Spectroscopic and stability studies on unsymmetrical 1,3-dialkyl-1,3-diphosphacyclobutane-2,4-diyls

Air‐tolerant 2,4‐bis(2,4,6‐tri‐t‐butylphenyl)‐1,3‐diphosphacyclobutane‐2,4‐diyl singlet biradicals can be prepared by utilizing the unique reactivity of a kinetically stabilized P≡C triple bond compound. In this procedure, we studied the spectroscopic properties of a fundamental unsymmetrical P‐heterocyclic biradical containing both PEt and PMe moieties, and the effects of the PCH2OMe group in relation to the stability of the P‐heterocyclic biradical skeleton. The experimentally observed nuclear magnetic resonance and photo‐absorption parameters of 1‐ethyl‐3‐methyl‐2,4‐bis(2,4,6‐tri‐t‐butylphenyl)‐1,3‐diphosphacyclobutane‐2,4‐diyl were discussed based on our previous findings and density functional theory calculations, suggesting particular structural characteristics of the P‐heterocyclic biradical skeleton and aromatic substituent effects on the sp2‐C atoms in the 4‐membered ring. Introduction of the methoxymethyl group in the P2C2 biradical moiety gave more stabilized 1,3‐diphosphacyclobutane‐2,4‐diyl derivatives. In comparison with considerably unstable biradicals bearing propargyl substituents, relatively higher lowest unoccupied molecular orbital energies suggest reluctant oxidation of the P‐heterocyclic skeleton. Copyright © 2011 John Wiley & Sons, Ltd. UV‐Vis photo‐absorption properties for air‐tolerant 1,3‐diphosphacyclobutane‐2,4‐diyls as singlet biradicals indicate that employed sterically bulky aromatic substituents such as Mes* on the sp2‐C atoms substantially conjugate with the cyclic skeleton to protect the biradical moiety effectively. The PR groups control the biradical stability by altering the electronic structures of the particular 4‐membered heterocyclic system.