A Metallo-molecular Cage That Can Close the Apertures with

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A metallo-molecular cage that can close the apertures by coordination bonds Shigehisa Akine, Masato Miyashita, and Tatsuya Nabeshima J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b00840 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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Journal of the American Chemical Society

A metallo-‐molecular cage that can close the apertures by coordina-‐ tion bonds Shigehisa Akine,* a Masato Miyashita,b and Tatsuya Nabeshima* b a

Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-‐machi, Kanazawa 920-‐1192, b Japan; Faculty of Pure and Applied Sciences, University of Tsukuba, 1-‐1-‐1 Tennodai, Tsukuba, Ibaraki 305-‐8571, Ja-‐ pan Supporting Information Placeholder ABSTRACT: We designed a novel tricobalt(III) metallo-‐ molecular cage in which its apertures can be closed with bifunctional ligands via coordination bonds. A closed-‐cage complex was easily formed by ligand exchange of its open-‐ cage analogue. Guest uptake/release by the open-‐cage complex was sufficiently fast to quickly reach guest-‐binding equilibrium, while that by the closed-‐cage complex was extremely slow and it took about one week to reach equilibrium. The guest uptake was retarded by at least 2000 times by introduction of the bifunctional ligands at the cage apertures.

Molecular cages have attracted much attention in recent years, because they can incorporate smaller molecules in the 1-‐3 cavity that is well separated from the outside environment. In general, molecular cages can efficiently wrap guest species so that the guest uptake and release are slow due to the 4 5 three-‐dimensional scaffold. Carcerands and fullerenes can completely imprison the guest species; the incarcerated guests cannot exit the cavity without destroying the capsular structure. In fact, whether or not the guest species are en-‐ trapped permanently depends on the size of the cage portals compared to the guest size. Hemicarcerands, which have a larger portal, allow some guest species to enter and exit the 6 cavity. While complete confinement of guest species is of interest from a scientific viewpoint, nano-‐sized molecular containers that can close their aperture depending on specif-‐ ic situations would be advantageous when we use these con-‐ tainers for storing and transporting functional molecules 7 (Scheme 1a). To date, some gated containers have been de-‐ veloped for controlling the guest binding processes based on 8 9 photo-‐ and redox reactions. We focused on the reversible feature of metal–ligand co-‐ ordination bonds, which allow us to introduce bifunctional ligands at the apertures of molecular cages to control enter-‐ ing of the guest species (Scheme 1b). Since the bifunctional ligands have to be introduced without destroying the cage skeleton, we should use a robust cage framework that can remain intact under the conditions for coordination bond 2 formation/cleavage. In this context, organic cages are ad-‐ vantageous to this study when compared to coordination

3,10

cages. The tris(saloph)-‐type organic cage H6L, which we previously designed for the synthesis of trinickel(II) metallo-‐ 11 host, fulfills the requirement. If we introduce hexacoordi-‐ 12 nate metal ions into its saloph tetradentate sites, we can introduce two additional ligands at the axial positions of each metal. This allows us to introduce a bifunctional ligand at each aperture of the molecular cage (Scheme 1c). The bi-‐ functional ligands can bridge neighboring metal centers to close the aperture, which should kinetically suppress the guest uptake. We now report a novel metallo-‐molecular cage that has bifunctional ligands to suppress guest molecules passing in through the apertures.

Scheme 1. (a) An open cage that can close the aper-‐ tures. (b) Concept of molecular cage that can close the aperture by coordination of bifunctional ligands. (c) Design of metallo-‐molecular cage.

6 In this study, we used the low-‐spin d cobalt(III) ion, which generally forms hexacoordinate inert complexes and undergoes very slow ligand exchange. This would enable

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facile introduction of the diamine ligand that is firmly fixed at the apertures of the molecular cage without affecting the organic cage skeleton. The closed-‐cage trinuclear complex with three hda (= 1,6-‐hexanediamine) ligands, [LCo3(hda)3](OTf)3, was synthesized in 92% yield by the complexation of H6L with cobalt(II) acetate in air in the 1 presence of hda ligand (Scheme 2a; Figures S1–S3). The H NMR spectrum showed six peaks assignable to the bridging 3+ hda ligand and confirmed the 1:3 stoichiometry ([LCo3] : hda). The formation of discrete species [LCo3(hda)3](OTf)3 was confirmed by ESI mass spectrometry (m/z = 709.8 for 3+ [LCo3(hda)3] ). An analogous complex [LCo3(oda)3](OTf)3, which have three longer diamine ligands oda (= 1,8-‐ octanediamine), was similarly synthesized (Figures S4, S5). However, when a shorter diamine ligand bda (= 1,4-‐ butanediamine) was used, the triply-‐bridged species [LCo3(bda)3](OTf)3 was not obtained (Figure S6), probably because the bda ligand is too short to bridge the cobalt ions without strain. We also synthesized the open-‐cage analogue, [LCo3(MeNH2)6](OTf)3, in a similar manner in 87% yield as the control compound without bifunctional ligands at the apertures (Scheme 2b, Figures S7, S8). This complex has six methylamine ligands at the axial positions of each cobalt(III) 1 center. The structure was confirmed by H NMR spectro-‐ scopy and mass spectrometry (ESI-‐MS, m/z = 655.8 for 3+ [LCo3(MeNH2)6] ).

membered –(Co–NH2–(CH2)6–NH2)3– macrocycle. The cavity is surrounded by two pivotal benzene rings in parallel (dis-‐ tance between them was 5.77 Å) as well as six phenoxo oxy-‐ gen atoms forming a triangular prism. In fact, there was a 3+ water molecule in the cavity of [LCo3(hda)3] in the crystal structure.

3+

Figure 1. Structure of [LCo3(hda)3] in the crystal of [LCo3(hda)3](TFPB)3•2H2O•1.5MeOH. (a,b) ORTEP drawings (30% probability level). The methoxy groups, tert-‐butyl groups, hydrogen atoms, and the minor components of dis-‐ ordered atoms are omitted for clarity. The Co3(hda)3 27-‐ membered ring is colored magenta. (c,d) Space filling-‐ models. The carbon atoms in hda ligands are colored orange. The cross section view is shown in (d) to show the inside cavity.

Scheme 2. Synthesis and structural conversion of complexes.

It is noteworthy that the aperture of this open-‐cage com-‐ 3+ plex [LCo3(MeNH2)6] can be easily closed by a reaction with the bifunctional ligands, hda. The reaction slowly proceeded with the concomitant release of methylamine. The open-‐ 3+ cage complex [LCo3(MeNH2)6] was almost perfectly con-‐ 3+ verted into the closed-‐cage complex, [LCo3(hda)3] (Scheme 1 2c), which was confirmed by H NMR spectroscopy. 3+

The X-‐ray crystallographic analysis of [LCo3(hda)3] re-‐ vealed a cage-‐like structure containing three octahedral co-‐ 13 balt(III) ions. Three hda ligands, which bridge the three cobalt ions, are effectively blocking the apertures to give a closed-‐cage (Figure 1a, c; Figures S19, S20, and Table S1). Nevertheless, there should remain enough space for guest 3+ inclusion inside the cage of [LCo3(hda)3] (Figure 1b,d) The three cobalt(III) ions and three hda ligands constitute a 27-‐

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Prior to evaluating the effect of the bifunctional ligands on the guest-‐uptake kinetics, we investigated the ion recogni-‐ tion behavior of the open-‐cage complex + + + [LCo3(MeNH2)6](OTf)3 toward alkali metal ions (Na , K , Rb , + 14 1 Cs ; TFPB salts ) by H NMR spectroscopy (see Figures S13– + S17). Upon the addition of Cs , a new set of signals for the 4+ inclusion complex [LCo3(MeNH2)6•Cs] separately appeared 3+ from those for the free host [LCo3(MeNH2)6] . Obviously, the rate of guest complexation and decomplexation was slow 1 on the H NMR timescale, while the guest inclusion quickly reached equilibrium on the laboratory timescale (within 5 min). The binding constant in CD3OD was determined to be –1 logKa = 4.23 ± 0.06 (Ka in M ) (Table 1). Based on these data, the rate constants for guest binding (k+, k–) were roughly –1 –1 –4 –1 estimated as follows: k+ > 4 sec M , k– > 2 × 10 sec (Scheme 3).

Scheme 3.

In fact, we had expected that the recognition of cationic 3+ guests by the tricationic host [LCo3(MeNH2)6] was unfavor-‐ able due to a strong electrostatic repulsion between the posi-‐ 4+ tive charges. However, the [LCo3(MeNH2)6•Cs] complex showed an unexpectedly high binding constant, which was + comparable to that for the dibenzo-‐24-‐crown-‐8/Cs complex 15 under similar conditions. This strong binding between the


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cationic species can be explained by the negatively polarized phenoxo groups that can bridge two or more metal ions as 16 17,18 seen in the macrocyclic and helical analogues. The trend of the binding constants for alkali metal ions was determined + + + + to be Cs > Rb > K >> Na , clearly indicating the preference for larger ions. The selectivity can be ascribed to the rigid host framework, which is well preorganized for larger ions but cannot shrink to fit smaller guest species.

species from passing in and out through the apertures (Scheme 1b) as expected from the molecular design.

Table 1. Association constantsa for the complexation between [LCo3(MeNH2)6](OTf)3 and alkali metal ionsb. –1

logKa (M )

Na+

K+

Rb+

Cs+

negligible

2.89 ± 0.04

3.70 ± 0.06

4.23 ± 0.06

a Determined by NMR spectroscopy in CD3OD at 20 °C. b The guests were added as a TFPB salt (TFPB = tetrakis(3,5-‐ bis(trifluoromethyl)phenyl)borate). 3+

+

Since the cage framework of [LCo3] showed high Rb and + Cs affinities, we then investigated the blocking effect of the diamine ligands in the closed-‐cage complex [LCo3(hda)3](OTf)3 on its binding kinetics (see Figures S9– 1 S11). The H NMR spectrum of [LCo3(hda)3](OTf)3, which was + recorded 1 h after the addition of 5 equiv of Cs , showed only + very small signals (approximately 4%) assignable to the Cs complex (Figure 2a i, ii). The signal slowly grew and it took + approximately 50 h for the 40% conversion to the Cs com-‐ plex (Figure 2b i). A similar slow uptake was observed for + + Rb , but the conversion to the Rb complex was less efficient (Figure 2a iii, iv). The rate constants (k+, k–) for the second-‐ order equilibrium reaction of the guest uptake were deter-‐ mined in CD3OD at 20 °C from the analysis of the time-‐ course of the mole fractions (Figure 2b, Table 2). It is note-‐ worthy that the uptake rate for the closed-‐cage complex (k+ ~ –3 –1 –1 2 × 10 M sec ) was lower by at least 2000 times than that –1 –1 for the open-‐cage complex (k+ > 4 M sec ). The guest re-‐ –6 –1 lease of the closed-‐cage complex (k– ~ 5 × 10 sec ) was also significantly retarded compared to the open-‐cage complex (k– –4 –1 ~ 2 × 10 sec ). A similar deceleration was observed for the + Rb binding. +

The blocking effect of the hda ligands in Cs encapsulation 3+ by [LCo3(hda)3] was also evidenced by mass spectrometry (Figure S18). While the mass spectrum of a mixture of 3+ + [LCo3(hda)3] and Cs recorded 5 min after mixing did not + show any peaks for Cs -‐containing species, the mass spec-‐ trum after 24 h exhibited intense peaks at m/z = 565.5 for 4+ + [LCo3(hda)3•Cs] . This clearly indicates that Cs was intro-‐ + duced into an environment where the uptake/release of Cs 3+ by [LCo3(hda)3] was significantly retarded. This result + strongly suggests the encapsulation of Cs into the cavity of 3+ [LCo3(hda)3] and confirmed the blocking effect of the hda + ligands that retard uptake/release of Cs . Probably, the guest uptake/release took place via partial dissociation mechanism, because the diamine molecules are completely occupying the + cage portals. The Cs ion was encapsulated almost at the center of the cavity, which was suggested by DFT calculation. It is noteworthy that the dimension of the cavity did not sig-‐ + nificantly change after inclusion of Cs , indicating that the 3+ + cavity of [LCo3(hda)3] is well preorganized for Cs inclusion. + Probably, the Cs ion binding is driven by cation-‐π interac-‐ tions with the two pivotal benzene rings as well as the coor-‐ dination of the phenoxo groups. Consequently, the bifunc-‐ tional ligands, 1,6-‐hexanediamine, efficiently block guest

1

Figure 2. (a) H NMR spectral changes of [LCo3(hda)3](OTf)3 after the addition of metal ions at 20 °C in CD3OD, [[LCo3(hda)3](OTf)3] = 0.5 mM, [MTFPB] = 2.5 mM, 400 MHz. Signals indicated by black circles are assigned to the inclusion complexes. (b) Changes in the mole fractions of 3+ 4+ [LCo3(hda)3] and [LCo3(hda)3•M] in CD3OD after the addi-‐ + + tion of 5 equiv of metal ions: (i) Cs , (ii) Rb .

Table 2. Rate constants and association constantsa for the complexation between [LCo3(diamine)3]3+ (diamine = hda, oda) and alkali metal ions (Rb+, Cs+). guest k+ (M–1sec–1)

host 3+

+

k– (sec–1) –4

Ka (M–1) –5

[LCo3(hda)3] Rb

(8.0 ± 0.6) × 10

[LCo3(hda)3]3+ Cs+

(2.02 ± 0.05) × 10–3 (4.9 ± 0.2) × 10–6

3+

+

[LCo3(oda)3] Cs

–3

(1.45 ± 0.04) × 10

(1.20 ± 0.10) × 10

67 410

–5

(1.04 ± 0.07) × 10 140

a Determined by NMR spectroscopy in CD3OD at 20 °C. Interestingly, the length in the bridging diamine has sig-‐ nificant influence on the guest binding kinetics and binding strength. When longer bifunctional ligand oda was intro-‐ duced, the guest uptake rate decreased by ca. 30% (Table 2; Figure S12). It is noteworthy that the association constants 3+ + for [LCo3(oda)3] with Cs became almost 1/3 of that for 3+ [LCo3(hda)3] . Thus, the increased number of methylene groups at the aperture has an influence on the uptake/release rates and the binding constant. In conclusion, we have designed a novel metallo-‐molecular cage to which we can introduce bifunctional ligands at the 3+ apertures. Both the open-‐cage complex [LCo3(MeNH2)6] 3+ + and close-‐cage complex [LCo3(hda)3] showed a Cs -‐ 3+ selectivity, but the hda ligands in [LCo3(hda)3] effectively



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block the guest uptake by more than three orders of magni-‐ tude. The concept of the open-‐close feature in this system would be applied to a molecular container in which the en-‐ trapped molecules can be released when required. Such an on-‐demand catch and release of guest species would be use-‐ ful for molecular functions such as drug delivery and chemi-‐ cal information relay systems.

ASSOCIATED CONTENT Supporting Information Synthetic procedure and the characterization data, com-‐ plexation studies, and crystallographic analysis of [LCo3(hda)3](TFPB)3. This material is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]‐u.ac.jp; [email protected]

ACKNOWLEDGMENT This work was supported in part by JSPS KAKENHI (Grant Number JP16H06510 (Coordination Asymmetry) and JP26288022) and Kanazawa University CHOZEN Project. This paper is dedicated to Professor Takayuki Kawashima on the occasion of his 70th birthday.

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Graphical abstract Graphical abstract 40x19mm (600 x 600 DPI)


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Figure 1 Figure 1 69x58mm (600 x 600 DPI)



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Figure 2 Figure 2 106x135mm (600 x 600 DPI)


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Scheme 1 Scheme 1 86x92mm (600 x 600 DPI)



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A Metallo-molecular Cage That Can Close the Apertures with

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