Synthesis of bis(semiquinone)s and their electrochemical and electron paramagnetic resonance spectral characterization

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 sp(3) carbon (5(..2-)), are presented, as well as the results of variable-temperature EPR CVT-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 greement with larger interelectron separation in 3(..2-), a biradical with quinone-methide pi-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-).