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Titanium-Oxide Host Clusters with Exchangeable Guests Guanyun Zhang, Wenyun Li, Caiyun Liu, Jiong Jia, Chen-Ho Tung, and Yifeng Wang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10565 • Publication Date (Web): 07 Dec 2017 Downloaded from http://pubs.acs.org on December 7, 2017
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Titanium-Oxide Host Clusters with Exchangeable Guests Guanyun Zhang, Wenyun Li, Caiyun Liu, Jiong Jia, Chen-Ho Tung and Yifeng Wang* Key Lab for Colloid and Interface Science of Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China Supporting Information Placeholder ABSTRACT: A novel family of water-soluble, polyoxocationic titanium-oxide host-guest clusters are reported herein. They exhibit an unprecedented hexagonal prismatic core structure + + + for hosting univalent cationic guests like K , Rb , Cs and + 133 H3O . Guest exchange has been studied using Cs NMR, showing the flexible pore of a host permits passage of a comparatively larger cation and giving an equilibrium constant of + + ca. 13 for displacing Rb by Cs . Attractive ion-dipole interaction, depending on host-guest size complementarity, plays a dominant role for the preferential encapsulation of larger alkali-metal cationic guests.
Since the discovery of crown ethers in 1967, syntheses of functional host compounds have become an important 1 branch in the field of supramolecular chemistry. Today, besides organic compounds represented by cryptands, cyclodextrins, and calixarenes, other hosts like soluble metal2 3 lacages and porous solids like zeolites and metal-organic 4,5 frameworks have also been studied extensively. One of the most intriguing features of these hosts is that they show very high affinity to certain guests according to size complementarity, and can preferentially uptake and reversibly exchange them. By contrast, water-soluble, inorganic hosts that can preferentially uptake and reversibly exchange their guests are very limited in the library of metal-oxide clusters, of which 6 7 {M@P5W30O110} (M = metal cation) and {Mo132} are the representative examples. Despite many other cluster compounds were synthesized with encapsulated ions, these ions are 8 structural templates and are not exchangeable. It still remains a great challenge to explore inorganic host-guest systems in water based on molecular compounds of other elements. Titanium-oxide clusters (TOCs) are a class of molecular analog of titanates and titanium-oxide minerals. They have been attracting increasing interests due to potential in con9 struction of organic-inorganic hybrid assemblies and applications in studying structures and electronic/photocatalytic 10 properties. To date there have been numerous examples of neutral Ti-oxo alkoxides/carboxylates synthesized from 9-11 nonaqueous systems and one polyoxocationic {Ti18O27} 12 cluster that is water-soluble. Although a few cage-like TOCs 13 that enclose guests were also reported, like the i − 13b [Br@Ti15O24(CoBr)6(O Pr)18] cluster, none of them show activity in guest-exchange. In the present study, we report a new class of water-soluble, cryptand-like, host-guest TOCs.
They not only exhibit an unprecedented hexagonal prismatic core structure, but more importantly, serve as water-soluble inorganic hosts for the reversible ion-dipole encapsulation of + + + + cationic guests such as K , Rb , Cs and H3O , which remarkably can be larger than the crystallographic size of the cluster’s flexible Ti6O6 pore. The {Ti12} clusters were synthesized in water by reactions of TiCl4 and metal chlorides in the presence of stabilizing 2– ligands like amino acids (AAs) and SO4 . Their structures were determined by X-ray diffraction and a variety of other complementary techniques (see sections 1-3 of SI). The struc13+ ture of [CsTi12O18(OH2)24Ser6] (Cs@Ti12Ser6; its counter – anion is Cl ) as a representative example of the {Ti12} class, is shown in Figure 1 (A and B). It possesses a unique hexagonal 12+ prismatic Ti12O18 framework (Figure 1, C and D), structurally related to the theoretically predicted yet not synthetically 14 isolated hexaprismane and unprecedented among any other compounds.
Figure 1. (A) Side and (B) top structural views of Cs@Ti12Ser6. 12+ (C) Side and (D) top views of the Ti12O18 framework. Color scheme: Cs, orange; TiO6 octahedron, green; Ti, blue; O, red; C, grey; N, purple. 12+
+
In the Ti12O18 framework of Cs@Ti12Ser6, a Cs ion is located at its center (site-a) and, in the crystal structure two + more Cs ions are “out-of-pocket” associated to the framework (site-b). All the six serine ligands are bound to the side surfaces, with each carboxylic group bridged to two nearest Ti-atoms. Besides AA, there are 24 aqua-ligands wrapping 12+ the Ti12O18 framework. In the side surfaces, the distance between the two O-atoms at opposite edges is 6.79 Å (d1 in
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Figure 1C). In the hexagonal bases, the distance between the two μ2-O at the parallel sides is 5.44 Å (d2 in Figure 1D). Sub2– tracting twice the radius of O from d1 and d2, i.e., 2.42 Å, 12+ gives the diameter of the Ti12O18 cavity (Figure 1C) and that of the Ti6O6 pore (Figure 1D) as 4.37 Å and 3.02 Å, respectively. Such a capsule-like structure makes it suitable for trap+ ping cations larger than its aperture, like Cs (d = 3.34 Å, larger than the diameter of the Ti6O6 pore by over 0.3 Å), and in turn provides capability for performing host-guest exchange. 12+
Remarkably, the [Ti12O18(OH2)24Pro6] (Ti12Pro6) host, 12+ isostructural to Cs@Ti12Ser6 but with a vacant Ti12O18 cavity, was obtained independently by using CaCl2 during the 2+ synthesis. Ca ions are found only at site-b of the Ti12Pro6 2+ clusters in the crystal structure, suggesting Ca is indeed important for the crystallization but formation of the other {Ti12} clusters that enclose guests is not necessarily associated 12+ with template effect of the cations trapped in the Ti12O18 13+ cavities. [(H3O)Ti12O18(OH2)24Ser6] ((H3O)@Ti12Ser6), 13+ [KTi12O18(OH2)24Ser6] (K@Ti12Ser6) and 13+ [RbTi12O18(OH2)24Ser6] (Rb@Ti12Ser6) were also prepared. A variety of AAs have been successfully utilized as the capping ligands to synthesize Cs@Ti12AA6 in high yields, including proline, serine, threonine, glycine, alanine, aspartic acid and cysteine. However, with common carboxylic acids like acetic acid, desired {Cs@Ti12} clusters could not be prepared, sug+ gesting an electron-withdrawing NH3 group at α-site is nec15 essary for a carboxylate species to passivate TOCs in water. Hence these Cs@Ti12AA6, the first examples of Ti-AA com15a plexes, may provide useful information for understanding binding of natural acids, peptides and proteins on the surface of titanates. Besides, in the reaction of TiCl4 with Cs2SO4, we have obtained Cs@Ti12(SO4)6 and Cs@Ti12(SO4)8, of which in 2– the former cluster all the six AAs are replaced with SO4 (Figure S1A), and the later one has two additional mono2– dentate SO4 ligands (Figure S1B). Both of the clusters could be crystalized together or independently, depending on the 2– amount of added SO4 . Nonetheless, an inorganic-organic mixed ligand-passivated cluster, Cs@Ti12(SO4)4Gly2 (Figure S1C) was also successfully prepared; either by one-pot reaction of TiCl4, Cs2SO4 and glycine, or by replacing ligands of 2– Cs@Ti12Gly6 with SO4 in a ligand exchange reaction. The versatilities of both guests and capping ligands have made the {Ti12} clusters a big family of molecular models for studying supramolecular chemistry. Before studying host-guest chemistry, aqueous solution chemistry of the {Cs@Ti12} cluster compounds was studied 17 133 1 17 with O NMR, Cs NMR, H NMR and IR. To perform O 17 NMR, ca. 0.05 M 10% O-enriched Cs@Ti12Pro6 was dissolved 17 in 10% O-enriched water with 1.0 M added CsCl. The area ratio of the three peaks at 681, 691 and 700 ppm (Figures 2A and S2) is near to 1:1:1, and hence the peaks are assigned to 12, the three different types of μ2-O of Cs@Ti12Pro6 (Figure 2B), 16 in agreement with the structural integrity of Cs@Ti12Pro6 in the aqueous solution. The strong peak at 6.5 ppm is assigned to both ligand aqua-O and solvent water-O, which rapidly are getting exchanged under the present experimental condi12 tions.
17
17
Figure 2. (A) O NMR of O-enriched Cs@Ti12Pro6 and (B) the three different types of μ2-O (O1, green; O2, purple; O3, brown; Cs@Ti12Pro6 contains six of each). For clarity, only the carboxylate groups of the proline ligands are shown. 133
In the Cs NMR spectrum of aqueous Cs@Ti12Ser6 (Figure + 3A), the single peak at 116.4 ppm is assigned to the Cs at site+ a. The peak at 29.9 ppm is assigned to the Cs both at site-b and in bulk solution, which are being exchanged rapidly at 1 room temperature. In the H NMR spectrum of a fresh Cs@Ti12Ser6 solution (Figure 3B), the three sets of peaks at 4.0 – 4.4 ppm (area ratio 1:1:1) are assigned to the three H1 atoms at α-C and β-C of the bound serine. To check H NMR of free serine, Cs@Ti12Ser6 was deliberately decomposed by incubating the solution at 80 °C for 3 h, by which serine was released after Cs@Ti12Ser6 had been converted into amorphous TiO2 precipitate. As indicated, the bound and the free 1 serine show distinct H NMR spectra and hence serine lig12+ ands are bound to the Ti12O18 framework in solution.
133
1
Figure 3. (A) Cs NMR and (B) H NMR of aqueous Cs@Ti12Ser6. In both cases 1.0 M CsCl was added. Additional evidences (Figures S3-S5 and the discussion therein) show that Cs@Ti12Ser6 and Rb@Ti12Ser6 remained stable in 40 vol% acetone/water solution (with added 1.2 M serine, pH = 0.3). Hence both are the second series of mem12-13, 17 bers of the water-soluble TOC family. By contrast, K@Ti12Ser6 and Ti12Pro6 decomposed quickly under comparable conditions. In addition, Rb@Ti12Ser6 was the major native species in its mother liquor, but K@Ti12Ser6 and Ti12Pro6 were
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not. Hence stability of the host-guest complexes depends on size of the guest in the following order: Cs@Ti12Ser6 > Rb@Ti12Ser6 > K@Ti12Ser6 ≈ Ti12Pro6. Based on the above solution characterizations, guest exchange experiments were carried out. The equilibrium constant, Keq, associated with this host-guest chemistry (Scheme 1) can be defined as eq 1. Scheme 1. Guest exchange in the Ti12O18
+
12+
host.
+
Keq = [Rb ]·[Cs@Ti12]/([Cs ]·[Rb@Ti12])
(1)
+
Upon addition of Cs to a solution of Rb@Ti12Ser6, rapid + + displacement of Rb by Cs and formation of Cs@Ti12Ser6 133 were evident by Cs NMR. To obtain the value of Keq, a set of ca. 0.1 M Rb@Ti12Ser6 solutions were independently reacted with various concentrations of CsCl. The exchange oc133 curred too fast to be monitored using Cs NMR (< 7 min, time for shimming the NMR magnet). A set of CsCl concen+ trations (equilibrated [Cs ] in the range of 0.04 – 0.5 M) gave consistent values of Keq, 10.3 ± 0.1 (Figure 4).
[Cs@Ti12] / [Rb@Ti12]
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1
0.1 0.01
0.1 1 [Cs+] / [Rb+]
Figure 4. Calculations of Keq for guest exchange. The squares + (■) are results of reactions of Cs and Rb@Ti12Ser6. Linear 2 fitting gives Keq = 10.3 ± 0.1 (the red line; R = 0.982). The dots + (●) are results of reaction of Rb and Cs@Ti12Ser6. Conditions: 40 vol% aqueous solution of acetone, [serine] = 1.2 M, pH = 0.30. To investigate the exchange of the guest of Cs@Ti12Ser6 by + Rb , RbCl was added to Cs@Ti12Ser6 (ca. 0.1 M). This reaction was also accomplished in less than 7 min, giving a value of Keq' of 16.2 ± 1.5 (the dots in Figure 4), slightly larger than the aforementioned value of Keq obtained by examining the reaction of Rb@Ti12Ser6 and CsCl. The resultant {Ti12} compound can be precipitated by adding methanol and subjected to repetitive exchange reactions, producing IR-pure Rb@Ti12Ser6 133 + after Cs NMR indicated no remaining Cs . The relative af+ + finity of K vs Cs to the host was compared based on the synthetic data. Since additional KCl did not decrease the 133 + yield of Cs@Ti12Ser6 (evident from Cs NMR), Cs is much + more favored as a guest than K . For the guest exchange reaction, additional kinetic data and analyses support a nonrupture mechanism. First of all, 17 time-resolved O NMR was used to study the exchange of μ2-
17
O of unmarked Cs@Ti12Ser6 with O-enriched water (see Figure S6 and the discussion). In this exchange reaction dissociation of a Ti–(μ2-O) bond is the rate-limiting step and thus the μ2-O exchange kinetics could be regarded as that of Ti–(μ2-O) dissociation. Since the μ2-O exchange reactions + + (completion time > 100 min) are much slower than Cs /Rb exchange shown in Figure 4 (< 7 min), dissociation of Ti–(μ2O) bonds should not be correlated with the fast kinetics of the latter. Then formation kinetics of Cs@Ti12Ser6 was meas133 ured by using Cs NMR (Figure S7). By mixing TiCl4, serine and CsCl, yield of Cs@Ti12Ser6 increased exponentially with time until approaching a plateau at ca. 73% after 60 min. + + Hence, formation of Cs@Ti12Ser6 is far slower than Cs /Rb exchange in Figure 4, and host rupture should lead to a much slower exchange kinetics than observed. Moreover, the guest exchange reactions showed reasonable mass balance, but a rupture-formation process would result in a yield of no more than 73%. Therefore, displacement of the guest from a {Ti12} host takes place via passing of the guest through the Ti6O6 pore of the host. While the solution-state diameter of the Ti6O6 pore remains immeasurable, it is clear that its crystal+ lographic diameter is substantially smaller than that of Cs (3.02 Å vs 3.34 Å), suggesting it be expanded to make passage + for a rigid Cs ion. This study and other reports about uptake 7a of organic molecules by the {Mo132} capsule as well as by the 18 metallacages, may suggest the aperture expansion phenomenon to be general in host-guest chemistry of soluble cagelike clusters. Next, information on host-guest interaction is provided. By a survey of the {Ti12} clusters in this study it is seen that without a guest (in the case of Ti12Pro6; entry 1 of Table 1), 12+ crystallographic dimension of the Ti12O18 cavity becomes larger. Such as, d1 of Ti12Pro6 is larger than that of Cs@Ti12Pro6 by 0.3 Å (entries 1–2) indicating attractive iondipole interaction between cationic guest and Ti–(μ2-O) di+ poles of the host. On the other hand, as guest varies from K + + to Rb to Cs , dimensions of the cavity and Ti6O6 pore remain unchanged (entries 3–5). Therefore, the established order of the univalent guests for their affinity to the {Ti12} + + + hosts (Cs > Rb > K ) indicates that the host-guest interac1c tion follows the “size complementarity” mechanism which was previously estabilished for explaining the higher selectivities of crown ethers and cryptands in capturing sizecomplementary alkali-metal cations (see Figure S8 and discussion).
Table 1. Key structural parameters of the TOCs abc a
Entry
TOCs
d1
1
Ti12Pro6
6.93
d2
a
5.51
d of pore
d + M
3.09
-
d
2
Cs@Ti12Pro6
6.64
5.48
3.06
3.34
3
K@Ti12Ser6
6.70
5.47
3.05
2.66
4
Rb@Ti12Ser6
6.76
5.41
2.99
2.94
5
Cs@Ti12Ser6
6.79
5.44
3.02
3.34
a
of
b
See Figure 1 for denotations. Average value for non-ideal 12+ c d hexagonal prismatic Ti12O18 shells. Unit is Å. Ionic diameter. In conclusion, we have developed a new class of watersoluble, host-guest TOCs based on a hexagonal prismatic,
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cryptand-like Ti12O18 framework. Based on structural information of the set of compounds, solution chemistry and guest exchange through their flexible Ti6O6 pores, we have clarified the attractive interaction between the hosts and the guests, and its effect on affinity of the guests to the hosts and the stabilities of the host-guest complexes, relying on size complementarity. Based on the solubility and stability in water, and the possibility to exchange guests and capping ligands, this class of TOCs are promising models for future research in supramolecular chemistry, especially those related to host-guest chemistry and bottom-up assembly of functional organic-inorganic hybrid materials.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/xxxxxxxxxx. Experimental details, crystallography, characterization and additional figures (PDF) X-ray crystallographic data (CIF)
AUTHOR INFORMATION Corresponding Author *
[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENTS Financial supports from NSFC (21401117), NSF of Shandong Province (ZR2014BQ003) and Shandong University (104.205.2.5) are gratefully acknowledged.
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