Transmetalation for Flexible Pendant-Armed Schiff-Base Macrocyclic

Jan 11, 2018 - (6) Template-directed synthesis is proved to be an effective method to form diverse Schiff-base macrocyclic metal complexes when combin...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Transmetalation for Flexible Pendant-Armed Schiff-Base Macrocyclic Complexes Influenced by Halide Effects Fei-Fan Chang, Lei Zhang, Pei-Chen Zhao, and Wei Huang* State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *

ABSTRACT: Three 46-membered [2 + 2] pendant-armed Schiff-base macrocyclic dinuclear CdII and CuII complexes (2a, 2b, and 3b) and one 23-membered [1 + 1] CuII macrocycle 4a were prepared from the template-directed condensation reactions between 1,2-bis(2-aminoethoxy)-ethane and extended Cl-dialdehyde in the presence of CdX2 and CuX2 (X = Cl and Br), in which halide effects play important roles in the organization of final macrocyclic complexes in addition to the dominant influence of metal cations. Transmetalation was intensively studied among these CdII and CuII complexes with large and flexible macrocyclic ligands, including two previously synthesized dinuclear ZnII macrocycles (1a and 1b). Our results indicate that ZnII → CuII and CdII → CuII transmetalation proceeds more quickly than that from CdII to ZnII, and all the processes are found to be irreversible. It is noted that a [2 + 2] heterodinuclear CdIIZnII macrocyclic intermediate could be detected by ESI-MS together with [2 + 2] homodinuclear CdII and ZnII macrocyclic complexes. Furthermore, distinct halide behavior was observed in the in situ CdII → CuII and ZnII → CuII metalion exchange. That is to say, [2 + 2] macrocycles (1a and 2a) could be converted to [1 + 1] macrocycles 4a and 4b under the reflux condition in the case of CuCl2, accompanied by the configurational transformation from half-folded dinuclear ZnII and CdII to unfolded CuII macrocyclic skeleton. In contrast, CuBr2 leads to a highly efficient transmetalation to corresponding [2 + 2] dinuclear CuII complex 3b from both 1b and 2b no matter the experimental condition used.

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INTRODUCTION Derived from the reversible condensation of aldehydes and amines and discovered by Hugo Schiff in 1864,1 Schiff-base compounds have attracted a great deal of attention because of their particular properties and intriguing applications in recognition,2 catalysis,3 magnetism,4 biological systems,5 and so on. Schiff-base imine bond formation and cleavage is one of a handful of reactions defined as dynamic covalent chemistry (DCC), which has been employed widely in the construction of exotic molecules and extended structures.6 Template-directed synthesis is proved to be an effective method to form diverse Schiff-base macrocyclic metal complexes when combined with DCC of imine covalent bonds such as metal−organic container molecules,7 imine macrocycles,8 helicates,9 catenanes,10 rotaxanes,11 Borromean rings,12 and Solomon knots.13 It is worth mentioning that the charge, ionic radius, redox property, coordination geometry, and coordination number of different template cations could impact the resultant Schiff-base macrocyclic metal complexes.14 However, lots of other experimental parameters such as counterion, solvent, concentration, pH, temperature, light, and mechanical stress could also affect the equilibriums of dynamic covalent imine bonds and the dynamic noncovalent metal−ligand coordinative bonds.15 As we have reported before, when the primary ZnII cation template is fixed, four experimental variables as the secondary © XXXX American Chemical Society

template (pendant arms, counterions, pH, and solvents) could further regulate the size of the final Schiff-base macrocycles.16 On the other hand, transmetalation of supramolecular assemblies is an emerging reaction strategy to synthesize multidentate metal−ligand complexes because the type of introduced metal ions has a definite influence on the structure and function of resultant materials.17 Among all the metal ions, d9-block metal CuII ion is preferentially selected as a targeting ion in the process of transmetalation because the Jahn−Teller distortion of CuII complexes could provide the exceptional thermodynamic stability.18 In addition, this synthetic strategy makes possible the access to certain metal complexes which could not be obtained by the direct template route.19 As for the in situ metal-ion exchange of Schiff-base metal macrocyclic complexes, some pioneering works have been done on rigid mononuclear and dinuclear complexes bearing 2,6-diformyl-4substituted phenol-,20 pyridine-,21 salphen-,22 or salen-23 based ligands. However, studies on Schiff-base macrocyclic complexes with larger and more flexible multidentate ligands have not been documented yet. As we have reported before, anioninduced24 and solvent-induced16a macrocyclic reconstruction as well as base-induced macrocyclic self-assembly25 could take Received: November 7, 2017

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DOI: 10.1021/acs.inorgchem.7b02835 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Scheme 1. Schematic Illustration for the Preparation of Schiff-Base Macrocyclic Complexes via in Situ Metal-Ion Exchange Influenced by Different Halide Counterions

Figure 1. ESI-MS spectrum of [2 + 2] macrocyclic dinuclear CuII complex 3b in CH3OH with the experimental and calculated IDPs (inset) that corresponded to the molecular ion peak for comparison.

place for pendant-armed Schiff-base ZnII macrocycles derived from flexible extended dialdehyde precursors, in which both coordination numbers and coordination modes for the flexible multidentate macrocyclic ligands are variable, and hence, ring degradation/expansion and additional metal-ion implantation have been successfully achieved. In view of the alterations of coordination numbers and modes for central metal ions as well as sizes for our large and flexible multidentate Schiff-base macrocyclic ligands, we aim to explore the possible in situ metal-ion exchange and macrocyclic

reconstruction as an extensive study. In this work, we select two pairs of pendant-armed Schiff-base macrocyclic dinuclear ZnII (1a and 1b) and CdII (2a and 2b) complexes as the starting materials, in which the common [2 + 2] macrocyclic ligand with 14 heteroatoms have versatile coordination modes and the central metal ions are both d10-block ions with distinct ionic radii, and the d9-block metal CuII is used as the targeting ion in the process of transmetalation. As a result, ZnII and CdII could be quickly replaced by CuII via in situ metal-ion exchange to generate corresponding 46-membered [2 + 2] (3a and 3b) and B

DOI: 10.1021/acs.inorgchem.7b02835 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. ESI-MS spectrum of [2 + 2] macrocyclic dinuclear CuII complex 4a in CH3OH with the experimental and calculated IDPs (inset) that corresponded to the molecular ion peak at 100% abundance for comparison.

among ZnII, CdII, and CuII macrocycles was extensively examined to compare their thermodynamic stability and reveal the transmetalation direction and possible intermediates and influencing factors. Our first transmetalation attempt was carried out by refluxing equivalent [2 + 2] CdII and ZnII macrocycles with CuCl2. The molecular ion peaks at m/z = 1259.17 in 1a and 1355.17 in 2a are changed to m/z = 744.08 and 744.17, corresponding to the species of [Cu2(HL2)Cl2]+ (Figures 2 and S4), indicating that the original [2 + 2] Schiff-base ZnII and CdII macrocycles 1a and 2a underwent complete CuII metal exchange and ringdegradation to form [1 + 1] dinuclear complexes 4a and 4b. In contrast, the original molecular ion peaks are shifted to m/z = 1257.00 under RT corresponding to the species of [Cu2(H2L1)Cl]+ of [2 + 2] dinuclear CuII macrocycle 3a (Figure S3). It should be mentioned that [2 + 2] macrocycle 3a could not be yielded through direct CuII template synthesis and was converted to [1 + 1] CuII complex 4a under the reflux condition. Next, CuBr2 was used in the transmetalation from [2 + 2] Schiff-base macrocyclic complexes 1b and 2b for comparison, and only [2 + 2] dinuclear CuII macrocycle 3b was isolated both under the RT and reflux conditions. This transmetalation was also confirmed by two obvious ESI-MS peaks at m/z = 1257.08 and 1301.33 corresponding to [Cu2(HL1)(H2O)2]+ and [Cu2(H2L1)Br]+, as shown in Figure 1. So, it is concluded that different halide counterions (Cl− and Br−) play important roles in the ZnII → CuII and CdII → CuII macrocyclic reconstruction under the RT and reflux conditions, which might be ascribed to the differences in the ion radii and coordination abilities of chloride and bromide anions. Further control experiments have uncovered that ZnII → CuII and CdII → CuII metal-ion exchange is irreversible. Finally, in situ metal-ion exchange was investigated between Schiff-base macrocyclic ZnII and CdII complexes 1a/2a and 1b/ 2b. Our results reveal that dinuclear CdII macrocycles 2a/2b were exchanged to dinuclear ZnII complexes 1a/1b completely and irreversibly, and no corresponding [1 + 1] product was isolated in the CdII → ZnII transmetalation under both RT and reflux conditions no matter if ZnCl2 or ZnBr2 was used. That is to say, impacts of both the halide effects and temperature were not noticed in the in situ CdII → ZnII one-way metal-ion

23-membered ring-contracted [1 + 1] (4a and 4b) dinuclear macrocyclic CuII complexes (Scheme 1). It is also noticed that all the CuII involved transmetalation is quick and irreversible. In contrast, slower in situ metal-ion exchange occurs only from CdII to ZnII macrocyclic complexes, but not vice versa. Furthermore, different halide counterions (Cl− and Br−) are found to play important roles in the ZnII → CuII and CdII → CuII macrocyclic reconstruction under room temperature (RT) and reflux conditions.

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RESULTS AND DISCUSSION Syntheses and Spectral Characterizations. In addition to our previously reported ZnII complexes (1a and 1b) prepared via template-assisted synthesis, it is found that larger d10-block CdII ion is also a suitable template for constructing [2 + 2] pendant-armed dinuclear macrocycles 2a and 2b bearing the same macrocyclic ligand [H2L1]2−. As for CuII ion template, the result is influenced by the halide effects. That is to say, CuBr2 and CuCl2 give rise to the formation of [2 + 2] dinuclear CuII macrocycle 3b and [1 + 1] dinuclear macrocycle 4a under the reflux condition, respectively. The successful formation of [2 + 2] macrocyclic dinuclear CdII and CuII complexes 2a, 2b, and 3b and [1 + 1] dinuclear CuII macrocycle 4a was assigned and simulated by electrospray ionization mass spectrometry ESI-MS (m/z = 1355.17 for [Cd2(H2L1)Cl]+, 1399.08 for [Cd2(H2L1)Br]+, 1310.25 for [Cu2(H2L1)Br]+, and 744.08 for [Cu2(HL2)Cl2]+, Figures 1 and 2 and S1 and S2). It should be pointed out that refluxing the reaction mixture is necessary in the template-directed synthesis because complicated and difficult to isolate products are produced at RT. Successful accommodation of certain central metal ions with distinguishable size and electronic configuration (ZnII, CdII, and CuII) could manifest the wide matching capability of flexible macrocyclic ligand [H2L1]2−. However, is there any discrepancy between their thermodynamic stability? What is the role of different halides in building macrocycles? Is it possible to achieve transmetalation among them? If so, is the transmetalation process reversible and are the central metal ions replaced one-by-one or simultaneously? Is there any way to monitor this process or capture certain intermediates such as through use of ESI-MS and UV−vis spectra? Bearing all the above-mentioned questions in mind, in situ metal-ion exchange C

DOI: 10.1021/acs.inorgchem.7b02835 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. ESI-MS spectra recorded in the metal-ion exchange from CdII to ZnII macrocycles corresponding to (A) 2a in CH3OH prior to reaction (1 equiv); (B) 10 min reflux, and (C) 2 h reflux after the addition of ZnCl2 (1 equiv) in A.

Schiff-base macrocyclic ZnII and CdII complexes 1a/2a and 1b/ 2b (λmax = 369−374 nm), only UV−vis titration involving CdII → CuII and ZnII → CuII transmetalation was done. As a result, UV−vis titration experiments were performed in EtOH by controlling different molar ratios of starting materials (1a, 1b, and 2b) and CuCl2/CuBr2 both under the RT and reflux conditions. First, in the CuCl2 titration experiment on 1a at RT, a clear bathochromic shift is observed from the absorption band at 369 nm in 1a to 379 nm in 3a, accompanied by a continuously increased absorption in the lower energy band (left in Figure 4A) as well as the color variation from yellow to green for the reaction mixture. Notably, an isosbestic point is present at 331 nm during the titration, indicating the existence of an equilibrium between 1a and 3a during the metal-ion exchange. Second, CuCl2 mediated ZnII → CuII transmetalation from 1a to 4a under the reflux condition follows an alternative process in which a new isoabsorptive point at 415 nm is found in addition to the one at 321 nm (middle in Figure 4A). The distinguishable ZnII → CuII transmetalation UV−vis titration results from 1a to 3a and 4a reflect different reaction routes under the RT and reflux conditions from [2 + 2] macrocyclic dinuclear ZnII complex to [2 + 2] and [1 + 1] macrocyclic dinuclear CuII complexes, respectively. Finally, CuBr2 directed

exchange. It is interesting to mention that the presence of a heterodinuclear [CdZn(H2L1)Cl]+ ion was monitored by ESIMS (m/z = 1309.25) as a key intermediate in the process of CdII → ZnII transmetalation with both the starting material 2a (m/z = 1355.17 corresponding to [Cd2(H2L1)Cl]+) and final product 1a (m/z = 1223.25 and 1259.17 corresponding to [Zn2(HL1)]+ and [Zn2(H2L1)(OH)(H2O)]+), as shown in Figure 3B. After 2 h reflux, it was found that the abundance of dinuclear ZnII complex 1a became dominating in the absence of above-mentioned heterodinuclear intermediate (Figure 3C). Further control experiment manifests only the coexistence of two homodinuclear complexes 2b and 3b when CdBr2 is participated in CdII → CuII transmetalation (Figure 4B), and the transmetalation process completed within half an hour under RT. It was concluded that CdII → ZnII metal-ion exchange reaction is step-by-step and the process is relatively slow, which was verified by the observation of a heterodinuclear CdIIZnII macrocyclic intermediate. In contrast, the replacement of CdII with CuII was rapid and is suggested to be simultaneous. The coexistence of multiple components of metal macrocycles observed in the ESI-MS prompts us to seek more proofs for the details of kinetic and thermodynamic processes in transmetalation. Because of the similarity of UV−vis spectra of D

DOI: 10.1021/acs.inorgchem.7b02835 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (A) UV−vis titration of compound 1a with CuCl2 in EtOH at RT (left) and under reflux (middle) conditions, respectively, and 1b with CuBr2 in EtOH both at RT and under reflux conditions (right). The starting concentration of 1a and 1b is 60 μM (black lines), and that of copper salts is 0, 15, 30, 45, and 60 μM (color lines). (B) ESI-MS spectrum recorded in the process of CdII → CuII transmetalation from 2b to 3b after the RT reaction for 1 min. Inset illustrates the UV−vis titration of 2b with CuBr2 in EtOH both under the RT and reflux conditions. The starting concentration of 2b is 60 μM and that of CuBr2 is 0, 10, 20, 30, 40, 50, and 60 μM.

Figure 5. ORTEP diagrams (30% thermal probability ellipsoids) of the molecular structures of 2−4, where all H atoms, uncoordinated anions, and solvent molecules are omitted for clarity.

ZnII → CuII and CdII → CuII transmetalation from 1b and 2b to 3b was performed for further comparisons. Similar bathochromic shifts from 371 and 374 nm in 1b and 2b to 377 nm in 3b as well as the color change from yellow to green for two reaction mixtures were found no matter what reaction temperature was used. In addition, the isosbestic points at 330

and 357 nm were observed for each reaction (right in Figure 4A and inset of Figure 4B). Crystal Structures of Schiff-Base Macrocyclic Complexes 2−4. ORTEP drawing of the structures, including the atomic numbering scheme, is given in Figure 5. Complexes 2a and 2b are [2 + 2] pendant-armed Schiff-base dinuclear CdII complexes with the same 46-membered macrocyclic skeleton E

DOI: 10.1021/acs.inorgchem.7b02835 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry consisting of [Cd2(H2L1)X]2+ cations and [Cd2X6]2− counterions (X = Cl and Br). Coordinated by two phenolic oxygen atoms, two Schiff-base nitrogen atoms, and one halide anion, each five-coordinate CdII center is severely distorted in a pyramidal way (τ = 0.423 in 2a and 0.368 in 2b) in which the apical position is occupied by the halide ion.26 The halide ion serves as a μ2-bridge linking two CdII centers with the same Cd−Cl and Cd−Br bond distances of 2.553(3) and 2.689(2) Å, and the nonbonded Cd···Cd separations are 3.470(2) Å in 2a and 3.471(1) Å in 2b. Half of the phenolic protons of pendantarmed macrocyclic ligand in 2a and 2b are retained, which can be verified by the electroneutrality principle for the whole molecule. It is worthwhile to point out that two alicylaldehyde rings from each dialdehyde component in the 46-membered [2 + 2] macrocyclic ligand are almost parallel with the dihedral angle of 1.9(4)° in 2a and 1.5(4)° in 2b, while the other two rings have a much larger dihedral angle of 58.6(2)° in 2a and 55.5(3)° in 2b, displaying half-folded configuration of the whole molecule. In addition, the bidentate 1,2-bis(2-aminoethoxy)-ethane components are folded with the N···N separations of 3.785(12) Å in 2a and 3.368 (13) Å in 2b, in which both oxygen atoms are uncoordinated. X-ray single-crystal diffraction analyses reveal that pendantarmed Schiff-base macrocyclic dinuclear CuII complexes 3a and 3b are isomorphous structures except that different halide ions (Cl− and Br−) serve as the μ2-bridging units and counterions, respectively. In comparison with 2a and 2b, 3a and 3b have the same 46-membered [2 + 2] half-folded and half-protonated macrocyclic skeleton. However, the coordination geometry of each five-coordinate CuII center in 3a and 3b is distorted trigonal bipyramidal (τ = 0.673 in 3a and 0.611 in 3b), which is different from the pyramidal configuration in 2a and 2b. It is noted that the above-mentioned dihedral angle of parallel alicylaldehyde rings is enlarged to 10.6(1)° in 3a and 11.7(4)° in 3b, and the other one is decreased to 81.2(2)° in 3a and 85.0(3)° in 3b, respectively, displaying the less half-folded conformation of the same macrocyclic ligand. On the other hand, the separations between the two CuII ions (3.757(1) Å in 3a and 3.903(1) Å in 3b) are longer than the Cd···Cd one (3.470(2) Å in 2a and 3.471(1) Å in 2b). Considering the smaller ionic radius of CuII ion, longer Cu···Cu separation and less half-folded conformation of macrocyclic ligand, it comes to the conclusion that CuII ion has preferential coordination environment than CdII ion, and the flexible [2 + 2] macrocyclic ligand herein has less steric hindrance and ring tension toward the former. Further comparisons with ZnII complexes 1a and 1b reveal the medium configurational parameters (dihedral angles: 8.7(2)/8.0(2)° and 80.9(2)/78.6(2)°; ZnII···ZnII separations: 3.556(4) and 3.476(4) Å), indicating the medium steric hindrance and ring tension of their macrocyclic ligand. So, the above-mentioned analyses and comparisons on [2 + 2] macrocyclic ZnII, CdII and CuII complexes are suggested to provide somewhat the structural proofs on the driving force of transmetalation among them. With regard to the 23-membered half-prontonated [1 + 1] macrocyclic dinuclear CuII complexes 4a and 4b, they are also isomorphous structures composed of two [Cu2(HL2)Cl2]+ cations countered by one [MCl4]2− (M = Cu and Cd) ion. The central CuII ions are in a NO2Cl2 donor set, and their coordination geometry is described as the slightly distorted pyramid with an apical chloride ion (τ = 0.027/0.022 and 0.207/0.003 in 4a, 0.088/0.045 and 0.055/0.001 in 4b). Each chloride ion in the NO2Cl2 donor set is μ2-bridged with two

CuII centers acting as the axial and one of the corner ligands of coordination polyhedron for each CuII ion simultaneously, and the axial Cu−Cl bond lengths (2.897(1)−2.585(2) Å) are significantly longer than the planar ones (2.261(5)−2.221(5) Å) in 4a and 4b, displaying typical Jahn−Teller distortion. Similarly, the apical Cu−Cl bond length (2.538(2) Å) in 3a is longer than the planar ones in 4a and 4b (see Table S1), and the Cu−Br bond length (2.774(2) Å) in 3b is also longer than the reported value of 2.6822(2) Å,27 manifesting again the above-mentioned Jahn−Teller distortion. Unlike the half-folded [2 + 2] macrocyclic ligand in 2a, 2b, 3a, and 3b, the identical 23-membered [1 + 1] one in 4a and 4b presents an unfolded style with the dihedral angle between two alicylaldehyde rings of one dialdehyde unit in the range of 36.6(1)−42.4(2)°, in which every 1,2-bis(2-aminoethoxy)ethane component is extended with the N···N separations falling within 6.629(6)−6.686(19) Å. So, the construction of planar configuration for [1 + 1] macrocyclic ligand with half of the atoms could still accommodate two metal centers, where the extended diamine component serves as a tetradentate ligand. However, the corresponding Cu−O bond lengths (2.116(13)−2.075(12) Å) are slightly longer than the other ones (1.913(11)−1.876(5) Å) with the phenolic oxygen atoms. It should be pointed out that the Cu···Cu separation in 4a and 4b is very short at 3.350(1)−3.267(4) Å, and the Cu−Cl− Cu bond angle (88.0(1)−79.8(1)°) is much smaller than that in 3a (95.5(1)°), which could be attributed to the size restriction of [1 + 1] macrocyclic ligand. In view of the formation of short Cu···Cu separation and small Cu−Cl−Cu bond angle in 4a and 4b, it seems to be less possible to form related [1 + 1] macrocyclic dinuclear CuII complexes having double μ2-bromide bridges because the bromide ion has a larger ionic radius, and it would be difficult to overcome the spatial crowding for such a multidentate macrocyclic ligand with very limited size and short Cu···Cu separation. Here, different halide effects are believed to be one reason why no corresponding [1 + 1] products can be isolated in the control experiments between 1b/2b and CuBr2.

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CONCLUSION In summary, three 46-membered [2 + 2] pendant-armed Schiffbase macrocyclic dinuclear CdII and CuII complexes (2a, 2b, and 3b) and one 23-membered [1 + 1] dinuclear CuII macrocycle 4a were prepared from the template-directed condensation reactions in which halide effects were found to play important roles in organizing the final macrocyclic complexes in addition to the dominant influence of metal ions. The successful construction of [1 + 1] and [2 + 2] pendant-armed Schiff-base dinuclear macrocycles having the same dialdehyde and diamine precursors implies that the configuration of our flexible macrocyclic ligands with distinguishable size and variable coordination modes could be adjusted to meet the stereochemical requirements of different host metal ions and make the transmetalation to be readily manipulated. Therefore, transmetalation was intensively studied among these large and flexible CdII and CuII macrocyclic complexes and two previously synthesized dinuclear ZnII macrocycles (1a and 1b). Our results have demonstrated that [2 + 2] and [1 + 1] macrocyclic dinuclear CuII complexes 3a and 4a could be yielded under the RT and reflux conditions, respectively, via CuCl2 directed syntheses from 1a and 2a, and only the one-way conversion from 3a to 4a is possible. It is worth mentioning that [2 + 2] macrocyclic CuII complex 3a F

DOI: 10.1021/acs.inorgchem.7b02835 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry could be merely produced by the ZnII → CuII or CdII → CuII transmetalation approach. Moreover, the irreversible ZnII → CuII and CdII → CuII transmetalation proceeds more quickly than the CdII → ZnII exchange. The former two processes undergo a simultaneous replacement, while the latter one experiences a step-by-step substitution evidenced by the presence of a heterodinuclear CdIIZnII macrocyclic intermediate in a related ESI-MS spectrum. It is believed that the lower stability of [2 + 2] Schiff-base macrocyclic dinuclear ZnII and CdII complexes compared with CuII ones that makes possible the ZnII → CuII and CdII → CuII transmetalation, and so does the CdII → ZnII transmetalation. In the one-way ZnII → CuII, CdII → CuII, and CdII → ZnII transmetalation, the cleavage of coordinative bonds, metal-ion exchange, and even the cleavage of dynamic covalent CN bonds and subsequent ring contraction take place, which could be impacted by halide effects and experimental conditions. Herein, the driving forces for transmetalation appear to be (1) the general reversibility of the imine units in macrocyclic skeleton; (2) the preferred coordination ability of the hard N and O binding groups to the harder metal centers according to hard soft acid−base (HSAB) theory;28 (3) the preferential match between central metal ions with distinct charge, radius, coordination ability, and spatial requirement for forming coordination sphere and macrocyclic ligands with suitable spatial conformation, minimized steric hindrance, and ring tension; and (4) the trend to form the most thermodynamically stable complexes. It is believed that the significant new advance of this work lies in the successfully transmetalation for Schiff-base macrocyclic complexes with larger and more flexible multidentate ligands, where in situ metal-ion exchange between ZnII, CdII, and CuII Schiff-base macrocyclic complexes and corresponding replacing mechanism have been carefully studied. We anticipate that the current transmetalation study on our flexible macrocyclic ligands could provide a new method to the selective preparation of Schiff-base macrocyclic complexes and a further insight into the DCC of Schiff-base imine bonds.

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ethane (0.016 g, 0.10 mmol) in 20 mL of ethanol was added dropwise. After being refluxed for additional 2 h, the reaction mixture was cooled to RT and filtered. The filtrate was concentrated to obtain compound 2a. Yield: 0.047 g (81%). 1H NMR (400 MHz, DMSO-d6): δ 8.23 (s, 2H), 8.10 (s, 2H), 7.56 (dd, J = 2.9, 4 Hz, 4H), 7.53 (d, J = 2.8 Hz, 2H), 7.39 (d, J = 2.8 Hz, 2H), 4.69 (d, J = 12 Hz, 2H), 4.19 (dd, J = 12.4, 12.8 Hz, 4H), 4.02 (dd, J = 10.4, 12.4 Hz, 2H), 3.72 (dd, J = 12, 9.6 Hz, 2H), 3.46 (s, 2H), 3.43 (d, J = 5.9 Hz, 2H), 3.40 (s, 4H), 3.26 (d, J = 7.4 Hz, 4H), 3.16 (d, J = 12.4 Hz, 4H), 2.99 (d, J = 10.0 Hz, 4H), 2.91 (d, J = 11.2 Hz, 4H), 2.55 (d, J = 10.8 Hz, 2H), 2.41 (d, J = 10.4 Hz, 2H), 2.32 (d, J = 6.0 Hz, 2H), 2.05−1.95 (m, 2H), 1.89 (s, 4H), 1.67−1.46 (m, 8H). Main FT-IR absorptions (KBr pellets, cm−1): 3436, 2920, 2856, 1633 (s, CHN), 1537, 1454, 1172, 1121, 1031, 775. UV−vis in MeOH, λmax = 374 nm. Anal. Calcd for C112H136Cl16N12O16Cd6: C, 42.73; H, 4.35; N, 5.34%. Found: C, 42.99; H, 4.48; N, 5.23%. ESI-MS (positive mode, m/z): 1355.17, [Cd2(H2L1)Cl]+ (100%). Light-yellow single crystals of complex 2a were grown by slow evaporation from a solution of MeCN/EtOH (v/ v = 1:2) in air at RT for 1 week. Synthesis of 2b [Cd2(H2L1)Br]2[Cd2Br6]. The synthetic procedure of complex 2b is the same as that of 2a except that CdBr2·4H2O (0.038 g, 0.11 mmol) was used. Yield: 0.049 g (76%). 1H NMR (400 MHz, DMSO-d6): δ 8.23 (s, 2H), 8.11 (s, 2H), 7.56 (dd, J = 2.8, 4.2 Hz, 4H), 7.53 (d, J = 2.6 Hz, 2H), 7.40 (d, J = 2.7 Hz, 2H), 4.69 (d, J = 12.3 Hz, 2H), 4.19 (dd, J = 13.3, 8 Hz, 4H), 4.03 (m, 2H), 3.73 (m, 4H), 3.44 (m, 8H), 3.27 (d, J = 7.3 Hz, 4H), 3.15 (d, J = 12.6 Hz, 4H), 2.99 (d, J = 10.0 Hz, 4H), 2.91 (d, J = 11.3 Hz, 4H), 2.57 (d, J = 10.3 Hz, 2H), 2.41 (d, J = 10.2 Hz, 2H), 2.33 (d, J = 8.7 Hz, 2H), 1.89 (s, 4H), 1.67−1.46 (s, 8H). Main FT-IR absorptions (KBr pellets, cm−1): 3448, 2937, 2858, 1631 (s, CHN), 1543, 1456, 1432, 1296, 1165, 1092, 1034, 766, 693. UV−vis in MeOH, λmax = 374 nm. Anal. Calcd for C112H136Cl8Br8N12O16Cd6: C, 38.39; H, 3.91; N, 4.80%. Found: C, 38.50; H, 4.06; N, 4.71%. ESI-MS (positive mode, m/z): 1399.08, [Cd2(H2L1)Br]+ (100%). Light-yellow single crystals of complex 2b were grown by slow evaporation from a solution of MeCN/EtOH (v/ v = 2:3) in air at RT for 1 week. Syntheses of 3a [Cu2(H2L1)Cl]Cl. Compound 3a was synthesized by the following two procedures: Methods 1 and 2 (in situ metal-ion exchange). CuCl2·2H2O (0.019 mg, 0.11 mmol) was dissolved in acetonitrile (20 mL) and added into a solution of 1a (0.128 g, 0.10 mmol) and 2a (0.157 g, 0.10 mmol) in ethanol (30 mL). The mixture was stirred for 2 h at RT and filtered. Dark-green single crystals of complex 3a were grown by slow evaporation from the filtrate in air at RT for 2 weeks. Yield: 0.081 g (63%) from 1a and 0.084 g (65%) from 2a. Main FT-IR absorptions (KBr pellets, cm−1): 3433, 2925, 2857, 1633 (s, CHN), 1550, 1460, 1435, 1325, 1223, 1122, 1031, 782, 698, 570. UV−vis in MeOH, λmax = 379 nm. Anal. Calcd for C56H68Cl6N6O8Cu2: C, 52.02; H, 5.30; N, 6.50%. Found: C, 51.78; H, 5.26; N, 6.43%. ESI-MS (positive mode, m/z): 1257.08, [Cu2(H2L1)Cl]+ (100%). Synthesis of 3b [Cu2(H2L1)Br]Br. Compound 3b was synthesized by the following three procedures: Method 1 (CuII-directed template synthesis). The synthetic procedure of complex 3b is the same as that of 2a except that CuBr2 (0.025 g, 0.11 mmol) was used. Yield: 0.060 g (79%). Main FT-IR absorptions (KBr pellets, cm−1): 3433, 2925, 2857, 1632 (s, CHN), 1549, 1460, 1435, 1320, 1219, 1121, 1037, 782, 698. UV−vis in MeOH, λmax = 377 nm. Anal. Calcd for C56H68Cl4Br2N6O8Cu2: C, 48.67; H, 4.96; N, 6.08%. Found: C, 48.78; H, 5.12; N, 5.98%. ESI-MS (positive mode, m/z): 1257.17, {[Cu2(H2L1)Br]−HBr+2H2O}+ (100%) and 1301.25, [Cu2(H2L1)Cl]+. Dark-green single crystals of complex 3b were grown by slow evaporation from a solution of MeCN/EtOH (v/v = 2:3) in air at RT for 2 weeks. Methods 2 and 3 (in situ metal-ion exchange). CuBr2 (0.025 g, 0.11 mmol) was dissolved in acetonitrile (20 mL) and added into a solution of 1b (0.132 g, 0.10 mmol) and 2b (0.175 g, 0.10 mmol) in ethanol (30 mL), respectively. The mixture was stirred for 2 h under reflux condition and filtered. The filtrate was concentrated to give the final product. Yield: 0.097 g (70%) from 1b and 0.101 g (73%) from 2b.

EXPERIMENTAL SECTION

Materials and Methods. Unless otherwise specified, solvents of analytical grade were purchased directly from commercial sources and used without any further purification. N-modified pendant-armed Cldialdehyde29 and macrocyclic ZnII complexes 1a and 1b were prepared via our previously reported method.30 Elemental analyses for carbon, hydrogen, and nitrogen were performed on a PerkinElmer 1400C analyzer. IR spectra (4000−400 cm−1) were collected on a Nicolet FT-IR 170X spectrophotometer at 25 °C using KBr plates. UV−vis spectra were recorded with a Shimadzu UV-3150 double-beam spectrophotometer using a quartz glass cell with a path length of 10 mm. Powder X-ray diffraction (PXRD) measurements were performed on a Philips X′pert MPD Pro X-ray diffractometer using Cu Kα radiation (λ = 0.15418 nm), in which the X-ray tube was operated at 40 kV and 40 mA at RT. 1H NMR spectral measurements were taken on Bruker AM 400 NMR spectrometer using TMS (SiMe4) as an internal reference at RT. None of the Schiff-base macrocyclic dinuclear ZnII and CdII complexes were soluble enough in any solvents, so high-quality 13C NMR spectra could not be recorded. Electrospray ionization mass spectra (ESI-MS) were recorded on a ThermoFisher Scientific LCQ Fleet mass spectrometer within the range 100−2000 amu. Synthesis of 2a [Cd2(H2L1)Cl]2[Cd2Cl6]. CdCl2·2.5H2O (0.025 g, 0.11 mmol) was dissolved in 10 mL ethanol and added into 15 mL of acetonitrile containing Cl-dialdehyde (0.043 g, 0.10 mmol). The mixture was refluxed for 15 min, and then 1,2-bis(2-aminoethoxy)G

DOI: 10.1021/acs.inorgchem.7b02835 Inorg. Chem. XXXX, XXX, XXX−XXX

H

a

C112H136Cl8Br8N12O16Cd6 3503.60 291(2) 0.71073 0.12 × 0.10 × 0.10 orthorhombic Cmca 30.323(2) 20.811(1) 22.857(2) 90 90 90 14424(1) 4/1.613 6896 3.290 −36/36 −23/24 −27/24 6465/371 R1 = 0.1075 wR2 = 0.2718 R1 = 0.1206 wR2 = 0.2797 1.175 1.947/−5.753

C112H136Cl16N12O16Cd6 3147.92 291(2) 0.71073 0.12 × 0.10 × 0.10 orthorhombic Cmca 30.039(4) 21.052(3) 22.725(3) 90 90 90 14371(3) 4/1.455 6320 1.226 −29/35 −25/24 −26/26 6437/371 R1 = 0.0945 wR2 = 0.2333 R1 = 0.1334 wR2 = 0.2549 0.989 2.442/−3.776

R1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = [∑[w(Fo2 − Fc2)2]/∑w(Fo2)2]1/2.

S Max/min Δρ/e Å−3

R1, wR2 (all data)a

empirical formula formula weight temperature/K wavelength/Å crystal size (mm) crystal system space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z/Dcalcd (g/cm3) F(000) μ/mm−1 hmin/hmax kmin/kmax lmin/lmax data/parameters R1, wR2 [I > 2σ(I)]a

2b

2a

Table 1. Crystal Data and Structural Refinements for Complexes 2−4 C56H68Cl6N6O8Cu2 1292.94 291(2) 0.71073 0.10 × 0.10 × 0.10 tetragonal I4̅2d 21.971(1) 21.971(1) 43.875(4) 90 90 90 21180(3) 8/0.811 5360 0.585 −17/26 −26/26 −51/52 9317/354 R1 = 0.0544 wR2 = 0.1120 R1 = 0.0865 wR2 = 0.1219 0.938 0.461/−0.218

3a C56H68Br2Cl4N6O8Cu2 1381.86 291(2) 0.71073 0.10 × 0.10 × 0.10 tetragonal I4̅2d 22.492(1) 22.492(1) 44.367(5) 90 90 90 22444(3) 8/0.818 5648 1.217 −26/26 −24/26 −44/52 9887/354 R1 = 0.0595 wR2 = 0.1479 R1 = 0.0974 wR2 = 0.1605 0.921 1.030/−0.360

3b C56H68Cl12N6O8Cu5 1696.26 291(2) 0.71073 0.12 × 0.12 × 0.10 monoclinic P21/n 13.315(16) 21.27(2) 23.37(3) 90 92.97(2) 90 6613(13) 4/1.704 3436 2.124 −9/15 −25/24 −26/27 11564/788 R1 = 0.1543 wR2 = 0.4106 R1 = 0.1775 wR2 = 0.4196 1.425 3.804/−1.371

4a

C56H68Cl12N6O8Cu4Cd 1745.12 291(2) 0.71073 0.12 × 0.12 × 0.10 monoclinic P21/n 13.340(1) 21.112(2) 23.211(2) 90 92.976(3) 90 6528.6(9) 4/1.775 3512 2.151 −17/16 −27/27 −30/23 15095/784 R1 = 0.0524 wR2 = 0.1099 R1 = 0.0829 wR2 = 0.1199 1.035 1.382/−0.896

4b

Inorganic Chemistry Article

DOI: 10.1021/acs.inorgchem.7b02835 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry 1

Syntheses of 4a [Cu2(HL2)Cl2]2[CuCl4]. Compound 4a was synthesized by the following two procedures. Method 1 (CuII-directed template synthesis). The synthetic procedure of complex 4a is the same as that of 3b except that CuCl2 (0.019 g, 0.11 mmol) was used. Yield: 0.028 g (76%). Main FT-IR absorptions (KBr pellets, cm−1): 3439, 2940, 2856, 1639 (s, CHN), 1550, 1454, 1301, 1108, 1019, 928, 773, 691, 487. UV−vis in MeOH, λmax = 376 nm. Anal. Calcd for C56H68Cl12N6O8Cu5: C, 39.65; H, 4.04; N, 4.95%. Found: C, 39.72; H, 4.12; N, 4.86%. ESI-MS (positive mode, m/z): 744.08, [Cu2(HL2)Cl2]+ (100%). Dark-green single crystals of [1 + 1] ring degradation product 4a were grown by slow evaporation from a solution of MeCN/EtOH (v/v = 2:3) in air at RT for 2 weeks. Method 2 (in situ metal-ion exchange). CuCl2·2H2O (0.019 g, 0.11 mmol) was dissolved in acetonitrile (20 mL) and added into a solution of 1a (0.128 g, 0.10 mmol) in ethanol (30 mL). The mixture was stirred for 2 h under reflux condition and filtered. The filtrate was concentrated to give the final product. Yield: 0.103 g (61%). Synthesis of 4b [Cu2(HL2)Cl2]2[CdCl4]. The synthetic procedure of complex 4b is the same as method 2 of 4a except that dinuclear CdII complex 2a was used as the starting material. Yield: 0.115 g (66%). Main FT-IR absorptions (KBr pellets, cm−1): 3439, 2940, 2856, 1639 (s, CHN), 1550, 1454, 1301, 1108, 1019, 928, 773, 691, 487. UV− vis in MeOH, λmax = 376 nm. Anal. Calcd for C56H68Cl12N6O8Cu4Cd: C, 38.54; H, 3.93; N, 4.82%. Found: C, 38.76; H, 3.78; N, 4.67%. ESIMS (positive mode, m/z): 742.17, [Cu2(HL2)Cl2]+ (100%). Darkgreen single crystals of complex 4b were grown by slow evaporation from a solution of MeCN/EtOH (v/v = 2:3) in air at RT for 1 week. X-ray Data Collection and Resolution. X-ray single-crystal diffraction data for macrocyclic complexes 2a, 2b, 3a, 3b, 4a, and 4b were measured on a Bruker SMART 1K CCD diffractometer using graphite monochromatic Mο Kα radiation (λ = 0.71073 Å). Data collection was performed by using the SMART program and cell refinement and data reduction were made with the SAINT program.31 The crystal system was determined by Laue symmetry, and the space groups were assigned on the basis of systematic absences by using XPREP. The structures were solved by the directed method and refined on F2 by using the full-matrix least-squares methods with SHELXTL version 6.10.32 All nonhydrogen atoms were refined on F2 by full-matrix least-squares procedure using anisotropic displacement parameters. In the case of complexes 2a, 2b, 3a, and 3b, it was found that the solvent molecules were highly disordered. Attempts to locate and refine the solvent peaks were unsuccessful. So, contributions to scattering due to these solvent molecules were removed using the SQUEEZE routine of PLATON. The structure was then refined again using the data generated. The contents of the solvent region are not represented in the unit cell contents in the crystal data. In the case of CdII complexes 2a and 2b, large residual electron density was found close to the heaviest central metal cations and bromide ions because of the conventional heavy atom effects. In addition, satisfactory X-ray diffraction data are difficult to be obtained for complex 4a even after several attempts. However, it is an isomorphous structure of 4b, and the powder XRD of 4a agrees well with the calculated data. Therefore, the structural mode of 4a is reliable, and the final refinement results were acceptable. Hydrogen atoms were inserted in the calculated positions assigned fixed isotropic thermal parameters at 1.2 times the equivalent isotropic U of the atoms to which they are attached (1.5 times for the oxygen atoms) and allowed to ride on their respective parent atoms. The summary of the crystal data, experimental details, and refinement results for six compounds is listed in Table 1; bond distances and angles are given in Table S1, and hydrogen bonding parameters are tabulated in Table S2.

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H NMR, powder XRD, FT-IR, and UV−vis spectra (PDF) Accession Codes

CCDC 1584108−1584113 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, 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

*Tel.: +86-25-89686526; Fax: +86-25-89682309; E-mail: [email protected]. ORCID

Wei Huang: 0000-0002-1071-1055 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of Jiangsu Province (Grant BK20171334), the Major State Basic Research Development Programs (Grant 2013CB922101), and the National Natural Science Foundation of China (Grant 21171088).

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REFERENCES

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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.7b02835. Tables of selected bond distances and angles and hydrogen-bonding interactions and figures of ESI-MS, I

DOI: 10.1021/acs.inorgchem.7b02835 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02835 Inorg. Chem. XXXX, XXX, XXX−XXX