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Palladium(II) driven self-assembly of a saturated quadruple-stranded metallo helicate† Debakanta Tripathy,a,b Amlan K. Pal,b Garry S. Hanan*b and Dillip Kumar Chand*a Received 29th April 2012, Accepted 27th July 2012 DOI: 10.1039/c2dt30937h

Complexation of the bridging bidentate ligand N,N′-( pyridine-2,6-diyl)dinicotinamide, L with palladium(II) resulted in a single discrete M2L4 self-assembly, 1, in a quantitative manner. The entropically-controlled assembly of 1 resulted in a rare saturated, quadruple-stranded metallo-helicate, in which both the left-handed (M) and right-handed (P) helicates exist in the crystal structure. Metal-driven self-assembly is a fascinating as well as an efficient tool for the synthesis of bigger molecular architectures by combining simple and easily accessible building blocks.1–5 Complexation of a metal salt or a partially protected metal salt with a designed ligand is a typical strategy to obtain the intended final assembly. The geometry of the resulting self-assembly depends upon the metal, the chosen ligand and depends equally on the reaction conditions applied. The interest in helicates has been in their use as structural components of functional materials as well as their inherent chirality.5,6 There are many reports on single, double and triple stranded metallo-helicates in the literature5–14 whereas reports on quadruple-stranded helicates are limited in number.15–22 All the reported quadruple-stranded metallo-helicates can be considered broadly under M2L4 family, however, all reported M2L4 compounds need not be considered as helicates when strictly envisioned by definitions incorporating the azimuthal angle θ. As can be seen in Fig. 1, the azimuthal angle23 θ, should be non zero in order to qualify a M2L4 structure in the helicate family. Metals like Cu(II),15 Fe(II),16 Ag(I),17 Th(IV)18 and Pd(II)19–22 form quadruple-stranded helicates of M2L4 composition with selected non-chelating bidentate ligand. Also, in some cases the helicates are saturated, in which the metal coordination sphere contains only the desired ligand,5 and in other they are unsaturated and contain additional solvent or counterions. There are only four examples of Pd2L4 helicates to the best of our knowledge where the azimuthal angle is either 45 degree19,21 or 90 degree22 and in another case it is a di-M2L4 catenated cage20 with a smaller azimuthal angle of the individual M2L4. a

Department of Chemistry, IIT Madras, Chennai, India. E-mail: [email protected]; Fax: +914422574202; Tel: +914422574224 b Department of Chemistry, University of Montreal, Montreal, Canada. E-mail: [email protected]; Fax: +1 514-343-7586; Tel: +1 514-340-5156 † Electronic supplementary information (ESI) available: Details of synthesis and characterizations of L and 1 and related data. CCDC 868361. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2dt30937h

This journal is © The Royal Society of Chemistry 2012

Fig. 1 Diagram showing the definition of azimuthal angle θ in M2L4 type cages. Only one of the four ligands is depicted for clarity. From left to right: Cage with θ = 0, right handed (P) and left handed (M) helicates with a definite azimuthal angle (θ > 0).

Chart 1 Selected conformers of L suitable for M2L4 cage formation upon complexation with palladium(II).

Recently the design and synthesis of a saturated quadruplestranded helicate was reported with a palladium(II) and a bridging bidentate ligand having a piperazine spacer.21 Here we report the synthesis and complete characterization of a quadruple-stranded helicate [Pd2(L)4](NO3)4,‡ 1 obtained from palladium(II) and a bridging bidentate ligand N,N′-( pyridine-2,6-diyl)dinicotinamide, L functionalized with an amide linkage. Both the left and right handed helicates are observed in the crystal structure of the complex, however, the two enantiomeric forms are most likely equilibrating in solution. We used the bis-pyridyl-bis-amide type ligand N,N′-( pyridine2,6-diyl)dinicotinamide, L for complexation with Pd(II) ion. The ligand L is appended with 3-pyridyl groups at both ends and the spacer backbone is decorated with amide functionality. Conformational flexibility of a similar type of ligand N,N′-(1,3-phenylene)dinicotinamide, L′ has been discussed in the literature.24,25 On the basis of the conformational analysis of the ligand L′ it is conceivable to propose the possible conformers of the chosen ligand L. Thus ten conformers are possible for the ligand out of which three (A, B and C) are shown here (Chart 1). The choice Dalton Trans., 2012, 41, 11273–11275 | 11273


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Fig. 2 1H NMR spectra (DMSO-d6, RT) for (i) ligand L and (ii) the helicate [Pd2(L)4](NO3)4, 1 at 5 mM concentration with respect to metal.

of these three conformers is attributed to the presence of parallel co-ordination vectors which is the required orientation of binding sites to result the expected and entropically favored M2L4 type cage. However, conformer C is found in the crystal structure of the complex [Pd2(L)4](NO3)4, 1. Complex 1 is remarkably a saturated quadruple-stranded helicate with 45 degree azimuthal angle. Ligand L was prepared by the condensation of 2,6-pyridinedicarbonyl chloride hydrochloride with 3-aminopyridine following the literature procedure.26 Reaction of the ligand L with Pd(NO3)2 at a ratio of 2 : 1 in DMSO or DMF at room temperature gave rise to a yellow solution. Finally, addition of excess EtOAc (DCM can also be used) gave a pale yellow precipitate which was separated by filtration, washed several times with EtOAc and dried under vacuum to obtain complex 1 as a pale yellow solid. The complex was also prepared in DMSO-d6 to check the identity of the product before isolation which is discussed below. Complex 1 is characterized by recording and analyzing 1H and 13C NMR spectra. All the protons are further completely and un-ambiguously assigned by H–H COSY and HMQC. The 1H NMR spectrum of 1 recorded at ∼5 mM concentration with respect to palladium (Fig. 2) shows a single set of sharp peaks which is an indication of the formation of a single and discrete species in the solution state. The simple pattern of the spectrum suggests the symmetric nature of the complex in the solution state. 13C NMR is also in agreement with the 1H NMR for the highly symmetric nature of the complex. The 1H NMR spectrum of the complex is compared with that of the ligand. The signal corresponding to the pyridine α-proton (Hb) is found to be downfield shifted (Δδ = +0.6 ppm) providing the signature of metal–ligand interaction.27,28 The other pyridine α-proton (Ha) is almost unchanged instead of the usual down-field shift. This is probably due to the spatial orientation of the proton Ha in the complex where a diamagnetic ring current is experienced by the proton from the terminal pyridine ring of one of the adjacent ligand moieties, albeit with overlapping Ha and Hb signals. The pyridine γ-proton (Hd) is also shifted down-field by 0.4 ppm. The signal for the pyridine β-proton (Hc) is as expected marginally shifted down-field whereas the signal for the amide proton is shifted down-field by Δδ = +0.4 ppm. A low-field shift of N–H 11274 | Dalton Trans., 2012, 41, 11273–11275

protons is probably due to hydrogen bonding with anions and solvent molecules. Complexation of L with Pd(II) was performed in situ in DMSO-d6 by taking a ∼5 mM solution of Pd(II) and the required amount of the ligand. A single and discrete compound was formed quantitatively as observed from the proton NMR spectrum. The pattern and position of the peaks observed for the isolated complex and in situ prepared complex are comparable. It is observed that the peak due to Ha is slightly shifted down-field when NMR is recorded at a concentration higher than 5 mM (recorded in the range of 10–30 mM) separating the signals of Ha and Hb whereas all other peaks are unchanged (see ESI†). This observation indicates a concentration dependent intermolecular interaction where the environment of Ha is somewhat influenced. Further characterization of the isolated complex 1 in solution state has been carried out by 1D NOESY and ROESY NMR (see ESI†). The existence of a single species in the solution state was further confirmed by DOSY NMR. The ratio of the diffusion coefficient for the ligand L to complex 1 is calculated as 1 : 0.4 (see ESI†). The size of the molecule was calculated from the diffusion co-efficient using Stokes–Einstein equation which was found to be approximately 9 Å in diameter. The formation of a single discrete complex as understood from NMR spectral data motivated us to find out whether or not the expected entropic M2L4 composition of the assembly is formed. High resolution ESI MS of the complex 1 showed signals at m/z 807.10547 and 516.74209 which were assigned to [1-(NO3)2]2+ and [1-(NO3)3]3+ in agreement with the calculated values of 807.10460 and 516.74028, respectively. The expansion of these envelopes of peaks showed a good agreement between the calculated and observed isotopic distribution for the above mentioned fragments. These signals indicate the formation of a complex with M2L4 composition (see ESI†). X-ray quality single crystals were obtained within a few days by slow diffusion of DCM into the DMSO solution of the complex at room temperature. The molecule crystallizes in P1ˉ space group as seen from crystallographic analysis, where four ligands are coordinated to two palladium(II) centers in bridging manner to constitute a binuclear M2L4 cage. All the four ligand units are disposed in a helical fashion subtending an azimuthal angle (Fig. 3) approximately equals to 45 degree with respect to

Fig. 3 Perspective views of the molecular structure of 1 (i) top view along helix axis showing azimuthal angle and also disposition of the ligands around metals in a helical manner, (ii) side view of M isomer showing encapsulated nitrates. Hydrogen atoms and solvent molecules are omitted for clarity.


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Notes and references

Fig. 4 Space filling model for complex 1, showing both left handed (M) and right handed (P) helicates. Hydrogen atoms, counter ions and solvent molecules are omitted for clarity.

the helix axis. Each palladium(II) unit forms a square plane with four terminal pyridines, one from each ligand. The coordinated pyridine rings are not perpendicular to the PdN4 plane and are disposed in a propeller shape. The molecule possesses pseudo D4 geometry, the C4 axis passes through both palladium(II) atoms and constitute the helix axis, and thus represents an example of homotopic, saturated quadruple-stranded, helicate.19–22 Fig. 3 shows the perspective views of the helix and helical disposition of the ligands. The asymmetric unit contains four molecules i.e. two pairs of both the enantiomers (M and P) of opposite twist (Fig. 4). Three nitrate molecules are encapsulated in the cavity. One of the encapsulated nitrates has a short contact with a palladium(II) center (2.97 Å). The remaining two nitrate ions are hydrogen bonded with the amide groups (2.86–3.06 Å). Ligand L is expected to display conformational dependent ligating topologies due to the meta-substitution in the pyridyl groups. The electronic repulsion resulted from the lone pair electrons present on nitrogen atom of the middle pyridine ring and the lone pair electrons located on oxygen atom of the amide carbonyl group can be one of the reasons for L to adopt this conformation. DFT calculations of conformers A, B and C gave energies of −1707.34, −1721.68 and −1738.82 Hartrees, respectively. Furthermore, some of the N–H protons are either H-bonded or in a short contact range with the encapsulated counter anions as favoured by the convergent orientation of the NH groups in conformation C. Complexation of N,N′-(1,3-phenylene)dinicotinamide, L′ with Pd(II) is reported in literature24 where the resulting assembly 1′ is M2L4 in composition, however, not a helicate. The ligand is found in conformer B and coordinated pyridine rings are perpendicular to PdN4 plane in 1′ unlike the existence of conformer C and propeller shaped disposition of the pyridine rings in 1. In conclusion, we have synthesized a saturated quadruplestranded amide functionalized helicate. The helicate is completely characterized by 1H, 13C and other 2D NMR experiments. The M2L4 composition of the molecule has been established by high-resolution ESI MS. Finally, the molecular structure of the helicate is established by single crystal X-ray diffraction technique. Currently we are trying to explore the effect of ligand topology on helicate formation and recognition of guest molecules by the cage for further application in host guest chemistry.


‡ [Pd2(L)4](NO3)4, 1: 1H NMR(400 MHz, DMSO-d6) δ 11.60 (s, 2H, N–H), 9.10 (m, 4H, Ha, Hb), 8.86 (d, 2H, J = 5.2 Hz, Hd), 8.34 (d, 2H, J = 7.2 Hz, He), 8.26 (dd, 1H, J = 6.6, 8.6 Hz, Hf ), 7.85 (dd, 2H, J = 6.0, 9.2 Hz, Hc). L: 1H NMR (300 MHz, DMSO-d6) δ 11.13 (s, 2H, N–H), 9.10 (d, 2H, J = 3.2 Hz, Ha), 8.45–8.40 (m, 4H, Hb, He), 8.36–8.31 (m, 3H, Hd, Hf ), 7.49 (dd, 2H, J = 11.2, 6.4 Hz, Hc). Crystal data for 1: 4(C68H52N20O8Pd2)·10(C2H6OS)·14(NO3)· 3(H2O), M = 7663.85, triclinic, a = 24.8177(8) Å, b = 29.4140(10) Å, c = 31.3524(11) Å, α = 67.883(2)°, β = 89.927(2)°, γ = 82.500(2)°, V = 20 992.8(12) Å3, T = 150(2) K, space group P1ˉ , Z = 4, 267 699 reflections measured, 79 363 independent reflections (Rint = 0.0709). The final R1 values were 0.0898 (I > 2σ(I)). The final wR(F2) values were 0.2519 (I > 2σ(I)). The final R1 values were 0.1224 (all data). The final wR(F2) values were 0.2716 (all data). (CCDC 868361). 1 J. W. Steed and J. L. Atwood, Supramolecular Chemistry, John Wiley and Sons, Ltd, 2nd edn, 2009. 2 M. Fujita, M. Tominaga, A. Hori and B. Therrien, Acc. Chem. Res., 2005, 38, 369–380. 3 R. Chakrabarty, P. S. Mukherjee and P. J. Stang, Chem. Rev., 2011, 111, 6810–6918. 4 N. B. Debata, D. Tripathy and D. K. Chand, Coord. Chem. Rev., 2012, 256, 1831–1945. 5 C. Piguet, G. Bernardinelli and G. Hopfgartner, Chem. Rev., 1997, 97, 2005–2062. 6 G. I. Pascu, A. C. G. Hotze, C. Sanchez-Cano, B. M. Kariuki and M. J. Hannon, Angew. Chem., Int. Ed., 2007, 46, 4374–4378. 7 G. C. Van Stein, G. Van Koten, K. Vieze, C. Brevard and A. L. Speck, J. Am. Chem. Soc., 1984, 106, 4486–4492. 8 A. E. Rowan and R. J. M. Nolte, Angew. Chem., Int. Ed., 1998, 37, 63–68. 9 J. Gregoliński and J. Lisowski, Angew. Chem., Int. Ed., 2006, 45, 6122–6126. 10 S. G. Telfer, T. Sato and R. Kuroda, Angew. Chem., Int. Ed., 2004, 43, 581–584. 11 J.-M. Lehn, A. Rigault, J. Siegel, J. Harrowfield, B. Chevrier and D. Moras, Proc. Natl. Acad. Sci. U. S. A., 1987, 84, 2565–2569. 12 J. Hamacek, S. Blanc, M. Elhabiri, E. Leize, A. V. Dorsselaer, C. Piguet and A. M. Albrecht-Gary, J. Am. Chem. Soc., 2003, 125, 1541–1550. 13 R. C. Scarrow, D. L. White and K. N. Raymond, J. Am. Chem. Soc., 1985, 107, 6540–6546. 14 M. Albrecht, Chem. Soc. Rev., 1998, 27, 281–288 and references therein. 15 F. Pan, J. Wu, H. Hou and Y. Fan, Cryst. Growth Des., 2010, 10, 3835–3837. 16 H. A. Burkill, R. Vilar, A. J. P. White and D. J. Williams, J. Chem. Soc., Dalton Trans., 2002, 837–839. 17 P. N. W. Baxter, J.-M. Lehn, G. Baum and D. Fenske, Chem.–Eur. J., 2000, 6, 4510–4517. 18 J. Xu and K. N. Raymond, Angew. Chem., Int. Ed., 2006, 45, 6480–6485. 19 D. A. McMorran and P. J. Steel, Angew. Chem., Int. Ed., 1998, 37, 3295–3297. 20 M. Fukuda, R. Sekiya and R. Kuroda, Angew. Chem., Int. Ed., 2008, 47, 706–710. 21 H. S. Sahoo and D. K. Chand, Dalton Trans., 2010, 39, 7223–7225. 22 S. Ø. Scott, E. L. Gavey, S. J. Lind, K. C. Gordon and J. D. Crowley, Dalton Trans., 2011, 40, 12117–12124. 23 For the definition of azimuthal angle see ref. 19. 24 N. L. S. Yue, D. J. Eisler, M. C. Jennings and R. J. Puddephatt, Inorg. Chem., 2004, 43, 7671–7681. 25 N. N. Adarsh, D. K. Kumar and P. Dastidar, Inorg. Chem. Commun., 2008, 11, 636–642. 26 A. J. Baer, B. D. Koivisto, A. P. Coté, N. J. Taylor, G. S. Hanan, H. Nierengarten and A. V. Dorsselaer, Inorg. Chem., 2002, 41, 4987–4989. 27 D. K. Chand, K. Biradha, M. Kawano, S. Sakamoto, K. Yamaguchi and M. Fujita, Chem.–Asian. J., 2006, 1–2, 82–90. 28 D. K. Chand, K. Biradha and M. Fujita, Chem. Commun., 2001, 1652–1653.

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