Substituent effects on the electronic structure of the flat Blatter radical: correlation analysis of experimental and computational data

Abstract

A series of C(10)-substituted derivatives of 2-Ph-3H-[1,2,4]triazino[5,6,1-kl]phenoxazin-3-yl was obtained using the aza-Pschorr, photochemical and radical-induced cyclization reactions, and through functional group transformations of the C(10)-amino and C(10)-iodo derivatives. The iodo derivative underwent Pd-catalyzed C–C cross coupling reactions leading to the installation of Ph, 2-thienyl and PhCC groups at the C(10) position effectively extending electronic conjugation. The substituent effect on the electronic properties of the 3H-[1,2,4]triazino[5,6,1-kl]phenoxazin-3-yl was investigated in twenty one derivatives with a diverse range of functional groups by spectroscopic (UV-vis and EPR) and electrochemical methods augmented with DFT calculations. Results show that the lowest energy electronic absorption and redox potentials correlate well with the σ_p substituent parameter, while a_N hfcc and the N–H bond dissociation energy (BDE) are well described by the σ_m parameter. In general, increasing the electron donating ability of the C(10)–X substituent lowers the π–π*(1) excitation energy, cathodically shifts the redox potentials, increases spin delocalization beyond the [1,2,4]triazinyl ring and lowers BDE. The latter two parameters are important indicators of the overall radical stability. Molecular and crystal structures of three radicals were established with the single crystal XRD method.