1,4-Phosphagermines: An Anionic 6π-heteroaromatic Biradicaloid and a Persistent π-Delocalized Radical

Dihetarenes with identical third-period or heavier elements are well established, whereas mixed-element analogues are not. Here, we report the synthesis and characterization of a 1,4-phosphagermine anion and its persistent delocalized radical. The unique synthetic protocol presented herein should be adaptable to other combinations of mixed-heavy main-group elements. Computational analyses (2D-XY NICS_π,zz^SOM scans and ELF_π) identify it as an aromatic P,Ge-heterocycle, which nonetheless possesses significant biradicaloid character as revealed by PW6B95-D3BJ/def2-QZVP(CPCM_THF)//TPSS-D3BJ/def2-TZVP(CPCM_THF) and CASSCF (CASSCF(X,Y)/NEVPT2/def2-TZVP) calculations. Electrochemical studies demonstrate the stability of the radical species, while mild chemical oxidation of the aromatic anion produces the radical, which was identified in solution by EPR spectroscopy. These findings expand the frontier of main-group chemistry, highlighting the potential of mixed-heavy-element aromatic systems. A detailed, comprehensive theoretical study (PW6B95-D3BJ/def2-TZVP(CPCM_THF) level of theory) on 16 combinations of heavy group 14/15 elements reveals an increasing tendency for a biradicaloid character but not the alternative Dewar structure.