Choua Sylvie, Dutan Cosmina, Cataldo Laurent, Berclaz Théo, Geoffroy Michel, Mézailles Nicolas, Moores Audrey, Ricard Louis, Le Floch Pascal
Department of Physical Chemistry, University of Geneva, 30 Quai Ernest Ansermet, 1211 Geneva 4, Switzerland.
Chemistry. 2004 Aug 20;10(16):4080-90. doi: 10.1002/chem.200400073.
EPR spectra show that one-electron reduction of bis(3-phenyl-6,6-(trimethylsilyl)phosphinine-2-yl)dimethylsilane (1) on an alkali mirror leads to a radical anion that is localized on a single phosphinine ring, whereas the radical anion formed from the same reaction in the presence of cryptand or from an electron transfer with sodium naphthalenide is delocalized on the two phosphinine rings. Density functional theory (DFT) calculations show that in the last species the unpaired electron is mainly confined in a loose P-P bond (3.479 A), which results from the overlap of two phosphorus p orbitals. In contrast, as attested by X-ray spectroscopy, the P-P distance in neutral 1 is large (5.8 A). As shown by crystal structure analysis, addition of a second electron leads to the formation of a classical P-P single bond (P-P 2.389 A). Spectral modifications induced by the presence of cryptand or by a change in the reaction temperature are consistent with the formation of a tight ion pair that stabilizes the radical structure localized on a single phosphinine ring. It is suggested that the structure of this pair hinders internal rotation around the C-Si bonds and prevents 1 from adopting a conformation that shortens the intramolecular P-P distance. The ability of the phosphinine radical anion to reversibly form weak P-P bonds with neutral phosphinines in the absence of steric hindrance is confirmed by EPR spectra obtained for 2,6-bis(trimethylsilyl)-3-phenylphosphinine (2). Moreover, as shown by NMR spectroscopy, in this system, which contains only one phosphinine ring, further reduction leads to an intermolecular reaction with the formation of a classical P-P bond.
电子顺磁共振光谱表明，双（3-苯基-6,6-（三甲基硅基）磷杂环戊二烯-2-基）二甲基硅烷（1）在碱金属镜上发生单电子还原反应会生成一种自由基阴离子，该阴离子定域在单个磷杂环戊二烯环上，而在穴醚存在下由同一反应形成的自由基阴离子或通过与萘钠进行电子转移形成的自由基阴离子则离域在两个磷杂环戊二烯环上。密度泛函理论（DFT）计算表明，在后一种物种中，未成对电子主要局限于一个松散的P-P键（3.479 Å）中，该键由两个磷原子的p轨道重叠形成。相比之下，如X射线光谱所证实的，中性的1中的P-P距离较大（5.8 Å）。晶体结构分析表明，再添加一个电子会导致形成一个经典的P-P单键（P-P 2.389 Å）。穴醚的存在或反应温度的变化所引起的光谱变化与形成紧密离子对相一致，该离子对稳定了定域在单个磷杂环戊二烯环上的自由基结构。有人认为，这种离子对的结构阻碍了围绕C-Si键的内旋转，并阻止1采取缩短分子内P-P距离的构象。2,6-双（三甲基硅基）-3-苯基磷杂环戊二烯（2）的电子顺磁共振光谱证实了在没有空间位阻的情况下磷杂环戊二烯自由基阴离子与中性磷杂环戊二烯可逆形成弱P-P键的能力。此外，如核磁共振光谱所示，在这个仅包含一个磷杂环戊二烯环的体系中，进一步还原会导致分子间反应并形成一个经典的P-P键。
