Serendipitous Synthesis Found in the Nuances of Homoleptic Zinc

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Serendipitous Synthesis Found in the Nuances of Homoleptic Zinc Complex Formation Dawid Jędrzkiewicz, Aleksandra Marszałek-Harych, and Jolanta Ejfler* Faculty of Chemistry, University of Wroclaw, 14 F. Joliot-Curie, 50-383 Wrocław, Poland

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S Supporting Information *

ABSTRACT: A series of aminophenolate and aminonaphtholate homoleptic zinc complexes were obtained using a simple and unique synthetic strategy. A rigorous analysis of the byproduct supported modifications of the main course of the bis-chelation reaction. Controlled alcoholysis was followed by alternation and controlled anaerobic hydrolysis of ethyl-zinc aminophenolate or aminonaphtholate complexes. This new and intriguing reaction yielded a new class of zinc corona complexes. All the synthesized complexes were fully characterized in the solid state and in solution using X-ray and spectroscopic methods as well as density functional theory calculations.

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INTRODUCTION The most exciting trend in contemporary catalysis is finding novel, simple compounds catalytically active and synthesized in clean, green reactions.1 However, even a compound with a relatively simple structure can demand a sophisticated synthetic strategy if it is to meet this challenge. One plentiful set of such compounds may be the fruit of rigorous analyses of unexpected products of a planned reaction. One example of such analysis is the optimization of the synthesis of simple homoleptic zinc complexes with aminophenols: L2Zn. In combination with an appropriate amount of alcohols, this type of complex forms efficient catalysts for the ring opening polymerization (ROP) of cyclic esters.2,3 The ROP reactions in the presence of the bifunctional catalytic system L2Zn/ROH selectively produce polyesters or linear alkylesters.4 Aliphatic polyesters for several years constitute a technologically important class of biodegradable and biocompatible polymers.5 They constitute a viable alternative to petrochemically derived plastics.6 Polylactide (PLA) has found global applications in biomedicine, pharmaceutical development, and the production of environmentally friendly disposable products.7 Alkyl lactate and lactyllactate esters are used as green solvents in the microelectronics and cosmetics industry and as valuable building blocks for fine chemicals production.7 In this paper, we report on both the simple and unique synthesis of homoleptic zinc aminophenolate and naphtholate complexes and efforts to achieve the crystallization of precisely designed complexes. The main aim is to elucidate the synthetic strategies for homoleptic zinc complex formation with the sole product being L2Zn, regardless of any structural perturbations in the ligands used in this simple reaction. The detailed analysis of the reaction details helped to elucidate the synthesis of cyclic zinc complexes using the novel anaerobic hydrolysis of ethyl© XXXX American Chemical Society

zinc complexes. The beauty of minor alternations in chemical reaction details presented here offers a step forward in our understanding of the hydrolytic processes of organometallic complexes.

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RESULTS AND DISCUSSION The classical synthetic strategy for the formation of L2Zn compounds is simple: a direct reaction between ZnEt2 and appropriate ligands with donor nitrogen/oxygen atoms in the molar ratio 1:2. However, a lack of comprehension of the reaction details could be a major impediment to successful synthesis and isolation of the desired product. For example, as it was reported in our previous papers, this bis-chelation reaction proceeds cleanly and smoothly for the aminophenolate ligands with sizable substitutions in the ortho positions of the phenol core. Applying a differentiating solvent is also necessary because the expected homoleptic L2Zn monomers could be synthesized without stoichiometry control (Scheme 1, the proligands (OdtBu,N-ox)-H, (O-dtBu,N-cy)-H , and their zinc complexes (OdtBu,N-ox)2Zn and (O-dtBu,N-cy)2Zn have been reported previously).2l,n The new proligands presented here, (O-ItBu, Ncy)-H and (O-dtBu, N−C12)-H, were obtained in one-pot Mannich condensation reactions between formaldehyde, appropriately substituted phenols, and amines. The synthesis of the new zinc complexes (O-ItBu, N-cy)2Zn, and (O-dtBu, NC12)2Zn) was carried out according to the synthetic strategy described for the previously obtained aminophenolate zinc complexes (O-dtBu,N-ox)2Zn and (O-dtBu,N-cy)2Zn.2l,n Reactions between ZnEt2 and the proligands proceed readily in n-hexane, yielding the expected homoleptic L2Zn compounds. Received: March 8, 2018

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DOI: 10.1021/acs.inorgchem.8b00550 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

presented in Figure 1 and Supporting Information (Table S3). The central zinc atoms adopt a distorted tetrahedral geometry

Scheme 1. Examples of Aminophenols That Can Be Used for the Direct Synthesis of Homoleptic L2Zn Monomersa

Figure 1. Molecular structures of (O-ItBu,N-cy)2Zn (top) and (OdtBu,N-C12)2Zn (bottom).

with Zn−N and Zn−O distances similar in ranges for related coordination compounds.2l,n,8−10 Although the structure of L2Zn compounds is clear in the solid state and can be determined by X-ray analysis, a mixture of isomers is observed in solution (Figure 2). In the aminophenolate bis-chelate zinc complexes described here and in the recent literature, this asymmetry is usually indicated as a consequence of the location of the prochiral auxiliary ligands around the metal center.2n We suppose that the chiral option of this ligand with a stereodirecting substituent can induce reduction of the number of potential isomers. Therefore, we used chiral ligand with ditert-butyl substituents (O-dtBu,N-mb)-H we had previously obtained and checked in the similar reaction.2m The expected product of the reaction between ZnEt2 and the chiral ligand should be the simple bis-chelate monomer (O-dtBu,N-mb)2Zn. The course of that reaction was unexpected because the heteroleptic dimer S-[(O-dtBu,N-mb)ZnEt]2 crystallized from the parent solution when a racemic mixture of proligands was used in the direct reaction with ZnEt2 (Figure 3).

a

The preparation of the proligands (O-dtBu,N-ox)-H and (OdtBu,N-cy)-H and their zinc complexes has been reported previously.2l,n.

The complexes are very soluble in chlorinated hydrocarbons, tetrahydrofuran (THF), or toluene, but insoluble in n-hexane or n-heptane. The proligands, on the other hand, are soluble in the most commonly used solvents, such as THF, benzene, toluene, hexanes, and chlorinated hydrocarbons. Therefore, n-hexane was the best option as a reaction medium. Both the new ligands, (O-dtBu,N-C12)-H and (O-ItBu,Ncy)-H, and the new zinc complexes, (O-dtBu,N-C12)2Zn and (O-ItBu,N-cy)2Zn, were obtained in high yields and characterized using standard elemental analysis and spectroscopic methods. Crystals of the new complexes suitable for structural assessment were grown via slow evaporation of concentrated dichloromethane solution obtained by dissolving powders of the appropriate complexes synthesized in n-hexanes. The details of the crystal structures of the two new bis-chelate compounds are B


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Inorganic Chemistry

Scheme 2. Formation of (O-dtBu,N-mb)2Zn via Ligand Redistribution

Figure 2. Fragment of 1H NMR spectrum of (O-dtBu,N-C12)2Zn (benzene-d6).

Figure 4. Fragment of 1H NMR spectrum of R-(O-dtBu,N-mb)2Zn (benzene-d6).

structures of all the optimized isomers and the relative energies are presented in Figure 5. Theoretical NMR spectra were also computed. The results of this analysis indicated the best fit between experimental and theoretical NMR spectra for RCRNΛZnRNRC, RCRNΔZnRNRC, and RCSNΔZnRNRC isomers (Table 1). The complex R-(O-dtBu,N-mb)2Zn exists in solution as a mixture of two symmetric isomers, RCRNΛZnRNRC and RCRNΔZnRNRC, and one asymmetric isomer, RCSNΔZnRNRC. However, the crystallization of one of them was still difficult to achieve. The idea for the rationalization of the synthesis of chiral homoleptic monomers was to carry out the ligand redistribution reaction while using strictly designed heterodimers.14b Combining the chiral ligands with the same amine arms but a different aryl core, S-(O-dtBu,N-mb)-H and R-(O-ptBu,N-mb)-H,14 in the necessary 1/1 stoichiometry, in the reaction with two equivalents of ZnEt2 yielded crystals of the heterodimer (R-(OptBu,N-mb))(S-(O-dtBu,N-mb))Zn2Et2 (Figure 6) with the aryl core substituents located in such a way that they influence the controlled decomposition of the dimer in toluene solution (Scheme 3). This is the basis of the unique synthetic strategy that enabled monocrystals of (O-dtBu,N-mb)2Zn to be obtained (Figure 7). The modification of hydroxyl group environments in the ortho position of the phenol core can be achieved through the introduction of a two-dimensional (2D) planar obstruction or the use of aminophenol/aminonaphthol ligands with a free ortho position. This involves stabilization of dimeric (LZnEt)2 compounds by bridging metal centers as we reported previously.2k We verified that methodology with the new proligand (O-2npt,N-cy)-H and its zinc complex (Scheme 4). The new aminonaphthol proligand, (O-2npt,N-cy)-H, and the corresponding heteroleptic alkyl zinc complex, [(O-2npt,Ncy)ZnEt]2, were synthesized in high yields and characterized

Figure 3. Molecular structure of S-[(O-dtBu,N-mb)ZnEt]2.

The structure represents ethyl groups located on the same side of the Zn2O2 core plane, while the methylbenzyl units are positioned on the opposite side of that plane. This syn-dimer (cisoid conformation) is rare in zinc ethyl chemistry.10a Within that class of ethyl zinc complexes, the ethyl groups are transoidal (anti-dimers). Therefore, to compare the obtained molecular structure via X-ray diffraction analysis, which is unusual for this type of complex, and to delineate the other possible isomers (syn- and anti-dimers) that are accessible in parent solution, the geometrical parameters and relative energies of the zinc complexes were investigated using density functional theory (DFT) calculations (see Supporting Information, Table S6, Figure S26). The analysis of all 22 optimized isomers revealed that syn-dimers, one of which was determined via X-ray diffraction, are more thermodynamically stable structures than anti-dimers. Although the syn-dimer was determined as the most stable structure via X-ray diffraction, and that was confirmed in a theoretical study, it is difficult to recognize its form as the dominant one, because the isolated crystals undergo a ligand redistribution process during dissolution in C6D6. The main product of the redistribution reaction is the monomeric (OdtBu,N-mb)2Zn compound (Scheme 2). However, using one enantiomer of the proligand R-(O-dtBu,N-mb)-H in that reaction, the desired bis-chelate R-(O-dtBu,N-mb)2Zn was the only isolated product. The NMR spectrum of R-(O-dtBu,N-mb)2Zn contains four sets of signals (Figure 4). To determine which isomers are detectable in the solution, all the possible structures of the isomers were optimized in a DFT study. The schematic C


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Inorganic Chemistry

Figure 5. Relative energies of R-(O-dtBu,N-mb)2Zn isomers.

Table 1. Selected 1H NMR Chemical Shifts for R-(O-dtBu,Nmb)2Zn Isomers in Benzene-d6

dichloromethane solution of the synthesized complex. The crystal structure is presented in Figure 8. We verified all the possible reaction conditions (molar ratio, temperature, solvents, excess of the ligand), and probably it is impossible to obtain homoleptic L2Zn compounds in all classical optimizations required. The modification of this classical synthetic strategy seems impossible. We have recently proved these results in a theoretical study that clearly indicated that dimers are the most stable structures for zinc complexes with “ortho-free” aminophenols.2k The new synthetic pathway applied here was inspired by the unsuccessful methanolysis of alkyl-aminophenolate zinc complex reported by Lappert et al. (Scheme 5A).8 The main product of methanolysis was the homoleptic L2Zn monomer formed by the ligand redistribution reaction of the planned LZnOMe species (Scheme 5A,B). The hypothetical reaction byproduct, a methoxy-zinc oligomeric compound, is indicated based on the reaction equation. The methanolysis of our aminophenolate heteroleptic zinc dimers now yielded the expected homoleptic monomers with a small fraction of linear oligomeric species. Both originate from the mutual reaction of flexible unstable methoxy-dimers formed in the first stage of the methanolysis (Scheme 5C).9 To understand the transformation of dimeric complexes to a homoleptic monomer and to find real methanolysis byproducts, we verified the experimental data in a theoretical study. These both computational and experimental data could indicate that the possible synthetic scenario to methanolysis leads to the main bis-chelation product, which easily crystallizes from the parent solution. The second product is a linear oligomer with zinc atoms surrounded by terminal aminophenolate ligands and bridged by methoxy groups. Examples of similar linear heteroleptic oligomers detected as an unexpected byproduct have been presented elsewhere in the literature.2i,11 We have recently described the theoretical and experimental details of alkyl-zinc dimer alcoholysis.9 Although at the time we detected only the linear oligomer in solution, our study indicated that it could be quite possible to obtain cyclic oligomeric entities under forced reaction conditions. Using the DFT method, we optimized all the possible linear and cyclic forms of alcoholysis products using aminonaphtholate ligand (O-1npt,N-cy)- and 1-phenylethoxy, benzyloxy, or methoxy groups (Figure 9). These data clearly indicate that for sizable bridging of 1-phenylethoxy or benzyloxy groups, linear oligomers are preferable, but smaller 1-methoxy-bridging

chemical shifts isomer

fragment

proton

EXPa

DFTb

RCRNΛZnRNRC

RCRNΛZn/RCRNΛZn

RCRNΔZnRNRC

RCRNΔZn/RCRNΔZn

RCSNΔZnRNRC

RCRNΔZn

N−CH-CH3 Ar−CH2-N Ar−CH2-N N−CH3 CH-CH3 N−CH-CH3 Ar−CH2-N Ar−CH2-N N−CH3 CH-CH3 N−CH-CH3 Ar−CH2-N Ar−CH2-N N−CH3 CH-CH3 N−CH-CH3 Ar−CH2-N Ar−CH2-N N−CH3 CH-CH3

4.05 4.21 3.18 2.53 1.71 4.70 4.58 3.39 2.23 1.40 2.93 3.83 2.75 2.01 1.29 4.63 4.98 3.67 2.36 1.10

4.24 4.55 3.30 2.43 1.64 4.63 4.81 3.17 2.60 1.63 2.82 3.94 2.41 2.25 1.53 4.90 5.25 3.79 2.65 1.44

RCSNΔZn

a

Experimental. bCalculated for optimized structures.

Figure 6. Molecular structure of (R-(O-ptBu,N-mb))(S-(O-dtBu,Nmb))Zn2Et2.

using standard spectroscopic methods. Crystals of the new zinc complex were grown via slow evaporation of concentrated D


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Inorganic Chemistry Scheme 3. Decomposition of (R-(O-ptBu,N-mb))(S-(O-dtBu,N-mb))Zn2Et2 into Hetero- and Homoleptic Complexes

Figure 8. Molecular structure of [(O-2npt,N-cy)ZnEt]2. Figure 7. Molecular structure of (S-(O-dtBu,N-mb))2Zn.

Although the DFT-optimized cyclic methoxy-oligomer seems to be available in the reaction mixture, it is difficult to isolate, and we had thus far not successfully crystallized it. Therefore, in the next step, we tested controlled hydrolysis of dimers (LZnEt)2 to obtain the elusive cyclic form of the zinc oligomer. In the direct reaction of dimers and a stoichiometric quantity of water, the new crystals appeared after several days. As per our assumption that the new products were cyclic oligomers, they contained four zinc atoms for the aminophenolate zinc compound (Figure 10) and six for the aminonaphtholate zinc compound (Figure 11). In a theoretical study, we assumed the cyclic compounds contained four zinc atoms because a similar structure had been previously described in literature.12 However, experimental verification indicated that a larger metal corona cycle is possible to crystallize. We verified the correlation between the bulk of the bridging groups and the size of the metal corona cyclic compounds. The results are presented in Figure 12. Note that the four/six zinc metal corona is a favorable structure for small substituents. Additionally, the stabilization of the cyclic form in comparison to the linear form is clearly visible for compounds with hydroxyl bridges. The fragment of the metal corona core with a different amount of metal centers is shown in Figure 13. The balance between various factors, simple like the solubility all species (substrates, byproducts, final compounds) and more advanced like their mutual reactivity during the reaction time, is decisive when it comes to the crystallization of more or less expected complexes. Here, we can control that process. The paradigm stemming from the literature describes ancillary ligands with bulky obstruction in the phenol core as (i) ancillary ligands for heteroleptic metal complex formation, and (ii) preventive ligands for homoleptic compound formation. Our study indicates that the bis-chelation product may represent a favorable reaction pathway under restricted and planned reaction conditions for these ancillary ligands. Searching for new synthetic strategies by analyzing byproduct formation opens new opportunities for the controlled anaerobic hydrolysis of organometallic complexes. This is very important for biologically active compounds.13a,b The obtained hydroxyl

Scheme 4. Examples of Aminophenol/Naphthol Ligands Applicable for Dimeric (LZnEt)2 Synthesisa2k

a

The preparation of the proligands (O-ptBu,N-cy)-H and (O-1npt,Ncy)-H and their zinc complexes has been reported previously.

ligands are able to form in a mixture of both linear and cyclic species. E


Article

Inorganic Chemistry Scheme 5. Formation of Homoleptic L2Zn Monomers through the Methanolysis of (LZnEt)2 Dimersa

a A − Methanolysis of (LZnEt)2 reported by Lappert et al.8 B − The suggested mechanism of L2Zn formation through the ligand redistribution reaction of alkoxy species; C − The proposed mechanism of L2Zn formation by reaction between flexible alkoxy dimers.

Figure 9. Relative energies for the geometrically optimized alkoxy zinc species.

zinc corona complexes possess a clean simple structural motif without unexpected oxygen atoms.13c,d The new cyclic zinc F


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Inorganic Chemistry

Figure 10. Molecular and schematic structures of the cyclic aminophenolate zinc compound [(O-IBu,N-cy)Zn(OH)]4.

Figure 11. Molecular and schematic structures of the cyclic aminonaphtholate zinc compound [(O-2npt,N-cy)Zn(OH)]6.

Figure 12. Relative energies for the geometrically optimized alkoxy/hydroxy zinc species.

serendipitous reactions (i.e., the structure of the product was entirely unexpected) to rationally designed reactions. Typically, an initial synthesis involves some serendipity, especially if structurally flexible components are involved, but then sensible variations and extensions of this initial result, e.g., through minor alterations to the ligand structure, allow predictable modifications to be made. The detailed analysis of this unexpected

complexes with precisely designed hydroxyl bridges are a new class of compounds that are highly relevant in biological chemistry.

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CONCLUSIONS

The results of the syntheses of simple compounds considered only in terms of structural motifs range from purely G


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Inorganic Chemistry

Figure 13. Size of the metal corona in aminophenolate/aminonaphtholate zinc complexes: [ZnO]2 − [(O-1npt,N-cy)Zn(OBn)]2,9 [ZnO]4 − [(OIBu,N-cy)Zn(OH)]4, [ZnO]6 − [(O-2npt,N-cy)Zn(OH)]6. and evaporated under reduced pressure. The crude product was obtained as a white solid. Yield 70% (2.8 g, 6.9 mmol). Anal. Calcd (Found) for C18H28INO: C, 53.87 (54.25); H 7.03 (6.94); N, 3.49 (3.44) %; ESI/MS: 402.1 [M+1]+. 1H NMR (500 MHz, CDCl3, RT): δ = 10.03 (br, s, 1H, OH), 7.60 (d, JHH = 2.3 Hz, 1H, ArH), 6.94 (d, JHH = 2.3 Hz, 1H, ArH), 3.78 (s, 2H, N−CH2−Ar), 2.60 (tt, JHH = 11.5, 3.3 Hz, 1H, N−CH), 2.28 (s, 3H, N−CH3), 1.98−1.61 (m, 5H, CH2), 1.42−1.18 (m, 4H, CH2), 1.25 (s, 9H, C(CH3)3), 1.17−1.05 (m, 1H, CH2); 13C NMR (75 MHz, CDCl3, RT): δ = 155.3 (s, 1C, ArC−OH), 143.6 (s, 1C, ArC-C), 134.7 (s, 1C, ArCH), 125.8 (s, 1C, ArCH), 121.3 (s, 1C, ArC−CH2), 85.2 (s, 1C, ArC-I), 62.8 (s, 1C, N−CH), 57.7 (s, 1C, N−CH2), 36.4 (s, 1C, N−CH3), 34.1 (s, 1C, C(CH3)3), 31.6 (s, 3C, C(CH3)3), 28.1 (s, 1C, CH2), 26.1 (s, 2C, CH2), 25.8 (s, 2C, CH2). N-[Methyl(2-naphthyl)]-N-methyl-N-cyclohexylamine, (O2npt,N-cy)-H. To a solution of 1.11 g (7.70 mmol) of 2-naphthol and 1 mL (7.70 mmol) of N-methylcyclohexylamine in MeOH (20 mL), 1.00 mL (13.30 mmol) of formaldehyde (37% solution in H2O) was added. After 15 min a crude product precipitated as a white solid. It was collected by filtration, washed with cold methanol, and dried in vacuo. Yield 97% (2.02 g, 7.50 mmol). Anal. Calcd (Found) for C18H23NO: C, 80.26 (80.11); H, 8.61 (8.78); N, 5.20 (5.19) %; ESI/ MS: 270.4 [M + 1]+; 1H NMR (500 MHz, CDCl3) δ = 13.16 (br, s, 1H, OH), 7.80 (d, JHH = 8.5 Hz, 1H, ArH), 7.77 (d, JHH = 8.5 Hz, 1H, ArH), 7.68 (d, JHH = 8.8 Hz, 1H, ArH), 7.44 (t, JHH = 7.6 Hz, 1H, ArH), 7.29 (t, JHH = 7.6 Hz, 1H, ArH), 7.10 (d, JHH = 8.8 Hz, 1H, ArH), 4.28 (s, 2H, NCH2), 2.67 (tt, JHH = 11.5, 3.3 Hz, 1H, NCH), 2.38 (s, 3H, NCH3), 2.05−1.06 (m, 10H, CH2). 13C NMR (126 MHz, CDCl3) δ = 157.4 (s, 1C, ArC−OH), 132.8 (s, 1C, ArC), 129.0 (s, 2C, ArCH), 128.4 (s, 1C, ArC), 126.3 (s, 1C, ArCH), 121.4 (s, 1C, ArCH), 120.9 (s, 1C, ArCH), 119.6 (s, 1C, ArCH), 111.2 (s, ArCH2), 62.4 (s, 1C, NCH), 52.9 (s, 1C, NCH2), 36.9 (s, 1C, NCH3), 28.3 (s, 1C, CH2), 25.8 (s, 4C, CH2). N-[Methyl(2-hydroxy-3,5-ditert-butylphenyl)]-N-methyl-N-dodecylamine, (O-dtBu,N−C12)-H. To a solution of 1.65 g (7.97 mmol) of 2,4-ditert-butylphenol and 2 mL (7.97 mmol) of N-methyldodecylamine in MeOH (20 mL), 0.90 mL (11.96 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 give white powder of (O-dtBu, N−C12)-H. Yield 78% (2.62 g, 6.27 mmol). Anal. Calcd (Found) for C28H51NO: C, 80.51 (80.55); H, 12.31 (12.63); N, 3.35 (3.85) %; ESI/MS: 418.4 [M+1]+. 1H NMR (500 MHz, CDCl3): δ = 11.27 (s, 1H, ArOH), 7.22 (d, JHH = 2.4 Hz, 1H, ArH), 6.82 (d, JHH = 2.4 Hz, 1H, ArH), 3.66 (s, 2H, Ar−CH2−N), 2.44 (t, JHH = 6.9 Hz, 2H, N−CH2), 2.28 (s, 3H, N−CH3), 1.57−1.49 (m, 2H, NCH2CH2), 1.43 (s, 9H, C(CH3)), 1.30 (s, 9H, C(CH3)), 1.34−1.23 (m, 18H, N−CH2CH2(CH2)9CH3), 0.90 (t, JHH = 7.0 Hz, 3H, CH3). 13C NMR (126 MHz, CDCl3): δ = 154.6 (s, 1C, ArC−OH), 140.3 (s, 1C, ArC), 135.6 (s, 1C, ArC), 123.3 (s, 1C, ArCH), 122.8 (s,

product of a planned reaction allowed the elucidation and then the discovery of the intriguing chemistry of the anaerobic hydrolysis of ethyl−zinc complex chemistry.

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EXPERIMENTAL SECTION

General Materials, Methods, and Procedures. All the reactions and operations were performed under an inert atmosphere of N2 using a glovebox (MBraun) or standard Schlenk apparatus and vacuum line techniques. The solvents were purified by standard methods: toluene, distilled from Na; CH2Cl2, distilled from P2O5; hexanes, distilled from Na; benzene distilled from NaH; THF, distilled from Na; deuterated solvents (C6D6, toluene-d8), distilled from NaH. ZnEt2 (1.0 M solution in hexanes), N-methyldodecylamine (98%), formaldehyde (37% solution in H2O), 4-tert-butylphenol, 2,4-di-tert-butylphenol, 2naphthol, KI, KIO3, zinc acetate dihydrate were purchased from Aldrich and used as received. 1H and 13C NMR spectra were detected at the temperature range from 200 to 300 K using Bruker ESP 300E or 500 MHz spectrometers. Chemical shifts are reported in parts per million and referenced to the residual protons in the deuterated solvents. Microanalyses were conducted with an ARL model 3410 + ICP spectrometer (Fisons Instruments) and a Vario EL III CHNS (inhouse). Syntheses. N-[Methyl(2-hydroxy-3,5-ditert-butylphenyl)]-Nmethyl-N-cyclohexylamine, (O-dtBu, N-cy)-H. This was synthesized according to a literature procedure.2n N-[Methyl(2-hydroxy-5-tert-butylphenyl)]-N-methyl-N-cyclohexylamine, (O-ptBu, N-cy)-H. This was synthesized according to a literature procedure.2k N-[Methyl(2-hydroxy-3,5-ditert-butylphenyl)]-N-methyl-N-(1phenylethyl)amine, (O-dtBu,N-mb)-H. This was synthesized according to a literature procedure.2m N-[Methyl(2-hydroxy-5-tert-butylphenyl)]-N-methyl-N-(1phenylethyl)amine, (O-ptBu,N-mb)-H. This was synthesized according to a literature procedure.14a N-[Methyl(2-hydroxy-3-iodo-5-tert-butylphenyl)]-N-methyl-Ncyclohexylamine, [(O-ItBu, N-cy)-H]. To a stirred solution of L-H 2.75 g (1.00 mmol) in MeOH (100 mL), 0.61 g of KI (3.7 mmol) and 1.38 g (6.5 mmol) of KIO3 in 30 mL of H2O was added. Next 1.6 mL of HCl (36%, 2.5 equiv) was added dropwise, and the solution was stirred at room temperature for 48 h. After completion of the reaction, the solution was neutralized by sodium bicarbonate, and the product was extracted with DCM (30 mL, 3 times). The organic layers were combined and NH4Cl, and HCl was added (until pH 6.6−7.0 of the solution was achieved), and next it was extracted with DCM (30 mL, 2 times). The organic layers were combined and Na2S2O3 was added until the color of the solution was changed from brown to yellow, and next it was washed by water. The organic layer was dried over MgSO4, filtered, H


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Inorganic Chemistry 1C, ArCH), 121.6 (s, 1C, ArC−CH2), 62.5 (s, 1C, N−CH2−Ar), 56.9 (s, 1C, NCH2), 41.2 (s, 1C, NCH3), 35.0 (s, 1C, C(CH3)3), 34.3 (s, 1C, C(CH3)3), 32.1 (s, 1C, CH2), 31.8 (s, 3C, C(CH3)3), 29.8 (s, 2C, CH2), 29.8 (s, 3C, C(CH3)3), 29.7 (s, 3C, CH2), 29.5 (s, 1C, CH2), 27.4 (s, 1C, CH2), 27.1 (s, 1C, N−CH2−CH2), 22.9 (s, 1C, CH2), 14.3 (s, 1C, CH3). (O-ItBu,N-cy)2Zn. To a stirred solution of 0.80 g (2.00 mmol) of (OItBu,N-cy)-H in n-hexane (50 mL), 1.00 mL of ZnEt2 (1 M solution in n-heptane) was added dropwise at ambient temperature. The solution was stirred for 2 h, and then it was placed at −15 °C until colorless crystalline product appeared. It was collected by filtration and dried in vacuo. Yield 83% (0.72 g, 0.83 mmol). Anal. Calcd (Found) for C36H54I2N2O2Zn: C, 49.93 (49.79); H, 6.28 (6.51); N, 3.23 (3.20) %. 1 H NMR (500 MHz, C6D6, RT): δ = 8.08 (d, JHH = 2.3 Hz, 2H, ArH), 6.84 (d, JHH = 2.3 Hz, 2H, ArH), 4.33 (br, s, 2H, N−CH2−Ar), 3.25 (br, s, 2H, N−CH), 3.15 (br, s, 2H, N−CH2−Ar), 1.79 (s, 6H, N−CH3), 1.70−0.60 (m, 20H, CH2), 1.27 (s, 18H, C(CH3)3); 13C NMR (75 MHz, C6D6, RT): δ = 163.0 (s, 2C, ArC-O), 138.5 (s, 2C, ArC-C), 136.7 (s, 2C, ArCH), 128.8 (s, 2C, ArCH), 119.7 (s, 2C, ArC−CH2), 92.8 (s, 2C, ArC-I), 63.4 (s, 2C, N−CH), 61.2 (s, 2C, N−CH2), 33.7 (s, 2C, N−CH3), 32.8 (s, 2C, C(CH3)3), 31.9 (s, 6C, C(CH3)3), 26.0 (s, 10C, CH2). (O-dtBu,N-C12)2Zn. To a stirred solution of 0.83 g (2.00 mmol) of (O-dtBu,N−C12)-H in n-hexane (5 mL), 1.00 mL of ZnEt2 (1 M solution in n-heptane) was added dropwise at ambient temperature. The solution was stirred for 2 h, and then it was placed at −15 °C until colorless crystals appeared as a product. It was collected by filtration and dried in vacuo. Yield 76% (0.68 g, 0.76 mmol). Anal. Calcd (Found) for C56H100N2O2Zn: C, 74.83 (74.78); H, 11.21 (11.65); N, 3.12 (3.16) %. 1H NMR (500 MHz, C6D6, 300 K): Isomer 1 (red color): δ = 7.59 (d, JHH = 2.8 Hz, 1H, ArH), 6.91 (d, JHH = 2.8 Hz, 1H, ArH), 3.60 (d, JHH = 12.6 Hz, 1H, N−CH2−Ar), 3.55 (d, JHH = 12.6 Hz, 1H, N−CH2−Ar), 3.01 (td, J = 12.6, 4.6 Hz, 1H, N−CH2), 2.38−2.30 (m, obscured, 1H, N−CH2), 2.01 (s, 3H, N−CH3), 1.72 (s, 9H, C(CH3)3), 1.47 (s, 9H, C(CH3)3), 1.37−1.14 (m, 18H, (CH2)9), 1.05−0.97 (m, 2H, N−CH2−CH2), 0.97−0.87 (m, 3H, CH2−CH3); Isomer 2 (blue color): δ = 7.58 (d, JHH = 2.7 Hz, 1H, ArH), 6.93 (d, JHH = 2.7 Hz, 1H, ArH), 4.06 (d, JHH = 12.3 Hz, 1H, N−CH2−Ar), 3.20− 3.16 (m, obscured, 1H, N−CH2), 3.09 (d, JHH = 12.3 Hz, 1H, N−CH2− Ar), 2.29−2.21 (m, obscured, 1H, N−CH2), 2.24 (s, 3H, N−CH3), 1.73 (s, 9H, C(CH3)3), 1.45 (s, 9H, C(CH3)3), 1.37−1.14 (m, 18H, (CH2)9), 1.14−1.04 (m, 2H, N−CH2−CH2), 0.97−0.87 (m, 3H, CH2−CH3); Isomer 3 (green color): δ = 7.57 (d, JHH = 2.6 Hz, 1H, ArH), 6.93 (d, JHH = 2.6 Hz, 1H, ArH), 4.01 (d, JHH = 12.3 Hz, 1H, N− CH2−Ar), 3.20 (d, JHH = 12.3 Hz, 1H, N-CH2−Ar), 2.61−2.53 (m,, 1H, N−CH2), 2.33 (s, 3H, N−CH3), 2.21−2.14 (m, 1H, N−CH2), 1.66 (s, 9H, C(CH3)3), 1.58 (m, 2H, N−CH2−CH2), 1.46 (s, 9H, C(CH3)3), 1.37−1.14 (m, 18H, (CH2)9), 0.97−0.87 (m, 3H, CH2−CH3); Isomer 4 (gray color): δ = 7.56 (d, JHH = 2.8 Hz, 1H, ArH), 6.90 (d, JHH = 2.8 Hz, 1H, ArH), 3.98 (d, JHH = 12.6 Hz, 1H, N−CH2−Ar), 3.39 (d, JHH = 12.6 Hz, 1H, N−CH2−Ar), 2.48 (td, JHH = 11.9, 4.6 Hz, 1H, N−CH2), 2.08 (s, 3H, N−CH3), 2.11−2.03 (m, obscured, 1H, N−CH2), 1.67 (s, 9H, C(CH3)3), 1.46 (s, 9H, C(CH3)3), 1.37−1.14 (m, 18H, (CH2)9), 1.14−1.04 (m, 2H, N−CH2−CH2), 0.97−0.87 (m, 3H, CH2−CH3). 13 C NMR (75 MHz, C6D6, 300 K): Isomer 1 (red color): δ = 163.7 (s, 1C, ArC-O), 138.1 (s, 1C, ArC), 135.5 (s, 1C, ArC), 125.8(s, 1C, ArCH), 124.6 (s, 1C, ArCH), 120.3 (s, 1C, ArC−CH2), 64.2 (s, 1C, ArC−CH2), 57.3 (s, 1C, N−CH2), 42.6 (s, 1C, N−CH3), 35.7 (s, 1C, C(CH3)3), 34.1 (s, 1C, C(CH3)3), 32.3 (s, 3C, C(CH3)3), 30.2 (s, 3C, C(CH3)3), 30.1 (s, 3C, CH2), 29.9 (s, 3C, CH2), 29.7 (s, 1C, CH2), 27.8 (s, 1C, CH2), 26.4 (s, 1C, N−CH2−CH2), 23.6 (s, 1C, CH2), 14.3 (s, 1C, CH2−CH3); Isomer 2 (blue color): δ = 163.9 (s, 1C, ArC-O), 138.4 (s, 1C, ArC), 135.6 (s, 1C, ArC), 125.8(s, 1C, ArCH), 124.5 (s, 1C, ArCH), 120.2 (s, 1C, ArC−CH2), 65.4 (s, 1C, ArC−CH2), 58.3 (s, 1C, N−CH2), 41.3 (s, 1C, N−CH3), 35.7 (s, 1C, C(CH3)3), 34.1 (s, 1C, C(CH3)3,), 32.3 (s, 3C, C(CH3)3), 30.3 (s, 1C, CH2), 30.2 (s, 3C, C(CH3)3), 30.0 (s, 5C, CH2), 29.6 (s, 1C, CH2), 28.0 (s, 1C, CH2), 27.5 (s, 1C, N−CH2−CH2), 24.9 (s, 1C, CH2), 14.3 (s, 1C, CH2− CH3); Isomer 3 (green color): δ = 163.9 (s, 1C, ArC-O), 138.5 (s, 1C, ArC), 135.6 (s, 1C, ArC), 125.8 (s, 1C, ArCH), 124.5 (s, 1C, ArCH),

120.2 (s, 1C, ArC−CH2), 65.2 (s, 1C, ArC−CH2), 60.3 (s, 1C, N− CH2), 41.7 (s, 1C, N−CH3), 35.7 (s, 1C, C(CH3)3), 34.1 (s, 1C, C(CH3)3), 32.3 (s, 3C, C(CH3)3), 30.1 (s, 1C, CH2), 30.0 (s, 5C, CH2), 29.9 (s, 1C, CH2), 29.8 (s, 3C, C(CH3)3), 27.9 (s, 1C, CH2), 26.5 (s, 1C, N−CH2−CH2), 23.1 (s, 1C, CH2), 14.3 (s, 1C, CH2− CH3); Isomer 4 (gray color): δ = 163.9 (s, 1C, ArC-O), 138.4 (s, 1C, ArC), 135.6 (s, 1C, ArC), 125.7 (s, 1C, ArCH), 124.5 (s, 1C, ArCH), 120.0 (s, 1C, ArC−CH2), 64.9 (s, 1C, ArC−CH2), 60.0 (s, 1C, N− CH2), 41.8 (s, 1C, N−CH3), 35.7 (s, 1C, C(CH3)3), 34.1 (s, 1C, C(CH3)3), 32.3 (s, 3C, C(CH3)3), 30.1 (s, 1C, CH2), 30.1 (s, 3C, C(CH3)3), 30.0 (s, 5C, CH2), 29.8 (s, 1C, CH2), 27.4 (s, 1C, CH2), 26.3 (s, 1C, N−CH2−CH2), 23.1 (s, 1C, CH2), 14.3 (s, 1C, CH2− CH3). [(O-dtBu,N-mb)ZnEt]2. To a stirred solution of 0.35 g (1.00 mmol) R-(O-dtBu,N-mb)-H and 0.35 g (1.00 mmol) of S-(O-dtBu,N-mb)-H in n-hexane (50 mL), 2.00 mL of (2.00 mmol) ZnEt2 (1 M solution in nheptane) was added dropwise at ambient temperature. The solution was stirred for 10 min, and then it was placed at −15 °C until colorless crystals appeared as a product. It was collected by filtration and dried in vacuo. Yield 87% (0.78 g, 1.74 mmol). Anal. Calcd (Found) for C52H78N2O2Zn2: C, 69.86 (69.26); H, 8.79 (9.12); N, 3.58 (3.44) %. 1 H NMR (500 MHz, toluene-d8, 240 K): δ = 7.62 (d, JHH = 2.3 Hz, 2H, ArH), 7.24−7.08 (m, 10H, ArH), 6.74 (d, JHH = 2.3 Hz, 2H, ArH), 5.25 (d, JHH = 11.3 Hz, 2H, N−CH2−Ar), 4.21 (q, JHH = 6.8 Hz, 2H, N− CH-Ar), 3.02 (d, JHH = 11.3 Hz, 2H, N−CH2−Ar), 1.98 (s, 6H, N− CH3), 1.84 (s, 18H, C(CH3)3), 1.70 (t, JHH = 8.1 Hz, 6H, CH2−CH3), 1.25−1.16 (m, 6H, CH3−CH), 1.13 (s, 18H, C(CH3)3), 0.74 (q, JHH = 8.1 Hz, 4H, CH2−CH3). 13C NMR (75 MHz, toluene-d8, 240 K): δ = 159.8 (s, 2C, ArC-O), 139.5 (s, 2C, ArC-C), 139.3 (s, 2C, ArC-C), 136.3 (s, 2C, ArC−CH), 128.4 (s, 10C, ArCH), 127.4 (s, 2C, ArCH), 125.6 (s, 2C, ArC−CH2), 125.1 (s, 2C, ArCH), 63.3 (s, 2C, N−CHAr), 61.9 (s, 2C, N−CH2−Ar), 36.0 (s, 2C, C(CH3)3), 34.7 (s, 2C, N− CH3), 34.1 (s, 2C, C(CH3)3), 31.7 (s, 6C, C(CH3)3), 31.2 (s, 6C, C(CH3)3), 19.5 (s, 2C, CH3−CH), 13.6 (s, 2C, CH2−CH3), 1.9 (s, 2C, CH2−CH3). (R-(O-ptBu,N-mb))(S-(O-dtBu,N-mb))Zn2Et2. To a solution of 0.30 g (1.00 mmol) of R-(O-ptBu,N-mb)-H and 0.35 g of (1.00 mmol) S(O-dtBu,N-mb)-H in n-hexane (50 mL), 2.00 mL (2.00 mmol) of ZnEt2 (1 M solution in n-heptane) was added dropwise at ambient temperature. The solution was stirred for 5 min, and then it was placed at −15 °C until colorless crystals appeared as a product. It was collected by filtration and dried in vacuo. Yield 76% (0.64 g, 1.52 mmol). Anal. Calcd (Found) for C48H70N2O2Zn2: C, 68.81 (68.62); H, 8.42 (8.51); N, 3.34 (3.51) %. R-(O-dtBu,N-mb(R))2Zn. To a stirred solution of 0.71 g (2.00 mmol) of R-(O-dtBu,N-mb)-H in n-hexane (50 mL), 1.00 mL (1.00 mmol) of ZnEt2 (1 M solution in n-heptane) was added dropwise at ambient temperature. The solution was stirred for 6 h, and then it was placed at −15 °C until a crude product precipitated as a white solid. It was collected by filtration and dried in vacuo. Yield 75% (0.5 g, 0.75 mmol). Anal. Calcd (Found) for C48H68N2O2Zn: C, 74.83 (74.66); H, 8.90 (9.06); N, 3.64 (3.67) %. 1H NMR (500 MHz, C6D6, 300 K): Isomer RCRNΛZnRNRC (red color): δ = 7.55 (d, JHH = 2.5 Hz, 2H, ArH), 7.15− 7.07 (m, 10H, ArH), 6,72 (d, JHH = 2.5 Hz, 2H, ArH), 4.21 (d, JHH = 11.7 Hz, 2H, N−CH2−Ar), 4.05 (q, JHH = 7.0 Hz, 2H, N−CH-Ar), 3.19 (d, JHH = 11.7 Hz, 2H, N−CH2−Ar), 2.53 (s, 6H, N−CH3), 1.71 (d, JHH = 7.0 Hz, 6H, CH3−CH), 1.66 (s, 18H, C(CH3)3), 1.28 (s, 18H, C(CH3)3; Isomer RCRNΔZnRNRC (blue color): δ = 7.60 (d, JHH = 2.5 Hz, 2H, ArH), 7.15−7.07 (m, 10H, ArH), 6.67 (s, 2H, ArH), 4.70 (q, JHH = 7.0 Hz, 2H, N−CH-Ar), 4.58 (d, JHH = 12.8 Hz, 2H, N−CH2− Ar), 3.39 (d, JHH = 12.8 Hz, 2H, N−CH2−Ar), 2.23 (s, 6H, N−CH3), 1.70 (s, 18H, C(CH3)3), 1.40 (d, JHH = 7.0 Hz 6H, CH3−CH), 1.34 (s, 18H, C(CH3)3; Isomer RCSNΔZnRNRC, fragment RCRNΔZn (green color): δ = 7.57 (s, 1H, ArH), 7.09−6.94 (m, 5H, ArH), 6.77 (d, JHH = 2.5 Hz, 1H, ArH), 3.83 (d, JHH = 12.0 Hz, 1H, N−CH2−Ar), 2.93 (q, JHH = 7.0 Hz, 1H, N−CH-Ar), 2.75 (d, JHH = 12.0 Hz, 1H, N−CH2− Ar), 2.01 (s, 3H, N−CH3), 1.78 (s, 9H, C(CH3)3), 1.29 (d, JHH = 7.0 Hz, 3H, CH3−CH), 1.24 (s, 9H, C(CH3)3; Isomer RCSNΔZnRNRC, fragment RCSNΔZn (gray color): δ = 7.58−7.49 (m, 5H, ArH), 6.97− 6.94 (m, 1H, ArH), 6.78 (s, 1H, ArH), 4.99 (d, JHH = 13.0 Hz, 1H, N− I


Article

Inorganic Chemistry CH2−Ar), 4.63 (q, JHH = 7.0 Hz, 1H, N−CH-Ar), 3.68 (d, JHH = 13.0 Hz, 1H, N−CH2−Ar), 2.36 (s, 3H, N−CH3), 1.50 (s, 9H, C(CH3)3), 1.34 (s, 9H, C(CH3)3), 1.10 (d, JHH = 7.0 Hz, 3H, CH3−CH). 13C NMR (75 MHz, C6D6, 300 K): Isomer RCRNΛZnRNRC (red color): δ = 164.3 (s, 2C, ArC-O), 137.9 (s, 2C, ArC-C), 135.9 (s, 2C, ArC-C), 130.6 (s, 2C, ArC−CH), 129.3 (s, 8C, ArCH), 128.6 (s, 2C, ArCH), 125.7 (s, 2C, ArCH), 124.5 (s, 2C, ArCH), 120.5 (s, 2C, ArC−CH), 64.8 (s, 2C, N−CH-Ar), 63.5 (s, 2C, N−CH2−Ar), 36.0 (s, 2C, N− CH3), 34.1 (s, 2C, C(CH3)3), 32.2 (s, 6C, C(CH3)3), 32.0 (s, 2C, C(CH3)3), 30.2 (s, 6C, C(CH3)3), 20.8 (s, 2C, CH3−CH); Isomer RCRNΔZnRNRC (blue color): δ = 164.4 (s, 2C, ArC-O), 138.2 (s, 2C, ArC-C), 135.1 (s, 2C, ArC-C), 130.9 (s, 2C, ArC−CH), 128.6 (s, 8C, ArCH), 128.5 (s, 2C, ArCH), 126.2 (s, 2C, ArCH), 124.8 (s, 2C, ArCH), 119.6 (s, 2C, ArC−CH), 63.7 (s, 2C, N−CH2−Ar), 63.6 (s, 2C, N−CH-Ar), 36.0 (s, 2C, N−CH3), 34.2 (s, 2C, C(CH3)3), 32.3 (s, 2C, C(CH3)3), 32.2 (s, 6C, C(CH3)3), 30.4 (s, 6C, C(CH3)3,), 20.5 (s, 2C, CH3−CH); Isomer RCSNΔZnRNRC, fragment RCRNΔZn (green color): δ = 163.9 (s, 1C, ArC-O), 138.0 (s, 1C, ArC-C), 135.8 (s, 1C, ArC-C), 131.1 (s, 1C, ArC−CH), 128.6 (s, 4C, ArCH), 128.5 (s, 1C, ArCH), 126.1 (s, 1C, ArCH), 124.7 (s, 1C, ArCH), 119.9 (s, 1C, ArC− CH), 64.1 (s, 1C, N−CH-Ar), 63.2 (s, 1C, N−CH2−Ar), 36.4 (s, 1C, N−CH3), 34.1 (s, 1C, C(CH3)3), 32.2 (s, 4C, C(CH3)3, C(CH3)3), 30.4 (s, 3C, C(CH3)3), 20.8 (s, 1C, CH3−CH); Isomer RCSNΔZnRNRC, fragment RCSNΔZn (gray color): δ = 163.7 (ArC-O, 1C), 138.5 (ArC-C, 1C), 135.4 (ArC-C, 1C), 130.4 (ArC−CH, 1C), 129.0 (s, 4C, ArCH), 128.9 (s, 1C, ArCH), 126.4 (s, 1C, ArCH), 124.6 (s, 1C, ArCH), 120.8 (s, 1C, ArC−CH), 64.1 (s, 1C, N−CH-Ar), 63.0 (s, 1C, N−CH2−Ar), 35.2 (s, 1C, N−CH3), 34.4 (s, 1C, C(CH3)3), 32.4 (s, 4C, C(CH3)3, 32.2 C(CH3)3), 30.8 (s, 3C, C(CH3)3), 12.4 (s, 1C, CH3−CH). [(O-2npt, N-cy)ZnEt]2. To a solution of (O-2npt, N-cy)-H (0.54 g, 2.00 mmol) in n-hexane (30 mL) ZnEt2 (2 mL, 2.00 mmol) was added dropwise 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 gave [(O-2npt, N-cy)ZnEt]2. Yield 92% (0.65 g, 0.9 mmol). Anal. Calcd (Found) for C40H54N2O2Zn2: C, 66.21 (66.15); H, 7.50 (7.62); N, 3.86 (3.98) %; 1 H NMR (500 MHz, C6D6, RT): δ = 7.94 (d, J = 8.5 Hz, 2H, ArH), 7.73 (d, J = 8.5 Hz, 4H, ArH), 7.45 (d, J = 7.4 Hz, 2H, ArH), 7.43 (d, J = 8.5 Hz, 2H, ArH), 7.25 (t, J = 7.4 Hz, 2H, ArH), 4.56 (d, J = 12.5 Hz, 2H, N−CH2−Ar), 4.44 (d, J = 12.5 Hz, 2H, N−CH2−Ar), 3.38 (t, J = 11.9 Hz, 2H, N−CH), 1.80 (s, 6H, N−CH3), 2.24−0.92 (m, 20H, CH2), 1.31 (t, J = 8.1 Hz, 6H, CH2−CH3), 0.33 (q, J = 8.1 Hz, 4H, CH2− CH3); 13C NMR (75 MHz, C6D6, RT): δ = 160.6 (s, 2C, ArC−OH), 132.2 (s, 2C, ArC), 131.8 (s, 2C, ArCH), 131.6 (s, 2C, ArCH), 129.8 (s, 2C, ArC), 127.4 (s, 2C, ArCH), 123.4 (s, 2C, ArCH), 122.7 (s, 2C, ArCH) 122.6 (s, 2C, ArCH), 115.1 (s, 2C, ArC−CH2), 64.1 (s, 2C, N− CH), 51.7 (s, 2C, N−CH2), 34.9 (s, 2C, N−CH3), 26.4 (s, 10C, CH2), 13.7 (s, 2C, CH2−CH3), −0.3 (s, 2C, CH2−CH3). [O-ItBu,N-cy)Zn(OH)]4. To a solution of 0.40 g (1.00 mmol) of (OItBu,N-cy)-H in benzene (20 mL), 1.00 mL of ZnEt2 (1 M solution in n-heptane) was added dropwise at ambient temperature. Next the solution of 0.21 g (1.00 mmol) of zinc acetate dihydrate in THF (30 mL) was added slowly. After several days, insoluble crystals appeared as a product. Yield 31% (0.15 g, 0.08 mmol). Anal. Calcd (Found) for C72H112I4N4O8Zn4: C, 44.79 (44.74); H, 5.85 (5.96); N, 2.90 (3.07) %. [(O-2npt,N-cy)Zn(OH)]6. To a solution of 0.36 g (0.50 mmol) of [(O-2npt,N-cy)ZnEt]2 in toluene (20 mL) the solution of 0.21 g (1.00 mmol) of zinc acetate dihydrate in THF (30 mL) was added dropwise. After several days, insoluble crystals appeared as product. Yield 26% (0.09 g, 0.04 mmol). Anal. Calcd (Found) for C108H138N6O12Zn6: C, 61.64 (61.69); H, 6.61 (6.52); N, 3.99 (4.23) %. Details of X-ray Data Analysis. X-ray diffraction data for a suitable crystal of each sample were collected using a KUMA KM4 CCD or Xcalibur CCD Ruby (see Supporting information Tables S1 and S2) with ω scan technique. The data collection and processing utilized CrysAlis suite of programs.15 The 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 least-squares on F2. All calculations were performed using the SHELX suite of programs.16 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 30% of probability displacements for non-hydrogen atoms by using Mercury 3.9 program.17 All data have been deposited with the Cambridge Crystallographic Data Centre CCDC-1587853 for (O-dtBu,N− C12)2Zn, -1587854 for (O-ItBu,N-cy)2Zn, -1587855 for [(O-ItBu,Ncy)Zn(OH)]4, -1587856 for S-(O-dtBu,N-mb)2Zn, -1587857 for S[(O-dtBu,N-mb)ZnEt]2, −1587858 for (R-(O-ptBu,N-mb))(S-(OdtBu,N-mb))Zn2Et2, -1587859 for [(O-2npt,N-cy)ZnEt]2, -1587860 for [(O-2npt,N-cy)Zn(OH)]6. 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 performed with the program suite Gaussian 03.18 The geometries of the complexes were optimized by using B3LYP density functional theory and the 6-31G** basis sets, implemented in the Gaussian 03 on all atoms.19,20 The starting geometries of complexes were generated from their crystal structures, whereas the starting geometries of other species were derived by the modification of optimized complexes. Structures of all isomers of calculated complexes were optimized in vacuo. Frequency calculations confirmed the stationary points to be minimal.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00550. Spectroscopic data (1H, 13C{1H}, COSY, NOESY NMR), X-ray data, computational details and checkCIF Alert B explanation for (R-(O-ptBu,N-mb)S-(O-dtBu,N-mb)Zn2Et2 (PDF) (PDF) Accession Codes

CCDC 1587853−1587860 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: jolanta.ejfl[email protected]. ORCID

Jolanta Ejfler: 0000-0002-7467-1312 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to express their gratitude to the National Science Centre in Poland (Grant 2017/25/B/ST5/00597) and the Wrocław Centre for Networking and Supercomputing (http://www.wcss.wroc.pl).

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REFERENCES

(1) (a) Lu, T.; Zou, J.; Zhan, Y.; Yang, X.; Wen, Y.; Wang, X.; Zhou, L.; Xu, J. Highly Efficient Oxidation of Ethyl Lactate to Ethyl Pyruvate Catalyzed by TS-1 Under Mild Conditions. ACS Catal. 2018, 8, 1287− 1296. (b) Wu, C.; Zhang, H.; Yu, B.; Chen, Y.; Ke, Z.; Guo, S.; Liu, Z. Lactate-Based Ionic Liquid Catalyzed Reductive Amination/Cyclization of Keto Acids under Mild Conditions: A Metal-Free Route To Synthesize Lactams. ACS Catal. 2017, 7, 7772−7776. (c) Ebrahimi, T.; Aluthge, D. C.; Patrick, B. O.; Hatzikiriakos, S. G.; Mehrkhodavandi, P. J


Article

Inorganic Chemistry

A. P. Stereocontrolled Ring-Opening Polymerisation of Lactide. Chem. Soc. Rev. 2010, 39, 486−494. (i) dos Santos Vieira, I.; Herres-Pawlis, S. Lactide Polymerisation with Complexes of Neutral N-Donors − New Strategies for Robust Catalysts. Eur. J. Inorg. Chem. 2012, 2012, 765− 774. (j) Sarazin, Y.; Carpentier, J.-F. Discrete Cationic Complexes for Ring-Opening Polymerization Catalysis of Cyclic Esters and Epoxides. Chem. Rev. 2015, 115, 3564−3614. (4) (a) Jędrzkiewicz, D.; Czeluśniak, I.; Wierzejewska, M.; Szafert, S.; Ejfler, J. Well-Controlled, Zinc-Catalyzed Synthesis of Low Molecular Weight Oligolactides by Ring Opening Reaction. J. Mol. Catal. A: Chem. 2015, 396, 155−163. (b) Grala, A.; Ejfler, J.; Jerzykiewicz, L. B.; Sobota, P. Chemoselective Alcoholysis of Lactide Mediated by a Magnesium Catalyst: an Efficient Route to Alkyl Lactyllactate. Dalton Trans. 2011, 40, 4042−4044. (5) For examples on application of PLA, see references: (a) Rokkanen, P. U.; Böstman, O.; Hirvensalo, E.; Mäkelä, E. A.; Partio, E. K.; Pätiälä, H.; Vainionpäa,̈ S.; Vihtonen, K.; Törmälä, P. Bioabsorbable Fixation in Orthopaedic Surgery and Traumatology. Biomaterials 2000, 21, 2607− 2613. (b) Langer, R.; Tirrell, D. A. Designing Materials for Biology and Medicine. Nature 2004, 428, 487−492. (c) Rickert, E. L.; Trebley, J. P.; Peterson, A. C.; Morrell, M. M.; Weatherman, R. V. Synthesis and Characterization of Bioactive Tamoxifen-conjugated Polymers. Biomacromolecules 2007, 8, 3608−3612. (d) Murariu, M.; Doumbia, A.; Bonnaud, L.; Dechief, A. L.; Paint, Y.; Ferreira, M.; Campagne, C.; Devaux, E.; Dubois, P. High-Performance Polylactide/ZnO Nanocomposites Designed for Films and Fibers with Special End-Use Properties. Biomacromolecules 2011, 12, 1762−1771. (e) Kim, H.; Park, H.; Lee, J.; Kim, T. H.; Lee, E. S.; Oh, K. T.; Lee, K. K. C.; Youn, Y. S. Highly Porous Large Poly(Lactic-co-Glycolic Acid) Microspheres Adsorbed with Palmityl-Acylated Exendin-4 as a Long-Acting Inhalation System for Treating Diabetes. Biomaterials 2011, 32, 1685−1693. (f) Liao, L.; Dong, J.; Wang, G.; Fan, Z.; Li, S.; Lu, Z. Microstructure−Property Relationship of L-Lactide/ Trimethylene Carbonate/Glycolide Terpolymers as Cardiovascular Stent Material. Eur. Polym. J. 2015, 66, 429−436. (g) Hu, Y.; Darcos, V.; Monge, S.; Li, S. Thermo-Responsive Drug Release from Self-Assembled Micelles of Brush-Like PLA/PEG Analogues Block Copolymers. Int. J. Pharm. 2015, 491, 152−161. (g) Wang, J.; Cheng, Y.; Fan, Z.; Li, S.; Liu, X.; Shen, X.; Su, F. Composites of Poly(L-Lactide-Trimethylene Carbonate-Glycolide) and Surface Modified Calcium Carbonate Whiskers as a Potential Bone Substitute Material. RSC Adv. 2016, 6, 57762−57772. (6) (a) Gross, R. A.; Kalra, B. Biodegradable Polymers for the Environment. Science 2002, 297, 803−807. (b) Miller, S. A. Sustainable Polymers: Opportunities for the Next Decade. ACS Macro Lett. 2013, 2, 550−554. (c) Auras, R.; Lim, L. T.; Selke, S. E. M.; Tsuji, H. Poly(Lactic Acid): Synthesis, Structure, Properties, Processing and Applications; Wiley: New York, 2017. (d) Tuck, C. O.; Perez, E.; Horvath, I. T.; Sheldon, R. A.; Poliakoff, M. Valorization of Biomass: Deriving More Value from Waste. Science 2012, 337, 695−699. (e) Myers, D.; Cyriac, A.; Williams, C. K. Polymer Synthesis: To react the Impossible Ring. Nat. Chem. 2016, 8, 3−4. (f) Chen, G. Q.; Patel, M. K. Plastics Derived from Biological Sources: Present and Future: A Technical and Environmental Review. Chem. Rev. 2012, 112, 2082−2099. (7) (a) Maki-Arvela, P.; Simakova, I. L.; Salmi, T.; Murzin, D. Yu. Production of Lactic Acid/Lactates from Biomass and Their Catalytic Transformations to Commodities. Chem. Rev. 2014, 114, 1909−1971. (b) Pereira, C. S. M.; Silva, V. M. T. M.; Rodrigues, A. E. Ethyl Lactate as a Solvent: Properties, Applications and Production Processes − a Review. Green Chem. 2011, 13, 2658−2671. (8) Farwell, J. D.; Hitchcock, P. B.; Lappert, M. F.; Luinstra, G. A.; Protchenko; Wei, X.-H. Synthesis and Structures of Some Sterically Hindered Zinc Complexes Containing 6-membered ZnNCCCN and ZnOCCCN Rings. J. Organomet. Chem. 2008, 693, 1861−1869. (9) Jędrzkiewicz, D.; Adamus, G.; Kwiecień, M.; John, Ł.; Ejfler, J. Lactide as the Playmaker of the ROP Game: Theoretical and Experimental Investigation of Ring-Opening Polymerization of Lactide Initiated by Aminonaphtholate Zinc Complexes. Inorg. Chem. 2017, 56, 1349−1365.

Air- and Moisture-Stable Indium Salan Catalysts for Living Multiblock PLA Formation in Air. ACS Catal. 2017, 7, 6413−6418. (2) For leading reviews on homoleptic complexes active in ROP catalysis, see: (a) Arnold, P. L.; Buffet, J.-C.; Blaudeck, R. P.; Sujecki, S.; Blake, A. J.; Wilson, C. C3-Symmetric Lanthanide Tris(alkoxide) Complexes Formed by Preferential Complexation and Their Stereoselective Polymerization of rac-Lactide. Angew. Chem., Int. Ed. 2008, 47, 6033−6036. (b) Naumann, S.; Scholten, P. B. V.; Wilson, J. A.; Dove, A. P. Dual Catalysis for Selective Ring-Opening Polymerization of Lactones: Evolution toward Simplicity. J. Am. Chem. Soc. 2015, 137, 14439−14445. (c) Horeglad, P.; Cybularczyk, M.; Litwińska, A.; Dąbrowska, A. M.; Dranka, M.; Ż ukowska, G. Z.; Urbańczyk, M.; Michalak, M. Controlling the Stereoselectivity of rac-LA Polymerization by Chiral Recognition Induced the Formation of Homochiral Dimeric Metal Alkoxides. Polym. Chem. 2016, 7, 2022−2036. (d) Cao, P.-F.; Mangadlao, J. D.; de Leon, A.; Su, Z.; Advincula, R. C. Catenated Poly(ε-caprolactone) and Poly(L-lactide) via Ring-Expansion Strategy. Macromolecules 2015, 48, 3825−3833. (e) Thomas, C.; Gladysz, J. A. Highly Active Families of Catalysts for the Ring-Opening Polymerization of Lactide: Metal Templated Organic Hydrogen Bond Donors Derived from 2-Guanidinobenzimidazole. ACS Catal. 2014, 4, 1134− 1138. (f) Song, S.; Zhang, X.; Ma, H.; Yang, Y. Zinc Complexes Supported by Claw-Type Aminophenolate Ligands: Synthesis, Characterization and Catalysis in the Ring-Opening Polymerization of rac-Lactide. Dalton Trans. 2012, 41, 3266−3277. (g) Huang, Y.; Wang, W.; Lin, C.-C.; Blake, M. P.; Clark, L.; Schwarz, A. D.; Mountford, P. Potassium, Zinc, and Magnesium Complexes of a Bulky OOO-Tridentate Bis(phenolate) Ligand: Synthesis, Structures, and Studies of Cyclic Ester Polymerization. Dalton Trans. 2013, 42, 9313− 9324. (h) Abbina, S.; Du, G. Chiral Amido-Oxazolinate Zinc Complexes for Asymmetric Alternating Copolymerization of CO2 and Cyclohexene Oxide. Organometallics 2012, 31, 7394−7403. (i) Wang, Y.; Zhao, W.; Liu, X.; Cui, D.; Chen, E. Y.-X. Ligand-Free Magnesium Catalyst System: Immortal Polymerization of L-Lactide with High Catalyst Efficiency and Structure of Active Intermediates. Macromolecules 2012, 45, 6957−6965. (k) Jędrzkiewicz, D.; Ejfler, J.; Gulia, N.; John, Ł.; Szafert, S. Designing Ancillary Ligands for Heteroleptic/Homoleptic Zinc Complex Formation: Synthesis, Structures and Application in ROP of Lactides. Dalton Trans. 2015, 44, 13700−13715. (l) Wojtaszak, J.; Mierzwicki, K.; Szafert, S.; Gulia, N.; Ejfler, J. Homoleptic Aminophenolates of Zn, Mg and Ca. Synthesis, Structure, DFT Studies and Polymerization Activity in ROP of Lactides. Dalton Trans. 2014, 43, 2424−2436. (m) Ejfler, J.; KrauzyDziedzic, K.; Szafert, S.; Jerzykiewicz, L. B.; Sobota, P. Synthesis, Characterization and Catalytic Study of Aryloxido Magnesium Complexes. Eur. J. Inorg. Chem. 2010, 2010, 3602−3609. (n) Ejfler, J.; Szafert, S.; Mierzwicki, K.; Jerzykiewicz, L. B.; Sobota, P. Homo- and Heteroleptic Zinc Aminophenolates as Initiators for Lactide Polymerization. Dalton Trans. 2008, 6556−6562. (3) For leading recent reviews on metal complexes and their use in ROP of cyclic esters, see: (a) O’Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. Polymerization of Lactide and Related Cyclic Esters by Discrete Metal Complexes. J. Chem. Soc., Dalton Trans. 2001, 2215−2224. (b) Albertsson, A. C.; Varma, I. K. Recent Developments in Ring Opening Polymerization of Lactones for Biomedical Applications. Biomacromolecules 2003, 4, 1466−1486. (c) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Controlled Ring-Opening Polymerization of Lactide and Glycolide. Chem. Rev. 2004, 104, 6147−6176. (d) Wu, J.; Yu, T. L.; Chen, C. T.; Lin, C. C. Recent Developments in Main Group Metal Complexes Catalyzed/Initiated Polymerization of Lactides and Related Cyclic Esters. Coord. Chem. Rev. 2006, 250, 602− 626. (e) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R. C.; Lohmeijer, B. G.; Hedrick, J. L. Organocatalytic Ring-Opening Polymerization. Chem. Rev. 2007, 107, 5813−5840. (f) Dove, A. P. Controlled Ring-Opening Polymerisation of Cyclic Esters: Polymer Blocks in Self-Assembled Nanostructures. Chem. Commun. 2008, 48, 6446−6470. (g) Wheaton, C. A.; Hayes, P. G.; Ireland, B. J. Complexes of Mg, Ca and Zn as Homogeneous Catalysts for Lactide Polymerization. Dalton Trans. 2009, 25, 4832−4846. (h) Stanford, M. J.; Dove, K


Article

Inorganic Chemistry (10) (a) Ikpo, N.; Saunders, L. N.; Walsh, J. L.; Smith, J. M. B.; Dawe, L. N.; Kerton, F. M. Zinc Complexes of Piperazinyl-Derived Aminephenolate Ligands Synthesis, Characterization and Ring-Opening Polymerization Activity. Eur. J. Inorg. Chem. 2011, 2011, 5347− 5359. (b) Zheng, Z.; Zhao, G.; Fablet, R.; Bouyahyi, M.; Thomas, C. M.; Roisnel, T.; Casagrande, O., Jr.; Carpentier, J.-F. Zinc and EnolatoMagnesium Complexes Based on Bi-, Tri- and Tetradentate Aminophenolate Ligands. New J. Chem. 2008, 32, 2279−2291. (11) (a) Chen, H.-Y.; Peng, Y.-L.; Huang, T.-H.; Sutar, A. K.; Miller, S. A.; Lin, C.-C. Comparative Study of Lactide Polymerization by Zinc Alkoxide Complexes with a Beta-Diketiminato Ligand Bearing Different Substituents. J. Mol. Catal. A: Chem. 2011, 339, 61−71. (12) Wang, J.-H.; Tsai, C.-Y.; Su, J.-K.; Huang, B.-H.; Lin, C.-C.; Ko, B.-T. Mono-, Di- and Tetra-Zinc Complexes Derived from an AminoBenzotriazole Phenolate Ligand Containing a Bulkier N-Alkyl Pendant Arm: Synthesis, Structure and Catalysis for Ring-Opening Polymerization of Cyclic Esters. Dalton Trans. 2015, 44, 12401−12410. (13) (a) Parkin, G. Synthetic Analogues Relevant to the Structure and Function of Zinc Enzymes. Chem. Rev. 2004, 104, 699−767. (b) Weston, J. Mode of Action of Bi- and Trinuclear Zinc Hydrolases and Their Synthetic Analogues. Chem. Rev. 2005, 105, 2151−2174. (c) Prochowicz, D.; Sokołowski, K.; Lewinski, J. Zinc Hydroxides and Oxides Supported by Organic Ligands: Synthesis and Structural Diversity. Coord. Chem. Rev. 2014, 270−271, 112−126. (d) Wróbel, Z.; Pietrzak, T.; Justyniak, I.; Lewiński, J. Oxygenation of RZn(N,O)-Type Complexes as an Efficient Route to Zinc Alkoxides Not Accessible via the Classical Alcoholysis Path. Chem. Commun. 2017, 53, 10808− 10811. (14) (a) Jędrzkiewicz, D.; Ejfler, J.; John, Ł.; Szafert, S. Zebra Reaction or the Recipe for the Synthesis of Heterodimeric Zinc Complexes. Dalton Trans. 2016, 45, 2829−2838. (b) Jędrzkiewicz, D.; Kantorska, D.; Wojtaszak, J.; Ejfler, J.; Szafert, S. DFT Calculations as a Ligand Toolbox for the Synthesis of Active Initiators for ROP of Cyclic Esters. Dalton Trans. 2017, 46, 4929−4942. (15) CrysAlisRED Software; Oxford Diffraction: Wrocław, Poland, 1995−2004. (16) Sheldrick, G. M. A Short History of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (17) Macrae, C. F.; Bruno, I. J.; Chisholm, J. A.; Edgington, P. R.; Mccabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor, R.; Van De Streek, J.; Wood, P. A. Mercury CSD 2.0 − New Features for the Visualization and Investigation of Crystal Structures. J. Appl. Crystallogr. 2008, 41, 466−470. (18) Gaussian 09, Revision B.01, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A.; J. Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian, Inc.: Wallingford, CT, 2010. (19) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (20) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula Into a Functional of the Electron Density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789.

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