Interactions of atoms, ions, molecules and clusters both at outer surface and inside pores of porous materials are of scientific and technological interests. Besides the well established applications of porous materials, i.e. ion exchange, adsorption and catalysis, emerging areas of application, for example, low-k dielectric films for use as electrical insulators, molecular recognition and separation membranes, chemical sensors, and optoelectronic devices, show great promise. Zeolite, which possesses hydrated, crystalline, microporous tectoaluminosilicate, is a representative class of the nanoporous materials. Progress in synthesis of zeolite has been made through experimentally trial-and-error routine; hence, the products crystallized in a particular system are hardly predicted. Moreover, designed synthesis of zeolites with tailored porosity has been impossible because of a lack of understanding in their crystallization mechanisms.

In the chapter 1, brief introduction of porous and cluster-built materials, and scope and structure of the present doctoral dissertation are presented. Bottom-up approach has been focused as a potential strategy to innovate a priori materials with desired structures and properties. Strategic building blocks can be designed to structure-direct the formation of objective materials, which leads to a wide spectrum of desired functions. In this doctoral dissertation, the bottom-up concept has been applied to synthesize inorganic-organic hybrid materials from well designed molecular building units. Cubic siloxane-based cages, which resemble to the double four-ring (D4R) unit recognized as a secondary building unit of many zeolite frameworks, have been chosen as the starting units due to their rigidity, high symmetry, and functionalization capability.

Considering intermolecular interactions as a primary parameter affecting the final structures of the hybrid materials, D4R cages with different organic moieties that can form relatively weak π-π interaction and hydrogen bond, moderately strong coordination bond, and strong covalent bond have been designed and synthesized mainly by Pt-catalyzed hydrosilylation and cross-metathesis reactions as described in the chapter 2. Mono-addition of organic groups that are able to provide weak intermolecular interaction (i.e., aromatic rings and urea group) and octa-addition of organic groups that can form stronger coordination (i.e., carboxy group) and covalent bonds (i.e., aryl bromide) into the terminal groups of D4R cages have been studied. D4R cages with Si-H or vinyl groups as terminal groups have been synthesized, and subsequently functionalized into desired organic moieties. The products with high regio- and stereoselectivity have been obtained under the investigated conditions. Such functionalized D4R cages have been used for the construction of inorganic-organic hybrid materials described in the chapter 3-5.

Siloxane D4R cages with the partially substituted end-groups has been considered as one of the suitable units for the formation of crystalline silica-based inorganic-organic hybrid materials because the unsubstituted end-groups (i.e., Si-H) can further be condensed to form siloxane bonds (Si-O-Si). In the chapter 3, monosubstituted D4R cages with aromatic rings can be crystallized into molecular crystals with the assistance of π-π interaction. The crystals have been modeled as the lamellar structures based on the X-ray diffraction patterns and computational structural minimization. The unsubstituted end-groups have been converted into alkoxy (Si-OR) or silanol (Si-OH) groups by Ir-catalyzed hydrolytic oxidation or alcoholysis. Subsequently, siloxane-based hybrid materials have been synthesized from the molecular crystals of the D4R cages via solid-state hydrolysis-polycondensation.

As order and crystallinity are not prerequisites for fine control over porosity of materials, porous poly(organosiloxane)s with relatively high surface area have been synthesized by connection of D4R cages via strong covalent as presented in the chapter 4 and 5. Synthesis of poly(organosiloxane) networks by Sonogashira cross-coupling reaction of bromophenylethenyl-terminated D4R cages and ethynyl compounds is reported in the chapter 4. The resulting carbon-carbon triple bonds can act as suitable linkers because of their rigidity. In the synthesis viewpoint, one of the advantages of polymer-based materials is that they can be synthesized in a modular manner; hence, their properties can be tuned or tailored by careful design and selection of monomers. Consequently, pore characteristics of the porous poly(organosiloxane) networks would be tailored by simply varying the length (e.g., phenyl and biphenyl) and the connectivity (e.g., di- and triethynyl) of the organic linkers. In comparison with other porous organic polymers reported previously, the poly(organosiloxane) networks synthesized in this work show relatively high surface area (apparent specific BET surface area of 850-1050 m2 g-1) and comparably high thermal stability. Moreover, the obtained porous poly(organosiloxane) networks can adsorb hydrogen molecules with relatively high isosteric heats of adsorption, raising the opportunity to utilize this porous hybrid materials as hydrogen storage materials. As very small ions such as fluoride can be encapsulated in the cubic D4R cages, the presence of such cages in the polymer networks can provide for the possibility to attain ionic porous polymers by post-synthetic modification.

In contrast to the co-polymerization (or cross-coupling) presented in the chapter 4, homo-polymerization (or homo-coupling) of functionalized D4R cages has been studied in the chapter 5. In other words, the D4R cages can serve as a single monomer for construction of the porous poly(organosiloxane) networks. In particular, microporous inorganic-organic hybrid material has simply been synthesized by homo-coupling of the bromophenylethenyl-terminated cubic siloxane cages as only building units via nickel-mediated Yamamoto reaction. The present work demonstrates that interpenetration or interweaving of the polymer network can be prevented by decreasing the linker distances, as evidenced as the sharp pore size distribution of the resuling hybrid. In comparison with the porous poly(organosiloxane) networks reported in the chapter 4, the present hybrid materials exhibit slightly high thermal stability with comparably high surface areas. Most of the D4R cages has retained in the resulting network, which is very useful for further post-synthetic modifications. As another alternative way, micro- and mesoporous inorganic-organic hybrid material has been synthesized by Friedel-Crafts alkylation of the benzylchloride-terminated cubic siloxane-based monomers. Although the D4R cages have been collapsed gradually occurring as the polymerization reaction proceeds, the resulting hybrid exhibits extremely high surface area (1860 m2 g-1) and pore volume (1.94cm3 g-1), which are among the highest values reported for the siloxane-based porous solids thus far. A series of porous poly(organosiloxane) networks has been achieved by combination of structural, synthetic organic, and materials chemistry, which can expand to synthesize other porous hybrid materials.

Finally, the chapter 6 presents general conclusions and future perspectives of this doctoral research. Overall, the present research has been devoted to develop the new synthesis route leading to designed framework materials from the strategic molecular building units. Considering secondary building units existing in zeolite frameworks, the cubic siloxane cage has been selected as a starting precursor. Utilization of intermolecular interaction ranging from weak interactions, i.e., hydrogen bond, π-π stacking, and van der Waals force, through coordination bond, to strong carbon-carbon covalent bond, siloxane-based inorganic-organic hybrid materials have been synthesized with partially targeted manners. By employing diversified interactions, carefully designed and synthesized cubic siloxane cages would shed a new light as molecular building units leading to functional materials; thus, the concept of this doctoral dissertation can be extended to other framework hybrid materials with "truly" designed structure and function.

原子、イオン、分子やクラスターと多孔性材料の内・外表面との相互作用の理解と制御は学術的にも実用的にも重要である。多孔性材料は、これまでによく知られているイオン交換剤、吸着剤及び触媒に加え、低誘電率膜、分子認識分離膜、化学センサー、光学材料など新たな分野での応用が期待されている。ゼオライトは代表的な多孔性材料であるが、その合成手法はtrial-and-errorの繰り返しによって進歩してきており、一連のシステムの中で得られる生成物を予測することは依然として困難である。さらに、結晶化メカニズムに関する知見は少なく、ゼオライト細孔の自在なデザインは現状では不可能である。

本博士論文では、戦略的に設計した分子ユニットをビルディングブロックとして用いることで、想定された骨格を持つ材料の合成を可能にする新しい合成ルートを開発することを目的としている。特に、ゼオライト骨格内に存在する2次構造ユニットに着目し、かご型シロキサン系分子(D4Rケージ)を出発前駆体モノマーとして選択している。各頂点の一箇所または八箇所に対し、多様な相互作用を持つ有機官能基を導入し、シロキサン系無機―有機ハイブリッド材料の合成の検討を行っている。

Chapter 1では、多孔性材料及びクラスターから組み上げられる材料について説明し、本研究の背景を述べている。目的とする材料に様々な機能を持たせるために、ビルディングブロックを戦略的に設計しボトムアップ手法により無機―有機ハイブリッド材料を合成する指針を示すとともに、本研究の意義について述べている。

Chapter 2では、白金触媒を用いたヒドロシリル化とクロスメタセシス反応による、D4Rケージの設計と合成について述べている。分子間相互作用は、最終的に得られるハイブリッド材料を形作る最も重要な因子であり、π-π相互作用や水素結合などの比較的弱い分子間相互作用を形成する有機基(芳香環や尿素結合など)やより強い配位結合を形成する有機基(カルボキシル基等)、そして強い共有結合を形成する有機基(ハロゲン化アリール等)のD4Rケージの各頂点末端への導入を検討している。

Chapter 3では、Chapter 2で得られた芳香環による一置換D4Rケージについて、π-π結合を用いた分子結晶の結晶化を行なっている。得られた結晶はX線回折と計算による構造最適化の結果、ラメラ構造をしていると考えられる。さらにその後、未修飾末端をイリジウム触媒による加水分解酸化またはアルコール分解によって変化させ、固相縮重合によってD4Rユニットからシロキサン結合を有するハイブリッド材料を合成している。

Chapter 4では、Chapter 2で得られたブロモフェニル基末端の八置換D4Rケージとエチニル化合物を用い、薗頭クロスカップリング反応による多孔性ポリオルガノシロキサンの合成について述べている。得られたポリマー材料の特性は、モノマーの構造に依存しており、その細孔特性は有機リンカー部の長さ(フェニル、ビフェニル基)と結合性(ジ、トリ‐エチニル)を変化させることで調整することができる。さらにこれまでに報告された他の多孔性材料に比べ、本研究で得られたポリオルガノシロキサンは高い表面積と熱安定性を有している。さらに水素分子に対する高い吸着熱が得られたことから、水素貯蔵材料として有用であることも示唆されている。

Chapter 5では、ニッケルを介したホモカップリング反応(山本ポリマー化反応)による多孔性ポリオルガノシロキサン合成について述べている。ブロモフェニル基末端の八置換D4Rケージのみを用いミクロ多孔性無機―有機ハイブリッド材料を合成している。得られたハイブリッド材料は狭い細孔径分布を有し、ポリマーネットワークの相互拡散や混ざり込みは、リンカー長の減少により抑制されていることを示唆している。大部分のD4Rケージはネットワーク中に残存し、さらなる後処理や修飾が可能である。また、ベンジルクロライド末端の八置換D4Rケージを用い、フリーデル‐クラフツアルキル化反応によりミクロ‐メソ多孔性無機―有機ハイブリッド材料の合成を検討している。得られたハイブリッド材料はこれまで報告されているシロキサン系多孔性材料の中では最も高い表面積と細孔容積を有している。

Chapter 6では本研究で得られた結果を総括している。

以上、本論文では多様な相互作用を利用することで、新たな無機―有機ハイブリッド材料の合成法を提案しており、これらの成果は、化学システム工学及び材料科学の発展に寄与するところが大きい。

よって本論文は博士(工学)の学位請求論文として合格と認められる。