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Article Cite This: Inorg. Chem. 2017, 56, 14206-14213

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Aggregates of Defined Stereochemical Scaffolds: A Study in Solution of a Zinc(II) Schiff Base Complex Derived from the Enantiopure trans1,2-Cyclopentanediamine Ivan Pietro Oliveri,† Giuseppe Forte,‡ Giuseppe Consiglio,*,† Salvatore Failla,† and Santo Di Bella*,† †

Dipartimento di Scienze Chimiche and ‡Dipartimento di Scienze del Farmaco, Università di Catania, I-95125 Catania, Italy S Supporting Information *

ABSTRACT: Molecular aggregation of bis(salicylaldiminato) ZnII Schiff base, ZnL, complexes is a topic of current interest. In this paper, we report a novel complex derived from the enantiopure trans-1,2-cyclopentanediamine, (R)-1, in order to probe the effect of the defined stereochemistry on its coordination geometry and aggregation properties, through detailed 1H NMR, DOSY NMR, UV/vis, and circular dichroism spectroscopic studies and accurate DFT calculations. This complex shows several peculiarities in solution, behaving very differently from ZnL complexes and more importantly from the related trans-1,2-diaminocyclohexane derivative. Unexpectedly, experimental data suggest the existence in DMSO of two species in equilibrium, the classical monomeric adduct and a dimer, indicating that the DMSO is not sufficiently strong Lewis basic to completely deaggregate the complex. Also, in chloroform an unusual behavior is observed with the existence of a single defined dimeric species characterized as a dinuclear double helicate Zn2L2 structure which does not deaggregate even with the addition of pyridine in large stoichiometric excess. The formation of mononuclear adducts occurs only when dissolving the complex in the stronger Lewis base pyridine. All these data indicate the helicate (R)-1-h complex has a higher thermodynamic stability than that of the cyclohexane analogue, leading to its unique aggregation properties.

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end, we have considered the ZnII complex derived from (1R,2R)-trans-1,2-cyclopentanediamine ((R)-1, Chart 1) and studied its properties in solution.

INTRODUCTION Recently, ZnII Schiff base complexes derived from N2O2 tetradentate bis(salicylaldimine) ligands, ZnL, have been extensively investigated as molecular materials with various catalytic,1−5 sensing,6−25 and optical properties.26−35 Peculiarities of these complexes are mostly connected to their Lewis acidic character which leads to interesting aggregation properties with formation of various supramolecular architectures and nanostructures.36−47 In turn, these are closely related to the structure of the bridging 1,2-diamine.48−51 In this regard, a singular example is provided by ZnL complexes derived from the 1,2-diaminocyclohexane. In fact, in the case of the cis-1,2diaminocyclohexane derivative an unusual asymmetric dimeric aggregate was found, either in solution of noncoordinating solvents or in the X-ray crystal structure.52 In contrast, the ZnL complexes derived from the enantiopure trans-1,2-diaminocyclohexane behave very differently, since there is evidence of an unusual aggregation mode which leads to species described as double helicate Zn2L2 structures with a tetrahedral coordination around the ZnII ions.53 In both cases, this can be related to the defined stereochemistry of the 1,2-diaminocyclohexane bridge. These results encouraged us to carry out further studies. Therefore, we asked ourselves the following question: Would an analogous 1,2-diamine bridge with the same trans stereochemistry give a complex with a same helical structure? To this © 2017 American Chemical Society

Chart 1

In this paper, we report an investigation involving detailed H NMR, DOSY NMR, UV/vis, and circular dichroism (CD) spectroscopic studies and DFT calculations on the structural and aggregation properties of (R)-1. The results are compared with previous data on related trans-1,2-diaminocyclohexane 1

Received: September 11, 2017 Published: November 7, 2017 14206

DOI: 10.1021/acs.inorgchem.7b02341 Inorg. Chem. 2017, 56, 14206−14213

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Inorganic Chemistry

Figure 1. 1H NMR spectra of (R)-1 (5.0 × 10−3 M) in CDCl3, DMSO-d6, and pyridine-d5. Asterisks indicate residual solvent peaks.

Figure 2. 1H NMR DOSY spectrum of (R)-1 in DMSO-d6 (1.0 × 10−2 M; 27 °C).

Surprisingly, the 1H NMR spectrum of (R)-1 in solution of the coordinating DMSO-d6 solvent (5.0 × 10−3 M) shows the presence of two sets of signals indicating the existence of two different species in solution (Figure 1). In fact, 1D T-ROESY analysis confirms the existence of two independent sets of signals, as the selective irradiation of the H1 or H1′ signals indicates that these protons are spatially close to the H4 and H2 or H4′ and H2′ protons, respectively (Figure S1). Moreover, in comparison with the 1H NMR spectrum of derivative (R)-2 in the same DMSO-d6 solvent,53 the set of signals at lower fields is in agreement with the presence of (R)-1·DMSO-d6 adduct, with chemical shifts almost identical to those of (R)-2 in the 3.5−8.5 ppm region (Figure S2). Instead, the set of signals at higher fields is consistent with the existence of (R)-1 in a new arrangement. Additionally, these signals show marked concentration dependence. In particular, the progressive dilution (down to 10−4 M) involves an increases of the intensity of the

derivative (R)-2. It is found that the different size of the cycloaliphatic ring of the trans-1,2-diamine bridge, cyclohexane versus cyclopentane, results in major differences in the aggregation properties of these ZnII Schiff base complexes.

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RESULTS

The synthesis of (R)-1 was achieved by the standard template method,49−53 using 2-hydroxy-4-methoxybenzaldehyde and chiral (1R,2R)-trans-1,2-cyclopentanediamine dihydrochloride in methanol solution and a stoichiometric excess of triethylamine, followed by complexation with the Zn(II) ion via its perchlorate salt. The isolated off-white solid of (R)-1 is soluble in most polar and nonpolar solvents, thus allowing performing suitable studies of its properties in solution. Unfortunately, all attempts to grow crystals suitable for X-ray structure determinations failed. 14207


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Inorganic Chemistry set of signals associated with the (R)-1·DMSO-d6 adduct with respect to the set of signals at higher fields (Figure S3). The ratio between the two sets of signals, considering those of the adduct as reference, varies from 0.45 at 1.0 × 10−2 to 14 at 1.0 × 10−4 M. This is in sharp contrast with previous studies on ZnL complexes in DMSO-d6 solution, always indicating the presence of a single set of signals associated with the ZnL· DMSO adduct.49−53 Conversely, the 1H NMR spectrum of (R)-1 in solution of a stronger Lewis base solvent, such as pyridine-d5 (py-d5), shows only one set of signals, independent from the concentration, according with the presence of the (R)1·py-d5 adduct, as inferred from the comparison with the set of signals at low fields observed in DMSO-d6 solution (Figure 1). On switching to the noncoordinating CDCl3 solvent, independently from the concentration in the 10−4−10−2 M range, one set of 1H NMR signals of (R)-1 is found, with a relevant upfield shift of H4 and H1 signals (0.69 and 0.48 ppm, respectively) with respect to those found for the (R)-1·DMSOd6 adduct (Figure 1). Moreover, in comparison with the 1H NMR spectrum of derivative (R)-2-h, obtained from chloroform solutions upon heating or standing,53 almost identical chemical shifts, except those related to the methylene hydrogens, are found (Figure S2). In order to further investigate about the structure of (R)-1 in solution, we performed diffusion-ordered NMR spectroscopy (DOSY) measurements in DMSO-d6 and CDCl3 solvents by using a known internal reference to estimate the molecular mass of species in solution.49−53 The 1H NMR DOSY spectrum of (R)-1 in DMSO-d6 (1.0 × 10−2 M) shows two components in the diffusion dimension (Figure 2) with D = 2.20 × 10−10 and 2.85 × 10−10 m2 s−1 (D = diffusion coefficient). By using DMSO-d6 solvent as internal reference (D = 7.30 × 10−10 m2 s−1), the estimated molecular mass (552 Da) of the component with the larger D is coherent with (R)-1·DMSO adduct, while the component with the lower D implies an estimated molecular mass (926 Da) consistent with a dimeric species. In contrast, adding (R)-2-h as internal reference species (D = 6.42 × 10−10 m2 s−1), the DOSY spectrum of (R)-1 in CDCl3 (1.0 × 10−2 M) shows a single component in the diffusion dimension with a comparable D value (D = 6.45 × 10−10 m2 s−1; Figure S4). This indicates an almost same molecular mass for (R)-1 (864 Da) and (R)-2-h in this solvent, the latter of which was characterized as a dimeric species (892 Da).53 Starting from 1.0 × 10−3 M CDCl3 solutions of (R)-1, we performed deaggregation studies by 1H NMR, using py-d5 as Lewis base. However, even after addition of 1000-fold mole excess of py-d5, neither spectral variations of (R)-1 nor appearance of new signals were observed. The UV/vis absorption spectrum of (R)-1 in CHCl3 consists of two defined bands at λ = 297 and 374 nm (Figure 3), independent from the concentration. On switching to the DMSO solvent, a blue-shift of the absorption bands, ca. 12 and 23 nm, respectively, and a broadening of the longer wavelength band are observed. Moreover, the absorption spectrum in DMSO exhibits appreciable concentration dependence (Figure S5). In particular, upon dilution (from 1.0 × 10−3 M to 1.0 × 10−5 M) the band at longer wavelength (λ = 352 nm) becomes blue-shifted (λ = 343 nm) and sharper. Finally, the absorption spectrum in pyridine consists of a band centered at λ = 348 nm which does not show concentration dependence. CD spectra of (R)-1 in the above solvents show the presence of bisignate signals in the range of 260−420 nm (Figure 3). In particular, the stronger signals at longer wavelengths reflect the

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

variations observed in the absorption spectra. Moreover, CD spectra in DMSO show that the broad negative signal centered at 377 nm, indicative of the existence of two species, which upon dilution becomes sharper, stronger, and blue-shifted (364 nm), suggesting the formation of a single species at lower concentrations (Figure S5). DFT calculations were used to optimize the possible structures of (R)-1, considering the effect of the solvent.54 The fully optimized structure of (R)-1 in chloroform, in a double helicate (R)-1-h (D2) and dimeric ((R)-1)2 (C2) structure, involves a different coordination geometry around the zinc atoms: a pseudotetrahedral and a distorted squarepyramidal coordination, respectively (Figure 4). As a consequence, while equivalent bond distances, Zn−O (1.96 Å) and Zn−N (2.02 Å), are calculated for the (R)-1-h structure, longer and nonequivalent Zn−O and Zn−N bond distances are found for ((R)-1)2. Moreover, the optimized (R)1-h structure is 8.8 kcal/mol lower in energy, in terms of enthalpy, with respect that of ((R)-1)2. The optimized structure of (R)-1-h can be useful compared with that of the related (R)-2-h. Although the two structures involve almost analogous Zn−N and Zn−O bond distances, a very different dihedral angle between nitrogen atoms of the diimine bridge (68.56 and 57.42°, for (R)-1-h and (R)-2-h, respectively) is found, because of the different cycloaliphatic ring size (Figure S6). This, in turn, is reflected in a very 14208


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Figure 4. Optimized geometry of the (R)-1 complex in chloroform in a double helicate (R)-1-h (left) and dimeric ((R)-1)2 (right) structure. Hydrogen atoms are omitted for clarity.

the (R)-1-h structure are strongly upfield-shifted (0.7 and 0.4 ppm, respectively) with respect to those calculated for the dimeric ((R)-1)2 structure. The comparison between the experimental and calculated 1H NMR spectrum of (R)-1 in chloroform solution indicates a good agreement with the calculated chemical shifts for the (R)1-h structure, while a complete disagreement is evident from the comparison with those calculated for the ((R)-1)2 structure (Figure 5).

different N−Zn−N bond angle and distance between the two Zn atoms (122.89°, 4.02 Å and 127.99°, 3.77 Å, for (R)-1-h and (R)-2-h, respectively). As a consequence, on switching from (R)-1-h to (R)-2-h, a different, almost square (4.82 × 4.02 Å) vs. rhomboid (5.18 × 3.77 Å), intrahelical cavity, is generated (Figure S6).

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DISCUSSION The study of (R)-1 in solution allowed us to further investigate the effect of the nonconjugated trans-1,2-diaminocyclo bridge on the coordination geometry of ZnII Schiff base complexes, providing new insight into their aggregation and Lewis acidic properties. The behavior of (R)-1 in solution of the noncoordinating chloroform solvent is rather unusual since independent from the concentration a single defined species is found. In contrast, for analogous ZnL complexes, we have previously established concentration-dependent aggregation properties in relation to the structure of the bridging diamine.49−53 In particular, for analogous (R)-2 complex in CDCl3 solution diverse aggregate species have been found.53 Upon heating or standing they have been irreversibly converted into a single species, (R)-2-h, which shows comparable 1H NMR spectral features to (R)-1 in the 3.5−8.5 ppm region. For this family of ZnL complexes, we have previously established that the chemical shifts of the H4 and H1 hydrogens are diagnostic to predict their aggregation mode.49−53 Actually, on going from pentacoordinated monomeric adducts in coordinating solvents to dimeric (ZnL)2 aggregates in chloroform solutions, an upfield shift of these signals has always been involved.48−53 A further upfield shift has been observed for the double helicate Zn2L2 structures with a tetrahedral coordination around the ZnII metal ion, as in (R)-2-h.53 Therefore, given the observed comparable chemical shifts of the H4 and H1 signals for (R)-1 and (R)-2-h in chloroform solution, we can hypothesize an analogous double helicate structure, (R)-1-h, for the former species. The observed strong bisignate signal at longer wavelengths in the CD spectrum is in agreement with this structure. This structural hypothesis for (R)-1 in chloroform solution is supported by DFT calculations. The calculated 1H NMR spectrum for the dimeric (R)-1 complex, considering the effect of the chloroform as solvent, either in a fully optimized double helicate (R)-1-h or dimeric ((R)-1)2 structure, indicates very different chemical shifts as far as the H4 and H1 hydrogens are concerned. In particular, the calculated H4 and H1 signals for

Figure 5. (a) 1H NMR spectrum of (R)-1 in CDCl3. Calculated 1H NMR spectrum of (R)-1 in CHCl3 for a double helicate (R)-1-h structure (b) and for a dimeric ((R)-1)2 structure (c).

Analogously, the calculated CD spectrum for the (R)-1-h structure, considering the effect of the chloroform as solvent, is in agreement with the experimental CD spectral features, unlike that calculated for the ((R)-1)2 structure (Figure S7). Therefore, all theoretical results also suggest for (R)-1 a dinuclear double helicate (R)-1-h structure. The observed behavior of (R)-1 in DMSO is rather singular because of the presence of two different species in this coordinating solvent, as deduced from 1H NMR, DOSY, UV/ vis, and CD spectroscopy, with a pronounced concentration 14209


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derivative is found. These observations together with the above chemical evidence point to a greater thermodynamic stability of the (R)-1-h complex.

dependence: (R)-1·DMSO adduct and a dimer. This is in contrast with all our previous investigations, since in coordinating solvents these Lewis acidic complexes were always stabilized as pentacoordinated monomeric adducts.49−53 As concerning the set of signals at high fields related to the dimer species, we can hypothesize a double helicate (R)-1-h structure similar to that assigned for (R)-1 in chloroform solution. This is further supported by DFT calculations. In fact, the comparison between the experimental and calculated 1H NMR spectra for the fully optimized (R)-1·DMSO adduct and double helicate (R)-1-h structure, considering the effect of the DMSO as solvent, shows an excellent agreement (Figure 6).

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CONCLUSIONS In order to further probe the effect of the defined trans-1,2diamine bridge on the coordination geometry and aggregation properties of ZnII Schiff base complexes, in this work we have studied (R)-1 derivative from (1R,2R)-trans-1,2-cyclopentanediamine. This complex shows unique characteristics in solution, behaving very different from ZnL complexes and, more importantly, from the analogue trans-1,2-diaminocyclohexane derivative. In particular, several peculiar aspects can be highlighted: (1) The existence in DMSO of two species in equilibrium, the classical monomeric adduct and a dinuclear double helicate dimer, indicate that the DMSO is not sufficiently strong Lewis basic to completely deaggregate the helicate complex. (2) The complete formation of monomeric adducts occurs only when dissolving the complex in the stronger Lewis base pyridine. (3) An unusual behavior in the noncoordinating chloroform solvent with the existence of a single defined dimeric species in a dinuclear double helicate Zn2L2 structure which does not deaggregate even with the addition of pyridine in large stoichiometric excess. In comparison, in chloroform solutions trans-1,2-cyclohexane (R)-2 derivative is mostly deaggregated after addition of a large stoichiometric excess of pyridine, while in DMSO solutions, it forms monomeric adducts. Theoretical calculations indicate that the switching from double helicate trans-1,2-cyclopentane to trans-1,2-cyclohexane derivative involves a more distorted tetrahedral coordination geometry around the zinc atoms. Therefore, chemical evidence and theoretical data indicate a higher thermodynamic stability of the trans-1,2-cyclopentane derivative than the cyclohexane analogue, leading to its unexpected, different aggregation properties. This study demonstrated that even a slight difference on the structure of the cycloaliphatic ring of the trans-1,2-diamine bridge plays a major role in the aggregation features, and hence, in their potential applications of these resourceful species.

Figure 6. (a) 1H NMR spectrum of (R)-1 in DMSO-d6 (5.0 × 10−3 M). (b) Calculated 1H NMR spectra of (R)-1 in DMSO for the (R)-1· DMSO adduct (red lines) and for a double helicate (R)-1-h structure (blue lines).

The concentration dependence for the two species of (R)-1 in DMSO suggests the existence of a slow equilibrium, with respect to the NMR time scale, between (R)-1·DMSO adduct and double helicate (R)-1-h. This allowed estimating the dimerization equilibrium constant (Kd, eq 1) from the integration ratio between the two sets of H4 and/or H1 signals.55 2(R)‐1·DMSO ⇆ (R)‐1‐h

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(1)

EXPERIMENTAL SECTION

Materials and General Procedures. All reactions were performed under a nitrogen atmosphere. Zinc perchlorate hexahydrate, 2-hydroxy-4-methoxybenzaldehyde, (1R,2R)-trans-1,2-cyclopentanediamine dihydrochloride, and triethylamine (NEt3) (Aldrich) were used without any further purification. Chloroform (Aldrich) stabilized with amylene was used for spectrophotometric measurements. CDCl3 (Aldrich) was stored over molecular sieves (3 Å), while DMSO-d6 and pyridine-d5 were used as obtained. Physical Measurements. Elemental analyses were performed on a Carlo Erba 1106 elemental analyzer. ESI-MS spectra were recorded on a Thermo Scientific (Linear Trap) LTQ-XL electrospray mass spectrometer (Bremen, Germany). The sample was dissolved either in methanol (∼1 × 10−5 M), acetonitrile (∼1 × 10−5 M), or in chloroform (1.0 × 10−4 M) and then diluted with methanol at ∼1 × 10−5 M. In all cases, analogous spectral patterns were obtained. All NMR experiments were recorded at 27 °C on a Varian Unity S 500 (499.88 MHz for 1H) spectrometer. For all 1D and 2D NMR experiments, tetramethylsilane (Si(CH3)4, TMS) was used as internal reference. 1H NMR DOSY measurements were performed as described elsewhere.49−52 Optical absorption and CD spectra were recorded using a UV/vis V650 Jasco spectrophotometer and a JASCO 810 spectropolarimeter, respectively.

The estimated value for the dimerization constant, Kd = 350, suggests that even a relatively strong Lewis base solvent, such as DMSO,56,57 is not sufficient enough to completely disaggregate (R)-1-h into monomeric adducts, as always observed for ZnL complexes.49−53 This evidences the very low Lewis acidic properties of this aggregate species. In fact, only when dissolving (R)-1 in py-d5, representing one of the stronger Lewis bases,56,57 does formation of (R)-1·py-d5 adduct occur (Figure 1). However, even after addition of a 103-fold mole excess of py-d5 to 1.0 × 10−3 M CDCl3 solutions of (R)-1 no deaggregation is observed, further confirming the stability of this aggregate species. In comparison, 1.0 × 10−3 M CDCl3 solutions of (R)-2-h are most deaggregated after addition of a 103-fold mole excess of py-d5. The switching from double helicate trans-1,2-cyclohexane to trans-1,2-cyclopentane derivative involves a larger dihedral angle between nitrogen atoms of the diimine bridge. As a consequence, a more distorted tetrahedral coordination geometry around the zinc atoms in the trans-1,2-cyclohexane 14210


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Synthesis of [N,N-Bis(4-methoxy-2-hydroxybenzylidene)(1R,2R)-trans-1,2-diaminocyclo-pentanediaminato]ZnII ((R)-1). 2-Hydroxy-4-methoxybenzaldehyde (0.304 g, 2.00 mmol) was dissolved in methanol (20.0 mL). Then, (1R,2R)-trans-1,2-cyclopentanediamine dihydrochloride (0.173 g, 1.00 mmol) and triethylamine (1.00 mL) were added. The mixture thus obtained was boiled with stirring for 2 h. Then, the yellow solution was treated with zinc perchlorate hexahydrate (0.372 g, 1.00 mmol) and more triethylamine (0.500 mL) and refluxed with stirring 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 with sulfuric acid. Off-white powder (0.378 g, 60%). C21H22N2O4Zn (431.80): Calcd C, 58.41; H, 5.14; N, 6.49. Found C, 58.58; H, 5.12; N, 6.48. ESI-MS: m/z = 861 [2 M + H]+, 883 [2 M + Na]+. 1H NMR (500 MHz, CDCl3): δ = 1.69 (br, 8H, cyclopentyl-H), 2.00 (br, 4H, cyclopentyl-H), 3.82 (s, 12H; OCH3), 3.84 (br, 4H, CH−NCH), 6.20 (dd, 3JHH = 9.0 Hz, 4JHH = 2.5 Hz, 4H; ArH), 6.31 (d, 4JHH = 2.5 Hz, 4H; ArH), 6.60 (d, 3JHH = 9.0 Hz, 4H; ArH), 7.50 (s, 4H; CH N). 13C NMR (125 MHz, CDCl3): δ = 18.90, 32.39, 55.2, 70.44, 104.21, 105.48, 112.63, 137.53, 166.03, 170.70, 172.44. Computational Methods. Geometries were fully optimized at the DFT level of theory by means of the Perdew−Burker−Ernzerhof (PBE) functional,58,59 adopting the 6-311+** basis set.60−64 All positive harmonic vibration frequencies were found thus ensuring the ground state of the structures. Calculations of 1H NMR chemical shifts were performed using the gauge independent atomic orbital (GIAO) approach,65−67 together with the dispersion corrected functional CAM-B3LYP68 combined with 6-311+g(2d,p) basis set.61 This level of theory gives a good performance of calculated NMR data. In fact, the calculated chemical shifts, rescaled multiplying by 0.94, are in excellent agreement with experimental data. The time-dependent density functional theory (TDDFT) approach was applied for the calculation of the electronic circular dichroism (ECD). The same basis set adopted for the geometry optimization was employed, while the hybrid functional of Truhlar and Zhao M06-2X,69 instead of PBE, was used. Such functional has been shown to be very accurate in the prediction of ECD characteristics.70 In all the calculations, the polarizable continuum model (PCM) was included to account for solvation effects.71,72 Calculations were performed using the G09 software.73

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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.7b02341. Additional 1H NMR, DOSY, UV/vis and CD spectroscopic data. Optimized geometrical parameters of all involved structures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Santo Di Bella: 0000-0002-7120-1817 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 Dr. V. Oliveri for the assistance in the CD measurements. 14211


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