Structure−Property Relationships in Trimethylenemethane-Type Biradicals. 2. Synthesis and EPR Spectral Characterization of Dinitroxide Biradicals

Abstract

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 "spinprotecting groups". In addition, the synthesis of a TMM-type dinitroxide having a planar π-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 ∆m s ) 1 and ∆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 π-coupling fragment. Biradical 5 •• , however, is a ground-state singlet species with the triplet lying about 140 cal/mol above the singlet.

Tuning the Spin Coupling Interactions in the Nitroxide‐Based Bisphenol‐Like Diradicals

“…Since no experimental reports of these designed diradicals are available, we here first give two analogues to confirm the feasibility of our design and usability of the calculational methods. Over the past years, Shultz and co‐workers synthesized stable trimethylenemethane analogues which are very similar to our designed diradicals of Etpp and investigated their magnetic properties . Two of them are shown in Figure S2.…”

Magneto‐Structural Characterization of Metallocene‐Bridged Nitronyl Nitroxide Diradicals by X‐Ray, Magnetic Measurements, Solid‐state NMR Spectroscopy, and Ab Initio Calculations

“…[17] Keeping the above considerations in mind, compounds a-2 and b-2 were analyzed. Thus, in a-2, the twist angles between the imidazoline and Cp rings are f = À118 and À128, whereas in b-2 both angles f are close to zero, thereby promoting a stronger magnetic exchange interaction between the radical units.…”

Structure-Property Relationships in New Semiquinone-Type Ligands: Past, Present, and Future Research Efforts

“…Likewise, one-electron reduction of metal complexes of 1 -5 will yield Downloaded by [The University of Manchester Library] at 08:43 09 December 2014 mixed-valent semiquinone-Linker-catecholate complexes, and we can test how conformation modulates delocalization. [46] We have shown that the molecular conformation is affected by the functional groups that cap the Linker: [56] clearly, the p-systems of the EG and Linker in 1 are mutually coplanar, while in 2 and 3 moderate torsion is induced by the norbornyl and quinone groups, respectively. Bis(semiquinone) 4 has even greater torsions, and 5 is a model compound that allows assessment of the C=C as Linker.…”

“…[29] However, relatively little is known about conformational exchange modulation in TMM-type biradicals. [29] To study this important structure-property relationship, we prepared a series of dinitroxide biradicals [56] and bis(semiquinone) ligands 1 -5. X-ray crystallographic analysis and magnetometry are nearly complete on the dinitroxide series of molecules and will be reported soon.…”