Introduction

Corannulene C(20)H(10), a fragment of buckminsterfullerene C(60), has attracted much attention due to its characteristic structure with a curved π-surface and unique features such as bowl-to-bowl interconversion and anisotropic structure. Such bowl-shaped aromatic compounds, namely buckybowls, have recently been extended over not only to organic modification but also to metal complexation. In contrast to planar polycyclic aromatic compounds, corannulene has both concave and convex faces due to its curvature so that it has much potential for forming many types of coordination structures with unique dynamic behaviors. While most of corannulene complexes reported so far are coordinated with π-coordination of their curved π-surfaces, metal complexes with σ-coordination at the rim are still rare.

In this study, I have designed and synthesized a corannulene-based ligand with a pyridyl pendant, 2-pyridylcorannulene (1) to construct a new class of corannulene metal complexes to bring out their unique coordination and association properties. Cyclometallation of ligand 1 was expected to produce C^N type metal complexes which have extended π-conjugated curved surfaces including metals. Moreover, stacked metallated corannulene derivatives would generate metal-metal interactions. Herein, I report two examples of cyclometallated corannulene complexes, [Pd(II)(H–11)(CH3CN)2]BF4 and Ir(III)(H–11)3, and their structures and chemical properties.

Synthesis and Structure of 2-Pyridylcorannulene Ligand 1

A corannulene ligand 1, 2-pyridylcorannulene, was synthesized from corannulene in two steps. Firstly, corannulene was brominated to afford monobromocorannulene according to a literature previously reported. Ligand 1 was then obtained by Suzuki-coupling between monobromocorannulene and 2-pyridylborate in 35% overall yield in two steps.

Ligand 1 was characterized by solution-phase NMR spectroscopy and DART mass spectrometry (m/z = 328.1 [H・1]+ and 655.2 [H・12]+). A pale-yellow block crystal for X-ray structural analysis was obtained from a solution of 1 in CHCl3/MeOH (3:1). The single-crystal X-ray analysis revealed that ligand 1 includes two enantiomers arising from the typical asymmetry of a singly substituted corannulene. The bowl depth of 1 (0.89 A) is almost the same as that of non-substituted corannulene (0.87 A) (Figure 2). In the crystal, the dihedral angle between corannulene and pyridyl moieties in 1 is relatively large (33°), suggesting that π-conjugation between these moieties is not extended. This is also supported by a UV-vis spectrum of 1 showing only a small amount of red shift by 3-6 nm compared with corannulene.

Synthesis of Cyclopalladated Complex, [Pd(II)(H–11)(CH3CN)2]BF4, and Its Self-assembly Behaviors

Cyclometallation of 1 was firstly examined using four-coordinate PdII with a view to a stacked structure. The reaction of 1 with 1 equiv. of [Pd(II)(CH3CN)4](BF4)2 was conducted in CD3CN at 60 °C for 8 days. The 1H NMR spectrum of the resulting solution suggested that two sets of structural isomers arising from different deprotonation positions (2a and 2b), [Pd(II)(H–11)(CH3CN)2]BF4, were produced in a ratio of 9:1 (Figure 3). A major species without a singlet signal for the α-position to the pyridyl moiety is assigned to 2a forming a 5-membered cyclopalladated ring, while the other minor species with two singlet signals is assigned to 2b forming a 6-membered cyclopalladated ring as the result of ß-deprotonation. The ESI-TOF mass spectrum strongly supports the formation of a cyclopalladated Pd(II)(H–11) structure (m/z = 473.0 [Pd(II)(H–11)(CH3CN)]+ and 514.1 [Pd(II)(H–11)(CH3CN)2]+).

Slow evaporation of a CH3CN solution containing a mixture of cyclopalladated complexes (2a and 2b) produced two different crystals, plates and needles, in 31% yield in total. 1H NMR of the mixed crystals indicated only one species, 2a. Conversion from 2a to 2b was too slow to be observed at room temperature in solution.

In the X-ray structure of 2a (plate), the central Pd(II) center is coordinated by a deprotonated C and an N atom of ligand 1 with a 5-membered chelate ring and two CH3CN molecules (Figure 4a). In the crystal, the Pd(II) complex is a racemate and two enantiomers arising from the inversion of the corannulene moiety are alternately stacked through π-π interactions to form a columnar structure (Figure 4b). The depth of corannulene moiety (0.82 A) is less than that of 1, and the dihedral angle (9 or 10°) between corannulene and pyridyl moieties is much smaller than that of 1. In the same column, the corannulene parts are stacked in a concave-convex fashion through π-π interactions (3.3 or 3.4 A). Moreover, there are two kinds of columns alternately arranged parallel to one another. One has a shorter Pd-Pd distance (3.8 A) while the other has a longer distance (4.5 A), which indicates there is no bonding between PdII centers in both cases. The Pd-Pd distances were expected to be adjusted by ligand exchange of the coordination sites bound by CH3CN ligands. Self-assembly of 2a with π-π stacking in solution was also suggested by upfield shift of 1H NMR signals with increasing concentrations of mixed 2a and 2b from 0.1 mM to 1 mM.

The electronic state of 2a in solution was studied by UV-vis spectroscopy (Figure 5). Compared with 1, an additional broad absorption band was observed with 2a in the range from 350 to 450 nm, which may be in part assigned to an MLCT transition. The absorption maxima at 271 and 306 nm in CH3CN are red-shifted from those of 1 (255 and 293 nm), which can be attributed mainly to the extended π-conjugation in the molecular structure of 2a, because only a small π-π stacking effect is expected under the highly dilute condition (9.9 µM).

Investigation of the bowl-to-bowl inversion of the corannulene ring was conducted by the replacement of two acetonitrile ligands on the Pd(II) center of 2a by a chiral bidentate ligand (1R,2R)-(–)-1,2-diaminocyclohexane (3), in expectation of the observation of the diastereomeric inversion. Unfortunately, although the ligand exchange quantitatively took place as shown by 1H NMR and ESI-TOF mass measurements, the diastereomeric inversion was not observed even at low temperatures.

Synthesis of Ir(III) Complex, mer-Ir(III)(H–11)3, and Its Coordination Structure

Tris(2-phenylpyridinato) Ir(III) complex, Ir(III)(ppy)3 (ppyH = 2-phenylpyridine), and its derivatives are attractive luminescent materials. A six-coordinate Ir(III)(H–11)3 complex was synthesized by the reaction of the bulky ligand 1 with Ir(III) in two steps according to the synthetic method of Ir(III)(ppy)3 through [Ir(III)(ppy)2(µ-Cl)]2. Firstly, a biscyclometallated Ir(III) complex was generated by the reaction of 1 with IrCl3・nH2O, which was characterized by 1H NMR spectroscopy and ESI-TOF mass spectrometry. The resulting biscyclometallated Ir(III) complex was then mixed with 1 at 140 °C to obtain IrIII(H–11)3, which was isolated as an orange solid after purification with silica gel column chromatography. The Ir(III)(H–11)3 complex was characterized by 1H NMR and ESI-TOF mass measurements (m/z = 1171.3 [Ir(H–11)3]+). It should be noted that some NMR signals for protons around the sterically crowded coordination sites were shifted to higher magnetic field by up to 6 ppm, indicating the shielding effect of the large π-surfaces. In the light of the symmetry of the NMR signals, the complex Ir(III)(H–11)3 was assigned to its meridional (mer) isomer (Figure 6a).

A UV-vis absorption spectrum of mer-Ir(III)(H–11)3 was significantly red shifted compared to that of mer-Ir(III)(ppy)3 (Figure 6b). Its absorption bands in the longer-wavelength region than 400 nm suggest the extended π-conjugation of the corannulene moiety due to the cyclometallation of 1 with Ir(III). According to the previously reported peak assignment for mer-Ir(III)(ppy)3, the strong absorption from 250 to 400 nm and the weaker absorption in the longer-wavelength region than 400 nm may be assigned to spin-allowed π-π* transition of the ligand moiety and MLCT, respectively. mer-Ir(III)(H–11)3 complex does not have strong luminescence, whereas facial (fac) isomer of Ir(III)(H–11)3 is expected to have strong luminescence as observed with fac-Ir(III)(ppy)3. Synthesis of fac-Ir(III)(H–11)3 is now underway.

Summary

A novel pyridyl-pendant corannulene ligand, 2-pyridylcorannulene (1), was synthesized from corannulene in two steps. Complexation of 1 with Pd(II) and Ir(III) produced [Pd(II)(H–11)(CH3CN)2]BF4 and mer-Ir(III)(H–11)3 complexes, respectively, with a cyclometallated structure. Their extended π-conjugation with metal complexation was established by structural and spectrochemical analyses of these complexes in solution and crystal states. The X-ray structure of the PdII complex suggests that the stacking interactions between the corannulene rings would provide an excellent tool to align metals attached around the rim of the rings in an extended π-conjugation system.

Scheme 1. Synthesis of 2-pyridylcorannulene (1) from corannulene in two steps.

Figure 2. Crystal structure of 2-pyridylcorannulene (1).

Figure 3. 1H NMR spectra (500 MHz, CD3CN, 300 K) of (a) 1 and (b) 2a and chemical structures of 1, 2a, and 2b.

Figure 4. X-ray structure of [Pd(II)(H–11)(CH3CN)2]BF4 (2a). BF4– ions are omitted for clarity.

Figure 6. (a) Structure and molecular model of mer-Ir(III)(H–11)3 and (b) absorption spectra of mer-Ir(III)(ppy)3 (CH2Cl2, [mer-Ir(III)(ppy)3] = 8 µM, 293 K) and mer-Ir(III)(H–11)3 (CH2Cl2, [mer-Ir(III)(H–11)3] = 8 µM, 293 K).

曲面状π共役化合物コラニュレンはπ曲面に由来する曲面反転挙動や構造異方性など特徴的な性質を有するため、平面状π共役化合物とは異なる性質や機能の発現が期待できる。このようなπ曲面は凹凸両面に多様な配位部位を有することから、有機化合物としてだけでなく錯体形成の配位子としても近年注目を集めてきた。コラニュレンから合成された錯体の多くはπ配位錯体で、曲面周縁部での金属配位によるσ配位錯体は比較的少ない。σ配位錯体形成はπ曲面の拡張や分子間相互作用などを誘導しうるため、σ配位錯体合成法の確立は新たな構造・機能制御法を提示できると考えられる。本研究では、周縁部によるσ配位形成法の一つとして、触媒能、発光など興味深い反応性や物性を示すシクロメタル化に着目し、コラニュレン周縁部の化学修飾による新規シクロメタル化錯体の開発を目指した。そのために、まず配位子としてピリジル基修飾型コラニュレンを設計・合成し、錯体合成および得られた錯体の構造について検討を行った。

本論文は全4章から成り、第1章においてはコラニュレン化合物およびシクロメタル化錯体に関する背景や本研究の目的が詳細に述べられている。

第2章では、新規シクロメタル化配位子の設計・合成およびモノシクロメタル化錯体の合成とその構造について述べられている。まず、π曲面を有する新規シクロメタル化配位子として2-ピリジルコラニュレン (1) を合成した。吸収スペクトルおよび結晶構造から、配位子1ではコラニュレンとピリジル環の間の共役が弱いことが見出された。次に、四配位平面型金属イオンの中で比較的配位子交換の速いPd(II)イオンを用いて、配位子1のモノシクロメタル化Pd(II)錯体を合成し、その構造解析を行った。その結果、配位子1とPd(II)イオンの反応からPd(II)錯体が2種類の化合物の9:1混合物として生成した。1H NMRおよびESI-TOF massスペクトルより、これらはシクロメタル化Pd(II)錯体[Pd(H–11)(CH3CN)]BF4の脱プロトン位の異なる2種類の構造異性体2aおよび2bであることが明らかとなった。この混合物の結晶化により錯体2aを板状結晶および針状結晶の混合物として収率31%で単離することに成功した。錯体2aの板状結晶の単結晶X線構造解析を行ったところ、四配位平面型のモノシクロメタル化Pd(II)錯体2aの構造が明らかとなった。この構造では、シクロメタル化によりπ共役が拡張し、π曲面が浅くなっていた。結晶構造中においては、Pd(II)錯体2aは二つのコラニュレン間のπ-π相互作用によりカラム状の一次元集積構造を構築していることが明らかとなった。このことは、配位子1が二つのピリジル環の間のπ-π相互作用およびCH-π相互作用によりネットワーク状構造を構築している結果と対照的である。また錯体2aの針状結晶の結晶構造解析を行ったところ、異なるパッキング構造が明らかとなった。この結晶中では、錯体2aがコラニュレンとピリジル環の間のπ-π相互作用によりカラム状構造を構築していた。したがって、板状結晶と針状結晶は錯体2aの二つの結晶多形であることが示された。また、錯体2aは溶液中でもπ-π相互作用による積層構造を形成していることが示唆された。さらに、キラル配位子(1R,2R)-(–)-1,2-diaminoccyclohexaneを用いてPd(II)錯体の反転速度計算を試みたところ、183 K以上ではNMRのタイムスケールより反転が速いことが確認された。また、この実験からは配位溶媒CH3CNの交換により更なる構造拡張が可能であることも示された。

第3章では、配位子1のトリスシクロメタル化錯体の合成について述べられている。六配位八面体型金属イオンの一つとして発光特性などに興味がもたれるIr(III)イオンを用いて、配位子1からmer-トリスシクロメタル化Ir(III)錯体を合成した。メリディオナル体の合成法は、mer-トリスフェニルピリジナトIr(III)錯体においてビスシクロメタル化Ir(III)錯体を用いる経路が確立されている。そこで、同様の合成法により配位子1からビスシクロメタル化Ir(III)錯体 [Ir(H–11)2(µ-Cl)]2を合成し、次にトリスシクロメタル化Ir(III)錯体Ir(H–11)3を合成した。得られたトリスシクロメタル化Ir(III)錯体は発光や1H NMRスペクトルの対称性からmer-体であることが強く示唆された。また、このIr(III)錯体はIr(III)イオン周りに三つのπ曲面が集合し、密な構造を構築していることが示された。

第4章では、本論文の総括と今後の展望が述べられている。

以上のように、本博士論文では、新規ピリジルコラニュレン配位子によるシクロメタル化錯体を開発した。ピリジルコラニュレンはコラニュレンπ曲面の新たな構造修飾・拡張の基盤となり、その錯体は構造・機能創出を介してコラニュレン化学の新しい領域を開拓し、理学の発展に大いに貢献するものである。よって、博士(理学)取得を目的とする学術研究として十分な意義を有する。なお、本論文における各章の研究は他の複数の研究者との共同研究によるものであるが、論文提出者が主体となって実験、解析および考察を行ったものであり、論文提出者の寄与が十分であると判断する。

したがって、博士(理学)の学位を受けるのに十分な資格を有するものと認める。