Introduction

Chemical bonds are fundamental components of every chemical compounds, and hence development of a novel type of bonding is a significant theme in chemistry. Novelty of the bonding depends on the coordination and oxidation states of the atoms that constitute the bond. When a main group element formally has more than eight electrons in its valence shell, the atom is called to be a hypervalent state. Although a hypervalent state of period 2 atoms (C, N etc.) is extremely unstable, that of silicon or phosphorus atom can be stabilized by proper ligands. In the cases of pentacoordinated phosphorus and silicon compounds, the central atom has five substituents and ten valence electrons, and compounds bearing a bond between two pentacoodinated atoms are expected to show unique structures, reactivities, and properties which are caused by severe steric and electronic congestion in the vicinity of the bond. However, it is difficult to synthesize such compounds due to aforementioned congestion and hence only few examples have been reported. The author has already synthesized the first disilicate 1 bearing a bond between two anionic pentacoordinated silicon moieties in master course (Figure 1). However, its physical property and electronic state are still unclear. In this work, the author carried out both experimental and theoretical investigations of 1 in order to elucidate its electronic state. And the author attempted to develop a bond which consists of pentacoordinated silicon and phosphorus atoms because such a bond has never been reported. Furthermore, the author also studied properties of hexacoordinated phosphorus compounds bearing P-H bonds, especially its tautomerization. In this study, an electron-withdrawing bidentate ligand that is called the Martin ligand has been used for stabilization of the hypervalent species.

Properties and electronic state of the disilicates

Electrochemical property of disilicate 1a was studied by cyclic voltammetry and compared with those of disilanes 2 and 3 (Table 1). In the case of 1a (M = (n-Bu)4N), two irreversible oxidation waves were observed (Epa = -0.22 and 0.83 V vs. Fc/Fc+, 0.1 M (n-Bu)4NClO4 in CH2Cl2). This result shows 1a has relatively higher electron donor ability than a simple disilane 3 (Epa = 0.95 V). On the other hand, in the case of 2, which was obtained by protonation of 1a, no oxidation waves were observed under the same conditions, showing that the electron donor ability was decreased by the protonation. The UV absorption spectra of 1-3 showed similar behavior to each other. The absorption maximum wavelength of 1a (λmax = 258 nm) is longer than that of 3 (λmax = 241 nm), and the protonation of 1 was found to cause blue shift, judging from the absorption maximum of 2. Change of the properties between 1 and 2 was reversible by protonation and deprotonation processes.

To elucidate the electronic state of the disilicate, theoretical calculation was carried out at the B3PW91/6-311+G(2d)[Si]:6-31G(d)[C,H,O,F] level. The theoretical investigation showed the high HOMO energy level and small HOMO-LUMO energy gap of 1b (M = Me4N), supporting the experimental results. In general, HOMO of a Si-Si bond compound is mainly presented by the bonding σ(Si-Si) orbital, and the HOMO of 1b considerably contained also p orbitals of the oxygen atoms in addition to it (Figure 2). The HOMO energy level would be raised by interaction of the Si-Si bonding orbital with occupied orbitals of the oxygen atoms in the vicinity of the bond. Changing of the properties by protonation would be caused by a decrease of the interaction. Reflecting the high HOMO energy level, addition of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone to disilicate 1c (M = Li+) afforded corresponding the hydroxysilicate. By contrast, 1c showed high thermal stability, and it could be sublimed without decomposition at 248 °C.

Construction of a bond between pentacoordinated silicon and phosphorus atoms

Considering a high reactivity of a Si-P bond, a synthesis of a compound bearing a bond between pentacoordinated silicon and phosphorus atoms would be difficult and it has been a big challenge. Therefore, phosphoranylsilicate 6 was designed as a target molecule whose phosphorus and silicon atoms are fixed at proper positions of a rigid naphthalene moiety. Phosphorane 5 was obtained by the reaction of cyclic phosphinate 4 with 8-(dimethylsilyl)naphthyllithium, but the reduction of 5 with lithium naphthalenide did not gave 6 (Scheme 1). Modification of the reduction condition is expected to afford phosphoranylsilicate 6.

Synthesis and properties of hexacoordinated phosphorus compounds bearing P-H bonds

A P-H bond is usually almost nonpolar because phosphorus and hydrogen has almost same electronegativity (xP(P) = 2.19,xP(H) = 2.20). However, some oxygen-substituted pentacoordinated hydrophosphoranes undergo proton exchange reaction via tautomerization to a trivalent phosphine because the hydrogen atom on the oxygen atom of the trivalent tautomer is protic. The tautomerization can interestingly change property of a hydrogen atom. Although hexacoordinated hydrophosphate with a similar framework is expected to show reversing polarity of the hydrogen atom through tautomerization, the property of such a compound has been still unclear due to difficulty of its synthesis and isolation. The author synthesized a stable hexacoordinated dihydrophosphate, which is an anionic species bearing two P-H bonds by taking advantage of the Martin ligands, and studied its tautomerization and reactivity.

Dihydrophosphate 8 was synthesized by successive treatment of hydrophosphorane 7 with lithium naphthalenide (3 equiv.), water (excess), and tetraethylammonium bromide (1.5 equiv.) (Scheme 2). The structure was determined by the X-ray crystallographic analysis, and the electronic structure was investigated by theoretical calculations. H-D (hydrogen-deuterium) exchange reaction of 8 with D2O was attempted in DMSO-d6 in anticipation of the protic character of the O-H group in a tautomer of 8. Although the hydrogen atoms on the phosphorus atom of 8 were hardly exchanged for the deuterium atoms of D2O under both basic and neutral conditions, the H-D exchange proceeded efficiently in the presence of AcOH (1 equiv.) to give 8-d2 bearing two P-D bonds (95%D) after 0.5 h. It is proved that hydrogen atoms on the phosphorus of 8 are exchangeable for protons of water. To clarify the exchange mechanism, the reaction of 8 with excess amount of AcOH was carried out, and formation of phosphine 9 was confirmed by 31P NMR spectroscopy. Judging from these results, tautomerization would proceed after protonation of 8 under the acidic condition, and the H-D exchange would be accomplished via the generated intermediate 9.

Dihydrophosphate 8 is also expected to behave as a hydride donor (Table 2). The reaction of 8 with 4-phenylbenzaldehyde (1 equiv.) in refluxing THF for 10 h, followed by treatment with aqueous NH4Cl solution, gave the corresponding alcohol 10 (85% yield) and 7, showing the hydridic reactivity of the P-H bond of 8. The reduction reaction was dramatically accelerated by addition of LiCl (1 equiv.) or AcOH (5 equiv.), which activated the aldehyde, and consequently the reaction was completed after 1 h at ambient temperature. It is interesting that addition of AcOH can promote both the proton exchange and hydride reduction.

Compound 8-d2, which had been prepared by the H-D exchange of 8 with D2O, naturally reduced 4-phenylbenzaldehyde to give the deuterated alcohol, 10-d (83% yield, 95%D) in the presence of LiCl. Furthermore, a one-pot reaction of the H-D exchange and reduction was successful. The H-D exchange from 8 using D2O with AcOH (1.5 equiv.), followed by addition of 4-PhC6H4CHO in THF, also gave 10-d (82% yield, 97%D). In these reactions, the deuterium from D2O behaves as deuteride, meaning achievement of reversing polarity from D5+ to D5-.

Conclusion

Properties of disilicate, which had an unprecedented chemical bond between two anionic pentacoordinated silicon moieties, were revealed. The characteristic electrochemical and optical properties could be turned off by protonation. Considering the unique properties and the high stability, such unusual Si-Si bonds are expected to be novel components to construct functional materials. The properties of hexacoordinated phosphorus compound bearing two P-H bonds were also investigated, and it could reverse the polarity of a hydrogen atom of water via the tautomerization. This method enabled reductive deuteration of carbonyl compounds using D2O under the mild conditions without careful handling.

Figure 1

Table 1. Oxidation potential (Epa), UV absorption maximum (λmax), and calculated HOMO energy level (E(HOMO)) and LUMO-HOMO energy gap (ΔE(LUMO-HOMO)).

Figure 2

Scheme 1. Attempted synthesis of phosphoranylsilicate 6

Figure 3

Scheme 2. Synthesis of dihydrophosphate 8 and the H-D exchange reaction.

Table 2. Reduction of an aldehyde with phosphate 8.

本論文は6章からなり、第1章は序論、第2章は5配位ケイ素原子同士の結合を有するジシリカートの性質、第3章は5配位リン-ケイ素結合を有するホスホラニルシリカートの合成検討、第4章は6配位ジヒドロホスフェートの合成、構造、および結合様式、第5章はジヒドロホスフェートを利用した水分子中の水素原子の極性変換、そして第6章は結論および今後の展望について述べている。

第1章では、超原子価化合物について、その成り立ちから始まり、特異な電子構造、安定化の手法および特徴的な反応性について述べている。さらに、超原子価原子同士が結合した化合物の特徴について言及しており、過去に合成例はあるもののその特異な化学結合の性質は未解明であると主張している。また、水分子中の水素原子の極性変換についてその困難さと潜在的な有用性を述べている。これらの背景から本論文の目的は二つ設定されており、一つ目が超原子価5配位原子同士の化学結合の性質解明であり、二つ目が6配位リン化合物の特性を活かした水分子中の水素原子の極性変換である。

第2章では、論文提出者が修士課程において合成した5配位ケイ素原子同士の結合を有するジシリカートの性質について述べている。サイクリックボルタンメトリーによってその低い酸化電位を明らかにするとともに、通常のジシランに比べて紫外吸収波長がレッドシフトすることを見出している。ジシリカートの分子軌道計算を行い、それらの特徴が中心のケイ素-ケイ素結合の結合性軌道と酸素原子の被占軌道の相互作用によって発現することを説明している。また、そのケイ素-ケイ素結合の酸化的開裂に成功し、5配位ケイ素原子同士の結合を可逆的に構築・切断できることを示している。一方でそのケイ素-ケイ素結合が熱的には非常に安定であることを見出している。

第3章では、異種の5配位原子同士の結合構築を目的とし、5配位リン原子と5配位ケイ素原子が結合したホスホラニルシリカートの合成検討について述べている。リン-ケイ素結合の構築には至らなかったが、ナフタレンの1,8位にリンおよびケイ素原子を導入した化合物の合成に成功しており、それらの化合物は目的化合物の前駆体として有望な構造であるとしている。

第4章では、リン-水素結合を有する6配位ジヒドロホスフェートの合成、構造および結合様式について述べている。まず5配位ヒドロホスホランへの形式的なヒドリド付加によって有機基を有する初めての6配位ジヒドロホスフェートの合成に成功している。X線結晶構造解析やNMRスペクトルによってその化合物が結晶状態でも溶液状態でも6配位状態を保っていることを確認している。また、その化合物が水や空気に対して高い安定性を示し、既知の6配位ジヒドロホスフェートなどと比べて格段に取り扱いやすい化合物であることを述べている。各種スペクトルおよび理論計算からそのリン-水素結合はs性が小さく分極の大きい結合であることを見出している。

第5章では、ジヒドロホスフェートを用いた水分子中の水素原子の極性変換について述べている。まずジヒドロホスフェートにD2Oを作用させてH-D交換反応を検討し、酢酸存在下で6配位リン原子上の水素がD化されることを見出している。さらにジヒドロホスフェートがカルボニル化合物を還元してアルコールを与えることを明らかにしている。これらの二つの反応性によって水分子中の水素原子の極性変換を実現している。本手法は安価なD2Oを用いて温和な条件下で簡便に還元的重水素化を行うことができるため、新規な同位体ラベル化法として有望である。

以上のように、本研究は5配位原子同士の化学結合の性質解明と水分子中の水素原子の極性変換という二つの目的について十分な成功を収めている。まず実験結果と理論計算の両面から5配位ケイ素原子同士の化学結合の性質を明らかにしており、これは新規な基礎化学的知識として意義深い。また、従来困難だった水分子中の水素原子の極性変換を6配位リン化合物の特性を利用することで実現している。その鍵となる6配位リン化合物の結合様式を各種スペクトルと理論計算から明らかにしており、この知見は今後の6配位典型元素化合物の設計・応用に対して大いに寄与すると考えられる。また重水という安価な重水素源を用いた新規かつ有用な同位体ラベル法を提供している。このように、有機典型元素化学のみならず有機合成化学に多大な貢献をしており高く評価できる。

なお、本論文は川島隆幸・狩野直和・佐々木啓史・永瀬茂・溝呂木直美との共同研究であるが、論文提出者が主体となって実験および解析を行ったものであり、論文提出者の寄与が充分であると判断する。

したがって、博士(理学)の学位を授与できるものと認める。