Trinuclear Intro-Vertere Circular Helicate and Its Columnar Hexagonal

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Trinuclear Intro-vertere Circular Helicate and its Columnar Hexagonal Stacking Aruna Panneerselvam, Rajamony Jagan, and Dillip Kumar Chand Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 13, 2017

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Crystal Growth & Design

Trinuclear Intro-vertere Circular Helicate and its Columnar Hexagonal Stacking Aruna P. Panneerselvam, Rajamony Jagan and Dillip K. Chand* Department of Chemistry, Indian Institute of Technology Madras, Chennai – 600036, India ABSTRACT: A self-assembled discrete Pd3L'3L3 type circular helicate of hitherto unknown architecture is disclosed in this work where L' stands for 1,10-phenanthroline (phen) and L for a triazole

appended

bidentate

nonchelating

ligand

i.e.

1,4-bis((1H-1,2,3-triazol-1-

yl)methyl)benzene. The coordination environment of palladium(II) in any given PdN4 coordination planes of the helicate is described by one “phen” moiety and two triazole moieties belonging to different ligand strands. All the three bound “phen” moeities are uniquely turned inside and lodged in the internal cavity of the molecule hence the term “intro-vertere circular helicate” is introduced. One of the PdN4 square planes is sandwiched between the other two via “phen”-inspired “intramolecular π-stacking” making a cylindrical stacking arrangement. A racemic pair designated by MPP and PMM isomers, where M and P stands for the handedness of the ligand L, is observed in the crystal structure. In the crystal packing, the trinuclear “introvertere circular helicate” molecules are arranged in a columnar hexagonal stacking. The two exposed “phen” moieties of a given molecule participated in the “intermolecular π-stacking”. The MPP and PMM isomers are arranged in an alternate manner in the stacks.

ACS Paragon Plus Environment

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Self-assembled coordination helicates are known to be either discrete1-4 or polymeric5,6 in builds and their architectures are quite intriguing. Self-assembled palladium(II) complexes of discrete compositions are though well explored,7-10 only a handful of such assemblies are established as helicates with due crystal structures.11-26 The compositions of these helicates are Pd1L1,11 Pd2L1,12,13 Pd2L2,14,15 Pd2L4,16-22 Pd2L'2L2,23 Pd2L2Cl224,25 and Pd3L1Cl626 where L' stands for a chelating bidentate ligand and L for a chelating/non-chelating bi- or polydentate ligand. The trinuclear Pd3L1Cl6 complex is the lone example of a circular helicate system among the above mentioned complexes. A typical circular helicate resembles the shape of a doughnut and equipped with a default central cavity that could be suitable for guest encapsulation as demonstrated in literature.1-4, 27-29 Circular helicates with narrow30-34 cavities are probably inapt for guest encapsulation whereas those with very wide35 cavities are rarely demonstrated36 for guest binding. We report here a circular helicate of hitherto unknown architecture that could be considered intro-vertere (turn inside), as explained next. Composition of this novel helicate is Pd3L'3L3, where the term L' stands for 1,10-phenanthroline (phen) and L for the triazole appended bidenate nonchelating ligand (1,4-bis((1H-1,2,3-triazol-1-yl)methyl)benzene), L. Three units of the ancillary ligand i.e. ‟phen” moeity, are uniquely turned inside and lodged in the internal cavity of the molecule. The phen moieties so lodged are held by “intramolecular π-stacking” as depicted in the cartoon diagram (Figure 1c) and compared with non-helical complexes of the same formulation. Relative positions of the three coordination square planes in the present Pd3L'3L3 type helicate is quite unlike the typical Pd3L'3L3 type “trigonal-planar”,37-39 (Figure 1a) “deep-bowl”,40,41 (Figure 1b) and shallow/partial bowl like architectures.26, 42-45 In fact, such a disposition is is not known in the literature for any complex of PdxL'xLy composition. In this disposition one of the PdN4 square


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planes is sandwiched between the other two via “phen”-inspired “intramolecular π-stacking” making a cylindrical stacking arrangement where the π-surfaces of two “phen” moieties are exposed and available. We have been exploring the self-assembled coordination complexes as tectons in crystal engineering by exploiting intermolecular π-stacking as the synthon.42, 45-48 The new molecule reported here provides an unique opportunity to evaluate the role of the exposed phen moieties in related crystal engineering aspects. In the crystal packing, the trinuclear “introvertere circular helicate” molecules are found to be arranged in a columnar hexagonal stacking (Figure 1d).

Figure 1. Cartoon representation of Pd3L'3L3 complexes showing (a) “trigonal-planar”, (b) “deep-bowl” and (c) hitherto unknown “intro-vertere circular helicate” type architectures. (d) columnar hexagonal stacking of the helicate.


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Scheme 1. Dynamic equilibrium of the binuclear [Pd2(phen)2(L)2](X)4, 1a/1b and trinuclear complexes [Pd3(phen)3(L)3](X)6, 2a/2b. (Sample-1, X=NO3-). The triazole appended bidentate nonchelating ligand, L was prepared via a typical azidealkyne cycloaddition protocol.49 To the best of our knowledge, the ligand L has never been used for complexation with any metal ion. To a solution of Pd(phen)(NO3)2 (0.050 g, 0.1217 mmol) in 1:1 acetonitrile-water (12.0 mL), ligand L (0.0290 g, 0.1217 mmol) was added and the mixture was stirred at room temperature for 5 h. The resulting solution was evaporated by standing at room temperature to obtain a pale yellow solid (sample-1) in quantitative yield (Scheme 1).


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Variable concentration 1H NMR spectra of the sample-1 in DMSO-d6 (Figure 2) exhibited two sets of peaks. Downfield shift of the triazole protons Ha (0.7 ppm) and Hb (0.8 ppm) as compared to the free ligand indicated the coordination of triazole units. The phenylene protons, Hc were up field shifted (0.3 ppm) in line with the behavior of a related binuclear complex prepared from imidazole appended ligands.23 The two sets of peaks were associated with two proposed complexes namely Pd2L'2L2 type binuclear [Pd2(phen)2(L)2](NO3)4, 1a and Pd3L'3L3 type trinuclear [Pd3(phen)3(L)3](NO3)6, 2a. The binuclear and trinuclear nature of the complexes was subsequently confirmed by electrospray ionization mass spectrometry (ESI-MS) and single crystal X-ray diffraction. Single crystals were achieved separately from the dynamic equilibrium under varied crystallization conditions.

Figure 2. 400 MHz 1H NMR spectra in DMSO-d6 at 25 oC (TMS as external standard) for (i) ligand, L and (ii) – (iv) dynamic equilibria of Pd2L'2L2 type binuclear and Pd3L'3L3 type trinuclear complexes 1a and 2a at (ii) 20 (iii) 40 and (iv) 160 mM. (Concentration: with respect to palladium(II)).


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At 20 mM concentration of palladium(II), in DMSO-d6, the percentage of ligand associated with 1a and 2a are approximately 90 and 10 (Figure 2). The proportion of 2a initially increased rapidly with concentration. At 120 mM concentration, the proportion of 2a increased to 50% in ligand component and no significant change could be observed at further higher concentration. The complexes (sample-1) were isolated from the mixed solvent system CH3CN:H2O (1:1), hence 1H NMR study was carried out in CD3CN:D2O (1:1). At 20 mM concentration, in CD3CN:D2O, the percentage of ligand associated with 1a and 2a are approximately 75 and 25 (Table S1). The presence of water in the mixed solvent seemingly favored the trinuclear complex 2a. This was further supported by the observation that at 20 mM concentration in DMSO-d6:D2O (1:1), the percentage of ligand associated with 1a and 2a are approximately 72 and 28. Increase of concentration, percentage of H2O in the solvent system (Figure S28) or lowering of temperature (in CD3NO2 at 25 ºC and -20 ºC (Figure S27)) also favored the trinuclear complex. Notably, all the signals of ‟phen” moeity based protons in the trinuclear complex are located significantly up field compared to the corresponding binuclear complex. It would be thus appropriate to assume here that the “phen” moieties are involved in π-stacking interactions in 2a. The proportion of trinuclear complex was favored in the equilibrium by employing H2O as one of the solvents of a mixed solvent system. Thus, it was decided to probe the dynamic equilibria of the complexes in the sole solvent H2O which can induce intramolecular π-π stacking. 1H NMR spectrum of sample-1 was recorded in D2O. Interestingly, at 20 mM concentration of palladium(II) the percentage of ligand associated with 1a and 2a are approximately 20 and 80. Thus the trinuclear complex was found to be hugely favored in H2O.


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Notably, when choice of L' is 2,2'-bipyridyl (bpy), the corresponding binuclear complex [Pd2(bpy)2(L)2](NO3)4, 3a (sample-2) was formed as an exclusive product in a variety of solvents like DMSO, DMSO:H2O or CH3CN:H2O as confirmed from 1H NMR study (Scheme 2). However, along with 3a, a proposed trinuclear complex [Pd3(bpy)3(L)3](NO3)6, 4a was formed when the solvent of choice was H2O. As mentioned earlier for sample-1, the hydrophobic nature of water facilitated intramolecular phen-phen π-π stacking to favour the formation of 2a. In sample-2 the intramolecular bpy-bpy stacking leading to formation of 4a could occur, however, only in water. At 20 mM concentration, the percentage of ligand associated with binuclear 3a and proposed trinuclear 4a are approximately 65 and 35 in D2O (Table S3). Thus, “bpy” moiety could form a trinuclear complex only when the solvent is exclusively water. Thus, “bpy” moiety is not suitable enough to favor a trinuclear complex unlike ‟phen” moeity. This is attributed to the smaller surface area of bpy as compared to phen and confirmed the vital role of π-surface. The augmented π-surface area in the cis-protecting group (as in ‟phen” moeity) and suitable solvent system that could garner hydrophobic interaction (as in H2O) are found to act in a synergy to afford the trinuclear complex. The differences in the roles played by “bpy” and “phen” are known in the literature.42,45,47,53 We have observed differences in the role of “bpy” and “phen” moieties in the solid-state packing behaviour of some Pd2L'2L247 and Pd3L'3L342,45 type complexes. Sun et al. has reported multinuclear palladium(II) complexes containing “bpy” or “phen” groups at the periphery of novel segmental architectures.53 While the “phen” containing complex retained its segmental architecture in the solid-state, the bpy containing complex reorganized during crystallization process to form continuous mesoporous metal-organic nanotubes.


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Scheme 2. Complexation of Pd(bpy)(X)2 with the ligand L in CH3CN:H2O followed by evaporation to isolate a solid (Sample-2, X=NO3-). Dissolution of Sample-2 in a variety of solvent afforded a single product i.e. [Pd2(bpy)2(L)2](NO3)4, 3a; however, in H2O a dynamic equilibrium of [Pd2(bpy)2(L)2](NO3)4, 3a and proposed [Pd3(bpy)3(L)3](NO3)6, 4a was observed.


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ESI-MS spectra of the sample-1 and sample-2 were collected to ascertain their molecular compositions. The data of sample-1 having the peak patterns at m/z = 371.74 and 425.97 corresponding to [1a – 3 NO3]3+ and [2a – 4 NO3]4+, conveyed the simultaneous presence of the binuclear [Pd2(phen)2(L)2](NO3)4, 1a and trinuclear [Pd3(phen)3(L)3] (NO3)6, 2a in sample-1. The data of sample-2 having m/z = 356.14 corresponding to [3a – 3 NO3]3+ endorsed the binuclear complex [Pd2(bpy)2(L)2](NO3)4, 3a in the sample-2. The equilibrium of 1a and 2a is given in equation 1 and the formula of Kapp in equation 2. Using the 1H NMR data of the equilibrium in different solvents and concentrations have been estimated and the Kapp values are presented in Table 1. Since the trinuclear complex 2a is largely favoured in aqueous condition, in consequence the estimated Kapp value is also significantly higher in D2O as compared to other solvents.

Table 1: Equilibrium constant (Kapp) values for the equilibrium of 1a and 2a at 298 K. Concentration of palladium(II)

Kapp (mmol-1L) in DMSO- d6

Kapp (mmol-1L) in CD3CN : D2O

Kapp (mmol-1L) in D2O

20 mM

1.1 x 10-3

6.5 x 10-3

2.5

40 mM

5.8 x 10-3

1.3 x 10-2

80 mM

1.2 x 10-2

2.0 x 10-2

120 mM

1.3 x 10-2

1.6 x 10-2

160 mM

1.8 x 10-2

2.5 x 10-2


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Our effort to crystallize 1a and 2a remained unsuccessful hence anion variation was performed. To a solution of Pd(phen)Cl2 (0.050 g, 0.1398 mmol) in 1:1 acetonitrile–water (14 mL), silver triflate was added (0.0710 g, 0.2796 mmol) at 60 oC, which lead to immediate precipitation of AgCl. The resulting mixture was heated for 30 min and centrifuged. The yellow colour solution of Pd(phen)(OTf)2 so formed was separated by decanting the centrifugate. To the clear yellow solution, ligand L (0.0335 g, 0.1398 mmol) was added and the mixture was stirred at room temperature for about 5 h. The resulting solution after centrifugation, was evaporated by standing at room temperature to obtain the mixture of complex [Pd2(phen)2(L)2](OTf)4, 1b + [Pd3(phen)3(L)3](OTf)6, 2b as a pale yellow solid. The equilibrium of [Pd2(phen)2(L)2](OTf)4, 1b and [Pd3(phen)3(L)3](OTf)6, 2b in CH3CN:H2O(1:1) was found to be suitable for crystallization (Table S2). Standing the equilibrium in CH3CN:H2O, and diffusion of ethyl acetate in separate trials afforded crystals of 1b·CH3CN and 2b, respectively. The 1H NMR spectra of the equilibria of 1b and 2b were obtained at different solvents and concentrations (ESI). This data was found to be closely comparable with the equilibria of 1a and 2a recorded under similar conditions. Thus, anions don’t have any influence on the architecture of the complexes. The cationic frame-work of 1b possesses a pair of PdN4 square planes in a given unit of the binuclear complex describing an overall step like arrangement.48 However, boat shape50,51 of the complex 1b, is attributed due to the conformation of bridging ligands in syn-syn orientation (Figure 3). The well separeted Pd---Pd nonbonded distance in 1b is approximately 12 Å (longer than that in trinuclear 2b).


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Figure 3. Crystal structure showing the (a) top and (b) side views for the complexed cation of 1b. Crystal structure of 2b is the focus of this work (Figure 4). Thus, it is necessary here to explain the shapes of known trinuclear Pd3L'3L3 type complexes so as to appreciate the uniqueness of the trinuclear complex of this work. The Pd3L'3L3 type complexes are usually prepared from cis-protected palladium(II) component, PdL' and selected nonchelating bidentate ligand L under appropriate conditions.52 The relative positions of the three coordination square planes of such trinuclear complexes are depicted in a cartoon diagram in Figure 1. As a reference, the three coordination square planes around the palladium(II) centres are located on the same plane (Figure 1a) and the imaginary array visualized by joining the three metal centres is approximately “trigonal-planar”. In most of the molecules, the three square planes are displaced from the reference “trigonal-planar” shape in such a manner that any given pair of square planes radially move either towards or opposite to each other. Slight or substantial radial displacement of all the three square planes towards each other would result a symmetrical arrangements resembling a “shallow-bowl” or “deep-bowl” shapes, respectively (Figure 1b).40-44 “Partial-bowl” is possible when one of the square planes is radially displaced in opposite direction from the other two.26,45


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Figure 4. Crystal structure (MPP/PMM enantiomeric pairs) of 2b depicting intramolecular πstacking as viewed from (a) top and (b) side of the complexed cation. The architecture shown in Figure 1c, corresponding to the crystal structure of 2b shown in Figure 4, is unique. This is due to the fact that the three coordination square planes of 2b are located inside the molecule in such a manner that one of the PdN4 square planes is sandwiched between the other two making a cylindrical stacking arrangement. This folding could be formally conceived by radially displacing one of the coordination square planes of a reference “trigonalplanar (see Figure 1a)” system at its own position by straight angle (180o). The other two square planes are axially displaced up and down, respectively, followed by flipping towards the inner side of the molecule. This need not be the mechanism but just a geometrical comparison. The architecture so resulted is claimed as unprecedented and that happened to be a trinuclear circular helicate as confirmed from the crystal structure of 2b. Since all three units of the ancillary ligands (‟phen” moeity) are uniquely turned inside hence the term “intro-vertere circular helicate” is introduced. Architecture of the reported trinuclear helicate of Pd3L1Cl6 is comparable to the “deep-bowl” and “partial deep-bowl”.10


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Our “intro-vertere circular helicate” is featured by (i) intramolecular π-π stacking fashioned by the phen moieties and (ii) the specific handedness of the bound-ligand units (Figure 4). The calculated inter-planar distances of the neighboring phen units span in the range of 3.54 to 3.58 Å, which in turn stabilizes the overall intricate structure of 2b. It should also be noted that the conformations of all the three ligand units in 2b are not equivalent as seen in the crystal structure. The conformation of one of the ligand units (say type-I) that bridges the farthest palladium centres (Pd1/Pd3) with Pd---Pd nonbonded distance of approximately 8 Å is different from the other two ligand units (say type-II) that bridges the nearest palladium centres (Pd1/Pd2 or Pd2/Pd3) with Pd---Pd nonbonded distances of approximately 6 Å. Also, the Pd---Pd distances of 2b are smaller than that of 1b. Pair of helical molecules that are mirror image of each other are seen in the packing diagram. Each molecule in the pair exhibited special ligand wrapping as explained next. For binuclear helicates comprising of non-chirogenic metal centres, the direction of ligand wrapping around the imaginary central axis, is used to name the left and right handed helicates as M and P, respectively.4,23 A pair of enantiomers is observed in the crystal structure of the trinuclear circular helicate 2b and their configurations are defined with respect to the ligand wrapping around the imaginary central axis. The central axis is the imaginary line that pierces through the centre of stacked phen units and run vertically. We noticed that the direction of ligand wrapping is not the same for all the three ligands (Figure 5). Accordingly, three alphabets (from M and P) are used in a sequence and each represents the handedness of a ligand strand for complete description of the configuration of a given isomer. The crystal structure of 2b exhibited the enantiomeric pair namely, MPP-Pd3L'3L3 and PMM-Pd3L'3L3 (Figure 5(a)-(c)). The first alphabet describes the


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type-I ligand and the next two for the type-II ligands; the sequence of the alphabets is chosen arbitrarily. The handedness of type-I and type-II ligands in a given enantiomer are just opposite. That could be necessary to restrain the observed architecture or vice versa.

Figure 5. Crystal structure (MPP/PMM enantiomeric pairs) of 2b (side views) emphasising the direction of ligand wrapping for each strands.

We are involved in the design and analysis of intermolecular interactions in selfassembled coordination binuclear and trinuclear complexes. 42, 45-48 In that context, packing of the molecules in the crystal structure of 2b was analysed so as to understand how the intro-vertere helicates are arranged with respect to one another. Since 2b is racemic, equal number of MPP and PMM helicates was observed in the packing. The “phen” moiety exhibited intermolecular ππ stacking apart from intramolecular π-π stacking, hence allowing the molecule to stack vertically in an alternate fashion of MPP and PMM. The calculated inter-planar distances of the neighboring phen units span in the range of 3.6 to 3.7 Å, indicating both face to face and offset


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intermolecular π−π stacking. Hence, the trinuclear “intro-vertere circular helicate” molecules are arranged through intermolecular and intramolecular π−π stacking of “phen” moieties in a columnar hexagonal stacking as shown in Figure 1d and Figure 6.

Figure 6. Molecular packing of 2b with alternate arrangement of MPP and PMM enantiomeric pairs showing intermolecular and intramolecular π−π stacking of “phen” moieties in 2b. The coordinates of complexed cation of 2b was taken from its crystal structure and then the density functional theory (DFT) optimization was carried out using Gaussian 09 package to show the solvent is essential to keep the intro-vertere circular helicate, 2b intact. The circular helicate like design was retained when the solvent environment employed was DMSO or H2O, where the energy was lowered in the aqueous environment. In gaseous state the phen units are flipped outside the cavity (Figure S43). In summary, complexation of a triazole appended bidentate non-chelating ligand with a selected cis-protected palladium(II) component resulted in a dynamic equilibrium of Pd2L'2L2 type boat-shaped binuclear complex and Pd3L'3L3 type trinuclear complex of a hitherto unreported helical architecture. The trinuclear architecture is named as intro-vertere circular


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helicate because the ancillary ‟phen” moeities are turned inside. Interestingly, the augmented πsurface in the cis-protecting group i.e. ‟phen” moeity favored the trinuclear helicate more so in aqueous conditions. This is attributed to the “intramolecular π-stacking” of the ‟phen” moeities. The corresponding bpy system was found to be much less amenable to the special π-stacking. Thus, we paved a way for preparation of intramolecular π-stacking driven self-assembled complexes of this variety. Further, the intermolecular interactions in the solid state exhibited a columnar hexagonal stacking. ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Experimental details, schemes and figures related to the synthesis and characterization of the complexes, and further details of the single crystal XRD data. AUTHOR INFORMATION Corresponding Author *Tel: +91-4422574224, Fax: +91-4422574202, e-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT D. K. C. thanks the SERB, Department of Science and Technology, Government of India (Project No. SB/S1/IC-05/2014) for financial support. A. P. P. thanks UGC, India for a research fellowship.


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Table of Contents Use Only Trinuclear Intro-vertere Circular Helicate and its Columnar Hexagonal Stacking Aruna P. Panneerselvam, Rajamony Jagan and Dillip K. Chand*

A Pd3L'3L3 type circular helicate of hitherto unknown architecture is disclosed where L' stands for phen, and L for a nonchelating bidentate ligand. Three phen units are uniquely turned inside (“intro-vertere”) and lodged in the internal cavity of the molecule via “intramolecular πstacking”. The two exposed “phen” moieties participated in “intermolecular π-stacking” to create a columnar hexagonal stacking.


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Trinuclear Intro-Vertere Circular Helicate and Its Columnar Hexagonal

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