Construction of Chiral [4 + 4] and [2 + 2] Schiff-Base Macrocyclic Zinc(II

Aug 9, 2016 - *E-mail: [email protected]. Synopsis. Chiral and racemic 68-membered [4 + 4] tetranuclear and 34-membered [2 + 2] dinuclear Schiff-base ...
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Construction of Chiral [4 + 4] and [2 + 2] Schiff-Base Macrocyclic Zinc(II) Complexes Influenced by Counterions and Pendant Arms Yong Hu, Lei Zhang, Fei-Fan Chang, Pei-Chen Zhao, Gen-Feng Feng, Kun Zhang, and Wei Huang* State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, Jiangsu Province 210093, P. R. China S Supporting Information *

while Zn(BF4)2·2H2O only generated a [2 + 2] macrocyclic zinc(II) complex directly.14 In our previous work, the selection of different pendant arms and halide counterions would generate different [1 + 1] mononuclear and [2 + 2] trinuclear macrocyclic zinc(II) complexes during the imine condensation between 1,3-propanediamine and extended dialdehydes (H2hpdd and H2pdd).3 As an extensive study, we first use chiral and racemic 1,2diaminocyclohexane precursors herein to react with a pair of extended dialdehydes with different pendant arms (H2hpdd and H2pdd) with the assistance of a primary zinc(II) ion template, paying particular attention to the influences of secondary template effects. As a result, 10 chiral/racemic [4 + 4] and [2 + 2] Schiff-base macrocyclic zinc(II) complexes 1−10 have been produced, as illustrated in Scheme 1. It is noted that regulation of the sizes of imine macrocycles could be effectively realized by the counterions of Cl− and NO3− for three pairs of [(S,S), (R,R), (±)] [4 + 4] and [2 + 2] macrocyclic zinc(II) complexes 1−6 when H2hpdd was used.

ABSTRACT: Chiral and racemic 68-membered [4 + 4] tetranuclear and 34-membered [2 + 2] dinuclear Schiffbase macrocyclic zinc(II) complexes 1−10 can be selectively synthesized based on the secondary template effects of counterions and pendant arms, when [(S,S), (R,R), (±)]-1,2-diaminocyclohexane precursors are first used to react with a pair of extended dialdehydes with different pendant arms via zinc(II) ion template-assisted imine condensation.

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he reversible nature of a dynamic covalent imine bond means that condensation between relative robust molecules could be governed by both thermodynamic and kinetic factors, where dynamic covalent chemistry operates and dominates the final products.1 So, the reversibility of one reaction can be influenced by the selection of an appropriate experimental condition. Namely, regulation of certain experimental parameters, such as anion,2 solvent,3 pH value,4 ligand,5 guest,6 hydrogen bond,7 concentration, and stoichiometry,8 can dramatically affect the state of equilibrium and determine the product distribution.9 Schiff-base macrocycles are an important family of compounds having dynamic covalent imine bonds that have been used in recognition, selective catalysis, biological system, sensing, and so on. New Schiff-base macrocycles possessing unusual topology and outstanding properties are constantly being constructed from relatively simple precursors.10 However, it is still a great challenge to selectively synthesize a macrocycle with controllable size, especially a larger one. The template effect, popularized by Busch in the mid-1960s,11 showed that the metal ions could preorganize the building blocks into a certain relative geometry, allowing for the surrounding of imine bonds. In the case of some template-directed reactions, [2 + 2], [3 + 3], [4 + 4], [6 + 6], and even larger condensation products have been produced by using the appropriate metal templates12 and even different stoichiometric amounts of cations.13 Apart from the cation templates, other approaches, such as anionic templates, have also been utilized for the construction of versatile Schiff-base macrocycles.2b,c Nevertheless, the influences of secondary template effects are rarely reported. Love et al. have reported one successful example by synthesizing [4 + 4] and [2 + 2] macrocyclic complexes between a [2 + 2] metal-free macrocyclic ligand and different zinc(II) salts. Namely, Zn(OAc)2·2H2O resulted in the formation of a [4 + 4] macrocyclic zinc(II) complex via a ring expansion reaction, © XXXX American Chemical Society

Scheme 1. Formation of [2 + 2] Dinuclear and [4 + 4] Tetranuclear Macrocyclic Zinc(II) Complexes

Received: June 22, 2016

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DOI: 10.1021/acs.inorgchem.6b01493 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry That is to say, [4 + 4] tetranuclear zinc(II) complexes 1−3 were obtained in a selective manner when ZnCl2 was used in the process of macrocyclic condensations, which could have originated from the stronger coordination capability and less steric hindrance of the Cl− anion. In contrast, [2 + 2] dinuclear zinc(II) complexes 4−6 were yielded in the presence of Zn(NO3)·6H2O. As for the influence of pendant arms, our results indicate that only [2 + 2] macrocyclic complexes 7−10 can be produced in the case of the −CH2CH2C6H5 pendant arm, while [2 + 2] or [4 + 4] macrocyclic complexes 1−6 were formed for the −CH2CH2C6H4OH one. By a comparison of the structures of [4 + 4] and [2 + 2] macrocyclic complexes 3 and 10, it is concluded that the impact of subtle alteration in the pendant arms for dialdehyde components cannot be neglected. The influence of pendant arms is suggested to originate from the simple reason that the pH value could be subtly adjusted by the pendant arms of the dialdehyde components. As we know, the successful isolation and structural characterization of large-sized macrocyclic complexes are very challenging. Actually, in many cases, the successful construction of large-sized macrocyclic complexes could be merely observed in the mass spectra. For example, Gregoliński et al. have reported the construction of large-sized multinuclear [6 + 6] and [8 + 8] macrocyclic cadmium(II) complexes via the metal-directed ringexpansion reaction between a metal-free [2 + 2] macrocycle with excess cadmium(II) ions, where the former was separated by filtration and the latter could only be indicated by mass spectral analysis.15 In our case, the formation of [4 + 4] macrocyclic zinc(II) complexes cannot be detected by mass spectroscopy because no molecular ion peaks could be found. However, the [4 + 4] macrocyclic complexes 1−3 with the same molecular weight of 2609.16 were the main products in our reactions, and they could be separated in high yields. Because of the similarity of the FTIR, UV−vis, and even 1H NMR spectra for all possible macrocyclic ring-closing products, single-crystal X-ray diffraction is used to reveal the molecular structure of the [4 + 4] macrocyclic complex 3, which is believed to be crystal-clear proof for the formation of this unique large-sized macrocyclic complex. Furthermore, the pure phase of this [4 + 4] macrocyclic zinc(II) complex 3 is confirmed by its powder X-ray diffraction pattern, which agrees well with the single-crystal diffraction simulative data (Figure SI1). In comparison with the former affirmed characteristic absorption peaks of aldehyde groups in H2ppd and H2hppd at 1660 and 1664 cm−1, a new strong single peak has been observed in the range of 1630−1640 cm−1 for every [2 + 2] and [4 + 4] macrocyclic zinc(II) complex (Figures SI2−SI11), verifying conversion from the aldehyde groups to CN imine components. Moreover, 1H NMR spectral comparisons have been made to explore variations of the chemical shifts between extended dialdehyde precursors and their respective macrocyclic zinc(II) complexes. After formation of the imine bonds in macrocyclic complexes 1−10, two new peaks were observed at 8.39−8.42 and 8.09−8.12 ppm (Figures SI12−SI19), while the aldehyde protons of H2hpdd and H2pdd at 10.01 ppm disappeared by contrast. The formation of all dinuclear [2 + 2] macrocyclic zinc(II) complexes 4−10 could be evidenced by their positive-mode electrospray ionization mass spectrometry (ESI-MS) spectra, as illustrated in Figures 1 and SI20−SI25. For example, complexes 4 and 7 having different extended dialdehyde components gave a positive peak for each at m/z 1165.08 and 1133.17, respectively. This peak can be assigned as the [M + CH3OH + H]+ species,

Figure 1. Representative ESI-MS of [2 + 2] macrocyclic complex 4 as well as the experimental and calculated isotopic distribution (insets) corresponding to the peak at 100% abundance.

which is consistent with the theoretical simulations. In addition, the circular dichroism spectra of three enantiomeric pairs of macrocyclic zinc(II) complexes, i.e., (S,S)-1 and (R,R)-2, (S,S)-4 and (R,R)-5, (S,S)-7 and (R,R)-8, have been recorded for comparison. The results reveal that the R,R isomers give Cotton effects that are mirror images of their respective S,S ones (Figures SI27−SI29), agreeing well with the enantiomeric nature of these chiral compounds. Structural analysis of 3 (Figure 2) reveals that it is a 68membered [4 + 4] tetranuclear macrocyclic zinc(II) complex.

Figure 2. (a) ORTEP drawing of the tetranuclear zinc(II) complex 3. (b) Perspective view of the tetranuclear zinc(II) complex 3.

Every metal center is five-coordinated by two oxygen atoms of the salicyladehyde fragment, two nitrogen atoms of the CN unit, and one halide ion. The coordination geometry for two crystallographically independent zinc(II) ions is a slightly distorted pyramid (τ = 0.067 and 0.073 for Zn1 and Zn2)16 with the axial position occupied by a chlorine ion for each. The spatial positions of four metal ions constitute a tetrahedron in 3, and the separations between every two zinc(II) centers are in the range of 7.637(4)−8.072(4) Å (Figure SI30). In contrast, complexes 4 and 7−9 (Figure 3) have folded 34membered [2 + 2] dinuclear macrocyclic zinc(II) backbones. It is noted that a pair of enantiomeric space groups (P41212 and P43212) is assigned in the cases of 7 and 8 with reasonable Flack parameters of −0.01(2) and 0.02(1), indicative of the use of chiral (S,S)- and (R,R)-1,2-diaminocyclohexane precursors. The coordination geometry for each zinc(II) center in dinuclear complexes 4 and 7−9 is a severely distorted pyramid (τ = 0.470 in 4, 0.448 in 7, 0.463 in 8, and 0.497/0.348 in 9), where the apical chlorine ion in 3 is replaced by the oxygen atom from methanol (4), H2O (7 and 8), or NO3− (9). In addition, the separations between two zinc(II) centers are much shorter than B

DOI: 10.1021/acs.inorgchem.6b01493 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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Communication

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01493. Synthetic details and characterization data (PDF) X-ray crystallographic data in CIF format for CCDC 1475880−1475884 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Major State Basic Research Development Programs (Grant 2013CB922101) and the NSFC of China (Grant 21171088) and Jiangsu Province (Grants BK20130054 and BE2014147-2).

Figure 3. ORTEP drawings of complexes 4 (a) and 7−9 (b−d) with the protons bonded with oxygen atoms shown as small spheres.

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REFERENCES

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those in 3, namely, 4.487(3) Å in 4, 4.406(5) Å in 7, 4.383(3) Å in 8, and 4.697(3) Å in 9. It is also found that the conformation of two salicylaldehyde rings in dialdehyde components is different in [4 + 4] and [2 + 2] macrocyclic zinc(II) complexes 3, 4, and 7−9. The dihedral angles between them are 77.2(5) and 86.3(5)° in 3, which are comparable with that in the free dialdehyde precursor [67.7(4)°].3,17 In contrast, the above-mentioned dihedral angles are much smaller in 4 and 7−9, which are in the range of 17.1(3)−22.5(3)°. Moreover, intramolecular π−π-stacking interactions are observed between adjacent two salicylaldehyde rings from different dialdehyde precursors in 3 with centroid-tocentroid separations of 3.515(4) and 3.367(4) Å (Figure SI31), respectively. In contrast, weaker intramolecular π−π-stacking interactions are found in 4 and 7−9 between two salicylaldehyde rings from the same dialdehyde precursor, where the centroid-tocentroid distances are in the range of 3.662(5)−3.905(3) Å (Figures SI32−SI35). In summary, chiral and racemic 68-numbered [4 + 4] tetranuclear (1−3) and 34-numbered [2 + 2] dinuclear (4− 10) pendant-armed Schiff-base macrocyclic zinc(II) complexes were obtained from the condensation reactions between [(S,S), (R,R), (±)]-1,2-diaminocyclohexane and extended dialdehydes (H2hpdd/H2pdd) in the presence of Zn(NO3)·6H2O and ZnCl2. We have demonstrated that two experimental variables, i.e., counterions and pendent arms, can regulate the ring size of the final macrocyclic products besides the primary zinc(II) ion template effect. Namely, the formation of large-sized [4 + 4] macrocycles could be controlled by the combination of a chloride ion and −CH2CH2C6H4OH pendant-armed effects, while only [2 + 2] macrocyclic complexes can be yielded using a nitrate or −CH2CH2C6H5 pendant arm. Considering that the successful construction, isolation, and structural characterization of largesized pendant-armed macrocycles are rarely reported, we think this work is interesting because we have demonstrated an unprecedented structural example for the selective formation of a pendant-armed [4 + 4] macrocyclic zinc(II) complex codominated by the combination of primary zinc(II) ion and secondary anion and pendant-armed template effects. C

DOI: 10.1021/acs.inorgchem.6b01493 Inorg. Chem. XXXX, XXX, XXX−XXX