Formation and structure of a stable monoradical cation by reduction of a diphosphafulvenium salt
Formation and structure of a stable monoradical cation by reduction of a diphosphafulvenium salt
复制标题
DOI：
10.1002/anie.200603009
复制
发表时间：
2006-01-01
影响因子：
16.6
通讯作者：
Geoffroy, Michel
中科院分区：
文献类型：
--
作者：
Biaso, Frederic;Cantat, Thibault;Geoffroy, Michel
在过去的20年里，物理化学家对有机自由基离子的关注从未停止过，[1，2]主要是因为这些物种经常充当反应中间体，并且因为它们的结构可以决定许多化学过程的结果。[3]相当大的努力已经投入到获得更好的了解他们的电子结构，不幸的是，他们的高反应性使他们的识别困难，和报告，他们的晶体结构仍然稀少。[4]由于有机离子自由基在材料科学中的实际应用，以及它们作为电子器件、有机磁体和有机导体的潜在用途，最近出现了对有机离子自由基行为的新兴趣。[5，6]富烯衍生物（A，方案1）在该领域中起着重要作用;例如，二硫富烯（B）构成导电、半导体和超导分子固体的基本单元之一。在这种情况下，四硫富瓦烯的一个关键性质在于其容易释放一个电子以可逆地产生相当持久的自由基阳离子的能力。[7]新的富勒烯为中心的系统，能够可逆地稳定自由基阳离子的发现是相当大的兴趣。本文的结果有助于该项目，正如将进一步看到的，磷化学可以在这一领域开辟新的和非常有吸引力的前景。基于1，4-二膦酰基环己-2，5-二烯化学的早期报道[8]，以及我们最近对单磷阳离子C+还原过程的EPR研究[9]，我们假设五价磷原子可以作为强电子受体，从而允许通过还原过程合成和稳定持久性自由基阳离子。我们验证了这一假设在这里的情况下，difulvenium二价D2+的还原导致一个稳定的自由基阳离子。据我们所知，二磷富静脉阳离子从来没有报道过。[10]反式-四苯基二磷富烯衍生物1，其合成最近被报道，被选为起始前体。[11]1与过量的三氟甲磺酸甲酯（MeOTf）在258 ℃下反应，干净地得到双阳离子22+，其分离为非常稳定的白色固体。22+的结构通过NMR数据和元素分析得到证实。不幸的是，尽管有许多尝试，22+被证明是不愿意结晶，从而排除了X射线晶体结构的记录。正如预期的那样，二价阳离子22+可以容易地被还原，如循环伏安法（CH 3CN，室温）所示，其显示在100 ° C下的四个波。54 V（可逆），101。10 V（不可逆），100 V。37（部分可逆）和101。72 V（不可逆）与SCE。重要的是，第一个还原过程被发现是可逆的，即使在低的扫描速率（50 mV slog 1），从而表明，这将是可能的，以隔离一个持久的自由基阳离子。减少22+是
The attention paid by physical chemists to organic radical ions has never ceased to increase over the last 20 years,[1, 2] mainly because these species often act as reaction intermediates and because their structure can determine the result of many chemical processes.[3] Considerable efforts have been devoted to gaining a better knowledge of their electronic structure; unfortunately their high reactivity make their identification difficult, and reports on their crystal structures remain sparse.[4] Renewed interest in the behavior of organic ion radicals has recently appeared owing to the practical applications predicted for some of these compounds in materials science and because of their potential use as electronic devices, organic magnets, and organic conductors.[5, 6] Fulvene derivatives (A, Scheme 1) play an important role in this field; for example, dithiafulvene (B) constitutes one of the elementary units of conducting, semiconducting, and superconducting molecular solids. In this context, a crucial property of tetrathiafulvalene lies in its capacity to easily release one electron to give, reversibly, a rather persistent radical cation.[7] The discovery of novel fulvene-centered systems that are able to reversibly stabilize a radical cation is of considerable interest. The results herein contribute to this project, and as will be seen further, phosphorus chemistry can open new and very attractive perspectives in this area. On the basis of early reports on the chemistry of 1, 4-diphosphoniacyclohexa-2, 5-dienes,[8] as well as our recent EPR study on the reduction processes of monophospholium cations C+,[9] we postulated that pentavalent phosphorus atoms could behave as strong electron acceptors and thus allow the synthesis and stabilization of persistent radical cations through reduction processes. We validate this hypothesis herein in the case of a diphosphafulvenium dication D2+ whose reduction leads to a stable radical cation. To the best of our knowledge, diphosphafulvenium cations have never been reported so far.[10]The trans-tetraphenyldiphosphafulvene derivative 1, whose synthesis was recently reported, was chosen as the starting precursor.[11] Reaction of 1 with excess methyl triflate (MeOTf) at 258C cleanly afforded dication 22+, which was isolated as a very stable white solid. The structure of 22+ was confirmed by NMR data and elemental analyses. Unfortunately, despite many attempts, 22+ proved to be reluctant towards crystallization, thus precluding the recording of an X-ray crystal structure. As expected, dication 22+ can be easily reduced, as indicated by cyclic voltammetry (CH3CN, room temperature), which revealed four waves at À0. 54 V (reversible), À1. 10 V (irreversible), À1. 37 (partially reversible), and À1. 72V (irreversible) versus SCE. Importantly, the first reduction process was found to be reversible even at low scan rates (50 mV sÀ1), thereby suggesting that it would be possible to isolate a persistent radical cation. Reduction of 22+ was