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Article

Supramolecular Aggregation of a New Substituted Bis(salicylaldiminato)zinc(II) Schiff-Base Complex Derived from trans-1,2-Diaminocyclohexane

by
Giuseppe Consiglio
*,
Ivan Pietro Oliveri
,
Salvatore Failla
and
Santo Di Bella
*
Dipartimento di Scienze Chimiche, Università di Catania, I-95125 Catania, Italy
*
Authors to whom correspondence should be addressed.
Inorganics 2018, 6(1), 8; https://doi.org/10.3390/inorganics6010008
Submission received: 1 December 2017 / Revised: 27 December 2017 / Accepted: 29 December 2017 / Published: 1 January 2018
(This article belongs to the Special Issue Schiff-Base Metal Complexes)
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Abstract

:
In this contribution is reported the synthesis, characterization, and aggregation properties in solution of a novel Zn(II) complex, (R)-2, derived from the enantiopure chiral trans-1,2-diaminocyclohexane and a substituted salicylaldehyde. Detailed 1H NMR, DOSY NMR, optical absorption, and circular dichroism spectroscopic studies and chemical evidence allowed to investigate the nature of aggregate species in solution. The high solubility of (R)-2 in solution of the non-coordinating chloroform solvent leads to formation of various aggregates, some of them consisting of large oligomers estimated to contain up to 27 monomeric units. The chiral trans-stereochemistry of the bridging diamine favors a different aggregation mode in these complexes, both in the oligomers and dimers, involving a tetrahedral coordination geometry around the metal center. Overall data suggest the formation of helical oligomers, (ZnL)n, in freshly prepared chloroform solutions which, by standing or heating, evolve towards a more thermodynamically stable, dinuclear double-helicate Zn2L2 dimer.

Graphical Abstract

1. Introduction

Molecular aggregation is a topic of current interest involving various properties and applications [1,2,3,4]. In recent years we have been involved in the study of the aggregation properties of bis(salicylaldiminato)zinc(II) Schiff-base complexes, derivatives from substituted salicylaldehydes and 1,2-diamines [5,6,7,8,9,10,11]. Their aggregation properties are related to the Lewis acidic character of the metal center which, in turn, is strongly connected to the nature of the bridging diamine [12]. Therefore, a variety of supramolecular architectures [13,14,15,16,17,18], mesomorphic [19,20,21,22], and self-assembled nanostructures [23,24,25,26,27] have been found, mostly because of intermolecular Zn···O axial interactions involving pentacoordinated square-pyramidal Zn(II) geometries. Moreover, these species exhibit interesting photophysical properties [28,29,30,31] and are sensors of various Lewis bases [32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53].
In this context, a singular behavior has been observed for Zn(II) complexes, ZnL, derived from the 1,2-diaminocyclohexane and the 4-methoxysalicylaldehyde. In fact, while in the case of the cis-1,2-diaminocyclohexane derivative an asymmetric dimeric aggregate with a typical Zn(II) pentacoordination has been found [7], complexes derived from the enantiopure (1S,2S)-(+)- or (1R,2R)-(−)-trans-1,2-diaminocyclohexane, 1, involved the existence of various species in solution [6]. In particular, chloroform solutions of 1 were characterized by the presence of three species, exhibiting a strong concentration dependence, the predominant of which consisting of large oligomeric aggregates. Heating or after standing chloroform solutions of 1 all species are irreversibly converted into a dimer, 1C, which has been described as a dinuclear, double-helicate Zn2L2 structure with a tetrahedral coordination around the Zn(II) atoms [6].
The peculiar aggregation features of these chiral complexes are doubtless related to the defined stereochemistry of the trans-1,2-diaminocyclohexane bridge. It is, therefore, of interest to further investigate on these complexes, to better understand the physicochemical features responsible for their unusual aggregation behavior. Thus, maintaining the skeleton structure of the chiral ligand we have considered a different substituent on the salicylidene rings, in order to improve the solubility of the related Zn(II) complex in the involved solvents. Accordingly, the complex with the 4-diethylamino substituent, (R)-2, has been synthesized (Chart 1) with a significant increase of solubility. This, in turn, leads to even more interesting aggregation characteristics in solution which are detailed described in this contribution.

2. Results

The synthesis of (R)-2 was carried out by standard template method [5,6,7,8,9,10], using 4-(diethylamino)-2-hydroxybenzaldehyde and (1R,2R)-trans-1,2-diaminocyclohexane in methanol solution and a stoichiometric excess of triethylamine. Then, complexation with the Zn(II) ion was accomplished by using zinc perchlorate. The isolated pale-yellow solid of (R)-2 is very soluble in chloroform and in most polar solvents.
The 1H NMR spectrum of (R)-2 in solution of the coordinating DMSO-d6 solvent (1.0 × 10−2 M) shows the presence of a single set of signals (Figure 1), independent from the concentration, indicating the existence of monomeric species in solution, as usually observed for Zn(II) Schiff-base complexes [6,7,8,9,10,11]. To further investigate about the structure of (R)-2 in solution, we performed diffusion-ordered NMR spectroscopy (DOSY) measurements, by using a known internal reference to estimate the molecular mass of species in solution [5,6,7,8,9,10]. The DOSY spectrum of (R)-2 in DMSO-d6 (1.0 × 10−2 M) confirms the presence of a single component in the diffusion dimension (D = 2.5 × 10−10 m2·s-1), with an estimated molecular mass (679 Da) consistent with the (R)-2∙DMSO adduct (Table 1).
The 1H NMR spectrum of (R)-2 in solution of the non-coordinating CDCl3 solvent (1.0 × 10−2 M), unlike the spectrum in DMSO-d6, appears much more complex for the presence of many signals (Figure 1), indicating the existence of various species in solution. In fact, the related DOSY spectrum is separated into six components, 2A–F, in the diffusion dimension (Figure 2, Table 1), three of them having definitely lower D values (D ~2 × 10−10 m2·s−1) than the remaining three (D ~6 × 10−10 m2·s−1). In particular, assuming 2F as internal reference dimeric species (vide infra) the molecular mass of remaining components was estimated. Thus, while 2D and 2E are also dimeric species, instead 2A–C result to be larger oligomeric aggregates, containing up to 27 monomeric units. In comparison with previous results on the (R)-1 complex in the same non-coordinating chloroform solvent, present data indicate the existence in solution of a greater number of species, some of them having larger estimated molecular masses. This behavior may be related to the greater solubility of (R)-2, thus allowing for a higher degree of aggregation in solution. Note that, despite the rather large oligomeric nature of aggregates 2A–C, they are characterized by sharp 1H NMR signals, whose resonance of the imine protons is comparable to that found for the (R)-2∙DMSO adduct (Figure 1). On the other hand, the resonance of the imine protons in 2F results to be up-field shifted (0.9 ppm) with respect that of 2A.
Chloroform solutions of (R)-2 exhibit a pronounced concentration dependence. In particular, starting from concentrated solutions (5.0 × 10−2 M), the progressive dilution leads to a decrease of 2A and an increase of the other species (Figure S2). In other terms, dilution favors fragmentation of the largest aggregate 2A into the other oligomers.
Addition of a Lewis base to solutions of non-coordinating solvents of ZnL aggregates generally leads to disaggregation with the formation of monomeric adducts [6,7,8,9,10,11]. 1H NMR studies of (R)-2 in mixtures of non-coordinating/coordinating (CDCl3/DMSO-d6) solvents further support the existence of various aggregate species in the former solvent (Figure S3). Actually, the successive addition of defined amounts of DMSO-d6 (up to ca. 230-fold mole excess) to a freshly prepared CDCl3 solution of (R)-2 leads mainly to the progressive disappearance of oligomer 2A and the appearance of a new set of signals consistent with the formation of the (R)-2·DMSO adduct. Only upon addition of ca. 1400-fold mole excess of DMSO-d6 the complete disappearance of all species is observed, and the resulting solution shows a 1H NMR spectrum almost comparable to that recorded in DMSO-d6 (Figure 1 and Figure S3). However, even in such large stoichiometric excess the species 2F remains almost unaltered, indicating a strong stability of this dimeric species.
The relative distribution of aggregates 2A–F exhibits remarkable changes after standing chloroform solutions of (R)-2 at room temperature for some time, as can be evaluated from 1H NMR signals of each species (Figure 3). In particular, freshly prepared chloroform solutions (5.0 × 10−2 M) show the predominant presence of 2A (82%), while 2B (6%) and 2C–F (3% each) are minor species. After standing, a progressive conversion of 2A into 2F is observed, while the relative percentage of the other species remain almost unchanged. After three weeks, a complete conversion of all species into 2F is obtained. Moreover, starting from more diluted CDCl3 solutions (5.0 × 10−3 M) the complete conversion of all species into 2F occurs in a shorter time (one week). An analogous result, that is the complete conversion of all species into 2F, is achieved by heating chloroform solutions of (R)-2 at 60 °C for four hours.
The isolated pale-yellow solid obtained from heated CHCl3 solutions of (R)-2 was characterized in chloroform by ESI and DOSY measurements as a dimeric species having identical 1H NMR spectroscopic characteristics of 2F. Specifically, DOSY experiments were performed using (R)-1C as an internal reference (Figure S1). The dimeric species 2F presents some characteristic features previously observed for 1C [6]. In fact, analogously to 1C, on passing from DMSO-d6 solutions of (R)-2 to CDCl3 solutions of 2F a strong up-field shift, ca. 0.8 ppm, is observed. Moreover, the chemical shift of the CH=N signal in 2F (7.22 ppm) is comparable to that found in 1C (7.35 ppm).
Optical absorption spectra of (R)-2 in DMSO and CHCl3 solutions are comparable to each other, consisting of a strong band centered at 341 nm and a shoulder at 366 nm (Figure 4), despite the different nature of the involved species in solution: monomeric adduct vs. oligomeric aggregates, respectively. In contrast, the UV–Vis spectrum of (R)-2F in CHCl3 is very different because, in addition to the absorption band at 341 nm, shows the appearance of a new intense band at longer wavelengths centered at 371 nm, consistent with the existence of strong interligand interactions. These results are indicative of a different aggregation mode on switching from oligomers to the (R)-2F dimer. On the other hand, no relevant interligand interactions are likely operating in (R)-2 oligomers given the comparable UV–Vis features to those of the monomeric (R)-2·DMSO adduct.
The differences observed in optical absorption spectra are reflected in circular dichroism (CD) spectra (Figure 4). Thus, while a comparable bisignate signal is observed for solutions of (R)-2 in DMSO and CHCl3, in contrast a stronger and redshifted bisignate signal is noticed for (R)-2F.

3. Discussion

For this family of complexes, we have established that the chemical shift of the imine hydrogens is diagnostic of their aggregation mode [5,6,7,8,9,10,11]. In particular, the switching from pentacoordinated monomeric adducts in coordinating solvents to dimeric aggregates in chloroform solutions has always accompanied by an up-field shift of these signals, because of the involved hydrogens lie under the shielding zone of the π electrons of a conjugated system [7,8,9,10,11]. Moreover, a further up-field shift has been observed for dinuclear double-helicate structures with a tetrahedral coordination around the Zn(II) metal center, as in 1C, because of the stronger shielding effects caused by the aromatic ring of the subunit of the other ligand [6]. Relevant chemical shifts for some 4-substituted bis(salicylaldiminato)Zn(II) Schiff-base complexes in CDCl3 solution and their aggregation properties are collected in Table 2. Therefore, given the observed comparable chemical shifts of the CH=N signal for 1C and 2F, we can hypothesize an analogous dinuclear double-helicate, Zn2L2, structure for the latter species. Moreover, 2F hardly deaggregates with the addition of a Lewis base, such as DMSO. In fact, starting from a 1.0 × 10−2 M CDCl3 solution of 2F no appreciable variation of 1H NMR signals is observed even after addition of 1.5 × 103-fold mole excess of DMSO-d6. This indicates a low Lewis acidic character of this dimeric species.
The high degree of aggregation of (R)-2 in freshly-prepared chloroform solutions with the prevalent formation of 2A, without any broadening of the 1H NMR signals and chemical shifts comparable to those of the monomeric (R)-2∙DMSO adduct, is in contrast to what is commonly observed for other bis(salicylaldiminato)zinc(II) Schiff-base complexes (Table 2). In fact, in the case of intermolecular interactions involving pentacoordinated square-pyramidal Zn(II) geometries, on switching from monomeric adducts to dimeric aggregates, chemical shifts are always up-field shifted, accompanied by a broadening of 1H NMR signals when an oligomerization occurs [7,8,9,10,11]. These observations suggest that in freshly prepared chloroform solutions of (R)-2 a different type aggregation occurs, likely involving a different coordination environment around the metal center, consequence of the preorganized structure of the chiral ligand derived from trans-1,2-diaminocyclohexane (Figure 5a). In particular, we hypothesize a tetrahedral coordination geometry in which each metal center is bonded to a bidentate subunit of two different ligands with formation of helical oligomers, (ZnL)n, resulting in no shielding effects on the chemical shift of CH=N and aromatic signals. An analogous structure is proposed for (R)-1 oligomers. By standing or heating, chloroform solutions of (ZnL)n oligomers are irreversibly converted in a more thermodynamically stable, dinuclear double-helicate Zn2L2 dimer (2F in Figure 5b) having a weak Lewis acidic character.
This picture is fully consistent with the optical absorption and circular dichroism spectroscopic results. In fact, (ZnL)n oligomers behave as monomeric (R)-2·DMSO adducts, with no evidence of relevant interactions between the subunits of two different ligands, since no shift of the UV–Vis spectral feature, related to π→π* transitions [54], is observed. In contrast, these interactions are operating in the double-helicate Zn2L2 dimer with consequent red-shift and hyperchromism of the longer wavelength absorption band. Due to the chiral trans-1,2-diaminocyclohexane bridge all involved species exhibit a bisignate CD signal characteristic of helical structures.

4. Experimental Section

4.1. Materials and General Procedures

All the reactions were executed under nitrogen. Zinc perchlorate hexahydrate, 4-(diethylamino)-2-hydroxybenzaldehyde, (1R,2R)-1,2-diaminocyclohexane, and triethylamine (Aldrich, Milan, Italy) were used as received. Chloroform (Aldrich, Milan, Italy) stabilized with amylene was used for UV–Vis and CD measurements. CDCl3 (Aldrich, Milan, Italy) was stored over molecular sieves (3 Å), while DMSO-d6 was used as obtained.

4.2. Physical Measurements

Elemental analyses were performed on a Carlo Erba 1106 elemental analyzer (Carlo Erba, Milan, Italy). ESI-MS spectra were recorded on a AB Sciex API 2000 LC/MS/MS System (AB Sciex Italia, Milan, Italy). All NMR experiments were recorded at 27 °C on a Varian Unity S 500 spectrometer (Varian, Palo Alto, CA, USA), using tetramethylsilane (Si(CH3)4, TMS) as an internal reference. DOSY experiments were performed as reported elsewhere [5,6,7,8,9,10]. Optical absorption and CD spectra were recorded at room temperature using a UV–Vis Jasco V-630 spectrophotometer (Jasco Europe, Cremella (LC), Italy) and a Jasco 810 spectropolarimeter (Jasco Europe, Cremella (LC), Italy), respectively. All UV–Vis and CD measurements were recorded using a 1 mm path length cuvette.

4.3. Computational Method

Geometry optimization for the ligand 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 [55,56]. The tight optimization criteria along with an ultrafine grid were adopted for the geometry optimization. Calculations were computed with the Gaussian 09 program [57] using the 6-31G basis set.

4.4. Syntheses

4.4.1. {N,N-Bis[4-(diethylamino)-2-hydroxybenzylidene]-(1R,2R)-trans-1,2-diaminocyclohexane-diaminato}Zn(II) (R)-2

(1R,2R)-1,2-Diaminocyclohexane (0.114 g, 1.00 mmol) was dissolved in methanol (20.0 mL). Then, 4-(diethylamino)-2-hydroxybenzaldehyde (0.387 g, 2.00 mmol) was added. The mixture obtained was heated at reflux with stirring for 2 h. The yellow solution so obtained was treated with zinc perchlorate hexahydrate (0.372 g, 1.00 mmol) and triethylamine (1.00 mL) to obtain a mixture that was refluxed and stirred overnight. After cooling to room temperature, the precipitated solid was collected by filtration, washed with methanol, and dried in a vacuum desiccator at 120 °C over sulfuric acid. Pale-yellow powder (0.502 g, 95%). C28H38N4O2Zn (528.02): Calcd. C, 63.69; H, 7.25; N, 10.61; Found C, 63.58; H, 7.24; N, 10.63. 1H NMR (500 MHz, DMSO-d6): δ = 1.10 (t, 3JHH = 7.0 Hz, 12H, NCH2CH3), 1.24 (br, 2H, cyclohexyl-H), 1.36 (br, 2H, cyclohexyl-H), 1.87 (br, 2H, cyclohexyl-H), 2.35 (br, 2H, cyclohexyl-H), 2.99 (br, 2H, CH–N=CH), 3.30 (q, 3JHH = 7.0 Hz, 8H; NCH2CH3), 5.77 (d, 4JHH = 2.0 Hz, 2H; ArH), 5.91 (dd, 3JHH = 8.5 Hz, 4JHH = 2.0 Hz, 2H; ArH), 6.91 (d, 3JHH = 8.5 Hz, 2H; ArH), 8.02 (s, 2H; CH=N). 13C NMR (125 MHz, DMSO-d6): δ = 12.86, 24.10, 27.80, 43.67, 64.30, 99.30, 101.86, 110.58, 136.51, 151.25, 161.82, 163.02.

4.4.2. (R)-2F

A sample of (R)-2 (0.0528 g, 0.10 mmol) was dissolved in chloroform (10 mL) and refluxed under nitrogen until all the other species 2A–E disappeared in the 1H NMR spectrum (typically four hours). The solvent was evaporated under vacuum to give 2F as pale-yellow powder in quantitative yield. ESI-MS: m/z = 1053 [M + H]+, 1075 [M + Na]+. 1H NMR (500 MHz, CDCl3): δ = 1.21 (t, 3JHH = 7.0 Hz, 24H; NCH2CH3), 1.25 (br, 4H, cyclohexyl-H), 1.56 (br, 4H, cyclohexyl-H), 1.65 (br, 8H, cyclohexyl-H), 3.31 (br, 4H, –CH–N=CH), 3.39 (q, 3JHH = 7.0 Hz, 16H; NCH2CH3), 6.02 (m, 8H; ArH), 6.55 (d, 3JHH = 10.0 Hz, 4H; ArH), 7.22 (s, 4H; CH=N). 13C NMR (125 MHz, CDCl3): δ = 12.95, 25.29, 37.87, 44.46, 68.61, 101.03, 101.43, 109.97, 138.37, 153.18, 169.23, 171.55.

5. Conclusions

This study further demonstrated the intriguing and variegate aggregation characteristics of bis(salicylaldiminato)zinc(II) Schiff-base complexes. Thanks to the 4-diethylamino substituent on the salicylidene rings, the greater solubility of (R)-2, in comparison with the (R)-1 analogue, allows a higher degree of aggregation in solution. The chiral trans-stereochemistry of the bridging diamine favors a different aggregation mode in these complexes, either in the oligomers and dimers, involving a tetrahedral coordination geometry around the metal center. Experimental data suggest the formation of helical oligomers, (ZnL)n, in freshly-prepared solutions of non-coordinating solvents which, by standing or heating, evolve towards a more thermodynamically stable, dinuclear double-helicate Zn2L2 dimer.

Supplementary Materials

The following are available online at www.mdpi.com/2304-6740/6/1/8/s1. Figure S1: 1H NMR DOSY spectrum of (R)-2F in the presence of (R)-1C used as internal reference; Figure S2: Concentration dependence of 1H NMR spectra of (R)-2 in CDCl3 solution; Figure S3: 1H NMR spectra of (R)-2 in CDCl3 (1.0 × 10−2 M; 6.0 × 10−6 mol) (a), and with addition of 2.8 × 10−4 mol (b), 7.0 × 10−4 mol (c), 1.4 × 10−3 mol (d), and 8.4 × 10−3 mol (e) of DMSO-d6.
Supplementary File 1

Acknowledgments

This research was supported by the MIUR and the University of Catania.

Author Contributions

Giuseppe Consiglio and Ivan Pietro Oliveri performed the experiments. Salvatore Failla and Santo Di Bella conceived the project and supervised the work. All authors discussed the results, interpreted the data, and contributed to the preparation of the final manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 06 00008 ch001 550
Chart 1. Investigated Zn(II) Schiff-base complexes.
Chart 1. Investigated Zn(II) Schiff-base complexes.
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Inorganics 06 00008 g001 550
Figure 1. 1H NMR spectra of (R)-2 (1.0 × 10−2 M) in DMSO-d6 and CDCl3. Asterisks indicate residual solvent peaks. The labeling of the 1H NMR signals related to the CDCl3 solution refers to species (R)-2A (denoted by the red squares) and (R)-2F (denoted by the black triangles). Inset: (a) expansion of the N-CH2-CH3 signal and (b) of the N-CH2-CH3 signal of (R)-2 in DMSO-d6.
Figure 1. 1H NMR spectra of (R)-2 (1.0 × 10−2 M) in DMSO-d6 and CDCl3. Asterisks indicate residual solvent peaks. The labeling of the 1H NMR signals related to the CDCl3 solution refers to species (R)-2A (denoted by the red squares) and (R)-2F (denoted by the black triangles). Inset: (a) expansion of the N-CH2-CH3 signal and (b) of the N-CH2-CH3 signal of (R)-2 in DMSO-d6.
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Figure 2. 1H NMR DOSY spectrum of (R)-2 in CDCl3 (1.0 × 10−2 M; 27 °C). Species 2A–F are shown with a different color.
Figure 2. 1H NMR DOSY spectrum of (R)-2 in CDCl3 (1.0 × 10−2 M; 27 °C). Species 2A–F are shown with a different color.
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Inorganics 06 00008 g003 550
Figure 3. 1H NMR spectra of (R)-2 (5.0 × 10−2 M) in CDCl3 recorded at different time intervals: (a) freshly prepared solution; (b) after 12 h; (c) after one week; and (d) after three weeks. The asterisk indicates the residual solvent peak. The labeling of the 1H NMR signals refers to species (R)-2A (denoted by the red squares) and (R)-2F (denoted by the black triangles).
Figure 3. 1H NMR spectra of (R)-2 (5.0 × 10−2 M) in CDCl3 recorded at different time intervals: (a) freshly prepared solution; (b) after 12 h; (c) after one week; and (d) after three weeks. The asterisk indicates the residual solvent peak. The labeling of the 1H NMR signals refers to species (R)-2A (denoted by the red squares) and (R)-2F (denoted by the black triangles).
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Figure 4. (Top) UV–Vis absorption spectra of (R)-2 (5.0 × 10−4 M) in CHCl3 and DMSO, and (R)-2F (5.0 × 10−4 M) in CHCl3. (Bottom) CD spectra of (R)-2 (5.0 × 10−4 M) in CHCl3 and DMSO, and (R)-2F (5.0 × 10−4 M) in CHCl3.
Figure 4. (Top) UV–Vis absorption spectra of (R)-2 (5.0 × 10−4 M) in CHCl3 and DMSO, and (R)-2F (5.0 × 10−4 M) in CHCl3. (Bottom) CD spectra of (R)-2 (5.0 × 10−4 M) in CHCl3 and DMSO, and (R)-2F (5.0 × 10−4 M) in CHCl3.
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Figure 5. (a) DFT optimized geometry (B3LYP) for the chiral ligand derived from trans-1,2-diaminocyclohexane. Hydrogens and ethyl groups are omitted for clarity. (b) Proposed helical structure for oligomeric aggregates of (R)-2 in chloroform solution. By heating, or after standing, oligomers are irreversibly converted into a thermodynamically stable, dinuclear double-helicate dimer 2F. Both the oligomers and the dimer involve a tetrahedral coordination geometry around the metal center.
Figure 5. (a) DFT optimized geometry (B3LYP) for the chiral ligand derived from trans-1,2-diaminocyclohexane. Hydrogens and ethyl groups are omitted for clarity. (b) Proposed helical structure for oligomeric aggregates of (R)-2 in chloroform solution. By heating, or after standing, oligomers are irreversibly converted into a thermodynamically stable, dinuclear double-helicate dimer 2F. Both the oligomers and the dimer involve a tetrahedral coordination geometry around the metal center.
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Table
Table 1. Diffusion coefficients, D, and estimated molecular mass, m, for (R)-2 in DMSO-d6 and CDCl3.
Table 1. Diffusion coefficients, D, and estimated molecular mass, m, for (R)-2 in DMSO-d6 and CDCl3.
CompoundSpeciesD × 1010/m2·s−1D (solvent) × 1010/m2·s−1m (n) 1/Dam (n) 2/Da
(R)-22⋅DMSO-d62.57.1(DMSO-d6)679 3612.2
(R)-22A1.726.3(CDCl3)14,046(26.6)14,256(27)
2B1.926.3(CDCl3)11,244(21.3)11,088(21)
2C2.226.3(CDCl3)8387(15.9)8448(16)
2D5.826.3(CDCl3)1207(2.3)1056(2)
2E6.026.3(CDCl3)1128(2.1)1056(2)
2F6.226.3(CDCl3)1056(2)1056(2)
(R)-2 42F5.924.8 (CDCl3)1017(1.9)1056(2)
1C6.324.8 (CDCl3)892(2) 891.7(2)
1 Estimated molecular mass using the species (R)-2F as internal reference. Values in parentheses (n) indicate the order of aggregation. 2 Expected molecular mass. Values in parentheses (n) indicate the order of aggregation. 3 Estimated molecular mass using the solvent as an internal reference. 4 CDCl3 solution of (R)-2F in the presence of (R)-1C used as internal reference (see Figure S1).
Table
Table 2. Comparison of the CH=N chemical shift for some 4-substituted bis(salicylaldiminato)Zn(II) Schiff-base complexes in CDCl3 solution and their aggregation properties.
Table 2. Comparison of the CH=N chemical shift for some 4-substituted bis(salicylaldiminato)Zn(II) Schiff-base complexes in CDCl3 solution and their aggregation properties.
Diamino Bridge4-substituentδ (ppm)∆δ (ppm) 1Zn(II) CoordinationAggregate StructureRef.
2,3-diamino-maleonitrile–OC11H218.350.03penta(ZnL)2[10]
benzene-1,2-diamine–OC10H208.470.39penta(ZnL)2[9]
ethane-1,2-diamine–OC16H337.610.67penta(ZnL)2[8]
cis-cyclohexane-1,2-diamine–OMe8.08; 8.330.22; −0.03penta(ZnL)2[7]
trans-cyclohexane-1,2-diamine–OMe8.33−0.12tetra(ZnL)n 2[6]
7.350.86tetra1C, Zn2L2
trans-cyclohexane-1,2-diamine–NEt28.14−0.11tetra2A, (ZnL)nThis work
7.220.81tetra2F, Zn2L2
trans-cyclopentane-1,2-diamine–OMe7.500.69tetraZn2L2[5]
1 Difference of the chemical shifts between DMSO-d6 and CDCl3 solutions. 2 Referred to (R)-1 oligomers.

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Consiglio, G.; Oliveri, I.P.; Failla, S.; Di Bella, S. Supramolecular Aggregation of a New Substituted Bis(salicylaldiminato)zinc(II) Schiff-Base Complex Derived from trans-1,2-Diaminocyclohexane. Inorganics 2018, 6, 8. https://doi.org/10.3390/inorganics6010008

AMA Style

Consiglio G, Oliveri IP, Failla S, Di Bella S. Supramolecular Aggregation of a New Substituted Bis(salicylaldiminato)zinc(II) Schiff-Base Complex Derived from trans-1,2-Diaminocyclohexane. Inorganics. 2018; 6(1):8. https://doi.org/10.3390/inorganics6010008

Chicago/Turabian Style

Consiglio, Giuseppe, Ivan Pietro Oliveri, Salvatore Failla, and Santo Di Bella. 2018. "Supramolecular Aggregation of a New Substituted Bis(salicylaldiminato)zinc(II) Schiff-Base Complex Derived from trans-1,2-Diaminocyclohexane" Inorganics 6, no. 1: 8. https://doi.org/10.3390/inorganics6010008

APA Style

Consiglio, G., Oliveri, I. P., Failla, S., & Di Bella, S. (2018). Supramolecular Aggregation of a New Substituted Bis(salicylaldiminato)zinc(II) Schiff-Base Complex Derived from trans-1,2-Diaminocyclohexane. Inorganics, 6(1), 8. https://doi.org/10.3390/inorganics6010008

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