Self-Assembly of Silver (I) and Ditopic Heteroscorpionate Ligands

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Self-Assembly of Silver(I) and Ditopic Heteroscorpionate Ligands. Spontaneous Chiral Resolution in Helices and Sequence Isomerism in Coordination Polymers Gema Durá,† M. Carmen Carrión,†,‡ Félix A. Jalón,† Ana M. Rodríguez,§ and Blanca R. Manzano*,† †

Departamento de Química Inorgánica, Orgánica y Bioquímica, Universidad de Castilla-La Mancha, Facultad de Ciencias y Tecnologías Químicas-IRICA, Avda. C. J. Cela, 10, 13071 Ciudad Real, Spain ‡ Fundación PCYTA, Paseo de la Innovación, 1, Edificio Emprendedores, 02006 Albacete, Spain § Departamento de Química Inorgánica, Orgánica y Bioquímica, Universidad de Castilla-La Mancha, Escuela Técnica Superior de Ingenieros Industriales, Avda. C. J. Cela, 3, 13071 Ciudad Real, Spain S Supporting Information *

ABSTRACT: The self-assembly of the ligands bis(pyrazol-1yl)(pyridine-4-yl)methane (L1) and bis(3,5-dimethylpyrazol-1yl)(pyridine-4-yl)methane (L2) with AgBF4 leads to three types of species: L1 gives rise to the formation of a boxlike cyclic dimer (1) while L2 leads to two different coordination polymers that are sequence isomers. With dependence on the crystallization conditions, either a homochiral helix (2) with spontaneous resolution or a zigzag polymer is formed (3). By means of solid-state circular dichroism studies, the presence of both enantiomers in 2 is observed, enriched locally in colonies of crystals. The homochiral motif is stable, even upon removal of guest solvent molecules. The noncovalent interactions in the crystalline structure are clearly affected by the presence of the methyl groups on the pyrazolyl rings. The behavior of the compounds in solution is discussed.

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The helices are especially interesting because they constitute one of the most attractive expressions of chirality,19 and hence, the construction of helical metal−organic structures through self-assembly20 is among the most challenging tasks in supramolecular chemistry and crystal engineering. However, the factors that control the formation of coordination helices are still not well-understood. Chiral predetermination is usually achieved by the use of enantiopure ligands21 and when achiral building blocks are employed, they usually give racemates of P and M helices.22 In these systems, spontaneous resolution, a phenomenon of great importance in a number of research areas23 and also of interest because of its implications in the origin of homochirality, is not frequently observed.24 This spontaneous chiral resolution, which yields homochiral helical conglomerates, requires an efficient transfer of stereochemical information between neighboring homochiral helices. A possibility of paramount importance for biological systems is based on noncovalent supramolecular interactions. Recently, we described25 the formation of di-, tri-, and polynuclear complexes in the reaction of the ditopic ligands bis(pyrazol-1-yl)(pyridine-4-yl)methane (L1) and bis(3,5-dimethylpyrazol-1-yl)(pyridine-4-yl)methane (L2) (Chart 1) with metals of different geometries. It was observed that the final

he coordination-driven self-assembly of metal−organic supramolecular architectures,1 including coordination polymers and metal−organic frameworks (MOFs), based on a combination of metal−ligand coordination and secondary noncovalent interactions, such as hydrogen-bonding,2 π−π stacking,3 and cation−π,4 anion−π,5 and XH−π interactions,6 is an area of intense current interest in inorganic crystal engineering.7 This interest arises not only because of their fascinating structures but also due to their multiple applications such as gas storage and separation,8 ion exchange,8e,9 catalysis,8e,f,10 storage and release of drugs,8f use as semiconductors, or as materials with metallic conductivity11 or sensors12 and also applications related to magnetic8f,13 or luminescent properties.8f,11 The use of metals that do not have a strong preference for a particular coordination geometry, such as silver, which can adopt coordination numbers between two and six and various geometries,14 can give rise to a certain number of structures that have similar stabilities, and the weak noncovalent interactions may exert a stronger influence on the molecular structure and the organization of the supramolecular network.15,16 Numerous examples of silver(I) coordination polymers have been described, including diverse structures like linear or zigzag chains, helices, ladders, and two- and three-dimensional networks of different natures.15,17,18 © 2013 American Chemical Society

Received: April 26, 2013 Revised: June 20, 2013 Published: June 25, 2013 3275

dx.doi.org/10.1021/cg400636a | Cryst. Growth Des. 2013, 13, 3275−3282

Crystal Growth & Design

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L2 does not give rise to dinuclear species but coordination polymers, a helix (2), and a zigzag polymer (3) with structures that depend on the crystallization conditions. 3 can be obtained either from 2 or directly from a mixture of L2 and AgBF4. In the IR spectra of the three complexes, the ν(B−F) bands of the BF4− group were split, indicating a possible decrease in the symmetry of the anion. Structures of 1−3. The structures of the three complexes were determined by X-ray diffraction analysis. See the Supporting Information and footnote27 for the crystallographic data and bond lengths and angles. The molecular structure of 1, [Ag(L1)]2(BF4)2, consists of a rectangular boxlike cyclic dimer formed by the self-assembly of two metal centers and two ligands in a head-to-tail disposition with an inversion center that makes only one-half of the structure unique (Figure 1). This type of dimer is relatively similar to that found with ligands L1 or L2 and octahedral or pentacoordinated metal centers.25 Examples of this type of structure with other ligands have been reported.28 The metal ions are coordinated by three nitrogen atoms, two from the pyrazolyl rings of one ligand, and the third from the pyridine ring of the other ligand, and there are also two Ag−F interactions from two different anions. The Ag−N distances follow the expected trend, considering the basicity of the heterocycles (py < pz* < pz, pz* = 3,5-dimethylpyrazole). This is a common characteristic for the three derivatives described in this paper and it will not be detailed further on. There are several noncovalent intramolecular interactions in this cage that were also found in previous examples, and these may have an influence on the stability and shape of the system. The two pyridine rings are parallel and exhibit a π−π stacking interaction that should be favored by the head-to-tail disposition of the two rings.3a,15 Besides, H3 and H5 of the pyridine ring give rise to CH−π interactions with the pyrazole heterocycles of the same ligand. See Supporting Information for the figure. More detailed data for these noncovalent interactions and others reported in this work are detailed in the Supporting Information. The aforementioned Ag−F interactions generate supramolecular zigzag chains of dimers that extend along the b

Chart 1. Ligands L1 and L2, Numbering, And Abbreviations

structure of the resulting complex mainly depended on the metal geometry. The ligands can be considered as being semirigid, and they always behave as a bridge, defining an angle between the two metal centers that relies on the axial orientation of the pyridine ring in the metallacycle formed after coordination.26 In this communication, we report the results of a study into the self-assembly of L1 and L2 and the nonstereochemically rigid silver cation in the presence of an anion of low coordinating ability (BF4−). These reactions led to the formation of three different polynuclear species, namely a helical polymer where spontaneous resolution is observed, a zigzag coordination polymer, which constitutes a rare case of a sequence isomer, and a boxlike cyclic dimer. Synthesis of 1−3. The processes that led to the formation of the new derivatives 1−3 are shown in Scheme 1. The Scheme 1. Synthesis of the New Complexes 1−3

reaction of L1 and AgBF4 yields 1, and this complex has a boxlike cyclic dimer structure. However, the methylated ligand

Figure 1. Structure of complex 1. (a) Supramolecular chains that extend along the b axis formed by the boxlike cyclic dimers connected through double asymmetric Ag−F contacts (blue) and F−F interactions (purple). (b) 3D structure viewed along the a axis. Columns formed through π−π stacking are indicated with black lines. Hydrogen atoms have been omitted for clarity. 3276



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Figure 2. (a) Section of the structure of 2 to show the coordination environment of the two silver centers. (b) and (c): side view of the structure of the helix of complex 2 that extends along the b axis. In (b), the Ag−F (blue), hydrogen bonding (pink), and CH−π (orange) interactions are indicated. Hydrogen atoms and solvent molecules have been omitted for clarity. In (a), the anions have also been omitted. Color of the atoms: boron (yellow), carbon (gray), fluorine (green), nitrogen (blue), and silver (pink).

ligands through the pyrazolyl rings. The four pyrazolyl rings are symmetrically different, and this means that, strictly speaking, these silver atoms are chiral. The other silver center is bonded to two pyridine rings of two different ligands in an almost linear geometry [N11−Ag2−N5 = 171.3(6)°] (Figure 2a), and it also exhibits two interactions with fluorine atoms of the anions in an approximate trans disposition and with an oxygen atom of a THF molecule. The helix is generated around a crystallographic 21 screw axis, and each coil of the helix, therefore, contains four silver centers and four ligands and has a pitch of 14.09 Å (Figure 2 (panels b and c). The helical polymer extends along the b axis and generates a chiral channel in which THF molecules are hosted (there are also THF molecules outside the channel). The volume of the channels is 335.2 A3, and this represents 14% of the unit cell.31 The channel has a rectangular shape. The corners of this rectangle are the Cα atoms of the nitrogenated ligands, and the sides are alternately occupied by the di- and tetra-coordinated silver centers (see Figure 4, where different helices are represented along the b axis). The dimensions of these rectangles are 6.5 and 6.8 Å. The anions are not hosted within the channels but are situated on the surface of the helix on both sides of the dicoordinated silver atoms and, in fact, the helix is stabilized by intrastrand Ag−F−B−F−H contacts (see Figure 2b). Each of these two anions gives rise to an interaction in each direction of the growing helix. Intramolecular CH−π contacts are also observed between the two ligands bonded to the same tetracoordinated silver center. It can be seen from Figure 3 that the contiguous helices are interdigitated.32 In that way, the structure of one helix determines the orientation of the contiguous helices, thus building the single crystal from the same homochiral helix enantiomer. The helical chains link each other through hydrogen bonds with the intermediacy of the BF4− anions, and regions of hydrophobic contacts where the methyl groups

axis (Figure 1a) in a similar way to that found for the difluorophospato complex described previously.25 In fact, complex 1 could be considered as an intermediate situation between structures with isolated cyclic dimers and structures with polymers constituted by these boxlike dimers. The two BF4− anions interact mutually through an F···F contact [dF3−F3 = 2.643(8) Å] (it is necessary to consider that because the BF4− anion is disordered in two positions, this interaction may not always be present). It has been argued that the halogen− halogen interaction is an electrostatic attractive force that can be used in supramolecular chemistry and crystal engineering.29 The two B−F···F angles are identical, with a value of 125.23°, and this is consistent with a type I interaction.29b,g The presence of this interaction between two anions is noteworthy. However, the negative charge should be partially delocalized in the silver atoms through the aforementioned interactions. Channels that extend along the a axis are formed where the pairs of BF4− anions are hosted (Figure 1b). The 3D supramolecular structure reflects the presence of other weak interactions such as hydrogen bonding, π−π stacking, and CH−π interactions. It is worth noting that the existence of pyrazole−pyridine π−π stacking interactions involving both pyridine rings of the dimer leads to the formation of small columns of pz−py−py−pz stacked rings. The relative disposition of the dimers in the bc plane allows this pyrazole−pyridine stacking interaction, as shown in Figure 1b. The derivative 2·0.5THF, [Ag(L2)]n(BF4)n·0.5nTHF, is a helical coordination polymer with spontaneous resolution that crystallizes in the chiral space group P21. The left-handed enantiomer (M) was present in the crystal studied by X-ray diffraction. The asymmetric unit contains two ligands, two anions, and two silver centers with coordination numbers of 4 and 2. The tetracoordinated silver atom has a seesaw environment [τ4 = 0.62−0.63, N9−Ag1−N1 = 157.2(4)°]30 (Figure 2a) formed by the chelating coordination of two 3277



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In order to further evaluate the spontaneous resolution process, crystals of 2 were characterized with solid-state circular dichroism (CD) spectroscopy in a KBr pellet. Although a monocrystal was used for the X-ray structure determination, compound 2 crystallizes as bunches of small crystals, and these bunches were used for the CD studies. Figure 5 gathers the CD spectra of two different bunches. It is observed that they are nearly mirror images of each other, and that one or the other enantiomers are preferentially present in the samples.24g,p This behavior reveals that locally one enantiomer is formed in excess, possible even exclusively, and suggests that growth of single colonies of homochiral crystals starting from single nucleation points may occur.24c,d The structure of complex 3, [Ag(L2)]n(BF4)n, is a zigzag chain that extends along the c axis and contains alternating ligands and tricoordinated silver centers (Figure 6). As in 1 and 2, the ligand coordinates in a chelate fashion through the pyrazolyl rings to one metal center and through the pyridine nitrogen atom to another. The coordination geometry around the silver center is T-shaped (irregular). Groups of a specific chain are connected through hydrogen bonds involving the anions and also through an intramolecular CH−π interaction that involves the pyridine ring and a methyl ring of another ligand. Both interactions may contribute to the twisting of the chain. The 3D supramolecular structure is established, thanks to the formation of hydrogen bonds with the anions and groups of different chains. There is also an intermolecular CH−π interaction. Further parameters for these interactions and a figure are given in the Supporting Information. Although the type of chain found in 3 is very different to that exhibited by the helical polymer, some similarities concerning noncovalent interactions and creation of polar and apolar regions are apparent when the 3D structure is analyzed. The chains of 3 are aligned along the b axis to form a sheet in the bc plane in such a way that the pyrazolyl groups are situated in the same region, possibly due to hydrophobic contacts. The aforementioned CH−π interaction also takes place along the b axis. These sheets interact with others along the a axis with the intermediacy of anions that form several hydrogen bonds. Once again, hydrophobic and polar regions can be distinguished in the crystal structure. In a similar way to 2, the adjacent sheets are displaced with respect to one another (by b/2) and each chain is surrounded by another six chains (see Figure 7 and Chart 2). The transformation 2 → 3, observed after recrystallization from MeOH/Et2O, warrants further comment. On the one hand, it is noteworthy that if the crystallization solvent is not considered, the coordination polymers 2 and 3 constitute a rare example of sequence isomerism, a well-known phenomenon in covalent polymers but unusual in coordination polymers.15 The formation of these isomers relies primarily on the fact that the ligand is asymmetric. If N and NN are the two coordination sites of ligand L2 (monodentate and chelate, respectively), in complex 2 the disposition is NAgN−NNAgNN−NAgN··· (head-to-head ligand orientation), leading to alternating coordination indexes for the silver centers. In 3, however, the disposition is NAgNN−NAgNN−NAgNN··· (head-to-tail) and, thus, all silver ions exhibit an identical tricoordination. Asymmetric ligands are more commonly arranged in a head-totail sequence within the coordination chain, which gives a polymer where all metal centers are identical33 and the repeating unit consists of one metal center and one ligand

Figure 3. View of two helices of 2. Hydrogen atoms have been omitted for clarity.

are gathered are also observed. In this way, sheets parallel to the (−101) plane are formed, and the chirality across a helix is preserved on both sides of the resulting sheet, with an interstrand distance of 14.5 Å. These sheets are also connected through interactions with the anions along the [−101] direction (perpendicular to the sheets) with a mutual offset of the sheets by 7.25 Å in the [101] direction (Figure 4). In this way, a sequence of ABAB··· planes is obtained and each polymer is surrounded by six others in an elongated hexagonal disposition. The formation of polar and apolar regions where the anions or pyrazolyl groups, respectively, are situated can be observed in Figure 4 and Chart 2.

Figure 4. Representation of the crystal packing in compound 2. View along the helical axis (b), showing the interlocking of the helices. Each helical chain is represented with a different color. Hydrogen atoms have been omitted for clarity. 3278



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Chart 2. Schematic Drawing of the Crystal Structures of Complexes (a) 2 and (b) 3.a

a

In (a) the helical polymer that extends along the b axis and (b) the zig-zag polymer that extends along the c axis are represented. The green and purple lines represent apolar (hydrophobic contacts) and polar regions (hydrogen bonds), respectively.

Figure 5. A solid-state CD spectrum of two bunches of crystals of 2. Figure 7. 3D supramolecular packing of 3. View along the c axis. Hydrogen bonds have been omitted for clarity.

Powder X-ray Diffraction and Thermal Analysis of 2·0.5 THF. It was verified by powder X-ray diffraction that the monocrystals of 2·0.5THF were representative of the bulk material (see the Supporting Information). The chemical stability of 2·0.5THF was assessed, and crystals of this compound were submerged in acetone during a 24 h period. A 1H NMR registered after filtration and solving into methanold4 reflected the presence of acetone and not THF. The crystals with acetone were then subjected to vacuum during 24 h, and it was observed (by powder X-ray diffraction) that they retained structural order. In fact, the final powder diffractogram was similar to that obtained for 2·0.5THF (see the Supporting Information). Thus, the homochiral motif in 2 is stable, even upon removal of guest solvent molecules. Crystals of 2·0.5THF were also submerged into dichloromethane for a 24 h period. The interchange of THF by this solvent was also verified by 1H NMR. The initial compound 2·0.5THF, when subjected to solvent evacuation at 110 °C (heating and vacuum overnight), lost part of the crystallinity. Thermogravimetric analysis indicated that all lattice solvent molecules of 2·0.5THF could be removed, up to about 200 °C, and that decomposition occurred at around 240 °C. Solution Behavior. A peak corresponding to the fragment [Ag2(L1)2]+ was observed in the mass spectrum of 1, and this could indicate that the boxlike dimer structure is maintained in solution, at least to some extent. In the spectrum of 2 and 3,

Figure 6. Zig-zag chain of 3 that extends along the c axis. Hydrogen bonds (pink) and CH−π interactions (orange) are indicated. The BF4− anions are in green. Hydrogen atoms have been omitted for clarity.

molecule. A case has been described in which a ligand containing 4-pyridyl and 2-pyridyl rings on the two sides gives rise to head-to-head and head-to-tail silver chains that differ in the counteranions (PF6− and NO3−).15 It was concluded that the NO3− anions, which have a greater affinity for silver centers than PF6−, interfered significantly in product formation. In our case, it is noteworthy that complexes 2 and 3 have the same counteranion. However, the complexes differ in that the helical polymer has solvent molecules (THF) hosted in the inner channel. It is proposed that these solvent molecules may behave as templates for the helical structures but, in any case, the two types of structures should have quite similar stabilities, and small changes may favor one over the other. 3279



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unexpected case of selective deuteration of the Hα position of L1 was observed for complex 1 in a methanol-d4 solution but not for complexes with L2.

peaks corresponding to fragments with one or two silver centers have been found (see the Supporting Information). The room temperature 1H NMR and 13C{1H} NMR spectra (methanol-d4) of the three complexes show the presence of a unique and symmetric nitrogenated ligand with chemical shifts different to those of the free ligand. A deshielding or shielding of 0.2−0.4 ppm was found for the pyrazolyl or pyridine resonances, respectively, with respect to those in the free ligand, and the position of the signals was very similar for the two complexes with L2, regardless of the solid structure. In addition, Me3-pz and H2/6-py signals, which are the nearest protons to the coordinating positions, are broad in 2 and 3. A variable temperature NMR experiment was carried out on complex 2. Above room temperature, the aforementioned resonances were less broad and at low temperature, they narrowed significantly before becoming sharp at −30 °C, an observation that could indicate the Ag−N bond-breaking process slowing down at low temperature. The existence of one set of signals is indicative of either the formation of a single species in solution or the presence of rapid equilibration between exchanging species. Bearing in mind the data discussed above and the labile nature of the silver(I) complexes in solution, as observed in related systems,15,18 we propose that a rapid equilibrium between species takes place in solution. The shifting of the resonances with respect to the free ligand and the broadening of some signals indicate that, at least in some of these species, the pyrazolyl and/or pyridine rings are coordinated. The signal of the Hα resonance disappeared with time in the 1 H NMR spectra of 1 due to a deuteration process from methanol-d4. We previously observed deuteration of the Hα of the ligand for Zn(II) and Ag(I) complexes with L1 and boxlike dimeric structures but not in derivatives with L2.25 This finding will be investigated further in the near future. In summary, self-assembly of the achiral ligands, L1 and L2, with AgBF4 gives rise to both discrete (1) and polymeric structures (2 and 3). The presence of methyl groups in the pyrazolyl rings leads to the formation of coordination polymers. The formation of a homochiral helix (2) that exhibits spontaneous resolution was also observed. Helices of Mchirality were found in the monocrystal used for the X-ray structure determination and both enantiomers were observed from solid-state circular dichroism (CD) spectroscopy applied to bunches of crystals. This reveals that locally, one enantiomer is formed in excess, possibly even exclusively, and suggests that growth of single colonies of homochiral crystals starting from single nucleation points may occur. The homochiral motif is stable even upon removal of guest solvent molecules. Significantly, this chiral architecture is generated from simple and inexpensive achiral building blocks. A change in the crystallization conditions led to a zigzag chain (3). These two polymers constitute a rare case of sequence isomerism. A clear change in the noncovalent interactions, depending on the presence of L1 or L2, was observed in the supramolecular structures. Hydrogen bonds are present in all cases, but columns of π−π stacking involving pyrazolyl and pyridinyl rings are present in the complex with L1, whereas the introduction of the methyl groups in L2 makes the pyrazolyl rings more apolar and hydrophobic contacts are observed in both derivatives with L2, which are coordination polymers with a relatively similar disposition in the crystal. On studying the solution chemistry, data pointed to the existence of rapid equilibrium between exchanging species. An

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files in CIF format. Experimental details. ORTEP representations of the structures. Tables with crystallographic data, with a selection of bond distances and angles, and with data for noncovalent interactions. Figures for the powder diffractograms. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: + 34 926295300. Fax: +34 926295318. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the MICINN of Spain (CTQ2011-24434, FEDER Funds). We thank the INCRECYT program of Castilla−La Mancha (contract to MCC) and the MINECO of Spain for an FPU grant (GD).

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DEDICATION This paper is dedicated to Prof. Antonio Laguna on the occasion of his 65th birthday. REFERENCES

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