Synthesis of Bis(semiquinone)s and Their Electrochemical and Electron Paramagnetic Resonance Spectral Characterization
摘要:
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-).
Preparation and some properties of 7,7-dilithionorbornane
作者:Cornelis P. Vlaar、Gerhard W. Klumpp
DOI:10.1016/0040-4039(91)80659-t
日期:1991.6
7,7-Dilithionorbornane 4a is formed from 7,7-dibromonorbornane 4b by reduction with 4,5 equiv of lithium p,p′-di-tert-butylbiphenyl at −100°C,at which temperature it adds to ethylene. On warming it rearranges to 1-1ithio-5-(2-lithioethyl)cyclopentene 5.
Structure−Property Relationships in Trimethylenemethane-Type Biradicals. 2. Synthesis and EPR Spectral Characterization of Dinitroxide Biradicals
作者:David A. Shultz、Andrew K. Boal、Hyoyoung Lee、Gary T. Farmer
DOI:10.1021/jo990061j
日期:1999.6.1
Several trimethylenemethane-type (TMM-type) dinitroxide biradicals have been prepared that differ in tert-butylaminoxylphenyl ring torsion angles by virtue of different steric demands of their "spin-protecting groups". In addition, the synthesis of a TMM-type dinitroxide having a planar pi-system is reported. The synthetic route employed for the planar TMM-type biradical is general and should therefore expand, the utility of TMM-type biradicals that are susceptible to bond torsions that attenuate exchange coupling. The compounds presented are designed to be effective to map out J-coupling/conformation space in TMM-type biradicals. EPR spectroscopic parameters (hfcc and D-values) were compared to determine if their values reflected differences in conformation in the series of biradicals. Interestingly, neither N- nor H-hfcc varied within our series of biradicals/monoradicals in a regular fashion, indicating that there is no apparent relationship between N- or H-hfcc and conformation (as judged by molecular mechanics calculations). However, D-values, estimated from relative intensities of Delta m(s) = 1 and Delta m(s) = 2 transitions, are consistent with a varied degree of delocalization in the dinitroxides: smaller D-values for biradicals having smaller aryl torsions (and greater delocalization), and larger D-values for dinitroxides having greater aryl torsions (and less delocalization). All the biradicals studied, except 5(..), exhibited linear Curie plots. The linear Curie plots are consistent with both triplet ground-states and singlet-triplet degeneracies. Interestingly, dinitroxide 6(..) exhibited a linear Curie plot, despite the lack of a pi-coupling fragment. Biradical 5(..), however, is a ground-state singlet; species with the triplet lying about 140 cal/mol above the singlet.