (s, 1C, ArC), 125.0 (s, 1C, ArCH), 123.0 (s, 1C, ArCH), 118.3 (s, 1C, ArCH), 114.6 (s, 1C, ArC-CH2), 62.2 (s, 1C, N-CH), 57.7 (s, 1C, N-CH2), 36.3 (s, 1C, N-CH3), 28.1 (s, 2C, CH2), 26.2 (s, 1C, CH2), 25.9 (s, 2C, CH2). 1 [1-Zn]. To a solution of L -H (0.55 g, 2.00 mmol) in hexanes (50 mL) ZnEt2 (2 mL, 2.00 mmol) was added drop-wise at room

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temperature. The solution was stirred until a white solid precipitated. It was filtered off, washed with hexanes (10 mL) and dried in vacuo. The resulting white solid was recrystallized from toluene at -15 °C to give colourless crystals. Yield 80% (0.59 g, 0.8 mmol). Anal. Calcd. (Found) for C40H68N2O2Zn2: C, 1 65.12 (64.96); H, 9.02 (9.12); N, 3.80 (3.77) %; H NMR (500 MHz, C6D6, RT): Major form: δ = 7.40 (dd, JHH = 8.4, 2.6 Hz, 2H, ArH), 7.13 (s, 2H, ArH), 7.12 (d, JHH = 8.6 Hz, 2H, ArH), 4.59 (d, JHH = 11.8 Hz, 2H, N-CH2-Ar), 3.33 (d, JHH = 11.9 Hz, 2H, N-CH2Ar), 3.27 (tt, JHH = 11.9, 9.1 Hz, 2H, N-CH), 1.84 (s, 6H, N-CH3), 2.24 – 0.81 (m, 20H, CH2), 1.35 (s, 18H, C(CH3)3), 1.34 (t, JHH = 8.1 Hz, 6H, CH2-CH3), 0.33 (q, JHH = 8.1 Hz, 4H, CH2-CH3); Minor form: δ = 7.36 – 7.33 (m, 2H, ArH), 7.09 – 7.04 (m, 4H, ArH), 4.20 (d, JHH = 13.0 Hz, 2H, N-CH2-Ar), 3.59 (d, JHH = 13.0 Hz, 2H, N-CH2-Ar), 2.51 (s, 6H, N-CH3), 2.41 (tt, JHH = 11.6, 8.9 Hz, 2H, N-CH), 2.34 – 0.73 (m, 20H, CH2), 1.35 (s, 18H, C(CH3)3), 1.39 (t, JHH = 7.9 Hz, 6H, CH2-CH3), 0.43 (q, JHH = 7.3 Hz, 4H, CH2-CH3); 13 C NMR (75 MHz, C6D6, RT): δ = 161.7 (ArC-OH, 2C), 140.2 (ArC-C, 2C), 128.6 (ArCH, 2C), 127.3 (ArCH, 2C), 125.1 (ArC-CH2, 2C), 120.0 (ArCH, 4C), 64.4 (N-CH, 2C), 59.6 (N-CH2, 2C), 35.9 (N-CH3, 2C), 34.1 (C(CH3)3, 2C), 32.0 (C(CH3)3, 6C), 26.4 (CH2, 10C), 13.5 (CH2-CH3, 2C), -1.4 (CH2-CH3, 2C). 2 [2-Zn]. To a solution of L -H (0.54 g, 2.00 mmol) in CH2Cl2 (30 mL) ZnEt2 (2 mL, 2.00 mmol) was added drop-wise at room temperature. The solution was stirred until a white solid precipitated. It was filtered off and dried in vacuo. Recrystallization of the product from toluene at -15 °C give [2Zn]. Yield 80% (0.58 g, 0.8 mmol). Anal. Calcd. (Found) for C40H54N2O2Zn2: C, 66.21 (66.05); H, 7.50 (7.55); N, 3.86 (3.81) 1 %; H NMR (500 MHz, C6D6, RT): Major form: δ = 8.79 (d, JHH = 8.3 Hz, 2H, ArH), 7.80 (d, JHH = 8.1 Hz, 2H, ArH), 7.50 – 7.37 (m, 2H, ArH), 7.36 – 7.29 (m, 4H, ArH), 7.09 (d, JHH = 8.2 Hz, 2H, ArH), 5.05 (d, JHH = 11.9 Hz, 2H, N-CH2-Ar), 3.53 (d, JHH = 12.0 Hz, 2H, N-CH2-Ar), 3.45 (tt, JHH = 11.9, 3.2 Hz, 2H, N-CH), 1.55 (s, 6H, N-CH3), 1.92 – 0.46 (m, 20H, CH2), 1.27 (t, JHH = 8.1 Hz, 6H, CH2-CH3), 0.34 – 0.21 (m, 4H, CH2 -CH3); Minor form: δ = 9.01 (d, JHH = 8.3 Hz, 2H, ArH), 7.74 (d, JHH = 8.1 Hz, 2H, ArH), 7.59 – 7.35 (m, 2H, ArH), 7.36 – 7.28 (m, 4H, ArH), 7.03 (d, JHH = 8.2 Hz, 2H, ArH), 4.43 (d, JHH = 11.6 Hz, 2H, N-CH2-Ar), 3.17 (d, JHH = 11.7 Hz, 2H, N-CH2-Ar), 2.70 (s, 6H, N-CH3), 2.15 (tt, JHH = 11.6, 3.1 Hz, 2H, N-CH), 2.33 – 0.48 (m, 20H, CH2), 0.90 (t, JHH = 13 7.0 Hz, 6H, CH2-CH3), -0.28 (q, JHH = 13.0, 4H, CH2-CH3); C NMR (75 MHz, C6D6, RT): δ = 160.2 (ArC-OH, 2C), 140.9 (ArC, 2C), 130.5 (ArCH, 2C), 129.3 (ArCH, 2C), 127.5 (ArCH, 2C), 125.7 (ArC, 2C), 124.5 (ArCH, 2C), 123.6 (ArCH, 2C) 117.8 (ArCH, 2C), 113.4 (ArC-CH2, 2C), 63.8 (N-CH, 2C), 58.8 (N-CH2, 2C), 35.4 (NCH3, 2C), 25.9 (CH2, 10C), 13.5 (CH2-CH3, 2C), -0.9 (CH2-CH3, 2C). Representative procedure for solution polymerization In a typical polymerization experiment, the solution of a metal complex (I) in CH2Cl2 were placed in a Schlenk flask and external alcohol in stoichiometric amount [I]/[ROH] = 0.5/1 was added. Then, after 10 minutes, monomer L-LA was added at a fixed molar ratio (0.5/n/1; n = 30, 60, 100). The resulted solution was stirred at room temperature for a prescribed

time. At certain time intervals (10 min), about 1 mL aliquots were removed, precipitated with hexanes and dried in vacuo. 1 The conversion was determined while observing H NMR resonances of the polymer and monomer by dissolving the precipitates in C6D6. After reaction was completed an excess of hexanes was added to the reaction mixture. Filtration and vacuum drying yielded a white polymer. The resulting solid was dissolved in CH2Cl2, and the polymer was precipitated with an excess of cold methanol. The polymer was collected by filtration, washed with methanol to remove the non-reacted monomer, and dried under reduced pressure. The reaction mixtures were prepared in a glove-box, then subsequent operations were performed by means standard Schlenk techniques. (representative procedure for 1-Zn: [I]/LLA/[MeOH] = 0.5/30/1; 1-Zn (0.09 g, 0.12 mmol), L-LA (1.01 g, 7.00 mmol), MeOH (10.00 µL, 0.24 mmol). Details of X-ray data collection and reduction X-ray diffraction data for a suitable crystal of each sample were collected using a KUMA KM4 CCD Saphire or Xcalibur CCD Onyx or Ruby (see Table S12) with ω scan technique at 100 K. The data collection and processing utilized CrysAlis suite 9 of programs. Space groups were determined, based on systematic absences and intensity statistics. Lorentz polarization corrections were applied. The structures were solved by direct methods and refined by full-matrix least2 squares on F . All calculations were performed using the 10 SHELXTL-2013 suite of programs. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atom positions were calculated with geometry and not allowed to vary. Thermal ellipsoid plots were prepared with 50% of probability displacements for non-hydrogen 11 atoms, by using Mercury 3.1 program. All data have been deposited with the Cambridge Crystallographic Data Centre 1 CCDC-1046687 for 2-Zn, 1046688 for 1-Zn, -1046691 for L -H. Copies of the data can be obtained free of charge by application to CCDC, 12 Union Road, Cambridge CB21EZ, UK or e-mail: [email protected]. Computational details All density functional theory (DFT) calculations were 12 performed with Gaussian 03 program suite. The geometries of the complexes and ligands were optimized (Tables, in the Supporting Information) by using B3LYP density functional theory and the 6-31G** basis sets, implemented in the 13,14 Gaussian 03 on all atoms. The starting geometries of complexes 1-Zn, 2-Zn and 3-Zn were generated from their crystal structures, whereas the starting geometries of adequate monomers were derived from their optimized complexes. Structures of all isomers of the calculated complexes were first optimized in vacuo. Next, to obtain results more relevant to the experiment, calculations were performed in the presence of a solvent, while using the Polarizable Continuum Model (PCM) as SCRF method. Frequency calculations confirmed the stationary points to be minimal on PES.

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1,2


The aminophenolate proligands L -H were synthesized through one-pot Mannich condensation reactions while using para- substituted phenols, formaldehyde and appropriate amines, as described in previous literature and in the 8 experimental section. The compounds were obtained in high yields (72 - 89 %) and characterized both by standard 1 elemental analysis, MS and spectroscopic methods. The H and

1

C { H}NMR techniques afforded well resolved resonances for all proton and carbon environments, indicating high quality and purity of isolated proligands (Fig. S1 – S4). Single crystals 1 of L -H, suitable for X-ray crystallography were grown from concentrated cold methanol solutions. The reactions of 1,2 stoichiometric amounts of L -H with ZnEt2 (1/1) in hexanes under ambient conditions led to isolation of molecular heteroleptic zinc complexes 1-Zn and 2-Zn (Scheme 1).

Scheme 1. Synthetic procedure for zinc complexes 1 – 3-Zn.

On the other hand our previously published study indicated that the similar aminophenolate proligand L3-H with ortho- and para positions, substituted with bulky tert-butyl groups, easily gave homoleptic complex 3-Zn.5c What is more under a wide range of experimental conditions (time, temperature, solvents, molar ratio) used for the reaction of L3-H with ZnEt2, brought about isolation of only homoleptic complexes. Moreover, new L1-H proligands immediately form exclusively heteroleptic (LZnEt)2 dimer, even if reactions are carried out in excess of ligand as has been done by us, and on this very basis conclusions have been drawn, namely, that it seems to be impossible to obtain homoleptic L2Zn compound. The method of complex isolation is undemanding because complex is wellsoluble in toluene, THF, dichloromethane and insoluble in aliphatic hydrocarbons. Precipitation of the desired product impels a proper reaction course. Whereas, free non-reacted ligand or ZnEt2 can be easily removed from the reaction mixture by filtration, when hexane is adopted as a solvent. The free orthoposition of aminophenolates is crucial for their high tendency to bridge metal centers, and stabilization of dimeric (LZnEt)2 complexes. It is, of course, a well-known behaviour for simple alkoxy compounds but the details for synthesis of appropriate ancillary ligands (LZnEt)2 “by design” are not so

clear. The balance between solubility of substrates/byproducts/final compounds and their mutual reactivity during the time of reaction decide about crystallization of more or less expected complexes. Yet, there is a difference between synthesis conducted in accordance with an assumed structural motive of the complex, and synthesis of the desired compound, obtained by chance in forced conditions of a reaction. The inability of some aminophenolate ligands to stabilize heteroleptic complexes renders them as unsuitable support for “single-site” initiators L-Zn-OR. Therefore, to determine how large substituents must be to suppress bis-chelatation, we have modified hydroxyl group environments in orthoposition, by introducing 2D planar bulky obstruction. New aminonaphtol ligand L2-H again forms only heteroleptic zinc complex 2-Zn. The structure of zinc complexes in a solution was characterized by 1H, 13C{1H} NMR spectroscopy. The 1H NMR spectrum of 1Zn showed the three broad resonances associated with aromatic protons at δ = 7.42, 7.13 and 7.12 ppm. The next sharp signals represent amine methyl and tert-butyl protons, appearing at 1.84 ppm and 1.35 ppm, respectively. The representative doublets for methylene (Ar-CH2-N) protons were observed at 4.59 ppm and 3.33 ppm, and were

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informative for establishing the formation of the complex between zinc center and aminophenolate ligands. Yet, between them there are additional signals indicating another form of 1-Zn. All possible dimer isomers have been generated through the transformation of zinc and nitrogen atoms configuration (∆, Λ and R, S) and are presented in Figure 1 (for more detailed schemes see Supplementary Information, S15 – S16). t-Bu t-Bu

N

O O

N N

t-Bu

O O

N

t-Bu

1-ZnA (

1-ZnB (

RS)

SR)

Figure 2. Fragment of 1H NMR spectrum of 1-Zn; major isomer 1-ZnA – red, minor isomer – green.

N

N

ZnN OO

t-Bu

t-Bu

1-ZnC (

RR,

SS)

ZnN OO

t-Bu

t-Bu

1-ZnD (

SS,

RR)

t-Bu

N

O O

N N

t-Bu 1-ZnE (

ZnN OO

t-Bu RR,

SS)

1-ZnF (

t-Bu RS,

RS)

Figure 1. Schematic structure of possible dimer isomers for 1-Zn; major isomer 1ZnA; red - oxygen atoms, green – zinc atoms, blue – nitrogen atoms

5.0

4.5

4.0

3.5 3.0 ppm

2.5

2.0

1.5

Figure 3. Fragment of 1H NMR spectrum of 2-Zn; major isomer 2-ZnA – red, minor isomer – green.

1

The H NMR spectroscopy revealed 1-Zn to exist in a solution, as a mixture of two isomers in the ratio of ca. 6:1, with the major component which is believed to be 1-ZnA (Fig. 1, 2, S5). 1 The H NMR spectra confirmed that both species have an identical skeleton. Zinc ethyl group resonances were observed in the spectrum as a quartet and triplet at 0.33 ppm and 1.34 ppm, respectively. 1 The H NMR spectra of complex 2-Zn are similar and confirm the mixture of isomers in the ratio of ca. 3:1 (Fig. 3, S7). The indicative doublets for methylene (Ar-CH2-N) protons were observed at 5.05 ppm and 3.53 ppm for the major form of 2-Zn and 4.43 and 3.17 ppm for minor isomer. All possible dimer isomers of 2-Zn are presented in Supplementary Information, S17 – S18.

The NOESY and DOSY NMR measurements were conducted to explore nuclearity of these complexes in a solution. The data show that zinc aminophenolate/aminonaphtalate 1-Zn and 2Zn present a dimeric structure in the solution. The NOESY spectrum shows interactions between aromatic protons and cyclohexyl ring which are possible only in dimeric structures (Fig. 4, S10, S12). Additionally there are chemical exchange signals between the major and minor isomer in the NOESY spectrum, which suggests possible transformation between them (orange peaks, signals d-d’ and f-f’. Fig 4). The diffusion coefficients obtained from DOSY experiments (Fig. S13, S14) are comparable to literature value for dimeric aminophenolate species.5d

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1

2

2

f

d'

f'

2

ppm

2

1

1

Figure 4. Fragment of 2D NOESY spectrum for 1, 2-Zn complexes.

Although molecular structure in the solution of the obtained zinc complexes could be more or less established, yet, the coordination motive in the solid state could not be deducted only by means of spectroscopic data support. Fortunately, we were able to grow suitable crystals for X-ray analysis. The procedure was simple and the X-ray quality crystals of the new complexes were grown via slow evaporation of saturated toluene solutions, obtained by redesolution of powders of appropriate complexes synthesized in hexanes. What is more, suitable monocrystals for crystallography experiments appeared only in several hours. The solid state structures of 1 proligand L -H and 1, 2-Zn complexes shown in Figures 5 – 6 and Table 1 summarizes crystal data for all zinc complexes structures reported in this paper.

Molecular structures of the crystalline centrosymmetric compounds 1-Zn, non-centrosymmetric 2-Zn are similar to each other, and form dimers with four coordinated zinc centers, bridged through phenolate oxygen atoms with two ethyl groups located trans to each other. All complexes show zinc fused six-membered chelating ring around the central Zn-O-Zn-O rhomboid. This core is a common structural motive in zinc coordination chemistry, frequently found in zinc-aminophenolate species. The chelating six-member ring includes metal surrounded by phenyl oxygen and nitrogen atoms of aminophenolate ligands with twisted boat-shaped conformation (Fig. 7). For reasons of clarity, ligands atoms are not shown in the figure. The central Zn2O2 core is nearly planar with a sum of internal angles equal to 360° (Table 1).

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Figure 5. Molecular structures of L1-H and 1-Zn. The thermal ellipsoids are drawn at 50% probability level, H atoms are excluded for clarity.

Table 1. Selected bond distances (A) and angles (°).

Atoms

Zn1-C21 Zn2-C51 Zn1-O1 Zn2-O2 Zn1-O1i Zn2-O1 Zn1-N1 Zn2-N2 Zn1-Zn1i,a i,a O1-O1

Figure 6. Molecular structure of 2-Zn. The thermal ellipsoids are drawn at 50% probability level, H atoms are excluded for clarity.

The central zinc atoms display a distorted tetrahedral coordination geometry. The bound distances between zinc and O, N, C atoms are typical and range 2.010(2)-2.077(2), 2.111(8)-2.144(8) and 1.975(11)-2.004(12) Å, respectively. In the core rhomboid Zn2O2 of the all complexes the non-bonding short contact interactions Zn-Zn 3.094(1)-2.941(1) and O-O 2.708(2)-2.838(2) Å have been observed. These distances and angles are all comparable with bond lengths, observed for the examples described in literature.5e-f, 5l, 6 The interposition of 2D-hindrance in 2-Zn provoked the drift between phenol and cyclohexyl rings in comparison to 1-Zn (Fig. 7).

1-Zn

2-Zn

Bond distance [Å] 1.997(2) 2.010(2) 2.077(2) 2.114(2) 2.941(1) 2.838(2) Angels [°] 123.91(8)

2.004(12) 1.975(11) 2.053(6) 2.049(6) 2.062(7) 2.058(7) 2.144(8) 2.111(8) 3.091(2) 2.708(9)

C21-Zn1-O1 128.2(4) C51-Zn2-O2 120.1(4) C21-Zn1-O1i 118.42(8) 124.1(4) C51-Zn2-O1 126.8(3) O1-Zn1-O1i 87.95(7) 82.3(2) O1-Zn2-O2 82.5(3) C21-Zn1-N1 129.74(8) 118.5(4) C51-Zn2-N2 121.1(4) O1-Zn1-N1 92.75(7) 92.0(3) O2-Zn2-N2 94.7(3) O1i-Zn1-N1 93.27(7) 102.9(3) O1-Zn2-N2 101.8(3) Zn1-O1-Zn1i 92.05(7) 97.5(3) Zn1-O2-Zn2 97.5(3) Sum of the interior angles of the core Zn2O2 [°] 360 359.8(1) i = −x+2, −y+1, −z+1 for 1-Zn; O1i = O2, Zn1i = Zn2 for 2-Zn a

Values in parentheses refer to the non-bonding interactions.

J. Name., 2013, 00, 1-3 | 7





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Journal Name

In order to acquire a more detailed picture of the processes which take place during the synthesis of zinc aminophenolate/aminonaphtolate complexes, theoretical calculations have been carried out (see the details in the Experimental Section and Supporting Information). The structures of zinc complexes 1, 2-Zn have been examined while using B3LYP density functional theory and the 6-31G** basis sets, implemented in the Gaussian 03 suite programs. Geometry optimizations have been performed while using the coordinates from X-ray data as the starting point for 1, 2-Zn complexes. Additionally, the potential monomeric form of zinc 1-2 compounds containing L ligands have been investigated by means of DFT calculations. The results of the optimization have been presented in the Tables S1 – S4 while the optimized geometrical structures have been displayed in Figs. S15 – S18 (Supporting Information). The optimized geometrical structures of 1-Zn and 2-Zn complexes are in agreement with their solid-state structures. The analysis of the optimized geometrical parameters of 1-Zn – 2-Zn complexes shows that Zn cations are four-coordinated and are surrounded by four (two O and one N and C atoms) the nearest neighbors in a distorted tetrahedral arrangement. The analysis of M-N and M-O distances and appropriate angles in Table S5, calculated and experimentally measured, confirmed these conclusions. The optimized geometry of complex 1-Zn displays a structure close to the crystallographic one, and we have also found other similar lower energy isomers, which differ mainly in cyclohexyl rings arrangement (see Fig. 8).

150 130 1-ZnF

110 1-ZnD

90

1-ZnB

70 1-ZnE

50 34.60

1-ZnC

30

26.72

1-ZnA

16.67 10.12

10 0.00

4.76

-10

Figure 8. The calculated relative energies in the gas-phase for the geometrically optimized isomers for dimeric 1-Zn species (1-ZnA – F).

8 | J. Name ., 2012, 00, 1-3




Figure 7. Superimposed molecular structures of 1-Zn (green), 2-Zn (blue).

∆ Ezpe (kJ/mol)


DFT calculation


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DOI: 10.1039/C5DT01553G

detected both in the solution and in the solid-state is mostly preferred. To evaluate the preference of dimer structure versus monomer species, the dimerization energy was calculated. The energies for monomers (1-ZnM) are over 141.87 kJ/mol larger than for dimeric complexes (Table S6 – S7, Supporting Information). The adequate energies for monomers and dimers of 2-Zn are lower (115.03 kJ/mol, Tab. S8 – S9) than mentioned before for 1-Zn. The large energy differences between dimers and monomers additionally confirm the proposal that 1, 2-Zn are dimeric in solution (Fig. 9).

280 240 2 L1ZnEt

200 160

2 L2ZnEt

143.87

120

115.03

80 2 L3ZnEt

40 0

0.00

0.00

6.46

0.00

-40 -80 -120 (L1ZnEt)2

(L2ZnEt)2

(L3ZnEt)2

Fig. 9. The calculated relative energies in the gas-phase for the geometrically optimized isomers for 1, 2-Zn species. The boxes (green, blue, red) include the gas-phase geometrically optimized structures, monomers and dimers for zinc complexes.

1,2

Next, possible homoleptic complexes for zinc with L ligands pose an important question stemming from the synthesis, namely, why the final product of the reaction between diethyl 1,2 zinc and L is independent of stoichiometry of the reaction.

The energies of DFT gas-phase geometry optimized structures of homoleptic complexes are presented in Tables S6 – S11 while structures are shown in Fig. 10, respectively.

J. Name ., 2013, 00, 1-3 | 9




ARTICLE

The relative energies ΔEZPE calculated in the gas-phase and in benzene for the geometrically optimized isomers of 1-Zn are presented in Tables S1 – S2, Fig. 8 (data for calculation obtained in benzene see Supporting Information). The lowest ΔE(1-ZnA) value found for a dimeric complex has been adequate to a solid-state structure determined by X-ray crystallography. Similar trends have been observed for all heteroleptic isomers of 2-Zn (data for 2-Zn see Supporting Information, Tab. S3 – S4). The difference between values of relative energies seems to be too small to explain the changes and dynamic behaviours observed in NMR signals correctly. However, the data proved that the dimeric form of complexes

Ezpe (kJ/mol)


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DOI: 10.1039/C5DT01553G

Fig. 10. DFT optimized structures of homoleptic complexes with L1-3.

The relative energy value between homoleptic compounds (L2Zn + ZnEt2, L = L1,2 ) and two monomers (LZnEt, L = L1,2) is again greater than for favourable heteroleptic dimers (L12 ZnEt)2 (see Fig. 11). In all the considered 1-Zn – 2-Zn isomers, theoretical studies significantly confirm experimental data, indicating that only heteroleptic dimers may act as potential pre-catalysts in ROP reactions. 3 The aminophenolate ligand L , with bulky substituents, located in ortho position of phenol rings form homoleptic complex

easily. The bis-chelate, homoleptic 3-Zn complex is also correctly described by a theoretical method as distorted tetrahedron, coordinated by two oxygen and nitrogen atoms of aminophenolate ligand. Thus, it was interesting to verify their stabilization versus possible generated heteroleptic 3 analoques. Surprisingly, the energies of monomers ΔE(L -Zn3 3 Et), ΔE[(L )2Zn + ZnEt2] and dimers ΔE(L ZnEt)2 are similar, however, the energy for a homoleptic complex 3-Zn is the lowest (Fig. 11, red spherules)

320 280 240 200 160

L12 Zn + ZnEt2 143.87

L2 2Zn + ZnEt2 139.87

120 2 L1ZnEt

115.03

112.76

80 2 L2ZnEt

40 0

0.00

0.00

L32Zn + ZnEt2 6.46

0.00

-1.80

3

-40

(L ZnEt)2 (L1ZnEt)2

(L2ZnEt)2

-80

2 L3ZnEt

Fig. 11. The calculated relative energies in the gas-phase for the geometrically optimized zinc homoleptic species with L1-3 ligands (in boxes) and heteroleptic species.

10 | J. Name ., 2012, 00, 1-3




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∆Ez pe (kJ/mol)


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ARTICLE

The calculated small difference between the energy of homoleptic monomers and heteroleptic dimers for zinc 3 complexes with L -H ligand explains their possible coexistence in the solution, and transformation into each other, which is easy even with a slightly smaller barrier. For example, precipitation of bis-chelate complex is enough to change the course of the reaction. The interposition of hindrance in orthoposition of phenol ring also provoked drift the cyclohexyl substituent located on nitrogen atom (Fig. 12), heteroleptic zinc complex with di-tert-butylphenol (red lines in the Fig. 11) generated by DFT presents the most “open” and labile structure. Whereas, aminonaphtalene species 2-Zn is exactly at the premium balance between labile 3-Zn and stabile 1-Zn.

adequate diffusion coefficients for 1-Zn and 2-Zn were estimated from their solid-state structures determined by Xray crystallography, as well as from DFT studies. In the first vdW stage we calculated theoretical diffusion coefficients D SES (using volume limited by van der Waals surface) and D (using volume limited by the Connolly surface), for details see experimental section and supporting information Fig. S19 – S23. The surfaces (van der Walls and Connolly) generated around structures (X-ray and its DFT optimized structure) of the discussed complexes have been presented in the Fig. 13, (data for all 1-Zn and 2-Zn species are presented in the supporting data Fig. S19 – S23).

Fig. 12. Superimposed structures of geometrically optimized heteroleptic dimers representing drift between phenol and cyclohexyl rings 1-Zn (green), 2-Zn (blue), (L3ZnEt)2 (red).

Moreover, the 1-Zn and 2-Zn complexes prefer heteroleptic dimeric structure, this dependence is reproduced by DFT calculations, the energy for monomeric forms is evidently higher. Additionally, we have found several preferred isomers in the DFT calculations, some of which in the solid state and the existing mixture of dimers in the solution, which was suggested by DFT calculations as well. The proposed dimeric structures of 1-Zn and 2-Zn compounds in the solution were confirmed from diffusion-ordered NMR spectroscopy data. The estimated translation diffusion coefficient Dexp for 1-Zn in deuterated benzene was equal to -10 2 6.65·10 m /s and the corresponding value for 2-Zn was -10 2 6.83·10 m /s. In order to compare these experimental data,

Figure 13. Van der Waals and Connolly surfaces generated around solid-state of 1-Zn and DFT optimized 1-ZnA structures with probe radius 2.7Ǻ. vdW

SES

Next, both D and D theoretical diffusion coefficients have been corrected by the shape factor because molecules should be regarded as prolate-shaped. All calculated theoretical vdW SES coefficients D , D and their corrected values marked as vdW SES Df and Df , compared with Dexp obtained from DOSY NMR experiments, are presented in Table 2.

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Table 2. Van der Waals volumes VvdW, Connolly volumes VSES, theoretical diffusion coefficients DvdW, DSES and theoretical diffusion coefficients corrected by shape factor DfvdW, DfSES for solid-state and DFT optimized strucures of 1, 2-Zn


V

3

[Å ]

SES

V

[Å ]

[10

-10

vdW

SES

2

m /s]

[10

-10

SES

Df

D

D

3

2

m /s]

[10

-10

2

m /s]

[10

-10

Df 2

m /s]

1-Znxray

619.28

751.41

6.87

6.90

6.73

6.76

1-ZnA

645.98

818.02

6.75

6.66

6.64

6.55

1-ZnB

646.76

822.19

6.75

6.64

6.58

6.48

1-ZnC

646.86

847.32

6.75

6.56

6.59

6.40

1-ZnD

648.06

823.73

6.74

6.64

6.69

6.59

1-ZnE

646.49

821.07

6.75

6.65

6.64

6.54

1-ZnF

647.34

831.87

6.75

6.61

6.60

6.47

1-ZnM

327.3

379.45

12.84

9.34

12.26

8.93

xray

583.6

709.85

7.10

7.07

7.05

7.02

2-ZnA

609.73

750.31

6.97

6.90

6.91

6.84

2-ZnB

610.11

746.74

6.96

6.92

6.85

6.81

2-ZnC

609.54

770.40

6.97

6.83

6.89

6.75

2-ZnD

610.84

754.94

6.96

6.89

6.81

6.74

2-ZnE

609.85

751.79

6.97

6.90

6.89

6.83

2-ZnF

610.04

759.90

6.96

6.87

6.83

6.74

2-ZnM

308.78

350.34

13.36

9.70

12.57

9.13

2-Zn

Both the calculated and experimentally obtained values of diffusion coefficients are similar, which suggests the presence of dimeric structures of the analyzed complexes in the solution. The comparison of the adequate data for dimers and hypothetic monomers is important for this “case study”. Therefore, we have calculated theoretical diffusion coefficients for 1-ZnM and 2-ZnM, generated by DFT study as model complexes. The higher values D(Df) calculated for monomers: -10 -10 1-ZnM, 12.84, (12.26) · 10 , 2-ZnM, 13.36 (12.57) · 10 (for -10 van der Walls) and 1-ZnM, 9.34 (8.93) · 10 , 2-ZnM, 9.70 -10 (9.13) · 10 (for Connolly) indicate that monomers are probably absent (unvisible) in the solution. The obtained values of diffusion coefficient for monomeric species are 5d comparable with examples described in literature (D=12.49). Then, comparable values of D(Df) for all possible dimers, generated by theoretical study and estimated by NMR and Xray study, suggest that the solid-state dimeric structure is retained in solution. In order to acquire a more detailed picture of the structures presented in the solution analyzed by NMR study, and to compare it with the view to checking whether it is the same in a solid state, the analysis of the calculated theoretical diffusion coefficients using DFT optimized structures proved to be better than the application of only X-ray data for this very purpose. Polymerization activity. The ability of the homoleptic 3-Zn to promote the living ROP of L-LA was tested and published on a previous occasion in our 5c,8c laboratory. The 3-Zn in the presence of alcohols, show high

exp

Df [10

-10

2

m /s]

6.65

-

6.83

-

catalytic activity and adequate control over ROP parameters. We expected that with the application of heteroleptic aminophenolate/aminonaphtolate complexes we could obtained initiators for ROP of lactides at a higher level. The new ligand design strategy focuses on modelling accessibility to zinc centre for stable heteroleptic dimer formation. These structural motive (L-Zn-R)2 is the most desirable and expected pre-initiator for ROP of lactide, proposed by other research groups working with ancillary ligands containing N,O-donor 5 atoms. However, modification is always proposed on nitrogen substituents with unchanged di-tert-butylphenol core. But subtle perturbation in proximity of hydroxyl group may be more interesting for designing of new ancillary ligand. Therefore, we have decided to retain cyclohexyl ring and estimate appropriate substituent in ortho position of phenol. 3 The sizable tert-butyl group gave ligand L capable of producing a mixture of complexes (homo/heteroleptic). 1 Whereas, ligand with free ortho positions (L ) prefers stable 2 dimers, and aminonaphthalene ligand L seems to be the optimum solution, which easily forms dimeric ethyl zinc complex 2-Zn with lower stability than 1-Zn. The classical living ROP of cyclic esters may proceed in the presence of pre-catalyst LnM-R which is transformed into active LnM-OR alkoxide species, during in situ alcoholysis reaction and our new complexes 1, 2-Zn fulfill these requirements. Benzyl alcohol is routinely applied in this type of polymerization and therefore we have decided to use this alcohol for polymerization tests of our pre-catalysts.

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For the determination of catalytic activity of 1, 2-Zn in ROP of the L-lactide, the reactions of zinc initiators [Zn] and monomer [L-LA]/[Zn]) in different molar ratios in solution of CH2Cl2 at room temperature have been studied. After the reaction time monitored by NMR spectroscopy the polymers precipitated in

cold methanol, which is a common way of isolation of polylactides. According to NMR study, all ROP reaction achieved full conversion, the obtained polymers showed narrow PDI and molecular weight values close to those expected from the [L-LA]/[Zn] molar ratio (see Table 3).

Entry/[I]

ROH

[I]/[L-LA]/ROH

1/1-Zn 2/1-Zn 3/1-Zn 4/1-Zn 5/1-Zn 6/2-Zn 7/2-Zn 8/2-Zn 9/2-Zn 10/2-Zn 11/3-ZnF 12/3-ZnF 13/3-ZnF

BnOH BnOH MeOH MeOH MeOH BnOH BnOH MeOH MeOH MeOH BnOH BnOH BnOH

0.5/30/1 0.5/100/1 0.5/30/1 0.5/60/1 0.5/100/1 0.5/60/1 0.5/100/1 0.5/30/1 0.5/60/1 0.5/100/1 1/30/1 1/50/1 1/100/1

Time [min] 20 80 20 50 80 30 60 10 40 60 20 45 60

C [%]A

103 Mn,calB

103 Mn,C

Mw/MnD

97 97 95 99 98 96 96 99 94 96 94 95 99

4.23 14.09 4.14 8.60 14.16 5.51 13.94 4.31 8.16 13.87 4.33 6.95 14.38

3.86 13.92 3.91 9.04 14.28 6.35 15.05 3.83 7.50 14.22 4.53 7.80 16.73

1.26 1.21 1.28 1.22 1.30 1.26 1.22 1.30 1.35 1.28 1.03 1.12 1.16

reaction conditions: Vsolvent = 25 mL, CH2Cl2; T = 25 ˚C. A Obtained from 1H NMR. B - Calculated from Mn,cal = [L-LA]0/[ROH]0 × C × 144.13 + MROH unless otherwise specified. C - Determined by GPC calibrated versus polystyrene standards and corrected by a factor of 0.58 according to literature recommendations.30 D - Obtained from GPC. F Published results5d

The accurate inspection of 1H NMR spectra for PLA, obtained in the presence of complexes 1, 2-Zn revealed resonances for both chain ends of the expected benzyl ester and hydroxy group suggesting that initiation occurred through the insertion of the benzyl alkoxy group from the metal complex into L-LA (Fig. 14, S26). Figure 14 (A-B) shows that after addition of 2 equiv. of benzyl alcohol to the zinc complex, a mixture of new zinc species appeared. The resulting mixture have been analyzed by DOSY 1H NMR experiment and three different levels of diffusion rate have been revealed (the values of diffusion coefficients: 10.2·10-10, 7.8·10-10 and 5.5·10-10, see Fig. 15). The data suggest that equilibrium between different monomeric, dimeric and oligomeric forms is possible. Ethyl groups coming from the started complex 2-Zn are absent, and the new signals of CH2-Ar protons are detected at range 4.4 – 5.7 ppm. This “cocktail” of zinc intermediate species (probably monomers, dimers, oligomers, etc.) proposed in the Scheme 2, after treatment of an excess of L-lactide (Fig. 14 C-D) undergoes transformation into active dominating zinc compound. What is more, during the reaction time no signals coming from free ancillary ligand have been found. The methine protons –CH of lactide monomers (green g signals Fig. 14 C-D) decrease while the new methine protons of oligomers

increase simultaneously. The assignment of oligolactide protons is based on our results published recently.8d What stems from the NMR analysis of this “living” ROP is that the real initiator LZnOBn emerges from the reaction between 2-Zn with BnOH in the presence of lactide as shown in Scheme 2. The linear correlation between the Mn of the formed PLA and monomer conversion during ROP reaction and a nearly constant PDI are in line with the controlled living polymerization process, Fig. S24 – 25. The polymerization process is slightly slower for pre-catalyst 1-Zn, 2-Zn than for 3-Zn, the difference could be attributed to the longer time of reaction between zinc alkyl compound and benzyl alcohol to achieve active alkoxide complex. In comparison, similar type complexes with piperazinyl-derived aminephenolate ligands polymerize lactide under higher temperature (60 °C), but zinc complexes 1-Zn and 2-Zn are active at room temperature.5l In the single-site initiators the end group is anchored to metal centre, therefore, those ones can produce only one time, only one type of PLA chains capped by a defined ester group. In this context 1, 2-Zn deprived of this defect are conducive to form PLA with other ester end groups.

J. Name ., 2013, 00, 1-3 | 13





Table 3. ROP of L-LA catalyzed by zinc complexes.



DOI: 10.1039/C5DT01553G

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Journal Name c

k

l

a O

N

j

O

c

i Zn

g


O

n

O

g5 g4

D a

O

e

b

d c

O

O

O

O

O

5

O

O g5

O g4

O g3

O g3

O g2

O g1

O

n3

n2

n1

e

n5

n4

n3

h

k

n5 n4

n3

g3 h

n1

g2 g1

n2 n

l

f gi

j

C

B

A

1

Figure 14. H NMR spectra of the “living” ROP of L-LA obtained in the presence of 2-Zn/2 BnOH: A: 2-Zn; B: 2-Zn and 2 equivalents of BnOH; C: 5 min. after addition of 14 equivalents of L-lactide; D: 30 min.

Scheme 2. The proposed mechanism of the ROP of lactides and the active initiator formation.

14 | J. Name ., 2012, 00, 1-3




b

d f

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moment. The findings of our research will be published in a different paper soon.

Figure 15. DOSY 1H NMR for products of the reaction 2-Zn and BnOH (Figure 14 B) in benzene-d6.

Therefore, at the next stage we have tested the polymerization reaction while using methanol as external alcohol. The results collected in (Table 3) show that 1-Zn, in the presence of MeOH, works less efficiently as an initiator than 2-Zn, and the obtained PLA achieved higher PDI = 1.30 in comparison to PLA with benzyl ester end group. The NMR spectrum of PLA, prepared while using 2-Zn with MeOH showed one methyl end group and one hydroxyl chain end with an integral ratio of 3/1 among protons, which suggests that initiation occurred through the insertion of a methoxy group from the complex into lactide, Fig. S27. The obtained results indicates that probably similar zinc complexes are formed in the reaction between zinc-alkyl complexes and alcohols (BnOH, MeOH) used. It is important because the mechanism of formation of active species during the alcoholysis reaction and the role of alcohol require closer examination. If there is an equilibrium between dimer - monomer; (LZnEt)2 - 2LZnEt then it is easy to explain the appearance of a suitable dimer with alkoxy bridges LZn(µ-OR)2ZnL or LZnOR monomer, which are generally considered to be initiators for ROP of lactides. For the more stable dimers presented here the process is more complex. One can suppose that the first stage of reaction refers to dimer with terminal alkoxy bridges. Further reactions may depend on 5l,v the application of alcohol. Thus, they can create monomers or undergo association through alkoxy bridges and form oligomeric structures, the possible scenarios are presented in the Scheme 2. While, the homoleptic complex 3-Zn together with alcohol forms active catalytic system, described in our 8d earlier papers. Experimental results indicated that zinc complexes 1-Zn and 2Zn initiate the polymerization of lactide, and PLA has been obtained with the expected molecular weight and low PDI. The polymerization process is slower for 1-Zn in comparison to 2Zn. The process of alcoholysis of 1, 2-Zn, as well as the detailed analysis of alkoxide complexes, based on those and similar ligands, constitute the subject of our intensive research at the

A family of new heteroleptic zinc complexes 1, 2-Zn has been synthesized via the reaction of diethyl zinc and corresponding 1,2 aminophenol/aminonaphthol ligands L -H. The binary catalyst systems 1-Zn, 2-Zn/ROH afforded satisfactory control over the reaction parameters (PDI 1.3 – 1.2, Mnobs ≈ Mncal). The heteroleptic zinc complexes exist in a dimeric form in the solid state and in the solution confirmed by single-crystal X-ray analysis, NMR study and DFT calculations. However, modulation of substituents around the ligand skeleton can strongly influence steric behaviour and appropriate complex formation. The bulky tert-butyl group located in orthoposition of phenyl ring induce the largest drift between phenol and cyclohexyl ring, therefore dimerization of LZnEt compounds as well as bis-chelatation are possible. As a consequence, in the solution the coexistence of the desired heteroleptic monomers LZnEt, dimers (LZnEt)2 and less expected homoleptic bis-helate complexes are observed. But it is difficult to recognize which one is capable of crystallization. During the reaction this equilibrium could be easily shifted to crystallization of homoleptic monomers or heteroleptic complexes. However, when the steric bulk is reduced, stabilization of dimeric alkyl heteroleptic forms are favored. The precisely simple synthetic protocol permits to obtain exactly one type of complexes without stoichiometry control. All of the zinc complexes are shown to be viable pre-catalysts for ROP of lactides but aminonaphtalene frame is a more interesting puzzle of ancillary ligands than classical di-tertbutylphenol.

Acknowledgements The author gratefully acknowledge the National Science Centre in Poland (Grant NN 204 200640) and the Wrocław Centre for Networking and Supercomputing (http://www.wcss.wroc.pl) and EU under the European Social Found (project “Academy of development as the key to strengthen human resources of the polish economy) for support of this research.

Notes and references 1

For leading and recent reviews, see: (a) B. J. O’Keefe, M. A. Hillmyer, W. B. Tolman, J. Chem. Soc., Dalton Trans. 2001, 2215; (b) A.-C. Albertsson, I. K. Varma, Biomacromolecules 2003, 4, 1466; (c) O. Dechy-Cabaret, B. Martin-Vaca, D. Bourissou, Chem. Rev. 2004, 104, 6147; (d) J. Wu, T.-L. Yu, C.T. Chen, C.-C. Lin, Coord. Chem. Rev. 2006, 250, 602; (e) K. Williams, Chem. Soc. Rev. 2007, 36, 1573; (f) B. N. Kamber, W. Jeong, R. M. Waymouth, R. C. Pratt, B. G. G. Lohmeijer, J. L. Hedrick, Chem. Rev. 2007, 107, 5813; (g) Dove, A. P. Chem. Commun. 2008, 48, 6446; (h) R. H. Platel, L. M. Hodgson, C. K. Williams, Polymer Rev. 2008, 48, 11; (i) C. A. Wheaton, P. G. Hayes, B. J. Irealand, Dalton Trans. 2009, 25, 4832; (j) N.

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Conclusions



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2

3

4

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ARTICLE


Published on 19 June 2015. Downloaded by Freie Universitaet Berlin

Page 17 of 17 Dalton Transactions

Graphical Abstract

Homoleptic aminophenolates of Zn, Mg and Ca. Synthesis, structure, DFT studies and polymerization activity in ROP of lactides.

Zinc Oxide-From Synthesis to Application: A Review.

A homoleptic η² hydroxylaminato Ce(IV) complex with S₄ symmetry.

Room-temperature synthesis of zinc oxide nanoparticles in different media and their application in cyanide photodegradation.

Reactions of a Zn(I) complex with group 14 azides--formation of zinc azide and zinc hexazene complexes.

P,O-Phosphinophenolate zinc(II) species: synthesis, structure and use in the ring-opening polymerization (ROP) of lactide, ε-caprolactone and trimethylene carbonate.

Homoleptic and heteroleptic N-alkylimidazole zinc(ii)-containing ionic liquids for high current density electrodeposition.

zinc oxide solar cell structures.

Further aspects of ochratoxin A-cation interactions: complex formation with zinc ions and a novel analytical application of ochratoxin A-magnesium interaction in the HPLC-FLD system.

Insights in ROP.

Spontaneous formation of complex structures made from elastic membranes in an aluminum-hydroxide-carbonate system.

Synthesis and characterization of zinc oxide-neem oil-chitosan bionanocomposite for food packaging application.

[Analysis of choroidal thickness in AP-ROP, threshold disease and ROP without laser photocoagulation].

Continuous synthesis of zinc oxide nanoparticles in a microfluidic system for photovoltaic application.

Model updating of complex structures using the combination of component mode synthesis and Kriging predictor.

Synthesis and structures of soluble magnesium and zinc carboxylates containing intramolecular NH···O hydrogen bonds in nonpolar solvents.

Synthesis, characterization and biological evaluation of Rutin-zinc(II) flavonoid -metal complex.

Synthesis, growth, structure and characterization of molybdenum zinc thiourea complex crystals.

Metal-ligand synergistic effects in the complex Ni(η(2)-TEMPO)2: synthesis, structures, and reactivity.

Bis (aspirinato) zinc (II) complex successfully inhibits carotid arterial neointima formation after balloon-injury in rats.

True boundary for the formation of homoleptic transition-metal hydride complexes.

Synthesis, structure and theoretical investigation into a homoleptic tris(dithiolene) tungsten.

Homoleptic borates and aluminates containing the difluorophosphato ligand - [M(O2PF2)x](y-) - synthesis and characterization.

Aminotrifluoromethylation of olefins via cyclic amine formation: mechanistic study and application to synthesis of trifluoromethylated pyrrolidines.