Double-Stranded Binuclear Helicates and Helicity Modulation - Crystal

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Double-Stranded Binuclear Helicates and Helicity Modulation Mili C. Naranthatta, Sreenivasulu Bandi, Rajamony Jagan, and Dillip K. Chand* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India S Supporting Information *

ABSTRACT: All of the reported self-assembled complexes of cis-Pd2L′2L2 formulation, where L′ stands for a chelating bidentate ligand and L for a nonchelating bidentate ligand, are “non-helical”. We disclose here the debut of the long awaited “helicate” type architecture of the cis-Pd2L′2L2 formulation. In the present work, the term L′ stands for tetramethylethylenediamine (tmeda) and L stands for a (1H-imidazolyl)methylappended bidentate nonchelating ligand having a rigid spacer. The torsion angle between the two coordination PdN4 planes of a given “helicate” is considered here as its magnitude of helicity. Length of spacer unit (i.e., 1,4-benzene, 4,4′-biphenyl and 4,4′-p-terphenyl) crafted in the backbone of L is found to impart such conformation to the bound ligand moieties that the magnitude of helicity is modulated.

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plateau architecture is actually a rare variety. Anyway, one of the square planes of the reference “plateau” type molecule is considered fixed, and the other displaced from its original position to formally perceive a variety of architectures. Thus, the approximated shapes of the cis-Pd2L′2L2 type molecules with relevant displacement of one of the planes (Figure 1a−e) are (a) “plateau” (no displacement); (b) “step” (axially displaced from plateau by θ1); (c) “bow” (radially displaced from plateau by acute θ2 i.e. < 90°); (d) “opened jaws” (radially displaced from plateau by obtuse θ2 i.e. > 90°); and (e) “zig-zag plateau” (parallel displaced from plateau by θ3). The “step” and “opened jaws” architectures are usually formed, whereas “plateau” is rare; the “bow” and “zig-zag plateau” shapes are probably not known. The “helicate” type design with a torsion angle of θ4 between the coordination square planes shown in Figure 1f,g is explained in the latter part of next paragraph. The present work describes the debut of the long awaited “helicate” type architecture in the family of cis-Pd2L′2L2 type complexes where L′ stands for tetramethylethylenediamine (tmeda) and L stands for a (1H-imidazolyl)methyl-appended bidentate nonchelating ligand. Cartoon representation of the double-stranded binuclear helicates of both handedness, i.e., cisM-Pd2L′2L2 and cis-P-Pd2L′2L2 are shown in Figure 1f,g. Such helicates could be formally conceived by restraining one of the PdN4 square planes of a “plateau” type cis-Pd2L′2L2 complex coinciding with a reference plane (xy-plane) and rotating the other PdN4 square plane by an angle θ4 around the Pd----Pd axis (x-axis) in either of the directions in a circular manner so that the ligands are helically wrapped to afford the M or P isomers of “helicate” type architecture. However, this is just a geometrical comparison only and not for mechanistic claims. In

elf-assembled coordination complexes of discrete designs are attractive due to their fascinating topological features and unique functions.1−10 Desirable coordination architectures, through the self-assembly routes, are often achieved by the combination of judiciously designed organic ligands and selected metal ions under suitable reaction conditions. Among the amazing variety of reported architectures, the helicates are quite intriguing and deserve special mention in the realm of supramolecular coordination chemistry.11−16 An essential structural feature of a helicate is the wrapping of ligand strand(s) about an imaginary central axis of the said molecule. The direction of ligand-wrapping in helicates could be visualized in two manners, namely, left-handed or righthanded. The number of metal centers (x), number of ligand strands (y), and handedness (M for left and P for right) in a helicate could be considered as the parameters that are required for the structural classification of these discrete architectures into M-MxLy and P-MxLy varieties. Helical compounds of diverse structural features are known, and a variety of applications/functions of the related molecules are also explored.17−20 The molecular formula cis-Pd2L′2L2 represents a particular family of self-assembled architectures where L′ stands for a chelating bidentate ligand and L stands for a nonchelating bidentate ligand.21,5 Although a large number of such binuclear complexes are prepared over a period of quarter century, surprisingly, all of these are nonhelical in architecture. Most of these binuclear nonhelical complexes could be classified, with reference to the relative positions of the two coordination square planes, under the approximated shapes (some shapes are discussed earlier by us)22 of the types specified next. In the reference “plateau” architecture (Figure 1a) both the coordination square planes are located on the same plane (say, xy-plane) where the nonbonded Pd----Pd axis which is a pseudo-C2 axis coincides with x-axis (same as the x′ axis). The © XXXX American Chemical Society

Received: September 30, 2016 Revised: November 4, 2016 Published: November 10, 2016 A

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Figure 1. Cartoon representation of self-assembled coordination complexes of cis-Pd2L′2L2 formulation showing the spatial orientation of the ligand units and the relative positions of the coordination square planes in a variety of architectures: (a) “plateau”, (b) “step”, (c) “bow”, (d) “opened jaws”, (e) “zig-zag plateau”, and (f)/(g) M and P isomers of “helicate” from plateau (circularly displaced from plateau in either direction by θ4, straight angle, i.e., 180° is used here for clarity).

principle, θ4 is the torsion angle between the two PdN4 planes. The value of θ4 is considered here as the magnitude of helicity and that is arbitrarily chosen as a straight angle (±180°), for clarity of representation only. The above-described helicates could be best named as “helicate” (from plateau). Helicates of cis-Pd2L′2L2 type having other three-dimensional arrangements, in case prepared, could be named as “helicate” (from a specific shape) on the basis of the primary formal shape considered to conceive the ensuing design through rotation along a chosen axis. The (1H-imidazolyl)methyl-appended bidentate nonchelating ligands 1,4-bis((1H-imidazol-1-yl)methyl)-benzene (L1),23 4,4′-bis((1H-imidazol-1-yl)methyl)biphenyl (L2),24 and 4,4′bis((1H-imidazol-1-yl)methyl)-p-terphenyl (L3)25,26 explored in this study are included in Figure 2. As can be perceived, there is a gradual increment of spacer length from L1 to L3 that are aromatic and rigid in nature. The 1H-imidazolyl units are appended at both ends of the rigid spacer via a methylene unit each, thus introducing some conformational flexibility. The other nitrogen centers of the imidazolyl moieties are the potential binding sites of the ligands. The direction of the coordination vectors of the imidazolyl moieties in the ligands could adapt to the requirement of coordination geometry of the metal centers due to the possibilities of conformational rotation around the flexible methylene moieties. On the basis of the design principles, we assumed that complexation of these ligands (L1, L2, and L3) with an equimolar amount of a cis-protected palladium(II) component, PdL′ should afford binuclear complexes of Pd2L′2L2 formulation. We also envisioned possibilities of double-stranded

Figure 2. Chemical structure of the ligands L1−L3.

binuclear helicates, though nonhelical complexes could not be ruled out. This was anticipated because single-stranded helical coordination polymers of the ligand L1, as prepared by complexation with cobalt(II),27 copper(II),28 or cadmium(II)29 are known, though the coordination sphere is fulfilled using ancillary ligands/counteranions. The ligand L2 upon complexation with zinc(II)30 resulted in a single-stranded helical coordination polymer, however, that also required ancillary ligands/counteranions. To the best of our knowledge, the coordination behavior of L3 is not explored so far. A literature search revealed that palladium(II) based helicates, be it of any composition but strictly helicate only, are very rare. Doublestranded binuclear helicates of Pd2L2 formulation are prepared, B

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one by Constable et al. using a pentadentate ligand31 and another by Hahn et al.,32 using a tetradentate ligand. Also, a helicate of trans-Pd2Cl2L2 was prepared by Mani et al.33 Quadruple-stranded binuclear helicates of Pd2L4 formulation was introduced by Steel et al.,34 whereas more examples added parallel by the Crowley group,35 Chand group,36,37 Yoshizawa group,38 and recently by the Sun group.39 Complexation of Pd(tmeda)(NO3)2 with an equimolar amount of the ligands L1, L2, and L3 were carried out in acetonitrile/water (1:1) at room temperature to afford the binuclear complexes [Pd 2 (tmeda) 2 (L1) 2 ](NO 3 ) 4 , 1a; [Pd2(tmeda)2(L2)2](NO3)4, 2a and [Pd2(tmeda)2(L3)2](NO3)4, 3a, respectively. Similarly, complexation of Pd(tmeda)(ClO4)2 with the ligands (L1−L3) were carried out to prepare the binuclear complexes [Pd2(tmeda)2(L1)2](ClO4)4, 1b; [Pd2(tmeda)2(L2)2](ClO4)4, 2b and [Pd2(tmeda)2(L3)2](ClO4)4, 3b, respectively. Also, a combination of Pd(tmeda)(BF4)2 with L2 afforded the binuclear complex [Pd2(tmeda)2(L2)2](BF4)4, 2c. All the complexes were formed spontaneously and isolated in quantitative yields by slow evaporation of the corresponding reaction mixtures. The chemical structures of the representative complexes are shown in Figure 3.

Figure 4. 500 MHz 1H NMR spectra in DMSO-d6 (TMS as external standard) for (i) L1; (ii) [Pd2(tmeda)2(L1)2](NO3)4, 1a; (iii) L2; (iv) [Pd2(tmeda)2(L2)2](NO3)4, 2a; (v) L3; and (vi) [Pd2(tmeda)2(L3)2](NO3)4, 3a.

Figure 2 for the labels of protons) of the complexes (1a−3a) are downfield shifted as compared to the signals corresponding to free ligands L1, L2, and L3, respectively. The magnitude of this complexation induced downfield shift Δδ (ppm) for the signals of Ha/Hb/Hc are 0.51/0.28/0.12 (for 1a), 0.78/0.50/ 0.27 (for 2a), and 0.78/0.52/0.22 (for 3a). The signals assigned to the protons (He, etc.) of rigid aromatic spacers are interestingly upfield shifted indicating some kind of intramolecular interaction of the aromatic rings of one strand of the bound ligand with the other. The magnitude of upfield shift Δδ (ppm) are 0.43 (for He in 1a), 0.69/0.60 (for He/Hf in 2a), and 0.33/0.30/0.40 (for He/Hf/Hg in 3a). Probably, the spacer units of both ligand strands are close by. However, the methylene protons (Hd) are marginally downfield shifted. Construction of the binuclear complexes of Pd2L′ 2L2 formulation was confirmed from ESI-MS data. The mass spectra of 1b, 2b, and 3a showed isotopic peak patterns corresponding to the multiply charged cations generated due to the loss of counteranions from the parent molecules. The experimentally observed peak patterns at m/z = 340.39 for [1b−3ClO4]3+; 391.14 for [2b−3ClO4]3+; and 306.58 for [3a− 4NO3]4+ closely matched with the calculated isotopic patterns. Crystal structures of the complexes 1b and 2c, prepared using the ligands L1 and L2, exhibit helical architectures (Figures 5 and 6). Although the crystal structure of the complex

Figure 3. Chemical structure (corresponding to the crystal structure) of the complexes [Pd2(tmeda)2(L1)2](ClO4)4, 1b; [Pd2(tmeda)2(L2)2](BF4)4, 2c; and [Pd2(tmeda)2(L3)2](ClO4)4, 3b.

The single discrete natures of the complexes (1a−3a) could be claimed from the observation of single set of peaks in their 1 H NMR spectra. The complexes were characterized using 1H NMR, 13C NMR, H−H COSY, and C−H COSY techniques. The electrospray ionization mass spectrometry (ESI-MS) technique established the Pd2L′2L2 composition of the complexes 1b, 2b, and 3a. Single crystal X-ray diffraction data of 1b, 2c, and 3b unambiguously supported the structure of the complexes. Particularly, the helicate nature and the magnitude of helicity are claimed by carefully analyzing the crystal structures. As mentioned above, the 1H NMR spectra (DMSO-d6) of the complexes indicated the formation of single discrete compounds in each case, and the peak patterns are compared with the corresponding free ligands as shown in Figure 4. The signals due to the imidazolyl protons Ha, Hb, and Hc (see C

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Figure 5. Crystal structures of the complexed cation of 1b displayed in suitable views to decipher the “helicate” from plateau architecture (M-isomer is shown): (a) top view and (b) side view. (c) Diagram showing molecular packing of the complexed cations of 1b (a poly-M column is shown here; neighboring poly-P columns and overall packing is shown in Supporting Information).

Figure 6. Crystal structures of the complexed cation of 2c displayed in suitable views to decipher the “helicate” from plateau architecture (M-isomer is shown): (a) top view and (b) side view. (c) Diagram showing molecular packing of the complexed cations of 2c (a poly-MP column is shown here; overall packing is shown in Supporting Information).

H2O (1:1). The complex crystallized in the monoclinic crystal system with C2/c space group. The asymmetric unit consists of half of the molecule of 1b. The complexed cation [Pd2(tmeda)2(L1)2]4+ is composed of two square planar metal centers that are cis-protected by tmeda units. The metal centers are connected by two strands of the nonchelating ligand moieties and that are disposed in a helical manner with a Pd----Pd nonbonded distance of 12.661 Å. The relative positions of the two PdN4 square planes and the spatial arrangement of the ligand units could be best described in terms of a “helicate” from plateau type architecture (see Figure 5) where the degree of helicity θ4 is around 120°. However, the

3b prepared from the longer ligand L3 did not exhibit the signature of a helicate (Figure 7), its architecture was found to be uncommon compared to the usual Pd2L′2L2 designs. Interestingly, the magnitude of helicity θ4 in our Pd2L′2L2 systems was found to be modulated by the length of the spacer crafted in the backbone of the ligands L1 and L2. To our gratification, coveted helical architectures could be introduced for the first time to the family of cis-Pd2L′2L2 type compounds. Relevant details of the structures are represented hereafter. Crystal structure of [Pd2(tmeda)2(L1)2](ClO4)4, 1b: Single crystals of 1b suitable for X-ray diffraction studies were obtained by slow evaporation of a solution of 1b in CH3CND

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Figure 7. Crystal structures of the complexed cation of 3b displayed in suitable views to decipher the “non-helicate” (zig-zag plateau) architecture with axial displacement (a) top view and (b) side view. (c) Diagram showing molecular packing of the complexed cations of 3b (a columnar arrangement is shown here, overall packing is shown in Supporting Information).

also formed. Each column (say poly-M column) is surrounded by six other parallel columns, two of its own type (poly-M) and four of the other type (poly-P). However, there is no direct contact between the neighboring columns. The packing diagram is shown in the Supporting Information (Figure S27), and a poly-M column is shown in Figure 5c. Crystal structure of [Pd2(tmeda)2(L2)2](BF4)4·1.5 dioxane, 2c·1.5 dioxane: Single crystals of 2c suitable for X-ray diffraction studies were obtained by slow diffusion of dioxane into a solution of 2c in CH3CN-H2O (1:1). The complex was crystallized in the triclinic crystal system with P1̅ space group. The asymmetric unit consists of one molecule of 2c and one and half molecules of dioxane. The overall composition of the complexed cation [Pd2(tmeda)2(L2)2]4+ is comparable with that of 1b where the metal centers are connected by two strands of the nonchelating ligand moieties and that are disposed in a helical manner with a Pd----Pd nonbonded distance of 16.599 Å. Although the axial displacement θ1 can be neglected for this structure, one of the PdN4 square planes is radially displaced (θ2) by an angle of around 10°, with respect to the other forming a bowlike arrangement. However, the relative positions of the two PdN4 square planes and the spatial arrangement of the ligand units could be best described in terms of a “helicate” from bow type architecture (see Figure 6) where the degree of helicity θ4 is around 170°. Both left-handed (M) and right-handed (P) helicates are present in the crystal lattice and presented in Figure 8. The packing of molecules (2c) in the crystal revealed the presence of equal number of left-handed (M) and right-handed (P) helicates. To understand the intermolecular interactions, packing of the molecules in the crystal structure was analyzed. Both of the 4,4′-biphenyl spacer moiety of a given molecule exhibit intermolecular π−π stacking by utilizing the biphenyl spacer of two different neighboring molecules where the M isomer stacks with P and vice versa generating a columnar packing to form a poly-MP column (intermolecular π−π stacking distance of 3.64 Å). Each column is surrounded by six other parallel columns. However, there is no direct contact

axial or radial displacement could be neglected for this structure. Both left-handed (M) and right-handed (P) helicates are, however, present in the crystal lattice and presented in Figure 8.

Figure 8. Crystal structures showing left handed (M) and right handed (P) helicates of the complexed cations of 1b and 2c shown in pairs (from left to right).

We are involved in the design of self-assembled coordination complexes keeping in mind the possibility of exotic intermolecular interactions in the solid state.22,40−43 In that context, packing of the molecules in the crystal structure of 1b was analyzed so as to understand the diversity of intermolecular interactions. Preliminary analysis revealed the presence of equal number of left-handed (M) and right-handed (P) helicates in the packing. Both of the 1,4-benzene spacer moiety of a given molecule exhibit intermolecular π−π stacking by utilizing the 1,4-benzene spacer of two different neighboring molecules of same handedness (i.e., M with M only and P with P only). The arrangement is such that a given molecule (say M isomer) got two neighbors, one above and the other below stacked in a columnar fashion to form a poly-M column (intermolecular π−π stacking distance of 3.66 Å); similarly, poly-P columns are E

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As described in the previous section, the torsion angle between the PdN4 planes in a given complex is defined as θ4 and considered as the magnitude of helicity. Accordingly, the calculated magnitudes of helicity for 1b and 2c are 120 and 170°, respectively. Interestingly, a higher torsion angle θ5 defined by N−Cben----Cben−N of a bound ligand unit also corresponded to higher magnitude of helicity as measured in terms of θ4. In addition to the torsion angles defined by N−Cben----Cben− N of a bound ligand, one other parameter, i.e., the extent of rotation of imidazolyl rings around the N − Cben bonds also has the potential to preserve or upset the formation of helicates in this design. For the bound ligands L1 and L2, the imidazolyl planes are faced toward the center of the ligand moiety, whereas one of the imidazolyl planes in the bound ligand L3 is disposed away from the center due to rotation around the corresponding N−Cben bond. The average torsion angle θ5 (N−Cben----Cben−N) for bound ligand L3 (in 3b) is 60°, that is probably favorable for helicate formation. However, the rotation of one of the imidazolyl moieties, around the N− Cben bond is actually responsible for the nonhelical structure of 3b. In summary, helicates of cis-Pd2L′2L2 compositions are prepared for the first time, and their design principles are deciphered. It is expected that systematic variations in the overall design of the type of complexes studied in this work should afford additional examples of such helicates and allow us to understand the phenomenon in more intricate manner. Nevertheless, this work has opened a new avenue in our research endeavors, and the related aspects are being pursued.

between the neighboring columns. The packing diagram is shown in the Supporting Information (Figure S28) and a polyMP column is shown in Figure 6c. Crystal structure of [Pd2(tmeda)2(L3)2](ClO4)4·2 DCM, 3b· 2 DCM: Single crystals of 3b suitable for X-ray diffraction studies were obtained by slow diffusion of tert-butanol into a solution of 3b in DCM. The complex was crystallized in triclinic crystal system with P1̅ space group. The asymmetric unit consists of half of the molecule of 3b and two DCM molecules. The overall composition of the complexed cation [Pd2(tmeda)2(L3)2]4+ is comparable with that of 1b and 2c. However, the metal centers are connected by two strands of the nonchelating ligand moieties and that are disposed in a nonhelical manner with a Pd----Pd nonbonded distance of 20.685 Å. The relative positions of the two PdN4 square planes and the spatial arrangement of the ligand units could be best described in terms of a “step from zig-zag plateau” type architecture (see Figure 7). The displacement of one of the PdN4 square planes in reference to the other is axial (θ1) and also parallel (θ3) by around 20 and 40°, respectively. The interplanar distance between two PdN4 planes of a given molecule, generated due to the formal axial displacement, is 5.5 Å. In the crystal packing one molecule is stacked with other neighboring molecule by π−π stacking interactions using the 4,4′-p-terphenyl units, and the distance between the two stacked units is 3.42 Å. The packing diagram is shown in the Supporting Information (Figure S29) and a columnar packing is shown in Figure 7c. Helicity modulation: The relative positions of the terminal 1H-imidazolyl moieties of the bound ligands (L1, L2, and L3) as observed in the crystal structures of the complexes 1b, 2c, and 3b (Figure 9) were analyzed. A given benzyl carbon (Cben)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01445. Experimental details, schemes and figures related to the synthesis and characterization of the complexes, and further details of the single crystal XRD data (PDF) Accession Codes

CCDC 1471415−1471417 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Figure 9. Relative positions of the terminal imidazolyl moieties of bound ligand strands in L1, L2, and L3 as observed in the crystal structures of 1b, 2c, and 3b, respectively (from left to right).

AUTHOR INFORMATION

Corresponding Author

*Tel: +91-4422574224. Fax: +91-4422574202. E-mail: dillip@ iitm.ac.in.

is directly bonded to a noncoordinating imidazolyl nitrogen (N) and a quaternary carbon atom (Cq) of the spacer unit. The relative positions of the imidazolyl units are viewed along the imaginary axis (Cben−Cq----Cq−Cben) created by joining the almost collinear pair Cben−Cq bonds as shown in Figure 9. The torsion angles θ5 defined by N−Cben----Cben−N arrangement are calculated for the bound ligand moieties. The magnitude of torsion angle θ5 corresponding to the average of both strands of bound L1 (in helical 1b) and bound L2 (in helical 2c) are approximately 65 and 100°, respectively; the same for the bound L3 (in nonhelical 3b) is 60°.

ORCID

Dillip K. Chand: 0000-0003-1115-0138 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.K.C. thanks the SERB, Department of Science and Technology, Government of India (Project No.SB/S1/IC-05/ 2014) for financial support. F

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(36) Sahoo, H. S.; Chand, D. K. Dalton Trans. 2010, 39, 7223−7225. (37) Tripathy, D.; Pal, A. K.; Hanan, G. S.; Chand, D. K. Dalton Trans. 2012, 41, 11273−11275. (38) Kishi, N.; Li, Z.; Yoza, K.; Akita, M.; Yoshizawa, M. J. Am. Chem. Soc. 2011, 133, 11438−11441. (39) Zhou, L.-P.; Sun, Q.-F. Chem. Commun. 2015, 51, 16767− 16770. (40) Naranthatta, M. C.; Das, D.; Tripathy, D.; Sahoo, H. S.; Ramkumar, V.; Chand, D. K. Cryst. Growth Des. 2012, 12, 6012−6022. (41) Naranthatta, M. C.; Ramkumar, V.; Chand, D. K. J. Chem. Sci. 2014, 126, 1493−1499. (42) Naranthatta, M. C.; Ramkumar, V.; Chand, D. K. J. Chem. Sci. 2015, 127, 273−280. (43) Dasary, H.; Jagan, R.; Chand, D. K. Chem. - Eur. J. 2015, 21, 1499−1507.

DEDICATION This communication is dedicated to Prof. N. Chandrakumar on the occasion of his 65th birthday.

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DOI: 10.1021/acs.cgd.6b01445 Cryst. Growth Des. XXXX, XXX, XXX−XXX