Synthesis, Crystal Structure, Magnetic, and Electron Paramagnetic Resonance Properties of a Spiroconjugated Biradical. Evidence for Spiroconjugation Exchange Pathway

A spiroconjugated nitronyl nitroxide biradical, 6,6‘-(4,4,5,5-tetramethylimidazolidine-3-oxide-1-oxyl)-3,3,3‘,3‘-tetramethyl-1,1‘-spirobisindane (1), has been prepared by functionalization of a 3,3,3‘,3‘-tetramethyl-1,1‘-spirobisindane framework followed by Ullman condensation and subsequent oxidation. The biradical
crystallizes in the monoclinic space group C2/c with four molecules in the unit cell of dimensions a = 24.861(10) Å, b = 12.129(3) Å, and c = 12.258(6) Å. X-ray analysis of a blue-plate single crystal has revealed
dihedral angles of 28° between the nitronyl nitroxide moiety and aromatic ring with intramolecular through
space radical−radical distances of 8.25 and 10.11 Å. In the solid state, the temperature dependence of the
molar magnetic susceptibility reveals antiferromagnetic interactions. These interactions are best fit using a
pair model, affording the value J = −4.0 cm-1 where J is the interaction parameter appearing in the spin
Hamiltonian H = −JS1·S2. The field dependence of the magnetization measured at 2 K is consistent with a
pair model. Frozen matrix EPR spectra of biradical 1 in CH2Cl2 at 100 K shows a half field transition at 1700
G. Temperature dependence of the half field transition intensity has been found to be consistent with a ground
singlet state and thermally accessible triplet state. The magnetic interaction observed in the solid state is also
observed in solution. Thus, room-temperature solution spectra display a nine-line pattern, with hyperfine coupling
to four “equivalent” nitrogen atoms and a hyperfine coupling constant aN = 3.8 G. Temperature dependence
of the solution EPR spectra of biradical 1 displays alternating line width effects caused by conformational
dynamics in solution. This behavior has been attributed to modulation of exchange and hyperfine interactions
most likely caused by rotational motion about the nitronyl nitroxide−phenyl bond. Biradical 1 therefore exists
as a ground-state singlet with a thermally accessible triplet at ca. 4 cm-1 higher in energy with a conformational
dependence of intramolecular exchange in solution. This coupling may present evidence for spiroconjugation
as an exchange pathway. Density functional calculations (B3/6-311G(D)) have been performed to investigate
this possibility.