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Chiral Recognition between Metallohelicates via Strong H Bonds: Homochiral Bishelical Coupling and Mesohelical Polymerization Jesús Sanmartín-Matalobos,* Cristina Portela-García, Matilde Fondo, and Ana M. García-Deibe* Departamento de Química Inorgánica, Facultad de Química, Campus Sur, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain S Supporting Information *

ABSTRACT: A rigid, one-dimensional, and monotopic ligand containing both a proton donor and a proton acceptor, 2-[(1Himidazol-2-yl)methyleneamino]phenol (H2L), has been used to attain three hierarchically assembled supramolecular polymers. The handedness of these hydrogen-bonded metallo-supramolecular structures is modulated by a selective coupling, via strong H bonds with significant covalent character. Thus, alternate Δ and Λ enantiomers of Cd(HL)2 form a one-dimensional supramolecular polymer via mutual strong N−H···O interactions (2.58 Å), which are associated with a hydrogen bond energy value of ∼11.5 kcal mol−1. In contrast, homochiral self-recognition of enantiomers of M(HL)2 and [M(H2L)2]2+ (M = CuII and NiII) gives rise to dinuclear bishelicates, whose metallohelical units are held together by two strong O−H···O bonds (2.41−2.44 Å), which are associated with hydrogen bond energies in the range of 25−35 kcal mol−1.

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INTRODUCTION Some hierarchical assemblies, like those occurring for proteins and nucleic acids, have attracted considerable attention over the past several decades.1−3 Helicity is a common motif in the secondary structure of biopolymers, and the specific onehandedness of the DNA double helix and protein’s α-helix is of key importance for their remarkable functions.4,5 The development achieved in the design of helical species has given rise to the construction of discrete metallohelicates, even from achiral components.6−9 It has been reported that the helical handedness of dynamic polymers can be modulated by using the selective coordination of different metal ions.9−12 Likewise, a hierarchically formed helicate, in which Li+ ions are connecting two mononuclear propellers that show the same twist, has recently been described, and it is forming Δ,Δ′ or Λ,Λ′ dimers.13,14 However, the stereoselective control in the synthesis of superstructures based on metallohelical units remains a major challenge. Some heterochiral superstructures based on metallohelical units are formed on the basis of interactions that are weaker than coordination bonds, such as hydrogen bonds,15 π−π stacking,16 and CH−π interactions.17 However, the homochiral assembly of metallohelicates by means of hydrogen contacts, which occurs in the DNA double helix, is much more infrequent. Despite this, a few examples can be mentioned, as a series of hydrogen-bonded helical dimers of the type [Co(L′) (HL′)X]2 [HL′ = pyridyne-2-methanol derivatives (X = Cl, Br, I, NO3, or SCN)],18,19 or more recently the trishelical complex [Ru(L″)3Ru(HL″)3]− [HL″ = 3-(2-pyridyl)-1H-pyrazole].20 © XXXX American Chemical Society

We have previously communicated that the reaction of 2[(1H-imidazol-2-yl)methyleneamino]phenol (H2L) with cadmium perchlorate gives rise to various complexes, such as [Cd10(L)4(HL)6(ClO4)2(CO3)](ClO4)2, Cd2(L)(HL)(ClO4)(MeOH)2, Cd2(L)(HL)(HO), and Cd(HL)2.21 These reactions allow one to deduce that H2L, in the absence of added proton acceptors, is acting as a tridentate ligand after losing its acidic protons, totally or partially. The monoprotonated ligands in [Cd10(L)4(HL)6(ClO4)2(CO3)](ClO4)2 allow the interaction through N−H···O bonds of some of the external imidazole N atoms with the perchlorate counterions (Figure 1).21 The length of these bonds was in the range of 2.52−2.86 Å, the shortest ones being very strong for this type of connection.24−28 This has aroused our interest because strong H bonding (assisted by charge and/or resonance) has been mostly studied in organic compounds, but not so thoroughly in metal−organic compounds. With this in mind, we have explored the use of the achiral ligand H2L for the construction of hydrogen-bonded supramolecular polymers, paying special attention to the recognition process between metallohelical units.22−26

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RESULTS AND DISCUSSION With the aim of knowing the ability of the ligand before its coordination for the construction of hydrogen bond networks, we will study the supramolecular structure of H2L·MeOH.21 Received: May 5, 2015 Revised: July 13, 2015

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persist after coordination when the ligand remains partially or totally protonated, and consequently, they could collaborate in the assembly of its complexes. [Cd(HL)2]n. Our understanding of the assemblage of this peculiar mesohelical supramolecular polymer, whose metallohelical units are held together by two strong N−H···O bonds, is summarized in Scheme 1. The assembly process can be understood as distinct steps that cannot proceed until the preceding step has been completed. The first step would be the formation of a racemic mixture of Λ and Δ enantiomers of twoblade helical complex Cd(HL)2 (Figure S2). Subsequently, after heterochiral recognition, a one-dimensional supramolecular polymer is formed with alternate Δ and Λ enantiomers. This metallo-supramolecular structure whose repeat Cd(HL)2 units are held together by head-to-tail linkages from imidazole to phenolate is additionally enforced by π−π stacking of the conjugated π systems. The bottom panel of Figure 3 shows four units of [Cd(HL)2]n with the Λ and Δ enantiomers in different colors. It is worth mentioning that this assembly led to a double-stranded infinite mesohelicate, whose threads are held together by Cd−O and Cd−N coordination bonds. Clear π−π interactions reinforce the interaction between neighboring ligands of contiguous repeat units, the distance between an imine group and the contiguous imidazole ring being ∼3.1 Å. The top panel of Figure 3 shows the zigzag arrangement in [Cd(HL)2]n via mutual strong N−H···O hydrogen bonds of each of the threads. In [Cd(HL)2]n, and in clear contrast with those moderate H bonds found in H2L·MeOH, two mutual strong N−H··O hydrogen bonds occur between the two contiguous helical units, which are mirror images. As seen for [Cd10(L)4(HL)6(ClO4)2(CO3)](ClO4)2,21 these H bonds can be considered strong, because their N···O lengths are as short as 2.58 Å (see Table S2a) and, therefore, have significant covalent character.22−28 According to Gilli correlation,25 the estimated energy for these resonance-assisted hydrogen bonds23−28 is ∼11.5 kcal mol−1, which could be considered exceptional. One might note that strong N−H···O bonds typically occur when some charge, positive and/or negative, is assisting the H bond.25−28 However, in [Cd(HL)2]n, in spite of the phenolate character of acceptor O atoms, they are not actually negatively charged, as this should be committed to the coordination to the cadmium(II) ions, and therefore, the repeat units are actually neutral. The strength of these N−H··O bonds is unusual, so it could be not only related to a resonance

Figure 1. Sticks diagram for [Cd10(L)4(HL)6(ClO4)2(CO3)](ClO4)2. Protonated imidazole nitrogen atoms are the only atoms represented as spheres, while CdII ions are represented in space-filling mode (90% probability).

After that, we will analyze the very interesting structural features of the hydrogen-bonded supramolecular polymer [Cd(HL)2]n, as well as those of dimeric [Ni(HL)2Ni(H2L)2](ClO4)2·4H2O and [Cu(HL)2Cu(H2L)2](ClO4)2·MeCO2Et· H2O. H2L·MeOH. Because the molecule of H2L contains an imidazole ring and a phenolic one, H2L can act as a donor and acceptor for H bonds. One might note that the NH group gives rise to very directional and planar interactions, while the OH group has a more conical scope, displaying an angular disposition.23 This can be observed in the crystal structure of H2L·MeOH (Figure S1), where a methanol molecule is acting as an angular bridge, between the two Schiff base molecules, to form infinite corrugated chains of the ligand (Figure 2). The conjugation and planarity of the free ligand facilitate π−π interactions that favor an effective stacking between pairs of these H-bonded chains. As the shortest D···A distance is ∼2.71 Å (see Table S1 for details), these H bonds can be considered moderate. In fact, according to Gilli correlation, the estimated hydrogen bond energy is in the range of 4.1−6.8 kcal mol−1.25 One might note that the abilities shown by the free ligand for H bonding could

Figure 2. Partial view of the packing scheme of H2L·MeOH, showing two zigzag chains of H-bonded molecules stacked via π−π interactions. H bonds are represented as pale blue dotted sticks. B

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Scheme 1. Hierarchical Self-Assembly of Two-Blade Helical Complex Cd(HL)2 To Form Mesohelical Supramolecular Polymer [Cd(HL)2]n by Coupling of Alternate Δ and Λ Isomersa

Figure 3. Space-filling representations (80%) of four units of [Cd(HL)2]n. The top structure is a view of the two mesohelical zigzag threads connected through both N−H···O bonds. The disposition is similar to that shown in Scheme 1, but the threads are represented in different colors for a better understanding. The bottom structure is a view of the Λ and Δ enantiomers of Cd(HL)2 (C atoms of the Λ enantiomer are colored blue, while those of the Δ enantiomer are colored gray). The H atoms of the imidazole ring that mutually connect the molecules are colored black, while the Cd atoms are colored pale yellow.

a The top of the figure explains the sketches used for the ligand. The handedness of the complexes is assigned according to that observed for the C2 axis of the mononuclear cadmium complexes.

Figure 4. Two interaction models that are possible between two Δ enantiomers of Cd(HL)2 with head-to-tail N−H··O hydrogen bonding: (left) phenol−phenol and imidazole−imidazole π−π stacking and (right) double phenol−imidazole π−π stacking. The significant steric hindrance that exists between both phenol rings (left) and the phenol−imidazole rings (right) appears to prevent its participation in the additional H bonds.

assistance of the π system of the ligands but also reinforced by the coordination to the cadmium ions, as the N−H bond could be more polarized. With the aim of checking the nature of the N−H···O or N··· H−O bond in [Cd(HL)2]n, some additional refinement cycles were conducted, to determine the real position of the H atoms involved in these bonds. These atoms could be located in the difference Fourier maps at 0.9−1.0 Å of the external Nimidazole atoms of the coordinated ligands, which is indicative of a N− H···O nature more than a N··H−O nature. This is also consistent with the C−O distances found (∼1.30 Å), which are more suitable for a coordinated phenolate group than for a protonated phenol one.29 To improve our understanding of how heterochiral recognition, rather than homochiral, could modulate the selfassembly process, we have simulated the rapprochement of two Δ enantiomers of Cd(HL)2 (Figure 4). For this purpose, we have kept in mind that only head-to-tail N−H··O interactions appear to be possible and that imidazole donor N−H bonds are very directed, in contrast to the rotating O−H bonds. As a result, instead of the two N−H··O interactions occurring between the two contiguous heterochiral blocks occurring in

the supramolecular polymer, only one N−H···O bond would be reasonable between two contiguous homochiral blocks, as a consequence of the patent steric hindrance. Therefore, the formation of a homochiral polymer appears to be less favorable than the heterochiral assembly found for [Cd(HL)2]n. As a continuation of our work, we have set the goal of strengthening the metallo-supramolecular structure. For this purpose, we have explored other possibilities of a “head-tohead” disposition of the metallo-helical units that would lead to supramolecular dimers of the type [M(HL)2M(H2L)2]2+. This new arrangement would involve Ophenol−H···Ophenolate bonds, which can be even stronger than the N−H···O bonds already observed for [Cd10(L)4(HL)6(ClO4)2(CO3)](ClO4)2 and Cd(HL)2.21 Because attempts to obtain crystals of [Cd(HL)2Cd(H2L)2]2+ were rather ineffective, we have chosen nickel and copper for these new experiments, because these metal ions are also adequate for the creation of the required two-blade helical complexes. Thus, the reaction of H2L with M(ClO4)2·6H2O (M = Ni and Cu) in a 2:1 molar ratio gave rise to complexes of the type [M(HL)2M(H2L)2](ClO4)2, although in low yields (see Experimental Section for details). C

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[Cu(HL)2Cu(H2L)2]2+ and [Ni(HL)2Ni(H2L)2]2+. Figure 5 shows the molecular structures of [Cu(HL)2Cu(H2L)2]2+ and

2.41−2.44 Å, they can be qualified as strong H bonds, according to Jeffrey’s categorization.22 This category is considered to have a quasi-covalent nature, and it has been associated with a estimated EHB in the range of 25−35 kcal mol−1.25 Because these two supramolecular dimers result from the association of M(HL)2 and [M(H2L)2]2+ species, the marked strength of these bonds seems to be associated with positive charge assistance, (+)CAHB.23−25 Another factor that could favor this strength could be the equalization principle proposed by Gilli et al.,25,27 as the neutral ligand and the monodeprotonated ligand are a conjugated acid−base pair and, hence, the difference between their pKa values is equal to 0. A similar strength had been also observed for supramolecular dimers of the type [Co(L′) (HL′)X]2.18,19 Both [Cu(HL)2Cu(H2L)2]2+ and [Ni(HL)2Ni(H2L)2]2+, as occurred for [Cd(HL)2]n, were also hierarchically formed by a multimediated multiple-interaction assembly, using metal− ligand bonds, hydrogen bonding, and π−π stacking interactions (Scheme 2). First, chiral M(HL)2 and [M(H2L)2]2+ species (M Scheme 2. Hierarchical Self-Assembly of H2L and M2+ To Form Helical Dimers of the Type [M(HL)2M(H2L)2]2+ (M = Ni or Cu) by Homochiral Recognitiona

Figure 5. Ellipsoid representations at the 50% probability level of the Λ,Λ′ enantiomer of [Cu(HL)2Cu(H2L)2]2+ (top) and the Δ,Δ′ enantiomer of [Ni(HL)2Ni(H2L)2]2+ (bottom).

a

The handedness values of the complexes are assigned according to that observed for the C2 axis of these molecules.

[Ni(HL)2Ni(H2L)2]2+. The H atoms potentially involved in the H bonding scheme were first found in the difference Fourier map and then freely refined. The final result shows that both H atoms appear to be clearly bonded to the two ligands corresponding to only one of the complexes present in the asymmetric units of their respective crystals. This is experimental evidence of the neutral and cationic nature of the M(HL)2 and [M(H2L)2]2+ species, respectively. As a consequence of the metal coordination, the phenol H atom radically changes its orientation toward the exterior in [M(H2L)2]2+ (when it is compared with the free ligand, where it is involved in an intramolecular H bond), and consequently, it can give rise to a firm head-to-head coupling. Because the O−H···O lengths found in [Cu(HL)2Cu(H2L)2]2+ and [Ni(HL)2Ni(H2L)2]2+ are in the range of

= Ni or Cu) are formed in solution as a racemic mixture of Δ and Λ enantiomers. Subsequently, reversible H bond-mediated coupling occurs between homochiral isomers of the neutral and cationic complexes, leading to a racemic mixture of both enantiomers Δ,Δ′ and Λ,Λ′ with locked stereochemistry (Figure S3). Furthermore, π−π stacking interactions are also evident between phenolic rings of M(HL)2 and [M(H2L)2]2+ species, the distance between their respective centroids being ∼3.8 Å. The combination of these interactions in both complexes leads to a M···M distance of ∼5.1 Å, with both metal ions situated along the helicate axis. The vast majority of crystals that correspond to racemic substances display the two typical mirror enantiomers, in many cases also through H bonds. On the other hand, these two bishelicates and other H-bonded dinuclear helicates are D

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associated as homochiral pairs, Δ,Δ′ and Λ,Λ.18−20 The coupling process described here is certainly homochirally selective, because only two-blade helical complexes M(HL)2 and [M(H2L)2]2+ with the same handedness can connect to give rise to these Δ,Δ′ or Λ,Λ′ isomers of [M2(HL)2(H2L)2]2+, as Figure 6 shows. This assertion can be easily confirmed by a

pressure of the brown solution yielded an oily product, from which an orange powder was obtained after the product had been stirred with 10 mL of ethyl acetate for ∼30 min. The orange powder in the resulting suspension was filtered off and dried under vacuum to give [Cu2(HL)(L)(H2O)2](ClO4), which has been spectroscopically characterized (Figure S5). Slow evaporation of the ethyl acetate solution gave rise to single crystals of [Cu(HL)2Cu(H2L)2](ClO4)2· MeCO2Et·H2O. [Cu(HL)2Cu(H2L)2](ClO4)2·MeCO2Et·H2O: yield 12%; FTIR (KBr, cm−1) ν(O−H) 3319(s), ν(CNimine) 1614(m), ν(COOa) 1595(m), ν(COOs) 1434(m), ν3(Cl−O) 1146(vs), 1115(vs) and 1090(vs), ν4(Cl−O) 637(s) and 628(s); MS (MALDI-TOF, DCTB) m/z 563.1 (100%) [Cu(HL)2(AcOEt) + K]+; λmax (MeOH) = 460 nm; μ = 1.9 μB. Elemental Anal. Found: C, 44.6; H, 3.3; N, 14.6. Calcd for C44H44Cl2Cu2N12O15: C, 44.8; H, 3.7; N, 14.3. [Cu2(HL)(L)(H2O)2](ClO4): yield 67%; FTIR (KBr, cm−1) ν(C Nimine) 1596(m), ν(COOa) 1586(m), ν(COOs) 1436(m), ν3(Cl−O) 1149(vs), 1121(vs), 1108(vs), and 1088(vs), ν4(Cl−O) 636(s) and 627(s). Elemental Anal. Found: C, 38.0; H, 3.1; N, 12.6. Calcd for C20H20ClCu2N6O8: C, 37.8; H, 3.2; N, 13.0. [Ni(HL)2Ni(H2L)2](ClO4)2·4H2O was obtained by stirring a methanol solution (40 mL) containing H2L (100 mg, 0.53 mmol) and Ni(ClO4)2·6H2O (96 mg, 0.26 mmol) at room temperature over a 6 h period. Concentration under reduced pressure of the orange solution yielded an oily product, from which a green powder was obtained after the product had been stirred with 10 mL of ethyl acetate for ∼30 min. The green powder in the resulting suspension was filtered off and dried under vacuum to give [Ni2(HL)(L)(MeOH)2](ClO4), which has been spectroscopically characterized (Figure S6). Slow evaporation of the methanol solution gave rise to single crystals of [Ni(HL)2Ni(H2L)2](ClO4)2·4H2O. [Ni2(HL)2(H2L)2](ClO4)2·4H2O: yield 15%; FTIR (KBr, cm−1) ν(O−H) 3330(m), ν(CNimine) 1611(m), ν3(Cl−O) 1145(s), 1110(vs), and 1089(vs), ν4(Cl−O) 636(m) and 627(m); MS (MALDI-TOF, DCTB) m/z 431.1 (100%) [Ni(HL)2 + H]+; μ = 3.1 μB. Elemental Anal. Found: C, 41.9; H, 3.5; N, 14.9. Calcd for C40H42Cl2N12Ni2O16: C, 42.3; H, 3.7; N, 14.8. [Ni2(HL)(L)(MeOH)2](ClO4): yield 65%; FTIR (KBr, cm−1) ν(O−H) 3333(m), ν(CNimine) 1610(m), ν3(Cl−O) 1145(s), 1111(vs), and 1087(vs), ν4(Cl−O) 634(m) and 625(m); MS (MALDI-TOF, DCTB) m/z 674.1 (100%) [Ni 2 (HL)(L)(MeOH)2(ClO4) + Na]+, 589.0 (62%) [Ni2(HL)(L)(ClO4) + H]+; λmax (MeOH) = 455 nm [3T1g(P) ← 3A2g(F)]. Elemental Anal. Found: C, 40.9; H, 3.3; N, 12.9. Calcd for C22H23ClN6Ni2O8: C, 40.5; H, 3.6; N, 12.9. X-ray Diffraction. Single-crystal diffraction data for [Cu2(HL)2(H2L)2](ClO4)2·MeCO2Et·H2O and [Ni2(HL)2(H2L)2](ClO4)2·4H2O were collected at 100(2) K, using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) from a fine focus sealed tube. Some significant crystal parameters and refinement data are listed in Table S5. Data were processed and corrected for Lorentz and polarization effects. Multiscan absorption corrections were performed using the SADABS routine.30 Structures were determined by standard direct methods using SIR200431 and then refined by full matrix least squares on F2 using SHELXL97.32 Both complexes presented some disorder for counterions and solvates. Non-H atoms were anisotropically treated, with the exception of disordered atoms showing low occupancies. Partial occupancies of solvate molecules were individually refined and then rounded to simplify the formula. H atoms attached to phenolic O atoms could be located on Fourier maps and then isotropically refined. Hydrogen atoms of organic groups were included at geometrically calculated positions with thermal parameters derived from the parent atoms. Most of the H atoms corresponding to water molecules were located on Fourier maps, and then thermal parameters were included as being dependent on the parent atoms. However, some of the hydrogen atoms from the water molecules in [Cu2(HL)2(H2L)2](ClO4)2·MeCO2Et·H2O (those with very low occupancies) could not be located. Final difference Fourier maps were almost flat.

Figure 6. Space-filling views (80%) of the four possible combinations of Λ and Δ enantiomers of Ni(HL)2 and [Ni(H2L)2]2+. Coupling of enantiomers with different handedness shows evident steric hindrance, and therefore the impossibility of forming the corresponding racemic or mesohelical Λ,Δ′ and Δ,Λ′ dimers.

mere rapprochement of two complexes of different handedness (Figure S4). For a head-to-head coupling, the double O−H···O interaction is possible only between two complexes of the same handedness, but impossible if different, as there is a patent steric hindrance between the phenolic rings of M(HL)2 and [M(H2L)2]2+.

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CONCLUSIONS We report here a feasible direct synthetic approach for obtaining robustly self-assembled supramolecular polymers via strong hydrogen bonding by chiral recognition from two-blade helical complexes. Furthermore, two short and strong O−H···O bonds per ligand unit, in conjunction with π−π interactions, are responsible for the frontal head-to-head dimerization of the two mononuclear species giving rise to double-stranded metallohelicates. A selective H bond-mediated coupling occurs only between homochiral isomers of neutral and cationic units to yield Δ,Δ′ and Λ,Λ′ dimers. In contrast, a heterochiral one-dimensional supramolecular polymer with infinite chains is obtained from alternate Δ and Λ enantiomers of neutral units. Unusual strong N−H··O hydrogen bonds (two per ligand unit) and π−π stacking stabilize the mesohelical head-to-tail polymerization of mononuclear species.

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EXPERIMENTAL SECTION

[Cu(HL)2Cu(H2L)2](ClO4)2·MeCO2Et·H2O was obtained by stirring a methanol solution (60 mL) containing H2L (100 mg, 0.53 mmol) and Cu(ClO4)2·6H2O (98 mg, 0.26 mmol) at room temperature over a 24 h period. Concentration under reduced E

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Samples of crystallized metal complexes were measured on a Philips powder diffractometer fitted with a Philips control unit (PW1710), a vertical Philips goniometer (PW1820/00), and an Enraf Nonius generator (FR590). The instrument was equipped with a graphitediffracted beam monochromator and a copper radiation source [λ(Kα1) = 1.5406 Å], operating at 40 kV and 30 mA. The X-ray powder diffraction patterns (XRPD) have been collected by measuring the scintillation response to Cu Kα radiation in the angular range 3° < 2θ < 27°, with a step size of 0.02° and a counting time of 4 s per step. XRPD of [Cd(HL) 2 ] n , [Ni 2 (HL) 2 (H 2 L) 2 ](ClO 4 ) 2 ·4H2 O, and [Cu2(HL)2(H2L)2](ClO4)2·MeCO2Et·H2O with their simulated powder diffraction patterns are shown in Figures S7−S9, respectively.

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(13) Albrecht, M.; Isaak, E.; Baumert, M.; Gossen, V.; Raabe, G.; Fröhlich, R. Angew. Chem., Int. Ed. 2011, 50, 2850−2853. (14) Albrecht, M.; Isaak, E.; Moha, V.; Raabe, G.; Fröhlich, R. Chem. - Eur. J. 2014, 20, 6650−6658. (15) Khatua, S.; Harada, T.; Kuroda, R.; Bhattacharjee, M. Chem. Commun. 2007, 3927−3729. (16) Dumitru, F.; Legrand, Y.-M.; Van der Lee, A.; Barboiu, M. Chem. Commun. 2009, 19, 2667−2669. (17) Otero, A.; Lara-Sánchez, A.; Fernández-Baeza, J.; AlonsoMoreno, C.; Tejeda, J.; Castro-Osma, J. A.; Márquez-Segovia, I.; Sánchez-Barba, L. F.; Rodríguez, A. M.; Gómez, V. Chem. - Eur. J. 2010, 16, 8615−8619. (18) Telfer, S. G.; Sato, T.; Kuroda, R. Angew. Chem., Int. Ed. 2004, 43, 581−584. (19) Telfer, S. G.; Kuroda, R. Chem. - Eur. J. 2005, 11, 57−68. (20) Metherell, A. J.; Cullen, W.; Stephenson, A.; Hunter, C. A.; Ward, M. D. Dalton Trans. 2014, 43, 71−84. (21) García-Deibe, A. M.; Portela-García, C.; Fondo, M.; Mota-Á vila, A. J.; Sanmartín-Matalobos, J. Chem. Commun. 2012, 48, 9915−9917. (22) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: Oxford, U.K., 1997. (23) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 48−76. (24) Gilli, G.; Gilli, P. The Nature of the Hydrogen Bond. Outline of a Comprehensive Hydrogen Bond Theory; Oxford University Press: Oxford, U.K., 2009. (25) Gilli, P.; Pretto, L.; Bertolasi, V.; Gilli, G. Acc. Chem. Res. 2009, 42, 33−44. (26) Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, J.; Alkorta, S.; Clary, I.; Crabtree, D. C.; Dannenberg, R. H.; Hobza, J. J.; Kjaergaard, P.; Legon, A. C.; Mennucci, B.; Nesbitt, D. J. Pure Appl. Chem. 2011, 83, 1619−1636. (27) Bertolasi, V.; Gilli, P.; Gilli, G. Cryst. Growth Des. 2011, 11, 2724−2735. (28) Gilli, G.; Bertolasi, V.; Gilli, P. Cryst. Growth Des. 2013, 13, 3308−3320. (29) Fondo, M.; García-Deibe, A. M.; Ocampo, N.; Sanmartín, J.; Bermejo, M. R.; Llamas-Saiz, A. L. Dalton Trans. 2006, 48, 4260− 4270. (30) Blesing, R. H. Acta Crystallogr., Sect. A: Found. Crystallogr. 1995, A51, 33−38. (31) Burla, M. C.; Caliandro, R.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; De Caro, L.; Giacovazzo, C.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 2005, 38, 381−388. (32) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112−122.

ASSOCIATED CONTENT

S Supporting Information *

CCDC 1043468−1043469 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00616. Ellipsoid views for H2L and [Cd(HL)2]n, geometric data and H bonding schemes for H2L, [Cd(HL)2]n, [Cu(HL)2·Cu(H2L)2](ClO4)2·AcOEt·H2O, and [Ni(HL)2· Ni(H2L)2](ClO4)2·4H2O, and some spectral data, powder X-ray patterns, and some other images of the homochiral coupling of these two bishelicates (PDF) Crystal data for [Cu(HL)2·Cu(H2L)2](ClO4)2·AcOEt· H2O (CIF) Crystal data for [Ni(HL)2·Ni(H2L)2](ClO4)2·4H2O (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Telephone: +34 881814237. *E-mail: [email protected]. Telephone: +34 881814396. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.cgd.5b00616 Cryst. Growth Des. XXXX, XXX, XXX−XXX