Exploration of Annulated Phosphorus Heterocycles Derived from Anionic Dicarbenes

吡嗪(1，4-二氮苯)是一种芳香族N-杂环化合物，广泛应用于配位聚合物、光电子材料和电池材料等领域。相比之下，1，4-二膦等较重的吡嗪类似物仍然是难以捉摸的物种。这是由于这些分子很容易形成Diels-Adler类型的二聚体。到目前为止，也没有苯环化的双环和三环杂环(例如9，10-二磷杂菲)作为单体存在。因此，这些物种作为单体的合成不可及性是探索它们的基础化学以及它们在材料科学中的潜在应用的主要障碍。为了克服这个问题，我们采用了用1，3-咪唑单元取代9，10-二磷杂菲的外周苯环的策略，得到了具有中心C4P2环的稠合三环杂环。1，3-咪唑单元在1，2，3-位上具有立体要求的芳基取代基。我们开发了所需的起始原料Li(ADCAr)，即所谓的阴离子二卡宾(ADCAr={：CN(DIPP)}2car；DIPP=2，6-iPr2C6H3，Ar=芳基)，通过相应的C2-芳基化的1，3-咪唑盐的双(C4/C5)去质子化很容易获得。例如，Li(ADCPh)与PCl3反应生成[(ADCPh)PCl2]2，该化合物在与KC8的4个当量化合物还原后得到具有8个π-电子的中心C4P2环的1，4-二膦-1，4-二胺化合物[(ADCPh)P]2。[(ADCPh)P]2体系容易被AgOTf氧化，生成具有中心6π电子芳香族C4P2环的[(ADCPh)P]2(OTf)2。在这些关于中心带有6π或8π电子C4P2环的环状磷杂环容易获得的初步结果的鼓舞下，我们现在的目标是制备新的衍生物，并系统地研究它们的结构、反应活性和光物理性质。目标化合物的稳定性和性质的目的是通过改变1，3-咪唑部分上C2-取代基的性质来进行修饰。稳定的自由基将通过相应的C4P2-物种的单电子氧化来制备。与N2O和有机叠氮化合物(RN3)的反应活性研究旨在分离含有P=X键的单体P-杂环(X=O或NR；R=烷基或芳基)。目标化合物的氧化还原行为和光物理性质将分别通过电化学和光谱(UV-Vis吸收/发射)研究。为了清楚地了解目标杂环的电子结构，我们还将进行量子化学计算。

Pyrazine (1,4-diazabenzene) is an aromatic N-heterocycle and has been used extensively as a key structural component in coordination polymers and materials for optoelectronics and battery applications. In contrast, heavier analogues of pyrazine such as 1,4-diphosphinines have remained elusive species. This is due to the high propensity of these molecules to form Diels-Adler type dimers. To date, no benzannulated bi- and tricyclic heterocycles (e.g., 9,10-diphosphaanthracene) exist as monomers either. Hence, the synthetic inaccessibility of these species as monomers is a major obstacle in exploring their fundamental chemistry as well as their potential applications in materials science. To overcome this problem, we pursue the strategy of formally replacing the peripheral benzene rings of 9,10-diphosphaanthracene with 1,3-imidazole units to obtain fused tricyclic heterocycles with a central C4P2 ring. The 1,3-imidazole units have sterically demanding aryl substituents at the 1,2,3-positions. The desired starting materials Li(ADCAr), the so-called anionic dicarbenes (ADCAr = {:CN(Dipp)}2CAr; Dipp = 2,6-iPr2C6H3, Ar = aryl) developed by us, are readily accessible by the double (C4/C5) deprotonation of the corresponding C2-arylated 1,3-imidazolium salts. For example, the reaction of Li(ADCPh) with PCl3 yields [(ADCPh)PCl2]2, which after reduction with 4 equivalents of KC8 gives the 1,4-diphosphinine-1,4-diide compound [(ADCPh)P]2 with a central C4P2 ring containing 8π-electrons. The antiaromatic [(ADCPh)P]2 system can be readily oxidized with AgOTf to give the [(ADCPh)P]2(OTf)2 with a central 6π-electron aromatic C4P2-ring. Encouraged by these preliminary results on the ready accessibility of annulated phosphorus heterocycles with a central 6π- or 8π-electron C4P2-ring, we now aim to prepare new derivatives and systematically investigate their structure, reactivity, and photophysical properties. The stability and properties of the target compounds are aimed to be modified by varying the nature of the C2-substituent on the 1,3-imidazole moiety. Stable radicals will be prepared by one-electron oxidations of the corresponding C4P2-species. Reactivity studies with N2O and organic azides (RN3) aim to isolate monomeric P-heterocycles containing P=X bonds (X = O or NR; R = alkyl or aryl). The redox behavior and photophysical properties of the targeted compounds will be studied by electrochemical and spectroscopic (UV-vis absorption/ emission) studies, respectively. To gain a clear insight into the electronic structures of the target heterocycles, we will also perform quantum chemical calculations.