学 位 の 種 類 博士（理学）

氏 名

学 位 の 種 類 博士（理学）

学 位 記 番 号 理工博 第

号 学位授与の日付 平成

年

月

日

課程・論文の別 学位規則第４条第１項該当

学 位 論 文 題 名

芳香族イミド配位ニオブ錯体の合成と反応性に関する研究（英文）

論 文 審 査 委 員 主査 教 授 野村 琴広 委員 准教授 佐藤 総一 委員 准教授 稲垣 昭子 委員 教 授 小宮 三四郎

【論文の内容の要旨】

Design of efficient molecular catalysts for olefin coordination/insertion polymerization is a promising target in terms of synthesis of new polymers and development of more efficient process.

Due to promising characteristics (remarkable reactivity toward olefins) displayed by classical Ziegler type vanadium catalyst systems, design of vanadium (including group 5) complex catalysts thus attracts considerable attention.¹ However, reported examples for the niobium complex catalysts² have been much less than those in group 4 transition metal catalysts employed in industry and those in the vanadium complex catalysts,³ although certain niobium catalysts systems (consisting of NbCl5

or Nb(O-2,6-Me2C6H3)nCl5-n(THF) and organometallic reagents) exhibited unique reactivity in polymerization of disubstituted acetylenes.⁴ This would be because the synthesis of the desired unsaturated Nb(V) complexes often face difficulties to avoid the solvent coordination that leads to a significant decrease in the activity in olefin polymerization. In this PhD thesis, synthesis of a series of aryloxo- or alkoxo-modified (imido)niobium(V) complexes and their reaction chemistry have thus been explored.⁵ The niobium(V) complexes containing ketamide ligands have been focused, because the related (imido)vanadium complexes displayed unique reactivity toward phenols.⁶

１. Synthesis of (imido)niobium(V) dichloride complexes containing phenoxy ligands and their

use as ethylene polymerization catalysts.

A series of (imido)niobium(V) dichloride complexes, Nb(NR)Cl2(OAr’)(dme) [Ar’ = 2,6-ⁱPr2C6H3,R = 2,6-Me2C6H3 (1a), 2,6-ⁱPr2C6H3 (1b), 1-adamantyl (1c); R = 2,6-Me2C6H3, Ar’

= 2,6-Ph2C6H3 (1d); dme = 1,2-dimethoxyethane], were prepared from Nb(NR)Cl3(dme) by treating with 1.0 equiv of LiOAr’ in Et2O.⁷ These complexes were identified by NMR spectra, elemental analysis and some structures were determined by X-ray crystallography. These complexes (1a and 1d) exhibited low catalytic activities for ethylene polymerization (11-23 kg-PE/mol-Nb·h, ethylene 8 atm, 0-50 ºC) in the presence of methylaluminoxane (MAO) or Et2AlCl cocatalysts. Since reason for the low activity could be ascribed to dme coordination to niobium,^2c and the removal was difficult, another synthetic route for synthesis of a series of (imido)niobium(V) complexes had to be thus developed for the purpose on this study.

２. Synthesis of ketimide-modified niobium(V) complexes and some reactions.

Two Nb(V) complexes containing three ketimide ligands, Nb(NAr)(N=C^tBu2)3 (3a) and trans-NbCl2(N=C^tBu2)3 (3b), were prepared from Nb(NAr)Cl3(dme) or NbCl5 by treating with Li(N=C^tBu2) in toluene, and the complexes were identified by NMR spectra and elemental analysis.

Reactions of 3a with one, two, and three equiv of 2,6-ⁱPr2C6H3OH in n-hexane afforded corresponding mono-, bis-, tris-phenoxide, respectively [Nb(NAr)(N=C^tBu2)3-n- (O-2,6-ⁱPr2C6H3)n, n = 1 (4a), 2 (5a), 3 (6a)]. Reaction of 3a with 3 equiv of Li(O-2,6-ⁱPr2C6H3) in Et2O also afforded 6a. In contrast, the reaction with 2,6-^tBu2C6H3OH in place of 2,6-ⁱPr2C6H3OH conducted under the same conditions recovered 3a; the reaction in toluene at 70 ºC afforded Nb(NAr)(N=C^tBu2)2(O-2,6-^tBu2C6H3) (4b). However, an attempted synthesis of the bis(phenoxide) failed even the solution was heated at 100 ºC, and complex 4b was the major product in the mixture.

It thus seems that this would be due to a difficulty of coordination of the phenol to niobium.⁶ In contrast, the reaction of 3a with C6F5OH afforded Nb(N=C^tBu2)2(OC6F5)3(HN=C^tBu2) (7) by reaction with the imido ligand, clearly indicating that (probably) an electronic nature of phenol affects the reactivity.

Reaction of 3a with 1 equiv of (CF3)3COH afforded Nb(NAr)(N=C^tBu2)2[OC(CF3)3]- (HN=C^tBu2) (8), although conversion of 3a was close to 50 % and was difficult to reach high conversion. The reaction with 2 equiv of (CF3)2CHOH yielded Nb(NAr)(N=C^tBu2)- [OCH(CF3)2]2(HN=C^tBu2) (9). As observed in the reaction with C6F5OH, one HN=C^tBu2

coordinates to Nb in these complexes. In order to prepare the “HN=C^tBu2 free” Nb complexes, complex 9 was treated with NiBr2 to afford Nb(NAr)(N=C^tBu2)[OCH(CF3)2]2 (10).

On the basis of the above results, it has thus been postulated that the reactions with phenols/alcohols proceed by coordination of phenol/alcohol without protonation in high oxidation state organometallic complexes with early transition metals.^6,7

Conclusion

In this thesis a possibility to establish a new synthetic route for preparation of “solvent free”

(arylimido)niobium complexes has been studied, since Nb(NR)Cl2(OAr’)(dme) showed low catalytic activities in ethylene polymerization due to dme coordination. Various (arylimido)niobium complexes, Nb(NAr)(N=C^tBu2)2(O-2,6-R2C6H3) [R = ⁱPr (4a), ^tBu (4b)], Nb(NAr)(N=C^tBu2)(O-2,6-ⁱPr2C6H3)2 (5a), Nb(NAr)(O-2,6-ⁱPr2C6H3)3 (6a) have been prepared from Nb(NAr)(N=C^tBu2)3 (3a) by reaction with 2,6- R2C6H3OH. In contrast, the reaction with arylimido ligand (not with ketamide) was occurred in the reaction of 3a with C6F5OH. Reaction of

3a with (CF3)3COH and (CF3)2CHOH (2 equiv) afforded

Nb(NAr)(N=C^tBu2)2[OC(CF3)3](HN=C^tBu2) (8), Nb(NAr)(N=C^tBu2)[OCH(CF3)2]2- (HN=C^tBu2) (9), respectively; the ketimine ligand could be cleaved by treatment with NiBr2. Since examples in reaction chemistry with unsaturated niobium(V) have been limited so far, the results should demonstrate important information in terms of the efficient synthesis and of better understanding in organometallic as well as inorganic chemistry that would be applied as catalysts for efficient organic transformations.

References

(1) Nomura, K.; Zhang, S. Chem. Rev. 2011, 111, 2342.

(2) For examples, (a) Mashima, K.; Oshiki, T.; Nakamura, A. et al. Organometallics 1995, 14 2633.

(b) Redshaw, C. et al. Inor. Chim. Acta 2005, 358, 4067. (c) Marchetti, F.; Pampaloni, G. et al. J.

Polym. Sci., Part A: Polym. Chem. 2011, 49, 1664. (d) Redshwa, C. et al. Chem. Eur. J. 2007, 13, 10129. (e) Redshaw, C.; Sun, W.-H. et al. Chem. Eur. J. 2013, 19, 8884.

(3) (a) Nomura, K. Hou, X. Dalton Trans. 2017, 46, 12. (b) Nomura, K.; Hou, X. Handbook of Transition Metal Polymerization Catalysts (2nd Ed.), Ray Hoff (Ed.); Wiley, in press (2018).

(4) For example, (a) Masuda, T.; Matsumoto, T.; Yoshimura, T.; Higashimura, T. Macromolecules 1990, 23, 4902. (b) Kozai, H.; Masuda, T.; Higashimura, T. J. Polym. Sci. PartA: Polym. Chem.

1994, 32, 2523. (c) Aoki, T.; Shinohara, K.; Kaneko, T.; Oikawa, E. Macromolecules 1996, 29, 4192.

(5) Synthesis of niobium(V) complexes containing tris(arylo)amine complexes and their use in ethylene polymerization: Omiya, T.; Srisupap, N.; Wised, K.; Tsutsumi, K.; Nomura, K. Polyhedron 2017, 125, 9. Special issue, Tridentate and tetradentate tripodal ligands.

(6) Yamada, J.; Fujiki, M.; Nomura, K. Organometallics 2007, 26, 2579.

(7) Wised, K.; Nomura, K. Organometallics.2016, 35, 2773.