Exploration of Annulated Phosphorus Heterocycles Derived from Anionic Dicarbenes

• Applicant: Privatdozent Dr. Rajendra S. Ghadwal
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 514566227

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.

• DFG Programme: Research Grants