Diastereoisomers of l-proline-linked trityl-nitroxide biradicals: synthesis and effect of chiral configurations on exchange interactions

Abstract

The exchange (J) interaction of organic biradicals is a crucial factor controlling their physiochemical properties and potential applications and can be modulated by changing the nature of the linker. In the present work, we for the first time demonstrate the effect of chiral configurations of radical parts on the J values of trityl-nitroxide (TN) biradicals. Four diastereoisomers (TNT₁, TNT₂, TNL₁ and TNL₂) of TN biradicals were synthesized and purified by the conjugation of a racemic (R/S) nitroxide with the racemic (M/P) trityl radical viaL-proline. The absolute configurations of these diastereoisomers were assigned by comparing experimental and calculated electronic circular dichroism (ECD) spectra as (M, S, S) for TNT₁, (P, S, S) for TNT₂, (M, S, R) for TNL₁ and (P, S, R) for TNL₂. Electron paramagnetic resonance (EPR) results showed that the configuration of the nitroxide part instead of the trityl part is dominant in controlling the exchange interactions and the order of the J values at room temperature is TNT₁ (252 G) > TNT₂ (127 G) ≫ TNL₂ (33 G) > TNL₁ (14 G). Moreover, the J values of TNL₁/TNL₂ with the S configuration in the nitroxide part vary with temperature and the polarity of solvents due to their flexible linker, whereas the J values of TNT₁/TNT₂ are almost insensitive to these two factors due to the rigidity of their linkers. The distinct exchange interactions between TNT_1,2 and TNL_1,2 in the frozen state led to strongly different high-field dynamic nuclear polarization (DNP) enhancements with ε = 7 for TNT_1,2 and 40 for TNL_1,2 under 800 MHz DNP conditions.