Self-Assembled Molecular Squares as Supramolecular Tectons

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Self-assembled molecular squares as supramolecular tectons Shobhana Krishnaswamy, Soumyakanta Prusty, Daniel Chartrand, Garry S Hanan, and Dillip Kumar Chand Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01425 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018

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Crystal Growth & Design

Self-assembled molecular supramolecular tectons

squares

as

Shobhana Krishnaswamy,† Soumyakanta Prusty,† Daniel Chartrand,‡ Garry S. Hanan,§ and Dillip K. Chand*† †

Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India

‡

LAMP – Laboratoire d’Analyse pour les Molécules et Matériaux Photoactifs – Laboratory

for the Analysis of Molecules’ and Materials’ Photoactivity, Université de Montréal, Montréal, Québec H3T 2B1, Canada §

Department of Chemistry, Université de Montreal, Montreal QC H3T-IJ4, Canada

ABSTRACT: Concentration-dependent equilibria of molecular squares [Pd4(L′)4(L)4](NO3)8 and triangles [Pd3(L′)3(L)3](NO3)6 were obtained when cis-protected Pd(II) units [PdL′(NO3)2] (L′ = tmeda, 2,2′-bpy, and phen) were combined independently with 4,4′bipyridine (L) in water. However, complexation of [PdL′(OTs)2] with L resulted in exclusive formation of the corresponding molecular squares. The addition of AgOTs to each mixture of square and triangle led to a shift in the equilibrium, resulting in the disappearance of the triangles and exclusive formation of the corresponding squares. The crystal structures of the molecular squares [Pd4(L′)4(L)4](OTs)8 revealed a pair of tosylate anions encapsulated in the hydrophobic

cavity

of

the

square.

Further,

[Pd4(2,2′-bpy)4(L)4](OTs)8

and

[Pd4(phen)4(L)4](OTs)8 exhibited solvatomorphism, yielding two crystalline forms each, respectively. The cationic units in these crystals associate through intermolecular π···π stacking interactions where in the cis-protecting unit (i.e. 2,2′-bpy and phen) of adjacent molecules overlap via side-on or end-on modes. Thus, the cations may be considered as ‘tectons’, each of which contains four peripheral 2,2′-bpy / phen units, which behave as ‘supramolecular synthons’ in the self-assembly of the squares. The tosylates interact with the cations through C-H···O and C-H···π interactions and play a role in the packing of the molecular squares.

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INTRODUCTION Crystal engineering is the design of functional molecular solids using intermolecular interactions between neutral or ionic building blocks.1,2 It involves analysis of noncovalent interactions between building blocks and the effect of variations in the structures of the building blocks upon the molecular assembly in the resulting crystalline forms. Crystal engineering of organic solids has been widely explored in recent decades, with the help of supramolecular synthons (structural units that contain information about the molecular recognition process) which can be reliably used for the construction of larger assemblies through noncovalent interactions.3-10 In contrast, the crystal structures of metal-organic compounds have not received as much attention, apart from coordination polymers and metal-organic frameworks. Metal-driven self-assembly is a convenient strategy for the construction of supramolecules of desired dimensions due to well defined metal-ligand coordination bonding which enables the use of simple synthetic procedures to obtain complex target molecules in high yields.11-16 For instance, transition metal ions such as Pd(II) and Pt(II) consistently adopt a square planar geometry upon coordination and are chemically stable, ideal for the creation of predesigned assemblies for functions such as molecular recognition, anion sensing, gas storage, electrochemical sensing and separation of mixtures.1722

As a part of our ongoing studies on metal-driven self-assembly in the solution and solid-state we had earlier investigated the structures of [Pd2(L′)2(LB)2](X)4, [Pd3(L′)3(LB)3](X)6, and [Pd3(L′)3(LT)2](X)6 type binuclear23,24 and trinuclear25-28 Pd(II) complexes respectively, containing cis-protecting agents (L′ = ethylenediamine (en), tetramethylethylenediamine (tmeda), 2,2′-bipyridine (2,2′-bpy), and 1,10-phenanthroline (phen)) and chosen ligands (LB = bidentate, LT = tridentate) coordinated to a Pd(II) metal centre (counteranion X = NO3¯ / PF6¯ / ClO4¯ / OTf¯). Analysis of the crystal structures of these complexes revealed the


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influence of the structure of the ligand and the cis-protected Pd(II) unit on the molecular selfassembly. The cations in the complexes which contain cis-protected Pd(II) units with aromatic rings (2,2′-bpy and phen), often present at the periphery of the molecule, tend to assemble through π···π and C-H···π interactions in their crystals.23-28 In continuation of this work, we sought to explore the crystal structures of tetranuclear [Pd4(L′)4(L)4](X)8 type complexes and examine the role of the cis-protected Pd(II) unit to direct the solid-state selfassembly of the molecules. Fujita’s molecular square [Pd4(en)4(4,4′-bpy)4](NO3)8, 1a shown in Scheme 1 is a tetranuclear Pd(II) complex, prepared by combining [Pd(en)(NO3)2] and the nonchelating bidentate ligand 4,4′-bipyridine (4,4′-bpy) in equimolar ratio.29,30 This molecular square can be represented using the general formula [Pd4(L′)4(L)4](NO3)8 where L′ and L represent ethylenediamine and 4,4′-bipyridine, respectively. The square planar geometry adopted by the cis-protected metal ion provides a 90° corner and the linear structure of L results in the formation of a symmetric square-like complex. Attempts towards the synthesis of similar square complexes using tmeda31 and 2,2′-bpy32 as the cis-protecting agents resulted in an equilibrium mixture of two symmetric species, identified as the corresponding molecular square and triangle, (2a/2a′ and 3a/3a′, Scheme 1) respectively. The combination of Pd(phen)(NO3)2 with L in water (this work) also yielded a similar equilibrium mixture (4a/4a′, Scheme 1). In our quest to study the structures of the molecular squares, we perturbed the counteranion during synthesis and could obtain the square exclusively using tosylate as the counter anion (2b-4b). Crystallization of 2b produced a single crystalline form, whereas two solvatomorphs each were obtained for complexes 3b and 4b. Metal-driven self-assembly in Pd(II) complexes can help predict the structure of the individual supramolecules based on the shape and geometry of their components, but information regarding their self-organization, or “self-assembly of self-assembly” of these supramolecules in their crystals is often encoded in the cis-protecting units and the ligand



used in their construction. The analysis of crystal structures of binuclear Pd(II) complexes comprising suitable ligands and cis-protected Pd(II) moieties i.e. [Pd(en)]2+, [Pd(tmeda)]2+, [Pd(2,2′-bpy)]2+ and [Pd(phen)]2+ revealed the influence of the cis-protecting agents upon the patterns of self-assembly of the complexes.33 The cations in the complexes which contain the cis-protecting agents 2,2′-bpy or phen, tend to self-assemble via π…π stacking interactions between the aromatic rings of the cis-protecting units in their crystals.33 Hence, we envisaged self-assembly of the cations in 3b and 4b through two different modes of self-assembly through π…π stacking interactions between their 2,2′-bpy / phen moieties as shown in Fig. 1. In the following discussion, we describe the syntheses and characterization of these complexes and the role of the cis-protecting units in the molecular assembly in their crystals.

Figure 1. Schematic representation of side-on and end-on modes of association of the cations in 3b and 4b through π…π stacking interactions between the 2,2′-bpy / phen moieties of the squares.

RESULTS AND DISCUSSION Equilibria of molecular squares and triangles. The combination of cis-protected Pd(II) units with non-chelating bidentate ligands having a rigid backbone often yields an equilibrating mixture of two symmetric species, the molecular square and triangle.16,34-42 A given square-triangle equilibrium depends upon the concentration, temperature, medium of complexation, cis-protecting unit, anions, guests and the ligand.32,34,37,41,43 An increase in the concentration favors the formation of the higher nuclear species due to the increased


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availability of building blocks, in accordance with Le Chatelier’s principle. The square is enthalpically favored due to less strain in its structure and the triangle is entropically favored due to the possibility of formation of more triangles than squares from the same number of building blocks.

Scheme 1. Synthesis of molecular squares / triangles with different counteranions. Complexation of L with cis-[Pd(en)(NO3)2] in water yields a single product, namely, the molecular square 1a (Scheme 1).29 However, the combination of 4,4′-bpy with cis[PdL′(NO3)2] (L′ = tmeda / 2,2′-bpy / phen) in aqueous media yields the corresponding equilibrium mixtures of molecular square and triangle (Scheme 1).31,32 The equilibrium mixtures of the complexes 2a/2a′, 3a/3a′ and 4a/4a′ show two sets of signals in D2O, some of which overlap (Fig. 2, Fig. S1-S15). Based on 1H NMR spectra (Fig. S16-S18) recorded at various concentrations (obtained by successive dilution), the two sets of peaks were tentatively designated as the molecular square and triangle complexes, respectively. Dilution



of the mixture results in a decrease in the peak intensity of the designated molecular square and a corresponding increase in that of the designated triangle. These data agree with the effect of concentration on the square-triangle ratio as per Le Chatelier’s principle. The triangle is the major species at lower concentration, whereas the square dominates at higher concentration. These results are depicted in plots (Fig. S19) of square-triangle percentage vs. concentration (in mM with respect to Pd(II)). Additional evidence for the assignment of the peaks as the square and triangle species was obtained from DOSY NMR spectra (Fig. S20S22).

Figure 2. Partial 500 MHz 1H NMR spectra in D2O (TMS as external standard) for (i) 4,4′bpy; (ii) 2a/2a′; (iii) 3a/3a′ and (iv) 4a/4a′. Prime labels refer to the peaks for molecular triangle. [Concentration with respect to Pd(II) source: 10 mM for 2 and 3 and 5 mM for 4].

Counter anion induced exclusive formation of molecular squares. The square 1a is known to recognize neutral aromatic compounds30 in water due to the presence of a hydrophobic cavity. Since 2a-4a could not be obtained as the exclusive products of the


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reaction of L with the corresponding cis-protected Pd(II) units, it was thought that the use of an aromatic moiety as a guest might favor the formation of the square. When the complexes were synthesized by replacing the nitrate anions with tosylates in aqueous medium, a single product was obtained in each case (Fig. 3). These were designated as the corresponding molecular squares [Pd4(L′)4(L)4](OTs)8 (2b-4b) (L′ = tmeda / 2,2′-bpy / phen, Scheme 1) based on comparison of peaks in their 1H NMR spectra with those of the complexes 2a-4a and characterized using 1D and 2D NMR spectroscopy (Fig. S23-S39).

Figure 3. 500 MHz 1H NMR spectra in D2O (TMS as external standard) for (i) 4,4′-bpy; (ii) 2b; (iii) 3b and (iv) 4b. [Concentration with respect to Pd(II) source: 10 mM for 2 and 3 and 1 mM for 4]

The addition of AgOTs to the equilibrium mixtures of 2a/2a′, 3a/3a′ and 4a/4a′ also resulted in a shift in the equilibrium towards the square, resulting in exclusive formation of the cores of the squares 2b, 3b and 4b, respectively (Fig. S40-S42). The 1H NMR spectra of 2b-4b



when recorded in D2O uniformly revealed a single set of signals, indicating the formation of one type of complex. Interestingly, the 1H NMR spectra of 2b-4b when recorded in [D6]DMSO, revealed two sets of signals, corresponding to the molecular square and triangle (Fig. S43), indicating that an aqueous medium favors the formation of the square. 1H NMR spectra recorded at various concentrations (obtained by successive dilutions) for 2b-4b, did not result in the significant appearance of any new signals, indicating that the square complex may be favored even at lower concentrations (Fig. S44-S46). Crystal structures of the molecular squares. Slow evaporation of acetonitrile-water solutions of 2b-4b yielded pale yellow crystals. Crystal structure solution established their identities as the corresponding molecular squares (2b, solvate 3bI and solvate 4bI), each containing a pair of ‘guest’ tosylate anions encapsulated in the hydrophobic cavity of the square (Fig. 4) along with many water molecules outside the cavity. Crystallization of 3b and 4b via diffusion of other suitable solvents into their acetonitrile-water solutions resulted in the formation of additional crystalline solvates 3bII and 4bIII. The results of crystallisation experiments are described in the following section. Correlation of the crystal structures with their 1H NMR spectra indicates fast exchange of tosylate moieties present inside and outside the square, resulting in the appearance of a single set of signals for the tosylate protons.


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Figure 4. The squares in the crystal structures of (a) 2b; (b) 3bI and (c) 4bI contain a pair of tosylates encapsulated in the hydrophobic cavity. Hydrogen atoms, tosylates outside the square cavity and solvent molecules are omitted for clarity. Molecular squares as tectons. The crystal structures of complexes 1a,32 and 2a31 and 2a′31 were reported earlier and the molecular packing in 1a and 2a is discussed below. Although the formation of the mixture 3a/3a′ has been proposed earlier32 crystal structures of the square/triangle have not been reported. The crystal structures of complexes 2b, 3b (two solvates - 3bI, 3bII) and 4b (two solvates - 4bI, 4bIII) are discussed in the following sections. The Pd···Pd distances, N(Py)-Pd-N(Py) angles and torsion angles between the rings of 4,4′-bpy molecules in the crystal structures of molecular squares are shown in Table 1. The crystallographic data are briefly summarized in Table 2 in the experimental section and details of structure solution are available in the Supporting Information. Table 1. Selected distances and angles in molecular square complexes. Crystal, Molecule 1a32 Mol. A

Pd···Pd (Å)

N(Py)-Pd-N(Py) (°)

11.078(7), 11.082(9)

89(2), 93(2)

Torsion angle between Py rings of 4,4′-bpy (°)a 2.64, 12.94

Mol. B

11.090(8), 11.095(7)

92(2), 87(2)

22.57, 26.37

Mol. C

10.964(9), 11.003(9)

85(2), 93(2)

27.61, 32.11

2a31

11.062(1), 11.0898(8)

86.8(3), 87.3(3)

31.86, 20.96

2b

11.105(1), 11.119(1)

88.6(1), 85.1(1)

8.81, 22.97

3bI

11.0716(7), 11.0780(6)

87.1(2), 87.6(2)

26.51, 27.58

3bII

11.1090(7), 11.1174(8)

87.7(2), 89.1(2)

5.23, 32.06

4bI

11.0404(7), 11.0532(6)

85.70(6), 89.22(6)

28.08, 33.02

4bIII Mol. A

11.066(1), 11.1230(7),

88.26(7), 88.35(7),

23.04, 26.98,

11.049(1), 11.0826(7)

88.81(6), 89.27(7)

31.67, 34.56

Mol. B

11.068(1), 11.0729(7)

88.93(7), 89.16(7)

34.60, 25.38

Mol. C

11.085(1), 11.1338(7)

88.54(7), 88.81(7)

31.77, 22.89

a

The torsion angle between the rings of a single 4,4′-bpy unit coordinated to two different Pd atoms.



The cationic units in the crystals of 3b and 4b are expected to assemble mainly via π stacking interactions of the aromatic rings in the cis-protecting units, in the absence of other strong hydrogen bond donors / acceptors. The tosylate anions can also interact with the aromatic rings of the cationic moiety of the complex through π···π, C-H···π and C-H···O interactions. Hence, we embarked upon a comprehensive analysis of the crystal structures of the solvates and examined the role of the anions and cis-protecting Pd(II) units in the self-assembly of these metallomacrocycles. A remarkable feature across all the solvates is the encapsulation of a pair of tosylate anions within the cavity of the molecular square. [Pd4(en)4(L)4](NO3)8, 1a. The molecular organization in 1a is unlike any of the other square complexes. It crystallized in the triclinic space group P-1 with three half molecules of the square (arbitrarily designated as mo lecule A, molecule B and molecule C in the asymmetric unit, Fig. S52).32 The molecular assembly when viewed along the ac-diagonal shows a herringbone pattern of organization of the B and C types of molecules (Fig. 5a, Fig. S52a). A portion of the molecular packing (marked by a rectangular box) when viewed along the baxis shows molecules A and B stacked atop each other in alternate fashion (driven by weak π···π stacking interactions (3.8 – 4.2 Å) between the 4,4′-bpy units), forming columns along the ac diagonal (Fig. 5b, Fig. S52b). The voids between two adjacent columnar assemblies of the A and B molecules along the b-axis are occupied by the squares of type C (Fig. 4a, Fig. S52a). The quality of the structure and absence of hydrogen atoms precludes further discussion and analysis of hydrogen bonding interactions. [Pd4(tmeda)4(L)4](NO3)8, 2a. In the case of tmeda as the Pd(II) cis-protecting unit, the structures of both the nitrate and tosylate complexes (i.e. 2a and 2b) are available for comparison. 2a crystallized in the monoclinic space group C2/c with half a molecule of the square and four nitrates in the asymmetric unit, electron density corresponding to the solvent was squeezed.31 The cationic units in the crystals of 2a assemble as layers in the ac-plane, but


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do not directly interact with each other. Nitrates (pink, blue and red, Fig. 6a, Fig. S53a) occupy the voids present between the cations and engage in C-H···O interactions with them. A slice of the molecular packing when viewed along the ac-diagonal shows the layers stacked along the b-axis and the anions (yellow) engaged in C-H···O interactions with the protons of the ligand and tmeda (Fig. 6b, Fig. S53b). The cationic units appear boat-shaped since the different Pd-(N)4 planes of the square are not coplanar and the dihedral angle between adjacent Pd-(N)4 planes is approximately 19°. Thus, each layer has a wavy shape and the molecular assembly appears like a corrugated sheet.



Figure 5. Views of the molecular packing in crystals of 1a show association of the cations along (a) the ac diagonal and (b) b-axis. Anions are omitted for clarity. Molecules of the A type are omitted in (a) and C type are omitted in (b) for clarity.

Figure 6. Views of the molecular packing in crystals of 2a show (a) assembly of cations in the form of a layer in the ac plane and (b) arrangement of the layers along the b-axis. Some hydrogen atoms are omitted for clarity.


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[Pd4(tmeda)4(L)4](C7H7O3S)8·11(H2O), 2b·11(H2O). The compound 2b crystallized in the triclinic space group P-1 with half a molecule of the square, four tosylates and eleven water molecules in the asymmetric unit (Fig. S47). The cations do not directly interact with each other and the Pd-(N)4 planes of the square are nearly coplanar; the dihedral angle between adjacent Pd-(N)4 planes of the square is approximately 8°. Each square also contains a pair of tosylates (yellow) in its hydrophobic cavity (Fig. 7a, Fig. S54a) which interact with the ligand through π···π stacking interactions (~ 3.9 Å). The cations assemble as layers in the bc-plane (Fig. 7b, Fig. S54b), creating channels along the b and c axes which are occupied by tosylates (red, blue and pink). A view of the molecular organization along the c-axis shows the arrangement of these layers along the a-axis. Tosylates (blue and pink) are present in between these layers and interact with the protons of the ligand and tmeda through C-H···O interactions (Table S2). [Pd4(2,2′-bpy)4(L)4] (C7H7O3S)8·13(H2O), 3bI·13(H2O). Crystallization of complex 3b from an acetonitrile-water (1:1 v/v) solution yielded a pair of solvates: 3bI (from slow evaporation of the solvent and with hexane / CHCl3 vapor diffusion) and 3bII (by diffusion of EtOAc / DCM / toluene vapor). In the latter case, crystals of 3bI and 3bII were obtained concomitantly in some trials. Crystal structure of the solvate 3bI is discussed here, and that of 3bII is discussed in a later paragraph. The solvate 3bI crystallized in the triclinic space group P-1 with half a molecule of the square in the asymmetric unit along with four tosylates and thirteen water molecules (Fig. S48), some of which are disordered. The dihedral angle between adjacent Pd-(N)4 planes of the square is approximately 6°. Unlike 2a and 2b, the molecular squares in the crystals of 3bI directly associate through π···π stacking interactions (~ 3.8 Å) involving the aromatic rings of all four 2,2′-bpy units. The molecular organization occurs via the end-on overlap mode, and results in a layered molecular arrangement in the bc plane (Fig. 8a, Fig. S55a).



Figure 7. Views of the molecular packing in crystals of 2b show (a) a layer-like assembly of the cations in the bc-plane and (b) the arrangement of these layers along the a-axis, respectively. Some hydrogen atoms, anions and water molecules are omitted for clarity.


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The pair of anions (yellow) encapsulated in the cavity of each square interact with the cationic units via C-H···π interactions. The assembly of four such square units results in the creation of a channel along the a-axis, which is occupied by tosylate anions (blue). A view of the molecular organization along the c-axis (Fig. 8b, Fig. S55b), shows the interactions (CH···O and C-H···π contacts) of the protons of the cationic units with the oxygen atoms and phenyl rings of tosylates (pink and red). [Pd4(phen)4(L)4] (C7H7O3S)8·13(H2O), 4bI·13(H2O). Crystallization of complex 4b from an acetonitrile-water (1:1 v/v) solution yielded a pair of solvates: 4bI (from slow evaporation and with diffusion of acetone / CHCl3 / DCM / hexane / ethylacetate into its solution) and 4bIII (by diffusion of EtOAc / toluene vapor). In the latter case, crystals of 4bI and 4bIII were obtained concomitantly in some trials. Crystal structure of the solvate 4bI is discussed here, and that of 4bIII is discussed in a later section. The crystals of 4bI contain half a molecule of the square in the asymmetric unit along with four tosylates and thirteen water molecules (Fig. S50). The dihedral angle between adjacent Pd-(N)4 planes of the square is approximately 2°. Cationic units in the bc plane associate through parallel, offset π···π stacking interactions (3.6 - 3.7 Å) between the aromatic rings of the four phenanthroline units in each square (Fig. 9a, Fig. S56a, Table S2), utilizing the end-on overlap mode, and form layers. A void is created by the assembly of four such cationic units, which is occupied by tosylates (blue) as seen previously in 3bI. The pair of tosylates (yellow) encapsulated in the cavity of the square interact with it through C-H···π interactions (Fig. 9a, Fig.S56a). A view of the molecular assembly along the c-axis (Fig. 9b, Fig. S56b) shows the arrangement of the layers in the ab plane. The tosylates (pink and red) are involved in C-H···O and C-H···π interactions with the protons of the ligand and cis-protecting units. Therefore, 3bI and 4bI are isostructural and isomorphous, with minor differences in the magnitude of the noncovalent interactions between the cations and the anions.



Figure 8. Views of the molecular packing in the crystals of 3bI show (a) association of the cations along the b and c axes through π···π stacking interactions (shown as dotted lines) between aromatic rings of the 2,2′-bpy units and (b) the interplay of the tosylates with the cations. Some anions, solvent molecules and hydrogens are omitted for clarity.


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Figure 9. Views of the molecular packing in the crystals of 4bI show (a) association of the cations along the b and c axes through π···π stacking interactions (shown as dotted lines) between aromatic rings of the phen units and (b) the interplay of the tosylates with the cations. Some anions, solvent molecules and hydrogens are omitted for clarity.



[Pd4(2,2′-bpy)4(L)4](C7H7O3S)8·5(H2O), 3bII·5(H2O). The molecules in the crystals of 3bII are packed in a totally different manner as compared to 3bI and 4bI. 3bII crystallized in the monoclinic space group P21/c with half a molecule of the square, four tosylates, five water molecules and a molecule of acetic acid in the asymmetric unit (Fig. S49). The source of the acetic acid was traced back to the acetonitrile used in the complexation. The dihedral angle between adjacent Pd-(N)4 planes of the square is approximately 9°. Adjacent cations in 3bII are linked by π···π stacking interactions (Fig. 10a, Fig. S57a) along the b-axis, utilizing the side-on overlap mode, giving rise to unidimensional molecular arrays. One pyridyl ring of each 2,2′-bpy unit is involved in π···π stacking interactions (~ 3.6 Å) with the adjacent square whereas the second pyridyl ring is involved in π···π stacking interactions (~ 3.6 Å) with tosylate anions (pink). The tosylates (yellow) trapped in the cavity of the square form CH···O and π···π stacking interactions (~ 3.8 Å) with the 4,4′-bpy units (Fig. 10a, Fig. S57a). The voids created between the squares are occupied by acetic acid molecules (brown) along the a-axis and by tosylates (pink) along the b-axis. The acetic acid molecules interact with the square via C-H···O interactions and self-assemble as dimers via hydrogen bonding between the carboxylic acid groups (Fig. 10a). A view of the molecular organization along the c-axis (Fig. 10b, Fig. S57b) reveals the zigzag arrangement of the arrays along the c-axis. Their antiparallel arrangement creates channels which are occupied by tosylate anions (red and blue), which engage in C-H···O and C-H···π interactions with the squares (Table S2).


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Figure 10. Views of the molecular packing in the crystals of 3bII show (a) cationic units in arranged in arrays along the b-axis through π···π stacking interactions and (b) the zigzag arrangement of molecular arrays along the c-axis, with tosylates sandwiched between them. Some anions, solvent molecules and hydrogens are omitted for clarity.



[Pd4(phen)4(L)4](C7H7O3S)8·39(H2O), 4bIII·39(H2O). The asymmetric unit in crystals of 4bIII consists of one full and two half molecules of the square along with sixteen tosylates and thirty-nine water molecules (Fig. S51). The three independent molecules are arbitrarily designated as molecules A, B and C (Fig. S58a). The dihedral angles between adjacent Pd(N)4 planes of the square lie in the range of 5-16°. Each square contains a pair of tosylates trapped in the hydrophobic cavity, (Fig. 11a, Fig. S58a, molecule A – cyan and light pink; molecule B – magenta; molecule C – yellow) which interact with the 4,4′-bpy units via CH···π and π···π stacking interactions (Table S2, Fig. S58a). Adjacent squares along the abdiagonal associate via π···π stacking interactions (3.4 – 3.9 Å) between the aromatic rings of their phen units to form unidimensional molecular arrays. Each square contains four phen units, of which two (diagonally opposite, Fig. 11a, Fig. S58a) are involved in π···π stacking interactions with adjacent squares, utilizing the end-on overlap mode, while the other pair is engaged in weak interactions with the tosylates. Therefore, the molecular arrays are connected by the tosylates which form π···π stacking and C-H···π interactions with the aromatic rings of the phen units belonging to adjacent cationic arrays (Fig. 11b, Fig. S58b). This pattern of molecular assembly is different from that observed in 4bI, where all four phen units of each square are involved in π···π stacking with those of adjacent squares. The side view of the molecular layers is shown in Fig. 12. Cations belonging to neighbouring layers do not directly interact with each other (Fig. S59). This is perhaps due to the tosylates (shown in purple, gold, turquoise and grey colors) sandwiched between the cationic layers along the b-axis which are involved in π···π stacking and C-H···O interactions with the squares (Fig. S59). The details of the interactions between the cations and anions are available in Table S2.


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Figure 11. Views of the molecular packing in the crystals of 4bIII show (a) arrays of cations linked by π stacking interactions along the ab-diagonal and (b) the tosylates which engage in weak interactions with the cationic arrays and form a molecular layer. Some anions, solvents and hydrogens are omitted for clarity.



Figure 12. A view of the molecular packing in the crystals of 4bIII shows the arrangement of the layers along the b-axis. Anions sandwiched between the layers engage in π···π stacking and C-H···O interactions with the squares. Some anions, solvent molecules and hydrogens are omitted. A grey rectangle depicts the side view of the molecular layer described in Fig. 11a.

Role of the counter anion and solvent. The combination of water as the complexation medium and tosylate as the counteranion is essential for the exclusive formation of the molecular squares described here. The tosylate anion in 2b-4b behaves as a ‘guest’ that induces a change in the structure of the ‘host’,45-47 as is evident from the addition of AgOTs to a mixture of the square-triangle complexes, resulting in the exclusive formation of the square. The creation of the square in water, perhaps, coincides with the occupation of the hydrophobic cavity by the anions, resulting in anion templated48-51 self-assembly of the molecular square complex. In the solution state, the tosylates in the square cavity are indistinguishable from the anions outside the cavity, due to fast exchange on the NMR timescale at ambient temperature.


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However, the crystal structures of the complexes reveal the role of the various tosylates in the molecular self-assembly. Crystalline forms 3bI and 4bI were obtained more frequently and often appeared concomitantly with 3bII and 4bIII, respectively, in crystallization trials. Other crystalline forms were not obtained in the case of 2b, possibly indicating a lack of alternative efficient modes of molecular packing, as compared to 3b and 4b. There is a close correspondence between the unit cell parameters and molecular organization in 2b, 3bI and 4bI. However, the cations in 2b do not directly interact with each other, perhaps due to the steric aspects associated with the proximity of the methyl groups of tmeda units of adjacent cations. The differences in the shape and mode of self-assembly of the cations in 2a and 2b reveal the effect of the counteranion upon the solid-state assembly in these structures.52-56 The encapsulation of the tosylates in aqueous solution could be driven by hydrophobic interactions and in solid-state via favorable noncovalent intermolecular interactions with the ‘host’ square. The formation of multiple solvated crystalline forms in the case of 3b and 4b can be attributed to the diverse modes of association of the cationic units as well as the interactions of the tosylates with the cationic units. The cations in 3bII exhibit self-assembly through the side-on overlap mode, whereas those in 3bI, 4bI and 4bIII self-assemble via the end-on overlap mode (Fig. 1). The cis-protected Pd(II) units containing 2,2′-bpy and phen moieties behave as ‘supramolecular synthons’ in the self-assembly of the squares, which themselves can be likened to ‘tectons’. The water molecules in these solvates tend to form pentameric and decameric (consisting of fused pentamers) water clusters57 (3bI, 4bI) and are also engaged in strong conventional hydrogen bonding interactions with the tosylate anions, creating an infinite hydrogen bonded network in the crystal. EXPERIMENTAL SECTION 4,4'-Bipyridine, PdCl2, and AgOTs were acquired from Aldrich Chemicals. All common solvents were obtained from Spectrochem, India. Deuterated solvent was obtained from



Aldrich and Cambridge Isotope Laboratories. NMR spectra were recorded on Bruker 400 MHz and 500 MHz FT-NMR spectrometers using tetramethylsilane as reference. The cisprotected Pd(II) units58,59 used were synthesized as per literature procedures. General procedure A: Synthesis of complexes with nitrate as counter anion: Equimolar quantitities of ligand L and the corresponding cis-protected Pd(II) units (with nitrate as counter anion) were stirred in distilled water (2 mL) for 24 h, at ambient temperature. The solvent was evaporated to obtain solid products in all cases. [Pd(tmeda)(4,4′-bpy)] 4(NO3)8 + [Pd(tmeda)(4,4′-bpy)] 3(NO3)6, (2a/2a′) The complex was synthesized using a modified version of the literature procedure31 by combining L (1.56 mg, 0.01 mmol) and [Pd(tmeda)(NO3)2] (3.46 mg, 0.01 mmol). Isolated yield = 4.3 mg, 85.6%. 2a: 1H NMR (500 MHz, D2O) δ [ppm] = 9.14 (d, J = 6.7 Hz, 16H, Ha), 7.85 (d, J = 6.8 Hz, 16H, Hb), 3.11 (s, 16H, N-CH2), 2.65 (s, 48H, N-CH3) 2a′: 1H NMR (500 MHz, D2O) δ [ppm] = 8.99 (d, J = 6.9 Hz, 12H, Ha′), 7.91 (d, J = 6.9 Hz, 12H, Hb′), 3.14 (s, 12H, N-CH2), 2.78 (s, 36H, N-CH3) 2a/2a′:

13

C NMR (125 MHz, D2O): δ [ppm] = 151.55, 151.48, 147.24, 145.25, 125.71,

124.43, 62.84, 62.70, 50.48, 50.28. [Pd(2,2′-bpy)(4,4′-bpy)] 4(NO3)8 + [Pd(2,2′-bpy)(4,4′-bpy)]3(NO3)6, (3a/3a′) The complex was synthesized according to general procedure A by combining L (1.56 mg, 0.01 mmol, 1 eq.) and [Pd(2,2′-bpy)(NO3)2] (3.86 mg, 0.01 mmol, 1 eq.). Isolated yield = 4.3 mg, 79.3%. 3a: 1H NMR (500 MHz, D2O) δ [ppm] = 9.31 (d, J = 6.6 Hz, 16H, Ha), 8.47-8.52 (m, 8H, Hg), 8.36-8.43 (m, 8H, Hh), 8.13 (d, J = 6.7 Hz, 16H, Hb), 7.59 (t, J = 6.7 Hz, 8H, Hf), 7.47 (d, J = 5.5 Hz, 8H, He).


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3a′: 1H NMR (500 MHz, D2O) δ [ppm] = 9.15 (d, J = 6.5 Hz, 12H, Ha′), 8.46-8.51 (m, 6H, Hg′), 8.36-8.44 (m, 6H, Hh′), 8.02 (d, J = 6.6 Hz, 12H, Hb′), 7.90 (d, J = 5.7 Hz, 6H, He′), 7.64 (t, J = 6.7 Hz, 6H, Hf′). 3a/3a′:

13

C NMR (125 MHz, D2O): δ [ppm] =156.68, 152.13, 151.98, 150.46, 149.78,

147.49, 146.75, 142.83, 128.12, 128.07, 126.02, 125.43, 124.43, 124.39. [Pd(phen)(4,4′-bpy)] 4(NO3)8 + [Pd(phen)(4,4′-bpy)] 3(NO3)6, (4a/4a′) The complex was synthesized according to general procedure A by combining L (2.46 mg, 0.006 mmol, 1 eq.) and [Pd(phen)(NO3)2] (0.9 mg, 0.006 mmol, 1 eq.). Isolated yield = 2.6 mg, 77.3%. 4a: 1H NMR (500 MHz, D2O) δ [ppm] = 9.42 (d, J = 6.7 Hz, 16H, Ha), 8.97 (t, J = 7.4 Hz, 8H, Hk), 8.28-8.33 (m, 8H, Hl), 8.23 (d, J = 6.7 Hz, 16H, Hb), 7.88-7.94 (m, 16H, Hj + Hi). 4a′: 1H NMR (500 MHz, D2O) δ [ppm] = 9.26 (d, J = 5.5 Hz, 12H, Ha′), 8.97 (t, J = 7.4 Hz, 6H, Hk′), 8.28-8.33 (m, 6H, Hl′), 8.10 (d, J = 6.50 Hz, 12H, Hb′), 7.96 (dd, J = 8.2 and 5.4 Hz, 6H, Hj′), 7.88-7.94 (m, 6H, Hi′). Attempts to record a 13C NMR spectrum for 4a/4a′ were unsuccessful because increasing the concentration of these complexes led to problems in solubilisation and a poor-quality spectrum was obtained at lower concentration. Procedure B: Synthesis of complexes with tosylate as counter anion: Equimolar quantities of ligand L and the corresponding in situ prepared cis-protected Pd(II) units (with tosylate as counter anion) were stirred in suitable solvent for 24 h at ambient temperature. The solvent was evaporated to obtain solid product. The complex 2b was prepared in distilled water, however, 3b and 4b were prepared in 1:1 (v/v) acetonitrile-water due to low solubility of [Pd(2,2′-bpy)(OTs)2] and [Pd(phen)(OTs)2] in water. [Pd(tmeda)(4,4′-bpy)] 4(OTs)8, (2b): [Pd(tmeda)Cl2] (0.0058 g, 0.01 mmol, 1 eq.) was combined with AgOTs (0.0111 g, 0.02 mmol, 2 eq.) in distilled water (2 mL) and the



suspension was warmed for 30 min at 60 °C. AgCl precipitated was discarded and the clear solution of [Pd(tmeda)(OTs)2] was added to a solution of 4,4′-bpy (0.0031 mg, 0.01 mmol, 1 eq.) in water (2 mL). The mixture was stirred for 24 h at ambient temperature and then evaporated to yield a yellow solid. Isolated yield = 12.5 mg, 86.8 %, mp. 230 °C, decomposed. 2b: 1H NMR (500 MHz, D2O) δ [ppm] = 9.19 (d, J = 6.5 Hz, 16H, Ha), 7.84 (d, J = 6.5 Hz, 16H, Hb), 7.05 (d, J = 8.0 Hz, 16H, HAr-OTs), 6.54 (d, J = 7.9 Hz, 16H, HAr-OTs), 3.08 (s, 16H, N-CH2), 2.61 (s, 48H, N-CH3), 1.93 (s, 24H, -CH3-OTs). 13

C NMR (125 MHz, D2O): δ [ppm] = 151.70, 146.32, 141.50, 139.39, 128.83, 125.59,

124.91, 62.69, 50.29, 20.29. [Pd(2,2′-bpy)(4,4′-bpy)] 4(OTs)8, (3b): [Pd(2,2′-bpy)Cl2] (0.0066 g, 0.01 mmol, 1 eq.) was combined with AgOTs (0.0111 g, 0.02 mmol, 2 eq.) in 1:1 (v/v) acetonitrile-water (3 mL) and the suspension was warmed for 30 min at 60 °C. AgCl precipitated was discarded and the clear solution of [Pd(2,2′-bpy)(OTs)2] was added to a solution of 4,4′-bpy (0.0031 mg, 0.01 mmol, 1 eq.) in 1:1 (v/v) acetonitrile-water (3 mL). The mixture was stirred for 24 h at ambient temperature and then evaporated to obtain a white solid. Isolated yield = 14.1 mg, 92.8 %, mp. 220 °C, decomposed. 3b: 1H NMR (500 MHz, D2O) δ [ppm] = 9.38 (d, J = 6.7 Hz, 16H, Ha), 8.40 (d, J = 8.3 Hz, 8H, Hh), 8.34 (t, J = 8.0 Hz, 8H, Hg), 8.09 (d, J = 6.9 Hz, 16H, Hb), 7.54 (t, J = 6.7 Hz, 8H, Hf), 7.34 (d, J = 5.7 Hz, 8H, He), 7.11 (d, J = 8.2 Hz, 16H, HAr-OTs), 6.64 (d, J = 8.0 Hz, 16H, HAr-OTs), 1.98 (s, 24H, -CH3-OTs). 13

C NMR (125 MHz, D2O): δ [ppm] = 156.50, 152.20, 149.70, 146.66, 142.77, 141.59,

139.48, 128.93, 128.08, 125.96, 124.96, 124.41, 20.38. [Pd(phen)(4,4′-bpy)] 4(OTs)8, (4b): [Pd(phen)Cl2] (0.0071 g, 0.01 mmol, 1 eq.) was combined with AgOTs (0.0111 g, 0.02 mmol, 2 eq.) in 1:1 (v/v) acetonitrile-water (4 mL) and the


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suspension was warmed for 30 min at 60 °C. AgCl precipitated was discarded and the clear solution of [Pd(phen)(OTs)2] was added to a solution of 4,4′-bpy (0.0031 mg, 0.01 mmol, 1 eq.) in 1:1 (v/v) acetonitrile-water (4 mL). The mixture was stirred for 24 h at ambient temperature and then evaporated to yield a white solid. Isolated yield = 12.5 mg, 79.8 %, mp. 272 °C, decomposed. 4b: 1H NMR (500 MHz, D2O) δ [ppm] = 9.51 (d, J = 6.5 Hz, 16H, Ha), 8.92 (d, J = 8.3 Hz, 8H, Hk), 8.25 (s, 8H, Hl), 8.19 (d, J = 6.7 Hz, 16H, Hb), 7.86 (dd, J = 8.2 and 5.4 Hz, 8H, Hj), 7.78 (d, J = 5.4 Hz, 8H, Hi), 7.15 (d, J = 6.5 Hz, 16H, HAr-OTs), 6.69 (m, 16H, HArOTs), 2.02 (s, 24H, -CH3-OTs). 13

C NMR (125 MHz, D2O): δ [ppm] = 152.52, 150.38, 146.81, 141.76, 139.41, 131.22,

128.92, 128.15, 125.95, 124.89, 20.39. Crystallographic details Single-crystal X-ray data were recorded for the crystals of 2b and 3b on a Bruker AXS Kappa Apex II CCD diffractometer with graphite monochromatized Mo-Kα (λ=0.71073 Å) radiation. The crystals were fixed at the tip of a glass fiber, mounted on the goniometer head and optically centered. The automatic cell determination routine, with 36 frames at three different orientations of the detector, was employed to collect reflections, and the APEXIISAINT program60 was used for determining the unit cell parameters. The data were corrected for Lorentz-polarization effects. Semi-empirical absorption correction (multi-scan) based on symmetry equivalent reflections was applied using the SADABS program.60 The structures were solved by direct methods and refined by full matrix least squares, based on F2 using SHELX-2014 software package61 and the program WinGX.62 Molecular and packing diagrams were generated using Mercury.63 Geometrical calculations were performed using PLATON.64 ORTEPs were prepared using ORTEP-3.62



For 4bI and 4bIII the X-ray measurements were performed on single crystal samples mounted using a fibre loop on a Bruker Venture Metaljet diffractometer equipped with an Oxford Cryosystem liquid N2 device, using Ga-Kα radiation (λ = 1.34139 Å). The cell parameters were determined from reﬂections taken from three sets of 100 frames. The structure was solved by direct methods using SHELX-2014 software package and the program Olex2.65 The H-atoms were included in calculated positions and treated as riding atoms using Olex2 default parameters. The crystallographic data for the crystals of 2b – 4b are summarized in Table 2. Details regarding the refinement of the structures are available in the Table S1. CONCLUSIONS In summary, we have described the exclusive formation and existence of the cis-protected Pd(II) molecular squares (cis-protecting agent = tmeda, 2,2′-bpy, phen) with 4,4′-bipyridine as the ligand, using tosylate as the counteranion and water as the medium of complexation. Crystallization of the 2,2′-bpy and phen squares produced two solvates of each complex wherein the cations self-assemble through aromatic stacking interactions between the 2,2′bpy / phen units, via side-on or end-on modes of association. The crystal structures of the complexes revealed a pair of tosylates encapsulated in the hydrophobic cavity of the square which is perhaps the driving force for the formation of the square.


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Table 2. Summary of crystallographic data for crystals of 2b-4b.

2b

3bI

3bII

4bI

Chemical Formula Mr

C120H152N16 O46Pd4S8 3236.65

C136H120N16 O47Pd4S8 3412.55

C140H128N16 O37.50Pd4S8 3316.66

C144H169.80N16 C144H158N16 O48.90Pd4S8 O43Pd4S8 3589.24 3482.94

Temp. (K)

293(2)

296(2)

296(2)

100

100

Crystal system

Triclinic

Triclinic

Monoclinic

Triclinic

Triclinic

Space group

P-1

P-1

P21/c

P-1

P-1

a (Å)

9.8546(9)

9.9562(2)

22.4777(9)

9.5734(4)

23.3123(11)

b (Å)

18.778(2)

18.5183(4)

17.5821(6)

17.7586(7)

23.5072(11)

c (Å)

20.755(2)

22.7992(5)

19.0678(7)

23.9228(9)

30.0241(14)

α (°)

75.766(3)

74.6070(10) 90

104.0070(10)

77.6191(13)

β (°)

87.336(3)

87.7830(10) 104.867(2)

91.9320(10)

68.0792(14)

87.509(3)

76.1340(10) 90

102.6910(10)

76.3777(14)

3716.7(6)

3933.25(15) 7283.4(5)

3833.5(3)

14687.8(12)

1

1

2

1

4

Dcalc (g cm )

1.446

1.441

1.512

1.555

1.575

Unique reflns

12937

13827

13353

19860

92959

Rint

0.0409

0.0218

0.0466

0.0371

0.0364

GoF

1.035

1.071

1.061

1.053

1.016

R1[I > 2σ(I)]

0.0415

0.0440

0.0535

0.0293

0.0412

wR2[I > 2σ (I)]

0.0965

0.1243

0.1334

0.0746

0.1052

R1_all data

0.0644

0.0555

0.0853

0.0302

0.0501

wR2_all data

0.1102

0.1398

0.1557

0.0751

0.1134

CCDC No.

1578158

1578159

1578160

1578161

1578162

γ (°) 3

V (Å ) Z –3


4bIII


ASSOCIATED CONTENT Supporting Information The supporting information includes 1D and 2D NMR spectra for complexes 2a/2a′, 3a/3a′ and 4a/4a′, 1H NMR spectra for 2a/2a′, 3a/3a′ and 4a/4a′ at different concentrations, plot of square-triangle percentage vs. concentration for 2a/2a′, 3a/3a′ and 4a/4a′, 2D DOSY NMR spectra for 2a/2a′, 3a/3a′ and 4a/4a′, 1D and 2D NMR spectra for complexes 2b-4b, 1H NMR spectra for 2a/2a′, 3a/3a′ and 4a/4a′ obtained upon addition of AgOTs, 1H NMR spectra for complexes 2b-4b recorded in [D6]DMSO, 1H NMR spectra for 2b-4b at different concentrations, table of crystallographic data for crystals of 2b4b, details of crystal structure refinement, ORTEPs of molecules in crystals of 2b-4b, figures showing the molecular packing in crystals of 1a, 2a and 2b-4b and table of intermolecular interactions in crystals of 2b-4b. This material is available free of charge via the Internet at http://pubs.acs.org. Accession Codes CCDC 1578158-1578162 AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +91-4422574224. Fax: +914422574202. Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS S.K. thanks IIT-Madras for an Institute Postdoctoral Fellowship. S.P. thanks CSIR, New Delhi for Senior Research Fellowship. We are grateful to Dr. B. Chandrika, SAIF, IITMadras for help with recording the 2D DOSY NMR spectra and Dr. B. Varghese for helpful discussions. G.S.H. and D.C. thank the Natural Sciences and Engineering Research Council (NSERC) of Canada for funding. D.K.C and G.S.H. thank the Shastri Indo-Canadian Institute and the Université de Montréal for travel grants. This work was also supported by a financial grant from SERB, Department of Science and Technology, Government of India (Project No. SB/S1/IC-05/2014).


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For Table of Contents Use Only

Self-assembled molecular squares as supramolecular tectons Shobhana Krishnaswamy,† Soumyakanta Prusty,† Daniel Chartrand,‡ Garry S. Hanan,§ and Dillip K. Chand*†

Complexation of cis-protected Pd(II) units, [PdL′(OTs)2], (where L′ = 2,2′-bipyridine / 1,10-phenanthroline) with 4,4′-bipyridine in aqueous medium resulted in exclusive formation of the corresponding molecular squares. The cationic units in their crystals selfassembled through π···π stacking interactions between the aromatic rings of the bpy and phen units via side-on or end-on modes of overlap between adjacent molecules.


Self-Assembled Molecular Squares as Supramolecular Tectons

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