・Introduction

The assembly of molecular or atomic building blocks into ordered solid compounds, which are applicable to selective sorption, separation, and heterogeneous catalysis, has been an attractive research area in material chemistry. Polyoxometalates (POMs) are nano-sized metal-oxide macroanions and have a growing interest as building blocks of ordered solid compounds. However, the combination of POM with a small sized and highly charged cation (e.g., first-row transition metal cation), usually forms a highly soluble solid. The complexation of POMs with appropriate organometallic complexes (macrocations) can create insoluble ionic crystals with specific structures. In this work, porous ionic crystals of polyoxometalate-organometallic complex are synthesized by the control of the shape, size, and charge of the constituent ions and the sorption properties are investigated (Figure 1).

1. Control of structures and sorption properties of ionic crystals of A2[Cr3O(OOCC2H5)6(H2O)3]2[α-SiW12O40] (A = Na, K, Rb, Cs, and TMA)

The complexation of Keggin-type polyoxometalate [α-SiW12O40](4-), macrocation [Cr3O(OOCC2H5)6(H2O)3]+, and monovalent cation A+ formed ionic crystals of A2[Cr3O(OOCC2H5)6(H2O)3]2[α-SiW12O40]・nH2O (A = Na [1a], K [2a], Rb [3a], Cs [4a], and tetramethylammonium (TMA) [5a]). Single crystal and powder X-ray analyses showed that the ionic crystals possess 2D-layers of polyoxometalates and macrocations. Compounds 2a-4a were isostructural, while the layers in 1a and 5a stacked in different ways. Among the isostructural guest free phases of 2b-4b, the lengths of the b-axes, which are twice the value of the interlayer distance, increased with the increase in the ionic radius of the monovalent cations (2b (K+: 1.52 A) < 3b (Rb+: 1.66 A) < 4b (Cs+: 1.81 A)), which resided between the layers (Table 1). Compounds 2b-4b possessed hydrophobic and hydrophilic channels, which existed between the layers and through the layers, respectively (Figure 2). The volumes of the hydrophobic channels increased in the order of 2b < 3b < 4b and those of the hydrophilic channels increased in the order of 2b <=3b < 4b (Table 1).

Figure 3 shows the vapor sorption isotherms of 2b-4b at 298 K. The amounts of water sorption increased with the increase in the water vapor pressure and were almost leveled off around P/P0 = 0.8 (Figure 3A). The amounts of sorption reached up to 19-22 μL g-1 at P/P0 = 0.8, and the values approximately agreed with the volumes of the hydrophilic channels (19-21 μL g-1). The water vapor sorption profiles of 2b-4b were reproduced by a linear driving force model, Mt = Me {1 - exp(-k1t)}, suggesting that water is sorbed only in the hydrophilic channel. As for amphiphilic n-propanol, the amounts of sorption increased in the order of 2b <= 3b < 4b (Figure 3B). The amounts of sorption at P/P0 > 0.6 exceeded the volumes of the hydrophilic channel, and reached up to 30, 45, and 55 μL g-1 for 2b, 3b, and 4b, respectively, of which the values approximately agreed with the sums of the volumes of hydrophilic and hydrophobic volumes. The n-propanol sorption profiles were reproduced by the summation of the linear driving force model, Mt = Me1 {1 - exp(-k1t)} + Me2 {1 - exp(-k2t)}, showing that two independent barriers exist in the n-propanol sorption. Therefore, amphiphilic n-propanol was sorbed into both hydrophilic and hydrophobic channels. Compound 4b sorbed hydrophobic dichloromethane, while the amounts for 2b and 3b were comparable to or smaller than those of surface adsorption (< 5 μL g-1) (Figure 3C). This is probably because the opening of the hydrophobic channel of 4b (4.0 x 5.2 A) was large enough to accommodate dichloromethane (d = 4.2 A), while the opening of the hydrophobic channels of 2b (2.5 x 5.1 A) and 3b (3.4 x 5.1 A) were too small. The amount of dichloromethane sorption for 4b around saturation pressure at 273 K was 25 μL g-1. The value fairly agreed with the volume of the hydrophobic channel of 4b (30.0 ± 3.0μL g-1), which is in accord with the idea that hydrophobic halocarbons are sorbed only in the hydrophobic channel.

When 4b was exposed to a gas flow containing water (P/P0 = 0.60) and dichloromethane (P/P0 = 0.40), the weight increased as shown in Figure 4. The best fits for the experimental data by the summation of the linear driving force model were given by k1 = 1.7 x 10-2 s-1, Me1 =16.4 μL g-1 (1.64wt%), k2 = 3.0 x 10-3 s-1, and Me2 = 7.0 μL g-1 (0.93wt%). The k1 and Me1 values were close to those of water sorption at P/P0 = 0.60 and the k2 and Me2 values were close to those of dichloromethane sorption at P/P0 = 0.40. Therefore, the rate and equilibrium amount of the dichloromethane sorption into the hydrophobic channel and those of water into the hydrophilic channel of 4b were independent of each other, and the phenomenon was different from those of zeolites and activated carbons, of which the amounts of dichloromethane sorption are decreased by the presence of water.

The collection of dichloromethane and water from the gas mixture was attempted with 4b according to Scheme 1. Compound 4b was exposed to the gas mixture of water and dichloromethane at 298 K for 12 h to form 4b・5.4±0.2H2O・0.8±0.05CH2Cl2. After the removal of the coexisting gases at 203 K, the sample was heated at 273 K and kept for 1.5 h. The amount of dichloromethane evolved was 9.9±0.5 μL g-1 (0.70±0.05CH2Cl2 per 4b). Then the sample was evacuated at 298 K for 1.5 h and the amount of water collected was 22±1 μL g-1 (5.2±0.2H2O per 4b). Thus, dichloromethane and water sorbed in 4b were successfully collected.

2. 3D-arrangements of [M3O(OOCC6H5)6 (H2O)3]4 [α-SiW12O40] (M = Cr, Fe) utilizing the π-π stacking

The complexation of [α-SiW12O40]4- with [M3O(OOCC6H5)6(H2O)3]+ (M = Cr, Fe) was attempted for the aim to utilize the π-π stacking in the arrangement of the macroions and to increase the hydrophobicity of the ionic crystals. Ionic crystals of [M3O(OOCC6H5)6(H2O)3]4[α-SiW12O40]・nH2O・mCH3COCH3 (M = Cr [6a], Fe [7a]) were formed from an acetone/water solution. Single crystal X-ray analyses showed that 6a and 7a were isostructural. As shown in Figure 1, the macrocations formed 8-membered rings, which incorporated polyoxometalates. The distances between the benzene groups of the neighboring macrocations were 3.4-3.6 A, and the π-π interaction probably stabilizes the crystal structure. Compounds 6a and 7a sorbed small organic molecules in the crystal lattice and heterogeneously catalyzed the pinacol rearrangement.

3. Monodispersed crystalline nano-particles of [Ni(tacn)2]2[α-SiW12O40]

The complexation of [α-SiW12O40]4- with [Ni(tacn)]2+ (tacn = triazacyclononane) yielded monodispersed crystalline nano-particles of [Ni(tacn)2]2[α-SiW(12)O(40)]・4H2O [8a] (Figure 1). The powder X-ray analysis of 8a showed that the macroions were closely packed into a trigonal cell. The BET surface area of anhydrous form 8b was 31 m2 g-1, and the αs-plot showed that 8b was non-porous. The average diameter of 8b calculated with the surface area and density was 51 nm and the value fairly agreed with that of the averaged particle size observed by SEM (59 nm). The formation of nano-particles was probably due to the strong ionic interaction between the multiple-charged macroions ([α-SiW12O40]4- with [Ni(tacn)]2+) and the hydrophobicity of the organic moiety which facilitated the nucleation and prevented the aggregation of the particles. The vapor sorption isotherms showed that 8b adsorbed hydrophobic tetrachloromethane as well as water and dinitrogen. The complexation of [y-SiV2W10O38(OH)2]4- with [Ni(tacn)]2+ yielded crystalline nano-particles of [Ni(tacn)2]2[y-SiV2W10O38(OH)2]・3H2O [9a], which was isostructural with 8a. Compound 9a heterogeneously catalyzed the epoxidation of olefins with H2O2 maintaining the stereoselectivity of the tetra-n-butylammonium salt of [y-SiV2W10O38(OH)2](4-) in the homogeneous reaction system.

Figure 1. Polyoxometalate-organometallic ionic crystals. (1) Crystal structures of A2[Cr3O(OOCC2H5)6(H2O)3]2[α-SiW12O40] (A = K, Rb, and Cs), (2) [M3O(OOCC6H5)6(H2O)3]4[α-SiW12O40] (M = Cr, Fe), and (3) SEM image and electron diffractogram of crystalline nano-particles of [Ni(tacn)2]2[α-SiW12O40] (tacn = 1,4,7-triazacyclononane).

Table 1. Structural Properties of 2b4b

Figure 2. Local structures of Rb2[Cr3O(OOCC2H5)6(H2O)3]2[α-SiW12O40] [3b] in the (a) bc- and (b) ab-plane. Dotted rectangles showed the layers of 3b. Hydrophilic and hydrophobic channels were indicated by the shadings.

Figure 3. Vapor sorption isotherms of 2b-4b at 298 K. (A) Water, (B) n-propanol, and (C) dichloromethane. Circle: 2b, triangle: 3b, and square: 4b. Open squares in (C) showed the results at 273

Figure 4. Changes in the weight of 4b by the exposure to a gas mixture of water and dichloromethane at 298 K. Solid line (a) showed the experimental data. Solid lines (e) and (f) showed the experimental sorption data of water and dichloromethane, respectively. Solid circle (b) showed the calculated data and open circles (c) and (d) showed the two components for the calculation.

本論文は「ポリオキソメタレート-有機金属錯体イオン性結晶の合成とその収着特性」と題し,全5章より構成されている.

第1章は序論であり,サブナノ空間を有する結晶性固体の概要とそれらの特徴を説明している.次に,新規な固体としてイオン性結晶を挙げ,構成ブロックとしてマクロイオン(ポリオキソメタレート(アニオン)と有機金属錯体(カチオン))を用いることの有用性と,イオン性結晶K3[Cr3O(OOCH)6(H2O)3][α-SiW12O40]が親水性分子を形状選択的に収着すること,マクロカチオンの架橋配位子をギ酸イオンからプロピオン酸イオンへと変えると親水性・疎水性チャネルを併せ持つイオン性結晶K2[Cr3O(OOCC2H5)6(H2O)3]2[α-SiW12O40]が生成することがこれまでに明らかになっていることを説明している.そして本研究の目的が,これらの知見を拡張し,適切な構成ブロックを組み合わせることにより,イオン性結晶の構造,分子収着・触媒特性を系統的に制御することであることを述べている.

第2章では,一価カチオン-マクロカチオン-ポリオキソメタレート(A2[Cr3O(OOCC2H5)6(H2O)3]2[α-SiW12O40], A = Na, K, Rb, NH4, Cs, TMA)の組み合わせにより構成されるイオン性結晶の構造と分子収着特性制御が可能であることを明らかにしている.イオン性結晶の親水性・疎水性チャネルの孔径及び体積は,一価カチオンのサイズにより系統的に制御される.さらに,分光学及び速度論的検討により,水(親水性分子),プロパノール(両親媒性分子),ジクロロメタン(疎水性分子)は,それぞれ,親水性チャネル,親水性及び疎水性チャネル,疎水性チャネルに収着されることを明らかにしている.また,疎水性チャネルの孔径が最も大きいCs2[Cr3O(OOCC2H5)6(H2O)3]2[α-SiW12O40]は,水/ジクロロメタン混合蒸気の分離回収能を有することも明らかにしている.

第3章では,芳香族配位子を有するマクロカチオン[M3O(OOCC6H5)6(H2O)3]+ (M = Cr, Fe)を構成ブロックとして用いると,芳香環のπ-πスタックにより形成されるマクロカチオン八員環の内部にポリオキソメタレートが取り込まれたイオン性結晶が生成し,このイオン性結晶がピナコール転位反応の不均一系触媒として機能することを明らかにしている.マクロカチオンの金属イオンをCr(III)からFe(III)に変えると反応速度が大きくなる,より分子サイズの大きなベンゾピナコールは反応しない,ことから,本反応はマクロカチオンが活性点として機能し,固体内部で反応が進行するものと推定している.

第4章では,多価のマクロカチオン[Ni(tacn)2]2+ (tacn = 1,4,7-triazacyclononane)を構成ブロックとして用いると,結晶性ナノ粒子が生成することを明らかにしている.ナノ粒子の生成は,多価かつ疎水性の高いマクロカチオンを用いることにより核生成が促進され、その結果粒子成長が抑制されるためと推定している.さらに,ポリオキソメタレートを[α-SiW12O40]4-から[y-H2SiV2W10O40]4-に変えると,得られたナノ粒子がアルケンのエポキシ化反応の不均一系触媒として機能することを明らかにしている.

第5章は全体の総括である.

以上,本論文では,適切な構成ブロックの組み合わせにより,イオン性結晶の構造・分子収着特性を系統的に制御できることを明らかにしている.これらの結果は,無機合成化学,物理化学,触媒化学において重要な知見である.従って,本論文は博士(工学)の学位請求論文として合格と認められる