Exploration of Annulated Phosphorus Heterocycles Derived from Anionic Dicarbenes

吡嗪（1，4-二氮杂苯）是一种芳香族氮杂环化合物，作为配位聚合物的重要结构组分，在光电子和电池领域有着广泛的应用。相比之下，较重的吡嗪类似物，如1，4-二膦仍然是难以捉摸的物种。这是由于这些分子形成Diels-Adler型二聚体的高倾向性。迄今为止，没有苯并环化的双环和三环杂环（例如，9，10-二磷蒽）也作为单体存在。因此，这些物种作为单体的合成不可及性是探索其基础化学及其在材料科学中的潜在应用的主要障碍。为了克服这个问题，我们追求的战略，正式取代外围苯环的9，10-二磷杂蒽与1，3-咪唑单元，以获得稠合三环杂环的中心C4 P2环。1，3-咪唑单元在1，2，3-位具有空间要求高的芳基取代基。 所需的起始原料Li（ADCAr），即我们开发的所谓阴离子二卡宾（ADCAr = {：CN（Dipp）}2CAr; Dipp = 2，6-iPr 2C 6 H3，Ar =芳基），可通过相应的C2-芳基化1，3-咪唑鎓盐的双（C4/C5）去质子化容易地获得。例如，Li（ADCPh）与PCl 3反应生成[（ADCPh）PCl 2]2，其在用4当量的KC 8还原后得到具有含8个π电子的中心C4 P2环的1，4-二膦-1，4-二化物化合物[（ADCPh）P]2。反芳香体系[（ADCPh）P]2可以很容易地被AgOTf氧化，得到中心具有6π电子芳香C4 P2-环的[（ADCPh）P]2（OTf）2。受这些初步结果的鼓舞，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，反应性和物理性质。目标化合物的稳定性和性质旨在通过改变1，3-咪唑部分上的C2-取代基的性质来改变。稳定的自由基将通过相应的C4 P2-物种的单电子氧化来制备。与N2 O和有机叠氮化物（RN 3）的反应性研究旨在分离含有P=X键（X = O或NR; R =烷基或芳基）的单体P-杂环。目标化合物的氧化还原行为和水物理性质将分别通过电化学和光谱（紫外-可见吸收/发射）研究进行研究。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.