Introduction

Low-valent group 6 metal centers surrounded by tertiary phosphine coligands have remarkable ability to activate the coordinated small molecules. Typical example is the [M(dppe)2] (M = Mo, W; dppe = Ph2PCH2CH2PPh2) site, which readily transforms numerous substrate molecules including N2, CO2, alkenes, alkynes, nitriles, isocyanides, etc. into a variety of attractive ligands or compounds under mild conditions. The linear tetraphosphine ligand meso-o-C6H4(PPhCH2CH2PPh2)2 (P4) has been discovered in the course of these studies by Mizobe's group. Formation of P4 is recognized as an example of unique bond rearrangement in the coordination sphere of low-valent Mo and W complexes, by which P4 and benzene are produced via the condensation of two dppe ligands. Tetradentate coordination of P4 to a metal center accumulates strain of three consecutive chelates, in which the P-M-P angles of five-membered rings are fairly smaller than 90°. This induces unusual structures that are distorted extremely from regular octahedron or facile change of the coordination mode from k4 to k3 and k2. The most easily prepared starting compounds for the Mo and W tetraphosphine complexes are [M(dppe)(k4-P4)] (1) but their metal centers are obstructed by steric bulk and strong M-P bonds of P4 and dppe. Therefore new precursor complexes that can effectively provide the reaction sites are anticipated. Polyhydride complexes are dependable candidates, since the reactivity types of polyhydride complexes are not limited to those of common hydride complexes but are also featured by their ability to generate coordinatively unsaturated intermediates via reductive elimination of H2.

Results and Discussion

1. Synthesis and properties of tetrahydride complexes containing P4 ligand

The tetrahydride complex [WH4(k4-P4)] (2b) was prepared in 77% yield by treatment of the W(II) complex [WBr2(k4-P4)] (3b) with excess NaBH4 in ethanol at 50 ℃ (Scheme 1). Formation of 2b was also observed by the reaction of 1b with 1 atm H2 at 80 ℃, but equilibrium with the dihydride complex [WH2(k2-dppe)(k3-P4)](4b) and free dppe in the ratio of 2b:4b = 1:1 hampered selective formation. The Mo analogue [MoH4(k4-P4)] (2a) was not formed at all by any routes shown in Scheme 1. The molecular structure of 2b was determined by X-ray crystallography, which revealed a distorted dodecahedral geometry of the W center. The k4-P4 ligand asymmetrically divided the coordination sphere into two. Three hydrido ligands were found at the wider part, and the other one existed at the opposite narrow side. The 1H NMR spectrum of 2b at 20 ℃ showed two hydrido signals at δ -3.06 and -5.46 with 3H and 1H intensities, indicating that one hydrido ligand is separated from the others also in solution.

The reaction of 2b with 3 equiv of RNC in toluene at 80 °C produced [W(CNR)2(k4-P4)] (R = t-Bu (5), Xy (6); Xy = 2,6-Me2C6H3) after 7 h (Scheme 1). Formation of the intermediates speculated as [WH2(CNR)(k4-P4)] was observed during this conversion. It is important to note that the related [WH4(dppe)2] shows no reactivity towards RNC under the same conditions. A highly distorted octahedral structure, in which two isocyanide ligands are oriented mutually cis, has been confirmed crystallographically for 5, and NMR spectra of both 5 and 6 are consistent with this structure. One of the isocyanide ligands in 5 (trans to the internal P atom) has a considerably bent C-N-C linkage (139.3(4)°) in solid state as a result of strong back-donation from the W(0) center, which is also indicated by low frequency shift of v(N≡C) in the IR spectra. From the lower v(N≡C) values for 6 (1837 and 1943 cm-1) than those for its Mo analogue (1851, 1879 and 1957 cm-1), electron-donating ability of the W(P4) moiety is estimated to be somewhat superior to that of the Mo(P4).

2. Transformations of heterocumulenes

Although the reaction of 2b with CO2 resulted in complicated mixture, from which no products could be fully characterized. Isoelectronic CS2 and RNCS molecules were found to be converted in the coordination sphere derived from 2b (Scheme 2). Treatment of 2b with 3 equiv of CS2 at 50 ℃ in toluene cleanly formed [W(k2-S2CH2)(k4-P4)] (7), which was obtained as green prismatic crystals in 76% yield. GC analysis of gas phase confirmed the concomitant formation of H2 in 0.73 equiv to W atom. In the 1H NMR spectrum of 7, no hydride signals were observed, and a singlet signal integrated to 2H appeared at δ 6.56, which was assigned to the CH2S2 moiety. The W center in 7 has a trigonal prismatic geometry, in which one of the three side rectangles is capped by the k4-P4 ligand. Although there have been many examples of CS2 insertion into a M-H bond to give dithioformate complexes, formation of a methanedithiolate ligand from a CS2 molecule and two hydride ligands are still rare. The reaction of 2b with 3 equiv of aryl isothiocyanate ArNCS (Ar = Ph, p-CH3C6H4 (Tol), p-ClC6H4) in toluene proceeded at 50 ℃ to form majorly two W complexes, and the less soluble [W(S2CNAr)(CNAr)(k4-P4)] (8) was separated out from the mixture as a brown precipitate (Scheme 2). X-ray crystallography of 8 (Ar = Tol) has revealed a distorted pentagonal bipyramidal structure, in which the isocyanide ligand and one of the sulfur atom of the dithiocarbonimidato ligand are occupying apical positions. The 31P{1H}

MR spectrum of 8 (Ar = Tol) in CDCl3 solution at 20 ℃ showed two very broad peaks at δ46 and 66, and these changed at -50 ℃ to two pairs of signals at (δ 49.9 and 66.9 vs δ 47.4 and 67.3 in a 2.5:1 ratio) probably corresponding to the syn- and anti-conformations around the C=N bond in the dithiocarbonimidato ligand. On the other hand, the liquid phase of the reaction mixture contained ArNHCH3, ArNH2, and unreacted ArNCS in addition to other W complexes. Yields of the hydrodesulfurization product ArNHCH3 were dependent on the Ar groups (0.45, 0.23, and 0.12 equiv to W for Ar = Tol, Ph, p-ClC6H4, respectively) and increased to about 1.5 times large (up to 0.65 equiv to W for Ar = Tol) when the reactions were carried out under H2 atmosphere.

Since the formation of 8 is recognized as disproportionation of two RNCS molecules on a W(0) center, the reactions of zero-valent complex 1b with aryl isothiocyanates were examined, and these proceeded at 50-80 ℃ to give 8 (Scheme 3). In contrast, the reactions of Mo complex 1a with 3 equiv of ArNCS in benzene at 60 ℃ have been found to afford [Mo(ArNC)2(n2-ArNCS)(k3-P4=S)] (9) and monosulfide of dppe. The formal oxidation states of the products, viz, W(II) for 8 and Mo(0) for 9, reflect higher electron-donating ability of W than Mo. The N atom of the dithiocarbonimidato ligand in 8 had a nucleophilic character, and it smoothly reacted with H+, MeI, and PhCOCl.

3. Transformations of nitriles

When 2b was heated with 3 equiv of TolCN in toluene at 80 ℃ for 3 h, the hydrido-alkylideneamido complex [WH(N=CHTol)(k4-P4)] (10Tol) was obtained as almost sole complex product (Scheme 4). X-ray crystallography has revealed that 10Tol has a highly distorted octahedral geometry with the hydrido and the alkylideneamido ligands at mutually trans positions. The N-C distance at 1.305(6) A and the N-C-C angle at 126.7(4)° are usual for an N=C(sp2) system.

The short W-N bond length of 1.863(3) A suggests multiple bond character that the alkylideneamido ligand acts as a 2σ2π-electron donor. Existence of the hydrido ligand was confirmed by the 1H NMR spectrum, which showed a broad quintet signal at δ -3.42. Prolonged heating of the above reaction mixture gradually produced another new complex, which became the sole product after 20 h and was identified as the imido complex [W(NCH2Tol)(k4-P4)] (11Tol). The conversion from 10(Tol) to 11Tol was unambiguously confirmed by heating isolated sample of 10(Tol). The metal center in 11Tol has a distorted square pyramidal geometry with the imido ligand at the apical position. The W-N bond length at 1.794(2) A is considerably shorter than that of 10(Tol) and typical of triple bond, that allows 18 electron counts around the W center.

Reactions of 2b with a variety of aromatic nitriles formed 10 similarly, while the isomerization from 10 to 11 proceeded fast with electron-rich Ar groups. Complexes 10 with considerably electron-deficient p-CF3C6H4 group as Ar did not convert to 11 at all. In contrast, 10 were never detected in the reactions of 2b with aliphatic nitriles AkCN (Ak = n-C7H15, c-Hex, t-Bu, PhCH2, p-ClC6H4CH2), and 11 were directly obtained.

Remarkably, highly electron-deficient nitriles ArFCN (ArF = 3,5-(CF3)2C6H3) reacted with 2b in a quite different way. Stirring 2b and 3-9 equiv of this nitrile in toluene at 80 ℃ for 20 h gave [W{NC(ArF)C(ArF)NC(ArF)NC(ArF)N}(k3-P4)] (12b) as the major product, but neither 10 nor 11 were observed at all (Scheme 5). The complex 12b was isolated as black prismatic crystals in 41% yield, and the X-ray crystallography has revealed that four nitrile molecules are linked linearly and that the resulting chain binds to the W center with three N atoms in a meridional fashion, forming the fused five- and six-membered chelate rings. By tridentate coordination of P4, the W center forms a distorted octahedral structure. The 31P{1H} NMR spectrum showed four signals at δ -10.3 (non-coordinated), 40.2, 46.8, and 73.8. Although 12b is considered to be produced via reductive bond forming reaction on a formally W(0) center, it could not be obtained from the reaction of 1b with ArFCN. Instead, the Mo analogue 12a was successfully obtained by the reaction of [Mo(cod)(k4-P4)] (13: cod = n4-1,5-cyclooctadiene) with ArFCN at 50 ℃.

Conclusion

The tetrahydride complex of W containing the linear tetraphosphine ligand P4 has been synthesized, and its reactions with organic molecules have been investigated. Highly strained geometry around the W center and ability of P4 to readily change hapticity probably increase the activity of [WH4(k4-P4)] so much higher than the related complex with diphosphine ligands [WH4(dppe)2]. It has been proved that [WH4(k4-P4)] is not only a strong hydride donor but also a good precursor of coordinatively unsaturated active species that facilitate specific bond formation and cleavage. Reactions involving zero-valent intermediates proceed more smoothly and cleanly with [WH4(k4-P4)] than [W(dppe)(k4-P4)], which may have a difficulty in dissociating dppe ligand. Transformation of organic molecules in this study are based on strong reducing power of W-H or W(0) species and quite unique in comparison with other transition metal complexes. Usability of recently prepared [Mo(cod)(k4-P4)] in related reactions has been also demonstrated, and albeit lower electron-donating ability than W, it may be a key precursor for extending above stoichiometric reactions to catalysis.

Scheme 1

Scheme 2

Scheme 3

Scheme 4

Scheme 5

前周期遷移金属の低原子価錯体は強い還元力と結合活性化能を有し,有機・無機小分子の変換反応において他の遷移金属錯体に見られない独特の反応性を示す。低原子価金属原子の高い電子密度を維持しつつ取扱い可能な化学種を得る上で,三級ホスフィンとの錯形成はきわめて有効である。さらに多座ホスフィン配位子は強固な配位によって錯体を安定化し,立体構造の制御によって高度な反応選択性をもたらす。これまでに二座ホスフィンを用いた研究は多いが,より高機能が期待できる三座以上の配位子はほとんど検討されていない。本論文は,先行研究で見いだされた特異な立体構造を有する直鎖型四座ホスフィン配位子P4の特性に着目し,その6族金属錯体の高活性化とそれらを用いた有機分子変換反応について述べたものであり,全6章で構成されている。

第一章では,序論として本研究の土台となっている項目を概観しており,多座ホスフィンの構造と配位化学,遷移金属ポリヒドリド錯体の性質,低原子価6族金属-二座ホスフィン錯体による分子変換,四座ホスフィンP4錯体の反応性が含まれている。これらの既知の研究を論じた上で,より活性と有用性の高いP4錯体としてポリヒドリド錯体を提案している。

第二章では,モリブデン,タングステンのポリヒドリド錯体の合成と物性評価が記述されている。P4配位子はこれら金属の二座ホスフィン錯体を原料として金属上でテンプレート合成されるため,それらの錯体としてのみ得ることができる。そのうち最も効率的に合成できるものを前駆体として,水素ガスとの反応によりジヒドリド錯体へと誘導した。さらにジブロモ-P4錯体を経由して水素化ホウ素ナトリウムで処理することにより,タングステンのテトラヒドリド錯体を合成した。これらの新規錯体を各種測定により同定し,従来の単座または二座ホスフィン類縁体との構造の違いについて議論している。電子的・立体的性質が近い二座ホスフィンを持つ類縁錯体に比べ,テトラヒドリドタングステン-P4錯体は反応性が非常に高いことも示されている。

第三章では,テトラヒドリドタングステン-P4錯体とヘテロクムレンとの反応が扱われている。二硫化炭素との反応では,ヒドリド配位子二つが炭素上に移行してメタンジチオラト配位子を形成し,残りのヒドリドは水素分子として解離した。金属-ヒドリド結合への二硫化炭素の挿入によるジチオホルマト配位子の生成はよく知られているが,上記のように還元がもう一段階進むことは珍しい。イソチオシアン酸アリールとの反応では,その水素化脱硫生成物であるN-メチルアニリン誘導体が得られ,触媒的ではないが水素雰囲気下で収率の向上が見られた。これと併発する別の機構で,二分子のイソチオシアン酸エステルが錯体上で不均化することによりイミノジチオ炭酸-イソシアニド錯体も生成した。後の反応にはヒドリド配位子が水素として解離することにより生成した0価配位不飽和中間体の関与が推定され,別の0価P4錯体を用いた実験で確認している。また,イミノジチオ炭酸配位子の窒素原子に様々な求電子剤が付加することが示された。

第四章では,テトラヒドリドタングステン-P4錯体とニトリルとの反応が検討されている。芳香族ニトリルとの反応では,タングステン-ヒドリド結合への挿入によって生じたアルキリデンアミド配位子とヒドリド配位子一つを有する錯体が初めに得られた。これはヒドリドの移行によりイミド錯体へと徐々に異性化した。一方,脂肪族ニトリルとの反応ではイミド錯体が直接得られた。ニトリル炭素にヒドリド二つが転移する機構でのイミド配位子の生成は,単核錯体では本反応以外の報告はない。本イミド錯体はタングステン中心が形式2価の極めて珍しい低原子価のものであり,イミド配位子と四座配位のP4からなる五配位構造を有する。

第五章では,電子不足性の高いニトリルが前章とは別経路で反応することが示されている。ここではヒドリド配位子は関与せず,形式0価のタングステン上でニトリル四分子の還元的カップリングにより鎖状の新規配位子が形成される。さらに,ジエン配位子の解離によって反応場を与える0価モリブデンP4錯体を新たに開発し,同じニトリルの四分子カップリング反応が起きることから高い活性が立証された。

第六章は全体を総括し,今後の研究の展望を述べている。

以上のように本論文では,特異な構造を持つ四座ホスフィンの錯体を用いて様々な有機不飽和分子の変換を検討している。ここで開発されたテトラヒドリド錯体は強いヒドリド供与体としてばかりでなく,低原子価配位不飽和種を簡便に発生させる前駆体としても有用であることが明示された。これらの反応性は四座配位子の強い配位力と構造制御能,動的な配位変化などの特性に起因すると見られており,錯体触媒の補助配位子設計において指針を与えるものである。これらの成果は,今後の有機金属化学,有機合成化学および触媒化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。