Singlet−Triplet Gap in Triplet Ground-State Biradicals Is Modulated by Substituent Effects

Three S = 1 bis(semiquinone) complexes have been prepared. To ensure ferromagnetic intramolecular exchange coupling, the two semiquinones are attached 1,3 to a 5-substituted phenylene ring. The biradical complexes differ in their meta-substituents:  1-NMe₂, X = N,N-dimethylamino; 1-t-Bu, X = tert-butyl; 1-NO₂, X = nitro. All three structures have been determined by X-ray crystallography. Results of structural studies indicate that the biradical ligands of all three complexes have nearly identical conformations with average semiquinone ring torsions of 32° ± 2° relative to the 5-substituted phenylene ring. The exchange parameter, J (Η = −2JŜ₁·Ŝ₂), ranges from +31.0 ± 0.6 cm^-1 for 1-NO₂ to +59.3 ± 1.2 cm^-1 for 1-t-Bu, with J = +34.9 ± 0.7 cm^-1 for 1-NMe₂. Since the conformations are nearly identical, the differences in exchange coupling parameter J are due to substituent effects. The experimental results are supported by Hückel theory arguments and previous computational work.