Self-Assembly of a Pd4L8 Double-Walled Square Takes Place

6 days ago - However, the self-assembly of a Pd4L8 DWS from rigid ditopic ligands took place with the formation of two kinds of metastable species ...
0 downloads 0 Views 1MB Size
Download PDF
Communication pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Self-Assembly of a Pd4L8 Double-Walled Square Takes Place through Two Kinds of Metastable Species Shumpei Kai, Tomoki Tateishi, Tatsuo Kojima, Satoshi Takahashi, and Shuichi Hiraoka* Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 10/20/18. For personal use only.

S Supporting Information *

ABSTRACT: The self-assembly process of a Pd4L8 double-walled square (DWS) was investigated. As was seen in Pd2L4 cages, Pd(II)-linked coordination selfassembly processes are generally affected by the rigidity of multitopic ligands. However, the self-assembly of a Pd4L8 DWS from rigid ditopic ligands took place with the formation of two kinds of metastable species [submicrometer-sized species and a Pd3L6 double-walled triangle (DWT)]. This result suggests that the self-assembly process of the DWS is largely affected by the geometry of the final product and not by that of the ditopic ligand.

S

pontaneous ordering of building blocks in molecular selfassembly is a mystic phenomenon in nature1 and one of the fundamental principles in life systems.2 “How do molecular building blocks recognize each other, bind together one by one in a proper way, and rearrange building blocks even if they assemble in an incorrect manner?” is still a question in modern molecular science. We have recently developed a method for the investigation of molecular self-assembly processes (QASAP: quantitative analysis of the self-assembly process)3 and applied this to Pd(II)- and Pt(II)-linked discrete coordination assemblies.4 One of the general trends in coordination selfassembly processes revealed by QASAP is that flexible multitopic ligands tend to produce submicrometer-sized intermediates during the self-assembly,5a while self-assemblies from rigid multitopic ligands take place faster not through such large species.5b This tendency is mainly because flexible ligands connect metal ions in an incorrect manner to kinetically produce divergent structures. The self-assembly process of the Pd418 double-walled square (DWS) from flexible ditopic ligands 1 is one of the examples (Figure 1).5c In the beginning of the selfassembly, the kinetic production of a Pd316 double-walled triangle (DWT) and 100-nm-sized large intermediates (IntL) is accompanied by formation of the Pd418 DWS. Thus, we wonder if the self-assembly of a Pd4L8 DWS from rigid ditopic ligands takes place without the formation of such large kinetic species as those seen in the self-assembly of a Pd3L6 DWT from rigid ditopic ligands.6 To investigate the effect of the ligand structure on the self-assembly process of the Pd4L8 DWS, the ditopic ligand 24d was chosen. Most of the Pd4L8 DWSs reported so far are selectively produced in the presence of template molecules.4f,h On the other hand, the DWSs from ditopic ligand 1 or 2 are the only examples where DWS is assembled without the help of template molecules.4d,e Therefore, ligands 1 and 2 are © XXXX American Chemical Society

Figure 1. (a) Chemical structures of ditopic ligands to produce Pd4L8 DWSs. (b) Schematic representation of the self-assembly process of Pd4L8 DWSs from PdPy*4 (Py* indicates 3-chloropyridine).

the best to purely evaluate the effect of the structure of the ligand. Here we report the self-assembly process of a Pd428 DWS4d revealed by QASAP (Figure 1). The conformational freedom in the ditopic ligand 2 is restricted rather than 1. Nevertheless, the self-assembly of the Pd428 DWS took place with transient formation of the DWT and large species. This result suggests that the self-assembly process of the Pd428 DWS is determined by the geometry of the final product and not significantly affected by the rigidity of the ditopic ligand. The self-assembly of the Pd428 DWS from PdPy*4(BF4)2 (Py* = 3-chloropyridine) and ditopic ligands 2 in CD3NO2 and CD2Cl2 (10:1, v/v) at 298 K was monitored by 1H NMR spectroscopy (Figures 2 and S1). Right after the self-assembly started, most of the substrates (2 and PdPy*4) were consumed and many weak 1H NMR signals appeared. These signals soon disappeared, and highly symmetrical signals (colored in red and blue in Figure 2) appeared. 1H NMR signals colored in blue are assigned to the Pd428 DWS by comparing them with the 1H NMR spectrum of the authentic sample of the Pd428 DWS. Indeed, these signals became dominant after convergence of the self-assembly. Considering that the Pd316 DWT is transiently produced during the self-assembly of the Pd418 DWS,5c 1H NMR signals colored in red, which finally disappeared after the convergence, suggest the formation of a metastable Pd326 DWT. These signals were characterized by H−H COSY (Figure S4) and 1H DOSY (Figure S5) measurements, and mass signals for Received: August 31, 2018

A

DOI: 10.1021/acs.inorgchem.8b02470 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Pd418 and Pd428 DWSs under almost the same conditions were compared (Figures S2 and S3 and Tables S6−S10). The Pd326 DWT, which was produced in about 25% in the beginning of the self-assembly (1 h), was stable for 1 day and slowly converted into the Pd428 DWS. Because submicrometer-sized large species are produced in the self-assembly of the Pd418 DWS,5c timedependent dynamic light scattering (DLS) measurement for the self-assembly of the Pd428 DWS was carried out to clarify whether such large species were produced during the selfassembly of the Pd428 DWS (Figure 4). At 10 min, 100-nm-sized

Figure 2. 1H NMR spectra [500 MHz, aromatic region, CD3NO2 and CD2Cl2 (10:1, v/v), 298 K] for the self-assembly of the Pd428 DWS from PdPy*4(BF4)2 and 2 ([2]0 = 1.5 mM) in CD3NO2 and CD2Cl2 (10:1, v/v) at 298 K. Figure 4. Time-dependent DLS data for the self-assembly of the Pd428 DWS from PdPy*4(BF4)2 and 2 ([2]0 = 1.5 mM) in CD3NO2 and CD2Cl2 (10:1, v/v) at 298 K.

the Pd326 DWT were detected by electrospray ionization timeof-flight mass spectrometry (Figure S6). Molecular modeling of the Pd326 DWT indicates slight distortion of the ditopic ligands 2 (Figure S7). These results indicate that the smaller Pd3L6 DWT is transiently produced regardless of the flexibility of the ditopic ligands. Changes in the existence ratios of the substrates, products, and all intermediates except for Pd326 (Int, whose existence ratio was calculated based on the ditopic ligand 2) are shown in Figure 3a,b (Tables S1−S5). The self-assembly processes of the

species appeared, and the size of the species increased with time to reach 250 nm. These results indicate that the self-assembly processes of the Pd418 and Pd428 DWSs are similar in that two kinds of metastable species (Pd3L6 DWT and IntL) are produced during the self-assembly. To compare the composition of IntL produced from 1 and from 2, n−k analysis was carried out (Figure 5).3,7 As to the selfassembly of the Pd428 DWS, the ⟨n⟩ value increased with a decrease in the ⟨k⟩ value (blue broken line), while the ⟨n⟩ value increased with a constant ⟨k⟩ value of 0.5 in the self-assembly of the Pd418 DWS (green broken line). This difference is because

Figure 3. Existence ratios of the substrates, products, and all intermediates (Int) except for the Pd326 DWT for the self-assembly of the Pd428 DWS from PdPy*4(BF4)2 and 2 ([2]0 = 1.5 mM) in CD3NO2 and CD2Cl2 (10:1, v/v) at 298 K: (a) from 0 to 60 min; (b) from 0 min to 26 days. Existence ratios of Pd4L8, Pd3L6, and Py* for the self-assembly of the DWS (Pd418 or Pd428) from PdPy*4(BF4)2 and a ditopic ligand (1 or 2) ([1]0 = 1.5 mM; [2]0 = 1.5 mM) in CD3NO2 (for 1) or CD3NO2 and CD2Cl2 (10:1, v/v; for 2) at 298 K: (a) from 0 to 60 min; (b) from 0 to 26 days.

Figure 5. n−k plots for the self-assembly of the DWS (Pd418 or Pd428) from PdPy*4(BF4)2 and a ditopic ligand (1 or 2) ([1]0 = 1.5 mM; [2]0 = 1.5 mM) in CD3NO2 (for 1) or CD3NO2 and CD2Cl2 (10:1, v/v; for 2) at 298 K. Green and blue broken lines indicate changes in the ⟨n⟩ and ⟨k⟩ values for the self-assembly of Pd418 and Pd428. Red crosshairs indicate the n and k values of species PdaLbPy*c, which is indicated by (a, b, c) in the (n, k) map. B

DOI: 10.1021/acs.inorgchem.8b02470 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

■

free ditopic ligand 2 remained in the early stages of the selfassembly, while all of the substrates were consumed within 5 min in the self-assembly of the Pd418 DWS. After 45 min, when the ⟨k⟩ value is constant at 0.5, the ⟨n⟩ value of Int in the selfassembly of Pd428 is slightly higher than that in the self-assembly of Pd418. This suggests that IntL produced in the self-assembly of Pd418 contain more Py* than those of Pd428, which is probably due to less efficient bridging of Pd(II) ions in IntL by the ditopic ligand 1 with higher flexibility. The formation of the Pd4L8 DWS and the release of Py* are compared between the self-assemblies of Pd418 and Pd428 (Figure 3c,d). The release rate of Py* in the self-assembly of Pd428 is slightly faster than that of Pd418 (green solid and broken lines), indicating that preorganization of the ligand 2 prompts ligand exchange. Nevertheless, the formation of the Pd428 DWS from 2 took place as slowly as that observed in the self-assembly of the Pd418 DWS from the ligand 1. Thus, faster ligand exchanges with the ligand 2 led to the metastable species, but their conversion to the Pd4L8 DWS was slower because of the high kinetic stability of the metastable species composed of the ligand 2. The formation of the Pd3L6 DWT and Pd4L8 DWS from 2 (red and blue solid lines in Figure 3c) started earlier than that from 1 (red and blue broken lines in Figure 3c), which is explained by the faster intramolecular bridging of Pd(II) ions by the preorganized ligand 2. The yield of the Pd326 DWT (25%, red solid line) is lower than that of the Pd316 DWT (34%, red broken line), while the yield of the Pd428 DWS (blue solid line) is higher than that of the Pd418 DWS (blue broken line) until 24 h. This indicates that 2 kinetically inhibits the formation of the Pd326 DWT, which is consistent with the slight distortion of 2 in the modeling structure of the Pd326 DWT. The formation ratio of the Pd428 DWS is higher than that of the Pd418 DWS until 1 day, but the yield of the Pd418 DWS increased faster than that of the Pd428 DWS after 1 day. This result suggests that IntL composed of 2 are kinetically stabilized, so their conversion into the Pd428 DWS becomes slower, and that some of IntL were kinetically trapped after convergence of the self-assembly. These differences between the two DWSs can be interpreted by differences in the rigidity of the ditopic ligands 1 and 2, but the contribution of other factors such as a change in the bridging unit that connects the two 4-pyridyl groups and the donor ability of the pyridyl groups is nonnegligible. However, it is worth emphasizing that the self-assembly process of the DWSs is not affected by the structure of the ditopic ligands, which is different from the self-assembly processes of other PdnL2n structures. In conclusion, in the self-assembly of the Pd4L8 DWSs from a ditopic ligand 1 or 2, submicrometer-sized species and metastable Pd3L6 DWTs are transiently produced. This result is different from the self-assembly processes of the Pd2L4 cage and Pd3L6 DWT, where flexible ditopic ligands tend to produce the final assemblies with the transient formation of large metastable species, while rigid ditopic ligands do not produce such species. Therefore, the metastable species (DWT and submicrometer-sized intermediates) are produced during the self-assembly of the Pd418 and Pd428 DWSs regardless of the rigidity of the ditopic ligand. In other words, the self-assembly processes of the two DWSs are largely affected by the geometry of the final product and not by the ligand structure. This finding is one of the keys to bridging the gap between the kinetic and thermodynamic phenomena in molecular self-assembly.

Communication

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02470. Experimental procedures, further figures and tables, and NMR and MS data (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shuichi Hiraoka: 0000-0002-9262-4747 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by JSPS Grants-in-Aid for Scientific Research on Innovative Areas “Dynamical Ordering of Biomolecular Systems for Creation of Integrated Functions” (Grants 25102001 and 25102005) and The Asahi Glass Foundation.

■

REFERENCES

(1) (a) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Molecular SelfAssembly and Nanochemistry: A Chemical Strategy for the Synthesis of Nanostructures. Science 1991, 254, 1312−1319. (b) Hartgerink, J. D.; Beniash, E.; Stupp, S. I. Self-Assembly and Mineralization of PeptideAmphiphile Nanofibers. Science 2001, 294, 1684−1688. (c) Whitesides, G. M.; Grzybowski, B. Self-Assembly at All Scales. Science 2002, 295, 2418−2421. (d) Whitesides, G. M.; Boncheva, M. Beyond Molecules: Self-Assembly of Mesoscopic and Macroscopic Components. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 4769−4774. (e) Zhang, S. Fabrication of Novel Biomaterials Through Molecular Self-Assembly. Nat. Biotechnol. 2003, 21, 1171−1178. (2) (a) Zhang, S. Emerging Biological Materials through Molecular Self-Assembly. Biotechnol. Adv. 2002, 20, 321−339. (b) Boncheva, M.; Whitesides, G. Making Things by Self-Assembly. MRS Bull. 2005, 30, 736−742. (c) Mendes, A. C.; Baran, E. T.; Reis, R. L.; Azevedo, H. S. Self-Assembly in Nature: Using the Principles of Nature to Create Complex Nanobiomaterials. WIREs Nanomed. Nanobiotechnol. 2013, 5, 582−612. (3) (a) Hiraoka, S. Unresolved Issues that Remain in Molecular SelfAssembly. Bull. Chem. Soc. Jpn. 2018, 91, 957−978. (b) Hiraoka, S. SelfAssembly Processes of Pd(II)- and Pt(II)-Linked Discrete SelfAssemblies Revealed by QASAP. Isr. J. Chem. DOI: 2018 DOI: 10.1002/ijch.201800073. (c) Hiraoka, S. What Do We Learn from the Molecular Self-Assembly Process? Chem. Rec. 2015, 15, 1144− 1147. (4) (a) Fujita, M.; Tominaga, M.; Hori, A.; Therrien, B. Coordination Assemblies from a Pd(II)-Cornered Square Complex. Acc. Chem. Res. 2005, 38, 369−378. (b) Northrop, B. H.; Zheng, Y.-R.; Chi, K.-W.; Stang, P. J. Self-Organization in Coordination-Driven Self-Assembly. Acc. Chem. Res. 2009, 42, 1554−1563. (c) Chakrabarty, R.; Mukherjee, P. S.; Stang, P. J. Supramolecular Coordination: Self-Assembly of Finite Two- and Three-Dimensional Ensembles. Chem. Rev. 2011, 111, 6810−6918. (d) Bloch, W. M.; Abe, Y.; Holstein, J. J.; Wandtke, C. M.; Dittrich, B.; Clever, G. H. Geometric Complementarity in Assembly and Guest Recognition of a Bent Heteroleptic cis-[Pd2LA2LB2] Coordination Cage. J. Am. Chem. Soc. 2016, 138, 13750−13755. (e) Chand, D. K.; Fujita, M.; Biradha, K.; Sakamoto, S.; Yamaguchi, K. Metal Driven Self-Assembly of Pyridine Appended Ligands with cisProtected/Naked Pd(II) Ion: A Comparative Study. Dalton Trans 2003, 2750−2756. (f) Suzuki, K.; Kawano, M.; Fujita, M. SolvatoControlled Assembly of Pd3L6 and Pd4L8 Coordination Boxes. Angew. C

DOI: 10.1021/acs.inorgchem.8b02470 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

process can be discussed by the time development of the (⟨n⟩, ⟨k⟩) value3.

Chem., Int. Ed. 2007, 46, 2819−2822. (g) Klein, C.; Gütz, C.; Bogner, M.; Topić, F.; Rissanen, K.; Lützen, A. A New Structural Motif for an Enantiomerically Pure Metallosupramolecular Pd4L8 Aggregate by Anion Templating. Angew. Chem., Int. Ed. 2014, 53, 3739−3742. (h) Zhang, T.; Zhou, L.-P.; Guo, X.-Q.; Cai, L.-X.; Sun, Q.-F. Adaptive Self-Assembly and Induced-Fit Transformations of Anion-Binding Metal-Organic Macrocycles. Nat. Commun. 2017, 8, 15898. (i) McMorran, D. A.; Steel, P. J. The First Coordinatively Saturated, Quadruply Stranded Helicate and Its Encapsulation of a Hexafluorophosphate Anion. Angew. Chem., Int. Ed. 1998, 37, 3295−3297. (j) Liao, P.; Langloss, B. W.; Johnson, A. M.; Knudsen, E. R.; Tham, F. S.; Julian, R. R.; Hooley, R. J. Two-Component Control of Guest Binding in a SelfAssembled Cage Molecule. Chem. Commun. 2010, 46, 4932−4934. (k) Wu, N.-W.; Zhang, J.; Ciren, D.; Han, Q.; Chen, L.-J.; Xu, L.; Yang, H.-B. Construction of Supramolecular Pyrene-Modified Metallacycles via Coordination-Driven Self-Assembly and Their Spectroscopic Behavior. Organometallics 2013, 32, 2536−2545. (l) Stang, P. J.; Persky, N. E.; Manna, J. Molecular Architecture via Coordination: SelfAssembly of Nanoscale Platinum Containing Molecular Hexagons. J. Am. Chem. Soc. 1997, 119, 4777−4778. (m) Kishi, N.; Li, Z.; Yoza, K.; Akita, M.; Yoshizawa, M. An M2L4 Molecular Capsule with an Anthracene Shell: Encapsulation of Large Guests up to 1 nm. J. Am. Chem. Soc. 2011, 133, 11438−11441. (n) Gütz, C.; Hovorka, R.; Schnakenburg, G.; Lützen, A. Homochiral Supramolecular M2L4 Cages by High-Fidelity Self-Sorting of Chiral Ligands. Chem. - Eur. J. 2013, 19, 10890−10894. (o) Chand, D. K.; Biradha, K.; Kawano, M.; Sakamoto, S.; Yamaguchi, K.; Fujita, M. Dynamic Self-Assembly of an M3L6 Molecular Triangle and an M4L8 Tetrahedron from Naked PdII Ions and Bis(3-pyridyl)-Substituted Arenes. Chem. - Asian J. 2006, 1, 82−90. (p) Fujita, D.; Yokoyama, H.; Ueda, Y.; Sato, S.; Fujita, M. Geometrically Restricted Intermediates in the Self-Assembly of an M12L24 Cuboctahedral Complex. Angew. Chem., Int. Ed. 2015, 54, 155− 158. (q) Ronson, T. K.; Fisher, J.; Harding, L. P.; Hardie, M. J. StarBurst Prisms with Cyclotriveratrylene-Type Ligands: A [Pd6L8]12+ Stella Octangular Structure. Angew. Chem., Int. Ed. 2007, 46, 9086− 9088. (5) (a) Kai, S.; Maddala, S. P.; Kojima, T.; Akagi, S.; Harano, K.; Nakamura, E.; Hiraoka, S. Flexibility of Components Alters the SelfAssembly Pathway of Pd2L4 Coordination Cages. Dalton Trans 2018, 47, 3258−3263. (b) Kai, S.; Marti-Centelles, V.; Sakuma, Y.; Mashiko, T.; Kojima, T.; Nagashima, U.; Tachikawa, M.; Lusby, P. J.; Hiraoka, S. Quantitative Analysis of Self-Assembly Process of a Pd2L4 Cage Consisting of Rigid Ditopic Ligands. Chem. - Eur. J. 2018, 24, 663−671. (c) Tateishi, T.; Zhu, W.; Foianesi-Takeshige, L. H.; Kojima, T.; Ogata, K.; Hiraoka, S. Self-Assembly of a Pd4L8 Double-Walled Square Partly Takes Place through the Formation of Kinetically Trapped Species. Eur. J. Inorg. Chem. 2018, 1192−1197. (6) Tateishi, T.; Kai, S.; Sasaki, Y.; Kojima, T.; Takahashi, S.; Hiraoka, S. Two Dominant Self-Assembly Pathways to a Pd3L6 Double-Walled Triangle. Chem. Commun. 2018, 54, 7758−5561. (7) The self-assembly of a Pd4L8 DWS from PdPy*4 ions and ditopic ligands, L, is expressed by the following equation: 4PdPy*4 + 8L V Pd4L8 + 16Py*

(1)

All intermediates in the self-assembly of DWS are expressed by PdaLbXc (a−c are the number of each component). The average composition of all of the intermediates, Pd⟨a⟩L⟨b⟩X⟨c⟩, is obtained by QASAP. In order to investigate the self-assembly processes from the time development of Pd⟨a⟩L⟨b⟩X⟨c⟩, the following two parameters are introduced: ⟨n⟩ =

4⟨a⟩ − ⟨c⟩ ⟨b⟩

(2)

⟨k⟩ =

⟨a⟩ ⟨b⟩

(3)

The ⟨n⟩ value indicates the average number of Pd(II) ions connected to a single ditopic ligand in all of the intermediates, while the ⟨k⟩ value indicates the ratio between Pd(II) ions and ditopic ligands in the average composition of all of the intermediates. The self-assembly D

DOI: 10.1021/acs.inorgchem.8b02470 Inorg. Chem. XXXX, XXX, XXX−XXX