A stable neutral hydrocarbon radical:: Synthesis, crystal structure, and physical properties of 2,5,8-tri-tert-butyl-phenalenyl
A stable neutral hydrocarbon radical:: Synthesis, crystal structure, and physical properties of 2,5,8-tri-tert-butyl-phenalenyl
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DOI：
10.1021/ja9836242
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发表时间：
1999-02-24
影响因子：
15
通讯作者：
Ouyang, JY
中科院分区：
文献类型：
--
作者：
Goto, K;Kubo, T;Ouyang, JY
苯丙基是具有高度对称性的奇数交替烃(D3h)，1具有形成阳离子、2a自由基、2b和阴离子三种氧化还原物种的能力。2C这种特征特征已被广泛用于探索新的共轭电子体系，如扩展的共轭自由基，3两性氧化还原物种，4和电学5和磁学6有趣的材料。其中，苯基自由基与含杂原子的中性自由基的固态性质的研究相联系，重新引起了人们对磁性材料的关注。7虽然碳氢自由基是自由基的原型，但由于易于二聚和/或空气氧化，在固体中没有苯基自由基的详细表征。在这种背景下，分离结晶状态下的持久性苯基一直是一个重要的问题。最近，5个异丙基稳定的环戊二烯自由基被报道，这是第一个用X射线晶体结构分析表征中性碳氢自由基的例子。8我们在三个位置引入了叔丁基，因为高活性的苯基需要取代基来稳定。本文报道了2，5，8-三叔丁基苯基自由基1b的合成、晶体结构和物理性质，重点介绍了1b在晶态下的电子结构特征。2的10溴化反应高产率地得到了3，通过锂离子物种转化为醛4。由4的Reformsky反应得到的酯5与三乙基硅烷还原得到6。6与LiI的水解得到高产率的酸7，尽管常规的水解是失败的。以7为原料，经低温Friedel-Craft环合反应，得到了定量产率为8的苯丙酮。关键前体菲10通过还原8和随后的脱水得到淡黄色晶体(方案1)。10在脱气甲苯中与对氯苯胺反应，得到蓝色中性自由基溶液。在己烷中进行类似的处理后，结晶产物会变成深蓝色的针状物。晶体中的基团1b在没有空气的情况下是稳定的。1b在室温下暴露于空气中2小时后发生轻微变化，花了一周多的时间才完全转变为黄色的苯丙酮衍生物和未知产物的混合物。我们首次成功地分析了一种奇数交替碳氢自由基的晶体结构。由脱气正己烷在-30℃重结晶得到适合于X射线晶体结构分析的1b单晶。苯环具有轻微扭曲的D3h对称性和接近平面的几何构型(图1)。与叔丁基团键合的碳原子偏离环的最小二乘平面+0.064~+0.070；环中碳-碳键的长度范围为1.374-1.421？，与萘的碳键长度相当。该分子以叔丁基团的交错排列形成二聚体对，以避免空间斥力。在这种排列中，单占据分子轨道(SOMO)中系数较大的R-碳原子之间有望实现有效的最大重叠。二聚体对中的非键距范围从3.201(8)到3.323(6)？，比碳原子范德华半径(3.4%)短两倍
Phenalenyl, the odd alternate hydrocarbon with high symmetry (D3h), 1 has the ability to form three redox species, cation, 2a radical, 2b and anion. 2c Such characteristic features have been widely utilized for exploring new conjugated electronic systems such as extended conjugated radicals, 3 amphoteric redox species, 4 and electrically5 and magnetically6 intriguing materials. Among them, phenalenyl radicals link to the studies on solid-state properties of neutral radicals with heteroatoms which have attracted renewed attention to search magnetically interesting materials. 7 Although hydrocarbon radicals are prototypes of radicals, no phenalenyl radicals have been characterized in detail in the solid state because of easy dimerization and/or air oxidation. In this context, isolation of persistent phenalenyls in the crystalline state has been an important issue. Recently, the cyclopentadienyl radical stabilized by five isopropyl groups has been reported, as the first example of the neutral hydrocarbon radical to be characterized by X-ray crystal structure analysis. 8 We have introduced tertiary butyl groups at three-positions because the highly reactive phenalenyl radical requires substituents for stabilization. We now report the synthesis, crystal structure, and physical properties of the 2, 5, 8-tri-tert-butyl-phenalenyl radical 1b, emphasizing sailent features of the electronic structure of 1b in the crystalline state.The radical 1b was synthesized from 2, 7-di-tert-butylnaphthalene 29 in 10 steps. 10 Bromination of 2 gave 3 exclusively in high yield, which was converted to the aldehyde 4 through a lithiated species. The ester 5, obtained by the Reformatsky reaction of 4, was reduced with triethylsilane to afford 6. 10 Hydrolysis of 6 with LiI gave the acid 7 in high yield, although the conventional hydrolysis failed. The Friedel-Crafts cyclization at low temperature of the acid chloride derived from 7 gave the phenalanone 8 in quantitative yield. The key precursor, the phenalene 10, was obtained as pale yellow crystals by reduction of 8 and subsequent dehydration (Scheme 1). Treatment of 10 with p-chloranil in degassed toluene gave a blue neutral radical solution. Similar treatment in hexane gave the crystalline products as deep blue needles. 11 The radical 1b in the crystals is stable in the absence of air. A slight change was observed after exposure of 1b to air for 2 h at room temperature, and it took more than one week for them to completely change to a mixture of yellow phenalenone derivatives and unidentified products. We have succeeded in crystal structure analysis of an odd alternant hydrocarbon radical for the first time. Single crystals of 1b suitable for X-ray crystal structure analysis were obtained by recrystallization from degassed hexane at-30 C. 12 The phenalenyl ring possesses a slightly distorted D3h symmetry and nearly planar geometry (Figure 1). The carbon atoms bonded to the tert-butyl group deviate by+ 0.064 to+ 0.070 Å from the least-squares plane of the ring. The carbon-carbon bond lengths in the ring range from 1.374 to 1.421 Å, comparable to those of naphthalene. The molecule forms a dimeric pair in a staggered arrangement of tert-butyl groups to avoid steric repulsion. In such an arrangement, effective maximum overlaps are expected between the R-carbon atoms having a large coefficient in the singly occupied molecular orbital (SOMO). The nonbonded distances in the dimeric pair range from 3.201 (8) to 3.323 (6) Å, which are shorter than twice the van der Waals radius of carbon atom (3.4