Ferrihydrite Particle Encapsulated within a Molecular Organic Cage

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Ferrihydrite Particle Encapsulated within a Molecular Organic Cage Masayuki Nihei, Hiromichi Ida, Takayuki Nibe, Adhitya Mangala Putra Moeljadi, Quang Thang Trinh, Hajime Hirao, Manabu Ishizaki, Masato Kurihara, Takuya Shiga, and Hiroki Oshio J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10957 • Publication Date (Web): 26 Nov 2018 Downloaded from http://pubs.acs.org on November 26, 2018

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Journal of the American Chemical Society

Ferrihydrite Particle Encapsulated within a Molecular Organic Cage Masayuki Nihei,*,†, Hiromichi Ida,† Takayuki Nibe,† Adhitya Mangala Putra Moeljadi,‡ Quang Thang Trinh,§ Hajime Hirao,‡ Manabu Ishizaki,‖ Masato Kurihara,‖ Takuya Shiga,† and Hiroki Oshio† † Faculty of Pure and Applied Sciences, Department of Chemistry, University of Tsukuba, Tennodai 1-1-1, Tsukuba 3058571, Japan ‡ Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China § Cambridge Centre for Advanced Research and Education in Singapore, Nanyang Technological University, 1 Create Way, Singapore 138602 ‖ Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata, 990-8560, Japan

ABSTRACT: Metal oxides with sizes of a few nm show variable crystal and electronic structures depending on their dimensions, and the synthesis of metal oxide particles with a desired size is a key technology in materials science. Although discrete metal oxide particles with the average diameter (d) smaller than 2 nm are expected to show size-specific properties, such ultrasmall metal oxide particles are significantly limited in number. In nature, on the other hand, nano-sized ferrihydrite (Fh), which is ferric oxyhydroxide, occurs as a result of biomineralization in ferritin, an iron storage protein cage. Here we describe the synthesis of Fh particle using a covalent molecular organic cage (MOC) derived from 8+12 cyclocondensation of triaminocyclohexane with a diformylphenol derivative. At the initial reaction stage, eight iron ions accumulated at the metal binding sites in the cage cavity, and Fh particles (d = 1.9 ± 0.3 nm) encapsulated within the cage (Fh@MOC) formed with quite a narrow size distribution. The formation process of the Fh particle in the organic cage resembles the biomineralization process in the natural iron storage protein, and the present method could be applicable to the synthesis of other metal oxide particles. Fh@MOC is soluble in common organic solvents and shows substantial redox activity in MeCN.

INTRODUCTION Metal oxides are ubiquitous in natural minerals and widely used in technological applications. In metal oxides with sizes of a few nm, the crystal and electronic structure and surfaceto-volume ratio change drastically with the particle size.1,2 It has been theoretically and experimentally evidenced that the size dependence of optical and magnetic properties become apparent as the particle sizes of semiconductor and metal oxide are reduced into the sub-2-nm range,3 and quantum confinement and spin-canting effects were observed for ultrasmall TiO2 and Fe2O3 particles with diameters smaller than 2 nm, respectively.4,5 The synthesis of metal oxide particles with sub-2-nm size is, therefore, critically important. Metal oxide nanoparticles have been synthesized by various methods such as sol-gel, reverse micelles, and thermal decomposition methods.6-8 However, discrete metal oxide particles with an average diameter (d) smaller than 2 nm have been scarcely reported,4,5,9 and the precise synthesis of metal oxide particles with a desired size is still challenging. Organic cages, which are discrete molecules with a nano-sized cavity constructed by covalent bonds, have been recently recognized as useful templates to prepare discrete noble metal particles with precisely controlled dimensions, where the interior space of the cage provides an environment for a size-constrained reaction for metal particles.10 Fujita et al. have reported the syntheses of highly monodisperse SiO2 and TiO2 particles using coordination cages, which are self-assembled spherical complexes with hollow structures.11,12 As molecular cages have well-defined structures and high designability of their size and shape, utilization of a molecular cage is a promising way to prepare metal

oxide particles with a precisely controlled size. However, there have been no reports on preparation of transition metal oxide particles other than TiO2 using molecular cages, and nucleation and the following growth of the metal oxide core within the cage structure are key to achieving the synthesis of highly monodisperse metal oxide particles. In nature, nano-sized ferrihydrite (Fh), which is a ferric oxyhydroxide, occurs as a result of biomineralization and forms the inorganic core of ferritin, an iron storage protein cage.13 Ferritin was the first protein to be utilized for metal oxide synthesis,14 and various protein cages have been reported as useful platforms for the synthesis of inorganic nanomaterials.15 The process of iron ion accumulation and iron oxide nanoparticles formation within a protein cage was investigated on LiDps, which is a DNA binding protein from starved cells from the Gram-negative bacterium Listeria innocua.16 LiDps is a member of the ferritin superfamily with an outer diameter of 9 nm and an inner cavity diameter of 5 nm, and produces an iron oxide core, similar to that of typical ferritins.17 In the proposed mechanism of mineralization in LiDps, the iron oxide formation within the protein cage proceeds in two steps: 1) initial accumulation of 12 iron ions at the binding sites within the protein cage, and 2) the following nucleation and autocatalytic condensation at the binding sites to provide iron oxide nanoparticles.16 If the bio-inspired synthesis of nano-sized metal oxides is achieved utilizing molecular cages, it is expected to provide a high degree of control over the metal oxide morphology, composition, and polymorph selection. For the bio-inspired synthesis of iron oxide particles smaller than 2 nm, the molecular cage should satisfy the following conditions,

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1) it should have a covalent framework to retain its cage structure during the iron oxide formation, 2) it should form a hydrophilic interior cavity with multiple binding sites for iron ions for the accumulation of iron ions, and 3) the interior cavity size should be smaller than 2nm. In this work, we focus on an amphiphilic molecular organic cage (MOC) reported by Skowronek et al., which was synthesized by a condensation reaction of 4-tert-butyl-2,6-diformylphenol with cis,cis-1,3,5triaminocyclohexane in 12:8 ratio.18 MOC is a covalent cuboctahedral cage constructed from twelve phenol and eight cyclohexane units bridged through imine bonds (Figure 1). The MOC possesses a hydrophilic interior cavity with a diameter of 1.87 nm and a hydrophobic exterior formed by butyl groups, and the MOC cavity is expected to be a suitable template for the preparations of metal oxide particles smaller than 2 nm (Figure S1 in the Supporting Information (SI)). The interior cavity contains twelve chelating units composed of phenol oxygen and imino nitrogen atoms. We have previously reported that an organic ligand with one phenolate and two imino nitrogen units act as chelating ligands to afford multinuculear molecular 3d metal oxides.19 The chelating units in the MOC cavity are, therefore, expected to act as a metal binding sites as do those in the LiDps protein cage. With that in mind, we demonstrated the accumulation of iron ions and the following formation of a Fh particle within the MOC cavity (Figure 1). At the initial reaction stage, eight iron ions accumulated at the chelating sites in the MOC cavity, and ultrasmall Fh particles (d = 1.9 ± 0.3 nm) encapsulated within MOC (Fh@MOC) formed with quite a narrow size distribution.

Figure 1. Synthesis of ultrasmall ferrihydrite (Fh) particles encapsulated within the molecular organic cages (MOC), Fh@MOC. In the proposed model of Fh@MOC, Fe centers are orange; N, blue; O, red; Cl, green; C, grey.

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RESULTS AND DISCUSSION Synthesis. Fh@MOC was prepared by following supplementary scheme 1. The mixture of MOC and FeCl2ꞏ4H2O in a 1:90 molar ratio in MeOH/CH2Cl2 (2:1) was stirred for 24 hours under a N2 atmosphere, and then a clear dark-brown solution was obtained. The solution turned into dark-brown suspension after addition of a methanol solution of NaOH. When the mixture was exposed to air, the precipitate slowly dissolved and a clear dark-brown solution resulted after 24 hours. The reaction solvent was then changed from MeOH/CH2Cl2 to benzonitrile, and the benzonitrile solution was heated at 180°C for 24 hours. The reaction solution was concentrated, and the addition of Et2O afforded brown precipitate. The precipitate was washed with water and extracted with methanol, and then the solution was evaporated to obtain Fh@MOC as a dark-brown powder. The isolated dark brown powder of Fh@MOC is soluble in common organic solvents such as MeOH and MeCN, and no precipitation was observed over a period of several months. These results suggested that the Fh core is effectively stabilized within the cavity of MOC, of which the exterior prevents the agglomeration of particles without use of any additional protecting groups. To ensure the role of MOC, the same reaction was carried out without MOC as the control experiment. The reaction of FeCl2ꞏ4H2O with NaOH followed by the air oxidation gave a clear yellow solution. The thermal treatment of the benzonitrile solution at 180°C gave an appreciable amount of brown insoluble material, which is in contrast to the reaction in the presence of MOC. The formation of insoluble material indicates that the heating process afforded nucleation of the iron oxide particles in solution, which rapidly agglomerated to form bulk iron oxide. The powder X-ray diffraction (XRD), atomic force microscopy (AFM) and IR measurements were conducted on the insoluble material formed in the control experiment (Figure S2 in the SI). The powder XRD profile of the insoluble material is characteristic of so-called 2-line ferrihydrite, which is a lowcrystalline disordered form of ferrihydrite.20 The IR spectrum agrees well with the spectrum for the 2-line ferrihydrite with adsorbed water and carbonate ions.21 The TEM image showed agglomerated bulk materials and no discrete particles were observed, indicating that the product in the control experiment is the bulk 2-line ferrihydrite. The result of the control experiment suggested that the growth of an iron oxide core in the presence of MOC is confined and stabilized inside the cavity of MOC during the reaction. IR spectra of Fh@MOC and MOC are shown in Figure S3 in the SI. In the IR spectrum of MOC, characteristic absorption peaks were observed at 29572862 cm-1 and 1635 cm-1, which are assignable to the alkyl CH and imine C=N stretching modes, respectively. The IR spectrum of Fh@MOC contains absorption peaks originating from MOC, and the additional broad absorption bands were observed at 400-800 cm-1, which are assigned to the Fe-O stretching bands characteristic of iron oxides.21 Strong broad absorption bands at 3000-3600 cm-1 in Fh@MOC are assigned to the OH stretching modes of adsorbed water molecules typically observed in the hydrated iron oxides.21 In Fh@MOC, a C=N absorption peak was observed at 1616 cm-1 with the original peak observed in MOC at 1635 cm-1 as a shoulder, suggesting a change in the chemical environment of a part of imine groups from that of the free MOC. The lower energy shift of the new peak compared with that in MOC indicates the coordination of a part of imino nitrogen atoms to the iron ions.22 These results strongly suggest that the iron oxide parti-


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cle was formed within the cavity of the MOC and was supported by the imino nitrogens of MOC through coordination bonds.

Figure 2. (a) MALDI-TOF-MS spectra of the reaction solution stirred for 24 hours (red) and 1 hour (blue). (b) A DFT-optimized structure of [Fe8O4@MOC], where Fe centers are orange; N, blue; O, red; and C, grey.

To probe the Fe ion accumulation behaviour of MOC at the initial stage of the synthetic reaction, matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS) measurements were conducted on the reaction solution. The starting materials of MOC and FeCl2ꞏ4H2O were mixed in MeOH/CH2Cl2 (2:1) under N2, and MS spectra were recorded on the solution after stirring for 1 and 24 hours (Figure 2a). In the spectrum obtained after 1 hour, the peaks showed broad distributions containing some prominent peaks which are assigned to [Fe5O3@MOC], [Fe7O3@MOC] and [Fe8O4@MOC] species by considering their isotope patterns, and no peaks originating from the free MOC were observed (Figure S4, S5 and Table S1 in the SI). After 24 hours, the peaks with smaller m/z values disappeared and the peak of [Fe8O4@MOC] became predominant. These results revealed that the iron ions accumulated within the interior cavity of MOC and [Fe8O4@MOC] was selectively formed as a stable product of the initial reaction stage. The formation of [Fe8O4@MOC] species is explained by considering the structure of the MOC cavity (Figure S6 in the SI). The MOC contains eight cyclohexane moieties with three imino nitrogens, which are linked by twelve phenol moieties. Be-

cause all salicyldiimine moieties adopt a syn,anti conformation, there are two sets of four cyclohexane moieties with different conformations, named A and B sites. Each of the A sites has three salicylimine moieties with syn conformations and both imino nitrogen and phenol oxygen atoms point to the interior side of MOC. By contrast, a B site is attached by three salicylimine moieties with anti conformations, and three imino nitrogen atoms point outward. Consequently, the four A sites in MOC can act as multiple metal chelating sites for the accumulation of iron ions inside of the MOC cavity. A possible structure of [Fe8O4@MOC] was obtained by DFT calculations (Figure 2b and Figure S15 in the SI). In the resultant structure, there are four oxo-bridged dinuclear [Fe2O] units at the four sets of metal chelating A sites inside the MOC cavity, which may act as the nuclei of the following iron oxide particle growth within the MOC cavity. The formation process of the Fh particle within MOC resembles the process of iron oxide formations in the natural iron storage protein of LiDPs. Size of Fh@MOC. The size of Fh particles in Fh@MOC was characterized using scanning transmission electron microscopy (STEM). The STEM image shows well-dispersed bright particles with quite a narrow size distribution (Figure 3a). The average diameter of the particles in Fh@MOC was estimated to be d = 1.9 ± 0.3 nm, which is consistent with the diameter (1.87 nm) of the MOC cavity (Figure S1 in the SI), and suggests the encapsulation of the Fh particle within MOC (Figure 3b). Energy dispersive X-ray spectrometry (EDS) analysis of the particles gave prominent peaks assigned to iron and oxygen atoms, indicating that the particles are composed of iron oxides (Figure S7 in the SI). The AFM image shows well-separated particles with the height of ca. 3 nm (Figure 3c and Figure S8a in the SI). In the height distribution histogram, there are two maxima at ca. 2.6 and 3.5 nm and the average height of the particles was estimated as 3.2 ± 0.7 nm (Figure S8b in the SI). The height distributions of the particles agree with the shape of the MOC framework with outer dimensions of 2.64 and 3.42 nm for the smallest and the largest lengths (Figure S1 in the SI), suggesting the accommodation of Fh particles within MOC. It should be noted that the AFM image of the free MOC showed particles with height distributions lower than ca. 2nm (Figure S9 in the SI), which might be due to the deformation of the empty cage structure on the substrate. Thus, according to the STEM and AFM measurements, the Fh@MOC has an iron oxide core with d = 1.9 ± 0.3 nm, and the core particle is encapsulated by MOC. Characterization of the Fh particle in Fh@MOC. The powder X-ray diffraction (XRD) pattern of Fh@MOC showed two broad peaks at 2 = 22.5 and 38.8° corresponding to dvalues of 2.56 and 1.51 Å, respectively (Figure 3d). The XRD profile is characteristic of 2-line ferrihydrite, which is lowcrystalline disordered form of ferrihydrite.20 The selected area electron diffraction (SAED) pattern of Fh@MOC showed two broad rings at 0.15 and 0.25 nm, consistent with the XRD data (Figure S10 in the SI). A high resolution TEM image of Fh@MOC shows well dispersed crystalline particles with lattice fringes, and the d-spacing was estimated as 2.5 Å (Figure 3e). 57Fe Mössbauer spectra of Fh@MOC were recorded at 150 K and 7 K (Figure 3f and Figure S11 in the SI) to investigate the electronic state of iron ions in the Fh core. The spectrum at 150 K was composed of one quadrupole doublet with Mössbauer parameters (relative to metallic iron) of  = 0.42 and EQ = 0.81 mm s-1, where  and EQ denote the isomer



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Figure 3. (a) STEM image of Fh@MOC, where the scale bar is 20 nm. (b) Size distribution histogram of Fh particles in Fh@MOC from STEM analysis. (c) AFM image of Fh@MOC on mica. (d) PXRD pattern of Fh@MOC. (e) High resolution TEM image of Fh@MOC. The scale bar is 2 nm. Inset of Figure 3e: magnified view of the Fh particle. (f) 57Fe Mössbauer spectrum of Fh@MOC at 150 K.

shift and the quadrupole splitting, respectively. The parameters are characteristic of high spin Fe(III) ions in the ferrihydrite.23 A magnetic hyperfine sextet was observed in the spectrum at 7 K, which is caused by the magnetic order in the Fh core. The ferrihydrite has been known to exhibit super paramagnetism, and the magnetic moment arises from the uncompensated spins on the particle surface and the disorder of cation vacancies.23 The magnetic hyperfine field in Fh@MOC was estimated as 47 T, which agrees well with the value for the ferrihydrite particle prepared within the natural protein.24 Thus, XRD, SAED, high resolution TEM, and 57Fe Mössbauer data revealed that the Fh particles encapsulated within MOC is 2-line ferrihydrite. Chemical composition and proposed structure of Fh@MOC. Although the crystal structure and chemical composition of ferrihydrite are still under debate, Michel et al. have proposed a single phase crystal structure of ferrihydrite, which was derived from the analyses of the pair distribution function of X-ray total scattering data.25 The resulting structure has hexagonal space group P63mc and a unit cell withaverage dimensions of a = ~5.95 Å and c = ~9.06 Å. They proposed that the compositions of disordered (2-line) and ordered (6-line) ferrihydrite are Fe8.2O8.5(OH)7.4 + 3H2O and Fe10O14(OH)2 + ~H2O, respectively, which have basically the same structural topology, and the different composition is caused by the random distribution of iron vacancies in two specific cation sites of the crystal structure. The XRD, XAED and TEM measurements of Fh@MOC revealed that the Fh particle formed within MOC is 2-line ferrihydrite, and thus the chemical formula of the particle is expected to be [Fe8.2O8.5(OH)7.4]n. The quantitative analyses of Fe, C, H, N,

and Cl in Fh@MOC were carried out using inductively coupled plasma atomic emission spectroscopy (ICP-AES) and elemental analyser, and the chemical composition of Fh@MOC was determined as [(Fe8.2O8.5(OH)5.4)11MOC]Cl22TA10, where 22 OH groups in Fh particles were replaced by Cl- anions by considering the charge valance, and TA is triaminocyclohexane possibly derived from the partial decomposition of MOC during the synthetic reaction. The result suggested that 2-line ferrihydrite particle with ca. 90 Fe ions, which corresponds to eleven unit cells of the single crystal phase proposed by Michel et al., are accommodated within the MOC cavity. The existence of 22 Cl atoms may be caused by the adsorbed chloride ions on the surface of the Fh particle. A proposed structure of the Fh@MOC was modelled by considering the previously reported single crystal structures of MOC and ferrihydrite,18,25 and suggested that the MOC cavity is capable of accommodating the Fh particle with ca. 90 Fe ions (Figure 1 and Figure S16 in the SI). Electronic structure and redox activity of Fh@MOC. The native ferrihydrite (d = 9.05 ± 1.44 nm) in ferritin shows an absorption band with an indirect band gap energy (Eg) of 2.09 eV, which is associated with a lower energy absorption band originating from a defect-related state.26 UV-vis absorption spectrum of Fh@MOC in methanol was measured to elucidate the Eg value of the Fh particles encapsulated within MOC. The absorption spectrum showed broad absorption bands over wavelengths shorter than 750 nm (Figure S12 in the SI). The gradual increase of absorption intensity at ca. 650 nm is ascribable to the onset of the indirect band gap transition in the Fh core of Fh@MOC. The Eg value of the Fh particle


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Figure 4. (a) Tauc plot of Fh@MOC in methanol (black circle). The Eg value was estimated using the relation of (h)1/2  (hEg) and the red line is a liner fitting of the band gap edge. (b) Cyclic voltamogram of Fh@MOC in MeCN at the scan rate of 0.05 – 1.00 V/s. (c) ip vs. v1/2 plot obtained from cyclic voltamograms of Fh@MOC with different scan rates.

was estimated as 2.29 eV by a linear extrapolation of the band gap edge observed in Tauc plot of Fh@MOC (Figure 4a), which is larger than for the native ferrihydrite. The ferrihydrite particles in ferritin have been known to exhibit size dependence of the indirect band gap, and smaller particles have larger indirect band gaps due to the quantum confinement effects.26 The larger value of Eg in Fh@MOC might be due to the quantum confinement effects. The redox behaviour of Fh@MOC in MeCN was investigated using cyclic voltammetry (CV) at the scan rate of 0.05 – 1.00 V/s (Figure 4b). Fh@MOC showed a reduction wave at E0’ = -1.09 V vs. Ag/Ag+ at the scan rate of 0.10 V/s. The reduction potentials of the free MOC were -1.96 and -2.23 V (Figure S13); therefore the reduction wave observed in Fh@MOC originates from the Fh core. Colloidal Fe2O3 particles adsorbed on an ITO electrode have been reported to show an Fe(III)/Fe(II) reduction at -0.66 V vs. SCE (ca. -0.96 V vs. Ag/Ag+),27 and the reduction wave in Fh@MOC is assigned to the Fe(III)/Fe(II) reduction process. The Faraday current of the reduction peak showed a linear correlation with the square root of the scan rate (Figure 4c), suggesting that the observed redox wave originates from the diffusion controlled redox process of the Fh@MOC dispersed in MeCN, and the redox reactions of the adsorbed Fh@MOC on the electrode surface can be excluded. The redox activity is known to be suppressed in the conventional iron oxide nanoparticles dispersed in organic solvent, since the densely packed surface by surfactants and the larger volume of the particles lead to slower electron transfer rates and smaller diffusion constants than those of small molecules.27,28 In Fh@MOC, the exposed surface of the Fh particle at the open sites of the MOC and the ultrasmall size comparable to the molecules may lead to the substantial redox activity in MeCN. CONCLUSION We have reported the synthesis of highly monodisperse ultrasmall Fh particles using a well-defined molecular organic cage, MOC, as a reaction template. The resulting Fh particle with d = 1.9 ± 0.3 nm was encapsulated within the MOC cavity, and the formation process of Fh particles resembles the biomineralization process in the natural iron storage protein. The present method could be applicable to the synthesis of other metal oxide particles smaller than 2 nm, and may pave the way to exploring new physical and chemical properties of

ultrasmall metal oxides. In addition, since Fh@MOC is soluble in common organic solvents, Fh@MOC is expected to find applications as redox active materials and homogeneous catalysts. Synthesis of new ultrasmall metal oxide particles and investigation of the catalytic activity of Fh@MOC are now in progress in our laboratory. EXPERIMENTAL SECTION Preparation of Fh@MOC. All reagents were obtained from commercial suppliers and were used without further purification unless otherwise noted. Anhydrous dichloromethane and methanol for the synthetic reaction were purchased from Wako Pure Chemical Corporation and were used after degassing by Freeze-Pump-Thaw cycling. MOC was synthesized following the literature methods.18 FeCl2ꞏ4H2O (232 mg, 1.17 mmol) and MOC (39.9 mg, 0.013 mmol) were dissolved in methanol (40 cm3) and dichloromethane (20 cm3) under nitrogen atmosphere. After stirring for 24 hours, NaOH (46.8 mg, 1.17 mmol) in methanol (5 cm3) was added to the resulting dark brown solution. Brown slurry was obtained after stirring for 24 hours, and the reaction mixture was exposed to the air. The reaction solution was oxidized by stirring for 24 hours under an aerobic condition to give a clear dark brown solution. Benzonitrile (300 cm3) was added to the reaction solution and methanol and dichloromethane was removed by evaporation under reduced pressure. The resulting benzonitrile solution was heated at 180 °C for 24 hours and a slight amount of precipitate was filtered off. The filtrate was concentrated, to which diethyl ether was added to obtain dark brown precipitate. The precipitate was washed with water and extracted with methanol, which was evaporated to obtain dark brown powder (17.3 mg, 0.014 mmol, 11 % yield). Analalysis (calcd., found for C252H377.4Cl22Fe90.2N54O164.9 ([Fh@MOC]Cl22ꞏTA10, Fh = (Fe8.2O8.5(OH)5.4)11, MOC = C192H228N24O12, TA = C6H9N3 (cyclohexanetriamine)): C (23.99, 24.54); H, (3.01, 2.82); Cl (6.18, 6.16); Fe (39.92, 40.2); N (5.99, 6.26); UV/Vis: max 360, 500 nm; IR (KBr): 2957, 1616 cm-1.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.



Experimental procedures, synthetic scheme, IR spectra, MALDITOF-MS spectra, EDS spectra, AFM data, SAED pattern, 57Fe Mössbauer spectrum, UV-vis absorption spectra, and structural modeling detail.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by JST, PRESTO (Grant Number JP10721 to MN and JPMJPR141B to HH), Japan, and by JSPS KAKENHI Grant Number JP18H01989 and JP16H06523 (Coordination Asymmetry). HH is grateful for financial support from City University of Hong Kong (7200534 and 9610369).

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TOC and Abstract graphic


Ferrihydrite Particle Encapsulated within a Molecular Organic Cage

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