Two spin-containing fragments connected by a two-electron one-center heteroatom π spacer. A new open-shell organic molecule witha singlet ground state

Abstract

The synthesis of 4,4-iminobis(2,2′,2″,4′,4″,6,6′,6″-octachlorotriphenylmethyl) diradical3, a stable organic magnetic molecule consisting of two spin-containing fragments linked by a nitrogen atom, is reported. Electron paramagnetic resonance (EPR) analysis in methyltetrahydrofuran solution (∼10^–3 m) at low temperatures showed a typical fine structure (g_xx=2.0042;g_yy=2.0048;g_zz=2.0027) of Δm_s=±1transition( | D/hc |=0.0071 cm^–1; | E/hc |=0.0006 cm^–1) as well as a broad (ΔH_pp=7.5 G) and weak signal of Δm_s=±2 transition (g=4.124), due to an asymmetric and excited triplet state corresponding to an intramolecular spin–spin interaction which diminishes with decreasing temperature. It also showed a pair of small peaks, that might be associated with a weaker dipole–dipole interaction which diminishes with increasing temperature, emerging at both sides of a central and single peak due to a doublet state resonance corresponding to 2,2′,2″,4′,4″,6,6′,6″-octachloro-4-{3,5-dichloro-4-[bis(2,4,6-trichlorophenyl)methylene]cyclohexa-2,5-dienylideneamino}triphenylmethyl radical 4, obtained by smooth oxidation of 3. From magnetic susceptibility measurements of the sample in the solid, a linear four-spin model was applied to establish that the singlet is the ground state of the molecule and that two triplets (J_intra=–286±30 K, J′_inter=–160±50 K) were the low-lying excited states. Organic solutions of 3 in air slowly oxidize to give 4, a much more persistent monoradical which is also obtained by a smooth oxidation of 3 with AgNO₃ in CHCl₃ . Cyclic voltammograms for the reduction of 3 and 4 in dimethylformamide (DMF) with tetra-n-butylammonium perchlorate exhibited three consecutive redox couples with standard potentials of –0.23, –0.58 and –0.74 V vs. SCE, indicating a reasonable stability of anions 3^–and3^2– in DMF solution.