Asymmetric and zwitterionic Blatter diradicals

Abstract

Asymmetric diradical molecular systems with different resonance mechanisms are largely unexplored. Herein, two conjugated asymmetric diradicals with Blatter and phenoxyl moieties (pBP and mBP) have been synthesized and studied in depth. A complete set of spectroscopic, X-ray crystallographic and magnetic techniques, together with quantum chemical calculations, have been used. The para-isomer (pBP) bears diradical and zwitterionic resonant forms, the latter by a electron delocalization mechanism, which are synergistically integrated by a sequence of nitrogen, provided by the Blatter moiety imine and amine (of different acceptor nature). In the meta-isomer (mBP), the zwitterionic form promoted in pBP by the lone-pair electron of the amine nitrogen is not available, yet it possesses a pseudo-hyperconjugation effect where the N lone pair mediates in a bonding coupling in a counter homolytic bond scission mechanism. Both electronic effects converge to promote medium diradical characters and narrow singlet-triplet gaps to the two electronic isomers. All these aspects delineate the subtle balance that shapes the electronic structure of open-shell molecules, which is even more challenging in the case of asymmetric systems, such as those described here with asymmetric phenoxyl-Blatter diradicals.

Scheme 1. Para-disubstituted benzoquinoid ring that transforms into a zwitterion by electron delocalization (A, ED) and into a diradical by electron-pair splitting (B, EPS). ED resonant forms have the same number of double bonds, while the diradical via EPS has one bond less, leading to the dominance of A over B if both effects act on the same π path. (C) Resonance structures of pBP (in brackets the relevant π-conjugation center is highlighted with the amine N acting as donor). (D) Resonance structures in mBP (in brackets the relevant π-conjugation center is highlighted with the amine N acting as a transmitter of the inter-radical coupling).

Scheme 2. Synthetic routes for pBP and mBP.

Fig. 1. Single-crystal structures of mBP (a) pBP (c) and packing diagrams of mBP (b) and pBP (d). Thermal ellipsoids are drawn at 50% probability. Hydrogen atoms and tert-butyl groups are omitted for clarity. Dihedral angles, bond lengths (Å) and short contacts (Å) are labeled. (e) Summary of bond lengths and BLA of rings A and B.

Fig. 2. ESR spectra of mBP (a) and pBP (b) in toluene, at 130 and 140 K, respectively. The black, red and blue lines indicate experimental, simulated and difference ESR (experimental-simulated) spectra, respectively. The inset shows the half-field ESR signal at 130 K. Magnetic susceptibilities (χT) versus T curve from the SQUID measurements on the powder of mBP (c) and pBP (d) and the fitting plot obtained with the Bleaney–Bowers equation. Calculated spin density of mBP (e) and pBP (f) based on single-crystal structure at the (u)m062x/6-311(d,p) level.

Fig. 3. UV-vis–NIR electronic absorption spectra of mBP (a) and pBP (b) in solvent of different polarity (green: DMF, blue: dichloromethane (DCM), red: CCl₄, and dark: 2-methyltetrahydrofuran (2-MeTHF)). Solution-state 785 nm excitation Raman spectra of mBP (c) and pBP (d) in several solvents.

Fig. 4. Electronic configurations for the asymmetric diradicals studied here, where A and B are the phenoxyl oxygen and imine nitrogen in the case of pBP: ionic forms (top) and neutral tetra-radical forms (bottom).