Protonation-induced self-assembly of bis-phenanthroline macrocycles into nanofibers arrayed with tetrachloroaurate, hexachloroplatinate or phosphomolybdate ions

Shohei Tashiro *, Shun Shimizu , Masumi Kuritani and Mitsuhiko Shionoya *
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: shionoya@chem.s.u-tokyo.ac.jp

Received 21st September 2020 , Accepted 5th October 2020

First published on 6th October 2020


One-dimensional self-assembly of macrocycles is one of the important strategies for constructing fibrous nanomaterials with anisotropic functions such as one-dimensional transport and accumulation of molecules and ions. Herein we report on the synthesis and properties of self-assembled nanofibers using macrocycles to develop a multipurpose template for one-dimensional array of noble metal ions. The nanofibers were prepared by protonation-induced self-assembly of bis-phenanthroline macrocycles, which have enabled the accumulation of some metal-containing anions, such as tetrachloroaurate, hexachloroplatinate and phosphomolybdate. Microscopic observations have demonstrated that the supramolecular nanofibers were reproducibly formed in a similar way, regardless of the structures and charge numbers of the anions. Moreover, the resulting nanofibers, arrayed with several metal ions, were chemically reduced, producing dispersible gold nanoparticles and mixed-valence nanofibers.


Introduction

In current supramolecular chemistry and molecule-based materials science, macrocyclic compounds are one of the unique building blocks for constructing several types of functional nanostructures.1 In particular, one-dimensional assemblies of macrocycles have been extensively studied to make nanofibers or nanotubes2 that exhibit some anisotropic functions such as molecular transport,3 conductivity4 and selective chemical reactions.5 Moreover, such a fibrous structure also serves as a template for one-dimensional metal accumulation.6 One of the typical methods is to use metallomacrocycles as a building block for self-assembly.7 However, since the type of metal is limited by the nature of the coordination site of the macrocyclic skeleton, the type of metal ion that can be accumulated by this method is also limited. Another method for arranging metal ions is a self-assembly method8 based on ion pair formation between metal species and oppositely charged macrocycles to form consecutive ion-pairs.8a–c,f,g However, a drawback of this method is that the ion-pair structure changes sensitively and sometimes collapses due to a mismatch in the molecular size and charge number of the ionic components.

We have previously reported that bis-phenanthroline macrocyclic ligands with two inward chelating sites9 have a macrocyclic cavity for metal arrangement and molecular recognition.9a–d In addition, macrocycle 1 with six dodecyl side chains (Fig. 1a) was found to form self-assembled nanofibers by π–π stacking in cyclohexane. Furthermore, a dinuclear PdII complex of 1 showed self-assembly into nanofibers in CHCl3/CH3CN.9e However, self-assembly into nanofibers has not previously been observed with other metal complexes of macrocycle 1. Meanwhile, preliminary experiments revealed that macrocycle 1 might be self-assembled when protonated.4d,10 Here we report that protonation of macrocycle 1 induces fiber formation in an organic solvent through ion pairing between protonated 1 and the counter anions, which allows the alignment of metal species contained in the counter anions on the nanofibers (Fig. 1b). As a typical example, the formation of nanofibers composed of 1 and HAuIIICl4·4H2O (Au) was suggested by 1H NMR, dynamic light scattering (DLS) and atomic force microscopy (AFM). Its detailed structure is also explained based on scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDS) and the crystal structure of a fragmental salt. Other metal-containing protic acids with different shapes and charge numbers, H2PtIVCl6·6H2O (Pt) and H3PMoVI12O40·nH2O (PMo), were also applied to this system, and those metal species were successfully arrayed along the self-assembled nanofibers of macrocycle 1. Furthermore, the metal-assembled fiber structures were used to synthesize dispersible metal nanoparticles and chemically-reduced nanofibers.


image file: d0dt03287e-f1.tif
Fig. 1 (a) Chemical structure of 1 and (b) schematic representation of acid-induced self-assembly of 1 into metal-encrusted nanofibers.

Results and discussion

First we investigated the protonation of bis-phenanthroline macrocycle 1 and its half macrocycle, 2,9-diphenyl-1,10-phenanthroline (2), using HAuCl4·4H2O (Au). When adding 2.0 equivalents of Au to a solution of 1 in CHCl3/CH3CN (10[thin space (1/6-em)]:[thin space (1/6-em)]1) at room temperature, electrospray ionization time-of-flight (ESI-TOF) mass analysis of the mixed solution showed the signals of protonated species of 1 (m/z = 2131.571 and 2471.456 for [H(1)]+ and [H2(1) + AuCl4]+, respectively), while no signals derived from AuCl3-coordinated species such as [Au(1)Cl3] and [Au2(1)Cl6] were observed at all (Fig. S1). The preferential protonation of 1 under this condition was also supported by single-crystal X-ray diffraction of the product of the reaction of 2 with Au under a similar condition. The structure of yellow crystals obtained from the reaction was solved as a protonated compound [H(2)][AuCl4], and due to electrostatic interaction between the protonated nitrogen atoms of 2 and [AuCl4] (N⋯Cl distance: 3.22 Å), two adjacent half macrocycle 2 were proven to be bridged (Fig. 2). In addition, another [AuCl4] anion also touched [H(2)]+ through Au–π interaction (Au⋯π-plane of [H(2)]+ distance: 3.38 Å).11 The protonated structure is in sharp contrast to the AuCl3-bonded structure of [Au(2)Cl3] previously synthesized by the reaction of 2 with KAuCl4·H2O in refluxing acetonitrile.12 These results suggest that under mild conditions 2,9-diaryl-1,10-phenanthroline does not complex with AuCl3 and only protonation of the coordination atoms occurs.
image file: d0dt03287e-f2.tif
Fig. 2 (a) Reaction scheme for the protonation of 2 by HAuCl4. (b) Crystal structure of [H(2)][AuCl4] as a dimer.

The protonation process of 1 was further investigated by a UV-vis titration experiment with Au. Titration of a solution of macrocycle 1 in CHCl3/CH3CN (10[thin space (1/6-em)]:[thin space (1/6-em)]1) with Au increased a new absorption band around 400 nm that can be assigned to the mono-protonation of the phenanthroline moiety.13 Based on the titration data, equilibrium constants, K1 ( = [H(1)AuCl4]/([1][HAuCl4]) M−1) and K2 ( = [H2(1)(AuCl4)2]/([H(1)AuCl4][HAuCl4]) M−1), at 20 °C were determined by the curve fitting analysis to be (5.7 ± 0.2) × 105 M−1 and (2 ± 1) × 105 M−1, respectively (Fig. S3). These values are almost comparable to the equilibrium constant of a reaction between 2,9-diaryl-1,10-phenanthroline and a strong acid.13a

The self-assembly of protonated 1 in a solution state was studied by 1H NMR and DLS analyses. When adding 2.5 equivalents of Au to a solution of 1 in CDCl3/CD3CN (10[thin space (1/6-em)]:[thin space (1/6-em)]1) ([1] = 8.4 × 10−5 M), the signals of the protons Ha–Hg in the macrocyclic skeleton and the protons Hh and Hi at the base of the side chain completely disappeared on the 1H NMR spectrum. On the other hand, the signals of the dodecyl side-chains were broadened but still remained (Fig. 3). These results suggest that self-assembly of the ring structure formed the molecular assembly without precipitation. It should be noted that, according to the equilibrium constants K1 and K2, almost all macrocycles are doubly protonated under this condition. This self-assembly that occurs in solution was also supported by DLS measurement. Significant light scattering was observed when the equivalent of Au exceeded 1.5 in CHCl3/CH3CN (10[thin space (1/6-em)]:[thin space (1/6-em)]1) ([1] = 1.2 × 10−4 M), and the hydrodynamic diameter (DH) and size distribution (polydispersity index; PDI) of aggregates were estimated to be 8–12 nm and 0.20–0.26, respectively (Fig. S5).


image file: d0dt03287e-f3.tif
Fig. 3 Reaction scheme for the protonation of 1 by HAuCl4, and 1H NMR spectra (500 MHz, 300 K, CDCl3/CD3CN = 10[thin space (1/6-em)]:[thin space (1/6-em)]1) of 1 with (a) 0, (b) 0.5, (c) 1.0, (d) 1.5 and (e) 2.0 equivalents of Au.

For direct observation of the self-assembled structure, AFM and STEM measurement of samples deposited on a mica surface or a carbon-coated Cu grid was conducted. As a result, micrometer-long nanofibers with a height of 5–7 nm were reproducibly observed in samples prepared from solutions containing 1 and Au (1[thin space (1/6-em)]:[thin space (1/6-em)]2) in CHCl3/CH3CN (10[thin space (1/6-em)]:[thin space (1/6-em)]1) (Fig. 4a). This height is comparable to a width of 1 (ca. 6 nm), so nanofibers are likely to have one-dimensional stacking of the protonated macrocycles. Although the lengths of nanofibers do not conflict with the smaller hydrodynamic diameters observed by DLS measurements (Fig. S5) as shown in some papers,8b it should be possible that the elongation of nanofibers proceeds upon evaporation of solvents. On the other hand, the thickness of the fibrous structures observed by STEM measurement was 30–50 nm (Fig. 4b), suggesting that the fibers tend to form bundles. In addition to the fibrous material, the addition of 4 equivalents of Au resulted in a large round body (Fig. S6). These results suggest that excessive amounts of Au entangle the nanofibers by inducing electrostatic interactions in the fibers.


image file: d0dt03287e-f4.tif
Fig. 4 (a) An AFM image, (b) a dark-field STEM image and EDS maps for (c) C (K-peak) and (d) Au (M-peak) of nanofibers composed of 1 and Au. (e) A possible structure of the resulting nanofibers composed of protonated 1 and [AuCl4] anions.

Further analysis of the nanofibers revealed the aggregate structure. EDS analysis by STEM measurement revealed that the nanofibers were covered with C and Au atoms derived from 1 and Au, respectively (Fig. 4c and d). This result suggests that [AuCl4] anions are included in the nanofiber to compensate the positive charges of protonated 1. Therefore, one of the possible structures is a stacking structure in which an anion bridges doubly protonated 1 (Fig. 4e), as observed in the crystal structure of [H(2)][AuCl4] (Fig. 2b). However, no diffraction peaks were obtained by powder X-ray diffraction analysis of the nanofibers, suggesting that the one-dimensional stacking structure of protonated 1 and [AuCl4] was not very ordered (Fig. S8).

We then extended this system into other metal-assembled nanofibers containing [PtCl6]2− or [PMo12O40]3− (Fig. 5a). As a result, similar nanofibers were observed by AFM analysis of samples prepared from 1 and appropriate amounts of H2PtCl6·6H2O (Pt) (1 equiv.) or H3PMo12O40·nH2O (PMo) (0.5 equiv.). In these cases, further addition of the acids formed spherical aggregates, probably due to the entanglement of the nanofibers as in the case with Au (Fig. S17 and 25). In addition, EDS mapping of both nanofibers confirmed the presence of platinum or molybdenum atoms along the fiber structures (Fig. 5b–e). This suggests the formation of nanofibers aligned with hexachloroplatinate or phosphomolybdate, as in the case of tetrachloroaurate. Electrostatic interactions between protonated 1 and their counter anions were also suggested by the crystal structures of the model salts, [H(2)]2[PtCl6] and [H(2)]2[PMo12O40]·[H(DMF)2]. Protonated half macrocycles 2 were linearly bridged by their counter anions via electrostatic interactions (Fig. 5f and g). These results suggest that macrocycle 1 is expected to self-assemble with acid to form nanofibers regardless of the type of counter anions.


image file: d0dt03287e-f5.tif
Fig. 5 (a) Reaction scheme for self-assembly of 1 in the presence of H2PtCl6 or H3PMo12O40 into nanofibers. (b) and (c) A dark-field STEM image and an EDS map for Pt (M-peak) of nanofibers composed of 1 and Pt, respectively. (d) and (e) A dark-field STEM image and an EDS map for Mo (L-peak) of nanofibers composed of 1 and PMo, respectively. (f) and (g) Crystal structures of [H(2)]2[PtCl6] and [H(2)]2[PMo12O40]·[H(DMF)2], respectively.

As an application of the metal-assembled nanofibers, we also examined chemical reduction of the nanofibers to produce reduced nanofibers or metal nanoparticles.14 For instance, the nanofibers composed of 1 and Au was successfully applied to the synthesis of dispersible gold nanoparticles in CHCl3 (Fig. 6a). When adding 10 equivalents of typical reductant NaBH4 to a solution of nanofiber (1[thin space (1/6-em)]:[thin space (1/6-em)]Au = 1[thin space (1/6-em)]:[thin space (1/6-em)]2) in CHCl3/CH3CN (10[thin space (1/6-em)]:[thin space (1/6-em)]1), the color of the solution quickly changed from yellow into purple, indicating the formation of gold nanoparticles. TEM analysis of the reduced sample (1/Au = 1[thin space (1/6-em)]:[thin space (1/6-em)]2) revealed formation of submicron-sized aggregates of 10–30 nm-sized gold nanoparticles (Fig. S30a–e). Of particular note is that the product was redissolved in CHCl3 after evaporation of the purple solution (Fig. S30f). This was in sharp contrast to the formation of insoluble materials in a control experiment without 1. The UV-vis spectrum of the obtained solution showed a broad absorption around 533 nm, which was assignable to surface plasmon resonance of 10–20 nm-sized gold nanoparticles (Fig. 6b).15 This result shows good agreement with the observed size of gold nanoparticles in the TEM analysis. Furthermore, the decrease in the absorption band around 400 nm was probably due to the neutralization of protonated 1 by excess NaBH4 (Fig. 6b). Consequently, it is believed that assembled or neutralized 1 would enhance the generation of gold nanoparticles with enhanced solubility.


image file: d0dt03287e-f6.tif
Fig. 6 (a) Schematic representation of chemical reduction of the Au-assembled nanofibers. (b) UV-vis absorption spectra of Au-assembled nanofibers before (dot line) and after (solid line) reduction (CHCl3, 298 K).

Finally, nanifibers composed of 1 and PMo were chemically reduced with L-ascorbic acid to produce mixed-valence nanofibers (Fig. 7a). When 3.0 equivalents of L-ascorbic acid were added to a solution containing nanofibers in CHCl3/CH3CN (10[thin space (1/6-em)]:[thin space (1/6-em)]1), the color of the solution gradually changed to greenish blue (Fig. 7b). UV-vis analysis revealed new broad absorption bands around 600–1200 nm (Fig. 7c), the color of which can be assigned to inter-valence charge-transfer between MoVI and MoV, indicating that [PMo12O40]3− was reduced to [PMo12O40]n (n = 4–5). Moreover, AFM observation of the reduced product clearly showed that the fiber structure was maintained after reduction (Fig. 7d). These results suggest that the phosphomolybdate-assembled nanofibers can be chemically reduced without significant collapse of the fiber structure.


image file: d0dt03287e-f7.tif
Fig. 7 (a) Schematic representation of chemical reduction of the Mo-assembled nanofibers. (b) Picture of the sample (1/PMo = 2[thin space (1/6-em)]:[thin space (1/6-em)]1) reduced by L-ascorbic acid. (c) UV-vis absorption spectra of Mo-assembled nanofibers before (dot line) and after (solid line) reduction (CHCl3/CH3CN = 10[thin space (1/6-em)]:[thin space (1/6-em)]1, 298 K). (d) AFM image (on mica) of the nanofibers after reduction.

Conclusions

In summary, we found that doubly protonated macrocycle 1 self-assembles into nanofibers. It serves as a template for one-dimensional assembly of several types of metal-containing anions such as tetrachloroaurate, hexachloroplatinate and phosphomolybdate with different charge numbers and shapes. The fiber structures were characterized by AFM, STEM and EDS measurements, and ion pairing between the protonated phenanthroline moieties and the counter anions was also supported by the crystal structure of the model salts of 2. Furthermore, it has become possible to prepare dispersible Au nanoparticles and mixed-valence nanofibers, respectively, by chemically reducing the tetrachloroaurate- and phosphomolybdate-assembled nanofibers. Thus, this method using macrocycle 1 as a general-purpose template for one-dimensional array of metal ions could be applied to molecular assembly type nanomaterials such as highly anisotropic metal clusters and nanoscale electrical wires.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by the JSPS KAKENHI Grant Numbers JP26248016 (Scientific Research (A)) and JP16H06509 (Coordination Asymmetry) to M.S., and JP26110504 and JP16H00956 (Molecular Architectonics) to S.T. We are grateful to Prof. E. Nakamura and Dr K. Harano for DLS and to Prof. S. Ohkoshi for AFM measurement. TEM, STEM and EDS measurement were conducted at the Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by the MEXT, Japan.

Notes and references

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Footnote

Electronic supplementary information (ESI) available: Experimental details and characterization data. CCDC 2017589 ([H(2)][AuCl4]), 2017590 ([H(2)]2[PtCl6]) and 2017591 ([H(2)]2[PMo12O40]·[H(DMF)2]). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0dt03287e

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