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Supramolecular Aggregates of Defined Stereochemical Scaffolds: Aggregation/Deaggregation in Schiff-Base Zinc(II) Complexes Derived from Enantiopure trans-1,2-Diaminocyclohexane Giuseppe Consiglio,† Ivan Pietro Oliveri,† Salvatore Failla,* and Santo Di Bella* Dipartimento di Scienze Chimiche, Università di Catania, I-95125 Catania, Italy S Supporting Information *

ABSTRACT: This contribution explores, through detailed 1H NMR, DOSY NMR, optical absorption, and circular dichroism spectroscopic studies, the aggregation properties in solution of noncoordinating solvents of some new ZnII Schiff-base complexes, (R)-1, (S)-1, and (R)-2, derived from the chiral trans-1,2-diaminocyclohexane. It is found that chloroform solutions of 1 are characterized by the presence of three species, the predominance of which consists of large oligomeric aggregates. For chloroform solutions of 1, upon heating or standing, all species are irreversibly converted into a dimer, 1C, which is very stable and hardly disaggregable. Analysis of 1H NMR, UV/vis, and CD spectroscopic data and chemical evidence allow proposing a double helicate Zn2L2 structure with a tetrahedral coordination around the ZnII ions for 1C, as a consequence of the defined stereochemistry of the trans-1,2-diaminocyclohexane chelate bridge. This represents a different, uncommon aggregation mode in ZnII complexes of tetradentate Schiff-bases.

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INTRODUCTION Molecular aggregation plays a crucial role in the design and development of new materials,1 with diverse optical, electrical, or magnetic properties with respect to those of constituent molecules,2 including mimicking biological systems.3 Bis(salicylaldiminato) zinc(II) Schiff-base complexes have recently been investigated for their interesting aggregation properties. In fact, these Lewis acidic complexes in the absence of Lewis bases aggregate via intermolecular Zn···O interactions, either in solution4,5 or in the solid state,6 with formation of various supramolecular structures7 and self-assembled nanostructures.8 In contrast, in the presence of Lewis bases remarkable changes of optical spectroscopic4,9,10 and secondorder nonlinear optical properties11 are involved, as a result of their deaggregation and formation of the Lewis acid−base adducts. Moreover, these features influence the cellular uptake and subcellular localization.12 In order to investigate how the definite stereochemistry of the nonconjugated bridging diamine influences the aggregation properties of this kind of complexes, we recently studied a ZnII Schiff-base complex derived from the cis-1,2-diaminocyclohexane.13 This complex is characterized by distinct aggregation © XXXX American Chemical Society

properties, as both the X-ray crystal structure and studies in solution indicate the formation of rigid asymmetric dimeric aggregates by the presence of the cis-1,2-diaminocyclohexane bridge.13 These results stimulated us to extend these studies also to the other isomers of the 1,2-diaminocyclohexane. The latter species, in fact, possessing two chiral stereocenters, give a couple of enantiomers, in addition to the meso cis-isomer. In this paper, we report a study in solution through detailed 1 H NMR, DOSY NMR, UV/vis, and circular dichroism (CD) spectroscopic investigations on the aggregation of some new ZnII Schiff-base complexes derived from the chiral (1S,2S)-(+)or (1R,2R)-(−)-trans-1,2-diaminocyclohexane (Chart 1). Spectroscopic and chemical evidence allow proposing a different aggregation mode in these ZnII complexes of tetradentate Schiff-bases. Received: July 1, 2016

A

DOI: 10.1021/acs.inorgchem.6b01580 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

reported only one example of an X-ray structure of a Schiff-base zinc(II) complex derivative from (1S,2S)-(+)-1,2-diaminocyclohexane, obtained from solutions of noncoordinating solvents, which gives a double helicate structure with a distorted tetrahedral geometry around the two zinc centers.17 1 H NMR Studies. The 1H NMR spectrum of (R)-1 in solution of DMSO-d6 (≈1 × 10−2 M) is consistent with the presence of monomeric species,4,13 presumably as (R)-1· DMSO adducts (Figure 1). On switching to noncoordinating solvents (CDCl3, ≈1 × 10−2 M), a substantial change of the 1H NMR spectrum, which becomes richer as far as the number of signals is concerned (Figure 1), is noted, indicating the existence of various species in solution. In fact, the low-field region (6−9 ppm) shows the presence of mainly three sets of signals with different intensity, the most intense of which, unexpectedly, have chemical shifts comparable to those found in DMSO-d6. Instead, the other two sets of signals are upfield shifted. The detailed assignment of each set of aromatic hydrogen signals of each species was made by means of selected 1D TROESY experiments, which indicate that the H1 protons are spatially close to the H4 and H2 hydrogens (see Figures 1 and S2). Therefore, 1H NMR and 1D T-ROESY analyses of (R)-1 in CDCl3 solution suggest the existence of three species, (R)1A−C, with different spectral characteristics: (R)-1A, relatively sharp signals, with chemical shifts comparable to those found in DMSO-d6; (R)-1B, broad signals, whose H4 and H1 signals are upfield shifted (≈0.15 ppm) with respect to those of the species (R)-1A; (R)-1C, sharp signals whose H4 and H1 signals are strongly upfield shifted (0.97 and 0.49 ppm, respectively) with respect to those of the species (R)-1A (Figure 1). Chloroform solutions of (R)-1 exhibit a strong concentration dependence. In particular, starting from concentrated solutions (1.0 × 10−2 M), the progressive dilution involves an increased complexity of the 1H NMR spectrum as far as the number of

Chart 1

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RESULTS The synthesis of 1 was simply accomplished by the template method as previously reported.13 MALDI-TOF/TOF mass spectrometry analysis of (R)-1 indicates the presence of defined signals corresponding to the dimer and trimer, in addition to the protonated molecular ion (Figure S1). For the sake of comparison with our previous study on the cis-1,2-diaminocyclohexane ZnII Schiff-base complex, 3,13 we considered the 4-methoxy-salicylidene derivatives of the (1S,2S)-(+)- or (1R,2R)-(−)-trans-1,2-diaminocyclohexane. Unfortunately, all attempts to grow suitable crystals of 1 from solutions of noncoordinating solvents for X-ray structure determination were unsuccessful. On the other hand, there are available X-ray structures of various substituted bis(salycilaldiminato)Zn(II) Schiff-base derivatives from trans1,2-diaminocyclohexane, achieved from solutions of coordinated solvents which always lead to the formation of the adduct with the solvent axially coordinated to the ZnII center.14 Analogously, pyridyl-functionalized salen derivatives of trans1,2-diaminocyclohexane give self-assembly metallacycles15 or chiral metallopolymers16 in which the pyridyl groups are axially coordinated to the ZnII centers. However, in the literature is

Figure 1. Comparison of 1H NMR spectra of (R)-1 in DMSO-d6 and CDCl3 solutions (1.0 × 10−2 M; 27 °C). The labeling of the 1H NMR signals related to the CDCl3 solution refers to species (R)-1A (denoted by the black triangles),(R)-1B (denoted by the blue circles), and (R)-1C (denoted by the red squares). B


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Inorganic Chemistry

Table 1. Diffusion Coefficients, D, and Estimated Molecular Mass, m, for Compounds (R)-1 and (R)-2 (1.0 × 10−2 M; 27 °C) Compd

D

Species

D (solvent) −1

×10 /m s 10

(R)-1 (R)-1

(R)-1d

(R)-1e (R)-2

1·DMSO-d6 1A 1B 1C 1A 1B 1C 1·DMSO-d6 1C 2A 2B 2C

2

×10 /m s 10

2

−1

m (n)a

m (n)b

Da

Da

2.82 2.00 6.20 6.41 1.91

7.35 25.01 25.01 25.01 24.20

(DMSO-d6) (CDCl3) (CDCl3) (CDCl3) (CDCl3)

572c 9162 (20.5) 953 (2.1) 892 (2) 8282 (18.5)

530.0 8917 (20) 891.7 (2) 891.7 (2) 8025.3 (18)

5.82 7.20 6.41 2.01 6.22 6.40

24.20 24.20 24.80 25.01 25.01 25.01

(CDCl3) (CDCl3) (CDCl3) (CDCl3) (CDCl3) (CDCl3)

892 (2) 583 (1.1) 892 (2) 8395 (20.3) 877 (2.1) 828 (2)

891.7 (2) 530.0 (1) 891.7 (2) 8277 (20) 827.7 (2) 827.7 (2)

a

Estimated molecular mass using the species (R)-1C (or (R)-2C) as internal reference. Values in parentheses (n) indicate the order of aggregation. Actually, the choice of (R)-1C as internal reference in CDCl3 gives an estimated molecular mass for the (R)-1·DMSO-d6 adduct consistent with that estimated in DMSO-d6 solvent. bExpected molecular mass. Values in parentheses (n) indicate the order of aggregation. cEstimated molecular mass using the solvent as internal reference. dCDCl3 solution of (R)-1 (1.0 × 10−2 M; 6.0 × 10−6 mol) upon addition of (7.0 × 10−5 mol) of DMSO-d6 (see Figure S6). eCDCl3 solution of (R)-1 upon heating (60 °C; 5 h) (see Figure S9).

Figure 2. 1H NMR DOSY spectrum of (R)-1 in CDCl3 (1.0 × 10−2 M; 27 °C). The labeling of 1H NMR signals refers to species (R)-1A (denoted by the black triangles), (R)-1B (denoted by the blue circles), and (R)-1C (denoted by the red squares).

the 1H NMR DOSY spectrum of (R)-1 in CDCl3 (1.0 × 10−2 M) is separated into three components in the diffusion dimension, associated with the sets of signals related to species (R)-1A−C (Figure 2). Both (R)-1B and (R)-1C species show comparable D values consistent with the existence of dimers (Table 1). Instead, the species (R)-1A is associated with a definitely lower D value (D = 2.0 × 10−10 m2 s−1) and, hence, a higher molecular mass, about 1 order of magnitude larger (Table 1). Dynamic light scattering (DLS) measurements of (R)-1 in CHCl3 (1 × 10−2 M) show the presence of species of ca. 50nm-size (average), further supporting the existence of larger, than simple dimeric, aggregates (Figure S4). Instead, DLS measurements at lower concentrations (≤10−3 M) do not give any signal.

signals is concerned, mainly because of the conversion of the initial predominant species (R)-1A in other new species (Figure S3). Despite the complexity of the 1H NMR spectrum at lower concentrations, it is possible to recognize the signals related to the species (R)-1C, which appear sharp and unaltered. Diffusion ordered NMR spectroscopy (DOSY) has been used to estimate the degree of aggregation and the molecular mass of species in solution, through the measurement of the relative diffusion coefficient, D, by using a known internal reference species.4,13 The 1H NMR DOSY spectrum of (R)-1 in DMSO-d6 (≈1 × 10−2 M) shows a single component in the diffusion dimension (D = 2.8 × 10−10 m2 s−1), whose estimated molecular mass (572 Da) is consistent with a monomeric species, as 1·DMSO adducts, with DMSO axially coordinated (Table 1). In contrast, C


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Inorganic Chemistry

Figure 3. 1H NMR spectra of (R)-1 in CDCl3 (1.0 × 10−2 M) recorded at 27 °C (a), heated up to 60 °C and then cooled at 27 °C (b), and heated at 60 °C for 5 h and then cooled at 27 °C (c). The labeling of 1H NMR signals refers to species (R)-1A (denoted by the black triangles), (R)-1B (denoted by the blue circles), and (R)-1C (denoted by the red squares). 1

surprisingly, under such conditions a partial and irreversible conversion of the species (R)-1A into (R)-1C is observed. In fact, on cooling down the solution to ambient temperature, the relative intensity ratio of the 1H NMR signals related to species (R)-1A and (R)-1C remains almost unaltered (Figure S7). Likewise, starting from a dilute CDCl3 solution of (R)-1 (5 × 10−4 M), an analogous behavior is observed (Figure S8). In fact, the species present in solution are partially converted into (R)-1C. These results suggested further investigations. Thus, the 1H NMR spectrum of a CDCl3 solution of (R)-1 (1 × 10−2 M) sealed in a NMR tube, heated at 60 °C for 5 h, and then cooled to room temperature indicates an complete conversion of species (R)-1A and (R)-1B into (R)-1C (Figure 3). In fact, the 2D DOSY spectrum of the heated solution shows a single component in the diffusion dimension related to species (R)1C (Figure S9, Table 1). An analogous behavior, in terms of complete conversion of species (R)-1A and (R)-1B into (R)1C, is observed after allowing CDCl3 solutions of (R)-1 (1 × 10−2 M) to stand at room temperature for a week. The isolated off-white solid obtained from heated chloroform solutions of (R)-1 was characterized by 1H NMR spectroscopy and identified as (R)-1C. 2D T-ROESY experiments of (R)-1C indicate that, analogously to 1D T-ROESY results, the H1 protons show dipolar interactions with the H4 and H2 hydrogens. In addition, the H4 protons are spatially close to hydrogens on the α- and β-carbons of the cyclohexane rings (Figure S10). The ESI mass spectrum of (R)-1C (Figure S11) shows a molecular peak [M + H]+ at m/z = 893, consistent with the molecular formula Zn2L2 (L = deprotonated Schiffbase ligand). Dilution studies indicate that the species (R)-1C does not show any concentration dependence in the 1 × 10−2 M to 5 ×

H NMR studies of (R)-1 in mixtures of noncoordinating/ coordinating (CDCl3/DMSO-d6) solvents further support the existence of a mixture of aggregate species in the former solvent. Actually, the addition of defined amounts of DMSO-d6 to a CDCl3 solution of (R)-1 leads to appreciable 1H NMR spectral changes (Figure S5). In particular, the addition of 10fold mole excess of DMSO-d6 leads to the complete disappearance of species (R)-1B and the appearance of a new set of signals consistent with the formation of the (R)-1·DMSO adduct. The successive addition of DMSO-d6 leads to the progressive disappearance of species (R)-1A and the increase of the signals related to the (R)-1·DMSO adduct. Upon addition of 100-fold mole excess of DMSO-d6, we observe the complete disappearance of species (R)-1A, and the resulting solution shows a 1H NMR spectrum almost comparable to that recorded in DMSO-d6 (Figure S5). However, even in such large stoichiometric excess the species (R)-1C remains almost unaltered, indicating a strong stability of this dimeric species. It is noteworthy that 1H NMR signals related to the formation of the (R)-1·DMSO adduct, recorded upon successive addition of DMSO-d6 to a CDCl3 solution of (R)1, are distinct with respect to those of the aggregate species (R)-1A and (R)-1C. This is confirmed by the 2D DOSY spectrum of a CDCl3 solution of (R)-1 after the addition of 10fold mole excess of DMSO-d6, which clearly shows the D values related to species (R)-1A and (R)-1C, in addition to that of the (R)-1·DMSO adduct, and the complete disappearance of the species (R)-1B (Figure S6, Table 1). In order to further investigate the behavior of the aggregate species (R)-1 in CDCl3 solution (1 × 10−2 M), we performed variable-temperature 1H NMR studies (Figure S7). No coalescence temperature is found until 333 K, the maximum temperature achievable with chloroform. Instead, rather D


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Inorganic Chemistry 10−4 M range considered. Moreover, 1H NMR studies of (R)1C in mixtures of noncoordinating/coordinating (CDCl3/ DMSO-d6) solvents indicate that the addition of 104-fold mole excess of DMSO-d6 to a CDCl3 5 × 10−4 M solution of (R)-1C gives rise to its partial deaggregation (Figure S12). On the other hand, the 1H NMR spectrum obtained by dissolving (R)1C in DMSO-d6 is identical to that recorded for (R)-1 in the same solvent (Figure 1). In order to evaluate the effect of a different substituent on the salicylidene rings on the aggregation of such complexes, we considered the 4-methyl derivative (R)-2. Actually, 1H NMR spectra of (R)-2 in both coordinating, DMSO-d6, and noncoordinating, CDCl3, solvents are fully comparable to those recorded for (R)-1, suggesting the presence mainly of three species, (R)-2A−C (Figure S13). Analogously, the 1H NMR DOSY spectrum of (R)-2 in CDCl3 (≈1 × 10−2 M) indicates that the species at lower diffusion coefficient, (R)-2A, is predominant, as found for (R)-1 (Table 1). As expected, the enantiomer ZnII complex derivative from the (1S,2S)-(+)-1,2-diaminocyclohexane, (S)-1, shows an identical behavior of (R)-1 in both coordinating and noncoordinating solvents. Optical Absorption and Circular Dichroism Spectroscopic Studies. The UV/vis absorption spectrum of (R)-1 in DMSO consists of two defined bands, at 282 and 343 nm, and a shoulder at ca. 363 nm. Changing to the CHCl3 solvent, the UV/vis absorption spectrum is analogous to the one in DMSO, except for a red-shift, ca. 4 nm, and a slight increase of the intensity of the band at 282 nm, while the band at longer wavelength is blue-shifted (336 nm) and less intense (Figure 4a). Moreover, no appreciable spectral changes are observed in the concentration range (1.0 × 10−3 M to 5.0 × 10−5 M) explored (Figure S14), considering that 1.0 × 10−3 M is the highest accessible concentration for optical spectroscopic measurements of (R)-1. CD spectra of (R)-1 and (S)-1 in both solvents indicate the presence of bisignate signals in the

range 250−400 nm (Figure 4b). In particular, the stronger signals at longer wavelengths reflect the variations observed in the absorption spectra. In fact, CD spectra in CHCl3 show the presence of a new signal, indicative of the existence of new species in this solvent. In agreement with UV/vis absorption spectra, CD spectra of both enantiomers in CHCl3 indicate no appreciable concentration dependence (Figure S15). The progressive addition of DMSO to a 1.0 × 10−3 M CHCl3 solution of (R)-1 leads to an optical absorption spectrum analogous to that observed in DMSO. In particular, a gradual increase of the band intensity at 343 nm with the presence of an isosbestic point at 332 nm and a blue-shift and decrease of the intensity of the band at 286 nm are observed. The saturation point is reached after addition of ca. 700-fold mole excess of DMSO (Figure 5).

Figure 5. UV/vis absorption spectra of (R)-1 (1.0 × 10−3 M; 2.5 × 10−7 mol) in CHCl3 and with the addition of DMSO. The absorption spectrum of (R)-1 in DMSO (dotted line) is reported for comparison.

After heating (60 °C, 5 h) of the chloroform solutions of (R)-1, either 1.0 × 10−3 M or 5.0 × 10−4 M, an appreciable variation of the UV/vis absorption spectra, associated with the formation of the (R)-1C species, is observed. In particular, the formation (R)-1C is accompanied by a general red-shift and increase of the intensity of the absorption spectrum, with the appearance of a new feature at 371 nm (Figure 6a). Moreover, a stronger bisignate signal, with a positive (negative for (S)-1C) Cotton effect at 357 nm, a negative (positive (S)-1C) Cotton effect at 386 nm, and a zero cross point at 369 nm, is observed for (R)-1C (Figure 6b). No appreciable deaggregation of this species is observed upon addition of up to 104-fold mole excess of DMSO, as the optical absorption spectra remain almost unaltered (Figure S16).

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DISCUSSION The synthesis of the chiral Schiff-base ZnII complexes 1 and 2, derived from (1S,2S)-(+)- or (1R,2R)-(−)-trans-1,2-diaminocyclohexane, soluble in both coordinating and noncoordinating solvents, allowed us to perform detailed spectroscopic studies in solution in relation to their aggregation/deaggregation properties. Thus, while in coordinating solvents these complexes, as usual, are found as monomeric adducts with the solvent axially coordinated, in noncoordinating solvents they exhibit an unusual behavior. 1 H NMR studies in CDCl3 solution at relatively high concentrations (≈10−2 M) suggest the existence of three different species, one of which is predominant (1A). DOSY measurements indicate that while the species present in a minor

Figure 4. (a) UV/vis absorption spectra of (R)-1 (1.0 × 10−3 M) in CHCl3 and DMSO. (b) CD spectra of (R)-1 and (S)-1 (1.0 × 10−3 M) in CHCl3 and DMSO. E


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Inorganic Chemistry

The isolated off-white solid obtained from heated CHCl3 solutions of 1 was characterized in chloroform solutions by ESI and DOSY measurements as a dimeric species having identical 1 H NMR spectroscopic characteristics of 1C. Moreover, 1C shows distinct optical absorption and CD features compared to those of chloroform solutions of 1. Unfortunately, all attempts to grow crystals 1C from noncoordinating solvents suitable for X-ray structure determination failed. Chloroform solutions of 1C are very stable, as they remain unaltered after several weeks and upon dilution. Moreover, even after the addition of 104-fold mole excess of DMSO-d6 to diluted CDCl3 solutions of 1C, only a partial deaggregation is observed. It is noteworthy that, compared to 1, the formation of the species 1C is accompanied by a significant enhancement of a bisignate CD signal. Overall, these results suggest that 1C in chloroform solution likely adopts a different coordination mode around the ZnII metal center. As a consequence of the stereochemistry of the trans-1,2-diaminocyclohexane chelate bridge, we hypothesize a tetrahedral, Zn2L2 coordination for the ZnII metal ion with a double helicate structure (Figure 7). This hypothesis is

Figure 6. (a) UV/vis absorption spectra of (R)-1 (1.0 × 10−3 M) in CHCl3 before and after heating (species (R)-1C). (b) CD spectra of (R)-1 and (S)-1 (1.0 × 10−3 M) in CHCl3 before and after heating (species (R)-1C and (S)-1C).

amount (1B, 1C) have a diffusion coefficient consistent with dimers, the predominant species 1A shows a relatively low diffusion coefficient, indicating the presence of larger oligomeric aggregates, further supported by DLS measurements. Unexpectedly, despite their large estimated molecular mass, 1A shows well-resolved 1H NMR signals, with a visible 3 JHH coupling of the aromatic hydrogens. This is in contrast with our previous investigations, because we found that larger aggregates were associated with a broadening of all 1H NMR signals.4 These observations suggest that 1A is likely operating in a different, uncommon aggregation mode than the typical intermolecular Zn···O interactions involved in Schiff-base ZnII complexes.4−7 Studies in mixtures of noncoordinating/coordinating solvents of aggregate species 1A−C in chloroform solution show a preferential deaggregation of the species present in solution. In particular, with addition of DMSO, the species 1B first undergoes deaggregation, followed by species 1A. In contrast, the species 1C remains almost unaltered, even with the addition of a 100-fold mole excess of DMSO-d6. This behavior indicates a different Lewis acidity of the various aggregate species 1A−C in solution. It is noteworthy that the formation of the 1·DMSO adduct involves the appearance in the 1H NMR spectrum of a new distinct set of signals, thus suggesting a slow equilibrium, compared to the NMR time-scale, between the monomeric adduct and the aggregate species. The observed bisignate CD signal for the (R)- and (S)-1·DMSO adducts is consistent with a monohelical structure.14c The above results suggested further studies. Dilution studies indicate that while the predominant oligomer 1A is converted into other species, 1C remains unaltered. On the other hand, by heating either diluted or concentrated chloroform solutions of 1, all species present in solution are irreversibly converted into 1C, as demonstrated by 1H NMR, DOSY, UV, and CD spectroscopic studies. The same result is obtained after standing chloroform solutions of 1 for several days.

Figure 7. DFT optimized geometry (B3LYP) for the (S)-1C enantiomer. Wireframe view (top); space filling model (bottom).

corroborated by the comparison with the aggregation properties of the cis-isomer Zn(II) derivative, 3, which exhibits the typical pentacoordination and aggregation mode.13 This proposed structure explains the unusual features observed for 1C. In particular, the significant upfield shift on switching from coordinating to noncoordinating solvents, ca. 1 ppm for H4 protons and ca. 0.5 ppm for the H1 protons (Figure 1), indicating that the involved hydrogens lie under the shielding zone of the π electrons of a conjugated system,4 is consistent with such a helicate structure (Figure S17). Moreover, the CHN chemical shift of 1C in CDCl3 is comparable (Table 2) with the upfield value measured for the F


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Inorganic Chemistry

1C and (S)-1C and their Lewis acidity. In particular, the tetrahedral coordination around the ZnII ions fulfills the coordination sphere of the metal centers, thus reducing their Lewis acidic character. In summary, the present results support a different aggregation mode than the typical intermolecular Zn···O interactions, in these ZnII complexes of tetradentate Schiffbases of defined stereochemistry, and offer new ideas for further investigations.

Table 2. Comparison of the CHN Chemical Shift for Some 1,2-Cyclohexanediamine ZnII Schiff-Base Complexes in CDCl3 Solution Compd 3 1C 4 Zn2L′2d a

Bridge cis-1,2cyclohexanediyl trans-1,2cyclohexanediyl trans-1,2cyclohexanediyl trans-1,2cyclohexanediyl

δ, ppm (CDCl3)

ZnII coordinationa

8.08; 8.33

penta

13

7.45b

tetrac

this work 17

7.30

tetrac

18

ref

7.35

b

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EXPERIMENTAL SECTION

Materials and General Procedures. Zinc perchlorate hexahydrate, 2-hydroxy-4-methoxybenzaldehyde, 2-hydroxy-4-methylbenzaldehyde, (1R,2R)-(−)-1,2-diaminocyclohexane, (1S,2S)-(+)-1,2-diaminocyclohexane, and triethylamine (NEt3) (Aldrich) were used as received. Chloroform (Aldrich) stabilized with amylene was used for spectrophotometric measurements. CDCl3 (Aldrich) was dried over molecular sieves (3 Å). Physical Measurements. Elemental analyses were performed on a Carlo Erba 1106 elemental analyzer. Fourier transform infrared (FTIR) spectra were recorded with KBr pellets with a PerkinElmer Spectrum 100 FT-IR spectrometer. ESI mass spectra were recorded on a Thermo Scientific (Linear Trap) LTQ-XL electrospray mass spectrometer (Bremen, Germany). All the samples were dissolved in chloroform (1.0 × 10−4 M) and diluted with acetonitrile at 2.0 × 10−5 M. Spectra were acquired in positive mode. MALDI mass spectra were obtained using an 5800 MALDI-TOF/TOF mass spectrometer (Sciex) equipped with an automated single-plate sample-loading system, 1 kHz OptiBeam On-Axis Laser Nd:YAG 349 nm wavelength, delayed-extraction (DE), two acceleration regions, QuanTis Precursor Ion Selector, CID cell, two-stage reflector mirror and a 1000 MHz digitizer. Sinapinic acid in THF solvent was used as matrix. Solution NMR experiments were carried out on a Varian Unity S 500 (499.88 MHz for 1H) spectrometer. 1D 1H NMR experiments were referenced to TMS. 1H DOSY measurements were performed as reported elsewhere.4 DLS measurements were performed at 25 °C with a Zetasizer NanoZS (Malvern Instruments, Malvern, UK) at a detection angle of 173° with a He−Ne laser (633 nm) as the incident beam. All the solutions used for DLS were filtered before measurements through a 0.2 μm PTFA filter. Optical absorption and CD spectra were recorded at room temperature using a UV/vis V650 Jasco spectrophotometer a JASCO J-710 and spectropolarimeter, respectively. All UV/vis and CD measurements were recorded using a 1 mm path length cuvette. Computational Method. Geometry optimization for (S)-1C was performed by means of first principle DFT calculations, using Becke’s three-parameter exchange functional supplemented with the Lee− Yang−Parr correlation functional, B3LYP.21 The tight optimization criteria along with an ultrafine grid were adopted for the geometry optimization. Calculations were computed with the Gaussian 09 program22 using the 6-31G basis set. General Procedure for the Synthesis of Complexes 1 and 2. The complexes were prepared by the template method used for the synthesis of complex 3.13 [N,N-Bis(4-methoxy-2-hydroxybenzylidene)-(1R,2R)-1,2diaminocyclohexanediaminato]ZnII ((R)-1). 2-Hydroxy-4-methoxybenzaldehyde (0.304 g, 2.00 mmol), (1R,2R)-(−)-1,2-diaminocyclohexane (0.114 g, 1.00 mmol), and zinc perchlorate hexahydrate (0.372 g, 1.00 mmol) were used. Off-white powder (0.378 g, 85%). C22H24N2O4Zn (445.82): Calcd C, 59.27; H, 5.43; N, 6.28; Found C, 59.20; H, 5.48; N, 6.33. MALDI-TOF/TOF MS: m/z = 445.16 [M + H]+, 893.22 [2 M + H]+, 1403.33 [3 M + 3Na−2H]+. 1H NMR (500 MHz, DMSO-d6): δ = 1.30 (br, 4H, cyclohexyl-H), 1.89 (br, 2H, cyclohexyl-H), 2.40 (br, 2H, cyclohexyl-H), 3.10 (br, 2H, −CH-N CH), 3.69 (s, 6H; OCH3), 6.04 (dd, 3JHH = 8.5 Hz, 4JHH = 3.0 Hz, 2H; ArH), 6.09 (d, 4JHH = 3.0 Hz, 2H; ArH), 7.08 (d, 3JHH = 8.5 Hz, 2H; ArH), 8.20 (s, 2H; CHN). 13C NMR (125 MHz, DMSO-d6): δ = 24.44, 28.21, 55.14, 64.87, 102.17, 104.92, 114.53, 136.98, 163.68, 163.97, 173.22.

c

Determined by X-ray analysis. See Experimental Section. Doublestranded helical structure. dL′ = bis(pyrrol-2-ylmethyleneamine)cyclohexane.

analogous Zn2L2 complex, 4, obtained from (1S,2S)-(+)-trans1,2-diaminocyclohexane and 3-tert-butyl salicylaldehyde, as a consequence of its double helicate structure with a tetrahedral geometry around the zinc centers.17 An analogous comparison can be done with the CHN chemical shift of the double helicate Zn2L′2 trans-1,2-diaminocyclohexane-(pyrrol-2-ylmethyleneamine) derivative.18 In contrast, the meso cis-isomer, 3, having an asymmetric dimeric structure with pentacoordinated Zn atoms, shows considerable CHN downfield shifted resonances (Table 2). The enhanced bisignate CD signals on switching from (R)-1 and (S)-1 to (R)-1C and (S)-1C, respectively, are also consistent with the suggested helicate structure, as found for the chiral Zn2L′2 complexes.19 Finally, the observed stability of 1C upon deaggregation with DMSO indicates a very low Lewis acidity of this species. Again, this is in accord with the proposed tetrahedral coordination around the ZnII ions, thus fulfilling the coordination sphere of the metal centers and, hence, reducing their Lewis acidic character.

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CONCLUSIONS This work explores the effect of a bridging diamine having a defined stereochemistry on the aggregation properties of some ZnII Schiff-base complexes. Actually, the Schiff-bases derived from the (1R,2R)-(−)- or (1S,2S)-(+)-trans-1,2-diaminocyclohexane establish a preorganized helical scaffold for the metal complexation.18,20 It is therefore expected that this can play a crucial role on the structural and aggregation properties of these complexes. Indeed, present chiral complexes in solution are characterized by a number of unusual features. 1H NMR data in CDCl3 solution at relatively high concentrations suggest the existence of three different species, 1A−C, the first of which is predominant. While 1B and 1C are dimeric species, 1A is consistent with an oligomeric species containing about 20 monomeric units. For chloroform solutions of 1, upon heating or standing, all species are irreversibly converted into 1C. The isolated (R)-1C and (S)-1C products are characterized as a Zn2L2 species having distinct optical absorption and CD features compared to those of chloroform solutions of (R)-1 and (S)-1. Chloroform solutions of 1C are very stable and hardly disaggregable, since they remain almost unaltered even after the addition of 104-fold mole excess of DMSO. Overall these data suggest a double helicate structure with a tetrahedral coordination around the ZnII metal ion for Zn2L2, as a consequence of the defined stereochemistry of the trans-1,2diaminocyclohexane chelate bridge. This proposed structure explains the unusual spectroscopic properties observed for (R)G


Inorganic Chemistry

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[N,N-Bis(4-methyl-2-hydroxybenzylidene)-(1R,2R)-1,2diaminocyclohexanediaminato]ZnII ((R)-2). 2-Hydroxy-4-methylbenzaldehyde (0.272 g, 2.00 mmol), (1R,2R)-(−)-1,2-diaminocyclohexane (0.114 g, 1.00 mmol), and zinc perchlorate hexahydrate (0.372 g, 1.00 mmol) were used. Pale-yellow powder (0.310 g, 75%). C22H24N2O2Zn (413.83): Calcd C, 63.85; H, 5.85; N, 6.77; Found C, 63.90; H, 5.79; N, 6.78. MALDI-TOF/TOF MS: m/z = 413.18 [M + H]+, 827.23 [2 M + H]+. 1H NMR (500 MHz, DMSO-d6): δ = 1.34 (br, 4H, cyclohexyl-H), 1.90 (br, 2H, cyclohexyl-H), 2.17 (s, 6H, ArCH3), 2.42 (br, 2H, cyclohexyl-H), 3.14 (br, 2H, −CH-NCH), 6.24 (dd, 3JHH = 8.0 Hz, 4JHH = 1.0 Hz, 2H; ArH), 6.42 (d, 4JHH = 1.0 Hz, 2H; ArH), 7.08 (d, 3JHH = 8.0 Hz, 2H; ArH), 8.26 (s, 2H; CHN). 13C NMR (125 MHz, DMSO-d6): δ = 21.92, 24.40, 28.19, 64.91, 114.23, 117.61, 123.10, 135.71, 143.03, 164.45, 171.36. 1C. A solution of (R)-1 or (S)-1 (0.0446 g, 0.10 mmol) in chloroform (10 mL) was refluxed under nitrogen until no more signals of 1A and 1B were observed in the 1H NMR spectrum (typically 5−6 h). The resulting solution was concentrated under vacuum to give 1C as an off-white powder in quantitative yield. C44H48N4O8Zn2 (891.68): Calcd C, 59.27; H, 5.43; N, 6.28; Found C, 59.28; H, 5.47; N, 6.29. ESI-MS: m/z = 853 [M − 2OCH3 + Na]+, 893 [M + H]+, 915 [M + Na]+. IR (KBr, cm−1): 2936(m), 2856(w), 1598(vs), 1524(s), 1486(w), 1464(w), 1441(m), 1422(m), 1403(w), 1309(w), 1217(s), 1170(w), 1148(w), 1120(m), 1030(w), 979(w), 840(w), 790(w), 600(w). 1H NMR (500 MHz, CDCl3): δ = 1.27 (br, 8H, cyclohexylH), 1.60 (br, 4H, cyclohexyl-H), 1.70 (br, 4H, cyclohexyl-H), 3.35 (br, 4H, −CH-NCH), 3.83 (s, 12H; OCH3), 6.22 (dd, 3JHH = 8.0 Hz, 4 JHH = 2.5 Hz, 4H; ArH), 6.29 (d, 4JHH = 2.5 Hz, 4H; ArH), 6.57 (d, 3 JHH = 8.0 Hz, 4H; ArH), 7.35 (s, 4H; CHN). 13C NMR (125 MHz, CDCl3): δ = 25.19, 37.93, 55.28, 69.04, 103.95, 105.14, 112.75, 138.10, 166.06, 170.72, 172.52. [N,N-Bis(3-tert-butyl-2-hydroxybenzylidene)-(1S,2S)-1,2diaminocyclohexanediaminato]ZnII (4). Prepared according to the literature procedure.17 Yellow powder (0.212 g, 85%). C56H72N4O4Zn2 (995.89): Calcd C, 67.53; H, 7.29; N, 5.63; Found C, 67.68; H, 7.32; N, 5.65. 1H NMR (500 MHz, CDCl3): δ = 1.24 (br, 4H; cyclohexylH), 1.45 (s, 36H; CH3), 1.58 (br, 4H; cyclohexyl-H), 1.65 (br, 4H; cyclohexyl-H), 1.72 (br, 4H; cyclohexyl-H), 3.58 (br, 4H; −CH-N CH), 6.56 (t, 2JHH = 8.5 Hz, 4H; ArH), 6.61 (dd, 3JHH = 8.5 Hz, 4JHH = 2.0 Hz, 4H; ArH), 7.40 (dd, 3JHH = 8.5 Hz, 4JHH = 2.0 Hz, 4H; ArH), 7.45 (s, 4H; CHN).

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01580. Additional 1H NMR, UV/vis, CD, and mass spectra (PDF)

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.F.). *E-mail: [email protected] (S.D.B.). Author Contributions †

G.C. and I.P.O. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the MIUR and FIR 2014 Cod. A19DBF. We gratefully thank A. Giuffrida for mass spectrometry measurements and A. D’Urso for the suggestions regarding CD measurements. H


Article

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