Curved aromaticity of a corannulene-based neutral radical: Crystal structure and 3D unbalanced delocalization of spin
Curved aromaticity of a corannulene-based neutral radical: Crystal structure and 3D unbalanced delocalization of spin
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DOI：
10.1002/anie.200704752
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发表时间：
2008-01-01
影响因子：
16.6
通讯作者：
Nakasuji, Kazuhiro
中科院分区：
文献类型：
--
作者：
Morita, Yasushi;Ueda, Akira;Nakasuji, Kazuhiro
碗状多环芳烃，如环烯，其本身具有一个非交替π共轭的富勒烯亚结构，近年来得到了广泛的研究这类研究着重从实验和理论两方面描述了它们的芳构性和在曲面上或曲面与金属离子之间的电子相互作用。与目前研究的闭壳体系相比，由于具有弯曲π共轭的中性开壳分子在空气中的稳定性较低，因此只在脱气溶液中研究过。[2,3]弯曲π共轭体系的自旋离域本质上是三维的，因此阐明晶体/电子-自旋结构以及晶体状态下的分子内和分子间交换相互作用是研究弯曲π体系（如富勒烯）中三维电子间交换和偶极相互作用的一个当前问题在本研究中，我们合成了一种含有苯氧基片段的corannul烯基中性自由基(1)，并首次确定了具有弯曲π共轭的中性自由基衍生物的晶体结构。corannulene部分1的高稳定性和广泛的自旋离域特性使我们能够研究这类具有非交替π共轭的弯曲中性自由基体系的弯曲芳香性和分子间相互作用，从而强调了在corannulene部分中自旋不平衡离域的发生。Scheme1中描述了1的合成路径。通过甲氧基甲基保护（MOM-protected）溴苯酚4的铃木偶联得到了自由基前体2作为无色板。用过量的PbO2处理2，再结晶得到1为黑板。晶体中的自由基1在À308C的空气中可以稳定几个星期，在脱气溶液中也非常稳定。我们成功地分析了自由基1的晶体结构（图1a）通过与苯酚2.[8]分子结构的比较，揭示了1的结构特征虽然1的碗形深度和POAV （π-轨道轴矢量）角[9,10]与2相似，但观察到苯氧基部分的键长以及环烯部分的C7ÀC8和C8ÀC17键长发生了显著变化（图1b，表1）。特别是1的OÀC1键（1.250(2)）更接近对苯醌（1.222）[11a]和对对苯二酚（1.231）]的C= O键长[11b]，说明1的CÀO键具有较强的双键性质。对1和2[12]的红外测量也证实了1的OÀC1键的双键性质。在
Bowl-shaped polycyclic aromatic hydrocarbons such as corannulene, which shares a fullerene substructure itself with a non-alternant π conjugation, have been studied extensively in recent years.[1] Such studies have emphasized the description of their aromaticity and electron interactions on curved surfaces or between a curved surface and a metal ion from both the experimental and theoretical perspectives. In contrast to the closed-shell systems studied so far, neutral openshell molecules with curved π conjugation have been studied only in degassed solution owing to their low stability in air.[2, 3] Spin delocalization on a curved π-conjugated system is intrinsically three-dimensional, and thus elucidation of the crystal/electronic-spin structures and intra-and intermolecular exchange interactions in the crystalline state is a current issue for studying the 3D interelectronic exchange and dipolar interactions in curved π systems such as fullerenes.[4] In this study, we have synthesized a corannulene-based neutral radical with a phenoxyl moiety (1) and determined for the first time the crystal structure of a neutral radical derivative with curved π conjugation. The high stability and extensively spin-delocalized nature of the corannulene moiety 1 have enabled us to investigate curved aromaticity and intermolecular interactions of this class of curved neutral radical systems with a non-alternant π conjugation,[5] thus emphasizing the occurrence of unbalanced delocalization of spin within the corannulene moiety. A synthetic route for 1 is depicted in Scheme1. The radical precursor 2 was obtained as colorless plates by Suzuki coupling of a pinacol boronate derivative 3 [6] with methoxymethyl-protected (MOM-protected) bromophenol 4 and subsequent deprotection. Treatment of 2 with an excess of PbO2 and recrystallization gave 1 as black plates. The radical 1 in the crystal is stable in air at À308C for a few weeks, and is extremely stable also in degassed solution. We have succeeded in the crystal structure analysis of radical 1 (Figure 1 a).[7] The structural features of 1 were revealed by comparison with the molecular structure of the phenol 2.[8] While the bowl depth and POAV (π-orbital axis vector) angles [9, 10] of 1 are similar to those of 2, significant changes in bond lengths of the phenoxyl moiety and C7ÀC8 and C8ÀC17 of the corannulene moiety were observed (Figure1b, Table1). Particularly, the OÀC1 bond of 1 (1.250 (2)) is closer to the C= O bond length of p-benzoquinone (1.222)[11a] and p-terphenoquinone (1.231),[11b] indicating that the CÀO bond of 1 has a substantial doublebond character. IR measurements of 1 and 2 [12] also demonstrated the double-bond character of the OÀC1 bond of 1. In