Structural characterization of p-benzosemiquinone radical in a solid state: the radical stabilization by a low-barrier hydrogen bond

Abstract

Semiquinone (p-benzosemiquinone), a transient organic radical, was detected in the solid state by EPR spectroscopy revealing four symmetrically equivalent protons. A variable-temperature X-ray diffraction analysis (293 and 90 K) and EPR data support a dynamical disorder of the proton. A low-barrier O-H...O hydrogen bond stabilizes the radical. The C-O bond length is 1.297 (4) A, corresponding to a bond order of ca 1.5. The geometry of the radical implies an electron delocalization throughout the benzenoid ring. Two polymorphs of semiquinone, monoclinic and triclinic, were observed and their structures determined. Their crystal packings were compared with those of quinhydrone polymorphs.

Stabilisation of tetrabromo- and tetrachlorosemiquinone (bromanil and chloranil) anion radicals in crystals

“…It has been a unique example of a reversible single crystalto-single crystal redox reaction. 33 Crystals of acetone solvates: KCl 4 Q˙ Me 2 CO, RbCl 4 Q˙ Me 2 CO and KBr 4 Q˙ Me 2 CO are very unstable. Thin plate-like crystals are formed 10-15 min after addition of solid alkali iodide into the solution of tetrahalogenoquinone (Fig.…”

“…Our recent crystallographic study showed that p-benzosemiquinone radical stabilised by two low-barrier hydrogen bonds is halfway between a quinone and a hydroquinone ring. 33 These variations affect the ring geometry (Table 2) but also redox properties of the system which is interrelated to electrontransfer capacity. …”

“…Recently, several osemiquinone complexes with transition metals have been prepared; 28-32 the crystal structure of unsubstituted p-benzosemiquinone radical stabilised by a low-barrier hydrogen bond was studied by X-ray diffraction and EPR spectroscopy. 33 The electron transfer process significantly affects the electronic structure of the semiquinone radical and its geometry, both, C-O bonds and quinoid ring skeleton. [33][34][35][36][37] We report crystal structures of a series of alkali salts of tetrachloro and tetrabromosemiquinone radical anion (Cl 4 Q˙ˉ and Br 4 Q˙ˉ) where the negative charge is balanced by an alkali cation (Scheme 2, Table 1).…”

“…33 The electron transfer process significantly affects the electronic structure of the semiquinone radical and its geometry, both, C-O bonds and quinoid ring skeleton. [33][34][35][36][37] We report crystal structures of a series of alkali salts of tetrachloro and tetrabromosemiquinone radical anion (Cl 4 Q˙ˉ and Br 4 Q˙ˉ) where the negative charge is balanced by an alkali cation (Scheme 2, Table 1). The radical anions are stabilised by spincoupling of unpaired electrons of contiguous tetrahalogenosemiquinone rings forming diamagnetic dimeric species as described by intermolecular association of charged π-radicals.…”

Dva školska sata kemije (slobodnih) radikala

“…[6][7][8] Biološki najvažniji radikal, semikinon, začudno je lako pripraviti. [9][10][11][12][13] U ovome je članku opisano nekoliko jednostavnih reakcija priprave semikinonâ, koje je lako izvesti u školskom kemijskom kabinetu. Sve zajedno ne bi trebale trajati dulje od 90 minuta i može ih se primijeniti za učenje kroz eksperimente ili mini projekte.…”