219948-13-7Relevant articles and documents
Synthesis of bis(semiquinone)s and their electrochemical and electron paramagnetic resonance spectral characterization
Shultz, David A.,Boal, Andrew K.,Farmer, Gary T.
, p. 9462 - 9469 (2007/10/03)
The syntheses of three new bis(semiquinone)s (2(··2-), 3(··2-), and 5(··2-)) linked through carbon-carbon double bonds in a geminal fashion (2(··2-) and 3(··2-)), and through an sp3 carbon (5(··2-)), are presented, as well as the results of variable-temperature EPR (VT-EPR) spectroscopy on these biradicals and two previously reported bis(semiquinone)s, 1(··2-) and 4(··2-). We suggest that the potential difference in redox couples associated with a biradical is useful for qualitatively assessing changes in the exchange parameter within an isostructural series. The zero-field-splitting parameters for 1(··2-) 5(··2-) are consistent with their electronic structures: biradicals 1(··2-)-3(··2-) which have conjugating groups attached to the semiquinone rings have D-values less than 5(··2-), a bis(semiquinone) that lacks such a conjugating group. Also, the D-value of 3(··2-) is significantly less than those of 1(··2-) and 2(··2-), in agreement with larger interelectron separation in 3(··2-), a biradical with quinone-methide π-system delocalization. Changes in counterion Lewis acidity are not manifested in the D-values of the biradical dianion 4(··2-). The EPR spectrum of biradical 5(··2-) is consistent with the existence of at least two rotamers having different zero-field-splitting parameters. Biradicals 1(··2-)-4(··2-) gave linear Curie plots, consistent with J > 0 (ferromagnetic coupling) or J = 0. The temperature-dependent intensity of EPR signals of 5(··2-) are characteristic of antiferromagnetic coupling. Best fit results give J = -114 ± 6 cal/mol for the |D/hc| = 0.01309 cm-1 rotamer of 5(··2-), and J = -76 ± 3 cal/mol for the |D/hc| = 0.01026 cm-1 rotamer of 5(··2-).
