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

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.

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 Zn 2 L 2 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.
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.
The 1 H 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 m 2 •s −1 ) than the remaining three (D 6 × 10 −10 m 2 •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 1 H NMR signals, whose resonance of the Chart 1. Investigated Zn(II) Schiff-base complexes.
The 1 H NMR spectrum of (R)-2 in solution of the non-coordinating CDCl 3 solvent (1.0 × 10 −2 M), unlike the spectrum in DMSO-d 6 , 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 m 2 •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 1 H 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.
Inorganics 2018, 7, 8 3 of 13 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.Table 1.Diffusion coefficients, D, and estimated molecular mass, m, for (R)-2 in DMSO-d6 and CDCl3. 1 Estimated molecular mass using the species (R)-2F as internal reference.Values in parentheses (n) indicate the order of aggregation. 2Expected molecular mass.Values in parentheses (n) indicate the order of aggregation. 3Estimated molecular mass using the solvent as an internal reference. 4CDCl3 solution of (R)-2F in the presence of (R)-1C used as internal reference (see Figure S1).Table 1.Diffusion coefficients, D, and estimated molecular mass, m, for (R)-2 in DMSO-d 6 and CDCl 3 .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.

Compound Species
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]. 1 H 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.1400fold mole excess of DMSO-d6 the complete disappearance of all species is observed, and the resulting solution shows a 1 H NMR spectrum almost comparable to that recorded in DMSO-d6 (Figures 1 and  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 1 H 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.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]. 1 H NMR studies of (R)-2 in mixtures of non-coordinating/coordinating (CDCl 3 /DMSO-d 6 ) solvents further support the existence of various aggregate species in the former solvent (Figure S3).Actually, the successive addition of defined amounts of DMSO-d 6 (up to ca. 230-fold mole excess) to a freshly prepared CDCl 3 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-d 6 the complete disappearance of all species is observed, and the resulting solution shows a 1 H NMR spectrum almost comparable to that recorded in DMSO-d 6 (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 1 H 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 CDCl 3 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 1 H 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.The isolated pale-yellow solid obtained from heated CHCl 3 solutions of (R)-2 was characterized in chloroform by ESI and DOSY measurements as a dimeric species having identical 1 H 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-d 6 solutions of (R)-2 to CDCl 3 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 CHCl 3 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 CHCl 3 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 CHCl 3 , in contrast a stronger and redshifted bisignate signal is noticed for (R)-2F.

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 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 1 H NMR signals is observed even after addition of 1.5 × 10 3 -fold mole excess of DMSO-d6.This indicates a low Lewis acidic character of this dimeric species.

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 CDCl 3 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, Zn 2 L 2 , 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 CDCl 3 solution of 2F no appreciable variation of 1 H NMR signals is observed even after addition of 1.5 × 10 3 -fold mole excess of DMSO-d 6 .This indicates a low Lewis acidic character of this dimeric species. 1 Difference of the chemical shifts between DMSO-d 6 and CDCl 3 solutions. 2Referred to (R)-1 oligomers.
The high degree of aggregation of (R)-2 in freshly-prepared chloroform solutions with the prevalent formation of 2A, without any broadening of the 1 H 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 1 H 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 Zn 2 L 2 dimer (2F in Figure 5b) having a weak Lewis acidic character.The high degree of aggregation of (R)-2 in freshly-prepared chloroform solutions with the prevalent formation of 2A, without any broadening of the 1 H 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 1 H 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-1,2diaminocyclohexane (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.

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.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 Zn 2 L 2 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.

Materials and General Procedures
All the reactions were executed under nitrogen.

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 Zn 2 L 2 dimer.

Figure 3 .
Figure 3. 1 H NMR spectra of (R)-2 (5.0 × 10 −2 M) in CDCl3 recorded at different time intervals: (a) freshly prepared solution; (b) after 12 hours; (c) after one week; and (d) after three weeks.The asterisk indicates the residual solvent peak.The labeling of the 1 H NMR signals refers to species (R)-2A (denoted by the red squares) and (R)-2F (denoted by the black triangles).

Figure 3 .
Figure 3. 1 H NMR spectra of (R)-2 (5.0 × 10 −2 M) in CDCl 3 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 1 H NMR signals refers to species (R)-2A (denoted by the red squares) and (R)-2F (denoted by the black triangles).

Figure 5 .
Figure 5. (a) DFT optimized geometry (B3LYP) for the chiral ligand derived from trans-1,2diaminocyclohexane.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 .
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.

Table 2 .
Comparison of the CH=N chemical shift for some 4-substituted bis(salicylaldiminato)Zn(II) Schiff-base complexes in CDCl 3 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.